JP2828251B2 - Optical fiber coupler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial application field" The present invention relates to a fusion stretching used for branching or merging an optical signal in a system using an optical fiber such as an optical fiber communication or an optical fiber sensor. Shape optical fiber coupler.

2. Description of the Related Art Conventionally, as one of optical fiber couplers, an optical fiber coupler having a configuration in which two or more optical fibers are arranged in parallel and fusion-stretched to form a fusion-stretched portion (fusion-stretched optical fiber). Known as couplers).

FIG. 13 and FIG. 14 are diagrams for explaining a method of manufacturing such a fusion-stretched optical fiber coupler,
To produce the optical fiber coupler 1, two optical fibers 2 are arranged in parallel as shown in FIG. 1, and each clad is fused together. It is formed by stretching to form a fusion-stretched portion 3. As the optical fiber 2, a normal optical communication fiber such as a silica-based single mode optical fiber is suitably used.

This optical fiber coupler 1 splits light incident from port A into ports C and D at a specified power ratio,
Light having a plurality of wavelengths incident on the port A can be branched to the ports C and D for each specific wavelength. Conversely, light separately incident from ports A and B can be multiplexed to port C.

The principle of light coupling of the optical fiber coupler 1 configured as described above will be described. As shown in FIG. 15, if only two optical fibers 2 are arranged in parallel before fusion and fused, only one of them is required. The mode of the optical fiber and the mode of the other optical fiber are strongly confined in the respective cores, and no coupling occurs between the two modes. Note that the symbol P in the figure indicates the power distribution of each optical fiber. Next, as each optical fiber 2 is extended, the core 4 of the optical fiber 2 becomes thinner, and the electromagnetic field in the core 4 seeps into the clad 5.

When the optical fiber 2 is elongated, the coupling distance between the two cores 4 is reduced.

As the optical fiber 2 is elongated, the length of the fiber involved in the coupling increases.

Due to these three effects, as shown in FIG. 16, the coupling between the energy of one optical fiber 2 and the energy of the other optical fiber 2 becomes strong.

[Problem to be Solved by the Invention] However, in the optical fiber coupler 1 configured as described above, the optical fiber 2 must be stretched considerably thinly, and the fusion-stretched portion 3 becomes thinner, and mechanical However, there has been a problem that the optical loss is increased and the loss of light is increased due to bending with a slight external force.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fusion-stretched optical fiber coupler which is easy to manufacture and has excellent optical coupling characteristics and mechanical strength.

"Means for Solving the Problems" As means for achieving the above object, the invention described in claim 1 is
Two or more optical fibers are arranged in parallel, the exposed cladding of these optical fibers is fused together, and the fused portion is stretched to form a fused stretched optical fiber coupler. In the fiber, the softening temperature of the core is higher than the softening temperature of the cladding, and the refractive index of the core of the fusion-stretched portion is reduced by the residual elastic strain applied to the core when the fusion-stretched portion is formed. Thus, the optical fiber coupler has a lower refractive index than the core of the non-stretched portion.

According to a second aspect of the present invention, in the optical fiber, the core is made of quartz glass containing no dopant or containing a dopant such that the refractive index change due to the dopant is 0.1% or less, and the cladding is made of fluorine. 2. The optical fiber coupler according to claim 1, wherein the optical fiber coupler is made of quartz glass whose refractive index is adjusted by at least one of boron.

In the invention according to claim 3, the core diameter of the optical fiber is:
3. The method as claimed in claim 1, wherein the two are different from each other.
2. The optical fiber coupler according to item 1.

[Operation] The invention according to claim 1 has the above configuration,
The power distribution of the mode in which the difference in the refractive index between the core and the clad is substantially reduced in the fusion-stretched portion can be widened, and the coupling between the optical fibers can be caused.

Also, in the case where the cladding of the optical fiber is fused to each other, and furthermore, the fusion-stretched portion is formed by twisting with a tension such that the refractive index of the core is reduced by the residual stress, the refractive index substantially between the core and the cladding is substantially reduced. By reducing the difference, the power distribution of the mode can be widened and the coupling between the optical fibers can be caused.

Further, as the optical fiber, the core is made of quartz glass substantially containing no dopant or containing a dopant so that the refractive index change due to the dopant is 0.1% or less, and the refractive index of the cladding is substantially What consists of quartz glass controlled by at least one of fluorine and boron is preferred.

Further, when an optical fiber coupler is formed by changing the core diameter of the optical fiber to be fused, a wide wavelength band type optical fiber coupler having a flat coupling-dependent wavelength dependence is obtained.

[Embodiment] FIG. 1 is a diagram showing an embodiment of the invention described in claim 1, and reference numeral 11 is an optical fiber coupler. This optical fiber coupler 11 arranges two optical fibers 14 having a core whose softening temperature is higher than the softening temperature of the cladding in parallel, and fuses the claddings of these optical fibers 14 together.
The fusion-bonded portion is stretched to form the fusion-bonded stretched portion 15, and when the fusion-bonded stretched portion 15 is formed, elastic strain is applied to the core, and the core is left. Is lower than the refractive index of the core of the non-stretched portion.

Suitable as the optical fiber 14 is, for example, a core 13
Is an optical fiber made of substantially pure quartz (SiO ₂ ), while the cladding 12 is made of a material having a refractive index lower than that of pure quartz by adding fluorine.
This optical fiber 14 has a core 13
Since there is a difference of about one digit between the core 13 and the cladding 12 (the viscosity coefficient of the core 13 is high), when the temperature is appropriately selected, the core 13 is substantially subjected to elastic strain. At this time, the clad 12 is still in the plastic region due to the low clay, and does not undergo elastic deformation even when a force is applied. On the other hand, the elastic strain applied to the core 13 can lower the refractive index of the core 13 through the photoelastic effect.

FIG. 2 shows the relationship between the heating stretching tension of the optical fiber 14 and the decrease in the relative refractive index difference due to the decrease in the refractive index of the core 13. The optical fiber 14 used had a core diameter of 11 μm and a clad outer diameter of 125 μm. As is clear from FIG. 2, when the stretching tension is increased, the relative refractive index difference between the core and the clad decreases. FIGS. 3 to 5 show this in terms of the refractive index distribution of the optical fiber.
As shown in these figures, in the single mode fiber, as the refractive index of the core (the protruding portion at the center in FIGS. 3 and 5) decreases, the power distribution P of the propagation mode greatly leaks outward. Become.

That is, in the conventional optical fiber coupler 1, the power distribution P of the mode is greatly expanded outside the core 4 by extending the optical fiber 2 and sufficiently reducing the core diameter. , Optical fiber
Although there is a slight reduction in the core diameter due to the stretching of 14, the refractive index difference between the core and the clad can be substantially reduced to widen the power distribution P of the mode, thereby causing coupling between the fibers. 6 and 7 are diagrams for explaining the power distribution in the fusion-spreading section 15, and FIG. 6 is a power distribution in a state where two optical fibers 14 are fused. FIG. 7 shows the power distribution of the fusion-stretched portion where slight stretching was performed after fusion.

From the above, in the optical fiber coupler 11, since sufficient optical coupling can be caused without extremely reducing the diameter of the fusion-stretched portion 15, the mechanical strength of the fusion-stretched portion 15 is improved. Can be done. In addition, the fusion stretching section
Since the diameter of the fibrous portion 15 is not reduced, the bending of the fusion-stretched portion 15 can be suppressed, and the loss of the optical fiber coupler can be reduced.

(Experimental example 1) An optical fiber coupler having the same configuration as that shown in Fig. 1 was produced.

Core diameter 10μm, cladding outer diameter 125μm, relative refractive index difference 0.3
% (Before stretching), two optical fibers consisting of pure quartz for the core material and fluorine-doped quartz for the cladding material are arranged in parallel,
Fused over a length of 100 μm and heated sufficiently to make the cross section almost circular, and further stretched to reduce the diameter of the fused part to about 125
μm.

Next, the fused portion was heated at a low temperature of the comparative example of about 1300 ° C., and pulled at a tension of about 50 g. At this time, the thickness of the fused portion was about 10%. Further, the heating temperature was quickly lowered while maintaining the above tension. By the above operation, a desired coupling was obtained between the two fibers. The fusion-stretched portion of the obtained optical fiber coupler had mechanical strength close to that of an optical fiber. The insertion loss of the obtained optical fiber coupler is about 0.
The loss was as low as 2dB.

FIG. 8 is a diagram showing another embodiment of the first aspect of the present invention, wherein reference numeral 16 denotes an optical fiber coupler 16. This optical fiber coupler 16 is formed by arranging two optical fibers 14 in parallel, fusing each clad to each other, and twisting and extending the fused portion to form a fused stretched portion 17. At the time of forming the fusion-stretched portion 17, elastic strain is applied to the core, and by this remaining, the refractive index of the core of the fusion-stretched portion 17 is lower than the refractive index of the core of the non-stretched portion. Is what it is.

In the optical fiber coupler 16 according to this example, as in the example shown in FIG. 1, the refractive index difference between the core and the clad is substantially reduced, the power distribution of the mode is widened, and the coupling between the fibers is caused. be able to. Therefore, sufficient optical coupling can be generated without extremely reducing the diameter of the fusion-bonded stretched portion 17, and the mechanical strength of the fusion-bonded stretched portion 17 can be improved. In addition, since the fusion-stretched portion 17 does not become thin, it is possible to suppress the bending of the fusion-stretched portion 17 to prevent a disadvantage that causes an increase in loss, and to reduce the loss of the optical fiber coupler 16.

(Experimental example 2) An optical fiber coupler having the same configuration as that shown in Fig. 8 was produced.

Core diameter 10μm, cladding outer diameter 125μm, relative refractive index difference 0.3
% (Before stretching), two optical fibers consisting of pure quartz for the core material and fluorine-doped quartz for the cladding material are arranged in parallel,
It was fused over a length of 100 μm and heated sufficiently to form a substantially circular cross section. Further, the fusion portion was twisted three times and stretched to make the diameter of the fusion portion approximately 80 μm.

Next, the fused portion was heated at a relatively low temperature of about 1300 ° C. and pulled with a tension of about 50 g. At this time, the thickness of the fused portion was about 10%. Further, the heating temperature was quickly lowered while maintaining the above tension. By the above operation, a desired coupling was obtained between the two fibers. The fusion-stretched portion of the obtained optical fiber coupler had mechanical strength close to that of an optical fiber. The insertion loss of the obtained optical fiber coupler is about 0.
The loss was as low as 2dB.

FIG. 9 is a diagram showing an embodiment of the invention described in claim 2, wherein reference numeral 18 denotes an optical fiber coupler. This optical fiber coupler 18 is made of quartz glass whose core is made of quartz glass containing fluorine so that the change in the refractive index is 0.1% or less, and whose refractive index of the cladding is adjusted to be lower than that of the core by fluorine. Are arranged in parallel, the claddings of these optical fibers 19 are fused together, and the fused portion is stretched to form a fused stretched portion 20. Elastic strain is applied to the core during the formation of the stretched portion 20, and the elastic strain remains, whereby the core refractive index of the fusion-bonded stretched portion 20 is lower than the refractive index of the core of the non-stretched portion.

Since the optical fiber 19 can be set to have a smaller refractive index difference between the core and the clad than the optical fiber 14 described above, the mechanical strength of the fusion-stretched portion 20 is smaller than that of the optical fiber coupler 11 shown in FIG. And loss reduction can be improved.

(Experimental Example 3) An optical fiber coupler having the same configuration as that shown in FIG. 9 was produced.

Core diameter 10μm, cladding outer diameter 125μm, relative refractive index difference 0.3
2% (before stretching), two optical fibers made of fluorine-doped quartz as core material and fluorine-doped quartz as clad material are arranged in parallel, fused over a length of 100 μm, and heated sufficiently to reduce the cross section. It was made into a circular shape, and further stretched to make the diameter of the fused portion about 125 μm.

Next, the fused portion was heated at a relatively low temperature of about 1300 ° C. and pulled with a tension of about 50 g. At this time, the thickness of the fused portion was about 10%. Further, the heating temperature was quickly lowered while maintaining the above tension.

By the above operation, a desired coupling was obtained between the two fibers. When the degree of coupling of the obtained optical fiber coupler was measured, the coupling from port E to port G shown in FIG.
51%, 49% coupling from port E to port H, port F
From port F to port G was 49% and from port F to port H was 51%. The insertion loss of the obtained optical fiber coupler was as low as 0.2 dB or less.

FIG. 10 is a diagram showing an embodiment of the invention according to claim 3, wherein reference numeral 21 denotes an optical fiber coupler. This optical fiber coupler 21 corresponds to the optical fiber coupler 18 shown in FIG.
The optical fiber 19 used in
An optical fiber 22 having a core whose core diameter is smaller than that of the optical fibers 19 and 22 is arranged in parallel, and the claddings of the optical fibers 19 and 22 are fused to each other, and the fused portion is stretched to form the fused stretched portion 23. An elastic strain is applied to the core at the time of forming the fusion-bonded stretched portion 23, and this remains, so that the refractive index of the core of the fusion-bonded stretched portion 23 is larger than the refractive index of the core of the non-stretched portion. Is also declining.

In this optical fiber coupler 21, each of the optical fibers 19, 2
Since the phase constants of the modes propagating through the two cores are different, for example, the wavelength dependence of the degree of coupling of light incident on port I to port L shown in FIG. 10 is a flat characteristic as shown in FIG. Become.

On the other hand, in the case of using two identical optical fibers, such as the respective optical fiber couplers 11, 16, and 20 shown in FIGS. 1, 8, and 9, respectively, as shown in FIG. In addition, the degree of coupling changes depending on the wavelength, and 100% coupling occurs at a specific wavelength. For example, when light of a plurality of wavelengths is incident on port E in FIG. It becomes a wavelength division type optical fiber coupler that can obtain the divided light.

In the optical fiber coupler 21 according to this example, since the phase constants of the respective optical fibers 19 and 22 are different, for example, the wavelength dependency of the degree of coupling of the light incident on the port I to the port L becomes flat. Therefore, a wide wavelength range optical fiber coupler that can reduce the range in which the degree of coupling changes depending on the wavelength is obtained.

In each of the above-described embodiments, an example in which an optical fiber coupler is configured using two optical fibers has been described. However, the number of optical fibers is not limited to this, and two or more optical fibers may be used. An optical fiber coupler may be configured.

(Experimental Example 4) An optical fiber coupler having the same configuration as that shown in FIG. 10 was produced.

Core diameter 10μm, cladding outer diameter 125μm, relative refractive index difference 0.3
2% (before stretching), an optical fiber whose core material is fluorine-doped quartz and cladding material is fluorine-doped quartz,
100 µm, cladding outer diameter 125 µm, relative refractive index difference 0.32% (before stretching), two optical fibers made of fluorine-doped quartz as core material and fluorine-doped quartz as clad material are arranged in parallel.
Weld over the length of μm and heat it sufficiently to make the cross section almost circular, then stretch it and make the diameter of the welded part about 125 μm
And

Next, the fused portion was heated at a relatively low temperature of about 1300 ° C. and pulled with a tension of about 50 g. At this time, the thickness of the fused portion was about 10%. Further, the heating temperature was quickly lowered while maintaining the above tension. An optical fiber coupler was obtained by the above operation. When the wavelength dependence of the coupling degree of the obtained optical fiber coupler was measured, the characteristics were as shown in FIG.

“Effects of the Invention” As described above, the optical fiber coupler according to the present invention has the following effects due to the above configuration.

According to the first aspect of the present invention, two or more optical fibers having a core whose softening temperature is higher than the softening temperature of the clad are arranged in parallel, the clads of these optical fibers are fused to each other, and the fused portion is further stretched. To form a fusion-stretched part,
An optical fiber coupler in which the refractive index of the core of the fusion-stretched portion is lower than the refractive index of the core of the non-stretched portion due to residual elastic strain applied to the core when the fusion-stretched portion is formed. Thereby, in the fusion-stretched portion, the refractive index difference between the core and the clad can be substantially reduced, the power distribution of the mode can be widened, and the coupling between the optical fibers can be caused.
Therefore, sufficient optical coupling can be caused without extremely reducing the diameter of the fusion-stretched portion, so that the mechanical strength of the fusion-stretched portion can be improved. Also, since the diameter of the fusion-stretched portion is not reduced, the bending of the fusion-stretched portion can be suppressed, and the loss of the optical fiber coupler can be reduced.

Further, the same effect can be obtained in an optical fiber coupler in which two or more optical fibers are arranged in parallel, the claddings of these optical fibers are fused together, and the fused portion is twisted and stretched to form a fused stretched portion. Can be obtained.

Furthermore, as in the second aspect of the present invention, the optical fiber is made of quartz glass whose core does not substantially contain a dopant or contains a dopant such that the refractive index change by the dopant is 0.1% or less. Become
An optical fiber made of quartz glass whose refractive index of the cladding is substantially controlled by at least one of fluorine and boron is particularly preferable, and a high-performance optical fiber coupler can be obtained.

Further, according to the third aspect of the present invention, by changing the core diameter of the optical fiber to be fused, it is possible to obtain a wide wavelength band type optical fiber coupler having a flat coupling characteristic with a flat wavelength dependence of the coupling degree. it can.

[Brief description of the drawings]

FIG. 1 is a side view of an optical fiber coupler according to an embodiment of the present invention, and FIG. 2 is a relative refractive index difference between a core and a clad of an optical fiber used in the optical fiber coupler shown in FIG. FIG. 3 to FIG. 5 are graphs showing the relationship between the core tension and the drawing tension.
FIGS. 6 and 7 show the relationship between the relative refractive index difference between the claddings and the stretching tension as the refractive index distribution of the optical fiber, and FIGS. 6 and 7 are diagrams for explaining the power distribution in the fused portion and the fused stretch portion. FIG. 8 is a side view of an optical fiber coupler according to another embodiment of the present invention, and FIG. 9 is a side view of an optical fiber coupler according to an embodiment of the present invention.
FIG. 10 is a side view of an optical fiber coupler showing an embodiment of the invention according to claim 3, FIG. 11 is a graph showing the wavelength dependence of the coupling degree of the optical fiber coupler shown in FIG. 10, and FIG. FIG. 10 is a graph showing the wavelength dependence of the degree of coupling of the optical fiber couplers shown in FIGS. 1, 8, and 9, respectively. 13 and 14 are views for explaining a method for manufacturing a conventional optical fiber coupler, FIG. 13 is a side view showing a state in which two optical fibers are fused, FIG. Side view of the optical fiber coupler, FIG. 15 is a cross-sectional view taken along the line II-II of FIG. 13,
FIG. 16 is an enlarged sectional view taken along the line III-III of FIG. 11, 16, 18, 21 ... optical fiber coupler 12 ... clad 13 ... core 14, 19, 22 ... optical fiber 15, 17, 20, 23 ... fusion spliced part.

Claims (3)

(57) [Claims] 1. An optical fiber having two or more optical fibers arranged in parallel, the exposed cladding of these optical fibers being fused together,
Further, in the fusion-stretched optical fiber coupler in which the fusion-bonded portion is stretched to form a fusion-bonded stretch, the softening temperature of the core of the optical fiber is higher than the softening temperature of the clad. An optical fiber, wherein the refractive index of the core of the fusion-stretched portion is lower than the refractive index of the core of the non-stretched portion due to the residual elastic strain applied to the core when the fusion-stretched portion is formed. Coupler. 2. The optical fiber according to claim 1, wherein the core is made of quartz glass containing no dopant or containing a dopant such that the refractive index change due to the dopant is 0.1% or less, and the cladding is made of fluorine or boron. 2. The optical fiber coupler according to claim 1, wherein the optical fiber coupler is made of quartz glass whose refractive index is adjusted by at least one of them. 3. The optical fiber coupler according to claim 1, wherein core diameters of said optical fibers are different from each other.