The definitive summary of my early work on noise cancellers: Six easily-constructed circuits that can improve the SNR of bright-field laser measurements by as much as 70 dB at low frequency and 40 dB out to 10 MHz. Bright field measurements at the shot noise limit become much easier.
The circuits are simple to construct (as simple as 1 dual op amp and 3 jellybean transistors), so anyone who can handle a soldering iron can improve the quality of his laser measurements enormously. Detailed applications advice is provided.
A word of caution: these circuits are easy to build but fairly hard to improve. Even if you want some custom tweaks, build the circuit as described, test it, and then change it around. You will have to add the usual 0.1 μF bypass caps on the supplies, and replace the discontinued Analog Devices MAT04 matched quad with the MAT14, but no other changes are necessary. Use ground plane construction, e.g. dead bug on a piece of Cu-clad FR4, or perf board style on a Vector 8007 protoboard, use nice quiet power supplies and/or capacitance multipliers, and don't put the photodiodes on cables. Bring the beams to them, instead.
Other important points: put a polarizer right at the laser to eliminate the spontaneous emission in the orthogonal polarization state, since it doesn't split the same way as the laser light, causing imbalance in the subtraction; and whatever you do, don't vignette the beams after splitting them.
(Applied Optics 36, 4 pp 903-920 [1 February 1997]).
In my consulting work, I've extended laser noise canceller technology out to higher speeds (10-100 MHz) and wider photocurrent ranges.
(Kurt L. Haller&Philip C. D. Hobbs)
Use of the log ratio version of the laser noise canceller to achieve 10-6 absorption sensitivity in current-tunable diode laser spectroscopy, even when the laser power changes by 30% over a scan. This is shown by the pictures at right. Using the well studied rovibrational spectrum of molecular iodine allowed us a simple demonstration system for a widely applicable technique, subsequently used in many instruments.
(Proc SPIE 1435, pp. 298-309 )
Popular article on the laser noise canceller, from Optics&Photonics News, April 1991. An updated and somewhat chattier version of the SPIE paper below, with more discussion of applications. (Optics & Photonics News 2 4, pp. 17-23 [April 1991])