Single mode optical fiber

• US 4,641,917 A
• Filed: 02/08/1985
• Issued: 02/10/1987
• Est. Priority Date: 02/08/1985
• Status: Expired due to Term

• Current Assignee: Fitel USA Corporation
• Original Assignee: AT&T, Inc.
• Inventors: Lenhan, Terrence A., Glodis, Paul F.
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): Wise, Robert E.
• Application Number:
  US06/699,641
• Time in Patent Office: 732 Days
• Field of Search: 350/96.33, 350/96.29, 350/96.30, 350/96.31
• US Class Current: 385/127
• CPC Class Codes: G02B 6/02047 Dual mode fibre G02B6/105 takes precedence
• CPC Class Codes: G02B 6/03655 arranged - + +
• CPC Class Codes: G02B 6/03666 arranged - + - +
• CPC Class Codes: G02B 6/03688 having 5 or more layers

Abstract

A single mode optical fiber comprises a core, a first cladding surrounding the core, and a second cladding surrounding the first cladding. It also comprises a third cladding region (or index "ring"). The core, has radius a and refractive index n_c, the first, second, and third cladding regions have inner radii, R_1i, R_2i, and R_3i, outer radii R_1o, R_2o, and R_3o, and indices n₁, n₂, n₃, respectively. The fiber has n₁ <n₃, R_1o ≦R_3i, R_3o ≦R_2i. In preferred embodiments, a=R_1i, R_1o =R_3i, R_3o =R_2i. Appropriate choice of ring parameters can result in a lowering of the cut-off wavelength of the fiber, or in fiber that is less sensitive to macrobending than similar prior art fiber lacking an index ring, without substantial change in fiber parameters that depend primarily on the waveform in the core. An optical fiber communications system comprising the inventive fiber is also disclosed.

Claims

1. Optical fiber adapted for guiding electromagnetic radiation, guided radiation comprising a LP₀₁ mode and, possibly, a LP₁₁ mode, the fiber being a single mode fiber for electromagnetic radiation of an operating wavelength, the optical fiber to be referred to as the first fiber, the first fiber comprising

(a) a core region having an equivalent radius a and an equivalent maximum refractive index n_c ;

(b) a first cladding region surrounding the core and having an equivalent outer radius R_1o, an equivalent inner radius R_1i, and an equivalent refractive index n₁, with n₁ <

n_c ; and

(c) a second cladding region surrounding the first cladding region and having an equivalent inner radius R_2i and an equivalent refractive index n₂, with n₂ ≧

n₁ ;

characterized in that

the first fiber further comprises a third cladding region having an equivalent inner radius R_3i, an equivalent outer radius R_3o, and an equivalent refractive index n₃, with R_3o ≦

R_2i, R_1o ≦

R_3i, and n₃ >

n₁ ;

wherein n₃, R_3i and R_3o are chosen such that

(d) at R_3i the square of the amplitude of the LP₀₁ mode is at most 10^-5 of the square of the amplitude of the LP₀₁ mode at the center of the core, for radiation of the operating wavelength of the optical fiber;

(e) at R_2i in the first fiber, the square of the amplitude of the LP₁₁ mode is at least 50% larger than the square of the amplitude of the LP₁₁ mode in a second fiber at a radius equal to R_2i, the second fiber being identical to the first fiber except that in the second fiber R_3i =R_3o, the third cladding region thus being absent in the second fiber, the second fiber having a cut-off wavelength, the squares of the amplitudes being compared at the cut-off wavelength of the second fiber; and

(f) at R_2i and at the operating wavelength, the square of the amplitude of the LP₀₁ mode in the first fiber is at most 50% larger than the square of the amplitude of the LP₀₁ mode in the second fiber, at the same wavelength and the same radius.

2. Fiber of claim 1, wherein R_3o =R_2i.

3. Fiber of claim 2, wherein R_1i =a, and R_1o =R_3i.

4. Fiber of claim 3, wherein n₁ <

n₂.

5. Fiber of claim 4, wherein R_3i ≧

4a.

6. Fiber of claim 5, wherein n₁ <

n₃ <

n₂.

7. Fiber of claim 6, wherein 0.1Δ

^- ≦

h≦

0.75Δ

^-, with h=(n₃² -n₁²)/(n₂² -n₁²), and Δ

^- =(n₂² -n₁²)/2n₂².

8. Fiber of claim 7, wherein R_3o -R_3i ≧

a.

9. Fiber of claim 7, wherein R_2i /a≧

6.5.

10. Fiber of claim 4, wherein the first fiber is a silica-based fiber, and at least the first and the third cladding regions comprise fluorine.

11. Fiber of claim 10, wherein the core comprises an element selected from the group consisting of Ge, Al, and P.

12. An optical fiber communication system comprising optical fiber according to claim 1, and further comprising a source of electromagnetic radiation of wavelength substantially equal to the operating wavelength, means for coupling the radiation into the optical fiber at a first fiber region, and means for detecting radiation emitted from the fiber at a second fiber region spaced from the first fiber region.

13. System according to claim 12, wherein R_3o =R_2i, R_1i =a, R_1o =R_3i, n_c ≧

n₂, n₁ ≦

n₂, and n₁ <

n₃.

14. System according to claim 13, wherein 0.1Δ

^- ≦

h≦

0.75Δ

^-, with h=(n₃² -n₁²)/(n₂² -n₁²), and Δ

^- =(n₂² -n₁²)/2n₂², and R_3o -R_3i ≧

a.

15. System according to claim 14, wherein R_2i /a≧

6.5.

16. System according to claim 15, wherein n₁ <

n₂.

17. System according to claim 12, wherein at least the first and the third cladding regions comprise fluorine.

18. System according to claim 12, wherein the core comprises an element selected from the group consisting of Ge, Al, and P.

19. System according to claim 12, wherein at R_2i and at the cut-off wavelength of the second fiber, the square of the amplitude of the LP₁₁ mode in the first fiber is at least 100% larger than the square of the amplitude of the LP₁₁ mode in the second fiber, at the same wavelength and the same radius.

20. System according to claim 19, wherein at R_2i and at the operating wavelength, the square of the amplitude of the LP₀₁ mode in the first fiber is at most 10% larger than the square of the amplitude of the LP₀₁ mode in the second fiber, at the same wavelength and the same radius.