The issue will not be at cold but at high temperature. VCSELs have very poor efficiency at high temperature and it’s possible to operate them where increasing current reduces light output. In a vehicle application the temperatures are very high and humidity can also be very high and condensing.
This couldn't be further from the truth. You can design the VCSEL cavity and top and bottom mirrors for peak efficiency at any temp, including very high temps. I wonder what we did...
Compared to the side emitter diode lasers used in legacy spinning lidar, VCSELS are cheaper, more efficient, more reliable, longer life, and better quality light sources to boot.
Unfortunately the gain falls as function of temperature so you also get a lot less light and you have to pump harder (more current). So while it’s possible to somewhat compensate somewhat with the mirrors the device still has this behavior at high temperatures as the device self heats. This behavior is widely documented in the literature.
VCSELs have a smaller current aperture and the current density is higher than in an edge emitting laser. As the reliability is a function of the junction temperature and the current density, VCSELs operating at high temperatures have significantly reduced lifetime compared to an edge emitting device due to the high current density.
See for example slide 5 which shows how lifetime scales as a function of temperature and current density. For high reliability your devices need to have low current density.
