Design and fabrication of long-period waveguide gratings
Student thesis: Doctoral Thesis
Related Research Unit(s)
A long-period grating is capable of coupling light between the guided mode and the co-propagating cladding modes at specific wavelengths and hence results in a series of sharp rejection bands in the transmission spectrum. Long-period grating in fibers, i.e., long-period fiber gratings (LPFGs) have been extensively studied and found numerous applications, such as filters, gain flatteners for erbium-doped fiber amplifiers, and sensors, etc. However, the geometry and material constraints of a fiber impose significant limitations on the functions that an LPFG can achieve, especially on the realization of active devices. Moreover, optical fiber is not suitable for making compact low-cost devices and does not satisfy the demand for mass production and integration. Therefore, to remove the constraints of the fiber and develop integrated-optical filters, long-period waveguide gratings (LPWGs), i.e., long-period gratings formed in planar waveguides have been proposed. Although the light-coupling mechanisms in an LPWG and an LPFG are basically the same, LPWGs exhibit much richer optical characteristics because of the additional degrees of freedom available in the design and fabrication of optical waveguides. The design, fabrication, and characterization of LPWGs in various waveguide geometries constitute the major part of this thesis. To start with, by using the coupled-mode theory, general formulae for the calculation of the characteristics of an LPWG, including the resonance wavelength, the coupling coefficient, the transmission spectrum, and the 3-dB bandwidth, are derived. The sensitivity characteristics of an LPWG, in particular, the temperature sensitivity of the resonance wavelength, are also investigated. The general theory is next applied to the analysis of LPWGs in slab waveguides, ridge waveguides, and channel waveguides. Extensive simulation results are presented to highlight the features of these LPWGs. It is found that LPWGs in different waveguide structures exhibit different characteristics. For instance, the cladding thickness is an effective parameter for the control of the temperature and polarization-dependence properties of LPWGs in slab and ridge waveguides, while for LPWGs in channel waveguides, the control of the dimensions of the waveguide core becomes more important. Various ways of fabricating LPWGs are proposed and demonstrated. In particular, LPWGs are fabricated in slab and channel waveguides using polymer materials and glass by conventional photolithography and reactive ion etching techniques. LPWGs are also written in slab and ridge waveguides by a direct UVwriting technique using a 248 nm excimer laser, which allows real-time monitoring of the growth of the transmission spectrum of the grating. The LPWGs can be designed to show excellent thermal-optic tuning characteristics and function as widely tunable filters. Moreover, it is possible to control their wavelength tunability and polarization dependence by controlling the waveguide dimensions. Finally, the possibility of controlling the transmission spectrum of an LPWG device by controlling the profile of the waveguide cladding along the grating (waveguide cladding apodization) is investigated. Waveguide cladding apodization can be used as a convenient post-processing technique to tune the characteristics of an LPWG. The idea is demonstrated experimentally with a phase-shifted LPWG and a chirped LPWG.
- Optical wave guides