Apparatus and method of compensating transmission property of light demultiplexer/light multiplexer

The present invention offers a compensation device having a transmission property for collectively compensating inclinations of accumulated transmission properties in a signal wavelength band of light signals of a plurality of channels configuring a wavelength-multiplexed light, among wavelength dependencies of transmission properties of a plurality of sets of light demultiplexers and light multiplexers.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an apparatus and a method of compensating the transmission property of a light demultiplexer/light multiplexer in a wavelength-division multiplexing (WDM) optical communication system.

FIG. 1Ashows a conventional WDM optical communication system. The WDM optical communication system ofFIG. 1Aincludes a transmission apparatus (TX)11, optical fibers12,17and22, and a reception apparatus (RX)23. Between the optical fibers12and17, the first drop and add node is arranged. Between the optical fibers17and22, the second drop and add node is arranged. The first drop and add node includes light amplifiers13and16, a light demultiplexer14and a light multiplexer15. The second drop and add node includes light amplifiers18and21, a light demultiplexer19and a light multiplexer20. Furthermore, a light repeater that amplifies a light signal is arranged in a light transmission path.

The transmission apparatus11includes a plurality of light transmission devices that respectively transmit light signals of a plurality of wavelengths (channels) and a light multiplexer that multiplexes the light signals. A reception apparatus23includes a light demultiplexer that demultiplexes a wavelength-multiplexed light and extracts the light signals of respective channels and a plurality of light reception devices that respectively receive light signals of a plurality of channels.

In the first drop and add node, the light demultiplexer14of multiple-outputs demultiplexes a wavelength-multiplexed light into a light signal of each channel and drops a part of the light signals. The light multiplexer15of multiple-inputs multiplexes the remaining light signals and the added light signal to be outputted. The second drop add node also implements operations same as those of the first drop and add node.

Generally, a plurality of drop and add nodes is arranged in a light transmission path for dropping and adding the light signal of each channel. In order to respond to the expansion of information networks, it is desirable to arrange equal to or more than dozens of drop and add nodes. As a light demultiplexer/light multiplexer, for example, an arrayed waveguide grating (AWG) is used (refer to, for example, patent literature 1).

Furthermore, a optical equalizer that compensates the gain wavelength dependence of a light amplifier is known (refer to, for example, patent literature 2).

[Patent literature 1] Japanese patent application laid-open disclosure number 2002-014243

[Patent literature 2] Japanese patent application laid-open disclosure number 2001-044935

However, there is the following problems in the above-mentioned conventional WDM optical communication system.

It is desirable that the transmission property of AWG that is used as a light demultiplexer/light multiplexer is flat in a signal wavelength band of each channel. Here, the transmission property of AWG indicates the dependence of the transmission rate of the light that passes through the AWG on the wavelength.

However, as the number of channels increases, the transmission property changes depending on wavelength in a signal wavelength band so that the property has an inclination in a channel of the shortest wave or the longest wave. When a plurality of AWGs are used, inclinations of this transmission property are accumulated to be large as shown inFIG. 1B, thereby deteriorating a light signal. Therefore, the number of the drop and add nodes is restricted in a communication system in order to restrain the deterioration of the light signal as much as possible.

SUMMARY OF THE INVENTION

The subject of the present invention is to compensate the accumulated inclinations of a transmission property of the light demultiplexer/light multiplexer in a WDM optical communication system.

A compensation apparatus of the present invention is used in a WDM optical communication system comprising a plurality of sets of light demultiplexers and light multiplexers and this apparatus is provided with a compensation device. This compensation device has a transmission property for collectively compensating the inclinations of accumulated transmission properties in a signal wavelength band of light signals of a plurality of channels configuring a wavelength-multiplexed light, among the wavelength dependencies of the transmission properties of the plurality of sets of light demultiplexers and light multiplexers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is the detailed explanation of the preferred embodiments for implementing the present invention in reference to the drawings.

FIG. 2Ashows the principle of a compensation apparatus of the present invention. The compensation apparatus ofFIG. 2Ais a compensation apparatus used in the WDM optical communication system comprising a plurality of sets of light demultiplexers and light multiplexers and this apparatus is provided with a compensation device101.

The compensation device101includes a transmission property for collectively compensating the inclinations of accumulated transmission properties in a signal wavelength band of light signals of a plurality of channels configuring a wavelength-multiplexed light, among the wavelength dependencies of the transmission properties of the plurality of sets of light demultiplexers and light multiplexers. As this transmission property, the following properties are used.

(1) Curve-shaped transmission property obtained by vertically flipping an envelope of the shape shown in each signal wavelength band in a drawing showing the wavelength dependency of an accumulated transmission property

(2) Transmission property having a shape that periodically changes according to a wavelength

These transmission properties have inclinations opposite to that of the accumulated transmission property in a signal wavelength band of the light signal of each channel.

By arranging a compensation apparatus equipped with the compensation device101in a WDM optical communication system, it becomes possible to compensate the accumulated inclinations of the transmission property of a light demultiplexer/light multiplexer.

According to the present invention, it becomes possible to compensate the accumulated inclination of the transmission property of a light demultiplexer/light multiplexer used in a WDM optical communication system. Therefore, the restriction of the number of drop and add nodes due to the accumulated transmission property of AWG can be significantly reduced.

According to this preferred embodiment, in order to compensate the inclination of a property in which the transmission properties in a signal wavelength band of each channel are accumulated, among the wavelength dependencies of the transmission properties of the plurality of sets of light demultiplexers and light multiplexer mounted on a plurality of drop and add nodes, a light component including a transmission property opposite to that inclination is arranged. Consequently, the restriction on the number of drop and add nodes can be reduced so that the number becomes double or more.

FIG. 2Bshows the compensation apparatus in which a light filter is used as a light component. The compensation apparatus ofFIG. 2Bis mounted every several nodes. This apparatus includes light amplifiers201and206, a light demultiplexer202, a gradient compensation filter203-i(i−1, 2, . . . , n) that is added for each channel and a light multiplexer204. Each gradient compensation filter203-ihas a transmission property of the inclination that is obtained by reversing the inclination of the accumulated transmission property of AWG in terms of positive and negative, in the signal wavelength band of each channel as shown inFIG. 3. In this way, the inclination of the accumulated transmission property can be compensated, thereby preventing signal deterioration.

In the configuration ofFIG. 2B, it is necessary to mount the gradient compensation filter203-ifor each channel. However, the inclinations of transmission properties of all the channels can be collectively compensated only with one compensation filter by utilizing the shape of the accumulated transmission property of AWG. The following is the detailed explanation of this compensation method.

As shown inFIG. 4, the inclination of the transmission property of AWG is generally small in channel ch n/2 of the center wavelength of a wavelength-multiplexed light but the inclination becomes gradually larger in the channel on the side of a shortwave or a longwave. InFIG. 4, line segments401-1,401-2,401-(n−1) and401-ncorrespond to the shapes of the upper sides of quadrangles of the accumulated transmission properties of channels ch1, ch2, ch n−1 and ch n, respectively. The absolute values of the inclinations of the line segments401-1and401-nare larger than those of the inclinations of the line segments401-2and401-(n−1).

Thereupon, if a light filter that has the transmission property having a shape of a curve403that is obtained by vertically flipping the envelope402of these line segments is used as a compensation filter, the inclinations of the transmission properties of all the channels can be collectively compensated.

The transmission property of AWG to be compensated can be measured by passing an ASE (Amplified Spontaneous Emission) light through the corresponding path or by passing the light while changing a wavelength of the variable wavelength source light. Otherwise, this property can be determined from the data of a component that is used in a light transmission path. It is sufficient to implement this operation once at the time of the initial operation of a system or after the route change is implemented.

The shape of the transmission property of a compensation filter can be determined by calculating the envelope of a shape in each signal wavelength band of the transmission property of AWG to be compensated, as mentioned above.FIG. 5shows the simulation results of the transmission property of AWG to be compensated.FIG. 6shows the shape of an envelope for the transmission property. In fact, the shape obtained by vertically flipping this envelope is used as the transmission property of a compensation filter.

The compensation filter that has such a transmission property can be materialized by a fiber grating and a light circulator as shown inFIG. 7. The compensation filter ofFIG. 7includes a light circulator701, a fiber grating702and a light end703. An input light704is guided to the fiber grating702via the light circulator701. The reflected light from the fiber grating702is outputted as an output light705via the light circulator701.

The fiber grating702generates a periodic refractive index change to the core of an optical fiber and reflects only the light that has the wavelength in the vicinity of a resonant wavelength of this grating. The refractive index change of the core is expressed with the following equation.
n′(z)=ncore(z)+½Δn(1*sin((2π/Λ)*z))

Here, z is a grating length and ncore(z) is the refractive index of the original core. n′ (z) is the refractive index after a change. Δn is the maximum refractive index change. Λ is a modulation cycle. The resonant wavelength (Bragg wavelength) that is expressed using a refractive index n is 2nΛ. The full width at half maximum is approximately proportional to Δn/n.

This resonant wavelength is used as the center wavelength of a signal light. The periodic refractive index change is materialized by projecting the interference pattern that occurs, for example, when an ultraviolet laser beam is transmitted into the diffraction grating substrate of quartz. By radiating the interference pattern as a light intensity to an optical fiber, the refractive index change equivalent to this interference pattern can be generated.

In addition, the compensation filter that has the above-mentioned transmission property can be materialized also using a Fabry-Perot-type light filter of a transmission-type.

Meanwhile, in consideration of the flatness of a transmission property of the current AWG, the number of nodes which allow the light transmission without compensation is approximately ten nodes. Therefore, a preferable WDM optical communication system is materialized if compensation filters each having the same compensation amount are arranged every several nodes.

In comparison with the case of the intensive compensation, the waveform deterioration due to a nonlinear optical phenomenon can be reduced by dispersing and arranging compensation filters in this way. It is ideal to arrange a compensation filter for each AWG to reduce the waveform deterioration. However, it makes deterioration of light SN ratio larger. Therefore, when the filter is arranged every several nodes, the performance becomes better.

The accumulated transmission property of AWG becomes a problem in case of the longest path. In some longest paths, the drop/add operations are implemented in the middle of the paths. By arranging a compensation filter every several nodes, even the light signal that is dropped/added in the middle of the longest path can be compensated with a fixed ratio so that an effective compensation scheme can be offered.

FIG. 8shows a WDM optical communication system in which a compensation filter is added every three nodes. The WDM optical communication system ofFIG. 8includes a transmission apparatus (TX)801, light demultiplexers802,805,808,812,815,818,822,825and828, light multiplexers803,806,809,813,816,819,823,826and829, optical fibers804,807,811,814,817,821,824,827and830, compensation filters810,820and840, and a reception apparatus (RX)841.

The configurations and operations of the transmission apparatus801and the reception apparatus841are similar as those of the transmission apparatus11and the reception apparatus23ofFIG. 1A. According to these configurations, one compensation filter inevitably acts on a path covering three nodes that include the set of a light demultiplexer/light multiplexer each of which uses AWG. With the drop and add operations of a light signal, the number of sets of the light demultiplexer/light multiplexer on which the compensation filter acts slightly varies depending on the path but it is conceivable that this variation does not cause significant waveform deterioration.

In the configuration ofFIG. 8, all the wavelength bands are compensated using one compensation filter. However, it is possible that the wavelength band is divided into three bands as a shortwave side, a center wavelength vicinity and a longwave side, and then compensation filters are respectively added to a shortwave side and a longwave side.

FIG. 9shows such a compensation apparatus. The compensation apparatus ofFIG. 9is mounted instead of each compensation filter ofFIG. 8and includes a light amplifier901, a band division filter902, a shortwave side compensation filter903, a longwave side compensation filter904and a band synthesis filter905.

The band division filter902demultiplexes the wavelength-multiplexed light that is outputted from the light amplifier901into three lights such as a light on the shortwave side, a light in the vicinity of a center wavelength and a light on the longwave side. The shortwave side compensation filter903and the longwave side compensation filter904compensate a light on the shortwave side and a light on the longwave side, respectively. Then, the band synthesis filter905multiplexes the lights outputted from the shortwave side compensation filter903and the longwave side compensation filter904, and a light in the vicinity of a center wavelength that is outputted from the band division filter902, thereby outputting the multiplexed light.

The division filter902and the band synthesis filter905are materialized by connecting, for example, dielectric filters in series.

In the above-mentioned preferred embodiment, a compensation filter that has the transmission property determined on the basis of an envelope of the transmission property of AWG is used but instead of this filter, another compensation filter with a transmission property that periodically changes regarding the wavelength can be used. In this case, the cycle and phase of the transmission property are adjusted so as to compensate the inclination of a property in which the transmission properties in the signal wavelength band of each channel are accumulated.

FIG. 10shows the compensation apparatus that uses such compensation filters. The compensation apparatus ofFIG. 10is mounted instead of each compensation filter ofFIG. 8. This apparatus includes a light amplifier1001, a band division filter1002, a shortwave side compensation filter1003, a longwave side compensation filter1004and a band synthesis filter1005. The operations of the band division filter1002and the band synthesis filter1005are the same as those of the band division filter902and the band synthesis filter905ofFIG. 9.

Each of the shortwave side compensation filter1003and the longwave side compensation filter1004includes a sine-wave-type transmission property1102as shown inFIG. 11. The sine wave of the transmission property1102has a cycle and an amplitude to delete the inclination of the accumulated transmission property1101of AWG. For example, when the cycle of the sine wave is the same as the channel interval of a light signal, the signal wavelength band of each channel is compensated by using the range from the maximum value to the minimum value of an inclination of the sine wave. In this case, the phase relation between the accumulated transmission property of AWG and the transmission property of the compensation filter1003and also the phase relation between that of AWG and the compensation filter1004are kept constant.

FIG. 12shows a compensation apparatus that uses still another compensation filter. The compensation apparatus ofFIG. 12is mounted instead of the respective compensation filters ofFIG. 8. This apparatus includes a light amplifier1201, interleavers1202and1211, band division filters1203and1204, shortwave side compensation filters1205and1207, longwave side compensation filters1206and1208, and band synthesis filters1209and1210.

The interleaver1202demultiplexes the wavelength-multiplexed light that is outputted from the light amplifier1201into two lights including light signals each having a double channel interval and it outputs the demultiplexed lights to the band division filters1203and1204. These lights include the light signals that are taken out every other light signal from among light signals of a plurality of channels that configure the wavelength-multiplexed light.

The operations of the band division filters1203and1204are same as that of the band division filter902ofFIG. 9. The operations of the band synthesis filters1209and1210are the same as that of the band synthesis filter905ofFIG. 9. An interleaver1211multiplexes two lights that are outputted from the band synthesis filters1209and1210.

Each of the shortwave side compensation filters1205and1207and the longwave side compensation filters1206and1208has a sine-wave type transmission property1302as shown inFIG. 13. The sine wave of a transmission property1302has a cycle and an amplitude to delete the inclination of an accumulated transmission property1301of AWG after interleaved.

The cycle of this sine wave is set, for example, almost twice the channel interval of a light signal before interleaved. By slightly deviating this cycle from the value obtained by doubling the channel interval, the inclination of the sine wave in the wavelength band matching with the signal wavelength band of each channel is gradually deviated for each channel and consequently it is set that the inclinations of the shortest wave and the longest wave become maximum.

InFIG. 13, line segments1303to1306correspond to shapes obtained by vertically flipping the upper sides of quadrangles of accumulated transmission properties of continuous four channels. The inclination of the line segment1303increases in accordance with the shape of the accumulated transmission property of the corresponding channel. On the other hand, the inclination of the line segment1306decreases in accordance with the shape of the accumulated transmission property of the corresponding channel. In this way, the inclination of the accumulated transmission property of AWG can be appropriately compensated by absorbing the inclination differences among channels.

The compensation filter that has such a periodic transmission property can be materialized using a Mach-Zehnder type light filter as shown inFIG. 14. The compensation filter ofFIG. 14includes an element1401having a light path ΔL and a heater1402. An input light1403is demultiplexed into two paths and the element1401adds a light path difference ΔL to each of the two paths. After that, lights of the two paths are multiplexed to be outputted as an output light1404.

The thus-multiplexed light output becomes in shape of a sine wave having a cycle c/ng*ΔL. Here, c is the velocity of light and ng is a group refractive index of the element1401. By matching the cycle of this sine wave with the intended cycle, a periodical transmission property can be materialized. Such a light filter can be produced using, for example, a quartz waveguide and the light path difference ΔL can be materialized by elongating the waveguide of either one of the two paths.

In order to materialize the inclination of the transmission property of a compensation filter with the intended frequency on a frequency axis, the fine adjustment of a light path difference is required. This fine adjustment can be materialized by heating either one of the two paths using the heater1402and changing the refractive index.

According to the configurations inFIGS. 9,10and12, a wavelength-multiplexed light is demultiplexed into lights of three bands by a band division filter but generally the number of divisions is optional so that the light can be demultiplexed into lights of five or seven bands.