Pumping ion is one way to get out of the red

Compact Blue-Green Lasers
March 5, 2004

This book springs from a series of meetings convened by the Optical Society of America in the early 1990s to discuss progress in developing new laser sources for the blue-green region of the visible spectrum, somewhat loosely defined as covering the wavelengths 450nm to 550nm.

From the late 1970s, the US military establishment financed research on powerful lasers capable of transmitting information via satellites to submerged submarines. Because the radiation has to penetrate some depth of seawater, the optimum wavelength for such lasers lies in the blue-green region of the spectrum. It is debatable whether or not this military interest was responsible for the burgeoning research in the following decade, but it is evident that efforts to produce efficient and compact blue-green lasers received renewed impetus.

Modern versions of the laser systems reviewed in this book fall well short of the required power capability for submarine communications.

Nevertheless, they are of great commercial interest because of their suitability for use in digital information storage (next-generation ultra-compact discs) and automated cell-sorting techniques, both of which require reliable, efficient and compact laser sources in this wavelength region.

Although aimed at specialists, Compact Blue-Green Lasers is a very readable text that deals with three quite different approaches to the generation of blue-green laser radiation. It covers theoretical and practical aspects in a commendably thorough fashion.

One method by which radiation from lasers operating in the near-infrared spectrum is converted to the visible region employs frequency doubling in a crystal with a non-linear optical response. For example, the output at 1064nm or 946nm from diode-pumped Nd:YAG lasers can be frequency-doubled to provide output at 532nm (green) or 473nm (blue). A second approach is to exploit the favourable arrangement of energy levels in certain crystals to provide a ladder by which stepwise absorption of pump photons can promote ions to upper laser levels much higher in energy than would be accessible by the absorption of a single photon in the near-infrared. Such "up-conversion" laser schemes, just as in the frequency-doubling scheme, convert input radiation from powerful arrays of near-infrared diode lasers to output at the blue end of the visible spectrum.

The last of the three technologies is the direct generation of laser radiation at the required wavelength by diode lasers constructed from semiconductor materials with band gaps of the appropriate range.

For any reader with a grasp of electromagnetic theory, this text presents an excellent review of the theory of second harmonic generation and the problems of the practical implementation of the technology. The review of the spectroscopy of rare-earth ions for up-conversion lasers is complete and thorough, but relies on considerable prior knowledge of quantum mechanics, atomic structure and spectroscopy. The section on blue-green semiconductor lasers, although detailed on the practical aspects of diode laser fabrication, likewise assumes the reader to be conversant with the concepts of band theory and conventional near-infrared semiconductor lasers.

This book will be invaluable for anyone setting out in laser research, or for established researchers whose work involves the use of solid-state lasers operating in the visible spectrum.

Colin Webb was formerly head of atomic and laser physics, University of Oxford, and is chairman, Oxford Lasers.

Compact Blue-Green Lasers

Author - William P. Risk, Timothy R. Gosnell, and Arto V. Nurmiko
Publisher - Cambridge University Press
Pages - 540
Price - £90.00 and £35.00
ISBN - 0 521 62318 9 and 52103 3

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