ELECTRONIC SUBTRACTER FOR TRACE-GAS DETECTION WITH INGAASP DIODE-LASERS

An electronic noise-cancellation scheme has been developed and tested for second-harmonic (2f) detection with short-external-cavity and dist ributed-feedback InGaAsP diode lasers and wavelength modulation. The 2 f background signal and noise from, e.g., optical feedback, optical fr inges, and power-supply pickup are effectively reduced by subtraction of a measure of the signal-beam photocurrent from a measure of the ref erence-beam photocurrent. The dynamic range required for the lock-in a mplifier is also reduced because the signal owing to modulation of the laser output at the first harmonic is canceled. Reduction of the 2f b ackground and dynamic range are important for atmospheric-pressure det ection where a large wavelength modulation is necessary. The detector noise was minimized by the use of zero-biased detectors in the subtrac tion circuit. A beam-noise level (defined as 2x the rms value) equival ent to a line-center absorption of 1.6 x 10(-6) was achieved with an e quivalent-noise bandwidth of 1.25 Hz for 2f detection at 10 kHz. The e lectronic circuit is easy to construct and low cost. (C) 1995 Optical Society of America