An efficient simulation method has recently been developed for multi-pulse ablation processes. This is based on pulse-by-pulse propagation of the machined surface according to one of several phenomenological models for the laser-material interaction. The technique allows quantitative predictions to be made about the surface shapes of complex machined parts, given only a minimal set of input data for parameter calibration. In the case of direct-write machining of polymers or glasses with ns-duration pulses, this data set can typically be limited to the surface profiles of a small number of standard test patterns. The use of phenomenological models for the laser-material interaction, calibrated by experimental feedback, allows fast simulation, and can achieve a high degree of accuracy for certain combinations of material, laser and geometry. In this paper, the capabilities and limitations of the approach are discussed, and recent results are presented for structures machined in SU8 photoresist.
Laser ablation using diode pumped solid state lasers shows great potential for a wide range of micromachining applications. We have been using a frequency quadrupled Nd:VO<sub>4</sub> laser (266 nm wavelength), with a pulse duration < 30 ns, to ablate a sol-gel Ormocer material. With a pulse energy of around 20 μJ, and a focal spot of the order of 10 μm diameter, single pulses were found to produce craters a few microns in depth and ~10 μm in diameter. A study of the variation of the crater profile with pulse energy and angle of incidence to the surface has enabled the development of an efficient method to simulate the ablation for a series of consecutive shots constituting a toolpath. Multiple pulses with varying degrees of overlap were simulated, and compared with experiment. Results show that the model accurately predicts the profiles of trenches and pocketed surfaces given parameters obtained from a single crater machined at normal incidence. The "self calibrating" feature of our approach significantly reduces the number of input parameters required for adequate simulations. In particular, it does not require knowledge of the beam profile or material ablation curve. The simplicity and practicality of the method make it promising for use in an industrial environment.
The principles of operation and general design criteria for PIN diode variable optical attenuators (VOAs) realized from silicon-on-insulator rib waveguide structures are described. We present as a benchmark the performance of devices based on the established VOA produced by Bookham Technology Plc, and demonstrate 25dB attenuation at less than 70mW with novel recessed dopant geometries. Optical and electrical simulation results for new, smaller cross-section VOA structures based on rib waveguides utilizing a 2mm high guiding layer are detailed and discussed. Experimental results demonstrating the successful fabrication of these structures and the significant improvements in performance attained are presented. In particular we show that the attenuation efficiency can be 30% higher than that of the larger structure, and that modulation bandwidths may approach 10MHz.