JP2003029063A - Optical fiber grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber grating characterized by that the center wavelength of loss does not vary against a manufacture error in grating cycle and signal light has no loss even outside a desired wavelength band during wide-band wavelength-multiplex communication, etc. SOLUTION: An optical fiber having a core 1 with a diameter of 2 to 6 μm and having >=1% relative refractive index difference of the core 1 from a clad 3 has grating parts formed in grating cycles of <=100 μm. Further, an optical fiber having an inner clad 2 with a diameter of 5 to 30 μm formed from the outer circumference of the core 1 to the inner circumference of the clad 3 and having >=1.5% relative refractive index difference of the inner clad 2 from the clad 3 has grating parts.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber grating useful in an optical communication system, an optical filter using the same, and an optical fiber type wavelength tunable filter.

[0002]

2. Description of the Related Art In an optical communication system, an optical fiber grating in which a grating portion is formed in an optical fiber is often used. Generally, in the grating part, there are a type in which a fundamental mode propagating in the same direction as the incident direction (positive direction) and a mode propagating in the cladding in the positive direction are combined, and a fundamental mode propagating in the core in the positive direction. There is a type that couples the core with a mode propagating in the negative direction. The former is generally called the radiation type and the latter is called the reflection type. Then, it can be used as, for example, an optical filter by coupling and attenuating light in a desired wavelength band. In general, a mode propagating in the core or the clad in the positive direction is called a transmission mode (transmitted light), and a mode propagating in the opposite negative direction is called a reflection mode (reflected light).

It is the perturbation formed in the core of the grating that enables such coupling between modes. In optical fibers, it is the refractive index change that creates this perturbation. In addition, a microbend may be used. For example, mode coupling can be achieved by periodic refraction changes of several hundred μm for the radiation type and 1 μm or less for the reflection type. This is why the radiation type is called the long period type and the reflection type is called the short period type. The period of such periodic changes in the refractive index and microbend is called the grating pitch.

At this time, what is important is the difference between the propagation constants of the two coupled modes. When this difference is large, the grating pitch becomes smaller,
When the difference is small, the grating pitch becomes large.
The reason why the grating portion coupled to the reflection mode has a short period is that the propagation constant difference between the transmission mode and the reflection mode propagating in the core is large.

[0005]

By the way, conventionally, a short period type optical fiber grating has been used for a "narrow band filter" having a narrow wavelength band of light to be attenuated.
However, since the short period type is a reflection type, the generated reflected light may adversely affect. Therefore, it is necessary to install an isolator in front of the device affected by the reflected light to block the reflected light. Therefore, there are problems that the number of parts for constructing the optical communication system increases, the system cost cannot be reduced, and excess loss (insertion loss and transmission loss) increases.

On the other hand, the radiation type has a relatively wide attenuation band, no reflected light exists, and is non-reflection. Therefore, it is used as a gain equalizer or a notch filter near an amplifier or a laser. However, the radiation type has a problem that the grating length (grating portion length) is generally long. In particular, when an emission type narrow band filter is to be produced, the length is about 200 times that of the short period type filter, for example, about 2 m, which is practically impossible. Further, since there are many transmission blocking peaks, there is a problem that the transmission loss in the transmission band becomes large.

The wavelength tunable filter whose attenuation band can be changed is advantageous because it can meet various required characteristics. The following characteristics are required to realize a wavelength tunable filter. (1) The central wavelength of the attenuation band can be actively controlled. (2) The attenuation band can be set freely. (3) The response speed is fast with respect to the control of the attenuation band. (4) High controllability. For example, in the conventional reflection type, the wavelength tunable filter is changed by actively changing the central wavelength of the reflected light by changing the tension applied to the grating part or by changing the temperature of the grating part (optical fiber) itself. Can be configured. However, in the former, there are problems in the mechanical reliability of the grating part (optical fiber) and the controllability of the center wavelength,
In the latter, there were problems in response speed and controllability. That is, the conventional reflection type has problems in (1) and (3). Further, the conventional radiation type could not satisfy the characteristic (2).

By the way, the notch filter is for removing unnecessary wavelength band light from the signal light in the wavelength multiplexing system. Specifically, it is used to remove the emitted light generated by the optical fiber amplifier and to improve the extinction ratio between the wavelengths of the multiplexed optical signals. Both the radiation type and the reflection type are used for the notch filter depending on the application, but in this case, there were the following problems. That is, the notch filter is desirably required to have the following characteristics. (1) The attenuation band is relatively wide and the bandwidth can be easily adjusted. (2) The transmission blocking rate is sufficiently large. Specifically 10
It is preferably at least dB. (3) The rise of the transmission blocking peak in the attenuation band is steep. (4) There is no reflected light.

Of these (1) to (4), the radiation type satisfies the condition (4), but does not satisfy the condition (3). In addition, the conditions (1) and (2) are often insufficient. On the other hand, the reflective type can satisfy the conditions (2) and (3) with relative accuracy and easily, but (1),
The condition of (4) cannot be satisfied.

The present invention has been made in view of the above circumstances, and is an optical fiber grating capable of forming an optical filter in which reflected light does not exist, which has characteristics that cannot be obtained by the conventional radiation type and reflection type. The challenge is to provide. Specifically, it is an object of the present invention to provide an optical fiber grating that does not require an isolator or the like and that can configure an optical filter having a non-reflection narrow band, which can construct an optical communication system with a small number of devices. Another object of the present invention is to provide an optical fiber grating capable of forming a non-reflection narrow band optical filter and having a short grating length. Another object of the present invention is to provide an optical fiber grating capable of forming a non-reflection narrow band optical filter having a small insertion loss or transmission loss. It is another object of the present invention to provide an optical fiber grating in which the central wavelength of loss is not greatly changed due to the manufacturing error of the grating pitch. Further, the present invention provides an optical fiber grating capable of reducing the number of loss peaks occurring in a wavelength band other than the desired wavelength band and preventing the signal light from being lost in the wavelength band other than the desired wavelength band in broadband wavelength division multiplexing communication. This is an issue. Another object of the present invention is to provide an antireflection narrow band optical filter that can construct an optical communication system at low cost, and an optical fiber grating that constitutes the optical filter. Another object of the present invention is to provide an optical fiber grating having preferable characteristics for use in a wavelength tunable filter. Specifically, it is an object of the present invention to provide a device that can actively control the central wavelength of the attenuation band, can freely set the attenuation band, and has a fast response speed with respect to the control of the attenuation band. Also,
An object of the present invention is to provide an optical fiber grating that can obtain an optical filter having preferable characteristics as a notch filter. Specifically, it is an object of the present invention to provide an attenuation band that is relatively wide, the bandwidth can be easily adjusted, the transmission rejection rate is sufficiently large, and the rise of the transmission prevention peak in the attenuation band is steep. To do.

[0011]

In order to solve the above problems, the present invention proposes the following optical fiber grating and an optical filter using the same. The optical fiber grating of the first invention is characterized in that the core of the optical fiber is provided with a radiation type grating portion in which a perturbation is formed at a grating pitch of 20 to 80 μm along the length direction thereof. An optical fiber grating of a second invention is the optical fiber grating of the first invention, in which a fundamental mode propagating in the core in a positive direction is coupled to a positive direction mode of 10th order or higher than the fundamental mode. It is characterized in that Third
The optical fiber grating of the invention of
In the optical fiber grating of the invention, the grating length is 30 mm or less. Fourth
The optical fiber grating of the present invention is characterized in that, in the optical fiber grating of any one of the first to third inventions, the grating pitch is a chirp pitch. The optical fiber grating of the fifth invention is an optical fiber having a core diameter of 2 μm to 6 μm and a relative refractive index difference of the core with respect to the cladding of 1% or more,
It is characterized in that a grating portion is formed with a grating pitch of 100 μm or less so that a fundamental mode propagating in the positive direction of the core is coupled to a clad mode of 10th order or less. An optical fiber grating of a sixth invention is the optical fiber grating of the fifth invention, wherein an inner clad is provided on the outer circumference of the core and the inner circumference of the clad, and the inner clad has a diameter of 5 μm to 30 μm.
m, and the relative refractive index difference of the inner cladding with respect to the cladding is 1.5% or more. A seventh invention is a method for producing an optical fiber grating according to any one of the first to sixth inventions by irradiating a laser beam with a predetermined grating pitch in a length direction of the optical fiber, and oscillating from a light source. The laser light is focused on an optical fiber through a lens and a spatial filter. An eighth invention is an optical filter using the optical fiber grating according to any one of the first to sixth inventions. A ninth invention is characterized in that, in the optical filter of the eighth invention, the transmission blocking rate is 5 dB or more. A tenth invention is characterized in that, in the optical filter of the ninth invention, the transmission blocking rate is 10 dB or more. An eleventh invention is a notch filter characterized by using the optical filter of the tenth invention. A twelfth invention is that a coating layer made of transparent plastic whose refractive index changes according to temperature change is provided on the outer periphery of the grating portion of any one of the first to sixth optical fiber gratings, and the coating layer is heated. The optical fiber type wavelength tunable filter is characterized in that a heater is provided. A thirteenth invention is characterized in that, in the optical fiber type wavelength tunable filter of the twelfth invention, the transmission blocking rate is 5 dB or more. A fourteenth invention is the optical fiber type wavelength tunable filter according to the thirteenth invention, wherein the transmission blocking rate is 1 or less.
It is characterized by being 0 dB or more.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below with reference to first, second, third and fourth embodiments. [First Embodiment] The first embodiment is an example of a non-reflection narrow band type. The inventors of the present invention found that an optical fiber grating capable of forming a narrow-band optical filter can be obtained by making the grating pitch of the emission optical fiber grating extremely smaller than the conventional one, and completed the present invention. Let Reducing the grating pitch increases the propagation constant difference between the modes used for coupling.

The high-order transmission mode propagating in the clad has a large difference in propagation constant from the fundamental mode propagating in the core. However, since the higher-order transmission mode generally has a poor coupling efficiency with the fundamental mode, it may not be possible to ensure a sufficient transmission blocking rate even if the transmission modes are indiscriminately coupled with the transmission mode.

Therefore, as a result of simple calculation by the present inventors in the LP mode, for example, in the case of a general 1.3 μm single mode optical fiber, the fundamental mode and LP (0,
22) When the optical fiber grating formed with a grating part having a grating pitch of 60 μm and a grating length of 25 mm is used as an optical filter by coupling with a higher-order transmission mode, the spectrum of the transmitted light shown in FIG. , The bandwidth is about 1.0
It was found that a transmission blocking peak having a wavelength of 10 nm and a transmission blocking rate of 10 dB or more was obtained. In this example, there are two transmission blocking peaks near 1320 nm and 1460 nm, but the transmission blocking rate is 20 dB or more at the peak near 1320 nm, and the transmission blocking rate is about 15 dB at the peak near 1460 nm. Also, unlike the past,
Since a narrow band type peak is obtained, the excess loss other than the transmission stop band is very small.

The 1.3 μm single mode optical fiber used in this example is composed of a core provided at the center and a clad provided on the outer periphery of the core and having a refractive index lower than that of the core. The core is made of germanium-doped quartz glass and the clad is made of pure quartz glass. The core refractive index is 1.448, the clad refractive index is 1.444, the core diameter is 9 μm, and the clad outer diameter is about 1.
It is 25 μm.

In the examination of the present invention, the mode is set to LP.
(0,1), LP (0,2), LP (0,3) ... LP
(0, n-1) and LP (0, n) (n is an integer) were simply calculated. The actual optical fiber mode is, for example, LP
Like (0,1), LP (1,1), LP (2,1) ..., it does not necessarily consist of only the mode represented by LP (0, n). Even if the mode represented by n) is limited, there is no problem in actual application to the optical fiber grating, and the calculation can be performed easily, which is advantageous for determining design conditions.

In practice, the grating pitch is 20 to
By setting the thickness to 80 μm, preferably 40 to 60 μm, it is possible to manufacture an optical fiber grating capable of forming a non-reflection narrow band optical filter. The optical fiber used may be, for example, a general 1.3 μm single-mode optical fiber, a dispersion-shifted optical fiber, or the like. Moreover, the grating pitch is 20 to 80 μm.
By this, the fundamental mode can be coupled to a transmission mode of 10th order or higher, substantially 10th to 30th order higher than the fundamental mode, and using this optical fiber grating, a non-reflection narrow band An optical filter can be provided. At this time, the tenth or higher order higher mode is
Unlike the above calculation, the mode actually propagated through the optical fiber is used as a reference. For example, LP (0,1), LP
In an optical fiber having modes of (1,1), LP (2,1) ..., LP (0,1) is used as a reference for LP.
This is a case where (1,1) is counted as the primary and LP (2,1) is counted as the secondary. By coupling to a sufficiently high-order transmission mode in this way, a non-reflection narrow band optical filter can be constructed using this optical fiber grating.

When the optical fiber grating of the present invention is used for an optical filter, the bandwidth is 2 nm or less, preferably, even if the grating length of the optical fiber grating is 30 mm or less, preferably 25 mm or less (substantially 5 mm or more). Is less than 1 nm and the transmission blocking rate is 5
A transmission blocking peak of not less than dB, preferably not less than 10 dB is obtained. Although the lower limit of the bandwidth is not particularly limited, it is substantially 0.1 nm. Although the upper limit of the transmission blocking rate is not particularly limited, it is substantially 40 dB.

Further, conventionally, a phase mask method utilizing a photorefractive effect, a metal mask method and the like have been used for manufacturing an optical fiber grating. The photorefractive effect is a phenomenon in which the refractive index changes when light of a predetermined wavelength is irradiated, and in an optical fiber grating, for example, when the core of an optical fiber made of germanium-doped quartz glass is irradiated with ultraviolet light near 240 nm, it is refracted. Take advantage of the phenomenon of increasing rates. An excimer laser or the like is used as the light source. The phase mask method is often used when manufacturing a reflective type having a grating pitch of 1 μm or less, but when the grating pitch is longer than that of the reflective type as in the present invention, an appropriate phase mask is commercially available. In addition, there is a problem that the characteristics vary even if the grating portion is manufactured using the same phase mask.
The same applies to the metal mask method. Therefore, it is preferable to use a manufacturing method in which the grating pitch and the like can be flexibly changed to finely adjust the grating characteristics.

Therefore, in the optical fiber grating of the present invention, after the laser light generated from the light source is condensed on the optical fiber by using the spatial filter as shown in FIGS. 2 (a) to 2 (c). It is preferable to manufacture the grating section by repeating the operation of stopping the irradiation of the laser beam and then moving the condensing position along the length direction of the optical fiber according to the grating pitch and irradiating the laser beam again. Conventionally, it has been considered difficult to narrow the laser light focusing to 40 μm or less, but it is possible to reduce the laser focusing to about 40 μm or less by using a spatial filter. In addition, in order to correspond to the grating pitch of the optical fiber grating of the present invention, the focusing is 40 μm.
The following is desirable.

FIG. 2 (a) shows an example of a method for manufacturing an optical fiber grating, in which reference numeral 1 is an optical fiber. An optical system is composed of the three convex lenses 2 to 4 and the spatial filter 5. Convex lens 2-4
Are sequentially arranged in parallel so as to cross the path of light from the light source to the optical fiber 1. The convex surface of the convex lens 2 is arranged on the light source side, and the convex surfaces of the convex lens 3 and the convex lens 4 are arranged on the optical fiber 1 side. At the center between the convex lens 2 and the convex lens 3, as shown in FIG.
A spatial filter 5 for forming a slit 5a of m is arranged. As shown in this figure, the spatial filter 5 includes two rectangular plates arranged in the same plane to form the slit 5
a may be formed, or the slit 5a may be formed in advance on one plate. The spatial filter 5 is usually made of metal such as stainless steel or ceramics.

In this example, the converging distance f of the convex lenses 2 and 3 is 2000 mm, and the converging distance f of the convex lens 4 is 100.
mm. The width of the slit 5a is 0.3 mm. The distance between the convex lens 2 and the spatial filter 5 is 20
00 mm, the distance between the spatial filter 5 and the convex lens 3 is 20
It is 00 mm. When laser light is oscillated from a light source such as an excimer laser, the laser light passes through the convex lens 2 and becomes a light with a width of 0.3 mm at the slit 5a. Focus on. In addition, as shown in FIG.
It is also possible to form the spatial filter using the convex lenses 2 and 4. In this example, the focusing distances f of the convex lenses 2 and 4 are 2000 mm and 100 mm, respectively, the distance between the convex lens 2 and the spatial filter 5 is 2000 mm, and the distance between the spatial filter 5 and the convex lens 4 is 3000 mm. The width of the slit 5a, the focusing distance f of each of the convex lenses 2 to 4, the arrangement distance of each component, and the like are not particularly limited, and can be appropriately changed as necessary. The width of the slit 5a is usually preferably about 100 to 800 μm.

When the optical fiber grating of the present invention is used as an optical filter (non-reflection narrow band optical filter), for example, the optical fiber grating is provided in a commonly known protective casing made of stainless steel, ceramics or the like. It is stored and inserted into an optical communication system for use. Further, the following optical fiber type wavelength tunable filter can be configured. FIG. 3 is a partial side sectional view showing an example of the configuration of an optical fiber type wavelength tunable filter using the optical fiber grating of the present invention. Reference numeral 1 in the figure
Reference numeral 1 is an optical fiber, and this optical fiber 11 is provided with a grating portion 12 in the middle of which a change in the refractive index is formed at a predetermined grating pitch. The grating portion 12 is housed in the center of a circular tubular heater 13, and a coating layer 14 made of transparent plastic whose refractive index changes with temperature is provided between the inner wall of the heater 13 and the outer surface of the grating portion 12. Has been. Therefore, when the heater 13 is operated to change the temperature of the coating layer 14, its refractive index changes, and the characteristics of the grating portion 12 can be changed.

In this example, the optical fiber grating is the same as that shown in the above example, and the outer diameter of the optical fiber 11 is about 125 μm. Also, the heater 1
Reference numeral 3 is made of ceramic and has an inner diameter of 250 μm and an outer diameter of 2 mm. Further, the refractive index of the plastic constituting the coating layer 14 is preferably lower than that of the cladding of the optical fiber, and for example, the refractive index changes in the range of 1.30 to 1.40 in the temperature range of 20 to 200 ° C. The thing etc. are preferable. For example, acrylic plastic can be exemplified. FIG. 4 shows changes in the spectrum when the temperature of the coating layer 14 is changed by the heater 13 in this optical fiber type wavelength tunable filter.
n = 1.35 is a transmission block peak when the refractive index of the coating layer 14 is 1.35, and n = 1.40 is a transmission block peak when the refractive index of the coating layer 14 is 1.40. Coating layer 14
It can be confirmed that the wavelength of the transmission blocking peak changes due to the change in the refractive index of.

In this case, since tension is not used unlike the conventional optical fiber type wavelength tunable filter using the reflection type, there is no problem concerning the mechanical strength of the optical fiber 11 and the grating portion 12. Also, not the optical fiber 11 itself,
The temperature of the coating layer 14 around it is changed,
Since the heater 13 and the coating layer 14 can be arranged in close contact with or close to each other, the response speed can be made faster than that of the conventional reflection type in which the wavelength is changed by the temperature change of the optical fiber itself.

[Second Embodiment] The second embodiment is an optical fiber grating suitable for a notch filter. The optical fiber grating of the second embodiment differs from the first embodiment in that the grating pitch is a chirp pitch. The chirp pitch means that the grating pitch changes in the length direction of the grating portion. For example, in the length direction of the grating part, the grating pitch gradually expands or contracts from the light incident side, or the grating pitch gradually expands from the incident side toward the center of the grating part, and further to the emitting side. Examples thereof include those in which the grating pitch gradually decreases toward the front. It is known that when a chirp pitch is applied to an optical fiber grating, the transmission stop band is widened. In the second embodiment, in the optical fiber grating according to the first embodiment, the chirp pitch is set as the grating pitch, so that, for example, a narrow band type transmission block having a steep rise as shown in FIG. It is possible to obtain a transmission blocking peak in which the bandwidth of the peak is widened. For example, such characteristics can be obtained by gradually changing the grating pitch in the range of 51 μm to 53 μm. The numerical range of the grating pitch in the case of the chirp pitch, the rate of change, etc. can be appropriately changed according to the required characteristics.

In this optical fiber grating, the following characteristics suitable for a notch filter can be realized. That is, since it is a radiation type,
There is no reflected light. In addition, the bandwidth can be easily adjusted by changing the numerical range of the grating pitch, the rate of change, and the like. Further, as described above, the transmission blocking rate of the optical fiber grating of the present invention is sufficiently large, and a transmission blocking rate of 10 dB or more suitable for a notch filter can be realized. Further, it is possible to obtain a transmission blocking peak having a sharp rise. Further, by using this optical fiber grating, an optical filter and an optical fiber type wavelength tunable filter can be constructed similarly to the first embodiment.

[Third Embodiment] An optical fiber grating according to the third embodiment is characterized in that the fundamental mode propagating in the positive direction of the core is coupled to a cladding mode of 10th order or less. Is. In this optical fiber grating, a core diameter is 2 μm to 6 μm, and a grating portion is formed at a grating pitch of 100 μm or less in an optical fiber having a relative refractive index difference of 1% or more with respect to a clad. In the first embodiment, the grating pitch is made small and the fundamental mode and the tenth or higher order modes are coupled.
This is because as the grating pitch becomes smaller, the order of the cladding mode coupled with the fundamental mode becomes larger. When the grating pitch is reduced, the difference in propagation constant, which is the difference between the propagation constant (effective refractive index) of the fundamental mode, that is, the normal signal light, and the propagation constant (effective refractive index) of the higher-order clad mode to be coupled to it, Need to be bigger. Of these propagation constants, the propagation constant of the fundamental mode is universal, so it was necessary to reduce the propagation constant of the higher-order modes in order to increase the difference in the propagation constants. There is a relationship of delta β = 2π / Δ between the propagation constant difference delta β and the grating pitch Δ, and the propagation constant is maximum in the fundamental mode and decreases as the order of the mode increases.

FIG. 5 shows the relationship between the grating pitch and the loss central wavelength of a commonly used transmission single-mode optical fiber. Each line in FIG. 5 shows a cladding mode that is coupled to the fundamental mode. The modes are first-order, second-order, etc. in order from the upper side, and the lower order indicates the higher-order mode. The circles in FIG. 5 indicate the grating pitch for each cladding mode, which is necessary for the wavelength 1500 nm band to be the center wavelength of the loss. In FIG. 5, as can be seen from the small slope of the line indicating each cladding mode, the center wavelength of the loss is sensitive to the fluctuation of the grating pitch. Then, the center wavelength of the loss is greatly changed due to the manufacturing error of the grating pitch when manufacturing the optical fiber grating. The manufacturing error of such a grating pitch also affects the yield when manufacturing a narrow band filter, and even when the grating pitch varies by 1% or less, the bandwidth becomes larger than the desired value, and the filter manufacturing The controllability of the bandwidth at the time is deteriorated.

The third embodiment is intended to improve the above-mentioned problems that occur when coupling with a higher-order mode. The core diameter is 2 μm to 6 μm, and the relative refractive index difference of the core with respect to the clad is reduced. 1% or more, more preferably, an optical fiber having a core diameter of 3 μm to 6 μm and a relative refractive index difference of the core with respect to the clad of 2% or more is formed with a grating portion with a grating pitch of 100 μm or less, It is possible to couple to the cladding mode of. When the core diameter is 3 μm to 6 μm, it is advantageous in that the connection loss with the transmission optical fiber becomes small and the dependence of the central wavelength of the loss on the fluctuation of the grating pitch can be made small. FIG. 6 shows the relationship between the grating pitch and the central wavelength of loss in the third embodiment. Also in FIG.
Each line shows a cladding mode that is coupled to the fundamental mode. The modes are first-order, second-order, ... Modes from the top, and the higher the mode is, the lower the mode is. As can be seen from comparison with FIG. 5, the inclination of the line indicating the cladding mode is particularly large for the cladding modes of the 10th order or less, and it is found that the central wavelength of the loss fluctuates greatly due to the manufacturing error of the grating pitch. Can be suppressed.

Table 1 shows an example of the refractive index and relative refractive index difference of the core and the cladding of the optical fiber grating of the third embodiment.

[Table 1] Specifically, the core diameter is 4 μm, the relative refractive index difference of the core with respect to the cladding is 2%, and the grating pitch is 75 μm.
As an example, an optical fiber grating was manufactured and coupled with the LP08 mode, which is an eighth-order cladding mode, to manufacture an optical filter having a stop band near 1550 nm, which is a general communication band. The → and ◯ in FIG. 6 show the relationship between the grating pitch and the central wavelength of loss in this optical fiber grating. FIG. 7 illustrates the transmittance of the optical fiber grating of the third embodiment over a wavelength range of 1300 nm to 1600 nm. It can be seen that it has several stop bands in addition to the communication band near 1550 nm. According to the third embodiment, the fundamental mode can be coupled to the cladding modes of 10th order or less, and therefore, the optical fiber that does not greatly change the central wavelength of loss due to the manufacturing error of the grating pitch. A grating can be realized. Further, it is possible to prevent the controllability of the bandwidth at the time of manufacturing the filter from being deteriorated due to the manufacturing error of the grating pitch, and it is possible to improve the yield at the time of manufacturing the narrow band filter.

[Fourth Embodiment] In the fourth embodiment, the cladding has a double structure, the propagation constants of the higher-order cladding modes are made discrete, and the loss peaks generated in other than the desired band are eliminated. It is an optical fiber grating with a reduced number. FIG. 8 shows the structure of the optical fiber used in the optical fiber grating of this example. Reference numeral 15 is a core, and an inner clad 16 is provided on the outer periphery of the core 15.
And a clad 17 is formed on the outer periphery of the inner clad 16. This optical fiber
The core 15 has a diameter of 2 μm to 6 μm, the relative refractive index difference of the core 15 with respect to the cladding 17 is 1% or more, the inner cladding 16 has a diameter of 5 μm to 30 μm, and the relative refractive index difference of the inner cladding 16 with respect to the cladding 17 is 1.5%. It is above, More preferably, the diameter of the core 15 is 3 μm to 6 μm.
m, the relative refractive index difference of the core 15 with respect to the clad 17 is 2%
As described above, the diameter of the inner clad 16 is 10 μm to 20 μm.
m, the relative refractive index difference of the inner cladding 16 with respect to the cladding 17 is 3% or more. The diameter of the inner clad 16 is 5 μm to 30 μm, more preferably 10 μm to 20 μm.
The reason for m is that if the diameter of the inner clad 16 is too large, the effect of providing the inner clad 16 to make the propagation constant of the clad mode discrete is weakened, and conversely if the diameter of the inner clad 16 is too small, the propagation This is because the discretization of the constant progresses, but the dependence of the central wavelength of the loss on the grating period becomes stronger.

In order to adjust the refractive index of the core 15, the inner clad 16, and the clad 17, for example, the core 15
Is formed of SiO ₂ added with GeO ₂ , the inner cladding 16 is formed of SiO ₂ , and the cladding 17 is formed of SiO ₂ added with B ₂ O ₃ or F. Table 2 shows the core 1 of this optical fiber.
5, an example of the refractive index and the relative refractive index difference of the inner clad 16 and the clad 17 is shown.

[Table 2] In the optical fiber grating of this example, the above-mentioned optical fiber is provided with grating portions with a grating pitch of 100 μm or less. This optical fiber grating is intended to reduce the number of loss peaks generated at wavelengths other than the desired band shown in FIG. The cause of such a loss peak is shown in FIG.
As shown in (3), the spacing between the lines showing the cladding modes is narrow, and the propagation constants of the cladding modes are close to each other.

In this example, as described above, the cladding is
As a double structure, as shown in FIG. 9, the propagation constants of the cladding modes of the 10th order and below are made more discrete. Also in FIG. 9, each line shows a cladding mode that is coupled with the fundamental mode, and the modes are first-order, second-order, ... Specifically, the core diameter is 4 μm, the inner cladding diameter is 20 μm, and the relative refractive index difference of the core with respect to the cladding is 2 μm.
%, The relative refractive index difference of the inner clad with respect to the clad was 3%, and the grating pitch was 68 μm to fabricate an optical fiber grating. Fig. 10 shows the transmission loss of this optical fiber grating from wavelength 1300nm to 1600nm.
nm. As can be seen from comparison with FIG. 7, there are only two loss peaks that occur outside the desired wavelength band of 1550 nm, and the number of loss peaks could be reduced to about 1/3. According to the optical fiber grating of this example, the double structure of the clad reduces the number of loss peaks generated in other than the desired wavelength band, and in the broadband wavelength division multiplexing communication, the signal light is not in the desired wavelength band. It can prevent you from receiving losses.

[0035]

As described above, in the present invention, an optical fiber grating capable of forming an optical filter in which reflected light does not exist, which has characteristics that cannot be obtained by the conventional radiation type and reflection type. Can be provided. That is, it is possible to provide a radiation type and narrow band type optical fiber grating. Therefore, there is no need for an isolator or the like for blocking reflected light when it is inserted into an optical communication system as an optical filter or the like. Therefore, the number of devices for constructing the optical communication system is small, the insertion loss and the transmission loss can be reduced, and the low cost optical communication system can be constructed. Also, the grating length can be shortened,
The installation space is small, and it is possible to prevent the mechanical strength from being lowered due to the long element length. Further, it is possible to provide an optical fiber grating capable of forming an optical filter suitable for a notch filter. In other words, it is possible to provide a light attenuation band having a relatively wide band, the bandwidth can be easily adjusted, the transmission blocking ratio is sufficiently large, and the rise of the transmission blocking peak in the attenuation band is steep. Further, since the fundamental mode can be coupled to the 10th-order or lower clad mode, it is possible to realize an optical fiber grating in which the center wavelength of loss is not largely changed due to the influence of the manufacturing error of the grating pitch. Further, it is possible to prevent the controllability of the bandwidth at the time of manufacturing the filter from being deteriorated due to the manufacturing error of the grating pitch, and it is possible to improve the yield at the time of manufacturing the narrow band filter. In addition, the double structure of the clad reduces the number of loss peaks that occur outside the desired wavelength band, and prevents signal light from being lost outside the desired wavelength band in broadband wavelength division multiplexing communication. can do.

[Brief description of drawings]

FIG. 1 is a graph showing a spectrum of transmitted light of an optical filter of the present invention.

FIG. 2A to FIG. 2C are explanatory views showing an example of a method for manufacturing an optical fiber grating of the present invention.

FIG. 3 is a partial side sectional view showing an example of a configuration of an optical fiber type wavelength tunable filter using the optical fiber grating of the present invention.

FIG. 4 is a spectrum showing a state in which the grating characteristic changes due to temperature change in the optical fiber type wavelength tunable filter shown in FIG.

FIG. 5 is a diagram showing a relationship between a grating period and a loss central wavelength of a transmission single mode optical fiber.

FIG. 6 is a diagram showing a relationship between a grating period and a central wavelength of loss in the third embodiment example.

FIG. 7 is a diagram showing the transmittance of the optical fiber grating according to the third embodiment.

FIG. 8 is an explanatory diagram illustrating a structure of an optical fiber used in the optical fiber grating according to the fourth embodiment.

FIG. 9 is a diagram showing a relationship between a grating period and a central wavelength of loss in the fourth embodiment example.

FIG. 10 is a diagram showing the transmittance of the optical fiber grating according to the fourth embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical fiber, 2, 3, 4 ... Convex lens, 5 ... Spatial filter, 11 ... Optical fiber, 12 ... Grating part, 1
3 ... Heater, 14 ... Coating layer, 15 ... Core, 16 ... Inner clad, 17 ... Clad

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] October 23, 2001 (2001.10.
23)

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] Full text

[Correction method] Change

[Correction content]

[Document name] Statement

Title of the invention Optical fiber grating

[Claims]

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical fiber grating used in an optical communication system.

[0002]

2. Description of the Related Art In an optical communication system, an optical fiber grating in which a grating portion is formed in an optical fiber is often used. Generally, in the grating part, a type in which the fundamental mode propagating in the core in the same direction as the incident direction (positive direction) and the mode propagating in the cladding in the positive direction are combined, or the fundamental mode propagating in the core in the positive direction , And a mode that couples the core with a mode propagating in the negative direction. The former is generally called the radiation type and the latter is called the reflection type. Then, it can be used as, for example, an optical filter by coupling and attenuating light in a desired wavelength band. In general, a mode propagating in the core or the clad in the positive direction is called a transmission mode (transmitted light), and a mode propagating in the opposite negative direction is called a reflection mode (reflected light).

It is the perturbation formed in the core of the grating that enables such coupling between modes. In optical fibers, it is the refractive index change that creates this perturbation. In addition, a microbend may be used. For example, mode coupling can be achieved by periodic refraction changes of several hundred μm for the radiation type and 1 μm or less for the reflection type. This is why the radiation type is called the long period type and the reflection type is called the short period type. The period of such periodic changes in the refractive index and microbend is called the grating period.

At this time, what is important is the difference between the propagation constants of the two coupled modes. When this difference is large, the grating period is small, and when the difference is small, the grating period is large. The reason why the grating portion coupled to the reflection mode has a short period is that the propagation constant difference between the transmission mode and the reflection mode propagating in the core is large.

[0005]

Conventionally, a short period type optical fiber grating has been used for a narrow band filter having a narrow wavelength band of light to be attenuated. However, since the short period type is a reflection type, the generated reflected light may adversely affect. Therefore, it is necessary to install an isolator in front of the device affected by the reflected light to block the reflected light. Therefore, there are problems that the number of parts for constructing the optical communication system increases, the system cost cannot be reduced, and excess loss (insertion loss and transmission loss) increases.

On the other hand, the radiation type has a relatively wide attenuation band, no reflected light exists, and is non-reflection. Therefore, it is used as a gain equalizer or a notch filter near an amplifier or a laser. However, the radiation type has a problem that the grating length (grating portion length) is generally long. In particular, when an emission type narrow band filter is to be produced, the length is about 200 times that of the short period type filter, for example, about 2 m, which is practically impossible. Further, since there are many transmission blocking peaks, there is a problem that the transmission loss in the transmission band becomes large.

The inventors of the present invention have filed a patent application No. 2000-31254.
4 and Japanese Patent Application No. 2000-379162, while the grating period of the conventional long-period optical fiber grating is set to 150 μm or more, the long-period optical fiber grating is reduced by reducing the grating period to about 20 μm to 80 μm. However, it realizes a narrow band filter. A wide band antireflection filter having an arbitrary bandwidth is realized by adopting a chirp type in which the grating period is changed in the grating direction. In this application, the fundamental mode and the higher-order modes such as LP016 and the like higher than tenth order are combined, but this is because the grating period is made small. In general, the smaller the grating period, the higher the order of the cladding mode coupled to the fundamental mode.

FIG. 6 shows the relationship between the grating period and the loss central wavelength of a commonly used transmission single-mode optical fiber. Each line in FIG. 6 shows a cladding mode that is coupled to the fundamental mode, which is a first-order mode, a second-order mode ... in order from the upper side, and a higher order mode is shown in the lower side. The circles in Fig. 6 indicate the wavelength.
It shows the grating period for each cladding mode that is necessary for the 1500 nm band to be the center wavelength of the loss.

When the grating period is shortened, the propagation is the difference between the propagation constant (effective refractive index) of the fundamental mode, that is, the normal signal light, and the propagation constant (effective refractive index) of the higher-order cladding mode coupled to this. It is necessary to increase the constant difference. Of these propagation constants, the propagation constant of the fundamental mode is universal, so that the propagation constant of the higher-order mode needs to be small in order to increase the difference in the propagation constants. There is a relationship of delta β = 2π / Δ between the propagation constant difference delta β and the grating period Δ, and the propagation constant is
The basic mode is the maximum, and decreases as the order of the mode increases. However, in FIG. 6, as can be seen from the fact that the slope of the line indicating each cladding mode is small, the center wavelength of the loss is sensitive to the fluctuation of the grating period, and in a general optical fiber for transmission, coupling in a higher order mode is performed. If this is attempted, the center wavelength of the loss will be greatly changed due to the manufacturing error of the grating period when manufacturing the optical fiber grating.

Such a manufacturing error of the grating period also affects the yield when manufacturing the narrow band filter, and the bandwidth becomes larger than the desired value even when the grating period varies by 1% or less. However, the controllability of the bandwidth at the time of manufacturing the filter is deteriorated. Further, in FIG. 6, as can be seen from the narrow spacing between the straight lines, the higher-order modes exceeding the tenth order have very close propagation constants to the higher-order modes. Therefore, there is also a problem that the loss of signal light is adversely affected in broadband wavelength division multiplexing transmission. The present invention has been made in consideration of such circumstances, and the center wavelength of the loss is not greatly changed due to the influence of the manufacturing error of the grating period. An object of the present invention is to provide an optical fiber grating that does not suffer loss outside the band.

[0011]

In order to solve the above problems, the invention according to claim 1 has a core diameter of 2 μm to 6 μm.
m and the relative refractive index difference of the core with respect to the clad is 1% or more, a grating portion is formed with a grating period of 100 μm or less, and the fundamental mode propagating in the positive direction of the core is 10th or less. It is an optical fiber grating characterized by coupling to a transmission cladding mode. As a result, the fundamental mode can be coupled to the 10th order or lower cladding mode,
It is possible to realize an optical fiber grating in which the central wavelength of loss is not greatly changed due to the manufacturing error of the grating period. Further, it is possible to prevent the controllability of the bandwidth at the time of manufacturing the filter from being deteriorated due to the manufacturing error of the grating period, and it is possible to improve the yield when manufacturing the narrow band filter.

According to a second aspect of the present invention, in the optical fiber grating according to the first aspect, an inner clad is provided on the outer periphery of the core and the inner periphery of the clad, and the inner clad has a diameter of 5 μm to 30 μm. An optical fiber having a relative refractive index difference of 1.5% or more with respect to the clad is manufactured. As a result, it is possible to reduce the number of loss peaks that occur outside the desired wavelength band and prevent the signal light from being lost outside the desired wavelength band in broadband wavelength division multiplexing and the like.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
First, a first example of the optical fiber grating of the present invention will be described. The first example of the optical fiber grating of the present invention has a fundamental mode of 1 that propagates in the positive direction of the core.
In order to couple to a 0th order or lower cladding mode, an optical fiber having a core diameter of 2 μm to 6 μm and a relative refractive index difference of the core with respect to the cladding of 1% or more is 1
The grating portion is formed with a grating cycle of 00 μm or less.

In order to reduce the connection loss with the transmission optical fiber and to reduce the dependence of the central wavelength of the loss on the fluctuation of the grating period, the core diameter is 3 μm to 6 μm.
It is more preferable to form the grating portion with a grating period of 100 μm or less in an optical fiber in which the relative refractive index difference between the core and the clad is 2% or more.
FIG. 1 shows the optical fiber grating of this example.
It shows the relationship between the grating period and the central wavelength of the loss. Also in FIG. 1, each line shows a cladding mode that is coupled to the fundamental mode.
Next, second, etc. modes are indicated, and the lower the mode, the higher the mode. As can be seen by comparing with FIG.
The gradient of the line indicating the cladding mode is particularly large for the cladding mode of 10th order or less, and it is possible to suppress the large fluctuation of the central wavelength of the loss due to the manufacturing error of the grating period.

Table 1 shows an example of the refractive index and the relative refractive index difference between the core and the clad of the optical fiber grating of this example.

[0016]

[Table 1]

Specifically, an optical fiber grating is manufactured with a core diameter of 4 μm, a relative refractive index difference of the core with respect to the cladding of 2%, and a grating period of 75 μm.
An optical filter having a stop band near 1550 nm, which is a general communication band, was produced by coupling with an LP08 mode which is an eighth-order cladding mode. The → and ◯ in FIG. 1 show the relationship between the grating period and the central wavelength of loss in this optical fiber grating. FIG. 2 shows the transmittance of the optical fiber grating thus produced as
It is illustrated from 1300 nm to 1600 nm.
It can be seen that it has several stop bands in addition to the communication band near 1550 nm.

According to the optical fiber grating of this example, the fundamental mode can be coupled with the cladding modes of the 10th order or less, so that the central wavelength of the loss is not greatly changed due to the manufacturing error of the grating period. An optical fiber grating can be realized. Furthermore, due to the manufacturing error of the grating period,
It is possible to prevent the controllability of the bandwidth at the time of manufacturing the filter from being deteriorated and improve the yield at the time of manufacturing the narrow band filter.

Next, a second example of the optical fiber grating of the present invention will be described. This example uses two cladding
An optical fiber grating having a heavy structure in which the propagation constant of a higher-order clad mode is made discrete to reduce the number of loss peaks occurring outside the desired band. Figure 3
The structure of the optical fiber used in the optical fiber grating of this example is shown in FIG. Reference numeral 1 is a core, an inner clad 2 is formed on the outer periphery of the core 1, and further,
A clad 3 is formed on the outer circumference of the inner clad 2. In this optical fiber, the diameter of the core 1 is 2 μm to 6 μm.
μm, the relative refractive index difference of the core 1 with respect to the cladding 3 is 1% or more, the diameter of the inner cladding 2 is 5 μm to 30 μm, the relative refractive index difference of the inner cladding 2 with respect to the cladding 3 is 1.5% or more, and more preferably, , The diameter of the core 1 is 3 μm to 6 μm, the relative refractive index difference of the core 1 with respect to the cladding 3 is 2% or more, and the diameter of the inner cladding 2 is 10 μm to
The inner clad 2 is formed to have a relative refractive index difference of 20 μm and 3% or more with respect to the clad 3.

The diameter of the inner clad 2 is 5 μm to 30 μm.
μm, more preferably 10 μm to 20 μm,
If the diameter of the inner clad 2 is too large, the effect of providing the inner clad 2 to make the propagation constant of the cladding mode discrete is weakened. Conversely, if the diameter of the inner clad 2 is too small, the discretization of the propagation constant proceeds. However, the dependence of the central wavelength of the loss on the grating period becomes stronger.

Core 1, inner clad 2, clad 3
In order to adjust the refractive index of the core 1, for example, the core 1 is made of GeO.
₂ is formed by a SiO ₂ was added, the inner cladding 2 is formed by SiO _2, cladding 3, B ₂
It is formed of SiO ₂ with O ₃ or F added. Table 2 shows an example of the refractive index and the relative refractive index difference of the core 1, the inner clad 2, and the clad 3 of this optical fiber.

[0022]

[Table 2]

In the optical fiber grating of this example, a grating portion is formed in the above-mentioned optical fiber with a grating period of 100 μm or less. This optical fiber grating is intended to reduce the number of loss peaks generated at wavelengths other than the desired band shown in FIG. The cause of such a loss peak is that, as shown in FIG. 1, the spacing between the lines indicating the cladding modes is narrow and the propagation constants of the cladding modes are close to each other.

In this example, as described above, the cladding has two layers.
As a double structure, as shown in FIG. 4, the propagation constants of the cladding modes of 10th order or lower are made more discrete. Also in FIG. 4, each line shows a cladding mode that is coupled with the fundamental mode, and the modes are first-order, second-order, etc. in order from the upper side, and the lower order indicates the higher-order mode. Specifically, the core diameter is 4 μm, the inner cladding diameter is 20 μm, and the relative refractive index difference of the core with respect to the cladding is 2 μm.
%, The relative refractive index difference of the inner clad with respect to the clad was 3%, and the grating period was 68 μm to fabricate an optical fiber grating. Figure 5 shows the transmission loss of the optical fiber grating fabricated in this way at wavelength 13
It is shown from 00 nm to 1600 nm. As can be seen from comparison with FIG. 2, there are only two loss peaks that occur outside the desired wavelength band of 1550 nm, and the number of loss peaks could be reduced to about 1/3.

According to the optical fiber grating of this example, since the clad has a double structure, the number of loss peaks generated in other than the desired wavelength band is reduced, and the desired signal light is used in wideband wavelength division multiplexing communication. It is possible to prevent loss from occurring outside the wavelength band.

[0026]

As described above, according to the present invention,
For an optical fiber having a core diameter of 2 μm to 6 μm and a relative refractive index difference of the core with respect to the cladding of 1% or more, 100
By forming the grating portion with a grating period of μm or less, the fundamental mode propagating in the positive direction of the core can be coupled to the transmission clad mode of 10th order or less, and therefore, due to the manufacturing error of the grating period,
It is possible to realize an optical fiber grating in which the central wavelength of loss is not greatly changed. further,
It is possible to prevent the controllability of the bandwidth at the time of manufacturing the filter from being deteriorated by the manufacturing error of the grating period, and it is possible to improve the yield when manufacturing the narrow band filter.

By providing an inner clad on the outer circumference of the core and on the inner circumference of the clad, the diameter of the inner clad is 5 μm to 30 μm, and the relative refractive index difference of the inner clad with respect to the clad is 1.5% or more. It is possible to reduce the number of loss peaks that occur outside the desired wavelength band and prevent the signal light from being lost outside the desired wavelength band in broadband wavelength division multiplexing communication and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a grating period and a central wavelength of loss in a first example of an optical fiber grating of the present invention.

FIG. 2 is a diagram showing the transmittance of the optical fiber grating of the first example of the optical fiber grating of the present invention.

FIG. 3 is an explanatory diagram for explaining a structure of an optical fiber used in a second example of the optical fiber grating of the present invention.

FIG. 4 is a diagram showing a relationship between a grating period and a central wavelength of loss in a second example of the optical fiber grating of the present invention.

FIG. 5 is a diagram showing the transmittance of the optical fiber grating of the second example of the optical fiber grating of the present invention.

FIG. 6 is a diagram showing a relationship between a grating period of a transmission single mode optical fiber and a central wavelength of loss.

[Explanation of symbols] 1 ... Core, 2 ... Inner clad, 3 ... Clad

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 4]

[Figure 6]

[Figure 5]

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yu Ishii             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office (72) Inventor Kenji Nishide             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office F-term (reference) 2H050 AA01 AB05X AC84 AD01                 2H079 AA06 AA12 CA07 CA24 DA14                       EA09 EB27 KA08

Claims (14)

[Claims] 1. An optical fiber grating comprising a core of an optical fiber provided with a radiation type grating portion in which a perturbation is formed at a grating pitch of 20 to 80 μm along a length direction thereof. 2. The optical fiber grating according to claim 1, wherein a fundamental mode propagating in the core in a positive direction is coupled to a transmission mode of 10th order or higher than the fundamental mode. And an optical fiber grating. 3. The optical fiber grating according to claim 1 or 2, wherein the grating length is 30 mm or less. 4. The optical fiber grating according to any one of claims 1 to 3, wherein the grating pitch is a chirp pitch. 5. An optical fiber having a core diameter of 2 μm to 6 μm and a relative refractive index difference of the core with respect to the clad of 1% or more is formed with a grating portion at a grating pitch of 100 μm or less, and a positive direction of the core is formed. An optical fiber grating, characterized in that a fundamental mode propagating to a substrate is coupled to a transmission cladding mode of 10th order or less. 6. An inner clad is provided on an outer circumference of the core and an inner circumference of the clad, the inner clad has a diameter of 5 μm to 30 μm, and a relative refractive index difference of the inner clad with respect to the clad is 1.5% or more. The optical fiber grating according to claim 5, wherein 7. A method for producing an optical fiber grating according to claim 1, which comprises irradiating a laser beam with a predetermined grating pitch in a length direction of the optical fiber, wherein A method for manufacturing an optical fiber grating, characterized in that oscillated laser light is focused on an optical fiber through a lens and a spatial filter. 8. An optical filter using the optical fiber grating according to claim 1. Description: 9. The optical filter according to claim 8, wherein
An optical filter having a transmission blocking rate of 5 dB or more. 10. The optical filter according to claim 9, wherein the transmission blocking rate is 10 dB or more. 11. A notch filter using the optical filter according to claim 10. 12. A coating layer made of transparent plastic, the refractive index of which changes with temperature changes, is provided on the outer periphery of the grating portion of the optical fiber grating according to claim 1. An optical fiber type wavelength tunable filter, characterized in that a heater for heating is provided. 13. The optical fiber type wavelength tunable filter according to claim 12, wherein the transmission blocking rate is 5 dB or more. 14. The optical fiber type wavelength tunable filter according to claim 13, wherein the transmission blocking rate is 10 dB or more.