INTRODUCTION

In the preceding chapter, we considered single-pass SHG and SFG. There, we saw that efficient frequency upconversion from infrared to blue-green wavelengths is generally possible only when the power at the fundamental wavelength is several watts. The approach to achieving such powers that we considered in Chapter 3 was a very direct and “brute force” one: build a more powerful laser. We examined several approaches that have been used for increasing the infrared power available for the nonlinear interaction, including:

• using a power amplifier to boost the output of a master oscillator;

• using high-power diode lasers that have poor spectral and spatial characteristics for pumping solid-state lasers which then act as sources for frequency-doubling;

• using pulsed, rather than cw, operation in order to achieve higher peak powers.

While these brute force approaches have the advantage of being conceptually straightforward, it has only been since about 1995 that they have succeeded in producing blue-green powers sufficient for some of the applications described in Chapter 1. In addition, these approaches suffer from a number of practical disadvantages. The powerful lasers required for efficient single-pass conversion tend to be complicated and expensive, and since they generate high powers they require substantial electrical power and thermal management. Furthermore, although pulsed configurations have succeeded in producing large average blue-green powers, the power generated by cw operation has been too low for many applications.