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Mode-Locked Laser Tackles Raman Spectroscopy

Photonics Spectra
Mar 2000
Kevin Robinson

Researchers at the University of Texas have taken the first steps toward developing a rugged Raman spectrometer based on a high-power laser diode -- steps that may lead to a device that can detect environmental pollutants as well as analyze gases, gas flows and jets. A combination of solutions enabled them to overcome the typical problems associated with using high-power diodes for Raman spectroscopy of gases.

Because its signal contains highly specific molecular information, Raman spectroscopy is an important analytical tool. The signal, however, is very weak: Only a few photons experience a Raman shift when they strike a molecule. The solution is to put more photons in; that is, to increase the power of the illumination source.

A mode-locked diode laser with a Littman-Metcalf external cavity stabilizes lasing in the Raman spectroscopy setup.

According to Ricardo Claps, a physicist at the university, traditional Raman spectroscopy uses pulsed lasers with powers of 8 to 10 W or more. As a result, he said, diode lasers that can be used for Raman analysis of liquids and colloidal materials (where the Raman intensity is large compared with the sample size) have not yet been proved reliable for analyzing samples with low concentrations, such as air.

Besides power considerations, diode lasers, which top out at around 2 W, "present multimode behavior in free operation and large bandwidths," he explained. "Moreover, the light beam from high-power laser diodes is highly astigmatic and non-Gaussian." Another problem is that the conventional filters used to remove Rayleigh scattering, which is at the same wavelength as the light source, remove the rotational and soft vibrational Raman bands.

In their experimental setup, Claps and fellow researchers J. Sabbaghzadeh and Manfred Fink, the principal investigator, used a 1.2-W broad-area emitter from SDL Inc. of Mountain View, Calif. They corrected the beam's astigmatism with cylindrical and spherical optics, and eliminated multimode lasing and instability using a mode-locked diode with a Littman-Metcalf external cavity. (The cavity also allowed them to tune the laser in the near-infrared, between 790 and 800 nm.) They have developed a multipass cavity that Claps said "increases the Raman signal by a factor of 30 or more, which is essential given the low levels of intensity of the diode laser."

To ensure that the Rayleigh filters didn't filter out useful rotational and vibrational information, the researchers employed two atomic vapor cells.

With this setup for their work, which was reported in Applied Spectroscopy, Vol. 53, No. 5, the researchers were able to collect Raman spectra for several gases, including N2 and CO2.

"Our system is a step toward implementation of a Raman-based gas sensor," Claps said. "The main drawback still is the low sensitivity of the spectrometer in parts per thousand at atmospheric pressure. This can be overcome with the use of newly available higher-power laser diodes, high-quantum-efficiency detectors and better collection optics."

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