A while back I put up a post explaining that you can add photons together under the right circumstance. The other day at work someone pointed me to an interesting doubling paper: High-efficiency frequency doubling of continuous-wave laser light (arXiv link)
Second harmonic generation (SHG, or doubling) is a nonlinear process, meaning it has a nonlinear dependence on the strength of the electric field. In other words, it becomes more efficient as you go to higher intensity. Naively, if you need two photons interacting at the same time, the odds of having two right where you need them is higher if you have more photons in your volume.
The higher intensity here is achieved with a resonant cavity, comprised of a mirror and the doubling crystal. The left-hand side of the cavity is the mirror which is mounted on a piezoelectric transducer (PZT) so the cavity length can be tuned to give you a standing wave inside the cavity, made up of an integral number of wavelengths of the light.
The crystal is periodically poled potassium titanyl phosphate crystal (PPKTP), and the right-hand surface is curved and has a dielectric coating to make it highly reflective at both the 1550 nm pump wavelength and the 775 nm output wavelength. Clever! This way you don’t have to worry about additional losses from reflection off of the crystal surface; the other surface is antireflection coated, but that’s never going to give you 100% transmittance.
Since the mirror is reflective at 1550 nm, these photons will bounce multiple times before leaving, so the power can build up. This is shown schematically with the dotted line in a loop, but the actual light profile will be a bowtie shape, so the intensity is even higher inside the crystal. However, the mirror is antireflection coated for 775 nm, so once the light is doubled it leaves. The input beamsplitter to the cavity is dichroic, so it only reflects at 1550 nm and the 775 nm light is free to pass through it.
With this they got 95% conversion efficiency with an input of 1.1 Watts, and think they can bump it to 98% with 1.3 Watts.