High positional freedom SOI subwavelength grating coupler (SWG) for 300 mm foundry fabrication.

We present an apodized, single etch-step, subwavelength grating (SWG) high positional freedom (HPF) grating coupler based on the 220 nm silicon-on-insulator (SOI) with 2µm BOX substrate. The grating coupler was designed for 1550 nm light with transverse electric (TE) polarization. It has a measured maximum coupling efficiency of -7.49 dB (17.8%) and a -1 dB/-3 dB bandwidth of ~14 nm/29.5 nm respectively. It was fabricated in a 300mm state of the art CMOS foundry. This work presents an SOI-based grating coupler with the highest-to the best of our knowledge- -1 dB single mode fiber lateral alignment of 21.4 µm × 10.1 µm.

High-Q-factor Al2O3 micro-trench cavities integrated with silicon nitride waveguides on silicon.

We report on the design and performance of high-Q integrated optical micro-trench cavities on silicon. The microcavities are co-integrated with silicon nitride bus waveguides and fabricated using wafer-scale silicon-photonics-compatible processing steps. The amorphous aluminum oxide resonator material is deposited via sputtering in a single straightforward post-processing step. We examine the theoretical and experimental optical properties of the aluminum oxide micro-trench cavities for different bend radii, film thicknesses and near-infrared wavelengths and demonstrate experimental Q factors of > 106. We propose that this high-Q micro-trench cavity design can be applied to incorporate a wide variety of novel microcavity materials, including rare-earth-doped films for microlasers, into wafer-scale silicon photonics platforms.

Whispering gallery germanium-on-silicon photodetector.

We design and demonstrate, to the best of our knowledge, the first whispering gallery germanium-on-silicon photodetector with evanescent coupling from a silicon bus waveguide in a CMOS-compatible process. The small footprint (63.6 µm2), high responsivity (∼1.04 A/W at 1530 nm), low bias voltage (-1 V), low dark current (2.03 nA), and large optoelectric bandwidth (32.9 GHz) of the detector enable simultaneous wavelength filtering and power detection, ideal for handling large network data traffic. In addition, with the resonant nature of the detector, we also optimize the design to enable long-wavelength detection, achieving a separate device with a detection range of up to 1630 nm with a >0.45 A/W responsivity, making it an important building block for optical communication networks.

Ultra-narrow-linewidth Al2O3:Er3+ lasers with a wavelength-insensitive waveguide design on a wafer-scale silicon nitride platform.

We report ultra-narrow-linewidth erbium-doped aluminum oxide (Al2O3:Er3+) distributed feedback (DFB) lasers with a wavelength-insensitive silicon-compatible waveguide design. The waveguide consists of five silicon nitride (SiNx) segments buried under silicon dioxide (SiO2) with a layer Al2O3:Er3+ deposited on top. This design has a high confinement factor (> 85%) and a near perfect (> 98%) intensity overlap for an octave-spanning range across near infra-red wavelengths (950-2000 nm). We compare the performance of DFB lasers in discrete quarter phase shifted (QPS) cavity and distributed phase shifted (DPS) cavity. Using QPS-DFB configuration, we obtain maximum output powers of 0.41 mW, 0.76 mW, and 0.47 mW at widely spaced wavelengths within both the C and L bands of the erbium gain spectrum (1536 nm, 1566 nm, and 1596 nm). In a DPS cavity, we achieve an order of magnitude improvement in maximum output power (5.43 mW) and a side mode suppression ratio (SMSR) of > 59.4 dB at an emission wavelength of 1565 nm. We observe an ultra-narrow linewidth of ΔνDPS = 5.3 ± 0.3 kHz for the DPS-DFB laser, as compared to ΔνQPS = 30.4 ± 1.1 kHz for the QPS-DFB laser, measured by a recirculating self-heterodyne delayed interferometer (R-SHDI).

Monolithically-integrated distributed feedback laser compatible with CMOS processing.

An optically-pumped, integrated distributed feedback laser is demonstrated using a CMOS compatible process, where a record-low-temperature deposited gain medium enables integration with active devices such as modulators and detectors. A pump threshold of 24.9 mW and a slope efficiency of 1.3 % is demonstrated at the lasing wavelength of 1552.98 nm. The rare-earth-doped aluminum oxide, used as the gain medium in this laser, is deposited by a substrate-bias-assisted reactive sputtering process. This process yields optical quality films with 0.1 dB/cm background loss at the deposition temperature of 250 °C, and therefore is fully compatible as a back-end-of-line CMOS process. The aforementioned laser's performance is comparable to previous lasers having gain media fabricated at much higher temperatures (> 550 °C). This work marks a crucial step towards monolithic integration of amplifiers and lasers in silicon microphotonic systems.

Wavelength division multiplexed light source monolithically integrated on a silicon photonics platform.

We demonstrate monolithic integration of a wavelength division multiplexed light source for silicon photonics by a cascade of erbium-doped aluminum oxide (Al2O3:Er3+) distributed feedback (DFB) lasers. Four DFB lasers with uniformly spaced emission wavelengths are cascaded in a series to simultaneously operate with no additional tuning required. A total output power of -10.9 dBm is obtained from the four DFBs with an average side mode suppression ratio of 38.1±2.5 dB. We characterize the temperature-dependent wavelength shift of the cascaded DFBs and observe a uniform dλ/dT of 0.02 nm/°C across all four lasers.

Mode-evolution-based coupler for high saturation power Ge-on-Si photodetectors.

We propose a mode-evolution-based coupler for high saturation power germanium-on-silicon photodetectors. This coupler uniformly illuminates the intrinsic germanium region of the detector, decreasing saturation effects, such as carrier screening, observed at high input powers. We demonstrate 70% more photocurrent generation (9.1-15.5 mA) and more than 40 times higher opto-electrical bandwidth (0.7-31 GHz) than conventional butt-coupled detectors under high-power illumination. The high-power and high-speed performance of the device, combined with the compactness of the coupling method, will enable new applications for integrated silicon photonics systems.

Large-scale silicon nitride nanophotonic phased arrays at infrared and visible wavelengths.

We demonstrate passive large-scale nanophotonic phased arrays in a CMOS-compatible silicon photonic platform. Silicon nitride waveguides are used to allow for higher input power and lower phase variation compared to a silicon-based distribution network. A phased array at an infrared wavelength of 1550 nm is demonstrated with an ultra-large aperture size of 4 mm×4 mm, achieving a record small and near diffraction-limited spot size of 0.021°×0.021° with a side lobe suppression of 10 dB. A main beam power of 400 mW is observed. Using the same silicon nitride platform and phased array architecture, we also demonstrate, to the best of our knowledge, the first large-aperture visible nanophotonic phased array at 635 nm with an aperture size of 0.5 mm×0.5 mm and a spot size of 0.064°×0.074°.

Ultra-compact and low-threshold thulium microcavity laser monolithically integrated on silicon.

We demonstrate an ultra-compact and low-threshold thulium microcavity laser that is monolithically integrated on a silicon chip. The integrated microlaser consists of an active thulium-doped aluminum oxide microcavity beside a passive silicon nitride bus waveguide, which enables on-chip pump-input and laser-output coupling. We observe lasing in the wavelength range of 1.8-1.9 µm under 1.6 µm resonant pumping and at varying waveguide-microcavity gap sizes. The microlaser exhibits a threshold as low as 773 µW (226 µW) and a slope efficiency as high as 24% (48%) with respect to the pump power coupled into the silicon nitride bus waveguide (microcavity). Its small footprint, minimal energy consumption, high efficiency, and silicon compatibility demonstrate that on-chip thulium lasers are promising light sources for silicon microphotonic systems.

C-band swept wavelength erbium-doped fiber laser with a high-Q tunable interior-ridge silicon microring cavity.

We demonstrate swept-wavelength operation of an erbium-doped fiber laser using a tunable silicon microring cavity. The microring cavity is designed to have 35 nm free spectral range, a high Q of 1.5 × 105, and low insertion loss of <0.05 dB. The resonance wavelength of the cavity is tuned efficiently (8.1µW/GHz) and rapidly (τr,f~2.2µs) using an embedded Si heater. The laser achieves single-mode continuous-wave emission over the C-band (1530 nm-to-1560 nm). A mean swept-wavelength rate of 22,600 nm/s or 3106 THz/s is demonstrated within 1532 nm-to-1542 nm wavelength range. Its linewidth is measured to be 16 kHz using loss-compensated circulating delayed self-heterodyne detection.

Integrated heterodyne interferometer with on-chip modulators and detectors.

We demonstrate, to our knowledge, the first on-chip heterodyne interferometer fabricated on a 300-mm CMOS compatible process that exhibits root-mean-square (RMS) position noise on the order of 2 nm. Measuring 1 mm by 6 mm, the interferometer is also, to our knowledge, the smallest heterodyne interferometer demonstrated to date and will surely impact numerous interferometric and metrology applications, including displacement measurement, laser Doppler velocimetry and vibrometry, Fourier transform spectroscopy, imaging, and light detection and ranging (LIDAR). Here we present preliminary results that demonstrate the displacement mode.

Generating and identifying optical orbital angular momentum with silicon photonic circuits.

Here, we propose and demonstrate a silicon nanophotonic phased array that is capable of generating light carrying optical orbital angular momentum (OAM). Optical beams carrying different orbital angular momenta have been generated. In addition, the generated OAM wavefronts are experimentally identified by interfering with another on-chip generated Gaussian beam, opening up opportunities of integrating conventional optical systems and functionalities on to a silicon chip.

Integrated phased array for wide-angle beam steering.

We demonstrate an on-chip optical phased array fabricated in a CMOS compatible process with continuous, fast (100 kHz), wide-angle (51°) beam-steering suitable for applications such as low-cost LIDAR systems. The device demonstrates the largest (51°) beam-steering and beam-spacing to date while providing the ability to steer continuously over the entire range. Continuous steering is enabled by a cascaded phase shifting architecture utilizing, low power and small footprint, thermo-optic phase shifters. We demonstrate these results in the telecom C-band, but the same design can easily be adjusted for any wavelength between 1.2 and 3.5 µm.

Monolithic erbium- and ytterbium-doped microring lasers on silicon chips.

We demonstrate monolithic 160-µm-diameter rare-earth-doped microring lasers using silicon-compatible methods. Pump light injection and laser output coupling are achieved via an integrated silicon nitride waveguide. We measure internal quality factors of up to 3.8 × 105 at 980 nm and 5.7 × 105 at 1550 nm in undoped microrings. In erbium- and ytterbium-doped microrings we observe single-mode 1.5-µm and 1.0-µm laser emission with slope efficiencies of 0.3 and 8.4%, respectively. Their small footprints, tens of microwatts output powers and sub-milliwatt thresholds introduce such rare-earth-doped microlasers as scalable light sources for silicon-based microphotonic devices and systems.

CMOS-compatible 75 mW erbium-doped distributed feedback laser.

On-chip, high-power, erbium-doped distributed feedback lasers are demonstrated in a CMOS-compatible fabrication flow. The laser cavities consist of silicon nitride waveguide and grating features, defined by wafer-scale immersion lithography and an erbium-doped aluminum oxide layer deposited as the final step in the fabrication process. The large mode size lasers demonstrate single-mode continuous wave operation with a maximum output power of 75 mW without any thermal damage. The laser output power does not saturate at high pump intensities and is, therefore, capable of delivering even higher on-chip signals if a stronger pump is utilized. The amplitude noise of the laser is investigated and the laser is shown to be stable and free from self-pulsing when the pump power is sufficiently above threshold.

Two-dimensional apodized silicon photonic phased arrays.

In this Letter, we demonstrate an 8×8 apodized silicon photonic phased array where the emission from each of 64 nanoantennas was tailored to exhibit Gaussian-shaped intensity distributions in the near field so that the sidelobes of the generated far-field optical beam were suppressed compared to that of a uniform phased array. With the aid of the 72 thermo-optic phase tuners directly integrated within the phased array, we dynamically shaped the generated optical beam in the far field in a variety of ways.

Four-port integrated polarizing beam splitter.

In this Letter, we report on the first integrated four-port polarizing beam splitter. The device operates on the principle of mode evolution and was implemented in a silicon-on-insulator silicon photonics platform and fabricated on a 300 mm CMOS line using 193 nm optical immersion lithography. The adiabatic transition forming of the structure enabled over a 150 nm bandwidth from λ~1350 to λ~1500 nm, achieving a cross-talk level below -10 dB over the entire band.

Silicon wavelength-selective partial-drop broadcast filter bank.

We propose an approach to a wavelength-selective 1×N port optical broadcast network demonstrating the approach in a 1×8 port parallel optical drop filter bank utilizing adiabatic micro-ring tunable filters. The micro-ring filters exhibit first-order 92.7±3.7 GHz full width at half-maximum bandwidths with a 36.2 nm free spectral range, low-drop power variation (0.11 dB), and aggregate excess loss of only 1.1 dB in all drop ports. Error-free operation at a 10 Gbit/s data rate is achieved for all eight drop ports with less than a 0.5 dB power penalty among the ports. This wavelength-selective parallel-drop approach serves as a building block for on-chip all-to-all communication networks.

Uniformly spaced λ/4-shifted Bragg grating array with wafer-scale CMOS-compatible process.

We report on an integrated λ/4-shifted Bragg grating array using a wafer-scale complementary metal-oxide semiconductor (CMOS) compatible process with silicon-nitride waveguides. A sidewall grating was used to simplify the fabrication process, and a sampled Bragg grating with equivalent phase-shift structure was employed to achieve an accurate λ/4 phase shift. A four-channel λ/4-shifted Bragg grating array with highly uniform channel spacing was demonstrated with a measured channel spacing variation below 10 pm (1.25 GHz). The high channel-spacing uniformity and the CMOS-compatibility of the demonstrated device hold promise for integrated distributed feedback laser arrays for various silicon photonic applications.

Method for characterization of Si waveguide propagation loss.

A new method for measuring waveguide propagation loss in silicon nanowires is presented. This method, based on the interplay between traveling ring modes and standing wave modes due to back-scattering from edge roughness, is accurate and can be used for on wafer measurement of test structures. Examples of loss measurements and fitting are reported.