What is coherence length?
Coherence length is the space over which a wave is ‘nicely’ sinusoidal. In other words, it is the space over which we can predict the phase of a wave of beam (if we are speaking of lasers). Coherence time is related to the classical uncertainty principle and is thus a little more complex.
In the natural sciences, coherence or coherent radiation refers to electromagnetic waves that have a fixed phase relationship in terms of their spatial and temporal propagation. In everyday life, this theoretical, somewhat cumbersome definition applies to the light beam generated by a laser source.
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.