GB2210873A - Optical devices - Google Patents

Abstract

An optical device comprises a glass host material doped with a lanthanide rare earth element soluble in the glass matrix, the doping being to a concentration at which the doped glass exhibits photochromic behaviour. In Figure 1, thulium doped silica optical fibre (1), when subject to light from an argon-ion laser source (2) at appropriate wavelengths, is shown to exhibit unexpected photochromic behaviour. The behaviour is of use to provide, for example, optical memory devices, erasable photoinduced optical components such as diffraction gratings and a novel defect centre laser. <IMAGE>

Description

OPTICAL DEVICES The present invention relates to optical devices and in particular to optical devices utilising a photochromic effect.

A material is considered to exhibit a photochromic effect if its optical transmissivity can be altered by irradiation with electromagnetic radiation of an appropriate wavelength and power. The photochromicity of certain halides of silver, copper and titanium, for example, has been demonstrated.

The photochromic effect has been utilised, for example, in spectacle lenses. In this case, the effect is observed as a "darkening" or lowering of transmissivity when the material is illuminated by ultra violet solar radiation, and a corresponding increase in transmissivity, or "bleaching" when the lenses are taken out of the sunlight.

Known photochromic materials nave been shown to form crystalline colloidal suspensions in bulk glasses and they may be incorporated by suitably impregnating a porous glass matrix. However, they ao not dissolve in the miass matrix and problems can arise from a lack of uniformity in the size and distribution of colloidal clusters.

It is also a disadvantage of known photochromic materials that they are not generally appropriate for use in optical components compatible with optical fibre communications systems. For example, the materials can exhibit inconvenient absorption effects in the major transmission windows around 1.3 and 1.5 micrometres.

Indeed, some of the chemical elements wnich make up known photochromic compounds are generally considered as undesirable impurities to be avoided in optical fibre manufacture.

Furthermore, owing to the requirements of optical fibre fabrication techniques such as, for example, MCVD, and because of their compound chemical nature, known photochromic materials are themselves not at all suitable for incorporation into optical fibres. The photochromic effect therefore cannot conveniently be exploited in conjunction with components comprising optical fibre.

According to the present invention, a photochromic optical device comprises a glass host material doped with a lanthanide rare earth element, the doping being to a concentration at which the doped glass exhibits photochromic behaviour having relatively less transmissive, darkened and relatively more transmissive, bleached photochromic states and such that the transmissivity can be altered optically.

Preferably, the lanthanide dopant has trivalent and divalent states 3+ & 2+, and has an absorption band in the shorter wavelength (blue) region of the optical spectrum. More preferably, the rare earth element dopant is thulium or a lanthanide series element, such as europium, with a similar absorption band profile to thulium.

In contrast with previously known photochromic materials, lanthanide series elements exhibiting the photccnromic effect may convenIently be dissolved into a glass matrix host material rather than provided as crystalline colloids. It is a particular advantage that techniques for efficiently incorporating rare earth dopants into glass compositions suitable for drawing into optical fibres are also available. This possibility was not available with previous photochromic materials which were not soluble in a glass host matrix. Lanthanide doped glasses thus offer the opportunity to provide optical components which take advantage of the photochromic effect and which are readily compatible with optical fibre communications systems.

The invention is based on the demonstration of an unexpected photochromic effect, in particular in a thulium doped silica optical fibre.

In a preferred embodiment, the device comprises a glass optical fibre having a core doped with a lanthanide rare earth element soluble in the glass matrix, the doping being to a concentration at which the doped fibre exhibits photochromic behaviour having relatively less transmissive, darkened and relatively more transmissive, bleached photochromic states and such that the transmissivity can be altered optically.

The concentration of rare earth element may conveniently be in the range from above 0 to 10 mole S.

An optical fibre device according to the invention is demonstrable with concentrations as low as a few hundred ppm of rare earth dopant in a monomode optical fibre core.

European Patent Application 87302674.4 dated 27.3.87, claiming priority from earlier GB Application 8610053 of 24.4.86 in the name of the present applicants, discloses a suitable technique for incorporating such a dopant in an optical fibre preform.

Methods for fabricating glass compositions with higher concentrations of rare earth dosants are aescribed, for example, in GE patent Application 8713698 dates 11.6.87 also in the name of the present applicants.

Concentrations up to 10 mole % of the rare earth dopant may conveniently be acnieved.

Preferably, the device includes means for altering the photochromic state of rare earth dopant in the glass nost.

Conveniently, the means for altering the photochromic state of rare earth dopant provides radiation at an appropriate darkening or bleaching wavelength accordIngly.

The means for altering the photochromic state may include means for providing thermal energy. to the doped glass thereby to induce a relative bleaching.

In various preferred embodiments, the device further includes means for probing the photochromic state of rare earth dopant in the glass host.

Preferably, the means for probing the photochromic state probes the transmissivity of the doped glass with an optical radiation which does not substantially alter the photochromic state during probing.

Such a probe is useful, for example, as a 'reading' technique for examining the state of the glass when the device is employed as an optical memory.

In a preferred embodiment the host material comprises silica based glass.

Alternatively, for example, the host material may comprise a multi-component glass.

Optical devices according to the invention have application in many areas, for example, in optical memories; in holographic optical devices eg to provide a photo-induced optical switch; and in phase conjugate mirrors.

Furthermore, lanthanide series eiements are capaDie of lasing action. This is an advantage not known in previous photochromic materials. Devices according to the Invention may thus exsit the newly aemonstrated pnotochromic effect te provide an optical fibre laser.

For the avoidance of doubt, in this specification the term "optical" includes not only the visible part of the electromagnetic spectrum, but also all parts extending into the infra-red and ultra violet regions which are known to be transmissible via dielectric optical waveguides.

Embodiments of the invention will now be described in detail by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic drawing of an experimental arrangement for demonstrating the photochromic behaviour of an optical fibre according to the present invention; Figure 2 is an absorption spectrum diagram for a thulium doped optical fibre as drawn from the preform; Figure 3 is a graph of the change in transmissivity with time of the fibre of Fig 1 at two particular wavelengths illustrating the photochromic behaviour of the fibre; Figure 4 shows the long wavelength absorption spectrum changes in the optical fibre of Fig 1 due to the photochromic behaviour at a particular time; Figure 5 illustrates the photoluminescence spectrum of the fibre of Fig 1 observed in the "dark" photochromic state;; Figures 6, 7 and 8 illustrate further embodiments of the present invention; and Figures 9 and 10 are energy level diagrams relevant to explanation of the observed pnotochromlc benaviour and to the operation of devices according to the present invention.

In the experimental arrangement of -igure 1, which enables the surprising effect upon which the present invention is based to be demonstrated, a thulium doped silica optical fibre 1 is arranged to receive light input from a tunable argon-ion laser source 2 via a lens 3. The light output from the fibre 1 is directed to a spectrum analyser 4. The optical transmissivity of the fibre 1 after irradiation by a particular wavelength from the argon-ion laser source 2 is probed using a second source 5, which is either an HeNe laser or a filtered white light source according to need, and as detailed below. Light from this second source 5 is directed into the fibre 1 via a beam deflector 6.

In a first example experiment, a thulium-doped silica optical fibre was made by a variation of the MCVD process which allows incorporation of a low dopant concentration of a few hundred ppm. The main components in the glass composition were: Ge203 5 mole % P205 0.5 mole % SiO2 94 mole % with, additionally, a Tm 3+ dopant content of about 0.01 mole %.

The core-cladding refractive index step in the example fibre was approximately 0.008. The core radius was 4.0 micrometres and the V-value at 500nm wavelength was about 5. An absorption spectrum of this thulium-doped fibre, as drawn from the preform, and before irradiation to induce photochromic alteration, is shown in Figure 2. An important feature of the spectrum is the narrow absorption band, 3H6 - 1G4, between 450nm and 485nm, and the relatively high transmissivity between 485nm and 650nm.

The argon-ion laser 2 can provide output lines which fail (a) in this absorption band (475nm line), (b) just outside this band (488nm line) and (c) in a region having little absorption (514nm line). These wavelengths were used to demonstrate the photochromic behaviour of the fibre. In addition, the second source 5, as eitner a ee laser probe or a filtered white light source, was provided to aliow changes in transmissivity to be measured at longer wavelengths.

When the fibre was illuminated with invariant cw launch power at a wavelength of 475nm from the laser 2, the transmissivity at this wavelength was found to decrease as a function of time, ie the fibre appeared to progressively "darken". This process is also known as "colouration". This is attributed to structural changes of the glass in the vicinity of the thulium ions, caused by the strong absorption of optical energy at this wavelength. Evidence suggests that the nature of this reversible optical effect is due to lattice restructuring rather than electronic changes in the Tm ions, but it will be understood that the invention does not rely on the correctness of this explanation.

The darkening rate depends on the launch power, the fibre length, and the optical and thermal history of the fibre. For the 4-metre length of fibre in the example, with approximately 70mW of launched power at a wavelength of 475nm, the transmissivity at this wavelength varied as shown in Figure 3a. The response shows a rapid darkening process, tending towards a saturation level of absorption. The functional form of the process follows t n where n has been found empirically to be 3.3. After pumping with 70mW of 475nm for 10 minutes, the proportion of this power actually transmitted by the fibre was lOdB down on the initial level at t=O.

The fibre was then pumped with a constant 90mA cw or the 488nm line from the argon-ion laser. In this case, the pump wavelength is outside the 1G absorption band 4 and the effect of the illumination was gradually to reverse the darkening (Figure 3b). Saturation of the bleaching did not occur within the timescale of this measurement. The observed rate of recovery is slower than the darkening process, being of the functional form where m=0.83.

That a wavelength change of only 13nm (from 475 to 488nm) has such a dramatically different chromatic effect is remarkable.

Using the darkening and bleaching wavelengths of 475nm and 488nm respectively, the change in the transmissivity of the fibre at longer wavelengths was measured; namely at 514nm (argon-ion line); 633nm (HeNe line); and between 60Ohm and 1600nm (filtered white-light).

After a 10 minute bleach with 90mW launched power of 488nm light, the transmission of approximately 3mW of the 514nm line was measured. The fibre was then subjected to 10 minutes of 70mW launched power of light at 475nm to induce darkening, and the level of the 514nm signal was measured at the output for the same 3mW launch power. The increase in attenuation of the signal at 514nm wavelength was 14.7dB +/- ldB. The process was reversible and repeatable, within the above ldB error margin. To achieve these measurements, the only variables controlled were the tuning position and output power of the Coherent Radiation Innova-90 argon-ion laser. The launch optics were stable throughout.

The experiment was repeated using a HeNe laser probe at 633cm, which showed an increase in attenuation on darkening of 13dB +/- 2dB. This experiment required stable alignment of two lasers and the launch optics.

Following the same bleaching and darkening conditions as before, using a white light source 5, the white-light transmission characteristics for both states were recorded on an Anritsu MSM96A Optical Spectrum Analyser 4 and the ratio taken.

Figure 4 snows tne comcined long-wavelength attenuation-change results from the above three sets of measurements, together with the H6 - 1G H6 absorption data from figure 2. The regions 67Onm to 770cm and 760nm to 800nm are missing because of the large 3H6 - 3Fj absorption bands in thulium.

The photochromic behaviour of the example length of fibre under the stated pump conditions can be identified.

As with previously known photochromic glasses, the darkening occurs in the visible to near-IR regions, and is initiated by pumping in the blue part of the spectrum.

The maximum change in transmitted power on colouration is of the order of 15dB at 50Onm to 55Onm, with a long loss-tail out to 90Onm. At 750nm and 820nm - wavelengths associated with GaAlAs semiconductor lasers - the changes are 5dB and 2dB respectively, in the 4 metres of fibre.

The lifetimes of the dark and bleached states of the fibre are long - at least 3 days for saturation conditions at room temperature.

When the example fibre was pumped at 514nm, at the peak of the absorption band generated in the darkened state, a photoluminescence effect was also observed. The extent of this photoluminescence is illustrated in Figure 5. This photoluminescence spectrum exhibits none of the characteristic peaks associated with the fluorescence spectrum of thulium, consistent with the fact that there is no atomic absorption at the pump wavelength. The observed photoluminescence spectrum follows the shape of the measured photo-induced absorption spectrum (cf Fig 4) although this is a measurement which is changing with time as the 514nm pump attempts to bleach the fiDre.

The present inventors have thus demonstrated an unexpected photochromic effect in a thulium doped silica glass fibre. It is be sieved that pumping with blue light (475nm) produces changes In tne structural characteristics of the glass in the vicinity of the Tm ions in the glass thereby giving rise to the photochromic darkening.

There appear to be two possible mechanisms for the change: (a) involving a transition from Tm3+ to Tm"+ (where n is less than 3) with a consequent change in the free electron density around the Tm ion sites. It appears most likely that n=2. The photo-induced darkening may be reversed by thermal repopulation of the defect sites. The energy for this process can be provided, for example, either by external heating or by pumping into the induced absorption band to create the necessary heat for the repopulation, thereby to "bleach" the fibre; and (b) involving the generation of high-energy phonons in the vicinity of the Tm ions which, when resonantly excited, cause ionic motion over short ranges to produce modifications of the host glass. The reverse process occurs when sufficient thermal energy is provided to reorder the lattice structure.The observed long lifetimes in the two states can be attributed to the structure being 'frozen in' at ambient temperatures.

These altered lattice sites for the Tm ions may be considered as defect centres (or f-centres). The creation of these defect centres produces a broad absorption band (cf Figure 4) characteristic of an inhomogeneous structural change.

In the Tm doped fibre of the specific example, the time constant for the darkening response is short, of the order of a few seconds, whilst the thermal lifetime of the dark state at room temperature is significantly longer (a few days).

The observed photochromic behaviour has use in various optical devices according to the present invention.

An optical memory, for examole, may be produced, since a doped fibre according to the invention "remembers" the state, transmissive or not, in which it is eft. The memory state is readable using, for example, 633nm or GaAlAs signals without causing a state reversal, given the observed characteristics of the Tm doped fibre. High power pump pulses may be used to speed up the response time if the memory is to be re-written.

Optical devices may use the photochromic behaviour in a holographic manner. The structural changes in the glass are accompanied by an induced refracted index change. In the commonly employed optical fibre transmission windows around 1300 and 1550nm there are no observed changes in absorption, since the effect of the darkening decays above about 9OOnm. However, the associated refractive index change may usefully be exploited.

A photoinduced longitudinal grating may be produced in an optical fibre by setting up a standing wave, for example, using the darkening pump wavelength.

Alternatively, such a grating may be produced by using a standing wave at a "bleaching" wavelength in a previously darkened fibre. The standing wave(s) may be produced by conventional reflection techniques.

Under these circumstances, the refractive index of the doped fibre core would vary longitudinally, as would the effective mode index at 1300 and 1550cm. Light at these longer wavelengths would therefore see a periodic structure in the waveguide. Where the Bragg conditions were satisfied, the light would be reflected. The photoinduced fibre grating would be erasable by appropriate irradiation of the optical fibre.

Figure 6 illustrates schematically one possible configuration in which a doped fiDre 12 is cooled to a second fibre 13 to form a directional coupler il. The doped fibre 12 is pumped at one end with a state altering wavelength from a source 14. A reflector 15 is provided at the other end of the doped fibre 12 and IS positioned so that a standing wave interference pattern is established in the doped fibre 12. A non-reflctive termination 16 is provided on corresponding enc of the second fibre 13.The parameters of the coupler 11 may be selected in a known manner such that light at the pump wavelength is substantially confined to the doped fibre 12 and does not couple out in any significant proportion into the second fibre 13, whilst light input I to the second fibre 13 at different wavelengths from that in fibre 12 is nevertheless enabled to cross-couple to an appropriately greater extent. Selected component wavelengths of the input I are then reflected by the photoinduced grating 17 to form a return output R.

The rare earth doped glass in bulk or fibre form may also be used to effect in four-wave mixing, such that phase conjugate mirrors may be provided utilising the photochromic behaviour.

An optical device using the photochromic effect may also be configured as an optical switch or an attenuator.

The device of Figure 6 may be adapted for the purpose.

Figure 7 illustrates one example configuration, comprising a directional coupler 11 formed from a Tm-doped fibre 12 and a second fibre 13, but in this case the reflector 15 of Figure 6 is omitted. A pump source 14 similarly provides light input to the doped fibre 12 at a state altering wavelength. Using appropriate darkening and/or bleaching light inputs to control the operation, the transmissivity of the doped fibre can be selectively changed sucn that signals I of an appropriate wavelengtn entering the second fibre 13 may be effectively switched on or off or partially attentuated at the coupled output T. Short high power control pump pulses are preferred to improve the speed of operation.

The photochromic behaviour may further provide a "colour centre" visible wavelength laser. The defect centres in the darkened state may be considered to provide a broad (absorption) energy band having characteristics similar to a valence band in a conventional lasing medium. The energy band can be populated by pumping and the pumped electrons have a finite and relatively long lifetime in the populated state.

Tnus, using a Tm-doped glass fibre host, lasing action may be promoted by (i) pumping at a darkening wavelength (eg 475nm) to produce the structural defects and the associated broad absorption band; (ii) subsequently, or simultaneously, at high energy, pumping into the induced absorption band; (iii) providing optical feedback to provoke the lasing action.

Without the feedback, the population inversion will act to provide a gain medium without lasing.

The broadness of the emission spectrum associated with the above process should provide gain over a wide proportion of the visible spectrum (eg 520-lOOOnm) subject to modification by the Tm absorption bands in the 700 and 800cm regions.

Further wavelength selective feedback (eg via an external cavity) would allow the fibre laser to be tuned over a large extent of the visible to near IR spectrum.

An argon-ion laser running on all lines should conveniently provide all the required pumping inputs.

Figure 8 shows an experimental arrangement to demonstrate this lasing action. The doped fibre 1 is positioned between two reflectors 7,8 which together define a resonant cavity of length L. The fibre 1 is pumped from the argon-ion source 2 via the collimating lens arrangement 3. The ar30n-ion source provides both the darkening (475cm) and the exciting (514nm) wavelength pump inputs. The lasing wavelength is selected by adjustment of the cavity length L.

Figure 9 is a diagram of the relvant energy states for thulium, showing the induced absorption band. Figure 10 is an energy level diagram for the "colour centre" laser.

In the example shown the argon-ion lines at 475nm and at 514nm are used to create the defects and to effect the population inversion respectively. The lasing wavelength is determined by selective feedback.

A visible-to-near IR laser as described provides an alternative to known dye lasers which have more restricted tuning ranges per dye.

Devices according to the invention make use of the novel photochromic glass with soluble lanthanide dopant to provide new embodiments of optical components.

Although the invention has been specifically described with reference to thulium dopant in a silica glass optical fibre host, alternative embodiments may be utilised. In particular analogous photochromic behaviour may occur with other lanthanide dopants. Cerium (Ce), samarium (Sm), europium (Eu) and dysprosium (Dy) have 3 and 2+ ionic states possibly enabling the creation of defect centres under appropriate conditions.

Europium and dysprosium have absorption bands at the blue end of the spectrum in the 3 state. Europium, most particularly, has an absorption band at a convenient wavelength.

Further, it will be appreciated that glass hosts other than silica may be used without eliminating the photochromic behaviour. A multiconponent or a fluoride glass matrix, for example, may be used.

Yet further, it will be apparent that the glass host need not be restricted to optical fibre form. So long as sufficient optical Dower density can be concentrated on tne glass to create the defects, then the photochromic behaviour may be exploited.

Claims (1)

1. An optical device comprising a glass host material doped with a lanthanide rare earth element soluble in the glass matrix, the doping being to a concentration at which the doped glass exhibits photochromic behaviour having relatively less transmissive, darkened and relatively more transmissive, bleached photochromic states and such that the transmissivity can be altered optically.

2. An optical device according to claim 1 including means for altering the photochromic state.

3. An optical device according to claim 2 wherein the means for altering the photochromic state provides radiation at an appropriate darkening or bleaching wavelength accordingly.

4. An optical device according to claim 2 wherein the means for altering the photochromic state includes means for providing thermal energy to the doped glass thereby to induce a relative bleaching.

5. An optical device according to any preceding claim including means for probing the photochromic state.

5. An optical device according to claim 5 wherein tne means aor probing the photochromic state proDes the transmissivlty of the doped glass with an optical radiation which does not substantially alter the photochromic state during probing.

7. An optical device according to any preceding claim wherein the host material comprises a glass optical fibre doped with a lanthanide rare earth element soluble in the glass matrix, the doping being to a concentration at which the doped fibre exhibits photochromic behaviour having relatively less transmissive, darkened ane relatively more transmissive, bleached photochromic states and such that the transmissivity can be altered optically.

8. An optical device according to claim 7 as it depends on claim 2 wherein the means for altering the photochromic state includes means for producing interference fringes in the doped glass thereby to induce a modulation in photochromic state.

9. An optical device according to claim 8 wherein the means for producing interference fringes produces a standing wave modulation in the optical fibre thereby to provide a photoinduced grating in the doped glass.

10. An optical device according to claim 7 as it depends on claim 2 wherein the doped optical fibre is coupled to a second optical fibre in a directional coupler configuration and the means for altering the photochromic state provides an altering optical input to the doped optical fibre.

11. An optical device according to claim 7 including means for pumping the optical fibre with light at a darkening wavelength to provide a photoinduced absorption band, means for pumping energy into the optical fibre within the Induced absorption band to achieve an energy level population inversion, and optical feedback means for provoking lasing action.

12. An optical device according to any Dreceåinr 0151:: claim wherein the lanthanide is thulium.

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13. An optical device according to any of claims 1 to 11 wherein the lanthanide is selected from one of cerium, samarium, europium, dysprosium.

14. An optical device according to any preceding claim wherein the host material comprises a silica glass.

15. An optical device according to any preceding claim wherein the host material comprises a multicomponent glass.

16. An optical device according to claim 12 wherein the glass host contains a proportion of Tm in the range from 0-10 mole %.