Classical and Low-Light-Level Detection and Pulse Characterization using Optical-Frequency Mixers

Classical all-optical signal processing for telecommunication applications greatly benefits from the availability of highly efficient optical-frequency (OF) mixers, the optical analogue of radio frequency mixers used in RF signal processing. The OF mixers presented in this dissertation are based on reverse-proton-exchange (RPE) periodically poled lithium niobate (PPLN) waveguides, one of the most efficient and versatile material systems in the field of nonlinear optics. Taking advantage of fabrication technologies developed in Prof. Martin Fejer's group over the past two decades, we expand the range of applications for these OF mixers to low-light-level signal detection and pulse characterization. We demonstrate high-speed high-efficiency single- photon counting at telecommunication wavelengths, used for the implementation of record-breaking quantum-key distribution (QKD) systems, which allow unconditionally secure data transfer. In collaboration with researchers at the MIT Lincoln Laboratory, we also show that the same technology can be used to achieve an order-of-magnitude improvement in the sensitivity of classical few-photon free-space communication links based on pulse-position modulation. These extremely sensitive receivers (1 photon/bit) are being developed to facilitate deep-space communication over several hundred million kilometers between Mars and Earth.

OF mixers can also be used to fully characterize, potentially weak, ultrashort pulses, as well as time-magnify segments of ultra-high-speed data streams to be detected in real time by conventional streak cameras and oscilloscopes. We will present a novel implementation of both collinear autocorrelation as well as parametric temporal imaging (in collaboration with the Lawrence Livermore National Laboratory) based on mode-multiplexing in integrated asymmetric Y-junctions in combination with linearly-chirped apodized quasi-phasematching gratings. For the first time, background-free autocorrelation, frequency-resolved optical gating, and temporal imaging can be realized in single-polarization-guiding collinear waveguide structures at sub-60-aJ (400 photons/pulse) levels.

Recently, guided-wave OF mixers have also become important for precision metrology applications based on frequency-comb generation (FCG) (i.e. optical ruler) using ultrashort pulses. The most compact and energy efficient FCG systems use fiber lasers. In collaboration with IMRA America, Inc., we demonstrate that RPE PPLN waveguides can be used to implement fully integrated fiber-laser-based FCG systems taking advantage of unprecedented octave-spanning spectral broadening of the input pulses in combination with simultaneous phase sensing inside the same waveguide.