In order to obtain a rapid estimate of the quality of a lens by observing its behaviour in the interferometer, it is necessary for the observer to be familiar with the characteristic patterns of the various primary aberrations under various conditions of adjustment. A method is described by which these patterns can be easily calculated for any required case; and series of typical patterns are shown together with actual interferograms for comparison.

The interferometer patterns due to the primary aberrations

Although new affordable high-power laser technologies enable many processing applications in science and industry, depth control remains a serious technical challenge. In this Letter we show that inline coherent imaging (ICI), with line rates up to 312 kHz and microsecond-duration capture times, is capable of directly measuring laser penetration depth, in a process as violent as kW-class keyhole welding. We exploit ICI’s high speed, high dynamic range, and robustness to interference from other optical sources to achieve automatic, adaptive control of laser welding, as well as ablation, achieving 3D micron-scale sculpting in vastly different heterogeneous biological materials.

Automatic laser welding and milling with in situ inline coherent imaging.

An apparatus, method, and process that includes a substantially transparent substrate having a first surface, a second surface, and edge extending around at least a portion of a perimeter of the substantially transparent substrate, wherein the edge being a laser induced channel edge having enhanced edge characteristics.

Apparatus, method, and process with laser induced channel edge

Femtosecond laser micromachining together with Laser Induced Breakdown Spectroscopy (LIBS) allows us to drill precise hole in materials to internal buried layers as well as characterize the materials while drilling. We report detection of a metal layer buried deep inside silicon by creating an access hole through the semiconductor. We used 800 nm femtosecond laser pulses to carry out the drilling while monitoring the plasma emission with a spectrometer system. Higher drilling rates of 1 μm per shot were achieved using a Gaussian laser beam profile with peak fluences of 42 J/cm2. Lower drilling rates of 30 nm per pulse with better accuracy could be achieved using lower intensity flat top beam profiles at fluences of 1.4 J/cm2.

Detection of buried layers in silicon devices using LIBS during hole drilling with femtosecond laser pulses

The micro structural morphological investigations of the laser exposed samples of Platinum are presented. Q-Switched Nd: YAG laser (1064 nm, 1.1 MW, 9 - 14 ns) represented by Gaussian profile, power density 3 × 1015 Watt/m2 and focal spot size 12 um is used to irradiate the targets (4 N, 1 × 1 × 0.3 cm3). Surface modifications are observed and examined for optimized 50 pulses in air (1 atm) as well as under vacuum (~10–3 torr) by analyzing SEM micrographs. Ripples, cones, crater and hillocks formation, splashing, sputtering, solidification and redeposition are observed as main modifications at the irradiated surface. It is explored that material is ejected with explosive expel. Motic digital microscope is used for the measurements of ablated micron sized droplets. The average distance between the adjacent cones is larger near the crater region. Topographical changes are characterized applying Atomic Force Microscopy. RMS surface roughness, hillocks and crater sizes on the irradiated surfaces are also calculated. The structural analysis is mainly focused on measurements of grain sizes, diffracted X-Rays intensity and interplanar distance. The results thus obtained determine that IR radiations are unable to change interplanar distance of the target where as changes in diffracted x-rays intensity and grain sizes for irradiated platinum are noticed.

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Microstructural Morphological Changes in Laser Irradiated Platinum

When laser-etching channels through solid targets, the etch-rate is known to decrease with increasing depth, partly because of absorption at the sides of the channel. For ultrafast-laser pulses at repetition rates >100MHz, we show that the etch-rate is also affected by optical properties of the beam: the channel acts as a waveguide, and so the pulses will decompose into dispersive normal modes. Additionally, plasma on the inner surface of the channel will cause scattering of the beam. These effects will cause a loss of spatial coherence in the pulse, which will affect the propagated intensity distribution and ultimately the etch-rate. We have characterized this effect for various foil thicknesses to determine the evolution of the beam while drilling through metal.

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The effects of degraded spatial coherence on ultrafast-laser channel etching.

The transverse coherence of a 1 ps pulsed laser beam was measured using a technique involving a modified Michelson interferometer and separate reference images. Using this technique, the transverse coherence of a selected plane in the laser beam was determined, in this case at the exit of a channel in a metal foil self-drilled by the laser. Images of each arm were used as references. Through this technique, it is possible to use the interference patterns produced with uneven intensity distributions and for pulsed lasers on a single-shot basis. The results of these measurements were then shown to be in agreement with those obtained using a Young's double-slit setup.

Transverse coherence measurement using a folded Michelson interferometer.

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 without 
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.

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Optical coherence and beamspread in ultrafast-laser pulsetrain-burst hole drilling

When etching channels with ultrafast laser pulses, the etch-rate is known to decrease with increasing depth. We show that this depth-saturation is partly due to loss of spatial coherence incurred during propagation through the channel.

Martin Bercx

Papers

The effects of degraded spatial coherence on ultrafast-laser channel etching.

Transverse coherence measurement using a folded Michelson interferometer.

Optical coherence and beamspread in ultrafast-laser pulsetrain-burst hole drilling

Optical Coherence Effects that Limit Ultrafast-Laser Channel Etching