JPH06337324A - Optical fiber coupler for optimum multiplexing and demultiplexing and production of the same - Google Patents

Abstract

PURPOSE:To improve the stability over the entire part of a system by providing the optical fiber coupler with polarization characteristics and the branching ratios to light of >=2 specific wavelengths within a specific range, thereby making it possible to improve the polarization characteristics of a fiber amplifier, to narrow the range to be controlled of a phase compensator and to obtain a stable optical output. CONSTITUTION:The polarization maintaining optical fiber 1 is formed by fusing and stretching to have the polarization characteristics and to have >=98/2 or <=2/98 branching ratio to the light of at least >=2 specific wavelengths. The polarization maintaining optical fiber 1 has the anisotropy of a refractive index distribution in the specific direction of the plane of polarization. The light propagation constant of the fiber varies with the one direction of the plane of polarization to be preserved and the direction of the plane of polarization orthogonal therewith. Namely, the polarization maintaining optical fiber 1 has the characteristics varying in the branching ratio to the light having the same wavelengths and the different direction of the plane of polarization by a difference in the light propagation constant depending upon the direction of the plane of polarization of the optical fiber 1, i.e., the optical fiber has the polarization characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber coupler for optical multiplexing and demultiplexing using a polarization maintaining optical fiber and a method for manufacturing the same. The optical fiber coupler of the present invention is an element that multiplexes and demultiplexes signal light and pumping light in an amplifier (fiber amplifier) used under conditions where polarization is controlled such as in coherent communication, or relay in constant polarization soliton transmission. It can be used as an element for multiplexing and demultiplexing in an operational amplifier.

[0002]

2. Description of the Related Art In an optical communication system, an optical amplification technique is becoming more important in order to compensate for a decrease in signal level due to optical branching. Amplifier (fiber amplifier) used under the condition that polarization is controlled such as coherent communication
A rare earth element-doped fiber amplifier provided at the input and output ends of the element for multiplexing and demultiplexing the signal light and the pumping light is used for the optical amplifier, and erbium (Er) is used for amplifying light in the 1.5 μm band.
A doped fiber is used, and E is excited by 1.48 μm excitation light.
Stimulated emission is performed by utilizing the transition within the r atom.

Conventionally, a fiber amplifier using a rare earth element-doped fiber has a structure as shown in FIG. That is, the signal light L _S in the 1.5 μm band and the pump light L _{E in the} 1.48 μm band are combined by the multiplexer 1 including an optical fiber coupler formed by the single mode fiber f having no polarization preservation property.
00, and is guided into the Er-doped fiber f _ER having no polarization maintaining property. The signal light L _S ′ amplified in the Er-doped fiber f _ER is wavelength-separated from the 1.48 μm pump light in the demultiplexer 102 including the optical fiber coupler formed of the single mode fiber f.

[0004]

At this time, the signal light L _S
Is usually a laser beam having a polarization characteristic, but in general, the polarization characteristic is disturbed in the multiplexer 100, the Er-doped fiber f _ER , and the demultiplexer 102, so that the demultiplexer 102 demultiplexes. The polarization characteristics of the output light that has been transmitted are eliminated, so that the phase-compensator 50 controls the polarization of the amplified signal light L _S ′ at the output stage.

However, in general, the phase compensator 50 has a drawback that it is complicated, is weak against a disturbance such as temperature fluctuation, and cannot rapidly follow a high speed or large polarization plane fluctuation. Further, if the control of the polarization characteristics in the phase compensator is disturbed, the optical output will not be finally obtained stably, and in the case of coherent optical transmission, noise will occur, limiting the transmission speed, which is a problem.

Therefore, an object of the present invention is to improve the polarization characteristics of the fiber amplifier, thereby reducing the range to be controlled in the phase compensator, obtaining a stable optical output, and improving the overall system. An object of the present invention is to provide an optical fiber coupler which can improve stability and can be used as a multiplexing / demultiplexing element in a relay amplifier in a constant polarization soliton transmission and a manufacturing method thereof.

[0007]

The above object can be achieved by an optical fiber coupler for optical multiplexing and demultiplexing and a manufacturing method thereof according to the present invention. In summary, the present invention is an optical fiber coupler formed by melting and extending a polarization-maintaining optical fiber, having a polarization characteristic, and having a branching ratio for light of at least two or more specific wavelengths. It is an optical fiber coupler for optical multiplexing / demultiplexing which is set to 98/2 or more or 2/98 or less. Preferably, the polarization maintaining optical fiber is a panda type polarization maintaining optical fiber.

Such an optical fiber coupler for optical multiplexing / demultiplexing is
(A) Prepare a polarization-maintaining optical fiber and remove a coating coating by a predetermined amount, (b) reduce the diameter of the polarization-maintaining optical fiber to a predetermined shape, and (c) parallel the polarization plane directions. The optical coupling portion is formed by melt-stretching the diameter-reduced portions of both fibers to form an optical coupling portion.

[0009]

The optical fiber coupler for optical multiplexing and demultiplexing and the method for manufacturing the same according to the present invention will be described in more detail with reference to the drawings.

An optical fiber coupler for optical multiplexing / demultiplexing according to the present invention is formed by melting and extending a polarization-maintaining optical fiber, has polarization characteristics, and is capable of responding to light of at least two or more specific wavelengths. The branching ratio is set to 98/2 or more or 2/98 or less. A polarization-maintaining optical fiber has anisotropy in the refractive index distribution in a specific polarization plane direction, and one optical polarization plane direction to be conserved and a polarization plane direction orthogonal thereto have different optical propagation constants.

That is, the optical fiber coupler for optical multiplexing and demultiplexing according to the present invention has a function of holding the polarization plane direction of polarized light having a polarization plane direction parallel to at least one fiber axis direction of the optical fiber. An optical fiber coupler in which an optical coupling part is formed by melting and extending a polarization-maintaining optical fiber having a wavelength difference of the optical propagation constant depending on the polarization plane direction of the polarization-maintaining optical fiber. It has a characteristic that the branching ratio is different for light having different polarization plane directions, that is, it has a polarization characteristic. In addition, when polarized light having a polarization plane direction parallel to at least one specific fiber axis direction of the polarization-maintaining optical fiber is incident, a branching ratio for light of at least two specific wavelengths is 98/2 or more, or It is an optical fiber coupler having a ratio of 2/98 or less, that is, a branching ratio of 100/0 or 0/100.

In the present invention, the polarization maintaining optical fiber is preferably a panda type polarization maintaining optical fiber.
In the panda-type polarization-maintaining optical fiber, light in a specific orthogonal polarization plane direction is polarization-maintained.

Next, an embodiment of a method for manufacturing the optical fiber coupler for optical multiplexing / demultiplexing according to the present invention will be described.

According to the present invention, preferably, the panda-type polarization-maintaining optical fiber is thinned to a predetermined shape with an etching agent, and then the thinned panda-type polarization-maintaining optical fiber is fused. An optical fiber coupler for optical multiplexing / demultiplexing is suitably manufactured by the optical fiber coupler.

To explain further, the panda-type polarization-maintaining optical fiber 1 may be a normal one having a structure as shown in FIG. 1A. For example, the diameter (D ₁ ) 4.8 μm, clad 3 diameter (D ₂ ) 125
A panda-type polarization-maintaining optical fiber 1 having a diameter of μm, a diameter (d ₁ ) of the stress applying portion 4 of about 35 μm, and an envelope diameter (d ₂ ) of the stress applying portion 4 of 95 μm can be preferably used.

The panda-type polarization-maintaining optical fiber is provided with respect to polarized light having a polarization direction parallel to either the fiber axis direction crossing the centers of the two stress applying portions 4 or the fiber axis direction perpendicular to the fiber axis direction. It holds the polarization plane direction.

As described above, in the polarization-maintaining optical fiber, with respect to the polarized light having the polarization plane direction parallel to the specific fiber axis direction, the fiber axis direction exhibiting the function of holding the polarization plane direction is polarized. It is called the polarization plane direction of the wave preservation optical fiber.

In the panda-type polarization-maintaining fiber, the above-mentioned two fiber axis directions are respectively the polarization plane directions of the polarization-maintaining optical fiber, but generally, the fiber axis direction crossing the centers of the two stress applying portions 4 is used. Is the X-axis polarization plane direction,
The fiber axis direction perpendicular to that is called the Y-axis polarization plane direction.

Further, the polarization plane direction of the polarization-maintaining optical fiber has anisotropy in the refractive index distribution in that direction, and a structure such as two stress applying portions 4 in the panda-type polarization-maintaining optical fiber is provided. It is brought about by something.

As the etching agent, a mixed solution of hydrofluoric acid and an aqueous solution of ammonium fluoride can be preferably used. The etching conditions can be variously changed depending on the desired etching amount of the panda-type polarization-maintaining optical fiber 1, but normally, the etching temperature is 30 to 60 ° C. and the processing time is 15 to 30.
Considered as minutes. By such an etching process, the panda-type polarization-maintaining optical fiber 1 has a major axis (a) in the direction in which the stress applying portion 4 is arranged and a minor axis in the orthogonal direction, as shown in FIG. 1 (B). The cladding 3 has a diameter (b) in a substantially elliptical shape, that is, in the direction in which the anisotropy of the refractive index distribution increases.
Is an optical fiber 1'having a reduced diameter. Usually, the major axis (a) is 50 to 80 μm and the minor axis (b) is 30 to 50 μm.
And the ratio a / b of the major axis (a) to the minor axis (b) is preferably 8/5 or more and 8/3 or less, more preferably 5 /.
Select 3 or more and 7/3 or less. When the ratio a / b is 8/5 or more and 8/3 or less, more preferably 5/3 or more and 7/3 or less, the major axis (a) generally has two stress-applying portions 4, and the minor axis In (b), most of the clad 3 is lost by etching, and as shown in FIG. 1 (B), stress is reduced during the fusion-stretching process between the parts of the minor axis (b). The anisotropy of the refractive index distribution formed by the imparting section 4 is maintained.

When the ratio a / b is larger than 8/3, the anisotropy of the refractive index distribution can be greatly increased by making the diameter smaller, but in the etching treatment for making the cladding 3 thin,
Very limited etching conditions or special etching methods must be used. When the ratio a / b is smaller than 8/5, for example, when the major axis (a) is 50 μm and the minor axis (b) is 40 μm, the etching conditions for reducing the diameter of the cladding 3 are limited. Alternatively, it is not necessary to use a special etching method, but the anisotropy of the refractive index distribution slightly increases due to the reduction in diameter.

Further, by the above-mentioned etching treatment, two stress-applying portions 4 are left in a substantially elliptic shape, that is, in the major axis (a), and most of the cladding 3 is lost due to the etching in the minor axis (b). The two optical fibers 1 ′ having a reduced diameter are aligned so that the polarization plane directions of the polarization-maintaining optical fibers are parallel to each other, and are fused to each other at the reduced-diameter portion short diameter (b). It is stretched.

The heating temperature in the fusion drawing is determined on the basis of the thickness (diameter or cross-sectional area) of the diameter-reduced portion of the optical fiber 1'having a substantially elliptical shape, and the normal panda-type polarization-maintaining method is used. The optical fiber 1 has a predetermined temperature within the range of 1200 to 1500 ° C. When the heating temperature is higher than 1500 ° C., the clad 3 is likely to be excessively softened, and the shape of the diameter-reduced portion of the optical fiber 1 ′ was a substantially elliptical shape before heating, but due to the surface tension, the fusion drawing It becomes a perfect circle inside, and the anisotropy of the refractive index distribution due to the stress applying portion 4 is significantly impaired,
Not preferable. If the heating temperature is lower than 1200 ° C,
The softening of the clad 3 does not proceed easily, and fusion drawing cannot be performed at a practical drawing speed, which is also not preferable. That is, as shown in FIG. 1B, when the cross section of the obtained optical coupling portion is viewed, each optical fiber 1 ′ is fused while maintaining its substantially elliptical shape, and the stress applying portion 4 after stretching. The heating temperature is selected so that the anisotropy of the refractive index distribution due to is maintained.

The optical fiber 1'having a substantially elliptical shape has a diameter (diameter or cross-sectional area) of its diameter-reduced portion such that the optical fiber 1'has a major axis (a) of 50 μm and a minor axis (b) of 30 μm. For the thin type, the heating temperature is selected in a relatively low range within the range of 1200 to 1500 ° C. Further, if the optical fiber 1 ′ has a large diameter (diameter or cross-sectional area) in its diameter-reduced portion, such as one having a major axis (a) of 80 μm and a minor axis (b) of 50 μm, The heating temperature is selected in a relatively high range of 1200 to 1500 ° C.

In the optical fiber coupler 10 obtained by the above-described manufacturing method, since the optical fiber 1'in the optical coupling portion retains the anisotropy of the refractive index distribution due to the stress imparting portion 4, polarization preservation is performed. The light propagation constant differs between the polarization plane direction of the optical fiber and the polarization plane direction orthogonal to the polarization plane direction.

In the present embodiment, it was immersed in a mixed saturated solution of hydrofluoric acid and an aqueous solution of ammonium fluoride at a temperature of 50 ° C. for 20 minutes. By this etching process, the panda-type polarization-maintaining optical fiber 1 has a major axis (a) of 60 μm and a minor axis (b) of 30.
The optical fiber 1 ′ has an elliptical shape of μm. The etched fiber length was about 30 mm.

Then, the panda-type polarization-maintaining optical fiber 1 ′ thus reduced in diameter is aligned in parallel with the X-axis polarization plane direction as shown in FIG. According to the method, for example, while heating to 1200 to 1500 ° C. using a heat source such as hydrogen or an oxyhydrogen burner, pulling is performed at a constant drawing speed of 0.5 to 10 mm / min, and a fusion drawing process is performed.
The optical fiber coupler 10 for optical multiplexing / demultiplexing is manufactured. In this example, the heating temperature was 1400 ° C. and the stretching speed was 1.0 mm.
/ Min.

In the process of melt drawing, white light is input from one end of one fiber, and the output light is guided to the optical spectro-analyzer from the end of the fiber opposite to the input side, and the light intensity of the output light is measured. After the stretching is started, light of a specific wavelength, for example, a wavelength of 1.48 μm band starts to be coupled, the light emitted to the other fiber side becomes maximum, and the optical branching ratio is substantially 1
The time when the completely bonded state of 00/0 or 0/100 is set as the stopping point of stretching. In this example, the stretching amount is 8 mm,
The optical coupling length formed by fusion drawing was 20 mm.

Further, when the optical fiber coupler 10 for optical multiplexing / demultiplexing according to the present invention is used in a fiber amplifier, the configuration shown in FIG.
As shown in FIG. 2, a panda-type polarization-maintaining optical fiber 1 having a predetermined fiber length used for fusion splicing with an optical amplification region and another panda-type polarization-maintaining optical fiber 1 are provided at two positions at both ends of the fiber amplifier. It is possible to form a pair of optical fiber couplers 200 serving as the input / output ends of the fiber amplifier.

Of the four terminals of the optical fiber coupler 200, three terminals are used for input / output terminals, and the remaining one terminal not used for input / output terminals is processed according to the purpose. A fiber amplifier is constructed by fusion-splicing the pair of optical fiber couplers 200 and the Er-doped polarization maintaining optical fiber 22 which is the optical amplification region of the fiber amplifier.

A fusion splicing portion 204 for fusion splicing the input / output ends of the optical fiber coupler 200 and the Er-doped polarization-maintaining optical fiber 22, and the input signal L _S or the amplified signal to be output. At the fusion splicing portion 206 that fusion splices the polarization maintaining optical fiber 20 that guides the light L _S 'and the other input / output ends of the optical fiber coupler 200, two polarization maintaining optical fibers that are fusion spliced respectively. Match the deflection plane directions of.

The optical coupling portion of the optical fiber coupler is housed and protected in a package suitable for the usage of the fiber amplifier.

FIG. 4 shows the wavelength separation (isolation) characteristics when polarized light is input from one incident end of the optical fiber coupler 10 for optical multiplexing / demultiplexing thus obtained in parallel to one polarization plane direction. Show.

From FIG. 4, when polarized light having a polarization plane direction parallel to one of the polarization plane directions of the panda-type polarization-maintaining optical fiber is input, at least a wavelength band of 1.48 μm and a wavelength band of 1.55 μm are different. For light of one wavelength, the branching ratio is 98/2 or more or 2/98 or less, and can be regarded as substantially 100/0 or 0/100,
It is possible to combine and demultiplex light having wavelengths of 1.55 μm band and 1.48 μm band. At the same time, at one output end,
It can be seen that the ratio of the branching ratio between the light in the wavelength band of 1.55 μm and the light in the wavelength band of 1.48 μm, that is, the wavelength separation degree is 30 dB or more.

On the other hand, when the polarized light having a polarization plane direction orthogonal to the polarized light for which the wavelength separation characteristic of FIG. 4 is measured and the wavelength separation characteristic is measured (not shown), the branching ratio is substantially 100 / The wavelengths that can be regarded as 0 or 0/100 are light in the wavelength 1.47 μm band and wavelength 1.53 μm band, and the wavelength dependence of the branching ratio was different depending on the polarization plane direction.

In addition, the light of wavelength 1.55 μm band having the same polarization plane direction as used in the measurement of the wavelength separation characteristic of FIG.
Table 1 shows the characteristics of the optical fiber coupler 10 for optical multiplexing and demultiplexing when using light in the 48 μm band.

[0037]

[Table 1] Note) Extinction ratio: The intensity of the polarization direction component parallel to the polarization plane direction of the incident light at the exit end when the polarization plane is aligned with the specific fiber axis direction from the entrance end, and perpendicular to it. It is the ratio of the light intensities of the various polarization direction components.

[0038]

The optical fiber coupler for optical multiplexing and demultiplexing according to the present invention configured as described above is formed by melting and extending a polarization maintaining optical fiber such as a panda type polarization maintaining optical fiber. Since it has wave characteristics and the branching ratio for light of at least two specific wavelengths is 98/2 or more or 2/98 or less, the polarization characteristics of the fiber amplifier are improved,
As a result, the range to be controlled in the phase compensator can be reduced, and a stable light output can be obtained,
It has the advantage of improving the stability of the entire system and being able to be used as a multiplexing / demultiplexing element in a relay amplifier in constant polarization soliton transmission.

Further, according to the manufacturing method of the present invention, the optical fiber coupler for optical multiplexing and demultiplexing can be manufactured with good reproducibility and very easily.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a panda type polarization maintaining optical fiber used for manufacturing an optical fiber coupler for optical multiplexing and demultiplexing according to the present invention and a manufacturing method thereof.

FIG. 2 is a diagram showing an example of a fiber amplifier using the optical fiber coupler for optical multiplexing / demultiplexing of the present invention.

FIG. 3 is a diagram showing an example of a conventional fiber amplifier.

FIG. 4 is a diagram showing isolation characteristics of the optical fiber coupler for optical multiplexing and demultiplexing according to the present invention.

[Explanation of symbols]

 1, 20 Polarization-maintaining optical fiber 10 Optical fiber coupler for optical multiplexing / demultiplexing 200 Optical fiber coupler

Claims (4)

[Claims] 1. An optical fiber coupler formed by melting and extending a polarization-maintaining optical fiber, having a polarization characteristic, and having a branching ratio of at least two with respect to light of a specific wavelength of 9 or more.
An optical fiber coupler for optical multiplexing and demultiplexing which is set to 8/2 or more or 2/98 or less. 2. The optical fiber coupler for optical multiplexing and demultiplexing according to claim 1, wherein the polarization maintaining optical fiber is a panda type polarization maintaining optical fiber. 3. (a) preparing a polarization maintaining optical fiber and removing a coating coating by a predetermined amount; (b) reducing the diameter of the polarization maintaining optical fiber to a predetermined shape;
(C) A method for manufacturing an optical fiber coupler for optical multiplexing / demultiplexing, wherein the diameter-reduced portions of the both fibers are melt-stretched with the polarization plane directions aligned in parallel to form an optical coupling portion. 4. The method for manufacturing an optical fiber coupler for optical multiplexing and demultiplexing according to claim 3, wherein the polarization maintaining optical fiber is a panda type polarization maintaining optical fiber.