Raman Soliton Self-frequency Shift Research Articles

Numerical simulation of coherent mid-infrared supercontinuum generation based on carbon chloride core tapered photonic crystal fibers

We numerically studied coherent mid-infrared (MIR) supercontinuum (SC) generated by filling carbon chloride (CCl4, C2Cl4) into the tapered photonic crystal fiber (PCF). The mechanism involves nonlinear soliton compression, Raman soliton self-frequency shift and dispersive wave generated in a carbon chloride core tapered PCF pumped by a 1.98 μm femtosecond laser. Numerical solution of the generalized nonlinear Schrodinger equation shows that 1.15–5.5 μm (CCl4-core tapered PCF) and 1.15–5.7 μm (C2Cl4-core tapered PCF) coherent MIR-SC sources can be realized. Our results show that the above liquid-core tapered PCFs are promising nonlinear media for constructing broadband coherent MIR-SC light sources.

Sub-100 fs all-fiber polarization maintaining widely tunable laser at 2 µm.

An all-fiber polarization maintaining (PM) laser tunable over 170 nm via Raman soliton self-frequency shift (SSFS) delivering sub-100 fs pulses with a central wavelength ranging from 1880 to 2050 nm is presented. The 40 MHz laser emits power ranging from 15 to 34 mW with pulse duration as short as 80 fs. It is based on exclusively commercially available standard fibers and has been designed to produce sub-100 fs pulse durations over the whole spectral range without the need for adjusting neither nonlinearities nor dispersion. The laser is versatile and a robust potential alternative to Tm and Tm:Ho oscillators.

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.

Near-infrared pumped, octave-tunable, on-chip mid-infrared Raman soliton source.

This article proposes and numerically demonstrates a widely tunable on-chip Raman soliton source based on a cascaded As2Se3 waveguide. The cascaded sub-waveguides (input and output) with varying widths act as nonlinear devices, while a tapered waveguide is arranged between them to achieve low-loss transmission. The input waveguide provides anomalous dispersion in the near-infrared band, thereby enabling the 1.96 µm source for Raman soliton self-frequency shift (SSFS) pumping. The output waveguide exhibits large anomalous dispersion and good mode confinement in the mid-infrared band thus supporting further SSFS process. A 2.29∼4.57 µm tunable Raman source is theoretically realized in this on-chip platform. This work presents a simple and easy-to-implement strategy to extend the tuning range of on-chip sources. Notably, to the best of our knowledge, this is the first demonstration of the cascading strategy for SSFS process in an on-chip platform. The proposed tunable source has great potential in integrated spectroscopy, gas sensing, and LiDAR applications.

Multi-Raman soliton self-frequency shifts and dissipative rogue wave trains induced by cubic-quintic-Raman contributions in a double-negative material

We examine the cubic-quintic-Raman influences on the system when the second-order dispersion and the cubic-quintic-nonlinearities act in a double-negative material. This research is done for both absorption and negative index regimes. Using the collective coordinates’ technique, the quintic-nonlinearity induces the generation of Kuznetsov-Ma breathers and Peregrine soliton. Thereafter, the cubic-Raman effect comes into play and breeds the chaotic Peregrine waves’ field, the complex rogue event, and the modified complex rogue event. Therefore, the quintic-Raman effect induces first-, second-, and third-forms of the dissipative rogue waves and the twin parallel wave trains. Some fascinating phenomena such as deletion of waves, damping effect, tree structure, and disintegration of the Raman soliton self-frequency shift leading to the multi-Raman soliton self-frequency shifts, are also scrutinized. Further, internal strife leading to these above-mentioned effects, are likewise presented in detail.

Study on soliton self-frequency shift in a Tm-doped fiber amplifier seeded by a Kelly-sideband-suppressed conventional soliton

We experimentally present mid-infrared Raman soliton self-frequency shift (SSFS) process in a Tm-doped fiber amplifier using sideband-suppressed conventional solitons as seed pulses. The strong Kelly sidebands of the soliton oscillator were efficiently suppressed (more than 21 dB) using a home-made all-fiber Lyot filter (AFLF). As a result, the Raman solitons with a continuously tunable wavelength of 1.95-2.34 µm were achieved, with a high soliton energy conversion of >93% over the range of 1.95-2.24 µm. The conversion efficiency and tunable range of Raman solitons were both significantly improved, comparing to the same amplifier seeded with sideband-unsuppressed pulses.

Tunable Supercontinuum Generated in a Yb3+-Doped Microstructure Fiber Pumped by Ti:Sapphire Femtosecond Laser**Supported by the National Natural Science Foundation of China under Grant No 61735011, the Natural Science Foundation of Hebei Province under Grant Nos F2016203389 and F2018105036, the Science and Technology Research Project of College and University in Hebei Province under Grant No BJ2017108, the Open Subject of Jiangsu

We experimentally demonstrate that a tunable supercontinuum (SC) can be generated in a Yb3+-doped microstructure fiber by the concept of wavelength conversion with a Ti:sapphire femtosecond (fs) laser as the pump. Experimental results show that an emission light around 1040nm in an anomalous dispersion region is first generated and amplified by fs pulses in the normal dispersion region. Then, SC spectra from 1100 to 1380nm and 630 to 840nm can be achieved by combined effects of higher-order soliton fission and Raman soliton self-frequency shift in the anomalous dispersion region and self-phase modulation, dispersive wave, and four-wave mixing in the normal dispersion region. It is also demonstrated that the 20nm change of pump results in a 280nm broadband shift of soliton and the further red-shift of soliton is limited by OH− absorption at 1380 nm.

Generation of sub-100 fs pulses tunable from 1700 to 2100 nm from a compact frequency-shifted Er-fiber laser

We report generation of sub-100 fs pulses tunable from 1700 to 2100 nm via Raman soliton self-frequency shift. The nonlinear shift occurs in a highly nonlinear fiber, which is pumped by an Er-doped fiber laser. The whole system is fully fiberized, without the use of any free-space optics. Thanks to its exceptional simplicity, the setup can be considered as an alternative to mode-locked Tm- and Ho-doped fiber lasers.

Soliton repetition rate in a silicon-nitride microresonator.

The repetition rate of a Kerr comb composed of a single soliton in an anomalous group velocity dispersion silicon-nitride microcavity is measured as a function of pump frequency. By comparing operation in the soliton and non-soliton states, the contributions from the Raman soliton self-frequency shift (SSFS) and the thermal effects are evaluated; the SSFS is found to dominate the changes in the repetition rate, similar to silica cavities. The relationship between the changes in the repetition rate and the pump frequency detuning is found to be independent of the nonlinearity coefficient and dispersion of the cavity. Modeling of the repetition rate change by using the generalized Lugiato-Lefever equation is discussed; the Kerr shock is found to have only a minor effect on repetition rate for cavity solitons with duration down to ∼50 fs.

Flat-Top Supercontinuum and Tunable Femtosecond Fiber Laser Sources at 1.9–2.5 μm

We report the high-energy flat-top supercontinuum covering the midinfrared wavelength range of 1.9–2.5 μm as well as electronically tunable femtosecond pulses between 1.98 and 2.22 μm directly from the thulium-doped fiber laser amplifier. Comparison of experimental results with numerical simulations confirms that both sources employ the same nonlinear optical mechanism—Raman soliton frequency shift occurring inside the Tm-fiber amplifier. To illustrate that we investigate two versions of the compact diode-pumped SESAM mode-locked femtosecond thulium-doped all-silica-fiber-based laser system providing either broadband supercontinuum or tunable Raman soliton output, depending on the parameters of the system. The first system operates in the Raman soliton regime providing femtosecond pulses tunable between 1.98 and 2.22 μm. Wide and continuous spectral tunability over 240 nm was realized by changing only the amplifier pump diode current. The second system generates high-energy supercontinuum with the superior spectral flatness of better than 1 dB covering the wavelength range of 1.9–2.5 μm, with the total output energy as high as 0.284 μJ, and the average power of 2.1 W at 7.5-MHz repetition rate. We simulate the amplifier operation in the Raman soliton self-frequency shift regime and discuss the role of induced Raman scattering in supercontinuum formation inside the fiber amplifier. We compare this system with a more traditional 1.85–2.53-μm supercontinuum source in the external highly-nonlinear commercial chalcogenide fiber using the Raman soliton MOPA as an excitation source. The reported systems 1 1 Patent pending, ATLA Lasers AS, Trondheim, Norway.

Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications.

Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 μm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fiber's infrared loss. A spectrally-flat 2-2.5 μm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 μm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 μm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.

Broadband tunable Raman soliton self-frequency shift to mid-infrared band in a highly birefringent microstructure fiber**Project supported by the National Natural Science Foundation of China (Grant Nos. 61405172, 61405173, and 61275093), the Natural Science Foundation of Hebei Province, China (Grant No. F2014203194), the College Science Research Program of Hebei Province, China (Grant No. QN20131044), and the Program of Independent Research for

Raman soliton self-frequency shifted to mid-infrared band (λ > 2 μm) has been achieved in an air-silica microstructure fiber (MF). The MF used in our experiment has an elliptical core with diameters of 1.08 and 2.48 μm for fast and slow axis. Numerical simulation shows that each fundamental orthogonal polarization mode has two wide-spaced λZDW and the λZDW pairs located at 701/2110 nm and 755/2498 nm along the fast and slow axis, respectively. Using 810-nm Ti:sapphire femtosecond laser as pump, when the output power varies from 0.3 to 0.5 W, the furthest red-shift Raman solitons in both fast and slow axis shift from near-infrared band to mid-infrared band, reaching as far as 2030 and 2261 nm. Also, mid-infrared Raman solitons can always be generated for pump wavelength longer than 790 nm if output pump power reaches 0.5 W. Specifically, with pump power at 0.5 W, the mid-infrared soliton in slow axis shifts from 2001 to 2261 nm when the pump changes from 790 nm to 810 nm. This means only a 20 nm change of pump results in 260 nm tunability of a mid-infrared soliton.

Dressed dynamics of two time-reversed shapes of Airy pulses in a relaxing nonlinear medium

We investigate the dressed dynamics of two kinds of asymmetric Airy pulses with time-reversed shapes in a relaxing nonlinear medium. Airy tails act as low intensity “dressing” pulses, refuel the Airy main lobe, and modify propagation dynamics of Airy pulses. It is demonstrated that a soliton sheds from the main lobe of Airy pulses and the spectrum maximum intensity is periodically manifested due to the effect of self-phase modulation. The intrinsically asymmetric nature of Airy pulses is revealed through the nonlinear generation of a Raman soliton self-frequency shift and the generation of static solitons driven by the soliton fission processes and the interactions with dressing Airy tails. The resulting Raman-induced frequency shift can be controlled by using time-reversed Airy pulses and varying truncation coefficients. Numerical results show that the frequency shift can be enhanced with a larger input peak power as well.

Improved intrapulse raman scattering control via asymmetric airy pulses.

We experimentally demonstrate the possibility of tuning the frequency of a laser pulse via the use of an Airy pulse-seeded soliton self-frequency shift. The intrinsically asymmetric nature of Airy pulses, typically featured by either leading or trailing oscillatory tails (relatively to the main lobe), is revealed through the nonlinear generation of both a primary and a secondary Raman soliton self-frequency shift, a phenomenon which is driven by the soliton fission processes. The resulting frequency shift can be carefully controlled by using time-reversed Airy pulses or, alternatively, by applying an offset to the cubic phase modulation used to generate the pulses. When compared with the use of conventional chirped Gaussian pulses, our technique brings about unique advantages in terms of both efficient frequency tuning and feasibility, along with the generation and control of multicolor Raman solitons with enhanced tunability. Our theoretical analysis agrees well with our experimental observations.

Design for photonic crystal fibers with high nonlinearity and flattened dispersion for self-frequency shift of Raman soliton

A new concept of photonic crystal fiber (PCF) with high nonlinearity and flattened dispersion was designed. The PCF structure is indeed a hexagon lattice. The bigger air holes in the outer rings are used to confine the light field into the core region for enhancing the nonlinearity. The flattened dispersion can be achieved by adjusting the diameters of six smaller air holes in the first ring, and six micro air holes are inserted between smaller air holes for higher nonlinearity and the better flattened dispersion. By optimizing the size of the smaller and micro holes, the PCF can reach to high nonlinearity of 23.3 W−1 · km−1 and the low dispersion of 51.32 ps/(nm·km) with the fluctuation range of 0.98 ps/(nm·km), which is within the wavelength range of 1 400 nm to 1 900 nm. The designed PCF can be used in important applications in realizing the Raman soliton self-frequency shift (RSSFS).

Efficient method of calculation of Raman soliton self-frequency shift in nonlinear optical media

We present a method to evaluate Raman soliton self-frequency shift of soliton light pulses solution for higher-order nonlinear Schrödinger equation with non-Kerr nonlinearity which propagate in high-bit-rate optical systems. We show that the conventional technique, known as collective coordinates theory, becomes inappropriate and leads to a qualitatively and unpredictable dynamics of collective coordinates. We resolve this changeableness by reformulating the conventional technique during which we add two appropriate pulse parameters called the simulated Raman scattering specific coordinates. We point out the use of these coordinates by applying them to a correct calculation of soliton self-frequency shift (SSFS) and temporal shift when cubic-quintic effects effectively act. This method of calculation of soliton self-frequency shift could be an interesting physical tool to those working on propagation of nonlinear pulses in optical media where the investigations of simulated Raman scattering with associated phenomena are required.

Suppression of Raman soliton self-frequency shift in photonic crystal fibers with tellurite subwavelength core

A new nonlinear evolution equation including the vector nature of the electromagnetic field and the frequency variation of the mode profile is derived. A kind of new nonlinearity is demonstrated. Its magnitude is strongly dependent on the waveguide geometrical parameters, which will lead to a suppression of the Raman soliton self-frequency shift in a photonic crystal fiber with a tellurite subwavelength core. Our results can be supported by the detailed numerical simulations.

Chirped pulse amplification in single mode Tm:fiber using a chirped Bragg grating

We report femtosecond pulse generation and chirped pulse amplification in Tm:fiber. A mode-locked oscillator operating in the soliton regime produced 800 fs pulses with 5 nm spectral bandwidth, at 40 pJ pulse energy. This oscillator seeded a pre-amplifier that utilizes a Raman soliton self-frequency shift to produce wavelength tunable pulses with 3 nJ energy, reduced pulse duration of 150 fs, and increased bandwidth of 30 nm. For further amplification, the pulses were stretched up to 160 ps using a chirped Bragg grating (CBG). Stretched pulses were amplified to 85 nJ after compression in single-mode Tm:fiber and recompressed with the CBG as short as 400 fs. Compressed pulses were coupled into a highly nonlinear tellurite fiber to investigate the potential of this ultrashort pulse 2-μm fiber source as a pump for mid-IR supercontinuum generation.

1 μJ, sub-500 fs chirped pulse amplification in a Tm-doped fiber system

We demonstrate a Tm-doped fiber laser system producing ~300 fs pulses with 1 μJ energy, corresponding to peak powers greater than 3 MW. Pulses of 150 fs with 30 nm spectral bandwidth and 3 nJ pulse energy are generated in a Raman-soliton self-frequency shift amplifier, then stretched to ~160 ps using a chirped Bragg grating. The 60 MHz oscillator repetition rate is reduced to 100 kHz using an electro-optic modulator. After a single-mode fiber preamplifier and a large-mode-area fiber power amplifier, pulses were compressed using a folded Treacy grating setup to below 500 fs with up to 1 μJ pulse energy. To the best of our knowledge, this is the highest energy yet demonstrated as well as the first demonstration of peak powers exceeding 1 MW from a Tm:fiber laser system.

Controllable Raman soliton self-frequency shift in nonlinear metamaterials

Controllable and dispersive magnetic permeability in the metamaterials (MMs) provides us more freedom to harness the propagation of ultrashort electromagnetic pulses at will. Here we discuss the controllability of the Raman soliton self-frequency shift (SSFS) in the MMs with a nonlinear electric polarization. First, we derive a generalized nonlinear Schr\odinger equation suitable for few-cycle pulse propagation in the MMs with delayed Raman response, and demonstrate the Raman effect, high-order Raman-related nonlinearity, and high-order nonlinear dispersion terms occurring in this equation. Second, we present a theoretical investigation on the controllability of the Raman SSFS in the MMs. In particular, we identify the combined effects of the anomalous self-steepening (SS), third-order dispersion (TOD), and Raman effect on SSFS. It is shown that the positive SS effect suppresses SSFS; however, the negative SS effect enhances SSFS, and the positive TOD leads to the deceleration of SSFS. Finally, the effects of SS on the SSFS of the second-order soliton are also discussed.