Ablation-cooled material removal at high speed with femtosecond pulse bursts

Abstract

We report exploitation of ablation cooling, a concept well-known in rocket design, to remove materials, including metals, silicon, hard and soft tissue. Exciting possibilities include ablation using sub-microjoule pulses with efficiencies of 100-µJ pulses.OCIS codes: 140. 3390, 060.1510, 320.7090 Femtosecond pulses hold great promise for high-precision material and tissue processing. It is well-known that use of such pulse durations allows very precise and virtually thermal-damage free material removal under appropriate conditions. However, two drawbacks remain. The first is the limited ablation rates, which is particularly limiting for biological tissue removal. The second is that at several microjoules, but often 10's to 100's of microjoules of pulse energy are required. Regarding speed of ablation, the physics of the laser-material interaction precludes a straightforward scaling up of the removal rate simply by sending pulses more frequently, since this leads to collateral damage due to heat accumulation.Here, we exploit the same physics to circumvent this limitation. We apply rapid succession of ultrafast pulses from a burst-mode fibre laser to ablate the target before residual heat deposited by the previous pulse can diffuse away from the interaction region. In addition to being able to process thermally sensitive materials, such as brain tissue, at high speed and without thermal damage, our approach reduces the required pulse energy by orders of magnitude, opening the route to highly efficient material removal with oscillator-level pulse energies.For a long time, it was commonly assumed that heat effects are nearly completely eliminated through the use of ultrafast pulses. Heat damage can indeed occur during ultrafast pulse processing as a result of pulse-to-pulse accumulation of residual heat that is deposited around the border of the ablated region by each pulse. While deposition of some residual heat by each pulse is unavoidable, we present here a laser system that can catch much of this heat before it can diffuse beyond the volume to be ablated by the next incoming pulse by operating at very high repetition rates. This brings three interrelated advantages: (1) Most of the residual heat left by the previous pulse has not yet diffused out of the volume to be ablated by the next pulse. Thus, each pulse targets an already hot volume, which lowers the required ablation energy and peak power with numerous side benefits, such minimizing plasma shielding, reducing shock wave, cavitation bubble formation and self-focusing. In addition, the quantity of residual heat is proportional to the pulse energy, thus reducing the magnitude of the problem to be solved. (3) Finally and most importantly, much of this residual heat is then carried away from the tissue in the form of ablated matter when ablated by the next pulse, reducing the build-up of heat from pulse to pulse. This is known as ablation cooling, which is very well known in the context of atmospheric entry of meteorites or reusable space rockets ...

Soliton Booting Dynamics in an Ultrafast Anomalous Dispersion Fiber Laser

“…The ability of generating ultrashort pulses enables fast development of ultrafast science and technology, which thus, in turn, motivates laser scientists to search for high-performance ultrafast lasers with desirable features for practical applications [1][2][3] . The ultrashort pulses can be generated from the lasers by the principle of passive mode-locking technologies [4][5][6][7] .…”

3.5-GHz intra-burst repetition rate ultrafast Yb-doped fiber laser

“…The system produces bursts of pulses at μ 1 m wavelength and is capable of generating as short as 15 ns-long bursts with intra-burst pulse repetition rate of 3.5 GHz with μ 215 J energy at 1 kHz repetition rate. This combination of parameters constitute a unique capability, which has been specifically developed to exploit ablation-cooled laser material removal [21,22]. We expect the laser architecture demonstrated here to find applications in ultrafast laser-material and laser-tissue processing.…”

“…Recently, we reported a detailed investigation on the limits of pulsepumped [19] and continuously pumped [20] all-fiber burst mode laser system. By focusing on the pulse-material interaction physics, we have identified a new regime of laser-material interaction, which we refer to as ablation-cooled laser material removal [21,22]. In this regime, the repetition rate has to be high enough that there is insufficient time for the targeted spot size cool down substantially by heat conduction into the rest (bulk) of the target material by the time the next pulse arrives.…”

Generation of h–Shaped pulse in a mode–Locked erbium-doped fiber laser

“…Passively mode-locked fiber lasers (MLFLs) have attracted a lot of interest in the area of academia and industry in recent decades, for instance, optical sensing [1,2], materials micromachining [3,4] and biomedicine [5]. A strong nonlinear effect exists in MLFLs as a result of the unique structure of the fiber core.…”