Development Of Sidewall Long-Period Grating Assisted Polymer Waveguide Directional Coupler

Directional coupler and integrated optical device based on directional coupler is widely used to transmit and process optical signals in optical fiber communication system, it is the basic composition unit of a wavelength division multiplexer, an optical matrix switch, an add-drop multiplexer, a filter, a power mixer/distributor, a modulator, etc. Mode division multiplexing technology, which uses individual spatial modes as independent optical signal channel to transmit optical signals, is a new technology proposed in recent years, to improve transmission capacity of optical fiber communication system. Mode converter and mode multiplexer/demultiplexer are indispensable core components in mode division multiplexing system. Mode converter and mode multiplexer/demultiplexer with directional coupler as basic unit have advantages of simple structure, low production cost, material variety, and so on. It is of great significance for the research of mode division multiplexing technology.On the background of the mode division multiplexing technology, with grating assisted directional coupler as the basic structure, a kind of sidewall long-period grating assisted polymer waveguide directional coupler has been put forward in this dissertation. The proposed directional coupler consists of a single-mode waveguide and a few-mode waveguide which supports three spatial modes. The objective of the proposed directional coupler is to achieve the coupling between the fundamental modes of the single-mode waveguide and the few-mode waveguide, while the fundamental mode of the single-mode waveguide will not couple with the high order modes in the few-mode waveguide. The sidewall long-period grating along the inner side of the few-mode waveguide is used to compensate for the mode refractive index difference of the fundamental modes between the few-mode waveguide and the single-mode waveguide. The directional coupler is made from polymer materials, EpoCore and EpoClad, both are used as core layer and cladding layer, respectively. To find the optimal device parameters, simulation works have been done for the proposed sidewall long-period grating assisted polymer waveguide directional coupler, then device samples have been fabricated and characterized, and the results prove the feasibility of the scheme in theory and in experiment. From the point of mode division multiplexing technology, the sidewall long-period grating assisted polymer waveguide directional coupler proposed in this dissertation has realized the multiplex/demultiplex of fundamental mode channel in the mode division multiplexer/demultiplexer. The main work is as follows:(1) From the perspective of coupled mode theory, the basic working principles of the symmetrical directional coupler, the asymmetric directional coupler, and the grating assisted directional coupler was analyzed.(2) Simulation works were done to determine all the design parameters of the proposed sidewall long-period grating assisted polymer waveguide directional coupler, such as the widths, the thicknesses of the single-mode waveguide and the few-mode waveguide, and the gap between the single-mode waveguide and the few-mode waveguide, the grating period, the grating depth, the coupling length, and so on.(3) By the process steps of spin-coating, lithography, development, and curing, sample of the sidewall long-period grating assisted directional coupler, which meets the requirements of the design parameters, was fabricated.(4) To characterize the fabricated devices, the output infrared near-field pattern, the optical spectral characteristics, and the thermal tuning characteristics of the fabricated sidewall long-period grating assisted polymer waveguide directional coupler were measured. Moreover, the propagation losses of the fundamental modes of the single-mode waveguide and the few-mode waveguide were measured by cutback method, respectively. Also the additional loss caused by the sidewall long-period grating was measured for the fundamental mode of the few-mode waveguide.