The use of high birefringence fiber interrogating interferometer for optical sensing applications was discussed. The method is of low cost and permits simple adjustment of the optical path difference and has much lower sensitivity to environmental perturbation. The polarization-maintaining (PM) fiber interferometer adopted a heterodyne approach using interferometric wavelength shift detection. The study showed that the inclusion of power amplifier driving a multi-element piezoelectric stack will enable the bandwidth to be pushed up into the kHz regime

Lai, Y.

Norman, D.C.C.

Webb, David J.

English

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High birefringence fibre interrogating interferometer for optical sensing applications

We describe the use of high birefringence fibre forming a differential path interferometer for heterodyne fibre optic sensing applications. We firstly recover a low frequency strain amplitude of 1µe at 1Hz applied to a fibre Bragg grating sensor demonstrating a noise limited resolution of around 100ne/vHz. Secondly we interrogate a Mach-Zehnder interferometer sensor using the dual wavelength technique to detect a change in the Mach-Zehnder OPD of 200µm

Webb, D.J.

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High Birefringence Fibre Interrogating Interferometer for OpticalSensing Applications D. C. C. Norman, Y. Lai and D. J. Webb*Photonics Research Group, Department of Engineering and Applied Science, Aston University, Birmingham, West Midlands, B4 7ET, United Kingdom.ABSTRACTWe describe the use of high birefringence fibre forming a differential path interferometer for heterodyne fibre optic sensingapplications. We firstly recover a low frequency strain amplitude of 1?? at 1Hz applied to a fibre Bragg grating sensordemonstrating a noise limited resolution of around 100n?/?Hz. Secondly we interrogate a Mach-Zehnder interferometersensor using the dual wavelength technique to detect a change in the Mach-Zehnder OPD of 200?m.Keywords: High Birefringence Fibre, Interferometer, Sensor, Interferometric, Fibre Optic Sensing 1. INTRODUCTIONInterferometric techniques are usually used when it is important to obtain the highest resolution in an optical sensing system.An interferometer can be used as part of the signal processing scheme to address both interferometric [1] and fibre Bragggrating [2] sensors. In many such applications an optical path difference of a few hundred microns is a typical requirement.From a technical point of view, an excellent way of implementing this is to use an integrated optic based Mach-Zehnderinterferometer. Unfortunately this usually requires a bespoke, and therefore very expensive, device, which does not facilitatethe provision of a low cost sensing solution. Alternatively, in the laboratory a fibre (or combined fibre and free-space) Mach-Zehnder interferometer is often used. Such devices employ low cost components however, precise adjustment of the opticalpath difference is very time consuming and the system is usually very susceptible to environmental perturbations that mightaffect the fibre in one arm, but not the other.FBG= ConnectorInterrogationElectronicSystemsBroadbandASESourcePZTPolariserPZTHi-Bi FibrePMF PMFAnalyser= SplicePhotoDiodeMach-ZehnderInterferometerAWGAdjustableAir GapPMF = Polarisation Maintaining FibreFig. 1. Experimental set-up.In this paper we demonstrate a solution which is of potentially low cost, permits simple adjustment of the optical pathdifference and has a much lower sensitivity to environmental perturbations than the fibre Mach-Zehnder interferometer. The17th International Conference on Optical Fibre Sensors, Marc Voet, Reinhardt Willsch,Wolfgang Ecke, Julian Jones, Brian Culshaw, eds., Proceedings of SPIE Vol. 5855(SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.623270615Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/24/2013 Terms of Use: http://spiedl.org/termsidea is to exploit the ability to obtain interference between the fast and slow modes of a polarisation maintaining (PM) fibre.This has been used directly as a sensing mechanism [3], but has not to our knowledge been used as a means of interrogatingother optical sensors.2. EXPERIMENTAL SET-UP The experimental set-up is shown in Fig. 1. A non-polarised erbium-doped fibre amplified spontaneous emission sourceprovided 12dBm over a 33nm bandwidth centred on 1545nm. The light passed through a polariser, into a single eigenaxis ofa length of PM fibre, through a connector (which could be rotated through 3600) and into a second piece of PM fibre formingthe processing interferometer. This connector was oriented to equally populate both eigenmodes of the second PM fibre. Thesecond PM fibre was wound on a piezoelectric fibre stretcher connected to a function generator able to produce, using a serrodyne waveform, a 2? phase shift between the two modes of the fibre. The light then passed through a second rotatingconnector into a third piece of PM fibre, with eigenaxes oriented at 45 degrees to those of the second fibre. An analyser wasthen used to permit light in just one eigenmode of the third piece of PM fibre to propagate to a connector, and from thereeither to an FBG  sensor or an interferometric sensor, depending on the system being studied. The FBG sensor consisted of a single Bragg grating mounted onto a second piezoelectric fibre stretcher so that dynamic strain signals could be applied;reflected light from the FBG sensor was directed through a coupler to a photodiode and relevant electronic interrogationsystem. The interferometric sensor consisted of a Mach-Zehnder interferometer with an air gap in one arm to permitadjustment of the optical path difference (OPD); the light output from the interferometer being directed to an arrayedwaveguide grating (AWG) interrogation scheme [4, 5].-120.00-100.00-80.00-60.00-40.00-20.000.0018.50 19.00 19.50 20.00 20.50 21.00 21.50Frequency (Hz)Power (dB)Fig. 2. Typical spectrum due to a FBG with applied sinusoidal strain modulation of 1?? at 1Hz on a 20Hz carrier (7.63mHz bandwidth).3. PM FIBRE INTERFEROMETERWith this configuration, the second piece of PM fibre effectively forms a Mach-Zehnder interferometer with the two opticalpaths corresponding to the two eigenmodes of the fibre with a differential delay determined by the birefringence, ?n and thelength of the PM fibre, L. The free spectral range (FSR) of this interferometer isnLFSR??2?  (1)where ? is the central wavelength of the light source and ?nL is the optical path difference (OPD) of the interferometer. Thedifferential nature of this type of interferometer means that both arms are subject to similar amounts of environmental616     Proc. of SPIE Vol. 5855Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/24/2013 Terms of Use: http://spiedl.org/termsperturbations reducing phase drifting of the interferometer output. A 21.56m length of PM fibre was selected to produce an FSR of 0.3nm and an associated OPD of 8mm; 20m of this fibre were wound on the piezoelectric stretcher used to generate a carrier for the signal processing schemes. We oriented the eigenmodes and demonstrated the operation of the PM fibreinterferometer by increasing the amplitude of a 0.25Hz serrodyne waveform driving the stretcher and monitoring the fringepositions at the output of the PM fibre with an optical spectrum analyser. The strain required to produce a 2? phase shiftbetween the eigenmodes with this set-up was approximately 6??.4. FBG SENSING The first demonstration using the PM fibre interferometer adopts a heterodyne approach using interferometric wavelengthshift detection [2]. We modulated the phase of the PM interferometer at 20Hz and applied a low frequency strain amplitudeof 1?? at 1Hz to the FBG sensor. The FBG had a centre wavelength of 1558.3 nm and a -3dB linewidth of 0.13nm. The output of the photodiode was bandpass filtered (at the carrier frequency) and connected to a lock-in amplifier (LIA). Atypical recovered signal seen at the output of the photodiode is shown in Fig. 2 demonstrating a noise limited resolution ofaround 100n?/?Hz. We used the serrodyne modulation frequency as the reference for the LIA and monitored the phase shiftusing an oscilloscope connected to the output of the LIA. The results are shown in Fig. 3 and show the recovered 1Hz strainsignal applied to the FBG sensor.-0.15-0.1-0.0500.050.10.150 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Voltage (mV)Time (s)Fig. 3. Recovered 1?? signal at 1Hz.0501001502002503003504000 25 50 75 100 125 150 175 200OPD Displacement (?m)Phase Difference (degrees)Fig. 4. Phase difference vs. Mach-Zehnder OPD. Experimental results (squares). Theoretical relationship indicated by the solid line.Proc. of SPIE Vol. 5855     617Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/24/2013 Terms of Use: http://spiedl.org/terms5. INTERFEROMETRIC SENSING In this demonstration we used the PM fibre interferometer to interrogate the Mach-Zehnder interferometric sensor, the twoOPDs being closely matched in a coherence tuned system [6]. We again modulated the phase of the PM interferometer at20Hz and used an AWG to implement a dual wavelength scheme [7] by using its outputs to synthesise two separate lowcoherent light sources illuminating the system [5]. We then monitored the phase differences corresponding to the two virtualsources using a LIA as we adjusted the OPD of the Mach-Zehnder interferometric sensor. The results are shown in Fig. 4. The unambiguous range in terms of a change in the Mach-Zehnder OPD is 200?m for the two selected synthesised lightsources. The rms deviation from linearity is 1.1?m.6. CONCLUSIONThe experimental work reported here has demonstrated that an interferometer formed in PM fibre can be used at low cost tointerrogate FBG and interferometric sensors. The main limitations of the demonstrated system are the long fibre length andlow carrier frequency, resulting from the simple PZT driven translation stage used. The inclusion of a power amplifierdriving a multi-element piezo electric stack should enable the bandwidth to be pushed up into the kHz regime whilst enablinghigher strains to be used with concomitant reductions in the fibre length and sensitivity to environmental perturbations.7. ACKNOWLEDGEMENTDavid Norman would like to thank the UK Engineering and Physical Sciences Research Council for financial supportREFERENCES1 F. T. S. Yu and S. Yin, Fiber Optic Sensors, Marcel Dekker Inc 2002, pp. 295-4802 A. D. Kersey, T. A. Berkoff and W. W. Morey, Fibre-optic Bragg grating strain sensor with drift-compensated high-resolution wavelength shift detection, Optics Letters, Vol. 18, No. 1, January 1993, pp. 72-74 3 A. D. Kersey, M. Corke and D. A. Jackson, Linearized polarimetric optical fiber sensor using a heterodyne-type signalrecovery, Electronics Letters, Vol. 20, No. 5, 1984, pp. 209-2114 D. C. C. Norman, D. J. Webb and R. D. Pechstedt, Extended range interrogation of wavelength division multiplexed fibreBragg grating sensors using arrayed waveguide grating, Electronics Letters, Vol. 39, Nov. 2003, pp. 1714-1716.5 D. C. C. Norman, D. J. Webb and R. D. Pechstedt, Interferometric sensor interrogation using an arrayed waveguidegrating, Proc. SPIE, Vol. 5459, Optical Sensing, September 2004, pp. 101-1096 R. A. Patten, Michelson interferometer as a remote gauge, Applied Optics, Vol. 10, No. 12, 1971, pp. 2717-27217 A. D. Kersey and A. Dandridge, Dual-wavelength approach to interferometric sensing, Proc. SPIE, Vol. 798, Fibre Optic Sensors 1987, pp. 176-181*d.j.webb@aston.ac.uk; phone+44(0)121 204 3541; fax +44 (0)121 359 0156 618     Proc. of SPIE Vol. 5855Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/24/2013 Terms of Use: http://spiedl.org/terms

 1 High Birefringence Fibre Interferometer for Optical Sensing Applications  D. C. C. Norman, Y. Lai and D. J. Webb  We describe the use of high birefringence fibre forming a differential path interferometer for heterodyne fibre optic sensing applications.   Introduction: Interferometric techniques in optical fibre sensor applications offer extremely high sensitivity measurements that allow the development of high performance optical sensor schemes. The ability to generate a carrier signal by modulating interferometers with a 2pi serrodyne waveform has been successfully applied in many interferometric fibre optic sensor systems [1]. Common arrangements for two-beam interferometers include the Michelson and the Mach-Zehnder interferometer, both of which can be used to generate a beat frequency. Differential path interferometers are analogous to the Mach-Zehnder interferometers but with this technique the two beams are carried within a single fibre. High birefringence (Hi-Bi) fibre can be used to form a differential path interferometer by exploiting the fast and slow axis in the core of the Hi-Bi fibre [2].  In this letter we report on the use of a differential path interferometer formed in Hi-Bi fibre modulated to create a carrier frequency and demonstrate its performance in two fibre optic sensor schemes. Firstly, fibre Bragg grating (FBG) sensor interrogation using a heterodyne approach adopting interferometric wavelength shift detection [3]. Secondly, interferometric sensor interrogation combining heterodyne processing and the dual wavelength approach [4].   Experimental Set-Up: The experimental set-up is shown in Fig. 1. A non-polarised erbium-doped fibre amplified spontaneous emission source provided a total output power of 12dBm over a spectral width of 33nm centred on a wavelength of 1545nm. The light passed through a polariser, into a length of Polarisation maintaining fibre (PMF), through a rotating connector arrangement and into the Hi-Bi fibre wound around a piezoelectric fibre stretcher forming a differential path interferometer. The piezoelectric fibre stretcher was connected to a function generator able to produce a 2pi amplitude serrodyne waveform. The light then passed through a second rotating connector, through a mixer (second polariser) and into a selective connector directing the light to either the FBG or interferometric sensor. The FBG sensor consisted of a single Bragg grating mounted onto a second piezoelectric fibre stretcher so that dynamic strain signals could be applied, reflected light from the FBG sensor was directed through a coupler to a photodiode and relevant electronic interrogation system. The interferometric sensor consisted of a Mach-Zehnder interferometer with an air gap in one arm to adjust the optical path difference (OPD). The light output from the interferometer was connected to an arrayed waveguide grating (AWG) interrogation scheme [5, 6].  Hi-Bi Fibre Interferometer: Polarised light input into Hi-Bi  fibre at an azimuth of 45 degrees with respect to the birefringence axis of the fibre will degenerate into two non-interfering orthogonal polarised modes which propagate within the core of the fibre with a differential delay determined by the birefringence, ∆n and the length of the fibre, L. The detected light at the output of the Hi-Bi fibre after passing through a mixer exhibits periodic intensity variations against the input wavelength [2]. The free spectral range (FSR) of the periodic interferometer output is given by                                      nLFSR2∆λ=                                      (1)  Where, λ denotes the central wavelength of the light source and ∆nL is the OPD of the interferometer. A 21.56m length of Hi-Bi fibre was selected to produce an FSR of 0.3nm and an associated OPD of 8mm. The carrier signal was created by winding 20m of Hi-Bi fibre around a piezoelectric fibre stretcher and applying a ramp waveform sufficient in amplitude to drive the interferometer over a single interference fringe. We demonstrated this by increasing the amplitude of a 0.25Hz ramp waveform and monitoring the fringe positions at the output of the Hi-Bi interferometer with an optical spectrum analyser. The differential nature of the Hi-Bi interferometer means that both arms are subjected to the modulation at differing degrees. This results in an increased amount of strain that we need to apply to the fibre to drive the interferometer over a fringe than that would be necessary if we were to use a two-beam interferometer such as a Mach-Zehnder. The strain we applied to drive the Hi-Bi interferometer over one fringe was approximately 6µε.   FBG= ConnectorInterrogationElectronicSystemsBroadbandASESourcePZTPolariserPZTHi-Bi FibrePMF PMF Mixer= SplicePhotoDiodeMach-ZehnderInterferometerAWGAdjustableAir GapPMF = Polarisation Maintaining Fibre(Polariser)Fig.  1 Experimental set-up.  2 FBG Sensing: The first demonstration using the Hi-Bi fibre interferometer adopts a heterodyne approach using interferometric wavelength shift detection [3]. We modulated the phase of the Hi-Bi interferometer at 20Hz and applied a low frequency strain amplitude of 1µε at 1Hz to the FBG sensor. The FBG had a centre wavelength of 1558.29nm and a -3dB linewidth of 0.13nm. The output of the photodiode was bandpass filtered (at the carrier frequency) and connected to a lock-in amplifier (LIA). We used the serrodyne modulation frequency as the reference and monitored the phase shift using an oscilloscope connected to the output of the LIA. The results are shown in Fig. 2 and show the recovered 1Hz strain signal applied to the FBG sensor.  Interferometric sensing: In this demonstration we used the Hi-Bi fibre interferometer as a processing interferometer away from the Mach-Zehnder interferometer sensor with its OPD closely matched to the OPD of the processing interferometer forming a composite, coherence tuned system [7]. We again modulated the phase of the Hi-Bi interferometer at 20Hz and used an AWG to synthesise two separate low coherent light sources illuminating the system [6]. We then monitored their phase difference using a LIA as we adjusted the OPD of the Mach-Zehnder interferometric sensor. The results are shown in Fig. 3. The unambiguous range in terms of a change in the Mach-Zehnder OPD is 200µm for the two selected synthesised light sources. The errors in the experimental values are predominantly due to the Hi-Bi and Mach-Zehnder interferometers, which are prone to fluctuations due to draughts or temperature changes in the laboratory.  Conclusion: The experimental work reported has demonstrated that an interferometer formed in Hi-Bi fibre can be modulated using a piezoelectric stretcher to produce a low frequency carrier signal for fibre optic sensing applications.  We demonstrated its capability using firstly an FBG sensing scheme then an interferometric sensing scheme. The differential nature of this type of interferometer means that for short lengths of Hi-Bi fibre where a larger FSR is required the Hi-Bi fibre needs to be stretched enough to drive the interferometer over one fringe, although not demonstrated in this work it could prove to be a problem.  D. C. C. Norman, Y. Lai and D. J. Webb (Photonics Research Group, Aston University, Birmingham, B4 7ET, United Kingdom)  E-mail: d.j.webb@aston.ac.uk  References   1   YU, F. T. S., and YIN, S.: ‘Fiber Optic Sensors’, Marcel Dekker Inc 2002, pp. 295-480 2   Y. LAI.: ‘Fibre-based devices for next generation photonics communication systems, Ph.D Thesis, Aston University, 2004 3   KERSEY, A. D., BERKOFF, T. A., and MOREY, W. W.: ‘Fibre-optic Bragg grating strain sensor with drift-compensated high-resolution wavelength shift detection’, Optics Letters, Vol. 18, No. 1, January 1993, pp. 72-74  4  KERSEY, A. D., and DANDRIDGE, A.: ‘Dual-wavelength approach to interferometric sensing’, Proc. SPIE, Vol. 798, Fibre Optic Sensors 1987, pp. 176-181 5  NORMAN, D. C. C., WEBB, D. J., and PECHSTEDT, R. D.: ‘Extended range interrogation of wavelength division multiplexed fibre Bragg grating sensors using arrayed waveguide grating’, Electronics Letters, Vol. 39, Nov. 2003, pp. 1714-1716. 6  NORMAN, D. C. C., WEBB, D. J., and PECHSTEDT, R. D.: ‘Interferometric sensor interrogation using an arrayed waveguide grating’, Proc. SPIE, Vol. 5459, Optical Sensing, September 2004, pp. 101-109 7  PATTEN, R. A.: ‘Michelson interferometer as a remote gauge’, Applied Optics, Vol. 10, No. 12, 1971, pp. 2717-2721 -0.15-0.1-0.0500.050.10.150 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Time (s)Voltage (mV)Fig.  2 Recovered 1Hz strain signal. Fig.  3 Phase difference vs. Mach-Zehnder OPD. Experimental results (squares). Theoretical relationship indicated by the solid line. 0501001502002503003504000 25 50 75 100 125 150 175 200OPD Displacement (µm)Phase Difference (degrees)

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High birefringence fibre interrogating interferometer for optical sensing applications

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