An optical transmission device according to the present invention reduces an optical surge by exploiting, for example, SRS (Stimulated Raman Scattering).
SRS is one of nonlinear effects occurring in optical fibers; with this effect, the power of the pump light decreases, and the optimum amplification wavelength region resides in a wavelength range shifted from the pump wavelength toward longer wavelengths by about 100 nm to 120 nm, thus providing gain to the longer wavelength region.
In the present invention, in order to reduce the optical surge by exploiting SRS, a dummy light source is used that emits light in the wavelength region (for example, 1650-nm to 1680-nm region) shifted from the main signal region (for example, 1550-nm region) toward longer wavelengths by about 100 nm to 120 nm (corresponding to about 13 THz in terms of frequency). Further, a highly non-linear fiber (HNLF) having an effective cross-sectional area (Aeff) of 50 μm2 or less, preferably about 10 μm2, and having a high nonlinear effect generation efficiency, is used as the medium for generating SRS. By using an optical fiber with reduced Aeff and increased optical power concentration, and thereby enhancing the nonlinear effect generation efficiency, fiber length can be reduced, which serves to reduce the size of the device, as well as the loss of optical power.
By exploiting SRS occurring in the optical fiber, an excessive power increase that momentarily occurs in the main signal band in the event of an optical surge can be reduced by providing gain to the light source in the longer wavelength region.
In the present invention, the main signal corresponds to the pump light in the Raman amplifier, and the dummy light corresponds to the signal light in the Raman amplifier.
The SRS gain is expressed by the following equation.
gr: Raman gain coefficient (m/W)
Aeff: Effective cross-sectional area (μm2)
Leff: Effective fiber length (km)
P: Pump light power (mW)
From the above equation, doubling of the pump light power in terms of mW leads to a Raman gain increase of two times in terms of dB.
In the above configuration, when the main signal is combined with the longer wavelength dummy light and introduced into the optical fiber, an SRS effect occurs between the generated optical surge and the dummy light, and the optical surge can be reduced by providing gain to the dummy light. This serves to prevent excessive power exceeding the rated power from being input to the optical receiver.
SRS is a nonlinear effect, and the transition time required for the SRS effect to become apparent is nearly zero. Accordingly, by utilizing this physical phenomenon, any instantaneous optical surge occurring in the main signal can be reduced without involving any time constant.
By providing the optical transmission device 44 within the transmitting end station 10, optical power exceeding the rated power can be prevented from being transmitted on the optical fiber transmission line 18 in the event of an instantaneous optical surge.
Since SRS provides optimum amplification in the wavelength region shifted toward longer wavelengths by about 100 nm to 120 nm from the pump wavelength, light from a light source that operates in the wavelength region shifted toward longer wavelengths by about 100 nm to 120 nm (about 13 THz) from the main signal wavelength region is used as the dummy light. For example, if the main signal wavelength is 1550 nm in the C-band, a U-band light source that emits light in the 1650 nm to 1680 nm region is used as the dummy light source to be combined. Preferably, a light source having a wide spectral linewidth, such as a Fabry-Perot laser, is used as the dummy light source in order to broaden the Raman gain band and reduce the optical surge as much as possible.
The fiber used as the medium for generating SRS is only used to introduce a loss during normal operation when no optical surge occurs. Since minimizing the loss is desirable from the standpoint of reducing the insertion loss, it is preferable to use an optical fiber with an Aeff of 50 μm2 or less and reduce the fiber length. By reducing the fiber length, it is possible to construct a smaller fiber module.
If the purpose is to protect the light detectors in the optical receiver, the optical transmission device 44 is provided only in the receiver 22.
If the purpose is to protect the output monitor section of the optical repeater, as well as the optical transmitter, the optical transmission device 44 should be provided in each device.
In the case of a system configuration containing a factor that can cause an optical surge, for example, a configuration that contains an optical switch for path switching, the optical transmission device 44 is provided in such a device to prevent the surge from being transmitted downstream.
In the configuration where the optical transmission device 44 is provided in the optical transmitter or in the optical repeater, the highly nonlinear fiber is used by optimizing the power of the dummy light source and adjusting parameters such as Aeff, loss coefficient, and fiber length, so that by using the SRS, power can be held within the rated input power to the transmission line, thereby preventing excessive optical power from being transmitted downstream.
In the configuration where the optical transmission device 44 is provided in the optical receiver, the highly nonlinear fiber is used by optimizing the power of the dummy light source and adjusting fiber parameters such as Aeff, loss coefficient, and fiber length so that the SRS effect can be produced reliably by taking into consideration the rated input level to the optical receiver (i.e., by inversely calculating from the rated optical power at which the receiver breaks down) thereby preventing excessive optical power from being input to the receiver.
The outputs of the optical amplifiers may differ between the optical transmitter, the optical repeater, and the optical receiver due to their design requirements; in that case, the fiber is used by optimizing the power of the dummy light source and optimally adjusting the parameters for each device.
The highly nonlinear fiber used as the medium for generating SRS is one of the key devices in a system that exploits a nonlinear effect. Fiber parameters can be designed so as to match the purpose of the nonlinear effect used, and the characteristic can be created that matches the purpose. Further, as the highly nonlinear fiber, use may be made of a PCF (Photonic Crystal Fiber) that is fabricated by adjusting its core diameter (effective cross-sectional area: Aeff) so as to exhibit the characteristic (nonlinearity) as designed.
It is also possible to employ a commonly used dispersion-compensating fiber (DCF) as the highly nonlinear fiber. Aeff is about 10 μm2 for the standard HNLF, about 10 μm2 for the PCF, and about 15 to 30 m2 for the DCF.
FIGS. 9 and 10-1 to 10-4 show the simulation results of the SRS gain in a highly nonlinear fiber having an Aeff of 10 μm2, a loss coefficient of 0.5 dB/km, and a length of 3 km. The system generates 88 wavelengths to carry main signals in the C-band. The gains of the main signal and dummy light are shown in
When the simulation results of
The SRS gain can be adjusted by adjusting the parameters for the highly nonlinear fiber or by varying the output power of the dummy light used. Accordingly, by varying the output power of the dummy light for each system having a different optical amplifier output, it becomes possible to address the optical surge more flexibly.
When a plurality of light sources that emit light at respectively different wavelengths in the wavelength range of about 1650 nm to about 1680 nm are used to produce the dummy light of the wavelengths for reducing the surge, the channel characteristics of the SRS gain can be uniformly held below a certain value or inter-channel difference can be reduced in order to reduce the optical surge.
The simulation results of
The wavelength and power of the dummy light for the respective cases are as shown below.
If the length of the fiber used as the medium for generating SRS is increased (for example, to 10 km), not only does the insertion loss during the normal operation when no surge occurs increase, but the level drop due to the SRS effect with the dummy light during the normal operation also increases.
Compared with the fiber length of 3 km, the level drop due to SRS becomes larger even in the normal operation, because the effective fiber length having a nonlinear effect becomes longer.
Therefore, it is preferable that the length of the highly nonlinear fiber be made shorter than 10 km, a more preferable length being about 3 km to 5 km.
The smaller the Aeff of the fiber used as the medium for generating SRS, the higher the generation efficiency of SRS, but if a highly nonlinear fiber with an extremely small Aeff is used, a loss due to connection with the standard transmission line (for example, SMF: Aeff of about 80 μm2) may increase. However, a highly nonlinear fiber whose Aeff is 15 μm2 or less, for example, about 10 μm2, and whose fusion connection loss with SMF is 0.1 dB or less, which is comparable to the conventional fiber, has already been commercially implemented. Therefore, by constructing a fiber module such as shown in
When a fiber having a high nonlinearity is used as the medium for generating SRS, a problem may occur in that four wave mixing (FWM) whose optical power threshold for causing a nonlinear effect is generally the lowest may pose a problem during normal operation. Here, the effect of the four wave mixing increases as the main signal wavelength becomes closer to the zero dispersion wavelength, but since the highly nonlinear fiber can be designed by controlling the zero dispersion wavelength, the effect of the four wave mixing (FWM) during the normal operation can be avoided.
If a highly nonlinear fiber whose Aeff is as small as about 10 μm2 is used as the medium for generating SRS, there is a possibility that SBS (Stimulated Brillouin Scattering) may occur before the SRS effect distinctly appears.
The threshold Pth of the input power at which SBS occurs is expressed by the following equation.
Aeff: Effective cross-sectional area of fiber core
gSBS: SBS gain coefficient
Leff: Effective fiber length
ΔVL: Spectral linewidth of signal
ΔVB: SBS gain bandwidth
α: Fiber loss coefficient
L: Fiber length
As can be seen from the above equation, if Aeff is reduced in order to generate SRS efficiently, the generation threshold of SBS also reduces, and when the optical power exceeds the threshold, SBS occurs and the level of the optical power passing through the fiber drops.
However, since the degree to which the optical surge is reduced by the present invention is also dependent on the optical power of the dummy light (for example, about 1650 nm to 1680 nm) to which the gain is provided, the optical surge can be reduced without causing SBS, by increasing the power of the dummy light while allowing the use of an Aeff of 15 m or larger, for example, about 30 μm2 which is equivalent to that of a DCF.
When the Aeff is equivalent to that of a DCF, the optical power threshold at which SBS occurs is about +14.0 dBm/ch for the case of the fiber length of 3 km (calculated with α=0.5 (dB/km), ΔVL=50 MHz, ΔVB=16 MHz, and gSBS=4.1×10−11 (mW)).
From the above results, it can be seen that when a fiber having an Aeff equivalent to that of a DCF is used as the medium for generating SRS, any optical surge occurring in the main signal can be reduced by exploiting the SRS effect, even when the optical power is +10.0 dBm/ch at the time of the occurrence of the optical surge. Accordingly, if an optical surge occurs, since the optical surge is reduced by the SRS effect before the optical power reaches the SBS generation threshold, the influence of SBS can be avoided.
Number | Date | Country | Kind |
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2006-252470 | Sep 2006 | JP | national |