Gas-filled Photonic Crystal Fiber Research Articles

High sensitivity all-optical sensor for detecting toxic gases using hollow-core photonic crystal fiber composed of magnesium fluoride

• A hollow-core PCF composed of magnesium fluoride was proposed to identify toxic and hazardous gases. • The structure is based on the quasi-crystal arrangement of air holes. • Numerical simulation results showed that the fiber proposed in this study has a sensitivity coefficient above 50%, relatively low loss, and low dispersion. • The device is suitable for identifying toxic and dangerous gases. • The detection of which using this tool prevents combustion and explosion in the drilling process. In this paper, a hollow-core photonic crystal fiber (PCF) composed of magnesium fluoride is designed to identify toxic and hazardous gases. Its band structure is calculated using the well-known plane wave expansion method. The numerical results show that the proposed structure has a relatively wide photonic bandgap (PBG) in the wavelength range of 1 to 3.3 μm covering the near- and mid-infrared wavelength ranges. Then, by applying optical pulses with a wavelength in the range of the PBG to the proposed PCF, its propagation behavior is calculated by solving the nonlinear Schrödinger equation using the split-step Fourier method. The type of gas can be identified by observing and comparing the frequency responses of the gas-filled PCF with the absence of gas injection. Simulation results show that the fiber proposed in this study has a sensitivity coefficient of more than 50%. The low loss, low dispersion, and high accuracy of the proposed fiber make it a suitable tool for identifying toxic and dangerous gases such as methane and hydrogen sulfide in the drilling and oil exploration industry. Identifying them using this tool prevents combustion and explosion in the drilling process.

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.

Narrow-Linewidth 2 μm All-Fiber Laser Amplifier with a Highly Stable and Precisely Tunable Wavelength for Gas Molecule Absorption in Photonic Crystal Hollow-Core Fibers.

In recent years, mid-infrared fiber lasers based on gas-filled photonic crystal hollow-core fibers (HCFs) have attracted enormous attention. They provide a potential method for the generation of high-power mid-infrared emissions, particularly beyond 4 μm. However, there are high requirements of the pump for wavelength stability, tunability, laser linewidth, etc., due to the narrow absorption linewidth of gases. Here, we present the use of a narrow-linewidth, high-power fiber laser with a highly stable and precisely tunable wavelength at 2 μm for gas absorption. It was a master oscillator power-amplifier (MOPA) structure, consisting of a narrow-linewidth fiber seed and two stages of Thulium-doped fiber amplifiers (TDFAs). The seed wavelength was very stable and was precisely tuned from 1971.4 to 1971.8 nm by temperature. Both stages of the amplifiers were forward-pumping, and a maximum output power of 24.8 W was obtained, with a slope efficiency of about 50.5%. The measured laser linewidth was much narrower than the gas absorption linewidth and the wavelength stability was validated by HBr gas absorption in HCFs. If the seed is replaced, this MOPA laser can provide a versatile pump source for mid-infrared fiber gas lasers.

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.

Narrowband Vacuum Ultraviolet Light via Cooperative Raman Scattering in Dual-Pumped Gas-Filled Photonic Crystal Fiber

Many fields such as biospectroscopy and photochemistry often require sources of vacuum ultraviolet (VUV) pulses featuring a narrow line width and tunable over a wide frequency range. However, the m...

Robust excitation and Raman conversion of guided vortices in a chiral gas-filled photonic crystal fiber.

The unique ring-shaped intensity patterns and helical phase fronts of optical vortices make them useful in many applications. Here we report for the first time, to the best of our knowledge, efficient Raman frequency conversion between vortex modes in a twisted hydrogen-filled single-ring hollow core photonic crystal fiber (SR-PCF). High-fidelity transmission of optical vortices in an untwisted SR-PCF becomes more and more difficult as the orbital angular momentum (OAM) order increases, due to scattering at structural imperfections in the fiber microstructure. In a helically twisted SR-PCF, however, the degeneracy between left- and right-handed versions of the same mode is lifted, with the result that they are topologically protected from such scattering. With launch efficiencies of ${\sim}{75}\% $∼75%, a high damage threshold and broadband guidance, these fibers are ideal for performing nonlinear experiments that require the polarization state and azimuthal order of the interacting modes to be preserved over long distances. Vortex coherence waves of internal molecular motion carrying angular momentum are excited in the gas, permitting the polarization and OAM of the Raman bands to be tailored, even in spectral regions where conventional solid-core waveguides are opaque or susceptible to optical damage.

Quasi-phase-matched high-harmonic generation in gas-filled hollow-core photonic crystal fiber

High-order harmonic generation (HHG) provides a promising tabletop source of coherent short wavelength radiation. However, the low generation efficiency limits the mean photon flux of HHG sources when driven with low average power, kHz repetition rate lasers. The HHG flux could be increased by employing MHz repetition rate driving lasers. However, the low pulse energy necessitates tight focusing, which limits the effective interaction volume. Generating harmonics in a gas-filled photonic crystal fiber (PCF) mitigates this problem, but for both free focus and PCF targets, the HHG conversion efficiency is optimized at technologically challenging multibar gas pressures. Here, we perform HHG with μJ-level driving pulses in a hollow-core PCF. We observe dramatic 60-fold enhancement of the HHG flux through a time-multiplexed multimodal quasi-phase-matching technique at low gas pressures. We also observe high harmonic photon energies up to 61.6 eV with continuous spectral tunability made possible by controlled ionization-induced blue shifting of the driving laser.

Continuously wavelength-tunable blueshifting soliton generated in gas-filled photonic crystal fibers.

We experimentally report the generation of wavelength-tunable blueshifting soliton in the visible spectral region through a gas-filled single-ring photonic crystal fiber (SR-PCF). In particular, in a He-filled SR-PCF, we observed a sharp narrow-band spectral peak at the first resonant spectral region of the SR-PCF, which results from phase-matched nonlinear processes. To the best of our knowledge, this is the first time investigating the influence of the core-cladding resonance on the blueshifting soliton. In addition, when Ar gas was filled into the SR-PCF, some interference fringes on the blueshifting soliton were observed at high pulse-energy levels due to plasma-induced pulse fission. These two experimental observations are confirmed by numerical simulations. Furthermore, through properly adjusting input pulse energy, we found that the blueshifting soliton can obtain a high conversion efficiency (∼84%) and its wavelength can be tuned over hundreds of nanometers (∼240 nm).

Effect of anti-crossings with cladding resonances on ultrafast nonlinear dynamics in gas-filled photonic crystal fibers

Spectral anti-crossings between the fundamental guided mode and core wall resonances alter the dispersion in hollow-core anti-resonant-reflection photonic crystal fibers. Here we study the effect of this dispersion change on the nonlinear propagation and dynamics of ultrashort pulses. We find that it causes emission of narrow spectral peaks through a combination of four-wave mixing and dispersive wave emission. We further investigate the influence of the anti-crossings on nonlinear pulse propagation and show that their impact can be minimized by adjusting the core-wall thickness in such a way that the anti-crossings lie spectrally distant from the pump wavelength.

Mid-infrared dispersive wave generation in gas-filled photonic crystal fibre by transient ionization-driven changes in dispersion

Gas-filled hollow-core photonic crystal fibre is being used to generate ever wider supercontinuum spectra, in particular via dispersive wave emission in the deep and vacuum ultraviolet, with a multitude of applications. Dispersive waves are the result of nonlinear transfer of energy from a self-compressed soliton, a process that relies crucially on phase-matching. It was recently predicted that, in the strong-field regime, the additional transient anomalous dispersion introduced by gas ionization would allow phase-matched dispersive wave generation in the mid-infrared—something that is forbidden in the absence of free electrons. Here we report the experimental observation of such mid-infrared dispersive waves, embedded in a 4.7-octave-wide supercontinuum that uniquely reaches simultaneously to the vacuum ultraviolet, with up to 1.7 W of total average power.

Dynamics of Dispersive Wave Generation in Gas-Filled Photonic Crystal Fiber with the Normal Dispersion

The absence of Raman and unique pressure-tunable dispersion is the characteristic feature of gas-filled photonic crystal fiber (PCF), and its zero dispersion points can be extended to the near-infrared by increasing gas pressure. The generation of dispersive wave (DW) in the normal group velocity dispersion (GVD) region of PCF is investigated. It is demonstrated that considering the self-steepening (SS) and introducing the chirp of the initial input pulse are two suitable means to control the DW generation. The SS enhances the relative average intensity of blue-shift DW while weakening that of red-shift DW. The required propagation distance of DW emission is markedly varied by introducing the frequency chirp. Manipulating DW generation in gas-filled PCF by the combined effects of either SS or chirp and three-order dispersion (TOD) provides a method for a concentrated transfer of energy into the targeted wavelengths.

Tunable frequency-up/down conversion in gas-filled hollow-core photonic crystal fibers.

Based on the interplay between photoionization and Raman effects in gas-filled photonic crystal fibers, we propose a new optical device to control frequency conversion of ultrashort pulses. By tuning the input-pulse energy, the output spectrum can be either down-converted, up-converted, or even frequency-shift compensated. For low input energies, the Raman effect is dominant and leads to a redshift that increases linearly during propagation. For larger pulse energies, photoionization starts to take over the frequency-conversion process and induces a strong blueshift. The fiber-output pressure can be used as an additional degree of freedom to control the spectrum shift.

Strong Raman-induced noninstantaneous soliton interactions in gas-filled photonic crystal fibers.

We have developed an analytical model based on the perturbation theory to study the optical propagation of two successive solitons in hollow-core photonic crystal fibers filled with Raman-active gases. Based on the time delay between the two solitons, we have found that the trailing soliton dynamics can experience unusual nonlinear phenomena, such as spectral and temporal soliton oscillations and transport toward the leading soliton. The overall dynamics can lead to a spatiotemporal modulation of the refractive index with a uniform temporal period and a uniform or chirped spatial period.

Raman-induced temporal condensed matter physics in gas-filled photonic crystal fibers

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means.

Ultraviolet Light Generation in Gas-Filled Kagome Photonic Crystal Fiber

Kagome hollow-core photonic crystal fibers were found to be ideal for the occurrence of ultrafast non-linear optics. This article reports the optimal conditions for the generation of ultraviolet light using a gas filled kagome hollow-core-photonic crystal fiber. It is shown that by changing the pressure of the gas and the input pulse characteristics, the efficiency of conversion and quality of ultraviolet light can be improved, as well as tuning its central frequency. Results suggest that a highly coherent and tunable ultraviolet light source can be constructed, which can find numerous applications.

Combined soliton pulse compression and plasma-related frequency upconversion in gas-filled photonic crystal fiber

We numerically investigate self-frequency blueshifting of a fundamental soliton in a gas-filled hollow-core photonic crystal fiber. Because of the changing underlying soliton parameters, the blueshift gives rise to adiabatic soliton compression. Based on these features, we propose a device that enables frequency shifting over an octave and pulse compression from 30 fs down to 2.3 fs.

Bright Spatially Coherent Wavelength-Tunable Deep-UV Laser Source Using an Ar-Filled Photonic Crystal Fiber

We report on the spectral broadening of ~1 μJ 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving <1 m of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8%. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.

Direct Observation of Self-Similarity in Evolution of Transient Stimulated Raman Scattering in Gas-Filled Photonic Crystal Fibers

A unique characteristic of transient stimulated Raman scattering, in which the spatiotemporal evolution of the fields and the molecular excitation follow a universal self-similarity law, is observed in gas-filled photonic crystal fibers. As the input laser power is increased, the coupled system "optical fields + molecular excitation" goes through the same phases of time evolution but at a higher rate. Using the self-similarity law we are able to completely reconstruct the evolution of the pump and Stokes fields from one measurement.

Optical frequency comb generation in gas-filled hollow core photonic crystal fibres

The efficiency of gas-based nonlinear processes is often limited by the diffraction of the pump laser as it propagates through the nonlinear medium. As a consequence, phenomena with strong nonlinear response requirements, such as high harmonic generation or Raman sideband generation, lack the required laser–matter interaction to fulfil their potential. Indeed, the conversion efficiency of these techniques is usually low and the experimental set-up cumbersome. The advent of hollow core photonic crystal fibre technology drafts new territories for nonlinear optics, and in particular offers new alternatives for sub-femtosecond pulse generation. The air-guiding fibre combines unprecedented laser confinement over long interaction lengths and, when filled with an adequate nonlinear gas, offers improved conversion efficiency and up to a million-fold reduction of the pump power threshold. This paper presents a review of the types of hollow core PCF available for nonlinear applications and the results obtained for efficient Raman conversion in H2-filled hollow core PCF that led to the observation of a multi-octave frequency comb spanning from ~325 to ~2300 nm using a single pump laser with relatively low power. The generated ultra-broad spectrum creates a simple route towards a compact source of attosecond pulses.