Zhen Sheng

Fabrication and Characterization of Small Optical Ridge Waveguides Based on SU-8 Polymer

Small SU-8 ridge optical waveguides with an air cladding and a SiO2 buffer on Si substrate have been realized by using a direct ultraviolet (UV) photolithography technology. The propagation loss measured by the cut-back method is about 0.1 dB/mm (@1550 nm) when the core width is 2.8 ¿m. The bending losses of the present SU-8 optical ridge waveguides are also characterized. The measured results show that the bending loss decreases exponentially as the bending radius increases and the total loss can be reduced effectively by introducing an appropriate offset between two connected sections with different curvatures. A small bending radius (as small as 75 ¿m) is still allowed for the requirement of a small bending loss (< 0.1 dB) when an offset of 0.1 ¿m is introduced. Finally, by using this kind of waveguide, a small 1 × 2 Y-branch power splitter is fabricated and characterized.

InGaAs PIN photodetectors integrated on silicon-on-insulator waveguides

InGaAs PIN photodetectors heterogeneously integrated on silicon-on-insulator waveguides are fabricated and characterized. Efficient evanescent coupling between silicon-on-insulator waveguides and InGaAs photodetectors is achieved. The fabricated photodetectors can work well without external bias and have a very low dark current of 10pA. The measured responsivity of a 40microm-long photodetector is 1.1A/W (excluding the coupling loss between the fiber and the SOI waveguide) at a wavelength of 1550nm and shows good linearity for an input power range of 40dB. Due to the large absorption coefficient of InGaAs and the efficient evanescent coupling, the fabricated photodetectors can cover the whole S, C and L communication bands.

Novel ultra-broadband polarization splitter-rotator based on mode-evolution tapers and a mode-sorting asymmetric Y-junction

A novel silicon-on-insulator (SOI) polarization splitter-rotator is proposed based on mode-evolution tapers and a mode-sorting asymmetric Y-junction. The tapers are designed to adiabatically convert the input TM0 mode into the TE1 mode, which will evolve into the TE0 mode in the wide output arm while the input TE0 mode excites the TE0 mode in the narrow arm. The numerical simulation results show that the mode conversion efficiency increases with the lengths of the tapers and the Y-junction for the output waveguide widths in a large range. This proposed device has < 0.4 dB insertion loss with > 12 dB extinction ratio in an ultra-broad wavelength range from 1350 nm to 1750 nm. With such a broad operating bandwidth, this device offers potential applications for polarization diversity operating across every communication bands. Fabrication tolerance analysis is also performed in terms of the device width variation, the slab height variation and the variation of the upper-cladding refractive index.

Compact Arrayed Waveguide Grating Devices Based on Small SU-8 Strip Waveguides

Compact 48 × 48 and 23 × 23 arrayed waveguide grating (AWG) devices are realized by using SU-8 strip waveguides fabricated with the process of direct ultraviolet (UV) photolithography. The demonstrated 48-channel and 23-channel AWG devices operating around 1550 nm have a channel spacing of 0.8 nm (100 GHz) and 3.2 nm (400 GHz), respectively. Due to the high index-contrast of the SU-8 strip waveguide, the fabricated AWG has a very compact size of only about 0.22 × 0.47 cm2. For the fabricated 100 GHz-spaced AWG, the crosstalk between adjacent channels is less than -15 dB, and the polarization-dependent wavelength is about 0.72 nm (@1543.5 nm). The fabricated 400 GHz-spaced AWG device exhibits a crosstalk of less than -20 dB, a polarization-dependent wavelength of 0.02 nm (@1559.6 nm). Finally the temperature tenability of the present AWG device is also characterized. A 12 nm tuning range is observed as the temperature changes from 25°C to 115°C.

Design of a SiO 2 top-cladding and compact polarization splitter-rotator based on a rib directional coupler

A compact polarization splitter-rotator based on a silicon-on-insulator rib asymmetrical directional coupler with SiO2 top-cladding is proposed. Unlike previously reported PSRs which specifically required the top-cladding material to be different from the bottom cladding in order to break the symmetry of the waveguide cross-section, our proposed PSR has no such limitation on the top-cladding due to the horizontal asymmetry of the rib waveguide. In addition, the device is highly compact and has a total length as short as 24 μm. Numerical simulation shows that a high conversion efficiency of ~97% is obtained at the wavelength of 1550 nm. With the width variation of ± 15 nm and the gap variation of ± 50 nm, the PSR still has high ER of 12 dB at the cross-port, showing large fabrication tolerance. This device can be cascaded to improve the performance at the through port and an example of a two-stage PSR is presented. The mode conversion between the strip waveguide and the rib waveguide is also discussed.

Low-loss and low-crosstalk 8 × 8 silicon nanowire AWG routers fabricated with CMOS technology

Low-loss and low-crosstalk 8 × 8 arrayed waveguide grating (AWG) routers based on silicon nanowire waveguides are reported. A comparative study of the measurement results of the 3.2 nm-channel-spacing AWGs with three different designs is performed to evaluate the effect of each optimal technique, showing that a comprehensive optimization technique is more effective to improve the device performance than a single optimization. Based on the comprehensive optimal design, we further design and experimentally demonstrate a new 8-channel 0.8 nm-channel-spacing silicon AWG router for dense wavelength division multiplexing (DWDM) application with 130 nm CMOS technology. The AWG router with a channel spacing of 3.2 nm (resp. 0.8 nm) exhibits low insertion loss of 2.32 dB (resp. 2.92 dB) and low crosstalk of -20.5~-24.5 dB (resp. -16.9~-17.8 dB). In addition, sophisticated measurements are presented including all-input transmission testing and high-speed WDM system demonstrations for these routers. The functionality of the Si nanowire AWG as a router is characterized and a good cyclic rotation property is demonstrated. Moreover, we test the optical eye diagrams and bit-error-rates (BER) of the de-multiplexed signal when the multi-wavelength high-speed signals are launched into the AWG routers in a system experiment. Clear optical eye diagrams and low power penalty from the system point of view are achieved thanks to the low crosstalk of the AWG devices.

A Compact and Low-Loss MMI Coupler Fabricated With CMOS Technology

We present the design, fabrication, and measurement of a compact and low-loss multimode interference (MMI) coupler based on the silicon nanowire waveguide. The device is carefully designed to achieve both a good performance and a compact size by using the mode matching method. The device is fabricated on silicon-on-insulator (SOI) with 0.13-μm CMOS technology. By measuring the MMI coupler with a cascaded configuration, a very low excess loss of 0.06 dB at the wavelength of 1550 nm is obtained. The device can also work well for a wide wavelength band. The present MMI coupler is very compact with a footprint of ~ 3.6 × 11.5 μm2 for the multimode region.

Comparative Study of Losses in Ultrasharp Silicon-on-Insulator Nanowire Bends

Ultrasharp silicon-on-insulator (SOI) nanowire bends (with a bending radius of R < 2 mu m) are analyzed numerically. It is shown that the calculated bending losses for ultrasharp bends are overestimated when using a modal analysis method based on finite-difference method. In this case, reliable estimation of the bending loss can be made with a 3-D finite-difference time-domain (3-D-FDTD) method. By using 3-D-FDTD simulation, the losses in SOI nanowire bends with different structures and parameters are studied. By increasing the core width or height of the waveguide, one can reduce the bending loss at longer wavelengths for TE mode while the bending performance at shorter wavelengths degrades due to the multimode effect. Increasing the core height is much more effective to reduce the bending loss of TM mode than increasing core width. The relationship between the intrinsic Q-factor of a microring resonator and the bending radius is also obtained.

Compact Microracetrack Resonator Devices Based on Small SU-8 Polymer Strip Waveguides

Compact microracetrack resonator (MRR) devices are presented with small SU-8 polymer strip waveguides. The SU-8 strip waveguide has an SU-8 polymer core (n~1.573) , a SiO2 buffer (n~1.445), and an air cladding. The fabricated straight waveguide has a low propagation loss of about 0.1 dB/mm. With such a high index-contrast optical waveguide, a compact MRR with a small bending radius (~150 mum) are designed and fabricated. The measured spectral responses of the through/drop ports show a Q-factor of 8000.

Proposal for fabrication-tolerant SOI polarization splitter-rotator based on cascaded MMI couplers and an assisted bi-level taper

A novel silicon-on-insulator (SOI) polarization splitter-rotator (PSR) with a large fabrication tolerance is proposed based on cascaded multimode interference (MMI) couplers and an assisted mode-evolution taper. The tapers are designed to adiabatically convert the input TM(0) mode into the TE(1) mode, which will output as the TE(0) mode after processed by the subsequent MMI mode converter, 90-degree phase shifter (PS) and MMI 3 dB coupler. The numerical simulation results show that the proposed device has a < 0.5 dB insertion loss with < -17 dB crosstalk in C optical communication band. Fabrication tolerance analysis is also performed with respect to the deviations of MMI coupler width, PS width, slab height and upper-cladding refractive index, showing that this device could work well even when affected by considerable fabrication errors. With such a robust performance with a large bandwidth, this device offers potential applications for CMOS-compatible polarization diversity, especially in the booming 100 Gb/s coherent optical communications based on silicon photonics technology.