Broadband Optical Frequency Comb Research Articles

Controlled dispersive waves emission via swallowtail-type catastrophe pulses

The dispersive waves (DWs) emission has emerged as an efficient way to extend the spectral range of a light source. However, the applications of the DWs are limited because of its narrowband and low power. It is crucial to find novel ways to manipulate the DWs radiation. We investigate a kind of catastrophe pulses, called swallowtail pulses, which can radiate DWs with broadband and high power. By exploiting the dependence of the temporal and spectral structure of swallowtail pulses on the initial control parameter within the catastrophe integral, we can effectively manipulate the emission of DWs in terms of resonant frequency and conversion efficiency. Furthermore, we observe that the swallowtail pulses can shed two solitons, and that both of them radiate DWs with different frequencies. In comparison to fundamental solitons, the minimum value of third-order dispersion required for the onset of DWs emission becomes significantly smaller for that of swallowtail pulses. The results of this study clearly reveal the impact of the inherent behaviors of swallowtail pulses on the DWs emission, which should be relevant for applications requiring broadband light sources and optical frequency combs based on the mechanism of DWs emission.

Broadband Optical Frequency Comb Generation Based on a Dual-Pumped Brillouin-Erbium Fiber Laser

Broadband Optical Frequency Comb Generation Based on a Dual-Pumped Brillouin-Erbium Fiber Laser

Flexible broadband optical frequency comb generation assisted by injection locking of semiconductor laser

A broadband optical frequency comb (OFC) generator is proposed and experimentally demonstrated. Based on the injection locking process in a semiconductor laser diode, multiple coherent carriers with large intervals can be generated, which are further combined to give a much broader OFC through the electro-optical modulation (EOM) technique. The corresponding theoretical analysis and simulation have been given in detail. In the experimental demonstration, an OFC with 9 comb lines can be generated on a single carrier by the cascaded MZMs with a multi-harmonic RF drive signal. After tripling the number of carriers, the number of the comb lines becomes 27. By properly adjusting the carrier’s interval and drive signal frequency, the generated OFC shows great flexibility on spacing and the number of comb lines, where a 27-line OFC with FSR of 5 GHz and 6.5 GHz have been demonstrated respectively. In addition, by increasing the number of RF sources, the OFC with up to 43 comb lines can be generated in this system. Compared with the traditional OFC generation implemented by the EOM technique on a single optical carrier, three times optical spectrum extension or the number of comb lines increasing can be achieved. The Multi-carrier broaden spectrum method retains all the advantages of EOM-based OFC generation and overcomes its disadvantages, such as limited spectrum width and the number of comb lines. It provides a new idea for broadband OFC generation.

Design of optical frequency comb based on dual frequency pumped normal dispersion silicon carbide microresonator

<sec>The scheme of generating optical frequency comb (OFC) mainly includes mode-locked laser, electro-optic modulation comb, and nonlinear Kerr micro-resonator comb. The OFC with frequency spacing on the order of 10–200 GHz can be employed in optical communication, microwave photonics, and other fields. Silicon carbide (SiC) has aroused the considerable research interest in integrated nonlinear photonics owing to its high second nonlinear coefficient and third order nonlinear coefficient, low optical loss, without multiphoton absorption loss owing to the wide bandgap. Single soliton microcomb in anomalous group velocity dispersion regime based on a 4H-SiC-on-insulator thin film has been demonstrated with the relative lower pump to comb efficiency, while the OFC in normal dispersion regime based on the SiC microresonator has not been reported. The pump conversion efficiency of OFC in the normal dispersion regime is high, and the pump frequency detuning range for the OFC generation is large, which is conducive to the OFC generation and long-term stable operation. Since there is no modulation instability effect in normal dispersion regime, the key to generating the OFC in normal dispersion regime is that the initial state needs the assistance of a multi-frequency laser (or four-wave mixing sideband). The phase-locked dual-frequency laser can be regarded as a pulse pump laser source with wide pulse duration, which can be realized by integrated distributed feedback laser.</sec><sec>In this paper, a scheme of generating OFC by pumping the normal dispersion SiC microresonator with phase locked dual-frequency laser is proposed. The flat normal dispersion in 1550 nm band is realized through dispersion engineering of the SiC microresonator. The effective mode field area of the TE<sub>0</sub> fundamental mode at 1550 nm in the optimized SiC ridge waveguide is about 0.94 μm<sup>2</sup>, and the nonlinear coefficient is about 3.69 <inline-formula><tex-math id="M2">\begin{document}$ {{\mathrm{W}}}^{-1}{\cdot} {{\mathrm{m}}}^{-1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231442_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231442_M2.png"/></alternatives></inline-formula>. Meanwhile, dispersion parameters of the microresonator with 100 GHz FSR are also obtained. The OFC generation pumped by a phase-locked dual-frequency laser based on normal dispersion SiC microresonator is simulated through using the Lugiato-Lefever equation. The evolution process of the OFC in time and frequency domain related to the pump detuning is studied. The effects of several parameters such as the pump power, microresonator waveguide loss, microresonator dispersion, proportion of the dual-frequency laser, and the frequency interval of dual-frequency laser on the performance of the OFC are also investigated. The conclusions can be obtained through the OFC generation simulation as follows, 1) When the microresonator waveguide loss is larger, the pump detuning range for the OFC generation becomes smaller, and the pulse peak power under the same pulse intensity filling rate decreases. 2) When the input pump power is larger, the pump detuning range for the OFC generation becomes larger, the pulse peak power under the same pulse intensity filling rate increases, and the corresponding spectrum becomes wider. 3) With the increase of absolute dispersion value, the spectrum bandwidth of the generated OFC decreases obviously. 4) The power proportion of dual-frequency laser has little influence on the OFC generation. 5) The frequency spacing of the generated OFC can be tuned through changing the frequency spacing of the two phase-locked lasers with integral multiple of free spectral range.</sec><sec>The OFC with spectrum bandwidth of about 70 nm can be generated in a range of 1500—1600 nm through the simulation. The simulation results are beneficial to promoting the research and practical application of high repetition rate broadband optical frequency comb in a 1550 nm band based on the normal dispersion silicon carbide microresonator.</sec>

Photorefraction‐Assisted Self‐Emergence of Dissipative Kerr Solitons

AbstractGenerated in high‐Q optical microresonators, dissipative Kerr soliton microcombs constitute broadband optical frequency combs with chip sizes and repetition rates in the microwave to millimeter‐wave range. For frequency metrology applications such as spectroscopy, optical atomic clocks, and frequency synthesizers, octave‐spanning soliton microcombs generated in dispersion‐optimized microresonators are required, which allow self‐referencing for full frequency stabilization. In addition, field‐deployable applications require the generation of such soliton microcombs to be simple, deterministic, and reproducible. Here, a novel scheme to generate self‐emerging solitons in integrated lithium‐niobate microresonators is demonstrated. The single soliton features a broadband spectral bandwidth with dual dispersive waves, allowing 2f–3f self‐referencing. Via harnessing the photorefractive effect of lithium niobate to significantly extend the soliton existence range, a spontaneous yet deterministic single‐soliton formation is observed. The soliton is immune to external perturbation and can operate continuously for over 13 h without active feedback control. Finally, via integration with a pre‐programmed distributed feedback (DFB) laser, turnkey soliton generation is demonstrated. With further improvement of microresonator Q and hybrid integration with chip‐scale laser chips, compact soliton microcomb devices with electronic actuation can be created, which can become central elements for future LiDAR, microwave photonics, and optical telecommunications.

Coupling dynamics of dissipative localized structures: From polarized vector solitons to soliton molecules

Dissipative solitons are well known as representative nonlinear localized wave packets resulting from the composite balance of dispersion, nonlinearity, gain and loss in optical cavities. Since the last three decades, dissipative solitons keep as one of the most active research topics in the framework of nonlinear physics, not only for fundamental researches but also for various industrial applications. From one hand, due to the rich nonlinear dynamics, dissipative solitons have been considered as an ideal testbed to study the pumping-dissipation-bifurcation complexities. On the other hand, the emergences of dissipative solitons are tightly linked with the emission of ultrashort laser pulses, as well as the formation of broadband optical frequency combs. Compared with single-soliton counterpart, the multi-component dissipative solitons have exhibited even more fascinating properties and much wider range of applications, therefore the coupling dynamics of dissipative solitons is the blooming and emergent research area in the last decade. From this point of view, this review article covers the latest developments of compound dissipative solitons, not only in passive Kerr resonators, but also in mode-locked laser cavities. Ranging from polarized vector solitons to soliton molecules, the rising degree of freedom of these multi-component localized structures is boosting the discovery of novel nonlinear dynamics and underpin the various photonic applications in light science.

Two-colour dissipative solitons and breathers in microresonator second-harmonic generation

Frequency conversion of dissipative solitons associated with the generation of broadband optical frequency combs having a tooth spacing of hundreds of giga-hertz is a topical challenge holding the key to practical applications in precision spectroscopy and data processing. The work in this direction is underpinned by fundamental problems in nonlinear and quantum optics. Here, we present the dissipative two-colour bright-bright and dark-dark solitons in a quasi-phase-matched microresonator pumped for the second-harmonic generation in the near-infrared spectral range. We also found the breather states associated with the pulse front motion and collisions. The soliton regime is found to be typical in slightly phase-mismatched resonators, while the phase-matched ones reveal broader but incoherent spectra and higher-order harmonic generation. Soliton and breather effects reported here exist for the negative tilt of the resonance line, which is possible only via the dominant contribution of second-order nonlinearity.

Broadband optical frequency comb covering spectral regions at UV, VIS, and NIR

Herein, we present a compact, robust, self-referenced optical frequency comb (OFC) ranging from ultraviolet (UV) to near-infrared (NIR) light. The OFC system employs an all-polarization-maintained fiber figure-9 fiber oscillation as a seed laser, which is separated into two branches at the same repetition rate of approximately 100 MHz. One branch is adopted to implement frequency stabilization. Herewith, the repetition rate and the carrier-envelope phase are separately stabilized while the residual phase noises are reduced to 504 μrad and 1.12 rad (ranging from 1 Hz to 3 MHz), corresponding to fractional frequency stabilities of 1.91 × 10−12 and 1.44 × 10−17, respectively. The other branch is used to implement supercontinuum generation and quadruplicate frequency nonlinear processes, thus achieving an OFC system in the ranges of 266, 532, and 600–2200 nm. The precise frequency transferring characteristics of this OFC system exhibit great potential for precision measurement applications and optical frequency synchronization.

High-density and broad band optical frequency combs generated by pseudo-random phase modulation of optically injected gain-switched semiconductor lasers

We have demonstrated optical frequency combs (OFCs) featuring low and continuously tunable line spacing in the kilohertz range. The OFCs are generated by pulsed gain-switching of optically injected commercial semiconductor lasers emitting at around 1.55 μm. After generation, the OFCs are efficiently densified by pseudo-random-binary-sequence phase modulation. The case of an OFC generated by gain-switching at a repetition rate of 1 GHz has been numerically and experimentally analyzed and the conditions for optimal densification have been found. In this case, the comb has been densified by a factor of 2047 and exhibits a 90 GHz flat and broad spectrum consisting of more than 184,000 spectral lines, separated 488 kHz. Although this densification factor is not the largest that can be achieved with the method, it is more than 100 times larger than previously reported factors for gain-switching OFCs. These results open prospects for the use of potentially integrable gain-switching OFCs in applications requiring sub-megahertz line spacing.

Low threshold Kerr solitons.

Pumping a nonlinear optical cavity with continuous wave coherent light can result in generation of a stable train of short optical pulses. Pumping the cavity with a non-degenerate resonant coherent dichromatic pump usually does not produce a stable mode-locked regime due to competition of the oscillations at the pump frequencies. We show that generation of stable optical pulses is feasible in a dichromatically pumped cavity characterized with group velocity dispersion optimized in a way that the group velocity value becomes identical for the generated pulses and the beat note of the pump harmonics. The power threshold of the process drops nearly four times in this case and the produced pulses become sub-harmonically locked to the dichromatic pump harmonics. The process is useful for generation of broadband optical frequency combs and optical time crystals.

A power-efficient integrated lithium niobate electro-optic comb generator

Integrated electro-optic (EO) frequency combs are essential components for future applications in optical communications, light detection and ranging, optical computation, sensing and spectroscopy. To date, broadband on-chip EO combs are typically generated in high-quality-factor micro-resonators, while the more straightforward and flexible non-resonant method, usually using single or cascaded EO phase modulators, often requires high driving power to realize a reasonably strong modulation index. Here, we show that the phase modulation efficiency of an integrated lithium niobate modulator could be enhanced by passing optical signals through the modulation electrodes for a total of 4 round trips, via multiple low-loss mode multiplexers and a waveguide crossing, reducing electrical power consumption by an experimentally measured factor of 15. Using devices fabricated from a wafer-scale stepper lithography process, we demonstrate a broadband optical frequency comb featuring 47 comb lines at a 25-GHz repetition rate, using a moderate radio frequency (RF) driving power of 28 dBm (0.63 W). Leveraging the tunability in repetition rate and operation wavelength, our power-efficient EO comb generator could serve as a compact low-cost solution for future high-speed data transmission, sensing and spectroscopy, as well as classical and quantum optical computation systems.

Unexpected phase-locked Brillouin Kerr Frequency comb in fiber Fabry Perot resonators

We report the observation of a stable and broadband optical frequency comb in a high-Q fiber Fabry Perot resonator. We evidence it arises from an unexpected mode-locking phenomena.

Kerr Frequency Comb and Stimulated Raman Comb Covering S+C+L+U Band Based on a Packaged Silica Spherical Microcavity

Optical microresonators supported whispering gallery modes (WGMs) are one of the most cost-effective platforms for optical frequency comb generated due to their advantages of ultra-high quality ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) factor and ultra-low mode volume. In this paper, we report a portable and robust packaged silica spherical microcavity by melting the end of a standard single-mode fiber with ultra-high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> factor up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${10}^8$</tex-math></inline-formula> , and theoretically and experimentally demonstrate a broadband optical frequency comb (OFC) generation including Kerr OFC and stimulated Raman comb. Kerr OFCs are separated by one, two, eight and twelve free spectral ranges corresponding to 1.4, 2.8, 11.2, and 16.8 nm, respectively. The transition between Kerr OFC and stimulated Raman comb is achieved by changing the pump laser power and the detuning frequency resulting from gain competition between modulation instability and Raman gain. Benefitting from the dispersion control and ultra-high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> factor persistence in our packaged microcavity, OFC can be adjusted continuously covering from S-band to C-band, L-band and U-band. We also explain these results by numerical simulations using a model framework combined with the Lugiato-Lefever equation and Raman response function.

Generation of Broadband Optical Frequency Comb by Interfering Dual Lasers in Electro-Optic Modulators

Generation of Broadband Optical Frequency Comb by Interfering Dual Lasers in Electro-Optic Modulators

Broadband optical frequency comb generation based on single electro-absorption modulation driven by radio frequency coupled signals

Broadband optical frequency comb (OFC) generation based on a single electro-absorption modulator (EAM) is proposed. The EAM is driven by a radio frequency (RF) multi-frequency signal generated by a multiplication coupler composed of an electrical power splitter and an arithmetic circuit. Thus the number of comb-lines of the generated OFC can be increased. A complete theoretical model of OFC generation by an EAM driven by nth power of the RF source is established, and the performance of the OFC is analyzed by using OptiSystem software. The results show that, the number of comb-lines of the OFC is positively correlated with the number of multiplication of the RF source signal. The frequency spacing of the comb-lines is twice the frequency of the RF source signal and is tunable by adjusting the frequency of the RF source signal. Increasing chirp factor and modulation index of EAM could increase the number of comb-lines of the generated OFC. The amplitude of the RF source signal had little impact on the flatness of the OFC and the average OFC power. The scheme developed is not only simple and low-cost, but also can produce a large number of comb-lines.Graphical

Ultralow Noise and Timing Jitter Semiconductor Quantum-Dot Passively Mode-Locked Laser for Terabit/s Optical Networks

Diode optical frequency comb lasers are promising compact solutions to generate high-speed optical pulses for applications in high spectral efficiency wavelength division multiplexing transmission with advanced modulation formats. In this paper, an InAs/InP quantum dot (QDot) C-band single-section passively mode-locked laser (MLL) based broadband optical frequency comb source with a free spectral range of 28.4 GHz is presented. The device exhibits less than 1.5 MHz optical linewidth (phase noise) over 56 channels and 2.1 fs pulse-to-pulse timing jitter with a central wavelength of 1550 nm. Using this comb, we demonstrate an aggregate data transmission capacity of 12.5 Terabit/s over 100 km of standard single mode fiber by employing dual-polarization with 16 QAM modulation format. This investigation shows the viability for semiconductor QDot MLLs to be used as low-cost optical source in Terabit/s or higher optical networks.

Third-harmonic-assisted four-wave mixing in a chip-based microresonator frequency comb generation.

Microcombs generated in photonic integrated circuits can provide broadband and coherent optical frequency combs with a high repetition rate from microwave to terahertz. Coherent microcombs formed in normal group velocity dispersion microresonators usually have a flat-top temporal profile, called platicon. Here, we propose a novel scheme to generate platicon in Si3N4 microresonator with the assistance of third-harmonic generation. The nonlinear coupling between the fundamental and the third-harmonic waves that draws support from third-order sum/difference frequency generation provides a new mechanism to achieve the phase matching of four-wave mixing in normal dispersion microresonators. We show that single or multiple platicons can be obtained by changing the third-harmonic nonlinear coupling strength and phase matching condition for third-order sum/difference frequency generation. Our work provides a promising solution to facilitate coherent and visible microcomb generation in a pure χ(3) microresonator, which is potential for self-referencing combs and optical clock stabilization.

Mid-infrared broadband optical frequency comb generated in MgF2 resonators

Microresonator-based optical frequency combs are broadband light sources consisting of equally spaced and coherent narrow lines, which are extremely promising for applications in molecular spectroscopy and sensing in the mid-infrared (MIR) spectral region. There are still great challenges in exploring how to improve materials for microresonator fabrication, extend spectral bandwidth of parametric combs, and realize fully stabilized soliton MIR frequency combs. Here, we present an effective scheme for broadband MIR optical frequency comb generation in a MgF 2 crystalline microresonator pumped by the quantum cascade laser. The spectral evolution dynamics of the MIR Kerr frequency comb is numerically investigated, revealing the formation mechanism of the microresonator soliton comb via scanning the pump-resonance detuning. We also experimentally implement the modulation instability state MIR frequency comb generation in MgF 2 resonators covering from 3380 nm to 7760 nm. This work proceeds microresonator-based comb technology toward a miniaturization MIR spectroscopic device that provides potential opportunities in many fields such as fundamental physics and metrology.

Platicon microcomb generation using laser self-injection locking

The past decade has witnessed major advances in the development and system-level applications of photonic integrated microcombs, that are coherent, broadband optical frequency combs with repetition rates in the millimeter-wave to terahertz domain. Most of these advances are based on harnessing of dissipative Kerr solitons (DKS) in microresonators with anomalous group velocity dispersion (GVD). However, microcombs can also be generated with normal GVD using localized structures that are referred to as dark pulses, switching waves or platicons. Compared with DKS microcombs that require specific designs and fabrication techniques for dispersion engineering, platicon microcombs can be readily built using CMOS-compatible platforms such as thin-film (i.e., thickness below 300 nm) silicon nitride with normal GVD. Here, we use laser self-injection locking to demonstrate a fully integrated platicon microcomb operating at a microwave K-band repetition rate. A distributed feedback (DFB) laser edge-coupled to a Si3N4 chip is self-injection-locked to a high-Q ( > 107) microresonator with high confinement waveguides, and directly excites platicons without sophisticated active control. We demonstrate multi-platicon states and switching, perform optical feedback phase study and characterize the phase noise of the K-band platicon repetition rate and the pump laser. Laser self-injection-locked platicons could facilitate the wide adoption of microcombs as a building block in photonic integrated circuits via commercial foundry service.

Broadband and tunable optical frequency comb based on 1550 nm verticalcavity surface-emitting laser under pulsed current modulation and optical injection

Optical frequency combs (OFCs) each consist of a set of equally spaced discrete frequency components, and they have been widely applied to many fields such as metrology, optical arbitrary waveform generation, spectroscopy, optical communication, and THz generation. In this work, we propose a scheme for generating broadband and tunable OFCs based on a 1550 nm vertical-cavity surface-emitting laser (VCSEL) under pulsed current modulation and optical injection. Firstly, a pulsed electrical signal is utilized to drive a 1550 nm-VCSEL into the gain-switching state with a broad noisy spectrum. Next, a continuous optical wave is further introduced for generating broadband and tunable OFC. Under injection light with power of 18.82 µW and wavelength of 1551.8570 nm, and pulsed electrical signal with a frequency of 0.5 GHz and pulse width of 200 ps, an OFC with a bandwidth of 82.5 GHz and CNR of 35 dB is experimentally acquired, and the single sideband phase noise at the 0.5 GHz reaches –123.3 dBc/Hz at 10 kHz. Moreover, the influences of injection light wavelength, frequency and width of pulse electrical signal on the performance of generated OFC are investigated. The experimental results show that OFCs with different comb spacings can be obtained by varying the frequency of pulsed electrical signal. For the frequency of pulsed current signal varying in a range of 0.25 GHz–3 GHz, the bandwidth of generated OFCs can exceed 60 GHz through selecting optimized injection optical wavelength and width of pulse electrical signal.