A team at Tokai University in Hiratsuka and Shizuoka, Japan, and from National Kaohsiung Normal University in Kaohsiung, Taiwan, has demonstrated that the intensity modulation in a laser can be used to estimate the size and concentration of organic and inorganic particles in a fluid. Applications of the technique could include the characterization of biological species, a possibility they established by capturing the motion of phytoplankton in seawater. In the scheme, the researchers monitor the beat signals due to interference between a lasing field and frequency-shifted scattered radiation from a sample that is reinjected into the laser to yield a self-mixing modulation effect. When no particles are present in the fused quartz sample chamber, the beats are fixed. When particles are present, they alter the scattering and so change the beats. The scientists used a 0.3-mm-thick slice of LiNdP4O12 as their lasing medium in a demonstration of the approach. They pumped the crystal with an 808-nm laser diode to generate 1048-nm radiation and sent 96 percent of the output through two acousto-optic modulators. One raised the frequency while the other lowered it, producing a 2-MHz shift in the optical carrier frequency at the end of the round trip. They directed this modulated beam to the sample chamber and extracted the other 4 percent of the output from the laser cavity for analysis of the laser’s power spectrum, using an InGaAs photodiode from New Focus Inc. of San Jose, Calif. The researchers measured polystyrene particles in water, varying the size and concentration of the particles. By measuring only the power spectrum of the laser, they could estimate the size of the particles to a fair degree of accuracy — obtaining, for example, a calculated average size of 113.1 nm for 115-nm particles. They discovered that they could estimate concentration and were able to distinguish different sizes of particles at different concentrations in the water. They also were able to differentiate self-propelled and randomly floating species of phytoplankton, suggesting potential applications of the technique in marine biology. To that end, they are collecting data on various species of plankton. Optics Express, Feb. 6, 2006, pp. 1044-1054.