Just add lasers
I don’t believe there’s anything that can’t be improved by adding a laser to it. And now a group of intrepid engineers have proven me right by creating 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 good question to ask why we should bother adding lasers to oscilloscopes since it’s fairly well-established technology. The answer is speed. An oscilloscope is designed to display changes in voltage or current over time. To do this, the oscilloscope must sample the voltage faster than it changes, which is problematic for today’s modern, high-frequency electronics, where it is often easier to generate rapid changes than to measure them.
This is where a laser can have some benefit. In principle, a light field can be modulated at a rate that is a large part of the basic frequency (~600THz). Provided we can measure that modulation, we can measure time-varying voltages much faster than any electronic method. But therein lies a mystery: how do we measure the modulation of a light field? Using electrons. And what’s the problem with electrons? They’re too damn slow.
To solve this problem, the researchers used a very clever trick. First, the voltage is sampled using a probe connected to a very sharp tip in a vacuum. When a laser pulse hits the tip, it jumps some electrons off 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 measure the voltage at very short intervals, the researchers used a laser that emits light pulses of less than 10 femtoseconds (a femtosecond is 10-15 seconds) long and about 7 nanoseconds apart. Scanning this laser across a mirror shifts the light pulses slightly so that they arrive earlier or later. This samples the voltage at slightly different relative times. With enough samples, a complete picture of the time-varying voltage is constructed, provided, of course, that the voltage variations repeat themselves regularly.
To demonstrate that this process really works, the researchers tested a voltage signal of 9 GHz with their oscilloscope. They showed that they could sample at intervals of about 20 femtoseconds, meaning they could accurately measure waveforms with frequency components up to 125THz. To put this in perspective, the fastest commercial oscilloscopes can handle up to about 80 GHz (1500 times slower).
All this, of course, has a downside. It takes about 200 seconds to measure a single point. It took about four hours to obtain one of the graphs in the researchers’ paper. That’s probably too slow for the average soldering iron driver, but it’s not too hard to imagine it being very useful in extremely high-end applications. However, it has another consequence: the signal you are trying to measure must be stable for many hours. Any passing behavior is completely missed.
Don’t take the 200 second number too seriously, though. This is the kind of song that will never get worse, only better. I bet that this will be reduced to 1 second per sample within a year.
You might also think that such an oscilloscope will never leave the lab because it is simply too expensive and complicated. But high-end oscilloscopes are already quite expensive, and the kind of laser used here is about the same price. An alternative laser source, already commercially available, is about a factor of ten cheaper, although you may only get 10 THz of bandwidth. The probes, which contain vacuum tubes, will be expensive and difficult to manufacture with the right characteristics for sampling high-speed electronics. Still, I think it’s all doable.
We can bet on which commercial oscilloscope vendor will pick up the technology first in the comments.
Optical letters2015, DOI: 10.1364/OL.40.000260