The lower laser noise value, the lower the probability that the output light intensity will have an instantaneous peak, the higher of power stability of the laser, and the less damage to the optical components in the laser cavity, thus extending the working life of the laser.

The essence of which is: When the laser operates continuously under the condition of multiple longitudinal modes, due to the sum frequency effect in the frequency doubling crystal and the cross-saturation effect in the working material, each longitudinal mode is coupled to each other, make the laser output intensity fluctuate greatly with time, chaotic motion characteristics, generally the frequency of laser light intensity fluctuation is tens of hertz, this is the well-known "green light problem" and "blue light problem".

At present, the method of eliminating laser noise and realizing low noise operation of lasers can be summarized into two categories:

(2) Forced laser single frequency operation: when the laser working in the state of single longitudinal mode, will not produce and frequency in the frequency doubling crystal effect, eliminate the "green light", laser output is stable produced.

Cavity configuration of intracavity frequency doubling with compansator.

Intracavity frequency doubling laser with fast axis of Nd:YAG and KTP into ψ.