Highly Efficient Optical Frequency Mixers

This dissertation describes the development of highly efficient optical frequency mixers that build on previous work at Stanford to produce the most efficient nonlinear devices reported to date. Frequency mixing is an integral part of any broadband communications system. The ability to transfer information between carrier frequencies allows for efficient use of the available bandwidth in a transmission medium. In the electrical domain, this mixing function is very efficiently implemented using compact semiconductor devices, whose nonlinear current-voltage relationship results in the generation of output signals with frequencies at linear combinations of those of the inputs. The development of optical frequency analogs to these devices is complicated by the combination of weak nonlinearities available at optical frequencies (which necessitate the use of distributed mixers), and material dispersion. The basic techniques suited to circumventing these difficulties have been understood for decades, however the practical implementation of reasonably efficient mixers has only been achieved in the last ten years. Proton-exchanged waveguides in periodically-poled lithium niobate have proven to be the material of choice for these devices; this technology has been used to demonstrate a variety of useful all-optical signal processing functions in the last few years.

We begin with a general description of optical frequency mixing interactions, followed by a discussion of device design and fabrication. Optimization of the annealed proton-exchange process and development of buried waveguides using reverse proton exchange have resulted in a more than three-fold improvement in conversion efficiency. The high quality of these waveguides has permitted observation of several interesting nonlinear optical phenomena for the first time, including 99% pump depletion in single-pass second harmonic generation and the distortion in device tuning behavior that accompanies this “extreme” nonlinear optical regime. Telecommunications applications of these mixers are also presented, where all-optical gating and wavelength conversion with record efficiency have been observed.