Laser soliton microcombs on silicon (2103.02725v1)

Abstract: Silicon photonics enables wafer-scale integration of optical functionalities on chip. A silicon-based laser frequency combs could significantly expand the applications of silicon photonics, by providing integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent LiDAR, or photonics-assisted signal processing. Here, we report on heterogeneously integrated laser soliton microcombs combining both InP/Si semiconductor lasers and ultralow-loss silicon nitride microresonators on monolithic silicon substrate. Thousands of devices are produced from a single wafer using standard CMOS techniques. Using on-chip electrical control of the microcomb-laser relative optical phase, these devices can output single-soliton microcombs with 100 GHz repetition rate. Our approach paves the way for large-volume, low-cost manufacturing of chip-based frequency combs for next-generation high-capacity transceivers, datacenters, space and mobile platforms.

• The paper introduces a scalable integration of InP/Si lasers with Si3N4 microresonators using CMOS-compatible techniques.
• It achieves coherent optical frequency comb generation with single soliton states at a 100 GHz repetition rate and reduces laser linewidth from 60 kHz to 25 Hz.
• The innovative method paves the way for advanced photonic integrated circuits in RF photonics, coherent communications, and LIDAR.

Advanced Integration of Laser Soliton Microcombs on Silicon

This paper investigates the heterogeneously integrated laser soliton microcombs on a silicon platform, a significant advancement in silicon photonics. The paper involves the amalgamation of InP/Si semiconductor lasers and ultralow-loss Si$_3$N$_4$ microresonators into a monolithic silicon substrate. This integration process utilizes wafer-scale manufacturing with conventional CMOS techniques, resulting in a novel approach that could profoundly influence the field of photonic integrated circuits (PICs).

The integration enables the generation of optical frequency combs (OFCs) with soliton microcomb structures. The role of OFCs in timing, spectroscopy, and metrology is well-documented, typically requiring high-precision setups such as mode-locked lasers and supercontinuum generation for achieving the requisite octave-spanning spectra. In this research, the authors demonstrate the feasibility of generating OFCs via Kerr-nonlinear optical microresonators, specifically leveraging bright dissipative Kerr solitons (DKS), to achieve coherence and broadband OFC capabilities. Such capabilities are essential for RF photonics, coherent communication, LIDAR, and other technological applications.

This paper showcases a groundbreaking achievement in monolithic integration on a common silicon wafer, which has historically posed considerable challenges due to the disparate optical properties and requirements of the materials involved (Si, Si$_3$N$_4$, and III-V compounds). The authors have accomplished this integration using a photonic Damascene process combined with direct SOI wafer bonding and heterogeneous III-V integration, resulting in a robust, scalable fabrication method that could potentially revolutionize PICs by incorporating soliton microcombs into existing silicon photonic systems.

Key experimental results include the successful realization of single soliton states with a 100 GHz repetition rate and the demonstration of laser frequency noise reduction through self-injection locking. The laser self-injection locking has notably reduced the laser linewidth from its free-running 60 kHz to a precision of 25 Hz in the single soliton state, thereby allowing enhanced spectral purity and multi-wavelength, narrow-linewidth laser sources. The paper further discusses the optical phase dynamics crucial for stable soliton generation, emphasizing the role of forward and backward phase relations in self-injection locking.

The implications of this research extend to the development of high-capacity transceivers and mobile and space platforms, necessitating small footprint, low-cost, low-power consumption solutions. Future research can explore enhancing optical tunability and incorporating additional nonlinear photonic functionalities, such as hybrid integration with platforms like AlGaAs and LiNbO$_3$, to broaden the wavelength range and enable electro-optic modulation functionalities.

In conclusion, this paper presents a scalable, integrated approach for producing laser soliton microcombs, paving the way for substantial advancements in photonics-based signal processing and opening avenues for extensive, low-cost applications across various technological domains.