A KTP crystal in a nonlinear optical device is heated to and operated at a temperature of at least 50.degree. C. and less than 350.degree. C., and more preferably about 90.degree. C. to about 200.degree. C., and most preferably to about 100.degree. C. to about 125.degree. C., to reduce drift and damage. The KTP crystal is placed in an oven or other heating device, which may be regulated or unregulated. The KTP may be cut at the room temperature phase matching angle and angle tuned for operation at the higher temperature, or the KTP may be cut at the correct angle for phase matching at the operating temperature.