If you looked at the Ophir catalog a year or two ago, you could quickly get a sense for the lowest powers that can be measured.
For photodiodes (UV, visible), that was about 10-20 pW. For thermal sensors (broadband), about 300 μW.
But we know there are femtowatt lasers. There are low power IR sources that can’t be measured by Si/Ge photodiodes. Applications such as spectroscopy, THz detection, free space
gas analysis, atmospheric studies, Raman scattering, and others were left without a viable options for measuring their lasers.
We knew something had to be done.
But it isn’t so simple. Filtering and averaging only help to a point. At a certain level, the signal just disappears under the noise floor.
That’s where the RM9 family of sensors come in. They utilize a lock-in amplifier to allow signals beneath the noise floor to emerge – and be measured accurately.
Here’s an article from Ophir Senior Physicist Shimon Elstein, that explains how this is possible:
Working in the Basement: Measuring Signals Below the Noise Floor with a Lock-In Amplifier
Flickr creative commons image via Pete Birkinshaw