JP4509035B2 - Optical fiber communication system using optical amplification. - Google Patents

Description

  The present invention relates to an optical fiber communication system that performs distributed Raman amplification of an optical signal in an optical fiber laid in a city as a transmission path.

  FIG. 1 shows a configuration example of a conventional distributed Raman amplification system (DRA system) used in a wavelength division multiplexing optical fiber communication system (see, for example, Non-Patent Document 1). This figure shows the case of backward pumping DRA in which signal light and pumping light propagate in opposite directions. In this DRA system, a dispersion shifted fiber (DSF) is used as the transmission line fiber 1, and the typical value of the zero dispersion wavelength (λ0) of the DSF is 1.54 to 1.56 μm (the standard value is a slightly wider wavelength range). Have). The wavelength of the wavelength division multiplexing (WDM) signal light is set to a so-called L band 1570 to 1600 nm (typical approximate value). The pumping light is introduced into the transmission line from the linear repeaters 2-1 and 2-2 (or the receiving terminal device) using the multiplexer 3 in the direction opposite to the signal light.

  Each linear repeater 2-1 and 2-2 has an erbium-doped fiber amplifier 4 (EDFA). The signal light that leaves the linear repeater 2-1 upstream of the DSF and propagates through the DSF undergoes distributed Raman amplification near the linear repeater 2-2 downstream of the DSF, and is distributed in the transmission path. And then lumped constant with EDFA. In the present DRA system, the optical SNR (signal to noise ratio) is improved by distributed Raman amplification.

H. Masuda et al. Electron. Lett. , Vol. 35, pp. 411-412, 1999

  FIG. 3 is a diagram showing Raman gain spectra of the prior art and the first embodiment of the present invention. The horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). The thin line shows the Raman gain spectrum of the prior art. The spectrum is a spectrum obtained as a result of arranging the excitation light wavelength so that a flat gain spectrum can be obtained in the signal light wavelength region (1570 nm to 1600 nm). The difference (so-called Stokes shift) between the Raman gain peak signal light wavelength (λs_peak) and the pumping light wavelength in the case of single wavelength pumping is about 105 nm in the L band.

  The excitation light wavelengths of this prior art are three wavelengths of 1455, 1480, and 1505 nm, and λs_peak corresponding to each of these excitation light wavelengths is 1560, 1585, and 1610 nm, respectively. In FIG. 3, a relatively flat gain spectrum having a slight peak at a wavelength corresponding to the λs_peak is obtained.

  As described above, in the related art, in order to obtain a flat gain spectrum in the signal light wavelength range (1570 to 1600 nm), a desired gain (Gflat, a level indicated by a dotted line in the figure) with a gain width of about 50 nm of about 1560 to 1610 nm. A gain close to) has occurred. The desired gain value decreases as the gain width (Δλ) increases. In addition, the pumping light power that can be used in the distributed Raman system is limited to a predetermined value from the viewpoint of the performance of the pumping light source, system reliability, and safety.

The pumping light powers corresponding to pumping light wavelengths 1, 2, and 3 (1455, 1480, and 1505 nm in the above example) are Pp1, Pp2, and Pp3, respectively. If the limit value of the sum of pumping light power is Pp_max,
Pp1 + Pp2 + Pp3 = Pp_max
There is a relationship.

The Raman gain increases with the pumping light power. The Raman gain at the signal light wavelength λs is G (λs). At this time, the value obtained by integrating G (λs) with respect to λs is proportional to Pp_max. That is, if the proportionality constant is C, ∫G (λs) dλ = CP _{p_max} (1)
There is a relationship.

  Therefore, with the limited pumping light power, the prior art has a drawback that the Raman gain is limited and the optical SNR improvement amount is limited.

  The above is the case where the transmission line fiber is a DSF, but the same can be said when the transmission line fiber is a NZ-DSF as specifically shown in the embodiment.

  The present invention has been carried out against such a background, and is an optical fiber communication that can solve the drawbacks of the prior art that the Raman gain is limited and the optical SNR improvement amount is limited. The purpose is to provide a system.

  In order to solve the above-described problems, the present invention uses the following means and configuration.

  The present invention amplifies multi-wavelength signal light, a transmission line fiber as a gain medium for Raman amplification, a pumping light source for sending pumping light co-propagating through the transmission path fiber in the opposite direction to the signal light, An optical fiber communication system that is installed between a transmission line fiber and the pumping light source, and includes a multiplexer that combines the signal light and the pumping light. The signal light wavelength is longer than the zero dispersion wavelength of the transmission line fiber, and the Raman gain at the short wavelength end wavelength of the signal light wavelength is larger than the Raman gain at the long wavelength end wavelength of the signal light wavelength. In addition, the signal light power at the long wavelength end of the signal light wavelength is greater than the signal light power at the short wavelength end of the signal light wavelength, and the optical SNR spectrum after transmission is flat. .

  For example, the transmission line fiber is a dispersion shifted fiber, and the signal light wavelength is in the L band. Alternatively, the transmission line fiber is a non-zero dispersion shifted fiber, and the signal light wavelength is in the C band.

  Alternatively, in the optical fiber communication system of the present invention, the signal light wavelength is on a shorter wavelength side than the zero dispersion wavelength of the transmission line fiber, and the Raman gain at the short wavelength end wavelength of the signal light wavelength is the signal light. The light after transmission is smaller than the Raman gain at the long wavelength end of the wavelength, and the signal light power at the long wavelength end of the signal light wavelength is smaller than the signal light power at the short wavelength end of the signal light wavelength. The SNR spectrum is flat.

  For example, the transmission line fiber is a non-zero dispersion shifted fiber, and the signal light wavelength is in the C band.

  In addition, the excitation light source emits two wavelengths of excitation light, a first spectrum peak wavelength region of the excitation light having a long wavelength, and a second spectrum peak wavelength region of the excitation light having a short wavelength of the excitation light; By arranging the wavelengths of the pumping light having the long wavelength and the short wavelength so as to overlap, it is effective in improving the Raman gain in the long wavelength region of the Raman gain spectrum. At this time, it is preferable that the excitation light source emits multi-wavelength excitation light, and the difference between the longest wavelength and the shortest wavelength of the multi-wavelength excitation light is 25 to 35 nm.

  According to the present invention, the disadvantage that the Raman gain is limited and the optical SNR improvement amount is limited, which has been a problem in the prior art, can be solved.

  Embodiments of the present invention will be described below with reference to the drawings.

(First Example)
The wavelength relationship and dispersion curve in the first example are shown in FIG. The horizontal axis represents wavelength (nm), and the vertical axis represents dispersion (ps / nm / km). The signal light wavelength is 1570 to 1600 nm (L band), and the center of the zero dispersion wavelength is about 1550 nm.

Further, the Raman gain spectrum in the present embodiment is shown by a thick line in FIG. The horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). The difference in configuration between the first embodiment and the prior art is mainly in the excitation light wavelength used. The excitation light wavelengths in the prior art were 1455, 1480, and 1505 nm as described above. On the other hand, the excitation light wavelengths in the first embodiment are 1470 and 1490 nm. Λs_peak corresponding to each of these excitation light wavelengths is 1570 and 1600 nm, respectively. If the pumping light powers at the pumping light wavelengths 1470 and 1490 are Pp1 and Pp2, respectively,
Pp1 + Pp2 = Pp_max
There is a relationship.

The distribution width of the excitation light wavelength in the prior art is 1505-1455 = 50 nm.
In contrast, in this example, 1490-1470 = 20 nm.
It has become. For this reason, the gain width in the prior art is about 50 nm as described above, whereas in this embodiment, the gain width is as narrow as about 20 nm.

  However, in the signal light wavelength range of 1570 to 1600 nm, the gain of the present embodiment is higher than that of the prior art. In particular, in this embodiment, the gain at 1570 nm is 13 dB, and the gain at 1600 nm is 11.5 dB.

  On the other hand, in the prior art, the gains at 1570 nm and 1600 nm are both about 10 dB. In this embodiment, the reason why the gain at the short wavelength is made larger than the gain at the long wavelength will be described later.

  Further, the gain spectrum of the EDFA is set so that the gain spectrum of a unit transmission section (section consisting of one transmission path and a linear repeater immediately after that) is flat. This can be easily performed by adjusting the EDFA spectrum shape by adjusting the excitation level of the EDFA and adjusting the gain equalizing optical filter installed in the EDFA.

  FIG. 5 shows the optical SNR improvement characteristics. The horizontal axis represents the Raman gain (dB), and the vertical axis represents the optical SNR improvement (dB). That is, the relationship between the optical SNR improvement amount and the Raman gain is shown for each of cases where the noise figure (NF) of the EDFA is 5, 7, and 9 dB. For example, in the case of NF = 9 dB, since the Raman gain (G) = 10 dB in the conventional technique, the optical SNR improvement amount (ΔOSNR) = 7.5 dB.

  On the other hand, in this embodiment, since the signal light wavelength is 1570 nm and G = 13 dB, ΔOSNR = 9.1 dB.

That is, in this embodiment, ΔOSNR that is 1.6 dB higher than 9.1-7.5 = 1.6 dB is obtained compared to the prior art. Similarly, at 1600 nm at the long wavelength end, in this embodiment, G = 11.5 dB at a signal light wavelength of 1600 nm, so ΔOSNR = 8.3 dB. Therefore, this example is compared with the prior art,
8.3-7.5 = 0.8 dB
From the relationship, ΔOSNR which is 0.8 dB higher is obtained.

  FIG. 6 shows the signal light power spectrum of the fiber input. The horizontal axis represents wavelength (nm) and the vertical axis represents signal light power (dBm). In the prior art, since the Raman gain spectrum is flat as shown in FIG. 3, the signal light power spectrum should be flat. FIG. 7 shows an optical SNR spectrum of the EDFA output. The horizontal axis represents wavelength (nm) and the vertical axis represents optical SNR (dB). In the prior art, since the Raman spectrum and the signal light power spectrum are both flat, a flat optical SNR spectrum is obtained. However, the value of the signal light power is assumed to be a value within a maximum value (non-linear limit, indicated by a dotted line in FIG. 6) determined by the non-linear characteristic in the DSF that is the transmission line fiber. However, the non-linear limit is an amount determined by system parameters such as a signal light channel interval, a bit rate, and a modulation code.

  On the other hand, in this embodiment, the signal light power spectrum is not flat as shown in FIG. That is, the signal light power at the short wavelength end 1570 nm is the same as that of the conventional technique, and the signal light power at the long wavelength end is made larger than that of the conventional technique by ΔPs. This ΔPs is a value within an allowable value ΔPs_max determined by the nonlinear characteristic. As shown in FIG. 6, the reason why the signal light power in the long wavelength region can be higher than the signal light power in the short wavelength region is that the long wavelength region (˜1600 nm) is the zero dispersion wavelength region (approximately 1540˜ This is because the local dispersion is large and the nonlinear characteristic is small compared to the short wavelength region (˜1570 nm). In this embodiment, ΔPs = 0.8 dB is set in consideration of the optical SNR improvement characteristics.

  The optical SNR spectrum after transmission in the unit transmission section in this example is shown in FIG. The optical SNR improvement at the short wavelength end of 1570 nm (increase from the prior art) is 1.6 dB due to the increase in Raman gain. On the other hand, the optical SNR improvement amount at the long wavelength end of 1600 nm is 1.6 dB by adding the contribution of increasing the Raman gain of 0.8 dB and the increase of the signal light power of 0.8 dB. In this way, as shown in FIG. 7, an optical SNR improvement amount of 1.6 dB was obtained over the signal light wavelength range of 1570 to 1600 nm.

  That is, according to the present embodiment, it was possible to avoid the disadvantages that the Raman gain was limited and the optical SNR improvement amount was limited, which was a problem in the prior art.

The present invention is generally effective when the signal light wavelength region and the zero dispersion wavelength of the transmission line fiber are close within a certain range, and the short wavelength end and the long wavelength end within the signal light wavelength region are effective. This is effective when the absolute value (D) of the variance value differs greatly. The difference of D (Dmax−Dmin) is 1/5 or more with respect to the maximum value of D (Dmax). That is,
(Dmax-Dmin) / Dmax> 1/5
It is.

(Second embodiment)
FIG. 4 shows the Raman gain spectrum in the second embodiment of the present invention. The horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). For comparison, the Raman gain spectrum of the first embodiment is also shown. The excitation light wavelengths in the second embodiment are 1470 and 1490 nm, the same as in the first embodiment. The main differences between the second embodiment and the first embodiment are as follows.

If the pumping light powers at the pumping light wavelengths 1470 and 1490 nm in this embodiment are Pp1 ^* and Pp2 ^* , respectively, the pumping light powers in the first embodiment are Pp1 and Pp2, respectively.
Pp1 ^* + Pp2 ^* = Pp_max, Pp1 ^* > Pp1, Pp2 ^* <Pp2
There is a relationship.

  That is, in this embodiment, compared with the first embodiment, the pumping light power of the short wavelength is increased, the pumping light power of the long wavelength is decreased, and the signal light gain in the short wavelength region (˜1570 nm) is improved. Yes. Further, the signal light gain in the long wavelength region (˜1600 nm) is almost the same as that of the conventional technology. In this embodiment, the gain at 1570 nm is 15 dB, and the gain at 1600 nm is 10 dB.

  With respect to the optical SNR improvement characteristics of FIG. 5, in this embodiment, the case where the NF of the EDFA is 5 dB will be described. In the prior art, Raman gain (G) = 10 dB, so the optical SNR improvement amount (ΔOSNR) = 5.0 dB. On the other hand, in this embodiment, since the signal light wavelength is 1570 nm and G = 15 dB, ΔOSNR = 6.8 dB.

That is, this example is compared with the prior art,
6.8−5.0 = 1.8 dB
As a result, ΔOSNR which is 1.8 dB higher is obtained. Further, at 1600 nm at the long wavelength end, the same Raman gain (G = 10 dB) is used in both the present embodiment and the conventional technique, and thus the optical SNR improvement amount is the same (ΔOSNR = 5.0 dB).

  With respect to the signal light power spectrum of the fiber input in FIG. 6, in this embodiment, the signal light power at the long wavelength end is made larger than that of the prior art by ΔPs = 1.8 dB.

  The optical SNR spectrum in the present example has the shape shown in FIG. 7, and the optical SNR improvement amount (increased amount from the prior art) at the short wavelength end of 1570 nm is the contribution of the above-mentioned Raman gain increase of 1.8 dB. On the other hand, the optical SNR improvement amount at the long wavelength end of 1600 nm is 1.8 dB of the signal light power increase. Therefore, an optical SNR improvement amount of 1.8 dB was obtained over the signal light wavelength range of 1570 to 1600 nm (L band).

  That is, according to the present embodiment, it was possible to avoid the disadvantages that the Raman gain was limited and the optical SNR improvement amount was limited, which was a problem in the prior art.

(Third embodiment)
A configuration example of the third embodiment of the present invention is shown in FIG. The transmission line fiber 1 is a non-zero DSF (NZ-DSF), and the erbium-doped fiber amplifier 4 is a C-band EDFA. The wavelength relationship and the dispersion curve in this example are shown in FIG. The horizontal axis represents wavelength (nm), and the vertical axis represents dispersion (ps / nm / km). The signal light wavelength is 1530 to 1560 nm (C band), and the center of the zero dispersion wavelength is about 1500 nm.

  Further, the Raman gain spectrum in this example is shown in FIG. The horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). The third embodiment is similar to the first embodiment except for the following points.

  The signal light wavelength range in the third embodiment is 1530 to 1560 nm, and the excitation light wavelengths corresponding to the signal light wavelength range are 1430 and 1450 nm. The signal light power spectrum of the fiber input is as shown in FIG. FIG. 11 is a diagram showing signal light power spectra in the prior art and the third embodiment. The horizontal axis represents wavelength (nm) and the vertical axis represents signal light power (dBm). Further, the optical SNR spectrum in this example is obtained by shifting the signal light wavelength to the 40 nm short wavelength side in FIG.

  As in the case of the first embodiment, the optical SNR improvement amount (increased amount from the prior art) at the short wavelength end of 1530 nm is 1.6 dB due to the increase in Raman gain. On the other hand, the optical SNR improvement at the long wavelength end of 1560 nm is 1.6 dB by adding the contribution of the above-mentioned Raman gain increase of 0.8 dB and the increase of the signal light power of 0.8 dB. In this way, an optical SNR improvement of 1.6 dB was obtained over a signal light wavelength of 1530 to 1560 nm.

  That is, according to the present embodiment, it was possible to avoid the disadvantages that the Raman gain was limited and the optical SNR improvement amount was limited, which was a problem in the prior art.

(Fourth embodiment)
A configuration example of the fourth embodiment is shown in FIG. The transmission line fiber 1 is a non-zero DSF (NZ-DSF), and the erbium-doped fiber amplifier 4 is a C-band EDFA. The wavelength relationship and the dispersion curve in this example are shown in FIG. The horizontal axis represents wavelength (nm), and the vertical axis represents dispersion (ps / nm / km). The signal light wavelength is 1530 to 1560 nm (C band), and the center of the zero dispersion wavelength is about 1590 nm.

  FIG. 14 shows the Raman gain spectrum in this example. The horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). For comparison, the Raman gain spectrum in the prior art is also shown. The third embodiment is similar to the first embodiment except for the following points.

  In this embodiment, the signal light wavelength range is 1530 to 1560 nm, and the excitation light wavelengths corresponding thereto are 1430 and 1450 nm.

  In this embodiment, the zero dispersion wavelength is on the long wavelength side of the signal light wavelength, which is the opposite of the case of the first embodiment. Therefore, in this embodiment, the Raman gain in the long wavelength region (˜1560 nm) close to the zero dispersion wavelength is made larger than the Raman gain in the short wavelength region (˜1530 nm). Also, the signal light power spectrum of the fiber input is as shown in FIG. 15, and the signal light power in the short wavelength region (˜1530 nm) far from the zero dispersion wavelength is made larger than the level of the prior art. In FIG. 15, the horizontal axis represents wavelength (nm) and the vertical axis represents signal light power (dBm). Further, the optical SNR spectrum in this example is obtained by shifting the signal light wavelength to the 40 nm short wavelength side in FIG.

  As in the case of the first embodiment, the optical SNR improvement amount (increased amount from the prior art) at the short wavelength end of 1530 nm is 1.6 dB due to the increase in Raman gain. On the other hand, the optical SNR improvement at the long wavelength end of 1560 nm is 1.6 dB by adding the contribution of the above-mentioned Raman gain increase of 0.8 dB and the increase of the signal light power of 0.8 dB. In this way, an optical SNR improvement amount of 1.6 dB was obtained over the signal light wavelength range of 1530 to 1560 nm.

  That is, according to the present embodiment, it was possible to avoid the disadvantages that the Raman gain was limited and the optical SNR improvement amount was limited, which was a problem in the prior art.

(Fifth embodiment)
The present embodiment has the same basic configuration as the first embodiment, but differs from the first embodiment in that the specific range of the multiple excitation light wavelengths is clarified. The transmission line fiber of this embodiment is a DSF, the EDFA is an extended L-band EDFA, and the signal light wavelength range is 1575 to 1620 nm in consideration of the nonlinearity of the DSF. However, the short wavelength end 1575 nm is mainly determined by the non-linearity of the DSF, and the long wavelength end 1620 nm is determined by the gain band of the extended L-band EDFA.

  However, when phosphorus co-added EDFA is used as the extended L-band EDFA, the gain band of EDFA is up to about 1620 nm, but when bismuth EDFA is used, the gain band of EDFA is up to 1615 nm. Further, when a normal silica EDFA is used, the gain band of the EDFA is up to about 1605 nm. However, the features and effects of this embodiment do not depend on the type of EDFA and the signal light wavelength range. The total pumping light power is a specified value determined by system constraints.

  FIG. 16 shows an example of a Raman gain spectrum in this embodiment. The horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). The case where the number of excitation light wavelengths is 1 to 3 is shown. The excitation light wavelength when the number of excitation light wavelengths is 1 is 1493 nm (wavelength arrangement 1). When the number of excitation light wavelengths is 2, the excitation light wavelengths are 1476 nm and 1501 nm (wavelength arrangement 2), the difference between the longest wavelength and the shortest wavelength (Δλp) is 25 nm, and further 1475 nm and 1505 nm (wavelength arrangement 3, Δλp = 30 nm) and 1470 nm and 1505 nm (wavelength arrangement 4, Δλp = 35 nm). The excitation light wavelengths when the number of excitation light wavelengths is 3 are 1455 nm, 1478 nm, and 1510 nm (wavelength arrangement 5, Δλp = 55 nm). For each wavelength arrangement, the pump light wavelength and the power of each pump light wavelength are adjusted so that the gain at the short wavelength end 1575 nm of the signal light wavelength region is 1 dB larger than the minimum gain (Gmin) in the signal light wavelength region. Yes.

  For the wavelength arrangements 1 to 5, when the wavelength difference Δλp increases, the Raman gain spectrum becomes a wide band, but the average Raman gain in the signal light wavelength region is lowered. Further, as the wavelength difference Δλp increases, the deviation of Raman gain, that is, the difference (ΔG) between the maximum gain (Gmax) and the minimum gain Gmin within the signal light wavelength region decreases.

  FIG. 17 shows the dependency of Gmin (existing in the wavelength region of 1620 nm to 1600 nm or more) and ΔG on the wavelength difference Δλp. The horizontal axis represents the wavelength difference (nm) and the vertical axis represents the minimum gain (dB). Gmin shows a maximum value in the vicinity of Δλp = 25 to 30 nm, and shows a significant decrease when Δλp is smaller than about 20 nm and in a wavelength region larger than about 40 nm.

  On the other hand, ΔG decreases with Δλp and falls within a small value (about 3 dB or less) in a region where Δλp is greater than about 20 nm. This small ΔG facilitates gain spectrum equalization in the present optical fiber communication system, resulting in higher performance and lower cost of the system. From the viewpoint of Gmin and ΔG, Δλp is preferably about 25 to 35 nm. That is, it was found that Δλp = about 25 to 35 nm is an allowable wavelength difference.

  The factors of the above characteristics are shown below. FIG. 18 shows Raman gain spectrum components in the wavelength arrangement 3 (excitation light wavelengths = 1475 nm and 1505 nm). The horizontal axis represents wavelength (nm), and the vertical axis represents gain (normalized value). However, the peak value of the dB unit gain of each component (components of 1475 nm and 1505 nm excitation light) is normalized to 100. According to FIG. 18, the first spectral peak wavelength range of the 1505 nm component is about 1605 to 1625 nm, while the first spectral peak wavelength range of the 1475 nm component is about 20 nm of about 1570 to 1590 nm, and the second spectral peak wavelength range. The area is about 15 nm, about 1610-1625.

  That is, in the wavelength arrangement 3, the first spectral peak wavelength region of the long wavelength component of the excitation light and the second spectral peak wavelength region of the short wavelength component of the excitation light overlap, and each component shown in FIG. Is effective in improving the Raman gain in the long wavelength region of the synthesized Raman gain spectrum. Further, the above operation was found to be the same in the wavelength arrangements 2 and 4.

  As described above, by setting Δλp to about 25 to 35 nm, a high minimum gain Gmin and a small gain deviation ΔG can be obtained. The excitation light wavelength arrangement method can be applied not only to the first embodiment but also to the second to fourth embodiments. That is, the transmission line fiber may be NZ-DSF, and the signal light wavelength band may be the C band. However, in the third and fourth embodiments using the Raman gain in the C band, the shape of the Raman gain spectrum is almost the same as in the L band. That is, the wavelength dependence is small for the wavelength difference between the C band and the L band.

  As described above, according to the present embodiment, it is possible to avoid the disadvantage that the Raman gain is limited and the optical SNR improvement amount is limited, which is a problem in the prior art.

  According to the present invention, the problem that the Raman gain is limited and the optical SNR improvement amount is limited, which is a problem in the prior art, can be solved. A fiber communication system can be realized.

The figure which shows the structural example (in the case of DSF) of a back excitation DRA system. The figure which shows wavelength relationship (in the case of DSF). The figure which shows the Raman gain spectrum in a prior art and a 1st Example. The figure which shows the Raman gain spectrum in a 2nd Example. The figure which shows the optical SNR improvement characteristic. The figure which shows the signal light power spectrum in a prior art and a 1st, 2nd Example. The figure which shows the optical SNR spectrum in a prior art and a 1st, 2nd Example. The figure which shows the structural example (in the case of NZ-DSF-1) of a back excitation DRA system. The figure which shows wavelength relationship (in the case of NZ-FDS-1). The figure which shows the Raman gain spectrum in a prior art and a 3rd Example. The figure which shows the signal light power spectrum in a prior art and a 3rd Example. The figure which shows the structural example (in the case of NZ-DSF-2) of a back excitation DRA system. The figure which shows wavelength relationship (in the case of NZ-FSD-2). The figure which shows the Raman gain spectrum in a prior art and 4th Example. The figure which shows the signal light power spectrum in a prior art and 4th Example. The figure which shows the Raman gain spectrum of 5th Example. The figure which shows the minimum gain and gain difference in the Raman gain spectrum of 5th Example. The figure which shows the Raman gain spectrum component in 5th Example.

Explanation of symbols

1 Transmission path fibers 2-1 and 2-2 Linear repeater 3 Multiplexer 4 Erbium-doped fiber amplifier

Claims (7)

A transmission line fiber as a gain medium for Raman amplification for amplifying multi-wavelength signal light, a pump light source for sending pump light co-propagating through the transmission line fiber in the opposite direction to the signal light, and the transmission line fiber An optical fiber communication system that is installed between the pumping light source and includes a multiplexer that multiplexes the signal light and the pumping light,
The signal light wavelength is longer than the zero dispersion wavelength of the transmission line fiber, and the Raman gain at the short wavelength end wavelength of the signal light wavelength is greater than the Raman gain at the long wavelength end wavelength of the signal light wavelength. big,
And the optical signal power at the long wavelength end of the optical signal wavelength is larger than the optical signal power at the short wavelength end of the optical signal wavelength, and the optical SNR spectrum after transmission is flat. Communications system. A transmission line fiber as a gain medium for Raman amplification for amplifying multi-wavelength signal light, a pumping light source for sending pumping light co-propagating through the transmission line fiber in the opposite direction to the signal light, and the transmission line fiber; An optical fiber communication system having a multiplexer that multiplexes the signal light and the excitation light installed between the excitation light source,
The signal light wavelength is shorter than the zero dispersion wavelength of the transmission line fiber, and the Raman gain at the short wavelength end of the signal light wavelength is greater than the Raman gain at the long wavelength end of the signal light wavelength. small,
And the optical signal power at the long wavelength end of the optical signal wavelength is smaller than the optical signal power at the short wavelength end of the optical signal wavelength, and the optical SNR spectrum after transmission is flat. Communications system.   2. The optical fiber communication system according to claim 1, wherein the transmission line fiber is a dispersion shifted fiber, and the signal light wavelength is in an L band.   2. The optical fiber communication system according to claim 1, wherein the transmission line fiber is a non-zero dispersion shifted fiber, and the signal light wavelength is in a C band.   The optical fiber communication system according to claim 2, wherein the transmission line fiber is a non-zero dispersion shifted fiber, and the signal light wavelength is in a C band.   The excitation light source emits two wavelengths of excitation light, and the first spectrum peak wavelength region of the excitation light having the long wavelength overlaps the second spectrum peak wavelength region of the excitation light having the short wavelength of the excitation light. The optical fiber communication system according to claim 1, wherein the wavelengths of the long wavelength and the short wavelength pumping light are arranged as described above.   2. The optical fiber communication system according to claim 1, wherein the pumping light source emits multiwavelength pumping light, and a difference between the longest wavelength and the shortest wavelength of the multiwavelength pumping light is 25 to 35 nm.

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