The invention relates to an arrangement for the resonant frequency doubling of multimode laser radiation. 
The use of a prism, which is moved by a rapid, short-stroke piczoactuator, for tuning the length of a passive resonator, is known (E. Zanger, R. Mxc3xcller, B. Liu, M. Kxc3x6tteritzsch and W. Gries xe2x80x9cDiode-pumped cw all solid-state laser at 266 nmxe2x80x9d in OSA Trends in Optics and Photonics, Vol.  26, Advanced Solid-State Lasers, pp.  104-111). 
However, this arrangement is not suitable for tuning a resonator for doubling a multimode laser, since the angular deflection at the tuning prism results in dispersion, as a result of which the resonator is not resonant simultaneously for all modes of the laser radiation. 
A resonant amplification of laser radiation is described in U.S. Pat.  No.  5,969,780.  From His, it is known that, in the case of a resonant doubling of the laser radiation from multimode lasers, the absolute length or the mode distance of the resonator must be adapted to the mode spectrum of the laser radiation, that is, the resonator lengths of the multimode laser and of the passive resonator must be equal, if the mode distances are to be the same.  For this purpose, the system described for converting the frequency of multimode lasers contains two resonators, formed in each case by at least two mirrors, an energy source/amplifying element being accommodated in the first resonator and an optically nonlinear material in the second resonator.  The output multimode radiation with frequency doubling of the input radiation, brought about in the first resonator, is supplied to the second resonator, where a portion of the multimode radiation is amplified resonantly. 
It is therefore an object of the invention to develop a generic arrangement, with which a dispersion-free tuning of the length of a passive resonator is ensured, in order to enable the frequency doubling of a multimode laser, which is resonant simultaneously for all modes of the laser radiation. 
Pursuant to the invention, the objective is accomplished as follows.  Pairs of oppositely disposed movable elements of an optically transparent material, such as prisms, are accommodated in the beam path of the resonator, which is formed from mirrors and the optically nonlinear material.  The movable elements are connected with adjusting elements, such as piezoactuators, so that the optical length of the resonator can be tuned and dispersion compensation is attainable. 
The dispersion-free adaptation of the resonator lengths ensures that the resonance- is attained for all modes of laser radiation.  Moreover, due to the use of at least two movable prisms, the resonator geometry is retained during a movement of the prisms.