As a part of my daily work, I have to trawl through a huge number of articles trying to keep up with relevant work in my field. The horrible thing is that many of the papers I have to go through are painfully dry, and you know that before you even click the link. In these papers the findings are precisely what you expect from the title, and it's really just a matter of checking the contents just in case. Every now and then, though, one jumps out and you think "Why didn't we do that?"

Such a paper, about generating short pulses of light, has just appeared on the arXiv. The cool thing about this bit of research is that it takes an old idea and reworks it to make something that I think is just plain fantastic.

Before I describe this new scheme, let's take a step back and take a look at how we generate very short pulses of light with a laser. These pulses are typically shorter than 50 picoseconds (one ps = 10-12 seconds). Electronically, it is simply not possible to modulate a light stream fast enough to produce these pulses. To make things even more unlikely, the time scales that govern lasers—things like how long it takes a photon to make a complete pass through the laser, and how long an excited molecule or ion stays in the excited state—are usually longer than the switching times of electronics. So, given this, it seems that making short pulses is impossible.

Making a fast laser

Yet, we have laser systems that produce pulses as short as 5 femtoseconds (1fs = 10-15 seconds), so clearly it is possible. The key to understanding this is to recognize that there is a relationship between the shortest possible pulse and the spectral bandwidth (the breadth of wavelengths emitted by the laser) of the light pulse. Essentially, to make a very short pulse, you need to put together a very broad spectrum.

Now, despite what you might have been told, there are many laser gain materials that have a rather broad wavelength range. For example, sapphire crystals doped with titanium will emit laser light with wavelengths spanning over 200nm (meaning the range of colors that we see span around 200nm). If bandwidth were the only thing, every flashlight in the world would emit very short pulses—in fact they do, but not in a regular sequence over time, making them impossible to observe.

To get a regular train of pulses, we require two things. First, each wavelength that contributes to the pulse has to be evenly separated from the rest in terms of photon energy (or, equivalently, frequency). Each of the waves also needs to have a fixed phase relationship.

Luckily, the design of a laser takes care of the first problem. A laser is a gain material (one that increases the amount of light available) placed between mirrors. Light that reflects off of these mirrors meets itself after every round trip. If the light has not traveled exactly the right distance, it destructively interferes and dies. This leaves only those wavelengths that fit to rule, which all happen to be evenly spaced in energy.

That leaves phase locking. To do this, you need some process in the laser that communicates the phase of one wavelength to the next. If this communication is strong enough, these wavelengths all add up in phase and a pulse is generated that sweeps through the gain medium and is strongly amplified. All the wavelengths with the wrong phase don't get amplified because the pulse has stolen all the gain, so they die away, leaving a single pulse traveling back and forth between the mirrors.

An alternate route to short pulses

Life isn't easy though. For very short pulses, a lot of wavelengths are required, and maintaining the phase locking becomes a technical challenge. What if you could avoid this challenge by taking a single wavelength of light and add energy in discrete units to it, progressively building up a comb-like series of wavelengths that were all naturally phase locked?

You would get pulses as well. This is exactly what Wu and coworkers art Purdue have done. To generate their pulses, the researchers took the output from a non-pulsed laser and passed it through an amplitude modulator and a phase modulator.

How does that generate pulses? One of the effects of amplitude modulation is broadening the spectral bandwidth of the light. Essentially, the amplitude modulator switches the light beam on and off at a rate of, say, 10GHz, making 100ps pulses. To support these pulses, the single frequency in the laser input gains sidebands in units of 10GHz.

Because a 10GHz switching rate isn't very fast, the researchers added a second amplitude modulator that is out of phase with the first. This acts to shorten the pulses by another factor of two, bringing the pulse duration down to around 50ps, which is still rather long.

The real trick is to add a phase modulator at the end of the chain. This rearranges the phases all the different frequency components that are already there, but it also redistributes light among the different frequency components to help make the each wavelength contribute equally to the intensity of the light. Finally, the act of phase modulation also adds sidebands to each frequency component, so that the spectral bandwidth is increased even further.

With that, the pulses shorten to about 2ps. The laser people among us will be saying "Psh, wake me up when they reach 100fs." They are right—2ps is nothing to write home about in terms of short pulse generation. But, what we have here are electrically generated light pulses that are just 2ps in duration. That is pretty awesome. More importantly, this scheme is much more flexible than the standard way to generate short pulses. Certainly, I can see this being scaled in terms of total optical power, pulse repetition rate, and the production of even shorter pulse durations on the horizon.

So, why am I a little annoyed? We have a similar research program underway. Even though our scheme takes a very different approach to generating short pulses, this would have been a great side track to go down.

ArXiV, 2010, Abstract number 1005.5373

Listing image by Patryk Buchcik