| 摘要 |
A gas detector of gas in a target zone has a transmitter of frequency modulated light and a receiver. In the receiver, a homodyne detection circuit mixes a detected signal with a reference signal at one of the modulation frequencies, where the reference signal has been derived from light that has passed through the target zone. The homodyne detection circuit includes a bandpass filter, amplifier, phase lock loop and phase shifter. Etalon fringes are reduced by moving them to high frequencies and filtering them out. Gas density is measured by comparison of the detected signal from the target zone with a signal from a gas reference cell. A method of detecting gas includes transmitting frequency modulated light through a target zone, receiving the light, and homodyne detection of the detected signal by mixing the detected signal with a reference signal at one of the modulation frequencies, where the reference signal has been derived from light that has passed through the target zone. Light from a laser transmitter propagates along several optical light guides from a laser transmitter to a laser receiver. The optical light guides form a guided light path traversing each of several target zones where unwanted gas may be present. An optical switch permits selection of one of the paths and hence one of the target zones for the detection of gas. A preferred light guide uses optical fibers with optical switches, or a combination of a splitter and a switch. A lens system provides for the collection of light from a transmitting fiber optic and receiving fiber optic. The gas detector is provided with means to eliminate phase sensitivity of the detector. Also, a gas detector for detecting a target gas in multiple target zones. A light collector receives light from the first scanning optical element that has returned from the multiple target zones and directs the light towards the receiver. A controller controls the position of the first scanning optical element and thereby controls which of the target zones is traversed by light from the laser transmitter.
|
