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

Abstract

The utility model relates to an optical fiber sensing demodulation system based on a phase-locked loop. It includes an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit; the optical path demodulation unit includes fiber grating FBGA as a reference fiber grating, fiber grating FBGB as a sensing fiber grating, and acousto-optic for modulating the light source into pulsed light. Modulator AOM, the first 3-port circulator for splitting the transmitted light and incident light of the fiber grating FBGA, and the second 3-port circulator for connecting the reflected light of the fiber grating FBGB to the photoelectric receiving sub-module ROSA; photoelectric conversion circuit The unit includes the photoelectric receiving sub-module ROSA for converting the reflected light of the fiber grating FBGB into a current signal, and converts it into a voltage signal through a series resistor; the signal processing unit includes a phase-locked loop PLL for collecting voltage signals and an integrated display MCU. This system has strong anti-noise ability and high demodulation accuracy. It avoids the use of spectrometer, optical power meter and demodulator. It has the advantages of simple structure, intuitiveness, low cost and portability.

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 in particular relates to an optical fiber sensing demodulation system based on a phase-locked loop.

Background technique

Fiber Bragg Gratin (FBG) sensors have the characteristics of passiveness, strong anti-interference, and strong corrosion resistance, and are suitable for distributed measurements in tunnels, pipe corridors and other power-hungry environments. Signal demodulation is the core technology for the application of fiber grating sensors. The traditional signal demodulation method is to demodulate the wavelength shift caused by the influence of temperature on the refractive index during the optical signal transmission process, which has high safety. However, the wavelength shift needs to be observed with a spectrometer, resulting in a large system size and high cost, which limits its practical promotion. Many studies have proposed using phase demodulation instead of wavelength demodulation. The phase interference demodulation method uses the reference light and the sensing light to produce a certain phase difference after interference. The photodetector detects the intensity of the interference light to obtain phase information. It has the advantages of high sensitivity and strong anti-interference. However, the interference phase demodulation method requires strict interference conditions and complex algorithms. It has problems such as small dynamic range, slow demodulation speed, complex structure, and difficult production, making it difficult to produce in large quantities. Compared with the first two, the light intensity demodulation method converts the relative wavelength shift of the sensing light and the reference light into light intensity, and achieves demodulation by measuring the light intensity change of the output spectrum, such as Sagnac edge filtering method and overlapping spectrum power monitoring. Law. The Sagnac edge filtering method system has a simple structure and low production cost, but its principle is to use interference filtering, and the stability of the system is still restricted by interference conditions. The overlapping spectrum power monitoring method realizes demodulation by monitoring the power of the overlapping spectrum of two fiber gratings. The system is simple and easy to implement. However, the traditional power monitoring method is a DC signal, which has poor anti-noise and is susceptible to interference from light sources and optical paths. Solution Adjustment accuracy is low. In order to improve the anti-noise capability and accuracy, the sensing head needs to use a sensitivity-enhancing fiber grating. The packaging structure of the sensitivity-enhancing fiber grating is special, which is not only expensive, but also has low flexibility and is not suitable for bending or immersion in liquid applications.

Contents of the invention

The purpose of this utility model is to solve the problems existing in the existing structure of demodulation methods such as spectrometers, optical power meters, and DC light intensity demodulation, which are expensive, impractical, and have poor anti-noise capabilities, and provide a method based on phase-locked loops. The optical fiber sensing demodulation system has strong anti-noise ability and high demodulation accuracy. It avoids the use of spectrometer, optical power meter and demodulator. It has the advantages of simple structure, intuitiveness, low cost and portability.

In order to achieve the above purpose, the technical solution of the present invention is: an optical fiber sensing demodulation system based on a phase-locked loop, including an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit;

The optical path demodulation unit includes fiber grating FBGA as a reference fiber grating, fiber grating FBGB as a sensing fiber grating, an acousto-optic modulator AOM for modulating the light source into pulsed light, and splitting of transmitted light and incident light of the fiber grating FBGA. The first 3-port circulator and the second 3-port circulator are used to connect the reflected light of the fiber grating FBGB to the photoelectric receiving sub-module ROSA;

The photoelectric conversion circuit unit includes a photoelectric receiving sub-module ROSA for converting the reflected light of the fiber grating FBGB into a current signal, and converts it into a voltage signal through a series resistor;

The signal processing unit includes a phase-locked loop (PLL) for collecting voltage signals and an MCU with integrated display.

In one embodiment of the present invention, one end of the acousto-optic modulator AOM is connected to the broadband light source ASE, and the other end is connected to port 1 of the first 3-port circulator; port 2 of the first 3-port circulator is connected to one end of the fiber grating FBGA, and port 3 Connect one end of the fiber grating FBGB; the 1 port of the second 3-port circulator is connected to one end of the fiber grating FBGA, the 2 port is connected to one end of the fiber grating FBGB, and the 3 port is connected to one end of the photoelectric receiving sub-module ROSA; the other end of the fiber grating FBGA is covered with a cap; the optical fiber is covered with a cap. The other end of the grating FBGB is covered with a cap; the other end of the optical receiving sub-module ROSA is connected to one end of the phase-locked loop PLL; the other end of the phase-locked loop PLL is connected to the MCU.

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

In one embodiment of the present invention, the frequency of the pulsed light is 10 KHz.

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

In one embodiment of the present invention, the bandwidths of the fiber grating FBGA and the fiber grating FBGB are both 10 nm, the center wavelength of the fiber grating FBGA is 1550 nm, and the center wavelength of the fiber grating FBGB is 1555 nm.

In one embodiment of the present invention, the phase-locked loop PLL is a dual-phase phase-locked PLL loop.

Compared with the existing technology, the utility model has the following beneficial effects: the system of the utility model has strong anti-noise ability, high demodulation accuracy, avoids the use of spectrometers, optical power meters and demodulators, and has a simple, intuitive and low-cost structure. Low, portable and other advantages.

Description of the drawings

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

Figure 2 is a phase-locked voltage-temperature characteristic curve of an embodiment of the present invention.

Figure 3 is a system signal-to-noise ratio comparison chart according to the embodiment of the present invention.

Figure 4 is a demodulation result diagram of the system of the present invention.

Detailed ways

The technical solution of the present utility model will be described in detail below with reference to the accompanying drawings. In order to make the features and advantages of this patent application more obvious and easy to understand, examples are given below and described in detail as follows:

As shown in Figure 1, this example provides an optical fiber sensing demodulation system based on a phase-locked loop, including an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit;

The optical path demodulation unit includes the fiber grating FBGA as the reference fiber grating, the fiber grating FBGB as the sensing head, the acousto-optic modulator AOM used to modulate the light source into pulsed light, and the fiber grating FBGA used to split the transmitted light and incident light. The first 3-port circulator (ie, the 3-port circulator 1 below), the second 3-port circulator (ie, the 3-port circulator 2 below) used to connect the reflected light of the fiber grating FBGB to the photoelectric receiving sub-module ROSA; optical path The demodulation unit uses the linear relationship between the power of the reflection spectrum of the two FBGs and the wavelength shift to monitor temperature changes;

The photoelectric conversion circuit unit includes a photoelectric receiving sub-module ROSA for converting the reflected light of the fiber grating FBGB into a current signal, and converts it into a voltage signal through a series resistor;

The signal processing unit includes a phase-locked loop (PLL) for collecting voltage signals and an MCU with integrated display.

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

In this embodiment, the acousto-optic modulator is used to modulate the continuous light source into 10KHz pulsed light. One end is connected to the ASE, and the other end is connected to the 3-port circulator 1.

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

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

In this embodiment, the chirped fiber grating FBGA is used as the reference fiber grating. One end is connected to port 2 of the 3-port circulator 1, and the other end is capped and left unconnected.

The chirped fiber grating FBGB is used as a sensing fiber grating. One end is connected to the 2 ports of the 3-port circulator 2, and the other end is covered with a cap and left unconnected.

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

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

In this embodiment, the MCU displays the temperature monitoring situation based on the experimentally calibrated voltage-temperature relationship.

In this embodiment, the light source is modulated by the AOM from port 1 of circulator 1 to port 2, port 2 is connected to the FBGA, and port 3 of circulator 1 is connected to port 1 of circulator 2. The optical signal is reflected by FBGA and then incident on FBGB through port 2 of circulator 2. The reflected light of FBGB is incident on ROSA through port 3 of circulator 2. ROSA converts the converted current into voltage through the series current and then gives it to the phase locked loop. When FBGA and FBGB are in the same temperature environment, the reflection spectrum of FBGB remains unchanged. When FBGA is in a certain fixed temperature state and FBGB is affected by temperature, the center wavelength of FBGB will shift (in the long or short wave direction). FBGA is used as the reference fiber grating, and FBGB is used as the sensing fiber grating. Due to the positive temperature characteristics of fiber grating wavelength shift, the center wavelength moves toward the long wavelength direction.

Affected by the circulator and photodetector, the sensing signal is generally a weak signal submerged in the noise. The lock-in amplifier can extract the weak signal from the noise and measure it accurately. This system uses a dual-phase phase-locked loop, which outputs a DC signal that is independent of the phase of the signal. The reference signal only needs to be consistent with the frequency of the signal to be measured, without synchronization.

In this embodiment, the physical test and setup test are performed according to the schematic diagram in Figure 1 . In the experiment, a constant-temperature water tank water bath heating method was used. The temperature is continuously heated from 10°C to 90°C, and is sampled every 5 seconds. A standard electronic temperature sensor is used to synchronously send the temperature in the water tank and the phase-locked voltage to the MCU in real time. In the experiment, the signal source is used to generate pulse signals and reference signals. We set a 10kHz pulse signal with a duty cycle of 50% as the modulation signal, and set the amplitude of the reference signal to 1V. The relationship between phase-locked sampling voltage and temperature is shown in Figure 2. As can be seen from Figure 2, 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 Figure 2. Among them, the R-squared of the linear fitness is approximately 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 sampled voltage from the noise. Temperature-sensitive fiber Bragg gratings may be required as sensor heads. However, the system's cost, flexibility, and temperature range are limited by its specialized packaging structure. In this system, we use a PLL to extract the sampled voltage from the noise when the sensor head is a regular fiber grating. Through comparative experiments with the DC light intensity signal demodulation system, the anti-noise performance of the system is verified. The results are shown in Figure 3. Figure 3(a) shows the noise as a function of temperature, while Figure 3(b) shows the signal-to-noise ratio as a function of temperature. As shown in Figure 3(a), the middle line represents the improved system, with a maximum noise value of 0.11mV. The lines with large fluctuations up and down represent the previous system, with a maximum noise value of 0.0188V. It can be seen that the PLL greatly suppresses the noise of the proposed system.

In this embodiment, the MCU sends data to the display module for display. Figure 4 shows the measured temperature of the system. Temperature is measured continuously in the range of 10°C to 90°C, including heating and cooling. As shown in Figure 4, the heating process and the cooling process are basically the same, so we only show the linear fitting data of the heating process in Figure 4. The linear fitness has an R-squared of 0.99985 and a slope of 1.00187. The test temperature value is close to the set temperature, and the maximum standard deviation is approximately 0.4981°C, indicating that the sensor has high accuracy.

The above are the preferred embodiments of the present utility model. Any changes made according to the technical solution of the present utility model and the resulting functional effects do not exceed the scope of the technical solution of the present utility model, all belong to the protection scope of the present utility model.

Claims (7)

1. An optical fiber sensing demodulation system based on phase-locked loop, characterized in that it includes an optical path demodulation unit, a photoelectric conversion circuit unit and a signal processing unit; The optical path demodulation unit includes fiber grating FBGA as a reference fiber grating, fiber grating FBGB as a sensing fiber grating, an acousto-optic modulator AOM for modulating the light source into pulsed light, and splitting of transmitted light and incident light of the fiber grating FBGA. The first 3-port circulator and the second 3-port circulator are used to connect the reflected light of the fiber grating FBGB to the photoelectric receiving sub-module ROSA; The photoelectric conversion circuit unit includes a photoelectric receiving sub-module ROSA for converting the reflected light of the fiber grating FBGB into a current signal, and converts it into a voltage signal through a series resistor; The signal processing unit includes a phase-locked loop (PLL) for collecting voltage signals and an MCU with integrated display. 2. The optical fiber sensing demodulation system based on phase-locked loop according to claim 1, characterized in that one end of the acousto-optic modulator AOM is connected to the broadband light source ASE, and the other end is connected to the first 3-port circulator port 1; The 2-port of the first 3-port circulator is connected to one end of the fiber grating FBGA, the 3-port is connected to the FBGB end of the fiber grating; the 1-port of the second 3-port circulator is connected to one end of the fiber grating FBGA, the 2-port is connected to the FBGB end of the fiber grating, and the 3-port is connected to the photoelectric receiver. One end of the module ROSA; the other end of the fiber grating FBGA is covered with a cap; the other end of the fiber grating FBGB is covered with a cap; the other end of the optical receiving sub-module ROSA is connected to one end of the phase-locked loop PLL; the other end of the phase-locked loop PLL is connected to the MCU. 3. The optical fiber sensing demodulation system based on phase-locked loop according to claim 2, characterized in that the broadband light source ASE generates a light source with a central wavelength of 1540nm-1560nm and a power of 20mW. 4. The optical fiber sensing demodulation system based on phase-locked loop according to claim 1, characterized in that the frequency of the pulsed light is 10 KHz. 5. The optical fiber sensing demodulation system based on phase-locked loop according to claim 1, characterized in that, the fiber grating FBGA and the fiber grating FBGB are chirped fiber gratings. 6. The optical fiber sensing demodulation system based on phase-locked loop according to claim 1, characterized in that the bandwidths of fiber grating FBGA and fiber grating FBGB are both 10nm, the center wavelength of fiber grating FBGA is 1550nm, and the fiber grating FBGB has a center wavelength of 1550nm. The center wavelength is 1555nm. 7. The optical fiber sensing demodulation system based on phase-locked loop according to claim 1, characterized in that the phase-locked loop PLL is a dual-phase phase-locked PLL loop.