Optical frequency combs offer an unrivalled degree of frequency measurement precision that underpins the advance of modern technology in both fundamental science and commercial contexts. Recent ...progress in integrated photonics provides an attractive route to realize optical frequency comb sources in a compact, low-cost and energy-efficient manner by leveraging tightly-confined waveguide platforms and wafer-scale mass-manufacturing in photonic foundries, potentially revolutionizing the fields of information processing, time–frequency metrology and sensing. In this Review Article, we comprehensively examine the strategies for optical frequency comb generation in integrated photonics and provide detailed appraisals of those strategies in the context of prospective applications. The progress of high-level integration of optical frequency combs in photonic integrated circuits is summarized and a roadmap is proposed for transferring advanced optical frequency comb systems from the laboratory to the wider world.An examination of the strategies for generating optical frequency combs using integrated photonics.
The emergence of parallel convolution-operation technology has substantially powered the complexity and functionality of optical neural networks (ONN) by harnessing the dimension of optical ...wavelength. However, this advanced architecture faces remarkable challenges in high-level integration and on-chip operation. In this work, convolution based on time-wavelength plane stretching approach is implemented on a microcomb-driven chip-based photonic processing unit (PPU). To support the operation of this processing unit, we develop a dedicated control and operation protocol, leading to a record high weight precision of 9 bits. Moreover, the compact architecture and high data loading speed enable a preeminent photonic-core compute density of over 1 trillion of operations per second per square millimeter (TOPS mm
). Two proof-of-concept experiments are demonstrated, including image edge detection and handwritten digit recognition, showing comparable processing capability compared to that of a digital computer. Due to the advanced performance and the great scalability, this parallel photonic processing unit can potentially revolutionize sophisticated artificial intelligence tasks including autonomous driving, video action recognition and image reconstruction.
Silicon lasers have long been a goal for semiconductor scientists, and a number of important breakthroughs in the past decade have focused attention on silicon as a photonic platform. Here we review ...the most recent progress in this field, including low-threshold silicon Raman lasers with racetrack ring resonator cavities, the first germanium-on-silicon lasers operating at room temperature, and hybrid silicon microring and microdisk lasers. The fundamentals of carrier transition physics in crystalline silicon are discussed briefly. The basics of several important approaches for creating lasers on silicon are explained, and the challenges and opportunities associated with these approaches are discussed.
Abstract
The rising demand for high scanning accuracy and resolution in sensors for self-driving vehicles has led to the rapid development of parallelization in light detection and ranging (LiDAR) ...technologies. However, for the two major existing LiDAR categories—time-of-flight and frequency-modulated continuous wave—the light sources and measurement principles currently used for parallel detection face severe limitations from time- and frequency-domain congestion, leading to degraded measurement performance and increased system complexity. In this work we introduce a light source—the chaotic microcomb—to overcome this problem. This physical entropy light source exhibits naturally orthogonalized light channels that are immune to any congestion problem. Based on this microcomb state, we demonstrate a new type of LiDAR—parallel chaotic LiDAR—that is interference-free and has a greatly simplified system architecture. Our approach also enables the state-of-the-art ranging performance among parallel LiDARs: millimetre-level ranging accuracy and millimetre-per-second-level velocity resolution. Combining all of these desirable properties, this technology has the potential to reshape the entire LiDAR ecosystem.
A novel ultra-short polarization beam splitter (PBS) based on a bent directional coupler is proposed by utilizing the evanescent coupling between two bent optical waveguides with different core ...widths. For the bent directional coupler, there is a significant phase-mismatch for TE polarization while the phase-matching condition is satisfied for TM polarization. Therefore, the TM polarized light can be coupled from the narrow input waveguide to the adjacent wide waveguide while the TE polarization goes through the coupling region without significant coupling. An ultra-short (<10 μm-long) PBS is designed based on silicon-on-insulator nanowires and the length of the bent coupling region is as small as 4.5 μm while the gap width is chosen as 200 nm (large enough to simplify the fabrication). Numerical simulations show that the present PBS has a good fabrication tolerance for the variation of the waveguide width (more than ± 60 nm) and a very broad band (~200 nm) for an extinction ratio of >10 dB.
A novel concept for an ultracompact polarization splitter-rotator is proposed by utilizing a structure combining an adiabatic taper and an asymmetrical directional coupler. The adiabatic taper ...structure is singlemode at the input end while it becomes multimode at the other end. When light propagates along the adiabatic taper structure, the TM fundamental mode launched at the narrow end is efficiently (close to 100%) converted to the first higher-order TE mode at the wide end because of the mode coupling between them. By using an asymmetrical directional coupler that has two adjacent waveguides with different core widths, the first higher-order TE mode is then coupled to the TE fundamental mode of the adjacent narrow waveguide. On the other hand, the input TE polarization does not change when it goes through the adiabatic taper structure. In the region of the asymmetrical directional coupler, the TE fundamental mode in the wide waveguide is not coupled to the adjacent narrow waveguide because of phase mismatch. In this way, TE- and TM- polarized light are separated while the TM fundamental mode is also converted into the TE fundamental mode. A design example of the proposed polarization splitter-rotator is given by using silicon-on-insulator nanowires and the total length of the device is less than 100μm. Furthermore, only a one-mask process is needed for the fabrication process, which is compatible with the standard fabrication for the regular photonic integrated circuits based on SOI nanowires.
Driven by narrow-linewidth bench-top lasers, coherent optical systems spanning optical communications, metrology and sensing provide unrivalled performance. To transfer these capabilities from the ...laboratory to the real world, a key missing ingredient is a mass-produced integrated laser with superior coherence. Here, we bridge conventional semiconductor lasers and coherent optical systems using CMOS-foundry-fabricated microresonators with a high Q factor of over 260 million and finesse over 42,000. A five-orders-of-magnitude noise reduction in the pump laser is demonstrated, enabling a frequency noise of 0.2 Hz2 Hz−1 to be achieved in an electrically pumped integrated laser, with a corresponding short-term linewidth of 1.2 Hz. Moreover, the same configuration is shown to relieve the dispersion requirements for microcomb generation that have handicapped certain nonlinear platforms. The simultaneous realization of this high Q factor, highly coherent lasers and frequency combs using foundry-based technologies paves the way for volume manufacturing of a wide range of coherent optical systems.Using CMOS-ready ultra-high-Q microresonators, a highly coherent electrically pumped integrated laser with frequency noise of 0.2 Hz2 Hz−1, corresponding to a short-term linewidth of 1.2 Hz, is demonstrated. The device configuration is also found to relieve the dispersion requirements for microcomb generation that have limited certain nonlinear platforms.
Chip-based frequency combs
The realization of optical frequency combs, light sources with precisely spaced frequencies across a broad spectrum of wavelengths, in dielectric microresonators has ...affected a range of applications from imaging and ranging to precision time keeping and metrology. Xiang
et al.
demonstrate that the entire system, the laser-pumping system and the comb-generating microresonators, can be combined into an integrated silicon-based platform. Compatibility with foundry fabrication methods will enable this innovation to have a major impact on coherent communications, optical interconnects, and low-noise microwave generation.
Science
, abh2076, this issue p.
99
Optical microresonator frequency combs are realized in an integrated Si-based platform.
Silicon photonics enables wafer-scale integration of optical functionalities on chip. Silicon-based laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection and ranging, or photonics-assisted signal processing. We report heterogeneously integrated laser soliton microcombs combining both indium phospide/silicon (InP/Si) semiconductor lasers and ultralow-loss silicon nitride (Si
3
N
4
) microresonators on a monolithic silicon substrate. Thousands of devices can be produced from a single wafer by using complementary metal-oxide-semiconductor–compatible techniques. With on-chip electrical control of the laser-microresonator relative optical phase, these devices can output single-soliton microcombs with a 100-gigahertz repetition rate. Furthermore, we observe laser frequency noise reduction due to self-injection locking of the InP/Si laser to the Si
3
N
4
microresonator. Our approach provides a route for large-volume, low-cost manufacturing of narrow-linewidth, chip-based frequency combs for next-generation high-capacity transceivers, data centers, space and mobile platforms.
We analyze optical phased arrays with aperiodic pitch and element-to-element spacing greater than one wavelength at channel counts exceeding hundreds of elements. We optimize the spacing between ...waveguides for highest side-mode suppression providing grating lobe free steering in full visible space while preserving the narrow beamwidth. Optimum waveguide placement strategies are derived and design guidelines for sparse (> 1.5 λ and > 3 λ average element spacing) optical phased arrays are given. Scaling to larger array areas by means of tiling is considered.
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