JPH09297328A - Optical transmission line and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the deterioration in light pulse waveforms by the nonlinear effect, dispersion and higher dispersion of optical fibers which arises at the time of embodying super-high speed optical transmission of several tens Gb/s or over. SOLUTION: This system is constituted by having plural pieces of transmission circuits connected with the optical fibers 3a, 3b,... of the dispersion values decreasing in a longitudinal direction, optical amplifiers 4a, 4b,... and optical filters 5a, 5b,... in this order. In such a case, the decrease in the dispersion values of the respective optical fibers 3a, 3b and the amplification degrees of the respective optical fibers 4a, 4b are set by every transmission circuit so as to obtain prescribed light pulse widths. The transmission line is so constituted that the respective central frequencies of the plural optical fibers 5a, 5b are slid successively to a lower frequency side with respect to the propagation direction and that the respective average zero dispersion wavelengths of the plural optical fibers 3a, 3b are successively increased in the propagation direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission line and an optical transmission system which suppresses optical pulse waveform deterioration due to nonlinear effects, dispersion, and high-order dispersion of an optical fiber in ultrahigh-speed optical transmission and reduces timing jitter. Regarding

[0002]

2. Description of the Related Art In ultra-high-speed optical transmission that requires short optical pulses of several ps or less, the optical pulse waveform is significantly deteriorated due to nonlinear effects, dispersion, and higher-order dispersion of an optical fiber. The deterioration of the pulse waveform due to the influence of higher-order dispersion occurs because most of the optical spectrum shifts to the low frequency side when the dispersion of the optical fiber is zero or very small.
The deterioration of the pulse waveform in this way limits the transmission distance. Conventionally, as a method of compensating for waveform deterioration due to nonlinear effects and dispersion, a special optical pulse called an optical soliton that propagates in an optical fiber while maintaining its waveform shape by utilizing the balance between dispersion and nonlinear effects is used. There is a method for long-distance transmission.

When actually constructing an optical soliton transmission system, amplified spontaneous emission light (ASE) emitted from an optical amplifier used for compensating the loss of an optical fiber is
It gives a random change to the carrier frequency of an optical soliton. This change is due to the Gordon House effect (Reference 1: A. Hasega
wa et al., “Solitons in Optical Communications” .O
xford Univ. Press (1995)), and it is known to change the propagation time of each optical soliton in the optical fiber and generate timing jitter. In addition, the carrier linewidth of the optical soliton light source (Reference 2: K. Iwatsukl et.
al., IEEE J. Lightwave Technol., 13, pp. 639-649 (1995)
(See also) causes timing jitter in a similar process.

Therefore, in the optical soliton transmission, since this timing jitter limits the transmission speed or the transmission distance, the timing jitter is reduced by using an optical filter whose center frequency is slid along with the transmission distance, an optical filter and an intensity modulator, or the like. A method of reducing the transmission rate is proposed, and the transmission rate is up to 20 Gb.
The principle of optical soliton transmission at / s has been confirmed by a loop experiment (see Reference 1). However, in the above-mentioned conventional method, the bandwidth of the optical filter needs to be about 4 to 5 times the signal light spectrum in order to preserve the waveform, and a narrow band optical filter having a narrower bandwidth than this is used. I can't.

[0005]

However, since the timing jitter accumulation increases as the transmission speed increases, a more effective timing is required to realize the ultra-high-speed optical soliton transmission exceeding several tens Gb / s. Reducing jitter is an important issue. Further, in the optical soliton transmission, it is necessary to make the interval for arranging the optical amplifier, the optical filter and the intensity modulator, etc. sufficiently shorter than the soliton period defined by the pulse width of the optical soliton and the average dispersion value of the optical fiber. However, if the soliton cycle is shortened as the transmission speed is increased, there arises a problem that a realistic relay interval cannot be secured.

A method devised to solve the above-mentioned problem of the repeater interval is a method of using a dispersion-decreasing fiber (DDF) whose transmission value is reduced in the same manner as the attenuation of optical intensity in a transmission line (Reference 3: AJStentz). et al., Opt. Lett., 20, pp. 1770-1
772 (1995)). In the DDF, since the nonlinear effect and the dispersion are locally balanced, an optical soliton of several ps or less can be propagated while maintaining the waveform.
The relay interval can be secured by designing the strip length and dispersion distribution of F.

However, in the case of the long distance transmission method using the conventional DDF, if the average variance is reduced in consideration of the accumulation of timing jitter, the DDF is reduced.
Since the dispersion value becomes smaller near the output end of, the pulse waveform deteriorates due to the effects of higher-order dispersion and nonlinear effects (Reference 4: K.Su.
zuki et al., OAA'95 FB3 (1995)), there was a problem that the transmission distance was limited. As mentioned above, the deterioration of the pulse waveform due to the influence of high-order dispersion and nonlinear effects occurs because most of the optical spectrum shifts to the low frequency side when the dispersion of the optical fiber is zero or very small. It is as follows. Up to now, no specific solution has been proposed for such waveform deterioration. Further, even in the method using the DDF in the transmission line, since the optical soliton effect is used, timing jitter occurs, but a method for effectively reducing it has not been proposed.

An object of the present invention is to suppress optical pulse waveform deterioration due to nonlinear effects, dispersion, and higher-order dispersion of an optical fiber, which occurs when realizing ultrahigh-speed optical transmission of several tens Gb / s or more. And to provide an optical transmission system.

[0009]

In order to solve the above-mentioned problems, the invention according to claim 1 provides a transmission circuit in which an optical fiber whose dispersion value decreases in the longitudinal direction, an optical amplifier, and an optical filter are connected in this order. In order to obtain a predetermined optical pulse width, a plurality of optical fibers are configured to have a dispersion value reduction of each optical fiber and an amplification degree of each optical amplifier, and each central frequency of the plurality of optical filters is set so as to obtain a predetermined optical pulse width. It may be characterized in that the average zero dispersion wavelength of each of the plurality of optical fibers is sequentially increased in the propagation direction while being sequentially slid toward the low frequency side with respect to the propagation direction.

The invention according to claim 2 is the transmission line according to claim 1, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the time of its input.
In addition, the dispersion value of each optical fiber and the amplification factor of each optical amplifier are set so as to satisfy the following conditions so that the spectral width of the optical signal output from the optical fiber is increased more than the spectral width at the time of its input. It is characterized by being done. (I) making the dispersion value decrease of the optical fiber larger than the intensity attenuation of the optical signal propagating in the optical fiber, and (II) the peak intensity P of the input optical signal of the optical fiber.
The _{0, P 0 = 0.776 (λ} 3 A eff / π 2 cn 2) to the light peak intensity is defined by D / τ ² (however, lambda light signal wavelength, A
_eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ is the pulse width of the optical signal).

The invention according to claim 3 is the same as claim 1.
The optical signal output from each of the optical fibers in the transmission line
The pulse width of the signal is smaller than the pulse width at the time of its input,
And the spectral width of the optical signal output from the optical fiber
To increase the spectral width when input of
To reduce the dispersion value of each optical fiber so that the condition is satisfied,
The feature is that the amplification degree of each optical amplifier is set.
You. (I) Reduction of the dispersion value of the optical fiber
Equal to the intensity attenuation of the optical signal propagating in the optical fiber, and
(II) Peak intensity P of the input optical signal of the optical fiber
₀To P₀= 0.776 (λ^ThreeA _eff/ Π^Twocn_Two) D / τ^TwoDefined by
Light peak intensity.

The invention according to claim 4 is, in the transmission path according to claim 1, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the time of input thereof.
In addition, the dispersion value of each optical fiber and the amplification factor of each optical amplifier are set so as to satisfy the following conditions so that the spectral width of the optical signal output from the optical fiber is increased more than the spectral width at the time of its input. A transmission line characterized by being performed. (I) making the dispersion value decrease of the optical fiber larger than the intensity attenuation of the optical signal propagating in the optical fiber, and (II) the peak intensity P of the input optical signal of the optical fiber.
₀ is made larger than the optical peak intensity defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ² .

According to a fifth aspect of the present invention, in the transmission line according to the first aspect, an optical fiber manufactured so that the dispersion value gradually decreases is used as the optical fiber. The described invention is characterized in that, in the transmission line according to claim 1, a plurality of optical fibers having different dispersion values are connected to form the optical fiber, and the dispersion value is gradually reduced in the propagation direction. .

Further, the invention according to claim 7 is constituted by providing a plurality of transmission circuits in which an optical fiber whose dispersion value decreases in the longitudinal direction, an optical amplifier, an optical frequency shifter, and an optical filter are connected in this order, To obtain a given light pulse width,
The dispersion value reduction of each optical fiber and the amplification degree of each amplifier are set for each transmission circuit, and the optical spectrum of each output light of the plurality of optical frequency shifters is on the high frequency side with respect to each input optical spectrum. It is characterized by that.

Further, the invention according to claim 8 is configured by including a plurality of transmission circuits each including an optical fiber which is a distributed optical amplifier having a gain in a propagation direction and an optical filter, and each of the plurality of optical filters. It is characterized in that the center frequency is sequentially slid to the low frequency side in the propagation direction, and each mean zero dispersion wavelength of the plurality of optical fibers is sequentially increased in the propagation direction.

According to a ninth aspect of the present invention, in the transmission line according to the eighth aspect, the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the input, and the optical fiber is output. The amplification degree of each of the optical fibers is set so as to satisfy the following conditions so that the spectrum width of the optical signal to be input becomes larger than the spectrum width at the time of input. (I) The amplification factors of the plurality of optical fibers are sequentially increased in the propagation direction, and (II) the peak intensity P ₀ of the input optical signal of the plurality of optical fibers is changed to P ₀ = 0.776.
The optical peak intensity is defined by (λ ³ A _eff / π ² cn ₂ ) D / τ ² .

The invention according to claim 10 is the same as claim 8
In the transmission line described above, the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the input, and the spectral width of the optical signal output from the optical fiber is made larger than the spectral width at the input. Thus, the amplification degree of each optical fiber is set so as to satisfy the following conditions. (I) Each amplification factor of the plurality of optical fibers is made constant in the propagation direction, and (II) the peak intensity P ₀ of the input optical signal of the plurality of optical fibers is set to P ₀ = 0.776.
The optical peak intensity is defined as (λ ³ A _eff / π ² cn ₂ ) D / τ ² .

The invention according to claim 11 is the invention according to claim 8.
In the transmission line described above, the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the input, and the spectral width of the optical signal output from the optical fiber is made larger than the spectral width at the input. Thus, the amplification degree of each optical fiber is set so as to satisfy the following conditions. (I) The amplification factors of the plurality of optical fibers are sequentially increased in the propagation direction, and (II) the peak intensities P ₀ of the input optical signals of the plurality of optical fibers are changed to P ₀ = 0.7.
It is made larger than the optical peak intensity defined by 76 (λ ³ A _eff / π ² cn ₂ ) D / τ ² .

Further, the invention according to claim 12 is constituted by providing a plurality of transmission circuits in which an optical fiber which is a distributed optical amplifier having a gain in a propagation direction, an optical frequency shifter, and an optical filter are connected in this order. , An amplification factor of each amplifier is set for each of the transmission circuits so as to obtain a predetermined optical pulse width, and an optical spectrum of each output light of the plurality of optical frequency shifters. Is on the high frequency side with respect to each input light spectrum.

According to a thirteenth aspect of the present invention, there is provided an optical transmitting device, the transmission line according to any one of the first to twelfth aspects connected to the optical transmitting device, and the transmission line. It is a transmission system characterized by comprising an optical receiving device.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram showing a multi-repeater optical transmission system using an optical transmission line according to a first embodiment of the present invention. The multi-relay optical transmission system uses dispersion-decreasing optical fibers (DDF) 3a to 3d for the transmission path from the optical pulse transmitter 1 to the optical receiver 2, and optical amplifiers 4a to 4c at predetermined intervals between them. And narrow band optical filter 5
It is configured to relay by a set of a to 5c. The optical amplifiers 4a to 4c are controlled so that the output after the optical filter becomes equal to the initial fiber input intensity (ALC control).
It is possible to use an optical amplifier provided with.

In the present embodiment, the multi-repeater optical transmission system is configured by cascade-connecting a plurality of optical transmission circuits configured by connecting the DDFs 3a to 3d, the optical amplifiers 4a to 4c, and the optical filters 5a to 5c in this order. Has been done.
The decrease in the dispersion value of the DDFs 3a to 3d in the longitudinal direction is adjusted to be larger than the attenuation of the light intensity. The center frequencies of the optical filters 5a to 5c are slid (down-slide) to the low frequency side together with the transmission distance. Further, the average zero-dispersion wavelength of the cascade-connected DDFs 3a to 3d is sequentially increased with the transmission distance. By adopting such a transmission method, the waveform deterioration of the optical pulse due to dispersion and non-linear effects can be suppressed, and the timing jitter caused by using the optical soliton effect can be greatly reduced and the transmission distance can be increased. Becomes

In the optical transmission line of this embodiment, the DDF 3a ...
As 3d, the optical soliton adiabatic compression is performed in one repeater section by using the one in which the decrease of the dispersion value is adjusted to be larger than the attenuation of the light intensity (Reference 5: PV Mamyshev et al., IEEE J. Quantum E
lectron, .vol.27, No.10, p.2347 (1991)), narrows the optical pulse, widens its spectral width, and then down-slides it to the center frequency of the incident optical pulse. The optical spectrum is band-limited using the filters 5a to 5c, and waveform shaping is performed. This suppresses the optical pulse waveform deterioration caused by the nonlinear effect of the optical fiber and the high-order dispersion.
Further, by band-limiting the spectrum of the optical pulse using the narrow band optical filters 5a to 5c, it is possible to significantly reduce the timing jitter caused by using the optical soliton effect.

FIG. 2 is a dispersion value (solid line) of the optical transmission line including the transmission distance A (position of the optical amplifier 4a) and the transmission distance B (position of the optical amplifier 4b) and the light intensity in the transmission system shown in FIG. It is the figure which showed decrease, ie, the change of the loss (broken line). The optical pulse signal amplified by the amplifier 4a at the transmission distance A is the optical filter 5 having a predetermined center frequency f _0.
The band is limited by a and shaped into a pulse waveform showing a temporal change as shown in FIG. 3A, and then output to the DDF 3b. At this time, the optical spectrum of the optical signal is centered on the frequency f ₀ as shown in FIG. Adjusting the reduction of dispersion value to be larger than the attenuation of light intensity within one relay section D
The optical pulse signal that has passed through the DF 3b has its optical pulse width compressed and its spectral width expanded by adiabatic optical soliton compression. Therefore, at the output end of the optical amplifier 4b, the optical pulse signal shown in FIG. The time waveform and the optical spectrum are as shown in. As shown in FIG. 3 (e), the optical spectrum has a shape that is split due to the nonlinear effect of the optical fiber and the higher-order dispersion and shifted to the low frequency side. The center frequency of the optical pulse signal output from the optical amplifier 4b is slid down as compared with the center frequency f ₀ of the incident optical pulse, and the bandwidth thereof is the bandwidth of the incident optical pulse (FIG. 3 (d)). The optical spectrum is band-limited and the waveform is shaped by using the narrow-band optical filter 5b having a bandwidth about 1 to 2 times. As a result, only the main part of the optical spectrum that is split due to the nonlinear effect and higher-order dispersion of the optical fiber and shifted to the low frequency side is transmitted, and the dispersed wave shifted to the high frequency side is band-limited, so that the optical pulse Waveform deterioration is suppressed. The waveform immediately after passing through the narrow band optical filter 5b has the time change and the optical spectrum as shown in FIGS. 3 (c) and 3 (f).

In the prior art using the optical filter in which the center frequency is slid along with the transmission distance, the direction of frequency sliding is arbitrary because the purpose is to remove noise components such as ASE. However, the present invention is , The purpose is to shape the waveform deterioration due to nonlinear effects and higher-order dispersion, and the direction of frequency sliding is not arbitrary. In the present invention, since the optical spectrum width is intentionally expanded by the optical soliton adiabatic compression and then transmitted through the narrow band filter, the signal band width is not reduced by the optical filter, and therefore, to the same extent as the signal light spectrum. The optical filter can have a narrow band.
By using such a narrow band optical filter, it is possible to significantly reduce the timing jitter as compared with the conventional method in which the optical filter can be narrowed only to about 4 to 5 times the signal light spectrum. .

Here, the optical soliton adiabatic compression will be described. Under the condition of | αZ ₀ | << 1 (where α is the gain coefficient and Z ₀ is the soliton length), the optical soliton adiabatic compression is
While maintaining the optical soliton condition (pulse energy (ε) × pulse width (Δτ) ∝dispersion (D)), the dispersion value D is reduced in the propagation direction using DDF. Since the optical soliton condition is given by Δτ∝D / ε,
If the decrease of the dispersion D is larger than the attenuation of the pulse energy ε, the pulse width Δτ is compressed by the optical soliton adiabatic compression.

In the above description, in order to perform the optical pulse adiabatic compression, the optical soliton adiabatic compression is performed by gradually decreasing the dispersion D in the longitudinal direction using the DDF, but it is clear from the soliton condition. Thus, as will be described later in detail, a distributed optical amplifying fiber is used as the optical fiber instead of the DDF, and the optical soliton adiabatic compression is also achieved by gradually increasing the amplification degree of the optical amplification in the longitudinal direction. It can be carried out.

In addition to the optical soliton adiabatic compression, pulse compression can be performed by making the light intensity larger than the soliton power P _{0 described} below.

Therefore, in order to perform optical pulse compression, the transmission line may be constructed so as to satisfy any one of the following conditions.

(1) The dispersion reduction is larger than the optical fiber loss (optical intensity amplification) (optical soliton adiabatic compression).

(2) The fiber input light intensity is larger than the soliton power (optical soliton compression).

(3) The amplification degree in each relay section is amplified in the propagation direction (optical soliton adiabatic compression).

4 to 7 are diagrams showing these conditions as schematic diagrams. FIG. 4 shows the case where the dispersion reduction amount is made larger than the intensity reduction amount using the DDF, and FIG. 5 shows the case where the amplification degree is increased in the longitudinal direction using the distributed amplifier. In these cases, optical pulse compression is performed by optical soliton adiabatic compression while the fiber input intensity is kept below the soliton power P ₀ . The characteristic indicated by the broken line in the figure is for the case where the optical pulse width is kept constant. On the other hand, FIGS. 6 and 7 both show the case where the optical pulse compression is performed by making the fiber input intensity larger than the soliton power P _0. FIG. 6 shows an example using DDF and FIG. 7 shows a distributed amplifier. An example of use is shown.

In the embodiment shown in FIG. 1, the center frequencies of the narrow band optical filters 5a to 5c are down-slided. However, as described in the following embodiments, the narrow band optical filters 5a to 5c. Instead of sliding down the center frequency of 5c, it is also possible to shape the waveform by sliding (upslide) the optical frequency to the high frequency side by using an optical filter and a frequency shifter whose center frequency is fixed.

The conventional technique using the DDF described above is as follows.
Since the dispersion value of the DDF decreases in accordance with the attenuation of the light intensity, the characteristic is that the optical pulse waveform is preserved, and the transmission method is characterized by performing the optical pulse waveform shaping as in the present invention. Are different in nature.

Detailed conditions for pulse width compression in the embodiment shown in FIG. 1 will be described. The reduction of the dispersion value of each DDF and the amplification degree of each optical amplifier in one repeater interval when performing pulse compression make the pulse width of the optical signal output from each DDF smaller than the pulse width at the time of its input, and The following conditions (I) and (II) are set so that the spectral width of the optical signal that outputs
It is desirable that each characteristic is set so as to satisfy at least one of the above.

(I) The reduction of the dispersion value of the optical fiber is made larger than the intensity attenuation of the optical signal propagating in the optical fiber (optical soliton adiabatic compression).

(II) The peak intensity P ₀ of the input optical signal of the optical fiber is P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ²
In defined as an optical peak intensity (fundamental soliton optical power needed to form a: soliton power) larger than (where, lambda is optical signal wavelength, A _{e ff} is the effective core area of the optical fiber, c is The speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ is the pulse width of the optical signal).

When only the condition (I) is satisfied, the input optical signal of the optical fiber is changed to the peak intensity P _0.
It is desirable to make it equal to the optical peak intensity given by the formula (1), and on the other hand, when only condition (II) is satisfied, the dispersion value reduction of the optical fiber and the intensity attenuation of the optical signal propagating in the optical fiber are performed. It is desirable to make them equal.

As a method of constructing each DDF, an optical fiber manufactured so that the dispersion value gradually decreases, or an optical fiber having different dispersion values so that the dispersion value gradually decreases in the propagation direction is used. A plurality of DDFs can be connected to configure each DDF.

The amplification degree of each of the optical amplifiers 4a-4c is
Although it depends on the order of the optical soliton, for example, N =
When the basic optical soliton transmission of 1 is used, the power ratio is 1 to
It can take a value of 2.25 times, and an amplification degree of about twice becomes a desirable value.

The waveform shaping effect of the present invention will be clarified below. First, therefore, the numerical calculation result of the single optical pulse propagation characteristic with a pulse width of 3.0 ps is shown. The parameters used are a fiber loss of 0.22 dB / km and a high-order dispersion of 0.07 ps / nm ² / km. One relay section 30k
The dispersion value of the DDF of m is changed stepwise as shown in FIG.
The band width of the optical filter was 1.5 nm. The input light intensity is N = 1.
The light intensity was adjusted to be equal to 4 solitons. FIG.
The initial waveform is shown in (a). FIG. 9B is a waveform after propagation of 300 km when the center frequency of the optical filter is not slid. It can be seen that the pulse waveform has deteriorated. In FIG. 9C, the center frequency of the optical filter is set to 30 km.
3 when sliding down 10.0 GHz for each propagation
The waveform after propagation of 00 km is shown. However, the average zero dispersion wavelength of each DDF was increased with the transmission distance so that the dispersion value at the center wavelength of each optical pulse transmitted through each DDF would not change. It can be seen that the downslide of the optical filter center frequency effectively suppresses the deterioration of the pulse waveform.

Next, the result when a pulse train of a random pattern with ASE noise superimposed is transmitted at a transmission rate of 80 Gb / s is shown. The population inversion parameter of the optical amplifier is 1.
8, the optical amplifier was controlled (ALC) so that the output after the optical filter became equal to the initial fiber input intensity. The pulse width is 4.0 ps and the center frequency is 7.0 every 30 km using an optical fiber having a bandwidth of 1.7 nm.
It was slid down at GHz. FIG. 10 (a), (b)
Is the relationship between the S / N and the timing jitter with respect to the transmission distance. Since the optical pulse is compressed to 2.2 ps at the DDF output end, the bandwidth of the optical filter is 1.5 times the spectral width of the optical pulse. Considering that the relationship between the bandwidth and the spectrum width of the conventional optical filter for reducing timing jitter is restricted to about 4 to 5 times due to the stability of the optical soliton (see Document 1), the present invention is applicable. The action of the optical filter is different from that of the optical filter for reducing timing jitter. The values of S / N and timing jitter that give an error rate of 10 ^-9 are 21.5 respectively.
It is dB and 0.68 ps, and it can be seen from FIGS. 10A and 10B that the transmission distance giving an error rate of 10 ⁻⁹ is 2760 km.

FIG. 11 is a diagram for explaining a second embodiment of the present invention, in which a DDF 3, an optical amplifier 4, an optical frequency shifter 6 including an acousto-optic modulator, and an optical filter 5 are connected in this order. Then, each transmission circuit is configured, and each transmission circuit is cascaded in multiple stages to form a multi-repeater optical transmission system. In the example of FIG. 1, the center frequencies of the optical filters 5a to 5c are slid down along with the transmission distance, whereas in the present embodiment example, the frequency is shifted to the high frequency side with the optical filter 5 whose center frequency is fixed. The difference is that upslide is performed using an optical frequency shifter 6 such as an acousto-optic modulator, but in the case of the present embodiment example, the waveform deterioration of the optical pulse due to the high-order dispersion and the nonlinear effect is suppressed in the same manner. , The timing jitter caused by using the optical soliton effect is significantly reduced, and the transmission distance can be increased.

In this embodiment, the center frequency of the optical pulse to be transmitted is fixed to that of the optical filter 5 in order to perform frequency sliding by the optical filter 5 and the optical frequency shifter 6 whose center frequency is fixed, and the transmission distance It is constant regardless of. Therefore, the dispersion value of the transmission line at the center frequency of the optical pulse being transmitted through each DDF 3 does not increase, and it is not necessary to increase the mean zero dispersion wavelength of the optical fiber with the transmission distance as in the embodiment example of FIG.

In this embodiment, an acousto-optic modulator is used as the optical frequency shifter 6, but a phase modulator driven by a sawtooth voltage so as to give a phase change of an integral multiple of 2π (reference: 6: KKWong et al., "Performance
of a Serrodyne optical frequency translator, "Topy
cal Meeting on Integrated and Guided-Wave Optics,
WA5, Pacific-Globe, CA, Jan. 1982) and SSB (Sin
gle sideband) Optical frequency converter (Reference 7: B.Desormiere
et al, IEEE J. LlghtwaveTechnoL, 8, pp, 506-513 (1990)
The optical frequency shifter can be configured by using the same as that of the acousto-optic modulator.

FIG. 12 is a diagram for explaining a third embodiment of the present invention, in which transmission circuits in which distributed optical amplification fibers 7a to 7d and optical filters 5a to 5c are connected in this order are cascaded in multiple stages. Form a multi-relay transmission system. The distributed optical amplification fiber 7 has a gain in the propagation direction. Therefore, no optical amplifier is required. Cascaded optical fibers 7a-7d
The average zero-dispersion wavelength of each is adjusted to increase sequentially with the transmission distance. The center frequencies of the optical filters 5a to 5c are slid down together with the transmission distance. In the example of FIG. 1, pulse compression is performed using the DDFs 3a to 3d (see FIG. 4), whereas in the present embodiment example, pulse compression is performed using the distributed optical amplification fibers 7a to 7d (see FIG. 5). However, in the same manner, the waveform deterioration of the optical pulse due to the dispersion and the nonlinear effect is suppressed, and the timing jitter caused by using the optical soliton effect is significantly reduced, and the transmission distance can be increased. . Also,
In the embodiment shown in FIG. 12, the pulse width of the optical signal output from each distributed optical amplification fiber 7 is made smaller than the pulse width at the time of its input, and the optical signal output from the distributed optical amplification fiber 7 is reduced. The following conditions (I) and (II) are set so that the spectrum width is made larger than the spectrum width at the time of input.
It is desirable that each characteristic is set so as to satisfy at least one of the above.

(I) The amplification degree in each relay section is amplified in the propagation direction (optical soliton adiabatic compression).

(II) The peak intensity P ₀ of the input optical signal of the optical fiber is P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ²
Greater than the optical peak intensity defined by (however, λ
Is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ
Is the pulse width of the optical signal).

When only the condition (I) is satisfied, the input optical signal of the optical fiber is changed to the peak intensity P _0.
It is desirable to make it equal to the optical peak intensity given by the equation (4), and on the other hand, if only condition (II) is satisfied, it is desirable to make the amplification factor in each relay section constant in the propagation direction.

FIG. 13 is a diagram for explaining a fourth embodiment of the present invention, in which a distributed optical amplification fiber 7, an optical frequency shifter 6 and an optical filter 5 are connected in this order to form a transmission circuit, They are cascaded in multiple stages to form a multi-relay transmission system. In the example of FIG. 12, the center frequency of the optical filters 5a to 5c is slid down along with the transmission distance, and the mean zero dispersion wavelength of the cascaded optical fibers 7a to 7d is sequentially increased along with the transmission distance. The example is different in that the optical filter 5 having a fixed center frequency and the optical frequency shifter 6 such as an acousto-optic modulator that shifts the frequency to the high frequency side are used for up-slide. The waveform deterioration of the optical pulse due to the effect is suppressed, and the timing jitter generated by using the optical soliton effect is significantly reduced, and the transmission distance can be increased.

In this embodiment, the center frequency of the optical pulse to be transmitted is fixed to that of the optical filter 5 in order to perform frequency sliding by the optical filter 5 and the optical frequency shifter 6 whose center frequency is fixed, and the transmission distance It is constant regardless of. Therefore, the dispersion value of the transmission line that the optical pulse feels does not increase, and the optical fiber 7 is different from the third embodiment shown in FIG.
It is not necessary to increase the mean zero dispersion wavelength of a to 7d with the transmission distance.

Although the embodiments of the present invention have been described with reference to the drawings, the present invention can be implemented in various other forms without departing from the spirit or the main features thereof. Therefore, the above-described embodiments are merely examples in all respects, and should not be limitedly interpreted. The scope of the present invention is defined by the scope of claims and is not bound by the text of the specification. Further, all modifications and changes belonging to the doctrine of equivalents of the claims are within the scope of the present invention.

[0054]

According to the present invention, it is possible to suppress the waveform deterioration of an optical pulse due to the nonlinear effect, dispersion and high-order dispersion of an optical fiber, and to significantly reduce the timing jitter caused by using the optical soliton effect. Therefore, the transmission distance can be increased.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a first exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a transmission distance of one relay interval, a dispersion value, and a loss in the embodiment shown in FIG.

3A to 3C are diagrams showing time-varying waveforms of an optical pulse signal at an input end of a DDF, an output end of an optical amplifier, and an output end of an optical filter, respectively, and FIGS.
(F) is a figure which shows the optical spectrum of the optical pulse signal in the input end of DDF, the output end of an optical amplifier, and the output end of an optical filter, respectively.

FIG. 4 is a schematic diagram for explaining one condition of optical pulse compression.

FIG. 5 is a schematic diagram for explaining one condition of optical pulse compression.

FIG. 6 is a schematic diagram for explaining one condition of optical pulse compression.

FIG. 7 is a schematic diagram for explaining one condition of optical pulse compression.

FIG. 8 is a diagram showing a variance value in one relay section according to the present invention.

9 (a) to 9 (c) are diagrams for explaining the effect of the present invention, FIG. 9 (a) is an initial waveform diagram, and FIG. 9 (b) is a waveform after 300 km propagation when the optical filter center frequency is not slid. FIG. 6C is a waveform diagram after 300 km transmission when the optical filter center frequency is downslide by 7.0 GHz every 30 km.

10A is a diagram showing a relationship between transmission distance and S / N in the nonlinear transmission of the present invention, and FIG. 10B is a diagram showing a relationship between transmission distance and timing jitter.

FIG. 11 is a diagram illustrating a second exemplary embodiment of the present invention.

FIG. 12 is a diagram illustrating a third exemplary embodiment of the present invention.

FIG. 13 is a diagram illustrating a fourth exemplary embodiment of the present invention.

[Explanation of symbols]

1 Optical pulse transmitter 2 Optical receiver 3,3a, 3b, 3c, 3d Dispersion decreasing optical fiber (DDF) 4, 4a, 4b, 4c Optical amplifier 5, 5a, 5b, 5c Narrow band optical filter (optical filter ) 6 Optical frequency shifter 7, 7a, 7b, 7c, 7d Distributed optical amplification fiber

Claims (13)

[Claims] 1. A transmission circuit comprising a plurality of transmission circuits in which a dispersion value is reduced in the longitudinal direction, an optical fiber, an optical amplifier, and an optical filter are connected in this order, and the transmission is performed so as to obtain a predetermined optical pulse width. The reduction of the dispersion value of each optical fiber and the amplification factor of each optical amplifier are set for each circuit, the center frequencies of the plurality of optical filters are sequentially slid toward the low frequency side in the propagation direction, and the plurality of optical filters are set. A transmission line characterized in that each mean-zero-dispersion wavelength of the fiber is sequentially increased in the propagation direction. 2. The transmission line according to claim 1, wherein the pulse width of the optical signal output from each optical fiber is made smaller than the pulse width at the time of input, and the spectral width of the optical signal output from the optical fiber. So as to increase the spectrum width at the time of input, the dispersion value reduction of each optical fiber and the amplification degree of each optical amplifier are set so as to satisfy the following conditions: (I) The dispersion value reduction of the optical fiber is made larger than the intensity attenuation of the optical signal propagating in the optical fiber, and
(II) The peak intensity P ₀ of the input optical signal of the optical fiber is set to the optical peak intensity defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ² (where λ Is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ is the optical signal. Pulse width). 3. The transmission line according to claim 1, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the time of its input, and the spectral width of the optical signal output from the optical fiber. So as to increase the spectrum width at the time of input, the dispersion value reduction of each optical fiber and the amplification degree of each optical amplifier are set so as to satisfy the following conditions: (I) Making the dispersion value reduction of the optical fiber the same as the intensity attenuation of the optical signal propagating in the optical fiber, and (II)
The peak intensity P ₀ of the input optical signal of the optical fiber is made larger than the optical peak intensity defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ² (however,
λ is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ
Is the pulse width of the optical signal). 4. The transmission line according to claim 1, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the time of its input, and the spectral width of the optical signal output from the optical fiber. So as to increase the spectrum width at the time of input, the dispersion value reduction of each optical fiber and the amplification degree of each optical amplifier are set so as to satisfy the following conditions: (I) The dispersion value reduction of the optical fiber is made larger than the intensity attenuation of the optical signal propagating in the optical fiber, and
(II) The peak intensity P ₀ of the input optical signal of the optical fiber is made larger than the optical peak intensity defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ² (however, ,
λ is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ
Is the pulse width of the optical signal). 5. The transmission line according to claim 1, wherein an optical fiber manufactured so that the dispersion value gradually decreases is used as the optical fiber. 6. The transmission line according to claim 1, wherein a plurality of optical fibers having different dispersion values are connected to form the optical fiber, and the dispersion value is gradually reduced in the propagation direction. Transmission line. 7. An optical fiber, an optical amplifier, an optical frequency shifter, and an optical filter, which are connected in this order in the order of decreasing the dispersion value in the longitudinal direction, are provided to obtain a predetermined optical pulse width. In addition, the dispersion value reduction of each optical fiber and the amplification degree of each amplifier are set for each transmission circuit, and the optical spectrum of each output light of the plurality of optical frequency shifters is higher than that of each input optical spectrum. A transmission line characterized by: 8. An optical fiber, which is a distributed optical amplifier having a gain in the propagation direction, and a plurality of transmission circuits each including an optical filter, and each center frequency of the plurality of optical filters is a low frequency in the propagation direction. A transmission line characterized in that the mean zero dispersion wavelength of each of the plurality of optical fibers is sequentially increased in the propagation direction. 9. The transmission line according to claim 8, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the input, and the spectral width of the optical signal output from the optical fiber is input. The transmission path is characterized in that the amplification factor of each of the optical fibers is set so as to satisfy the following condition so as to increase the spectrum width more than the time. (I) Each amplification factor of the plurality of optical fibers propagates. Direction, and (II) the peak intensity P ₀ of the input optical signals of the plurality of optical fibers is defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ^2. Optical peak intensity (where λ is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, and D is the input of the optical fiber. The dispersion value at the edge, τ is the pulse width of the optical signal). 10. The transmission path according to claim 8, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the input, and the spectral width of the optical signal output from the optical fiber is input. The transmission path is characterized in that the amplification factor of each of the optical fibers is set so as to satisfy the following condition so as to increase the spectrum width more than the time. (I) Each amplification factor of the plurality of optical fibers propagates. Direction, and (II) the peak intensity P ₀ of the input optical signals of the plurality of optical fibers is defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ^2. Greater than optical peak intensity (however, λ
Is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ
Is the pulse width of the optical signal). 11. The transmission line according to claim 8, wherein the pulse width of the optical signal output from each of the optical fibers is made smaller than the pulse width at the input, and the spectral width of the optical signal output from the optical fiber is input. The transmission path is characterized in that the amplification factor of each of the optical fibers is set so as to satisfy the following condition so as to increase the spectrum width more than the time. (I) Each amplification factor of the plurality of optical fibers propagates. Direction, and (II) the peak intensity P ₀ of the input optical signals of the plurality of optical fibers is defined by P ₀ = 0.776 (λ ³ A _eff / π ² cn ₂ ) D / τ ^2. Greater than optical peak intensity (however, λ
Is the optical signal wavelength, A _eff is the effective core area of the optical fiber, c is the speed of light in vacuum, n ₂ is the nonlinear constant of the optical fiber, D is the dispersion value at the input end of the optical fiber, and τ
Is the pulse width of the optical signal). 12. A distributed optical amplifier having a gain in the propagation direction, an optical fiber, an optical frequency shifter, and an optical filter are connected in this order to form a plurality of transmission circuits, and a predetermined optical pulse width is obtained. Thus, the amplification degree of each amplifier is set for each of the transmission circuits, and the optical spectrum of each output light of the plurality of optical frequency shifters is on the high frequency side with respect to each input optical spectrum. Transmission line. 13. An optical transmitter, and any one of claims 1 to 12 connected to the optical transmitter.
And a light receiving device connected to the transmission line.