JPH02259706A - Optical fiber coupler - Google Patents

Abstract

PURPOSE:To obtain a ground type optical fiber coupler which has small dependency on coupling rate by wavelength and small difference of dependency between X polarization and Y polarization by providing matching agent whose refractive index is 1.4-1.5 between the ground surfaces of two polarized wave maintaining optical fibers. CONSTITUTION:The polarized wave maintaining optical fiber 4 consisting of a core 5 which is a center, two stress adding parts 8 disposed on both side parts of the core 5 and a clad part 9 surrounding all of them are buried and fixed in the groove 3 of a grooved substrate 2 and the surface of the groove side is ground to the vicinity of the core 5 so as to form the ground surface 6. Two grooved substrates 2 are allowed to abut on each other so that the ground surfaces 6 are overlapped and the matching agent 7 is interposed between the respective ground surfaces 6 so as to bond both substrates. A material whose refractive index is 1.4-1.5, desirably 1.45-1.46, which is nearly equal to the refractive index of glass, is used as the matching agent 7. Thus, the dependency on the coupling rate by the wavelength is made small and also the difference of the dependency between the X polarization and the Y polarization is made small.

Description

Detailed Description of the Invention "Field of Industrial Application" The present invention relates to an optical fiber coupler used for branching and coupling light in an optical fiber communication system 1, an optical fiber sensor, etc. This invention relates to a device using a polarization-maintaining optical fiber.

"Prior art and its problems" The degree of coupling of commonly used end-fiber couplers has wavelength dependence, and in the case of polarization-maintaining optical fiber couplers, the dependence differs depending on the polarization. It's coming.

The difference in the coupling ratio between X-polarized waves and Y-polarized waves at a given wavelength is usually 10% or more, except when the relational curve between wavelength and coupling ratio has an opposite slope and intersects. . An example of a relationship curve between the wavelength of such X and Y polarized waves and the coupling ratio is shown in FIG.

Incidentally, there are known methods for manufacturing optical fiber couplers, such as the fusion-drawing method and the polishing method, but the above-mentioned tendency to cause differences in wavelength dependence due to polarization is observed in optical fiber couplers manufactured by the fusion-drawing method. It tends to be more noticeable in products with a longer fused and stretched tapered region length. FIG. 7 is a diagram showing the relationship between the coupling ratio and one wavelength of a polarization-maintaining optical fiber coupler (tapered region length: 9 ms) produced by the fusion-stretching method.

This difference in coupling ratio between polarized waves means that when used in an actual device, there is either no difference in the degree of coupling due to polarization at a single wavelength, or, on the contrary, each polarization is used as a polarized beam splitter. It is useful if the waves are completely separated as shown in FIG.

The present invention provides a polished optical fiber coupler in which the dependence of coupling ratio on wavelength is small in commonly used polarization-maintaining optical fibers, and the difference in dependence between X-polarized waves is also small. It is an object.

"Means for Solving the Problems" In order to solve the above problems, the present invention provides polished surfaces of two polarization-maintaining optical fibers in which a part of the polarization-maintaining optical fiber is polished to the vicinity of the core. It is an optical fiber coupler made by joining, and the refractive index between both polished surfaces is 1.4~!
, 5 matching agents were provided.

Hereinafter, the present invention will be described in detail with reference to the drawings.

1 and 2 are diagrams showing an example of the optical fiber coupler of the present invention, and reference numeral 1 in these figures indicates the optical fiber coupler.

This optical fiber coupler l embeds and fixes a stress-applying polarization-maintaining optical fiber 4 in the WI 13 of a grooved substrate 2, and connects the groove-side surface of the grooved substrate 2 to the core of the polarization-maintaining optical fiber 4. 4 to form a polished surface 6, and butt the two grooved substrates 2 so that their respective polished surfaces 6 overlap, and interpose a matching agent 7 between each polished surface 6 to bond both substrates. It is constructed by joining.

As the matching agent 7, a material having a refractive index of 1.4 to 1.5, preferably 1.45 to 1.46, which is approximately equivalent to the refractive index of glass, is used. The matching agent 7 can be made of an oil-like material or a transparent synthetic resin that is also used as an adhesive, but when considering the stability of its properties against temperature, an oil-like material has poor stability and is not practical. Not.

Therefore, a solid matching agent such as an adhesive is preferably used.

Further, as shown in FIG. 3, the polarization-maintaining optical fiber 4 is composed of a central core 5, two stress-applying parts 8 arranged on both sides of the central core 5, and a cladding part 9 surrounding them. ing. Examples of suitable materials for these parts include: the core 5 is made of G e Ot doped 5ift, the cladding part 9 is made of 5ift, and the stress applying part 8 is made of B, 0. Doped Sin.

When the optical fiber coupler 1 is constructed using this silica-based polarization-maintaining optical fiber 4, the distance between the refractive index of the matching agent 7 and the x mm and X polarization of the optical fiber coupler l is as shown in FIG. There is a relationship as shown.

In other words, in order to keep the difference in the coupling ratio between X polarized waves and It is necessary to do so. Furthermore, in order to keep the difference in the coupling ratio between the X-polarized wave and the .45~1
．． It needs to be 46.

By the way, most commonly used transparent synthetic resin adhesives have a refractive index of 1.5 or more;
．． When a material of 5 or more is used as the matching agent 7, the difference in the coupling ratio between the X polarization and the -Y polarization becomes 10% or more, which is not preferable. The refractive index of the adhesive used as the matching agent 7 is 1
．． In order to make it 4 to 1.5, the C-H bond in the molecule must be
This is achieved by using a fluorine-based adhesive containing fluorine atoms replaced by -F bonds. Further, the refractive index of such an adhesive molecule can be adjusted by adjusting the number of C-F bonds in the molecule.

The above optical fiber coupler 1 has a refractive index of 1.4~! between the polished surfaces 6 of each polarization maintaining optical fiber 4! By providing the matching agent No. 5, the difference in the coupling ratio between the X polarized waves and the X polarized waves at a single wavelength can be suppressed to within 10%.

Hereinafter, the effects of the present invention will be clarified through Examples.

"Example" An optical fiber coupler having the same structure as that shown in FIGS. 1 and 2 was prepared using a quartz-based stress-applied polarization-maintaining optical fiber having a core diameter of 8 μm and a fiber outer diameter of 125 Bm.

This polarization-maintaining optical fiber was inserted into the groove of a grooved substrate made of quartz, and an adhesive was injected and solidified to embed and fix the polarization-maintaining optical fiber in the grooved substrate. Next, this grooved substrate was polished to polish a portion of the polarization maintaining optical fiber within the groove to the vicinity of the core. Next, an ultraviolet curing adhesive (matching agent) with a refractive index of 1.438 is applied to this polished surface, and another grooved substrate is placed on the polished surface so that the polished surfaces overlap, and the adhesive is solidified to bond both surfaces together. A grooved substrate was bonded.

When the X polarization and the coupling ratio of the X polarization at a wavelength of 1.37 zm of the obtained optical fiber coupler were measured, the coupling ratio of the X polarization was 49.5%, and the coupling ratio of the X polarization was 50.7%. The difference was very small at 1.2%.

In addition, the X-polarized wave and the coupling ratio of the X-polarized wave were also measured at other wavelengths, and the relationship between the wavelength and the coupling ratio was investigated. The results are shown in Figure 5.

As is clear from FIG. 5, the difference in coupling ratio between polarized waves is extremely small.

"Effects of the Invention" As described above, the optical fiber coupler according to the present invention has a refractive index between the polished surfaces of two polarization-maintaining optical fibers! , 4~
1.5, the dependence of the coupling ratio on wavelength is small, and the difference in dependence between X and Y polarizations can also be made small.

[Brief explanation of drawings]

1 and 2 are diagrams showing an example of the optical fiber coupler of the present invention, in which FIG. 1 is a side view of the optical fiber, FIG. 2 is a cross-sectional view of the central part, and FIG. 3 is a polarization FIG. 4 is a cross-sectional view showing an example of a holding optical fiber, FIG. 4 is a graph showing an example of the relationship between the coupling ratio of X, Graph showing an example of the relationship between X1Y polarization and wavelength in an optical fiber, No. 6
Figures 8 through 8 show X in conventional optical fiber couplers,
It is a graph for explaining wavelength dependence between X polarized waves. 1... Optical fiber coupler, 4... Polarization maintaining optical fiber, 5... Core, 6...
Polished surface, 7... Matching agent.

Claims (1)

[Claims] An optical fiber coupler in which the polished surfaces of two polarization-maintaining optical fibers are joined together by polishing a part of the polarization-maintaining optical fiber to the vicinity of the core, and the refractive index between both polished surfaces is 1. An optical fiber coupler characterized in that a matching agent of .4 to 1.5 is provided.