CN102494702B - Long period fiber grating sensor and remote-sensing demodulating system - Google Patents

Abstract

The invention discloses a long-period optical fiber grating sensor, which has the following structure: it is composed of an ordinary single-mode optical fiber and a hollow-core optical fiber; The length of the core fiber; the inner diameter of the hollow-core fiber is larger than the mode field diameter of the common single-mode fiber, and the long-period fiber grating is written on the common single-mode fiber. The present invention also discloses a telemetry demodulation system based on the foregoing sensor; the beneficial technical effect of the present invention is: the long-period fiber grating sensor structure of the present invention enables the LPFG with transmission-type band-stop characteristics to have a band-pass function; benefiting from the long-term Periodic fiber grating sensor structure, the telemetry demodulation system of the present invention can make the light source and the demodulation system at the same end, which is convenient for subsequent processing, and the demodulation system can use the tunable FP demodulation method for demodulation processing, avoiding the The use of spectrometers in the system reduces the cost of the telemetry system.

Description

Long period fiber grating sensor and telemetry demodulation system

technical field

The present invention relates to a fiber optic telemetry technology, in particular to a long-period fiber grating sensor and its telemetry technology.

Background technique

The long-period fiber grating (LPFG) can couple the core fundamental mode to the cladding mode, and it has higher sensitivity to external physical quantities (such as refractive index, strain, bending and twist, etc.) than the fiber Bragg grating (FBG). Therefore, LPFG has important application value in the fields of biochemical detection and structural health monitoring. So far, the demodulation methods for LPFG sensing mainly include two types, namely the wavelength drift method and the edge filter method; these two demodulation methods have the following problems in practical applications:

The spectrometer is a necessary instrument when using the wavelength drift method for demodulation, which not only complicates the structure of the measurement device, slows the measurement speed, but also increases the cost of the measurement system. At the same time, it also limits the field application of LPFG.

When the edge filtering method is used for demodulation, the linear region of the LPFG transmission stop band is mainly used, and its negative effect is that the range of LPFG's measurement of external physical quantities is limited.

In addition, LPFG itself is a transmission-type device. When LPFG is used to directly measure physical quantities, the demodulation instrument and input light source are located at both ends of the entire system, which is not convenient for subsequent processing, especially for long-distance sensing. The problem is even more serious for the system.

Contents of the invention

Aiming at the problems in the background technology, the present invention proposes a low-cost bandpass type long-period fiber grating sensor, its structure is: it is made up of common single-mode fiber and hollow-core fiber; The length of ordinary single-mode fiber is longer than that of hollow-core fiber; the inner diameter of hollow-core fiber is larger than the mode field diameter of ordinary single-mode fiber, and long-period fiber gratings are written on ordinary single-mode fiber.

The solid part on the bare end face of the hollow-core fiber is provided with a reflective film; the material of the reflective film can be selected from silver Ag, gold Au or Ti ₂ O ₃ according to the temperature environment when the device works.

The axial length of the hollow core fiber is greater than 0 and less than 3mm.

The mode field diameter of ordinary single-mode fiber is 10.4 μm, and the inner diameter of hollow core fiber is 11~13 μm.

On the basis of the long-period fiber grating sensor of the aforementioned structure, the present invention also proposes a telemetry demodulation system, which is structured as follows: it consists of a long-period fiber grating sensor, a pump source, a wavelength division multiplexer, an erbium-doped optical fiber, Composed of isolator, three-port circulator, coupler, demodulation system and polarization controller;

Both the output end of the pumping source and the output end of the polarization controller are connected to the optical path of the input end of the wavelength division multiplexer, and the output end of the wavelength division multiplexer is connected to the optical path of the input end of the isolator through an erbium-doped optical fiber; The output end is connected to the optical path of the input end of the three-port circulator, and the multiplexing end of the three-port circulator is connected to the ordinary single-mode fiber optical path of the long-period fiber grating sensor; the output end of the three-port circulator is connected to the optical path of the input end of the coupler The output end of the coupler is respectively connected with the demodulation system and the polarization controller optical path.

The demodulation system uses the most commonly used tunable F-P demodulation system for further demodulation.

The beneficial technical effects of the present invention are: the long-period fiber grating sensor structure of the present invention enables the LPFG with transmission-type band-stop characteristics to have a band-pass function; thanks to the long-period fiber Bragg grating sensor structure, the telemetry demodulation system of the present invention, The light source and the demodulation system can be at the same end, which is convenient for subsequent processing, and the demodulation system can use the tunable F-P demodulation method for demodulation processing, which avoids the use of spectrometers in the system and reduces the cost of the remote measurement system.

Description of drawings

Fig. 1, the long-period fiber grating sensor structure schematic diagram of the present invention;

Figure 2. Ordinary LPFG transmission spectrum;

Fig. 3, long period fiber grating sensor transmission spectrum of the present invention;

Fig. 4, the telemetry demodulation system structure schematic diagram of the present invention;

Fig. 5, the output spectral line of the telemetry demodulation system of the present invention.

Detailed ways

A long-period fiber grating sensor, the structure of which is: it consists of a common single-mode fiber 1 and a hollow-core fiber 2; The length of optical fiber 1 is longer than that of hollow-core optical fiber 2; the inner diameter of hollow-core optical fiber 2 is larger than the mode field diameter of ordinary single-mode optical fiber 1, and long-period fiber gratings (hereinafter referred to as LPFG) are written on the entire length of ordinary single-mode optical fiber 1.

The principle of ordinary LPFG on light is: when the light transmitted in the core of the transmission fiber passes through the LPFG, the LPFG couples the energy of a specific wavelength λ _res in the input broadband light source to the cladding of the fiber according to the phase matching condition In the mode, a stop band with λ _res as the center wavelength will be formed in the transmission spectrum of LPFG (its transmission spectrum is shown in Figure 2);

The working principle of the long-period fiber grating sensor of the present invention is: when the light transmitted in the core of the transmission fiber touches the common single-mode fiber 1 provided with the LPFG, the part of the light that meets the phase matching condition (the part of the light marked Part A) is coupled into the cladding of ordinary single-mode fiber 1 by LPFG and forms a cladding mode, and part A continues to propagate forward in the form of cladding mode, and the part of light that does not meet the phase matching condition (this part of light Part B) continues to propagate forward in the core, when Part A and Part B are transmitted to the junction of LPFG and HCF, Part A is transmitted into the cladding of HCF and excites the cladding in the cladding of HCF Mode (this cladding mode is marked as c cladding mode), part B is also transmitted into the hollow core of HCF; the part of light entering the hollow core of HCF is either scattered into the cladding and finally injected into the air to be lost, Either continue to transmit forward in the hollow core of HCF and finally consume, in a word, part B is finally consumed; the aforementioned c-cladding mode excited by part A in HCF is transported to the end face of HCF, and is absorbed by HCF The end face is reflected back and re-transmitted into the ordinary single-mode fiber 1, and the light re-transmitted into the ordinary single-mode fiber 1 meets the phase matching condition, so this part of the light is recoupled back to the core by the LPFG to form a passband output (I _out ) (its transmission spectrum is shown in Figure 3); therefore, the long-period fiber grating sensor of the present invention can also be called reflective LPFG (Re-LPFG).

When the external measured physical quantity (such as temperature, strain, bending, refractive index, etc.) changes, the effective refractive index and grating period of the LPFG cladding mode will also change, which will eventually lead to the shift of the LPFG resonance wavelength. Re-LPFG through The central wavelength of the band will also drift, therefore, the output wavelength of the laser will also drift, so as to achieve the purpose of measurement.

In order to improve the reflection efficiency of the end face of the HCF, the present invention also makes the following improvements: the solid part on the exposed end face of the hollow-core optical fiber 2 is provided with a reflective film 2-1; the material of the reflective film 2-1 depends on the working temperature The environment can choose silver Ag, gold Au or Ti ₂ O ₃ .

The axial length of the hollow-core fiber 2 can be adjusted in the range of greater than 0 and less than 3mm; the shorter the length of the hollow-core fiber 2, the smaller the energy loss of the device, but in actual operation, the shorter the hollow-core fiber 2 The higher the precision requirement is, when the axial length of the hollow-core fiber 2 is about 3mm, it can ensure that the energy loss of the device is within a reasonable range, and the ordinary cutting equipment can meet the processing requirements; if the hollow-core fiber If the axial length of 2 is shorter or even less than 1 mm, high-precision cutting equipment is necessary, which increases the processing cost of the device.

An optimal parameter selection proposed by the inventor is as follows: the mode field diameter of the ordinary single-mode fiber 1 is 10.4 μm, and the inner diameter of the hollow-core fiber 2 is 11-13 μm.

On the basis of the structure of the aforementioned long-period fiber grating sensor, the present invention also proposes a telemetry demodulation system using the aforementioned long-period fiber grating sensor. Its structure is: it consists of a long-period fiber grating sensor, a pumping source , a wavelength division multiplexer 4, an erbium-doped fiber 5, an isolator 6, a three-port circulator 7, a coupler 8, a demodulation system 9 and a polarization controller 10;

The specific connection structure of the above-mentioned multiple devices is as follows: the output end of the pump source 3 and the output end of the polarization controller 10 are all connected to the input end of the wavelength division multiplexer 4 through an optical path, and the output end of the wavelength division multiplexer 4 is connected through a doped The erbium optical fiber 5 is connected to the input end optical path of the isolator 6; the output end of the isolator 6 is connected to the input end optical path of the three-port circulator 7, and the sending and receiving multiplexing end of the three-port circulator 7 is connected to the common unit of the long-period fiber grating sensor. Mode optical fiber 1 is optically connected (transmission optical fiber is used, and the length of the transmission optical fiber can be extended to tens of kilometers); the output end of the three-port circulator 7 is optically connected to the input end of the coupler 8, and the output end of the coupler 8 is respectively connected to the The demodulation system 9 is optically connected to the polarization controller 10 .

The working principle of the telemetry demodulation system is: the output of the pump source 3 is coupled into the erbium-doped fiber 5 through one end (980nm) of the 980/1550nm wavelength division multiplexer 4, and the erbium-doped fiber 5 is pumped by the pump source 3 to form The population is reversed and amplified spontaneous emission (ASE) occurs. ASE is equivalent to the input broadband light source, which enters port 1 of the three-port circulator 7 through the isolator 6. The isolator 6 can ensure the unidirectional transmission of light in the ring cavity and form a traveling wave field, thus avoiding the spatial hole burning effect. Then, light enters port 2 from port 1 of the three-port circulator 7, and is filtered by Re-LPFG to obtain a band-pass signal. The central wavelength of the pass band is the resonance wavelength of the long-period fiber grating. The band-pass signal enters port 3 through port 2 of the three-port circulator 7 and then passes through a 99:1 coupler 8. The 99% output of the coupler 8 provides feedback, and the 1% output provides laser output to the demodulation system 9. The band-pass signal enters the annular cavity through the other end (1550nm) of the wavelength division multiplexer 4 to complete a cycle. The energy of the light wave is amplified in each cycle, and the central wavelength of the passband is the first to form the laser output. During the whole process, the polarization state of the light can be controlled by adjusting the polarization controller 10, so as to achieve the purpose of suppressing side modes and optimizing laser performance. When the external measured physical quantity causes the period of the long-period fiber grating or the effective refractive index of the cladding mode to change, the center wavelength of the passband of the sensor will drift, which will cause the wavelength of the laser to drift accordingly.

From the structure of the telemetry demodulation system, it can be seen that the light source and the demodulation system 9 are located at the same end of the LPFG sensor, but in the prior art, because the LPFG has a band-stop characteristic, the light source and the demodulation system 9 must be located at the LPFG sensor. This brings considerable inconvenience to the post-processing, and the present invention just solves this problem.

Demodulation system 9 can adopt spectrometer, the output spectral line obtained by it is referring to Fig. 5, as can be seen from Fig. 5, the output spectral line of the present invention is similar to the reflection spectral line of FBG, therefore when demodulating, can fully Using the tunable F-P demodulation method widely used at present, that is, when the demodulation system 9 adopts the tunable F-P demodulation system, the use of a spectrometer can be avoided, the cost of the measurement system can be significantly reduced, and the flexibility of the system can be improved at the same time.

Claims (6)

1. A long-period fiber grating sensor, characterized in that: it consists of a common single-mode fiber (1) and a hollow-core fiber (2); one end of the common single-mode fiber (1) and one end of the hollow-core fiber (2) For fusion splicing, the length of ordinary single-mode fiber (1) is greater than that of hollow-core fiber (2); the inner diameter of hollow-core fiber (2) is greater than the mode field diameter of ordinary single-mode fiber (1), and ordinary single-mode fiber (1) reads A long-period fiber grating is incorporated. 2. The long-period fiber grating sensor according to claim 1, characterized in that: the solid part on the exposed end face of the hollow-core optical fiber (2) is provided with a reflective film (2-1); the reflective film (2-1) The material used is silver Ag, gold Au or Ti ₂ O ₃ . 3. The long period fiber grating sensor according to claim 1, characterized in that: the axial length of the hollow core fiber (2) is greater than 0 and less than 3mm. 4. The long-period fiber grating sensor according to claim 1, characterized in that: the mode field diameter of the ordinary single-mode fiber (1) is 10.4 μm, and the inner diameter of the hollow-core fiber (2) is 11-13 μm. 5. A telemetry demodulation system adopting the long-period fiber grating sensor as claimed in claim 1, characterized in that: it consists of the long-period fiber grating sensor, pumping source (3), wavelength division multiplexer ( 4), composed of erbium-doped fiber (5), isolator (6), three-port circulator (7), coupler (8), demodulation subsystem (9) and polarization controller (10); Both the output end of the pumping source (3) and the output end of the polarization controller (10) are optically connected to the input end of the wavelength division multiplexer (4), and the output end of the wavelength division multiplexer (4) passes through an erbium-doped optical fiber (5) connected to the optical path of the input end of the isolator (6); the output end of the isolator (6) is connected to the optical path of the input end of the three-port circulator (7), and the transceiver multiplexing end of the three-port circulator (7) is connected to The ordinary single-mode optical fiber (1) of the long-period fiber grating sensor is connected to the optical path; the output end of the three-port circulator (7) is connected to the optical path of the input end of the coupler (8), and the output end of the coupler (8) is respectively connected to the The demodulation subsystem (9) is connected to the optical path of the polarization controller (10). 6. The telemetry demodulation system according to claim 5, characterized in that the demodulation subsystem (9) adopts a tunable F-P demodulation system.

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