FEATURE ULTRAFAST AMPLIFIERS
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
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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.
Vulcan has staged amplification based on a master oscillator power amplifier architecture
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
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Coherent’s Astrella laser amplifier is HALT and HASS tested for a highly reliable system APRIL 2014 l ELECTRO OPTICS 25
Coherent
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