Abstract:
Improved laser sensors ( 10 ) employing doped laser crystals ( 24 ) for transducing output proportional to forces impinging upon the sensors. The disclosed sensors are compact, low powered and may be constructed relatively inexpensively from readily available materials. The disclosed sensors eliminate the need for costly, optical power-sapping fiber connections at the laser crystals. According to certain embodiments, the disclosed sensors are configured for local recovery of output signals using conventional digital telemetry. According to other embodiments, the sensors generate output through a dense wavelength division multiplexing (DWDM) laser ( 28 ), thereby allowing remote recovery without the need for frequency division multiplexing and issues involved with preloading the sensors to produce beat frequencies in unique bands.

Description:
This application claims the benefit of U.S. provisional application No. 60/689,554, filed Jun. 13, 2005 and entitled “Acoustic Sensor for Towed Array.” 
    
    
     BACKGROUND 
     Doped lasing crystals, such as Neodymium Doped Yttrium Aluminum Garnet (Nd:YAG) crystals lase at their natural response wavelength when energized with light at their pump wavelength. Optically isotropic lasing crystals produce a single polarization of light at the natural response wavelength when pumped, whereas optically anisotropic lasing crystals produce two orthogonally oriented polarizations of light at the natural response wavelength when pumped. The generation of two orthogonally oriented polarizations is known as “birefringence.” 
     The phenomenon of birefringence associated with doped lasing crystals enables the development of photonic sensors for simultaneous detection of various measurands. These measurands include, among others, acoustic field, depth (i.e., the imposition of a static pressure), multi-axis acceleration, particle velocity sensors, orientation sensors. It is known that the application of force to one face of an isotropic crystal creates a small shift in the lattice of the crystal, and thereby results in the temporary creation of an anisotropic crystal which results in a small change in the wavelength of the polarized light associated with the direction of the force. As the force is applied to the crystal, the unforced direction of the lasing crystal also generates light at the natural response frequency of the crystal. Thus, the crystal simultaneously produces two wavelengths of light in orthogonal polarizations that are nearly the same wavelength. The difference in wavelength can be measured as a beat frequency, which is proportional to the force imposed on the crystal. 
       FIG. 1  is a schematic representation of a force transducer apparatus taken from Holzapfel et al. (Holzapfel, W.; Neuschaefer-Rube, St.; Kobusch, M., “High-resolution, very broadband force measurements by solid-state laser transducers”,  Measurement , vol. 28, pp. 277-291, (2000)). In the Holzapfel apparatus, a small diode laser produces a light at the pump wavelength (808 nm in the case of a Nd:YAG lasing crystal). The pump light is focused onto a first coated optical surface of the lasing crystal. A second coated surface of the lasing crystal then generates 1064 nm response light in two orthogonal polarizations. When a force is imposed on a top surface of the crystal, the vertical polarization wavelength of the response light is shifted slightly from its natural frequency. The polarized wavelengths of the response light are then combined with an optical polarizer and the difference frequency, or beat frequency, between the two polarizations are thereafter detected with a photodiode. 
     Holzapfel et al. used the apparatus of  FIG. 1  to measure the beat frequencies for static and dynamic forces placed on Nd:YAG lasing crystals of various sizes with various loading rates. Holzapfel et al. showed that the change in the beat frequency for both statically and dynamically loaded crystals is linear for at least nine decades of force load. Furthermore, the results of the Holzapfel et al. experiments were consistent regardless of the size of the crystal or the method of loading the crystal. The results of the Holzapfel experiment lead to several observations, including: 1) the dynamic range of a sensor employing this concept is strictly limited by the ability to measure a change in the beat frequency of the polarized light; 2) the materials needed to manufacture such sensors are widely available, low-cost components; 3) accurate measurements can be made from DC to the limit of modern phase or frequency detector circuits; and 4) such sensors can be configured to measure force, acceleration, pressure and orientation. 
     The disclosure of U.S. Pat. No. 6,693,848 to Ambs et al. builds upon the concepts disclosed by Holzapfel. In the &#39;848 patent, Ambs et al. disclose a hydrophone that is an optically-pumped microchip laser that is powered by light at the pump frequency, and produces different beat frequencies related to a pressure field impinging on the microchip laser. Several microchip laser cavities are placed in a single fiber array using fiber splitters. Each crystal is precisely preloaded to produce beat frequencies in a unique band (analogous to wavelength division multiplexing). The pump laser is located near a dry side receiver inside a seismic exploration ship. The frequency of each sensor is measured by placing a linear polarizer oriented at 45 degrees to one of the polarization axes of the sensor&#39;s output laser signal. The signal resulting from the polarizer is the beat frequency. Signals from the microchip lasers are reflected onto the single fiber and are converted to electrical energy by photodiodes. The beat frequency of each sensor is measured by a dry side FM receiver. The Ambs et al. approach is advantageous in comparison with conventional, passive optical fiber hydrophones which rely on intensity and/or phase modulation of a reference laser signal. In contrast to signals in passive optical fiber hydrophones, the frequency-modulated signals generated by the Ambs et al. microchip laser sensors are not affected by mechanical perturbations of the fiber telemetry link. Further, intensity fluctuations which can be problematic in interferometer-based passive techniques have no effect on the frequency modulated signals of the Ambs et al. system since the data is not encoded in the intensity. 
     There remains a need for laser sensors which provide improvements beyond the concepts disclosed by Holzapfel et al. and Ambs et al. In particular, one limiting factor of the seismic array design disclosed by Ambs et al. is the lack of sufficient optical power to create large, many-element arrays. Additionally, the use of fiber coupled to the laser crystals in the Ambs et al. design is costly and requires relatively high optical power for transmitting light through the fiber. Additionally, frequency division techniques are not scalable to larger arrays. It is therefore desirable to provide compact, low power laser sensors that eliminate the need for coupling fiber to the laser crystals. It is also desirable to provide laser sensors that eliminate the need for frequency division multiplexing and issues involved with preloading the sensors to produce beat frequencies in unique bands. It is further desirable to provide improved monitoring systems employing such laser sensors. 
     SUMMARY 
     The disclosure concerns improved laser sensors employing doped lasing crystals for generating output proportional to forces impinging upon the laser crystals. The disclosed sensors are compact, low powered and may be constructed relatively inexpensively from readily available materials. The disclosed sensors eliminate the need for long, costly, optical power-sapping fiber optic cables between the lasing crystal and a pumping device for the lasing crystal and between the lasing crystal and an output detecting device for the lasing crystal. According to certain embodiments, the disclosed sensors are configured for local recovery of output signals using conventional digital telemetry. According to other embodiments, the disclosed sensors generate output signals through a dense wavelength division multiplexing (DWDM) laser, thereby eliminating the need for frequency division multiplexing and issues involved with preloading the sensors to produce beat frequencies in unique bands. 
     The disclosure further concerns seismic and sonar monitoring systems employing arrays of improved laser sensors. The sensor arrays are low powered and require a single fiber optic line connecting multiple sensors. Furthermore, the design of the sensor arrays is relatively simple, as there is no outboard logic in the sensor arrays and only passive electronics are employed. 
     According to one embodiment, a laser sensor comprises:
         a doped lasing crystal arranged to generate a light at two different frequencies with two orthogonal polarizations, said light exhibiting a beat frequency between said two different frequencies which varies in accordance with a force incident upon the lasing crystal;   a pump diode generating an excitation light to drive said lasing crystal, said pump diode adjacent to and directly coupled to a first end of said lasing crystal by a direct butt-coupling integrated optic; and   a photodetector directly coupled to a second end of said lasing crystal by a direct butt-coupling integrated optic.       

     According to another embodiment, a monitoring system comprises:
         a fiber optic transmission line;   a sensor array comprising at least two laser sensors, wherein each of said at least two laser sensors comprises:
           a doped lasing crystal arranged to generate a light at two different frequencies with two orthogonal polarizations, said light exhibiting a beat frequency between said two different frequencies which varies in accordance with a force incident upon the lasing crystal;   a pump diode generating an excitation light to drive said lasing crystal, said pump diode adjacent to and directly coupled to a first end of said lasing crystal by a direct butt-coupling integrated optic;   a photodetector directly coupled to a second end of said lasing crystal by a direct butt-coupling integrated optic;   a dense wavelength division multiplexing laser in communication with said photodetector, said dense wavelength division multiplexing laser generating an output modulated by said output current and exhibiting frequency shifts proportional to said force; and   a fiber optic cable connected to said dense wavelength division multiplexing laser for transmitting said output, wherein said fiber optic cable is in communication with said fiber optic transmission line; and   
           a receiving unit arranged to process said output, said receiving unit comprising:
           an optical demultiplexer connected to said fiber optic transmission line to demultiplex output light from said at least two laser sensors and to produce demultiplexed outputs; and   optical receivers in communication with said optical demultiplexer and arranged to process said demultiplexed outputs from said optical demultiplexer, wherein each of said optical receivers includes a photodiode for converting light into an electrical signal and an analog-to-digital converter for converting an output of said photodiode to digital data.   
               

     Other features and advantages will become apparent from the description and drawings that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a conventional microchip laser sensor for measuring forces impinging upon the sensor. 
         FIG. 2  is a side sectional view of a novel laser sensor according to one embodiment. 
         FIG. 3  is an end view of the sensor of  FIG. 2 . 
         FIG. 4  is a partial top plan view of the sensor of  FIG. 2 . 
         FIG. 5  is a partial side view of the sensor of  FIG. 2 . 
         FIG. 6  is a partial top plan view of a novel laser sensor according to another embodiment. 
         FIG. 7  is a side sectional view of the laser sensor of  FIG. 7 . 
         FIG. 8  is a partial top plan view of a novel laser sensor according to yet another embodiment. 
         FIG. 9  is a side sectional view of the laser sensor of  FIG. 8 . 
         FIG. 10  is a partial top plan view of a novel laser sensor according to still another embodiment. 
         FIG. 11  is a side sectional view of the laser sensor of  FIG. 10 . 
         FIG. 12  is a graph providing an example of the response from a dense wavelength division multiplexing laser used in the sensors of  FIGS. 2-7 . 
         FIG. 13  is a graph showing an example of a typical channel response of a dense wavelength division multiplexing laser. 
         FIG. 14  is a graph showing applied pressure plotted against change in beat frequency for the disclosed laser sensors. 
         FIG. 15  is a graph showing the spectrum level associated with various sea-states. 
         FIG. 16  is a schematic drawing of a monitoring system according to another embodiment, wherein an array of laser sensors is parasitically powered. 
         FIG. 17  illustrates the connection of two laser sensors in the system of  FIG. 16 . 
         FIG. 18  is a schematic drawing of a monitoring system having a remote power source supplying power to an array of laser sensors. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2-5  show a laser sensor  10  according to one embodiment. The laser sensor  10  may be used, for example as an acoustic sensor in a towed sensor array of an aquatic seismic or sonar monitoring system. Referring to  FIGS. 1 and 2 , the sensor  10  includes a housing  12  encasing an internal sensing unit  14 . The sensing unit  14  is supported in the housing  12  by polymer potting  16 . As shown in  FIGS. 1 and 2 , the housing  12  may be cylindrical. However, other shapes are possible for the housing. 
     Referring to  FIGS. 2 ,  4  and  5 , the internal sensing structure  14  includes an active sensing unit  17 , a baseboard  18  and an electrical and photonic circuit  20  supported by the baseboard  18 . The circuit  20  includes a low-power pump diode  22 , an anisotropic (birefringent) crystal laser  24 , a compact, low-power photodetector (e.g., photodiode)  26  and a dense wavelength division (DWDM) laser  28 . A low-pass filter  29  may also be provided. 
     As shown in  FIGS. 2 ,  3  and  5 , a capacitor  44  is provided for storing electrical power to be supplied to the circuit  20 . As will be described in a further embodiment directed to a seismic and sonar monitoring system, the capacitor  44  enables powering of the sensor  10 . The capacitor  44  may be attached on a side of the baseboard  18  opposite the side on which the other elements of the circuit  20  are attached. Such an arrangement allows the capacitor  44  to be relatively large to ensure adequate power storage. A DC power regulator and trickle charger unit  46  is also provided for controlling charging of the capacitor as well as the supply of electrical power to the circuit  30 . 
     The lasing crystal  24  is constructed to emit an output light signal at a natural response wavelength. The lasing crystal  24  may have a cross-sectional area of 1 mm 2  or less and may be, for example, an Nd:YAG lasing crystal having a pump wavelength of 808 nm and a response wavelength of 1064 nm. Other possible lasing materials for the crystal  24  include, but are not limited to, Nd:YV04, Nd:YLF, Ti:Sapphire, Yb:YAG, Er:YAG ND:Ce:YAG and Ho:Cr:Tm:YAG. The pump diode  22  is located adjacent to and directly coupled to a first side of the lasing crystal  24  by a first direct butt-coupling integrated optic  30 . The size and materials of the crystal  24  can be tailored to meet the specific needs of an application. The photodetector  26  is located adjacent to and directly coupled to a second side of the lasing crystal  24  opposite the first side by a second direct butt-coupling integrated optic  32 . The direct butt-coupling integrated optics  30  and  32  may include glass feral mounts and alignment optics bonded to the crystal by a UV epoxy. When a Nd:YAG lasing crystal is used, the pump diode  22  generates pump light at a wavelength of 808 nm and the photodetector  26  converts 1064 nm wavelength light to electrical energy. 
     The DWDM laser  28  is connected in the circuit  20  such that it is driven by output current of the photodetector  26 . As shown in  FIG. 4 , the DWDM laser  28  is connected to a fiber optic cable  38  in the sensor by a fiber branch  40  at junction  41 . 
     The sensor  10  further includes an input fiber connector  50  connected to an input end  38   a  of the fiber optic cable  38  and an output fiber connector  52  connected to an output end  38   b  of the fiber optic cable  38 . The connectors  50 ,  52  may be surface mounted on the baseboard  18  and extend outside the housing  12  for connection to upstream and downstream segments of an external fiber optic transmission line. 
     In operation of the sensor, the pump diode  22  focuses an optical beam on the laser crystal  24 , causing the laser crystal  24  to lase a response light signal at the response wavelength of the crystal  24 . The response light signal has two orthogonal wavelengths of slightly different frequency. The difference in the frequencies of the two polarizations is the beat frequency of the response light signal. 
     The photodetector  26  receives the response light signal and converts the energy from the response light signal into electrical energy in the form of an output current which is proportional to the beat frequency of the response light signal. The DWDM laser  28  generates an output light signal based on the output current of the photodetector  26 . In particular, the output current of the photodetector  26  is used as a modulation signal which is applied to the DC bias current on the DWDM laser  28 . Thus, the frequency of the output light signal from the DWDM laser  28  is proportional to the output current of the photodetector  26 . Since the photodetector  26  is capable of generating frequencies above the frequency range in which useful information is carried, the low-pass filter  29  cuts off frequencies from the photodetector above a specified threshold. 
     The output light signal from the DWDM laser  28  is transmitted to the fiber optic cable  38 , from which the output light signal can be transmitted into an external fiber optic transmission line (i.e., fiber telemetry link) of a seismic and sonar monitoring system. As will be described in greater detail in further embodiments, output light signals from one or more additional sensors in a sensor array may be passed through the fiber optic cable  38  of the sensor  10  via the input connector  50  and the output connector  52 . 
     The low-pass filter  29  is a passive circuit used to limit the transmission bandwidth to the maximum bandwidth of the DWDM laser. 
     The active sensing element  17  couples the measurand to the lasing crystal  24  with gain that is related to its mass, impedance, and surface area. When a dynamic force is applied to the sensor  10  (e.g, acoustic pressure), the force is acts on the crystal  24  via the active sensing element  17 , and causes a shift in the beat frequency of the crystal&#39;s response light signal. The beat frequency of the response light signal is proportional to the force applied to the crystal  24 . The shift in beat frequency in proportion to applied force has been shown to be linear for over nine decades for Nd:YAG laser crystals. Since the output current of the photodetector  26  is proportional to the beat frequency of the response light signal, and the output light signal of the DWDM laser  28  is proportional to the output current of the photodetector  26 , one can determine the magnitude of dynamic forces acting on the sensor  10  based on changes in the frequency of the output light signal of the DWDM laser  28 . 
     A static force applied to the sensor  10  causes a DC shift in the beat frequency of the response light signal. Thus, because static pressure is directly proportional to depth, the depth of the sensor  10  can also be determined based on DC frequency changes in the output light signal due to DC shifts in the beat frequency. In other versions of the sensor  10 , one can determine the orientation of the sensor  10  with respect to the gravitational field by coupling a mass (not shown) to the crystal  24 , decoupling the mass from the pressure sensing function and reading the change in the output frequency of the DWDM laser  28 . Thus, acoustic pressure, sensor depth and the orientation of the sensor  10  relative to the gravitational field of the Earth can be measured simultaneously. 
     The sensitivity of the sensor  10  increases inversely in proportion to the cross-sectional area of the lasing crystal  24 . For a common crystal size of 0.1 mm 2  and using a typical design for a coupling to the measurand using an impedance-matched surface with a pin or lever coupled to the crystal transducer surface, a sensitivity of 10 GHz of change in beat frequency per Newton can be achieved. According to a conservative expectation for the sensor  10 , a pressure field of 8,000 Pa would translate to 1 N of force applied to the crystal. This is the equivalent of transferring the force applied to a flexible disc of 1 cm radius to the crystal  24 . As shown in experiments by Holzapfel et al., the stability of the beat frequency is 3 Hz over a 60-second sample period. With a sensitivity of 10 GHz of change in beat frequency per Newton, 3 Hz equates to about 0.2 μPa or −12 dB re 1 μPa.  FIG. 14  shows pressure versus change in beat frequency for a crystal  24 . 
     For comparison,  FIG. 15  illustrates the spectrum level associated with various sea-states (taken from “Principles of Underwater Sound, 3 rd  Edition, Urick, 1983, pg. 210).  FIG. 15  shows that, at the frequency range of interest (DC to less than 25 kHz), even sea state 0 is typically above 30 dB re 1 μPa, although 20 dB is often used as a good sensor noise floor. Referring back to  FIG. 14 , the target 20 dB noise floor is indicated by the horizontal line, and a minimum of 120 dB dynamic range is indicated by the vertical arrow line. The dynamic range and noise floor of the sensor  10  is dictated by the ability to resolve the change in beat frequency. Therefore, the dynamic range of the sensor  10  is really only limited by the bandwidth of the photodetector  26  and the sampling rate of the frequency detection device being used to read the sensor output. 
     The sensor  10  provides significant advantages over conventional, passive optical-fiber hydrophones, which rely upon intensity and/or phase modulation of a reference laser signal, and conventional PZT sensors which have relatively low sensitivity. In contrast to the output signals of passive optical-fiber hydrophones, the frequency-modulated output signals from the DWDM laser  28  in the sensor  10  are unaffected by perturbations of the fiber telemetry link to which the sensor  10  is connected. Intensity fluctuations which can plague interferometer-based passive techniques have no effect on the frequency modulated signals of the DWDM laser  28 , since the data is not encoded in the intensity of the signal. Because the frequency modulation of the output signal is encoded at the sensor  10  and cannot be shifted or otherwise compromised by the telemetry system in which the sensor  10  is employed, the sensor  10  provides a robust noise rejection scheme that is somewhat analogous to common mode rejection in electrical circuits. 
     Additionally, measurement or sampling of the carrier frequency from the crystal laser  24  is not required, resulting in lower cost and simpler design of transmission and receiving systems incorporating the sensor  10 . When multiple sensors  10  are used in a sensor array, the use of the DWDM laser  28  eliminates the need for frequency division multiplexing and issues associated with pre-loading each sensor to offset the frequency response range of each sensor  10 , because each sensor  10  generates an output light signal in its own discrete wavelength. Where a 1550 nm International Telecommunications Union (ITU) grid telemetry laser is used for the DWDM laser  28 , 25 GHz channel spacing with 166 channels on the grid is widely available for use in a sensor array.  FIGS. 12 and 13  show the C-Band response and typical channel response, respectively, of a commercially available ITU grid laser having 100 GHz channel spacing (ITU G.692). 
     The sensor  10  is also advantageous in that it is small in size, requires no digital logic in the sensor itself and can be constructed relatively inexpensively from widely available components. The sensor  10  may be as small as 12 mm in diameter and 25 mm in length. The baseboard  18  may be as small as 10 mm wide by 25 mm long, including the fiber connectors  50  and  52 . 
       FIGS. 6 and 7  show a laser sensor  10   a  according to another embodiment, wherein reference numbers repeated from  FIGS. 2-5  indicate similar components. Laser sensor  10   a  is similar to laser sensor  10 , except that sensor  10   a  lacks the capacitor  44  and DC power regulator and trickle charger unit  46  included in the sensor  10 . Thus, the sensor  10   a  must be powered by an external power supply. 
       FIG. 16  shows a seismic and sonar monitoring system or telemetry system  100  including a towed sensor array  110  a receiving unit  150 . The sensor array  110  is located on the wet side of the system  100  (i.e., in a body of water). The receiving unit  150  is located on the dry side of the system  100  (i.e., aboard a ship). 
     The sensor array  110  includes a plurality (S 1 -S n ) of laser sensors  10  connected in series along a fiber optic transmission line or telemetry backbone  112 . The fiber optic transmission line  112  may be a single-mode optical fiber and is only required to carry output signals from the sensors  10  to the receiving unit  150 . The input connector  50  of each sensor  10  (except for the last sensor in the line  112 ) is connected to a downstream portion of the transmission line  112  by the input connector  50 , while the output connector  52  of each sensor  10  is connected to an upstream portion of the transmission line  112 . Thus, the output light signal of each sensor  10  is carried by the transmission line and is passed through any upstream sensors (via the fiber optic cable  38  in each sensor) to the receiver unit  150 . Where 1550 nm ITU grid telemetry lasers are used for the DWDM lasers  28 , many unique 2.5 Gbps channels are available, meaning that the number of sensors  10  that can be used per transmission line  112  is limited only by the number of ITU grid channels. 
     Referring to  FIGS. 16 and 17 , piezoelectric (PZT) fibers  114  extend between adjacent sensors  10  and are operatively connected to the capacitor  44  of each sensor. Vibration and tension in the fibers  114  parasitically powers the sensors  10 , thereby eliminating the need for a dry-side power supply. As shown in  FIG. 17 , slack  113  is provided in the fiber optic transmission line  112  in order to prevent stress in the fiber connection between sensors  10 . Additionally, strength members  116  that are slightly longer than the PZT fibers  114  are placed between adjacent sensors  10  in order to limit tension in the PZT fibers. 
     The receiver unit  150  includes a main receiver  152 , a plurality of optical receivers  154  in communication with the main receiver  152 , and an optical demultiplexer  156  connected to the optical receivers  154  and the fiber optic transmission line  112 . The optical demultiplexer demultiplexes the light output signals from each sensor  10  and transmits demultiplexed light signals to the optical receivers  154 . An optical receiver  154  is provided for each sensor  10  in the array  110 . Each optical receiver  154  includes photodiodes  158  for converting the demultiplexed light signals to electrical signals and further includes analog-to-digital converters (ADCs) or frequency modulation (FM) receiver devices  160  for converting the electrical signals from the photodiodes  158 . Other frequency detection means, including frequency-to-voltage circuits such as the commercially available Phase Locked Loops (PLLs), may be used instead of the ADCs and FM receivers  160 . The main receiver  152  decodes the output of the optical receivers  154  and communicates the decoded information to sonar equipment. 
       FIG. 18  illustrates another seismic or sonar monitoring system  200 , which is similar to system  100 , except that system  200  includes a towed sensor array  210  with sensors  10   a  and does not employ parasitic powering of the array  210 . Instead of PZT fibers  114  and the associated tension limiting strands  116 , a power supply  170  is provided on the dry side of the system  200 . The power supply  170  is connected to each sensor by an electrical power line  172  and branch lines  174 , thereby supplying the necessary power to each sensor  10   a.    
     The arrays  110  and  210  require three orders less power than conventional towed arrays, due in part to the location of individual, low-power pump diodes  22  in each sensor  10 , and due to the elimination of fiber connections between the pump diodes  22  and the respective lasing crystals  24  and between the photodetectors  26  and the respective lasing crystals  24 . The pump diode  22  and photodetector  26  require only about 5 μW each. The DWDM laser  28  also contributes to low power consumption, as a 166-channel ITU grid laser requires only about 10 μW of power. Due to the high bandwidth of DWDM lasers  28  and the low power consumption of the sensors  10 , the disclosed towed sensor arrays can be scaled up to extremely large sizes that are not possible with conventional towed sensor arrays. Furthermore, the disclosed arrays require much fewer optical fiber connections and electrical connections than conventional arrays, resulting to less expense and simpler design. Additionally, completely redundant arrays can be achieved by using two fiber optic transmission lines  112 . In the redundant configuration, two uplink fibers  112  will be used and a single ITU grid laser will be capable of driving two fibers in parallel. 
       FIGS. 8 and 9  show laser sensor  60  according to an additional embodiment. The laser sensor  60  shares components with the sensor  10  of  FIGS. 2-5 , except that the sensor  60  is modified for local recovery of the sensor&#39;s output signal. The sensor  60  includes a FM receiver  54  which decodes the output signal of the photodetector  26 . Information may be recovered locally from the FM receiver  54  by know digital telemetry techniques in order to determine sensor depth, sensor orientation and acoustic pressure acting on the sensor  60 . As there is no optical output signal in the sensor, there is no need for an optical fiber or optical inputs/outputs. The sensor  60  may be powered parasitically in the same manner as the sensors  10  in the embodiment of  FIGS. 16 and 17 . 
       FIGS. 10 and 11  show yet another embodiment of a laser sensor  60   a . Sensor  60   a  is similar to sensor  60 , except that is lacks the capacitor  44  and DC power regulator and trickle charger unit  46  included in the sensor  60 . Thus, the sensor  60   a  must be powered by an external power supply. 
     The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments, not explicitly defined in the detailed description.