Blue-green light can be generated by using nonlinear crystals to “upconvert” the infrared wavelengths produced by high-power semiconductor diode lasers. In secondharmonic generation (SHG), a single infrared laser with frequency ω1 is passed through a nonlinear crystal and blue-green light emerges with frequency 2ω1. In sum-frequency generation (SFG), two infrared lasers with frequencies ω1 and ω2 are combined in the crystal; the generated blue-green beam then has frequency ω1 + ω2. These “second-order” nonlinear effects are relatively weak, yet it is still possible to use them to generate blue-green radiation at power levels suitable for the applications described in Chapter 1. In fact, of the three basic approaches to blue-green light generation discussed in this book, nonlinear frequency upconversion has so far been the most extensively developed and the most prolific in spawning commercial blue-green laser products.
The inherent weakness of these nonlinear effects has forced researchers and laser engineers to explore a variety of techniques for enhancing the efficiency of these interactions. In Chapters 3–6, we will discuss these different approaches, which include such things as intracavity frequency-doubling, resonant enhancement, and guided-wave interactions. However, all of these different embodiments exploit the same basic nonlinear interactions, and this chapter is devoted to explaining the essential nature of those processes. In it, we will give a qualitative explanation of the physical process underlying SHG and SFG, we will present some of the basic equations necessary for understanding and designing blue-green lasers based on these effects, we will discuss techniques for providing “phasematching”, which we will see is a crucial requirement for efficient generation of blue-green light, and we will examine some of the nonlinear materials that can be used for frequency conversion of near-infrared light.