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Use of Single-Mode Optical Fiber As a Resonant Ultrasonic Sensor

IP.com Disclosure Number: IPCOM000061007D
Original Publication Date: 1986-Jun-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 56K

Publishing Venue

IBM

Related People

Levenson, MD: AUTHOR [+2]

Abstract

Single-mode optical fibers can be used for detection of ultrasonic waves which are resonant with one of the acoustic mode frequencies of the optical fiber. Because the fiber is essentially a high Q acoustic resonator, the sensitivity of such a sensor is greatly enhanced compared to current fiber-optic acoustic sensors which do not use this resonant effect. A fiber-optic acoustic sensor in which the sensitivity is enhanced by tuning one of the fiber resonances to the frequency that one desires to detect is here disclosed. Because fibers have many resonances over the range from a few MHz to several hundred MHz, essentially this entire band can be covered. The sensitivity is maximized by choosing a fiber jacket material which does not dampen the fiber resonant modes, lowering their Q.

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Use of Single-Mode Optical Fiber As a Resonant Ultrasonic Sensor

Single-mode optical fibers can be used for detection of ultrasonic waves which are resonant with one of the acoustic mode frequencies of the optical fiber. Because the fiber is essentially a high Q acoustic resonator, the sensitivity of such a sensor is greatly enhanced compared to current fiber-optic acoustic sensors which do not use this resonant effect. A fiber-optic acoustic sensor in which the sensitivity is enhanced by tuning one of the fiber resonances to the frequency that one desires to detect is here disclosed. Because fibers have many resonances over the range from a few MHz to several hundred MHz, essentially this entire band can be covered. The sensitivity is maximized by choosing a fiber jacket material which does not dampen the fiber resonant modes, lowering their Q. When a laser beam propagates down a single-mode optical fiber, the beam is phase modulated by thermally excited acoustic modes of the fiber. The resulting light scattering can be detected by heterodyne spectroscopy, i.e., the beat between the scattered light and the phase shifted laser beam is detected with a photodiode and dispersed by a RF spectrum analyzer. Such a spectrum is shown in Fig. 1 for a 100 m diameter aluminum jacketed fiber. Each peak in this spectrum is due to light scattered from one of the thermally excited acoustic eigenmodes of this fiber. Thus, the spectrum also gives the frequency dependence of the s...