CA2133556A1 - Improvements to optical phase shifting - Google Patents
Improvements to optical phase shiftingInfo
- Publication number
- CA2133556A1 CA2133556A1 CA002133556A CA2133556A CA2133556A1 CA 2133556 A1 CA2133556 A1 CA 2133556A1 CA 002133556 A CA002133556 A CA 002133556A CA 2133556 A CA2133556 A CA 2133556A CA 2133556 A1 CA2133556 A1 CA 2133556A1
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- Prior art keywords
- polarisation
- light
- paths
- propagation
- wavelength
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/09—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect
- G02F1/093—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect used as non-reciprocal devices, e.g. optical isolators, circulators
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2589—Bidirectional transmission
- H04B10/25891—Transmission components
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- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Nonlinear Science (AREA)
- Electromagnetism (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Power Engineering (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
Abstract
A non-reciprocal optical phase shifter is disclosed, together with various applications. The phase shifter includes direction dependent and independent polarisation rotating devices (52, 53) in parallel optical paths which are recombined, so that the relative phase shift of the output signal depends on the direction of propagation. The device can be employed to form simple isolators, bidirectional wavelength dependent isolators, circulators, and enables bidirectional communications down a single fibre at different wavelengths without polarisation selection. Bulk optics and fibre implementations are disclosed.
Description
wo93/2n47s ~ i5~ PCI/A1193/00146 IMPP~OVE~MENTS TO OPTICAL PHASE SHIFTING
Technical Field The present inv~ntion relates to optical systems, including phas~
shifters, isolators, circulators and bi-directional ~ommunication systems, 5 particularly but not exclusively for optical fibre communication systems.
Background Art An opticai isolator is an important componen~ in many optical systems, including communications applications and bulk lasers. The role of the isolator is to allow transmission of light in only one dir~ction.
10 Isolators are used in communications system~ to prevent feedback resultin~ from refi~ctions returning to a laser dio~e, and are used in conjunction with optical amplifiers to ensure thers is no lasing or noise degradations due to feedback. A typical amplifier may have a conventional isolator at its input, its output or both input and output. It is 15 not practically possible t~ operatB a very high gain amplifier without isolation because of residual refiections and scattering.
The most common type of prior art isolator relies on the Faraday ~ffect to achieve non reciprocal rotation o~ the polarisation state of light. Ih the presence of an applied magnetic field the pslarisation of 20 light is rotated in a direction independ~nt of the propagation direction, and proportional to the Verde constant of the material in whi~h the light propagates. ~ By using a crystal~ with a high V~rde constant, a rotation of 45 ~degrees can be effected in a short distance. A single polarisation isolator can be constructed~ as shown in Figurs 1. Light travelling from 25 1 eR to right is polarised in the vertical direction, th~n rotated by 45 degrees by the Faraday rotator~ A second polariser is placed at this an~le, allowing the light to pass undisturbed, Light travelling from right ~to Ieft is polarised in the 45 degres direction, then rotated by the ~ ~ Faraday elemsnt a further 45 degrees, so that it is now in the horizontai 30 plane. This light will be completely block~d by the vertical polarisation so there will be no transmission in the right to left direction.
One technique for achi~ving polarisalion independent isolation is WO 93/21)475 ~ 1 3 3 5 5 1~ PCl ~AU93/00 to split incident light in~o two poiarisa~ion components, isolating ~ach of ths components, and then recombining the two polarisation ccmponents.
The polarisatîon splitt?rs th~mselves may act as the polarisers for the isolators, so a polarisation independent isolator can bs constructed as 5 shown in Figure 2.
Whilst such iselators are essentTal in many amplification applications, they negate the possibility of two w~y communicatlon down a single fibre, and of bi-directionai ampli~ication. It wolJld be advantageous to be able to provide singl~ fibre two way comrnunication 10 ~ in systems which use optical amplifiers.
It is one obj~ct of the pres~nt inv~ntion to provi~e an improved isolator which at least ameliorates the disadvantag~s of the prior art.
According to one aspect ~he present inv~ntion proYides a non-reciprocal phass shifter, comprising at least one first polarisa~ion rotating 15 means, the direction of polarisation rotation being dependant on the direction of propagation~ of transmTtted light, and at least one s~ccnd ~ means for altering polaris~ation, the direction of polarisation alteratlon : ~ being independent of the direction of propagation of transmitted light,c hara~terised in that substantially half of any light propagating trom either 20 end of the phase shifter havin~ an arbitrary polarisation travels through : each of: the first polarisation rotating means and s~cond means for a5tering poiarisation.
According to:~ another aspect the invBntion comprises a non-reciprocal opticai phase shitter, comprising m~ans tor transmitting incident 25 light through first and second optical paths and recombining said paths at an output, said paths including respPctively first ~nd second means for altering the polarisatlon of: incident light having arbitrary polarisation, at ieast one of said means for altering polarisation having a first rotation in one direction of propagation and another rotation in a second direction of 30 propagation, and the o~her: or one of said rneans for aitering having a polarisation change independent of the direction of propagation, the arrangement being such that a first reiative phase shift wo s3~2047s 2 1 3 3 S ~ li PCI /AU93/001~6 between the paths occurs for light propagating in on~ direction, and a second relative phase shift betw~an the paths occurs tor light propagating in a second direction.
According to a Surthsr aspect the invention ~omprises a non-5 reciprocal optical phase shifter, cemprising means for transmitting incidentlight through first and s~cond optical paths and r~combining said paths at an output, said first path includin~ In a tirst p~opagation direction su~cessively first polarisatlon rotating m~ans having a rotation dependent on propagation dire~tion and second polarisation altering means having a : : 10 change independent of the dir~ction of propasatl~n~ said second path including in said first propagation direction successively thTrd p~larisation : alteriny means haYing a change independQnt of the direction of propagation and ~ourth polarisation rotating means having a rotation dependent on propagation directionj tS ~ ~ ~ the arrangement being ~ such that substantially half of the incident: light trav~ls through each of said first and second paths, and the ; relative: phase shi~t of ~ ~output light is dependent on the dir~ction of pr~pagation of the: incident light.
A :furth~r aspect of the present invention provides a non-: 20~ reciprocal phase shifter,; comprising means for transmitting substantiallyh~alf of incident ~ light into two optical paths each having arbitrary : polarisation, and recombining: the paths to produce an outputl each of said: ~paths: comprising ~ means for altering the polarisation of the t ransmitted light,: characterise~ in that in a first direction of propagation 2S the~paths have outputs~:which ars have a first relatiY6 phase shift, and in th reverse :direction the :paths have outputs which have a different : relative phase shift.
:~ : : Another aspect of the present invention provides an optical , : isolator, comprising ~a non-reciprocal phase shifter characterised in that 30 ~ for a selected waveiength, in a first propa9ation dirsction the output ~ight is substantially transmitted,~ and in the reverse direction the output light is : substantially attenuated.
WO 93/2047~ 1 3 3 5 5 6 PCI /A U93/00!
A further aspect of the present invention proYides bidirectional optical isolator, cornprising m~ans for ~ransmi~ting substantially half of incident light through each of ~irst and second optical paths, at least said first path including p~larisation rotating means having a rotation 5 dependent on propagation dir~c~ion and at least said second path including second polarisation altering means having a rotation independent of tha diraction of propagation, sàid first and second paths having a path length difference, and means for re~ornbining said first and second paths, 10the arrangement being such that for llght haYing a first wavelength propagating in a tirst direction total attenuation occurs, while for light having said first wavelength propagating in a second reverse direction substantia3 transmission occurs; and for Jight having a second :: wavelength propagating in said first direction substantial transmission ~15 occurs, while ~r iight having said second wavelength propagating in ~: : said second reverse direction substantial attenuatiQn occurs.
: ~
A further aspect of the present invention provides an optical circulator9 comprising at~ieast one input to an non-reciprocal isolator, and means for coupling the: ~isolator to two output~, th~ arrangement being 20;: such that~light having one: wavelength is output through onB output, and ght~ having a second wavelength is output through the second output.
A further aspect of the present invention provides bidirectional optlcal fibre communications~ system, in which signals travelling in a tirst direction ~have a first wavelength, and signals travelling in the other 25~: direction have a second w~velength, both signais travelling in the same optical fibre, characierised~ in that the system includes one or mors wavelength selective bidirectional isolators~ .
A further aspect of ~the present in~ention provides a bidirectional ~-optical amplifier for allowing amplification of signals at a first wavelength :30 in a first direction, and at a second waveien9th in the second, reverse direction, comprising means for inducing gain at said ~irst and second wavelengths, and bldirectional wavelength dependent isolation means :.' ' . .
~)13~S5~
arranged such that signals at said first wavelength travelling in said first direction, and signals at said sscond wavelength travelling in said second direction, are transmitted, and signals at saict firs~ wavelength travelling in said second direction, and signals at said second wave,ength trav~lling in said first direction, are attenuated, and such that undasired teedback at said first and second wav~lengths is substantially suppr0ssed.
A further aspect of the present invention provides an optica isolator, comprising means for transmitting substantially half ot incident light into two op~ical paths each halving arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for alterin~ the polarisation of the transmitted light, t 5 characterised in that in a first direction of propagation the paths have outputs which are in phase, ~nd so transmit the incident light, and in the r~verse direction the paths have outputs whic~ are 180~ out of phase9 and ;so do not transmit tha incident light.
: 20 Br~e~ Description of Drawings Several ernbodiments of the invention will be described with ~- ~ r eference to the drawings, in which:
:~ Figure 1 is a schematic view of a prior art isolator;
~: ~ Figure 2 is a schematic view of another prior art isolator;
Figure 3 is a sch~matic view of an optical circulator;
Figure 4 iilustrates a first technique for bi-directional isolation in a wavelength diversity transmission system;
Figure 5 i!lustrates a second techniqus for bi-directional isolation in a wavelength diversity transmission system;
Figure 6 illustrates conceptually a techniqu~ for bi-directional isolation in a wavelength diversity system;
Figure 7A and 7B illustrate interference within the system ~f wo s3/2047s PCr/AUg3JOO-~33556 6 figure 1 0, Figure 8 illustrates ona implem~nta~ion of the systsm ~ figure 10;
Figure 9 illustrat~s anoth~r impl~mentation of the system of 5 figure 10;
Figur~s 10A and 10B illus~rate a preferred implem~ntation of a non-reciprocai phase shifter;
Figure 11 illustrates a polarisation~ dispersion fr~s isolator;
Figure 12 illustrates a balanced polarisation dispersion free ~0 :design;
Figure 13 illustrat~s an integrated impiementathn o~ a ci~ulator/isolator according :to tho pres~nt inventi~n;
Figure 14 is a graph showing wavelangth dependence in an exp~rimental device;::
15 : ~ Figure 15 illustrates a prefsrred implementati~n of the system of `
figure 10;
~: Figure 16 illustrates schematically a bidire~ional amplifier. : -Figure 17 illustrates schematically a fibr~ ~mbedd~d circulat~r 20~ according to th~ present invention; ~:
Figure 18 ~ illustrates an implementation ~f the device of Figure Figure 19 illustrates~ a mod~&onv~rting circulator;
:: : Figure 20 iilustrates a fused Mach Zender irnplementatlon of an :25 ~ isolato~/circulator: according to the preseni invention; : ::
- ~.
~; Figure 21 illustrates a filtered isolator accordin~ to the pr~sent inYention;
Figure 22 illustrates a high isolation devica;: ~;j : : Figure 23~ illustrates ~ a network utilising a bi-dlr~ctional i::
30 ~ amplifier,and ` ;;
Figure 24 illustra~es schematically a low palarisation dispersion amplifler.
W093/20475 ~13~55~ PCI/AU93/00146 Detailed description This aspect of the pres~nt invention is particularly adapted to be implemented in bi-dir~ctional networks. In tha transmission system envisaged ~sing this form of isoîation, transmission in one direction takes 5 place at one wavelength, and in tha opposite direction a~ a second wavelength, as is illustrat~d in principle in Figuro 23. A partlcular difficul~yin this arrangemen~ is in cons~tuc~in~ a simple isolatQr wh}ch achi3ves wavelength s~lectiv0 isolation. H such an isolator could be constructed, ~t is clear that the amplifie-: would be adequately isolated from refl~ctions, 10 as such refiections would in general maintain the wavalength of the light : and so be passed out of the system upon meetin~ an isolator.
There are several approaches that can be taken to achieve the desired end. A first embodimen~ of this type o~ isolator depends upon ~ ~ splRting the light into the:two wavelengths using a grating or wavelength 5 ~ division~: multiplexer 34, isolating each wavelength separately in each direction using isolators~ 32, 33, th~n recombining the light afterwards 31, as shown in Figure ~4. A: slightly more refin~d approach can be constructe:d with an optical clrculator 42 and a single waveiength multiplexing elemsnt 41 as :shown in Figure ~.
20~ This aspect :of the: pr~sent invention is based on a new approach to isolation~ which is inter~erometric instead Ql being polarisation dependent. One immediate ~advantage that arises from this is that all of the embodiments~ described~: are in general polarisation independent, :
with~ut the need t~ split ~ and: recombine polarisatJons. Sorne o~ the 25~ mplementations illustrafed ~aré for single moded waYeguiding applications only, but the genera3 concepi is~ equalîy applicabls to bulk Isoia~ors. ~ -A general: form of this aspect of the invention is shown in ~: ; Figure~ 6.~ A ~ingle ~beam o f light entering at: point 1 is split by -beamsp!itter 51 into two paths ~formed using mirrors 54, 55. In one path 30~ ~a~ 90 degree Faraday rotator ~3 is placed, and in~ the second path a pair of half-waYe retardation~ plates 52 at a relative angle of 4~ degrees ~s ::
placed. The pair of reta ders 52 rotates ~he polarisation of any arbitrary ::
:
,,,:
WO 93/21)47~ PCl/AU93/00 2133S5~
polarisation of incident light by 9Q d~grees.
This can be d~monstrat~d by the appropriate Jones' Matrix multiplications~ The two polarisations are now in ~he same direction, and the optical path length can now be adjusted to b~ Qt:lU~II in both arms 5 ensuring constructive interfererlce on ~h~ throvgh pa~h ~1) to (4). 3n the reverse path with light incident at por~ 4, ths polarlsatlon of the two b~ams will be opposits because the F~raday rotation Is independent of the prvpagation direction, whereas ~he birefringent ro~a~ion is reciprocal.
This corresponds to a 1 8û degrees phase chan~e in the electric field 10 vector between the two be~rns and dsstructivs int~rfsrence wTII oc~ur at port 1. The light will instead exit thfough port 2 formiQg an isslator.
.
The d~vice created is r~lated to a Mach-Zender interferometer.
It will be apparent that wavelength dependence in the isolation can be ;achieved. If an optical ;path length difference is introduced between the 15; two arms of the Mach-Zender, then there will be only certain wavelengths ;~ ` for which th~ int~rference will be constructiv~ in light travelling ~rom port to port 4. This will give a sin2 de,oQndencs to the intensity of light emerging In port 4 as~ a function ~of ~, with a peri~)dicity proportional to 1, the difference in optical path lengths, with the remaining light being ~20 ~ redirected to port 3. Because of the 180 degree phas~ shift between beams travelling right~to left, the tfansmission curve at port 1, Figure 7A, wili ~ be complimentary ~to that obtained at port 4, Figure 7B. For wavelengths having ~destructive intPr~erence at port 4 for forward propagation, constructive interference wili be obtained at port 1 for reverse 25 propagation.
This is precisely the characteristics required ~or a wavelength dependent isolator. ;For instance, by using an optical path difference of 0.12 mm a device can be constructed to be forward propagating at 1535 nn (corresponding to the first gain peak of the erbium-doped fibre 3û amplifier) and revers~ propagating at 1555 nrn (the second gain peak of the erbium doped fibre arnplifier). Isolation is provided for both wavelengths.
wo 93/2047s ~ ~ 3 ~ 5 ~ 6 Pcr/Aug3/l)0146 The main obsta~le to be overcome is the stabillty of the device to external perturbations. As an int~rfQrometric device i~ is potentially much more sensitive to any ~herrnally Qt mechanically induced variations in the relative path lengths. Althou~h th~rmai and rnechanical 5 stabilisation is possible, it is unlik~ly tha~ any bulk d~vice built accoFdirlg to Figurs 6 would have the required stability tor a ~ield devlce.
One solution is to build .the entire device from s~lid componants, with a balance in ~he construction of ~he arms such that any therrnal expansion affects each arm equally (excspt for the very small :: 10 path length difference 1/ 1. This is shown in Figure 8. Incident light 62 is incident on silvered face ~3, and passes either through rotat~r 67 and half wav~ plate 68, or rotator 64 and half wav~ plate 65, to eventually exit 66. It will be apparent that an isolator similar to figure 6 is created.
It is :noted that instead of a~ 90 degree Faraday ro~ation in one arm we : :t5 now:hav~ a 4~ degree~rotation in one arrn ~nd minus 45 degr~es in the second arm,and one of the hal~ wave plates ts Tn one arm and the second i5 in the second arm. It can a~ain be shown by Jones matrices ~: : ; :that this is equivalent :to ~the~ previous arrangement. It is, however, very difficult t o produce a ~50% beam-splitting cube which is polarisation 20~ ~insensitive~ at~ many of the wavelengths of particular interest to optical : communi~ations. ~:
A:second :solution is to physicaOly split the beam into the top half ~and bottom half and to~ use:~the same rotators and birefringent plates as ~ before ,as~ shown in ~Figure 9. Lens 73, 71 ~collimate/focus the : 25~: incident~: 1ight to~ and from~ beam 75 and project the light onto mirror set 70. It is now much less obvious that this should work as an isolator, or : even that there shQuld ~ be~ any interfersnce at all. In fact this will no : longer :act as a bulk isolatot, and it is only when the~input 74 and output ~ ~ 72 are both single mode: fibres that the interference or isolation will take :: ; 30 place. This is because ~of the quantum nature o~ the modes of a fibre.
A qualitùtive theoretical understanding can be obtainad by : considering the device as a black box, which provides ~two paths ~f equal WO 93/20475 P(:~/AU93/00' 213 3~ ~fi lo probability and the sante phase for photons travelling in the forward direction. The expectation value of a photon arriving at a point in the second fibre is the sum of ~the diftsr~nt possible wave functions times the probabiiity of that wave ~unction~. In the forward case the wave functions 5 will be in phase for a ~iven wav~leng~h photon and the normalised expectation value will be ~ (100% probabili~.of the photon arriving). In the reverse direction for that same wavel~nQih the waYe functions will be opposite sign and so the probability of the photon arriving will be 0.
The aboYe argum~nt is easily v~rified by using the formalism ~f t 0 Fourier optic~. An assumed Gaussian moda is transformed by the l~ns into a Gaussian beam in Fourier space. In the absence o~ any phase delay, the second lens transforms the be~m back to the original Gaussianf which eXCitBS ~ully the ~undamental mode of the second fibre~
In quantum mechanical terms, the overlap int~gral of the mode and the 15 exciting~light is 1. We~have 100% transmiss!on~ If a phase delay of ~
is: incurred in the top half: of th~ beam, then this is equivaient to ~: multiplying by -t the top half ot the Fourier transform. Upon performing ~ ths Fourier transform corresponding t~ the second lans, an odd function is :: : obtained. The overlap integral of an odd functiQn with an e~ en function 20 ~ is~ necessarily zero. it is ~now clear what is happening to the photons which are lost: to the system. They are simply trying to excite higher order modes which are cut off and so cannot propagate along the ~ibre.
These photons are lost very~quickly into the cladding ~t the flbre.
This principla has been demonstrated in the laboratory by 25 constructing a very simple interferometer, which has proved surprisingly stable. An optical flat was inserted so as to occupy ha~ of the beam in a beam expander between two single mode fibres. Ther~ is a clear modul:ation of the received light as a function of wavelength, corresponding to the two paths being in phase and out of phase. This 30 is shown in Figure 14.
This leads us to another device ~or bi-directional isolation~ In this the expanded beam is nsver physically separated. A compound ~133SS6 element consisting of a 90 ~egree Faraday 7~ ro~a~or and a pair of crossed half wave plates 7~ is made up, by polishing and joining on one side. This cornpound element is now located in ~he bRam so as ~o occupy exactly one half of the beam as shown in Figure ~ ~. The path S difference can ba achi~ved by having a slightly different optical path between tha Faraday rotator and the birefringent elemont. This is conceptually identical to the davice outlined in Figure 9. The exact wavelength of transmission can be tuned by varying slightly the angie at which the elemen~ is inserted into the beam. Ths thickness of this 10 compound element need not be much larger than 500 llm with stat~of-th~art Faraday rotators. Insertion loss should be negligible (c0.5 dB) with good beam expansion optics, and stabiliîy is maximised because there is only a small length of crystal (~1 mm) generating the path difference.
:1 5One disadvantage of She embodiments de~cribed is that the ~: construction of the non-reciprocal phase shi~ter used different materials to act upon: each half of the beam which is split. The device is ther~fore .
susceptible to temperature dependence as the thermal expansion coefficients nd thermal refractive index coefficients for each half will be 20 dfflerent, thus shifting the~ wavelength of maximum isolatlon.
Figures tûA, 10B illustrate a preferred stabl2 design for the non- :
reciprocal phase shifting element.
:: :
:: This implementation: uses a 45 degree Faraday rotator 101 in ~: both::halves and a haif-wave plate 100, 102 in both halves. In one half : 25 the half waveplate 100 is to the left of the Faraday rotator 101, and in the second half, the half waveplate 102 is to the right of tha Faraday rotator 101. The ~second waveplate 102 is orientated to have its QptiC
axis at 45 degree relative to the first waveplate 100.
To understand the operation of the device ~onsider the path of 30 light travelllng left to right in the vertical polarisation 104 for a wavelength independent device~ In the top half of the beam, tha light tra~els through the fast axis of the half-wave plate 100 then through the Faraday rotator WO 93/2047~ PCr/AU93/001 ~ ;~ '3 S S 6 12 101 to rotate the polarisation 45 degre~ c~ockwise. The light in the bottom half of th~ beam passss ~irst lhrough the Faraday rotator 101, to be rotated cleckwise into line with the fast axis of th~ half wave plate 102. The two beams upon recombining wil~ be in phase. Simitarly, for 5 light in the horizontal polarisa~ion 105, light travelling through both the top and bottom halves wil~ travel through . !hè slow axis of the half wave plates 100, 102 and so will be in phase. ~ight travelling from right to left, incident at 45 dagree clockwise to ~he verti~al in the top halt wlll travel through the Faraday rotator 101 first to ~e rotated to the horizontal 10~ axis and then pass through the slow axis of the half wava plate 100.
Light travelling from right to left, incident at 45 d~gree clockwise to the vertical in the bottom half wilt pass first through the fast axis of the half wave pla~e 102 then through the Faraday rotator 101 to be in the : horkontal axis. As: the top beam has trav~lled through the fast axis and 15 the bottom beam has~ travelled through tha slow axls th~re is a 180 : degree relative: phase shifl~ between both halves of the beam. In the forward direction, recombination of the light leads to constructive interference and hence ~ transmission, and in the revers2 direction recombination of the light~ leads to destructive interf~rencs and hence 20~ attenuatlon. This is~the:~basis:for~non-reciprocal transmission, or isolation.
:This~may~ be readily ~extended ~to a circulator application, as will be apparent to the reader. ~
The isolator describ2d with reference to Fi~ure 10 has a very small~ (yet :: finite~ polari~s~ation:dispersion of one wa~elength difference 25~:::between :the~ two polarisaiion states in the forward directiQn. This is because one polarisation travels in the fast axis ~top and bottom halves) : ~ and the other polarisation travels in the slow axis of ths half wave plates.~The -~resultant polarisation~ dispersion is for 1.5 ~m light equal to 5 fs.
: ~` : Although this value is much iower than even the best commercially ~:: ;30 a~railable isolators, it is: possible to design the present isolator to have intrinsically zero polarisation dispersion. This is done by making the forward transmissive palh equivalent to a fast axis and a slow axis transit.
;
Wo 93/2047s ~ 5 5 ~ PCI/AU93iO0146 Figure 1t shows hvw this can b~ achieY~d. The top half consists of for the vertical polarisation.
fast axis: Faraday rotator: slow axis for th~ hwizon~al pol~fisa~ion slow axis: Faraday rotator: fast axis The bottom half consists for both polarisations ot an optical path length equivalent to fast axis + Faraday rotatQr + slow axis.
There is no intrinsic polarisation disp~rsion.
Figure 12 is equivalQnt to Figure 11 except that the design is 10 entir~ly balanced again.
A simpie opti~al circulatQr using the b~am splitting isolator as the basis may be readily implernented as shown in Fi~ure 19. In the isolator the light which is rejected in the non-transmission direction is ; ~ antisymmetric about the~ axis of the interface and so cannot excite th~
15 fundamental mode o~ a single mode fibre. It can however excite the tirst higher~order mode~of a~ mul~imode fibre 110 which can have the same symmetry. By using~a ~fibre ~which supports this hi~her otder mode and then~produGin g a coupler~t~t5 which coup~es the higher order mode to a se~ond~fibre~ we can~ per{~rm the ~unction of a thre~ or four port circulator.
20 ~ Single~ ~ mode fibres 116 ar~ coupled 114, 115 to multimode fibre t 18, 110.~ ~ ~Lens ~111, 113 and~ phase shifter 1t2 fofm~ a beam-expanded Isolator~ as descrlbed above.~ Such a coupl~r ~an ~uple light ~rom the ;correct~ symmetry higher~ order~ mode ~o the fund~mental mode of a sin~le mode fibre, where~ tl~e~ ~ propagation coefficients are matched in the 25~coupling ~region. Ei~her~fused or polished ~coupling techniqu~s can be used. Note that with the crystal interface in tha parall~l ~xpanded beam, non-op~imal excitation o f~ the higher order mode is achieved with ths remaining light lost to the system This can be improved by having the interfa~e where the beam of light is in transîtion from the near-field to far-30 field~ image. This is achieved by~focussing ~he light between the lens~s ~ ~ snd placing the non reciprocal phas~ shifting crystals at an appropri~t~
; ~ ~position.
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WO 93~2047S P~/AU93/0~ .
Technical Field The present inv~ntion relates to optical systems, including phas~
shifters, isolators, circulators and bi-directional ~ommunication systems, 5 particularly but not exclusively for optical fibre communication systems.
Background Art An opticai isolator is an important componen~ in many optical systems, including communications applications and bulk lasers. The role of the isolator is to allow transmission of light in only one dir~ction.
10 Isolators are used in communications system~ to prevent feedback resultin~ from refi~ctions returning to a laser dio~e, and are used in conjunction with optical amplifiers to ensure thers is no lasing or noise degradations due to feedback. A typical amplifier may have a conventional isolator at its input, its output or both input and output. It is 15 not practically possible t~ operatB a very high gain amplifier without isolation because of residual refiections and scattering.
The most common type of prior art isolator relies on the Faraday ~ffect to achieve non reciprocal rotation o~ the polarisation state of light. Ih the presence of an applied magnetic field the pslarisation of 20 light is rotated in a direction independ~nt of the propagation direction, and proportional to the Verde constant of the material in whi~h the light propagates. ~ By using a crystal~ with a high V~rde constant, a rotation of 45 ~degrees can be effected in a short distance. A single polarisation isolator can be constructed~ as shown in Figurs 1. Light travelling from 25 1 eR to right is polarised in the vertical direction, th~n rotated by 45 degrees by the Faraday rotator~ A second polariser is placed at this an~le, allowing the light to pass undisturbed, Light travelling from right ~to Ieft is polarised in the 45 degres direction, then rotated by the ~ ~ Faraday elemsnt a further 45 degrees, so that it is now in the horizontai 30 plane. This light will be completely block~d by the vertical polarisation so there will be no transmission in the right to left direction.
One technique for achi~ving polarisalion independent isolation is WO 93/21)475 ~ 1 3 3 5 5 1~ PCl ~AU93/00 to split incident light in~o two poiarisa~ion components, isolating ~ach of ths components, and then recombining the two polarisation ccmponents.
The polarisatîon splitt?rs th~mselves may act as the polarisers for the isolators, so a polarisation independent isolator can bs constructed as 5 shown in Figure 2.
Whilst such iselators are essentTal in many amplification applications, they negate the possibility of two w~y communicatlon down a single fibre, and of bi-directionai ampli~ication. It wolJld be advantageous to be able to provide singl~ fibre two way comrnunication 10 ~ in systems which use optical amplifiers.
It is one obj~ct of the pres~nt inv~ntion to provi~e an improved isolator which at least ameliorates the disadvantag~s of the prior art.
According to one aspect ~he present inv~ntion proYides a non-reciprocal phass shifter, comprising at least one first polarisa~ion rotating 15 means, the direction of polarisation rotation being dependant on the direction of propagation~ of transmTtted light, and at least one s~ccnd ~ means for altering polaris~ation, the direction of polarisation alteratlon : ~ being independent of the direction of propagation of transmitted light,c hara~terised in that substantially half of any light propagating trom either 20 end of the phase shifter havin~ an arbitrary polarisation travels through : each of: the first polarisation rotating means and s~cond means for a5tering poiarisation.
According to:~ another aspect the invBntion comprises a non-reciprocal opticai phase shitter, comprising m~ans tor transmitting incident 25 light through first and second optical paths and recombining said paths at an output, said paths including respPctively first ~nd second means for altering the polarisatlon of: incident light having arbitrary polarisation, at ieast one of said means for altering polarisation having a first rotation in one direction of propagation and another rotation in a second direction of 30 propagation, and the o~her: or one of said rneans for aitering having a polarisation change independent of the direction of propagation, the arrangement being such that a first reiative phase shift wo s3~2047s 2 1 3 3 S ~ li PCI /AU93/001~6 between the paths occurs for light propagating in on~ direction, and a second relative phase shift betw~an the paths occurs tor light propagating in a second direction.
According to a Surthsr aspect the invention ~omprises a non-5 reciprocal optical phase shifter, cemprising means for transmitting incidentlight through first and s~cond optical paths and r~combining said paths at an output, said first path includin~ In a tirst p~opagation direction su~cessively first polarisatlon rotating m~ans having a rotation dependent on propagation dire~tion and second polarisation altering means having a : : 10 change independent of the dir~ction of propasatl~n~ said second path including in said first propagation direction successively thTrd p~larisation : alteriny means haYing a change independQnt of the direction of propagation and ~ourth polarisation rotating means having a rotation dependent on propagation directionj tS ~ ~ ~ the arrangement being ~ such that substantially half of the incident: light trav~ls through each of said first and second paths, and the ; relative: phase shi~t of ~ ~output light is dependent on the dir~ction of pr~pagation of the: incident light.
A :furth~r aspect of the present invention provides a non-: 20~ reciprocal phase shifter,; comprising means for transmitting substantiallyh~alf of incident ~ light into two optical paths each having arbitrary : polarisation, and recombining: the paths to produce an outputl each of said: ~paths: comprising ~ means for altering the polarisation of the t ransmitted light,: characterise~ in that in a first direction of propagation 2S the~paths have outputs~:which ars have a first relatiY6 phase shift, and in th reverse :direction the :paths have outputs which have a different : relative phase shift.
:~ : : Another aspect of the present invention provides an optical , : isolator, comprising ~a non-reciprocal phase shifter characterised in that 30 ~ for a selected waveiength, in a first propa9ation dirsction the output ~ight is substantially transmitted,~ and in the reverse direction the output light is : substantially attenuated.
WO 93/2047~ 1 3 3 5 5 6 PCI /A U93/00!
A further aspect of the present invention proYides bidirectional optical isolator, cornprising m~ans for ~ransmi~ting substantially half of incident light through each of ~irst and second optical paths, at least said first path including p~larisation rotating means having a rotation 5 dependent on propagation dir~c~ion and at least said second path including second polarisation altering means having a rotation independent of tha diraction of propagation, sàid first and second paths having a path length difference, and means for re~ornbining said first and second paths, 10the arrangement being such that for llght haYing a first wavelength propagating in a tirst direction total attenuation occurs, while for light having said first wavelength propagating in a second reverse direction substantia3 transmission occurs; and for Jight having a second :: wavelength propagating in said first direction substantial transmission ~15 occurs, while ~r iight having said second wavelength propagating in ~: : said second reverse direction substantial attenuatiQn occurs.
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A further aspect of the present invention provides an optical circulator9 comprising at~ieast one input to an non-reciprocal isolator, and means for coupling the: ~isolator to two output~, th~ arrangement being 20;: such that~light having one: wavelength is output through onB output, and ght~ having a second wavelength is output through the second output.
A further aspect of the present invention provides bidirectional optlcal fibre communications~ system, in which signals travelling in a tirst direction ~have a first wavelength, and signals travelling in the other 25~: direction have a second w~velength, both signais travelling in the same optical fibre, characierised~ in that the system includes one or mors wavelength selective bidirectional isolators~ .
A further aspect of ~the present in~ention provides a bidirectional ~-optical amplifier for allowing amplification of signals at a first wavelength :30 in a first direction, and at a second waveien9th in the second, reverse direction, comprising means for inducing gain at said ~irst and second wavelengths, and bldirectional wavelength dependent isolation means :.' ' . .
~)13~S5~
arranged such that signals at said first wavelength travelling in said first direction, and signals at said sscond wavelength travelling in said second direction, are transmitted, and signals at saict firs~ wavelength travelling in said second direction, and signals at said second wave,ength trav~lling in said first direction, are attenuated, and such that undasired teedback at said first and second wav~lengths is substantially suppr0ssed.
A further aspect of the present invention provides an optica isolator, comprising means for transmitting substantially half ot incident light into two op~ical paths each halving arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for alterin~ the polarisation of the transmitted light, t 5 characterised in that in a first direction of propagation the paths have outputs which are in phase, ~nd so transmit the incident light, and in the r~verse direction the paths have outputs whic~ are 180~ out of phase9 and ;so do not transmit tha incident light.
: 20 Br~e~ Description of Drawings Several ernbodiments of the invention will be described with ~- ~ r eference to the drawings, in which:
:~ Figure 1 is a schematic view of a prior art isolator;
~: ~ Figure 2 is a schematic view of another prior art isolator;
Figure 3 is a sch~matic view of an optical circulator;
Figure 4 iilustrates a first technique for bi-directional isolation in a wavelength diversity transmission system;
Figure 5 i!lustrates a second techniqus for bi-directional isolation in a wavelength diversity transmission system;
Figure 6 illustrates conceptually a techniqu~ for bi-directional isolation in a wavelength diversity system;
Figure 7A and 7B illustrate interference within the system ~f wo s3/2047s PCr/AUg3JOO-~33556 6 figure 1 0, Figure 8 illustrates ona implem~nta~ion of the systsm ~ figure 10;
Figure 9 illustrat~s anoth~r impl~mentation of the system of 5 figure 10;
Figur~s 10A and 10B illus~rate a preferred implem~ntation of a non-reciprocai phase shifter;
Figure 11 illustrates a polarisation~ dispersion fr~s isolator;
Figure 12 illustrates a balanced polarisation dispersion free ~0 :design;
Figure 13 illustrat~s an integrated impiementathn o~ a ci~ulator/isolator according :to tho pres~nt inventi~n;
Figure 14 is a graph showing wavelangth dependence in an exp~rimental device;::
15 : ~ Figure 15 illustrates a prefsrred implementati~n of the system of `
figure 10;
~: Figure 16 illustrates schematically a bidire~ional amplifier. : -Figure 17 illustrates schematically a fibr~ ~mbedd~d circulat~r 20~ according to th~ present invention; ~:
Figure 18 ~ illustrates an implementation ~f the device of Figure Figure 19 illustrates~ a mod~&onv~rting circulator;
:: : Figure 20 iilustrates a fused Mach Zender irnplementatlon of an :25 ~ isolato~/circulator: according to the preseni invention; : ::
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~; Figure 21 illustrates a filtered isolator accordin~ to the pr~sent inYention;
Figure 22 illustrates a high isolation devica;: ~;j : : Figure 23~ illustrates ~ a network utilising a bi-dlr~ctional i::
30 ~ amplifier,and ` ;;
Figure 24 illustra~es schematically a low palarisation dispersion amplifler.
W093/20475 ~13~55~ PCI/AU93/00146 Detailed description This aspect of the pres~nt invention is particularly adapted to be implemented in bi-dir~ctional networks. In tha transmission system envisaged ~sing this form of isoîation, transmission in one direction takes 5 place at one wavelength, and in tha opposite direction a~ a second wavelength, as is illustrat~d in principle in Figuro 23. A partlcular difficul~yin this arrangemen~ is in cons~tuc~in~ a simple isolatQr wh}ch achi3ves wavelength s~lectiv0 isolation. H such an isolator could be constructed, ~t is clear that the amplifie-: would be adequately isolated from refl~ctions, 10 as such refiections would in general maintain the wavalength of the light : and so be passed out of the system upon meetin~ an isolator.
There are several approaches that can be taken to achieve the desired end. A first embodimen~ of this type o~ isolator depends upon ~ ~ splRting the light into the:two wavelengths using a grating or wavelength 5 ~ division~: multiplexer 34, isolating each wavelength separately in each direction using isolators~ 32, 33, th~n recombining the light afterwards 31, as shown in Figure ~4. A: slightly more refin~d approach can be constructe:d with an optical clrculator 42 and a single waveiength multiplexing elemsnt 41 as :shown in Figure ~.
20~ This aspect :of the: pr~sent invention is based on a new approach to isolation~ which is inter~erometric instead Ql being polarisation dependent. One immediate ~advantage that arises from this is that all of the embodiments~ described~: are in general polarisation independent, :
with~ut the need t~ split ~ and: recombine polarisatJons. Sorne o~ the 25~ mplementations illustrafed ~aré for single moded waYeguiding applications only, but the genera3 concepi is~ equalîy applicabls to bulk Isoia~ors. ~ -A general: form of this aspect of the invention is shown in ~: ; Figure~ 6.~ A ~ingle ~beam o f light entering at: point 1 is split by -beamsp!itter 51 into two paths ~formed using mirrors 54, 55. In one path 30~ ~a~ 90 degree Faraday rotator ~3 is placed, and in~ the second path a pair of half-waYe retardation~ plates 52 at a relative angle of 4~ degrees ~s ::
placed. The pair of reta ders 52 rotates ~he polarisation of any arbitrary ::
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WO 93/21)47~ PCl/AU93/00 2133S5~
polarisation of incident light by 9Q d~grees.
This can be d~monstrat~d by the appropriate Jones' Matrix multiplications~ The two polarisations are now in ~he same direction, and the optical path length can now be adjusted to b~ Qt:lU~II in both arms 5 ensuring constructive interfererlce on ~h~ throvgh pa~h ~1) to (4). 3n the reverse path with light incident at por~ 4, ths polarlsatlon of the two b~ams will be opposits because the F~raday rotation Is independent of the prvpagation direction, whereas ~he birefringent ro~a~ion is reciprocal.
This corresponds to a 1 8û degrees phase chan~e in the electric field 10 vector between the two be~rns and dsstructivs int~rfsrence wTII oc~ur at port 1. The light will instead exit thfough port 2 formiQg an isslator.
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The d~vice created is r~lated to a Mach-Zender interferometer.
It will be apparent that wavelength dependence in the isolation can be ;achieved. If an optical ;path length difference is introduced between the 15; two arms of the Mach-Zender, then there will be only certain wavelengths ;~ ` for which th~ int~rference will be constructiv~ in light travelling ~rom port to port 4. This will give a sin2 de,oQndencs to the intensity of light emerging In port 4 as~ a function ~of ~, with a peri~)dicity proportional to 1, the difference in optical path lengths, with the remaining light being ~20 ~ redirected to port 3. Because of the 180 degree phas~ shift between beams travelling right~to left, the tfansmission curve at port 1, Figure 7A, wili ~ be complimentary ~to that obtained at port 4, Figure 7B. For wavelengths having ~destructive intPr~erence at port 4 for forward propagation, constructive interference wili be obtained at port 1 for reverse 25 propagation.
This is precisely the characteristics required ~or a wavelength dependent isolator. ;For instance, by using an optical path difference of 0.12 mm a device can be constructed to be forward propagating at 1535 nn (corresponding to the first gain peak of the erbium-doped fibre 3û amplifier) and revers~ propagating at 1555 nrn (the second gain peak of the erbium doped fibre arnplifier). Isolation is provided for both wavelengths.
wo 93/2047s ~ ~ 3 ~ 5 ~ 6 Pcr/Aug3/l)0146 The main obsta~le to be overcome is the stabillty of the device to external perturbations. As an int~rfQrometric device i~ is potentially much more sensitive to any ~herrnally Qt mechanically induced variations in the relative path lengths. Althou~h th~rmai and rnechanical 5 stabilisation is possible, it is unlik~ly tha~ any bulk d~vice built accoFdirlg to Figurs 6 would have the required stability tor a ~ield devlce.
One solution is to build .the entire device from s~lid componants, with a balance in ~he construction of ~he arms such that any therrnal expansion affects each arm equally (excspt for the very small :: 10 path length difference 1/ 1. This is shown in Figure 8. Incident light 62 is incident on silvered face ~3, and passes either through rotat~r 67 and half wav~ plate 68, or rotator 64 and half wav~ plate 65, to eventually exit 66. It will be apparent that an isolator similar to figure 6 is created.
It is :noted that instead of a~ 90 degree Faraday ro~ation in one arm we : :t5 now:hav~ a 4~ degree~rotation in one arrn ~nd minus 45 degr~es in the second arm,and one of the hal~ wave plates ts Tn one arm and the second i5 in the second arm. It can a~ain be shown by Jones matrices ~: : ; :that this is equivalent :to ~the~ previous arrangement. It is, however, very difficult t o produce a ~50% beam-splitting cube which is polarisation 20~ ~insensitive~ at~ many of the wavelengths of particular interest to optical : communi~ations. ~:
A:second :solution is to physicaOly split the beam into the top half ~and bottom half and to~ use:~the same rotators and birefringent plates as ~ before ,as~ shown in ~Figure 9. Lens 73, 71 ~collimate/focus the : 25~: incident~: 1ight to~ and from~ beam 75 and project the light onto mirror set 70. It is now much less obvious that this should work as an isolator, or : even that there shQuld ~ be~ any interfersnce at all. In fact this will no : longer :act as a bulk isolatot, and it is only when the~input 74 and output ~ ~ 72 are both single mode: fibres that the interference or isolation will take :: ; 30 place. This is because ~of the quantum nature o~ the modes of a fibre.
A qualitùtive theoretical understanding can be obtainad by : considering the device as a black box, which provides ~two paths ~f equal WO 93/20475 P(:~/AU93/00' 213 3~ ~fi lo probability and the sante phase for photons travelling in the forward direction. The expectation value of a photon arriving at a point in the second fibre is the sum of ~the diftsr~nt possible wave functions times the probabiiity of that wave ~unction~. In the forward case the wave functions 5 will be in phase for a ~iven wav~leng~h photon and the normalised expectation value will be ~ (100% probabili~.of the photon arriving). In the reverse direction for that same wavel~nQih the waYe functions will be opposite sign and so the probability of the photon arriving will be 0.
The aboYe argum~nt is easily v~rified by using the formalism ~f t 0 Fourier optic~. An assumed Gaussian moda is transformed by the l~ns into a Gaussian beam in Fourier space. In the absence o~ any phase delay, the second lens transforms the be~m back to the original Gaussianf which eXCitBS ~ully the ~undamental mode of the second fibre~
In quantum mechanical terms, the overlap int~gral of the mode and the 15 exciting~light is 1. We~have 100% transmiss!on~ If a phase delay of ~
is: incurred in the top half: of th~ beam, then this is equivaient to ~: multiplying by -t the top half ot the Fourier transform. Upon performing ~ ths Fourier transform corresponding t~ the second lans, an odd function is :: : obtained. The overlap integral of an odd functiQn with an e~ en function 20 ~ is~ necessarily zero. it is ~now clear what is happening to the photons which are lost: to the system. They are simply trying to excite higher order modes which are cut off and so cannot propagate along the ~ibre.
These photons are lost very~quickly into the cladding ~t the flbre.
This principla has been demonstrated in the laboratory by 25 constructing a very simple interferometer, which has proved surprisingly stable. An optical flat was inserted so as to occupy ha~ of the beam in a beam expander between two single mode fibres. Ther~ is a clear modul:ation of the received light as a function of wavelength, corresponding to the two paths being in phase and out of phase. This 30 is shown in Figure 14.
This leads us to another device ~or bi-directional isolation~ In this the expanded beam is nsver physically separated. A compound ~133SS6 element consisting of a 90 ~egree Faraday 7~ ro~a~or and a pair of crossed half wave plates 7~ is made up, by polishing and joining on one side. This cornpound element is now located in ~he bRam so as ~o occupy exactly one half of the beam as shown in Figure ~ ~. The path S difference can ba achi~ved by having a slightly different optical path between tha Faraday rotator and the birefringent elemont. This is conceptually identical to the davice outlined in Figure 9. The exact wavelength of transmission can be tuned by varying slightly the angie at which the elemen~ is inserted into the beam. Ths thickness of this 10 compound element need not be much larger than 500 llm with stat~of-th~art Faraday rotators. Insertion loss should be negligible (c0.5 dB) with good beam expansion optics, and stabiliîy is maximised because there is only a small length of crystal (~1 mm) generating the path difference.
:1 5One disadvantage of She embodiments de~cribed is that the ~: construction of the non-reciprocal phase shi~ter used different materials to act upon: each half of the beam which is split. The device is ther~fore .
susceptible to temperature dependence as the thermal expansion coefficients nd thermal refractive index coefficients for each half will be 20 dfflerent, thus shifting the~ wavelength of maximum isolatlon.
Figures tûA, 10B illustrate a preferred stabl2 design for the non- :
reciprocal phase shifting element.
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:: This implementation: uses a 45 degree Faraday rotator 101 in ~: both::halves and a haif-wave plate 100, 102 in both halves. In one half : 25 the half waveplate 100 is to the left of the Faraday rotator 101, and in the second half, the half waveplate 102 is to the right of tha Faraday rotator 101. The ~second waveplate 102 is orientated to have its QptiC
axis at 45 degree relative to the first waveplate 100.
To understand the operation of the device ~onsider the path of 30 light travelllng left to right in the vertical polarisation 104 for a wavelength independent device~ In the top half of the beam, tha light tra~els through the fast axis of the half-wave plate 100 then through the Faraday rotator WO 93/2047~ PCr/AU93/001 ~ ;~ '3 S S 6 12 101 to rotate the polarisation 45 degre~ c~ockwise. The light in the bottom half of th~ beam passss ~irst lhrough the Faraday rotator 101, to be rotated cleckwise into line with the fast axis of th~ half wave plate 102. The two beams upon recombining wil~ be in phase. Simitarly, for 5 light in the horizontal polarisa~ion 105, light travelling through both the top and bottom halves wil~ travel through . !hè slow axis of the half wave plates 100, 102 and so will be in phase. ~ight travelling from right to left, incident at 45 dagree clockwise to ~he verti~al in the top halt wlll travel through the Faraday rotator 101 first to ~e rotated to the horizontal 10~ axis and then pass through the slow axis of the half wava plate 100.
Light travelling from right to left, incident at 45 d~gree clockwise to the vertical in the bottom half wilt pass first through the fast axis of the half wave pla~e 102 then through the Faraday rotator 101 to be in the : horkontal axis. As: the top beam has trav~lled through the fast axis and 15 the bottom beam has~ travelled through tha slow axls th~re is a 180 : degree relative: phase shifl~ between both halves of the beam. In the forward direction, recombination of the light leads to constructive interference and hence ~ transmission, and in the revers2 direction recombination of the light~ leads to destructive interf~rencs and hence 20~ attenuatlon. This is~the:~basis:for~non-reciprocal transmission, or isolation.
:This~may~ be readily ~extended ~to a circulator application, as will be apparent to the reader. ~
The isolator describ2d with reference to Fi~ure 10 has a very small~ (yet :: finite~ polari~s~ation:dispersion of one wa~elength difference 25~:::between :the~ two polarisaiion states in the forward directiQn. This is because one polarisation travels in the fast axis ~top and bottom halves) : ~ and the other polarisation travels in the slow axis of ths half wave plates.~The -~resultant polarisation~ dispersion is for 1.5 ~m light equal to 5 fs.
: ~` : Although this value is much iower than even the best commercially ~:: ;30 a~railable isolators, it is: possible to design the present isolator to have intrinsically zero polarisation dispersion. This is done by making the forward transmissive palh equivalent to a fast axis and a slow axis transit.
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Wo 93/2047s ~ 5 5 ~ PCI/AU93iO0146 Figure 1t shows hvw this can b~ achieY~d. The top half consists of for the vertical polarisation.
fast axis: Faraday rotator: slow axis for th~ hwizon~al pol~fisa~ion slow axis: Faraday rotator: fast axis The bottom half consists for both polarisations ot an optical path length equivalent to fast axis + Faraday rotatQr + slow axis.
There is no intrinsic polarisation disp~rsion.
Figure 12 is equivalQnt to Figure 11 except that the design is 10 entir~ly balanced again.
A simpie opti~al circulatQr using the b~am splitting isolator as the basis may be readily implernented as shown in Fi~ure 19. In the isolator the light which is rejected in the non-transmission direction is ; ~ antisymmetric about the~ axis of the interface and so cannot excite th~
15 fundamental mode o~ a single mode fibre. It can however excite the tirst higher~order mode~of a~ mul~imode fibre 110 which can have the same symmetry. By using~a ~fibre ~which supports this hi~her otder mode and then~produGin g a coupler~t~t5 which coup~es the higher order mode to a se~ond~fibre~ we can~ per{~rm the ~unction of a thre~ or four port circulator.
20 ~ Single~ ~ mode fibres 116 ar~ coupled 114, 115 to multimode fibre t 18, 110.~ ~ ~Lens ~111, 113 and~ phase shifter 1t2 fofm~ a beam-expanded Isolator~ as descrlbed above.~ Such a coupl~r ~an ~uple light ~rom the ;correct~ symmetry higher~ order~ mode ~o the fund~mental mode of a sin~le mode fibre, where~ tl~e~ ~ propagation coefficients are matched in the 25~coupling ~region. Ei~her~fused or polished ~coupling techniqu~s can be used. Note that with the crystal interface in tha parall~l ~xpanded beam, non-op~imal excitation o f~ the higher order mode is achieved with ths remaining light lost to the system This can be improved by having the interfa~e where the beam of light is in transîtion from the near-field to far-30 field~ image. This is achieved by~focussing ~he light between the lens~s ~ ~ snd placing the non reciprocal phas~ shifting crystals at an appropri~t~
; ~ ~position.
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WO 93~2047S P~/AU93/0~ .
2 i3~S~
Referring to Figure 13, in th~s impl~m~nta~ion a Mach-Zander wave guide is mad~ in integr~tsd optics. Thes~ ars commercially available. A slot is c~tt thr~ugh bo~h ~tms of the MachaZender 113 and the two halves of the non-r~ciprocal phase shifter t 12 are insert3d into the slot so that light trQm one arm passes through one half and light from the second arm passes through th~ second half. Beam ~xpansion techniques can be used if nacessary:lo reduce the toss due to the non-wave~uiding propag~tion through thls ragion. As usual a magne~ic field mus~ b~ supplied around the F~rad~y rotator. This device could be 10 ccmbir~ed with other integrat~d op~ic device~ in a useful fashion, such as combining it with an inlegrated splitt~r.
Figure 20 illustrates a further Ma~h~;~ender implementation involving two fibres :using ~used coupler t~chnolo~y. The implemerltation illustrated insert two fibres 115 Into a glass tube 114 of lower refractive 15 index than th~ fibr~ :cladding Tndex, and tap~r the tuba down in ~w~
closely spaced rsgions 113~ to form two 50:50 couplers. ~th the fibres 115 held firmly: in place ~y the surrounding collapsed glass tubing 114, : the ~ devi~ :can be cut and ~ polished, ba~ore the n~n-reciprocal phase shifting elements t12 ~ are placed between the fibre waveguides.
20 ~AIternatl~ely, a s!ot~ l~16 ~can be~ used as for the integrated optic impiementation :shown:~::; in Figure 13. This implementation has ths potentiai :to provide ~low~ loss coupling of the light through the non-reciproca! phase :shifter~ ::: By expanding the ~undamentai mode, the di~fraction~ eflects: are ~reduced and propagation through non-waveguiding 25~ regions can be achieved with iow loss. Beam expansion is achisved ~: through: using a tapere~ region of the fibre, or by core diffusion techniques~ :
A single polarisation ~ibre embedded isolator has be0n described in the :s~ientifi~ literature~ The application ot single polarisation ~30 devices :is, however, extremely limit~d. The davice shown in Figure 15 provides a polarisation~ insensitive isolator. Light frvm a singl~ mode ~ibre 74 is expanded via lens 73 into an expanded beam. Non-reciprocal :
:: ' wo 93/2047~ 2 1 ~ b PCI/AU93/~0~46 phase shifter 112 is formed from a Faraday ro~ator 7~ in one half of beam 75 and a pair of half wav~ pla~es 77 in the other half. Lens 71 conveys the light into fibre 7~ will be appreciated that the principle of operation is analogous to Ihe devi~e of Figuro 6. These could be 5 potentially made into isola~ing connsctors. Such deYices can be sither wavelength independent or wav~length selective Another feature which can be incorporated into a isoiator ~f the split beam typ5 (or a standard isolator for ~hat matter) is filt~ring using the split beam technique. By splitting the beam with a non-bire~ringent 10 reciproc~l ~lement for instance a non-bire~ring~nt wave plata 117 with different optical path iengths on both sides a sinusoidal tiltering can be , applied. This can be seen in Figure 21. This is particularly U~BfLII in amplifier applications to equalise the gain over a certain band width. To combine this with an isoiator of the typa dls~ussed above th~ int~rface of 1:5 th~ reciproçal tiltering elem~nt should be ortho~onal to the interface o~ the non-reciprocal phase shifter 112. By splitting the beam using ratios other than 50:50 the required degree of extinctîon can be obtained.
The ability: to casoade these d~Yk:eS to ~orm higher i505ation or temperature independent features is remarkably simple. Two non~
20 : reciprQcal phase shifters 11 2 1 22 can be cascaded in the same beam expander by ensuring that the~ interfaces of the devices are perpendicular to each :other. This is:~ shown in Figure 22. Some tunirlg ot th~
characterîstics of the ;device can also be achiev~d with small variations from ~9O degree relative: orientation. Two identical devices can be ~ ~ , : 25~ : cascaded to increase: ~the peak isolation and isolation bandwidth of thodevice or two devices; with siightly different c~ntral wavelengths can bs cascaded to achievs a broàder isolation bandwidth~
~Peak isolation wavelength tuning It is useful in: manufacture to be able to tune the wavelength of 30 peak isolation to be: different to the waYelengths of 45 degree Faraday rotation. This can be achieved according to the present invention by choosing the relative angle between the optic axis ~f the half wave plates :::
' WO 93~20475 PCI /A U93JI)0 ' 2133~56 to be equal to the Faraday rotation ang~e at the wavelengths of desired peak isolation. This featur~ can ba combined with th~ previous ~eature for broadband isolation, or to achieve hi~h isolation in opposite directions for wavelengths which ars wsll saparated.
5 Tuning of device One important practical issue, distinct from the wavelength of peak isolation, is the tuning ot ~he devîce ~o ensure that the maximum extlnction occurs at the wavalength(s~ which are re~lJired. This i5 achieved by ehsuring that tha light from sach half of the non-reciprocal :10 phase element is exac~ly in phase at the wavelen~th(s) of interest for the transmission direction. The relative phase between each half is most easily tuned by an angul~r ~wariation ~f the crystals. Where the optical path length îs different or there is a difference in the angular orientation of both halves, this: can be ~achieved by rotating the whole device. This ~:15~ should be rotated in a plane such that the interta~e of the crystals remains perpendicular to ~he beam, for the split beam implementation.
A separate ~uning mechanism is possible~ A small phase shiR
in haîf of the Fourier plane (ie :in th~ expand~d bsam) is equivalent to :~: ; a lateral displacement in the~direction perpendicular to the interface in the : 20 image plane :So an alternative is to laterally displace the fibre from the :: ~
true~ image position to ~tune the phase. This ~uning mechanism can incur some~ small losses, but :is useful for fine tuning.
Température independence of phase change through material alancing ~::
:~ 25 : It should be noted that although the balance design has exaGtly the sama material on~ both: haives of the non-reciprocal phase shifter for the broad band case, the bi-directional design requires an additional :: :
:: : optlcal path length, and: so there will be potsntially son ~ temperatura : dependence to the phase, due to the thermal expansiorl, and therrnal ~; ~: 30 refractive index dependence in this unbalanced portion. At least two approaches could be taken. One is to uso a material with a very small temperature dependent phase shifter Another is to use two dNferent :
: :~
wo ~3~2047s ~ .i 5 b PCr/AUg3/0014~
materials in each haif of the non-reciprocal phase shifter, to produce the optical path difference. These materials are chosen to have slightly different thermal coefficients such that the smaller optical length has the larger therrnal coeffici~n~s. Usîng this m~thod the relative phase can be 5 made to remain essentially cons~ant ov~r a wids tamperature range.
Fibre embeddod optical circulator Figu~e 17 illustrat~ a circulator using the principle of the devices described sarlier, for instanca Figure 6, except that the beam splitting will be done in fibre using an optical coùpl~r. A 90 degree 10 Faraday rotator 88 is embedded in ona arm of the l~lach-Zender and a 90 degree birefringent rotator 88 in the o~her arm (~r other combinations as described earlier), to form a circulator.
One way to manufacture thls and keap the arm lengths down to an absolute rninimum is described with ref~rence to Figure 18. A fused :: : 15 silica V-grove 90, 92 is used to align each of the ~ibres, and a 90 : : desree Faraday rotator 93 is embedded into one fibre 130 half way along the fused silica and a 90 degree polarisation rotating birefringent plate 94 is embedded into the other fibr~ 131. The silica V-grooves are then used to make a polished coupler, by continually polishing unSil 100 20i~ :percent coupling is achieved in the forward direction. This should correspond to 50% coupling before and after the rotating devices. The ~; ~: device~ is ~ :now identical in operation to that shown in Figure 17, but is extremely compact and resistant to the environment. The device will behave as a circulator and is capable of mass pro~uction. This is of 25 cours~ an alternative implementation of the isolator, as all ~irculators are also isolators. It may be necessary to bury slightly the fibre in the silica at the point wher~ the cr~stal embedding takes plac~, but this will havo no reai effect on the operation of the d~vice.
Figure 24 illustrates an amplifier arrangement using an isolator 30 according to the present invention . This arrangernent is low polarisation dispersive. Pump source 131 and input signal 142 enter wavelength division muitipiexer 135, pass through low birefringencs erbium doped .- ~..
WO 93/20475 PCl/AU93/00 3556 1~
~ibre 132, and enter polarisation dispersion isola~or 138 (as described with referencs to figures 11 and 1 ~ for example). The signal then passes through low birefringence erbium doped fibre 134, wavelength division multiplexer 136, and is outpu~ 143. Pump source 137 drives the erbium 5 fibrs amplifier.
Figure 25 i!lus~rat~s a bidirectional ampiifier, of the type which enables bidirectional communications down a single fibre, utilising the ` ~ same amplifiers for both wavelengths. The arrangement is similar tv figure 24, but incorporates a bidirectional wavel~ngth dependant isolator 133 10 (as described, for example, with refer6nce to tigure 6). The isolator 133 is positioned between two erbium dop~d fibre amplifiers 132, 134. At t40, signals at ~ are input, and at ~2 are output. At 141, signais at ~2 are input, and at ~ are output. Signals counterpropagating to the allowed directions at the different frequencies, f~r instance at ~l input at .141, are 15 suppressed. Thus, a true bidirectional system is possible, as is a true bidirectional amplifier, without undesired feedback to the amplifiers .~
:~ ~ becoming a probiem.:
: ~ :
lt will be appreciated that variations and additions are possible within the spirit and scope of the invention.
: :
. :
.
Referring to Figure 13, in th~s impl~m~nta~ion a Mach-Zander wave guide is mad~ in integr~tsd optics. Thes~ ars commercially available. A slot is c~tt thr~ugh bo~h ~tms of the MachaZender 113 and the two halves of the non-r~ciprocal phase shifter t 12 are insert3d into the slot so that light trQm one arm passes through one half and light from the second arm passes through th~ second half. Beam ~xpansion techniques can be used if nacessary:lo reduce the toss due to the non-wave~uiding propag~tion through thls ragion. As usual a magne~ic field mus~ b~ supplied around the F~rad~y rotator. This device could be 10 ccmbir~ed with other integrat~d op~ic device~ in a useful fashion, such as combining it with an inlegrated splitt~r.
Figure 20 illustrates a further Ma~h~;~ender implementation involving two fibres :using ~used coupler t~chnolo~y. The implemerltation illustrated insert two fibres 115 Into a glass tube 114 of lower refractive 15 index than th~ fibr~ :cladding Tndex, and tap~r the tuba down in ~w~
closely spaced rsgions 113~ to form two 50:50 couplers. ~th the fibres 115 held firmly: in place ~y the surrounding collapsed glass tubing 114, : the ~ devi~ :can be cut and ~ polished, ba~ore the n~n-reciprocal phase shifting elements t12 ~ are placed between the fibre waveguides.
20 ~AIternatl~ely, a s!ot~ l~16 ~can be~ used as for the integrated optic impiementation :shown:~::; in Figure 13. This implementation has ths potentiai :to provide ~low~ loss coupling of the light through the non-reciproca! phase :shifter~ ::: By expanding the ~undamentai mode, the di~fraction~ eflects: are ~reduced and propagation through non-waveguiding 25~ regions can be achieved with iow loss. Beam expansion is achisved ~: through: using a tapere~ region of the fibre, or by core diffusion techniques~ :
A single polarisation ~ibre embedded isolator has be0n described in the :s~ientifi~ literature~ The application ot single polarisation ~30 devices :is, however, extremely limit~d. The davice shown in Figure 15 provides a polarisation~ insensitive isolator. Light frvm a singl~ mode ~ibre 74 is expanded via lens 73 into an expanded beam. Non-reciprocal :
:: ' wo 93/2047~ 2 1 ~ b PCI/AU93/~0~46 phase shifter 112 is formed from a Faraday ro~ator 7~ in one half of beam 75 and a pair of half wav~ pla~es 77 in the other half. Lens 71 conveys the light into fibre 7~ will be appreciated that the principle of operation is analogous to Ihe devi~e of Figuro 6. These could be 5 potentially made into isola~ing connsctors. Such deYices can be sither wavelength independent or wav~length selective Another feature which can be incorporated into a isoiator ~f the split beam typ5 (or a standard isolator for ~hat matter) is filt~ring using the split beam technique. By splitting the beam with a non-bire~ringent 10 reciproc~l ~lement for instance a non-bire~ring~nt wave plata 117 with different optical path iengths on both sides a sinusoidal tiltering can be , applied. This can be seen in Figure 21. This is particularly U~BfLII in amplifier applications to equalise the gain over a certain band width. To combine this with an isoiator of the typa dls~ussed above th~ int~rface of 1:5 th~ reciproçal tiltering elem~nt should be ortho~onal to the interface o~ the non-reciprocal phase shifter 112. By splitting the beam using ratios other than 50:50 the required degree of extinctîon can be obtained.
The ability: to casoade these d~Yk:eS to ~orm higher i505ation or temperature independent features is remarkably simple. Two non~
20 : reciprQcal phase shifters 11 2 1 22 can be cascaded in the same beam expander by ensuring that the~ interfaces of the devices are perpendicular to each :other. This is:~ shown in Figure 22. Some tunirlg ot th~
characterîstics of the ;device can also be achiev~d with small variations from ~9O degree relative: orientation. Two identical devices can be ~ ~ , : 25~ : cascaded to increase: ~the peak isolation and isolation bandwidth of thodevice or two devices; with siightly different c~ntral wavelengths can bs cascaded to achievs a broàder isolation bandwidth~
~Peak isolation wavelength tuning It is useful in: manufacture to be able to tune the wavelength of 30 peak isolation to be: different to the waYelengths of 45 degree Faraday rotation. This can be achieved according to the present invention by choosing the relative angle between the optic axis ~f the half wave plates :::
' WO 93~20475 PCI /A U93JI)0 ' 2133~56 to be equal to the Faraday rotation ang~e at the wavelengths of desired peak isolation. This featur~ can ba combined with th~ previous ~eature for broadband isolation, or to achieve hi~h isolation in opposite directions for wavelengths which ars wsll saparated.
5 Tuning of device One important practical issue, distinct from the wavelength of peak isolation, is the tuning ot ~he devîce ~o ensure that the maximum extlnction occurs at the wavalength(s~ which are re~lJired. This i5 achieved by ehsuring that tha light from sach half of the non-reciprocal :10 phase element is exac~ly in phase at the wavelen~th(s) of interest for the transmission direction. The relative phase between each half is most easily tuned by an angul~r ~wariation ~f the crystals. Where the optical path length îs different or there is a difference in the angular orientation of both halves, this: can be ~achieved by rotating the whole device. This ~:15~ should be rotated in a plane such that the interta~e of the crystals remains perpendicular to ~he beam, for the split beam implementation.
A separate ~uning mechanism is possible~ A small phase shiR
in haîf of the Fourier plane (ie :in th~ expand~d bsam) is equivalent to :~: ; a lateral displacement in the~direction perpendicular to the interface in the : 20 image plane :So an alternative is to laterally displace the fibre from the :: ~
true~ image position to ~tune the phase. This ~uning mechanism can incur some~ small losses, but :is useful for fine tuning.
Température independence of phase change through material alancing ~::
:~ 25 : It should be noted that although the balance design has exaGtly the sama material on~ both: haives of the non-reciprocal phase shifter for the broad band case, the bi-directional design requires an additional :: :
:: : optlcal path length, and: so there will be potsntially son ~ temperatura : dependence to the phase, due to the thermal expansiorl, and therrnal ~; ~: 30 refractive index dependence in this unbalanced portion. At least two approaches could be taken. One is to uso a material with a very small temperature dependent phase shifter Another is to use two dNferent :
: :~
wo ~3~2047s ~ .i 5 b PCr/AUg3/0014~
materials in each haif of the non-reciprocal phase shifter, to produce the optical path difference. These materials are chosen to have slightly different thermal coefficients such that the smaller optical length has the larger therrnal coeffici~n~s. Usîng this m~thod the relative phase can be 5 made to remain essentially cons~ant ov~r a wids tamperature range.
Fibre embeddod optical circulator Figu~e 17 illustrat~ a circulator using the principle of the devices described sarlier, for instanca Figure 6, except that the beam splitting will be done in fibre using an optical coùpl~r. A 90 degree 10 Faraday rotator 88 is embedded in ona arm of the l~lach-Zender and a 90 degree birefringent rotator 88 in the o~her arm (~r other combinations as described earlier), to form a circulator.
One way to manufacture thls and keap the arm lengths down to an absolute rninimum is described with ref~rence to Figure 18. A fused :: : 15 silica V-grove 90, 92 is used to align each of the ~ibres, and a 90 : : desree Faraday rotator 93 is embedded into one fibre 130 half way along the fused silica and a 90 degree polarisation rotating birefringent plate 94 is embedded into the other fibr~ 131. The silica V-grooves are then used to make a polished coupler, by continually polishing unSil 100 20i~ :percent coupling is achieved in the forward direction. This should correspond to 50% coupling before and after the rotating devices. The ~; ~: device~ is ~ :now identical in operation to that shown in Figure 17, but is extremely compact and resistant to the environment. The device will behave as a circulator and is capable of mass pro~uction. This is of 25 cours~ an alternative implementation of the isolator, as all ~irculators are also isolators. It may be necessary to bury slightly the fibre in the silica at the point wher~ the cr~stal embedding takes plac~, but this will havo no reai effect on the operation of the d~vice.
Figure 24 illustrates an amplifier arrangement using an isolator 30 according to the present invention . This arrangernent is low polarisation dispersive. Pump source 131 and input signal 142 enter wavelength division muitipiexer 135, pass through low birefringencs erbium doped .- ~..
WO 93/20475 PCl/AU93/00 3556 1~
~ibre 132, and enter polarisation dispersion isola~or 138 (as described with referencs to figures 11 and 1 ~ for example). The signal then passes through low birefringence erbium doped fibre 134, wavelength division multiplexer 136, and is outpu~ 143. Pump source 137 drives the erbium 5 fibrs amplifier.
Figure 25 i!lus~rat~s a bidirectional ampiifier, of the type which enables bidirectional communications down a single fibre, utilising the ` ~ same amplifiers for both wavelengths. The arrangement is similar tv figure 24, but incorporates a bidirectional wavel~ngth dependant isolator 133 10 (as described, for example, with refer6nce to tigure 6). The isolator 133 is positioned between two erbium dop~d fibre amplifiers 132, 134. At t40, signals at ~ are input, and at ~2 are output. At 141, signais at ~2 are input, and at ~ are output. Signals counterpropagating to the allowed directions at the different frequencies, f~r instance at ~l input at .141, are 15 suppressed. Thus, a true bidirectional system is possible, as is a true bidirectional amplifier, without undesired feedback to the amplifiers .~
:~ ~ becoming a probiem.:
: ~ :
lt will be appreciated that variations and additions are possible within the spirit and scope of the invention.
: :
. :
.
Claims (14)
1. A non-reciprocal optical phase shifter, comprising at least one first polarisation rotating means, the direction of polarisation rotation being dependant on the direction of propagation of transmitted light, and at least one second means for altering polarisation, the polarisation change being independent of the direction of propagation of transmitted light, characterized in that substantially half of any light propagating from either end of the phase shifter having an arbitrary polarisation travels through each of the first polarisation rotating means and second means for altering polarisation.
2. A non-reciprocal optical phase shifter, comprising means for transmitting incident light through first and second optical paths and recombining said paths at an output, said paths each including respectively first and second means for altering the polarisation of incident light having arbitrary polarisation, at least one of said means for altering having a first rotation in one direction of propagation and another rotation in a second direction of propagation, and the other or one of said means for altering having a change independent of the direction of propagation, the arrangement being such that a first relative phase shift between the paths occurs for light propagating in one direction, and a second relative phase shift between the paths occurs for light propagating in a second direction.
3. A non-reciprocal optical phase shifter, comprising means for transmitting incident light through first and second optical paths and recombining said paths at an output, said first path including in a first propagation direction successively first polarisation rotating means having a rotation dependent on propagation direction and second means for altering polarisation having a rotation independent of the direction of propagation, said second path including in said first propagation direction successively third means for altering polarisation having a change WO 93/20475 PCT/AU93/00?
independent of the direction of propagation and fourth polarisation rotating means having a rotation dependent on propagation direction, the arrangement being such that substantially half of the incident light travels through each of said first and second paths, and the relative phase shift of output light is dependent on the direction of propagation of the incident light.
independent of the direction of propagation and fourth polarisation rotating means having a rotation dependent on propagation direction, the arrangement being such that substantially half of the incident light travels through each of said first and second paths, and the relative phase shift of output light is dependent on the direction of propagation of the incident light.
4. A non-reciprocal phase shifter according to claim 3, wherein the first and third polarisation rotating means are provided by a single faraday rotator extending across both optical paths.
5. A non-reciprocal phase shifter, comprising means for transmitting substantially half of incident light into two optical paths each having arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for altering the polarisation of the transmitted light, characterised in that a first direction of propagation the paths have outputs which are have a first relative phase shift, and in the reverse direction the paths have outputs which have a different relative phase shift.
6. A bidirectional optical isolator, comprising means for transmitting substantially half of incident light through each of first and second optical paths, at least said first path including polarisation rotating means having a rotation dependent on propagation direction and at least said second path including second means for altering polarisation having a change independent of the direction of propagation, said first and second paths having a path length difference, and means for recombining said first and second paths, the arrangement being such that for light having a first wavelength propagating in a first direction total attenuation occurs, while for light having said first wavelength propagating in a second reverse direction substantial transmission occurs; and for light having a second wavelength propagating in said first direction substantial transmission occurs, while for light having said second wavelength propagating in said second reverse direction substantial attenuation occurs.
7. An optical circulator, comprising a least one input to an isolator according to claim 13 , and means for coupling the isolator to two ports, the arrangement being such that light travelling in one direction travels through the isolator and one port, and light travelling in the other direction travels through the other port and the isolator.
8. An optical circulator, comprising at least one input to an isolator according to claim 13, and means for coupling the isolator to two ports, the arrangement being such that light having one mode is output through one ports, and light having a second mode is output through the second ports.
9. A bidirectional optical fibre communications system, in which signals travelling in a first direction have a first wavelength, and signals travelling in the other direction have a second wavelength, both signals travelling in the same optical fibre, characterised in that the system includes one or more wavelength selective bidirectional isolators .
10. A bidirectional optical fibre communications system according to claim 9, wherein the isolators are in accordance with claim 12.
11. A bidirectional optical amplifier for allowing amplification of signals at a first wavelength in a first direction, and at a second wavelength in the second, reverse direction, comprising means for inducing gain at said first and second wavelengths, and bidirectional wavelength dependent isolation means arranged such that signals at said first wavelength travelling in said first direction, ????????????
and signals at said second wavelength travelling in said second direction, are transmitted, and signals at said first wavelength travelling in said second direction, and signals at said second wavelength travelling in said first direction, are attenuated, and such that undesired feedback at said first and second wavelengths is substantially suppressed.
and signals at said second wavelength travelling in said second direction, are transmitted, and signals at said first wavelength travelling in said second direction, and signals at said second wavelength travelling in said first direction, are attenuated, and such that undesired feedback at said first and second wavelengths is substantially suppressed.
12. An optical isolator, comprising means for transmitting substantially half of incident light into two optical paths each having arbitrary polarisation, andrecombining the paths to produce an output, each of said paths comprising means for altering the polarisation of the transmitted light, characterised in that in a first direction of propagation the paths have outputs which are in phase, and so transmit the incident light, and in the reverse direction the paths have outputs which are 180° out of phase, and so do not transmit the incident light.
13. An optical isolator, comprising a non-reciprocal phase shifter characterised in that for a selected wavelength, in a first propagation direction the output light is substantially transmitted, and in the reverse direction the output light is substantially attenuated.
14. An optical isolator, comprising means for expanding incident light having arbitrary polarisation into a beam, and means for re-focusing the light to produce an output, characterised in that intermediate the beam is positioned a non-reciprocal phase shifter having at least a first portion and a second portion, said first portion including polarisation rotating means having a rotation dependent on propagation direction and at least said second path including second means for altering polarisation having a change independent of the direction of propagation, the arrangement being such that in a first direction of propagation of incident light the output is substantially attenuated, and in thereverse direction the output is substantially transmitted incident light.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPL171392 | 1992-04-03 | ||
| AUPL1713 | 1992-04-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2133556A1 true CA2133556A1 (en) | 1993-10-14 |
Family
ID=3776073
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002133556A Abandoned CA2133556A1 (en) | 1992-04-03 | 1993-04-02 | Improvements to optical phase shifting |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0634025A4 (en) |
| JP (1) | JPH07508355A (en) |
| CA (1) | CA2133556A1 (en) |
| WO (1) | WO1993020475A1 (en) |
Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5905823A (en) * | 1996-02-28 | 1999-05-18 | Nippon Telegraph And Telephone Corporation | Polarization independent optical nonreciprocal circuit based on even mode to odd mode conversion |
| DE19653276A1 (en) * | 1996-12-20 | 1998-06-25 | Bosch Gmbh Robert | Bi-directional optical amplifier with optical isolator |
| JP2000066137A (en) * | 1998-08-19 | 2000-03-03 | Fujitsu Ltd | Optical device usable as optical isolator as well as optical amplifier and system including this optical device |
| EP1315007A1 (en) * | 2001-11-09 | 2003-05-28 | Corning Incorporated | Polarisation dependence compensation |
| DE102010019134B4 (en) * | 2010-04-30 | 2019-08-08 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | THz measuring system and sensor arrangements and their use |
| CN104917040B (en) * | 2015-06-08 | 2018-03-09 | 湖北航天技术研究院总体设计所 | A kind of annular multi-wavelength light fiber amplifier |
| CN106019617B (en) * | 2016-07-28 | 2018-11-06 | 中国工程物理研究院应用电子学研究所 | A kind of optically isolated beam merging apparatus |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US1252512A (en) * | 1916-12-12 | 1918-01-08 | Hilger Ltd Adam | Method and apparatus for finishing prisms or lenses or combinations of the same. |
| US4627731A (en) * | 1985-09-03 | 1986-12-09 | United Technologies Corporation | Common optical path interferometric gauge |
| JPH0367231A (en) * | 1989-08-07 | 1991-03-22 | Oki Electric Ind Co Ltd | Optical repeating and transmitting system |
| US5033830A (en) * | 1989-10-04 | 1991-07-23 | At&T Bell Laboratories | Polarization independent optical isolator |
| NL9100292A (en) * | 1991-02-19 | 1992-09-16 | Nederland Ptt | OPTICAL TRANSMITTER AND RECEIVER WITH OPTICAL CIRCULATOR. |
-
1993
- 1993-04-02 EP EP93907651A patent/EP0634025A4/en not_active Withdrawn
- 1993-04-02 WO PCT/AU1993/000146 patent/WO1993020475A1/en not_active Application Discontinuation
- 1993-04-02 JP JP5516909A patent/JPH07508355A/en active Pending
- 1993-04-02 CA CA002133556A patent/CA2133556A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| WO1993020475A1 (en) | 1993-10-14 |
| EP0634025A1 (en) | 1995-01-18 |
| JPH07508355A (en) | 1995-09-14 |
| EP0634025A4 (en) | 1995-05-17 |
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