JP2001053679A - Optical automatic equalizer - Google Patents

Abstract

(57) [Problem] To provide a preset dispersion equalization function and an adaptive dispersion equalization function, and to simultaneously correct both timing jitter and intensity fluctuation of a signal. SOLUTION: It is composed of two function units of a dispersion equalization unit 1 and a waveform shaping unit 4. The dispersion equalization unit 1 controls a coarse dispersion unit 2 for selecting a plurality of dispersion media and roughly performing dispersion equalization, and a variable dispersion equalizer combining two types of chirped fiber gratings to perform fine dispersion equalization. And the fine movement unit 3 that performs the above. The waveform shaping unit 4 includes a timing jitter correction unit 5 that narrows a pulse width and reduces timing jitter by optical modulation synchronized with a clock signal, and amplifies signal light to a larger power by a basic soliton, and further provides highly nonlinear dispersion. It comprises an intensity fluctuation correction unit 6 which converts the intensity fluctuation into a spectral spread fluctuation by inputting the signal into a medium and outputs the signal light from which the intensity fluctuation of the signal light is removed by passing through an optical bandpass filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a terminal station or a relay station of an ultra-high-speed and large-capacity optical communication system, which automatically equalizes chromatic dispersion or waveform distortion which causes deterioration of transmission characteristics and shapes a signal waveform. The present invention relates to an automatic optical equalizer.

[0002]

2. Description of the Related Art Dispersion equalization of an optical transmission line includes a preset dispersion equalization performed when a system is introduced and an adaptive dispersion equalization performed when a system is operated.

The preset dispersion equalization includes a method of equalizing the dispersion of an optical fiber transmission line by tuning the wavelength of a light source, and a method of using a variable dispersion equalizer on the receiving side. As a method of detecting the optical fiber dispersion value at the time of the preset dispersion equalization, a method using a special signal and a method using the data main signal itself have been proposed.

As a method using a special signal, there is a method of estimating a zero dispersion wavelength from a reception amplitude of a monitor signal (Reference 1: M. Tomizawa et al., "Automatic Dispersion Equali
zationfor Installing High Speed Optical Transmissi
on Systems ", J. Lightwave Technol., vol.16, no.2,
p.184, 1998) and a method of monitoring phase shift (Reference 2: A. Sano et al., "Adaptive dipersion equalizati
on of 8-ps pulses in 400-km transmission line by m
onitoring relative phase shift between spacing-fix
ed WDM signals ", Proc. OFC'99, WJ4, p.165, 1999). The method using the main data signal is optimized by branching and monitoring the clock output from the timing extraction circuit. There is a method of detecting an operating point (Reference 3: Oi et al., "40 Gbit / s Automatic Dispersion Equalization Experiment Using Tunable Laser", IEICE Communications Society Conference 1998, B-10-96).

As a variable dispersion equalizer, an optical switch is used to select a dispersion compensating fiber having an optimum dispersion value from a plurality of dispersion compensating fibers (Reference 4: A. Sano et al., "Automatic
dispersion equalization by monitoring extracted c
lock power level in a 40-Gbit / s, 200-km transmissi
on line ", ECOC'96, TuD.3.5, 1996) and a method of applying tension to the fiber grating (Reference 5: T. Imai e
t al., "Dispersion Tuning of a Linearly Chirped Fib
er Bragg Grating without a Center Wavelength Shift
by Applying a Strain Gradient ", IEEE Photo. Techn
ol. Lett., vol. 10, no. 6, p. 845, 1998) or a method of giving a temperature gradient (Ref. 6: D. Gartheet al., "Adjust")
able dispersion equalizer for 10 and 20Gbit / s ove
r distances up to 160km ", Electron. Lett., vol. 30,
no.25, p.2159, 1994) and a method using a planar waveguide (PLC) (Ref. 7: K. Takiguchi et al., "Disper
sion Slope Equalizer for Dispersion Shifted Fiber
Using a Lattice-Form Programmable Optical Filter o
na Planar Lightwave Circuit ", IEEE J. Lightwave Te
chnol., vol.16, no.9, p.1647, 1998).

The tunable dispersion equalizer using the dispersion compensating fiber has an advantage that the dispersion variable range is wide, but there is a problem that it is difficult to perform dispersion equalization with high precision because the dispersion value cannot be changed continuously. . On the contrary, the variable dispersion equalizer using the fiber grating has the advantage that the dispersion value can be continuously changed and the dispersion equalization with high accuracy can be achieved, but there is a problem that the band becomes narrower when the dispersion value is increased. . The variable dispersion equalizer using a planar waveguide has an advantage that a dispersion value can be set with high accuracy, but has a problem that a narrow band has a large insertion loss and is difficult to control.

For adaptive dispersion equalization, there is a method of continuously changing the wavelength of a light source, but a method using a variable dispersion equalizer has not been proposed so far. At the time of adaptive variance equalization, a method for detecting a fluctuation direction of an increase / decrease of a variance value due to an environmental change is required. These include a method of setting a monitor wavelength (Reference Documents 1 and 2) and a method of detecting a light source wavelength by constantly shaking it (Reference Document 3). However, there is a problem that the system becomes complicated.

[0008]

As described above, in the method of tuning the wavelength of the light source in the preset dispersion equalization and the adaptive dispersion equalization method, devices are required on both the transmission side and the reception side, which makes the device complicated and complicated. There is a problem of high cost. Also, in the method using the variable dispersion equalizer, only the receiving side can cope, and the apparatus can be simplified and the cost can be reduced. However, there is no effective variable dispersion equalizer for adaptive dispersion equalization. .

In an optical communication system, there is a method of performing signal shaping on a signal pulse for the purpose of further increasing the speed. In particular, in soliton transmission, a technique called “soliton control” has been developed (Ref. 8: H. Kubota et a).
l., "Soliton Transmission Control in Time and Frequ
ency Domains ", IEEE J. Quantum Electron., vol.29, n
o.7, p.303, 1993), soliton waveform distortion and timing jitter can be reduced. For example, when synchronous modulation is used, timing jitter of a signal pulse can be corrected, and when an optical filter is used, soliton pulse energy can be stabilized and intensity fluctuation can be reduced. However, this method has been proposed for an optical transmission line, and there is no example of using it for stabilizing energy as one element of an optical equalizer.

An object of the present invention is to provide a high-performance automatic optical equalizer having both a preset dispersion equalization function and an adaptive dispersion equalization function and capable of simultaneously correcting both timing jitter and intensity fluctuation of a signal. With the goal.

[0011]

FIG. 1 shows the basic structure of an automatic optical equalizer according to the present invention. In the figure, the optical automatic equalizer of the present invention is composed of two functional units, a dispersion equalizing unit 1 and a waveform shaping unit 4.

The dispersion equalizer 1 includes a coarse moving unit 2 for roughly performing dispersion equalization by taking advantage of a wide dispersion variable range of a variable dispersion equalizer for selecting a plurality of dispersion media, and two types of chirped fibers. It is composed of a fine movement unit 3 that performs a dispersion equalization finely taking advantage of an advantage that a dispersion value of a variable dispersion equalizer combined with a grating can be continuously varied and high-precision dispersion equalization can be performed. By combining the coarse moving unit 2 and the fine moving unit 3, a variable dispersion equalizer having a large dynamic range is configured.

The coarse movement section 2 detects the clock signal level from the signal light after passing through the dispersion medium, and selects a dispersion medium having a dispersion value at which the clock signal level is maximized.

In the fine movement unit 3, the signal light after passing through the variable dispersion equalizer is branched into two paths, an optical fiber having a positive dispersion value is arranged in one path, and a negative fiber is arranged in the other path. In this configuration, an optical fiber having a dispersion value is arranged, a difference between clock signal levels of signal light passing through each optical fiber is detected, and a variable dispersion equalizer is tuned so that the difference is minimized. Alternatively, higher-order components of the clock signal may be compared with each other to detect a clock signal level difference between the two paths.

The tuning means of the variable dispersion equalizer of the fine movement unit 3 is configured such that two chirped fiber gratings are fixed to a piezo element to control the amount of expansion or contraction, or to a heater to control the temperature gradient. It is.

In such a fine movement unit 3, since the equalization region of the variable dispersion equalizer can be continuously controlled in the time domain, dispersion fluctuation due to environmental temperature and waveform distortion due to perturbation are equalized. be able to.

Further, the waveform shaping unit 4 receives the signal light dispersed and equalized by the dispersion equalizing unit 1 and narrows the pulse width and adjusts the timing jitter by optical modulation synchronized with the clock signal extracted from the signal light. And the signal jitter is amplified by the basic soliton to a larger power, and further input to a highly nonlinear dispersion medium to convert the intensity fluctuation into a fluctuation of the spectrum spread and pass through an optical bandpass filter. And an intensity fluctuation correction unit 6 that outputs the signal light from which the intensity fluctuation of the signal light has been removed.

Further, in the timing jitter correction unit 5, since the pulse width is narrowed by the synchronous modulation, it is possible to suppress the waveform spread due to the polarization mode dispersion (PMD) (Reference 9: A). .Saha
ra et al., "Ultra high speed soliton transmission i
n the presence of polarization mode dispersion usi
ng in-line synchronous modulation ", Proc. OFC'99,
WC3, p.38, 1999).

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Example of Configuration of Coarse Moving Unit 2 of Dispersion Equalization Unit 1) FIG.
In the figure, a coarse motion unit 2 includes a plurality of dispersion media 21-1 to 21-n having different dispersion values arranged in parallel, and an optical switch 22-1 connected to an input side and an optical switch 22-1 connected to an output side. 22-2 is to select one of them. The clock extraction circuit 23 inputs a part of the output signal light of the optical switch 22-2 via the optical coupler 24, and measures the clock signal level. The control circuit 25 controls the optical switches 22-1 and 22-2 so as to select the dispersion medium that maximizes the clock signal level measured by the clock extraction circuit 23.
Switch 2-2.

(Example of configuration of fine movement unit 3 of dispersion equalization unit 1) FIG.
Shows a configuration example of the fine movement unit 3 of the dispersion equalization unit 1. In the figure, the fine movement unit 3 has a configuration using a variable dispersion equalizer 31 combining two types of chirped fiber gratings. The clock signal level difference detection circuit 32 converts a part of the output signal light of the variable dispersion equalizer 31 into the optical couplers 33-1 and 33-3.
3-2, and the difference between the clock signal levels of the two paths is measured by a configuration described later. The control circuit 34 controls the variable dispersion equalizer 31 such that the difference between the clock signal levels of the two paths measured by the clock signal level difference detection circuit 32 is minimized.

FIG. 4 shows an example of the configuration of the variable dispersion equalizer 31. In the figure, the dispersion variable equalizer 31 includes a first chirped fiber grating 311 that receives light from the long wavelength side of the reflection wavelength and a second chirped fiber grating 312 that receives light from the short wavelength side. As shown in FIG. 4 (a), the cascade-connected three-port optical circulators 313 and 314 may be connected to the second ports in any order, or as shown in FIG.
5 is connected to the second port and the third port in random order to form a transmission filter. Then, the tuning means moves the reflection wavelength range of each chirped fiber grating equally in the same direction.

FIG. 5 shows an example of the configuration of tuning means for a chirped fiber grating. FIG. 5A shows that two chirped fiber gratings 311 and 312 are bonded and fixed to a piezo element 316, and a voltage is applied to the piezo element 316 to control the amount of expansion and contraction so that the reflection wavelength range of each chirped fiber grating is the same. This is a configuration for shifting in the direction. FIG. 5B shows that two chirped fiber gratings 311 and 312 are bonded and fixed to the heater 317.
The configuration is such that a current is applied to the heater 317 to control the temperature gradient, and the reflection wavelength range of each chirped fiber grating is shifted in the same direction. In FIG. 5C, the piezo elements 316-1 and 316-2 are arranged on both sides of two chirped fiber gratings 311 and 312, and the amount of expansion and contraction is increased by pulling each chirped fiber grating from both sides.

First Chirped Fiber Grating 3
The distribution λ (z) of the reflection wavelengths in the longitudinal direction of the eleventh grating is represented by L for the grating length, λ _{S for} the shortest reflection wavelength, and λ for the longest wavelength
_L , Δλ = λ _L −λ _S , where z is the coordinate of a coordinate axis taken in the longitudinal direction with the shortest wavelength portion as the origin, λ (z) = λ _S + Δλ (z / L) ^1/2 (1) ).

Second Chirped Fiber Grating 3
Similarly, the distribution λ (z) of the 12 reflected wavelengths in the longitudinal direction is expressed by the following formula: λ (z) = λ _S + Δλ ｛1- (1-z / L) ¹ /2… (2)

FIG. 6 shows a model and group delay characteristics of the first chirped fiber grating 311 represented by the equation (1). Although the pitch of the grating changes stepwise, it is schematically shown here with the short wavelength side being fine and the long wavelength side being coarse. As shown in FIG. 6A, when light having a wavelength λ is incident from the long wavelength side of the chirped fiber grating, a group delay characteristic as shown in FIG. 6B is obtained. The group delay amount in this case is expressed as DelayA (λ) = − (τ ₀ / Δλ ² ) (λ−λ _S ) ² + τ ₀ (3) Here, Δλ = λ _L −λ _S (4) τ ₀ = 2 nL / c (5), where n is the effective refractive index of the core and c is the speed of light.

FIG. 7 shows a model and a group delay characteristic of the second chirped fiber grating 312 represented by the equation (2). However, the notation is the same as that of the first chirped fiber grating shown in FIG. As shown in FIG. 7A, when light having a wavelength λ is incident from the short wavelength side of the chirped fiber grating, a group delay characteristic as shown in FIG. 7B is obtained. The amount of group delay in this case is represented as DelayB (λ) = − (τ ₀ / Δλ ² ) (λ−λ _L ) ² + τ ₀ (6)

Next, a three-port optical circulator 31
Both chirped fiber gratings 31 coupled by a 3,314 or 4-port optical circulator 315
When the group delay characteristics of 1,312 are added, an upwardly convex quadratic curve as shown in FIG. 8 is obtained. The group delay in this case is

[0028]

(Equation 1)

## EQU2 ## The chromatic dispersion function obtained by differentiating equation (7) with respect to wavelength is

[0030]

(Equation 2)

The wavelength dispersion characteristics are as shown in FIG. In the chromatic dispersion characteristics shown here, the chromatic dispersion linearly changes from positive to negative as the wavelength becomes longer, which is opposite to the chromatic dispersion characteristics of a normal dispersion-shifted fiber as shown in FIG. The maximum value of the chromatic dispersion shown in FIG. 9 is 2τ ₀ / Δλ, and the minimum value is −2τ ₀ / Δλ.
Λ ₀ in FIG. 10 is a zero dispersion wavelength.

The chromatic dispersion characteristics (center wavelength of the equalization region) shown in FIG. 9 are obtained by moving the reflection wavelength regions of the chirped fiber gratings 311 and 312 equally in the same direction by the tuning means shown in FIG.
Tunable linearly. The wavelength dispersion characteristic at this time moves in parallel as shown in FIG.

In FIG. 11A, the solid line shows the characteristic when tuning is performed by Δλ / 2 on the short wavelength side with the broken line as the initial characteristic. Focusing on a certain wavelength λsig, the chromatic dispersion of λsig is 0 in the initial state, but the chromatic dispersion changes to −2τ ₀ / Δλ by this tuning. on the other hand,
In FIG. 11 (b), if the characteristic obtained when tuning is performed by Δλ / 2 on the long wavelength side with the broken line as the initial characteristic is shown by the solid line, the chromatic dispersion of the wavelength λsig changes to 2τ ₀ / Δλ by this tuning.

[0034] Thus, by the reflection wavelength range of the chirped fiber gratings 311 and 312 is equal tuning in the same direction, is possible to change the wavelength dispersion for a wavelength λsig from -2τ ₀ / Δλ to 2.tau ₀ / [Delta] [lambda] it can. Therefore, the dispersion compensation amount for the wavelength λsig required for a certain optical transmission line is −2τ ₀ /
If it is between Δλ and 2τ ₀ / Δλ, dispersion compensation can be performed by tuning the chirped fiber gratings 311 and 312. That is, the transmission filter as shown in FIG. 4 operates as a variable dispersion equalizer in which the wavelength dispersion of transmitted light changes by tuning the reflection wavelength range of the chirped fiber gratings 311 and 312.

FIG. 12 shows a configuration example of the clock signal level difference detection circuit 32 of the fine movement unit 3. The clock signal level difference detection circuit 32 is configured to detect the optimum point in the reflection wavelength range of the chirped fiber gratings 311 and 312 constituting the variable dispersion equalizer 31.

In the figure, the signal light branched by the optical coupler 33-1 is input to the optical fiber 321 having positive dispersion (path 1), and the signal light branched by the optical coupler 33-2 is light having negative dispersion. The signal is input to the fiber 322 (path 2). The signal light that has passed through the optical fibers of each path is converted into an electric signal by photodiodes (PD) 323-1 and 323-2, respectively, and a bandpass filter (BPF) 324-
RF detector 325-1,3
25-2, and the clock signal level is detected.
The clock signal level of each path is compared by a differentiator 326, and the difference signal is input to the control circuit 34.

Here, since the dispersion equalization is not performed with high accuracy before the fine movement section 3 operates, the clock signal level of either path increases. This is shown in FIG. Here, the control circuit 34 tunes the reflection wavelength range of the chirped fiber gratings 311 and 312 constituting the variable dispersion equalizer 31 so that the clock signal level of each path becomes the same value (the difference signal is minimum). . At this time, comparison between higher-order clock signals may be performed.
For example, comparison of a basic clock of 10 Gbit / s is performed at 10 GHz, while comparison of a higher-order clock is performed at 20 GHz, 30 GHz, or the like. With such a method, the clock signal level of the signal light can be maximized at the output of the fine movement unit 3, and the preset dispersion equalization with high accuracy can be performed. The state at this time is shown in FIG.

(Operation at the time of preset dispersion equalization of the dispersion equalization unit 1) The coarse movement unit 2 and the fine movement unit 3 of the dispersion equalization unit 1 change the group delay and the chromatic dispersion during the preset dispersion equalization. Will be described with reference to FIG. FIG. 14A shows a state before preset equalization. Dispersion shift fiber (DS
The dispersion value D and the relative group delay τ _{g of} F) are the signal wavelength λ _sig
, And D _a and τ _a , respectively. FIG.
4 (b) shows a state after the equalizing process by the coarse moving unit 2. By selecting an appropriate dispersion medium (DCF) in the coarse motion section 2, the dispersion value D and the relative group delay τ _g for the signal wavelength λ _sig become D _b and τ _b , respectively. FIG. 14C shows a state after the equalization processing by the fine movement unit 3. The center wavelength of the equalization region of the variable dispersion equalizer 31 of the fine movement unit 3 is set to the zero dispersion wavelength λ.
₀ by controlling so as to coincide with the dispersion amount in the whole equalization region be 0, the preset dispersion equalization is performed.

(Operation at the time of adaptive dispersion equalization of dispersion equalizer 1)
Next, adaptive distributed equalization performed during system operation will be described with reference to FIG. During system operation, the zero dispersion wavelength of the optical fiber transmission line (dispersion shift fiber DSF) changes due to environmental changes such as distortion and temperature changes. By tuning the reflection wavelength range of the chirped fiber gratings 311 and 312 so as to follow the change of the zero dispersion wavelength, the zero dispersion wavelength and the tunable dispersion equalizer 3 are adjusted.
The center wavelength of one equalization region can be matched. Thereby, dispersion fluctuation can be equalized.

Here, the variance value Df generated when the variance fluctuates is calculated based on FIG. 16 as follows. First, the group delay characteristic of the dispersion-shifted fiber (DSF) is expressed as τ _g (DSF) = (1/2) D ₀ (λ−λ ₀ ) ² (9). Where τ _g is the group delay, D ₀ is the dispersion slope,
λ is the wavelength and λ ₀ is the zero-dispersion wavelength.

On the other hand, chirped fiber grating 3
The group delay characteristics τ _g (NLCFG) of the variable dispersion equalizer 31 using the BERs 11 and 312 are as follows: τ _g (NLCFG) = − (1/2) D ₀ (λ−λ ₀ ) ² (10) Is done.

If the zero-dispersion wavelength of the dispersion-shifted fiber changes by Δλ toward the longer wavelength side due to a change in environment, the group delay characteristic of the dispersion-shifted fiber is τ _g (DSF) = (1/2) D ₀ {Λ− (λ ₀ + Δλ)} ² (11) Common tangent coordinate of the contact A and B of this equation (10) and equation (11) becomes _{A (λ 0, D 0 Δλ} 2/2) B (λ 0 + Δλ, -D 0 Δλ 2/2) . The slope of the common tangent AB ^{_{is, - (D 0 Δλ 2/}} 2 + D 0 Δλ 2/2) / Δλ = -D 0 Δλ ... (12) and a demand, since the dispersion value Df is equal to the slope of the straight line AB Df = −D ₀ Δλ (13)

Therefore, the clock extraction circuit 32 of the fine movement unit 3
Signals passing through the paths 1 and 2 are affected by different variance values as shown in FIG. The zero-dispersion wavelength and the center wavelength of the equalization region of the tunable dispersion equalizer 31 are matched so that the dispersion is equalized. Thereby, dispersion fluctuation can be equalized.

(Configuration Example of Timing Jitter Correction Unit 5 of Waveform Shaping Unit 4) FIG. 18 shows a configuration example of the timing jitter correction unit 5 of the waveform shaping unit 4. In the figure, a timing jitter correction unit 5 includes a photoelectric conversion circuit 52 that converts a signal light branched by an optical coupler 51 into an electric signal, a clock extraction circuit 53 that extracts a clock signal from the electric signal, and a modulation synchronized with the clock signal. A modulation signal phase adjusting circuit 54 for generating a signal and an optical modulator 55 for synchronously modulating the input signal light with the modulation signal are provided.

As shown in FIG. 19, the pulse width of the signal light input to the timing jitter correction unit 5 having such a configuration is forcibly narrowed in the time domain by modulation synchronized with the extracted clock signal. That is, the timing jitter is suppressed. In addition, since the pulse width can be forcibly narrowed by synchronous modulation, there is also a function of suppressing waveform spread due to PMD, and compensating for waveform spread due to PMD, which is a problem in a high bit rate transmission system, and extending the transmission distance. be able to.

(Example of Configuration of Intensity Fluctuation Correcting Unit 6 of Waveform Shaping Unit 4) FIG. 20 shows an example of the configuration of the intensity fluctuation correcting unit 6 of the waveform shaping unit 4. In the figure, the intensity fluctuation correction unit 6 includes an optical amplifier 61 and a highly nonlinear dispersion-shifted fiber (DSF).
62 and an optical band-pass filter (optical BPF) 63 are sequentially connected. The input signal light amplified to a power larger than the basic soliton by the optical amplifier 61 has intensity fluctuation as shown in FIG. When this is passed through the highly nonlinear DSF 62, the fluctuation of the intensity of the signal pulse is converted into the fluctuation of the spectrum spread by the self-phase modulation. This fluctuation of the spectrum spread is removed by passing through the optical BPF 63. Thereby, the intensity fluctuation of the input signal light is corrected.

The intensity fluctuation compensator 6 is characterized in that an optical fiber is used as an element for an optical equalizer, and the function of soliton stabilization is used not as soliton transmission but as an energy stabilizing element.

[0048]

Embodiment (Embodiment of Distributed Equalization Unit 1) FIG. In the figure, the distributed equalizer 1
The basic configuration of the coarse moving section 2 is shown in FIG. 2, and here, the configuration of the clock extracting circuit 23 is specifically shown. The clock extracting circuit 23 converts the signal light branched by the optical coupler 24 into an electric signal by a photodiode (PD) 231, and converts the electric signal to an R signal through a bandpass filter (BPF) 232.
The signal is input to the F detector 233 to detect the clock signal level, and is input to the control circuit 25. The basic configuration of the fine movement unit 3 of the dispersion equalizer 1 is as shown in FIG. 3, the configuration of the variable dispersion equalizer 31 is as shown in FIGS. 4 and 5, and the configuration of the clock extraction circuit 32 is as shown in FIG. As shown.

(Preset dispersion equalization) An embodiment of a preset dispersion equalization method when a system is introduced will be described. The dispersion equalizer 1 is arranged at a portion of a linear repeater at an interval of 80 km. A dispersion compensating fiber and a dispersion slope compensating fiber are used as the dispersion media 21-1 to 21-4 of the coarse motion section 2. The reason why the dispersion slope compensating fiber is used is to make the dispersion in the coarse movement section 2 flat. Instead of the dispersion compensating fiber, a linear chirped fiber grating may be used. The variable dispersion equalizer 31 in the fine movement unit 3 includes:
A fiber having a group delay profile completely opposite to the group delay profile of the optical fiber transmission line (dispersion shift fiber) is used. Here, when the dispersion values of the dispersion media 21-1 to 21-4 are -40 ps / nm to +40 ps / nm at a signal light wavelength of 1550 nm, the coarse movement unit 2 can roughly perform dispersion compensation, and the fine movement unit 3 -30ps / nm, -10ps / nm, + 10ps / nm, + 30ps / nm
Optical fiber is used.

From the equation (8), the dispersion slope of the variable dispersion equalizer 31 of the fine movement section 3 is given by D ₀ = −8 _ng L / (cΔλ ² ) (14). Here, _ng is the group refractive index, L is the grating length, c is the speed of light in vacuum, and Δλ is the band of each chirped fiber grating. Since the dispersion slope of the dispersion shifted fiber is 0.07 ps / km / nm ² , the dispersion slope of the 80 km dispersion shifted fiber is 5.6 ps / nm ² . The variable dispersion equalizer 31 (L = 10 m) that compensates for this dispersion slope
(as calculated by 14), Δλ = (- 8n g L / (cD 0) band [Delta] [lambda] of m) ^{formula) 1/2 = (8 × 1.47 ×} 10 [mm] / (3 × 10 8 [m / s ] × 5.6 [ps / nm ² ])) ^1/2 ≒ 8.4 [nm].

By the way, it is known that when the two chirped fiber gratings are subjected to apodization in order to reduce unnecessary group delay ripples, the effective band becomes narrow (Ref. 10: D. Pastor et al.). ., "Des
ign of Apodised Linearly Chirped Fiber Grating for
Dispersion Compensation ", IEEE J. Lightwave Techno
l., vol.14, no.11, p.2581, 1996). Effective band is 8.4n
Assuming a change from m to 6.7 nm, the dispersion value variable width is about
38 ps / nm. Considering the variance margin,-
The range from 10 ps / nm to +10 ps / nm is set as the dispersion value variable region of the fine movement unit 3.

At this time, a voltage is applied to the piezo element 316 of the variable dispersion equalizer 31, and a pair of chirped fiber gratings (each grating length is 10 mm) 311, 3
When 0.3 is extended by 0.3%, the center wavelength of the equalization region becomes 3.5 mm
Can be changed.

In the above configuration, when the system is introduced,
So that the clock signal level of the signal light becomes the highest
Switching the optical switches 22-1 and 22-2 of the coarse movement unit 2,
One of the dispersion media 21-1 to 21-4 is selected. In addition,
The higher the clock signal level, the smaller the pulse width due to dispersion equalization.

After coarse dispersion equalization is performed in the coarse movement section 2, the path 1 of the optical fiber 321 having a positive dispersion (+5 ps / nm) and the negative dispersion (-5 ps / nm) are changed in the fine movement section 3. The clock signal level is measured from the signal that has passed through each of the paths 2 of the optical fiber 322. At this time, as shown in FIG. 13, since the dispersion equalization has not been performed with high accuracy yet, the clock signal level of either path increases. Therefore, the chirped fiber gratings 311 and 312 constituting the variable dispersion equalizer 31 are set so that the clock signal level of each path becomes the same value (the error signal is minimized).
Tune the reflection wavelength range. As a result, the clock signal level of the output signal light of the dispersion equalizer 1 can be maximized, and dispersion equalization with high accuracy can be achieved.

(Adaptive Distributed Equalization) Next, an embodiment of an adaptive distributed equalization method during system operation will be described. This is performed by the fine movement unit 3 of the dispersion equalization unit 1. Similarly to the preset equalization, the path 1 of the optical fiber 321 having a positive dispersion (+5 ps / nm) and the optical fiber 3 having a negative dispersion (−5 ps / nm)
The clock signal level is measured from the signal that has passed through each of the 22 paths 2. During system operation, the zero dispersion wavelength of the optical fiber transmission line changes due to environmental changes such as distortion and temperature changes, and a dispersion value Df is generated as shown in FIG. Due to the dispersion fluctuation, as shown in FIG. 13, the clock signal level of either path increases. The chirped fiber grating 31 constituting the tunable dispersion equalizer 31 has the same clock signal level.
The reflection wavelength range of 1,312 is tuned. Thereby, the center wavelength of the equalization region and the zero-dispersion wavelength can be matched, and the dispersion fluctuation can be equalized with high accuracy.

(Embodiment of Waveform Shaping Unit 4) FIG. 23 shows an embodiment of the waveform shaping unit 4. In the figure, the basic configuration of the timing jitter correction section 5 of the waveform shaping section 4 is as shown in FIG. 18, and here, an embodiment configuration of each section is shown. The signal light branched by the optical coupler 51 is a photodiode (P
D) The signal is converted into an electric signal in 521, and the electric signal is
1. Bandpass filter (BPF) 523, electric amplifier 522
The signal is branched into two by the divider 524 via -2. One electric signal branched into two by the divider 524 is input to the RF detector 531 to extract a clock signal, and the other electric signal is input to the phase shifter 541. Control circuit 53
2 controls the phase shifter 541 based on the clock signal extracted by the RF detector 531 and generates a modulation signal synchronized with the clock signal. The modulation signal output from the phase shifter 541 is amplified by the electric amplifier 542, and drives the optical modulator 55 to modulate the input signal light. Thereby, the pulse width of the signal light can be narrowed in the time domain, and the timing jitter can be suppressed.

The configuration of the intensity fluctuation correction unit 6 of the waveform shaping unit 4 is as shown in FIG. The input signal light is amplified by the optical amplifier 61 to a power larger than the basic soliton (1.6 basic soliton power). This is converted to a highly nonlinear dispersion-shifted fiber 62 (MFD = 5.2 μm, D = + 3.2 ps / km / nm, L = 1.
3 Z ₀ ), the intensity fluctuation of the signal pulse is converted into the fluctuation of the spectrum spread by the self-phase modulation. This broadened pulse is passed through an optical bandpass filter 63.
(Bandwidth = 1 nm), the spectrum spread is removed, and the intensity fluctuation can be corrected.

(Other Embodiments) Since the reflection wavelength range of the chirped fiber grating of the tunable dispersion equalizer 31 is wide, a plurality of channels can be allocated within the band. Therefore, the automatic optical equalizer of the present invention can be applied to wavelength division multiplexing (WDM) transmission. For example, it is assumed that a bit rate of 40 Gbit / s is set to four channels in a dispersion-shifted fiber transmission line having a relay interval of 80 km. The assigned wavelength of each channel is 1547.6, 154
9.2, 1550.8 and 1552.4 nm. Variable dispersion equalizer 31
2nm tuning of the center wavelength of the equalization region of
Assuming a bandwidth of 11 nm (no apodization),
Equation (14) shows that a chirped fiber grating having a grating length of 17.4 mm may be used.

[0059]

As described above, the automatic optical equalizer of the present invention can preset equalize the chromatic dispersion of an optical fiber transmission line at the time of system introduction, and at the time of system operation, the optical transmission due to environmental changes. It is possible to automatically adaptively equalize the dispersion fluctuation of the road.

Further, by adding a waveform shaping function, it is possible to correct timing jitter and intensity fluctuation of a signal pulse and to suppress PMD by narrowing a pulse width.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of an automatic optical equalizer of the present invention.

FIG. 2 is a block diagram showing a configuration example of a coarse movement unit 2 of the dispersion equalization unit 1;

FIG. 3 is a block diagram showing a configuration example of a fine movement unit 3 of the dispersion equalization unit 1;

FIG. 4 is a diagram showing a configuration example of a variable dispersion equalizer 31.

FIG. 5 is a diagram showing a configuration example of tuning means of a chirped fiber grating.

FIG. 6 is a diagram showing a model and group delay characteristics of a chirped fiber grating 311 represented by Expression (1).

FIG. 7 is a diagram showing a model and group delay characteristics of a chirped fiber grating 312 represented by Expression (2).

FIG. 8 shows chirped fiber gratings 311, 31.
FIG. 6 is a diagram showing the sum of group delay characteristics of FIG.

FIG. 9 shows chirped fiber gratings 311, 31.
2 is a diagram showing the sum of wavelength dispersion characteristics of FIG.

FIG. 10 is a diagram showing chromatic dispersion characteristics of a normal optical fiber.

FIG. 11 is a diagram showing an example of tuning the chromatic dispersion characteristics of the variable dispersion equalizer 31.

FIG. 12 is a clock signal level difference detection circuit 3 of the fine movement unit 3;
FIG. 2 is a block diagram showing a configuration example of FIG.

FIG. 13 is a view for explaining a dispersion equalization fine movement method in the fine movement unit 3;

FIG. 14 is a diagram illustrating changes in group delay and chromatic dispersion during preset dispersion equalization.

FIG. 15 is a view for explaining adaptive equalization to dispersion fluctuation due to an environmental change.

FIG. 16 is a view for explaining a method of calculating a variance value generated by variance fluctuation.

FIG. 17 is a clock signal level difference detection circuit 3 of the fine movement unit 3;
FIG. 6 is a diagram for explaining a change due to dispersion fluctuation of a dispersion value affected by a signal passing through paths 2 and 2 of FIG.

FIG. 18 is a block diagram showing a configuration example of a timing jitter correction section 5 of the waveform shaping section 4.

FIG. 19 is a diagram illustrating a method for suppressing timing jitter.

FIG. 20 is a block diagram illustrating a configuration example of an intensity fluctuation correction unit 6 of the waveform shaping unit 4.

FIG. 21 is a diagram illustrating a method for removing intensity fluctuation.

FIG. 22 is a diagram showing an embodiment configuration of the dispersion equalization unit 1;

FIG. 23 is a diagram showing an example configuration of a waveform shaping unit 4.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Dispersion equalization part 2 Coarse movement part 21 Dispersion medium 22 Optical switch 23 Clock extraction circuit 231 Photodiode (PD) 232 Bandpass filter (BPF) 233 RF detector 24 Optical coupler 25 Control circuit 3 Fine movement part 31 Variable dispersion equalizer 311 , 312 Chirped fiber grating 313, 314 3-port optical circulator 315 4-port optical circulator 316 Piezo element 317 Heater 32 Clock signal level difference detection circuit 321, 322 Optical fiber 323 Photodiode (PD) 324 Bandpass filter (BPF) 325 RF Detector 326 Differentiator 33 Optical coupler 34 Control circuit 4 Waveform shaping section 5 Timing jitter correction section 51 Optical coupler 52 Photoelectric conversion circuit 521 Photodiode (PD) 522 Electric amplifier 523 Bandpass filter (BPF) 524 Divider 53 Clock extraction circuit 531 RF detector 532 Control circuit 54 Modulation signal phase adjustment circuit 541 Phase shifter 542 Electric amplifier 55 Optical modulator 6 Intensity fluctuation correction unit 61 Optical amplifier 62 Highly nonlinear dispersion shift fiber (DSF) 63 Optical Bandpass Filter (Optical BPF)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G02F 1/365 (72) Inventor Masataka Nakazawa 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone In-house F term (reference) 2H041 AA02 AA21 AB10 AB18 AC08 2H050 AC82 AC84 AD00 2H079 AA06 AA07 BA03 CA04 CA24 EA09 EB26 EB27 FA01 2K002 AA02 AB40 CA15 DA10 EA07 GA10 HA16 5K002 AA05 AA06 BA02 BA01 BA01 CA01

Claims (6)

[Claims] 1. An automatic optical equalizer for automatically equalizing waveform spread and distortion due to chromatic dispersion of an optical fiber transmission line, wherein one of a plurality of dispersion media having different dispersion values is switched by an optical switch. A coarse movement section for selecting and roughly correcting the waveform spread / distortion, and two types of chirped fiber gratings for respectively entering light from the long wavelength side and the short wavelength side of the reflection wavelength via an optical circulator; It has a variable dispersion equalizer that realizes a group delay profile opposite to the group delay profile of the transmission line, and finely corrects the waveform spread and distortion by tuning the reflection wavelength range of the chirped fiber grating pair to move equally in the same direction. An automatic optical equalizer comprising a fine movement unit. 2. The automatic optical equalizer according to claim 1, wherein the coarse movement unit detects a clock signal level from the signal light after passing through the dispersion medium, and a dispersion value at which the clock signal level becomes maximum. An automatic optical equalizer characterized by having a configuration for selecting a dispersion medium having: 3. The optical automatic equalizer according to claim 1, wherein the fine movement unit converts the signal light after passing through the variable dispersion equalizer into two.
The optical fiber has a positive dispersion value on one path, an optical fiber with a negative dispersion value on the other path, and a clock for the signal light passing through each optical fiber. An automatic optical equalizer, characterized by detecting a difference between signal levels and tuning the variable dispersion equalizer so that the difference is minimized. 4. The optical automatic equalizer according to claim 3, wherein the high-order components of the clock signal are compared to detect a clock signal level difference between two paths. Equalizer. 5. The automatic optical equalizer according to claim 1, wherein the tuning means of the variable dispersion equalizer in the fine movement unit comprises:
An automatic optical equalizer having a configuration in which two chirped fiber gratings are fixed to a piezo element to control the amount of expansion and contraction, or a configuration in which the temperature gradient is controlled by fixing to a heater. 6. The optical automatic equalizer according to claim 1, wherein the signal light dispersed and equalized by the coarse moving unit and the fine moving unit is input, and light synchronized with a clock signal extracted from the signal light. A timing jitter correction unit for narrowing a pulse width and reducing a timing jitter by modulation; amplifying the signal light to a larger power by a basic soliton, and further inputting the signal light to a highly nonlinear dispersion medium to change the intensity fluctuation to a spectrum spread fluctuation. And an intensity fluctuation corrector for outputting a signal light in which the intensity fluctuation of the signal light has been removed by passing the signal light through an optical bandpass filter.