Active engineering of four-wave mixing spectral correlations in multiband hollow-core fibers.

We demonstrate theoretically and experimentally a high level of control of the four-wave mixing process in an inert gas-filled inhibited-coupling guiding hollow-core photonic crystal fiber. The specific multiple-branch dispersion profile in such fibers allows both correlated and separable bi-photon states to be produced. By controlling the choice of gas and its pressure and the fiber length, we experimentally generate various joint spectral intensity profiles in a stimulated regime that is transferable to the spontaneous regime. The generated profiles may cover both spectrally separable and correlated bi-photon states and feature frequency tuning over tens of THz, demonstrating a large dynamic control that will be very useful when implemented in the spontaneous regime as a photon pair source.

Mid IR hollow core fiber gas laser emitting at 4.6 µm.

Emission at 4.6 µm was observed from an N2O filled hollow core fiber laser. 8-ns pump pulses at 1.517 µm excited a vibrational overtone resulting in lasing on an R and P branch fundamental transition from the upper pump state. At optimum gas pressure of 80 Torr, photon conversion efficiency of 9% and slope efficiency of 3% were observed from a mirrorless laser. The laser threshold occurred at absorbed pump energy of 150 nJ in a 45-cm long fiber with 85 µm core diameter. The observed dependence of the laser output on gas pressure was shown to be a result of line broadening and relaxation rates.

Gas mixture for deep-UV plasma emission in a hollow-core photonic crystal fiber.

We report on the first deep-ultraviolet/ultraviolet (DUV/UV) emission using a highly compact microwave-driven plasma-core photonic crystal fiber. The latter consists of a few centimeter long micro-plasma column of a gas mixture in the core of Kagome hollow-core photonic crystal fiber. The plasma is generated by nonintrusively exciting a ternary gas mixture of argon, nitrogen, and oxygen (Ar/N2/O2) with a microwave resonator. Several spectral lines in the wavelength range of 200-450 nm were produced, guided by an Ar-N2-O2 plasma-filled fiber, and controlled by simply varying the gas ratio of this gas mixture. An optimum gas mixture ratio was experimentally and theoretically identified for the strongest emission in the DUV range of 200-275 nm. The developed DUV emitting plasma-core fiber represents an important milestone towards the development of tunable and miniaturized DUV/UV laser sources.

Raman gas self-organizing into deep nano-trap lattice.

Trapping or cooling molecules has rallied a long-standing effort for its impact in exploring new frontiers in physics and in finding new phase of matter for quantum technologies. Here we demonstrate a system for light-trapping molecules and stimulated Raman scattering based on optically self-nanostructured molecular hydrogen in hollow-core photonic crystal fibre. A lattice is formed by a periodic and ultra-deep potential caused by a spatially modulated Raman saturation, where Raman-active molecules are strongly localized in a one-dimensional array of nanometre-wide sections. Only these trapped molecules participate in stimulated Raman scattering, generating high-power forward and backward Stokes continuous-wave laser radiation in the Lamb-Dicke regime with sub-Doppler emission spectrum. The spectrum exhibits a central line with a sub-recoil linewidth as low as ∼14 kHz, more than five orders of magnitude narrower than conventional-Raman pressure-broadened linewidth, and sidebands comprising Mollow triplet, motional sidebands and four-wave mixing.

X-SEA-F-SPIDER characterization of over octave spanning pulses in the infrared range.

We show a practical implementation of a pulse characterization method for sub-cycle pulse measurements in the infrared spectral range based on spectral shearing interferometry. We employ spatially-encoded arrangement filter-based spectral phase interferometry for direct electric field reconstruction with external ancila pulses (X-SEA-F-SPIDER). We show merits and limitations of the setup and an in-depth comparison to another widely used temporal characterization technique - Second-Harmonic Generation Frequency Resolved Optical Gating (SHG-FROG). The X-SEA-F-SPIDER implementation presented in this paper allows measurement of sub-cycle pulses with over one octave wide spectrum spanning the 900-2400 nm range without adding any extra dispersion due to the pulse characterization apparatus.

CW hollow-core optically pumped I2 fiber gas laser.

Continuous wave lasing of a hollow-core fiber gas laser (HOFGLAS) is achieved with molecular iodine in the 1280-1340 nm region when optically pumped at 532 nm.

A strong-field driver in the single-cycle regime based on self-compression in a kagome fibre.

Over the past decade intense laser fields with a single-cycle duration and even shorter, subcycle multicolour field transients have been generated and applied to drive attosecond phenomena in strong-field physics. Because of their extensive bandwidth, single-cycle fields cannot be emitted or amplified by laser sources directly and, as a rule, are produced by external pulse compression-a combination of nonlinear optical spectral broadening followed up by dispersion compensation. Here we demonstrate a simple robust driver for high-field applications based on this Kagome fibre approach that ensures pulse self-compression down to the ultimate single-cycle limit and provides phase-controlled pulses with up to a 100 µJ energy level, depending on the filling gas, pressure and the waveguide length.

Ultra low-loss hypocycloid-core Kagome hollow-core photonic crystal fiber for green spectral-range applications.

We report on the development of a hypocycloidal-core Kagome hollow-core photonic crystal fiber guiding, with low transmission loss in the 450-650 nm visible spectral range. Transmission loss records have been achieved with 70 dB/km at 600 nm, and 130 dB/km at 532 nm. As a demonstration of the fiber potential applications, we report on a compact 600 THz wide Raman comb generator, centered around 532 nm, and on a 10 W average power frequency-doubled Yb-fiber picosecond laser beam delivery, along with its use for organic material laser micro-processing.

Multi-meter fiber-delivery and pulse self-compression of milli-Joule femtosecond laser and fiber-aided laser-micromachining.

We report on damage-free fiber-guidance of milli-Joule energy-level and 600-femtosecond laser pulses into hypocycloid core-contour Kagome hollow-core photonic crystal fibers. Up to 10 meter-long fibers were used to successfully deliver Yb-laser pulses in robustly single-mode fashion. Different pulse propagation regimes were demonstrated by simply changing the fiber dispersion and gas. Self-compression to ~50 fs, and intensity-level nearing petawatt/cm(2) were achieved. Finally, free focusing-optics laser-micromachining was also demonstrated on different materials.

High temperature stable PbS quantum dots.

For the fabrication of nanoparticle containing optical fibers by melt and draw technique, nanoparticle stability at high temperatures is a requirement. We report the synthesis of quantum dots at temperatures as high as 1000 °C, compatible with fiber drawing, stabilized for the first time by a prior low temperature heating step. It is observed that quantum dots formed by this two step heating leads to a better emission stability at high powers associated with a reversible phenomenon, making these nanomaterials suitable for further technological applications.

Generation and confinement of microwave gas-plasma in photonic dielectric microstructure.

We report on a self-guided microwave surface-wave induced generation of ~60 µm diameter and 6 cm-long column of argon-plasma confined in the core of a hollow-core photonic crystal fiber. At gas pressure of 1 mbar, the micro-confined plasma exhibits a stable transverse profile with a maximum gas-temperature as high as 1300 ± 200 K, and a wall-temperature as low as 500 K, and an electron density level of 10¹4 cm⁻³. The fiber guided fluorescence emission presents strong Ar⁺ spectral lines in the visible and near UV. Theory shows that the observed combination of relatively low wall-temperature and high ionisation rate in this strongly confined configuration is due to an unprecedentedly wide electrostatic space-charge field and the subsequent ion acceleration dominance in the plasma-to-gas power transfer.

Supercritical xenon-filled hollow-core photonic bandgap fiber.

We demonstrate that filling a hollow-core photonic-bandgap fiber with supercritical xenon creates a medium with a controllable density up to several hundred times that at STP, while working at room temperature. The high compressibility of the supercritical fluid allows rapid tuning of the spectral guidance window by making small changes of gas pressure near the critical point. We discuss potential applications of this system in linear and nonlinear optics.

Hypocycloid-shaped hollow-core photonic crystal fiber Part I: arc curvature effect on confinement loss.

We report on numerical and experimental studies showing the influence of arc curvature on the confinement loss in hypocycloid-core Kagome hollow-core photonic crystal fiber. The results prove that with such a design the optical performances are strongly driven by the contour negative curvature of the core-cladding interface. They show that the increase in arc curvature results in a strong decrease in both the confinement loss and the optical power overlap between the core mode and the silica core-surround, including a modal content approaching true single-mode guidance. Fibers with enhanced negative curvature were then fabricated with a record loss-level of 17 dB/km at 1064 nm.

Hypocycloid-shaped hollow-core photonic crystal fiber Part II: cladding effect on confinement and bend loss.

We report on numerical and experimental studies on the influence of cladding ring-number on the confinement and bend loss in hypocycloid-shaped Kagome hollow core photonic crystal fiber. The results show that beyond the second ring, the ring number has a minor effect on confinement loss whereas the bend loss is strongly reduced with the ring-number increase. Finally, the results show that the increase in the cladding ring-number improves the modal content of the fiber.

Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression.

We report on the recent design and fabrication of kagome-type hollow-core photonic crystal fibers for the purpose of high-power ultrashort pulse transportation. The fabricated seven-cell three-ring hypocycloid-shaped large core fiber exhibits an up-to-date lowest attenuation (among all kagome fibers) of 40 dB/km over a broadband transmission centered at 1500 nm. We show that the large core size, low attenuation, broadband transmission, single-mode guidance, and low dispersion make it an ideal host for high-power laser beam transportation. By filling the fiber with helium gas, a 74 µJ, 850 fs, and 40 kHz repetition rate ultrashort pulse at 1550 nm has been faithfully delivered at the fiber output with little propagation pulse distortion. Compression of a 105 µJ laser pulse from 850 fs down to 300 fs has been achieved by operating the fiber in ambient air.

Millijoule laser pulse delivery for spark ignition through kagome hollow-core fiber.

We report on power handling oriented design of kagome lattice hollow-core fiber and demonstrate through it for the first time nanosecond laser pulses induced spark ignition in a friendly manner. Two different core designs and transmission bands are investigated and evaluated. The energy threshold damage was measured to be in excess of the 10 mJ level and the output power density is approaching the TW/cm2 after focusing; demonstrating the outstanding ability of such fiber for high power delivery.

Matched cascade of bandgap-shift and frequency-conversion using stimulated Raman scattering in a tapered hollow-core photonic crystal fibre.

We report on a novel means which lifts the restriction of the limited optical bandwidth of photonic bandgap hollow-core photonic crystal fiber on generating high order stimulated Raman scattering in gaseous media. This is based on H(2)-filled tapered HC-PCF in which the taper slope is matched with the effective length of Raman process. Raman orders outside the input-bandwidth of the HC-PCF are observed with more than 80% quantum-conversion using a compact, low-power 1064 nm microchip laser. The technique opens prospects for efficient sources in spectral regions that are poorly covered by currently existing lasers such as mid-IR.

Control of surface modes in low loss hollow-core photonic bandgap fibers.

We report on the fabrication and characterization of hollow-core photonic bandgap fibers that do not suffer from surface mode coupling within the photonic bandgap of the cladding. This enables low attenuation over the full spectral width of the bandgap--we measured a minimum loss of 15 dB/km and less than 50 dB/km over 300 nm for a fiber operating at 1550 nm. As a result of the increased bandwidth, the fiber has reduced dispersion and dispersion slope--by a factor of almost 2 compared to previous fibers. These features are important for several applications in high-power ultrashort pulse compression and delivery. Realizing these advances has been possible due to development of a modified fabrication process which makes the production of low-loss hollow-core fibers both simpler and quicker than previously.

High power tunable femtosecond soliton source using hollow-core photonic bandgap fiber, and its use for frequency doubling.

We report a high power tunable femtosecond soliton-based source using a simple combination of fiber-amplified pulses at 1064 nm and hollow-core photonic bandgap fiber. Compression of 5.5 ps input pulses, strongly chirped by self phase modulation in the amplifier, results in stable 520 fs-soliton formation with 77% conversion efficiency after only 8m propagation in the hollow-core fiber. The Raman self-frequency shift of the solitons was used to provide 33 nm wavelength tuneability. The transform-limited output pulses were frequency doubled using a nonlinear crystal with high conversion efficiency of 60% to demonstrate a femtosecond green laser tunable from 534 nm to 548 nm with 180 nJ pulse energy.

Delivery of sub-100fs pulses through 8m of hollow-core fiber using soliton compression.

We report soliton compression in a tapered hollow-core photonic bandgap fiber. We compress unchirped 195fs input pulses at 800 nm wavelength to less than 100fs after single-mode propagation through 8m of fiber, at pulse energies of around 50nJ.