JP2004109775A - Optical variable filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical variable filter capable of reducing the loss of the gain of signal light, freely controlling loss wavelength characteristics and maintaining a stable state even against an environment temperature change. <P>SOLUTION: In the optical variable filter for correcting and equalizing the gain wavelength characteristics of an optical amplifier whose gain changes corresponding to a wavelength, a filter body 1 is constituted of a multilayer film filter 11 arranged at a prescribed position on an optical path and an actuator 14 connected to the multilayer film filter 11 and the loss wavelength characteristics of the filter body 1 are formed into the same shape as the gain wavelength characteristics of the optical amplifier by compositing the loss wavelength characteristics of the plurality of multilayer film filters 11. The arranging state to the optical path of the multilayer film filter 11 is changed by the actuator 14 so as to turn the loss wavelength characteristics of the filter body 1 to the same shape corresponding to the fluctuation of the gain wavelength characteristics of the optical amplifier. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical filter for correcting a gain wavelength characteristic of an optical amplifier used in the field of optical communication, and more particularly, to an optical variable filter capable of variably and accurately correcting a gain wavelength characteristic.
[0002]
[Prior art]
As a technique for increasing the transmission capacity of an optical fiber, there is a wavelength division multiplexing (WDM) optical transmission system. The WDM optical transmission system is a transmission system capable of increasing communication capacity by transmitting signal light of a plurality of wavelengths through one optical fiber.
[0003]
In such a WDM optical transmission system, an optical amplifier for amplifying signal light is used in order to correct a loss in an optical fiber and enable long-distance transmission.
However, since this optical amplifier has a gain-wavelength characteristic in which the gain changes according to the wavelength, if the transmission system is composed of only the optical amplifier and the optical fiber, the gain of the transmission system also changes according to the wavelength. It shows gain wavelength characteristics. The gain wavelength characteristic is such that the greater the number of optical amplifiers provided in the transmission system, the higher the wavelength dependence of the gain.
[0004]
To solve such a problem that occurs because the gain of the transmission system depends on the wavelength, an optical filter is inserted in the transmission system so that the gain of the transmission system (optical amplifier) does not depend on the wavelength. (Gain equalization) is performed. That is, the gain of the transmission system is flattened by inserting an optical filter having a loss wavelength characteristic having the same shape as the gain wavelength characteristic of the transmission system into the transmission system.
[0005]
By the way, when the loss of the optical transmission line upstream of the optical amplifier fluctuates for some reason and the power of the signal light input to the optical amplifier fluctuates, the power of the signal light output from the optical amplifier is kept constant. In order to maintain the gain, it is necessary to change the gain of the optical amplification of the signal light in the optical amplifier. When the gain is changed, the gain wavelength characteristic changes, and as a result, the flatness of the gain is impaired. Also, other factors may cause fluctuations in the gain wavelength characteristic of the optical amplifier.
[0006]
As a prior art of an optical filter that solves such a problem, there is an optical filter described in JP-A-2002-49013. In the optical filter described in this publication, three Mach-Zehnder interferometers are connected between a first optical input / output terminal and a second optical input / output terminal, and each Mach-Zehnder interferometer is composed of two optical paths and two optical couplers. Be composed. In each Mach-Zehnder interferometer, the optical characteristic is changed by heating the optical path with a heater, and the loss wavelength characteristic of the optical filter can be controlled. As a result, even when the gain wavelength characteristic of the optical amplifier fluctuates, the loss wavelength characteristic of the optical filter can be changed in accordance with the fluctuation, and the gain of the optical amplifier can be flattened.
[0007]
[Problems to be solved by the invention]
However, in such an optical filter, even if the gain wavelength characteristic of the optical amplifier fluctuates, the gain can be flattened, but the signal light passes through a plurality of optical paths, so that the loss of the gain of the signal light is lost. Is as large as several dB. In addition, since the loss wavelength characteristic is controlled by heating the optical path with a heater, the loss wavelength characteristic of the optical filter changes sensitively even when the environmental temperature changes. Furthermore, in order to maintain the state where the loss wavelength characteristic is changed, the optical path must be constantly maintained at a constant temperature by the heater. Therefore, it is necessary to always supply power to the heater control system, and the power consumption is severe. Become.
[0008]
The present invention has been made in view of the above points, and has a small loss of signal light gain, can freely control loss wavelength characteristics, and has a stable state with respect to environmental temperature changes. An object of the present invention is to provide a variable optical filter for maintaining.
[0009]
[Means for Solving the Problems]
To solve the above problems, the optical variable filter according to the present invention is an optical variable filter that corrects and equalizes the gain wavelength characteristic of an optical amplifier whose gain changes according to the wavelength,
A plurality of multilayer filters arranged at predetermined positions on the optical path, and a filter body is configured from an actuator connected to the multilayer filter,
Each of the plurality of multilayer filters has a loss wavelength characteristic in a predetermined wavelength band, and the loss wavelength characteristic of the filter body is formed into the same shape as the gain wavelength characteristic of the optical amplifier by combining these loss wavelength characteristics. And
The actuator changes the arrangement state of the multilayer filter with respect to the optical path by the actuator so that the loss wavelength characteristic of the filter body has the same shape in accordance with the change in the gain wavelength characteristic of the optical amplifier. Is deformed.
[0010]
According to the present invention, since the filter main body is composed of the multilayer filter and the actuator connected to the multilayer filter, the loss wavelength characteristic of the filter main body can be freely changed as necessary.
[0011]
The variable optical filter according to the present invention is characterized in that the arrangement of the multilayer filter with respect to the optical path is changed by inclining the multilayer filter with respect to the optical path.
[0012]
According to the present invention, since the multilayer filter is inclined with respect to the optical path, the loss wavelength characteristic of the multilayer filter can be changed with high accuracy.
[0013]
The variable optical filter according to the present invention is characterized in that the multilayer filter is moved in a direction substantially perpendicular to the optical path to change the arrangement state of the multilayer filter with respect to the optical path.
[0014]
According to the present invention, since the multilayer filter is moved in a substantially vertical direction, a portion of the multilayer filter through which the signal light passes can be moved. Alternatively, the multilayer filter can be removed from the optical path.
[0015]
The variable optical filter according to the present invention is characterized in that the multilayer filter is formed such that the film configuration is different depending on the portion.
[0016]
According to the present invention, the multilayer filter is formed so that the film configuration is different depending on the portion, so that one multilayer filter can have various loss wavelength characteristics.
[0017]
The variable optical filter according to the present invention is characterized in that the multilayer filter is a gradient index filter.
[0018]
According to the present invention, since the multilayer filter is a gradient index filter, higher-order ripple components can be eliminated, and a gain correction curve as intended can be obtained.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present embodiment, a case where flattening is performed in a band of wavelengths 1530 to 1560 nm used in the WDM optical transmission system will be described as an example.
FIG. 1 is a schematic diagram of an optical variable filter according to an embodiment of the present invention. In this figure, optical fibers 31 and 32 serve as signal light input / output ports of the filter main body 1 or are connected to the input / output ports. The two optical fibers 31 and 32 are optically coupled with a collimated beam by a lens 21 and a lens 22 provided at the end of the filter body 1.
A plurality of movable filters 10 each including a multilayer filter 11 and a rotary actuator 14 coupled to the multilayer filter 11 via a coupling shaft 15a or 15b are disposed between the lenses 21 and 22. In FIG. 1, the number of the movable filters 10 is two, but this number differs depending on the object whose gain wavelength characteristic is to be corrected. Although a rotary actuator is used here, any actuator may be used as long as it can move the multilayer filter 11 accurately.
[0020]
The multilayer filter 11 is formed by laminating a dielectric thin film 13 on a substrate 12, each having a specific loss wavelength characteristic, and it is possible to create a desired loss wavelength characteristic by combining a plurality of multilayer filters 11 having different characteristics. it can. Here, since the gain is flattened by the multilayer filter 11, the change in the optical characteristics with respect to the environmental temperature change is smaller than when the optical characteristics are changed by the heater. Also, the multilayer filter 11 itself can further reduce the change in the optical characteristics with respect to the environmental temperature change by matching the expansion coefficients of the materials of the substrate 12 and the dielectric thin film 13.
[0021]
When the rotary actuator 14 is coupled to the multilayer filter 11 via a coupling shaft 15 a formed linearly, the coupling formed in a substantially L-shape so that the rotation axis intersects the optical path 2 substantially perpendicularly. When connected to the multilayer filter 11 via the shaft 15 b, it is arranged such that the rotation axis is substantially parallel to the optical path 2. Thus, a change in a desired arrangement state described later can be given to the multilayer filter 11. This combination of connections also differs depending on the object whose gain wavelength characteristic is to be corrected.
[0022]
Next, control of the loss wavelength characteristic in such a variable optical filter will be described. First, the movable filter 10 in which the rotary actuator 14 is connected to the multilayer filter 11 via the coupling shaft 15a will be described. FIG. 2 is a top view of the movable filter 10 in which the rotary actuator 14 is connected to the multilayer filter 11 via the coupling shaft 15a in FIG. In this case, as described above, the rotation axis of the rotary actuator 14 intersects the light path 2 substantially perpendicularly and is connected to the multilayer filter 11 via the linearly formed coupling shaft 15a. When the actuator 14 is rotated, as shown in FIGS. 2A and 2B, the multilayer filter 11 is inclined with respect to the optical path 2 by the rotation angle of the rotary actuator 14. When the multilayer filter 11 is tilted in this manner, the optical path length when the signal light passes through the multilayer filter 11 becomes relatively long, and the loss wavelength characteristic of the multilayer filter 11 becomes long as shown in FIG. Transition to the wavelength side. By shifting the loss wavelength characteristic to the longer wavelength side by tilting the multilayer filter 11 in this manner, the loss wavelength characteristic of the filter body 1 can be changed.
[0023]
In addition, since the signal light is passed through the multilayer filter 11 on one optical path 2 to flatten the gain, the signal light passes through many optical paths as in the prior art document to reduce the gain. The loss of gain in the variable optical filter can be reduced as compared with the case of flattening.
Further, since the rotary actuator 14 maintains the state after rotation, it is not necessary to keep energizing to maintain the state after changing the arrangement of the multilayer filter 11, and power consumption can be suppressed.
[0024]
Next, the movable filter 10 in which the rotary actuator 14 is connected to the multilayer filter 11 via the coupling shaft 15b will be described. FIG. 4 is a top view of the movable filter 10 in which the rotary actuator 14 is connected to the multilayer filter 11 via the coupling shaft 15b in FIG. In this case, the rotation axis of the rotary actuator 14 is substantially parallel to the optical path 2 and is connected to the multilayer filter through a coupling shaft 15b formed in a substantially L-shape. When rotated, the multilayer filter 11 moves in a direction substantially perpendicular to the optical path 2 as shown in FIGS. Therefore, when the rotation angle of the rotary actuator 14 is increased, the multilayer filter 11 is removed from the optical path 2. As described above, by removing the multilayer filter 11 from the optical path 2, the loss wavelength characteristic of the multilayer filter 11 off the optical path 2 can be removed from the loss wavelength characteristic of the filter body 1. Thereby, the loss wavelength characteristic of the filter body 1 can be changed.
[0025]
The loss wavelength characteristic of the filter body 1 can also be changed by changing the film configuration of the multilayer filter 11 depending on the portion. That is, as shown in FIG. 5, the dielectric thin films 13 of the multilayer filter 11 are stacked so that the film configuration differs depending on the location. Such a multilayer filter 11 is connected to a rotary actuator 14 via a coupling shaft 15b formed in a substantially L shape. When the rotary actuator 14 is rotated, the multilayer filter 11 moves in a direction substantially perpendicular to the optical path 2, and the film configuration of the dielectric thin film 13 on the optical path 2 changes.
[0026]
As described above, by changing the film configuration of the dielectric thin film 13 on the optical path 2, the same effect as when a plurality of multilayer filters 11 are arranged on the optical path 2 can be obtained. When the multilayer filter 11 is sequentially moved in the direction in which the film configuration is different from B to C, the loss wavelength characteristic increases as shown in FIG. 6, and when the multilayer filter 11 is moved in the reverse direction, the loss wavelength characteristic increases. Becomes smaller. Thereby, the loss wavelength characteristic of the filter body 1 can be changed.
Although the dielectric thin film 13 is formed in three stages in FIG. 5, the number of stages may be changed as necessary. Further, the loss wavelength characteristic may be changed by laminating the dielectric thin films 13 constituting each stage in combination with different materials. By doing so, various loss wavelength characteristics can be created with one multilayer filter 11.
[0027]
The embodiments of the present invention have been described above. In the above embodiments, the control of the loss wavelength characteristic of the multilayer filter 11 has been described individually. However, these are not used separately, and an arbitrary loss wavelength characteristic is formed by combining them as needed.
In the above embodiment, the multilayer filter 11 is preferably a gradient index filter as shown in FIG. That is, the dielectric thin film 13 of the multilayer filter 11 is formed by a refractive index gradient laminated portion 13A for changing the refractive index inclining with respect to the laminating direction, a high refractive index dielectric thin film layer and a low refractive index dielectric thin film layer. It comprises an alternately laminated portion 13C which is alternately laminated, and an adjusting portion 13B which adjusts a difference in refractive index between the refractive index gradient laminated portion 13A and the alternately laminated portion 13C. With such a configuration, ripples, which are high-order loss components existing in the smooth portion of the entire transmission band, are removed, and the multilayer filter 11 having a loss wavelength characteristic of an arbitrary shape can be obtained.
[0028]
【The invention's effect】
As described above, according to the variable optical filter according to the present invention, since the filter body is composed of the multilayer filter and the actuator connected to the multilayer filter, it is possible to freely change the loss wavelength characteristic of the filter body as necessary. it can. Further, since the gain is flattened by passing the signal light through the multilayer filter on one optical path, the loss of gain in the variable optical filter can be reduced. Furthermore, since the gain is flattened by the multilayer filter, the change in the optical characteristics with respect to the environmental temperature change is smaller than when the optical characteristics are changed by the heater. Further, the multilayer filter itself can further reduce the change in the optical characteristics with respect to the environmental temperature change by matching the expansion coefficients of the material of the substrate and the dielectric thin film. Furthermore, since the actuator maintains the state after the movement, it is not necessary to keep energizing to maintain the state after changing the arrangement of the multilayer filter, and the power consumption can be reduced.
[0029]
According to the variable optical filter of the present invention, since the multilayer filter is inclined with respect to the optical path, the loss wavelength characteristic of the multilayer filter can be changed with high accuracy. As a result, the loss wavelength characteristic of the filter body can be flexibly changed with respect to the fluctuation of the gain wavelength characteristic of the optical amplifier, and can be corrected with high accuracy.
[0030]
Further, according to the variable optical filter of the present invention, the multilayer filter is moved in a substantially vertical direction, so that the portion of the multilayer filter through which the signal light passes can be moved. Thereby, the loss wavelength characteristic of the multilayer filter for the signal light can be changed. Alternatively, the multilayer filter can be removed from the optical path. As a result, the loss wavelength characteristics of the removed multilayer filter can be removed from the loss wavelength characteristics of the filter body, so that loss wavelength characteristics of various shapes can be formed.
[0031]
Further, according to the variable optical filter of the present invention, the multilayer filter is formed so that the film configuration is different depending on the portion, so that one multilayer filter can have various loss wavelength characteristics. Thereby, the loss wavelength characteristic of the filter body can be changed over a wide range.
[0032]
Further, according to the variable optical filter according to the present invention, since the multilayer filter is a gradient index filter, higher-order ripple components can be eliminated, and a gain correction curve as intended can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a variable optical filter according to an embodiment of the present invention.
FIG. 2 is a schematic diagram showing a case where a multilayer filter is inclined with respect to an optical path to change a loss wavelength characteristic.
FIG. 3 is a graph showing a change in a loss wavelength characteristic when a multilayer filter is inclined with respect to an optical path.
FIG. 4 is a schematic diagram showing a case where a multilayer filter is removed from an optical path to change a loss wavelength characteristic.
FIG. 5 is a schematic diagram in a case where the film configuration of a multilayer filter is formed differently depending on a portion to change a loss wavelength characteristic.
FIG. 6 is a graph showing a change in a loss wavelength characteristic when a film configuration of a multilayer filter is different depending on a portion.
FIG. 7 is a diagram showing a relationship between a refractive index and a film thickness of a gradient index filter, and a cross-sectional structure.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Filter main body 2 Optical path 10 Movable filter 11 Multilayer filter 12 Substrate 13 Dielectric thin film 14 Rotary actuator 15a, 15b Coupling shaft 21, 22 Lens 31, 32 Optical fiber

Claims (5)

In an optical tunable filter that corrects and equalizes the gain wavelength characteristic of an optical amplifier whose gain changes according to the wavelength,
A plurality of multilayer filters arranged at predetermined positions on the optical path, and a filter body is configured from an actuator connected to the multilayer filter,
Each of the plurality of multilayer filters has a loss wavelength characteristic in a predetermined wavelength band, and the loss wavelength characteristic of the filter body is formed into the same shape as the gain wavelength characteristic of the optical amplifier by combining these loss wavelength characteristics. And
The actuator changes the arrangement state of the multilayer filter with respect to the optical path by the actuator so that the loss wavelength characteristic of the filter body has the same shape in accordance with the change in the gain wavelength characteristic of the optical amplifier. A variable optical filter, characterized by deforming the optical filter. The optical variable filter according to claim 1, wherein the arrangement of the multilayer filter with respect to the optical path is changed by inclining the multilayer filter with respect to the optical path. 2. The variable optical filter according to claim 1, wherein the arrangement of the multilayer filter with respect to the optical path is changed by moving the multilayer filter in a direction substantially perpendicular to the optical path. 4. The optical tunable filter according to claim 3, wherein the multilayer filter is formed so that a film configuration is different depending on a portion. 5. The variable optical filter according to claim 1, wherein the multilayer filter is a gradient index filter.

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