JPH11202139A - Optical fiber for grating, optical fiber preform for grating, and manufacture of same optical fiber preform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce transmission loss in gradation mode at the time of grating writing by suppressing the rounding of the refractive index distribution of the clad part of the optical fiber for grating. SOLUTION: The optical fiber consists of three layers of a core part 1, and a 1st clad part 2 and a 2nd clad part 3. In this case, germanium is added to the core part 1 and 1st clad part 2 and pure quartz or fluorine is added to the 2nd clad part 3 to obtain a stepwise refractive index distribution radially and give the 1st clad part 2a a <=0.15% relative index difference from the pure quartz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber for a grating, a preform for the optical fiber, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a 1.3 μm zero dispersion fiber (S
MF) and 1.55 μm zero dispersion fiber (DSF) are widely used as transmission optical fibers. A grating is provided in these fibers, and studies utilizing the wavelength selectivity of the grating have been actively conducted. The writing of the grating utilizes a phenomenon in which the fiber is masked with a mask substrate, ultraviolet light is irradiated from above, and the refractive index changes between a portion irradiated with the ultraviolet light and a portion not irradiated with the ultraviolet light. The mechanism is that when germanium doped in quartz glass is irradiated with ultraviolet rays, lattice defects increase in that portion, and the refractive index increases only in that portion. That is, the grating can be written only in the portion where germanium is doped.
When a grating is written in the SMF or DSF, a grating 4 is formed in the core 1 doped with germanium, as shown in FIG. However, the actual propagating light 5 does not propagate with the light confined in the core portion 1 but propagates slightly into the cladding portion 6. for that reason,
The manner in which light propagates depends on the part where the grating is written and the part where it is not. For example, if light of a certain wavelength is reflected at the grating portion and exhibits an effect as a filter, light of the wavelength propagating in the cladding will be transmitted without being reflected as it is, and as transmission loss It causes deterioration of characteristics. For the purpose of solving such a problem, Japanese Patent Application Laid-Open No. 8-290932 reports a method of adding germanium and fluorine to a cladding part to adjust the refractive index to a level of pure quartz. Since the increase in the refractive index is canceled out by fluorine, if the germanium concentration is high, the concentration distribution difference between the center and the outside increases, and the refractive index distribution becomes dull as shown in FIG. There is a problem that it is not possible to obtain something.

[0003]

An object of the present invention is to provide an optical fiber for a grating, which can suppress the dulling of the refractive index profile of the first cladding portion and can suppress the transmission loss due to the radiation mode at the time of writing the grating.
It is an object of the present invention to provide an optical fiber preform for grating and a method of manufacturing the optical fiber preform.

[0004]

According to the present invention, there is provided a core part, a first part.
It consists of three layers, a clad part and a second clad part. Germanium is added to the core part and the first clad, and pure silica or fluorine is added to the second clad part. A grating optical fiber, a core member, a first clad member and a second clad member, wherein the relative refractive index difference between the first clad portion and the pure quartz is 0.15% or less. Germanium is added to the core member and the first clad member, pure quartz or fluorine is added to the second clad member, and has a stepwise refractive index distribution in a radial direction, and pure quartz of the first clad member. An optical fiber preform for grating characterized by having a relative refractive index difference of 0.15% or less, and a core member that provides a desired refractive index in advance, and a glass member is provided around the outer periphery of the core member. Relative refractive index difference between pure silica after reduction is 0.15
% Or less of germanium-doped quartz glass fine particles to be the first clad member is deposited, vitrified by hydrogen gas treatment, and further, silica glass fine particles to be the second clad member are deposited and vitrified. The gist of the present invention is a method for manufacturing an optical fiber preform for grating.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.
FIG. 1A is a cross-sectional view in the radial direction of the optical fiber for grating of the present invention, which comprises three layers of a core part 1, a first clad part 2, and a second clad part 3, and the core part 1 and the first clad part. 2 is doped with germanium, and the second clad 3
Fig. 1 (b) is pure quartz or fluorine added thereto.
Has a step-like refractive index distribution in the radial direction as shown in FIG.
The cladding has a relative refractive index difference from pure quartz of 0.15% or less. FIG. 2 is a diagram showing the relationship between the writing range of the grating 4 and the propagation range of the propagation light 5 of the grating optical fiber of the present invention. Germanium is formed not only on the core 1 of the optical fiber but also around it. By doping the first cladding portion 2 with a small amount, the range of the grating 4 is expanded to cover the propagation range of the propagating light 5, and as a result, the transmission loss in the radiation mode due to the grating of the optical fiber is suppressed. In addition, the refractive index of the first cladding portion is suppressed to a specific refractive index difference of 0.15% or less from that of pure quartz and is made as uniform as possible, so that the refractive index profile can be made substantially rectangular. Was. 0.15% relative refractive index difference from pure quartz
If it exceeds, it will not be rectangular and will not function as a filter. The outer diameter ratio R ₁ / R ₂ of the outer diameter (R ₂₎ of the outer diameter (R ₁₎ and the first cladding portion of the core portion is preferably in the _{1 / 2~1 / 15, R 1} / R _{If 2} is more than 1/2, the transmission loss due to the radiation mode in the grating cannot be suppressed low, and if it is less than 1/15, the effect of further reducing the transmission loss cannot be obtained, and the inner cladding portion is further increased. Thickening is uneconomical.

An optical fiber preform for obtaining the optical fiber for grating by drawing is composed of three layers of a core member, a first clad member and a second clad member having the same structure as the optical fiber. Germanium is added to the first clad member, pure quartz or fluorine is added to the second clad member, has a stepwise refractive index distribution in the radial direction, and has a relative refraction of the first clad member with pure quartz. An optical fiber preform for a grating, wherein a rate difference is 0.15% or less. The outer diameter of the core member (R ₁ )
When the outer diameter ratio R ₁ / R ₂ of the outer diameter (R ₂₎ of the first cladding member is preferably set to 1 / 2-1 / 15 by the above reasons.

In the method of manufacturing a preform for an optical fiber for a grating according to the present invention, a core member having a desired refractive index is prepared in advance, and the relative refractive index difference between the vitrified pure quartz and the pure silica after the vitrification is 0.15%. Germanium-doped quartz glass fine particles are deposited so as to be the first clad member described below, and hydrogen gas treatment is performed when vitrifying the fine particles. Then, silica glass fine particles to be the second clad member or fluorine is added thereto to deposit the fine particles. And vitrify. In the above, the core member is
The first clad member and the second clad member can be manufactured by a method such as a VAD method, an OVD method, an MCVD method, or the like, and a method such as a VAD method, an OVD method, or jacketing.

[0008] The hydrogen gas treatment is performed to facilitate writing of the grating when it is made into a fiber. After the silica glass fine particles serving as the first cladding member are deposited, this is treated in an atmosphere of helium gas and hydrogen gas. 100 ° C
It may be carried out at a temperature of at least 1000 ° C., preferably at 200 to 800 ° C.

[0009]

EXAMPLES (Example) A core member having a relative refractive index difference of 0.4% from pure quartz was produced by doping germanium by the VAD method, and a first clad member was deposited on the outer periphery of the core member by the VAD method. . Germanium was doped so that the relative refractive index difference of the first clad member was 0.3% smaller than that of the core member and 0.1% larger than that of pure quartz. The outer diameter ratio of the core member to the first clad member was about 1/7.
Further, a second portion made of pure quartz on the outer periphery so as to have a desired outer diameter.
The clad member was deposited and vitrified to obtain an optical fiber preform. This preform was drawn to produce an optical fiber for grating having an outer diameter of 125 μm. FIG. 1 (b) shows the refractive index distribution in the radial direction of the fiber. The refractive index of the first cladding is a uniform rectangle 0.1% larger than that of pure quartz. When a grating was written on this fiber, the refractive index changed around the core to which germanium was added, and the grating was also written on the first clad.
Transmission loss due to the radiation mode could be suppressed.

Comparative Example 1 Quartz glass fine particles were deposited on the outer periphery of the same core member as in the example by a VAD method to form a first cladding layer. The relative refractive index difference of the first clad member was adjusted to the level of pure quartz by adding germanium and fluorine. The outer diameter ratio of the core member and the first clad member is about 1
/ 7. A second clad member was deposited and vitrified in the same manner as in the example to obtain an optical fiber preform. This was drawn to produce an optical fiber for grating having an outer diameter of 125 μm. Fig. 4 shows the refractive index distribution in the radial direction of the fiber. However, the refractive index distribution became dull and a uniform one could not be obtained. Did not.

Comparative Example 2 Fine silica glass particles were deposited on the outer periphery of the same core member as in the example by a VAD method to form a first clad member. The relative refractive index difference of the first clad member is:
Germanium was doped to obtain a refractive index 0.25% larger than that of pure quartz. The outer diameter ratio of the core member to the first clad member was about 1/7. A second clad member was deposited and vitrified in the same manner as in the example to obtain an optical fiber preform. Subsequently, this was drawn to produce an optical fiber for grating having an outer diameter of 125 μm. However, this fiber did not function sufficiently as a filter after grating because the relative refractive index difference of the first clad exceeded 0.15% with respect to pure quartz.

[0012]

According to the present invention, when light propagates through the portion of the fiber where the grating is not written, the light propagates to the core, the first cladding acts as a cladding, and the portion where the grating is written is formed. In the light propagation, since the grating is written in the core and the first clad, transmission loss due to the radiation mode can be suppressed. Further, in the conventional method of adding germanium and fluorine to the clad, the concentration distribution of germanium affects the refractive index profile and the profile becomes dull, but in the present invention, the refractive index profile is formed into a uniform rectangular shape. Since the shape can be made close to that and no fluorine is added, the cost can be reduced even in terms of raw materials.

[Brief description of the drawings]

FIG. 1A is a radial cross-sectional view of a grating optical fiber of the present invention, and FIG. 1B is a graph showing an example of a radial refractive index distribution of the grating optical fiber of the present invention.

FIG. 2 is a diagram showing a relationship between a grating writing range and a light propagation range of the optical fiber for grating of the present invention.

FIG. 3 is a diagram showing a relationship between a grating writing range and a light propagation range of a conventional grating optical fiber.

FIG. 4 is a graph showing an example of a radial refractive index distribution of a conventional grating optical fiber.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Core part 2 ... First clad part 3 ... Second clad part 4 ... Grating part 5 ... Propagation light 6 ... Clad part

Claims (5)

[Claims] 1. A core comprising a core, a first cladding and a second cladding, wherein the core and the first cladding are doped with germanium, and the second cladding is pure quartz or fluorine is added thereto. Has a stepped refractive index distribution in the radial direction, and has a relative refractive index difference from pure quartz of the first cladding portion.
An optical fiber for gratings, wherein the content is 0.15% or less. 2. An outer diameter ratio of a core part / a first clad part is 1 /.
2. The optical fiber for grating according to claim 1, wherein the ratio is 2 to 1/15. 3. A core member, a first clad member and a second clad member.
The cladding member is composed of three layers, the core member and the first cladding member are doped with germanium, the second cladding member is doped with pure quartz or fluorine, and has a radially stepped refractive index distribution. An optical fiber preform for a grating, wherein a relative refractive index difference between the first clad member and pure quartz is 0.15% or less. 4. A core member providing a desired refractive index is prepared in advance, and germanium-doped quartz glass is formed on the outer periphery of the core member as a first clad member having a relative refractive index difference of 0.15% or less from pure quartz after vitrification. An optical fiber preform for a grating, characterized in that fine particles are deposited, treated with hydrogen gas and vitrified, and further, silica glass fine particles of a second clad member are deposited and vitrified to form an optical fiber preform. Manufacturing method. 5. The method of manufacturing an optical fiber preform for a grating according to claim 4, wherein the hydrogen gas treatment comprises exposing the substrate to a temperature of 100 ° C. or more and less than 1000 ° C. in an atmosphere of helium gas and hydrogen gas.