Etched Diffraction Grating Simulation And Optimization Design

With the use of Wavelength Division Multiplexing (WM) technology, the information capacity of an optical communication network is greatly increased.Wavelength multiplexers and demultiplexers are key components in a WDM optical network. Among various technologies to implement the multiplexing/demultiplexing functionality, etched diffraction gratings (EDGs) have shown great potential due to their compactness and high spectral finesse. Conventional numerical method of grating simulations cannot be used to simulate a concave grating with a large size in terms of the wavelength. Moreover, the essential polarization dependent characteristics of the diffraction grating cannot be treated with the scalar method.Therefore, one needs a more accurate simulation tool which can take the polarization effects of EDGs into consideration.In this thesis, the polarization dependent characteristics of an etched diffraction grating demultiplexer are analyzed using the boundary element method (BEM) for both an echelle grating coated with a metal and a dielectric grating with retro-reflecting facets. For EDGs with a metal coated, a more effective method of moment (MoM) is also presented to calculate the surface current, which produces the diffracted field at the image plane, for both polarizations. Futhermore, a fast simulation method for EDGs with retro-reflecting facets is presented based on the Kirchhoff-Huygens principle and the Goos-Hanchen shift. The simple method has a good agreement with a BEM for a wide range of practical device parameters. Using these numerical methods, many performances of the device (e.g., loss, polarization dependent loss, crosstalk, chromatic dispersion, return loss) are analyzed detailed and an insightful physical explanation for the numerical results are also given.Some novel designs are presented in order to improve performances of EDGs. These designs are as follows: (1) A flat-topped EDG demultiplexer with low chromatic dispersion is designed. A parabolic multimode interference (MMI) section with a reshaping taper is connected at the end of the input waveguide. The field distribution at the end of the taper is reshaped optimally to have sharp transitions. A genetic algorithm is used to optimize the parabolic MMI section. The designed EDG demultiplexer has an excellent fiat-topped spectral response and a very low chromatic dispersion characteristic. (2) By making the surface of shaded facets with an appropriate roughness, the PDL of the demultiplexer can effectively be improved in a large bandwith. The PDL of an EDG demultiplexer can be reduced near the central wavelength at some special values for the radius of the rounded comers (or with special values for the tilting of shaded facets). (3)An EDG demultiplexer with high sidelobe suppression is designed. Sidelobes resulting from two adjacent wavelengths are suppressed by etching two optimized rectangular air trenches in front of each output waveguide, which can induce large resonance loss to the adjacent wavelength whereas have little influence on the operation wavelength. The designed EDG demultiplexer can present a crosstalk as small as 50 dB in theory. (4) A design for EDG demultiplexers is presented to reduce the return loss. The input waveguide is placed on the minimal intensity position of the diffraction envelope. Then, by further chirping the diffraction order for each facet, we minimize the envelope intensity for other adjacent diffraction orders, which can contribute to a negligible return loss for a large spectral width. (5) A design for EDG demultiplexers is presented to obtain both large grating facets and a larger free spectral range (FSR) using the optimal chirped diffraction orders for different facets. The large grating facets by using a large diffraction order contribute to lowering the manufacturing difficulty, and can provide lower polarization dependent loss. However, usually the large diffraction order must lower the FSR of the device by all means. Based on the present design, the FSR can overlay the whole diffraction envelop for a special order, but avoid the influence of the adjacent diffraction envelops. Calculations indicate that the extinction ratio between the operated order and adjacent orders can attain a 35 dB or so, which makes the crosstalk from channels in adjacent envelops to those in the operated diffraction envelop acceptable.The fabrication tolerances (e.g., rounded effect, surface roughness and point defect in the waveguid) to the performance (such as the insertion loss, the polarization dependent loss and the chromatic dispersion) of an EDG demultiplexer are detailed analyzed in the thesis. Based on silica on silicon, all the manufacture processes are implemented. At last the fabricating of some EDG chip samples is made. Some measurements are helpful for imporved fabrications in future.