CN107290042B - Fiber Bragg Grating Sensing Modulation and Demodulation Method and Device Based on Phase Shift Signal Modulation - Google Patents

Abstract

The invention discloses a kind of optical fiber grating sensing modulation-demo-demodulation methods and device based on phase shift signal modulation, including laser light source, isolator, photoswitch, phase-modulator, circulator, fiber grating group, signal generator, photoelectric converter, data acquisition module and data processing module, utilize photoswitch, laser light source is modulated, generate pulsed light beam, phase-modulator is recycled to carry out high-speed pulse phase-modulation to pulsed light beam, pulsed light beam interferes in circulator, it realizes the generation of multichannel phase shift signal and is exported by the 3rd port, cooperate the demodulation method of multichannel Phase-shifting algorithm, realize the vibration phase demodulation of fibre optical sensor.The modulation-demo-demodulation method optical path is simple, forms multiple sensing modules with can be convenient, and modulation requires lower, and demodulator circuit is simple, and real-time is high, and precision is high, and dynamic range is wide, and environment resistant interference performance is strong.

Description

Fiber Bragg Grating Sensing Modulation and Demodulation Method and Device Based on Phase Shift Signal Modulation

technical field

The invention belongs to the technical field of optical fiber sensing, and in particular relates to a modulation and demodulation method and device for fiber grating sensing based on phase-shift signal modulation.

Background technique

With the rapid development of science and technology, human society has entered the information age of rapid development. Internet of things technology, communication technology and computer have become rapidly developing information technology. On this basis, the sensor network in the Internet of Things has attracted more and more attention. In the development of information technology, various industries are inseparable from sensors, such as industrial control, automated production processes, environmental monitoring, oil pipeline monitoring, biomedicine, security monitoring, bridge and tunnel monitoring, etc. There are traces of sensors. Compared with traditional electronic sensors, fiber grating sensors have many advantages, such as small size, light weight, low cost, anti-electromagnetic interference, high sensitivity, safety and reliability, corrosion resistance, distributed measurement, and easy networking. It is widely used in the measurement of temperature, vibration, stress and other parameters.

U.S. Patent US5903350 (Demodulator and Method useful for MultiplexedOptical Sensors), proposes a "five-step modulation method", which needs to generate 5 phase changes in one modulation pulse, namely 0, π/2, π, 3π/2, 2π . This modulation and demodulation method needs to generate 5 stepped modulation signals, the 5 phases need to be uniformly improved, and the width of each phase step is equal. The modulation and demodulation method has high requirements on the hardware and methods used to generate the modulated signal. Due to the small width of the phase-modulated laser pulse beam, when five phase steps of the same width are generated, the height of the five phase steps of the same width cannot be maintained uniformly, and the single phase is not easy to maintain. Vibration is easy to occur. The modulation and demodulation method also requires double pulsed beams, using a pulsed beam that has undergone the "five-step modulation method" to interfere with another pulsed beam that has not been modulated to provide an interference light intensity signal. Therefore, before the pulse beam enters the fiber grating group, it needs to pass through a compensator, so that the two pulse beams can obtain a corresponding time difference, so that the two pulse beams can interfere after returning from the fiber grating group. However, due to the accuracy of the compensator, errors may occur when the two pulsed beams interfere, reducing the final detection accuracy.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a fiber grating sensing modulation and demodulation method and device based on phase-shifted signal modulation, which solves the problem that the traditional phase detection technology is difficult to modulate, susceptible to interference, high hardware system requirements, slow in solution speed, and many other problems. Hardware complex issues during area detection.

The technical solution for realizing the purpose of the present invention is: a fiber grating sensing modulation and demodulation method based on phase-shift signal modulation, the method steps are as follows:

Step 1. Turn on the laser light source and make the fiber grating group inductively vibrate. The signal generator is connected to the optical switch and the phase modulator, respectively. The signal generator generates a pulse signal that acts on the optical switch to generate continuous optical pulses, which act on the phase modulator. , to generate continuous phase pulse modulation, the light emitted by the laser light source passes through the isolator, the optical switch and the phase modulator in sequence to form a phase-modulated laser pulse beam;

Step 2. The phase-modulated laser pulse beam enters the first port of the circulator, and enters the fiber of the fused fiber grating group through its second port. When the pulse beam passes through different fiber gratings in the fiber grating group, there are some pulse beams. Returning to the circulator, the pulse beams reflected by the adjacent two fiber gratings generate interference signals in the circulator, and output from the third port of the circulator to the photoelectric converter, and the photoelectric converter receives the interference signals. The time difference of the reflected light is Δt, then the interference light intensity I(t) generated by two adjacent fiber gratings is as follows:

Let the vibration period of the vibration signal be T _S , the width of the optical pulse beam formed by the optical switch is T ₁ , the optical pulse period formed by the optical switch is the same as the phase modulation period of the modulation signal, both are T, and T << T _S , so It is considered that in the period of 0<t<T, the vibration signal remains unchanged in the time domain; k is an integer, let the combined phase in is the initial phase, is the drift phase, Indicates the vibration signal phase; A and B are both constants;

Step 3: The photoelectric converter converts the interference signal into a light intensity electrical signal, and the data acquisition module collects the light intensity electrical signal and sends it to the data processing module. The data processing module obtains the delay by calibrating the phase modulation signal and the received signal. The three-way phase-shifted light intensity signals I ₁ , I ₂ , and I ₃ are generated after the interference of two adjacent fiber gratings, so we have

According to the three-step phase-shifting algorithm, the solution is

Step 4. Remove the initial DC phase and drift phase of θ, and demodulate the continuous vibration signal phase

A device for fiber grating sensing modulation and demodulation method based on phase-shift signal modulation, including laser light source, isolator, optical switch, phase modulator, circulator, fiber grating group, signal generator, photoelectric converter, data acquisition Module and data processing module, wherein the laser light source, isolator, optical switch, phase modulator and circulator are connected in sequence, the fiber grating group is spliced with the fiber, the second port of the circulator is connected with the fiber of the fused fiber grating group, and the signal generator It is respectively connected with the optical switch and the phase modulator, the third port of the circulator is connected with the photoelectric converter through the optical fiber, and the data acquisition module is respectively connected with the photoelectric converter and the data processing module.

The light emitted by the laser light source passes through the isolator, the optical switch and the phase modulator in sequence, and the signal generator generates a pulse signal to act on the optical switch, so that the optical switch modulates the laser light, generates optical pulses and sends them to the phase modulator, while the signal The generator generates a high-speed pulse signal, and provides π/2 phase modulation to the phase modulator, so that the laser pulse beam passes through the phase modulator to form a phase-modulated laser pulse beam; the phase-modulated laser pulse beam enters the first port of the circulator , enter the fiber of the fusion-spliced fiber grating group through its second port, and return to the circulator after being reflected by the fiber grating group, generate an interference signal in the circulator, and output from the third port of the circulator to the photoelectric converter, the photoelectric converter The interference signal is converted into a light intensity electrical signal, and the data acquisition module collects the light intensity electrical signal and sends it to the data processing module to demodulate the continuous vibration signal phase.

Compared with the prior art, the present invention has the following significant advantages: (1) using an optical switch to generate a laser pulse, using a phase modulator to modulate the laser pulse beam in phase, using a fiber grating to reflect a specific wavelength beam and a circulator to unidirectionally The special type of ring transmission realizes the generation of multi-channel phase-shifted signals in the circulator, which is conducive to the fast solution of phase information.

(2) The structure is simple, the size is small, the precision is high, the signal-to-noise ratio is high, the anti-environmental interference ability is strong, and the requirement for the modulation signal is low, which improves the real-time performance of the algorithm.

(3) The number and spacing of fiber gratings can be adjusted according to requirements to adjust the detection range and the number of detection areas.

Description of drawings

FIG. 1 is a schematic diagram of the device structure of the fiber grating sensing modulation and demodulation method based on phase-shift signal modulation of the present invention.

Fig. 2 is the reflected light signal diagram when two separate fiber gratings (6-1, 6-2) constitute the sensing module according to the present invention, wherein (a) and (b) are the phase and time relationship diagrams in the reflected light pulse, (c) ) is the light intensity and time relationship diagram output by the third port (5-3) of the circulator 5, the large rectangle on the time axis represents the reflected light pulse, and the small rectangle above represents the phase information.

Figure 3 is a diagram of reflected light signals when four fiber gratings (6-1, 6-2, 6-3, 6-4) of the present invention form a sensing module, wherein (a), (b), (c), ( d) is the relationship between phase and time in a single reflected light pulse, (e) is the relationship between light intensity and time output from the third port of circulator 5, the large rectangle on the time axis represents the reflected light pulse, and the small rectangle on it represents the phase information.

FIG. 4 is a phase diagram of a demodulated vibration signal in an embodiment of the present invention. information.

Detailed ways

The present invention will be described in further detail below with reference to the accompanying drawings.

1, a fiber grating sensing modulation and demodulation method based on phase-shift signal modulation, the method steps are as follows:

Step 1. Turn on the laser light source 1, and let the tester walk around the optical fiber where the fiber grating group 6 is welded, so that the fiber grating group 6 is induced to vibrate, and the signal generator 7 is respectively connected with the optical switch 3 and the phase modulator 4. The device 7 generates a pulse signal, which acts on the optical switch 3, generates continuous laser pulses, and acts on the phase modulator 4 to generate continuous phase pulse modulation with a phase of π/2. The light emitted by the laser light source 1 passes through the isolator 2 and the optical switch 3 in turn. and the phase modulator 4 to form a phase-modulated laser pulse beam.

The phase-modulated laser pulse beam, its pulse beam width T ₁ is determined by the distance between two adjacent fiber gratings, which is Within one pulse, the laser beam can be expressed as

where E ₀ represents the power amplitude of the laser light source 1, v ₀ represents the center frequency of the laser light source 1, t represents the propagation time, Represents the phase modulated by high-speed pulses, resulting in a phase modulation of π/2, and the modulated pulse width in one cycle is τns.

Step 2. The phase-modulated laser pulse beam enters the first port of the circulator 5, and enters the fiber of the fusion-spliced fiber grating group 6 through its second port. After the pulse beam is reflected by the fiber grating group 6, it returns to the circulator 5. The interference signal is generated in the circulator 5, and is output from the third port of the circulator 5 to the photoelectric converter 8. The photoelectric converter 8 receives the interference signal, and the distance between two adjacent fiber gratings is ΔL, and the light beam reaches the adjacent two fiber gratings. and the returning times are t ₁ and t ₂ respectively, then the reflected light has a time difference Δt

where n is the refractive index of the medium and C is the propagation speed of the laser in vacuum.

The interference light intensity I(t ₁ , Δt) received by the photoelectric converter 8 can be expressed as

Among them, A and B are constants, and B=KA, K<1, K is the visibility of interference fringes, is the initial phase, is the drift phase, represents the vibration signal phase, for the modulation phase.

is the π/2 phase modulation generated by the high-speed pulse signal, and its specific expression is

in k is an integer, and T represents the period of the pulse signal.

When there is external vibration, the change in the equivalent fiber length caused by the vibration can be ignored compared with the initial length difference between two adjacent fiber gratings, that is, the change in Δt can be ignored, and the duty cycle of the pulse signal is controlled so that make phase Then the interference light intensity I(t) can be equivalent to the following formula

Suppose the vibration period of the vibration signal is T _S , and the period of the pulse modulation signal T<<T _S , so it is considered that in the period of 0<t<T, the vibration signal remains unchanged in the time domain.

Step 3: The photoelectric converter 8 converts the interference signal into a light-intensity electrical signal, and the data acquisition module 9 collects the light-intensity electrical signal and sends it to the data processing module 10. The data processing module 10 calibrates the phase modulation signal and the received signal. Delay to obtain the three-way phase-shifted interference light intensity signals I ₁ , I ₂ , I ₃ , so we have

According to the three-step phase-shifting algorithm, the solution is

Step 4. Remove the initial DC phase and drift phase of θ, and demodulate the continuous vibration signal phase

With reference to Fig. 1, a device for a fiber grating sensing modulation and demodulation method based on phase-shift signal modulation includes a laser light source 1, an isolator 2, an optical switch 3, a phase modulator 4 and a circulator 5, a fiber grating group 6, The signal generator 7, the photoelectric converter 8, the data acquisition module 9 and the data processing module 10, wherein the laser light source 1, the isolator 2, the optical switch 3, the phase modulator 4 and the circulator 5 are connected in turn through the optical fiber, and the fiber grating group 6 With the optical fiber fusion, the optical fiber of the fusion fiber grating group 6 is connected with the second port of the circulator 5, the signal generator 7 is respectively connected with the optical switch 3 and the phase modulator 4, and the third port of the circulator 5 is connected with the photoelectric converter through the optical fiber. 8 is connected, and the data acquisition module 9 is connected with the photoelectric converter 8 and the data processing module 10 respectively.

The light emitted by the laser light source 1 passes through the isolator 2, the optical switch 3, and the phase modulator 4 in turn. The signal generator 7 is respectively connected to the optical switch 3 and the phase modulator 4, and the signal generator 7 generates a pulse signal to act on the signal generator 7. The optical switch 3 generates continuous laser pulses, which act on the phase modulator 4 to generate continuous phase pulse modulation with a phase of π/2; the phase-modulated laser pulse beam enters the first port of the circulator 5, and enters the fusion splicing through the second port The optical fiber of the fiber grating group 6 is reflected by the fiber grating group 6, and then returns to the circulator 5, where an interference signal is generated in the circulator 5, and is output from the third port of the circulator 5 to the photoelectric converter 8. The interference signal is converted into a light intensity electrical signal, and the data acquisition module 9 collects the light intensity electrical signal and sends it to the data processing module 10 to demodulate the continuous vibration signal phase.

The circulator 5 adopts a three-port circulator, and the fiber grating group 6 adopts a fiber Bragg grating.

The data processing module 10 obtains the combined phase θ through a three-step phase-shifting algorithm, removes the initial DC phase and drift phase of θ, and demodulates the continuous vibration signal phase.

Example 1

The vibration information of a fiber grating sensor is tested experimentally. The distance between the two grating fibers is 24m, the sampling time is 22s, the laser pulse period and the frequency of the phase modulation pulse are 1MHz, and the pulse width of the phase modulation in one cycle is 160ns. As shown in Figure 1, the measurement steps are:

Step 1. Turn on the laser light source 1 and let the tester walk around the optical fiber where the fiber grating group 6 is welded, so that the fiber grating group 6 is induced to vibrate, and the light emitted by the laser light source 1 passes through the isolator 2, the optical switch 3 and the phase modulator in turn. 4. The signal generator 7 generates a high-speed pulse signal with a frequency of 1MHz, which acts on the optical switch 3 to generate continuous laser pulses, and simultaneously provides the phase modulation of π/2 to the phase modulator 4 to form a phase-modulated laser pulse beam;

Step 2. The phase-modulated laser pulse beam enters the first port of the circulator 5, and enters the fiber of the fusion-spliced fiber grating group 6 through the second port. After being reflected by the fiber grating group 6, an interference signal is generated in the circulator 5, and the photoelectric conversion is performed. The device 8 receives the interference signal.

Step 3. The data acquisition module 9 collects the light intensity electrical signal generated by the photoelectric converter 8 and sends it to the data processing module 10. The data processing module 10 obtains the three-way signal by calibrating the phase modulation signal and the delay of the received signal. Phase-shifted interference light intensity signal.

Step 4. Remove the initial DC phase and drift phase of θ, and demodulate the continuous vibration signal phase

With reference to FIGS. 2 to 4 , this embodiment realizes the measurement of the phase vibration signal through a series of measures. The phase diagram finally obtained, the larger fluctuation is generated when the tester crosses the optical fiber, and its quantity and amplitude are consistent with the actual walking situation of the tester, reflecting that the fiber grating sensor can accurately detect the vibration signal change within the working range. It shows that the method is simple and practical, has high measurement accuracy, strong anti-interference ability and high real-time performance.

Claims (4)

1. a fiber grating sensing modulation and demodulation method based on phase-shift signal modulation, is characterized in that, the method steps are as follows: Step 1. Turn on the laser light source (1), and make the fiber grating group (6) vibrate. The signal generator (7) is connected to the optical switch (3) and the phase modulator (4) respectively, and the signal generator (7) The pulse signal is generated to act on the optical switch (3) to generate continuous optical pulses, which act on the phase modulator (4) to generate continuous phase pulse modulation, and the light emitted by the laser light source (1) passes through the isolator (2), the optical switch ( 3) and a phase modulator (4) to form a phase-modulated laser pulse beam; Step 2. The phase-modulated laser pulse beam enters the first port of the circulator (5), and enters the fiber of the spliced fiber grating group (6) through its second port, and the pulse beam passes through different fibers in the fiber grating group (6). In the case of fiber gratings, part of the pulsed beams returns to the circulator (5), and the pulsed beams reflected by two adjacent fiber gratings generate interference signals in the circulator (5), and are output from the third port of the circulator (5) to the circulator (5). The photoelectric converter (8), the photoelectric converter (8) receives the interference signal, and assuming that the time difference between the reflected light of two adjacent fiber gratings is Δt, the interference light intensity I(t) generated by the adjacent two fiber gratings is as follows: Suppose the vibration period of the vibration signal is T _S , the width of the optical pulse beam formed by the optical switch (3) is T ₁ , the optical pulse period formed by the optical switch (3) is the same as the phase pulse modulation period of the modulation signal, both are T, and T<<T _S , so it is considered that in the period of 0<t<T, the vibration signal remains unchanged in the time domain; k is an integer, so that the combined phase in is the initial phase, is the drift phase, Indicates the vibration signal phase; A and B are both constants; Step 3, the photoelectric converter (8) converts the interference signal into a light intensity electrical signal, the data acquisition module (9) collects the light intensity electrical signal, and sends it to the data processing module (10), and the data processing module (10) passes the Calibrate the delay of the phase modulation signal and the received signal, and obtain the three-way phase-shifted light intensity signals I ₁ , I ₂ , and I ₃ generated by the interference of two adjacent fiber gratings, so we have According to the three-step phase-shifting algorithm, the solution is Step 4. Remove the initial DC phase and drift phase of θ, and demodulate the continuous vibration signal phase 2. The fiber grating sensing modulation and demodulation method based on phase-shift signal modulation according to claim 1, characterized in that: in the step 1, the signal generator (7) generates a pulse signal to act on the optical switch (3) , generate continuous optical pulses, the optical pulse beam width T1 formed by the optical switch ( ₃ ) is determined by the spacing between two adjacent fiber gratings, and the optical pulse beam period is determined by the number of fiber gratings; The number of gratings is N, then the optical pulse period T>(N+1)Δt, and the optical pulse beam width is where n is the refractive index of the fiber medium, and c is the speed of light traveling in a vacuum. 3. The fiber grating sensing modulation and demodulation method based on phase-shift signal modulation according to claim 1, characterized in that: in the step 1, a high-speed pulse signal is generated by a signal generator (7), and sent to the phase modulator (4) Provide a phase modulation of π/2, the pulse width of the phase modulation in one cycle is τns, and 4. The fiber grating sensing modulation and demodulation method based on phase-shift signal modulation according to claim 1, wherein the circulator (5) adopts a three-port circulator, and the fiber grating group (6) adopts a three-port circulator. ) is composed of several fiber Bragg gratings arranged in sequence, and the spacing between two adjacent fiber Bragg gratings is equal.