Application Cases

Experiment Name: Acoustic Sensing Tests of Ring Resonators

Testing Equipment: High-voltage amplifier, signal generator, oscilloscope, lock-in amplifier, photodetector, phase modulator, spectrum analyzer, etc.

Experiment Process:

Figure 1: (a) Experimental test system for the ring resonator: PM, phase modulator; SG, signal generator; PD, photodetector; LIA, lock-in amplifier; PI, lock-in frequency controller; HVA, high-voltage amplifier; OSC, oscilloscope; SA, spectrum analyzer; PA, power amplifier. Here, the blue lines represent the optical paths, and the black lines represent the electrical circuits; (b) Spectral lines and frequency locking curves of the ring resonator of Sensor 2; (c) Response of the demodulated signal when an acoustic signal is added to Sensor 2

A sound signal detection system based on phase-modulated spectral technology was constructed, as shown in Figure 1. The frequency response range of the acoustic signal was divided into frequencies from 50Hz to 20kHz generated by a loudspeaker and frequencies above 20kHz generated by a piezoelectric ceramic. Due to the frequency limitations of the acoustic source, ultrasonic signals above 20kHz were tested by switching to the piezoelectric ceramic. In the frequency response test experiment, the output amplitude of the SG was kept at 10V to maintain the stability of the acoustic pressure generated by the acoustic source. The performance of the polymer ring resonator] was calibrated at 40kHz, and the response frequency points generated by the SOI micro-ring resonator were tested at 40kHz, 58kHz, 200kHz, and 300kHz to indicate the frequency range. Based on this, the frequency interval was reduced, with a step of 10Hz in the range of 50Hz to 100Hz, a step of 100Hz in the range of 100Hz to 1kHz, and a step of 1kHz in the range of 1kHz to 20kHz. The ultrasonic frequency range was appropriately increased to test the frequency range that the designed sensor could respond to. The frequency response range of the ring resonator was tested using a spectrum analyzer: data on the acoustic signal response were collected at a certain frequency point until the amplitude of the acoustic signal could no longer be distinguished from the background noise. The test results are shown in Figure 2. Each sensor has a flatness of 2dB, with the widest frequency range reaching 160kHz. When the Q value is higher, the resonator performs better, achieving a wider frequency response. However, during the ultrasonic frequency measurement process, some high-frequency responses with stable amplitude can still be observed but cannot be distinguished from the background noise signal. The reason for the missing high frequencies and difficulty in testing may be due to high system noise, dense electrical connections, and a low signal-to-noise ratio, which can be addressed later through a digital signal processing module for noise reduction.

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

Experimental Results:

Figure 3: Sensitivity comparison chart of acoustic sensors

The signal generator produces a 1kHz sine signal and keeps it constant, which is amplified by the power amplifier and then connected to the acoustic source system. By controlling the amplitude of the SG, the acoustic source outputs signals with different sound pressure levels. In the experiment, the amplitude of the SG is increased from 1 to 10V in steps of 1V. The voltage output of the sensor on the oscilloscope and the sound pressure value of the standard sound level meter are recorded, respectively. As shown in Figure 3, with the increase of the acoustic source driving voltage, the sound pressure level gradually increases, leading to a significant change in the refractive index of air, resulting in an increase in the detected voltage amplitude. The experimental results show that all R2 values are greater than 0.97, indicating good linearity. In addition, the higher the Q value, the greater the output voltage of the sensor. The sensitivity of the electroacoustic sensor is very high, but its frequency response is limited by the resonant characteristics of the diaphragm, and fiber optic acoustic sensors based on diaphragms are similar. Currently, although the sensing technology of ring resonators based on detecting changes in the refractive index of air due to sound pressure is not as sensitive as membrane-based acoustic sensors, its advantage lies in the good linear frequency response without the influence of frequency dependence and mechanical resonance frequencies.

High-Voltage Amplifier Recommendation: ATA-2082

Figure: Specification Parameters of the ATA-2082 High-Voltage Amplifier

This material is organized and released by Aigtek. For more case studies and product details, please continue to follow us. Xi'an Aigtek has become a widely recognized supplier of instruments and equipment in the industry with a broad range of products and considerable scale. Free trials of demo units are supported.