I am currently going back through my textbook Laser Systems Engineering by Keith Kasunic. Chapter 1.2 Laser Engineering says the following:

One of the primary goals of laser engineering is to meet system requirements by controlling the unique features of lasers that determine the properties of coherence length and beam quality, namely, the degree of phase coherence. That is, the wavefronts must be in phase along the propagation direction (temporal coherence, Fig. 1.7); in addition, the wavefronts must also be in phase across the beam diameter (spatial coherence, Fig. 1.8). There are also variations on these themes, where specialized lasers have low temporal coherence but good spatial coherence (white-light or super-continuum lasers), or high temporal coherence but poor spatial coherence (“random” lasers).

Is spatial coherence more important for interferometry, or is temporal coherence more important for interferometry? Everything I've read usually discusses the importance of spatial coherence for interferometers, which is why we have things like distributed-feedback lasers, but, based on what I've read, the temporal coherence clearly also has a substantial effect on the phase of the light, and we know that the phase is of central importance in interferometry, so I wonder about this. How should I think about these two when designing interferometers in experiments?


I found a good explanation of these concepts here.

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    $\begingroup$ This is a rather broad question. Which matters more depends on what kind of interferometry you are dealing with. Large delta path lengths? wide field-of-view? and so on. $\endgroup$ Aug 10, 2021 at 14:49
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    $\begingroup$ @CarlWitthoft Hmm, I see. What do you mean by "delta path lengths"? $\endgroup$ Aug 10, 2021 at 14:54
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    $\begingroup$ Some configurations allow the user to "trombone" the reference leg, and if that leg gets drastically longer or shorter than the sampling leg, you run into temporal coherence limits $\endgroup$ Aug 10, 2021 at 16:13


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