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QUB fires up massive ultrafast laser system

24 Jan 2007

The UK's largest-ever commercially built laser system, TARANIS, was recently installed at Queen's University Belfast, UK. OLE talks with the system's designers to find out more.

Queen's University Belfast (QUB) is home to a unique ultrafast laser system that can deliver focused spot intensities in excess of 1019 W/cm2. Installed in November, TARANIS (Terawatt Apparatus for Relativistic and Non-linear Interdisciplinary Science) is now set to play a key role in the research of warm dense matter and laser-accelerated protons. By every definition, including peak power, pulse energy and footprint, engineers claim that this is the largest commercially built laser system ever delivered in the UK.

Wish list

The process began when QUB scientists Ciaran Lewis, Matthew Zepf, Marco Borghesi and Dave Riley began to define an ultrafast amplifier system that would jointly meet their research needs (see table "QUB's requirements"). "From the outset we knew that we needed flexibility, but at the same time we wanted a highly reliable, workhorse-like system for performing hard science," Zepf told OLE. "We wanted hands-free operation, so that as little time as possible would be spent on adjusting and maintaining the laser."

In addition, the team needed to tightly synchronize the output pulses of the new system to an existing home-built Ti:sapphire multistage amplifier. "This home-built set-up produces hundreds of millijoules with a pulse width compressible to 50 fs for a peak power of 8 TW," explained Zepf. "We wanted to use the two systems in pump-probe-type experiments."

Keen to focus on its research, the group wanted an off-the-shelf solution and selected Coherent, US, as the vendor.

Timing issues

To maximize the temporal resolution of the final set-up, it was important to keep the timing jitter to below 800 fs, which corresponds to the minimum compressed pulse width of the new high-energy system. The firm's solution was to supply a new 800 nm Ti:sapphire oscillator (Coherent Mira 900F) equipped with a SynchroLok system to seed the existing QUB home-built Ti:sapphire multistage amplifier. The SynchroLok locks the pulsing of the 800 nm Ti:sapphire oscillator with the 1053 nm Ti:sapphire oscillator used to seed the new glass phosphate amplifier system (see figure "Schematic view").

The oscillators are passively mode-locked using Kerr-lens mode-locking, which allows very tight synchronization to another ultrafast oscillator or to an external clock. Synchronization is accomplished by the use of fast photodetectors and automated real-time control of up to three cavity mirrors in each oscillator. The end result is a laser–laser jitter of typically less than 100 fs with a guaranteed specification of <200 fs.

Technical hurdles

A number of significant challenges had to be overcome before the final system could be assembled, some of which involved trade-offs dictated by fundamental limitations of laser physics.

"Pulse width and spectral bandwidth are generally the major trade-offs in any ultrafast system, but with a chirped pulse amplifier [CPA] chain at high power, the situation is complicated by gain-narrowing and gain-saturation effects in the various amplifiers," explained Steve Edstrom, product line manager at Coherent. "One of QUB's most critical needs was the ability to generate a focused spot with a very high intensity, which simultaneously requires high pulse energy and short-pulse duration coupled with high beam quality."

QUB had asked for a flexible system with parallel amplifier chains each delivering a final output of up to 20 J/pulse. To suit the various research needs, the group wanted turnkey access to transform-limited pulses from various locations between the regenerative amplifier and the four successive multipass amplifiers and final compressor.

Edstrom adds that it is important to define spectral bandwidth early on in the design process. "Typical commercial CPA systems based on glass amplifiers deliver a bandwidth of 6–7 nm at the pulse stretcher stage, but initially QUB asked for a 14 nm bandwidth," he recalled. "Because of the geometry of the stretcher and the size of the diffraction gratings and other key optical components, this proved to be extremely difficult."

To deliver higher bandwidth requires larger optics (plus mounts), which limits the bandwidth through the stretcher. In the end, the joint Coherent/QUB team settled on a 12 nm bandwidth through the stretcher. To put this in perspective, even this compromise needed the focusing mirror in the stretcher to have a diameter of 12 inches.

But why not save costs and settle for the usual bandwidth of 6–7 nm? "QUB was concerned about any spectral clipping that might produce modulation of the spectrum," answered Edstrom. "Having as much stretcher bandpass as possible helps to minimize this effect."

Another challenging aspect of the QUB design was matching the amplifier wavelength. Because of its upper-state lifetime, Nd:glass is the best material for producing high pulse energies, albeit at modest repetition rates due to its limited thermal conductivity. But the gain of this material is centred at 1053 nm, whereas Ti:sapphire (used in the oscillator and regenerative amplifier) has its gain peak near 800 nm. Although 1053 nm is within the range of Ti:sapphire, the low gain at this wavelength could have been an issue. Weak oscillator output would potentially allow the amplifiers to produce unacceptably high levels of amplified stimulated emission (ASE).

To get round this problem, Coherent specified a very high power continuous-wave pump laser, the 18 W Verdi, which enabled the Mira 900F oscillator to deliver more than 400 mW of average power at 1053 nm. This output is then fed into a Ti:sapphire regenerative amplifier that was optimized for operation at 1053 nm.

Two final vacuum-pulse compressors each compress 20 J pulses to a near-transform pulse width of only 750 fs, determined by the final amplified 3 nm bandwidth. Weighing over 1.5 tonnes each, these identical stainless-steel units contain a pair of enormous (200 × 500 mm) diffraction gratings. The final output beam has a top-hat beam spatial profile due to the relay-imaging techniques employed in the design of this amplifier chain.

The modular system architecture enables access to the beam(s) at several target stations with different beam-combination options including: an uncompressed (0.75 ns) beam and a compressed (750 fs) beam combination from the glass amplifiers; a 0.75 ns/750 fs combination with the home-built 50 fs Ti:sapphire beam; a compressed/compressed combination; and a single compressed 750 fs beam plus the home-built Ti:sapphire output.

Accelerating research

Several different QUB research programmes will benefit from what the group calls its dream system. According to Zepf, these include proton-acceleration experiments, the investigation of warm dense matter and the generation of ultrashort X-ray pulses.

In the area of proton beams, TARANIS will allow acceleration up to energies as high as tens of megaelectron volts in the space of 1 μm. Specifically, irradiating a thin film or wire target causes massive expulsion of relativistic electrons from the rear of the target. This results in a high transient field that accelerates protons off the target in picosecond bursts. By comparison, a conventional linear accelerator would need to be hundreds of metres long and have an equipment budget that is orders of magnitude higher than TARANIS to reach these energies – albeit with much longer pulses.

Warm dense matter, in theoretical terms, is difficult to characterize due to its highly non-ideal behaviour. TARANIS will help to generate important experimental data by creating near-solid density and modest temperature (104 K) conditions in iron, aluminium and other targets. "These plasmas are not well understood at present, with models based on hot solids and cold plasma disagreeing on predictive observables by up to an order of magnitude," Zepf said. "The usual PV=nRT equation does not hold at these conditions and there is no agreement on what equation of state is applicable to date. Hopefully, TARANIS will help provide answers to some of these questions."

Other work will involve research into ultrashort X-ray pulses for applications such as investigating plasma physics on ultrashort timescales. This includes the simultaneous use of long and short pulses to create X-ray laser output from targets such as silver and molybdenum. Here, a relatively long nanosecond beam will be used to create an equilibrated plasma that acts as the X-ray gain medium when pumped with a femtosecond pulse.

For more information contact steve.edstrom@coherent.com, john.mackay@coherent.com (California, US) or adam.drysdale@coherent.com (UK).

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