CLIAMS:
An optical fiber refractive index profile comprising a core region and a cladding region, said core region being divided into at least four sub-regions, each sub-region characterized by a relative refractive index and a slope, wherein,
a first sub-region has a relative refractive index ?n_1, a radius r1 and a slope S0 ;
a second sub-region has a relative refractive index ?n_2, a radius r2and a slope S1;
a third sub-region has a relative refractive index ?n_3, a radius r3 and a slope S2; and
a fourth sub-region has a relative refractive index ?n_4, a radius r4 and a slope S3;
said relative refractive indices and radii of said four sub-regions satisfy the following equations:
4.7 x 10-3 = ?n_1 = 6.0 x 10-3;
4.7 x 10-3 = ?n_2 = 6.0 x 10-3;
3.5 x 10-3 = ?n_3 = 4.15 x 10-3;
0.1 x 10-3 = ?n_4 = 1.0 x 10-3;
0.5 ?m = r1 = 2 ?m;
2 ?m = r2 = 4 ?m;
3.5 ?m = r3 = 5 ?m;
7 ?m = r4 = 12 ?m;
wherein each of said slopes S0, S1, S2 and S3 are defined as rate of change of relative refractive index of the respective sub-region as a function of radius of the respective sub-region and said slopes S1, S2 and S3 satisfy the inequality |S2 | > |S1 | > | S3|;
1.5 > (|S2|/| S3|)> 269.2;
0.3 > (|S1|/| S3|) > 61.5; and
a loss in volume? ??_2^', of said second sub-region is less than or equal to an additional volume ?_4 incorporated in said fourth sub-region, wherein volume of said regions is defined as ?_i=2?_(x_1)^(x_2)¦?|?n_i (r)|.rdr?, where i takes values 2 and 4 and x_(1 )and x_(2 )are the starting and ending points of said sub-regions.

The optical fiber as claimed in claim 1, wherein slope S1 of said second sub-region is in the range from -1.5 x 10-4 to -8.0 x 10-4 (1/?m).

The optical fiber as claimed in claim 1, wherein slope S2 of said third sub-region is in the range from -8.3 x 10-4 to -34.9 x 10-4 (1/?m).

The optical fiber as claimed in claim 1, wherein slope S3 of said fourth sub-region is in the range from -0.13 x 10-4 to -5.7 x 10-4 (1/?m).

The optical fiber as claimed in claim 1, wherein the ratio of the absolute value of said slope S2 of said third sub-region to the absolute value of said slope S1 of the second sub-region is in the range from 1.03 to 23.3.

The optical fiber as claimed in claim 1, wherein said first sub-region is sub-divided into two parts, a central part and a periphery part, wherein relative refractive index ?n_0 of said central part is in the range from 1.5 x 10-3 to 4.0 x 10-3 and relative refractive index ?n_1 said periphery part is in the range from 4.7 x 10-3 to 6.0 x 10-3.

The optical fiber as claimed in claim 1, wherein said cladding region is divided into two portions, a first cladding portion having a relative refractive index ?n_51 in contact with said fourth sub-region and a second cladding portion, surrounding and in contact with said first cladding portion, having a relative refractive index ?n_52,

The optical fiber as claimed in claim 7, wherein relative refractive index ?n_51 of said first cladding portion is in the range from -3.5 x 10-3 to -7.0 x 10-3.

The optical fiber as claimed in claim 1, wherein the ratio of ? ??_2^' to ?_4 is in the range from 0.6 to 1.0.

The optical fiber as claimed in any of preceding claims, wherein said optical fiber exhibits at least one of following properties:
a mode field diameter (MFD) in the range from 8.2 ?m to 9.5 ?m at and around 1310 nm wavelength region;
an effective area (Aeff) of greater than or equal to 70 ?m2 at and around 1550 nm wavelength region;
a chromatic dispersion (CD) of less than 18 ps/nm/km at and around 1550 nm wavelength region;
a cabled cutoff wavelength (?c) of less than 1260 nm;
a zero dispersion wavelength (?0) in the range from 1304 nm to 1324 nm;
an attenuation loss of 0.190 dB/km or less at and around 1550 nm wavelength region;
a macro-bend loss of 0.5 dB/turn on a mandrel of 32 mm at and around 1550 nm wavelength region.

The optical fiber as claimed in claim 8, wherein said optical fiber exhibits a macro-bend loss of less than about 0.15 dB/loop for one loop of 5 mm radius at and around 1550 nm wavelength region.

The optical fiber as claimed in claim 8, wherein said optical fiber exhibits a macro-bend loss of less than about 0.2 dB/loop for one loop of 7.5 mm radius at and around 1550 nm wavelength region.
,TagSPECI:FIELD OF DISCLOSURE
The present invention generally relates to the field of optical fibers, and more particularly, to an optical fiber having reduced macro-bending losses along with desired optical parameters including cabled cut-off wavelength, zero dispersion wavelength, chromatic dispersion, mode field diameter and effective area.

BACKGROUND
Optical fibers are typically used for long distance data communication. Rapidly increasing telecommunication infrastructure has increased the demand of optical fibers significantly. High bandwidth capability of the optical fibers has made optical fibers the backbone of communication networks for voice, video and data transmission. Furthermore, rapid growth in information technology has fueled the demand for higher bandwidths leading to exponential increase in use of optical fibers.

An optical fiber's refractive index profile is a graph of (relative) refractive index of various regions of the optical fiber cross-section plotted against the radial distance from the center of the optical fiber. The radial distance (r) from the center of the optical fiber is plotted along the x axis, and the relative refractive index at a distance (r) is plotted along the y axis. A typical refractive index profile 100 of a conventional single mode optical fiber is depicted in FIG. 1. The refractive index profile 100 comprises a core region (101a, 101b) and a cladding region 102. The core region (101a, 101b) allows propagation of an optical signal, while the cladding 102 limits the optical signal in the core (101a, 101b). To enable the propagation of the optical signal in accordance with the principle of total internal reflection, the core region (101a, 101b) is generally doped with a refractive index raising dopant (such as germanium dioxide) thereby increasing the refractive index of the core region with reference to pure silica, whereas the cladding region is typically made of pure silica but may be doped with one or more dopants. Thus the relative refractive index of core region (101a, 101b) is greater than that of the cladding region 102.

It is well-known in the art that the propagation of an optical signal in a single-mode optical fiber comprises a fundamental mode (LP01) which is guided in the core region (101a, 101b), and a few secondary modes that may be guided over a certain distance in the core region (101a, 101b) and the optical cladding 102.

The refractive-index profile 100 may have a “step” profile 101a, a “parabolic” profile 101b or any other shape. Generally, single-mode optical fibers (SMF) with step-index profile 101a are used in an optical fiber transmission system. Such optical fibers typically have a chromatic dispersion and a chromatic-dispersion slope that comply with specific telecommunications standards.

In order to ensure that the optical fibers manufactured by different manufacturers are compatible with each other, the International Telecommunication Union (ITU) has defined a standard reference ITU-T G.652 according to which a standard optical transmission fiber (i.e., a standard single-mode fiber or SSMF) should comply. The ITU-T G.652 recommendations and each of its attributes (i.e., A, B, C, and D) as approved in November 2011 are hereby incorporated by reference. In accordance with the ITU-T G.652 standard, a SMF should have (i) a mode field diameter (MFD) in the range from 8.6 µm to 9.5 ?m at a wavelength of 1310 nm, (ii) a maximum cable cut-off wavelength (?C) of 1260 nanometers (nm), (iii) a zero-dispersion wavelength (?0) of between 1300 nm and 1324 nm, and (iv) a maximum zero-dispersion slope (ZDS) of 0.092 picoseconds per square nanometer kilometer (ps/(nm2•km).

A typical transmission system using optical fibers comprises extra lengths of optical fiber that are stored in containers. The extra-fiber lengths are provided for possible future use. These containers are desired to have small dimension which leads to winding of the optical fibers on small diameter mandrels particularly for FTTH (fiber-to-the-home) or FTTC (fiber-to-the-curb) applications, wherein the diameter of the mandrels may be of the order of 15 mm or less. Further, while installing the optical fibers in offices and/or homes, bending diameters of 15 mm or 10 mm are encountered. Hence, it is desirable to have fibers that exhibits low bend losses at 15 mm or even less bending diameters. Typically, the standard single mode optical fiber (complying with G652D standard) exhibits significant bending losses at radii of less than about 30 mm at 1550 nm wavelength.

A parameter called MAC for an optical fiber which is defined as the ratio of the mode field diameter of the fiber at 1550 nm and the cut-off wavelength ?C is known to have impact on the macro-bending losses of the optical fiber. The macro-bending losses may be reduced by reducing MAC value. Reduction in MAC value may be achieved by reducing the mode field diameter and/or by increasing the cut-off wavelength. This may improve the bending performance of the optical fiber. However, the optical fiber with mode field diameter less than that recommended by ITU-T G652D standard will make such a low mode field diameter fiber incompatible with existing transmission networks. The low mode field diameter fiber might have high coupling losses with the standard single mode fiber which is installed in the networks worldwide.

Notably, the value of the cut-off wavelength ?C cannot be increased beyond a pre-determined value, typically, 1260 nm. This is because, for transmission at around 1310 nm wavelength region, the power launched in the optical fiber with cutoff wavelength greater than 1260 nm, will be also distributed in higher order modes. However, higher order modes are not at all desired in single mode optical transmission systems.

Alternately, bending performance of the optical fiber can be improved by increasing the relative refractive index of the core region. The relative refractive index of the core region is achieved by increasing the concentration of dopant (GeO2) in the core region. However, increasing the concentration of the dopant tends to increase the Rayleigh scattering loss and hence the attenuation of the optical fiber which is undesirable.

Furthermore, the bending performance of the optical fiber can be improved by providing a down doped region around the core region. Typically the down doped region is achieved by doping the cladding region (partially or completely) with dopants such as fluorine or boron. Typically, the optical fiber with an optimal depth and width of the down doped region may achieve a bend loss of 0.2 dB/turn or less at 1550 nm on a mandrel of 15 mm diameter.

A major drawback of providing such down doped region around the core region is that the mode field diameter of the optical fiber decreases which may again make the optical fiber incompatible with the standard single mode fiber (or the ITU-T G652D standard). Further, in an optical fiber with down doped region, the cutoff wavelength increases which again would lead to the situation wherein the power is distributed in higher order modes, which is not desired. Still further, in an optical fiber with down doped region, the zero dispersion wavelength decreases. Particularly, if the zero dispersion wavelength is below 1308 nm, then the dispersion at 1550 nm wavelength increases to a value greater than 18 ps/km.nm, which is again incompatible with the ITU-T G652D standards.

There have been several endeavors to develop optical fibers having reduced bend loss and attenuation. For instance, US patent 7676129 [referred to as the US‘129 hereinafter] discloses a bend resistant single mode optical fiber having a core region and a cladding region, wherein the core region consists of a central core region and an annular core region surrounding the central core region; and the cladding region consists of an annular ring region and an annular outer region.

US patent application 2008/0056654 [referred to as US‘654 hereinafter] discloses a bend resistant optical fiber having a glass core and a glass cladding surrounding the core, wherein the cladding consists of an annular inner region an annular ring region and an annular outer region.

US patent application 2010/0296783 [referred to as US‘783 hereinafter] discloses an optical fiber having a first core at the center a second core covering the first core, a third core covering the second core and a cladding covering the third core.

However, a major drawback of the optical fibers of US‘129, US‘654 and US‘783 is that width of the GeO2 dopant in the central core region is high which leads to increased Rayleigh scattering and hence increased attenuation. Another drawback of the optical fibers of US‘129, the US‘654 and the US‘783 is that if value of the slope of the region surrounding and adjacent to the core region is zero, then the mode field diameter reduces, and macro-bending loss and zero dispersion wavelength increases. Furthermore, the effective area of the optical fibers of US‘129, US‘654 and US‘783 is about 80 ?m2 or less. One more drawback is that the optical fibers of US‘129, US‘654 and US‘783 have low dispersion at 1550 nm and low cutoff wavelength value owing to relatively high GeO2 width of the central core region.

Hence there is a need for an optical fiber which complies with ITU-T G652D standards and has reduced macro-bending losses in comparison to standard single mode optical fiber. Furthermore, there is need for an optical fiber which complies ITU-T G652D standard (including the optical parameters such as the cabled cut-off wavelength, zero dispersion wavelength, chromatic dispersion, mode field diameter and effective area) along with a macro-bending losses of less than about 0.2 dB/turn on a mandrel of 15 mm diameter and 0.15 dB/turn on a mandrel of 10 mm diameter at 1550 nm wavelength region.

OBJECTS
Some of the objects of the present invention aimed to ameliorate the problems of the prior art of at least provide a useful alternative are listed herein below.

An object of the present invention is to provide an optical fiber that exhibits reduced macro-bending loss.

Another object of the present disclosure is to provide an optical fiber that exhibits desired optical parameters including cabled cut-off wavelength, zero dispersion wavelength, chromatic dispersion, mode field diameter and effective area.

Another object of the present invention is to provide an optical fiber that complies with the G652D ITU-T standards.

Still another object of the present invention is to provide an optical fiber that exhibits a macro-bending loss of less than about 0.2 dB/turn on a mandrel of 15 mm diameter.

Yet another object of the present invention is to provide an optical fiber that exhibits a macro-bending loss of less than about 0.15 dB/turn on a mandrel of 10 mm diameter at 1550 nm wavelength region.

Other objects and advantages of the present invention will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present invention.

SUMMMARY
In accordance with a first embodiment of the present invention, there is provided an optical fiber comprising a core region and a cladding region, the core region being divided into at least four sub-regions, each sub-region characterized by a relative refractive index and a slope, wherein, a first sub-region has a relative refractive index ?n_1 and slope S0, a second sub-region has a relative refractive index ?n_2 and slope S1, a third sub-region has a relative refractive index ?n_3 and slope S2, and a fourth sub-region has a relative refractive index ?n_4 and slope S3, wherein the slopes are defined as rate of change of relative refractive index of the respective sub-regions as a function of radius of the respective sub-regions, wherein the slope S0 of the first sub-region is a real number and an absolute value of the slope S2 of the third sub-region is greater than an absolute value of the slope S1 of the second sub-region which in turn is greater than an absolute value of the slope S3 of the fourth sub-region, the ratio of the absolute value of the slope S2 of the third sub-region to the absolute value of the slope S3 of the fourth sub-region is in the range from 1.5 to 269.2, the ratio of the absolute value of the slope S1 of the second sub-region to the absolute value of the slope S3 of the fourth sub-region is in the range from 0.3 to 61.5 and volume loss? ??_2^', of the second sub-region is less than or equal to an additional volume ?_4 incorporated in the fourth sub-region, wherein volume (loss or incorporated) of the sub-regions is defined as ?_i=2?_(x_1)^(x_2)¦?|?n_i (r)|.rdr?, where i takes values 2 and 4 and x_(1 )and x_(2 )are the starting and ending points of the sub-regions.

In accordance with the first embodiment of the present invention the ratio of ? ??_2^' to ?_4 is in the range from 0.6 to 1.0.

In accordance with a second embodiment of the present invention, there is provided an optical fiber comprising a core region and a cladding region, the core region being divided into at least four sub-regions, each sub-region characterized by a relative refractive index and a slope, wherein, a first sub-region has a relative refractive index ?n_1 and slope S0, a second sub-region has a relative refractive index ?n_2 and slope S1, a third sub-region has a relative refractive index ?n_3 and slope S2; and a fourth sub-region has a relative refractive index ?n_4 and slope S3, wherein the slopes are defined as rate of change of relative refractive index as a function of radius of the optical fiber wherein, the slope S0 of the first sub-region is a real number and absolute value of the slope S2 of the third sub-region is greater than absolute value of the slope S1 of the second sub-region which in turn is greater than absolute value of the slope S3 of the fourth sub-region, the ratio of the absolute value of the slope S2 of the third sub-region to the absolute value of the slope S3 of the fourth sub-region is in the range from 1.5 to 269.2, the ratio of the absolute value of the slope S1 of the second sub-region to the absolute value of the slope S3 of the fourth sub-region is in the range from 0.3 to 61.5, and volume loss? ??_2^', of the second sub-region is less than or equal to an additional volume ?_4 incorporated in the fourth sub-region, wherein volume (loss or incorporated) of the sub-regions is defined as ?_i=2?_(x_1)^(x_2)¦?|?n_i (r)|.rdr?, where i takes values 2 and 4 and x_(1 )and x_(2 )are the starting and ending points of the sub-regions, wherein the cladding region is divided into two, a first cladding portion having a relative refractive index ?n_51 surrounding and in contact with the fourth sub-region and a second cladding portion, surrounding and in contact with the first cladding portion, having a relative refractive index ?n_52, wherein the first cladding portion is down-doped with a refractive index decreasing dopant such a boron or fluorine thereby forming an optical trench.

The optical fiber, in accordance with the first and/or second embodiments of the present invention, exhibits a mode field diameter (MFD) in the range from 8.2 m to 9.5 ?m at and around 1310 nm wavelength region, an effective area (Aeff) of greater than or equal to 70 ?m2 at and around 1550 nm wavelength region, a chromatic dispersion (CD) of less than 18 ps/nm/km at and around 1550 nm wavelength region, a cabled cutoff wavelength (?c) of less than 1260 nm, a zero dispersion wavelength (?0) in the range from 1304 nm to 1324 nm, an attenuation loss of 0.190 dB/km or less at and around 1550 nm wavelength region, and a macro-bend loss of 0.5 dB/turn on a mandrel of 30 mm radius at and around 1550 nm wavelength region.

In accordance with the second embodiment of the present invention the optical fiber optical fiber exhibits a macro-bend loss of less than about 0.15 dB/loop of 10 mm diameter at and around 1550 nm wavelength region and a macro-bending loss of less than about 0.2 dB/loop on a 15 mm diameter at and around 1550 nm wavelength region.

Other objects, advantages and embodiments of the present invention will be apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit scope of the present invention, but are incorporated merely for illustrating the present invention.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The optical fiber of the present invention will now be described with the help of accompanying drawings in which:

FIG. 1 illustrates a typical refractive index profile of an optical fiber as known in the art;

FIG. 2 illustrates a refractive index profile of an optical fiber in accordance with a first embodiment of the present invention depicting a refractive index profile with three slopes;

FIG. 3 illustrates a refractive index profile of an optical fiber in accordance with a second embodiment of the present invention comprising three slopes and a down-doped region;

FIG. 4A is a graph of variation of cutoff wavelength with respect to the ratio S1/S2 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 4B is a graph of variation of zero dispersion wavelength with respect to the ratio S1/S2 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 4C is a graph of variation of chromatic dispersion at 1550nm wavelength region with respect to the ratio S1/S2 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 4D is a graph of variation of macro-bending loss at 1550 nm mandrel on a mandrel of 32 mm diameter with respect to the ratio S1/S2 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 4E is a graph of variation of mode field diameter with respect to the ratio S1/S2 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 5A is a graph of variation of cutoff wavelength with respect to the ratio S2/S3 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 5B is a graph of variation of zero dispersion wavelength with respect to the ratio S2/S3 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 5C is a graph of variation of chromatic dispersion at 1550nm wavelength region with respect to the ratio S2/S3 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 5D is a graph of variation of macro-bending loss at 1550 nm mandrel on a mandrel of 32 mm diameter with respect to the ratio S2/S3 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 5E is a graph of variation of mode field diameter with respect to the ratio S2/S3 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 6A is a graph of variation of cutoff wavelength with respect to the ratio S1/S3 of an optical fiber in accordance with the embodiments of the present invention;

FIG. 6B is a graph of variation of macro-bending loss at 1550 nm mandrel on a mandrel of 32 mm diameter with respect to the ratio S1/S3 of an optical fiber in accordance with the embodiments of the present invention; and

FIG. 6C is a graph of variation of mode field diameter with respect to the ratio S1/S3 of an optical fiber in accordance with the embodiments of the present invention.

DETAILED DESCRIPTION
The optical fiber of the present invention will now be described with reference to the embodiments which do not limit the scope and ambit of the invention. The description relates purely to the exemplary preferred embodiments of the disclosed system and its suggested applications.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The terms ‘relative refractive index’ or ‘relative refractive indices’ of various regions as used hereinafter in this invention refer to the difference in refractive index of a region of the optical fiber profile under consideration with reference to refractive index of pure silica. For example, if the relative refractive index of a second sub-region in FIG. 1 is 4 x 10-3, then this means that the difference between the refractive indices of the second sub-region and pure cladding is 4 x 10-3.

Other parameters such as the mode field diameter, the cutoff wavelength, the zero dispersion wavelength, chromatic dispersion, macro-bending loss, attenuation etc., are well-known in the art.
FIG. 2 illustrates a refractive index profile 200 of an optical fiber with three slopes, in accordance with the first embodiment of the present invention. It is observed that dividing the core region of the optical fiber in at least four regions, wherein each region is characterized by a predetermined slope of relative refractive index and a relative refractive index profile, overcomes one or more limitations of the prior art discussed hereinabove.

In accordance with the first embodiment , the optical fiber has a relative refractive index profile 200 comprising a core region and a cladding region 205, wherein the core region is divided into at least four sub-regions and each sub-region characterized by a relative refractive index and a slope. The four sub-regions being a first sub-region 201 having a relative refractive index ?n_1 and slope S0; a second sub-region 202b having a relative refractive index ?n_2 and slope S1; a third sub-region 203 having a relative refractive index ?n_3 and slope S2; and a fourth sub-region 204 having a relative refractive index ?n_4 and slope S3.

The slopes S0, S1, S2 and S3 of the first, second, third and fourth sub-regions respectively are defined as follows:
S_i=((??n?_(i+1)- ??n?_i))/((r_i- r_(i-1)))
wherein, ?n_(i+1) is the highest value of the relative refractive index of the sub-region under consideration, and ?n_i is the lowest value of the relative refractive index of the same sub-region and r_i,and r_(i-1) are respectively the radii at which the relative refractive index is highest and lowest for the sub-region under consideration.

The slope S0 of the first sub-region is a real number and the absolute value of the slope S2 of the third sub-region is greater than absolute value of the slope S1 of the second sub-region which in turn is greater than absolute value of the slope S3 of the fourth sub-region. Further, the ratio of the absolute value of the slope S2 of the third sub-region to the absolute value of the slope S3 of the fourth sub-region, (S2/S3), is in the range from 1.5 to 269.2 and the ratio of the absolute value of the slope S1 the second sub-region to the absolute value of the slope S3 of the fourth sub-region, (S1/S3), is in the range from 0.3 to 61.5.

It is observed that the values of the slopes (S0 through S3) for the four sub-regions influences one or more optical parameters of the optical fiber including the mode field diameter, chromatic dispersion, effective area, zero dispersion wavelength , cutoff wavelength and the macro-bending loss.
Table I
S.No. Slopes Optical Parameters
Symbol Trend MFD ?c ?0 Bend Loss C.D Aeff
1 |S0| - - - - - - -
2 |S1|

3 |S2|
4 |S3|
|S3|*

Table 1 above provides the influence of the absolute values of the slopes (S0 through S3) on the mode field diameter (MFD), the cutoff wavelength (?c), the zero dispersion wavelength (?0), the bend loss, the chromatic dispersion (C.D) and the effective area (Aeff). For example, as the absolute value of slope S1 decreases (indicated by a down-pointing arrow in the table), the mode field diameter, the zero dispersion wavelength, the bend loss and effective area of the optical fiber, decreases (indicated by down-pointing arrow in the table), whereas the cutoff wavelength and the chromatic dispersion increases with decreasing absolute value of S1. Similarly the MFD, zero dispersion wavelength, bend loss, effective area, cutoff wavelength and chromatic dispersion increase/decrease based on increase/decrease in absolute values of slopes S2 and S3. However, slope S3 shows a peculiar behavior when value of slope of S3 is zero. The peculiar behavior of S3 is provided in table and marked as |S3|*. The absolute value of slope S2 is chosen such that it has minimal effect on the mode field diameter. Similarly, S3 is chosen such that it increases the MFD and hence the effective area of the optical fiber.

Further, the ranges of the slopes of the second sub-region S1, the third sub-region S2, and the fourth sub-region S3 are provided in the Table II herein below.

Table II
S.No. Slope Values ( x 10-4)
Min Max
1 S0 Real number
2 S1 -1.50 -8.00
3 S2 -8.30 -34.90
4 S3 -0.13 -5.70

The provision of the slope S1, results in loss of a volume? ??_2^' (indicated by patched region in FIG. 2 by numeral 202a) in the second sub-region 202b. The loss of volume? ??_2^' (202a), of the second sub-region 202b leads to decrease in the MFD of the optical fiber. In order to compensate the decrease in the MFD due to the loss of volume ? ??_2^' (202a) in the second sub-region 202b, additional volume ?_4 is provided which is volume of the fourth sub-region 204. The loss in volume of the second sub-region? ??_2^' is less than or equal to an additional volume ?_4 incorporated in the fourth sub-region. In accordance with present invention the volume of the sub-regions is defined as ?_i=2?_(x_1)^(x_2)¦?|?n_i (r)|.rdr?, where i takes values 2 and 4 and x_(1 )and x_(2 )are the starting and ending points of the sub-regions.

The ratio of ? ??_2^' to ?_4 is in the range from 0.6 to 1.0. It is observed that if the ratio of ? ??_2^' to ?_(4 )is greater than 1.0, then the macro-bending loss of the optical fiber increases beyond the desired limits, whereas if the ratio of ? ??_2^' to ?_4 is less than 0.6 cabled cutoff of the optical fiber may fail.

In accordance with the first embodiment of the present invention, the ranges of the refractive indices and radii of sub-regions of the core and the cladding are provided in Table III-A below. As used herein the radii of various sub-regions of the core region and cladding are distances from the center of the fiber to the interface of the two sub-regions. For example, r1 is radius of first sub-region, which means first sub-region ends at distance r1 microns and second sub-region starts at r1 thereby forming an interface at r1.
Table III-A
Sub-regions Relative refractive indices Radii (?m)
Symbol Values ( x 10-3) Symbol Min Max Typical
Min Max Typical
First ?n_1 4.7 6.0 5.0 r1 0.5 2 0.9
Second ?n_2 4.7 6.0 5.0 r2 2 4 3.6
Third ?n_3 3.5 4.15 4.0 r3 3.5 5 4.1
Fourth ?n_4 0.1 1.0 0.5 r4 7 12 8.6
Cladding ?n_5 0 0 0 r5 NA 62.5 62.5

Table III-B provides the values of the volumes of ? ??_2^' and ?_4 in accordance with the embodiments of the present invention.
Table III-B
Sub-region Index volume (loss/ gain) µm2
Symbol Max Typical
Second ?n_2 Loss 0.0125 0.00344
Fourth ?n_4 Gain 0.0267 0.00628

Table IV provides the ranges of the ratios of the slopes S2/S1, S2/S3 and S1/S3 in accordance with embodiments of the present invention.
Table IV
S.No. Ratio Values
Min Max
1 S2/S1 1.1 23.3
2 S2/S3 1.5 269.2
3 S1/S3 0.3 61.5

The ranges of the ratios of the slopes are chosen such that the desired values of the optical parameters are achieved in the optical fiber. FIGS. 4A to 4E illustrate graphs of variation of the cutoff wavelength (?c), the zero dispersion wavelength (?0), the chromatic dispersion (C.D) at and around 1550 nm wavelength, the bending loss at 1550 nm wavelength and the mode field diameter (MFD) at 1310 nm wavelength with the ratio S2/S1. It is observed that the cutoff wavelength (?c) and the chromatic dispersion (C.D) at and around 1550 nm wavelength increases with increasing value of S2/S1, whereas zero dispersion wavelength (?0), the bending loss at 1550 nm wavelength and the mode field diameter (MFD) at 1310 nm wavelength decreases with increasing value of S2/S1.

FIG. 5A to 5E illustrate graphs of variation of the cutoff wavelength (?c), the zero dispersion wavelength (?0), the chromatic dispersion (C.D) at and around 1550 nm wavelength, the bending loss at 1550 nm wavelength and the mode field diameter (MFD) at 1310 nm with the ratio S2/S3. It is observed that the cutoff wavelength (?c), the chromatic dispersion (C.D) at and around 1550 nm and the mode field diameter at 1310 nm wavelength decreases with increasing value of the ratio S2/S3, whereas the zero dispersion wavelength (?0) and the bending loss at 1550 nm wavelength decreases with increasing value of the ratio S2/S3.

FIG. 6A to 6C illustrate graphs of the cutoff wavelength (?c), the bending loss at 1550 nm wavelength and the mode field diameter (MFD) at 1310 nm wavelength with the ratio S1/S3. It is observed that the bending loss at 1550 nm wavelength decreases while the cutoff wavelength (?c) and the mode field diameter (MFD) at 1310 nm increases with increasing value of the ratio S1/S3.

In accordance with the first embodiment of the present invention the first sub-region 201 may be sub-divided into two parts, namely, a central part 201a and a periphery part 201b, wherein relative refractive index ??n?_0 of the central part 201a is in the range from 1.5 x 10-3 to 4.0 x 10-3 and relative refractive index ??n?_1 the periphery part is in the range from 4.7 x 10-3 to 6.0 x 10-3.

FIG.3 illustrates a refractive index profile 300 of an optical fiber in accordance with the second embodiment of the present invention comprising three slopes and a down-doped region. In accordance with a second embodiment of the present invention there is provided an optical fiber comprising a core region and a cladding region 305, the core region being divided into at least four sub-regions, each sub-region characterized by a relative refractive index and a slope, wherein, a first sub-region 301 has a relative refractive index ?n_1 and slope s0, a second sub-region 302 has a relative refractive index ?n_2 and slope S1, a third sub-region 303 has a relative refractive index ?n_3 and slope S2; and a fourth sub-region 304 has a relative refractive index ?n_4 and slope S3, and wherein the slopes are defined as rate of change of relative refractive index as a function of radius of the optical fiber. The slope S0 of the first sub-region 301 is a real number and absolute value of the slope S2 of the third sub-region 303 is greater than absolute value of the slope S1 of the second sub-region 302 which in turn is greater than absolute value of the slope S3 of the fourth sub-region 304. The ratio of the absolute value of the slope S2 of the third sub-region 303 to the absolute value of the slope S3 of the fourth sub-region 304 is in the range from 1.5 to 269.2, and the ratio of the absolute value of the slope S1 the second sub-region 302 to the absolute value of the slope S3 of the fourth sub-region 304 is in the range from 0.3 to 61.5. Volume loss? ??_2^', of the second sub-region is less than or equal to an additional volume ?_4 incorporated in the fourth sub-region, wherein volume (loss or incorporated) of the sub-regions is defined as ?_i=2?_(x_1)^(x_2)¦?|?n_i (r)|.rdr?, where i takes values 2 and 4 and x_(1 )and x_(2 )are the starting and ending points of the sub-regions.The cladding region 305 is divided into two portions, a first cladding portion 305a having a relative refractive index ?n_51 surrounding and in contact with the fourth sub-region 304 and a second cladding portion 305b, surrounding and in contact with the first cladding portion 305a, having a relative refractive index ?n_52, wherein the first cladding portion 305a is down-doped with a refractive index decreasing dopant such a boron or fluorine thereby forming an optical trench.

The provision of the trench region 305a further reduces the macro-bending losses in the optical fiber. The incorporation of the trench region 305a can cause the optical parameters including the mode field diameter, the cutoff wavelength, the zero dispersion wavelength and the chromatic dispersion to be shifted from the desired values. However, the provision of the slopes (S0 through S3) having ranges provided in the Table II hereinabove mitigates the shift caused by the incorporation of the trench region 305a.

Table V
Sub-region Relative refractive indices Radii (?m)
Symbol Values ( x 10-3) Symbol Min Max Typical
Min Max Typical
?n_0 1.5 4.0 3.0 r11 0.2 1.0 0.6
First ?n_1 4.7 6.0 5.0 r12 0.5 2 0.9
Second ?n_2 4.7 6.0 5.0 r2 2 4 3.6
Third ?n_3 3.5 4.15 4.0 r3 3.5 5 4.1
Fourth ?n_4 0.1 1.0 0.5 r4 7.0 12 8.6
At point (a) 0 0 0 r4 7.0 12 8.6
Fifth ?n_51 -3.5 -7.0 -4.0 r51 14 22 16
Sixth ?n_52 0 0 0 r52 - 62.5 62.5

In accordance with second embodiment of the present invention the values of various sub-regions of the core region and the cladding region are provided in Table V above. The volume loss 302a in the second sub-region 302b is compensated by providing an additional volume in the fourth sub-region 304.

Further, in accordance with the second embodiment of the present invention the first sub-region may be divided into two parts, namely, a central part 301a and the periphery part 301b.

The central part 301a or 201a as described herein above may be present in the relative refractive index profile of an optical fiber when such optical fiber is made by outside chemical vapor deposition (OVD) or modified chemical vapor deposition (MCVD) techniques as these techniques involve a central hole formation which results in a central dip in the relative refractive index. The central dip is indicated by the numeral 301a or 201a and named as central portion.
The optical fiber having the refractive index profile in accordance with the present invention exhibits a mode field diameter (MFD) in the range from 8.2 ?m to 9.5 ?m at and around 1310 nm wavelength region; an effective area (Aeff) of greater than or equal to 70 ?m2 at and around 1550 nm wavelength region; a chromatic dispersion (CD) of less than 18 ps/nm/km at and around 1550 nm wavelength region; a cabled cutoff wavelength (?c) of less than 1260 nm; a zero dispersion wavelength (?0) in the range from 1304 nm to 1324 nm; an attenuation loss of 0.190 dB/km or less at and around 1550 nm wavelength region; a macro-bend loss of 0.5 dB/turn on a mandrel of 32 mm at and around 1550 nm wavelength region.

The optical fiber having a refractive index profile in accordance with the second embodiment of the present invention exhibits a macro-bend loss of less than about 0.15 dB/loop for one loop of 5 mm radius at and around 1550 nm wavelength region and a macro-bend loss of less than about 0.2 dB/loop for one loop of 7.5 mm radius at and around 1550 nm wavelength region.

In accordance with the present invention, the optical fibers can be manufactured by modified chemical vapor deposition (MCVD) process, plasma chemical vapor deposition (PCVD) process, vapor axial deposition (VAD) process and outside chemical vapor deposition (OVD) process.

Particularly, the optical fiber with the relative refractive index of FIG. 2 or FIG. 3 may be manufactured by the OVD process in the following manner. In accordance with the OVD method a mandrel (or deposition substrate) is taken and fixed in a lathe in close proximity to soot (SiO2) deposition burners. The burners are supplied with silicon containing precursor wherein the silicon containing precursor is hydrolyzed in oxy-hydrogen flame to form SiO2 particles. These SiO2 particles are deposited on the mandrel in form of soot. The lathe is capable of rotating and translating the mandrel, wherein the soot deposition burners deposit soot onto the mandrel in form of multiple layers. First few layers of the soot being deposited are doped with GeO2 to increase the refractive index of the core region, wherein the burners are supplied with germanium containing precursor, the precursor being hydrolyzed in oxy-hydrogen flame forming GeO2. The slopes of various segments in the core are achieved by varying the concentration of GeO2 in the soot layers being deposited. The concentration of GeO2 is varied by varying the flows of germanium containing precursor in the burners. Once the core portion soot is deposited on the mandrel, the cladding soot which is essentially pure SiO2 (devoid of any dopant) is deposited on the core soot portion. The mandrel is then removed forming a soot body. The soot body so formed is then dehydrated, to remove the OH content, in a dehydration furnace at a temperature of 1150?C in presence of dehydrating gases such as chlorine and thermal conductivity increasing gases such as helium. The dehydrated soot body is then sintered in a sintering furnace at a temperature of 1550?C in atmosphere of chlorine and helium gas to form a solid glass body. The glass body is then drawn into optical fiber.

Examples:
Example 1 (Comparative example): An optical fiber was manufactured with a core region comprising four core sub-regions, and a cladding region as shown in FIG. 2 that violates the norms specified in the first embodiment of the present invention, which results in the failure of optical fiber. The values of refractive indices and radii of the core sub-regions and the cladding region are as provided in tables VI-A below. The values of the slopes S0 to S3 are provided in Table VI-B, and the ratios of the slopes S2/S1. S2/S3 and S1/S3 are provided in Table VI-C. It is observed that the volume loss ?_2^'of the second sub-region is equal to 0.0115 ?m2 and that of the fourth sub-region ?_4 is 0.005 ?m2, thus loss of volume ?_2^'is not less than or equal to? ??_4. From the Table VI-D it is clear that the optical fiber fails in the cutoff wavelength (a value of 1277 nm) which is greater than 1260 nm.
Table VI-A
Sub-regions Relative refractive indices Radii (?m)
Symbol Values
( x 10-3) Symbol Values
First ?n_1 5.2 r1 1.4
Second ?n_2 5.2 r2 4.3
Third ?n_3 4.0 r3 5.1
Fourth ?n_4 0.7 r4 5.9
Cladding ?n_5 0.0 r5 62.5

Table VI-B
S.No. Slope Values ( x 10-4)
1 S0 0
2 S1 -15
3 S2 -41.25
4 S3 -8.75

Table VI-C
S.No. Ratio Values
1 S2/S1 2.75
2 S2/S3 4.71
3 S1/S3 1.72

Table VI-D
S.No. Optical Parameters Values (achieved)
1 Dispersion at 1550 nm 17.73 ps/km.nm
2 Zero dispersion wavelength 1302.4 nm
3 Mode field diameter 9.38 ?m
4 Effective area (Aeff) 85 ?m2
5 Cabled cutoff wavelength 1277 nm
6 Bend loss at 1550 nm on a 32 mm radius mandrel 0.03 dB/(100 loops)

Example 2 (in accordance with the present invention): An optical fiber was manufactured with a core region comprising five core sub-regions, and a cladding region with trench 305a as shown in FIG. 5, which follows the slope range specified in this embodiment. The values of refractive indices and radii of the core sub-regions and the cladding region along with the trench are as provided in tables VII-A. Tables VII-B & VII-C provide the values of the slopes S0 through S3 and the ratios of the slopes respectively. Further, Table VII-D provides values of the optical parameters achieved by the optical fiber with relative refractive index profile of FIG. 3. It is observed that the volume loss ?_2^'of the second sub-region is equal to 0.0027 ?m2 and that of the fourth sub-region ?_4 is 0.0040 ?m2, thus loss of volume ?_2^'is less than ? ??_4. From the Table VI-D it is clear that the optical fiber complies to all the values of the optical parameters.

Table VII-A
Sub-region Relative refractive indices Radii (?m)
Symbol Values ( x 10-3) Symbol Values
Zeroth ?n_0 1.5 r11 0.2
First ?n_1 4.7 r12 0.5
Second ?n_2 4.7 r2 2
Third ?n_3 3.5 r3 3.5
Fourth ?n_4 0.05 r4 7.0
Fifth ?n_51 -3.9 r51 14
Sixth ?n_52 0 r52 -

Table VII-B
S.No. Slope Values ( x 10-4)
1 S0 Real value
2 S1 -8.0
3 S2 -22.7
4 S3 -1.43

Table VII-C
S.No. Ratio Values
1 S2/S1 2.83
2 S2/S3 15.87
3 S1/S3 28.5

Table VII-D
S.No. Optical Parameters Values (achieved)
1 Dispersion at 1550 nm 17.8 ps/(km.nm)
2 Zero dispersion wavelength 1310 nm
3 Mode field diameter 8.7 ?m
4 Effective area (Aeff) 71 ?m2
5 Cabled cutoff wavelength 1250 nm
6 Bend loss at 1550 nm on a 5 mm radius mandrel 0.06 dB/loop
7 Bend loss at 1550 nm on a 7.5 mm radius mandrel
Thus the optical fiber of the present invention having multi-sloped refractive index profile achieves the desired optical parameter values of the optical fiber including mode field diameter, cabled cutoff wavelength, zero dispersion wavelength, chromatic dispersion, bend loss and attenuation. The optical fiber of the present invention facilitates total internal reflection of light traveling through the fiber, thereby making the optical fiber effective carrier of information.

TECHNICAL ADVANCEMENTS AND ECONOMIC SIGNIFICANCE
Technical advancements offered by the optical fiber of the present disclosure include the realization of:

reduced bend loss;
desired optical parameters including cabled cut-off wavelength, zero dispersion wavelength, chromatic dispersion, mode field diameter and effective area;
compliance with the G652D ITU-T standards;
macro-bending loss of less than about 0.2 dB/turn on a mandrel of 15 mm diameter;
macro-bending loss of less than about 0.15 dB/turn on a mandrel of 10 mm diameter at 1550 nm wavelength region
minimization of attenuation loss to less than 0.18 dB/km; and

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer step, or group of elements, integers or steps, but not the exclusion of any other element, integer of step, or group of elements, integers or steps.

The use of the expression “at least” or at least one” suggests the use of more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.

Wherever a range of values is specified, a value up to 10% below and above the lowest and highest numerical value respectively, of the specified range, is included in the scope of the invention.

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.