Showing papers by "Philip St. J. Russell published in 2016"

Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes.

Abstract: We report a hollow-core photonic crystal fiber that is engineered so as to strongly suppress higher-order modes, i.e., to provide robust LP01 single-mode guidance in all the wavelength ranges where the fiber guides with low loss. Encircling the core is a single ring of nontouching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes. We show that the resulting modal filtering effect depends on only one dimensionless shape parameter, akin to the well-known d/Λ parameter for endlessly single-mode solid-core PCF. Fabricated fibers show higher-order mode losses some ∼100 higher than for the LP01 mode, with LP01 losses 110 THz bandwidth.

High-resolution wavefront shaping with a photonic crystal fiber for multimode fiber imaging.

Abstract: We demonstrate that a high-numerical-aperture photonic crystal fiber allows lensless focusing at an unparalleled resolution by complex wavefront shaping. This paves the way toward high-resolution imaging exceeding the capabilities of imaging with multi-core single-mode optical fibers. We analyze the beam waist and power in the focal spot on the fiber output using different types of fibers and different wavefront shaping approaches. We show that the complex wavefront shaping technique, together with a properly designed multimode photonic crystal fiber, enables us to create a tightly focused spot on the desired position on the fiber output facet with a subwavelength beam waist.

Hybrid photonic-crystal fiber for single-mode phase matched generation of third harmonic and photon triplets

Abstract: All-fiber systems for third harmonic generation are of great interest because they can be used for the inverse process, namely, the generation of entangled photon triplets. Usually, chromatic dispersion prevents phase matching between the incident and generated radiation when they are both guided in an LP01-like mode. Here, we present a hybrid photonic crystal fiber that has been designed for phase matched third harmonic generation from 1596 to 532 nm in single-lobed modes. The third harmonic radiation is guided by an all-solid bandgap microstructure, while the pump frequency is confined by conventional total internal reflection. The fiber is also suitable for the generation of photon triplet states.

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores.

Abstract: Photonic crystal fibers (PCFs) made from ZBLAN glass are of great interest for generating broadband supercontinua extending into the ultraviolet and mid-infrared regions. Precise sub-micrometer structuring makes it possible to adjust the modal dispersion over a wide range, making the generation of new frequencies more efficient. Here we report a novel ZBLAN PCF with six cores, each containing a central nanobore of a diameter ∼330 nm. Each nanobore core supports several guided modes, and the presence of the nanobore significantly modifies the dispersion, strongly influencing the dynamics and the extent of supercontinuum generation. Spectral broadening is observed when a single core is pumped in the fundamental and first higher order core modes with 200 fs long pulses at a wavelength of 1042 nm. Frequency-resolved optical gating is used to characterize the output pulses when pumping in the lowest order mode. The results are verified by numerical simulations.

Hollow-core fibre and method of manufacturing thereof

Abstract: A hollow-core fibre ( 100 ) of non-bandgap type comprises a hollow core region ( 10 ) axially extending along the hollow-core fibre ( 100 ) and having a smallest transverse core dimension (D), wherein the core region ( 10 ) is adapted for guiding a transverse fundamental core mode and transverse higher order core modes, and an inner cladding region ( 20 ) comprising an arrangement of anti-resonant elements (AREs) ( 21, 21 A, 21 B) surrounding the core region ( 10 ) along the hollow-core fibre ( 100 ), each having a smallest transverse ARE dimension (d i ) and being adapted for guiding transverse ARE modes, wherein the core region ( 10 ) and the AREs ( 21, 21 A, 21 B) are configured to provide phase matching of the higher order core modes and the ARE modes and the ARE dimension (d i ) and the core dimension (D) are selected such that a ratio of the ARE and core dimensions (d i /D) is approximated to a quotient of zeros of Bessel functions of first kind (u lm,ARE /u lm,core ), multiplied with a fitting factor in a range of 0.9 to 1.5, with m being the m-th zero of the Bessel function of first kind of order l, said zeros of the Bessel functions describing the LP lm ARE modes and LP lm higher order core modes, respectively. Furthermore, an optical device ( 200 ) including the hollow-core fibre ( 100 ) and a method of manufacturing the hollow-core fibre are described.

Coherent octave-spanning mid-infrared supercontinuum generated in As2S3-silica double-nanospike waveguide pumped by femtosecond Cr:ZnS laser

Abstract: A more than 1.5 octave-spanning mid-infrared supercontinuum (1.2 to 3.6 μm) is generated by pumping a As2S3-silica “double-nanospike” waveguide via a femtosecond Cr:ZnS laser at 2.35 μm. The combination of the optimized group velocity dispersion and extremely high nonlinearity provided by the As2S3-silica hybrid waveguide enables a ~100 pJ level pump pulse energy threshold for octave-spanning spectral broadening at a repetition rate of 90 MHz. Numerical simulations show that the generated supercontinuum is highly coherent over the entire spanning wavelength range. The results are important for realization of a high repetition rate octave-spanning frequency comb in the mid-infrared spectral region.

RF-dressed Rydberg atoms in hollow-core fibres

Abstract: The giant electro-optical response of Rydberg atoms manifests itself in the emergence of sidebands in the Rydberg excitation spectrum if the atom is exposed to a radio-frequency (RF) electric field. Here we report on the study of RF-dressed Rydberg atoms inside hollow-core photonic crystal fibres (HC-PCF), a system that enables the use of low modulation voltages and offers the prospect of miniaturised vapour-based electro-optical devices. Narrow spectroscopic features caused by the RF field are observed for modulation frequencies up to 500 MHz.

Broadband electric-field-induced LP 01 and LP 02 second harmonic generation in Xe-filled hollow-core PCF.

Abstract: Second harmonic (SH) generation with 300 fs pump pulses is reported in a xenon-filled hollow-core photonic crystal fiber (PCF) across which an external bias voltage is applied. Phase-matched intermodal conversion from a pump light in the LP01 mode to SH light in the LP02 mode is achieved at a particular gas pressure. Using periodic electrodes, quasi-phase-matched SH generation into the low-loss LP01 mode is achieved at a different pressure. The low linear dispersion of the gas enables phase-matching over a broad spectral window, resulting in a measured bandwidth of ∼10 nm at high pump energies. A conversion efficiency of ∼18%/mJ is obtained. Gas-filled anti-resonant-reflecting hollow-core PCF uniquely offers pressure-tunable phase-matching, ultra-broadband guidance, and a very high optical damage threshold, which hold great promise for efficient three-wave mixing, especially in difficult-to-access regions of the electromagnetic spectrum.

Single-shot reconstruction of spectral amplitude and phase in a fiber ring cavity at a 80 MHz repetition rate.

Abstract: Femtosecond pulses circulating in a synchronously driven fiber ring cavity have complex amplitude and phase profiles that can change completely from one round-trip to the next. We use a recently developed technique, combining dispersive Fourier transformation) with spectral interferometry, to reconstruct the spectral amplitude and phase at each round-trip and, thereby, follow in detail the pulse reorganization that occurs. We focus on two different regimes: a period-two regime in which the pulse alternates between two distinct states and a highly complex regime. We characterize the spectral amplitude and phase of the pulses in both regimes at a repetition rate of 75.6 MHz and find good agreement with modeling of the system based on numerical solutions of the generalized nonlinear Schrodinger equation with feedback.

Broadband, Lensless and Optomechanically Stabilised Coupling into Microfluidic Hollow-Core Photonic Crystal Fiber Using Glass Nanospike

Abstract: We report a novel technique for launching broadband laser light into liquid-filled hollow-core photonic crystal fiber (HC-PCF). It uniquely offers self-alignment and self-stabilization via optomechanical trapping of a fused silica nanospike, fabricated by thermally tapering and chemically etching a single mode fiber into a tip diameter of 350 nm. We show that a trapping laser, delivering ~300 mW at 1064 nm, can be used to optically align and stably maintain the nanospike at the core center. Once this is done, a broadband supercontinuum beam (~575 to 1064 nm) can be efficiently and close to achromatically launched in the HC-PCF. The system is robust against liquid-flow in either direction inside the HC-PCF and the Fresnel back-reflections are reduced to negligible levels compared to free-space launching or butt-coupling. The results are of potential relevance for any application where the efficient delivery of broadband light into liquid-core waveguides is desired.

Soft glass microstructured fibers and their applications

Abstract: We report recent progress on the fabrication of microstructured fibres from different soft glasses and their use in generating supercontinua in the UV and mid-IR from short-pulse infrared pump lasers at 1 and 2 μm.

Twist-tuning of higher-order mode suppression in single-ring hollow-core photonic crystal fibers

Abstract: In straight single-ring hollow-core PCFs, dramatic higher-order-mode suppression occurs for a strict geometrical condition. Twisting permits this condition to be relaxed, reducing fabrication tolerances. The twisted fiber also exhibits weak circular birefringence.

Reducing losses in solid-core photonic crystal fibers using chlorine dehydration

Abstract: The fabrication of photonic crystal fibers (PCFs) involves the stacking of multiple preform elements, providing many opportunities for contamination by water vapor or dust particles and causing increased fiber loss. Even after manufacture, diffusion of water vapor into the hollow channels is known to cause a slow increase in loss if the fibers are stored in a humid environment. In this paper we report a systematic study of three methods to reduce OH-related loss in solid-core PCFs: (1) treating the stack (primary preform) with chlorine or oxygen; (2) treating the cane (intermediate preform) with chlorine or oxygen; and (3) using a dry gas for pressurization of the hollow channels during the final step of fiber drawing. Each treatment is independently found effective in reducing OH-related loss, although stack treatment alone is not sufficient if the canes are subsequently stored for a longer time. On the other hand, chlorine-treatment of the canes and/or using a suitably dry gas using fiber drawing significantly lowers the loss even when the canes have been stored for more than two years in a closed tube at room temperature and at relative humidities in the range ~20% to ~50%.

Long-range optical binding in a hollow-core photonic crystal fiber using higher order modes

Abstract: We report long-range optical binding of multiple polystyrene nanoparticles (100-600 nm in diameter) at fixed interparticle distances that match multiples of the half-beat-lengths between the lowest order modes of a hollow-core photonic crystal fiber. Analysis suggests that each nanoparticle converts the incoming optical mode into a superposition of co-propagating modes, within the beat pattern of which further particles can become trapped. Strikingly, the entire particle arrangement can be moved over a distance of several cm, without changing the inter-particle spacing, by altering the ratio of backward-to-forward optical power. Potential applications are in multi-dimensional nanoparticle-based quantum optomechanical systems.

PHz-wide spectral interference through plasma-induced fission of higher order solitons

Abstract: We experimentally demonstrate plasma-induced soliton fission in a gas-filled photonic crystal fiber. Launching few μJ, 30 fs pulses at 1030 nm, PHz-wide spectral beating is observed as a result of pulse splitting and spectral interference.

Coherent Raman gain suppression in a gas-filled hollow-core PCF pumped in the deep ultraviolet

Abstract: DUV-pumped hydrogen-filled kagome-PCF displays coherent Raman gain suppression at much higher values of dephasing than for visible pumping. This will impair the performance of gas-based Raman amplifiers and lasers, especially at higher pump powers.

Adiabatic silica microspike with high mechanical Q-factor

Abstract: A novel mechanical resonator, based on a tapered fibre microspike, is engineered to yield Q-factors as high as 217,000 while offering adiabatic single mode optical guidance.

A quarter century of photonic crystal fibre

Abstract: Since their appearance two decades ago, PCFs have triggered a range of unique advances in light-matter interactions, including ultrabroadband supercontinuum generation, enhanced optomechanical nonlinearities, OAM-preserving twisted PCFs and diffraction-free pulse compression and nonlinear frequency conversion in gases.

Hybrid photonic crystal fiber for efficient single-mode third-harmonic and triplet photon generation

Abstract: We present a hybrid photonic-crystal fiber for phase-matched third-harmonic generation. Third-harmonic radiation is confined in a Gaussian-like mode by an all-solid bandgap microstructure, while outer hollow channels ensure guidance of pump radiation in fundamental mode.

Optically Driven Self-Oscillations of a Silica Nanospike at Low Gas Pressures

Abstract: We report light-driven instability and optomechanical self-oscillation of a fused silica “nanospike” at low gas pressures. The nanospike (tip diameter 400 nm), fabricated by thermally tapering and HF-etching a single mode fiber (SMF), was set pointing at the endface of a hollow-core photonic crystal fiber (HC-PCF) into the field created by the fundamental optical mode emerging from the HC-PCF. At low pressures, the nanospike became unstable and began to self-oscillate for optical powers above a certain threshold, acting like a phonon laser or "phaser". Because the nanospike is robustly connected to the base, direct measurement of the temporal dynamics of the instability is possible. The experiment sheds light on why particles escape from optical traps at low pressures.

Twin Beams from Noble Gas Filled Kagomé-PCF

Abstract: We generate twin beams through modulational instability in Ar-filled hollow-core kagome-PCF. The observed sidebands show photon-number correlations below the shot-noise level, no Raman noise, a single spatial mode, and tunable frequency mode content.

Quasi-phase-matched electric-field-induced second-harmonic in gas-filled hollow-core PCF

Abstract: Quasi-phase-matched electric-field-induced second-harmonic generation is demonstrated in Xe-filled hollow-core kagome photonic crystal fiber. The system is used to frequency double femtosecond near-infrared pulses, all signals being in the low-loss fundamental mode.

Sub-100 fs pulses from Er-fiber laser passively mode-locked at 1.872 GHz by acoustic resonance in solid-core PCF

Abstract: We report a dispersion-managed all-fiber laser delivering ∼85 fs pulses at 1550 nm, passively mode-locked at the 178th harmonic (repetition rate 1.872 GHz) by intense optoacoustic interactions in a photonic crystal fiber (PCF) core.

Photoionization-induced emission of mid-IR dispersive waves in gas-filled photonic crystal fibers

Abstract: We experimentally demonstrate photoionization-induced mid-infrared dispersive wave emission in gas-filled hollow-core PCF. Launching few μJ, 30 fs pulses at 1030 nm, we generated a 4.2-octave-spanning spectrum, including plasma-induced resonant radiation between 3.2 and 3.8 μm.

Stable GHz-Rate Mode-Locking of Fiber Lasers by Optoacoustic Effects in Photonic Crystal Fibers

Abstract: Tightly-trapped optoacoustic interactions in solid-core photonic crystal fibers have been successfully used for harmonically mode-locking fiber lasers at GHz repetition rate and storing encoded GHz-rate soliton sequence in the laser cavity over many hours.