Application Cases

Experiment Name: High-Frequency Acoustic Emission Signal Excitation Experiment

Test Purpose: To verify the functionality of the entire multi-channel fiber Bragg grating high-frequency demodulation system, with a focus on whether the system control software can perform real-time control of the tunable laser light source to compensate for the central wavelength shift caused by temperature and stress on the fiber Bragg grating. This ensures that the output wavelength of the tunable laser remains near the half-bandwidth wavelength of the fiber Bragg grating, thereby increasing the detection sensitivity of the fiber Bragg grating. The system should be capable of detecting high-frequency acoustic emission signals and collecting the direct current voltage signals from the photodetector that these signals induce.

Testing Equipment: High-voltage amplifier, signal generator, oscilloscope, photodetector, piezoelectric ceramic disc, etc.

Figure 1: Optical Path Connection Diagram for High-Frequency Acoustic Emission Signal Experiment

Experiment Process:

A signal with a specific frequency and voltage amplitude is generated by the signal generator and connected to the high-voltage amplifier via a BNC cable. The high-voltage amplifier amplifies the signal amplitude without altering the high-frequency acoustic emission signal, with an amplification factor that can reach several tens of times. In the experiment, a continuous signal type is selected with a frequency set at 250 kHz and an amplitude of 10 V. The amplification factor of the high-voltage amplifier can be set using a knob. To verify the functionality of the signal generator and high-voltage amplifier, as well as whether the piezoelectric ceramic disc can be properly excited, a higher amplification factor is chosen to increase the output voltage amplitude of the high-voltage amplifier. After setting, the amplified ultrasonic signal is output to the piezoelectric ceramic disc for excitation. The laser light source is matched with the detection fiber Bragg grating and connected to the photodetector, with the output voltage signal connected to the oscilloscope. First, the detection system, consisting of a broadband light source, an optical spectrum analyzer, and spectrum viewing software, reads the spectral data of the detection fiber Bragg grating. Figure 2 shows the spectral diagram of the detection fiber Bragg grating in the spectrum viewing software.

Figure 2: Spectral Diagram of the Detection Fiber Bragg Grating

From the spectrum viewing software, it can be concluded that the central wavelength of this fiber Bragg grating is 1549.722 nm, with a 3 dB bandwidth of 0.511 nm. To comply with the matching principle of the fiber Bragg grating, the output wavelength of the tunable laser light source should be the central wavelength of the fiber Bragg grating minus or plus the 3 dB half-bandwidth of the fiber Bragg grating. In this matching process, the output wavelength of the tunable laser light source needs to be set to 1549.467 nm. After matching, the sensitivity of the fiber Bragg grating to acoustic emission signals theoretically reaches its maximum. When the parameters of the photodetector remain unchanged, the wavelength shift detected by the fiber Bragg grating due to the acoustic emission signal is converted into the maximum change in the voltage amplitude of the photodetector. After being connected to the oscilloscope via an attenuator cable, the voltage signal displayed on the oscilloscope is shown in Figure 3 in the Origin software.

Figure 3: Display of Continuous High-Frequency Signal Detected by Fiber Bragg Grating

Experimental Results:

The continuous signal voltage curve is subjected to spectral analysis to determine whether the main frequency of the continuous signal corresponds to the excitation signal frequency. The spectral analysis diagram of the continuous signal voltage curve is shown in Figure 4.

Figure 4: Spectral Analysis Diagram of Continuous Signal Voltage Curve

From the spectral analysis diagram of the continuous signal voltage curve, it can be seen that during the experiment, the main frequency of the continuous signal detected by the detection grating is around 250 kHz, consistent with the excitation signal frequency.

High-Voltage Amplifier Recommendation: ATA-7010

Figure: Specification Parameters of the ATA-7010 High-Voltage Amplifier

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