Close

Search

Search Menu
Photonics Media Photonics Buyers' Guide Photonics EDU Photonics Spectra BioPhotonics EuroPhotonics Industrial Photonics Photonics Showcase Photonics ProdSpec Photonics Handbook
More News
SPECIAL ANNOUNCEMENT
2016 Photonics Buyers' Guide Clearance! – Use Coupon Code FC16 to save 60%!
share
Email Facebook Twitter Google+ LinkedIn Comments

Laser Effect Used to Detect Particles in Suspension

Photonics Spectra
Apr 2006
Intensity modulation in self-mixing microchip laser identifies plankton.

Hank Hogan

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.


Comments
Terms & Conditions Privacy Policy About Us Contact Us
back to top

Facebook Twitter Instagram LinkedIn YouTube RSS
©2016 Photonics Media
x Subscribe to Photonics Spectra magazine - FREE!