Optical Device Guide

Measuring Gas with Light (Methane Gas Sensing)

Methane Gas Sensing with Light

Demand for observing the composition of specific gases with an environment impact has been increasing in recent years. In particular, methane in the atmosphere is a factor that impacts global warming. It is necessary to examine the distribution and variation over time of specific gases in the atmosphere, and there is demand for measurement methods that enable remote detection with high-sensitivity. Gas detection using laser absorption spectroscopy is a sensing technology that provides remote detection and other superior qualities, such as enabling measurement in a short time.

Measurement Principle

Gas molecules have unique infrared absorption spectral lines. A gas detector for measuring methane selects a wavelength (1,653 nm) suitable for measurement, which is absorbed by methane but not affected by other gases such as water vapor, and stabilizes the emission wavelength of the DFB-LD to that wavelength.

The gas detector then modulates the emission wavelength of the LD so that the absorption line peak is kept at the center with 10 kHz, and irradiates the laser light to the space. This generates a 1f (10 kHz) component proportional to the light intensity, and a 2f (20 kHz) component via gas absorption if methane exists in the optical path. The gas detector detects the 1f component and 2f component and determines the gas concentration by dividing 2f by 1f.

Absorption Line Wavelengths of Major Gases

Anritsu Light Sources for Gas Sensing

Anritsu has technologies for designing and manufacturing laser diodes for optical fiber communication, and develops laser diodes (LDs) for methane detection, based on these technologies. We stably manufacture 1.65 μm band LDs for the methane absorption line using an InGaAsP/InP type LD with an InGaAsP4 quaternary mixed crystal on an InP substrate as the luminescent layer and adopting a fine strained quantum well structure. The wavelength of our LDs is precisely adjusted near the methane absorption line wavelength using: structure which makes it easier to adopt the oscillation wavelength design, a DFB-LD that obtains excellent single mode light, and internal diffraction grating of which intervals are micromachined.

Gas Sensing for Various Applications

Anritsu DFB-LD (Distributed Feedback Laser Diode) >