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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.
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
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.
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.
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.