Soliton Self-frequency Shift Research Articles

Composite wavelength tuning for precision Raman resonance in soliton self-frequency shift-based coherent anti-Stokes Raman scattering

Composite wavelength tuning is proposed and demonstrated in a soliton self-frequency shift (SSFS)-based coherent anti-Stokes Raman scattering (CARS) system in order to realize a fast and precision measurement of the Raman spectrum. The Stokes wavelength can be continuously tuned over 300 nm, corresponding to a molecular vibrational range of approximately 3600 cm−1. The CARS three-dimensional spectrogram of cyclohexane around 2900 cm−1 has been measured and the corresponding Raman spectrum was recovered with 25 cm−1 spectral resolution. The composite wavelength tuning method strikes a balance between wavelength scanning speed and tuning precision during the measurement of the Raman spectrum in the SSFS-based CARS system, which can effectively solve the larger wavelength tuning step problem caused by the min-repeatable optical power adjustment.

Microstructured Fibers Based on Tellurite Glass for Nonlinear Conversion of Mid-IR Ultrashort Optical Pulses

Compact fiber-based sources generating optical pulses with a broadband spectrum in the mid-IR range are in demand for basic science and many applications. Laser systems producing tunable Raman solitons in special soft-glass fibers are of great interest. Here, we report experimental microstructured tellurite fibers and demonstrate by numerical simulation their applicability for nonlinear soliton conversion in the mid-infrared (-IR) range via soliton self-frequency shift. The fiber dispersion and nonlinearity are calculated for experimental geometry. It is shown numerically that there are two zero dispersion wavelengths for the core size of 2 μm and less. In such fibers, efficient Raman soliton tuning is attained up to a central wavelength of 4.8 μm using pump pulses at 2.8 μm.

Tree-like structures and Freak waves generation induced by quintic-nonlinearity and cubic-Raman effect in a nonlinear metamaterial

The goal of this investigation is based on electromagnetic wave behavior which is modeled by a nonlinear Schrodinger equation in a nonlinear metamaterial optical system. Two regimes are under consideration known as negative index regime and absorption regime. During the propagation, second-order dispersion, cubic-quintic nonlinearities and cubic-Raman effect are taken into account. The characterization of electromagnetic wave is assured by collective coordinates method and conventional ansatz function. The soliton light pulse is progressively modified by quintic-nonlinearity and cubic-Raman effect in order to provoke freak waves, tree structures and soliton self-frequency shift generation. Hence, the coefficients of the Schrodinger equation are plotted in order to improve the explanation of these strange phenomena. Moreover, all those above mentioned phenomena are strongly influenced by frequency and the regime under consideration. Thus, some specific freak waves such as Kuznetsov–Ma, Peregrine and Riemann waves are found. It is clearly pointed out the key role of internal and external disturbances due to cubic-Raman effects. That leads to freak waves, tree structures and soliton self frequency shift generation for both regimes. The so-called self-compensation process as technique of cancellation of these phenomena is also investigated.

Soliton detuning of 68.5 THz in the near-infrared in a highly nonlinear suspended core tellurite fiber

We demonstrate an isolated phenomenon of soliton self-frequency shift over a bandwidth of 68.5 THz in the near-infrared spectral range. We designed, fabricated, and characterized a suspended core microstructured fiber made of highly nonlinear tellurite glass. The fiber is designed for a flat anomalous dispersion to leverage on Raman scattering-assisted frequency shift of a fundamental-order soliton. We demonstrate a prominent soliton with an input-power-based tunability in the spectral range from the 1560 nm central pump wavelength up to around 2400 nm central wavelength of the frequency-shifted soliton, when the fiber is pumped with 90 fs pulses from a commercial, fiber-based, mode-locked laser.

In vivo deep-brain blood flow speed measurement through third-harmonic generation imaging excited at the 1700-nm window.

Measurement of the hemodynamic physical parameter blood flow speed in the brain in vivo is key to understanding brain physiology and pathology. 2-photon fluorescence microscopy with single blood vessel resolution is typically used, which necessitates injection of toxic fluorescent dyes. Here we demonstrate a label-free nonlinear optical technique, third-harmonic generation microscopy excited at the 1700-nm window, that is promising for such measurement. Using a simple femtosecond laser system based on soliton self-frequency shift, we can measure blood flow speed through the whole cortical grey matter, even down to the white matter layer. Together with 3-photon fluorescence microscopy, we further demonstrate that the blood vessel walls generate strong THG signals, and that plasma and circulating blood cells are mutually exclusive in space. This technique can be readily applied to brain research.

Manipulating Soliton Polarization in Soliton Self-Frequency Shift and Its Application to 3-Photon Microscopy in Vivo

Soliton self-frequency shift (SSFS) enables energetic femtosecond pulse generation and deep-brain 3-photon microscopy (3PM) in animal models in vivo. The signal level in 3PM is currently limited by the available soliton energy. Here we propose a new maneuverable physical parameter that enables more energetic soliton generation through SSFS in a photonic-crystal (PC) rod. Through both theory and experiment, we demonstrate that circularly-polarized solitons are 1.5 times more energetic than previously reported linearly-polarized solitons. We further demonstrate comparative 3PM of the white matter layer in the mouse brain in vivo with both circularly-polarized and linearly-polarized solitons. The 3.7 times higher 3PM signal excited with circularly-polarized solitons indicates that this novel nonlinear optical technique is promising for 3PM.

All-fiber nonlinear optical wavelength conversion system from the C-band to the mid-infrared.

We demonstrate an all-fiber wavelength conversion system from the C-band to the wavelength range of 2.30-2.64 µm of the mid-infrared (MIR). A series of nonlinear processes is used to perform this spectral shift in excess of 80 THz; from optical pulses in the C-band, self-phase modulation spectral broadening and offset filtering generate probe pulses in the C- and L-band. In parallel to this, Raman-induced soliton self-frequency shift converts pulses from the C-band into pump pulses in the 2 µm wavelength band. The resulting synchronized probe and pump pulses interact via degenerate four-wave mixing to produce wavelength-converted idler pulses in the MIR. Silica fiber is used for nonlinear processes at wavelengths $ {\lt} 2\;{\unicode{x00B5}{\rm m}}$<2µm whereas chalcogenide glass is used for nonlinear processes at wavelengths $ {\ge} 2\;{\unicode{x00B5}{\rm m}}$≥2µm. This system is a major step toward the development of compact MIR optical sources generated from widespread pump lasers of the C-band.

Compact all-fiber 2.1-2.7 μm tunable Raman soliton source based on germania-core fiber.

Although ultrafast rare-earth-doped fiber lasers mode-locked at near-infrared and ∼3 μm wavelengths have been well developed, it is relatively difficult to achieve ultrafast fiber laser emitting in the 2.1-2.7 μm spectral gap between ∼2 μm (Tm fiber) and ∼2.8 μm (Er or Ho fluoride fiber). In this paper, we report the generation of 2.1-2.7 μm tunable femtosecond Raman solitons from a compact fusion-spliced all-fiber system using a home-made 1.96 μm ultrafast pump source and a MIR-available germania-core fiber. At first, a Tm-doped double-clad fiber amplifier is used to not only boost up the power of 1957 nm femtosecond seed laser, but also to generate the first-order soliton self-frequency shift (SSFS). The first-order Raman solitons can be tuned from 2.036 to 2.152 μm, have a pulse duration of ∼480 fs and can reach a pulse energy of 1.07 nJ. The first-order Raman solitons are further injected into a 94 mol.% germania-core fiber to excite the second-order SSFS. The second-order solitons can be tuned to longer wavelengths, i.e. from 2.157 μm up to 2.690 μm. Our work could provide an effective way to develop compact, all-fiber ultrafast MIR laser sources with the continuous wavelength tuning of 2.1-2.7 μm.

Deep-brain 2-photon fluorescence microscopy in vivo excited at the 1700 nm window.

Here we demonstrate deep-brain 2-photon fluorescence microscopy in mouse in vivo, excited at the 1700nm window. Through signal versus power measurement, we show that indocyanine green (ICG) is a promising 2-photon fluorescent dye excitable at the 1700nm window. In order to excite ICG circulating in the vasculature in the deep brain, we employ a circular-polarization soliton self-frequency shift technique to generate energetic femtosecond pulses at 1617nm. Combining the labeling and laser technologies, we achieve a record 2-photon fluorescence brain vasculature imaging depth of 2000μm in vivo. Both the effective attenuation length measurement and signal-to-background ratio measurement indicate that we have reached the theoretical depth limit in 2-photon fluorescence microscopy.

Soliton supermode transitions and total red shift suppression in multi-core fibers.

Soliton propagation in seven-core fibers is studied numerically with the frequency dependence of linear coupling taken into account. It is shown that under certain conditions, solitons can undergo spontaneous supermode transitions upon abrupt red shift, analogous to the recently discovered phenomenon of soliton self-mode conversion in step-index multi-mode fibers. When the core separation is increased, the dynamics change drastically, leading to total suppression of soliton self-frequency shift and the emergence of spatially oscillating solitonic structures with resemblance to solitons in graded-index multi-mode fibers. The observed phenomena are general multi-core effects and not specific to seven-core fibers.

On the efficiency of different numerical methods for the calculation of intrapulse Raman scattering of optical solitons

In this paper we compare the performance of different numerical methods for the calculation of the asymptotic evolution of soliton self-frequency shift in the presence of intrapulse Raman scattering (IRS) in optical fibers. First we have calculated the order of global accuracy for the fundamental soliton and the second-order bound state of the unperturbed nonlinear Schrodinger equation for the following numerical methods: the simple split step (SS) method, the full SS method, the reduced SS method, the reduced SS method with fourth order Runge–Kutta (RK4), the Blow–Wood method, the fourth order Runge–Kutta in the interaction picture (RK4IP) method and the Agrawal SS method with one and two iterations. We have shown that the asymptotic evolution of soliton self-frequency shift in the presence of IRS in optical fiber can be best described by the Agrawal SS method (compared to the Blow–Wood method and the RK4IP method). The obtained numerical results for the soliton position and frequency are in agreement with the predictions for these parameters according to the perturbation theory. We have shown that in the presence of IRS the fundamental soliton quickly develops an oscillating tail on its left. The generated tail hardly influences the soliton propagation at large distances. At large distances the form of the soliton gradually becomes asymmetric. The increase of the frequency resolution leads to a notable increase of the maximum propagation distance of the numerical soliton under the influence of IRS.

Optical quantization based on soliton self-frequency shift and a flexible spectrum compression scheme utilizing time-dependent filtering.

We propose and experimentally demonstrate a novel optical spectrum compression scheme utilizing time-dependent filtering effect, which can be used to improve the resolution of the optical quantization based on soliton self-frequency shift (SSFS). The spectrum compression is realized through power-dependent filtering utilizing polarizers and nonlinear polarization rotation effect in a spool of high-nonlinear fiber (HNLF) after mapping the soliton spectrum to the time domain in a section of dispersive fiber. Theoretical analysis and numerical simulation indicate that, compared with the conventional spectrum compression scheme based on chirp compensation, the proposed scheme does not have a restrictive length match requirement of the dispersive fiber and the HNLF. In addition, the output spectral width variation is small for various pulse peak power, which is favorable for the subsequent optical coding. In the experiment, the spectra after SSFS are compressed to an average spectral width of 1.65 nm, which is 45% of that using comb-like fibers. The quantization resolution using the proposed scheme is 5.95 bits for a maximum wavelength shift of 100 nm, which is 1.13 bits higher than that using comb-like fibers.

Air-core fiber or photonic-crystal rod, which is more suitable for energetic femtosecond pulse generation and three-photon microscopy at the 1700-nm window?

Energetic femtosecond pulses at the 1700-nm window are a prerequisite for deep-tissue three-photon microscopy (3PM). Soliton self-frequency shift (SSFS) in photonic-crystal (PC) rod has been the only technique to generate such pulses suitable for 3PM. Here we demonstrate through SSFS in an air-core fiber, we can generate most energetic femtosecond soliton pulses at the 1700-nm window, 5.2 times higher than that from PC rod. However, the air-core soliton pulse width is 5.9 times longer than that of PC rod soliton. Based on comparative 3PM excited with both air-core and PC rod solitons, we propose the more suitable source for 3PM. We further elucidate the challenge of generating shorter soliton pulses from air-core fibers through numerical simulation.

Robust 1.7-μm, all-polarization-maintaining femtosecond fiber laser source based on standard telecom fibers

We demonstrate a compact 1.7-μm all-polarization-maintaining ultrafast fiber laser source, which employs soliton self-frequency shift in an optical fiber as the dominant spectral-shifting mechanism. The laser generates stable pulses with 384 fs pulse width and 102 MHz repetition rate centered at 1.7 μm with 7 nm spectral bandwidth. The average power is about 35 mW with a high conversion efficiency of 66%. The measured root mean square output power stability over 10 h is as low as 0.22%. Both experimental and simulation results demonstrate that this laser has a potential to improving multiphoton imaging depth.

Theoretical simulations of the soliton self-frequency shift of mid-infrared femtosecond pulses in step-index tellurite optical fibers: broadband tunability and high efficiency

Broadband tunable ultrafast lasers using mid-infrared (MIR) fibers operating at a 3-5 µm atmospheric transmission window are attractive sources because of their numerous applications. Tellurite fibers possess the merits of large linear and nonlinear refractive indices, sufficient chemical stability, and wide transparency range up to ∼5 µm; also, they are highly suitable for high efficiency MIR ultrafast fiber laser sources based on soliton self-frequency shift (SSFS). We numerically simulate SSFS of MIR femtosecond pulses in step-index tellurite optical fibers. A femtosecond erbium-doped fluoride fiber laser at 3.5 µm is employed as the pump source. Parameters including the peak power of the input pulse and nonlinear fiber length are optimized for high efficiency broadband tunable MIR ultrafast laser performance. Our results show that a high-efficiency 3.5-6 µm wavelength-tunable femtosecond laser can be realized by employing SSFS in a 22-cm-long segment of tellurite step-index fiber pumped by femtosecond pulses with 10-300 kW peak powers at 3.5 µm. Ultra-high energy ratios of the most redshifted solitons to the input pulses of >50% are obtained across the 3.5-5 µm tuning range. The presented numerical study provides valuable guidance for SSFS of MIR femtosecond pulses in step-index tellurite fibers and is valuable for future high efficiency wavelength-tunable MIR ultrafast fiber laser development.

Dual-comb spectroscopy of methane based on a free-running Erbium-doped fiber laser.

Dual-comb spectroscopy has been developed into a high-precision technique that is capable of sensing many important species of samples, such as methane. Recent studies on single-cavity, dual-comb light sources further reduce the system complexity of such schemes. In contrast to the previous demonstrations around the lasing spectrum, this work significantly expands the spectral coverage of a dual-comb spectroscopy setup using one free-running laser to a region far beyond the laser's emission wavelengths. Nonlinear wavelength conversion based on soliton self-frequency shift is adopted to convert and tune the wavelengths of both dual-comb pulses to ~1650nm. It is shown that this process has introduced little additional intensity noise. The 2ν3 absorption band of methane from 1647 nm to 1663nm is measured with very good agreement with HITRAN, and the standard deviation of the residual is < ~0.006 after averaging ~1.96 seconds of data. Our results further elucidate the potential of dual-comb spectroscopy using one laser, and could pave the way for the development of low-cost, power-efficient, and compact dual-comb instrument targeting more spectral regions.

Fate of a Soliton in a High Order Spatial Mode of a Multimode Fiber.

We show numerically that under certain conditions noise-induced soliton self-mode conversion dominates over soliton self-frequency shift for a soliton in a high order spatial mode of a multimode optical fiber. The input soliton has to be group index matched to a lower order mode for a frequency separation for which the Raman gain is non-negligible, and this condition determines the wavelength of the pulse growing from noise. The phenomenon has no known analogs in single-mode or graded-index fibers. The results demonstrate that it is possible for a noise-induced physical process to dominate over a seeded one even for noise levels at the fundamental limit.

In-cavity soliton self-frequency shift ultrafast fiber lasers.

Lasing wavelengths are generally limited at the gain bands of active materials. In this Letter, we propose and demonstrate a new concept of ultrafast fiber laser based on in-cavity soliton self-frequency shift (SSFS). Benefiting from SSFS, the wavelength of ultrafast fiber lasers can be beyond the gain band of active fibers. SSFS was previously stimulated by transmitting pulses in passive fibers. Triggering SSFS inside a laser cavity has never been reported. We successfully stimulate SSFS in a laser by employing a piece of dispersion-shifted fiber. In-cavity SSFS is expected to be implemented in various ultrafast lasers and is conducive to develop compact wavelength versatile laser sources.

Control of soliton self-frequency shift dynamics via Airy soliton interaction.

We investigate Airy-soliton interaction in a nonlinear fiber with Raman effect. We find that Airy solitons may fuse upon interaction at a position that can be controlled by a proper engineering of the Airy tail direction. This control allows us to generate Airy solitons with varying deceleration. At variance with the case of two solitons interaction, Raman-induced soliton self-frequency shift (SSFS) is strongly enhanced when the leading soliton is replaced with the accelerating Airy pulse and slightly suppressed for the decelerating one. These notable features are ascribed to the unique properties of asymmetrical Airy pulses with a switchable direction of the oscillatory tails. We show the way these processes are uncovered unambiguously by cross-correlation frequency resolved optical gating. We also investigate the impact of chirp imposed on the input pulse on the SSFS dynamics. Our results not only provide a new way to manipulate the SSFS, but may help to improve the control of soliton fusion events during supercontinuum generation, optical rogue waves and giant dispersive waves formation.

Tunable mid-infrared Raman soliton generation from 1.96 to 2.82 μm in an all-solid fluorotellurite fiber

We demonstrate tunable mid-infrared (MIR) Raman soliton generation in an all-solid fluorotellurite fiber pumped by a 1960 nm femtosecond fiber laser. All-solid fluorotellurite fibers are fabricated by using a rod-in-tube method. The core and cladding materials are TeO2-BaF2-Y2O3 (TBY) and AlF3-based glasses. Since the refractive index of AlF3-based glass is much lower than that of TBY glass, the zero-dispersion wavelength of all-solid fluorotellurite fiber can be tuned in a wide range by simply varying the core diameter of the fiber. By using a 0.5 m long all-solid fluorotellurite fiber with a core diameter of ∼ 2.7 μm as the nonlinear medium and a 1960 nm femtosecond fiber laser as the pump source, tunable MIR Raman soliton generation from 1.96 to 2.82 μm is obtained via soliton self-frequency shift by increasing the pump power. For the soliton laser source operating at 2.82 μm, the obtained optical-to-optical conversion efficiency is about 18% and the calculated pulse width is about 93 fs. Our results show that all-solid fluorotellurite fibers are promising nonlinear media for constructing tunable MIR ultrafast laser sources.