Coherent terabit communications with microresonator Kerr frequency combs (1307.1037v3)

Abstract: Optical frequency combs enable coherent data transmission on hundreds of wavelength channels and have the potential to revolutionize terabit communications. Generation of Kerr combs in nonlinear integrated microcavities represents a particularly promising option enabling line spacings of tens of GHz, compliant with wavelength-division multiplexing (WDM) grids. However, Kerr combs may exhibit strong phase noise and multiplet spectral lines, and this has made high-speed data transmission impossible up to now. Recent work has shown that systematic adjustment of pump conditions enables low phase-noise Kerr combs with singlet spectral lines. Here we demonstrate that Kerr combs are suited for coherent data transmission with advanced modulation formats that pose stringent requirements on the spectral purity of the optical source. In a first experiment, we encode a data stream of 392 Gbit/s on subsequent lines of a Kerr comb using quadrature phase shift keying (QPSK) and 16-state quadrature amplitude modulation (16QAM). A second experiment shows feedback-stabilization of a Kerr comb and transmission of a 1.44 Tbit/s data stream over a distance of up to 300 km. The results demonstrate that Kerr combs can meet the highly demanding requirements of multi-terabit/s coherent communications and thus offer a solution towards chip-scale terabit/s transceivers.

• The paper demonstrates coherent terabit communications using Kerr frequency combs with 392 Gbit/s and 1.44 Tbit/s experimental transmissions.
• It details the precise control of pump conditions and feedback stabilization to achieve low phase noise essential for QPSK and 16-QAM modulation.
• The research underscores CMOS-compatible integration of high-Q silicon nitride resonators for scalable, energy-efficient optical networks.

Coherent Terabit Communications with Microresonator Kerr Frequency Combs

The paper presents a detailed account of utilizing microresonator Kerr frequency combs to achieve coherent terabit communications. This work addresses a critical challenge in optical communications: the integration of high-speed and scalable transmission solutions compatible with chip-scale photonics. The goals are achieved through Kerr frequency combs that promise to fulfill the exacting demands of coherence and low-phase noise necessary for advanced modulation schemes.

Key Contributions and Findings

Kerr frequency combs, generated in nonlinear microresonators, offer a substantial potential to support terabit communications. These combs, capable of spanning hundreds of equidistant wavelength channels, are pivotal in wavelength-division multiplexing (WDM) systems. The paper details two significant experiments:

1. 392 Gbit/s Transmission: Utilizing a microresonator Kerr comb, the authors successfully encode a data stream of 392 Gbit/s, employing quadrature phase shift keying (QPSK) and 16-QAM modulation. This experiment leverages the comb's ability to maintain low phase noise and singlet spectral lines, essential for coherent transmission.
2. 1.44 Tbit/s Transmission over 300 km: A second experimental setup shows feedback stabilization of the Kerr comb, achieving a 1.44 Tbit/s data transmission over 300 km. Modulation formats and advanced spectral efficiency strategies, such as Nyquist WDM, are crucial to maintaining data integrity across such a distance.

Technical Insights

• Microresonator Technology: The use of high-Q silicon nitride (SiN) microresonators represents a significant technical choice, enabling dense integration and compatibility with CMOS fabrication processes. This compatibility is vital for the co-integration of photonic and electronic circuits using fabless CMOS processing.
• Comb Characteristics and Optimization: The research elaborates on the systematic tuning of pump conditions, integral to achieving the desired comb characteristics. Low phase noise and frequency stability are attained by careful control of detuning and the implementation of feedback loops to stabilize the pump wavelength in extended operations.
• Transmission Performance: The notable spectral efficiencies of up to 6 bit/s/Hz for 16QAM channels highlight the potential of Kerr combs not only for laboratory demonstrations but as realistic solutions for future high-capacity optical networks. The research emphasizes that the observed phase and amplitude stability satisfy the stringent requirements of coherent WDM systems.

Implications and Future Directions

The findings of this research set a foundation for ongoing advancement in photonic technologies. Kerr frequency combs, as demonstrated, are poised to be key enablers of scalable, high-bandwidth communication systems suitable for data-intensive applications at data centers and beyond. The paper suggests future exploration in:

• Further Q-Factor Enhancements: As comb bandwidth is related inversely to resonator Q-factors, advancements in resonator design may significantly lower the required pump power, facilitating the integration of compact, low-power systems.
• Integration and Scalability: With the demonstrated compatibility of microresonators with silicon photonics, efforts could steer towards monolithic integration to overcome hybrid assembly challenges and improve scalability.
• Multiplexing Techniques: Exploration into mode-division multiplexing (MDM) could further leverage Kerr combs for increased data capacities, potentially surpassing present SSMF bandwidth limitations.

In conclusion, the research showcases significant advancements in coherent optical communications, providing valuable insights that merge the realms of photonics and electronics within scalable and high-performing communication paradigms. It sets a promising trajectory for the deployment of integrated, energy-efficient terabit/s transceivers utilizing Kerr frequency comb technology.