Guidance of ultraviolet light down to 190 nm in a hollow-core optical fibre.

We report an anti-resonant hollow core fibre with ultraviolet transmission down to 190 nm, covering the entire UV-A, UV-B and much of the UV-C band. Guidance from 190 - 400 nm is achieved apart for a narrow high loss resonance band at 245 - 265 nm. The minimum attenuation is 0.13 dB/m at 235 nm and 0.16 dB/m at 325 nm. With an inscribed core diameter of ∼12 µm, the fibre's bend loss at 325 nm was 0.22 dB per turn for a bend radius of 3 cm at 325 nm.

Measurement of resonant bend loss in anti-resonant hollow core optical fiber: erratum.

An error on the part of the authors in drafting resulted in Eq. (3) being incorrect in the published paper [Opt. Express25, 20612 (2017)10.1364/OE.25.020612]. We present a corrected version of the equation. It should be noted that this does not affect the presented results or conclusions of the paper.

Molecular detection of Gram-positive bacteria in the human lung through an optical fiber-based endoscope.

PURPOSE: The relentless rise in antimicrobial resistance is a major societal challenge and requires, as part of its solution, a better understanding of bacterial colonization and infection. To facilitate this, we developed a highly efficient no-wash red optical molecular imaging agent that enables the rapid, selective, and specific visualization of Gram-positive bacteria through a bespoke optical fiber-based delivery/imaging endoscopic device. METHODS: We rationally designed a no-wash, red, Gram-positive-specific molecular imaging agent (Merocy-Van) based on vancomycin and an environmental merocyanine dye. We demonstrated the specificity and utility of the imaging agent in escalating in vitro and ex vivo whole human lung models (n = 3), utilizing a bespoke fiber-based delivery and imaging device, coupled to a wide-field, two-color endomicroscopy system. RESULTS: The imaging agent (Merocy-Van) was specific to Gram-positive bacteria and enabled no-wash imaging of S. aureus within the alveolar space of whole ex vivo human lungs within 60 s of delivery into the field-of-view, using the novel imaging/delivery endomicroscopy device. CONCLUSION: This platform enables the rapid and specific detection of Gram-positive bacteria in the human lung.

Experimental measurement of supercontinuum coherence in highly nonlinear soft-glass photonic crystal fibers.

We present experimental measurements illustrating the power-dependent coherence evolution for supercontinuum generated in highly nonlinear SF6 photonic crystal fibers. The measurements were performed for fiber lengths close to and much longer than the soliton fission length. Simulations of the spectral evolution were also carried out to accompany the experimental observation. Many parameters were estimated by matching the simulated and the measured evolution. Both the measured and the simulated coherence evolution confirm the association between coherence degradation and soliton fission.

Measurement of resonant bend loss in anti-resonant hollow core optical fiber.

We present the results of measurements of resonant spectral bend loss using a novel apparatus in a series of hollow core anti-resonant optical fibers, important for their applications in the delivery of industrial power ultra-short laser pulses. The measured bend losses exhibit clear wavelength-bend diameter resonances. We demonstrate, in good agreement with theoretical analysis, that the sensitivity to bend diameter (in terms of minimum bend radii) is dependent on the ratio between cladding and core structure size. By decreasing the cladding capillary diameter: core diameter ratio from 0.70 to 0.43 the minimum bend diameter is decreased from >160 mm to ~15 mm at a wavelength of 800 nm. Furthermore it is demonstrated that the exact position of the loss bands is highly dependent on the orientation of the fiber structure with the bend plane.

Quantitative characterization of endoscopic imaging fibers.

We describe a technique for the quantitative characterization of endoscopic imaging fibers using an interference pattern as the standard object to be imaged. The visibility of the pattern at the other end of the fiber is then analyzed as wavelength and fringe period are varied. We demonstrate the use of the technique by comparing three fibers: two fabricated in-house from the same preform, designed to minimize inter-core coupling at visible wavelengths less than 650 nm, and a commercial imaging fiber. The techniques discussed are currently being used to optimize fibers for fluorescence bronchoscopy to be used in intensive care clinics.

Experimental study of low-loss single-mode performance in anti-resonant hollow-core fibers.

Anti-resonant hollow-core fibers are optical fiber waveguides which exhibit very low dispersion, high damage threshold and ultra-low nonlinear response. However, they typically deliver the light in several spatial modes, whereas their application usually requires that they support a single spatial mode. We report the principles, fabrication, demonstration and characterization of anti-resonant hollow-core fibres with strong differential modal attenuations and low overall attenuations. These fibers perform as single-mode and are eminently suitable for delivery of powerful ultrashort optical pulses in machining, cutting, welding and multiphoton microscopy applications.

Adaptive multiphoton endomicroscopy through a dynamically deformed multicore optical fiber using proximal detection.

This paper demonstrates multiphoton excited fluorescence imaging through a polarisation maintaining multicore fiber (PM-MCF) while the fiber is dynamically deformed using all-proximal detection. Single-shot proximal measurement of the relative optical path lengths of all the cores of the PM-MCF in double pass is achieved using a Mach-Zehnder interferometer read out by a scientific CMOS camera operating at 416 Hz. A non-linear least squares fitting procedure is then employed to determine the deformation-induced lateral shift of the excitation spot at the distal tip of the PM-MCF. An experimental validation of this approach is presented that compares the proximally measured deformation-induced lateral shift in focal spot position to an independent distally measured ground truth. The proximal measurement of deformation-induced shift in focal spot position is applied to correct for deformation-induced shifts in focal spot position during raster-scanning multiphoton excited fluorescence imaging.

High energy green nanosecond and picosecond pulse delivery through a negative curvature fiber for precision micro-machining.

In this paper we present an anti-resonant guiding, low-loss Negative Curvature Fiber (NCF) for the efficient delivery of high energy short (ns) and ultrashort (ps) pulsed laser light in the green spectral region. The fabricated NCF has an attenuation of 0.15 dB/m and 0.18 dB/m at 532 nm and 515 nm respectively, and provided robust transmission of nanosecond and picosecond pulses with energies of 0.57 mJ (10.4 kW peak power) and 30 µJ (5 MW peak power) respectively. It provides single-mode, stable (low bend-sensitivity) output and maintains spectral and temporal properties of the source laser beam. The practical application of fiber-delivered pulses has been demonstrated in precision micro-machining and marking of metals and glass.

Hollow antiresonant fibers with low bending loss.

We first use numerical simulations to show that bending losses of hollow antiresonant fibers are a strong function of their geometrical structure. We then demonstrate this by fabricating a hollow antiresonant fiber which presents a bending loss as low as 0.25 dB/turn at a wavelength of 3.35 µm and a bend radius of 2.5 cm. This fiber has a relatively low attenuation (<200 dB/km) over 600 nm mid-infrared spectral range.

Efficient diode-pumped mid-infrared emission from acetylene-filled hollow-core fiber.

We report 3.1-3.2 µm mid-infrared emission from acetylene-filled low loss antiresonant hollow-core fiber pumped with an amplified, modulated, narrowband, tunable 1.5 µm diode laser. The maximum power conversion efficiency of ~30%, with respect to the absorbed pump power, is obtained with a 10.5 m length of fiber at 0.7 mbar. The maximum efficiency with respect to the total incident pump power (~20%) and the minimum pump laser energy required (<50 nJ) are both improved compared to similar work reported previously using an optical parametric oscillator as a pump source. This paper provides an effective route to obtain compact mid-infrared fiber lasers.

Hollow antiresonant fibers with reduced attenuation.

An improved design for hollow antiresonant fibers (HAFs) is presented. It consists of adding extra antiresonant glass elements within the air cladding region of an antiresonant hollow-core fiber. We use numerical simulations to compare fiber structures with and without the additional cladding elements in the near- and mid-IR regimes. We show that realizable fiber structures can provide greatly improved performance in terms of leakage and bending losses compared to previously reported antiresonant fibers. At mid-IR wavelengths, the adoption of this novel fiber design will lead to HAFs with reduced bending losses. In the near-IR, this design could lead to the fabrication of HAFs with very low attenuation.

Spectral attenuation limits of silica hollow core negative curvature fiber.

In this paper we discuss the limits of attenuation of silica hollow core negative curvature fibers in the wavelength range from 800 nm up to 4.5 µm. Both numerical and experimental results are presented and show good agreement. A minimum attenuation of 24.4 dB/km was measured at around 2400 nm wavelength, while 85 dB/km was measured at 4000 nm.

Effect of core boundary curvature on the confinement losses of hollow antiresonant fibers.

We use numerical simulations to investigate how the curvature of the fiber core boundary influences the attenuation of hollow antiresonant fibers. We show the importance of a "negative" curvature core boundary in reducing confinement losses and also how, for certain curvatures, optical power is coupled resonantly to cladding modes. We simulate bending losses and find results in agreement with previously-reported experiments.

Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature Fiber for micro-machining applications.

We present high average power picosecond and nanosecond pulse delivery at 1030 nm and 1064 nm wavelengths respectively through a novel hollow-core Negative Curvature Fiber (NCF) for high-precision micro-machining applications. Picosecond pulses with an average power above 36 W and energies of 92 µJ, corresponding to a peak power density of 1.5 TWcm⁻² have been transmitted through the fiber without introducing any damage to the input and output fiber end-faces. High-energy nanosecond pulses (>1 mJ), which are ideal for micro-machining have been successfully delivered through the NCF with a coupling efficiency of 92%. Picosecond and nanosecond pulse delivery have been demonstrated in fiber-based laser micro-machining of fused silica, aluminum and titanium.

Temperature response of an all-solid photonic bandgap fiber for sensing applications.

The spectral shift due to temperature in the photonic bandgap (PBG) of an all-solid PBG fiber is investigated, aiming at discrete and distributed temperature sensing. A temperature rise induces a red shift in the bandgap spectra, which can be easily and precisely monitored by measuring the fiber transmission near one of the band edges. Two different situations that are potentially compatible with distributed and quasi-distributed sensing were investigated: heating a 2 m section of a longer (~10 m) fiber, and heating the whole extension of a fiber that is tens of centimeters in length and was spliced to conventional fibers on both sides. The latter setup yielded bandgap spectral shifts up to ~35 pm/°C. Aiming at discrete sensing, a short (~50 mm) fiber section was subjected to a tight bend so as to exhibit increased temperature sensitivity. Choosing the position of the bend allows for reconfiguration, on demand, of the sensor. A semi-analytical method to identify the spectral position of bandgaps was used to model the fiber transmission, as well as the bandgap shift with temperature, with consistent results.

Low loss silica hollow core fibers for 3-4 µm spectral region.

We describe a silica hollow-core fiber for mid-infrared transmission with a minimum attenuation of 34 dB/km at 3050 nm wavelength. The design is based on the use of a negative curvature core wall. Similar fiber designed for longer wavelengths has a transmission band extending beyond 4 µm.

Highly-efficient, octave spanning soliton self-frequency shift using a specialized photonic crystal fiber with low OH loss.

We report the first demonstration of efficient, octave spanning soliton self-frequency shift. In order to achieve this we used a photonic crystal fiber with reduced OH absorption and widely spaced zero-dispersion wavelengths. To our knowledge, this is the largest reported frequency span for a tunable, fiber-based source. In addition, we observe the generation of light above 2 µm directly from a Ti:Sapphire laser in the form of Cerenkov emission by the soliton when the red-shift saturates at the edge of the anomalous dispersion region.

Higher order guided mode propagation in solid-core photonic bandgap fibers.

We investigate higher order core-modes of solid-core photonic bandgap fibers experimentally and theoretically. We observe that for some wavelengths ranges the second mode is guided while the fundamental mode is not. We interpret this behavior in terms of the band diagrams and full numerical simulations, in good agreements with experiments. The sole guidance of the second, ring shaped modes observed at the edges of bandgaps could be of use for generation of vortex beams.

High power red and near-IR generation using four wave mixing in all integrated fibre laser systems.

We demonstrate high power generation of visible red and near IR light by four wave mixing in photonic crystal fibres (PCFs) pumped at 1064 nm with picosecond pulses (30 - 80 ps). 30% conversion efficiency is demonstrated in a single pass using fibre lengths less than 1 m, with signal wavelengths from 650 nm to 820 nm selectable by choice of PCF. An all fibre integrated system delivers 2.16 W at 740 nm with a pulse repetition frequency of 20 MHz. We discuss the overall parameter space for this type of wavelength conversion in PCF with different fibre designs suitable for delivering a particular wavelength at low or high power.