Fuwan Gan

Reconfigurable radio-frequency arbitrary waveforms synthesized in a silicon photonic chip

Photonic methods of radio-frequency waveform generation and processing can provide performance advantages and flexibility over electronic methods due to the ultrawide bandwidth offered by the optical carriers. However, bulk optics implementations suffer from the lack of integration and slow reconfiguration speed. Here we propose an architecture of integrated photonic radio-frequency generation and processing and implement it on a silicon chip fabricated in a semiconductor manufacturing foundry. Our device can generate programmable radio-frequency bursts or continuous waveforms with only the light source, electrical drives/controls and detectors being off-chip. It modulates an individual pulse in a radio-frequency burst within 4 ns, achieving a reconfiguration speed three orders of magnitude faster than thermal tuning. The on-chip optical delay elements offer an integrated approach to accurately manipulating individual radio-frequency waveform features without constraints set by the speed and timing jitter of electronics, and should find applications ranging from high-speed wireless to defence electronics.

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.

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.

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.

Optimization and Demonstration of a Large-bandwidth Carrier-depletion Silicon Optical Modulator

We present the design, fabrication, and measurement of a high-speed carrier-depletion silicon optical modulator based on Mach-Zehnder Interferometer structure. Based on an equivalent circuit model, the traveling-wave electrode size and doping concentration of the PN junction are optimized to achieve a large modulation bandwidth. The modulation efficiency and optical loss at different positions of the PN junction are also simulated. The device is fabricated on silicon-on-insulator (SOI) with 0.13 μm CMOS technology. An insertion loss of 3.9 dB (resp. 6.2 dB) and a VπLπ of 1.62-2.05 V·cm (resp. 1.47-1.97 V·cm) are experimentally realized for 1 mm (resp. 2 mm) long phase shifter. By small signal measurement, the modulator exhibits a 3 dB bandwidth of 30 GHz and 19 GHz for 1 mm and 2 mm long phase shifter, respectively, which agrees well with the simulation results. The optical eye diagram with data rate up to 44 Gb/s is also demonstrated, showing potential in the application of high-speed optical interconnects.

Nonlinear response and optical limiting in SrBi(2)Ta(2)O(9) thin films.

SrBi(2)Ta(2)O(9) (SBT) thin films on quartz substrates were prepared by use of the pulsed-laser deposition technique. The nonlinear refractive indices, n(2) , of the SBT films were measured by use of z-scan techniques with picosecond pulses. Large negative nonlinear refractive indices of 3.84 and 3.58cm(2)/GW were obtained for the wavelengths 532 nm and 1.064mum , respectively. The two-photon absorption coefficient was determined to be 7.3 cm/GW for 532 nm. The limiting behavior of SBT thin film on a quartz substrate was investigated in an f/5 defocusing geometry by use of 38-ps-duration, 532-nm, 1.064mum laser excitation.

Fabrication, Characterization and Loss Analysis of Silicon Nanowaveguides

Low loss silicon waveguides are the key to the realization of high performance photonic integrated circuits. In this paper, fabrication, characterization and loss analysis of silicon nanowaveguides are presented. Silicon nanowaveguides are fabricated on silicon-on-insulator (SOI) wafers with 0.13 μm complementary metal-oxide-semiconductor (CMOS) technology. To reduce the propagation loss, both photolithography and etching processes are optimized to make the waveguide sidewalls smooth. Propagation losses of 2.4 ± 0.2 and 0.59 ± 0.32 dB/cm are obtained at 1550 nm wavelength for TE and TM modes, respectively. A theoretical method is used to estimate the propagation losses for TE and TM modes. Scattering losses from both sidewalls and top/bottom surface are considered. The calculated results show that loss comes from sidewall roughness is the main source of propagation loss for TE mode while for TM mode, losses from both sidewall and top/bottom surface contribute comparably to the total propagation loss. The theoretically estimated propagation loss agrees well with the measured results.

Experimental demonstration of in-plane negative-angle refraction with an array of silicon nanoposts.

Controlling an optical beam is fundamental in optics. Recently, unique manipulation of optical wavefronts has been successfully demonstrated by metasurfaces. However, these artificially engineered nanostructures have thus far been limited to operate on light beams propagating out-of-plane. The in-plane operation is critical for on-chip photonic applications. Here, we demonstrate an anomalous negative-angle refraction of a light beam propagating along the plane, by designing a thin dielectric array of silicon nanoposts. The circularly polarized dipoles induced by the high-permittivity nanoposts at the scattering resonance significantly shape the wavefront of the light beam and bend it anomalously. The unique capability of a thin line of the nanoposts for manipulating in-plane wavefronts makes the device extremely compact. The low loss all-dielectric structure is compatible with complementary metal-oxide semiconductor technologies, offering an effective solution for in-plane beam steering and routing for on-chip photonics.