The problem with scientific papers is that they hide about half of the interesting stuff. Recently, a group of scientists set out to directly measure a property of a laser, something that goes by the exciting-sounding name of “spatial hole burning.” In the process, though, they discovered how to turn a laser into a very high-speed microwave device, a discovery that may make the next generations of Wi-Fi and mobile data much easier to implement.
Was this discovery an accident, or did the scientists know that the application potential was there before they started? The paper is silent on the issue. But are we seriously going to put a laser in every cell phone?
Your laser is full of holes
When you form a picture of light from a laser, you might imagine something like light from a laser pointer: red or green, a nice directed beam, possibly a bit sparkly when you shine it on the wall. Lasers emit light with a single pure color, right? Unfortunately, life is not that simple.
A laser needs two things: a material that produces light and a couple of mirrors that keep the emitted light circulating through the material. Most materials will emit light over a broad range of colors, so the purity of laser light is usually not a natural property of the material that emits the light.
The laser’s purity is induced by the light itself. The mirrors keep the light passing back and forth through the material, and the presence of the light induces the material to emit more light of the same color. The brighter the color, the more easily it induces the material to emit light of the same color. In the ensuing race, tiny advantages quickly become big advantages. A color with a tiny headstart wins the race and kills off all other colors except for itself.
The victory ensures the color’s own death, though. Light is a wave, and when stuck between two mirrors, the light forms a standing wave. This is exactly the same as the wave you get from plucking a guitar string. In this sort of wave, there are fixed physical locations where the wave’s amplitude is large. These places of high brightness are also where the material is induced to emit light.
After a short time, the ability of the material to emit light is burnt out. The color’s brightness can no longer grow. Indeed, it starts to dim because light is emitted (otherwise we wouldn’t have an output and the cat would get bored). The place where the gain is burnt out is called a hole, and the whole process is called “spatial hole burning.”
Once spatial hole burning starts, it allows a new color to start lasing, and it, too, burns out. In the meantime, the material recovers in the regions where the first color had burned it out. The result is that the burnt and recovered material form their own wave that has a wavelength and frequency that corresponds to the difference between the two (or more) colors that lase.
That’s how it should work, at least. But no one has directly observed spatial hole burning before. In this research, scientists used a specific type of laser that uses electrons that emit light in well-confined locations to track spatial hole burning. The electrons essentially enter a material that looks like a set of buckets. Each bucket has a little shelf at the top and a hole in the bottom.
The electrons enter a bucket by falling on the bucket’s shelf. The electrons sit there for a short time and then fall to the bottom of the bucket, emitting light as they do. The electrons then drain out of the hole in the bottom of the bucket and land on the shelf of the next bucket.
In this context, spatial hole burning means that there are no electrons sitting on the shelves in some of the buckets. This imbalance creates a voltage between buckets that oscillates at the frequency with which spatial holes are burnt and recover—this is the frequency difference between two colors of light that the laser emits.
In the researchers’ case, the frequency difference between two colors corresponds to microwave radiation (in fact, radiation near the 5GHz Wi-Fi band). They were able to measure this frequency by scanning a probe along the length of the material (from bucket to bucket, as it were) and measuring the voltage between the scanned probe and a fixed probe.
Amazingly enough, the physics worked, and spatial hole burning was directly observed.
Making lasers into microwave mixers
What the researchers then realized is that they could manipulate spatial hole burning by injecting microwaves through the probes. The microwave signal is locked to the spatial hole burning, so the nature of the mixing (or more specifically, the phase between two mixed signals) could be changed by altering the location of the probes.
The microwave radiation gently moves electrons around, changing the places where the laser can emit light. In doing so, the injected microwave signals are mixed with the spatial hole burning microwave signal. The mixed signal is then re-emitted to the probe.
This mixing process is exactly what your cell phone does. It takes the voice and data signals and encodes them in some intermediate frequency signal. This signal is then mixed with a higher frequency that is transmitted to the network (or received from the network).
The critical point is the mixing process. Encoding the information at the intermediate frequency is reasonably straightforward. Generating a carrier wave at high (even very high) frequencies is sometimes difficult but doable. The problem is the electrical element that mixes the two. This is the point at which the information that we want to transmit is put into the broadcast signal. Without that step, cell phone technology would still involve tin cans and pieces of string.
Using spatial hole burning, the high frequency signal is just the difference between two light colors in the laser. That can be 5GHz, 10GHz, or 1,000GHz depending on how the laser is constructed. And no matter what that frequency is, the mixing process will work.
In one step, this has opened up an entirely new way to generate very high frequency microwave data transmitters. Even better, all the modulation techniques that make the current generation of transmitters so efficient in their use of spectrum can be used directly.
Is that a laser in your pocket or are you just a little hot?
In this setup, the encoded information would be emitted back into the probe—the laser’s output becomes irrelevant here. That raises the question of what to do with the laser light. Currently, the emitted light is simply thrown away.
These lasers typically operate at wavelengths that are much longer than that visible and are perfect for heating things. So I’ll be the first to propose a combination cellphone/toasted sandwich maker. Because why wouldn’t you want that?
More seriously, the most likely solution is to not have any laser output at all, which would conserve as much power as possible. A wasted opportunity if you ask me.
Optica, 2018, DOI: 10.1364/OPTICA.5.000475 (About DOIs).