Title of Invention	METHOD OF BUILDING TRULY FUSED 1x4 COUPLERS WITH ULTRA BROADBAND SPECTRAL PERFORMANCE, LOW POLARIZATION SENSITIVITY AND HIGH IMPACT RESISTANCE AND COUPLERS BUILD THEREFORE
Abstract	A method for making truly fused lx4 coupler with ultra broadband spectral performance, and low polarization sensitivity, where only the primary fiber is pre- processed and four fibers are braided together to get a symmetric coupling from primary fiber to each secondary fiber and there is no mutual coupling between secondary fibers, is disclosed. Method of making the product impact resistant and thermally stable, for outdoor applications, is also claimed. lx4 Wavelength Independent Couplers are of particular interest.

Title of Invention

METHOD OF BUILDING TRULY FUSED 1x4 COUPLERS WITH ULTRA BROADBAND SPECTRAL PERFORMANCE, LOW POLARIZATION SENSITIVITY AND HIGH IMPACT RESISTANCE AND COUPLERS BUILD THEREFORE

Abstract

A method for making truly fused lx4 coupler with ultra broadband spectral performance, and low polarization sensitivity, where only the primary fiber is pre- processed and four fibers are braided together to get a symmetric coupling from primary fiber to each secondary fiber and there is no mutual coupling between secondary fibers, is disclosed. Method of making the product impact resistant and thermally stable, for outdoor applications, is also claimed. lx4 Wavelength Independent Couplers are of particular interest.

Full Text	DESCRIPTION OF INVENTION A) Field Of Technology This invention in general relates to the manufacturing of fused fiber optic couplers where more than two fibers are fused together, using the fused bi-conical taper technology. More particularly this invention relates to the manufacturing of ultra broadband 1x4 truly fused couplers with reduced polarization dependence and considerable impact resistance. This product is ideal for Fiber-To-The-Home Passive Optical Network applications, where data and analog video signals arc transmitted over the same fiber distribution network. B) Prior Art Disclosure Fused couplers are, key elements in multi access fiber optic networks, used to distribute optical signals over different routes. In Passive Optical Distribution Networks for Fiber-To-The-Home applications, multiple wavelengths are assigned to carry different signals such as data, video and monitoring signals, which demands ultra broadband spectral performance of such fused couplers. Cjcncrally, the couplers made with fused bi-conical taper technique are wavelength dependent. But, it is a well-known technique that by controlling the propagation constant difference and the degree of fusion between the interacting fibers, it is possible to reduce the wavelength dependence. Though this technique has been well established for making wavelength insensitive 1x2 couplers, the same has not been effectively realized in case of IxN couplers, where N is greater than two. Usually IxN couplers are realized by concatenating multiple 1x2 couplers, but truly fused IxN couplers is a better alternative for concatenated couplers in FTTTI applications. Wavelength insensitive truly fused 1x4 couplers are of particular interest in three wavelength (1310, 1490 & 1550) FTlTI passive optical networks where analog, video and data communication services are converged. 1x4 couplers should also support the transmission at 1625nm, for network monitoring applications thus necessitating the bandwidth coverage of such components from 1250 to 1650 nm. Several methods have been proposed for the realization of 1x4 truly fused devices. In one such solution, N-1 identical fibers are symmetrically positioned around a central fiber for fabricating a wavelength flattened IxN coupler. Mortimore suggested (R^* US Patent US 5, 175^799) using the capillary tube to keep the fibers parallel to the central core in a symmetrical fashion around the central core to get identical coupling to each of the surrounding fibers, during the process of fusion and elongation. But the patent does not describe how 3 fibers can be symmetrically placed using a capillary tube, which may require dummy fibers, without light guiding cores. Moreover, the model suggested by Mortimore shows a complete power transfer from the central core to the surrounding fiber, for 1x4 couplers and thus extra care must be taken to control the fabrication parameters to get the wavelength-flattened performance. Another way for realizing IxN couplers is by fusing fibers with dissimilar propagation constants to make it less wavelength sensitive. But here the coupling between the wave-guides is in such a fashion that complex coupling signatures exist between the interacting fibers {Kef: US Patent US 5,355,426). This leads to lot of complexities in the fabrication of wavelength independent IxN couplers. Here it also becomes necessary to control the characteristics of individual fibers to get the required spectral flatness, which makes the manufacturing process highly complicated. Moreover, this adds to the polarization sensitivity of the product. US Patent US 5,883,992 describes a method of making IxN couplers from four identical fibers, in which the secondary fibers are kept at predefined positions -apexes of an equilateral triangle — around a central fiber using special fiber holding jigs. Though the patent claims that, after twisting the structure has the same cross- section, as before, there can be variations in the relative positions. Moreover the process requires asymmetric longitudinal profile to make the coupler wavelength independent, which requires complex process control measures. This patent does not disclose the power-coupling pattern with elongation. The primary packaged coupler has a length of 62mm and the splitting ratio is not even at 1620nm range. Further this patent recommends tuning after fusion to get the required wavelength flatness. C) Brief Description of the Itiventioti In the present invention, a method of building 1x4 couplers using the same platform of 1x2 fused bi-conical taper coupler station is described. No special jigs are required to preposition the fibers. The fibers are kept parallel to one another, in a plane, in a suitable mechanical groove and are braided together manually to form a bundle, in such a way that there exists no interaction among the secondary fibers, but at the same time each of the secondary fiber shares equal area of interaction with the central fiber. This helps in getting a coupling of equal strength to each of the secondar)^ fibers over a broad range of wavelengths, which ensures the ultra-broadband spectral performance. Here effect of mutual coupling between the secondary fibers is eliminated through the special braiding pattern, which also controls amount of light coupled to the secondary fibers. All the secondary fibers are having the identical propagation characteristics and only the central fiber through which the power is launched for monitoring during fusion process is preprocessed to have a different propagation constant to realize the 1x4 coupler, according to the present invention. The symmetry of the structure and the low degree of fusion helps in controlling the polarization performance to a great extent, l^he couplcrvS so formed have a length 49mm, after packaging, and a typical uniformity of IdB over the wavelength range of 1250 to 1650nm. Thus the complexity of the process is reduced considerably. In general, fused couplers are ver^^ weak towards impact and thermal shocks and hence are not suitable for outdoor applications, such as passive optical distribution networks. Fused couplers are protected by fixing it to the quartz substrate using epoxies and subsequendy covering it in a metal tube. This existing packaging method is not sufficient to provide good impact resistance to the fused couplers. Hence in another aspect of the present invention, method of packaging the couplers to suitable quartz substrate to get good impact resistance is disclosed. In this packaging technique, the adhesive application points are defined in such a way to secure the fibers twists firmly, to prevent any relative motion between the fibers, l^urther multiple epoxy application points are suggested in the packaging method. To further enhance the impact performance, coating of fibers between the epoxy application points is described. These techniques improve the impact resistance as well as the thermal stability of truly fused couplers. D) Objects of Invention • It is the primary object of the invention to design a novel manufacturing process for ultra broadband truly fused 1x4 couplers with reduced polarization dependence, which is unique. • It is another object of the invention to invent and design a novel coupling profile and braiding pattern for the 1x4 fused couplers with symmetric coupling profile to all secondary fibers, in which there is no mutual coupling between the secondary fibers exists. • It is another object of the invention to invent and design a novel impact resistant 1x4 coupler for outdoor optical distribution networks where in a new method of packaging technique to hold the fiber twists properly using adhesive application is suggested, which in general applicable for all t)^pes of fused couplers. P^urther objects of the invention will be clear from the following description in detail. The description will extensively deal with the nature of the invention and manner in which it is to be performed referring to the accompanying drawings. E) Detailed Description of Invention Brief Description of Diagrams The statement of the drawings, which accompany this complete specification, is as follows - Figure 1 of the drawings shows the complex coupling profile during the fusion and elongation process for 1x4 couplers described in the previous art. Figure 2 of the drawings shows the novel coupling profile of 1X4 couplers according to the new invention, where the coupling to all secondarj- fibers is identical. Figure 3 of the drawings shows the coupling profile, as per the new invention, where all the fibers have identical propagation constants Figure 4 of the drawings shows the cross-section of the fused Nx4 couplers, where the fibers are placed at the corners of a parallelogram. Figure 5 of the drawings shows the waist cross-section of 1x4-fused coupler according to the present invention. The center fiber 1 has a different propagation constant compared to the three surrounding fibers. Figure 6 of the drawings shows the cross-section of the 1x4 coupler as per the new invention, at five different locations (a) cross-section at waist (b) cross-section at 2mm right to the waist (c) cross-section at 2mm left to the waist (d) cross-section at 6mm right to the waist, (e) cross-section at 6mm left to the waist. Figure 7 of the drawings shows the position of the flame with respect to the four braided fibers. ¥igure 8 of the drawings shows the spectral response of 1x4 couplers built in the new process. Figure 9 of the drawings shows the dimensional details of the quartz substrate used for packing the fused region. Figure 10 of the drawings shows the schematic of epoxy application points prescribed for improving the impact resistance of fused couplers, (a) Epoxy application point redefined to cover the twist points (b) Epoxy bead suggested to improve the impact resistance as well as thermal stabilit}^ (c) Coating between the epoxy bead and initial epoxy using a soft epoxy resin to further enhance the impact resistance Truly fused, ultra broadband 1x4 couplers are of particular interest in FTrfl networks. As described in the previous section, there are several methods being proposed for manufacturing 1x4 couplers, but not ideal manufacturing methods due to complex coupling signatures involved or due to the requirement of special jigs or asj^mmetric taper profile. This invention describes a simple and straightforu^ard solution for the manufacturing of truly fused 1x4 couplers: In a preferred embodiment according to this invention, four bare fibers (approximately 2 meters in length) are stripped off the coating for a length of around 25mm and are cleaned using acetone or suitable cleaning solutions. One of the fibers is preprocessed to have a slightly different propagation constant over the coupling region. The desired dissimilarity in the primary fiber can be achieved through methods such as etching, heating, or using suitable dopants. Alternatively, fibers with different propagation constants may be used. The level of dissimilarity is so chosen that the power being distributed evenly among the four fibers, over the required wavelength range. The primary fiber is having a diameter difference of around \\xxxi from the secondary fibers. The four fibers arc placed parallel to one another in a groove, such that all the four fibers lie in the same horizontal plane, with the primary fiber at the second position from one end. The four fibers are braided together in a particular pattern to form a bundle, where all the secondary fibers are in close proximity to the primary fiber and shares equal area with the primary fiber along the coupling region. Also, there shall not be any interaction between the secondary fibers to avoid the complex coupling coefficients and to control the maximum power being coupled to the secondary fibers. With such patterns, it is obser\^ed that the coupling from the central fiber to the surrounding fibers occurs in an identical manner, and no complex coupling coefficients are involved. The coupling behavior in 1x4 couplers according to the previous art is depicted in Vigure /. This shows that the coupling to the secondary^ fibers is not identical and is difficult to get true wavelength independent perfoHTiance. ¥igure 2 depicts the coupling profile according to the present invention, where the coupling to the secondary fibers is identical and thus is very easy to control the process. Figure 3 shows the coupling profile of the braided pattern, when all the four fibers have same propagation characteristics. As shown in l^igure i, the coupled power maximum is around 60%, when the propagation constants are same, and hence is not suitable for 1x4 splitter. The primary fiber is pre-tapered to a level where the maximum of the oscillator)^ response coincide to get the 25% coupling ratio as well as the ultra broadband spectral response. Vigure 4 shows the cross-section of truly fused 1x4 couplers, reported previously {Kef, US Patent: US 5,355y42&). Here the four fibers are numbered 1, 2, 3 and 4 respectively and 1 being the primary fiber. Only the boundary of the fiber cores is indicated in Figure 4, Figure 5 shows the cross-section at the waist of the coupling region as per the new invention, where all the fibers are continuous along the coupling region. The cross-section of the braided pattern, after fusion, is shown at five different locations in Figure 6, From the figure, it is clear that the secondar}' fibers are disposed around the central fiber, but not symmetrical with respect to each of the secondary fibers. It is observed that the cross-section pattern gets slightly rotated around the central fiber axis. The cross-section is taken by cleaving the fiised region and is inspected with a microscope of suitable magnification. 7\s shown in Figure 6 of cross-section, there exists an acute angle between the two secondary fibers 1& 2. Both these fibers make an obtuse angle with the other secondar}^ fiber 3. At the end of the coupling region, the four fibers will again be in the same parallel plane. To get identical coupling profile as in Figure 2, it is not necessary that each of the secondary fibers shall be symmetrically disposed over the primary fiber, but it is necessary that secondary fibers be separated by at least a distance to avoid the interaction between them. The braided pattern helps to maintain the secondaiy fibers in close proximity to the primary fiber, along the coupling region, with a waist cross-section as shown in the Figure 5. Though the braiding can be done in many ways, we suggest two methods. In the first t}^pe, three such fibers are singly twisted three times, among which one fiber is the primary fiber. After that the fourth fiber is twisted over the three-fiber bundle with full turn, so that the relative positions of the four fibers at both ends are 1, 2, 3, 4 and 1, 2, 3, 4. I'his structure is sufficient to get a coupling profile as shown in Figure Z However, there may be a chance that a small deviation in the relative tension can contribute to the interaction among a pair of fibers, thus slightly affecting the uniformity and repeatability of the process. In another embodiment of the braiding pattern, three such fibers are singly twisted three times, among which one fiber is the priman' fiber. Then the fourth fiber is fuU twisted, followed by a half turn over this bundle. The waist cross-section of such a device is shown in the Figure 5. In this case the relative positions of the fibers are 1, 2, 3, 4 and 4, 1, 2, 3 at the ends. (Coupling in structures with the latter twist pattern are found to be more efficient than the former twist and offers good repeatability and better uniformity. The four-fiber bundle is properly secured to keep the structure during the fusion and elongation. For the above described fiber braiding patterns no special jigs is required and can be done manually. Thus the same platform used for the manufacturing of basic FBT coupler can be used for the fabrication of 1x4 couplers. The groove, which is used to hold the fiber, shall have sufficient width to hold the four fibers. The fused coupler station has axially moving stages that move over precision slides, driven by stepper motors, During the fusion process the coupling behavior is online monitored by injecting power to the central fiber, at the input, and monitoring the power at each output simultaneously. I'he process is monitored for the coupled power at a single or multiple wavelengths simultaneously and is stopped at that point where we get an equal coupling ratio over the required wavelength range. The heat source can use either hydrogen or deuterium gas or any suitable gas. The flame has a width of 6mm and is placed approximately 1cm above the fiber. The flame can be moved in three axes, XYZ and is controlled in a known manner. The flame for fusion process is applied in the downward direction to provide a constant temperature :^one, during the fabrication. The position of the flame with respect to the braided fibers is shown in Figure 7. The coupling profile as per the new invention is very simple; the coupling to the secondary fibers is symmetric with respect to each other. The graph in Figure 3 shows the coupling profile when aU the four fibers have similar propagation constant. Here the minimum power remaining in the primary fiber is 40% and the m^aximum power at the secondary fibers is 20%. This condition is not ideal for making a 1x4 splitter. Figure 2 shows the coupling signature, when the central fiber has a different propagation constant. Here the maximum of the oscillator)^ response curve coincide at a coupled power of 25% and provide ultra broadband 1x4 splitter characteristics. At the waist of the fused region, the distance between the fiber cores of each of the secondary fiber and the primary fiber typically ranges from 35±3 \|um, where the primary fiber is having a slighdy reduced diameter (of the order of 1 \|am) compared to the secondary fibers. There exits physical separation between the secondary fibers and the minimum separation is between the secondary fibers 1 & 2 as shown in the Figure 5, which is typically in the range of 40+7 \|im. At the waist, the lateral area of contact is t^^pically around 10\|J,m. It is YCVj easy to achieve wavelength independent performance for 1x4 couplers over 400nm range, from 1250 to 1650nm, with this new method. The spectral flatness of the products made using the new method, over the range of 1250-1650 is shown in the Figure 8. More over the structure at coupling region makes the process less sensitive to changes in the polarization states, because of the low degree of fusion and the symmetry being preserved, where the length of the fusion region is typically around 14 - 16.5 mm. It is also possible to preprocess one or more secondary fibers to achieve wavelength flatness. The polarizatic^n performance of 1x4 couplers manufactured with the new invention offers polarization dependent loss as small as 0.15 dB. The power coupled to the any one of the secondary fiber can be represented by ?,= P3= P4= 0.25 sin" (2C2) where C is the coupling coefficient and 2 is the distance along the fiber axis And the power remaining in the throughput fiber (P^) is P,= 1-(P2+P3+P4) For the present investigation the fiber used is Corning SMF 28e. However this is achievable with any standard single mode fibers and even with specialty fibers. Further the use of water peak suppressed fibers is recommended to ensure the wide operating spectral range and better uniformity. Here the achieved spectral flatness is around 1 clB, over the band of 400nm, which shows a clear performance improvement over the products manufactured in other process. The fused region is protected inside quartz substrate, whose thermal expansion is matched to that of fiber, with dimensions as shown in the Figure 9, The design of the substrate is in such a way that the fiber is fully seated inside the substrate even at the twisted regions. The width of quartz substrate groove is designed in such a way that it is sufficient to accommodate four parallel fibers. The depth is so designed that the braided structure is fully with in the groove at all positions of the bundle. The structure is fixed to the substrate using suitable epoxies, either UV curable or thermal epoxies, in such a way that the fused region is safeguarded from external perturbations. The surface of the quartz groove at both the ends of the substrate is roughened to make the epoxy adhesive bonding stronger. The material used for the quartz substrate is GE214. The t}^pical range of groove roughness is 80\|a". Also the epoxies can be mixed with quartz powders to minimize its thermal expansion. The unutilized fibers at the input side of the coupler are terminated suitably as in the case of 1x2 couplers with an index matching epoxy. I'he summary of performance parameters according to the new invention is Maximum Insertion Loss: 7.2 dB Uniformity over a range of 1250-1650nm: IdB Excess Loss: 0.2dB Polarization Dependent Loss: 0.15dB Back Reflection: 60 dB Mainly these couplers are intended for passive optical networks and are likely to be used at outside room conditions. Thus it is necessary to make such couplers impact resistant and less temperature sensitive. In the weU-known procedure of fixing the couplers to quartz substrate, the epoxies are placed up to those points before the twisting starts, which is enough for securing the fused region to the substrate. But these couplers are found to be prone to breakages when dropped from 2 feets on to a concrete surface. In the new packaging technique, the region up to the starting point of the coupling region is fixed with suitable adhesives as indicated in the Vigure /(), so as to cover the fiber crossings partially or fully. The typical distance between the epoxies at the two ends is 26-30mm. In another embodiment of the packaging style, it is suggested to use two different kinds of epoxies. One epoxy is used to form beads at the starting and ending points of the tapered structure. I'he epoxy beads are usually separated by a distance (16-20mm). This technique is found to be improving the reliability of the product to a great extent against repeated thermal stressing. The epoxy that is used to form the bead has low coefficient of thermal expansion. In another embodiment, the region between the epoxy bead and the initial epoxy is coated with a soft epoxy resin as shown in Figure 10, The above described methods, restricts the freedom of twists, thus reduces the thermal strains to the fused region, improving the thermal stabilit}', as well as the impact resistance by a considerable amount. The impact resistance of the product is verified by dropping it from a suitable height and thus subsequently subjecting the product for continuous temperature cycling from -40 to +85, for up to 1000 cycles. These products are found to withstand an impact when directly dropped from a height of 6 feets. Thus the suitability of the product towards outdoor application is ensured. Patent Keferences Cited: 1. US 5,175,779 Method of forming an Optical Fiber Coupler and a coupler so formed — British Telecommunications 2. US 5,355,426 Broadband MxN Optical Fiber Couplers and Method of Making - Gould Electronics 3. US 5,883,992 Method for Making Optical Wave guide Couplers with low wavelength sensitivit)^ and couplers thereby produced — Framatome Connectors We claim: - 1. A Method of making truly fused 1x4 couplers, starting from four identical fibers, among which only the primary fiber is preprocessed to change the propagation constant, and braided and fused to get a coupling profile as described in the specification and shown in Figure 2. 2. 1x4 couplers, where the secondary fiber locations are around the central fiber and the angle between two such fibers is always acute, but still maintains a gap of 2-15\|im between those fibers to avoid mutual coupling among them. 3. A method of braiding four fibers together to get a braided stmcture with waist cross-section as described in claim 2, where the cross-sectional pattern rotates with respect to the primary fiber axis. 4. A method of achieving a twisted structure as mentioned in Claim no. 3 where multiple stages are involved. a. Twisting three fibers together followed by a full turn of fourth fiber with the three fiber bundle b. Twisting three fibers together and finally full twisting fourth fiber more than a full turn over the three-fiber bundle. 5. The process by which the fusion process is stopped at 25% splitting ratio, over the desired wavelength range, after monitoring the four fibers, for identical coupling, simultaneously at single or multiple wavelengths during the fusion process. 6. The process of protecting the fused region inside a quartz groove with dimensions as mentioned in figure 9 and bottom surface with a roughness of 80u". 7. A novel impact resistant 1x4 coupler for outdoor optical distribution networks wherein a new method of packaging to hold the fiber crosses using epoxy application as described in the specification and as shown in figure 10 (a) is suggested. 8. A new method to bond coupler to quartz substrate where an epoxy bead is used just before the tapering section as in Figure 10 (b) to improve the thermal stability and impact resistance of the product. The initial epoxy and the epoxy bead can vary in type; preferably the epoxy bead is having low thermal expansion. 9. A new method of coating the fiber bundle between the adhesives with a soft epoxy resin as in Figure 10 (c), to further enhance the impact resistance of the product.

Full Text

DESCRIPTION OF INVENTION
A) Field Of Technology
This invention in general relates to the manufacturing of fused fiber optic couplers where more than two fibers are fused together, using the fused bi-conical taper technology. More particularly this invention relates to the manufacturing of ultra broadband 1x4 truly fused couplers with reduced polarization dependence and considerable impact resistance. This product is ideal for Fiber-To-The-Home Passive Optical Network applications, where data and analog video signals arc transmitted over the same fiber distribution network.
B) Prior Art Disclosure
Fused couplers are, key elements in multi access fiber optic networks, used to distribute optical signals over different routes. In Passive Optical Distribution Networks for Fiber-To-The-Home applications, multiple wavelengths are assigned to carry different signals such as data, video and monitoring signals, which demands ultra broadband spectral performance of such fused couplers.
Cjcncrally, the couplers made with fused bi-conical taper technique are wavelength dependent. But, it is a well-known technique that by controlling the propagation constant difference and the degree of fusion between the interacting fibers, it is possible to reduce the wavelength dependence. Though this technique has been well established for making wavelength insensitive 1x2 couplers, the same has not been effectively realized in case of IxN couplers, where N is greater than two. Usually IxN couplers are realized by concatenating multiple 1x2 couplers, but truly fused IxN couplers is a better alternative for concatenated couplers in FTTTI applications. Wavelength insensitive truly fused 1x4 couplers are of particular interest in three wavelength (1310, 1490 & 1550) FTlTI passive optical networks where analog, video and data communication services are converged. 1x4 couplers

should also support the transmission at 1625nm, for network monitoring applications thus necessitating the bandwidth coverage of such components from 1250 to 1650 nm.
Several methods have been proposed for the realization of 1x4 truly fused devices. In one such solution, N-1 identical fibers are symmetrically positioned around a central fiber for fabricating a wavelength flattened IxN coupler. Mortimore suggested (R^* US Patent US 5, 175^799) using the capillary tube to keep the fibers parallel to the central core in a symmetrical fashion around the central core to get identical coupling to each of the surrounding fibers, during the process of fusion and elongation. But the patent does not describe how 3 fibers can be symmetrically placed using a capillary tube, which may require dummy fibers, without light guiding cores. Moreover, the model suggested by Mortimore shows a complete power transfer from the central core to the surrounding fiber, for 1x4 couplers and thus extra care must be taken to control the fabrication parameters to get the wavelength-flattened performance.
Another way for realizing IxN couplers is by fusing fibers with dissimilar propagation constants to make it less wavelength sensitive. But here the coupling between the wave-guides is in such a fashion that complex coupling signatures exist between the interacting fibers {Kef: US Patent US 5,355,426). This leads to lot of complexities in the fabrication of wavelength independent IxN couplers. Here it also becomes necessary to control the characteristics of individual fibers to get the required spectral flatness, which makes the manufacturing process highly complicated. Moreover, this adds to the polarization sensitivity of the product.
US Patent US 5,883,992 describes a method of making IxN couplers from four identical fibers, in which the secondary fibers are kept at predefined positions -apexes of an equilateral triangle — around a central fiber using special fiber holding jigs. Though the patent claims that, after twisting the structure has the same cross-

section, as before, there can be variations in the relative positions. Moreover the process requires asymmetric longitudinal profile to make the coupler wavelength independent, which requires complex process control measures. This patent does not disclose the power-coupling pattern with elongation. The primary packaged coupler has a length of 62mm and the splitting ratio is not even at 1620nm range. Further this patent recommends tuning after fusion to get the required wavelength flatness.
C) Brief Description of the Itiventioti
In the present invention, a method of building 1x4 couplers using the same platform of 1x2 fused bi-conical taper coupler station is described. No special jigs are required to preposition the fibers. The fibers are kept parallel to one another, in a plane, in a suitable mechanical groove and are braided together manually to form a bundle, in such a way that there exists no interaction among the secondary fibers, but at the same time each of the secondary fiber shares equal area of interaction with the central fiber. This helps in getting a coupling of equal strength to each of the secondar)^ fibers over a broad range of wavelengths, which ensures the ultra-broadband spectral performance. Here effect of mutual coupling between the secondary fibers is eliminated through the special braiding pattern, which also controls amount of light coupled to the secondary fibers. All the secondary fibers are having the identical propagation characteristics and only the central fiber through which the power is launched for monitoring during fusion process is preprocessed to have a different propagation constant to realize the 1x4 coupler, according to the present invention. The symmetry of the structure and the low degree of fusion helps in controlling the polarization performance to a great extent, l^he couplcrvS so formed have a length 49mm, after packaging, and a typical uniformity of IdB over the wavelength range of 1250 to 1650nm. Thus the complexity of the process is reduced considerably.

In general, fused couplers are ver^^ weak towards impact and thermal shocks and hence are not suitable for outdoor applications, such as passive optical distribution networks. Fused couplers are protected by fixing it to the quartz substrate using epoxies and subsequendy covering it in a metal tube. This existing packaging method is not sufficient to provide good impact resistance to the fused couplers. Hence in another aspect of the present invention, method of packaging the couplers to suitable quartz substrate to get good impact resistance is disclosed. In this packaging technique, the adhesive application points are defined in such a way to secure the fibers twists firmly, to prevent any relative motion between the fibers, l^urther multiple epoxy application points are suggested in the packaging method. To further enhance the impact performance, coating of fibers between the epoxy application points is described. These techniques improve the impact resistance as well as the thermal stability of truly fused couplers.
D) Objects of Invention
• It is the primary object of the invention to design a novel manufacturing process for ultra broadband truly fused 1x4 couplers with reduced polarization dependence, which is unique.
• It is another object of the invention to invent and design a novel coupling profile and braiding pattern for the 1x4 fused couplers with symmetric coupling profile to all secondary fibers, in which there is no mutual coupling between the secondary fibers exists.
• It is another object of the invention to invent and design a novel impact resistant 1x4 coupler for outdoor optical distribution networks where in a new method of packaging technique to hold the fiber twists properly using adhesive application is suggested, which in general applicable for all t)^pes of fused couplers.

P^urther objects of the invention will be clear from the following description in detail. The description will extensively deal with the nature of the invention and manner in which it is to be performed referring to the accompanying drawings.
E) Detailed Description of Invention
Brief Description of Diagrams
The statement of the drawings, which accompany this complete specification, is as
follows -
Figure 1 of the drawings shows the complex coupling profile during the fusion and elongation process for 1x4 couplers described in the previous art.
Figure 2 of the drawings shows the novel coupling profile of 1X4 couplers according to the new invention, where the coupling to all secondarj- fibers is identical.
Figure 3 of the drawings shows the coupling profile, as per the new invention, where all the fibers have identical propagation constants
Figure 4 of the drawings shows the cross-section of the fused Nx4 couplers, where the fibers are placed at the corners of a parallelogram.
Figure 5 of the drawings shows the waist cross-section of 1x4-fused coupler according to the present invention. The center fiber 1 has a different propagation constant compared to the three surrounding fibers.
Figure 6 of the drawings shows the cross-section of the 1x4 coupler as per the new invention, at five different locations (a) cross-section at waist (b) cross-section at 2mm right to the waist (c) cross-section at 2mm left to the waist (d) cross-section at 6mm right to the waist, (e) cross-section at 6mm left to the waist.
Figure 7 of the drawings shows the position of the flame with respect to the four braided fibers.

¥igure 8 of the drawings shows the spectral response of 1x4 couplers built in
the new process.
Figure 9 of the drawings shows the dimensional details of the quartz
substrate used for packing the fused region.
Figure 10 of the drawings shows the schematic of epoxy application points
prescribed for improving the impact resistance of fused couplers, (a) Epoxy
application point redefined to cover the twist points (b) Epoxy bead
suggested to improve the impact resistance as well as thermal stabilit}^ (c)
Coating between the epoxy bead and initial epoxy using a soft epoxy resin to
further enhance the impact resistance
Truly fused, ultra broadband 1x4 couplers are of particular interest in FTrfl networks. As described in the previous section, there are several methods being proposed for manufacturing 1x4 couplers, but not ideal manufacturing methods due to complex coupling signatures involved or due to the requirement of special jigs or asj^mmetric taper profile. This invention describes a simple and straightforu^ard solution for the manufacturing of truly fused 1x4 couplers:
In a preferred embodiment according to this invention, four bare fibers (approximately 2 meters in length) are stripped off the coating for a length of around 25mm and are cleaned using acetone or suitable cleaning solutions. One of the fibers is preprocessed to have a slightly different propagation constant over the coupling region. The desired dissimilarity in the primary fiber can be achieved through methods such as etching, heating, or using suitable dopants. Alternatively, fibers with different propagation constants may be used. The level of dissimilarity is so chosen that the power being distributed evenly among the four fibers, over the required wavelength range. The primary fiber is having a diameter difference of
around \\xxxi from the secondary fibers.

The four fibers arc placed parallel to one another in a groove, such that all the four fibers lie in the same horizontal plane, with the primary fiber at the second position from one end. The four fibers are braided together in a particular pattern to form a bundle, where all the secondary fibers are in close proximity to the primary fiber and shares equal area with the primary fiber along the coupling region. Also, there shall not be any interaction between the secondary fibers to avoid the complex coupling coefficients and to control the maximum power being coupled to the secondary fibers.
With such patterns, it is obser\^ed that the coupling from the central fiber to the surrounding fibers occurs in an identical manner, and no complex coupling coefficients are involved. The coupling behavior in 1x4 couplers according to the previous art is depicted in Vigure /. This shows that the coupling to the secondary^ fibers is not identical and is difficult to get true wavelength independent perfoHTiance. ¥igure 2 depicts the coupling profile according to the present invention, where the coupling to the secondary fibers is identical and thus is very easy to control the process. Figure 3 shows the coupling profile of the braided pattern, when all the four fibers have same propagation characteristics. As shown in l^igure i, the coupled power maximum is around 60%, when the propagation constants are same, and hence is not suitable for 1x4 splitter. The primary fiber is pre-tapered to a level where the maximum of the oscillator)^ response coincide to get the 25% coupling ratio as well as the ultra broadband spectral response.
Vigure 4 shows the cross-section of truly fused 1x4 couplers, reported previously {Kef, US Patent: US 5,355y42&). Here the four fibers are numbered 1, 2, 3 and 4 respectively and 1 being the primary fiber. Only the boundary of the fiber cores is indicated in Figure 4, Figure 5 shows the cross-section at the waist of the coupling region as per the new invention, where all the fibers are continuous along the coupling region. The cross-section of the braided pattern, after fusion, is shown at five different locations in Figure 6, From the figure, it is clear that the secondar}'

fibers are disposed around the central fiber, but not symmetrical with respect to each of the secondary fibers. It is observed that the cross-section pattern gets slightly rotated around the central fiber axis. The cross-section is taken by cleaving the fiised region and is inspected with a microscope of suitable magnification.
7\s shown in Figure 6 of cross-section, there exists an acute angle between the two secondary fibers 1& 2. Both these fibers make an obtuse angle with the other secondar}^ fiber 3. At the end of the coupling region, the four fibers will again be in the same parallel plane.
To get identical coupling profile as in Figure 2, it is not necessary that each of the secondary fibers shall be symmetrically disposed over the primary fiber, but it is necessary that secondary fibers be separated by at least a distance to avoid the interaction between them. The braided pattern helps to maintain the secondaiy fibers in close proximity to the primary fiber, along the coupling region, with a waist cross-section as shown in the Figure 5. Though the braiding can be done in many ways, we suggest two methods. In the first t}^pe, three such fibers are singly twisted three times, among which one fiber is the primary fiber. After that the fourth fiber is twisted over the three-fiber bundle with full turn, so that the relative positions of the four fibers at both ends are 1, 2, 3, 4 and 1, 2, 3, 4. I'his structure is sufficient to get a coupling profile as shown in Figure Z However, there may be a chance that a small deviation in the relative tension can contribute to the interaction among a pair of fibers, thus slightly affecting the uniformity and repeatability of the process. In another embodiment of the braiding pattern, three such fibers are singly twisted three times, among which one fiber is the priman' fiber. Then the fourth fiber is fuU twisted, followed by a half turn over this bundle. The waist cross-section of such a device is shown in the Figure 5. In this case the relative positions of the fibers are 1, 2, 3, 4 and 4, 1, 2, 3 at the ends. (Coupling in structures with the latter twist pattern are found to be more efficient than the former twist and offers good repeatability and better uniformity. The four-fiber

bundle is properly secured to keep the structure during the fusion and elongation. For the above described fiber braiding patterns no special jigs is required and can be done manually. Thus the same platform used for the manufacturing of basic FBT coupler can be used for the fabrication of 1x4 couplers. The groove, which is used to hold the fiber, shall have sufficient width to hold the four fibers.
The fused coupler station has axially moving stages that move over precision slides, driven by stepper motors, During the fusion process the coupling behavior is online monitored by injecting power to the central fiber, at the input, and monitoring the power at each output simultaneously. I'he process is monitored for the coupled power at a single or multiple wavelengths simultaneously and is stopped at that point where we get an equal coupling ratio over the required wavelength range. The heat source can use either hydrogen or deuterium gas or any suitable gas. The flame has a width of 6mm and is placed approximately 1cm above the fiber. The flame can be moved in three axes, XYZ and is controlled in a known manner. The flame for fusion process is applied in the downward direction to provide a constant temperature :^one, during the fabrication. The position of the flame with respect to the braided fibers is shown in Figure 7.
The coupling profile as per the new invention is very simple; the coupling to the secondary fibers is symmetric with respect to each other. The graph in Figure 3 shows the coupling profile when aU the four fibers have similar propagation constant. Here the minimum power remaining in the primary fiber is 40% and the m^aximum power at the secondary fibers is 20%. This condition is not ideal for making a 1x4 splitter. Figure 2 shows the coupling signature, when the central fiber has a different propagation constant. Here the maximum of the oscillator)^ response curve coincide at a coupled power of 25% and provide ultra broadband 1x4 splitter characteristics.

At the waist of the fused region, the distance between the fiber cores of each of the secondary fiber and the primary fiber typically ranges from 35±3 |um, where the primary fiber is having a slighdy reduced diameter (of the order of 1 |am) compared to the secondary fibers. There exits physical separation between the secondary fibers and the minimum separation is between the secondary fibers 1 &
2 as shown in the Figure 5, which is typically in the range of 40+7 |im. At the waist, the lateral area of contact is t^^pically around 10|J,m.
It is YCVj easy to achieve wavelength independent performance for 1x4 couplers over 400nm range, from 1250 to 1650nm, with this new method. The spectral flatness of the products made using the new method, over the range of 1250-1650 is shown in the Figure 8. More over the structure at coupling region makes the process less sensitive to changes in the polarization states, because of the low degree of fusion and the symmetry being preserved, where the length of the fusion region is typically around 14 - 16.5 mm. It is also possible to preprocess one or more secondary fibers to achieve wavelength flatness. The polarizatic^n performance of 1x4 couplers manufactured with the new invention offers polarization dependent loss as small as 0.15 dB.
The power coupled to the any one of the secondary fiber can be represented by
?,= P3= P4= 0.25 sin" (2C2)
where C is the coupling coefficient
and 2 is the distance along the fiber axis And the power remaining in the throughput fiber (P^) is
P,= 1-(P2+P3+P4)
For the present investigation the fiber used is Corning SMF 28e. However this is achievable with any standard single mode fibers and even with specialty fibers. Further the use of water peak suppressed fibers is recommended to ensure the wide operating spectral range and better uniformity. Here the achieved spectral

flatness is around 1 clB, over the band of 400nm, which shows a clear performance improvement over the products manufactured in other process.
The fused region is protected inside quartz substrate, whose thermal expansion is matched to that of fiber, with dimensions as shown in the Figure 9, The design of the substrate is in such a way that the fiber is fully seated inside the substrate even at the twisted regions. The width of quartz substrate groove is designed in such a way that it is sufficient to accommodate four parallel fibers. The depth is so designed that the braided structure is fully with in the groove at all positions of the bundle. The structure is fixed to the substrate using suitable epoxies, either UV curable or thermal epoxies, in such a way that the fused region is safeguarded from external perturbations. The surface of the quartz groove at both the ends of the substrate is roughened to make the epoxy adhesive bonding stronger. The material used for the quartz substrate is GE214. The t}^pical range of groove roughness is
80|a". Also the epoxies can be mixed with quartz powders to minimize its thermal expansion. The unutilized fibers at the input side of the coupler are terminated suitably as in the case of 1x2 couplers with an index matching epoxy.
I'he summary of performance parameters according to the new invention is Maximum Insertion Loss: 7.2 dB Uniformity over a range of 1250-1650nm: IdB Excess Loss: 0.2dB Polarization Dependent Loss: 0.15dB Back Reflection: 60 dB
Mainly these couplers are intended for passive optical networks and are likely to be used at outside room conditions. Thus it is necessary to make such couplers impact resistant and less temperature sensitive. In the weU-known procedure of fixing the couplers to quartz substrate, the epoxies are placed up to those points before the twisting starts, which is enough for securing the fused region to the substrate. But

these couplers are found to be prone to breakages when dropped from 2 feets on to a concrete surface. In the new packaging technique, the region up to the starting point of the coupling region is fixed with suitable adhesives as indicated in the Vigure /(), so as to cover the fiber crossings partially or fully. The typical distance between the epoxies at the two ends is 26-30mm.
In another embodiment of the packaging style, it is suggested to use two different kinds of epoxies. One epoxy is used to form beads at the starting and ending points of the tapered structure. I'he epoxy beads are usually separated by a distance (16-20mm). This technique is found to be improving the reliability of the product to a great extent against repeated thermal stressing. The epoxy that is used to form the bead has low coefficient of thermal expansion. In another embodiment, the region between the epoxy bead and the initial epoxy is coated with a soft epoxy resin as shown in Figure 10, The above described methods, restricts the freedom of twists, thus reduces the thermal strains to the fused region, improving the thermal stabilit}', as well as the impact resistance by a considerable amount.
The impact resistance of the product is verified by dropping it from a suitable height and thus subsequently subjecting the product for continuous temperature cycling from -40 to +85, for up to 1000 cycles. These products are found to withstand an impact when directly dropped from a height of 6 feets. Thus the suitability of the product towards outdoor application is ensured.
Patent Keferences Cited:
1. US 5,175,779 Method of forming an Optical Fiber Coupler and a coupler so formed — British Telecommunications
2. US 5,355,426 Broadband MxN Optical Fiber Couplers and Method of Making - Gould Electronics
3. US 5,883,992 Method for Making Optical Wave guide Couplers with low
wavelength sensitivit)^ and couplers thereby produced — Framatome
Connectors

We claim: -
1. A Method of making truly fused 1x4 couplers, starting from four identical fibers, among which only the primary fiber is preprocessed to change the propagation constant, and braided and fused to get a coupling profile as described in the specification and shown in Figure 2.
2. 1x4 couplers, where the secondary fiber locations are around the central fiber and the angle between two such fibers is always acute, but still
maintains a gap of 2-15|im between those fibers to avoid mutual coupling among them.
3. A method of braiding four fibers together to get a braided stmcture with waist cross-section as described in claim 2, where the cross-sectional pattern rotates with respect to the primary fiber axis.
4. A method of achieving a twisted structure as mentioned in Claim no. 3 where multiple stages are involved.
a. Twisting three fibers together followed by a full turn of fourth fiber
with the three fiber bundle
b. Twisting three fibers together and finally full twisting fourth fiber
more than a full turn over the three-fiber bundle.
5. The process by which the fusion process is stopped at 25% splitting ratio, over the desired wavelength range, after monitoring the four fibers, for identical coupling, simultaneously at single or multiple wavelengths during the fusion process.
6. The process of protecting the fused region inside a quartz groove with dimensions as mentioned in figure 9 and bottom surface with a roughness of
80u".
7. A novel impact resistant 1x4 coupler for outdoor optical distribution
networks wherein a new method of packaging to hold the fiber crosses

using epoxy application as described in the specification and as shown in figure 10 (a) is suggested.
8. A new method to bond coupler to quartz substrate where an epoxy bead is used just before the tapering section as in Figure 10 (b) to improve the thermal stability and impact resistance of the product. The initial epoxy and the epoxy bead can vary in type; preferably the epoxy bead is having low thermal expansion.
9. A new method of coating the fiber bundle between the adhesives with a soft epoxy resin as in Figure 10 (c), to further enhance the impact resistance of the product.

Documents:

1115-CHE-2004 AMENDED PAGES OF SPECIFICATION 04-07-2011.pdf

1115-CHE-2004 AMENDED CLAIMS 04-07-2011.pdf

1115-CHE-2004 AMENDED CLAIMS 11-11-2013.pdf

1115-CHE-2004 AMENDED PAGES OF SPECIFICATION 11-11-2013.pdf

1115-CHE-2004 EXAMINATION REPORT REPLY RECEIVED 11-11-2013.pdf

1115-che-2004 form-1 04-07-2011.pdf

1115-CHE-2004 FORM-3 04-07-2011.pdf

1115-CHE-2004 FORM-5 04-07-2011.pdf

1115-CHE-2004 CORRESPONDENCE OTHERS 11-11-2013.pdf

1115-CHE-2004 EXAMINATION REPORT REPLY RECEIVED 04-07-2011.pdf

1115-CHE-2004 FORM-13 11-11-2013.pdf

1115-che-2004-abstract.pdf

1115-che-2004-assignement.pdf

1115-che-2004-claims.pdf

1115-che-2004-correspondnece-others.pdf

1115-che-2004-description(complete).pdf

1115-che-2004-drawings.pdf

1115-che-2004-form 1.pdf

1115-che-2004-form 26.pdf

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Patent Number

261028

Indian Patent Application Number

1115/CHE/2004

PG Journal Number

23/2014

Publication Date

06-Jun-2014

Grant Date

30-May-2014

Date of Filing

26-Oct-2004

Name of Patentee

NeST RESEARCH & DEVELOPMENT CENTRE

Applicant Address

PLOT NO 43, COCHIN SPECIAL ECONOMIC ZONE,KAKKANAD,COCHIN-682 037

Inventors:

#	Inventor's Name	Inventor's Address
1	NAIR KRISHNAN NAIR RAJAMMA SURESH	PLOT NO 43, COCHIN SPECIAL ECONOMIC ZONE,KAKKANAD,COCHIN-682 037
2	VARGHESE SAMUEL	PLOT NO 43, COCHIN SPECIAL ECONOMIC ZONE,KAKKANAD,COCHIN-682 037
3	IQBAL MUHAMMED	PLOT NO 43, COCHIN SPECIAL ECONOMIC ZONE, KAKKANAD,COCHIN-682 037
4	KOSHIBA YOSHITAKA	TOKODO BLDG. 6F' 3-21-12, AKASAKA, MINATOKU TOKYO 107-0052

PCT International Classification Number

G02B 6/26

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA