Excimer laser with improved beam uniformity and stability
An excimer laser includes a chamber for containing a gas mixture between a rear end and a front end of a linear laser resonator, a convex end-mirror defining the rear end, and a plurality of output-coupling mirrors collectively defining the front end. The output-coupling mirrors are distributed along an optical axis of the resonator and have respective normal vectors pointing toward the rear end. The normal vectors include four mutually non-parallel normal vectors, wherein no three of the four mutually non-parallel normal vectors are coplanar. These different orientations of the output-coupling mirrors cooperate with the convex end-mirror to blur fine structure in the output beam of the resonator.
1 . An excimer laser, comprising:
a chamber configured to contain a gas mixture between a rear end and a front end of a linear laser resonator;
a convex end-mirror defining the rear end, the convex end-mirror having adjustable curvature; and
a plurality of output-coupling mirrors that (a) are distributed along an optical axis of the linear laser resonator, (b) collectively define the front end, and (c) have a respective plurality of normal vectors pointing toward the rear end, the plurality of normal vectors including four mutually non-parallel normal vectors, wherein no three of the four mutually non-parallel normal vectors are coplanar.
2 . The excimer laser of claim 1 , wherein each of the at least four mutually non-parallel normal vectors is tilted away from the optical axis in a different respective tilt direction.
3 . The excimer laser of claim 2 , wherein the respective tilt directions of the at least four mutually non-parallel normal vectors are distributed about the optical axis such that each 135-degree azimuthal-angle-interval about the optical axis contains at least one of the tilt directions.
4 . The excimer laser of claim 1 , wherein the output-coupling mirrors include a plurality of output-coupling mirror pairs, each output-coupling mirror pair being rear-facing and front-facing surfaces of a wedge.
5 . The excimer laser of claim 4 , wherein each wedge has a symmetry axis located in a mirror plane between the rear-facing and front-facing surfaces and coplanar with the normal vectors of the rear-facing and front-facing surfaces, the respective symmetry axes of the wedges being mutually non-parallel and oriented at different respective azimuthal angles about the optical axis.
6 . The excimer laser of claim 4 , wherein, for each wedge, an angle between the rear-facing and front-facing surfaces is between 0.02 and 0.5 milliradians.
7 . The excimer laser of claim 1 , wherein each non-rearmost one of the output-coupling mirrors is tilted away from the normal vector of the rearmost one of the output-coupling mirrors by at least 0.02 milliradians.
8 . The excimer laser of claim 1 , wherein a longitudinal distance between a frontmost one of the output-coupling mirrors and a rearmost one of the output-coupling mirrors is no more than 10% of a length of the linear laser resonator between the convex end-mirror and the frontmost one of the output-coupling mirrors.
9 . The excimer laser of claim 1 , the convex end-mirror has an on-axis curvature, on the optical axis, characterized by a radius of curvature that exceeds a length of the linear laser resonator between the convex end-mirror and a frontmost one of the output-coupling mirrors.
10 . The excimer laser of claim 1 , wherein curvature of the convex end-mirror is rotationally symmetric about the optical axis.
11 . The excimer laser of claim 1 , wherein the convex end-mirror has stronger curvature in a first transverse dimension than in a second transverse dimension.
12 . The excimer laser of claim 1 , further comprising a holder for the convex end-mirror, the holder including an actuator configured to adjust the curvature by applying pressure to a rear surface of the convex end-mirror facing away from the linear laser resonator.
13 . The excimer laser of claim 1 , wherein reflectivity of each of the output-coupling mirrors, at a wavelength of a laser beam generated by the excimer laser, is between 1% and 10%.
14 . The excimer laser of claim 1 , wherein the convex end-mirror is at least 99% reflective at a wavelength of a laser beam generated by the excimer laser.
15 . An excimer laser, comprising:
a chamber configured to contain a gas mixture between a rear end and a front end of a linear laser resonator;
a convex end-mirror defining the rear end; and
a plurality of output-coupling mirrors that (a) are distributed along an optical axis of the linear laser resonator, (b) collectively define the front end, and (c) have a respective plurality of normal vectors pointing toward the rear end, the plurality of normal vectors including four mutually non-parallel normal vectors, wherein no three of the four mutually non-parallel normal vectors are coplanar;
wherein the output-coupling mirrors include a plurality of output-coupling mirror pairs, each output-coupling mirror pair being rear-facing and front-facing surfaces of a wedge having a symmetry axis located in a mirror plane between the rear-facing and front-facing surfaces and coplanar with the normal vectors of the rear-facing and front-facing surfaces, the respective symmetry axes of the wedges being mutually non-parallel and oriented at different respective azimuthal angles about the optical axis, the wedges being held against each other in a stack with a spacer between each pair of adjacent wedges.
16 . The excimer laser of claim 15 , wherein each spacer is planar.
17 . The excimer laser of claim 15 , further comprising a holder for adjusting orientation of the stack.
18 . The excimer laser of claim 15 , wherein the output-coupling mirror pairs include four output-coupling mirror pairs.
19 . The excimer laser of claim 15 , wherein the output-coupling mirror pairs include six output-coupling mirror pairs.