Design and fabrication of polarization-insensitive polymer optical waveguide devices
Student thesis: Doctoral Thesis
Related Research Unit(s)
Optical integrated circuit plays an important role in modern optical communication systems. However, the performance of integrated-optic devices is usually sensitive to the polarization state of light, because the two orthogonal polarization components of light propagating in the device may have different propagation constants and different propagation losses. When light is launched into a polarization-sensitive device from a single-mode fiber, because the polarization state at the output light of the fiber varies randomly, the performance of the device becomes unstable. This issue is particularly serious for application in optical fiber communications. The most direct way to circumvent this problem is to design inherently polarization-insensitive devices. This thesis is concerned with the design and fabrication of some basic polarization-insensitive waveguide devices by control of the geometric parameters of the waveguides. The thesis starts with the design and fabrication of zero-birefringence strip and rib waveguides with benzocyclobutene (BCB) polymer, which attracts a lot of attention as a competitive waveguide material. Polymer waveguides permit mass production at a low cost, and are readily integratable on a wide range of substrate materials, such as glass, silicon dioxide, and silicon. However, the large thermal stress in polymer films leads to stress-induced birefringence that makes the design of zero-birefringence waveguides more complicated. To take into account the stress-induced birefringence, the design formulas that were derived originally for isotropic strip waveguides are generalized. The measurement results agree well with theoretical calculations, which confirms the feasibility of achieving zero birefringence with such a waveguide structure. It is also demonstrated that zerobirefringence or near zero-birefringence conditions can be maintained over a wide range of temperature. An immediate application of zero-birefringence waveguides is in the design of polarization-insensitive Bragg waveguide gratings. Corrugation Bragg gratings are fabricated on BCB channel waveguides by excimer laser ablation through a phase mask. The polarization dependence of the Bragg wavelengths of the gratings are measured and compared with simulation results. Polarization-insensitive Bragg waveguide gratings with a wide tuning range are demonstrated. Directional coupler in the form of two parallel waveguides is also studied theoretically and experimentally. The structure is analyzed by the effective-index method with built-in perturbation correction. It is shown that polarizationinsensitive operation of the coupler can be achieved by controlling the physical parameters of the waveguides. A number of couplers with different waveguide dimensions are fabricated with BCB polymer. Experimental results are compared with theoretical calculations with the thermal stress in the waveguides taken into account. Furthermore, thermal tuning is shown to be a practical method of maintaining the polarization-insensitive condition of the coupler. The results presented in the thesis are expected to be useful for the realization of a wide range of polarization-insensitive polymer devices.
- Polymers, Optical properties, Optical wave guides