Optimization And Fabrication Of Polymer/Si Arrayed Waveguide Grating Device

Human society is entering into an information age powered by rapidly evolving technologies in areas of communications electronics and computing. As the rapid increase of global communication services, it will be a certain tendency of the broadband integrated services network, which is of the increase of speed and the enlargement of capacity in modern communication networks. The application of the wavelength division multiplexing (WDM) technology could solve the problem about the massive increase of speed and the enlargement of capacity broadband information. The all-optical network bases on WDM system is full of agility and environmental stability, and perfectly fit into optical fiber communication systems in the future. Therefore, the WDM technology has already become one of the most important technologies in optical fiber communication. Recently, considerable attention and great efforts have been focused on the development of arrayed waveguide grating (AWG) devices that based on the WDM technology. AWG devices can offer some basic functions including multiplexing, demultiplexing, wavelength routing and wavelength detecting, and possesses some advantages, such as easy optical integrating, narrow wavelength spacing, much more signal channels and compact structure. Also, AWG devices are the most parts of optical add/drop multiplexer (OADMs) and optical cross connectors (OXC) modules in optical fiber network. Polymer AWG's excellent particular features, such as low cost, simple fabrication process, low propagation loss, small birefringence and easy control of the refractive index, evoke the researchers'interest.In this thesis, the optimization and fabrication of two fluorinated polymer/Si AWG multiplexers are reported, which consist of 33×33- and 49×49-channels, respectively. The wavelength spacing of the AWGs is 0.8 nm, the insertion loss of is lower than 10dB and the crosstalk less than–21 dB. The main contents are as follows:1. The research history and current situation about AWG are reviewed. This thesis expounds the advantages of polymer AWG over inorganic AWG, and put forward the need of studying polymer AWG.2. Some principle theories and academic formulae about AWG were given out. And some important characteristics about AWG are discussed and analyzed, in order to offer theoretic basis for following parameter optimization and layout design. Furthermore, the main applications about AWG in optical fiber communication system are generalized.3. When AWG devices are fabricated by the chemical etching or reaction ion etching (RIE), usually trapezoid core cross-sections but originally designed rectangular ones are formed because of the side-etching in experiment. The manufacturing errors can result in the shift of the transmission spectrum, and lead to the increase of the crosstalk compared with the device originally designed with exact rectangular core cross-sections. In order to eliminate the bad influence of manufacturing errors on the transmission characteristics, we perform the parameter optimization and the structural design of the AWG device with trapezoid core cross-sections.First, we optimize the parameters of the original AWG device with rectangular core cross-sections to do some essential preparations for the following characteristics analysis, parameter improvement, and layout redesign of an AWG with trapezoid core cross-sections. These parameters include the width and thickness a and b of the guide core, mode effective refractive index nc, diffraction order m, length difference of adjacent arrayed waveguides ? L, focal length of slab f, free spectrum range FSR, the minimum number 2Mmin+1 of arrayed waveguides, and the maximum number Nmax of input/output channels. Second, we present a technique named"equivalent rectangular waveguide method"to analyze the trapezoid waveguide. Based on the perturbation theory, the main idea is that we use an equivalent rectangular waveguide with width a and thickness b to replace the trapezoid waveguide with bottom width a1, top width a2, and thickness b, and let both propagation constants be equal to each other by changing the equivalent width aeq, which is called"the equivalent core width". In this way, we analyze the influence of trapezoid core cross-sections on the mode effective refractive index, and modify the values of some parameters, including the length difference of adjacent arrayed waveguides ? L, focal length of slab f, free spectrum range FSR, and carry on layout redesign. Furthermore, we analyze the transmission spectrum and crosstalk of the AWG device with trapezoid core cross-sections. Investigated results show that the bad influence of manufacturing errors on the characteristics of AWG device can be eliminated effectively by the"equivalent rectangular waveguide method".4. Cross-linkable 2,3,4,5,6-pentafluorostyrene-co-glycidylmethacrylate (PFS- co-GMA) as a waveguide material has been synthesized, and the styrene (St) is used to regulate the PFS-co-GMA to form the core material with higher refractive index. The refractive index of copolymers can be easily controlled from 1.46 to 1.55. The material's solidification temperature is 125℃. The root mean square surface roughness (Rrms) of the film is 0.69nm.Based on the PFS-co-GMA polymer material and the above equivalent rectangular waveguide method, parameter values are optimized, and the layout designed for a 33×33-channel fluorinated polymer/Si AWG. The relative refractive index difference between the core and cladding is 0.5%, central wavelength is 1550.918 nm, wavelength spacing is 0.8 nm, the diffraction order is 56, the number of arrayed waveguides is 201, and the number of I/O channels is 33.A fabrication technology by using photoresist and aluminum film as mask in O2 RIE process is presented. Also, a vapor-redissolution technique is proposed, which can smooth the sidewall roughness resulting from side-etching. The mechanism of this technique is as: By means of the action of saturated steam pressure, solvent molecules adsorb on and get into the sidewalls of waveguides and enhance their fluidity. Then the surface tension of sidewalls smoothes their corrugation. By carefully selecting the redissolution temperature, time and solvent, the sidewall loss of the polymer waveguide is decreased by 2.1 dB/cm, the waveguide propagation loss is 0.68dB/cm at 1550nm. The insertion loss of our AWG device is reduced by 5.5 dB for the central channel and 6.7 dB for the edge channels, and the crosstalk is reduced by 2.5 dB after the vapor-redissoluton treatment.Furthermore, a waveguide end-face polishing technique is also introduced. We choose the glass slide as the covering layer, and UV-light-sensitive glue as the bond. The end-face of waveguide is polished at the optimized rate of 60rmp. The obtained the near-field mode spot of the AWG is light and rounded. Using the fiber-waveguide coupling method, the crosstalk is less than–22dB, and the insertion loss of AWG device is 9.0 dB, which involves the propagation loss causing by material absorption of 3.4dB, the diffraction loss of 2.9dB, and the optical fiber–waveguide coupling loss of 2.7dB.The AWG package technique is exploringly investigated. After packaging the AWG, we get the polarization-dependent wavelength shift of 3.3nm and the birefringence of 0.0031. The temperature-dependent wavelength shift of package AWG multiplexer is about–0.12 nm/K, which measured by a Peltier temperature controller. The wavelength adjustment range is about 6.6nm.5. For the first time in China, a 49-channel AWG device operating around the 1550 nm wavelength has been designed and fabricated using highly fluorinated polyethers based upon the preceding basic theory and technique method. The waveguide propagation loss is measured to be 0.45dB/cm by the cut-back method. The insertion loss is 8.1~13.5dB, crosstalk is–21~–23dB, and free spectrum range FSR is 40.69nm by the fiber-waveguide coupling method. In the central channel, the total loss is 8.1dB, which contain the absorption loss of 2.7dB, the diffraction loss of 2.82dB, and the coupling loss 5.28dB. The polarization- dependent wavelength is 3.8 nm, and the birefringence is 0.0037.The research about polymer AWG is being in the ascendant in the worldwide. Our following research will focus on further optimizing the device structure, synthesizing some lower loss polymer materials, reducing manufacturing errors, improving manufacturing precision, and ameliorating the vapor-redissolution technique. We are devoting ourselves to fabricate the polymer AWG device with low loss, low crosstalk, low polarization-independence and high thermal stability.