Mitigation of the harmful effects of stray-light reflections in high-energy laser systems

Abstract

Reduction or elimination of negative consequences of reflected stray light from lens surfaces is achieved by propagating a laser beam through an eccentric pupil that excludes the optical axis of the system, which is rotationally symmetric. In such systems, stray light reflections eventually are focused onto the unique optical axis of the system, in either a real or virtual focal region. By using an eccentric pupil, all damage due to focusing of the stray light lies outside of the beam. These focal regions can, e.g., be physically blocked to eliminate beam paths that lead to optical damage, re-pulse beams and parasitic lasing.

Claims

1. An apparatus, comprising: at least one lens having an optical axis, wherein said at least one lens is rotationally symmetric about said optical axis, wherein a beam of light directed at said at least one lens will produce stray light; and a pupil eccentric to said optical axis, wherein said pupil is configured such that at least a portion of said stray light will be focused or directed by said at least one lens to a location outside of said beam.

2. The apparatus of claim 1, wherein said pupil is configured such that at least a portion of said stray light will be focused, in a real or a virtual focal region, onto said optical axis outside of said beam.

3. The apparatus of claim 1, wherein said portion of said stray light comprises all of said stray light.

4. The apparatus of claim 1, further comprising means for blocking or absorbing said stray light.

5. The apparatus of claim 4, wherein said means for absorbing said stray light comprises an absorber.

6. The apparatus of claim 5, wherein said absorber is tilted to reduce fluence from said stray light incident upon said absorber.

7. The apparatus of claim 5, wherein said absorber is cooled.

8. The apparatus of claim 5, further comprising a prism, positioned such that said stray light is first refracted by said prism before it reaches said absorber.

9. The apparatus of claim 1, further comprising means for blocking or absorbing said stray light to eliminate beam paths that lead to negative consequences selected from the group consisting of optical damage, a pre-pulse beams, parasitic lasing, generation of plasma within the path of said beam, generation of parasitic or extraneous laser beams (e.g., “pencil beams”), and back-reflection of light into the source of said beam, wherein said source comprises a laser.

10. The apparatus of claim 1, wherein said at least one lens comprises a plurality of lenses, wherein said plurality of lenses are rotationally symmetric about said optical axis, wherein said beam of light directed at said plurality of lenses will produce said stray light, wherein said pupil is configured such that at least a portion of said stray light will be focused by said plurality of lenses onto said optical axis outside of said beam.

11. The apparatus of claim 10, wherein said plurality of lenses are configured as a Keplerian telescope.

12. The apparatus of claim 1, wherein said at least one lens is a partial lens.

13. The apparatus of claim 1, wherein said stray light propagates in a stray light propagation pathway, wherein said apparatus further comprises a diagnostic positioned in said stray light propagation pathway.

14. The apparatus of claim 13, further comprising a prism, positioned such that said stray light is first refracted by said prism before it reaches said diagnostic.

15. The apparatus of claim 13, further comprising a collimating lens positioned to collimate said stray light to produce collimated stray light, said apparatus further comprising a static corrector plate positioned between said collimating lens and said diagnostic.

16. The apparatus of claim 1, wherein said stray light propagates in a stray light propagation pathway, wherein said apparatus further comprises a source of laser light configured to direct laser light in said pathway back to said at least one lens.

17. The apparatus of claim 16, further comprising a focusing lens positioned to direct said laser light into said pathway, said apparatus further comprising a static corrector plate positioned between said focusing lens and said source of laser light.

18. The apparatus of claim 10, wherein said stray light propagates in a stray light propagation pathway unique to each of said plurality of lenses, wherein said apparatus further comprises a diagnostic positioned in at least one said stray light propagation pathway.

19. The apparatus of claim 10, wherein said stray light propagates in a stray light propagation pathway unique to each of said plurality of lenses, wherein said apparatus further comprises at least one source of laser light configured to direct laser light in at least one said pathway back towards at least one lens of said plurality of lenses.

20. The apparatus of claim 19, further comprising a focusing lens positioned to direct said laser light into at least one said pathway.

21. The apparatus of claim 20, said apparatus further comprising a static corrector plate positioned between said focusing lens and said source of laser light.

22. The apparatus of claim 1, wherein said pupil is configured such that when said beam of light is directed parallel to the optical axis through said pupil and to the at least one lens, at least a portion of said stray light will be focused or directed by said at least one lens to a location outside of said beam.

23. The apparatus of claim 1, including a second pupil eccentric to the optical axis, wherein the at least one lens is between the first pupil and the second pupil.

24. A method, comprising: providing an apparatus that includes at least one lens having an optical axis, wherein said at least one lens is rotationally symmetric about said optical axis, wherein a beam of light directed at said at least one lens will produce stray light, wherein said apparatus further includes a pupil eccentric to said optical axis, wherein said pupil is configured such that at least a portion of said stray light will be focused or directed by said at least one lens to a location outside of said beam; and directing said beam of light through said pupil and through said lens, wherein stray light is produced by said lens and wherein said stray light is focused or directed by said at least one lens to a location outside of said beam.

25. The method of claim 24, further comprising blocking or absorbing said stray light.

26. The method of claim 24, further comprising absorbing said stray light with an absorber that is tilted to reduce fluence from said stray light incident upon said absorber.

27. The method of claim 24, further comprising absorbing said stray with an absorber, further comprising cooling said absorber.

28. The method of claim 24, further comprising absorbing said stray with an absorber, further comprising positioning a prism such that said stray light is first refracted by said prism before it reaches said absorber.

29. The method of claim 24, wherein said at least one lens is a partial lens.

30. The method of claim 24, wherein said stray light propagates in a stray light propagation pathway, the method further comprising operatively positioning a diagnostic in said stray light propagation pathway.

31. The method of claim 30, further comprising positioning a prism such that said stray light is first refracted by said prism before it reaches said diagnostic.

32. The method of claim 30, further comprising operatively positioning a collimating lens to collimate said stray light to produce collimated stray light, the method further comprising operatively positioning a static corrector plate between said collimating lens and said diagnostic.

33. The method of claim 24, wherein said stray light propagates in a stray light propagation pathway, the method further comprising directing laser light in said pathway back to said at least one lens.

34. The method of claim 33, further comprising positioning a focusing lens to direct said laser light into said pathway, further comprising positioning a static corrector plate between said focusing lens and said source of laser light.

35. The method of claim 24, wherein said beam of light propagates parallel to the optical axis to said lens.

36. The method of claim 24, including directing the beam of light through said pupil, through said lens, and through a second pupil that is eccentric to the optical axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

(2) FIG. 1 shows a beam propagating through a Keplerian telescope.

(3) FIG. 2 shows a Keplerian telescope used to increase the beam diameter as the beam propagates from one amplifier stage to another.

(4) FIG. 3 illustrates how a lens brings a beam to a focus onto a target plane.

(5) FIG. 4 shows examples of lowest-order stray-light reflections and their foci, often called “ghost foci.”

(6) FIG. 5 shows a parallel beam that is multiply reflected inside a lens.

(7) FIG. 6 shows a parallel beam passing through a component and then being reflected by a concave mirror.

(8) FIG. 7 illustrates that by tilting the lenses, it is often possible to block stray reflected light from an odd number of reflections that would otherwise be passed through the spatial filter.

(9) FIG. 8 illustrates the use of a wedged focusing lens used on the National Ignition Facility (NIF) at LLNL to isolate and separate the 1ω, 2ω and 3ω focal regions so that only 3ω light enters the target capsule.

(10) FIGS. 9A and 9B illustrate that “pencil beams” generated by reflections from spatial filter lenses can damage optics and produce pre-pulses on a target.

(11) FIG. 10 illustrates locations of ghost foci, which are also generated by reflections from lens surfaces, and can produce high-intensity light that directly damages optical components.

(12) FIGS. 11A-11F illustrate embodiments of the present invention that use eccentric pupils to remove stray light from the main beam.

(13) FIG. 12 shows an embodiment which eliminates ghost foci and pencil beam problems.

(14) FIG. 13 shows an embodiment similar to the embodiment of FIG. 12, except that is utilizes a partial lens to eliminate ghost foci and pencil beam problems.

DETAILED DESCRIPTION

(15) The present technology was created considering many of the problems described above and propagates the laser beam through an eccentric pupil that excludes the optical axis of the system, which is rotationally symmetric. Rotationally-symmetric systems are favored because they simplify construction, reduce costs and offer efficient use of space. In such systems, all stray light reflections eventually are focused onto the unique optical axis of the system, in either a real or virtual focal region. By using such an eccentric pupil, all ghost foci and their attendant high-risk damage zones lie outside of the beam. These focal regions can be physically blocked to eliminate beam paths that lead to optical damage, pre-pulse beams, back-pulses returning to the front end and parasitic lasing.

(16) More specifically, FIG. 11A shows a main beam 210 propagating in series through a first eccentric or off-optical-axis pupil 212, through lens 214, across optical axis 216, through lens 218, through a second eccentric or off-optical-axis pupil 220 to continue through other optical components 222. The figure shows that lens 214 produces stray reflected light 224 which passes through a first ghost focus 226 at optical axis 216. The figure further shows that lens 218 produces stray light 228 which passes through a second ghost focus 230 on optical axis 216.

(17) FIG. 11B illustrates a modification to the system of FIG. 11A so that it uses the ghost reflections off the lenses to either inject the laser beam from the front end of a laser or to sample the laser beam for diagnostic or alignment purposes. Using ghost reflections in this way is not feasible with centered pupils since the ghost reflections would overlap the main beam path and cause interference with the main beam. Specifically, either the front-end laser or the diagnostic package would block the main laser beam, or optics directing the beam from the front-end laser or to the diagnostic package would block the main laser beam. In the figure, a beam can be injected into the main beam path from any of front-end lasers 217, 223, 233 and/or 240. For example, a beam from laser 217 can be directed through static corrector plate 215 and then lens 213 through ghost foci 226 to enter the system through multiple reflections within lens 214 such that the beam is directed toward aperture 220. A beam from laser 223 can be directed through static corrector plate 221 and then lens 219 to lens 214. A beam from laser 240 can be directed through static corrector plate 239 and then lens 237 to lens 218. A beam from laser 233 can be directed through static corrector plate 231 and then lens 229 through ghost foci 230. In the case where the lasers 217, 223, 233 and/or 240 are included in the system, an alternate embodiment can omit beam 210 as well as aperture 212. Further, the second surface of lens 212 that is encountered by the beam counter-propagating on stray light path 224 may include a partially or fully reflective coating or other coating. Alternately, any of the front-end lasers can be replaced by diagnostic packages 217, 223, 233 and 240.

(18) Four options for sampling the beam for diagnostics are illustrated in FIG. 11B.

(19) 1. A beam propagating from the focal plane of the telescope is reflected by the outer surface of lens 214 back toward the focal plane. This beam comes to a focus 226 of the stray light 224, is collimated by a lens 213, passes through a static corrector plate 215 and enters the diagnostic package 217.

(20) 2. A beam propagating from the focal plane of the telescope is reflected by the inner surface of one of lens 214 back toward the focal plane. This beam is collimated by lens 219, passes through a static corrector plate 221 and enters the diagnostic package 223.

(21) 3. A beam propagating from outside the telescope (parallel to the optical axis and from right to left in FIG. 11B) is reflected by the inner surface of lens 218 in a direction away from the focal plane. This beam comes to a focus 230, is collimated by lens 229, passes through a static corrector plate 231 and enters the diagnostic package 233.

(22) 4. A beam propagating from outside the telescope (parallel to the optical axis and from right to left in FIG. 11B) is reflected by the outer surface of one of the lenses in a direction away from the focal plane. This beam comes to the upper right in FIG. 11B) is collimated by a lens 237, passes through a static corrector plate 239 and enters the diagnostic package 240.

(23) Static corrector plates are used to compensate for aberrations imposed on the sampled beam by its interactions with the eccentric-aperture lenses. Aberrations are caused by reflection and refraction of the beam at the curved surfaces of the eccentric-aperture lenses. It is standard practice today to use modern optics codes to model such aberrations, to determine the lens designs that minimize the aberrations, and to find shapes that can be applied to static corrector plates to reduce remaining aberrations, if needed. The arbitrary shape specified by the code would be achieved by using a deterministic finishing processes, such as magneto-rheological finishing (MRF).

(24) A feature of options 1 and 3 discussed above is that the beam passes through the lens once, is reflected by a curved surface, then passes through the lens a second time. Due to the curvature of the reflective and refractive surfaces, the ghost reflection comes to a focus, typically at a distance from the lens that is short compared with the focal length of the lens (only ˜23% of the focal length for a symmetric bi-convex lens). The beam is then recollimated using another lens, as shown in the diagram of FIG. 11B. In options 2 and 4 discussed above, the beam is reflected directly from the lens surface without propagating through the lens. Thus, aberrations due to refraction at the surfaces of the eccentric lens are eliminated. Additionally, aberrations due the curvature of the reflecting surface can be minimized by making the reflecting surface nominally flat, i.e., by making the eccentric lens a plano-convex lens. On this case, option 2 tends to produce smaller aberrations on the main beam than option 4 since the flat surfaces of the plano-convex lenses face the focal plane, consistent with best practice.

(25) With some modifications, the options for sampling the beam described correspond to four options for injecting front-end laser beams into the main beam path. For each of the options 1 through 4, these modifications are: 1) to replace the diagnostic package with a laser front-end; and 2) to replace the sampled beam with an injected beam propagating in the opposite direction. The discussion above on the beam aberrations imparted on the sampled and main beams for the four options 1 through 4 also applies to four corresponding injected-beam cases. Therefore, just as for the sampling options, the beam-injection options 2 and 4 will tend to impart smaller aberrations on the injected beam than beam-injection options 1 and 3.

(26) An alternative method currently used for sampling the beam is to place a beam splitter in the beam to divert a fraction of the beam energy to the diagnostic package. However, the method described here has the advantage of eliminating the need for inserting an extra component into the main beam path. Adding additional transmissive components tends to increase beam losses and to increase the non-linear phase shift (B-integral), which is usually undesirable.

(27) Another alternative method currently used for sampling the beam is to design the reflective coating on a mirror at some location in the beam line to be partially transmitting, so that a portion of the beam energy is transmitted through the mirror to the diagnostic package. The method described here has the advantage of eliminating the need to produce a high-quality optical finish on the back, transmissive side of the partially-transmitting mirror, of eliminating the need to modify the back side of the mirror mount to allow the diagnostic beam to be propagated, and to make space in the vicinity of the back side of the mirror for the diagnostic package. The method described here gives the laser designer options to place the diagnostic package(s) at alternative locations, which, depending on the circumstances, may be useful or even necessary for fitting the overall laser system into the allocated space.

(28) An alternative method currently used for injecting the beam is to place an injection mirror near the focal plane of a spatial-filter telescope, as is currently used on large fusion lasers such as the National Ignition Facility (NIF) in the United States and the Laser Mégajoule (LMJ) in France. A feature of method used by NIF and the LMJ is that the location of the focal spot must be moved transversely from pass to pass, so that the injection mirror does not block the main beam on any of its subsequent passes through the telescope. This angle offset at the focal plane also causes a transverse offset of the beam at the amplifier aperture, such that the amplifier aperture needs to be larger than the outer beam dimensions. As the angle offsets increase as more passes are accumulated, the number of passes taken is ultimately limited by the size of the amplifier aperture. The present technique has no such limitation, as the beam needs no angle offset to keep the main beam from hitting an injection mirror. The laser designer has the option of using no angle offset (in which case a Pockels-cell switch is recommended for holding off the gain, to prevent parasitic lasing), or a small angle offset (smaller than needed when using the injection mirror) to limit the number of passes taken by stray light and to reduce parasitic laser risk. The beam can be passed through the amplifier more times to make up for the increased loss of energy of the injected beam that results from the low reflectance of the lens. Note that since the ghost foci have been moved out of the beam due to eccentric-aperture lenses, the ghost foci are now more benign, and the reflectance of the reflecting lens surface might be increased to reduce the energy loss of the injected beam. The optimum reflectance will represent a tradeoff between injected-energy loss, losses experienced by the main beam each time it passes through the telescope, and the number of passes needed to produce the desired energy.

(29) FIG. 11C illustrates use of absorbers 241, 243, 245 and/or 247 to terminate ghost reflections. Use of eccentric-pupil lenses puts the ghost reflections out of the main beam path so that absorbers can be used without interfering with the main beam. Absorbers might be actively cooled in the event that the laser is operated at high average power, either continuously or with repetitive pulsing. FIG. 11D illustrates tilting the absorbers of FIG. 11C to reduce fluences incident on the absorbers. FIG. 11E illustrates the use of the absorbers of FIG. 11C with the addition of prisms 251, 253, 255 and/or 257 to divert ghost foci at high fluence to more desirable absorber locations, or to change the fluence at the absorbers.

(30) FIG. 11F shows the system of FIG. 11A modified to inject the pump beam 300 into the system. Pump beam 300 is reflected by a pair of mirrors (302, 304) through aperture 220 and lens 218 and then into the main beam line 210. The beam then passes through lens 214, aperture 212, amplifier 206 and is reflected by deformable mirror 208. The beam can make multiple passes through the system and can then be switched out to the optical components 222. Alternately, pump 300 may be injected into the system at other locations, e.g., as discussed above regarding the NIF.

(31) FIG. 12 illustrates an embodiment which eliminates ghost foci and pencil beam problems as a result of using the eccentric, or off-axis pupil. The figure shows beam 450 after it has passed through an off-axis pupil 461. The beam passes through optic(s) 452 and then propagates through lens 454 to be focused at focal spot 456 on optical axis 458. Stray light 460 is focused to the first-order ghost focus 462 on optical axis 458. Stray light 464 is focused to the second-order ghost focus 466 on optical axis 458. Notice that stray light 460 does not propagate onto or focus onto optic(s) 452. Notice also that neither stray light 460 nor stray light 464 continues to propagate within beam 450. Optic 452 can be replaced with a laser diode array. Thus, a use of the invention is in the design of telescopes for transporting diode light from diode arrays to laser slabs or other possible targets for the diode light. Use of eccentric-pupil optics would serve to protect the diode arrays from back-reflected light. Diode arrays can be highly sensitive to back reflections and are often damaged by back-reflected light.

(32) FIG. 13 shows an embodiment where only the portion of the lens that passes the beam need be used. The elements and description of FIG. 13 are identical to that of FIG. 12 except that full lens 454 of FIG. 13 is replaced in this figure with a partial lens 455. Thus, the figure shows beam 450 after it has passed through an off-axis pupil 461. The beam passes through optic(s) or element 452 and then propagates through partial lens 455 to be focused at focal spot 456 on optical axis 458. Stray light 460 is focused to the first-order ghost focus 462 on optical axis 458. Stray light 464 is focused to the second-order ghost focus 466 on optical axis 458. Notice that stray light 460 does not propagate onto optic(s) or element 452. Notice also that neither stray light 460 nor stray light 464 continues to propagate within beam 450.

(33) When this technology utilizes lenses that are thicker on one side than the other (e.g., a plano-convex lens), a consequence will be that the nonlinear phase shift accumulated by intense laser beams will be greater on the thick side of the lens than on the thin side. Nonlinear phase shift leads to several undesirable beam characteristics, including growth of small-scale intensity features as the beam propagates. To limit this growth, it will be desirable to minimize the maximum nonlinear phase shift that is accumulated by the beam. The nonlinear phase shift can be minimized by alternating the side of the beam on which the lenses are thickest.

(34) Similarly, in short-pulse cavities, relay telescopes using pairs of wedged lenses could be used to compensate for material dispersion, thereby eliminating the need for prism pairs.

(35) A use of the present technology is to generate reflected light that can be used to diagnose the laser beam. Reflected light from lenses can be directed to detectors to determine the pulse shape, energy, wavefront or fluence distribution of the beam.

(36) A use of the present technology is to couple light into a beam path from a laser front end. Just as light can be coupled out of a beam path and delivered to equipment to diagnose the beam, the process can be reversed to inject a beam into a beam path. In multi-pass system, light will tend to be reflected back to the front end. In such systems, back-pulse protection will need to be provided.

(37) In some embodiments, beams are injected in the far field, near the focal plane of a spatial filter telescope. In these cases, angle multiplexing is used in the far field to get the beam into and out of the amplifier cavity.

(38) In some embodiments, beams are injected in the near field. There are at least two categories of near-field injection:

(39) 1. Angle multiplexing is used in the near field to get the beam into and out of the cavity; and

(40) 2. Polarization multiplexing is used in the near field to get the beam into and out of the cavity. Polarization multiplexing can be implemented in several ways, including: a. Use of active polarization switches, in which a Pockels cell is used in combination with a polarizer; and b. Use of passive polarization switches, in which a Faraday rotator, an active rotator (such as active quartz rotators), a half-wave plates, or a quarter-wave plate, which may be used alone or combined one with another(s), with one or more polarizers.

(41) There are embodiments in which more than one spatial-filter telescope using eccentric-pupil lenses are used.

(42) There are embodiments in which one or more spatial-filter telescopes using eccentric-pupil lenses are used, in which a spatial-filter telescope that does not use eccentric-pupil lenses is also used. Possible or likely advantages of using some spatial filters without eccentric-pupil lenses are reduced cost and reduced aberrations.

(43) There are embodiments in which one or more spatial-filter telescopes using eccentric-aperture lenses are used, in which one or more spatial filter telescopes using cylindrical lenses and slit spatial filters are also used. Possible or likely advantages of using one or more telescopes with cylindrical lenses and slit filters is the ability to better filter out high-frequency amplitude and phase noise without generating plasmas and ablating spatial filter materials over time, as shots are accumulated. This is especially important for high-energy lasers operated at high repetition rates. Being able to better filter out noise at the slit filters can enable designers to relax spatial filtering requirements for telescopes in the system that use eccentric-pupil lenses. This is a way to mitigate some of the effects of increased aberration due to use of the eccentric pupils, which tend to increase focal-spot size at the pinhole filter plane and make tight spatial filtering in the eccentric-aperture telescopes undesirable (increased ablation at the pinhole, plasma production and “pinhole closure” effects, increased energy loss).

(44) There are embodiments in which various types of absorbing materials, often referred to as baffles, are placed close to but outside the main beam path to absorb light reflected from the surfaces of the eccentric-aperture lenses. These absorbing materials can be placed at various locations along the main beam path. In some instances, especially in high-energy pulsed laser systems in which the reflected light can attain high intensities, it may be necessary to take steps to reduce the intensity of the reflected light to prevent damage to the absorbing materials. Several such steps are as follows:

(45) 1. Using volumetric absorbers, such as glass doped with absorbing ions, which can have higher damage thresholds than surfaces absorbers. Higher damage thresholds are attained by spreading the absorbed energy over more mass, thus reducing the peak temperatures obtained;

(46) 2. Orienting the absorbers to achieve oblique angles of incidence, thus spreading the absorbed light over larger surface areas and reducing the peak temperatures obtained;

(47) 3. Using reflecting or refracting elements made from high-damage-threshold materials, such as high-purity fused silica, that absorb little or none of the reflected light, to redirect and spread the reflected light so as to reduce intensities before the reflected light is incident on the absorbing material.

(48) 4. Successful implantation includes ray-trace modeling of reflected-light ray paths and calculation of ray intensities to identify optimum locations and orientations for absorbing materials, to design the reflecting or refracting elements and to verify that intensities incident on the absorbing materials are below damage thresholds.

(49) There are embodiments in which various types of lens shapes are used, including: biconvex (symmetric and asymmetric), planar-convex (oriented either way), meniscus, aspherical shapes (designed to minimize spherical aberrations, which become more important for eccentric pupils), and achromatic lenses (designed to minimize chromatic aberrations, which become more important for eccentric pupils.

(50) All elements, parts and steps described herein are preferably included. It is to be understood that any of these elements, parts and steps may be replaced by other elements, parts and steps or deleted altogether as will be obvious to those skilled in the art.

(51) Broadly, this writing discloses at least the following: Reduction or elimination of negative consequences of reflected stray light from lens surfaces is achieved by propagating a laser beam through an eccentric pupil that excludes the optical axis of the system, which is rotationally symmetric. In such systems, stray light reflections eventually are focused onto the unique optical axis of the system, in either a real or virtual focal region. By using an eccentric pupil, all damage due to focusing of the stray light lies outside of the beam. These focal regions can, e.g., be physically blocked to eliminate beam paths that lead to optical damage, pre-pulse beams and parasitic lasing.

(52) Concepts:

(53) This writing also presents at least the following concepts:

(54) 1. An apparatus, comprising: at least one lens having an optical axis, wherein said at least one lens is rotationally symmetric about said optical axis, wherein a beam of light directed at said at least one lens will produce stray light; and a pupil eccentric to said optical axis, wherein said pupil is configured such that at least a portion of said stray light will be focused or directed by said at least one lens to a location outside of said beam.

(55) 2. The apparatus of concepts 1 and 3-21, wherein said pupil is configured such that at least a portion of said stray light will be focused, in a real or a virtual focal region, onto said optical axis outside of said beam.

(56) 3. The apparatus of concepts 1, 2, and 4-21 wherein said portion of said stray light comprises all of said stray light.

(57) 4. The apparatus of concepts 1-3, further comprising means for blocking or absorbing said stray light.

(58) 5. The apparatus of concept 4, wherein said means for absorbing said stray light comprises an absorber.

(59) 6. The apparatus of concept 5, wherein said absorber is tilted to reduce fluence from said stray light incident upon said absorber.

(60) 7. The apparatus of concept 5, wherein said absorber is cooled.

(61) 8. The apparatus of concept 5, further comprising a prism, positioned such that said stray light is first refracted by said prism before it reaches said absorber.

(62) 9. The apparatus of concepts 1-3, further comprising means for blocking or absorbing said stray light to eliminate beam paths that lead to negative consequences selected from the group consisting of optical damage, a pre-pulse beams, parasitic lasing, generation of plasma within the path of said beam, generation of parasitic or extraneous laser beams (e.g., “pencil beams”), and back-reflection of light into the source of said beam, wherein said source comprises a laser.

(63) 10. The apparatus of concepts 1-5, wherein said at least one lens comprises a plurality of lenses, wherein said plurality of lenses are rotationally symmetric about said optical axis, wherein said beam of light directed at said plurality of lenses will produce said stray light, wherein said pupil is configured such that at least a portion of said stray light will be focused by said plurality of lenses onto said optical axis outside of said beam.

(64) 11. The apparatus of concept 10, wherein said plurality of lenses are configured as a Keplerian telescope.

(65) 12. The apparatus of concepts 1-5 wherein said at least one lens is a partial lens.

(66) 13. The apparatus of concepts 1-5 wherein said stray light propagates in a stray light propagation pathway, wherein said apparatus further comprises a diagnostic positioned in said stray light propagation pathway.

(67) 14. The apparatus of concept 13, further comprising a prism, positioned such that said stray light is first refracted by said prism before it reaches said diagnostic.

(68) 15. The apparatus of concept 13, further comprising a collimating lens positioned to collimate said stray light to produce collimated stray light, said apparatus further comprising a static corrector plate positioned between said collimating lens and said diagnostic.

(69) 16. The apparatus of concepts 1-5, wherein said stray light propagates in a stray light propagation pathway, wherein said apparatus further comprises a source of laser light configured to direct laser light in said pathway back to said at least one lens.

(70) 17. The apparatus of concept 16, further comprising a focusing lens positioned to direct said laser light into said pathway, said apparatus further comprising a static corrector plate positioned between said focusing lens and said source of laser light.

(71) 18. The apparatus of concept 10, wherein said stray light propagates in a stray light propagation pathway unique to each of said plurality of lenses, wherein said apparatus further comprises a diagnostic positioned in at least one said stray light propagation pathway.

(72) 19. The apparatus of concept 10, wherein said stray light propagates in a stray light propagation pathway unique to each of said plurality of lenses, wherein said apparatus further comprises at least one source of laser light configured to direct laser light in at least one said pathway back towards at least one lens of said plurality of lenses.

(73) 20. The apparatus of concept 19, further comprising a focusing lens positioned to direct said laser light into at least one said pathway.

(74) 21. The apparatus of concept 20, said apparatus further comprising a static corrector plate positioned between said focusing lens and said source of laser light.

(75) 22. A method, comprising: providing an apparatus that includes at least one lens having an optical axis, wherein said at least one lens is rotationally symmetric about said optical axis, wherein a beam of light directed at said at least one lens will produce stray light, wherein said apparatus further includes a pupil eccentric to said optical axis, wherein said pupil is configured such that at least a portion of said stray light will be focused or directed to a location outside of said beam; and directing said beam of light through said pupil, wherein and at said lens, wherein stray light is produced by said lens and wherein said stray light is focused or directed by said at least one lens to a location outside of said beam.

(76) 23. The method of concepts 22, 24-28 and 31, further comprising blocking or absorbing said stray light.

(77) 24. The method of concepts 22, 23-28 and 31, further comprising absorbing said stray light with an absorber that is tilted to reduce fluence from said stray light incident upon said absorber.

(78) 25. The method of concepts 22-24, 26-28 and 31, further comprising absorbing said stray with an absorber, further comprising cooling said absorber.

(79) 26. The method of concepts 22-25, 27, 28 and 31, further comprising absorbing said stray with an absorber, further comprising positioning a prism such that said stray light is first refracted by said prism before it reaches said absorber.

(80) 27. The method of concepts 22-26, 28 and 31, wherein said at least one lens is a partial lens.

(81) 28. The method of concepts 22-27 and 31, wherein said stray light propagates in a stray light propagation pathway, the method further comprising operatively positioning a diagnostic in said stray light propagation pathway.

(82) 29. The method of concept 28, further comprising positioning a prism such that said stray light is first refracted by said prism before it reaches said diagnostic.

(83) 30. The method of concept 28, further comprising operatively positioning a collimating lens to collimate said stray light to produce collimated stray light, the method further comprising operatively positioning a static corrector plate between said collimating lens and said diagnostic.

(84) 31. The method of concepts 22-28, wherein said stray light propagates in a stray light propagation pathway, the method further comprising directing laser light in said pathway back to said at least one lens.

(85) 32. The method of concept 31, further comprising positioning a focusing lens to direct said laser light into said pathway, further comprising positioning a static corrector plate between said focusing lens and said source of laser light.

(86) The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.