Communications – electrical: acoustic wave systems and devices – Testing – monitoring – or calibrating
Reexamination Certificate
2001-10-10
2003-07-15
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Testing, monitoring, or calibrating
C367S149000
Reexamination Certificate
active
06594198
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention described herein relates generally to hydrophone signal processing and, more particularly, to systems and methods for calibrating optical hydrophone detector systems.
(2) Description of the Prior Art
A typical optical hydrophone has a reference leg and a sensing leg. The sensing leg is formed by wrapping a fiber optic cable around a compliant mandrel. The reference leg is formed by wrapping a length of fiber optic cable around a noncompliant mandrel. During operation, light is pulsed down both fiber legs and reflected by mirrors imbedded in the ends of the fibers. The output of both legs, the reference and sensing legs, are summed at a node forming an interferometer. This summation produces a phase modulating signal of the form
O=A+B
cos &thgr;(
t
) (1)
where
A & B=Constants proportional to the input power, and
&thgr;(t)=Phase difference between the interferometer sensor and reference leg.
Typically, a sinusoidal modulating frequency is injected through a piezoelectric element on the reference leg of the interferometer. The output signal is given by
O=A+B
cos(
C
cos &ohgr;
o
(
t
)+
x
(
t
)) (2)
where
x(t)=Signal of interest,
C=Modulating signal amplitude, and
&ohgr;
o
=Modulating signal frequency.
Analog demodulators are used to process the output signal. These demodulators are complex custom-built hardware, requiring both expensive and time-consuming calibration. What is needed is an improved system for using programmable digital signal processor for demodulation and for calibration.
Patents that show attempts to solve the above and other related problems are as follows:
U.S. Pat. No. 4,977,546, issued Dec. 11, 1990, to Flatley et al., discloses a system for signal stabilizing in-phase modulated optical hydrophone arrays employing 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.
U.S. Pat. No. 5,313,266, issued May 17, 1994, to Keolian et al., discloses a highly sensitive optical fiber interferometer sensor comprising a laser light source, a [2×2] optical fiber coupler to split the beam in two, a differential transducer which converts a signal of interest into optical phase shift in the laser light transmitted through the two optical fibers in the interferometer and a [3×3] optical fiber complex which recombines the two beams, producing interference which can be electronically detected. The use of the [3×3] coupler permits Passive Homodyne demodulation of the phase-modulated signals provided by the interferometer without feedback control or modulation of the laser itself and without requiring the use of electronics within the interferometer.
U.S. Pat. No. 5,345,172, issued Sep. 6, 1994, to Taguchi et al., discloses a means to accomplish double-slice imaging by a nuclear magnetic resonance (NMR) imaging apparatus having an ordinary radio frequency magnetic field generator, two radio frequency magnetic field waveforms are used and slices are separated by subsequent calculation. More definitely, two slice portions are excited in a REAL direction by a COS waveform and are excited in an IMAG direction by a SIN waveform. When one of the slices is S1 with the other being S2, the signal SC when the COS waveform is used is S1+S2 while the signal SS when the SIN waveform is used is i.S1−i.S2. Therefore, the calculation for separating the slices proves SC+i.SS and SC−i.SS.
U.S. Pat. No. 5,809,087, issued Sep. 15, 1998, to Ashe et al., discloses an architecture for remote calibration of coherent systems using coherent reference and calibration signals that contain the relative amplitude and phase information desired in the calibration process. Circuitry extracts the relevant amplitude and phase information needed for the calibration while compensating for non-synchronized clocks and the effects of Doppler shifts due to relative motion of the transmitting and receiver platforms. The coherent detection architectures can be used effectively with any scheme designed to determine the relative amplitudes and phases of the signals emitted from the different elements of the phased array. These architectures are particularly applicable to coherent encoding calibration procedures that enhance the effective SNR by using coherent transmission of orthogonal transform encoded signals from N elements of the phased array. In an example calibration architecture, coherent elemental signals are encoded using controlled switching of the delay phase control circuits themselves to effectively generate a perfect orthogonal transform encoding of the signal vectors, even though the control circuits may be imperfect; no additional encoding hardware is required. The switching is dictated by matrix elements of an N×N invertible binary matrix, with the most preferred embodiment being an orthogonal binary matrix, i.e., a Hadamard matrix. The coherent signals are decoded with the inverse of the same binary matrix used in the control circuit encoding.
U.S. Pat. No. 5,894,280, issued Apr. 13, 1999, to Ginetti et al., discloses a digital to analog converter (DAC) offset autocalibration system in a digital synthesizer integrated circuit. The present invention includes a DAC coupled to a filter. The input of the DAC accepts digital values for conversion to an analog signal. The output of the DAC is coupled to the input of the filter. The filter smoothes the analog signal received from the DAC. A switch is coupled to the filter output to receive the analog signal. A comparator is coupled to the switch. The input of the comparator receives the analog signal from the filter output via the switch. An autocalibration control circuit is coupled to the output of the comparator, to the switch, and to the DAC. The autocalibration control circuit is adapted to input a value to the DAC in order to determine an offset correction from the output of the comparator and adjust the analog signal using the offset correction.
U.S. Pat. No. 5,903,350, issued May 11, 1999, to Bush et al., discloses an apparatus and method providing wide dynamic range measurements of the input phase to an interferometer using a phase generated carrier. This invention is useful when utilizing time multiplexing to demodulate a series of interferometers. A modulation drive output is provided by the invention and maintained under operation at the optimum amplitude by an internal feedback loop. The resulting highly stable system can be fabricated from an analog to digital converter, a digital signal processor, and a digital to analog converter making low cost open loop demodulators a reality.
U.S. Pat. No. H1619, issued Dec. 3, 1996, to McCord et al., discloses a frequency modulated monitor hydrophone system which is used to monitor low frequency sound signals where cross-talk coupling is a problem. The invention consists of a hydrophone, preamplifier and receiver which includes a control group. The hydrophone comprises an acoustic sensor and low-noise preamplifier utilizing dynamic range compression to condition the electrical acoustic sensor
Amaral Michael
Ames Gregory H.
Deus III Antonio L.
Hansen Christopher M.
Moretti David J.
Kasischke James M.
Nasser Jean-Paul A.
Oglo Michael F.
Pihulic Daniel T.
The United States of America as represented by the Secretary of
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