Double-beam laser absorption spectroscopy: shot noise-limited performance at baseband with a novel electronic noise canceler

Abstract

A novel electronic circuit has been developed that detects absorption in one of two (reference and signal) laser beams with shot noise-limited sensitivity. The authors demonstrate its use as a simple shot noise limited spectroscopy by measuring several absorption lines of I2 vapor near 670 nm with a tunable diode laser. The noise-equivalent absorption in this double-beam method is 2 times the shot-noise-to-signal ratio of the signal beam. In this case, for 1.2 mW of power, the noise-equivalent absorption in a 1-Hz bandwidth was 4.2 × 10-8. This noise level was confirmed, although measurements were sometimes limited by interference fringes and laser tuning drift. These are technical, not fundamental, problems arising from unsophisticated optical equipment. This baseband method provides absorption spectra directly, eliminating laser excess noise without the somewhat elaborate signal generation and processing equipment needed in laser frequency modulation methods. The only modulation used in the system was the 0.5 to 1.0-kHz sweep of the diode laser current used to tune the laser output wavelength. The noise-cancelling circuit itself is not complex: workable versions can be constructed from readily available components at a cost of about 10 dollars.