CN111811554A - Optical cavity ring-down-based large-range high-precision fiber grating sensing method and device - Google Patents
Optical cavity ring-down-based large-range high-precision fiber grating sensing method and device Download PDFInfo
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Abstract
The invention relates to a cavity ring-down based large-range high-precision fiber grating sensing method and device. The device comprises an LED broadband light source, a signal generator, a 2 x 1 optical fiber coupler, a transmission optical fiber, an optical fiber delay line, a circulator, an optical fiber grating FBG, an edge filter, a photoelectric detector, an optical fiber grating, a PC and an oscilloscope. The present invention uses a fiber ring cavity ring down (FLRD) technique. The spectrum of the broadband light source has a certain range, and when the fiber bragg grating is influenced by external parameters and wavelength conversion occurs, effective reflected light can be generated. The edge filter can make the light intensity after passing through the edge filter and the wavelength of the fiber grating in a linear one-to-one correspondence relationship, so that the output loss in the cavity and the wavelength of the fiber grating are in a linear relationship. Because the ring-down time is determined by the loss in the cavity, the effective measurement of the parameters acting on the fiber grating can be realized by measuring the ring-down time, thereby realizing the measurement with large range and high precision.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a large-range high-precision optical fiber grating sensing method and device based on cavity ring-down.
Background
The fiber grating is formed by writing a coherent field pattern of incident light into a fiber core by ultraviolet light exposure using the photosensitivity of the fiber material, and generating a periodic variation of the refractive index in the fiber core along the axial direction of the fiber core, which essentially forms a narrow-band (transmissive or reflective) edge filter or mirror in the fiber core. In practical measurement, the fiber grating is equivalent to form a narrow-band edge filter in the fiber to reflect light with a specific central wavelength, when the fiber grating is subjected to external physical quantity change, the effective refractive index and the grating period of the fiber grating can be changed, the central wavelength is changed accordingly, and the change of the external physical quantity can be measured by detecting the offset of the central wavelength of the fiber grating. The fiber grating has the characteristics of small volume, good wavelength selectivity, no influence of nonlinear effect, insensitive polarization, easy connection with a fiber system, convenient use and maintenance, large bandwidth range, small additional loss, miniaturized device, good coupling, capability of being fused with other fiber devices and the like, and the fiber grating has mature manufacturing process, easy formation of mass production and low cost, so the fiber grating has good practicability and the superiority that other devices cannot be replaced. This makes fiber gratings and fiber grating-based devices important as photoelectric sensing devices.
The fiber ring cavity ring-down (FLRD) technology is a new fiber sensing technology. The technology simulates the concept of ring-down, light passes through the substance to be detected for multiple times to form a circular ring-down, then an output optical signal is converted into an electrical signal by using a photoelectric detector, a ring-down pulse curve can be seen in an oscilloscope, and the information of the substance to be detected can be obtained by detecting the ring-down time of the pulse curve. Because the ring-down time is detected, the influence of the fluctuation of the light source on the detection can be avoided. The fiber ring cavity consists of a fiber coupler and a section of long fiber, a sample to be measured is placed in the ring cavity, and the relevant information of the parameter to be measured can be calculated by measuring the attenuation time. Compared with the traditional spectrum technology, the technology avoids the use of a spectrometer, so that the whole system is low in cost, short in response time and capable of having strong advantages in rapid detection and line detection. The fiber ring cavity ring-down technology has the advantages of no influence of light source fluctuation, high response speed, high sensitivity, good stability and the like. Through development and research in the last decade, the technology gradually matures, and can be applied to measurement of parameters such as liquid refractive index, gas concentration, temperature, strain and the like.
The Fiber Bragg Grating (FBG) -based FLRD sensing system also has the advantages of the Fiber ring-down (FLRD) technology, the detection sensitivity of the sensing system is in direct proportion to the round-trip times of optical pulses transmitted in an optical Fiber loop, and the optical Fiber loop can be as long as several kilometers, so that the long-distance sensing detection is facilitated; unlike direct measurement of light intensity attenuation or wavelength drift, the sensing system is insensitive to light source fluctuations, thus also ensuring accuracy. However, the measuring range of the sensing system is not large, and some measuring requirements cannot be met.
Disclosure of Invention
The problem that the sensing system of the fiber ring cavity ring-down (FLRD) technology cannot measure in a large range is solved. The invention provides a fiber grating sensing method and a device which are simple in structure, good in stability and based on cavity ring-down large-range high-precision.
The method adopted for solving the technical problem comprises the following steps:
the method comprises the following steps: selecting an LED broadband light source, a signal generator, a circulator, a fiber bragg grating FBG with a certain length, an edge filter, an optical fiber delay line, a plurality of couplers, an oscilloscope, a PC and a photoelectric detector. The central wavelength of the FBG and the edge filter is required to be within the coverage range of the LED broadband light source, and the central wavelength of the FBG is required to be within the coverage range of the edge filter.
Step two: the electric pulse emitted by the signal generator is loaded on the LED broadband light source to obtain a modulated optical pulse signal, the optical pulse signal emitted by the broadband light source enters an optical fiber delay line through a first coupler, passes through the optical fiber delay line and then passes through a second coupler, a part of optical signal output at the second coupler is detected and amplified by a photoelectric detector, then the waveform is demodulated and output by an oscilloscope, and the other part of optical signal returns to enter an annular cavity and enters the FBG through the circulator.
The central wavelength of the fiber grating is related to the effective refractive index and the period of the fiber grating, and the specific expression is as follows:
λB＝2neffΛ， (1)
the formula (1) represents the center wavelength lambda of the fiber gratingBAnd the effective refractive index n of the fiber coreeffThe period Λ of the fiber grating; wherein n iseffAnd Λ will vary with temperature and stress. The expression (2) represents the variation Delta lambda of the central wavelength of the fiber gratingBTemperature T and stress. α is the coefficient of thermal expansion of the fiber material, pijIs the elastic optical tensor component of the optical fiber material, xi is the thermo-optic coefficient of the optical fiber material, delta is the strain variation, delta T is the temperature variation, and ν is the poisson ratio coefficient of the optical fiber material. The reflection spectrum of a fiber grating can be approximated by a gaussian function.
The formula (3) represents the reflection intensity f and the center wavelength v of the fiber grating in the fiber grating reflection spectrum1The relationship (2) of (c). λ is the wavelength, and Δ λ represents the offset of the center wavelength of the sensing fiber grating (FBG).
Step three: the signal reflected by the FBG enters the edge filter through the circulator, and enters the first coupler again after demodulation and filtering of the edge filter to complete the circulation of an optical cavity.
The edge filter filtering curve is:
F(λ)＝K(λ-λ0) (4)
in the formula (3), K is the slope of the edge filter, and lambda0At zero output wavelength, i.e. F (lambda)0) 0. When the reflection spectrum of the FBG moves in the linear region of the filter curve.
The signal strength after passing through the edge filter is:
Ii＝I0·F(λ)·f (5)
in formula (5), F (λ) is the filter curve of the edge filter, F is the reflection spectrum function of the Fiber Bragg Grating (FBG), I0Is the initial light intensity input to the system.
In the FLRD system, the light intensity that can be detected by the detector is:
i is the real-time light intensity detected by the detector, I0Is the initial light intensity input to the system. t is the time, the ring down time τ of the cavity is defined as the time required for the light to decay to 1/e of its initial intensity, n is the refractive index of the ring down cavity, and L is the length of the ring down cavity. And c is the speed of light. And A is a fixed loss. For a given fiber loop, n, L, c, A in the formula are all constants. The additional loss B is determined by the linear filter. When an external parameter (e.g. changing pressure, temperature, etc.) acts on the fiber grating FBG, its reflected wavelength changes, thereby causing a change in the intensity of the transmitted light through the linear filter. As can be expressed by equation (4), the greater the λ shift, the lower the transmittance, and the greater the extra loss B caused. The smaller the ring-down time τ is with the increase in excess loss B, and by measuring the change in ring-down time τ, the change in the external parameter can be detected.
The invention adopts the following devices for solving the technical problems:
the cavity ring-down large-range high-precision fiber grating sensing device comprises: the device comprises an LED broadband light source, a signal generator, a 2X 1 optical fiber coupler, a transmission optical fiber, an optical fiber delay line, a circulator, an optical fiber grating FBG, an edge filter, a photoelectric detector, an optical fiber grating, a PC and an oscilloscope.
The 2 x 1 fiber coupler includes a first coupler and a second coupler.
The broadband light source is connected with an output port of the signal generator through a cable, two ports on one side of the first coupler are respectively connected with the broadband light source and one end of the optical fiber delay line, the other end of the optical fiber delay line is connected with one end of a port of the second coupler, port 1 of the circulator is connected with one end of the second coupler, port 2 of the circulator is connected with the Fiber Bragg Grating (FBG), port 3 of the circulator is connected with one end of the edge filter, the other end of the edge filter is connected with one end of the first coupler, an input port of the photoelectric detector is connected with one end of the second coupler, the PC is connected with the photoelectric detector through a USB line, and an output port of the photoelectric detector is connected with the oscilloscope through an optical cable. The first coupler, the optical fiber delay line, the second coupler, the circulator, the Fiber Bragg Grating (FBG) and the edge filter form a ring-down cavity structure, the Fiber Bragg Grating (FBG) is used as a sensing area, and the photoelectric detector, the PC and the oscilloscope are used as demodulators of the sensors; monitoring the attenuation change curve of light intensity when different light intensity is to be measured, and realizing the measurement to be measured by measuring the ring-down time of the curve.
The invention has the beneficial effects that:
the light source uses a broadband light source, an edge filter is added, the broadband light source has an energy spectrum with a certain width, and after signal linear modulation of the edge filter is carried out, the measurement range of the device system is enlarged, and the measurement precision is more accurate.
The change of the light source intensity has no influence on the measurement result of the ring-down cavity system, and the light intensity is within the detection range. And the ring-down cavity technology measures a ring-down time value which is a relative value, does not need calibration and is not easy to be submerged by background light.
Drawings
FIG. 1 is a schematic diagram of a large-scale high-precision fiber grating sensing device using cavity ring-down technology according to the present invention.
Fig. 2 is a schematic diagram of how the present invention can realize a wide-range high-precision measurement.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
As shown in fig. 1, the cavity ring-down technique large-range high-precision fiber grating sensing device includes a signal generator 1, a broadband light source 2, a first coupler 3, a fiber delay line 4, a second coupler 5, a circulator 6, a fiber grating (FBG)7, an edge filter 8, a photodetector 9, a PC 10, and an oscilloscope 11. The broadband light source 2 is connected with an output port of the signal generator 1 through a cable, a port 301 of the first coupler 3 is connected with the broadband light source 2, a port 302 of the first coupler 3 is connected with an optical fiber delay line 4, the other end of the optical fiber delay line 4 is connected with a port 501 of the second coupler, a port 601 of the circulator is connected with a port 503 of the second coupler, a port 602 of the circulator is connected with an optical fiber grating (FBG)7, a port 603 of the circulator is connected with one end of an edge filter 8, the other end of the edge filter 8 is connected with a port 303 of the first coupler, an input port of the photoelectric detector 9 is connected with a port 502 of the second coupler 3, the PC 10 is connected with the photoelectric detector 9 through a USB line, and an output port of the photoelectric detector 9 is connected with the oscilloscope 11 through an optical cable. The first coupler 3, the optical fiber delay line 4, the second coupler 5, the circulator 6, the Fiber Bragg Grating (FBG)7 and the edge filter 8 form a ring-down cavity structure, the Fiber Bragg Grating (FBG)7 serves as a sensing area, and the photoelectric detector 9, the PC 10 and the oscilloscope 11 serve as demodulators of sensors.
The working mode of the invention is as follows: the light wave of the broadband light source 2 generates time sequence pulse signal light through the signal generator 1, the time sequence pulse signal light is input into the first coupler 3 through the optical fiber, then is transmitted to the optical fiber delay line 4 from the port 302 of the first coupler 3, and is transmitted to the second coupler 5 through the optical fiber delay line 4, one part of the pulse signal light is transmitted to the circulator 6 through the port 503 of the second coupler 5, and the other part of the light signal is transmitted to the photoelectric detector through the port 502. The signal light passing through the circulator 6 is input from the port 601, is sensed by the Fiber Bragg Grating (FBG)7 through the port 602, the sensed signal light is transmitted to the edge filter 8 through the port 603 for linear selection processing, and the signal passing through the edge filter 8 enters the next ring-down cycle through the port 303 of the first coupler 3. The electric signal passing through the photodetector 9 is converted into an optical signal, amplified by a parameter adjusting signal of the PC 10, and finally displayed on the oscilloscope 11.
The device for sensing the large-range high-precision fiber bragg grating by cavity ring-down is characterized in that an LED broadband light source with a certain width is used, a spectrum of the LED broadband light source has a certain range as shown in a figure 2-1, and when a system device carries out measurement, the fiber bragg grating is influenced by external parameters and can generate effective reflected light when wavelength is changed. By adding the edge filter into the device, the light intensity after passing through the edge filter and the wavelength of the fiber grating are in linear one-to-one correspondence, and the output loss in the cavity and the wavelength of the fiber grating are in linear relationship. The energy spectrum output by the broadband light source after being acted on by the edge filter can be seen from fig. 2-2, and the output energy is linearly selected, as can be seen from fig. 2-3. The accuracy of the finally detected signal after linear processing is further improved. The signal generator controls the broadband light source to output a pulse sequence, and detects that the output light intensity is changed into the input light intensity under the conditions of the same pulse sequence and different to-be-measured values
The time required. Different losses and ring-down times correspond to different quantities to be measured. The loss in the ring-down cavity is only related to the measurement to be measured and is unrelated to other factors, and the influence of other factors on the measurement is eliminated, so that the accurate measurement of the band measurement is completed. The device can enlarge the measuring range and improve the accuracy.
The key technology of the device for realizing the optical cavity ring-down large-range high-precision fiber grating sensing method and the device is as follows:
1. a broadband light source is chosen instead of a single wavelength light source. The spectrum of the broadband light source has a certain range, and the fiber bragg grating is influenced by external parameters and can generate effective reflected light when the wavelength is changed.
2. The edge filter is added in the device, so that the light intensity after passing through the edge filter and the wavelength of the fiber grating are in linear one-to-one correspondence, and the output loss in the cavity and the wavelength of the fiber grating are in linear relationship. Because the ring-down time is determined by the loss in the cavity, the effective measurement of the parameters acting on the fiber grating can be realized by measuring the ring-down time, thereby realizing the measurement with large range and high precision.
In a specific embodiment of the invention, the broadband light source SLED-1550 has a spectral width of 55 nm; the modulation frequency of the pulse wave of the signal generator is 10 kHz. The sensing Fiber Bragg Grating (FBG) is prepared on a G.652 single-mode fiber, the length is 15mm, the working wavelength is 1547.3nm, the bandwidth is 3dB, and the loss is 0.01 dB. An edge filter with an action range of 1540nm to 1555nm and a slope K of 0.68,1540nm, and the zero output wavelength lambda of the edge filter0(ii) a The method comprises the following steps that axial stress is measured, two ends of a sensing Fiber Bragg Grating (FBG) are fixed on a clamp of a moving platform, the sensing Fiber Bragg Grating (FBG) moves towards two sides through the clamp, and the axial stress is applied to the sensing Fiber Bragg Grating (FBG); the optical fiber is G.652 single-mode optical fiber; the photoelectric detector is an Avalanche Photodiode (APD), the working wavelength is 1.0-2.9 μm, the range to be measured is 0-300 μm, and the measurement precision can reach 0.005 μm.
What has been described above and shown in the drawings is merely a preferred embodiment of the invention. It should be noted that variations and modifications can be made by those skilled in the art without departing from the principle of the present invention, and these should also be construed as falling within the scope of the present invention.
Claims (2)
1. The optical cavity ring-down based large-range high-precision fiber grating sensing method is characterized by comprising the following steps of:
the method comprises the following steps: selecting an LED broadband light source, a signal generator, a circulator, a Fiber Bragg Grating (FBG) with a certain length, an edge filter, an optical fiber delay line, a plurality of couplers, an oscilloscope, a Personal Computer (PC) and a photoelectric detector; the central wavelength of the FBG and the edge filter is required to be within the coverage range of the LED broadband light source, and the central wavelength of the FBG is required to be within the coverage range of the edge filter;
step two: the electric pulse sent by the signal generator is loaded to the LED broadband light source to obtain a modulated optical pulse signal, the optical pulse signal sent by the broadband light source enters an optical fiber delay line through a first coupler, passes through the optical fiber delay line and then passes through a second coupler, after a part of optical signal output at the second coupler is detected and amplified by a photoelectric detector, the waveform is demodulated and output by an oscilloscope, and the other part of optical signal returns to enter an annular cavity and enters the FBG through the circulator;
the central wavelength of the fiber grating is related to the effective refractive index and the period of the fiber grating, and the specific expression is as follows:
λB＝2neffΛ， (1)
the formula (1) represents the center wavelength lambda of the fiber gratingBAnd the effective refractive index n of the fiber coreeffThe period Λ of the fiber grating; wherein n iseffAnd Λ will vary with temperature and stress; the expression (2) represents the variation Delta lambda of the central wavelength of the fiber gratingBTemperature T and stress; α is the coefficient of thermal expansion of the fiber material, pijIs the elastic optical tensor component of the optical fiber material, xi is the thermo-optic coefficient of the optical fiber material, delta is the strain variation, delta T is the temperature variation, and v is the Poisson ratio coefficient of the optical fiber material; the reflection spectrum of the fiber grating can be approximately expressed by a Gaussian function;
the formula (3) represents the reflection intensity f and the center wavelength v of the fiber grating in the fiber grating reflection spectrum1The relationship of (1); λ is the wavelength, Δ λ represents the offset of the center wavelength of the sensing fiber grating (FBG);
step three: the signal reflected by the FBG enters the edge filter through the circulator, and enters the first coupler again after demodulation and filtering of the edge filter to complete the circulation of an optical cavity, certain loss is generated due to the fact that the intensity of the optical signal changes after passing through the fiber bragg grating and the edge filter, when the optical signal passes through the circulation of the optical cavity for many times, a periodic optical pulse signal of cavity ring-down is formed, each output signal is detected by the photoelectric detector, and therefore a periodic attenuated signal is formed on the oscilloscope;
the edge filter filtering curve is:
F(λ)＝K(λ-λ0) (4)
in the formula (3), K is the slope of the edge filter, and lambda0At zero output wavelength, i.e. F (lambda)0) 0; when the reflection spectrum moving range of the FBG is in the linear interval of the filter curve;
the signal strength after passing through the edge filter is:
Ii＝I0·F(λ)·f (5)
in formula (5), F (λ) is the filter curve of the edge filter, F is the reflection spectrum function of the Fiber Bragg Grating (FBG), I0Is the initial light intensity input to the system;
in the FLRD system, the light intensity that can be detected by the detector is:
i is the real-time light intensity detected by the detector, I0Is the initial light intensity input to the system; t is time, the ring down time τ of the cavity is defined as the time required for the light to decay to 1/e of its initial intensity, n is the refractive index of the ring down cavity, and L is the length of the ring down cavity; c is the speed of light; a is the fixed loss; for a given optical fiber loop, n, L, c and A in the formula are constants; the additional loss B is determined by the linear filter; when an external parameter (e.g., changing pressure, temperature, etc.) acts on the fiber grating FBG, its reflected wavelength changes, thereby causing a change in the intensity of transmitted light through the linear filter; as can be expressed by equation (4), the greater the λ shift, the lower the transmittance, and the greater the extra loss B caused; the smaller the ring-down time τ, with the increase in excess loss B, by pairing the ring-down timeThe change of the external parameter can be detected by measuring the change of the tau.
2. An apparatus for implementing the method of claim 1, comprising: the system comprises an LED broadband light source, a signal generator, a 2X 1 optical fiber coupler, a transmission optical fiber, an optical fiber delay line, a circulator, an optical fiber grating FBG, an edge filter, a photoelectric detector, an optical fiber grating, a PC and an oscilloscope;
the broadband light source is connected with an output port of the signal generator through a cable, two ports on one side of the first coupler are respectively connected with the broadband light source and one end of an optical fiber delay line, the other end of the optical fiber delay line is connected with one end of a port of the second coupler, a port 1 of the circulator is connected with one end of the second coupler, a port 2 of the circulator is connected with an optical fiber grating (FBG), a port 3 of the circulator is connected with one end of an edge filter, the other end of the edge filter is connected with one end of the first coupler, an input port of the photoelectric detector is connected with one end of the second coupler, the PC is connected with the photoelectric detector through a USB line, and an output port of the photoelectric detector is connected with the oscilloscope through an optical cable; the first coupler, the optical fiber delay line, the second coupler, the circulator, the Fiber Bragg Grating (FBG) and the edge filter form a ring-down cavity structure, the Fiber Bragg Grating (FBG) is used as a sensing area, and the photoelectric detector, the PC and the oscilloscope are used as demodulators of the sensors; monitoring the attenuation change curve of light intensity when different light intensity is to be measured, and realizing the measurement to be measured by measuring the ring-down time of the curve.
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|---|---|---|---|---|
| CN114777823A (en) * | 2022-05-24 | 2022-07-22 | 华中科技大学 | FLRD sensor system and FLRD sensing device based on phase drift |
| CN117091722A (en) * | 2023-08-25 | 2023-11-21 | 南方电网科学研究院有限责任公司 | Optical monitoring method and optical monitoring device for thermal runaway of energy storage battery |
-
2020
- 2020-08-10 CN CN202010792864.7A patent/CN111811554A/en active Pending
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114777823A (en) * | 2022-05-24 | 2022-07-22 | 华中科技大学 | FLRD sensor system and FLRD sensing device based on phase drift |
| CN114777823B (en) * | 2022-05-24 | 2024-01-05 | 华中科技大学 | FLRD sensor system and FLRD sensing device based on phase drift |
| CN117091722A (en) * | 2023-08-25 | 2023-11-21 | 南方电网科学研究院有限责任公司 | Optical monitoring method and optical monitoring device for thermal runaway of energy storage battery |
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