Power-efficient silicon nitride thermo-optic phase shifters for visible light.

We demonstrate power-efficient, thermo-optic, silicon nitride waveguide phase shifters for blue, green, and yellow wavelengths. The phase shifters operated with low power consumption due to a suspended structure and multi-pass waveguide design. The devices were fabricated on 200-mm silicon wafers using deep ultraviolet lithography as part of an active visible-light integrated photonics platform. The measured power consumption to achieve a π phase shift (averaged over multiple devices) was 0.78, 0.93, 1.09, and 1.20 mW at wavelengths of 445, 488, 532, and 561 nm, respectively. The phase shifters were integrated into Mach-Zehnder interferometer switches, and 10 - 90% rise(fall) times of about 570(590) µs were measured.

Two-dimensional multi-layered SiN-on-SOI optical phased array with wide-scanning and long-distance ranging.

Silicon based optoelectronic integrated optical phased array is attractive owing to large-dense integration, large scanning range and CMOS compatibility. In this paper, we design and fabricate a SiN-on-SOI two-dimensional optical phased array chip. We demonstrate a two-dimensional scanning range of 96°×14.4° and 690 mW peak power of the main lobe. Additionally, we set up the time of flight (ToF) and frequency-modulated continuous-wave (FMCW) ranging systems by using this optical phased array chip, and achieve the objects detection at the range of 20 m in the ToF system and 109 m in the FMCW system, respectively.

Investigation and demonstration of a high-power handling and large-range steering optical phased array chip.

The optical power handling of an OPA scanning beam determines its targeted detection distance. So far, a limited number of investigations have been conducted on the restriction of the beam power. To the best of our knowledge, we for the first time in this paper explore the ability of the silicon photonics based OPA circuit for the high power application. A 64-channel SiN-Si based one-dimensional (1D) OPA chip has been designed to handle high beam power to achieve large scanning range. The chip was fabricated on the standard silicon photonics platform. The main lobe power of our chip can reach 720 mW and its peak side-lobe level (PSLL) is -10.33 dB. We obtain a wide scanning range of 110° in the horizontal direction at 1550 nm wavelength, with a compressed longitudinal divergence angle of each scanning beam of 0.02°.

Broadband couplers for hybrid silicon-chalcogenide glass photonic integrated circuits.

We report on the design, fabrication and testing of three types of coupling structures for hybrid chalcogenide glass Ge23Sb7S70-Silicon (GeSbS-Si) photonic integrated circuit platforms. The first type is a fully etched GeSbS grating coupler defined directly in the GeSbS film. Coupling losses of 5.3 dB and waveguide-to-waveguide back-reflections of 3.4% were measured at a wavelength of 1553 nm. Hybrid GeSbS-to-Si butt couplers and adiabatic couplers transmitting light between GeSbS and Si single-mode waveguides were further developed. The hybrid butt couplers (HBCs) feature coupling losses of 2.7 dB and 9.2% back-reflection. The hybrid adiabatic couplers (HACs) exhibit coupling losses of 0.7 dB and negligible back-reflection. Both HBCs and HACs have passbands exceeding the 100 nm measurement range of the test setup. GeSbS grating couplers and GeSbS-to-Si waveguide couplers can be co-fabricated in the same process flow, providing, for example, a means to first couple high optical power levels required for nonlinear signal processing directly into GeSbS waveguides and to later transition into Si waveguides after attenuation of the pump. Moreover, GeSbS waveguides and HBC transitions have been fabricated on post-processed silicon photonics chips obtained from a commercially available foundry service, with a previously deposited 2 µm thick top waveguide cladding. This fabrication protocol demonstrates the compatibility of the developed integration scheme with standard silicon photonics technology with a complete back-end-of-line process.

Thermal annealing study of the mid-infrared aluminum nitride on insulator (AlNOI) photonics platform.

Aluminum nitride on insulator (AlNOI) photonics platform has great potential for mid-infrared applications thanks to the large transparency window, piezoelectric property, and second-order nonlinearity of AlN. However, the deployment of AlNOI platform might be hindered by the high propagation loss. We perform thermal annealing study and demonstrate significant loss improvement in the mid-infrared AlNOI photonics platform. After thermal annealing at 400°C for 2 hours in ambient gas environment, the propagation loss is reduced by half. Bend loss and taper coupling loss are also investigated. The performance of multimode interferometer, directional coupler, and add/drop filter are improved in terms of insertion loss, quality factor, and extinction ratio. Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction spectroscopy suggest the loss improvement is mainly attributed to the reduction of extinction coefficient in the silicon dioxide cladding. Apart from loss improvement, appropriate thermal annealing also helps in reducing thin film stress.

Aluminum nitride on insulator (AlNOI) platform for mid-infrared photonics.

We report an aluminum nitride on insulator platform for mid-infrared (MIR) photonics applications beyond 3 µm. Propagation loss and bending loss are studied, while functional devices such as directional couplers, multimode interferometers, and add/drop filters are demonstrated with high performance. The complementary metal-oxide-semiconductor-compatible aluminum nitride offers advantages ranging from a large transparency window, high thermal and chemical resistance, to piezoelectric tunability and three-dimensional integration capability. This platform can have synergy with other photonics platforms to enable novel applications for sensing and thermal imaging in MIR.

Efficient and broadband subwavelength grating coupler for 3.7 µm mid-infrared silicon photonics integration.

A grating coupler is an essential building block for compact and flexible photonics integration. In order to meet the increasing demand of mid-infrared (MIR) integrated photonics for sensitive chemical/gas sensing, we report a silicon-on-insulator (SOI) based MIR subwavelength grating coupler (SWGC) operating in the 3.7 µm wavelength range. We provide the design guidelines of a uniform and apodized SWGC, followed by numerical simulations for design verification. We experimentally demonstrate both types of SWGC. The apodized SWGC enables high coupling efficiency of -6.477 dB/facet with 3 dB bandwidth of 199 nm, whereas the uniform SWGC shows larger 3dB bandwidth of 263.5 nm but slightly lower coupling efficiency of -7.371 dB/facet.

Experimental realization of an O-band compact polarization splitter and rotator.

We experimentally realize a compact wideband polarization splitter and rotator (PSR) with CMOS compatibility. The fabricated PSR is then tested by utilizing a fabrication-tolerant TE-pass on-chip polarizer we propose to practically solve the issue of accurately aligning the polarizations in fiber and modes on chip. Both of these polarization handling devices take the advantage of bend structure that confines TE mode better than TM mode. The fabricated PSR has a high TM-TE and TE-TE mode conversion efficiency of -0.4 dB and -0.2 dB at 1310 nm, while the extinction ratio is better than 18 dB and the broad bandwidth exceeds 100 nm.

Aluminum nitride electro-optic phase shifter for backend integration on silicon.

An AlN electro-optic phase shifter with a parallel plate capacitor structure is fabricated on Si using the back-end complementary metal-oxide-semiconductor technology, which is feasible for multilayer photonics integration. The modulation efficiency (Vπ⋅Lπ product) measured from the fabricated waveguide-ring resonators and Mach-Zehnder Interferometer (MZI) modulators near the 1550-nm wavelength is ∼240 V⋅cm for the transverse electric (TE) mode and ∼320 V⋅cm for the transverse magnetic (TM) mode, from which the Pockels coefficient of the deposited AlN is deduced to be ∼1.0 pm/V for both TE and TM modes. The methods for further modulation efficiency improvement are addressed.

Compact highly-efficient polarization splitter and rotator based on 90° bends.

We propose a compact highly-efficient CMOS-compatible polarization splitter and rotator (PSR) with a wide bandwidth covering the whole O-band. It benefits from the different confinement capability of TE and TM modes in bend structure. This bend structure helps shorten the PSR and maintain high efficiency, achieving the bending, polarization splitting, rotating of light beam at the same time. Numerical simulations utilizing Lumerical 3-D FDTD solutions demonstrate that the present PSR has a high TM-TE conversion efficiency of -0.11 dB and high TE-TE conversion efficiency of -0.09 dB at 1310 nm, while the extinction ratio is 27.36 dB and 30.61 dB respectively.

Passively mode-locked III-V/silicon laser with continuous-wave optical injection.

We demonstrate electrically pumped two-section mode locked quantum well lasers emitting at the L-band of telecommunication wavelength on silicon utilizing die to wafer bonding techniques. The mode locked lasers generate pulses at a repetition frequency of 30 GHz with signal to noise ratio above 30 dB and 1 mW average output power per facet. Optical injection-locking scheme was used to improve the noise properties of the pulse trains of passively mode-locked laser. The phases of the mode-locked frequency comb are shown to be coherent with that of the master continuous-wave (CW) laser. The radio-frequency (RF)-line-width is reduced from 7.6 MHz to 150 kHz under CW optical injection. The corresponding pulse-to-pulse jitter and integrated RMS jitter are 29.7 fs/cycle and 1.0 ps, respectively. The experimental results demonstrate that optical injection can reduce the noise properties of the passively mode locked III-V/Si laser in terms of frequency linewidth and timing jitter, which makes the devices attractive for photonic analog-to-digital converters and clock generation and recovery.

Hybrid III-V/silicon laser with laterally coupled Bragg grating.

In this paper, we demonstrate a compact electrically pumped distributed-feedback hybrid III-V/silicon laser with laterally coupled Bragg grating for the first time to the best of our knowledge. The hybrid laser structure consists of AlGaInAs/InP multi-quantum-well gain layers on top of a laterally corrugated silicon waveguide patterned on a silicon on insulator (SOI) substrate. A pair of surface couplers is integrated at the two ends of the silicon waveguide for the optical coupling and characterization of the ouput light. Single wavelength emission of ~1.55µm with a side-mode-suppression- ratio larger than 20dB and low threshold current density of 1.54kA/cm(2) were achieved for the device under pulsed operation at 20 °C.

Low-loss partial rib polarization rotator consisting only of silicon core and silica cladding.

We present a low-loss and small-footprint polarization rotator based on mode evolution. The polarization rotator is composed of an asymmetric-rib waveguide and a tapered waveguide, both of which consist only of a silicon core and a silica cladding. The rotator is fabricated under the same design rules as other device blocks, such as rib-waveguide phase shifters for photonic integration. The polarization rotator is fabricated using CMOS-based processes and provides polarization rotations with an on-chip insertion loss lower than 0.5 dB from transverse-electric (TE) to transverse-magnetic (TM) polarization and a loss lower than 1.0 dB from the TM to TE polarization in a 200 nm wavelength range extending over C and L bands.

Guided mode resonance enabled ultra-compact Germanium photodetector for 1.55 µm detection.

We propose a novel technique of enhancing the photodetection capabilities of ultrathin Ge films for normally incident light at 1.55 µm through the guided mode resonance (GMR) phenomenon. Specifically, by suitably patterning the surface of a Ge thin film, it is possible to excite guided modes which are subsequently coupled to free space radiative modes, resulting in spectral resonances that possess locally enhanced near fields with a large spatial extent. Absorption is found to be enhanced by over an order of magnitude over a pristine Ge film of equal thickness. Furthermore, attenuation of incident light for such a structure occurs over very few grating periods, resulting in significantly enhanced theoretical 3 dB bandwidth-efficiency products of ~58 GHz. The nature of the enhancement mechanism also produces spectrally narrow resonances (FWHM ~30 nm) that are polarization sensitive and exhibit excellent angular tolerance. Finally, the proposed device architecture is fully compatible with existing Si infrastructure and current CMOS fabrication processes.

CMOS compatible monolithic multi-layer Si3N4₋ on-SOI platform for low-loss high performance silicon photonics dense integration.

We demonstrated a low-loss CMOS-compatible multi-layer platform using monolithic back-end-of-line (BEOL) integration. 0.8dB/cm propagation loss is measured for the PECVD Si3N4 waveguide at 1580nm wavelength. The loss is further reduced to 0.24dB/cm at 1270nm wavelength, justifying the platform's feasibility for O-band operation. An inter-layer transition coupler is designed, achieving less than 0.2dB/transition loss across 70nm bandwidth. This is the lowest inter-layer transition loss ever reported. A thermally tuned micro-ring filter is also integrated on the platform, with performance comparable to similar device on SOI platform.

Thermal characterization of electrical tracing-assisted dual-microring optical sensors.

In this paper, we study the temperature sensitivity of an electrical tracing-assisted dual-microring optical sensor, which consists of a sensing ring to detect the refractive index change on its surface and a tracing ring to trace the resonance wavelength shift of the sensing ring by the thermo-optic effect with a heating electrode on it. The wavelength shift measurement is therefore changed to electrical power variation measurement. Thanks to the real-time compensation effect of the tracing ring, the temperature dependence of the sensor is found to be intrinsically low. The resonance wavelength temperature sensitivity difference between the two rings is measured to be as low as 10.1 pm/°C, showing that the temperature dependence of the sensor in terms of wavelength per degree is reduced by ~6 times compared to that of a single ring sensor. The temperature sensitivity of the sensor in terms of electrical power per degree is measured to be -0.077 mW/°C. By using tracing ring with enhanced tuning efficiency, this value can be further decreased to -0.0057 W/°C. The experimental results agree well with the expectation. This type of sensors with low temperature dependence has great potential to be deployed in various practical point-of-care diagnostic applications.

High-efficiency Si optical modulator using Cu travelling-wave electrode.

We demonstrate a high-efficiency and CMOS-compatible silicon Mach-Zehnder Interferometer (MZI) optical modulator with Cu traveling-wave electrode and doping compensation. The measured electro-optic bandwidth at Vbias = -5 V is above 30 GHz when it is operated at 1550 nm. At a data rate of 50 Gbps, the dynamic extinction ratio is more than 7 dB. The phase shifter is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (Vπ·Lπ) of ~18.5 V·mm. Such a Cu-photonics technology provides an attractive potentiality for integration development of silicon photonics and CMOS circuits on SOI wafer in the future.

Novel integration technique for silicon/III-V hybrid laser.

Integrated semiconductor lasers on silicon are one of the most crucial devices to enable low-cost silicon photonic integrated circuits for high-bandwidth optic communications and interconnects. While optical amplifiers and lasers are typically realized in III-V waveguide structures, it is beneficial to have an integration approach which allows flexible and efficient coupling of light between III-V gain media and silicon waveguides. In this paper, we propose and demonstrate a novel fabrication technique and associated transition structure to realize integrated lasers without the constraints of other critical processing parameters such as the starting silicon layer thicknesses. This technique employs epitaxial growth of silicon in a pre-defined trench with taper structures. We fabricate and demonstrate a long-cavity hybrid laser with a narrow linewidth of 130 kHz and an output power of 1.5 mW using the proposed technique.

Microring resonator photodetector for enhancement in L-band performance.

We proposed a microring resonator (MRR) enhanced photodetector (PD) structure. Resonance wavelength enhanced by the MRR amplifies the PD response. At L-band wavelengths, responsivity was doubled for an ultra-short germanium PD of 4 µm employing the MRR structure. Data rates of up to 40 Gb/s were also demonstrated at 1600 nm.

Three-dimensional (3D) monolithically integrated photodetector and WDM receiver based on bulk silicon wafer.

We propose a novel three-dimensional (3D) monolithic optoelectronic integration platform. Such platform integrates both electrical and photonic devices in a bulk silicon wafer, which eliminates the high-cost silicon-on-insulator (SOI) wafer and is more suitable for process requirements of electronic and photonic integrated circuits (ICs). For proof-of-concept, we demonstrate a three-dimensional photodetector and WDM receiver system. The Ge is grown on a 8-inch bulk silicon wafer while the optical waveguide is defined in a SiN layer which is deposited on top of it, with ~4 µm oxide sandwiched in between. The light is directed to the Ge photodetector from the SiN waveguide vertically by using grating coupler with a Aluminum mirror on top of it. The measured photodetector responsivity is ~0.2 A/W and the 3-dB bandwidth is ~2 GHz. Using such vertical-coupled photodetector, we demonstrated an 8-channel receiver by integrating a 1 × 8 arrayed waveguide grating (AWG). High-quality optical signal detection with up to 10 Gbit/s data rate is demonstrated, suggesting a 80 Gbit/s throughput. Such receiver can be applied to on-chip optical interconnect, DRAM interface, and telecommunication systems.