Silicon Photonic Chip Research Articles

Silicon Photonics Rectangular Universal Interferometer

AbstractUniversal multiport photonic interferometers that can implement any arbitrary unitary transformation between input and output optical modes are essential to support advanced optical functions. Integrated versions of these components can be implemented by means of either a fixed triangular or a fixed rectangular arrangement of the same components. We propose the implementation of a fixed rectangular universal interferometer using a reconfigurable hexagonal waveguide mesh circuit. A suitable adaptation synthesis algorithm tailored to this mesh configuration is provided and the experimental demonstration of a rectangular multiport interferometer by means of a fabricated silicon photonics chip is reported. The 7‐hexagonal cell chip can implement 2 × 2, 3 × 3 and 4 × 4 arbitrary unitary transformations. The proposed hexagonal waveguide mesh operates in a similar way as a Field Programmable Gate Array (FPGA) in electronics. We believe that this work represents an important step‐forward towards fully programmable and integrable multiport interferometers.

Label-free detection of the IL-6 and IL-8 interleukines through monolithic silicon photonic chips and simultaneous dual polarization optics

A silicon microphotonic chip with ten monolithically integrated interferometers and an equal number of light sources is employed for the label-free and multiplexed detection of two proteins of the cytokine family. The chip implements broad-band Mach-Zehnder interferometry and is used for the detection of the IL-6 and IL-8 interleukins. The silicon transducer is made through mainstream silicon technology and integrates avalanche diode based white light LEDs and monomodal waveguides forming Mach-Zehnder interferometers. The light spectrum is modulated through the interferometer and the spectrally resolved interference fringes are monitored by a digital spectrometer. Biomolecular built-up on the sensing arm causes pattern shifts that are evaluated through Fourier Transform. The microsystem level set-up includes microfluidic components, direct contacting of the chip contact pads through spring loaded pins and alignment guides and a spectrometer. The dual polarization optics allows the simultaneous derivation of the TE and TM sensitivity plots and the calculation of the refractive indices of either protein.

Compact high-efficiency perfectly-vertical grating coupler on silicon at O-band.

A compact, high-efficiency grating coupler is demonstrated for interfacing a silicon waveguide and a perfectly-vertical fiber at O-band. The grating lies on a tilted silicon membrane for minimizing the reflections. Circular grating lines are adopted to shorten the overall device length to about 60μm. 57% peak coupling efficiency and >28nm 1-dB coupling bandwidth are obtained experimentally. Back reflections of 1% to the silicon waveguide and the single mode fiber are theoretically estimated. The processing flow to realize the proposed structure is discussed in detail. The fabrication control over the tilted angle of the silicon membrane is investigated. The approach by applying an oxide cladding to improve the stability of the membrane is also introduced. The present grating coupler is compatible to common fabrication processes for silicon photonic chips.

High-Thermal Performance 3D Hybrid Silicon Lasers

A 3D integrated hybrid silicon laser was realized for high-thermal performance by integrating silicon photonic (SiPh) chips and indium phosphide (InP) chips. The optical gain is provided by the InP chip with a total internal reflection mirror for surface emission. The surface grating couplers on the SiPh chip couples light into a silicon waveguide. The InP chips were flip-chip bonded P-side down to metal pads on the silicon chips. Two lasers are reported. For laser A, the InP chip was bonded on the top cladding oxide of the silicon waveguide. For laser B, the InP chip was bonded to the silicon substrate in an etched recess. Both lasers demonstrate milliwatt-level light coupled into the silicon waveguide. Laser B demonstrated three times better thermal performance with a measured thermal impedance of 6.2 °C/W.

Chip-scale demonstration of hybrid III–V/silicon photonic integration for an FBG interrogator

Silicon photonic integration is a means to produce an integrated on-chip fiber Bragg grating (FBG) interrogator. The possibility of integrating the light source, couplers, grating couplers, de-multiplexers, photodetectors (PDs), and other optical elements of the FBG interrogator into one chip may result in game-changing performance advances, considerable energy savings, and significant cost reductions. To the best of our knowledge, this paper is the first to present a hybrid silicon photonic chip based on III–V/silicon-on-insulator photonic integration for an FBG interrogator. The hybrid silicon photonic chip consists of a multiwavelength vertical-cavity surface-emitting laser array and input grating couplers, a multimode interference coupler, an arrayed waveguide grating, output grating couplers, and a PD array. The chip can serve as an FBG interrogator on a chip and offer unprecedented opportunities. With a footprint of 5 mm×3 mm, the proposed hybrid silicon photonic chip achieves an interrogation wavelength resolution of approximately 1 pm and a wavelength accuracy of about ±10 pm. With the measured 1 pm wavelength resolution, the temperature measurement resolution of the proposed chip is approximately 0.1°C. The proposed hybrid silicon photonic chip possesses advantages in terms of cost, manufacturability, miniaturization, and performance. The chip supports applications that require extreme miniaturization down to the level of smart grains.

(Invited) Rare Earth Doped Light Emitting Thin Film Materials for Silicon Photonics

Silicon photonic technology is becoming ubiquitous for data center, sensing and advanced photonic applications. However, one of the key challenges for silicon photonic microsystems is integrating materials which can provide optical gain. The leading approach involves hybrid bonding of III-V materials to silicon chips, which is expensive and difficult to scale. Alternatively, rare-earth-doped materials are promising for active device applications on silicon. Rare earth doped oxide thin films can be deposited using standard, low-cost, and wafer-scale methods directly on silicon and emit light in important bands for communications and other emerging applications. This presentation will cover recent progress on integrating rare-earth-doped materials into silicon photonic microsystems. It will focus on fabrication and integration methods, particularly reactive magnetron co-sputtering, which yields low-loss, high-gain thin films which can be deposited using a single post-processing step onto silicon photonic chips that have been fabricated in a silicon foundry. The fabrication challenges, spectroscopic properties, and device design requirements related to prospective materials, including ytterbium-, erbium- and thulium-doped oxides will be discussed. The talk will review the application of such films in light-emitting rare-earth-doped devices, including amplifiers and continuous-wave and pulsed lasers.

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Circular core single-mode polymer optical waveguide fabricated using the Mosquito method with low loss at 1310/1550 nm.

We fabricate low-loss single-mode (SM) polymer optical waveguides using a photomask-free simple technique named the Mosquito method. The insertion losses of a 5-cm long SM polymer waveguide fabricated are 2.52 dB and 4.03 dB at 1310- and 1550-nm wavelengths, respectively. The coupling loss between a single-mode fiber and the waveguide is as low as 0.5 dB including the Fresnel reflection. The 0.5-dB misalignment tolerance in the radial direction is ± 2.0 μm at 1550 nm. The Mosquito method is promising for fabricating SM polymer optical waveguides compatible with silicon photonics chips.

Integration of an O-band VCSEL on silicon photonics with polarization maintenance and waveguide coupling.

We demonstrate the hybrid integration of an O-band vertical-cavity surface-emitting laser (VCSEL) onto a silicon photonic chip using a grating coupler that is optimized to simultaneously provide feedback to maintain the single emission polarization and efficient in-plane coupling. The grating coupler was fabricated on silicon-on-insulator using a standard silicon photonics foundry process, and integrated with a commercially available VCSEL. A transparent VCSEL submount was fabricated with femtosecond laser templating and chemical etching to simplify the passive and active alignment steps. A record-high VCSEL-to-chip coupling efficiency of -5 dB was obtained at a bias current of 2.5 mA. The slope efficiency and output power are competitive with microcavity hybrid silicon lasers. The results show the feasibility of VCSELs as low threshold current on-chip sources for silicon photonics.

Optimization of PAM-4 transmitters based on lumped silicon photonic MZMs for high-speed short-reach optical links.

We demonstrate how to optimize the performance of PAM-4 transmitters based on lumped Silicon Photonic Mach-Zehnder Modulators (MZMs) for short-reach optical links. Firstly, we analyze the trade-off that occurs between extinction ratio and modulation loss when driving an MZM with a voltage swing less than the MZM's Vπ. This is important when driver circuits are realized in deep submicron CMOS process nodes. Next, a driving scheme based upon a switched capacitor approach is proposed to maximize the achievable bandwidth of the combined lumped MZM and CMOS driver chip. This scheme allows the use of lumped MZM for high speed optical links with reduced RF driver power consumption compared to the conventional approach of driving MZMs (with transmission line based electrodes) with a power amplifier. This is critical for upcoming short-reach link standards such as 400Gb/s 802.3 Ethernet. The driver chip was fabricated using a 65nm CMOS technology and flip-chipped on top of the Silicon Photonic chip (fabricated using IMEC's ISIPP25G technology) that contains the MZM. Open eyes with 4dB extinction ratio for a 36Gb/s (18Gbaud) PAM-4 signal are experimentally demonstrated. The electronic driver chip has a core area of only 0.11mm2 and consumes 236mW from 1.2V and 2.4V supply voltages. This corresponds to an energy efficiency of 6.55pJ/bit including Gray encoder and retiming, or 5.37pJ/bit for the driver circuit only.

Electronic-Photonic Integrated Circuit for 3D Microimaging

An integrated electronic-photonic phase-locked loop (PLL) modulates the frequency of a tunable laser for use in frequency-modulated continuous-wave (FMCW) lidar 3D imaging. The proposed lidar can perform 180k range measurements per second. The rms depth precision is 8 $\mu \text{m}$ at distances of ±5 cm from the range baseline. The range window is 1.4 m, with a precision of 4.2 mm at the edges of the window. Optical circuitry, including input light couplers, waveguides, and photodiodes, is realized on a 3 mm $\times $ 3 mm silicon-photonic chip. The 0.18- $\mu \text{m}$ CMOS ASIC of the same area comprises the front-end transimpedance amplifier, analog electro-optical PLL, and digital control circuitry consuming 1.7 mA from a 1.8 V supply and 14.1 mA from a 5-V supply. The latter includes 12.5-mA bias current for the distributed Bragg reflector section of the tunable laser. The two chips are integrated using through-silicon-vias implemented in the silicon-photonic chip.

Driver circuit for a PAM‐4 optical transmitter using 65 nm CMOS and silicon photonic technologies

A push–pull silicon photonic Mach–Zehnder modulator (MZM) driver is presented which uses a switched capacitor approach to generate a ∼2 V peak-to-peak differential 4-level pulse amplitude modulation (PAM-4) signal. The driver chip includes a Gray encoder and retiming flip-flops. The switched capacitor approach allows driving the lumped silicon photonic MZM with reduced power consumption compared with the conventional approach of driving MZMs (with transmission line based electrodes) with a power amplifier. This is critical for upcoming short-reach link standards such as 400 Gbit/s 802.3 Ethernet. The chip was fabricated using a 65 nm CMOS technology and flip-chipped on top of the silicon photonic chip (fabricated using IMEC's ISIPP25G technology) that contains the MZM. Open eyes with 4 dB extinction ratio for a 36 Gbit/s (18 Gbaud) PAM-4 signal are experimentally demonstrated. The electronic driver chip has a core area of 0.11 mm2 and consumes 236 mW from 1.2 to 2.4 V supply voltages. This corresponds to an energy efficiency of 6.55 pJ/bit including Gray encoder and retiming, or 5.37 pJ/bit for the driver circuit only.

Research on Optical Transmitter and Receiver Module Used for High-Speed Interconnection between CPU and Memory

ABSTRACTWith the development of the multicore processor, the bandwidth and capacity of the memory, rather than the memory area, are the key factors in server performance. At present, however, the new architectures, such as fully buffered DIMM (FBDIMM), hybrid memory cube (HMC), and high bandwidth memory (HBM), cannot be commercially applied in the server. Therefore, a new architecture for the server is proposed. CPU and memory are separated onto different boards, and optical interconnection is used for the communication between them. Each optical module corresponds to each dual inline memory module (DIMM) with 64 channels. Compared to the previous technology, not only can the architecture realize high-capacity and wide-bandwidth memory, it also can reduce power consumption and cost, and be compatible with the existing dynamic random access memory (DRAM). In this article, the proposed module with system-in-package (SiP) integration is demonstrated. In the optical module, the silicon photonic chip is included, which is a promising technology to be applied in the next-generation data exchanging centers. And due to the bandwidth–distance performance of the optical interconnection, SerDes chips are introduced to convert the 64-bit data at 800 Mbps from/to 4-channel data at 12.8 Gbps after/before they are transmitted though optical fiber. All the devices are packaged on cheap organic substrates. To ensure the performance of the whole system, several optimization efforts have been performed on the two modules. High-speed interconnection traces have been designed and simulated with electromagnetic simulation software. Steady-state thermal characteristics of the transceiver module have been evaluated by ANSYS APLD based on finite-element methodology (FEM). Heat sinks are placed at the hotspot area to ensure the reliability of all working chips. Finally, this transceiver system based on silicon photonics is measured, and the eye diagrams of data and clock signals are verified.

Expanding the Silicon Photonics Portfolio With Silicon Nitride Photonic Integrated Circuits

The high index contrast silicon-on-insulator platform is the dominant CMOS compatible platform for photonic integration. The successful use of silicon photonic chips in optical communication applications has now paved the way for new areas where photonic chips can be applied. It is already emerging as a competing technology for sensing and spectroscopic applications. This increasing range of applications for silicon photonics instigates an interest in exploring new materials, as silicon-on-insulator has some drawbacks for these emerging applications, e.g., silicon is not transparent in the visible wavelength range. Silicon nitride is an alternate material platform. It has moderately high index contrast, and like silicon-on-insulator, it uses CMOS processes to manufacture photonic integrated circuits. In this paper, the advantages and challenges associated with these two material platforms are discussed. The case of dispersive spectrometers, which are widely used in various silicon photonic applications, is presented for these two material platforms.

Stabilization and Frequency Control of a DFB Laser With a Tunable Optical Reflector Integrated in a Silicon Photonics PIC

We investigate the effect of tunable optical feedback on a commercial DFB laser edge coupled to a Silicon Photonics planar integrated circuit in which a tunable reflector has been implemented by means of a ring resonator based add-drop multiplexer. Controlled optical feedback allows for fine-tuning of the laser oscillation frequency. Under certain conditions it also allows suppression of bifurcation modes triggered by reflections occurring elsewhere on the chip. A semi-analytical model describing laser dynamics under combined optical feedback from the input facet of the edge coupler and from the tunable on-chip reflector fits the measurements. Compensation of detrimental effects from reflections induced elsewhere on a transceiver chip may allow moving isolators downstream in future communications systems, facilitating direct hybrid laser integration in Silicon Photonics chips, provided a suitable feedback signal for a control system can be identified. Moreover, the optical frequency tuning at lower feedback levels can be used to form a rapidly tunable optical oscillator as part of an optical phase locked loop, circumventing the problem of the thermal to free carrier effect crossover in the FM response of injection current controlled semiconductor laser diodes.

Wideband dynamic microwave frequency identification system using a low-power ultracompact silicon photonic chip.

Photonic-based instantaneous frequency measurement (IFM) of unknown microwave signals offers improved flexibility and frequency range as compared with electronic solutions. However, no photonic platform has ever demonstrated the key capability to perform dynamic IFM, as required in real-world applications. In addition, all demonstrations to date employ bulky components or need high optical power for operation. Here we demonstrate an integrated photonic IFM system that can identify frequency-varying signals in a dynamic manner, without any need for fast measurement instrumentation. The system is based on a fully linear, ultracompact system based on a waveguide Bragg grating on silicon, only 65-μm long and operating up to ∼30 GHz with carrier power below 10 mW, significantly outperforming present technologies. These results open a solid path towards identification of dynamically changing signals over tens of GHz bandwidths using a practical, low-cost on-chip implementation for applications from broadband communications to biomedical, astronomy and more.

Ultrahigh Q whispering gallery mode electro-optic resonators on a silicon photonic chip.

Crystalline whispering gallery mode (WGM) electro-optic resonators made of LiNbO₃ and LiTaO₃ are critical for a wide range of applications in nonlinear and quantum optics, as well as RF photonics, due to their remarkably ultrahigh Q(>10⁸) and large electro-optic coefficient. Achieving efficient coupling of these resonators to planar on-chip optical waveguides is essential for any high-yield and robust practical applications. However, it has been very challenging to demonstrate such coupling while preserving the ultrahigh Q properties of the resonators. Here, we show how the silicon photonic platform can overcome this long-standing challenge. Silicon waveguides with appropriate designs enable efficient and strong coupling to these WGM electro-optic resonators. We discuss various integration architectures of these resonators onto a silicon chip and experimentally demonstrate critical coupling of a planar Si waveguide and an ultrahigh QLiTaO₃ resonator (Q∼10⁸). Our results show a promising path for widespread and practical applications of these resonators on a silicon photonic platform.

Broadband and Polarization-Independent Efficient Vertical Optical Coupling With 45° Mirror for Optical I/O of Si Photonics

Efficient, broadband, and polarization-independent optical input/output (I/O) coupling of silicon photonics chips is significant for its practical application, and single-mode vertical optical I/O coupling will be necessary for high-density optical I/O connections required in future high-performance computing systems. We demonstrated such optical coupling using a 45° mirror, which was integrated by a cost-effective dicing technique with a polishing process. The 45° mirrors were integrated into Si-rich silica (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) waveguides on a Si substrate. To evaluate the light beam profile, which was important factor for the single-mode optical coupling, we measured near field patterns and far field patterns of light beams reflected by the 45° mirror. Owing to the polishing process, high-quality single-mode beams were successfully obtained as the vertical optical output. Using the fabricated 45° mirrors, the efficient single-mode vertical optical coupling with loss penalty of around 0.3 dB was demonstrated. The wavelength-dependent loss was less than 0.6 and 0.2 dB for TE and TM polarization, respectively. The polarization-dependent loss was also very small and it was less than 0.2 dB in 1500–1600 nm.

Glass interposer for short reach optical connectivity.

We propose a glass interposer containing femtosecond laser-scribed waveguides to interconnect silicon photonic chips. The glass interposer has an insertion loss of about 1.5 dB/cm, and simplifies alignment of silicon photonic chips. Our experiment shows that the insertion loss for the grating coupler/inscribed glass interface was only 0.5 dB higher than the estimated coupling loss of grating coupler to SMF. The 3 dB coupling degradation occurs after 5 µm of in-plane displacement between the laser-inscribed waveguide and the grating coupler.