Error-free operation of a polarization-insensitive 4λ x 25 Gbps silicon photonic WDM receiver with closed-loop thermal stabilization of Si microrings.

We report the first closed-loop operation of a 100 Gbps polarization-insensitive, 4-channel wavelength-tracking WDM receiver in silicon photonics platform. Error-free operation is achieved with input polarization scrambling over input wavelength change of 4.5 nm using efficient thermal tuning of Si microring demux, corresponding to greater than 60°C fluctuation in temperature.

Efficient, tunable flip-chip-integrated III-V/Si hybrid external-cavity laser array.

We demonstrate a surface-normal coupled tunable hybrid silicon laser array for the first time using passively-aligned, high-accuracy flip chip bonding. A 2x6 III-V reflective semiconductor optical amplifier (RSOA) array with integrated total internal reflection mirrors is bonded to a CMOS SOI chip with grating couplers and silicon ring reflectors to form a tunable hybrid external-cavity laser array. Waveguide-coupled wall plug efficiency (wcWPE) of 2% and output power of 3 mW has been achieved for all 12 lasers. We further improved the performance by reducing the thickness of metal/dielectric stacks and achieved 10mW output power and 5% wcWPE with the same integration techniques. This non-invasive, one-step back end of the line (BEOL) integration approach provides a promising solution to high density laser sources for future large-scale photonic integrated circuits.

Demonstration of 12.2% wall plug efficiency in uncooled single mode external-cavity tunable Si/III-V hybrid laser.

A Si/III-V hybrid laser has been a highly sought after device for energy-efficient and cost-effective high-speed silicon photonics communication. We present a high wall-plug efficiency external-cavity hybrid laser created by integrating an independently optimized SOI ring reflector and a III-V gain chip. In our demonstration, the uncooled integrated laser achieved a waveguide-coupled wall-plug efficiency of 12.2% at room temperature with an optical output power of ~10 mW. The laser operated single-mode near 1550 nm with a linewidth of 0.22 pm. This is a tunable light source with 8 nm wavelength tuning range. A proof-of-concept laser wavelength stabilization technique has also been demonstrated. Using a simple feedback loop, we achieved mode-hop-free operation in a packaged external-cavity hybrid laser as bias current was varied by 60mA.

Microring-based multi-chip WDM photonic module.

We report on a packaged prototype of a WDM photonic transceiver. It is an all-solid state hybrid assembly based on 130nm SOI photonic circuitry integrated with a 40nm CMOS VLSI driver. Our prototype supports eight tunable WDM channels operating at 10Gb/s, each capable of both transmitting and receiving data on the same chip. We discuss two options to close the link using the optical fiber or a waveguide bridge chip. We provide integration details and supporting link measurement data to describe packaged photonic module and its power efficient functionality with its on-chip power per channel averaging 1.3pJ/bit, excluding off-chip laser electrical power.

An all-solid-state, WDM silicon photonic digital link for chip-to-chip communications.

We describe a multiwavelength hybrid-integrated solid-state link on a 3 µm silicon-on-insulator (SOI) nanophotonic platform. The link spans three chips and employs germanium-silicon electroabsorption waveguide modulators, silicon transport waveguides, echelle gratings for multiplexing and demultiplexing, and pure germanium waveguide photo-detectors. The 8λ WDM Tx and Rx components are interconnected via a routing "bridge" chip using edge-coupled optical proximity communication. The packaged, retimed digital WDM link is demonstrated at 10 Gb/s and 10(-12) BER, with three wavelength channels consuming an on-chip power below 1.5 pJ/bit, excluding the external laser power.

A high-speed, tunable silicon photonic ring modulator integrated with ultra-efficient active wavelength control.

We report the first complete 10G silicon photonic ring modulator with integrated ultra-efficient CMOS driver and closed-loop wavelength control. A selective substrate removal technique was used to improve the ring tuning efficiency. Limited by the thermal tuner driver output power, a maximum open-loop tuning range of about 4.5nm was measured with about 14mW of total tuning power including the heater driver circuit power consumption. Stable wavelength locking was achieved with a low-power mixed-signal closed-loop wavelength controller. An active wavelength tracking range of > 500GHz was demonstrated with controller energy cost of only 20fJ/bit.

High power and widely tunable Si hybrid external-cavity laser for power efficient Si photonics WDM links.

A highly efficient silicon (Si) hybrid external cavity laser with a wavelength tunable ring reflector is fabricated on a complementary metal-oxide semiconductor (CMOS)-compatible Si-on-insulator (SOI) platform and experimental results with high output power are demonstrated. A III-V semiconductor gain chip is edge-coupled into a SOI cavity chip through a SiN(x) spot size converter and Si grating couplers are incorporated to enable wafer-scale characterization. The laser output power reaches 20 mW and the highest wall-plug efficiency of 7.8% is measured at 17.3 mW in un-cooled condition. The laser wavelength tuning ranges are 8 nm for the single ring reflector cavity and 35 nm for the vernier ring reflector cavity, respectively. The Si hybrid laser is a promising light source for energy-efficient Si CMOS photonic links.

Efficient, high-power, narrow-linewidth, continuous-wave quantum-dot semiconductor comb laser.

We report a continuous-wave, O-band quantum-dot semiconductor comb laser for WDM optical interconnects exhibiting a 2.2 THz optical bandwidth with up to 89 comb wavelengths spaced at 25 GHz, over 30% peak ex-facet electrical-to-optical power conversion efficiency, up to 270 mW of usable laser power, relative intensity noise below - 135 dB/Hz per individual mode, individual laser mode linewidth of 140 kHz, mode beating linewidths of 50 kHz across all modes, and stable far-field output with 75% coupling efficiency to PM fiber in a butterfly package.

Vertical-coupled high-efficiency tunable III-V- CMOS SOI hybrid external-cavity laser.

We demonstrate a hybrid III-V/SOI laser by vertically coupling a III-V RSOA chip with a SOI-CMOS chip containing a tunable wavelength selective reflector. We report a waveguide-coupled wall-plug-efficiency of 5.5% and output power of 10 mW. A silicon resistor-based microheater was integrated to thermally tune a ring resonator for precise lasing wavelength control. A high tuning efficiency of 2.2 nm/mW over a range of 18 nm was achieved by locally removing the SOI handler substrate. C-band single mode lasing was confirmed with a side mode suppression ratio of 35 dB. This grating coupler based vertical integration approach can be scaled up in two dimensions for efficient multi-wavelength sources in silicon photonics.

Enhanced optical bistability from self-heating due to free carrier absorption in substrate removed silicon ring modulators.

We show enhanced optical bistability induced by free carrier absorption from junction doping in substrate-removed silicon ring modulators. Such linear thermal effects dominate the loss in high-speed depletion silicon ring modulators. Optical bistability was observed with about 100 µW of input optical power. We further show that such thermal interactions causes data-dependent ring resonance shifts, and consequently severely degrade the data modulation quality at low speeds. The frequency response of this effect was measured to be about 100~200 kHz.

High speed GeSi electro-absorption modulator at 1550 nm wavelength on SOI waveguide.

We demonstrate a high speed GeSi electro-absorption (EA) modulator monolithically integrated on 3 µm silicon-on-insulator (SOI) waveguide. The demonstrated device has a compact active region of 1.0 × 55 µm(2), an insertion loss of 5 dB and an extinction ratio of 6 dB at wavelength of 1550 nm. The modulator has a broad operating wavelength range of 35 nm and a 3 dB bandwidth of 40.7 GHz at 2.8 V reverse bias. This compact and energy efficient modulator is a key building block for optical interconnection applications.

Improving CMOS-compatible Germanium photodetectors.

We report design improvements for evanescently coupled Germanium photodetectors grown at low temperature. The resulting photodetectors with 10 µm Ge length manufactured in a commercial CMOS process achieve >0.8 A/W responsivity over the entire C-band, with a device capacitance of <7 fF based on measured data.

Ultralow-loss, high-density SOI optical waveguide routing for macrochip interconnects.

We report optical waveguides up to one meter long with 0.026 dB/cm loss fabricated in a 300nm thick SOI CMOS process. Combined with tight bends and compact interlayer grating couplers, we demonstrate a complete toolbox for ultralow-loss, high-density waveguide routing for macrochip interconnects.

25Gb/s 1V-driving CMOS ring modulator with integrated thermal tuning.

We report a high-speed ring modulator that fits many of the ideal qualities for optical interconnect in future exascale supercomputers. The device was fabricated in a 130 nm SOI CMOS process, with 7.5 µm ring radius. Its high-speed section, employing PN junction that works at carrier-depletion mode, enables 25 Gb/s modulation and an extinction ratio >5 dB with only 1V peak-to-peak driving. Its thermal tuning section allows the device to work in broad wavelength range, with a tuning efficiency of 0.19 nm/mW. Based on microwave characterization and circuit modeling, the modulation energy is estimated ~7 fJ/bit. The whole device fits in a compact 400 µm2 footprint.

30GHz Ge electro-absorption modulator integrated with 3 µm 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.

Design and fabrication of 3µm silicon-on-insulator waveguide integrated Ge electro-absorption modulator.

We present the design and fabrication of a waveguide-based Ge electro-absorption (EA) modulator integrated with a 3 µm silicon-on-isolator (SOI) waveguide. The proposed Ge EA modulator employs a butt-coupled horizontally-oriented p-i-n structure. The optical design achieves a low-loss transition from Ge to Si waveguides. The interaction between the optical mode of the waveguide and the bias induced electric field in the p-i-n structure was maximized to achieve high modulation efficiency. By balancing the trade-offs between the extinction ratio and the insertion loss of the device, an optimal working regime was identified. The measurement results from a fabricated device were used to verify the design. Under a -4Vpp reverse bias, the device demonstrates a total insertion loss (including the transition loss) of 2.7-5.2 dB and an extinction ratio of 4.9-8.2 dB over the wavelength range of 1610-1640 nm. Subtracting the contribution of the transition loss, the Δα/α value for the fabricated device was estimated to be between 2.2 and 3.2 with an electric field around 55 kV/cm.

Ultra-efficient 10 Gb/s hybrid integrated silicon photonic transmitter and receiver.

Using low parasitic microsolder bumping, we hybrid integrated efficient photonic devices from different platforms with advanced 40 nm CMOS VLSI circuits to build ultra-low power silicon photonic transmitters and receivers for potential applications in high performance inter/intra-chip interconnects. We used a depletion racetrack ring modulator with improved electro-optic efficiency to allow stepper optical photo lithography for reduced fabrication complexity. Integrated with a low power cascode 2 V CMOS driver, the hybrid silicon photonic transmitter achieved better than 7 dB extinction ratio for 10 Gbps operation with a record low power consumption of 1.35 mW. A received power penalty of about 1 dB was measured for a BER of 10(-12) compared to an off-the-shelf lightwave LiNOb3 transmitter, which comes mostly from the non-perfect extinction ratio. Similarly, a Ge waveguide detector fabricated using 130 nm SOI CMOS process was integrated with low power VLSI circuits using hybrid bonding. The all CMOS hybrid silicon photonic receiver achieved sensitivity of -17 dBm for a BER of 10(-12) at 10 Gbps, consuming an ultra-low power of 3.95 mW (or 395 fJ/bit in energy efficiency). The scalable hybrid integration enables continued photonic device improvements by leveraging advanced CMOS technologies with maximum flexibility, which is critical for developing ultra-low power high performance photonic interconnects for future computing systems.

Submilliwatt, ultrafast and broadband electro-optic silicon switches.

We present a broadband 2x2 electro-optic silicon switch with an ultralow switching power and fast switching time based on a Mach-Zehnder interferometer (MZI). Forward-biased p-i-n junctions are employed to tune the phase of silicon waveguides in the MZI, to achieve a π-phase switching power of 0.6 mW with a drive voltage 0.83 V with a MZI arm length of 4 mm. The 10%-90% switching time is demonstrated to be 6 ns. Optical crosstalk levels lower than -17 dB are obtained for an optical bandwidth of 60 nm. The free carrier induced silicon refractive index change is extracted from the experimental results for the concentration range from 10(16) to 10(17) cm(-3). We find that at the concentration of 10(16) cm(-3), the index change is about twice that calculated by the commonly used index change equation.

Low power and compact reconfigurable multiplexing devices based on silicon microring resonators.

We present thermally reconfigurable multiplexing devices based on silicon microring resonators with low tuning power and low thermal crosstalk. Micro-heaters on top of the rings are employed to tune the resonant wavelengths through the thermo-optic effect of silicon. We achieve a low tuning power of 21 mW per free spectral range for a single ring by exploiting thermal isolation trenches close to the ring waveguides. Negligible thermal crosstalk is demonstrated for rings spaced by 15 microm, enabling compact multiplexing devices. The tuning time constant is demonstrated to be less than 10 micros.

Thermally tunable silicon racetrack resonators with ultralow tuning power.

We present thermally tunable silicon racetrack resonators with an ultralow tuning power of 2.4 mW per free spectral range. The use of free-standing silicon racetrack resonators with undercut structures significantly enhances the tuning efficiency, with one order of magnitude improvement of that for previously demonstrated thermo-optic devices without undercuts. The 10%-90% switching time is demonstrated to be ~170 µs. Such low-power tunable micro-resonators are particularly useful as multiplexing devices and wavelength-tunable silicon microcavity modulators.