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 
  • G02F1/0128—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 electro-mechanical, magneto-mechanical, elasto-optic effects
  • G02F1/0131—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 electro-mechanical, magneto-mechanical, elasto-optic effects based on photo-elastic effects, e.g. mechanically induced birefringence
  • G02F1/0134—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 electro-mechanical, magneto-mechanical, elasto-optic effects based on photo-elastic effects, e.g. mechanically induced birefringence in optical waveguides
• • 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
  • G02F1/29—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 position or the direction of light beams, i.e. deflection
  • G02F1/31—Digital deflection, i.e. optical switching
  • G02F1/313—Digital deflection, i.e. optical switching in an optical waveguide structure
  • G02F1/3131—Digital deflection, i.e. optical switching in an optical waveguide structure in optical fibres

Definitions

• This invention relates to optical fibre devices.
• 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.
• 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] .
• 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 pan of the coupling region being mechanically twisted substantially about a longitudinal axis of the coupling region.
• 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.
• birefringence arises from the difference in the propagation constants of the two orthogonally polarised fibre modes, caused m turn by a combination of core ellipticity (form birefringence) [6] and an associated thermal stress asymmetry (stress birefringence) [7].
• V- Value is given by
• ⁇ is the optical wavelength
• the polarisation splitting, for a null coupler can be expressed as:
• 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 wavelength- dependent response of the device for the other input polarisation and resulting from the other of the applied acoustic frequencies.
• 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.
• 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.
• 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.
• 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 5b 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 ⁇ m. 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 ⁇ m. For the chosen acoustic frequency of 1 MHz the required waist diameter can be calculated as 12.7 ⁇ m [1].
• 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
• 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.
• PZT piezoelectric
• 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 2a)
• 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
• the clamps 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.
• the resonant wavelength at 1540 nm is at an intermediate value in relation to the two eigen polarisation states in Figures 2a and 2b.
• null couplers 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 ⁇ m and 15 ⁇ m 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 5a and 5b 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 ⁇ s, which compares well with the 46 ⁇ s predicted in [1] .
• the net effect is a time delay of
• 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.
• 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 ⁇ m 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.
• filters and frequency shifters where high acoustic frequency operation, and so a narrow taper waist, is generally required
• 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 po ⁇ s. 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.
• 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.
• 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 / t ( ⁇ ) and v ( ⁇ ), 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.)
• 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 Af[ ⁇ ) 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.
• 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.
• the optical bandwidth for single pass was 13.5 nm and 12.5 nhi 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.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
• Optical Couplings Of Light Guides (AREA)

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• 1996
  • 1996-09-10 GB GB9618860A patent/GB2317236A/en not_active Withdrawn
• 1997
  • 1997-09-10 AU AU41310/97A patent/AU4131097A/en not_active Abandoned
  • 1997-09-10 WO PCT/GB1997/002449 patent/WO1998011463A2/en not_active Ceased
  • 1997-09-10 EP EP97939091A patent/EP0925522A2/de not_active Ceased

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