Control scheme for long wavelength channels in wideband WDM optical fiber transmission system

There is provided in an optical amplifier device used for a system in which a power transfer takes place from comparatively short wavelength signal to a comparatively long wavelength signal. The device includes an amplifier stage coupled to an optical transmission medium, a monitor monitoring a status of a first band; and a pump light source unit supplying at least one first pump light to the optical transmission medium on the basis of the status of the first band monitored, so that the above at least one first pump light supplies additional power to longer wavelength channels related to the status of the first band.

TECHNICAL FIELD

The present invention relates to wide-band WDM (Wave Division Multiplexing) transmission systems and a protection scheme for the long wavelength channels in such systems against failure in the absence of the short wavelength signals.

BACKGROUND ART

The optical communication system of the related art comprises a transmitting terminal for generating a WDM optical signal formed by wavelength-division multiplexing of a plurality of optical signals of different wavelengths, an optical transmission line for transmitting the WDM optical signal transmitted from the transmitting terminal and a receiving terminal for receiving the transmitted WDM optical signal. Moreover, this optical communication system comprises, as required, one or a plurality of optical repeaters having the function to amplify the WDM optical signal in the course of the optical transmission line.

In such an optical communication system, the waveform of each optical signal is deteriorated due to non-linear optical effects in the optical transmission line. In order to eliminate the deterioration of the waveform, it is effective to reduce the optical power of the optical signals launched into the transmission line, but a reduction of the optical power results in an increase of the optical signal to noise ratio (OSNR) due to noise accumulation in the optical amplifiers.

For this purpose, it has been proposed to use a combination of discrete optical amplifiers provided within repeaters and distributed optical amplifier using the optical transmission line in common as the optical amplifying medium. In a discrete optical amplifier the amplifying medium and pump light source are centralized in one area. In contrast, the amplifying medium of a distributed optical amplifier is laid between two remote places and pump light source is provided in one or both places.

Fiber doped optical amplifiers represent one group of optical fiber amplifiers. In doped fiber amplifiers a lanthanide rare-earth element is added to the optical fiber. The structure of electronic excitation levels of lanthanide rare-earth atoms allows for amplification by stimulated emission in the low-absorption wavelength domain of optical fibers. The operation bandwidth is limited to certain wavelength ranges: Neodymium (Nd) amplifies in the 1060 nm wavelength band, Praseodymium (Pr) in the 1300 nm wavelength band, Thulium (Tm) in the 1450 nm wavelength band and Erbium (Er) in the 1550 nm band.

The other group of optical fiber amplifier takes advantage of stimulated Raman scattering (SRS) an inelastic scattering process between photons and optical phonons of lattice vibrations. It has a wide gain width and a gain shift of 13.3 THz (about 100 nm), as will be described later with reference to FIG.4. In contrast to erbium doped fiber amplifiers, the SRS effect occurs also in ordinary optical fibers. Moreover, the pumping wavelength can be set for any amplification wavelength.

The low loss transmission window in silica-based optical fibers covers the wavelength range from 1450-1650 nm with a minimum around 1550 nm. Until recently, only Erbium doped fiber amplifiers (EDFA) which cover the so-called C-band (1530-1565 nm) and the gain-shifted EDFA which cover the so-called L-band (1570-1605 nm) were employed. In these systems the pump wavelengths for distributed Raman amplification (DRA) are much shorter than the signal wavelengths.

The increasing demand for transmission capacity of optical fiber systems requires the expansion of the optical bandwidth in a single fiber. Extension to longer wavelengths has several drawbacks. The loss profile in this wavelength domain varies strongly among installed fibers, which makes system design more difficult and materials and technologies for optical components (e.g. photodiodes) yet have to be developed. Raman amplification is in principle available for this wavelength domain. However, the pump wavelengths would partly overlap with the short wavelengths signals in the C-band.

On the short wavelength side below 1530 nm, the low loss region of silica-based fibers extends to 1450 nm. Raman pump wavelengths for this region do not overlap with signals; however, they are located at the water-peak of optical fibers, where the absorption loss is high. Nevertheless, due to the availability of high-power pump lasers, Raman amplification is a feasible technology for this wavelength domain. Besides, Thulium doped amplifiers and gain-shifted Thulium doped amplifiers are candidates as amplifiers for wavelength bands below 1530 nm. The additional wavelength regions are referred to as S+ band (1450-1490 nm) and S band (1490-1530 nm). In these new wide-bandwidth systems, the short wavelength signals act as DRA pump light with respect to the long wavelength signals. S+ and S band wavelengths transfer optical power to the C and L band channels via SRS. Distributed Raman pumping of the S+ and S band channels compensates the power depletion due to SRS as well as the increased fiber loss at S+ and S wavelengths. If all wavelengths are in service, the power transfer is balanced.

A further description will be given of the conventional optical communication system with reference to the accompanying drawings.

FIG. 1is a graph of a typical optical loss spectrum of silica-based optical fibers in which the low loss region covers the wavelength range from 1450 to 1650 nm. Optical amplifiers allow simultaneous amplification of a group of wavelengths. The C and L band correspond to the wavelength ranges of Erbium doped and the gain-shifted Erbium doped amplifiers. The S+ and S band are related to the wavelength ranges of Thulium doped and gain-shifted Thulium doped fiber amplifiers. When light of 1450 nm and 1550 nm travels 100 km through an optical fiber with a loss of 0.26 dB/km, it experiences a loss of 26 dB and 20 dB, respectively. Thus, light with a wavelength of 1450 nm experiences a loss of about 0.06 dB/km higher than the lowest loss wavelength.

FIG. 2Ashows a conventional WDM transmission system. Symbols of optical components in the accompanying drawings includingFIG. 2Aare defined as shown inFIGS. 3A through 3F.FIG. 3Ashows various types of optical amplifiers. The C and L band can be amplified either separately by means of a broadband C/L band amplifier. Accordingly, S and S+ bands can be amplified either by separate doped fiber amplifiers or Raman amplifiers, or by amplifiers covering the whole S+ and S band wavelength range. For this group of amplifiers a double lined triangle is used in this specification. Variable optical attenuators (VOA) can be added to the amplifiers as means for adjusting the amplifier output power.

Turning toFIG. 2Aagain, the WDM transmission system includes a transmitter, a transmission fiber connecting remote locations, discrete optical amplifier to compensate for the fiber loss, and a receiver. Multiple wavelengths transmission enhances the transmission capacity. The optical amplifiers add noise in the form of amplified spontaneous emission, which reduces the optical signal-to-noise ratio, thus giving rise to errors in the signal detection. Distributed Raman amplification can improve the signal to noise ratio because it amplifies the signals along the transmission fiber. Moreover, the stimulated Raman scattering tilt, which will be described later in detail, can be compensated for in the system. There are control schemes that allow adjusting the spectral tilt under changing conditions of C and L band channel usage (OECC'99, “Optical SNR degradation due to stimulated Raman scattering in dual-band WDM transmission systems and its compensation by optical level management”, T. Hoshida, T. Terahara, J. Kumasako and H. Onaka).

Distributed Raman amplification generally is not high enough to make discrete amplifiers obsolete. As shown inFIG. 2B, counter-propagating amplification is used to average out bit-pattern dependent amplification causing power fluctuations. Commercial systems employ C and L band amplifiers. In the laboratory triple band (S, C, L) transmission has been demonstrated (ECOC2000, “Experimental Study on SRS loss and its compensation in three-band WDM transmission”, Yutaka Yano, Tadashi Kasamatsu, Yoshitaka Yokoyama and Takashi Ono), as shown in FIG.2C.

In dense WDM systems, channel-interleaved bi-directional transmission as shown inFIG. 2Dcan reduce impairments due to nonlinear interaction between adjacent channels (cross phase modulation, four wave mixing) and thus allows increasing the spectral efficiency of the system. At the amplifier stage, optical circulators (directional coupling elements) separate forward and backward propagating channels.

FIG. 4shows the optical power depletion due to stimulated Raman scattering and fiber loss. In wide-band WDM transmission systems with high channel count, SRS causes a strong power transfer from the short wavelengths to long wavelengths. The Raman gain depends on the frequency shift between the shorter and the longer wavelength. It has a maximum around 13.3 THz in silica-based fibers. Thus, for distributed Raman pumping it is most effective to allocate the pump wavelength shifted about 100 nm to shorter wavelength with respect to the signal wavelengths. In wide band WDM systems the short wavelength signals become efficient pump light sources for the long wavelength channels.

FIG. 5shows SRS-spectral tilt compensation using DRA and pre-emphasis (repeater output level control). Using pre-emphasis and distributed Raman amplification of the short wavelength channels, the higher absorption loss and the SRS power depletion can be compensated (ECOC2000, “Experimental Study on SRS loss and its compensation in three-band WDM transmission”, Yutaka Yano, Tadashi Kasamatsu, Yoshitaka Yokoyama and Takashi Ono). It is to be noted that the power transfer is balanced if all channels are on.

However, in wide-bandwidth systems, an interruption of the operation of the short wavelength channels (either by failure or for the purpose of maintenance) or a reduced number of active short wavelength channels result in less or no power transfer to the C and L band signals. As a consequence, the C and L band signal output power drops and the OSNR degrades, making these channels more error-prone.

Thus, a general object of the present invention is to provide a control scheme for long wavelength channels in wideband WDM optical fiber transmission system in which the above problem is overcome.

A more specific object of the present invention is to provide an optical amplifier device capable of protecting long wavelength channels of wideband optical fiber transmission systems in which the power transfer from the short to the long signal wavelengths due to stimulated Raman scattering is essential for the transmission of the long wavelength signals.

Another object of the present invention is to provide an optical communication system utilizing the above protection scheme.

DISCLOSURE OF THE INVENTION

The above objects of the present invention are achieved by an optical amplifier device used for a system in which a power transfer takes place from comparatively short wavelength signal to a comparatively long wavelength signal, the optical amplifier device comprising: an amplifier stage coupled to an optical transmission medium; a monitor monitoring a status of a first band; and a pump light source unit supplying at least one first pump light to the optical transmission medium on the basis of the status of the first band monitored, so that the above at least one first pump light supplies additional power to longer wavelength channels related to the status of the first band.

The above objects of the present invention are also achieved by an optical amplifier device used for a bi-directional system in which a power transfer takes place from comparatively short wavelength signal to a comparatively long wavelength signal, the optical amplifier device comprising: first and second amplifier systems; and directional coupling elements coupling the first and second amplifier stages to an optical transmission medium. Each of the first and second amplifier systems comprises: an amplifier stage coupled to the optical transmission medium; a monitor monitoring a status of a first band; and a pump light source unit supplying at least one first pump light to the optical transmission medium on the basis of the status of the first band monitored, so that the above at least one first pump light supplies additional power to longer wavelength channels related to the status of the first band.

The above objects of the present invention are also achieved by a method of controlling an optical amplifier device, comprising the steps of: monitoring a status of a first band; and supplying at least one first pump light to an optical transmission medium on the basis of the status of the first band monitored, so that the first pump light supplies additional power to longer wavelength channels related to the status of the first band.

The above objects of the present invention are also achieved by an optical transmission system comprising: optical amplifier devices; and an optical transmission medium coupling the optical amplifier devices. One of the optical amplifier devices comprises: an amplifier stage coupled to the optical transmission medium; a monitor monitoring a status of a first band; and a pump light source unit supplying at least one first pump light to the optical transmission medium on the basis of the status of the first band monitored, so that the above at least one first pump light supplies additional power to longer wavelength channels related to the status of the first band.

The above objects of the present invention are achieved by a bi-directional optical transmission system in which a power transfer takes place from comparatively short wavelength signal to a comparatively long wavelength signal, the bi-directional optical transmission system comprising: optical amplifier devices; and an optical transmission medium coupling the optical amplifier devices. One of the optical amplifier devices comprises: first and second amplifier systems; and directional coupling elements coupling the first and second amplifier stages to the optical transmission medium. Each of the first and second amplifier systems comprises: an amplifier stage coupled to the optical transmission medium; a monitor monitoring a status of a first band; and a pump light source unit supplying at least one first pump light to the optical transmission medium on the basis of the status of the first band monitored, so that the above at least one first pump light supplies additional power to longer wavelength channels related to the status of the first band.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will first be given of the principle of the present invention.

According to one aspect of the present invention, additional Raman pump light sources are provided in the amplifier stages. These sources are off when the S+ and the S band are in full service. Photodiodes monitor the power levels of the S+ and S bands. Depending on the position of the photodiodes the control signal is used to switch on/off the substitute Raman pump laser diodes in the same repeater node or in the previous or next one node. The photodiodes can either be located before or behind the amplifier. Thus, it is possible to maintain a reliable C/L band transmission also without full operation of the S+ and S band channels.

Referring toFIG. 6, in the case of the absence of all S+ and/or S band channels or parts of the channels, the power transfer from the shorter to the longer wavelengths is reduced or completely interrupted. As a consequence, the power of C and L band channels drops and the OSNR suffers, leading to a higher error probability for these channels. Therefore, a back-up system and a control mechanism are required, which provide optical pump power to the C and L band channels in the case of the absence of the S+ and/or S band or parts of these bands. In addition, an increase of the C and L band repeater output power of each amplifier stage can improve the system performance in the absence of the short wavelength bands or parts of these bands.

More particularly, if the S+ band is totally or partly off or the power level of the S+ band drops, one or more substitute pump lights of different wavelengths can be switched on so that power can be supplied to longer wavelength channels (which can be referred to as longer wavelength lights or regions), as shown in part (a) of FIG.6. As has been described with reference toFIG. 4, for distributed Raman pumping it is most effective to allocate the pump wavelength shifted about 100 nm to shorter wavelength with respect to the signal wavelengths. Hence, the substitute pump lights most efficiently pump wavelength channels that are located 100 nm longer than the wavelengths of the substitute pump lights. Thus, pump power can be supplied to the C and L bands.

The number of substitute pump lights and the wavelength values thereof can arbitrarily be selected. Generally, more substitute pump lights, more efficiently longer wavelength channels can be pumped. Preferably, the wavelengths of the substitute pump lights are selected so as not to overlap with those of the S+ band signal channels. This is essential to the case where the substitute pump lights are switched on if the power level of the S+ band drops (not totally off). Some substitute pump lights may have the wavelengths that overlap with the S+ band signal channels in a case where the substitute pump lights are switched on only when the S+ band is totally off.

In part (a) ofFIG. 6, two S+ substitute pump lights are illustrated as an example. Alternatively, a single S+ substitute pump light or three or more S+ substitute pump lights can be used.

If the S band is totally or partly off or the power level of the S band drops, one or more substitute pump lights of different wavelengths can be switched on so that power can be supplied to longer wavelength channels, as shown in part (b) of FIG.6. The wavelengths of the substitute pump lights shown in part (b) ofFIG. 6are longer than those of the substitute pump lights shown in part (a) of FIG.6.

If both the S+ and S bands are totally or partly off or the power levels of both the bands drop, the substitute pump lights shown in parts (a) and (b) ofFIG. 6are switched on as shown in part (c) of FIG.6.

If a part of a short wavelength band is off, this part can be substituted with a single substitute pump wavelength, as shown in part (d) of FIG.6. That is, channel-by-channel based control can be achieved.

In short, according to the present invention, substitute pump lights are supplied in a case where the power transfer from the shorter to longer wavelengths due to stimulated Raman scattering is not sufficient for the transmission of the longer wavelength channels. Therefore, even if a reduced number of active short wavelength channels results in less or no power transfer to the longer wavelengths such as C and L bands, the substitute pump lights of wavelengths in the shorter bands such as S+ and/or S band can be supplied to compensate for the less or no power transfer. The number of substitute pump lights or the power levels thereof can be adjusted based on the power levels of the longer wavelength bands such as C and L bands.

FIG. 7Ais a block diagram of a WDM transmission system according to a first embodiment of the present invention. Optical repeater nodes10and20formed so as to include amplifier devices are coupled through an optical fiber14, which is one of optical transmission media. The amplifier device10includes an amplifier stage11, a central processing unit (CPU)12, and an electrically erasable and programmable read only memory (EEPROM)13. Similarly, the repeater node20includes an amplifier stage16, a CPU17, and an EEPROM18. The amplifier stages11and16are coupled through the optical fiber14. Signal light and pump light are propagated through the optical fiber14. The CPUs12and17exchange control data via an optical supervisory channel15, which is also transmitted over the optical fiber14.

FIG. 8is a block diagram of an example of the structure of the repeater node10. The symbols of the components shown inFIG. 8are defined inFIGS. 3A through 3F. The amplifier stage11is made up of WDM couplers31and32, a C/L optical amplifier stage33, optical taps34and35, photodiodes (PD)36and37, optical amplifiers38and39, and WDM optical couplers40and41. An optical circulator43is provided in the optical transmission line14. A Raman pump light source unit (LD)42, which is controlled by the CPU12, is coupled to the optical circulator43. The Raman pump light source unit42is provided in front of the WDM coupler31. Raman pump lights emitted by the Raman pump light source unit42are counter-propagated with respect to signal lights.

The WDM coupler31demultiplexes a multiplexed signal light into signals in the C and L bands (C/L signals) and signals in the S+ and S bands (S+/S signals). The amplifier stage33amplifies the C/L signals. The repeater output powers of the amplifier stage33can be adjusted by the CPU12. The WDM coupler32demultiplexes the S+/S signals into the S+ signal and the S signal. The amplifier38amplifies the S signal. The amplifier39amplifies the S+ signal. The monitor photodiode36monitors the status (level) of the whole S band by referring to a part of the S signal from the optical tap34. The monitor37monitors the status (level) of the whole S+ band by referring to a part of the S+ signal from the optical tap35. The repeater output powers of the amplifiers38and39can be adjusted by the CPU12. The WDM coupler40multiplexes the amplified S and S+ signals into a multiplexed S+/S signal. The WDM coupler41multiplexes the amplified C/L signal and the S+/S signal into a multiplexed signal light, which is transmitted over the optical fiber14.

The Raman pump light source unit42is configured as shown in FIG.9. The Raman pump light source unit42includes a coupling unit45, an S+ Raman pump light source unit46, an S Raman pump light source unit47, an S+ signal substitution (additional) Raman pump light source unit48, and an S signal substitution Raman pump light source unit49. The S+ Raman pump light source unit46generates S+ band pumping lights of wavelengths λ1and λ2for pumping the S+ band (see FIG.5). The unit46includes a laser diode LD emitting the pump light of λ1, and another laser diode LD emitting the pump light of λ2. These laser diodes are controlled by the CPU12via digital-to-analog (D/A) converters. The levels of the pumping lights are monitored by photodiodes via optical taps, and are supplied to the CPU12via analog-to-digital (A/D) converters.

The S Raman pump light source unit47emits S band pumping lights of wavelengths λ3and λ4for pumping the S band (see FIG.5). The unit47has the same structure as the unit46.

The S+ and S signal substitution Raman pump light source units48and49are newly provided according to the first embodiment of the present invention. The unit48generates two substitute pump lights of wavelengths λ5and λ6in the region between the S band Raman pump wavelengths and the S band signal wavelengths. The unit48has the same structure as the units46and47. The unit49generates two substitute pump lights of wavelengths λ7and λ8in the region between the S band Raman pump wavelengths and the S band signal wavelengths. The unit49has the same structure as the units46-8.

The coupler45includes seven WDM couplers, and multiplexes the eight Raman pump lights of λ1through λ8. The multiplexed Raman pump light is applied to the circulator43, which allows it to be propagated through the optical fiber14in the direction opposite to the direction in which signal light is propagated therethrough (counter-propagating). That is, the multiplexed pump light thus generated is applied to the signal light coming from the previous repeater node (not shown in FIG.9). The circulator43is capable of coupling the multiplexed Raman pump light into the optical fiber14even if they are spectrally overlapping with the S+/S signal wavelengths.

The CPU12controls the pump light source units46-49in accordance with the levels of the S+ and S bands respectively monitored by the photodiodes36and37. The output signals of the photodiodes36and37are applied to the CPU12via A/D converters51and52. Further, the CPU12receives information concerning the status of the amplifiers and controls them, as will be described later. The EEPROM13stores programs executed by the CPU12, and pre-set parameter values of the gains of amplifiers (or of the attenuators behind the amplifiers) and the levels of the pump light sources. The parameter settings for the Raman pump powers and the output powers of the C and L band channels for the various scenarios (S+ band off, S band off, etc.) are determined at the installation of the system.

FIG. 10Ais a flowchart of a control operation executed by the CPU12, andFIG. 10Bshows a table describing possible status changes of S+ and S bands. As shown inFIG. 10B, 16 status changes of the S+ and S bands are possible. Each of the status changes is identified by serial number #i. For example, for #i=1, there is no status change, and for #i=4, both the S+ and S bands change from on to off. For #i=7, the S+ band changes from on (existence of signal lights) to off (absence of signal lights) status, and the S band from off to on status. The CPU12switches on/off the Raman pump light source units48and49in accordance with the control sequence when a status change of the S+ and S bands takes place.

Referring toFIG. 10A, the CPU12reads the monitor signals of the S+ and S bands supplied from the photodiodes36and37via the A/D converters51and52(step S11). Then, the CPU12determines whether there is a status change of the S+ and S bands. There is no change for #i=1, 6, 11 and 16. In this case, the control sequence proceeds to step S18, which will be described later. In contrast, if a status change of the S+ and S bands occurs, the CPU12switches off the discrete amplifier(s) of the on-to-off band(s) (step S13). Then, the CPU12switches off the substitute Raman pump(s) for the off-to-on band(s) (step S14). For case of #i=7, the CPU12switches off the amplifier39shown inFIG. 8, and switches off the S signal substitution Raman pump light source unit49. At step S13, depending on the optimum conditions determined at the installation of the system, the Raman pumps of the on-to-off bands might also be switched off.

Then, the CPU12sends, via the control channel15, a control signal to the previous repeater node to adjust the C and L band powers according to the pre-set values of the parameters stored in the EEPROM13for case #i (step S15). The process of step S15may be omitted, if required. The C and L band powers can be adjusted by controlling the optical amplifiers or the attenuators behind the amplifiers.

Thereafter, the CPU12switches on the substitute Raman pump (s) for the on-to-off band(s) (step S16), and switches on the discrete amplifier(s) of the off-to-on bands(s) (step S17). For case of #i=7, the CPU12switches on the S+ signal substitution Raman pump light source unit48shown inFIG. 8, and switches on the optical amplifier38.

Finally, the CPU12receives a control signal from the next repeater node and adjusts the local C and L band amplifiers accordingly (step S18). That is, the amplifier stage33of the repeater node10shown inFIG. 8is adjusted by the control signal sent by the repeater node20shown in FIG.7. It is to be noted that regardless of status changes, the C and L band amplifiers (or the attenuators behind these amplifiers) are adjusted according to the control signal received from the next repeater node.

The repeater node20shown inFIG. 7operates in the same manner as the repeater node10, and therefore a description thereof will be omitted.

A description will be given of a second embodiment of the present invention.

FIG. 11is a block diagram of a WDM transmission system according to a second embodiment of the present invention. InFIG. 11, parts that are the same as those shown inFIG. 7are given the same reference numbers. Two repeater nodes100and200employ optical spectrum analyzers (OSA) for monitoring the S+ and S bands. The advantage of the photodiodes for monitoring the S+ and S bands in the first embodiment of the present invention is that their response is much faster than the optical spectrum analyzers. In contrast, the use of the spectrum analyzers can realize finer control, as will be described later.

The repeater node100is equipped with optical spectrum analyzers (OSA)20and22, which are coupled with the optical fiber14via optical taps19and21, respectively. The optical spectrum analyzer20monitors the optical spectra of all bands at the input side of the amplifier stage11and supplies the spectral data to the CPU12. The optical spectrum analyzer22monitors the optical spectra of all bands at the output side of the amplifier stage11and supplies the spectral data to the CPU12. The amplifier stage11and the Raman pump light source unit42are configured, for example, as shown in FIG.8.

Similarly, the repeater node200includes optical spectrum analyzers24and26, which are respectively coupled to the input and output sides of the amplifier stage16via optical taps23and25.

FIG. 12Ais a flowchart of a control operation of the CPU12of the repeater node100, andFIG. 12Bshows a table describing possible status changes of S+ and S bands. The contents of the table shown inFIG. 12Bare the same as those of the table shown in FIG.10B. The control sequence shown inFIG. 12Aincludes steps S12through S18which have been described with reference toFIG. 10A, and particular steps S20and S21resulting from the use of the optical spectrum analyzers. In steps S12through S18, the spectral data rather than the photodiode monitor outputs is used.

The CPU12executes step S20at first. At step S20, the CPU12reads input and output spectral data from the optical spectrum analyzers20and22. In addition, the CPU12receives span input spectral data from the previous repeater node (repeater node200), and provides the input spectral data of the repeater node100to the next repeater node. Then, the CPU12executes step S12. If #i=1, 6, 11 or 16, the CPU12adjusts the pump powers by controlling the laser diodes of the working pumping power units among the units46through49via the corresponding D/A converters. This adjustment refers to the input and output spectral data and the input spectral data received from the previous repeater node. The input and output spectral data show the status of all the channels in each of the bands. Thus, it is possible to finely adjust the pump powers so that all the channels can be set at given pre-set levels stored in the EEPROM13. Further, at step S21, the CPU12sends a control signal to the previous repeater node for adjustment of the signal input powers. Then, the CPU12executes step S18, which is followed by step S20.

The first and second embodiments of the present invention may be modified so that three or more Raman pumping lights can be used. The Raman pump light sources may be a multiple wavelength pump light source as disclosed, for instance, in WO 00/5622. The monitor photodiodes36and37used in the first embodiment of the present invention monitor the whole S+ and S bands, respectively. Alternatively, a set of monitor photodiodes can be combined with a WDM coupler device to monitor multiple wavelength groups (i.e. subbands) within a band simultaneously. The monitor photodiodes can be placed behind the amplifiers as well.

A description will be given of other embodiments of the present invention.

FIG. 13Ais a block diagram of a repeater node10C according to a third embodiment of the present invention. The repeater node10C differs from the repeater node10in the position of the optical circulator43. If the coupling loss of the WDM coupler31is sufficiently low at the short Raman pump wavelengths, the optical coupler43can be placed behind the WDM coupler31, as shown in FIG.13A. This has an advantage that the loss for the signal lights in the C and L bands in the transmission line is reduced.

FIG. 13Bis a block diagram of a variation of the repeater node10C. A repeater node10D shown inFIG. 13Bis arranged so that the C/L signal lights are counter-propagated with respect to the S+/S signal lights. That is, the S+/S Raman pump lights emitted by the Raman pump light source unit42are co-propagated with the C/L signal lights.

FIG. 13Cis a block diagram of a modification10E of the repeater node10C. The Raman pump light source unit42used in the previously described embodiments of the present invention is separated into two units42aand42b. The unit42acorresponds to the combination of the S+ and S Raman pump light source units46and47and the associated WDM coupler shown in FIG.9. The unit42bcorresponds to the combination of the S+ and S signal substitution Raman pump light source units48and49and the associated WDM couplers shown in FIG.13C. The S+/S Raman pump light source unit42ais coupled to the optical fiber14in front of the WDM coupler31by a WDM coupler55. The S+ and S signal substitution Raman pump light source unit42bis coupled, by the optical circulator43, to the corresponding inner optical fiber so as to be located behind the WDM coupler31. With the above arrangement, the coupling losses for the S+/S Raman pump light can be reduced in comparison to repeater node13A.

A variation of the repeater node10E is illustrated as a repeater node10F shown in FIG.13D. The C/L signal lights are co-propagated with the Raman pump lights, and are counter-propagated with respect to the S+/S signal lights.

FIG. 14Ais a block diagram of a repeater node10G according to a fourth embodiment of the present invention. The S signal substitution Raman pump light source unit49is coupled to the corresponding inner S-signal transmission line so as to be located behind the WDM coupler32. The monitor photodiode36monitors part of the S signal coming from the optical tap34. The unit49is controlled based on the status of the S band. The S+ signal substitution Raman pump light source unit48is coupled to the corresponding inner S+-signal transmission line so as to be located behind the WDM coupler32. The monitor photodiode37monitors part of the S+ signal coming from the optical tap35. The unit48is controlled based on the status of the S+ band.

FIG. 14Bis a block diagram of a variation10H of the repeater node10G. The C/L signal lights are co-propagated with the Raman pump lights, and are counter-propagated with respect to the S+/S signal lights.

FIG. 15Ais a block diagram of an optical repeater node10I according to a fifth embodiment of the present invention, in which the signal wavelengths extend only to the S band. In this case, the Raman pump wavelengths can be allocated without spectrally overlapping with the signal wavelengths. This allows using a WDM device for coupling the S+ and S signal substitution Raman pumps into the transmission line. An integrated S Raman pump light/S+ and S signal substitution Raman pump light source unit42cis coupled to the optical fiber14by means of a WDM coupler55located in front of the WDM coupler31. The Raman pump lights emitted by the unit42care counter-propagated with respect to the S/C/L signal lights. The unit42cincludes the Raman pump light source units47,48and49and the associated WDM couplers of the coupling unit45shown in FIG.9. The S band is monitored by the monitor photodiode37coupled to the inner S signal transmission line extending from the WDM coupler31.

The Raman pump light source unit42cis controlled by the status of the S band monitored by the S signal monitor37under the control of the CPU12. For example, the Raman pump light source unit48is maintained in the ON state, and Raman pump light source unit49is switched on/off based on the status of the S band. Alternatively, the S+ and S signal substitution light source units48and49may be simultaneously switched on/off based on the status of the S band.

FIG. 15Bis a block diagram of a variation10J of the repeater node10I. The S band Raman pump light is co-propagated. The Raman pump light source unit42is separated into the S Raman pump light source unit47and an S+ and S signal substitution Raman pump light source unit42b. The unit42bis coupled to the optical fiber14forming the transmission line by a WDM coupler60located in front of the WDM coupler.41. The S+ and S signal substitution Raman pump lights are counter-propagated with respect to the C/L signal lights. The S pump light is co-propagated with the C/L signal lights. The S+ and S signal substitution Raman pump light source unit42bis controlled by the status of the S band monitored by the monitor photodiode37.

FIG. 16Ais a block diagram of an optical repeater node10K, which is another variation of the repeater node10I. A WDM coupler61is located behind the WDM coupler31. The S+ and S signal substitution Raman pump light source42bis coupled to the inner S signal transmission line by the WDM coupler61. The S Raman pump light and the S+/S signal substitution Raman pump lights are counter-propagated with respect to the S/C/L signal lights.

FIG. 16Bis a block diagram of a variation10L of the repeater node10K. The WDM coupler61, to which the S+ and S signal substitution Raman pump light source42is coupled, is located behind the S-band amplifier38. The S+ and S signal substitution Raman pump lights are counter-propagated with respect to the C/L signal lights. The S Raman pump lights are co-propagated with the C/L signal lights.

FIG. 17Ais a block diagram of an optical repeater node17A according to a sixth embodiment of the present invention. In this embodiment, a pair of optical switches65and66is used to realize a protection for the C/L band transmission in the case of an interruption of the S+ and/or S band transmission. The optical switches65and66are coupled to respective inner transmission lines via WDM couplers75and76. Each of the optical switches65and66allows choosing between two optical paths. The pair of optical switches65and66are located in front of the WDM coupler31, and selectively connects either the S+/S Raman pump light source unit42aor the S+ and S signal substitution Raman pump light source unit42bto the transmission line.

The optical switches65and66are controlled together with the S+ and S signal substitution Raman pump light source unit42bby the CPU12, as shown in FIG.9. The monitor output status is supplied from the previous repeater node. The switching control of the optical switches65and66is the same as the aforementioned on/off control of the Raman pump light source units. That is, the optical switches65and66are operated in accordance with the table shown in FIG.10B.

It is to be noted that the optical switches65and66do not permit the simultaneous transmission of the S+ and S signal substitution-Raman pump lights and the S+/S signal lights. Therefore, it is required to perform switching between the S+/S Raman pump lights and the S+ and S signal substitution Raman pump lights. This means that only the whole S+ and S band can be substituted. The Raman pump lights are counter-propagated with the signal lights.

An S+/S amplifier stage62is used to amplify the signal lights in the S+ and S bands. A monitor photodiode63monitors the state of the S+ and S bands. The monitor output for controlling the S+ and S signal substitution Raman pump light source unit of the next repeater node is sent to the next repeater node. The S+/S/C/L signal lights are co-propagated.

FIG. 17Bis a block diagram of a variation10N of the optical repeater node10M. A pair of optical switches67and68is provided in front of the WDM coupler41. The S+ and S signal substitution Raman pump light source unit42bis selectively coupled to the optical fiber14by the switches67and68controlled by the CPU12. The C/L signal lights are counter-propagated with respect to the S+/S signal lights, and are co-propagated with the S+/S Raman pump lights. The S+ and S signal substitution Raman pump lights are counter-propagated with respect to the C/L signal lights. The monitor output of the photodiode63located behind the S+/S amplifier stage62is used by the CPU12to control the switches67and68and the S+ and S signal substitution Raman pump light source unit42b.

FIG. 18Ais a block diagram of an optical repeater node10P according to a seventh embodiment of the present invention. An optical switch69for selectively coupling the S+ and S signal substitution Raman pump light source unit42bwith the transmission line is provided behind the WDM coupler31. The optical switch69is switched on/off by the status of the S+ and S bands monitored by the monitor photodiode63under the control of the CPU12. The S+/S Raman pump light source unit42ais provided in front of the WDM coupler31and is coupled to the optical fiber via the WDM coupler55. The S+/S/C/L signal lights are co-propagated, while the S+/S Raman pump lights and S+/S signal substitution Raman pump lights are counter-propagated with respect to the signal lights. The CPU12controls the S+ and S signal substitution Raman pump light source unit42band the optical switch69on the basis of the status of the S+/S bands monitored by the photodiode63.

FIG. 18Bis a block diagram of a variation10Q of the repeater node10P. An optical switch70is provided behind the WDM element31as the repeater node shown inFIG. 18A, but is located in a different position. The S+ and S signal substitution Raman pump light source unit42bis selectively coupled to the WDM coupler41by the switch70based on the status of the S+/S bands under the control of the CPU12. The S+ and S signal substitution Raman pump lights, which are switched based on the status of the S+/S bands, are counter-propagated with respect to the C/L signal lights and the S+/S Raman pump lights.

FIG. 19Ais a block diagram of an optical repeater node10R according to an eighth embodiment of the present invention. The Raman pump light source unit48for substitution for S+ signal lights is coupled behind the WDM coupler56by an optical switch72. The Raman pump light source unit49for substitution for S signal lights is coupled behind the WDM coupler56by an optical switch71. The CPU12controls the Raman pump light source unit48and the switch72on the basis of the status of the S+ band monitored by the photodiode37. Similarly, the CPU12controls the Raman pump light source unit49and the switch71on the basis of the status of the S band monitored by the photodiode36. The S+/S Raman pump light source unit42ais coupled to the optical fiber14via the WDM coupler55. The S+/S Raman pump lights and the /S+ and S signal substitution Raman pump lights are co-propagated, and counter-propagated with respect to the S+/S/C/L signal lights.

FIG. 19Bis a block diagram of a variation10S of the optical amplifier10R. The optical switches73and74are provided behind the optical amplifiers38and39, respectively. The optical switch73selectively couples the Raman pump light source49with the WDM coupler40based on the status of the S band under the control of the CPU12. Similarly, the optical switch74selectively couples the Raman pump light source48with the WDM coupler40based on the status of the S+ band under the control of the CPU12. The S+ and S signal substitution Raman pump lights are counter-propagated with respect to the C/L signal lights and the S+/S Raman pump light source unit42a.

FIG. 25Ashows an optical setup for monitoring part of the forward and the backward propagating lights S+S/CL bi-directional transmission systems by using optical spectrum analyzers. The forward propagating lights are in the S+ and S bands, and the backward propagating lights are in the C and L bands. Optical taps160and161are provided in front and behind the amplifier stage. A WDM coupler162couples parts of incoming signals of the amplifier stage. A WDM coupler163couples parts of outgoing signals. An optical spectrum analyzer (OSA)164connected to the WDM coupler162monitors the power levels of the incoming signals. An optical spectrum analyzer (OSA)165connected to the WDM coupler163monitors the power levels of the outgoing signals. The optical spectrum analyzers164and165communicate with the CPU12.

A description will be given of an optical repeater node according to a ninth embodiment of the present invention. This repeater node is used in DWDM systems as shown in FIG.2D. In DWDM systems, impairments due to nonlinear interaction between neighboring channels can be reduced by channel-interleaved bi-directional transmission. The channel-interleaving is illustrated in a graph in FIG.2D. The channels of the opposite (forward and backward) propagating directions respectively illustrated by solid and broken lines are interleaved. At the amplifier stage, optical circulators are used to separate forward and backward propagating channels. The amplifier structure for each direction is similar to that in the aforementioned first through eighth embodiments of the present invention. The difference is that all lights in one branch are propagated in the same direction.

FIG. 20Ais a block diagram of an optical repeater node100A according to the ninth embodiment of the present invention, which can be applied to the channel-interleaved bi-directional S/C/L transmission systems. The repeater node100A processes three bands of S, C and L. The repeater node100A is coupled to the optical transmission line formed of the optical fiber14via optical circulators43and121, which make two amplifier systems. One of the two systems includes a first optical amplifier involved in the forward propagation and made up of the aforementioned components. Similarly, the other system includes a second optical amplifier having the same structure as the first optical amplifier. The second amplifier involved in the backward propagation is made up of a WDM coupler122, a C/L amplifier stage123, an S band amplifier124, an optical tap125, a monitor photodiode126, a S+ and S signal substitution Raman pump light source unit127, a WDM coupler129, an S band Raman pump light source unit130, and a WDM coupler131.

Multiplexed light passes through the circulator43, and is applied to the WDM coupler31. The C/L signal lights are applied to the C/L amplifier stage33. The S signal and S-band pump lights are applied to the S-band amplifier38. The S+ and S signal substitution Raman pump source unit42bis controlled by the CPU12on the basis of the status of the S band in the forward propagation monitored by the photodiode37via the optical tap34. The S+ and S signal substitution Raman pump lights are multiplexed with the amplified S-band signal lights and the C/L signal lights via the WDM coupler60and the WDM coupler41. Further, the S Raman pump lights are coupled with the output of the WDM coupler41by the WDM coupler55. Then, the multiplexed light is sent to the optical fiber14via the circulator121.

Similarly, multiplexed light passes through the circulator121, and is applied to the WDM coupler122. The C/L signal lights and the S+/S signal substitution Raman pump lights, if any, are applied to the C/L amplifier stage123. The S signal and S-band pump lights are applied to the S-band amplifier124. The S+ and S signal substitution Raman pump source unit127is controlled by the CPU12on the basis of the status of the S band in the backward propagation monitored by the photodiode126via the optical tap125. The S+ and S signal substitution Raman pump lights are multiplexed with the amplified S-band signal lights and the C/L signal lights via the WDM coupler128and the WDM coupler129. Further, the S Raman pump lights are coupled with the output of the WDM coupler129by the WDM coupler131. Then, the multiplexed light is sent to the optical fiber14via the circulator43.

FIG. 20Bis a block diagram of a variation100B of the optical repeater node10A shown in FIG.20A. The S Raman pump light /S+ and S signal substitution Raman pump light source unit42cis provided behind the WDM coupler41via the WDM coupler55. The CPU12controls the unit42cbased on the status of the S band monitored by the monitor photodiode37coupled to the inner S-band forward transmission line extending from the S-band amplifier38via the WDM tap34. Similarly, an integrated S Raman pump/S+ and S signal substitution Raman pump light source unit130ais provided behind the WDM coupler129via the WDM coupler131. The CPU12controls the unit130aon the basis of the status of the S band monitored by the monitor diode126coupled to the inner-S band backward transmission line extending from the S-band amplifier124.

FIG. 21is a block diagram of an optical repeater node100C according to a tenth embodiment of the present invention. The device100C can be applied to the channel-interleaved bi-directional transmission systems which have the S+/S/C/L bands. The forward amplifier system includes the aforementioned WDM coupler31, the C/L amplifier stage33, the S+/S amplifier stage62, the WDM coupler41, the optical tap69, the S+/S band monitor photodiode63, the S+/S Raman pump light source unit42a, the WDM coupler55, the S+ and S signal substitution Raman pump light source unit42b, the WDM coupler76, and the optical switches67and68. Similarly, the backward amplifier system includes the aforementioned WDM coupler122, the C/L amplifier stage123, an S+/S amplifier stage134, the WDM coupler129, an optical tap135, an S+/S band monitor photodiode136, an S+/S Raman pump light source unit138, a WDM coupler131, an S+ and S signal substitution Raman pump light source unit137, a WDM coupler132, and optical switches139and140.

The switches67and68and the Raman pump light source units42aand42bare controlled by the CPU12on the basis of the status of the S+/S bands in the forward propagation monitored by the photodiode63. Similarly, the switches139and140and the Raman pump light source units137and138are controlled by the CPU12on the basis of the status of the S+/S bands in the backward propagation monitored by the photodiode136. If the forward S+ and/or S band fails, the S+ and S signal substitution Raman pump light source unit42is selected by the optical switches67and68. Similarly, if the backward S+ and/or S band fails, the S+ and S signal substitution Raman pump light source unit137is selected by the optical switches139and140.

FIG. 22is a block diagram of an optical repeater node100D according to an eleventh embodiment of the present invention. The first (forward) amplifier system is made up of the WDM coupler31, the C/L amplifier stage33, the S+/S amplifier stage62, the optical tap64, the monitor photodiode63, the S+and S signal substitution Raman pump light source unit42b, the optical switch70, the WDM coupler41, the S+/S Raman pump light source unit42aand the WDM coupler55. The S+ and S signal substitution Raman pump light source unit42bcontrolled based on the status of the S+/S bands monitored by the photodiode63is selectively coupled to the WDM coupler41via the optical switch70. The optical switch70is controlled by the CPU12to select the S+ and S signal substitution Raman pump light source unit42bfor protection of the C/L band transmission in the forward propagation. The S+/S Raman pump light source unit42ais coupled to the circulator121via the WDM coupler55.

The second (backward) amplifier system is made up of the WDM coupler122, the C/L amplifier stage123, the S+/S amplifier stage134, the optical tap135, the monitor photodiode136, the S+ and S signal substitution Raman pump light source unit137, the optical switch140, the WDM coupler129, the WDM coupler131, and the S+/S Raman pump source unit138. The S+ and S signal substitution Raman pump light source unit137controlled on the basis of the status of the S+/S bands monitored by the photodiode136is selectively coupled to the WDM coupler129via the optical switch140. The optical switch140is controlled by the CPU12to select the S+ and S signal substitution Raman pump light source unit137for protection of the C/L band transmission in the backward propagation. The S+/S Raman pump light source unit138is coupled to the circulator43via the WDM coupler131.

FIG. 23is a block diagram of an optical repeater node100E according to a twelfth embodiment of the present invention. The first (forward) amplifier system is configured as shown in FIG.19B except for the position of the WDM coupler55to which the S+/S Raman pump light source unit42ais connected. The WDM coupler55of the repeater node100E is connected to the output of the WDM coupler41. The second (backward) amplifier system has the same structure as the first amplifier system. More particularly, the second amplifier system includes the WDM coupler122, the C/L amplifier stage123, a WDM coupler1340, a WDM coupler1290, a monitor photodiode140, an S-band amplifier141, an optical tap142, an S signal substitution Raman pump light source unit143, an optical switch144, a monitor photodiode145, an S+-band amplifier146, an optical tap147, an S+ signal substitution Raman pump light source unit148, an optical coupler149, the S+/S Raman pump light source unit138and the WDM coupler129.

If the S band fails, the CPU12controls the optical switch144to select the Raman pump light source unit143rather than the S amplifier141. If the S+ band fails, the CPU12controls the optical switch149to select the S+ signal substitution Raman pump light source unit148rather than the S+ amplifier146.

FIG. 24is a block diagram of a variation of any of the optical amplifier units100and100B-100E. If the Raman pump lights does not spectrally overlap with the signal wavelengths, the associated Raman pump light sources can be outsides of the optical circulators43and121between which two amplifier stages150and151are formed. A Raman pump light source unit152which emits Raman pump lights that do not spectrally overlap with the signal wavelengths is coupled to the optical transmission line via a WDM coupler153. Similarly, a Raman pump light source unit154which emits Raman pump lights that do not spectrally overlap with the signal wavelengths is coupled to the optical transmission line via a WDM coupler155.

In the optical repeater nodes100and100B-100E, the monitor photodiodes are used. Rather, optical spectrum analyzers may be employed.

FIG. 25Bshows an optical setup for monitoring part of the forward and the backward propagating lights in channel-interleaved bi-directional transmission systems by using optical spectrum analyzers. The forward propagating lights are involved in even channels, and the backward propagating lights are involved in odd channels. Rather than the WDM couplers162and163, optical switches166and167are used as shown in FIG.25B.

The present invention is not limited to the specifically described first through twelfth embodiments, variations and modifications thereof.

For example, a different number of Raman pumping lights may be used. The Raman pump sources can be multiple wavelengths pump sources as disclosed, for instance, WO00/05622. Also, wavelength tunable pump light sources may be used. A single photodiode can be either used to monitor the is whole optical band, or a set of photodiodes can be combined with a WDM coupler device to monitor the multiple wavelength groups (i.e. subbands) within a band simultaneously. The monitor photodiodes can be placed behind the amplifiers as well.

The present invention includes systems with other combinations of co-and counter-propagating signal and pump lights such as a system in which the S+/S pump lights are co-propagated with the signal lights. The present invention includes systems which use a different set of bands, such as a system using S+/C/L bands, as well as systems including the wavelength region beyond the L band (i.e. L+ band).

It is to be noted that the present invention provides a scheme which allows in-service upgrading of wideband WDM systems, in which the power transfer from comparatively short wavelength channels to comparatively long wavelength channels for reliable transmission of the long wavelength channels. If in such systems first a small number of short wavelength channels is employed, but the addition of further short wavelength channels should be kept optional, a provision for later in-service upgradability has to be made. For this purpose, a small number of short wavelengths substitute Raman pumps provide the power to comparatively longer wavelength channels. Each substitute pump wavelength replaces a group of short wavelength channels to be installed later.