GB2250606A - Interferometer having twin-core optical fibre - Google Patents

Abstract

A non-linear Mach-Zehnder interferometer comprises a twin core optical fibre 8 whose ends have been tapered to form non-50:50 four-port couplers 4 and 8. The use of twin-core fibre reduces the differential optical path length changes due to random external perturbations. A polarisation controller 34 may be used to adjust the differential optical path length and polarisation of the signals at the output coupler. The twin cores may have the same or different diameters. Output ports 28, 28, microscope objective 30 and detectors 32 are shown. <IMAGE>

Description

INTERFERONETER This invention relates to Mach-Zehnder interferometers comprising a first and a second four-port directional coupler, each of two ports of the first coupler being optically coupled to a respective port of the second coupler. The other ports of each coupler constitute input and output ports of the interferometer respectively.

An optical signal coupled to an input port of such an interfermeter will be split into two portions which then propagate along separate optical paths to the two ports of the second coupler. The two signals interfere at the second coupler in known manner. The optical signals exiting the output ports are dependent on the relative phase of the two portions when they recombine and therefore on the difference of the two optical path lengths between the couplers.

Changes in the differential optical path lengths can be monitored by monitoring the optical power exiting a port of the second coupler.

The ultrafast optical Kerr effect in silica fibres is increasingly being exploited for all-optical switching and logic applications. Silica fibres have a small nonlinearity but unlike other nonlinear materials have the significant advantage of extremely low loss (0.2 dB/Km). This allows long lengths of fibre, in order of 100s of metres, to be used for fabricating devices so that switching powers can be minimised. All-optical switching in silica fibres has been demonstrated in nonlinear Mach-Zehnder interferometers. N. Imoto, S. Watkins and Y.

Sasaki, "A Non-linear optical fibre interferometer for non-demolitional measurement of photon number", Optics Comm Vol 61, No.2, pp 159-163; I.H. White, R.V. Penty, N.R. E. Epworth, "Demonstration of the optical Kerr effect in an all-fibre Mach-Zehnder interferometer at laser diode power levels", Elec.

Lett Vol 24 No.6 pp 340-341. A major constraint is the need to provide stability to the device against external acoustic and thermal perturbations. To date the non-linear optical loop mirror (NOLM) is the only interferometer having long fibre lengths to exhibit insensitivity to linear refractive index changes arising from external acoustic and thermal perturbations see for example N.J. Doran, D.S. Forrester and B.K. Nayar, Experimental investigation of all-optical switching in fibre loop-mirror device" Electron, Lett., 25, 267(1989) and K.J. Blow, N.J.Doran and B.K. Nayar, "Experimental demonstration of optical soliton switching in an all-fibre non-linear Sagnac interferometer", Opt. Lett., 14, 754(1989).

However, there are other relevant considerations for such devices. An important requirement for implementation of all-optical logic operations such as AND, OR, NAND and NOR gates, is a sharp on-off switching or threshold characteristic. One way this can be achieved is to concatenate a number of nonlinear devices. The NOLM is essentially a two-port device and to prevent any feedback into the cavity it is necessary to incorporate an optical isolator between it and the laser.

Optical isolators are also required between the concatenated elements. Four-port operation of the NOLM can be achieved by rotating the input polarisation by 900 within the loop using a polarization controller and using polarising prisms in the input and output arms. However, the inclusion of either optical isolators or extra polarisation optics increases both overall loss and complexity.

In contrast the nonlinear Mach-Zehnder interferometer (NLMZ) is a simple four-port device and can be directly concatenated for all-optical switching/logic applications. However, in NLMZs reported to date instabilities occurred having time constants of the order of a millisecond. As a result it is necessary to use active stabilisation to maintain the linear operating point.

Temporal overlap is not an issue if-high power control pulses are used to switch weak cW signals. In many applications, however, temporal coincidence of pulses at the interferometer output is necessary. In consequence, optical path lengths in the interferometer arms must be kept closely matched.

According to the present invention an interferometer according to the preamble of claim 1 is characterised in that the optical coupling means comprises a single optical fibre having two cores.

It has been found that the core proximity of the twin core fibre provides a significant reduction in the differential phase change experienced by signals in the two waveguides due to random external perturbations.

The cores should be spaced apart sufficient to ensure substantially no cross-coupling of signals propagating along one of the optical cores to the other. A core radius and spacing sufficient to produce less than 10/o coupling over the length of the interferometer has been found to be adequate but less than about 0.50/o preferable. Typical lengths of silica fibre required to obtain optical switching at reasonable optical power levels are around 200m.

The twin-core optical fibre may be fabricated by any convenient technique. One method is to fabricate an optical fibre pre-form, remove a longitudinal portion to produce a flat along its length, cut it in two, and place the flats together to form a dual core preform. This can then be pulled to form an optical fibre in the usual way. Other multicore fibres having other usused cores, could of course, also be used.

The optical couplers are preferably optical fibre couplers to reduce losses between them and the optical fibre. Splice losses between the couplers and the optical fibre can be eliminated by forming the optical couplers from the optical fibre itself by tapering. This can be achieved by a' method of fabricating the interferometer including the steps of a) providing an optical fibre having two cores; b) locally heating a first region of the optical fibre and tapering the fibre by pulling until a predetermined coupling ratio a a predetermined wavelength is obtained; and c) locally heating a second region of the fibre and tapering the fibre by pulling through the same distance as when forming the first coupler.

An embodiment of the invention and its principle of operation will now be described with reference to the following drawings of which: Figure 1 is a schematic diagram of an interferometer according to the present invention; Figure 2 is a diagrammatic longitudinal cross-sectional view of a coupler of the embodiment of Figure 1, Figures 3 and 4 are diagrammatic sectioned views in the direction III-III and IV-IV of Figure 2, respectively.

Figure 4 is a graph of the transmittance of the bar and cross output ports of the interferometer of Figure 1 as a function of time for a constant input power; Figures 5 and 6 are graphs showing the experimental and theoretical transmittance of the cross and bar states of the interferometer of Figure 1 as a function of the input peak power for an initial linear differential phase change of fL = 0.2r (box enclosure used) fL = 0.8 (no box enclosure), respectively; and Figure 7 is a series of oscillograms showing the temporal pulse profile of the bar and cross outputs as a function of peak input power.

Referring to Figure 1, an interferometer 2 comprises four-port directional couplers 4 and 6 each of two ports of which (not referenced) are optically coupled by a twin-core optical fibre 8.

The interferometer 2 was fabricated from a 200 m length of twin core fibre 8 having approximately uniform and identical cores 10 and 12 with 3.8um diameter, a core centre-centre separation of 37 ssm, and a higher mode cut-off at l.O7um wavelength. The large ratio of the core centre-centre separation to core radius and small mode spot size results in an estimate of the directional coupling beat length of greater than 1000 km so that substantially no coupling occurs between the two waveguides over the 200 metres length of the interferometer 2. The waveguides were not designed to be polarisation-maintaining. The interferometer 2 is stored by wrapping it round a drum 14.

The directional couplers 4 and 6 at each end of the interferometer 2 were fabricated using a standard fusion coupler fabrication rig to locally heat and pull the fibre 8. Coupler 4 is shown in detail in Figures 2, 3 and 4. It has a tapered central region 16 about lcm long where the cores 10 and 12 are pulled to about 500/0 of their original, size with a similar reduction in the core diameter and separation (to about 18;m), to allow evanescent coupling (see Figure 4) and untapered end regions 18 and 20 (see Figure 4). The input coupler 4 was pulled until it had a power splitting ratio of 34:66 at 1.32#m wavelength determined by direct measurement of a test signal coupled from one of the cores 10 and 12 to the other.The second coupler was fabricated by heating and pulling the fibre over approximately the same distance so as to obtain a similar splitting ratio to the first as accurate monitoring is not possible after the first coupler 4 has been formed. However, the power splitting ratio was measured to be 23:77 (a2=0.23) on cut-back after completion of the experiments. The difference in the power splitting ratio is attributed to tapering in the fibre dimensions.

The coupling ratio of the second coupler 6 could be made closer to the desired ratio by removing a section of the end of the fibre, forming a test coupler from this section by monitoring the changing coupling ratio during formation (as with coupler 4) and then forming the coupler 6 by pulling to the same length as this test coupler. This should provide more accuracy as the fibre characteristics at the coupler 6 will more closely match the test coupler than the first coupler 4.

The coupling ratios were polarisation independent. A virtue of this design is that nearly equal optical paths are obtained and temporal overlap for ultrafast pulses is assured. The insertion loss of the interferometer 2 was measured to be 0.4 dB.

The end of coupler 4 is shown in Figure 3. The ends of the cores 10 and 12 provide the input ports to the interfermeter 4 and are referenced 22 and 24 in Figure 2 but not separately discernible in Figure 1. Similarly the coupler 6 has two output ports 26, 28, not separately discernible in Figure 1, provided by the ends of the cores 10 and 12 at the coupler 6.

It is envisgaged that etching or cutting techniques will permit separation of the output ports to allow splicing to the individual cores 10 and 12.

An alternative method of coupling the core 10 and 12 to external fibres is to fuse the end regions of two separate D-fibres to form a fused twin-core fibre at one end, only, having a core spacing equal to that of the core 10 and 12 at the end of the coupler 4 and 6. This can then be fusion spliced to the coupler 4 or 6.

Output ports 26 and 28 can be designated the bar and cross ports respectively. The input coupler with its asymmetric power splitting ratio allows the intensity in both arms of the interferometer to differ so that a differential phase change can be induced between the pulses in the two interferometer arms as a function of the signal intensity. The pulses are then coherently combined by the output coupler 6.

A 76 MHz mode-locked Nd:YAG laser 28 operating at l.32#m wavelength was used as the test source. The laser pulse width was measured to be 135 ps (FWHM) using a fast InGaAs photodiode with a 75 ps response time. Thus, the deconvoluted laser pulse width corresponds to about 110 ps (FWHM). The laser light was launched into port 22 of the interferometer and a microscope objective 30 was used at the output to collimate the light from both output ports 26 and 28. The two outputs were focused onto detectors 32 situated approximately 2m from the fibre end. Pulse shaping effects were monitored with fast InGaAs photodiodes whilst large area germanium photodiodes were used to measure the stability of the linear operating point and the time-averaged response of the device.

The fibre was looped through a- mechanical polarisation controller 34 in order to obtain similar polarisation states in both the waveguides prior to recombination in the second coupler. This device is generally used with single core non-polarisation-maintaining fibres to give any desired output polarisation by virtue of the bend induced birefringence in the fibre. In the present case, bending of the fibre 8 in the plane defined by both cores (10, 12) has been found to give rise to an additional effect whereby one core is elongated whilst the other is compressed. Consequently, the mechanical polarisation controller 34 serves both to match the polarisation states in the two arms and to impose a linear phase difference.Naximum transmission was obtained at either output port 26, 28 by simply changing the polarisation controller setting to obtain an initial linear phase change of ff between two arms. The device output was stable over a period of few minutes without any external stabilization measures. This increased to periods of over an hour when a box enclosure (not shown) was placed over the interfermeter 2 to prevent air draughts.

In Figure 4 the transmittance of the two output ports 26, 28 of the interferometer 2 is plotted as a function of time for a fixed input power condition well into the nonlinear regime. The duration of the scan in this case was 14 minutes. Long term stability is clearly demonstrated. The noise shown in these response curves arises partly from the source laser and the detectors as well as the device itself.

The average nomalised transmitted pulse energy, T1 and T2, from the two output ports of the NLNZ can be computed using

where T (t) is the input temporal pulse shape (e.g. for a sech shaped pulse 9 (t) = A sech(t) and T1 is the CW transmittance of the bar port, given by

fL is the linear differential phase-change due to unequal optical paths, XNL is the nonlinear phase-change given by

X is the optical wavelength, n2 is the optical err coefficient which is silica fibre has a value 3.2 x 10'20 m2/W, Pi is the input. power, and Aeff is the effective mode area.

Switching occurs between the output ports of the interferometer 2 whenever there is an increase in the input power of WAeff/[2(l-2al)n2L], a quantity which is independent of the second coupler splitting ratio, a2.

Experimental and theoretical plots of the output port transmittances as a function of the peak input pulse power are shown in Figures 5 and 6. Arbitrary settings of the polarisation controller 34 were used to bias the device with different values of linear phase fL for the two data sets. The theoretical curves were calculated using the measured coupler splitting ratios, with the linear phase jL being a free fitting parameter. The laser pulse was assumed to be sech shaped.

Excellent agreement is obtained between theory and experiment.

Bias control is carried out quite simply with this device, therefore, and furthermore the coupler 4 and 6 splitting ratios can be deduced from the nonlinear response. These plots also show clearly the stabilizing influence of the box enclosure, for the curves in Figures 5 and 6 were obtained with and without the box respectively. The data in Figure 6 is demonstrably noisier.

Temporal pulse response at the two output ports 26, 28 corresponding to Figure 5 as a function of increase in peak input power is shown in Figure 7. At low input powers the output is predominately from the cross port 28. On increasing the input power the output from the bar port 26 increases nonlinearly whilst there is a decrease in the output from the cross port 28.

Due to the non-square nature of the pulses the high power part of the pulse switches while the wings remain unswitched. The first maximum dip in the cross port pulse profile corresponds to an induced nonlinear phase shift of ir. The photodiode response time limits the dip going down to zero. This occurs at a peak (average) input power of approximately 7.7 W (65 mew). On further increasing the input power the dip disappears at the cross port 28 and appears at the bar port 26. This clearly illustrates the incomplete switching behaviour obtained with nonsquare pulses in ultrafast interferometric devices. Similar device behaviour was obtained when the initial condition was set up to obtain low power output from predominately the bar port 26.The onset of stimulated Raman scattering (SRS) was investigated using an optical spectrum analyzer, however, even at the maximum input power available no SRS was evident.

In conclusion, the present invention provides all-optical switching for picosecond optical pulses in a 200 metre low loss NLMZ without the use of active stabilisation. It has been shown that mechanical polarisation controllers can be used to operate the NLMZ with an arbitrary but controllable phase difference without compromising the requirement of temporal pulse overlap for picosecond pulses. This can permit an easy realisation of all the basic logic functions. The optical switching was, as expected, incomplete due to the non-square pulses used. Complete switching will result when either square or soliton-like pulses are used or alternatively a high power control pulse is used to switch a weak signal pulse. The results demonstrate that NLMZs could be successfully used for all-optical switching and the implementation of pipeline logic. It is possible also that the interferometer design could well be advantageous for other purposes such as sensing and quantum optics.

An alternative way of providing asymmetry in the Mach-Zehnder is to provide a twin-core fibre having different non-linearities, for example by virtue of having different core diameters. In this case 50:50 couplers may be formed at each end of the interferometer if desired.

Claims (10)

1. An optical interferometer comprising a first and a second four-port directional coupler (4, 6), each of two ports of the first coupler (4) being optically coupled to a respective port of the second coupler (16) by an optical coupling means, characterised in that the optical coupling means comprises a single optical fibre (8) having two cores (10, 12).

2. An interferometer as claimed in claim 1 in which the couplers (4, 6) are optical fibre couplers.

3. An interferometer as claimed in claim 2 in which each optical fibre coupler comprises a tapered region (16) of the optical fibre (8).

4. An interferometer as claimed in any preceding claim in which the cores have substantially the same diameter.

5. An interferometer as claimed in claim 4 in which the couplers (4, 6) are non-50:50 couplers.

6. An interferometer as claimed in any preceding claim in which the coupling beat length between the cores (10, 12) is less than about 0.50/0.

7. An interferometer as claimed in claim 6 in which the optical fibre (8) is a silica fibre, the cores (10, 12) have a diameter of approximately 3.8cm and are separated by a centre-to-centre distance of approximately 37 m.

8. An interferometer as claimed in claim 7 in which the optical fibre (8) is about 200m long.

9. An interferometer as claimed in any preceding claim including a polarisation controller (34) mechanically coupled to the optical fibre (8).

10. A method of fabricating an interferometer including the steps of a) providing an optical fibre (8) having two cores (10, 12); b) locally heating a first region (16) of the optical fibre (8) and tapering the fibre (8) by pulling until a predetermined coupling ratio a predetermined wavelength is obtained; and c) locally heating a second region of the fibre and tapering the fibre (8) by pulling through the same distance as when forming the first coupler (4).