[0037] Example one
[0038] figure 1 It is a structural block diagram of the phase demodulator 10 provided by an embodiment of the present invention, and the details are as follows:
[0039] A phase demodulator 10 adopts the structure of a Sagnac optical fiber ring, is connected with a diaphragm type optical fiber sound pressure sensor 20, includes a broadband light source 101, a photodetector 102, is connected to the photodetector 102, and receives the photodetector 102 The signal analysis module for demodulating the output electrical signal is characterized in that the phase demodulator 10 further includes:
[0040] Connect the broadband light source 101 and the photodetector 102 respectively, and split the light emitted by the broadband light source 101 into a first fiber coupler 103 that transmits CW light clockwise and CCW light counterclockwise;
[0041] Connect the first fiber coupler 103 to adjust the intensity of the CW light, or a tunable optical attenuator 104 that adjusts the intensity of the CCW light;
[0042] The optical fiber delay line 105 connected to the first optical fiber coupler 103;
[0043] Respectively connect the optical fiber delay line 105 and the tunable optical attenuator 104 to couple the second optical fiber coupler 106 of the light reflected by the diaphragm type optical fiber sound pressure sensor 20;
[0044] Connect the second optical fiber coupler 106 to provide a stable π/2 quadrature phase offset optical frequency shifter 107;
[0045] The optical frequency shifter 107 is connected, and the optical fiber depolarizer 108 of the diaphragm type optical fiber acoustic pressure sensor 20 is externally connected through an optical fiber.
[0046] Among them, the end face of the optical fiber is formed by oblique cutting, so that the reflectivity of the end face of the optical fiber is much lower than that of the ordinary vertical optical fiber end face, which is negligible, so it cannot form an effective Fabry-Perot interference cavity with the graphite film 201; optical fiber 204 It is only used to transmit incident light to the graphite film 201 and receive the light reflected by the film; when there is sound pressure from the outside, the graphite film 201 deforms and modulates the phase of the light back to the optical fiber. By detecting this phase change, the sound pressure signal can be obtained.
[0047] Among them, the optical frequency shifter 107 is used to provide a stable quadrature phase offset for the optical fiber Sagnac ring, which can effectively reduce the disturbance of the optical fiber sound pressure sensor due to changes in the external environment, so that the sensor can work stably in the best linearity and sensitivity state for a long time. . The quadrature phase offset of the demodulation system does not depend on the sensor cavity length and the length of the transmission fiber 109, which greatly improves the applicability of the system.
[0048] Further, in the phase demodulator 10, the first fiber coupler 103 and the second fiber coupler 106 are fiber couplers with the same splitting ratio.
[0049] Further, the optical fiber coupler is a 3dB optical fiber coupler.
[0050] Further, in the phase demodulator 10, the optical frequency shifter 107 is located between the optical fiber coupler and the optical fiber depolarizer 108.
[0051] Further, in the phase demodulator 10, the time delay τ provided by the optical fiber delay line 105 satisfies τ=1/2f, where f is the center frequency of the sound pressure signal to be detected.
[0052] Further, in the phase demodulator 10, the fiber depolarizer 108 is formed by fusion splicing two sections of polarization-maintaining fiber with a length ratio of 1:2 and a fast axis at a 45-degree angle; or,
[0053] It is formed by fusion splicing of two polarization-maintaining optical fibers with a length ratio of 1:2 and the slow axis at a 45 degree angle.
[0054] reference figure 2 with image 3 , figure 2 It is an implementation flowchart of the phase demodulator 10 provided by an embodiment of the present invention, and the details are as follows:
[0055] S201: The light emitted by the broadband light source 101 enters through the a port of the first fiber coupler 103, and is split into the CW light for clockwise transmission and CCW light for the counterclockwise transmission through the first fiber coupler 103. The CW light is output from the c port of the first fiber coupler 103, and the CCW light is output from the d port of the first fiber coupler 103;
[0056] S202. The CW light passes through the tunable optical attenuator 104 and the second optical fiber coupler 106 to reach the optical frequency shifter 107, and after passing through the optical frequency shifter 107 and the optical fiber depolarizer , Reach the diaphragm type optical fiber sound pressure sensor 20;
[0057] S203, the CCW light passes through the optical fiber delay line 105, the second optical fiber coupler 106, and reaches the optical frequency shifter 107, and after passing through the optical frequency shifter 107 and the optical fiber depolarizer, Reach the diaphragm type optical fiber sound pressure sensor 20;
[0058] S204, the light reflected by the graphite film in the diaphragm type optical fiber acoustic pressure sensor 20 is coupled back to the optical fiber link by the optical fiber 109 through the second optical fiber coupler 106;
[0059] S205, the CW light travels along the 2 ports of the second fiber coupler 106, passes through the fiber delay line 105 to the d port of the first fiber coupler 103, and the CCW light travels along the 1 port of the second fiber coupler 106 Go through the tunable optical attenuator 104 to the c port of the first fiber coupler 103;
[0060] S206, the CW and CCW light after passing through the first fiber coupler 103 converge and interfere at the b port of the first fiber coupler 103, and are converted into electrical signals by the photodetector 102, and the electrical signals can be directly output to The signal analysis module;
[0061] S207: The signal analysis module analyzes and processes the electrical signal output by the photodetector 102.
[0062] Its working principle is detailed as follows:
[0063] The graphite film sound pressure probe 200 is connected to the demodulation system in the reflection mode and is used as a part of the improved Sagnac optical fiber loop. By introducing a fiber delay line into the d port of the coupler 103 and the 2 port of the coupler 106, there will be a time difference τ in the time when the CW and CCW light in the Sagnac fiber loop reach the film. Without considering the pigtail length of the tunable optical attenuator, the time difference is determined by the length L of the optical fiber delay line 105, and the following relationship is satisfied:
[0064] τ=nL/c (1)
[0065] Therefore, when a dynamic sound pressure signal from the outside causes a time-varying deformation of the graphite film 201, since the CW and CCW light in the Saganc optical fiber loop reach the graphite film at different times, the film deformation felt is also different. Suppose the sound pressure of the diaphragm is deformed into u(t)=u 0 cos(w s t), where u 0 Is the amplitude of diaphragm deformation caused by sound wave, w s Is the angular frequency of the dynamic sound pressure; the equivalent phase change induced by the diaphragm deformation on the Sagnac fiber loop can be expressed as among them Is the amplitude of phase change caused by sound wave, which is proportional to the amplitude of diaphragm deformation caused by sound wave u 0; So the phase change difference between CW and CCW light Satisfy the following relationship,
[0066]
[0067] among them Is the offset phase generated by the optical frequency shifter frequency 107, and satisfies the following relationship,
[0068]
[0069] Where L is the length of the optical fiber delay 105, n is the refractive index of the optical fiber, c is the speed of light in vacuum, and δf is the frequency shift size of the optical frequency shifter. When the amplitude of the external sound pressure signal Change, CW and CCW optical phase difference The size changes, causing a change in the light intensity I detected by the photodetector. Light intensity I and phase difference Satisfy the following relationship,
[0070]
[0071] Where I 0 Is the intensity of the input light. Putting formula (2) into formula (4), the output light intensity can be obtained as,
[0072]
[0073] According to formula (5), by adjusting the frequency of the optical frequency shifter or the length of the optical fiber delay line, the offset phase generated by the optical frequency shifter can satisfy It can make the sensor work at the best working point to obtain the maximum linear working area and the highest sensitivity. When the amplitude of the acoustic signal is small, the optical phase change caused by the deformation of the film Then the communication part in formula (5) is,
[0074]
[0075] Therefore, by detecting the change in output light intensity I ac , The phase change of the sensor film under the action of sound pressure can be obtained, and the sound pressure signal can be demodulated. It can be obtained from formula (6) that the light intensity change I ac The magnitude is related to the frequency, when the sound pressure signal frequency w s Satisfy relation w s =π/τ, I ac Maximum, the system has the maximum signal output. When the signal frequency is much smaller than π/τ, I ac Decrease, the output signal of the system also decreases accordingly. Therefore, the system is suitable for signal detection within a certain frequency range. Adjust the time difference τ between CW and CCW light reaching the film through the fiber extension 105 to make it equal to the frequency w of the sound wave to be detected s Satisfy relation w s =π/τ, can maximize the system sensitivity. When the sound wave is not a single frequency signal, the responsivity of different frequency components will be different, but with a certain sin(w s τ/2) dependence, so it can be corrected by subsequent processing.
[0076] In this embodiment, the beneficial effects of using the phase demodulator 10 are as follows:
[0077] 1. By adopting the SLD low-coherence light source 101, the coherent noise caused by the reflected light in the system can be effectively reduced;
[0078] 2. By using the fiber depolarizer 108, it can effectively eliminate the signal instability caused by the disturbance of the light polarization state by the environment;
[0079] 3. The phase offset of the sensor is realized by frequency shifting the CW and CCW light in the Sagnac optical fiber ring through an optical frequency shifter. Since the optical frequency shifter 107 is placed between the 4 ports of the second fiber coupler 106 and the fiber depolarizer 108, the stable phase offset provided by the optical frequency shifter can be prevented from being affected by the sound pressure of the diaphragm type optical fiber. The influence of the length of the connecting fiber 109 of the sensor 20;
[0080] 4. Since the Sagnac fiber optic ring CW and CCW light are transmitted in the same fiber path, it has the ability to resist low-frequency interference. At the same time, the orthogonal phase offset provided by the Sagnac fiber optic ring to the diaphragm optical fiber sound pressure sensor 20 has nothing to do with the deformation of the film. , Thus greatly reducing the sensor's operating point drift caused by the disturbance of the external environment (such as air flow, temperature, etc.) on the highly sensitive graphite film.
[0081] In summary, this method of providing phase offset eliminates the need for complex dynamic phase control, and enables the diaphragm-type optical fiber sound pressure sensor 20 based on a high-sensitivity film to work stably for a long time.
