I don't believe there is anything that can't be improved by adding a laser to it. And now a group of intrepid engineers has proven me right by making an oscilloscope. An oscilloscope with lasers.

Of course, not everyone shares my obsession with lasers—such people are strange and have sad little lives, but we forgive them. But it's a fair question to ask why we should bother adding lasers to oscilloscopes given that they are pretty well-established tech. The answer is speed. An oscilloscope is designed to display changes in voltage or current with respect to time. To do this, the oscilloscope needs to sample the voltage faster than it changes, which is problematic for today's modern, high-frequency electronics, where it's often easier to generate fast changes than it is to measure them.

This is where a laser may have some benefit. In principle, a light field can be modulated at a rate that is a large fraction of its base frequency (~600THz). Provided we can measure that modulation, we can measure time-varying voltages much faster than we could using any electronic method. But therein lies a conundrum: how do we measure the modulation of a light field? Using electrons. And what is the problem with electrons? They are too damn slow.

To overcome this problem, the researchers used a very clever trick. First, the voltage is sampled using a probe that connects to a very sharp tip in a vacuum. When a laser pulse hits the tip, it pops some electrons from the tip into the vacuum. The energy of the electrons depends on the voltage applied to the tip at the time of the laser pulse. The electron energy is then measured and used to calculate the voltage at the tip.

To be able to sample the voltage at very short intervals, the researchers used a laser that emits light pulses that are less than 10 femtoseconds (a femtosecond is 10-15 seconds) long, and are spaced by about 7 nanoseconds. By scanning this laser across a mirror, the light pulses are shifted slightly so that they arrive earlier or later. This samples the voltage at slightly different relative times. Given sufficient samples, a complete picture of the time-varying voltage is assembled—provided, of course, that the voltage variations repeat regularly.

To demonstrate that this process actually works, the researchers tested a 9GHz voltage signal with their oscilloscope. They showed that they could sample at intervals of about 20 femtoseconds, which means that they can accurately measure waveforms that have frequency components up to 125THz. To put this in perspective, the fastest commercial oscilloscopes can manage up to about 80GHz (1,500 times slower).

There is, of course, a downside to all of this. It takes about 200 seconds to measure a single point. One of the graphs in the researchers' paper took about four hours to obtain. That's probably too slow for the average soldering iron driver, but it's not too difficult to imagine it being very useful in extremely high-end applications. It does, however, have another consequence: the signal that you are trying to measure must be steady for many hours. Any transient behavior will be missed completely.

Don't take the 200 second number too seriously, though. This is the sort of number that is never going to get worse, only better. My bet is that within a year, it will be down to 1 second per sample.

You might also be thinking that such an oscilloscope will never get out of the lab because it's just too expensive and complicated. But high-end oscilloscopes are quite expensive already, and the sort of laser used here is about the same price. One alternative laser source, which is already commercially available, is about a factor of ten cheaper—though you might only get 10THz of bandwidth. The probes, which involve vacuum tubes, will be expensive and hard to manufacture with the right properties for sampling super-fast electronics. Still, I think it's all doable.

We can bet on which commercial oscilloscope supplier will pick up the technology first in the comments.

Optics Letters, 2015, DOI: 10.1364/OL.40.000260