JP3380111B2 - Optical waveguide grating - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide grating, and more particularly to a radiation mode coupling type optical waveguide grating.

[0002]

2. Description of the Related Art An optical waveguide grating forms a constant periodic change in the length direction of an optical fiber or a planar optical waveguide, for example, a periodic change in the refractive index of the core or a periodic change in the core diameter. It was done.

Generally, there are a radiation mode coupling type and a reflection mode coupling type in the grating, and the radiation mode coupling type grating couples the mode propagating in the core and the mode propagating in the clad to each other to emit light of a specific wavelength. The characteristic is that radiation is performed outside the optical waveguide to be attenuated. Further, the reflection mode coupling type grating reflects light of a specific wavelength by coupling a mode propagating in the core in the positive direction and a mode propagating in the core in the opposite direction (negative direction). The characteristics are obtained. The main difference in structure between the radiative mode coupling type grating and the reflection type grating lies in the difference in the cycle of periodical change (hereinafter sometimes referred to as the grating pitch). For example, in the case of an optical fiber grating formed by forming a periodic change in the refractive index in the core of an optical fiber, a radiating grating is obtained by setting the grating pitch to several hundreds μm,
The reflection type grating has a grating pitch of 1μ
It is obtained by setting it to about m.

In the radiation mode coupling type grating, for example, the wavelength-transmission loss characteristic as shown in FIG. 2 is obtained, and the transmission loss of light in a specific wavelength band is selectively increased. The width of the wavelength band in which the transmission loss increases is called the stop band width, the center wavelength is called the center wavelength of the stop band, and the magnitude of the change in the transmission loss is called the stop ratio. A radiation mode coupling type optical fiber grating having such characteristics is
For example, it is used in the field of optical communications, especially in optical communication systems that use erbium-doped optical fiber amplifiers to suppress spontaneous emission from erbium-doped optical fibers and reduce the wavelength dependence of the gain of erbium-doped optical fiber amplifiers. It is used.

By the way, as one of the conventional methods for manufacturing an optical waveguide grating, the phenomenon that the refractive index rises according to the irradiation amount when strong ultraviolet rays are irradiated to the silica glass to which germanium is added, is used to make the core A method of forming a periodic refractive index change is known. For example, when manufacturing a radiation mode coupling type optical fiber grating, a germanium-doped core / silica clad optical fiber or a germanium-doped core / fluorine-doped clad optical fiber is used. After hydrogenation treatment in a container (about 100 atm),
A method of irradiating ultraviolet light at a constant cycle in the optical fiber length direction using a photomask, a method of successively irradiating ultraviolet light at equal intervals in the optical fiber length direction, and the like are used.

However, in the conventional radiation mode coupling type optical fiber grating produced by using the germanium-doped core / silica clad optical fiber or the germanium-doped core / fluorine-doped clad optical fiber, the center of the stop band is There was an unfavorable property that the temperature dependence of the wavelength was large. Specifically, such an optical fiber grating has a temperature characteristic of about 0.05 nm / ° C., and the temperature rises (falls) by 10 ° C.
Then, the center wavelength of the stop band has a property of moving to the long wavelength side (short wavelength side) of about 0.5 nm, and there was concern about the stability and reliability of the optical component.

[0007]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to obtain a radiation mode coupling type optical waveguide grating having a small temperature dependence of the center wavelength of the stop band and excellent temperature characteristics.

[0008]

[Means for Solving the Problems ]
Therefore, the invention according to claim 1 is a radiation mode coupling type optical waveguide grating using an optical waveguide made of a silica-based glass material , wherein GeO ₂ and B ₂ are contained in at least the core.
O ₃ was added, and the concentration of B ₂ O _{3 was} the same as that of GeO ₂ .
The optical waveguide grating is characterized by being set to 0.5 to 2.0 times . The invention according to claim 2 provides an optical waveguide
3. The optical fiber according to claim 1, wherein the path is an optical fiber.
It is an optical waveguide grating.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
FIG. 1 shows an example of the optical waveguide grating of the present invention.
FIG. 3 is a partial cross-sectional view schematically showing an example of a radiation mode coupling type optical fiber grating (hereinafter, also simply referred to as an optical fiber grating). In the figure, reference numeral 1 is an optical fiber, 2 is a core, 3 is a clad, and 4 is a coating layer. This fiber optic grating
The refractive index of the core 2 is part of the optical fiber 1
The grating portion 5 that is periodically changing in the length direction is formed.

The core 2 of the optical fiber 1 is made of quartz glass containing at least germanium and boron. Other than this, the core 2 may be appropriately added with aluminum, erbium, titanium or the like. The clad 3 is made of silica glass having a lower refractive index than the core, and for example, pure silica glass, fluorine-containing silica glass, or the like is preferably used. For example, when the optical fiber 1 is manufactured using the vapor phase reaction method, germanium is actually GeO.
₂ is added to the core 2 and boron is added to the core 2 as B ₂ O ₃ . GeO ₂ concentration in core 2, preferably a core of the optical fiber 1 - with cladding relative refractive index difference is obtained by irradiating ultraviolet light to the optical fiber 1, the desired refractive index change in the core 2 is obtained Is set as follows. As will be described later, B ₂ O
₃ was added to Core 2 to give GeO ₂
The temperature characteristic of the optical fiber grating is improved by canceling the change in the refractive index of the added quartz glass with respect to the temperature change. Further, when B ₂ O ₃ is added to the quartz glass, the refractive index of the quartz glass decreases. Therefore, B _{2 in} core ₂
The O ₃ concentration is set according to the GeO ₂ concentration in the core 2 so that the temperature characteristics of the optical fiber grating can be preferably improved and a preferable core-clad relative refractive index difference in the optical fiber 1 can be obtained. However, if the B ₂ O ₃ concentration in the core 2 is too high, the temperature characteristic of the optical fiber grating deteriorates to the opposite characteristic side as described later.
Therefore, the concentration of B ₂ O ₃ in the core 2 depends on the GeO ₂ in the core _2.
The concentration is set to 2.0 times or less. Further, B ₂ O ₃ can improve the temperature characteristics of the optical fiber grating to some extent by adding even a little. Therefore, the lower limit of the B ₂ O ₃ concentration in the core 2 is G
It is added so that the concentration of eO ₂ is 0.5 times or more.

The change amount of the core refractive index in the grating portion 5, the grating pitch, the grating shape (profile of the change in the core refractive index), the length of the grating portion 5 in the length direction of the optical fiber 1 and the like depend on these parameters. Characteristics of the grating,
That is, since the central wavelength of the stop band, the stop band width, and the stop ratio change, they are appropriately set according to the characteristics of the optical fiber grating to be obtained. Further, in order to obtain the characteristics of the radiation mode coupling type, the grating pitch is set within the range of several tens to several hundreds μm.

To manufacture such an optical fiber grating, for example, an optical fiber is first manufactured by using a known method such as the VAD method or the MCVD method. At this time, GeO ₂ and B ₂ O ₃ are added to the core. Optical fiber 1
The coating layer 4 is partially removed, and the portion where the coating layer 4 is removed is
The grating portion 5 is formed by periodically irradiating ultraviolet light in the length direction of the optical fiber 1. The wavelength of the ultraviolet light with which the optical fiber 1 is irradiated is preferably about 240 to 250 nm. As a method of forming the grating portion 5, several tens to several hundreds μm
After irradiating the optical fiber 1 with ultraviolet light having a relatively large spot width, or after irradiating the optical fiber 1 with ultraviolet light having a small spot width for a certain period of time through a photomask having slits at constant intervals, A well-known method such as a method of irradiating the optical fiber 1 with ultraviolet light at a constant interval is appropriately used by repeating the operation of stopping the irradiation, moving the irradiation position in the length direction of the optical fiber 1, and irradiating again. be able to.

The operation of the optical fiber grating of this embodiment will be described below. Although there are a plurality of stop bands in the radiation mode coupling type optical fiber grating, the condition expressed by the following formula (1) is satisfied at the center wavelength λc of one stop band. βco−βcl = 2π / Λ (1) where βco is the propagation constant of the (core) guided mode, βcl
Is the propagation constant of the cladding mode, and Λ is the grating pitch. Then, rewriting the above formula (1) gives the following formula (2). λc = Λ · (neco−necl) (2) where neco and necl are the effective refractive indices of the guided mode and the cladding mode, respectively (2π / Λ · effective refractive index = propagation constant).

When this equation (2) is differentiated with respect to temperature, the following equation (3) is obtained.

[Equation 1] Then, in order to prevent the center wavelength λc from shifting with respect to the temperature change, both sides of the mathematical expression (3) may be set to zero. Here, the thermal expansion coefficient of quartz-based glass whose main component is quartz is very small, so that ∂Λ / ∂T (the same value as the thermal expansion coefficient of glass) in the above formula (3) can be regarded as almost zero. . Therefore, (∂ / ∂T)
(Neco-necl) should be set to 0.

By the way, regarding the quartz glass used for the cladding of the conventional radiation mode coupling type optical fiber grating and the germanium-doped quartz glass used for the core, the refractive index also rises as the temperature rises. It has the property. In addition, in an optical fiber, generally, when the refractive index of the material increases, the effective refractive index of the guided mode also increases. Since the temperature dependence of the refractive index change of germanium-doped silica glass is larger than the temperature dependence of the refractive index change of silica glass, it is ∂nec in the conventional radiation mode coupling type optical fiber grating.
o / ∂T> ∂necl / ∂T, and the right side of the above equation (3) did not become 0. On the other hand, the silica glass to which boron is added has a property that the refractive index decreases as the temperature rises. Therefore, by adding not only germanium but also an appropriate amount of boron to the core 2, the temperature dependence of the refractive index of the core 2 and the temperature dependence of the refractive index of the clad 3 can be made substantially equal. As a result, ∂neco / ∂T−∂necl / ∂T≈0 can be obtained, and a radiation mode coupling type optical fiber grating in which the center wavelength of the stop band has small temperature dependence can be obtained.

When the concentration of boron in the core 2 increases, the temperature characteristic of the radiation mode coupling type optical fiber grating shifts to the characteristic side opposite to the conventional one, and as the temperature rises (falls), the central wavelength of the stop band becomes shorter. It has the property of moving to the wavelength side (long wavelength side). Then, the concentration of boron in the core 2 is 2 times that of germanium.
If it exceeds twice, the temperature characteristics deteriorate to the opposite side, which is not preferable. The method of improving the temperature characteristics of the optical fiber grating by adding boron to the core 2 is not limited to the optical fiber grating of the present embodiment, but the core and the clad are made of a silica glass material,
Germanium is added to the core, and the temperature dependence of the refractive index change of the cladding material is smaller than the temperature dependence of the refractive index change of the core material, and it is an optical fiber grating with radiation mode coupling characteristics. Is valid,
It can be applied to an optical fiber grating having any structure.

[0017]

【Example】

Example 1 First, an optical fiber was prepared in which the core was made of silica glass to which 12% mol of GeO ₂ and 8% mol of B ₂ O ₃ were added, and the cladding was made of (pure) silica glass. Next, after partially removing the coating layer of the optical fiber, ultraviolet light having a wavelength of 248 nm was radiated to the optical fiber through a photomask having a slit to form a grating portion in the portion where the coating layer was removed. The grating pitch Λ was 400 μm, and the length of the grating portion was 20 mm. The optical fiber grating thus obtained had characteristics as a radiation mode coupling type grating, and the center wavelength of the stop band was 1560.0 nm at room temperature. With respect to this optical fiber grating, the temperature dependence of the central wavelength was examined in the temperature range of −20 to 80 ° C., and the result was 0.01 nm / ° C.

(Embodiment 2) As an optical fiber, the core is
Example 1 except that an optical fiber made of silica glass to which 8% mol of GeO ₂ and 16% mol of B ₂ O ₃ were added and the cladding made of (pure) silica glass was used.
An optical fiber grating was produced in the same manner as. The obtained optical fiber grating had characteristics as a radiation mode coupling type grating, and the center wavelength of the stop band was 1538 nm at room temperature. As a result of investigating the temperature dependence of the center wavelength in the temperature range of −20 to 80 ° C. for this optical fiber grating, −0.04
It was 5 nm / ° C., and had characteristics opposite to those of Example 1 above.

Comparative Example 1 As a comparative example, B ₂ O ₃ was added to the core.
An optical fiber grating was prepared using an optical fiber without addition of. That is, as the optical fiber, the same as in Example 1 except that an optical fiber having a core made of silica glass to which 4.0% mol of GeO ₂ was added and a clad made of (pure) silica glass was used. An optical fiber grating was produced. The obtained optical fiber grating has characteristics as a radiation mode coupling type grating, and the center wavelength of the stop band is 1490.
was nm. With respect to this optical fiber grating, the temperature dependence of the central wavelength was examined in the temperature range of −20 to 80 ° C., and the result was 0.052 nm / ° C.

From the results of Examples 1 and 2 and Comparative Example 1,
By using a silica-based optical fiber in which B ₂ O ₃ is added to the core in addition to GeO ₂ , the temperature characteristic of the center wavelength of the stop band of the optical fiber grating is better than that in the case where B ₂ O ₃ is not added to the core. It is recognized that it can be improved. Further, as shown in Example 2, it was confirmed that an optical fiber grating exhibiting a temperature characteristic of the center wavelength of the stop band having an inverse characteristic to the conventional one can be realized depending on the concentration of B ₂ O ₃ added to the core.

In the above description, an example in which a radiation mode coupling type optical waveguide grating is constructed by using an optical fiber as an optical waveguide has been described. However, even when a planar type optical waveguide is used as an optical waveguide, it is prevented by the same principle. The temperature characteristic of the central wavelength of the band can be improved.

[0022]

As described above, according to the radiation mode coupling type optical waveguide grating of the present invention, the temperature dependence of the central wavelength of the stop band is dependent on the use of the silica glass in which germanium and boron are added to the core. Can be reduced. Therefore, a radiation mode coupling type optical waveguide grating having excellent temperature characteristics and excellent stability and reliability as an optical component can be obtained. Moreover, the radiation mode coupling type optical waveguide grating of the present invention can improve the temperature characteristics of the optical waveguide grating only by adding boron to the core in addition to germanium,
Since the conventional grating manufacturing method can be applied as it is, the manufacturing is simple, new equipment is not required, and it is economically advantageous.

The concentration of B ₂ O ₃ in the core is
Since the concentration of GeO ₂ is 0.5 to 2.0 times, the temperature dependence of the center wavelength of the stop band is small, whereby a radiation mode coupling type optical waveguide grating having excellent temperature characteristics can be obtained, and the conventional core It is also possible to realize a radiative mode coupling type grating having an opposite characteristic to the case where only germanium is added to. Further, in the radiation mode coupling type optical fiber grating using the optical fiber as the optical waveguide, since it has good connectivity with optical components using other optical fibers, in the optical communication field using the optical fiber. It can be effectively used, and temperature characteristics, stability, and reliability of the optical communication system can be improved.

[Brief description of drawings]

FIG. 1 is a partial sectional view showing an embodiment of a radiation mode coupling type optical waveguide grating of the present invention.

FIG. 2 is a graph showing an example of characteristics of a radiation mode coupling type optical waveguide grating.

[Explanation of symbols]

1 ... Optical fiber (optical waveguide), 2 ... Core, 5 ... Grating part

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoshi Okude               1440 Rokuzaki, Sakura City, Chiba Prefecture               Zikura Sakura Factory (72) Inventor Akira Wada               1440 Rokuzaki, Sakura City, Chiba Prefecture               Zikura Sakura Factory (72) Inventor Ryozo Yamauchi               1440 Rokuzaki, Sakura City, Chiba Prefecture               Zikura Sakura Factory                (56) Reference JP-A-7-218712 (JP, A)                 JP-A-7-283786 (JP, A)                 Special table flat 7-508358 (JP, A)

Claims (2)

(57) [Claims] 1. A radiation mode coupling type optical waveguide grating using an optical waveguide made of a silica glass material, comprising:
At least GeO ₂ and B ₂ O ₃ are added to the core ,
The concentration of B ₂ O ₃ is 0.5 to 2.0 times the concentration of GeO _2.
An optical waveguide grating characterized by being made . 2. The optical waveguide is an optical fiber.
The optical waveguide grating according to claim 1.