Patent Publication Number: US-2013229649-A1

Title: Optical brillouin sensing systems

Description:
BACKGROUND 
     1. Field 
     The present specification generally relates to optical sensing systems and, more specifically, to optical Brillouin sensing systems for interacting with sensing optical fibers. 
     2. Technical Background 
     Distributed sensors based on Brillouin scattering are attractive for forming optical fiber sensing systems used to measure the structural integrity of buildings, bridges, tunnels, dams and pipelines, as well as ships and airplanes. The most popular Brillouin optical fiber sensing system is Brillouin Optical Time Domain Reflectometry (BOTDR). This technique is similar to Rayleigh-based OTDR, where spontaneous Brillouin light backscattered from an intense pulse is recorded as a function of time. The frequency distribution of the backscattered signal is measured for each time step to determine a strain or a temperature change at each location. Like a conventional OTDR, a BOTDR requires access to a single fiber end only, which is convenient for many applications. 
     Another optical fiber sensing system utilizes Brillouin Optical Time Domain Analysis (BOTDA). This technique takes advantage of the Stimulated Brillouin Scattering (SBS) based on a pump-probe technique wherein an intense pump pulse interacts locally during its propagation with a weak counter-propagated continuous-wave (CW) probe. The gain experienced by the probe at each location can be analyzed by recording the probe amplitude in the time domain. The frequency difference between the pump and the probe is scanned step-by-step, and the local amplification can be retrieved for a given pump-probe frequency difference. The local gain spectrum can then be reconstructed by analyzing the gain at a given location as a function of frequency. BOTDA can require access to both optical fiber ends since the pump pulse and CW probe counter-propagate in the sensing fiber, which is a limitation in some situations. 
     High spatial resolution BOTDR/BOTDA fiber sensors with a long sensing range is desired by many applications. The spatial resolution of a BOTDR/BOTDA can be improved by using relatively short pump/probe pulses (i.e., short pump pulses for BOTDR, and pump/probe pulses for BOTDA). However, because the pulse repetition rate is correlated with the sensing range, shortening the width of the pulses, without changing the sensing range, will decrease the duty cycle of the pulse train. A shortened duty cycle can cause two issues for a BOTDR/BOTDA system. One is that the shortened the pulses can weaken the sensing signal. Moreover, the signal-to-noise ratio (SNR) of sensing optical signal can be degraded due to the finite extinction ratio of the pulses. Therefore, a tradeoff exists between spatial resolution and sensing sensitivity. 
     Furthermore, it is noted that a significant broadening and lowering of the Brillouin gain spectrum can be caused as the pulse width is reduced to the values comparable with the acoustic relaxation time (˜10 ns). Moreover, BOTDR or BOTDA sensitivity of Brillouin frequency can exhibit sensitivity to parameters that are not being measured. This sensitivity can lead to ambiguity in the measurement, as one does not know whether the observed Brillouin frequency shift is caused by the change of one parameter or another. 
     Accordingly, a need exists for alternative optical sensing systems for interacting with a sensing optical fiber. 
     SUMMARY 
     According to one embodiment, an optical sensing system for interacting with a sensing optical fiber may include a light source, an optical modulator, a gated optical amplifier, one or more triggering devices, a first optical coupler, a second optical coupler, and an optical detector. The optical modulator can be optically coupled to the light source. The optical modulator can receive at least a portion of the optical energy of the light source and can transform the optical energy that is received into a pulse signal comprising a baseline portion and a peak portion having a greater amplitude than the baseline portion. The gated optical amplifier can be optically coupled to the optical modulator having a lossy state that attenuates signal and a gain state that amplifies signal. The gated optical amplifier can receive at least a portion of the pulse signal of the optical modulator and can transform the pulse signal that is received into an amplified pulse signal having an amplified peak portion. The one or more triggering devices can be communicatively coupled to the optical modulator and the gated optical amplifier. The one or more triggering devices can transmit an amplifier trigger signal to the gated optical amplifier to control the gated optical amplifier such that the gated optical amplifier is in the lossy state while the baseline portion of the pulse signal is transformed and the gated optical amplifier is in the gain state while the peak portion of the pulse signal is transformed. The first optical coupler can be optically coupled to the gated optical amplifier. The first optical coupler can transmit the amplified pulse signal to the sensing optical fiber when the sensing optical fiber is connected to the first optical coupler. The second optical coupler can be optically coupled to the first optical coupler. The second optical coupler can receive a sensed optical signal from the sensing optical fiber when the sensing optical fiber is connected to the first optical coupler. The optical detector can be optically coupled to the second optical coupler. 
     In another embodiment, an optical sensing system may include a light source, a sensing optical fiber, an optical modulator, an optical coupler, and an optical detector. The light source can output optical energy. The sensing optical fiber can be optically coupled to the light source. At least a portion of the optical energy of the light source can be transmitted into the sensing optical fiber. The sensing optical fiber can generate a sensed optical signal from the optical energy that is received. The optical modulator can be optically coupled to the light source. The optical modulator can receive at least a portion of the optical energy of the light source and can transform the optical energy that is received into a pulse signal comprising a baseline portion and a peak portion having greater amplitude than the baseline portion. The optical coupler can be optically coupled to the optical modulator and the sensing optical fiber. The optical coupler can combine the pulse signal of the optical modulator and the sensed optical signal of the sensing optical fiber into a combined optical signal. The optical detector can be optically coupled to the optical coupler. The optical detector can receive at least a portion of the combined optical signal of the optical coupler. 
     In a further embodiment, a method for Brillouin based sensing may include transforming a first pulse signal comprising a first baseline portion and a first peak portion having a greater amplitude than the first baseline portion into an amplified pulse signal having an amplified peak portion with a gated optical amplifier. The gated optical amplifier can have a lossy state that attenuates signal and a gain state that amplifies signal. The gated optical amplifier can be controlled such that the gated optical amplifier is in the lossy state while the first baseline portion of the first pulse signal is transformed and the gated optical amplifier is in the gain state while the first peak portion of the first pulse signal is transformed. The amplified pulse signal can be transmitted into a sensing optical fiber. A sensed optical signal can be received with an optical coupler. The sensed optical signal can be emitted from a point of interest along the sensing optical fiber. A second pulse signal comprising a second baseline portion and a second peak portion having greater amplitude than the second baseline portion can be received with the optical coupler. The sensed optical signal can be received contemporaneously with the second peak portion of the second pulse signal. 
     Additional features and advantages of the embodiments described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts an optical sensing system according to one or more embodiments shown and described herein; 
         FIG. 2  schematically depicts an optical sensing system according to one or more embodiments shown and described herein; 
         FIG. 3  schematically depicts an optical sensing system according to one or more embodiments shown and described herein; 
         FIG. 4  schematically depicts an optical sensing system according to one or more embodiments shown and described herein; and 
         FIG. 5  graphically depicts radio frequency spectra of probe pulse according to one or more embodiments shown and described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One embodiment of the optical sensing system for interacting with a sensing optical fiber of the present disclosure is schematically depicted in  FIG. 1 , and is designated generally throughout by the reference numeral  10 . 
     Throughout the present disclosure, reference will be made to the term light. The term “light” as used herein refers to various wavelengths of the electromagnetic spectrum, particularly wavelengths in the ultraviolet (UV), infrared (IR), and visible portions of the electromagnetic spectrum. 
     Referring now to  FIG. 1 , the optical sensing system  10  generally includes a pulsed light assembly  20  for generating a first pulse signal  30  and a second pulse signal  40 . As used herein, the term “pulse” refers to a light signal having a baseline portion that is at a lower signal amplitude than a peak portion. Moreover, the peak portion of the each optical pulse is generally delineated by the full width at half maximum of the optical pulse. Accordingly, the pulsed light assembly  20  can generate a first pulse signal  30  comprising a first peak portion  32  and a first baseline portion  34 , and a second pulse signal  40  comprising a second peak portion  42  and a second baseline portion  44 . 
     Additionally, the optical pulses described herein generally include a rapid transient portion as the signal changes from the baseline portion to the peak portion. Accordingly, each pulse can include a substantially triangular waveform, a substantially square waveform, a substantially Gaussian waveform, or any other waveform having a peak that is distinguishable from a baseline. In some embodiments, it may be desirable for the baseline portion to be substantially equal to about zero amplitude. However, it is not necessary for the baseline portion to have an amplitude that is substantially equal to about zero. Furthermore, it is noted that the term “signal” means a waveform (e.g., electrical, optical, magnetic, mechanical or electromagnetic), such as a continuous wave, a pulsed wave, or the like, capable of traveling through a medium. 
     Referring still to  FIG. 1 , the optical sensing system  10  can comprise an optical circulator  50  for optically coupling the pulsed light assembly  20  with a sensing optical fiber  60 . The optical circulator  50  is an optical coupler that can be used to control bi-directional propagation of light. For example, the optical circulator  50  can include any number of ports having non-reciprocal optics, i.e., changes in the properties of light passing through one port to another port are not reversed when the light passes through in the opposite direction. Specifically, the optical circulator  50  can be a three-port optical circulator designed such that light entering any port exits from the next port. Accordingly, if light enters a first port it can be emitted from a second port. If some of the emitted light is reflected back to the second port, the reflected light can exit from a third port without exiting the first port. 
     The optical sensing system  10  may further comprise a coupler  52  for combining the second pulse signal  40  with a sensed optical signal  62 . For example, the coupler  52  can be a 50:50 2×2 coupler. Accordingly, the 2×2 coupler can be configured to receive two optical input signals, split each optical input signal into two output signals having about 50% of its respective input signal level, and combine one output signal corresponding to each of the optical input signals into a combined signal. The resulting combined signal would then be the superimposed combination of about 50% of each of the input optical signals. The ratio of the coupler  52  can be set to any desired level for a 2×2 coupler such as, for example, 90:10, 80:20 or any other ratio that sums into a value less than or equal to about 100. 
     In some embodiments, it may desirable to further include one or more optical isolators to control the propagation direction of light. Specifically, an optical isolator can be utilized to allow light to propagate in a forward direction for transmission or reception, while absorbing or displacing light propagating in the reverse direction to mitigate undesired feedback. Optical isolators can be polarization-dependent and polarization-independent throughout the electromagnetic spectrum. Thus, it should now be understood that, the optical sensing system  10  may include one or more optical couplers for transmitting optical signals, receiving optical signals, combining optical signals, splitting optical signals, or combinations thereof. Accordingly, while specific optical couplers are described herein, the functions performed by any of the optical couplers described herein can be replaced with an equivalent alternative combination of optical couplers such as, for example, an optical isolator, an optical circulator, optical splitter, an optical combiner, a coupler, or the like. 
     The optical sensing system  10  can comprise an optical detector  54  that is optically coupled to the coupler  52  for detecting the combined optical signal  64  from the coupler  52 . The optical detector  54  can be any device configured to detect light and transform the detected light into a signal indicative of a characteristic of the detected light (e.g., optical power). Accordingly, the optical detector  54  can include one or more photodetectors such as, for example, a photodiode, photoresistor, a phototransistor, or the like. In one embodiment, the optical detector  54  can include two matched photodetectors for coherent detection of two coherent combined signals from the coupler  52 . Without being bound to any particular theory, it is believed that coherent balanced detection can provide improved sensing sensitivity, and thus improved spatial resolution. It is noted that, the phrase “optically coupled,” as used herein, means that components are capable of exchanging light with one another via one or more intermediary mediums such as, for example, electromagnetic signals via air, optical signals via optical waveguides, optical signals via optical couplers, or the like. 
     Referring still to  FIG. 1 , the optical sensing system  10  can comprise a signal processor  56  that is communicatively coupled to the optical detector  54 . The signal processor  56  can include one or more processors, which can be any device capable of executing machine readable instructions. Accordingly, the processor can be a controller, an integrated circuit, a microchip, a computer, or any other computing device. The each processor can further include and/or be communicatively coupled to a memory such as, for example, RAM, ROM, a flash memory, a hard drive, or any device capable of storing machine readable instructions. As used herein, the term “communicatively coupled” means that the components are capable of exchanging data signals with one another such as, for example, electrical signals via conductive medium, electromagnetic signals via air, optical signals via optical waveguides, and the like. 
     Embodiments of the present disclosure can comprise logic for transforming a signal indicative of a characteristic of the detected light into a physical parameter (e.g., temperature or a strain) using Brillouin based analysis such as, but not limited to, Brillouin optical time domain analysis (BOTDA) and Brillouin optical time domain reflectometry (BOTDR). Accordingly, the logic can include machine readable instructions or an algorithm written in any programming language of any generation (e.g., 1GL, 2GL, 3GL, 4GL, or 5GL) such as, e.g., machine language that may be directly executed by the processor, or assembly language, object-oriented programming (OOP), scripting languages, microcode, etc., that may be compiled or assembled into machine readable instructions and stored on a machine readable medium. Alternatively, the logic or algorithm may be written in a hardware description language (HDL), such as logic implemented via either a field-programmable gate array (FPGA) configuration or an application-specific integrated circuit (ASIC), and their equivalents. Accordingly, the logic may be implemented in any conventional computer programming language, as pre-programmed hardware elements, or as a combination of hardware and software components. 
     As is noted above, the optical sensing system  10  can optionally include a sensing optical fiber  60 , i.e., some embodiments of the optical sensing system  10  can be integral with the sensing optical fiber  60 , selectively optically coupled to the sensing optical fiber  60 , or may not include any sensing optical fiber  60 . The sensing optical fiber  60  is generally a flexible, substantially transparent fiber that operates as an optical waveguide and can be made from a transmissive material such as silica. The sensing optical fiber  60  is generally configured to modulate the intensity, phase, polarization, wavelength, or transit time of light in the fiber when acted upon by the property that is to be detected by the sensing optical fiber  60 . Accordingly, the sensing optical fiber  60  can be utilized to measure strain, temperature, or pressure. The embodiments described herein, can include sensing optical fiber  60  configured to support any number of modes. Accordingly, the sensing optical fiber  60  can be a single-mode fiber (SMF), a few-mode fiber (FMF), or a multi-mode fibers (MMF). 
     Referring now to  FIG. 2 , an embodiment of an optical sensing system  110  that can be utilized for BOTDR is schematically depicted. The optical sensing system  110  comprises a pulsed light assembly  120  for generating a first pulse signal  30  and a second pulse signal  40 . The pulsed light assembly  120  can include a continuous wave laser  112  that operates as a light source for the optical sensing system  110  by emitting a substantially continuous signal of optical energy. A 1×2 optical splitter  114  can be optically coupled to the continuous wave laser  112  and configured to split the optical energy emitted by the continuous wave laser  112  into two portions. 
     The first portion  116  of the optical energy can be utilized as pump light. Specifically, a first optical modulator  122  can be optically coupled to the 1×2 optical splitter  114 . The first optical modulator  122  can convert the first portion  116  of the optical energy into a pump pulse signal  130  having a pump peak portion  132  and a pump baseline portion  134 . As used herein, the phrase “optical modulator” refers to a device that can be utilized to modulate a parameter of a light signal such as, but not limited to, amplitude modulators, phase modulators, polarization modulators, or the like. Suitable optical modulators include but are not limited to electro-optic modulators, acousto-optic modulators, magneto-optic modulators, mechano-optical modulators, or combinations thereof. Additionally, it is noted that while  FIG. 2  depicts external modulation, in some embodiments modulation can be performed by direct modulation of the light source. 
     The pulsed light assembly  120  may further include a gated optical amplifier  124  optically coupled to the first optical modulator in order to transform the pump pulse signal  130  into the first pulse signal  30 . The gated optical amplifier can be any optical device having a selectively activated lossy state that attenuates signal and a selectively activated gain state that amplifies signal. In some embodiments, the gated optical amplifier  124  can be a semiconductor optical amplifier. The semiconductor optical amplifier can be an electrically controlled optical amplifier that includes a semiconductor gain medium made from a direct band gap semiconductor such as, but not limited to, group III-V compounds (e.g., GaAs/AlGaAs, InP/InGaAs, InP/InGaAsP and InP/InAlGaAs). In further embodiments, the semiconductor optical amplifier can a have a lossy state with a very high loss such as, for example, a loss of greater than about 35 dB, or in another embodiment a loss of up to about 70 dB. 
     The pulsed light assembly  120  may further include one or more triggering devices  126  for controlling modulation and amplification. Specifically, the one or more triggering devices  126  can be communicatively coupled to the first optical modulator  122  and the gated optical amplifier  124 . The one or more triggering devices  126  can each be a signal generator configured to generate any desired triggering signal such as, for example, an electronic signal, or an optical signal. 
     Additionally, the one or more triggering devices  126  can be communicatively coupled to one or more delay controllers. Specifically, the one or more delay controllers can each be a processor that executes machine readable instructions to control the timing of each signal output from the one or more triggering devices  126 . Accordingly, the one or more triggering devices  126  can be configured to control the modulation of the first portion  116  of the optical energy into the pump pulse signal  130  (i.e., amplitude modulation). The one or more triggering devices  126  can also be configured to control the gated optical amplifier  124 . 
     Specifically, the one or more triggering devices  126  can transmit an amplifier trigger signal  140  to the gated optical amplifier  124  to control the gated optical amplifier  124  such that the gated optical amplifier  124  is in the lossy state while pump baseline portion  134  of the pump pulse signal  130  is transformed by the gated optical amplifier  124 , and the gated optical amplifier  124  is in the gain state while the pump peak portion  132  of the pump pulse signal  130  is transformed by the gated optical amplifier  124 . Accordingly, the gated optical amplifier  124  can be utilized to increase signal-to-noise ratio by amplifying the pump peak portion  132  of the pump pulse signal  130  and attenuating the pump baseline portion  134  of the pump pulse signal  130 . 
     In one embodiment, the delay controller can execute machine readable instructions to cause the one or more triggering devices  126  to transmit the amplifier trigger signal  140  to the gated optical amplifier  124  a gate delay time period after the pump peak portion  132  of the pump pulse signal  130  is transmitted from the first modulator. Specifically, the gate delay time period is substantially equal to the travel time required for the pump peak portion  132  of the pump pulse signal  130  to travel from the first optical modulator  122  to the gated optical amplifier  124 . When the one or more triggering devices  126  transmit a first modulation signal  142  to the first optical modulator  122  in order to control the modulation, the gate delay time period can be determined based upon the timing of the first modulation signal  142 . In further embodiments, the gate delay time period can be determined based upon detection of the pump pulse signal  130 . 
     The optical sensing system  110  may further comprise an optical coupler or an optical circulator  50  optically coupled to the gated optical amplifier  124 . Accordingly, when the sensing optical fiber  60  is optically coupled to the optical circulator  50 , the first pulse signal  30  can be transmitted into the sensing optical fiber  60 . Moreover, a sensed optical signal  62  can be generated within the sensing optical fiber  60  by the first pulse signal  30  and received by the optical circulator  50 . For example, the sensed optical signal  62  can be a back reflected Brillouin scattered signal. It is noted that, while the sensing optical fiber  60  is depicted as a SMF, the sensing optical fiber  60  can be a FMF or a MMF. 
     Referring still to  FIG. 2 , the second portion  118  of the optical energy can be utilized as local oscillator light. Specifically, the pulsed light assembly  120  may further comprise a second optical modulator  128  optically coupled to the 1×2 optical splitter  114 . The second optical modulator  128  can transform the second portion  118  of the optical energy into a second pulse signal  40  having a second peak portion  42  and a second baseline portion  44 . 
     In one embodiment, the delay controller can execute machine readable instructions to cause the one or more triggering devices  126  to transmit a local trigger signal  144  to the second optical modulator  128  in order to control parameters of the second pulse signal  40  such as the timing of the transmission of the second pulse signal  40  and the duration of the second peak portion  42  of the second pulse signal  40 . Specifically, the second optical modulator  128  can be controlled such that the second pulse signal  40  is transmitted a calculated time period after the first peak portion  32  of the first pulse signal  30  is transmitted from the gated optical amplifier  124 . Accordingly, a point of interest along the sensing optical fiber  60  can be detected by setting an appropriate calculated time period. The calculated time period can be based upon the round trip travel time needed for the first peak portion  32  of the first pulse signal  30  to travel from the gated optical amplifier  124  to the point of interest along the sensing optical fiber  60 . The round trip travel time can be equal to the sum of the time required for the first peak portion  32  of the first pulse signal  30  to travel from the gated optical amplifier  124  to the point of interest along the sensing optical fiber  60  and the amount of time required for the sensed optical signal  62  to travel from the point of interest along the sensing optical fiber  60  to the coupler  52  minus the travel time required for the second peak portion  42  of the second pulse signal  40  to travel from the second optical modulator  128  to the coupler  52 . It is noted that, while the calculated time period is described herein as being derived from the round trip travel time, the calculated time period can be set to any value such that the second peak portion  42  of the second pulse signal  40  arrives at the coupler  52  contemporaneous with the sensed optical signal  62  from the point of interest along the sensing optical fiber  60 . 
     Furthermore, it is noted that a range of detection along the sensing optical fiber  60  can be controlled by the duration of the second peak portion  42  of the second pulse signal  40 . For a relatively long duration of the second peak portion  42  a relative large length of the sensing optical fiber  60  can be detected, and for a relatively short duration of the second peak portion  42  a relative small length of the sensing optical fiber  60  can be detected. For example, a pulse width of about 100 ns can correspond to a range of about 10 m. 
     The optical sensing system  110  can comprise an optical detector  54  comprising balanced photodetectors. The coupler  52  can combine the second pulse signal  40  and the sensed optical signal  62  into a first combined signal  164  and a second combined signal  166 . The first combined signal  164  and the second combined signal  166  can each include a superposition of about 50% of the second pulse signal  40  and about 50% of the sensed optical signal  62 , i.e., the coupler  52  can be a 2×2 coupler. Accordingly, the output of the coupler  52  can be detected using a balanced photodetector. Once detected, the optical detector  54  can transform the first combined signal  164  and the second combined signal  166  into a data signal. The data signal can then be transmitted to the signal processor  56  and processed according to Brillouin based analysis algorithms. 
     As is noted above, it is believed that coherent balanced detection can provide improved spatial resolution. For example, in an embodiment optical detector  54  comprises balanced photodetectors and transforms light into an output photocurrent, the photocurrent of the balanced photodetectors can be expressed as: 
         I= 2 R √{square root over ( P   s   P   LO )} cos [2π(ν s −ν LO ) t +(φ s −φ LO +π/2)],  (1)
 
     where R is the balanced photodetector responsivity, P s  is the optical power level of the sensed optical signal, P LO  is the optical power level of the second pulse signal, ν s  is the optical frequency of the sensed optical signal and ν LO  the optical frequency of the second pulse signal, and φ s  is the phase of sensed optical signal, and   LO  is the phase of the second pulse signal. From Eq. 1, the photocurrent of the balanced photodetectors can be increased by increasing the optical power level of the second pulse signal. Such an increase in photocurrent can enhance spatial resolution, sensing distance, sensitivity, acquisition time, or a combination thereof with BOTDR and/or BOTDA. 
     Referring now to  FIG. 3 , an embodiment of the optical sensing system  210  that can be utilized for BOTDA is schematically depicted. The optical sensing system  210  comprises a pulsed light assembly  220  for generating a first pulse signal  30 , a second pulse signal  40 , and a continuous wave light signal  218 . The pulsed light assembly  220  is similar to the pulsed light assembly  120  ( FIG. 2 ), with the addition of components configured to emit the continuous wave light signal  218 . 
     Specifically, the pulsed light assembly  220  may further comprise a wavelength shifter  212  such as, for example, an optical phase modulator or a single sideband electro-optic modulator, for shifting the frequency of the second portion  118  of the optical energy. The frequency shift can be set to be about equal to the Brillouin frequency shift induced by the sensing optical fiber  60 . In one embodiment, the wavelength shifter  212  can be optically coupled to the 1×2 optical splitter  114 , such that the second portion  118  of the optical energy is transformed into a frequency shift signal  214 . The wavelength shifter  212  can also be optically coupled to a 1×2 optical splitter  216 , such that frequency shift signal  214  is divided into the continuous wave light signal  218  and a second shifted frequency signal  222 . 
     The 1×2 optical splitter  216  can be optically coupled to a first end  66  of the sensing optical fiber  60 , such that the continuous wave light signal  218  is transmitted into the first end  66  of the sensing optical fiber  60 . Additionally, the optical circulator  50  can be optically coupled to a second end  68  of the sensing optical fiber  60 , such that the first pulse signal  30  is transmitted into the second end  68  of the sensing optical fiber  60 . The 1×2 optical splitter  216  can also be optically coupled to the second optical modulator  128 , such that the second shifted frequency signal  222  is transformed by the second optical modulator  128  into the second pulse signal  40 . The detection and operation of the optical sensing system  210  are similar to that of the optical sensing system  110  ( FIG. 2 ), as is described above. 
     Referring now to  FIG. 4 , an embodiment the optical sensing system  310  that can be utilized for BOTDR is schematically depicted. The optical sensing system  310  comprises a pulsed light assembly  320  for generating a first pulse signal  30 , a second pulse signal  40 , and a continuous wave light signal  218 . The pulsed light assembly  320  is similar to the pulsed light assembly  220  ( FIG. 3 ), with the addition of components configured to emit the first pulse signal  30  and the continuous wave light signal  218  into a sensing optical FMF  360 . 
     The pulsed light assembly  320  may comprise a tunable continuous wave laser  312  that operates as a light source for the optical sensing system  310  by emitting a substantially continuous signal of optical energy. A 1×2 optical splitter  114  can be optically coupled to the tunable continuous wave laser  312  and configured to split the optical energy emitted by the tunable continuous wave laser  312  into a first portion  116  of the optical energy and a second portion  118  of the optical energy. The first portion  116  of the optical energy and the second portion  118  of the optical energy are transformed by the pulsed light assembly  320  into the first pulse signal  30  and the second pulse signal  40  in a manner similar to that of the pulsed light assembly  120  ( FIG. 2 ). 
     The pulsed light assembly  320  may further comprise a continuous wave laser  314  that operates as a light source for generating the continuous wave light signal  218 . In one embodiment, the tunable continuous wave laser  312  and the continuous wave laser  314  can be configured to generate different frequencies of light. Specifically, the frequencies can differ to account for the phase matching frequency difference between LP 01  and LP 11  modes of the sensing optical FMF  360 . 
     Referring still to  FIG. 4 , the optical sensing system  310  may further comprise an optical mode converter  316  for converting the first pulse signal  30  from a fundamental mode into a higher mode (e.g., from LP 01  to LP 11 ). Accordingly, the optical mode converter  316  can be coupled to the gated optical amplifier  124 . The first pulse signal  30  can then be launched into the sensing optical FMF  360 . 
     In one embodiment, the continuous wave laser  314  and the optical mode converter  316  can be optically coupled to a 2×1 optical coupler  318 . The 2×1 optical coupler  318  can combine the first pulse signal  30  and the continuous wave light signal  218  such that the first pulse signal  30  travels along a different mode of the sensing optical FMF  360  than the continuous wave light signal  218 . Specifically, the 2×1 optical coupler  318  can be optically coupled to the optical circulator  50 . Accordingly, when the sensing optical FMF  360  is optically coupled to the optical circulator  50 , the first pulse signal  30  and the continuous wave light signal  218  can be transmitted into the sensing optical FMF  360 . A different port of the optical circulator  50  can be optically coupled to an optical mode converter  322  for converting the sensed optical signal  62  from the higher mode to the fundamental mode (e.g., from LP 11  to LP 01 ). The sensed optical signal  62  can then be filtered by an optical filter  324  and transmitted to the optical detector  54 . Detection and operation of the optical sensing system  310  are similar to that of the optical sensing system  110  ( FIG. 2 ), as is described above. 
     EXAMPLE 
     The embodiments described herein will be further clarified by the following example. 
     An embodiment of the optical sensing system  310  was constructed and tested. In the tested embodiment, a continuous wave narrow-linewidth fiber laser and an erbium doped fiber amplifier (EDFA) were utilized to form the continuous wave laser  314 . The tunable continuous wave laser  312  was modulated externally with the first optical modulator  122 . A semiconductor optical amplifier (SOA) was used as the gated optical amplifier  124  to optically gate and amplify the pump pulse signal  130  output from the first optical modulator  122 . Another SOA was used as the second optical modulator  128  to output a second pulse signal  40  having a pulse width of about 7 ns. An electrical pulse generator was utilized as the one or more triggering devices  126  to generate the amplifier trigger signal  140 , the first modulation signal  142  and the local trigger signal  144 . Accordingly, the timing between the SOA&#39;s, and the first optical modulator  122  was controlled by the electrical pulse generator to synchronize the first pulse signal  30  and the second pulse signal  40 . 
     The first pulse signal  30  having a 100 kHz repetition rate was launched into the LP 11  mode of a 15.96 km two-mode fiber, which was utilized as the sensing optical FMF  360 . The continuous wave light signal  218  was launched into the LP 01  mode of the 15.96 km two-mode fiber. The sensed optical signal  62  (i.e., the signal reflected by the two-mode fiber) was converted from high order mode LP 11  mode into the fundamental mode LP 01  by the optical mode converter  322 . The sensed optical signal  62  and second pulse signal  40  were combined and detected by a coherent balanced photodetector with a bandwidth of 20 GHz. The output of the balanced photodetector was monitored by an electrical spectral analyzer. 
       FIG. 5  depicts experimental results of the radio frequency spectra of the first pulse signal  30  without use of the SOA (non-amplified spectrum  500 ) and with use of the SOA (amplified spectrum  502 ). The non-amplified signal-to-noise ratio  504  corresponding to the non-amplified spectrum  500  was about 22 dB and amplified signal-to-noise ratio  506  corresponding to the amplified spectrum  502  was about 48 dB. It was experimentally determined that the improvement in signal-to-noise ratio can reduce the confusion in specifying the location of the sensing region (i.e., the portion of the sensing fiber to detect) and the error in detecting the Brillouin frequency. Accordingly, improvements were made in measuring strain and temperature. 
     It should now be understood that the embodiments described herein can be utilized to improve the performance of BOTDR/BOTDA fiber sensors. For example, the use of a pulsed local oscillator to interact with the sensing optical signal in coherent balanced detection can improve performance compared to regular sensing systems where the local oscillator light is continuous wave light. Specifically, a pulsed local oscillator having equivalent average power to a continuous wave local oscillator can have higher peak powers than the continuous wave local oscillator. Accordingly, the embodiments described herein can be used to interact with the pulsed local oscillator light with the sensing optical signal without saturating the balanced photodetectors, and improve spatial resolution, sensing distance, sensitivity, and/or acquisition time of a BOTDR/BOTDA fiber sensor. Moreover, a gated optical amplifier (e.g., SOA) can be used to gate and amplify the probe/pump pulses to increase the signal-to-noise ratio of probe/pump pulses. Such improvements in the signal-to-noise ratio of probe/pump pulses, can increase the signal-to-noise ratio of the sensing signal to improve the performance of the BOTDR/BOTDA fiber sensor. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.