Patent Number: 
Section: claims

1. A radiation source, comprising:a free electron laser configured to emit radiation having a wavelength of at most 30 nm; anda device configured to feed back the emitted radiation to the free electron laser, the device comprising:a first optical component configured to couple out of a beam path the emitted radiation; anda second optical component configured to couple into the free electron laser at least a portion of the coupled-out radiation,wherein the first optical component comprises a diffractive optical component configured to separate an order of diffraction of the emitted radiation and send the separated order of diffraction of the emitted radiation to the second optical component without sending at least one other order of diffraction to the second optical component, andwherein the separated out order of diffraction of the emitted radiation comprises the zeroth diffraction order of the emitted radiation. 2. The radiation source of claim 1, wherein the first optical component comprises a grazing incidence mirror. 3. The radiation source of claim 2, wherein the grazing incidence mirror comprises at least one member selected from the group consisting of silicon carbide (SiC), silicon (Si), copper (Cu), ruthenium (Ru), aluminum (Al), and diamond. 4. The radiation source of claim 2, wherein the second optical component comprises a grazing incidence mirror. 5. The radiation source of claim 1, wherein the second optical component comprises a grazing incidence mirror. 6. The radiation source of claim 5, wherein the grazing incidence mirror comprises at least one member selected from the group consisting of silicon carbide (SiC), silicon (Si), copper (Cu), ruthenium (Ru), aluminum (Al), and diamond. 7. The radiation source of claim 1, wherein the diffractive optical component is drivable. 8. The radiation source of claim 1, wherein the diffractive optical component comprises lines perpendicular to a direction of the radiation incident on the diffractive optical component. 9. The radiation source of claim 1, wherein the diffractive optical component comprises lines parallel to a direction of the radiation incident on the diffractive optical component. 10. The radiation source of claim 1, wherein the diffractive optical component comprises a cooling unit. 11. The radiation source of claim 1, wherein the device comprises two grazing incidence mirrors and two normal incidence mirrors. 12. A method, comprising:providing a radiation source according to claim 1;using the FEL to generate radiation;using the diffractive optical component to separate an order of diffraction of the radiation; andcoupling into the FEL at least a portion of the separated order of diffraction of the radiation without coupling into the FEL at least one other order of diffraction of the radiation. 13. The method of claim 12, further comprising driving the diffractive optical component to control the radiation guided to an image field. 14. An illumination system, comprising:a radiation source according to claim 1,wherein the illumination system is selected from the group consisting of a microlithographic illumination system and a metrology illumination system. 15. A system, comprising:an illumination system comprising a radiation source according to claim 1, the illumination system configured to illuminate an object field; anda projection optical unit configured to image the object field into an image field,wherein the system is a microlithographic projection exposure system. 16. A method of using a microlithographic projection exposure system comprising an illumination system and a projection optical system, the method comprising:using the illumination system to illuminate a reticle in an object field with radiation;using the projection optical system to image the reticle onto a light-sensitive material in an image field,wherein the illumination system comprises a radiation source according to claim 1. 17. The radiation source of claim 1, wherein the diffractive optical component comprises a blazed grating. 18. The radiation source of claim 1, wherein the diffractive optical component comprises a grating having a line density of at least 30 lines per millimeter. 19. A radiation source, comprising:a free electron laser configured to emit radiation having a wavelength of at most 30 nm; anda device configured to feed back the emitted radiation to the free electron laser, the device comprising first, second, third and fourth mirrors,wherein:the first mirror comprises a diffractive optical element;the first mirror is configured to separate an order of diffraction of the emitted radiation and send the separated order of diffraction of the emitted radiation to the second mirror without sending at least one other order of diffraction to the second mirror;the separated out order of diffraction of the emitted radiation comprises the zeroth diffraction order of the emitted radiation;the second mirror is configured to reflect at least a portion of the separated order of diffraction of the emitted radiation to the third mirror;the third mirror is configured to reflect at least a portion of the separated order of diffraction of the emitted radiation to the fourth mirror; andthe fourth mirror is configured to couple into the free electron laser at least a portion of the separated order of diffraction of the emitted radiation. 20. The radiation source of claim 19, wherein:the first mirror is a grazing incidence mirror;the second mirror is a normal incidence mirror;the third mirror is a normal incidence mirror; andthe fourth mirror is a grazing incidence mirror. 21. The radiation source of claim 19, wherein the diffractive optical element is drivable.