Optical coherence and beamspread in ultrafast-laser pulsetrain-burst hole drilling Jesse Dean a , Paul Forrester a , Martin Bercx a , David Graper a , Luke McKinney a , Felix Frank a,c , Marc Nantel a,b , Robin Marjoribanks a a Department of Physics and Institute for Optical Sciences, University of Toronto 60 St George St., Toronto, ON M5S-1A7, Canada b Centre for Photonics, Ontario Centres of Excellence, Inc. 156 Front Street West, Suite 200, Toronto, Ontario M5J 2L6 ABSTRACT Pulsetrain-burst machining has been shown to have advantages over single-pulse laser processing of materials and biological tissues. Ultrafast lasers are often able to drill holes in brittle and other difficult materials with- out cracking or swelling the target material, as is sometimes the case for nanosecond-pulse ablation; further, pulsetrain-bursts of ultrafast pulses are able to recondition the material during processing for instance, making brittle materials more ductile and striking advantages can result. In the work we report, we have investigated hole-drilling characteristics in metal and glass, using a Nd:glass pulsetrain-burst laser (1054 nm) delivering 1 - 10 ps pulses at 133 MHz, with trains 3 - 15 μs long. We show that as the beam propagates down the channel being drilled, the beam loses transverse coherence, and that this affects the etch-rate and characteristics of channel- shape: as the original Gaussian beam travels into the channel, new boundary conditions are imposed on the propagating beam principally the boundary conditions of a cylindrical channel, and also the effects of plasma generated at the walls as the aluminum is ablated. As a result, the beam will decompose over the dispersive waveguide modes, and this will affect the transverse coherence of the beam as it propagates, ultimately limiting the maximum depth that laser-etching can reach. To measure transverse beam coherence, we use a Youngs two-slit interference setup. By measuring the fringe visibility for various slit separations, we can extract the transverse coherence as a function of displacement across the beam. However, this requires many data runs for different slit separations. Our solution to this problem is a novel approach to transverse coherence measurements: a modified Michelson interferometer. Flipping the beam left-right on one arm, we can interfere the beam with its own mirror-image and characterise the transverse coherence across the beam in a single shot. Keywords: materials processing, ultrafast laser, pulsetrain-burst, laser micromachining, coherence, beamspread, interferometer 1. INTRODUCTION It has been shown that lasers often confer special advantages in making tiny holes and vias. These tiny channels are used in many applications (interconnections between layers in used in the semiconductor industry, fabrication of microfluidics, fuel injectors, etc.). Ultrafast lasers do a particularly good job for many special materials or needs, particularly glasses, ceramics, fine processing of metals, and transparent biotissues. Among methods of ultrafast-laser processing, it has been shown that pulsetrain-burst processing, using a series of ultrafast laser pulses, at a repetition rate of 1 MHz or greater, is less likely to leave cracks or residual stress in the target material, and will instead typically result in an atomically smooth locally re-melted surface. It is important to identify what factors determine the limits for hole-size and depth, for these lasers. A number of other experimenters have looked at this issue, considering, for example, the relation between hole Send correspondence to: R.S. Marjoribanks, E-mail: marj@physics.utoronto.ca, Telephone: 416 978 6769 c current address: Universit¨at Heidelberg, Seminarstr. 2, 69117 Heidelberg Photonics North 2006, edited by Pierre Mathieu, Proc. of SPIE Vol. 6343, 63432A, (2006) 0277-786X/06/$15 · doi: 10.1117/12.707967 Proc. of SPIE Vol. 6343 63432A-1