Electro-optic long-period gratings on lithium-niobate waveguides
Student thesis: Doctoral Thesis
Related Research Unit(s)
Long-period waveguide grating (LPWG) is a useful optical waveguide structure for wavelength selection and control because of its capability of coupling light from the core to the cladding at specific wavelengths. A number of LPWG-based devices, such as variable attenuators, optical filters, and add/drop multiplexers, have been demonstrated experimentally, where the operating wavelength and/or the grating strength can be tuned over a wide range thermo-optically. The thermo-optic gratings, however, are suitable only for low-speed applications. To achieve a tuning speed much higher than that achievable with the thermo-optic effect, it is necessary to employ a much faster effect, such as the electro-optic (EO) effect, to drive the LPWG. Therefore, it is of great practical importance to develop techniques to form LPWGs in EO crystals, especially lithium niobate (LiNbO3), which is the most mature EO material used in the photonics industry. The challenge of using LiNbO3 for such an application rests on the formation of the waveguide structure required for the operation of the grating. The waveguide required must contain a cladding to support a discrete set of cladding modes, so that light can be coupled from the core mode to a cladding mode at a particular wavelength. The cladding refractive index must be lower than that of the core and higher than that of the substrate. The traditional LiNbO3 waveguide fabrication techniques, such as metal in-diffusion and proton exchange, produce a core in a LiNbO3 substrate directly. Because of the large index of LiNbO3 and the small index difference between the core and the substrate, there is virtually no material with the right index that can be placed on the surface of the waveguide to create the required cladding. The present thesis provides a solution to this problem, which leads to the very first experimental demonstration of an EO LPWG formed on a LiNbO3 waveguide. The thesis starts with a discussion of the coupled-mode theory for the calculation of the optical characteristics of an LPWG. The theory is applied to the analysis of an EO LPWG based on a simplified LiNbO3 waveguide model. The electric-field distribution generated by the electrode placed on the LiNbO3 waveguide is calculated with an electrostatics solver. The analysis shows that the performance of an EO LPWG depends sensitively on the electrode configuration, the waveguide parameters, and the EO coefficient distribution. A set of optimal parameters are given as guidance for the experiments. The experimental study focuses on the fabrication of a special single-mode LiNbO3 waveguide suitable for the realization of an LPWG. The waveguide consists of a channel core embedded in a thin slab cladding and is fabricated on a z-cut LiNbO3 substrate with a two-step proton-exchange process. The suitability of the waveguide structure for LPWG applications is verified experimentally with a photoresist grating placed on the waveguide surface. To recover the EO effect lost in the proton-exchange process, the clad LiNbO3 waveguide goes through a reverse proton-exchange process, which creates a thin cover layer on top of the clad channel waveguide. An EO LPWG is finally formed by depositing a SiO2 buffer layer and an Al electrode on the waveguide. Many EO LPWG samples are fabricated with different conditions. The effects of using different cladding modes and waveguide parameters on the grating performance are demonstrated and the experimental results compare well with the theoretical analysis. The temperature sensitivity of the EO LPWG is also investigated. A typical 10-mm long sample shows a 27-dB rejection band at a driving voltage of 95 V with a center wavelength tunable thermally at a sensitivity of −0.4 nm/ºC. Finally, the photorefractive and poling effects and their influences on the performance of the EO LPWG are discussed. The LiNbO3 LPWG provides an EO control of the grating strength and a thermo-optic control of the operating wavelength and thus opens up many new opportunities for high-speed applications.
- Bragg gratings, Optical wave guides, Lithium niobate