Introduction

In this chapter we shall continue our discussion of molecular gas lasers. Many of the lasers to be discussed here provide substantial CW and pulsed power output, or have unusual and innovative technical features. Some of the lasers to be discussed have already been encountered in another context in earlier chapters. For many lasers a change in the method of excitation enhances some important aspects of laser performance, for example providing higher power output or operation in a new wavelength range. Radical departures from traditional methods of gas-discharge excitation have been particularly important in allowing the development of many of the laser systems to be described in the present chapter.

Gas transport lasers

In many laser systems a fundamental limit to the average output power is set by the buildup of waste heat that results from inefficient laser operation. Even in the relatively highefficiency CW CO2 and CO lasers, collisions that destroy vibrationally excited molecules, rather than just leading to energy exchange from one molecule to another, cause the temperature of the laser medium to rise. In these lasers the temperature rise reduces the population inversion through thermal excitation of the lower laser level. The rise in temperature also reduces the gain through an increase in the Doppler width. Waste heat can also produce changes in the optical properties of the laser medium in a spatially inhomogeneous way. This leads to a phenomenon called thermal lensing.