JP2618544B2 - Optical fiber connection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for connecting optical fibers having different mode field diameters, and more particularly to a method for connecting a rare earth element-doped optical fiber having a small mode field diameter to another optical fiber with low loss. It is.

[0002]

2. Description of the Related Art A single mode fiber in which a core is doped with a rare earth element such as neodymium (Nd) or erbium (Er) as a laser active material is used for an optical amplifier such as an optical fiber laser or an optical power amplifier, and for optical communication. It is used in the field. In particular, the optical fiber doped with erbium has a wavelength at which the transmission loss in the silica-based optical fiber is minimized,
Since the excitation wavelength of the laser active substance matches, the optical signal can be directly amplified, which is useful. As a method of efficiently operating an optical fiber type optical amplifier using such a rare earth element-doped optical fiber, for example, M.N.
akazawa, et al., Topical Meeting on Optical Amplif
As described in iers and Their Applications, PD-1, (1990), etc., the relative refractive index difference between the core and the clad of a rare earth element-doped optical fiber is increased, and the mode field diameter of the optical fiber (hereinafter referred to as MFD) It is known to reduce the abbreviation.). For example, the MFD of an erbium-doped optical fiber suitably used for an optical fiber type optical amplifier is 6 μm
It is as follows.

[0003]

Since the above-mentioned rare earth element-doped optical fiber is used as an amplification medium in an optical amplifier, ordinary optical fibers are used for inputting pumping light and signal light and outputting amplified light. used. Fusion splicing is widely used for connecting rare earth element-doped optical fibers to ordinary optical fibers. In general, the connection loss in fusion splicing of optical fibers depends on the alignment of the core axes of the optical fibers to be connected, and it is required that the cores of the optical fibers to be connected be connected to each other without eccentricity.

However, generally used optical fibers are
While the MFD is about 8 to 10 μm, the MFD of the rare-earth-doped optical fiber is designed to be very small for the purpose of increasing the efficiency of the optical amplification function as described above. Therefore, a large MFD difference occurs at the connection between these two optical fibers. When optical fibers having a large difference in FMD are connected to each other by a conventional fusion method, there is a disadvantage that the difference in MFD causes an increase in transmission loss.

The present invention has been made to solve the above-mentioned problem, and has as its object to provide a method of connecting optical fibers having a large MFD difference with low transmission loss.

[0006]

According to a method for connecting an optical fiber of the present invention, a first optical fiber comprising a core and a cladding having a softening temperature lower than that of the core is formed in a mode different from that of the first optical fiber. A method of connecting to a second optical fiber having a field diameter, wherein after fusing the first optical fiber and the second optical fiber, the first optical fiber is heated to a temperature higher than the softening temperature of the cladding of the first optical fiber. The solution is to heat the fused portion to a temperature equal to or lower than the softening temperature of the core of the optical fiber, and then to cool the fused portion while applying stress.

[0007]

When the fused portion of the first optical fiber and the second optical fiber is heated at a temperature equal to or higher than the softening temperature of the cladding of the first optical fiber and equal to or lower than the softening temperature of the core of the first optical fiber, Only the cladding of the first optical fiber softens. When the softened clad is cooled while applying stress, the first
A stress residual portion is formed in the core of the optical fiber. Since the refractive index of the core decreases due to the residual stress, the MFD of the first optical fiber expands.

Hereinafter, the present invention will be described in detail in the order of steps. A first optical fiber including a quartz core and a clad having a softening temperature lower than the softening temperature of the core is prepared. A rare earth element such as neodymium or erbium may be added to the core of the first optical fiber according to the purpose of use. In order to make the softening temperature of the core and the cladding of the first optical fiber different, use high-viscosity quartz for the core and use low-viscosity quartz having a softening temperature lower than that of the core. Can be. Further, a dopant for changing the softening temperature of quartz may be added to one or both of the core and the clad.
Note that the difference in softening temperature between the core and the clad only needs to exist at the connection portion of the first optical fiber, and need not be formed over the entire length of the first optical fiber.

Next, after the axes of the first optical fiber and the second optical fiber are made to coincide with each other, the connecting portion is heated to a temperature equal to or higher than the softening temperature of the core of the first optical fiber, and the first optical fiber is connected to the first optical fiber. The second optical fiber is fused to the second optical fiber. For the axis alignment step and the fusion step, a general-purpose optical fiber fusion method can be used. The vicinity of the fusion portion of the first optical fiber and the second optical fiber thus fusion-spliced to each other is set to a temperature equal to or higher than the softening temperature of the cladding of the first optical fiber.
Is heated below the softening temperature of the core of the optical fiber. By this heating, only the fused portion of the clad of the first optical fiber can be softened.

Next, the softened first optical fiber is cooled while applying stress. The method of applying stress to the first optical fiber is not particularly limited, but a method of applying tension along the longitudinal direction of the optical fiber, for example, can be suitably used. This stress is preferably applied continuously until the cladding of the first optical fiber is completely solidified. By thus cooling and solidifying the cladding of the first optical fiber while applying stress,
A stress residual portion can be formed in the core of the first optical fiber.

Regarding the effect of the residual stress of the optical fiber core on the characteristics of the optical fiber, see, for example, R. Yamauchi, e.
t, al., OFC'89, THI3, (1989) and the like, and it is known that the refractive index of the stress residual portion decreases. Therefore, by forming the stress residual portion, the refractive index of the core of the fusion spliced portion of the first optical fiber decreases, and as a result, the first
The MFD of the optical fiber will be expanded. On the other hand, since this residual stress portion is not formed in the second optical fiber, the second optical fiber is not affected at all by this step. Therefore, through this cooling step, the MFD difference between the first optical fiber and the second optical fiber can be reduced. As a result, it is possible to prevent an increase in the transmission loss of the fusion splice due to the MFD.

[0012]

Example A quartz rod to which erbium was added was prepared by a known method. On the outer periphery of this core rod, a relative refractive index difference from the core becomes Δ = 0.7%,
A cladding made of fluorine-doped quartz was formed. During the formation of the clad, the thickness of the clad was adjusted so that the MFD of the optical fiber obtained after drawing the rod was a minimum value. Thereafter, the rod was drawn to obtain a first optical fiber having a core diameter of 5.3 μm, an MFD of 7.06 μm, and a cutoff wavelength of 1.14 μm. On the other hand, as the second optical fiber, an optical fiber having a pure silica cladding and an MFD of 10 μm was prepared.

Next, the first optical fiber and the second optical fiber were heated and fused. Then, with 15 g of tensile stress applied from both ends of the connected optical fiber,
The fused portion was heated for 10 seconds in a temperature range where the pure quartz did not soften, and then cooled to solidify the fused portion while maintaining the stress in the erbium-added core.

When the connection loss between the first optical fiber and the second optical fiber thus connected was examined,
It turned out to be very small at 0.3 dB. As a result,
According to the connection method of the present invention, it was confirmed that the refractive index of the core of the erbium-doped optical fiber was reduced and the MFD thereof was expanded. Further, based on the strain applied to the first optical fiber, that is, the tensile stress of 15 g, the refractive index of the core becomes 0.7.
% To 0.43%, and from this change in the refractive index, the MFD is from 7.06 μm to 9.95 μm.
Was estimated to be increasing.

Further, an erbium-doped optical fiber amplifier was constructed using the optical fibers connected in this manner, and its output conversion efficiency was measured. As a result, a high value of 55% was obtained.

Comparative Example A first optical fiber and a second optical fiber exactly the same as those used in the examples were prepared. These optical fibers were connected by a usual fusion splicing method. That is, at the time of fusing the first optical fiber and the second optical fiber, they were connected without applying any stress and solidifying without forming a residual stress portion at the fused portion. When the connection loss of the fusion spliced portion was examined, it was very large, 1 dB. Further, an erbium-doped optical fiber amplifier is configured using the optical fibers connected in this manner,
When the output conversion efficiency was measured, only a low value of 40% was obtained.

[0017]

As described above, according to the optical fiber connection method of the present invention, when the optical fibers having different MFDs are fusion-spliced, the fusion spliced portion is solidified in a state where a stress is applied. Since a stress residual portion is formed at the connection portion, thereby increasing the MFD diameter of one optical fiber, optical fibers having different MFDs can be connected with low transmission loss.

Claims (2)

(57) [Claims] 1. A method for connecting a first optical fiber comprising a core and a cladding having a softening temperature lower than that of the core to a second optical fiber having a mode field diameter different from that of the first optical fiber. Then, after the first optical fiber and the second optical fiber are fused, the fusion is performed at a temperature equal to or higher than the softening temperature of the cladding of the first optical fiber and equal to or lower than the softening temperature of the core of the first optical fiber. A method for connecting an optical fiber, comprising heating a portion and then cooling the fused portion while applying stress. 2. The optical fiber connection method according to claim 1, wherein the first optical fiber is a rare earth element-doped optical fiber obtained by adding a rare earth element to a core.