Method of making a short-pass fiber with controlled cut-off wavelength
View Patent ↗A method of producing a short-pass fiber by drawing a preform for a depressed cladding fiber at a predetermined drawing ratio. The preform has a core of refractive index n 0 , a depressed cladding surrounding the core and having a refractive index n 1 , and a secondary cladding surrounding the depressed cladding and having a refractive index n 2 . The core has a core cross-section and the depressed cladding has a depressed cladding cross-section that is larger than the core cross-section. The drawing ratio is determined based on the parameters of the preform measured along the axis of the preform and based on a desired minimum fundamental mode cutoff wavelength λ m . The final core cross-section defines a fundamental mode cutoff wavelength λ c such that λ c ≧λ m along the axis. Then the preform is pulled by the thus-determined drawing ratio to produce the short-pass fiber. In some embodiments a test section of the fiber is pulled first before pulling the short pass fiber.
1. A method of producing a short-pass fiber comprising the following steps:
a) producing a preform comprising:
1) a core having a core cross-section and a refractive index n 0 ;
2) a depressed cladding surrounding said core and having depressed cladding cross-section larger than said core cross-section and a refractive index n 1 ;
3) a secondary cladding surrounding said depressed cladding and having a refractive index n 2 ;
b) measuring said core cross-section;
c) measuring said depressed cladding cross-section;
d) setting a minimum fundamental mode cutoff wavelength λ m ;
e) measuring a variation of said refractive index n 0 and said refractive index n 1 along said axis;
f) deriving from measurements of said core cross-section, said depressed cladding cross-section, and said variation a drawing ratio at which said core cross-section has a final value defining a fundamental mode cutoff wavelength λ c such that λ c ≧λ m along said axis;
g) pulling said preform by said drawing ratio to produce said short-pass fiber.
2. The method of claim 1 , wherein said core cross-section is circular and said final value is a final core radius.
3. The method of claim 1 , further comprising adjusting a secondary cladding cross-section before said pulling step.
4. The method of claim 3 , wherein said adjusting is performed by a technique selected from the group consisting of etching, rod-in-tube and outside vapor deposition.
5. The method of claim 1 , wherein said minimum fundamental mode cutoff wavelength λ m is set at least 5 nm below a lowest value of said fundamental mode cutoff wavelength λ c along said axis.
6. The method of claim 1 , wherein said step of pulling said preform comprises:
a) pulling a test section of said preform by said drawing ratio;
b) determining said fundamental mode cutoff wavelength λ c in said test section;
c) adjusting said drawing ratio to an adjusted drawing ratio based on the value of said fundamental mode cutoff wavelength λ c in said test section; and
d) pulling a remainder of said preform by said adjusted drawing ratio.
7. The method of claim 6 , wherein said test section comprises less than 20% of said preform.
8. The method of claim 1 , further comprising:
a) measuring an average of said core cross-section along said axis;
b) adjusting said drawing ratio to achieve an average final value of said core cross-section in said short-pass fiber.
9. The method of claim 8 , wherein said core cross-section is circular and said average final value is an average final core radius.
10. A method of producing a short-pass fiber comprising:
a) producing a preform having a core having a core cross-section and a refractive, index n 0 , a depressed cladding surrounding said core having a depressed cladding cross-section larger than said core cross-section and a refractive index n 1 , a secondary cladding surrounding said depressed cladding and having a refractive index n 2 ;
b) setting a minimum fundamental mode cutoff wavelength λ m ;
c) deriving from said core cross-section, said depressed cladding cross-section and said refractive indices n 0 , n 1 a drawing ratio at which said core cross-section has a final value defining a fundamental mode cutoff wavelength λ c that remains above said minimum fundamental mode cutoff wavelength λ m ;
d) pulling a test section of said preform by said drawing ratio;
e) determining said fundamental mode cutoff wavelength λ c in said test section;
f) adjusting said drawing ratio to an adjusted drawing ratio based on said fundamental mode cutoff wavelength λ c in said test section; and
g) pulling a remainder of said preform by said adjusted drawing ratio to produce said short-pass fiber.
11. The method of claim 10 , further comprising:
a) determining a variation of said refractive index n 0 and said refractive index n 1 along an axis of said preform; and
b) correcting said adjusted drawing ratio based on said variation.
12. The method of claim 10 , further comprising adjusting a secondary cladding cross-section before pulling said remainder of said preform.
13. The method of claim 12 , wherein said adjusting is performed by a technique selected from the group consisting of etching, rod-in-tube and outside vapor deposition.
14. The method of claim 10 , wherein said minimum fundamental mode cutoff wavelength λ m is set at least 5 nm below a lowest value of said fundamental mode cutoff wavelength λ c along an axis of said preform.
15. The method of claim 10 , wherein said test section comprises less than 20% of said preform.
16. The method of claim 10 , wherein said core cross-section is circular and said final value is a final core radius.
17. The method of claim 16 , further comprising:
a) selecting said final core radius;
b) changing said adjusted drawing ratio to achieve said final core radius in said short-pass fiber.