Deep-UV to Mid-IR Supercontinuum Generation driven by Mid-IR Ultrashort Pulses in a Gas-filled Hollow-core Fiber

Optimization of stimulated rotational Raman scattering over vibrational scattering in a hydrogen-filled fiber

We present the first, to the best of our knowledge, investigation of the gain competition between rotational and vibrational stimulated Raman scattering (SRS) in the transient regime for a hydrogen (H2)-filled antiresonant fiber (ARF) with the aim of generating multispectral emission composed of only rotational SRS. We show numerically and experimentally that purely rotational emission requires optimization of ARF length and spectral transmission, pump power and polarization, and H2 pressure. In this work, the H2-filled ARF is pumped by 40 kW, 7 ns pulses at λ = 1.06 µm to produce six discrete rotational lines from 1.1 to 1.7 µm with unique temporal profiles and pulse energies up to tens of microjoules.

Unified and vector theory of Raman scattering in gas-filled hollow-core fiber across temporal regimes

Raman scattering has found renewed interest owing to the development of gas-filled hollow-core fibers, which constitute a unique platform for exploration of novel ultrafast nonlinear phenomena beyond conventional solid-core-fiber and free-space systems. Much progress has been made through models for particular interaction regimes, which are delineated by the relation of the excitation pulse duration to the time scales of the Raman response. However, current experimental settings are not limited to one regime, prompting the need for tools spanning multiple regimes. Here, we present a theoretical framework that accomplishes this goal. The theory allows us to review recent progress with a fresh perspective, makes new connections between distinct temporal regimes of Raman scattering, and reveals new degrees of freedom for controlling Raman physics. Specific topics that are addressed include transient Raman gain, the interplay of electronic and Raman nonlinearities in short-pulse propagation, and interactions of short pulses mediated by phonon waves. The theoretical model also accommodates vector effects, which have been largely neglected in prior works on Raman scattering in gases. The polarization dependence of transient Raman gain and vector effects on pulse interactions via phonon waves is investigated with the model. Throughout this Perspective, theoretical results are compared to the results of realistic numerical simulations. The numerical code that implements the new theory is freely available. We hope that the unified theoretical framework and numerical tool described here will accelerate the exploration of new Raman-scattering phenomena and enable new applications.

Soliton and dispersive wave generation with third-order dispersion and temporal boundary

We investigate the pulse evolution and energy conservation condition at the temporal boundary under third-order dispersion. When the fundamental soliton crosses the temporal boundary and forms two reflected pulses and one transmitted pulse, the power of the transmitted pulse first increases and then decreases as the incident spectrum shifts toward the blue side. If the transmitted spectrum lies in the anomalous group-velocity dispersion region, second-order soliton is formed and dispersive wave is radiated. We present a modified phase-matching condition to predict the resonance frequencies. The predicted results are in good agreement with the results obtained by numerically solving the nonlinear Schrödinger equation.

Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air

Supercontinuum (SC) light source has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics, and chemistry. Compared to the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for supercontinuum generation. A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level. We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy. This was achieved by applying an external DC electric field to the air plasma filament. Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold. The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

Deep ultraviolet hydrogel based on 2D cobalt-doped titanate

Birefringent optical elements that work in deep ultraviolet (DUV) region become increasingly important these years. However, most of the DUV optical elements have fixed birefringence which is hard to be tuned. Here, we invent a birefringence-tunable optical hydrogel with mechano-birefringence effect in the DUV region, based on two-dimensional (2D) low-cobalt-doped titanate. This 2D oxide material has an optical anisotropy factor of 1.5 × 10^-11 C² J^-1 m^-1, larger than maximum value obtained previously, leading to an extremely large specific magneto-optical Cotton-Mouton coefficient of 3.9 × 10⁶ T^-2 m^-1. The extremely large coefficient enables the fabrication of birefringent hydrogel in a small magnetic field with an ultra-low concentration of 2D oxide material. The hydrogel can stably and continuously modulate 303 nm DUV light with large phase tunability by varying the strain (compression or stretching) from 0 to 50%. Our work opens the door to design and fabricate new proof-of-concept DUV birefringence-tunable element, as demonstrated by optical hydrogels capable of DUV modulation by mechanical stimuli.

Analytical Lah-Laguerre optical formalism for perturbative chromatic dispersion

We present a generalized perturbative analytical formalism for evaluation and optimization of the chromatic dispersion of complex ultrafast optical systems. Notably, we identify polynomial and recursive relations associated with the chromatic dispersion orders that are identical to the Lah and Laguerre transforms. We explicitly outline the first ten dispersion terms and dispersion slope parameters and visualize the significance of the chromatic dispersion orders for several advanced ultrafast optical and photonic systems consisting of various optical materials and nanostructures, grating and prism-pair compressors, and hollow-core photonic anti-resonant fibers. The derived simple hypergeometric transforms are applicable for evaluation of infinitely high orders for any type of frequency-dependent phase and can facilitate the optimization of complex optical systems with controlled dispersion balance at the single-cycle waveform extreme.

Random misalignment and anisotropic deformation of the nested cladding elements in hollow-core anti-resonant fibers

Hollow-core anti-resonant fibers (HC-ARFs) are en route to compete with and surpass the transmission performance of standard single-mode fibers (SSMFs). Recently, nested cladding elements emerged as a key enabler in reaching ultra-low transmission losses over a wide bandwidth. However, implementing nested geometry features poses a great challenge even in the current state-of-the-art fiber fabrication technology, often leading to structural imperfections, which ultimately worsen overall fiber performance. This article provides insights into the impact of fabrication-based perturbations of the cladding elements on the transmission performance and identifies areas of highest susceptibility. The impact of random outer and nested cladding tube misalignments as well as their anisotropic deformation on the propagation loss is analyzed based on observations of experimentally fabricated fibers. A dominance of the deformation effect over the misalignment effect is observed, with higher-order modes (HOMs) being affected one order of magnitude stronger than the fundamental mode (FM). The impact on propagation loss by structural perturbations is highly wavelength dependent, ranging from negligibly small values up to loss increases of 65% and 850% for FM and HOM propagation, respectively. The investigations are directly linked to fabrication metrics and therefore pave the way for assessing, predicting, and improving the transmission quality of fabricated hollow-core fibers.

Efficient soliton self-frequency shift in hydrogen-filled hollow-core fiber

We report a study of soliton self-frequency shifting in a hydrogen-filled hollow-core fiber. The combination of hydrogen and short 40-fs input pulses underlies clean and efficient generation of Raman solitons between 1080 and 1600 nm. With 240-nJ input pulses, the Raman soliton energy ranges from 110 to 20 nJ over that wavelength range, and the pulse duration is approximately 45 fs. In particular, 70-nJ and 42-fs pulses are generated at 1300 nm. Numerical simulations agree reasonably well with experiments and predict that microjoule-energy tunable pulses should be possible with higher-energy input pulses.

Impact of Mode-Area Dispersion on Nonlinear Pulse Propagation in Gas-Filled Anti-Resonant Hollow-Core Fiber

We numerically investigate the effect of mode-area dispersion in a tubular-type anti-resonant hollow-core fiber by using a modified generalized nonlinear Schrödinger equation that takes into account the wavelength-dependent mode area in its nonlinear term. The pulse evolution dynamics with and without the effect of mode-area dispersion are compared and analyzed. We show that strong dispersion of the mode area in the proximity of the cladding wall thickness-induced resonances has a significant impact on the soliton pulse propagation, resulting in considerable changes in the conversion efficiencies in nonlinear frequency mixing processes. The differences become more prominent when the pump has higher energy and is nearer to a resonance. Hence, the mode-area dispersion must be accounted for when modeling such a case.

Far-IR to deep-UV adaptive supercontinuum generation using semiconductor nano-antennas via carrier injection rate modulation

Supercontinuum generating sources, which incorporate a non-linear medium that can generate a wideband intensity spectrum under high-power excitation, are ideal for many applications of photonics such as spectroscopy and imaging. Supercontinuum generation using ultra-miniaturized devices is of great interest for on-chip imaging, on-chip measurement, and for future integrated photonic devices. In this study, semiconductor nano-antennas are proposed for ultra-broadband supercontinuum generation via analytical and numerical investigation of the electric field wave equation and the Lorentz dispersion model, incorporating semiconductor electron dynamics under optical excitation. It is shown that by a rapid modulation of the carrier injection rate for a semiconductor nano-antenna, one can generate an ultra-wideband supercontinuum that extends from the far-infrared (Far-IR) range to the deep-ultraviolet (Deep-UV) range for an infrared excitation of arbitrary intensity level. The modulation of the injection rate is achieved by high-intensity pulsed-pump irradiation of the nano-antenna, which has a fast nonradiative electron recombination mechanism that is on the order of sub-picoseconds. It is shown that when the pulse period of the pump irradiation is of the same order with the electron recombination time, rapid modulation of the free electron density occurs and electric energy accumulates in the nano-antenna, allowing for the generation of a broad supercontinuum. The numerical results are compared with the semiempirical second harmonic generation efficiency results for validation and a mean accuracy of 99.7% is observed. The aim of the study is to demonstrate that semiconductor nano-antennas can be employed to achieve superior supercontinuum generation performance at the nanoscale and the process can be programmed in an adaptive manner for continuous spectral shaping via tuning the pulse period of the pump irradiation.

CO2-based hollow-core fiber Raman laser with high-pulse energy at 1.95 µm

In this Letter, we present a high-pulse energy (>10µJ) Raman laser at 1946 nm wavelength directly pumped with a 1533 nm custom-made fiber laser. The Raman laser is based on stimulated Raman scattering (SRS) in an 8 m carbon dioxide (CO₂)-filled nested anti-resonant hollow-core fiber. The low-energy phonon emission combined with the inherent SRS process along the low-loss fiber allows the generation of high-pulse energy up to 15.4 µJ at atmospheric CO₂ pressure. The Raman laser exhibits good long-term stability and low relative intensity noise of less than 4%. We also investigate the pressure-dependent overlap of the Raman laser line with the absorption band of CO₂ at the 2 µm spectral range. Our results constitute a novel, to the best of our knowledge, and promising technology towards high-energy 2 µm lasers.

Temporal characterization of two-octave infrared pulses by frequency resolved optical switching

We present the temporal characterization of infrared pulses with spectra extending from 0.55 to 2.5 μm by using the frequency resolved optical switching (FROSt) technique. The pulses are obtained by broadening femtosecond pulses at 1.75 μm central wavelength in a two-stage hollow core fiber setup. This work demonstrates the capability of the FROSt technique to temporally characterize pulses with ultra-broadband spectra. Being free of phase-matching constraints, it enables the characterization of pulses with very low energy at the limit of the detection threshold and with arbitrary long pulse duration. This strength of the FROSt technique is illustrated by the characterization of supercontinua pulses whose spectra span over two octaves and with only 150 nJ energy that is spread temporally over almost 40 ps. The FROSt capabilities provide a versatile tool for the characterization of sub-cycle pulses and to study nonlinear processes such as supercontinuum generation.

Deep-UV-enhanced supercontinuum generated in a tapered gas-filled photonic crystal fiber

We present the use of a linearly down-tapered gas-filled hollow-core photonic crystal fiber in a single stage, pumped with pulses from a compact infrared (IR) laser source, to generate a supercontinuum (SC) carrying significant spectral power in the deep ultraviolet (UV) [200-300 nm]. The generated SC extends from the near IR down to ∼213nm with 0.58 mW/nm and down to ∼220nm with 0.83 mW/nm in the deep UV.

Understanding bending-induced loss and bending-enhanced higher-order mode suppression in negative curvature fibers

We present a numerical analysis on bending-induced loss and bending-enhanced higher-order mode suppression in negative curvature fibers. We provide underlying mechanisms on how geometrical parameters affect the bending properties. We find that fiber parameters influence the bending performance by altering the resonant coupling conditions, as well as light leakage through inter-tube gaps. We identify regions in the parameter space that exhibit excellent bending properties and offer general guidelines for designing negative curvature fibers that are less sensitive to bending. Moreover, we explore the possibility of enhancing higher-order core mode suppression through mechanical bending. We find that up to nine-fold increase in the higher-order mode extinction ratio can be achieved by bending the fiber.

Ultra-broadband supercontinuum generation in gas-filled photonic-crystal fibers: the epsilon-near-zero regime

In this Letter, we show theoretically that the nonlinear photoionization process of a noble gas inside a hollow-core photonic-crystal fiber can be exploited in obtaining broadband supercontinuum generation via pumping close to the mid-infrared regime. The interplay between the Kerr and photoionization nonlinearities is strongly enhanced in this regime. Photoionization continuously modifies the medium dispersion, in which the refractive index starts to significantly decrease and approach the epsilon-near-zero regime. Subsequently, the self-phase modulation induced by the Kerr effect is boosted because of the accompanied slow-light effect. As a result of this interplay, an output spectrum that comprises a broadband light with multiple dispersive wave emission is obtained.

Enhanced birefringence in conventional and hybrid anti-resonant hollow-core fibers

A hollow-core anti-resonant fiber (HC-ARF) design based on hybrid silica/silicon cladding is proposed for single-polarization, single-mode and high birefringence. We show that by adding silicon layers in a semi-nested HC-ARF, one of the polarization states can be strongly suppressed while simultaneously maintaining low propagation loss for other polarization states, single-mode and high birefiringence. The optimized HC-ARF design exhibits propagation loss, high birefringence, and polarization-extinction ratio of 0.05 dB/m, 0.5 × 10^-4, >300 respectively for y-polarization while the loss of x-polarization is >5 dB/m at 1064 nm. The fiber also has low bend-loss and thus can be coiled to a small bend radii of 5 cm having ≈0.06 dB/m bend loss.

Mid-Infrared Ultra-Short Pulse Generation in a Gas-Filled Hollow-Core Photonic Crystal Fiber Pumped by Two-Color Pulses

We show numerically that ultra-short pulses can be generated in the mid-infrared when a gas filled hollow-core fiber is pumped by a fundamental pulse and its second harmonic. The generation process originates from a cascaded nonlinear phenomenon starting from a spectral broadening of the two pulses followed by an induced phase-matched four wave-mixing lying in the mid-infrared combined with a dispersive wave. By selecting this mid-infrared band with a spectral filter, we demonstrate the generation of ultra-short 60 fs pulses at a 3–4 µm band and a pulse duration of 20 fs can be reached with an additional phase compensator.

Mid-infrared photoacoustic gas monitoring driven by a gas-filled hollow-core fiber laser

Development of novel mid-infrared (MIR) lasers could ultimately boost emerging detection technologies towards innovative spectroscopic and imaging solutions. Photoacoustic (PA) modality has been heralded for years as one of the most powerful detection tools enabling high signal-to-noise ratio analysis. Here, we demonstrate a novel, compact and sensitive MIR-PA system for carbon dioxide (CO₂) monitoring at its strongest absorption band by combining a gas-filled fiber laser and PA technology. Specifically, the PA signals were excited by a custom-made hydrogen (H₂) based MIR Raman fiber laser source with a pulse energy of ⁓ 18 μJ, quantum efficiency of ⁓ 80% and peak power of ⁓ 3.9 kW. A CO₂ detection limit of 605 ppbv was attained from the Allan deviation. This work constitutes an alternative method for advanced high-sensitivity gas detection.

Impact of cladding elements on the loss performance of hollow-core anti-resonant fibers

Understanding the impact of the cladding tube structure on the overall guiding performance is crucial for designing a single-mode, wide-band, and ultra low-loss nested hollow-core anti-resonant fiber (HC-ARF). Here we thoroughly investigate on how the propagation loss is affected by the nested elements when their geometry is realistic (i.e., non-ideal). Interestingly, it was found that the size, rather than the shape, of the nested elements has a dominant role in the final loss performance of the regular nested HC-ARFs. We identify a unique 'V-shape' pattern for suppression of higher-order modes loss by optimizing free design parameters of the HC-ARF. We find that a 5-tube nested HC-ARF has wider transmission window and better single-mode operation than a 6-tube HC-ARF. We show that the propagation loss can be significantly improved by using anisotropic nested anti-resonant tubes elongated in the radial direction. Our simulations indicate that with this novel fiber design, a propagation loss as low as 0.11 dB/km at 1.55 μm can be achieved. Our results provide design insight toward fully exploiting a single-mode, wide-band, and ultra low-loss HC-ARF. In addition, the extraordinary optical properties of the proposed fiber can be beneficial for several applications such as future optical communication system, high energy light transport, extreme non-nonlinear optics and beyond.

Band-edge mediated frequency down-conversion in a gas-filled anti-resonant hollow-core fiber

We demonstrate frequency down-conversions of femtosecond pulses through dispersive wave generation and degenerate four-wave mixing in a gas-filled anti-resonant hollow-core fiber. These are achieved by exploiting the rapid variation of the dispersion in the fiber's transmission band edge. In this approach, the wavelength of the down-shifted radiation is governed solely by the thickness of the dielectric wall at the core-cladding interface, while other system parameters are accountable only for inducing sufficient nonlinear phase shifts. With the right choice of cladding wall thickness, the concept can be applied directly for generating high-power mid-infrared femtosecond pulses.

Spectral broadening of ultraviolet dispersive waves in gas-filled hollow-core fiber using pump pulse modulation

UV supercontinuum laser sources based on resonant dispersive wave (RDW) generation in gas-filled hollow-core (HC) fibers offer an attractive architecture for numerous applications. However, the narrow UV spectral peak inherent to RDW generation limits the suitability for applications that require broad spectral coverage within the UV region such as spectroscopic scatterometry. In this Letter, we demonstrate how the UV spectrum can be shaped by modulating the peak power of the pump pulses driving the RDW generation, thereby creating a broadened and flattened UV spectrum. Using an argon-filled anti-resonant HC fiber, we generate a UV spectrum with a center wavelength of 323.6 nm with an FWHM of 51.7 nm, corresponding to a relative bandwidth of 16.1%.

Low-noise tunable deep-ultraviolet supercontinuum laser

The realization of a table-top tunable deep-ultraviolet (UV) laser source with excellent noise properties would significantly benefit the scientific community, particularly within imaging and spectroscopic applications, where source noise has a crucial role. Here we provide a thorough characterization of the pulse-to-pulse relative intensity noise (RIN) of such a deep-UV source based on an argon (Ar)-filled anti-resonant hollow-core (AR HC) fiber. Suitable pump pulses are produced using a compact commercially available laser centered at 1030 nm with a pulse duration of 400 fs, followed by a nonlinear compression stage that generates pulses with 30 fs duration, 24.2 μJ energy at 100 kHz repetition rate and a RIN of < 1%. Pump pulses coupled into the AR HC fiber undergo extreme spectral broadening creating a supercontinuum, leading to efficient energy transfer to a phase-matched resonant dispersive wave (RDW) in the deep-UV spectral region. The center wavelength of the RDW could be tuned between 236 and 377 nm by adjusting the Ar pressure in a 140 mm length of fiber. Under optimal pump conditions the RIN properties were demonstrated to be exceptionally good, with a value as low as 1.9% at ~ 282 nm. The RIN is resolved spectrally for the pump pulses, the generated RDW and the broadband supercontinuum. These results constitute the first broadband RIN characterization of such a deep-UV source and provide a significant step forward towards a stable, compact and tunable laser source for applications in the deep-UV spectral region.

Birefringent large-mode-area anti-resonant hollow core fiber in the 1.9 µm wavelength window

We report on the development and characterization of a birefringent large-mode-area anti-resonant silica fiber. The fiber structure is composed of six non-touching capillaries. The birefringence results from the breaking of the circular symmetry of an air core with increasing of the diameters of two capillaries located across the fiber diameter. We depart from earlier designs of polarizing hollow core fibers, in which coupling of the guided modes was intentionally facilitated with the cladding layout. Instead, with the help of numerical simulations, we enhance birefringence in our design by varying the capillary wall thickness between the larger- and smaller-diameter capillary sections of the cladding. The fiber has a large, elliptical core with semi-axes of ∼55 and 41 µm in diameter, an effective area of the fundamental mode of 1200µm², and a total outer diameter of 127 µm. The cladding is composed of two pairs of smaller capillaries, which are 18 µm in diameter with 1.66 µm thick walls, and two larger capillaries with a 24 µm diameter and 1.14 µm thick walls, located across the diagonal of the fiber. Measured group birefringence over 1820-1920 nm wavelengths is monotonically increasing from 0.4×10^-4 to 2.0×10^-4, while its phase birefringence is from 5×10^-6 to 1.1×10^-5. Despite this, the fiber holds polarization with a 12 dB polarization extinction ratio at 1900 nm over a 1.5 m long sample.

Photoionization-assisted, high-efficiency emission of a dispersive wave in gas-filled hollow-core photonic crystal fibers

We demonstrate that the phase-matched dispersive wave (DW) emission within the resonance band of a 25-cm-long gas-filled hollow-core photonic crystal fiber (HC-PCF) can be strongly enhanced by the photoionization effect of the pump pulse. In the experiments, we observe that as the pulse energy increases, the pump pulse gradually shifts to shorter wavelengths due to soliton-plasma interactions. When the central wavelength of the blueshifting soliton is close to the resonance band of the HC-PCF, high-efficiency energy transfer from the pump light to the DW in the visible region can be obtained. During this DW emission process, we observe that the spectral center of the DW gradually shifts to longer wavelengths leading to a slightly increased DW bandwidth, which can be well explained as the consequence of phase-matched coupling between the pump pulse and the DW. In particular, at an input pulse energy of 6 µJ, the spectral ratio of the DW at the fiber output is measured to be as high as ∼53%, corresponding to an overall conversion efficiency of ∼19%. These experimental results, well accompanied by theoretical simulations and analysis, offer a practical and effective method of generating high-efficiency tunable visible light sources and provide a few useful insights into the fields of soliton-plasma interaction and resonance-induced DW emission.

Femtosecond supercontinuum generation around 1560 nm in hollow-core photonic crystal fibers filled with carbon tetrachloride

We investigated experimentally supercontinuum generation in hollow-core photonic crystal fibers with cores infiltrated with carbon tetrachloride. As a pump source, we used a standard fiber-based femtosecond laser with a central wavelength at 1560 nm and a pulse duration of 90 fs. The first investigated fiber has a zero-dispersion wavelength at 1740 nm and generates a supercontinuum in the wavelength range from 1350 to 1900 nm. The second fiber has a zero-dispersion wavelength at 1440 nm, and the observed supercontinuum spectrum ranges from 1000 to 1900 nm. We numerically analyzed coherence of simulated supercontinuum pulses and noted that the observed supercontinuum spectra had a potential for high coherence. While the dynamics of supercontinuum generation in each of the investigated cases was revealed to be in agreement with the established state of the art in nonlinear fiber optics, our results are the first demonstration of such dynamics, to the best of our knowledge, leading up to octave spanning supercontinuum spectra in liquid-filled hollow-core silica fibers under pumping with a small-footprint femtosecond laser.

Noise and spectral stability of deep-UV gas-filled fiber-based supercontinuum sources driven by ultrafast mid-IR pulses

Deep-UV (DUV) supercontinuum (SC) sources based on gas-filled hollow-core fibers constitute perhaps the most viable solution towards ultrafast, compact, and tunable lasers in the UV spectral region, which can even also extend into the mid-infrared (IR). Noise and spectral stability of such broadband sources are key parameters that define their true potential and suitability towards real-world applications. In order to investigate the spectral stability and noise levels in these fiber-based DUV sources, we generate an SC spectrum that extends from 180 nm (through phase-matched dispersive waves - DWs) to 4 μm by pumping an argon-filled hollow-core anti-resonant fiber at a mid-IR wavelength of 2.45 μm. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity noise (RIN) of the DW at 275 nm. The results indicate no sign of spectral degradation over 110 hours, but the RIN of the DW pulses at 275 nm is found to be as high as 33.3%. Numerical simulations were carried out to investigate the spectral distribution of the RIN and the results confirm the experimental measurements and that the poor noise performance is due to the high RIN of the mid-IR pump laser, which was hitherto not considered in numerical modelling of these sources. The results presented herein provide an important step towards an understanding of the noise mechanism underlying such complex light-gas nonlinear interactions and demonstrate the need for pump laser stabilization.

Highly-tunable, visible ultrashort pulses generation by soliton-plasma interactions in gas-filled single-ring photonic crystal fibers

Ultrashort laser pulses, featuring remarkable spectral tunability, are highly demanded for nonlinear light-matter interactions in a variety of molecules. Here, we report on the generation of soliton-plasma-driven ultrashort pulses with both bandwidth- and wavelength-tunability in the visible spectral region. Using He-filled single-ring photonic crystal fiber (SR-PCF), we demonstrate in the experiments that the spectral bandwidths of blueshifting solitons can be manipulated by adjusting the input pulse energy, gas pressure and core diameter of the SR-PCF, while the central wavelengths of these solitons can be continuously tuned over 200 nm. We found that in a large-core SR-PCF (24.6-µm core diameter), the bandwidths of blueshifting solitons can be effectively broaden to near 100 nm, pointing out the possibility of generating few-cycle, wavelength-tunable visible pulses using this set-up. In addition, we observed in the experiments that in a small-core SR-PCF (with a core diameter of 17 µm), the blueshifting solitons show little residual light near the pump wavelength, resulting in a high-efficiency frequency up-conversion process. These experimental results, confirmed by numerical simulations, pave the way to a new generation of compact, ultrashort light sources with excellent tunability at visible wavelengths, which may have many applications in the fields of time-resolved spectroscopy and ultrafast nonlinear optics.

Nested capillary anti-resonant silica fiber with mid-infrared transmission and low bending sensitivity at 4000 nm

We report a silica glass nested capillary anti-resonant nodeless fiber with transmission and low bending sensitivity in the mid-infrared around 4000 nm. The fiber is characterized in terms of transmission over 1700-4200 nm wavelengths, revealing a mid-infrared 3500-4200 nm transmission window, clearly observable for a 12 m long fiber. Bending loss around 4000 nm is 0.5 dB/m measured over three full turns with 40 mm radius, going up to 5 dB/m for full turns with 15 mm radius. Our results provide experimental evidence of hollow-core silica fibers in which nested, anti-resonant capillaries provide high bend resistance in the mid-infrared. This is obtained for a fiber with a large core diameter of over 60 μm relative to around 30 μm capillaries in the cladding, which motivates its application in gas fiber lasers or fiber-based mid-infrared spectroscopy of CO_x or N_xO analytes.

Multi-octave visible to long-wave IR femtosecond continuum generated in Cr:ZnS-GaSe tandem

We report a technique for generation of broad and coherent femtosecond (fs) continua that span several octaves from visible to long-wave IR parts of the spectrum (0.4-18 µm). The approach is based on simultaneous amplification of few-cycle pulses at 2.5 µm central wavelength at 80 MHz repetition rate, and augmentation of their spectrum via three-wave mixing in a tandem arrangement of polycrystalline Cr:ZnS and single crystal GaSe. The obtained average power levels include several mW in the 0.4-0.8 µm visible, 0.23 W in the 0.8-2 µm near-IR, up to 4 W in the 2-3 µm IR, and about 17 mW in the 3-18 µm long-wave IR bands, respectively. High brightness and mutual coherence of all parts of the continuum was confirmed by direct detections of the carrier envelope offset frequency of the master oscillator.