JPH08286050A - Optical waveguide type diffraction grating and its production - Google Patents

Abstract

PURPOSE: To provide optical waveguide type diffraction gratings which are less changed 10 Bragg wavelength and have high reliability despite of a change in the temp. of an atmosphere where the optical waveguide type diffraction grating is installed and a change in the temp. of the diffraction gratings themselves. CONSTITUTION: The optical waveguide type diffraction gratings 43, 44 have cores 41 which are optical waveguide parts contg. germanium oxide and clads 43 which have the refractive index lower than the refractive index of these cores 41 in contact with the cores 41. These diffraction gratings include titanium oxide in the cores 41 or the clads 42. The process for producing the optical waveguide type diffraction gratings 43, 44 include a first stage of preparing glass optical waveguides including the titanium oxide in the cores 41 or the clads 42 and a second stage for changing the refractive index by subjecting the plural prescribed parts of the cores 41 and clads 42 of the glass optical waveguides to irradiation 10, 11, 20, 30 with UV rays.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type diffraction grating having a diffraction grating formed in an optical fiber or a thin film optical waveguide, and a method for manufacturing the same.

[0002]

2. Description of the Related Art There are various types of diffraction gratings, which are a type of optical element, but when used in an optical communication system, they are easy to connect with an optical waveguide and have a low insertion loss. A diffraction grating is preferred.

As a conventional method for producing an optical waveguide type diffraction grating, the method described in Japanese Patent Publication No. 62-500052 is known. This is a method of forming a diffraction grating by irradiating a silica-based optical fiber in which germanium oxide is added to form a core with a high refractive index with strong ultraviolet rays to cause a periodic change in the refractive index in the core. is there.

[0004]

However, in the above-mentioned method, the obtained diffraction grating has a change in the spacing between the fringes of the change in the refractive index due to thermal expansion, and the effective refractive index changes due to the temperature change. There is a problem that the reflection wavelength (hereinafter, referred to as Bragg wavelength) changes under the influence of the temperature change of the atmosphere in which the diffraction grating is installed.

Specifically, the Bragg wavelength λ _B
Can be described by the effective refractive index n of the core and the interval Λ of the period of the refractive index change of the diffraction grating as in the equation (1). Further, by differentiating this equation with respect to the temperature T, the influence of the Bragg wavelength λ _B on the change of the temperature T is expressed by the equation (2).

[Equation 1] Where n is the effective refractive index of the core Λ is the interval of the refractive index change period of the diffraction grating ΔT is the temperature change α is the coefficient of thermal expansion of the core (= Λ ⁻¹ · δΛ / δT) ξ is the refractive index Temperature coefficient (= n ^-1 · δn / δT)

Now, the coefficient of thermal expansion α of silicon oxide (quartz glass) is 0.5 × 10 ⁻⁶ / ° C., the temperature change ξ of the effective refractive index n of the core is 6.8 × 10 ⁻⁶ / ° C., Assuming that the Bragg wavelength λ _B at room temperature is 1550 nm, the rate of temperature change is 0.0113 nm / ° C from the above equation. This change amount is 0.9n at the maximum operating environment temperature of -20 ° C to 60 ° C.
also m

[0007]

In order to solve the above problems, an optical waveguide type diffraction grating of the present invention comprises a core which is an optical waveguide portion containing at least germanium oxide, and a core which is in contact with the core and With a low refractive index cladding,
An optical waveguide type diffraction grating having a diffraction grating region in which a plurality of refractive index changing portions are formed along the optical axis direction, characterized in that both or one of the core and the clad contains titanium oxide. .

The method for producing an optical waveguide type diffraction grating according to the present invention further comprises a core containing at least germanium oxide and a clad in contact with the core and having a refractive index lower than that of the core. A first step of preparing a glass optical waveguide containing titanium oxide in both or one of them, and irradiating ultraviolet light to a plurality of predetermined portions of the core and the clad of the glass optical waveguide to change the refractive index. It is characterized by comprising two steps.

[0009]

In other words, the optical waveguide type diffraction grating of the present invention and the method for manufacturing the same utilize the fact that the thermal expansion coefficient of titanium oxide is negative, and the titanium oxide is mixed into the core or the clad of the glass optical waveguide. By offsetting the thermal expansion of silicon oxide or germanium oxide having a positive coefficient of thermal expansion,
It suppresses a change in Bragg wavelength due to a change in temperature. That is, according to the present invention, by mixing titanium oxide into the core or the clad of the glass optical waveguide, the thermal expansion coefficient α of the core of the formula (2) and the temperature coefficient ξ of the refractive index are calculated.
By reducing the influence of the change of the temperature T on the Bragg wavelength λ _B to solve the above-mentioned problems, to provide a highly reliable optical waveguide type diffraction grating, and to enable further multiplexing in WDM communication. To do.

[0010]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

The first embodiment will be described. In FIG.
An outline of the refractive index distribution and composition distribution of the optical waveguide used in this example is shown. In this example, first, a silica optical fiber containing silica (SiO ₂ ) glass as a main component was prepared as an optical waveguide used for forming a diffraction grating. This optical fiber
20 wt% germanium oxide (GeO) on quartz glass
₂ ) Doped core and quartz glass with 10 wt% germanium oxide and 10 wt% titanium oxide (Ti
O ₂ ) and a clad co-doped with 20 wt% of boron oxide (B ₂ O ₃ ). Here, germanium oxide is a dopant that raises the refractive index by increasing the refractive index while irradiating with ultraviolet light, titanium oxide is a dopant that lowers the thermal expansion coefficient, and boron oxide is a dopant that lowers the refractive index. It's a punt. Since titanium oxide increases the refractive index, boron oxide is mixed with the titanium oxide in the clad to lower the refractive index from that of the core.

The optical fiber may be manufactured by any manufacturing method. For example, the well-known MCVD
It can be prepared by heating an optical fiber preform prepared by the method, the VAD method, the OVD method, the rod-incube method, or the like in an electric furnace to draw it. For example, in the VAD method, first, a rotating center rod is used as a target, and glassy fine particles containing silicon oxide and germanium oxide produced in a flame are deposited thereon to form a soot preform. . Specifically, silicon tetrachloride (SiCl ₄ ) and germanium tetrachloride (GeC), which are raw materials for the core burner, are used.
l ₄ ), hydrogen as a fuel, and oxygen (the above are all gases) at the same time. On the other hand, the clad bar
In addition to the above gases, the boron further contains boron bromide (BBr).
₃ ) and titanium bromide (TiBr ₄ ) are fed. And
Apply the flame of each burner due to the combustion of hydrogen to the center rod,
Form a soot preform.

Next, the formed soot preform is completely dehydrated at a high temperature to scatter impurities and then further heated to sinter. Since the preform (optical fiber preform) is completed as described above, an optical fiber can be obtained by melting at a higher temperature in a drawing furnace and melting. The optical fiber used in this example has a core diameter of 10 μm and a cladding diameter of 125.
μm.

Next, the optical fiber is irradiated with ultraviolet light. Here, the irradiation of the ultraviolet light may be performed on the optical fiber cut into a desired length, or may be performed during the winding of the drawn optical fiber, for example, by incorporating it into the optical fiber manufacturing process. May be.

In the present embodiment, the diffraction gratings of equal pitch are formed in the optical fiber, and for this reason, ultraviolet light is applied to the optical fiber while generating interference fringes at equal intervals. Hereinafter, the method of irradiating the ultraviolet light will be described in detail.

FIG. 1 is a diagram for explaining the irradiation method. As shown in FIG. 1, the ultraviolet light output from the ultraviolet light source 10 is interfered by the interference means 20 and is applied to the optical fiber 40 while generating interference fringes.

In this embodiment, ultraviolet light is caused to interfere by the holographic interferometry method. In this method, the interference means 20
Is a beam splitter 21a and a reflecting mirror 2 as shown in FIG.
1b and 21c. Further, an argon laser light source 11 is used as the ultraviolet light source 10.

The argon laser light source continuously oscillates coherent ultraviolet light of 244 nm. This ultraviolet light is
The beam splitter 21a splits the light into two light beams of transmitted light and reflected light. The branched light fluxes are respectively reflected by the reflecting mirror 21.
74 ° (α in FIG. 1), which are reflected by b and 21c and are complementary to each other with respect to the axial direction of the core 41, 1
The optical fiber is irradiated at an angle of 06 ° (180 ° -α in FIG. 1). Each of the branched light beams has an interference region 30.
And is irradiated onto the optical fiber 40 while forming interference fringes at predetermined intervals. The irradiated ultraviolet light enters the core 41 and the clad 42, and periodically changes the refractive index of the incident part in the optical axis direction. The mechanism by which the refractive index of the glass changes due to the incidence of ultraviolet light has not been completely clarified, but it is considered that the important cause is the generation of oxygen-deficient defects related to germanium in the glass. ing.

FIG. 2 is a diagram showing the irradiation of the optical fiber 40 with ultraviolet light. The incident angle of the ultraviolet light with respect to the radial direction of the optical fiber 40 is θ (= 90 ° -α), and the wavelength of the ultraviolet light is λ.
Then, the periodic interval Λ of the interference fringes is expressed as Λ = λ / (2sinθ) equation (3). Therefore, the core 41 and the clad 42
In the region where the ultraviolet light is incident, since the refractive index changed portions are arranged along the optical axis direction of the optical fiber 40 with the periodic interval Λ of the interference fringes as the period, the diffraction grating 4 with the pitch Λ is arranged.
3 and 44 are formed on the core 41 and the clad 42, respectively. Thus, the optical fiber type diffraction grating having the diffraction grating region in the core 41 and the clad 4 is formed.

When the refractive index n of the core 41 and the pitch Λ of the diffraction grating 43 are used, the reflection wavelength λ _B of this fiber type diffraction grating can be calculated from the equations (1) and (3) according to the well-known Bragg diffraction condition. , Λ _B = λ · n / sin θ Expression (4) In this embodiment, the reflection wavelength λ _B is set to be 1550 nm at room temperature.

During the irradiation of ultraviolet light by the above method, LE
The formation of the diffraction grating can be monitored in real time by allowing the light from the D light source to enter from one end of the optical fiber and measuring the transmission spectrum of this light with a spectrum analyzer connected to the other end. Here, the spectrum analyzer detects the relationship between the wavelength and the light intensity of the light transmitted through the diffraction gratings 43 and 44. Since the formation of the diffraction gratings 43 and 44 progresses when the irradiation of the ultraviolet light is started, the intensity of the transmitted light in the transmission spectrum decreases around the reflection wavelength. If there is no change in the transmission spectrum,
Since the formation of the diffraction gratings 43 and 44 is considered to be saturated, the irradiation of ultraviolet light is stopped at this point. In this example, the saturation time is 40 to 50 minutes.

The temperature of the atmosphere of the optical fiber type diffraction grating in which the diffraction gratings 43 and 44 are formed is 20 ° C. and 40 ° C., respectively.
The change in the Bragg wavelength λ _B is measured while changing the temperature from 60 ° C. to 60 ° C. This is shown in FIG. When the rate of temperature change of the Bragg wavelength is calculated from this value, it becomes 0.0098 nm / ° C. For comparison, FIG. 5 shows that the optical fiber type diffraction grating having the same lattice constant and containing no titanium oxide has a temperature of 40.degree.
The values of Bragg wavelength at 60 ° C and 60 ° C are shown. The ratio of the Bragg wavelength to the temperature change obtained from FIG. 5 is 0.011n
m / ° C. Therefore, by mixing titanium oxide into the optical fiber, the temperature change of the Bragg wavelength can be suppressed.

In the above-mentioned embodiment, the ultraviolet fringes are formed by using the Holographic-Frack interferometry, but the phase grating method may be used instead. FIG. 3 is a diagram for explaining the phase grating method. First, the phase grating 22, which is the interference means 20, is closely fixed to the optical fiber 40. Phase grating 2
For 2, a quartz plate having grooves formed at equal intervals can be used. Since the groove of the phase grating 22 can be formed by lithography and chemical etching, the diffraction grating can be freely selected and a complicated shape can be formed.

Next, for example, a KrF excimer laser light source 1
2 (ultraviolet light source 10) as a pulse light source
A pulsed light of a predetermined intensity of 48 nm is output at a predetermined frequency, and irradiation is performed from the upper surface of the phase grating for a predetermined time as shown in the figure.
The ultraviolet light may be continuously oscillated. When the ultraviolet light passes through the phase grating 22, interference fringes are formed at predetermined intervals. Since the light enters the core 41 and the clad 42 while forming the interference fringes, the periodical refractive index change, that is, the diffraction grating 43,
44 occur in the core 41 and the clad 42, respectively. Thus, an optical fiber type diffraction grating in which the diffraction grating is formed on both the core 41 and the clad 42 is manufactured.

In the first embodiment, the core does not contain titanium oxide, but it can be contained. In this case, the effect of suppressing the influence of temperature change on the Bragg wavelength naturally increases. However, since the cladding diameter is 125 μm with respect to the core diameter of 10 μm, the effect of titanium oxide in the cladding is more dominant in the effect of suppressing the influence of temperature change on the Bragg wavelength. Further, although germanium oxide is contained in the clad in the above embodiment, it is possible not to include germanium oxide. However, in this case, since the diffraction grating is not formed in the clad, the reflectance at the Bragg wavelength is slightly lowered. Although boron oxide is used as the refractive index lowering agent in the above embodiment, fluorine may be used. In this case, the addition of fluorine may be carried out by keeping the soot preform in fluorine gas at a constant temperature and pressure.
Alternatively, the raw material quartz glass powder may be impregnated with fluorine before forming the glass.

The second embodiment will be described. FIG. 7 shows a schematic distribution of the refractive index of the optical waveguide and the concentration of germanium oxide and the concentration of titanium oxide according to this example. In the first embodiment, an optical waveguide including a core, which is an optical waveguide portion containing germanium oxide, and a clad having a refractive index lower than that of the core, which is in contact with the core, is used. A first clad that is in contact with the core and has a refractive index lower than that of the core;
An optical waveguide composed of a second clad located outside the D field is used. Here, the mode field is 1 which is the maximum value of the intensity distribution of the single mode optical fiber.
/ E (= 2.71) is the inner part of the diameter, and the diameter has a property that depends on the wavelength of the light used, but it is generally approximately the same as the diameter of the core. Regarding the specific composition concentration, the composition of the core portion is the same as that in the above-mentioned embodiment, and the silica glass is
0 wt% germanium oxide is doped. First
The clad is made of quartz glass. The composition of the second clad is such that 15 wt% titanium oxide is added to quartz glass. Since this portion is outside the mode field and is substantially unrelated to the transmission of light, it is not necessary to lower the refractive index below the refractive index of the core, so that the refractive index lowering agent is not mixed. .

This optical fiber is manufactured by the MVCD method or the like in almost the same manner as in the first embodiment. Core diameter is 10μ
m, the first cladding diameter is 30 μm, and the second cladding diameter is 125 μm. The mode field has a diameter of about 10 .mu.m and protrudes in the first cladding in the immediate vicinity of the outside of the core. After irradiating ultraviolet rays to form a diffraction grating in substantially the same manner as in Example 1, the temperature of the atmosphere of the optical fiber is changed to 20 ° C., 40 ° C., and 60 ° C., and the change in Bragg wavelength is measured. This is shown in FIG. From this value, the rate of change in Bragg wavelength with temperature is calculated to be 0.0078n.
m / ° C. This value is 20 ° C., 40 ° C. for the optical fiber type diffraction grating in the case of not containing titanium oxide.
This is a value lower than 0.011 nm / ° C., which is the rate of change with respect to temperature change, which is determined from the Bragg wavelength values at 60 ° C. and 60 ° C. (FIG. 5). That is, also in the optical fiber having the configuration of FIG. 7, it is possible to suppress the temperature change of the Bragg wavelength by mixing titanium oxide into the optical fiber.

In the second embodiment, germanium oxide is not included in the first cladding, but it may be included. In this case, since the diffraction grating is also formed on the first clad, the reflectance is improved.

[0029]

As described above, since the optical waveguide type diffraction grating of the present invention contains titanium oxide in the core or the clad, the temperature of the atmosphere in which the diffraction grating is installed changes,
Even if the temperature of the diffraction grating itself changes, the change in Bragg wavelength is small, and as a result, it is possible to provide an optical waveguide type diffraction grating with high reliability in WDM communication and further enable multiplexing.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an irradiation method of ultraviolet light used in Example 1.

FIG. 2 is a diagram showing irradiation of an optical fiber with ultraviolet light in the method of FIG.

FIG. 3 is a diagram illustrating an ultraviolet light irradiation method by a phase grating method.

FIG. 4 is a diagram showing a refractive index distribution of an optical fiber used in Example 1 and a schematic distribution of respective concentrations of germanium oxide, titanium oxide, and boron oxide.

FIG. 5 shows an optical fiber type diffraction grating for comparison which does not contain titanium oxide, and the atmosphere temperature is 20 ° C., 40 ° C.
It is a figure which shows the value of the Bragg wavelength which changed and changed into 60 degreeC and 60 degreeC.

FIG. 6 shows the optical fiber type diffraction grating according to the present invention in Example 1, in which the ambient temperature is 20 ° C., 40 ° C.
It is a figure which shows the value of the Bragg wavelength which changed and changed into 60 degreeC and 60 degreeC.

FIG. 7 is a diagram showing a radial distribution of a refractive index of an optical fiber type diffraction grating used in Example 2 and a schematic radial distribution of respective concentrations of germanium oxide and titanium oxide.

FIG. 8 shows the optical fiber type diffraction grating according to the present invention in Example 2, in which the ambient temperature is 20 ° C. and 40 ° C.
It is a figure which shows the value of the Bragg wavelength which changed and changed into 60 degreeC and 60 degreeC.

[Explanation of symbols]

 10: Ultraviolet light source 20: Interference means 21a: Beam splitters 21b, 21c: Reflector 22: Phase grating 30: Interference region 40: Optical fiber 41: Core 42: Clad 43, 44: Diffraction grating

Claims (13)

[Claims] 1. A core, which is an optical waveguide including germanium oxide, and a cladding, which is in contact with the core and has a refractive index lower than that of the core, are formed, and a plurality of refractive index changing portions are formed along the optical axis direction. An optical waveguide type diffraction grating having a diffraction grating region, wherein both or one of the core and the clad contains titanium oxide. 2. The optical waveguide type diffraction grating according to claim 1, wherein the cladding contains germanium oxide. 3. The optical waveguide type diffraction grating according to claim 1 or 2, wherein a refractive index lowering agent is contained together with titanium oxide in both or one of the core and the clad. 4. The optical waveguide type diffraction grating according to claim 3, wherein the refractive index lowering agent is boron oxide or fluorine. 5. The first cladding, which is in contact with the outside of the core and has a lower refractive index than the core, and the second cladding, which is in contact with the first cladding and is outside the mode field. 5. The clad of claim 1 to claim 4.
Optical waveguide diffraction grating described in 6. A glass optical waveguide comprising a core containing germanium oxide and a clad in contact with the core and having a refractive index lower than that of the core, and a glass optical waveguide containing titanium oxide in at least one of the core and the clad. Fabrication of an optical waveguide type diffraction grating comprising a first step of preparing and a second step of irradiating ultraviolet rays to a plurality of predetermined portions of the core and the clad of the glass optical waveguide to change the refractive index. Method 7. The method for manufacturing an optical waveguide type diffraction grating according to claim 6, wherein the clad contains germanium oxide. 8. The optical waveguide type diffraction grating according to claim 6 or 7, wherein both or one of the core and the clad contains a titanium oxide and a refractive index lowering agent. Manufacturing method 9. The method of manufacturing an optical waveguide type diffraction grating according to claim 8, wherein the refractive index lowering agent is boron oxide or fluorine. 10. The first clad is in contact with the outside of the core and has a lower refractive index than the core, and the second clad is in contact with the first clad and is outside the mode field. 10. The method for manufacturing an optical waveguide type diffraction grating according to claim 6, further comprising: 11. The method for producing an optical waveguide type diffraction grating according to claim 6, wherein the glass optical waveguide is irradiated with interference fringes generated by causing ultraviolet light to interfere with each other. 12. The glass optical waveguide is irradiated with interference fringes generated by interfering two coherent ultraviolet rays with each other from an angle which is a complementary angle to the optical axis of the glass optical waveguide. A method of manufacturing an optical waveguide type diffraction grating according to claim 11. 13. The optical waveguide type diffraction grating according to claim 11, wherein the glass optical waveguide is irradiated with interference fringes generated by irradiating the phase grating with ultraviolet light and transmitting the ultraviolet light. Manufacturing method