JPH087284B2 - Leaked optical fiber and its manufacturing method - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a leaky optical fiber and a method for manufacturing the same.

[Prior art]

Conventionally, a leaky coaxial cable is known as a signal transmission line using a conductor. This leaky coaxial cable is one in which a slit-shaped hole is intermittently provided in the outer metal jacket of the coaxial line over the entire length of the cable, and the signal propagating in the coaxial cable is leaked little by little in the length direction by this hole, It is designed so that a signal from the outside can be taken into a coaxial cable at an arbitrary point and propagated.

[Problems to be Solved by the Invention]

However, regarding the optical fiber that is the transmission path of the optical signal, the light propagating in the optical fiber is extracted from any position in the length direction of the optical fiber, or the light from the outside is propagated in the propagation mode. No effective one has been established as a leaky optical fiber that can be coupled to the optical fiber. This is because the optical fiber has been studied so far only in the direction of reducing the loss.
That is, the optical signal propagating in the optical fiber has been mainly studied only in the optical fiber that can be propagated with a low loss from the transmitting end to the receiving end without leaking as much as possible during the propagation. The present invention provides a leaky optical fiber capable of leaking light propagating in an optical fiber at an arbitrary position in the length direction of the optical fiber and coupling external light to a propagation mode of the optical fiber. It is an object of the present invention to provide a manufacturing method capable of easily and reliably manufacturing a leaky optical fiber.

[Means for solving problems]

The leaky optical fiber according to the present invention is characterized in that a layer having a non-uniform fluorine concentration is formed near the boundary between the core and the cladding. Further, the method for producing a leaky optical fiber according to the present invention comprises a step of depositing glass particles containing glass particles having a particle size of 0.5 μm or more on a transparent glass rod, and then the obtained glass rod and glass particle deposition. Heat treating the composite preform of the layer in a fluorine-containing atmosphere.

[Action]

In an optical fiber, if a layer having a non-uniform fluorine concentration is formed near the boundary between the core and the cladding, fluctuations in the refractive index due to the non-uniform fluorine concentration remain and scattering loss increases. The scattered light leaks to the outside, and the outside light is coupled with the propagation mode in the optical fiber. Therefore, an optical fiber in which such a layer having a non-uniform concentration of fluorine is formed in the length direction can be used as a leaky optical fiber. Optical fiber with fluorine added to the clad (of course,
It does not prevent adding fluorine to the core,
Conventionally, the following method is known as one effective method for producing (if necessary, fluorine may be added to both the core and the clad). First of all, a transparent glass rod to be the core is prepared. The material of this glass rod is pure quartz glass,
Alternatively, the required amount of additive (eg, germanium,
It is possible to use a silica glass to which phosphorus, a small amount of fluorine, or the like which is generally used for producing a silica glass optical fiber) is added. Next, fine glass particles are generated by a flame hydrolysis method or a thermal oxidation method, and these are deposited on the above glass rod. Further, a composite preform obtained by depositing such glass particles (a preform having a glass particle layer around a transparent glass rod at the center) is heat-treated in a high-temperature fluorine-containing atmosphere to form glass particles in the glass particles. In addition to the addition of fluorine, in the next step, a high-temperature atmosphere is created, and a transparent glass rod is entirely formed, thereby obtaining an optical fiber preform. While the inventors of the present invention have been engaged in research on the conventional method for producing a fluorine-doped optical fiber as described above, as a result of numerous experiments, the particle diameter of deposited glass fine particles and the final loss of the optical fiber are extremely low. It has been found to have a large correlation with. That is, a glass rod and a glass particle deposition layer are deposited by changing the particle diameter of glass particles to form a composite preform, and an optical fiber is produced via this composite preform, and the loss with respect to the particle diameter of the glass particles is measured. As a result, the results shown in FIG. 8 were obtained. From FIG. 8, there is a correlation between the loss of the optical fiber and the particle size of the deposited glass particles, and the loss of the optical fiber can be rapidly increased by setting the average particle size to 0.15 μm or more. Do you get it. The reason can be interpreted as follows. Considering the case where the glass particle layer 84 is deposited on the transparent glass rod 81 as shown in FIG. 9, depending on the size of each glass particle 85 and the method of adhesion or fusion bonding to the glass rod 81, It seems that the state of fluorine diffusion in the process of adding fluorine is different. For example, as shown in FIG. 9, when the glass fine particles 85 have different sizes, fluorine cannot penetrate into the inside of the glass particles having a very large particle diameter, and the fluorine concentration is shown in a shaded area. It becomes high at the outside, that is, it is high on the outside and low on the inside, so it is not uniform. This is because the diffusion distance of fluorine in quartz glass is at most 0.1μ under normal heating conditions.
This is because it is estimated to be about m. Therefore, when glass particles with a large particle size adhere to the surface of the transparent glass rod,
At the interface between the core and the clad of the finally obtained optical fiber, the fluctuation of the refractive index due to the non-uniform concentration of the added fluorine remains. If such fluctuations in the refractive index at the boundary between the core and the clad remain, so-called scattering loss increases, and it can be used as a leaky optical fiber. As described above, in the experiments by the present inventors (FIG. 8), it was concluded that the amount of scattered light from the optical fiber can be increased by depositing glass particles having a large particle size. When observing the glass particles deposited in the immediate vicinity of the glass rod with an electron microscope, light spun from a preform manufactured under the manufacturing conditions that many glass particles with a particle size of 0.5 μm or more are deposited near the core-clad boundary surface. It was found that in the fiber, the propagation mode and the external light are fairly uniformly coupled, and the average particle size is not necessarily large and the particle size distribution may be wide. Of course, the transmission loss of an optical fiber manufactured under such a large grain size is larger than that of an optical fiber that has no refractive index fluctuation near the core-clad boundary. This is natural as long as it is a fiber. The deposition of glass particles having such a large particle size is sufficient only in the vicinity of the glass rod, and it is not necessary to increase the particle size of the entire deposited layer. For example, when the final optical fiber has a core diameter of 12 μm, the thickness of the clad glass layer formed from the deposited layer of glass particles having a large particle diameter is half the core diameter or the same thickness as the core diameter. All you have to do is The glass outside the thickness of the clad portion may be formed from a glass fine particle deposition layer having an average particle size of 0.1 μm or less under normal conditions.

【Example】

FIG. 1 shows a leaky optical fiber 1 according to an embodiment of the present invention. As shown in FIG. 1, a layer in which the fluorine additive concentration is made non-uniform near the boundary between the core 11 and the clad 12. 13 are provided. In such a non-uniform additive concentration layer 13, fluctuations in the refractive index occur, thereby increasing the loss and leaking the light propagating through the core 11 of the optical fiber 1. On the contrary, external light can be coupled to the propagation mode in the optical fiber 1 through the additive concentration nonuniform layer 13. Therefore, such a leaky optical fiber 1 can take out light at an arbitrary position in the optical fiber length direction as shown in FIG. That is, in FIG. 2, when the light from the light source 2 is incident on one end of the leaky optical fiber 1 through the lens 3, the light propagating in the optical fiber 1 is
Since the light leaks in the length direction, the leaked light can be focused from an arbitrary position through the lens 4 and the like, guided to the light receiver 5, and a signal can be sent to the light receiving circuit 6. On the contrary, as shown in FIG. 3, the light source 2 connected to the transmission circuit 7 is placed at an arbitrary position in the length direction of the leaky optical fiber 1, and light is incident from around the optical fiber 1 via the lens 3 and the like. And can be coupled to the propagation mode in the optical fiber 1. The light thus propagating in the optical fiber 1 is emitted from one end and guided to the light receiver 5 via the lens 4. Since light leaks around the entire circumference of the leaky optical fiber 1, it is possible to efficiently receive light by using a cylindrical light receiver 51 as shown in FIG. Although not shown, the same applies to the case of using a cylindrical light source in the case of FIG. Further, in the leaky optical fiber 1 of FIG. 1, the layer 13 having a non-uniform fluorine additive concentration is formed on the entire circumference of the optical fiber 1, but as shown in FIG. You may do it. In this case, since optical coupling is possible only in the direction in which the nonuniform concentration layer 13 is formed, efficient optical coupling can be achieved by arranging a light receiver or a light source in that direction. In such a leaky optical fiber, the larger the transmission loss is, the larger the amount of light leaked to the outside is. Therefore, the attenuation of the light in the course of propagation becomes large, and the transmission distance becomes shorter. Therefore, how much light is leaked to the outside is determined by the transmission distance and the receiving ability of the light receiver outside the optical fiber. In a very simple approximation, Po> Pi × exp (−αL) × α · ΔL · C must be assumed because most of the transmission loss of the optical fiber is due to the scattering loss near the core / cladding boundary. Where Po is the minimum received power, Pi is the incident power, α is the optical fiber transmission loss coefficient (neper /
m), L is the distance from the incident end, ΔL is the length of the optical fiber that contributes to the reception of light, and C is the proportion of light that is actually coupled to the light receiver. For example, in the second example, the incident power
Pi = 1 mW, α of leaky optical fiber 1 = 20 neper / km, L = 0.1
When km, ΔL = 0.0001 km, and C = 0.1, the minimum power Po that can be received by the light receiver 5 is Po = −45 dBm. Next, a method of manufacturing such a leaky optical fiber will be described. First, as shown in FIG. 6, glass fine particles generated in the flame 83 of the burner 82 are attached to the periphery of the transparent glass rod 81 to form a glass fine particle deposition layer 84. In this example, as the glass rod 81, a pure quartz glass rod having a diameter of 10 mm and having a transparent and smooth surface was used. Burner 82
Hydrogen, oxygen, silicon tetrachloride, and argon were fed into the reactor, and the burner 82 was traversed a plurality of times in parallel with the axis of the glass rod 81 to deposit large glass particles having a particle size of 15 mm. The flow rate condition of each gas at this time is
Hydrogen: 8 l / min, oxygen: 15 l / min, silicon tetrachloride: 300 cc / min, argon: 500 cc / min. Hydrogen is a fuel for combustion, oxygen is a combustion improver, and silicon tetrachloride is a raw material gas that becomes glass. The maximum surface temperature of the glass particulate deposition layer during the deposition was measured and found to be 1320 ° C. Next, the flow rates of oxygen and hydrogen are decreased together with the flow rate of the raw material gas to successively deposit glass fine particles, and finally, a composite plate having a diameter of 100 mm (consisting of the glass rod 81 and the glass fine particle deposition layer 84 thereon) is formed. A reform was made. This composite preform was heat-treated in a heating furnace at a furnace temperature of 1000 ° C. in a mixed gas atmosphere of fluorine-containing gas, sulfur hexafluoride, and helium gas. The heat treatment time was about 2 hours. This heat treatment is the first stage, and its function is to infiltrate the fluorine-containing gas into the porous glass portion in order to add fluorine into the clad. Also,
At the temperature of the first stage, a considerable part of the fluorine-containing gas such as sulfur hexafluoride is decomposed, and it is considered that activated fluorine atoms are generated to some extent. It also has the effect of removing the residual OH groups. In the next second step, the temperature of the heating furnace is raised to about 1520 ° C., and the composite preform which has been subjected to the first step heat treatment as described above is made into transparent glass. At this time as well,
A fluorine-containing atmosphere is set, specifically, an atmosphere in which 10 parts of sulfur hexafluoride is set to 90 parts of helium. When an optical fiber was spun from the preform thus produced, its transmission loss wavelength characteristic was as shown in FIG. Although several methods are known as a method for depositing glass fine particles, in the above, oxide fine particles are generated by an oxidation reaction or a hydrolysis reaction in a flame obtained by burning hydrogen or natural gas. A method was adopted in which a raw material that would be produced was sent as a gas to generate glass fine particles. In this case, as the glass raw material gas, a metal halide of Group III, Group IV, Group V, such as silane trichloride, germanium tetrachloride, phosphorus oxychloride, boron tribromide, etc., or a partial hydrogen compound is used. can do. In the case of such a glass particle deposition method, generally, between the particle diameter of the generated glass particles and the condition of the burner, the particle diameter tends to increase as the temperature of the burner increases. The particle size tends to increase as the concentration of the generated source gas increases. The smaller the distance to the surface of the growing composite preform that is the deposition target, the smaller the particle size tends to be. Since the particle size tends to decrease as the flow rate of the glass particles generated in the burner increases, the particle size of the deposited glass particles can be controlled by using this relationship.

【The invention's effect】

According to the leaky optical fiber of the present invention, the light propagating in the optical fiber is leaked at an arbitrary position in the length direction of the optical fiber, or the external light is coupled to the propagation mode of the optical fiber. Further, according to the manufacturing method of the present invention, such a leaky optical fiber can be manufactured simply and reliably.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of a leaky optical fiber according to the present invention, FIGS. 2, 3, and 4 are schematic perspective views showing examples of use of this leaky optical fiber, and FIG. FIG. 6 is a cross-sectional view of another embodiment, FIG. 6 is a schematic perspective view of one embodiment of the manufacturing method according to the present invention, and FIG. 7 is a graph showing loss wavelength characteristics of the optical fiber obtained in the same embodiment. FIG. 8 is a graph showing the correlation between the particle size of deposited glass particles and the loss, and FIG. 9 is an enlarged cross-sectional view showing the fluorine concentration distribution. 1 ... Leakage optical fiber, 2 ... Light source, 3, 4 ... Lens, 5 ... Photoreceiver, 6 ... Photodetector circuit, 11 ... Core, 12 ...
… Cladding, 13 …… Layer with non-uniform concentration of fluorine additive, 81 ……
Glass rod, 82 …… Burner, 83 …… Fire flame, 84 …… Glass particle deposit layer, 85 …… Glass particle.

Claims (4)

[Claims] 1. A leaky optical fiber, wherein a layer having a non-uniform concentration of fluorine is formed near the boundary between the core and the cladding. 2. The leaky optical fiber according to claim 1, wherein a layer having a non-uniform fluorine concentration near the boundary between the core and the clad is provided in the entire circumferential direction of the optical fiber. 3. The leaky optical fiber according to claim 1, wherein a layer having a non-uniform concentration of fluorine added near the boundary between the core and the cladding is provided in a part of the optical fiber circumferential direction. . 4. A step of depositing glass particles containing glass particles having a particle size of 0.5 μm or more on a transparent glass rod,
Next, a step of heat-treating the obtained composite preform of the glass rod and the glass particle deposition layer in a fluorine-containing atmosphere is provided.