Publications

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Passive silicon photonic waveguides are exposed to gamma radiation to understand how the performance of silicon photonic integrated circuits is affected in harsh environments such as space or high energy physics experiments. The propagation loss and group index of the mode guided by these waveguides is characterized by implementing a phase sensitive swept-wavelength interferometric method. We find that the propagation loss associated with each waveguide geometry explored in this study slightly increases at absorbed doses of up to 100 krad (Si). The measured change in group index associated with the same waveguide geometries is negligibly changed after exposure. Additionally, we show that the post-exposure degradation of these waveguides can be improved through heat treatment.

Silicon photonics is a platform that enables densely integrated photonic components and systems and integration with electronic circuits. Depletion mode modulators designed on this platform suffer from a fundamental frequency response limit due to the mobility of carriers in silicon. Lithium niobate-based modulators have demonstrated high performance, but the material is difficult to process and cannot be easily integrated with other photonic components and electronics. In this manuscript, we simultaneously take advantage of the benefits of silicon photonics and the Pockels effect in lithium niobate by heterogeneously integrating silicon photonic-integrated circuits with thin-film lithium niobate samples. We demonstrate the most CMOS-compatible thin-film lithium niobate modulator to date, which has electro-optic 3 dB bandwidths of 30.6 GHz and half-wave voltages of 6.7 V×cm. These modulators are fabricated entirely in CMOS facilities, with the exception of the bonding of a thin-film lithium niobate sample post fabrication, and require no etching of lithium niobate.

We present a 30 GHz heterogeneously integrated silicon photonic/lithium niobate Mach-Zehnder modulator simultaneously utilizing the strong Pockels effect in LiNbO3 while also taking advantage of the ability for photonic/electronic integration and mass production associated with silicon photonics. Aside from the final step of bonding the LiNbO3, this modulator can be entirely fabricated using CMOS facilities.

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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.

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We present a quantitative analysis of the correlation of resonant wavelength variation with process variables, and find that 50% of the resonant wavelength variation for microrings is due to systematic process conditions. We also discuss the improvement of device uniformity by mitigating these systematic variations.

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We demonstrate a silicon photonic transceiver circuit for high-speed discrete variable quantum key distribution that employs a common structure for transmit and receive functions. The device is intended for use in polarization-based quantum cryptographic protocols, such as BB84. Our characterization indicates that the circuit can generate the four BB84 states (TE/TM/45°/135° linear polarizations) with >30 dB polarization extinction ratios and gigabit per second modulation speed, and is capable of decoding any polarization bases differing by 90° with high extinction ratios.

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We demonstrate a silicon photonic transceiver circuit to implement polarization encoding/decoding for DV-QKD. The circuit is capable of encoding BB84 states with >30 dB PER and decoding with >20 dB ER.

We demonstrate an on-chip polarization beam splitter (PBS), which is adiabatic for the transverse magnetic mode, and diabatic for the transverse electric mode. The PBS has a simple structure that is tolerant to manufacturing variations and exhibits high polarization extinction ratios over a wide bandwidth.

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