A General Purpose Fiber-Optic Hydrophone Made of Castable Epoxy

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ic.D > .-) C Cr) C 0 .-) 0 a) a) 0 e. 6.0 4.0 2.0 liii ZI1II 1ZI ZI 1. x xXZ) . --;-;- T 0.0 40 ido . > Figure 10. Axial acceleration sensitivity. The filled dots represent the measurement when the capsule stem was supported and show an average acceleration sensitivity between 70 Hz and 900 Hz of 1.4 0.6 rad/g, represented by the dashed line. For comparison, the predicted single-plate, dual-coil acceleration sensitivity, based on equation (4) is shown as a solid line at 7.4 rad/g. The X's represent the measured acceleration sensitivity of the capsule when the stem containing the coupler and about 1.5 cm of optical fiber is not supported on the shaker table. In a vibrationally noisy environment, the ratio of the acceleration sensitivity to the acoustic sensitivity to is a good figure-of-merit for comparison of their noise-limited performance. Since this ratio removes the output parameter (volts or radians), it is also useful for comparing fiber-optic hydrophones to piezoelectric hydrophones. Reporting this ratio in decibels, the hydrophone capsule discussed in this paper has a figure-of-merit is -134 dB re g/Pa. This is 43 dB better than the Naval Research Laboratory's planar flexible fiber-optic interferometric hydrophone and 57 dB better than the piezoelectric polymer (PVF2) hydrophone of similar geometry3. An overall comparison of the NRL and NPS designs is given in Table 2. TABLE 2. NPS and NRL Planar Hydrophones Performance measurement NRL NPS NPS Advantage Normalized acoustic sensitivity (dB re: lpPa1) -321 -297 Normalized acceleration sensitivity (dB re: lj.iPa1) -144 -163 Acoustic-to-acceleration FOM (dB re: g/jiPa) -177 -134 x Table 2. The NPS flexural disk hydrophone is 16 times more sensitive to pressure and 9 time less sensitive to acceleration than the NRL spiral3 giving the NPS design a factor of 43 dB improvement over the spiral in a vibrationally noisy application. . >( Freauenc, Kertz . : iiiIziiziizI Iz1 1ZI I 11 1 cT . : 1000 (dB) +24 +19 +43 26 1 SPIE Vol. 1367 Fiber Optic and Laser Sensors VIII (1990)
5. DISCUSSIONOF RESULTS The results reported here represent the highest normalized sensitivity ever reported for a fiber-optic hydrophone designed to operate under conditions of static pressure higher than 500 psi. It also represents the best acceleration cancellation figure of merit for any fiber-optic hydrophone. One additional feature of the design which should be considered equally important is that its performance is in excellent agreement with simple elastic theory so that hydrophone designers can scale these results as necessary for other applications requiring differing materials, bandwidth, depth tolerance, etc., with a good deal of confidence. The fact that this hydrophone capsule can be fabricated as a single unit cast out of epoxy suggests that its cost for mass production may be significantly lower that other hydrophone designs requiring machined, thin, air-backed, cylindrical mandrils which may require subsequent fabrication steps that make internal access for fiber winding, and thus push-pull performance impractical. Thus far, the discussion of acoustic performance has concentrated on the normalized sensitivity. That parameter provides a comparison between design strategies on a meter-for-meter basis. We would like to conclude this discussion with comparison of the NPS flexural disk and NRL flexible planar designs which also introduces the effects of "winding density" since this allows comparison of the actual available phase modulation (i.e., signal). The overall available phase modulation amplitude determines the detection threshold if one assumes a given demodulator detection threshold. Similarly, one could use an acoustic basis to set the detection threshold and allow the hydrophone sensitivity to dictate the required demodulator performance, which then determines the cost and complexity of the demodulator, laser diodes, etc. 6. ACKNOWLEDGEMENTS Over the past six years this work has been supported by the Office of Naval Research - Physics Division, the Office of Naval Technology, the Space and Naval Warfare Systems Command, the Naval Sea Systems Command, and the Naval Postgraduate School Direct Funded Research Program. 7. REFERENCES a) Lieutenant, U. S. Navy 1 . T. G. Giallorenzi, A. Dandridge, and A. D. Kersey, "Interferometric and intensity sensors become more sophisticated" Laser Focus World 27(7), 137-143 (July, 1989). 2. N. Lagakos, A. Dandridge, and J. A. Bucaro, "Optimizing fiber coatings for interferometric acoustic sensing (U)", U. S. Navy J. Underwater Acoustics 37(3), 233-254 (July,1987). 3 . N. Lagakos, P. Ehrenfeuchter, T. R. Hickman, A. Tveten, and J. A. Bucaro, "Planar flexible fiberoptic interferometric acoustic sensor", Opt. Lett. 13(9), 788-790; OFC '90 Tech. Digest (22-26 Jan 1990, San Francisco, CA) p. 46; N. Lagokos, J. A. Bucaro, P. A. Ehrenfeuchter, T. R. Hickman, A. B. Tveten, and A. Dandridge, "Planar-conformal fiber optic acoustic sensing element", in Fiber Optic Smart Structures and Skins II, Proc. Soc. Photo-Optical Instrumentation Eng. (SPIE) 1170, 472-477, (1989). SPIE Vol. 1367 Fiber Optic and Laser Sensors VIII (1990) / 27
Page 1 and 2: A General Purpose Fiber-Optic Hydro
Page 3 and 4: ATMOSPHERIC PRESSURE Sensing Plate
Page 5 and 6: The reader is reminded that M is th
Page 7 and 8: on the order of one kilohertz. The
Page 9 and 10: 3 .4 Capsule Fabrication The task o
Page 11 and 12: Acoustic calibrations in water were
Page 13: 4 . 5 Acceleration Sensitivity and
Page 17: 23. D. Ricketts, "Model for a piezo

A General Purpose Fiber-Optic Hydrophone Made of Castable Epoxy

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