Fiber-optic communications with microresonator frequency combs

Modern data communication links target ever-higher information throughput. To utilize the available bandwidth in a single strand of fiber, optical communication links often require a large number of lasers, each operating at a different wavelength. A microresonator frequency comb is a chip-scale multi-wavelength laser source whose spectrum consists of multiple evenly spaced lines. As the line spacing of a microresonator comb is on the order of several tens of GHz, it provides a promising light source candidate for implementing an integrated multi-wavelength transceiver. The interest for using microresonator combs in communications applications has therefore increased greatly in the last five years. The application-related developments have been complemented with an increased exploration and understanding of the operating principles behind these devices.This thesis studies microresonator frequency combs in both long-haul and high data-rate (multi-terabit per second) fiber communications systems. The results specifically include the longest demonstrated communications link with a microresonator light source as well as the highest order modulation format demonstration using any integrated comb source. The used microresonators are based on a high-Q silicon nitride platform provided by our collaborators at Purdue University. Part of the results are enabled by the high line powers resulting from a recently demonstrated novel comb state. This state bears similarities with dark solitons in fibers in that it corresponds to a train of dark pulses circulating inside the microresonator cavity. Overall, the results in this thesis provide a promising pathway towards enabling a future chip-scale multi-wavelength coherent transmitter

Laser Frequency Combs for Coherent Optical Communications

Frequency combs can replace multiple lasers in WDM systems. We highlight performance requirements for frequency combs in coherent communications and present recent results using Kerr combs for multi-Tb/s transmission using advanced modulation formats

Low-loss high-Q silicon-rich silicon nitride microresonators for Kerr nonlinear optics

\ua9 2019 Optical Society of America. Silicon nitride is a dielectric material widely used for applications in linear and nonlinear optics. It has an ultra-broad transparency window, low intrinsic loss, and a refractive index that allows for moderate optical field confinement in waveguides. The chemical composition of this material can be precisely set during the fabrication process, leading to an extra degree of freedom for tailoring the optical and mechanical properties of photonic chips. Silicon-rich silicon nitride waveguides are appealing for nonlinear optics, because they have a higher nonlinear Kerr coefficient and refractive index than what is possible with stoichiometric silicon nitride. This is a direct consequence of the increased silicon content. However, silicon-rich silicon nitride waveguides typically display higher absorption losses. In this Letter, we report low-loss (∼0.4 dB∕cm) silicon-rich silicon nitride waveguides. The structures feature high optical confinement and can be engineered with low anomalous dispersion. We find an optimum silicon composition that, through an annealing process, overcomes optical losses associated to N-H bonds in the telecom band. Based on this technology, we successfully fabricate microresonators with mean quality factors (Q) ∼0.8 7 106 in the C and L bands. Broadband coherent microresonator frequency combs are generated in this platform, indicating its potential for efficient Kerr nonlinear optics

Low Loss Silicon-Rich Silicon Nitride for Nonlinear Optics

We demonstrate low loss (~ 0.4 dB/cm) silicon-rich silicon nitride waveguides and highQ microresonators (Qi ~ 1 000 000) featuring broadband anomalous dispersion. Microresonator combs aregenerated for the first time in this emerging material platform

Optical linewidth of soliton microcombs

Understanding noise dynamics in frequency combs is crucial for applications. Here, the authors study the phase noise dynamics and the linewidth of soliton microcombs, revealing that some comb lines can be more quiet than the pump laser itself. Soliton microcombs provide a versatile platform for realizing fundamental studies and technological applications. To be utilized as frequency rulers for precision metrology, soliton microcombs must display broadband phase coherence, a parameter characterized by the optical phase or frequency noise of the comb lines and their corresponding optical linewidths. Here, we analyse the optical phase-noise dynamics in soliton microcombs generated in silicon nitride high-Q microresonators and show that, because of the Raman self-frequency shift or dispersive-wave recoil, the Lorentzian linewidth of some of the comb lines can, surprisingly, be narrower than that of the pump laser. This work elucidates information about the physical limits in phase coherence of soliton microcombs and illustrates a new strategy for the generation of spectrally coherent light on chip

Superchannel Engineering with Microresonator Combs

We study segmenting the available bandwidth of a WDM system into microcombdrivensuperchannels. This solution improves the power per line while using a fraction of thepump power, making it potentially more favorable for integration

Laser Frequency Combs for Coherent Optical Communications

Laser frequency combs with repetition rates on the order of 10 GHz and higher can he used as multi-carrier sources in wavelength-division multiplexing (WDM). They allow replacing tens of tunable continuous-wave lasers by a single laser source. In addition, the comb\u27s line spacing stability and broadband phase coherence enable signal processing beyond what is possible with an array of independent lasers. Modern WDM systems operate with advanced modulation formats and coherent receivers. This introduces stringent requirements in terms of signal-to-noise ratio, power per line, and optical linewidth which can be challenging to attain for frequency comb sources. Here, we set quantitative benchmarks for these characteristics and discuss tradeoffs in terms of transmission reach and achievable data rates. We also highlight recent achievements for comb-based superchannels, including >10 Tb/s transmission with extremely high spectral efficiency, and the possibility to significantly simplify the coherent receiver by realizing joint digital signal processing. We finally discuss advances with microresonator frequency combs and compare their performance in terms of flatness and conversion efficiency against state-of-the-art electro-optic frequency comb generators. This contribution provides guidelines for developing frequency comb sources in coherent fiber-optic communication systems

High spectral efficiency coherent superchannel transmission with soliton microcombs

Spectral efficiency (SE) is one of the key metrics for optical communication networks. An important building block for its maximization are optical superchannels, channels that are composed of several subchannels with an aggregate bandwidth larger than the bandwidth of the detector electronics. Superchannels which are routed through the network as a single entity, together with flex-grid routing, allow to more efficiently utilize available bandwidth and eliminate the guard-bands between channels, thus increasing spectral efficiency. In contrast to traditional wavelength division multiplexing (WDM) channels, subchannel spacing and thus superchannel SE is governed by the linewidth and stability of the frequency spacing of the transmitter lasers. Integrated optical frequency combs, particulary the parametrically generated so-called microcombs, which provide optical lines on a fixed frequency grid are a promising solution for low power superchannel laser sources that allow to minimize the SE loss from suboptimal channel spacing. However, it is extremely challenging to realize micro-combs with sufficient line power, coherence and line spacing that is compatible with electronic bandwidths. Because the line-spacing generated by most devices is above 40 GHz, demonstrations often rely on additional electro-optic frequency shifter or divider stages to avoid digital-to-analog-converter (DAC) performance degradation when operating at high symbol rates. Here we demonstrate a 50-line superchannel from a single 22 GHz line spacing soliton microcomb. We demonstrate 12 Tb/s throughput with > 10 bits/s/Hz SE efficiency after 80 km transmission and 8 Tb/s throughput (SE > 6 bits/s/Hz) after 2100 km, proving the feasibility and benefits of generating high signal quality, broadband waveforms directly from the output of a micro-scale device with a symbol rate close to the comb repetition rate

Superchannel engineering of microcombs for optical communications

Microresonator frequency combs (microcombs) are a promising technology for generating frequency carriers for wavelength division multiplexing (WDM) systems. Multi-terabit per second WDM coherent transmitters have recently been demonstrated using both dissipative Kerr solitons and mode-locked dark pulses in optical microresonators. These experiments have focused on microcombs designed to cover a large portion of the erbium-doped fiber window. However, the question of optimum bandwidth for microcombs in WDM systems has not been addressed. Here we show that segmenting the bandwidth into smaller microcomb-driven superchannels results in an improvement of power per line. Through numerical simulations we establish a quantitative comparison between dark-pulse and soliton microcombs in WDM systems, including aspects such as conversion efficiency, tolerance to intrinsic cavity loss, and group velocity dispersion engineering. We show that the improvement of minimum line power scales linearly with the number of superchannels for both types of microcombs. This work provides useful guidelines for the design of multi-terabit per second microcomb-based superchannel systems

PM-64QAM Coherent Optical Communications Using a Dark-Pulse Microresonator Frequency Comb

Dark-pulse microresonator combs exhibit efficient pump-to-comb power conversion. Using on-chip pump powers of 21 dBm, we show 20-channel PM-64QAM-based data transmission. These results represent the highest-order modulation format encoded onto any integrated comb