Application Cases

Experiment Name: Performance Testing of the Sensing System

Test Equipment: High-voltage amplifier, function generator, low-pass filter, lock-in amplifier, photodetector, computer, etc.

Figure 1: Cavity-enhanced photoacoustic sensing system. f-EOM: fiber-coupled electro-optic modulator; f-AM: fiber-coupled LiNbO₃ intensity modulator; L1: mode-matching lens; λ/2: half-wave plate; PBS: polarizing beam splitter; λ/4: quarter-wave plate; L2 and L3: focusing lenses; PD: photodetector; SG: signal generator; LPF: low-pass filter; PID: proportional-integral-derivative controller; HV: high-voltage amplifier; LIA: lock-in amplifier; PC: computer

Experimental Procedure:
To verify the performance of the constructed cavity-enhanced photoacoustic sensing system, acetylene (C₂H₂) gas was selected as the target gas. The absorption line of C₂H₂ at 1530.98 nm, with an absorption line strength of $4.00\times 10^{-21}\text{cm}\cdot {\text{molecule}}^{-1}$, was chosen as the target detection line. Real-time monitoring of acetylene gas holds significant importance in various fields, including chemical production, power systems, and industrial process control.

Initially, the wavelength of the whispering gallery mode laser was measured using a wavemeter. By adjusting the laser control software and scanning current, the emission wavelength was tuned to sweep across the selected gas absorption line. The Pound-Drever-Hall (PDH) locking technique was then employed to lock the laser emission wavelength to the center of the cavity mode. Since this experiment primarily focused on the amplification factor of the photoacoustic signal achieved by using the optical cavity, it was unnecessary to ensure that the laser emission wavelength was precisely centered on the gas absorption line; it only needed to fall within the broadening range of the absorption line.

To generate the photoacoustic signal, a fiber-coupled LiNbO₃ intensity modulator was used to modulate the laser intensity. A high-voltage amplifier provided a DC bias, and a signal generator (SG3) produced a square wave with a 50% duty cycle to drive the intensity modulator. The intensity modulator provided a DC extinction ratio of 20 dB, meeting the requirements of this experiment. The square wave frequency from SG3 was set to the resonance frequency of the photoacoustic cell, 1781.0 Hz. Due to optical losses from intensity modulation and fiber coupling, the power incident on the optical cavity was 0.7 mW.

The signal detected by the microphone was processed through a custom differential amplification circuit and then fed into the lock-in amplifier. The TTL signal from SG3 served as the reference. The lock-in amplifier operated in 1-f demodulation mode with a time constant of 1 s and a filter slope of 12 dB/oct, corresponding to a detection bandwidth of 0.25 Hz. The demodulated signal from the lock-in amplifier was acquired and processed in real-time using a custom LabVIEW program on a computer. A 500 ppm standard concentration of C₂H₂ gas and pure nitrogen (N₂) were used with a gas dilution system to produce C₂H₂/N₂ mixtures of varying concentration ratios.

Experimental Results:

Figure 2: (a) Scanning voltage signal of the laser; (b) Transmission signal of the optical cavity; (c) Error signal

In the cavity mode signal and error signal test, a signal generator (SG1) produced a 10 Hz triangular wave to scan the laser emission wavelength to observe the optical cavity modes. To ensure the wavelength scanning range covered a full free spectral range (FSR) (~0.9 GHz), the positive and negative peaks of the triangular wave were amplified to ±40 V using a high-voltage amplifier. The laser scanning voltage signal, optical cavity transmission signal, and error signal are shown in Figure 2(a), (b), and (c), respectively. From Figure 2(b), the ratio of FSR to cavity mode linewidth was obtained, and the intra-cavity power enhancement factor was calculated to be 175. This implies that, under conditions without absorption saturation, the photoacoustic signal obtained using the optical cavity should theoretically be 175 times stronger than without it.

Figure 3: Comparison of photoacoustic signals with and without the optical cavity

In the comparative test of photoacoustic signals with and without the optical cavity, a standard 500 ppm acetylene gas mixture was introduced into the gas cell. All experiments were conducted under atmospheric pressure and room temperature. The photoacoustic signals obtained with the optical cavity (cavity-enhanced photoacoustic sensor) and without it (conventional photoacoustic sensor) are shown in Figure 3. The experimental results indicate that the photoacoustic signal increased from 44.3 µV to 7366.8 µV with the use of the optical cavity, representing an enhancement factor of 166. Considering the laser-to-optical cavity coupling efficiency of ~95% and the theoretical enhancement factor of 175, the experimentally obtained enhancement factor aligns well with the theoretical calculation.

High-Voltage Amplifier Recommendation: ATA-2022B

Figure: ATA-2022B High-Voltage Amplifier Specifications

This document is compiled and published by Aigtek.