Polymer based multimode interference optical devices

Abstract

A research on polymer based optical waveguides and devices have been a topic of great interest in optical communications due to its pertinent advantages which include versatility and reduction in fabrication cost. This thesis is significantly devoted towards the first ever development of single mode optical waveguides and multimode interference (MMI) interconnecting devices based on photosensitive BenzoCyclobutene (BCB 4024-40) polymer. The developed MMI optical devices are splitters, splitter-combiner, cross couplers and Wavelength Division Multiplexing (WDM) coupler. These development process can be divided into four essential stages; material characterization, design and modelling, fabrication and device characterization. In each stage, several important techniques and equipments have been employed. The devices are fabricated on BK7 glass substrate and thin film of silica as a clad using soda lime glass mask of ±0.2 µm resolution. A relatively high propagation loss of 3.55 dB/cm has been observed for single mode waveguide structure, which is due to the resulting sidewall roughness. The splitting uniformity of symmetric MMI splitters were found to be better than 0.6 dB and the insertion loss for all splitter structures were measured to be less than 1.5 dB at 1550 nm wavelength. The 1×2 splitters were interconnected to function as a splitter-combiner which is ideally used as a basic building block for Mach-Zehnder Interferometer. The measured structure yielded an insertion loss of 1.85 dB for device size of 4.2 mm. The insertion loss of the 2×2 and 3×3 cross couplers based on general and paired interference were measured and found to be between 2.5 to 3.5 dB for 6 mm to 10 mm of cavity size. A 1310 nm and 1550 nm WDM coupler is demonstrated in which the device is designed based on a combination of general interference and paired interference mechanisms. The measured crosstalk at 1310 nm is -14.42 dB and at 1550 nm is -20.61 dB. The measured insertion losses were in the range of 3.2 to 3.5 dB for MMI cavity size of 7 mm. A novel 1×2 MMI thermal photonics switch is proposed. The switch uses a ridge waveguide of BCB 4024-40 polymer on silica clad. The proposed structure works well with crosstalk level of -28 dB and low switching power. Significantly, this research has successfully demonstrated the possibility of applying a photosensitive BCB 4024-40 polymer in the low cost development of integrated optics components.