GB2317236A - Optical fibre device - Google Patents

Abstract

An optical fibre null coupler is formed of two or more optical fibres fused together in a coupling region, the coupling region being twisted about its longitudinal axis. Coupling is brought by exciting the coupling region with an acoustic wave. The twist is applied to the fused fibres while the coupling region is being heated and twist maintained as the coupling region cools. The twist alleviates polarisation dependence. Polarisation dependence can also be alleviated by exciting the coupling region at more than one acoustic frequency.

Description

OPTICAL FIBRE DEVICE This invention relates to optical fibre devices.

Recently a new type of acousto-optic device based on four port null taper couplers has been demonstrated, and has been shown to be efficient as an optical switch, frequency shifter and tunable filter; see [1-3] and WO95/22783.

The null coupler is a special type made from two fibres with diameters or other optical or physical properties so mismatched that it does not actually couple any light.

Light input via one fibre excites just the fundamental mode in the narrow waist of the coupler whereas light input via the other fibre excites just the second mode. In both cases the light propagates along the waist and returns to the original fibres at the output end of the coupler. Thus, each output fibre is an output for light which propagated along the coupling region in a respective one of the two modes.

However, a flexural acoustic wave propagating along the coupler causes a periodic refractive index perturbation in the waist. If a resonance condition is met (by which the acoustic wavelength matches the optical beat length between the modes) then light can couple between the modes, and so the proportions of light output from the two output fibres can be varied by applying the flexural acoustic vibration to the coupling region. In this way, the device can act as a switch or a modulator.

The main advantages of the null coupler are that it is a monolithic four-port device with a low insertion loss, low drive power requirement and a high conversion efficiency when compared to other acousto-optic devices [4,5].

However, The null coupler acousto-optic devices are inherently polarization dependent since the optical beat length of the two relevant spatial modes are different for the two eigen-polarization states [10]. This polarization dependence reduces the usefulness of the device as an optical switch or as a practical filter for WDM network applications.

This invention provides an optical fibre device comprising an optical fibre null coupler formed of two or more optical fibres fused together at a coupling region, at least a part of the coupling region being mechanically twisted substantially about a longitudinal axis of the coupling region.

In embodiments of the invention the effect of birefringence in a null coupler can be overcome by twisting the waist (coupling region) of the device during manufacture and/or after fabrication.

The birefringence is caused by very different physical features to those giving rise to birefringence in a single mode optical fibre. In a single mode optical fibre, birefringence arises from the difference in the propagation constants of the two orthogonally polarised fibre modes, caused in turn by a combination of core ellipticity (form birefringence) [6] and an associated thermal stress asymmetry (stress birefringence) [7].

In contrast, because of the coupler's narrow waist region, in a null coupler there exists a large refractive index variation (silica to air) across the waveguide and this in turn means that the taper waist is not "weakly" guiding. Therefore, while ellipticity and thermal stress can be contributing factors to the birefringence, the principal effect can be attributed to the change in the so-called V-value, which is determined by the waist diameter of the null coupler.

The V-Value is given by: V 2 II a NA 3e where a is the fibre radius, NA is the numerical aperture, and X is the optical wavelength.

In the circular limit, the polarisation splitting, a), for a null coupler can be expressed as:

where X (nm) is the optical wavelength, V the V-value of the cladding-air waveguide at the taper waist, na the refractive index of the surrounding medium and j01 = 2.405 and j11 = 3.832 are the solutions of the Bessel functions defining the cut-off for the fundamental and higher order mode.

From this equation it is clear that the effect of polarisation splitting will increase for narrower taper waists.

From this analysis, the effect of twisting the null coupler should overcome or alleviate the problem of polarisation sensitivity, particularly within the limit that the twist rate exceeds the birefringence between the polarisation states and for the case that the final taper cross-section is slightly elliptical.

This invention also provides an optical fibre device comprising: an optical fibre null coupler formed of two or more optical fibres fused together at a coupling region; and means for exciting acoustic vibration of at least a part of the coupling region at at least two acoustic frequencies, so that an optical wavelength-dependent response of the device for one input polarisation and resulting from one of the applied acoustic frequencies occurs at substantially the same optical wavelengths as an optical wavelengthdependent response of the device for the other input polarisation and resulting from the other of the applied acoustic frequencies.

In other embodiments of the invention, substantially polarization-insensitive operation of a null coupler acousto-optic tunable filter is obtained by simultaneously applying two acoustic waves. The two waves provide phase-matched coupling for each of the individual eigen-polarization states.

The polarisation dependence of the device to excitation at one of the acoustic frequencies leads to two different filter responses, one corresponding to each polarisation.

One of these two responses will be at a higher optical wavelength than the other, and if a particular input polarisation gives rise to, say, the higher optical wavelength filter response of the pair at a particular acoustic frequency it will do so at other acoustic frequencies.

So, if two acoustic frequencies are applied, the acoustic frequencies can be selected so that the higher wavelength response of the two filter responses from one acoustic frequency overlies in optical wavelength the lower wavelength response of the two filter responses from the other acoustic frequency, then the combination of the two overlying filter responses (which relate to the two different input polarisations) provides a substantially polarisation independent filter response.

Preferably, any potential problems associated with resultant unequal frequency shifts for the individual polarization components are overcome by a simple double-pass arrangement, in which light experiencing (say) an up-shift on its first pass through the coupler is down-shifted equally on the second, resulting in no net frequency shift for both polarization components.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of a null coupler; Figures 2a and 2b are graphs showing throughput and coupled spectra for an untwisted null coupler; Figure 3 is a graph showing the retardance of a null coupler against applied twist; Figures 4a and 4b are graphs showing throughput and coupled spectra for a twisted null coupler; Figure 5a is a graph of throughput and coupled spectra against input polarisation for an untwisted null coupler; Figure Sb is a graph of throughput and coupled spectra against input polarisation for a twisted null coupler; Figure 6 illustrates the time variation of optical power in a null coupler used as an optical switch; Figure 7 is a schematic diagram of an acousto-optic tunable filter using a null coupler; Figure 8 is a graph illustrating the variation in centre wavelength for the two eigenpolarisations of the device of Figure 7; Figure 9 is a graph illustrating spectra for the device of Figure 7; and Figure 10 illustrates spectral characteristics of the device of Figure 7.

Figure 1 is a schematic diagram of a null coupler 10, produced using standard telecommunications fibre, with a diameter of 125 ,um. The coupler was made by stretching two fibres 20, 30 together in an oxybutane flame, where one fibre 20 had initially been pre-tapered to a diameter of 90 clam. For the chosen acoustic frequency of 1 MHz the required waist diameter can be calculated as 12.7 ym [1].

For the interaction to be resonant along the length of the coupler the waist should be uniform in diameter. Uniformity and diameter control were achieved by using a travelling flame as the heat source.

The final coupler waist 40 was 8 mm long and had short taper transitions. The excess loss of the passive coupler was 0.1 dB and the maximum splitting ratio was 1:1000.

An acoustic wave was generated by a piezoelectric (PZT) disk 50 driven by a rf electrical supply 60 and coupled to the fibres by a conical horn 70.

With light of a given polarisation state and a wavelength of 1550 nm launched in one fibre (e.g. the fibre 30 as illustrated), acousto-optic coupling was observed at an rf drive frequency of 1 MHz. For a drive power of - (about) 1 mW, 98 % conversion efficiency was possible into the output port of the other fibre 20, corresponding to a cross-talk of -17 dB and an insertion loss of 0.15 dB.

Optical spectra were measured by launching white light into the unpre-tapered arm of the device (i.e. the fibre 30) and measuring the normalised throughput and coupled output spectrum using an optical spectrum analyser.

Figures 2a and 2b are graphs showing a typical pair of spectra for a drive frequency of 1 MHz (without any mechanical twisting of the coupler). The throughput spectra (Figure 2a) shows the amount of light emerging at the output port corresponding to the fibre 30, and has two principle dips. The complementary coupled spectra (Figure 2b) shows the amount of light emerging at the output port corresponding to the fibre 20 and demonstrates two peaks at corresponding wavelengths.

The spacing of 60 nm between the peaks, implying a beat length of 9 mm between the polarisation states, is close to the expected polarisation splitting of 50 nm calculated from equation 2. (The side peaks obtained experimentally are attributed to lack of uniformity along the final interaction length).

The effects of applying a mechanical twist to the fused coupling region 40 of the null coupler will now be discussed.

The theoretical results for the retardance, R(z), representing the birefringence of the coupling region in terms of the phase difference between the fast and slow axes of the coupling region, are shown schematically in Figure 3, for increasing twist, in the case that the interaction length is 8 mm long. The effect of twisting the null coupler and the rapid reduction of the overall birefringence is clear.

This beneficial effect of twisting the coupling region is again illustrated in Figures 4a and 4b, where the wavelength dependence of the device with a static applied mechanical twist of two revolutions is monitored.

The mechanical twist can be applied by clamping one end of the coupler waist, twisting the coupler waist (e.g. rotating the free end of the waist to apply a mechanical twist substantially about the longitudinal axis of the waist), and then clamping the other end of the coupler waist to maintain the twist. The clamps (e.g. glue spots) are positioned just beyond the extreme ends of the waist on the coated portion of the fibres.

This avoids any problems of acoustic reflections at the clamps, because the coating applied to the fibre is acoustically absorbing.

In contrast to the "untwisted" spectra of Figures 2a and 2b, the spectra now reveals a single dip for the throughput (Figure 4a) with the coupled spectra (Figure 4b) giving a single peak at the same corresponding wavelength.

The resonant wavelength at 1540 nm is at an intermediate value in relation to the two eigen polarisation states in Figures 2a and 2b.

Final confirmation that the twisted device is substantially polarisation insensitive was achieved by first launching laser light at 1550 nm through a bulk optic half-wave plate and polarisation controller, and then monitoring the light output via the throughput and coupled output ports at a detector. The results of this experiment are shown in Figures 5a and Sb. In each of Figures 5a and 5b, throughput light is represented by open (non-shaded) graph points, and coupled light is represented by closed (shaded) points.

The effect of polarisation sensitivity for the untwisted device is shown in Figure 5a, where rotating the half-wave plate by 45 degrees reduces the coupled output light by 17 dB on account of exciting the orthogonal polarisation state. This corresponds to a 17 dB increase in the light output via the throughput port.

However, in the case that the device is rotated by two revolutions of twist, Figure 5b, light monitored at the coupled output port is reduced only by 0.2 dB when the orthogonal polarisation state is excited and is an indication of the relative polarisation insensitivity of the device.

Whilst null couplers have many applications, the benefits of twisting the device in order to overcome or alleviate polarisation sensitivity are expected to be particularly relevant when the device is used as a broadband (50 nm) re-routing switch. In this case waist diameters between 10 ym and 15 ym are required which is in contrast to the narrower taper waists needed for narrowband filters and frequency shifters.

The results obtained with the device used as a substantially polarisation insensitive switch are shown in Figure 6. The acoustic drive frequency was first tuned from 1 MHz to 1.01 MHz such that the resonant wavelength in Figures Sa and Sb was 1550 nm. The time response of the switch was then measured by modulating the rf drive with a square wave. The switch changes state in 40 ys, which compares well with the 46 ys predicted in [1]. The time lag between the rf signal being turned on (at t=0 in Figure 6) and the start of switching is the time that the acoustic wave takes to travel from the transducer through the horn and along the fibres to the coupler. The net effect is a time delay of 100 s between the electrical signal and the completion of optical switching. Any fluctuation in the output on account of rotating the half-wave plate and thus exciting the orthogonal polarisation state was less than 5% in the prototypes constructed and tested.

A substantially polarisation independent acousto-optic device based on a null coupler has therefore been described. It has been shown that by twisting the fused interaction region the polarisation sensitivity is reduced typically from 17 dB to 0.2 dB.

It is likely that the effect will be restricted to waist diameters 2 10 ym, otherwise the requirements of the polarisation beat length being greater than the spin pitch, and a non-perfect circular waist are difficult to satisfy. However, for switching applications where frequency shifts 5 2 MHz are sufficient, it is thought that twisting will prove fundamental in order to overcome the problem of polarisation sensitivity.

In other embodiments, the twist need not be applied after manufacture, but instead, a permanent twist could be fused within the taper waist as it is being fabricated.

The effect is attributed to a combination of ellipticity of the taper waist and the requirement that the polarisation beat length between the higher order modes exceeds the twist pitch. The fact that both of these conditions are violated (in devices formed of standard single mode fibre) for taper waists having a diameter < 6 ym means that the technique is less suitable for filters and frequency shifters (where high acoustic frequency operation, and so a narrow taper waist, is generally required), and tends to be more suitable for devices used as switches. However, if other fibre materials are used these limits can be overcome.

In further embodiments of the invention, the polarisation sensitivity of a null coupler acousto-optic tunable filter can be alleviated by simultaneously applying two acoustic waves. The two waves provide phase-matched coupling for each of the individual eigen-polarization states.

The null coupler on which these embodiments of the invention are based is made from two fibres with diameters mismatched to the extent that the resultant coupler gives an extremely small passive coupling efficiency. It can be made by pre-tapering one of two identical single mode fibres along a short length before both fibres are fused and elongated together to form the coupler. This gives a device with identical ports. Input light in the fibre that was not pre-tapered excites only the fundamental mode in the narrow waist of the coupler. Light in the other fibre excites only the second-order mode in the waist. In both cases, the light propagates along the waist without further interactions and returns to the original fibre at the other end of the coupler. A flexural acoustic wave propagating along the waist effectively causes a periodic index modulation.

When the acoustic wavelength matches the optical beat length of the two modes in the waist, resonant coupling takes place between them. Spectral filtering arises from the wavelength-dependent characteristics of the beat length. The centre wavelength of the filtered spectrum can be tuned by control of the acoustic frequency.

The eigen-polarization states of the device are determined by the symmetry of the null coupler; one eigen-state is linearly polarized parallel to the plane of the null coupler (Y-pol.) and the other is orthogonally polarized (X-pol.). In a coupler waist with a circular cross-section, X-pol. has a larger beat length than Y-pol. This results in a polarization splitting of affix) = fy(X)-f,(X) in the acoustic frequency required to couple a given optical wavelength; where fy(X) and fx(X) are the phase-matching acoustic frequencies for Y-pol. and X-pol., respectively, at the wavelength X.

The null coupler acousto-optic tunable filter and associated test apparatus are illustrated schematically in Figure 7.

The device comprises a null coupler 90, connected via a conical horn 95 to an acoustic transducer 100 driven by electrical signals 110 with frequencies of fx(X) and fy(X), respectively. Input light of arbitrary polarization enters the device in port 1. Both polarization components are coupled to port 2, but undergo different up-shifts in frequency. (Any uncoupled light emerging from port 3 is rejected by an isolator 120.) After propagating through the loop, light of each polarization re-enters the coupler and is coupled a second time, exiting through port 4 with a frequency down-shift.

(Again, any uncoupled light emerging at port 1 is blocked by an isolator 130.) Providing it is ensured that the birefringence within the loop is such that the two eigen-polarizations are not mixed, the frequency shifts associated with the two acoustooptic couplings cancel to give zero net frequency shift for each input polarization state.

Furthermore, since spectral filtering occurs twice in this double-pass configuration additional benefits are obtained: the spectral bandwidth of the filter is reduced by a factor of 0.75 and, more significantly, a much enhanced spectral side lobe suppression of up to -18.6 dB can be obtained.

A prototype device has been demonstrated experimentally as follows. The null coupler 90 with a uniform waist of 8 mm long was fabricated using standard single mode telecommunication fibre. The excess loss of the passive null coupler was ~ 0.1 dB and the maximum coupling efficiency was -25 dB. The acoustic transducer, formed with a piezoelectric element 100 and an aluminium concentrator horn 95, was used to excite a flexural acoustic wave to at least a fused coupling region 140 of the coupler. The horn was bonded to the null coupler transversely at some distance from the waist.

The single pass polarization characteristics of the acousto-optic switch were measured by launching broad band polarized light into the device and measuring the coupled spectrum at port 2 with an optical spectrum analyzer. The centre wavelength of both X-pol. and Y-pol. filter responses were obtained and are plotted against acoustic frequency in Figure 8. The acoustic frequency splitting affix) required to couple a given wavelength was determined to be ~ 1.5 MHz in agreement with the calculations carried out for a circular coupler waist.

Figure 9 shows the optical spectra of both eigen-polarization states at (a) the input, (b) in the loop and (c) at the output, when narrow band ( < 100 kHz) light with X= 1545 nm is launched into the device with equal intensities in both polarizations.

Electrical signals with frequencies offx= 10.675 MHz andfy= 12.139 MHz were applied to the transducer. The optical spectrum was measured with a Fabry-Perot scanning interferometer (2 MHz resolution). It is clear that the two polarizations have the same optical frequency at the filter output, whereas within the loop the spectrum is split due to the different frequency shifts for the two polarization components.

Figure 10 shows the spectral filtering characteristics measured with a polarized broad band LED source. Figure 4(a) and (b) show single-pass coupled spectra centred at X=1560 nm for X-pol (fx=l0.835 MHz) and Y-pol (fy=12.348 MHz), respectively.

These are compared to the double-pass result shown in Figure 4(c) when the input consists of both eigen-polarization states with equal intensities. The optical bandwidth for single pass was 13.5 nm and 12.5 nm for X-pol. and Y-pol., respectively. This was reduced to 9.5 nm for the double pass. The sidelobe suppression was about -8 dB for single pass and was enhanced to -17 dB for the double pass. The centre wavelength of the filter could be tuned, by control of the acoustic frequencies, over a region covering the entire gain bandwidth of erbium-doped fibres, with similar optical bandwidth and sidelobes.

The total loss of the device was measured to be - 6 dB. This relatively large loss was mainly due to the imperfect acousto-optic coupling efficiency ( ~ 70%) owing to poor electrical impedance matching of the transducer. Losses of 2 dB, dominated by the loss of the two isolators (0.5 dB each), should be realistically achievable with an improved transducer. The polarization-dependent loss of the double-pass device was measured to be < 0.1 dB, clearly validating the principle of the polarization desensitisation.

The principal drawback to the technique relates to polarization crosstalk between the two eigen-polarizations within the null coupler. Even though the polarization frequency splitting was quite large (1.5 MHz, corresponding to a 100 nm separation of resonant optical wavelengths), a significant polarization intensity crosstalk of between -20 dB and -13 dB was observed at a given wavelength depending on the required operating frequencies. The origin of this effect lies in resonant coupling within the taper transitions which were relatively long (25 mm) for this device. This cross-talk can give rise to signal beating in the time domain. The same problem has been observed in multichannel operations of conventional acousto-optic tunable filters [11]. It should prove possible to reduce the effect significantly in future devices by making the waist of the null coupler more uniform and by shortening the taper transitions.

In conclusion, we have demonstrated an all-fibre, polarization-insensitive acoustooptic tunable filter based on a null coupler simultaneously excited by two acoustic waves.

A polarization dependent loss of less than 0.1 dB is achieved with zero net frequency shift, 9.5 nm bandwidth and sidelobe suppression better than -17 dB. With advanced coupler design and higher frequency acoustic drives, we fully anticipate the future development of low loss ( < 2 dB), low-polarization cross talk, polarization insensitive acousto-optic tunable filters with < 1 nm bandwidth and side-lobe suppression better than -30 dB.

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Claims (10)

1. An optical fibre device comprising an optical fibre null coupler formed of two or more optical fibres fused together at a coupling region, at least a part of the coupling region being mechanically twisted substantially about a longitudinal axis of the coupling region.

2. A device according to claim 1, comprising means for exciting acoustic vibration of at least a part of the coupling region.

3. A device according to claim 1 or claim 2, comprising means for clamping at least two portions of the coupling region to maintain an applied twist of the coupling region between the clamped portions.

4. A method of fabricating an optical fibre device in which two optically or physically dissimilar optical fibres are fused together at a coupling region to form a null coupler, the method comprising the steps of: applying a mechanical twist, substantially about a longitudinal axis of the coupling region, to the coupling region while the coupling region is heated; maintaining the applied twist while the coupling region is allowed to cool.

S. An optical fibre device comprising: an optical fibre null coupler formed of two or more optical fibres fused together at a coupling region; and means for exciting acoustic vibration of at least a part of the coupling region at at least two acoustic frequencies, so that an optical wavelength-dependent response of the device for one input polarisation and resulting from one of the applied acoustic frequencies occurs at substantially the same optical wavelengths as an optical wavelengthdependent response of the device for the other input polarisation and resulting from the other of the applied acoustic frequencies.

6. A device according to claim S, the device being connected so that, in operation, (i) an input optical signal is launched into a first optical fibre of the null coupler and through the coupling region; and (ii) throughput light emerging at the other end of the first optical fibre is returned via the second optical fibre through the coupling region in a reverse direction with respect to the input optical signal; (iii) throughput light emerging at the other end of the second optical fibre forms an optical output signal.

7. A method of operating an optical fibre device comprising an optical fibre null coupler formed of two or more optical fibres fused together at a coupling region, the method comprising the step of: exciting acoustic vibration of at least a part of the coupling region at at least two acoustic frequencies, so that an optical wavelength-dependent response of the device for one input polarisation and resulting from one of the applied acoustic frequencies occurs at substantially the same optical wavelengths as an optical wavelength-dependent response of the device for the other input polarisation and resulting from the other of the applied acoustic frequencies.

8. An optical fibre device substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of fabricating an optical fibre device, the method being substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of operating an optical fibre device, the method being substantially as hereinbefore described with reference to the accompanying drawings.