Temperature variation

Paul Leyland

With the red laser I suspect it’s thermal expansion of the cavity which changes the resonant frequency.  The LED is a broadband emitter but constructive interference occurs only when the cavity is an integral number of wavelengths.  Change the length and the resonant wavelength changes.

The Nd transition is at a frequency governed by an electronic transition involving a 4f orbital.  Those orbitals are largely isolated from their chemical and thermal surroundings so the transition is very much narrow band which doesn’t change greatly with temperature.  The pumping diode is a broadband source but that doesn’t matter as long as it emits strongly enough at the laser frequency.

In a previous life I used to play with a tunable laser — a dye laser pumped by a fixed frequency argon-ion laser in that instance. It was tuned by changing the length of the cavity.