Motorised Beam Reducers for Vulcan High Power Laser

The STFC Central Laser Facility (CLF) is one of the world’s leading laser facilities providing scientists from the UK and Europe with state-of-the-art laser technology. Vulcan is a versatile high power Nd: Glass laser system capable of delivering up to 2.6kJ of laser energy in long pulse mode (nanosecond pulses) and up to 1 PW (1015 W) peak power in a short pulse (500 fs duration). The CLF’s wide ranging applications include experiments in physics, chemistry and biology, accelerating subatomic particles to high energies, probing chemical reactions on the shortest timescales and studying biochemical and biophysical process critical to life itself.

An STFC spokesman commented "Target Area West uses 108mm diameter beam lines for the long-pulse delivery to target. There are 6 of these beam lines which operate at 1053nm with frequency doubling options to convert to 527nm. The diagnosis of these beams requires the beam diameter to be reduced by a factor of 10 following a leakage through a final turning mirror. The motorised beam reducers, to be supplied by Optical Surfaces Ltd for each of the beam lines, will allow operation in 2 modes. One to be used for continuous wave (cw) alignment and a second mode for “on shot” diagnostics with maximum of 1% transmission at 1053 and 527nm and beam energies of up to 50J at the input of the beam reducers. The mode switching makes their precise, high accuracy design and construction critical. The on-shot mode requires a carefully designed “dump” to prevent the high energy beam propagating and interacting with the transmitted output which would result in high background noise and stray reflections..

Dr Aris Kouris, sales manager for Optical Surfaces Ltd. commented “The project has posed significant design challenges for us both optically and mechanically. The main challenges were related to dimensional constraints, maintenance of pointing accuracy and repeatability when switching between the 2 modes, and designing the system to be able to deal with the high power densities involved during the on shot diagnostics mode. The Beam Reducer will be based upon a Dall-Kircham off-axis mirror design with a number of folding mirrors between the primary and secondary. A final periscope pair at the output allows the input and output to be co-axial and maintain centre height for in-line operation. The switching between the 2 modes is achieved by two “switched” folding mirrors operated by an Optical Surfaces developed mechanism that will allow a high degree of repeatability of the order of just few arc seconds. This will allow maintenance of high optical performance and pointing accuracy during operation. Furthermore, the 2 mirrors are located in a position that will allow the coating to be able to withstand the energy densities present during the on-shot diagnostic operation and before attenuating the beam to suitable levels. Selection of appropriate coatings was also a great challenge and specific coating limitations greatly dictated the final optical design”.

Optical Surfaces Ltd has been producing optical components, mounted optical components and systems for more than 50 years and is now accepted as one of the world’s leading manufacturers of high-precision optics and optical systems for High Power laser applications. The company’s ISO 9001-2008 approved manufacturing workshops and test facilities are deep underground in a series of tunnels excavated in solid chalk where temperature remains constant and vibration is practically non-existent. With such stable conditions testing, particularly with long path lengths, becomes quantifiable and reliable. Working with these natural advantages is a highly skilled team of craftsmen with a commitment to excellence in both product quality and customer service.