Apparatus and method for sensing parameters using Fiber Bragg Grating (FBG) sensor and comparator

Various implementations of an apparatus for sensing one or more parameters are disclosed herein. The apparatus includes a sweeping wavelength laser configured to generate a sweeping wavelength optical signal; 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; 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 threshold; and a processor configured to generate an indication of the parameter based on the pulse. The comparator may be configured as a Schmitt trigger.

FIELD

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.

BACKGROUND

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.

SUMMARY

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.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1Aillustrates a block diagram of an exemplary apparatus100in accordance with an aspect of the disclosure. In summary, the apparatus100includes a comparator for digitizing a reflected optical signal from a Fiber Bragg Grating (FBG) structure formed within an optical fiber. The use of a comparator, instead of a complex peak-searching algorithm, allows the determination of the time of arrival of the reflected optical signal to be performed substantially faster and with greater accuracy and precision. This allows the apparatus100to perform higher quality measurements at substantially greater rates, as well as to employ substantially more FBG structures for sensing many parameters of a system.

In particular, the apparatus100comprises a swept wavelength laser (SWL)101, which may comprise a light source102, an optical amplifier104, and a tunable filter106. The apparatus100also comprises a scan generator108for generating a scan signal for the SWL101, as discussed in more detail herein. The apparatus100further comprises a coupler110, and an optical fiber112, which includes at least one FBG structure114. Additionally, the apparatus100comprises a photo detector (PD)116, a comparator118, and a processor120configured to process any reflected wavelength signal from FBG structure114.

The SWL101is configured to generate an optical signal having a wavelength that is periodically and continuously swept between a minimum wavelength λ1and a maximum wavelength λ2. In particular, the light source102generates a broadband optical signal with wavelengths at least between the minimum wavelength λ1and a maximum wavelength λ2of the sweeping optical signal. The optical amplifier104amplifies the broadband optical signal to a defined power level suitable for performing parameter measurements using the optical fiber112. The tunable filter106filters the amplified broadband optical signal based on a scan signal generated by the scan generator108in order to produce the sweeping optical signal (e.g., SWL ofFIG. 1B).

The sweeping optical signal (λ1-λ2) is applied to an input end of the optical fiber112by way of the coupler110. The sweeping optical signal propagates within the optical fiber112towards the other end of the optical fiber. As previously discussed, the FBG structure114of the optical fiber112reflects 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 λBpropagates back to the input end of the optical fiber112and to the coupler110, where it directs the reflected optical signal λBto the photo detector116.

The photo detector116generates an electrical signal based on the reflected optical signal λB. The electrical signal from the photo detector116is applied to the comparator118, 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 detector116before the electrical signal is applied to the comparator118. Such additional components may include an amplifier for amplifying the electrical signal and a filter for removing noise from the electrical signal. The comparator118generates a signal including a high logic-level if the electrical signal from the photo detector116exceeds the threshold (TH), and a low logic-level signal if the electrical signal from the photo detector116does not exceed the threshold (TH).

The processor120receives the scan signal from the scan generator108and the signal from the comparator118. The scan signal is processed to establish the time of the start of the wavelength scan, and the signal from the comparator118indicates the time of arrival of the reflected optical signal. Thus, the processor120is able to determine the wavelength λBof 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 comparator118. Since, as previously discussed, the wavelength λBof the reflected optical signal depends on the parameter being sensed by the FBG structure114(e.g., temperature, strain, pressure, etc.), the processor120is able to generate an indication or output indicative of the sensed parameter based on the determined wavelength λB.

FIG. 1Billustrates a timing diagram of the signals associated with the operation of the apparatus100in accordance with another aspect of the disclosure. The upper graph is a timing diagram of an exemplary scan signal generated by the scan generator108. The middle diagram is a timing diagram of an exemplary electrical signal generated by the photo detector116. And, the lower graph is a timing diagram of an exemplary signal generated by the comparator118.

According to the upper graph, the scan signal generated by the scan generator108may 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 SWL101to generate an optical signal with a wavelength linearly increasing from the minimum wavelength λ1to the maximum wavelength λ2. Similarly, the falling portion of the scan signal causes the SWL101to generate an optical signal with a wavelength linearly decreasing from the maximum wavelength λ2to 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 detector116in response to receiving the optical signal reflected by the FBG structure114of the optical fiber112. As previously discussed, the wavelength λBof the reflected optical signal depends on the configuration of the FBG structure114as 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 λ1and λ2in a defined (predictable) manner, the time of arrival of the reflected optical signal may be mapped to the particular wavelength λBof the reflected optical signal. Thus, in this example, the times t1and t2in the first period corresponds to the wavelength λB, the times t3and t4in the second period corresponds to the wavelength λB, and the times t5and t6in the third period corresponds to the wavelength λB.

With reference to the middle and lower graphs, the comparator118compares the electrical signal from the photo detector116with 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 comparator118, which includes substantially square-wave pulses that coincides with times t1to t6, all of which coincide with the wavelength λBof the reflected optical signal.

The processor120receives the signal from the comparator118and maps the timings t1to t6to the wavelength λB(or wavelengths λB1to 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 generator108. As previously discussed, the wavelength λBof the reflected optical signal depends on the one or more parameters being sensed by the FBG structure114of the optical fiber112, which may vary over time. Thus, the processor120generates an output or indication of the one or more sensed parameters based on the wavelengths λB1to λB6.

FIG. 2Aillustrates a block diagram of another exemplary apparatus200in accordance with another aspect of the disclosure. The apparatus200is similar to that of apparatus100, and includes many of the same or similar elements as indicated by the same reference numbers, but with a “2” as the most significant digit rather than a “1”. The detail discussion of such same or similar elements has been provided above with reference to apparatus100. The apparatus200differs from that of apparatus100in that apparatus200includes a different type of comparator. That is, the apparatus200includes a comparator employing a defined threshold and hysteresis function (e.g., essentially mimicking upper and lower thresholds TH1and TH2) that govern an output state of the comparator. An example of such comparator is a Schmitt trigger.

Although a single input threshold comparator (e.g., comparator118) and a Schmitt trigger (e.g., comparator219) 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 toFIG. 2A, the apparatus200comprises an SWL201including light source202, optical amplifier204, and tunable filter206. The apparatus200further includes a scan generator208, coupler210, and optical fiber212including an FBG structure214. Additionally, the apparatus200comprises a photo detector216, a comparator219, and a processor220.

The comparator219generates a high logic-level from a low logic-level in response to the electrical signal from the photo detector216initially exceeding the upper threshold TH2. The comparator219continuous to generate the high logic-level as long as the electrical signal exceeds the lower threshold TH1. Once the electrical signal from the photo detector216falls below the lower threshold TH1, the comparator219generates the low logic-level. The comparator219is useful in combating noise that may be present in the electrical signal generated by the photo detector216.

FIG. 2Billustrates a timing diagram of the signals associated with the operation of the apparatus200in accordance with another aspect of the disclosure. The signal timing diagram of apparatus200is similar to that of signal timing diagram of apparatus100, except that noise is present in the electrical signal from the photo detector216. Because of the dual threshold hysteresis response of the comparator219, the comparator generates a single square wave pulse even though the electrical signal of the photo detector216crosses the upper threshold TH1multiple times due to noise. The single pulse per reflected optical signal makes it easier for the processor220to determine the time of arrival of the reflected optical signal, and therefrom, the wavelength λBof the reflected optical signal. As in the previous embodiment, the processor220generates an output or indication of one or more parameters sensed by the FBG structure214based on the determined wavelength λBor time of arrival of the reflected optical signal.

For additional protection against noise, the processor220may be configured to employ a discrimination filtering of the signal generated by the comparator219. In particular, the discrimination filtering may reject pulses from the comparator219having width shorter a defined minimum duration. This keeps any fast noise from setting off nuisance false positive detections. If the hysteresis effect of the comparator219is not sufficient to suppress all the unwanted noise, the discrimination filtering employed by the processor220provides the additional noise suppression.

FIG. 3Aillustrates a block diagram of another exemplary apparatus300in accordance with another aspect of the disclosure. The apparatus300is similar to that of apparatuses100and200, and includes many of the same or similar elements as indicated by the same reference numbers, but with a “3” as the most significant digit rather than a “2” or “1”. The detail discussion of such same or similar elements has been provided above with reference to apparatuses100and200. The apparatus300differs from that of apparatus200in that apparatus300includes an optical fiber that has a plurality of FBG structures. This allows a single optical fiber to be configured to measure a plurality of parameters located at different parts of a system.

More specifically, the apparatus300comprises an SWL301, a scan generator308, a coupler310, and an optical fiber312including FBG structures313,314, and315. Additionally, the apparatus300comprises a photo detector316, a comparator319, and a processor320.

The FBG structures313,314, and315are 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 λB3of the FBG structures should be sufficiently spaced apart such that the corresponding electrical signals generated by the photo detector316do not interfere with each other in a manner that results in error in the measurement of the corresponding sensed parameters.

FIG. 3Billustrates a timing diagram of the signals associated with the operation of the apparatus300in accordance with another aspect of the disclosure. The signal timing diagram of apparatus300is similar to that of signal timing diagrams of apparatuses100and200, except that for each “half” cycle of the scan signal, there are three (3) electrical signals generated (one for each of the reflected optical signals from FBG structures313,314, and315, instead of one in the case of single FBG structure114and214in apparatuses100and200, respectively).

The upper graph illustrates that a wavelength range is associated with a corresponding natural wavelength. For instance, wavelength ranges ΔλB1, ΔλB2, and ΔλB3are associated with natural wavelengths λB1, λB2, and λB3of the FBG structures313,314, and315, respectively. The wavelength ranges ΔλB1, ΔλB2, and ΔλB3are the possible wavelengths of optical signals reflected by the FBG structures313,314, and315as affected by the one or more parameters sensed by the FBG structures, respectively. As the upper graph illustrates, the wavelengths corresponding to times t1to t6are: at, below, above, at, below, and below the corresponding natural wavelengths, respectively. Similarly, the wavelengths corresponding to times t7to t12are: 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 structures313,314, and315affect the time of arrival of the signals at the photo detector319. Thus, the time of arrivals t1-t12depend on the wavelengths of the optical signals reflected by the FBG structures313,314, and315in the illustrated two scan cycle example shown inFIG. 3B. The comparator319“cleans” the output of the photo detector316to generate substantially square wave pulses at times t1to t12. The processor320receives the pulses and generates an output or indication of the one or more parameters sensed by the FBG structures313,314, and315based on the times t1to t12and the scan signal, as previously discussed.

FIG. 4illustrates a block diagram of yet another exemplary apparatus400in accordance with another aspect of the disclosure. The apparatus400is similar to that of apparatus300, and includes many of the same or similar elements as indicated by the same reference numbers, but with a “4” as the most significant digit rather than a “3”. The detail discussion of such same or similar elements has been provided above with reference to apparatus300. The apparatus400differs from that of apparatus300in that apparatus400includes two optical fibers connected in parallel, each having a plurality of FBG structures. Thus, optical fibers with FBG structures may be connected in parallel to increase the number of parameters of a system being sensed.

The SWL401generates the sweeping optical signal (λ1-(λ2) in accordance with the scan signal generated by the scan generator408. The optical splitter403splits the sweeping optical signal into a first sweeping optical signal for optical fiber412aand a second sweeping optical signal for optical fiber412b. The first and second sweeping optical signals are applied to inputs of the optical fibers412aand412bby way of couplers410aand410b, respectively. In response to the sweeping optical signals, the FBG structures413a,414a,415a,413b,414b, and415bproduce reflected optical signals with wavelengths (λB1, (λB2, (λB3, (λB4, (λB5, and (λB6, respectively.

The reflected optical signals from FBG structures413a,414a, and415apropagate to photo detector416aby way of coupler410a. Similarly, the reflected optical signals from FBG structures413b,414b, and415bpropagate to photo detector416bby way of coupler410b. As previously discussed, the photo detector416aand corresponding comparator419aare configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures413a,414a, and415aare received by the photo detector416a. Similarly, the photo detector416band corresponding comparator419bare configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures413b,414b, and415bare received by the photo detector416b.

The processor420is configured to receive the square wave pulses from the comparators419aand419b. The processor420generates an output including indications related to the one or more parameters sensed by the FBG structures413a,414a,415a,413b,414b, and415bbased on the times the processor receives the pulses from the comparators419aand419b, and the scan signal.

FIG. 5illustrates a block diagram of still another exemplary apparatus500in accordance with another aspect of the disclosure. The apparatus500is similar to that of apparatus400, and includes many of the same or similar elements as indicated by the same reference numbers, but with a “5” as the most significant digit rather than a “4”. The detail discussion of such same or similar elements has been provided above with reference to apparatus400. The apparatus500differs from that of apparatus400, in that one of the optical fibers of apparatus500is used as a reference subjected to a controlled temperature TRand mechanical strain εRenvironment (e.g., TRand εRare substantially constant (e.g., εR˜0)).

The SWL501generates the sweeping optical signal ((λ1-(λ2) in accordance with the scan signal generated by the scan generator508. The optical splitter503splits the sweeping optical signal into a first sweeping optical signal for sensing optical fiber512aand a second sweeping optical signal for reference optical fiber512b. The first and second sweeping optical signals are applied to inputs of optical fibers512aand512bby way of couplers510aand510b, respectively. In response to the sweeping optical signals, the FBG structures513a,514a,515a,513b,514b, and515bproduce reflected optical signals with wavelengths λB1, λB2, λB3, λB4, λB5, and λB6, respectively.

The FBG structures513a,514a, and515aof the sensing optical fiber512aare configured to sense the temperatures T1, T2, and T3of certain components550,552, and554of a system, respectively. In this example, the components550,552, and554do not impart any significant mechanical strain (e.g., εR˜0) on the FBG structures513a,514a, and515a, respectively. The reference optical fiber512bis situated within a separate temperature and strain controlled environment560. Thus, the wavelengths λB1, λB2, and λB3of the reflected optical signals of the sensing optical fiber512avaries with the temperatures T1, T2, and T3of the components550,552, and554, which depend on heat emitted by the components. The wavelengths λB4, λB5, and λB6of the reflected optical signals of the reference optical fiber512bis substantially constant due to the controlled temperature and strain environment560.

The reflected optical signals from FBG structures513a,514a, and515apropagate to photo detector516aby way of coupler510a. Similarly, the reflected optical signals from FBG structures513b,514b, and515bpropagate to photo detector516bby way of coupler510b. As previously discussed, the photo detector516aand corresponding comparator519aare configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures513a,514a, and515aare received by the photo detector516a. Similarly, the photo detector516band corresponding comparator519bare configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures513b,514b, and515bare received by the photo detector516b.

The processor520is configured to receive the square wave pulses from the comparators519aand519b. The processor520generates 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 generator508, and the time differences between the times the processor520receives the pulses from comparator519aand the times the processor520receives the pulses from comparator519b, respectively.

FIG. 6illustrates a block diagram of an additional exemplary apparatus600in accordance with another aspect of the disclosure. The apparatus600is similar to that of apparatus500, and includes many of the same or similar elements as indicated by the same reference numbers, but with a “6” as the most significant digit rather than a “5”. The detail discussion of such same or similar elements has been provided above with reference to apparatus500. The apparatus600differs from that of apparatus500, in that one of the optical fibers of apparatus600is used as a reference subjected to substantially the same reference temperature TRas a sensing optical fiber, but not subjected to any mechanical strain (e.g., a reference strain εR˜0).

The SWL601generates the sweeping optical signal (λ1-λ2) in accordance with the scan signal generated by the scan generator608. The optical splitter603splits the sweeping optical signal into a first sweeping optical signal for sensing optical fiber612aand a second sweeping optical signal for reference optical fiber612b. The first and second sweeping optical signals are applied to inputs of optical fibers612aand612bby way of couplers610aand610b, respectively. In response to the sweeping optical signals, the FBG structures613a,614a,615a,613b,614b, and615bproduce reflected optical signals with wavelengths λB1, λB2, λB3, λB4, λB5, and λB6, respectively.

The FBG structures613a,614a, and615aof the sensing optical fiber612aare configured to sense mechanical strain ε1, ε2, and ε3upon certain components650,652, and654of a system, respectively. The sensing and reference optical fibers612aand612bare situated within an environment660that subjects both optical fibers to substantially the same reference temperature TR. Thus, the wavelengths λB1, λB2, and λB3of the reflected optical signals of the sensing optical fiber612avary with the mechanical strain ε1, ε2, and ε3upon components650,652, and654of a system and the reference temperature. The wavelengths λB4, λB5, and λB6of the reflected optical signals of the reference optical fiber612bvaries only with the reference temperature TR, as the reference mechanical strain εRis substantially zero (0).

The reflected optical signals from FBG structures613a,614a, and615apropagate to photo detector616aby way of coupler610a. Similarly, the reflected optical signals from FBG structures613b,614b, and615bpropagate to photo detector616bby way of coupler610b. As previously discussed, the photo detector616aand corresponding comparator619aare configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures613a,614a, and615aare received by the photo detector616a. Similarly, the photo detector616band corresponding comparator619bare configured to generate substantially square wave pulses corresponding to the times the reflected optical signals from FBG structures613b,614b, and615bare received by the photo detector616b.

The processor620is configured to receive the square wave pulses from the comparators619aand619b. The processor620generates an output including indications related to the strain ε1, ε2, and ε3upon components650,652, and654based on the scan signal from the scan generator608, and the time differences between the times the processor620receives the pulses from comparator619aand the times the processor620receives the pulses from comparator619b, respectively.