You can reduce the spectral bandwidth using narrow bandpass filters. That increases the temporal coherence. Spatial coherence can be increased by reducing the spot size, either using a pinhole or a single mode fiber.

Thanks you for answering. I will try to do it.

One way is to increase the more round trip radiation in the resonator which emphasizes the reflectivity of the mirrors and increases the saturation of the phase coherence.

The next point is to be more accurate in fixing the distance of the mirrors to the correct ratio of the net wavelength generated to create some kind of static wave mode.

Thank you so much for answering the question.