system
Given the following post, respond as if you're commenting on Reddit:
human
How were sufficiently coherent light sources generated for optics experiments before lasers?

All the optics and photonics lab work I did as an undergrad a decade ago used a laser as the source due to its coherence.  

I still don’t understand how the famous optics experiments (thin slit interferences, interferometers...) that predated the laser were achieved without that advantage.
gpt
For light sources to be coherent, they must be monochromatic (one colour) and have a constant phase difference between them.

The monochromatic-ness is easy to achieve, just use a source which produces only one wavelength of light.

For the constant phase difference, a single slit was often used before the gratings, or double slit, or whatever equipment the experiment was using. This works because the single slit acts as a point source, and therefore, as long as it is not moving relative to the other  gratings/slits, will mean there is a constant phase difference between the sources.

Hope this is correct and answers your question :)