530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system
Published in Optics Letters, Vol. 38 Issue 13, pp.2283-2285 (2013)
Spotlight summary: Challenges to improve the performance of laser amplifiers have led both to the discovery of new physics and to the development of novel technology. One of the most exciting challenges has been to generate higher intensity pulses at a higher average power with higher stability, since this would open the door for new fundamental and industrial research such as high intensity physics and laser materials processing. Recently, it has been demonstrated that fiber laser amplifiers have better properties than solid state ones in terms of high stability and average output power, due to less drifting and the ease of reducing thermal load, respectively. Therefore, fiber laser amplifiers are good candidates for this challenge. However, as always, there are some limitations. In the case of femtosecond fiber laser amplifiers, nonlinear effects in the fiber determine the ultimate limitations, since they cause temporal and spatial pulse distortion as well as changes in the spectrum of the laser emission. Reducing these effects is therefore essential.
In this Letter, to overcome these limitations, the authors cleverly assemble a few existing technologies: chirped pulse amplification (CPA), coherent combination of spatially separated amplifiers, and large-pitch fibers (LPFs) that significantly reduce the peak power of pulses in the amplifier both in the temporal and spatial domains and prevent nonlinear effects. A brief description of the method is as follows. The authors first use a mode-locked solid-state oscillator to generate femtosecond seed pulses for amplification, and then stretch the seed pulses to about 2 ns using a grating stretcher. This significantly reduces the peak power of pulses prior to amplification. Once the pulses are stretched in the time domain, three pre-amplifiers and four main amplifiers are employed to amplify the stretched seed pulses. Particularly, the authors use six LPFs for two pre-amplifiers and four main amplifiers following one fiber-based pre-amplifier. Since LPFs can have a large effective mode area with single mode operation based on preferential gain only for the fundamental mode, a higher pulse energy can be held in them without compromising the quality of the pulses. Additionally, coherent combination of four spatially separated main amplifiers is used to further increase the pulse energy. This combination method consists of first dividing the pre-amplified pulse into four pulses, then amplifying each in a spatially separated amplifier, and finally recombining coherently these four pulses into one pulse. The separation into four pulses effectively increases the total beam size four times and also reduces thermal load, consequently leading to a higher pulse energy and average power in the amplifier. Following all the amplification processes, the amplified pulses are compressed in the time domain using a grating compressor, eventually leading to a gigawatt peak power with an average power of 530W.
In summary, the authors demonstrate the generation of gigawatt femtosecond pulses with half a kilowatt average power using a femtosecond fiber CPA system based on LPFs with coherent combination of spatially separated amplifiers. As the authors note, it is clearly expected that the coherent combination with the help of the development of fiber laser technology can further improve the performance of femtosecond laser amplifier in the future.
--Taek Yong Hwang
Technical Division: Optoelectronics
ToC Category: Fiber Optics and Optical Communications
|OCIS Codes:||(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators|
|(140.7090) Lasers and laser optics : Ultrafast lasers|
|(140.3298) Lasers and laser optics : Laser beam combining|
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