Gas laser for emitting light modulated at two different wavelengths and an arrangement incorporating the gas laser to detect a gaseous substance

An arrangement for detecting a substance present in a measuring path, the arrangement comprising:

• a gas laser including:
• a waveguide in which a laser gas having a predetermined composition is present;
• said waveguide defining an optical axis and having first and second ends;
• an excitation source for exciting said laser gas to produce laser light;
• an out-coupling mirror having a predetermined reflectivity and being mounted on said optical axis adjacent said first end;
• a grating arranged on said optical axis adjacent said second end and having a number of grating lines and a predetermined resolution and an angular position relative to said optical axis;
• said waveguide, said out-coupling mirror and said grating conjointly defining a resonator having a length (L) measured along said axis from said out-coupling mirror to said grating;
• a modulator to modulate said length (L) sinusoidally from a first length within a first interval of length to a second length within a second interval of length measured along said axis;
• said grating structure, said angular position, said reflectivity, said length (L), said composition of said laser gas being selected so as to cause said gas laser to emit laser light along said optical axis at first and second wavelengths (.lambda..sub.1, .lambda..sub.2) when said length (L) is within first and second intervals, respectively, and to suppress emission of said laser light during transition between said intervals of length;
• and,
• an evaluation device for computing a ratio of light absorption by said substance for said two wavelengths (.lambda..sub.1, .lambda..sub.2) to determine therefrom the concentration of said substance.