Jonathan P. Parry

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.

Application of cooled spatial light modulator for high power nanosecond laser micromachining

The application of a commercially available spatial light modulator (SLM) to control the spatial intensity distribution of a nanosecond pulsed laser for micromachining is described for the first time. Heat sinking is introduced to increase the average power handling capabilities of the SLM beyond recommended limits by the manufacturer. Complex intensity patterns are generated, using the Inverse Fourier Transform Algorithm, and example laser machining is demonstrated. The SLM enables both complex beam shaping and also beam steering.

Towards practical gas sensing with micro-structured fibres

In this work we realize a compact, low volume gas sensor utilizing commercially available hollow core photonic band-gap fibre (PBGF) guiding at around 1550 nm. The device is demonstrated for the detection of methane and acetylene gas. Hollow core PBGF offers near free-space beam guidance. This gives an advantage for gas sensing applications providing a longer interaction length than conventional gas cells and the potential to detect very low analyte concentrations whilst also being inherently safe for operation in remote or hazardous environments. However, one difficulty with such a device is filling a long fibre core with the test gas. Allowing the test gas to enter by diffusion alone can be a lengthy process and forced gas flow may be undesirable. A compact sensor has been designed and built to provide gas detection in an enclosed environment. Standardized connectors incorporating single mode fibre are used to couple light to and from the device. This is spliced to PBGF which has been arranged in sections connected by specially designed splices allowing gas into the fibre and therefore reducing the diffusion length. The fill time by diffusion is minimized while maintaining a long (~1 m) interaction length. Results from demonstration gas concentration measurements are presented and the performance of the device is evaluated. Results are limited by the presence of interferometric noise which is believed to arise from the interaction between core and surface modes within the PBGF; despite this, acetylene concentrations down to 0.05% are measured.

Application of a liquid crystal spatial light modulator to laser marking

Laser marking is demonstrated using a nanosecond (ns) pulse duration laser in combination with a liquid crystal spatial light modulator to generate two-dimensional patterns directly onto thin films and bulk metal surfaces. Previous demonstrations of laser marking with such devices have been limited to low average power lasers. Application in the ns regime enables more complex, larger scale marks to be generated with more widely available and industrially proven laser systems. The dynamic nature of the device is utilized to improve mark quality by reducing the impact of the inherently speckled intensity distribution across the generated image and reduce thermal effects in the marked surface.

Analysis of optical damage mechanisms in hollow-core waveguides delivering nanosecond pulses from a Q-switched Nd:YAG laser.

Hollow-core waveguides consisting of a glass capillary tube with an internal reflective coating are capable of delivering pulse energies of tens of millijoules with improved focusability compared to step index fibers of similar core diameter. We demonstrate the capability of these fibers to deliver high-power Q-switched pulses at the fundamental (1064 nm), second (532 nm), and third (355 nm) harmonics of a Nd:YAG laser, both in terms of peak power and beam quality delivered. In terms of peak power delivery, the primary limitation is the occurrence of bend-induced optical damage to the reflective coating. The damage mechanism and the influential factors are analyzed, in particular, the dependence upon the number of guided modes, core diameter, coating thicknesses, and input polarization alignment.

Optical fiber array for the delivery of high peak-power laser pulses for fluid flow measurements

Fiber delivery of 64.7 mJ laser pulses (approximately 6 ns duration) from a Q-switched Nd:YAG laser operating at 532 nm is demonstrated. A custom diffractive optical element was used to shape the laser beam and facilitate coupling into a linear fiber array. This launch arrangement achieves an improvement in launch efficiency compared with a circular fiber bundle evaluated in previous work and the delivery of higher pulse energies is demonstrated. The bundle is capable of delivering light of sufficient pulse energy and, importantly, with suitable focusability, to generate a thin light sheet for the fluid flow measurement technique of particle image velocimetry (PIV). Fiber delivery offers an advantage, in terms of optical access, for the application of PIV to enclosed measurement volumes, such as the cylinder of a combustion engine.

Hollow-core waveguides for particle image velocimetry

The use of a hollow-core fibre waveguide to deliver a light sheet for particle image velocimetry (PIV) inside an optically accessed internal combustion engine is presented. Fibre delivery applied to such small scale, high-speed fluid flow applications gives the potential to minimize the optical access required to an enclosed measurement volume. A 0.54 mm internal diameter hollow fibre was used to deliver 13 mJ, 8 ns pulses from a frequency-doubled (532 nm) Nd:YAG laser. The output from the fibre was focused into a thin light sheet and used to take PIV measurements as the test engine was cycled. Comparative measurements were also taken using a conventionally (bulk optic) delivered light sheet with closely similar properties. The PIV data taken using the two techniques are compared to demonstrate that the use of a hollow-core fibre generates similar data quality to conventional measurement techniques and is a viable alternative when complex access is required.

Speckle contrast reduction in a large-core fiber delivering Q-switched pulses for fluid flow measurements

Due to their capability for supporting high-peak powers, large-core hollow optical fibers may be used to deliver high-peak-power nanosecond pulses for the fluid flow measurement technique of particle image velocimetry [Meas. Sci. Technol. 16, 1119 (2005)]. One drawback of using such fibers for fluid flow measurements is that the output suffers from a speckled interference pattern due to the fibers multimode nature, which can lead to a loss of spatial information and reduced data quality. Presented here is a technique to reduce the speckle contrast from these fibers when delivering nanosecond pulses. Significant smoothing of the output intensity distribution is demonstrated, giving an improved source of illumination for fluid flow measurements and other imaging techniques requiring pulsed laser illumination.

Adaptive extracavity beam shaping for application in nanosecond laser micromachining

The typical Gaussian intensity distribution generated at focus of a laser machining workstation is not always ideal for the application; instead other shapes such as ellipses, flat-tops (circular or square), or doughnuts can in some cases give better results. Also, other more complex beam profiles might be beneficial for surface micro structuring. In order to realise, and rapidly change between such beam shapes, we are investigating an adaptive optics approach based on using an iterative simulated annealing algorithm to control the actuators of a deformable mirror. A 37-element piezoelectric deformable mirror and a 37-element bimorph mirror were applied in an extracavity arrangement. Beam shaping results with these systems are presented and example laser machining is demonstrated in this paper. The results enabled by the deformable mirrors are compared to previous results using a spatial light modulator (SLM) based on a liquid crystal microdisplay. The SLM has a much higher resolution and enables complex beam shapes to be generated, however is much slower in response. Having an active beam shaping element incorporated in a laser machining workstation adds increased flexibility and improves process control.

Compensation for time fluctuations of phase modulation in a liquid-crystal-on-silicon display by process synchronization in laser materials processing.

We demonstrate the adverse influence of temporal fluctuations of the phase modulation of a spatial light modulator (SLM) display device on nanosecond laser micromachining. We show that active cooling of the display reduces the amplitude of these fluctuations, and we demonstrate a process synchronization technique developed to compensate for these fluctuations when applying the SLM to laser materials processing. For alternative SLM devices developed specifically for laser wavefront control (which do not exhibit such flickering problems), we show that our process synchronization approach is also beneficial to avoid machining glitches when switching quickly between different phase profiles (and hence beam patterns).