Description:FIELD OF THE INVENTION
The present disclosure relates to optical sensing techniques and more particularly, relates to a system and a method to perform an intensity based optical interrogation.
BACKGROUND
Optical sensing involves using light (visible or invisible) to detect and measure various parameters without direct physical contact. Typically, optical sensing involves converting light signals into electrical signals, enabling precise and multifunctional processing of data from optical sensors. The optical sensors, such as Fiber Bragg Grating (FBG) sensors, are commonly employed for tasks such as detection of changes in physical parameters such as strain and temperature distance measurements, and even more complex applications like biochemical detection.
Optical sensing with FBG sensors offers several advantages compared to conventional techniques, particularly for remote sensing in harsh environments. A ratiometric optical interrogation is a commonly employed technique for optical sensing using the FBG sensors, as discussed in the forthcoming paragraphs in conjunction with Figure 1. The FBG sensors provide Electromagnetic Interference (EMI)-free sensing with a linear response, thereby reducing the complexity of measurement systems. The FBG sensors detect changes in environmental parameters (for example, strain and temperature) by monitoring shifts in Bragg wavelengths using an Optical Spectrum Analyzer (OSA). While the OSAs are effective and widely used for monitoring wavelength shifts, the OSAs are generally bulky, expensive, and not easily portable. Moreover, the OSAs often require considerable time for initial optical alignment and wavelength calibration, which introduces a warm up time. In addition, an intermittent calibration ‘zeroing’ takes place in the OSAs, during which the OSAs do not accept any input signal.
Figure 1 illustrates a schematic diagram of a ratiometric optical interrogator 100 for monitoring an applied strain variations using an FBG sensor 112, in accordance with the related art. The ratiometric optical interrogator 100 (hereinafter referred to as the “interrogator 100”) includes a broadband source (BBS) 102, a circulator 108, the FBG sensor 112, a 1x2 signal splitter 114, an edge filter 116, two photodetectors (PD) 118-1 and 118-2, two Analog-To-Digital Converters (ADCs) 120-1 and 120-2, and two Digital Signal Processing (DSP) units 122-1 and 122-2.
The circulator 108 is a passive optical device configured to route optical signals in a specific direction through its one or more ports, thereby allowing for a unidirectional flow of the optical signals. As shown in Figure 1, the circulator 108 has three ports, viz., a first port 110-1, a second port 110-2, and a third port 110-3. The BBS 102 is connected to the first port 110-1 of the circulator and is configured to generate one or more pulsed signals. The BBS 102 may be driven by an electrical input 104, providing a wide spectral output through a broadband emitter such as a light-emitting diode (LED) 106. The FBG sensor 112 is connected to the second port 110-2 of the circulator 108 and is configured to receive the one or more pulsed signals from the BBS 102 through the circulator 108 via the second port 110-2. For each of the one or more pulsed signals, the FBG sensor 112 reflects a signal pulse based on the applied strain at the Bragg wavelength to the second port 110-2, which is then internally passed by the circulator 108 to the third port 110-3. The 1x2 splitter 114 is connected to the third port 110-3 and is configured to split the reflected signal pulse into two equal parts, which comprises a reference path (XR) and a signal path (XS). The 1x2 splitter 114 may be a 3 decibel (dB) splitter. From the reflected signal pulse, the 1x2 splitter 114 derives a reference signal along the path XR, and a sensed signal along the path XS. The sensed signal is directed through the edge filter 116 having an associated characteristics curve. The edge filter 116 is configured to convert the shift in the Bragg wavelength into corresponding power changes. The reflected Bragg wavelength is selected to fall on a slope region of a characteristics curve of the edge filter 116. When the edge filter 116 has a positive slope, the intensity of an output signal of the edge filter 116 increases proportionally with the shift in the Bragg wavelength. Further, the reference signal and the sensed signal from the reference path and the signal path are independently detected by the PD 118-2 and the PD 118-1, respectively. The PD 118-1 and the PD 118-2 convert the output of the edge filter 116 into electrical signals. Thereafter, the ADC 120-1 and the ADC 120-2 convert the electrical signals received from the PD 118-1 and the PD 118-2 respectively into digital signals. Finally, the DSP 122-1 and the DSP 122-2 analyze the digital signals received from the ADC 120-1 and the ADC 120-2 respectively to extract information about the applied strain.
Assuming a total input power across all wavelengths remains constant, denoted as Pi, a reflected power from the FBG sensor 112 is PFS, which is determined by an equation (1) provided below:
P_FS= R_S P_i … (1)
In the equation (1), RS is a reflection coefficient of the FBG sensor 112. A photocurrent I generated by a photodetector (PD) is dependent on a responsivity R of the PD and an optical power incident on the PD. The photocurrent I is determined by an equation (2) provided below:
I= R.P_FS … (2)
Thus, photocurrents at the PD 118-1 and the PD 118-2 corresponding to the paths XS and XR are given by equations (3a) and (3b), respectively, as provided below:
I_S= R.P_FS .(1- A) m_S … (3a)
I_R= R.P_FS .A … (3b)
In the equations (3a)-(3b), A: (1- A) is 1x2 coupler splitting ratio indicating fractional optical powers directed to the reference and signal paths. The mS is a transmission coefficient of the edge filter 116.
The photocurrents IS and IR are converted to voltages using a current-to-voltage conversion circuit with a load resistance RL, given by an equation (4) as provided below:
V_R= R.P_FS.A .R_L=I_R .R_L
〖 V〗_S= R.P_FS.(1- A).m_S .R_L =I_S .R_L … (4)
The optical powers PR and PS received in the reference and signal paths at an optical receiver, or the optical power analyzed via an OSA, are given by an equation (5) provided below:
P_R= V_R .I_R
P_S= V_S .I_S … (5)
The ratio (RT) of the voltages VR and VS is used to determine the applied strain using an equation (6) as provided below:
R_T= V_S/V_R = ((1- A) )/A m_S … (6)
It should be noted that the reflected power PFS is linearly dependent on the input power Pi from the equation (1), and any fluctuations in the input power equally affect both the VR and the VS. Consequently, the ratio RT remains unaffected according to the equation (6).
In a first example scenario, a 3 dB splitter with a splitting ratio A = 0.5 is used. Thus, the equation (6) becomes:
R_T= m_s … (7)
In a second example scenario, a 3 dB splitter with the splitting ratio A: (1- A) = 0.3:0.7 is used. Thus, equation (6) becomes:
R_T= 7/3 m_s … (8)
Further, in a third example scenario, a 3dB splitter with the splitting ratio A: (1- A) = 0.1:0.9 is used. Thus, the equation (6) becomes:
R_T= 9 m_s … (9)
Among the above three example scenarios, the ratio RT is highest in the third example scenario due to the VS being the highest of all the example scenarios.
The reflected Bragg wavelength (λ_M) is also calculated using an equation (10) as provided below:
λ_M= m .W+ λ_I … (10)
In the equation (10), W is a weight applied to the FBG sensor 112, m is a slope of plot λ versus W, λI is an initial (intercept) wavelength before applying the strain. Thus, the weight is given by the equation (11) provided below:
W= ((λ_M-λ_I ))/m … (11)
In an example scenario, weights ranging from 50 grams to 2000 grams are applied to a strain-sensing FBG sensor, which resulted in an average sensitivity or wavelength shift of 1.134 pm/gram.
As the optical power (PR) in the reference path (XR) is derived from the reflected sense signal, the optical power (PS) available in the signal path (XS) is reduced depending on the splitter ratio. Such a reduction in the PS adversely affects a signal power in the signal path (XS). Further, in the interrogator 100, the third port 110-3 of the circulator 108 accommodates both the reference and signal paths. Such a configuration requires the 1x2 splitter 114, the two photodetectors 118-1 and 118-2, the two ADCs 120-1 and 120-2, and two DSP units 122-1 and 122-2, which leads to increased power consumption, higher cost, and added design complexity.
Thus, there is a need for improved optical sensing techniques which address the above shortcomings. Further, there is a need for enhanced optical sensing techniques which are cost-effective, compact, and reliable.
The information disclosed in this background of the disclosure section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art already known to a person skilled in the art.
SUMMARY
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention nor is it intended for determining the scope of the invention.
In an embodiment of the present disclosure, a system to perform an intensity-based optical interrogation is disclosed. The system includes a transmitting module configured to generate one or more pulsed signals. The system also includes a Fiber Bragg Grating (FBG) module configured to receive the one or more pulsed signals. The FBG module includes a sensing FBG configured to reflect a sensing signal pulse at a sensing Bragg wavelength corresponding to each of the one or more pulsed signals. The FBG module also includes a reference FBG configured to reflect a reference signal pulse at a reference Bragg wavelength corresponding to each of the one or more pulsed signals. The FBG module further includes an optical waveguide to interconnect the sensing FBG and the reference FBG. The system further includes an edge filter configured to convert a shift in the sensing Bragg wavelength into an intensity variation based on a characteristics curve, for optical interrogation. The sensing Bragg wavelength and the reference Bragg wavelength are positioned on a slope region and on a flat region of the characteristics curve, respectively. Further, the shift in the sensing Bragg wavelength and the corresponding intensity variation after passing through the edge filter are induced by one or more physical parameters acting on the sensing FBG.
In another embodiment of the present disclosure, a method to perform an intensity based optical interrogation is disclosed. The method includes generating, by a transmitting module, one or more pulsed signals. The method also includes receiving, by a Fiber Bragg Grating (FBG) module, the one or more pulsed signals. The FBG module includes a sensing FBG, a reference FBG, and an optical waveguide to interconnect the sensing FBG and the reference FBG. The method further includes reflecting, by the sensing FBG, a sensing signal pulse at a sensing Bragg wavelength corresponding to each of the one or more pulsed signals. Furthermore, the method includes reflecting, by the reference FBG, a reference signal pulse at a reference Bragg wavelength corresponding to each of the one or more pulsed signals. Still further, the method includes converting, by an edge filter, a shift in the sensing Bragg wavelength into an intensity variation based on a characteristics curve for optical interrogation. The sensing Bragg wavelength and the reference Bragg wavelength are positioned on a slope region and on a flat region of the characteristics curve, respectively. Moreover, the shift in the sensing Bragg wavelength and the corresponding intensity variation after passing through the edge filter are induced by one or more physical parameters acting on the sensing FBG.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features and characteristics of the disclosure are set forth in the appended claims. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:
Figure 1 illustrates a schematic diagram of a ratiometric optical interrogator for monitoring applied strain variations using a Fiber Bragg Grating (FBG) sensor, in accordance with the related art;
Figure 2 illustrates a schematic diagram of a system to perform the intensity based optical interrogation, in accordance with an embodiment of the present disclosure;
Figure 3 illustrates a characteristics curve associated with an edge filter, in accordance with an embodiment of the present disclosure;
Figures 4A-4B illustrate exemplary plots depicting a linear relationship between an applied weight and wavelength shifts, in accordance with an embodiment of the present disclosure;
Figure 5 illustrates a wavelength versus strain plot, in accordance with an embodiment of the present disclosure;
Figures 6A-6B illustrate exemplary plots of optical power variation detected at a photodetector with respect to the applied weight, in accordance with an embodiment of the present disclosure;
Figures 7A-7B illustrate exemplary plots of a ratio RT with respect to the applied weight, in accordance with an embodiment of the present disclosure;
Figures 8A-8D illustrate exemplary wavelength spectrums of a sensing FBG and a reference FBG with respect to the applied weight, in accordance with an embodiment of the present disclosure;
Figures 9A-9D illustrate exemplary pulses reflected from the sensing FBG and the reference FBG with respect to the applied weight, in accordance with an embodiment of the present disclosure; and
Figure 10 illustrates a flowchart for a method to perform the intensity based optical interrogation, in accordance with an embodiment of the present disclosure.
It should be appreciated by those skilled in the art that any block diagram herein represents conceptual views of illustrative systems embodying the principles of the present subject matter. Similarly, it will be appreciated that any flowcharts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and executed by a computer or processor, whether or not such computer or processor is explicitly shown.
DETAILED DESCRIPTION OF FIGURES
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.
The term “some” as used herein is defined as “none, or one, or more than one, or all.” Accordingly, the terms “none,” “one,” “more than one,” “more than one, but not all” or “all” would all fall under the definition of “some.” The term “some embodiments” may refer to no embodiments or to one embodiment or to several embodiments or to all embodiments. Accordingly, the term “some embodiments” is defined as meaning “no embodiment, or one embodiment, or more than one embodiment, or all embodiments.”
The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict or reduce the spirit and scope of the claims or their equivalents.
More specifically, any terms used herein such as but not limited to “includes,” “comprises,” “has,” “consists,” and grammatical variants thereof do NOT specify an exact limitation or restriction and certainly do NOT exclude the possible addition of one or more features or elements, unless otherwise stated, and furthermore must NOT be taken to exclude the possible removal of one or more of the listed features and elements, unless otherwise stated with the limiting language “MUST comprise” or “NEEDS TO include.”
Whether or not a certain feature or element was limited to being used only once, either way, it may still be referred to as “one or more features” or “one or more elements” or “at least one feature” or “at least one element.” Furthermore, the use of the terms “one or more” or “at least one” feature or element do NOT preclude there being none of that feature or element, unless otherwise specified by limiting language such as “there NEEDS to be one or more . . .” or “one or more element is REQUIRED.”
Unless otherwise defined, all terms, and especially any technical and/or scientific terms, used herein may be taken to have the same meaning as commonly understood by one having ordinary skills in the art.
Reference is made herein to some “embodiments.” It should be understood that an embodiment is an example of a possible implementation of any features and/or elements presented in the attached claims. Some embodiments have been described for the purpose of illuminating one or more of the potential ways in which the specific features and/or elements of the attached claims fulfil the requirements of uniqueness, utility and non-obviousness.
Use of the phrases and/or terms such as but not limited to “a first embodiment,” “a further embodiment,” “an alternate embodiment,” “one embodiment,” “an embodiment,” “multiple embodiments,” “some embodiments,” “other embodiments,” “further embodiment”, “furthermore embodiment”, “additional embodiment” or variants thereof do NOT necessarily refer to the same embodiments. Unless otherwise specified, one or more particular features and/or elements described in connection with one or more embodiments may be found in one embodiment, or may be found in more than one embodiment, or may be found in all embodiments, or may be found in no embodiments. Although one or more features and/or elements may be described herein in the context of only a single embodiment, or alternatively in the context of more than one embodiment, or further alternatively in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.
Any particular and all details set forth herein are used in the context of some embodiments and therefore should NOT be necessarily taken as limiting factors to the attached claims. The attached claims and their legal equivalents can be realized in the context of embodiments other than the ones used as illustrative examples in the description below.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or an apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus.
The present disclosure relates to a system and a method to perform an intensity-based optical interrogation. The optical interrogation may refer to a method of extracting information from an optical sensor, such as a Fiber Bragg Grating (FBG) sensor. FBG sensors are passive dielectric components that are used to sense physical parameters like strain, temperature, pressure, and vibration. Particularly, an FBG is a periodic modulation of the refractive index along the core of an optical fiber, which creates a wavelength-specific mirror inside the optical fiber. When an optical signal including multiple wavelengths is directed into the optical fiber, the FBG sensor reflects a narrow band of wavelengths, called Bragg wavelength (λB), and the remaining wavelengths are transmitted through the FBG sensor. The Bragg wavelength of the FBG sensor is given by an equation (12) provided below:
λ_B=2 n_eff∧ … (12)
In the equation (12), ∧ is a pitch of the FBG sensor, n_eff is an effective refractive index of the core of the optical fiber. When the optical fiber is exposed to strain, the optical fiber stretches or compresses, which in turn changes the pitch and the effective refractive index. Additionally, when the optical fiber is exposed to temperature, thermal expansion and refractive index shift are observed in the optical fiber. Such changes cause a shift in the Bragg wavelength (λB), which is given by an equation (13) provided below:
(Δλ_B)/λ_B =("1- " "P" _"e" ).ε+ (α_e - α_o )Δt … (13)
In the equation (13), Pe is a strain-optic coefficient of a silica optical fiber (i.e., Pe = 0.22), ε is an applied strain, α_e is a thermal expansion coefficient of the silica optical fiber, α_o is a thermo-optic coefficient of the silica optical fiber, and Δt is the change in the temperature.
In an example scenario, only the strain on the FBG sensor is varied by applying weight, and hence the shift in the Bragg wavelength due to the applied strain is given by an equation (14). As there is no change in the temperature, the effect of the temperature is neglected.
Δλ_B=〖("1- " "P" _"e" ).ε .λ〗_B … (14)
The present disclosure provides a system and a method to perform an intensity based optical interrogation using an edge filter and a reference FBG to translate wavelength shifts into linear intensity variation to solve the above-mentioned problems associated with conventional techniques, as discussed in detail in the forthcoming paragraphs in conjunction with Figures 2-10.
Figure 2 illustrates a schematic diagram of a system 200 to perform the intensity based optical interrogation, in accordance with an embodiment of the present disclosure. The system 200 may include a transmitting module 202, a circulator 208, an FBG module 211, an edge filter 218, a PD 220, an ADC 222, and a DSP unit 224. In an embodiment, the system 200 may also include an Optical Spectrum Analyzer (OSA) 226.
In an embodiment, the transmitting module 202 may be configured to generate the one or more pulsed signals. The transmitting module 202 may include a pulse BBS, modulated by a signal having a pulse width (TW) of 90 ns and a period of 200 KHz (therefore, a pulse repetition rate (TR) of 1/200 KHz = 5 µs). The pulse BBS may correspond to the BBS 102 and may be driven by an electrical input 204, providing a wide spectral output through an LED 206.
In an embodiment, the circulator 208 may include at least a first port 210-1, a second port 210-2, and a third port 210-3. The transmitting module 202, the FBG module 211, and the edge filter 218 may be connected to the first port 210-1, the second port 210-2, and the third port 210-3, respectively. The circulator 208 may be configured to route the one or more pulsed signals from the transmitting module 202 to the FBG module 211. The circulator 208 may correspond to the circulator 108.
In an embodiment, the FBG module 211 may be configured to receive the one or more pulsed signals through the circulator 208. The FBG module 211 may include a sensing FBG 212 configured to reflect a sensing signal pulse at a sensing Bragg wavelength (λS) corresponding to each of the one or more pulsed signals. The FBG module 211 may also include a reference FBG 214 configured to reflect a reference signal pulse at a reference Bragg wavelength (λR) corresponding to each of the one or more pulsed signals. The circulator 208 may be configured to route the sensing signal pulse and the reference signal pulse corresponding to each of the one or more pulsed signals from the FBG module 211 to the edge filter 218. In the present disclosure, the reference signal is obtained from the reference FBG 214, which is distinct from the sensing FBG 212. Therefore, for a single pulsed signal generated by the transmitting module 202, both the sensing signal pulse and the reference signal pulse are generated.
In an embodiment, the FBG module 211 may include an optical waveguide 216 to interconnect the sensing FBG 212 and the reference FBG 214. In a non-limiting example, the optical waveguide 216 may interconnect the sensing FBG 212 and the reference FBG in series. A length (d) of the optical waveguide 216 may be selected based on the pulse width (TW) and the pulse repetition rate (TR) associated with the one or more pulsed signals. Examples of the optical waveguide (216) may include, but are not limited to, a Single-Mode Fiber (SMF), a Multimode Fiber (MMF), and similar fiber optic components for coupling the light.
In an example embodiment, the pulse width (TW) may be 90 ns and the pulse repetition rate (TR) may be 5 µs (1/200KHz). Particularly, the length (d) may be selected such that a round-trip delay time (t) falls within an interval defined in an equation (15) provided below, 90 ns < t < 4.91 µs, (5 µs – 90 ns= 4.91 µs). Accordingly, the length d = 12 m is selected, which corresponds to the round-trip delay time (t) of 120 ns.
T_W 9.306 m. Thus, selecting d = 12 m between the two FBGs 212 and 214 satisfies the above equations.
In an embodiment, the edge filter 218 may be configured to convert a shift in the sensing Bragg wavelength (λS) into an intensity variation based on a characteristics curve, for optical interrogation. The characteristics curve may be associated with the edge filter 218 and refer to a graphical representation describing a response of the edge filter 218 to different wavelengths.
Figure 3 illustrates a characteristics curve 300 associated with the edge filter 218, in accordance with an embodiment of the present disclosure. As shown in Figure 3, the sensing Bragg wavelength (λS) and the reference Bragg wavelength (λR) may be positioned on a slope region (a positive slope region 302 and a negative slope region 304) and on a flat region 306 of the characteristics curve 300, respectively. The slope region of the characteristics curve 300 may include the positive slope region 302 and the negative slope region 304. Further, the sensing Bragg wavelength (λS) may be positioned on one of a lower end of the positive slope region 302 or an upper end of the negative slope region 304.
In an example scenario, the characteristics curve 300 may span 1538–1562 nm wavelength range and include the positive slope region 302, the flat region 306, and the negative slope region 304, as shown in Figure 3. In the curve 300, the positive slope region of approximately 10 dB/nm between 1538–1542 nm may be utilized for sensing. Further, the sensing Bragg wavelength (λS) of 1539 nm, with a reflectivity of 74.34% may be selected, which is positioned at the lower end of the positive slope region. The positioning of the sensing Bragg wavelength (λS) ensures that any wavelength shift due to, e.g., strain variations remains within a linear response region of the edge filter 218. Furthermore, the reference Bragg wavelength (λR) of 1546 nm, with the reflectivity of 74.82%, may be selected, which lies within the flat region of the curve 300, thereby ensuring minimal intensity variation. As the applied weight increases, the sensing Bragg wavelength (λS) may shift toward higher wavelengths along the positive slope, resulting in an increased intensity level of the sensing signal pulse after passing through the edge filter 218, while the reference signal pulse may maintain a consistent intensity for a given input pulse power. In an example scenario, the output power measured at each wavelength may demonstrate a linear relationship with the applied weight in the range of 400 to 1650 grams. For weights between 1700 and 2000 grams, correction factors may be applied to account for the nonlinearity of the edge filter’s response in the upper portion of the positive slope. Such a correction effectively extends a measurement range, as explained in detail in the forthcoming paragraphs in conjunction with Figures 6-7.
Referring to Figure 2, the shift in the sensing Bragg wavelength (λS) and the corresponding intensity variation after passing through the edge filter 218 may be induced by one or more physical parameters acting on the sensing FBG 212. The one or more physical parameters acting on the sensing FBG 212 may include strain, temperature, pressure, and vibration. In a non-limiting example, the edge filter 218 may correspond to a fused-fiber demultiplexer.
In an embodiment, the PD 220 may be operatively connected to an output of the edge filter 218. The PD 220 may also be configured to convert the intensity variation into one or more electrical signals. Further, the ADC 222 may be operatively connected to an output of the PD 220 and may be configured to convert the one or more electrical signals into one or more digital signals. Furthermore, the DSP unit 224 may be operatively connected to an output of the ADC 222 and may be configured to determine one or more insights associated with the sensing FBG 212 based on the one or more digital signals. The one or more insights may correspond to information associated with the one or more physical parameters acting on the sensing FBG 212.
In an example scenario, when a weight is applied at the sensing FBG 212, the sensing Bragg wavelength (λS) is shifted due to strain created in the sensing FBG because of the applied weight. When the reflected signal with shifted sensing Bragg wavelength (λS) is passed through the edge filter 218, an intensity variation is observed, which is detected by the PD 220. The intensity variation is further processed by the ADC 222 and the DSP unit 224 to determine the one or more insights, as explained above.
In an alternative embodiment, the OSA 226 may be operatively connected to the output of the edge filter 218. Further, the OSA 226 may be configured to measure at least one of the sensing Bragg wavelength (λS) or an optical power associated with the sensing Bragg wavelength (λS).
In an example scenario, the system 200 with a sensing FBG 212 having the sensing Bragg wavelength (λS) of 1539 nm and the reference FBG 214 having the reference Bragg wavelength (λR) of 1546 nm, which are interconnected by the optical waveguide 216 having the length d = 12 m is used to measure the strain on the sensing FBG 212 by applying the weight, which varies in terms of optical intensity measurement by using the edge filter 218.
In an embodiment, the intensity variation in reflected sensing signal pulses, after passing through the edge filter 218, as detected by the PD 220, may either be due to a change in the one or more physical parameters acting on the sensing FBG 212, or due to fluctuations in an input source power. However, for accurate sensing, the intensity variation is required to only be due to the one or more physical parameters acting on the sensing FBG 212. The reference signal pulse nullifies the intensity variation due to fluctuations in the input source power. The reference FBG 214 acts as an instantaneous source power. Since any fluctuation in the input source power impacts both the sensing pulse signal and the reference pulse signal proportionally, only the shifts in the sensing Bragg wavelength (λS) will cause changes in the sensing pulse signal after passing through the edge filter 218. As a result, the reference FBG 214 compensated for the input source power fluctuations. Consequently, any change in the optical power of the sensing FBG 212 is only due to the change in the one or more physical parameters acting on the sensing FBG 212, which is achieved with reduced hardware and better signal power as compared to the ratiometric optical interrogator 100.
In an embodiment, the input source power to the FBG module 211 may be represented as Pi , PFS, and PFR may be optical powers reflected from the sensing FBG 212 and the reference FBG 214 respectively. The PFS and the PFR may be determined using an equation (17) provided below:
P_FS= R_FS P_i ,
P_FR= R_FR P_i … (17)
In the equation (17), RFS and RFR may be reflection coefficients of the sensing FBG 212 and the reference FBG 214, respectively. Since, the optical powers reflected from the sensing FBG 212 and the reference FBG 214 are detected by the same PD 220, a value of the responsivity ℛ may be the same for both the reference signal pulse and the sense signal pulse.
Further, photocurrents IR and IS generated by the PD 220 corresponding to the reference signal pulse and the sense signal pulse respectively may be determined using an equation (18) provided below:
I_R= R.P_FR,
I_S= R.P_FS .m_S, IS increases with an increase in λS. … (18)
In the equation (18), mS is the transmission coefficient of the edge filter 218 for the positive slope region.
Furthermore, voltages VR and VS at the PD 220 corresponding to the reference signal pulse and the sense signal pulse respectively may be determined using an equation (19) provided below:
V_R= 〖R .P〗_FR .R_L
V_S= 〖R .P〗_FS .m_S R_L … (19)
In the equation (19), RL is the load resistance of the current-to-voltage conversion circuit.
Therefore, the ratio (RT) of the VS and the VR may be determined using the equation (20), as provided below:
R_T= (P_FS 〖.m〗_S)/(P_FR )= (R_FS .P_i. m_S)/(R_FR .P_i )= (R_FS m_S)/(R_FR ) (20)
As shown by the equation (20), the ratio RT is unaffected by any fluctuations in the input source power Pi, as the input source power Pi affects both the reflected powers PFS and PFR equally.
Figures 4A-4B illustrate exemplary plots depicting a linear relationship between the applied weight and wavelength shifts, in accordance with an embodiment of the present disclosure. In an example scenario, the applied weight on the sensing FBG 212 is varied, and the reflected sensing Bragg wavelength (λS) and the optical power associated with the sensing Bragg wavelength (λS) is measured using the OSA 226. As the applied weight is increased from 0 grams to 2000 grams in steps of 50 grams, the intensity level of the sensing signal pulse increases when passed through the positive slope region of the characteristics curve 300.
Figure 4A illustrates a plot 402 that depicts the reflected sensing Bragg wavelength (λS) measured on the OSA 226, with variation in the applied weight. A linear fit to data is represented as a solid line. In the example scenario, a total wavelength shift of 1.7131 nm is observed for the applied weight of 2000 grams, resulting in an average wavelength sensitivity (Δλ / gram) of 1.134 pm/ gram, with an R2 = 0.9933 indicating the linear relationship between the applied weight and the shift in the sensing Bragg wavelength (λS). Referring to the plot 402, the applied weight (W) may be back-calculated from the measured wavelength (λM) using equations (21)-(23) provided below, where λI is the initial wavelength. Although the example scenario is explained with respect to monitoring the strain, it should be noted that a similar process may also be applicable to other physical parameters such as temperature.
λ_M= m .W+ λ_I … (21)
W= (λ_M-λ_I )nm/m … (22)
W= (〖(λ〗_M-1540.0766))/(0.83714 〖* 10〗^(-3) ) … (23)
Figure 4B illustrates a plot 404 that depicts the shift sensing Bragg wavelength (λS) corresponding to successive weight increments, with an R2 = 0.9933 again indicating the linear relationship between the applied weight and the shift in the sensing Bragg wavelength (λS).
Figure 5 illustrates a wavelength versus strain plot 500, in accordance with an embodiment of the present disclosure. Particularly, the plot 500 depicts the reflected sensing Bragg wavelength (λS) measured on the OSA 226 with strain, calculated using the equation (14). A linear fit to data is represented as a solid line.
Figures 6A-6B illustrate exemplary plots of optical power variation detected at the PD 220 with respect to the applied weight, in accordance with an embodiment of the present disclosure. Figure 6A illustrates a plot 602 that depicts the optical power variation with respect to the applied weight from 0 to 2000 grams. As shown by the plot 602, the relationship between the optical power variation and the applied weight is linear between 400 to 1650 grams. In order to maintain the linear relationship, a correction factor may be applied to the optical power received for the applied weight from 1700 to 2000 grams. In an example scenario, the correction factor may be selected based on observing the optical power, which coincides with a linear fit represented by the solid line and given by an equation (24), where 230 nw is the correction factor. The linearity is observed with R2 = 0.9942. Figure 6B illustrates a plot 604 that depicts the optical power variation with respect to the applied weight with the correction factor applied after 1700 grams weight and the linearity obtained between 400 to 2000 grams.
P_FS (corrected)= P_FS+230 nw … (24)

Figures 7A-7B illustrate exemplary plots of the ratio RT with respect to the applied weight (W), in accordance with an embodiment of the present disclosure. Figure 7A illustrates a plot 702 that depicts a linear relationship between the ratio (RT) and the applied weight in the range from 400 to 1650 grams. Figure 7B illustrates a plot 704 that depicts the linear relationship between the ratio (RT) and the applied weight in the range from 400 to 2000 grams, and with the correction factor after 1700 grams weight. The linearity is observed with R2 = 0.99421. The ratio (RT) and the applied weight may be determined using the equations (25)-(26) provided below:
R_T= 1.9893*10^(-3) W-0.81741 … (25)
W= (R_T + 0.81741)/(1.9893 〖* 10〗^(-3) ) … (26)
In the equations (25)-(26), 1.9893 * 10-3 slope of the RT versus W plot, and -0.81741 is a value of y-intercept. In an embodiment, if reflection from the reference FBG 214 varies, variation in the ratio (RT) remains linear with respect to the applied weight.
Figures 8A-8D illustrate exemplary wavelength spectrums of the sensing FBG 212 and the reference FBG 214 with respect to the applied weight, in accordance with an embodiment of the present disclosure. Figure 8A illustrates a plot 802 that depicts the wavelength spectrum when there is no applied weight at the sensing FBG 212. Figure 8B illustrates a plot 804 that depicts the wavelength spectrum at the applied weight of 500 grams. Figure 8C illustrates a plot 806 that depicts the wavelength spectrum at the applied weight of 1000 grams. Figure 8D illustrates a plot 808 that depicts the wavelength spectrum at the applied weight of 2000 grams. As shown in Figures 8A-8D, as the applied weight increases, the sensing Bragg wavelength (λS) is shifted along the positive slope of the characteristics curve 300, however, the reference Bragg wavelength (λR) remains constant.
Figures 9A-9B illustrate exemplary pulses reflected from the sensing FBG 212 and the reference FBG 214 with respect to the applied weight, in accordance with an embodiment of the present disclosure. The reflected pulses may be spaced apart by a fiber delay line. Further, the reflected pulses may be observed using a Digital Storage Oscilloscope (DSO). Figure 9A illustrates a plot 902 that depicts the reflected pulses when no weight is applied at the sensing FBG 212. Figure 9B illustrates a plot 904 that depicts the reflected pulses at the applied weight of 500 grams. Figure 9C illustrates a plot 906 that depicts the reflected pulses at the applied weight of 1000 grams. Figure 9D illustrates a plot 908 that depicts the reflected pulses at the applied weight of 2000 grams. As shown in Figures 9A-9D, as the applied weight increases, an amplitude of the pulse reflected from the sensing FBG 212 increases, however, an amplitude of the pulse reflected from the reference FBG 214 remains constant.
Figure 10 illustrates a flowchart for a method 1000 to perform the intensity based optical interrogation, in accordance with an embodiment of the present disclosure. The method 1000 may be implemented by the system 200.
At step 1002, the method 1000 may include generating, by the transmitting module 202, one or more pulsed signals.
At step 1004, the method 1000 may include receiving, by the FBG module 211, the one or more pulsed signals. The FBG module 211 may include the sensing FBG 212 and the reference FBG 214. The FBG module 211 may further include the optical waveguide 216 to interconnect the sensing FBG 212 and the reference FBG 214 in series. The length (d) of the optical waveguide 216 may be selected based on the pulse width (TW) and the pulse repetition rate (TR) associated with the one or more pulsed signals. In an embodiment, the optical waveguide (216) may include one of a Single-Mode Fiber (SMF), a Multimode Fiber (MMF), and any other fiber optic component for coupling the light.
At step 1006, the method 1000 may include reflecting, by the sensing FBG 212, the sensing signal pulse at the sensing Bragg wavelength (λS) corresponding to each of the one or more pulsed signals.
At step 1008, the method 1000 may include reflecting, by the reference FBG 214, the reference signal pulse at the reference Bragg wavelength (λR) corresponding to each of the one or more pulsed signals.
At step 1010, the method 1000 may include converting, by the edge filter 218, the shift in the sensing Bragg wavelength (λS) into the intensity variation based on the characteristics curve for the optical interrogation. The sensing Bragg wavelength (λS) and the reference Bragg wavelength (λR) may be positioned on the slope region and on the flat region of the characteristics curve, respectively. The slope region of the characteristics curve may include the positive slope region and the negative slope region. Further, the sensing Bragg wavelength (λS) may be positioned on one of the lower end of the positive slope region, or the upper end of the negative slope region. Furthermore, the shift in the sensing Bragg wavelength (λS) and the corresponding intensity variation after passing through the edge filter 218 may be induced by the one or more physical parameters acting on the sensing FBG 212. In a non-limiting example, the edge filter 218 may correspond to the fused-fiber demultiplexer.
In an embodiment, the method 1000 may include routing, by the circulator 208, the one or more pulsed signals from the transmitting module 202 to the FBG module 211. The method 1000 may also include routing, by the circulator 208, the sensing signal pulse and the reference signal pulse corresponding to each of the one or more pulsed signals from the FBG module 211 to the edge filter 218. The circulator 208 may include at least the first port 210-1, the second port 210-2, and the third port 210-3. The transmitting module 202, the FBG module 211, and the edge filter 218 may be connected to the first port 210-1, the second port 210-2, and the third port 210-3 respectively.
In an embodiment, the method 1000 may further include converting, by the PD 220 operatively connected to the output of the edge filter 218, the intensity variation into the one or more electrical signals. The method 1000 may also include converting, by the ADC 222 operatively connected to the output of the PD 220, the one or more electrical signals into the one or more digital signals. Further, the method 1000 may include determining, by the DSP unit 224 operatively connected to the output of the ADC 222, the one or more insights correspond to the sensing FBG 212 based on the one or more digital signals. The one or more insights may include the information associated with the one or more physical parameters acting on the sensing FBG 212.
In an alternative embodiment, the method 1000 may include measuring, by the OSA 226 operatively connected to the output of the edge filter 218, at least one of the sensing Bragg wavelength (λS) or the optical power associated with the sensing Bragg wavelength (λS).
The present disclosure provides various advantages. Particularly, the present disclosure provides an improved system and method that utilize two FGBs (i.e., the sensing FBG 212 and the reference FBG 214), which nullifies the effect of fluctuations in the input source power, particularly during computation of the ratio (RT). The present disclosure is applicable for sensing a variety of physical parameters such as strain, temperature, pressure, etc. The present disclosure provides an improved system that is highly reliable, and cost-effective, has a portable and compact design, and supports multi-path signal processing. Further, the present disclosure provides improved signal power with reduced hardware over the conventional ratiometric method. For example, the present disclosure achieves signal power improvement with the system comprising an edge filter, a PD, an ADC, and a DSP unit, as compared to one 1x2 splitter, an edge filter, two PDs, two ADCs, and two DSP units used in the conventional ratiometric method. The reduced hardware requirement further results in lower power consumption, thereby improving the overall performance.
Further, the disclosed systems and methods are capable of analyzing signals from fiber optic sensors with high precision, providing critical data for industries such as aerospace, civil engineering, oil and gas, healthcare, and structural health monitoring. The disclosed systems and methods play a crucial role in transforming the huge amount of data captured by distributed fiber optic sensing systems into actionable insights, enabling condition monitoring, asset management, and early detection of potential problems. Accordingly, the present disclosure may be implemented in various sectors, including telecommunications, aerospace and defense, energy and utilities, medical, industrial, and others. In the telecommunications sector, the disclosed systems and methods are used for monitoring and maintaining the integrity of fiber optic networks. In the aerospace and defense sector, the disclosed systems and methods are used for structural health monitoring of aircraft and defense equipment. In the energy and utilities sector, the disclosed systems and methods are used for monitoring the performance of energy infrastructure such as power grids, pipelines, and renewable energy systems. In the medical sector, the disclosed systems and methods are used for various diagnostic and monitoring applications.
While specific language has been used to describe the present subject matter, any limitations arising on account thereto, are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein. The drawings and the foregoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. , Claims:1. A system (200) to perform an intensity-based optical interrogation, the system (200) comprising:
a transmitting module (202) configured to generate one or more pulsed signals;
a Fiber Bragg Grating (FBG) module (211) configured to receive the one or more pulsed signals, the FBG module (211) comprising:
a sensing FBG (212) configured to reflect a sensing signal pulse at a sensing Bragg wavelength corresponding to each of the one or more pulsed signals;
a reference FBG (214) configured to reflect a reference signal pulse at a reference Bragg wavelength corresponding to each of the one or more pulsed signals; and
an optical waveguide (216) to interconnect the sensing FBG (212) and the reference FBG (214); and
an edge filter (218) configured to convert a shift in the sensing Bragg wavelength into an intensity variation based on a characteristics curve (300), for optical interrogation, wherein the sensing Bragg wavelength and the reference Bragg wavelength are positioned on a slope region (302 or 304) and on a flat region (306) of the characteristics curve (300), respectively,
wherein the shift in the sensing Bragg wavelength and the corresponding intensity variation after passing through the edge filter (218) are induced by one or more physical parameters acting on the sensing FBG (212).

2. The system (200) as claimed in claim 1, wherein the optical waveguide (216) is configured to interconnect the sensing FBG (212) and the reference FBG (214) in series, wherein a length of the optical waveguide (216) is selected based on a pulse width and a pulse repetition rate associated with the one or more pulsed signals, and wherein the optical waveguide (216) comprises one of a Single-Mode Fiber (SMF) and a Multimode Fiber (MMF).

3. The system (200) as claimed in claim 1, wherein the slope region of the characteristics curve (300) comprises a positive slope region (302) and a negative slope region (304), and wherein the sensing Bragg wavelength is positioned on one of:
a lower end of the positive slope region (302); or
an upper end of the negative slope region (304).

4. The system (200) as claimed in claim 1, further comprising:
a circulator (208) comprising at least a first port (210-1), a second port (210-2), and a third port (210-3), wherein the transmitting module (202), the FBG module (211), and the edge filter (218) are connected to the first port (210-1), the second port (210-2), and the third port (210-3), respectively,
wherein the circulator (208) is configured to:
route the one or more pulsed signals from the transmitting module (202) to the FBG module (211), and
route the sensing signal pulse and the reference signal pulse corresponding to each of the one or more pulsed signals from the FBG module (211) to the edge filter (218).

5. The system (200) as claimed in claim 1, further comprising:
a photodetector (220) operatively connected to an output of the edge filter (218) and is configured to convert the intensity variation into one or more electrical signals;
an Analog-to-Digital Converter (ADC) (222) operatively connected to an output of the photodetector (220) and is configured to convert the one or more electrical signals into one or more digital signals; and
a Digital Signal Processing (DSP) unit (224) operatively connected to an output of the ADC (222) and configured to determine one or more insights associated with the sensing FBG (212) based on the one or more digital signals, wherein the one or more insights correspond to information associated with the one or more physical parameters acting on the sensing FBG (212).

6. The system (200) as claimed in claim 1, further comprising:
an Optical Spectrum Analyzer (OSA) (226) operatively connected to an output of the edge filter (218) and is configured to measure at least one of the sensing Bragg wavelength or an optical power associated with the sensing Bragg wavelength.

7. The system (200) as claimed in claim 1, wherein the edge filter (218) corresponds to a fused-fiber demultiplexer.

8. A method (1000) to perform an intensity based optical interrogation, the method (1000) comprising:
generating (1002), by a transmitting module (202), one or more pulsed signals;
receiving (1004), by a Fiber Bragg Grating (FBG) module (211), the one or more pulsed signals, the FBG module (211) comprising a sensing FBG (212), a reference FBG (214), and an optical waveguide (216) to interconnect the sensing FBG (212) and the reference FBG (214);
reflecting (1006), by the sensing FBG (212), a sensing signal pulse at a sensing Bragg wavelength corresponding to each of the one or more pulsed signals;
reflecting (1008), by the reference FBG (214), a reference signal pulse at a reference Bragg wavelength corresponding to each of the one or more pulsed signals; and
converting (1010), by an edge filter (218), a shift in the sensing Bragg wavelength into an intensity variation based on a characteristics curve (300) for optical interrogation, wherein the sensing Bragg wavelength and the reference Bragg wavelength are positioned on a slope region (302 or 304) and on a flat region (306) of the characteristics curve (300), respectively,
wherein the shift in the sensing Bragg wavelength and the corresponding intensity variation after passing through the edge filter (218) are induced by one or more physical parameters acting on the sensing FBG (212).

9. The method (1000) as claimed in claim 8, wherein the optical waveguide (216) is configured to interconnect the sensing FBG (212) and the reference FBG (214) in series, wherein a length of the optical waveguide (216) is selected based on a pulse width and a pulse repetition rate associated with the one or more pulsed signals, and wherein the optical waveguide (216) comprises one of a Single-Mode Fiber (SMF) and a Multimode Fiber (MMF).

10. The method (1000) as claimed in claim 8, wherein the slope region of the characteristics curve (300) comprises a positive slope region (302) and a negative slope region (304), and wherein the sensing Bragg wavelength is positioned on one of:
a lower end of the positive slope region (302); or
an upper end of the negative slope region (304).

11. The method (1000) as claimed in claim 8, further comprising:
routing, by a circulator (208), the one or more pulsed signals from the transmitting module (202) to the FBG module (211); and
routing, by the circulator (208), the sensing signal pulse and the reference signal pulse corresponding to each of the one or more pulsed signals from the FBG module (211) to the edge filter (218),
wherein the circulator (208) comprises at least a first port (210-1), a second port (210-2), and a third port (210-3), and wherein the transmitting module (202), the FBG module (211), and the edge filter (218) are connected to the first port (210-1), the second port (210-2), and the third port (210-3) respectively.

12. The method (1000) as claimed in claim 8, further comprising:
converting, by a photodetector (220) operatively connected to an output of the edge filter (218), the intensity variation into one or more electrical signals;
converting, by an Analog-to-Digital Converter (ADC) (222) operatively connected to an output of the photodetector (220), the one or more electrical signals into one or more digital signals; and
determining, by a Digital Signal Processing (DSP) unit (224) operatively connected to an output of the ADC (222), one or more insights correspond to the sensing FBG (212) based on the one or more digital signals, wherein the one or more insights comprise information associated with the one or more physical parameters acting on the sensing FBG (212).

13. The method (1000) as claimed in claim 8, further comprising:
measuring, by an Optical Spectrum Analyzer (OSA) (226) operatively connected to an output of the edge filter (218), at least one of the sensing Bragg wavelength or an optical power associated with the sensing Bragg wavelength.

14. The method (1000) as claimed in claim 8, wherein the edge filter (218) corresponds to a fused-fiber demultiplexer.