How to Measure Extremely Low Power Lasers (pW, fW)

Small signal, drowned in noise

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

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