Methane as the main component of natural gas is an important industrial raw material and fuel gas with flammability and explosion. Real-time monitoring the generation and leakage of methane has great practical significance for using natural gas resources safely and effectively. Many detection technologies can be used for methane gas detection, such as infrared spectroscopy, gas chromatography, electrochemical sensors and so on. Tunable diode laser absorption spectroscopy (TDLAS) technology as a kind of infrared spectroscopy technology, utilizes the narrow linewidth and wavelength tunability of semiconductor lasers to focus single or several absorption lines of gas molecules. When the lasing wavelength is matched with the center frequency of the methane absorption line, the gas molecules will absorb photons and move to a higher energy level, and the laser power will decay at the same time. Methane gas detection can be realized by comparing the output optical power after absorption with the initial output optical power. This technology has many unique advantages, such as intrinsic safe, good stability, good selectivity, and long working life etc..
Methane gas molecules have absorption lines in both near-infrared and mid-infrared bands. The absorption coefficient of the absorption lines of carbon dioxide and water molecules near 1.65 μm is much smaller than that of methane molecules. Therefore, the 1.65 μm light source is less affected by carbon dioxide and water molecules in the atmosphere, and its transmission loss is small. It is suitable for the long-distance detection of methane gas molecules. Compared with other lasers in near-infrared or mid-infrared bands, 1.65 μm lasers are more mature due to near the optical communication band.
The device introduced by Prof. Yongzhen Huang's research group from Institute of Semiconductors Chinese Academy of Sciences in Chinese Optics Letters Volume 20, Issue 6, 2022 (Yingrun Fan, et al., 1.65 µm square-FP coupled cavity semiconductor laser for methane gas detection) is a square-FP coupled cavity semiconductor laser operating at about 1654 nm. A gas detection system based on TDLAS technology using the coupled cavity laser was demonstrated and successfully used for methane gas detection.
The lasing wavelength tuning range is 2 nm by adjusting the FP cavity injection current, covering the methane absorption line at 1653.72 nm. The wavelength can also be tuned by adjusting the square microcavity injection current or temperature, respectively. The laser can operate single mode lasing with the side-mode suppression ratio (SMSR) about 40 dB and the linewidth of 5 MHz, as shown in Figure 1. In addition, the optical power coupled into an optical fiber is 7.4 mW.
Figure 1 (a) Microscope image of the square-FP coupled cavity laser. (b) Optical spectra and SMSR with different FP cavity injection currents when the square microcavity injection current is 20 mA.
This work presented the application prospect of the square-FP coupled cavity laser in the gas sensor field. As a high-performance laser source for methane gas detection, the coupled cavity laser has the advantages of stable output wavelength, high SMSR, narrow linewidth, low power consumption, small volume, simple structure and so on. From the viewpoint of fabrication cost, the laser based on generic InP-based quantum well platforms and standard photolithographic technique with re-growth free and grating free is a potential low cost and compact single mode laser source.
Future work will focus on other gas detection by optimizing the coupled cavity structure parameters to adjust its lasing wavelength tuning range covering the absorption line of other gas molecules. Multi-channel coupled cavity laser array will be a noteworthy structure that can cover absorption lines of multiple gases to achieve multi-gas detection.