ABSTRACT
Recently a lot of activity has been observed around mode coupling in nonlinear waveguides, aiming to efficient generation of blue (or green) light with infrared pumping. As discussed in Chapter II, the light confinement due to the small region occupied by the waveguide mode enables to drastically reduce the pump power required, from 20-1000 W to less than 100 mW. To this aim, it is first necessary to couple the pump light into a waveguide mode, second, to phase-match this mode with a mode at the second harmonic frequency, and third to couple out the generated blue or green light. The main problem appears in equalizing the propagation constants of the modes at the pump and second harmonic frequencies, i.e. in their phase matching. Due to the material dispersion, it is always possible to choose optogeometrical parameters of the waveguide in such a way as to have equal phase velocities of, say, the fundamental (0-th) mode at the pump frequency ω and one of the higher modes at the double frequency 2ω. However, for highly efficient second harmonic generation, it is desirable to obtain coupling between modes of equal order (namely, the fundamental ones), because their transverse field distribution is almost identical and their coupling is the strongest.
