Structure of an input end of an optical fiber

1. A structure of an input end of an optical fiber, comprising a first optical fiber and a second optical fiber;

wherein the first optical fiber and the second optical fiber are coaxial, one end of the first optical fiber is used to receive light beam, and the other end of the first optical fiber is engaged with the second optical fiber;

wherein the first optical fiber comprises a fiber core and a first cladding surrounding its fiber core; the second optical fiber comprises a fiber core and a first cladding surrounding its fiber core;

wherein a diameter of the first cladding of the first optical fiber is larger than a diameter of the first cladding of the second optical fiber and a difference between them is larger than a first preset threshold; and

wherein a diameter of the fiber core of the first optical fiber is smaller than or equal to a diameter of the fiber core of the second optical fiber and a difference between them is smaller than a second preset threshold.

2. The structure according to claim 1 , further comprising a second cladding of the first optical fiber surrounding the first cladding of the first optical fiber.

3. The structure according to claim 1 , wherein the diameter of the first cladding of the first optical fiber gradually and uniformly decreases from a preset position until the engagement of the first optical fiber and the second optical fiber.

4. The structure according to claim 1 , further comprising an end cap;

wherein one end of the end cap is coated with an antireflection film, and the other end of the end cap is engaged with the one end of the first optical fiber, a material of the end cap is quartz or glass, and a diameter of the end cap is larger than the diameter of the fiber core of the first optical fiber.

5. The structure according to claim 1 , further comprising a sleeve;

wherein the sleeve surrounds the first optical fiber and the second optical fiber, and a material of the sleeve is metal or glass;

wherein when the material of the sleeve is metal, a gap formed between an inner surface of the sleeve and an outer surface of the first cladding of the second optical fiber is filled with glue; and

wherein when the material of the sleeve is glass, the gap formed between the inner surface of the sleeve and the outer surface of the first cladding of the second optical fiber is filled with glue, or the inner surface of the sleeve and the outer surface of the first cladding of the second optical fiber are welded by laser or electric arc.

6. The structure according to claim 1 , wherein

an end face of the one end of the first optical fiber is coated with an antireflection film.

7. The structure according to claim 1 , wherein

the other end of the first optical fiber is engaged with the second optical fiber by way of electrode discharge fusion, laser heating fusion or adhesive bonding.

8. The structure according to claim 1 , wherein

a ratio of the diameter of the first cladding of the first optical fiber to the diameter of the fiber core of the first optical fiber is 1.1˜50.

9. The structure according to claim 1 , wherein

a length of the first optical fiber is 1 mm˜3000 mm.

10. The structure according to claim 1 , wherein

the first optical fiber is a single-mode optical fiber or a multimode optical fiber; the second optical fiber is a single-mode optical fiber or a multimode optical fiber;

the first optical fiber is a shaped optical fiber with D-shaped or square shaped fiber core and cladding, or the first optical fiber is a photonic crystal optical fiber; and

the second optical fiber is a shaped optical fiber with D-shaped or square shaped fiber core and cladding, or the second optical fiber is a photonic crystal optical fiber.