WO2009050727A2

WO2009050727A2 - Optical fiber having high sbs threshold

Info

Publication number: WO2009050727A2
Application number: PCT/IN2008/000340
Authority: WO
Inventors: Saurav Dutta; Reena Harnal; Hrudaya Ranjan Sahu; Pankaj Khanna
Original assignee: Sterlite Technologies Ltd
Priority date: 2007-05-31
Filing date: 2008-05-29
Publication date: 2009-04-23
Also published as: WO2009050727A3

Abstract

An optical fiber with reduced Stimulated Brillouin Scattering (SBS) and increased SBS threshold value, comprises a ore region and a clad region (15) surrounding the core region. The core region is divided into at least three sub-core regions, namely inner sub-core region (11), outer sub-core region-I (12) and outer sub-core region-II (13). The relative refractive indices of the inner sub-core region (11), the outer sub-core region-I (12), the outer sub-core region-II (13) and the clad region (15) are Δ1,Δ 2, Δ3 and Δ4, respectively. The refractive indices of the three sub-core regions and the clad region are related by the equation: Δ1> Δ2>Δ3>Δ4. Wherein, the inner sub-core region (11) is surrounded by the outer sub-core region-I (12), the outer sub-core region-I (12) is surrounded by the outer sub-core region-II (13), and he outer sub-core region-II (13) is surrounded by the clad region (15).

Description

OPTICAL FIBER HAVING HIGH SBS THRESHOLD

Field of the Invention:

The present invention generally relates to optical fiber having high Stimulated Brillouin Scattering [SBS] threshold. Particularly, the present invention relates to single mode optical fiber having high SBS threshold suitable for long haul transmission and CATV systems. More particularly, it relates to single mode optical fiber having high SBS threshold suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm.

Background of the Invention:

Over the last decade, the optical fibers have been developed and installed as the backbone of networks for voice, video and data transmission applications. The optical fiber is becoming vital with growing and expanding telecommunication infrastructure.

An optical fiber comprises a core and a cladding surrounding the core. The refractive index of the core is generally higher than that of the cladding. The difference in refractive index between the core and the cladding is responsible for guiding the light in the optical fiber. The refractive index profile, which is graph of change of refractive index of core and or clad radially, is changed to obtain desired optical parameters, such as dispersion, dispersion slope, cutoff wavelength, and mode field diameter among other optical parameters in the optical fiber. In the most common type of the optical fiber the refractive index of the core is constant with cladding having refractive index less than that of the core. Such type of optical fiber wherein the refractive index of the core is constant over the radius of the core is called step-indexed optical fiber. The other optical fibers may have the refractive index of the core varying radially outward [parabola shape]. Such optical fibers are called graded indexed optical fibers. The number of modes (light) they support also classifies the optical fibers. A fiber supporting a single mode is called single mode optical fiber, whereas the optical fiber supporting more than one mode is called multimode optical fiber. The most widely used optical fiber in the networks is the step indexed single mode optical fibers. Typical such optical fibers have core diameter of about 9 microns and cladding diameter of about 125 microns. Optical fibers with various refractive index profiles and supporting one or plurality of modes are well known in prior art [referred to a conventional optical fiber or simply optical fiber hereafter].

The conventional optical fibers are used in long distance information transmission applications. The long distance transmission of information has become possible using optical fibers having high bandwidth of 40 teraHz or greater and low attenuation loss. In order to achieve long-distance information transmission, it is not only essential to lessen the transmission loss of the optical fiber, but also essential to increase signal power launched into optical fiber so that less number of amplifiers are required to achieve reduction in cost of networking.

It has been observed that increase in signal power launched into conventional optical fiber is limited. It has been observed that if signal power launched into conventional optical fiber exceeds certain threshold, for example, if it exceeds 7 dBm (read as dB relative to milli-watt), a non-linear optical phenomenon occurs which produces acoustic waves in the optical fiber. It appears that acoustic waves are generated due to intense optical field of signal power launched into the optical fiber. The generation of acoustic waves has been found to cause changes in material density which in-turn has been found to result in refractive index variations. The variations in refractive index in-turn backscatter the signal or say signal power back towards the source. Therefore, above a certain threshold [for example more than 7 dBm] value of signal power, the amount of back scattered light is very high, which in-turn limits the signal power to be transmitted through the optical fiber and in addition reduces signal to noise ratio, which should be higher to achieve better signal transmission. This phenomenon of back scattering of signal or say signal power above a certain threshold value of signal power and is known as "Stimulated Brillouin Scattering [SBS]" has been found to restrict suitability of optical fiber for long haul transmission and CATV systems, particularly at the wavelength of about 1310 nm to about 1625 nm. Therefore, it is understood that the conventional fibers have limited signal power launch due to limited SBS threshold, i.e. equal to or less than 7dBm, and hence have restricted suitability for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm. It is also understood that SBS threshold of the conventional optical fiber puts an upper limit to signal power transmitted therethrough. Therefore, if requirement is to have increased signal power launched in optical fiber, it is desirable to increase SBS threshold value thereof to have its suitability for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm.

Need of the Invention:

Therefore, there is a need to have optical fiber having high SBS threshold value meaning thereby having high signal power launched characteristic as compared to conventional optical fibers, and hence, being suitable for long haul transmission and CATV systems,

Objects of the Present Invention:

Accordingly, main object of the present invention is to provide optical fiber having high SBS threshold value so that high signal power can be launched into the optical fiber.

It is also an object of the present invention to provide optical fiber having high SBS threshold value, and hence, having high signal power launched capacity and being suitable for long haul transmission and CATV systems, particularly at wavelength varying from about 1310 nm to about 1625 nm.

It is also an object of the present invention to provide optical fiber having high

SBS threshold value, which is more than about 7dBm, preferably more than about

1OdBm so as to achieve high signal power launched characteristic of optical fiber so as to make it suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm,

It is also an object of the present invention to provide optical fiber which does not suffer from above-described problems of prior art, that is, even if the signal power launched is increased , the non-linear optical phenomenon is greatly reduced meaning thereby acoustic waves is greatly avoided, and hence, changes in material density is also avoided, therefore, variations in refractive index are greatly avoided. Accordingly, backscattering of signal or say signal power is greatly reduced even at higher signal power value, and therefore, signal power to be transmitted through the optical fiber is greatly increased.

It is also an object of this invention to provide optical fiber having high SBS threshold value and still having increased signal to noise ratio, meaning thereby better signal transmission is achieved in optical fiber of present invention.

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.

Brief Description of the Invention;

The present invention relates to a single mode optical fiber having high SBS threshold, wherein the high SBS threshold is realized without loosing the characteristics of a single mode optical fiber including dispersion, dispersion slope, high effective area, attenuation, cutoff and low PMD and being suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm.

In accordance with the present invention a single mode optical fiber having high SBS threshold, wherein the high SBS threshold is realized without loosing the characteristics of a single mode optical fiber including dispersion, dispersion slope, high effective area, attenuation, cutoff and low PMD and being suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm is disclosed comprising core and clad regions, the core region is divided into three sub-core regions, namely inner sub-core region, outer sub- core region-I and outer sub-core region-H The inner sub-core region is surrounded by the outer sub-core region-I, which in turn is surrounded by the outer sub-core region- II, which in turn is surrounded by the cladding. In accordance with the present invention the refractive indices of the various regions of the presently disclosed fiber are related in a manner that the refractive indices of the core region is higher than that of the cladding region, the core region being divided into three sub-core regions with the refractive indices such that the refractive index of the inner core region is higher than that of the outer core region-I which in turn is higher than that of the outer core region-II, which has been observed to provide the high SBS threshold without loosing the characteristics of a single mode optical fiber including dispersion, dispersion slope, high effective area, attenuation, cutoff and low PMD and being suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm. The refractive indices of the various regions of the presently disclosed fiber are related by ni > n₂ > m Eqn. 1 wherein ni is refractive index of inner sub-core region, n₂ is refractive index of outer sub-core region-I and n₃ is refractive index of outer sub-core region-II, which has been found to provide high SBS threshold value, which is more than about 7dBm, preferably more than about 1OdBm and which has been found to have high signal power launched characteristic and suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm without loosing the characteristics of a single mode optical fiber including dispersion, dispersion slope, high effective area, attenuation, cutoff and low PMD and being suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm.

In accordance with present invention, the core region is surrounded by a clad region and fiber is characterized by comprising an inner sub-core region surrounded by an outer sub-core region-I which in-turn is surrounded by an outer sub-core region- II and said outer sub-core region II is surrounded by said clad region and having respective refractive index of each region related by equation 2:-

ni > n₂ > 11₃ > n₄ Eqn. 2 wherein rii is refractive index of inner sub-core region, n₂ is refractive index of outer sub-core region-I, n₃ is refractive index of outer sub-core region-II and n₄ is refractive index of clad region.

In one embodiment of the present invention, the clad region surrounding the core region is divided into two sub-clad regions, namely inner sub-clad region and outer sub-clad region and fiber is characterized by comprising a core region divided into three sub-core regions in a manner that inner sub-core region is surrounded by outer sub-core region-I which in-turn is surrounded by outer sub-core region-II and said outer sub-core region II is surrounded by said inner sub-clad region which is then surrounded by outer sub-clad region and having respective refractive index of each region related by equation 3:-

ni > n₂ > n₃ > ns > n₄ Eqn. 3

wherein ni is refractive index of inner sub-core region, n₂ is refractive index of outer sub-core region-I, n₃ is refractive index of outer sub-core region-II, n₄ is refractive index of inner sub-clad region and n₅ is refractive index of outer sub-clad region. According to this embodiment, the inner sub-clad region is depressed clad having refractive index lower than outer sub-clad region.

The other objects, embodiments 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 embodiments and are not intended to limit scope of the present invention.

Brief Description of the Accompanying Figures:

Figure 1 shows a cross sectional view of design of optical fiber in accordance with one of the preferred embodiments of the present invention.

Figure 2 shows a cross sectional view of design of optical fiber in accordance with another preferred embodiment of the present invention. Figure 3 shows a cross sectional view of design of optical fiber in accordance with first embodiment of the present invention to illustrate measurement of thickness of each sub-core region of core region of optical fiber in accordance with first embodiment of this invention.

Figure 4 shows refractive index profile of the optical fiber in accordance with the one embodiment of the present invention to illustrate the relation between the refractive indices of the three sub-core regions and the cladding region.

Figure 5 shows refractive index profile of the optical fiber in accordance with another embodiment of the present invention to illustrate the relation between the refractive indices of the three sub-core regions, the depressed cladding and the cladding region.

Figure 6 shows a SBS measurement setup to measure the SBS threshold value.

Description [nature] and preferred embodiments of the Invention;

With aim to overcome above-described drawbacks and limitations of prior art, inventors of the present invention have found that if in a fiber comprising core and clad regions, the core region is divided into three sub-core regions, namely inner sub- core region, outer sub-core region-I and outer sub-core region-II having respective refractive index related by equation 1:-

ni > n₂ > n₃ Eqn, 1

wherein ni is refractive index of inner sub-core region, n₂ is refractive index of outer sub-core region-I and n₃ is refractive index of outer sub-core region-II, the fiber thus produced has been surprisingly found to have high SBS threshold value, which is more than ab,out 7dBm, preferably more than about 1OdBm and which has been surprisingly found to have high signal power launched characteristic and suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm. Accordingly, the present invention relates to optical fiber comprising core and clad regions, characterized in that the core region is divided into three sub-core regions, namely inner sub-core region, outer sub-core region-I and outer sub-core region-II having respective refractive index related by equation 1 :-

ni > n₂ > n₃ Eqn. 1

wherein ni is refractive index of inner sub-core region, n₂ is refractive index of outer sub-core region-I and n₃ is refractive index of outer sub-core region-II.

In accordance with present invention, the core region divided into three sub- core regions is characterized by comprising an inner sub-core region surrounded by an outer sub-core region-I which in-turn is surrounded by an outer sub-core region-II and having respective refractive index related by equation 1:-

ni > n₂ > n₃ Eqn. 1

ni > n₂ > n₃ > n₄ Eqn. 2

wherein m is refractive index of inner sub-core region, n₂ is refractive index of outer sub-core region-I, n₃ is refractive index of outer sub-core region-II and n₄ is refractive index of clad region.

m > n₂ > n₃ > n₅ > n₄ Eqn. 3

Now referring to accompanying Figure 1 which illustrates configuration of optical fiber in accordance with one of the preferred embodiments of the present invention, the present invention relates to optical fiber having high SBS threshold value, and having high signal power launch characteristic and being suitable for long haul transmission and CATV systems, particularly at the wavelength varying from about 1310 nm to about 1625 nm, wherein fiber comprising core region and clad region is characterized by a core region divided into three sub-core regions, namely inner sub-core region (11), outer sub-core region-I (12) and outer sub-core region-II (13) having respective refractive index related by equation 1:-

ni > n₂ > n₃ Eqn. 1

In accordance with present invention, the core region divided into three sub- core regions is characterized by comprising an inner sub-core region (11) surrounded by an outer sub-core region-I (12) which in-turn is surrounded by an outer sub-core region-II (13) and having respective refractive index related by above equation 1. In accordance with present invention, said core region is surrounded by a clad region (15) and fiber is characterized by comprising an inner sub-core region (11) surrounded by an outer sub-core region-I (12) which in-turn is surrounded by an outer sub-core region-II (13) and said outer sub-core region II (13) is surrounded by said clad region (15) and having respective refractive index of each region related by above equation 2,

In one embodiment of the present invention, the clad region (15) surrounding the core region [comprising three sub-regions (11), (12) and (13)] is divided into two sub-clad regions, namely inner sub-clad region (14) and outer sub-clad region (16)

[Figure 2] and fiber is characterized by comprising a core region divided into three sub-core regions in a manner that inner sub-core region (11) is surrounded by outer sub-core region-I (12) which in-turn is surrounded by outer sub-core region-II (13) and said outer sub-core region II (13) is surrounded by said inner sub-clad region (14) which is then surrounded by outer sub-clad region (16) and having respective refractive index of each region related by above equation 3.

The refractive indices ni, m, n₃, rμ and n₅ may also be referred to as relative refractive indices, that is, relative refractive indices of the inner sub-core region (11), outer sub-core region-I (12), outer sub-core region-II (13), inner sub-clad region (14) and outer sub-clad region (15) is identified by symbols Δ,, A₂, A₃, A₄ and Δ₅ respectively, Therefore in accordance with the present invention, the relative refractive indices of the presently disclosed fiber are characterized by equations Ia, 2a and 3a respectively corresponding to equations 1, 2 and 3 respectively as below: -

Δ_j > A₂ > Δ₃ Eqn. Ia

A₁ > A₂ > Δ₃ > Δ₄ Eqn. 2a

Δ, > A₂ > A₃ > A₅ > A₄ Eqn. 3a The relative refractive indices of the various regions can be calculated from the refractive indices of that region and the refractive index of the outer glass region by applying the following equation Ib: - Δ_x = [(n_x ² - n_y ²) xl00]/(2xn_x ²) Eqn, Ib

Wherein, 'y' is the number of regions in the optical fiber, and x = 1, 2, 3, 4 and 5 for inner core region (11), outer core region-I (12), outer core region-II (13), inner sub- clad region (14), and outer sub-clad region (16) respectively.

In accordance with one embodiment of the present invention, an optical fiber with reduced Stimulated Brillouin Scattering (SBS) and increased SBS threshold value is provided. The optical fiber comprises a core region and a clad region surrounding the core region. The core region is divided into at least three sub-core regions, namely inner sub-core region, outer sub-core region-I and outer sub-core region-II. The relative refractive indices of the inner sub-core region, the outer sub- core region-I, the outer sub-core region-II and the clad region are A₁, Δ₂, Δ₃ and Δ₄ respectively. The refractive indices of the three sub-core regions and the clad region are related by the equation: Δ_j > A₂ > Δ₃ > Δ₄ Eqn. 2a

The inner sub-core region is surrounded by the outer sub-core region-I, the outer sub- core region-I is surrounded by the outer sub-core region-II, and the outer sub-core region-II is surrounded by the clad region.

The fiber of the present invention has the refractive index profile along the radial axis of the fiber as illustrated in figure 4, wherein the core region is sub-divided into three sub-regions namely the inner core region (11), the outer core region-I (12) and the outer core region-// (13), wherein the inner core region (11) is surrounded by the outer core region-I (12) which in turn is surrounded by the outer core region-II (13) which in turn is surrounded by cladding (14).

In accordance with one embodiment of the present invention, the relative refractive indices of the various regions (equation Ia, and equation 2a) of the optical fiber core rod as measured by PK 2600 refractive index profiler are selected within the range as specified in table 1. Table 1

The refractive index profile is such as shown in figure 4. The refractive index of the inner core region (U) is maximum at the center (0.54 %) and decreases to 0.27 % at the edge of the inner core region (11). Similarly the refractive index of the outer core region-I (12) is 0.27 % and decreases to 0.25 % and thereafter the refractive index of the outer core region-II (13) starts with 0.25 % and decreases to 0 which is

^• equal to refractive index of the cladding.

In one embodiment the decrease of refractive index within the various regions may be linear, logarithmic, parabolic or any other.

In accordance with another embodiment of the present invention, the relative refractive indices (equation 3a) of the various regions of the optical fiber core rod as measured by PK 2600 refractive index profiler are selected within the range as specified in table 2.

Table 2

The refractive index profile is such as shown in figure 5. The refractive index of the inner core region (U) is maximum at the center (0.54 %) and decreases to 0.27 % at the edge of the inner core region (11). Similarly the refractive index of the outer core region-I (12) starts with 0.27 % and decreases to 0.25 % and thereafter the refractive index of the outer core region-II (13) starts with 0.25 % and decreases to 0, the inner sub-clad (14) has a refractive index that is less than 0 (-0.02 %) and reaches this value in a single step, whereas the outer sub-clad has a refractive index 0 which is equal to refractive index of the cladding. The decrease of refractive index within the various regions may be linear, logarithmic, parabolic or any other. The decrease of refractive index within the various regions may be linear, logarithmic, parabolic or any other.

According to this embodiment, the inner sub-clad region (14) is depressed clad having refractive index lower than outer sub-clad region (16).

In accordance with one of the preferred embodiments of this invention, the thickness of each sub-core region is so selected that it does not result in reduction of SBS threshold value which has been increased by novel and inventive division of core region into three sub-core regions.

In accordance with one of the preferred embodiments of present invention, the thickness of each sub-core region is related by equation 4a:-

t₂ > ti -) Eqn, 4a t₃ > t, J

wherein ti is thickness of inner sub-core region (11) which is equivalent to its radius, t₂ is thickness of outer sub-core region-I (12) which is equivalent to difference of its radius minus radius of inner sub-core region (11) and t₃ is thickness of outer sub-core region-II (13) which is equivalent to difference of its radius minus radius of inner sub-core region (1 1) and radius of outer sub-core region-I (12) and as illustrated in accompanying Figure 3. The thickness of various regions as measured in the optical fiber with the help of the Photon Kinetics instrument S 14 are tabulated in table 3. Table 3

In accordance with one of the preferred embodiments of present invention, the thickness of each sub-core region and sub-clad region is related by equation 4b:-

t₂ > h > U Eqn. 4a t₃ > ti > U

wherein ti is thickness of inner sub-core region (11) which is equivalent to its radius, t₂ is thickness of outer sub-core region-I (12) which is equivalent to difference of its radius minus radius of inner sub-core region (11), t₃ is thickness of outer sub-core region-II (13), U is thickness of the inner sub-clad region (14) which is equivalent to difference of its radius minus radius of inner sub-core region (11) and radius of outer sub-core region-I (12). The thickness of various regions as measured in the optical fiber with the help of the Photon Kinetics instrument S 14 are tabulated in table 4.

Table 4

The optical fibers having configuration either of equation 1, 2 or 3, were checked for SBS threshold value and were surprisingly found to have SBS threshold value more than 7dBm, preferably more than 10 dBm.

In one of the preferred embodiments of the present invention, the optical fibers having configurations described herein were checked for optical attenuation loss which was found to be equal to or less than about 0.35 dB/Km at about 1310 nni, equal to or less than about 0.21 dB/Km at about 1550 nm and equal to or less than about 0.30 dB/Km at about 1383 nm in the fiber of each of above-described configuration. Typical values of the various optical parameters obtained for the optical fiber of the present invention is tabulated in table 5 below.

Table 5

In one of the preferred embodiments of the present invention, the optical fibers having configurations described herein were checked for cutoff wavelength, dispersion, Mode Field Diameter [MFD] and MAC number, and cutoff wavelength was found to be equal to or greater than about 1 160 nm, dispersion was found to be zero at about 1300 nm to about 1325 nm, mode field diameter [MFD] was found to be equal to or less than about 9.4 μm and MAC number was found to be equal to or less than about 7.6 in the fiber of each of above-described configuration.

Another advantage of present invention includes providing optical fiber which not only has high SBS threshold value, but also complies with the ITU G652D specifications.

Measurement of SBS threshold - Method and Apparatus: The SBS threshold was measured using the instrument (setup) described below:

The SBS threshold measurement setup in accordance with the ITU specifications is shown in figure 6. The measurement setup comprises a 81662A DFB laser at 1550 nm, a 4x1 switch and a power meter. The narrow linewidth 1550nm optical radiation from the DFB laser is amplified using a Keopsys Erbium Doped Fiber Amplifier (EDFA) module. The EDFA module also has a tap coupler at its output side, which is used to measure the reference power fed into the test fiber through a circulator. The transmitted power from the test fiber, the back-reflected power extracted through the circulator, and the input reference power from the tap coupler are all connected to a 4x1 switch. The output of the switch is connected to a calibrated power meter, thereby enabling all the power measurements to be done without manual intervention.

In order to measure the SBS threshold, the power injected into the test fiber was varied by changing the gain of EDFA module. The DFB laser provided the input to the EDFA module and was held at a constant power of -10 dBm (100 μW). The incident power is varied in 0.5 dBm steps and the corresponding values of transmitted and back-reflected power are noted down. For our measurements, we considered the peak back-reflected value at each incident power. In accordance with ITU procedure, the SBS threshold is measured as the incident power at which the back-reflected power is equal to the transmitted power.

Example 1: (Conventional single mode stepped indexed optical fiber)

A conventional single mode stepped index optical fiber, comprising a core with constant refractive index radially and cladding surrounding the core with refractive indices and thickness as tabulated in table 6 was taken and the SBS threshold value of the optical fiber was measured using the above method and instrument. The value of the SBS threshold was found to be 6.8 dBm. It is clear that the SBS threshold of the optical fiber was quite low and is not suitable for use in long haul transmission and CATV systems with high launch power. The optical parameters and SBS threshold are tabulated in table 7 below. Table 6

Table 7

Example: 2 (One Embodiment)

An optical fiber with the refractive index profile as illustrated in figure 4 was fabricated by the MCVD technique. The core region is subdivided into three regions namely, the inner sub-core region surrounded by the outer sub-core region-I which in turn is surrounded by the outer sub-core region-II which is surrounded by cladding and the refractive indices are related by equation 2a. The refractive indices of the various regions and the thickness are as tabulated in table 8 below. It was observed that the SBS threshold for the optical fiber as measured with the method and apparatus described above was 12.04 dBm, which is twice as compared with the conventional optical fiber of example 1. The other optical parameters of the fiber under consideration are listed in table 9 below. Table 8

Table 9

Example: 3 (Another Embodiment)

An optical fiber with the refractive index profile as illustrated in figure 5 was fabricated by the MCVD technique. The core region is subdivided into three regions namely, the inner sub-core region surrounded by the outer sub-core region-I which in turn is surrounded by the outer sub-core region-II which is surrounded by inner sub- clad region, which is surrounded by outer sub-clad and the refractive indices are related by equation 3a. The refractive indices of the various regions and the thickness are as tabulated in table 10 below. It was observed that the SBS threshold for the optical fiber as measured with the method and apparatus described above was 13.27 dBm, which is twice as compared with the conventional optical fiber of example 1. The other optical parameters of the fiber under consideration are listed in table 11 below.

Table 10

Table 11

It may be noted that present invention is not restricted by selection of method of production of presently disclosed optical fiber having high SBS threshold value which can be produced by any method known in art, for example, can be prepared by Modified Chemical Vapor Deposition [MCVD], Outside Vapor Deposition [OVD], Vapor Axial .Deposition [VAD], Plasma Chemical Vapor Deposition [PCVD] processes, etc, Accordingly the present invention is not restricted by selection of method for producing the optical fiber, but restricted by configuration and refractive index profile as described herein.

For example, the high SBS threshold optical fiber of the present invention can be prepared by MCVD process to have cladding with or without depressed cladding, outer core region-II, outer core region-I and inner core region in the defined sequence inside the glass tube while controlling refractive index of each region in a manner that it satisfies the relationship defined in either of equation 1, 2 or 3. The deposited layers are subjected to steps of dehydration, sintering and collapsing to form optical fiber preform, which in turn is then subjected to step of draw to draw the high SBS threshold optical fiber of the present invention.

In accordance with one of the preferred embodiments of the present invention, the refractive index of each region can be varied with the help of one or more dopants, for example with the help of germanium, phosphorous, fluorine etc.

It may be noted that present invention is not restricted by nomenclature of sub- regions of core region, which have been described as inner sub-core region, outer sub- core region-I and outer sub-core region-II merely for understanding configuration of presently provided fiber and are not intended to limit scope of this invention.

Similarly, it may be noted that present invention is not restricted by nomenclature of sub-regions of clad region, which have been described as inner sub- clad region and outer sub-clad region merely for understanding configuration of presently provided fiber and are not intended to limit scope of this invention.

Claims

We claim:

1. An optical fiber with reduced Stimulated Brillouin Scattering (SBS) and increased SBS threshold value, the optical fiber comprising: a core region; and a clad region surrounding the core region; characterized in that, the core region is divided into at least three sub-core regions, namely inner sub- core region, outer sub-core region-I and outer sub-core region-II; the relative refractive indices of the inner sub-core region, the outer sub-core region-I, the outer sub-core region-II and the clad region are Δi, Δ₂, Δ₃ and _Δ₄ respectively, wherein the refractive indices of the three sub-core regions and the clad region are related by the equation:

Δi > Δ₂ > Δ₃ > Δ₄ wherein, the inner sub-core region is surrounded by the outer sub-core region-I, the outer sub-core region-I is surrounded by the outer sub-core region-II, and the outer sub-core region-II is surrounded by the clad region

2. The optical fiber according to claim 1, wherein the inner sub-core region has a minimum relative refractive index of about 0.5238 and maximum relative refractive index of about 0,5562, the outer sub-core region-I has a minimum relative refractive index of about 0.2592 and maximum relative refractive index of about 0.2808, the outer sub-core region-II has a minimum relative refractive index of about 0.2375 and maximum relative refractive index of about 0.2625, and the clad region has a relative refractive index of zero.

3. The optical fiber according to claim 1, wherein the thickness of the three sub- core regions are related by the equations: t₃ > ti; and t₂> ti wherein the thickness of inner sub-core region, outer sub-core region-I and outer sub-core region-II are tj, t_∑, and t₃ respectively,

4. The optical fiber according to claim 3, wherein the inner sub-core region has minimum thickness of about 1.5801μm and maximum thickness of about 1.7998 μm, the outer sub-core region-I has minimum thickness of about 2.1616 μm and maximum thickness of about 2.2184 μm, and the outer sub-core region-II has minimum thickness of about 2.1213 μm and maximum thickness of about 2.1386 μm.

5. The optical fiber according to claim 1, wherein the SBS threshold value is greater than 7 dBm.

6. The optical fiber according to claim 1, wherein the SBS threshold value is greater than 10 dBm.

7. The optical fiber according to claim 1, wherein the optical attenuation loss in the optical fiber is equal to or less than about 0.35 dB/Km at about 1310 nm, equal to or less than about 0.21 dB/Km at about 1550 nm, and equal to or less than about 0.30 dB/Km at about 1383 nm.

8. The optical fiber according to claim 1, wherein the optical fiber has, cutoff wavelength equal to or greater than about 1160 nm, zero dispersion at about 1300 nm to about 1325 nm, Mode Field Diameter [MFD] is less than about 9.4 μm and MAC number is equal to or less than about 7.6.

9. The optical fiber according to claim 1, wherein the clad region is divided into at least two sub-clad regions, namely inner sub-clad region and outer sub-clad region having relative refractive indices Δ4 and Δ₅ respectively, wherein, the outer sub-core region II is surrounded by the inner sub-clad region and the inner sub-clad region is surrounded by outer sub-clad region.

10. The optical fiber according to claim 9, wherein the relative refractive indices of the three sub-core region and the relative refractive indices of the two sub-clad regions are related by the equation:

Δi > Δ₂ > Δ₃ > Δ₅ > Δ₄.

11. The optical fiber according to claim 10, wherein the inner sub-core region has a minimum relative refractive index of about 0.5238 and maximum relative refractive index of about 0.5562, the outer sub-core region-I has a minimum relative refractive index of about 0.2592 and maximum relative refractive index of about 0.2808, the outer sub-core region-II has a minimum relative refractive index of about 0.2375 and maximum relative refractive index of about 0.2625, the inner sub-clad region has a minimum relative refractive index of about -0.028 and maximum relative refractive index of about -0.012, and the outer sub-clad has a relative refractive index of zero.

12. The optical fiber according to claim 10, wherein the thickness of the three sub- core regions and inner sub-clad region are related by the equations: t₂ > ti > W, and t₃ > t, > U wherein the thickness of inner sub-core region, outer sub-core region-I, outer sub-core region-II, and inner sub-clad region are tj, tz, t₃, and U respectively.

13. The optical fiber according to claim 12, wherein the inner sub-core region has minimum thickness of about 1.58Olμm and maximum thickness of about 1.7998 μm, the outer sub-core region-I has minimum thickness of about 2.1616 μm and maximum thickness of about 2.2184 μm, the outer sub-core region-II has minimum thickness of about 2.1213 μm and maximum thickness of about 2.1386 μm, and the inner sub-clad region has minimum thickness of about 1.6022 μm and maximum thickness of about 1.6977 μm.

14. The optical fiber according to claim 10, wherein the inner sub-clad region is depressed clad having refractive index lower than outer sub-clad region.

15. The optical fiber according to claim 10, wherein the SBS threshold value is greater than 7 dBm.

16. The optical fiber according to claim 10, wherein the SBS threshold value is greater than 10 dBm.

17. The optical fiber according to claim 10, wherein the optical attenuation loss in the optical fiber is equal to or less than about 0.35 dB/Km at about 1310 nm, equal to or less than about 0.21 dB/Km at about 1550 nm, and equal to or less than about 0.30 dB/Km at about 1383 nm.

18. The optical fiber according to claim 10, wherein the optical fiber has, cutoff wavelength equal to or greater than about 1160 nm, zero dispersion at about 1300 nm to about 1325 nm, Mode Field Diameter [MFD] is less than about 9.4 μm and MAC number is equal to or less than about 7.6 .