So when we talk about the emission wavelength of a laser, what we actually mean most of the time is the central, brightest emission wavelength, e.g. 852 nm on some diode laser. When we look at this laser light more closely with a high-finesse cavity, we can see modes emerge from our single wavelength, i.e., multiple sharp lines that repeat after we scan for more than one free spectral range $\Delta \nu_{\text{FSR}} = c/2L$. These modes may drift a bit or vary in intensity over time, but what I would like to know is: can you ever have a laser run in single-mode operation without some kind of active stabilization?

By active stabilization, I mean you take some output light of your laser, feed it into some detector, then analyze it and send some sort of correction signal to e.g. a Piezo actuator affixed to one of your cavity mirrors to shorten/lengthen the cavity "on-the-go" to counter drifts due to thermal expansion of other cavity elements, vibrations, etc.

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    $\begingroup$ A HeNe laser whose cavity is short enough that the free spectral range is longer than the gain curve will lase in one (longitudinal) mode. It will drift, but that's a different question. $\endgroup$ – garyp Mar 7 at 12:15

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