Patent Application
20240258760
The present invention relates to techniques of a distributed optical amplifier, a distributed optical amplifier system, and a distributed optical amplification method.
In design of high-speed and large-capacity distributed optical amplifier systems, it is important to reduce an SN degradation of received signals caused due to transmission line loss. Therefore, various configurations in which optical amplification is performed on the optical transmission line itself to compensate for the transmission line loss have been devised. Among them, distributed Raman amplification has the great advantage of using already installed optical fibers as a gain medium, and is therefore expected to be applied to high-speed and large-capacity optical communication of the next generation.
Signal light is sent from an optical transmitter. In the case of Raman amplification, a wavelength of pump light is shorter than a wavelength of signal light by about 100 nm. Since the Raman amplification in the optical fiber has a wide gain band, the wavelength difference of 100 nm is not so strict, and the gain can be obtained even if the wavelength difference is slightly different.
Generally, because the pump light propagates through the core of the optical transmission line like signal light, in the forward pumping, it is necessary to multiplex the signal light and pump light traveling in the same direction as the signal light. On the other hand, in the backward pumping, there is a need to send out the pump light traveling in an opposite direction to the signal light, and separate only the signal light and send it to the optical receiver. This multiplexing and demultiplexing can be realized by a wavelength multiplexing coupler or a circulator. While the bidirectional pumping has been described in
Since the Raman amplification is an optical effect having polarization dependency, the pump light output from the pump light source for forward pumping and the pump light source for backward pumping need to be depolarized. As a specific method for making the depolarized light, for example, a method described in NPL 1 can be used. Although the depolarization is essential, a detailed description thereof will be omitted. The following description of the background technique is based on the premise that the pump light source for forward pumping and the pump light source for backward pumping are subjected to depolarization processing.
When the noise in the pump light cannot be ignored, the noise is transferred to the signal light to be amplified. It is known that noise transfer becomes more remarkable in the forward pumping. This is because the pump light and the signal light propagate in the same direction.
Here, a group velocity νs of the signal light and a group velocity νp of the pump light in the optical transmission line are considered.
This is because, even if the intensity of the pump light periodically fluctuates at a high speed, the signal light will receive Raman gain from both the high-power pump light and the low-power pump light when viewed from the signal lights with different group velocities. So then, the signal light ultimately receives an averaged Raman gain.
However, since the time during which the signal light can receive the Raman gain in the course of propagation through the optical transmission line is finite, if the period of the intensity fluctuation of the pump light is long, the above-mentioned averaging is not completely performed, and the Raman gain also fluctuates with time. That is, even if the group velocity of the signal light and the pump light is different, the low-frequency component of the noise in the pump light is transferred to the signal light, and degradation of the signal quality occurs to some extent. The details thereof are disclosed in, for example, in NPL 2.
However, as shown in
In view of the above circumstances, an object of the present invention is to provide a technology that can suppress quality degradation of the transmitted signal associated with the distributed optical amplifier.
An aspect of the present invention is a distributed optical amplifier including: a pump light generator which generates pump light for optically exciting an optical transmission line; and a multiplexer which multiplexes n kinds (n is a positive integer) of light to be amplified and the pump light output from the pump light generator, and outputs the multiplexed light to the optical transmission line, wherein, when a center wavelength of the n kinds of light to be amplified is denoted as λS1 to λSn and a zero-dispersion wavelength of the optical transmission line is denoted as λ0, intensity of the pump light generated by the pump light generator is suppressed at n kinds of wavelengths represented by 2×λ0−λS1 to 2×λ0−λSn in comparison to intensity of other wavelengths of the pump light generated by the pump light generator.
An aspect of the present invention is a distributed optical amplifier system including an optical transmitter which transmits n kinds (n is a positive integer) of signal light, a distributed optical amplifier which amplifies the signal light transmitted by the optical transmitter in an optical transmission line as light to be amplified, and an optical receiver which receives the signal light amplified by the distributed optical amplifier, wherein the distributed optical amplifier includes a pump light generator which generates pump light for optically exciting the optical transmission line; and a multiplexer which multiplexes n kinds of light to be amplified and the pump light output from the pump light generator, and outputs the multiplexed light to the optical transmission line, and wherein, when a center wavelength of the n kinds of light to be amplified is denoted as λS1 to λSn and a zero-dispersion wavelength of the optical transmission line is denoted as λ0, intensity of the pump light generated by the pump light generator at n kinds of wavelengths ranging from 2×λ0−λS1 to 2×λ0−λSn is suppressed in comparison to intensity of other wavelengths of the pump light generated by the pump light generator.
An aspect of the present invention is a distributed optical amplifier system including an optical transmitter which transmits n kinds (n is a positive integer) of signal light, a distributed optical amplifier which amplifies the signal light transmitted by the optical transmitter in an optical transmission line as light to be amplified, and an optical receiver which receives the signal light amplified by the distributed optical amplifier, wherein the distributed optical amplifier includes a pump light generator which generates pump light for optically exciting an optical transmission line; and a multiplexer which multiplexes n kinds of light to be amplified and the pump light output from the pump light generator, and outputs the multiplexed light to the optical transmission line, wherein the optical transmission line from the distributed optical amplifier to the optical receiver is configured by connecting a plurality of optical waveguides having different wavelength dispersions in columns, and wherein, when a center wavelength of the n kinds of light to be amplified is denoted as λS1 to λSn and a zero-dispersion wavelength of the optical waveguide at which intensity of the propagating pump light is the highest among the plurality of optical waveguides is denoted as λ0in, intensity of the pump light at n kinds of wavelengths ranging from 2×λ0in−λS1 to 2×λ0in−λSn is suppressed in comparison to intensity of other wavelengths of the pump light generated by the pump light generator.
An aspect of the present invention is a distributed optical amplification method including: generating pump light for optically exciting an optical transmission line; and multiplexing n kinds (n is a positive integer) of light to be amplified and the pump light output, and outputting the multiplexed light to the optical transmission line, wherein, when a center wavelength of the n kinds of light to be amplified is denoted as λS1 to λSn and a zero-dispersion wavelength of the optical transmission line is denoted as λ0, intensity of the pump light generated at n kinds of wavelengths ranging from 2×λ0−λS1 to 2×λ0−λSn is suppressed in comparison to intensity of other wavelengths of the pump light generated.
According to the present invention, it is possible to provide a technology that can suppress quality degradation of transmitted signals associated with the distributed optical amplifier.
An embodiment of the present invention will be described in detail with reference to the diagrams.
In the present embodiment, a plurality of Configuration Examples of a distributed optical amplifier system will be described, prior to that, content common to the respective Configuration Examples will be described.
In the distributed optical amplifier system of this embodiment, an amplification action by Raman amplification is generated in the optical transmission line. In many cases, the group velocity of the optical transmission line is symmetrical with respect to the zero-dispersion wavelength. If νp and νs have values close to each other as shown in the graph of
λS−λ0=λ0−λp can be rewritten as λp=2×λ0−λS . . . (1). Therefore, the pump light that satisfies equation (1) should be prohibited, or if the optical spectrum of the pump light is wide, the wavelength range including the wavelength λp that satisfies equation (1) should be cut off with a filter.
Next, the range of the wavelength region to be cut off will be described. The problem of how much the wavelength region should be cut off comes down to a problem of how much the group velocity νs of the signal light and the group velocity νp of the pump light should differ to suppress the signal quality degradation due to the noise transfer from the pump light to the signal light to a negligible level.
However, the amount of noise in the pump light before being transmitted to the signal light largely depends on the design of the pump light source, and the penalty due to noise transfer to the signal light depends on a signal baud rate and a signal format. Therefore, it cannot be said unconditionally how much difference between νp and νs is acceptable as a transmission system.
Therefore, first, the results of calculation of how many noise components of the pump light are transferred to the signal light are shown to make an allowable measure.
In the graph, three curves A, B and C are shown. Curve A shows the transfer function of a case of |1−νs/νp|=0.00001. Curve B shows the transfer function of a case of |1−νs/νp|=0.0001. Curve C shows the transfer function of a case of |1−νs/νp|=0.001.
As shown in these curves A, B, and C, if |1−νs/νp| is greater than 0.0001, it is understood that the noise transfer is rapidly reduced. Therefore, |1−νs/νp|>0.00001 can be used as a standard of design.
Hereinafter, each Configuration Example will be described.
The optical transmitter 40 outputs signal light to the optical receiver 60. A carrier wavelength of this signal light is denoted as λS. The distributed optical amplifier 10 outputs the signal light from the optical transmitter 40 and pump light to the optical transmission line 50. The optical receiver 60 receives amplified signal light from the optical transmission line 50.
The distributed optical amplifier 10 is made up of a pump light generator 20 and a multiplexer 30. The pump light generator 20 generates pump light for optically exciting the optical transmission line. The pump light generator 20 is made up of a pump light source 21 for forward pumping and an optical notch filter 22. The pump light source 21 for forward pumping outputs the pump light having a wide spectrum width. This wide spectrum can be generated from white noise. As the pump light source 21 for forward pumping, a multi-mode laser which oscillates a large number of longitudinal modes in a wide wavelength region may be used as the pump light source.
In the pump light output from the pump light source 21 for forward pumping, a part of spectrum is cut off by an optical notch filter 22. The multiplexer 30 multiplexes the pump light output from the pump light generator 20 and outputs the result to the optical transmission line 50. That is, the optical output of the optical notch filter 22 is multiplexed with the signal light by the multiplexer 30 and is made incident on the optical transmission line 50.
The optical notch filter 22 cuts off light of a wavelength in a wavelength region including 2×λ0−λS. That is, the intensity of the pump light having a wavelength of 2×λ0−λS among the pump light generated by the pump light generator 20 is suppressed in comparison with the intensity of the other wavelengths of the pump light generated by the pump light generator 20. Thus, since the worst condition that νp and νs coincide with each other can be avoided, the quality degradation of the transmitted signal can be suppressed. Since the gain band of the Raman amplification is wide, the Raman amplification can be performed even if a part of the pump light is cut off. Here, as described above, the wavelength region to be cut off is desirably set to a value that satisfies |1−νs/νp|>0.00001.
The first optical transmitter 40-1 outputs signal light. The carrier wavelength of an optical signal is denoted as λS1. The second optical transmitter 40-2 outputs signal light. A carrier wavelength of this signal light is denoted as λS2. The signal light multiplexer 70 multiplexes the signal light output from the first optical transmitter 40-1 and the signal light output from the second optical transmitter 40-2. The distributed optical amplifier 10 outputs the signal light from the signal light multiplexer 70 and pump light to the optical transmission line 50. The optical receiver 60 receives amplified signal light from the optical transmission line 50.
The distributed optical amplifier 10 is made up of an pump light generator 20 and a multiplexer 30. The pump light generator 20 generates pump light for optically exciting the optical transmission line. The pump light generator 20 is made up of an pump light source 21 for forward pumping, a first optical notch filter 22-1, and a second optical notch filter 22-2. The pump light source 21 for forward pumping outputs pump light having a wide spectrum width. As described above, this wide spectrum can be generated from white noise. As the pump light source 21 for forward pumping, a multi-mode laser which oscillates a large number of longitudinal modes in a wide wavelength region may be used as the pump light source.
In the pump light output from the pump light source 21 for forward pumping, a part of spectrum is cut off by the first optical notch filter 22-1 and the second optical notch filter 22-2. The multiplexer 30 multiplexes the light to be amplified and the pump light output from the pump light generator 20, and outputs it to the optical transmission line 50. That is, the optical output of the optical notch filter 22 is multiplexed with the signal light by the multiplexer 30, and is made incident on the optical transmission line 50.
As described above, the difference between Configuration Example 1 and Configuration Example 2 is that two optical transmitters of the first optical transmitter 40-1 and the second optical transmitter 40-2 are used, signal lights with different wavelengths output from these optical transmitters are multiplexed by the signal light multiplexer 70 and then multiplexed with the pump light by the distributed optical amplifier 10.
Further, the difference between the Configuration Example 1 and the Configuration Example 2 is that two filters of the first optical notch filter 22-1 and the second optical notch filter 22-2 are provided. The first optical notch filter 22-1 cuts off light of a wavelength in a wavelength region including 2×λ0−λS1. The second optical notch filter 22-2 cuts off light of a wavelength in a wavelength region including 2×λ0−λS2. λ0 represents the zero-dispersion wavelength of the optical transmission line 50 as described above. λS represents the wavelength of the signal light as described above.
As described above, the pump light generator 20 in Configuration Example 2 is made up of the pump light source 21 for forward pumping and two optical notch filters (a first optical notch filter 22-1, and a second optical notch filter 22-2), and the light output from the pump light source 21 for forward pumping includes two kinds of cut-off light bands ranging from 2×λ0−λS1 to 2×λ0−λS2, and the two optical notch filters cut off light of two kinds of wavelengths ranging from 2×λ0−λS1 to 2×λ0−λS2 among light output from the pump light source 21 for forward pumping.
That is, Configuration Example 2 is a configuration example of “the pump light generator includes a light source, and n or less optical notch filter. The spectrum of the light output from the light source includes one or more of n kinds of wavelengths represented by 2×λ0−λS1 to 2×λ0−λSn. And the optical notch filters of the number equal to or less than n cut off all wavelengths corresponding to n kinds of wavelengths represented by 2×λ0−λS1 to 2×λ0−λSn from the spectrum of the light output from the light source”, where n=2 and the number of optical notch filters is two.
In Configuration Example 2, a configuration using two optical transmitters was described. However, the number of optical transmitters in an actual wavelength multiplex transmission system is much more than 2, and many signal lights from these optical transmitters are often transmitted in a dense wavelength arrangement. Therefore, a configuration that uses n (n is a positive integer) optical transmitters will be described as a modified example of Configuration Example 2.
A k-th optical transmitter 40-k (k=1 to n) outputs signal light. The carrier wavelength of an optical signal is denoted as λSk. A signal light multiplexer 70 multiplexes the signal light output from the first optical transmitter 40-1, the signal light output from the second optical transmitter 40-2, . . . , and the signal light output from the n-th optical transmitter 40-n. The distributed optical amplifier 10 outputs the signal light from the signal light multiplexer 70 and pump light to the optical transmission line 50. The optical receiver 60 receives amplified signal light from the optical transmission line 50.
The distributed optical amplifier 10 is made up of a pump light generator 20 and a multiplexer 30. The pump light generator 20 generates pump light for optically exciting the optical transmission line. The pump light generator 20 is made up of a pump light source 21 for forward pumping and an optical notch filter 22. That is, the pump light generator 20 is made up of an pump light source 21 for forward pumping and optical notch filter 22 of number (one) smaller than n. The pump light source 21 for forward pumping outputs pump light having a wide spectrum width. As described above, this wide spectrum can be generated from white noise. As the pump light source 21 for forward pumping, a multi-mode laser which oscillates a large number of longitudinal modes in a wide wavelength region may be used as the pump light source.
In the pump light output from the pump light source 21 for forward pumping, a part of spectrum is cut off by the optical notch filter 22. The optical output of the optical notch filter 22 is multiplexed with the signal light by the multiplexer 30 and made incident on the optical transmission line 50. In the modified example, the pump light generator 20 is made up of a pump light source 21 for forward pumping, and an optical notch filter 22 of number (one) smaller than n.
Thus, the distributed optical amplifier system 100 includes the distributed optical amplifier 10 which amplifies n kinds of light to be amplified in the optical transmission line. The multiplexer 30 multiplexes the n kinds of light to be amplified and the pump light output from the pump light generator 20 and outputs the multiplexed light to the optical transmission line.
Although the optical notch filter corresponding to each optical transmitter was provided in Configuration Example 2, one optical notch filter 22 is provided in the modified example. Although the optical notch filter may be provided for every 2×λ0−λSk (k=1 to n) as in Configuration Example 2, an optical notch filter 22 for cutting off light of a wavelength in a single wavelength region including any of 2×λ0−λSk (k=1 to n) is used in the modified example.
Attention should be paid to the configuration in which the light is cut off collectively in that the wavelength region to be cut off is not always located at the center of the optical spectrum of the pump light. In the case of an optical fiber, although the Raman gain becomes the highest at λS−λp=100 nm, since this relational expression is not directly related to λ0, in order to obtain the maximum Raman gain, a configuration in which the region of the cut-off wavelength in
Further, as a result of selecting the wavelength arrangement for obtaining the maximum Raman gain, the pump light may not exist on the longer wavelength side than 2×λ0−λSm or on the shorter wavelength side than 2×λ0−λSm with m being a positive integer equal to or less than n. In such a case, it is also possible to narrow the cut-off region of the optical notch filter, for example, so that the cutoff region should not include wavelengths where no pump light is present.
The pump light generator 20 may be constituted by an optical notch filter of number smaller than n (n≥3). For example, two filters are provided, and one optical notch filter cuts off r kinds of wavelengths of k=1 to r, in 2×λ0−λSk (k=1 to n). The other optical notch filter cuts off n−r kinds of wavelengths of k=r+1 to n, in 2×λ0−λSk (k=1 to n). In this way, at least one of the band optical notch filter may be configured to simultaneously cut off two or more among n kinds of wavelengths ranging from 2×λ0−λS1 to 2×λ0−λSn.
As shown in
The optical transmitter 40 transmits signal light. The carrier wavelength of the optical signal is denoted as λS. The distributed optical amplifier 10 outputs the signal light from the optical transmitter 40 and pump light to the optical transmission line 50. The optical receiver 60 receives amplified signal light from the optical transmission line 50.
The distributed optical amplifier 10 is made up of an pump light generator 20 and a multiplexer 30. The pump light generator 20 is made up of a first pump light source 21-1 for forward pumping, a pump light source 21-2 for forward pumping, and a pump light multiplexer 24.
The first pump light source 21-1 for forward pumping outputs pump light to the pump light multiplexer 24. The center wavelength of the pump light is denoted as λp1. The second pump light source 21-2 for forward pumping outputs pump light to the pump light multiplexer 24. The center wavelength of the pump light is denoted as λp2. In Configuration Example 3 as well, λp1 and λp2 are selected to satisfy λp2<2×λ0−λS<λp1 to prevent light having a wavelength in the wavelength region including 2×λ0−λS from being multiplexed. Here, λ0 represents the zero-dispersion wavelength of the optical transmission line 50 as described above. λS represents the wavelength of the signal light as described above.
The pump light multiplexer 24 multiplexes the pump light output from the first pump light source 21-1 for forward pumping and the pump light output from the second pump light source 21-2 for forward pumping, and outputs them to the multiplexer 30. The multiplexer 30 multiplexes the pump light and the signal light output from the pump light multiplexer 24, and outputs them to the optical transmission line 50.
As described above, in Configuration Example 3, the pump light generator 20 is made up of a plurality of light sources (the first pump light source 21-1 for forward pumping and the second pump light source 21-2 for forward pumping), and the light output from the plurality of light sources does not include light of one kind of wavelength of 2×λ0−λS. Further, as shown in
As shown in
The optical transmitter 40 outputs signal light. The distributed optical amplifier 10 outputs the signal light from the optical transmitter 40 and pump light to the first optical waveguide 50-1. A second optical waveguide 50-2 is connected to a rear stage of the first optical waveguide 50-1, and the second optical waveguide 50-2 is connected to the optical receiver 60. In this way, the optical transmission line is constituted by connecting a plurality of optical waveguides in columns.
The optical receiver 60 receives the signal light from the second optical waveguide 50-2. The distributed optical amplifier 10 is made up of a pump generator 20 and a multiplexer 30. The pump light generator 20 is made up of a pump light source 21 for forward pumping.
Since the first optical waveguide 50-1 is closer to the multiplexer 30 than the second optical waveguide 50-2, pump light having intensity higher than that of the second optical waveguide 50-2 propagates. In Configuration Example 4, wavelength dispersion is selected so that νp and νs have sufficiently different values in the first optical waveguide 50-1. In order to satisfy this condition, when the zero-dispersion wavelength in the first optical waveguide 50-1 is denoted as λ0in, the pump light may be set so as not to have a wavelength component of 2×λ0in−λS. More generally, when the center wavelength of the light to be amplified is λS1 to λSn, the pump light may not include n kinds of wavelengths ranging from 2×λ0in−λS1 to 2×λ0in−λSn.
Since the intensity of the pump light is attenuated in the process of propagating through the transmission line and the Raman gain viewed locally is also reduced, even if νp and νs in the second optical waveguide 50-2 have close values, since the noise transfer from the pump light to the signal light can be suppressed, the quality degradation of the transmitted signal can be suppressed.
In each of the above-described embodiments, the configuration in which the pump light amplifies the signal light has been described. However, in a system for performing high-order Raman amplification, a multi-stage configuration may be adopted in which the first pump light amplifies the second pump light and the second pump light amplifies the signal light, using the first pump light and the second pump light whose wavelengths are separated by 100 nm. In such a configuration, the same configuration as in each embodiment can be used, after the group velocity of the second pump light is replaced by νs and the wavelength of the second pump light is replaced by λS. At this time, all of the first pump light, the second pump light, and the signal light may be propagated in the same direction, or only one of the three kinds may be propagated in the opposite direction to the other two, using the backward pumping.
In each embodiment, the optical amplification using Raman amplification has been described. However, as long as the distributed optical amplifier amplifies light of another wavelength by the pump light in the optical transmission line, it is not limited to Raman amplification, but another nonlinear optical effect may be used.
Although the embodiment of the present invention has been described in detail with reference to the drawings, a specific configuration is not limited to this embodiment, and design within the scope of the gist of the present invention, and the like are included.
The present invention is applicable to a distributed optical amplifier system that performs transmission through an optical fiber transmission line.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2021/037046 | Oct 2021 | WO |
| Child | 18627950 | US |
