Downhole Interferometry: Cavity-Stabilized 1550-nm Laser

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.