April 2001 Meeting

2001 Eastman Lecture featuring
Professor A. E. Siegman:
"Defining and Measuring Laser Beam Quality"

The Optical Society of San Diego and this evening's host, Maurice Pessot, are pleased to present this year's Eastman Lecture by A. E. Siegman, McMurtry Professor of Engineering Emeritus, Stanford University and 1999 President, Optical Society of America.

Abstract: Laser beam quality is a concept that is often discussed, but less often defined or measured with any rigor. I will review some of the problems in trying to characterize real laser beams by any single beam quality measure, and will describe one particular approach for defining and measuring a rigorous M-squared value, or beam propagation factor, that turns out to be useful both for practical laser applications and for studies of laser physics.

Speaker: As of the new millennium Anthony Siegman is Professor Emeritus of Electrical Engineering at Stanford and Past President for the year 2000 of the Optical Society of America.

During his 42-year career at Stanford he was elected to the National Academies of Engineering and of Science; received the OSA's R. W. Wood Prize and Frederick Ives Medal and the IEEE LEOS Quantum Electronics Award; and became known as the author of the world's heaviest LASERS text. During his four years in the OSA leadership from 1997 through 2000 he helped to guide the society through a particularly difficult and tumultuous period, during which he managed to make essentially half the OSA membership (48.6%, to be precise) his strong supporters, and the other half his fervent opponents. His current activities include technical writing and consulting and low-energy backcountry skiing.

Eastman Lecture: The Eastman Lecture is a bi-annual grant underwritten by the Eastman Kodak Company for bringing speakers of national stature to local chapters of the Optical Society of America.

 

April Meeting Review:

M2 with Tony Siegman
by Harry Rieger, JMAR Technology

On April 19, 2001 Professor Tony Siegman lectured the members of the Optical Society of San Diego about qualifying laser beams using a popular measure known as M2.

The meeting was held at the Acapulco restaurant in Miramar. About 40 people attended the talk. Professor Siegman remarked on the role of the Optical Society of America in serving professional optics and the controversial issue of the attempted merge with the SPIE before his talk.

Although all of us know what laser beams are all about, but quantifying "real" laser beams is very subjective. The Gaussian beam maintains its Gaussian profile as it propagates, and its shape easy to quantify in terms of a single parameter-the beam waist. There are simple propagation equations describing the size of the beam waist as a function of distance. Any other beam profile will undergo changes as it propagates. What simple parameter can be used to specify a real beam size? One way of quantifying a "real" beam is by looking at the intensity only and defining the beam size by its standard deviation. By replacing the waist of a Gaussian beam in the propagation equation with the standard deviation of the "real" beam and a fudge factor of M2 to match the actual propagation we can relate beam quality to the value of M2. A perfect Gaussian beam will have M2 =1. Beams with higher order transverse modes will have M2 greater than 1.

Professor Siegman did a great job explaining M2 and putting it in prospective.