PIERS ONLINE, VOL. 3, NO. 3, 27 329 Applications of Cladding Stress Induced Effects for Advanced Polarization Control in licon Photonics D.-X. Xu, P. Cheben, A. Delâge, S. Janz, B. Lamontagne, M.-J. Picard E. Post, P. Waldron, and W. N. Ye Institute for Microstructural Sciences, National Research Council Canada (NRC) Ottawa, Ontario, Canada Abstract The applications of cladding stress in SOI waveguide components to enhance device functionality and improve fabrication tolerance are reviewed. Assisted by FEM design tools, characteristics of stress-induced effects are studied in depth. Design strategies are developed for using stress engineering to achieve a variety of functions. Polarization insensitive arrayed waveguide gratings (AWGs) and ring resonators, and polarization splitters and filters are demonstrated using these design principles. DOI: 1.2529/PIERS61616357 licon photonics is a rapidly growing research field with many advances in recent years. Motivated by the potential of high integration density, compatibility with mature CMOS technologies, silicon-on-insulator (SOI) has been the main material platform for silicon photonic waveguide components [1]. Along with these benefits, control and utilization of polarization dependent properties arise as new challenges, as well as opening new possibilities of novel designs and functionalities. Polarization dependent properties have long been an important issue in integrated optical systems. In applications such as wavelength demultiplexing and high resolution spectroscopy, the shift in channel wavelength with the polarization state of the incoming optical signal often limits the device spectral resolution. One approach to handle this issue is to produce components or systems with polarization insensitive performance. In some cases this approach may not be practical, and then polarization diversity may be adopted where the signal is divided into orthogonal polarization states and processed separately. In this contribution, we review the solutions we have developed to manage the polarization, namely using cladding stress-induced effects to control the waveguide polarization properties in SOI waveguides. Design methodologies for using this technique to achieve polarization-insensitive arrayed waveguide gratings (AWGs) and ring resonators, Mach-Zehnder interferometer (MZI) based polarization splitters and filters are presented. In planar waveguides, the modal birefringence results from a combination of geometrical, compositional and stress-induced effects. In low index contrast glass waveguides, the geometrical effect is minimal and the birefringence is primarily controlled by the material residual stress. A large body of research has been devoted to this subject, where the main goal is to reduce the residual stress in the waveguide core region by adjusting the thermal expansion coefficients of the cladding and core layers. In high index contrast systems such as SOI where the light confinement is strong, electromagnetic boundary conditions dictated by the cross-section geometry of the waveguides have the largest impact on the waveguide effective index and also the birefringence. These geometrical dependencies have been used to obtain single mode waveguides with large cross-sections, photonic wires with submicron dimensions which can afford bending radius on the order of microns, and to adjust the waveguide (geometrical) birefringence n geo. Although it is possible to minimize the birefringence by tailoring the waveguide aspect ratio [2 4], this method only works well for waveguides with large cross-sections. As the core size is reduced, typically to 2 µm, it becomes increasingly difficult to maintain the designed birefringence values. nce ridge dimensions also determine the number of waveguide modes, the minimum usable bend radius, the mode size, and the coupling between adjacent waveguides, it is often impossible to simultaneously meet these different design objectives. When a stress with axial anisotropy is imposed on the originally isotropic material, a stress contribution n s to the modal birefringence is induced. The total modal birefringence can then be expressed as n eff = n TM n TE = n geo + n s, provided the stress induced index change is much smaller than the core-cladding index step, which is the case for high index contrast SOI waveguides. Cladding layers such as silicon dioxide deposited or grown over the silicon core generally
PIERS ONLINE, VOL. 3, NO. 3, 27 33 y cladding waveguide 2 substrate film film x y 5 MPa -5-1 M Pa x Figure 1: Cross-section of a SOI ridge waveguide; Stress distributions in the x-direction, and y-direction. The ridge height is 2.2 µm, the width is 1.8 µm and the etch depth is 1.47 µm. The cladding oxide is 1 µm thick and the stress is film = 32 MPa. retain a stress, mainly due to the thermal mismatch between the materials, and in turn create an anisotropic stress field inside the waveguide. Due to the photoelastic effect, the material birefringence is given by [5 7]: (n y n x ) = (C 2 C 1 )( y x ). Here i (i = x, y) are the stress tensor components, n i is the material refractive index, n is the refractive index without stress, and C 1 and C 2 are the stress-optic constants. We have evaluated the stress-induced effects using a finite-element differential equation solver (FEMLAB) and include stress equations that modify the permittivity tensor via the photoelastic effect. The stress distributions in a SOI ridge waveguide (Fig. 1) are shown in Figs. 1 and. The corresponding local material birefringence (n y n x ) is shown in Fig. 2, with values as large as 4 1 3. These modifications in the material cause n TM eff to increase with the oxide thickness and the stress magnitude, and nte eff to decrease (Figs. 2 and ). Two parameters control the stress-induced birefringence: the oxide thickness t c and the film stress level film. The stress-induced index variations resulted from commonly used cladding films are of comparable magnitude to the geometrical birefringence found in typical SOI ridge waveguides. Depending on the value of the geometrical birefringence n geo, the total modal birefringence may be designed to be zero or other desired values. Fig. 3 gives the calculated results of maximum birefringence that can be induced by a cladding of 3 MPa in waveguides with 2 µm ridge height and different aspect ratios. These characteristics are the bases for stress engineering in SOI waveguides, and they can be advantageously employed in many applications. D W H.4.2 -.2 3.446 3.4455 3.445 3.4445 3.444 3.4435 3.443 3.4425 TE TM.5 1 1.5 2 Birefringence neff (x1-3 ) 2.5-3 M Pa 1.5.5-2 M Pa -.5-1 MPa -1.5 MPa -2.5.5 1 1.5 2 2.5 Oxide thickness ( µm m) Oxide thickness ( µm ) Figure 2: Stress-induced material birefringence (n y n x ) corresponding to the stress distributions shown in Figs. 1, ; Effective index vs. the oxide upper cladding thickness, for different cladding stress levels film of MPa (dotted lines), 2 MPa (dashed lines), and 4 MPa (solid lines), respectively, n eff as a function of t c and film. In the following, we present several examples of using stress engineering for the control of polarization properties. First we discuss the case of polarization insensitive AWG demultiplexers. In AWGs (Fig. 4) the polarization dependent wavelength shift λ = λ TM λ TE is mainly determined by the birefringence of the arrayed waveguides as λ = λ n eff /n g, where n g is the waveguide group index and λ is the free-space operating wavelength. Fig. 4 shows the TE and TM spectra of an AWG with a waveguide ridge height of 2.2 µm. The geometrical birefringence of the as fabricated arrayed waveguides was n geo 1.3 1 3, giving a polarization dependent
PIERS ONLINE, VOL. 3, NO. 3, 27 331 Figure 3: Stress-induced birefringence n s for ridges with different aspect ratios, with H = 2 µm, t c = 2 µm and film = 3 MPa. wavelength shift of λ.6 nm. Here the waveguides were mainly designed to support only a single mode, while achieving reasonable coupling and desired bending radius. A PECVD oxide upper cladding film was then deposited, with the stress level and thickness properly chosen ( 32 MPa and.6 µm) using calculations similar to that presented in Fig. 2. The λ was then reduced to below.4 nm (corresponding to n eff < 1 1 4 ) [8]. Polarization dependent loss was also negligible in these devices. The cladding stress level can also be adjusted by post-processing such as thermal annealing, providing a practical method of post-process tuning [9]. This technique for polarization management is also applicable to high resolution microspectrometers developed in our laboratory. Compared to demultiplexers, spectrometers require a better resolution and a larger free spectral range, while crosstalk and band flatness are relatively minor factors in the specifications. A silica-on-silicon demultiplexer used as a high resolution and high bandwidth microspectrometer would lead to prohibitively large footprint; while the use of SOI can considerably reduce its size. Our present spectrometer prototype has 1 output channels designed with 1.5 2 µm 2 arrayed waveguide cross-sections and occupies 8 8 mm 2, two orders of magnitude less than a similar device based on glass waveguides [1]. Polarization dependence is again an important issue here, which can be managed with the assistance of stress engineering. -1 Intensity (dbm) -15-2 -25-3 -35-4 1528 1532 1536 154 1544 Wavelength (nm) Figure 4: Optical image of an fabricated AWG, Measured spectra for an SOI AWG with λ compensated using.6 µm thick cladding with film = 32 MPa. TM (solid) and TE (dashed). We also applied the approach of stress engineering to design polarization-insensitive ring resonators (RR) [11]. Ring resonators can be used as the building block in applications including add-drop, switching, modulation, and sensing. However, polarization sensitivity is often an obstacle that limits their use. Polarization-independent RR designs using directional couplers were investigated [4], but showed very stringent fabrication requirements. We proposed the use of multimode-interference (MMI) couplers for light transfer between the bus and ring waveguides (Figs. 5
PIERS ONLINE, VOL. 3, NO. 3, 27 332 and ). Compared to the commonly used directional couplers, MMI couplers provide broadband and low polarization sensitivity in coupling ratios. The polarization dependent shift in resonator phase accumulation caused by waveguide geometrical birefringence n geo in the different sections can be compensated by applying the appropriate cladding stress-induced birefringence n s. We have fabricated MMI-based ring resonators in SOI wafers with 1.5 µm thick silicon at the Canadian Photonics Fabrication Center (CPFC). A standard in-house PECVD process produces oxide films with 25 MPa stress. mulations similar to that in Fig. 2 show that an oxide thickness of.8 µm will compensate the overall birefringence. Preliminary measurement results are in shown in Fig. 5. Although there is some polarization dependent loss, the birefringence is well compensated in these devices. Transmission (db, normalized) -5-1 -15 TE TM 1512 1513 1514 1515 1516 Wavelength (nm) Figure 5: A ring resonator using an MMI coupler, a close-up of the MMI section, TE and TM transmission spectra of the ring resonator, with an upper cladding of.8 µm thick and film stress of film = 25 MPa. Stress-induced effects can also be used to produce high level of birefringence in selected areas. Polarization splitters and filters can be made by applying a cladding patch on one arm of a Mach- Zehnder interferometer (MZI). The cladding stress induces a phase difference ϕ. between the two interference paths. When the phase relations between the two arms are such that ϕ TE = 2Mπ and ϕ TM = (2N + 1)π (M and N are integers), the device can operate as either a polarization splitter or filter, depending on the number of output ports of the combining coupler. An extinction ratio of 35 db is predicted for a polarization filter [12]. As we have demonstrated, stress-induced modifications of the effective indices for the TE and TM modes and the associated birefringence are important for a wide range of commonly employed SOI waveguide geometries. The significance of these effects is starting to be recognized in the research community. If not taken into consideration, these effects can lead to large deviations in device characteristics from the designed specifications. Fortunately, the use of cladding stress to correct the birefringence allows a considerable degree of freedom in designing SOI waveguides to meet other performance criteria such as relaxed dimensional tolerance, reduced loss at waveguide bends, and overall improved device performance. The stress-induced modifications to the effective indices in SOI waveguides are mainly controlled by the stress level and the thickness of the upper oxide cladding. As we have demonstrated, polarization insensitivity in AWGs, spectrometers and ring resonators can be achieved using this technique, leaving the freedom of optimizing the waveguide geometry for considerations other than the birefringence. nce the effect of a cladding on mode shape is negligible, there is little mode mismatch loss or polarization dependent loss at the junctions between waveguide sections with and without the claddings. Therefore tailored cladding patches can be applied at discrete locations in a planar waveguide circuit with negligible insertion loss penalty, which we demonstrated in the design of broadband polarization splitters and filters. The applications of cladding stress induced effects can be envisioned in a variety of situations to enhance device functionality, simplify fabrication and improve operation tolerance.
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