Patent Application
20160103017
The present disclosure relates generally to optical measurement apparatuses, and in particular, to an apparatus and method for sensing parameters using Fiber Bragg Grating (FBG) sensor and comparator.
Many monitoring and control systems employ a plurality of sensors for measuring different parameters associated with a system. For instance, such sensed parameters include temperature, mechanical strain, pressure, as well as other. Traditionally, such sensors use mechanical and electrical principles for performing parameter sensing and measurement. For example, traditional temperature sensors employ thermocouples, thermistors, resistance temperature detectors (RTDs), and infrared sensors. Similarly, traditional strain sensors may include strain gauges, piezo-resistive pressure sensors, and capacitive pressure sensors.
Although such traditional sensors are useful in many applications, in other applications, such sensors may not be suitable. For instance, in applications where the sensing environment is harsh, such traditional sensors may not be suitable since they may degrade over time, or not work at all. For instance, in an underwater application, electronic-based sensors may not be suitable as the water generally causes shorts in the electronic components. Other harsh environments include corrosive environments, radiation environments, harsh chemical environments, high and low temperature environments, vacuum environments, and others potentially in combination.
One type of sensor useful for harsh environment sensing is an optical-based sensor that employs a Fiber Bragg Grating (FBG). A FBG sensor typically comprises an optical fiber that includes one or more FBG structures formed within the fiber. Each FBG structure is configured to reflect light at a particular wavelength (e.g., a narrowband wavelength range) and pass through light at other wavelengths. The FBG structure is sensitive to temperature (e.g., the structure expands and contracts with increasing and decreasing temperature, respectively) and to mechanical strain (e.g., the structure expands and contracts with strain). Accordingly, the wavelength of the optical signal that the FBG structure reflects depends on the stressed induced from applied strain, either caused by temperature and/or externally applied forces.
In the past, FBG-based sensors used a system to convert the reflected wavelength into a particular time of receiving the reflected signal. Accordingly, the time of receiving the signal is a function of the wavelength which, in turn, is a function of the sensed parameter (e.g., temperature, strain, pressure, etc.). Typically, such sensors employ a complex process for determining the time of receiving the reflected signal, which consists of converting the reflected optical signal into an electrical signal, digitizing the electrical signals, and performing an algorithm on the digitized signal for determining the peak of the signal. Such complex peak-searching algorithm and analog-to-digital conversion electronics generally limit the speed in which measurements may be made, as well as the number of FBG structures that may be employed on a single optical fiber.
An aspect of the disclosure relates to an apparatus, such as a sensor or an apparatus that includes a sensor, for sensing one or more parameters. The apparatus comprises a sweeping wavelength laser (SWL) configured to generate a sweeping wavelength optical signal. Additionally, the apparatus comprises an optical fiber including a Fiber Bragg Grating (FBG) structure configured to sense a parameter, wherein the optical fiber is configured to receive the sweeping wavelength optical signal, wherein the FBG structure is configured to produce a reflected optical signal with a particular wavelength in response to the sweeping wavelength optical signal, and wherein the particular wavelength varies as a function of the parameter. The apparatus further comprises a photo detector configured to generate an electrical signal based on the reflected optical signal, a comparator configured to generate a pulse based on a comparison of the electrical signal to a first threshold, and a processor configured to generate an indication of the parameter based on the pulse.
In another aspect of the disclosure, the comparator is configured to generate the pulse by producing a high logic level in response to the electrical signal from the photo detector exceeding the first threshold. In another aspect, the comparator is configured to generate the pulse by at least producing a high logic level in response to the electrical signal rising above the first threshold, and producing a low logic level in response to the electrical signal falling below a second threshold. In yet another aspect, the comparator comprises a Schmitt trigger.
In another aspect of the disclosure, the indication of the parameter is based on a timing at which the processor receives the pulse. In another aspect, the apparatus further comprises a scan generator configured to generate a scan signal for controlling the sweeping wavelength optical signal. In still another aspect, the processor is configured to generate the indication of the parameter based on the scan signal and the pulse. In an additional aspect, the sweeping wavelength laser (SWL) comprises a light source configured to generate a light having a defined range of wavelengths, and a tunable filter configured to generate the sweeping wavelength optical signal by wavelength filtering the light in accordance with the scan signal. In a further aspect, the SWL may further include an optical amplifier to increase the level of the light generated by the light source.
In another aspect of the disclosure, the apparatus further comprises a second optical fiber including a second FBG structure configured to sense a second parameter, wherein the second optical fiber is configured to receive the sweeping wavelength optical signal, wherein the second FBG structure is configured to produce a second reflected optical signal with a second particular wavelength in response to the sweeping wavelength optical signal, and wherein the second particular wavelength varies as a function of the second parameter. Additionally, the apparatus comprises a second photo detector configured to generate a second electrical signal based on the second reflected optical signal, and a second comparator configured to generate a second pulse based on a comparison of the second electrical signal to a second threshold. In yet another aspect, the processor is configured to generate a second indication of the second parameter based on the second pulse. In still another aspect, the processor is configured to generate the indication of the parameter based on the second pulse.
In another aspect of the disclosure, the optical fiber includes other one or more FBG structures for sensing other one or more parameters, wherein the other one or more FBG structures are configured to produce other one or more reflected optical signals with other one or more particular wavelengths in response to the sweeping wavelength optical signal, and wherein the other one or more particular wavelengths varies as a function of other one or more parameters, respectively. According to this aspect, the photo detector is configured to generate other one or more electrical signals based on the other one or more reflected optical signals, respectively; the comparator is configured to generate other one or more pulses based on a comparison of the other one or more electrical signals to the first threshold; and the processor is configured to generate other one or more indications of the other one or more parameters based on the other one or more pulses, respectively.
Other aspects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
In particular, the apparatus 100 comprises a swept wavelength laser (SWL) 101, which may comprise a light source 102, an optical amplifier 104, and a tunable filter 106. The apparatus 100 also comprises a scan generator 108 for generating a scan signal for the SWL 101, as discussed in more detail herein. The apparatus 100 further comprises a coupler 110, and an optical fiber 112, which includes at least one FBG structure 114. Additionally, the apparatus 100 comprises a photo detector (PD) 116, a comparator 118, and a processor 120 configured to process any reflected wavelength signal from FBG structure 114.
The SWL 101 is configured to generate an optical signal having a wavelength that is periodically and continuously swept between a minimum wavelength λ1 and a maximum wavelength λ2. In particular, the light source 102 generates a broadband optical signal with wavelengths at least between the minimum wavelength λ1 and a maximum wavelength λ2 of the sweeping optical signal. The optical amplifier 104 amplifies the broadband optical signal to a defined power level suitable for performing parameter measurements using the optical fiber 112. The tunable filter 106 filters the amplified broadband optical signal based on a scan signal generated by the scan generator 108 in order to produce the sweeping optical signal (e.g., SWL of
The sweeping optical signal (λ1-λ2) is applied to an input end of the optical fiber 112 by way of the coupler 110. The sweeping optical signal propagates within the optical fiber 112 towards the other end of the optical fiber. As previously discussed, the FBG structure 114 of the optical fiber 112 reflects the incident optical signal at a particular wavelength λB (e.g., the Bragg wavelength), and passes other wavelengths of the sweeping optical signal. The reflected optical signal λB propagates back to the input end of the optical fiber 112 and to the coupler 110, where it directs the reflected optical signal λB to the photo detector 116.
The photo detector 116 generates an electrical signal based on the reflected optical signal λB. The electrical signal from the photo detector 116 is applied to the comparator 118, where it compares the electrical signal to a defined threshold (TH). It shall be understood that additional components may be provided to process the electrical signal from the photo detector 116 before the electrical signal is applied to the comparator 118. Such additional components may include an amplifier for amplifying the electrical signal and a filter for removing noise from the electrical signal. The comparator 118 generates a signal including a high logic-level if the electrical signal from the photo detector 116 exceeds the threshold (TH), and a low logic-level signal if the electrical signal from the photo detector 116 does not exceed the threshold (TH).
The processor 120 receives the scan signal from the scan generator 108 and the signal from the comparator 118. The scan signal is processed to establish the time of the start of the wavelength scan, and the signal from the comparator 118 indicates the time of arrival of the reflected optical signal. Thus, the processor 120 is able to determine the wavelength λB of the reflected optical signal based on the time elapsed from the start of the wavelength scan and the rising edge of the asserted signal from the comparator 118. Since, as previously discussed, the wavelength λB of the reflected optical signal depends on the parameter being sensed by the FBG structure 114 (e.g., temperature, strain, pressure, etc.), the processor 120 is able to generate an indication or output indicative of the sensed parameter based on the determined wavelength λB.
According to the upper graph, the scan signal generated by the scan generator 108 may be triangular- or sawtooth like. That is, the scan signal includes a substantially linear rising portion and a substantially linear falling portion. The slope of the rising portion may be not the same as the slope of the falling portion. The rising portion of the scan signal causes the SWL 101 to generate an optical signal with a wavelength linearly increasing from the minimum wavelength λ1 to the maximum wavelength λ2. Similarly, the falling portion of the scan signal causes the SWL 101 to generate an optical signal with a wavelength linearly decreasing from the maximum wavelength λ2 to the minimum wavelength λ1. Also, as shown, the scan signal may be continuous and periodic; the graph illustrating three (3) periods of the scan signal. It shall be understood that the scan signal may be configured differently including a non-linear rise and linear fall, a linear rise and a non-linear fall, a non-linear rise and fall, and other variations.
As illustrated, the middle graph depicts the electrical signal generated by the photo detector 116 in response to receiving the optical signal reflected by the FBG structure 114 of the optical fiber 112. As previously discussed, the wavelength λB of the reflected optical signal depends on the configuration of the FBG structure 114 as well as the one or more parameters that it is sensing. Because the scan signal is rising and falling between the minimum and maximum wavelengths λ1 and λ2 in a defined (predictable) manner, the time of arrival of the reflected optical signal may be mapped to the particular wavelength λB of the reflected optical signal. Thus, in this example, the times t1 and t2 in the first period corresponds to the wavelength λB, the times t3 and t4 in the second period corresponds to the wavelength λB, and the times t5 and t6 in the third period corresponds to the wavelength λB.
With reference to the middle and lower graphs, the comparator 118 compares the electrical signal from the photo detector 116 with a defined threshold (TH), and generates a high logic-level signal if the electrical signal exceeds the threshold, and a low logic-level signal if the electrical signal does not exceed the threshold. Thus, the lower graph illustrates the signal generated by the comparator 118, which includes substantially square-wave pulses that coincides with times t1 to t6, all of which coincide with the wavelength λB of the reflected optical signal.
The processor 120 receives the signal from the comparator 118 and maps the timings t1 to t6 to the wavelength λB (or wavelengths λB1 to λB6, as the wavelength may vary over time depending on the sensed parameter) of the reflected optical signal using the scan signal received from the scan generator 108. As previously discussed, the wavelength λB of the reflected optical signal depends on the one or more parameters being sensed by the FBG structure 114 of the optical fiber 112, which may vary over time. Thus, the processor 120 generates an output or indication of the one or more sensed parameters based on the wavelengths λB1 to λB6.
Although a single input threshold comparator (e.g., comparator 118) and a Schmitt trigger (e.g., comparator 219) are used to exemplify the invention, it shall be understood that other types of comparators may be used. Some examples include, but are not limited to: (1) a Schmitt-type input, with a fixed threshold and a fixed hysteresis; (2) a comparator with a single threshold input and externally configurable hysteresis; (3) a custom comparator using multiple inputs; and (4) any known or future configurations of comparing an analog input value to one or more known threshold values for the purpose of creating a digital output.
Referring again to
The comparator 219 generates a high logic-level from a low logic-level in response to the electrical signal from the photo detector 216 initially exceeding the upper threshold TH2. The comparator 219 continuous to generate the high logic-level as long as the electrical signal exceeds the lower threshold TH1. Once the electrical signal from the photo detector 216 falls below the lower threshold TH1, the comparator 219 generates the low logic-level. The comparator 219 is useful in combating noise that may be present in the electrical signal generated by the photo detector 216.
For additional protection against noise, the processor 220 may be configured to employ a discrimination filtering of the signal generated by the comparator 219. In particular, the discrimination filtering may reject pulses from the comparator 219 having width shorter a defined minimum duration. This keeps any fast noise from setting off nuisance false positive detections. If the hysteresis effect of the comparator 219 is not sufficient to suppress all the unwanted noise, the discrimination filtering employed by the processor 220 provides the additional noise suppression.
More specifically, the apparatus 300 comprises an SWL 301, a scan generator 308, a coupler 310, and an optical fiber 312 including FBG structures 313, 314, and 315. Additionally, the apparatus 300 comprises a photo detector 316, a comparator 319, and a processor 320.
The FBG structures 313, 314, and 315 are configured to reflect optical signals at natural (when the sensed parameter is not influencing the wavelength) wavelengths λB1, λB2, and λB3, respectively. The natural wavelengths λB1, λB2, and λB3 of the FBG structures should be sufficiently spaced apart such that the corresponding electrical signals generated by the photo detector 316 do not interfere with each other in a manner that results in error in the measurement of the corresponding sensed parameters.
The upper graph illustrates that a wavelength range is associated with a corresponding natural wavelength. For instance, wavelength ranges ΔλB1, ΔλB2, and ΔλB3 are associated with natural wavelengths λB1, λB2, and λB3 of the FBG structures 313, 314, and 315, respectively. The wavelength ranges ΔλB1, ΔλB2, and ΔλB3 are the possible wavelengths of optical signals reflected by the FBG structures 313, 314, and 315 as affected by the one or more parameters sensed by the FBG structures, respectively. As the upper graph illustrates, the wavelengths corresponding to times t1 to t6 are: at, below, above, at, below, and below the corresponding natural wavelengths, respectively. Similarly, the wavelengths corresponding to times t7 to t12 are: below, above, at, above, at, and below the corresponding natural wavelengths, respectively.
As previously discussed, the particular wavelengths of the reflected optical signals from the FBG structures 313, 314, and 315 affect the time of arrival of the signals at the photo detector 319. Thus, the time of arrivals t1-t12 depend on the wavelengths of the optical signals reflected by the FBG structures 313, 314, and 315 in the illustrated two scan cycle example shown in
In particular, the apparatus 400 comprises an SWL 401, scan generator 408, optical splitter 403, couplers 410a and 410b, and optical fibers 412a and 412b including respective FBG structures 413a and 413b, 414a and 414b, and 415a and 415b. The apparatus 400 further comprises photo detectors 416a and 416b, comparators 419a and 419b, and processor 420.
The SWL 401 generates the sweeping optical signal (λ1-(λ2) in accordance with the scan signal generated by the scan generator 408. The optical splitter 403 splits the sweeping optical signal into a first sweeping optical signal for optical fiber 412a and a second sweeping optical signal for optical fiber 412b. The first and second sweeping optical signals are applied to inputs of the optical fibers 412a and 412b by way of couplers 410a and 410b, respectively. In response to the sweeping optical signals, the FBG structures 413a, 414a, 415a, 413b, 414b, and 415b produce reflected optical signals with wavelengths (λB1, (λB2, (λB3, (λB4, (λB5, and (λB6, respectively.
The reflected optical signals from FBG structures 413a, 414a, and 415a propagate to photo detector 416a by way of coupler 410a. Similarly, the reflected optical signals from FBG structures 413b, 414b, and 415b propagate to photo detector 416b by way of coupler 410b. As previously discussed, the photo detector 416a and corresponding comparator 419a are configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures 413a, 414a, and 415a are received by the photo detector 416a. Similarly, the photo detector 416b and corresponding comparator 419b are configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures 413b, 414b, and 415b are received by the photo detector 416b.
The processor 420 is configured to receive the square wave pulses from the comparators 419a and 419b. The processor 420 generates an output including indications related to the one or more parameters sensed by the FBG structures 413a, 414a, 415a, 413b, 414b, and 415b based on the times the processor receives the pulses from the comparators 419a and 419b, and the scan signal.
In particular, the apparatus 500 comprises an SWL 501, scan generator 508, optical splitter 503, couplers 510a and 510b, a sensing optical fiber 512a including FBG structures 513a, 514a, and 515a, and a reference optical fiber 512b including FBG structures 513b, 514b, and 515b. The apparatus 500 further comprises photo detectors 516a and 516b, comparators 519a and 519b, and processor 520.
The SWL 501 generates the sweeping optical signal ((λ1-(λ2) in accordance with the scan signal generated by the scan generator 508. The optical splitter 503 splits the sweeping optical signal into a first sweeping optical signal for sensing optical fiber 512a and a second sweeping optical signal for reference optical fiber 512b. The first and second sweeping optical signals are applied to inputs of optical fibers 512a and 512b by way of couplers 510a and 510b, respectively. In response to the sweeping optical signals, the FBG structures 513a, 514a, 515a, 513b, 514b, and 515b produce reflected optical signals with wavelengths λB1, λB2, λB3, λB4, λB5, and λB6, respectively.
The FBG structures 513a, 514a, and 515a of the sensing optical fiber 512a are configured to sense the temperatures T1, T2, and T3 of certain components 550, 552, and 554 of a system, respectively. In this example, the components 550, 552, and 554 do not impart any significant mechanical strain (e.g., εR˜0) on the FBG structures 513a, 514a, and 515a, respectively. The reference optical fiber 512b is situated within a separate temperature and strain controlled environment 560. Thus, the wavelengths λB1, λB2, and λB3 of the reflected optical signals of the sensing optical fiber 512a varies with the temperatures T1, T2, and T3 of the components 550, 552, and 554, which depend on heat emitted by the components. The wavelengths λB4, λB5, and λB6 of the reflected optical signals of the reference optical fiber 512b is substantially constant due to the controlled temperature and strain environment 560.
The reflected optical signals from FBG structures 513a, 514a, and 515a propagate to photo detector 516a by way of coupler 510a. Similarly, the reflected optical signals from FBG structures 513b, 514b, and 515b propagate to photo detector 516b by way of coupler 510b. As previously discussed, the photo detector 516a and corresponding comparator 519a are configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures 513a, 514a, and 515a are received by the photo detector 516a. Similarly, the photo detector 516b and corresponding comparator 519b are configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures 513b, 514b, and 515b are received by the photo detector 516b.
The processor 520 is configured to receive the square wave pulses from the comparators 519a and 519b. The processor 520 generates an output including indications related to the temperatures T1, T2, and T3 (e.g., Δ1=T1−TR, Δ2=T2−TR, and Δ3=T3−TR) based on the scan signal from the scan generator 508, and the time differences between the times the processor 520 receives the pulses from comparator 519a and the times the processor 520 receives the pulses from comparator 519b, respectively.
In particular, the apparatus 600 comprises an SWL 601, scan generator 608, optical splitter 603, couplers 610a and 610b, a sensing optical fiber 612a including FBG structures 613a, 614a, and 615a, and a reference optical fiber 612b including FBG structures 613b, 614b, and 615b. The apparatus 600 further comprises photo detectors 616a and 616b, comparators 619a and 619b, and processor 620.
The SWL 601 generates the sweeping optical signal (λ1-λ2) in accordance with the scan signal generated by the scan generator 608. The optical splitter 603 splits the sweeping optical signal into a first sweeping optical signal for sensing optical fiber 612a and a second sweeping optical signal for reference optical fiber 612b. The first and second sweeping optical signals are applied to inputs of optical fibers 612a and 612b by way of couplers 610a and 610b, respectively. In response to the sweeping optical signals, the FBG structures 613a, 614a, 615a, 613b, 614b, and 615b produce reflected optical signals with wavelengths λB1, λB2, λB3, λB4, λB5, and λB6, respectively.
The FBG structures 613a, 614a, and 615a of the sensing optical fiber 612a are configured to sense mechanical strain ε1, ε2, and ε3 upon certain components 650, 652, and 654 of a system, respectively. The sensing and reference optical fibers 612a and 612b are situated within an environment 660 that subjects both optical fibers to substantially the same reference temperature TR. Thus, the wavelengths λB1, λB2, and λB3 of the reflected optical signals of the sensing optical fiber 612a vary with the mechanical strain ε1, ε2, and ε3 upon components 650, 652, and 654 of a system and the reference temperature. The wavelengths λB4, λB5, and λB6 of the reflected optical signals of the reference optical fiber 612b varies only with the reference temperature TR, as the reference mechanical strain εR is substantially zero (0).
The reflected optical signals from FBG structures 613a, 614a, and 615a propagate to photo detector 616a by way of coupler 610a. Similarly, the reflected optical signals from FBG structures 613b, 614b, and 615b propagate to photo detector 616b by way of coupler 610b. As previously discussed, the photo detector 616a and corresponding comparator 619a are configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures 613a, 614a, and 615a are received by the photo detector 616a. Similarly, the photo detector 616b and corresponding comparator 619b are configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures 613b, 614b, and 615b are received by the photo detector 616b.
The processor 620 is configured to receive the square wave pulses from the comparators 619a and 619b. The processor 620 generates an output including indications related to the strain ε1, ε2, and ε3 upon components 650, 652, and 654 based on the scan signal from the scan generator 608, and the time differences between the times the processor 620 receives the pulses from comparator 619a and the times the processor 620 receives the pulses from comparator 619b, respectively.
While the invention has been described in connection with various embodiments, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptation of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within the known and customary practice within the art to which the invention pertains.
