The organizing idea of the previous chapters is that nonlinear, nonresonant, properties of insulating materials can be exploited to convert long-wavelength coherent radiation into short-wavelength radiation. As was discussed at length, this compels the device designer to simultaneously satisfy two demanding operational constraints: (1) Optical intensities at a first-harmonic frequency must be sufficiently high that the electromagnetic response of dielectric media is pushed into the nonlinear regime. (2) A travelling wave thus generated at the second-harmonic frequency must propagate at the same phase velocity as the fundamental wave lest the second harmonic switch roles from receiver to donor of optical power in the device. However, nonlinear frequency generation hardly requires that the operative interactions take place off resonance. Photon adding functions can be accomplished equally well with the aid of resonant optical processes in insulating materials, in turn wholly eliminating the two challenging constraints just named. We are, of course, referring to upconversion lasers. In this chapter and the next, we present this second approach to the nonlinear generation of short visible wavelengths and discuss what different operational challenges arise in creating a practical upconversion device.
INTRODUCTION TO UPCONVERSION LASERS AND RARE-EARTH OPTICAL PHYSICS
Upconversion lasers function just as ordinary lasers do, at least insofar as the mechanism by which their output beams are generated: A population inversion is created between two widely separated states thus making possible optical gain and laser oscillation in whatever media play host to the atoms, ions, or molecules possessing those states. The difference comes in the pumping mechanism.