Abstmcf-The design and implementation of a multiplexing scheme ... nique, PSD performance, and nonlinear transduction mechanism, are discussed.
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. I, NO. 3, MARCH 1989
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Multiplexing of Nonlinear Fiber-optic Interferometric Sensors F. BUCHOLTZ, A. D. KERSEY,
Abstmcf-The design and implementation of a multiplexing scheme for nonlinear fiber-optic interferometric sensors is presented. The scheme is demonstrated with a three sensor array consisting of magnetic field, pressure, and displacement sensors exhibiting resolutions at 1 Hz of 20 pOe/& (limited by ambient noise), 0.1 kPa@, and 30 nm/& respectively. Multiplexing is accomplished by dithering each transducer at a different frequency and using parallel phase-sensitive detectors (PSD) at the interferometer output to demultiplex the information. A single Mach-Zehnder interferometer driven by a single unmodulated laser was used. Factors affecting the performance of the multiplexing scheme, including interferometric demodulation technique, PSD performance, and nonlinear transduction mechanism, are discussed.
I. INTRODUCTION WO IMPORTANT issues in the research and development of interferometric fiber-optic sensors are multiplexing and low-frequency measurement capability. Multiplexing schemes reported to date include time- [13, frequency- [2], and coherence-division multiplexing [3]. In addition to providing some means for separating the information from each sensor in an array (with minimum crosstalk and numbers of sources, fibers, and interferometers), a multiplexing system must also successfully demodulate each interferometric output. Some multiplexing/demodulation schemes which have been demonstrated include coherence multiplexing with active homodyne demodulation [3], and time- and frequency-division multiplexing with phase-generated carrier demodulation [4][6]. A detailed review of multiplexing schemes has been compiled by Kersey et al. [7]. All of the schemes described above have been applied to sensors in which the strain induced in the fiber is linearly proportional to the measurand. It is well known that dc and low-frequency measurements are difficult with this type of sensor due to the high susceptibility of fiber interferometers to environmentally induced mechanical and thermal fluctuations. High-resolution low-frequency measurements can be performed with interferometric fiber sensors, however, by using nonlinear transduction mechanisms. Here, a dither is employed to up-convert measurand information away from the noisy low-frequency regime. Nonlinear transducers have been demonstrated using two types of mechanisms: 1) nonlinear transducing
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Manuscript received February 16, 1988; revised June 22, 1988. The authors are with the Naval Research Laboratory, Optical Sciences Division, Washington, DC 20375. IEEE Log Number 8823580.
AND
A. DANDRIDGE
material in which the only example is the magnetostrictive alloy used in the fiber optic magnetometer [8], and 2) nonlinear displacement-to-strain conversion (NDSC) using linear transducing materials. This technique has been successful in the measurement of low-ftequency displacement [9], acceleration [lo], and pressure [ 111. In this paper we present the inherent multiplexing capabilities of nonlinear transducing mechanisms and demonstrate this capability by multiplexing a displacement, pressure, and magnetic field sensor on a single interferometer driven by a single CW laser source. Relevant aspects of the nonlinear response of both the transducer and the interferometer are described as well as the advantages and disadvantages of this technique. Direct comparisons to other multiplexing techniques are generally not useful since this technique can be implemented only with nonlinear transducers and is not applicable to arrays of linear sensors. 11. NONLINEAR TRANSDUCTION MECHANISMS Consider a parameter Q consisting of low-frequency (measurand) and high-frequency (dither) terms
+
COS wt (1) Q = Qn COS Qt where Q 60 dBfor(f, -f,) >> ( 1 / 7 ) b u t o n l y r i < 2 0 d B f o r ( f , - f,) < ( 1 / T ) , wheref, andf, are the frequencies of the asynchronous and synchronous signals, respectively. Typical input dynamic range values, Ri = (rms full scale sensitivity) /(minimum resolvable rms synchronous signal), lie in the range Ri = (60 - 80) dB. Therefore, if thejth LIA provides Ri = 60 to 80 dB of input dynamic range with 1-prad minimum resolvable phase shift, then it will tolerate phase shifts in adjacent channels from a worst case 10 mrad (i.e., 1-mrad full-scale sensitivity corresponding to Rj = 60 dB, plus an additional 20 dB corresponding to ri = 20 dB, giving 10 mrad) to a best case 1 0 rad (Ri= 80 dB, ri = 60 dB), depending on channel frequency separation and LIA operating parameters. Determination of the optimum carrier frequency allocation for this scheme is similar to the analysis by Dandridge et al. [6] for multiplexing using phase carrier techniques. Let A w be the channel separation required for an N-sensor array and let w 1 be the lowest dither frequency usable with no overlap. A w will depend not only on the measurand spectral bandwidth but on LIA dynamic reserve as well. Even though no information is carried in the sidebands of the 2w signals, we shall require no overlap between the highest 2 w component 2wN, and the low-
&.
- 1)Aw.
(12)
This analysis yields the optimum dither frequency allocation for N sensors utilizing the lowest possible dither frequency band. In practice, the dither frequencies often cannot be chosen with complete freedom and the required frequency band will be larger than optimum. This was the case in the three-sensor array described in Table I where w3 (magnetic) >> wl, w2, 2wl, 2w2 was chosen for optimum C value. Crosstalk in this scheme will be determined by the dynamic reserve capabilities of the LIA and the dither frequency allocation. Even with optimum frequency alloca1 0 / 7 should provide at least 60 tion, taking Aw/27r dB of crosstalk suppression. Typical channel separation required for these types of sensors will be in the range 3100 Hz for measurand spectral bandwidth of 1-10 Hz. We observed -50-dB crosstalk (between sensors 1 and 2) for the three displacement sensor array with ( w l - w2)/27r = 3/7 ( 7 = 100 ms). We conclude this section with a brief discussion of the advantages and disadvantages of the nonlinear multiplexing scheme. The chief disadvantage is the need to provide dithering at each transducer and, as such, represents a disadvantage of the transduction technique and not of the multiplexing scheme itself. Dithering can be provided either via direct electrical connections or, as demonstrated by Kersey et al. [ 1 13, via multimode fiber carrying intensity-modulated light. The latter method preserves the all-dielectric nature of the link between the interferometer and the drive/signal processing module. The capability to incorporate N sensors into an array using a single CW laser source and a single fiber interferometer is a clear advantage of this scheme. Each transducer receives full optical power (except for splice losses) and, with proper demodulation, no fundamental noise penalty is suffered upon expansion of the array. The noise floor is determined mainly by the characteristics of the interferometer and not by the nature or number of transducers on the interferometer.
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V. SUMMARY We have described a technique for multiplexing nonlinear fiber interferometric sensors by allocating different dither (carrier) frequencies to each sensor. The technique was demonstrated with an array consisting of displacement, pressure, and magnetic field sensors on a single Mach-Zehnder interferometer driven by a single laser source. Less than -50-dB crosstalk and greater than 80dB dynamic range was observed for low-frequency mea-
BUCKHOLTZ
et a l . :
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MULTIPLEXING OF NONLINEAR FIBER-OPTIC INTERFEROMETRIC SENSORS
surands. The noise level was independent of the number of sensors and depended only on the intrinsic noise floor of the single interferometer. The effects of demodulation scheme, phase-sensitive detection performance, and intrinsic transduction limitations on the performance of the multiplexing scheme were considered. Phase-sensitive detector dynamic reserve is the critical performance parameter affecting crosstalk and frequency allocation. Large 2w phase shifts can be troublesome in systems using active demodulation but can be handled with passive demodulation. Immediate applications for this multiplexing scheme include three-axis fiber-optic magnetometer and accelerometer arrays as well as magnetometer/accelerometer/pressure configurations. REFERENCES [ l ] J . L. Brooks, B. Y. Kim, M. Tur, and H. J. Shaw, “Sensitive fiberoptic interferometric sensor arrays,” SPIE Fiber Optic and Laser Sensors IV, vol. 718, pp. 174-181, 1986. [2] I. Sakai, G. Parry, and R. C. Youngquist, “Multiplexing fiber-optic sensors by frequency modulation: cross-term considerations,” Opt. Lett., vol. 11, no. 3, pp. 183-185, 1986. [3] J. L. Brooks, R. H. Wentworth, R. C. Youngquist, M. Tur, B. Y. Kim, and H. J. Shaw, “Coherence multiplexing of fiber-optic interferometric sensors,” J . Lightwave Technol., vol. LT-3, no. 5 , pp. 1062-1072, 1985. [4] J. L. Brooks, B. Moleshi, B. Y. Kim, and H . J. Shaw, “Time-domain addressing of remote fiber-optic interferometric sensor arrays,” J . Lightwave Technol., vol. LT-5, no. 7, pp. 1014-1023, 1987. [5] A. D. Kersey, A . Dandridge, and A. B. Tveten, “Time-division multiplexing of interferometric fiber sensors using passive phase-generated carrier interrogation,” Opt. L a . , vol. 12, no. 10, pp. 775-777, 1987. [6] A. Dandridge, A. B. Tveten, A. D. Kersey, and A. M. Yurek, “Multiplexing of interferometric sensors using phase carrier techniques,” J . Lightwave Technol., vol. LT-5, no. 7 , pp. 947-952, 1987. [7] A. D. Kersey, A. Dandridge, and A. B. Tveten, “Overview of multiplexing techniques for interferometric fiber sensors,’’ presented at Fiber Optic and Laser Sensors V , San Diego, CA, Aug. 1987. [8] K. P. Koo, A. Dandridge, A. B. Tveten, and G. H. Sigel, Jr., “A fiber-optic dc magnetometer,” J . Lightwave Technol., vol. LT-1, no. 3, pp. 524-525, 1983. [9] A. D. Kersey, F. Bucholtz, and A. Dandridge, “New nonlinear phase transduction method for dc measurand interferometric fiber sensors,” Electron. Lett., vol. 22, no. 2, pp. 75-76, 1986. [lo] F. Bucholtz, A . D. Kersey, and A. Dandridge, “DC fiber-optic accelerometer with sub-pg sensitivity,” Electron. Lett., vol. 22, no. 9 , pp. 451-453, 1986. [ l l ] F. Bucholtz, A. D. Kersey, and A. Dandridge, “Multiplexed nonlinear interferometric fiber sensors,” in OFS ’86 Tech. D i g . , 1986, pp. 63-64. [12] A . D. Kersey, F. Bucholtz, K. Sinansky, and A. Dandridge, “Interferometric sensors for dc measurands-A new class of fiber sensors,” SPIE Fiber Optic and Laser Sensors IV, vol. 718, pp. 198-202, 1986. [13] F. Bucholtz, A. M. Yurek, K. P. Koo, and A. Dandridge, “Lowfrequency, submicrogauss fiber-optic magnetometer, ’’ Electron. Lett., vol. 23, no. 19, pp. 985-987, 1987. [14] C. D. Butler and G. B. Hocker, “Fiber optic strain gauge,” Appl. Opt., vol. 17, pp. 2867-2864, 1978. [15] A . Dandridge, A . B. Tveten, and T. G. Giallorenzi, “Homodyne demodulation scheme for fiber optic sensors using phase generated camer,” IEEEJ. Quantum Electron., vol. QE-18, no. 10, pp. 16471653, 1982.
[16] D. A. Jackson, A. D. Kersey, M. Corke, and J. D. C. Jones, “Pseudo-heterodyne detection scheme for optical interferometers,” Electron. Lett., vol. 18, p. 1081-1083, 1982. [17] K. P. Koo, A. B. Tveten, and A. Dandridge, “Passive stabilization scheme for fiber interferometry using ( 3 X 3 ) fiber directional couplers,’’ Appl. Phys. Lett., vol. 41, no. 7 , pp. 616-618, 1982. [18] M. L. Meade, Lock-In AmpliJers: Principles and Applications. London, U.K.: Peter Peregrinus Ltd., 1983.
* Frank Bucholtz was born in Detroit, MI, on April 14, 1953. He received the B.S. degree in physics from Wayne State University, Detroit, MI, in 1975, and the Ph.D. degree in physics from Brown University, Providence, RI, in 1981. His postgraduate work included structural studies of magnetic and fast ionic conducting borate glasses using nuclear magnetic resonance. From 1981 to 1983 he was an NRC Postdoctoral Research Associate at the Naval Research Laboratory, Washington, DC, where he conducted research in the area of ferromagnetic devices for microwave signal processing. Since 1983 he has been a member of the Optical Sciences Division at Naval Research Laboratory. His current research interests include fiber-optic sensors and the magnetic properties of materials.
* Alan D. Kersey was born in Milford Haven, Wales, on June 15, 1956. He received the B.Sc. deg;ee in physical electronics from the University of Warwick, and the Ph.D. degree from the University of Leeds, England. In his graduate research he studied magnetooptical effects in vapors. He has worked in the area of optical fiber sensors since 1981, first at the University of Kent, England, as a Postdoctoral Research Fellow, and since late 1984 as a Research Physicist with the fiber-optic sensor research program at the U.S. Naval Research Laboratory, Washington, DC. Presently, he leads a team in the Optical Sensor Section at NRL researching multiplexed fiber-optic sensors, interferometric phase detection techniques, polarization phenomena in fiber-optic systems, and optical communications. Dr. Kersey has coauthored over 100 journal and conference publications in the area of fiber optics.
* Anthony Dandridge was born in Kent, England. He received the B.Sc. and Ph.D. degrees in physics from the Sir John Cass School of Science and Technology, City of London Polytechnic, England. His postgraduate and postdoctoral research work included flow birefringence, viscometric, and light-scattering studies of short-chain polymers. In 1979 he was a Lecturer in Physics at the University of Kent, Canterbury, England. Since 1980, he has been associated with Georgetown University, Washington, DC, John Carroll University, Cleveland, OH, and the Naval Research Laboratory, Washington, DC. In 1984 he became Head of the Optical Sensor Section at the Naval Research Lab. His research work covers fiber-optic sensory systems as well as the noise and spectral characteristics of semiconductor lasers. He has authored and co-authored over one hundred and fifty journal and conference publications. Dr. Drandridge is a Fellow of the Royal Astronomical Society.
