Photon Budget and Optical Data Receiver
This one was a somewhat similar application for the Navy.
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 results in a noise floor asymptotically only 10 log(1.5) ≈ 1.76 dB above the shot noise of the signal photocurrent, instead of 3 dB for straight photocurrent feedback.
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.
The stabilization strategy was one I patented in about 1992: Send the beam through a fixed etalon; detect both the reflected and transmitted beams; form a linear combination C = T-αR for some convenient value 0 < α < 1; and servo the laser tuning to null out C, which can be done very accurately, without needing a high finesse cavity. The key observation is that by choosing α correctly, you can completely eliminate the coupling between AM and FM laser noise, so that besides excellent laser stability, you can also get outstandingly stable amplitude measurements by forming the combination A = T+αR. If you choose the right value of α, namely
α = -(dT/dν) / (dR/dν), then dA/dν=0,
so none of the FM noise of the laser gets turned into AM noise. (I'm not entirely certain that I was the first one to do this, but that was pretty early days for diode laser based instruments.) When combined with laser noise cancellation to get rid of the actual AM noise of the laser, this scheme lets you do shot-noise limited measurements inside a passive resonant cavity, which is a very useful trick.
A modern telecom DFB laser doesn't current-tune very far. It's easy to say, but a lot of design effort has gone into making this happen. DWDM channels are very closely spaced; when you current-modulate one laser to send some data, you don't want it to scribble all over the adjacent channels. That's excellent for telecoms, but inconvenient for laser stabilization, because the tuning range is too narrow. Thus this design needed a combination temperature- and current-tuning loop. Only current-tuning could achieve the required feedback loop bandwidth, and only temperature tuning could cover the required wavelength range. The breadboard prototype worked very well, in fact well enough to advance the state of the measurement art, but funding ran out before the actual downhole version could be completed. There were a few very interesting temperature-control concepts that came out of this work as well. I'd very much like to revisit it if I have the opportunity.
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.
Their scheme uses a really cool technique: solar heterodyne detection. It's a good illustration of the importance of a photon budget.