By Shimon Elstein, Senior Physicist
The measurement of optical signals in the femtowatt to nanowatt range can be extremely difficult as signal levels this low are typically lost in detector noise levels and swamped by background light. Photodiode detectors that operate within small bandwidths have a noise floor of only 1 picowatt. Any attempt to narrow the bandwidth by filtering or averaging will only provide a small additional reduction. In order to significantly improve the noise reduction in a photodiode, a lock-in amplifier can be used to improve noise rejection by 3 orders of magnitude or more. This article presents the advantages of using lock-in amplifiers for measuring signals below the noise floor, how they work, and available devices from Ophir. Download the article for more information.