Project Report – Polymeric waveguide devices

Integrated optics, or guided wave optics, is playing an increasingly important role in optical communication networks and optical sensor systems today. Optical devices with diverse functionality based on different materials have been demonstrated. Recently, polymeric waveguide devices have attracted a great deal of attention because polymer materials have many unique properties, such as ease of fabrication, low-production costs, and compatibility with Si and GaAs fabrication technologies as compared to materials such as silica and III-V compound semiconductors. All these are important factors for the practical implementation of complex, high-density interconnects and circuits. The low-temperature fabrication process of polymer devices gives the designer a large degree of freedom. In addition, polymer materials can be thermally tuned over a wider spectral range as they have thermo-optic coefficients an order of magnitude larger than that of silica. These polymers also exhibit low thermal conductivities and large thermal index changes. Unlike most rigid waveguide materials, polymers can be deposited directly on any kind of flat or curved substrates so that blend or foldable devices can be achieved easily. Tailored polymer materials offer high structural flexibility through different combinations of the monomers, and an accurate control of the material’s refractive indices and properties can be obtained. Various techniques, such as reactive ion beam etching, photo-bleaching and ion-implantation processes, have been used to fabricate polymeric channel waveguides and devices. These methods involve many processing steps and have a long fabrication time and low yield. Strong efforts are hence focused on developing effective, novel and simple fabrication technologies for polymer device fabrication. In my Ph.D. study, electron beam direct writing for optical devices has been developed. It is advantageous over existing techniques because fewer steps are involved. It has the advantage of being mask-less, allowing rapid and inexpensive prototyping, in contrast to a conventional mask-based photolithographic approach in which a mask must be made before the waveguide devices can be fabricated. Nanometer patterns with flexibility in writing complex structures are also possible. The technique has high flexibility because all the patterns are designed using computer files, and simply changing the contents of the files can complete the design modification. Moreover, for the sake of achieving low cost and high throughput, an ultraviolet (UV) direct printing methodology for active devices has also been developed. UV direct printing takes advantage of UV photolithography with the utilization of a photo-sensitive polymer for the realization of active devices, and it is suitable for mass production with high yield in industries. However, the selection of suitable polymer in both fabrication methodologies is a crucial factor. Among all polymers, Epoxy novolak resin polymer (ENR) (also known as NANOTM SU8 2000) from MicroChem Corporation was chosen as the waveguiding core due to its distinct properties. With the aid of acid catalysis, ENR is a negative tone polymer sensitive to both UV and electron beam radiation, thus optical waveguides and devices can be achieved directly upon exposure and development without any further processing. In this thesis, a discussion of the ENR polymer, particularly on its curing chemistry and processing parameters, is included. A comprehensive coverage of various material characterization techniques for the analysis of the optical, thermal, spectroscopic and lithographic properties of ENR polymer are also included. To extend the application of polymers to active devices, rare earth ions have to be incorporated in host materials as amplification medium. However, the incorporation of rare earth ions in polymers is difficult because most of the rare earth ions are in inorganic salt forms. Inorganic salts do not mix well with polymers and coagulation usually occurs. As a result, a new approach has been developed for the synthesis of the rare earth doped polymer, in which organic rare earth derivatives were used for mixing with ENR polymer. Er3+-Yb3+ codoped as well as Nd3+ doped compounds with different doping parameters were synthesized for realization of active devices. A comprehensive study on the properties of newly synthesized Er3+-Yb3+ codoped and Nd3+ doped polymers was carried out to determine their use as active guiding layers. Investigations were made on two sets of doping series with different concentrations and different weight percentage combinations. Based on the material characteristics, it is quite obvious that pure and doped ENR polymers are appropriate candidates for the production of various photonic components for applications in telecommunications. Once the appropriate optical material is found, it is natural to study the realization of functional components. Thus, a variety of optical devices, both passive and active, have been demonstrated on ENR polymer using either electron beam direct writing or UV direct printing. As a fundamental component, optical waveguide plays an important role in integrated optical circuits. Low-loss optical waveguides based on the ENR polymer have been fabricated using electron beam direct writing. The single mode channel waveguides fabricated have low propagation losses at telecommunication wavelengths, smooth surface, sharp profile and high environmental stability. Waveguide grating devices are important too, and they are key components for constructing transmitter and receiver terminals in WDM systems. They are small and can be integrated with other components on the same substrate. Application examples include add/drop multiplexers and dispersion compensators. Polymeric waveguide wavelength filters based on ENR polymer have been fabricated using electron beam direct writing. Both the waveguides and the gratings were exposed simultaneously, which effectively eliminate the alignment errors that can be generated during fabrication. A 6μm channel waveguide with a 5mm long first order Bragg grating and a transmission peak of –27dB was demonstrated. The effect of temperature on the wavelength dependence of the device response was also characterized. A linear shift has been obtained with the heating power, and dλ/dT is found to be ~-0.14nm/oC. As alignment problems are eliminated using the electron beam direct writing process, the tuning capability of the device can further increase the flexibility of channel selection in WDM systems. These results are comparable to those reported using multiple processing steps. Besides, binary optics elements like Fresnel lens have also been fabricated. Another interesting research area is the demonstration of active devices like optical waveguide amplifiers. In metropolitan area networks (MANs) and local area networks (LANs), losses in interconnections or from device components can affect the overall system performances. To allow recovery and maintain efficient signal transfer, waveguide amplifiers are inevitable and essential components in the optical communication systems. In order to achieve optical amplification in optical waveguides, the guiding core is doped with rare-earth ions. Among all the rare earth ions demonstrated, erbium (Er3+) ions have received special attention because the 4I13/2 – 4I15/2 transition near 1540nm wavelength matches one of the fiber low-loss windows. However, only the simulated gain has been studied in Er3+ doped polymeric devices up to now. Planar waveguide amplifiers doped with Er3+ and Yb3+ ions based on ENR polymer were also demonstrated using electron beam direct writing. Signal enhancement ~13 dB at 110 mW pump power was measured in an 18 mm long device using an input signal power of >-18 dB m. In addition, polymer channel waveguide amplifier arrays were fabricated using ultraviolet (UV) direct printing methodology. Large area and low cost patterning of polymer active devices can be obtained using the UV direct printing methodology, which speeds up and simplifies the fabrication and production of polymer active devices, and has its own intrinsic advantages over other direct write techniques that rely on more sophisticated equipment. Hence, UV direct printing is a suitable technique for mass production of active polymer devices, and one of the potential applications is the fabrication of low cost laser arrays. To summarize, cost-effective functional optical devices are required for expanding optical communication networks, and integrated optics or planar waveguide technology is expected to play a major role. From material characterization to processing to device fabrication, the work described in this thesis provides a comprehensive and systemic study on the realization of a new generation of polymer devices, thus opening up a new platform for more novel polymer planar lightwave circuits.

Author: Wong, Wing Han

Source: City University of Hong Kong

Download URL 2: Visit Now

Leave a Comment