JPH09325229A - Optical waveguide grating and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To narrow a blocking band width without lengthening the length of a grating by forming a radiation mode coupling type grating in an optical waveguide whose difference of average specific refractive index of a core is larger than that of an optical waveguide for communication. SOLUTION: In a radiation mode coupling type grating using an optical fiber 1, the optical fiber 1 is composed of a core 1a and a clad 1b having a refractive index lower than the core 1a and a grating part 2 whose refractive index is periodically changed in the direction of length of the optical fiber 1 is formed in a part of the core 1a. The optical fiber used here is formed so that the difference of the average specific refractive index is larger than that of an optical fiber for communication. The optical fiber 1 having preferably a single mode type transfer characteristic in the operating wavelength area is used. The reason why is that a single mode optical fiber is mostly available in an optical communication system and a connection loss to an existing system remains small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation mode coupling type optical waveguide grating, and more particularly to an optical waveguide grating capable of achieving a narrow stop band width without increasing the grating length.

[0002]

2. Description of the Related Art Optical waveguide gratings are obtained by forming a constant periodic change, for example, a periodic change in core refractive index, in the length direction of an optical fiber or a planar optical waveguide. 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, so that light of a specific wavelength is emitted from the optical fiber. It is designed to obtain the characteristic of radiating to outside and attenuating. The reflection mode coupling type grating reflects light of a specific wavelength by coupling a mode propagating in the core in a positive direction and a mode propagating the core in the opposite direction (negative direction). The characteristic is obtained.

For example, in the case of a grating realized in an optical fiber, a radiation type grating is obtained by setting the cycle of the change in the refractive index of the core (hereinafter sometimes referred to as the grating pitch) to several hundred 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, a wavelength-transmission loss characteristic (transmission spectrum) as shown in FIG. 6 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 change in the transmission loss is called the stop ratio.

And, these grating characteristics are
It is known that each parameter of the grating changes depending on the amount of change in core refractive index, grating pitch, grating shape (profile of core refractive index change), grating length in the optical fiber length direction, effective refractive index, and the like. Table 1 below shows the effect of each parameter in the grating on the grating characteristics. In the table, x has no effect,
○ indicates that there is an influence, and △ indicates that the influence is small. Further, ↑ (↓) indicates that as the parameter value increases, the value of the grating characteristic increases (decreases) accordingly.

[0006]

[Table 1]

The radiation mode coupling type optical fiber grating is used, for example, in the field of optical communication, and is suitable for reducing the wavelength dependence of the gain of an erbium-doped optical fiber amplifier, especially in an optical communication system for wavelength division multiplexing transmission. Can be used for. In this case, it is preferable to design the emission mode coupling type optical fiber grating so that the stop band width is equal to the wavelength band used for transmission. For example, FIG. 7 shows the wavelength dependence of the gain of a general erbium-doped optical fiber amplifier,
This optical fiber amplifier is used in an optical communication system that performs wavelength division multiplexing transmission between wavelength A and wavelength B. In the optical fiber grating used in this optical communication system, the stop band width matches the wavelength range between wavelength A and wavelength B, and the wavelength-transmission loss characteristics in this wavelength range are gains in the same wavelength range. It is preferable to design so as to draw a curve similar to the wavelength dependency of (curve in graph), and by designing in this way, the gain in this wavelength range can be efficiently equalized.

By the way, heretofore, as a method of controlling the stop band width in the radiation mode coupling type grating,
Only the method of adjusting the grating length as shown in Table 1 above has been known. However, in general, the wavelength band used in the wavelength division multiplex transmission is determined by the gain band of the erbium-doped optical fiber amplifier, which is the wavelength band between wavelength A and wavelength B in FIG. The bandwidth is about 15 to 20 nm, but there is a problem that the grating length becomes large when an attempt is made to fabricate a radiation mode coupling type grating having a relatively narrow stop bandwidth that matches this bandwidth. . For example, conventionally, the same optical fiber as a communication optical fiber has been used to manufacture an optical fiber grating. When manufacturing such an optical fiber having a stop band width of 15 nm or less, the grating is used. A length of at least 50 mm was required.

If the grating length of the optical fiber grating becomes large, it is disadvantageous in terms of downsizing the optical parts, and it cannot be housed in an existing repeater. An optical fiber grating is usually used in a state where both sides of the grating part are fixed to a substrate with an adhesive.However, as the grating length becomes longer, the resonance frequency value of the grating part becomes smaller. When the repeater is installed, the grating section may resonate and be damaged.

[0010]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to make it possible to narrow the stop band width without increasing the length of the grating of the optical waveguide grating.

[0011]

In order to solve the above problems, the invention according to claim 1 forms a radiation mode coupling type grating in an optical waveguide having a larger average relative refractive index difference of the core than that of the communication optical waveguide. It is an optical waveguide grating characterized by the following. The invention according to claim 2 is characterized in that the optical waveguide has a single mode type transmission characteristic in an operating wavelength range of the grating.
It is the described optical waveguide grating. In the invention according to claim 3, the relationship between the grating length, the average relative refractive index difference of the core, and the stop band width in the grating is obtained in advance, and based on this relationship, the grating length condition and stop band width to be obtained are obtained. A method for producing an optical waveguide grating, characterized in that an average relative refractive index difference of cores that simultaneously satisfy the conditions is obtained, and a radiation mode coupling type grating is formed in an optical waveguide having the average relative refractive index difference of the cores.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
FIG. 1 is a plan view showing an embodiment of the optical waveguide grating of the present invention. Here, a radiation mode coupling type optical fiber grating using an optical fiber (hereinafter, also simply referred to as an optical fiber grating) will be described as an example of the optical waveguide. In the figure, reference numeral 1 is an optical fiber, 1a is a core, 1b is a clad, 2 is a grating portion, 3 is a substrate, 4 is an adhesive, and 5 is a coating layer.

The optical fiber 1 comprises a core 1a and a clad 1b having a refractive index lower than that of the core. The refractive index of a part of the core 1a is periodically changed in the length direction of the optical fiber 1. The grating portion 2 is formed. In this embodiment, as the optical fiber 1, the coating layer 5 of the optical fiber core wire is used.
Those obtained by partially removing are preferably used. Further, in this embodiment, in order to obtain the characteristics of the radiation mode coupling type, the grating pitch in the grating portion 2 is preferably set within a range of about several tens to several hundreds of μm. In the present embodiment, the core 1a of the optical fiber 1 is made of a material whose refractive index changes according to the intensity of the ultraviolet light and the irradiation time when the ultraviolet light is irradiated, and preferably germanium oxide-doped quartz. It is made of glass. In addition to germanium oxide, aluminum, erbium, titanium, etc. may be appropriately added to the core 1a. The clad 1b of the optical fiber 1 is preferably made of pure silica glass or fluorine-doped silica glass.

The optical fiber 1 used for producing the optical fiber grating in the present invention may be a single mode optical fiber or a multimode optical fiber, but preferably has a single mode type transmission characteristic in the operating wavelength range of the grating. Things are used. The reason is that single mode optical fiber is often used in the existing optical communication system,
The use of a single-mode optical fiber requires less connection loss with the existing system, and does not need to consider the peak of transmission loss that occurs in higher-order modes.

The optical fiber 1 used in the present invention is formed so that the average relative refractive index difference of the core 1a is larger than that of the communication optical fiber. Here, the average relative refractive index difference of the core in the present invention means a value obtained by averaging the core-clad relative refractive index difference in the cross-sectional direction, and is defined by the following mathematical formula (I).

[Equation 1] Here, Score represents the cross-sectional area of the core, and δ is a function representing the distribution of the relative refractive index difference. The numerator of the above formula (I) is
This function δ is obtained by integrating only the inside of the core by dS (area fragment of the core cross section). Here, when the distance from a center of the optical fiber to a certain point in the optical fiber is r, δ is a function of r, and δ = δ (r) is a relative refractive index difference at a point of radius r from the center of the core. Indicates the size. When δ is cylindrically symmetric, the average relative refractive index difference of the core is represented by the following mathematical formula (II), where R is the core diameter.

[Equation 2]

Specifically, in the present invention, the optical fiber 1
A 1.3 μm zero-dispersion optical fiber, a cutoff shift optical fiber, or a dispersion-shifted optical fiber can be preferably used as the optical fiber. However, when the 1.3 μm zero-dispersion optical fiber is used, 1.3 μm zero-dispersion optical fiber for communication is used. While the average relative refractive index difference of the core of the fiber is usually about 0.35%, the 1.3 μm zero-dispersion optical fiber used in the present invention is formed such that the average relative refractive index difference of the core is larger than this. ing. When using a cutoff shift optical fiber,
While the average relative refractive index difference of the core of the cutoff shift optical fiber for communication is usually about 0.4 to 0.45%, the average ratio of the core of the cutoff shift optical fiber used in the present invention is The refractive index difference is formed larger than this. When using a dispersion-shifted optical fiber, the average relative refractive index difference of the core of the dispersion-shifted optical fiber for communication is usually about 0.4%, while the dispersion-shifted optical fiber used in the present invention is The average relative refractive index difference of the core is formed larger than this.

The difference in the average relative refractive index difference between the core of the optical fiber 1 used in the present invention and that of the core for communication is obtained because the stop band width in the optical fiber grating is changed as described later. Is set according to the length (grating length) of the grating portion 2 and the stop band width in the optical fiber grating. When the grating length is constant, the larger the average relative refractive index difference of the core of the optical fiber 1 is, the smaller the stop band width becomes, which is desirable as will be described later. However, in the single mode optical fiber, the average relative refractive index of the core is small. If the difference is made too large, the mode field diameter of the optical fiber 1 becomes small, so that there arises a problem that the connection loss becomes large when connecting to an existing system system or the like. Therefore,
It is desirable that the average relative refractive index difference of the core of the optical fiber 1 used in the present invention is increased as long as the above-described problem of connection loss does not occur in the communication optical waveguide to be connected. Therefore, the upper limit is determined by the allowable range of the splice loss. For example, when the allowable range of the splice loss is about 0.2 dB or less, the average relative refractive index difference of the core 1a of the optical fiber 1 is 1.5.
It is preferably set within the range of not more than%. In this embodiment,
A 1.3 μm zero-dispersion optical fiber with an average relative refractive index difference of 0.7% of the core 1a is used, and the grating length is 20 mm,
A radiation mode coupling type optical fiber grating having a stop band width of 20 nm is constructed.

The optical fiber 1 on which the grating portion 2 is formed is fixed on the substrate 3. The material of the substrate 3 is preferably one having a small difference in coefficient of thermal expansion from the optical fiber 1, and a quartz substrate is preferably used. The shape of the substrate 3 can be arbitrary. An adhesive 4 is preferably used for fixing the optical fiber 1 to the substrate 3, and for example, an epoxy adhesive, an ultraviolet curable adhesive, or the like is preferably used. In the radiation mode coupling type grating, when the adhesive 4 comes into contact with the grating portion 2, the grating characteristics are changed. Therefore, the portions other than the grating portion 2, preferably, 5 mm or more apart from both ends of the grating portion 2, respectively. Adhesive is applied to both sides. In this embodiment, the length of the grating portion 2 is 20 m.
m, the distance a between the grating portion 2 and the adhesive 4 is 5 mm on both sides, and the adhesive 4 is applied to the optical fiber 1
The length b required to fix the board to the substrate 3 is 5 on each side.
mm, and a quartz tube having a length of 40 mm is used as the substrate 3.

The method of manufacturing the optical fiber grating of the present invention will be described below. First, as shown in FIG. 2, the relationship between the grating length, the average relative refractive index difference of the core, and the stop band width is obtained in advance by the calculation method described later, and the grating length and stop band to be obtained based on this are calculated. The average relative refractive index difference of the core that can simultaneously achieve the width is obtained. Figure 2 shows that the grating length is 15, 20, 2
5 is a graph exemplifying the relationship between the average relative refractive index difference of each core and the stop band width at 5, 30, 35, 40 mm, where the horizontal axis represents the average relative refractive index difference of the core and the vertical axis represents the stop band. The calculation result of the width (theoretical limit value) is shown.

A method of calculating the stop band width (theoretical limit value) of the radiation mode coupling type optical fiber will be described below. As described above, the radiative mode coupled grating is configured to attenuate the guided mode light by coupling the guided mode (of the core) with the mode propagating in the cladding. The attenuation factor 2α of the optical power of the mode is expressed by the following mathematical expression (1).

(Equation 3) Here, m represents the mode number of the clad to be coupled, and C
m represents the coupling constant when coupling to the m-th mode of the clad. Φ (Δβm) is the square of the Fourier transform of the grating pattern, and corresponds to the grating period (or the basic period if the grating change is not sinusoidal) when the grating pattern is evenly spaced. Φ (Δβm) is expressed by the following mathematical expression (2).

(Equation 4) Here, Δβm is the difference in propagation constant between the m-th mode of the clad and the mode guided in the core. Further, Λ represents the pitch of the grating, and ν represents the number of pitches of the grating. FIG. 3 shows the relationship between Φ and Δβ in the vicinity of Δβ = 2π / Λ.

As shown in this figure, Δβ is 2 (ν-
1) When π / νΛ and Δβ is 2 (ν + 1) π / ν
Assuming that the wavelengths for Λ are λl and λs, respectively, when the destination wavelength is λl to λs (that is, Δβ is 2 (ν−
1) From π / νΛ to 2 (ν + 1) π / νΛ), the value Φ (Δβm) of the equation (2) becomes finite, and the attenuation rate 2α becomes a large value according to the equation (1). λl to λs
Outside the range of, Φ (Δβm) becomes almost 0, and the attenuation rate 2α becomes negligible. That is, the loss due to the grating increases between λl and λs to form a stop band, and the interval between λl and λs is the stop band width Δ.
It is defined as λ. Also, the formula Δβ (λl) −Δβ (λ
s) = 4π / νΛ (3) holds, where λc = (λ
l + λs) / 2, the above equation (3) can be rewritten as the following equation (4),

(Equation 5) By approximating the expansion up to the first order, the above equation (4) can be rewritten as the following equation (5).

(Equation 6) If this is further modified, it becomes the following formula (6),

(Equation 7) Further, if the grating length is L, then L = νΛ, and therefore the following formula (7), that is, the formula for calculating the stop bandwidth is obtained.

(Equation 8)

Here, in the above equation (7), Δβ (λ
c) is the difference between the propagation constant of the guided mode of the core and the propagation constant of the cladding mode at the center wavelength λc of the stopband. As explained below, the larger the average relative refractive index difference of the core, the more Δβ It can be seen that the wavelength derivative of (λc), that is, the denominator of Expression (7) becomes large, so that the right side of Expression (7) becomes small, and thus the stop bandwidth (Δλ) becomes small.

That is, in FIG. 4, the horizontal axis represents the frequency of light (ω) and the vertical axis represents the effective refractive index (neff) of the mode.
The relationship (dispersion curve) between the two is shown. In this figure, the dispersion curve of the guided mode of the core of the optical fiber having a large average relative refractive index difference of the core, is the dispersion curve of the guided mode of the core of the optical fiber of the small average relative refractive index difference of the core,
Shows the dispersion curves of the cladding modes, respectively.
As shown in this figure, when light of the same frequency (ω) is used, the effective refractive index (neff) of the guided mode of the core is differentiated by the frequency (ω) (corresponding to the slope of the dispersion curve). That is, the frequency dependence of the effective refractive index is larger in an optical fiber having a larger average relative refractive index difference in the core than in an optical fiber having a smaller average relative refractive index difference in the core. But,
The effective refractive index of the cladding mode is hardly affected by the difference in average relative refractive index difference of the core. Therefore, the optical fiber having a large difference in average relative refractive index of the core has a greater frequency dependence of the difference in effective refractive index between the waveguide mode and the cladding mode of the core. Since the frequency is proportional to the reciprocal of the wavelength (ω = 2πν = 2πC / λ), the difference in the effective refractive index between the waveguide mode and the cladding mode of the core is larger in the optical fiber having a larger average relative refractive index difference of the core. This means that the absolute value of the wavelength dependence of is large. Since the propagation constant is equal to the effective refractive index multiplied by the wave number of light in vacuum, the absolute value of the wavelength derivative of the difference (Δβ) in the propagation constant between the guided mode and the cladding mode of the core is It can be said that the optical fiber having a larger average relative refractive index difference is larger.

The condition of the grating length of the optical fiber grating to be obtained is determined mainly in consideration of the accommodation property and vibration resistance of the optical fiber grating. In this embodiment, the grating length is set to 20 mm in order to obtain an optical fiber grating that can be housed and laid in a submarine repeater. That is, in order to accommodate the optical fiber grating as shown in FIG.
Must be 0 mm or less. Further, in the case of the radiation mode coupling type optical fiber grating, in order to fix this on the substrate 3 with the adhesive 4, it is necessary to have fixing stands of 10 mm or more on both sides of the grating portion 2. Therefore, in order to fix the substrate 3 having a length of 40 mm or less, the grating length is limited to 20 mm or less.

The diameter of the optical fiber 1 is usually 125 μm.
m, the tension of the quartz glass string having a diameter of 125 μm and the resonance frequency have a relationship as shown in FIG. FIG. 5 shows that the length is 10, 15, 20, 25, 30,
It shows the relationship between the tension and the resonance frequency of a 35-40 mm quartz glass string. Generally, when performing a vibration test on optical components, the upper limit of vibration is set to 2000 Hz because the vibration generated when laying a cable or a repeater is 2000 Hz or less. Regarding the vibration resistance of an optical fiber grating, its resonance frequency is 2
By designing the frequency to be 000 Hz or higher, it is possible to prevent the optical fiber grating from resonating and being damaged during a vibration test or during installation.

By the way, in order to accommodate the optical fiber grating in the submarine repeater as described above, the length of the substrate 3 is limited to 40 mm or less. Then, an adhesive 4 is used to fix the optical fiber grating.
Since the length b of the portion to be coated with must be at least 5 mm on each side, the length of the optical fiber 1 between the adhesive fixing points cannot be longer than about 30 mm. On the other hand, if the tension applied to the optical fiber grating is too large, a problem occurs in the reliability of the optical fiber 1. Therefore, the tension is 1/4 of 200 gf which is the proof strength.
It is preferably used in an environment of ⅕ or less. Therefore, the tension applied to the optical fiber grating is 40-
It should be 50 gf or less. Thus, the upper limit of the tension applied to the optical fiber grating is 40 to 50.
In order to set the resonance frequency to 2000 Hz or higher under the condition that the length is gf and the length of the optical fiber grating between the adhesive fixing points cannot be longer than 30 mm, as shown in FIG. It is necessary to set the length of the optical fiber grating between the points to about 30 mm or less. Since the distance a between the grating portion 2 and the adhesive 4 is preferably at least 5 mm, the grating length is preferably 20 mm or less.

The condition of the stop band width of the optical fiber grating to be obtained is determined by the application of the optical fiber grating. For example, in an optical communication system that performs wavelength division multiplexing transmission, in the case of a radiation mode coupling type optical fiber grating used to reduce the wavelength dependence of the gain of an erbium-doped optical fiber amplifier, the stop bandwidth is the wavelength range used for transmission. And the stop band width is preferably set to 20 nm in this embodiment.

Under the conditions of the grating length and the stop band width thus determined, a suitable value of the average relative refractive index difference of the core 1a of the optical fiber 1 used for manufacturing the grating is determined. In this embodiment, in order to obtain a radiative mode coupling type optical fiber grating having a grating length of 20 mm or less and a stop band width of 20 nm, FIG.
From the graph, it is understood that the average relative refractive index difference of the core 1a should be 0.7% or more.

When the suitable average relative refractive index difference of the core 1a of the optical fiber 1 is determined, the optical fiber 1 satisfying this is prepared, and the refractive index of the core is periodically changed by a part of the optical fiber 1 to make the grating part. Form 2. When manufacturing the grating portion 2, parameters other than the grating length of the optical fiber grating to be obtained change the center wavelength and the blocking rate, so they are appropriately set according to the grating characteristics to be obtained.

The method of forming the grating portion 2 is not particularly limited, but in the present embodiment, the core 1a of the optical fiber 1 is made of a material whose refractive index changes by irradiation with ultraviolet light, so that the optical fiber 1 has a predetermined shape. The grating part 2 can be preferably formed by periodically irradiating the region of 1 with ultraviolet light in the length direction of the optical fiber 1. The wavelength of the ultraviolet light applied to the optical fiber 1 is 200 to
About 300 nm is preferable, and the light source is, for example, Kr.
An F laser (wavelength 248 nm) is preferably used. Then, a method of irradiating the optical fiber 1 with ultraviolet light having a relatively large spot width, or a method of irradiating the ultraviolet light with a reduced spot width through a photomask in which slits with constant intervals of several tens to several hundreds of μm are cut A method of irradiating the optical fiber 1 with ultraviolet light at a constant interval by repeating the operation of irradiating No. 1 for a certain period of time, stopping the irradiation, moving the irradiation position in the length direction of the optical fiber 1 and irradiating again. Such,
The grating section 2 can be formed by appropriately using a known method.

Further, the hydrogenation treatment of the optical fiber 1 may be performed prior to the irradiation of the ultraviolet light. In an optical fiber in which the germanium concentration in the core 1a is at most several percent or less, it is preferable to carry out hydrogenation treatment in advance in order to obtain a sufficient change in the refractive index of the core due to irradiation with ultraviolet light. This hydrogenation treatment is performed, for example, by using the optical fiber 1 at 100 atm,
It is achieved by holding in a hydrogen pressure vessel adjusted to about 50 ° C. for about 48 hours. However, this hydrogenation treatment is not essential, and when the germanium concentration in the core 1a is about 30% and the rejection rate of the optical fiber grating may be relatively low, it may be configured not to perform it. . When such a hydrogenation treatment is performed before irradiation with ultraviolet light, it is preferable to desorb hydrogen in the optical fiber 1 after forming the grating portion 2. This dehydrogenation step is performed, for example, by leaving the optical fiber 1 under a temperature condition of room temperature to 100 ° C. for several days. This dehydrogenation step is intended to prevent a change in the refractive index due to the hydrogen itself added to the optical fiber 1 prior to the irradiation of the ultraviolet light and the grating characteristics after the grating portion 2 is manufactured to change over time. It is valid.

After forming the grating portion 2 in this way, the optical fiber 1 is bonded and fixed onto the substrate 3 to obtain an optical fiber grating.

According to the optical fiber grating of the present embodiment, the stop band width can be narrowed by increasing the average relative refractive index difference of the core of the optical fiber 1 without increasing the grating length. You can Further, when manufacturing an optical fiber grating, the relationship between the grating length, the average relative refractive index difference of the core and the stop band width can be obtained by the above calculation, and based on this, a suitable average relative refractive index difference of the core is calculated. Since the value can be obtained, an optical fiber grating having a desired grating length and stop band width can be easily and surely obtained.

[0034]

【Example】

(Example 1) First, a 1.3 μm zero-dispersion optical fiber having a core made of silica glass doped with germanium oxide and a cladding made of pure silica glass, having an average relative refractive index difference of 1.0%. An optical fiber was prepared. Ultraviolet light with a spot width of 200 μm (wavelength 248n
After irradiating m) for a certain period of time, the irradiation was stopped, the irradiation position was moved in the length direction of the optical fiber, and the operation of irradiating again was repeated to form a grating portion.
The grating pitch was 400 μm, and the grating length was 20 mm. The obtained optical fiber grating had characteristics as a radiation mode coupling type grating, and the stop band width was 15 nm.

COMPARATIVE EXAMPLE 1 First, a 1.3 μm zero-dispersion optical fiber for communication having a core made of quartz glass doped with germanium oxide and a clad made of pure silica glass was prepared. The average relative refractive index difference of the core of this optical fiber was about 0.35%. A grating portion was formed on this optical fiber by the same method as in Example 1 above. The grating pitch was 400 μm, which was the same as in Example 1 above.
Even if the grating length was set to 40 mm, which is twice the grating length in Example 1, the stop band width was narrowed to about 20 nm.

In the above embodiment, the example in which the grating portion is formed by periodically changing the refractive index of the core has been described, but the structure of the grating portion is not limited to this, and the above formula (1) is used. Any configuration can be adopted as long as it is a radiation mode coupling type grating in which Further, the present invention can be similarly applied not only to the optical fiber grating but also to the case where a planar optical waveguide is used as the optical waveguide.

[0037]

As described above, the optical waveguide grating of the present invention is characterized in that the radiation mode coupling type grating is formed in the optical waveguide having a larger average relative refractive index difference of the core than the communication optical waveguide. To do. Therefore, the stop band width can be narrowed without increasing the grating length. If the optical waveguide having a single mode type transmission characteristic in the operating wavelength range of the grating is used, the connection loss with the existing optical communication system using the single mode optical waveguide can be small, and This is preferable because it is not necessary to consider the peak of transmission loss that occurs in the higher-order mode.

In the method of manufacturing an optical waveguide grating of the present invention, the relationship between the grating length, the average relative refractive index difference of the core, and the stop band width in the grating is obtained in advance, and the relationship is obtained based on this relationship. It is characterized in that the average relative refractive index difference of the core that simultaneously satisfies the grating length condition and the stop bandwidth condition is obtained, and the radiation mode coupling type grating is formed in the optical waveguide having the average relative refractive index difference of the core. . Therefore, an optical waveguide grating having a desired grating length and stop band width can be easily and surely obtained.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of an optical waveguide grating of the present invention.

FIG. 2 is a graph showing an example of the relationship between the grating length, the average relative refractive index difference of the core, and the stop band width according to the present invention.

FIG. 3 is a graph for explaining a formula for calculating a stop bandwidth according to the present invention.

FIG. 4 is a graph showing the relationship between the frequency of light and the effective refractive index in order to explain the relationship between the average relative refractive index difference of the core and the stop band width in the optical waveguide grating.

FIG. 5 is a graph showing the relationship between the optical fiber length, tension, and resonance frequency in the optical fiber grating according to the present invention.

FIG. 6 is a graph showing characteristics of a radiation mode coupling type optical waveguide grating.

FIG. 7 is a graph showing an example of wavelength characteristics of gain of an optical fiber amplifier.

[Explanation of symbols]

 1 ... Optical fiber, 1a ... Core, 2 ... Grating part.

Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location G02B 6/34 G02B 6/28 D (72) Inventor Satoshi Okude 1440 Rosaki, Sakura City, Chiba Fujikura Ltd. Sakura Factory (72) Inventor Masaaki Sudo 1440 Rokuzaki, Sakura City, Chiba Prefecture Fujikura Co., Ltd.Sakura Factory (72) Inventor Tetsuya Sakai 1440 Rokuzaki, Sakura City, Chiba Prefecture Fujikura Sakura Factory (72) Inventor Akira Wada 1440 Rokuzaki, Sakura-shi, Chiba Fujikura Co., Ltd.Sakura factory (72) Inventor Ryozo Yamauchi 1440 Rokuzaki, Sakura-shi, Chiba Fujikura Sakura factory, Ltd.

Claims (3)

[Claims] 1. An optical waveguide grating, characterized in that a radiation mode coupling type grating is formed in an optical waveguide having a core having a larger average relative refractive index difference than that of a communication optical waveguide. 2. The optical waveguide grating according to claim 1, wherein the optical waveguide has a single mode type transmission characteristic in an operating wavelength range of the grating. 3. The relationship between the grating length, the average relative refractive index difference of the core and the stop band width in the grating is obtained in advance, and the grating length condition and the stop band width condition to be obtained are simultaneously determined based on this relationship. A method for manufacturing an optical waveguide grating, characterized in that a difference in average relative refractive index of the cores to be satisfied is obtained and a radiation mode coupling type grating is formed in the optical waveguide having the average relative refractive index difference of the core.