23-24 September 2013, Laserlab Workshop 'Characterisation of ultra-short high energy laser pulses', Abingdon, UK

Venue: The Cosener's House, Abingdon

Workshop on characterisation of ultra-short high-energy laser pulses

On 23-24 September 2013, a two-day workshop was held in Abingdon, UK, on the characterisation of ultra-short high-energy laser pulses. The workshop was hosted by the Central Laser Facility (CLF) on behalf of Laserlab-Europe and attended by over thirty participants from eleven institutions. It enabled the delegates to discuss the challenges surrounding the characterisation of laser pulses used in laser-matter interactions, which is fundamental to the understanding and interpretation of the resultant data.

The workshop was attended by delegates from established facilities and those that are developing their own systems. The sentiment throughout the presentations and discussions was one that enabled common problems to be aired and potential solutions identified. The format of the workshop was arranged so that the overall topic was divided into sessions, with a series of talks that were then used as discussion points for the rest of the session.

The first session concentrated on measuring the pulse durations of high energy Nd:Glass lasers, where the biggest problem is to ensure that there is a reliable pulse length measurement on a shot-to-shot basis. The discussion revolved around the issues of the B-integral (the nonlinear phase shift along the beam's optical axis, ed.) and its impact on the pulse shape and pulse length tuning and on pointing stability into the measuring devices. The methods used to attenuate the incident energy on the shot to reduce the B-integral for the diagnostics arms fell into two camps, with the relative merits of reflective and transmissive schemes being discussed. In addition, the technique of using a sub-aperture sample beam to measure the pulse length was explored.

The second session was targeted at Ti:Sapphire laser systems; since those have higher repetition rates than Nd:Glass lasers it was felt that the average pulse length measurement was appropriate for diagnosing the laser. With the shorter pulse lengths operated on these systems, the pulse-front tilt is an additional problem and the talks in the session led to discussions on the use of inverting interferometers for pulse-front tilt measurements and the availability and reliability of commercially available diagnostics. There was a later session dedicated to different techniques for measuring the pulse-front tilt, and this enabled a thorough discussion as to the limitations and advantages to the different schemes being developed by the speakers.

The third session discussed the difficulty associated with measuring the contrast of laser pulses. The talks from the session showed that whilst there are reliable schemes for determining the nanosecond contrast 'on the shot', the contrast within picoseconds of the arrival of the pulse still requires the use of a scanning device for stable measurement. It was also discussed that with the difficulty measuring the pulse duration on high-energy systems, measuring the contrast would be an even greater challenge. Measurement of the spatial beam quality on the shot was also an area that brought together a consensus that it was a difficult measurement to make; the best approach seemed to be an equivalent-plane measurement or one that recreated a spot from a wave-front measurement.

With the advent of multi-PW laser systems, the diagnostics challenges associated with these schemes led to an interesting discussion on the relative merits of using parabolas for beam expansion for very broad bandwidths and how to maintain their alignment. Techniques for measuring and characterising damage to the final gratings were also discussed.

The problems associated with higher repetition rate laser systems led to a discussion about automated processing of diagnostics and the potential problem with large amounts of laser diagnostic data and whether an average or sampling approach should be taken. Other highlights from this discussion included the use of reflective optical systems for polarisation control and the potential benefits of a dark-field imaging system for damage detection.

The workshop ended with a tour of some of the laser facilities operated by the CLF.

Ian Musgrave (CLF)

>> Workshop agenda and talks