Cascaded long-period fiber grating multiplexing demodulation device based on low-coherence interference principle

Abstract

The present invention provides a cascaded long period fiber grating multiplexing demodulation device based on the principle of low coherence interference. The device comprises a broadband light source, a first coupler, a cascaded long period fiber grating, a second coupler, a Fizeau interferometer, a linear array InGaAs and a data storage and processing device. The broadband light source is connected to the port optical signal of the first fiber coupler, the first fiber coupler is connected to at least two cascaded long period fiber grating sensors, the cascaded long period fiber grating sensor is connected to the second fiber coupler through an optical signal, the other interface of the second fiber coupler is optically connected to the input end of the Fizeau interferometer, the output optical signal of the Fizeau interferometer reaches the linear array InGaAs, and the linear array InGaAS is connected to the data storage and processing device. The present invention provides a cascaded long period fiber grating multiplexing demodulation device based on the principle of low coherence interference. The fiber grating multiplexing demodulation device has the advantages of stability, high spatial resolution, large number of multiplexing, and low cost.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensing, in particular to a cascaded long-period optical fiber grating multiplexing demodulation device based on the principle of low-coherence interference. Background Art

[0002] Cascaded long-period fiber gratings and their sensing principle

[0003] Cascaded long-period fiber gratings are composed of two long-period fiber gratings with exactly the same parameters written on the same optical fiber. As a special structure of long-period fiber gratings, cascaded long-period fiber gratings are essentially long-period fiber gratings. The sensing and measurement based on cascaded long-period fiber gratings are based on the changes in the grating wavelength or the transmission depth at the resonant wavelength caused by the external environment to obtain sensing information. The sensing mechanisms of cascaded long-period fiber gratings include deformation and thermo-optical effects caused by temperature, deformation and elastic-optical effects caused by strain, and changes in effective refractive index caused by refractive index. Based on these basic sensing mechanisms, the temperature, strain, bending, refractive index and other parameters of the structure can be monitored.

[0004] Cascaded long-period fiber gratings are the unidirectional coupling between the fundamental mode of the fiber core layer and multiple cladding modes, and work in the transmission mode with a large loss peak bandwidth. Due to the lack of reflection characteristics, the time division multiplexing scheme using the measurement of pulse light reflection delay is not suitable for long-period fiber grating arrays; and the multiple wide-spectrum loss band characteristics of the transmission spectrum of long-period fiber gratings make the wavelength division multiplexing scheme impossible to implement. These factors have caused difficulties in the multiplexing modulation of cascaded long-period fiber grating sensing signals.

[0005] Low Coherence Interferometry

[0006] Low-coherence interferometry technology is an interference system that uses broad-spectrum light as a light source, and uses zero-order interference fringes as a reference position based on the principle of white light coherence, so as to obtain accurate measurement of absolute position information within a large range. The sensor structure used by the low-coherence interferometry system to obtain the detection signal is an interferometer, and the demodulation part is a scanning interferometer. Different sensors use different optical path difference designs to achieve multi-channel sensor multiplexing. Since the low-coherence system uses an interferometer to demodulate the information of each sensor, it can avoid environmental noise introduced by factors such as unstable light sources, and has high anti-interference performance; it makes full use of all the data of the interference pattern to evaluate the measured signal, and has the effect of the average effect favored by the precision measurement theory, and obtains higher measurement accuracy; the construction of the optical fiber low-coherence interferometry system does not require the tunable laser light source used in the high-coherence interferometry system, and the signal analysis and processing does not require expensive precision equipment such as spectrometers, network analyzers, and vector analyzers. The system cost is much lower than the currently widely used wavelength division multiplexing and time division multiplexing systems. Summary of the invention

[0007] In order to solve the above-mentioned problems, the present invention provides a cascaded long period fiber grating multiplexing demodulation device based on the principle of low coherence interference, and the fiber grating multiplexing demodulation device has the advantages of stability, high spatial resolution, large number of multiplexing and low cost. To achieve this purpose, the present invention provides a cascaded long period fiber grating multiplexing demodulation device based on the principle of low coherence interference, the device comprises a broadband light source, a first coupler, a cascaded long period fiber grating, a second coupler, a Fizeau interferometer, a linear array InGaAs and a data storage and processing device, the broadband light source is connected to the port optical signal of the first fiber coupler, the first fiber coupler is connected to at least two cascaded long period fiber grating sensors, the cascaded long period fiber grating sensor is connected to the second fiber coupler through an optical signal, the other interface of the second fiber coupler is optically connected to the input end of the Fizeau interferometer, the output optical signal of the Fizeau interferometer reaches the linear array InGaAs, and the linear array InGaAS is connected to the data storage and processing device.

[0008] According to a further improvement of the present invention, there are N cascaded long-period fiber gratings, where N is a positive integer and N≥2, and the first fiber coupler is connected to at least two cascaded long-period fiber grating sensors.

[0009] According to a further improvement of the present invention, the central wavelength position of the broadband light source is 1565 nm, and the spectral range is 60 nm, covering the bandwidth of the resonance loss peak of the long-period fiber grating sensor.

[0010] According to a further improvement of the present invention, the grating of the cascaded long-period fiber grating sensor is composed of two identical long-period fiber gratings with a coupling strength of 3 dB, which is designed to ensure that the interference fringes have a higher visibility.

[0011] According to a further improvement of the present invention, the optical path difference introduced by the cascaded long-period fiber grating and the Fizeau interferometer should be smaller than the coherence length of the broadband light source, so that the design satisfies the low-coherence interference condition.

[0012] In a further improvement of the present invention, the distance l between the long-period fiber gratings of each cascaded long-period fiber grating should be preset. i This is designed to prevent the interference fringes of different sensors from overlapping.

[0013] The present invention provides a cascaded long-period fiber grating multiplexing demodulation device based on the principle of low-coherence interference. The device measures and calculates the resonance loss peak wavelength position of the sensor, and then demodulates the information to be measured. By arranging a series of sensors with different optical path differences in parallel, the multiplexing of multiplexed sensors can be achieved. The device of the present invention has the advantages of stability, high spatial resolution, large number of multiplexed sensors, and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the present invention;

[0015] Illustration Description:

[0016] 1. Broadband light source; 2. First coupler; 3. Cascaded long-period fiber grating; 4. Second coupler; 5. Fizeau interferometer;

[0017] 6. Linear array InGaAs; 7. Data storage and processing equipment. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments:

[0019] The invention provides a cascaded long-period fiber grating multiplexing demodulation device based on the principle of low-coherence interference. The fiber grating multiplexing demodulation device has the advantages of stability, high spatial resolution, large multiplexing number and low cost.

[0020] As an embodiment of the present invention, the present invention provides Figure 1 The cascaded long period fiber grating multiplexing demodulation device based on the principle of low coherence interference shown in the figure comprises a broadband light source 1, a first coupler 2, a cascaded long period fiber grating 3, a second coupler 4, a Fizeau interferometer 5, a linear array InGaAs 6 and a data storage and processing device 7, and is characterized in that: the broadband light source 1 is connected to the port optical signal of the first fiber coupler 2, the first fiber coupler 2 is connected to at least two cascaded long period fiber grating sensors 3, the cascaded long period fiber grating sensor 3 is connected to the second fiber coupler 4 through an optical signal, the other interface of the second fiber coupler 4 is optically connected to the input end of the Fizeau interferometer 5, the output optical signal of the Fizeau interferometer 5 reaches the linear array InGaAs 6, and the linear array InGaAS6 is connected to the data storage and processing device 7.

[0021] There are N cascaded long-period fiber gratings, N is a positive integer, N≥2, the first fiber coupler is connected to at least two cascaded long-period fiber grating sensors, the central wavelength position of the broadband light source 1 is 1565nm, the spectral range is 60nm, and the bandwidth of the resonance loss peak of the long-period fiber grating sensor is covered.

[0022] In order to ensure that the interference fringes have a high visibility, the grating of the cascaded long-period fiber grating sensor 3 is composed of two identical long-period fiber gratings with a coupling strength of 3 dB.

[0023] In order to meet the low coherence interference condition, the optical path difference introduced by the cascaded long period fiber grating 3 and the Fizeau interferometer 5 should be smaller than the coherence length of the broadband light source 1 .

[0024] In order to prevent the interference fringes of different sensors from overlapping, the distance l between the long-period fiber gratings of each cascaded long-period fiber grating 3 should be preset. i different.

[0025] This application works as follows:

[0026] Monochromatic light of a wavelength λ emitted from a broadband light source is split by coupler 1 into N beams of light with equal amplitude and are incident on LPGP-1, LPGP-2, ..., LPGP-N respectively.

[0027] By presetting the optical path difference introduced by the optical signal in each cascaded long-period fiber grating sensor, the optical path difference introduced by each sensor is different. Multiple light paths are transmitted in it, one of the optical paths of the cascaded long-period fiber grating sensor is the reference arm, and the other optical paths including the cascaded long-period light and the long-period fiber grating distance l i Different from the conventional fiber grating sensor, the optical path of the fiber grating sensor is used as a sensor arm for parameter measurement. Each sensor arm has a different optical path difference with the reference arm, and they are all greater than the coherence length of the light source, so there is no interference when the light from each path is combined through the fiber coupler.

[0028] The light field radiated by the broadband light source is split by the beam splitter and then enters the electric vector of a cascaded long-period fiber grating sensor. According to the coupled mode theory and the transmission matrix, the electric vector expression after the cascaded long-period fiber grating sensor is:

[0029]

[0030] Where T0 represents the transmission coefficient of the long-period fiber grating core layer, T j represents the transmission coefficient of the LP0j-th cladding mode of the long-period fiber grating, and Δφ represents the phase difference introduced by the two arms of the cascaded long-period fiber grating sensor:

[0031]

[0032] and denote the effective refractive index of the core guided mode and the effective refractive index of the j-th order cladding mode, respectively, l i represents the spacing between two long-period fiber gratings.

[0033] When the light beam enters the Fizeau interferometer, the optical path difference caused by the Fizeau interferometer matches the optical path difference caused by the cascaded long-period fiber grating sensor, and obvious low-coherence interference fringes are generated in the corresponding local area of ​​the linear array InGaAs. The electric vector of the light field obtained on the linear array InGaAs can be expressed as:

[0034]

[0035] Where: r represents the reflectivity of the beam splitter in the Fizeau interferometer, represents the phase difference introduced by the Fizeau interferometer:

[0036]

[0037] Where: n is the refractive index of the Fizeau interferometer, and h is the thickness of the Fizeau interferometer.

[0038] The expression of the interference light intensity of a certain wavelength λ detected on the linear array InGaAs is:

[0039]

[0040] The interference signal collected by the linear array InGaAS is the superposition of the monochromatic wavelength interference signals of each wavelength (λ1~λ2) of the broadband light source:

[0041]

[0042] The linear array InGaAs outputs the received signal to the signal processing device, which demodulates the interference fringe signal. There is a connection between the information of the interference fringe and the transmission spectrum information of the cascaded long-period fiber grating. The interference signal can be fast Fourier transformed by a computer to restore the transmission spectrum of the cascaded long-period fiber grating and obtain the drift of the resonant wavelength caused by the change of external parameters (temperature, stress, refractive index, etc.).

[0043] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A cascaded long-period fiber grating multiplexing demodulation device based on the principle of low-coherence interference, characterized in that: The device comprises a broadband light source (1), a first optical fiber coupler (2), a cascaded long-period fiber grating sensor (3), a second optical fiber coupler (4), a Fizeau interferometer (5), a linear array InGaAs (6) and a data storage and processing device (7), wherein the broadband light source (1) is connected to a port optical signal of the first optical fiber coupler (2), the first optical fiber coupler (2) is connected to at least two cascaded long-period fiber grating sensors (3), the cascaded long-period fiber grating sensor (3) is connected to the second optical fiber coupler (4) via an optical signal, another interface of the second optical fiber coupler (4) is optically connected to an input end of the Fizeau interferometer (5), the output optical signal of the Fizeau interferometer (5) reaches the linear array InGaAs (6), and the linear array InGaAs (6) is connected to the data storage and processing device (7); The number of the cascaded long-period fiber grating sensors (3) is N, where N is a positive integer and N≥2; The central wavelength of the broadband light source (1) is 1565 nm, and the spectral range is 60 nm, covering the bandwidth of the resonance loss peak of the long-period fiber grating sensor; The grating of the cascaded long-period fiber grating sensor (3) is composed of two identical long-period fiber gratings with a coupling strength of 3 dB; The optical path difference introduced by the cascaded long-period fiber grating sensor (3) and the Fizeau interferometer (5) is smaller than the coherence length of the broadband light source (1); It is preset that the distances li between the long-period fiber gratings of each cascaded long-period fiber grating sensor (3) are different; The light field radiated by the broadband light source is split by the beam splitter and then enters the electric vector of a cascaded long-period fiber grating sensor. express; According to the coupled mode theory and transmission matrix, the electric vector expression after cascading long-period fiber grating sensors is: Where T0 represents the transmission coefficient of the long-period fiber grating core layer, Tj represents the transmission coefficient of the LP0j-th order cladding mode of the long-period fiber grating, and Δφ represents the phase difference introduced by the two arms of the cascaded long-period fiber grating sensor: and They represent the effective refractive index of the core guided mode and the effective refractive index of the j-th order cladding mode respectively; when the light beam enters the Fizeau interferometer, the optical path difference caused by the Fizeau interferometer matches the optical path difference caused by the cascaded long-period fiber grating sensor, and obvious low-coherence interference fringes will be generated in the corresponding local area of ​​the linear array InGaAs. The electric vector of the light field obtained on the linear array InGaAs is expressed as: Where: r represents the reflectivity of the beam splitter in the Fizeau interferometer, represents the phase difference introduced by the Fizeau interferometer: Where: n is the refractive index of the Fizeau interferometer, h is the thickness of the Fizeau interferometer; The expression of the interference light intensity of a certain wavelength λ detected on the linear array InGaAs is: The interference signal collected by the linear array InGaAS is the superposition of the monochromatic wavelength interference signals of the broadband light source at wavelengths λ1 to λ2:

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