We began in Chapter 1 by discussing applications that required compact blue-green lasers; hence, it seems only fitting to end by examining the extent to which these requirements have been fulfilled. In this final chapter, then, we attempt to gather up some of the diverse topics that this book has treated and establish the current state of the art in the application of compact blue-green lasers.
The preceding chapters have covered three principal approaches to creating compact blue-green lasers. In the first approach, blue-green light is generated through nonlinear frequency conversion of infrared semiconductor diode lasers or diode-pumped solid-state lasers. We saw that since these nonlinear processes tend to be rather weak, the desire for efficient generation of blue-green light has stimulated the development of high-power infrared lasers as well as the invention of a host of device configurations intended to boost the nonlinear conversion efficiency. These configurations include resonator-enhancement schemes, intracavity SHG, and waveguide implementations.
An alternative approach – the “upconversion laser” – directly excites a blue-green laser transition by combining the energy of two or more lower-energy pump photons through excited state absorption or cooperative energy transfer processes. Upconversion lasers using both bulk and fiber-optic media have been demonstrated.
Finally, we examined semiconductor diode lasers that are pumped by electrical injection and directly produce blue-green photons. Two main materials systems have been used to fabricate these devices: GaN and ZnSe.