Optical communication system, optical transmitter, and optical receiver

Abstract

To relax the limitation of a transmission distance when transmitting a QKD optical signal in a wavelength multiplexing optical communication system.SOLUTION: An optical communication system includes an optical transmission line that does not include an optical amplifier, an optical transmitter that transmits a wavelength-multiplexed optical signal including one or more first optical signals of a first wavelength band and a second optical signal of a second wavelength band to the optical transmission line, and an optical receiver that performs wavelength demultiplexing on the wavelength-multiplexed optical signal received from the optical transmission line. An optical receiving device, configured to output one or more first optical signals of a first waveband and a second optical signal of a second waveband, where the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for quantum key distribution, wavelengths of the first waveband are shorter than wavelengths of the second waveband, a loss of the first waveband is greater than a loss of the second waveband, and a frequency difference between the one or more first optical signals and the second optical signal is greater than or equal to 15THz.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical communication systems. [Background technology]

[0002] Non-Patent Document 1 discloses an optical communication system that uses the O-band (1260 nm to 1360 nm) for quantum key distribution (QKD) and the C-band (1530 nm to 1565 nm) for data transmission. The power of the optical signal transmitted for QKD is kept low to prevent eavesdropping. In the following description, an optical signal for QKD will be referred to as a "QKD optical signal," an optical signal for controlling QKD will be referred to as a "control optical signal," and an optical signal unrelated to QKD will be referred to as a "general optical signal." A restriction is imposed on the QKD optical signal such that it cannot be optically amplified in the optical transmission path.

[0003] In optical communications, Raman scattering can cause noise in optical signals on the longer wavelength (lower frequency) side of the optical signal. When QKD optical signals and general optical signals are wavelength-multiplexed and transmitted, as disclosed in Non-Patent Document 1, by using the C band for transmitting the general optical signals and the O band for transmitting the QKD optical signals, it is possible to prevent noise caused by Raman scattering in the general optical signals from affecting the transmission of the QKD optical signals. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] P.Gavignet et al., “Co-Propagation of QKD & 6 Tb / s(60×100G) DWDM Channels With ~17 dBm Total WDM Power in Single and Multi-Span Configurations”, in Journal of Lightwave Technology, vol.42, no.4, pp.1321-1327, February 15, 2024 Summary of the Invention [Problem to be solved by the invention]

[0005] However, because the O-band has higher loss than the C-band, using the O-band to transmit QKD optical signals limits the transmission distance of optical communication systems due to the low transmission power of the QKD optical signals and the high loss of the O-band. Using the C-band to transmit QKD optical signals and the O-band, which is on the shorter wavelength side (higher frequency side) than the C-band, to transmit general optical signals can alleviate the transmission distance limitation, but the optical noise generated by Raman scattering of the general optical signals can still limit the transmission distance of QKD optical signals.

[0006] The present disclosure provides a technique for alleviating limitations on transmission distance when transmitting QKD optical signals in a wavelength-multiplexed optical communication system. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, an optical communication system includes an optical transmission line that does not have an optical amplifier, an optical transmitting device that transmits a wavelength-multiplexed optical signal to the optical transmission line, the wavelength-multiplexed optical signal including one or more first optical signals in a first wavelength band and a second optical signal in a second wavelength band, and an optical receiving device that performs wavelength demultiplexing of the wavelength-multiplexed optical signal received from the optical transmission line, and outputs the one or more first optical signals in the first wavelength band and the second optical signal in the second wavelength band, wherein the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band, loss in the first wavelength band is greater than loss in the second wavelength band, and a frequency difference between the one or more first optical signals and the second optical signal is 15 THz or more. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to alleviate the limitations on the transmission distance when transmitting a QKD optical signal in a wavelength-multiplexed optical communication system. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of an optical communication system according to one embodiment. [Figure 2] 1 is a schematic diagram of an optical communication system according to one embodiment. [Figure 3] FIG. 3 is a diagram illustrating the relationship between pump light, a first optical signal, and a second optical signal. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. Furthermore, the same reference numerals are used for the same or similar components, and redundant explanations will be omitted.

[0011] First Embodiment 1 is a configuration diagram of an optical communication system according to this embodiment. The optical communication system includes an optical transmitting device, an optical receiving device, and an optical transmission path from the optical transmitting device to the optical receiving device. The optical communication system of this embodiment wavelength-multiplexes and transmits one or more first optical signals in a first wavelength band and one or more second optical signals in a second wavelength band.

[0012] In this embodiment, the first wavelength band is used for transmitting general optical signals and control optical signals, and the second wavelength band is used for transmitting QKD optical signals. In the following description, the first wavelength band is referred to as the O-band and the second wavelength band is referred to as the C-band. However, as long as the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band and the loss in the optical transmission path of the first wavelength band is greater than the loss in the optical transmission path of the second wavelength band, this embodiment is not limited to the first wavelength band being the O-band and the second wavelength band being the C-band. For example, the first wavelength band may be a wavelength band that includes the O-band as part thereof. Alternatively, the first wavelength band may be a wavelength band that is a part of the O-band. Furthermore, the second wavelength band may be a wavelength band that includes the C-band as part thereof. Alternatively, the second wavelength band may be a wavelength band that is a part of the C-band.

[0013] The optical transmitting device has an amplifier 11. One or more first optical signals are input to the amplifier 11. The first optical signals are general optical signals, control optical signals, or optical signals obtained by wavelength-multiplexing these. The amplifier 11 amplifies the one or more first optical signals and outputs them to the multiplexing unit 10. The multiplexing unit 10 wavelength-multiplexes one or more first optical signals from the amplifier 11 with one or more second optical signals and outputs a wavelength-multiplexed optical signal to the optical transmission path. The second optical signal is a QKD optical signal. As described above, optical amplification cannot be applied to QKD optical signals, so no optical amplifier is provided in the optical transmission path.

[0014] A wavelength-multiplexed optical signal from an optical transmission line is input to a demultiplexer 20 of an optical receiving device. The demultiplexer 20 demultiplexes the input wavelength-multiplexed optical signal into a first wavelength band (O-band in this example) and a second wavelength band (C-band in this example) and outputs the separated signals. Therefore, the demultiplexer 20 outputs one or more first optical signals and one or more second optical signals. The one or more first optical signals output by the demultiplexer 20 are amplified by an amplifier 21.

[0015] In the optical transmission line, Raman scattering of the first optical signal can generate noise on the longer wavelength side (lower frequency side) than the first optical signal. In particular, when the first wavelength band is the O band, the first wavelength band includes wavelengths where chromatic dispersion is zero, so the noise generated by Raman scattering becomes strong. Therefore, it is necessary to suppress the effect of noise caused by Raman scattering of the first optical signal on the second optical signal (QKD optical signal).

[0016] Here, the noise due to Raman scattering of the first optical signal increases as the transmission power of the first optical signal increases. Therefore, in this embodiment, the transmission power of the first optical signal, i.e., the gain of the amplifier 11, is set so that the noise due to Raman scattering of the first optical signal does not significantly affect the second optical signal.

[0017] Furthermore, noise due to Raman scattering of the first optical signal occurs on the longer wavelength side (lower frequency side) than the first optical signal, but this noise mainly occurs in a range up to approximately 15 THz with respect to the frequency of the first optical signal. Therefore, in this embodiment, the frequency difference between the first optical signal having the longest wavelength among the one or more first optical signals and the second optical signal having the shortest wavelength among the one or more second optical signals is set to 15 THz or more.

[0018] By setting the first optical signal and the second optical signal in this way, it is possible to suppress the influence of noise due to Raman scattering on the QKD optical signal, and to alleviate the restriction on the transmission distance of the QKD optical signal.

[0019] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. FIG. 2 is a configuration diagram of an optical communication system according to this embodiment. As described in the first embodiment, the transmission power of the first optical signal, i.e., the gain of the amplifier 11, is set to a low value so that noise due to Raman scattering of the first optical signal does not significantly affect the second optical signal. For this reason, in this embodiment, one or more first optical signals are Raman amplified in the optical transmission line. In FIG. 2, a coupler 22 is provided between the demultiplexer 20 and the amplifier 21 of the optical receiving device, and pump light generated by a light source 23 is transmitted to the optical transmission line via the coupler 22 and the demultiplexer 20. In other words, this embodiment performs backward pumping. However, it is also possible to provide a coupler between the multiplexer 10 and the amplifier 11 of the optical transmitting device, and transmit pump light generated by the light source of the optical transmitting device to the optical transmission line via the coupler and the multiplexer 10, i.e., perform forward pumping. In addition, in the case of backward pumping, the first optical signal can be amplified so as to compensate for losses in the optical transmission path, which has the advantage that the noise level due to Raman scattering of the first optical signal can be reduced compared to the case of forward pumping.

[0020] In Raman amplification, optical signals that are primarily amplified are on the longer wavelength side (lower frequency side) than the pump light and in the range of up to 15 THz relative to the frequency of the pump light. For this reason, as shown in Fig. 3(A), a configuration can be adopted in which pump light 31 is generated by light source 23, which has a shorter wavelength side than one or more first optical signals and a frequency difference from the first optical signal with the longest wavelength among the one or more first optical signals is within 15 THz. In this case, all of the one or more first optical signals can be amplified.

[0021] Furthermore, the loss of one or more first optical signals in the first wavelength band is not constant, and the shorter the wavelength (the higher the frequency), the greater the loss. Therefore, as shown in FIG. 3B, a configuration can be adopted in which the light source 23 generates pump light 31 that is shorter in wavelength than one or more first optical signals, has a frequency difference of 15 THz or less with some of the one or more first optical signals, and has a frequency difference of 15 THz or more with the remaining first optical signals. In this case, the some of the one or more first optical signals can be amplified. Note that the some of the first optical signals to be amplified are determined according to the loss of each wavelength in the first wavelength band.

[0022] As described above, in this embodiment, by performing Raman amplification in the optical transmission line, the gain of the amplifier 11 can be made smaller than in the configuration of the first embodiment. Therefore, the gain of the amplifier 11 can be set to a value that makes it possible to ignore noise caused by Raman scattering of the first optical signal in the optical transmission line. Therefore, the effect of noise caused by Raman scattering on the QKD optical signal can be suppressed, and restrictions on the transmission distance of the QKD optical signal can be relaxed.

[0023] 3, a band of 15 THz or more is secured between the first optical signal and the second optical signal as a region where no optical signal is placed, as in the first embodiment. However, in this embodiment, the gain of amplifier 11 can be set to a value where noise caused by Raman scattering of the first optical signal can be ignored due to Raman amplification in the optical transmission line, so it is not necessary to secure a band of 15 THz or more between the first optical signal and the second optical signal.

[0024] This configuration can alleviate the limitations on transmission distance when transmitting QKD optical signals in wavelength-multiplexed optical communication systems, thereby contributing to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which is to "Build resilient infrastructure, promote sustainable industrialization, and foster innovation." [Explanation of symbols]

[0025] 10: Multiplexing section, 20: Separation section, 11, 21: Amplifiers

Claims

1. an optical transmission line that does not have an optical amplifier; an optical transmitting device that transmits a wavelength multiplexed optical signal including one or more first optical signals in a first wavelength band and a second optical signal in a second wavelength band to the optical transmission line; an optical receiving device that performs wavelength demultiplexing of the wavelength-multiplexed optical signal received from the optical transmission line, and outputs the one or more first optical signals in the first wavelength band and the second optical signal in the second wavelength band; Equipped with the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for the quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band; a loss in the first wavelength band is greater than a loss in the second wavelength band; An optical communication system, wherein the frequency difference between the one or more first optical signals and the second optical signal is 15 THz or more.

2. an optical transmission line that does not have an optical amplifier; an optical transmitting device that transmits a wavelength multiplexed optical signal including one or more first optical signals in a first wavelength band and a second optical signal in a second wavelength band to the optical transmission line; an optical receiving device that performs wavelength demultiplexing of the wavelength-multiplexed optical signal received from the optical transmission line, and outputs the one or more first optical signals in the first wavelength band and the second optical signal in the second wavelength band; Equipped with the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for the quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band; a loss in the first wavelength band is greater than a loss in the second wavelength band; The optical receiving device or the optical transmitting device further transmits pump light to the optical transmission line for Raman amplifying at least one of the one or more first optical signals in the optical transmission line.

3. a wavelength of the pump light is shorter than a wavelength of the one or more first optical signals; The optical communication system according to claim 2 , wherein a frequency difference between the frequency of the pump light and the frequency of the one or more first optical signals is less than 15 THz.

4. the one or more first optical signals are a plurality of first optical signals; the wavelength of the pumping light is shorter than the wavelengths of the plurality of first optical signals; 3. The optical communication system according to claim 2, wherein a frequency difference between the pump light and some of the first optical signals among the plurality of first optical signals is smaller than 15 THz, and a frequency difference between the pump light and the remaining first optical signals among the plurality of first optical signals is 15 THz or more.

5. 3. The optical communication system according to claim 2, wherein the frequency difference between the one or more first optical signals and the second optical signal is 15 THz or more.

6. The optical communication system according to claim 1 , wherein the first wavelength band includes wavelengths at which chromatic dispersion is zero.

7. The optical communication system according to claim 1 , wherein the first wavelength band includes an O-band.

8. The optical communication system according to claim 1 , wherein the second wavelength band includes a C-band.

9. The optical communication system according to claim 1 , wherein the optical transmitter comprises an amplifier that amplifies the one or more first optical signals.

10. The optical communication system according to claim 1 , wherein the one or more first optical signals are optical signals for controlling the quantum key distribution or optical signals unrelated to the quantum key distribution.

11. a multiplexing unit that wavelength-multiplexes one or more first optical signals in a first wavelength band and a second optical signal in a second wavelength band to generate a wavelength-multiplexed optical signal to be transmitted to an optical transmission line that does not have an optical amplifier; the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for the quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band; a loss in the first wavelength band is greater than a loss in the second wavelength band; an optical transmitting device, wherein the frequency difference between the one or more first optical signals and the second optical signal is 15 THz or more;

12. a multiplexing means for wavelength-multiplexing one or more first optical signals in a first wavelength band with a second optical signal in a second wavelength band to generate a wavelength-multiplexed optical signal to be transmitted to an optical transmission line that does not have an optical amplifier; a light source that generates pump light for Raman amplifying at least one of the one or more first optical signals in the optical transmission line; Equipped with the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for the quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band; An optical transmission device, wherein the loss in the first wavelength band is greater than the loss in the second wavelength band.

13. a demultiplexing unit that performs wavelength demultiplexing of a wavelength-multiplexed signal in which one or more first optical signals in a first wavelength band and a second optical signal in a second wavelength band are wavelength-multiplexed, the wavelength-multiplexed signal being received from an optical transmission line that does not have an optical amplifier, and outputs the one or more first optical signals in the first wavelength band and the second optical signal in the second wavelength band, the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for the quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band; a loss in the first wavelength band is greater than a loss in the second wavelength band; an optical receiving device, wherein the frequency difference between the one or more first optical signals and the second optical signal is 15 THz or more;

14. a demultiplexing unit that performs wavelength demultiplexing of a wavelength-multiplexed signal in which one or more first optical signals in a first wavelength band and a second optical signal in a second wavelength band are wavelength-multiplexed, the wavelength-multiplexed signal being received from an optical transmission line that does not have an optical amplifier, and outputs the one or more first optical signals in the first wavelength band and the second optical signal in the second wavelength band; a light source that generates pump light for Raman amplifying at least one of the one or more first optical signals in the optical transmission line; Equipped with the second optical signal is an optical signal for quantum key distribution, the one or more first optical signals are not optical signals for the quantum key distribution, the wavelength of the first wavelength band is shorter than the wavelength of the second wavelength band; An optical receiving device, wherein the loss in the first wavelength band is greater than the loss in the second wavelength band.

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