FEATURE
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Powering the pulse

Greg Blackman on the amplifier technology needed to generate petawatt-class, ultrashort laser pulses

Ultrafast science has been an active area of research ever since Ahmed Zewail won the Nobel Prize in Chemistry in 1999 for his work studying chemical reactions using femtosecond spectroscopy. Today, commercial ultrafast amplifiers are available offering millijoule outputs. The large, research-grade laser facilities, however, are able to amplify to hundreds of joules of energy in ultrashort pulses, which requires highly specialised amplification techniques.

‘Typically you can maybe get millijoules of pulse energy out of commercially available parametric oscillators or OPAs [optical parametric amplifiers], but that tends to be limited by the pump lasers available,’ said Dr Ian Musgrave, head of the Vulcan laser at the UK Science and Technology Facilities Council’s Central Laser Facility (CLF). ‘What we would do with Vulcan would be to build up a kilojoule pump laser system that would enable us to pump an OPA process on a 20 x 20cm aperture to reach 300J of energy [at 30fs].’

Musgrave is commenting on the plans, currently under review, to extend the already petawatt Vulcan laser facility at the Rutherford Appleton Laboratory in Oxfordshire, UK to 10 petawatt power levels (300J in 30fs). Vulcan has been operational for 10 years and uses Nd:glass amplifiers pumped with flash lamps to produce 500J of energy in 500fs pulse durations. To reach 10PW, the scientists at the facility would employ a technique called optical parametric chirped pulse amplification (OPCPA). They would use the existing Vulcan Nd:glass laser, frequency double the beam to the second harmonic, and then use that light to pump optical parametric amplifiers (OPAs).

To amplify ultrafast pulses shorter than 50ps or 100ps requires a technique called chirped pulse amplification. Here, an initial 100fs pulse, for example, is stretched by 1,000 times to around 100ps, which can then be amplified without destroying the active medium needed for the amplification process. After it has been amplified, the pulse is recompressed to its original duration.

The optical parametric amplification (OPA) technique is effectively difference frequency generation, in which there is an intense pump beam and a weaker signal beam, and the difference between the two is the idler beam. In this process, the signal beam is amplified. This is different from standard laser amplification via stimulated emission.

OPA is ideal for generating very short pulse durations or for having flexible, tuneable sources. The technique can produce gain over several hundreds of nanometres with special phase matching conditions, rather than the limited spectral bandwidth with traditional gain media.

‘The CLF has been pioneering and developing the technique of optical parametric amplification for use for short pulse generation,’ said Musgrave. ‘Within the Vulcan petawatt facility, we generate laser pulses with an OPCPA pre-amplifier because it has this very broad bandwidth.’

Vulcan has staged amplification based on a master oscillator power amplifier architecture. The energy and aperture of pulses from an oscillator are increased to the order of 10J in rod amplifiers (a cylindrical rod of Nd:glass surrounded by flash lamps). Flash lamp-pumped disk amplifiers are also used to get a uniform gain profile with larger apertures. ‘The biggest amplifiers that we currently have on Vulcan are designed for a 20cm diameter beam,’ said Musgrave. ‘Before the compressor, we generate 600J of energy.’ There is a whole chain of these amplifiers: a rod chain containing rod amplifiers from 9mm in diameter up to 45mm, and disk amplifiers from 45mm diameter to 20cm beams.

The 10PW project would see the CLF build two additional kilojoule-level beamlines, which would be available to scientists as long-pulse beams or frequency-doubled to pump stages of amplification via OPCPA.

Another petawatt laser facility, currently under construction, is the High Repetition-Rate Advanced Petawatt Laser System (HAPLS), which is part of the European Extreme Light Infrastructure (ELI) project. The EU is investing €850 million in the project, which will see three laser facilities for fundamental research built: HAPLS in the Czech Republic and sites in Hungary and Romania, with a fourth site under discussion.

At Photonics West earlier in the year, it was announced that Austrian company Femtolasers will supply the front-end laser source for HAPLS; the laser facility’s project manager Dr Constantin Haefner and Femtolasers president Dr Andreas Stingl described key aspects of the partnership at the conference.

When built, HAPLS will deliver peak power of one petawatt at a repetition rate of 10Hz, with each pulse lasting less than 30fs. ‘That is a unique laser system because it will allow scientists to experiment at petawatt peak powers with a high fidelity,’ said Haefner, speaking to Electro Optics. ‘The laser will deliver 10 pulses per second, so the quality of the data will be much higher compared to other petawatt systems, where the laser is fired every 20 minutes or every hour.

‘Going to petawatt peak power is challenging, but has been done in the past,’ Haefner continued. ‘But nobody has done this at high repetition rates.’ Lawrence Livermore National Laboratory (LLNL) in the US has been contracted by ELI to build the HAPLS laser system.

The short-pulse beamline is based on a double chirped pulse amplification (CPA) scheme. Here, the system begins with a very short pulse, which is stretched in time, amplified and compressed, and then cleaned in the time domain to remove noise before and after the pulse. The process is then repeated to generate the high peak power.

‘The reason for double CPA is that what the researchers want is a very clean pulse,’ explained Haefner. ‘When any laser pulse is amplified, you have a signal-to-noise problem, i.e. you always amplify noise as well as the signal. The noise generates energy sitting ahead of the pulse. With these very high peak power lasers, that noise will arrive at the target before the main pulse, which will destroy the signal.’

The laser system is based on a Ti:sapphire gain medium, pumped by an Nd:glass laser. This Nd:glass laser is energised by powerful laser diode arrays, outputting 3.2MW of laser power.

Amplifying the laser will deposit heat in the gain medium, which has to do with how the amplifiers are pumped. Flash lamps, which have a broad emission spectrum, put a lot of heat into the gain medium and at high repetition rates can cause thermal stress in the material. In order to avoid that, the HAPLS lasers are pumped with a laser diode array, which emits only at a certain wavelength to maximise the laser transition and minimise the amount of heat dumped into the gain medium.

The laser system also employs specialised cooling methods developed at Lawrence Livermore National Laboratory, whereby helium is flowed at almost the speed of sound over the face of these amplifiers to extract the heat. ‘By extracting the heat from the face, the residual thermal stress is equally distributed over the surface and the amount of distortion in the beam from the thermal impact is much smaller,’ explained Haefner. Helium is inert, has good heat conductivity, and there’s no distortion when light passes through it.

Haefner commented that industry can learn from the HAPLS diode-pumped technology: ‘Commercial diode-pumped solid-state lasers typically run at low energies and high repetition rates. However, using this technology for high energy systems, and especially with Nd:glass has not been explored widely. We hope that industry will learn from this, as there are several applications where you can use this kind of laser other than for fundamental research.’ HAPLS is in the preliminary design phase. The whole system is scaling up from technology developed at LLNL around 10 years ago, called the mercury laser system, a 10Hz diode-pumped system. Along with the helium cooling, LLNL is providing the diffraction gratings needed to recompress the pulse.

Ultra-reliable

Reliability of the components making up large laser facilities like Vulcan and HAPLS is key to maximising the uptime of the beamlines for experimentation. US laser manufacturer, Coherent, has supplied ultrafast lasers to the Stanford Linac Coherent Light Source (LCLS) in the USA and the Elettra Sincrotrone in Trieste in Italy. ‘In these installations they want a laser that runs for thousands of hours, because if the laser fails they waste beamline and time for researchers running very specific experiments,’ commented Marco Arrigoni, director of strategic marketing, scientific segment at Coherent. ‘The downtime cost is something like $30-40k an hour,’ he said.

Coherent is employing an industrial testing process of highly accelerated life testing (HALT) and highly accelerated stress screening (HASS) testing in its more recent ultrafast products. ‘This hasn’t been the case in the high-performance ultrafast laser market to any great degree,’ commented Coherent’s director of marketing for research laser systems, Steve Butcher. ‘We’ve been deploying that testing both in the design and production phases to screen out potential problems with the systems before they get to the field.’

Coherent’s Vitara femtosecond oscillator, used at Stanford LCLS and Elettra Sincrotrone, is HALT/HASS-tested, as is the company’s Astrella laser amplifier, which produces 6mJ pulse energy at 1kHz and sub-35fs. ‘The focus is on reliability and lifetime; it is a one-box amplifier, including the oscillator, amplifier and laser,’ commented Butcher, referring to the Astrella. ‘The laser is water cooled, keeping complexity down. We want to offer a high-performance, reliable laser at an attractive price.

‘Scientists are interested in minimising the cost of acquiring data,’ he continued. ‘Because Coherent designs and manufactures Astrella using HALT/HASS techniques, the reliability will be there and we’re able to offer attractive extended warranties of up to five years, which have not been available in the past. That means a lot for customers, especially in the US, where scientists might find it difficult to get funds for maintenance.’

The HALT and HASS tests are designed to weed out all the opto-mechanical weaknesses – mounts that are not rigid or might break, sub-micron scale misalignment of parts, as well as how the components shift with changing temperature.

‘Scientists want an amplifier that behaves like a black box,’ said Arrigoni. ‘They don’t want to have to become specialists in laser systems to run their experiments. The reliability and the reproducibility of the performance day after day have become as important as the advanced performance. This is why we see the HALT/HASS approach is of paramount importance.’

Attosecond science

At the extreme end of ultrafast science are attosecond lasers, systems delivering pulses of light so short they can image electrons moving or molecular bonds forming. In June, Trumpf Scientific Lasers will supply its first customer with a parametric amplifier that has pulses a few femtoseconds in duration, and which is a suitable beam source for generating attosecond pulses.

The laser is an optical parametric chirped pulse amplifier (OPCPA) offering high pulse energies, high repetition rate and pulses less than 5fs, known as few-cycle laser pulses. The branch of Trumpf uses standard Ti:sapphire oscillators and amplifies them using an in-house developed pump source based on Trumpf thin disk picosecond technology especially tailored for pumping OPCPAs.

Dr Thomas Metzger, head of technology at Trumpf Scientific Lasers, explained that when a Ti:sapphire broadband seed pulse is overlapped in a nonlinear crystal with a pump laser in space and time, then the seed can be amplified to multi-millijoule pulse energies at kilohertz repetition rates. This is an order of magnitude higher than standard Ti:sapphire amplifier technology, he said.

Metzger said the TruMicro series 5000 picosecond laser is the ideal pump source for parametric amplification. He said the short pump pulses equate to high intensities when the light is focused into a nonlinear crystal. Therefore, with picosecond pulses, 107 or 108 amplification can be achieved in a crystal a few millimetres in size.

The damage threshold intensities of dielectric materials, such as the nonlinear crystals used in OPAs, are much higher for picosecond pulse durations. ‘It’s a complex process. It’s not visible at first glance why we use picosecond lasers for pumping OPAs. It all boils down to the increased damage threshold of this nonlinear crystal for picosecond pulses,’ said Metzger. Trumpf is able to use thin nonlinear crystals due to the high damage threshold and still able to achieve large amplification values while maintaining a broad bandwidth.

‘The technology can produce very short pulses of 5fs and peak powers of terawatt-level,’ said Metzger. ‘This can be achieved fairly simply, with one or two nonlinear crystals. Other terawatt lasers based on Ti:sapphire are huge; they probably need an entire laser laboratory. We can do this now in a compact parametric amplifier on an optical table.’

Trumpf Scientific Lasers is working on reaching higher pulse energies and also at taking this parametric amplifier technology out of the laboratory and using it to build a reliable tool for scientists. ‘The goal is to equip the attosecond laser community with reliable lasers, for areas like attosecond spectroscopy or for observing how electrons move under real-time conditions,’ Metzger said. ‘There is a market for attosecond lasers,’ he concluded. ‘The attosecond community is very active right now.’

About the author

Greg Blackman is the editor for Electro Optics, Imaging & Machine Vision Europe and Laser Systems Europe.

You can contact him at greg.blackman@europascience.com or on +44 (0) 1223 275 472.

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