Systems and methods for optically generated trigger multiplication

Systems and methods for providing trigger signals in an optical interrogator, wherein multiple triggers are generated within each period of a varying reference signal, and wherein the triggers are evenly spaced according to the wavenumber of the reference signal. In one embodiment, an optical frequency domain reflectometry system provides a laser beam to a reference interferometer to produce a reference signal. This signal is passed through a 4×4 optical coupler which splits the signal into a first signal and a second signal that is 90 degrees out of phase with the first signal. These signals are converted to electrical signals, and a trigger unit generates triggers at points at which the two electrical signals have zero-crossings, and at which the magnitudes of the signals are equal. The resulting triggers remain evenly spaced within the period of the reference signals, even when the period is changed.

BACKGROUND

1. Field of the Invention

The invention relates generally to interferometry and more particularly to systems and methods for generating multiple triggers for each cycle of a sampling signal in an optical interrogator.

2. Related Art

The commercial success of optical fiber telecommunications has fostered the growth of optical fiber sensing applications by providing a ready supply of low cost, high quality components and test equipment. Another enabling technology for fiber sensing was the discovery of the ultraviolet (UV) photosensitivity in optical fiber. Photosensitivity allows the alteration of the internal structure of a fiber waveguide. Modification of the waveguide can be employed for a number of useful purposes, one of which is to induce a periodic modulation of the refractive index along the fiber core to create a wavelength selective reflector called a fiber Bragg grating (FBG). FBG's can be produced conveniently and very inexpensively during the fiber draw process. The period of the modulation of the refractive index determines the wavelength reflected by the FBG. After the FBG is formed, the grating period of the FBG can be physically altered by changing the mechanical load on the fiber, or by changing its temperature. By monitoring the wavelength reflected from a FBG, the FBG can be used as a transducer for both strain and temperature.

An important recent development in the use of FBGs is the use of optical frequency domain reflectometry (OFDR), which is a sensing technique that can be used to monitor FBGs or other sensors. This technique can be used to interrogate hundreds or thousands of FBG's distributed along the length of a single optical fiber. The OFDR technique has many applications where light weight, immunity to electromagnetic interference, high sensor density, and remote readout are important considerations. These applications include monitoring sensors such as FBGs, providing diagnostics on optical fiber networks and cables, including the intrinsic Rayleigh scatter of the optical fiber, monitoring the condition of aerospace structures, monitoring industrial processes, and monitoring sub-marine and oil well systems.

One of the problems with conventional OFDR systems of the type described above is that the sinusoidal reference signal provides sampling triggers only once per period, for instance at rising zero-crossings. When a reference interferometer is used for sampling, the frequency of the sampling limits the frequency of signals from the device under test that can be resolved by the system (because of the Nyquist criteria), which in turn limits the length of the device under test. If it is desired to increase the sampling frequency and thereby increase the possible length of the device under test, the path length difference of the reference interferometer must be increased. This may present practical difficulties, however. It would therefore be desirable to provide systems and methods for increasing the rate of the sampling signal without increasing the length of the reference interferometer.

SUMMARY OF THE INVENTION

This disclosure is directed to systems and methods for providing trigger signals in an optical interrogator that solve one or more of the problems discussed above. In particular, the systems and methods provide for the generation of multiple triggers within each period of a varying reference signal, where the triggers are substantially evenly spaced according to the wavenumber of the reference signal.

In one embodiment, an OFDR system is used to interrogate a sensor array that is embedded in an optical fiber. A laser illuminates both the sensor array and a reference interferometer. The reference interferometer produces a reference signal that is passed through a 4×4 optical coupler which splits the signal and provides output signals that include a first signal and a second signal that is 90 degrees out of phase with the first signal. Optical detectors are used to convert these optical signals to electrical signals. A trigger unit then detects points at which the two electrical signals have zero-crossings, and at which the magnitudes of the signals are equal. This produces eight triggers per period of the reference signals, rather than the single trigger that is normally produced at the rising zero-crossing of the reference signal. Further, because the triggers are generated from events that correspond to fixed positions within the period of the reference signals, the triggers remain approximately evenly spaced within the period of the reference signals, even when the period changes as a result of non-linearities in the sweeping of the frequency of the laser.

An apparatus for triggering uniform wavenumber sampling in an optical frequency domain reflectometer system. The apparatus includes an optical coupler, at least one reference optical detector, and a trigger unit. The optical coupler is configured to receive a reference optical signal from a reference interferometer and to provide at least one output optical signal having the same period as the reference optical signal. The reference optical detector is configured to receive the output optical signal and to convert this signal into at least one electrical signal having the same period as the output optical signal. The trigger unit is configured to receive the electrical signals and to generate a trigger signal that contains more than one trigger per period of the electrical signals, where the triggers have uniform wavenumber spacing. In one embodiment, the optical coupler is configured to split the reference optical signal into two optical signals that are 90 degrees out of phase with each other (as are the two resulting electrical signals). The optical coupler may be, for example, a 4×4 optical coupler. The trigger unit may be configured to generate triggers at zero-crossings of the first and second electrical signals and at times at which the first and second electrical signals have equal magnitudes. For instance, the trigger unit may include four comparator-differentiator pairs and a summing unit, where a first comparator-differentiator pair receives the first and second electrical signals as inputs, a second comparator-differentiator pair receives the first electrical signal and an inverse of the second electrical signal as inputs, a third comparator-differentiator pair receives the first electrical signal and ground as inputs, and a fourth comparator-differentiator pair receives the second electrical signal and ground as inputs. The output of each comparator-differentiator pair can then be provided as an input to the summing unit, the output of which is the trigger signal. The apparatus may also include a reference interferometer and a laser, where the reference interferometer is configured to receive a laser light beam from the laser and to produce the reference optical signal.

Another embodiment comprises a method for triggering uniform wavenumber sampling in an optical frequency domain reflectometer system. The method includes providing a reference optical signal, converting the reference optical signal into at least one electrical signal having the same period as the reference optical signal, and generating a trigger signal based on the at least one electrical signal, where the trigger signal contains more than one trigger per period of the at least one electrical signal, and where the triggers have uniform wavenumber spacing. The reference optical signal may be generated by providing a laser beam to a reference interferometer, the output of which is passed through a 4×4 optical coupler to produce a pair of optical signals that are 90 degrees out of phase with each other. These optical signals are then converted by optical detectors to electrical signals that are 90 degrees out of phase. The triggers may then be generated at zero-crossings of the two electrical signals and at times at which the two electrical signals have equal magnitudes.

Another embodiment comprises an OFDR system that includes a laser and a first optical coupler (e.g., a 4×4 coupler) that couples the laser to a reference interferometer. The reference interferometer uses the laser light beam to produce a reference optical signal, which is returned to the first optical coupler. Reference optical detectors receive the returned output optical signals (which are 90 out of phase with each other) and convert them into corresponding electrical signals having the same period as the output optical signals. A trigger unit receives the electrical signals and generates a trigger signal that contains more than one trigger per period of the at least one electrical signal. The triggers have uniform wavenumber spacing. For instance, the trigger unit may use comparator-differentiator pairs to generate triggers corresponding to zero-crossings of the first and second electrical signals and times at which the first and second electrical signals have equal magnitudes. The system also includes a sensor array that receives the beam from the laser and produces an optical sensor signal. A sensor array optical detector receives the optical sensor signal and the trigger signal. The sensor array optical detector samples the optical sensor signal in response to occurrences of the triggers in the trigger signal. The sensor array may be a sensor array interferometer. The sensor array interferometer may, for example, include a 2×2 optical coupler and a plurality of selectively reflective sensors, such as fiber Bragg gratings. A first port of the 2×2 optical coupler receives the laser light beam, a second port of the 2×2 optical coupler is coupled to a first optical fiber that terminates at a broadband reflector, and a third port of the 2×2 optical coupler is coupled to a second optical fiber that incorporates the sensors.

Numerous other embodiments are also possible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As described herein, various embodiments of the invention comprise systems and methods for providing trigger signals in an optical interrogator, wherein multiple triggers are generated within each period of a varying reference signal, and wherein the triggers are evenly spaced according to the wavenumber of the reference signal.

In one embodiment, an OFDR system is used to interrogate a sensor array that is embedded in an optical fiber. A laser provides a beam to both the sensor array and a reference interferometer. The reference interferometer produces a reference signal that is passed through a 4×4 optical coupler which splits the signal and provides output signals that are 90 degrees out of phase with each other. Optical detectors are used to convert these optical signals to electrical signals. A trigger unit then detects points at which the two electrical signals have zero-crossings, and at which the magnitudes of the signals are equal. This produces eight triggers per period of the reference signals, rather than the single trigger that is normally produced at the rising zero-crossing of the reference signal. Further, because the triggers are generated from events that correspond to fixed positions within the period of the reference signals, the triggers remain evenly spaced within the period of the reference signals, even when the period changes as a result of sweeping the frequency of the laser.

OFDR is a technique that can be used to monitor, or interrogate, sensors such as FBGs that have reflectivity which is wavelength-dependent. FBGs are devices that utilize periodic modulations of the refractive index of an optical fiber to achieve wavelength-selective reflection. The period of the modulation determines the wavelength that is reflected, but the period of the modulation (hence the wavelength reflected) can be altered by changing the mechanical load on the fiber or the temperature of the fiber. The FBG can be used as a sensor for strain and/or temperature by monitoring the wavelength that is reflected by the FBG.

Because of the availability of techniques such as OFDR, many FBGs (e.g., hundreds or even thousands) can be distributed along the length of a single fiber. In one configuration, OFDR system generates a sampling signal, for example, by using a frequency-swept laser with an in-fiber reference interferometer. The light from the laser is also provided to the fiber that incorporates the FBGs. Each of the FBGs effectively provides an interferometer output which is sampled according to the sampling signal produced by the reference interferometer. The light reflected by each FBG is modulated by a unique frequency that is dependent upon the FBG's location along the length of the fiber.

A basic, conventional OFDR system is shown inFIG. 1. This system includes a wavelength-tunable laser110, three 2-by-2 optical couplers120-122, two photodiode detectors130-131, an FBG-array-based interferometer140, and an in-fiber interferometer150. The laser light is split by coupler120and travels to couplers121and122. The port of coupler120that is not used is terminated (as noted in the figure by an “X”).

Each of the 2-by-2 optical couplers splits the light that passes through the coupler, so light entering one side of the coupler is split and output on the two ports on the opposite side. The coupler works the same in both directions. Coupler121is used to form an in-fiber interferometer with the light reflected from reflectors151and152. This light is detected by a detector130, such a photodiode. This reference interferometer has an optical path length difference of 2nL, where n is the effective refractive index of the fiber and L is the path difference of the two paths through the interferometer (from coupler121to reflector151, and from coupler121to reflector152).

Coupler122is used to form what is effectively a sequence of similar overlapping interferometers. The first path of each interferometer is formed between coupler122and reflector141. The second path of each interferometer is formed between coupler122and the respective one of the FBGs (e.g.,142,143or144). The light reflected from reflector141and each grating (142-144) is detected by detector131.

The signal detected by detector130is sinusoidal. The phase of this signal is a linear function of the wavenumber of the laser. The signal detected by detector130is converted to an electrical signal by trigger unit160. This signal is used to trigger sampling of the sensor signals arriving at detector131. Typically, the rising zero-crossings of the sinusoidal signal detected by detector130are used as triggers. This is shown inFIG. 2. The reference signal I detected by detector130is depicted at the top ofFIG. 2. The trigger signal T produced by trigger unit160is shown at the bottom of the figure. As indicated in this figure, rising zero-crossings210and211are detected by trigger unit160, which generates corresponding triggers220and221in the trigger signal.

The data detected by detector131is forwarded to a data processing unit170. Using the signal detected by detector130to trigger sampling of the optical signals arriving at detector131provides high resolution, and also provides uniform wavenumber sampling of these signals. It should be noted that, because the frequency of the laser is swept, time-synchronous sampling may not provide uniform sampling, if the tuning is non-linear with respect to time. The high-resolution, uniform sampling allows discrete Fourier analysis of the signals received at detector131.

The system ofFIG. 1can easily be extended to include additional sensor modules. Each additional sensor module may be constructed in the same manner as the first, including a detector (e.g.,131) and a sensor-array-based interferometer (e.g.,140). If optical coupler120is changed, for example, to a 4×4 coupler, the light from laser110can be provided to one or two additional sensor modules. The detector of each additional sensor module can be triggered using the same trigger signal generated by trigger unit160, and the data can be provided to and processed by the same data processing unit

As noted above, the light reflected by each FBG is modulated by a unique frequency that is dependent upon the FBG's location along the length of the fiber. The farther an FBG is located from the coupler, the higher the frequency associated with the FBG. In order to increase the length of the fiber (hence the distance of the FBGs from the coupler), the system ofFIG. 1must also increase the path length difference of the reference interferometer. Because it may not be practical or convenient to increase the length of the reference interferometer, it may be preferable to maintain the length of the reference interferometer and multiply the triggers that are generated based on the reference signal detected by detector130.

The OFDR signals are driven by the wavelength tuning of the laser. As the laser is tuned, the signal at detector130is given by
D1=cos(k2nL)   (1)

The frequency of this signal is proportional to L, the interferometer path length difference. The constant k is the wavenumber of the light, and is related to the light's wavelength, λ, by
k=2π/λ  (2)

The interferometer cycles once for a wavenumber change, Δk, of
Δk=π/nλ(3)

This is a constant. The interferometer therefore cycles linearly as a function of wavenumber. The positive-going zero crossing of the signal at detectcor130are used to trigger the sampling of the signal at detector131. This guarantees that the signal at131is sampled at the constant wavenumber interval given by Equation (3).

The signals corresponding to all of the FBGs are present at detector131. Each of these signals is similar to the signal at detector130, bur the response of each FBG is limited to the narrow wavelength range, or spectrum, over which it reflects. In other words, the individual interferometer corresponding to each FBG can only produce an output signal when it is reflecting light. The signal at detector131is the sum of these individual interferometer responses, so the signal at detector131can be written as

where Ri is the spectrum of the i'th grating and Li is the path length difference of the corresponding i'th interferometer. This equation shows, as noted above, that the spectrum of each grating is modulated by a signal with a unique frequency which is governed by the grating's position, Li, in the fiber. By bandpass filtering around a specific frequency (location) via fast Fourier transform, the spectrum of each grating can be independently measured and strain or temperature inferred.

Referring toFIG. 3, an OFDR system that provides multiple triggers per period of the reference interferometer signal is shown. This system includes a wavelength-tunable laser310, three optical couplers320-322, three photodiode detectors330-332, an FBG-array-based interferometer340, and an in-fiber interferometer350.

As in the system ofFIG. 1, the laser light is split by coupler320and travels to couplers321and322. Couplers320and321are 2×2 couplers. Coupler322, however, is a 4×4 coupler, so light entering one side of this coupler is split and output on four ports on the opposite side. Two of the ports on the right side of coupler322are connected to optical fibers that terminate at broadband reflectors351and352, forming a reference interferometer which is essentially the same as the reference interferometer ofFIG. 1. The reference interferometer ofFIG. 3has an optical path length difference of 2nL, where n is the effective refractive index of the fiber and L is the path difference of the two paths through the interferometer. In the system ofFIG. 3, however, the interferometer's output signal is returned to two detectors,330and331, as well as to port325, which is unused. The output signal received by detector330is 90 degrees out of phase with the signal received by detector331. As inFIG. 1, coupler321is used to form effectively overlapping interferometers340. The light reflected from reflector341and each FBG (342-344) is detected by detector332.

The signals detected by detectors330and331are sinusoidal and have phases that are linear functions of the wavenumber of the laser. As noted above, the signals are 90 degrees out of phase. The optical signals detected by detectors330and331are converted to electrical signals by trigger unit360. Trigger unit360makes several comparisons of these electrical signals with each other and with reference voltages, and generates trigger signals based on the comparisons. The trigger signals are used to trigger sampling of the optical signals arriving at detector332. The data detected by detector332is forwarded to a data processing unit370, which processes the data to determine the wavelength of the light reflected from each of the FBGs.

In one embodiment, trigger unit360is configured to generate eight trigger pulses uniformly distributed throughout the period of the sinusoidal optical signals. An exemplary structure for the trigger unit is shown inFIG. 4. The trigger unit ofFIG. 4receives the electrical signals, I and Q, which correspond to the optical signals received by detectors330and331, and generates a trigger signal, T, which is used to trigger sampling at detector332.

The trigger unit identifies zero-crossings (both rising and falling) of signals I and Q, and also identifies points at which the magnitudes of I and Q are equal. The zero-crossings occur every 90 degrees during the period of the signals. Similarly, the magnitudes of the signals are equal every 90 degrees during the period of the signals, but these occur with a 45 degree phase difference from the zero-crossings. When combined, these events occur every 45 degrees throughout the period of the signals, resulting in eight evenly spaced trigger events during the period of the signals.

Referring toFIG. 4, the trigger unit includes a set of comparators410-413, a set of differentiators420-423, and a summing unit430. In this embodiment, comparators410-413comprise Schmidt triggers with negligible hysteresis. Comparators410-411and differentiators420-421are used to produce triggers corresponding to points at which the optical signals have equal magnitude, while comparators412-413and differentiators422-423are used to produce triggers corresponding to zero-crossings.

Comparator410receives signals I and Q as inputs. When I is greater than Q, the output of comparator410is high. When I is less than Q, the output of comparator410is low. Thus, the output of comparator410transitions between high and low whenever I and Q are equal. The output of comparator410is input to differentiator420. Differentiator420generates an output pulse whenever there is a transition in the input signal. Consequently, differentiator420produces an output pulse whenever I and Q are equal. The output of differentiator420is then provided as an input to summing unit430. Comparator411receives signal Q and the inverse of signal I as inputs. Comparator411could alternatively receive I and the inverse of Q. When signals I and Q are equal in magnitude, but opposite in sign, the output of comparator411transitions between high and low. The output of comparator411is provided as an input to differentiator421, which converts each transition of the input signal to a pulse at the output of the differentiator. The output the differentiator421is provided as an input to summing unit430.

Signal I is provided as an input to comparator412. The other input of comparator412is tied to ground. When signal I is greater than zero, the output of comparator412is high, and when the signal is less than zero, the output of the comparator is low. The output of comparator412transitions between high and low whenever signal I crosses zero. The output of comparator412is provided as an input to differentiator422. Differentiator422produces a pulse at its output whenever there is a transition between high and low in the input signal. The output of differentiator422is provided as an input to summing unit430. Signal Q is provided as a first input to comparator413. The second input of comparator413is tied to ground, so that the output of the comparator transitions between high and low whenever signal Q crosses zero. The output of comparator413is provided as an input to differentiator423. Differentiator423produces an output pulse whenever there is a high-low or low-high transition in the input signal, so in output pulse is generated for each zero-crossing of signal Q.

As noted above, the outputs of differentiators420-423are provided as inputs to summing unit430. Each of these inputs is low, except when a pulse is generated for the corresponding trigger conditions (i.e., I=Q for differentiator420, I=−Q for differentiator421, I=0 for differentiator422, and Q=0 for differentiator423). Consequently, the output of summing unit430is low, except when a pulse is received from one of differentiators420-423.

Referring toFIG. 5, a diagram illustrating the relationship between signals I and Q and the output of the trigger unit is shown. In the upper part of the figure, signals I and Q are depicted. The output of the trigger unit is shown in the lower part of the figure. Signals I and Q are the electrical signals that are output by detectors330and331. Signals I and Q track the optical signals that are produced by reference interferometer350and optical coupler322—they have the same period, phase and waveform. When signals I and Q are input to the trigger unit shown inFIG. 4, the trigger unit produces a trigger signal, T, as shown at the bottom ofFIG. 5. The pulses (triggers) in trigger signal T correspond to the events detected by the comparator-differentiator pairs in the trigger unit. Comparator-differentiator pair410/420detects the points at which I and Q are equal (e.g.,513,517) and generates corresponding triggers (e.g.,523,527). Comparator-differentiator pair411/421detects the points at which I and Q are equal in magnitude, but opposite in sign (e.g.,511,515) and generates corresponding triggers (e.g.,521,525). Comparator-differentiator pair412/422detects the zero-crossings of signal I (e.g.,510,514,518) and generates triggers for these events (e.g.,520,524,528). Comparator-differentiator pair413/423detects zero-crossings of signal Q (e.g.,512,516) and generates the corresponding triggers (e.g.,522,526).

Using the signal detected by detector330to trigger sampling of the optical signals arriving at detector331provides higher resolution than conventional sampling based on rising zero-crossings, and also provides uniform wavenumber sampling, even when the frequency of the laser is swept. It should be noted that, because the frequency of the laser is swept, time-synchronous sampling would not provide uniform sampling. The high-resolution, uniform sampling allows discrete Fourier analysis of the signals received at detector331.

The embodiment ofFIGS. 3-5is intended to be exemplary. It is contemplated that there may be many alternative embodiments which incorporate variations of the elements described above and still fall within the scope of the invention. For instance, while the foregoing embodiment generates eight triggers per period of the reference signal, other embodiments may generate less (e.g., two or four) or more (e.g., 16) triggers per period. Further, while the foregoing embodiment employs a particular arrangement of electronic components (comparators, differentiators, summing unit) to generate the triggers in the trigger signal, it should be understood that many alternative components and arrangements may be used to achieve the desired result (triggers that provide uniform wavenumber sampling). Such alternative embodiments are believed to be within the scope of the present disclosure. Still further, while the embodiments above are used in connection with a sensor array that employs FBGs, the disclosed systems and methods for optical interrogation may be useful with other types of sensors or devices, and are not limited to use with FBGs.

Those of skill will further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software (including firmware,) or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Similarly, the particular hardware or software components that are chosen to implement the described functionality may be selected to achieve specific design goals. Those of skill in the art may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.