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Berkeley Lab mixes different color fiber lasers to form ultrashort pulses

25 Jul 2023

New technique set to boost the potential of laser-plasma accelerators.

A team of researchers from the Accelerator Technology & Applied Physics (ATAP) Division at Berkeley Lab, California, has developed a new technique for combining fiber lasers operating at different wavelengths to produce ultrashort laser pulses.

The team says the work could advance the development of laser-plasma accelerators (LPAs), which have the potential to push the frontiers of high-energy physics and enable discoveries in materials science, fusion research, and many other areas.

LPAs use intense, ultrafast laser pulses passing through a plasma to accelerate charged particles up to a thousand times faster than current technologies. They promise more compact and powerful machines that are less expensive to build and operate than conventional accelerators.

The research was published in Optics Express.

Currently, most LPAs use laser pulses with repetition rates of only a few Hertz; however, realizing the full potential of LPAs “will require high-power laser systems capable of generating ultrashort, high-energy laser pulses at repetition rates in the kHz range or higher,” said Siyun Chen, a researcher at ATAP’s BELLA Center, who led the experimental demonstration of the new technique.

These constraints, added Chen, place demanding requirements on the laser systems that generate such pulses. So, the researchers turned to fiber lasers, which she explained are the “most efficient high-power laser technology demonstrated to date and also have extensive industrial development that could be leveraged in our work.”

Although the energy and power of pulses produced by fiber lasers can be scaled up by combining multiple pulses in space and in time, these pulses, however, are currently limited to about 100 fs, which are not short enough to drive LPAs.

“While fiber laser systems offer the highest wall-plug efficiencies—the electrical-to-optical power efficiency—the spectrum of ultrashort laser pulses amplified in these systems narrows,” explained Tong Zhou, a research scientist in ATAP’s BELLA Center who led the development of the new technique.

Spectral combination

“This gain narrowing is a fundamental effect when laser pulses are amplified in this way; the narrower the pulse’s spectrum is, the longer its duration. Consequently, it is very challenging for high-power fiber lasers to generate pulses shorter than about a hundred fs.”

However, by spectrally combining multiple laser pulses operating at adjacent wavelength ranges, the team, which also included Qiang Du from the Engineering Division and Dan Wang and Russell Wilcox from ATAP, achieved an ultra-broad combined spectrum able to support very short pulses at tens of femtoseconds.

To increase the bandwidth and produce tens-of-fs-long pulses, the researchers first used a mode-locked oscillator and ytterbium-doped fiber amplifier to generate pulses of 120 fs at 100 MHz repetition rates. These were sent to a photonic-crystal fiber, where their spectrum was broadened from 27 nm to 90 nm.

“This ultra-broadband spectral combining with synthesized pulse shaping produced pulses of only 42 fs in duration, which is significantly shorter than the pulses generated from each of the three fiber channels,” said Chen. “We believe this is the shortest pulse duration ever achieved from a spectrally combined ytterbium fiber laser system.”

Zhou said, “While the work has demonstrated ultrafast pulses that are so far at low energy, it demonstrates the key principles of ultra-broadband spectral combining and coherently spectrally synthesized pulse shaping and provides a path forward for using fiber lasers to drive LPAs.”

The team plans to add more amplification stages and implement multidimensional techniques capable of spatially, temporally, and spectrally combining fiber lasers to produce high-energy, tens-of-fs laser pulses.

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