Non-thermal material and tissue processing with 100 MHz and 500 MHz repetition rate bursts

Abstract

There are a number of applications that would avail a pulse pattern in the form of closely grouped and uniformly spaced pulses, i.e., bursts [1]. Closely grouped pulses with pulse to pulse separation in the order of a few nanoseconds have a potential for increasing material removal rates [2] and thereby reducing the thermal effects. Besides, keeping the burst repetition period in the order of thermal relaxation time has the advantage of keeping the overall average power at lower levels in order to prevent the cumulative heating of the material.Here, we demonstrate results on ablation efficiency for three different pulse repetition rates for Cu targets and results on human dentine samples to emphasize the detrimental thermal effects for low repetition rate mode. We used custom built Yb fiber amplifier, which produces bursts at an overall repetition rate up to 1 kHz comprising of 25 pulses each with up to tens of μJs of energy, ∼1ps pulse duration and intra-burst repetition rate of 100 MHz and 500 MHz ( Fig. 1(a)). The system can also produce pulses with 25 kHz uniform repetition rate (not bursts), which we used it for our low repetition rate mode. We kept the individual pulse energy, average power, pulse width, spot size and total number of incident pulses (hence total processing time) strictly constant. Fig. 1(b) shows results obtained on a 100 μm thick Cu targets in ambient atmosphere at room temperature using the three modes of operations, where we have characterized the depth of ablation per pulse. Fig. 1(c) shows results obtained on human dentine, where areas of 1 mm 2 in each sample were raster scanned with scan velocity of 0.1 mm/s. Comparison of material ablation rate per pulse for copper target using 25 pulses at repetition rates of 25 kHz, 100 MHz, and 500 MHz; showing that ablation per pulse increases by ∼6 times at higher repetition rates. (c) Human dentine samples processed using uniform repetition rate of 25 kHz (i), and 25-pulse bursts at 1-kHz burst repetition rate with 100-MHz intra-burst (ii) and 500-MHz intra-burst (iii) repetition rate. Average power, processing time, pulse width, pulse energy, spot size are the same in each case.In conclusion, we have developed a mJ-level burst-mode integrated fiber amplifier with high in-burst repetition rates, and compressed pulses of ∼1 ps. Our micromachining results demonstrate almost an order of magnitude increase in volume of ablated material per pulse at higher repetition rates, along with much reduced thermal effects. References

Generation of picosecond pulses directly from a 100 W, burst-mode, doping-managed Yb-doped fiber amplifier

“…This was followed by [9], which broke the 1 mJ energy per barrier, with individual pulse energies of 40 μJ. The expected increase in ablation rate has been confirmed with this system [10]. Both of these systems were built using off-theshelf components and step-index fibers, operated at 1 kHz, corresponding to a modest average power of 1 W. Using rod-type fibers, Breitkopf et al, have obtained 58 mJ burst energy at 20 Hz repetition frequency with the average power of ∼1.6 W [11].…”

Ultrafast micromachining of Cu and Si at ultra-high repetition rates with pulse bursts

“…The amplified bursts contain about 15 -35 pulses of 5 μJ average and 8 μJ maximum individual energy. Previously, we have observed significant increases of efficiency with in-burst rates of 100 and 500 MHz compared to the uniform repetition rate [2]. Fig.2 shows ablation depth and volume per burst energy (bursts constituting ~15 pulses for copper and ~30 pulses for silicon) for different pulse repetition rates inside a burst.…”

Efficient, high-speed ablation of soft tissue with few-microjoule, femtosecond pulse bursts

“…Through the use of ultrafast pulses delivered in burst mode, a completely new interaction regime opens up that holds great potential for use of the laser as a precise scalpel for tissue removal applications, and the possibility to overcome the collateral damage and ablation rate challenges simultaneously 15,16 . This mode of operation has already been shown to increase the ablation rate for metals 17,18 and hard tissue, e.g. tooth samples 15,18 , as compared to the application of pulses at a lower repetition rate.…”

“…This mode of operation has already been shown to increase the ablation rate for metals 17,18 and hard tissue, e.g. tooth samples 15,18 , as compared to the application of pulses at a lower repetition rate. Here, we report on a novel Yb-doped fiber laser operating in the burst mode, as well as uniform repetition rate and assess the ablation performance of burst mode on soft-tissue from a medical perspective.…”