What’s that smell? If you can’t tell, maybe a new laser system from CU Bolder and NIST can help. The device is simple and sensitive enough to detect gasses at concentrations down to parts per trillion.

The laser at the system’s heart is a frequency comb laser, originally made for optical atomic clocks. The laser has multiple optical frequencies in its output. The gas molecules absorb light of different wavelengths differently, giving each type of molecule a unique fingerprint.

Unlike traditional lasers, which emit a single frequency,  a frequency comb laser can emit thousands or millions of colors at once. The inventor picked up the Nobel prize in 2005 for that work.

The gas is placed between two highly-reflective mirrors. The beam bounces in this optical cavity, although previous attempts were difficult because the cavity has a particular affinity for frequencies. The answer was to jiggle the mirrors to change the size of the cavity during measurments.

This is one of those things that doesn’t seem very complicated except — whoops — you need an exotic comb laser. But if those ever become widely available, you could probably figure out how to replicate this.

This could revolutionize air quality instruments. Small quantities of hydrogen sulfide can be detected easily (although, paradoxically, too much is hard to smell).