A 50 W sub-picosecond fiber chirped pulse amplification system generating 50 µJ pulses at a repetition rate of 1 MHz is demonstrated. As required for precision high speed micro-machining, this system has a practical system configuration enabled by the fiber stretcher and 1780 l/mm dielectric diffraction grating compressor and is capable of ablation rates >0.17 mm3/s metal, ceramic, and glass.
©2006 Optical Society of America
Despite the well-documented potential of ultrashort pulse laser systems to greatly improve processing precision, practical and economic issues have limited wide scale commercial application of this technology. To be practical, an ultrashort laser must provide nearly diffraction-limited beam quality in addition to reliable and stable performance. Due to their inherent complexity and cost, such a laser must also be capable of rapid processing in order to be economically viable. The search for higher speed processing has motivated a wide range of research efforts to scale up average power without sacrificing beam quality or system reliability. Ultrafast fiber lasers have many practical advantages over conventional Ti:sapphire lasers, due to their robust and compact nature  and high power handling capabilities [2, 3]. While the commercial reliability and stability of femtosecond fiber laser oscillators and low energy (≤10 µJ) amplified systems are well known , the development of practical high energy ultrafast fiber lasers has been limited by the high nonlinearities inherent in fibers and hence the large dispersion manipulation required for chirped pulse amplification (CPA). The generation of >100 µJ femtosecond pulses from fiber lasers was demonstrated by Galvanauskas et al. more than five years ago , however the >1000:1 pulse stretching ratios were produced using stretchers and compressors based on bulk optical gratings with matched dispersion compensation. The size and alignment tolerances of bulk gratings stretchers make them impractical for use in ultrashort pulse fiber lasers.
Here we report on a Yb-fiber CPA system which overcomes these limitations, producing sub-picosecond laser pulses with 50 µJ at 1 MHz and ≤1.4 M2. The key to the stability of this system is the integrated all-polarization maintaining (PM) fiber architecture. The power amplifier is designed to support cubicon pulse formation [6, 7], which is implemented to reduce third-order dispersion induced pulse wings in the compressed pulses arising from the highly mismatched stretcher and compressor. As a result the power fiber amplifier enables the generation of pulses with acceptable pulse quality up to nonlinearities of 10π. Compression to high average powers is enabled by the multilayer dielectric grating providing >94% diffraction efficiency, ~3.0 J/cm2 damage threshold, and high power handling capability .
Several high power ultrashort pulse laser systems have been previously reported, however many are limited to relatively low pulse energy [3, 9–11] or are incapable of generating sub-picosecond laser pulses . The system reported here has a robust, integrated fiber architecture and uniquely produces moderate energy sub-picosecond laser pulses and high average power with nearly diffraction-limited beam quality.
2. System performance
Utilizing a design similar to that described in our previously published work [6, 7], here we further investigate the power scalability of high energy cubicon amplification, increasing the laser system repetition rate to 1 MHz. As in our previous integrated PM fiber CPA systems [7, 13], 50 µJ was the maximum compressible pulse energy. While the available pump power limited our maximum repetition rate to 1 MHz, with sufficient pump power and appropriate pump coupling optics it should be possible to achieve the same pulse energy at the oscillator repetition rate of 45 MHz. Although the system was not optimized for compactness, the amplifier occupies an area less than 50×75 cm2 and the folded Treacy compressor fits within 30×60 cm2.
Figure 1 shows autocorrelation traces for varying output power, and the inset shows a representative spectrum with ~5 nm (FWHM) and 1038 nm peak wavelength. The stretched pulse is ~500 ps with a peak power of ~175 kW during amplification. As is characteristic of cubicon amplification, the pulse duration and pedestal decrease with increasing pulse energy such that the pulse duration is ~550 fs (FWHM) at 50 W given the 800 fs autocorrelation width and ~1.5 deconvolution factor . However, due to the relatively large 1780 l/mm compressor grating groove density, it was not possible to completely compensate the TOD mismatch between the stretcher and compressor. As we have previously shown [6, 7, 13] this residual TOD pulse pedestal can be considerably reduced by using a compressor grating with 1480 l/mm groove density; however such a dielectric compressor grating was not available for these experiments.
As shown in Fig. 2, the system is extremely efficient due to ~65% slope efficiency of the power amplifier and the >80% compressor throughput. As such, the power amplifier provides ~50% optical-to-optical efficiency from the diode pump to the compressed output. Figure 3 plots the M2 as a function of diode pump power, confirming that the beam quality remains ≤1.4 independent of laser power.
3. Micromachining experiments
In order to investigate the utility of this system for high speed high precision micromachining, a galvanometric scanning system was used to perform laser milling of aluminum, alumina, and glass targets. The scanning system includes a 125 mm focal length F-Theta lens, which produces a focal beam diameter of ~25 µm. Figure 4 shows the ablation rate versus the incident power measured after a single laser milling pass over a 5×5 mm2 area at 500 mm/s scan speed. Notably, the ablation rate is relatively independent of target material and is nearly linear with respect to laser power; however further investigation of these relationships are required as pulse energy and laser repetition rate could not be freely varied during these experiments.
Optical images of the laser milling results are shown in Fig. 5, where the individual scan lines are clearly visible with a 20 µm center-to-center separation. At 40 W incident power, with average fluence and intensity of ~8.0 J/cm2 and ~1.4×1013 W/cm2 respectively, the ablation rate is >0.17 mm3/s in all three materials. This is a significant increase over our previously published results  and, to the best of our knowledge, significantly exceeds the highest ablation rate published for a compact 4 MHz, 12 ps laser system producing 50 W at 532 nm . An SEM image of the aluminum target is shown in Fig. 6. The average depth is 35 µm and the milled region is irregular, except areas such as the line entering from the left that were only scanned once. The milling technique was not optimized for surface roughness or edge quality, thus it should be possible to significantly improve the milling quality with appropriate choice of laser parameters and debris management. Relative to our previous high-power femtosecond micromachining efforts [7, 13], the issue of debris management is more severe. Thus, it is difficult to determine the optimal machining technique/parameters and the corresponding optimized ablation rate at this initial point.
We demonstrate a robust ultrafast laser system which generates sub-picosecond laser pulses with 50 µJ pulse energy and 50 W average power while maintaining nearly diffraction-limited beam quality. As proof of this system’s utility, we investigate high speed precision laser milling of aluminum, alumina, and glass targets.
This laser system demonstrates techniques to resolve many of the problems that have prevented the large scale industrial acceptance of ultrashort laser processing through the use of an integrated PM fiber architecture. Cubicon amplification allows for the generation of high energy sub-picosecond laser pulses. The >50 W nearly diffraction-limited ultrashort pulse output enables truly high speed precision micromachining, as required for high volume industrial processing.
We would like to thank Zhendong Hu and Bing Liu for their assistance in obtaining SEM images, and Alan Arai, Ingmar Hartl and Oleg Bouevitch from IMRA America for their helpful discussions.
References and links
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