Abstract

The theoretical performance of many types of optical measurements is extremely good. With a 5 mW He–Ne laser, the shot noise limit (1 Hz bandwidth) corresponds to an extinction uncertainty of 2 parts in 10⁸ or, in a Michelson interferometer, to a position uncertainty of 2 fm. Practical measurements do not approach this accuracy, except by great effort, because of excess laser noise and environmental perturbations. This paper describes several types of optical extinction and homodyne measurements performed at baseband (without choppers or modulators), that achieve shot-noise limited performance, even though the lasers used are up to 55 dB noisier than the laser described above. The measurements include diode-laser optical-absorption spectroscopy, measurement of aerosol particles by optical extinction, coherent lidar, and phase-sensitive microscopy. The measurement techniques all rely on a novel, all-electronic noise cancellation scheme. Although related to earlier differential and ratiometric techniques, it has much better noise cancellation performance, is effective at modulation frequencies up to 100 MHz, and requires no critical adjustments. With this system, it is often possible to achieve the performance of a complex heterodyne system by means of a much simpler homodyne approach.

© 1990 Optical Society of America