Optical Parametric Oscillators (OPOs):
OPOs are remarkable because they use nonlinear optics to convert one laser beam, called the pump, into two new beams of light, called the signal and idler beams. A schematic of what happens is below.


The in the center of the curved mirrors above represents the nonlinear optics medium which is the basis of the whole OPO.

I'm interested in OPOs because you can use them to do quantum optics and quantum information. In fact, the light output by an OPO has properties unlike anything you can see from a laser or any other linear source of light. These properties involve noise reduction, which can allow you to make measurements with increased precision over what you could do with a normal laser. But beyond that, the quantum properties of the light make it possible to use OPOs in quantum information experiments. One day, OPOs could be used to form the basis of quantum computers and quantum communication protocols.

Despite looking deceptively simple, achieving a device like the one in the diagram above is quite difficult. Anyone can throw two mirrors together and put something in between them in hopes of making a laser, but the steps after that to align it and get it lasing are the true tests of an optician. Once that's done, you've got to actually be able to operate it and use it, too. A whole host of electronics for stabilization and detection have to come into play as well just to get it working. Below, you can see how to build an OPO, in pictures:

This OPO was built in 2004, that was three years ago. So, don't expect to get any secrets out of this! What this will show you is a general idea of the structure of the device.

To the right you can see one of the first components you need to make. Those copper blocks are heatsinks. You need heatsinks because OPOs need to be temperature controlled. The little white square to the right of the black screwdriver is a peltier element. It's glued onto the heatsink in this photo. This heatsink is pretty big compared to the actual size of the nonlinear crystal that was used. The bigger the heatsink, the more stability you have in your temperature control efforts.

On the left here you can see the various pieces of the OPO. The heatsink is on top of a mount which has extremely fine control over displacement in terms of height, transverse translation, and angle.

The whole thing is sitting on top of an aluminum mount. On the left that large shiny structure is the entire resonator assembly. Inside those large metal slabs on the left and right are holes to mount the mirrors into. In the background you can see metal screws where the mirrors are going to be glued. Those screws go into the metal plates. The large metal bars connecting the plates are basically what's responsible for the incredible stability of the resonator. The whole thing is made of a special alloy that has very low thermal expansion. The weight makes it very resistant to vibrations also. Those four big nuts are what you use to control the distance between the cavity mirrors.

Below you can see the partially assembled structure. The resonator connects to the aluminum mount via two long screws on the left side. The right hand side is free, since it has to be movable in order to adjust the mirror position and angle.

Below you can see a picture of a fully assembled OPO with a crystal oven in the center of the resonator.
In this particular photo you can see green light on the left side of the OPO, and nothing on the right.
In fact, infrared light is input into the OPO on the right, and visible light is being created inside the cavity and exiting to the left. This is called second harmonic generation. It's just one of the many fascinating things that nonlinear optics crystals can do.