CN219935149U

CN219935149U - Optical fiber sensing demodulation system based on phase-locked loop

Info

Publication number: CN219935149U
Application number: CN202321709722.5U
Authority: CN
Inventors: 魏晓菲; 黄淑燕; 胡晓华; 张昊; 陈伟娟
Original assignee: Fujian Jiangxia University
Current assignee: Fujian Jiangxia University
Priority date: 2023-07-03
Filing date: 2023-07-03
Publication date: 2023-10-31
Anticipated expiration: 2033-07-03

Abstract

The utility model relates to an optical fiber sensing demodulation system based on a phase-locked loop. The device comprises an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit; the optical path demodulation unit comprises an optical fiber grating FBGA serving as a reference optical fiber grating, an optical fiber grating FBGB serving as a sensing optical fiber grating, an acousto-optic modulator AOM for modulating a light source into pulse light, a first 3-port circulator for splitting transmission light and incident light of the optical fiber grating FBGA, and a second 3-port circulator for connecting reflection light of the optical fiber grating FBGB into a photoelectric receiving sub-module ROSA; the photoelectric conversion circuit unit comprises a photoelectric receiving submodule ROSA used for converting reflected light of the fiber bragg grating FBGB into a current signal and converting the current signal into a voltage signal through a series resistor; the signal processing unit comprises a phase-locked loop PLL for collecting voltage signals and an MCU for integrating display. The system has the advantages of strong noise immunity, high demodulation precision, simple and visual structure, low cost, portability and the like, and avoids the use of a spectrometer, an optical power meter and a demodulator.

Description

Optical fiber sensing demodulation system based on phase-locked loop

Technical Field

The utility model belongs to the field of optical fiber temperature sensors, and particularly relates to an optical fiber sensing demodulation system based on a phase-locked loop.

Background

The fiber bragg grating (Fiber Bragg Gratin, FBG) sensor has the characteristics of being passive, strong in interference resistance, strong in corrosion resistance and the like, and is suitable for distributed measurement in the electricity-fear environments such as tunnels and pipe galleries. The signal demodulation is a core technology applied to the fiber grating sensor, and the traditional signal demodulation method demodulates wavelength shift caused by the influence of temperature on the refractive index in the optical signal transmission process, so that the optical fiber grating sensor has higher safety. However, the wavelength shift needs to be observed by a spectrometer, so that the system is large in size and high in cost, and practical popularization is limited. Many studies have proposed replacing wavelength demodulation with phase demodulation. The phase interference demodulation method utilizes the reference light and the sensing light to generate a certain phase difference after interference, and the light intensity of the interference light is detected by the photoelectric detector to obtain phase information, so that the phase interference demodulation method has the advantages of high sensitivity, strong anti-interference performance and the like. However, the interferometric phase demodulation method requires strict interferometric conditions and complex algorithms, and has the problems of small dynamic range, low demodulation speed, complex structure, difficult manufacture and the like, and is difficult to mass production. In comparison with the former two, the light intensity demodulation method converts the relative wavelength shift of the sensing light and the reference light into light intensity, and the demodulation is realized by measuring the light intensity change of the output spectrum, such as a Sagnac edge filtering method and an overlap spectrum power monitoring method. The Sagnac edge filtering method has the advantages of simple structure and lower manufacturing cost, but the principle is that interference filtering is adopted, and the stability of the system is still limited by interference conditions. The overlap spectrum power monitoring method is to realize demodulation by monitoring the power of the overlap spectrum of two paths of fiber gratings, the system is simple and easy to realize, but the traditional power monitoring is direct current signals, noise resistance is poor, the interference of a light source and a light path is easy to occur, and the demodulation precision is low. In order to improve noise resistance and accuracy, the sensor head needs to adopt a sensitization type fiber bragg grating, and the sensitization type fiber bragg grating has a special packaging structure, is high in price and low in flexibility, and is not suitable for bending or soaking in liquid.

Disclosure of Invention

The utility model aims to solve the problems of high price, impractical performance, poor noise immunity and the like existing in the prior structure in the demodulation methods such as using a spectrometer, an optical power meter, and direct current light intensity demodulation, and provides an optical fiber sensing demodulation system based on a phase-locked loop.

In order to achieve the above purpose, the technical scheme of the utility model is as follows: an optical fiber sensing demodulation system based on a phase-locked loop comprises an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit;

the optical path demodulation unit comprises an optical fiber grating FBGA serving as a reference optical fiber grating, an optical fiber grating FBGB serving as a sensing optical fiber grating, an acousto-optic modulator AOM for modulating a light source into pulse light, a first 3-port circulator for splitting transmission light and incident light of the optical fiber grating FBGA, and a second 3-port circulator for connecting reflection light of the optical fiber grating FBGB into a photoelectric receiving sub-module ROSA;

the photoelectric conversion circuit unit comprises a photoelectric receiving submodule ROSA used for converting reflected light of the fiber bragg grating FBGB into a current signal and converting the current signal into a voltage signal through a series resistor;

the signal processing unit comprises a phase-locked loop PLL for collecting voltage signals and an MCU for integrating display.

In an embodiment of the utility model, one end of the AOM is connected with a broadband light source ASE, and the other end is connected with a port 1 of the first 3-port circulator; the 2 ports of the first 3-port circulator are connected with one end of the fiber bragg grating FBGA, and the 3 ports are connected with one end of the fiber bragg grating FBGB; the second 3-port circulator is connected with one end of the fiber bragg grating FBGA, the 2-port circulator is connected with one end of the fiber bragg grating FBGB, and the 3-port circulator is connected with one end of the photoelectric receiving sub-module ROSA; the other end of the fiber bragg grating FBGA is sleeved by a cap; the other end of the fiber bragg grating FBGB is sleeved by a cap; the other end of the optical receiving sub-module ROSA is connected with one end of a phase-locked loop PLL; the other end of the phase-locked loop PLL is connected with the MCU.

In one embodiment of the utility model, the broadband light source ASE generates a light source with a central wavelength of 1540nm-1560nm and a power of 20 mW.

In one embodiment of the present utility model, the pulsed light has a frequency of 10KHz.

In an embodiment of the present utility model, the fiber grating FBGA and the fiber grating FBGB are chirped fiber gratings.

In an embodiment of the present utility model, the bandwidths of the fiber bragg grating FBGA and the fiber bragg grating FBGB are both 10nm, the center wavelength of the fiber bragg grating FBGA is 1550nm, and the center wavelength of the fiber bragg grating FBGB is 1555nm.

In an embodiment of the present utility model, the PLL is a two-phase PLL.

Compared with the prior art, the utility model has the following beneficial effects: the system has strong noise immunity and high demodulation precision, avoids the use of a spectrometer, an optical power meter and a demodulator, and has the advantages of simple and visual structure, low cost, portability and the like.

Drawings

FIG. 1 is a schematic system diagram of an embodiment of the present utility model.

Fig. 2 is a phase-locked voltage-temperature characteristic curve according to an embodiment of the utility model.

Fig. 3 is a diagram illustrating comparison of signal to noise ratios of a system according to an embodiment of the present utility model.

Fig. 4 is a graph of the demodulation results of the system of the present utility model.

Detailed Description

The technical scheme of the utility model is specifically described below with reference to the accompanying drawings. In order to make the features and advantages of the present application more comprehensible, embodiments accompanied with figures are described in detail below:

as shown in fig. 1, the present example provides an optical fiber sensing demodulation system based on a phase-locked loop, which includes an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit;

the optical path demodulation unit comprises an optical fiber grating FBGA serving as a reference optical fiber grating, an optical fiber grating FBGB serving as a sensing head, an acousto-optic modulator AOM for modulating a light source into pulse light, a first 3-port circulator (namely a 3-port circulator 1 below) for splitting transmission light and incident light of the optical fiber grating FBGA, and a second 3-port circulator (namely a 3-port circulator 2 below) for connecting reflection light of the optical fiber grating FBGB into a photoelectric receiving sub-module ROSA; the optical path demodulation unit monitors temperature change by utilizing the linear relation between the power and the wavelength offset of the reflection spectrums of the two FBGs;

In this embodiment, the broadband light source ASE is used to generate a light source with a center wavelength of 1540nm-1560nm and a power of 20 mW;

in this embodiment, the acousto-optic modulator is used to modulate the continuous light source into a pulse light of 10KHz, one end is connected with ASE, and the other end is connected with the 3-port circulator 1.

In this embodiment, the 3-port circulator 1 is used for splitting the transmitted light and the incident light of the chirped fiber grating FBGA, and the 1-port circulator is connected to the AOM, the 2-port circulator is connected to the chirped fiber grating FBGA, and the 3-port circulator is connected to the chirped fiber grating FBGB.

In this embodiment, the 3-port circulator 2 is used to couple reflected light of the FBGB into the ROSA. The port 1 of the 3-port circulator 2 is connected with the FBGA, the port 2 is connected with the chirped fiber grating FBGB, and the port 3 is connected with the ROSA.

In this embodiment, the chirped fiber grating FBGA is used as a reference fiber grating, one end is connected to the 2 port of the 3-port circulator 1, and the other end is sleeved by a cap and is not connected in the air.

The chirped fiber grating FBGB is used as a sensing fiber grating, one end of the chirped fiber grating FBGB is connected with the 2 port of the 3-port circulator 2, and the other end of the chirped fiber grating FBGB is sleeved by a cap and is not connected in the air.

In this embodiment, the ROSA is configured to convert the reflected light of the FBGB into a current signal, and after being connected in series with a resistor, the ROSA is converted into a voltage signal for the PLL to collect. One end is connected with the 3 port of the 3-port circulator 2, and the other end is connected with a phase-locked loop PLL.

In this embodiment, the PLL is configured to lock and extract a 10KHz signal, so as to effectively suppress the photoelectric noise and improve the signal-to-noise ratio of the system. The phase-locked loop PLL is a two-phase-locked loop, and has a signal source and a multiplication filter built in. The signal source is used to generate a reference signal and another reference signal with a 90 degree phase offset, the frequency of which can be set. The multiplying filter is used for multiplying and filtering the two paths of reference signals with the input signals respectively to obtain direct current signals. One end of the PLL is connected with the ROSA, the other end of the PLL is connected with the MCU, and a display module is integrated in the MCU.

In this embodiment, the MCU displays a temperature monitoring condition according to the voltage-temperature relationship calibrated by the experiment.

In this embodiment, the light source is AOM modulated to reach 2 ports from 1 port of the circulator 1, 2 ports are connected to the FBGA, and 3 ports of the circulator 1 are connected to 1 port of the circulator 2. The optical signal is reflected by the FBGA and then enters the FBGB through the 2 ports of the circulator 2, and the reflected light of the FBGB enters the ROSA through the 3 ports of the circulator 2. The ROSA converts the converted current into voltage through serial current and then provides the voltage to the phase-locked loop. When the FBGA and the FBGB are in the same temperature environment, the reflection spectrum of the FBGB is unchanged. When the FBGA is in a certain fixed temperature state and the FBGB is affected by temperature, the center wavelength of the FBGB is shifted (in the long wave or short wave direction). The FBGA is used as a reference fiber grating, and the FBGB is used as a sensing fiber grating. The center wavelength shifts in the long-wavelength direction due to the positive temperature characteristic of the fiber grating wavelength shift.

The sensing signal is a weak signal which is submerged in noise under the influence of the circulator and the photoelectric detector, and the phase-locked amplifier can extract the weak signal from the noise and accurately measure the weak signal. The system uses a dual-phase-locked loop, outputs a direct current signal which is irrelevant to the phase of the signal, and the reference signal only needs to be consistent with the frequency of the signal to be detected without synchronization.

In this embodiment, a physical test setup test is performed according to the schematic diagram of fig. 1. In the experiment, a mode of heating the constant-temperature water tank in a water bath is adopted. The temperature is continuously heated to 90 ℃ from 10 ℃, sampling is carried out every 5s, and the temperature in the water tank and the phase-locked voltage are synchronously transmitted to the MCU in real time by using a standard electronic temperature sensor. In experiments, a signal source was used to generate a pulse signal and a reference signal. We set a pulse signal of 10kHz with a duty cycle of 50% as the modulation signal and set the amplitude of the reference signal to 1V. The phase-locked sampled voltage versus temperature is shown in fig. 2. From fig. 2 it can be seen that the linear fits of the heating process and the cooling process are very close, so we only show the linear fit of the heating process in fig. 2. Wherein the R square of the linear fitness is about 0.99935 and the Temperature Coefficient (TC) of the proposed sensor is 0.796mV/°c.

When the voltage temperature coefficient is so small, it is difficult to accurately identify the sampling voltage from noise. A temperature sensitive fiber bragg grating may be required as a sensor head. However, the cost, flexibility and temperature range of the system are limited by its particular packaging configuration. In this system, when the sensor head is a plain fiber grating, we use a PLL to extract the sampled voltage from the noise. The anti-noise performance of the system is verified through a comparison experiment with a direct current light intensity signal demodulation system. The results are shown in fig. 3, where fig. 3 (a) shows the noise as a function of temperature, and fig. 3 (b) shows the signal-to-noise ratio as a function of temperature. As shown in FIG. 3 (a), the middle line represents the modified system, the maximum noise value is 0.11mV, and the line with large fluctuation is the previous system, and the maximum noise value is 0.0188V. It can be seen that the PLL suppresses the noise of the proposed system greatly.

In this embodiment, the MCU sends the data to the display module for display. Fig. 4 shows the measured temperature of the system. The temperature is measured continuously in the range of 10 ℃ to 90 ℃, including heating and cooling. As shown in fig. 4, the heating process and the cooling process are substantially identical, so we only show the linear fitting data of the heating process in fig. 4. The R square of the linear fitness is 0.99985 and the slope is 1.00187. The measured temperature value is close to the set temperature, and the maximum standard deviation is about 0.4981 ℃, which shows that the sensor has higher precision.

The above is a preferred embodiment of the present utility model, and all changes made according to the technical solution of the present utility model belong to the protection scope of the present utility model when the generated functional effects do not exceed the scope of the technical solution of the present utility model.

Claims

1. The optical fiber sensing demodulation system based on the phase-locked loop is characterized by comprising an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit;

2. The phase-locked loop-based optical fiber sensing demodulation system according to claim 1, wherein one end of the acousto-optic modulator AOM is connected with a broadband light source ASE, and the other end is connected with a 1 port of the first 3-port circulator; the 2 ports of the first 3-port circulator are connected with one end of the fiber bragg grating FBGA, and the 3 ports are connected with one end of the fiber bragg grating FBGB; the second 3-port circulator is connected with one end of the fiber bragg grating FBGA, the 2-port circulator is connected with one end of the fiber bragg grating FBGB, and the 3-port circulator is connected with one end of the photoelectric receiving sub-module ROSA; the other end of the fiber bragg grating FBGA is sleeved by a cap; the other end of the fiber bragg grating FBGB is sleeved by a cap; the other end of the optical receiving sub-module ROSA is connected with one end of a phase-locked loop PLL; the other end of the phase-locked loop PLL is connected with the MCU.

3. The phase locked loop based optical fiber sensing demodulation system of claim 2 wherein the broadband light source ASE produces a light source with a center wavelength of 1540nm-1560nm and a power of 20 mW.

4. The phase locked loop based fiber sensing demodulation system of claim 1 wherein the pulsed light frequency is 10KHz.

5. The phase-locked loop based optical fiber sensing demodulation system according to claim 1, wherein the optical fiber grating FBGA and the optical fiber grating FBGB are chirped optical fiber gratings.

6. The phase-locked loop-based optical fiber sensing demodulation system according to claim 1, wherein bandwidths of the optical fiber grating FBGA and the optical fiber grating FBGB are 10nm, a center wavelength of the optical fiber grating FBGA is 1550nm, and a center wavelength of the optical fiber grating FBGB is 1555nm.

7. The phase-locked loop based optical fiber sensing demodulation system of claim 1 wherein the phase-locked loop PLL is a two-phase-locked PLL loop.