WO2007043060A1

WO2007043060A1 - Optical fiber having higher bandwidth and method for producing the same

Info

Publication number: WO2007043060A1
Application number: PCT/IN2005/000354
Authority: WO
Inventors: Pannaga Bangalore Krishnaswamy; Saurav Dutta; Sathis Ram Panneerselvam; Senthil Kumar Nageswaran
Original assignee: STERLITE OPTICAL TECHNOLOGIES Ltd
Current assignee: STERLITE OPTICAL TECHNOLOGIES Ltd
Priority date: 2005-10-07
Filing date: 2005-10-28
Publication date: 2007-04-19
Anticipated expiration: 2008-04-07
Also published as: CN101052596A

Abstract

A method for manufacturing a multimode optical fiber having higher bandwidth comprising applying correction factor to obtain desired layerwise alpha refractive index profile thereby resulting in formation of the preform to form an optical fiber having higher bandwidth. The correction factor is determined from difference between ideal layerwise mode delay and actual layerwise mode delay. The process also comprises determining the correction factor from difference between ideal layerwise alpha and actual layerwise alpha.

Description

TITLE OF THE INVENTION:-

Optical fiber having higher bandwidth and method for producing the same.

FIELD OF THE INVENTION:

The present invention generally relates to the field of manufacturing a multimode optical fiber having higher bandwidth and multimode optical fiber. More particularly it relates to a method for manufacturing a multimode optical fiber having higher bandwidth by refractive index profile measurement technique.

BACKGROUND OF THE INVENTION:

Fiber-based light wave communication systems are playing an important role in voice and data transmission. Optical fiber for use in communications systems can be either single mode optical fiber or multimode optical fiber. The present invention is primarily concerned with the method for manufacturing multimode optical fiber. However, the disclosed method can also be extended to manufacture single mode optical fiber.

Multimode optical fiber comprises a core, to which essentially the entire signal is confined, and a clad surrounding the core.

Frequently a barrier layer is interposed between core and clad.

Multimode optical fiber is drawn from preform, which can be manufactured by different techniques, such as modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD), outer vapor deposition (OVD), vapor axial deposition (VAD), etc. The performance of optical fibers for communications is primarily determined by optical loss or attenuation and by dispersion. Optical losses are caused by absorption, scattering, and imperfect geometry or structural defects and dispersion, which cause a smearing of light pulses leading to noise.

Signal dispersion in multimode fiber is due to chromatic dispersion and to mode dispersion. The chromatic dispersion is a variation in the refractive index with wavelength due to the material itself, and the mode dispersion is due to differences in the optical path lengths for different transmission modes. The dispersion is expressed in terms of the fiber bandwidth per unit length or in terms of maximum bit rate per unit length.

It is possible to reduce the mode dispersion by designing the fiber having radially varying refractive index profile in the core region.

The magnitude of refractive index (n) is known to vary as a function of radial distance (r) from the center of the fiber according to the following equation 1,

n(r) = ni (1 - 2 Δ (r/a)^α )i/a (1) n(r) = n₂ (1 -2 Δ )ⁱ/a

Herein, ni represents the refractive index of the center of the core; n2 represents refractive index of clad, α represents the core shape index (alpha) and Δ represents relative index difference as shown in equation 2,

Δ = (ni-na)/ ni (2) It has been observed that the multimode fiber bandwidth critically depends on the value of alpha (α). Accordingly, the core profile shape for graded index multimode fiber is parabolic and can have higher bandwidth when the alpha (α) is selected to be varying from about 1.9 to about 2, which is depended on the wavelength. This observation is drawn from figure 34, pp 43 in "Optical Fiber Communications vol.1, Fiber Fabrication, Academic Press Inc., 1985, - Tingye Li". This figure illustrates that the bandwidth of the multimode fiber will be less than 500 MHz-Km when the alpha value is either below 1.9 or above 2. If the alpha value is selected to be equal to 2 (1 -Δ) where Δ is the relative refractive index difference between core and clad, then the bandwidth of the multimode fiber will be higher. Accordingly, it is observed that achieving this value of alpha or at the maximum achieving the alpha varying from about 1.9 to about 2.0 will result in maximization of the bandwidth of the multimode optical fiber.

It is the above observations, which led to the development of the presently disclosed method and the multimode fiber manufactured thereby.

The prior art [US Patent No. 4,286,979] describes a method to manufacture a multimode optical fiber having improved mode dispersion resulting in higher bandwidth. This method involves differential mode-group delay measurement technique to optimize dispersion by means of appropriate index of refraction configuration. According to this technique, light is launched into a multimode fiber using a single mode fiber which is caused to radially scan the face of the multimode fiber. In this method the differential group delay measurements of the manufactured fiber are taken and the index configuration of the subsequently manufactured fibers is designed to minimize differential mode group delay. The subsequently manufactured fibers are then again measured using this technique and the radial refractive index gradation is again changed accordingly.

Accordingly, the major drawback of this method is that it is applicable on the manufactured fiber and not during the method of manufacturing the fiber, that is, it cannot be applied as on-line process of manufacturing for rendering the technique more efficient and to improve or control the manufacturing process to enhance the production capacity and the economics of the manufacturing process to make it economically more viable.

The major limitation of the known method is that the efficacy of the technique is restricted by exciting mode groups which cannot be easily predicted. In this method, the single mode fiber should have properties to result in the excitation of only a limited subgroups of modes within the multimode fiber. This limitation is due to its dependency upon the excitation of small subgroups of modes, that is it relies on the differential group delay measurement using single mode fiber excitation.

The another limitation of the known method is that it gives location of variation at a particular radial position in 0.1 microns in fiber. Identifying corresponding layer with respect to the radial position of fiber measurement is taken into some assumption that will lead to variation in identifying the right deposited layer position. It has been observed that the resolution of measurement in fiber stage by using known technique is about 0.1 micron, which is comparatively very low than the desired resolution.

Further, as stated above, in accordance with the known method, the local variation in refractive index [RI] profile in the core rod cannot be identified before drawing the fiber. The fiber drawn may not meet the desired bandwidth thereby resulting in the wastage of fiber production.

Still further, the known method, in addition to requiring the RI profile measurement using profile analyzer also requires analysis by using differential mode group delay equipment to correct the RI profile configuration of the manufactured fiber. Accordingly, this method adds onto cost of manufacturing by requiring additional equipment and technical manpower. This additional step of manufacturing process also adds onto time of manufacturing.

NEED OF THE INVENTION:

Accordingly, there is a need to develop a method for manufacturing a multimode optical fiber which can improve the RI profile even during the method of manufacturing the fiber, that is, can be applied as on-line process of manufacturing. There is also a need to have a method which does not require excitation of small subgroups of modes, and can still give location of variation at a particular radial position upto about 5 micron in the core rod stage without variations in identifying the right deposited layer position. There is also a need to develop a method to rectify the RI profile without analyses by differential mode group delay equipment thereby enhancing the economical viability of the developed method.

BRIEF DESCRIPTION OF THE INVENTION:

The value of alpha (α), as described hereinabove, effects the value of bandwidth of the multimode fiber. The bandwidth is observed to be higher, if alpha is selected to be varying from about 1.9 to about 2.0. Accordingly, it is observed that achieving the value of alpha varying from about 1.9 to about 2.0 will result in maximization of the bandwidth of the multimode optical fiber. It is this observation, which led to the development of the presently disclosed method and the multimode fiber manufactured thereby.

It has been surprisingly observed that when refractive index (RI) of each deposited layer is measured layerwise in the radial direction, it gives indication of local variation in the RI profile of each deposited layer. It has been further observed that if the RI profile is measured before drawing the fiber, that is immediately after preparation of the optical fiber preform, it substantially results in reduction of wastage of the fiber produced. The conventional method involving differential mode-group delay technique is capable of providing resolution of about 0.1 micron. However, the layerwise measurement of RI profile in radial direction before drawing the fiber has been observed to surprisingly result in resolution of about 0.02 micron.

Accordingly, in one embodiment, the present invention relates to a method for manufacturing a multimode optical fiber having higher bandwidth comprising preparation of optical fiber preform in the manner known in the art, subjecting the preform prepared to measurement of refractive index of core, diameter of core, alpha value of core, refractive index of clad and diameter of clad by employing preform profiler to determine the preform profile, characterized in that:-

a) the refractive index profile of the core is determined in each layer of the deposition from the preform profile of the core; b) the crests and troughs in the refractive index profile are determined along the radial direction of the preform from the refractive index profile of the core obtained in step-a; c) the refractive index profile of each layer is determined from the average height of the crest and trough obtained in step-b); d) the layerwise alpha is determined in the radial direction from the refractive index profile of each deposited layer obtained in step-c); e) the layerwise delay is calculated from the layerwise alpha obtained in step-d); f) the correction factor is determined empirically from the difference between the ideal layerwise mode delay and the actual layerwise mode delay to obtain the desired alpha refractive index profile of each layer; and g) the desired alpha refractive index profile is obtained by varying reaction parameters, such as dopant flow rates or concentration or reaction temperature, etc. based on the correction factor thereby resulting in formation of the preform to form an optical fiber having higher bandwidth.

Accordingly, in another embodiment, the present invention relates to a method for manufacturing a multimode optical fiber having higher bandwidth comprising preparation of optical fiber preform in the manner known in the art, subjecting the preform prepared to measurement of refractive index of core, diameter of core, alpha value of core, refractive index of clad and diameter of clad by employing preform profiler to determine the preform profile, characterized in that:-

a) the refractive index profile of the core is determined in each layer of the deposition from the preform profile of the core; b) the crests and troughs in the refractive index profile are determined along the radial direction of the preform from the refractive index profile of the core obtained in step- a; c) the refractive index profile of each layer is determined from the average height of the crest and trough obtained in step-b); d) the layerwise alpha is determined in the radial direction from the refractive index profile of each deposited layer obtained in step-c); e) the correction factor is determined empirically from the difference between the ideal layerwise alpha and the actual layerwise alpha to obtain the desired layerwise alpha refractive index profile of each layer; and f) the desired layerwise alpha refractive index profile is obtained by varying reaction parameters, such as dopant flow rates or concentration or reaction temperature, etc. based on the correction factor thereby to result in formation of the preform to form an optical fiber having higher bandwidth. OBJECTS OF THE PRESENT INVENTION:

The main object of the present invention to provide a method of manufacturing a multimode optical fiber having higher bandwidth wherein the corrections of manufacturing parameters can be carried out on-line, that is before the fiber is manufactured and the index configuration can be corrected on-line to maximize bandwidth of the fiber.

The another object of the present invention is to provide a method for manufacturing a multimode optical fiber which can be applied as on-line process of manufacturing for rendering the technique more efficient and to improve or control the manufacturing process to enhance the production capacity and the economics of the manufacturing process to make it economically more viable.

Yet another object of the present invention is to provide a method for manufacturing a multimode optical fiber wherein the efficacy of the method is not restricted by exciting the mode groups, that is it is not depend upon the excitation of small subgroups of modes.

Still another object of the present invention is to provide a method for manufacturing a multimode optical fiber which can give location of variation at a particular radial position in 0.02 microns in fiber.

Accordingly, the present invention aims at making a complete disclosure of a method for manufacturing a multimode optical fiber wherein the local variation in refractive index [RI] profile in the core rod can be identified before drawing the fiber and the correction factor can be applied on-line thereby resulting in a fiber having higher bandwidth simultaneously avoiding wastage of fiber production.

This is yet an object of the present invention to provide a method for manufacturing a multimode optical fiber which does not require analysis by employing differential mode group delay equipment to correct the RI profile configuration of the manufactured fiber thereby resulting in savings on the cost of production by avoiding requirement of additional equipment and technical manpower. This advantage of the present method also save on the time of production of the fiber thereby increases the production capacity of the plant.

The other objects, advantages and preferred embodiments of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings, which are not intended to restrict scope of the present invention, but are incorporated for illustration of preferred embodiments of the present invention.

BREIF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:

Figure 1 shows the schematic cross sectional view of the graded index multimode optical fiber.

Figure 2 shows a schematic representation of deposition process inside the deposition tube.

Figure 3 shows the refractive index (RI) profile of a multimode optical fiber respectively representing the ideal profile, conventional profile and profile of the fiber in accordance with the present invention.

Figure 4 shows the layerwise alpha of each deposited layer in accordance with the conventional method and the ideal layerwise alpha of each deposited layer.

Figure 5 shows the layerwise alpha of each deposited layer in accordance with one of the embodiments of the present invention and the ideal layerwise alpha of each deposited layer.

Figure 6 shows the calculated time delay (ns) across the radial direction of core in accordance with the conventional method and the ideal delay time (ps) for ideal alpha profile.

Figure 7 shows the calculated time delay (ns) across the radial direction of core in accordance with one of the embodiments of the present invention and the ideal delay time (ps) for ideal alpha profile.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention discloses a method to identify layerwise refractive index of core along the radial direction thereby determining layerwise alpha [α] and determining layerwise time delay (ns) of light signal. This layerwise differential mode delay is employed to correct the refractive index profile configuration thereby to obtain the desired alpha varying from about 1.9 to about 2.0 for the wavelength range at 850 nm and at 1300 nm which governs center and peripheral area of core respectively which thereby reduces the mode dispersion to improve the bandwidth above 200/500 MHz-Km, preferably 500/800 MHz-km or above at 850 nm and 1300 nm wavelength respectively.

In Figure 1 of the accompanying drawings, the core 3 diameter of the multimode optical fiber is in the range of about 40-70 μm and surrounded clad 1 has a diameter of about 125 μm and the corresponding refractive index of core and clad is represented by nl and n.2. The refractive index of the core nl is of parabolic shape, which is called as graded index profile. The curve 3a of the core 3 represented by a parameter alpha α is required to be rectified to improve the bandwidth performance of the fiber produced.

In accordance with one embodiment of the present invention, the multimode optical fiber preform can be fabricated by any method known in the art, such as by chemical vapor deposition as shown in Figure 2. The chemical reactants are injected from its entry side

7 inside the deposition tube 4 and the tube 4 is heated by heat source 5 in order to react the chemical inside the tube to form soot particle along with rotation of tube 4. The reacted soot 6 is moved away from the hot zone towards the direction of flow and falls inside the tube to get deposited as shown in Figure 2. The reaction occurring inside the tube can be represented as follows by the equation (3):-

SiCl₄ + O₂ > SiO₂ + 2Cl₂ (3)

The above deposition of SiO₂ occurs in successive layers for cladding deposition layer. The cladding layer 2 is provided between tube and core to minimize the OH diffusion inside the core. After completion of desired clad layers 2, the core deposition starts and is deposited successively in the manner that the cross sectional area of each layer decreases until the center of the core layers 3.

In order to increase the refractive index nl of the core 3, the doping chemicals, preferably selected from the group comprising germanium tetrachloride GeCl₄, P0CI3, Freon may be varied in each layers in a manner to obtain the index profile as shown in Figure 1

(3a). The SiCl₄ flow rate and GeCl₄ flow rate may also be varied in each deposition layer to obtain the profile shape 9 as shown in Figure 3.

The dopant halide is varied in the core deposition layers based on the layerwise alpha thereby to result in radially varying index profile. The particular dopant halide phosphorous halide is varied in the core deposition layers accordingly silicon halide flow rates or dopant halide flow rates.

After completion of formation of the desired layer thickness of the core, the supply of halides may be stopped and collapsing process started to collapse the tube to form preform. During collapsing process, Freon gases preferably at 10 to 50 seem (standard cubic centimeter per minute) flow rate are passed inside the tube to etch the surface of core thereby reducing central dip.

The preform prepared in the manner described herein is subjected to the method of the present invention to rectify the RI profile based on the correction factor determined empirically from the difference between the ideal layerwise mode delay and the actual layerwise mode delay by determining the refractive index profile of the core along the radial direction which gives crests and troughs in the refractive index profile to determine the layerwise refractive index profile of each deposited layer, which in-turn gives the layerwise alpha in the radial direction for each deposited layer to calculate the layerwise delay in the radial direction for each deposited layer. The rectified RI profile guides the variations in the reaction parameters.

In one embodiment, the present invention provides to a method for manufacturing a multimode optical fiber having higher bandwidth comprising preparation of optical fiber preform in the manner described hereinabove or as known in the art, subjecting the preform prepared to measurement of refractive index of core, diameter of core, alpha value of core, refractive index of clad and diameter of clad by employing preform profiler to determine the preform profile, characterized in that:-

The layerwise alpha in step-d) is determined in the radial direction from the refractive index profile of each deposited layer by employing equation (1) :

n(r) = ni (1 - 2 Δ (r/a)^α )ⁱ/2 (1)

wherein ni represents the refractive index of the center of the core; Ώ.2 represents refractive index of clad, α (alpha) represents the core shape index and Δ represents relative index difference as shown by equation 2:

The layerwise delay in step-e) is calculated from the layerwise alpha obtained in step-d) by employing mode delay equation 4:

D = l+( α - 2 )/( α + 2).Δ.F+(3α -2)/( α+ 2).(Δ.f )²/2+ ..eqn (4)

wherein D is propagation delay

G = (α/α+2) α is alpha parameter of refractive index core. Δ is refractive index difference between core and clad as shown in equation (2).

The correction factor in step-f) is determined by using the following empirically equation (5):-

Correction factor = 1+ K (Target delay - Actual delay) /Target delay eqn. (5)

wherein K is constant range from 0.5 to 1.

The Target delay is calculated by using the above equation (4) and the Target alpha and Actual delay are calculated by using above equation (4) and Actual alpha in each deposited layer. From the equation (5), the correction factor is obtained for each layer of deposition according to actual delay determined in each layer and reaction parameter is changed by employing the correction factor in the following manner: -

New reaction parameter = correction factor X old reaction parameter in each layer in each layer

In another embodiment, the present invention provides a method for manufacturing a multimode optical fiber having higher bandwidth comprising preparation of optical fiber preform in the manner as described hereinabove or as known in the art, subjecting the preform prepared to measurement of refractive index of core, diameter of core, alpha value of core, refractive index of clad and diameter of clad by employing preform profiler to determine the preform profile, characterized in that:- a. the refractive index profile of the core is determined in each layer of the deposition from the preform profile of the core; b. the crests and troughs in the refractive index profile are determined along the radial direction of the preform from the refractive index profile of the core obtained in step-a; c. the refractive index profile of each layer is determined from the average height of the crest and trough obtained in step-b); d. the layerwise alpha is determined in the radial direction from the refractive index profile of each deposited layer obtained in step-c); e. the correction factor is determined from the difference between the ideal layerwise alpha and the actual layerwise alpha to obtain the desired layerwise alpha refractive index profile of each layer; and f. the desired layerwise alpha refractive index profile is obtained by varying reaction parameters, such as dopant flow rates or concentration or reaction temperature, etc. based on the correction factor thereby to result in formation of the preform to form an optical fiber having higher bandwidth.

n(r) = ni (1 - 2 Δ (r/a)^α )ⁱ/² (1) wherein m represents the refractive index of the center of the core; n2 represents refractive index of clad, α (alpha) represents the core shape index and Δ represents relative index difference as shown by equation 2:

The correction factor in step-e) is determined by using the following empirically equation (6):-

Correction factor = 1+ K (Target alpha - Actual alpha) /Target alpha eqn. (6)

wherein K is constant range from 0.5 to 1.

The Actual alpha is calculated by using above equation (1) and the Target alpha is in the ranges from about 1.9 to 2. From equation (6), the correction factor is obtained for each layer of deposition according to actual alpha determined in each layer and reaction parameters are changed by employing the correction factor in the following manner: -

The refractive index profile shape 9 according to conventional method is shown in Figure 3. The RI profile obtained from the profile analyzer gives an overall alpha value based on the best- fitted curve for the actual deposited RI profile. Even though the overall alpha value achieved is nearly equal to ideal value, but the variation during manufacturing conditions lead to locally varying the RI profile along the radial direction. Due to these local variations desired bandwidth cannot be achieved. Therefore, it becomes a challenge to identify the exact locations of these variations in the radial direction and then to reduce the local variation in the radial direction in RI thereby improving the bandwidth of multimode optical fiber, which is successfully achieved by the method of the present invention.

The preform profiler conventionally provides the over all alpha α value of the core for any positions of the core rod along the length of core rod by considering the entire shape of the core profile based on the best- fitted curve ranges from 15 percent to 95 percent of the actual refractive index profile of each side as shown by dashed curve in Figure 3 of prior art.

The fabricated core rod is loaded on the chuck of the preform profiler to measure the RI Profile. The preform profiler basically consists of tank with index matching liquid in to which the core rod is immerse, an optical unit that has a laser with typically 632.8 nm wavelength and detector. As the measurement starts, the laser beam scans the entire radial direction of the core rod. Due to change in RI in core rod, the light is deflected and deflected angle is measured by the detector. The deflection units are expressed in radians. The deflection data is reconstructed in to refractive index profile by using a reconstruction algorithm with 5-micron step size. The refractive index can be measured in the order of 0.0002 by using profile analyzer for example PK2600 PK Technology Instrument.

The profile analyzer measures the refractive index of core at an interval of 5 microns in core rod stage, which is equivalent to 0.02 micron in fiber stage. For example, in order to make 50 micron multimode fiber, the core diameter in core rod should be approximately 13mm which 13000 micron diameter and hence it is equivalent to measure in fiber stage in the order of 0.02 micron resolution.

From the preform profile data, present inventive technique is to identify the refractive index profile of core in each layerwise of deposition. This refractive index profile data is used to identify crests and troughs of RI along the radial direction. The refractive index profile of core deposited in MCVD process is not smooth radially and has ripples. Each such ripple corresponds to a deposited layer and can be characterized by a crest and trough. Hence one crest and one trough (peak and valley) denote one deposited layer, the average height of one crest and one trough gives the RI of a particular layer.

Similarly RI of all layers is calculated using present method, which can also give information of layer thickness when the difference between successive crests or troughs is arrived radially. Using the above layer wise RI data layer wise alpha is calculated using the above equation (1). The layerwise alpha measured from the layerwise refractive index is depicted in figure 4 in which dotted points line 12 represents each layer of deposition and ideal alpha along the radial axis is depicted in line 11 of figure 4. From the layerwise alpha data, layerwise delay is calculated using the mode delay equation cited in the prior art "Introduction to optical fiber communication systems" by William B. Jones which is modified to give layerwise delay as shown in Figure 5 in accordance with the present invention: -

D = l + ( α - 2 )/( α + 2).Δ.F + (3α -2)/( α+ 2).(Δ.f )²/2 + .. eqn (4) wherein D is propagation delay

G = (α/α+2) α is alpha parameter of refractive index core.

Δ is refractive index difference between core and clad as shown in equation (2).

The time delay plot 16 shows the calculated time delay across the radial direction of the fiber made according to the conventional method. In this time delay plot, dotted point represents the each layer of deposition. Similarly an ideal delay plot 17 for each layer is calculated using a target ideal alpha of each layer for example value of 1.96. The deviation of the actual delay plot 15 from the ideal delay plot 21 gives an indication of local variations that have occurred during deposition.

In one embodiment of the present invention, a correction factor is obtained empirically based on the difference of ideal mode delay from the actual mode delay, which is used to obtain the optimum alpha refractive index profile for each layer by varying chemicals in each layer for example dopant flow rates or concentration or reaction temperature there by achieving actual delay plot nearly equal to the ideal delay plot in order to maximize the bandwidth of the optical fiber.

In an another embodiment of the present invention, correction factor can also be obtained empirically based on the difference of the ideal layerwise alpha and actual layerwise alpha, which is used to obtain the optimum alpha layerwise refractive index profile by altering the chemicals which is deposited inside the tube for example flow rates or concentration or reaction temperature therby achieving the optimized alpha in each layer in order to maximize the bandwidth of the multimode optical fiber.

The layerwise refractive index profile of core is used to obtain the layerwise alpha value as well as the layerwise time delay plot and refractive index profile configuration is altered to minimize the mode dispersion thereby maximizing the bandwidth of multimode optical fiber. Subsequent preform are made after correcting the chemical flow rates and again layerwise RI profile and time delay are measured to correct the refractive index configuration. The core rod prepared according to present invention is loaded on to the preform profiler to measure the refractive index profile. The refractive index profile 10 as shown in figure 3 is according present invention. From profile profiler, alpha measurement by using best- fitted curve for the both the RI profile 9 and 10 are 1.95 and 1.96 respectively that may seems to be equal, but the layerwise alpha 12 is deviated from the target alpha 11 in the central region compare to peripheral region of core for RI profile 9 which will lead to signal delay at the central region of the core. According to present invention, the deviation of layerwise alpha 14 from the target alpha 13 is reduced by using inventive technique of identifying the layerwise RI profile, determining layerwise alpha and determining layerwise delay plot.

The layerwise alpha measured from the layerwise refractive index is depicted in figure 5 in which dotted points line 14 represents each layer of deposition and ideal alpha along the radial axis is depicted in line 13 of Figure 5. In Figure 5, layerwise alpha dotted line 14 according to present invention is improved to nearly equal to optimum alpha 13. From the layerwise alpha data, layerwise delay is calculated using the mode delay equation as shown above equation (4). The layerwise delay plot obtained according to present invention is noted in dotted lines 18 in figure 7 and another line 17 shows the ideal delay time. The layerwise delay and as well as layerwise alpha can be achieved nearly equal to the targeted alpha and delay of layerwise by continuously incorporating the present invention technique. By this approach, refractive index profile configuration is optimized to achieve maximum bandwidth of multimode fiber.

According to the present invention, the multimode optical fiber preform made is further drawn in to optical fiber of desired dimension. The multimode optical fiber manufactured in accordance with the present invention has been found to have substantially higher bandwidth, for example the bandwidth is increased from about 120 and 225 MHz-Km to about 490 and 350 MHz-Km, preferably to about 1050 and about 1200 MHz-Km at 850 and 1300 nm wavelengths respectively.

The multimode optical fiber manufactured in accordance with the present invention has been found to have maximum refractive index along the axis of said fiber and a lower refractive index at its periphery.

The present invention will now be more clear from the following examples which are not indented to limit scope of the present invention.

The present invention has been described for manufacture of multimode optical fiber, but can also be applied for manufacturing a single mode optical fiber to improve the profile characteristics, which also included in the scope of the present invention. Accordingly, the present invention is generally concerned with the processes for fabricating optical fibers having maximum refractive index in the core.

Example 1:

Preform was deposited as per conventional method to achieve a target alpha value between 1.9 to 2 as described in prior art, and corresponding RI profile was measured by using profile analyzer. This core had an alpha value of 1.96 which was very close to the target, but the fiber drawn from this core rod gave very low bandwidth of 123 and 225 Mhz-Km at both 850 and 1300 nm wavelengths respectively.

Example 2:

The above profile was analyzed as per our present invention method to identify the reasons for low bandwidth with target alpha. The layerwise delay plot 16 dotted lines of the above rod demonstrated huge variations from the ideal delay plot 15 as shown in fig 6. Based on this deviation from the ideal delay a correction factor for each layer was calculated using an empirical equation. This correction factor was used to change the dopant flow of each layer arid a second rod was deposited. The RI profile of this rod was measured and it had an alpha value of 1.955. This profile was analyzed layerwise as per present inventive method prior to draw to check if the local variations. The time delay plot 18 dotted lines along the radial direction had reduced as shown in fig 7. Comparing the time delay plot 18 of second rod with that of ^' example 1 time delay plot 16, it is evident that the difference between ideal delay plot and actual delay plot has reduced but still there is some variation. When fiber of the second -preform was drawn it gave bandwidth of 491 and 352 MHz-Km at 850 and 1300 nm respectively.

Example 3:

Another core rod was made, by further correcting the layerwise dopant flows based on the correction factor obtained from the second core rod. This rod gave an alpha value of 1.96 and layerwise delay plot was obtained as per present inventive step. This plot was very close to the ideal plot except for the periphery and center due to central dip. This rod when drawn gave bandwidth of 750 and 880 MHz-Km at 850 and 1300nm respectively.

Example 4:

The third core rod was made by the method as explained in above example 3, thereafter, the third core rod was analyzed for its refractive index profile by using profile analyzer in accordance with the method of the present invention. According to the method of the present invention, the layerwise refractive index of each deposited layer was determined to arrive at the correction factor for each layer based on the layerwise alpha and target alpha, by using the following equation: -

Correction factor = l+K(Target alpha - Actual alpha) /Target alpha

The dopant halide flow rate of each layer is modified by using the following equation based on the correction factor: -

New dopant flow rates = correction factor X old dopant flow rates in each layer in each layer

The fourth core rod was prepared based on the new dopant halide flow rates calculated above and the preform was drawn into optical fiber, which was found to have bandwidth of 950 and 1100 MHz- Km at 850 and 1300 nm respectively.

Example 5:

The fifth core rod was made by a method as explained in above example 3, thereafter, the fifth core rod was analyzed for its refractive index profile by using profile analyzer in accordance with the method of the present invention. According to the method of the present invention, the layerwise refractive index of each deposited layer was determined to arrive at the correction factor for each layer based on the layerwise time delay and target time delay, by using the following equation: -

Correction factor = l+K(Target delay - Actual delay) /Target delay

The dopant halide concentration of each layer was modified by using the following equation based on the correction factor: -

New dopant halide flow rates = correction factor X old dopant halide flow in each layer rates in each layer

The fifth core rod was prepared based on the new dopant halide flow rates as calculated above and the preform was drawn into optical fiber, which was found to have bandwidth of 1050 and 1200 MHz-Km at 850 and 1300 nm respectively.

The present invention has been described with the help of the accompanying drawings and the foregoing figures. It is obvious for the persons skilled in the art to modify the present method without deviating from the scope of the present invention, and such modifications are included in the scope of this invention.

Claims

1. A method for manufacturing a multimode optical fiber having higher bandwidth comprising preparation of optical fiber preform, subjecting the preform prepared to measurement of refractive index of core, diameter of core, alpha value of core, refractive index of clad and diameter of clad by employing preform profiler to determine the preform profile, characterized in that:-

a. the refractive index profile of the core is determined in each layer of the deposition from the preform profile of the core; b. the crests and troughs in the refractive index profile are determined along the radial direction of the preform from the refractive index profile of the core obtained in step- a; c. the refractive index profile of each layer is determined from the average height of the crest and trough obtained in step-b); d. the layerwise alpha is determined in the radial direction from the refractive index profile of each deposited layer obtained in step-c); e. the layerwise delay is calculated from the layerwise alpha obtained in step-d); f. the correction factor is determined empirically from the difference between the ideal layerwise mode delay and the actual layerwise mode delay to obtain the desired alpha refractive index profile of each layer; and g. the desired alpha refractive index profile is obtained by varying reaction parameters, such as dopant flow rates or concentration or reaction temperature, etc. based on the correction factor thereby resulting in formation of the preform to form an optical fiber having higher bandwidth.

2. A method as claimed in claim 1, wherein the layerwise alpha in said step-d) is determined in the radial direction from the refractive index profile of each deposited layer by employing equation (1) :

n(r) = ni (1 - 2 Δ (r/a)^α )i/2 (1)

wherein m represents the refractive index of the center of the core; Ώ.2 represents refractive index of clad, α (alpha) represents the core shape index and Δ represents relative index difference as shown by equation 2:

3. A method as claimed in claim 1 or 2, wherein the layerwise delay in said step-e) is calculated from the layerwise alpha obtained in said step-d) by employing mode delay equation 4:

D = l+( α - 2 )/( α + 2).Δ.F+(3α -2)/( α+ 2).(Δ.f )²/2+ ..eqn (4) wherein D is propagation delay F = (m/M)^β = (r/a)²ε

G = (α/α+2) α is alpha parameter of refractive index core.

Δ is refractive index difference between core and clad as shown in equation (2) .

4. A method as claimed in any one of the preceding claims 1 to 3, wherein said correction factor in said step-f) is determined by employing following empirical equation (5):-

Correction factor = l+K(Target delay- Actual delay) /Target delay eqn.

(5) wherein K is constant in the range from 0.5 to 1.

5. A method as claimed in claim 4, wherein said target delay is calculated by said equation (4), and target alpha and actual delay are calculated by said equation (4) and actual alpha in each deposited layer.

6. A method as claimed in any one of preceding claims, wherein . said reaction parameters are changed by employing the correction factor in the following manner: -

7. A method for manufacturing a multimode optical fiber having higher bandwidth comprising preparation of optical fiber preform, subjecting the preform prepared to measurement of refractive index of core, diameter of core, alpha value of core, refractive index of clad and diameter of clad by employing preform profiler to determine the preform profile, characterized in that:-

a. the refractive index profile of the core is determined in each layer of the deposition from the preform profile of the core; b. the crests and troughs in the refractive index profile are determined along the radial direction of the preform from the refractive index profile of the core obtained in step-a; c. the refractive index profile of each layer is determined from the average height of the crest and trough obtained in step-b); d. the layerwise alpha is determined in the radial direction from the refractive index profile of each deposited layer obtained in step-c); e. the correction factor is determined empirically from the difference between the ideal layerwise alpha and the actual layerwise alpha to obtain the desired layerwise alpha refractive index profile of each layer; and f. the desired layerwise alpha refractive index profile is obtained by varying reaction parameters, such as dopant flow rates or concentration or reaction temperature, etc. based on the correction factor thereby to result in formation of the preform to form an optical fiber having higher bandwidth.

8. A method as claimed in claim 4, wherein the layerwise alpha in said step-d) is determined in the radial direction from the refractive index profile of each deposited layer by employing equation (1) :

n(r) = ni (1 - 2 Δ (r/a)^α )ⁱ/a (1)

9. A method as claimed in claim 7 or 8, wherein said correction factor in said step-e) is determined by employing following empirical equation (6):-

Correction factor = l+K(Target alpha - Actual alpha) /Target alpha eqn.

(6) wherein K is constant in the range from 0.5 to 1.

10. A method as claimed in claim 9, wherein said actual alpha is calculated by said equation (1), and target alpha is in the ranges from about 1.9 to 2.

11. A method as claimed in any one of preceding claims 7 to 10, wherein said reaction parameters are changed by employing the correction factor in the following manner: -

12. A method as claimed in any one of the preceding claims, wherein the dopant is halide gas, preferably germanium halide and phosphorous halide gas.

13. A method as claimed in any one of the preceding claims, wherein the dopant halide is varied in said core deposition layers based on the correction factor thereby to result in radially varying index profile.

14. A method for manufacturing a multimode optical fiber substantially as herein described with the help of foregoing examples and as illustrated with the help of accompanying drawings.

15. A multimode optical fiber having higher bandwidth, which is increased from about 120 and 225 MHz-Km to about 490 and 350 MHz-Km at 850 and 1300 nm wavelengths respectively.

16. A multimode optical fiber having higher bandwidth, which is increased from about 120 and 225 MHz-Km to about 750 and 880 MHz-Km at 850 and 1300 nm wavelengths respectively.

17. A multimode optical fiber having maximum refractive index along the axis of said fiber and a lower refractive index at its periphery.