Ph.D Thesis

Ph.D StudentWillinger Amnon
SubjectParametric Processes in Dispersion Engineered Photonic
Crystal Waveguides
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein


In optical signal processing, information is carried on an electromagnetic wave and is being amplified, filtered or manipulated, usually before arriving at an opto-electrical receiver. A Photonic Crystal (PhC) waveguide (PCW) is a device that has attractive nonlinear properties for signal processing purposes. It is also chip-scale in size, making it a good candidate for integration in future compact communication systems.

A Photonic Crystal (PhC) is a periodic structure composed of materials with different optical properties. Depending on its design, a PhC allows light in specific spectral range to propagate through it in some directions, while rejecting propagation in other wavelengths and directions. The devices we experiment with and simulate comprise a 170nm to 180nm thick membrane made of GaInP, patterned with air holes having radii between 100nm to 120nm. To match the optical wavelengths of the telecom band (1550nm), the holes are ordered in a hexagonal array with a periodicity of about 500nm. A missing line of holes, called the defect line, constitutes a waveguide core in which light can propagate. The exact formation determines the propagation parameters in the waveguide. This is termed dispersion-engineering since by designing different types of hole and defect line geometries, it is possible to fashion numerous types of propagation properties with interesting dispersion functions, which can suppress or enhance nonlinearities.

In PCWs the spatial distribution of the optical fields has a very small cross section, leading to highly nonlinear properties. This contributes to the use of the PCWs for parametric amplification, where a strong pumping wave would amplify a phase-matched signal by means of four-wave mixing (FWM). Moreover, the band-gap of GaInP (1.9eV) prevents two-photon absorption at 1550nm and therefore GaInP is an ideal semiconductor for nonlinear PCWs. Since phase-matching is crucial for effective parametric interactions, dispersion engineering plays a role as it affects the dispersion in propagation coefficients of the interacting waves.

In this thesis we examine the potential PCWs have for parametric interactions. We measure for the first time narrowband parametric interactions in a semiconductor waveguide, obtained by achieving phase matching between largely detuned waves. The interaction can be tuned, in both optimal signal wavelength and bandwidth, by changes of the pump wavelength, thus acting similar to an optically tuned filter. Another interesting type of interaction is an advance parametric process of phase-sensitive amplification. In such operation mode, the signal gain depends on its input relative phase compared to the phases of two equally detuned high-intensity pumping waves. In coherent communication systems, when data is encoded on the signal phase, this is advantageous since out-of-phase noise accompanying the signal is reduced.

The research combines experiments with comprehensive modeling. We have developed an advanced numerical tool which accounts for the severe dispersion in propagation parameters and losses in these nano-structure PhC waveguide devices. In order to account for the dispersive nature of the nonlinear parameters, we developed a computation algorithm called Multiple-envelope Split-Step Fourier Transform which is superior in the cases we study over the common propagation algorithms.