JP2024058536A - Forward Raman amplifier, bidirectional Raman amplification system, and forward Raman amplification system - Google Patents

Abstract

[Problem] To suppress XPM degradation of signal light in accordance with the type of fiber of an optical transmission line. [Solution] An optical transmission device 110 includes a forward pumping unit 140 and a forward pumping control unit 150. The forward pumping unit 140 has a plurality of pumping light sources with different wavelengths. The optical transmission line 120 uses various types of fiber such as SMF and DSF. The forward pumping control unit 150 changes the number of pumping light sources to be emitted by the forward pumping unit 140 according to the type of fiber. For example, when the type of fiber is DSF, the forward pumping control unit 150 prevents signal degradation of the signal light S by extinguishing the power of the pumping light source corresponding to a primary pumping light PF1x located on the long wavelength side of a portion of the primary pumping light PF1x that causes XPM degradation for the signal light S among the primary pumping light PF1 of the plurality of wavelengths of the forward pumping unit 140. [Selected Figure] FIG. 1A

Description

The present invention relates to a forward Raman amplifier, a bidirectional Raman amplification system, and a forward Raman amplification system.

Optical transmission using wavelength division multiplexing (WDM) allows the transmission of large-capacity signals, and Raman amplification allows the optical signal to be optically amplified and transmitted over long distances. For example, single mode optical fiber (SMF) or dispersion shifted fiber (DSF) is used for the optical transmission path.

Prior art includes, for example, a technique for stably Raman amplifying signal light in a bidirectionally pumped transmission line by optimizing the power of forward pumping light and backward pumping light based on the measurement results of the OSNR of the Raman-amplified signal light at the start of the system. There is also a technique for forward pumping with wavelength λp0, backward pumping with N channels of wavelengths λp1 to λpN, and controlling the forward pumping light so that the output power level of each signal channel becomes a predetermined value, thereby improving the transient response characteristics without depending on the increase or decrease of signal channels (see, for example, Patent Documents 1 and 2 below). Also disclosed is a technique for pumped Raman amplification using non-coherent pumping light (see, for example, Non-Patent Document 1 below).

JP 2005-303070 A JP 2004-258622 A

"Co-Propagating Dual-Order Distributed Raman Amplifier Utilizing Incoherent Pump", Morimoto et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, No. 7, APRIL 1, 2017

The inventors discovered through actual measurements and simulations that while forward pumping Raman is effective in improving signal quality in SMF, for example, when transmitting in the L band using DSF, signal quality deteriorates. For example, in DSF, the WDM signal and forward pumping light are arranged on either side of the fiber's zero dispersion wavelength (around 1550 nm). In this case, a portion of the signal light that has an equal delay to the forward pumping light is affected by cross phase modulation (XPM), degrading signal quality (details will be described later).

In one aspect, the present invention aims to suppress XPM degradation of signal light by adapting it to the fiber type of the optical transmission path.

According to one aspect of the present invention, in a forward Raman amplifier having multiple pump light sources with different wavelengths, the number of pump light sources that emit light must be changed depending on the type of fiber.

According to one aspect of the present invention, it is possible to suppress XPM degradation of signal light in accordance with the fiber type of the optical transmission path.

FIG. 1A is a diagram illustrating an overview of a forward Raman amplifier according to an embodiment. FIG. 1B is an explanatory diagram illustrating an overview of a forward Raman amplifier according to an embodiment. FIG. 1C is an explanatory diagram illustrating an overview of a forward Raman amplifier according to an embodiment. FIG. 1D is an explanatory diagram illustrating an overview of a forward Raman amplifier according to an embodiment. FIG. 1E is an explanatory diagram illustrating an overview of a bidirectional Raman amplification system according to an embodiment. FIG. 1F is a flowchart illustrating an example of pump light control according to the embodiment. FIG. 2 is a graph showing the power profile due to co-propagating Raman amplification. FIG. 3 is a diagram showing the wavelength relationship between the signal light and the pump light. FIG. 4 is a diagram showing Raman amplification by non-coherent pump light. FIG. 5 is a chart showing chromatic dispersion characteristics for each type of fiber. FIG. 6 is a diagram illustrating the problems that arise in co-propagating Raman amplification for DSF. FIG. 7A is a graph showing the degradation of the Q factor and Raman gain due to XPM. (SMF) FIG. 7B is a graph showing the degradation of the Q factor and Raman gain due to XPM. (DSF) 7C is a graph showing the degradation of the Q factor and the Raman gain due to XPM. FIG. 8 is a diagram illustrating a first configuration example of an optical transmission system according to an embodiment. FIG. 9 is a diagram showing an example of an information table. FIG. 10 is a diagram showing an example of the pumping light power ratio table. FIG. 11 illustrates an example of a hardware configuration of a control unit of an optical transmission device. FIG. 12 is a flowchart showing an example of pump light control according to the first configuration example. FIG. 13 is a diagram illustrating a second configuration example of an optical transmission system according to an embodiment. FIG. 14 is a flowchart showing an example of pump light control according to the second configuration example. FIG. 15 is a diagram illustrating a third configuration example of an optical transmission system according to an embodiment. As illustrated in FIG. FIG. 16 is a diagram for explaining extraction of signal light and pump light degraded by XPM. FIG. 17 is a diagram showing an example of an information table according to the third configuration example. FIG. 18A is a diagram showing a transmitting side pumping light power ratio table according to the configuration example 3. (Part 1) FIG. 18B is a diagram showing a transmitting side pumping light power ratio table according to the configuration example 3. (Part 2) FIG. 18C is a diagram showing a transmitting side pumping light power ratio table according to the configuration example 3. FIG. 19 is a diagram showing a receiving side pumping light power ratio table according to the third configuration example. FIG. 20A is a flowchart showing an example of pump light control according to configuration example 3. FIG. 20B is a flowchart showing an example of pump light control according to configuration example 3. FIG. 21 is a diagram showing the chromatic dispersion characteristics of the TWRS and the ELEAF. FIG. 22A is a diagram illustrating the problems that arise in co-propagating Raman amplification for a TWRS. (Part 1) FIG. 22B is a diagram illustrating the problems that arise in co-propagating Raman amplification for a TWRS. (Part 2) FIG. 22C is a diagram illustrating the problems that arise in co-propagating Raman amplification for a TWRS (part 3). FIG. 23 is a graph showing the degradation of the Q value due to XPM. FIG. 24A is a diagram illustrating the problems that arise in co-propagating Raman amplification for ELEAF. (Part 1) FIG. 24B is a diagram illustrating the problems that arise in forward Raman amplification for ELEAF. (Part 2) FIG. 24C is a diagram illustrating the problems that arise in forward Raman amplification for ELEAF. (Part 3) FIG. 25 is a graph showing the degradation of the Q value due to XPM. FIG. 26 is an explanatory diagram showing an example of excitation control for the TWRS. FIG. 27 is a graph showing the Raman gain before and after the XPM countermeasure. FIG. 28 is a chart showing the Q values before and after the XPM countermeasure. FIG. 29 is an explanatory diagram showing an example of excitation control for ELEAF. FIG. 30 is a graph showing the Raman gain before and after the XPM countermeasure. FIG. 31 is a chart showing the Q values before and after the XPM countermeasure. FIG. 32 is a flowchart illustrating an example of pumping light control corresponding to various fibers according to the embodiment. FIG. 33 is a waveform diagram showing the spectrum before and after the occurrence of XPM. FIG. 34 is a diagram showing an example of the configuration of an optical transmission system that takes measures against XPM by using an OSNR monitor. FIG. 35 is a flowchart of a process example for detecting wavelengths with XPM degradation by an OSNR monitor. FIG. 36 is a flow chart of an example of excitation control for wavelengths affected by XPM. FIG. 37 is a chart showing an example for explaining the determination of the amount of power reduction of the primary excitation light that causes XPM.

(Excitation Control Example for DSF)
Hereinafter, with reference to the drawings, the disclosed embodiments of a forward Raman amplifier, a bidirectional Raman amplification system, and a forward Raman amplification system will be described in detail. In the embodiments, in order to cope with cases where different types of fibers are used in an optical transmission line, the signal quality of the signal light can be maintained regardless of the type of fiber. For example, in addition to SMF, DSF and dispersion shifted fiber may be used in the optical transmission line. In the embodiments, the influence of XPM degradation on the signal light, which occurs in the case of DSF and dispersion shifted fiber, is suppressed, and degradation of the signal quality is suppressed. For this reason, in the case of DSF, for example, the embodiments perform control to optimize the output of forward pumping light, or the output of bidirectional pumping light consisting of forward pumping light and backward pumping light, based on the optical transmission characteristics (profile) unique to DSF.

(Outline of Excitation Control of the Embodiment)
1A to 1D are explanatory diagrams illustrating an overview of a forward Raman amplifier according to an embodiment.

(Control Example 1)
First, a first control example of forward pumping will be described with reference to Fig. 1A. Fig. 1A(a) shows a configuration of forward Raman amplification using an optical transmission device 110. The optical transmission device 110 transmits a signal light S through an optical transmission line 120.

Although the internal configuration is simplified in FIG. 1A(a), the optical transmission device 110 includes an optical amplifier 130 such as an erbium doped fiber amplifier (EDFA), a forward pumping unit 140, and a forward pumping control unit 150. The forward pumping unit 140 outputs pumping light of multiple wavelengths from a pumping light source 142 to the optical transmission line 120 via a multiplexer 141, and forward pumps the WDM signal light S. The forward pumping control unit 150 controls the output of the forward pumping pumping light.

Next, control example 1 of forward pumping will be explained using Figures 1A(b) and (c). In control example 1, in a forward Raman amplifier that has multiple pumping light sources with different wavelengths, the number of pumping light sources that emit light is changed depending on the fiber type, thereby suppressing XPM degradation. The horizontal axis of Figures 1A(b) and (c) is the wavelength, and the vertical axis is the pumping power.

Figure 1A (b) shows the excitation control state of the forward excitation light when the optical transmission line 120 is SMF. When the fiber type of the optical transmission line 120 is SMF, the forward excitation control unit 150 controls the forward excitation of the forward excitation unit 140 by the forward excitation light PF including the primary excitation light PF1 and the secondary excitation light PF2 having a shorter wavelength than the primary excitation light PF1.

Figure 1A (c) shows the excitation control state of the forward excitation light when the optical transmission line 120 is DSF. When the fiber type of the optical transmission line 120 is DSF, the forward excitation control unit 150 changes the number of excitation light sources to be emitted from the multiple primary excitation light PF1 for the excitation light sources 142 of the forward excitation unit 140.

For example, the forward pumping control unit 150 turns off (extinguishes) the power of the primary pumping light PF1x of some wavelengths among the multiple pumping wavelengths of the primary pumping light PF1, and performs forward pumping by the primary pumping light PF1 of wavelengths other than the primary pumping light PF1x. In the example of FIG. 1A(c), the forward pumping control unit 150 extinguishes the primary pumping light PF1x located on the longest wavelength side among the multiple pumping wavelengths of the primary pumping light PF1.

As shown in FIG. 1A(b), when the type of fiber used in the optical transmission line 120 is SMF, the forward excitation control unit 150 performs forward excitation of all wavelengths of the primary excitation light PF by the excitation light source 142. In contrast, as shown in FIG. 1A(c), when the type of fiber used in the optical transmission line 120 is DSF, the forward excitation control unit 150 performs forward excitation by wavelengths excluding some wavelengths of the primary excitation light PF1x.

Here, when DSF is used for the optical transmission line 120, some wavelengths of the primary excitation light PF1x are wavelengths that cause XPM degradation of the signal light S. For example, when the forward excitation control unit 150 acquires information that the fiber type is DSF, it performs forward excitation with wavelengths other than some wavelengths of the primary excitation light PF1x for the primary excitation light PF, thereby suppressing XPM degradation of the signal light S and preventing signal degradation of the signal light S. Details of the XPM degradation that occurs when DSF is used for the optical transmission line 120 will be described later.

In addition, in relation to control example 1, the optical transmission device 110 may be equipped with an excitation light source having a wavelength that does not correspond to the wavelength band obtained by folding back the wavelength band of the signal light with respect to the zero dispersion wavelength of the fiber on the wavelength axis where forward excitation is performed. For example, the forward excitation unit 140 may be equipped with an excitation light source other than the excitation light source corresponding to the primary excitation light PF1x of a wavelength that causes XPM degradation for the signal light S, among the multiple primary excitation light sources.

(Control Example 2)
Next, a control example 2 of forward pumping will be described with reference to FIG. 1B. In the control example 2 of FIG. 1B, the optical transmission device 110 has a configuration similar to that of FIG. 1A(a) and controls forward Raman amplification. In the control example 2, in a forward Raman amplifier having multiple pumping light sources with different wavelengths, the power ratio between the multiple pumping light sources with different wavelengths is changed according to the fiber type to suppress XPM degradation. The horizontal axis of FIG. 1B is the wavelength, and the vertical axis is the pumping power.

Figure 1B (a) shows the excitation control state of the forward excitation light when the optical transmission line 120 is SMF. When the fiber type of the optical transmission line 120 is SMF, the forward excitation control unit 150 controls the forward excitation of the forward excitation unit 140 by the forward excitation light PF including the primary excitation light PF1 and the secondary excitation light PF2 having a shorter wavelength than the primary excitation light PF1, as in Figure 1A (b).

Figure 1B (b) shows the excitation control state of the forward excitation light when the optical transmission line 120 is DSF. When the fiber type of the optical transmission line 120 is DSF, the forward excitation control unit 150 changes the power ratio between multiple excitation light sources PF1 with different wavelengths for the excitation light source 142 of the forward excitation unit 140.

For example, the forward pumping control unit 150 performs forward pumping to reduce the power of the primary pumping light PF1x at some wavelengths among the multiple pumping wavelengths of the primary pumping light PF1 compared to the other wavelengths. In the example of FIG. 1B(b), the forward pumping control unit 150 makes the power of the primary pumping light PF1x located at the longest wavelength side among the multiple pumping wavelengths of the primary pumping light PF1 lower than the power of the primary pumping light PF at other wavelengths.

Here, as shown in FIG. 1B(a), when the type of fiber used in the optical transmission line 120 is SMF, the forward pumping control unit 150 performs forward pumping with a constant power ratio for all wavelengths of the primary pumping light PF for the pumping light source 142. On the other hand, as shown in FIG. 1B(b), when the type of fiber used in the optical transmission line 120 is DSF, the forward pumping control unit 150 performs forward pumping with reduced power of the primary pumping light PF1x for some wavelengths of the primary pumping light PF.

Here, when DSF is used for the optical transmission path 120, some wavelengths of the primary excitation light PF1x are wavelengths that cause XPM degradation of the signal light S. For this reason, when the fiber type is set to DSF, the forward excitation control unit 150 performs forward excitation with reduced power of some wavelengths of the primary excitation light PF1x for the primary excitation light PF, thereby suppressing XPM degradation of the signal light S and preventing signal degradation of the signal light S.

(Control Example 3)
Next, a control example 3 of forward pumping will be described with reference to FIG. 1C. In the control example 3 of FIG. 1C, the optical transmission device 110 has a configuration similar to that of FIG. 1A(a) and controls forward Raman amplification. In the control example 3, in a forward Raman amplifier having a plurality of pumping light sources with different wavelengths, the wavelength characteristic of the gain, for example, the inclination of the gain with respect to the wavelength, is changed according to the type of fiber to suppress XPM degradation. The horizontal axis of FIG. 1C is the wavelength of the signal light S in the L band, and the vertical axis is the Raman gain.

Figure 1C (a) shows the pumping control state of the forward pumping light when the optical transmission line 120 is SMF. When the fiber type of the optical transmission line 120 is SMF, the forward pumping control unit 150 controls the forward pumping of the forward pumping unit 140 so that a constant Raman gain is achieved over the entire wavelength band of the signal light S. The forward pumping unit 140 performs forward pumping including the primary pumping light and the secondary pumping light, similar to Figure 1A (a).

Figure 1C (b) shows the pumping control state of the forward pumping light when the optical transmission line 120 is DSF. When the fiber type of the optical transmission line 120 is DSF, the forward pumping control unit 150 changes the gain slope with respect to the wavelength for the pumping light source 142 of the forward pumping unit 140.

In the example of FIG. 1C(b), the forward pumping control unit 150 changes the slope of the gain with respect to wavelength by making the gain highest on the short wavelength side and gradually reducing the gain toward the long wavelength side. This slope of the gain can be obtained, for example, by making the power of the primary pumping light on the short wavelength side of the multiple pumping wavelengths of the primary pumping light PF1 described in FIG. 1A(c) the highest, and gradually reducing the power of the primary pumping light toward the long wavelength side.

Here, when the type of fiber used in the optical transmission line 120 is SMF, the forward pumping control unit 150 performs forward pumping with a constant power ratio for all wavelengths of the primary pumping light PF for the pumping light source 142, for example, to obtain the gain characteristics shown in FIG. 1C(a). On the other hand, when the type of fiber used in the optical transmission line 120 is DSF, the forward pumping control unit 150 performs forward pumping in which the power of the primary pumping light on the short wavelength side of the pumping wavelengths of the primary pumping light PF1 is maximized and the power of the primary pumping light PF1 is gradually reduced toward the long wavelength side. This results in the gain characteristics shown in FIG. 1C(b).

Here, when DSF is used for the optical transmission path 120, the long wavelength PF1x of the primary pumping light PF1 is a wavelength that causes XPM degradation of the signal light S. Therefore, when the fiber type is set to DSF, the forward pumping control unit 150 suppresses XPM degradation of the signal light S and prevents signal degradation of the signal light S by setting the gain characteristics shown in FIG. 1C(b).

In the above-described control examples 1 to 3, excitation control may be performed according to the zero-dispersion wavelength of the fiber. For example, in control example 1, the number of excitation light sources to be emitted may be changed according to the zero-dispersion wavelength of the fiber. When the fiber type is DSF, the zero-dispersion wavelength is around 1550 nm. Then, when XPM degradation occurs in part of the signal light S according to the zero-dispersion wavelength of the fiber, for example, the number of excitation light sources to be emitted is changed by quenching a part of the wavelength PF1x of the primary excitation light PF1 corresponding to the degradation, thereby suppressing XPM degradation of the signal light S.

Similarly, in control example 2, the power ratio between multiple pump light sources with different wavelengths may be changed according to the zero dispersion wavelength of the fiber, and in control example 3, the slope of the gain with respect to the wavelength may be changed according to the zero dispersion wavelength of the fiber.

(Control Example 4)
Next, a fourth control example of forward pumping will be described with reference to FIG. 1D. In the fourth control example of FIG. 1D, the optical transmission device 110 controls forward Raman amplification with the same configuration as that of FIG. 1A(a). In the fourth control example, the optical transmission device 110 determines the amount of change in the forward pumping power described in the first to third control examples based on whether or not the signal light is present within the wavelength band obtained by folding back the pumping light wavelength on the wavelength axis with respect to the zero-dispersion wavelength, thereby suppressing XPM degradation. The horizontal axis of FIG. 1D is the wavelength, and the vertical axis is the power.

Figure 1D(a) is a diagram showing the wavelengths of signal light and pump light with XPM degradation. Figure 1D(a) shows the signal band (L band) of signal light S and the arrangement of pump light when the optical transmission line 120 is DSF, with the horizontal axis representing wavelength and the vertical axis representing optical power. For convenience, the numbers λ1 to λ5 representing wavelengths (λ) from the short wavelength side to the long wavelength side of the secondary pump light, and the numbers λ6 to λ8 representing the short wavelength side to the long wavelength side of the primary pump light are given. Figure 1D(a) shows the zero dispersion wavelength λ0 (1550 nm) of DSF. The zero dispersion wavelength of SMF is around 130 nm.

Figure 1D(b) shows the signal band (L band) of the signal light S and the pump light shown in Figure 1D(a) rearranged on the wavelength axis in terms of distance from the zero-dispersion wavelength when the optical transmission line 120 is DSF. The horizontal axis shows the distance (wavelength) from the zero-dispersion wavelength, with the left end being wavelength 0.

As shown in FIG. 1D(b), when the signals of each wavelength are folded back at the zero-dispersion wavelength λ0, the L-band wavelength on the long wavelength side of the signal light S overlaps with the primary pump light of λ8 on the long wavelength side of the primary pump light PF1.

Figure 1D(c) shows the positions on the wavelength axis of the pump light wavelength, signal band, and zero-dispersion wavelength when the optical transmission line 120 is SMF. Also, Figure 1D(c) shows the optical transmission line 120 is SMF, and the signal band (L band) of the signal light S and the pump light are rearranged to the distance from the zero-dispersion wavelength on the wavelength axis. In the case of SMF, there is no overlap of any pump light PF with the signal light S.

In this way, when the optical transmission line 120 is a DSF, the amount of change in the forward pumping power described in control examples 1 to 3 can be determined by whether or not the signal light is present within the wavelength band obtained by folding back the pumping light wavelength with respect to the zero-dispersion wavelength on the wavelength axis shown in FIG. 1D(b). For example, when the signal light S is present within the wavelength band obtained by folding back the wavelength of the pumping light PF1 with respect to the zero-dispersion wavelength λ0 on the wavelength axis, the forward pumping control unit 150 suppresses XPM degradation of the signal light S by changing the forward pumping power of the primary pumping light λ8 that overlaps with the signal light S.

For example, in an application example to control example 1, when signal light S exists within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength λ0, the power of the pump light source for that wavelength PF1x is turned off, as shown in FIG. 1A(c). In addition, in an application example to control example 2, when signal light S exists within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength λ0, as shown in FIG. 1B(b), the pump light power ratio of the existing wavelength PF1x is lowered below the pump light power ratio of the non-existent wavelength. In addition, in an application example to control example 3, when signal light S exists within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength λ0, the gain is tilted so that the gain of that signal wavelength becomes smaller, as shown in FIG. 1C(b).

In control example 4, when the pump light is folded back at the zero-dispersion wavelength λ0 in the DSF, there are multiple possible patterns of overlap with the band of the signal light S, depending on the band of the signal light S and the bands of the wavelengths λ1 to λ8 of the multiple pump lights PF. The multiple patterns of overlap of the wavelengths of the signal light S and the pump lights PF will be described later.

(Control Example 5)
1E is an explanatory diagram illustrating an overview of a bidirectional Raman amplification system according to an embodiment of the present invention. A control example 5 of a bidirectional Raman amplification system using forward pumping and backward pumping will be described with reference to FIG. 1E.

In control example 5, backward Raman pumping is performed to generate a tilt of the gain in the opposite direction with respect to the wavelength so as to compensate for the tilt of the gain with respect to the wavelength generated by the forward Raman pumping described in each of control examples 1 to 4 above.

Figure 1E (a) shows an example of the configuration of an optical transmission system using a bidirectional Raman amplification system according to an embodiment. In the example of Figure 1E, an example of a pair of optical transmission devices 110A, 110B is shown in which the section of the optical transmission path 120 that is the target of pump light control is one span. The upstream first optical transmission device 110A is connected to one end of the DSF 120, and the downstream second optical transmission device 110B is connected to the other end of the DSF 120.

In the first optical transmission device 110A, a forward pumping unit 140A is arranged after the optical amplifier 130A, similar to the configuration of FIG. 1A(a) described above. The forward pumping unit 140A outputs the pumping light of the pumping light source 142A to the DSF 120 via the multiplexer 141A, and forward pumps the WDM signal light S. The forward pumping control unit 150A controls the output of the forward pumping pumping light.

In the second optical transmission device 110B, a multiplexer 141B of the backward pumping unit 140B is arranged in front of the optical amplifier 130B, and the pumping light of the pumping light source 142B is supplied to the DSF 120 via the multiplexer 141B to backward pump the signal light. The backward pumping control unit 150B controls the output of the backward pumping pumping light.

The forward pumping control unit 150A of the first optical transmission device 110A and the backward pumping control unit 150B of the second optical transmission device 110B are electrically connected to each other via a network. For example, the forward pumping control unit 150A notifies the backward pumping control unit 150B of control information such as the wavelength of the pumping light output by the pumping light source 142A. The network may include an optical transmission path.

Although not shown in FIG. 1E, a general control unit that generalizes the control of the forward excitation control unit 150A and the backward excitation control unit 150B may be separately arranged (details will be described later). In this case, the general control unit generalizes and manages control information such as the wavelength of the excitation light of the excitation light source 142A of the forward excitation unit 140A and the excitation light source 142B of the backward excitation unit 140B.

Here, an example of control for suppressing XPM degradation will be described with reference to Figs. 1E(b) and (c). Fig. 1E(b) is a diagram showing an overview of pumping light control of the forward pumping unit 140A. In the diagram, the horizontal axis is wavelength, and the vertical axis is optical power. In the first optical transmission device 110A, the wavelength of the forward pumping light PF is arranged on the shorter wavelength side than the zero dispersion wavelength λ0 centered on the zero dispersion wavelength (for example, near 1550 nm) λ0 of the DSF 120, and the wavelength of the L-band signal light S is arranged on the longer wavelength side than the zero dispersion wavelength. The forward pumping unit 140A includes, as the forward pumping light PF, a primary pumping light PF1 on the wavelength side close to the zero dispersion wavelength, and a secondary pumping light PF2 on the wavelength side away from the zero dispersion wavelength λ0.

In the case of the wavelength arrangement shown in FIG. 1E(b), the delay of the forward pumping light PF, particularly the primary pumping light PF1, and the signal light S are equal. In the case of excitation by the forward pumping light PFx of a part (1500 nm) on the long wavelength side close to the zero dispersion wavelength λ0 of the forward pumping light PF, the long wavelength side Sx of the signal light S is affected by XPM, and the signal quality deteriorates.

Because light with the same wavelength interval from the zero dispersion wavelength λ0 has the same delay amount, the long wavelength side Sx of the signal light S has the same wavelength interval from the zero dispersion wavelength λ0 as the primary pump light PF1x and is affected by XPM. The relationship between the mutual delays of the primary pump light PF1 and the signal light S based on the optical transmission characteristics (profile) specific to the DSF120, for example, the zero dispersion wavelength characteristics, will be described in detail later.

The forward pumping control unit 150A of the first optical transmission device 110A controls forward pumping by the pumping light source 142A. The forward pumping control unit 150A controls the optical power of the primary pumping light PF1x of a portion of the primary pumping light PF1 that is on the long wavelength side close to the zero dispersion wavelength. For example, in the example of FIG. 1E(b), as described in control example 1, the optical power of the primary pumping light PF1x is turned off (extinguished). The forward pumping control unit 150A suppresses XPM degradation of the signal light S (Sx) by controlling forward pumping for the DSF 120.

Figure 1E (c) is a diagram showing an overview of the pumping light control of the backward pumping unit 140B. In the diagram, the horizontal axis is wavelength, and the vertical axis is optical power. The second optical transmission device 110B outputs, for example, backward pumping light PB having a wavelength band similar to that of the primary pumping light PF of forward pumping.

The first optical transmission device 110A extinguishes the forward pumping light PF1x at some wavelengths of the forward pumping light PF, resulting in a shortage of Raman gain. For this reason, in the embodiment, the backward pumping control unit 150B of the second optical transmission device 110B compensates for the Raman gain in one span by increasing the gain of the backward pumping light PBx, which corresponds to the wavelength of the extinguished forward pumping light PF1x, among the backward pumping light PB.

As a result, even if, for example, DSF is used in the optical transmission path 120, XPM degradation of the signal light S can be suppressed by controlling forward pumping. In addition, the signal light S can be restored to a predetermined power level by controlling backward pumping, thereby ensuring the quality of the transmitted signal.

In FIG. 1E, the first optical transmission device 110A quenches the forward pumping light PF1x of some wavelengths among the forward pumping light PF, which corresponds to control example 1. The backward pumping control unit 150B of the second optical transmission device 110B compensates for the Raman gain in one span by increasing the gain of the backward pumping light PBx, which corresponds to the wavelength of the quenched forward pumping light PF1x, among the backward pumping light PB.

Furthermore, the bidirectional pumping Raman amplification system shown in FIG. 1E may correspond to a configuration in which the power ratio between multiple pumping light sources with different wavelengths is changed according to the fiber type in the forward pumping light PF, which corresponds to control example 2, and the gain of the backward pumping light PBx corresponding to the wavelength of the forward pumping light PF1x with a lower power ratio may be increased. Also, the inclination of the gain with respect to the wavelength may be changed according to the fiber type in the forward pumping light PF, which corresponds to control example 2. For example, the gain may be increased at the wavelength of the primary pumping light PF1x, which corresponds to the configuration in which the gain is inclined to decrease, and compensation may be performed by inclining the gain of the wavelength of the backward pumping light PBx corresponding to the wavelength of the primary pumping light PF1x.

Figure 1F is a flowchart showing an example of excitation light control according to the embodiment. In the embodiment, the control content of forward excitation is changed depending on the fiber type, as described in Control Examples 1 to 4.

Figure 1F shows the control contents performed by the forward excitation control unit 150A of the first optical transmission device 110A and the backward excitation control unit 150B of the second optical transmission device 110B in cooperation with each other in the bidirectional excitation control system described in control example 5.

First, the forward pumping control unit 150A and the backward pumping control unit 150B each receive fiber type information (step S101). The fiber type information may be manually input by a maintenance person or may be input from a management server or the like via a network.

Next, the forward pumping control unit 150A and the backward pumping control unit 150B each determine the input fiber type (step S102). Here, it is assumed that the above-mentioned SMF or DSF is input as the fiber type of the optical transmission path 120.

When the input fiber type is SMF (step S102: SMF), the forward pumping control unit 150A performs forward pumping applying the pumping power ratio for SMF (step S103). Similarly, the backward pumping control unit 150B performs forward pumping applying the pumping power ratio for SMF. For example, the forward pumping control unit 150A and the backward pumping control unit 150B control forward pumping using a table of pumping power ratios for SMF. The pumping power ratio table for SMF has flat gain characteristics with respect to wavelength for both forward pumping and backward pumping (see FIG. 1E(b)).

On the other hand, if the input fiber type is DSF (step S102: DSF), the forward pumping control unit 150A and the backward pumping control unit 150B each perform forward pumping using the pumping power ratio for DSF (step S104). For example, the forward pumping control unit 150A and the backward pumping control unit 150B control the forward pumping by referring to a table of pumping power ratios for DSF that have been prepared in advance.

For example, 1. In the DSF pumping power ratio table, the extinction of the long wavelength pumping light source (corresponding to control example 1) or the power reduction (corresponding to control example 2) for forward pumping is set, and the forward pumping control unit 150A refers to it to control the forward pumping. Correspondingly, for backward pumping, the DSF pumping power ratio table is set to increase the power of the long wavelength pumping light source, and the backward pumping control unit 150B refers to it to control the backward pumping.

In addition, 2. the DSF pumping power ratio table is set with gain wavelength characteristics for forward pumping where the gain decreases on the long wavelength side, and the forward pumping control unit 150A refers to it to control forward pumping. Correspondingly, for backward pumping, the DSF pumping power ratio table is set with gain wavelength characteristics for backward pumping where the gain increases on the long wavelength side, and the backward pumping control unit 150B refers to it to control backward pumping (control example 5 (see FIG. 1E(c)).

The forward pumping control unit 150A and the backward pumping control unit 150B end the bidirectional pumping control corresponding to the fiber type by the control of step S103 or step S104. Note that the control in FIG. 1F means changing the setting of the bidirectional pumping power based on the fiber type of the optical transmission line 120, and the forward pumping control unit 150A and the backward pumping control unit 150B continue to execute the pumping control based on the changed setting of the bidirectional pumping power while the optical transmission line 120 is in operation.

(XPM Degradation Due to Forward Excitation)
Here, conventional XPM degradation will be described. First, Raman amplification will be described. In WDM transmission, in order to increase the power of signal light transmitted through one span of fiber (for example, about 100 km), a method of backward pumping Raman amplification is used, which increases the input to an erbium doped fiber amplifier (EDFA). In addition, a method of forward pumping Raman amplification is also used, in which pumping light is transmitted to the transmitting side in the same direction as the signal light to perform Raman amplification.

Figure 2 is a diagram showing the power profile of signal light by forward Raman amplification. The horizontal axis is the transmission distance (km), and the vertical axis is the optical power (dBm). The solid line in the figure shows the transmission characteristics when forward Raman amplification is not performed, and the dotted line in the figure shows the transmission characteristics when forward Raman amplification is performed. Forward Raman amplification has the advantage of suppressing fiber input power and ensuring optical signal to noise ratio (OSNR) while suppressing signal degradation due to optical nonlinearity.

Figure 3 is a diagram showing the wavelength relationship between the signal light and the pump light. The horizontal axis is wavelength, and the vertical axis is optical power. For Raman amplification, the pump light must be placed about 100 nm (13 THz) shortwave side of the wavelength of the signal light S. In the case of a WDM signal, multiple pump lights are required to correspond to the multiple wavelengths of signal light. When the power of the pump light is small, the secondary pump light PF2, which Raman amplifies the pump light (primary pump light PF1), is placed 100 nm shortwave side of the primary pump light PF1.

Raman pump light has noise (RIN: Relative Intensity Noise). In forward pumping Raman amplification, the pump light and the signal light wavelength are transmitted in the same direction through the fiber of the optical transmission line 120. For this reason, noise is carried over from the pump light to the signal light through Raman amplification, and when attempting to obtain sufficient gain, a problem occurs in which the signal quality deteriorates.

Regarding this problem, for example, a technology has been disclosed that uses incoherent pump light, which has a small RIN and a gently spreading spectrum (see, for example, Non-Patent Document 1 above).

Figure 4 is a diagram showing Raman amplification by non-coherent pump light. Since the non-coherent pump light PF1 shown in Figure 4 has a gently spreading wavelength characteristic, by receiving Raman amplification from different wavelength parts, the noise is averaged and the noise carried by the signal light S can be reduced. For example, Raman amplification is performed from different wavelengths Δλ1, Δλ2, Δλ3 of the non-coherent light PF1 to the signal light S, but the noise carried by these wavelengths Δλ1, Δλ2, Δλ3 is different. As a result, noise is averaged in Raman amplification by non-coherent pump light, and the noise can be reduced to 1/10 or less compared to Raman amplification using pump light with a line spectrum. The forward pump light used in the embodiment may be a combination of non-coherent pump light and coherent light.

The forward pumping Raman amplification described above is effective in improving signal quality in SMF, but the inventors discovered through actual measurements and simulations that when signal light is transmitted through dispersion-shifted fiber such as DSF, the signal quality of L-band signal light deteriorates.

Figure 5 is a chart showing the chromatic dispersion characteristics for each type of fiber. The SMF shown by the solid line in the figure has zero dispersion near a wavelength of 1310 nm, and in the 1550 nm band where propagation loss is smallest, has a dispersion of, for example, about 17 ps/nm/km. In contrast, DSF is designed to reduce dispersion in the 1550 nm band. Next, we will explain the problems that arise when using fibers such as DSF with shifted chromatic dispersion characteristics.

Figure 6 is a diagram that explains the problems that occur with forward pumping Raman amplification for DSF. Figure 6 shows the relationship between the zero dispersion wavelength in the case of DSF, the wavelength of the WDM signal light (L-band), and the pump wavelength.

Figure 6(a) shows the wavelength vs. delay for DSF. The speed of light waves propagating through optical fiber varies depending on the wavelength (corresponding to chromatic dispersion). When the equation for wavelength vs. chromatic dispersion is integrated with respect to wavelength, the equation for the wavelength vs. delay is obtained, and as shown in Figure 6(a), the characteristic line for wavelength vs. delay is approximately a quadratic function. This wavelength vs. delay is centered around the zero-dispersion wavelength (1550 nm) λ0, and light on the short-wave side and long-wavelength side with the same wavelength interval has the same delay.

Figure 6(b) is a diagram showing the effect of XPM on the signal light when DSF is used. As described above, in order to perform efficient Raman amplification, the signal light S and the pump light (primary pump light PF1) are spaced apart by a wavelength interval (arrangement) of about 100 nm. As shown in Figure 6(b), the signal light S and the primary pump light PF1 are arranged on either side of the zero-dispersion wavelength λ0. The primary pump light PF1x of a portion of the primary pump light PF1 that is on the long wavelength side close to the zero-dispersion wavelength λ0, and the long wavelength side Sx of the L-band signal light S are the same distance (wavelength interval) from the zero-dispersion wavelength λ0.

As described above, light with equal wavelength intervals from the zero dispersion wavelength λ0 has the same delay amount, so the long wavelength side Sx of the signal light S is affected by XPM due to the long wavelength side PF1x of the primary pump light PF1. The long wavelength side primary pump light PF1x has the role of amplifying the long wavelength side Sx of the signal light S, but the long wavelength side primary pump light PF1x has the same delay as the band of the long wavelength side Sx of the signal light S. For example, PF1x with a wavelength of 1500 nm and the signal light Sx with a wavelength of 1600 nm have the same delay. As a result, the signal light Sx on the long wavelength side of the signal light S is affected by XPM, causing signal degradation.

It is known that XPM occurs between two adjacent signal lights, but because the pump light PF has a power density several times higher than that of the signal light S, it imparts a large amount of XPM to the signal light S, distorting the waveform of the signal light S and degrading the signal quality.

In contrast, in the embodiment, as described in FIG. 1, when transmitting signal light using a dispersion-shifted fiber such as DSF, the arrangement of forward pumping light with the same distance (wavelength interval) as the L-band signal band centered on the zero-dispersion wavelength is avoided. For example, signal degradation due to XPM is avoided by reducing the power of the long-wavelength side PF1x of the primary pumping light PF1 by quenching or the like.

The amplification gain by forward pumping is reduced by the reduction or disappearance of the pumping light power of the long wavelength side PF1x, and the gain of the long wavelength side Sx of the signal light S decreases. For example, a tilt occurs in the signal light S such that the gain of the long wavelength side Sx becomes smaller. Although the long wavelength side Sx receives the gain of the pumping light on the short wavelength side, it is less than the required amount, so the input level of the receiving amplifier decreases, and due to the insufficient OSNR, the signal quality deteriorates if this continues. For this reason, in the embodiment, the power level of the long wavelength side Sx of the signal light S is restored by increasing the gain on the long wavelength side PBx of the backward pumping. In this way, in the embodiment, signal deterioration due to XPM of the forward pumping light is avoided and signal quality is ensured. At this time, a tilt is generated by backward pumping such that the gain of the long wavelength side Sx of the signal light S becomes large.

Figures 7A to 7C are graphs showing the degradation of the Q factor and Raman gain due to XPM. The horizontal axis of each graph in Figures 7A to 7C is wavelength. The vertical axis of (a) in Figures 7A to 7C is Q factor, and the vertical axis of (b) is gain.

Figure 7A (a) shows the channel dependence of the Q value during SMF transmission. In the case of SMF transmission, XPM due to pump light does not occur, and it is shown that there is no channel dependence in the Q value. Figure 7A (b) shows the channel dependence of the Raman gain during SMF transmission. In the case of SMF transmission, the gain is flat across the entire wavelength band in all cases of forward pumping only, backward pumping only, and forward pumping + backward pumping, and it is shown that there is no channel dependence in the Raman gain.

Figure 7B (a) shows the channel dependence of the Q value during DSF transmission. In the case of DSF transmission, XPM occurs due to the pump light, and the Q value gradually deteriorates toward the longer wavelength side, with a deterioration of -ΔQ occurring at wavelengths of 1600 nm or more. Figure 7B (b) shows the channel dependence of the Raman gain during DSF transmission. In DSF transmission, the gain is flat across the entire wavelength band in all cases of forward pumping only, backward pumping only, and forward pumping + backward pumping, and it is shown that there is no channel dependence in the Raman gain.

Figure 7C shows characteristics corresponding to the Raman amplification control of the embodiment, and Figure 7C (a) shows the channel dependency of the Q value during DSF transmission. In order to deal with the degradation of the Q value shown in Figure 7B (a), the degradation of the Q value is eliminated and flattened by quenching the long wavelength side PF1x of the forward pumping light PF1, which is causing XPM. Figure 7C (b) shows the channel dependency of the Raman gain during DSF transmission. By quenching the long wavelength side PF1x of the forward pumping light PF1, a tilt occurs in which the Raman gain gradually decreases along the long wavelength side of the forward pumping light PF1. In the case of Figure 7C (b), the Raman gain decreases by -ΔD in the long wavelength side PF1x of 1600 nm or more.

In response to this, in the embodiment, the Raman gain of the long wavelength side PBx of the backward pumping light PB is increased by +ΔD. A tilt is generated that gradually increases the Raman gain according to the long wavelength side PBx of the backward pumping light PB. This ensures flatness in the Raman gain due to forward pumping and backward pumping (solid lines in the figure) and eliminates deterioration of the Q value.

(Configuration Example of the Embodiment)
Next, examples of the configuration of the bidirectional Raman amplification system according to the embodiment will be described. In each of the following examples 1 to 3, the DSF 120 is used as the dispersion shifted fiber.

Configuration example 1. A configuration compatible with DSF using general-purpose forward pumping Raman amplification.
Configuration Example 2: Configuration of co-propagating Raman amplification compatible with DSF (a configuration example of co-propagating Raman in which a laser module on the long wavelength side is not mounted, taking into consideration the influence of XPM generated in DSF).
Configuration example 3. A configuration in which general-purpose forward pumping Raman amplification is used to perform pumping control for multiple spans based on various information in multiple spans throughout the optical transmission line, including information on the zero-dispersion wavelength.

(Configuration Example 1)
In the first configuration example, pumping control corresponding to the DSF is performed using general-purpose forward pumping Raman amplification. In the first configuration example, pumping control is performed to suppress the influence of XPM based on the optical transmission characteristics (profile) specific to the DSF 120.

Figure 8 is a diagram showing a first example of a configuration of an optical transmission system according to an embodiment. The optical transmission system 800 of the first example of configuration shown in Figure 8 shows an example having a pair of optical transmission devices 110A, 110B for one span on the optical transmission path 120. The upstream first optical transmission device 110A is connected to one end of the DSF 120, and the downstream second optical transmission device 110B is connected to the other end of the DSF 120. The signal light S has a bandwidth of the L band.

The first optical transmission device 110A has a forward excitation unit 140A. The second optical transmission device 110B has a backward excitation unit 140B and a general control unit 810.

The configuration of the first optical transmission device 110A side will be described. The forward pumping unit 140A of the first optical transmission device 110A includes a primary pumping light source 842a that emits primary pumping light of multiple wavelengths according to the bandwidth of the signal light S, a polarization coupler 844a that combines orthogonal polarizations of the primary pumping light, and a multiplexing filter 845a that combines multiple primary pumping lights.

The forward pumping unit 140A of the first optical transmission device 110A includes a secondary pumping light source 842b that emits multiple secondary pumping lights according to the bandwidth of the primary pumping light, and a polarization coupler 844b that combines orthogonal polarizations of the primary pumping light. The forward pumping unit 140A also includes multiplexing filters 845b and 845c that combine multiple secondary pumping lights.

The primary pump light and the secondary pump light are multiplexed by the multiplexing filter 845d and then multiplexed as forward pump light for the signal light S on the DSF 120 via the multiplexer 841A.

The output of the multiplexing filter 845d is split by the splitting filter 846a and output to the excitation light monitor 847a. The excitation light monitor 847a detects the output level of the forward excitation light and outputs it to the excitation light control unit 848a.

The excitation light control unit 848a controls the output levels of the primary excitation light source 842a, which is the forward excitation light, and the secondary excitation light source 842b, based on the output level detected by the excitation light monitor 847a.

The forward pumping control unit 150A controls the pumping light control unit 848a to variably control the output level of the forward pumping light by the forward pumping unit 140A. For example, the forward pumping control unit 150A performs control to reduce the gain of the long wavelength side PF1x of the forward pumping light PF1 shown in FIG. 7C(b).

The configuration of the second optical transmission device 110B side will be described. The rear excitation unit 140B of the second optical transmission device 110B includes an excitation light source 842c that emits excitation light of multiple wavelengths according to the bandwidth of the signal light S, a polarization coupler 844c that polarization-combines the excitation light, and a multiplexing filter 845c that combines the excitation light.

The multiple pump lights are multiplexed by the multiplexing filter 845f and then multiplexed via the multiplexer 841B as backward pump lights for the signal light S on the DSF 120.

The output of the multiplexing filter 845f is split by the splitting filter 846c and output to the excitation light monitor 847c. The excitation light monitor 847c detects the output level of the backward excitation light and outputs it to the excitation light control unit 848b.

The excitation light control unit 848b controls the output level of the excitation light source 842c based on the output level detected by the excitation light monitor 847c.

The backward pumping control unit 150B controls the pumping light control unit 848b to variably control the output level of the backward pumping light by the backward pumping unit 140B. For example, the backward pumping control unit 150B performs control to increase the gain of the long wavelength side PBx of the backward pumping light PB shown in FIG. 7C(b).

The forward pumping control unit 150A and the backward pumping control unit 150B are connected to a general control unit 810. The general control unit 810 generalizes the variable control of the Raman gain performed by the forward pumping control unit 150A of the first optical transmission device 110A and the backward pumping control unit 150B of the second optical transmission device 110B. For example, the general control unit 810 is placed in a terminal station for every span or every few spans.

In the example of FIG. 8, the overall control unit 810 is disposed in the second optical transmission device 110B. However, the overall control unit 810 may be disposed in the first optical transmission device 110A, or its functions may be divided and installed in both the first optical transmission device 110A and the second optical transmission device 110B.

8, the forward pumping control unit 150A of the first optical transmission device 110A and the backward pumping control unit 150B of the second optical transmission device 110B are electrically connected to the general control unit 810 via a network other than the DSF 120. In addition, the forward pumping control unit 150A and the backward pumping control unit 150B can also include control information in supervisory signal (OSC: Optical Supervisory Channel) light and superimpose it on the signal light S on the optical transmission path 120 for optical transmission. In this case, the forward pumping control unit 150A and the backward pumping control unit 150B transmit and receive OSC information by dropping/inserting the OSC light using a multiplexer/demultiplexer (not shown) on the optical transmission path 120.

The overall control unit 810 monitors the power of the signal light S on the transmitting side for the first optical transmission device 110A. In the example of FIG. 1, the output level of the demultiplexed output of the demultiplexer 861A arranged before (upstream from) the multiplexing position of the forward pump light on the first optical transmission device 110A side is monitored by the transmitting side signal light monitor 862A.

The overall control unit 810 also monitors the power of the signal light S on the receiving side for the second optical transmission device 110B. In the example of FIG. 1, the output level of the demultiplexed output of the demultiplexer 861B arranged downstream of the multiplexing position of the backward pumping light on the second optical transmission device 110B side is monitored by the receiving side signal light monitor 862B.

The overall control unit 810 receives information such as the monitor outputs of the transmitting side signal light monitor 862A and the receiving side signal light monitor 862B, and the transmission characteristics (profile) specific to the DSF 120. It also receives information on the span section to be controlled, the fiber type, the zero dispersion wavelength, the signal light band, and the designed Raman gain.

The general control unit 810 has the functions of monitoring input/output power wavelength characteristics, calculating gain wavelength characteristics, and calculating gain.

Based on these calculation results, the general control unit 810 controls the forward excitation power for the forward excitation control unit 150A and the backward excitation power for the backward excitation control unit 150B.

The general control unit 810 outputs control information such as fiber type, gain wavelength characteristics, and gain change instructions to the forward pumping control unit 150A on the first optical transmission device 110A side and the backward pumping control unit 150B on the second optical transmission device 110B side. Specifically, the pumping ratio table is set so that different pumping ratios are set for the forward pumping light source depending on the fiber type.

Figure 9 is a diagram showing an example of an information table. The general control unit 810 holds information such as conditions, values, units, etc. for each of the following items in the information table 900: the span number of one span, the signal band of the signal light S, Raman gain, forward pumping Raman gain, backward pumping Raman gain, fiber zero dispersion wavelength, forward pumping wavelength, and forward pumping wavelength spread.

This information table 900 is stored in the memory of the general control unit 810. The signal band to backward pumping Raman gain shown in the example of FIG. 9 is set to a predetermined value corresponding to the C band and L band of the signal light S. Also, in the example of FIG. 9, it is shown that the forward pumping wavelengths of the primary pumping light are eight wavelengths, λ1 to λ8, and each of the wavelengths λ1 to λ8 has a forward pumping wavelength spread Δλ1 to Δλ8, respectively.

The overall control unit 810 also calculates the span loss, forward pumping Raman gain, backward pumping Raman gain, and total Raman gain (Raman gain) based on the monitor outputs of the transmitting side signal light monitor 862A and the receiving side signal light monitor 862B. The overall control unit 810 sets the calculated values in the corresponding records of the information table 900.

The overall control unit 810 calculates, for example, the span loss and Raman gain as follows. Here, the transmitting side signal light monitor 862A monitors and outputs the transmitting signal light level (TX_mon), and the receiving side signal light monitor 862B monitors and outputs the receiving signal light level (Rx_mon).

The general control unit 810 calculates the span loss from the received signal light level (Rx_mon) - the transmitted signal light level (TX_mon) before starting up the forward pumping unit 140A and the backward pumping unit 140B.

The general control unit 810 calculates the backward pumping Raman gain by subtracting the span loss from the received signal light level (Rx_mon) - the transmitted signal light level (TX_mon) when only the backward pumping unit 140B is activated.

The general control unit 810 calculates the forward pumping Raman gain by subtracting the backward pumping Raman gain and span loss from the received signal light level (Rx_mon) - the transmitted signal light level (TX_mon) after starting up the forward pumping unit 140A and the backward pumping unit 140B.

The overall control unit 810 calculates the total Raman gain by subtracting the span loss from the received signal light level (Rx_mon) - the transmitted signal light level (TX_mon) after starting up the forward pumping unit 140A and the backward pumping unit 140B. The overall control unit 810 calculates the total Raman gain after extinguishing or applying a low power ratio to the pumping light (forward pumping unit 140A) that causes XPM.

After calculating the information related to the pumping control, the general control unit 810 outputs control information to the forward pumping unit 140A and the backward pumping unit 140B. The control information includes, for example, the fiber type, the calculated span loss, the forward pumping Raman gain, the backward pumping Raman gain, and an instruction for the pumping ratio table to be used.

The overall control unit 810 outputs a gain change instruction to the forward pumping unit 140A to reduce (extinguish) the pumping light power of a portion (L band) of the forward Raman pumping light. After outputting the control information, the overall control unit 810 also outputs a gain change instruction to the backward pumping unit 140B to increase the power of the backward Raman pumping light until the total Raman gain reaches the required magnitude and deviation.

It is also possible to lower the excitation power of some of the forward excitation Raman excitations and to increase the excitation light power of some of the backward excitation Raman excitations by changing the settings of the excitation lasers individually. In addition, this can also be done by applying an excitation light power ratio table that sets the ratio of the output power of each excitation laser that has been designed in advance.

The forward pumping control unit 150A of the first optical transmission device 110A has a pumping light power ratio table that sets the ratio of pumping power on the short wavelength side (C band) and the long wavelength side (L band). When the general control unit 810 outputs control information, the forward pumping control unit 150A refers to the pumping light power ratio table to determine the pumping ratio and gain on the short wavelength side and the long wavelength side, and controls the pumping light control unit 848a.

Similarly, the backward pumping control unit 150B of the second optical transmission device 110B has a pumping light power ratio table that sets the ratio of pumping power on the short wavelength side (C band) and the long wavelength side (L band). When the general control unit 810 outputs control information, the forward pumping Raman backward pumping control unit 150B refers to the pumping light power ratio table to determine the pumping ratio and gain on the short wavelength side and the long wavelength side, and controls the pumping light control unit 848b.

FIG. 10 is a diagram showing an example of an excitation light power ratio table. The excitation light power ratio table 1000 shown in FIG. 10 is held in advance by the general control unit 810. However, the excitation light power ratio table 1000 may be held in the storage unit of the forward excitation control unit 150A on the first optical transmission device 110A side and the backward excitation control unit 150B on the second optical transmission device 110B side.

The pumping light power ratio table 1000 includes information such as table number, fiber type, span loss, fiber input signal power, Raman gain, fiber zero dispersion wavelength, forward pumping power ratio, and backward pumping power ratio for each span number.

As described above, in the case of DSF120, in forward pumping, the power of the pumping light on the long wavelength side is quenched or the pumping power ratio is set small, and in backward pumping, the pumping power ratio on the long wavelength side is set large to compensate for the lack of gain in forward pumping. The forward pumping power ratio and backward pumping power ratio are set for each of the multiple wavelengths λ1 to λ8 in the band.

In addition, to accommodate cases where the forward Raman gain and backward Raman gain are not flat, as in the case of DSF120, the forward Raman gain is set to include a forward Raman gain regulation wavelength that specifies the flat portion of the band. The backward Raman gain is set to include a backward Raman gain regulation wavelength that specifies the flat portion of the band.

The general control unit 810 refers to the pumping light power ratio table 1000 and outputs a gain control instruction to the forward pumping control unit 150A to gradually reduce the gain on the long wavelength side in accordance with the forward pumping power ratio of each of the wavelengths λ1 to λ8 for each table number.

The general control unit 810 also refers to the pumping light power ratio table 1000 and outputs a gain control instruction to the backward pumping control unit 150B to gradually increase the gain on the long wavelength side in accordance with the backward pumping power ratio of each of the wavelengths λ1 to λ8 for each table number.

(Example of Hardware Configuration of a Control Unit of an Optical Transmission Device)
11 is a diagram illustrating an example of a hardware configuration of a control unit of an optical transmission device. For example, the forward pumping control unit 150A of the first optical transmission device 110A, the backward pumping control unit 150B of the second optical transmission device 110B, and the general control unit 810 can each be configured with the hardware illustrated in FIG.

For example, the forward excitation control unit 150A has a processor 1101 such as a CPU (Central Processing Unit), a memory 1102, a network IF 1103, a recording medium IF 1104, and a recording medium 1105. In addition, each component is connected to each other by a bus 1100.

Here, the processor 1101 is a control unit that controls the entire forward excitation control unit 150A. The processor 1101 may have multiple cores. The memory 1102 has, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), and a flash ROM. Specifically, for example, the flash ROM stores a control program, the ROM stores an application program, and the RAM is used as a work area for the processor 1101. The program stored in the memory 1102 is loaded into the processor 1101 to cause the processor 1101 to execute the coded process.

The network IF 1103 serves as an interface between the network NW and the inside of the device, and controls the input and output of information between the other backward excitation control units 150B and the general control unit 810.

The recording medium IF 1104 controls the reading/writing of data from/to the recording medium 1105 under the control of the processor 1101. The recording medium 1105 stores the data written under the control of the recording medium IF 1104.

In addition to the components described above, the forward excitation control unit 150A may also be capable of connecting, for example, an input device, a display, etc. via an IF.

The processor 1101 shown in FIG. 11 can realize the functions of the forward excitation control unit 150A shown in FIG. 8 etc. by executing a program.

The network IF 1103 shown in FIG. 11 may include an optical transmission path in addition to communication by electrical signals. The forward pumping control unit 150A and the backward pumping control unit 150B shown in FIG. 8 may include the optical transmission path 120 as part of the transmission and reception of control information between the general control unit 810. For example, the control information output by the general control unit 810 may be included in the OSC, and the OSC may be multiplexed with the signal light S by a multiplexer on the optical transmission path 120 toward the forward pumping control unit 150A and the backward pumping control unit 150B side for transmission.

Also, as in the example shown in FIG. 8, when the optical transmission path 120 in the target section is one span, the function of the overall control unit 810 may be integrated, for example, into the backward excitation control unit 150B of the second optical transmission device 110B. In this case, the control unit on the second optical transmission device 110B side controls the forward excitation control unit 150A of the first optical transmission device 110A. Furthermore, when the optical transmission path 120 in the target section is multiple spans, the function of the overall control unit 810 may be integrated, for example, into the backward excitation control unit 150B of the final-stage (most downstream) optical transmission device 110.

(Excitation Light Control Example of Configuration Example 1)
Fig. 12 is a flowchart showing an example of pumping light control according to configuration example 1. Fig. 12 shows overall control mainly performed by the overall control unit 810. Through the overall control of the overall control unit 810, the forward pumping control unit 150A and the backward pumping control unit 150B cooperate to control the pumping light power.

As a preliminary step, the general control unit 810 creates an excitation light power ratio table 1000 for each fiber type (step S1201). As shown in FIG. 10, in the case of DSF 120, the excitation light power ratio table 1000 includes a forward excitation power ratio that reduces the power on the long wavelength side of forward excitation Raman, and a backward excitation power ratio that compensates for the gain wavelength characteristic in which the long wavelength of forward excitation Raman is reduced.

Then, the overall control unit 810 sets basic information for pump light control for the target span (see FIG. 8) (step S1202). The overall control unit 810 sets the basic information based on the input information input to the overall control unit 810. The input information is the fiber type, fiber zero dispersion wavelength, signal light band, etc. Based on the input information, the overall control unit 810 sets the span number, signal band, Raman gain, forward pumping Raman gain, backward pumping Raman gain, fiber zero dispersion wavelength, forward pumping wavelength, and backward pumping wavelength in the information table 900 of FIG. 9. For example, the overall control unit 810 sets the Raman gain to a calculated value based on the measurement as described above.

Next, the overall control unit 810 calculates the forward Raman gain, the backward Raman gain, and the pumping power ratio from the fiber type and span loss. Then, the overall control unit 810 outputs pumping light control information to the forward pumping control unit 150A and the backward pumping control unit 150B (step S1203).

Next, the overall control unit 810 controls the optical amplifier 130A on the transmitting side (first optical transmission device 110A) to start up (step S1204). Next, the overall control unit 810 controls the forward pumping unit 140A to start up (step S1205). At this time, the forward pumping control unit 150A performs forward pumping using the pumping power ratio.

Next, the general control unit 810 controls the startup of the backward excitation unit 140B (step S1206). At this time, the backward excitation control unit 150B performs backward excitation using the excitation power ratio.

Next, the central control unit 810 controls the startup of the optical amplifier 130B on the receiving side (second optical transmission device 110B) (step S1207), thereby completing the startup process for one span.

According to configuration example 1, while using general-purpose forward pumping Raman amplification, it is possible to avoid the effects of XPM when the optical transmission line is DSF120, and to prevent degradation of the signal quality of the signal light S.

(Configuration Example 2)
FIG. 13 is a diagram showing a second example of the optical transmission system according to the embodiment. In the optical transmission system 1300 of the second example, the same components as those of the first example shown in FIG. 8 are denoted by the same reference numerals, and the description thereof is omitted. In the second example, the laser module on the long wavelength side of the forward pumping unit 140A is not mounted in consideration of the influence of XPM generated in the DSF 120. As shown in FIG. 13, in the second example, the forward pumping unit 140A has only four primary pumping light sources 842a on the short wavelength side (λ1 to λ4) of the band, and the four primary pumping light sources 842a on the long wavelength side (λ5 to λ8) are not provided. As described in the first example, the long wavelength side of the primary pumping light sources 842a is extinguished, so in the second example, the primary pumping light source 842a on the long wavelength side is not provided.

The number of light sources in each of the primary excitation light source 842a of the forward excitation section 140A and the secondary excitation light source 842b of the backward excitation section 140B is eight (the same as in configuration example 1).

In configuration example 2, the overall control unit 810 may be placed in the second optical transmission device 110B, may be placed in the first optical transmission device 110A, or may have its functions split and installed in both the first optical transmission device 110A and the second optical transmission device 110B.

(Excitation Light Control Example of Configuration Example 2)
Fig. 14 is a flowchart showing an example of pump light control according to configuration example 2. The process shown in Fig. 14 is almost the same as that of configuration example 1, and steps S1402 to S1407 of configuration example 2 (Fig. 14) are the same as steps S1202 to S1207 of configuration example 1 (Fig. 12).

In configuration example 2, the processing of step S1401 differs from configuration example 1. The general control unit 810 creates an excitation light power ratio table 1000 for each fiber type as a preparation for step S1401. When the fiber type is DSF120, the excitation light power ratio table 1000 shown in FIG. 10 includes a forward excitation power ratio that reduces the power on the long wavelength side of forward excitation Raman, and a backward excitation power ratio that compensates for the gain wavelength characteristic in which the long wavelength of forward excitation Raman is reduced.

In configuration example 2, the primary excitation light source 842a on the long wavelength side is not provided. Therefore, the general control unit 810 performs forward excitation by applying the forward excitation power ratio only to the forward excitation ratio power on the short wavelength side (λ1 to λ4) of the forward excitation Raman shown in FIG. 10 in the excitation light power ratio table 1000.

According to configuration example 2, the influence of XPM when the optical transmission path is DSF120 can be avoided, and deterioration of the signal quality of the signal light S can be prevented. In addition, in configuration example 2, the influence of XPM generated in DSF120 is taken into consideration, and a configuration is adopted in which the primary pumping light source 842a on the long wavelength side is not provided, so that the number of primary pumping light sources in the forward pumping can be reduced compared to configuration example 1.

(Configuration Example 3)
The above configuration examples 1 and 2 are configurations in which pumping is controlled by applying a pumping light ratio using the fiber parameters of a typical optical transmission line 120 and preset values of the Raman pumping configuration. In contrast, configuration example 3 is a configuration in which the wavelength to be controlled is actually determined from zero-dispersion wavelength information, etc., and pumping is controlled in response to various parameter variations.

In configuration example 3, the error rate of the main signal (signal light S) is measured to determine whether the pump light ratio is optimal, and the system is configured with multiple spans including the most upstream transmitter to the most downstream receiver arranged on the optical transmission path 120.

Figure 15 is a diagram showing a third example of the optical transmission system according to the embodiment. The optical transmission system 1500 shown in Figure 15 shows an example in which the spans shown in the first example (Figure 8) are multiple spans, and for convenience, a forward excitation unit 140A (first optical transmission device 110A) and a backward excitation unit 140B (second optical transmission device 110B) are arranged for each span. In Figure 15, components similar to those in the first example (Figure 8) are given the same reference numerals, and the components of spans 2 to n-1 are omitted.

In configuration example 3, a network management system (NMS) network control unit 810C is placed in the second optical transmission device 110B of span 1 and in the second optical transmission device 110B of span n.

In FIG. 15, at the most upstream position of the optical transmission path 120, i.e., before (upstream from) span 1, there are multiple transmitters 1501 and a multiplexer 1502 that multiplexes the signal light of different wavelengths output by the multiple transmitters 1501 and outputs it to the optical transmission path 120 as signal light S.

In addition, at the most downstream of the optical transmission path 120, that is, after (downstream) of span n, a splitter 1503 that splits the optical transmission path 120 by wavelength, and a receiver 1504 that receives the signal light of each split wavelength are arranged. In addition, each of the multiple receivers 1504 is provided with an error rate measurement unit 1505 that measures the signal error rate when receiving a signal, and information on the signal error rate of the error rate measurement unit 1505 is output to the general control unit 810. Each receiver 1504 may have an error rate measurement unit 1505.

The overall control unit 810 receives the monitor outputs of the transmitting side signal light monitor 862A and the receiving side signal light monitor 862B for multiple spans shown in configuration example 1. The overall control unit 810 performs excitation light control sequentially for the upstream span 1 to the downstream span n based on the same input information as in configuration example 1, the monitor information of the transmitting side signal light monitor 862A and the receiving side signal light monitor 862B for each span, and information such as the signal error rate.

In the Raman amplification method in configuration example 3, the general control unit 810 generally performs the following processes 1 to 4.

1. Creation of an excitation light power ratio table for each span 2. Extraction of excitation light causing XPM degradation for each span 3. Application and adjustment of the excitation light power ratio table for each span 4. Evaluation and optimization using signal error rate (application of a new excitation light ratio table when improvement is insufficient)

Each process will be described below.
(1. Creation of a pumping light power ratio table for each span)
In the third configuration example, the general control unit 810 creates a plurality of pump light power ratio tables 1000 for each span.

(2. Extraction of Excitation Light Causing XPM Degradation of Each Span)
16 is a diagram for explaining extraction of signal light and pump light degraded by XPM. Fig. 16(a) shows the signal band (L band) of the signal light S and the arrangement of the pump light, with the horizontal axis representing wavelength and the vertical axis representing optical power. For convenience, the numbers λ1 to λ5 are assigned according to wavelength (λ) from the short wavelength side to the long wavelength side of the secondary pump light, and the numbers λ6 to λ8 are assigned from the short wavelength side to the long wavelength side of the primary pump light.

In addition, FIG. 16(b) shows the signal band (L band) of the signal light S and the pump light shown in FIG. 16(a) rearranged in terms of distance from the zero-dispersion wavelength. The horizontal axis shows the distance (wavelength) from the zero-dispersion wavelength, with the left end being wavelength 0.

As shown in FIG. 16(b), the signals of each wavelength in FIG. 16(a) are folded back at the zero dispersion wavelength λ0. In the example of FIG. 16(b), it is shown that the long wavelength L-band wavelength of the signal light S overlaps with the long wavelength λ8 primary pump light of the primary pump light PF1.

Figure 16 (c) shows patterns 1 to 4 for different overlap states between the signal light S and the primary excitation light PF1. It is shown that the primary excitation light (λ8) overlaps with the band of the signal light S, and is a wavelength that causes XPM. When the primary excitation light PF1 is folded back at the zero-dispersion wavelength λ0, there are four possible overlap patterns with the band of the signal light S. These overlap states can be determined by comparing the band of the signal light S with the spectral spread of the primary excitation light PF1.

The lower and upper limits of the band of the signal light S are sig_L and sig_H depending on the distance from the zero dispersion wavelength λ0, and the lower and upper limits of the spectrum of the primary pump light PF1 (λ8) are pump_L and pump_H depending on the distance from the zero dispersion wavelength λ0. In this case, any of the following overlapping patterns 1 to 4 occurs.

Pattern 1 is a state in which the primary pump light PF1 overlaps with the short wavelength side of the signal light S. The overlap state is such that the lower limit (pump_L) of the primary pump light PF1 (λ8) is less than the lower limit (sig_L) of the signal light S, and the upper limit (pump_H) of the primary pump light PF1 (λ8) is greater than the lower limit (sig_L) of the signal light S. Furthermore, the upper limit (pump_H) of the primary pump light PF1 (λ8) is less than the upper limit (sig_H) of the signal light S.

Pattern 2 is a state in which the primary pump light PF1 overlaps with the long wavelength side of the signal light S. In this overlapping state, the lower limit (pump_L) of the primary pump light PF1 (λ8) is greater than the lower limit (sig_L) of the signal light S, and the lower limit (pump_L) of the primary pump light PF1 (λ8) is less than the upper limit (sig_H) of the signal light S. Furthermore, the upper limit (pump_H) of the primary pump light PF1 (λ8) is greater than the upper limit (sig_H) of the signal light S.

Pattern 3 is a state in which the bandwidth of the primary excitation light PF1 is wider than that of the signal light S, and the primary excitation light PF1 covers the bandwidth of the signal light S. The overlap state is such that the lower limit (pump_L) of the primary excitation light PF1 (λ8) is < the lower limit (sig_L) of the signal light S, and the upper limit (pump_H) of the primary excitation light PF1 (λ8) is > the upper limit (sig_H) of the signal light S.

Pattern 4 is a state in which the bandwidth of the primary excitation light PF1 is narrower than the bandwidth of the signal light S. The overlap state is such that the lower limit (pump_L) of the primary excitation light PF1 (λ8) is greater than the lower limit (sig_L) of the signal light S, and the upper limit (pump_H) of the primary excitation light PF1 (λ8) is less than the upper limit (sig_H) of the signal light S.

As shown in patterns 1 to 4, different overlap states occur between the signal light S and the primary excitation light PF1 (λ8) depending on their respective center wavelengths and the spectral spread state of the primary excitation light PF1.

The general control unit 810 can find the primary excitation light PF1 that causes XPM by calculating the states of patterns 1 to 4. In reality, in terms of the band of the signal light S and the spectral spread of the primary excitation light PF1, the above patterns 1 and 2 apply in most cases.

Figure 17 is a diagram showing an example of an information table according to configuration example 3. In configuration example 3, the general control unit 810 holds, as information table 1700, information similar to that of configuration example 1 (Figure 9) for each span number. For example, information table 1700 holds information such as conditions, values, units, etc. for each of the following items: span number, signal band of signal light S, Raman gain, forward pumping Raman gain, backward pumping Raman gain, fiber zero dispersion wavelength, forward pumping wavelength, and forward pumping wavelength spread.

Here, the general control unit 810 checks the above-mentioned conditions, for example,
1. The lower and upper limits of the band of the signal light S are the distances from the zero dispersion wavelength λ0: sig_L, sig_H,
2. When the lower limit and upper limit of the spectrum of the primary pump light PF1 are distances from the zero dispersion wavelength: pump_L, pimp_H, in the information table 1700 shown in FIG. 17, when the signal band of the signal light S is the c-band,
c-band_sig_L=band1_low-λ_0,
Set c-band_sig_H=band1_high-λ_0.

In addition, for each forward pumping wavelength of the primary pumping light PF1 of the forward pumping, for example, for the wavelength λ1,
λ1_pump_L=λ_1−Δλ_1−λ_0,
Set λ1_pump_H=λ_1+Δλ_1−λ_0.

(3. Application and Adjustment of Pumping Light Power Ratio Table to Each Span)
18A to 18C are diagrams showing transmitting side pumping light power ratio tables according to configuration example 3. The transmitting side pumping light power ratio table 1800a in FIG. 18A shows an example in which only one primary pumping light of forward pumping that causes XPM is detected for the band of the signal light S. The transmitting side pumping light power ratio table 1800a includes information on the symbol (forward pumping light power ratio), value, remarks (information on coherent light/non-coherent light), and the presence or absence of pumping light that causes XPM (◯: pumping light that causes XPM). For reference, FIG. 18A shows the distance from the zero dispersion wavelength λ0.

As shown in FIG. 18A, when there is one primary excitation light (λ8) that causes XPM in the band of the signal light S, the integrated control unit 810 selects the wavelength λ8 (FWR_PR_λ8) that has the smallest value indicating the power ratio of the primary excitation light (the smallest value that can be designed).

The transmitting side excitation light power ratio table 1800b in FIG. 18B shows an example in which two primary excitation lights (λ7, λ8) that cause XPM in the band of the signal light S are detected. When multiple primary excitation lights are detected, the general control unit 810 changes the power ratio of the primary excitation light (λ7) that is closest to the zero dispersion wavelength λ0, for example, to 50% (value 80→40).

After the settings in FIG. 18B, the overall control unit 810 may again determine the power ratio based on the improvement state of the degradation due to XPM, which will be described later. For example, assume that the degradation due to XPM has not been improved with the settings in FIG. 18B. In this case, the overall control unit 810 changes the power ratio of the primary excitation light (λ7) closest to the zero dispersion wavelength λ0 from 50% to 25% for the two primary excitation lights (λ7, λ8), as shown in the transmitting side excitation light ratio power table 1800c in FIG. 18C. For example, the value is changed from 40 to 20.

The closer the primary pumping light is to the zero dispersion wavelength λ0, the greater the possibility of XPM degradation. For this reason, the general control unit 810 reduces the influence of primary pumping light with wavelengths close to the zero dispersion wavelength λ0 as much as possible, but for the primary pumping light, which has the second greatest effect, it changes it from 50% to 25% taking into account the decrease in gain and judges the improvement state. In this case, the multiple pumping light power ratio tables 1800a to 1800c shown in Figures 18A to 18C are differentiated by assigning table numbers according to the power ratio of the primary pumping light. The general control unit 810 selects the pumping light power ratio table 1800 with the table number corresponding to the power ratio of the relevant primary pumping light.

Figure 19 is a diagram showing a receiving side excitation light power ratio table according to configuration example 3. The receiving side excitation light power ratio table 1900 in Figure 19 is created in advance to correspond to the transmitting side excitation light power ratio table 1800 shown in Figure 18 and to compensate for the gain wavelength characteristics. The general control unit 810 selects the receiving side excitation light power ratio table 1900 corresponding to the selected transmitting side excitation light power ratio table 1800.

In the example of the receiving side excitation light power ratio table 1900 shown in FIG. 19, in response to the case where there is one primary excitation light (λ8) in forward excitation that causes XPM, the power ratio of the backward excitation wavelengths λ5 and λ6 corresponding to the wavelength λ8 is set large.

(4. Evaluation and Optimization Using Signal Error Rate)
There is a demand for checking the improvement of the signal error rate characteristic in the adjustment of the primary pump light. However, when forward pumping Raman and backward pumping Raman are applied to multiple spans, the error rate may be too bad to measure unless the evaluation using the pumping light power ratio tables 1800 and 1900 (FIGS. 18A to 19) is applied. For this reason, in the configuration example 3, the general control unit 810 performs evaluation using the pumping light power ratio table for the forward and backward pumping Raman of all spans, and then performs evaluation and optimization using the signal error rate when there is room for optimizing the pumping light ratio.

Specifically, when there are two primary excitation wavelengths that cause XPM, the overall control unit 810 extinguishes the primary excitation wavelength that causes XPM and is closest to the zero-dispersion wavelength λ0 in the transmitting side excitation light power ratio table 1800 (makes the power ratio sufficiently small). However, the overall control unit 810 changes the power ratio of the primary excitation wavelength that is second closest to the zero-dispersion wavelength from 50% to 25%, optimizing the signal error rate.

(Excitation Light Control Example of Configuration Example 3)
20A and 20B are flowcharts showing an example of pumping light control according to configuration example 3. 20A and 20B show an example of overall control mainly performed by the overall control unit 810. The overall control unit 810 controls the pumping light power for the forward pumping control unit 150A and the backward pumping control unit 150B of span 1 in ascending order of the multiple spans 1 to n. Thereafter, the overall control unit 810 controls the pumping light power for spans 2 and onward, and finally controls the pumping light power for span n.

As a preliminary step, the overall control unit 810 creates an excitation light power ratio table for each fiber type (step S2001). At this time, the overall control unit 810 creates an excitation light power ratio table for each fiber type and span 1 to n. The excitation light power ratio table consists of a transmitting side excitation light power ratio table 1800 shown in FIG. 18A etc. and a receiving side excitation light power ratio table 1900 shown in FIG. 19.

Next, the overall control unit 810 calculates the excitation light that causes XPM in the signal band of the target span (step S2002). The overall control unit 810 calculates the excitation light that causes XPM in the signal band of the target span by the process described in FIG. 16.

Next, the overall control unit 810 creates an excitation power ratio table for each span (step S2003). Here, the overall control unit 810 creates the transmitting side excitation light power ratio table 1800 shown in FIG. 18A and the receiving side excitation light power ratio table 1900 shown in FIG. 19.

In step S2003, the general control unit 810 creates an excitation power ratio table for each span corresponding to: 1. the case where there is one excitation wavelength that causes XPM; and 2. the case where there are two excitation wavelengths that cause XPM.

1. When there is one pump wavelength causing XPM, the general control unit 810 creates a power ratio table for forward pumping Raman in which the pump wavelength ratio of the wavelength is sufficiently small. For backward pumping Raman, the general control unit 810 creates a power ratio table that compensates for the gain wavelength characteristic of forward pumping.

2. When there are two pumping wavelengths causing XPM For forward pumping Raman, the general control unit 810 sets the pumping wavelength ratio for the wavelength close to the zero dispersion wavelength to be sufficiently small, and the power ratios for the wavelengths second closest to zero dispersion to, for example, 50%, 25%, and 10%. Here, the general control unit 810 sets the power ratio of 50% to table number 1, the power ratio of 25% to table number 2, and so on, in separate tables. For backward pumping Raman, the general control unit 810 creates a power ratio table that compensates for the gain wavelength characteristics of forward pumping.

Next, the overall control unit 810 selects the excitation power ratio of the target span of the target span and the initial value of the table (step S2004). For example, when starting processing, the overall control unit 810 initializes the span number and correspondingly selects table numbers #1 for the information table 1700, the transmitting side excitation light power ratio table 1800, and the receiving side excitation light power ratio table 1900. Thereafter, the overall control unit 810 increments the span number when selecting a span again.

Next, the overall control unit 810 starts up the transmitting side optical amplifier 130A (step S2005). Next, the overall control unit 810 judges whether the span in question includes forward pumping Raman (step S2006). If the span in question includes forward pumping Raman (step S2006: Yes), the overall control unit 810 proceeds to processing of step S2007, and if the span in question does not include forward pumping Raman (step S2006: No), the overall control unit 810 proceeds to processing of step S2008.

In step S2007, the general control unit 810 performs control to start up the forward pumping unit 140A of the corresponding span (step S2007). At this time, the forward pumping control unit 150A performs forward pumping applying the pumping power ratio.

After that, in step S2008, the overall control unit 810 determines whether the span in question includes backward Raman (step S2008). If the span in question includes backward Raman (step S2008: Yes), the overall control unit 810 proceeds to processing in step S2009, and if the span in question does not include backward Raman (step S2008: No), the overall control unit 810 proceeds to processing in step S2010.

In step S2009, the overall control unit 810 controls the startup of the backward pumping unit 140B (step S2009). At this time, the backward pumping control unit 150B performs backward pumping using the pumping power ratio. After that, in step S2010, the overall control unit 810 starts up the receiving side optical amplifier 130B (step S2010).

Next, the central control unit 810 determines whether the span number has reached the total number of spans (step S2011). If the span number has not reached the total number of spans (step S2011: No), the central control unit 810 increments the span number (step S2012) and returns to the processing of step S2005. On the other hand, if the span number has reached the total number of spans (step S2011: Yes), the central control unit 810 proceeds to the processing of step S2013.

In the process from step S2013 onward shown in FIG. 20B, the excitation power is optimized based on the error rate of the received signal. First, the general control unit 810 measures the error rate characteristics of the received signal using the error rate measurement unit 1505 (step S2013).

Then, the central control unit 810 judges whether the error rate characteristic is within a predetermined tolerance range (step S2014). If the error rate characteristic is not within the predetermined tolerance range (step S2014: No), the central control unit 810 proceeds to the process of step S2015 and onwards. On the other hand, if the error rate characteristic is within the predetermined tolerance range (step S2014: Yes), the central control unit 810 ends the above process.

In step S2015, the overall control unit 810 initializes the span number (step S2015), and then determines whether the span includes forward Raman (step S2016). If the span includes forward Raman (step S2016: Yes), the overall control unit 810 proceeds to processing in step S2017, and if the span does not include forward Raman (step S2016: No), the overall control unit 810 proceeds to processing in step S2022.

In step S2017, the overall control unit 810 determines whether the table number is less than the maximum table number (step S2017). A number of table numbers are provided for the transmitting side excitation light power ratio table 1800 and the receiving side excitation light power ratio table 1900, corresponding to the number of different forward excitation and backward excitation powers in order to vary the excitation light power. In the following control, for example, the overall control unit 810 performs control to lower the excitation power for each increment of the table number within the allowable range of the error rate.

If the current table number is not less than the maximum table number (step S2017: No), the central control unit 810 proceeds to step S2022. On the other hand, if the table number is less than the maximum table number (step S2017: Yes), the central control unit 810 proceeds to step S2018.

In step S2018, the central control unit 810 increments the table number to change the corresponding span excitation ratio table (step S2018). Then, the central control unit 810 measures the error rate characteristic again (step S2019) and determines whether the error rate has improved (step S2020). If the error rate has improved (step S2020: Yes), the central control unit 810 proceeds to the process of step S2021. On the other hand, if the error rate has not improved (step S2020: No), the central control unit 810 returns to the process of step S2017.

In step S2021, the central control unit 810 decrements the table number to change the corresponding span excitation ratio table (step S2021). Next, the central control unit 810 determines whether the span number has reached the total number of spans (step S2022). If the span number has not reached the total number of spans (step S2022: No), the central control unit 810 increments the span number (step S2023) and returns to the processing of step S2016. On the other hand, if the span number has reached the total number of spans (step S2022: Yes), the above processing ends.

According to configuration example 3, the pumping light that causes XPM degradation for each span is extracted by actual measurement, and the span loss and power wavelength characteristics at the time of a specified pumping control are determined by measuring the forward pumping and backward pumping power. In addition, based on the error rate of the received signal, control is performed to vary the pumping light power in accordance with the table number. This makes it possible to accommodate different transmission characteristics for each span, avoid the effects of XPM when the optical transmission path is DSF120, improve the error rate of the received signal, and prevent degradation of signal quality.

(Examples of pumping control for other dispersion shifted fibers)
In the above-mentioned embodiment, the excitation control for DSF has been described. In the following description, the excitation control for suppressing XPM degradation for TWRS, which is NZ-DSF, and ELEAF will be described. TWRS is an abbreviation for True Wave RS (Reduced Slope), and ELEAF is an abbreviation for Enhanced Large Effective Area Fiber.

(Wavelength dispersion characteristics of TWRS and ELEAF)
Fig. 21 is a diagram showing the chromatic dispersion characteristics of TWRS and ELEAF. For reference, Fig. 21 also shows the chromatic dispersion characteristics of SMF, in which the dispersion becomes zero near a wavelength of 1310 nm, and DSF, in which the dispersion becomes zero near a wavelength of 1550 nm. TWRS has a chromatic dispersion characteristic in which the dispersion becomes zero near a wavelength of 1452 nm, and ELEAF has a chromatic dispersion characteristic in which the dispersion becomes zero near a wavelength of 1499 nm. These NZ-DSFs are widely installed fibers, and C-band and L-band are used as signal bands. Next, problems when these TWRS and ELEAF are used in the optical transmission line 120 will be described.

(XPM degradation caused by forward excitation of TWRS)
22A to 22C are diagrams for explaining problems that occur in forward pumping Raman amplification for a TWRS. FIG. 22A shows the relationship between the zero dispersion wavelength in the case of a TWRS, the signal bands of WDM (C-Band, L-Band), and the pumping wavelength. The horizontal axis is the wavelength, and the vertical axis is the pumping power. For convenience, numbers 1 to 5 are assigned according to wavelength (λ) from the short wavelength side to the long wavelength side of the secondary pumping light PF2, and numbers 1 to 3 are assigned from the short wavelength side to the long wavelength side of the primary pumping light PF1.

Figure 22B shows the relationship between the zero dispersion wavelength in the case of TWRS, the wavelength of the WDM signal light (C-band, L-band), and the pump wavelength. As shown in Figure 22B(a), the TWRS has a quadratic delay characteristic on either side of zero dispersion (wavelength 1452 nm). When this delay characteristic is superimposed on the arrangement of the signal band and the pump light wavelength axis, it becomes as shown in Figure 22B(b). As shown in Figure 22B(b), almost the entire band of the C-band and part of the band of the L-band have the same delay amount as wavelengths 1, 2, 3, and 4 of the secondary pump light PF2, and they are transmitted through the fiber at the same speed. XPM that occurs in the fiber of the TWRS occurs strongly between lights transmitted at the same speed, so the signal light is affected by a large XPM from the secondary pump light PF2.

Figure 22C is a rearrangement of Figure 22B(b) in terms of distance from zero dispersion to make it easier to understand the distance from zero dispersion of the signal and pump light. The horizontal axis shows the distance (wavelength) from the zero dispersion wavelength, with the left end being wavelength 0.

As shown in Figure 22C, the signals of each wavelength in Figure 22B(b) are folded back at the zero-dispersion wavelength λ0. In the example of Figure 22C, the entire signal band of the C-band and the short wavelength side of the L-band overlap with wavelengths 1, 2, 3, and 4 of the secondary excitation light PF1, which are wavelengths that cause XPM.

Figure 23 is a chart showing the degradation of the Q value due to XPM. Figure 23 shows the wavelength characteristic of the Q value for the wavelength arrangement of the pump light shown in Figure 22C. The vertical axis is the Q value, and the horizontal axis is the wavelength. As shown in Figure 23, a dip occurs in the Q value in the signal band where the delay amount is the same as that of the secondary pump light PF2 (wavelengths 1, 2, 3, 4), resulting in a significant degradation. Since the secondary pump light PF2 has a large power density, the effect of XPM becomes very large, which causes a problem of significant degradation in the signal quality of the signal light S.

(XPM Degradation Caused by Forward Pumping of ELEAF)
24A to 24C are diagrams for explaining problems that occur in forward pumping Raman amplification for ELEAF. Fig. 24A shows the relationship between the zero dispersion wavelength in the case of ELEAF, the signal bands of WDM (C-Band, L-Band), and the pumping wavelength. The horizontal axis is the wavelength, and the vertical axis is the pumping power. For convenience, numbers 1 to 5 according to wavelength (λ) are assigned from the short wavelength side to the long wavelength side of the secondary pumping light PF2, and numbers 1 to 3 are assigned from the short wavelength side to the long wavelength side of the primary pumping light PF1.

Figure 24B shows the relationship between the zero dispersion wavelength in the case of ELEAF, the wavelength of the WDM signal light (C-band, L-band), and the pump wavelength. As shown in Figure 24B (a), the TWRS has a delay characteristic of a quadratic function on either side of zero dispersion (wavelength 1499 nm). When this delay characteristic is superimposed on the arrangement of the signal band and the pump light wavelength axis, it becomes as shown in Figure 24B (b). As shown in Figure 24B (b), part of the C-band band and part of the L-band band have the same delay amount as the primary pump light 1, 2 and the wavelength 5 of the secondary pump light PF2, and they are transmitted through the fiber at the same speed. XPM that occurs in the TWRS fiber occurs strongly between lights transmitted at the same speed, so the signal light is greatly affected by XPM from these primary pump light PF1 and secondary pump light PF2.

Figure 24C is a rearrangement of Figure 24B(b) in terms of distance from zero dispersion to make it easier to understand the distance from zero dispersion of the signal and pump light. The horizontal axis shows the distance (wavelength) from the zero dispersion wavelength, with the left end being wavelength 0.

As shown in Figure 24C, the signals of each wavelength in Figure 24B(b) are folded back at the zero-dispersion wavelength λ0. In the example of Figure 24C, the long wavelength side of the C-band overlaps with wavelength 2 of the primary excitation light PF1, and the short wavelength side of the C-band overlaps with wavelength 1 of the primary excitation light PF1. In addition, the long wavelength side of the L-band overlaps with wavelength 1 of the primary excitation light, and the short wavelength side of the L-band overlaps with wavelength 5 of the secondary excitation light, indicating that these are wavelengths that cause XPM.

Figure 25 is a chart showing the degradation of the Q value due to XPM. Figure 25 shows the wavelength characteristic of the Q value for the wavelength arrangement of the pump light shown in Figure 24C. The vertical axis is the Q value, and the horizontal axis is the wavelength. As shown in Figure 25, a dip occurs in the Q value in the signal band where the delay amount is the same for wavelengths 1 and 2 of the primary pump light PF1 and wavelength 5 of the secondary pump light PF2, resulting in significant degradation. In particular, since the secondary pump light PF2 has a higher power density than the primary pump light, the effect of XPM becomes very large, resulting in a problem of significant degradation of the signal quality of the signal light S.

As described above, in the TWRS of NZ-DSF and ELEAF, the signal quality of the signal light S is degraded by the pump light that causes XPM. In particular, the secondary pump light has a higher power density than the primary pump light, and therefore significantly degrades the signal quality. For this reason, in the embodiment, the pump light that causes XPM is quenched or its optical power is reduced to prevent degradation. In addition, a certain degree of forward pumping Raman gain is ensured, so that improvements in transmission characteristics due to forward pumping Raman can be expected.

For example, in the embodiment, when the secondary pumping light PF2 of the forward pumping causes XPM, the amount of degradation is very large, so the light of the corresponding wavelength is extinguished. Also, when the primary pumping light PF1 causes XPM, if there is one wave of pumping light of the corresponding wavelength, it is extinguished, and if there are multiple waves, the wavelength closest to zero dispersion is extinguished, and the power of the wavelength farthest from zero dispersion is reduced by a predetermined amount, for example to 50% or less.

The appropriate amount of power reduction in forward pumping can be calculated in advance, but it may also be predicted based on actual measurement data. The reduction in gain caused by reducing the pumping power of forward pumping is compensated for by adjusting the gain between the forward pumping light sources and adjusting the gain wavelength characteristics of the backward Raman. For example, the gain adjustment between the forward pumping light sources includes compensating for the lack of gain caused by the extinction of the secondary pumping light PF2 by increasing the power of the corresponding primary pumping light PF1.

(Example of excitation control for TWRS)
26 is an explanatory diagram showing an example of pumping control for a TWRS. The horizontal axis is wavelength, and the vertical axis is optical power. Fig. 26(a) is a diagram showing the relationship between pumping light and signal band before XPM countermeasures are taken, and Fig. 26(b) is a diagram showing the relationship between pumping light and signal band after XPM countermeasures are taken.

As shown in FIG. 26(a), in TWRS, wavelengths 1, 2, 3, and 4 of the secondary pump light PF2 cause XPM in the signal band (the entire C-band and part of the L-band). For this reason, as shown in FIG. 26(b), the optical transmission device 110 controls the extinction of wavelengths 1, 2, 3, and 4 of the secondary pump light PF2 to suppress signal degradation. In addition, to compensate for the decrease in Raman gain caused by the extinction of wavelengths 1, 2, 3, and 4 of the secondary pump light PF2, the optical transmission device 110 controls to increase the power of the primary pump light PF1 (wavelengths 1, 2, and 3).

In addition, Figures 26(c) and (d) show the signal band of the signal light S and the pump light shown in Figures 26(a) and (b), respectively, rearranged on the wavelength axis in terms of distance from the zero-dispersion wavelength. The horizontal axis is the distance (wavelength) from the zero-dispersion wavelength, and the vertical axis is the optical power. Figure 26(c) shows the relationship between the pump light and the signal band before XPM countermeasures are taken, and Figure 26(d) is a chart showing the relationship between the pump light and the signal band after XPM countermeasures are taken.

Figure 27 is a graph showing the Raman gain before and after XPM countermeasures. The horizontal axis is wavelength, and the vertical axis is gain. Figure 27(a) shows the channel dependence of the Raman gain before XPM countermeasures, and Figure 27(b) shows the channel dependence of the Raman gain after XPM countermeasures.

As shown in Figure 27(a) before the XPM countermeasures were taken, in TWRS transmission, the gain is flat across the entire wavelength band in all cases of forward pumping only, backward pumping only, and forward pumping + backward pumping, and it is shown that there is no channel dependence in the Raman gain. In contrast to Figure 27(a), after the XPM countermeasures were taken as shown in Figure 27(b), the wavelength dependence of the Raman gain has changed.

As shown in FIG. 27(b), the optical transmission device 110 according to the embodiment compensates by increasing the power of the primary pump light PF1 because a gain deficiency occurs due to the extinction of the secondary pump light PF2. Here, compared to the case in which the secondary pump light PF2 is present as shown in FIG. 27(a), in FIG. 27(b) the gain gradually becomes insufficient (-ΔD) toward the shorter wavelength side, and there is also a slight gain deficiency on the longer wavelength side, so the gain is compensated (+ΔD) by the backward Raman PB in response to this deficiency (-ΔD). This makes it possible to ensure the flatness of the Raman gain (solid line in the figure) due to pumping control (forward pumping and backward pumping) after XPM countermeasures are implemented.

Figure 28 is a graph showing the Q value before and after XPM countermeasures. The horizontal axis is wavelength, and the vertical axis is Q value. Figure 28(a) shows the wavelength dependence of the Q value before XPM countermeasures, and Figure 28(b) shows the wavelength dependence of the Q value after XPM countermeasures.

Before the XPM countermeasure shown in FIG. 28(a) is implemented, in TWRS transmission, a dip occurs in the Q value in the signal band where the delay amount is the same as that of the secondary excitation light PF2 (wavelengths 1, 2, 3, and 4), causing significant degradation. In contrast, the optical transmission device 110 according to the embodiment is able to suppress significant degradation in the Q value, as shown in FIG. 28(b).

(Excitation control example for ELEAF)
Fig. 29 is an explanatory diagram showing an example of pumping control for ELEAF. The horizontal axis is wavelength, and the vertical axis is optical power. Fig. 29(a) is a diagram showing the relationship between pumping light and signal band before XPM countermeasures are taken, and Fig. 29(b) is a diagram showing the relationship between pumping light and signal band after XPM countermeasures are taken.

As shown in FIG. 29(a), in ELEAF, waves 1 and 2 of the primary excitation light PF1 and wavelength 5 of the secondary excitation light PF2 cause XPM in the C-band and L-band signal bands. For this reason, the optical transmission device 110 performs extinction control of wavelength 5 of the secondary excitation light PF2 as shown in FIG. 29(b). The optical transmission device 110 also performs control to extinguish wavelength 2 of the primary excitation light OF1, which is close to zero dispersion in the primary excitation light PF1, and to reduce the power of wavelength 1 of the primary excitation light PF1 to a predetermined amount, for example, 50% or less. In addition, to compensate for the decrease in Raman gain due to the extinction of wavelength 5 of the secondary excitation light PF2, the optical transmission device 110 performs control to increase the power of wavelength 3 of the primary excitation light PF1.

In addition, Figures 29(c) and (d) show the signal band of the signal light S and the pump light shown in Figures 29(a) and (b), respectively, rearranged on the wavelength axis in terms of distance from the zero-dispersion wavelength. The horizontal axis is the distance (wavelength) from the zero-dispersion wavelength, and the vertical axis is the optical power. Figure 29(c) shows the relationship between the pump light and the signal band before XPM countermeasures are taken, and Figure 29(d) is a chart showing the relationship between the pump light and the signal band after XPM countermeasures are taken.

Figure 30 is a graph showing the Raman gain before and after XPM countermeasures. The horizontal axis is wavelength, and the vertical axis is gain. Figure 30(a) shows the channel dependence of the Raman gain before XPM countermeasures, and Figure 30(b) shows the channel dependence of the Raman gain after XPM countermeasures.

As shown in Figure 30(a) before the XPM countermeasures were taken, in ELEAF transmission, the gain is flat across the entire wavelength band in all cases of forward pumping only, backward pumping only, and forward pumping + backward pumping, and it is shown that there is no channel dependence in the Raman gain. In contrast to Figure 30(a), after the XPM countermeasures were taken as shown in Figure 30(b), the wavelength dependence of the Raman gain has changed.

As shown in FIG. 30(b), the optical transmission device 110 according to the embodiment compensates for the gain (+ΔD) with backward Raman PB in response to the gain deficiency (-ΔD) at the center of the signal band due to the extinction of wavelength 2 of the secondary pump light PF2. This makes it possible to ensure the flatness of the Raman gain (solid line in the figure) due to pumping control (forward pumping and backward pumping) after XPM countermeasures are implemented.

Figure 31 is a graph showing the Q value before and after XPM countermeasures. The horizontal axis is wavelength, and the vertical axis is Q value. Figure 31(a) shows the wavelength dependence of the Q value before XPM countermeasures, and Figure 31(b) shows the wavelength dependence of the Q value after XPM countermeasures.

As shown in Figure 31 (a) before the XPM countermeasure, in ELEAF transmission, a dip occurs in the Q value in the signal band where the delay amount is the same for wavelengths 1 and 2 of the primary pump light PF1 and wavelength 5 of the secondary pump light PF2, resulting in significant degradation. In contrast, as shown in Figure 31 (b) in the optical transmission device 110 according to the embodiment, the Q value is slightly degraded in the center of the signal band due to the insufficient gain of the forward pumping Raman, but significant degradation of the Q value is suppressed.

(Examples of excitation control for various fibers)
Fig. 32 is a flowchart showing an example of pumping light control corresponding to various fibers according to the embodiment. Fig. 32 also shows the above-mentioned control example for DSF (see Fig. 1F). In the embodiment, the pumping control is changed according to the fiber type of DSF, TWRS, or ELEAF.

The process in FIG. 32 can be performed in cooperation with the forward excitation control unit 150A of the first optical transmission device 110A and the backward excitation control unit 150B of the second optical transmission device 110B in the bidirectional excitation control system 100 described in FIG. 1F (control example 5).

First, the forward pumping control unit 150A and the backward pumping control unit 150B each receive fiber type information (step S3201). The fiber type information may be manually input by a maintenance person or the like, or may be input from a management server or the like via a network.

Next, the forward pumping control unit 150A and the backward pumping control unit 150B each determine the input fiber type (step S3202). Here, it is determined whether the fiber type of the optical transmission path 120 is SMF or not.

When the input fiber type is SMF (step S3202: Yes), the forward pumping control unit 150A performs forward pumping applying the pumping power ratio for SMF (step S3203). Similarly, the backward pumping control unit 150B performs forward pumping applying the pumping power ratio for SMF. For example, the forward pumping control unit 150A and the backward pumping control unit 150B use a table of pumping power ratios for SMF that are prepared in advance to control the forward pumping and the backward pumping. The pumping power ratio table for SMF has flat gain characteristics with respect to wavelength for both forward pumping and backward pumping (for example, see FIG. 1E(b)).

On the other hand, if the input fiber type is other than SMF (step S3202: No), the forward pumping control unit 150A and the backward pumping control unit 150B each determine the input fiber type (step S3204).

If the input fiber type is DSF in step S3024 (step S3204: DSF), the forward pumping control unit 150A and the backward pumping control unit 150B perform forward pumping applying the pumping power ratio for DSF (step S3205). The process of step S3205 is the same as step S104 in FIG. 1F. For example, in the control of forward pumping by the forward pumping control unit 150A and backward pumping by the backward pumping control unit 150B, pumping is controlled by referring to a table of pumping power ratios for DSF that have been prepared in advance.

For example, 1. In the DSF pumping power ratio table, the extinction of the long wavelength pumping light source (corresponding to control example 1) or the power reduction (corresponding to control example 2) for forward pumping is set, and the forward pumping control unit 150A refers to it to control the forward pumping. Correspondingly, for backward pumping, the DSF pumping power ratio table is set to increase the power of the long wavelength pumping light source, and the backward pumping control unit 150B refers to it to control the backward pumping.

In addition, 2. the DSF pumping power ratio table is set with gain wavelength characteristics for forward pumping where the gain decreases on the long wavelength side, and the forward pumping control unit 150A refers to it to control forward pumping. Correspondingly, for backward pumping, the DSF pumping power ratio table is set with gain wavelength characteristics for backward pumping where the gain increases on the long wavelength side, and the backward pumping control unit 150B refers to it to control backward pumping (control example 5 (see FIG. 1E(c)).

Also, in step S3024, if the input fiber type is TWRS (step S3204: TWRS), the forward pumping control unit 150A and the backward pumping control unit 150B perform forward pumping applying the pumping power ratio for TWRS (step S3206).

For example, the forward pumping control unit 150A controls the extinction of wavelengths 1 to 4 of the secondary pumping light PF2. The forward pumping control unit 150A also controls the power of the primary pumping light PF1 to increase in order to compensate for the decrease in Raman gain caused by the extinction of the secondary pumping light PF2 (see FIG. 26). The backward pumping control unit 150B also controls the gain compensation with the backward pumping PB based on the gain wavelength characteristic (see FIG. 27) that compensates for the gain wavelength characteristic of the forward pumping.

Also, in step S3024, if the input fiber type is ELEAF (step S3204: ELEAF), the forward pumping control unit 150A and the backward pumping control unit 150B perform forward pumping applying the pumping power ratio for ELEAF (step S3207).

For example, the forward pumping control unit 150A performs extinction control of wavelength 5 of the secondary pumping light PF2. The forward pumping control unit 150A also performs extinction control of wavelength 2 of the primary pumping light PF1 that is close to zero dispersion among the primary pumping light PF1, and performs control to reduce the power of wavelength 1 of the primary pumping light PF1 to a predetermined amount, for example, 50% or less. The forward pumping control unit 150A also performs control to increase the power of wavelength 3 of the primary pumping light PF1 in order to compensate for the decrease in Raman gain due to the extinction of the secondary pumping light PF2 (see FIG. 29). The backward pumping control unit 150B also performs control to compensate for the gain with backward pumping PB based on the gain wavelength characteristic (see FIG. 30) that compensates for the gain wavelength characteristic of forward pumping.

The forward pumping control unit 150A and the backward pumping control unit 150B end the bidirectional pumping control corresponding to the fiber type by the control of step S3203 or steps S3205 to S3207. The forward pumping control unit 150A and the backward pumping control unit 150B continue to execute the pumping control based on the changed bidirectional pumping power setting while the optical transmission path 120 is in operation.

The pumping light wavelength that causes XPM depends on the zero-dispersion wavelength, and the zero-dispersion wavelength of NZ-DSF varies depending on the type of fiber, and the pumping light that causes XPM differs. In an actual optical transmission system, fibers that were manufactured at similar times are installed, and the zero-dispersion wavelength can be roughly determined from the fiber type information, so degradation due to XPM can be avoided by the process shown in Figure 32.

(An example of pumping control to accommodate variations in the zero-dispersion wavelength)
On the other hand, there is a possibility that the zero-dispersion wavelength may vary in fibers that are manufactured at different times or in an early stage of manufacture. In the following, an example of pumping control will be described in which the pumping light wavelength that causes XPM in a fiber with a varying zero-dispersion wavelength is identified, and degradation due to XPM is avoided.

The pumping light wavelength that causes XPM depends on the zero-dispersion wavelength, and there is a possibility that the zero-dispersion wavelength of the transmission line fiber varies. In the embodiment described below, after identifying the pumping light wavelength that causes XPM, the same pumping light control as described above is performed to avoid degradation due to XPM.

Figure 33 is a waveform diagram showing the spectrum before and after XPM occurs. The horizontal axis is wavelength, and the vertical axis is optical power. Figure 33(a) is a waveform diagram of the signal light S before XPM, and Figure 33(b) is a waveform diagram of the signal light S after XPM. When XPM occurs to the signal light S due to pump light, as shown in Figure 33(b), the signal quality (Q value) of the signal light S deteriorates, and at the same time, the noise level N also deteriorates. In particular, at the wavelength λx of the signal light S that has suffered XPM, the bandwidth (signal spectrum) W widens, the noise level N increases, and the OSNR deteriorates.

Figure 34 is a diagram showing an example of the configuration of an optical transmission system that uses an OSNR monitor to take measures against XPM. In the optical transmission system 3400 shown in Figure 34, the same components as those in the configuration described above (see Figure 8) are given the same reference numerals and will not be described.

As described in FIG. 8, the optical transmission system 3400 includes a first optical transmission device 110A that performs forward excitation of the signal light S, and a second optical transmission device 110B that performs backward excitation. In addition, the general control unit 81 is disposed in the second optical transmission device 110B.

The difference between the configuration in FIG. 34 and that in FIG. 8 is that in the first optical transmission device 110A, a WSS 3401 and an ASE light source 3402 are arranged at the input stage of the signal light S, and the output of the WSS 3401 is input to the optical amplifier 130A. In addition, in the second optical transmission device 110B, a splitter 3403 is arranged at the output stage of the optical signal (before the optical amplifier 130B), and the optical signal S is detected by the OCM 3404 via the splitter 3403. A DGEQ may be arranged instead of the WSS 3401.

The WSS3401, ASE light source 3402, and OCM3404 are connected to the overall control unit 810. For example, when the optical signal S is not in operation (not being transmitted), the overall control unit 810 controls the WSS3401 and ASE light source 3402 on the first optical transmission device 110A side, and generates and transmits comb-shaped dummy light (waveform equivalent to that in FIG. 33(a)) by the WSS3401 using the ASE light.

Then, the overall control unit 810 measures the OSNR when the dummy light is received by the OCM 3404 on the second optical transmission device 110B side. The overall control unit 810 can obtain a waveform equivalent to FIG. 33(b) by monitoring the OSNR. When a wave of forward Raman pump light is emitted and a change in the spectrum occurs as shown in FIG. 33(b) and deterioration of the OSNR is observed, it is found that XPM is occurring due to the pump light. The XPM deterioration is eliminated by performing pump light control on the forward pump in the first transmission device 110A and the backward pump in the second optical transmission device 110B.

Figure 35 is a flowchart of a process example for detecting wavelengths with XPM degradation using an OSNR monitor. The process shown in Figure 35 is the processing content of the control unit of the overall control unit 810, and the overall control unit 810 controls each unit of the first transmission device 110A and the second optical transmission device 110B, for example, when the signal light S is not in operation.

In addition, in order to simplify the explanation in FIG. 35, for forward pumping, the wavelengths (numbers) of the secondary pumping light PF2 are 1, 2, 3, 4, and 5, and the wavelengths (numbers) of the primary pumping light PF1 are 6, 7, and 8, and these are represented as pumping light #N (N=1, 2, 3, ..., 8, Nmax=8).

First, the overall control unit 810 controls the ASE light source 3402 and the WSS 3401 to generate, for example, dummy light in the C-band and L-band of a 50 GHz grid and transmit it to the optical transmission path (step S3501). Next, the overall control unit 810 records the results of measuring the OSNR of all the dummy light with the OCM 3403 (step S3502).

Next, the overall control unit 810 sets the number of detected wavelengths N to an initial value of 0 (step S3503). Then, the overall control unit 810 selects and emits the forward excitation light #N+1 of the first optical transmission device 110A (step S3504).

Next, the overall control unit 810 records the results of measuring the OSNR of all dummy light with the OCM 3404 (step S3505). Next, the overall control unit 810 compares the OSNR when there is no pump light with the result of step S3503, and if there is degradation, sets a flag for the wavelength (number #) of the corresponding pump light (step S3506).

Then, the central control unit 810 judges whether N=Nmax (step S3507). If N has not reached Nmax (step S3507: No), the central control unit 810 returns to the process of step S3504. On the other hand, if N has reached Nmax (step S3507: Yes), the central control unit 810 stops the transmission of dummy light by the ASE light source 3402 and WSS 3401 (step S3508), and ends the above process.

Figure 36 is a flowchart of an example of excitation control for wavelengths with XPM degradation. This explains an example of control that the general control unit 810 performs on the forward excitation unit 140A of the first transmission device 110A and the backward excitation unit 140B of the second optical transmission device 110B based on the information on the wavelengths that cause XPM degradation described in Figure 35.

First, the overall control unit 810 acquires information on the wavelength of the excitation light that causes XPM degradation detected in FIG. 35 (step S3601). Next, based on the information acquired in step S3601, the overall control unit 810 performs extinction control on the secondary excitation light source 842b of the secondary excitation light PF2 (number #) that causes XPM for forward excitation. In addition, the overall control unit 810 performs extinction control on the primary excitation light source 842a of the wavelength (number #) closest to zero dispersion for the primary excitation light PF1 (number #) that causes XPM. In addition, the primary excitation light source 842a that is not extinguished is controlled to reduce its power by a predetermined amount (for example, 50% to 100%) (step S3602).

Next, for forward pumping, when the secondary pumping light PF2 (number #) is quenched in step S3603 for pumping light (number #) that does not cause XPM, the integrated control unit 810 performs control to flatten the gain by increasing the power of the primary pumping light PF1 (number #) that corresponds to the quenched secondary pumping light PF2 (number #) (step S3603).

Furthermore, the general control unit 810 controls the gain for the backward pumping unit 140B to compensate for the gain of the forward pumping Raman performed in step S3603 (step S3604), and ends the above control.

Figure 37 is an example chart explaining the determination of the amount of power reduction of the primary pump light that causes XPM. The horizontal axis of Figure 37 is the amount of power reduction of the primary pump light, and the vertical axis is the amount of OSNR degradation. At a pump light power set to obtain Raman gain, XPM causes a 1.2 dB OSNR degradation, but this OSNR degradation is eliminated by reducing the pump light power by 100%.

As explained in FIG. 36 etc., in the embodiment, the primary pump light PF1 that causes XPM is extinguished when it is closest to the zero dispersion wavelength. In addition, for other pump lights, the influence of XPM is relatively small, so the power is reduced by a predetermined amount, for example, about 50% to 100%, in order to ensure Raman gain while suppressing the influence of XPM. A guideline for power reduction is, for example, a 0.3 dB OSNR reduction due to XPM. The power reduction that results in a 0.3 dB OSNR reduction can be adjusted while monitoring the OSNR, but the amount of power reduction required to achieve a 0.3 dB OSNR reduction can be predicted in advance based on the actual measured amount of OSNR degradation.

The relationship between the amount of power reduction of the primary excitation light PF1 and the amount of OSNR degradation measured as shown in FIG. 37 is stored in advance in the memory 1102 of the general control unit 810 as a setting table 3700. The general control unit 810 then refers to the setting table 3700 to determine the amount of power reduction of the primary excitation light PF1 that corresponds to the amount of OSNR degradation measured.

The forward Raman amplifier of the embodiment described above is a forward Raman amplifier that has multiple pump light sources with different wavelengths, and changes the number of pump light sources to be emitted depending on the fiber type. This corresponds to the type of fiber used in the optical transmission line, and when DSF and dispersion-shifted fiber are used, the number of pump light sources can be reduced by, for example, extinguishing pump light sources with wavelengths that cause XPM degradation, thereby suppressing XPM degradation of the signal light and maintaining the signal quality of the signal light. Note that when SMF is used in the optical transmission line, XPM degradation does not occur, so the number of pump light sources to be emitted is not changed.

The forward Raman amplifier of the embodiment has multiple pumping light sources with different wavelengths, and changes the power ratio between the multiple pumping light sources with different wavelengths depending on the fiber type. This makes it possible to maintain the signal quality of the signal light by suppressing XPM degradation of the signal light, for example by reducing the power ratio of the pumping light sources with wavelengths that cause XPM degradation, in response to the type of fiber used in the optical transmission line, when DSF and dispersion-shifted fiber are used in the optical transmission line.

The forward Raman amplifier of the embodiment is a forward Raman amplifier that has multiple pumping light sources with different wavelengths, and changes the wavelength characteristics of the gain depending on the fiber type. This makes it possible to maintain the signal quality of the signal light by suppressing XPM degradation of the signal light, for example, by providing a characteristic that reduces the gain of the wavelength portion of the pumping light source that degrades XPM in response to the type of fiber used in the optical transmission line, when DSF and dispersion-shifted fiber are used in the optical transmission line.

The forward Raman amplifier of the embodiment changes the number of pump light sources to be emitted depending on the zero dispersion wavelength of the fiber. For example, when DSF and dispersion shifted fiber are used in the optical transmission path, the number of pump light sources is reduced by extinguishing the pump light sources of wavelengths that cause XPM degradation. In addition, in the case of DSF, it is also possible to not install pump light sources of wavelengths that cause XPM degradation in advance. This makes it possible to suppress XPM degradation of the signal light in accordance with the type of fiber used in the optical transmission path, and to maintain the signal quality of the signal light.

The forward Raman amplifier of the embodiment changes the power ratio between multiple pumping light sources with different wavelengths according to the zero-dispersion wavelength of the fiber. The zero-dispersion wavelengths are different between SMF and DSF, and when the optical transmission line is DSF, XPM degradation occurs in part of the signal light according to the zero-dispersion wavelength. Therefore, the power ratio between multiple pumping light sources with different wavelengths is changed according to the zero-dispersion wavelength of the DSF fiber. For example, XPM degradation of the signal light can be suppressed by reducing the power ratio of the pumping light sources with wavelengths that cause XPM degradation, and the signal quality of the signal light can be maintained.

The forward Raman amplifier of the embodiment also changes the wavelength characteristic of the gain according to the zero-dispersion wavelength of the fiber. The zero-dispersion wavelength is different between SMF and DSF, and when the optical transmission line is DSF, XPM degradation occurs in part of the signal light according to the zero-dispersion wavelength. Therefore, the slope of the gain with respect to the wavelength is changed according to the zero-dispersion wavelength of the DSF fiber. For example, by lowering the gain of the wavelength that causes XPM degradation, it is possible to suppress XPM degradation of the signal light, and the signal quality of the signal light can be maintained.

In addition, the forward Raman amplifier of the embodiment may be configured to turn off the power of the pump light source of a wavelength when the signal light is present within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength. This folding back quenches the pump light of a wavelength that overlaps with the band of the signal light, making it possible to easily suppress XPM degradation of the signal light and maintain the signal quality of the signal light.

In addition, in the forward Raman amplifier of the embodiment, when signal light is present within a wavelength band obtained by folding back the pump light wavelength on the wavelength axis with respect to the zero-dispersion wavelength, the pump light power ratio of that wavelength may be lowered below the pump light power ratio of a wavelength where no signal light is present within the wavelength band obtained by folding back the pump light wavelength with respect to the zero-dispersion wavelength. By this folding back, the pump light power ratio of a wavelength that overlaps the band of the signal light is lowered below the pump light power ratio of a wavelength that does not overlap with the signal light, which makes it possible to easily suppress XPM degradation of the signal light and maintain the signal quality of the signal light.

The bidirectional Raman system of the embodiment may also include the forward Raman amplifier described above, and a backward Raman amplifier that generates a wavelength characteristic of gain in the opposite direction so as to compensate for the wavelength characteristic of the gain generated by the forward Raman amplifier. This makes it possible to suppress XPM degradation caused by forward Raman pumping, and to compensate for the gain of the signal light reduced by the forward Raman pumping by backward Raman pumping, thereby maintaining signal quality.

The forward Raman amplifier of the embodiment is equipped with a pump light source of a wavelength that does not fall into the wavelength band obtained by folding back the wavelength band of the signal light on the wavelength axis with respect to the zero dispersion wavelength of the fiber. This folding back allows only pump light sources of wavelengths that do not overlap with the pump light wavelengths to easily prevent XPM degradation. In this case, costs can be reduced by not providing a pump light source.

In addition, in the forward Raman amplifier of the embodiment, when there are multiple pumping light sources in which the signal light exists within a wavelength band in which the pumping light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength, the power ratio may be reduced for pumping light sources closer to the zero-dispersion wavelength. This makes it possible to ensure Raman gain while suppressing the effects of XPM.

The forward Raman amplifier of the embodiment is further configured to use non-coherent pump light and coherent light as pump light, and further, for both the coherent pump light source and the non-coherent pump light source, when signal light is present within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength, the coherent pump light source is turned off, and the power ratio of the non-coherent pump light source may be reduced as the pump light source is closer to the zero-dispersion wavelength. This makes it possible to ensure Raman gain while suppressing the effects of XPM by using non-coherent pump light and coherent light as pump light.

In addition, the forward Raman amplification system of the embodiment is a bidirectional Raman amplification system including an upstream station having a forward Raman amplifier, a downstream station connected to the upstream station via an optical transmission line and having a backward Raman amplifier, and a general control unit, in which the upstream station transmits dummy light in which ASE light is shaped into the spectral shape of signal light, and the downstream station measures the OSNR of the dummy light, and identifies the zero-dispersion wavelength of the optical transmission line based on the measurement result. The zero-dispersion wavelength of the fiber used in the optical transmission line may vary depending on the manufacturing time, but even with this variation, the zero-dispersion wavelength for each type of optical transmission line can be easily measured, making it possible to perform Raman amplification suited to the type.

In addition, in the embodiment, the control unit may be disposed in one of the multiple optical transmission devices. The control unit may be disposed outside the optical transmission device, but by disposing the control unit in one of the multiple optical transmission devices, costs can be reduced.

100, 800, 1300, 1500, 3400 Optical transmission system 110 Optical transmission device 120 Optical transmission line 130 Optical amplifier 140 Pumping section 140A Forward pumping section 140B Backward pumping section 141A, 141B Multiplexer 142A, 142B Pumping light source 150A Forward pumping control section 150B Backward pumping control section 810 General control section 841A, 841B Multiplexer 842a Primary pumping light source 842b Secondary pumping light source 842c Pumping light source 844a to 844c Polarization coupler 845a to 845f Multiplexing filter 846a, 846c Demultiplexing filter 847a, 847c Pumping light monitor 848a, 848b Pumping light control section 861A, 861B Demultiplexer 862A Transmitting side signal light monitor 862B Receiving side signal light monitor 900, 1700 Information table 1000 Pumping light power ratio table 1100 Bus 1101 CPU
1102 Memory 1103 Network IF
1105 Recording medium 1501 Transmitter 1502 Multiplexer 1503 Demultiplexer 1504 Receiver 1505 Error rate measurement unit 1800 Transmitting side pumping light power ratio table 1900 Receiving side pumping light power ratio table 3401 WSS
3402 ASE light source 3404 OCM
λ0 Zero dispersion wavelength NW Network PB Backward pumping light PF Forward pumping light S Signal light

Claims (13)

A forward Raman amplifier that has multiple pump light sources with different wavelengths, and is characterized by changing the number of pump light sources that emit light depending on the fiber type. A forward Raman amplifier that has multiple pumping light sources with different wavelengths, and is characterized by changing the power ratio between the multiple pumping light sources with different wavelengths depending on the fiber type. A forward Raman amplifier having a plurality of pumping light sources with different wavelengths, characterized in that the wavelength characteristic of gain is changed according to the type of fiber.
A forward Raman amplifier that changes the number of pump light sources that emit light according to the zero dispersion wavelength of the fiber. A forward Raman amplifier that changes the power ratio between multiple pump light sources with different wavelengths according to the zero dispersion wavelength of the fiber. The forward Raman amplifier according to claim 3, characterized in that the wavelength characteristic of the gain is changed according to the zero dispersion wavelength of the fiber. A forward Raman amplifier that turns off the power of the pump light source for a wavelength when the signal light is present within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero dispersion wavelength. A forward Raman amplifier characterized in that, when signal light is present within a wavelength band in which the pump light wavelength is folded back on the wavelength axis relative to the zero-dispersion wavelength, the pump light power ratio of that wavelength is lower than the pump light power ratio of a wavelength in which no signal light is present within the wavelength band in which the pump light wavelength is folded back on the wavelength axis relative to the zero-dispersion wavelength. A forward Raman amplifier according to any one of claims 1 to 6,
a backward Raman amplifier that generates a wavelength characteristic of gain in an opposite direction so as to compensate for the wavelength characteristic of gain generated by the forward Raman amplifier;
A bidirectional Raman amplification system comprising: A forward Raman amplifier equipped with a pump light source with a wavelength that does not fall into the wavelength band obtained by folding back the wavelength band of the signal light on the wavelength axis with respect to the zero dispersion wavelength of the fiber. In claim 8, the forward Raman amplifier is further characterized in that, when there are multiple pumping light sources in which the signal light exists within a wavelength band in which the pumping light wavelength is folded back on the wavelength axis with respect to the zero dispersion wavelength, the pumping light source closest to the zero dispersion wavelength is extinguished. In claim 8, further comprising: using non-coherent excitation light and coherent light as excitation light;
Furthermore, the forward Raman amplifier is characterized in that, for both a coherent pump light source and a non-coherent pump light source, when signal light is present within a wavelength band in which the pump light wavelength is folded back on the wavelength axis with respect to the zero-dispersion wavelength, the coherent pump light source is extinguished, and the pump light source that is closest to the zero-dispersion wavelength among the non-coherent pump light sources is extinguished. an upstream station having a forward Raman amplifier with a pump source of a plurality of pump wavelengths, an ASE source, and a device for shaping the ASE source;
a downstream station connected to the upstream station via an optical transmission line and having a device for measuring an OSNR;
A general control unit;
In a forward Raman amplified system comprising:
The upstream station transmits dummy light of multiple wavelengths obtained by shaping the ASE light into a spectrum shape of the signal light,
the downstream station measures a first OSNR wavelength characteristic of the dummy light in a state where all pump lights of the forward Raman amplifiers are turned off;
Further, a second OSNR wavelength characteristic of the dummy light is measured in a state where only a pump light source having a first pump wavelength among a plurality of pump wavelengths in the forward Raman amplifier is irradiated;
A forward Raman amplification system, comprising: determining a power of a pumping light source having a first pumping wavelength, or a pumping light power ratio, based on a first OSNR-wavelength characteristic and a second OSNR-wavelength characteristic.

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