Cheng-Chih Kung

Low V_pp, ultralow-energy, compact, high-speed silicon electro-optic modulator

We present a high-speed silicon optical modulator with a low V(pp) (peak-to-peak driving voltage) and ultralow energy consumption based on a microring resonator, with the refractive index modulation achieved by electric-field-induced carrier depletion in a reverse-biased lateral pn diode embedded in the ring structure. With a V(pp) of 2 V, we demonstrate a silicon modulator with a 3 dB bandwidth of 11 GHz, a modulation depth of 6.5 dB together with an insertion loss of 2 dB, ultralow energy consumption of 50 fJ per bit, and a small device active area of approximately 1000 microm(2).

High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide

We demonstrate a compact, high speed Ge photodetector efficiently butt-coupled with a large cross-section silicon-on-insulate (SOI) waveguide in which the Ge p-i-n junction is placed in the horizontal direction to enable very high speed operation. The demonstrated photodetector has an active area of only 0.8×10 μm2, greater than 32 GHz optical bandwidth, and a responsivity of 1.1 A/W at a wavelength of 1550 nm. Very importantly the device can readily be integrated with high performance wavelength-division-multiplexing filters based on large cross-section SOI waveguide to form monolithic integrated silicon photonics receivers for multichannel terabit data transmission applications.

30GHz Ge electro-absorption modulator integrated with 3μm silicon-on-insulator waveguide

We demonstrate a compact waveguide-based high-speed Ge electro-absorption (EA) modulator integrated with a single mode 3 µm silicon-on-isolator (SOI) waveguide. The Ge EA modulator is based on a horizontally-oriented p-i-n structure butt-coupled with a deep-etched silicon waveguide, which transitions adiabatically to a shallow-etched single mode large core SOI waveguide. The demonstrated device has a compact active region of 1.0 × 45 µm(2), a total insertion loss of 2.5-5 dB and an extinction ratio of 4-7.5 dB over a wavelength range of 1610-1640 nm with -4V(pp) bias. The estimated Δα/α value is in the range of 2-3.3. The 3 dB bandwidth measurements show that the device is capable of operating at more than 30 GHz. Clear eye-diagram openings at 12.5 Gbps demonstrates large signal modulation at high transmission rate.

Low loss shallow-ridge silicon waveguides

We demonstrate low loss shallow-ridge silicon waveguides with an average propagation loss of 0.274 + or - 0.008 dB/cm in the C-band (1530 nm - 1565 nm). These waveguides have a cross section of 0.25 microm by 2 microm and are fabricated by standard photolithography and dry etching. We also investigate a compact double-level taper which adiabatically couples light from these waveguides to silicon strip waveguides enabling tight bends.

36 GHz submicron silicon waveguide germanium photodetector

We present two effective approaches to improve the responsivity of high speed waveguide-based Ge photodetectors integrated on a 0.25 μm silicon-on-insulator (SOI) platform. The main cause of poor responsivity is identified as metal absorption from the top contact to Ge. By optimizing Ge thickness and offsetting the contact window, we have demonstrated that the responsivity can be improved from 0.6A/W to 0.95 A/W at 1550 nm with 36 GHz 3 dB bandwidth. We also demonstrate that a wider device with double offset contacts can achieve 1.05 A/W responsivity at 1550 nm and 20 GHz 3 dB bandwidth.

Optical proximity communication using reflective mirrors

Optical proximity communication (OPxC) with reflecting mirrors is presented. Direct optical links are demonstrated for silicon chips with better than -2.5dB coupling loss, excluding surface losses. OPxC is a true broadband solution with little impairment to the signal integrity for high-speed optical transmission. With wavelength division multiplexing (WDM) enabled OPxC, very high bandwidth density I/O, orders of magnitude higher than the traditional electrical I/O, can be achieved for silicon chips.

Optical Proximity Communication With Passively Aligned Silicon Photonic Chips

We report high-fidelity 10-Gb/s optical-proximity-communication using reflecting mirrors micro-machined into silicon and co-integrated with low-loss silicon-on-insulator waveguides for packaged chip-to-chip communication. Device integration was carried out by dry etching a rib waveguide 8 mum wide that was tapered to a width of 13 mu m and subsequently truncated with a wet-etched, micro-mirror facet forming a 54deg angle with the (100) surface. Light in waveguides on a bottom chip can couple to waveguides on a top chip upon face-to-face positioning so that the reflecting mirrors form a coupled pair and complete an optical proximity hop. High-speed link measurements were accomplished with chips aligned with a six-axis nano-positioner stage and compared with results for a new precision alignment approach that packages silicon chips for proximity signaling. Our new packaging approach is based on the co-integration of pyramidal etch pits micro-machined into silicon that match a precision micro-sphere for accurate chip alignment. Assemblies of chips can self-align in the package using chip placement that is initially coarse. The final chip alignment accuracy of our new packaging approach is limited by photolithographic resolution. Additionally, multichip arrays can be aligned together with global positioning having similar precision. Nonreturn-to-zero data was fiber launched into the waveguides and transported across a package consisting of a three-chip assembly with two optical proximity hops for inter-chip communication. Continuous wave optical losses, eye diagrams, bit error rates, and power penalties were measured. A passively aligned chip-to-chip optical proximity hop in the package was measured to have optical coupling loss of 4.0 dB, which was 1 dB more than when measured with precision-aligned chips with a nano-positioning stage. RMS jitter and amplitude metrics for the eye quality are shown to be nearly identical (to within 1%) when OPxC hops associated with the package is inserted into 10-Gb/s links. This self-aligned mechanism enables chip packages for several classes of proximity communication.

Silicon-on-Insulator-Based Planar Circuit for Passive Optical Network Applications

A novel design of a silicon-on-insulator (SOI) filter for passive optical network applications is described. The SOI filter comprises a monolithically integrated planar reflective grating and a multistage Mach-Zehnder interferometer. The fabricated device showed low insertion losses and high optical isolation. To the best of our knowledge, this letter describes the first demonstration of an SOI circuit comprising monolithically integrated planar reflective grating and cascaded Mach-Zehnder interferometer

Fabrication Insensitive Echelle Grating in Silicon-on-Insulator Platform

We demonstrate a compact, low crosstalk, low loss, and flat passband demultiplexer based on an echelle grating fabricated in the silicon-on-insulator (SOI) platform. The demonstrated 12 channel demultiplexer has an 8-nm channel spacing, 5.5-nm flat passband, 1.7-dB on-chip loss, and better than 25-dB optical crosstalk. By arranging the output waveguides very close to the zero degree angle of the echelle grating, the performance of the demonstrated device is made insensitive to the vertical angle of the grating facet. The fabricated devices have a very small footprint and have the potential to provide a low-cost demultiplexer solution for multichannel data transmission applications.

Low loss silicon waveguides for application of optical interconnects

We present low loss shallow-ridge silicon waveguides with an average propagation loss of 0.274 dB/cm in the C-band, which can find applications in chip-level optical interconnects.