  Phase-matched nonlinear processes exhibit a tradeoff between the conversion
efficiency and the acceptance bandwidth. Adiabatic nonlinear processes, in
which the phase mismatch varies slowly along the interaction length, enable us
to overcome this tradeoff, allowing an efficient frequency conversion with
broad spectral and thermal bandwidths. Until now, the variation in the phase
mismatch condition was mainly based on quasi-phase matching in ferroelectric
crystals. However, this solution is limited to low power sources. Here,
instead, we study the adiabatic second harmonic in birefringently phase-matched
lithium triborate crystal, enabling us to handle much higher power levels. The
variation in the phase mismatch is achieved by inducing a temperature gradient
along the crystal. By using a 50 mm long crystal, the adiabatic process
provided a temperature bandwidth of 18-celsius degrees, 5.4 times wider than
what is achieved when the same crystal is held at the fixed phasematching
temperature. The conversion efficiency exceeded 60% for a 0.9 millijoule pump
pulse.
