PURPOSE: To obtain a miniature and portable detector, available for real-time constant-amount measurements, whose operation is simple and which obtain the high-reliability detection amount, by composing a semiconductor laser, having an oscillation wavelength in a single longitudinal mode in an infrared-ray band of 2.3μ, of a mixed crystal of III and V group semiconductors.

CONSTITUTION: On indium-arsenic substrate 11, 1st layer 12 and 3rd layer 14 of the composition of InP0.35As0.50Sb0.15 and 2nd layer 13 of the composition of InP0.28As0.60Sb0.12 are sequentially grown by a liquid-phase epitaxial method and a stripe structure is added to constitute a semiconductor laser which has an oscillation wavelength of approximate 2.3μ and oscillates in a single mode. Rectangular current pulse 21 is applied to semiconductor laser 22 of the above-mentioned constitution and emitted light is passed through cell 23 containing 1W10% of CO gas to detect light passing through by PbS photoelectric cell 24. Obtained electric signal 25 has characteristic dips 26 because CO is absorbed.