I've been thinking about laser fundamentals recently and I recalled a conversation I had with Sam Goldwasser almost 2 decades ago where we discussed how the cavity size (that is, the linear distance between the two mirrors that make up the Fabry-Perot resonator, not just the linear length of the gain medium) needs to be an integer multiple of the output wavelength to reach optimal gain in the cavity.

Obviously with external mirrors you can adjust for this. And even with hard-sealed, non-adjustable optics (thinking HeNe tubes here), the tube length (and thus the cavity length) is still quite long when compared to the fundamental output wavelength, so even a tiny shift in the output wavelength (minute fractions of a nm, which would be well within quantum uncertainty limits for the electron energy levels involved) would still add up to enough of a total shift in length along the cavity to allow for a perfect standing wave to form between the mirrors. No problem so far...

My question then, is thus: in the case of a low power single-mode direct-injection laser diode (which has an almost comically short cavity size), does this short cavity end up broadening the output spectrum between different diodes? And is there a point where the cavity becomes so short that lasing action can't happen because you can't get a standing wave to fit in the cavity at the desired output wavelength? (That is, the cavity is not a convenient multiple of the wavelength and you can't get enough waves in the cavity to allow for a tiny shift in wavelength to add up to a large enough distance change between the mirrors...)

Adam