Application Cases

Experiment Name: Application of Linear F-P Cavity in CH₄ Gas Detection Using OF-CEAS

Research Direction:
Methane (CH₄), as a significant greenhouse gas, holds great importance for human production and life due to its online high-sensitivity detection. Utilizing OF-CEAS for measuring atmospheric CH₄ concentration not only achieves high sensitivity and precision but also avoids interference from other gases such as H₂O and CO₂. This chapter introduces a CH₄ detection system based on the linear F-P cavity OF-CEAS technology. The experiment was conducted using a linear two-mirror F-P resonant cavity, selecting a DFB laser with a central wavelength of 1.65 μm as the light source, employing an optical attenuator to adjust feedback light intensity, and using a horizontal displacement stage and piezoelectric ceramics to adjust the distance between the laser and the F-P cavity.

Test Equipment: High-voltage amplifier, function generator, semiconductor laser, piezoelectric ceramics, etc.

Experimental Procedure:

Figure 1: Schematic Diagram of OF-CEAS Based on a Linear F-P Cavity

As shown in Figure 1, an OF-CEAS system based on a linear F-P cavity was established. The system employs a continuous single-mode output semiconductor laser with a central wavelength of 1.65 μm and a maximum output power of 10 mW. The laser is mounted on a two-dimensional translation stage to achieve horizontal position adjustment. A laser driver controller is used to regulate the laser's temperature and injection current, setting the temperature to 26°C and the central current to 75 mA. A function generator outputs a triangular wave signal with a frequency of 10 Hz and an amplitude of 200 mV to achieve continuous tuning of the laser's output wavelength. The laser output beam is split into two parts: one is transmitted to a wavelength meter via a coupler for laser frequency monitoring, while the other passes through an optical attenuator, two reflectors, and a matching lens before being coupled into the optical cavity. The attenuator is used to adjust the light intensity passing through the laser, thereby modifying the feedback rate. Piezoelectric ceramics are installed behind the reflector near the F-P cavity, which can undergo controlled length changes under high-voltage input. A high-voltage amplifier is used to generate the required high-voltage current.

Experimental Results:

Figure 2: Transmission Signal of the Linear Cavity in a N₂ Background

As shown in Figure 2, the transmitted cavity mode signal under non-absorbing conditions was obtained by filling the cavity with N₂ gas at one atmosphere. Each individual transmitted cavity mode in the figure exhibits an "arch" shape, with broadened and symmetrical cavity modes, indicating that feedback phase and feedback rate largely meet the experimental requirements.

Figure 3: Absorption Signal of 20 ppm CH₄ Gas at 689.2 Torr Pressure

Figure 3 shows the OF-CEAS absorption spectral signal of CH₄ gas obtained by filling the cavity with CH₄ at one atmosphere. The triangular wave scanning frequency was 10 Hz with a scanning amplitude of 200 mV, corresponding to a laser frequency tuning range of approximately 6046.1–6047.5 cm⁻¹. During the scanning process, the DFB laser was locked to 86 consecutive cavity modes, and the CH₄ absorption signal covered approximately 24 FSRs (free spectral ranges).

High-Voltage Amplifier Recommendation: ATA-2042

Figure: ATA-2042 High-Voltage Amplifier Specifications

The experimental materials in this article were compiled and published by Xi'an Aigtek Electronics.