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

Ultrafast laser pulses ( ≤ 1 ps) are qualitatively different in the nature of their interaction with materi- als, including biotissues, as compared to nanosecond or longer pulses. This can confer pronounced advan- tages in outcomes for tissue therapy or laser surgery. At the same time, there are distinct limitations of their strong-field mode of interaction. As an alternative, it is shown here that ultrafast laser pulses delivered in a pulse-train burst mode of radiant exposure can access new degrees of control of the interaction process and of the heat left behind in tissues. Using a laser system that delivers 1 ps pulses in 20 μ s pulse-train bursts at 133 MHz repetition rates, a range of heat and energy-trans- fer effects on hard and soft tissue have been studied. The ablation of tooth dentin and enamel under various conditions, to assess the ablation rate and character- ize chemical changes that occur, are reported. This is compared to ablation in agar gels, useful live-cell-cul- ture phantom of soft tissues, and presenting different mechanical strength. Study of aspects of the optical science of laser-tissue interaction promises to make qualitative improvements to medical treatments using lasers as cutting and ablative tools.

Ablation and thermal effects in treatment of hard and soft materials and biotissues using ultrafast-laser pulse-train bursts

A 3D living-cell culture in hydrogel has been developed as a standardized low-tensile-strength tissue proxy for study of ultrafast, pulsetrain-burst laser-tissue interactions. The hydrogel is permeable to fluorescent biomarkers and optically transparent, allowing viable and necrotic cells to be imaged in 3D by confocal microscopy. Good cell-viability allowed us to distinguish between typical cell mortality and delayed subcellular tissue damage (e.g., apoptosis and DNA repair complex formation), caused by laser irradiation. The range of necrosis depended on laser intensity, but not on pulsetrain-burst duration. DNA double-strand breaks were quantified, giving a preliminary upper limit for genetic damage following laser treatment.

Pulsetrain-burst mode, ultrafast-laser interactions with 3D viable cell cultures as a model for soft biological tissues.

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

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.

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.

/pdf/optical-coherence-and-beamspread-in-ultrafast-laser-2ldiwn6da3.pdf

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

Ultrafast-laser micromachining has promise as an approach to trimming and 'healing' small laser-produced damage sites in laser-system optics--a common experience in state-of-the-art high-power laser systems. More-conventional approaches currently include mechanical micromachining, chemical modification, and treatment using cw and long-pulse lasers. Laser-optics materials of interest include fused silica, multilayer dielectric stacks for anti-reflection coatings or high-reflectivity mirrors, and inorganic crystals such as KD*P, used for Pockels cells and frequency-doubling. We report on novel efforts using ultrafast-laser pulsetrain-burst processing (microsecond bursts at 133 MHz) to mitigate damage in fused silica, dielectric coatings, and KD*P crystals. We have established the characteristics of pulsetrain-burst micromachining in fused silica, multilayer mirrors, and KD*P, and determined the etch rates and morphology under different conditions of fluence-delivery. From all of these, we have begun to identify new means to optimize the laser-repair of optics defects and damage.

Mitigating intrinsic defects and laser damage using pulsetrain-burst (>100 MHz) ultrafast laser processing

Ultrafast-laser pulsetrain-burst processing (microsecond bursts at 100 MHz) is an option for fluence-delivery which leads to exceptional characteristics of morphology and processing-rates; we describe the background science and application to mitigation of high-power laser-induced damage.

Ultrafast laser pulsetrain-burst (≫100 MHz) processing of glasses for laser-damage mitigation and photonic applications

Damage to optics caused by lasers may be mitigated by material-processing with ultrafast lasers. Ultrahigh (<100 MHz) repetition-rate pulsetrains afford special control of residual heat. We describe studies in fused silica and crystals.

Felix Frank

Papers

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

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

Mitigating intrinsic defects and laser damage using pulsetrain-burst (>100 MHz) ultrafast laser processing

Ultrafast laser pulsetrain-burst (≫100 MHz) processing of glasses for laser-damage mitigation and photonic applications

Mitigating laser damage in glasses and crystals, using pulsetrain-burst (<100 MHz) ultrafast laser processing