JP3157000B2 - Optical waveguide - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide using quartz glass suitable for a fiber type optical amplifier or the like.

[Conventional technology]

An erbium-doped optical fiber is known as a fiber-type optical amplifier. However, the erbium-doped optical fiber has a relatively large wavelength dependence in its operation characteristics, and there is a disadvantage that the operation gain is largely changed by a slight change in the wavelength of the communication light source.

For this reason, recently, an optical fiber doped with both aluminum and erbium has been developed as one having a small wavelength dependency and capable of solving the above-mentioned disadvantages.

As a preferred method for producing such quartz glass doped with aluminum and erbium, the present inventors have
He has previously proposed Japanese Patent Application No. 2-49100.

However, in the case of quartz glass doped with aluminum, the melt viscosity increases, and the sintering temperature for sintering the aggregate of quartz glass fine particles (soot) to form a transparent glass must be increased. In the sintering furnace used, the amount of aluminum added was 0.5w from the upper limit of the sintering temperature.
t% or less. Further, even when the addition amount of aluminum is 0.5 wt% or less, the sintering temperature is high as described above, so that there is a disadvantage that the life of the quartz muffle is shortened.

[Problems to be solved by the invention]

Therefore, an object of the present invention is to provide an optical waveguide using quartz glass obtained by a method capable of reducing the sintering temperature when producing quartz glass in which erbium and aluminum are co-added.

[Means for solving the problem]

This problem can be solved by using a quartz glass obtained by doping the aggregate of silica glass fine particles with aluminum and erbium and then sintering the aggregate of fine particles in a fluorine atmosphere.

Hereinafter, the present invention will be described in detail along with the operation of the manufacturing method suitable for the optical waveguide of the present invention.

First, an aggregate of silica glass fine particles to be a base is prepared. This aggregate of silica glass fine particles is obtained by a well-known gas phase synthesis method such as a VAD method or an OVD method, and is made of silicon oxide or silicon oxide and germanium oxide, boron oxide synthesized by a thermal oxidation reaction or a flame hydrolysis reaction. It is a porous body formed by depositing fine particles made of a mixed oxide with a dopant such as

The shape of this aggregate of silica glass fine particles is a rod-like shape in which quartz glass fine particles are deposited in the axial direction at the tip of a rod-shaped starting base material if the VAD method is used, and a rod-like shape if the OVD method is used. After the silica glass particles are deposited on the outer peripheral surface of the starting base material in the radial direction, the starting base material is drawn out, but is not limited thereto.

This aggregate of silica glass fine particles has a bulk density of 0.4 to
It is desirable to be within the range of 0.7 g / cm ³ , and it is preferable that the bulk density is uniform between the central part and the surface part.
Bulk density insufficient mechanical strength is less than 0.4 g / cm ^3, not withstand the handling of such penetration operations alcohol solution of aluminum erbium chloride in the next step, the alcohol exceeds 0.7 g / cm ³ The solution does not quickly and sufficiently penetrate into the central part of the aggregate. Bulk density 0.4 g / cm ³
If it is less than 4, it can be increased to a range of 0.4 to 0.7 g / cm ³ by performing a heat treatment in an inert atmosphere such as helium or argon. The heating temperature for this depends on the type of glass constituting the aggregate of silica glass fine particles.
In the range of 00 ° C, 70% for glass composed of quartz glass with germanium oxide, boron oxide, etc. added.
Heat treatment is performed in the range of 0 to 1100 ° C. By this heat treatment, the surface of the glass fine particles forming the aggregate of silica glass fine particles is melted, and the gap between the glass fine particles is reduced, and accordingly, the bulk density is increased to about 0.4 to 0.7 g / cm ³ .

In addition, in the aggregate of silica glass fine particles obtained by the ordinary VAD method, the bulk density of the central portion is high (0.4 to 0.4).
About 5 g / cm ³ ), and the surface area tends to be low (about 0.25 g / cm ³ ). Therefore, good results are not obtained in performing uniform penetration of the alcohol solution. For this reason, the VAD which can obtain the bulk density of the aggregate of silica glass fine particles between the central part and the surface part
The method is preferable, for example, when forming an aggregate of silica glass fine particles, the temperature difference between the temperature of the central portion and the temperature of the surface portion of the glass fine particle deposition site of the aggregate, by using an additional heating burner or the like, By adopting a method of keeping the temperature within 100 ° C., a quartz glass particle aggregate having a uniform bulk density can be obtained. It is also possible to increase the bulk density of only the surface portion by heating the aggregate of silica glass fine particles obtained by the ordinary VAD method several times at a temperature close to the melting temperature of the glass in a short time.

Separately, an alcohol solution prepared by mixing and dissolving aluminum chloride and erbium chloride is prepared. As aluminum chloride, anhydrous or hydrated salts of trichloride are used. As erbium chloride, an anhydrous salt of trichloride is used. Further, as the alcohol, methanol, ethanol, isopropyl alcohol, butanol and the like are used. Preferably, a monohydric alcohol having 2 to 5 carbon atoms is used. The concentration of aluminum chloride and erbium chloride is determined by the amount of dopant added, and is not limited, but is usually 1 to 30% by weight for aluminum chloride and 0.1 to 1% by weight for erbium chloride.
Degree. Also, aluminum chloride and erbium chloride are dissolved in the same or different alcohols, respectively.
These two solutions may be mixed at an appropriate mixing ratio.

Next, a mixed alcohol solution of the thus obtained aluminum chloride and erbium chloride is permeated into the aggregate of silica glass fine particles. As the infiltration operation, the simplest method is to immerse the aggregate in a mixed alcohol solution for about 0.5 to 24 hours. It is also possible to drop, apply, or spray a mixed alcohol solution on the aggregate. Further, before the infiltration operation, the aggregate is left in a vacuum, and the gas in the void of the aggregate,
The penetration of the mixed alcohol solution may be promoted by sucking and eliminating water and the like.

The aggregate impregnated with the mixed alcohol solution in this manner is then placed in an inert gas atmosphere at 70 to 10
Heat at 0 ° C for 96 hours or more to remove alcohol sufficiently. Of course, drying under reduced pressure can be used in combination.

Next, the dried aggregate is subjected to a heat treatment in a heating furnace such as an electric furnace while flowing chlorine gas to remove water (hydroxyl groups) remaining in the aggregate.

Next, this assembly is heat-treated in a fluorine atmosphere,
The quartz glass fine particles are doped with fluorine, and the quartz glass fine particles are melted and made vitrified to obtain a quartz glass body made of quartz glass doped with erbium, aluminum and fluorine.

As the fluorine compound for forming the fluorine atmosphere, SiF ₄ , CF ₄ , SF ₆ , C ₂ Cl ₂ F _{2 and the} like are used.
A fluorine atmosphere can be formed by introducing the seed or mixture as it is or by diluting it with an inert gas such as argon or helium into a heating furnace. Heating temperature is 1400
A temperature in the range of ℃ 1500 ° C. is sufficient, and transparent vitrification can be performed even if the temperature does not exceed 1500 ° C.

In such a method for producing quartz glass, the aggregate of quartz glass particles to which erbium and aluminum are added is heated in a fluorine atmosphere to add fluorine to lower the melt viscosity of the glass. Sintering temperature can be lowered.

By the way, the erbium, aluminum, and fluorine-doped quartz glass obtained in this manner have a refractive index that is reduced according to the doping amount due to fluorine doping.
For this reason, in the optical fiber in which the silica glass is used as the core and the fluorine-doped silica glass is used as the clad, the relative refractive index difference cannot be increased.
It is difficult to obtain a small mode field diameter (MFD) required for obtaining a high gain fiber optical amplifier.

Therefore, in the optical waveguide according to the present invention, in order to solve such inconvenience, an inner core body is formed from quartz glass doped with aluminum, erbium, and fluorine, and the three-component doped quartz is formed around the inner core body. An outer core body made of glass having a higher refractive index than glass is provided, and a clad body made of glass having a lower refractive index than glass forming the outer core body is provided around the outer core body.

Specifically, the aluminum, erbium, or fluorine-doped quartz glass body obtained as described above is stretched into a stretched rod, and a glass having a high refractive index serving as an outer core body is formed on the stretched rod by an external method or the like. For example, pure quartz glass or the like is provided. Next, a glass having a low refractive index, for example, a fluorine-doped quartz glass, which becomes a clad body, is provided on the outer core body by an external method or the like. Next, this rod is spun into a fiber by a conventional method.

FIG. 1 shows an example of a refractive index distribution of a fiber obtained by such a method, wherein reference numeral 1 denotes an inner core made of aluminum, erbium and fluorine-doped quartz glass, and reference numeral 2 denotes a pure silica glass. Reference numeral 3 denotes an outer core, and reference numeral 3 denotes a clad made of fluorine-doped quartz glass, and the relative refractive index difference between the inner core 1 and the outer core 2 is -0.0.
The relative refractive index difference between the outer core 2 and the clad 3 is -0.7%.

In such a fiber, the effective relative refractive index difference between the core and the clad can be made large, and a high-efficiency and high-gain fiber optical amplifier can be configured.

 Hereinafter, specific examples will be described.

(Example) A quartz glass particle aggregate (weight: 208 g) having a uniform bulk density of 0.45 g / cm ³ was obtained by the VAD method using a quartz glass particle deposition burner and a heating burner. Next, Al
When the above-described aggregate of silica glass particles was immersed in a solution obtained by mixing 200 g of an ethanol solution of 20 wt% of Cl ₃ and 250 g of an ethanol solution of 0.54 wt% of ErCl ₃ , 141 g of the mixed solution was absorbed. Then, the mixture was reduced in pressure to about 30 Torr at room temperature and left to dry for 3 days. This was put into a heating furnace, and the inside of the furnace was heated to 900 ° C in a helium atmosphere with a chlorine gas concentration of 0.5% and dehydrated, then heated to 1500 ° C in a helium atmosphere with a 10% concentration of SiF ₄ , sintered and transparent. When vitrification was performed, a transparent glass body was obtained. In this transparent glass body
Er was 0.1 wt% and Al was 2 wt%. This transparent glass body is drawn into a drawn rod, and an outer core body made of pure quartz glass and a clad body made of fluorine-doped quartz glass are provided on the drawn rod by an external method, and spun by a conventional method. A fiber having a refractive index distribution as shown in FIG. 1 was obtained.

When laser light of 1.48 μm and 45 mW was input to this fiber as pump light and light of −40 dBm was input as signal light, optical amplification was performed at a high efficiency of 1.7 dB / mW at a wavelength of 1.532 μm. I understood. Also, 1.552μm
The gain variation could be suppressed within 3 dB within a range of 15 nm centered on. The gain at this time was 30.5 dB.

(Conventional example) In the example, the atmosphere for transparent vitrification was 10
When performed in a 0% helium atmosphere, the sintering temperature was 155
Even when the temperature was raised to 0 ° C. (the limit temperature of the quartz muffle), a transparent glass body could not be obtained.

(Comparative Example) A fluorine-doped quartz glass serving as a clad was directly externally provided on the drawn rod obtained in the example and spun by a conventional method to obtain a fiber having a refractive index distribution as shown in FIG. Obtained. In FIG. 2, reference numeral 11 denotes a core, and reference numeral 12 denotes a clad. When this fiber was subjected to optical amplification operation in the same manner as in the example, the gain was as low as 0.8 dB / mW.

〔The invention's effect〕

As described above, the waveguide of the present invention has an inner core body made of quartz glass doped with aluminum, erbium and fluorine, and a quartz glass provided outside the inner core body and forming an inner core body. An outer core body made of glass having a high refractive index and a clad body provided outside the outer core body and having a lower refractive index than the glass forming the outer core body, so that an effective relative refraction is obtained. The rate difference can be increased, and highly efficient optical amplification can be performed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing an example of a refractive index distribution of an optical waveguide according to the present invention, and FIG. 2 is a graph showing a refractive index distribution of a conventional optical waveguide.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification code FI G02B 6/00356 G02B6 / 00356A (56) References JP-A-4-50131 (JP, A) JP-A-3-45525 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C03B 37/014 C03B 8/04 C03C 13/04

Claims (1)

(57) [Claims] 1. An inner core made of quartz glass doped with aluminum, erbium and fluorine, and a glass provided outside the inner core and having a higher refractive index than the quartz glass forming the inner core. An outer core body,
An optical waveguide provided outside the outer core body and made of a clad body having a lower refractive index than glass constituting the outer core body.