Folded sagnac sensor array

Details Folded sagnac sensor array Folded sagnac sensor array

2001-07-24

2003-03-04

Pihulic, Daniel T. (Department: 3662)

Communications, electrical: acoustic wave systems and devices

Signal transducers

Underwater type

Reexamination Certificate

active

06529444

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of fiber optic acoustic sensor arrays wherein light is propagated in the arrays and the effects of acoustic signals on the light returning from the arrays are analyzed to determine the characteristics of the acoustic signals.
2. Description of the Related Art
Fiber optic based acoustic sensors are promising alternatives to conventional electronic sensors. Included among their advantages are a high sensitivity, large dynamic range, light weight, and compact size. The ability to easily multiplex a large number of fiber optic sensors onto common busses also makes fiber optic sensors attractive for large-scale arrays. The recent successful incorporation of multiple small-gain erbium doped fiber amplifiers (EDFAs) into a fiber optic sensor array to increase the number of sensors that can be supported by a single fiber pair has made large-scale fiber optic sensor arrays even more competitive.
For acoustic detection, the fiber optic sensor of choice has been the Mach-Zehnder interferometric sensor. In any interferometric sensor, phase modulation is mapped into an intensity modulation through a raised cosine function. Because of this nonlinear transfer function, a sinusoidal phase modulation will generate higher order harmonics. An interferometer biased at quadrature (interfering beams &pgr;/2 out of phase) has a maximized response at the first order harmonic and a minimized response at the second order harmonic. For this reason, quadrature is the preferred bias point. As the bias point drifts away from quadrature (for example, due to external temperature changes), the response at the first order harmonic decreases and the response at the second order harmonic increases. When the interferometer is biased at 0 or &pgr; out of phase, the first order harmonic disappears completely. This decreased response at the first order harmonic (resulting from the bias points away from quadrature) is referred to as signal fading.
Because Mach-Zehnder interferometric sensors have an unstable bias point, they are especially susceptible to the signal fading problem just mentioned. In order to overcome signal fading, a demodulation of the returned signal is required. The typical demodulation technique is the Phase-Generated Carrier (PGC) scheme, which requires a path-mismatched Mach-Zehnder interferometric sensor. (See, for example, Anthony Dandridge, et al.,
Multiplexing of Interferometric Sensors Using Phase Carrier Techniques, Journal of Lightwave Technology
, Vol. LT-5, No. 7, July 1987, pp. 947 -952.) This path imbalance also causes the conversion of laser phase noise to intensity noise, which limits the performance of the Mach-Zehnder interferometric sensor arrays at low frequencies and places stringent requirements on the linewidth of the source. This narrow linewidth requirement has slowed the development of amplified Mach-Zehnder interferometric sensor arrays at 1.55 &mgr;m.
The Sagnac interferometer has found widespread use in the fiber optic gyroscopes. (See, for example, B. Culshaw, et al.,
Fibre optic gyroscopes, Journal of Physics E
(
Scientific Instruments
), Vol. 16, No. 1, 1983, pp. 5-15.) It has been proposed that the Sagnac interferometer could be used to detect acoustic waves. (See, for example, E. Udd,
Fiber
-
optic acoustic sensor based on the Sagnac interferometer, Proceedings of the SPIE
-
The International Society for Optical Engineering
, Vol. 425, 1983, pp. 90-91; Kjell Kråkenes, et al.,
Sagnac interferometer for underwater sound detection: noise properties, OPTICS LETTERS
, Vol. 14, No. 20, Oct. 15, 1989, pp. 1152-1145; and Sverre Knudsen, et al.,
An Ultrasonic Fiber
-
Optic Hydrophone Incorporating a Push
-
Pull Transducer in a Sagnac Interferometer, JOURNAL OF LIGHTWAVE TECHNOLOGY
, Vol. 12, No. 9, September 1994, pp. 1696-1700.) Because of its common-path design, the Sagnac interferometer is reciprocal and therefore has a stable bias point, which eliminates signal fading and prevents the conversion of source phase noise into intensity noise. Therefore, the Sagnac interferometer is immune to the phase noise which limits the Mach-Zehnder interferometric sensors at low frequencies.
SUMMARY OF THE INVENTION
One aspect of the present invention is an acoustic sensor that comprises a source of light pulses, a first coupler, a polarization dependent second coupler, an optical delay path and at least one detector. The first coupler couples the light pulses to a first optical path having a first optical length and to an array of sensors. The array of sensors comprises at least a first sensor. The first sensor is in a second optical path having a second optical length different from the first optical length. The polarization dependent second coupler couples light pulses received from the first optical path in a first polarization to the optical delay path and couples light pulses received from the array in a second polarization to the optical delay path. The light pulses coupled to the optical delay path in the first polarization return from the optical delay path to the second coupler in the second polarization. The light pulses coupled to the optical delay path in the second polarization return from the optical delay path to the second coupler in the first polarization. The second coupler couples the light pulses returning to the second coupler from the optical delay path in the first polarization to the first optical path to propagate therein to the first coupler. The second coupler couples light pulses returning to the second coupler from the optical delay path in the second polarization to the array to propagate therein to the first coupler. The first coupler combines the light pulses from the first optical path and the light pulses from the array to cause light pulses traveling equal distances through the first optical path and the array to interfere and to generate a detectable output signal. The detectable output signal varies in response to acoustic energy impinging on the first sensor. The detector detects the detectable output signals to generate a detector output signal responsive to variations in the detectable output signal from the first coupler. Preferably, the array includes a second sensor. The second sensor is in a third optical path having a third optical length different from the first optical length and the second optical length. Also preferably, the polarization dependent second coupler comprises a polarization beam splitter. In preferred embodiments, the optical delay path comprises a length of optical waveguide and a polarization rotating reflector. The reflector causes light incident on the reflector in the first polarization to be reflected as light in the second polarization, and causes light incident on the reflector in the second polarization to be reflected as light in the first polarization. the reflector advantageously comprises a Faraday rotating mirror. In particularly preferred embodiments, the first optical path includes a non-reciprocal phase shifter which causes light propagating through the first optical path in a first direction and light propagating through the first optical path in a second direction to experience a relative phase shift such that light combined in the first coupler has a phase bias. Preferably, In such embodiments, a third optical path is positioned in parallel with the first optical path. One of the first optical path and the third optical path includes an optical delay to cause the first optical path to have an optical path length different from an optical path length of the third optical path, such that light propagating through the first optical path has a propagation time different from a propagation time of light propagating through the second optical path to thereby time multiplex the light pulses. Preferably, the non-reciprocal phase shifter comprises a first Faraday rotator, a quarter-wave plate and a second Faraday rotator, the first Faraday rotator. The quarter-wave plate and the second Faraday rotator are positioned

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