JP2002223189A - Simultaneous compensation method and device for higher order dispersion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simultaneously compensate third-order and fourth-order dispersion in optical fiber transmission. SOLUTION: An optical pulse is propagated through a second-order dispersion application fiber 11, in which a linear chirp is applied to the pulse and the resulting pulse is made incident onto a phase modulator 13, an extract device 15 extracts a cosine wave signal synchronously with the branched optical pulse, the cosine wave signal is given to the phase modulator 13 as a modulation signal via a phase sifter 17 and a variable attenuator 18, and an emitted light from the phase modulator 13 is made incident onto a transmission optical fiber 16 via a dispersion compensating optical fiber 14. A linearly synthesized signal obtained from a sine phase modulation signal to compensate the third-order dispersion of the optical fiber 16 and the cosine phase modulation signal to compensate the fourth-order dispersion together with the order-order dispersion is applied to the phase modulator 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for compensating waveform deterioration of a signal light pulse due to higher-order (wavelength) dispersion of an optical fiber transmission line by phase modulation.

[0002]

2. Description of the Related Art As a prior art of a high-order dispersion compensator using a phase modulator, a technique of compensating only a fourth-order dispersion or only a third-order dispersion of an optical fiber transmission line is described.
There are reports for each. First, the operation principle of the fourth-order dispersion compensation and the third-order dispersion compensation of the optical fiber transmission line according to the conventional technique will be specifically described. The deterioration of the signal light pulse waveform due to the wavelength dispersion is caused by a phase change acting on each angular frequency component of the signal light pulse. The angular frequency dependence of the phase change is expressed by the following Taylor expansion formula.

[0003]

(Equation 1) However,

[0004]

(Equation 2)

Where L is the length of the optical fiber transmission line, and ω ₀ is the central angular frequency of the signal light pulse. Non-linear terms after the term including β _{2 in} the right-hand side of the Taylor expansion formula give waveform changes of the optical signal pulse. The variances caused by the terms β ₂ , β ₃ , and β ₄ are called second-order, third-order, and fourth-order variances, respectively.
In particular, the dispersion caused by the terms after the term of β ₃ is generally called higher-order dispersion. Note that the linear term on the right side of the Taylor expansion does not change the pulse waveform. From the Taylor expansion formula, the magnitude of the change in the pulse waveform due to the second, third, and fourth variances is the square of the angular frequency band of the signal light pulse, three,
It can be seen that they are proportional to the power of 4 and the power of 4, respectively.

Since the angular frequency band of a signal light pulse is inversely proportional to the pulse width, for example, when the pulse width is reduced to 1/10, the magnitudes of the waveform changes due to the second, third and fourth dispersions are 100 times, respectively. 1000 times, 10000 times.
Therefore, as the transmission speed of optical communication increases and the pulse width decreases, the effect of higher-order dispersion increases sharply, and compensation for the effect becomes important. The second-order dispersion and the third-order dispersion of the transmission optical fiber can be simultaneously compensated by combining different types of optical fibers.

However, it is not possible to simultaneously compensate for the second, third and fourth dispersions by combining different types of optical fibers. On the other hand, if an appropriate phase change is given to each angular frequency component of the signal light pulse and the angular frequency dependence of the phase change shown on the right side of the equation (1) can be linearized, the signal light pulse due to dispersion can be obtained. Waveform changes can be compensated. Higher-order dispersion compensation using a phase modulator is based on this principle. FIG. 8 illustrates the principle of fourth-order dispersion compensation using a phase modulator, using the calculation result of the angular frequency dependence of the amount of phase change.

A quartic curve C1 represented by a thin solid line in FIG.
Indicates the phase change due to the fourth-order dispersion of the transmission line.
This quartic curve C1, substantially cosine curve when added together appropriate quadratic curve C2, the curve is not shown in Figure 8 took bias in the vertical axis direction in the vicinity of omega ₀ shown by a broken line in FIG. 8 It is represented by Here, the quadratic curve C2 to be added is
Can be set by adjusting the magnitude of the secondary dispersion of the entire optical fiber transmission line.

In this state, cosine phase modulation (dotted line C3 in FIG. 8) having an appropriate amplitude is applied to the angular frequency component of the signal light pulse, thereby giving a curve representing the entire phase change amount (FIG. 8). A thick solid line (C4) in the middle has a linear region wider than the original quartic curve C4. As a result, within the angular frequency range of the signal light pulse, the range in which the angular frequency dependence of the amount of phase change becomes substantially linear is widened, thereby reducing the influence of fourth-order dispersion.

In this method, it is necessary to apply cosine phase modulation to the signal light pulse in the angular frequency domain by the phase modulator, but the phase modulator is originally an element that applies modulation in the time domain. Therefore, first, an optical fiber having a large second-order dispersion is inserted before the phase modulator, and a linear chirp is applied to the signal light pulse. This linear chirp means that the angular frequency component of the pulse is a linear function with respect to time, and as a result, the cosine phase modulation applied in the time domain by the phase modulator is automatically converted to the cosine phase modulation in the angular frequency domain. Becomes

The repetition frequency of cosine phase modulation is determined by the repetition frequency of the signal light pulse. FIG. 9 illustrates the principle of third-order dispersion compensation using a phase modulator based on the calculation result of the angular frequency dependence of the amount of phase change. A cubic curve C5 represented by a thin solid line in FIG. 9 shows a phase change due to tertiary dispersion of the optical fiber transmission line. In this case, by giving sinusoidal phase modulation (dotted line C6 in FIG. 9) having an appropriate amplitude to the angular frequency component of the signal light pulse, a curve representing the entire amount of phase change (thick solid line in FIG. 9) ) C7 has a linear region wider than the original cubic curve C5, and the effect of cubic dispersion is reduced.

As in the case of the fourth-order dispersion compensation, a linear chirp is applied to the signal light pulse by an optical fiber having a large second-order dispersion, and thereafter, the signal light pulse is incident on the phase modulator and is subjected to sinusoidal phase modulation in the angular frequency domain. Enable application. As a specific example of the prior art of high-order dispersion compensation for compensating only the fourth-order dispersion of an optical fiber transmission line, IEEE Photonics Technology Letters, twelfth
FIG. 10 shows the basic configuration of the higher-order dispersion compensator shown in Vol.

The higher-order dispersion compensator 10 described in this document includes a second-order dispersion applying optical fiber 11, an optical coupler 1
2. It is composed of a phase modulator 13, a dispersion compensating optical fiber 14, and a clock extractor 15, and is installed at a stage (incident side) before the transmission optical fiber 16. First, a linear chirp is given to the light pulse incident on the high-order dispersion compensator 10 by widening the pulse width by the second-order dispersion applying optical fiber 11.
Subsequently, a part of the optical pulse is branched by the optical coupler 12 and is incident on the clock extractor 15, and the clock extractor 15 generates a cosine wave signal synchronized with the optical pulse.

The cosine wave signal is incident on the optical phase modulator 13, and the optical pulse given a linear chirp is subjected to phase modulation. The phase modulation timing is set so that the intensity peak of the optical pulse and the peak of the phase shift due to the modulation temporally coincide with each other. The optical pulse after the phase modulation is incident on the dispersion compensating fiber 14. The length of the dispersion compensating fiber 14 is set such that a quadratic curve C2 indicated by a broken line in FIG. 8 is obtained. That is, the sum of the curves of the angular frequency dependence of the second-order dispersion and the fourth-order dispersion in the entire optical fiber including the optical fibers 11 and 14 and the transmission optical fiber 16 constituting the high-order dispersion compensator 10 can be approximated by a cosine curve. Thus, the length is adjusted by the length of the dispersion compensating optical fiber 14.

The effect on dispersion is the same even if the dispersion compensating optical fiber 14 is inserted after the transmission optical fiber 16 instead of before the transmission optical fiber 16. With such a configuration, the phase change generated by the dispersion of the transmission optical fiber 16 and the opposite phase change are performed by the higher-order dispersion compensator 1.
Generation by the optical fibers 11 and 14 and the phase modulator 13 within 0 compensates for waveform distortion based on the fourth-order dispersion of the transmission optical fiber 16. As a specific example of the prior art of high-order dispersion compensation for compensating only the third-order dispersion of the transmission optical fiber 16, IEEE Photonics Technology Letters (IEEE
Photonics Technology Letters), Volume 11, Volume 14
Pages 61 to 1463 are listed.

The basic configuration of the high-order dispersion compensator shown in this document is the same as that shown in FIG. An optical pulse having a pulse width expanded by the secondary dispersion applying optical fiber 11 to give a linear chirp is incident on the phase modulator 13. However, the timing of the phase modulation is set so that the intensity peak of the light pulse and the point where the phase shift due to the modulation becomes 0 coincide with each other in time. The optical pulse after the phase modulation is incident on the dispersion compensating optical fiber 14.

At this time, the length of the dispersion compensating optical fiber 14 is adjusted so that the second-order dispersion in the entirety of the optical fibers 11 and 14 constituting the higher-order dispersion compensating device and the transmission optical fiber 16 becomes zero. In the third-order dispersion compensation, similarly to the fourth-order dispersion compensation, a phase change opposite to the phase change caused by the dispersion of the transmission optical fiber 16 is generated in the fibers 11 and 14 and the phase modulator 13 in the higher-order dispersion compensator 10. By doing so, waveform distortion based on the third-order dispersion of the transmission optical fiber 16 is compensated.

[0018]

The cosine phase modulation is applied to the phase modulator 13 using the high-order dispersion compensator shown in FIG. 10 so that the time of the peak of the signal light pulse coincides with the time of the peak of the amplitude of the phase modulation. By giving, 4 of the transmission optical fiber
The distortion based on the third-order dispersion can be compensated, but in this case, the distortion based on the third-order dispersion is not compensated. Further, by applying sine phase modulation to the phase modulator 14 so that the peak of the signal light pulse coincides with the time when the amplitude of the phase modulation becomes 0, distortion based on third-order dispersion can be compensated. In this case, distortion due to fourth-order dispersion is not compensated.

That is, in the method of applying the cosine phase modulation or the sine phase modulation in which the timing is synchronized with the peak of the signal light pulse using the high-order dispersion compensator shown in FIG. There is a problem that only distortion based on one of the dispersions can be compensated. SUMMARY OF THE INVENTION An object of the present invention is to provide a method and an apparatus that can simultaneously compensate waveform distortion based on third-order dispersion and fourth-order dispersion of a transmission optical fiber using a phase modulator.
If this is realized, it becomes possible for the first time to transmit an ultrashort optical pulse of 200 fs (femtosecond) or less over a long distance.

[0020]

According to the present invention, there is provided a method for simultaneously compensating for high-order dispersion, wherein a linear chirp is applied to a signal light pulse before transmission, and then the signal light pulse to which the linear chirp is applied is applied to the transmission optical fiber. By applying a sine (herein, sine and cosine are represented as sine) wave phase modulation in which the amplitude and timing are optimized according to the values of the fourth-order dispersion and the fourth-order dispersion, the third and fourth order of the transmission optical fiber are applied. Compensate for waveform distortion based on second order dispersion. That is, conventionally, a sine wave is used for sine phase modulation for compensating for waveform distortion based on third-order dispersion,
The phase modulation by a single sine wave obtained by linearly combining the cosine wave of the cosine phase modulation for compensating the waveform distortion based on the fourth-order dispersion is the optimized sine wave phase modulation.

The high-order dispersion simultaneous compensator according to the present invention includes means for applying a linear chirp to a signal light pulse, an optical coupler for branching the signal light pulse, and extracting a sine wave signal synchronized with the branched signal light pulse. A high-order dispersion compensator comprising a clock extractor, a phase modulator that modulates the phase of a signal light pulse to which a linear chirp is applied by a sine wave signal, and a dispersion compensating optical fiber. The variable phase shifter and the variable amplitude adjuster are inserted in series in the path of the sine wave signal. By adjusting these variable phase shifters and variable amplitude adjusters, the amplitude and timing of the sine wave signal incident on the phase modulator can be adjusted to 3
Next, it can be set according to the fourth-order dispersion.

Another high-order dispersion simultaneous compensator according to the present invention is a means for applying a linear chirp to a signal light pulse, an optical coupler for branching the signal light pulse, a sine wave signal synchronized with the branched signal light pulse. A high-order dispersion compensator comprising: a clock extractor for extracting a signal, a phase modulator for phase-modulating a signal light pulse to which a linear chirp is applied by a sine wave signal, and a dispersion-compensating optical fiber. A variable amplitude adjuster and a variable phase shifter are inserted in series in the sine wave signal path between the sine wave signal paths, and the layered optical thin film total transmission dispersion equalizer (Layered Optical Thin-fi)
lm Allpass Dispersion Equalizer: LOTADE) is inserted. Waveform distortion based on the third-order dispersion of the transmission optical fiber is reduced by the laminated optical thin film total transmission dispersion equalizer, and waveform distortion based on the residual third-order dispersion and fourth-order dispersion is compensated by the phase modulation of the phase modulator. It is configured.

[0023]Action Angular frequency dependence of phase change due to fourth order dispersion of transmission optical fiber
Presence-based waveform distortion applies linear chirp before transmission
By applying cosine phase modulation to the modulated signal light pulse
Can compensate. Also, the angular frequency of the phase change due to third-order dispersion
Waveform distortion based on the dependence can be compensated by sinusoidal phase modulation.
Here, as shown in FIG. 1, the cosine level having the same frequency
Phase modulation φ_c(t) =-φ _ccos (2πR₀t) (in FIG. 1)
Solid line C11) and sinusoidal phase modulation φ_s(t) =-φ_ss
in (2πR₀t) Linear sum φ of (dotted line C12 in FIG. 1)
(t) is a generalized cosine wave curve as shown below (Fig.
1 is represented by a thick solid line C13). φ (t) = − sign (φ_c) √ (φ_c ^Two+ Φ_s ^Two) cos [2πR₀t-arctan (φ_s/ Φ_c)] (3) This equation (3) is given by 2 according to the fourth order dispersion of the transmission optical fiber.
Minimize the length of the second dispersion applying optical fiber and dispersion compensating optical fiber.
From the state where the signal light pulse is given the optimized and sinusoidal phase modulation,
Amplitude of phase modulated signal according to third-order dispersion of transmission optical fiber
And third-order dispersion and fourth-order
Shows that simultaneous compensation of each waveform distortion based on
are doing.

FIG. 2 shows a third order using one phase modulator,
The principle of the fourth-order dispersion compensation will be described using the calculation result of the angular frequency dependence of the amount of phase change. In this figure, the sum of phase changes due to second, third, and fourth order dispersion (thin solid line C14 in FIG. 2), and phase modulation in which the amplitude and timing are optimized for the third, fourth order dispersion compensation (FIG. Dotted line C in 2
15), and the phase change amount (thick solid line C16 in FIG. 2) obtained by summing them. Second, third, the curve C14 representing the sum of the quartic curve, by adding the one of the sinusoidal curve C15, it is seen that the linear phase characteristic is obtained at the center angular frequency ω around _0.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIG. 3 shows the configuration of a first embodiment of a simultaneous higher-order dispersion compensation method and apparatus according to the present invention. In the first embodiment, 10
A Gbit / s femtosecond signal light pulse is multiplexed 64 by time multiplexing and further multiplexed by polarization multiplexing to 1.2.
A description will be given of a result of applying the third- and fourth-order dispersion simultaneous compensation of the present invention to an experiment in which an 8 Tbit / s signal light is generated and transmitted through an optical fiber of 70 km.

The simultaneous high-order dispersion compensator 10 of the present invention comprises:
An optical coupler 12 for splitting input light into two, a secondary dispersion applying optical fiber 11 having one end connected to one output port of the optical coupler 12, and a phase modulator 13 connected to the other end of the secondary dispersion applying optical fiber 11 A dispersion compensating optical fiber 14 having one end connected to the output side of the phase modulator 13, a clock extractor 15 connected to the other output port of the optical coupler 12, an output side of the clock extractor 15, A variable phase shifter 17 and a variable attenuator 18 are connected in series with the modulation signal input side.

A chirp-free 10 Gbit / s signal light pulse is incident on the high-order dispersion simultaneous compensator 10, and a chirp reverse to the nonlinear chirp generated by the second-order, third-order, and fourth-order dispersion of the transmission optical fiber 16 is given in advance. After that, multiplexing is performed by the time multiplexing device 19. By the application of chirp in the simultaneous high-order dispersion compensator 10, the deterioration of the waveform of the signal light pulse due to the high-order dispersion when passing through the transmission fiber 16 can be greatly reduced.

The configuration in the high-order simultaneous dispersion compensator 10 and the setting of each parameter will be described. The pulse width of the 10 Gbit / s signal light pulse incident on the high-order dispersion simultaneous compensator 10 is expanded to T _s by the second-order dispersion applying optical fiber 11 having the second-order dispersion β _2s and the length L _s , and the linear chirp is generated. Is given. Due to this linear chirp, the angular frequency component of the optical signal pulse becomes a linear function at time t as follows.

The _{ω-ω 0 = -sign (β} 2s) (2πF BW / T s) t (4) where omega ₀ is the center angular frequency of the pulse, F _BW is the bandwidth of the optical signal pulse. When phase modulation for changing the phase with time is performed on the optical signal pulse given the linear chirp in this way, a different phase change is given for each angular frequency component of the optical signal pulse. As an example, consider the case where cosine phase modulation is performed by the phase modulator 13, that is, when the phase modulation is performed on a linearly chirped optical signal pulse with a cosine wave, the angular frequency dependence of the phase change is expressed by a cosine curve. become.

A part of the signal light pulse input to the high-order dispersion simultaneous compensation device 10 is input to the clock extractor 15 after being branched by the optical coupler 12, and the clock extractor 15
Outputs a 10 GHz sinusoidal electric signal synchronized with the signal light. After optimizing the timing and amplitude of the 10 GHz sinusoidal electric signal obtained in this way by the variable phase shifter 17 and the variable attenuator 18 according to the third-order dispersion and fourth-order dispersion of the transmission optical fiber 16, respectively, The signal is input to the modulator 13 as a modulation signal, and phase modulation is applied to the linearly chirped signal light pulse.

The dispersion compensating optical fiber 14 having the second-order dispersion β _2c and the length L _c adjusts the amount of the second-order dispersion intentionally applied for compensating the waveform distortion based on the fourth-order dispersion of the transmission optical fiber 16. To insert. When the signal light pulse incident on the high-order dispersion simultaneous compensation device 10 has a Gaussian waveform, 2
The length L _{s of the} second-order dispersion applying optical fiber 11, the length L _c of the dispersion compensating optical fiber 14, the amplitude φ _c of cosine phase modulation required for fourth-order dispersion compensation, and the amplitude of sine phase modulation required for third-order dispersion compensation φ _s is obtained by solving a simultaneous equation composed of the following four equations.

[0032]

(Equation 3)

[0033] _{R 0 = (0.94πF BW / T} s) (- β 4 / 6β 2) 1/2 (6) φ c = 3β 2 2 L / 4β 4 (7) φ s = - (F BW 3 ^{_{π 3 β 3 L / 24R 0}} 3 T s 3) (8) _{where, β n = β ns L s} + β nc L c + β ntf L tf / L, (n = 2,3,4) (9) L = L _s + L _c + L _tf (10), where T _in is the pulse width of the signal light pulse incident on the high-order dispersion simultaneous compensator 10, R ₀ is the repetition frequency of the signal light pulse, and F _BW is the signal light pulse. The band, L _tf, is the length of the transmission optical fiber 16, β _ns , β _nc , β _ntf (n = 2, 3, 4)
Denote second-, third- and fourth-order dispersions of the second dispersion applying optical fiber 11, the dispersion compensating optical fiber 14, and the transmission optical fiber 16, respectively.

In the experiment described in the first embodiment, a zero-dispersion single mode optical fiber for 1.3 μm wavelength was used as the secondary dispersion applying optical fiber 11, and an inverse dispersion optical fiber was used as the dispersion compensating optical fiber 14. The transmission optical fiber 16 having a length of 69.4 km has a length of 39.7 km.
km 1.3 μm wavelength zero-dispersion single mode optical fiber, 4.6 km dispersion-shifted optical fiber, 25.1 km
Were constructed by connecting reverse dispersion optical fibers. Where T _in
= 380 fs, T _s = 46 ps, β _2s = −23.0
ps ² / km, β _3s = 1.23 × 10 ⁻¹ ps ³ / km,
β _4s = −1.10 × 10 ⁻³ ps ⁴ / km, β _2c = 37.
8 ps ² / km, β _3c = −2.12 × 10 ⁻¹ ps ³ / k
m, β _4c = 2.43 × 10 ⁻³ ps ⁴ / km, β _2tf =
7.76 × 10 ⁻³ ps ² / km, β _3tf = 3.40 × 1
0 ⁻⁴ ps ³ / km, β _4tf = 8.68 × 10 ⁻⁴ ps ⁴ /
km, L _tf = 69.5 km, the numerical values are substituted into the above equations (5)-(8), and the calculation results are as follows: The length of the second dispersion applying optical fiber 11 is L _s = 276.2 m, and the dispersion is The length of the compensating optical fiber 14 is L _c = 139.2 m, 4
The amplitude of cosine phase modulation required for the second order dispersion compensation is φ _c = 1.2
The amplitude of the sinusoidal phase modulation required for 0π and third-order dispersion compensation is φ _s
= −0.18π is obtained.

From this calculation result and equation (3), it can be seen that the 10 GHz cosine wave modulation necessary for compensating the third and fourth order dispersion is given by the following equation. φ (t) = − 1.21πcos [2πR ₀ t + 0.048π] (11) The timing and amplitude of the 10 GHz sine wave signal incident on the phase modulator 13 are optimized by the variable phase shifter 17 and the variable attenuator 18, respectively. Do. The dispersion value (β _2tf , β _3tf , β _4tf ) of the transmission optical fiber 16 is _added to the time multiplexing device 19.
And the dispersion of each intensity of the erbium-doped optical fiber amplifier inserted before and after the transmission optical fiber 16 are also included.

FIG. 4 shows how the signal light pulse waveform is degraded by the third and fourth order dispersion of the transmission optical fiber 16 of the first embodiment, and the waveform distortion is simultaneously compensated by the third and fourth order dispersion by phase modulation. The calculation result about the improvement of the waveform when performing it is shown. The values of the parameters used for the calculation are as described above. FIG. 4A shows a state of waveform deterioration due to only the third-order dispersion of the transmission optical fiber 16. A thin line C20 is an optical pulse before transmission having an optical pulse width of 380 fs (femtosecond) before transmission, and a thick line C21 is an optical pulse after the waveform is deteriorated due to the third-order dispersion.

It can be seen that an asymmetric waveform distortion occurs due to the third-order dispersion, and a vibration component is observed at the bottom. FIG. 4 (b)
Shows the state of waveform deterioration due to only the fourth-order dispersion of the transmission optical fiber 16. The thick line C22 is an optical pulse after the waveform has been deteriorated due to the fourth-order dispersion. It can be seen that the symmetric optical pulse waveform spreads due to the fourth-order dispersion, and a long tail is seen.

On the other hand, FIG. 4 (c) shows the same setting as in the above-described experimental system, and shows the waveform after transmission when compensating for the waveform distortion based on the third and fourth order dispersion of the transmission optical fiber 16, by using a thick line C2.
Indicated by 3. It can be seen that the waveform deterioration shown in FIGS. 4A and 4B is sufficiently suppressed by the dispersion compensation by the phase modulation based on the first embodiment. FIG. 5 shows a measurement result of an autocorrelation waveform when an optical pulse of 380 fs is actually transmitted using the above-described experimental system.

FIG. 5A shows a waveform before the light enters the high-order dispersion simultaneous compensator 10, and FIG. 5B shows a waveform after the dispersion is compensated and passed through the transmission optical fiber 16. The experiment described in the first embodiment is performed at 1.28 Tbit / s.
5B. In FIG. 5B, the signal light is transmitted at 160 Gbit / s so that one entire light pulse waveform can be seen while avoiding the overlap of adjacent light pulses.
The time multiplexed signal light was transmitted to observe the waveform. Since the optical pulse width after transmission is 400 fs and the pulse spread due to transmission is only 20 fs, the effectiveness of the third and fourth order dispersion compensation according to the first embodiment is experimentally shown.

In this experiment, an error rate was measured, and it was confirmed that an error rate of 1 × 10 ⁻⁹ or less was obtained for all 128-channel 10 Gbit / s signals after transmission at a received light intensity of −21 dBm or more. I have. This is the first
As described in the embodiment, the third and fourth order simultaneous compensation method and apparatus of the present invention suppress deterioration of the signal light pulse waveform in transmission of ultra-high-speed time-multiplexed signal light,
It will be a very important technology in the ultra-high speed optical communication required for the future information technology society. Second Embodiment FIG. 6 shows a configuration of a second embodiment of a method and apparatus for simultaneous compensation of higher-order dispersion according to the present invention.

The high-order dispersion simultaneous compensator 10 shown as the second embodiment is similar to the first embodiment shown in FIG.
Optical coupler 12, Secondary dispersion applying optical fiber 11, Phase modulator 13, Dispersion compensating optical fiber 14, Clock extractor 1
5, the phase shifter 17 and the variable attenuator 18 are provided, or in the second embodiment, a layered optical thin film total transmission dispersion equalizer (Layered Optical Th
in-film Allpass Dispersion Equalizer: LOTAD
E) 20 are inserted in series. The signal light pulse is incident on the high-order dispersion simultaneous compensator 10 and transmitted by the transmission optical fiber 16
After a chirp inverse to the non-linear chirp generated by the second, third and fourth order dispersion is given in advance in the high-order simultaneous dispersion compensator 10, multiplexing is performed by the time multiplexing device 19.

In the second embodiment, the transmission optical fiber 1
Consider a dispersion-shifted optical fiber as 6. In the first embodiment, the transmission optical fiber 16 is composed mainly of a zero-dispersion single-mode optical fiber for 1.3 μm wavelength and an inverse dispersion optical fiber, and is added with a dispersion-shifted optical fiber. The values of not only the second-order variance but also the third-order variance of the whole are set as small as possible. As a result, the third-order dispersion in the transmission optical fiber 16 of the first embodiment was a small value of 3.40 × 10 ⁻⁴ ps ³ / km.

On the other hand, in the second embodiment in which a dispersion-shifted optical fiber is used as the transmission optical fiber 16, the second-order dispersion is a small value, but the third-order dispersion is about 0.1 ps ³ / km. Yes, it is a very large value compared to the first embodiment. In this case, it is difficult to compensate for waveform distortion based on third-order dispersion only by phase modulation. Therefore, in the second embodiment, the laminated optical thin film total transmission dispersion equalizer 20 is inserted after the phase modulator 13, and the waveform distortion based on the third-order dispersion of the transmission optical fiber 16 is reduced. Most of the compensation is performed by the total transmission dispersion equalizer 20, and the remaining waveform distortion due to the third-order dispersion and the waveform distortion due to the fourth-order dispersion are compensated by phase modulation by the phase modulator 13. is there.

As a specific example of the basic configuration of the laminated optical thin film total transmission dispersion equalizer 20, Electronics Letters (Elec)
tronics Letters), Vol. 36, Nos. 1139-1141
See FIG. The configuration shown in FIG. 1 is shown as FIG.
The laminated optical thin film total transmission dispersion equalizer 20 in the above document is
In this configuration, a dual optical fiber ferrule assembly 22 for input and output is coupled to a coupling dual resonator total transmission filter 21. The coupled bi-resonator total transmission filter 21 has mirrors 23, 24, and 25 having reflectivities of r ₁ , r ₂ , and r ₃ , respectively, at intervals d.
₁ , d ₂ , and r ₁ , r ₂ and d ₁ , d
_By setting ₂ appropriately, 3 of the desired size
As a result, the third-order dispersion of the transmission optical fiber 16 can be compensated.

The reflectance r ₃ is set to 100%.
By connecting the laminated optical thin film total transmission dispersion equalizers 20 having different center wavelengths in tandem, the band can be expanded for an ultrafast time multiplexed signal light pulse. When a dispersion-shifted optical fiber is used as the transmission optical fiber 16, in wavelength multiplex transmission, the wavelength is set to avoid the zero dispersion wavelength of the transmission optical fiber 16 in order to prevent signal deterioration due to four-wave mixing between channels. This is normally done, and the vicinity of the zero dispersion wavelength is not used.

On the other hand, by using the method and the apparatus shown in the second embodiment, the vicinity of the zero dispersion wavelength of the dispersion-shifted optical fiber can be used for the time-multiplexed signal light, and the effective use of the wavelength can be improved. It becomes possible. In FIG. 6, the laminated optical thin film total transmission dispersion equalizer 20 may be provided at the subsequent stage of the phase modulator 13. Therefore, in FIG.
It may be inserted between the phase modulator 13 and the dispersion compensating optical fiber 14, or may be inserted on the incident side or the output side of the transmission optical fiber 16.

In each of the embodiments shown in FIGS. 3 and 6, the secondary dispersion applying optical fiber 11 may be any fiber that can apply a linear chirp to a signal light pulse, such as a fiber grating. Others may be used. The means for applying a linear chirp to this signal light pulse is an optical coupler 1 as shown in FIG.
It may be provided at a stage prior to 2. That is, the signal light pulse branched by the optical coupler 12 and supplied to the clock extractor 15 may or may not have a linear chirp applied thereto.

The variable attenuator 18 may be any as long as it can adjust the amplitude of the sine wave signal applied to the phase modulator 13, and may use a variable gain device. That is, a variable amplitude adjuster may be used. Further, the dispersion compensating optical fiber 14 may be provided at the input end or the output end of the transmission optical fiber 16.
Equations (5) to (8) are for the case where the signal light pulse has a Gaussian waveform, and the equations are different for other waveforms, but in many cases, equations (5) to (8) may be approximately used.

[0049]

As described above, the method and apparatus for simultaneous compensation of higher-order dispersion of the present invention can suppress the deterioration of the signal light pulse waveform due to higher-order dispersion, which is a serious problem in speeding up optical communication. it can. According to the present invention, ultrahigh-speed optical communication using ultrashort pulse light of picosecond to femtosecond becomes possible. Further, according to the second embodiment, the zero-dispersion wavelength of the dispersion-shifted optical fiber is used for time-division multiplexed signal light transmission, so that the wavelength use efficiency can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cosine phase modulation curve, a sine phase modulation curve, and a phase modulation curve representing the sum of the two.

FIG. 2 is a diagram for explaining the principle of simultaneous third-order and fourth-order dispersion compensation using one phase modulator using a calculation result of angular frequency dependence of a phase change amount.

FIG. 3 is a diagram showing a configuration of a first embodiment of a high-order dispersion simultaneous compensation method and apparatus according to the present invention;

4A and 4B show calculation results for explaining the effect of the present invention. FIG. 4A is a diagram showing a state of deterioration of a signal light pulse waveform due to third-order dispersion, and FIG. 4B is a diagram showing a signal due to fourth-order dispersion. FIG. 3C is a diagram showing how the optical pulse waveform is degraded, and FIG. 3C is a diagram showing a waveform when the third- and fourth-order dispersion simultaneous compensation is performed according to the present invention.

FIG. 5 is a diagram showing a case where 380f is actually used by using the experimental system of the first embodiment.
7A is a measurement result of an autocorrelation waveform when a pulse of s is transmitted, and FIG. 7A is a diagram illustrating a waveform before being incident on the high-order simultaneous dispersion compensator 10, and FIG. FIG. 9 is a diagram showing a waveform after the secondary dispersion simultaneous compensation has been performed and the transmission optical fiber 16 has been passed.

FIG. 6 is a diagram showing a configuration of a second embodiment of a high-order dispersion simultaneous compensation method and apparatus according to the present invention;

FIG. 7 is a diagram showing a basic configuration of a laminated optical thin film total transmission dispersion equalizer 20 in FIG. 6;

FIG. 8 is a diagram for explaining the principle of fourth-order dispersion compensation using a phase modulator, using a calculation result of the angular frequency dependence of the amount of phase change.

FIG. 9 is a diagram for explaining the principle of third-order dispersion compensation using a phase modulator using a calculation result of the angular frequency dependence of a phase change amount.

FIG. 10 is a diagram showing a conventional configuration of high-order dispersion compensation.

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Claims (3)

[Claims] 1. applying a linear chirp to a signal light pulse;
Sinusoidal phase modulation is performed on the signal light pulse to which the linear chirp has been applied so that the time of the intensity peak of the light pulse coincides with the time of zero phase shift.
A method for compensating for the waveform distortion of the signal light pulse due to the next dispersion, and performing cosine phase modulation on the signal light pulse to which the linear chirp is applied so that the time of the intensity peak and the phase shift peak of the light pulse coincide with each other. And a second-order dispersion applying optical fiber for applying a second-order dispersion to compensate for a waveform distortion of a signal light pulse due to a second-order dispersion and a fourth-order dispersion of the optical fiber transmission line. By applying a phase change to the signal light pulse to which the linear chirp is applied by a single sine wave represented by a linear sum of the phase change with the modulation, the third-order dispersion and the fourth-order dispersion of the optical fiber transmission line are simultaneously performed. A high-order dispersion simultaneous compensation method characterized by performing compensation. A second dispersion applying means for applying a linear chirp to the signal light pulse, an optical coupler for branching out a part of the signal light pulse, and a sine wave synchronized with the branched and extracted signal light pulse A clock extractor for extracting a signal, a phase modulator for performing phase modulation on a signal light pulse to which a linear chirp is applied by the sine wave signal, and a dispersion compensating fiber for adjusting a magnitude of a secondary dispersion received by the signal light pulse And a variable amplitude adjuster and a variable phase shifter are inserted in series in a path of the sine wave signal between the clock extractor and the phase modulator. High-order dispersion simultaneous compensation device. 3. A second-order dispersion applying means for applying a linear chirp to a signal light pulse, an optical coupler for branching and extracting a part of the signal light pulse, and a sine wave synchronized with the branched and extracted signal light pulse A clock extractor for extracting a signal, a phase modulator for performing phase modulation on a signal light pulse to which a linear chirp is applied by a sine wave signal, and a dispersion compensating fiber for adjusting a magnitude of a secondary dispersion received by the signal light pulse. A variable amplitude adjuster and a variable phase shifter inserted in series in the path of the sine wave signal between the clock extractor and the phase modulator; and Layered optical thin-film allpass dispersion equalizer (Layered Optical Thin-film Allpass Dispe
(Rsion Equalizer: LOTADE), the waveform distortion of the signal light pulse due to the third order dispersion of the optical fiber transmission line is reduced by the laminated optical thin film total transmission dispersion equalizer, and the signal light due to the minute residual third order dispersion and fourth order dispersion. A high-order simultaneous dispersion compensator, wherein a pulse waveform distortion is compensated by phase modulation of a phase modulator.