Michael Gehl

The Atmospheric Boundary Layer (ABL) is the portion of the troposphere that is directly influenced by the Earth’s surface and responds to combined action of mechanical and thermal forcing. Most of the energy exchange with respect to solar heating and evaporation that drive the atmosphere and the ocean occur within the ABL, yet it is one of the most poorly observed and modeled regions of the atmosphere. Conventional passive microwave systems fall well short of being optimized for near surface sensing due to limited number of spectral channels and coarse spectral resolution covering only a small portion of the spectrum of interest for ABL sensing. The so called “window regions” of the microwave spectrum between and on the shoulders of the strong oxygen and water vapor absorption lines carry the information on the near surface thermodynamic structure in the boundary layer. Sampling these regions requires new spectrometers capable of resolving >50 GHz spectral regions at modest spectral resolution (~1GHz). The ultra-wideband photonic spectro-radiometer instrument is funded by NASA ESTO under ACT-20 program to combine low-noise wideband RF technology with a novel photonic integrated circuit (PIC) design for obtaining large bandwidth (>50 GHz) with enhanced channel resolution (

Phase errors in large optical phased arrays degrade beam quality and must be actively corrected. Using a novel, low-power electro-optic design with matched pathlengths, we demonstrate simplified optimization and reduced sensitivity to wavelength and temperature.

We demonstrate the first silicon photonic single-sideband (SSB) modulator with dual-parallel Mach-Zehnder modulators (MZMs) operating near 1550 nm with a measured carrier suppression of 27 dB and at least 12 dB sideband suppression at 1 GHz.

Measurement uncertainties in the techniques used to characterize loss in photonic waveguides becomes a significant issue as waveguide loss is reduced through improved fabrication technology. Typical loss measurement techniques involve environmentally unknown parameters such as facet reflectivity or varying coupling efficiencies, which directly contribute to the uncertainty of the measurement. We present a loss measurement technique, which takes advantage of the differential loss between multiple paths in an arrayed waveguide structure, in which we are able to gather statistics on propagation loss from several waveguides in a single measurement. This arrayed waveguide structure is characterized using a swept-wavelength interferometer, enabling the analysis of the arrayed waveguide transmission as a function of group delay between waveguides. Loss extraction is only dependent on the differential path length between arrayed waveguides and is therefore extracted independently from on and off-chip coupling efficiencies, which proves to be an accurate and reliable method of loss characterization. This method is applied to characterize the loss of the silicon photonic platform at Sandia Labs with an uncertainty of less than 0.06 dB/cm.

Significant progress has been made in the field of silicon photonics over the past two decades. Silicon photonics provides substantial performance advantages in many data processing and transfer applications. Until now, the benefits of active silicon photonics in cryogenic systems have remained unexplored. Here we demonstrate the operation of a high-speed, CMOS compatible silicon micro-disk modulator transmitting data at rates up to 10 Gb/s and at temperatures down to 4.8 K. This opens the door for the use of silicon photonics as an interface to low temperature, bandwidth intensive systems such as high-performance superconducting-based computing and integrated quantum optics circuits interfacing to superconducting single-photon detectors.

Arrayed waveguide gratings provide flexible spectral filtering functionality for integrated photonic applications. Achieving narrow channel spacing requires long optical path lengths which can greatly increase the footprint of devices. High index contrast waveguides, such as those fabricated in silicon-on-insulator wafers, allow tight waveguide bends which can be used to create much more compact designs. Both the long optical path lengths and the high index contrast contribute to significant optical phase error as light propagates through the device. Therefore, silicon photonic arrayed waveguide gratings require active or passive phase correction following fabrication. Here we present the design and fabrication of compact silicon photonic arrayed waveguide gratings with channel spacings of 50, 10 and 1 GHz. The largest device, with 11 channels of 1 GHz spacing, has a footprint of only 1.1 cm². Using integrated thermo-optic phase shifters, the phase error is actively corrected. We present two methods of phase error correction and demonstrate state-of-the-art cross-talk performance for high index contrast arrayed waveguide gratings. As a demonstration of possible applications, we perform RF channelization with 1 GHz resolution. Additionally, we generate unique spectral filters by applying non-zero phase offsets calculated by the Gerchberg Saxton algorithm.

Currently, superconducting qubits lead the way in potential candidates for quantum computing. At the same time, transferring quantum information over long distances typically relies on the use of photons as the elementary qubit. Converting between stationary electronic qubits in superconducting systems and traveling photonic qubits is a challenging yet necessary goal for the interface of quantum computing and communication. One promising path to achieving this goal appears to be the integration of superconductivity with optically active semiconductors, with quantum information being transferred between the two by means of the superconducting proximity effect. Obtaining good interfaces between superconductors and semiconductors is the next obvious step for improving these hybrid systems. Here, we report on our observation of superconductivity in a 2.3 μm diameter self-assembled indium structure grown epitaxially on the surface of a semiconductor material.