Optical fiber

1. An apparatus for receiving input radiation and broadening a frequency range of the input radiation so as to provide broadband output radiation, the apparatus comprising:

an optical fiber comprising:

a hollow core;

a cladding portion surrounding the hollow core, the cladding portion comprising a plurality of anti-resonance elements for guiding radiation through the hollow core; and

a support portion surrounding and supporting the cladding portion, wherein the support portion comprises an inner support portion, an outer support portion, and a deformable connecting portion connecting the inner support portion to the outer support portion; and

a gas disposed within the hollow core configured to broaden a frequency range of a received input radiation so as to provide the broadband output radiation, wherein:

the broadband output radiation comprises a continuous range of radiation frequencies, or

the gas comprises a noble gas, and

wherein the deformable connecting portion is configured to deform thereby reducing stress in the optical fiber.

2. The apparatus of claim 1 , wherein the inner support portion, the outer support portion, and the deformable connecting portion are formed from the same material.

3. The apparatus of claim 2 , wherein the material comprises glass.

4. The apparatus of claim 1 , wherein the deformable connecting portion comprises a ring structure arranged around the inner support portion, the ring structure comprising a plurality of apertures interspersed with a plurality of generally radially extending portions of material extending generally between the inner support portion and the outer support portion.

5. The apparatus of claim 4 , wherein the deformable connecting portion comprises a plurality of ring structures arranged around the inner support portion, each ring structure comprising a plurality of apertures interspersed with a plurality of generally radially extending portions of material extending generally between the inner support portion and the outer support portion.

6. The apparatus of claim 4 , wherein in cross-section the apertures are generally rectangular.

7. The apparatus of claim 4 , wherein in cross-section the apertures are generally of the form of a sector of an annulus.

8. The apparatus of claim 4 , wherein the thickness of the inner support portion is sufficiently large that the apertures in the deformable connecting portion do not act as anti-resonance elements.

9. The apparatus of claim 1 , wherein at least one of:

the plurality of anti-resonance elements of the cladding portion are disposed in a ring structure around the hollow core,

each of the plurality of anti-resonance elements comprises a capillary, and

the plurality of anti-resonance elements is arranged so that each of the anti-resonance elements is not in contact with any of the other anti-resonance elements.

10. A radiation source for providing broadband output radiation, the radiation source comprising:

an apparatus configured to receive input radiation and broadening a frequency range of the input radiation so as to provide broadband output radiation, the apparatus comprising:

an optical fiber comprising:

a hollow core;

a cladding portion surrounding the hollow core, the cladding portion comprising a plurality of anti-resonance elements for guiding radiation through the hollow core; and

a support portion surrounding and supporting the cladding portion, wherein the support portion comprises an inner support portion, an outer support portion, and a deformable connecting portion connecting the inner support portion to the outer support portion;

a gas disposed within the hollow core configured to broaden a frequency range of a received input radiation so as to provide the broadband output radiation, wherein:

the broadband output radiation comprises a continuous range of radiation frequencies, or

the gas comprises a noble gas; and

an input radiation source configured to provide input radiation to the apparatus,

wherein the apparatus is configured to broaden the input radiation to provide the broadband output radiation, and

wherein the deformable connecting portion is configured to deform thereby reducing stress in the optical fiber.

11. A metrology arrangement for determining a parameter of interest of a structure on a substrate, the metrology arrangement comprising:

the radiation source of claim 10 ;

an illumination sub-system for illuminating the structure on the substrate using the broadband output radiation; and

a detection sub-system for detecting a portion of radiation scattered and/or reflected by the structure, and for determining the parameter of interest from said portion of radiation.

12. A lithographic apparatus comprising:

a metrology arrangement configured to determine a parameter of interest of a structure on a substrate, the metrology arrangement comprising:

the radiation source of claim 10 ;

an illumination sub-system configured to illuminate the structure on the substrate using the broadband output radiation; and

a detection sub-system for detecting a portion of radiation scattered and/or reflected by the structure, and for determining the parameter of interest from said portion of radiation.

13. An optical fiber comprising:

a hollow core; and

a cladding portion surrounding the hollow core, wherein the cladding portion comprises an inner support without microstructures, an outer portion, and a deformable connecting portion connecting the inner portion to the outer portion,

wherein the hollow core comprises no microstructures and extends to an inner surface of the cladding portion.

14. The optical fiber of claim 13 , wherein the microstructures are anti-resonance elements for guiding radiation through the hollow core.

15. A method of forming an optical fiber, the method comprising:

providing a manufacturing intermediate having an elongate body comprising:

a hollow core;

a cladding portion surrounding the hollow core; and

a support portion surrounding the cladding portion, wherein the support portion comprises an inner support portion, an outer support portion, and a deformable connecting portion connecting the inner support portion to the outer support portion, the deformable connecting portion comprising a ring structure arranged around the inner support portion, the ring structure comprising a plurality of apertures arranged generally between the inner support portion and the outer support portion; and

heating and drawing the manufacturing intermediate so as to form the optical fiber,

wherein the deformable connecting portion is configured to deform thereby reducing stress in the optical fiber.