Hollow-core photonic crystal fibre for high power laser beam delivery

We review the use of hollow-core photonic crystal fibre(HC-PCF)for high power laser beam delivery.A comparison of bandgap HC-PCF with Kagome-lattice HC-PCF on the geometry,guidance mechanism,and optical properties shows that the Kagome-type HC-PCF is an ideal host for high power laser beam transportation because of its large core size,low attenuation,broadband transmission,single-mode guidance,low dispersion and the ultra-low optical overlap between the core-guided modes and the silica core-surround.The power handling capability of Kagome-type HC-PCF is further experimentally demonstrated by millijoule nanosecond laser spark ignition and^100μJ sub-picosecond laser pulse transportation and compression.

Low-loss single-mode hybrid-lattice hollow-core photonic-crystal fibre

Remarkable recent demonstrations of ultra-low-loss inhibited-coupling(1C)hollow-core photonic-crystal fibres(HCPCFs)established them as serious candidates for next-generation long-haul fibre optics systems.A hindrance to this prospect and also to short-haul applications such as micromachining,where stable and high-quality beam delivery is needed,is the difficulty in designing and fabricating an IC-guiding fibre that combines ultra-low loss,truly robust single-modeness,and polarisation-maintaining operation.The design solutions proposed to date require a trade-off between low loss and truly single-modeness.Here,we propose a novel IC-HCPCF for achieving low-loss and effective single-mode operation.The fibre is endowed with a hybrid cladding composed of a Kagome-tubular lattice(HKT).This new concept of a microstructured cladding allows us to significantly reduce the confinement loss and,at the same time,preserve truly robust single-mode operation.Experimental results show an HKT-IC-HCPCF with a minimum loss of 1.6dB/km at 1050 nm and a higher-order mode extinction ratio as high as 47.0 dB for a 10 m long fibre.The robustness of the fibre single-modeness is tested by moving the fibre and varying the coupling conditions.The design proposed herein opens a new route for the development of HCPCFs that combine robust ultra-low-loss transmission and single-mode beam delivery and provides new insight into IC guidance.

Sub-50 fs pulses at 2050 nm from a picosecond Ho:YLF laser using a two-stage Kagome-fiber-based compressor

The high-energy few-cycle mid-infrared laser pulse beyond 2μm is of immense importance for attosecond science and strong-field physics.However,the limited gain bandwidth of laser crystals such as Ho:YLF and Ho:YAG allows the generation of picosecond(ps)long pulses and,hence,makes it challenging to generate few-cycle pulse at 2μm without utilizing an optical parametric chirped-pulse amplifier(OPCPA).Moreover,the exclusive use of the near-infrared wavelength has limited the generation of wavelengths beyond 4μm(OPCPA).Furthermore,high harmonic generation(HHG)conversion efficiency reduces dramatically when driven by a long-wavelength laser.Novel schemes such as multi-color HHG have been proposed to enhance the harmonic flux.Therefore,it is highly desirable to generate few-cycle to femtosecond pulses from a 2μm laser for driving these experiments.Here,we utilize two-stage nonlinear spectral broadening and pulse compression based on the Kagome-type hollow-core photonic crystal fiber(HC-PCF)to compress few-ps pulses to sub-50 fs from a Ho:YLF amplifier at 2μm at 1 kHz repetition rate.We demonstrate both experimentally and numerically the compression of 3.3 ps at 140μJ pulses to 48 fs at 11μJ with focal intensity reaching 10^(13)W/cm^(20.Thereby,this system can be used for driving HHG in solids at 2μm.In the first stage,the pulses are spectrally broadened in Kagome fiber and compressed in a silicon-based prism compressor to 285 fs at a pulse energy of 90μJ.In the second stage,the 285 fs pulse is self-compressed in air-filled HC-PCF.With fine-tuning of the group delay dispersion(GDD)externally in a 3 mm window,a compressed pulse of 48 fs is achieved.This leads to a 70-fold compression of the ps pulses at 2050 nm.We further used the sub-50 fs laser pulses to generate white light by focusing the pulse into a thin medium of YAG.

Strong nonlinear optical effects in micro-confined atmospheric air

Historically, nonlinear optical phenomena such as spectral broadening by harmonic generation have been associated with crystals owing to their strong nonlinear refractive indices, which are in the range of 10-14cm^2∕W.This association was also the result of the limited optical power available from early lasers and the limited interaction length that the laser–crystal interaction architecture could offer. Consequently, these limitations disqualified a large number of materials whose nonlinear coefficient is lower than n210-16cm^2∕W as suitable materials for nonlinear optics applications. For example, it is a common practice in most of optical laboratories to consider ambient or atmospheric air as a "nonlinear optically" inert medium due to its very low nonlinear coefficient(10.10^-19cm^2∕W) and low density. Today, the wide spread of high-power ultra-short pulse lasers on one hand, and low transmission loss and high-power handling of Kagome hollow-core photonic crystal fiber on the other hand, provide the necessary ingredients to excite strong nonlinear optical effects in practically any gas media, regardless of how low its optical nonlinear response is. By using a single table-top 1 m J ultra-short pulse laser and an air exposed inhibited-coupling guiding hollow-core photonic crystal fiber, we observed generation of supercontinuum and third harmonic generation when the laser pulse duration was set at 600 fs and Raman comb generation when the duration was 300 ps. The supercontinuum spectrum spans over 1000 THz and exhibits a typical spectral-density energy of 150 n J/nm. The dispersion profile of inhibited-coupling hollow-core fiber imprints a distinctive sequence in the supercontinuum generation, which is triggered by the generation of a cascade of four-wave mixing lines and concluded by solitonic dynamics. The Raman comb spans over 300 THz and exhibits multiple sidebands originating from N2 vibrational and ro-vibrational Raman transitions. With the growing use of hollow-core photonic crystal fiber in different fields, the results can be applied to mitigate air nonlinear response when it is not desired or to use ambient air as a convenient nonlinear medium.