JPH03154010A - Manufacture of optical fiber coupler - Google Patents

Abstract

PURPOSE:To obtain the optical coupler with slight variance of the branch characteristic by increasing symmetrically the heating area length by following an increase of the drawing quantity. CONSTITUTION:Glass parts 1, 2 of two pieces of optical fibers are arranged, a burner 5 of a microtorch is used as a heating source, the fiber is moved in the axial direction at a speed of 2mm/sec, and the part of about 3.5mm is heated and welded. Subsequently, the adding a weight of 3g to both ends and monitoring it by a displacement sensor, the drawing quantity DELTAL is measured, and by allowing the burner movement at the time of this drawing to escape by + or -2.5mm in the axial direction, and about 0.3mm to both ends to the upper part so as to go away from the fiber, a heating area is increased. As a result, an optical coupler in which the polarized wave dependency is small, and with slight variance of a branch characteristic is obtained, and it is used usefully for an optical amplifier and an optical multiplexer/demultiplexer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of manufacturing an optical fiber coupler used in communication systems and sensor systems.

(Prior Art) In constructing an optical communication system, an optical data link network, etc., an optical branching device that distributes optical signals emitted from a light source at a desired ratio is an important component. One type of optical splitter is an optical fiber coupler, which usually consists of twisting multiple optical fibers together or fusing them in parallel, then heating and melting them using a heat source such as an acetylene burner. It is made by stretching this under constant tension.

In the drawing process, the optical fibers are fused by heating,
In the drawing process, in the method for manufacturing an optical fiber coupler described in JP-A-1-154008, one end side of a pair of parallel optical fibers is relatively fed toward a heating means, and the The optical fibers are fused and stretched to each other while the other ends of the optical fibers are relatively separated from the heating means at a speed higher than the feeding speed, and the heating means and the optical fibers move relatively in one direction. is to be carried out.

In such conventional multiplexing/demultiplexing couplers, when the wavelength interval for multiplexing and demultiplexing is narrowed, the branching state often varies greatly depending on the polarization direction of the light incident on the coupler.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned circumstances, and has been made for the purpose of providing a method for manufacturing an optical fiber coupler with less variation in branching characteristics.

(Means for Solving the Problems) The present invention provides a method for manufacturing an optical fiber coupler in which a plurality of optical fibers are heated, fused and stretched, in which the heating region length is symmetrically increased as the amount of stretching increases. It is characterized by:

The length of the heated region can be increased by reciprocating the heating source during stretching in the axial direction while maintaining a predetermined distance from the optical fiber, and increasing the distance of the reciprocating motion as the amount of stretching increases.

The heating region length can be increased by increasing the length of the heating source in the axial direction during stretching as the amount of stretching increases.

(Function) The present invention provides a method for manufacturing an optical fiber coupler in which a plurality of optical fibers are heated, fused and stretched, by symmetrically increasing the heating region length as the amount of stretching increases.
A coupler with a long taper part can be obtained, and an optical fiber coupler with a narrow wavelength spacing for multiplexing and demultiplexing and less variation in branching characteristics can be manufactured.

The branching characteristics of optical fiber couplers will be explained.

It is well known that when two single mode optical fibers are thermally fused and stretched, the branching ratio changes in a sinusoidal manner with respect to wavelength.

That is, as shown in FIG. 2, the optical power propagated from the input end of one optical fiber 1 is PO, the optical power propagated to the output end is Pl, and the optical power propagated to the other optical fiber 2 is branched to the output end. When the optical power to be propagated is P2 and the insertion loss of the coupler is shown, the crosstalk is Pl-P2 knee 10 log, (sin2(CL)
)P2-po=-1010gto (1-5in”
(CL) ) However, λ Here, n2: cladding refractive index n3: cladding peripheral refractive index a: minimum diameter L of Kabra: coupling length λ: wavelength.

According to this, it can be seen that in order to set the wavelength spacing for multiplexing and demultiplexing to be relatively narrow, it is sufficient to make the minimum diameter a of the coupler extremely thin or to lengthen the coupling length.

From this consideration, the minimum diameter a (in terms of the outer diameter of the fused/stretched part of the coupler,
It becomes 2a. ), and one of the characteristics of the single mode converter is measured.
FIG. 4 shows the results of measuring the polarization dependence at a wavelength of 1.294 μm.

From this result, the minimum outer diameter 2a of the cover part is 20μ
m or less, the branching ratio often exceeds 2% depending on the direction of polarized waves incident on the coupler. As mentioned above, when setting the wavelength spacing for multiplexing and demultiplexing to be relatively narrow by decreasing the minimum diameter a of the coupler or increasing the coupling length, the polarization dependence can be reduced to within ±1% in practical terms. Since the minimum diameter a to satisfy the requirement is 10 μm or more,
It can be said that it is sufficient to make the minimum diameter a 10 μm or more and increase the bond length.

Taking as an example the case of designing a multiplexing/demultiplexing coupler with a wavelength spacing of 50 nm for 1, 48, and 1.53 μm used in optical amplifiers, etc., the wavelength spacing of multiplexing/demultiplexing is calculated when the minimum diameter a is 10 μm. As seen from Figure 5, the bond length is approximately 18
It can be seen that mm is required. In this way, from the viewpoint of polarization dependence, it can be said that for an optical fiber coupler with a narrow wavelength interval for multiplexing and demultiplexing, it is better to increase the minimum diameter a and increase the coupling length.

Further, the polarization dependence of the branching ratio is influenced by the cross-sectional shape at the minimum diameter of the coupler. The branching ratio Q% of the optical fiber coupler of the coupling length in Fig. 2 is Q=100/2 (1+c
It is expressed as os (CxL) ・cos (CyL) ).

Here, if the shape of the XX cross section is a square shape as shown in FIG. 3(A), then the following equation is obtained.

Moreover, if the shape of the XX cross section is a circular shape as shown in FIG. 5(B), then the following equation is obtained.

In addition, λ U=2.045 CXyCY: Coupling coefficient It is.

From this result, FIG. 6 shows the difference between the coupling coefficients in the X direction and the Y direction (Cx Cy) on the vertical axis and the minimum outer diameter 2a on the horizontal axis. The solid line represents data at a wavelength of 1.3 μm, and the dotted line represents data at a wavelength of 1.55 μm. In the figure, A
is a square cross section, and B is a circular cross section. It can be seen that when the side surfaces of the fused portion are not so fused and the circular shape is not distorted, the difference in coupling coefficients between orthogonally polarized waves is smaller, and the polarization dependence is smaller.

To summarize the above effects, the shape of an optical fiber coupler with a relatively narrow wavelength interval for multiplexing and demultiplexing is such that the fused cross section does not collapse much, the minimum diameter is 10 μm or more, and the taper length is long. can be said to be desirable.

It has been found that an optical fiber coupler having this structure can be obtained by gradually increasing the heating area during stretching in accordance with the amount of stretching.

(Example) FIG. 1 is a schematic diagram for explaining an example of the method for manufacturing an optical fiber coupler of the present invention. In the figure, 1 and 2 are the glass parts of the optical fiber, 3 and 4 are the coating parts, 5 is the burner, and 6
is a light source, and 7 and 8 are light receiving elements.

The glass parts 1 and 2 from which the coatings 3 and 4 of two optical fibers have been removed by about 25 mm are arranged in parallel, and a microtorch using propane and oxygen as combustion gas is used as the heat source as the burner 5 to heat the fibers in the axial direction. was moved at a speed of 2 mm/sec, and a portion of approximately 3.5 mm was heat-fused. At this time, no tension is applied to the optical fiber. The temperature was measured by infrared rays emitted from the optical fiber during fusion and found to be 1480'C. Next, a weight of 3 g was added to each end of the fiber, and the amount of stretching was monitored by an optical displacement sensor to measure the amount of stretching ΔL. During stretching, the burner was moved so as to move away from the fiber by ±2.5 mm in the axial direction of the fiber and heat the fiber by about 0.3 mm upward at both ends. The stretching ratio at that time was α=1. Thereafter, the gas flow rate was adjusted so that the amount of heat in the fiber reached approximately 1300° C., and the amount of burner movement was increased by an amount corresponding to the amount of stretching. The stopping of the stretching was monitored by the LD light source 6 and light receiving elements 7 and 8 which were connected in advance to the optical fiber.

FIG. 7 shows the longitudinal cross-sectional shape of the optical fiber coupler manufactured by the above method. The horizontal axis of the figure is the center of the taper part at 0.
This is a dot. Note that the cross-sectional shape at 2 mm on the horizontal axis is shown in FIG.

FIG. 8 shows the results of measuring the wavelength dependence of insertion loss. The variation in branching ratio with respect to the polarization direction is at wavelength 1.493μ
1.7% at m, 1.9% at 1.545 μm
It was good.

In the above-described embodiment, as a means for symmetrically increasing the heating region length as the amount of stretching increases, the heating source during stretching is moved back and forth in the axial direction while maintaining a predetermined distance from the optical fiber. The distance of reciprocating motion is
Although the length of the heating region was increased as the amount of stretching increased, the length of the heating region can be increased by an appropriate method, such as increasing the length of the heating source in the axial direction during stretching as the amount of stretching increases.

(Effects of the Invention) As is clear from the above description, according to the present invention, a product with low polarization dependence can be obtained in an optical fiber coupler with a relatively narrow wavelength interval for multiplexing and demultiplexing, and a pump for an optical amplifier can be obtained. This has the effect of providing an effective manufacturing method for light input and multiplexer/demultiplexer used for wavelength division multiplexing communication.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram for explaining one embodiment of the manufacturing method of the present invention, FIGS. 2 to 6 are explanatory diagrams of the operation, and FIGS. 7 to 9 are optical fibers in the embodiment. FIG. 3 is an explanatory diagram of characteristics of a coupler. 1.2... Glass portion of optical fiber, 3, 4... Covering portion, 5... Burner, 6... Light source, 7, 8... Light receiving element.

Claims (1)

[Claims] A method for manufacturing an optical fiber coupler in which a plurality of optical fibers are heated, fused and stretched, the method comprising increasing the length of the heated region symmetrically as the amount of stretching increases.