Hollow-core photonic crystal fiber and method of manufacturing thereof

1. A hollow-core photonic crystal fiber (HC-PCF), being configured for guiding at least one mode of a light field along a mode guiding section of the HC-PCF, comprising:

an outer jacket, an inner cladding, and a hollow core, that extend along the HC-PCF, wherein:

the inner cladding is arranged on an interior surface of the outer jacket and comprises anti-resonant structures surrounding the hollow core, and

the hollow core has a mode guiding core diameter (d) provided along the mode guiding section of the HC-PCF,

wherein at least one fiber end of the HC-PCF has a light field coupling section in which the hollow core is tapered over an axial coupling section length from a fiber end core diameter (D) at the at least one fiber end to the mode guiding core diameter (d),

wherein the outer jacket has a constant thickness, and

wherein the inner cladding thickness decreases towards an opening of the at least one fiber end.

2. The hollow-core photonic crystal fiber of claim 1 , wherein:

the anti-resonant structures have a cross-sectional dimension that gradually increases in the light field coupling section towards the mode guiding section.

3. The hollow-core photonic crystal fiber of claim 1 , wherein:

the fiber end core diameter (D) and the axial coupling section length are selected such that an overlap of the inner cladding and the light field to be focused to the hollow core for guiding by the HC-PCF is excluded or negligible at the fiber end.

4. The hollow-core photonic crystal fiber of claim 1 , wherein:

the axial coupling section length of the light field coupling section is at least the mode guiding core diameter (d) and/or at most a transition dimension (π(D 2 d 2 −d 4 ) 0.5 /(4λ)), where λ is the central wavelength of the light field.

5. The hollow-core photonic crystal fiber of claim 4 , wherein:

the axial coupling section length of the light field coupling section is at least 20 μm and/or at most 5000 μm.

6. The hollow-core photonic crystal fiber of claim 1 , wherein:

the anti-resonant structures have rounded ends facing toward the at least one fiber end.

7. The hollow-core photonic crystal fiber of claim 1 , wherein:

the inner cladding extends to an opening of the at least one fiber end.

8. The hollow-core photonic crystal fiber of claim 1 , wherein:

the light field coupling section is provided at an incoupling end of the HC-PCF only.

9. The hollow-core photonic crystal fiber of claim 1 , wherein:

the HC-PCF is configured to subject a light field to an optically nonlinear process, in particular spectral broadening, or

the HC-PCF is configured to deliver a light field to an application site.

10. The hollow-core photonic crystal fiber of claim 1 , wherein

the light field coupling section is formed by thermal treatment of the HC-PCF.

11. The hollow-core photonic crystal fiber of claim 10 , wherein:

resonant structures of the inner cladding are softened and the light field coupling section is formed by the effect of surface tension in the softened anti-resonant structures.

12. The hollow-core photonic crystal fiber of claim 11 , wherein:

the thermal treatment comprises a heating of the HC-PCF such that the resonant structures of the inner cladding are softened and the light field coupling section is formed by the combined effect of surface tension in the softened anti-resonant structures and an applied vacuum in at least one of the anti-resonant structures.

13. The hollow-core photonic crystal fiber method of claim 11 , wherein:

the thermal treatment comprises a heating of the HC-PCF such that the resonant structures of the inner cladding are softened and the light field coupling section is formed by the combined effect of surface tension in the softened anti-resonant structures and an applied pressure in the mode guiding core.

14. A hollow-core photonic crystal fiber (HC-PCF), being configured for guiding at least one mode of a light field along a mode guiding section of the HC-PCF, comprising:

an outer jacket, an inner cladding, and a hollow core, that extend along the HC-PCF, wherein:

the inner cladding is arranged on an interior surface of the outer jacket and comprises anti-resonant structures surrounding the hollow core, and

the hollow core has a mode guiding core diameter (d) provided along the mode guiding section of the HC-PCF,

wherein at least one fiber end of the HC-PCF has a light field coupling section in which the hollow core is tapered over an axial coupling section length from a fiber end core diameter (D) at the at least one fiber end to the mode guiding core diameter (d), and

wherein an axial transition length over which the fiber core diameter dimension reduces in the light field coupling section from the fiber end core diameter (D) to (0.5*(D+d)) is at least 0.5 times the mode guiding core diameter (d) and/or at most 0.5 times a transition dimension (π(D 2 d 2 −d 4 ) 0.5 /(4λ)), where λ is the central wavelength of the light field.

15. The hollow-core photonic crystal fiber of claim 14 , wherein:

the axial transition length is at least 10 μm and/or at most 1000 μm.

16. A hollow-core photonic crystal fiber (HC-PCF), being configured for guiding at least one mode of a light field along a mode guiding section of the HC-PCF, comprising:

an outer jacket, an inner cladding, and a hollow core, that extend along the HC-PCF, wherein:

the inner cladding is arranged on an interior surface of the outer jacket and comprises anti-resonant structures surrounding the hollow core, and

the hollow core has a mode guiding core diameter (d) provided along the mode guiding section of the HC-PCF,

wherein at least one fiber end of the HC-PCF has a light field coupling section in which the hollow core is tapered over an axial coupling section length from a fiber end core diameter (D) at the at least one fiber end to the mode guiding core diameter (d), and

wherein the inner cladding does not extend to an opening of the at least one fiber end.