Active feedback stabilization of super-efficient microcombs in photonic molecules.

We demonstrate seamless channel multiplexing and high bitrate superchannel transmission of coherent optical orthogonal frequency division multiplexing (CO-OFDM) data signals utilizing a dissipative Kerr soliton (DKS) frequency comb generated in an on-chip microcavity. Aided by comb line multiplication through Nyquist pulse modulation, the high stability and mutual coherence among mode-locked Kerr comb lines are exploited for the first time, to the best of our knowledge, to eliminate the guard intervals between communication channels and achieve full spectral density bandwidth utilization. Spectral efficiency as high as 2.625bit/Hz/s is obtained for 180 CO-OFDM bands encoded with 12.75Gbaud 8-QAM data, adding to the total bitrate of 6.885Tb/s within a 2.295THz frequency comb bandwidth. This Letter confirms that high coherence is the key superiority of Kerr soliton frequency combs over independent laser diodes, as a multi-spectral coherent laser source for high-bandwidth high-spectral-density transmission networks.

Dissipative Kerr solitons (DKS) in photonic molecules (PMs) have enabled deterministic generation of super-efficient microcombs [1]. In photonic molecules, the linear coupling between auxiliary and main cavities feature crossings that distort the microcomb spectrum and allow for controlling the dispersive-wave (DW) emission [2]. This is particularly relevant to balance the Raman self-frequency shift and attain an operating point in pump detuning [3] that suppresses the transduction of the pump phase noise into the repetition rate [4]. In a recent work [5], we demonstrated that a quiet point can also be attained in normal-dispersion microcombs in photonic molecules, even in absence of the Raman self-frequency shift. Here, we observe a similar dynamics in anomalous-dispersion microcombs, resulting in low-phase noise repetition rates and high optical conversion efficiency.

The dissipative Kerr soliton (DKS) microcomb has been regarded as a highly promising multi-wavelength laser source for optical fiber communication, due to its excellent frequency and phase stability. However, in some specific application scenarios, such as direct modulation and direct detection (DM/DD), the relative intensity noise (RIN) performance of Kerr optical combs still fails to meet the requirements. Here, we systematically investigate the key factors that contribute to the power fluctuations in DKS combs. By exploiting the gain saturation effect of the semiconductor optical amplifier (SOA), the RIN of an on-chip DKS microcomb is effectively suppressed, achieving a maximum reduction of about 30 dB (@600 kHz offset frequency) for all comb lines. Moreover, such DKS comb RIN suppression technology based on an SOA chip can eliminate the need for additional complex feedback control circuits, showcasing the potential for further chip integration of the ultra-low-RIN DKS microcomb system.

Dissipative Kerr solitons in resonant frequency combs offer a promising route for ultrafast mode-locking, precision spectroscopy and time-frequency standards. The dynamics for the dissipative soliton generation, however, are intrinsically intertwined with thermal nonlinearities, limiting the soliton generation parameter map and statistical success probabilities of the solitary state. Here, via use of an auxiliary laser heating approach to suppress thermal dragging dynamics in dissipative soliton comb formation, we demonstrate stable Kerr soliton singlet formation and soliton bursts. First, we access a new soliton existence range with an inverse-sloped Kerr soliton evolution—diminishing soliton energy with increasing pump detuning. Second, we achieve deterministic transitions from Turing-like comb patterns directly into the dissipative Kerr soliton singlet pulse bypassing the chaotic states. This is achieved by avoiding subcomb overlaps at lower pump power, with near-identical singlet soliton comb generation over twenty instances. Third, with the red-detuned pump entrance route enabled, we uncover unique spontaneous soliton bursts in the direct formation of low-noise optical frequency combs from continuum background noise. The burst dynamics are due to the rapid entry and mutual attraction of the pump laser into the cavity mode, aided by the auxiliary laser and matching well with our numerical simulations. Enabled by the auxiliary-assisted frequency comb dynamics, we demonstrate an application of automatic soliton comb recovery and long-term stabilization against strong external perturbations. Our findings hold potential to expand the parameter space for ultrafast nonlinear dynamics and precision optical frequency comb stabilization.

We study the dynamics and stability of soliton optical frequency comb generation in a dissipative, coherently pumped cavity with both second- and third-order nonlinearity. Cavity sweep simulations and linear stability analysis based on path continuation reveal the existence of bistable solitons. These families of solitons represent a continuous transition between a purely quadratic and a Kerr cavity soliton frequency comb. Perspective demonstration of these novel, to the best of our knowledge, optical sources is an ongoing relevant subject within the frequency comb community.

Self-referenced dissipative Kerr solitons (DKSs) based on optical microresonators offer prominent characteristics allowing for various applications from precision measurement to astronomical spectrometer calibration. To date, direct octave-spanning DKS generation has been achieved only in ultrahigh-Q silicon nitride microresonators under optimized laser tuning speed or bi-directional tuning. Here we propose a simple method to easily access the octave-spanning DKS in an aluminum nitride (AlN) microresonator. In the design, two modes that belong to different families but with the same polarization are nearly degenerate and act as a pump and an auxiliary resonance, respectively. The presence of the auxiliary resonance can balance the thermal dragging effect, crucially simplifying the DKS generation with a single pump and leading to an enhanced soliton access window. We experimentally demonstrate the long-lived DKS operation with a record single-soliton step (10.4 GHz or 83 pm) and an octave-spanning bandwidth (1100–2300 nm) through adiabatic pump tuning. Our scheme also allows for direct creation of the DKS state with high probability and without elaborate wavelength or power schemes being required to stabilize the soliton behavior.

Dissipative Kerr solitons can be generated within an existence region defined on a space of normalized pumping power versus cavity-pump detuning frequency. The contours of constant soliton power and constant pulse width in this region are studied through measurement and simulation. Such isocontours impart structure to the existence region and improve understanding of soliton locking and stabilization methods. As part of the study, dimensionless, closed-form expressions for soliton power and pulse width are developed (including Raman contributions). They provide isocontours in close agreement with those from the full simulation, and, as universal expressions, can simplify the estimation of soliton properties across a wide range of systems.

Phase-shifted gratings are expected to play a significant role in wavelength division multiplexing, lasers, and optical signal processing due to their exceptional performance, including an ultra-narrow bandwidth, a high extinction ratio, and a sharp line shape. The key to introducing an accurate phase shift is to implement high-precision lithography, which is currently not available. However, this bottleneck can be addressed by employing the method of equivalent phase shift. Compared to implementing a true phase shift, the requirement for an equivalent phase shift can be reduced by three orders of magnitude. Currently, most phase-shifted gratings are fabricated in silicon-on-insulator (SOI) thanks to their compatibility with CMOS technology. Since the grating's periodic sides affect the phase shift, they also impact the full width at half maximum of the transmission peak. Ultra-low-loss waveguides based on silicon nitride are a favorable platform for researching microwave photonics and their applications in a wide variety of fields at present. Nevertheless, to date, a survey of phase-shifted gratings reveals a paucity of those fabricated on silicon nitride platforms. In this paper, we propose an optical filter based on a silicon nitride platform utilizing equivalent phase shift technology. This component under consideration comprises sampled gratings and true phase shift, with a length of 5 mm on a 350-nmthick Si₃N₄ platform. This component has a pitch of 480 nm and a sampling length of 100 μm, utilizing side-wall modulation. For this device, a narrow and sharp passband window of approximately 0.75 nm was observed over a stopband region at the target wavelength of 1566.35 nm. The reflection spectrum also shows good repeatability of the passband response for both the 1595.43 nm and 1598.12 nm wavelengths. Moreover, this scheme has broad potential applications in optical frequency combs, optical communication, and optical sensing.

Wave instability-the process that gives rise to turbulence in hydrodynamics1-represents the mechanism by which a small disturbance in a wave grows in amplitude owing to nonlinear interactions. In photonics, wave instabilities result in modulated light waveforms that can become periodic in the presence of coherent locking mechanisms. These periodic optical waveforms are known as opticalfrequency combs2-4. In ring microresonator combs5,6, an injected monochromatic wave becomes destabilized by the interplay between the resonator dispersion and the Kerr nonlinearity of the constituent crystal. By contrast, in ring lasers instabilities are considered to occur only under extreme pumping conditions7,8. Here we show that, despite this notion, semiconductor ring lasers with ultrafast gain recovery9,10 can enter frequency comb regimes at low pumping levels owing to phase turbulence11-an instability known to occur in hydrodynamics, superconductors and Bose-Einstein condensates. This instability arises from the phase-amplitude coupling of the laser field provided by linewidth enhancement12, which produces the needed interplay of dispersive and nonlinear effects. We formulate the instability condition in the framework of the Ginzburg-Landau formalism11. The localized structures that we observe share several properties with dissipative Kerr solitons, providing a first step towards connecting semiconductor ring lasers and microresonator frequency combs13.

Continuous-wave-driven Kerr nonlinear microresonators give rise to self-organization in terms of dissipative Kerr solitons, which constitute optical frequency combs that can be used to generate low-noise microwave signals. Here, by applying either amplitude or phase modulation to the driving laser we create an intracavity potential trap to discipline the repetition rate of the solitons. We demonstrate that this effect gives rise to a novel spectral purification mechanism of the external microwave signal frequency, leading to reduced phase noise of the output signal. We experimentally observe that the microwave signal generated from disciplined solitons is injection locked by the external drive at long timescales, but exhibits an unexpected suppression of the fast timing jitter. Counterintuitively, this filtering takes place for frequencies that are substantially lower than the cavity decay rate. As a result, while the long timescale stability of the Kerr frequency comb's repetition rate is improved by more than 4 orders of magnitude, the purified microwave signal shows a reduction of the phase noise by 30dB at offset frequencies above 10kHz.

The research work carried out within the framework of this PhD has focused on the generation of Optical Frequency Combs (OFC) in semiconductor lasers using the gain-switching (GS) technique. The work has been mainly experimental, although simulations have been used to contribute to a better understanding of the internal physics of the OFCs generation process. Various types of lasers have been used, specially distributed feedback (DFB) lasers and discrete mode lasers (DML). An exhaustive experimental characterisation of the properties of the OFCs generated by GS in a full range of operating conditions has been performed. Two ranges of repetition frequencies have been explored: the GHz range with applications in optical communications, and the range of hundreds of MHz for application in spectroscopy. The effect of the optical injection (OI) on the properties of the OFCs has been investigated, and interesting results have been obtained, such as the improvement or deterioration of the quality of the spectra depending on the GS and OI conditions. The optimal operating conditions to generate high quality OFCs at the different frequencies considered have been found. In order to improve the understanding of the different physical processes involved in the generation of the OFCs generated by GS, a simulation tool based on the rate equation model for semiconductor lasers has been developed, with detailed treatment of the noise sources. For a correct simulation of the OFCs the values of a set of internal laser parameters is are required. They were obtained through an original procedure based on the measurement of the relative intensity noise (RIN) spectra. Using this procedure, the uncertainty in the simulated results has been reduced, since it allows to extract more parameters than other procedures due to an adequate analytical treatment of the RIN equations. The simulations have completely reproduced the temporal and spectral response of different lasers in a wide range of electrical and optical injection conditions. From the simulations, the predominant physical processes that contribute to the generation of OFCs, dynamic chirp at high repetition frequencies, and adiabatic chirp at low frequencies, have been identified. It has been shown that it is possible to improve the characteristics of low frequency OFCs by driving the laser with electric pulses, instead of using sinusoidal driving. High quality 100 MHz OFCs (up to 866 tones in 10 dB) with good carrier-to-noise (CNR) ratio have been generated. It has also been shown that step-recovery diodes (SRDs) can be used as a source of electrical pulses to generate OFCs, thus reducing the cost and size of the equipment involved, while maintaining the quality of the OFCs. The role of the “jitter” of the electrical source in the quality of the OFCs has been studied theoretically and experimentally. Finally, the OFCs generated by gain-switching and optical injection have been used to measure the absorption spectra of different elements, using three different architectures: heterodyne detection, and dual-comb spectroscopy and a new implementation of this technique. In this implementation the dual-comb is generated from a single GS laser by alternating the repetition frequency. Promising results for the application of these OFCs in gas spectroscopy have been obtained.

Soliton Frequency Combs in Dual Microresonators

The monolithic integration of soliton microcomb devices with active photonic components and high-frequency electronics is highly desirable for practical applications. Among many materials, silicon nitride ( SiN x ) waveguide layers prepared by low-pressure chemical vapor deposition (LPCVD) have been the main platform for on-chip optical frequency comb generation. However, the high temperatures involved in LPCVD render it incompatible as a back-end process with complementary metal oxide semiconductor (CMOS) or active III-V compound semiconductor fabrication flows. We report the generation of coherent soliton frequency combs in micro-ring resonators fabricated in deuterated silicon nitride ( SiN x : D ) waveguides with a loss of 0.09 dB/cm. Deposited at 270°C by an inductance-coupled plasma chemical vapor deposition (ICP-CVD) process, the material preparation and fabrication flow are fully CMOS-compatible. These results enable the integration of silicon-nitride-based optical combs and photonic integrated circuits (PICs) on prefabricated CMOS and/or III-V substrates, therefore marking a major step forward in SiN x photonic technologies.

Optical frequency combs (OFCs) generated in microresonators can substitute for widely employed power-hungry laser arrays, ensuring spectral and energy efficiency in wavelength-division multiplexed passive optical networks (WDM-PONs). Here, we propose a realistic design of a WDM-PON based on a silica microresonator generating an OFC in the dissipative Kerr soliton regime and present a corresponding theoretical study, paying particular attention to the impact of the comb linewidth on the characteristics of the communication system. Using intensive numerical simulation, data transmission performance in an eight-channel 100 GHz spaced WDM-PON is investigated for OFC carrier linewidths of 100 kHz, 1 MHz, 10 MHz, and 100 MHz. We show that microresonator-based OFCs with linewidths of up to 100 MHz can be used for a non-return-to-zero on-off keying modulated data transmission system and give an acceptable bit error rate (BER). Results show that the narrower the linewidth, the lower the BER is. The data transmission results show that error-free data transmission is possible with a BER of 4.4 × 10−12 for 100 kHz OFC carrier linewidth, providing 80 Gbps of total data rate on eight OFC generated carriers.

We propose that the problem of tuning to soliton comb state and frequency comb generation in normal dispersion regime can be efficiently solved by the self-injection locking of the pump laser to the nonlinear microresonator. We developed original model describing the process of frequency comb generation in the self-injection locking regime and performed numerical modelling of this process.