Choice of light sources for a  simple fizeau intererometer.

It should have narrow bandwith, but  to narrow bandwith can cause unwanted interference from other surfaces.

Green  frequency doubled Nd:YAG  seems to be very suited to this.

typical  green DPSS laser

It is

• very frequecy stable compared to a diodelaser.
•  resonable  coherence length of appr 20 mm.
• inexpensive 50euro range.
• good TEM00 no elipticity in the beam.
• Polarized, if phase contrast imaging is needed (not all types are polarised)

typical data  for simple  "pentype" green laser:

Wavelength 532 nm CW
output power (mW) 5
Output power stability < 2% over 2 hours;
Noise(10 Hz to 50 MHz) < 0.5%
Transverse beam mode TEMoo,
Beam diameter (1/e²) 0.36 mm
Beam divergence 2 mrad
Operation temperature 10 to 35 degree C
Warm-up time < 2 min
Expected life time > 10,000 hours
Bandwith 0.2 nm
Coherence length 20 mm