High-data-rate deep space optical communication (DSOC) links are desired by NASA and other space agencies to support future advanced science instruments, live high-definition (HD) video feeds, telepresence, and human exploration of Mars and beyond. Optical communications can provide a 10 to 100× increase in data rates from deep space for equivalent spacecraft mass and power as compared to state-of-the-art deep space Ka-band RF communication systems. One of the key technologies for DSOC is a large-area photon-counting detector array for the ground-based receiver.
A new type of SNSPD array based on tungsten silicide (WSi) instead of NbN was developed. WSi is amorphous, so it does not have the strict dependence on the crystal phase affecting NbN, which greatly simplifies the material deposition. Furthermore, WSi has a lower superconducting energy gap than NbN, easing the need for ultra-narrow wires and allowing wires to be reliably fabricated on a large scale using photolithography. The pixel yield is far higher than in NbN, with 93% of the pixels functioning on a recently measured device, and overall device yield of ≈70% across a wafer. This technology is readily scalable to the telescope aperture sizes required for future deep space ground stations.
A gold mirror layer was deposited on an oxidized silicon wafer, and amorphous-state WSi was sputtered from a compound target at a thickness of 5 nm. The WSi nanowire was embedded at the center of a three-layer vertical optical cavity consisting of two silica layers and a titanium oxide antireflective coating. The layer thicknesses were chosen to optimize efficiency at the target communication wavelength of 1,550 nm, and to minimize the polarization dependence of the detector response.
The detector modules differ from the previous state of the art in several key respects. First, the superconducting material used for the nanowires was amorphous WSi, rather than a refractory metal such as NbN or NbTiN. This allowed a much greater active area than has been possible due to the lower incidence of nanowire constrictions. In the future, this will allow coupling to a much larger telescope aperture than has previously been demonstrated in an optical communication experiment.
This work was done by Matthew D. Shaw, Francesco Marsili, Andrew D. Beyer, and Jeffrey A. Stern of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49463