Laser noise is very often the primary limiting factor in making high-accuracy optical intensity measurements. There are ways of making your laser quieter, but they won't get to the shot noise level. On the other hand, what we actually measure is the photocurrent, not the laser power, and that we can improve.
Laser Noise Cancellers are extremely powerful devices that allow us to make shot-noise limited measurements at baseband, even with very noisy lasers. With zero adjustments, they will reliably suppress the effects of laser residual intensity noise (RIN) by 55 or 60 dB from dc to several megahertz, and with a bit of (optical) tweaking, will do 70 dB or more at low frequency, which is where it's most needed (see the picture above, which shows > 70 dB suppression of noise intermodulation). There's a New Focus app note which surveys applications of noise cancellers.
The laser noise canceller has two operating modes, linear and log-ratio. The linear mode produces a replica of the photocurrent minus the noise. The log ratio mode also suppresses the intermodulation of the laser noise with the signal, allowing (for example) tunable diode laser spectroscopy to achieve 1-ppm sensitivities even when the laser power is varying by >30% over a scan line, as shown here.
Laser noise canceller performance, showing the intermodulation suppression action of the log ratio output. The desired signal is the 50 kHz central peak, but the laser has additional unwanted 5 kHz AM, which puts intermodulation sidebands on the desired signal.
Upper trace: Single-beam TIA mode (comparison beam replaced with constant current to show actual photocurrent spectrum);
Lower trace: Normal dual-beam ratiometric operation (comparison beam unblocked), showing spurious intermod suppressed by 60-70 dB.
Note that the sideband energy has been returned to the main peak, as expected.
Elimination of sloping baseline and peak-height variations. Here's one way the previous figure translates into real measurement benefits.
Top: Raw detected photocurrent, comparison beam blocked. Spectral features are barely detectable bumps on a huge nonlinear sloping baseline, even in the detail view.
Bottom: the spectral region of the detail (about 0.5 cm-1) with the log ratio output of the noise canceller. The noise intermodulation suppression of 70 dB or thereabouts makes the peak heights independent of the laser power variations, so the lower trace gives sample absorbance directly, at a sensitivity better than 10-6.
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 [1991])
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])
First description of the laser noise canceller, including comparisons with heterodyne systems and use with helium-neon lasers showing mode beat suppression by >50 dB.
(Proc SPIE 1376, pp. 216-221 [1990])