Patent Application
20090162059
The present application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-327229 filed on Dec. 19, 2007, with the Japanese Patent Office, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The disclosures herein relate to a technology for providing additional channels in a WDM (wavelength division multiplexing) transmission apparatus.
2. Description of the Related Art
There is a demand for an increase in the transmission capacity of a submarine optical cable system in response to an increase in communication traffic. Generally, countermeasures as follows may be taken to meet such demand.
A new submarine optical cable is laid down, and a submarine line terminal is constructed.
A new submarine line terminal is added to a submarine optical cable that is laid down but unused (generally referred to as a “dark fiber”).
A new channel is added to an optical communication equipment that is already installed (which is generally referred to as an “upgrading method”). This upgrading method does not require an additional submarine optical cable or additional submarine line terminal, and is thus preferable from the viewpoint of increasing transmission capacity at low cost.
Further, the upgrading method includes:
a method of adding a transponder to an unused port for multiplexing/demultiplexing provided on the transmission and reception side; and
providing an optical branch on both the transmission side and the reception side of an already operational optical communication equipment to install a new optical terminal.
The method of adding a transponder to an unused port for multiplexing/demultiplexing is applicable only when such an unused port is in existence. Such method thus cannot serve as a universally applicable method for increasing transmission capacity. It follows that the method of installing a new optical terminal by providing an optical branch is preferable. The method (i) described above is of course effective if there is an unused port. The applicability of this method should not be entirely discarded.
While an NRZ (Non Return to Zero) scheme is typically employed as an encoding scheme in land optical communication systems, a RZ-OOK (Return to Zero On Off Keying) scheme is typically employed as an optical modulation scheme for use in submarine communication. The RZ scheme may require a transmitter having a complex configuration, but provides advantages such as superior receiver sensitivity and relatively small signal degradation (transmission degradation) for a long distance transmission through an optical fiber.
Here, nonlinear characteristics of an optical fiber cause transmission degradation. Such causes include self phase modulation (SPM) and cross phase modulation (XPM). SPM refers to a phenomenon in which the refractive index of optical fiber changes in response to the channel optical power to cause phase modulation. Such phase modulation causes the optical spectrum to spread, resulting in the distortion of optical waveform due to the dispersion characteristics of the transmission fiber. XPM refers to a phenomenon in which the refractive index of optical fiber changes in response to the optical power of an adjacent channel to cause phase modulation. This phase modulation causes a distortion in the optical waveform due to the dispersion characteristics of the transmission fiber. There is another nonlinear effect of optical fiber referred to as four wave mixing (FWM). This effect is avoidable by creating a difference in propagation speed between WDM-signal channels. Specifically, FWM can be avoided by using an optical fiber having a chromatic dispersion of −2 [ps/nm/km] more or less.
In recent years, the application of a RZ-DPSK (Differential Phase Shift Keying) scheme using optical phase to carry signals has been studied for the purpose of further improving receiver sensitivity (see Patent Document 1). In the RZ-DPSK scheme, the transmitter may have a more complex configuration that that of the RZ-OOK scheme. Receiver sensitivity of the RZ-DPSK scheme, however, is expected to be 3 dB higher than receiver sensitivity of the RZ-OOK scheme. In detail, receiver sensitivity is approximately doubled by use of a configuration in which a 1-bit delay optical interferometer is provided on the reception side to divide the output path according to “0/1” of the optical signal, and a pair of balanced photodiodes is used to receive light.
As described above, providing an optical branch on the transmission side and reception side of an existing optical communication equipment to install an additional optical terminal is a preferable upgrading method for adding a channel to an existing submarine optical cable system. It is further preferable to use the RZ-DPSK scheme for the additional channel. It should be further noted that the use of the RZ-DPSK scheme for an additional channel is preferable even when the upgrading method that adds a transponder to an unused port for multiplexing/demultiplexing is employed.
The use of the RZ-DPSK scheme in an upgrading method is expected to provide the following advantages.
It is possible to achieve high channel density because spectrum broadening is smaller than the RZ-OOK scheme.
Even when the RZ-OOK scheme suffers large penalty (degradation in error rate) and cannot guarantee transmission quality because of large cumulative dispersion, the RZ-DPSK scheme may properly be employed due to its superior receiver sensitivity.
In the above-described upgrading methods ((i) and (ii)), an existing transponder that is adjacent to an additional transponder in the wavelength domain may employ the RZ-OOK scheme rather than the RZ-DPSK scheme. In such a case, the characteristics of the additional transponder may degrade due to interaction between the two transponders. While the RZ-DPSK scheme may be used for a new installment, most of the existing transponders employ the RZ-OOK scheme. It is thus highly likely that a channel using the RZ-OOK scheme is situated adjacent to a channel using the RZ-DPSK scheme in the wavelength domain. A risk of suffering degradation is rather high.
XPM causing this problem is dependent on relative polarization between two optical signals. XPM becomes minimum when the polarizations are perpendicular to each other, and becomes maximum when the polarizations are parallel to each other. Relative polarization between two optical signals exhibits extremely slow fluctuation (in a cycle of a few seconds or more) due to changes in the environmental conditions of a terminal equipment. The worst polarization condition may last more than a few seconds to cause burst errors.
Forward error correction (FEC) is used to compensate for signal quality degradation. It is well known, however, that FEC cannot correct error if a poor error rate condition (i.e., a condition in which the error rate exceeds a correctable burst error rate) lasts more than a certain time period (e.g., the length of an FEC correction frame). Under such circumstances, the error rate of the FEC output may not be improved, or may even be worse. In order to correct burst errors, a correction frame needs to have at least two portions where no burst error is present. This is because the correction of a portion suffering burst errors requires adjacent areas suffering no burst errors ahead of and behind the portion.
As previously described, error rate fluctuation occurs due to changes in relative polarization between two optical signals resulting from changes in the environmental conditions of a terminal equipment. It is thus difficult to predict and control the error rate fluctuation. Since such fluctuation often has a period of a few seconds or more, it is desirable to provide a countermeasure to prevent the occurrence of error-uncorrectable state. The above description has been provided with respect to a case in which XPM responsive to the optical intensity of an RZ-OOK channel affects an RZ-DPSK channel. A channel using any other intensity modulation scheme (e.g., NRZ scheme) in place of the RZ-OOK scheme may also have similar effects on a channel using a phase modulation scheme (e.g., DPSK, DQPSK (Quadrature Phase Shift Keying), or RZ-DQPSK) other than the RZ-DPSK scheme.
Patent Document 1 discloses a technology for scrambling the polarization of incident light in order to achieve high-density wavelength multiplexing and also to prevent S/N fluctuation and polarization-dependent fading induced by optical devices. This technology does not take into account a situation in which a channel is added to an existing optical terminal, and, thus, cannot obviate the problems described above.
Patent Document 2 discloses a technology for generating orthogonal polarization WDM signals for which dispersion compensation is made in advance for the purpose of adding a new channel to an optical transmission apparatus. This technology does not take into account the effect that XPM responsive to the optical intensity of an intensity-modulated channel has on another channel utilizing phase modulation, and, thus, cannot obviate the problems described above.
Patent Document 3 discloses a technology for reducing penalty caused by polarization mode dispersion, polarization-dependent loss, and polarization-dependent gain in optical communication systems. This technology does not take into account the effect that XPM responsive to the optical intensity of an intensity-modulated channel has on another channel utilizing phase modulation, and, thus, cannot obviate the problems described above.
There is thus a need to provide a wavelength division multiplexing transmission system that can prevent the occurrence of a time period in which error correction cannot be performed to correct errors caused by XPM between a channel using an intensity modulation scheme such as the RZ-OOK scheme and a channel using a phase modulation scheme such as the RZ-DPSK scheme in the environment in which the bit error rate exhibits extremely slow fluctuation.
[Patent Document 1] Japanese Patent Application Publication No. 10-285144
[Patent Document 2] Japanese Patent Application Publication No. 2001-103006
[Patent Document 3] Japanese Patent Application Publication No. 2005-65273
[Non-patent Document 1] JORNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005, pp 95-103, “RZ-DPSK Field Trial Over 13100 km of Installed Non-Slope-Matched Submarine Fibers.”
It is a general object of the present invention to provide a wavelength division multiplexing transmission system and a method of controlling a wavelength division multiplexing transmission system that substantially obviate one or more problems caused by the limitations and disadvantages of the related art.
In one embodiment, a wavelength division multiplexing transmission system in which a first channel using an intensity modulation scheme and a second channel using a phase modulation scheme are present includes a polarization scrambler inserted into a signal path of either one of the first channel and the second channel to perform polarization scrambling, and a drive unit configured to drive the polarization scrambler at frequency greater than or equal to a value defined as: (bit rate of phase modulated signal)/(error correction frame length)×2.
In another embodiment, a method of controlling a wavelength division multiplexing transmission system in which a first channel using an intensity modulation scheme and a second channel using a phase modulation scheme are present includes driving a polarization scrambler inserted into a signal path of either one of the first channel and the second channel at frequency greater than or equal to a value defined as: (bit rate of phase modulated signal)/(error correction frame length)×2.
The system described above is not only applicable to the upgrading method of installing a new optical terminal by providing an optical branch in an existing optical communication equipment on the transmission side and the reception side, but also applicable to the upgrading method of adding a transponder to an unused port for multiplexing/demultiplexing.
According to at least one embodiment, in a wavelength division multiplexing transmission system in which a channel using an intensity modulation scheme and a channel using a phase modulation scheme are present, a length of burst errors can be set shorter than an error correction frame period even in an environment in which a bit error rate exhibits extremely slow fluctuation due to an effect of the intensity-modulated channel on the phase-modulated channel. Further, the number of portions suffering no bust error in one error correction frame becomes two or more, which can prevent the occurrence of error-uncorrectable state and signal quality degradation.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:
In the following, embodiments of the present invention will be described with reference to the accompanying drawings.
In
A new optical terminal 3A on the transmission side has new channels ch5 through ch8 employing the phase-modulated RZ-DPSK scheme. The relationships between these new channels and the existing channels ch1 through ch4 are supposed to be the same as those illustrated in
The frequency of the drive signal supplied from the drive unit 303 to the polarization scramblers 301-2 and 301-3 is determined as follows. It suffices for the polarization scramblers 301-2 and 301-3 to change polarization to create at least two portions suffering no burst error in an error correction frame, thereby making it possible to perform error correction. Thus, the following relationship suffices.
Drive Frequency>(Bit Rate of Phase Modulated Signal)/(Error Correction Frame Length)×2
Here, (Bit Rate of Phase Modulated Signal)/(Error Correction Frame Length) represents the frequency at which the error correction frame is repeated because the error correction frame is carried by the bit rate of a phase-modulated signal. Doubling the above-noted value is intended to create polarization changes equivalent to at least two cycles within one error correction frame. With this arrangement, the length of bust errors is set shorter than the error correction frame period, and, also, the number of portions having no burst error is two or more in one error correction frame. The occurrence of error-uncorrectable state can thus be prevented.
What is supposed to be done by use of the polarization scramblers 301-2 and 301-3 is to change relative polarization between the intensity-modulated channels ch1 and ch4 and the phase-modulated channels ch6 and ch7, respectively. In place of the polarization scramblers 301-2 and 301-3 in the new optical terminal 3A, polarization scramblers may be inserted immediately after the transponders 21-1A and 21-4A corresponding to the respective channels ch1 and ch4 in the existing optical terminal 2A. In this case, the polarization scramblers 301-2 and 301-3 may be retained in the new optical terminal 3A so that both the new optical terminal 3A and the existing optical terminal 2A perform polarization scrambling. With such arrangement, there is a need to use different drive frequencies. It should be noted that the provision of a polarization scrambler after combining the phase-modulated signal and the intensity-modulated signal at the optical coupler 36A does not provide the intended effect.
Although the RZ-DPSK scheme is used as an example of a phase modulation scheme, other schemes such as DPSK, DQPSK, or RZ-DQPSK can properly be used. Further, although the RZ-OOK scheme is used as an example of an intensity modulation scheme, other schemes such as NRZ can properly be used.
In the above description, only the effect of the channel ch1 on the channel ch6 and the effect of the channel ch4 on the channel ch7 have been taken into consideration. If cumulative chromatic dispersion is small, however, an effect on other new channels ch5 and ch8 may also have to be considered. Such case can be taken care of similarly to the manner described above to achieve the same result. Further, the above description has been given with respect to a case in which the channel ch5 through ch8 using the RZ-DPSK scheme are arranged on both sides of the channels ch1 through ch4 using the RZ-OOK scheme. Even if the channels using the RZ-OOK scheme are arranged on both sides of the channels using the RZ-DPSK scheme, the above-described configuration or similar configuration can be used to achieve the same result. Any arrangement of wavelengths can be taken care of similarly to the manner as described above.
The above description has been given with respect to a case in which two groups of signals inclusive of a group of one or more phase modulated signals and a group of one or more intensity modulated signals are present. This is not a limiting example, and the present embodiment is applicable to a case in which three or more groups of signals are present. When three more groups are present, the present embodiment is applied similarly to the manner described above by focusing attention on the combination of a phase modulated signal and an intensity modulated signal.
In
Instead of providing the polarization scrambler 301 in the new optical terminal 3A, a polarization scrambler may be inserted after the optical amplifier 23A in the existing optical terminal 2A. In this case, the polarization scrambler 301 may be retained in the new optical terminal 3A so that both the new optical terminal 3A and the existing optical terminal 2A perform polarization scrambling. With such arrangement, there is a need to use different drive frequencies. It should be noted that the provision of a polarization scrambler after combining the phase-modulated signal and the intensity-modulated signal at the optical coupler 36A does not provide the intended effect.
Although the RZ-DPSK scheme is used as an example of a phase modulation scheme, other schemes such as DPSK, DQPSK, or RZ-DQPSK can properly be used. Further, although the RZ-OOK scheme is used as an example of an intensity modulation scheme, other schemes such as NRZ can properly be used.
In the above description, only the effect of the channel ch1 on the channel ch6 and the effect of the channel ch4 on the channel ch7 have been taken into consideration. If cumulative chromatic dispersion is small, however, an effect on other new channels ch5 and ch8 may also have to be considered. Such case can be taken care of similarly to the manner described above to achieve the same result. Further, the above description has been given with respect to a case in which the channel ch5 through ch8 using the RZ-DPSK scheme are arranged on both sides of the channels ch1 through ch4 using the RZ-OOK scheme. Even if the channels using the RZ-OOK scheme are arranged on both sides of the channels using the RZ-DPSK scheme, the above-described configuration or similar configuration can be used to achieve the same result. Any arrangement of wavelengths can be taken care of similarly to the manner as described above.
The above description has been given with respect to a case in which two groups of signals inclusive of a group of one or more phase modulated signals and a group of one or more intensity modulated signals are present. This is not a limiting example, and the present embodiment is applicable to a case in which three or more groups of signals are present. When three more groups are present, the present embodiment is applied similarly to the manner described above by focusing attention on the combination of a phase modulated signal and an intensity modulated signal.
The third embodiment is directed to a configuration in which the receiver unit of a transponder in the new optical terminal 3B on the reception side is configured to cope with polarization dependency.
In the case where a polarization scrambler is inserted into the path after the multiplexing performed in the new optical terminal 3A on the transmission side as illustrated in
If there is polarization dependency in the polarization scramblers used in the new optical terminal 3A on the transmission side, polarization scrambling may not be properly performed, which may result in a failure to perform sufficient cancellation in the new optical terminal 3B on the reception side. In such a case, a polarization-independent polarization scrambler as shown in
In the case where a polarization scrambler is provided in the existing optical terminal 2A (either for each channel or for the multiplexed signal), the arrangement as described above is not necessary because the new additional channels do not experience polarization changes.
In
A similar configuration may also be employed in the new optical terminal 3B on the reception side to cancel the effect of polarization dependency.
Further, a similar configuration may be applied to the existing optical terminal 2A.
In
The monitor circuit 5 then measures a error rate and stores the measured error rate in memory (step S4). The monitor circuit 5 checks whether the above-described settings are made with respect to every point within the range of drive amplitude (step S5). If the settings are not made with respect to every point (NO in step S5), the monitor circuit 5 sets a new drive amplitude (step S3).
If the settings are made with respect to every point within the range of drive amplitude (YES in step S5), the monitor circuit 5 checks whether the above-described settings are made with respect to every point within the range of drive frequency (step S6). If the settings are not made with respect to every point (NO in step S6), the monitor circuit 5 sets a new drive frequency (step S2).
If the settings are made with respect to every point within the range of drive frequency (YES in step S6), the monitor circuit 5 sets a drive frequency and drive amplitude to the optimum drive frequency and drive amplitude (step S7). With this, the procedure comes to an end (step S8).
The above-described procedure may be performed at constant intervals thereby to maintain optimum conditions responsive to changes in the environmental conditions.
Embodiments of the present invention have been described heretofore for the purpose of illustration. The present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. The present invention should not be interpreted as being limited to the embodiments that are described in the specification and illustrated in the drawings.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-327229 | Dec 2007 | JP | national |
