New Technology Combines Fiber Lasers Of Different Wavelengths To Produce Ultrashort Laser Pulses

Recently, a group of researchers from Berkeley Lab's Accelerator Technology and Applied Physics (ATAP) division announced the development of a new technique that can combine fiber lasers of different wavelengths to produce ultrashort laser pulses.
This work could advance the development of laser plasma gas pedals (LPAs), which have the potential to push the frontiers of high-energy physics and enable discoveries in materials science, fusion research and many other fields.
Laser plasma gas pedals (LPAs) use intense pulses of ultrafast laser light to accelerate charged particles through a plasma at speeds up to 1,000 times faster than current technology, enabling the creation of machines that are more compact and powerful than conventional gas pedals, and less expensive to build and operate.
Currently, most laser plasma gas pedals (LPAs) use laser pulses with repetition frequencies of only a few hertz (Hz). However, realizing the full potential of LPAs requires high-power laser systems capable of generating ultrashort, high-energy laser pulses at repetition frequencies of kHz or higher.
These limitations place very demanding requirements on the laser systems that generate such pulses. As a result, the researcher turned to fiber lasers, which she says are "by far the most efficient high-power laser technology and also have extensive industrial development that can be exploited in our work."
While the energy and power of pulses generated by fiber lasers can be amplified by combining multiple pulses in space and time, these pulses are currently limited to about 100 femtoseconds (fs), which is not enough to drive laser plasma gas pedals (LPAs).
Research scientists at the BELLA Center, part of ATAP, explain, "While fiber laser systems provide the highest electro-optical power efficiencies - the spectrum of ultrashort laser pulses amplified in these systems narrows."
"When laser pulses are amplified in this way, gain narrowing is a fundamental effect; the narrower the spectrum of the pulse, the longer the duration. As a result, it is very challenging for high-power fiber lasers to generate pulses shorter than 100 seconds."
However, by spectrally combining multiple laser pulses operating in adjacent wavelength ranges, the team (including Qiang Du of Engineering and Dan Wang and Russell Wilcox of ATAP) achieved an ultra-wide combined spectrum capable of supporting very short pulses of tens of seconds.
To increase the bandwidth and generate pulses tens of seconds long, the researchers first used a mode-locked oscillator and a ytterbium-doped fiber amplifier (YDFA) to generate 120-second pulses at a repetition rate of 100 MHz. They were sent into a photonic crystal fiber, and the spectrum of these laser pulses expanded from 27 nanometers (nm) to 90 nm.
They then used a dichroic mirror, which allows the laser pulses to be separated or combined without significant loss of intensity, thus splitting the pulses spectrally. They were then sent to two pulse shapers to shape the intensity and phase of the respective pulse spectra. When the reflected pulses are sent to the first shaper, the emitted pulses are amplified by the YDFA, the pulses are formed by the second shaper, and further split by another dichroic mirror. The three chirped pulses from the fiber laser are then amplified and recombined using additional dichroic mirrors.
This ultra-broadband spectrum combined with synthetic pulse shaping produces pulses with a duration of only 42 seconds, which is much shorter than the pulses produced by the three fiber channels. According to the team, this is the shortest pulse laser duration obtained from a spectrally combined ytterbium fiber laser system.
While this work has demonstrated ultrafast pulses at low energies so far, it shows the key principles of ultrawideband spectral combining and coherent spectral synthesis pulse shaping, and provides a breakthrough in the use of fiber lasers to drive laser plasma gas pedals (LPAs). In the future, the team plans to add more amplification stages and implement multidimensional techniques capable of combining fiber lasers spatially, temporally, and spectrally to generate tens of seconds of high-energy laser pulses.