4678 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 21, NOVEMBER 1, 2009 Stress-Induced Birefringence Characteristics of Polymer Optical Rib Waveguides M. Faruque Hossain, Hau Ping Chan, Member, IEEE, Mohammad Afsar Uddin, and R. K. Y. Li Abstract—We report the detailed numerical investigation of stress-induced material birefringence in polymer rib waveguide for the design of nonbirefringent waveguide devices. To accurately simulate the stress-induced effects we propose a more realistic model in the finite element analysis which considers the stresses induced over the entire sequential fabrication process. It is ob- served that the birefringence is nonuniform, and it is different for different etch depth and core width. The maximum birefrin- gence in the core layer is observed near the lower cladding which decreases to zero toward the top surface. The influence of this material anisotropy on the modal birefringence is analyzed also for different rib structures. We found the stress effects on the modal birefringence to be largely affected by etch depth, while core width has small effect. It is also found that the deeply etched core has better birefringence stability. Finally, an accurate design of the zero-birefringence waveguide is illustrated by taking the stress effects into account, and the results are compared with experimental data. Excellent agreement between calculated and experimental results confirms the potential application of this work to aid in the design of polarization-insensitive waveguide devices. Index Terms—Birefringence, optical waveguide, polymer wave- guide, stress. I. INTRODUCTION P LANAR optical waveguides are basic components in many functional devices of optical communication system such as, arrayed waveguide gratings (AWG), Bragg grating filters, etc. Polymer materials are very attractive in the fabrication of such waveguide devices due to their advan- tages, such as simple fabrication processes using embossing or UV techniques, easy formation of multilayer structures by spin-coating on any surface of interest, and potential low cost at mass production [1]–[3]. One bottleneck to the widespread ap- plication of these devices is polarization-dependent properties [4], [5]. An important originating source of polarization depen- dence is stress, which is usually generated in the fabrication process due to polymerization shrinkage and thermal-expansion mismatch among dissimilar materials. This stress causes an anisotropic change in the material refractive index, and thus Manuscript received October 29, 2008; revised April 08, 2009. First pub- lished June 16, 2009; current version published September 10, 2009. M. F. Hossain, H. P. Chan, and M. A. Uddin are with the Department of Electronic Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong (e-mail: eehpchan@cityu.edu.hk). R. K. Y. Li is with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Hong Kong. This work was supported by the Research Grants Council of Hong Kong Spe- cial Administrative Region, China, under project CityU 1109/05E. Digital Object Identifier 10.1109/JLT.2009.2025518 material birefringence. The combination of stress-induced ma- terial birefringence and geometrical birefringence results modal birefringence, which leads to the two orthogonal polarization modes to travel at slightly different velocities and mode field distribution. Thus adversely affect the transmission of light and performance of devices [6], [7]. To reduce this polarization dependence, a few methods, such as using stress-releasing grooves or introducing some additional layers to reduce the stress [8] or using compensating devices [5], have been used. Generally, those methods increase the com- plexity and cost of the devices. A simple and practical solu- tion is to use a polarization-insensitive (i.e., nonbirefringent) planar waveguide device. An important building block of po- larization-insensitive devices is a zero-birefringence waveguide. Theoretical works have been carried out in the design of such waveguide devices by tailoring the waveguide geometry [9], [10]. However, it is often found discrepancies between theoret- ical design and experimental results due to the effects of induced stresses [11], [12]. In order to minimize or control the bire- fringence, therefore, it is paramount that the design engineers have a good quantitative understanding of stress distributions in planar waveguides, and their impact on the optical properties of components. Several empirical techniques have been developed to study the stress-birefringence in optical waveguides [13], [14]. These techniques are limited to uniform stress distribution only, and not sufficient for the growing complicated optical circuit design. On the other hand, it is impractical to measure the stresses as well as the optical properties inside the complex structure of tiny optical waveguide devices. A number of works have analyzed the stress and the induced birefringence numerically using the finite element (FE) method [15]–[17]. These works employed simplified model in which the entire structure is assumed to be stress free at certain temperature (e.g., SiO or glass depo- sition temperature) and then cool down to room temperature. This conventional approach may not be suitable to handle the multilayer polymer devices where the material layers are de- posited and cured at different temperature. As a result, all the layers cannot be stress free at a certain temperature. A signif- icant amount of stress may generate due to curing shrinkage of polymer which cannot be explain also by simplified model. However, although the stress-induced effects have been studied extensively in glass waveguide [15]–[17], not many people have analyzed stress-birefringence in polymer waveguides. This is partly because most of the commercially available numerical software intended for optical analysis has the limitations in the modeling of previously mentioned practical steps. In particular, the quantitative predictions of stress in polymer waveguides in detail and sequential manner as well as its effects on the device performance have not been reported yet. 0733-8724/$26.00 © 2009 IEEE