JP2843654B2 - Thermal diffusion method of optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to effectively and homogeneously and extensively thermally diffusing a core dopant in an optical fiber to increase the mode field diameter of the optical fiber. The present invention relates to a method for thermally diffusing an optical fiber.

(Prior Art) Conventionally, as a method of connecting optical fibers, a fusion splicing method and a connector connecting method are generally used. In both connection techniques, the common point is that the axis alignment accuracy of the core inside the optical fiber mainly depends on the outer diameter accuracy of the optical fiber.

Therefore, when optical fibers having different outer diameters are connected to each other, a certain degree of axis deviation occurs, and it can be said that the magnitude of the connection loss is substantially determined by the amount of the axis deviation.

Marcuse analyzed analytically the relationship between the amount of misalignment of the optical fiber core and the splice loss.

FIG. 2 is a graph showing the relationship between the amount of misalignment of the optical fiber core and the connection loss with respect to a commonly used single mode optical fiber based on the Mercus theory. In FIG. 2, the horizontal axis represents the amount of axis deviation, and the vertical axis represents the connection loss. As the parameter, the mode field diameter w of the single mode optical fiber is selected.

As can be seen from FIG. 2, in order to reduce the connection loss, it is necessary to reduce the amount of axial deviation, and when a constant amount of axial deviation occurs, use an optical fiber having a mode field diameter w as large as possible. It is important to use.

By the way, in order to reduce the amount of axis deviation of the optical fiber, it is only necessary to improve the manufacturing accuracy of the optical fiber itself to geometrically improve the axial alignment accuracy.
It is extremely difficult to suppress the outer diameter deviation to 5 ± 1 μm or less. Also, the use of an optical fiber having a large mode field diameter w inevitably reduces the connection loss, but from the viewpoint of transmission characteristics and reliability, the size of the mode field diameter w is specified to be about 10 μm. .

Therefore, conventionally, attempts have been made to reduce the connection loss by heating only the vicinity of the connection point of the optical fiber and thermally diffusing the dopant in the optical fiber core to increase the mode field diameter w.

The expansion of the mode field diameter due to the thermal diffusion has a constant mode order because the value of the normalized frequency V is constant in principle, and it is possible to reduce the connection loss without changing the transmission characteristics. .

As a conventional specific thermal diffusion method, a method using electric discharge, a method using a gas burner, and the like are widely used.

FIG. 3 is an explanatory view of a method of heating an optical fiber by electric discharge. In the figure, reference numeral 1 denotes an optical fiber, 2 denotes a discharge electrode, and 3 denotes a discharge path. Discharge heating of the optical fiber 1 is widely used in a fusion splicing method of an optical fiber as is well known since a stable heating state can be obtained.

FIG. 4 is an explanatory view of a method of directly heating an optical fiber using a gas burner.
Is a gas burner and 5 is a flame. In this method, the flame 5
It is possible to heat the optical fiber 1 in a relatively wide range in the axial direction of the optical fiber 1 by the spread.

(Problems to be Solved by the Invention) Incidentally, in order to actually heat the optical fiber and enlarge the mode field diameter w, the optical fiber is heated to a certain extent over the axial direction of the optical fiber and uniformly. Further, it is necessary to keep the optical fiber at a temperature around the softening point at which heat diffusion occurs for a certain period of time.

However, for example, in the above-described method using discharge heating, the width of the discharge path 3 is relatively narrow, so the heated portion is extremely narrow, and the dopant can be thermally diffused only in this narrow range. Therefore, the propagation power passes through the connection point while maintaining the original shape without expanding the mode field in this portion. That is, there is a problem that it is difficult to reduce the influence of the axis deviation at the connection point.

In order to solve this problem, the optical fiber 1
The method using discharge heating cannot be a practical method, for example, it is necessary to move the discharge path 3 along the axis or to repeat the discharge intermittently.

In the method using direct heating with a gas burner, uniform diffusion is performed because the flame 5 itself is unstable, the temperature distribution inside the flame is large, and the temperature changes greatly due to a slight change in the combustion gas flow rate. Was difficult.

Further, when heating is performed by the gas burner 4, impurities contained in the combustion gas are removed by the gas flow,
That is, there is a problem that the surface of the optical fiber 1 is eroded, and the reliability of the optical fiber 1 is reduced.

The present invention has been made in view of such circumstances,
It is an object of the present invention to provide a method for thermally diffusing an optical fiber that can uniformly and widely expand the mode field diameter of the optical fiber.

(Means for Solving the Problems) In order to achieve the above object, in claim (1), the optical fiber is heated to a temperature at which the core causes thermal diffusion, and the temperature is maintained for a certain period of time. In a method for thermally diffusing an optical fiber in which a core dopant in a fiber is thermally diffused and its mode field diameter is enlarged, a heating plate is disposed near the optical fiber, and the optical fiber is heated by radiant heat from the heating plate. I made it.

In claim (2), the optical fiber is sandwiched between a plurality of heating plates while maintaining a constant interval.

Further, in claim (3), the heating plate has a cylindrical shape,
An optical fiber was inserted into the heating plate.

(Operation) According to claim (1), the temperature of the heating plate is kept equal to or higher than the temperature at which heat diffusion occurs in the core of the optical fiber throughout the heating state. The heating plate emits radiant heat from its surface according to the holding temperature.

Accordingly, the optical fiber is exposed to a uniform atmosphere over a wide range of the radiant heat of the heating plate, and is uniformly and stably heated over a predetermined length in the axial direction.

According to claim (2), the optical fiber is exposed to a mixed atmosphere of radiant heat generated by the plurality of heating plates.

According to claim (3), the optical fiber is exposed not only to a predetermined length in the axial direction but also to the uniform temperature atmosphere over the entire circumferential direction.

(Embodiment) FIG. 1 is a view for explaining a first embodiment of a method for thermally diffusing an optical fiber according to the present invention, and the same components as those in FIG. Is represented by That is, 1 is an optical fiber, 4 is a gas burner as a heating source,
5 is a flame of the gas burner 4.

Reference numeral 6 denotes a heating plate, in which the optical fiber 1 is disposed in the vicinity of the upper surface 61, for example, within several mm, and the gas burner 4
Is arranged. As the heating plate 6, for example, a plate made of metal such as platinum or tungsten, glass, or an inorganic material such as graphite or zirconia, or a ceramic plate is used. The range is selected to be the optimal range.

In the above configuration, the flame 5 of the gas burner 4
The lower surface 62 of the heating plate 6 is directly heated. The entire temperature of the heating plate 6 is raised to, for example, 1500 ° C. or more by heat conduction, and radiant heat is generated accordingly.

As a result, the optical fiber 1 is exposed to a uniform atmosphere due to the radiant heat from the upper surface 61 of the heating plate 6, for example, at 1300 ° C.
Heated to a degree.

This heating state is maintained for a predetermined time (for example, 10 to 15 minutes), and the dopant (for example, Ge) of the core in the optical fiber 1 is used.
Are thermally diffused. As a result, the mode field diameter of the optical fiber 1 is increased.

As described above, according to the first embodiment, the optical fiber 1 is not directly heated by the gas burner 4 by the flame 5, but is heated by the flame 5. Since the radiant heat is exposed to a uniform atmosphere over a wide range, even if the flame 5 of the gas burner 4 is slightly unstable in time, the optical fiber 1 is uniformly spread over a predetermined length in the axial direction. , And can be stably heated. That is, heat diffusion can be caused uniformly over a wide range.

Further, since the heating plate 6 is interposed, there is an advantage that an adverse effect such as erosion of the surface of the optical fiber 1 due to impurities in the combustion gas can be removed.

Also, as shown in FIG. 5, the method of the present invention can be applied to a multi-core optical fiber tape 1a. In this case, it is possible to heat the plurality of optical fibers 1 uniformly at once, which is extremely efficient.

In the first embodiment, the gas burner 4 is used as a heating source. However, the present invention is not limited to this.
It goes without saying that the same effect as described above can be obtained even if a resistor heater or a laser is used.

Further, in FIG. 5, the heating plate 6 is a flat plate, but the shape thereof may be a curved surface.
The same effect as the above effect can be obtained.

FIG. 6 is a view for explaining a second embodiment of the optical fiber heat diffusion method according to the present invention.

The second embodiment differs from the first embodiment in that a set of heating plates 6a, 6b and gas burners 4a, 4b are arranged at positions symmetrical about the axis of the optical fiber 1. is there.

In such a configuration, the optical fiber 1 can be exposed to the radiant heat atmosphere of the heating plates 6a and 6b from both sides of the optical fiber 1 and heated, so that in addition to the effect of the first embodiment, the output of the optical fiber 1 is small. Even if a gas burner is used, a high temperature state can be realized, and the temperature distribution can be kept more uniform.

It goes without saying that the second embodiment can be applied to a multi-core optical fiber tape.

FIG. 7 is a view for explaining a third embodiment of the optical fiber heat diffusion method according to the present invention, wherein 7 denotes a heating plate.

The difference between the third embodiment and the first embodiment is that a heating plate 7 processed into a cylindrical shape is used. Therefore, the optical fiber 1 is arranged so as to be inserted into the cylindrical heating plate 7.

According to the third embodiment, in addition to the effects of the first and second embodiments, the optical fiber 1 is exposed to a uniform temperature atmosphere not only in the axial direction but also in the entire circumferential direction. Therefore, more uniform heating is possible.

(Effects of the Invention) As described above, according to claim (1), the heating plate is disposed near the optical fiber, and the optical fiber is heated by radiant heat from the heating plate. Over a predetermined length in the axial direction, uniformly and
Heating can be stably performed, and a plurality of optical fibers can be heated uniformly and stably at once. Therefore, the mode field diameter can be easily and stably increased by the thermal diffusion of the optical fiber.

According to claim (2), since the optical fiber is sandwiched between the plurality of heating plates while maintaining a constant interval, in addition to the effect of claim (1), the temperature distribution can be kept more uniform. .

According to claim (3), the heating plate is formed in a cylindrical shape,
Since the optical fiber is inserted into the heating plate, the optical fiber can be exposed to a uniform temperature atmosphere not only in the axial direction but also in the entire circumferential direction, so that more uniform heating is possible.

[Brief description of the drawings]

FIG. 1 is a view for explaining a first embodiment of the optical fiber heat diffusion method according to the present invention, and FIG. 2 is a diagram showing the relationship between the axial deviation of the optical fiber core with respect to the single mode optical fiber and the connection loss. FIG. 3 is a diagram for explaining a conventional heat diffusion method by electric discharge, FIG. 4 is a diagram for explaining a conventional heat diffusion method by a gas burner, and FIG. 5 is a multi-core light of the method of the present invention. FIG. 6 is a view showing an example of application to a fiber tape, FIG. 6 is a view for explaining a second embodiment of the method of the present invention, and FIG. 7 is a view for explaining a third embodiment of the method of the present invention. is there. 1 in the figure: optical fiber, 1a: multi-core optical fiber tape,
4,4a gas burner, 5 flame, 6, 6a, 6b, 7 heating plate.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroaki Hanafusa Nippon Telegraph and Telephone Corporation, 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo (58) Field surveyed (Int. Cl. ⁶ , DB name) C03B 37 / 00-37/16 G02B 6/24

Claims (3)

(57) [Claims] (1) By heating an optical fiber to a temperature at which a core causes thermal diffusion, and maintaining the temperature for a certain period of time,
In a method for thermally diffusing an optical fiber, in which a core dopant in the optical fiber is thermally diffused and its mode field diameter is enlarged, a heating plate is disposed near the optical fiber, and the optical fiber is heated by radiant heat from the heating plate. A method for thermally diffusing an optical fiber, comprising: 2. A method for thermally diffusing an optical fiber according to claim 1, wherein said optical fiber is sandwiched between a plurality of heating plates at a predetermined interval. 3. The method according to claim 1, wherein the heating plate has a cylindrical shape, and an optical fiber is inserted into the heating plate.