Signal stabilization in optical hydrophones

• US 4,977,546 A
• Filed: 01/29/1982
• Issued: 12/11/1990
• Est. Priority Date: 01/29/1982
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Flatley, James P., Shajenko, Peter
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Pihulic, Daniel T.
• Application Number:
  US06/347,112
• Time in Patent Office: 3,238 Days
• Field of Search: 367/140, 367/149, 367/12, 250/227.19, 250/227.27, 350/96.1
• US Class Current: 367/140
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125

Abstract

A system for signal stabilizing in-phase modulated optical hydrophone arr employs interferometry with homodyne detection. Phase stabilization is accomplished by modulating the input laser signal in proportion to variations in the output of an optical transducer to balance the output phase so that the fringes are kept at optimum position. Additionally, fluctuations in light intensity are compensated for so that a photodetector responds only to phase shift variations. The technique used is to split the input beam into signal and reference beams using a beam divider, exposing the signal beam to the acoustic pressure of interest, recombining the signal beam with the reference beam, detecting the combined beams and filtering the resulting signal to separate out the acoustic information of interest from the phase shift and light intensity portions used to stabilize the input beam. The acoustic information is processed and the phase shift and light intensity information provides a feedback signal for use in input beam stabilization.

Claims

1. A signal stabilized optical hydrophone comprising:

interferometer means for providing a reference beam and a fringe pattern;

a first photodetector optically connected to receive said fringe pattern and provide an output electrical signal having the fringe pattern information, said output electrical signal comprised of a signal current and a direct current;

a high pass filter connected to receive said output electrical signal and conducting the signal current and inhibiting the flow of the direct current;

a low pass filter connected to receive said output electrical signal and conducting the direct current and inhibiting the flow of the signal current;

a second photodetector optically connected to receive said reference beam and providing an output electrical signal having said reference beam information;

a comparator connected to receive the output electrical signal from said second photodetector and the direct current from said low pass filter and providing an output signal of the difference;

a frequency shifter connected to receive the output signal of said comparator and providing an output signal suitable for shifting the frequency of a laser; and

a laser connected to receive the output signal of said frequency shifter and for optically providing a beam to said interferometer.

2. A signal stabilized optical hydrophone according to claim 1 wherein said interferometer means further comprises:

a beam divider receiving the beam from said laser and providing a pair of identical beams;

a signal leg receiving one of said pair of identical beams;

a reference leg receiving the other of said pair of identical beams;

an acoustic sensing element connected into said signal leg for modulating said one of said pair of identical beams;

a beam divider connected into said reference leg for dividing in two the other of said pair of identical beams into two identical reference beams, one of said reference beams being conducted to said second photodetector;

a beam recombiner connected to receive the modulated one of said pair of beams and the other of said reference beams and conbining the received beams into a fringe pattern; and

an aperture aligned with said beam recombiner for transmitting a portion of said fringe pattern to said first photodetector.

3. A signal stabilized optical hydrophone comprising:

interferometer means for providing a fringe pattern;

a photodetector connected to receive said fringe pattern and provide an output electrical signal having the fringe pattern information, said output electrical signal comprised of a signal current and a ten kHz current;

a high pass filter connected to receive said output electrical signal and conducting the ten kHz current and inhibiting the flow of the signal current;

a low pass filter connected to receive said output electrical signal and conducting the signal current and inhibiting the flow of the ten kHz current;

a second harmonic detector connected to said high pass filter for detecting the second harmonic of said ten kHz current;

a third harmonic detector connected to said high pass filter for detecting the third harmonic of said ten kHz current;

a ten kHz oscillator connected to said third harmonic detector for receiving the third harmonic signal and providing a ten kHz signal;

an offset control connected to said second harmonic detector for receiving the second harmonic signal and to said ten kHz oscillator for receiving the ten kHz signal;

a summer connected to said ten kHz oscillator and said offset control for receiving their respective outputs;

a frequency shifter connected to receive the output signal of said summer and providing an output signal suitable for shifting the frequency of a laser; and

a laser connected to receive the output signal of said frequency shifter and for providing a beam to said interferometer.

4. A signal stabilized optical hydrophone array system comprising:

a plurality of interferometer means for providing a plurality of respective fringe patterns and provide a plurality of respective output electrical signals having the respective fringe pattern information, each of said output electrical signals comprised of a signal current and a ten kHz current;

a plurality of high pass filters connected to respectively receive each of said output electrical signals and conducting the ten kHz current and inhibiting the flow of the signal current;

a plurality of low pass filters to respectively receive each of said output signals and conducting the signal current and inhibiting the flow of the ten kHz current;

a plurality of second harmonic detectors connected to respective members of said plurality of high pass filters for detecting the second harmonic of said ten kHz current;

a plurality of third harmonic detectors connected to respective members of said plurality of high pass filters for detecting the third harmonic of said 10 kHz current;

a plurality of ten kHz oscillators interconnected for synchronization and connected to respective members of said plurality of third harmonic detectors for receiving the third harmonic signal and each of said plurality of ten kHz oscillators providing respective ten kHz signals;

a plurality of offset controls connected to respective members of said plurality of second harmonic detectors for receiving the second harmonic signal and to respective members of said plurality of ten kHz oscillators for receiving the respective ten kHz signals;

a plurality of summers connected to respective members of said plurality of ten kHz oscillators and respective members of said plurality of offset controls for receiving their respective outputs;

a plurality of frequency shifters connected to receive respective output signals of said plurality of summers and providing respective output signals suitable for shifting the frequency of a laser; and

a plurality of lasers connected to receive the respective output signals of said plurality of frequency shifters for providing a plurality of respective beams to respective members of said plurality of interferometers.