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Title: Realization of long-period waveguide grating coupler
Other Titles: Chang zhou qi guang bo dao guang shan ou he qi de zhi zuo
Authors: Chow, Chi Kin (周梓健)
Department: Department of Electronic Engineering
Degree: Master of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Optical wave guides.
Bragg gratings.
Notes: CityU Call Number: TK7871.65 .C45 2009
xiv, 132 leaves : ill. 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2009.
Includes bibliographical references.
Type: thesis
Abstract: Long-period fiber grating (LPFG) allows the coupling of light from the fundamental core mode to the selected cladding modes in a single-mode optical fiber at specific resonance wavelengths. The light that coupled to the cladding modes is usually attenuated or radiated out during transmission, thus resulting in a series of rejection bands in the transmission spectrum. An LPFG can basically function as a band-rejection filter. To turn such grating into a band-pass filter or add/drop multiplexer, grating couplers based on parallel LPFGs have been demonstrated recently. In an LPFG coupler, two or more identical LPFGs are packaged together in parallel, and the light that is coupled to the cladding modes by the grating of the launching core can be collected by the grating of the other cores. However, due to the geometry and material inherent limitations of the optical fiber, the packaging of multi LPFGs in a perfect parallel for providing a practical add/drop function is still a major technique challenge. To relax the limitations of the LPFG, long-period waveguide gratings (LPWGs) are proposed. A thin-film optical waveguide provides more flexibility in the grating structure and material design. Parallel LPWGs can be easily integrated together on the same substrate. A number of benzocyclobutene (BCB) polymer LPWG devices in various grating structures have been reported recently. Due to the UV and thermal-optical sensitive properties of the BCB polymer, LPWG can be either formed permanently by UV-writing (index modulation) or thermal-optically induced dynamically by electrode grating. In this thesis, we present our in-depth study on the attainment of a polymer-based LPWG coupler device. The thesis begins with a discussion on the theory of the LPWG coupler using a design example. The detailed fabrication techniques and procedures for a polymer channel waveguide and gratings are also outlined. A passive LPWG coupler is experimentally demonstrated by the UV-writing technique, where the gratings are written into two parallel BCB waveguide cores by UV irradiation after the waveguide had been fabricated and fully characterized. With this technique, the grating pitch can be determined more accurately and the grating strength can be optimized by controlling the UV dosage. The experimental results show that the coupler can offer a wavelength thermal-tuning range exceeding 120 nm with a coupling efficiency as high as ~80% and ~60% for the TE and TM polarizations, respectively. A polarization-independent resonance wavelength property at a specific temperature is also observed and characterized. This coupler has the potential to be developed into a practical broadband add/drop multiplexer. Finally, the possibility of creating an active tunable LWPG coupler by forming a metal electrode grating onto a two-core BCB waveguide is investigated. The idea behind an active LPWG coupler is that the gratings can be induced by the electrode grating using the thermo-optic effect. In our study, we find that the cladding modes degeneracy problem must be avoided; otherwise, light coupling may not occur between the two gratings of the coupler through the common cladding. This problem is mainly due to a large number of cladding modes appearing in the cladding layer. By using a finite width cladding to reduce the number of cladding modes, the problem of cladding modes degeneracy can be overcome.
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