Optical element for focusing approximately collimated rays

Abstract

A one-piece optical element for focusing an input bundle of collimated rays around an optical axis in a focal region around a focal point. The optical element is bounded on the entry side by a truncated cone centered relative to the optical axis with a top surface pointing toward the light entry and bounded on the exit side by a cone with a cone tip pointing toward the light exit on the optical axis and a rotationally symmetric aspheric boundary surface arranged around the cone. The cone is formed as a complementary cone to the truncated cone. The aspheric boundary surface is formed as a partial surface of the convex surface of a plano-convex aspheric converging lens with a focal point located behind the light exit of the optical element on the optical axis. The lateral surfaces of the truncated cone and of the cone are formed reflecting inwardly and spaced apart along the optical axis so that the approximately collimated input bundle is directed from the inner side of the lateral surface of the cone to the inner side of the lateral surface of the truncated cone.

Claims

1. A one-piece optical element made of an optically transparent material for focusing an input bundle of collimated rays around an optical axis in a focal region around a focal point bounded on an entry side by a truncated cone centered relative to the optical axis with a top surface pointing toward a light entry and bounded on an exit side by a cone with a cone tip pointing toward the light exit on the optical axis and an aspheric boundary surface arranged rotationally symmetrically around the cone, wherein the cone is formed as a complementary cone to the truncated cone, wherein the aspheric boundary surface is formed as a partial surface of the convex surface of a plano-convex aspheric converging lens with a focal point located behind the light exit of the optical element on the optical axis, and wherein the lateral surfaces of the truncated cone and of the cone are spaced apart inwardly reflecting and along the optical axis so that the collimated input bundle is directed from the inner side of the lateral surface of the cone to the inner side of the lateral surface of the truncated cone.

2. The one-piece optical element according to claim 1, wherein the lateral surfaces of the truncated cone and of the cone are mirrored inwardly.

3. The one-piece optical element according to claim 1, wherein the opening angle of the cone is smaller than an angle of 180 degrees, reduced by double the limiting angle of the total reflection for a transition from the optically transmissive material of the optical element to air.

4. The one-piece optical element according to claim 1, wherein a diffractive optical element, whose transmission function is selected so that the group velocity dispersion of an ultrashort laser pulse along the ray path through the optical element is minimized, is located on the top surface of the truncated cone.

5. The one-piece optical element according to claim 4, wherein the transmission function of the diffractive optical element is selected so that the group velocity dispersion and higher-order dispersions of an ultrashort laser pulse along the ray path through the optical element are minimized.

6. A method for focusing an input bundle of collimated rays with a one-piece optical element according to claim 1, wherein an irradiance above a minimum irradiance is achieved within the focal region around the focal point for a physical effect that is completely absent outside the focal region.

7. The method according to claim 6, wherein the physical effect is based on a polymerization above the minimum irradiance.

8. The method according to claim 6, wherein the physical effect is based on an optical perforation above the minimum irradiance.

9. The method according to claim 6, wherein the physical effect is based on the melting of a solid material above the minimum irradiance.

10. A system for carrying out a method according to claim 6, wherein the one-piece optical element is illuminated with collimated light on the entry side.

11. The system according to claim 10, wherein the one-piece optical element is connected to the focal region around the focal point by an immersion liquid with a refractive index greater than 1.

12. The system according to claim 10, wherein an optically transmissive protective element is disposed between the one-piece optical element and the focal region.

13. The system according to claim 12, wherein the protective element is exchangable.

14. The optical element of claim 1, wherein the lateral surfaces of the truncated cone mirror the lateral surfaces of the cone about a plane perpendicular to the optical axis.

15. An optical element, comprising: a single piece, monolithic lens having a entrance face receiving a light beam, a exit face opposite the entrance face, and an optical axis; wherein a central portion of the exit face forms a cone about the optical axis with a lateral conical surface, wherein the entrance face forms a truncated cone about the optical axis, wherein a central portion of the entrance face directly opposite the central portion of the exit face is planar and perpendicular to the optical axis, wherein the cone is formed as a complementary cone to the truncated cone, wherein the light beam is split by an internal surface of the lateral conical surface of the cone by reflection, the light beam reflecting away from the optical axis towards a peripheral portion of the entrance face, wherein the peripheral portion of the entrance face reflects the split light beam internally to be parallel to the optical axis, the peripheral portion of the entrance face forming a first aspheric boundary surface, wherein the split light beam is then refracted by a peripheral portion of the exit face towards a focal point, the peripheral portion of the exit face being a second aspheric boundary surface.

16. The optical element of claim 15, wherein after the light beam enters the lens, all light paths remain in an optical material of the lens until the split light beam exits at the exit face.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

(2) FIG. 1 shows schematically the ray path through a plano-convex aspheric lens;

(3) FIG. 2 shows schematically the profile of the irradiance through a focal region along the distance from the optical axis; and

(4) FIG. 3 shows schematically the ray path through a one-piece optical element for ray focusing.

DETAILED DESCRIPTION

(5) FIG. 1 shows schematically the path of light rays S through a plano-convex aspheric lens L with an optical axis OA according to the prior art. An input bundle EB of incoming rays, collimated to optical axis OA, at light entry LE enters the plano-convex aspheric lens L at the convex surface thereof L.k. An output bundle AB of convergent outgoing rays exits again at light exit LA on planar surface L.p of plano-convex aspheric lens L. Convex surface L.k is formed rotationally symmetric to optical axis OA so that according to the laws of geometrical optics the outgoing rays would intersect in a focus or focal point F located on the exit side of plano-convex aspheric lens L.

(6) The diffraction to be considered because of the wave nature of light, however, has the effect that contrary to the laws of geometrical optics the entire radiant flux, distributed over all incoming light rays S, is not concentrated at focal point F, but rather is distributed rotationally symmetric to optical axis OA in a focal plane, whereby the focal plane is the plane that contains focal point F and is intersected perpendicularly by optical axis OA at said point.

(7) FIG. 2 shows schematically the distribution of the irradiance in the focal plane along any imaginary line through focal point F as an irradiance profile, whereby the distance of a position x to focal point F is plotted on the position axis X and the value of the irradiance, measured at said distance x, is plotted on the irradiance axis E. Positions located at the same distance but in the opposite direction from focal point F along the imaginary line through focal point F have opposite signs.

(8) If a plano-convex aspheric lens L is illuminated with an input bundle EB of collimated rays S, as shown schematically in FIG. 1, and if said input bundle EB has a Gaussian irradiance distribution rotationally symmetric around optical axis OA, thus an irradiance distribution rotationally symmetric around optical axis OA and an irradiance profile E_L with a maximum at position x=0, therefore at the location of focal point F, which is surrounded by two zero points, result in the focal plane. A mirror-symmetrical course of irradiance profile E_L relative to the perpendicular line through the position x=0 follows from the rotational symmetry of the irradiance distribution relative to optical axis OA.

(9) If a specific minimum irradiance E_min is needed to achieve a specific physical effect, for example, a polymerization, an optical perforation, or melting of a material, thus this physical effect is achieved in the focal plane only within an effect circle around optical axis OA whose radius is given by the distance value x_L_min, at which minimum irradiance E_min is just achieved by irradiance profile E_L.

(10) The position of the two zero points, surrounding the maximum at focal point F, of irradiance profile E_L is determined by the numerical aperture of the plano-convex aspheric lens L. An increase in the numerical aperture causes these zero points to move closer and, therefore, because the entire radiation power as an area below irradiance profile E_L remains unchanged, also brings about a greater maximum and a steeper decline around this maximum. Systems and methods according to the prior art therefore attempt to increase the numerical aperture of the plano-convex aspheric lens L in order to attain a higher minimum irradiance E_min for achieving a physical effect and/or for improving the accuracy when a physical effect is being achieved.

(11) FIG. 3 shows schematically the ray path through a one-piece optical element OE, which is formed rotationally symmetric to an optical axis OA. On the entry side ES, optical element OE is delimited by a truncated cone KS, which has a conical lateral surface KS.m, which runs to a planar top surface KS.p. The conical lateral surface KS.m is mirrored inwardly, so that a light ray S, striking the conical lateral surface KS.m from the interior of optical element OE, is reflected. The planar top surface KS.p points in the direction of the entry side ES and is oriented centered and perpendicular to optical axis OA. The planar top surface KS.p represents the entry port of optical element OE.

(12) On the exit side AS, optical element OE is delimited by a central cone K, whose cone tip lies on optical axis OA and points in the direction of exit side AS. Cone K is formed as a complementary cone to truncated cone KS; in other words: cone K completes truncated cone KS to form a complete cone K. Lateral surface K.m of cone K is mirrored inwardly, so that a light ray S, striking lateral surface K.m from the interior of optical element OE, is reflected.

(13) As an alternative to the mirroring of lateral surfaces K.m, KS.m of cone K and of truncated cone KS, it is possible that depending on the refractive index of the material for optical element OE an opening angle for cone K, and therefore for truncated cone KS as well, is selected so that the reflection of a light ray S on the inner side of lateral surface K.m of cone K and also on the inner side of lateral surface KS.m of truncated cone KS occurs via total reflection.

(14) On exit side AS, optical element OE is delimited further by an aspheric boundary surface AG, which is rotationally symmetric to optical axis OA and borders cone K and encloses it. In the area of aspheric boundary surface AG, therefore outside cone K, the exit-side boundary surface of optical element OE coincides with the convex area L.k of an imaginary plano-convex aspheric lens L, whose focal point on the exit side lies on optical axis OA.

(15) It is possible that the planar top surface KS.p and/or the aspheric boundary surface AG are made anti-reflective in order to limit transmission losses during the entry and/or exit of light into and/or out of optical element OE.

(16) The ray path through optical element OE will be explained below. An input bundle EB of rays S, collimated to optical axis OA, with a circular cross section penetrates planar top surface KS.p without a change in direction and is reflected inwardly on lateral surface K.m of cone K. Because of the incline of lateral surface K.m to optical axis OA, the ray bundle after the reflection on lateral surface K.m has a ring-shaped cross section concentric to optical axis, whereby the inner and outer diameter of light ring LR with this ring-shaped cross section widens uniformly in the direction of the ray path, therefore, in the direction to lateral surface KS.m of truncated cone KS.

(17) The widened light ring LR strikes the inner side of lateral surface KS.m of truncated cone KS at the same angle of incidence at which collimated rays S, parallel to optical axis OA, strike the inner side of lateral surface K.m of truncated cone K, because cone K forms a complementary cone to truncated cone KS. Consequently, the widened light ring LR is reflected on the inner side of lateral surface K.m in a bundle of light rays S, collimated to the optical axis, with a ring-shaped cross section in the direction of the aspheric boundary surface AG. Said reflected light ring LR with an unchanged ring-shaped cross section therefore strikes aspheric boundary surface AG.

(18) The aspheric boundary surface AG is curved rotationally symmetrically, so that according to the laws of geometrical optics incident rays S, collimated to optical axis OA, would strike an exit-side focal point F.

(19) In fact, the result here as well, because of the bending of the light, is a distribution of the irradiance in the focal plane, which is not concentrated in an infinitesimally small focal point F but reaches beyond it in a rotationally symmetrically manner. FIG. 2 schematically represents the irradiance profile E_OE achieved with optical element OE of the invention, next to the irradiance profile E_L for a plain plano-convex aspheric lens L according to the prior art without use of axicons A1, A2 for beam expansion, whereby both systems have the same numerical aperture.

(20) As is evident from FIG. 2, the central maximum, therefore the region between the two minima or zero points enclosing the maximum, of the irradiance profile E_OE, achieved with optical element OE, is narrower than the central maximum of the irradiance profile E_L according to the prior art. In particular, the region, in which the predefined minimum irradiance E_min is exceeded, with a ray formed by optical element OE is also narrower and corresponds to a circle around optical axis OA, whereby the radius of the circle is given by the distance value x_SF_min at which minimum irradiance E_min is just achieved by irradiation profile E_OE.

(21) Therefore, a more accurate processing of a workpiece or a material can occur in an advantageous manner with optical element OE, when this processing is based on a physical effect that is triggered only above such a predefined minimum irradiance E_min, without the numerical aperture with the negative effects known from the prior art having to be increased for this purpose.

(22) It is apparent to the skilled artisan that an irradiance profile that is basically similar to the shown course of irradiance profile E_OE, particularly with respect to the height and width of the central maximum, results for an input bundle EB, comprising not solely precisely collimated rays S, but slightly divergent or convergent, therefore generally slightly inclined rays S, for example, by less than 5 degrees, relative to optical axis OA. Therefore, optical element OE can also be used advantageously for light sources that are not precisely collimated.

(23) The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.