Part of my professional job is to develop new light sources. So a recent Physical Review Letter on the development of a very special optical parametric oscillator caught my interest. Of course, many of you are now saying "a what?" Relax. As usual, I will bore you with the details.

Almost everyone is familiar with lasers. A laser is just a gain source—a way to get a medium glowing with a particular wavelength of light—surrounded by mirrors to provide feedback. We give the gain medium energy—a lot of energy, in fact—which puts most of the medium into an excited state. Nothing likes being excited for very long, so the atoms or ions or molecules that make up the gain medium relax back to their ground state. As these molecules relax, they emit light, some of which is captured by the mirrors and reflected back and forth through the gain medium. As the light field builds up, this causes the excited gain to emit in sympathy with the existing light field. This process of stimulated emission is one of the things that makes a laser special.

But there are some limitations to this approach. For instance, to get laser light with a particular color, you need to find a material that emits light at that color. And, worse, the material's excited state has to have some pretty special properties. So, basically, you are stuck with what nature has given you, and nature hasn't given us everything we might want.

Oscillating optics

The optical parametric oscillator avoids these problems by not coupling directly to the excited states of the gain medium. Instead, we put a strong light field into the medium, and this causes the electrons to oscillate in sympathy. If the light field is really strong, it will try to drive the electrons too far from their parent atoms. The electrons resist, and, as a result, don't precisely follow the driving field, which means that the electrons radiate lots of different colors.

By itself, this doesn't get you far, because it isn't very efficient. But over a long distance, you can get efficient wavelength conversion. So, like a laser, we place the gain medium between two mirrors so that the light passes through the gain medium many times, turning a short gain medium into a long one.

Unfortunately, this isn't the full story, because the parametric part of an optical parametric oscillator means that a strict relationship between the various light fields must hold.

To understand this, let's take two tiny slices of our gain material that are separated by some longish distance. In the first slice of material, our strong pump field generates a range of colors—all at longer wavelengths than the original pump wavelength—but they are all rather weak. Those propagate through the medium until they reach the next slice. Unfortunately, since all the different colors of light see a different refractive index, they have all traveled slightly different lengths to the next slice. That means that, although the peaks of all the light waves lined up when they were generated, they don't line up by the time they reach the next slice.

The end result is that the light generated in the second slice of material destructively interferes with the light generated in the first slice—no new colors are generated. The saving grace is that this dephasing effect takes a certain amount of distance, so the trick is to modify the properties of the gain medium every so often. If done correctly, the waves retain a phase relationship that oscillates back and forth about an optimum, but never quite goes over the edge into destructive interference. As a result, frequency conversion (converting a light field of one color into others with different colors) is a go.

Tunable colors on a disk

The nice thing about this trick is that you choose the color of light you want to generate by choosing the length between each modification. In other words, an optical parametric oscillator can produce light over a very broad spectrum, and we can determine which part of the spectrum by our engineering choices. This basic process has been around for many years, and many labs have tables full of equipment dedicated to delicate instruments that are gently coaxed into life every once in a while to produce light.

The big issues with the approach are that a strong pump is required, high reflectivity mirrors are required, and the optical alignment must be very precise. The end result is an instrument that only its mother could love. And that's what makes this latest development so interesting. A multinational team of researchers used a small disk of material as their gain medium, while the periodic modification consisted of pie segments radiating out from slightly off the disk's center.

How does this help? The disk combines both high reflectivity mirrors and the gain medium—the light travels around the disk by total internal reflection, exactly like the whispering galleries that are found in some cathedrals. This means that the optical alignment is rather robust. The whispering gallery mode resonator doesn't care what color of light is put in, so a weak pump field can used—because the light circulates for many trips around the disk, it will build up to high intensity.

The end result is that their optical parametric oscillator can run with just a few milliwatts of pump light. Furthermore, the off-center piechart approach to gain medium modifications means that a very wide range of wavelengths can be generated; choosing the exact ones is just a matter of changing the temperature of the disk. Any light path around the resonator will see a range of different spatial periods, which allows many different colors to build up. The method by which light gets into and out of the disk picks off particular colors at particular angles for use elsewhere.

This is not exactly a world-changing bit of work, but it is a nice, tidy piece of technology development. Funnily enough, its very strength is also its weakest point. You see, it's rather difficult to get light into and out of one of these disk resonators.

The problem is that you need to place a prism or fiber optic cable within a micrometer or so of the surface of the disk. Light then leaks back and forth between the prism and the disk. But this is very sensitive to the distance between the disk and the prism. Any mechanical motion (or thermal expansion, for example) would have a drastic effect on the performance of the optical parametric oscillator.

An alternative, which we are working on in our group, is to use waveguide rings to resonate the light. The advantage here is that the coupling between the resonator and the outside world is a good deal more robust, but we pay for that with the quality of the resonator. Whispering gallery mode resonators are extremely good, while ring resonators are not, so we will be needing to use higher powered sources to drive our optical parametric oscillators. What I like best, though, is that we are clearly not the only people working on these ideas, so, the danger of ending up in an uninteresting corner of the physics world is greatly reduced.

Physical Review Letters, 2011, DOI:10.1103/PhysRevLett.106.143903