Report Reveals Pitfalls of Laser Astigmatism Measurement

Kathleen G. Tatterson

AUBURN, Calif. -- Scientists using a common method of measuring the astigmatism, or optimum focal length, of laser diodes have been led astray, according to a report released by researchers at Coherent Inc.
Measuring astigmatism is vital to the applications of laser diodes in fields such as optical disc data reading and high-resolution bar-code reading. However, in a paper published in Optical Engineering, Coherent optical engineer Haiyin Sun said, "Previous measurement with the measured data being processed by geometric optics was wrong."
The geometric optics method is traditionally used to interpret the measurement data of laser diode astigmatism, an aberration causing the tangential and sagittal image planes to separate axially. Although this method can be useful in some situations, researchers tend to rely on it exclusively because it is easy. Often researchers must treat laser diode beams as Gaussian beams because the size of astigmatism is comparable to the source size of the beam. "In these cases, geometric optics will lead to a wrong and useless measurement as a result," said Sun.

More accurate method
Sun recommends using the moving-diode method, which reduces a laser diode Gaussian beam to a geometric beam. Using this method, scientists move the diode in the direction of the fixed-beam axis. Only then can researchers use geometric optics to calculate measurements that are correct in magnitude and arithmetic sign.
Another method, the moving-profiler technique, will yield a measurement that is accurate to within a few percentage points. However, it requires intricate calculations that scientists tend to skip, leading to inaccuracies.
"It's a little surprising that no such research work had been published before [because] the existence and effects of laser diode astigmatism have been well known for many years to many people in the field of manufacturing and application of laser diodes," Sun said.
Experts advise end users to measure astigmatism after the diode is in place with all of its optics.
Users should "concentrate on measuring the beam where they need it. Don't try to back-calculate to the diode. That's where the rat's nest happens," and users get bogged down in calculations, said Lew Brown of Photon Inc., a Santa Clara, Calif.-based maker of measurement instruments.