GB2180368A - Increasing the power density in the focus of a laser beam - Google Patents

Abstract

A method of increasing the power density in the focus of a laser beam 11 which is coupled out from a confocal unstable resonator and which is spherically straightened in its far-field wave front 14, as well as equipment therefor are based on the recognition that in the wave front 14 phase shifts of about 1/4 of the wave length occur where the radially oscillating intensity distribution (l/r) over the cross-section of the beam 11 respectively becomes zero. Those cross-sectional regions 13.2, 13.1 which do indeed have a considerably lesser radiation intensity than the central region region 13.12, but by reason of the surface area content of these radially graduated ring regions 13.2, 13.1 nevertheless have a considerable power content, likewise contribute to the intensity in the beam focus when the phase shifts are compensated for, so that a corrected flat wave front 14.6 with as a whole a gaussian phase distribution occurs. For the phase correction there is effected a regionally shifted reflection at a graduated corrected mirror, the mirror-surface steps 17 of which extend wherever in the cross-section of the uncorrected incident beam 11 in each case the minima of the radiation intensity (l) extend. <IMAGE>

Description

SPECIFICATION Improvements in or relating to laser beams This invention relates to improvements in or relating to laser beams and is more particularly concerned with the power density in a focussed high-energy laser beam.

In the technology of high-energy gas lasers a so-called unstable resonator is used in order to be able to seemingly utilise the largest possible excited volume of gas. The annular cross-sectional structure, existing in the laserbeam short-range or close field of the intensity distribution about a beam-free centre-with a ring geometry determined by the geometry of the resonator decoupling/coupling out mirrors (Resonator-Auskoppelspiegel)-- leads in the long-range or. far field to an intensity which pulsates over the radius of the beam cross-section. The intensity maximum, which is smaller than when focussing a gaussian-shaped beam cross-section, lies in the beam centre and oscillates, after a steep drop to the value zero, several times with an amplitude which becomes increasingly smaller.

However, since the ring surface increases quadratically over the radius, despite smaller amplitudes in the first ring surfaces surrounding the centre, as a whole, approximately the same power is present as in the high-energy central beam cross-section. It can now be shown that in the long-range wave front a phase shift by about a quarter of the wave length apparently always occurs where the radial intensity distribution over the beam crosssection extends through the value zero. Upon focussing, however, primarily only those radiation intensities which have a steady, gaussianshaped distribution of the phase change in the wave front make a contribution in or at the focal point.

An object of the present invention is to increase the focus intensity of a high-energy laser beam (having an unstable mode and with low or seemingly towards the least possible expenditure on apparatus).

According to the present invention there is provided a method of increasing the power density in the focus of a laser beam decoupled or coupled out from a confocal unstable resonator having a spherically straightened wave front, in which ring-shaped cross-sectional regions of the beam experience a delay relative in each case to the neighbouring region situated towards the beam centre, in order for example to substantially compensate for phase shifts in the intensity distdbhtion over the radius of the beam cross-section.

Accordingly, in the long-range field beam cross-section the phase shift lying at an intensity zero location is cancelled in that the individually-defined ring-shaped beam cross-section regions experience a delay which is graduated or staggered by about a quarter of their wave length; whereby in the thus compensated corrected beam range a steady gaussian-similar phase distribution emerges.

On focussing, now also those partial powers contribute to the intensity integral which lay radially beyond the first phase shift (namely had absolutely a relatively small intensity) and upon integration, by reason of the ring-shaped geometry of these individual beam cross-sectional regions, make a considerable intensity contribution in the focus.

Preferably, the regional delay for the compensation of the phase shift is effected purely reflectively, in other words for example with coolable metal laser mirrors. These have a regionally stepped reflector surface, so that the individual cross-sectional regions of the incident beam are reflected with differing travel paths and thus displaced in phase. The precision manufacture of the stepped mirror surface is relatively uncritical, since the individual plateau heights lie in the order of magnitude of a few microns and are thus at least one power of ten higher than the manufacturing accuracy of laser mirror surfaces which can be achieved nowadays, for example by an etching process.

Thus, it is to be noted that the present invention is not concerned with the known time-variable deformation of the wave front of a beam by means of the deformable mirror surface regions of a so-called called MDA-mirror, in order to compensate for actual atmospheric disturbance effects on the propagation of an already focussed beam, derived from instantaneously reflected radiation energy.

Additional alternatives and further developments as well as further features and advantages of the present invention will become apparent from the claims and from the following description.

An embodiment of a method of increasing the power density in a laser and equipment therefor in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative much simplified drawings in which: Figure 1 shows in qualitative representation a typical intensity distribution in the case of a laser beam in the unstable mode, with phase shifts at intensity zero positions, plotted over the cross-sectional radius of a beam in the spherically corrected long range field, and Figure 2 shows in cross-section a mirror with a graduated reflection surface for compensation of the phase shifts existing in a wave front in accordance with Fig. 1.

Referring to the FIGURES in the drawings, in the long-range or far field of the beam 11 of a laser with a confocal unstable resonator, after spherical straightening or shortening (begradigter) of the wave front 14, from the beam centre 12 outwardly (in other words over the cross-sectional radius 4 no steady drop of the beam intensity 1 is present; but rather there is a ring-shaped intensity distribution, afforded point-symmetrically with regard to the centre 12, in such a way that the intensity I initially drops away steeply from a relatively high value similar to a gaussian distribution curve, and after that still fluctuates beyond the first intensity zero value over the radius r repeatedly with an amplitude which becomes increasingly smaller, as shown in a solid line in Fig. 1.

In the vicinity of the centre 12 the phase relationship of the electrical field is initially approximately constant, in order then to drop away steeply along the radius r. At the beam cross-sectional locations where the intensity I reaches the value zero, a phase shift by about a quarter of the wave length of the light takes place, indicated by a broken line in the qualitative diagram in accordance with Fig. 1 as a discontinuous course of the phase P over the radius r of the beam cross-section.

To be striven after, over the entire beam cross-section, is a steady course of the phase P(r) (as is already followed in the central ring region 13.12 of the gaussion distribution function) so that also the radiation powers in the ring regions 13.2, 13.1 contribute in a seemingly optimum manner to the intensity upon the focussing of the beam 11.

For the phase correction the individual longrange field ring regions 13.2, 13.1 with phases leading relative to the central ring region 13.12 experience an appropriate delay, so that in the corrected beam 11.6 there occurs a wave front 14.6, flat over the beam cross-section, with a steady or constant phase distribution. In addition to the intensity integral of the central ring region 13.12, therefore, then in a focussed beam the integrals of the ring regions 13.2, 13.1 contribute to the focus intensity; which thereby lies orders of magnitude higher than when focussing an only spherically corrected beam 11 without correction of the phase shifts existing therein.

The phase correction is, preferably, effected purely reflectively. This is because then a cooled mirror can be used, whereby losses, and detrimental influences on the beam geometry by reason of thermal heating of the optical correction means, can be avoided. Corresponding to the geometry of the ring regions 13 between the intensity zero positions, in other words to the locations of the respectively occurring phase shift, for such a reflection correction is the splitting up of the mirror surface 16 of a mirror 13 into axially mutually staggered planes; with steps 17 which extend along the location of an intensity zero position in the beam cross-section, and in other words, in the case of the axially-symmetrical beam 11 of circular cross-section, describe circles about the mirror centre 15.12.The steps 17 indeed do not reflect; however, ther mal losses nevertheless do not occur here, since the steps are indeed fashioned at locations at which the beam intensity amounts at least approximately to zero.

The angle of incidence 18 of the beam 11 that is to be corrected is if possible to be selected in order not to heat up the side surfaces of the steps 17 and in order if possible to be able to design the individual regions 13 of the graduated mirror surface 16 so as to be mutually parallel. With reference to the direction of the incident beam 11, the central mirror surface 16.12 lies in front of the stepped ring surfaces 16.2, 16.1. In this way the outermost ring region 13.1 of the beam 11 experiences at the correction mirror 15 the greatest phase shift in relation to the beam centre 11.12-as is to be required in order to achieve a phase distribution which is as free as possible of shift locations, in other words is steady, over the radius r of the beam cross-section in the case of the corrected departure beam 11.6.

To summarise, there is provided a method of increasing the power density in the focus of a laser beam (11) which is coupled out from a confocal unstable resonator and which is spherically straightened in its far-field wave front (14), as well as equipment for carrying out the method are based on the recognition that in the wave front (14) phase shifts of about 1/4 of the wave length occur where the radially oscillating intensity distribution (I/r) over the cross-section of the beam (11) respectively becomes zero.Those cross-sectional regions (13.2, 13.1) which do indeed have a considerably lesser radiation intensity then the central region region (13.12), but by reason of the surface area content of these radially graduated ring regions (13.2, 13.1) nevertheless have a considerable power content, likewise contribute to the intensity in the beam focus when the phase shifts are compensated for, so that a corrected flat wave front (14.6) with as a whole a gaussian phase distribution occurs. For the phase correction there is effected a regionally shifted reflection at a graduated corrected mirror, the mirrorsurface steps (17) of which extend wherever in the cross-section of the uncorrected incident beam (11) in each case the minima of the radiation intensity (I) extend.

It is to be understood that the scope of the present invention is not to be unduly limited by the particular choice of terminology and that a specific term may extend to or be replaced by any equivalent or generic term where sensible. For example the term equip-, ment may extend to "apparatus" or "device" and undue limitation is not to be placed on the term. Further individual features, methods, use, or functions related to the equipment for increasing the power density in the form of a laser beam or combinations thereof might be patentably inventive.

Further according to the present invention there is provided a mirror, said mirror being of generally circular form with concentric ring portions being axially raised relative to one another or stepped towards a raised central disc region, said ring portions and said disc portion being capable of reflecting a laser beam and being so dimensioned relative to one another that, in use, phase shifts in the intensity distribution of the radius of a laser beam coupled out from a conocal unstable resonator can be substantially corrected or compensated, by placing the mirror in the beam path. The mirror may have at least one central disc region and two or more concentric ring portions. The diameter of the central portion may correspond to the mirror surface ring width of one or more of said concentric rings.

Preferably, the rings and the central region lie at heights within a few microns of one another and/or the surfaces of the ring portions and central region will usually be parallel.

Further according to the present invention there is provided a method of increasing the power density in the focus of a laser beam using the mirror in the immediately preceding paragraph (with or without optional features).

The angle of incidence of the beam to the mirror is, preferably, such as not to heat up the side surfaces of the steps. Further according to the present invention there is provided a laser beam when corrected by said method.

Still further according to the present invention there is provided the combination of a laser and a mirror in accordance with the penultimate paragraph (with or without optional features) said mirror being positioned relative to the laser to correct phase shifts in the intensity distribution of the radius of laser beam.

Claims (7)

1. A method of increasing the power density in the focus of a laser beam, decoupled or coupled out from a confocal unstable resonator, having a spherically straightened/corrected wave front, in which ring-shaped crosssectional regions of the beam experience a delay relative in each case to the neighbouring region situated towards the beam centre, in order for example to substantially compensate for phase shifts in the intensity distribution over the radius of the beam cross-section.

2. A method as claimed in Claim 1, in which the delay is effected by reflection at mirror surface regions shifted in the beam pro pagation direction.

3. A method as claimed in Claim 2, in which the reflection is effected at a graduated or stepped mirror surface, the steps of which correspond with the course of the phase shifts in the cross-sectional surface area of the uncorrected beam.

4. Equipment for increasing the power den sity in the focus of a laser beam, decoupled or coupled out from a confocal unstable resonator, having a spherically corrected wave front, for carrying out the method as claimed in any one of the preceding claims, in which arranged in the propagation path of the beam is a mirror, the mirror surface of which has individual regions which rise in step-shaped manner to the central region.

5. Equipment as claimed in Claim 4, in which the steps extend between the mirrorsurface regions where the incident beam has in the mirror surface approximately a zero intensity.

6. A method as claimed in Claim 1 and substantially as herein described with reference to the accompanying drawings.

7. Equipment as claimed in Claim 4 and substantially as herein described with reference to Fig. 2 of the accompanying drawings