Fei Yu

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.

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.

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.

Flexible delivery of Er:YAG radiation at 2.94 µm with negative curvature silica glass fibers: a new solution for minimally invasive surgical procedures

We present the delivery of high energy microsecond pulses through a hollow-core negative-curvature fiber at 2.94 µm. The energy densities delivered far exceed those required for biological tissue manipulation and are of the order of 2300 J/cm2. Tissue ablation was demonstrated on hard and soft tissue in dry and aqueous conditions with no detrimental effects to the fiber or catastrophic damage to the end facets. The energy is guided in a well confined single mode allowing for a small and controllable focused spot delivered flexibly to the point of operation. Hence, a mechanically and chemically robust alternative to the existing Er:YAG delivery systems is proposed which paves the way for new routes for minimally invasive surgical laser procedures.

Cavity-based mid-IR fiber gas laser pumped by a diode laser

Mid-infrared (IR) lasers are currently an area of rapid development, with several competing technologies. In traditional gas lasers, the effective interaction length is limited and the system as a whole is bulky and inflexible, limiting their applications. Standard gain fibers cannot be used in the mid-IR because the glass forming the fiber core is not transparent at these longer wavelengths. In this Letter, we report the demonstration of a mid-IR fiber gas laser using feedback in an optical cavity. The laser uses acetylene gas in a high-performance silica hollow-core fiber as the gain medium, and lases either continuous wave or synchronously pumped when pumped by telecom-wavelength diode lasers. We have demonstrated lasing on a number of transitions in the spectral band of 3.1–3.2 μm. The system could be extended to other selected molecular species to generate output in the spectral band up to 5 μm, and it has excellent potential for power scaling.

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.

Negative Curvature Hollow-Core Optical Fiber

The background, optical properties, and applications of low-loss negative curvature hollow-core fiber are reviewed. Data on spectral attenuation are collated and extended.

Efficient 1.9 μm emission in H2-filled hollow core fiber by pure stimulated vibrational Raman scattering

We report here efficient 1.9 μm emission by pure stimulated vibrational Raman scattering in a hydrogen-filled anti-resonant hollow-core fiber pumped with a 1064 nm microchip laser. A maximum quantum conversion efficiency ~48% was achieved by using a 6.5 m length of fiber filled with 23 bar hydrogen, with a maximum peak output power >2 kW. By properly designing the transmission bands of the fiber, selecting alternative pump sources and active gases, the emission wavelength could be extended into the mid-infrared. This provides a potential route for generating efficient, compact, broadly tunable, high power, and narrow linewidth mid-infrared fiber gas lasers with broad application in defense, environmental, and medical monitoring.

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.

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.