Optical amplifying apparatus for amplifying wide-wavelength-band light, optical sending apparatus, optical transmission system, and optical amplifying method

There are provided a plurality of optical adjusting sections, a wavelength-multiplexing section, and a control section. The plurality of optical adjusting sections, which are provided for respective wavelength bands, amplifies light beams in the respective wavelength bands. The wavelength-multiplexing section wavelength-multiplexes amplified light beams in the respective wavelength bands. The control section controls the outputs of the respective optical amplifying sections so that optical powers of the respective wavelength bands will become approximately identical at a predetermined point when wavelength-multiplexed light of the light beams in the respective wavelength bands travels to the predetermined point. This configuration makes it possible to eliminate optical power deviations between wavelength bands that would otherwise occur when an optical signal of a plurality of wavelength bands is transmitted, and to thereby make optical SNRs uniform.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical amplifying apparatus that has optical amplifiers corresponding to respective wavelength bands and can almost equalize optical powers of the respective wavelength bands at a point that an optical signal reaches after traveling a predetermined distance by controlling the outputs of the respective optical amplifiers, as well as to a related optical sending apparatus. The invention also relates to an optical transmission system using these apparatuses as an optical repeater. The invention further relates to an optical amplifying method in an optical amplifying apparatus that has optical amplifiers corresponding to respective wavelength bands.

To construct future multimedia networks, ultra-long-distance, large-capacity optical transmission systems are now required. The wavelength-division multiplexing (hereinafter abbreviated as WDM) is now researched and developed as a scheme for realizing such increase in capacity, because of its capability of effectively utilizing wide bandwidth and large capacity of optical fibers and other advantages.

In particular, with demand for enhancement of the WDM in bandwidth and the number of usable wavelengths, optical amplifying apparatuses for amplifying WDM optical signals that are increased in bandwidth and the number of wavelengths are now researched and developed extensively.

2. Description of the Related Art

An optical amplifying apparatus for amplifying WDM optical signals that are increased in bandwidth and the number of wavelengths was reported (“Trinal-wavelength-band WDM transmission over dispersion-shifted fiber”, Jun-ichi Kani et al., 1999 IEICE General Conference).

Referring toFIG. 23that is drawn based on the above report, 16 laser diodes (hereinafter abbreviated as LDS)150-1to150-16emit laser beams having wavelengths that correspond to channel-1to channel16of the S+band, respectively. The emitted laser beams are input to an arrayed waveguide grating (hereinafter abbreviated as AWG)151-1. The AWG151-1generates WDM light by wavelength-multiplexing the laser beams of channel-1to channel-16. The WDM light is input to a Mach-Zehnder interferometer type optical modulator (hereinafter abbreviated as “MZ modulator”)152-1, where it is modulated with information to be transmitted and thereby converted into a WDM optical signal. The WDM optical signal is input to a thulium-doped fiber amplifier (hereinafter abbreviated as TDFA)153. Being a rare-earth-element-doped fiber amplifier that amplifies light in a 1,450-nm band, the TDFA153can amplify an S+-band WDM optical signal. The amplified WDM optical signal is input to a wavelength-multiplexing coupler (hereinafter referred to as “WDM coupler”)156that is a dielectric multilayer optical filter.

A C-band WDM optical signal is generated by a block that is similar to the above block and is composed of LDs150-17to150-32, an AWG151-2, an MZ modulator152-2, and an erbium-doped fiber amplifier (hereinafter abbreviated as EDFA)154that is provided instead of the TDFA153and performs amplification in a 1,550 nm band. The C-band WDM optical signal is input to the WDM coupler156.

An L-band WDM optical signal is generated by a block that is similar to the above block and is composed of LDs150-33to150-48, an AWG151-3, an MZ modulator152-3, and a gain-shifted erbium-doped fiber amplifier (hereinafter abbreviated as GS-EDFA)155that is provided instead of the TDFA153and performs amplification in a 1,580 nm band. The L-band WDM optical signal is input to the WDM coupler156.

The WDM coupler156generates a three-wavelength-band WDM optical signal by wavelength-multiplexing the S+-band, C-band, and L-band WDM optical signals. The three-wavelength-band WDM optical signal is sent to an optical transmission line, that is, a dispersion-shifted fiber (hereinafter abbreviated as DSF)157.

In the above optical transmission system, WDM optical signals that are channel-allocated to the wavelength bands of the S+band (1,450-1,490 nm), the C band (1,530-1,570 nm), and the L band (1,570-1,610 nm), respectively, are generated, amplified by the rare-earth-element-doped fiber amplifiers on a wavelength band basis, and wavelength-multiplexed into a three-wavelength-band WDM optical signal, which is sent to the optical transmission line.

Incidentally, it is known that crosstalk occurs between WDM optical signals traveling through an optical transmission line owing to nonlinear optical phenomena such as stimulated Raman scattering, four-wave mixing, and cross-phase modulation.

In particular, the stimulated Raman scattering makes optical powers of respective channels non-uniform because it causes shorter-wavelength optical power to be transferred to a longer-wavelength side through interaction with optical phonons in the optical transmission line. This causes a gain gradient and hence deteriorates the optical signal-to-noise ratios (hereinafter abbreviated as “optical SNRs”) of WDM optical signals of shorter-wavelength channels.

Where a WDM optical signal is in a frequency band of 15 THz having Raman gain, the proportion D of optical power that is removed from the shortest-wavelength channel of the WDM optical signal is given by
D=∑i=1N-1⁢⁢(λ⁢⁢iλ⁢⁢O)×(Pi⁢⁢γ⁢⁢iLe2⁢Aeff)(1)
where N is the number of channels and λi, Pi, and γi are the wavelength, optical power, and Raman gain coefficient, respectively, of an i-th channel. Le is the effective length of the optical transmission line and is given by Le={1-exp(−α)}/α where α is the loss coefficient of the optical transmission line. Aeff is the effective core cross section of the optical transmission line.

In general, the Raman gain coefficient is triangle-approximated and given byγ⁢⁢i=i⁢⁢Δ⁢⁢f⁢⁢γ⁢⁢p⁢1.5×1′⁢013(2)
where Δf is the space between channels and γp is the peak gain coefficient that is the maximum value of Raman gain coefficients that are obtained by the triangle approximation.

Formulae relating to the stimulated Raman scattering including the above equations are described on pp. 276-278 of “Optical Fiber Communication Technology” (supervised by Yoshihiro Konishi, The Nikkan Kogyo Shinbun, Ltd.).

It is known that if a 32-wave WDM optical signal is transmitted over a certain distance through an optical fiber, stimulated Raman scattering causes part of the optical power of channel-1to be transferred to longer-wavelength channels and hence causes a gain gradient in the WDM signal. That is, it is known that a gain gradient due to stimulated Raman scattering occurs in a WDM optical signal in a single wavelength band.

Incidentally, it is calculated that the range of the interaction of the stimulated Raman scattering in a wavelength band around 1,550 nm covers a wide wavelength band of 130 nm or more. Therefore, when a three-wavelength-band WDM optical signal whose channels are set in three wavelength bands are transmitted over 100 km in the optical transmission system ofFIG. 23, it is expected that at point X, which is the point where the transmission ends, the optical SNRs deteriorate because stimulated Raman scattering causes part of the optical power of the S+band that is a shorter-wavelength band to be transferred to the C band and the L band that are longer-wavelength bands.

Based on the above understanding, a measurement was performed to evaluate how the stimulated Raman scattering influences a two-wavelength-band WDM optical signal that is transmitted in both of the C band and L band.

Referring toFIG. 17,32LDs120-1to120-32emit laser beams having wavelengths that correspond to channel-1to channel-32of the C-band, respectively. The emitted laser beams are input to an AWG121-1, where they are wavelength-multiplexed into WDM light. The WDM light is input to an EDFA122and amplified there. The amplified WDM light is input to an attenuator (hereinafter abbreviated as ATT)123that attenuates optical power. The WDM light whose optical power has been attenuated to a predetermined level is input to a WDM coupler126.

L-band WDM light is generated by a block that is similar to the above block and is composed of LDs120-33to120-64, an AWG121-2, a GS-EDFA124that is provided instead of the EDFA122, and an ATT125. The generated L-band WDM light is input to a WDM coupler126.

The WDM coupler126generates a two-wavelength-band WDM light by wavelength-multiplexing the C-band and L-band WDM light beams, and sends it to a single-mode fiber (hereinafter abbreviated as SMF)127.

After being transmitted through the SMF127over 80 km, the two-wavelength-band WDM light is input to an optical spectrum analyzer128that measure the wavelength and the power of light entered.

The attenuation amounts of the respective ATTs123and125are so adjusted that the optical power of each channel in the C band and that in the L band are equalized at a point immediately downstream of the output point of the WDM coupler126, that is, at point Y shown in FIG.17.

In the above measurement system, WDM light beams having channels in the wavelength bands of the C band and the L band are generated, the optical powers are then adjusted on a wavelength band basis by the rare-earth-element-doped fiber amplifiers122and124and the ATTs123and125, and resulting WDM light beams are wavelength-multiplexed into two-wavelength-band WDM light, which is sent to the SMF127. Two-wavelength-band WDM light that has been transmitted through the SMF127over 80 km is measured by the optical spectrum analyzer128.

Comparison between measurement results ofFIGS. 18 and 19shows that whereas inFIG. 18the optical powers of the C band and the L band are approximately identical, inFIG. 19(after transmission over 80 km) the optical powers of the C-band are smaller than those of the L band.

InFIGS. 18 and 19, the vertical axis represents the optical power in dBm and the horizontal axis represents the wavelength in nm. InFIG. 20, the vertical axis represents the Raman gain in dB and the horizontal axis represents the wavelength in nm.

FIG. 20is a graph that is drawn based onFIGS. 18 and 19to clarify the above finding. InFIG. 20, mark “x” represents optical powers that are obtained when only C-band WDM light s transmitted over 80 km, marks “▾” represent optical powers that are obtained only L-band WDM light is transmitted over 80 km, and marks “♦” represent optical powers that are obtained when both of C-band and L-band WDM light beams are transmitted over 80 km.

It is seen fromFIG. 20that when both of C-band and L-band WDM light beams are transmitted, the optical power of the C band decreases and the optical power of the L band increases, that is, the stimulated Raman scattering causes part of the power of the C band to be transferred to the L band.

The above measurement is directed to the case where C-band and L-band WDM light beams are transmitted in the same direction. A similar measurement was performed for a case where C-band and L-band WDM light beams are bidirectionally transmitted.

FIG. 21shows a measurement system for the latter case. This measurement system is the same as the measurement system ofFIG. 17except that the block inFIG. 17for generating L-band WDM light that is composed of the LDs120-33to120-64, the AWG120-2, the GS-EDFA124, and the ATT125is provided on the side that is opposite, with respect to the SMF127, to the side where C-band WDM light is generated and that an optical spectrum analyzer130for measuring a spectrum of L-band WDM light is added. Therefore, a description of the configuration of this measurement system is omitted.

In this measurement system, the optical power of generated C-band WDM light is adjusted by the EDFA122and the ATT123and resulting C-band WDM light is sent to the SMF127. C-band WDM light that has been transmitted through the SMF127over 80 km is measured by the optical spectrum analyzer128. On the other hand, the optical power of generated L-band WDM light is adjusted by the GS-EDFA124and the ATT125and resulting L-band WDM light is sent to the SMF127. L-band WDM light that has been transmitted through the SMF127over 80 km is measured by the optical spectrum analyzer130.

The ATTs123and125make such adjustments as to equalize the optical power of each channel in the C band at point Z1(seeFIG. 21) and that in the L band at point Z2(see FIG.21).

It is seen from a measurement result ofFIG. 22that the phenomenon that part of the optical power of the C band is transferred to the L band occurs in the same manner in the unidirectional transmission and the bidirectional transmission.

InFIG. 22, the vertical axis represents the Raman gain in dB and the horizontal axis represents the wavelength in nm. Marks “♦” represent optical powers in the unidirectional transmission that were transferred from FIG.20and marks “▪” represent optical powers in the bidirectional transmission.

It is seen fromFIGS. 19,20, and22that when two-wavelength-band WDM light is transmitted, stimulated Raman scattering causes part of the power of C-band WDM light to be transferred to L-band WDM light. That is, the stimulated Raman scattering causes part of the power of WDM light in a shorter-wavelength band to be transferred to WDM light in a longer-wavelength band. As a result, when n-wavelength-band WDM light is transmitted, optical power deviations occur between the wavelength bands and the optical SNRs of WDM light beams in shorter-wavelength bands are lowered.

In particular, as is understood from Equation (1), the optical SNRs deteriorate more in the case of ultra-long-distance transmission because Pi and Le become larger in that case.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical amplifying apparatus, an optical sending apparatus, and an optical transmission system that are free of deviations between optical powers of respective wavelength bands after transmission of wide-wavelength-band light.

Another object of the invention is to provide an optical transmission system that increases the optical SNRs after transmission of optical signals when transmitting wide-wavelength-band light.

A further object of the invention is to provide an optical amplifying method that is free of deviations between optical powers of respective wavelength bands after transmission of wide-wavelength-band light.

The above objects are attained by an apparatus having a plurality of optical adjusting sections, a wavelength-multiplexing section, and a control section in which the control section controls the outputs of the optical adjusting sections so that optical power of light in a shorter,wavelength band becomes larger than optical power of light in a longer-wavelength band.

Examples of an optical adjusting section are optical amplifiers and optical attenuators. A control section, for example, may adjust the outputs of the plurality of optical adjusting sections by referring to optical powers of light beams in the respective wavelength bands of wavelength-multiplexed light that has been transmitted by a predetermined distance. Alternatively, the control section, as another example, may adjust the outputs of the optical adjusting sections by referring to optical powers of part of light beams in the respective wavelength bands of wavelength-multiplexed light that has been transmitted by a predetermined distance.

Since the above apparatus can control the outputs of the optical adjusting sections, it can eliminate deviations between the wavelength bands that would otherwise occur due to wavelength-dependent amplification and losses such as stimulated Raman scattering and a loss in an optical transmission line, a loss in a wavelength-demultiplexing section, and a loss in the wavelength-multiplexing section, and hence can increase the optical SNRS. Therefore, the above apparatus can improve the performance of the entire optical transmission system.

Here the further objects and features of the invention will become apparent from the following description to be made with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described with reference to the accompanying drawings. In these Figures, the same constructions are designated by the same reference numerals, and their repeated description may be omitted.

As shown inFIG. 1, a composite optical amplifying apparatus1is composed of a wavelength-demultiplexing section8, a plurality of optical amplifying sections5-1to5-n, a wavelength-multiplexing section6, and a control section7.

Input light that has traveled through an optical transmission line11is input to the wavelength-demultiplexing section8. The wavelength-demultiplexing section8demultiplexes the input light into light beams in predetermined wavelength bands and outputs the demultiplexed light beams in a separated manner.

The demultiplexed light beams in the respective wavelength bands are input to the respective optical amplifying sections5-1to5-n. Provided for the respective wavelength bands of light beams to receive, the optical amplifying sections5-1to5-n amplify the light beams in the respective wavelength bands.

The control section7controls the optical amplifying sections5-1to5-n so that the output of an optical amplifying section among the optical amplifying sections5-1to5-n that amplifies light in a shorter-wavelength band becomes larger than the output of an optical amplifying section among the optical amplifying sections5-1to5-n that amplifies light in a longer-wavelength band.

Light beams in the respective wavelength bands that have been amplified by the respective optical amplifying sections5-1to5-n are input to the wavelength-multiplexing section6and wavelength-multiplexed there. The wavelength-multiplexed light beams in the respective wavelengths are output to an optical transmission line10that is connected to the wavelength-multiplexing section6.

The optical transmission line10is not limited to an optical fiber and may be a space filled with a gas.

Next, the operation principle and the advantageous effects of the first embodiment will be described.

The factors of varying the optical power of light being transmitted through the optical transmission line10are the stimulated Raman scattering (SRS), the loss in the optical transmission line10, the loss in the wavelength-demultiplexing section8, and the loss in the wavelength-multiplexing section6. Those factors depend on the wavelength.

The reason why the loss in the wavelength-demultiplexing section8is taken into consideration is that the optical SNR of an optical signal that has been transmitted over a certain distance depends on the input power and the noise figure of each of the optical amplifying sections5-1to5-n. Since the loss in the wavelength-demultiplexing section8and the loss in the wavelength-multiplexing section6exhibit the same characteristic, they will be dealt with as the loss in a wavelength-multiplexing/demultiplexing section. The dependence on the noise figure of the optical SNR of an optical signal that has been transmitted over a certain distance will be described later with reference to FIG.2E.

Although the following description will be directed to a case of three wavelength bands to simplify the description, a case of an arbitrary number of wavelength bands can be explained in a similar manner.

FIG. 2Ashows light that is transmitted through the optical transmission line10, for example, a three-wavelength-band WDM optical signal consisting of a WDM optical signal in a first wavelength band, a WDM optical signal in a second wavelength band, and a WDM optical signal in a third wavelength band.

FIG. 2Bshows an example of the loss due to the stimulated Raman scattering in the above wavelength bands. As seen fromFIGS. 2A and 2B, the stimulated Raman scattering causes part of the optical power of the first wavelength band to be transferred to the second and third wavelength bands and also causes part of the optical power of the second wavelength band to be transferred to the third wavelength band. As a result, the optical powers of the respective wavelength bands increase or decrease.

FIG. 2Cshows an example of the loss in the optical transmission line10. As shown inFIG. 2C, the loss in the optical transmission line10depends on the wavelength of light transmitted. In general, optical transmission lines have a loss-wavelength characteristic that the loss has a minimum value at a particular wavelength.

FIG. 2Dshows an example of the loss in the wavelength-multiplexing/demultiplexing section. In particular, an interference filter type optical multi/demultiplexer such as a dielectric multilayer optical filter multiplexes light beams (or demultiplexes a light beam) on a wavelength band basis in a step-like manner. Therefore, light that is added (or separated) at the first stage passes through a different number of interference filters and hence is given a different loss than light that is added (or separated) at the last stage.

FIG. 2Eshows an example of the noise figures (NF) of the optical amplifying sections5-1to5-n. In the optical amplifying sections5-1to5-n, the intensity of amplified spontaneous emission (ASE) depends on the wavelength. Therefore, the noise figure of each of the optical amplifying sections5-1to5-n also depends on the wavelength. The noise figure is a value obtained by dividing the input optical SNR by the output optical SNR in each of the optical amplifying sections5-1to5-n and relates to the noise power.

When the three-wavelength-band WDM optical signal shown inFIG. 2Ais transmitted, since the optical transmission line10and the wavelength-multiplexing/demultiplexing section exhibit the wavelength-dependent losses shown inFIGS. 2B-2D, the optical powers of the respective wavelength bands of the three-wavelength-band WDM optical signal have deviations after the transmission.

That is, referring toFIG. 1, the losses shown inFIG. 2Doccur in the WDM optical signals in the first to third wavelength bands when they are wavelength-demultiplexed by the wavelength-demultiplexing section8and when they are wavelength-multiplexed by the wavelength-multiplexing section6after being amplified by the respective optical amplifying sections5-1to5-3. While being transmitted through the optical transmission line10to point A, the losses shown inFIGS. 2B and 2Coccur in the three-wavelength-band WDM optical signal. Because the losses depend on the wavelength, if the outputs of the optical amplifying sections5-1to5-n are not adjusted, deviations occur between the optical powers of the respective wavelength bands at point A. Therefore, the SNRs vary from one wavelength band to another.

In view of the above, the optical powers of the respective wavelength bands are pre-emphasized on a wavelength band basis by adjusting the outputs of the optical amplifying sections5-1to5-3before transmission of a three-wavelength-band WDM optical signal so as to compensate for deviations that will occur in the optical powers of the respective wavelength bands.

By performing inter-wavelength-band preemphasis in this manner, losses that occur during transmission through the optical transmission line10can be compensated for and hence deviations between the optical powers of the respective wavelength bands after the transmission can be reduced or even be eliminated. Therefore, the optical SNR of the entire three-wavelength-band WDM optical signal can be increased.

Where the stimulated Raman scattering loss is so large that the loss in the optical transmission line10and the loss in the wavelength-multiplexing/demultiplexing section are negligible, the amounts of inter-wavelength-band preemphases can be calculated according to Equations (1) and (2). In this case, since the stimulated Raman scattering causes parts of the optical powers of shorter-wavelength bands to be transferred to longer-wavelength bands, satisfactory results are obtained by setting the optical powers in such a manner that the optical power of the first wavelength band is largest, the optical power of the second wavelength band is intermediate, and the optical power of the third wavelength band is smallest.

On the other hand, where the loss in the optical transmission line10and the loss in the wavelength-multiplexing/demultiplexing section are not negligible, drawings corresponding toFIGS. 2C and 2Dare produced by actually measuring the loss in the optical transmission line10and the loss in the wavelength-multiplexing/demultiplexing section. The amounts of inter-wavelength-band preemphases can be calculated by using those drawings. In this case, the manner of preemphasis varies depending on the magnitude relationship among the losses. The optical powers may be set in such a manner that the optical power decreases in order of the first wavelength band, the second wavelength band, and the third wavelength band as shown in FIG.3A. Alternatively, the optical powers may be set in such a manner that the optical power decreases in order of the first wavelength band, the third wavelength band, and the second wavelength band as shown in FIG.3B.

Where the noise powers in the optical amplifying sections5-1to5-n have no differences or have negligible differences, inter-wavelength-band preemphasis is performed in the above manner in consideration of the stimulated Raman scattering, the loss in the optical transmission line10, and the loss in the wavelength-multiplexing/demultiplexing section. Since the composite optical amplifying apparatus1can almost equalize the optical powers of respective wavelength bands after transmission in this manner, the optical SNRs can be increased.

On the other hand, where noise powers in the optical amplifying sections5-1to5-n have non-negligible differences, inter-wavelength-band preemphasis is performed in the following manner. Since noise powers in the optical amplifying sections5-1to5-n are transmitted as they are, the inter-wavelength-band preemphases are so adjusted as to equalize optical powers obtained by eliminating noise powers in the optical amplifying sections5-1to5-n for amplifying WDM optical signals in the respective wavelength bands from optical powers of the respective wavelength bands after transmission. In this manner, the optical SNRs can further be increased. The elimination of noise powers in the optical amplifying sections5-1to5-n can be performed by subtracting the noise powers in the optical amplifying sections5-1to5-n from optical powers of the respective wavelength bands by expressing the two kinds of powers in dB.

The optical power of each wavelength band is larger when the number of channels in each wavelength band is larger or each wavelength band is wider. Therefore, the inter-wavelength-band preemphasis is performed in such a manner that the differences between the optical powers of shorter-wavelength bands and those of longer-wavelength bands are made larger when the number of channels in each wavelength band is larger or each wavelength band is wider.

Where as described above the stimulated Raman scattering, the loss in the optical transmission line10, and the loss in the wavelength-multiplexing/demultiplexing section can be measured in advance, satisfactory results are obtained by the control section7's performing control according to the above principle.

In the composite optical amplifying apparatus1, the control section7controls in advance the outputs of the respective optical amplifying sections5-1to5-n in consideration of optical power variations that will occur in the optical transmission line10from the composite optical amplifying section1to a point that is a predetermined distance away from the composite optical amplifying section1. Therefore, optical powers of the respective wavelength bands after transmission over the predetermined distance are made approximately identical. Therefore, where input light beams are WDM optical signals, deterioration in optical SNR at the predetermined point can be reduced. If an optical receiving apparatus for receiving and processing WDM optical signals is provided at the predetermined point where the optical SNRs of the respective wavelength bands are made uniform, the performance of the entire optical transmission system having the composite optical amplifying apparatus1and the optical receiving apparatus can be improved.

In the first embodiment, it is preferable that as indicated by a broken line inFIG. 1the composite optical amplifying apparatus1further has a pump source9for supplying optical power to input light that is connected to the wavelength demultiplexing section8, and that the wavelength-demultiplexing section8inputs, to the optical transmission line11, light that is supplied from the pump source9. In this case, the composite optical amplifying apparatus1can stimulated-Raman-amplify input light in the optical transmission line11with pump light of the pump source9, making it possible to compensate for attenuation amounts that occur in the wavelength-demultiplexing section8in demultiplexing the input light.

A description will now be made of a case where input light is a two-wavelength-band WDM optical signal consisting of a WDM optical signal in a first wavelength band and a WDM optical signal in a second wavelength band that is longer in wavelength than the first wavelength band and the number of channels of a WDM optical signal is increased or decreased.

FIG. 4Ashows a case where the number of channels in the first wavelength band is changed from m to m+k, andFIG. 4Bshows a case where the number of channels in the first wavelength band is changed from m to m−k.

FIG. 5Ashows a case where the number of channels in the second wavelength band is changed from m to m+k, andFIG. 5Bshows a case where the number of channels in the second wavelength band is changed from m to m−k.

InFIGS. 4A and 4BandFIGS. 5A and 5B, the vertical axis represents the optical power and the horizontal axis represents the wavelength.

Control to be performed by the control section7when the number of channels of a WDM optical signal is increased or decreased will be described with reference toFIGS. 4A and 4BandFIGS. 5A and 5B.

First, control to be performed when the number of channels in the first wavelength band is increased or decreased will be described.

It is assumed that as shown in the left-hand parts ofFIGS. 4A and 4Ban m-wave WDM optical signal is set in the first wavelength band, an L-wave WDM optical signal is set in the second wavelength band, and the optical power of each channel is P0.

When the number of channels in the first wavelength band is increased from m to m+k in this state, since the optical power of the first wavelength band increases, the following three kinds of control are available as control to be performed by the control section7as shown in the right-hand parts of FIG.4A.

In the first control, as shown in the top-right part ofFIG. 4A, the optical power per channel of the first wavelength band is decreased from P0and the optical power per channel of the second wavelength band is kept at P0. In the second control, as shown in the middle-right part ofFIG. 4A, the optical power per channel of the first wavelength band is kept at P0and the optical power per channel of the second wavelength band is increased from P0. In the third control, as shown in the bottom-right part ofFIG. 4A, the optical power per channel of the first wavelength band is decreased from P0and the optical power per channel of the second wavelength band is increased from P0.

When the number of channels in the first wavelength band is decreased from m to m−k in the state as shown the left-hand parts ofFIG. 4B, since the optical power of the first wavelength band decreases, the following three kinds of control are available as control to be performed by the controlling section7as shown in the right-hand parts of FIG.4B.

In the first control, as shown in the top-right part ofFIG. 4B, the optical power per channel of the first wavelength band is increased from P0and the optical power per channel of the second wavelength band is kept at P0. In the second control, as shown in the middle-right part ofFIG. 4B, the optical power per channel of the first wavelength band is kept at P0and the optical power per channel of the second wavelength band is decreased from P0. In the third control, as shown in the bottom-right part ofFIG. 4B, the optical power per channel of the first wavelength band is increased from P0and the optical power per channel of the second wavelength band is decreased from P0.

Next, control to be performed when the number of channels in the second wavelength band is increased or decreased will be described.

It is assumed that as shown in the left-hand parts ofFIGS. 5A and 5Ban m-wave WDM optical signal is set in the first wavelength band, an L-wave WDM optical signal is set in the second wavelength band, and the optical power of each channel is P0.

When the number of channels in the second wavelength band is increased from L to L+h in this state, since the optical power of the second wavelength band increases, the following three kinds of control are available as control to be performed by the controlling section7as shown in the right-hand parts of FIG.5A.

In the first control, as shown in the top-right part ofFIG. 5A, the optical power per channel of the first wavelength band is increased from P0and the optical power per channel of the second wavelength band is kept at P0. In the second control, as shown in the middle-right part ofFIG. 5A, the optical power per channel of the first wavelength band is kept at P0and the optical power per channel of the second wavelength and is decreased from P0. In the third control, as shown in he bottom-right part ofFIG. 5A, the optical power per channel f the first wavelength band is increased from P0and the optical power per channel of the second wavelength band is decreased from P0.

When the number of channels in the second wavelength band is decreased from L to L−h in the state as shown the left-hand parts ofFIG. 5B, since the optical power of the second wavelength band decreases, the following three kinds of control are available as control to be performed by the control section7as shown in the right-hand parts of FIG.5B.

In the first control, as shown in the top-right part ofFIG. 5B, the optical power per channel-of the first wavelength band is decreased from P0and the optical power per channel of the second wavelength band is kept at P0. In the second control, as shown in the middle-right part ofFIG. 5B, the optical power per channel of the first wavelength band is kept at P0and the optical power per channel of the second wavelength band is increased from P0. In the third control, as shown in the bottom-right part ofFIG. 5B, the optical power per channel of the first wavelength band is decreased from P0and the optical power per channel of the second wavelength band is increased from P0.

As described above, three kinds of control are available for each of the cases ofFIGS. 4A and 4BandFIGS. 5A and 5B. The control section7selects and performs one of the three kinds of control.

In each of the above cases, the amount of increase or decrease from P0of the optical power per channel is determined based on the number k or h of increased or decreased channels, the wavelengths of the first and second wavelength bands, the reference optical power P0, the transmission distance to a predetermined point where the optical powers of the respective wavelength bands are to be equalized, and other factors.

As described above, when the number of channels of a WDM optical signal in the first wavelength band has been increased or decreased or the number of channels of a WDM optical signal in the second wavelength band has been increased or decreased, satisfactory results are obtained in such a manner that the control section7increases or decreases the output of the optical amplifying section5-1that amplifies the WDM optical signal in the first wavelength band or increases or decreases the output of the optical amplifying section5-2that amplifies the WDM optical signal in the second wavelength band so that the optical powers will become approximately identical when the WDM optical signals in the respective wavelength bands are transmitted to a predetermined point. With this control, deterioration in optical SNR at the predetermined point can be reduced.

As shown inFIG. 6, a composite optical amplifying apparatus2is composed of a plurality of optical amplifying sections5-1to5-n, a wavelength-multiplexing section6, and a control section7.

Input light in a first wavelength band is input to the optical amplifying section5-1for amplifying light in the first wavelength band. Input light in a second wavelength band is input to the optical amplifying section5-2for amplifying light in the second wavelength band. Similarly, input light in an n-th wavelength band is input to the optical amplifying section5-n for amplifying light in the n-th wavelength band. In this manner, the optical amplifying sections5-1to5-n are provided for the respective wavelength bands of the input light and amplify the respective input light to predetermined optical powers under the control of the control section7. The amplified light beams in the respective wavelength bands are input to the wavelength-multiplexing section6, where they are wavelength-multiplexed and then output to an optical transmission line10that is connected to the wavelength-multiplexing section6.

In the composite optical amplifying apparatus2according to the second embodiment, even if input light in a plurality of wavelength bands are input to the composite optical amplifying apparatus2, they are input to the optical amplifying sections5-1to5-n for amplifying input light in the respective wavelength bands to predetermined optical powers. Therefore, the optical powers of the light beams of the respective wavelength bands can be adjusted in a reliable manner. Therefore, even if input light in a plurality of wavelength bands are wavelength-multiplexed and then output from the composite optical amplifying apparatus2, the optical powers of the light beams in the respective wavelength bands can be made approximately uniform at a predetermined point.

As shown inFIG. 7, a wide-wavelength-band optical sending apparatus3is composed of a plurality of optical signal generating sections13-1to13-n, a plurality of optical amplifying sections5-1to5-n, a wavelength-multiplexing section6, and a control section7.

The optical signal generating sections13-1to13-n are provided for respective wavelength bands and generate WDM optical signals in the respective wavelength bands.

The generated WDM optical signals in the respective wavelength bands are amplified to predetermined power levels by the optical amplifying sections5-1to5-n that are controlled by the control section7. The amplified WDM optical signals are wavelength-multiplexed by the wavelength-multiplexing section6and then output to an optical transmission line10as an n-wavelength-band WDM optical signal.

That is, the wide-wavelength-band optical sending apparatus3according to the third embodiment is constructed by adding, to the composite optical amplifying apparatus2according to the second embodiment, the optical signal generating sections13-1to13-n in such a manner that they correspond to the respective optical amplifying sections5-1to5-n of the composite optical amplifying apparatus2.

The control section7performs one of the various kinds of control that were described in the first and second embodiments.

Therefore, the operation principle and the advantageous effects of the wide-wavelength-band optical sending apparatus3that relate to making optical powers of the respective wavelength bands at a predetermined point approximately identical and the operation principle and the advantageous effects of the wide-wavelength-band optical sending apparatus3that relate to adjusting optical powers of the respective wavelength bands at a predetermined point in consideration of the noise figures of the respective optical amplifying sections5-1to5-n are the same as in the first and second embodiments and hence are not described here.

In the first to third embodiments, the control section7may control the outputs of the respective optical amplifying sections5-1to5-n so that when light beams in the respective wavelength bands amplified by the respective optical amplifying sections5-1to5-n travel to a predetermined point, for example, point A, powers obtained by eliminating noise powers in the respective optical amplifying sections5-1to5-n from optical powers of the respective wavelength bands at point A will become approximately identical.

In the first to third embodiments, the control section7may control the outputs of the respective optical amplifying sections5-1to5-n so that when light beams in the respective wavelength bands amplified by the respective optical amplifying sections5-1to5-n travel to a predetermined point, for example, point A, on the optical transmission line10that is a predetermined distance away from the composite optical amplifying apparatus1or2or the wide-wavelength-band optical sending apparatus3, optical powers of the respective wavelength bands at point A will become approximately identical.

In the first to third embodiments, the control amounts of the control section7may be determined based on at least one of the stimulated Raman scattering in the optical transmission line10, the loss in the optical transmission line10, the loss in the wavelength demultiplexing section8, and the loss in the wavelength multiplexing section6.

In the first to third embodiments, the control section7may have a detecting section for detecting optical powers of respective wavelength bands at a predetermined point when light beams in the respective wavelength bands amplified by the respective optical amplifying sections5-1to5-n travel to the predetermined point, and may adjust the outputs of the respective optical amplifying sections5-1to5-n based on outputs of the detecting section so that the optical powers of respective wavelength bands at the predetermined point become approximately identical.

In this case, the control section7actually detects optical powers of the respective wavelength bands at the predetermined point and feedback-controls the outputs of the respective optical amplifying sections5-1to5-n based on detection results. Therefore, the composite optical amplifying apparatuses1and2and the wide-wavelength-band optical sending apparatus3can reliably make the optical powers of the respective wavelength bands at the predetermined point approximately identical.

In the first to third embodiments, the control section7may have a detecting section for detecting optical powers of respective wavelength bands at a predetermined point when light beams in the respective wavelength bands amplified by the respective optical amplifying sections5-1to5-n travel to the predetermined point, and may adjust the outputs of the respective optical amplifying sections5-1to5-n based on outputs of the detecting section so that powers obtained by eliminating noise powers in the respective optical amplifying sections5-1to5-n from the optical powers of respective wavelength bands at the predetermined point become approximately identical.

In this case, the control section7actually detects optical powers of the respective wavelength bands at the predetermined point and feedback-controls the outputs of the respective optical amplifying sections5-1to5-n based on detection results and noises in the respective optical amplifying sections5-1to5-n. Therefore, the composite optical amplifying apparatuses1and2and the wide-wavelength-band optical sending apparatus3can reliably make the optical powers of the respective wavelength bands at the predetermined point approximately identical in consideration of noises in the respective optical amplifying sections5-1to5-n.

The above configuration is particularly effective when the stimulated Raman scattering, the loss in the optical transmission line10, and the loss in the wavelength-multiplexing section6are not the only factors that vary the optical powers of the respective wavelength bands.

Where the degree of stimulated Raman scattering, the loss in the optical transmission line10, and the loss in the wavelength-multiplexing section6can be measured in advance, target values of preemphases to be provided for the respective wavelength bands based on the principle described above with referenceFIGS. 2A-2Eare calculated based on measurement values and the control section7stores calculation results. The control can be performed quickly if the control section7refers to target values corresponding to outputs of the detecting section and feedback-controls the outputs of the respective optical amplifying sections5-1to5-n based on the target values.

In the first to third embodiments, input light may be an n-wavelength-band WDM optical signal consisting of WDM optical signals in respective wavelength bands, and the control section7may have a detecting section for detecting optical power of the shortest-wavelength channel at a predetermined point, for example, point A, and may adjust the outputs of the respective optical amplifying sections5-1to5-n based on an output of the detecting section so that optical powers of WDM optical signals in the respective wavelength bands at the predetermined point become approximately identical.

This control section7actually detects optical power of the shortest-wavelength channel at the predetermined point and feedback-controls the outputs of the respective optical amplifying sections5-1to5-n based on a detection result. Since as indicated by Equation (1) the stimulated Raman scattering is the phenomenon that optical power of a shorter wavelength side is transferred to a longer wavelength side, optical powers of the respective wavelength bands at the predetermined point can be calculated according to Equation (1) etc. based on the optical power of the shortest-wavelength channel. Therefore, the composite optical amplifying apparatuses1and2and the wide-wavelength-band optical sending apparatus3can reliably make the optical powers of the respective wavelength bands at the predetermined point approximately identical.

In the first to third embodiments, input light may be an n-wavelength-band WDM optical signal consisting of WDM optical signals in respective wavelength bands, and the control section7may have a detecting section for detecting optical power of the shortest-wavelength channel at a predetermined point, for example, point A, and may control the outputs of the respective optical amplifying sections5-1to5-n based on an output of the detecting section so that powers obtained by eliminating noise powers in the respective optical amplifying sections5-1to5-n from optical powers of WDM optical signals in the respective wavelength bands at the predetermined point become approximately identical.

This control section7actually detects optical power of the shortest-wavelength channel at the predetermined point and feedback-controls the outputs of the respective optical amplifying sections5-1to5-n based on a detection result and noise powers in the respective optical amplifying sections5-1to5-n. Therefore, the composite optical amplifying apparatuses1and2and the wide-wavelength-band optical sending apparatus3can reliably make the optical powers of the respective wavelength bands at the predetermined point approximately identical in consideration of noises in the respective optical amplifying sections5-1to5-n.

Although in the first to third embodiments the optical powers of the respective wavelength bands are adjusted while light beams in the respective wavelength bands are amplified, they may be adjusted while light beams in the respective wavelength bands are attenuated. In the latter case, an optical attenuator or the like can be used.

As shown inFIG. 8, an optical transmission system according to a fourth embodiment is comprised of an optical sending apparatus4that generates and sends optical signals, an optical transmission line10, and an optical receiving apparatus14that receives and processing optical signals.

The optical sending apparatus4is comprised of a plurality of optical signal generating sections15-1to15-n, a plurality of optical amplifying sections5-1to5-n, a control section7, and a wavelength-multiplexing section6.

The optical signal generating sections15-1to15-n are provided for respective wavelength bands. The optical signal generating sections15-1to15-n generate WDM optical signals in the respective wavelength bands. Each WDM optical signal is an optical signal obtained by wavelength-multiplexing optical signals whose optical powers have been adjusted based on detection results of each of spectrum detecting sections17-1to17-n.

The optical amplifying sections5-1to5-n are connected to the respective optical signal generating sections15-1to15-n and amplify WDM signals generated by the respective optical signal generating sections15-1to15-n.

The control section7controls the outputs of the respective optical amplifying sections5-1to5-n so that when the WDM optical signals in the respective wavelength bands amplified by the respective optical amplifying sections5-1to5-n are transmitted to a predetermined point, optical powers of the WDM optical signals in the respective wavelength bands at the predetermined point will become approximately identical.

The wavelength-multiplexing section6wavelength-multiplexes the amplified WDM optical signals in the respective wavelength bands.

The optical transmission line10is connected to the optical sending apparatus4and transmits the wide-wavelength-band WDM optical signal to the optical receiving apparatus14.

The optical receiving apparatus14is comprised of a wavelength-demultiplexing section18, a plurality of spectrum detecting sections17-1to17-n, and a plurality of optical receiving sections19-1to19-n. The spectrum detecting sections17-1to17-n detect spectra of the WDM optical signals and outputs detection results to the optical sending apparatus4.

Where the transmission distance is so long that the power levels of wide-wavelength-band WDM optical signals that are detected by the optical receiving apparatus14are low, optical amplifying apparatuses for amplifying a wide-wavelength-band WDM optical signal, for example, the above-described composite optical amplifying apparatuses1, may be provided on the optical transmission line10. In particular, where the composite optical amplifying apparatuses1are provided, by installing the next repeater station or the optical receiving apparatus14at the above-mentioned predetermined point, the optical powers of the respective wavelength bands of the wide-wavelength-band WDM optical signals at each repeater station or the optical receiving apparatus14can be made approximately identical. Therefore, deterioration in the optical SNR of the wide-wavelength-band WDM optical signal that is received by each repeater station or the optical receiving apparatus14is reduced and hence ultra-long distance transmission is enabled.

Next, the operation principle and the advantageous effects of the fourth embodiment will be described.

FIG. 9Aschematically shows the optical transmission system according to the fourth embodiment.FIG. 9Bshows spectra at respective points on the optical transmission line10, the points being the output end of the optical sending apparatus4, the input end of a composite optical amplifying apparatus1A that is the first repeater station, the output end of the composite optical amplifying apparatus1A, and the input end of the optical receiving apparatus14that are arranged in order from the left in FIG.9A.FIG. 9C, which is prepared to show the advantageous effects of the optical transmission system according to the fourth embodiment, shows spectra at the same points as inFIG. 9Bthat are obtained when inter-wavelength-band preemphasis is performed but in-wavelength-band preemphasis is not performed by the optical sending apparatus4. InFIGS. 9B and 9C, the vertical axis represents the optical power and the horizontal axis represents the wavelength.

Although to simplify the description the following description will be directed to a case of a two-wavelength-band WDM optical signal consisting of a WDM optical signal in a first wavelength band and a WDM optical signal in a second wavelength band, a case of an n-wavelength-band WDM optical signal formed by wavelength-multiplexing WDM optical signals in n respective wavelength bands can be explained in a similar manner.

Referring to FIGS.8and9A-9C, a WDM optical signal in the first wavelength band is subjected, in the optical sending apparatus4, to in-wavelength-band preemphasis that adjusts the optical powers of respective optical signals. Similarly, a WDM optical signal in the second wavelength band is subjected to inner-wavelength band preemphasis in the optical sending apparatus4.

In an optical transmission system having an optical amplifier, noise due to ASE is necessarily superimposed on an optical signal. Since the ASE depends on the wavelength, in a WDM optical signal produced by wavelength-multiplexing a plurality of optical signals having different wavelengths the levels of noises that are superimposed on the respective optical signals are different from each other. Therefore, the optical signals of the WDM optical signal have different optical SNRs. Since an optical receiving apparatus receives and processes the optical signals having different optical SNRs, it is forced to receive and process the optical signals in such a manner as to adjust itself to an optical signal having the smallest optical SNR. Such optical SNR deviations can be compensated for if the optical sending apparatus4adjusts the optical powers of the respective optical signals so as to make the optical power of an optical signal having the smallest optical SNR largest to thereby eliminate the optical SNR deviations between the optical signals. In particular, if the optical sending apparatus4adjusts the optical powers of the respective optical signals so that when received by the optical receiving apparatus14the optical signals will have optical SNRs that are equal to the largest optical SNR, the optical receiving apparatus14can receive optical signals having optical SNRs that are approximately equal to each other and equal to the largest optical SNR.

As for the inner-wavelength band preemphasis, satisfactory results are obtained if the optical sending apparatus4adjusts the optical powers of the respective optical signals in the above-described manner based on detection results of the spectrum detecting sections17-1and17-2of the optical receiving apparatus14.

After performing the in-wavelength-band preemphasis on each WDM optical signal, the optical sending apparatus4causes the optical amplifying sections5-1and5-2, which are gain-controlled by the control section7, to perform inter-wavelength-band preemphasis.

Since the inter-wavelength-band preemphasis is the same as described above with reference toFIGS. 2A-2Eand3A-3B, it is not described here. Adjustments of the inter-wavelength-band preemphasis may be made by using either the sum or the average (seeFIG. 9B) of the optical powers of the optical signals of each WDM optical signal.

After being subjected to the inner-wavelength band preemphasis and the inter-wavelength-band preemphasis, the first wavelength band WDM optical signal and the second wavelength band WDM optical signal are wavelength-multiplexed by the wavelength-multiplexing section6and output from the optical sending apparatus4to the optical transmission line10in the form of a two-wavelength-band WDM optical signal as shown in the leftmost part of FIG.9B.

At the input end of the composite optical amplifying apparatus1A, the optical powers of the WDM optical signals of the two-wavelength-band WDM optical signal are varied by the stimulated Raman scattering and the loss in the optical transmission line10and other factors. However, since the inter-wavelength-band preemphasis was performed, the optical power of the first wavelength band WDM optical signal is approximately equal to that of the second wavelength band WDM optical signal as shown in the second part (from the left) of FIG.9B. Therefore, the optical SNRs of the respective optical signals are made larger than in the case where the inter-wavelength-band preemphasis is not performed.

As shown in the third part (from the left) ofFIG. 9B, the two-wavelength-band WDM optical signal is subjected to inter-wavelength-band preemphasis and amplification in the composite optical amplifying section1A and then output to the optical transmission line10at the output end of the composite optical amplifying apparatus1A.

At the input end of the optical receiving apparatus14, the two-wavelength-band WDM optical signal that was subjected to the inter-wavelength-band preemphasis and amplification sequentially in the composite optical amplifying sections1is input to the optical receiving apparatus14. Since the optical signals of the incident two-wavelength-band WDM optical signal were subjected to the in-wavelength-band preemphasis, deterioration in optical SNR due to ASEs generated by the optical amplifiers in the optical sending apparatus4, the composite optical amplifying sections1, and the optical receiving section14can be reduced as shown in the rightmost part of FIG.9B. Further, since the inter-wavelength-band preemphasis was also performed, deterioration in optical SNR due to deviations between the wavelength bands caused by the stimulated Raman scattering etc. can also be reduced.

In general, where the wavelength characteristics of the gains and the noise figures of the respective optical amplifiers are uniform, the transmission intervals of the optical transmission line10are constant, and the optical SNRs of the respective channels are made uniform on the receiving side by the in-wavelength-band preemphasis, the gradients of the optical powers are made such that the directions of the gradients are opposite to those in the sending side and the amplitudes of the gradients are approximately equal to those in the sending side.

On the other hand, where the optical sending apparatus4performs inter-wavelength-band preemphasis but does not perform inner-wavelength band preemphasis, at the output end of the optical sending apparatus4the optical powers of the respective optical signals of the WDM optical signal in each wavelength band are identical as shown in the leftmost part of FIG.9C.

Such a two-wavelength-band WDM optical signal is transmitted after being subjected to inter-wavelength-band preemphasis in the composite optical amplifying apparatuses1as shown in the second and third parts (from the left) ofFIG. 9C, at the input end of the optical receiving apparatus14deterioration's in optical SNR due to deviations between the wavelength bands are reduced but deterioration in optical SNR within each wavelength band remains as shown in the rightmost part of FIG.9C.

In conclusion, the optical transmission system according to the fourth embodiment can further increase the distance of ultra-long distance transmission.

An optical transmission system according to a fifth embodiment is such that a three-wavelength-band WDM optical signal sending apparatus generates a three-wavelength-band WDM optical signal, composite optical amplifying apparatuses relay the three-wavelength-band WDM optical signal plural times, and a three-wavelength-band WDM optical signal receiving apparatus receives and processes the three-wavelength-band WDM optical signal. The three-wavelength-band WDM optical signal consists of a WDM optical signal having s channels that is set in the S+band, a WDM optical signal having t channels that is set in the C band, and a WDM optical signal having u channels that is set in the L band.

First, the entire configuration of the optical transmission system according to the fifth embodiment will be described.

As shown inFIG. 10, s optical senders (OSs)20-1to20-s generate optical signals corresponding to channel-1to channel-s of the S+band, respectively. For example, each of the optical senders20-1to20-s can be comprised of a semiconductor laser for emitting laser having a wavelength that is assigned to the associated channel, an MZ modulator for modulating the laser beam with information to be sent out, and a control circuit for driving and controlling the semiconductor laser and the MZ modulator. The semiconductor laser can be any of various semiconductor lasers such as a distributed feedback laser and a distributed Bragg reflection laser.

The optical signals generated by the respective optical senders20-1to20-s are input to a WDM coupler (WDMCPL)21-1. The WDM coupler21-1converts the optical signals into a WDM optical signal by wavelength-multiplexing those. In this manner, a WDM optical signal is generated in which a plurality of optical signals having different wavelengths are wavelength-multiplexed. And an optical sending section that has the optical senders20-1to20-s and the WDM coupler21-1and generates an S+-band WDM optical signal is formed. The WDM optical signal that is output from the WDM coupler21-1is input to a TDFA22and amplified there. For example, the WDM coupler21-1is an AWG or a dielectric multilayer optical filter that is an interference filter type optical multi/demultiplexer.

The TDFA22controls the optical power of the S+-band WDM optical signal while its output is controlled by a monitoring/control circuit28(MCC). The optical-power-controlled S+-band WDM optical signal is input to a WDM coupler25.

An optical-power-controlled C-band WDM optical signal is generated in a similar manner by t optical senders20-s+1 to20-s+t, a WDM coupler21-2, and an EDFA23.

An optical-power-controlled L-band WDM optical signal is generated in a similar manner by u optical senders20-s+t+1 to20-s+t+u, a WDM coupler21-3, and a GS-EDFA24.

As described above with reference toFIG. 2D, the WDM coupler25have different insertion losses for the respective wavelength bands. Therefore, to increase optical SNRS, where the TDFA22, the EDFA23, and the GS-EDFA24have different noise figures, it is preferable that a WDM optical signal amplified by a rare-earth-element-doped fiber amplifier having the worst noise figure be input to the WDM coupler25in a wavelength band of the smallest insertion loss.

The configurations of the TDFA22, the EDFA23, and the GS-EDFA24are approximately the same as in a composite optical amplifying apparatus shown inFIG. 11, they will be described in describing the composite optical amplifying apparatus.

The numbers s, t, and u may each be any number. The maximum value of s is determined by the gain-wavelength characteristic of the TDFA22that performs amplification in this wavelength band and the space between channels. The maximum value of t is determined by the gain-wavelength characteristic of the EDFA23that performs amplification in this wavelength band and the space between channels. The maximum value of u is determined by the gain-wavelength characteristic of the GS-EDFA24that performs amplification in this wavelength band and the space between channels.

The S+-band WDM optical signal, the C-band WDM optical signal, and the L-band WDM optical signal are input to the WDM coupler25, where they are wavelength-multiplexed into a three-wavelength-band WDM optical signal. The three-wavelength-band WDM optical signal is output to an optical fiber48-1as an optical transmission line and transmitted through it to the next-stage repeater.

The optical fiber48-1is connected to a WDM coupler31A in the next-stage repeater. The three-wavelength-band WDM optical signal that has been transmitted through the optical fiber48-1is input to the WDM coupler31A, where it is wavelength-demultiplexed into WDM optical signals in the respective bands.

The demultiplexed S+-band WDM optical signal is input to a coupler37-1for branching light into two parts at an optical power ratio of 10:1, for example. The branched WDM signal having the smaller optical power is input to an optical power meter (OPM)36-1for measuring optical power, where the optical power of the S+-band WDM optical signal is measured. A measurement result is sent to the monitoring/control circuit28at the preceding stage. On the other hand, the branched WDM optical signal having the larger optical power is input to a TDFA32A.

The power of the demultiplexed C-band WDM optical signal is measured by a block that is similar to the above and is composed of a coupler37-2, an optical power meter36-2and an EDFA33A. A measurement result is sent to the monitoring/control circuit28at the preceding stage. The branched WDM optical signal having the larger optical power is input to an EDFA33A.

The power of the demultiplexed L-band WDM optical signal is measured by a block that is similar to the above and is composed of a coupler37-3, an optical power meter36-3and a GS-EDFA34A. A measurement result is sent to the monitoring/control circuit28at the preceding stage. The branched WDM optical signal having the larger optical power is input to a GS-EDFA34A.

The monitoring/control circuit28receives the outputs of the optical power meters36-1to36-3. The monitoring/control circuit28calculates the differences between the optical powers of the respective bands and adjusts the outputs of the TDFA22, the EDFA23, and the GS-EDFA24that perform amplification in the respective bands so as to eliminate the differences.

On the other hand, the S+-band WDM optical signal having the larger optical power is amplified by the TDFA32A and input to a WDM coupler35A. The TDFA32A controls the optical power of the S+-band WDM optical signal while its output is controlled by a monitoring/control circuit38A.

The C-band WDM optical signal having the larger optical power is amplified by the EDFA33A whose output is controlled by the monitoring/control circuit38A, and is then input to the WDM coupler35A.

The L-band WDM optical signal having the larger optical power is amplified by the GS-EDFA34A whose output is controlled by the monitoring/control circuit3BA, and is then input to the WDM coupler35A.

The WDM optical signals in the respective bands are wavelength-multiplexed by the WDM coupler35A and thereby returned to a three-wavelength-band WDM optical signal. The three-wavelength-band WDM optical signal is input to an optical fiber48-2and transmitted through it to the next-stage repeater.

The three-wavelength-band WDM optical signal that has been transmitted through the optical fiber48-2is input to a WDM coupler31B in the next-stage repeater, where it is wavelength-demultiplexed into WDM optical signals in the respective bands.

The demultiplexed S+-band WDM optical signal is input to a coupler37A-1for branching light into two parts at an optical power ratio of 10:1, for example. The branched WDM signal having the smaller optical power is input to an optical power meter36A-1for measuring optical power, where the optical power of the S+-band WDM optical signal is measured. A measurement result is sent to the monitoring/control circuit38A at the preceding stage. On the other hand, the branched WDM optical signal having the larger optical power is input to a TDFA32B.

The power of the demultiplexed C-band WDM optical signal is measured by a block that is similar to the above and is composed of a coupler37A-2, an optical power meter36A-2and an EDFA33B. A measurement result is sent to the monitoring/control circuit38A at the preceding stage. The branched WDM optical signal having the larger optical power is input to an EDFA33B.

The power of the demultiplexed L-band WDM optical signal is measured by a block that is similar to the above and is composed of a coupler37A-3, an optical power meter36A-3and a GS-EDFA34B. A measurement result is sent to the monitoring/control circuit38A at the preceding stage. The branched WDM optical signal having the larger optical power is input to a GS-EDFA34B.

The monitoring/control circuit38A receives the outputs of the optical power meters36A-1to36A-3. The monitoring/control circuit38A calculates the differences between the optical powers of the respective bands and adjusts the outputs of the TDFA32A, the EDFA33A, and the GS-EDFA34A that perform amplification in the respective bands so as to eliminate the differences.

On the other hand, the S+-band WDM optical signal having the larger optical power is amplified by the TDFA32B and input to a WDM coupler35B. The TDFA32B controls the optical power of the S+-band WDM optical signal while its output is controlled by a monitoring/control circuit38B.

The C-band WDM optical signal having the larger optical power is amplified by the EDFA33B whose output is controlled by the monitoring/control circuit38B, and is then input to the WDM coupler35B.

The L-band WDM optical signal having the larger optical power is amplified by the GS-EDFA34B whose output is controlled by the monitoring/control circuit38B, and is then input to the WDM coupler35B.

The WDM optical signals in the respective bands are wavelength-multiplexed by the WDM coupler35B and thereby returned to a three-wavelength-band WDM optical signal. The three-wavelength-band WDM optical signal is input to an optical fiber48-3and transmitted through it to the next-stage repeater.

Subsequently, in similar manners, the three-wavelength-band WDM optical signal is demultiplexed into WDM optical signals in the respective bands, subjected to optical power amplification and control, and wavelength-multiplexed. The three-wavelength-band WDM optical signal is relayed plural times in this manner.

As described above, the optical power amplification and control on WDM optical signals in the respective bands are performed by the TDFA32, the EDFA33, and the GS-EDFA34. The outputs of the TDFA32, the EDFA33, and the GS-EDFA34are controlled based on the outputs of the optical power meters36-1to36-3in the next-stage composite optical amplifying apparatus, respectively.

A three-wavelength-band WDM optical signal that is output from the final-stage composite optical amplifying apparatus is input to a WDM coupler41, where it is demultiplexed into WDM optical signals in the respective bands.

The demultiplexed S+-band WDM optical signal is input to a coupler37Z-1for branching light into two parts at an optical power ratio of 10:1, for example. The branched WDM signal having the smaller optical power is input to an optical power meter36Z-1for measuring optical power, where the optical power of the S+-band WDM optical signal is measured. A measurement result is sent to the monitoring/control circuit38Z at the preceding stage. On the other hand, the branched WDM optical signal having the larger optical power is input to a WDM coupler45-1.

The power of the demultiplexed C-band WDM optical signal is measured by a block that is similar to the above and is composed of a coupler37Z-2, an optical power meter36Z-2and a WDM coupler45-2. A measurement result is sent to the monitoring/control circuit38Z at the preceding stage. The branched WDM optical signal having the larger optical power is input to the WDM coupler45-2.

The power of the demultiplexed L-band WDM optical signal is measured by a block that is similar to the above and is composed of a coupler37Z-3, an optical power meter36Z-3and a WDM coupler45-3. A measurement result is sent to the monitoring/control circuit38Z at the preceding stage. The branched WDM optical signal having the larger optical power is input to the WDM coupler45-3.

The S+-band WDM optical signal is wavelength-demultiplexed by the WDM coupler45-1into optical signals of channel-1to channel-s. The wavelength-demultiplexed optical signals of the respective channels are input to respective optical receivers (ORs)46-1to46-s, where they are received and processed.

Similarly, the C-band optical signal is wavelength-demultiplexed by the WDM coupler45-2into optical signals of channel-1to channel-t. The wavelength-demultiplexed optical signals of the respective channels are input to respective optical receivers46-s+1 to46-s+t, where they are received and processed. The L-band optical signal is wavelength-demultiplexed by the WDM coupler45-3into optical signals of channel-1to channel-u. The wavelength-demultiplexed optical signals of the respective channels are input to respective optical receivers46-s+t+1 to46-s+t+u, where they are received and processed.

Next, the configuration of each composite optical amplifying apparatus in the optical transmission system according to the fifth embodiment will be described.

As shown inFIG. 11, a three-wavelength-band WDM optical signal is transmitted through the optical fiber48and thereby input from the pre-stage composite optical amplifying apparatus to the WDM coupler31. The three-wavelength-band WDM optical signal is wavelength-demultiplexed by the WDM coupler31into WDM optical signals in the respective bands. The demultiplexed S+-band WDM optical signal is input to the coupler37-1in the TDFA32. The demultiplexed C-band WDM optical signal is input to the coupler37-2in the EDFA33. The demultiplexed L-band WDM optical signal is input to the coupler37-3in the GS-EDFA34.

The configurations of the TDFA32, the EDFA33, and the GS-EDFA34are identical except that they are different from each other in the rare-earth-element-doped optical fiber as the medium for amplifying light and the pump source for pumping it. Therefore, basically the configuration of only the TDFA32will be described below. The configurations of the EDFA33and the GS-EDFA34will be described only in different points than in the TDFA32.

The TDFA32will be described below.

A WDM optical signal having the smaller optical power separated by the coupler37-1is input to a photodiode (hereinafter abbreviated as PD)54, where it is subjected to photoelectric conversion. A resulting current value corresponds to the optical power of the S+-band WDM optical signal. The PD54outputs the current to an operation unit (OPU)58and an analog-to-digital converter (hereinafter abbreviated as A/D)73. The A/D73converts the input from an analog value to a digital value and outputs the digital conversion result to the monitoring/control circuit38. The monitoring/control circuit38converts the received digital value into an optical signal having information corresponding to the digital value, and sends the optical signal to the monitoring/control circuit in the pre-stage repeater via a control line.

On the other hand, a WDM optical signal having the larger optical power separated by the coupler37-1is input to a thulium-doped fiber (hereinafter abbreviated as TDF)52.

When the TDF52absorbs laser beam that is emitted from an LD55, electrons in the TDF52are excited and population inversion of electrons is produced. If a WDM optical signal is input to the TDF52in a state that population inversion exists, the WDM optical signal is amplified through stimulated emission. In the LD55, first a semiconductor laser is laser-oscillated by supplying a drive current to it from an LD driving circuit56. A solid-state laser is then oscillated by using laser beam that is emitted from the semiconductor laser, whereby laser beam for pumping the TDF52is emitted from the LD55.

The WDM optical signal amplified by the TDF52is input to a coupler53for branching light into two parts at an optical power ratio of 10:1, for example. The WDM optical signal having the smaller optical power that has been separated by the coupler53is input to a PD57, where it is subjected to photoelectric conversion. The PD57outputs a resulting current to the operation unit58.

The operation unit58converts the currents that are input from the PDs54and57into voltages by using resistors (not shown in FIG.11), respectively. The operation unit58compares the voltages corresponding to the respective PDs54and57and supplies an output corresponding to the difference between the two voltages to the LD driving circuit56. The LD driving circuit56judges the gain of the WDM optical signal being amplified by the TDF52based on the output of the operation unit56, and adjusts the drive current for the LD55so that the gain becomes a predetermined value. The predetermined value can be changed by adjusting the ratio between the resistance values of the resistors that convert currents that are received from the PDs54and57into voltages.

The coupler37-1and PD54have two functions. The first function is detecting the optical power of a WDM optical signal that is input to the TDF52, to make the gain of the TDF52constant. The second function is a function of detecting the optical power of a WDM optical signal that has been transmitted to this repeater, to send information of the optical power to the monitoring/control circuit in the pre-stage repeater. The PD54and the A/D73correspond to the optical power meter36-1shown in FIG.10.

On the other hand, the WDM optical signal having the larger optical power that has been separated by the coupler53is input to a variable optical attenuator (hereinafter abbreviated as VAT)59. The VAT59attenuates the optical power of input light and outputs attenuated light. The amount of the attenuation by VAT59may be variable. The amount of attenuation is controlled by a VAT driving circuit70.

The WDM optical signal that is output from the VAT59is input to a dispersion compensator (hereinafter abbreviated as DC)60that compensates for chromatic dispersion. The DC60may be a dispersion compensation fiber, a dispersion compensation grating, or the like. The DC60compensates for chromatic dispersion that occurs in the optical fiber between the pre-stage repeater and the repeater concerned and chromatic dispersion that occurs in the optical fiber between the repeater concerned and the next-stage repeater. That is, chromatic dispersion that occurs in the optical fiber between adjacent repeaters is compensated for by both repeaters rather than only one of those.

The WDM optical signal that is output from the DC60is input to a TDF62. Like the TDF52, the TDF62amplifies the WDM optical signal by absorbing laser beam that is emitted from an LD65that is supplied with a drive current from an LD driving circuit66.

The WDM optical signal amplified by the TDF62is input to a coupler that branches light into four parts.

A first WDM optical signal separated by the coupler63is input to a PD64via a fiber grating filter (hereinafter abbreviated as FBG)76that is a band-pass filter. The PD64performs photoelectric conversion on the first WDM optical signal. The central wavelength (central frequency) of the pass-band of the FBG76is so set that that the FBG76passes only light of channel-s of the S+-band WDM optical signal, that is, light of the longest-wavelength channel in the S+band. Therefore, a current value obtained by the PD64through photoelectric conversion corresponds to the optical power of the longest-wavelength channel in the S+band. The PD64outputs the current to an operation unit68.

A second WDM optical signal separated by the coupler63is input via an FBG77to a PD67, where it is subjected to photoelectric conversion. The central wavelength of the pass-band of the FBG77is so set that that the FBG77passes only light of channel-1of the S+-band WDM optical signal, that is, light of shortest-wavelength channel in the S+band. Therefore, a current value obtained by the PD67through photoelectric conversion corresponds to the optical power of the shortest-wavelength channel in the S+band. The PD67outputs the current to the operation unit68.

A third WDM optical signal separated by the coupler63is input to a PD61, where it is subjected to photoelectric conversion. A current value obtained by the PD61through photoelectric conversion corresponds to the optical power of the S+-band WDM optical signal as amplified by the TDFA32. The PD61supplies the current to an operation unit71.

A fourth WDM optical signal separated by the coupler63is input to a WDM coupler35. Since the fourth WDM signal becomes an optical signal to be transmitted to the next-stage repeater, setting should be so made that the optical power of the fourth WDM optical signal is larger than the optical powers of the first to third WDM optical signals.

The operation unit68converts the currents that are supplied from the PDs64and67into voltages by using resistors (not shown in FIG.11). The operation unit68compares the voltages corresponding to the respective PDs64and67and supplies an output corresponding to the difference between the two voltages to the LD driving circuit66. The LD driving circuit66judges a gain gradient of the WDM optical signal being amplified by the TDF62based on the output of the operation unit68. The operation unit68compensates for the gain-wavelength characteristic of the TDF62by adjusting, based on a judgment result, the drive current for the LD66so as to eliminate the gain gradient.

The monitoring/control circuit38receives, from the monitoring/control circuit of the next-stage repeater, signals indicating optical powers of the respective bands of the three-wavelength-band WDM optical signal that was transmitted from the repeater concerned.

Based on the received signals, the monitoring/control section38calculates the differences between the optical powers of the respective bands after transmission. By referring to a correlation table between the sending optical power and the reception optical power that is stored in a ROM51in the monitoring/control circuit38, the monitoring/control circuit38calculates, based on the differences, a target value of the optical power of the S+-band WDM optical signal.

The correlation table is a table formed in advance for each band by determining, through theoretical calculation or actual measurement, a relationship between the optical power of a WDM optical signal to be output from the repeater concerned and the optical power of a WDM optical signal to be input to the next-stage repeater based on the stimulated Raman scattering in the optical fiber existing between the repeater concerned and the next-stage repeater, the loss in the optical fiber, and the losses in the WDM coupler35in the repeater concerned and the WDM coupler31in the next-stage repeater.

The monitoring/control circuit38converts the target value into a control value of the VAT driving circuit70based on a relationship between the target value and the control value of the VAT driving circuit70that is stored in a ROM74. The monitoring/control circuit38outputs the resulting control value to a digital-to-analog converter (hereinafter abbreviated as D/A)72. The D/A converts the control value from a digital value to an analog value and outputs the resulting analog control value to the operation unit71.

The operation unit71compares the outputs of the D/A72and the PD61and supplies an output corresponding to the difference to the VAT driving circuit70. Based on the output of the operation unit71, the VAT driving circuit70adjusts the attenuation amount of the VAT59. As a result, the optical power of the S+-band WDM optical signal that is output from the repeater concerned is adjusted to a control value.

The configuration of the EDFA33is the same as that of the TDFA32except that the TDF52is replaced by an erbium-doped fiber and the LD55is replaced by a semiconductor laser. Various settings of the dispersion compensation fiber60and the operation units58,68, and71and the contents of the ROM74are adjusted so as to be suitable for handling of a C-band WDM optical signal.

The configuration of the GS-EDFA34is the same as that of the TDFA32except that the TDA52is replaced by a long, erbium-doped fiber and the LD55is replaced by a semiconductor laser. Various settings of the dispersion compensation fiber60and the operation units58,68, and71and the contents of the ROM74are adjusted so as to be suitable for handling of an L-band WDM optical signal.

The EDFA33that performs amplification in a 1,550-nm wavelength band is different from the GS-EDFA34that performs amplification in a 1,580-nm wavelength band in the length of the erbium-doped fiber. Although erbium-doped fibers inherently have the 1,550-nm wavelength band and the 1,580-nm wavelength band as amplification bands, the amplification factor in the 1,580-nm wavelength band is smaller than that in the 1,550-nm wavelength band. Therefore, to realize optical amplification in the 1,580-nm wavelength band in approximately the same degree as in the 1,550-nm wavelength band, it is necessary to make the fiber length of the fiber amplifier in the 1,580-nm wavelength band about 10 times that of the fiber amplifier in the 1,550-nm wavelength band.

The configuration of each composite optical amplifying apparatus has been described above in detail with reference to FIG.11. The configuration of the three-wavelength-band WDM optical signal sending apparatus shown inFIG. 10is similar to that of each composite optical amplifying apparatus.

AS for the corresponding relationship between the two apparatuses, the S+-band WDM optical signal that is output from the WDM coupler21-1corresponds to that output from the WDM coupler31, the C-band WDM optical signal that is output from the WDM coupler21-2corresponds to that output from the WDM coupler31, and the L-band WDM optical signal that is output from the WDM coupler21-3corresponds to that output from the WDM coupler31. The TDFA22corresponds to the TDFA32, the EDFA23corresponds to the EDFA33, and the GS-EDFA24corresponds to the GS-EDFA34. The WDM coupler25corresponds to the WDM coupler35, couplers27-1to27-3correspond to the couplers37-1to37-3, and optical power meters26-1to26-3correspond to the optical power meters36-1to36-3.

In the following description, when a component of the three-wavelength-band WDM optical signal sending apparatus is referred to with reference toFIG. 11under the above corresponding relationship, a suffix “os” will be added to the reference symbol of the component. For example, the VAT in the TDFA22will be referred to as “VAT59os.” “TDF52os” will mean the TDF in the TDFA22and “TDF52” will mean the TDF in the TDFA32.

Next, the operation principle and the advantageous effects of the composite optical amplifying apparatus according to the fifth embodiment will be described.

A three-wavelength-band WDM optical signal that has been transmitted from the pre-stage repeater is demultiplexed into WDM optical signals in the respective bands by the WDM coupler31. Since the operation principle and the advantageous effects are the same for the WDM optical signals in the respective bands, they will be described below for the S+-band WDM optical signal.

The demultiplexed S+-band WDM optical signal is amplified by the TDF52at a predetermined gain. Optical powers of the S+-band WDM optical signal before and after the amplification by the TDF52are detected by the respective PDs54and57. Since the optical power of pump light for the TDF52is adjusted by the operation unit58's controlling the LD driving circuit56based on detection results, the gain of the TDF52is kept approximately constant. Since the gain of the TDF52depends on the optical power of the pump light, the gain of the TDF52can be kept constantly at a predetermined value by adjusting the optical power of the pump light for the TDF52.

The power of the S+-band WDM optical signal that has been amplified at the predetermined gain is attenuated by the VAT59.

At this time, the operation unit71adjusts the attenuation amount of the VAT59so as to eliminate the difference between the outputs of the PD61and the D/A72. The output of the PD61corresponds to the optical power of the S+-band WDM optical signal that is actually output from the repeater concerned to the next-stage repeater. The output of the D/A72is a control value that is used to equalize the optical powers of the respective bands in the next-stage repeater as well as a control value for the optical power of an S+-band WDM optical signal to be output from the repeater concerned to the next-stage repeater. Since the attenuation amount of the VAT59is adjusted in the above manner, the optical powers of the respective bands can be made approximately identical in the next-stage repeater.

The S+-band WDM optical signal that is output from the VAT59is dispersion-compensated by the DC60and then subjected to adjustment of a gain gradient in the S+band in the TDF62. At this time, a gain gradient of the S+-band WDM optical signal that is output from the TDF62is detected by the respective PDs64and67. Since the optical power of pump light for the TDF62is adjusted by the operation unit68's controlling the LD driving circuit66based on detection results, the monitoring/control circuit38can almost eliminate the gain gradient in the S+band. Since the gain-wavelength characteristic of the TDF62depends on the optical power of the pump light, the gain gradient can be almost eliminated by adjusting the optical power of the pump light for the TDF62.

The reason whey the TDFs52and62are provided in cascade is that both of the gain and the gain gradient cannot be controlled by a single TDF because both of the gain and the w gain-wavelength characteristic depend on the optical power of the pump light.

As described above, in the optical transmission system according to the fifth embodiment, the gain gradient in each wavelength band can almost be eliminated in each repeater interval by the two-step configuration of rare-earth-element-doped fibers in each of the TDFA32, the EDFA33, and the GS-EDFA34. Further, in the optical transmission system according to the fifth embodiment, since preemphases are provided for the respective bands by means of the respective VATs in the TDFA32, the EDFA33, and the GS-EDFA34, the gain gradients between the three wavelength bands can almost be eliminated in the next-stage repeater.

Next, the operation principle and the advantageous effects of the three-wavelength-band WDM optical signal sending apparatus in the optical transmission system according to the fifth embodiment will be described. Since the operation principle and the advantageous effects are the same for the WDM optical signals in the respective bands that are output from the WDM couplers21, they will be described below for the S+-band WDM optical signal.

The S+-band WDM optical signal that is outputted from the WDM coupler21-1is amplified by the TDF52os at a prescribed gain. Optical powers of the S+-band WDM optical signal before and after the amplification by the TDF52os are detected by the respective PDs54os and57os. Since the optical power of pump light for the TDF52os is adjusted by the operation unit58os's controlling the LD driving circuit56os based on detection results, the gain of the TDF52os is kept approximately constant.

The power of the S+-band WDM optical signal that has been amplified at the prescribed gain is attenuated by the VAT59os.

At this time, the operation unit71os adjusts the attenuation amount of the VAT59os so as to eliminate the difference between the outputs of the PD61os and the D/A72os. The output of the PD61os corresponds to the optical power of the S+-band WDM optical signal that is actually output from the three-wavelength-band WDM optical signal sending apparatus concerned to the first-stage repeater. The output of the D/A72os is a control value that is used to equalize the optical powers of the respective bands in the first-stage repeater as well as a control value for the optical power of an S+-band WDM optical signal to be output from the three-wavelength-band WDM optical signal sending apparatus concerned to the first-stage repeater. Since the attenuation amount of the VAT59os is adjusted in the above manner, the optical powers of the respective bands can be made approximately identical in the next-stage repeater.

The S+-band WDM optical signal that is output from the VAT59os is dispersion-compensated by the DC60os and then subjected to adjustment of a gain gradient in the S+band in the TDF62os. At this time, a gain gradient of the S+-band WDM optical signal that is output from the TDF62os is detected by the respective PDs64os and67os. Since the optical power of pump light for the TDF62os is adjusted by the operation unit68os's controlling the LD driving circuit66os based on detection results, the monitoring/control circuit28can almost eliminate the gain gradient in the S+band. Since the gain-wavelength characteristic of the TDF62os depends on the optical power of the pump light, the gain gradient can be almost eliminated by adjusting the optical power of the pump light for the TDF62os.

As described above, in the optical transmission system according to the fifth embodiment, the gain gradient in each wavelength band can almost be eliminated in the interval of the three-wavelength-band WDM optical signal sending apparatus and the first repeater by the two-step configuration of rare-earth-element-doped fibers in each of the TDFA22, the EDFA23, and the GS-EDFA24. Further, in the optical transmission system according to the fifth embodiment, since preemphases are provided for the respective bands by means of the respective VATs in the TDFA22, the EDFA23, and the GS-EDFA24, the gain gradients between the three wavelength bands can almost be eliminated.

Since the gain gradients between the three wavelength bands can almost be eliminated in each repeater, the optical SNRs of received WDM optical signals in the respective bands are made uniform and hence the performance of the entire optical transmission system can be improved.

In the above configuration, the monitoring/control circuit28or38adjusts the outputs of the TDFA22or23, the EDFA23or33, and the GS-EDFA24or34in consideration of the stimulated Raman scattering, the loss in the optical fiber48, and the loss in the WDM coupler25or35. As indicated by a broken line inFIG. 11, another configuration is possible in which the composite optical amplifying apparatus or the three-wavelength-band WDM signal sending apparatus is additionally provided with a ROM75that stores data relating to noise figures and the monitoring/control circuit28or38adjusts the outputs while referring to the data stored in the ROM75.

In this case, the data relating to noise figures are noise figure-wavelength characteristics for each optical power value of laser beam emitted from the LD55or65as the pump source for each of the TDFA22or32, the EDFA23or33, and the GS-EDFA24or34.

The monitoring/control circuit38corrects, based on the noise figure-wavelength characteristics that are stored in the ROM75, a target value that was calculated by referring to the correspondence table between the sending optical power and the reception optical power that is stored in the ROM51. The monitoring/control circuit38outputs a corrected target value to the ROM74.

In the above configuration, the monitoring/control circuit38adjusts the optical powers of WDM optical signals in the respective wavelength bands to be transmitted from the repeater concerned by comparing the optical powers of WDM optical signals in the respective wavelength bands to be transmitted from the repeater concerned and the optical powers of WDM optical signals in the respective wavelength bands in the next-stage repeater. Another configuration is possible in which another EBG whose central frequency of the pass-band is so set that only light of the shortest-wavelength channel in the band is passed is provided between the coupler37and PD54and an output of PD54, rather than the PD61, is supplied to the operation unit71. This makes it possible to reduce the number of parts and thereby simplify the circuit. The ROM75may further be provided in this case.

A sixth embodiment is directed to an optical transmission system.

This optical transmission system is such that a two-wavelength-band WDM optical signal sending apparatus generates a two-wavelength-band WDM optical signal, composite optical amplifying apparatuses relay the two-wavelength-band WDM optical signal plural times, and a WDM optical signal receiving apparatus receives and processes the two-wavelength-band WDM optical signal. The two-wavelength-band WDM optical signal consists of a WDM optical signal having t channels that is set in the C band and a WDM optical signal having u channels that is set in the L band.

As shown inFIG. 12, t optical senders80-1to80-t generate optical signals corresponding to channel-1to channel-t of the C band, respectively. For example, each of the optical senders80-1to80-t can be composed of a semiconductor laser for emitting laser beam having a wavelength that is assigned to the associated channel, an MZ modulator for modulating the laser beam with information to be sent out, and a control section for driving and controlling the semiconductor laser and the MZ modulator. Each of the optical senders80-1and80-t is controlled, as to whether to generate an optical signal, through a control signal that is supplied from a monitoring/control circuit82.

The optical signals generated by the respective optical senders80-1to80-t are wavelength-multiplexed by a WDM coupler21-2into a WDM optical signal, which is input to an EDFA23.

The output of the EDFA23is controlled by the monitoring/control circuit82, whereby the optical power of the C-band WDM optical signal is controlled. The optical-power-controlled C-band WDM optical signal is input to a WDM coupler25.

An L-band WDM optical signal is generated by a block that is similar to the above and is composed of u optical senders80-t+1 to80-t+u, a WDM coupler21-3, and a GS-EDFA24. The generated L-band WDM optical signal is input to the WDM coupler25.

The C-band WDM optical signal and the L-band WDM optical signal are wavelength-multiplexed by the WDM coupler25into a two-wavelength-band WDM optical signal, which is input to an optical fiber48-1as an optical transmission line and transmitted through it to the next-stage repeater.

After being transmitted through the optical fiber48-1, the two-wavelength-band WDM optical signal is input to a WDM coupler84A in the next-stage repeater, where it is wavelength-demultiplexed into WDM optical signals in the respective bands. The WDM optical signals in the respective bands are input to couplers27-2and27-3.

A pump source86A is comprised of an LD for emitting laser beam, a driving circuit for supplying a drive current to the LD, and a control circuit for keeping the wavelength and the optical power of the laser beam at predetermined values by controlling the driving circuit. Laser beam emitted from the pump source86A is input to the optical fiber48-1via the WDM coupler84A. The predetermined wavelength is a wavelength at which a C-band WDM optical signal and an L-band WDM optical signal can be amplified in the optical fiber48-1by stimulated Raman scattering. The predetermined optical power is optical power with which losses of a C-band WDM optical signal and an L-band WDM optical signal in the WDM coupler84A can be compensated for by stimulated Raman scattering in the optical fiber48-1.

Where the loss in the WDM coupler84A is compensated for by amplification through stimulated Raman scattering, it is necessary to take into consideration that the amplification through stimulated Raman scattering and the loss in the WDM coupler84A have wavelength dependence as shown inFIGS. 2B and 2D.

The C-band WDM optical signal is branched by a coupler27-2into two parts, which are input to an optical power meter26-2and an EDFA33A. Similarly, the L-band WDM optical signal is branched by a coupler27-3into two parts, which are input to an optical power meter26-3and a GS-EDFA34A. The monitoring/control circuit85A sends, to the pre-stage monitoring/control circuit82, a result of measurement of the optical power of the C-band WDM optical signal by the optical power meter26-2and a result of measurement of the optical power of the L-band WDM optical signal by the optical power meter26-3.

The monitoring/control circuit82receives the outputs of the optical power meters26-2and26-3, calculates the difference between the optical powers of the respective bands, and adjusts the outputs of the EDFA23and the GS-EDFA24so that the difference will be eliminated.

On the other hand, the C-band WDM optical signal that is input to the EDFA33A is amplified by the EDFA33A and input to a WDM coupler35A. The output of the EDFA33A is controlled by the monitoring/control circuit85A, whereby the optical power of the C-band WDM optical signal is controlled.

The L-band WDM optical signal that is input to the GS-EDFA34A is amplified by the EDFA34A whose output is controlled by the monitoring/control circuit85A. The amplified L-band WDM optical signal is input to the WDM coupler35A.

The WDM optical signals of the respective bands are wavelength-multiplexed by the WDM coupler35A and thereby returned to a two-wavelength-band WDM optical signal. The two-wavelength-band WDM optical signal is input to an optical fiber48-2and transmitted to the next-stage repeater through it.

After being transmitted through the optical fiber48-2, the two-wavelength-band WDM optical signal is input to a WDM coupler84B in the next-stage repeater, where it is wavelength-demultiplexed into WDM optical signals in the respective bands. The WDM optical signals in the respective bands are input to respective couplers37A-2and37A-3.

A pump source86B has the same configuration as the pump source86A. Laser beam emitted from the pump source86B is input to the optical fiber48-2via the WDM coupler84B.

The C-band WDM optical signal is branched by a coupler37A-2into two parts, which are input to an optical power meter36A-2and an EDFA33B. Similarly, the L-band WDM optical signal is branched by a coupler37A-3into two parts, which are input to an optical power meter36A-3and a GS-EDFA34B. The monitoring/control circuit85B sends, to the pre-stage monitoring/control circuit85A, a result of measurement of the optical power of the C-band WDM optical signal by the optical power meter36A-2and a result of measurement of the optical power of the L-band WDM optical signal by the optical power meter36A-3.

The monitoring/control circuit85A receives the outputs of the optical power meters36A-2and36A-3, calculates the difference between the optical powers of the respective bands, and adjusts the outputs of the EDFA33A and the GS-EDFA34A so that the difference will be eliminated.

On the other hand, the C-band WDM optical signal that is input to the EDFA33B is amplified by the EDFA33B and input to a WDM coupler35B. The output of the EDFA33B is controlled by the monitoring/control circuit85B, whereby the optical power of the C-band WDM optical signal is controlled.

The L-band WDM optical signal that is input to the GS-EDFA34B is amplified by the GS-EDFA34B whose output is controlled by the monitoring/control circuit85B. The amplified L-band WDM optical signal is input to the WDM coupler35B.

The WDM optical signals of the respective bands are wavelength-multiplexed by the WDM coupler35B and thereby returned to a two-wavelength-band WDM optical signal. The two-wavelength-band WDM optical signal is input to an optical fiber48-3and transmitted to the next-stage repeater through it.

Subsequently, in similar manners, the two-wavelength-band WDM optical signal is demultiplexed into WDM optical signals in the respective bands, subjected to optical power amplification and control, and wavelength-multiplexed. The three-wavelength-band WDM optical signal is relayed plural times in this manner. As described above, the optical power amplification and control on WDM optical signals in the respective bands are performed by the EDFA33and the GS-EDFA34.

A two-wavelength-band WDM optical signal that is output from the final-stage composite optical amplifying apparatus is input to a WDM coupler41, where it is demultiplexed into WDM optical signals in the respective bands. The WDM optical signals in the respective bands are input to respective couplers36Z-2and36Z-3.

The C-band WDM optical signal is branched by the coupler37Z-2into two parts, which are input to an optical power meter36Z-2and an EDFA43. A result of measurement of the optical power of the C-band WDM optical signal by the optical power meter36Z-2is sent to the pre-stage monitoring/control circuit85Z. Similarly, the L-band WDM optical signal is branched by the coupler37Z-3into two parts, which are input to an optical power meter36Z-3and a GS-EDFA44. A result of measurement of the optical power of the L-band WDM optical signal by the optical power meter36Z-3is sent to the pre-stage monitoring/control circuit85Z.

The C-band WDM optical signal amplified by the EDFA43is input to a WDM coupler45-2, where it is wavelength-demultiplexed into optical signals of channel-1to channel-t. The wavelength-demultiplexed optical signals of the respective channels are input to respective optical receivers87-1to87-t, where they are received and processed.

Similarly, the L-band WDM optical signal amplified by the GS-EDFA44is wavelength-demultiplexed by a WDM coupler45-3into optical signals of channel-1to channel-u, which are input to respective optical receivers87-t+1 to87-t+u, where they are received and processed.

Since the EDFAs23and33and the GS-EDFAs24and34are the same as in the fifth embodiment except for the contents of the RON51os and51and the manners of control of the monitoring/control circuits82and85, they are not described here.

The contents of each of the ROMS51os and51are a decrease amount of optical power per channel of a C-band WDM optical signal that should be used to make the optical powers of the two respective bands approximately identical in the next-stage repeater when the number of channels of a C-band WDM optical signal has increased by k (0≦k≦t−1) and an increase amount of optical power per channel of a C-band WDM optical signal that should be used to make the optical powers of the two respective bands approximately identical in the next-stage repeater when the number of channels of a C-band WDM optical signal has increased by k (0≦k≦t−1).

Increase and decrease amounts are determined in advance for each k value through theoretical calculation or actual measurement in consideration of the stimulated Raman scattering in the optical fiber between the repeater concerned and the next-stage repeater, the loss in the optical fiber, the losses in the WDM coupler35in the repeater concerned and the WDM coupler84in the next-stage repeater.

Optical power increase and decrease amounts per channel of an L-band WDM optical signal for k=0 (i.e., the number of channel is not changed) are the same as in the correspondence table between the sending optical power and the reception optical power in the fifth embodiment.

Next, the operation principle and the advantageous effects of the sixth embodiment will be described. Since the control of the gain gradient in each wavelength band and the control of the gain gradients between each wavelength band that are performed after the number of channels of the C-band has been increased or decreased are the same as in the fifth embodiment, they are not described here.

To set the number of channels of the C band to m (1≦m≦t) in the above optical transmission system, the monitoring/control circuit82assigns them channels to optical senders80-1to80-m and causes the optical senders80-1to80-m to generate optical signals.

The generated optical signals are wavelength-multiplexed by the WDM coupler21-2and a resulting C-band WDM optical signal is amplified by the EDFA23. The amplified C-band WDM optical signal consisting of m waves is wavelength-multiplexed by the WDM coupler25with an L-band WDM optical signal consisting of u waves into a two-wavelength-band WDM optical signal, which is transmitted through the optical fiber48-1.

In this case, in the optical fiber48-1, the two-wavelength-band WDM optical signal is subjected to stimulated Raman scattering that involves pump light that is emitted from the pump source86A, whereby the loss in the WDM coupler84A is compensated for. Since the pump light of the pump source86A amplifies the WDM optical signals in both bands, it does not serve to equalize the optical powers of the respective bands after the transmission through the optical fiber48-1.

To increase the number of channels of the C-band by, for example, three, the monitoring/control circuit82newly assigns three channels to optical senders80-m+1 to80-m+3 and causes the optical senders80-m+1 to80-m+3 to generate optical signals.

The generated optical signals are wavelength-multiplexed by the WDM coupler21-2and amplified by the EDFA23.

At this time, the monitoring/control circuit82adjusts the output of the EDFA23by referring to a decrease amount (stored in the ROM51os) of optical power per channel of a C-band WDM optical signal that should be used when the number of channels of a C-band optical signal has been increased by three (0≦k≦t−1). That is, whereas before the increase in the number of channels the optical power of the first wavelength band was controlled by referring to the correspondence table between the sending optical power and the reception optical power by using P0(seeFIG. 4A) as a reference, after the addition of three channels it is controlled by using, as a reference, a new optical power value that is smaller than P0by the decrease amount.

After being amplified, the C-band WDM optical signal consisting of m+3 waves is wavelength-multiplexed by the WDM coupler25with an L-band WDM optical signal consisting of u waves into a two-wavelength-band WDM optical signal, which is transmitted through the optical fiber48-1.

On the other hand, to decrease the number of channels, in the same manner as in the above case, the monitoring/control circuit82sets a new reference by referring to an increase amount (stored in the ROM51os) of optical power per channel that should be used when the number of channel has been decreased.

With the above control, in the optical transmission system according to the sixth embodiment, preemphases are provided for the respective bands quickly in response to increase or decrease in the number of channels and hence gain gradients between the two wavelength bands can almost be eliminated.

Further, in the optical transmission system according to the sixth embodiment, since gain gradients between the two wavelength bands are almost eliminated in each repeater, the optical SNRs of received WDM optical signals in the respective bands are made uniform and hence the performance of the entire optical transmission system can be improved.

A seventh embodiment is directed to an optical transmission system.

This optical transmission system is such that a two-wavelength-band WDM optical signal sending apparatus generates a two-wavelength-band WDM optical signal, composite optical amplifying apparatuses relay the two-wavelength-band WDM optical signal plural times, and a WDM optical signal receiving apparatus receives and processes the two-wavelength-band WDM optical signal. The two-wavelength-band WDM optical signal sending apparatus performs inner-wavelength-band preemphasis and inter-wavelength-band preemphasis on WDM optical signals. Each composite optical amplifying apparatus performs inter-wavelength-band preemphasis while performing optical amplification. The two-wavelength-band WDM optical signal consists of a WDM optical signal having t channels that are set in the C band and a WDM optical signal having u channels that are set in the L band.

As shown inFIG. 13, t optical senders20-s+1 to20-s+t generate optical signals corresponding to channel-1to channel-t of the C band, respectively. The generated optical signals are input to respective VATs90-1to90-t.

The VATs90-1to90-t attenuate the optical powers of input light beams and output attenuated light beams. The attenuation amounts are variable. The attenuation amounts are controlled by VAT driving circuits91-1to91-t, setting of which is performed by a VAT control circuit94.

Since the optical powers of respective optical signals in the C band are adjusted by the respective VATs90-1to90-t, inner-wavelength band preemphasis can be performed.

The optical signals that are output from the respective VATs90-l to90-t are input to a WDM coupler21-2, where they are wavelength-multiplexed into a C-band WDM optical signal. The C-band WDM optical signal is input to a coupler92-2for branching a WDM optical signal into two parts at an optical power ratio of 10:1, for example.

The C-band WDM optical signal having the smaller optical power that has been separated by the coupler92-2is input to an optical spectrum analyzer93-2for detecting wavelengths of input light and optical power values at the detected wavelengths, where a spectrum of the C-band WDM optical signal is measured. The optical spectrum analyzer93-2outputs a measurement result to the VAT control circuit94.

The C-band WDM optical signal having the larger optical power that has been separated by the coupler92-2is input to an EDFA23. The output of the EDFA23is controlled by a monitoring/control circuit95, whereby the optical power of the C-band WDM optical signal is controlled. The C-band WDM optical signal is input to a WDM coupler25.

An L-band WDM optical signal is generated by a block that is similar to the above block and is composed of u optical senders20-s+t+1 to20-s+t+u, VATs90-t+1 to90-t+u, VAT driving circuits91-t+1 to91-t+u, a WDM coupler21-3, a coupler92-3, an optical spectrum analyzer93-3, a VAT control circuit94, and a GS-EDFA24. The generated L-band WDM optical signal is input to the WDM coupler25.

The C-band WDM optical signal and the L-band WDM optical signal are wavelength-multiplexed by the WDM coupler25into a two-wavelength-band WDM optical signal, which is input to an optical fiber48-1as an optical transmission line and transmitted through it to the next-stage repeater.

After being transmitted through the optical fiber48-1, the two-wavelength-band WDM optical signal is input to a WDM coupler31A in the next-stage repeater, where it is wavelength-demultiplexed into WDM optical signals in the respective bands. The WDM optical signals in the respective bands are input to couplers27-2and27-3.

The C-band WDM optical signal is branched by a coupler27-2into two parts, which are input to an optical power meter26-2and an EDFA33A. Similarly, the L-band WDM optical signal is branched by a coupler27-3into two parts, which are input to an optical power meter26-3and a GS-EDFA34A. The monitoring/control circuit85A sends, to the pre-stage monitoring/control circuit95, a result of measurement of the optical power of the C-band WDM optical signal by the optical power meter26-2and a result of measurement of the optical power of the L-band WDM optical signal by the optical power meter26-3.

The monitoring/control circuit95receives the outputs of the optical power meters26-2and26-3, calculates the difference between the optical powers of the respective bands, and adjusts the outputs of the EDFA23and the GS-EDFA24so that the difference will be eliminated.

On the other hand, the C-band WDM optical signal that is input to the EDFA33A is amplified by the EDFA33A and input to a WDM coupler35A. The output of the EDFA33A is controlled by the monitoring/control circuit85A, whereby the optical power of the C-band WDM optical signal is controlled.

The L-band WDM optical signal that is input to the GS-EDFA34A is amplified by the EDFA34A whose output is controlled by the monitoring/control circuit85A. The amplified L-band WDM optical signal is input to the WDM coupler35A.

The WDM optical signals of the respective bands are wavelength-multiplexed by the WDM coupler35A and thereby returned to a two-wavelength-band WDM optical signal. The two-wavelength-band WDM optical signal is input to an optical fiber48-2and transmitted to the next-stage repeater through it.

The two-wavelength-band WDM optical signal is relayed plural times by the composite optical amplifying apparatuses in such a manner as to be demultiplexed into WDM optical signals in the respective bands, subjected to optical power amplification and control, and wavelength-multiplexed in the same manner as in a case where no S+-band WDM optical signal exists in the fifth embodiment. The optical power amplification and control on WDM optical signals in the respective bands are performed by the EDFA33and the GS-EDFA34that are controlled based on the outputs of the optical power meters36-2and36-3in the next-stage composite optical amplifying apparatus.

A two-wavelength-band WDM optical signal that is output from a composite optical amplifying apparatus as the final-stage repeater is input to a WDM coupler41, where it is demultiplexed into WDM optical signals in the respective bands. The WDM optical signals in the respective bands are input to respective couplers36Z-2and36Z-3.

The C-band WDM optical signal is branched by the coupler37Z-2into two parts, which are input to an optical power meter36Z-2and an EDFA43. A result of measurement of the optical power of the C-band WDM optical signal by the optical power meter36Z-2is sent to the pre-stage monitoring/control circuit38Z.

The C-band WDM optical signal amplified by the EDFA43is input to a coupler96-2for branching light into two parts at an optical power ratio of 10:1, for example.

The C-band WDM optical signal having the smaller optical power that has been separated by the coupler92-2is input to an optical spectrum analyzer97-2, where a spectrum of the C-band WDM optical signal is measured. The optical spectrum analyzer97-2outputs a spectrum measurement result to the monitoring/control apparatus95, which supplies the received spectrum measurement result to the VAT control circuit94as it is.

The C-band WDM optical signal having the larger optical power that has been separated by the coupler92-2is input to a WDM coupler45-2, where it is wavelength-demultiplexed into optical signals of channel-1to channel-t. The optical signals of the respective channels are input to respective optical receivers46-s+1 to46-s+t, where they are received and processed.

The L-band WDM optical signal is received and processed by a block that is similar to the above block and is composed of a coupler37Z-3, an optical power meter36Z-3, a GS-EDFA44, a coupler96-3, an optical spectrum analyzer97-3, a WDM coupler45-3, and optical signal receiving sections46-s+t+1 to46-s+t+u.

Next, the configuration of each composite optical amplifying apparatus will be described in detail.

The configuration of each of the EDFA33and the GS-EDFA34of each composite optical amplifying apparatus are the same as in the fifth embodiment except that an optical filter98that equalizes the gain of the fiber amplifier is provided in place of the TDF62, the FBGs76and77, the PDs64and67, the operation unit68, the LD driving circuit66, and the LD65that equalize the gain of the fiber amplifier.

The above configuration is employed for the following reason. That is, the method in which the gain-wavelength w characteristic of the EDF78is equalized by measuring gains for a shortest-wavelength optical signal and a longest-wavelength optical signal cannot easily equalize the gain-wavelength characteristic of the EDF78because a WDM optical signal that is input to the composite optical amplifying apparatus concerned was subjected to in-wavelength-band preemphasis.

As shown inFIG. 14, a two-wavelength WDM optical signal that has been transmitted from the pre-stage composite optical amplifying apparatus through the optical fiber48is input to the WDM coupler31, where it is wavelength-demultiplexed into WDM optical signals in the respective bands. The demultiplexed C-band WDM optical signal is input to the coupler37-2of the EDFA33. The demultiplexed L-band WDM optical signal is input to the coupler37-3of the GS-EDFA34.

Since the configurations of the EDFA33and the GS-EDFA34are different from each other only in the rare-earth-element-doped fiber and the pump source for pumping it, only the configuration of the EDFA33will be described in detail and the configuration of the GS-EDFA34will be described only for different points.

The EDFA33will be described below.

A WDM optical signal having smaller optical power separated by the coupler37-2is input to a PD54, where it is subjected to photoelectric conversion. The PD54outputs a resulting current value to an operation unit58and an A/D73. The A/D73converts the input current value from an analog value to a digital value and outputs the digital current value to the monitoring/control circuit38. The monitoring/control circuit38converts the received digital value into an optical signal and sends the optical signal to the monitoring/control circuit in the pre-stage repeater via a control line.

On the other hand, a WDM optical signal having larger optical power separated by the coupler37-2is input to an erbium-doped fiber (hereinafter abbreviated as EDF)78. Population inversion is formed in the EDF78through absorption of laser beam that is emitted from an LD55, and the EDF78amplifies the WDM optical signal through stimulated emission. Supplied with a drive current from an LD driving circuit56, the LD55emits laser beam for pumping the EDF78.

The WDM optical signal amplified by the EDF78is input to a coupler53. A WDM optical signal having smaller optical power that has been separated by the coupler53is input to a PD57, where it is subjected to photoelectric conversion. The PD57outputs a resulting current to the operation unit58.

The operation unit58converts the currents that are input from the PDs54and57into voltages by using resistors (not shown in FIG.14), respectively. The operation unit58compares the voltages corresponding to the respective PDs54and57and supplies an output corresponding to the difference between the two voltages to the LD driving circuit56. The LD driving circuit56judges the gain of the WDM optical signal being amplified by the EDF78based on the output of the operation unit56, and adjusts the drive current for the LD55so that the gain becomes a predetermined value.

On the other hand, a WDM optical signal having larger optical power that has been separated by the coupler53is input to a VAT59. The VAT59attenuates the optical power of the input WDM optical signal and outputs the attenuated WDM optical signal, the attenuation amount being controlled by a VAT driving circuit70.

The WDM optical signal whose optical power has been attenuated by the VAT59is input to a DC60, where chromatic dispersion is compensated for.

The WDM optical signal that is output from the DC60is input to an optical filter98. The optical filter98is a gain equalizer for making the gain-wavelength characteristic of the EDF78approximately flat.

The WDM optical signal that is output from the optical filter98is input to a coupler99for branching light into two parts.

One WDM optical signal separated by the coupler99is input to a PD61, where it is subjected to photoelectric conversion. The PD61outputs a resulting current to an operation unit71. The other WDM optical signal separated by the coupler99is input to the WDM coupler35, where it is converted into an optical signal to be transmitted to the next-stage repeater.

The monitoring/control circuit38receives, from the monitoring/control circuit of the next-stage repeater, signals indicating the optical powers of the respective bands of the two-wavelength-band WDM optical signal that was transmitted from the repeater concerned. Based on the received signals, the monitoring/control section38calculates the difference between optical powers of the respective bands after transmission. By referring to a correlation table between the sending optical power and the reception optical power that is stored in a ROM51in the monitoring/control circuit38, the monitoring/control circuit38calculates, based on the difference, a target value of the optical power of a C-band WDM optical signal to be output from the repeater concerned. The monitoring/control circuit38converts the target value into a control value of the VAT driving circuit70based on a relationship between the target value and the control value of the VAT driving circuit70that is stored in a ROM74. The monitoring/control circuit38outputs the resulting control value to a D/A72. The D/A converts the control value from a digital value to an analog value and outputs the resulting analog control value to the operation unit71.

The operation unit71compares the output of the D/A72with a voltage obtained through conversion of the current of the PD61by a resistor (not shown in FIG.14), and supplies an output corresponding to the difference to the VAT driving circuit70. Based on the output of the operation unit71, the VAT driving circuit70adjusts the attenuation amount of the VAT59. As a result, the optical power of the C-band WDM optical signal that is output from the repeater concerned is adjusted to a control value.

The configuration of the GS-EDFA34is the same as that of the EDFA33except that the EDF78is replaced by a long, erbium-doped fiber. Various settings of the dispersion compensation fiber60and the operation units58and71and the contents of the ROM74are adjusted so as to be suitable for handling of an L-band WDM optical signal.

The configuration of each composite optical amplifying apparatus has been described above in detail with reference to FIG.14. The configurations of the EDFA23and the GS-EDFA24of the two-wavelength-band WDM optical signal sending apparatus shown inFIG. 13are similar to those of each composite optical amplifying apparatus.

As for the corresponding relationship between the two apparatuses, the C-band WDM optical signal that is output from the WDM coupler21-2corresponds to that output from the WDM coupler31and the L-band WDM optical signal that is output from the WDM coupler21-3corresponds to that output from the WDM coupler31. The EDFA23corresponds to the EDFA33and the GS-EDFA24corresponds to the GS-EDFA34. The WDM coupler25corresponds to the WDM coupler35, couplers27-2and27-3correspond to the couplers37-2and37-3, and optical power meters26-2and26-3correspond to the optical power meters36-2and36-3.

Next, the operation principle and the advantageous effects of the optical transmission system according to the seventh embodiment will be described. Since the control relating to the inter-wavelength-band preemphasis that is performed after the inner-wavelength band preemphasis is the same as in the fifth embodiment, it is not described here.

First, the inner-wavelength band preemphasis will be described.

A two-wavelength-band WDM optical signal that is output from the two-wavelength-band WDM optical signal sending apparatus is amplified and relayed by the repeaters that are composite optical amplifying apparatuses while being transmitted through the optical fibers48, and then the two-wavelength-band WDM optical signal is input to the WDM optical signal receiving apparatus.

In the WDM optical signal receiving apparatus, a spectrum of the C-band WDM optical signal is measured by the optical spectrum analyzer97-2. A measurement result is sent to the monitoring/control circuit95in the two-wavelength-band WDM optical signal sending apparatus via the control line that is dedicated to control signal transmission. Alternatively, one of the optical signals of the two-wavelength-band WDM optical signal may be used instead of using the control line. In this case, for example, undefined bytes of a section overhead of the SDH (synchronous digital hierarchy) are used. The section overhead is a portion in the SDH to accommodate information that is necessary to operate a network, such as maintenance information and a status monitor.

The monitoring/control circuit95outputs the measurement result to the VAT control circuit94.

Based on the measurement result, the VAT control circuit94determines an optical signal having the best optical SNR value among the optical signals of the C-band WDM optical signal. The VAT control circuit94supplies signals to the VAT driving circuits91-1to91-t corresponding to the respective optical signals and controls the attenuation amounts of the respective VATs90-1to90-t so that the optical SNRs will be made equal to the best optical SNR value.

The optical signals whose optical powers have been adjusted by the respective VATs90-1to90-t are wavelength-multiplexed by the WDM coupler21-2and a resulting C-band WDM optical signal is input to the coupler92-2. After being separated by the coupler92-2, part of the C-band WDM optical signal is input to the optical spectrum analyzer93-2, where its spectrum is measured. The optical spectrum analyzer93-2outputs a measurement result to the VAT control circuit94.

The VAT control circuit94judges, based on the measurement result supplied from the optical spectrum analyzer93-2, whether the optical powers of the respective optical signals are equal to the intended values of the adjustment, and then finely adjusts the VAT driving circuits91-1to91-t.

The inner-wavelength band preemphasis on the C-band WDM optical signal is performed in this manner.

Similarly, the inner-wavelength band preemphasis on the L-band WDM optical signal is performed by the coupler96-3, the optical spectrum analyzer97-3, the monitoring/control circuit95, the VAT control circuit94, the VAT driving circuits91-t+1 to91-t+u, the VATs90-t+1 to90-t+u, the coupler92-3, and the optical spectrum analyzer93-3.

The WDM optical signals that have been subjected to the inner-wavelength band preemphasis are then subjected to the inter-wavelength-band preemphasis in the manner described in the fifth embodiment, and the resulting WDM signals are output to the optical fiber48-1.

In the first repeater, the optical powers of the WDM optical signals of the two-wavelength-band WDM signal are varied owing to the stimulated Raman scattering and the loss in the optical transmission line. However, since the inter-wavelength-band preemphasis was performed, the optical power of the C-band WDM optical signal is made approximately equal to that of the L-band WDM optical signal. The two-wavelength-band WDM signal is subjected to the inter-wavelength-band preemphasis and amplification in the repeater, and output to the optical fiber48-2.

The two-wavelength-band WDM signal is similarly subjected to the inter-wavelength-band preemphasis and amplification in each of the subsequent repeaters. The two-wavelength-band WDM signal is relayed plural times by the plurality of repeaters and then input to the WDM optical signal receiving apparatus.

Since the received two-wavelength-band WDM signal was subjected to the inner-wavelength band preemphasis as described above, deterioration in optical SNRs due to ASEs caused by the EDFA23or33and the GS-EDFA24or34in each apparatus can be reduced. Further, since the inter-wavelength-band preemphasis was also performed, deterioration in optical SNRs due to deviations between the wavelength bands caused by the stimulated Raman scattering etc. in the optical fibers48can also be reduced.

As such, the optical transmission system according to the seventh embodiment can further increase the distance of ultra-long distance transmission because the optical SNRs of the respective optical signals can greatly be increased.

Although in the seventh embodiment the in-wavelength-band preemphasis is adjusted by using the VATS, the invention is not limited to such a case and any device capable of adjusting optical power can be used. For example, a fiber amplifier and a semiconductor optical amplifier can be used. In the case of a fiber amplifier, the in-wavelength-band preemphasis can be performed by adjusting the gain by adjusting the output of pump light for pumping an optical fiber. In the case of a semiconductor laser optical amplifier, in-wavelength-band preemphasis can be performed by adjusting the gain by adjusting a bias current (drive current).

In the composite optical amplifying apparatus in each of the fifth to seventh embodiments, the gain gradient in each wavelength band is controlled by the next-stage rare-earth-element-doped fiber. Instead, a gain equalizer that is an optical filter can be used.

FIG. 15Ashows a case where a gain equalizer is provided in each optical amplifying unit, andFIG. 15Bshows a case where a gain equalizer is separated from the corresponding optical amplifying unit.

Referring toFIG. 15A, in this composite optical amplifying apparatus, a WDM coupler31demultiplexes an input three-wavelength-band WDM optical signal into WDM optical signals in respective bands. The demultiplexed WDM optical signals are input to optical amplifiers101-1ato101-3aof respective amplifying units101-1to101-3. The optical amplifier101-1ahas the same configuration as the TDFA32shown inFIG. 11except that the TDF62, the LD65, the LD driving circuit.66, the operation unit68, the FBGs76and77, and the PDs64and67are not provided. The same applies to the other optical amplifiers101-2a and101-3a. The WDM optical signals in the respective bands whose optical powers have been adjusted by the respective optical amplifiers101-1ato101-3aare input to respective gain equalizers101-1bto101-3b. The gain equalizers101-1bto101-3balmost flatten the gains of the WDM optical signals in the respective bands. The resulting WDM optical signals in the respective bands are input to a WDM coupler35, where they are wavelength-multiplexed and thereby returned to a three-wavelength-band WDM optical signal, which is transmitted to the next-stage repeater.

The composite optical amplifying apparatus ofFIG. 15Bis composed of optical amplifiers103-1to103-3and gain equalizers104-1to104-3all of which are independent optical parts. The optical amplifiers103-1to103-3correspond to the optical amplifiers101-1ato101-3ain FIG.15A and the gain equalizers104-1to104-3correspond to the gain equalizers101-1bto101-3bin FIG.15A.

The fifth to seventh embodiments were directed to the case where the composite optical amplifying apparatus is fixed in the number of wavelength bands in constructing an optical system. However, by providing optical adapters for each of the WDM couplers31and35, the fifth to seventh embodiments can adapt to a case of decreasing or increasing the number of wavelength bands after construction of an optical system.

To enable attachment/detachment of optical amplifiers, as shown inFIGS. 16A and 16B, optical adapters106-1to106-3are provided between the WDM coupler31and the TDFA32, EDFA33, and GS-EDFA34and optical adapters107-1to107-3are provided between the TDFA32, EDFA33, and GS-EDFA34and the WDM coupler35. The contents of the ROM51in the monitoring/control circuit38are written in accordance with decrease or increase in the number of wavelength bands.

The fifth embodiment was directed to the case of handling a three-wavelength-band WDM optical signal, and the seventh embodiment was directed to the case of handling a two-wavelength-band WDM optical signal. However, the invention is not limited to such cases and can be applied to a case of handling an n-wavelength-band WDM optical signal of an arbitrary number of wavelength bands.

The fifth to seventh embodiments were directed to the case of unidirectional transmission in which WDM optical signals in respective wavelength bands are transmitted in the same direction. However, the invention can also be applied to the case of bidirectional transmission because same results are obtained in unidirectional transmission and bidirectional transmission as was described with reference toFIGS. 21 and 22.