4.1 INTRODUCTION

This chapter is a summary of visible lasers, with the primary emphasis upon lasers that are commercially available. Although the first laser ever discovered was a visible solid-state laser, operating in a ruby crystal at 694 nm (just barely in the visible spectral region), most visible lasers that have been discovered have occurred in gaseous media (including metal vapors and high-density gaseous plasmas) or in liquid media (organic dyes). For the visible lasers that occur in gaseous media, the output beam generally consists of one or more discrete wavelengths, each having a relatively narrow spectral bandwidth ( ? ?/ ? < 0.0001) corresponding to the width of the atomic ormoleculartransition associated with that wavelength. For organic dye lasers, the spectral distribution or bandwidth of the gain occurs over a broad wavelength range (of the order of up to 60 to 70 nm with ? ?/ ? < 0.05 to 0.1). Consequently, the narrow tunable spectral output that is available from such a laser has to be generated by frequency-selective components associated with the laser cavity.

The other two types of visible lasers are solid-state and semiconductor lasers, which are more commonly identified with infrared laser output. However, there are several specific laser materials in these two laser categories that provide laser output that extends into the visible spectral region. These lasers include specially doped solid-state crystal and glass lasers, and bandgap engineered semiconductor lasers.

This chapter first reviews visible gaseous lasers,...