Abstract:We report on a quartz-enhanced photoacoustic (QEPAS) gas sensing system for hydrogen sulphide (H 2 S) detection. The system architecture is based on a custom quartz tuning fork (QTF) optoacoustic transducer with a novel geometry and a quantum cascade laser (QCL) emitting 1.1 mW at a frequency of 2.913 THz. The QTF operated on the first flexion resonance frequency of 2871 Hz, with a quality factor Q = 17,900 at 20 Torr. The tuning range of the available QCL allowed the excitation of a H 2 S rotational absorption line with a line-strength as small as S = 1.13·10−22 cm/mol. The measured detection sensitivity is 30 ppm in 3 seconds and 13 ppm for a 30 seconds integration time, which corresponds to a minimum detectable absorption coefficient α min = 2.3·10 −7 cm −1 and a normalized noise-equivalent absorption NNEA = 4.4·10 −10 W·cm −1 ·Hz -1/2 , several times lower than the values previously reported for near-IR and mid-IR H 2 S QEPAS sensors.

A topic of great current interest is the harnessing and enhancement of optical tweezer forces for trapping small objects of different sizes and shapes at relatively small powers. Here we demonstrate the stable trapping, inside the core of a hollow-core photonic crystal fiber (HC-PCF), of a mechanically compliant fused silica nanospike, formed by tapering a single-mode fiber (SMF). The nanospike is subwavelength in diameter over its similar to W50 mu m insertion length in the HC-PCF. Laser light, launched into the SMF core, adiabatically evolves into a mode that extends strongly into the space surrounding the nanospike. It then senses the presence of the hollow core, and the resulting optomechanical action and back-action results in a strong trapping force at the core center. The system permits lens-less, reflection-free, self-stabilized, and self-aligned coupling from SMF to HC-PCF with a demonstrated efficiency of 87.8%. The unique configuration also provides an elegant means of investigating optomechanical effects in optical tweezers, especially at very low pressures. (C) 2016 Optical Society of Americ

Appropriately designed optomechanical devices are ideal for making ultra-sensitive measurements. Here we report a fused-silica microspike that supports a flexural resonance with a quality factor greater than 100 000 at room temperature in vacuum. Fashioned by tapering single-mode fiber (SMF), it is designed so that the core-guided optical mode in the SMF evolves adiabatically into the fundamental mode of the air-glass waveguide at the tip. The very narrow mechanical linewidth (20 mHz) makes it possible to measure extremely small changes in resonant frequency. In a vacuum chamber at low pressure, the weak optical absorption of the glass is sufficient to create a temperature gradient along the microspike, which causes it to act as a microscopic Knudsen pump, driving a flow of gas molecules towards the tip where the temperature is highest. The result is a circulating molecular flow within the chamber. Momentum exchange between the vibrating microspike and the flowing molecules causes an additional restoring force that can be measured as a tiny shift in the resonant frequency. The effect is strongest when the mean free path of the gas molecules is comparable with the dimensions of the vacuum chamber. The system offers a novel means of monitoring the behavior of weakly absorbing optomechanical sensors operating in vacuum.

No abstract

Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core-guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-centimeter distances-orders of magnitude farther than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and may also permit precise delivery of light to remote locations.

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.

Nowadays fiber biphoton sources are nearly as popular as crystal-based ones. They offer a single spatial mode and easy integrability into optical networks. However, fiber sources lack the broad tunability of crystals, which do not require a tunable pump. Here, we report a broadly tunable biphoton source based on a suspended core fiber. This is achieved by introducing pressurized gas into the fibers hollow channels, changing the step index. The mechanism circumvents the need for a tunable pump laser, making this a broadly tunable fiber biphoton source with a convenient tuning mechanism, comparable to crystals. We report a continuous shift of 0.30 THz/bar of the sidebands, using up to 25 bar of argon.Optical fibers are an ideal platform for the generation of entangled photon pairs (biphotons) via spontaneous four-wave-mixing (FWM), due to their long light-matter interaction length. In particular, solid core fibers offer high effective nonlinearity. However, they typically lack the convenient tuning mechanism present in crystal-based biphoton sources, beside the trivial but costly scheme of tuning the pump wavelength. A limited amount of tuning has been demonstrated by stretching and heating the fiber [1], however, these approaches are limited by fiber damage. Meanwhile, gas filled hollow-core fibers offer broad dispersion tuning, but implementing a biphoton source in these fibers remains a challenging task, due to their low nonlinearity. Here, we combine the high nonlinearity of a solid core fiber with the convenient tuning scheme offered by gas filled fibers, to implement a tunable source of entangled photons. This is achieved by filling the channels surrounding the core of a suspended-core fiber (SCF). SCFs are a class of index-guiding microstructured fibers, where light is guided in a glass core suspended by three glass nano-membranes. SCFs have been used in a variety of applications, ranging from supercontinuum generation [2] to gas absorption spectroscopy [3], or chemical sensing [4].In FWM, two photons of an incident beam (denoted pump) are annihilated and the energy is transferred to two daughter photons (denoted signal and idler) symmetrically spaced around the pump. The signal and idler frequencies ω S , ω I are determined by the * jonas.hammer@mpl.mpg.de

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.