DE4438283C2 - Laser for generating narrow-band radiation - Google Patents

Description

The invention relates to a laser for generating narrowband radiation with the characteristics of the preamble of Claim 1.

Such a laser, in particular excimer laser, is known from the US-A-5 150 370 is known.

There are a variety of arrangements in the literature for generation supply of narrow-band laser radiation, especially excimer lasers radiation, known (see e.g. SPIE, Vol. 1463; Optical / Laser Microlithography IV (1991), 604).

DE 44 01 131 A1 also describes an excimer laser narrowband emission.

In the prior art are called wavelength selective elements to narrow the bandwidth in the resonator usually the fol aids, either alone or in combination set

• - Grid with beam expansion
• - Prism arrangements with beam expansion and
• - Fabry-Perot etalons.

Essential criteria for the selection of one or the other Component are the effectiveness with regard to the Bandwidth narrowing and secondly long-term stability or the radiation resistance.

In this regard, an etalon compared to lattice and Prism arrangement a very good effectiveness with the mash has a constriction, but on the other hand only a relative one has low radiation resistance. Grids and prisms, however are long-term stability and beam dielectric strength significantly better than etalons.

The above-mentioned US-A-5 150 370 takes advantage of etalons arranged in the resonator ("intra-cavity") Use and be effective in reducing bandwidth limits the radiation exposure of the etalon by means of a be special coupling of the emitted radiation by means of a polarizing beam splitter. With this known arrangement becomes a relatively high output at a relatively low Radiation exposure to the intra-cavity etalon reached.

In this known arrangement, by rotating one Quarter wavelength plate the output of the decoupled Radiation can be maximized. In that resonator section, in which the etalon is arranged, the performance of the Radiation in the resonator kept relatively low, so that Etalon is not overloaded. With this well-known An order, however, no way is shown how the range of emitted radiation can be further reduced. The one Building additional etalons is due to the high beam performance in other sections of the resonator closed and also the installation of an additional wavelength selective element in the form of a grid with a beam expander  in the resonator is not easily possible because the Radiation in this resonator branch a special Polarisa state (the radiation is circularly polarized there) and a grid arrangement (the same applies to one Prism arrangement) the polarization behavior of the radiation decisively influenced, so that the decoupling with a polarizing beam splitter is no longer functional kidney.

In the arrangement according to the above DE 44 01 131 A1 who a grating and a Fabry-Perot etalon as a wavelength lective elements used. There the radiation with a decoupled split mirror consisting of two mirror elements pelt. Due to the small gap widths (e.g. 0.35 to 0.7 mm) undesirable diffraction effects occur that affect the lateral Adversely affect intensity distribution. It also causes narrow gap width a comparatively poor utilization of the discharge cross section.

The invention has for its object a laser gangs mentioned type so that very narrow-band High intensity radiation is emitted.

The inventive solution to this problem is in the patent saying 1 marked.

Preferred embodiments of the invention are in the Unteran sayings described.

The invention is preferably used in excimer lasers.

As such, Faraday rotators are known in the art knows. They contain when using the invention for UV lasers (e.g. excimer laser) an optically transparent in the UV range  isotropic material, e.g. B. so-called Suprasil (trade name). DE 41 39 395 A1 describes a Faraday rotator in one Solid-state ring laser shown. There the Faraday rotator but not to improve the narrow band in one Lasers of the type mentioned here are used.

The laser arrangement according to the invention has the advantage that the magnetic field strength in the Faraday rotator and the effective Length of the optically isotropic material (e.g. the Suprasilsta bes) can be chosen so that the polarization plane of the Light with a simple passage through the rotator a certain angle is rotated, this angle is set so that the plane of incidence of the beam splitter which decouples the radiation by reflection to exactly that mentioned angle with respect to the plane of incidence of the grid / prism men arrangement is rotated.

The angle of rotation can in particular be selected so that optimal coupling of the beam results from the resonator.

Instead of reflection, the radiation can also be transmitted sion can be coupled out by the beam splitter.

Another embodiment of the laser arrangement according to the invention provides that two slit diaphragms, two polarizing beam divider and at least one widening optic in the beam path in the right sonator are arranged.

The plane of incidence of the prisms / mirrors is preferred order parallel to the plane of incidence of the beam splitter.

Another preferred embodiment of the invention Laser arrangement provides that the defi by slit diaphragms The resonator axis is at an angle to the longitudinal direction of the elec tread. The latter variant applies to a gas discharge line water, especially an excimer laser.  

In the following, exemplary embodiments become more inventive Laser arrangements described with reference to the drawing. It shows:

Fig. 1 shows schematically a first embodiment of a laser for generating narrow-band radiation;

FIG. 2 shows a modification of the exemplary embodiment according to FIG. 1;

Fig. 3 shows a second embodiment of a laser for generating narrow-band radiation; and

Fig. 4 shows a third embodiment of a laser for generating narrow-band radiation.

In the embodiments described below the invention is implemented in excimer lasers.

A laser-active medium 10 is therefore by means of a gas discharge in a corresponding chamber by means of electrodes. The laser resonator has a highly reflecting mirror 12 on one side and an optical grating 14 on the other side. A beam expander 16 in the form of prisms is arranged in front of the grating 14 in a manner known per se. The optical axis of the laser is marked with "A".

A Faraday rotator 18 is arranged between the grating / prism arrangement 14 , 16 and the laser-active medium 10 . Between the laser-active medium 10 and the highly reflecting mirror 12 , a polarizing beam splitter 20 and a Fabry-Perot etalon 22 are arranged in the order according to FIG. 1.

UV radiation is generated in the laser-active medium 10 (plasma of the gas discharge). The radiation traveling in the direction of the highly reflecting mirror 12 passes through the polarizing beam splitter 20 . The beam splitter 20 is a thin film polarizer here. P-polarized light (polarized in the paper plane in the exemplary embodiment) is transmitted by the beam splitter 20 with a transmittance of more than 98% and S-polarized light (in the example perpendicular to the paper plane) is reflected by the beam splitter 20 to more than 98% ( less than 2% of the S-polarized light is therefore transmitted). The radiation transmitted through the beam splitter 20 , ie essentially P-polarized radiation, passes through the etalon 22 , is reflected at the mirror 12 and in turn passes through the etalon 22 , so that a very effective bandwidth narrowing (filtering) takes place. The radiation of greatly reduced bandwidth passes through the beam splitter 20 (from right to left in FIG. 1) without significant attenuation and is then amplified in the laser-active medium 10 . The amplified radiation passes through the Faraday rotator 18 (from right to left in FIG. 1). The Faraday rotator 18 has, for example, an optically active material. B. Suprasil, which is casual in the UV range. Furthermore, a Faraday rotator contains means for generating a magnetic field, ie either a permanent magnet or an electromagnet. Depending on the strength of the applied magnetic field and the effective length of the permeable material (here a Suprasil rod), the polarization direction of the radiation is rotated by an angle (α).

The assembly of prisms / beam expander 16 and grating 14 serves as a further wavelength-selective element. So that this assembly reflects the radiation to the maximum, the entire assembly (consisting of grating 14 and prism beam expander 16 ) must be rotated about axis A in such a way that the plane of incidence of this assembly is parallel to the polarization plane of the radiation, which in the exemplary embodiment according to FIG. 1 is rotated by the above-mentioned angle α from the plane of the drawing. The radiation reflected by the grating 14 passes through the Faraday rotator 18 again and is rotated again by the same angle α ge. The returning radiation (from left to right in FIG. 1) is further amplified in the laser-active medium 10 and now strikes the beam splitter 20 with a polarization plane rotated by 2 × α relative to the original polarization direction.

The reflectivity R and the transmission T of the polarizing beam splitter 20 are given by R = sin² (2 α) and T = cos² (2 α). The reflectivity R determines in the exemplary embodiment according to FIG. 1 the degree of decoupling of the resonator, ie the proportion of the decoupled radiation 24 of the total radiation incident on the beam splitter 20 . In the case of excimer lasers, the optimum degree of decoupling is typically 85% to 90% depending on the amplification per cycle and also on the internal resonator losses. This means that the angle a explained above is selected so that there is a reflectivity R of 0.85 to 079 for the radiation impinging on the beam splitter 20 . An angle α of 34 ° to 36 ° thus results for these degrees of decoupling. Since the polarizing beam splitter 20 has a transmission T of only 0.1 to 0.15, the intensity of the radiation in the resonator at the location of the Fabry-Perot etalon 22 is a factor of 6 to 10 less than on the other side (in Fig. 1 left) of the beam splitter 20th The etalon is therefore only slightly exposed to radiation.

From the above explanation it follows that the polarizing beam splitter 20 in the illustrated exemplary embodiment is adjusted in the beam path in the resonator such that its plane of incidence forms an angle of -α with the plane of incidence of the arrangement of grating and beam expander 14 , 16 (the In a single pass, the Faraday rotator rotates the polarization plane by the angle + α).

In the above described embodiment shown in FIG. 1, the beam splitter 20 couples the emitted radiation 24 by reflection.

FIG. 2 shows a modification of the exemplary embodiment according to FIG. 1, the radiation 24 not being decoupled by reflection but by transmission through the beam splitter 20 . Fig. 2 shows only the parts of interest to that extent. The Faraday rotator 18 , the prism beam expander 16 and the grating 14 correspond to the embodiment of FIG. 1 and are therefore only indicated in Fig. 2 with the corresponding reference numerals.

Fig. 2 shows a further embodiment of an Excimerla sers with extremely narrow-band emitted radiation in the ultra violet spectral range.

In the figures, parts provided with the same reference numerals have a corresponding function, so that reference can be made to the above description. In addition, two further polarizing beam splitters 26 , 38 are provided in the exemplary embodiment according to FIG. 3, and a half-wave length plate 18 and a further beam expander, consisting of prisms 30 , 36 and mirrors 32 , 34 . Furthermore, two slit diaphragms 40 , 42 are arranged in the resonator.

The function of this arrangement according to FIG. 3 is as follows: To improve the bandwidth reduction by means of the grating 14 and the beam expander 16 , slit diaphragms are installed in the resonator. The slits 40 , 42 serve to reduce divergence. However, this results in a relatively poor utilization of the discharge cross section in the laser-active medium 10 by the radiation to be amplified. In order to overcome this disadvantage, the two polarizing beam splitters 38 , 26 and the beam optics from the prisms 30 , 36 and the mirrors 32 , 34 are additionally seen in the arrangement according to FIG. 3. The polarizing beam splitters 26 , 38 are arranged in the beam path so that their planes of incidence are parallel to the plane of incidence of the polarizing beam splitter 20 . The half-wavelength plate 28 is installed in such a way that the radiation after passing through the components 36 , 34 , 32 , 30 and 28 (that is to say from left to right in FIG. 3) is S-polarized and is accordingly completely reflected on the beam splitter 26 .

Fig. 4 shows another embodiment of an Excimerla sers with very narrow-band emission. It is known in the prior art to guide the radiation in the resonator obliquely to the direction of the electrodes of the excimer laser, in particular in the case of transversely excited lasers. Such beamlines have also been successfully used in solid-state lasers.

Fig. 4 shows an arrangement with a beam directed obliquely to the electrode direction. In Fig. 4, the Faraday rotator 18 , the prism beam expander 16 and the grating 14 are not Darge, but are indicated by reference numerals. In this respect, the exemplary embodiment according to FIG. 4 corresponds to that of FIG. 3.

The direction of the resonator axis is determined by gap blades 42 , 46 and is somewhat obliquely (approximately 10) aligned with the electrode direction. Radiation reflected by the polarizing beam splitter 20 passes through a beam expander 44 (e.g. according to components 30 , 32 , 34 and 36 according to FIG. 3) and is reflected back to the beam splitter 20 via a mirror 50 and then runs again at an angle to the electrode direction (this time inclined by approximately 2) through the laser-active medium 10 . The slit aperture 46 is designed to couple out the radiation 24 as a highly reflecting mirror according to FIG. 4.

Claims (14)

1. Laser for generating narrow-band radiation

• - a laser resonator, between the mirrors ( 12 , 14 ) of which a laser-active medium ( 10 ) is arranged,
• a first wavelength-selective element ( 22 ) on a first side of the medium ( 10 ) in the beam path between the medium ( 10 ) and a first, highly reflecting mirror ( 12 ) of the resonator,
• - A polarizing beam splitter ( 20 ) for coupling out radiation ( 24 ) from the resonator, and
• - a component ( 18 ) rotating the polarization plane of polarized radiation in the resonator,

characterized in that

• - A Faraday rotator is provided as the component rotating the polarization plane ( 18 ) and
• - A grating or a prism arrangement as a second wavelength-selective element ( 14 ) is provided on the second side of the medium ( 10 ) in the beam path of the resonator.

2. Laser according to claim 1, characterized in that a grating ( 14 ) is assigned as a second wavelength-selective element, a beam expansion optics ( 16 ). 3. Laser according to one of claims 1 or 2, characterized in that the magnetic field strength in the Faraday rotator and the optical effective length of the optically transparent material of the Faraday rotators are dimensioned so that the polarization plane of the Rotator once through radiation through an angle (α) is rotated, which corresponds to the angle through which the plane of incidence of the beam splitter with respect to the plane of incidence of the grating is rotated. 4. Laser according to claim 3, characterized in that the angle of rotation (α) is chosen so that an optimal beam coupling takes place. 5. Laser according to one of the preceding claims, characterized in that the polarizing beam splitter ( 20 , 26 ) is arranged such that it reflects the s component of the radiation to the maximum and is maximally transmissive to the p component of the radiation. 6. Laser according to one of the preceding claims, characterized in that the Faraday rotator ( 18 ) contains an optically isotropic material which is transparent in the UV region. 7. Laser according to one of the preceding claims, characterized in that the radiation by reflection at the beam splitter from the laser is decoupled. 8. Laser according to one of claims 1 to 6, characterized in that the radiation is coupled out by transmission through the beam splitter ( 20 ). 9. Laser according to one of the preceding claims, characterized in that two slit diaphragms ( 40 , 42 ), two polarizing beam splitters ( 20 , 26 ) and a widening optics ( 30 , 32 , 34 , 36 ) for better utilization of the discharge cross section in the laser-active medium ( 10 ) are arranged in the beam path of the resonator. 10. Laser according to one of the preceding claims, characterized in that the plane of incidence of the beam splitter ( 20 ) is parallel to the plane of incidence of a beam splitter prism mirror arrangement ( 30 , 32 , 34 , 36 ) as an optical widening. 11. Laser according to one of the preceding claims, characterized in that slit diaphragms ( 42 , 46 ) determine a resonator axis and that the resonator axis is arranged obliquely to the electrode direction, a gas discharge being provided between the electrodes as the laser-active medium. 12. Laser according to one of the preceding claims, characterized in that through the beam splitter ( 20 ) reflected radiation is expanded by means of a beam expander ( 44 ) and after reflection on the beam splitter ( 20 ) passes through the laser-active medium in a gas discharge chamber obliquely to the electrode direction. 13. Laser according to one of the preceding claims, characterized in that he is an excimer laser.