Microcomb-driven silicon photonic systems.

Microcombs have sparked a surge of applications over the past decade, ranging from optical communications to metrology1-4. Despite their diverse deployment, most microcomb-based systems rely on a large amount of bulky elements and equipment to fulfil their desired functions, which is complicated, expensive and power consuming. By contrast, foundry-based silicon photonics (SiPh) has had remarkable success in providing versatile functionality in a scalable and low-cost manner5-7, but its available chip-based light sources lack the capacity for parallelization, which limits the scope of SiPh applications. Here we combine these two technologies by using a power-efficient and operationally simple aluminium-gallium-arsenide-on-insulator microcomb source to drive complementary metal-oxide-semiconductor SiPh engines. We present two important chip-scale photonic systems for optical data transmission and microwave photonics, respectively. A microcomb-based integrated photonic data link is demonstrated, based on a pulse-amplitude four-level modulation scheme with a two-terabit-per-second aggregate rate, and a highly reconfigurable microwave photonic filter with a high level of integration is constructed using a time-stretch approach. Such synergy of a microcomb and SiPh integrated components is an essential step towards the next generation of fully integrated photonic systems.

Athermal, fabrication-tolerant Si-SiN FIR filters for a silicon photonics foundry platform.

A means of athermalizing unbalanced Mach-Zehnder interferometers on a 300 mm silicon photonics foundry platform utilizing Si and SiN layers to produce the path imbalance is demonstrated. This technique can be applied to all other forms of finite impulse response filters, such as arrayed waveguide gratings. Wafer scale performance of fabricated devices is analyzed for their expected performance in the target application: odd-even channel (de)-interleavers for dense wavelength division multiplexing links. Finally, a method is proposed to improve device performance to be more robust to fabrication variations while simultaneously maintaining athermality.

Effects of nonlinear loss in high-Q Si ring resonators for narrow-linewidth III-V/Si heterogeneously integrated tunable lasers.

High-Q Si ring resonators play an important role in the development of widely tunable heterogeneously integrated lasers. However, while a high Q-factor (Q > 1 million) is important for ring resonators in a laser cavity, the parasitic high-power density in a Si resonator can deteriorate the laser performance at high power levels due to nonlinear loss. Here, we experimentally show that this detrimental effect can happen at moderate power levels (a few milliwatts) where typical heterogeneously integrated lasers work. We further compare different ring resonators, including extended Si ring resonators and Si3N4 ring resonators and provide practical approaches to minimize this effect. Our results provide explanations and guidelines for high-Q ring resonator designs in heterogeneously integrated tunable lasers, and they are also applicable for hybrid integrated butt-coupled lasers.

2 λ switch.

We demonstrate an elastic multi-wavelength selective switch with up to two wavelength switching capability per crosspoint. We fabricated the switch in a silicon photonics foundry and demonstrated a 17 nm tuning range for ring resonators, with a mean path loss of 2.43 dB. This is a 70% reduction in path loss as compared to previous generations, and we demonstrate a high-speed pulse-amplitude-modulation-4 transmission at 111 Gbps through different paths of the switch.

Multi-wavelength selective crossbar switch.

Here we demonstrate an 8x4 multi-wavelength selective ring resonator based crossbar switch matrix implemented in a 220-nm silicon photonics foundry for interconnecting electronic packet switches in scalable data centers. This switch design can dynamically assign up to two wavelength channels for any port-port connection, providing almost full connectivity with significant reduction in latency, cost and complexity. The switch unit cell insertion loss was measured at 0.8 dB, with an out-of-band rejection of 32 dB at 400 GHz channel separation. All the ring resonator heaters were thermally tuned, with heaters controlled by a custom 64-channel DAC driver. Detailed measurements on the whole switch showed standard deviation of 2 dB in losses across different paths, standard deviation of 0.33 nm in resonant wavelength and standard deviation of 0.01 nm/mW in ring heater tuning efficiency. Data transmission experiments at 40 Gbps showed negligible penalty due to crosstalk paths through the switch.

On-chip wavelength locking for photonic switches.

We present an on-chip wavelength reference with a partial drop ring resonator and germanium photodetector. This approach can be used in ring-resonator-based wavelength-selective switches where absolute wavelength alignment is required. We use the temperature dependence of heater resistance as a temperature sensor. Additionally, we discuss locking speed, statistical variation of heater resistances, and tuning speed of the switches.

Broadband quantum-dot frequency-modulated comb laser.

Frequency-modulated (FM) laser combs, which offer a quasi-continuous-wave output and a flat-topped optical spectrum, are emerging as a promising solution for wavelength-division multiplexing applications, precision metrology, and ultrafast optical ranging. The generation of FM combs relies on spatial hole burning, group velocity dispersion, Kerr nonlinearity, and four-wave mixing (FWM). While FM combs have been widely observed in quantum cascade Fabry-Perot (FP) lasers, the requirement for a low-dispersion FP cavity can be a challenge in platforms where the waveguide dispersion is mainly determined by the material. Here we report a 60 GHz quantum-dot (QD) mode-locked laser in which both the amplitude-modulated (AM) and the FM comb can be generated independently. The high FWM efficiency of -5 dB allows the QD laser to generate FM comb efficiently. We also demonstrate that the Kerr nonlinearity can be practically engineered to improve the FM comb bandwidth without the need for GVD engineering. The maximum 3-dB bandwidth that our QD platform can deliver is as large as 2.2 THz. This study gives novel insights into the improvement of FM combs and paves the way for small-footprint, electrically pumped, and energy-efficient frequency combs for silicon photonic integrated circuits (PICs).

Professional workforce training needs for Health Impact Assessment in spatial planning: A cross sectional survey.

Objectives: Heath Impact Assessment (HIA) in spatial planning can influence wider determinants and prevent ill-health, promote good health and reduce inequalities. They remain underutilised in England, partly due to inadequate workforce training. This study sought to identify the training needs of Public Health Professionals, Planners and Impact Assessment Practitioners in England in 2021. Study design: Cross-sectional survey. Methods: A survey was undertaken of Public Health Professionals, Planners and Impact Assessment Practitioners to ascertain their training needs. Descriptive statistics, frequency tables and charts summarised feedback. Results: 76% (68) of 90 total respondents, never received any training in HIAs. Quantifying impact of planning on health, especially in monetary terms and monitoring success of HIA recommendations, were common challenges for all professions. Musculoskeletal health and infectious disease were among specific health impacts where professionals welcomed further training. Public Health professionals requested support to identify high-impact interventions and work more collaboratively with Planners and a need to justify budgets for undertaking HIAs. Planners expressed training needs around justifying requirement for HIAs, data collection; and identifying local public health priorities. Impact Assessment Practitioners expressed uncertainty about policy triggers needed to undertake HIAs citing that the scale of HIAs must be proportional to project scope, the challenge of identifying planning interventions and a need to apply consistent methodological approaches in HIAs. Conclusion: There is need for baseline training to upskill all professionals, alongside tailored-training on key topic areas to address profession- and locality-specific needs to ensure spatial planning decisions better influence wider determinants and address local health priorities.