JP2000252920A - Optical equalizer - Google Patents

Abstract

(57) [Problem] In a conventional optical equalizer, the grating pitch is changed in the longitudinal direction of the optical waveguide and written in the optical waveguide, so the dispersion amount is a fixed value and cannot be changed. There was a problem. Further, there is a problem that the active dispersion compensation amount cannot be controlled according to a slight change in the state of the transmission line. SOLUTION: A temperature gradient applying means is provided in the grating to change an equivalent refractive index of the grating in a longitudinal direction. Changing the temperature gradient can change the amount of dispersion that occurs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system, and more particularly to a technique for compensating dispersion of an ultra-high-speed optical signal.

[0002]

2. Description of the Related Art There is a phenomenon that an optical signal propagates at a different speed on a transmission line depending on its wavelength. This is because the transmission medium has a different refractive index for each wavelength. Further, in an optical transmission line represented by an optical fiber, since light is transmitted in a state where light is confined in a part of a transmission medium, the propagation speed of a signal varies depending on the state of confinement of light in the transmission medium. . Since the state of confinement differs depending on the wavelength, it causes a difference in propagation speed for each wavelength. The difference in propagation speed due to the wavelength of light caused by the above factors is called dispersion. Dispersion is given as the derivative of delay time as a function of wavelength,
It has a unit of ps / nm.

The effect of dispersion is a phenomenon that cannot be avoided even in a single mode fiber (hereinafter, referred to as SMF) mainly used for high-speed optical transmission, and causes signal waveform deterioration due to transmission. For example, 10Gb /
When transmitting a signal of s, the acceptable dispersion value is about 1
000 ps / nm, which corresponds to a dispersion of about 70 km SMF. Therefore, it is very important to compensate for dispersion for long-distance transmission. This technique is called dispersion compensation or optical equalization. As a device for dispersion compensation,
Dispersion compensating fiber (DCF) is commercially available. A dispersion compensating fiber (DCF) is a device having a dispersion value having a sign opposite to that of SMF used for a transmission line. However, DCF
Is a long optical fiber, and has a problem that the mounting volume is large.

Conventional Example 1 As another dispersion compensation technique, a technique called a chirped grating is known, and for example, Japanese Patent Application No. 56-137.
No. 2, JP-A-59-126336 discloses details. FIG. 8 is a diagram illustrating the concept of a chirped grating. 8, 17 is an optical signal input terminal, 1
8 is an optical circulator, 19 is an optical signal output terminal, 3 is an optical waveguide input / output terminal, 4 is an optical waveguide output terminal, 1 is an optical waveguide, and 2 is a diffraction grating. The optical circulator outputs an optical signal input from the optical signal input terminal to the optical waveguide input / output terminal 3 and outputs an optical signal input from the optical waveguide input / output terminal 3 to the optical signal output terminal 19. It is. The diffraction grating 2 has a refractive index changed at a Bragg wavelength period, and is called a grating. The optical signal input from the optical signal input terminal 17 is
Is input to the optical waveguide 1 from the optical waveguide input / output terminal 3. The optical signal reflected by the diffraction grating 2 written in the optical waveguide 1 is output from the optical waveguide input / output terminal 3 via the optical circulator 18 to the optical signal output terminal 19.

Here, dispersion compensation by a Bragg grating written in an optical waveguide will be described. Assuming that the grating pitch (grating interval) is Λ and the equivalent refractive index of the grating portion is Neff, the wavelength λB (Bragg wavelength) reflected by the Bragg grating is expressed by the following equation (1). λB = 2 Neff Λ (1)

Therefore, in a chirped grating in which the grating pitch is linearly changed in the longitudinal direction of the optical waveguide, the Bragg wavelength is 1 with respect to the longitudinal direction of the optical waveguide.
It is distributed as a quadratic function. This is equivalent to the fact that the reflection points of light corresponding to each Bragg wavelength are distributed in a linear function in the longitudinal direction of the waveguide. Also, the light reflected back by the grating is delayed according to the distance to the reflection point, and the amount of delay is expressed as a linear function of the wavelength.

For example, as shown in FIG. 8, in a chirped grating 2 in which the grating pitch linearly increases from the optical signal incident side, the wavelengths λ1, λ2, λ3
The delay increases linearly in the order of (λ1 <λ2 <λ3). As a result, the dispersion represented by the wavelength derivative of the group delay is shown in FIG.
Is a straight line having a constant value with respect to the wavelength. Thus, a desired dispersion value can be obtained by the chirped grating.

[0008]

However, since the chirp and the grating are written in the optical waveguide by changing the grating pitch in the longitudinal direction of the optical waveguide as described above, the amount of dispersion becomes a fixed value. There is a problem that can not be. Since the dispersion value required for the optical equalizer is the amount of dispersion that cancels the dispersion of the transmission line, the dispersion value differs depending on the use situation. Therefore, the restriction that the amount of dispersion cannot be changed is a major problem in use. In the case of an ultra-high-speed optical signal of 10 Gb / s or more, it is necessary to increase the accuracy of dispersion compensation.
It has been argued that it is necessary to control the active dispersion compensation amount in response to a slight change in the state of the transmission line for an optical signal of b / s or more. However, the conventional optical equalizer has a problem that it is impossible to control the amount of active dispersion compensation.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems. An optical equalizer according to the first invention has an input / output terminal for inputting / outputting an optical signal, and an optical signal input / output terminal. And a temperature gradient applying means for applying a temperature gradient in the optical signal propagation direction of the optical waveguide.

The optical equalizer according to the second invention has an electric field gradient applying means for applying an electric field gradient to an optical waveguide which causes Bragg reflection by a diffraction grating in an optical signal propagation direction.

The optical equalizer according to the third invention has a stress gradient applying means for applying a stress gradient in an optical signal propagation direction to an optical waveguide which causes Bragg reflection by a diffraction grating.

According to a fourth aspect of the present invention, there is provided an optical equalizer, comprising: temperature detecting means for detecting the temperature at a plurality of portions of the optical waveguide; and temperature control for controlling the temperature of the optical waveguide based on the detection result of the temperature detecting means. Means for applying a temperature gradient based on the control of the temperature control means.

In an optical equalizer according to a fifth aspect of the present invention, the temperature gradient detecting means is disposed at a central portion of the diffraction grating, and the temperature gradient control means controls the temperature at the central portion of the diffraction grating to be constant. Is what you do.

An optical equalizer according to a sixth aspect of the present invention comprises the above-mentioned waveguide having unequally spaced comb-shaped heat conductors.

According to a seventh aspect of the present invention, an optical equalizer includes an input / output terminal for inputting / outputting an optical signal, an optical coupler for optically multiplexing / demultiplexing the optical signal from the input / output terminal, and a first output of the optical coupler. , A second optical waveguide connected to the second output of the optical coupler, and first temperature gradient applying means for applying a temperature gradient in the longitudinal direction of the first optical waveguide. And second temperature gradient applying means for applying a temperature gradient in the longitudinal direction of the second optical waveguide, wherein the first optical waveguide and the second optical waveguide have a diffraction grating that causes Bragg reflection.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a block diagram showing the configuration of an optical equalizer according to the present invention. In FIG.
17 is an optical signal input terminal, 18 is an optical circulator, 19
Is an optical signal output terminal, 3 is an optical waveguide input / output terminal, 4 is an optical waveguide output terminal, 1 is an optical waveguide, 2 is a diffraction grating, 5a, 5a
b is a temperature applying means. As the optical waveguide 1, an optical fiber, a silica-based waveguide, a semiconductor waveguide, or the like can be used.
As the temperature applying means 5a and 5b, a heater, a Peltier element, or the like can be used.

Next, the operation will be described. Optical signal input terminal 17
The input optical signal passes through the optical circulator 18 and
The light is input to the optical waveguide 1 from the optical waveguide input / output terminal 3. The optical signal whose grating pitch (grating interval) of the diffraction grating 2 and the equivalent refractive index of the grating portion match the Bragg wavelength determined by Neff is reflected by the diffraction grating 2 and the optical signal that does not match the Bragg wavelength is Output from the optical waveguide output terminal 4. That is, the diffraction grating 2 functions as an optical filter. The optical signal reflected by the diffraction grating 2 is output from an optical signal output terminal 19 via an optical circulator 18 from an optical waveguide input / output terminal 3.

Heat is applied to the optical waveguide 1 by the temperature applying means 5a, 5b. At this time, by setting the amount of heat generated by the temperature applying unit 5a and the amount of heat generated by the temperature applying unit 5b to different amounts, a thermal gradient is generated in the optical waveguide 1 and the diffraction grating 2. Since the refractive index of the optical waveguide 1 is a function of temperature, the equivalent refractive index Neff of the diffraction grating 2 is also a function of temperature. The thermal gradient applied by the temperature applying means 5a, 5b also gives the equivalent refractive index Neff a distribution in the longitudinal direction of the waveguide. As shown in the equation (1), the Bragg wavelength is a function of the equivalent refractive index Neff, so that the Bragg wavelength is distributed in the longitudinal direction of the optical waveguide 1. Therefore, an effect similar to that of the chirped grating is produced by the thermal gradient. If the wavelength is different, the distance to the reflection point is different, and dispersion occurs. For example, if the amount of heat generated by the temperature applying unit 5b is larger than the amount of heat generated by the temperature applying unit 5a, FIG.
As a result, the distance from the optical waveguide input / output terminal 3 to the reflection point varies depending on the wavelength. The solid line in FIG. 1C represents a function of the wavelength and the delay time. Since the dispersion is a wavelength derivative of the delay time, the dispersion as shown by the solid line in FIG. 1D occurs.

Now, let us consider reducing the difference between the amount of heat generated by the temperature applying unit 5a and the amount of heat generated by the temperature applying unit 5b. The thermal gradient and the equivalent refractive index gradient generated in the diffraction grating 2 become smaller as shown by the dotted line in FIG. 1B, and the wavelength dependence of the generated delay time becomes larger as shown by the dotted line in FIG. Therefore, the variance value increases as shown by the dotted line in FIG. Furthermore, if the amount of heat generated by the temperature applying unit 5b is smaller than the amount of heat generated by the temperature applying unit 5a, the sign of the dispersion that occurs is negative. In this way,
By changing the thermal gradient applied to the diffraction grating 2, it is possible to change the amount of generated dispersion. That is, optical equalization can be performed. Note that the sign relationship between the temperature gradient and the equivalent refractive index gradient may be opposite to the above example depending on the temperature coefficient of the medium forming the optical waveguide. Further, by controlling the temperature gradient, not only the dispersion but also the dispersion slope corresponding to the wavelength derivative of the dispersion value can be controlled.

As the diffraction grating 2, an equally spaced grating or a chirped grating can be used. Similar effects can be obtained by using a non-uniform grating pitch called an apodized grating. In FIG. 1, two temperature applying means are used, but it goes without saying that three or more temperature applying means can be used.

Embodiment 2 FIG. FIG. 2 shows a configuration diagram of another embodiment according to the present invention. The difference from FIG. 1 is that the electrodes 6a, 6b, 6c, 6d, 6e, 6f and the voltage source 7
a, 7b, 7c, 7d, and 7e. Since the refractive index of the optical waveguide 1 changes according to the applied electric field, a distribution in the longitudinal direction occurs in the equivalent refractive index of the diffraction grating 2 as in FIG. Voltage sources 7a, 7b, 7c, 7
By setting d and 7e to a voltage satisfying 7a <7b <7c <7d <7e, dispersion similar to that in FIG. 1 occurs. By changing the distribution of this voltage, desired dispersion can be obtained.

The phenomenon that the refractive index changes due to the electric field applied to the optical waveguide 1 is called an electro-optic effect. The voltage of the voltage sources 7a, 7b, 7c, 7d, and 7e can be reduced by using an optical waveguide that easily causes the electro-optic effect. Semiconductors and dielectric crystals are known as materials having a large electro-optic effect. Although five electrodes and five voltage sources are used in FIG. 2, the number of electrodes and voltage sources is arbitrary as long as the purpose of generating an equivalent refractive index distribution in the diffraction grating is satisfied. Monitoring and controlling the applied voltage or electric field using a voltmeter or an electric field sensor is effective for realizing a stable optical equalizer. In FIG. 2, the optical circulator 18 to be connected to the waveguide input / output terminal 3 in FIG. 1 is omitted.

Embodiment 3 FIG. FIG. 3 is a block diagram showing another embodiment of the present invention. In FIG. 3, 3 is an optical waveguide input / output terminal, 4 is an optical waveguide output terminal, 1 is an optical waveguide, 2 is a diffraction grating, 8a, 8b, 8c, 8d, 8e are piezo elements, 7a, 7b, 7c, 7d, 7e is a voltage source, 9 is a fixed base. The piezo elements 8a, 8b, 8c, 8d, 8e operate as stress applying means.

Next, the operation will be described. A stress is applied to the optical waveguide 1 by the piezo element. As a result, the distance to the Bragg wavelength changes because the optical path length changes,
Dispersion occurs. By changing the applied stress, a desired dispersion can be obtained. In FIG. 3, the optical circulator 18 to be connected to the waveguide input / output terminal 3 in FIG. 1 is omitted.

Embodiment 4 FIG. 4 is a block diagram showing another embodiment of the present invention. 3 is an optical waveguide input / output terminal, 4 is an optical waveguide output terminal, 1 is an optical waveguide, 2 is a diffraction grating,
Reference numerals a and 5b denote temperature applying means, 10a and 10b denote temperature detecting means, and 11a and 11b denote temperature control circuits. The temperature control circuits 11a and 11b include temperature detection circuits 12a and 12b, reference voltage generation circuits 15a and 15b, comparators 13a and 13b,
It is composed of A thermistor, a semiconductor device, or the like is used as the temperature detecting means 10a, 10b.

Next, the operation will be described with reference to FIG. The temperature measured by the temperature detecting means 10a is converted into a voltage by the temperature detecting circuit 12a,
is compared with the voltage generated by the comparator 13a. Since the temperature control means 11a controls the temperature application means 5a according to the error signal generated by the comparator 13a, the voltage generated by the temperature detection circuit 12a is equal to the reference voltage generation circuit 15a.
It operates so as to be equal to the voltage output from. Similarly, the temperature measured by the temperature detecting means 10b is converted into a voltage by the temperature detecting circuit 12b, and is compared with the voltage generated by the reference voltage generating circuit 15b by the comparator 13b. Since the temperature control unit 11b controls the temperature application unit 5a based on the error signal generated by the comparator 13b,
The operation is performed so that the voltage generated by the temperature detection circuit 12b becomes equal to the voltage output from the reference voltage generation circuit 15b.
The difference from FIG. 1 is that the provision of the temperature detecting means 10a and 10b makes it possible to apply a temperature gradient with higher accuracy and stability. Needless to say, providing three or more temperature detecting means is effective for further improving the accuracy and stabilizing. In FIG. 4, the optical circulator 18 to be connected to the waveguide input / output terminal 3 in FIG. 1 is omitted.

Embodiment 5 FIG. FIG. 5 is a configuration diagram showing another embodiment of the present invention. 3 is an optical waveguide input / output terminal, 4 is an optical waveguide output terminal, 1 is an optical waveguide, 2 is a diffraction grating, 5
Reference numerals a and 5b denote temperature applying means, 10 denotes a temperature detecting means, and 11 denotes a temperature control circuit. The temperature control circuit 11 includes a temperature detection circuit 12, a first reference voltage generation circuit 15a, a comparator 13, a second reference voltage generation circuit 15b, and an adder 14.

FIG. 5 is characterized in that the temperature detecting means 10 is attached to the center of the diffraction grating 2. The temperature control circuit 11 keeps the temperature detected by the temperature detecting means 10 at a constant value determined by the voltage provided by the first reference voltage generation circuit 15a, and also adjusts the temperature gradient to the second reference voltage generation circuit 15b.
Is performed in accordance with the voltage value given by. The temperature measured by the temperature detecting means 10 is converted into a voltage by the temperature detecting circuit 12, and is compared with the voltage generated by the first reference voltage generating circuit 15a by the comparator 13. The temperature control means 11 controls the temperature application means 5a according to the error signal generated by the comparator 13.

The adder 14 adds the voltage generated by the comparator 13 and the voltage generated by the second reference voltage generation circuit 15b, and the temperature control means 11 controls the temperature application means 5b. The operation is performed so that the voltage generated by the temperature detection circuit 12 becomes equal to the voltage output from the first reference voltage generation circuit 15a. Even when the applied temperature gradient is changed, the temperature at the center of the diffraction grating 2 is controlled to be a constant value, so that the center wavelength of the Bragg reflection wavelength of the diffraction grating 2 is constant. In FIG. 5, the optical circulator 18 to be connected to the waveguide input / output terminal 3 in FIG. 1 is omitted.

Embodiment 6 FIG. FIG. 6 is a block diagram showing another embodiment of the present invention. The difference from FIG. 1 is that a heat conductor 16 is added. The heat conductor 16 has a non-equidistant comb shape, and can give a thermal gradient to the diffraction grating 2.
The operation is the same as in FIG.

Embodiment 7 FIG. 7 is a configuration diagram showing another embodiment of the present invention. In FIG. 7, 17 is an optical signal input terminal, 19 is an optical signal output terminal, 23 is an optical multiplexer / demultiplexer, 1a and 1b are optical waveguides, 2a and 2b are diffraction gratings,
Reference numerals a and 5b denote temperature applying means. Integrating an optical multiplexer / demultiplexer and an optical waveguide is effective for miniaturization and high precision of an optical equalizer. Usually, a 3 dB optical coupler is used as the optical multiplexer / demultiplexer 23. In this embodiment, an effect similar to that of FIG. 1 can be obtained without using an optical circulator.

The operation according to the present embodiment will be described. Power is input from the optical signal input terminal 17 to the two adjacent waveguides in the optical multiplexer / demultiplexer 23, each having a power of 各 々 and a phase difference of π.
The light demultiplexed in the state of / 2 undergoes dispersion in the optical waveguides 1a and 1b including the diffraction gratings 2a and 2b to which a temperature gradient is applied, respectively, and is then reflected back by the optical multiplexer / demultiplexer 23 and returned. . The optical signal of each waveguide input to the optical multiplexer / demultiplexer 23 makes one round trip through the optical multiplexer / demultiplexer 23 in order to further obtain a phase difference of π / 2 when passing through the optical multiplexer / demultiplexer 23. The phase difference of light in each waveguide is π. as a result,
The light returning to the optical multiplexer / demultiplexer 23 is output to the optical signal input terminal 17 and the optical signal output terminal 19.

[0033]

As described above, according to the present invention, according to the present invention, an optical transmission line having a grating causing Bragg reflection is provided with a means for applying a temperature gradient in an optical signal propagation direction. Can generate dispersion. Further, according to the present invention, the variance value can be changed to a desired value.

According to the next invention, the optical transmission line having the grating causing Bragg reflection is provided with means for applying an electric field gradient in the optical signal propagation direction, and the dispersion can be generated by applying the electric field gradient. it can. Further, according to the present invention, the variance value can be changed to a desired value.

According to the next invention, the optical transmission line having a grating that causes Bragg reflection is provided with a means for applying a stress gradient in the direction of optical signal propagation, and the dispersion can be generated by applying the stress gradient. it can. Further, according to the present invention, the variance value can be changed to a desired value.

According to the next invention, since two or more temperature control circuits are provided, a desired temperature gradient can be given, so that the dispersion compensation amount can be controlled.

According to the next invention, since there is provided means for applying a temperature gradient to the transmission line so that the Bragg center wavelength of the grating does not change, it is possible to provide an optical equalizer that is stable with respect to the signal wavelength. .

According to the next invention, since a non-equidistant comb-shaped heat conductor is provided, it is possible to apply a temperature gradient with a simple configuration, and it is possible to easily provide an optical equalizer.

According to the next invention, the optical coupler, the first optical waveguide connected to the first output of the optical coupler, the second optical waveguide connected to the second output of the optical coupler, ,
A first temperature gradient applying means for applying a temperature gradient in a longitudinal direction of the first optical waveguide, and a second temperature gradient applying means for applying a temperature gradient in a longitudinal direction of the second optical waveguide; Since the waveguide and the second optical waveguide are characterized by having a grating that causes Bragg reflection, a compact and simple optical equalizer can be provided without using an optical circulator.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment.

FIG. 2 is a block diagram showing a second embodiment.

FIG. 3 is a block diagram showing a third embodiment.

FIG. 4 is a block diagram showing a fourth embodiment.

FIG. 5 is a block diagram showing a fifth embodiment.

FIG. 6 is a block diagram showing a sixth embodiment.

FIG. 7 is a block diagram showing a seventh embodiment.

FIG. 8 is a diagram showing a configuration of Conventional Example 1.

[Explanation of symbols]

1, 1a, 1b Optical waveguide 2, 2a, 2b Diffraction grating 3 Optical waveguide input / output terminal 4 Optical waveguide output terminal 5, 5a, 5b Temperature applying means 6, 6a, 6b, 6c, 6d, 6e, 6f Electrode 7, 7a , 7b, 7c, 7d, 7e Voltage source 8, 8a, 8b, 8c, 8d, 8e Piezo element 9 Fixed base 10, 10a, 10b Temperature detection means 11, 11a, 11b Temperature control circuit 12 Temperature detection circuit 13 Comparator 14 Adder 15, 15a, 15b Reference voltage generating circuit 16 Thermal conductor 17 Optical signal input terminal 18 Optical circulator 19 Optical signal output terminal 20 Polarization separator 23 Optical multiplexer / demultiplexer

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Claims (7)

[Claims] An input / output terminal for inputting / outputting an optical signal, an optical waveguide having a diffraction grating for causing a Bragg reflection of the input / output optical signal, and a temperature for applying a temperature gradient in the optical signal propagation direction of the optical waveguide. An optical equalizer comprising: a gradient applying unit. 2. An optical waveguide according to claim 1, further comprising an electric field gradient applying means for applying an electric field gradient to an optical waveguide causing Bragg reflection by a diffraction grating in an optical signal propagation direction, in place of said temperature gradient applying means. An optical equalizer as described. 3. The optical waveguide according to claim 1, further comprising a stress gradient applying means for applying a stress gradient in an optical signal propagation direction to the optical waveguide causing Bragg reflection by the diffraction grating, instead of the temperature gradient applying means. Optical equalizer. 4. A temperature detecting means for detecting a temperature at a plurality of positions of the optical waveguide, and a temperature controlling means for controlling a temperature of the optical waveguide based on a detection result of the temperature detecting means; 2. The optical equalizer according to claim 1, wherein the gradient applying means gives a temperature gradient based on the control of the temperature control means. 5. The apparatus according to claim 1, wherein said temperature gradient detecting means is arranged at a central portion of said diffraction grating, and said temperature gradient control means controls said temperature gradient at a central portion thereof to be constant. 5. The optical equalizer according to any one of 4. 6. The optical equalizer according to claim 1, wherein said waveguide has a non-equidistant comb-shaped heat conductor. 7. An input / output terminal for inputting / outputting an optical signal, an optical coupler for optically multiplexing / demultiplexing an optical signal from the input / output terminal, and a first optical waveguide connected to a first output of the optical coupler. A second optical waveguide connected to a second output of the optical coupler; first temperature gradient applying means for applying a temperature gradient in a longitudinal direction of the first optical waveguide; An optical equalizer, comprising: second temperature gradient applying means for applying a temperature gradient in a longitudinal direction, wherein the first optical waveguide and the second optical waveguide have a diffraction grating that causes Bragg reflection.