Highly repetitive laser system having a compact design

#The invention relates to a highly repetitive laser system having a compact design according to the precharacterizing clause of claim 1.

#For many applications, such as, for example, in biomedicine or material processing, high intensities of laser pulses are required. With good beam quality (M2<1.2) and sub-500 fs pulses, this means as a rule pulse energies in the range of 1-10 μJ, in order to surpass the typical ablation or processing threshold energy densities and to permit an efficient process.

#In the area of material processing, the writing of arbitrary and very precisely formed waveguides in glass or plastic represents an example of use, this technology being of considerable importance for the future telecommunication networks.

#Laser systems according to the invention are described in more detail purely by way of example below with reference to working examples shown schematically in the drawing. Specifically,

#FIG. 1 shows the diagram of the cavity of a laser system according to the principle of the regenerative amplifier according to the prior art;

#FIG. 2 shows the diagram of the cavity of a first embodiment of the laser system according to the invention;

#FIG. 1 shows the cavity with the system components of the generic type for a laser system according to the principle of the regenerative amplifier according to the prior art. A polarized seed laser pulse from a seed laser source is input via a polarizer 1 into the arrangement and is reflected by a mirror 4 after passing through an electro-optical modulator 3, such as, for example, a Pockels cell, and a quarter-wave plate 2. Depending on the voltage applied to the electro-optical modulator 3, the polarization plane of the laser beam is rotated so that the polarizer 1 can be passed. By appropriate switching, it is thus possible both to input pulses into the arrangement and to output them as laser pulse S. The electro-optical modulator 3, together with the polarizer 1, thus forms an externally controllable switch by means of which a laser pulse can be alternatively input and output and the resonator quality can be controlled. If a pulse is input and hence trapped within the resonator, it is amplified by the laser medium 6 in each pass, multiple reflections taking place at folding mirrors 5. The laser medium 6 is optically pumped by an external laser diode source which is not shown here. After a certain number of resonator cycles and passes through the amplifying laser beam 6, the pulse is output as laser pulse S via the polarizer 1 by a further rotation of the polarization by switching the electro-optical modulator 3. This arrangement represents only one example of a restorable or regenerative amplifier.

#In FIG. 2, the cavity of the first embodiment of the laser system according to the invention, shown in FIG. 1, is modified by introducing a dispersion-influencing element as pulse stretcher 7. In this example, a pulse stretcher 7 which is in the form of a block of glass of the type SF57 is introduced into the beam path between two folding mirrors 5. In order to achieve as far as possible loss-free input into the pulse stretcher 7, the latter is formed with two Brewster windows. According to the invention, however, other elements of other materials with other shapes or in other positions can also be used. Thus, for example, Gires-Tournois interferometers or dispersive layer structures can be positioned instead of one of the folding mirrors 5 in addition to or as an alternative to the pulse stretcher 7 comprising a dispersive glass block. Furthermore, the glass block can be formed with a reflective side so that it simultaneously performs the function of a folding mirror 5. A further possible design is given by the multiple reflection in the interior of the glass block or of another dispersive medium, which can take place at a reflective surface or at the interface with the air.

#FIG. 3 shows the diagram of the cavity of a second embodiment of the laser system according to the invention. The basic principle corresponds to the design of a regenerative amplifier, shown in FIG. 1. A polarized seed laser pulse is input via a polarizer 1 into the arrangement comprising quarter-wave plate 2, electro-optical modulator 3 and mirror 4. The cavity is now defined by folding mirror 5′ and a laser medium 6′ in an arrangement modified compared with FIG. 1 or FIG. 2. A combination of two reflective elements 8a and 8b having dispersive properties is used as a pulse stretcher, the surfaces being slightly inclined relative to one another so that an incident pulse is multiply reflected and thus fed several times to the elements 8a and 8b having dispersive properties. In addition to general folding of the beam path in the cavity frequent interaction with the dispersive elements 8a and 8b and hence corresponding pulse stretching can be achieved thereby even during a single cycle. If the angle between the reflective surfaces of the elements 8a and 8b is designed so as to be adjustable or the angle of incidence is changed, for example by means of an adjustable input mirror, the dispersion achievable per cycle and hence the pulse stretching can also be varied by means of the number of reflections. Here, merely for the sake of simplicity, the pulse stretcher is shown in a simple variant comprising the two elements 8a and 8b. Both the pulse stretcher and an optical system possibly used for input into the pulse stretcher can, however, also be supplemented by further elements for beam guidance. Likewise, the pulse stretcher may also be in the form of a monolithic element in which reflection takes place at interfaces inclined relative to one another. The dispersive effect can then be achieved by the medium and/or by an optionally applied reflective layer. A concept suitable for realising such a pulse stretcher with multiple reflections and associated embodiments are described in U.S. 60/442,917. This document is hereby considered as being incorporated in this Application by reference.