Lian-Wee Luo

WDM-compatible mode-division multiplexing on a silicon chip

Significant effort in optical-fibre research has been put in recent years into realizing mode-division multiplexing (MDM) in conjunction with wavelength-division multiplexing (WDM) to enable further scaling of the communication bandwidth per fibre. In contrast, almost all integrated photonics operate exclusively in the single-mode regime. MDM is rarely considered for integrated photonics because of the difficulty in coupling selectively to high-order modes, which usually results in high inter-modal crosstalk. Here we show the first microring-based demonstration of on-chip WDM-compatible mode-division multiplexing with low modal crosstalk and loss. Our approach can potentially increase the aggregate data rate by many times for on-chip ultrahigh bandwidth communications.

High Coupling Efficiency Etched Facet Tapers in Silicon Waveguides

We demonstrate a platform based on etched facet silicon inverse tapers for waveguide-lensed fiber coupling with a loss as low as 0.7 dB/facet. This platform can be fabricated on a wafer scale enabling mass-production of silicon photonic devices with broadband, high-efficiency couplers.

High bandwidth on-chip silicon photonic interleaver

We demonstrate a 120 GHz 3-dB bandwidth on-chip silicon photonic interleaver with a flat passband over a broad spectral range of 70 nm. The structure of the interleaver is based on an asymmetric Mach-Zehnder interferometer (MZI) with 3 ring resonators coupled to the arms of the MZI. The transmission spectra of this device depict a rapid roll-off on the band edges, where the 20-dB bandwidth is measured to be 142 GHz. This device is optimized for operation in the C-band with a channel crosstalk as low as -20 dB. The device also has full reconfiguration capability to compensate for fabrication imperfections.

High quality factor etchless silicon photonic ring resonators

We demonstrate high-Q silicon ring resonators fabricated by selective oxidation without any silicon etching. We achieve an intrinsic quality factor of 510,000 in 50 μm-radius ring resonators with ring losses of 0.8 dB/cm.

Power insensitive silicon microring resonators

We demonstrate power insensitive silicon microring resonators without the need for active feedback control. The passive control of the resonance is achieved by utilizing the compensation of two counteracting processes, free carrier dispersion blueshift and thermo-optic redshift. In the fabricated devices, the resonant wavelength shifts less than one resonance linewidth for dropped power up to 335 μW, more than fivefold improvement in cavity energy handling capability compared to regular microrings.

Fully reconfigurable silicon photonic lattice filters with four cascaded unit cells

We present a fully-reconfigurable CMOS-compatible silicon-photonic lattice-filter with four cascaded unit cells consisting of resonant rings and Mach-Zehnder interferometers. The measurements show high-quality filter responses including IIR and FIR filter characteristics matching theoretical predictions.

Fast Wavelength Locking of a Microring Resonator

Microring resonator devices require thermal initialization to match their intended operating wavelengths in a process called wavelength locking. We use the dither technique, where we apply a small periodic signal to the microrings integrated heater, to demonstrate reliable wavelength locking of a microring resonator filter to an operating wavelength at a wavelength shift rate of 0.012 nm/μs, an order of magnitude faster than previous demonstrations. We also identify and characterize fundamental speed limits of the wavelength locking process, showing that the integrated-heater bandwidth is the limiting factor.

Colorless Optical Network Unit Based on Silicon Photonic Components for WDM PON

We demonstrate a low-cost colorless optical network unit (ONU) utilizing silicon photonic components for wavelength division multiplexed passive-optical-networks. At the ONU, a waveguide-coupled microring works as a demultiplexer for separating the downstream signal from the centrally distributed continuous-wave (CW) light. The 10-Gb/s downstream signal is received using a waveguide-integrated germanium photodetector while the CW light is further modulated at 5 Gb/s using a silicon microring modulator for upstream signal generation. Error-free transmission over 25-km single mode fiber is achieved with 0.2- and 0.4-dB power penalties for the downstream and upstream signals, respectively. Complementary metal-oxide semiconductor-compatible silicon photonic technology offers the potential for monolithic integration and mass production.

Fast wavelength locking of a microring resonator

We experimentally investigate the latency for wavelength locking and thermally stabilizing a microring resonator to its operating wavelength, showing that the integrated-heater bandwidth is the limiting factor.

FPGA controlled microring based tunable add-drop filter

We demonstrate a microring based add-drop filter controlled by a field programmable gate array (FPGA). Specialized heater design maintains tuning performance of the device under non-optimized applied voltages, thus avoiding complicated control circuitry.