| 摘要 |
PURPOSE: To remove background noises of drifting nature and compensate the change with time in the laser by sensing the gas concentration from the difference in the detected wave component between a gas atmosphere transmitted light and direct beams of light. CONSTITUTION: A laser beam of a branching feedback type semiconductor laser 2 is branched 4. A part thereof is passed through a forward path optical fiber 5c, fed to a cell 6 for gas to be sensed, and received 8c upon penetrating gas of unknown concentration, while another part is passed through a fiber 5a and received directly 8a. A signal processing part 20 makes phase sensitive wave detection 14 (15) of the sensed signal 8a, and the resultant is used as a fundamental wave (double wave) phase sensitive wave detection signal. On the other hand, the sensing signal 8c is subjected to a phase sensitive wave detection 16 (17), and the resultant is used as a fundamental wave (double wave) phase sensitive wave detection signal. Therein, the ratio of the double wave/fundamental wave phase sensitive detection signals subjected to detections 15, 14 represents a drift noise component, while the ratio of the fundamental wave/ double wave phase sensitive detection signals subjected to detections 17, 16 represents a gas signal component plus drift noise component. Accordingly the gas signal component is obtained by taking difference between them, and the gas concentration is determined from its wave height. |
