Raman Soliton Research Articles

Mid-infrared enhanced Raman soliton generation in an erbium-doped ZBLAN with record energy and peak power

The advancement of tunable ultrafast sources in the mid-infrared spectrum would bring about significant progress in various scientific fields. This study suggests a gain-modulation technique to increase the peak power and pulse energy of mid-infrared tunable Raman solitons. By utilizing a high-power 2-2.23 µm Raman soliton pulse as a pump source, mid-infrared Raman solitons within the 2-3.2 µm tuning range can be generated in a segment of Er3+: ZBLAN fiber. Additionally, the introduction of 976 nm pump light into the Er3+: ZBLAN fiber leverages the gain of Er3+ to amplify the frequency shift velocity and pulse energy of the Raman soliton. Consequently, the frequency shift range of the Raman soliton is extended to 3.5 µm, with peak power and pulse energy reaching 0.93 MW and 107 nJ, respectively. These achievements represent the highest peak power and energy levels of Raman solitons generated in mid-infrared fibers to date.

Generation of a MW-class tunable Raman soliton up to 3.8 µm in a fluorotellurite fiber.

We report the generation of MW-class tunable Raman solitons of 3.3-3.8 µm in a fluorotellurite fiber. The pump source of the mid-infrared fiber is a 2.15-2.3 µm Raman soliton source which has the highest peak-power Raman soliton (3.8 MW) ever achieved in a silica fiber. Fluorotellurite fiber is characterized by its short length and high nonlinearity, preventing the Raman soliton pulse broadening that occurs with fiber dispersion. The Raman soliton shifted to 3.8 µm while still maintaining an ultra-narrow pulse width of 70.4 fs with 1.36 W output power and 1.17 MW peak power. To the best of our knowledge, this is the highest peak-power Raman soliton obtained in a mid-infrared fiber.

Role of the inverse Čerenkov effect in the formation of ultrashort Raman solitons in silica microspheres.

We theoretically demonstrate a new regime, to the best of our knowledge, of the formation of ultrashort optical solitons in spherical silica microresonators with whispering gallery modes. The solitons are driven by a coherent CW pump at the frequency in the range of normal dispersion, and the energy is transferred from this pump to the solitons via two channels: the Raman amplification and inverse Čerenkov effect. We discuss three different regimes of soliton propagation and we also show that these Raman solitons can be controlled by weak coherent CW signals.

Automating physical intuition in nonlinear fiber optics with unsupervised dominant balance search.

Identifying the underlying processes that locally dominate physical interactions is the key to understanding nonlinear dynamics. Machine-learning techniques have recently been shown to be highly promising in automating the search for dominant physics, adding important insights that complement analytical methods and empirical intuition. Here we apply a fully unsupervised approach to the search for dominant balance during nonlinear and dispersive propagation in an optical fiber and show that we can algorithmically identify dominant interactions in cases of optical wavebreaking, soliton fission, dispersive wave generation, and Raman soliton emergence. We discuss how dominant balance manifests both in the temporal and spectral domains.

Continuous-wave driving elucidates the desynchronization dynamics of ultrashort dissipative Raman solitons generated in dispersive Kerr resonators.

Phase-coherent pulsed driving of passive optical fiber resonators enables the generation of ultrashort dissipative Raman solitons with durations well below 100 fs. The existence and characteristics of such solitons critically depend on the desynchronization between the pulsed driving source and the resonator round trip time, yet the full mechanism through which these dependencies arise remains unclear. Here, we numerically demonstrate that Raman solitons can exist even under conditions of continuous-wave (CW) driving, and by numerically examining the existence and characteristics of Raman solitons under such conditions, we elucidate the role of desynchronization in pulse-driven systems. In addition to providing new insights into the existence and characteristics of ultrashort Raman solitons, our analysis yields a qualitative explanation for the range of desynchronizations over which the solitons can exist.

Optical solitons in multimode fibers: recent advances

Optical solitons in multimode fibers were predicted 40 years ago and extensively investigated theoretically. Transmission experiments in nonlinear multimode fibers have gained renewed interest, motivated by their potential to extend the capacity of long-distance transmission systems; only in the last few years, new experiments have revealed unexpected properties of optical solitons propagating in graded-index and step-index multimode fibers, partially re-writing the existing theory. Here we provide an overview of the recent experimental, numerical, and theoretical studies that revealed those new properties. It will be shown that multimode fiber solitons form with specific pulse width and energy dependent on the wavelength, and that they naturally evolve toward fundamental-mode Raman solitons. New soliton fission mechanisms, governed by the modal dispersion, will be explained. Possible applications in space-division multiplexed systems will be discussed. A recent thermodynamic approach to soliton condensation will be described.

790–1000 nm continuously-tunable fiber-based femtosecond laser source

We report a 790–1000 nm tunable femtosecond laser source by second harmonic generation (SHG) of a 1580–2000 nm soliton self-frequency shift (SSFS) fiber laser. The SSFS fiber laser with an all polarization-maintaining fiber configuration consists of passively mode-locked figure-9 Er-doped fiber oscillator, two-stage fiber amplifiers and a nonlinear fiber, delivering femtosecond Raman solitons in a tunable range of 1580 to 2016 nm. The tunable Raman solitons are further injected into a specifically-designed chirped periodically-poled lithium niobite (CPPLN) crystal for efficient SHG, and therefore femtosecond pulses with wide wavelength tuning from 790 to 1000 nm are obtained. The shortest pulse duration is < 100 fs at 890 nm, and the average power is 7.55 mW with a repetition rate of 44.9 MHz. This is, to the best of our knowledge, the first demonstration of a < 1000 nm broadband tunable fiber-based femtosecond source almost covering the whole short-wavelength near-infrared spectrum, which may have important applications in the nonlinear microscopy, biomedical imaging, and material processing.

Generation of sub-150 fs, 10 W-class, pedestal-free 2.07 µm ultrashort pulses using a cascade compression system.

We report high-power pedestal-free ultrashort pulses in a cascade compression system. In the self-compression stage, the 2 µm ultrashort pulses with 123 fs duration and up to 21.7 W output power were obtained in a 0.3 m 50 µm core diameter fiber. It is the highest self-compressing power ever obtained in a silica fiber with an all-fiber 2 µm laser amplifier as the pump source. To obtain purer pedestal-eliminated pulses, we further increase the fiber length to 1 m to trigger the soliton self-frequency shifting (SSFS) effect. By employing an enhanced SSFS technique based on third-order dispersion (TOD) and filtering out the unshifted signal light, we finally achieved sub-150 fs, 10 W-class, 1.2-MW peak-power, pedestal-free 2.07 µm ultrashort pulses. This is also the highest, to the best of our knowledge, power and energy of Raman soliton obtained by SSFS in an ordinary silica fiber.

Supercontinuum generation optimization in a dispersion-varying chalcogenide fiber

We experimentally demonstrate that a variable longitudinal dispersion profile can enhance supercontinuum generation in a chalcogenide tapered fiber, compared to tapered fibers with a standard, constant dispersion profile. For this purpose, nonlinear As2S3 tapered fibers are compared experimentally and numerically based on their dispersion profile. The optimal taper with non-uniform dispersion profile simultaneously enhances the energy transfer to both dispersive waves and Raman solitons towards shorter and longer wavelengths, respectively. The generated supercontinuum spans within the spectral range of 1.20–2.98 μm with an out-of-pump power enhancement of 4.3 dB at shorter wavelengths, 2.5 dB at longer wavelengths, and a bandwidth extension of 262 nm, compared to the supercontinuum generated with an optimal but constant dispersion profile.

High-Power Raman Soliton Generation at 1.7 μm in All-Fiber Polarization-Maintaining Erbium-Doped Amplifier

An all-fiber polarization maintaining high-power laser system operating at 1.7 μm based on the Raman-induced soliton self-frequency shifting effect is demonstrated. The entirely fiberized system is built by erbium-doped oscillator and two-stage amplifiers with polarization maintaining commercial silica fibers and devices, which can provide robust and stable soliton generation. High-power soliton laser with the average power of 0.28 W, the repetition rate of 42.7 MHz, and pulse duration of 515 fs is generated directly from the main amplifier. Our experiment provides a feasible method for high-power all-fiber polarization maintaining femtosecond laser generation working at 1.7 μm.

Bidirectional Raman soliton-like combs with unidirectional pump in a spherical microresonator.

We experimentally demonstrate bidirectional Raman soliton-like combs in a whispering gallery mode microresonator with a unidirectional pump for the first time, to the best of our knowledge. We develop a relatively simple theoretical model and find an analytical solution for forward- and backward-propagating Raman sech2-shaped solitons in an anomalous dispersion region under unidirectional pumping in a normal dispersion region. Raman solitons exist, thanks to the balance between losses and Raman gain from a CW wave (which is equal in both directions) as well as between dispersion and Kerr nonlinearity.

Efficient mid-infrared Raman soliton frequency shift in a tellurite microstructured optical fiber

Efficient mid-infrared Raman soliton frequency shift in a tellurite microstructured optical fiber

High-power tunable Raman soliton generation in large core diameter passive fibers via a precise fundamental-mode matching technique.

We demonstrate tunable high-power, high-energy Raman solitons with the range of 1.9-2.3 µm in large mode area (LMA) fibers and an optimized fundamental-mode matching technique for coupling LMA silica fibers. Finally, we obtained Raman solitons with a maximum output power of 5.8 W and a maximum pulse energy of 105 nJ in a LMA passive fiber with 32 µm core diameter, the tuning range of Raman soliton is 1.96-2.35 µm. In addition, we obtained Raman solitons with a maximum output power of 7.3 W and a maximum pulse energy of 126 nJ in a LMA passive fiber with 48 µm core diameter, the tuning range of Raman soliton is 1.96-2.27 µm. The output power of 7.3 W is the highest Raman soliton power currently available in silica fibers, and the result fills a gap in the generation of both high-power and high-energy Raman solitons in a LMA silica fiber.

Comparative study on fluoride fibers for mid-infrared generation pumped by a Cr:ZnS oscillator

We demonstrate a comparative study on single-mode fluoride fibers for mid-IR generation. We took into consideration examples of ZBLAN fibers from all leading manufacturers. The fluoride fibers were pumped with the 30-fs pulses from a Cr:ZnS oscillator which allowed for Raman soliton generation up to 4 μm and supercontinuum generation up to 5.3 μm.

Generation of Supercontinuum in a Nonlinear Fiber Light Guide. Controlling the Raman Soliton Wavelength by Controlling the Polarization State of the Pump Pulse

Generation of Supercontinuum in a Nonlinear Fiber Light Guide. Controlling the Raman Soliton Wavelength by Controlling the Polarization State of the Pump Pulse

Ultrashort dissipative Raman solitons in Kerr resonators driven with phase-coherent optical pulses

Ultrashort dissipative Raman solitons in Kerr resonators driven with phase-coherent optical pulses

Supercontinuum generation in As2S3 chalcogenide waveguide pumped by all-fiber structured dual-femtosecond solitons.

Supercontinuum sources with high compactness are essential for applications such as optical sensing, airborne detection and communication systems. In the past decades, the adoption of bulky optical parametric amplifier to pump various chalcogenide glass waveguides are widely reported for on-chip mid-infrared supercontinuum generation, but this usually leads to a large volume of the whole system, and is not practical. Therefore, integrating advanced femtosecond fiber lasers with optical waveguides using nano-fabrication technology are highly desired. However, the scarcity of compact pump sources and the dispersion-matched high-nonlinearity waveguide in short wavelength regions have hindered the advancement of integrated supercontinuum source performances in the near and mid-infrared region. In this study, we demonstrate a broadband supercontinuum source from As2S3 waveguide pumped by a compact dual-femtosecond solitons pulse source. The laser is completely fiber structured, and its wavelength can be readily tuned from 2 to 2.3 µm using Raman soliton self-frequency shift technology in a Tm3+-doped fiber amplifier. Furthermore, the As2S3 waveguide is designed with controllable dispersion and high nonlinearity for a broadband supercontinuum generation. These results will advance the development of on-chip supercontinuum sources based on chalcogenide waveguides.

Spontaneous soliton mode-locking of a microcomb assisted by Raman scattering.

We successfully control the interaction dynamics between optical parametric oscillation (OPO) and stimulated Raman scattering, leading to the generation of distinct frequency comb states in a microresonator. Through Raman-scattered photons, a Raman comb with a sech2 envelope is demonstrated having a broad RF beat note linewidth of several hundred kHz. Moreover, under a specific coupling regime, we successfully generate self-locked Raman single-solitons which is confirmed by a narrow RF beat note of 25 Hz. Remarkably, this spontaneous Raman soliton is deterministically generated through adiabatic pump frequency detuning without the requirement of external locking mechanisms. Additionally, we identify a frequency comb with an unconventional envelope that can be fitted with a Lorentzian × sech2 function, generated via an anti-Stokes process with respect to the Raman comb.

Numerical Investigation of Polarization-Dependent Ultrashort Pulse Amplification in Erbium-Doped Fluoride Fibers

We have theoretically investigated the polarization-dependent ultrashort pulse amplification in erbium-doped fluoride fibers. The numerical model is based on the coupled generalized nonlinear Schrödinger equations and the rate equations. It is found that the Raman effect can be weakened by converting the seed polarization from linear to circular. Compared with the linearly polarized seed pulses, amplification of the circularly polarized seed pulses can generate Raman solitons with a larger pulse energy of the same central wavelength, although more pump power is needed. Furthermore, the amplification dynamics when no pulse splitting occurs have also been investigated. We demonstrate that amplification of the circularly polarized seed pulses allows the main pulse to accumulate more energy while maintaining a single-pulse profile. It is predicted that sub-two-cycle laser pulses at 2.8 μm with megawatt peak power and hundreds of nanojoules pulse energy can be directly generated from the erbium-doped fluoride fiber amplifiers. The investigations conducted in this paper can provide guidelines for the design of Raman-soliton-based tunable fiber laser systems and high-power few-cycle fiber laser systems.

High-power, sub-100-fs, 1600-1700-nm all-fiber laser for deep multiphoton microscopy.

The 1600-1700-nm ultrafast fiber lasers attract great interests in the deep multiphoton microscopy, due to the reduced levels of the tissue scattering and absorption. Here, we report on the 86.7-MHz, 717-mW, 91.2-fs, all-fiber laser located in the spectral range from 1600 nm to 1700nm. The soliton self-frequency shift (SSFS) was introduced into the Er:Yb co-doped fiber amplifier (EYDFA) to generate the high-power, 1600-1700-nm Raman soliton. Detailed investigations of the nonlinear fiber amplification process were implemented in optimizing the generated Raman soliton pulses. The miniature multiphoton microscopy was further realized with this home-built laser source. The clearly imaging results can be achieved by collecting the generated harmonic signals from the mouse tail skin tissue with a penetration depth of ∼500 µm. The experimental results indicate the great potential in utilizing this 1600-1700-nm fiber laser in the deep multiphoton microscopy.