Lee Carroll

Broad parameter optimization of polarization-diversity 2D grating couplers for silicon photonics

Polarization-diversity couplers, which are designed to couple the unknown polarization state of an optical fiber into the TE-polarized modes of integrated waveguides, are important for the development of practical all-optical circuits. We describe the use of a full 3D finite difference time domain (FDTD) calculation campaign to rigorously optimize the 2D photonic crystal grating that couples a single-mode telecom fiber to the silicon waveguides of a Silicon-on-Insulator (SOI) platform. With this approach we identify the unique optimum combination of etch-depth, hole-radius, and grating-pitch of the photonic crystal array for best performance at 1550 nm. The mean (polarization-averaged) coupling efficiency of 48% (-3.2dB) exceeds reported efficiencies of analogous couplers, and has only a marginal dependence on the polarization state of the input fiber (48 ± 3%). In addition, 3D-FDTD calculations are used to characterize the propagation direction, mode-profile, and polarization of light coupled from the fiber into the SOI slab. Such information is crucial for component design and goes beyond previously available results from existing approximations and simulations of 2D-grating coupler performance. Calculations of photonic mode dispersion in the grating coupler, by means of guided-mode expansion, indicate that the coupling is due to an optically active resonant guided mode in the photonic crystal array. This points towards a fast optimization scheme that enhances both the performance and the physical interpretation of 3D-FDTD simulations.

Ultra-broadband infrared pump-probe spectroscopy using synchrotron radiation and a tuneable pump.

Synchrotron infrared sources have become popular mainly because of their excellent broadband brilliance, which enables spectroscopically resolved spatial-mapping of stationary objects at the diffraction limit. In this article we focus on an often-neglected further advantage of such sources - their unique time-structure - to bring such broadband spectroscopy to the time domain, for studying dynamic phenomenon down to the 100 ps limit. We describe the ultra-broadband (12.5 to 1.1 μm) Fourier transform pump-probe setup, for condensed matter transmission- and reflection-spectroscopy, installed at the X01DC infrared beam-line of the Swiss Light Source (SLS). The optical pump consists of a widely tuneable 100 ps 1 kHz laser system, covering 94% of the 16 to 1.1 μm range. A thorough description of the system is given, including (i) the vector-modulator providing purely electronic tuning of the pump-probe overlap up to 1 ms with sub-ps time resolution, (ii) the 500 MHz data acquisition system interfaced with the experimental physics and industrial control system (EPICS) based SLS control system for consecutive pulse sampling, and (iii) the step-scan time-slice Fourier transform scheme for simultaneous recording of the dual-channel pumped, un-pumped, and difference spectra. The typical signal/noise ratio of a single interferogram in a 100 ps time slice is 300 (measured during one single 140 s TopUp period). This signal/noise ratio is comparable to that of existing gated Globar pump-probe Fourier transform spectroscopy, but brings up to four orders of magnitude better time resolution. To showcase the utility of broadband pump-probe spectroscopy, we investigate a Ge-on-Si material system similar to that in which optically pumped direct-gap lasing was recently reported. We show that the mid-infrared reflection-spectra can be used to determine the optically injected carrier density, while the mid- and near-infrared transmission-spectra can be used to separate the strong pump-induced absorption and inversion processes present at the direct-gap energy.

Quantum-confined direct-gap transitions in tensile-strained Ge/SiGe multiple quantum wells

Tensile-strained Ge/Si1−xGex (x = 0.87) multiple quantum wells (MQWs) on a Ge-on-Si virtual substrate are investigated with Brewster transmission and photo-reflectance, to identify quantum-confined direct-gap transitions and their light/heavy-hole splitting. Strain is deduced from optical splitting and x-ray diffraction measurements. As-prepared MQWs have an exciton at ≈ 820 meV, close to the 810 meV edge of the telecommunication C-band. The effect of rapid thermal annealing, to red-shift this feature into the C-band via increased strain, is investigated and interpreted with a tight-binding model. Annealing is observed to red-shift bulk absorption, but MQW transitions experience a net blue-shift due to interdiffusion.

Flip-chip integration of tilted VCSELs onto a silicon photonic integrated circuit.

In this article we describe a cost-effective approach for hybrid laser integration, in which vertical cavity surface emitting lasers (VCSELs) are passively-aligned and flip-chip bonded to a Si photonic integrated circuit (PIC), with a tilt-angle optimized for optical-insertion into standard grating-couplers. A tilt-angle of 10° is achieved by controlling the reflow of the solder ball deposition used for the electrical-contacting and mechanical-bonding of the VCSEL to the PIC. After flip-chip integration, the VCSEL-to-PIC insertion loss is -11.8 dB, indicating an excess coupling penalty of -5.9 dB, compared to Fibre-to-PIC coupling. Finite difference time domain simulations indicate that the penalty arises from the relatively poor match between the VCSEL mode and the grating-coupler.

Meeting the Electrical, Optical, and Thermal Design Challenges of Photonic-Packaging

Integrated photonics is a promising route toward high-performance next-generation ICT and sensing devices. Although fiber-packaging is perhaps the most widely discussed obstacle to low-cost photonic devices, electronic-photonic integration and thermal-stabilization are also significant design considerations that need to be properly managed. Using a state-of-the-art Si-photonic optical-network-unit as a worked example, we illustrate some key challenges and solutions in the field of photonic-packaging. Specifically, we describe a novel solder-reflow bonding process for the face-to-face three-dimensional (3-D) integration of photonic and electronic integrated circuits, discuss current and future multichannel fiber-alignment techniques, and investigate the coefficient-of-performance of the thermo-electric cooler that stabilizes the temperature of the photonic components. The challenge of photonic-packaging is to simultaneously satisfy these electrical, optical, and thermal design requirements on small-footprint devices, while establishing a route to scalable commercial implementation.

Grating Couplers in Silicon-on-Insulator: The Role of Photonic Guided Resonances on Lineshape and Bandwidth

Most grating couplers for silicon photonics are designed to match the approximately 10 μm mode-field diameter (MFD) of single-mode telecom fibres. In this letter, we analyse grating-coupler designs in the Silicon-on-Insulator (SOI) platform in a wide range of MFDs (4–100 μm) and related footprints, to give a physical understanding of the trends in efficiency and lineshape of the corresponding coupling spectra. We show that large-footprint grating couplers have an intrinsic Lorentzian lineshape that is determined by the quasi-guided photonic modes (or guided resonances) of the corresponding photonic crystal slab, while small-footprint grating couplers have a Gaussian lineshape resulting from the k-space broadening of the incident mode. The crossover between the two regimes is characterized by Voigt lineshapes. Multi-objective particle-swarm optimisation of selected small-footprint apodized grating-couplers is then used to locate the “Pareto fronts;” along which the highest coupling efficiency is achieved f...

Transmitter Made up of a Silicon Photonic IC and its Flip-Chipped CMOS IC Driver Targeting Implementation in FDMA-PON

We report on the design, fabrication, and characterization of a reflective transmitter targeting implementation in passive optical networks (PON) with frequency division multiplexed access (FDMA). It is made up of a Silicon photonic integrated circuit (Si-PIC) comprising a reflective Mach Zehnder modulator and its flip-chipped CMOS electronic integrated circuit driver, the two ICs being interconnected by means of high density and low parasitic copper micro pillars. Several transmissions, in an FDMA PON context, are successfully demonstrated using 500 MBaud QPSK and 16-QAM modulated subcarriers, achieving bit error rate below 2.10-3. For QPSK-modulated subcarriers (respectively, 16-QAM), the available access frequency bandwidth is measured to be 1-7 GHz (respectively, 2-4 GHz) with an available loss budget of 9 dB (respectively, 5 dB). Improvements of the Si-PIC are further identified to achieve compliancy with 31 dB ODN loss.

Hybrid integration of VCSELs onto a silicon photonic platform for biosensing application

This paper presents a technology of hybrid integration vertical cavity surface emitting lasers (VCSELs) directly on silicon photonics chip. By controlling the reflow of the solder balls used for electrical and mechanical bonding, the VCSELs were bonded at 10 degree to achieve the optimum angle-of-incidence to the planar grating coupler through vision based flip-chip techniques. The 1 dB discrepancy between optical loss values of flip-chip passive assembly and active alignment confirmed that the general purpose of the flip-chip design concept is achieved. This hybrid approach of integrating a miniaturized light source on chip opens the possibly of highly compact sensor system, which enable future portable and wearable diagnostics devices.

Optimizing grating couplers for silicon photonics

Grating couplers realized on a Silicon-on-Insulator (SOI) platform are an efficient means of coupling light from a single-mode optical fiber into silicon photonic waveguides. Grating couplers can be 1D, coupling light with a fixed polarization into a single waveguide, or 2D, in order to couple light with unknown polarization into a pair of silicon waveguides. In this work we review computational approaches, based on the finite-difference time domain (FDTD) method, that allow optimizing 1D and 2D grating couplers in order to maximize the coupling efficiency with respect to the various structural parameters. In general, we find that the gap between the performance of 1D and 2D grating couplers can be reduced with proper optimization. Relevant factors like polarization-dependent loss (for 2D couplers), bandwidth, tolerances, and fabrication constraints are addressed.

Thermal challenges for packaging integrated photonic devices

In this work, we present thermal imaging and analysis of a silicon photonic optical network unit (ONU) for next generation passive optical networks (NG-PONs). A high-speed thermal camera is used to capture both the dynamic and steady-state temperatures of the photonic integrated circuit (PIC) and electric integrated circuit (EIC) at the core of the ONU. The dynamic temperature measurements of the PIC and EIC on powering-up the ONU show several distinct “steps” before the steady-state temperature is reached, and we show that these features can be predicted and fitted using a simple lumped-element rate-equation model. Steady-state temperature measurements made by mounting the ONU module on a thermally-stabilized stage allow the coefficient of performance (COP) of the thermoelectric cooler (TEC) in the module to be evaluated for simulated ambient temperatures ranging from 15-45 °C. This provides valuable information on the operational cost of the ONU in the field, where temperature-stabilization of the PIC represents a significant fraction of the overall power-budget.