JP2002082366A - Raman amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that specific numeric value of a excitation light wavelength interval suitable for realizing a Raman amplifier, having a wide-band and a suitable gain flatness is not clear. SOLUTION: When three or more excitation light wavelengths are used in the Raman amplifier and they are grouped into a shorter wavelength side and a longer wavelength side excitation light, by using an exciting light wavelength of which the adjoining wavelength interval is the widest, the group of the shorter wavelength side includes two or more excitation wavelengths having almost equal wavelength intervals; while the group of the longer wavelength side is composed of two or fewer excitation wavelengths. When the excitation wavelength is defined as a 1st channel, and a 2nd through n-th channels are defined at about 1 THz intervals, starting from there toward the longer wavelength side, the excitation light of the wavelengths corresponding to the 1st through the n-th channels are multiplexed, and in addition to these, excitation light having a wavelength of 2 THz or larger separated apart from the n-th channel toward the longer wavelength side is multiplexed, and the resultant excitation light is used as a pumping light source.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical amplifier, and more particularly to a Raman amplifier.

[0002]

2. Description of the Related Art In a wavelength division multiplexing (WDM) system used in current optical communication systems, there are two methods of expanding a transmission capacity, a method of expanding a signal wavelength band to increase the number of multiplexed wavelengths, and a method of expanding one wavelength. There is a method of improving the transmission rate (bit rate) per unit. In a typical WDM system, an erbium-doped optical fiber amplifier (EDFA)
Most commonly, the wavelength of the signal light is selected from a wavelength band of about 1530 to 1565 nm called the C band from the restriction of the gain wavelength band. On the other hand, even in a wavelength band of about 1575 to 1610 nm called an L band,
Since optical amplification by an EDFA is possible, development of a WDM system in this band has recently been advanced.

By expanding the wavelength band as described above, the capacity that can be transmitted by one WDM system can be expanded. Since the WDM system has begun to be introduced from the C-band WDM system, in order to expand the transmission capacity of the WDM system, the facilities of the existing C-band WDM system are utilized, and the L-band WDM system is added to the WDM system in stages. It is desirable to increase the capacity. Conventionally,
In a WDM system using an EDFA, the transmission rate per wavelength has been improved (increased) by technical improvement of various elements constituting the transmission system. However, in a system using a centralized amplifier such as an EDFA, the speed has been increased. Approaching its limits. It is said that the use of a distributed amplifier such as a Raman amplifier is indispensable for further increasing the speed and increasing the distance, and development is being actively conducted for practical use.

As shown in FIG. 20, the Raman amplifier comprises an optical fiber as an amplifying medium and an excitation light source for causing stimulated Raman scattering therein. When a silica-based optical fiber is used as the amplifying medium, a gain peak appears on a longer wavelength side than the wavelength of the pumping light and a frequency band about 13.2 THz lower in frequency. For example, the wavelengths of 1450 nm and 15
Since the difference between the frequencies of 50 nm is 13.2 THz, C
Excitation light having a wavelength around 1450 nm is used to Raman-amplify the band, and excitation light having a wavelength around 1490 nm is used to Raman-amplify the L band (FIG. 21).

However, the Raman gain of single-wave excitation has a large wavelength dependence, and as is apparent from FIG. 22, when the Raman gain exceeds about 5 dB, the gain deviation is reduced to 1 dB or less with respect to an operating bandwidth of 30 nm. It can no longer be suppressed. In order to solve this problem, it is effective to apply a plurality of pump lights having appropriate wavelength intervals to the Raman amplifier (wavelength multiplex pumping). According to this method, it is possible to realize a Raman amplifier having a wider band and better gain flatness than conventional ones.

Such a concept itself is disclosed in Japanese Patent Publication No. 7-99.
It can be intuitively understood as described in the description of FIG. Japanese Patent Application No. 10-20845
No. 0 and Japanese Patent Application No. 11-34833 refer to specific numerical values of the wavelength interval, and claim that a suitable value is 6 nm to 35 nm.

FIGS. 23 (a) and 23 (b) show examples of Raman gain profiles when the interval between pumping lights is 4.5 THz and 5 THz, respectively, and DSF is used as an amplification fiber. As is clear from FIGS. 23A and 23B, when the interval between the pump lights increases, the valley of the gain becomes deep and the gain deviation increases. In FIG. 23A, the values shown in Table 1 below are used as the frequencies (wavelengths) of the excitation light.
In (b), the values shown in Table 2 below were used as the frequency (wavelength) of the excitation light. In this case, the excitation light interval 4.5 THz is 3
3 nm corresponds to 5 THz corresponds to 36.6 nm.
That is, it is shown by this example that the gain flatness is not so good when the pumping light interval is 35 nm or more.

[0008]

[Table 1]

[0009]

[Table 2]

Next, FIG. 24 shows a gain profile in a case where three wavelengths are used with an interval of the pumping light of 4.5 THz. From this figure, it can be seen that when the third pumping wavelength is added, the gain valley becomes deep when the pumping light interval is 4.5 THz. FIG. 25 shows a gain profile in a case where three wavelengths are used with the interval between the pumping lights of 2.5 THz and 4.5 THz, and the valley of the gain is shallower than that of FIG. Since the frequency interval of 2.5 THz used here is about 18 nm in wavelength interval, it can be said that also in this case, it falls within the range of 6 nm to 35 nm. However, if the reverse approach is taken, it can be said that even within this range, a flat gain cannot be obtained unless the wavelength interval is properly set.

In the conventional design of an optical amplifier for WDM, it is a problem to minimize the gain flatness, and an optical amplifier in which all signal lights receive the same gain has been considered ideal. This design philosophy was sufficient when the number, power, and bandwidth of the signals used were small, but the Raman amplification effect between the signal lights became a problem as the bandwidth of the signal lights expanded. Is coming. This phenomenon has been described in the literature (eg, S. Bigo et al., IEEE Photonic
s Technology Letters, pp. 671-673, 1999), the WDM signal that had been set to the same power when it entered the transmission path becomes smaller on the short wavelength side and larger on the long wavelength side after propagation. It has such a linear inclination. This inclination is determined by various factors such as the number of signal lights, power, bandwidth, characteristics of a fiber constituting a transmission path, and transmission distance. As a means to address this problem, a tilt compensator (T. Naito et al., OAA'99,
Using WC5) and the wavelength dependence of Raman gain to compensate for tilt by giving a relatively large gain to the shorter wavelength signal
(M. Takeda et al., OAA'99, ThA3) have been proposed.
Since the former method is disadvantageous in terms of noise,
The latter is better. However, Takeda et al.
Since the slope of the Raman gain is not linear, the gain flatness after compensation for the slope is relatively large at 1 dB or more.

In a WDM system using a Raman amplifier, similarly to the above-described WDM system using only the EDFA, where a WDM system for the C band is introduced, the L band of the L band is used while keeping its facilities. WDM
It is desirable to have a configuration that can add a system.

In a Raman amplifier using wavelength-division multiplexing pumping, when considering the expansion of a gain band typified by the C band to the C + L band, all the pumping wavelengths used before the expansion are used, and after the expansion, It must be designed to operate for the C + L band. That is, it must be designed to operate for the C + L band by adding a new excitation wavelength to the excitation wavelength used for the C band. In this case, the wavelength arrangement for flattening the gain in the C band and the C + L band needs to be shared.

[Problems to be solved by the invention]

Since the gain deviation is proportional to the magnitude of the peak gain, when the gain is large, it is necessary to set the interval between pumping lights small. Further, as described above, when the pump light is arranged at equal intervals, the gain deviation may not be sufficiently reduced even if the wavelength interval is 35 nm or less. Even in such a case, it becomes necessary to use a narrower wavelength interval. In principle, the gain deviation can be reduced by reducing the interval between the pumping lights, but there is a practical lower limit for the interval between the pumping lights due to the problem of the multiplexing technique and the cost. In Japanese Patent Application Nos. 10-208450 and 11-34833, the lower limit is 6 nm based on the multiplexing technique.

[0015] However, Japanese Patent Application No. Hei 10-208450.
In Japanese Patent Application No. Hei 11-34833, it is disclosed that the interval between two adjacent excitation wavelengths is preferably in the range of 6 nm to 35 nm, but sufficient information is disclosed about detailed design values. Absent. In addition, known literature (Y. Emori et a
l., OFC'99 PD19), the gain deviation is 1 dB, which is not applicable when a smaller gain flatness is required. Therefore, in the present invention, a first object is to show a method of selecting a wavelength arrangement in a Raman amplifier using three or more pump wavelengths, to provide a Raman amplifier with good gain flatness, and to further provide a peak value of Raman gain. It is an object of the present invention to provide a Raman amplifier having a gain deviation of about 0.1 dB for 10 dB. It is another object of the present invention to provide a Raman amplifier suitable for compensating for a Raman effect between signals, which is a problem in wideband WDM transmission.

The present invention also provides a Raman amplifier in which, when a new pump light is added to extend the gain band, the gain and the flatness of the gain after extension are not significantly deteriorated compared to those before the extension. To consider.

[0017]

Prior to the development of the present invention, the present inventor studied the gain profile of a Raman amplifier using wavelength division multiplex pumping and clarified the wavelength arrangement for flattening the gain. The principle is as follows.

The gain profile of a Raman amplifier using wavelength-division multiplexing pumping is based on superposition of gains generated by each pumping wavelength. Therefore, combining gain slopes that cancel each other is a requirement for obtaining a flat gain. That is, by combining a downward-sloping curve in which the gain decreases from the short-wavelength side to the long-wavelength side and an upward-sloping curve in which the gain increases from the short-wavelength side to the long-wavelength side, a gain with good wavelength flatness is obtained. Characteristics are obtained. When the number of pumping wavelengths is two, the slope on the long wavelength side of the gain peak of the gain curve caused by the short wavelength pumping light, and the slope on the short wavelength side of the gain peak of the gain curve caused by the long wavelength pumping light. Will be combined. The Raman gain curve of one-wavelength excitation, which is the basis of superposition, has two gain peaks instead of a single peak, as is apparent from FIG.
0 nmm, the second downward-sloping peak is 1560n
m. In the L band, the first peak with the upper right slope is 1595 nm, and the second peak with the lower right slope is 1605 nm. Therefore, in any case, the two excitation wavelengths need to be separated by at least 10 nm. Curve A in FIG. 21 shows that the center wavelength of the excitation wavelength is 1450 nm,
Curve B is for the case where the center wavelength of the excitation wavelength is 1490 nm.

FIG. 26 shows an example of a gain profile of a Raman amplifier designed for the C band, and Table 3 shows pumping wavelengths used at that time. The assumed fiber is a normal single mode fiber, and the gain band is designed to cover from 1530 nm to 1565 nm. The gain profile of a Raman amplifier using wavelength division multiplexing pumping is realized by superposition of gains generated by each pumping wavelength. In FIG. 26, the gain distribution of each wavelength is optimized so that the flatness of the added gain is minimized. The number of excitation wavelengths is appropriately selected according to the required gain flatness. Similarly, FIG. 27 shows an example of the gain profile of the Raman amplifier designed for the L band.
Table 4 shows the excitation wavelengths used at that time.

[0020]

[Table 3]

[0021]

[Table 4]

When there are two pumping wavelengths, the gain bandwidth can be increased by increasing the distance between them,
If they are too far apart, there will be a gain valley in the band. Therefore, the gain flatness and the gain band have a trade-off relationship. FIG.
6, in FIG. 27, the pump wavelength interval is determined so as to optimize in a predetermined gain band, and the interval is 27n.
m and 29 nm (see Tables 3 and 4).

As shown in FIG. 21, the Raman gain curve of one-wavelength excitation, which is the basis of superposition, has a steeper gain slope on the long wavelength side than the gain peak on the longer wavelength side than the gain slope on the shorter wavelength side. The available bandwidth is narrow. In order to widen the band by making the slope of the downward slope gentle, it is necessary to create a downward-sloping gain curve using a plurality of pump wavelengths.

When a gain curve having a downward-sloping gain slope is created using three or more pumping wavelengths, similarly to the case where there are two pumping wavelengths, the pumping wavelength for creating a downward-sloping gain slope and the right-hand side are used. It is necessary to separate the pump wavelengths at least 10 nm or more to create an upward gain gradient. However, since the pumping light for forming the downward-sloping gain tilt is composed of a plurality of wavelengths, the longest pumping wavelength among them is separated from the pumping wavelength for generating the upward-sloping gain tilt by 10 nm or more. . 1435 nm and 1460 of three wavelength excitation in Table 3
nm spacing, 1438 nm and 1462 nm for 4 wavelength excitation
Corresponds to this. In Table 4, an interval between 1475 nm and 1500 nm for three-wavelength excitation corresponds to an interval between 1478 nm and 1501 nm for four-wavelength excitation.

When three or more pumping wavelengths are used, and the intervals between the pumping wavelengths are close to equal intervals, ripples appearing in a downward-sloping gain slope caused by superposition become small.
When flattening is performed in combination with a curve rising to the right, this ripple determines the final gain flatness. FIG.
The case of four-wavelength excitation in 6, 27 corresponds to this example. As shown in Tables 3 and 4, the optimized wavelength is 1
423nm, 1430nm, 1438nm and 1462n
m, 1470 nm, and 1478 nm, and are arranged at nearly equal intervals.

FIGS. 28 to 30 show the behavior of the Raman gain curve when the interval between the pumping lights is equal. FIG. 28 shows an example in which the peak gain is adjusted to 10 dB under the condition that the gain generated from each pump light is the same. From FIG. 28, the smaller the interval between the excitation lights,
It can be seen that the unevenness of the gain is reduced. FIG.
Is an example in which the gain by each pump light is adjusted so that the gain becomes flat. Also in this case, as in FIG. 28, the smaller the interval between the pumping lights, the smaller the unevenness of the gain.
It can also be seen that the undulation of the gain curve in FIG. 28 determines the maximum gain deviation in FIG. From the above, it can be said that the excitation light interval of 2 THz is too large to make the gain deviation about 0.1 dB, but 1 THz is sufficient.

Next, FIG. 30 shows the behavior when the excitation light interval is 1 THz and the multiplex number is changed. As can be seen from the gain curve of 1ch pumping, in the case of a silica-based fiber, there is a smooth curve without irregularities on the short wavelength side from the gain peak, but there are three relatively large irregularities on the long wavelength side. However, this is a factor that determines the limit of flattening.
This unevenness decreases as the number of multiplexes increases. For example, looking at the gain curve of one channel, there is a protrusion near 1 dB near 187 THz, but this becomes smaller as the number of multiplexes is increased. This is because the peak gain is set to the same value, so that as the number of multiplexes increases, the gain per wave decreases, the size of the protrusion itself decreases, and the same irregularities are slightly spaced at equal intervals. This is due to the fact that they are added together. That is, by adding the convex portion of the gain curve at a certain excitation wavelength and the concave portion of the gain curve at another excitation wavelength, the overall unevenness is reduced. The numerical value of about 1 THz according to claims 2 to 4 is based on this principle. In the gain curve of the 1-channel excitation shown in FIG. 30, a protrusion near 187 THz and a dent near 188 THz immediately adjacent thereto are shown. The basis is that the frequency difference is about 1 THz. Therefore, depending on the fiber used, the gain curve of 1ch pumping may be slightly different, and the numerical value described as about 1 THz in claims 2 to 4 may be changed. In any case, in order to reduce the gain deviation, it is necessary to cancel out the unevenness of the gain curve that is the basis of the addition.

Since the limit of the gain deviation is determined by the undulation and unevenness of the gain curve on which the superposition is based, it is considered that a flat gain profile with a small gain deviation can be obtained by combining gain curves with small unevenness. Therefore, this is achieved by combining a gain curve by pump light multiplexed at an interval of about 1 THz and a gain curve by pump light on a longer wavelength side than the pump wavelength. At this time, it is desirable that the peaks of the two gain curves be appropriately separated from each other in terms of broadening the bandwidth.

The effect described so far was for the purpose of reducing the gain flatness. However, by reducing the gain due to the pump light on the long wave side, the gain is linearly changed from the short wave side to the long wave side. It is also possible to achieve a decreasing gain profile. If this is combined with the level gradient generated by the Raman effect between the signal lights, the level of the signal lights can be made flat. By adjusting the distribution of gains on the short wave side and the long wave side, an arbitrary inclination can be realized, so that any Raman inclination can be compensated.

When attempting to extend the gain bands of the C band and the L band, it is considered that the optimal method is to use both pump wavelengths for the C band and the L band at the same time.

However, the excitation light for the C band shown in FIG.
FIG. 31 shows a gain profile when the gain distribution of each pump wavelength is optimized by simultaneously using the L-band pump light in FIG. 27. Table 5 shows the excitation wavelengths used at that time.
Shown in In this case, even if an attempt is made to make the gain flatness equal to those in FIGS. 26 and 27 over the entire C + L band,
As shown in FIG. 31, there is a problem that a large depression is formed in the band of the C band, and the gain flatness is worse than before expansion.

[0032]

[Table 5]

For the same reason as described above,
When operating with good gain flatness in the C + L band,
A downward-sloping gain curve is created by using a plurality of pump wavelengths arranged at substantially equal intervals, and the gain curve is set to be 1
It is necessary to make a gain curve that rises to the right using pumping light on the longer wavelength side than 0 nm. However, since only the excitation wavelengths of FIGS. 26 and 27 are used at the same time as the excitation wavelength used in FIG. 31, the condition of arrangement near equidistant in the excitation wavelength band for the C band cannot be satisfied. Table 5 shows the excitation wavelengths used in FIG. According to this, it is understood that the excitation wavelength is insufficient in the excitation wavelength band for the C band in order to arrange the short wavelength side close to the equal intervals. It has been found that this shortage is a factor that causes the gain depression shown in FIG.

From the above results, it has been clarified that the means necessary to achieve the above object are as follows. One means of the present invention is a Raman amplifier using three or more pumping wavelengths, in which the adjacent wavelength interval has the widest pumping wavelength as a boundary, and is divided into short wavelength side and long wavelength side pumping wavelength groups. The glue on the short wavelength side includes two or more excitation wavelengths, and the wavelength intervals are substantially equal, and the group on the long wavelength side is configured by two or less excitation wavelengths. According to another means of the present invention, when the shortest excitation wavelength is defined as the first channel and the second to n-th channels are defined therefrom at intervals of about 1 THz on the longer wavelength side, the wavelength corresponding to the first to n-th channels is defined. In addition to the multiplexed pumping light, pumping light having a wavelength separated from the n-th channel by 2 THz or more on the longer wavelength side is multiplexed and used as pumping light for the Raman amplifier. Also,
When the shortest excitation wavelength is defined as the first channel, and the second to n-th channels are defined on the longer wavelength side at intervals of about 1 THz, wavelengths corresponding to channels other than the n-1 and n-2 channels are defined. The combined light was used as the pump light of the Raman amplifier. Alternatively, a combination of all wavelengths corresponding to channels other than the (n-2) th and (n-3) th channels was used as pumping light for the Raman amplifier.

Another aspect of the present invention is a Raman amplifier for extending a gain wavelength band, wherein the pump wavelengths before extension are two or more, and two or more pump wavelengths are added to extend the gain wavelength band. One or more excitation wavelengths to be used are different from the excitation wavelength used before extension, and the one or more excitation wavelengths are arranged in the excitation wavelength band used before extension.

Another aspect of the present invention is a Raman amplifier for extending a gain wavelength band, wherein the pump wavelengths before extension are two or more, and two or more pump wavelengths are added to extend the gain wavelength band. One or more excitation wavelengths to be used are different from the excitation wavelength used before extension, and the one or more excitation wavelengths are arranged in a wavelength band with insufficient gain in the excitation wavelength band used before extension.

Another aspect of the present invention is a Raman amplifier for extending a gain wavelength band, wherein the pump wavelength before extension is two or more, and one or more pump wavelengths are included in the pump wavelength band before extension for gain wavelength band extension. Are added so that the excitation wavelengths in the excitation wavelength band before expansion are at equal intervals or nearly equal intervals.

Another aspect of the present invention is to simultaneously use two or more excitation wavelengths for amplifying the C band and two or more excitation wavelengths for amplifying the L band to obtain the C band and the L band. Is a Raman amplifier that adds one or more pumping wavelengths different from the pumping wavelength of the C-band used before extension into the pumping wavelength band of the C-band when simultaneously amplifying the C-band.

Another aspect of the present invention is to simultaneously use two or more excitation wavelengths for amplifying the C band and two or more excitation wavelengths for amplifying the L band to obtain the C band and the L band. Is a Raman amplifier that adds one or more pumping wavelengths different from the C-band pumping wavelength used before extension to the gain-deficient wavelength band in the C-band pumping wavelength band when simultaneously amplifying the C-band.

Another aspect of the present invention is to simultaneously use two or more excitation wavelengths for amplifying the C band and two or more excitation wavelengths for amplifying the L band to obtain the C band and the L band. At the same time, one or more excitation wavelengths different from the excitation wavelength of the C band used before extension are added to the excitation wavelength band of the C band, and the excitation wavelength within the excitation wavelength band before extension is added by the addition. Are Raman amplifiers having equal or nearly equal intervals.

[0041]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiments described below, an example is shown in which the first channel is set to 211 THz. This is because the amplification band is assumed to be 1530 nm or more (about 196 THz or less in frequency display) used in the current WDM transmission system. Therefore, the amplification band is 1580 nm or more called L band (about 190 THz in frequency display).
If the following is assumed, the excitation band may be shifted by 6 THz, and the first channel may be set to 205 THz. The first channel can be determined for other amplification bands in the same way.

FIG. 1 shows a first embodiment of the present invention, which is an embodiment corresponding to claim 2. The frequency of the first channel is 211 THz (wavelength 1420.8 nm), and the frequency of the second channel and lower is 210 THz (wavelength 1427.6 nm).
It is arranged at 1 THz intervals up to 7 THz (wavelength 1448.3 nm). The pump light having a wavelength 2 THz away from the fifth channel on the longer wavelength side (frequency 205 THz, wavelength 146
2.4 nm) to form a wavelength multiplexing pump light source for Raman amplification. As a pumping light source 10 of each wavelength shown in FIG. 1, a Fabry-Perot type semiconductor laser whose wavelength is stabilized by a fiber Bragg grating (FBG) is used as a polarization synthesizer (P).
BC). Polarization combining is a measure to increase the pump power of each wavelength and at the same time reduce the polarization dependence of Raman gain. When the pump power is sufficient with the output of one laser, the laser output may be depolarized and then connected to a wavelength multiplexer. The Mach-Zehnder interferometer type combiner 20 shown in FIG. 1 is suitable for multiplexing pump lights of a plurality of wavelengths whose adjacent frequency intervals are constant. The dielectric multilayer filter 30 shown in FIG. 1 is suitable for multiplexing two relatively wide wavelength bands, and can multiplex longer and shorter wavelengths than a specific wavelength. In this embodiment, 2
A frequency higher than 07 THz (wavelength shorter than 1448.3 nm) and a frequency lower than 205 THz (1462.4 n
(wavelength longer than m).
In FIG. 1, the excitation light multiplexed by the dielectric multilayer filter 30 is sent to the dielectric multilayer filter 50 through the isolator 40, and the WDM signal is amplified in the optical fiber.

FIG. 2 shows a Raman gain profile when the pumping light source of FIG. 1 is used. Curve A represents the total gain, curve B is the sum of the gains by the pump light of the first to fifth channels, curve C is the gain of the sixth channel, and the thin lines are the respective pump wavelengths of the first to fifth channels. It represents the gain. As described in the effect section, by multiplexing the short-wavelength pump light at 1 THz intervals, a smooth curve with a downward slope is formed, and this is combined with the upward-sloping gain curve due to the long-wavelength pump light. , The total Raman gain becomes flat. From FIG. 2, it can be seen that the use of the interval of 1 THz successfully cancels out the unevenness of the plurality of gain curves. FIG. 3 shows an enlarged view of the total gain. The peak gain is 10 dB and the gain band is 196 THz (wavelength 1
The characteristic is around 529.6 nm) to 193 THz (wavelength 1553.3 nm), and the gain deviation is about 0.1 dB.

FIG. 4 shows a gain profile when the wavelength of the sixth channel is set to a wavelength 2.5 THz away from the fifth channel by 2.5 THz (frequency: 204.5 THz, wavelength: 1465.5 nm). . As in FIG. 2, curve A represents the total gain, curve B is the sum of the gains due to the pump light of the first to fifth channels, curve C is the gain of the sixth channel, and the thin line is the first to fifth channels. It shows the gain at each pump wavelength of the channel. Also in this case, the total curve of Raman gain is flattened by the addition of the downward-sloping curve due to the short-wavelength pump light and the upward-sloping gain curve due to the long-wavelength pump light. FIG. 5 shows an enlarged view of the total gain. Peak gain is 10dB and gain band is 196THz
(Wavelength 1529.6 nm) to 192 THz (wavelength 1561.4 n
m), and the characteristic that the gain deviation is about 0.1 dB is realized. Although the gain band is wider than in FIG. 3, the gain dip in the middle of the band is slightly larger. This is because the distance between the fifth and sixth channels has been increased.

FIG. 6 shows a second embodiment of the present invention, which is an embodiment corresponding to claims 2 and 3. The frequency of the first channel is 211 THz (wavelength 1420.8 nm)
The frequency below the second channel is 210 THz (wavelength 142
1 THz from 7.6 nm) to 204 THz (wavelength 1469.6 nm)
They are lined up at intervals. The total number of channels is 8, and the excitation light source is configured using six wavelengths excluding the sixth channel and the seventh channel. The configuration of the excitation light of each channel is selected as necessary as described in the first embodiment. FIG.
As the pumping light source 10 of each wavelength shown in FIG. 1, a Fabry-Perot type semiconductor laser wavelength-stabilized by a fiber Bragg grating (FBG) multiplexed by a polarization combiner (PBC) is used. Polarization combining is a measure to increase the pump power of each wavelength and at the same time reduce the polarization dependence of Raman gain. When the pump power is sufficient with the output of one laser, the laser output may be depolarized and then connected to a wavelength multiplexer. The Mach-Zehnder interferometer type multiplexer 20 shown in FIG. 6 is suitable for multiplexing pump lights of a plurality of wavelengths in which adjacent frequency intervals are constant. The dielectric multilayer filter 30 shown in FIG. 6 is suitable for multiplexing two relatively wide wavelength bands.
It is possible to combine a longer wavelength and a shorter wavelength than a specific wavelength. In FIG. 6, the pump light output from the Mach-Zehnder interferometer type combiner 20 is sent to the dielectric multilayer filter 50 through the isolator 40, and the WDM signal is amplified in the optical fiber. FIG. 7 shows a Raman gain profile when the pumping light source of FIG. 6 is used. Curve A represents the total gain, curve B represents the sum of the gains by the pump light of the first to fifth channels, curve C represents the gain of the eighth channel, and thin lines represent the respective pump wavelengths of the first to fifth channels. It represents the gain. even here,
The right-downward curve due to the short-wavelength pumping light and the right-upward gain curve due to the longwave-side pumping light are added to make the total Raman gain flat. FIG. 8 shows an enlarged view of the total gain. Peak gain is 10dB, gain band is 19
6 THz (wavelength 1529.6 nm) to 191 THz (wavelength 156
9.6 nm), and a characteristic with a gain deviation of about 0.1 dB is realized. As compared with FIGS. 3 and 5, the gain band is further widened. This is because the longest excitation wavelength is set to a longer wavelength.

FIG. 9 shows a third embodiment of the present invention, which is an embodiment corresponding to claims 2 and 4. As in the second embodiment, the frequency of the first channel is 211TH.
z (wavelength 1420.8 nm), the frequency below the second channel is from 210 THz (wavelength 1427.6 nm) to 204 THz (wavelength 1
469.6 nm) at 1 THz intervals. The total number of channels is 8, excluding the fifth and sixth channels.
An excitation light source is configured using the wavelength. The configuration of the excitation light of each channel is selected as necessary as described in the first embodiment. As the pump light source 10 of each wavelength shown in FIG. 9, a Fabry-Perot type semiconductor laser wavelength-stabilized by a fiber Bragg grating (FBG) multiplexed by a polarization combiner (PBC) is used. Polarization combining is a measure to increase the pump power of each wavelength and at the same time reduce the polarization dependence of Raman gain. When the pump power is sufficient with the output of one laser, the laser output may be depolarized and then connected to a wavelength multiplexer. Mach-Zehnder interferometer type multiplexer 2 shown in FIG.
0 is suitable for multiplexing pump lights of a plurality of wavelengths whose adjacent frequency intervals are constant. The dielectric multilayer filter 30 shown in FIG. 9 is suitable for multiplexing two relatively wide wavelength bands, and can multiplex longer and shorter wavelengths than a specific wavelength. In FIG. 9, the excitation light output from the Mach-Zehnder interferometer type combiner 20 is passed through an isolator 40 to a dielectric multilayer filter 50.
And amplify the WDM signal in the optical fiber. FIG. 10 shows a Raman gain profile when the pump light source of FIG. 9 is used. Curve A represents the total gain, curve B represents the sum of the gains of the first to fourth channels by the pump light, curve C represents the sum of the gains of the seventh and eighth channels, and the thin line represents the gain by each pump wavelength. Represents. Again, the downward-sloping curve due to the short-wave side pump light and the upward-sloping gain curve due to the long-wave side pump light are added,
The total Raman gain is flat. FIG. 11 shows an enlarged view of the total gain. The peak gain is 10 dB, the gain band is around 196 THz (wavelength 1529.6 nm) to 191 THz (wavelength 1569.6 nm), and the gain deviation is 0.1 dB.
The characteristic of degree is realized. What should be noted here is the difference in the magnitude of the gain depending on each pump wavelength in the second and third embodiments. In FIG. 7, there is a channel close to 8 dB at the maximum, whereas in FIG. It is the largest. This is because while the second embodiment forms the gain on the long wave side shown by the curve C with the gain of one channel, the third embodiment forms the gain with the sum of the gains of two channels. is there. This means that the maximum value of the pump light power required per wave can be reduced, which is very effective from a practical viewpoint.

FIGS. 12 to 15 show 211 THz (wavelength 142).
1 THz from 0.8 nm) to 199 THz (wavelength 1506.5 nm)
This is a gain profile when 11 channels are used from among 13 channels at intervals. FIG.
And the pump light other than 201 THz and 200 THz is used. Curve A represents the total gain, curve B is the sum of the gains by the pump light of the first to tenth channels, curve C is the gain of the thirteenth channel, and the thin line is each of the pump wavelengths of the first to tenth channels. Represents the gain due to Also in this case, the total curve of Raman gain is flattened by the addition of the downward-sloping curve due to the short-wavelength pump light and the upward-sloping gain curve due to the long-wavelength pump light. FIG. 13 shows an enlarged view of the total gain. The peak gain is 10 dB, the gain band is around 196 THz (wavelength 1529.6 nm) to 186 THz (wavelength 1611.8 nm), and the characteristic that the gain deviation is about 0.1 dB is realized.

FIG. 14 uses the configuration according to claim 4 and uses pump light other than 202 THz and 201 THz.
Curve A represents the total gain, curve B represents the sum of the gains of the first to ninth channels by the pumping light, curve C represents the sum of the gains of the twelfth and thirteenth channels, and the thin line represents the respective pumping wavelengths. It represents the gain. Also in this case, the total curve of Raman gain is flattened by the addition of the downward-sloping curve due to the short-wavelength pump light and the upward-sloping gain curve due to the long-wavelength pump light. FIG. 15 shows an enlarged view of the total gain. The peak gain is 10 dB and the gain band is 196 THz (wavelength 1
The characteristic is around 529.6 nm to 186 THz (wavelength 1611.8 nm), and the gain deviation is about 0.1 dB. As can be seen by comparing FIGS. 12 and 14, the second embodiment forms the long-wave-side gain indicated by the curve C by the gain of one channel, while the third embodiment forms the gain on the long-wave side by two channels. Since it is formed by the sum of the gains of two channels, the maximum value of the gain required per wave is smaller in FIG. This means that the maximum value of the pump light power required per wave can be reduced, which is very effective from a practical viewpoint.

FIG. 16 shows a gain profile when the same pumping light source as that shown in FIG. 12 is used. By using such a Raman amplifier, it is possible to compensate for the upward slope due to the Raman effect between signal lights as shown in the literature of Bigo et al., And to keep the WDM signal at a flat level in the optical amplification repeater system. Can be. For example, in FIG. 2 of the document of Bigo et al., A gain slope of 2.3 dB occurs at 25 nm, and the level slope due to the Raman effect between signals is canceled by adjusting the reverse slope so as to reduce 7.4 dB when converted to 80 nm. be able to. FIG. 16 shows 3 dB at 80 nm and 5 dB.
The gain slope is shown to decrease by 7 dB.
Since it is 2.2 dB, 1.6 dB, and 0.9 dB when converted to 5 nm, it is considered that the gain tilt can be compensated for even under various conditions shown in FIG.

(Embodiment 1) The Raman amplifier of the present invention is
A case where the band and the L band are combined and extended to the C + L band will be described as an example. FIG. 17 shows an example in which the excitation light before expansion has two wavelengths, and the excitation wavelengths before and after expansion are as shown in Table 6. By setting one of the excitation wavelengths to be added for extension to 1439 nm, at least one of the excitation lights to be added for extension is arranged in a band (1426-1453 nm) of the excitation light before extension. I do. The presence of the pump light realizes expansion while maintaining gain flatness.

[0051]

[Table 6]

FIG. 18 shows an example in which the excitation light before expansion has three wavelengths. The excitation wavelengths before and after expansion are as shown in Table 7. By setting one of the pumping wavelengths to be added for extension to 1446 nm, at least one of the pumping lights to be added for extension is set to the band of the pumping light before extension (1424-146).
0 nm). The presence of the pump light realizes expansion while maintaining gain flatness.

[0053]

[Table 7]

FIG. 19 shows an example in which the excitation light before expansion has four wavelengths, and the excitation wavelengths before and after expansion are as shown in Table 8. Two of the excitation wavelengths added for extension are 1445 nm and 1
By setting the wavelength to 453 nm, at least one of the excitation lights added for extension is in the band (14) of the excitation light before extension.
23-1462 nm). The presence of the pump light realizes expansion while maintaining gain flatness. In this example, since the excitation wavelength of 1462 nm is used for both the C band and the L band independently, it is not necessary to add the excitation wavelength when extending.

[0055]

[Table 8]

(Embodiment 2) This embodiment is an example in which the pumping light before extension is a two-wavelength pump, and the wavelength of the pumping light before extension is longer than that in the first embodiment. In this example,
The gain band of the C band is designed to be 1535 to 1570 nm. Table 7 shows the excitation wavelengths before and after the extension. One of the excitation wavelengths to be added for extension is 14
By setting the wavelength to 44 nm, at least one of the pump lights added for the extension is in the band (143) of the pump light before the extension.
0-1457 nm). The presence of the pump light realizes expansion while maintaining gain flatness.

[0057]

[Table 9]

(Embodiment 3) This embodiment is an example in which the pumping light before extension is a two-wavelength pump, and the wavelength of the pumping light before extension is shorter than that in the first embodiment. In this example,
The C band is designed to have a gain band of 1525 to 1560 nm. Table 10 shows the excitation wavelengths before and after the extension. One of the excitation wavelengths to be added for extension is 14
When the wavelength is set to 38 nm, at least one of the pump lights added for the extension is in the band (142) of the pump light before the extension.
2-1450 nm). The presence of the pump light realizes expansion while maintaining gain flatness.

[0059]

[Table 10]

[0060]

According to the first aspect of the present invention, in the Raman amplifier using three or more pumping wavelengths, a group of pumping wavelengths on the short wavelength side and the long wavelength side with the boundary between the pumping wavelengths having the widest interval between adjacent wavelengths. When divided into two groups, the short wavelength side group includes two or more excitation wavelengths, and the wavelength intervals are almost equal, and the long wavelength side group is configured with two or less excitation wavelengths. A Raman amplifier with a wide band and good gain flatness is formed by forming a downward-sloping gain curve with small irregularities in a wide band by the short-wavelength group and combining it with a rising-slop gain curve formed by the long-wavelength group. Is realized. In the Raman amplifier according to claims 2 to 4, since the excitation wavelength interval of the group on the short wavelength side is about 1 THz, a Raman amplifier having a gain deviation of about 0.1 dB with respect to a Raman gain peak value of 10 dB is realized. . In the Raman amplifier according to claim 5, when the pump wavelengths before the gain band extension are two or more, two or more new pump wavelengths different from the pump wavelengths used before the extension are added, and one or more of the pump wavelengths are added. Since the excitation wavelength is different from the excitation wavelength used before the extension and the additional excitation wavelength is arranged in the excitation wavelength band used before the extension, the additional band is excited and the gain of the band is increased, and the gain is flat over a wide band. And the gain band is extended.

In the Raman amplifier according to claim 6, one or more of the additional pumping wavelengths are different from the pumping wavelength used before extension, and the one or more pumping wavelengths have a gain shortage in the pumping wavelength band used before extension. Since the band is arranged in the wavelength band, the pumping wavelength insufficient band is pumped, the gain of the band is increased, the gain is flattened over a wide band, and the gain band is extended.

In the Raman amplifier according to claim 7, one or more pump wavelengths to be added are different from the pump wavelengths used before extension, and by adding one or more pump wavelengths, the pump wavelength within the pump wavelength band before extension is increased. Since the excitation wavelengths are set at equal or nearly equal intervals, the entire excitation wavelength band is excited, the gain is flattened over a wide band, and the gain band is extended.

In the Raman amplifier of claim 8, when simultaneously amplifying the C band and the L band, one or more excitation wavelengths different from the excitation wavelength of the C band used before extension are set within the excitation wavelength band of the C band. Because of the addition, an additional band in the C band is excited to increase the gain of the band, flatten the gain over a wide band, and extend the gain band.

In the Raman amplifier according to the ninth aspect, when simultaneously amplifying the C band and the L band, one or more excitation wavelengths different from the excitation wavelength of the C band used before extension are used as gains in the excitation wavelength band of the C band. Since the band is added to the shortage wavelength band, the excitation wavelength short band in the C band is excited to increase the gain, the gain is flattened over a wide band, and the gain band is extended.

In the Raman amplifier according to claim 10 of the present invention,
When simultaneously amplifying the C band and the L band, one or more excitation wavelengths different from the excitation wavelength of the C band used before extension are added to the excitation wavelength band before extension to excite the C band before extension. Since the pump wavelengths in the wavelength band are set to be at equal intervals or nearly equal intervals, the entire pump wavelength band of the C band is excited, and the gain is flattened over a wide band,
The gain band is extended.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a Raman amplifier according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a Raman gain profile when the pumping light source shown in FIG. 1 is used.

FIG. 3 is an enlarged view of the total gain shown in FIG.

FIG. 4 shows the Raman amplifier shown in FIG.
The figure which shows the Raman gain profile at the time of making a wavelength z away.

FIG. 5 is an enlarged view of the total gain shown in FIG.

FIG. 6 is an explanatory diagram showing a Raman amplifier according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a Raman gain profile when the pumping light source shown in FIG. 6 is used.

FIG. 8 is an enlarged view of the total gain shown in FIG. 7;

FIG. 9 is an explanatory diagram illustrating a Raman amplifier according to a third embodiment of the present invention.

FIG. 10 is a diagram showing a Raman gain profile when the pumping light source shown in FIG. 9 is used.

11 is an enlarged view of the total gain shown in FIG.

FIG. 12: 1 from 211 THz to 199 THz at 1 THz intervals
FIG. 9 is a diagram illustrating a Raman gain profile when using 11 channels out of 3 channels and using pump light other than 201 THz and 200 THz.

FIG. 13 is an enlarged view of the total gain shown in FIG.

FIG. 14: 1 at 1 THz intervals from 211 THz to 199 THz
FIG. 9 is a diagram illustrating a Raman gain profile when 11 channels are used from among 3 channels and pump light other than 202 THz and 201 THz is used.

15 is an enlarged view of the total gain shown in FIG.

FIG. 16 is a diagram showing a Raman gain profile when the same pump light source as in FIG. 12 is used.

FIG. 17 is an explanatory diagram of gain characteristics when the pump light before expansion has two wavelengths.

FIG. 18 is an explanatory diagram of gain characteristics when the pump light before expansion has three wavelengths.

FIG. 19 is an explanatory diagram of gain characteristics when the pump light before extension has four wavelengths.

FIG. 20 is an explanatory diagram of a Raman amplifier.

FIG. 21 is an explanatory diagram of gain characteristics when the C band and the L band are each excited by one wavelength.

FIG. 22 is a diagram showing a relationship between a gain magnitude and a gain bandwidth.

23 (a) and (b) show the case where the interval between the excitation lights is 4.5 THz,
FIG. 5 is a diagram showing a Raman gain profile when 5 THz is used and DSF is used as an amplification fiber.

FIG. 24 is a diagram illustrating a Raman gain profile when three wavelengths are used with an excitation light interval of 4.5 THz.

FIG. 25 is a diagram showing a Raman gain profile when three wavelengths are used with an excitation light interval of 2.5 THz and 4.5 THz.

FIG. 26 is an explanatory waveform diagram of an example in which the Raman amplification method of the present invention is applied to the C band.

FIG. 27 is an explanatory waveform diagram of an example in which the Raman amplification method of the present invention is applied to the L band.

FIG. 28 shows the behavior of a Raman gain curve when the intervals of the pump light are equal, and the peak gain is 10 under the condition that the gain generated from each pump light is the same.
The figure when adjusted so that it might become dB.

FIG. 29 shows the behavior of a Raman gain curve when the intervals between pumping lights are equal, and is a diagram in which the gains of individual pumping lights are adjusted so that the gains become flat.

FIG. 30 shows the behavior of the Raman gain curve when the interval between the pumping lights is equal, and the interval between the pumping lights is 1TH.
The figure when changing the multiplex number as z.

FIG. 31 is an explanatory diagram of gain characteristics when the Raman amplification method of the present invention is applied to the C + L band.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/16 F term (Reference) 2K002 AA02 AB30 BA04 CA15 DA10 GA10 HA23 5F072 AB07 AK06 JJ20 KK30 MM07 PP07 QQ07 RR01 YY17 5K002 AA06 BA05 BA13 CA13 DA02 FA01

Claims (10)

[Claims] In a Raman amplifier using three or more pumping wavelengths, when a short wavelength side and a long wavelength side pumping wavelength are divided into groups with a pumping wavelength having the widest interval between adjacent wavelengths as a boundary, a short wavelength is used. A Raman amplifier, wherein the group on the side includes two or more pump wavelengths, the wavelength intervals of which are substantially equal, and the group on the long wavelength side includes two or less pump wavelengths. 2. An excitation wavelength is defined as a first channel.
From there, when defining the second to n-th channels at an interval of about 1 THz on the longer wavelength side, in addition to the multiplexed excitation light of the wavelength corresponding to the first to n-th channels, the n-th A Raman amplifier, which combines pump light having a wavelength of 2 THz or more away from a channel and uses this as a pump light source. 3. A method according to claim 1, wherein a certain excitation wavelength is defined as a first channel.
From there, when defining the second to n-th channels at an interval of about 1 THz on the long wavelength side, a multiplexed excitation light having a wavelength corresponding to channels other than the n-1 and n-2 channels is used as the excitation light source. A Raman amplifier characterized in that it is used. 4. A method according to claim 1, wherein a certain excitation wavelength is defined as a first channel.
From there, when defining the second to n-th channels at an interval of about 1 THz on the long wavelength side, a multiplexed excitation light having a wavelength corresponding to channels other than the n-2 and n-3 channels is used as an excitation light source. A Raman amplifier characterized in that it is used. 5. A Raman amplification method for extending a gain wavelength band, wherein the pump wavelengths before extension are two or more, and two or more pump wavelengths are added for gain wavelength band extension, and one or more pump wavelengths to be added are added. Is different from the pump wavelength used before extension, and one or more pump wavelengths are arranged in the pump wavelength band used before extension. 6. A Raman amplification method for extending a gain wavelength band, wherein the pump wavelengths before extension are two or more, and two or more pump wavelengths are added to extend the gain wavelength band, and one or more of the additional pump wavelengths are added. Wherein the Raman amplifier is different from the pump wavelength used before the extension, and one or more of the pump wavelengths are arranged in a wavelength band with insufficient gain in the pump wavelength band used before the extension. 7. A Raman amplification method for extending a gain wavelength band, wherein the pump wavelength before extension is two or more, and one or more pump wavelengths are added in the pump wavelength band before extension for gain wavelength band extension. A Raman amplifier characterized in that the pump wavelengths in the pump wavelength band before extension are made at equal intervals or nearly equal intervals by the addition thereof. 8. A method for simultaneously amplifying a C band and an L band by simultaneously using two or more excitation wavelengths for amplifying a C band and two or more excitation wavelengths for amplifying an L band. A Raman amplifier characterized in that one or more pump wavelengths different from the C-band pump wavelength used before extension are added to the C-band pump wavelength band. 9. A method for simultaneously amplifying the C band and the L band by simultaneously using two or more excitation wavelengths for amplifying the C band and two or more excitation wavelengths for amplifying the L band. A Raman amplifier characterized in that one or more pump wavelengths different from the C-band pump wavelength used before extension are added to a gain-deficient wavelength band in the C-band pump wavelength band. 10. A method for simultaneously amplifying a C band and an L band by simultaneously using two or more excitation wavelengths for amplifying a C band and two or more excitation wavelengths for amplifying an L band. One or more excitation wavelengths different from the C-band excitation wavelength used before the extension are added to the C-band excitation wavelength band, so that the excitation wavelengths in the excitation wavelength band before the extension become equidistant or equidistant. A Raman amplifier characterized by having close intervals.