Claims:CLAIMS
What is claimed is:

1. An optical fiber (100) comprising:

a first region (104) defined from a longitudinal axis to a first pre-defined radius r1 from the longitudinal axis, wherein the first region (104) is a core region, wherein the first region (104) is made of pure silica and wherein the first region (104) has a first pre-defined refractive index a1;

a second region (106) defined from the first pre-defined radius r1 to a second pre-defined radius r2 from the longitudinal axis, wherein the second region (106) having a down dopant in silica, wherein the second region (106) has a second pre-defined refractive index a2 varying from the first pre-defined radius r1 to the second pre-defined radius r2 and wherein the second pre-defined refractive index a2 varies with a second pre-defined concentration of the down dopant in silica from the first pre-defined radius r1 to the second pre-defined radius r2;

a third region (108) defined from the second pre-defined radius r2 to a third pre-defined radius r3 from the longitudinal axis, wherein the third region (108) having the down dopant in silica, wherein the third region (108) has a third pre-defined concentration c3 of the down dopant in silica and a third pre-defined refractive index a3;

a fourth region (110a-b) comprising:
a first stress application part (110a) defined from the third pre-defined radius r3 to a fourth pre-defined radius r4 from the longitudinal axis; and
a second stress application part (110b) defined from the third pre-defined radius r3 to the fourth pre-defined radius r4 from the longitudinal axis, wherein the first stress application part (110a) and the second stress application part (110b) are made of boron trioxide doped silica, wherein the first stress application part (110a) and the second stress application part (110b) are positioned symmetrically along the longitudinal axis and wherein the first stress application part (110a) and the second stress application part (110b) have a fourth pre-defined concentration c4 of boron trioxide and wherein the first stress application part (110a) and the second stress application part (110b) have a circular cross-section lying along the longitudinal axis;

a fifth region (112) is an area defined from the third pre-defined radius r3 to a fifth pre-defined radius r5 excluding an area covered by the fourth region, wherein the fifth region (112) is having the down dopant in silica and wherein the fifth region (112) has a fifth pre-defined concentration c5 of the down dopant and a fifth predefined refractive index a5; and

a sixth region (114) defined from the fifth pre-defined radius r5 to a sixth pre-defined radius r6 from the longitudinal axis, wherein the sixth region (114) is made of pure silica and wherein the sixth region (114) has a sixth pre-defined refractive index a6.

2. The optical fiber (100) as recited in claim 1, wherein the optical fiber (100) has an attenuation of 0.46 decibel per kilometer (dB/km) for a fiber diameter of 125 µm.

3. The optical fiber (100) as recited in claim 1, wherein the optical fiber (100) has an attenuation of 1.96 decibel per kilometer (dB/km) for a fiber diameter of 80 µm.

4. The optical fiber (100) as recited in claim 1, wherein the optical fiber (100) has an effective area in a range of 75 µm2 –95 µm2.

5. The optical fiber (100) as recited in claim 1, wherein the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm, the second pre-defined radius r2 is in a range of 2.5 µm-25 µm, the third pre-defined radius r3 is in a range of 2.6 µm-22.5 µm, the fourth pre-defined radius r4 is in a range of (r3+3) µm-(r3+15) µm, the fifth pre-defined radius r5 is in a range of 37 µm-38 µm and the sixth pre-defined radius r6 is 39 µm-41 µm when the optical fiber has a fiber diameter of 80 µm and when dimension of each region is measured along a first axis.

6. The optical fiber (100) as recited in claim 1, wherein the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm, the second pre-defined radius r2 is in a range of 2.5 µm-25 µm, the third pre-defined radius r3 is in a range of 2.6 µm-22.5 µm, the fourth pre-defined radius r4 is in a range of (r3+3) µm-(r3+20) µm, the fifth pre-defined radius r5 is in a range of 55 µm-58 µm and the sixth pre-defined radius r6 is 62 µm-63 µm when the optical fiber has a fiber diameter of 125 µm and when dimension of each region is measured along a first axis.

7. The optical fiber (100) as recited in claim 1, wherein the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm, the second pre-defined radius r2 is in a range of 2.5 µm-25 µm, the third pre-defined radius r3' is in a range of 37 µm-38 µm and the sixth pre-defined radius r6 is 39 µm-41 µm when the optical fiber (100) has a fiber diameter of 80 µm and when dimension of each region is measured along a second axis.

8. The optical fiber (100) as recited in claim 1, wherein the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm, the second pre-defined radius r2 is in a range of 2.5 µm-25 µm, the third pre-defined radius r3' is in a range of 55 µm-58 µm and the sixth pre-defined radius r6 is 62 µm-63 µm when the optical fiber (100) has a fiber diameter of 125 µm and when dimension of each region is measured along a second axis.

9. The optical fiber (100) as recited in claim 1, wherein the first pre-defined refractive index a1 is 1.444, the second pre-defined refractive index a2 varies in a range of 1.438-1.441, the third pre-defined refractive index a3 is in a range of 1.438-1.440, the fifth pre-defined refractive index a5 is in a range of 1.438-1.440 and the sixth pre-defined refractive index a6 is 1.444 when the optical fiber (100) has a fiber diameter of 80 µm and when the refractive index is measured along a first axis at a wavelength of 1550 nanometers.

10. The optical fiber (100) as recited in claim 1, wherein the first pre-defined refractive index a1 is 1.444, the second pre-defined refractive index a2 varies in a range of 1.438-1.441, the third pre-defined refractive index a3 is in a range of 1.438-1.440, the fifth pre-defined refractive index a5 is in a range of 1.438-1.440 and the sixth pre-defined refractive index a6 is 1.444 when the optical fiber (100) has a fiber diameter of 125 µm and when the refractive index is measured along a first axis at a wavelength of 1550 nanometers.

11. The optical fiber (100) as recited in claim 1, wherein the first pre-defined refractive index a1 is 1.444, the second pre-defined refractive index a2 varies in a range of 1.438-1.441, the third pre-defined refractive index a3' is in a range of 1.438-1.440 and the sixth pre-defined refractive index a6 1.444 when the optical fiber (100) has a fiber diameter of 80 µm and when the refractive index is measured along a second axis at a wavelength of 1550 nanometers.

12. The optical fiber (100) as recited in claim 1, wherein the first pre-defined refractive index a1 is 1.444, the second pre-defined refractive index a2 varies in a range of 1.438-1.441, the third pre-defined refractive index a3' is in a range of 1.438-1.440 and the sixth pre-defined refractive index a6 is 1.444 when the optical fiber (100) has a fiber diameter of 125 µm and when the refractive index is measured along a second axis at a wavelength of 1550 nanometers.

13. The optical fiber (100) as recited in claim 1, wherein the third pre-defined concentration c3 of fluorine in silica is in a range of 0.9 mol%-1.26 mol%, the fourth pre-defined concentration c4 of boron trioxide in silica is in a range of 5 mol%-25 mol% and the fifth pre-defined concentration c5 of fluorine is 0.9 mol%-1.26 mol% when the optical fiber has a fiber diameter of 80 µm and 125 µm and wherein the concentration is measured along a first axis.

14. The optical fiber (100) as recited in claim 1, wherein the second pre-defined concentration is in a range of 0.68 mol%-1.26 mol%, the third pre-defined concentration c3' of fluorine in silica is in a range of 0.9 mol%-1.26 mol% when the optical fiber (100) has a fiber diameter of 80 µm and 125 µm and wherein dimension of each region is measured along a first axis.

15. The optical fiber (100) as recited in claim 1, wherein the first region, the second region, the third region, the fifth region and the sixth region are concentrically arranged and wherein the first pre-defined refractive index a1 is more than the second pre-defined refractive index a2, the third pre-defined refractive index a3 is more than the fourth pre-defined refractive index a4, the fifth pre-defined refractive index a5 is equal to the third pre-defined refractive index a3 and the sixth pre-defined refractive index a6 is equal to the first pre-defined refractive index a1.

16. The optical fiber (100) as recited in claim 1, wherein the down dopant added in silica is fluorine.

Dated: 31st Day of March, 2016 Signature
Arun Kishore Narasani Patent Agent

, Description:TECHNICAL FIELD
[0001] The present disclosure relates to the field of fiber optics and, in particular, relates to a design and structure of a polarization maintaining optical fiber.
BACKGROUND
[0002] A standard single mode fiber supports two orthogonally polarized modes. These two orthogonally polarized modes have same propagation constant. Thus, the two orthogonally polarized modes have same phase velocity. In practical situations, these two modes propagate with slightly different phase and group velocities due to inherent asymmetries of core ellipticity. Further, the factors such as bend, twist, and stress produce external birefringence in the fiber. These factors lead to coupling of optical energy from one mode to the other mode of the fiber. In order to preserve the state of polarization inside the optical fiber, polarization maintaining fiber is used. In a polarization maintaining fiber, two stress application parts are introduced symmetrically alongside the core at some finite distance. Traditionally, these two stress application parts are made of materials having high thermal coefficient of expansion compared to the optical fiber cladding. As the fiber is cooled in the fiber drawing process, these stress application parts lose their heat quicker than the cladding and thereby shrink. The stress application parts following their shrink apply a stress on the attached core and cladding. This produces high internal birefringence in the core of the polarization maintaining optical fiber and makes it anisotropic. Thus, the traditional polarization maintaining fiber shows different propagation constant and different velocities for different modes of polarization.
[0003] In some of the conventional polarization maintaining fiber designs, the core is made of SiO2 and GeO2, the cladding is made of pure SiO2 and the stress application parts (SAPs) are made from doping of B2O3 in SiO2. These conventional optical fiber designs have very high attenuation of the order of 0.5 dB/km for 125 microns and 2-5 dB/km for 80 microns clad diameter. Thus, the use of high power lasers is required to compensate for the attenuation. However, the use of high power laser leads to non-linearity and escalation of cost. The high effective area facilitates reduction of non-linearity and splice loss as compared to conventional single mode fiber designs. The conventional polarization maintaining optical fibers have low effective area of 28-30 µm2 which eventually results in low transmission efficiency.

[0004] In light of the above stated discussion, there is a need for a polarization maintaining optical fiber that overcomes the above stated disadvantages.
OBJECT OF THE DISCLOSURE
[0005] A primary object of the present disclosure is to provide an optical fiber having high internal birefringence and polarization maintaining property.

[0006] Another object of the present disclosure is to provide an optical fiber having low attenuation.

[0007] Yet another object of the present disclosure is to provide an optical fiber having high effective area.
SUMMARY
[0008] In an aspect, the present disclosure provides an optical fiber. The optical fiber includes a first region defined from a longitudinal axis to a first pre-defined radius r1 from the longitudinal axis. The term ‘defined’ refers to derived/starts. Further, the optical fiber includes a second region defined from the first pre-defined radius r1 to a second pre-defined radius r2 from the longitudinal axis. The optical fiber includes a third region defined from the second pre-defined radius r2 to a third pre-defined radius r3 from the longitudinal axis. Furthermore, the optical fiber includes a fourth region. The fourth region includes a first stress application part defined from the third pre-defined radius r3 to a fourth pre-defined radius r4 from the longitudinal axis. In addition, the fourth region includes a second stress application part defined from the third pre-defined radius r3 to the fourth pre-defined radius r4 from the longitudinal axis. The optical fiber includes a fifth region is an area defined from the third pre-defined radius r3 to a fifth pre-defined radius r5 excluding an area covered by the fourth region. Further, the optical fiber includes a sixth region defined from the fifth pre-defined radius r5 to a sixth pre-defined radius r6 from the longitudinal axis. The first region is a core region. The first region is made of pure silica and the first region has a first pre-defined refractive index a1. Moreover, the second region includes a down dopant in silica. The second region has a second pre-defined refractive index a2 in a range of 1.438-1.441. The second pre-defined refractive index a2 varies with a second pre-defined concentration in a range of 0.68 mol%-1.26 mol% from the first pre-defined radius r1 to the second pre-defined radius r2. Further, the third region includes the down dopant in silica. The third region has a third pre-defined concentration c3 of the down dopant and a third pre-defined refractive index a3. Moreover, the first stress application part and the second stress application part are made of boron trioxide doped silica. The first stress application part and the second stress application part are positioned symmetrically along the longitudinal axis. In addition, the first stress application part and the second stress application part have a fourth pre-defined concentration c4 of boron trioxide (B2O3). The first stress application part and the second stress application part have a circular cross-section lying along the longitudinal axis. Moreover, the fifth region is made from the doping of the down dopant in silica. The fifth region has a fifth pre-defined concentration c5 of the down dopant and a fifth predefined refractive index a5. In addition, the sixth region is made of pure silica and the sixth region has a sixth pre-defined refractive index a6.
[0009] In an embodiment of the present disclosure, the optical fiber has an attenuation of 0.46 decibel per kilometer (dB/km) for a fiber diameter of 125 µm.

[0010] In another embodiment of the present disclosure, the optical fiber has the attenuation of 1.96 decibel per kilometer (dB/km) for the fiber diameter of 80 µm.

[0011] In an embodiment of the present disclosure, the optical fiber has an effective area in a range of 75 µm2 –95 µm2.

[0012] In an embodiment of the present disclosure, the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm and the second pre-defined radius r2 is in a range of 2.5 µm-25 µm when the optical fiber has a fiber diameter of 80 µm and when dimensions of each region is measured along a first axis. The third pre-defined radius r3 is in a range of 2.6 µm-22.5 µm and the fourth pre-defined radius r4 is in a range of (r3+3) µm-(r3+15) µm when the optical fiber has the fiber diameter of 80 µm and when the dimensions of each region is measured along the first axis. The fifth pre-defined radius r5 is in a range of 37 µm-38 µm and the sixth pre-defined radius r6 is 39 µm-41 µm when the dimension of each region is measured along the first axis.

[0013] In an embodiment of the present disclosure, the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm when the optical fiber has the fiber diameter of 125 µm and when the dimension of each region is measured along the first axis. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and the third pre-defined radius r3 is in a range of 2.6 µm-22.5 µm when the optical fiber has the fiber diameter of 125 µm. In addition, the fourth pre-defined radius r4 is in a range of (r3+3) µm-(r3+20) µm and the fifth pre-defined radius r5 is in a range of 55 µm-58 µm when the optical fiber has the fiber diameter of 125 µm. Further, the sixth pre-defined radius r6 is 62 µm-63 µm when the dimension of each region is measured along the first axis.

[0014] In an embodiment of the present disclosure, the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm when the optical fiber has a fiber diameter of 80 µm and when dimension of each region is measured along a second axis. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and the third pre-defined radius r3' is in a range of 37 µm-38 µm when the optical fiber has the fiber diameter of 80 µm. In addition, the sixth pre-defined radius r6 is 39 µm-41 µm when the dimension of each region is measured along the second axis.

[0015] In an embodiment of the present disclosure, the first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm when the optical fiber has the fiber diameter of 125 µm and when the dimension of each region is measured along the second axis. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and the third pre-defined radius r3' is in a range of 55 µm-58 µm when the optical fiber has the fiber diameter of 125 µm. In addition, the sixth pre-defined radius r6 is 62 µm-63 µm when the dimension of each region is measured along the second axis.

[0016] In an embodiment of the present disclosure, the first axis and the second axis are planar and orthogonal in a two dimensional plane. The first axis symmetrically divides the first stress application part and the second stress application part. Accordingly, the second axis symmetrically divides a cross-section of the optical fiber orthogonally to the first axis.

[0017] In an embodiment of the present disclosure, the first pre-defined refractive index a1 is 1.444 when the optical fiber has the fiber diameter of 80 µm and when refractive index is measured along the first axis. The second pre-defined refractive index a2 is in a range of 1.438-1.441 and the third pre-defined refractive index a3 is in a range of 1.438-1.440 when the optical fiber has the fiber diameter of 80 µm. In addition, the fifth pre-defined refractive index a5 is in a range of 1.438-1.440 when the optical fiber has the fiber diameter of 80 µm. Further, the sixth pre-defined refractive index a6 is 1.444 at a wavelength of 1550 nanometer when the optical fiber has the fiber diameter of 80 µm and when the refractive index of each region is measured along the first axis.

[0018] In an embodiment of the present disclosure, the first pre-defined refractive index a1 is 1.444 when the optical fiber has the fiber diameter of 125 µm and when refractive index is measured along the first axis. The second pre-defined refractive index a2 is in a range of 1.438-1.441 and the third pre-defined refractive index a3 is in a range of 1.438-1.440 when the optical fiber has the fiber diameter of 125 µm. In addition, the fifth pre-defined refractive index a5 is in a range of 1.438-1.440 when the optical fiber has the fiber diameter of 125 µm and when the refractive index is measured along the first axis. Further, the sixth pre-defined refractive index a6 is 1.444 at the wavelength of 1550 nanometer when the optical fiber has the fiber diameter of 125 µm and when the refractive index is measured along the first axis.

[0019] In an embodiment of the present disclosure, the first pre-defined refractive index a1 is 1.444 when the optical fiber has the fiber diameter of 80 µm and when the refractive index is measured along the second axis. The second pre-defined refractive index a2 is in a range of 1.438-1.441 and the third pre-defined refractive index a3' is in a range of 1.438-1.440 when the optical fiber has the fiber diameter of 80 µm. In addition, the sixth pre-defined refractive index a6 is 1.444 at the wavelength of 1550 nanometer when the optical fiber has the fiber diameter of 80 µm.

[0020] In an embodiment of the present disclosure, the first pre-defined refractive index a1 is 1.444 when the optical fiber has the fiber diameter of 125 µm and when the refractive index of each region is measured along the second axis. The second pre-defined refractive index a2 is in a range of 1.438-1.441 and the third pre-defined refractive index a3’ is in a range of 1.438-1.440 when the optical fiber has the fiber diameter of 125 µm. The sixth pre-defined refractive index a6 is 1.444 at the wavelength of 1550 nanometer when the optical fiber has the fiber diameter of 125 µm and the refractive index is measured along the second axis.

[0021] In an embodiment of the present disclosure, the second pre-defined concentration c2 of fluorine is between 0.68 mol%-1.26 mol% when the optical fiber has the fiber diameter of 80 µm. The second pre-defined concentration is from the first pre-defined radius r1 to the second pre-defined radius r2. The third pre-defined concentration c3 of fluorine in silica is in a range of 0.9 mol%-1.26 mol% when the optical fiber has the fiber diameter of 80 µm. The fourth pre-defined concentration c4 of boron trioxide (B2O3) in silica is in a range of 5 mol%- 25 mol% when the optical fiber has the fiber diameter of 80 µm. In addition, the fifth pre-defined concentration c5 of fluorine is 0.9 mol%-1.26 mol% when the optical fiber has the fiber diameter of 80 µm and 125 µm and wherein concentration of each region is measured along the first axis.

[0022] In an embodiment of the present disclosure, the second pre-defined concentration of fluorine is in a range of 0.68 mol%-1.26 mol% when the optical fiber has the fiber diameter of 80 µm and 125 µm and the concentration of each region is measured along the second axis. Furthermore, the third pre-defined concentration c3’ of fluorine in silica is in a range of 0.9 mol%-1.26 mol% when the optical fiber has the fiber diameter of 80 µm and 125 µm. In addition, the concentration of each region is measured along the second axis.

[0023] In an embodiment of the present disclosure, the second pre-defined concentration c2 of fluorine is between 0.68 mol%-1.26 mol% when the concentration of each region is measured along the first axis. The third pre-defined concentration c3 is in a range of 0.9 mol%-1.26 mol% and the fourth pre-defined concentration c4 of boron trioxide is in a range of 5 mol%-25 mol% when the concentration of each region is measured along the first axis. Further, the fifth pre-defined concentration of fluorine c5 is in a range of 0.9 mol%-1.26 mol% when the optical fiber has the fiber diameter of 80 µm and 125 µm. In addition, the dimension of each region is measured along the first axis.
[0024] In an embodiment of the present disclosure, the first region, the second region, the third region, the fifth region and the sixth region are concentrically arranged. The first pre-defined refractive index a1 is more than the second pre-defined refractive index a2 and the third pre-defined refractive index a3 is more than the fourth pre-defined refractive index a4. In addition, the fifth pre-defined refractive index a5 is equal to the third pre-defined refractive index a3. Also, the first pre-defined refractive index a1 is equal to the sixth pre-defined refractive index a6.

[0025] In an embodiment of the present disclosure, the down dopant added in silica is fluorine.
STATEMENT OF THE DISCLOSURE
[0026] The present disclosure relates to an optical fiber. The optical fiber includes a first region defined from a longitudinal axis to a first pre-defined radius r1 from the longitudinal axis. Further, the optical fiber includes a second region defined from the first pre-defined radius r1 to a second pre-defined radius r2 from the longitudinal axis. The optical fiber includes a third region defined from the second pre-defined radius r2 to a third pre-defined radius r3 from the longitudinal axis. Furthermore, the optical fiber includes a fourth region. The fourth region includes a first stress application part defined from the third pre-defined radius r3 to a fourth pre-defined radius r4 from the longitudinal axis. In addition, the fourth region includes a second stress application part defined from the third pre-defined radius r3 to the fourth pre-defined radius r4 from the longitudinal axis. The optical fiber includes a fifth region is an area defined from the third pre-defined radius r3 to a fifth pre-defined radius r5 excluding an area covered by the fourth region. Further, the optical fiber includes a sixth region defined from the fifth pre-defined radius r5 to a sixth pre-defined radius r6 from the longitudinal axis. The first region is a core region. The first region is made of pure silica and the first region has a first pre-defined refractive index a1. Moreover, the second region includes a down dopant in silica. The second region has a second pre-defined refractive index a2 in a range of 1.438-1.441. The second pre-defined refractive index a2 varies with a second pre-defined concentration is between 0.68 mol%-1.26 mol% from the first pre-defined radius r1 to the second pre-defined radius r2. Further, the third region includes the down dopant in silica. The third region has a third pre-defined concentration c3 of the down dopant and a third pre-defined refractive index a3. Moreover, the first stress application part and the second stress application part are made of boron trioxide doped silica. The first stress application part and the second stress application part are positioned symmetrically along the longitudinal axis. In addition, the first stress application part and the second stress application part have a fourth pre-defined concentration c4 of boron trioxide. The first stress application part and the second stress application part have a circular cross-section lying along the longitudinal axis. Moreover, the fifth region is made from the doping of the down dopant in silica. The fifth region has a fifth pre-defined concentration c5 of the down dopant and a fifth predefined refractive index a5. In addition, the sixth region is made of pure silica and the sixth region has a sixth pre-defined refractive index a6.
BRIEF DESCRIPTION OF FIGURES
[0027] Having thus described the disclosure in general terms, reference will now be made to the accompanying figures, wherein:

[0028] FIG. 1A illustrates a cross-sectional view of an optical fiber, in accordance with an embodiment of the present disclosure;
[0029] FIG. 1B illustrates a perspective view of the optical fiber of FIG. 1A, in accordance with an embodiment of the present disclosure;

[0030] FIG. 2A illustrate a first refractive index profile of a 80 µm diameter optical fiber and a 125 µm diameter optical fiber of FIG. 1A along a first axis, in accordance with an embodiment of the present disclosure;

[0031] FIG. 2B illustrates a table listing a range of radii for each region of the 80 µm diameter optical fiber of FIG. 1A along the first axis, in accordance with an embodiment of the present disclosure;

[0032] FIG. 2C illustrates the table listing the range of radii for each region of the 125 µm optical fiber of FIG. 1A along the first axis, in accordance with another embodiment of the present disclosure;

[0033] FIG. 2D illustrates the table listing a range of refractive indices for each region of the 80 µm optical fiber of FIG. 1A along the first axis, in accordance with an embodiment of the present disclosure;

[0034] FIG. 2E illustrates the table listing the range of refractive indices for each region of the 125 µm optical fiber of FIG. 1A along the first axis, in accordance with another embodiment of the present disclosure;

[0035] FIG. 3A illustrate a second refractive index profile of the 80 µm optical fiber and the 125 µm optical fiber of FIG. 1A along a second axis, in accordance with an embodiment of the present disclosure;

[0036] FIG. 3B illustrates a table listing a range of radii for each region of the 80 µm optical fiber of FIG. 1A along the second axis, in accordance with an embodiment of the present disclosure;

[0037] FIG. 3C illustrates the table listing the range of radii for each region of the 125 µm optical fiber of FIG. 1A along the second axis, in accordance with another embodiment of the present disclosure;

[0038] FIG. 3D illustrates the table listing a range of refractive indices for each region of the 80 µm optical fiber of FIG. 1A along the second axis, in accordance with an embodiment of the present disclosure; and

[0039] FIG. 3E illustrates the table listing the range of refractive indices for each region of the 125 µm optical fiber of FIG. 1A along the second axis, in accordance with another embodiment of the present disclosure;

[0040] It should be noted that the accompanying figures are intended to present illustrations of exemplary embodiments of the present disclosure. These figures are not intended to limit the scope of the present disclosure. It should also be noted that accompanying figures are not necessarily drawn to scale.

DETAILED DESCRIPTION
[0041] Reference will now be made in detail to selected embodiments of the present disclosure in conjunction with accompanying figures. The embodiments described herein are not intended to limit the scope of the disclosure, and the present disclosure should not be construed as limited to the embodiments described. This disclosure may be embodied in different forms without departing from the scope and spirit of the disclosure. It should be understood that the accompanying figures are intended and provided to illustrate embodiments of the disclosure described below and are not necessarily drawn to scale. In the drawings, like numbers refer to like elements throughout, and thicknesses and dimensions of some components may be exaggerated for providing better clarity and ease of understanding.

[0042] It should be noted that the terms "first", "second", and the like, herein do not denote any order, ranking, quantity, or importance, but rather are used to distinguish one element from another. Further, the terms "a" and "an" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

[0043] FIG. 1A illustrates a cross-sectional view of an optical fiber 100, in accordance with an embodiment of the present disclosure. The optical fiber 100 is polarization maintaining optical fiber. The optical fiber 100 is designed to maintain a state of polarization (hereinafter “SOP”) across orthogonal polarization modes. The optical fiber 100 is a single mode fiber.

[0044] The optical fiber 100 is utilized for polarization sensitive applications that demand a maintained state of polarization of input signal along a length of optical fiber 100. In an example, the optical fiber 100 is used for polarization sensitive application in space technology and space communication. The optical fiber 100 maintains the SOP based on principle of stress induced polarization maintenance.

[0045] The cross-section of the optical fiber 100 is symmetrically described in a 2-D plane. The 2-D plane is characterized by a first axis 102a and a second axis 102b. The first axis 102a and the second axis 102b are planar and orthogonal in the 2-D plane. The first axis 102a and the second axis 102b are mutually defined from a longitudinal axis 102c (as shown in FIG. 1B). The term ‘defined’ refers to derived/starts. The longitudinal axis 102c is non-planar and is defined to be orthogonal to the first axis 102a and the second axis 102b. The optical fiber 100 includes a first region 104, a second region 106, a third region 108, a fourth region 110a-b, a fifth region 112 and a sixth region 114. The first region 104 corresponds to a core region of the optical fiber 100. Moreover, the second region 106, the third region 108 and the fifth region 112 correspond to a cladding region of the optical fiber 100. Further, the fourth region 110a-b corresponds to a first stress application part 110a and a second stress application part 110b. The first axis 102a is defined from the longitudinal axis 102c and symmetrically divides the first stress application part 110a and the second stress application part 110b. The second axis 102b is defined from the longitudinal axis 102c and orthogonally divides the cross-section of the optical fiber 100. The above mentioned six regions are selectively doped with dopant elements for desired effect. In addition, the optical fiber 100 is symmetrically distributed along the longitudinal axis 102c.

[0046] In an embodiment of the present disclosure, the fiber diameter of the optical fiber 100 is 125 µm. In another embodiment of the present disclosure, the fiber diameter of the optical fiber 100 is 80 µm. In an embodiment of the present disclosure, the optical fiber 100 has the attenuation of 1.96 decibel per kilometer (dB/km) for fiber diameter of 80 µm. In another embodiment of the present disclosure, the optical fiber 100 has the attenuation of 0.46 decibel/kilometer (dB/km) for the fiber diameter of 125 µm.

[0047] The first region 104 is defined from the longitudinal axis 102c to a first pre-defined radius r1 from the longitudinal axis 102c. Moreover, the first region 104 carries two orthogonal polarization modes of light signal throughout the length of the optical fiber 100. The first region 104 is made of pure silica (SiO2). In addition, the first region 104 has a first pre-defined refractive index a1. The first region 104 of the optical fiber 100 has relatively higher refractive index compared to all regions except the sixth region 114 of the optical fiber 100. The relatively higher refractive index creates condition for propagation of light inside the optical fiber 100.

[0048] The second region 106 is defined from the first pre-defined radius r1 to a second pre-defined radius r2 from the longitudinal axis 102c. Moreover, the second region 106 is made of down doped silica. In general, the down doped silica corresponds to a silica region doped with a down dopant which is used to decrease the refractive index. In an embodiment of the present disclosure, the down dopant introduced in the silica is fluorine. The second region 106 has a second pre-defined refractive index a2 in a range of 1.438-1.441. The second pre-defined refractive index a2 varies between a second pre-defined concentration c2 of the down dopant. In an embodiment of the present disclosure, the second pre-defined concentration c2 of the fluorine in silica increases is between 0.68 mol% to 1.26 mol% from the first pre-defined radius r1 to the second pre-defined radius r2. Fluorine is used as the down dopant to decrease the second pre-defined refractive index a2. Fluorine is used as the down dopant owing to property of fluorine to dissolve well in the glass matrix of the silica with less effect on material properties of glass. Further, the second pre-defined refractive index a2 decreases with the second pre-defined radius r2. In general, the decrease in refractive index follows the increase in concentration of fluorine in silica in the second region 106. In addition, the second region 106 has a negative gradient of refractive index from the first pre-defined radius r1 to the second pre-defined radius r2. The negative gradient in the second region 106 improves an effective area of the optical fiber 100. In an embodiment of the present disclosure, the optical fiber 100 has an effective area in a range of 75 µm2-95 µm2. In another embodiment of the present disclosure, the optical fiber 100 has the effective area of 81.2 µm2. In yet another embodiment of the present disclosure, the optical fiber 100 may have any suitable effective area. In general, the effective area (hereinafter “Aeffective”) is defined as:
Aefective = (? E2 dA) / (? E4 dA)
[0049] Here, E is amplitude of electric field and integration is done over the whole plane. For example, the effective area for a Gaussian beam with mode field radius w is pw2. Moreover, the higher effective area facilitates a reduction in splice related loss between two optical fibers and reduces non-linearity in optical fiber 100.

[0050] The third region 108 is defined from the second pre-defined radius r2 to a third pre-defined radius r3 from the longitudinal axis 102c. Moreover, the third region 108 is made of down doped silica. In an embodiment of the present disclosure, the down dopant introduced in the third region 108 is fluorine. The third region 108 has a third pre-defined concentration c3 of fluorine in silica. The third pre-defined concentration c3 is constant from the second pre-defined radius r2 to the third pre-defined radius r3. The third region 108 has a third pre-defined refractive index a3. In addition, fluorine doping in silica facilitates confinement of light inside the first region 104 of the optical fiber 100.

[0051] Further, the fourth region 110a-b includes the first stress application part 110a and the second stress application part 110b. The first stress application part 110a and the second stress application part 110b are circular regions. The first stress application part 110a and the second stress application part 110b are equally separated and positioned symmetrically from the longitudinal axis 102c. The first stress application part 110a is defined from the third pre-defined radius r3 to a fourth pre-defined radius r4 from the longitudinal axis 102c. Similarly, the second stress application part 110b is defined from the third pre-defined radius r3 to the fourth pre-defined radius r4 from the longitudinal axis 102c. In an embodiment of the present disclosure, the first stress application part 110a and the second stress application part 110b are made from doping of boron trioxide (B2O3) in silica. In another embodiment of the present disclosure, the first stress application part 110a and the second stress application part 110b are made from doping of aluminum trioxide (Al2O3) doping in Silica. In yet another embodiment of the present disclosure, the first stress application part 110a and the second stress application part 110b are made from doping of any suitable metal or metal oxide. Examples of the suitable metal include but may not be limited to chromium (Cr), cobalt (Co), Iron (Fe), Zinc (Zn) and Nickel (Ni). The doping of boron trioxide is preferred owing to lower attenuation in the optical fiber 100. The first stress application part 110a and the second stress application part 110b have a fourth pre-defined concentration c4 of boron trioxide. Moreover, the first stress application part 110a and the second stress application part 110b have a circular cross section along the longitudinal axis 102c.
[0052] The first stress application part 110a and the second stress application part 110b are separated from the longitudinal axis 102c by a pre-defined separation. The pre-defined separation is equal to the third pre-defined radius r3. The first stress application part 110a and the second stress application part 110b are made from the material having a high coefficient of thermal expansion. Accordingly, the first stress application part 110a and the second stress application part 110b shrink relatively more than the first region 104, the second region 106 and the third region 108. The shrinkage in size of the first stress application part 110a and the second stress application part 110b produces compressive stress in the optical fiber 100. In addition, the compressive stress from the first stress application part 110a and the second stress application part 110b are symmetrical along the longitudinal axis 102c and opposite in direction placed along the first axis 102a. Accordingly, the compressive force of first stress application part 110a and the second stress application part 110b is propagated to the first region 104, the second region 106 and the third region 108.

[0053] Further, the compressive stress induces significantly high internal birefringence in the optical fiber 100. In general, the internal birefringence is an optical property of a material having a refractive index that depends on the state of polarization and propagation direction of light inside the optical fiber 100. The internal birefringence is a property of anisotropic materials. For example, a standard single mode fiber is isotropic. The isotropic nature of the standard single mode fiber shows no birefringence. In addition, the application of compressive stress makes the optical fiber 100 anisotropic.

[0054] Going further, the fifth region 112 is an area defined from the third pre-defined radius r3 to a fifth pre-defined radius r5 excluding an area covered by the fourth region 110a-b. Moreover, the fifth region 112 is made of the down doped silica. In an embodiment of the present disclosure, the down dopant introduced in the silica is fluorine. The fifth region 112 has a fifth pre-defined concentration c5 of the down dopant. The fifth region 112 has a fifth predefined refractive index a5. The sixth region 114 is defined from the fifth pre-defined radius r5 to a sixth pre-defined radius r6 from the longitudinal axis 102c. The sixth region 114 is made of pure silica. Moreover, the sixth region 114 has a refractive index a6. The first region 104, the second region 106, the third region 108, the fifth region 112 and the sixth region 114 of the optical fiber 100 are concentrically arranged. The first pre-defined refractive index a1 is more than the second pre-defined refractive index a2. The third pre-defined refractive index a3 is more than the fourth pre-defined refractive index a4 of the optical fiber 100. In addition, the fifth pre-defined refractive index a5 is equal to the third pre-defined refractive index a3 of the optical fiber 100. Also, the first pre-defined refractive index a1 is equal to the sixth pre-defined refractive index a6.

[0055] FIG. 2A illustrate a refractive index profile 200 of a 80 µm optical fiber and a 125 µm optical fiber along the first axis 102a, in accordance with an embodiment of the present disclosure. The refractive index profile 200 describes a graphical plot of a variation of refractive index of each region as a function of radius measured from the longitudinal axis 102c (shown as Refractive Index). The refractive index profile 200 is characterized by the first pre-defined refractive index a1, the second pre-defined refractive index a2 and the third pre-defined refractive index a3 on the longitudinal axis 102c. In addition, the refractive index profile 200 is characterized by the fourth pre-defined refractive index a4, the fifth pre-defined refractive index a5 and the sixth pre-defined refractive index a6 on the longitudinal axis 102c. The graphical plot of the refractive index profile 200 corresponds to the first region 104, the second region 106, the third region 108, the fourth region 110a-b, the fifth region 112 and the sixth region 114 of the optical fiber 100. Each refractive index value corresponding to each region is plotted against radius of each region. Further, the refractive index profile 200 is plotted against the first pre-defined radius r1, the second pre-defined radius r2, the third pre-defined radius r3 and the fourth pre-defined radius r4 on the first axis 102a. In addition, the refractive index profile 200 is plotted against the fifth pre-defined radius r5 and the sixth pre-defined radius r6 on the first axis 102a.

[0056] The graphical plot of the first pre-defined refractive index a1 for the first region 104 represents the core region spanning parallel from the longitudinal axis 102c to the first pre-defined radius r1. Further, the graphical plot of the second pre-defined refractive index a2 for the second region 106 represents a concentric cladding region spanning with a negative slope from first predefined region r1 to the second pre-defined radius r2. The graphical plot of the third pre-defined refractive index a3 for the third region 108 represents another concentric cladding region spanning parallel from the second pre-defined radius r2 to the third pre-defined radius r3. The graphical plot of the fourth pre-defined refractive index a4 for the fourth region 110a-b represents stress applying parts spanning parallel from the third pre-defined radius r3 to the fourth pre-defined radius r4. The graphical plot of the fifth pre-defined refractive index a5 for the fifth region 112 excluding the fourth region 110a-b represents the concentric cladding region. The graphical plot of the fifth pre-defined refractive index spans parallel from the third predefined radius r3 to the fifth pre-defined radius r5. In addition, the graphical plot of the sixth pre-defined refractive index a6 for the sixth region 114 represents the concentric cladding region spanning parallel from the fifth pre-defined radius r5 to the sixth pre-defined radius r6.

[0057] FIG. 2B illustrates a table 202 listing a range of radii measured along the first axis 102a for each region of the 80 µm optical fiber, in accordance with an embodiment of the present disclosure. The first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm and is 2.3 µm for the first region 104. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and is 7.3 µm for the second region 106. The third pre-defined radius r3 is in a range of 2.6 µm-22.5 µm and is 20.3 µm for the third region 108. The fourth pre-defined radius r4 is in a range of (r3+3) µm-(r3+15) µm and is 26.3 µm for the fourth region 110a-b. The fifth pre-defined radius r5 is in a range of 37 µm-38 µm and is 37 µm for the fifth region 112. The sixth pre-defined radius r6 is in a range of 39 µm-41 µm and is 40 µm for the sixth region 114.

[0058] FIG. 2C illustrates a table 204 listing the range of radii measured along the first axis 102a for each region of the 125 µm optical fiber, in accordance with another embodiment of the present disclosure. The first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm and is 2.3 µm for the first region 104. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and is 7.3 µm for the second region 106. The third pre-defined radius r3 is in a range of 2.6 µm-22.5 µm and is 20.3 µm for the third region 108. The third pre-defined radius r3 may have any suitable radius. The fourth pre-defined radius r4 is in a range of (r3+3) µm-(r3+20) µm and is 26.3 µm for the fourth region 110a-b. The fifth pre-defined radius r5 is in a range of 55 µm-58 µm and is 57 µm for the fifth region 112. The sixth pre-defined radius r6 is in a range of 62 µm-63 µm and is 62.5 µm for the sixth region 114.

[0059] FIG. 2D illustrates a table 206 listing a range of refractive indices and dopant concentrations measured along the first axis 102a for regions of the 80 µm optical fiber, in accordance with an embodiment of the present disclosure.

[0060] The first pre-defined refractive index a1 is 1.444 for the first region 104. The second pre-defined refractive index a2 is in a range of 1.438-1.441 for the second region 106. The second pre-defined concentration c2 of Fluorine in Silica is between 0.68 mol%-1.26 mol% for the second region 106. The third pre-defined refractive index a3 is in a range of 1.438-1.440 and is 1.439 for the third region 108. Each refractive index value is evaluated for each region at a wavelength of 1550 nanometers.

[0061] The third pre-defined concentration c3 is in a range of 0.9 mol%-1.26 mol% and is 1.13 mol%. The fourth pre-defined concentration c4 is in a range of 5 mol%-25 mol% and is 10 mol% for the fourth region 110a-b. The fifth pre-defined refractive index a5 is in a range of 1.438-1.440 and is 1.439 for the fifth region 112. The fifth pre-defined concentration c5 is in a range of 0.9 mol%-1.26 mol% and is 1.13 mol%. The sixth pre-defined refractive index a6 is 1.444 for the sixth region 114. Each concentration value is evaluated for each region at the wavelength of 1550 nanometers.

[0062] FIG. 2E illustrates a table 208 listing the range of refractive indices and concentrations measured along the first axis 102a for regions of the 125 µm optical fiber, in accordance with another embodiment of the present disclosure.

[0063] The first pre-defined refractive index a1 is 1.444 for the first region 104. The second pre-defined refractive index a2 is in a range of 1.438-1.441 for the second region 106. The second pre-defined concentration c2 of fluorine in silica is between 0.68 mol%-1.26 mol% for the second region 106. The third pre-defined refractive index a3 is in a range of 1.438-1.440 and is 1.439 for the third region 108. Each refractive index value is evaluated for each region at the wavelength of 1550 nanometers.

[0064] The third pre-defined concentration c3 is in a range of 0.9 mol%-1.26 mol% and is 1.13 mol% for the third region 108. The fourth pre-defined concentration c4 is in a range of 5 mol%-25 mol% and is 10 mol% for the fourth region 110a-b. The fifth pre-defined refractive index a5 is in a range of 1.438-1.440 and is 1.439 for the fifth region 112. The fifth pre-defined concentration c5 is in a range of 0.9 mol%-1.26 mol% and is 1.13 mol%. The sixth pre-defined refractive index a6 is 1.444 for the sixth region 114. Each concentration value is evaluated for each region at the wavelength of 1550 nanometers.

[0065] FIG. 3A illustrate a second refractive index profile 300 measured along a second axis 102b of the optical fiber 100, in accordance with an embodiment of the present disclosure. The refractive index profile 300 describes the graphical plot of the variation of refractive index along the longitudinal axis 102c. The refractive index profile 300 is characterized by the first pre-defined refractive index a1, the second pre-defined refractive index a2 and the third pre-defined refractive index a3' on the longitudinal axis 102c. In addition, the refractive index profile 300 is characterized by the sixth pre-defined refractive index a6 on the longitudinal axis 102c. The graphical plot of the refractive index profile 300 corresponds to the first region 104, the second region 106, the third region 108 and the sixth region 114 of the optical fiber 100. Each refractive index value corresponding to each region is plotted against radius of each region. The refractive index profile 300 is plotted against the first pre-defined radius r1, the second pre-defined radius r2, the third pre-defined radius r3' and the sixth pre-defined radius r6 along the second axis 102b.

[0066] The graphical plot of the first pre-defined refractive index a1 for the first region 104 represents the core region spanning parallel from the longitudinal axis 102c to the first pre-defined radius r1. The graphical plot of the second pre-defined refractive index a2 for the second region 106 represents the concentric cladding region. In addition, the graphical plot of the second pre-defined refractive index spans from the first pre-defined radius r1 to the second pre-defined radius r2 with the negative slope. The graphical plot of the third pre-defined refractive index a3' for the third region 108 represents the concentric cladding region spanning parallel from second pre-defined radius r2 to the third pre-defined radius r3'. The graphical plot of the sixth pre-defined refractive index a6 for the sixth region 114 represents the concentric cladding region spanning from the third pre-defined radius r3' to the sixth pre-defined radius r6.

[0067] FIG. 3B illustrates a table 304 listing a range of radii measured along the second axis 102b for each region of the 80 µm optical fiber, in accordance with an embodiment of the present disclosure. The first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm and is 2.3 µm for the first region 104. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and is 7.3 µm for the second region 106. The third pre-defined radius r3' is in a range of 37 µm-38 µm and is 40 µm for the third region 108. The sixth pre-defined radius r6 is in a range of 39 µm-41 µm and is 40 µm for the sixth region 114.

[0068] FIG. 3C illustrates a table 306 listing the range of radii measured along the second axis 102b for each region of the 125 µm optical fiber, in accordance with another embodiment of the present disclosure. The first pre-defined radius r1 is in a range of 0.75 µm-4.5 µm and is 2.3 µm for the first region 104. The second pre-defined radius r2 is in a range of 2.5 µm-25 µm and is 7.3 µm for the second region 106. The third pre-defined radius r3' is in a range of 55 µm-58 µm and is 57 µm for the third region 108. The sixth pre-defined radius r6 is in a range of 62 µm-63 µm and is 62.5 µm for the sixth region 114.

[0069] FIG. 3D illustrates a table 308 listing a range of refractive indices and concentrations measured along the second axis 102b for regions of the 80 µm optical fiber, in accordance with an embodiment of the present disclosure.

[0070] The first pre-defined refractive index a1 is 1.444 for the first region 104. The second pre-defined refractive index a2 is in a range of 1.438-1.441 for the second region 106. The second pre-defined concentration c2 of fluorine in silica is between 0.68 mol%-1.26 mol% for the second region 106. The third pre-defined refractive index a3' is in a range of 1.438-1.440 and is 1.439 for the third region 108. The optical fiber 100 has the third pre-defined concentration c3'. The third pre-defined concentration c3' is in a range of 0.9 mol%-1.26 mol% and is 1.13 mol% for the third region 108. The sixth pre-defined refractive index a6 is 1.444 for the sixth region 114. Each concentration value and refractive index value is evaluated for each region at the wavelength of 1550 nanometers.

[0071] FIG. 3E illustrates a table 310 listing the range of refractive indices and concentrations measured along the second axis 102b for regions of the 125 µm optical fiber, in accordance with another embodiment of the present disclosure.

[0072] The first pre-defined refractive index a1 is 1.444 for the first region 104. The second pre-defined refractive index a2 is in a range of 1.438-1.441 for the second region 106. The second pre-defined concentration c2 of fluorine in silica is in between 0.68 mol%-1.26 mol%. The second pre-defined concentration of the down dopant increases from the first pre-defined radius r1 to the second pre-defined radius r2 for the second region 106. The third pre-defined refractive index a3' is in a range of 1.438-1.440 and is 1.439 for the third region 108. Further, the third pre-defined concentration c3' is in a range of 0.9 mol%-1.26 mol% and is 1.13 mol% for the third region 108. The sixth pre-defined refractive index a6 is 1.444 for the sixth region 114. Each concentration value and refractive index value is evaluated for each region at the wavelength of 1550 nanometers.

[0073] The optical fiber described in the present disclosure has numerous advantages. The disclosed optical fiber uses a pure silica core providing a lower attenuation compared to conventional polarization maintaining fibers. The lower attenuation reduces input power requirement. Further, the optical fiber has a negative gradient doping of fluorine along the radius to provide higher effective area. The use of Boron Trioxide as dopant in the fourth region generates compressive stress and high internal birefringence. The usage of sixth region of silica provides required reliability to the optical fiber.

[0074] The foregoing descriptions of pre-defined embodiments of the present technology have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present technology to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present technology and its practical application, to thereby enable others skilled in the art to best utilize the present technology and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present technology.