Application of Ultra-high Precision Off-Axis Parabolic Mirror for Electron Acceleration Experiments

Topics Covered

Role of Optical Surfaces’ Solution in RAL Experiment
Off-Axis Paraboloids
Applications of Off-Axis Paraboloids
About Optical Surfaces


The Central Laser Facility at the STFC Rutherford Appleton Laboratory (RAL) in Didcot, the United Kingdom, has procured an ultra-high precision off-axis parabolic mirror from Optical Surfaces to build a focusing element, which is an essential component for RAL’s ongoing research involving the Vulcan Nd:Glass High Energy Laser.

Role of Optical Surfaces’ Solution in RAL Experiment

With the help of its highly experienced and skilled production team, Optical Surfaces manufactured the 320mm diameter off-axis parabolic mirror (Figure 1) with a surface gradient of better than λ/20 P-V, surface accuracy of better than λ/8 P-V, and smoothness of 20/10 scratch/dig, using its proprietary production techniques.

Figure 1. Optical Surfaces’ ultra-high precision off-axis parabolic mirror will form a crucial focusing element in RAL’s ongoing experimentation using the Vulcan Nd:Glass High Energy Laser.

The combination of the superior surface quality achieved during production and the remarkable high reflectivity coatings will facilitate RAL researchers to realize the high optical performance and efficiency demanded by their pioneering experiments. The off-axis mirror was produced in ClearCeram HS and coated with a special high performance coating to yield high reflectivity (R>99%) at 1053 and 527nm. 

According to Trevor Winstone, Operations Manager for the Vulcan Laser at RAL, the F/15 off-axis parabola will be employed for experiments demanding long Rayleigh focal length such as electron acceleration experiments. The development in laser technology to ultra-high intense (>1018Wcm-2) short-pulse (<1ps) lasers has enabled studying laser-plasma interactions at ultra-high intensities. Applications include inertial confinement fusion schemes, particle accelerators, and biological and medical technologies. The selection of Optical Surfaces to provide this precison focusing optic is based on their proven track record in producing and delivering high quality, demanding optics to ultra high power laser facilities all across the world.

Dr Aris Kouris of Optical Surfaces informed that the most challenging part of the fabrication of this demanding optic was to realize the surface gradients. An iterative process was required to achieve the needed smoothness. The use of an iterative process to attain λ/20 P-V surface gradients specification involved a substantial amount of handling during the production that increased the risks for markings. Fortunately, the company’s skilled workers have vast experience in handling optics and maintaining superior surface quality.

Off-Axis Paraboloids

Off-axis paraboloids (Figure 2) provide the advantage of an unhindered aperture and access to the focal plane. They are ideal for broadband or multiple wavelength applications, thanks to their complete achromatic performance. Off-axis paraboloids often deliver the most cost-effective solution without compromising performance to many problems faced by optical and system designers.

Figure 2. Off-axis paraboloids. 

The following are the specifications of off-axis paraboloids:

  • Off-axis angles as large as 45°
  • Non-circular shapes
  • Up to 600mm in diameter
  • Micro-roughness typically below 1.2nm RMS
  • Surface accuracy up to λ/20 at 633nm
  • Availability of a comprehensive range of coatings. 

Applications of Off-Axis Paraboloids

Off-axis paraboloids are used in:

  • Spectrograph mirrors
  • MRTD test systems
  • MTF reference surfaces
  • Target Projection
  • High power laser beam focusing
  • FLIR testing
  • Beam Expanders
  • Collimators. 

About Optical Surfaces

Optical Surfaces Ltd, has been producing optical components and systems for more than 30 years and is now accepted as one of Europe's leading manufacturers of high-precision optics. Our list of products include:

  • Etalons and Windows
  • Off-Axis Mirrors
  • Laser Beam Expanders
  • Collimators
  • Lenses and Prisms
  • Sphericals and Flat Mirrors
  • Mounted/Custom Optics. 

The main 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 become 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.

Interferometers routinely used include Zygo, Twyman Green, Scatterplate, Fizeau up to 600mm dia. Laser unequal path (LUPI) and point diffraction. They are able to test surface geometry, interfacing with our Wyko computer fringe analysis to confirm final wavefront.

This information has been sourced, reviewed and adapted from materials provided by Optical Surfaces.

For more information on this source, please visit Optical Surfaces.

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