What is coherence length and coherence time?
Temporal coherence tells us how monochromatic a source is. Suppose the laser emits wavelength λ and λ + Δλ. These waves with slightly different wavelengths would continue to interfere constructively and destructively in space at points dependant on the magnitude of Δλ. Smaller is the value of Δλ, larger will be the distance between points where constructive and destructive interference will take place. This optical path between these two positions is called coherence length lc. This is mathematically represented as: lc =λ2/ Δ λ In terms of time, Δt, the time taken by the waves to cover the above-mentioned two positions, it can be given as: lc = c Δtc : where c is the speed of the light wave.
