Method and apparatus for automatic generation of phase-adapted coherent secondary radiation in a non-linear crystal

DETAILED DESCRIPTION It is therefore an object of this invention to provide a method of and a device for the automatic re-tuning of the phase adaption of the secondary radiation to a frequency-variable primary radiation.
According to the invention this object is solved in that the change in the angle between the direction of the secondary radiation and of the optical axis of the crystal, occurring in dependence on the wavelength of the primary radiation, is used for the control of a control loop for re-tuning of the phase adaption of the secondary radiation to the primary radiation.
In order to obtain tunable secondary radiation in a wide emission range, it is advantageous to use a laser of wavelength-variable frequency in the visible range as a source for the primary radiation.
With the method of frequency doubling in the crystal it is then possible to obtain a tunable radiation source for the UV-range.
In order to control the control loop for re-tuning of the phase adaption, a portion of the secondary radiation is directed onto a laminar detector such that by the local displacement of the direction of the secondary radiation occurring in dependence on the wavelength of the primary radiation an electric signal is generated in the detector whose absolute value is representative of the magnitude of the wavelength change and indicates by its sign whether the wavelength of the primary radiation has become greater or smaller.
If the phase adaption shall be obtained by a change in the crystal orientation with respect to the direction of incidence of the primary radiation, according to the invention the signal generated in the detector is used for the control of a servomotor which corrects the position of the crystal until the phase adaption between primary and secondary radiation has been re-established and the signal re-assumes the value zero.
If the phase adaption shall be obtained by a change in the refractive index in the crystal, the signal generated in the detector is expediently used for the control of a heating furnace surrounding the crystal, which changes the temperature and therewith the refractive index until the phase adaption between primary and secondary radiation has been re-established and the signal re-assumes the value zero