Development of a highly efficient compact x-ray laser…

Beating the bend.

Using a series of demanding high precision optics supplied by Optical Surfaces Ltd. the Japan Atomic Energy Research Institute (JAERI) in Kyoto (Japan) have demonstrated the possibility of a compact pumping system for x-ray lasers in the shorter wavelength region.

Scientists at JAERI have demonstrated gain saturation of nickel-like ion x-ray lasers at wavelengths of 13.9 and 12.0 nm using a compact chirped pulse amplification Nd:glass laser with an input energy of c. 14J. Employing a quasitraveling-wave pumping system using a special six-step mirror enabled the pumping energy to achieve gain saturation to be reduced to 2.5J/mm. The result demonstrates the possibility of a compact pumping system for x-ray lasers in the shorter wavelength region.

High precision optics from Optical Surfaces Ltd. were used for two critical elements of the experimental design.

To generate a quasitraveling wave Optical Surfaces produced a complex step mirror consisting of six blocks, each connected to a base plate by optical contact. The blocks were manufactured parallel to each other with an accuracy of 0.05 mrad in the vertical direction and of 0.1 mrad in the horizontal position. The size of the blocks was around 25 mm wide, 150 mm long and only 25mm thick, creating a very flexible aspect ratio. Neighbouring blocks were required to have a difference of thickness between them of 600 microns. Using an angle of incidence of 11 degrees with respect to the surface normal produced an optical delay of 4.07 ps between the laser beams reflected from neighbouring blocks

The variables included in the bending phenomenon from both contacting and hard coating created an interesting problem for Optisurf opticians. This was solved by a series of interations using exact methods to characterise the surfaces. Using these techniques we overcame the effects of bending to achieve the target of lambda / 5 flatness .

To provide the precise beam focusing required placed great demands on the quality of the optical components, a 150 mm diameter off-axis paraboloid (772 mm focal length) and a 203 mm diameter off-axis spherical mirror (curvature of 1000 mm). Based upon flexible lapping technology and their uniquely stabilised production environment, Optical Surfaces were able to produce superbly smooth optics fully mounted and tested to have better than lambda /10 flatness across the working surface.

Optical Surfaces is well known for producing off-axis mirrors, but this case highlights our ability to tackle non-aspheric prototype work using iterative thinking.