Viewing posts from September, 2017
This was a collaboration with Chris Wieland of Della Enterprises on an Army Research contract.
The standard problem with conventional nanowatt photoreceivers is that in order to get near the shot noise, you have to use feedback resistors so gigantic that you can't maintain decent bandwidth.
This one has what I think is a completely novel photo-feedback architecture, i.e. rather than using a feedback resistor in the TIA, it uses two secondary photocurrents to cancel the input current. Putting the two secondary photodiodes in series makes the cancellation current 3 dB quieter than the shot noise, and a feedback system prevents them from fighting, as series-connected current sources normally would.
This was in collaboration with a start-up in New Mexico called Symphony Acoustics. Downhole measurements are notoriously difficult, and this one was no exception: building a laser that could achieve an Allan variance of 10-10 at 10,000 seconds, and do it 5000 feet down a 2-inch cased drillhole. Due to the casing thickness, the maximum outer diameter of the instrument package was 38 mm, including its own casing and two concentric zones of thermal control.
This was in cooperation with Mesa Photonics of Santa Fe NM. It's part of a DOE program, an advanced deployable solar occultation spectrometer for detecting volatile plumes from clandestine uranium enrichment.
A follow-on to the single channel version. This one had to work at very much lower power, which required a new amplifier topology based on local feedback around a very low noise JFET. This was a very fruitful development, which has been used in a number of follow-on designs.