Single-ring Hollow-core Photonic Crystal Fiber Research Articles

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.

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.

Fabrication and non-destructive characterization of tapered single-ring hollow-core photonic crystal fiber

We report on the properties of tapered single-ring hollow-core photonic-crystal fibers, with a particular emphasis on applications in nonlinear optics. The simplicity of these structures allows the use of non-invasive side-illumination to assess the quality of the tapering process, by observing the scattered far-field spectrum originating from excitation of whispering-gallery modes in the cladding capillaries. We investigate the conditions that ensure adiabatic propagation in the up- and down-tapers, and the scaling of loss-bands (created by anti-crossings between the core mode and modes in the capillary walls) with taper ratio. We also present an analytical model for the pressure profile along a tapered hollow fiber under differential pumping.

Strong circular dichroism for the HE11 mode in twisted single-ring hollow-core photonic crystal fiber

We report a series of experimental, analytical, and numerical studies demonstrating strong circular dichroism for the HE11-like core mode in helically twisted hollow-core single-ring photonic crystal fiber (SR-PCF), formed by spinning the preform during fiber drawing. In the SR-PCFs studied, the hollow core is surrounded by a single ring of nontouching capillaries. Coupling between these capillaries results in the formation of helical Bloch modes carrying orbital angular momentum. When twisted, strong circular birefringence appears in the ring, so that coupling to the core mode is possible for only one circular polarization state. The result is a SR-PCF that acts as a circular polarizer, offering 1.4 dB/m for the low-loss polarization state and 9.7 dB/m for the high-loss state over a 25 nm band centered at 1593 nm wavelength. In addition, we report for the first time that the vector fields of the helical Bloch modes are perfectly periodic when evaluated in cylindrical coordinates. Such fibers have many potential applications, such as generating circularly polarized light in gas-filled SR-PCF and realizing polarizing elements in the deep and vacuum ultraviolet.

Generation of microjoule pulses in the deep ultraviolet at megahertz repetition rates

Although ultraviolet (UV) light is important in many areas of science and technology, there are very few if any lasers capable of delivering wavelength-tunable ultrashort UV pulses at MHz repetition rates. Here we report the generation of deep-UV laser pulses at MHz repetition rates and \mu J-energies by means of dispersive wave (DW) emission from self-compressed solitons in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF). Pulses from an ytterbium fiber laser (~300 fs) are first compressed to ~25 fs in a SR-PCF-based nonlinear compression stage, and subsequently used to pump a second SR-PCF stage for broadband DW generation in the deep UV. The UV wavelength is tunable by selecting the gas species and the pressure. At 100 kHz repetition rate, a pulse energy of 1.05 \mu J was obtained at 205 nm (average power 0.1 W), and at 1.92 MHz, a pulse energy of 0.54 \mu J was obtained at 275 nm (average power 1.03 W).

Generation of broadband mid-IR and UV light in gas-filled single-ring hollow-core PCF.

We report generation of an ultrafast supercontinuum extending into the mid- infrared in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) pumped by 1.7 µm light from an optical parametric amplifier. The simple fiber structure offers shallow dispersion and flat transmission in the near and mid-infrared, enabling the generation of broadband spectra extending from 270 nm to 3.1 µm, with a total energy of a few µJ. In addition, we demonstrate the emission of ultraviolet dispersive waves whose frequency can be tuned simply by adjusting the pump wavelength. SR-PCF thus constitutes an effective means of compressing and delivering tunable ultrafast pulses in the near and mid-infrared spectral regions.

Photochemistry in a soft-glass single-ring hollow-core photonic crystal fibre.

A hollow-core photonic crystal fibre (HC-PCF), guided by photonic bandgap effects or anti-resonant reflection, offers strong light confinement and long photochemical interaction lengths in a microscale channel filled with a solvent of refractive index lower than that of glass (usually fused silica). These unique advantages have motivated its recent use as a highly efficient and versatile microreactor for liquid-phase photochemistry and catalysis. In this work, we use a single-ring HC-PCF made from a high-index soft glass, thus enabling photochemical experiments in higher index solvents. The optimized light-matter interaction in the fibre is used to strongly enhance the reaction rate in a proof-of-principle photolysis reaction in toluene.