Abstract:
A fiber optic fault detector and generic fiber optic sensor system for detecting breaks in an optical fiber using a low coherence interferometric technique. The system comprises a light source configured to produce light traveling along the optical path, a modulator optically coupled to the light source configured to modulate at least a portion of the light as a function of a modulation signal, a detector optically coupled to the modulator configured to produce a detector output based upon a sensed intensity of the light, and an electronic array configured to receive the detector output and determine the optical fault. The low coherence interferometric technique allows for detection of a fault in the fiber with a minimal amount of test equipment and with higher measurement sensitivity and resolution. The system may alternatively include a transducer, positioned in place of the fiber under test, having a response which changes in reflective or optical path length. The system can be used in a LIDAR system, wherein telescope optics are used in place of the fiber under test, to transmit light and collect light scattered from objects or from the air.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates generally to optical sensors. More particularly, the invention relates to interferometric sensors for determining optical path length. 
     BACKGROUND OF THE INVENTION 
     As fiber optics become more prevalent, various types of optical sensors have become increasingly common. Indeed, various types of sensors can be used to detect fiber lengths, locations of breaks, cracks or inconsistencies in optical fibers, temperature, pressure, fiber expansion, attributes of chemical species, etc. 
     Optical fibers may be subjected to various external effects that produce geometric (e.g., size, shape) and/or optic (e.g., refractive index, mode conversion) changes to the fiber depending upon the nature and the magnitude of the perturbation. While these effects are often considered to be parasitic (i.e. noise-causing) in communications applications, the response of the fiber to external influence may be increased in sensing applications so that the resulting change in optical characteristics can be used as a measure of the external effect. Therefore, optical fibers may act as transducers that convert effects such as temperature, stress, strain, rotation or electric and magnetic currents into corresponding changes in optical effects. 
     Since amplitude or intensity, phase, frequency, and polarization typically characterize light, any one or more of these parameters may undergo a change due to external effects. The usefulness of the fiber optic sensor therefore depends upon the magnitude of this change and upon the ability to measure and quantify the change reliably and accurately. 
     Different types of sensors based upon fiber optic technologies are known. Among such sensor technologies are interferometers, which typically detect various phenomena by sensing phase changes or interference patterns between multiple optical signals passing through the sensor. In fact, interferometers can be used to determine distance, slope, rotation, and the like. Specifically, since about 1980, interferometric fiber optic gyroscopes (IFOGs) have been widely used to detect rotation, because such sensors have proven to be particularly useful for generating inertial navigation data that can be used to guide aircraft, automobiles, downhole drilling apparatus, and robots. Various embodiments of IFOGs are generally described in U.S. Pat. Nos. 6,211,963 and 6,175,410, which are incorporated herein by reference. In addition, techniques for sensing proper frequency used in conjunction with IFOGs are generally described in U.S. Pat. No. 5,734,469, which is incorporated herein by reference. 
     In practice however, interferometers are often complex and difficult to design and manufacture, and interferometers are typically not suitable for low-cost applications such as fiber optic length sensors. Thus, the present invention solves this problem by presenting a relatively simple and low-cost interferometric sensor that is accurate, has a high resolution, is useful for a variety of applications. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a sensor for determining a length of an optical path, comprises a light source, a modulator configured to direct light along said optical path, and an electronic system. The modulator is optically coupled to the light source, and the modulator is configured to modulate at least a portion of the light as a function of a modulation signal. The detector is optically coupled to the modulator and is configured to produce a detector output based upon a sensed intensity of the light at the end of the optical path. The electronic system is configured to receive the detector output, whereby the optical path length is determined by the detector output. 
     In accordance with another aspect of the invention, a method of determining a length of an optical path comprises the following: generating a light along the optical path; splitting the light into a first beam and a second beam; modulating at least one of the first beam and the second beam in response to a modulation signal to induce a difference between the first beam and the second beam; re-combining the first beam and the second beam to generate a recombined signal; detecting an output intensity of the recombined signal at a detector; adjusting the modulation signal as a function of the output intensity; and computing the length of the optical path as a function of the modulation signal. 
     In accordance with a further aspect of the invention, a system for determining the length of an optical fiber under test comprises a low coherence white light source configured to send light along an optical path defined by an optical fiber, a phase modulator optically coupled to the light source for modulating at least a portion of the light in a first path relative to a second path, a detector optically coupled to the optical path for producing a detector output based upon a length of the optical path, and a processor for receiving the detector output and for producing an output based upon the length of the optical path, wherein the light source has a coherence length shorter than a difference in path length between the first and the second paths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
     FIG. 1 is a block diagram of an exemplary sensor according to the present invention; 
     FIG. 2A is a block diagram of a second exemplary embodiment of a sensor according to the present invention; 
     FIG. 2B is a block diagram of a third exemplary embodiment of a sensor according to the present invention; 
     FIG. 2C is a block diagram of a fourth exemplary embodiment of a sensor using birefringence modulation in polarization modes according to the present invention; 
     FIG. 3 is a plot of various performance characteristics for an exemplary sensor according to the present invention; 
     FIG. 4 is a plot of various exemplary modulation signals at a proper frequency according to the present invention; 
     FIG. 5 is a plot of various exemplary modulation signals that are not at a proper frequency; and 
     FIG. 6 is a plot of various performance characteristics for an exemplary sensor that is not operating at the proper frequency. 
    
    
     DETAILED DESCRIPTION 
     The present invention may be described herein in terms of functional block components and various processing steps. It should be appreciated that such functional blocks may be realized by any number of hardware and/or software components configured to perform the specified functions. For example, the present invention may employ various integrated circuit or optical components, e.g., memory elements, processing elements, logic elements, look-up tables, and the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. Similarly, the software elements of the present invention may be implemented with any programming or scripting language, such as C, C++, Java, or Assembly, with various algorithms being implemented with any combination of data structures, objects, processes, routines or other programming elements. 
     Further, it should be noted that the present invention could employ any number of conventional techniques for electronics configuration, optical configuration, signal processing, and data processing. 
     It should be appreciated that the particular implementations shown and described herein are examples of the invention and are not intended to otherwise limit the scope of the present invention in any way. For the sake of brevity, conventional electronics, optics, software development and other functional aspects of the present invention, and components of the individual operating systems of the invention, may not be described in detail herein. 
     Moreover, no item or component is essential to the practice of the present invention unless the present apparatus and method claim elements are specifically described herein as essential or critical. 
     According to various exemplary embodiments of the present invention, a fiber optic sensor and its associated method of operation, is produced that provides a highly reciprocal light path for two or more light beams in an interferometer. Indeed, the paths taken by the various beams propagating through the optical portion of the sensor may be identical, except for a portion of the optical circuit that induces a modulation between the beams. In various embodiments, a modulation technique based upon the proper frequency may be sensitive to minute changes in the length of the optical path taken by the light in the interferometer. Such sensor may be useful in a variety of applications including, a fiber break tester, a LIDAR system, an optical transducer circuit, or in any number of pressure, temperature or chemical sensing applications. 
     In addition, various conventional techniques such as manufacturing techniques, modulation techniques and signal processing techniques, used in conjunction with interferometric sensors (such as IFOGs), may be used in conjunction with the present invention. Moreover, bulk optics components such as couplers and the like, may be substituted for any of the components described herein. 
     FIG. 1 is a schematic of a first exemplary embodiment of an interferometric sensor. 
     With reference to FIG. 1, an exemplary sensor system  100  includes a light source  102 , a first optical coupler  104 , a photodetector circuit  108  that may be coupled to a photodiode  106 , an integrated optics chip (IOC)  107 , a second optical coupler  120 , an optional delay loop  122 , an interface  124  and a suitable electronic system  126  providing a sensor output  130 . Light generated by light source  102  suitably passes through sensor  100  to a device  150 , such as an optical fiber under test, a telescopic lens, or any other device coupled to interface  124 . 
     Integrated optics chip (IOC)  107  may include a Y-junction  110  and one or more phase modulators  116 ,  118 , as shown in FIG.  1 . In such embodiments, Y-junction  110  separates light into two components traveling on paths  112  and  114 , respectively that may be individually modulated to induce phase differences between the two beams. The separated beams may be re-combined at coupler  120  so that the only non-reciprocal portion of sensor  100  is that portion between Y-junction  110  and coupler  120 . 
     After light is provided to the sensing device coupled to interface  124 , reflected light from the device may be returned through the optical portion of sensor  100  to detector  106 , which produces a signal indicative of the phase difference between the separate beams as appropriate. This phase difference may be observed and processed by electronic system  130  to determine the total path length of the light traveling through the device. This phase difference may also be used to compute a modulation signal  128 , as described more fully below. 
     Light source  102  is any device capable of producing light in sensor  100  such as a laser diode (LD), a light-emitting-diode (LED), a super-luminescent diode (SLD), or the like. 
     Although coherent light or light having any coherence length could be used, various types of light source  102  produce white light with a relatively low coherence length, which is typically on the order of several hundred microns or less, to create desired interference patterns at detector  106 , as described more fully below. Light generated by light source  102  is split into at least four components in sensor  100  corresponding to: (1) path  112  out, path  112  returning; (2) path  112  out, path  114  returning; (3) path  114  out, path  112  returning; and (4) path  114  out, path  114  returning. 
     If the coherence of light source  102  is properly selected such that the coherence length of light source  102  is significantly shorter than the differential path length between paths  112  and  114 , then only the interference of paths (2) and (3) above will produce a desired signal at photodetector  108 . The specific bandwidth of light source  102  is relative to the particular application, but in various exemplary embodiments light source  102  is a fiber light source, laser diode (LD), or super-luminescent diode (SLD). Light source  102  is coupled to optical f 5  fiber  136  through any conventional technique. 
     Optical fibers  132 ,  134 ,  136 ,  138  interconnecting the various components in sensor  100  may be any sort of optical fiber capable of directing light between the components. 
     In another embodiment, the optical fibers are single mode fibers capable of directing a single optical mode, such that various filters are not required in sensor  100  to isolate desired modes for signal processing. Optical fibers may also be polarization maintaining fibers or polarizing fibers, particularly in embodiments that do not include a polarizer in the optical circuit such as the embodiment shown in FIG.  1 . If polarization maintaining optical fiber is not used, various alternate embodiments might include an optical polarizer anywhere in the optical circuit such as IOC  107  or between coupler  104  and IOC  107 . 
     Couplers  104 ,  120  may be any coupling devices capable of joining optical signals propagating on separate fibers. Exemplary couplers include conventional 2×2 couplers, such as ones available from the Sifam Instruments, Ltd. of Devon, England. Alternatively, fibers  136 ,  138  and  132 ,  134  may be joined to form a coupler by stripping the cladding off of each fiber in the relevant position for the coupler, placing the two fiber cores together, and melting the cores together with the application of heat and/or tensile pressure. Light entering couplers  104 ,  120  from either port in a first direction are divided into two portions, with each portion exciting the coupler on a port on the opposite side of the coupler. In another embodiment, the light is split approximately equally between the two opposing ports. In yet other embodiments, one of the ports receives more or even all of the light passing through the coupler. 
     The IOC  107  includes a Y-junction and at least one modulator  116 ,  118 . In another embodiment, IOC  107  is formed from lithium niobate (LiNO 3 ) or another material that affects the speed of light in response to an applied electric potential. Alternatively, IOC  107  may be any conventional optical splitter/modulator combination, such as a model #SG-150-11=k IOC available from JDS Uniphase Corporation of San Jose, Calif. IOC  107  suitably includes a waveguide, shown as a solid line in FIG. 1, for guiding light from source  102  through the chip. The path may include a Y-junction  110  that splits light from coupler  104  into two paths  112  and  114 . The Y-junction  110  may also re-combine light received upon paths  112  and  114 , as appropriate. 
     One or more optical phase modulators  116 ,  118 , which may be implemented as electrodes in IOC  107  near paths  114 ,  112 , may be provided to produce phase shifts in light passing through paths  114 ,  112 , respectively, in response to modulation signals produced by electronic system  130 . In various alternate embodiments and as described more fully below, IOC  107  may be replaced with different but equivalent components such as couplers, splitters, modulators, such as piezoelectric modulators, etc. 
     Optional delay loop  122  may be included in various embodiments. Delay loop  122  may be a physical loop or coil of optical fiber that adds to the optical path length traveled by light in sensor  100 . 
     Interface  124  is any interface to a device  150  being sensed. For example, device  150  can be a lens arrangement, such as a telescopic lens, used with a LIDAR system, or interface  124  could be an interface to an external optical fiber under test to detect a fault, or as a fiber length measurement sensor. Interface  124  may be a mere fiber splice, or interface  124  can be omitted in embodiments where sensor  100  is formed as an integral part of an optical fiber. 
     Photodetector  108  may be any circuit capable of detecting the amplitude or intensity of light emanating from fiber  138 . In various embodiments, photodetector circuit  108  suitably includes a photodiode or avalanche photodiode  106  that conducts an electric current in response to the intensity of incident light. Photodetector circuit  108  may also include circuitry or other components to generate a digital or analog signal provided to electronic system  130 , as appropriate. Numerous conventional photodetector circuits  108  have been developed for use with fiber optic gyroscopes or other sensors that may be applicable to sensor  100 . In another embodiments, photodetector  108  is a model PN 03000040-999 photodiode available from the Epitaxx Corporation of West Nepian, Ontario, Canada. 
     Photodetector  108  response may be dependent upon the wavelength of incident light, so photodetector  108  may be selected to correspond to the wavelength of light propagating through sensor  100 . 
     Electronic system  126  includes processing circuitry suitable for calculating sensor output  130  and feedback signal  128 , can be a microprocessor, a microcontroller, a digital signal processor, a programmed array logic (PAL), an application specific integrated circuit (ASIC), or other such device. Electronic system  126  suitably includes a digital signal processor, which will typically be provided in conjunction with an associated memory and circuitry for addressing, input/output. 
     Electronic system  126  integrates, filters and processes the output of photodetector  108  to produce an output signal  130 . It should be appreciated that even though FIG. 1 shows sensor  100  operating as a feedback driven or “closed loop” sensor, alternate embodiments may use an “open loop” (i.e., no feedback) configuration that generates modulation signal  128  without regard to the output of photodetector  108 . However, while closed loop operating may add to the stability and resolution of sensor  100 , closed loop operating may be more complex than open loop operation in many embodiments. Thus, various embodiments of electronic system  126  could be readily adapted for use with the present invention. 
     The sensor system  100  of the present invention functions by passing light generated from light source  102  through coupler  104  to IOC  107 . The light is split by Y-junction  110  into a beam passing through waveguide  112  and a beam passing through waveguide  114 . At least one of the beams is modulated by phase modulator  116  in response to modulation signal  128 , as described more fully below, to create a shift in the phase of the light beam. The two beams are re-combined at coupler  120 , where light is passed through optional delay loop  122  to interface  124  and onto the sensed device  150 . Light reflected from the sensed device reenters sensor  100  at interface  124 , where the light passes through optional delay loop  122  before being split at coupler  120 . The reflected light is split into a component passing on fiber  132  and waveguide  112 , and a component passing on fiber  134  and waveguide  114 , where a modulation may be applied by modulator  116 . The two components are suitably rejoined at Y-junction  110  and passed through coupler  104  to detector  108 . 
     As noted above, light generated by light source  102  is split into at least four components corresponding to: (1) path  112  out, path  112  returning; (2) path  112  out, path  114  returning; (3) path  114  out, path  112  returning; and (4) path  114  out, path  114  returning. It should be noted that the two components of light passing through path (2) and light passing through path (3) will traverse identical distances. Moreover, light passing on these paths will receive identical modulations from modulator  116 , although the modulation will be shifted in time by an amount related to the time delay for the beam to pass through the sensed device. Hence, the difference in modulations applied to the two beams is due to the time delay, which is related to the length of the device. By adjusting the modulation applied to account for this delay, the length of the path traversed by the beams can be calculated. This path length can be used to determine the length of an optical fiber or the location of a break in an optical fiber. Alternatively, the path length could be used in any other type of sensor such as a LIDAR system, or transducer sensor. 
     The basic concepts described above and below may be applied to any number of equivalent devices that produce a sensor output  130  based upon a sensed interference of two light signals. 
     FIGS. 2A,  2 B and  2 C are schematics of exemplary alternate embodiments of sensors. 
     With reference to FIG. 2A, various embodiments of sensor  100  may eliminate coupler  120  and replace IOC  107  shown in FIG. 1 with a modified IOC  107  as shown in FIG.  2 . With reference to FIG. 2B, IOC  107  is eliminated altogether and replaced with a coupler  202  and a phase modulator  204 . Phase modulator  204  may be a piezoelectric modulator or any other type of phase-modifying device. Coupler  202  is any conventional coupler such as described above in conjunction with couplers  104  and  120 . Moreover, it is not necessary to modulate the phase of light passing through path  112  to produce an appropriate output signal  130 . With reference to FIG. 2C, IOC  107  is replaced with a polarizer  230 , a birefringence modulator  232 , and a depolarizer  234 . In such embodiments, the polarization of light passing through modulator  232  is modulated by signal  128 , using the techniques described herein, to produce a transverse magnetic (TM) mode and a transverse electric (TE) mode having different (e.g. orthogonal) polarizations. Interference between the TM and TE signals can then be detected at photodiode  106 , as described above. 
     Polarizer  230  is nominally shown in FIG. 2C as a 45-degree polarizer, although any angle of polarization except zero or ninety degrees could be used. Sensor  100  may also include a portion  236  of polarization-maintaining fiber to transmit the two modes from modulator  232  to depolarizer  234 . 
     Hence, an interference pattern may be produced at detector  106  even though sensor  100  includes but a single physical path  112 ,  114 . The terms “splitter” or “splitting means” as used herein, may refer not only to a fiber splitter but also to any structure that creates two or more optical paths. Other embodiments of sensor  100  modulate signal amplitude, frequency or other characteristics of light passing through the sensor to produce distinct but interfering light paths or modes. Any of the components described herein as part of sensor  100  may be replaced by equivalent bulk optics components such as modulators, couplers, and the like. 
     With reference now to FIG. 3, an exemplary output characteristic  300  for a sensor operating at a proper frequency is shown. With reference now to FIG. 3, an interferogram  302  suitably plots the intensity of light impinging on photodetector  108  versus the phase shift observed between the two light beams propagating in sensor  100 . The intensity of the light is suitably maximized when the light beams are in phase, such as corresponding to zero phase shift or any integer multiple of +/−2π phase shift. Similarly, the intensity of the light is minimized when the light beams are out of phase, in correspondence to a π phase shift, or any odd integer multiple of +/−π. When the sensor operates near a maximum or minimum point on interferogram  302 , however, changes in phase (Δφ) produce only small changes in the intensity of light (I). 
     Moreover, it may be difficult to detect the magnitude of changes in phase from the intensity of light at such operating points, since the curve decreases in both directions departing from the maximum points and increases in both directions departing the minimum points. Various embodiments may therefore bias the gyro to a more sensitive operating point, such as point  310  or point  312  on interferogram  302 , which correspond to phase shifts of π/2 and −π/2, respectively. Of course any odd integer multiple of +/−2π would produce a similar result. 
     This modulation may be produced with modulation  304 , which corresponds to modulation signal  128  in FIG.  1  and FIG. 2, as described more fully below in connection with FIG.  4 . FIG. 3 shows bias modulation  304  is an alternating bias signal that produces phase biases of +/−π/2 radians between the two beams propagating in sensor  100 . It will be appreciated that any modulation  304  could be provided according to the particular characteristics and needs of the particular embodiment. 
     As the two beams are biased in accordance with modulation  304 , then the output intensity of light incident upon detector  108  over time may be as shown in plot  306  in FIG.  3 . Plot  306  shows that the output intensity of the light (I) is relatively constant at the level  316  corresponding to points  310  and  312  on interferogram  302 , with momentary spikes  314  resulting from the transition in operating points from point  310  to point  312 , and vice versa. Level  316  may also include a component from the two paths, paths  112  and  114 , that do not interfere. Spikes  314  may be filtered, ignored, or otherwise processed by an electronic system  126  shown in FIG. 1, such that the relatively constant output level  316  may be observed. 
     FIG. 4 is a plot of an exemplary modulation technique applied at a proper frequency that may be used to generate modulation  304 . With reference to FIGS. 1 and 4, bias modulation signal  128  is generated by electronic system  126  and provided to modulator  116  to modulate light traveling on waveguide  114 . Each beam of light involved in creating interferogram  302 , shown in FIG. 3, passes through waveguide  114 , but at a different time (e.g. a first beam passes through waveguide  114  on the way to the sensed device, the second beam passes through waveguide  114  after being reflected by the sensed device). The modulations applied to the two beams, then, are suitably identical but shifted in time according to a delay constant (τ) of the sensor, which is related to the path length of the light beam. 
     With reference to FIG. 4, an exemplary modulation signal  128  is a sawtooth waveform having an amplitude tailored to modulator  116 , such that the desired phase shift is produced, and having a frequency that is tuned to the proper frequency of sensor  100 . Any technique for sensing proper frequency could be used with the sensors disclosed herein. Similarly, modulation signal  128  may be any digital or analog serrodyne, triangle, ramp, dual ramp, pulse, step or other waveform, as appropriate, or may incorporate characteristics of multiple waveforms. 
     As shown in FIG. 4, the modulations applied to the two beams counter-propagating in sensor  100  are identical but shifted in time by delay constant τ. The difference between these two signals is shown as signal Δφ  304 , which corresponds to signal  304  in FIG.  3 . Any modulation signal  128  that produces a desired phase modulation  304  may be used in various embodiments of sensor  100 . 
     FIG. 5 is a plot of an exemplary modulation technique that is not applied at a proper frequency. A modulation signal  128  is applied to modulator  116 , but the frequency of signal  128  is not tuned to a proper frequency related to the delay constant τ. Hence, the difference in phase (Δφ)  304  between beam  1  and beam  2  does not produce a balanced phase modulation signal like that described above. Rather, the difference  304  between the two beams may be characterized by relatively long periods of bias at level  502  interspersed by relatively short periods corresponding to time τ of bias  504  in an opposite direction and with much larger magnitude than level  502 . 
     An exemplary output characteristic  600  corresponding to the modulation Δφ  304  shown in FIG. 5 is shown in FIG.  6 . 
     In FIG. 6, modulation  304  applied to interferogram  302  produces an output characteristic  606  at a photodetector  108 . As shown in FIG. 6, points  504  on modulation  304  correspond to point  604  on interferogram  302  and output plot  606 . Points  502  on modulation  304  correspond to points  602  on interferogram  302  at output plot  606 . Hence, the light intensity observed at photodetector  108  suitably alternates between levels  602  and  604 . 
     By comparing and contrasting plot  606  with plot  306  in FIG. 3, it will be appreciated that the light intensity incident upon photodetector  108  is dependent upon the frequency of the modulation signal, and that the proper frequency of the modulation signal, such as the frequency that produces a relatively constant output at photodetector  108 , is dependent upon the time that light takes to pass through sensor  100 . It therefore follows that the proper modulation frequency is related to the length of the light path. Consequently, the output  130  may be determined from the modulation that results in relatively constant output at detector  108 , or in any other suitable desired effect upon the detector output. Stated another way, the length of an optical path can be readily determined as a function of the proper modulation frequency that produces a relatively constant output at detector  108 . 
     Adjustments to modulation signal  128  may be performed by a microcontroller, microprocessor, digital signal processor or other controller associated with electronic system  126 . 
     The detector output is sampled at a frequency at least as high as the frequency of modulation signal  128  such that changes in the detector output may be identified. 
     As the frequency of modulation signal  128  approaches the proper frequency for the length of the optical path, changes in the detector output are suitably reduced. When an exemplary sensor  100  is modulated at a proper frequency, the output characteristic appears as in FIG.  3 . When the sensor is modulated at a frequency that is not proper for the particular path length, output characteristics observed at detector  108  may be skewed, as shown in FIG.  6 . By attempting to maintain the detector output at a desired level, the length of the optical path traveled by the light in sensor  100  may be readily calculated from said proper frequency using a lookup table, mathematical formula, or other technique. This concept may be used to create various sensor devices such as LIDARS, break or fault testers, fiber length testers, range finders, or the like. 
     Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.