Application Cases

Experiment Name: Acoustic Sensing Test for Frequency Response Range and Sensitivity

Test Equipment: Power amplifier, Oscilloscope, Signal generator, Spectrum analyzer, Lock-in amplifier, Phase modulator, Photodetector, etc.

Experimental Process:

Figure 1: (a) Experimental test system for the ring resonator: PM, Phase modulator; SG, Signal generator; PD, Photodetector; LIA, Lock-in amplifier; PI, Locked frequency controller; HVA, Power amplifier; OSC, Oscilloscope; SA, Spectrum analyzer; PA, Power amplifier. Here, the blue lines represent the optical path, and the black lines represent the electrical circuit; (b) Spectral line and frequency locking curve of the ring resonator for Sensor 2; (c) Response of the demodulated signal of Sensor 2 when an acoustic signal is applied.

An acoustic signal detection system based on phase modulation spectroscopy technique was constructed. The test system includes a laser and phase modulator, photodetector, lock-in amplifier, signal generator, power amplifier (HVA), spectrum analyzer, oscilloscope, and proportional-integral (PI) module, as shown in Figure 1.

Figure 2: Frequency response and flatness of the sensor. (a) Frequency response of the speaker; (b) Frequency response of the piezoelectric ceramic.

In the frequency response test experiment, the output amplitude of the SG was maintained at 10 V to ensure the stability of the sound pressure generated by the acoustic source. Response frequency points generated by the SOI microring resonator were tested at 40 kHz, 58 kHz, 200 kHz, and 300 kHz to demonstrate the frequency range. Based on this, the frequency intervals were reduced: from 50 Hz to 100 Hz, the interval was narrowed to 10 Hz; from 100 Hz to 1 kHz, the interval was 100 Hz; and from 1 kHz to 20 kHz, the interval was 1 kHz. The ultrasonic frequency range was appropriately expanded to test the frequency range to which the designed sensor could respond. The frequency response range of the ring resonator was tested using a spectrum analyzer: at each frequency point, the acoustic signal response data were collected until the amplitude of the acoustic signal could no longer be distinguished from the background noise. The test results are shown in Figure 2. The flatness of each sensor was 2 dB, with the widest frequency range reaching 160 kHz. When the Q-factor was higher, the resonator performance improved, achieving a wider frequency response.

Experimental Results:

Figure 3: Comparison of acoustic sensor sensitivity.

Sensitivity is one of the primary indicators of acoustic performance, defined as the fitting slope of the sensor's output voltage amplitudes to the corresponding sound pressure values measured by a standard sound level meter, with units of mV/Pa. The SG generated a 1 kHz sinusoidal signal and remained unchanged. After amplification by the power amplifier, the signal was connected to the acoustic source system. By controlling the amplitude of the SG, the acoustic source output signals with different sound pressure levels. In the experiment, the SG amplitude was sequentially increased from 1 V to 10 V in steps of 1 V. The voltage output of the sensor and the sound pressure values from the standard sound level meter were recorded separately. As shown in Figure 3, as the driving voltage of the acoustic source increased, the sound pressure level gradually enhanced, leading to significant changes in the air refractive index, and the detected voltage amplitude increased. The experimental results showed that all R² values were greater than 0.97, indicating excellent linearity. Additionally, with a higher Q-factor, the output voltage of the sensor showed a significant increasing trend. Therefore, the sensor with a Q-factor of 3.44×10⁶ achieved a high sensitivity of 60.075 mV/Pa.

Power Amplifier Recommendation: ATA-309C

Figure: ATA-309C Power Amplifier Specifications

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