DE69032900T2 - Fiber optic coupler - Google Patents

Description

BACKGROUND OF THE INVENTION [FIELD OF THE INVENTION]

The present invention relates to fiber optic couplers used in communication systems.

[STATE OF THE ART]

In one type of prior art fiber optic coupler, two or more fibers are aligned side by side in a plane and thermally fused and stretched longitudinally to form a fused longitudinal region.

In the fused longitudinal region thus formed, the diameter of each component optical fiber is reduced as is the diameter of the core of each fiber. As the diameter of the cores of the optical fibers is reduced, a proportionately larger portion of the light propagating in them escapes through the cladding enclosing the core of each fiber. In addition, as the component optical fibers are pulled and thus stretched, the distance between the cores of adjacent optical fibers is reduced and, due to this, the coupling between the propagation modes of the individual fibers becomes extremely large. In this way, the light transmitted through one fiber branches and is caused to be transmitted through two or more optical fibers simultaneously.

However, the fiber optic coupling method described above has the following disadvantage:

1. When conventional fused silica single mode fibers are used to make the fiber optic coupler, since very little light energy is lost through the side face of the fiber due to the properties of the material, the fused longitudinal region must be quite long to achieve sufficient optical coupling. Furthermore, in order to create a fused longitudinal region of sufficient length, the resulting diameters of the component optical fibers in the fused longitudinal region are reduced quite considerably. For example, assuming fused silica optical fibers with an outside diameter of 125 µm, it is quite possible for the fibers in the fused longitudinal region of the resulting coupler to have a final outside diameter of the order of 20 µm. Due to this effect, even a relatively small bend in the optical fiber will cause light to leak out through the side of the fiber, resulting in a loss of light energy. As the optical fiber is bent more severely, the bend losses become significant.

2. When it is desired to limit the amount of elongation in forming the fused longitudinal portion and still achieve the desired degree of optical coupling, the length of the fused portion must be considerable. In such a case, even if the elongation of the optical fiber has been limited, the coupler becomes quite long. Consequently, in applications where miniaturization is required, such an optical fiber coupler cannot be practically used.

EP-A-0093460 relates to a method for producing a fiber optic coupling element which comprises fusing two single-mode fibers whose The fibre core is made of core glass having an American softening temperature at least 8ºC higher than that of the cladding glass, the fibres being heated to a temperature of between 520 and 560ºC.

SUMMARY OF THE INVENTION

In response to the difficulties described above, it is an object of the present invention to provide a fiber optic coupler that can be used in applications where miniaturization is required.

The present invention provides a fiber optic coupler which can be created by aligning a portion of each of two or more of the component optical fibers next to each other, the primary coating material having been removed from that portion so as to expose the cladding, thermally fusing the exposed cladding of adjacent optical fibers together, then stretching the fused portion so as to form a fused longitudinal portion which constitutes the fiber optic coupler, using fiber optic material in which the softening temperature of the core is higher than that of the cladding, and further applying tension to stretch the fused portions such that the refractive index of the core is reduced due to the residual stress.

When the optical fiber coupler is formed in this way, it is possible to reduce the difference in refractive index between the core and the cladding, broaden the mode power distribution and thereby achieve optical coupling between adjacent optical fibers. In addition, since the amount of elongation or stretching of the fibers carried out after fusing the fibers need not be extreme while achieving adequate optical coupling, it is possible to improve the mechanical strength of the fused longitudinal region. In addition, since the amount of stretching of the component fibers after fusing and consequently the reduction in diameter of the respective cores are limited, it is possible to provide optical fiber couplers with low optical loss. By limiting the formation of bends in the coupler, the losses can be further reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of the fiber optic coupler of the first preferred embodiment of the present invention.

Fig. 2 is a graph illustrating the relationship between the relative difference in the refractive index for the core and that for the cladding and the tension exerted during elongation of the optical fiber used in the fiber optic coupler shown in Fig. 1.

Figures 3 to 5 are views of key points in the graph of Figure 2 in which the relationship between the relative difference in the refractive index for the core and that for the cladding and the tension exerted during elongation is expressed in terms of the refractive index distribution for the optical fiber.

Fig. 6 and 7 are illustrations of the power distribution in a fused section and in a fused longitudinal section in a fiber optic coupler, respectively.

Fig. 8 is a side view of the fiber optic coupler of the second preferred embodiment of the present invention.

Fig. 9 is a side view of the fiber optic coupler of the third preferred embodiment of the present invention.

Fig. 10 is a side view of the fiber optic coupler of the fourth preferred embodiment of the present invention.

Figures 11 and 12 are graphs illustrating the dependence of the degree of optical coupling on wavelength for the fiber optic couplers shown in Figures 11 and 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS [FIRST PREFERRED EMBODIMENT]

In the following section, the first preferred embodiment of the present invention is described with reference to Fig. 1.

The fiber optic coupler 21 of the present embodiment, as shown in Fig. 1, consists of a fused longitudinal region 25 formed by thermally fusing the cladding of a portion of each of two component optical fibers 24 and then stretching the fused portion, both of the optical fibers 24 being those in which the softening temperature of the core 23 is higher than that of the cladding 22. In order to stretch the fused portions, a tension is applied such that the refractive index of the cores of the fused portions is lowered due to the remaining mechanical stress.

For the optical fibers 24 of the present embodiment, a suitable example is an optical fiber material having a core 23 of substantially pure quartz (SiO2) and a cladding 22 to which fluorine has been added so as to cause the cladding to have a lower refractive index than the core. In an optical fiber 24 of this type, at high temperatures, the viscosity coefficient of the core 23 and that of the cladding 22 differ by an order of magnitude (the viscosity coefficient of the core 23 is higher). Since the viscosity coefficient of the cladding 22 is lower, by appropriately selecting the temperature used during elongation of the fused sections, at which only the cladding 22 is in a deformable state, one can introduce a degree of elastic strain only in the core 23, which can be controlled by the amount of tension exerted during elongation. Due to the strain-dependent optical properties of the core material, the refractive index of the core 23 is reduced by the elastic strain exerted on the core 23, while that of the cladding 22 is not affected.

In Fig. 2, a graph is shown illustrating the relationship between the amount of tension applied during elongation of the heated optical fiber 24 and the reduction in the difference in refractive index by lowering the refractive index of the core material. For the graph in Fig. 2, a fiber optic material with a core diameter of 11 µm and an outer diameter of the cladding of 125 µm was used. As can be seen from Fig. 2, the difference in refractive index between the core and the cladding decreases as the amount of tension applied during elongation of the optical fiber 24 is increased.

In Figs. 3 through 5, the refractive index distribution for the optical fiber 24 is shown. As shown in these drawings, for the single mode optical fiber 24, the propagation mode power distribution (P) becomes wider and shorter and is substantially peripherally shifted as the refractive index of the core (the upwardly projecting central part of each drawing) and hence the refractive index difference for the optical fiber 24 decreases. Consequently, due to the effect of elongation on the refractive index difference and hence on the propagation mode power distribution, a reasonably wide mode diameter and hence good optical coupling can be achieved with the optical fiber 24 of the present embodiment, which would have required considerably greater elongation and reduction of the core diameter in the case of the optical fibers used in prior art fiber optic couplers.

In Fig. 6 and 7, the power distribution for the fused longitudinal section 25 of the fiber optic coupler 21 of the present embodiment is schematically illustrated. Fig. 6 shows the power distribution of the fused optical fibers 24 before elongation, and Fig. 7 is the power distribution for the fused and stretched optical fibers 24 constituting the fused longitudinal section 25 of the fiber optic coupler 21. As can be seen from Fig. 6, there is no superposition of the power distribution of the two component optical fibers 24, and consequently no optical coupling can occur. In the case of the fused and also stretched optical fibers 24 shown in Fig. 7, there is a good superposition of the power distributions emanating from the slightly narrowed cores 23 in the fused longitudinal region 25, so that the optical signal from one of the component optical fibers 24 can penetrate into the adjacent optical fiber and thus couple with it.

As described above, in the optical fiber coupler 21 of the present embodiment, adequate optical coupling can be achieved without extreme elongation of the component optical fibers 24 and hence without extreme reduction in the diameter of the cores. As a result, the mechanical strength of the optical fiber coupler 21 can be improved. In addition, the optical loss caused by bending of the optical fiber coupler 21 can be reduced.

[FIRST EXPERIMENTAL EXAMPLE]

Using two optical fibers 24, each having a core diameter of 10 µm, an outer cladding diameter of 125 µm, a difference in refractive index before elongation of 0.3%, a core of pure quartz and a cladding with fluorine added, a 100 µm section of each of the two was aligned side by side and thermally fused using a sufficiently high temperature to form a roughly cylindrical fused section which was then stretched to form a fused region having a diameter of about 125 µm.

Then, the fused portion formed in the above-described manner was heated to a relatively low temperature on the order of 1300°C and stretched at a tensile stress of 50 g, thereby reducing the diameter of the fused portion by about 10%. Finally, while maintaining the tensile stress described above, the heating temperature was rapidly lowered so as to obtain a fiber optic coupler 21 similar to that of the first preferred embodiment shown in Fig. 1. By means of the method described above, an adequate degree of optical coupling between the two component optical fibers 24 can be achieved. The fiber optic coupler 21 thus produced had a mechanical strength close to that of the component optical fibers 24. In addition, the optical losses for the fiber optic coupler 24 produced were low at about 0.2 dB.

In the present experimental example, the length of the portion of the initially fused optical fibers 24 was 100 µm, but this length may be as short as 1 to 2 mm for the same type of optical fiber coupler 21. As an example, an optical fiber coupler 21 was manufactured in which the length of the portion of the initially fused optical fibers 24 was 1 mm while all other conditions were the same. The optical fiber coupler 21 manufactured in this way had optical losses of 0.3 dB.

[SECOND PREFERRED EMBODIMENT]

In the following section, the second preferred embodiment of the present invention will be described with reference to Fig. 8.

The fiber optic coupler 26 of the present embodiment, as shown in Fig. 8, consists of a twisted fused longitudinal portion 27 formed by thermally fusing the cladding of a portion of each of two optical component fibers 24 identical to those of the above-mentioned first preferred embodiment and then stretching the fused portion while twisting the pair of optical fibers 24. In stretching and twisting the fused portions, tension is applied such that due to the residual stress, the refractive index of the cores of the fused portions is lowered.

In the optical fiber coupler 26 of the present embodiment, as was the case with the optical fiber coupler 21 of the first preferred embodiment shown in Fig. 1, adequate optical coupling between the fused portions of the optical fiber 24 can be achieved essentially by reducing the difference in refractive index between the core and the cladding and thus broadening the mode power distribution. Accordingly, adequate optical coupling is possible without extremely reducing the diameter of the longitudinal fused portion 27. Consequently, the mechanical strength of the optical fiber coupler 26 can be improved. In addition, the optical loss caused by bending of the optical fiber coupler 26 can be reduced.

[SECOND EXPERIMENTAL EXAMPLE]

Using two optical fibers 24, each having a core diameter of 10 µm, an outer cladding diameter of 125 µm, a difference in refractive index before elongation of 0.3%, a core of pure quartz and a cladding with fluorine added, a 100 µm section of each of the two was aligned side by side and thermally fused using a sufficiently high temperature to form a roughly cylindrical fused section which was then was stretched by twisting three full turns to form a fused region with a diameter of approximately 80 µm.

Then, the fused portion formed in the above-described manner was heated to a relatively low temperature on the order of 1300°C and stretched under a tension of 50 g, thereby reducing the diameter of the fused portion by about 10%. Finally, while maintaining the above-described tension, the heating temperature was rapidly lowered to thereby provide an optical fiber coupler 26 similar to that of the second preferred embodiment as shown in Fig. 8. By the above-described method, an appropriate degree of optical coupling between the two component optical fibers 24 can be achieved. The thus-produced optical fiber coupler 26 had a mechanical strength close to that of the component optical fibers 24. In addition, the insertion loss for the produced optical fiber coupler 24 was low at about 0.2 dB.

In the present experimental example, the length of the portion of the originally fused optical fibers 24 was 100 µm, but this length may be as short as 1 to 2 mm for the same type of optical fiber coupler 26. As an example, a fiber optic coupler 26 was manufactured in which the length of the portion of the originally fused optical fibers 24 was 1 mm while all other conditions were the same. The fiber optic coupler 26 manufactured in this way had an insertion loss of 0.3 dB.

[THIRD PREFERRED EMBODIMENT]

In the following section, the third preferred embodiment of the present invention will be described with reference to Fig. 9.

The fiber optic coupler 28 of the present embodiment, as shown in Fig. 9, consists of a fused longitudinal region 30 formed by thermally fusing the cladding of a portion of each of two component optical fibers 29 having a silica glass core whose refractive index has been lowered to 0.1% or less by the addition of fluorine and a silica glass cladding whose refractive index has been adjusted to a value lower than that of the core by the addition of fluorine, and then stretching the fused portion. To stretch the fused portions, tension is applied such that the refractive index of the cores of the fused portions is lowered due to the remaining mechanical stress. The reason for using an optical fiber in which the refractive index of the core has been reduced to 0.1% or less as described above is to facilitate the formation of a fiber optic coupler 28 having an appropriate difference in refractive index between the core and the cladding.

Since the optical component fibers 29 used in the present preferred embodiment have a core-cladding difference in refractive index which is even smaller than that of the optical component fibers 24 used in the first and second preferred embodiments, an effective optical coupling between the fused portions of the optical fiber 29 can be achieved with an even smaller elongation and thus a reduction in diameter. Consequently, the mechanical strength of the fiber optic coupler 28 can be further improved, and the Insertion loss caused by bending of the fiber optic coupler 28 can be further reduced.

[THIRD EXPERIMENTAL EXAMPLE]

Using two optical fibers 29 each having a core diameter of 10 µm, an outer cladding diameter of 125 µm, a difference in refractive index before elongation of 0.32%, a core of fluorine-added quartz (enough fluorine is added to lower the refractive index of the core to 0.05%), and a fluorine-added quartz cladding, a 100 µm section of each of the two component optical fibers 29 was aligned side by side and thermally fused using a sufficiently high temperature to form a roughly cylindrical fused section which was then stretched to form a fused region having a diameter of about 125 µm.

Then, the fused portion formed in the above-described manner was heated to a relatively low temperature on the order of 1300°C and stretched at a tension of 50 g, thereby reducing the diameter of the fused portion by about 10%. Finally, while maintaining the above-described tension, the heating temperature was rapidly lowered to thereby provide an optical fiber coupler 28 similar to that of the third preferred embodiment, as shown in Fig. 9. By the method described above, an appropriate degree of optical coupling between the two component optical fibers 29 can be achieved. Then, the degree of optical coupling was measured for the thus-fabricated fiber optic coupler 28, and it was found that an optical coupling of 51% was achieved between port 2E and port 2G as shown in Fig. 9, 49% was achieved between port 2E and port 2H, an optical coupling of 49% was achieved between port 2F and port 2G, and an optical coupling of 51% was achieved between port 2F and port 2H. In addition, the optical loss for the thus-fabricated fiber optic coupler 28 was as low as about 0.2 dB.

In the present experimental example, the length of the portion of the initially fused optical fibers 29 was 100 µm, but this length may be as short as 1 to 2 mm for the same type of optical fiber coupler 28. As an example, an optical fiber coupler 28 was manufactured in which the length of the portion of the initially fused optical fibers 29 was 1 mm while all other conditions were the same. The optical fiber coupler 28 manufactured in this way had losses of 0.3 dB.

[FOURTH PREFERRED EMBODIMENT]

In the following section, the fourth preferred embodiment of the present invention will be described with reference to Fig. 10.

The fiber optic coupler 31 of the present invention, as shown in Fig. 10, consists of a fused longitudinal portion 33 formed by aligning and thermally fusing the cladding of a portion of each of two fiber optic components, an optical fiber 29 identical to that used in the above-mentioned third preferred embodiment and an optical fiber 32 having a core of smaller diameter than the optical fiber 29, and then stretching the fused portion. To stretch the interconnected A tension is applied to the fused sections such that the refractive index of the cores of the fused sections is reduced due to the remaining mechanical stress.

In the fiber optic coupler 31 of the present embodiment, the phase constant of the two component optical fibers 29, 32 is different. For this reason, the optical coupling in the fiber optic coupler 31 is essentially independent of wavelength, as illustrated by the fairly flat curve in Fig. 11, which represents the degree of optical coupling between ports 21 and 2L as a function of the wavelength of light for the fiber optic coupler 31 shown in Fig. 10.

In contrast, in the case of the above-described fiber optic couplers 21, 26 and 28 shown in Figs. 1, 8 and 9, respectively, which are made from two identical optical fibers, the coupling ratio differs with the changing wavelength of the transmitted light, with virtually 100% coupling occurring at certain specific wavelengths of the transmitted light. As shown in Fig. 12, it was found that when an optical signal containing two wavelength components was input to port 2E of the fiber optic coupler 28 shown in Fig. 9, the optical signal was separated into its two components, one emitted from port 2G and the other from port 2H. Consequently, unlike the optical fiber coupler 31 of the present embodiment, the optical fiber couplers that operate with two identical optical fibers can be used as selective wavelength splitter type optical fiber couplers.

In the fiber optic coupler 31 of the present preferred embodiment, the optical coupling in the fiber optic coupler 31 is largely independent of wavelength due to the different phase constant for the individual component optical fibers 29, 32. Consequently, the fiber optic coupler 31 can be used as a wavelength-independent fiber optic coupler (broadband type fiber optic coupler) over a wide range of wavelengths.

[FOURTH EXPERIMENTAL EXAMPLE]

Using an optical fiber 29 having a core diameter of 10 µm, an outer cladding diameter of 125 µm, a difference in refractive index before elongation of 0.32%, a core of fluorine-added quartz and a fluorine-added quartz cladding, and an optical fiber 32 having a core diameter of 9 µm, an outer cladding diameter of 125 µm, a difference in refractive index before elongation of 0.32%, a core of fluorine-added quartz and a fluorine-added quartz cladding, 100 µm sections of each of the component optical fibers 29, 32 were aligned side by side and thermally fused using a sufficiently high temperature to form a roughly cylindrical fused section, which was then stretched to form a fused region having a diameter of about 125 µm.

Then, the fused portion formed in the above-described manner was heated to a relatively low temperature of the order of 1300°C and stretched at a tension of 50 g, thereby reducing the diameter of the fused portion by about 10%. Finally, while maintaining the above-described tensile stress, the heating temperature is rapidly lowered to thereby provide a fiber optic coupler 31 similar to that of the fourth preferred embodiment as shown in Fig. 10.

In the present experimental example, the length of the portion of the originally fused optical fibers 29, 32 was 100 µm, but this length may be as long as 1 to 2 mm for the same type of optical fiber coupler 31. As an example, an optical fiber coupler 31 was manufactured in which the length of the portion of the originally fused optical fibers 29 was 1 mm while all other conditions were the same. The optical fiber coupler 31 manufactured in this way had losses of 0.3 dB.

The various examples of the present invention illustrated in the above preferred embodiments are only examples and are not to be considered as limiting the present invention in any way. For example, it is possible to use three or more optical fibers in the fiber optic coupler of the present invention with acceptable results. It is to be understood that the fiber optic coupler of the present invention includes all forms encompassed by the appended claims.

Claims (5)

1. A fiber optic coupler which can be created by aligning a section of each of two or more component optical fibers next to each other, the primary coating material having been removed from each of these sections so as to expose the cladding, thermally fusing the exposed claddings together to form a fused section, and then elongating the fused section to form a fused longitudinal region, wherein the component optical fibers are optical fibers in which the softening temperature of the core is higher than that of the cladding and in which tension is applied to elongate the fused section such that the refractive index of the cores of the optical fibers is reduced due to the residual mechanical stress. 2. The fiber optic coupler of claim 1, wherein the fused longitudinal region is twisted during elongation to form a twisted fused longitudinal region. 3. A fiber optic coupler according to claim 1 or 2, wherein the cores of the two or more component optical fibers are made of dopant-free fused silica and wherein the claddings of the two or more component optical fibers are made of fused silica, the refractive index of which has been tuned by the addition of at least one additive selected from the group comprising fluorine and boron. 4. A fiber optic coupler according to claim 1 or 2, wherein the cores of the two or more component optical fibers are made of fused silica containing a dopant such that the refractive index has been changed by up to 0.1%, and wherein the claddings of the two or more component optical fibers are made of fused silica whose refractive index has been tuned by the addition of at least one additive selected from the group comprising fluorine and boron. 5. A fiber optic coupler according to any one of claims 1 to 4, wherein the two or more component optical fibers include optical fibers having at least two different diameters.