Abstract:
The present invention provides a method and device for to implement time division multiplexing of a fiber optic serial Bragg grating sensor array containing more than one Bragg grating. The device provides a pulse read-out system that allows for a reduction in system noise and an increase in sensor resolution and flexibility. The optical signals reflected from the Bragg grating sensors are gated by an electronically controlled optical modulator before any wavelength measurement is performed to determine the sensor information. This offers significant advantages since the sensor information is encoded into the wavelength of the optical signal and not its intensity. Therefore the sensor signal information is not distorted by the gating. Since the gating or switching of the optical modulator between transmission and attenuating states is performed on the optical signal, the speed of the electronic processing needs only to be performed at the speed of variation of the sensor information and the choice of methods of wavelength measurement is not influenced by the gating action.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for optical demultiplexing multiple Bragg grating sensors in a serial array in optical fibers. 
     BACKGROUND OF THE INVENTION 
     Fiber optic Bragg gratings may be used as sensors to monitor perturbations in their environment. A Bragg grating is formed in a single mode optical fiber by creating a periodic refractive index perturbation in the fiber core as described by Kawaski, Hill, Johnson and Fuhjii in Optics Letters, Vol. 3, pp. 66-68, 1978. The diffraction grating in the fiber core will reflect optical frequencies within a narrow bandwidth around the Bragg wavelength of the optical grating. The Bragg wavelength of the diffraction grating can be altered by changing the grating pitch. If an external influence alters the grating pitch then the reflection spectrum of the grating can be monitored to determine the magnitude of the external influence. If the grating is subject to varying strain or temperature, the pitch of the grating is altered as described by Morey, Meltz and Glenn in the Proceeding of the IEEE, vol. 1169, pp. 98-107, 1989. By coupling the grating to an appropriate transducer, the grating can be used to monitor a wide variety of parameters including but not limited to strain, temperature, vibration, pressure, and acceleration. 
     Fiber optic Bragg grating sensors offer many advantages over traditional electrical sensors for monitoring the various parameters. They provide inherent immunity to electromagnetic interference and provide a reliable signal with very little noise. They can also withstand large variations in temperature and pressure and are compact in size allowing them to be used in locations where conventional sensors are impractical. Bragg grating fiber sensors have the additional advantage that the signal is encoded directly into an absolute wavelength shift of the optical signal, so the signal is insensitive to optical power fluctuations and other signal perturbations. 
     Unfortunately, the design of Bragg grating sensor systems is often more costly than the conventional electrical sensor alternatives and this has prevented their widespread adoption in many applications. To increase the utility of Bragg grating sensors, it would be advantageous to be able to multiplex many grating sensors in the same optical fiber in order share expensive resources such as the optical source and the sensor measurement unit among the many sensors thereby dramatically reducing the cost per sensor. The placement of many sensors in the same fiber often simplifies the installation of the sensors in structures or systems by reducing bulk and complexity. It is also desirable that the functionality and performance of the system not be degraded by the multiplexing technique. 
     These potential advantages have motivated significant efforts into developing methods of multiplexing Bragg grating sensors. It would be very beneficial to be able to multiplex a hundred sensors or more in a single optical fiber using only one light source and spectral measurement system. Current systems have fallen short of this goal with about ten sensors per fiber in demonstrated systems that do not severely restrict the sensor&#39;s application. As the number of sensors grows there is an increased demand on the optical source power and the complexity of the multiplexing and/or demultiplexing. For a very large number of sensors the cross talk between the sensors can become a significant problem. 
     Many different multiplexing techniques have been developed for Bragg grating sensors. The most successful techniques for use with a large number of sensors have been wavelength division and time division multiplexing. Examples of these systems are described in the paper by Kersey et al. in the Journal of Lightwave Technology vol. 15, pp.1442-1462, 1997. 
     In wavelength division multiplexing, the Bragg wavelength of each sensor is set at a separate and unique wavelength. The separations of the Bragg wavelengths are made to be far enough apart so that any reasonable external influence to the grating sensors will not be sufficient to cause the Bragg wavelengths of any two sensors to overlap. Thus each sensor is given a unique wavelength band or slot for its Bragg wavelength. In many situations, the size of each wavelength slot may need to be very large. This requirement can result from the necessity to be able to detect a large range of the parameter being sensed or due to the fact there may be uncertainty in the nominal Bragg wavelength of the sensors. Uncertainty may arise from variations in the fabrication process of the gratings, by static strains or uncertain operating temperatures when the sensor is used. The variability can necessitate a wavelength slot for each sensor in excess of 15 nm for Bragg wavelengths near 1550 nm. When the number of multiplexed sensors is large, the bandwidth requirement on the optical source can become intractable thus limiting wavelength division multiplexing to well controlled sensors that are subject to small external influences. 
     To overcome the aforementioned problems associated with limited optical bandwidth, the Bragg wavelengths of the sensors may be fabricated with nearly identical Bragg wavelengths and multiplexed with time division multiplexing. In this method a short optical pulse is sent along the fiber containing the Bragg sensors. The pulse will partially reflect off of each sensor and return the sensor information from each grating. The signals from each sensor can be distinguished by their time of arrival. Previous demonstrations of time division multiplexing have determined the time of arrival of the signal by converting the optical pulses into an electrical signal and then gating the electrical signal with a known time delay. Only the pulse that is passing through the electronic detector at the time of the gate is measured. By varying the time delay of the gate, the signals from each of the sensors can be read out. 
     A previous method used in the art to identify the sensor signals is to electrically gate the sensor signals as disclosed in U.S. Pat. No. 5,680,489. Since the sensors are now identified by time discrimination instead of wavelength, bandwidth requirements of the source will not limit the number of sensors. However, different problems can be encountered in time division multiplexing that can limit the performance of the system. Time division multiplexed systems generally experience more noise than wavelength division multiplexed systems. A significant contribution of the noise is from multiple reflection between the different grating sensors that cause a pulse to arrive back from the sensor array at a time later than expected. Noise is also be contributed by the optical source which may not be pulsed in an ideal manner so that there is a finite level of optical power between successive pulses. 
     Bragg grating sensor systems often require a very high dynamic range of eighty to a hundred and twenty decibels. Therefore any small sources of noise can be significant. To optimize the performance of the system it is necessary to perform the signal gating in as short a time period as possible. This allows the system to reject a large portion of the noise that does not return at the same time as a sensor pulse. With the method of gating used previously in the art, the performance of the system is limited. An electronic circuit performs the gating action after an optical detector has detected the optical signal. Therefore the electronic circuit must be operated at the speed of arrival of the optical pulses. It is difficult to operate electronic circuits at very high speed and still maintain very high signal fidelity due to noise and distortion. Since the gating is done after the optical signal is detected, the wavelength measurement on the signals must be done before the gating. Therefore any noise or distortions in the gating process will create errors in the sensor signal. Furthermore, the limited operation of this gating method will reduce the spatial resolution of the sensor system since the pulses from the sensor array must be spaced far apart in time. 
     It would therefore be very advantageous to provide a method and apparatus for time division optical multiplexing multiple serial Bragg gratings in optical fibers which reduces noise associated with the gating process and allows for very fast gating times. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a method and apparatus to facilitate multiplexing many Bragg grating sensors along an optical fiber that can all share the same optical source and sensor processing unit. 
     The present invention provides a pulse read-out system to implement time division multiplexing of a fiber optic Bragg grating sensor array. The pulse read-out system allows for a reduction in system noise and an increase in sensor resolution and flexibility. The essential idea of the invention is that the optical signal from the grating sensors is gated by an electronically controlled optical modulator before any wavelength measurement is performed to determine the sensor information. This offers significant advantages since the sensor information is encoded into the wavelength of the optical signal and not its intensity. Therefore the sensor signal information is not distorted by the gating. Since the gating is performed on the optical signal, the speed of the electronic processing needs only to be performed at the speed of variation of the sensor information and the choice of methods of wavelength measurement is not influenced by the gating action. 
     The gating or switching action of the optical modulator will modify the optical power transmitted to the sensor information-processing portion of the system, but will not modify the spectral content of the optical signal. Therefore distortion and noise in the gating signal will not alter the sensor reading thus providing a more robust read-out system. This allows the system to operate at very short gating times and provides a measure of immunity from unwanted signals returning from the sensor array and provides superior sensor spatial resolution. 
     The present invention provides a means for evaluating the sensor configuration of the network to high degree of precision if it is not known beforehand. A means is also provided to implement synchronous detection of the sensor signal in combination with the gating action of the optical signal. 
     An additional advantage of the present method is its flexibility with sensor signal decoding techniques. Depending on the application of the sensors, different demands may be required of the system. For example, one may want to measure rapidly varying signals or quasi-static signals. One may require a large dynamic range or a large sensing range. Many different techniques of decoding the sensor information of Bragg gratings have been developed but all of them must measure the wavelength of the returned signal. Therefore the present sensor read-out technique can be easily integrated with a wide variety of sensor measurement methods since the optical gating does not alter the wavelength information of the optical signal. This is in contrast to previous techniques where the electronic gating is performed after the wavelength detection making it more difficult to integrate the demultiplexing with the sensor decoding technique. 
     The present invention provides an optical fiber serial Bragg grating sensor device, comprising: 
     a) a light source adapted to produce optical pulses; 
     b) an optical fiber network including an optical fiber optically coupled to said light source, the optical fiber including a Bragg sensor array having at least two spaced apart Bragg gratings; and 
     c) an optical transmission element connected to a section of said optical fiber network adapted to receive optical pulses reflected from said at least two Bragg gratings, a wavelength detection means optically coupled to said optical transmission element, switch means connected to said optical transmission element for switching said optical transmission element between an attenuating state in which said optical transmission element attenuates light and a transmission state in which light is transmitted through said optical transmission element to said wavelength detection means, said switch means being activated at selectively adjustable times after production of said optical pulses. 
     The present invention also provides a device for time domain demultiplexing serial optical fiber Bragg grating sensor networks, the network including a light source adapted to produce optical pulses connected to an optical fiber network with the optical fiber network including a sensor array having at plurality of spaced Bragg gratings. The device comprises 
     an optical transmission element connected to a section of said optical fiber network adapted to receive optical pulses reflected from said at least two Bragg gratings, switch means connected to said optical transmission element for switching said optical transmission element between a transmission state in which said optical transmission element transmits light therethrough and an attenuating state in which said optical transmission element attenuates light, said switch means being activated at selectively adjustable times after production of said optical pulses; and 
     wavelength detection means connected to said optical transmission element. 
     The present invention also provides a method for time domain demultiplexing a serial fiber Bragg grating sensor network, the sensor network including an optical fiber having at least two spaced Bragg gratings and a light source for producing light pulses that propagate along the sensor network and are incident on said at least two Bragg gratings. The method comprises: 
     directing optical pulses reflected by said at least two Bragg gratings to an optical transmission element; 
     spectrally analyzing optical pulses reflected from a selected Bragg grating by switching said optical transmission element to a state of transmission at effective periods of time after preselected optical pulses are produced, said periods of time being equal to a transit time of said optical pulses from a light source to said selected Bragg grating and to said optical transmission element; and 
     maintaining said optical transmission element in the state of transmission for an effective period of time to permit light pulses to be transmitted through said optical transmission element to a wavelength detection means and thereafter switching said optical transmission element to a state of attenuation to block optical pulses reflected from all other Bragg gratings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The method and apparatus for time division optical demultiplexing Bragg gratings in optical fibers will now be described, by way of example only, reference being had to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a system for time division optical demultiplexing of multiple Bragg gratings in an optical fiber; 
     FIG. 2 is a block diagram of a pulsed read-out system forming part of a time-division multiplexed fiber optic Bragg grating sensor array; 
     FIG. 3 is a more detailed block diagram of the pulsed read-out unit of FIG. 2; 
     FIG. 4A shows the optical spectrum of a sensor array having two multiplexed Bragg gratings without use of time division demultiplexing; 
     FIG. 4B shows the optical spectrum of the sensor array of FIG. 4A using a pulsed read-out system using a delay of a gating pulse so that only the optical spectrum from the first Bragg grating sensor in the sensor array is detected; 
     FIG. 4C is similar to FIG. 4B but using a differently delayed gating pulse so that only the optical spectrum from the second Bragg grating sensor in the sensor array is detected; 
     FIG. 5 illustrates a method of determining the configuration of the sensors; and 
     FIG. 6 is a second embodiment of the invention to implement synchronous detection. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, an apparatus used for time division optical demultiplexing multiple Bragg gratings in optical fibers is shown generally at  10 . A light source  12  launches optical pulses  13  into a optical fiber  14  containing a fiber splitter  16  and a serial array of Bragg grating sensors  18  located on the other side of splitter  16  from source  12 . The optical fiber used is preferably a single mode silica optical fiber however any other optical fiber or waveguide in which a Bragg grating can be written may be used. Each sensor in sensor array  18  will return an optical pulse with wavelength encoded information, producing a train of pulses that are directed towards an optical demultiplexing system  20 . The sensors in array  18  are coupled to one or more external parameters that they are to monitor so that changes in these parameters will modify the Bragg wavelength of the sensors. The coupling may be achieved by embedding or bonding the fiber sensors  18  to the structure or apparatus to be monitored so that changes in temperature or strain are also experienced by the sensors. The sensors may also be coupled to an appropriate transducer known in the art to convert other parameters into a shift in the sensor&#39;s Bragg wavelength. The optical fiber near the sensors has the protective buffer removed to permit the sensors to be directly coupled to the appropriate structure, apparatus or transducer. 
     The optical demultiplexing system  20  is essentially an optical transmission device that can be rapidly switched between a transmission state in which light is transmitted through it and an attenuation state in which light is attenuated. The optical transmission device includes an optical modulator  22 , a preferred optical modulator is a commercial lithium niobate opto-electronic modulator that is gated (switched) using a switching mechanism comprising an electrical signal from a short pulse generator  24  so that light is only allowed to pass through the modulator  22  to a wavelength detection system  40  when the gating voltage signal is applied. The switch also includes a variable electrical delay generator  24  connected to the short pulse generator  24 . By varying the time delay of the gating signal using the variable electrical delay generator  24 , the individual reflected optical pulses transmitted through the modulator to the wavelength detection system are selected. The optical demultiplexing system  20  may include a polarization control  28 . The polarization control is useful for adjusting the polarization of the sensor signals to a preferred polarization state if the optical modulator  22  is sensitive to the polarization of the optical signal. The polarization control may be performed by inducing birefringence into the optical fiber after the fiber splitter  16  or by other methods known in the art. 
     Referring to FIG. 2, Bragg grating sensor array  18  includes several Bragg gratings  30 A,  30 B . . .  30 N are written at separate locations in the single mode optical fiber  14 . Optical pulse  13  from light source  12  (containing sufficient optical bandwidth to cover the expected range of Bragg wavelengths of any given Bragg grating sensor in array  18 ) is launched into the serial sensor array  18  through the optical coupler  16 . The Bragg grating sensors  30  are each fabricated to be reflective within the bandwidth of the optical source and wavelength measurement capability of the system for any reasonable perturbations of the sensors. 
     The reflectivity of each Bragg grating sensor in array  18  at each of their respective Bragg wavelengths is designed to be a few percent or less so that only a small portion of the pulse  13  launched into the array is back-reflected at each sensor. The rest of the optical pulse is allowed to propagate to sensors further down the array  18  and be likewise reflected. The arrows  32  indicate the possible paths of the optical signal. Thus, from the single optical pulse  13  launched into the sensor array  18 , a train of pulses  36  are returned from the sensor array through the fiber path  38  after passing through coupler  16 . Each returned pulse has a spectral content corresponding to the spectral reflectivity of the Bragg grating sensor that it originated from. In general the duration of the pulses must be shorter than the duration of the optical gate and the repetition rate must be lower than the time for the pulse to traverse the fiber and return to the pulse read-out system. 
     The minimum physical spacing of the Bragg sensors in the array  18  is given by the temporal duration of the optical gate. The time for the optical pulse to travel twice the distance between the two nearest sensors must be longer than the gating time. The maximum number of sensors is limited to the ratio of the total physical length of the sensor array, from the first sensor to the last, to the minimum physical spacing between sensors. The maximum number can also be expressed as the ratio of twice the time for an optical pulse to travel from the first sensor to last, to the temporal duration of the optical gate. 
     In a preferred embodiment the pulses from the source are made to be shorter in duration than the time for a pulse to travel twice the distance between the two spatially closest sensors on the sensor array. In this preferred embodiment a mode-locked fiber laser producing sub-picosecond pulses with a bandwidth &gt;10 nm may be used. However those skilled in the art will understand that other light sources may be used as long as they meet the requirements described above. Each of the individual pulses making up pulse train  36  from the sensor array  18  will return from the sensor array at unique times. The pulses containing the sensor information in the optical fiber branch  38  are directed towards the pulse read-out system  20 . The optical source  10  launches a series of pulses at a fixed repetition rate into the sensor array to repeat the process described above. The period between pulses is greater than the time for a pulse to travel twice the distance from the first sensor to the last sensor in the array. 
     The sensor information contained within each pulse of pulse train  36  may be identified as coming from the appropriate Bragg grating sensor by the time of arrival of the pulse at the pulse read-out unit  20 . The pulse read-out unit  20  allows the optical signal to propagate to the wavelength detection unit  40  for a short period of time and acts as an optical gate on the returned optical signal. The duration of the optical gate is chosen to be longer than the temporal duration of the pulse response from any one Bragg grating and shorter than the time between two pulses arriving from spatially adjacent Bragg grating sensors of array  18 . 
     The timing of the optical modulator is determined by a timing signal derived from the pulses from the optical source  12 . The timing signal may be generated by the optical detector  44  and passed to the pulse read out unit  20  through path  46 . The signal may also be generated directly at the optical source  12 . For example, if the optical source  12  is pulsed directly using an electrical control signal, then this signal may be used for timing by the pulse read-out unit  20 . 
     The timing signal is delayed in the pulse read-out unit  20  and used to trigger the optical gate. The delay is chosen so that only one pulse is allowed to pass through the optical gate for each pulse of pulse train  36  returning from the sensor array  18 . Thus, only the signal from one Bragg grating sensor will reach the wavelength detection unit  40 , and the wavelength detection can be performed as if only one Bragg grating sensor was being monitored. The wavelength detection unit  40  may be of any standard design that is suitable for measuring the sensor signal and interrogation of the optical pulse may be performed using techniques known in the art. 
     The operation of the pulse read-out unit  20  is more closely detailed in FIG.  3 . The pulse read-out unit  20  includes electronic delay generator  26  connected to short electrical pulse generator  24  which is connected to electro-optical modulator  22  that modifies the transmission of light in accordance with the electrical signal applied to it. 
     The train of pulses  36  along path  38  of the fiber is shown at the input to the optical modulator  22  in FIG.  3 . Each individual pulse has a central wavelength, denoted by λ B1 , λ B2  . . . λ Bn  corresponding to the Bragg grating wavelength of the sensor from which the pulse originates. By choosing a suitable delay of the trigger pulse with the electrical delay generator  26 , the short pulse generator can be triggered to produce an electrical pulse to the electro-optical modulator  22  when one of the pulses, for example the pulse containing λ B2 , is passing through the modulator. The gating of the optical pulses is demonstrated graphically by  23 . The top set of pulses in  23  shows the progression in time of the set of pulses  36 . The gating action of the modulator is shown below these pulses. The gating is synchronized with the pulses containing λ B2 . Below the gating pulses, the selected optical pulses are shown containing only λ B2 . The short pulse generator  24  produces a very short electrical pulse that is wider than the temporal width of the pulse to be gated. It is found that if the pulse from the optical source  12  is several picoseconds or less in temporal duration, then the reflected pulses typically have a temporal width of fifty to a hundred picoseconds. The temporal gate width of the optical modulator  22  should be slightly larger than the width of the pulse, however the lower limit may be restricted by the dynamic response of the modulator or the speed of the electrical pulse generator  24  that produces the gating signal  50 . Typical gating times may be from five hundred to a thousand picoseconds. The optical modulator  22  can be implemented, among other methods known in the art, by a Mach-Zehnder integrated optic modulator that is controlled through the electro-optic effect or by a semiconductor electro-absorption modulator. 
     The process described above is repeated at the repetition rate of the optical source  12  so that only the pulse from one Bragg grating sensor is allowed to pass through the modulator  22  for each pulse launched into the system. This is shown in FIG. 3 by the single pulse  54  that exits from the modulator  22  for the train of pulses incident on the modulator  22 . A train of pulses will then arrive at the wavelength detection unit  40  at the repetition rate of the optical source  12 . This repetition rate is made to be greater than the electrical bandwidth of the wavelength detection unit  40 . The lower bandwidth of the detection electronics will make the train of pulses appear as a continuous signal that varies at the rate of perturbations to the Bragg grating sensors. The average level of detected signal is given by the average optical power from the pulse read-out unit. In this way, the wavelength detection unit effectively is decoding a sensor signal as if there was only one sensor in the system. Thus, any one of the numerous methods known in the art for signal decoding a single Bragg grating sensor may be used. 
     Different sensors may be monitored by altering the pulse  54  that is selected by the pulse read-out unit from the train of pulses  36  corresponding to each Bragg grating sensor in array  18 . This selection is achieved by altering the delay in the electrical delay generator  26  so the gating pulse  50  is applied to the optical modulator  22  when the desired pulse passes through the modulator. 
     The gating pulse  50  is made to be slightly longer than the optical pulses returning from the sensors. The time between pulses from the optical source will typically be much longer then the gating time. For example if the length of the sensor array  18  is made to be a hundred meters and the gating time was 1 nanosecond, then the optical gate would be open 0.1% of the time. This enables the sensor system to reject a large portion of unwanted signals from sensor array  18 . Such unwanted signals include multiple reflections between grating sensors, reflections from fiber splices and other components and noise from the optical source that may be caused by a small continuous light output in addition to the pulsed output. In this way, the pulse read-out system  20  helps to reject erroneous signals from the sensor array  18 . 
     It is to be noted that electrical noise in the gating pulse  50  does not affect the sensor reading. Variations in the gating pulse amplitude will cause variations in the optical signal at the output of the pulse read-out unit  20 , but will not affect the spectral content of the optical signal. Therefore the sensor information can still be recovered despite imperfections in the high speed gating pulse. 
     FIG. 4 shows the result of the operation of the pulsed read-out unit with a time multiplexed sensor array using two Bragg grating sensors. These figures show the optical spectrum from the sensor array as obtained on a standard optical spectrum analyzer. The optical spectrum from the sensor array without the pulse read-out system is shown in FIG.  4 A. In FIG. 4A there are clearly two peaks corresponding to the reflection from the two sensors and some background optical signal. With the use of the pulse read-out system only the optical spectrum from the first Bragg grating sensor in the sensor array is seen at the spectrum analyzer as shown in FIG.  4 B. In FIG. 4C the delay of the gating pulse is set so that the spectrum analyzer only measures the spectrum from the second Bragg grating sensor. The pulse read-out unit allows one to identify and isolate the sensor information from each of the Bragg grating sensors. 
     FIG. 5 illustrates a method of using the pulse read-out unit to identify each of the sensor gratings to determine their location in the sensor array and to choose the correct delay to read-out each sensor. An arbitrary starting delay is chosen for the delay generator  26  of FIG.  3 . The value of the delay, denoted by the τ axis of FIG. 5 is swept from the starting point given by τ equal to zero to the time for one repetition of the optical source. The optical power at the output of the optical modulator  22  in FIG. 3 versus the delay τ reveals the pulse response of the sensor array. By calibrating the distance along the sensing fiber that the optical signal will travel for a given delay τ, the physical location of each sensor may be determined. Therefore the gratings may be placed in the sensor without detailed knowledge of their positions. By determining the positions of each sensor, and by calculating their Bragg wavelengths, the effects of cross talk due to multiple reflections may also be reduced since the occurrences of multiple reflections can be predicted if the configuration and state of the sensor array is known. 
     An alternative embodiment of the invention is shown at  80  in FIG.  6 . The operation of the pulse read-out system  80  in FIG. 6 is similar to the system  20  of FIG. 3 except a low frequency modulating signal  82  is multiplied with the timing signal to the modulator at junction  84 . This junction  84  may be placed before the delay generator  26  as shown or between the delay generator  26  and the pulse generator  24  (not shown). The modulating signal  82  alternately turns the timing signal on and off at a rate of a few kilohertz. This allows the output from the pulse read-out unit  80  to be modulated at the same rate. The modulation signal  82  is also passed to the wavelength detection unit for reference. The modulation allows for synchronous detection to be used in measuring the sensor signal. Synchronous detection permits the system to obtain higher sensitivity by rejecting noise such as the dark current from optical detectors and noise in electrical amplifiers. 
     The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.