Zheng Kuang

Ultrafast laser parallel microdrilling using multiple annular beams generated by a spatial light modulator

Ultrafast laser parallel microdrilling using diffractive multiple annular beam patterns is demonstrated in this paper. The annular beam was generated by diffractive axicon computer generated holograms (CGHs) using a spatial light modulator. The diameter of the annular beam can be easily adjusted by varying the radius of the smallest ring in the axicon. Multiple annular beams with arbitrary arrangement and multiple annular beam arrays were generated by superimposing an axicon CGH onto a grating and lenses algorithm calculated multi-beam CGH and a binary Dammann grating CGH, respectively. Microholes were drilled through a 0.03mm thick stainless steel foil using the multiple annular beams. By avoiding huge laser output attenuation and mechanical annular scanning, the processing is ∼200 times faster than the normal single beam processing.

Multi-point laser spark generation for internal combustion engines using a spatial?light modulator

This paper reports on a technique demonstrating for the first time successful multi-point laser-induced spark generation, which is variable in three dimensions and derived from a single laser beam. Previous work on laser ignition of internal combustion engines found that simultaneously igniting in more than one location resulted in more stable and faster combustion – a key potential advantage over conventional spark ignition. However, previous approaches could only generate secondary foci at fixed locations. The work reported here is an experimental technique for multi-point laser ignition, in which several sparks with arbitrary spatial location in three dimensions are created by variable diffraction of a pulsed single laser beam source and transmission through an optical plug. The diffractive multi-beam arrays and patterns are generated using a spatial light modulator on which computer generated holograms are displayed. A gratings and lenses algorithm is used to accurately modulate the phase of the input laser beam and create multi-beam output. The underpinning theory, experimental arrangement and results obtained are presented and discussed.

NUV femtosecond laser inscription of volume Bragg gratings in poly(methyl)methacrylate with linear and circular polarizations

Large, high efficiency, volume Bragg gratings with dimensions of 5 mm × 5 mm and thickness between 1 and 7 mm with 20 μm pitch have been inscribed in poly(methyl)methacrylate (PMMA) with 180 fs, 387 nm parallel beams using both linear and circular polarizations. Linear polarization (perpendicular to the scan direction) produced the highest refractive index contrast, while circular polarization produced the lowest. The measured first-order diffraction efficiency with grating thickness L agrees well with theoretical expectations, and reached a maximum of 94% near L = 4 mm, the highest yet observed in pure PMMA. The source of the variation in refractive index contrast was investigated, and it was found to be due to the polarization-dependent nonlinear filamentation, the first such observation in a pure polymer.

Imaging-based amplitude laser beam shaping for material processing by 2D reflectivity tuning of a spatial light modulator

We have demonstrated an imaging-based amplitude laser-beam-shaping technique for material processing by 2D reflectivity tuning of a spatial light modulator. Intensity masks with 256 gray levels were designed to shape the input laser beam in the outline profile and inside intensity distribution. Squared and circular flattop beam shapes were obtained at the diffractive near-field and then reconstructed at an image plane of an f-theta lens (f∼100  mm). The observed intensity distribution inside the beam-shaping geometry was much more even than using binary masks. The ablation footprint well matches the desired beam shape.

Femtosecond laser micro-structuring of materials in the nir and UV regime

Ultrafast laser micro-structuring of ceramic alumina, titanium and stainless steel are presented confirming that precision, melt free micro-structuring is indeed possible with intense femtosecond optical pulses. While metals are best machined at a fluence a few times ablation threshold, ceramic alumina, on the other hand, shows excellent structuring with no evidence of melt at high fluence F ∼ 21Jcm−2. In PMMA, at 387nm with pulselength ∼ 200fs, we demonstrate refractive index modification within the bulk induced by two photon absorption to generate phase gratings. By using a holographic technique combined with 387nm femtosecond radiation and 0.5NA UV objective, sub-micron pitch periodic structures were ablated on stainless steel, titanium and silicon. With a lower (0.15NA) objective and focussing the periodic optical field distribution within bulk PMMA, micron pitch phase gratings can be generated.Ultrafast laser micro-structuring of ceramic alumina, titanium and stainless steel are presented confirming that precision, melt free micro-structuring is indeed possible with intense femtosecond optical pulses. While metals are best machined at a fluence a few times ablation threshold, ceramic alumina, on the other hand, shows excellent structuring with no evidence of melt at high fluence F ∼ 21Jcm−2. In PMMA, at 387nm with pulselength ∼ 200fs, we demonstrate refractive index modification within the bulk induced by two photon absorption to generate phase gratings. By using a holographic technique combined with 387nm femtosecond radiation and 0.5NA UV objective, sub-micron pitch periodic structures were ablated on stainless steel, titanium and silicon. With a lower (0.15NA) objective and focussing the periodic optical field distribution within bulk PMMA, micron pitch phase gratings can be generated.

Multiple Pulse Laser Ignition in GDI Lean Combustion

Laser Ignition (LI) is a new, innovative technology which overcomes several limitations of conventional spark ignition; it presents many potential advantages for engine dynamic control, especially for gasoline direct injection engine. This paper highlights a new approach, using principle component analysis of the pressure curve, to dynamically control multiple pulse laser ignition (MPLI). In the results, experimental data presents how the control of MPLI improves the performance of the engine and demonstrates the effectiveness of MPLI in lean combustion. This work is part of an Engineering and Physical Sciences Research Council funded research project which is to investigate how optimised control strategies, including multiple stratified fuel injection (MSFI); MPLI and multiple location laser ignition (MLLI), can help to improve the stability of leaner operation and reduce the emissions. The possible future of LI dynamical control strategies is discussed.

Parallel patterning of indium tin oxide with ultra short pulses

Abstract Indium tin oxide (ITO) is a commonly used material in the manufacture of several photonic devices such as flat panel displays, solar cells and electronic ink. Direct laser patterning of ITO films offers a viable alternative to these methods. An ultrashort pulse laser system with pulse duration of 10 ps and variable repetition rate (5–50 kHz) was applied to ITO removal from a glass substrate. Removal of the film was achieved through determination of the ablation threshold (Φth) of both the ITO and glass. By varying the traverse speed, fluence and offset, a processing window was identified. In addition, a spatial light modulator was introduced into the optical path and used for diffractive multiple beam processing; this increased throughput by increasing the ‘effective’ repetition rate.

Picosecond Laser Beam Shaping Using a Spatial Light Modulator

A beam shaping technique, which is capable of converting a Gaussian beam to an arbitrary 3D beam profile through the implementation of a phase-only Spatial Light Modulator (SLM) combined with polarising optics, is presented. A beam profiling camera provides visualisation of beam alterations, which allowed for control over beam shape and intensity distribution in real time. Various beam profiles can be created for different applications. An investigation into the production of laser induced periodic surface structures (LIPSS) with a square flat-top beam shows potential improvement of LIPSS uniformity over a Gaussian beam. It is believed that the combination of accurate beam shape and top-hat intensity allows uniform LIPSS production in a region of ∼0.4mm along optical axis, which is well over the Rayleigh length. Benefits and limitations and of this technique are also discussed.A beam shaping technique, which is capable of converting a Gaussian beam to an arbitrary 3D beam profile through the implementation of a phase-only Spatial Light Modulator (SLM) combined with polarising optics, is presented. A beam profiling camera provides visualisation of beam alterations, which allowed for control over beam shape and intensity distribution in real time. Various beam profiles can be created for different applications. An investigation into the production of laser induced periodic surface structures (LIPSS) with a square flat-top beam shows potential improvement of LIPSS uniformity over a Gaussian beam. It is believed that the combination of accurate beam shape and top-hat intensity allows uniform LIPSS production in a region of ∼0.4mm along optical axis, which is well over the Rayleigh length. Benefits and limitations and of this technique are also discussed.

Multiple Pulse Laser Ignition Control Application in GDI Lean Combustion

This paper highlights a new innovative approach to create and control multiple pulse laser ignition applied on a GDI engine. The results present the dual pulse effectiveness in super lean condition and it indicates the dual pulse has significantly improved the engine stability.

Diffractive Multi-point Laser ignition of internal combustion engines using a spatial light modulator

We generated diffractive multi-beams for laser ignition using a spatial light modulator (SLM). Breakdown sparks with arbitrary geometrical location were created. The multi-beam patterns were delivered to engine for ignition tests without clipping.