Duncan P. Hand

High energy nanosecond laser pulses delivered single-mode through hollow-core PBG fibers

We report on the development of hollow-core photonic bandgap fibers for the delivery of high energy pulses for precision micromachining applications. Short pulses of (65ns pulse width) and energies of the order of 0.37mJ have been delivered in a single spatial mode through hollow-core photonic bandgap fibers at 1064nm using a high repetition rate (15kHz) Nd:YAG laser. The ultimate laser-induced damage threshold and practical limitations of current hollow-core fibers for the delivery of short optical pulses are discussed.

Ultra-Wideband Bandpass Filter With Multiple Notch Bands Using Nonuniform Periodical Slotted Ground Structure

A novel ultra-wideband (UWB) bandpass filter that is capable of integrating multiple notch bands is proposed in this paper. A multilayer nonuniform periodical structure, which can generate multiple transmission zeros, is deployed in the filter design to improve the selectivity and upper stopband performance of the filter. Short-circuited stub resonators are then integrated to obtain multiple notch bands. The compact footprint of the filter is achieved. UWB filters of this type without a notch and with single-, double-, and triple-notch bands are designed to meet the Federal Communications Commission-defined UWB indoor limit. The designed filters are verified by experiments and fabricated by using multilayer liquid-crystal polymer lamination technology. Both full-wave simulated and measured results are presented and good agreement between them is observed. The proposed filter has good performance and is attractive for UWB communication and radar systems.

Single-mode mid-IR guidance in a hollow-core photonic crystal fiber

We report, for the first time, bandgap guidance above 3 mum in a silica based air-core photonic crystal fiber. The peak of the bandgap is at 3.14mum with a typical attenuation of ~ 2.6 dB m-1. By further optimization of the structure, modeling suggests that a loss below 1 dB m-1 should be achievable, greatly extending the useful operating range of silica-based single-mode fibers. Such fibers have many potential applications in the mid-IR, offering an alternative to fluoride, tellurite or chalcogenide glass based optical fibers for chemical and biological sensing applications.

Transient deformation measurement with electronic speckle pattern interferometry and a high-speed camera

To the best of our knowledge, transient deformations have been measured in real time with microsecond temporal resolution for the first time with speckle pattern interferometry. The short exposure period and high framing rate of a high-speed camera at as many as 40,500 frames per second allow low-power continuous-wave laser illumination and fiber-optic beam delivery to be used. We have applied the technique to measure both harmonic vibration and transient deformation.

Mid-infrared methane detection in a photonic bandgap fiber using a broadband optical parametric oscillator

We demonstrate methane sensing using a photonic bandgap fiber-based gas cell and broadband idler pulses from a periodically-poled lithium niobate femtosecond optical parametric oscillator. The hollow core of the fiber was filled with a methane:nitrogen mixture, and Fourier transform spectroscopy was used to measure transmission spectra in the 3.15-3.35 mum methane absorption region. The method has applications in gas sensing for remote or hazardous environments and potentially at very low concentrations.

Melt ejection during laser drilling of metals

Abstract In laser drilling of metals, melt ejection can be a significant mechanism of material removal. Vaporisation within the hole creates high pressure gradients, which expel molten material from the hole. Results are presented for a range of metals drilled with single pulses with durations of 0.1 and 0.5 ms, using a Nd:YAG laser. Power intensities across the focussed beam were of the order of 0.2 MW mm −2 . Ejected droplets were collected and characterised, using several experimental techniques. The particle size distribution, angle of trajectory, molten layer thickness and temporal variation of melt ejection were determined. Two complementary methods, high speed photography and a particle stream interruption technique, were used to determine the ejection velocity. The experimental results obtained have been used to gain insight into the overall process of melt ejection. Melt ejection commences with the ejection of small (∼10 μm) droplets, moving at velocities of up to 30 m s −1 . This is followed towards the end of the process by the ejection of larger (∼100 μm), slower-moving droplets, with velocities of ∼1 m s −1 . Increasing the pulse intensity increases the ejection velocity and decreases the average particle size. This is attributed to the molten layers around the cavity being thinner, as a consequence of the higher thermal gradients. To a first approximation, typical particle diameters appear to be of the order of the molten layer thickness during drilling.

Optical techniques for real-time penetration monitoring for laser welding

Optical techniques for real-time full-penetration monitoring for Nd:YAG laser welding have been investigated. Coaxial light emission from the keyhole is imaged onto three photodiodes and a camera. We describe the spectral and statistical analyses from photodiode signals, which indicate the presence of a full penetration. Two image processing techniques based on the keyhole shape recognition and the keyhole image intensity profile along the welding path are presented. An intensity ratio parameter is used to determine the extent of opening at the rear of a fully opened keyhole. We show that this parameter clearly interprets a hole in formation or a lack of penetration when welding is performed on workpieces with variable thicknesses at constant laser power.

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.

Fibre optic beam delivery system for high peak power laser PIV illumination

Diffractive optical elements (DOEs) are used to couple Q-switched and frequency-doubled Nd:YAG laser beams into optical fibres to achieve significantly increased damage thresholds, enabling fibre optic beam delivery for high-speed particle image velocimetry (PIV) measurements. For single fibre delivery systems, the maximum pulse energy density that can be transmitted is increased by a factor of 5-10 compared with the best that can be achieved using conventional optics, of up to 10 mJ in a m core diameter fibre at 532 nm. We also applied the DOE arrangement to our previously developed bundle delivery system, comprising nineteen m core diameter fibres. On testing, over 1000 pulses with energies of 30 mJ were successfully transmitted, with no indication of damage. This allows fibre delivery to become a practical option for many air-flow PIV applications, as demonstrated here with measurements of flow in a square duct.

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.