JP2001051134A - Optical waveguide type filter and optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide type filter excellent in a cutoff characteristic, allowing dissolution of ripple. SOLUTION: In this optical waveguide type filter 10, a grating 12 is formed in an optical fiber 11 that is an optical waveguide. The optical fiber 11 is basically made of silica glass, and has a high-refractive index core area 13 added with GeO2 and a low-refractive index clad area 14 surrounding the core area 13. The grating 12 is formed in the core area 13. The normal direction to grating stripes of the grating 12 is inclined by an angle θ to the optical axis of the optical fiber 11. A grating stripe interval of the grating 12 varies in the longitudinal direction of the optical fiber 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type filter for blocking a propagation mode light by coupling a propagation mode light of a predetermined wavelength to a reflection leakage mode light by a diffraction grating formed in the optical waveguide, and The present invention relates to an optical fiber amplifier that performs gain equalization using the optical waveguide filter.

[0002]

2. Description of the Related Art An optical waveguide type filter has a diffraction grating formed on an optical waveguide such as an optical fiber, and couples propagation mode light having a predetermined wavelength corresponding to the grating interval of the diffraction grating to leak mode light. , Thereby acting as an optical filter that blocks the propagation mode light. This optical waveguide filter is also used as a gain equalizing means in an optical fiber amplifier.

[0003] For example, in Reference 1, "AM Vengsarkar, et a
l., J. of Lightwave Tech., Vol. 14, No. 1, pp. 58-64
(1996) "describes the operating principle and an example of fabrication of an optical waveguide filter in which a long-period diffraction grating having a grating interval of several hundred μm is formed. This long-period optical waveguide filter has a lattice spacing of Λ, a propagation constant of forward propagation mode light β _f, and a propagation constant of forward leakage mode light β _b , where β _f −β _b = 2π / Λ (1) The propagation mode light having a wavelength satisfying the phase matching condition represented by the following equation is blocked.

[0004] Reference 2 "R. Kashyap, et al., Elec
tron. Lett., Vol.29, No.2, pp.154-156 (1993) ", an optical waveguide type filter (hereinafter, referred to as a diffraction grating whose vertical direction to the grating fringes is inclined with respect to the optical axis of the optical waveguide). A technique for realizing gain equalization in an optical fiber amplifier using an "inclined optical waveguide filter" is described. Reference 3 “T. Erdogan, et al., Opt. L
ett., Vol. 20, No. 18, pp. 1838-1839 (1995) "describes a theoretical study on the characteristics of this tilted optical waveguide filter and a verification by experiment. Such a tilted optical waveguide filter is formed with a short-period diffraction grating having a grating interval of sub-μm to several μm, and this diffraction grating leaks forward propagation mode light of a predetermined wavelength backward. The light is coupled to the mode light and cut off.

[0005]

An optical waveguide type filter in which a diffraction grating is formed in such an optical waveguide and blocks propagation mode light of a predetermined wavelength is compared with other optical filters such as an etalon having the same function. There is an advantage that the coupling with the optical fiber is excellent and the insertion loss is low. However, the optical waveguide type filter has the following problems.

[0006] In an optical waveguide filter in which a long-period diffraction grating as described in Document 1 is formed, the cutoff characteristic varies depending on the temperature. That is, when the temperature fluctuates, the values of the lattice spacing and the propagation constant under the phase matching condition represented by the above equation (1) fluctuate. As a result, the center wavelength of the propagation mode light cut off by the optical waveguide filter is changed. fluctuate.

[0007] In addition, the inclined optical waveguide type filters described in References 2 and 3 have the advantage that the temperature dependence of the cutoff characteristics is small, but the cutoff wavelength at which the cutoff ratio is maximized. , A fine cutoff peak ripple occurs, and the cutoff characteristics are poor. That is, not only the coupling from the forward propagation mode light to the backward leakage mode light but also the coupling from the backward leakage mode light to the forward propagation mode light occur, Fabry-Perot resonance occurs, and as a result, a fine cutoff peak ripple occurs. By immersing the optical waveguide filter in matching oil or coating the periphery of the optical waveguide filter with a refractive index matching polymer, backward leak mode light can be emitted to the outside of the optical waveguide, and this ripple is reduced. Elimination is theoretically possible. But,
Since high-precision index matching is required, it is difficult to completely remove the ripple.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an optical waveguide type filter having excellent cutoff characteristics by eliminating ripples, and gain equalization using the optical waveguide type filter. It is an object to provide an optical fiber amplifier.

[0009]

A first optical waveguide type filter according to the present invention is characterized in that a propagation mode light having a predetermined wavelength is coupled to a reflection leakage mode light by a diffraction grating formed in the optical waveguide. An optical waveguide filter for blocking light, wherein a diffraction grating is formed at least in a part of a light propagation region of the optical waveguide, and a direction perpendicular to the grating stripes of the diffraction grating is inclined with respect to an optical axis of the optical waveguide, and the diffraction grating is formed. Is characterized in that the lattice spacing changes in the longitudinal direction of the optical waveguide. In the first optical waveguide type filter, a part of the propagation mode light of a predetermined wavelength that has propagated in the core region of the optical waveguide includes:
The light is converted into reflection leakage mode light in a diffraction grating in which the direction perpendicular to the lattice fringes is inclined with respect to the optical axis of the optical waveguide. Even if the reflected leakage mode light is reflected on the outer boundary surface of the cladding region and returns to the diffraction grating, since the grating interval of the diffraction grating changes in the longitudinal direction of the optical waveguide,
It is not converted into propagation mode light again. That is,
There is no Fabry-Perot resonance, and the ripple in the transmission spectrum is small and smooth.
Excellent blocking properties.

A second optical waveguide type filter according to the present invention is an optical waveguide for blocking a propagation mode light by coupling a propagation mode light of a predetermined wavelength to a reflection leakage mode light by a diffraction grating formed in the optical waveguide. Type filter, wherein the diffraction grating is formed in at least a part of the light propagation region of the optical waveguide, the direction perpendicular to the grating fringes of the diffraction grating is inclined with respect to the optical axis of the optical waveguide, and the grating spacing of the diffraction grating is Changes in the longitudinal direction, and the rate of change of the lattice spacing is 0.05 to
It is 3.0 nm / mm. In the second optical waveguide filter, since the change rate of the grating interval of the diffraction grating is 0.05 to 3.0 nm / mm, the ripple in the transmission spectrum is sufficiently eliminated and the cutoff amount at a predetermined wavelength is reduced. Becomes larger. Note that the rate of change of the lattice spacing is a unit length (m) in the longitudinal direction of the optical waveguide.
m) is the rate of change (nm) in the grating spacing of the diffraction grating per m).

A third optical waveguide type filter according to the present invention is an optical waveguide for blocking propagation mode light by coupling propagation mode light of a predetermined wavelength to reflection leakage mode light by a diffraction grating formed in the optical waveguide. Type filter, wherein the diffraction grating is formed in at least a part of the light propagation region of the optical waveguide, the direction perpendicular to the grating fringes of the diffraction grating is inclined with respect to the optical axis of the optical waveguide, and the grating spacing of the diffraction grating is And the periphery of the optical waveguide is covered with a refractive index matching material. The third optical waveguide filter is suitable in terms of fiber strength because the periphery of the optical waveguide is covered with a refractive index matching material. In addition, ripples in the transmission spectrum are further eliminated and smooth. Become.

A fourth optical waveguide filter according to the present invention is characterized in that the rate of change of the lattice spacing in the third optical waveguide filter is 0.05 to 3.0 nm / mm. The ripple in the transmission spectrum is sufficiently eliminated, and the amount of cutoff at a predetermined wavelength is increased.

A fifth optical waveguide filter according to the present invention is characterized in that any two or more of the first to fourth optical waveguide filters are connected in cascade.
According to the fifth optical waveguide filter, the same operation and effect as those of the first optical waveguide filter can be obtained.
In addition to the effect, a desired transmission spectrum can be obtained.

An optical fiber amplifier according to the present invention comprises: (1) an amplification optical fiber for optically amplifying and outputting a signal light input when a rare earth element is added and pumping light is supplied;
(2) pumping means for supplying pumping light to the amplification optical fiber,
(3) a cascade-connected optical fiber for amplification, and any one of the first to fifth optical waveguide filters for equalizing the gain of optical amplification for signal light in the optical fiber for amplification. . According to this optical fiber amplifier,
When the pumping light is supplied to the amplification optical fiber by the pumping means, the input signal light is optically amplified by the amplification optical fiber, gain-equalized by the optical waveguide filter, and output.

[0015]

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

First, an optical waveguide filter according to a first embodiment of the present invention will be described. FIG. 1 is an explanatory diagram of an optical waveguide filter 10 according to the first embodiment. The optical waveguide filter 10 according to the present embodiment is obtained by forming a diffraction grating 12 on an optical fiber 11 which is an optical waveguide. The optical fiber 11 includes a core region 13 having a high refractive index based on silica glass and doped with GeO ₂ ,
Low refractive index cladding region 1 surrounding core region 13
And 4. The diffraction grating 12 is formed in the core region 13. The direction perpendicular to the lattice fringes of the diffraction grating 12 is inclined by an angle θ with respect to the optical axis of the optical fiber 11. The grating interval of the diffraction grating 12 changes in the longitudinal direction of the optical fiber 11.

In this optical waveguide filter 10, a part of the propagation mode light having a predetermined wavelength that has propagated through the core region 13 of the optical fiber 11 has an angle θ with respect to the optical axis of the optical fiber 11 perpendicular to the lattice fringes. The light is converted into leakage mode light in the inclined diffraction grating 12. This leakage mode light may be reflected on the outer boundary surface of the cladding region 14 and return to the diffraction grating 12. However, since the grating interval of the diffraction grating 12 changes in the longitudinal direction of the optical fiber 11, the propagation mode It is not converted to light. That is, Fabry-Perot resonance does not occur.

The optical waveguide filter 10 configured as described above has excellent cut-off characteristics by eliminating ripples. In particular, the rate of change of the grating interval of the diffraction grating 12 is 0.1.
It is preferably from 0.05 to 3.0 nm / mm. Also,
It is further preferable that the periphery of the optical fiber 11 is covered with a refractive index matching material in order to remove a ripple. The refractive index matching material is, for example, a matching oil or a polymer.

The refractive index matching agent preferably has a refractive index whose difference from the refractive index of the outermost layer of the cladding region 14 is ± 1% or less. That is, when the difference between the refractive index outside the cladding region 14 and the refractive index of the cladding region 14 is large, Fresnel reflection due to the refractive index exists at the interface. The optical fiber 11 has a Fresnel reflection surface that is axially symmetric about the core region 13. In such a case, a cladding mode is established in the cladding region 14 as well as in the core region 13. This cladding mode is a discrete mode due to boundary conditions inside / outside the cladding region 14.
Coupling from the core-propagating light into such discrete modes is discrete and thus results in a type of ripple spectrum with respect to wavelength. On the other hand, when the difference between the refractive index outside the cladding region 14 and the refractive index of the cladding region 14 is small, the Fresnel reflection becomes sufficiently small, and the cladding mode is not discrete but continuous, so that ripples are eliminated. ,
A continuous spectrum is obtained. Therefore, in order to sufficiently reduce the ripple to 0.3 dB or less, the refractive index in the visible light region of the refractive index matching agent covering the periphery of the cladding region 14 should be 1.459 to 1.471. From this, it is preferable that the difference from the refractive index of the outermost layer of the cladding region 14 is ± 1% or less.

FIG. 2 is an explanatory diagram of a method of manufacturing the optical waveguide filter 10 according to the first embodiment. The manufacturing method shown in this figure uses a phase grating plate 30. Ultraviolet light is applied to the optical fiber 11 via the phase grating plate 30. At this time, the direction perpendicular to the lattice fringes of the phase grating plate 30 is inclined with respect to the optical axis of the optical fiber 11. The lattice spacing of the phase grating plate 30 changes in the normal direction. The ultraviolet light applied to the optical fiber 11 is spatially intensity-modulated by ± 1st-order diffracted lights generated by the phase grating plate 30 interfering with each other. Then, in the core region 13 to which GeO ₂ has been added, a change in the refractive index according to the power of the irradiated ultraviolet light is induced, and the diffraction grating 12 is formed. Further, the optical waveguide filter 10 according to the present embodiment uses the two-beam interference method,
It can also be manufactured by making the two branched ultraviolet lights cross each other at a predetermined angle by the optical fiber 11 and interfere with each other.

Next, a description will be given of a second embodiment of the optical waveguide filter according to the present invention. FIG. 3 is an explanatory diagram of the optical waveguide filter 20 according to the second embodiment. The optical waveguide filter 20 according to the present embodiment is obtained by forming a diffraction grating 22 on an optical fiber 21 which is an optical waveguide. The optical fiber 21 has a high refractive index core region 23 doped with GeO ₂ based on silica glass,
Ge surrounding the core region 23 and containing GeO ₂
Addition cladding region 24A and Ge addition cladding region 24
Low refractive index G surrounding A and not containing GeO ₂
e non-doped cladding region 24B. Diffraction grating 22
Is formed over both the core region 23 and the Ge-doped cladding region 24A. The diameter of the Ge-doped cladding region 24A is sufficient as long as the evanescent wave of the propagation mode light substantially propagating in the core region 23 is present, for example, about several times the outer diameter of the core region 23. The direction perpendicular to the lattice fringes of the diffraction grating 22 is inclined by an angle θ with respect to the optical axis of the optical fiber 21. The grating interval of the diffraction grating 22 changes in the longitudinal direction of the optical fiber 21.

In the optical waveguide filter 20, a part of the propagation mode light having a predetermined wavelength that has propagated through the core region 23 of the optical fiber 21 has a direction perpendicular to the lattice stripe with respect to the optical axis of the optical fiber 21 by an angle θ. The light is converted into leakage mode light in the inclined diffraction grating 22. The leakage mode light may be reflected on the outer peripheral boundary surface of the Ge non-doped cladding region 24 </ b> B and return to the diffraction grating 22.
Since the lattice spacing of No. 2 changes in the longitudinal direction of the optical fiber 21, it is not converted into propagation mode light again. That is, Fabry-Perot resonance does not occur.

The thus configured optical waveguide filter 20 also has excellent cut-off characteristics because ripples are removed. In particular, in the present embodiment, the diffraction grating 22 includes not only the core region 23 of the optical fiber 21 but also the Ge-doped cladding region 2.
4A, the efficiency of conversion of propagation mode light of a predetermined wavelength into leakage mode light increases,
The cutoff ratio at the predetermined wavelength increases. The rate of change of the grating interval of the diffraction grating 22 is 0.05 to 3.0 n.
Preferably, it is m / mm. In addition, the optical fiber 21
Is preferably covered with a refractive index matching material in order to remove ripples.

Next, five examples of the optical waveguide filter 20 according to the second embodiment will be described together with two comparative examples.

The optical fiber used in the first comparative example is:
This was a single mode optical fiber in which GeO ₂ was added only to the core region. The relative refractive index difference between the core region and the cladding region was 0.35%, and the diameter of the core region was 4 μm. The optical fiber used in each of the second comparative example and the first to fifth examples is a single mode optical fiber in which GeO ₂ is added to each of the core region 23 and the Ge-doped cladding region 24A. The relative refractive index difference of the core region 22 with respect to the non-doped cladding region 24B is 0.35%, the diameter of the core region 23 is 4 μm,
The diameter of the Ge-added cladding region 24A was three times the diameter of the core region 22.

Each optical fiber is subjected to a hydrogen treatment under a hydrogen atmosphere of 200 atm for 2 weeks, after which a part of the coating in the longitudinal direction is removed, a diffraction grating is formed by a phase grating method, and the refractive index matching is performed. Recoated with polymer. In forming a diffraction grating by the phase grating method, laser light having a wavelength of 248 nm output from an excimer laser light source was used as ultraviolet light for inducing a change in the refractive index, and the irradiation time was set to 5 minutes. The direction perpendicular to the lattice fringes of the phase grating plate was inclined by an angle of 6 degrees with respect to the optical axis of the optical fiber, whereby the direction perpendicular to the lattice fringes of the diffraction grating was inclined by an angle of 6 degrees with respect to the optical axis of the optical fiber. . Diffraction grating length L is 5mm
Met.

In each of the first comparative example and the second comparative example, the lattice spacing was a constant spacing of 537 nm. First
In the embodiment of the present invention, the lattice spacing is from 536.925 nm to 53
It changed continuously to 7.075 nm, and the rate of change of the lattice spacing was 0.03 nm / mm. In the second embodiment, the lattice spacing is from 536.875 nm to 537.75 nm.
It changed continuously to 125 nm, and the rate of change of the lattice spacing was 0.05 nm / mm. In the third embodiment, the lattice spacing is from 536.75 nm to 537.25 n
m, and the rate of change of the lattice spacing was 0.1 nm / mm. In the fourth embodiment, the lattice spacing changes continuously from 536.5 nm to 537.5 nm, and the rate of change of the lattice spacing is 0.2 nm /.
mm. In the fifth embodiment, the lattice spacing is 5
It changed continuously from 35.0 nm to 539.0 nm, and the rate of change of the lattice spacing was 0.8 nm / mm.

FIG. 4 is a graph showing a transmission spectrum, a reflection spectrum and a transmission spectrum after recoating of the optical waveguide filter according to the first comparative example. FIG.
Transmission spectrum of the optical waveguide filter according to the comparative example of,
It is a graph which shows a reflection spectrum and a transmission spectrum after recoating. FIG. 6 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the first example. FIG. 7 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the second embodiment. FIG. 8 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the third example. FIG.
11 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the fourth example. FIG. 10 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the fifth embodiment. 4 to 10, each figure (a) shows a transmission spectrum before recoating, each figure (b) shows a reflection spectrum before recoating, and each figure (c) shows a transmission spectrum after recoating. Is shown.

As can be seen from these graphs, the transmission spectra of the optical waveguide filters according to the first and second comparative examples before recoating (FIGS. 4A and 5A).
Is a set of many ripples, and each ripple has a sharp shape. The transmission spectra after recoating (FIGS. 4 (c) and 5 (c)) are such that the transmission spectra before recoating are averaged, but ripples still remain. .

On the other hand, the transmission spectra of the optical waveguide filters according to the first to fifth embodiments before recoating (FIGS. 6 to 10A) have smaller ripples. A smooth one was obtained. In addition, the transmission spectrum after the recoating (FIGS. 6 to 10 (c)) further reduced the ripples and became smoother.

As can be seen by comparing FIGS. 6 to 10, the optical waveguide filter according to the present embodiment has
If the change amount or change rate of the grating interval is small, ripples remain in the transmission spectrum. It should be noted that the change rate of the lattice spacing is 0.
In the first embodiment having a thickness of 03 nm / mm, the transmission spectrum after the recoating does not always sufficiently eliminate the ripple (FIG. 6C). Conversely, as the amount of change or the rate of change of the grating interval is larger, the ripple of the transmission spectrum is eliminated, but the cutoff amount is reduced, and the cutoff spectrum is broadened. This indicates that there is a suitable range of the rate of change of the lattice spacing. From the above, it was found that the preferable range of the change rate of the lattice spacing was 0.05 to 3.0 nm / mm.

Further, the optical waveguide filter according to the present embodiment has a lower reflectance than those according to the first and second comparative examples. Therefore, it is not necessary to add an optical component such as an optical isolator. Further, in the optical waveguide filter according to the present embodiment, the transmission spectrum does not greatly change depending on the propagation direction of the propagation mode light.

Next, the relationship between the inclination angle between the direction perpendicular to the lattice fringes of the diffraction grating and the optical axis of the optical fiber, and the reflection characteristics and transmission characteristics will be described. FIG. 11 is a graph showing the relationship between the inclination angle and the reflection and transmission characteristics. FIG. 9A shows a case where the inclination angle is 3 degrees,
FIG. 6B shows a case where the inclination angle is 6 degrees, FIG. 7C shows a case where the inclination angle is 9 degrees, and FIG.
Shows a case where the inclination angle is 11 degrees, and FIG. 3E shows a case where the inclination angle is 12 degrees. Each of them has a diffraction grating length L of 5 mm and a grating interval of 53 mm.
It changes continuously from 5 nm to 539 nm. As can be seen from this figure, it is possible to adjust the transmission spectrum characteristics by adjusting the tilt angle even for the same fiber.

Next, an embodiment of the optical fiber amplifier according to the present invention will be described. FIG. 12 is a schematic configuration diagram of the optical fiber amplifier 1 according to the present embodiment. The optical fiber amplifier 1 includes an amplifying optical fiber 43, an optical coupler 44, and an optical waveguide filter 50 between an input end 41 and an output end 42.
An excitation light source 46 is connected via the reference numeral 5.

The amplification optical fiber 43 is doped with an Er element as a rare earth element, and pump light (for example, a wavelength of 0.98 μm) output from the pump light source 46 is transmitted to the optical fiber 4.
5 and an optical coupler 44. The amplification optical fiber 43 has a wavelength of 1.5
The signal light in the 5 μm band is optically amplified and output. Optical coupler 44
The optical waveguide filter 50 cascade-connected to the amplification optical fiber 43 via the optical amplifier 43 equalizes the gain of the optical amplification for the signal light in the amplification optical fiber 43 and outputs the signal light to the emission end 42. The optical waveguide filter 50 may be the one according to the above-described first or second embodiment, or may be one in which a plurality of filters are cascaded.

FIG. 13 is an explanatory diagram of the optical waveguide filter 50 in the optical fiber amplifier 1 according to the present embodiment. The optical waveguide filter 50 is formed on an optical fiber 51 similar to the optical fiber 21 in the second embodiment. The length of the phase grating plate 30 in the longitudinal direction of the optical fiber 51 is 25 mm. , The lattice spacing changes continuously from 1.064 μm to 1.084 μm. The irradiation amount of ultraviolet light that induces a change in the refractive index is the largest in the region near the lattice spacing of 1.064 μm, the second largest in the region near the lattice spacing of 1.084 μm, and smaller in the region between these.

FIG. 14 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter 50. FIG. 7A shows the transmission spectrum before recoating, FIG. 7B shows the reflection spectrum before recoating, and FIG. 7C shows the transmission spectrum after recoating. As can be seen from this graph, the transmission spectrum of the optical waveguide filter 50 before recoating (FIG. 14A)
Has a small ripple and is smooth. Further, the transmission spectrum after the recoating (FIG. 14C) further eliminates ripples and becomes smoother. In particular, the transmission spectrum of the optical waveguide filter 50 is characterized by having two cutoff peaks.

FIG. 15 is a graph showing gain characteristics of the optical fiber amplifier 1 according to the present embodiment. In this figure, the solid line indicates the gain characteristic when the optical waveguide type diffraction grating 50 is removed, that is, the gain characteristic of the amplification optical fiber 43. A broken line indicates a gain characteristic of the optical fiber amplifier 1 including the optical waveguide type diffraction grating 50. As can be seen from FIGS. 14 and 15, the gain characteristic of the amplification optical fiber 43 (solid line in FIG. 15) and the transmission spectrum of the optical waveguide filter 50 are substantially opposite to each other. Therefore, the gain characteristic (broken line in FIG. 15) of the optical fiber amplifier 1, which is a combination of the gain characteristic of the amplification optical fiber 43 and the transmission spectrum of the optical waveguide filter 50, is substantially flat over a bandwidth of about 20 nm. And good gain equalization is achieved.

[0039]

As described above in detail, according to the optical waveguide type filter according to the present invention, a part of the propagation mode light of a predetermined wavelength propagating in the core region of the optical waveguide is reduced by the optical axis of the optical waveguide. Is converted into reflection leakage mode light by a diffraction grating whose vertical direction with respect to the lattice fringes is inclined.
Even if the reflected leakage mode light is reflected on the outer boundary surface of the cladding region and returns to the diffraction grating, since the grating interval of the diffraction grating changes in the longitudinal direction of the optical waveguide, the reflected leakage mode light is again transmitted to the propagation mode light. It is not converted. That is, the Fabry-Perot resonance does not occur, the ripple in the transmission spectrum becomes small and the transmission spectrum becomes smooth, and the blocking characteristics are excellent. Since the diffraction grating is formed not only in the core region but also in the cladding region of the optical waveguide, the efficiency of converting the propagation mode light of a predetermined wavelength into the leakage mode light increases, and the cutoff rate at the predetermined wavelength increases.

Further, the rate of change of the grating interval of the diffraction grating is 0.
According to the optical waveguide type filter having the wavelength of 0.05 to 3.0 nm / mm, the ripple in the transmission spectrum is sufficiently eliminated, and the cutoff amount at a predetermined wavelength is increased.
In addition, according to the optical waveguide filter in which the periphery of the optical waveguide is covered with the refractive index matching material, the ripple in the transmission spectrum is further eliminated and the optical waveguide becomes smooth. further,
According to the optical waveguide filter in which any two or more of these are cascaded, a desired transmission spectrum can be obtained.

Further, according to the optical fiber amplifier of the present invention, the input signal light is optically amplified by the amplifying optical fiber, gain-equalized by the optical waveguide filter of the present invention, and output. Therefore, the gain characteristic of this optical fiber amplifier becomes substantially flat over a wide band, and excellent gain equalization is performed.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an optical waveguide filter according to a first embodiment.

FIG. 2 is an explanatory diagram of a method of manufacturing the optical waveguide filter according to the first embodiment.

FIG. 3 is an explanatory diagram of an optical waveguide filter according to a second embodiment.

FIG. 4 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the first comparative example.

FIG. 5 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the second comparative example.

FIG. 6 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the first example.

FIG. 7 is a graph showing a transmission spectrum, a reflection spectrum and a transmission spectrum after recoating of the optical waveguide filter according to the second embodiment.

FIG. 8 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the third example.

FIG. 9 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the fourth embodiment.

FIG. 10 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter according to the fifth embodiment.

FIG. 11 is a graph showing a relationship between an inclination angle and reflection characteristics and transmission characteristics.

FIG. 12 is a schematic configuration diagram of an optical fiber amplifier according to the present embodiment.

FIG. 13 is an explanatory diagram of an optical waveguide filter in the optical fiber amplifier according to the embodiment.

FIG. 14 is a graph showing a transmission spectrum, a reflection spectrum, and a transmission spectrum after recoating of the optical waveguide filter in the optical fiber amplifier according to the present embodiment.

FIG. 15 is a graph showing gain characteristics of the optical fiber amplifier according to the embodiment.

[Explanation of symbols]

10 ... optical waveguide filter, 11 ... optical fiber, 12 ...
Diffraction grating, 13: core region, 14: cladding region, 20
... optical waveguide filter, 21 ... optical fiber, 22 ... diffraction grating, 23 ... core region, 24A ... Ge-doped clad region, 24B ... Ge-doped clad region, 30 ... phase grating plate, 41 ... incident end, 42 ... outgoing Ends, 43: amplification optical fiber, 44: optical coupler, 45: optical fiber, 46: excitation light source, 50: optical waveguide filter.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Seiichi Mobara 1st Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Michiko Harumoto 1st Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works F-term (reference) 2H038 BA25 2H049 AA02 AA33 AA59 AA62 2H050 AB04Y AB05X AB05Y AC09 AC36 AC82 AC84 AD00 5F072 AB09 AK06 JJ05 KK07 PP07 RR01 YY17

Claims (6)

[Claims] An optical waveguide type filter for blocking propagation mode light by coupling propagation mode light of a predetermined wavelength to reflection leakage mode light by a diffraction grating formed in an optical waveguide, wherein the diffraction grating is Formed at least in part of the light propagation region of the optical waveguide, the direction perpendicular to the lattice fringes of the diffraction grating is inclined with respect to the optical axis of the optical waveguide, and the lattice spacing of the diffraction grating changes in the longitudinal direction of the optical waveguide. An optical waveguide filter characterized by: 2. An optical waveguide type filter for blocking a propagation mode light by coupling a propagation mode light of a predetermined wavelength to a reflection leakage mode light by a diffraction grating formed in the optical waveguide, wherein the diffraction grating is Formed at least in part of the light propagation region of the optical waveguide, the direction perpendicular to the lattice fringes of the diffraction grating is inclined with respect to the optical axis of the optical waveguide, and the lattice spacing of the diffraction grating changes in the longitudinal direction of the optical waveguide. And
An optical waveguide filter characterized in that the rate of change of the lattice spacing is 0.05 to 3.0 nm / mm. 3. An optical waveguide type filter for blocking propagation mode light by coupling propagation mode light of a predetermined wavelength to reflection leak mode light by a diffraction grating formed in the optical waveguide, wherein the diffraction grating is Formed at least in part of the light propagation region of the optical waveguide, the direction perpendicular to the lattice fringes of the diffraction grating is inclined with respect to the optical axis of the optical waveguide, and the lattice spacing of the diffraction grating changes in the longitudinal direction of the optical waveguide. And
An optical waveguide type filter, wherein the periphery of the optical waveguide is covered with a refractive index matching material. 4. The method according to claim 1, wherein the rate of change of the lattice spacing is 0.05 to 3.
The optical waveguide filter according to claim 3, wherein the optical waveguide filter has a thickness of 0 nm / mm. 5. An optical waveguide filter, wherein at least two of the optical waveguide filters according to claim 1 are connected in cascade. 6. An amplification optical fiber for optically amplifying and outputting an input signal light when a rare earth element is added and excitation light is supplied, and excitation means for supplying the excitation light to the amplification optical fiber. The optical waveguide filter according to any one of claims 1 to 5, which is cascaded with the amplification optical fiber, and equalizes a gain of optical amplification with respect to the signal light in the amplification optical fiber. An optical fiber amplifier, characterized in that: