Fluctuations and Noise of the Optical Output Power of Laser Diodes and the Effect on Optical Particle Size Determination

Laser diodes are increasingly used as a light source in optical particle measurement technology. They offer the advantages of very small size, very low weight, and have recently become commercially available with a sufficient optical output power and at an acceptable price. However, they have the disadvantages of needing a matched current source and of having a generally higher noise level than gas lasers. In optical particle measurement technology, the amplitude of the light scattered by the particles is very often used to get information about the particles. The scattered light intensity is directly proportional to the incident light intensity. So the noise of the light source directly affects the quality of the scattered light, and can produce errors in the particle size determination. This paper investigates the constancy of the optical output power of laser diodes in combination with specific current sources, and calls attention to possible problems in optical particle size determination. A measurement method is introduced with example laser diodes and current sources to find the best combination of laser diode and current source with the lowest noise level in the optical output power. The optimum range of operation is determined with measurements. Further, the influence of the noise in the optical output power on particle size determination is estimated theoretically with an error propagation calculation. The measurements show a very low rms noise in the optical power output of laser diodes. The quantitative estimation of the error in particle size determination shows that the error below a particle diameter of 1 /-Lm for polystyrene latex (PSL) and for laser diode wavelengths of 670 and 780 nm is insignificant. Above this particle diameter, error can increase due to the slight slope of the scattered light curve.