Control of laser light power output for use in light scattering instruments by inducing mode hopping and averaging result

A method for controlling a laser source, which is inherently noisy due to mode hopping instabilities, for use in a light scattering measurement comprising the steps of:

• a) modulating the laser drive current at a frequency approximately 10 to 1000 times greater than the frequency of measurement of the scattered light signals and with a maximum amplitude less than the peak current level of the laser;
• b) splitting a fraction of the laser output beam so-produced and detecting said split fraction with a light sensitive monitor detector to obtain thereby a representative measure of said output beam power;
• c) averaging the signal produced by said monitor detector with a signal averaging filter selected to smooth the power fluctuations due to said modulation;
• d) detecting the light scattered by a sample illuminated by said modulated laser beam at one or more angles and averaging each said detected scattered light signal at each said angle with a signal averaging filter approximately equal to the signal averaging filter selected to smooth said power fluctuations of said monitor detector, each said signal averaging filter smoothing the detected scattered light fluctuations due to said modulation of said illuminating laser beam;
• e) dividing the so-smoothed scattered light signals by the similarly smoothed laser monitor signal to produce an output ratio of the light scattered by said sample to the light incident thereon for each of said angles measured.