FEATURE ULTRAFAST AMPLIFIERS
A laser scientist aligning a mirror in the Vulcan laser amplifier chain
Powering the pulse U
Greg Blackman on the amplifier technology needed to generate petawatt-class, ultrashort laser pulses
ltrafast 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
24 ELECTRO OPTICS l APRIL 2014
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.
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