JP2006014360A - Optical transmission system - Google Patents

Abstract

The present invention relates to an optical transmission system in which chromatic dispersion in an optical fiber transmission line is compensated, and enables long-distance transmission of wavelength multiplexed optical signals by compensating primary dispersion and secondary dispersion in the optical fiber transmission line.
[Configuration] An optical fiber transmission line 1 between a transmission unit 6 and a reception unit 7, an optical amplifier 5, and a dispersion compensator 2 are connected in cascade, and the dispersion compensator 2 is connected to the primary of the optical fiber transmission line 1. A first dispersion compensator 3 having first-order dispersion with opposite sign and dispersion, and a second-order dispersion having opposite sign and second-order dispersion of optical fiber transmission line 1, and optical fiber transmission line 1 and first dispersion compensator 3 and the second dispersion compensator 4 for compensating the first-order dispersion and the second-order dispersion. Each of the first dispersion compensator 3 and the second dispersion compensator 4 can be composed of an optical fiber.
[Selection] Figure 1

Description

  The present invention relates to an optical transmission system that enables long-distance transmission by performing dispersion compensation on an optical fiber transmission line.

  An erbium (Er) -doped optical fiber amplifier has been developed as an optical amplifier, and its application to an optical transmission system is being studied. Such an optical amplifier can be applied to a linear transmission repeater, a booster amplifier that increases the transmission output of the transmitter, or a preamplifier that improves the reception sensitivity of the receiver. In particular, when an optical amplifier is used as a linear repeater, an ultrahigh-speed electronic circuit required in a regenerative repeater is not required, so that the configuration is simple and small.

  However, in a relay transmission system in which a linear repeater using an optical amplifier is connected to an optical transmission line, the noise generated in each linear repeater and the nonlinear effect of the optical fiber constituting the optical transmission line accumulate. There is a problem that the transmission capacity and transmission distance are limited. That is, the maximum transmission speed allowed as a function of the maximum transmission distance, optical power level diagram, repeater spacing, etc. is determined. In a carefully designed optical transmission system, for example, it is known that a maximum transmission distance of about 10,000 km can be obtained at a transmission speed of 5 Gb / s. As described above, the application of the optical amplifier leads to an increase in optical power and an increase in the linear relay distance, but the noise generated in the optical amplifier, the chromatic dispersion of the optical fiber, and the accumulation of nonlinear effects become new technical issues. ing.

  Chromatic dispersion is a phenomenon in which the propagation speed of an optical pulse depends on the wavelength (frequency) of light. A light pulse modulated at high speed has a wide spectrum in the frequency domain. When such a light pulse propagates through an optical fiber, the short wavelength component and the long wavelength in the spectrum are affected by the chromatic dispersion of the optical fiber. The propagation speed differs from the component, and the waveform of the optical pulse changes. In order to reduce the influence of such chromatic dispersion, it is known that the optical signal wavelength may be set to a wavelength at which the dispersion value of the optical fiber becomes substantially zero.

  Currently, a large number of 1.3 μm band zero-dispersion single mode fibers (hereinafter abbreviated as SMF) are generally laid, and an optical communication system using a light source of 1.3 μm band has been put into practical use. The amplification band of the above-described erbium-doped optical fiber amplifier (hereinafter abbreviated as EDFA) is 1.5 μm, and an optical signal is transmitted by an optical transmission system combining SMF and EDFA with the signal wavelength as this wavelength band. In this case, the SMF has a large dispersion value of about +18 ps / nm / km in the 1.55 μm band. Therefore, when transmitting an optical signal through the SMF at a transmission speed of about several Gb / s or more, a technique for compensating for chromatic dispersion is required.

  In ultra-long distance optical transmission systems such as across the continent, a dispersion shifted fiber (hereinafter abbreviated as DSF) in which the zero dispersion wavelength is shifted to the 1.5 μm band of the EDFA amplified optical signal band is used. However, in such an ultra-long-distance optical transmission system, when the signal wavelength is set in the vicinity of the zero-dispersion wavelength, a nonlinear effect (four-waves) between spontaneous emission light noise generated from an optical amplifier such as an EDFA and signal light is generated. (Mixing) has a problem of deteriorating transmission quality.

  Such a problem can be avoided by managing the dispersion value in the longitudinal direction of the optical fiber. In other words, the degradation of the spontaneous emission optical noise near the signal light band due to the nonlinear effect of the optical fiber can be avoided by increasing the dispersion value of the optical fiber, and the change of the optical pulse waveform due to the dispersion is caused by the power of the optical pulse. If it is small, it is affected by linear accumulation.

  Therefore, as shown in FIG. 10, the optical transmission path between the optical signal transmission unit OS and the optical signal reception unit OR is constituted by a dispersion shifted fiber DCF and an erbium-doped optical fiber amplifier EDFA, and the cumulative dispersion of the DCF. A configuration is known in which SMFs having dispersion values of opposite signs to values are connected as dispersion compensators, and the average dispersion is zero as shown by a curve of dispersion values [ps / nm]. By providing such chromatic dispersion management means, it is possible to improve waveform degradation due to dispersion and waveform degradation due to four-wave mixing.

  The dispersion compensator interval in the ultra-long distance optical transmission system has an optimum interval. FIG. 11 is a diagram for explaining the relationship between the transmission distance and the dispersion compensator interval. The eye pattern penalty is 1 dB, the group delay compensation is 100%, the noise figure (NF) of the optical amplifier is 6 dB, and the dispersion value of the optical fiber transmission line is shown. As a parameter, the relationship between the transmission distance [km] and the distance between the dispersion compensators [km] is shown. The dispersion values of the optical fiber transmission line are −10.0 ps / nm / km, −1.0 ps / nm. / Km, -0.1 ps / nm / km. For example, if the dispersion value of the optical fiber transmission line is −1.0 ps / nm / km, the optimum dispersion compensator interval is about 500 km, and in this case, the transmission possible distance is about 9,000 km (for example, non-transmission distance) Patent Document 1).

  Even if the dispersion compensator interval is shorter or longer than the optimum interval, the transmission distance becomes shorter. This is because when the dispersion compensator interval is shortened, the frequency of returning to zero dispersion by the dispersion compensator increases, so that transmission degradation due to four-wave mixing between the spontaneous emission optical noise and the optical signal by the optical amplifier is remarkable. On the contrary, if the dispersion compensator interval is increased, the waveform deteriorates due to the interaction between the optical frequency change (frequency chirp) and dispersion at the rise and fall of the optical pulse due to the self-phase modulation effect of the optical signal. This is because the transmission quality deteriorates.

  As the dispersion compensator, (1) a configuration using a grating, (2) a configuration using an optical interferometer, (3) a configuration using an optical fiber, and the like have been proposed. Among these, the dispersion compensator using an optical fiber does not require a control circuit or the like, and can be passively operated, and the wavelength band used is extremely wide compared to dispersion compensators of other configurations. It is considered the most practical.

  FIG. 12 is an explanatory diagram of dispersion characteristics, where the horizontal axis is the wavelength [nm], the vertical axis is the dispersion value [ps / nm · km], a is SMF that becomes zero dispersion at a wavelength of 1300 nm, and b is the wavelength. A DSF obtained by performing zero dispersion shift at 1540 nm is shown (for example, see Non-Patent Document 2). In this case, both SMF and DSF have a dispersion value of about 0.08 ps / nm / km in the vicinity of the zero-dispersion wavelength. It becomes a problem.

For example, in the case of a transmission distance of about 9,000 km, the transmission distance is constituted by a DSF and an SMF as a dispersion compensator, and each dispersion slope (dispersion value of the dispersion value = second order dispersion value) is both 0.08 ps / nm ² / km, when the wavelength interval between the longest wavelength and the shortest wavelength is 5 nm, between the longest wavelength optical signal and the shortest wavelength optical signal,
5 [nm] × 0.08 [ps / nm ² / km] × 9000 [km]
= 3600 [ps / nm]
A difference in the chromatic dispersion will occur. That is, the accumulated dispersion value differs by 3600 ps / nm between the optical signals of the longest wavelength and the shortest wavelength.

  In FIG. 11 described above, when the dispersion of the optical fiber transmission line is −1 ps / nm / km, the optimum dispersion compensator interval is about 500 km in order to perform transmission of about 9,000 km. It is shown. That is, it shows that dispersion compensation is performed at a position where the cumulative dispersion value becomes −500 ps / nm.

  However, in wavelength multiplexing transmission, it is impossible to perform dispersion compensation for all optical signal wavelengths due to the influence of the dispersion slope of the optical fiber. For example, as described above, when dispersion compensation is optimized for the longest wavelength, even if dispersion compensation can be completely performed for the optical signal with the longest wavelength, the optical signal with the shortest wavelength can be compensated. Will cause cumulative dispersion of about 3600 ps / nm, and the transmission distance cannot be increased.

FIG. 13 is an explanatory diagram of dispersion compensation of a conventional example. In the optical transmission system having a transmission distance of 5,000 km, the horizontal axis is the distance [km] and the vertical axis is the dispersion value [ps / nm]. This shows a case where dispersion compensation is performed. The dispersion value (primary dispersion value) of the optical fiber transmission line by DSF is −2 ps / nm / km, and the dispersion slope (secondary dispersion value) of DSF is 0.07 ps / nm ² / In this example, the dispersion value (primary dispersion value) of the dispersion compensator using km and SMF is +18 ps / nm / km, and the dispersion slope (secondary dispersion value) is 0.07 ps / nm ² / km.

For the design wavelength (Δλ = 0 nm), as shown by a dotted line, the cumulative dispersion value of 900 km DSF (900 km × −2 ps / nm / km = −1800 ps / nm) is changed to the cumulative dispersion value of 100 km SMF (100 km). X18 ps / nm / km = 1800 ps / nm), and the dispersion value can be made zero every 1,000 km in total, but for a wavelength 5 nm apart (Δλ = −5 nm), As indicated by the solid line, a dispersion compensation error of −1750 ps / nm (−5 nm × 0.07 ps / nm ² / km × 5000 km−1750 ps / nm) occurs at the receiver end 5,000 km away due to the influence of the dispersion slope. I understand that.

  Therefore, in order to reduce the difference in the accumulated dispersion value between the optical signal wavelengths, it is conceivable to narrow the wavelength interval. However, when the wavelength interval is narrowed, four-wave mixing between the optical signals becomes remarkable. There arises a problem that transmission characteristics deteriorate.

In order to avoid such a problem, it has been proposed to perform dispersion compensation for each wavelength. For example, as shown in FIG. 14, the optical signal of the wavelength lambda ₁ to [lambda] ₅ from the signal source ₅₃ 1 to 53 _5, the multiplexer 54 via a dispersion compensator 52 consisting of the optical fiber ₅₂ 1 to 52 ₅ are multiplexed, transmitted by the optical fiber transmission line 51 as a wavelength-multiplexed optical signal is wavelength lambda ₁ to [lambda] ₅ demultiplexed by the demultiplexer 55, is detected by the photodetector 56 _{1-56 5} respectively. That is, at the transmission side of the optical signal, indicating the case where the dispersion compensation by the optical fiber ₅₂ 1 to 52 ₅ to a wavelength lambda ₁ to [lambda] ₅ corresponding (e.g., see Patent Document 1).

  Further, as shown in FIG. 15, the wavelength is demultiplexed by the arrayed waveguide type demultiplexer 61 and compensated so that the accumulated dispersion value becomes zero by the wavelength-dedicated arrayed waveguide type optical delay line 63. A configuration is known in which the light is multiplexed and transmitted by the waveguide multiplexer 62 (see, for example, Patent Document 2).

  FIG. 16 is an explanatory view of dispersion compensation by overcompensation in the conventional example, and shows a case where dispersion compensation is performed in advance as in the optical signal transmission side in FIG. In this case, as in FIG. 13, dispersion compensation is performed so that the cumulative dispersion value becomes zero every 1,000 km at the design wavelength (Δλ = 0 nm), but the wavelength (Δλ = 5 nm), dispersion compensation for 1750 ps / nm is given in advance by the transmitter. As a result, the cumulative dispersion value can be made zero at the design wavelength (Δλ = 0 nm) or at a wavelength 5 nm away (Δλ = 5 nm).

In the optical transmission system having the transmission distance of 9,000 km, as described above, 5 [nm] × 0.08 [ps / nm ² ] on the transmission side with respect to a wavelength 5 nm away from the design wavelength. / Km] × 9000 [km] = 3600 [ps / nm] in advance, the cumulative dispersion value can be made zero on the receiving side. When post-compensation on the receiving side is used in combination, and half-address compensation is performed on the transmitting side and the receiving side, dispersion of 1800 ps / nm is given on the transmitting side and 1800 ps / nm on the receiving side. It is better to give a variance of.
JP 62-18131 A Japanese Patent Laid-Open No. 5-346515 Saito, IEICE Technical Report OCS94-26, 1994 S. E. Miller, I.M. P. Kaminow "OPTICAL FIBER TELECOMMUNICATIONS II" Academic Press, 1988, p35

  In ultra-long distance optical transmission systems, the accumulation of chromatic dispersion and nonlinear effects in optical fibers is a problem, and as described above, dispersion compensation and the like have been proposed in the conventional example. For example, as described with reference to FIG. 14 or FIG. 16, when dispersion compensation is performed on the transmission side in advance, excessive dispersion is given, which causes a problem of transmitting an optical signal having a greatly distorted waveform. As described above, in an optical transmission system with a distance of about 9,000 km, it is necessary to perform dispersion compensation within a cumulative dispersion value of about −500 ps / nm, but this cannot be satisfied. Further, due to excessive dispersion compensation, an anomalous dispersion region (dispersion value is positive) is transmitted in the first half of the transmission line. These factors cause the transmission characteristics to deteriorate due to the influence of various nonlinear effects in the optical fiber. Therefore, in the ultra-long-distance wavelength division multiplexing transmission, there is a problem that the accumulated dispersion value for all wavelengths cannot be set to a predetermined value by dispersion compensation in one or both of the transmitter and the receiver. is there.

  In wavelength division multiplexing transmission, after wavelength separation in an optical repeater, dispersion compensation is performed for each wavelength. For example, in the means shown in FIG. 15, it is arranged at intervals of several tens to several hundreds km. Since the second-order dispersion can be compensated in the optical amplifying repeater, it is possible to compensate so that the accumulated dispersion value is within −500 ps / nm for all wavelengths. However, demultiplexing and multiplexing of wavelength-division multiplexed optical signals multiplexed at a narrow interval requires a precise optical circuit using an optical waveguide or the like, which causes a very expensive configuration and is not practical.

  It is an object of the present invention to compensate for primary dispersion and secondary dispersion of an optical fiber transmission line with a simple configuration and to enable long-distance transmission of an optical signal.

  The optical transmission system of the present invention includes an optical fiber transmission line, an optical amplifier, a first dispersion compensator having a first order dispersion opposite to the first order dispersion of the optical fiber transmission line, and the optical fiber transmission line. A configuration in which a second-order dispersion having a second-order dispersion and a second-order dispersion having an opposite sign, and a first dispersion compensator and a second dispersion compensator for compensating for the second-order dispersion of the first dispersion compensator are cascade-connected. It has.

Also, residual dispersion values D1 and D2 set in advance for different wavelengths λ1 and λ2 in or near the signal wavelength band of the optical fiber transmission line, cumulative dispersion values of the first and second dispersion compensators, and 2 For the second variance value,
(Transmission path cumulative dispersion value at design wavelength) + (Cumulative dispersion value of first dispersion compensator at design wavelength) + (Cumulative dispersion value of second dispersion compensator at design wavelength) ≈D1
(Secondary dispersion value of transmission path × Transmission path length) + (Secondary dispersion value of first dispersion compensator) + (Secondary dispersion value of second dispersion compensator) ≈D2 / (λ1-λ2)
The cumulative dispersion value and the secondary dispersion value of the first and second dispersion compensators are set so as to satisfy the above condition.

Further, the first and second dispersion compensators are first and second dispersion compensating fibers configured by optical fibers, and the lengths of the first and second dispersion compensating fibers are L _1stDCF , L _2ndDCF , and the light The length of the fiber transmission line is L _Trans , the second-order dispersion value of the optical fiber transmission line is S _Trans , the second-order dispersion value of the first dispersion-compensating fiber is S _1stDCF , and the second-order dispersion value of the second dispersion-compensating fiber S _2ndDCF , the first-order dispersion value of the optical fiber transmission line is D _Trans , the first-order dispersion values of the first and second dispersion-compensating fibers are D _1stDCF and D _2ndDCF , and the residual dispersion value at the wavelength λ 1 is D1, The residual dispersion value at wavelength λ2 is D2, the wavelength band of the difference between wavelength λ1 and wavelength λ2 is B _WDM , and the total length is L _TOTAL .
L _Trans + L _{1st DCF} + L _{2nd DCF} = L _{TOTAL
(D Trans × L Trans) +} (D 1stDCF × L 1stDCF) + (D 2ndDCF × L 2ndDCF) ≒ D1
B _WDM [(S _Trans × L _Trans ) + (S _1stDCF × L _1stDCF ) + (S _2ndDCF × L _2ndDCF )] _≈D2
The lengths L _1stDCF and L _2ndDCF of the first and second dispersion compensating fibers, the primary dispersion values D _1stDCF and D _2ndDCF, and the secondary dispersion values D _1stDCF and D _2ndDCF are set so as to _satisfy the following condition: It is to set.

  The optical fiber transmission line can be constituted by a dispersion-shifted fiber having a negative sign of dispersion value within the signal wavelength band.

  The optical fiber transmission line having a negative sign of the dispersion value, and the accumulated dispersion value of the optical signal multiplexed optical signals and the first and second dispersion compensators multiplexed with a plurality of wavelengths are positive signs. The length and the dispersion value can be set so as not to become.

  By compensating the primary dispersion and the secondary dispersion generated in the optical fiber transmission line 1 by the dispersion compensator 2, it is possible to compensate for the chromatic dispersion over a wide band and enable high-speed and large-capacity optical communication. There are advantages. In addition, the dispersion compensator 2 uses a first dispersion compensator 3 that mainly compensates for the first-order dispersion of the optical fiber transmission line, and a second dispersion compensator 4 that mainly compensates for the second-order dispersion of the optical fiber transmission line 1. Thus, there is an advantage that the first and second dispersion compensators 3 and 4 can be configured by an optical fiber that is relatively easy to manufacture.

  Referring to FIG. 1, the optical transmission system of the present invention is an optical fiber transmission line 1, an optical amplifier 5, and a first dispersion compensator having a first order dispersion opposite in sign to the first order dispersion of the optical fiber transmission line 1. 3 and a second-order dispersion having the opposite sign to the second-order dispersion of the optical fiber transmission line 1 and a second dispersion compensating the first-order dispersion and the second-order dispersion of the optical fiber transmission line 1 and the first dispersion compensator 3. The dispersion compensator 4 is connected in cascade.

  FIG. 1 is an explanatory diagram of Embodiment 1 of the present invention, where 1 is an optical fiber transmission line, 2 is a dispersion compensator, 3 is a first dispersion compensator, 4 is a second dispersion compensator, 5 is an optical amplifier, A transmitting unit 7 is a receiving unit. As described above, the optical amplifier 5 can be an erbium-doped optical fiber amplifier (EDFA). The optical fiber transmission line 1 can be a DSF (dispersion shifted fiber), for example, and the first and second dispersion compensators 3 and 4 can be SMF (zero dispersion single mode fiber).

  In an optical transmission system having a transmission distance of, for example, about 9,000 km, when DSF is used as an optical fiber transmission line 1, transmission characteristics by four-wave mixing of signal light and spontaneously emitted optical noise generated by the optical amplifier 5 In order to suppress the deterioration of the image, it is desirable to transmit in the normal dispersion region (dispersion value is negative). This is also shown in Non-Patent Document 1 described above. In particular, when performing wavelength division multiplexing, it is necessary to set the dispersion value to a large value of about −1 to −2 ps / nm / km in order to avoid the influence of four-wave mixing between signals. In this regard, when DSF is used as the optical fiber transmission line 1, the dispersion value is set to −1 to −2 ps in order to avoid the influence of four-wave mixing between the signal light and the spontaneous emission light noise generated in the optical amplifier 5. It has been proposed that it is necessary to set a value of about / nm / km (see, for example, RW Tkach et al. ECOC'94 PD, p45-49, 1994).

  Therefore, in order to compensate the dispersion of the optical fiber transmission line 1 using the DSF, the dispersion compensator 2 having a positive sign of dispersion and a large absolute value is required. When the dispersion compensator 2 is constituted by an optical fiber, SMF can be used. This SMF has a low loss and is relatively inexpensive, and has a primary dispersion value of about +18 ps / nm / km in the 1.55 μm band. Accordingly, if the primary dispersion value of the optical fiber transmission line 1 is set to -2 ps / nm / km, the SMF has a dispersion value about 10 times this, so a length of about 1/10 of the transmission distance is used as a dispersion compensator. The first-order dispersion value of the optical fiber transmission line 1 can be compensated by adding or laying it as a part of the transmission line. In this case, as described above, in the case of a transmission distance of about 9,000 km, about 1,000 km may be replaced with SMF, and the remaining about 8,000 km may be used as the optical fiber transmission line 1.

  If an optical fiber having a positive dispersion value sign and a large absolute value is used, the length of the dispersion compensating fiber can be shortened in principle, but the dispersion characteristics and waveguide structure of quartz, which is the material of the optical fiber, can be reduced. Considering the resulting dispersion characteristics, there is a limit to increasing the absolute value of the dispersion value. Further, when the dispersion value is increased, there is a problem that propagation is not performed in a single mode.

In addition, the DSF as the optical fiber transmission line 1 and the SMF as the dispersion compensator 2 usually use a second-order dispersion value (hereinafter, the dispersion slope and the second-order dispersion value have the same meaning, and are simply referred to as dispersion values). The case shows a primary dispersion value) having a value of about 0.06 to 0.08 ps / nm ² / km. Therefore, the dispersion compensator for the second-order dispersion compensation needs to have a characteristic in which the sign is opposite to that of the second-order dispersion of the DSF as the optical fiber transmission line 1 and has a large absolute value. In this case, the sign of the primary dispersion value is preferably a positive sign in view of assisting the primary dispersion compensation.

As described above, when the DSF is the optical fiber transmission line 1 and the first-order dispersion and the second-order dispersion are compensated 100% when the transmission distance is 9,000 km, the first and second dispersion compensations are used. By the first dispersion compensating fiber, the second dispersion compensating fiber, and the dispersion value and dispersion slope constituting the devices 3 and 4,
(DSF transmission path length + SMF length + second dispersion compensating fiber length) = 9000 km
... (1)
(DSF dispersion value × DSF transmission line length) + (SMF dispersion value × SMF length) + (dispersion value of second-order dispersion compensation fiber × second dispersion compensation fiber length) = 0 (2)
(DSF dispersion slope × DSF transmission path length) + (SMF dispersion slope × SMF length) + (dispersion slope of second dispersion compensation fiber × second dispersion compensation fiber length)
= 0 (3)
It satisfies the following conditions.

  The expression (1) indicates a condition for the transmission line length, the expression (2) indicates a condition for primary dispersion compensation, and the expression (3) indicates a condition for secondary dispersion compensation. Among these, since the dispersion value and dispersion slope of the DSF and the dispersion slope of the SMF are known, when the dispersion value and dispersion slope of the second dispersion compensating fiber are determined, DSF transmission is performed from the equations (1) to (3). The path length, the first dispersion compensating fiber length, that is, the SMF transmission path length, and the second dispersion compensating fiber length can be obtained.

A general formula when the dispersion compensator 2 is formed of an optical fiber is expressed as follows.
L _Trans + L _{1st DCF} + L _{2nd DCF} = L _TOTAL (4)
_{(D Trans × L Trans) +} (D 1stDCF × L 1stDCF) + (D 2ndDCF × L 2ndDCF) = D1 ... (5)
B _WDM [(S _Trans × L _Trans ) + (S _1stDCF × L _1stDCF ) + (S _2ndDCF × L _2ndDCF )] = D2 (6)

However, the lengths of the first and second dispersion compensating fibers are L _1stDCF and L _2ndDCF , the length of the optical fiber transmission line 1 is L _Trans , the secondary dispersion value of the optical fiber transmission line 1 is S _Trans , and the first The second-order dispersion value of the dispersion-compensating fiber is S _1stDCF , the second-order dispersion value of the second dispersion-compensating fiber is S _2ndDCF , the first-order dispersion value of the optical fiber transmission line 1 is D _Trans , and the first and second dispersion-compensating fibers are The primary dispersion values are D _1stDCF , D _2ndDCF , the residual dispersion value at wavelength λ1 is D1, the residual dispersion value at wavelength λ2 is D2, the wavelength band of the difference between wavelength λ1 and wavelength λ2 is B _WDM , and the total length is L _TOTAL Shows the case.

When the design wavelength is λ1, the residual dispersion value D2 at the wavelength λ2 separated by the wavelength band B _WDM and the residual dispersion value D1 at the design wavelength λ1 are set to zero, and the above-described equations (4) to (6) By solving the simultaneous equations consisting of the following equation, the transmission path length L _Trans , the first dispersion compensating fiber length L _1stDCF , and the second dispersion compensating fiber length L _2ndDCF can be obtained. However, the residual dispersion value D1, depending on the size of the pre-chirp (phase modulation component generated simultaneously with the intensity modulation) generated in the modulator in the transmission unit 6, the size of the waveform chirp due to the nonlinear effect, the band of the reception unit 7, etc. In some cases, D2 is preferably not zero. In that case, depending on these parameters, the aforementioned D1 and D2 are selected to be optimum values.

The dispersion compensator 2 can be applied to various configurations in addition to the configuration using an optical fiber, and generalized equations in this case are shown below.
(Cumulative dispersion value of transmission line at design wavelength) + (Cumulative dispersion value of first dispersion compensator at design wavelength)
+ (Cumulative dispersion value of second dispersion compensator at design wavelength) ≈D1 (residual dispersion value of wavelength λ1) (7)
(Secondary dispersion value of transmission path × Transmission path length) + (Secondary dispersion value of first dispersion compensator)
+ (Secondary dispersion value of the second dispersion compensator)
≈D2 (residual dispersion value of wavelength λ2) / (λ1-λ2) (8)

  Wavelengths λ1 and λ2 are wavelengths within or near the band of the wavelength multiplexed signal, desired residual dispersion values D1 and D2 for the two wavelengths λ1 and λ2, the accumulated dispersion value of the transmission path, and the dispersion slope (secondary dispersion). Value) and the dispersion slopes of the first and second dispersion compensators 3 and 4 are assumed, the accumulated dispersion values necessary for the first and second dispersion compensators 3 and 4 can be obtained. Then, the accumulated dispersion value is distributed at every appropriate relay interval, and is compensated for each transmission distance at which the optimum accumulated dispersion value is obtained.

FIG. 2 is an explanatory diagram of dispersion characteristics of an embodiment of the present invention. The optical fiber transmission line 1 is composed of a dispersion shifted fiber (DSF), the first dispersion compensator 3 is composed of a single mode fiber (SMF), 2 When the dispersion compensator 4 is constituted by a dispersion compensation fiber (DCF), the dispersion shift fiber (DSF) length, the single mode fiber (SMF) length, and the dispersion compensation fiber (DCF) length are plotted on the vertical axis [km]. In addition, the secondary dispersion value [ps / nm ² / km] of the dispersion compensating fiber (DCF) is shown on the horizontal axis.

Also, DSF dispersion value D = -2 ps / nm / km, dispersion slope = 0.08 ps / nm ² / km, SMF dispersion value D = + 18 ps / nm / km, dispersion slope = 0.08 ps / nm ² / km Further, when the DCF dispersion value D = + 1 ps / nm / km, when the DCF dispersion slope (secondary dispersion value) is about −0.5 ps / nm ² / km, the DSF length is 7,045 km, By setting the SMF length to 715 km and the DCF length to 1,240 km, the primary dispersion and the secondary dispersion can be compensated. Further, as described above, the dispersion compensation described above may be performed by dividing the accumulated dispersion value every several hundred km so that the accumulated dispersion value is, for example, about −500 ps / nm.

FIG. 3 is an explanatory diagram of dispersion compensation according to an embodiment of the present invention, and shows a case where primary dispersion and secondary dispersion are compensated at intervals of 1,000 km in an optical transmission system having a transmission distance of 5,000 km. The DSF length of one section is 795.6 km, the first-order dispersion value is -2 ps / nm / km, the second-order dispersion value is +0.07 ps / nm ² / km, and the first-order dispersion compensation fiber (first dispersion compensator 3) The length of the SMF is 81.6 km, the first dispersion value is +18 ps / nm / km, the second dispersion value is +0.07 ps / nm ² / km, and the second-order dispersion compensation fiber (second dispersion compensator 4) The length was 122.8 km, the primary dispersion value was +1.0 ps / ns / km, and the secondary dispersion value was −0.5 ps / nm ² / km.

  At the design wavelength (Δλ = 0 nm), as shown by the dotted line, dispersion compensation is performed accurately, and the wavelength (Δλ = 5 nm) and the design distance from the design wavelength (Δλ = 0 nm) to the shorter wavelength side by 5 nm. Even at a wavelength (Δλ = −5 nm) that is 5 nm longer than the wavelength (Δλ = 0 nm), dispersion compensation was accurately performed every 1,000 km, as indicated by the solid line. That is, dispersion compensation can be performed over a wide wavelength band by simultaneously compensating the primary dispersion and the secondary dispersion.

  In addition, the transmission characteristics may deteriorate due to four-wave mixing between the signal light and the spontaneous emission light noise generated by the optical amplifier. In order to reduce the deterioration of the transmission characteristics, the transmission path for all signal wavelengths is used. It is desirable to set the accumulated dispersion values of the first and first and second dispersion compensators 3 and 4 to be negative. In this case, it is desirable to use on the short wavelength side from the design wavelength (Δλ = 0 nm).

FIG. 4 is an explanatory diagram of a wavelength division multiplexing optical transmission system according to an embodiment of the present invention, showing a case where it is applied to land transmission, 11 _{1 to} 11 _n are electro-optic converters (E / O), 12 is a multiplexer, 13 is a dispersion shifted fiber (DSF) constituting an optical fiber transmission line, 14 is an optical repeater, 15 is a preamplifier, 16 is an optical amplifier such as an erbium-doped optical fiber amplifier, and 17 is a single mode constituting a first dispersion compensator. A fiber (SMF), 18 is a dispersion compensating fiber (DCF) constituting the second dispersion compensator, 19 is a multiplexer, and 20 _{1 to} 20 _n are photoelectric converters (O / E).

  The interval between the optical repeaters 14 is assumed to be about 80 km, for example, and the first dispersion value of the DSF constituting the optical fiber transmission line 13 is set to −2 ps / nm / km, and the first and second dispersion compensators are configured. The case where the SMF 17 and the DCF 18 are built in the optical repeater 14 and the preamplifier 15 together with the optical amplifier 16 is shown. Therefore, in the case of this configuration, the first and second dispersion compensators can be provided in the optical repeater 14 of the existing optical fiber transmission line. In the optical repeater 14 or preamplifier 15, it is amplified by the preceding optical amplifier 16 and input to the SMF 17 and DCF 18 to perform dispersion compensation, and is amplified by the subsequent optical amplifier 16 to become the DSF of the optical fiber transmission line 13. Stable dispersion compensation can be performed by transmitting an optical signal.

FIG. 5 is a diagram for explaining the relationship between the secondary dispersion value and the length in the wavelength division multiplexing optical transmission system according to the embodiment of the present invention. As shown in FIG. As described above, the dispersion value is −2 ps / nm / km, the second-order dispersion value (dispersion slope) is 0.08 ps / nm ² / km, and 1 of the SMF 17 as the first dispersion compensator 3 (see FIG. 1). The first order dispersion value is +18 ps / nm / km, the second order dispersion value (dispersion slope) is 0.08 ps / nm ² / km, and the first order dispersion value of the DCF 18 as the second dispersion compensator 4 (see FIG. 1) is +1 ps. / Nm / km is shown, and when the secondary dispersion value (dispersion slope) of the DCF 18 as the second dispersion compensator 4 is about −0.5 ps / nm ² / km, the SMF length is 8.1 km. The DCF length is about 14 km Ri can together compensate primary dispersion and second-order dispersion and the.

FIG. 6 is an explanatory diagram of the wavelength dependence of the dispersion value. As described above, the length L of the DSF as the optical fiber transmission line 13 is 80 km, and the length L of the SMF 17 as the first dispersion compensator is L = 8. 1 km, the length L of the DCF 18 as the second dispersion compensator L = 14.1 km, the dispersion slope (secondary dispersion value) of the DSF 13 and the SMF 17 is 0.08 ps / nm ² / km of the same code, and the DSF 13 When the SMF 17 has a characteristic indicated by a solid line, when combined, the dispersion value can be made zero at one point of wavelength, for example, near 1563 nm, as indicated by the dotted line of DSF + SMF.

However, at other wavelengths, the dispersion value cannot be compensated to be zero. Therefore, the dispersion slope (secondary dispersion value) of the DCF 18 as the second dispersion compensator is −0.5 ps / characteristic, for example, indicated by a dotted line of the dispersion slope (secondary dispersion value) opposite to the sign of the DSF 13 and the SMF 17. It is set to nm ² / km. As a result, the dispersion value can be made zero over the entire wavelength band, as indicated by the one-dot chain line of DSF + SMF + DCF.

  In each of the above-described embodiments, the explanation is made on the assumption that the dispersion of the optical fiber transmission line is compensated 100%. However, the number of wavelengths in wavelength multiplexing, the wavelength chirp in the transmitter, The optimum dispersion compensation amount may vary depending on the wavelength chirp due to the non-linear effect, the characteristics of the receiver, etc. (for example, G. Ishikawa et al. “10-Gb / s Repeaterless Transmission ...” IEICE TRANS. ELECTRON, Vol. E78-C, No. 1, Jan.). That is, it is necessary to select the residual dispersion values D1 and D2 in the above formulas (5) to (8) as optimum values.

  Further, like the above-mentioned DCF, it is desirable that the secondary dispersion compensating fiber has a secondary dispersion value having a large absolute value with a sign opposite to that of the secondary dispersion of SMF or DSF in the 1550 nm band, for example. In general, the total dispersion of an optical fiber propagating in a single mode is the sum of material dispersion and structural dispersion. Of these, the material dispersion is almost determined by quartz, which is the material of the optical fiber, and controls the refractive index. It is hardly affected by the type and concentration of the dopant. On the other hand, structural dispersion can be controlled to some extent by changing the refractive index distribution of the optical fiber.

  Therefore, as the second dispersion compensator 4 (see FIG. 1), for example, an optical fiber called a W-type or a quadruple-clad type can be applied (for an optical fiber having a W-type or a quadruple-clad type structure). For example, see BJ Ainslie et al., “A Review of Single-Mode Fibers with Modified Dispersion Characteristics”, JOLT., Vol. LT-4, No. 8, p967-979, 1986).

FIG. 7 is an explanatory diagram of dispersion characteristics of the W-type structure. The refractive index differences Δn ₁ (0.006) and Δn ₂ (−0.008) with respect to the reference value are constant, and the ratio of the radii a1 and a2 is used as a parameter. The relationship between the wavelength [μm] and the dispersion value [ps / nm / km] is shown. For example, when a2 / a1 = 1, the dispersion value becomes zero near 1.35 μm (1350 nm), and the dispersion slope (secondary dispersion value) becomes a small negative value. If a2 / a1 = 1.91, an optical fiber having a positive value with a primary dispersion value of approximately zero and a large dispersion slope (secondary dispersion value) at 1550 nm can be obtained. The dispersion slope (secondary dispersion value) at this time is about −0.5 ps / nm ² / km.

FIG. 8 is an explanatory diagram of the dispersion characteristics of the quadruple clad structure. With respect to the refractive index differences Δn _{1 to} Δn _{4 with} respect to the reference values of the regions having the radii a1 to a4, the dispersion with respect to the wavelength [μm] is used with the core radius a1 as a parameter. The value [ps / nm / km] is shown. In this case, when the radius a1 of the core is changed, the dispersion value becomes zero in the 1.3 μm band, and on the longer wavelength side, the wavelength at which the dispersion value becomes zero becomes longer as the radius a1 is increased. . The dispersion slope (secondary dispersion value) has a sign opposite to that of the DSF or SMF described above. In this case, the dispersion slope (secondary dispersion value) in the 1.5 μm band was about −0.25 ps / nm ² / km.

  As described above, a W-type structure or quadruple that has a primary dispersion value that compensates for the accumulated dispersion value of, for example, DSF as an optical fiber transmission line, has a large dispersion slope (secondary dispersion value), and has an opposite sign. By using a dispersion compensation fiber or the like having a clad structure as a dispersion compensator, it is possible to compensate for the primary dispersion and the secondary dispersion of the optical fiber transmission line, and to enable wavelength division multiplexing transmission over a very long distance. In this case, as described above, the primary dispersion value, the secondary dispersion value, and the length of the dispersion compensating fiber are selected so as to optimize the residual dispersion values D1 and D2.

  The above-mentioned W-type optical fiber and quadruple-clad type optical fiber have been developed in order to make the dispersion value almost zero over a wide band from 1300 nm to 1550 nm, for example. Due to the large value of about 0.3 to 0.4 dB / km and the dispersion value changing sensitively due to the variation of the structural parameters, the dispersion value may vary greatly even when manufactured under the same conditions.

  Therefore, as described above, the first-order dispersion value and the second-order dispersion value of the optical fiber transmission line 1 are mainly compensated for the first-order dispersion value by the first dispersion compensator 3, and the second dispersion compensator 4 is mainly used. By compensating the secondary dispersion value, dispersion compensation with excellent reproducibility is made possible. In that case, the length of the dispersion compensation fiber can be relatively shortened by using a dispersion compensation fiber having a large first-order dispersion value and a second-order dispersion value, so that the increase in loss can be negligible. . Further, it is important that the optical fiber for secondary dispersion compensation has a large dispersion slope (secondary dispersion value), and the absolute value of the primary dispersion value is not so important. Accordingly, a dispersion compensating fiber having desired characteristics can be easily realized by designing and manufacturing separately from the optical fiber for primary dispersion compensation.

FIG. 9 is a diagram for explaining the relationship between the dispersion value and the length of the embodiment of the present invention. In the transmission line having the total length L _TOTAL = 9000 km, the primary dispersion value D = −2 ps / nm / as an optical fiber transmission line. km, dispersion slope (secondary dispersion value) = 0.07 ps / nm ² / km DSF as first dispersion compensator, first dispersion value D = + 18 ps / nm / km, dispersion slope (secondary dispersion) Value) = 0.07 ps / nm ² / km SMG is used, and as the second dispersion compensator, dispersion compensation (secondary dispersion value) = − 0.5 ps / nm ² / km DCF is used for dispersion compensation. In the case, the result of calculating the lengths of the SMF and the DCF as a function of the primary dispersion value of the DCF is shown.

  It can be seen that the dispersion value of the entire transmission line is determined by the DSF and SMF having a large total dispersion amount, and hardly affects the absolute value of the dispersion value of the DCF for secondary dispersion compensation. Even if the dispersion value of the DCF for secondary dispersion compensation varies greatly in manufacturing, the dispersion value can be easily adjusted to a desired value by adjusting the dispersion value of the SMF for primary dispersion compensation. it can.

  The present invention is not limited to the above-described embodiments. For example, a time-division multiplex transmission system in which the second-order dispersion of the optical fiber transmission line limits the transmission capacity or transmission distance, or phase conjugation is used. The present invention can also be applied to methods.

It is explanatory drawing of the Example of this invention. It is dispersion characteristic explanatory drawing of the Example of this invention. It is dispersion compensation explanatory drawing of the Example of this invention. It is explanatory drawing of the wavelength division multiplexing optical transmission system of the Example of this invention. FIG. 4 is a diagram illustrating the relationship between the secondary dispersion value and the length in the wavelength division multiplexing optical transmission system according to the embodiment of the present invention. It is explanatory drawing of the wavelength dependence of a dispersion value. It is a dispersion | distribution characteristic explanatory drawing of a W-type structure. It is dispersion characteristic explanatory drawing of a quadruple clad type structure. It is explanatory drawing of the relationship between the dispersion value and length of the Example of this invention. It is explanatory drawing of the wavelength dispersion management of a prior art example. It is a relationship explanatory drawing of transmission distance and a dispersion compensator space | interval. It is dispersion characteristic explanatory drawing. It is dispersion compensation explanatory drawing of a prior art example. It is explanatory drawing of the dispersion compensation means of a prior art example. It is explanatory drawing of the dispersion compensation means of a prior art example. It is dispersion | distribution compensation explanatory drawing by the excess compensation of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber transmission line 2 Dispersion compensator 3 1st dispersion compensator 4 2nd dispersion compensator 5 Optical amplifier 6 Transmitter 7 Receiver

Claims (5)

  An optical fiber transmission line; an optical amplifier; a first dispersion compensator having a first order dispersion of the opposite sign to the first order dispersion of the optical fiber transmission line; And a second dispersion compensator for compensating for the first-order dispersion and the second-order dispersion of the optical fiber transmission line and the first dispersion compensator. Optical transmission system. Residual dispersion values D1 and D2 set in advance for different wavelengths λ1 and λ2 in or near the signal wavelength band of the optical fiber transmission line, cumulative dispersion values and second order of the first and second dispersion compensators About the variance value
(Transmission path cumulative dispersion value at design wavelength) + (Cumulative dispersion value of first dispersion compensator at design wavelength) + (Cumulative dispersion value of second dispersion compensator at design wavelength) ≈D1
(Secondary dispersion value of transmission path × Transmission path length) + (Secondary dispersion value of first dispersion compensator) + (Secondary dispersion value of second dispersion compensator) ≈D2 / (λ1-λ2)
2. The optical transmission system according to claim 1, wherein the cumulative dispersion value and the second-order dispersion value of the first and second dispersion compensators are set so as to satisfy the following condition. The first and second dispersion compensators are first and second dispersion compensating fibers configured by optical fibers, and the lengths of the first and second dispersion compensating fibers are L _1stDCF , L _2ndDCF , and the light The length of the fiber transmission line is L _Trans , the second-order dispersion value of the optical fiber transmission line is S _Trans , the second-order dispersion value of the first dispersion-compensating fiber is S _1stDCF , and the second-order dispersion value of the second dispersion-compensating fiber S _2ndDCF , the first-order dispersion value of the optical fiber transmission line is D _Trans , the first-order dispersion values of the first and second dispersion-compensating fibers are D _1stDCF and D _2ndDCF , and the residual dispersion value at the wavelength λ 1 is D1, The residual dispersion value at wavelength λ2 is D2, the wavelength band of the difference between wavelength λ1 and wavelength λ2 is B _WDM , and the total length is L _TOTAL .
L _Trans + L _{1st DCF} + L _{2nd DCF} = L _{TOTAL
(D Trans × L Trans) +} (D 1stDCF × L 1stDCF) + (D 2ndDCF × L 2ndDCF) ≒ D1
B _WDM [(S _Trans × L _Trans ) + (S _1stDCF × L _1stDCF ) + (S _2ndDCF × L _2ndDCF )] _≈D2
The lengths L _1stDCF and L _2ndDCF of the first and second dispersion compensating fibers, the primary dispersion values D _1stDCF and D _2ndDCF, and the secondary dispersion values D _1stDCF and D _2ndDCF are set so as to _satisfy the following condition: The optical transmission system according to claim 1, wherein the optical transmission system is set.   4. The optical transmission system according to claim 1, wherein the optical fiber transmission line is configured by a dispersion shifted fiber having a negative sign of a dispersion value within a signal wavelength band. 5.   The accumulated dispersion value of the optical fiber transmission line having a negative dispersion value sign and the optical fiber transmission line of the optical signal multiplexed with a plurality of wavelengths and the first and second dispersion compensators is a positive sign. The optical transmission system according to any one of claims 1 to 4, wherein a length and a dispersion value are set so as not to occur.

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