Optical Frequency Mixers Using Three-wave Mixing for Optical Fiber Communications

The rapid evolution of broadband telecommunication systems is creating enormous demands for optoelectronic components with capacities beyond those currently available for transmission, multiplexing/demultiplexing, and switching. Examples of devices that could have major impact on future optical communication systems include all-optical wavelength converters in wavelength-division multiplexed (WDM) systems, all-optical gated mixers in optical time-division multiplexed (TDM) systems, and components for eliminating dispersion and nonlinear effects in fiber links. This dissertation describes the development of optical frequency (OF) mixers, analogous to the RF mixers in microwave signal processing technology, fabricated in periodically poled LiNbO 3 (PPLN) waveguides for optical fiber communications and other all-optical signal processing applications. Such OF mixers would be an enabling technology to realize high capacity and transparency in WDM and in high speed TDM systems.

The OF mixers developed in this dissertation are based on three-wave mixing using nonlinear optical effects based on the second-order nonlinear susceptibility chi(2) . In an OF mixer, the (usually weak) signal at frequency s is mixed with a strong local oscillator at frequency LO to generate an output at frequency out = LO - s. Such an OF mixer is well suited to communication applications. It can easily accommodate terahertz signal bandwidth, has negligible spontaneous emission noise, has no intrinsic frequency chirp, and can up and down convert multiple wavelengths with equal efficiency. Phasematching in the nonlinear frequency-mixing process can be accomplished by quasi-phasematching (QPM) structures, which can be engineered to design devices for various novel functions.

The main challenge with OF mixers is to fabricate a highly efficient device, capable of operating at local oscillator powers of tens of mW. With the development of efficient PPLN waveguides and integrated waveguide structures, several potentially important devices have been demonstrated, including efficient WDM wavelength converters within the 1.5-um-band or between the 1.3-um-band and the 1.5-um-band, and spectral inverters for dispersion compensation in fiber links. By use of non-uniform QPM structures, multiple-channel wavelength converters for dynamic reconfiguration and broadcasting, as well as very broadband converters have also been demonstrated. Applications of OF mixers as optical gated mixers for TDM systems are investigated theoretically.