Abstract:
A direct detection method and apparatus for a fiber optic acoustic sensor array systems using an in-line Michelson sensor TDM array and an interferometric section having two acousto-optic modulators that produce optical pulses that are frequency shifted with respect to each other. Direct detection is accomplished according to the equation:
 
 I ( t )= A+B  cos [φ 1 −φ 2 +2π( f   1   −f   2 ) t], 
 
with the phase shift difference φ 1 −φ 2  between two paths containing the acoustic phase information and the frequency f 1 −f 2  being the difference between the RF frequencies for the two acousto-optic modulators.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to improvements in fiber optic acoustic sensor array systems and more particularly pertains to a new and improved method and apparatus for detecting the signals from an array of fiber optic interferometric sensors for determining changes in a physical parameter measured by the individual sensors. 
     2. Description of Related Art 
     Mismatched fiber optic interferometers are commonly used as sensing elements in fiber optic sensor arrays for measuring changes in parameters such as fluid pressure, acceleration or magnetic field intensity, for example. Such sensing elements measure the phase delay between two optical paths having unequal path lengths. Typically, in time division multiplex (TDM) systems, a modulated optical signal is input to the sensor array and various demodulation techniques have been proposed and are used for correlating the signals from the array of sensors that produce the signals. 
     Common to all demodulation methods for fiber optic interferometric arrays, is the acquisition of an in-phase term proportional to the cosine of the interferometer phase shift and a quadrature term proportional to the sine of the interferometer phase shift. The sine of the sensor phase shift, is known as the quadrature term Q; and the cosine of the sensor phase shift is referred to as the in-phase term I. The angle of the phase shift is determined by calculating the ratio of Q/I, which is the tangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the successful implementation of an arc tangent routine to find the sensor phase shift. 
     An interrogation method called differential delay heterodyne is one of a variety of methods used for fiber optic acoustic sensor array systems. It uses an in-line Michelson sensor time division multiplexed (TDM) array structure and a compensating interferometer section. 
     An example of such a system is shown in  FIG. 1  as having a continuous wave laser  11  supplying signal energy to a pair of acousto-optic modulators  13 ,  15  that act as optical gates to produce pulses like the two optical pulses  1  and  2 . Each acousto-optic modulator puts a unique frequency shift on the light energy from laser  11 . This provides, for example, pulse  1  at a frequency shift of 105 MHz and pulse  2  at a frequency shift of 95 MHz for a difference frequency of 10 MHz between them. A delay coil  17  in series with the acousto-optic modulator  13  creates an optical path length difference between the pulse signal paths for the signal&#39;s output from the two acousto-optic modulators  13  and  15 . 
     These output pulses travel down optical transmission line  16  in direction  43  to an in-line Michelson array with two hydrophone sections  19  and  21  located between mirrors  23 (A),  24 (B) and  25 (C). Mirrors  23 (A) and  24 (B) are coupled to transmission line  16  by tap couplers having appropriate coupling ratios. The spacing between the three mirrors  23 (A),  24 (B) and  25 (C) is selected to produce a reflected sequence of four pulses  35 ,  39 ,  41  and  37  traveling out of the Michelson array, in direction  45 . 
     Return pulse  35  is effectively pulse  1  reflected from mirror  23 (A). Pulse  35  can be labeled  1 (A). Pulse  39  is an interference pulse signal that is a combination of pulse  1  reflected from mirror  24 (B) and pulse  2  reflected from mirror  23 (A). Return pulse  39  can be labeled ( 2 A/ 1 B). This pulse contains acoustic phase information from the hydrophone section. Pulse  41  is another interference pulse containing information from the combination of pulse  1  being reflected from mirror  25 (C) and pulse  2  being reflected from mirror  24 (B). Pulse  41 , therefore, could be labeled ( 2 B/ 1 C). Pulse  37  is simply pulse  2  reflected from mirror  25 (C). (Pulse  37  could be labeled pulse  2 C.) The middle two pulses,  39  and  41 , are the interference pulses that contain acoustic phase information  40  and  42 , respectively from the two hydrophone sections. The system could be expanded to any number of hydrophones, so that for N hydrophones there are N+2 return pulse signals. 
     Pulses  35  and  37  contain no useful information. 
     These return pulse signals are demodulated in a receiver that contains a local oscillator  31  set at the difference frequency between the modulator frequencies of the two acousto-optic modulators  13  and  15 . The local oscillator signal is mixed with the output signal from the photodiode detector  27  in mixer  29  to produce the cosine I and sine Q components of the optical signal. These cosine and sine components of the optical signal are then processed in the demodulator  33  to produce the signals representative of the change in parameter measured by the Michelson array. 
     The above example is only one of many available methods of demodulation known in the prior art. Other methods are shown and described in U.S. Pat. No. 6,154,308, U.S. Pat. No. 6,122,057, and U.S. Pat. No. 5,917,597. These patents show other examples of demodulation methods for fiber optic sensor arrays. 
     The present invention provides an improved method and apparatus for demodulating signal&#39;s from a fiber optic sensor array, by eliminating the need for a mixer and local oscillator, as shown in  FIG. 1 . 
     SUMMARY OF THE INVENTION 
     An apparatus and method for direct detection of signals from a differential delay heterodyne in line interferometric system that receives pulsed optical signals from an optical signal source, the pulsed optical signals being frequency shifted with respect to each signal path before being provided to the interferometric system. The interference signals from the interferometric system are detected and the signal intensity of each interference signal is measured at a plurality of points along the interference signal interval. The desired phase information is then calculated using the measured signal intensity points for each interference signal interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exact nature of this invention, as well as its objects and advantages, will become readily apparent from consideration of the following specification when considered in conjunction with the accompanying drawings which illustrates and describes a preferred embodiment of the invention, and in which like reference numerals designate like parts throughout the figures thereof and wherein: 
         FIG. 1  is a schematic of a prior art differential delay heterodyne interferometer apparatus used with a fiber optic acoustic sensor array system; 
         FIG. 2  is a schematic illustration of a preferred embodiment of the present invention; 
         FIG. 3  is a block diagram of a direct detection apparatus used in  FIG. 2 ; 
         FIG. 4  is a signal diagram illustrating detection of a signal by the apparatus of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in  FIG. 2 , the direct detection method and apparatus of the present invention embodied in detector  47 , receives the output signal from the photodiode detector  27  and proceeds to demodulate the received signals to extract the acoustic phase information without using a mixer, or a local oscillator. 
       FIG. 3  illustrates a preferred structure for a direct detector  47  that could be used in  FIG. 2 , as comprising a transimpedence amplifier  49  receiving the signals from the photodiode detector  27  ( FIG. 2 ), providing the amplified signals to a low pass filter  51 . The signals are then passed to a variable gain amplifier  53 . The amplified signals are provided to an analog to digital (A/D) converter  55 . The output of A/D converter  55  is provided to a digital demodulator  57 . 
     Either output of the two beam interferometer of  FIG. 2  pulse  39  ( 2 A/ 1 B) or pulse signal  41  ( 2 B/ 1 C), at the photodiode, is expressed by the equation:
 
 I ( t )= A+B  cos [φ 1 −φ 2 +2π( f   1   −f   2 ) t )]  (1)
 
     The phase shift difference φ 1 −φ 2  between the two paths of the beam interferometer contains the acoustic phase information. The frequency difference f 1 −f 2  is the frequency difference between the RF frequencies for the two acousto-optic modulators  13  and  15 , 10 MHz in our example. 
     The pulse signal length or duration for each sensor return is typically 100 to 200 nanoseconds. With a frequency difference f 1 −f 2  of 10 MHz, there will be one to two cycles of the 10 MHz waveform riding on the top  40  of return pulse  39  and on the top  42  of return pulse  41 . 
     The pulse intensity I(t) can be captured every quarter cycle (π/2) to generate a series of values for the acquisition of the desired acoustic data according to the following equations:
 
 I   0   =A+B  cos [φ 1 −φ 2 ]  (2)
 
 I   1   =A+B  cos [φ 1 −φ 2 +π/2 ]=A−B  sin [φ 1 −φ 2 ]  (3)
 
 I   2   =A+B  cos [φ 1 −φ 2   +π]=A−B  cos [φ 1 −φ 2 ]  (4)
 
 I   3   =A+B  cos [φ 1 −φ 2 +3π/2 ]=A+B  sin [φ 1 −φ 2 ]  (5)
 
 I   4   =A+B  cos [φ 1 −φ 2 +2 π]=I   0    (6)
 
     Ratios of the various sums and differences of the five pulse intensities I 0  to I 4  can be used to acquire the acoustic phase information according to one of the following equations:
 
φ 1 −φ 2   =a  tan [( I   3   −I   1 )/( I   0   −I   2 )]  (7)
 
φ 1 −φ 2   =a  tan [( I   3   −I   1 )/( I   1   +I   3 −2 I   2 )]  (8)
 
φ 1 −φ 2   =a  tan [( I   0   +I   2 −2 I   1 )/( I   0   −I   2 )]  (9)
 
       FIG. 4  illustrates two cycles of a 10 MHz waveform riding on top of one of the return pulse signals. The two cycle waveform  40 ,  42  provides ample sampling points  61 ,  63 ,  65 ,  67 ,  69 . The points I 0    61  through I 4    69  can be obtained in one cycle by sampling at π/2 intervals. 
     Four pulse intensities are required by equation (7). Only three pulse intensities are needed for equations (8) and (9). For a 10 MHz waveform with a period of 100 nanoseconds, the minimum of three signal levels can be determined from a 50 nanosecond half cycle. This is sufficient to determine the acoustic phase shift. By utilizing the multiple solutions for φ 1 −φ 2  provided by equations (7), (8) and (9) and averaging these solutions, accuracy is increased. 
     I 0 (t)  61  and L 4 (t)  69 , as can be seen from  FIG. 4 , are one cycle apart and should have the same signal level. Certain systems utilizing a larger number of sensors or having distances from the source or receiver to the sensor arrays that exceed certain limits, require the use of erbium doped fiber amplifiers. A characteristic of erbium doped fiber amplifiers is to place a ramp on top of each sensor return pulse. 
     This ramp on each sensor return pulse signal adds complexity to the demodulation process of the present invention. This ramp is usually quite small and can be modeled as a linear slope as indicated in the following equations:
 
 I   0   =A+B  cos [φ 1 −φ 2 ]  (10)
 
 I   1 =(1 +x )( A−B  sin [φ 1 −φ 2 ])  (11)
 
 I   2 =(1+2 x )( A−B  cos [φ 1 −φ 2 ])  (12)
 
 I   3 =(1+3 x )( A+B  sin [φ 1 −φ 2 ])  (13)
 
 I   4 −(1+4 x )( A+B  cos [φ 1 −φ 2 ])  (14)
 
     The linear slope factor x is only a few percent and can be expressed as follows:
 
 X =( I   4   −I   0 )/4 I   0 
 
     Substituting x into equations (11) through (14), we obtain:
 
 S   0   =I   0   =A+B  cos [φ 1 −φ 2 ]  (16)
 
 S   1 =4( I   0   I   1 )/(3 I   0   +I   4 )= A−B  sin [φ 1 −φ 2 ]  (17)
 
 S   2 =4( I   0   I   2 )/(2 I   0 +2 I   4 )= A−B  cos [φ 1 −φ 2 ]  (18)
 
 S   3 =4( I   0   I   3 )/( I   0 +3 I   4 )= A+B  sin [φ 1 −φ 2 ]  (19)
 
 S   4   =I   0   =A+B  cos [φ 1 −φ 2 ]  (20)
 
     The quantities S 0  through S 4  in equations (16) through (20) reduce to the quantities I 0  through I 4  in equations (2) through (6) at the limit of slope factor x=0. 
     Ratios of the various sums and differences of the five modified pulse intensities S k  can be used to acquire the acoustic phase information. In utilizing equations (7), (8) and (9) with the modified pulse intensities we obtain:
 
φ 1 −φ 2   =a  tan [( S   3   −S   1 )/( S   0   −S   2 )]  (21)
 
φ 1 −φ 2   =a  tan [( S   3   −S   1 )/ S   1   +S   3 −2 S   2 )]  (22)
 
φ 1 −φ 2   =a  tan [( S   0   +S   2 −2 S   1 )/( S   0   −S   2 )]  (23)
 
     Thus, even when erbium doped fiber amplifiers are required for the system, the direct detection demodulation method of the present invention can be used to create the advantage of the multiple solutions provided by equations (21), (22) and (23) to allow for averaging of φ 1 −φ 2  to increase accuracy of the result. 
     The mathematical manipulation of the return pulse signals discussed above are performed by the direct detector  47  of the present invention and specifically by the digital demodulator  57 . 
     The direct detection demodulation method of the apparatus and method of the present invention eliminates the requirement to use a mixer and a local oscillator, thereby considerably simplifying the receiver architecture for differential delay heterodyne interferometer systems.