The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific, medical and industrial applications. However, its potential is limited by the low speeds at which material can be removed and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk.

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Ablation-cooled material removal with ultrafast bursts of pulses

Ultrafast lasers operating at high repetition rates, in particular the GHz range, enable new possibilities in laser-material processing, particularly accessing the recently demonstrated ablation-cooled regime. We provide a unified perspective of the unique opportunities created by operating at high repetition rates together with our efforts into the development of enabling laser technology, including new results on further scaling up the capabilities of the laser systems. In order to access GHz repetition rates and microjoule-level pulse energies without requiring kilowatts of average power, we implement burst-mode operation. Our results can be grouped into two distinct directions: low- and high-power systems. Pulsed pumping is employed in the later stages of low-power systems, which have low burst repetition rates to achieve high pulse energies, whereas the technique of doping management is developed for the continuously pumped power amplifier stage of high power systems. While most of the developments have been at 1- $\mu$ m wavelength range due to the relative maturity of the laser technology, we also report the development of Tm-fiber lasers around the 2- $\mu$ m region specifically for tissue processing and laser-surgery applications.

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High-Repetition-Rate Ultrafast Fiber Lasers for Material Processing

We report on silicon ablation with a 20 W GHz amplified femtosecond laser source. This novel laser delivers burst energies up to 400 μJ, providing flexible intra-pulse repetition rates of 0.88 or 3.52 GHz, up to 200 pulses with ∼350  fs pulse duration. High-efficiency, high-quality ablation can be achieved through optimally determining the number of pulses, intra-pulse repetition, and average pulse energy within a burst. Due to such optimization, we demonstrate a specific ablation rate of 2.5  mm3/min/W with a burst containing 200 pulses at 0.88 GHz, which is the highest one reported so far for fs laser ablation, to the best of our knowledge. GHz ablation is sensitive to the selection of laser parameters. We conceptually discuss the contributions of the pulses within a burst to heat-accumulation-based incubation and material ablation.

High-efficiency femtosecond ablation of silicon with GHz repetition rate laser source.

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

High power optical pulses generating methods and laser oscillators are provided. A light generating module generates seed optical pulses having predetermined optical characteristics. A spectrum tailoring module is then used to tailor the spectral profile of the optical pulses. The spectral tailoring module includes a phase modulator which imposes a time-dependent phase variation on the optical pulses. The activation of the phase modulator is synchronized with the passage of the optical pulse therethough, thereby efficiently reducing the RF power necessary to operate the device.

Spectrally tailored pulsed fiber laser oscillator

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

Energy absorption and heat transfer are important factors for regulating the effects of ablation of biological 
tissues. Heat transfer to surrounding material may be desirable when ablating hard tissue, such as teeth or bone, 
since melting can produce helpful material modifications. However, when ablating soft tissue it is important to 
minimize heat transfer to avoid damage to healthy tissue - for example, in eye refractive surgery (e.g., Lasik), 
nanosecond pulses produce gross absorption and heating in tissue, leading to shockwaves, which kill and thin the 
non-replicating epithelial cells on the inside of the cornea; ultrafast pulses are recognized to reduce this effect. 
Using a laser system that delivers 1ps pulses in 10μs pulsetrains at 133MHz we have studied a range of heat- and 
energy-transfer effects on hard and soft tissue. We describe the ablation of tooth dentin and enamel under 
various conditions to determine the ablation rate and chemical changes that occur. Furthermore, we characterize 
the impact of pulsetrain-burst treatment of collagen-based tissue to determine more efficient methods of energy 
transfer to soft tissues. By studying the optical science of laser tissue interaction we hope to be able to make 
qualitative improvements to medical treatments using lasers.

Effects of heat transfer and energy absorption in the ablation of biological tissues by pulsetrain-burst (>100 MHz) ultrafast laser processing

Effects of irradiating dental hard tissue with an ultrashort pulsetrain-burst (≫100 MHz) laser are studied. The ablation rate is investigated as a function of the pulsetrain duration. Material modification is characterized using micro-Raman spectroscopy.

Ablation of hard dental tissue using ultrashort pulsetrain-burst (≫100MHz) laser

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

Effects of irradiating dental hard tissue with an ultrashort pulsetrain-burst (>100MHz) laser were studied. The ablation rate was measured and the effects of dividing the pulsetrains were examined. Material modification was measured using micro-Raman spectroscopy.

Paul Forrester

Papers

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

Effects of heat transfer and energy absorption in the ablation of biological tissues by pulsetrain-burst (>100 MHz) ultrafast laser processing

Ablation of hard dental tissue using ultrashort pulsetrain-burst (≫100MHz) laser

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

Ablation of Dental Hard Tissue with an Ultrashort Pulsetrain-Burst (>100MHz) Laser