JP2002228841A - Chirped grating type optical filter and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a chirped grating type optical fiber which can be easily manufactured, has flat wavelength characteristics and can be precisely adjusted for the wavelength dependence of the transmittance, and to provide a method for manufacturing the filter. SOLUTION: The optical filter can be easily designed and the transmittance characteristics depending on the wavelength can be arbitrarily controlled by dividing the whole chirped grating 11 into a plurality of regions A1, A2, A3, etc., so as to make use of the characteristics of a chirped fiber grating in which the Bragg wavelength varies and by making one-to-one correspondence between the transmittance at the center Bragg wavelength of each region and the amplitude of the chirped grating 11 in each region A1, A2, A3, etc. Moreover, since the coupling between the ground level modes propagating the core is used, the temperature dependence is reduced to 0.01 nm/ deg.C. In the manufacture of the grating 11, a method using excimer laser light 12, a phase mask 14 or the like can be used similarly in the manufacture of a narrow band fiber grating. This means no requirement for large changes in the manufacture techniques.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chirped grating type optical filter and a method for manufacturing the same.

[0002]

2. Description of the Related Art EDFA (erbium-doped)
There are various types of gain equalizers for fiber amplifiers (erbium-doped optical fiber amplifiers).

For example, there is a gain equalizer using a long-period fiber grating (Harumoto, Mobara, Kanamori: Transmission characteristics ripple suppression of a long-period grating, Transactions of the 1998 IEICE Electronics Society Conference,
C-3-50, p. 184).

FIG. 10A is a block diagram of an EDFA and a conventional gain equalizer, and FIG. 10B is a block diagram of FIG.
FIG. 10C is a graph showing the wavelength transmittance of the gain equalizer of FIG. 10A, and FIG. 10D is a graph showing the output spectrum of FIG. 10A. FIG. 10 (b) and 10 (d), the horizontal axis indicates the wavelength axis, the vertical axis indicates the gain axis, and in FIG. 10 (c), the horizontal axis indicates the wavelength axis, and the vertical axis indicates the transmittance axis. . FIG.
At 0 (c), curve L1 shows the wavelength transmittance characteristic of the gain equalizer at the preceding stage, curve L2 shows the wavelength transmittance characteristic of the gain equalizer at the middle stage, and curve L3 shows the wavelength transmittance characteristic of the gain equalizer at the subsequent stage. 3 shows wavelength transmittance characteristics.

Here, the long-period grating is a grating of a refractive index modulation type having a period of 300 μm to 500 μm, which causes a coupling between a fundamental mode propagating in the core and a cladding mode leaking to the cladding, resulting in loss of loss. It controls the wavelength dependence.

A gain equalizer 1 shown in FIG. 10 (a) has a long-period fiber grating 2, 3, 4 connected in multiple stages, and
DFA5 gain equalization is performed.

As described above, while the gain of the EDFA 5 is equalized, light of a plurality of wavelengths is propagated in one optical fiber, and the development of the optical wavelength division multiplexing transmission which drastically increases the transmission capacity is actively developed. However, when the EDFA 5 is applied to a wavelength division multiplexing transmission system, a difference in gain between signal wavelengths during amplification becomes a problem.

For example, EDF in which Er is added to quartz glass
The gain characteristic with respect to the wavelength of A5 is generally a bimodal type having gain peaks near 1532 nm and 1552 nm. When Al is co-added to quartz glass, the peak at 1552 nm widens, and the gain can be flattened to some extent in the band of 1545 to 1560 nm.

However, the peak of 1532 nm still exists, and it is difficult to completely flatten the EDFA 5 in the gain wavelength band of 30 nm or more. Particularly, in the case of a long-distance transmission system in which the EDFAs 5 are connected in multiple stages, the difference in gain appears remarkably, and greatly affects the transmission characteristics depending on the signal wavelength.

[0010] Therefore, it is necessary to correct the gain difference between the signals by some method.

As a method of realizing a so-called gain equalizer for reducing the wavelength dependence of the gain, a long-period fiber grating has been studied.

When the period of the change in the refractive index is relatively long, from several tens of μm to several hundreds of μm, the following equation is used in the light propagating in the core of the optical fiber.

[0013]

The light having the wavelength λ satisfying the condition of λ = (ng−ncl) す る is coupled from the mode (guiding mode) propagating through the core to the cladding mode.

Here, ng is the effective refractive index for the waveguide mode, ncl is the effective refractive index for the coupled cladding mode, and Λ is the period of change in the refractive index.

The light coupled to the cladding mode is absorbed by the coating resin layer of the optical fiber and attenuated. Thus, a long-period fiber grating emits light of a specific wavelength out of the core of an optical fiber, causing loss,
Unnecessary reflected return light is not generated, and a relatively wide band (10 to 10)
It functions as a transmission type optical filter that blocks light of several tens of nm).

[0016]

As described above, the gain characteristic of an EDFA is generally 1532 nm and 1 dB.
Although it is a bimodal type having a gain peak near 552 nm, in an actual long distance transmission system, EDFAs having different gain characteristics are connected in multiple stages. Therefore, the optical output characteristics with respect to the wavelength of the signal light differ depending on the transmission system, and a gain equalizer having a more complicated filter characteristic suitable for each transmission system must be inserted.

The transmission spectrum of a long-period fiber grating has a characteristic that changes smoothly. Therefore, long-period fiber gratings with different transmission loss characteristics are connected in multiple columns, and the superimposed transmission loss spectra are designed and manufactured so as to flatten the gain deviation in the transmission system, that is, to have the inverse characteristic spectrum. Must.

In a long-period grating connected in cascade, the cladding mode generated in the preceding grating may not be sufficiently attenuated and may be recombined with the latter grating. In this case, the total transmission characteristic is not the product of the transmittances of the individual long-period gratings, but shows a complicated characteristic. In order to realize a desired loss spectrum characteristic by combining a plurality of long-period fiber gratings in this way, not only the design but also the manufacturing process becomes considerably complicated, and it is difficult to control the filter characteristic.

The characteristics of the long-period grating are sensitively affected by the optical characteristics of the coating layer coated on the cladding. In particular, when the refractive index of the cladding layer and the refractive index of the coating layer are close to each other, the transmission loss is small and largely depends on the environment around the cladding. Therefore, in many cases, a grating is formed without removing the coating layer and used without recoating. Such a long-period grating that must be used without recoating not only has a problem in mechanical reliability but also has a structure whose optical characteristics are easily affected by an external environment.

Further, the temperature dependence of the long-period fiber grating is dominated by the temperature dependence of the effective refractive index difference (ng-ncl) between the guided mode and the clad mode propagating in the core, and The coefficient is about 0.05 nm /
There is about ° C.

On the other hand, the temperature coefficient of the short-period fiber grating used as a reflection type narrow band filter is
Since the temperature dependence of the refractive index of quartz glass becomes dominant,
Usually, it is about 0.01 nm / ° C.

As described above, the temperature dependency of the long-period type fiber grating is about five times larger than that of the short-period type fiber grating, and there is a problem that reduction of the temperature dependency is strongly required.

Accordingly, an object of the present invention is to solve the above-mentioned problems, to provide a chirped grating type optical filter which is easy to manufacture, has a flat wavelength characteristic, and allows fine adjustment of transmittance wavelength dependency. It is to provide a manufacturing method.

[0024]

To achieve the above object, a chirped grating type optical filter according to the present invention comprises:
In a chirped grating type optical filter in which the Bragg wavelength increases from the input side to the output side of the optical fiber or the core of the optical waveguide, the entire grating is divided into a plurality of regions, and the center of each region corresponds to the Bragg wavelength. The transmittance and the amplitude of the grating in that region have a one-to-one correspondence.

In the chirped grating type optical filter of the present invention, a grating in which the period of change of the refractive index gradually changes in the longitudinal direction is formed in the optical fiber or the core of the optical waveguide. The size is locally changed.

In addition to the above configuration, in the chirped grating type optical filter of the present invention, it is preferable that the average refractive index of the grating forming portion corresponding to the used wavelength band is constant.

In addition to the above-described configuration, the chirped grating type optical filter of the present invention has a refractive index modulation of the grating that changes so as to cancel the fluctuation of the output with respect to the wavelength of the optical signal passing through the optical amplifier. Preferably, the output for the wavelength of the optical signal is flattened.

In addition to the above configuration, in the chirped grating type optical filter of the present invention, it is preferable that the grating is formed in an optical fiber to which an optical amplification medium is added or in a core of an optical waveguide.

In addition to the above configuration, the chirped grating type optical filter of the present invention is preferably provided with an isolator for removing reflected return light generated by the grating at a stage preceding the grating.

The chirped grating type optical filter according to the present invention is a chirped grating type optical filter in which the Bragg wavelength increases from the input side to the output side of the optical fiber or the core of the optical waveguide. Is equal to the shortest wavelength in the band used,
Alternatively, a grating having a Bragg wavelength on the output side that is equal to or longer than the longest wavelength in the operating band, or the amplitude of the chirped grating is modulated in the light propagation direction, and has a Bragg wavelength included in the operating wavelength band. Is determined only by the desired transmittance at the Bragg wavelength.

A method of manufacturing a chirped grating type optical filter according to the present invention is directed to a method of manufacturing a chirped fiber grating filter for increasing a Bragg wavelength from an input side to an output side of an optical fiber or a core of an optical waveguide. A phase mask divided into a plurality of regions in the longitudinal direction is arranged in parallel on the core, and a laser beam is irradiated through the phase mask to form a grating on the core. The laser irradiation amount is adjusted so as to obtain the rate.

The method of manufacturing a chirped grating type optical filter of the present invention is directed to a method of manufacturing a chirped grating type optical filter for increasing a Bragg wavelength from an input side to an output side of an optical fiber or a core of an optical waveguide. After passing the laser beam through the aperture whose shape has changed along the light propagation axis of the core and performing the desired grating amplitude modulation, the laser beam cut out by the aperture is focused on the core by a focusing lens. is there.

According to the present invention, taking advantage of the characteristics of a chirped fiber grating in which the Bragg wavelength changes, the entire grating is divided into a plurality of regions, and the transmittance at the center Bragg wavelength of each region and the amplitude of the grating in each region Is made one-to-one, design becomes easy and the transmittance wavelength characteristic can be freely controlled. Further, since the coupling between the fundamental modes propagating in the core is used, the temperature dependency is 0.01 nm / ° C.
Further, as in the manufacture of the narrow band fiber grating, a method using an excimer laser, a phase mask, or the like can be applied to the manufacture of the chirped grating, and there is no need to largely change the manufacturing technology.

According to the present invention, a so-called chirp grating having a short period in which the period of change of the refractive index gradually changes in the longitudinal direction is used, and the magnitude of the refractive index modulation in the longitudinal direction is locally changed. Desired transmission loss spectral characteristics are obtained. Even if a grating is formed, by keeping the average refractive index of the optical transmission line constant within the used wavelength band, unnecessary ripples are generated in the spectral characteristics even when filter characteristics in which the transmission loss spectrum changes sharply are required. Is suppressed, and a desired filter characteristic can be faithfully realized. By changing the refractive index modulation of the grating so as to cancel the fluctuation of the output with respect to the wavelength of the optical signal transmitted through the optical amplifier, the optical output with respect to the wavelength after passing through the grating can be flattened. By forming a grating directly on an optical fiber to which an optical amplification medium is added, or on a planar optical waveguide, and integrating it with a transmission line having optical amplifier capability to flatten the gain characteristics of each optical amplifier, An optical amplifier having a gain equalizer can be obtained, and the size of the device can be reduced. By inserting an isolator before the grating, unnecessary reflected light generated by the grating unit can be removed, and the influence on the gain characteristics of the optical amplifier can be suppressed.

According to the present invention, the amplitude of the chirped grating whose Bragg wavelength changes in a region including the used band is modulated in the direction of the light propagation axis, and the desired filter transmittance and the amplitude modulation degree of the grating at the Bragg wavelength of the grating are obtained. Is made one-to-one, design and manufacture are facilitated, and fine control of transmittance is facilitated. Since the coupling between the fundamental modes propagating through the core is used, the temperature dependency becomes 0.01 nm / ° C. As in the manufacture of the narrow band fiber Bragg grating, a method using an excimer laser, a phase mask or the like can be applied to the manufacture of the chirped grating, and there is no significant change in the manufacturing technology.

[0036]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1A is a structural view showing one embodiment of a chirped grating type optical filter of the present invention, and FIG. 1B is a chirped grating type optical filter shown in FIG. 1A. FIG. 1C is a diagram showing the relationship between the Bragg wavelength and the grating amplitude, and FIG. 1C shows the relationship between the wavelength region and the wavelength and the transmittance of the chirped grating type optical filter shown in FIG. FIG. In FIG. 1B, the horizontal axis is the Bragg wavelength axis, and the vertical axis is the grating amplitude axis. In FIG. 1C, the horizontal axis is the wavelength axis, and the vertical axis is the transmittance axis.

The chirped grating type optical filter 20 includes a chirped grating 1
1 is formed (FIG. 1A). The chirped grating 11 has a plurality of areas A1, A2, A
, And the amplitude of each grating is modulated as represented by the relationship between the Bragg wavelength and the grating amplitude at the center of each region A1, A2, A3,... (FIG. 1B). ing. The horizontal axis of the relationship between the Bragg wavelength and the grating amplitude at the center of each of the regions A1, A2, A3,... Corresponds to the desired transmittance characteristic (FIG. 1C). The chirped grating 11 is manufactured based on a design method in which the amplitude of a region having the Bragg wavelength is determined by a desired transmittance at the Bragg wavelength.

FIG. 2A is a conceptual diagram of an apparatus to which the method for manufacturing a chirped grating type optical filter shown in FIG. 1A is applied, and FIG. 2B is a view shown in FIG. FIG. 2C is a diagram showing the relationship between the region of the optical fiber and the irradiation time when the device is used, and FIG. 2C shows the relationship between the region of the optical fiber and the wavelength when the device shown in FIG. It is a figure showing a relation. In FIG. 2B, the horizontal axis is the position axis, and the vertical axis is the irradiation time axis. In FIG. 2C, the horizontal axis is the wavelength axis, and the vertical axis is the transmittance axis.

In FIG. 2A, reference numeral 12 denotes an excimer laser beam (which may be an ultraviolet laser beam), and reference numeral 13 denotes a beam which bends the excimer laser beam 12 toward the optical fiber 10 and is movable along the longitudinal direction of the optical fiber 10. The movable reflection mirror 14 is a phase mask arranged in parallel with the optical fiber 10.

The excimer laser beam 12 is divided into the divided areas A
Have substantially the same beam diameter as 1, A2, A3,... The excimer laser light 12 is reflected by the movable reflection mirror 13, passes through the phase mask 14, and irradiates the optical fiber 10. The irradiation amount of the excimer laser beam 12 to each area is adjusted by the scanning speed of the movable reflection mirror 13.
To obtain desired transmittance characteristics (FIG. 2C), FIG.
Irradiation is performed for an irradiation time as shown in FIG. After the formation of the chirped grating 11, the phase mask 14 is removed, and a process of irradiating the excimer laser beam 12 so that the average refractive index becomes constant in the propagation axis direction is performed.

FIG. 3 (a) is a connection diagram between the gain equalizer and the EDFA shown in FIG. 1 (a), and FIG. 3 (b) is a wavelength gain characteristic diagram of the EDFA shown in FIG. 3 (a). 3 (c) is FIG.
FIG. 3 is a wavelength transmittance characteristic diagram of the gain equalizer shown in FIG.
FIG. 3D is a wavelength gain characteristic diagram showing the output spectrum of FIG. 3B to 3D, the horizontal axis is the wavelength axis, in FIGS. 3B and 3D, the vertical axis is the gain axis, and in FIG. 3C, the vertical axis is the transmittance axis.

While the conventional example shown in FIG. 10 requires a three-stage long-period grating, the chirped grating type optical filter 20 has only one long-period grating 11 and has a simple structure. You can see that there is. The chirped grating type optical filter 2
0 allows fine control of transmittance wavelength dependence.
Not only the wavelength dependence of the gain of the EDFA 5 but also the loss wavelength dependence due to components such as filters and couplers inserted into the system can be canceled and the wavelength can be flattened.

As described above, in the present embodiment, the entire grating is divided into a plurality of regions by making use of the characteristics of the chirped grating in which the Bragg wavelength changes, and the transmittance at the Bragg wavelength at the center of each region and the grating in that region are determined. By making the amplitude correspond one-to-one, the design becomes simple, and the transmittance-wavelength characteristic can be freely controlled. Further, since the coupling between the fundamental modes propagating through the core is used, the temperature dependency can be set to 0.01 nm / ° C. In addition, a method using an excimer laser, a phase mask, or the like can be applied to the production of a chirped grating, similarly to the production of a narrow-band fiber Bragg grating, and there is no need to significantly change the production technique.

FIG. 4A is a structural view showing another embodiment of the chirped grating type optical filter of the present invention, and FIG. 4B is a longitudinal direction of the optical filter shown in FIG. 4A. FIG. 4 is a diagram showing the relationship between the position of the laser beam and the induced refractive index.
FIG. 5C is a diagram illustrating a relationship between the wavelength and the transmission loss of the optical filter illustrated in FIG. In FIG. 4B, the horizontal axis is the position axis, and the vertical axis is the induced refractive index axis. FIG. 4 (c)
In, the horizontal axis is the wavelength axis, and the vertical axis is the transmission loss axis.

Here, the band showing the filter characteristic of the so-called chirp grating, in which the period of the change of the refractive index formed in the core of the optical fiber (or the planar optical waveguide) gradually changes in the longitudinal direction, is the chirp rate of the grating. (Change in grating pitch per unit length)
And can be realized from a narrow band of 1 nm or less to a wide band of several tens nm. In a chirped grating, the position in the longitudinal direction where the grating is written corresponds to each wavelength. That is, light having a short wavelength is reflected in a portion having a short grating period, and light having a long wavelength is reflected in a portion having a long grating period.

In this embodiment, a chirp grating is used, and the magnitude of the refractive index modulation is changed in the longitudinal direction.

In the chirped grating type optical filter 20a, a chirped grating 11a whose refractive index changes in the longitudinal direction is formed in the optical transmission line (core) of the optical fiber (plane optical waveguide or the like) 10 in the longitudinal direction. Have been. The chirp rate and grating length are determined by the band of the filter characteristic used.

In the chirped grating type optical filter 20a, the band of the filter characteristics to be used is 1525 nm to 1565 nm, which is the gain band of the EDFA, and the chirped grating length is 50 mm. FIG. 4
As shown in (b), according to a desired filter characteristic,
The magnitude of the induced refractive index modulation of the chirped grating 11a was changed. The distribution of the induced refractive index modulation in FIG. 4B is an example, but if a chirped grating having the induced refractive index modulation is formed as described above, as shown in FIG. For short wavelength region,
A portion having a long refractive index period shows transmission loss characteristics corresponding to a long wavelength region.

Therefore, by arbitrarily changing the magnitude of the refractive index modulation, a chirped grating type optical filter having desired transmission characteristics can be easily formed.

In this embodiment, as shown by the broken line in FIG. 4B, the average value of the induced refractive index is constant within the wavelength band to be used. Generally, when the magnitude of the refractive index modulation is changed in the longitudinal direction, an average refractive index distribution occurs,
Because of Fabry-Perot resonance, multiple reflections occur and unnecessary ripples occur, steep spectral characteristics cannot be realized. In order to avoid this unnecessary ripple, in the present embodiment, even if a grating whose refractive index modulation changes in the longitudinal direction is formed, the average refractive index is constant within the wavelength band used.

FIG. 5 is an explanatory diagram of a grating forming method for forming the chirped grating shown in FIG.

The formation of the chirped grating 11a by the present grating forming method utilizes ultraviolet light-induced change in refractive index. That is, the present grating forming method utilizes a phenomenon in which the refractive index increases when ultraviolet laser light is applied to quartz glass to which germanium is added. For example, a phase mask 14 is attached to an optical fiber (or a planar optical waveguide) 10a doped with germanium.
It can be easily formed by irradiating the excimer laser beam 15 through the substrate. Excimer laser light 1 which is an ultraviolet laser
When 5 enters the phase mask 14, ± 1st-order diffracted lights are generated on the exit side of the phase mask 14, and these diffracted lights interfere with each other to form an interference pattern. The period of this interference pattern is determined by the period of the phase mask 14, and by appropriately designing this period, an interference pattern of ultraviolet light equal to the grating period can be obtained.

Therefore, if an optical transmission line such as an optical fiber or an optical waveguide is arranged directly below the phase mask 14, a desired pitch can be obtained by utilizing a change in the refractive index induced by the excimer laser light in the core which is a light propagation region. Can be formed.

In the present embodiment, the phase mask 14 having a chirp ratio that can obtain the desired filter characteristics described above is used. The magnitude of the induced refractive index modulation of the grating can be controlled by the intensity of the irradiation beam or the irradiation time.

The beam of the excimer laser beam 15 is scanned in the longitudinal direction on the phase mask 14 by the movable reflection mirror 13 using the grating forming apparatus shown in FIG.
The magnitude of the induced refractive index modulation of the grating written in the optical fiber 10a was controlled by locally changing the irradiation time by controlling the scanning speed.

Also, in the present embodiment, the aforementioned FIG.
As shown by the dashed line in (b), the average refractive index within the used wavelength band was made uniform. This is to cancel the change in the refractive index rise due to the writing of the grating.
By controlling the speed of scanning in the longitudinal direction with the phase mask 14 removed, a refractive index distribution having reverse characteristics is generated, and the average refractive index is made uniform. By this scan speed control, even when filter characteristics in which the transmission loss spectrum changes sharply are required, generation of unnecessary ripples in the spectrum characteristics can be suppressed, and desired filter characteristics can be faithfully realized.

FIG. 6A is a schematic diagram showing an application example of the chirped grating type optical filter shown in FIG. 4A, and FIG. 6B is a diagram showing the optical amplifier of the optical amplifier shown in FIG. FIG. 6C is a transmission characteristic diagram of the chirped grating type optical filter shown in FIG.
FIG. 6C is an output characteristic diagram of the optical amplifier shown in FIG. 6A after gain flattening. 6B to 6C, the horizontal axis is the wavelength axis, the vertical axis is the gain axis in FIG. 6B, and the vertical axis is the transmittance axis in FIG. 6C.
In (d), the vertical axis is the light output axis.

FIG. 6 (a) shows an insertion using the chirped grating type optical filter 20a of the present invention so as to cancel the fluctuation of the gain with respect to the wavelength of the EDFA 5. By using such a chirped grating type optical filter 20a, the output of each optical signal with respect to the wavelength after transmission through the grating is flattened.

In this embodiment, not only a normal optical fiber but also an optical fiber EDF added with an optical amplification medium is used.
Or an optical waveguide EDW formed on a planar optical waveguide
Can be similarly formed. This is because the refractive index change can be easily induced by excimer laser light because germanium is added to the core region of the optical transmission line to which the optical amplification medium is added.

If the present grating type optical filter is formed directly on the optical transmission line to which such an optical amplifying medium is added, an optical amplifier having a gain equalizer can be realized.

Further, in this embodiment, an isolator is inserted before the grating in order to eliminate the influence of the return light reflected by the grating. This insertion of the isolator prevents the operation of the optical amplifier from becoming unstable due to the reflected return light.

In the above, by inserting a fiber grating having a loss wavelength characteristic opposite to the gain wavelength characteristic of the EDFA into the output of the EDFA, an optical transmission system having a flat wavelength characteristic in the use wavelength region as a whole is obtained. Can be realized. The present grating type optical filter functioning as such a gain equalizer can easily realize any filter characteristics with low loss and little temperature dependency as compared with a thin film filter or a long period grating, and can be downsized.

FIG. 7A is a structural view showing another embodiment of the chirped grating type optical filter of the present invention, and FIG. 7B is a diagram showing the grating of the optical filter shown in FIG. 7A. FIG. 7 is a diagram showing a state of amplitude modulation, and FIG.
FIG. 8C is a diagram illustrating the transmittance wavelength dependence of the optical filter illustrated in FIG. In FIG. 7B, the horizontal axis is the position axis, and the vertical axis is the photo-induced refractive index axis. In FIG. 7C, the horizontal axis is the wavelength axis, and the vertical axis is the transmittance axis.

The chirped grating type optical filter 20b shown in FIG. 7A is obtained by forming a chirped grating 11b on an optical fiber 10b. The period of the chirped grating 11b is about 530 nm (the figure is only an image diagram, and the period is not accurate). The Bragg wavelength ranges from 1520 nm to 1570 n from the input side to the output side (left to right in the figure).
m. Here, the amplitude of the chirped grating 11b is modulated in the propagation axis direction according to the transmittance desired for the optical filter, but the average refractive index is constant in the propagation axis direction. FIG. 7C shows that the grating amplitude modulation is reflected.

FIG. 8A is an explanatory view showing another embodiment of the method for manufacturing a chirped grating type optical filter according to the present invention, and FIG. 8B is a diagram showing the aperture of the aperture shown in FIG. It is a top view.

This manufacturing method is an application of the phase mask method, which is one of the generally used methods of manufacturing a fiber Bragg. That is, the excimer laser light 15 is irradiated onto the optical fiber 10c disposed directly below the phase mask 14.

The feature of this manufacturing method is that the excimer laser beam 1
By inserting an aperture 16 and a condenser lens (cylindrical lens) 17 in the middle of the optical path 5, the power of the excimer laser light 15 irradiated in the propagation axis direction of the optical fiber 10c is changed.

The aperture 16 is configured to transmit the excimer laser light 15 only in the white portion 16a.
The aperture 16 is formed by processing an aluminum plate, and a white portion (through hole) 16a of the aperture 16 is formed.
Can be changed. Since the grating amplitude can be controlled by the power of the irradiated excimer laser beam 15, the amplitude modulation can be easily controlled.
After the grating was formed, the phase mask 14 was removed, and an aperture (not shown) giving a beam power distribution opposite to that of the white portion 16a of the aperture 16 was used to make the average refractive index constant in the propagation axis direction.

Instead of using the aperture 16 for manufacturing a chirped grating type optical filter, a beam scanning method generally used for manufacturing a fiber Bragg grating for a narrow band filter is also possible. The beam scanning method is a method of performing apodization by scanning UV light having a beam diameter of about 1 mm in the direction of the propagation axis of an optical fiber and changing a scanning speed or the like. Note that another ultraviolet laser may be used instead of the excimer laser light.

FIG. 9A shows an ED using the chirped grating type optical filter shown in FIG. 7A as a gain equalizer.
FIG. 9 is an explanatory diagram of gain equalization when connected to an FA.
FIG. 9A is a connection diagram between a gain equalizer and an EDFA, and FIG.
FIG. 9C is a diagram illustrating a wavelength gain characteristic of the EDFA, FIG. 9C is a diagram illustrating a wavelength transmittance characteristic of the gain equalizer, and FIG. 9D is a diagram illustrating an output spectrum.

Conventionally, as shown in FIG. 10, a three-stage long-period grating was required. However, the chirped grating type optical filter 20b requires only one stage, so that the configuration is simplified. In addition, fine control of transmittance wavelength dependence is possible, which cancels not only wavelength dependence of gain of EDFA5 but also loss wavelength dependence due to components such as filters and couplers inserted in the system, and flattens wavelength. Is possible.

As described above, according to the present embodiment, the amplitude of the chirped grating is modulated in the light propagation axis direction.
By making the desired filter transmittance at the Bragg wavelength of the grating correspond to the amplitude modulation degree of the grating on a one-to-one basis, design and manufacture become easy, and fine control of the transmittance becomes possible. Since the coupling between the fundamental modes propagating through the core is used, the temperature dependency can be set to 0.01 nm / ° C. A method using an excimer laser, a phase mask, or the like can be applied to the production of the chirped grating similarly to the production of the narrow-band fiber Bragg grating, and there is no significant change in the production technique.

[0074]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to provide a chirped grating type optical filter which is easy to manufacture, has a flat wavelength characteristic, and allows fine adjustment of transmittance wavelength dependence, and a method of manufacturing the same.

[Brief description of the drawings]

FIG. 1A is a structural view showing one embodiment of a chirped grating type optical filter of the present invention, and FIG.
4A is a diagram showing a relationship between the region of the chirped grating optical filter shown in FIG. 5A and the Bragg wavelength and the grating amplitude, and FIG. 5C is a diagram showing the wavelength region of the chirped grating optical filter shown in FIG. FIG. 4 is a diagram showing a relationship between wavelength, and transmittance.

2A is a conceptual diagram of an apparatus to which the method for manufacturing a chirped grating type optical filter shown in FIG. 1A is applied, and FIG. 2B is a diagram when the apparatus shown in FIG. It is a diagram showing the relationship between the area of the optical fiber and the irradiation time,
(C) is a diagram showing the relationship between the region of the optical fiber and the wavelength when the device shown in (a) is used.

FIG. 3 (a) shows a gain equalizer and an ED shown in FIG. 1 (a).
(B) is a wavelength gain characteristic diagram of the EDFA shown in (a), (c) is a wavelength transmittance characteristic diagram of the gain equalizer shown in (a), and (d) is (a) 3 is a wavelength gain characteristic diagram showing an output spectrum of FIG.

FIG. 4A is a structural view showing another embodiment of the chirped grating type optical filter of the present invention;
(B) is a diagram showing the relationship between the position in the longitudinal direction of the optical filter shown in (a) and the induced refractive index, and (c) is the relationship between the wavelength of the optical filter shown in (a) and the transmission loss. FIG.

FIG. 5 is an explanatory diagram of a grating forming method for forming the chirped grating shown in FIG.

6A is a schematic diagram showing an application example of the chirped grating type optical filter shown in FIG. 4A;
(B) is a gain characteristic diagram of the optical amplifier shown in (a),
(C) is a transmission characteristic diagram of the chirped grating type optical filter shown in (a), and (c) is an output characteristic diagram after gain flattening of the optical amplifier shown in (a).

FIG. 7A is a structural diagram showing another embodiment of the chirped grating type optical filter of the present invention;
(B) is a diagram showing a state of amplitude modulation of the grating of the optical filter shown in (a), and (c) is a diagram showing a transmittance wavelength dependence of the optical filter shown in (a).

FIG. 8A is an explanatory view showing another embodiment of the method for producing a chirped grating optical filter of the present invention, and FIG. 8B is a plan view of the aperture shown in FIG.

9A is an explanatory diagram of gain equalization when the chirped grating type optical filter shown in FIG. 7A is connected to an EDFA as a gain equalizer, and FIG. (B) is a wavelength gain characteristic diagram of the EDFA, (c) is a wavelength transmittance characteristic diagram of the gain equalizer,
(D) is a diagram showing an output spectrum.

10A is a block diagram of an EDFA and a conventional gain equalizer, FIG. 10B is a gain wavelength characteristic diagram of the EDFA of FIG. 10A, and FIG. 10C is a gain equalization of FIG. It is a figure which shows the output spectrum of (a) of FIG.

[Explanation of symbols]

 Reference Signs List 10 optical fiber 11 chirped grating 20 chirped grating type optical filter

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shinobu Sato 5-1-1 Hidakacho, Hitachi City, Ibaraki Prefecture F-term in Opto-Systems Research Laboratory, Hitachi Cable, Ltd. 2H038 BA25 2H049 AA02 AA33 AA45 AA59 AA66 2H050 AA01 AC84 AD00

Claims (9)

[Claims] In a chirped grating type optical filter in which a Bragg wavelength increases from an input side to an output side of an optical fiber or a core of an optical waveguide, the entire grating is divided into a plurality of regions, and the center of each region is divided into a plurality of regions. Wherein the transmittance at each Bragg wavelength and the amplitude of the grating in the region correspond to each other on a one-to-one basis. 2. A grating in which the period of change of the refractive index gradually changes in the longitudinal direction in the optical fiber or the core of the optical waveguide, and the magnitude of the refractive index modulation in the longitudinal direction changes locally. A chirped grating type optical filter. 3. The chirped grating type optical filter according to claim 2, wherein the average refractive index of the grating forming portion corresponding to the used wavelength band is constant. 4. The method according to claim 1, wherein the refractive index modulation of the grating is changed so as to cancel the fluctuation of the output with respect to the wavelength of the optical signal passing through the optical amplifier, and the output with respect to the wavelength of the optical signal after passing through the grating is flattened. Item 3. A chirped grating optical filter according to item 2. 5. The chirped grating type optical filter according to claim 2, wherein the grating is formed in an optical fiber to which an optical amplification medium is added or in a core of an optical waveguide. 6. The chirped grating optical filter according to claim 2, wherein an isolator for removing reflected return light generated in the grating is inserted in a stage preceding the grating. 7. A chirped grating type optical filter in which the Bragg wavelength increases from the input side to the output side of the optical fiber or the core of the optical waveguide, wherein the Bragg wavelength on the input side is the shortest wavelength in the used band. Equal or shorter, the Bragg wavelength on the output side is equal to or longer than the longest wavelength in the band used, the amplitude of the chirped grating is modulated in the propagation direction of light,
A chirped grating type optical filter, wherein an amplitude modulation degree of a grating having a Bragg wavelength included in a used wavelength band is determined only by a desired transmittance at the Bragg wavelength. 8. A method of manufacturing a chirped fiber grating filter for increasing a Bragg wavelength from an input side to an output side of an optical fiber or a core of an optical waveguide, wherein a phase mask divided into a plurality of regions in a longitudinal direction. Are arranged in parallel on the core, a laser beam is irradiated through the phase mask to form a grating on the core, and the laser irradiation amount is adjusted so that a desired transmittance at each Bragg wavelength is obtained at the center of each region. Manufacturing method of chirped grating type optical filter to be adjusted. 9. A method of manufacturing a chirped grating type optical filter for increasing a Bragg wavelength from an input side to an output side of an optical fiber or a core of an optical waveguide, wherein the shape is along the light propagation axis of the core. A chirped grating type optical filter, characterized in that, after performing a desired grating amplitude modulation by passing a laser beam through the changed aperture, the laser beam cut out by the aperture is focused on the core by a focusing lens. Manufacturing method.