Combined spectral lasers can unlock the potential of laser plasma accelerators

A team of researchers in Berkeley Lab's Accelerator Technology and Applied Physics (ATAP) division has developed a new technique that combines fiber lasers of different wavelengths to generate ultra-short laser pulses. The research is in the journal Optics Letters.

This work could advance the development of laser plasma accelerators (LPA), which have the potential to push the frontiers of high-energy physics and facilitate discoveries in materials science, fusion research, and many other fields.

LPA uses strong ultrafast laser pulses passing through plasma to accelerate charged particles a thousand times faster than current technology. They promise to deliver more compact and powerful machines that are cheaper to build and operate than traditional accelerators.

Currently, most Lpas use laser pulses with a repetition rate of just a few Hertz (Hz); However, to realize the full potential of LPA, "high-power laser systems capable of producing ultra-short, high-energy laser pulses in the kHz range or higher repetition rates are required," said Siyun Chen, a research scientist at ATAP's Bella Center, who led the experimental demonstration of the new technology.

Chen added that these limitations put very demanding demands on laser systems that generate such pulses. So the researchers turned to fiber lasers, which she says are "the most efficient high-power laser technology demonstrated to date, and also have a wide range of industrial developments that can be exploited in our work."

While the energy and power of pulses produced by fiber lasers can be amplified by combining multiple pulses on space (space) and time (time), these pulses are currently limited to about a hundred femtoseconds (fs), which is not enough to drive LPA.

"Although fiber laser systems offer the highest electro-optic conversion efficiency (i.e., electro-optic power efficiency), the spectrum of ultrashort-short laser pulses amplified in these systems gets narrower," explains Tong Zhou, a research scientist at the ATAP BELLA Center.

"When the laser pulse is amplified in this way, the gain narrowing is a fundamental effect; The narrower the spectrum of the pulse, the longer its duration. Therefore, it is very challenging for high-power fiber lasers to generate pulses shorter than about a hundred femtoseconds."

However, by combining the spectra of multiple laser pulses operating in the adjacent wavelength range, the team, which also includes Qiang Du from the engineering department and Dan Wang and Russell Wilcox from ATAP, achieved ultra-wide combined spectra capable of supporting very short pulses of tens of fs.

To increase the bandwidth and generate pulses tens of fs long, the researchers first used mode-locked oscillators and ytterbium-doped fiber amplifiers (YDFA) to generate pulses of 120 fs at a 100 MHz repetition rate. These are sent to the photonic crystal fiber, whose spectrum widens from 27 nanometers (nm) to 90 nanometers.

They then used dichroic mirrors to separate or combine laser pulses without a significant loss of intensity, splitting the pulses spectroscopically. They are then sent to two pulse shapers to shape the intensity and phase of the corresponding pulse spectrum. When the reflected pulse is sent to the first shaper, the transmitted pulse is amplified by the YDFA, pulse shaper by the second shaper, and further split by another dichroic mirror. An additional dichroic mirror is then used to amplify and recombine the three chirped pulses from the fiber laser.

"This ultra-wideband spectrum combined with synthetic pulse shaping produces pulses with a duration of only 42 fs, which is significantly shorter than the pulses generated in each of the three fiber channels," Chen said. "We believe this is the shortest pulse duration ever achieved by a spectrally combined ytterbium fiber laser system."

Zhou notes, "While this work demonstrates ultrafast pulses at low energies so far, it demonstrates a key principle of ultra-wideband spectral combination and coherent spectral synthesis pulse shaping, and provides a way forward for using fiber lasers to drive LPA."

The team plans to add more amplifiers and implement multidimensional techniques that can combine fiber lasers in space, time, and spectrum to produce high-energy, tens of femtosecond laser pulses.

Commenting on the work, Cameron Geddes, ATAP Division Director, said, "It demonstrates how ATAP researchers are driving the development of advanced particle accelerators that hold the promise of making discoveries in basic science research and breakthroughs in fusion, medicine, materials science and many other fields."

Source: Laser Network