FORM 2
The Patent Act 1970
(39 of 1970)
&
The Patent Rules, 2005

COMPLETE SPECIFICATION
(SEE SECTION 10 AND RULE 13)

TITLE OF THE INVENTION

“AN OPTICAL FIBER WAVEGUIDE HAVING REDUCED BEND LOSSES”

APPLICANTS:

Name Nationality Address
Sterlite Technologies Limited Indian E1/E2/E3, MIDC Waluj, Aurangabad- 431136, India

The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed:-

This is a divisional application in furtherance to the first application number 312/MUM/2010 dated 06/02/2010.

FIELD OF INVENTION
Embodiments herein generally relate to an optical fiber waveguide, and more particularly but not exclusively, to an optical fiber waveguide fiber which has reduced macro-bending loss and comply with G652 and G657 (A1, A2, B1 & B2) ITU-T standards.
BACKGROUND
Optical fiber waveguide plays an important role in the field of modern communications. A momentous increase in usage of optical fiber waveguide is being observed. It is expected that the use of optical fiber waveguide will continue to increase in order to deliver greater amount of information in the form of data, audio, and video signals to residential and commercial users.
A simple optical fiber waveguide has a core and a cladding surrounding the core. The refractive index of the core is higher as compared with the refractive index of the cladding in order to achieve light transmission inside the optical fiber waveguide, by a phenomenon known as total internal reflection.
An optical signal traveling through the optical fiber waveguide, apart from undergoing losses such as attenuation, dispersion and scattering, also suffers from other types of losses which are caused due to physical factors. One such loss is the macro-bending loss of the fiber, which is induced due to bending of the fiber due to improper handling or during installation.
Macro-bending loss of the fiber is particularly large when the fiber is laid with multiple bends or turns in its path. Additionally, due to harsh restrictions during installations, the optical fiber waveguide is always at the risk of being accidentally bent thereby inducing macro-bending losses., Such macro-bending losses can be detrimental to good quality communication.
Since the optical fiber waveguides are fast replacing copper wires (even in short distance transmissions like in Local area networks, FTTX applications, etc.), situations of an optical fiber being laid with bends at corners, around the wall, corridors, etc. are becoming very common. An optical fiber which is not sufficiently immune to macro-bending losses may not perform satisfactorily under these conditions.
Ways and means have been devised to reduce the macro-bending loss of an optical fiber waveguide. A value called the MAC number, defined as the ratio of the mode field diameter (MFD) (measured in micrometer) of the optical fiber waveguide at 1310 nm to the effective cut-off wavelength, is of considerable significance in this regard. It is well known that the MAC number influences the macro-bending losses. Reducing the MAC number, either by reducing the MFD and/or by increasing the effective cut-off wavelength, can effectively reduce the macro-bending losses.
Optical fiber waveguides are generally divided into two classes, namely, single mode optical fiber (SMF) waveguide and multimode optical fiber (MMF) waveguide. In SMF waveguide, the core diameter is small (say, in the range of 8 µm to 10 µm) thereby allowing only a single mode to propagate, whereas in MMF waveguide the core diameter is large (say, in the range of 40 µm to 70 µm), thereby allowing multiple modes to propagate through the fiber. Generally, the overall diameter (sum of core and cladding) of both SMF and the MMF waveguides is about 125 µm. The SMF waveguide is widely used in transmission lines due to its high bandwidth and low attenuation.
The International Telecommunication Union (ITU) has laid down a standard known as ITU-T G652, wherein G652 is the standard that must be fulfilled by a Standard Single Mode Fiber (SMF). Details of specifications for various fiber parameters for a single mode optical fiber are listed in table 1 below.
TABLE 1
SR.NO. PARAMETERS VALUE
1 Mode field diameter (MFD) @ 1310 nm 8.6 to 9.5 µm
2 Cutoff wavelength [?CC] 1260 nm
3 Zero dispersion wavelength [?0] 1300 nm to 1324 nm
4 Chromatic dispersion slope = 0.093 ps/nm2-km

As mentioned above, in order to reduce the macro-bending losses in a SMF, if the MFD and MAC number are decreased, non-linear phenomenon such as four wave mixing would increase. Also, if the cut-off wavelength is increased then operating wavelength (that is the wavelength above which the optical fiber waveguide exhibits a single mode character) of the optical fiber waveguide gets shifted. These effects are undesirable.
Various types of optical fibers which are aimed at reducing macro-bending losses have been proposed. In order to have compatibility of standards among different manufacturers with respect to the macro-bending losses of optical fiber waveguide, the ITU-T has laid down G657 (A1, A2, B1 & B2) standard.
In accordance with the ITU-T G657 (A1, A2, B1 & B2) standard, at a wavelength of 1310 nm, the SMF waveguide should have a mode field diameter [MFD] in the range from about 6.3 to 9.5 µm; a fiber cutoff wavelength [?C] not more than 1260 nm; a zero dispersion wavelength [?0] in the range from 1300 nm to 1324 nm and a maximum chromatic dispersion slope of 0.093 ps/nm2-km. The macro-bending losses along with the optical parameters for said G657 (A1, A2, B1 & B2) fiber in accordance with ITU-T specifications are summarized in table 2 below.
TABLE 2
SR.NO. PARAMETER VALUE
1 Mode field diameter (MFD) @ 1310 nm 6.3 to 9.5 µm
2 Cutoff wavelength [?C] = 1260 nm
3 Zero dispersion wavelength [?0] 1300 nm to 1324 nm
4 Chromatic dispersion slope = 0.093 ps/nm2-km
5 Macro-bending losses @ 1550 nm
15 mm radius mandrel & 10 turns 0.03 dB
10 mm radius mandrel & 1 turn 0.1 dB
7.5 mm radius mandrel & 1 turn 0.5 dB

To achieve the specifications of ITU-T G657 (A1, A2, B1 & B2) standard and to reduce the macro-bending losses in the optical fiber waveguide, either the MFD of the optical fiber waveguide may be reduced and/or the cutoff wavelength of the optical fiber waveguide may be adjusted so that the MAC number gets reduced. However, this method has a disadvantage, that is if the MFD is reduced, the splicing of the optical fiber waveguide becomes difficult, whereas if cutoff wavelength is increased, the operating wavelength (that is the wavelength above which the optical waveguide fiber exhibits a single mode character) of the optical fiber waveguide gets shifted. This may render the optical fiber waveguide useless for any application.
Thus, there is a need for an optical fiber waveguide that would exhibit a reduced macro-bending loss even at bending radii of about 7.5 mm while adhering to the G652 standard. More particularly, there is a need for an optical fiber waveguide which would have a MFD and cut-off wavelength comparable to an optical fiber waveguide complying ITU-T G652 standard and at the same time should have reduced macro-bending losses even at bending radii of about 7.5 mm and 5 mm.
In other words, it is desirable to have an optical fiber waveguide which, apart from being compatible with the ITU-T G652 specifications (particularly the MFD and cutoff wavelength of the fiber must be as per ITU-T G652 specification), exhibits reduced bend sensitivity and macro-bend losses, meaning thereby, it is desired to have an optical fiber waveguide, which complies with both the ITU-T G652 and G657 (A1, A2, B1 & B2) standards.
OBJECTS OF THE INVENTION
An object of the invention is to provide an optical fiber waveguide having reduced macro-bending loss.
Another object is to provide an optical fiber waveguide with reduced macro-bending loss, which is compatible with both ITU-T G652 and G657 (A1, A2, B1 & B2) standards.
Yet another object is to provide an optical fiber waveguide with reduced macro-bending loss, wherein the optical waveguide fiber has desired values of the optical parameters, including the cutoff wavelength, zero dispersion wavelengths, the mode field diameter and the dispersion slope.
These and other objects and advantages of the present invention will be apparent from the following description when read in conjunction with the accompanying drawings which are incorporated for illustration of preferred embodiments of the present invention and are not intended to limit the scope thereof.
STATEMENT OF THE INVENTION
The present invention discloses an optical fiber waveguide which has reduced macro-bending losses and is compatible with both ITU-T G652 and G657 (A1, A2, B1 & B2) standards.
In accordance with embodiments of the present invention, the optical fiber waveguide has at least one layer of depressed refractive index (or a trench layer), provided around a core. Refractive index values and functions in the refractive index profile of the optical fiber waveguide are chosen so that a certain set of conditions are always complied with.
In accordance with one embodiment of the present invention, the optical fiber waveguide has a core having refractive index n1 and radius a1. The optical fiber waveguide further has a first cladding layer surrounding the core, a second cladding layer surrounding the first cladding layer and a third cladding layer surrounding the second cladding layer. The first cladding layer has a refractive index n2 and a thickness a2. The second cladding layer has a refractive index n3 and a thickness a3. Further, the third cladding layer has a refractive index n4 and a thickness a4. A particular set of values of n1, n2, n3, n4, a1, a2 and a3 for the optical fiber waveguide is selected from a group having a plurality of sets of values for n1, n2, n3, n4, a1, a2 and a3. Within each set of values for n1, n2, n3, n4, a1, a2 and a3, n1 > n2 = n4 > n3. The zero dispersion wavelength of the optical fiber waveguide vary inversely with [(n1- n2) x a1]/[(n1/2 - n2/2) x (a2 + a3)], and within the group, the values of mode field diameter and fiber cutoff wavelength corresponding to various sets of values for n1, n2, n3, n4, a1, a2 and a3, vary proportionally with [(n1- n2) x a1]/[(n1/2 - n2/2) x (a2 + a3)].
These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It is understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by the way of illustration and not of limitation. Many changes and modifications may be made within the scope to the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the present invention is illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:
FIG. 1 illustrates the refractive index profile of an optical fiber waveguide constructed according to first embodiment as disclosed herein;
FIG. 2 illustrates the refractive index profile of an optical fiber waveguide constructed according to second embodiment as disclosed herein;
FIG. 3 illustrates a graph of the cutoff wavelength of the optical fiber waveguide versus the ratio R in accordance with the embodiments of the present invention;
FIG. 4 is a graph of mode field diameter of the optical waveguide versus the ratio R in accordance with the embodiments of the present invention; and
FIG. 5 is a graph of zero dispersion wavelength of the optical fiber waveguide versus the ratio R in accordance with the embodiments of the present invention.
Definitions as used herein:
Refractive index profile means a graph or a plot of variation of refractive indices of various regions of an optical fiber waveguide as a function of the radial distance from center of the optical fiber waveguide.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
In accordance with the present invention, an optical fiber waveguide is disclosed wherein such optical fiber waveguide fabricated according to certain conditions, would not only have reduced macro-bending losses but would also be compatible with both ITU-T G652 and G657 (A1, A2, B1 & B2) standards.
In accordance with the embodiments of the present invention, the optical fiber waveguide has a core region, a cladding region surrounding the core region, at least one layer of depressed refractive index (or a trench layer) around the core embedded in the cladding region. Further, among the multiple possible refractive index profiles of various regions of the waveguide, a refractive index profile is chosen such that, the value of refractive index at any location of the core of the optical fiber waveguide is greater than the refractive index of any other location lying out of the core of the optical fiber waveguide, and the value of refractive index at any location in the trench layer(s) of the optical fiber waveguide is lesser than the value of refractive index at any location lying outside the trench layer(s) of the optical fiber waveguide. The zero dispersion wavelength of the optical fiber waveguide vary inversely with a ratio R, and the mode field diameter and the optical fiber waveguide cutoff wavelength vary proportionally with the ratio R. In a graph of a refractive index profile of various regions of the optical fiber waveguide plotted against the radial distance from the center of the cross section of the waveguide, the ratio R is defined as
[(n1- n2) x a1]/[(n1/2 - n2/2) x (a2 + a3)]
where, n1, n2 and n3 are the refractive indices of the core region, the adjacent region to the core and that of the trench region respectively, and a1 ,a2 and a3 are the radius of the core region, thickness of the first layer and thickness of the trench region respectively.
FIG. 1 illustrates the refractive index profile 101 of an optical fiber waveguide constructed according to first embodiment as disclosed herein. In accordance with the present invention the optical fiber waveguide has a core region 102. The core region 102 is surrounded by three layers namely, a first layer 103, a second layer that is a trench layer 104 and a third layer 105. The refractive indices of all three layers 103, 104 and 105 are lesser than the refractive index of the core region 102, and the refractive index of the second layer 104 that is the trench layer is lower than the refractive index of the layers 103 and 105.
In accordance with an exemplary embodiment the refractive indices of the core region 102, the first layer 103, the second layer 104 and third layer 105 are substantially constant along the radial distance from the center of the core region 102 (FIG. 1).
In accordance with another exemplary embodiment the refractive indices of the core region , the first layer, the second layer and third layer are not constant along the radial distance from the center of the core region and vary as some function of the radial distance. For example, in FIG. 2 which illustrates the refractive index profile 201 of an optical fiber waveguide constructed according to second embodiment as disclosed herein the optical fiber waveguide has a core region 202. The core region 202 is surrounded by three layers namely, a first layer 203, a second layer that is a trench layer 204 and a third layer 205. The refractive indices of all three layers 203, 204 and 205 are lesser than the refractive index of the core region 202, and the refractive index of the second layer that is the trench layer 204 is lower than the refractive index of the layers 203 and 205. As indicated herein the refractive index of the core region 202 varies in a linearly decreasing manner with the radial distance r from the center.
In other embodiments the variation of the refractive index of the core region 202 may vary as a function of r, wherein the variation may be other than a linear one.
In still other embodiments the refractive indices of other regions including 203, 204 and 205 may also vary as a function of radial distance r either linearly or in any other suitable manner.
In accordance with the embodiments of the present invention the optical fiber waveguide exhibits a reduced macro-bending loss, and is compatible with both ITU-T G652 and G657 (A1, A2, B1 & B2) standards, the refractive indices and thickness of various regions of optical fiber waveguide are chosen such that:
n1 > n2 = n4 > n3.

In order to comply with G657 (A1, A2, B1 & B2) standard, the zero dispersion wavelength of the optical fiber waveguide vary inversely with a ratio R, and the mode field diameter and the optical fiber waveguide cutoff wavelength vary proportionally with the ratio R, where the ratio R is defined as
[(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)]
where, n1, n2 and n3 are the refractive indices of the core region, the adjacent region to the core and that of the trench region respectively, and a1 ,a2 and a3 are the radius and thickness of the core region, the first layer and that of the trench region respectively
Examples (present invention):
Few optical fiber waveguides were prepared in accordance with the first embodiment of the present invention. The optical fiber waveguide had a core region. The core region was surrounded by three layers namely, a first layer, a second layer that is a trench layer and a third layer. The refractive indices of each of the core region, the first, second and third layers was n1, n2, n3 and n4 respectively. A particular set of values of n1, n2, n3, n4, a1, a2 and a3 for the optical fiber waveguide were selected from a group having a plurality of sets of values for n1, n2, n3, n4, a1, a2 and a3, wherein within each set of values for n1, n2, n3, n4, a1, a2 and a3, with n1 > n2 = n4 > n3, it was observed that the values the zero dispersion wavelength of the optical fiber waveguide varied inversely with [(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)], whereas within the group, the values of the mode field diameter and the optical fiber waveguide cutoff wavelength varied proportionally with [(n1- n2) × a1]/[(n1/2 - n22/2) × (a2 + a3)]. The values of n1, n2, n3, n4, a1, a2 and a3 are listed in Table 3 below for various optical fiber waveguide samples.
These optical fiber waveguides were tested for various parameters including zero dispersion wavelength, cutoff wavelength, mode field diameter and macro-bending losses and are listed in table 4 below for the samples.
FIG. 3 illustrates a graph of the cutoff wavelength of the optical fiber waveguide versus [(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)] . FIG. 4 is a graph of mode field diameter versus [(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)], whereas FIG. 5 is a graph of zero dispersion wavelength versus [(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)], which confirms the facts that the zero dispersion wavelength of the optical fiber waveguide vary inversely with [(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)], whereas within the group, the values of the mode field diameter and the optical fiber waveguide cutoff wavelength vary proportionally with [(n1- n2) × a1]/[(n1/2 - n2/2) × (a2 + a3)].
TABLE 3

TABLE 3 (continued …)

TABLE 4

TABLE 4 (continued …)

It is to be noted that though the embodiments are explained with the help of the few exemplary refractive index profiles, additional ways for practicing the methods of the present invention will be evident to persons skilled in the art from the disclosure herein. Similar logic may be applied to any other optical fiber waveguides types and/or refractive index profiles. Such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments.
The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

We claim:

1. An optical fiber waveguide comprising:
a core region; and
a cladding region surrounding said core region, said cladding region having
at least one trench region with a depressed refractive index ,
wherein among a plurality of possible refractive index profiles of various regions of the waveguide, a refractive index profile is chosen such that;
a value of refractive index at any location of the core of the optical fiber waveguide is greater than a value of refractive index of any other location lying out of the core of the optical fiber waveguide; and
a value of refractive index at any location in the trench region of the optical fiber waveguide is lesser than a value of refractive index at any location lying outside the trench region of the optical fiber waveguide,
wherein a zero dispersion wavelength of the optical fiber waveguide vary inversely with a ratio R; and
a mode field diameter and an optical fiber waveguide cutoff wavelength vary proportionally with the ratio R,
wherein the ratio R is defined as
[(n1- n2) x a1]/[(n1/2 - n2/2) x (a2 + a3)]
wherein, n1, n2 and n3 are the refractive indices of the core region, a region adjacent to the core and the trench region respectively, and a1 ,a2 and a3 are the radius of the core region, thickness of the region adjacent to the core and thickness of the trench region respectively.

2. The optical fiber waveguide of claim 1, wherein the cladding region further comprises a layer other than the region adjacent to the core, wherein said layer is adjacent to the trench region and has a refractive index n4.

3. The optical fiber waveguide of claim 1, wherein the refractive index n1of said core region is substantially constant.

4. The optical fiber waveguide of claim 1, wherein the refractive index n1 of said core region varies as a function of radial distance r.

5. The optical fiber waveguide of claim 2, wherein the refractive indices of n1, n2, n3 and n4 are constraint by the inequality n1 > n2 = n4 > n3.

6. The optical fiber waveguide of claim 1, wherein the value of the ratio R is in the range of 0.3 to 0.7.

7. The optical fiber waveguide of claim 2, wherein the refractive indices of n1, n2, n3 and n4 are constraint by the inequalities:
1.440 = n1 = 1.470;
1.440 = n2 = 1.460;
1.440 = n3 = 1.455; and
1.440 = n4 = 1.460.

8. The optical fiber waveguide of claim 1, wherein the values of a1, a2 and a3 are constrained by the following inequalities:
6 µm = a1 = 10 µm;
2 µm = a2 = 15 µm; and
2 µm = a3 = 10 µm.

9. The optical fiber waveguide of claim 2, wherein the refractive indices n2, n3 and n4 are substantially constant.

10. The optical fiber waveguide of claim 2, wherein the refractive indices n2, n3 and n4 varies as a function of radial distance r.

11. The optical fiber waveguide of claim 1, wherein said optical waveguide exhibits a bend loss of less than or equal to 0.03 dB with 10 turns on a mandrel of 15 mm radius.

12. The optical fiber waveguide of claim 1, wherein said optical waveguide exhibits a bend loss of less than or equal to 0.1 dB with 10 turns on a mandrel of 10 mm radius.

13. The optical fiber waveguide of claim 1, wherein said optical waveguide exhibits a bend loss of less than or equal to 0.5 dB with 10 turns on a mandrel of 7.5 mm radius.

Dated 04th FEB, 2011

Dr. Kalyan Chakravarthy
Patent Agent

ABSTRACT
The optical fiber waveguide having reduced macro-bending losses and in compliance with ITU-T G652 and G657 (A1, A2, B1 & B2) standards has a core region, a cladding region surrounding the core, at least one layer of depressed refractive index (or a trench layer) around the core embedded in the cladding. A refractive index profile is chosen from multiple profiles such that, the value of refractive index at any location of the core is greater than the refractive index of any other location lying out of the core, and the value of refractive index at any location in the trench layer(s) is lesser than the value of refractive index at any location lying outside the trench layer(s). The zero dispersion wavelength of the waveguide vary inversely with a ratio R, and the mode field diameter and the cutoff wavelength vary proportionally with the ratio R. FIG-1