The superconducting nanowire single-photon detector (SNSPD) arrays created in this innovation were fabricated using a WSi nanowire process. A gold mirror layer is deposited on an oxidized silicon wafer, and amorphous-state WSi is sputtered from a compound target at a thickness of 5 nm. The WSi nanowire is embedded at the center of a three-layer vertical optical cavity consisting of two silica layers and a titanium oxide anti-reflective coating. The layer thicknesses were chosen, on the basis of simulations and measured material parameters, to optimize efficiency at the target communication wavelength of 1,550 nm, and to minimize the polarization dependence of the detector response.
Two sets of six WSi nanowires are co-wound together to fill a round active area of 66 microns in diameter. This active area was chosen to match the 62.5-micron mode field diameter of standard GIF-625 graded-index multimode fiber. The current biases for the individual nanowires are connected in such a way that the current in each adjacent wire flows in opposite directions in order to mitigate the effects of crosstalk in adjacent channels due to mutual inductance between adjacent wires. Detector yield is high, with all 12 nanowires functioning on the delivered device.
The nanofabricated detector element is coupled to the multimode fiber using a variation on a self-aligned cryogenic packaging technique. The chip is patterned into a shape compatible with the coupling scheme using deep reactive ion etching. The 12 wires are connected to coplanar microwave waveguides and connected to bond pads at the edge of the chip. The chip is then mounted in a custom-developed copper microwave sample mount to thermalize the detector chip at sub-Kelvin temperatures. The 12 nanowire lines are wire-bonded to a printed circuit board that routes the signals through coplanar waveguides to 12 SMP (sub-miniature push-on) connectors mounted on the board. This detector module can then be mounted in a sub-Kelvin cryostat such as a closed-cycle helium-4 sorption refrigerator. The signals are then sent to custom cryogenic amplifier boards where the signal is amplified and the current bias is added with a bias tee. Cryogenic signal routing is performed using low thermal conductivity CuNi coaxial cables.
Once the amplified signals are passed out of the cryostat using standard RF SMA (Subminiature Type-A) feedthroughs, they are filtered, amplified further, and discriminated using commercially available ECL (emitter-coupled logic) comparators. These signals are then combined using an analog resistive combiner network, amplified in a final stage, and digitized using a high-speed data acquisition system to implement a communication receiver. With this innovation, single-pixel devices with 93% system detection efficiency at 1.55 microns (a standard telecommunication wavelength), 140 ps timing jitter, 40 ns recovery time, and sub-hertz intrinsic dark counts have been reported. These detector arrays have been successfully fielded in late 2013 in a ground terminal for the Lunar Laser Communication Demonstration, and were used to downlink optical communication data from Lunar Orbit at 79 Mbps.
This work was done by Matthew D. Shaw, Andrew D. Beyer, Kevin M. Birnbaum, William H. Farr, Francesco Marsili, and Jeffrey Stern of Caltech; and Sae Woo Nam of National Institute of Standards and Technology for NASA’s Jet Propulsion Laboratory. NPO-49420