Gas-filled Hollow-core Photonic Crystal Fiber Research Articles

Acoustically-switchable nonlinear frequency conversion in gas-filled hollow-core photonic crystal fibers

We found that an acoustically-induced perturbation can support a different type of quasi-phase matching for nonlinear frequency conversions in gas-filled hollow-core photonic crystal fibers. A gas pressure grating, a periodic spatial modulation of gas pressure, which is reversibly constructed by the acoustically-induced perturbation, compensates a nonzero phase mismatch and enhances the nonlinear optical process. The field amplitude oscillates about the averaged field amplitude which lineally grows with distance. While group velocity walk-off is suppressed by manipulating the gas pressure inside the fiber, a nonzero wavevector mismatch is inevitable, which can be compensated by the acoustically-induced perturbation. The frequency conversion is switchable by turning on/off the acoustically-induced perturbation. We exemplarily consider third-harmonic generation in gas-filled hollow-core photonic crystal fibers, however the conception of acoustically-induced quasi-phase matching is applicable to other kinds of nonlinear optical processes in gas-filled structures such including four-wave mixing and sum frequency generation and it would provide a new route for nonlinear optics and acoustooptics.

Effect of rotational Raman response on ultra-flat supercontinuum generation in gas-filled hollow-core photonic crystal fibers.

We experimentally and numerically investigate flat supercontinuum generation in gas-filled anti-resonant guiding hollow-core photonic crystal fiber. By comparing results obtained with either argon or nitrogen we determine the role of the rotational Raman response in the supercontinuum formation. When using argon, a supercontinuum extending from 350 nm to 2 µm is generated through modulational instability. Although argon and nitrogen exhibit similar Kerr nonlinearity and dispersion, we find that the energy density of the continuum in the normal dispersion region is significantly lower when using nitrogen. Using numerical simulations, we find that due to the closely spaced rotational lines in nitrogen, gain suppression in the fundamental mode causes part of the pump pulse to be coupled into higher-order modes which reduces the energy transfer to wavelengths shorter than the pump.

Temporal Self-Compression and Self-Frequency Shift of Submicrojoule Pulses at a Repetition Rate of 8 MHz

We combine soliton dynamics in gas-filled hollow-core photonic crystal fibers with a state-of-the-art fiber laser to realize a turn-key system producing few-fs pulses at 8 MHz repetition rate at pump energies as low as 220 nJ. Furthermore, by exploiting the soliton self-frequency shift in a second hydrogen-filled hollow-core fiber, we efficiently generate pulses as short as 22 fs, continuously tunable from 1100 nm to 1474 nm.

Spectrally pure photons generated in a quasi-phase matched xenon-filled hollow-core photonic crystal fiber.

Spectrally pure photons heralded from unentangled photon pair sources are crucial for any quantum optical system reliant on the multiplexing of heralded photons from independent sources. Generation of unentangled photon pairs in gas-filled hollow-core photonic crystal fibers specifically remains an attractive architecture for integration into quantum-optical fiber networks. The dispersion design offered by selection of fiber microstructures and gas pressure allows considerable control over the group-velocity profile which dictates the wavelengths of photon pairs that can be generated without spectral entanglement. Here, we expand on this design flexibility, which has previously been implemented for four-wave mixing, by modeling the use of a static, periodically poled electric field to achieve an effective quasi-phase-matched three-wave mixing nonlinearity that creates spontaneous parametric downconversion. Electric-field-induced quasi-phase-matched spontaneous parametric downconversion enables control of phase matching conditions that is independent of the group velocity, allowing phase matching at arbitrary wavelengths without affecting the entanglement of photons at those wavelengths. This decoupling of entanglement engineering and phase matching facilitates spectrally pure photon pair generation with efficiency and wavelength-tunability that is otherwise unprecedented.

Beam profile and pulse width assessment in an engineered D-shaped hollow-core photonic crystal fiber

In this work, a theoretical model is discussed to investigate the performance of a core-engineered gas-filled hollow-core photonic crystal fiber (HC-PCF). To gain the shortest pulse width and the best beam quality at the output, the core geometry of the fiber is modified within four specific types referred to as type I to type IV fibers. It is found that, by using type III and type IV HC-PCF devices, a 5 ps laser pulse in the input can be respectively compressed to 18.5 fs and 13.7 fs at the output. It is foundthat, a 5-ps laser pulse in the input can be reduced to 18.5 fs and 13.7 fs if type III- and type IV of modified HC-PCF device are respectively used for compression. The structural similarity (SSIM) index is used to evaluate the quality of the beam cross-section that ultimately emerges from the end of the fiber. The results suggest that the highest SSIM value of 0.76 can be obtained if type III HC-PCF is employed for pulse compression.

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.

Study of gas dynamics in hollow-core photonic crystal fibers

The unique design of hollow-core photonic crystal fibers (HC-PCFs) has attracted a lot of researchers’ attention. Their hollow-core structure with low transmission loss allow strong light-gas interaction inside the fiber, making HC-PCF a strong candidate as portable gas cells for sensing or spectroscopic purposes. Gas-filled HC-PCFs also have applications in the study of Raman scattering (SRS) transient regime and waveguides research. Considering the design of the cell, it is important to understand the gas flow dynamic, for example, estimate the evacuation or gas filling time for a certain length of fiber. In this paper, the gas dynamics theory and the comparison between single- and double-end pumping in special HC-PCFs is investigated. For two different types of HC-PCFs, our experimental data verified the trend for pressure decrease during the evacuation process was consistent with the theoretical prediction. After adding a correction factor, which represents the hollow-core structure of the fiber, to the existing model, the simulation results matched well with the experimental observation.

Linearity and Nonlinearity in Hollow-Core Antiresonant Fiber Sensors in the Terahertz Regime

In nonlinear optics, a major challenge is to maximize the interaction between the light from laser sources and low-density media such as gases. An ultrafast laser beam can be focused to form a highly intense spot over a small concentrated area. The ultrafast laser beam has a short pulse width of less than one picosecond and high peak power in the beam profile (Fig. 1). The beam profile of the laser source describes the energy density and distribution of light, and the beam profile of a laser source is usually affected during the propagation and collimation of the beam. An efficient nonlinear optical sensor also requires high peak power at low energy (low average power) over short duration of laser pulse, a high beam profile and long interaction length. These requirements of a nonlinear optical sensor with low attenuation constant can be achieved in hollow-core photonic crystal fiber (HC-PCF). Gas-filled HC-PCF exhibits optical nonlinearity for an ultrashort temporal and spectral broadening of NIR pulses [1], [2]. The nonlinearity in HC-PCF can be achieved by tuning the gas pressure. Gas-filled HC-PCF nonlinear media are low-cost, replenishable, reconfigurable and exhibit sharp spectral lines [3]. Linear and nonlinear responses are also found in other types of optical fibers.

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.

Photoionization-Induced Broadband Dispersive Wave Generated in an Ar-Filled Hollow-Core Photonic Crystal Fiber

The resonance band in hollow-core photonic crystal fiber (HC-PCF), while leading to high-loss region in the fiber transmission spectrum, has been successfully used for generating phase-matched dispersive wave (DW). Here, we report that the spectral width of the resonance-induced DW can be largely broadened due to plasma-driven blueshifting soliton. In the experiment, we observed that in a short length of Ar-filled single-ring HC-PCF the soliton self-compression and photoionization effects caused a strong spectral blueshift of the pump pulse, changing the phase-matching condition of the DW emission process. Therefore, broadening of DW spectrum to the longer-wavelength side was obtained with several spectral peaks, which correspond to the generation of DW at different positions along the fiber. In particular, we numerically used the super-Gauss windows with different central wavelengths to filter out these DW spectral peaks and studied the time-domain characteristics of these peaks respectively using Fourier transform method. We observed that these multiple-peaks on the DW spectrum have different delays in the time domain, which is in good agreement with our theoretical prediction. More interestingly, we found that the broadband DW with several spectral peaks can be compressed to ~29 fs after proper dispersion compensation. The results reported here, on the one hand, provide some useful insights into the resonance-induced DW generation process in gas-filled HC-PCFs. On the other hand, the DW-emission mechanism could be used to generate the ultrashort light sources with a wide spectral range through using the proper design of the resonance bands of the HC-PCFs, which has many applications in the ultrafast related experiments.

Modulational-instability-free pulse compression in anti-resonant hollow-core photonic crystal fiber.

Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient nonlinear temporal compression of femtosecond laser pulses, two main schemes being direct soliton-effect self-compression and spectral broadening followed by phase compensation. To obtain stable compressed pulses, it is crucial to avoid decoherence through modulational instability (MI) during spectral broadening. Here, we show that changes in dispersion due to spectral anti-crossings between the fundamental-core mode and core wall resonances in anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum, enabling MI-free pulse compression for optimized fiber designs. The results are important, since MI cannot always be suppressed by pumping in the normal dispersion regime.

Efficient Generation of Coherent Stokes Field in Hydrogen Gas-Filled Hollow Core Photonic Crystal Fibres

In this paper, we study of the coherent Stokes generation in a transient Raman regime by Hydrogen gas-filled hollow-core photonic crystal fibres (HC-PCFs) configuration. The temporal and spatial evolution of the pump and Stokes field envelopes as well as the coherence and population inversion is numerically observed. The influence of the pump pulse width and gas pressure on the energy exchange along fiber and Stokes generation efficiency is investigated.

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.

Pump-Probe Study of Plasma Dynamics in Gas-Filled Photonic Crystal Fiber Using Counterpropagating Solitons

We present a pump-probe technique for monitoring ultrafast polarizability changes. In particular, we use it to measure the plasma density created at the temporal focus of a self-compressing higher-order pump soliton in gas-filled hollow-core photonic crystal fiber. This is done by monitoring the wavelength of the dispersive wave emission from a counter-propagating probe soliton. By varying the relative delay between pump and probe, the plasma density distribution along the fiber can be mapped out. Compared to the recently introduced interferometric side-probing for monitoring the plasma density, our new technique is relatively immune to instabilities caused by air turbulence and mechanical vibration. The results of two experiments on argon- and krypton-filled fiber are presented, and compared to numerical simulations. The technique provides an important new tool for probing photoionization in many different gases and gas mixtures as well as ultrafast changes in dispersion in many other contexts.

Ionization-induced adiabatic soliton compression in gas-filled hollow-core photonic crystal fibers.

We investigate in the experiments the ionization-induced adiabatic soliton compression process in a short length of He-filled single-ring photonic crystal fiber. We observe that the plasma-driven blueshifting solitons show little residual light near the pump wavelength in a certain pulse energy region, leading to a high-efficiency frequency upconversion process. In contrast, at high pulse energy levels, we observe that the quality of the frequency upshifting process is impaired due to the existence of a dynamical loss channel induced by the coupling of the soliton to linear modes near the pump wavelength. In addition, through adjusting the input pulse energy, the central wavelength of blueshifting solitons can be continuously tuned over 300nm. These experimental results, confirmed by numerical simulations, not only offer a deep insight into ionization-induced soliton-plasma dynamics in gas-filled hollow-core photonic crystal fibers, but also develop highly tunable ultrafast light sources at visible wavelengths, which may have many applications in ultrafast spectroscopy.

Highly efficient deep UV generation by four-wave mixing in gas-filled hollow-core photonic crystal fiber.

We report on a highly efficient experimental scheme for the generation of deep-ultraviolet (UV) ultrashort light pulses using four-wave mixing in gas-filled kagomé-style photonic crystal fiber. By pumping with ultrashort, few microjoule pulses centered at 400nm, we generate an idler pulse at 266nm and amplify a seeded signal at 800nm. We achieve remarkably high pump-to-idler energy conversion efficiencies of up to 38%. Although the pump and seed pulse durations are ∼100 fs, the generated UV spectral bandwidths support sub-15fs pulses. These can be further extended to support few-cycle pulses. Four-wave mixing in gas-filled hollow-core fibers can be scaled to high average powers and different spectral regions such as the vacuum UV (100-200nm).

Carrier-envelope-phase-stable soliton-based pulse compression to 4.4 fs and ultraviolet generation at the 800 kHz repetition rate.

In this Letter, we report the generation of a femtosecond supercontinuum extending from the ultraviolet to the near-infrared spectrum and detection of its carrier-envelope-phase (CEP) variation by f-to-2f interferometry. The spectrum is generated in a gas-filled hollow-core photonic crystal fiber, where soliton dynamics allows the CEP-stable self-compression of the optical parametric chirped-pulse amplifier pump pulses at 800nm to a duration of 1.7 optical cycles, followed by dispersive wave emission. The source provides up to 1μJ of pulse energy at the 800kHz repetition rate, resulting in 0.8W of average power, and it can be extremely useful, for example in strong-field physics, pump-probe measurements, and ultraviolet frequency comb metrology.

Optical pulse structuring in gas-filled hollow-core kagomé PCF for generation and detection of phase-locked multi-THz pulses [Invited

A gas-filled hollow-core photonic crystal fiber is used to structure near-infrared ultrashort pulses and enable the generation and time-resolved detection of multi-terahertz radiation. Due to self-phase modulation, near-infrared pulses launched into the fiber experience spectral broadening characterized by the appearance of side lobes at the edges of the spectrum. Phase-locked terahertz generation between 10 and 18 terahertz is achieved by difference-frequency mixing of these spectral side lobes. This method allows for an implementation of time-resolved spectroscopy in the multi-terahertz range through the efficient production of near-infrared pulses with tailored spectra, without requiring ultrabroadband optical sources.

Pump-probe multi-species CARS in a hollow-core PCF with a 20 ppm detection limit under ambient conditions.

We report coherent anti-Stokes Raman spectroscopy (CARS) in a gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) using a pump-probe configuration. The long collinear path length offered by an SR-PCF strongly enhances the efficiency of the Raman interactions. Pressure tuning the zero-dispersion wavelength (ZDW) of the SR-PCF allows the Raman coherence prepared by seeded pumping at 515nm to be used in the visible for phase-matched generation of an anti-Stokes signal from a probe in the ultraviolet. The unique dispersion profile in the vicinity of the ZDW enables simultaneous phase matching of all known Raman transitions. We demonstrate that simultaneous multi-species CARS with a detection limit of 20ppm is possible with only 20kW of peak pump power delivered by a single laser source.

Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow-Core Photonic Crystal Fibers.

Broadband-tunable sources of circularly polarized light are crucial in fields such as laser science, biomedicine, and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components and are limited by the availability of suitably transparent crystals and glasses. Although a gas-filled hollow-core photonic crystal fiber provides pressure-tunable dispersion, long well-controlled optical path lengths, and high Raman conversion efficiency, it is unable to preserve a circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on a helically twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining a circular polarization state against external perturbations. This makes it possible to generate pure circularly polarized Stokes and anti-Stokes signals by rotational Raman scattering in hydrogen. The polarization state of the frequency-shifted Raman bands can be continuously varied by tuning the gas pressure in the vicinity of the gain-suppression point. The results pave the way to a new generation of compact and efficient fiber-based sources of broadband light with a fully controllable polarization state.