Optical phase conjugator and optical communication system

Abstract

An optical phase conjugator and an optical communication system capable of long-distance transmission with low power consumption are provided. [Solution] The optical phase conjugator 3 includes a pump light source 33 optically coupled to a nonlinear optical element 31, to which a first signal light Sin is input and which outputs a second signal light Sout that has undergone phase conjugation. The optical filter 32 is a phase conjugator that selectively transmits the second signal light. The first and second optical fibers 2 and 5 that transmit the first and second signal lights are each single-mode optical fibers. The first signal light has multiple channels within its wavelength band, and each channel contains pulse-amplitude-modulated signal light. When the frequency interval Δfch between adjacent channels satisfies 50 (GHz)≦Δfch≦76.25 (GHz), the wavelength λp of the pump light and the wavelength λs of the channel closest to the wavelength λp within the wavelength band of the first signal light satisfy 5 (nm)≦|λp-λs|≦10 (nm).

Description

[Technical Field]

[0001] The present disclosure relates to an optical phase conjugator and an optical communication system. [Background technology]

[0002] Non-Patent Documents 1 and 2 disclose optical communication systems using optical phase conjugate transmission. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Satoshi Yoshima et al., “Mitigation of Nonlinear Effects on WDM QAM Signals Enabled by Optical PhaseConjugation With Efficient Bandwidth Utilization”, Journal of LightwaveTechnology Vol. 35, Issue 4, pp.971-978 (2017) [Non-patent document 2] KRH Bottrill et al., “PAM4transmission over 360 km of fiber using optical phase conjugation”, OSAContinuum, Vol.2, pp.973-982, No.3 / 15 March (2019) Summary of the Invention [Problem to be solved by the invention]

[0004] In optical communications between data centers and the like, extension of transmission distances and reduction of power consumption are expected. The present disclosure provides an optical phase conjugator and an optical communications system capable of long-distance transmission with low power consumption. [Means for solving the problem]

[0005] An optical phase conjugator according to a first disclosure comprises a nonlinear optical element, a pump light source optically coupled to the nonlinear optical element, an optical input end for inputting first signal light to the nonlinear optical element, and an optical output end for outputting second signal light phase conjugated by the nonlinear optical element via an optical filter that selectively transmits second signal light, wherein the first optical fiber transmitting the first signal light and the second optical fiber transmitting the second signal light are each single-mode optical fibers, the first signal light has a plurality of channels within its wavelength band, and includes signal light that is pulse amplitude modulated in each channel, and the frequency interval Δfch between adjacent channels satisfies 50 (GHz)≦Δfch≦76.25 (GHz), and the wavelength λp of the pump light output from the pump light source satisfies the relationship 5 (nm)≦|λp-λs|≦10 (nm) with respect to the wavelength λs that is closest to the wavelength λp within the wavelength band of the first signal light.

[0006] An optical phase conjugator according to a second disclosure comprises a nonlinear optical element, a pump light source optically coupled to the nonlinear optical element, an optical input end for inputting first signal light to the nonlinear optical element, and an optical output end for outputting second signal light phase conjugated by the nonlinear optical element via an optical filter that selectively transmits second signal light, wherein the first optical fiber transmitting the first signal light and the second optical fiber transmitting the second signal light are each dispersion-flattened optical fibers, the first signal light has a plurality of channels within its wavelength band, and includes signal light that is pulse amplitude modulated in each channel, and the frequency interval Δfch between adjacent channels satisfies 100 (GHz)≦Δfch≦300 (GHz), and the wavelength λp of the pump light output from the pump light source satisfies the relationship 11 (nm)≦|λp-λs|≦12.5 (nm) with respect to the wavelength λs that is closest to the wavelength λp within the wavelength band of the first signal light.

[0007] The optical communication system of the present disclosure includes an optical phase conjugator, a transmitter that transmits the first signal light to the first optical fiber, and a receiver that receives the second signal light output from the second optical fiber. [Effects of the Invention]

[0008] The optical phase conjugator and optical communication system according to the present disclosure enable long-distance transmission with low power consumption. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing the overall configuration of an optical communication system. [Figure 2] FIG. 2 is a diagram illustrating the structure of an example of a transmitter. [Figure 3] FIG. 3 is a diagram showing an optical phase conjugator according to a first example. [Figure 4] FIG. 4 is a graph showing the relationship between the frequency (f) and the intensity (I) of the first signal light (Sin), the pump light (SPUMP), and the idler light (SIDLER). [Figure 5] FIG. 5 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a single-mode optical fiber is used. [Figure 6] FIG. 6 is a graph showing the relationship between channel frequency spacing (Δfch (GHz)) and TDECQ (dB) when a single-mode optical fiber is used. [Figure 7] FIG. 7 is a graph showing the relationship between channel frequency spacing (Δfch (GHz)), |λp−λs| (nm), and TDECQ (dB) when a single-mode optical fiber is used, by interpolating data. [Figure 8] FIG. 8 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a single-mode optical fiber is used. [Figure 9] FIG. 9 is a graph showing the relationship between the transmission distance L (km) and the TDECQ (dB) when a single-mode optical fiber is used. [Figure 10] FIG. 10 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a single-mode optical fiber is used. [Figure 11] FIG. 11 is a graph showing the relationship between the transmission distance L (km) and the TDECQ (dB) when a single-mode optical fiber is used. [Figure 12] FIG. 12 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a single-mode optical fiber is used. [Figure 13] FIG. 13 is a graph showing the relationship between the transmission distance L (km) and the TDECQ (dB) when a single-mode optical fiber is used. [Figure 14] FIG. 14 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a single-mode optical fiber is used. [Figure 15] FIG. 15 is a graph showing the relationship between the transmission distance L (km) and the TDECQ (dB) when a single-mode optical fiber is used. [Figure 16] FIG. 16 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a dispersion-flattened optical fiber is used. [Figure 17] FIG. 17 is a graph showing the relationship between the transmission distance L (km) and the TDECQ (dB) when a dispersion-flattened optical fiber is used. [Figure 18] FIG. 18 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a dispersion-flattened optical fiber is used. [Figure 19] FIG. 19 is a graph showing the relationship between the transmission distance L (km) and the TDECQ (dB) when a dispersion-flattened optical fiber is used. [Figure 20] FIG. 20 is a diagram showing an optical phase conjugator according to the second example. DETAILED DESCRIPTION OF THE INVENTION

[0010] Various exemplary embodiments will be described in detail below with reference to the drawings. Note that the same or equivalent parts in the drawings will be denoted by the same reference numerals, and redundant explanations will be omitted.

[0011] FIG. 1 is a diagram showing the overall configuration of an optical communication system 100. As shown in FIG.

[0012] The optical communication system 100 includes a transmitter 1, a first optical fiber 2, an optical phase conjugator 3, an optical amplifier 4, a second optical fiber 5, and a receiver 6.

[0013] The transmitter 1 outputs a first signal light. The first signal light is wavelength division multiplexed (WDM) and has signal light of multiple channels (wavelengths). The signal light in each channel is pulse amplitude modulated (PAM). An example of pulse amplitude modulation is PAM4, but modulation methods such as PAM6 and PAM8 can also be applied to this system.

[0014] The first optical fiber 2 is connected to the signal light output terminal of the transmitter 1. The first optical fiber 2 transmits the first signal light (Sin) output from the transmitter 1 and inputs it to the optical phase conjugator 3.

[0015] The optical phase conjugator 3 performs optical phase conjugation on the input first signal light (Sin) to generate a second signal light (Sout). The optical phase conjugator 3 is disposed at a midpoint in the optical transmission path between the transmitter 1 and the receiver 6. The optical phase conjugator 3 performs a conversion that imparts time reversibility to the signal light, so that the group velocity dispersion of the signal light generated in the first half of the optical transmission path is compensated for by the inverse group velocity dispersion that the signal light experiences while propagating in the second half of the optical transmission path.

[0016] The optical amplifier 4 amplifies the second signal light (Sout) output from the optical phase conjugator 3. An optical fiber amplifier can be used as the optical amplifier 4. An optical fiber amplifier is a device that amplifies an optical signal within an optical fiber, and optical fiber amplifiers doped with rare earth elements are particularly used. Laser light from a pumping laser diode is input into the rare-earth doped optical fiber together with the signal light, and the signal light is amplified. Known rare-earth elements that can be doped into optical fibers include Pr (1.3 μm band), Er (1.5 μm band), Tm (1.4 μm band), and Yb (1.0 μm band). When using signal light in the 1.5 μm band in optical communications, Er can be used as the dopant for rare-earth doped optical fiber.

[0017] The second optical fiber 5 is connected to the signal light output terminal of the optical phase conjugator 3 or the optical amplifier 4. The second optical fiber 5 transmits the second signal light (Sout) output from the optical phase conjugator 3 or the optical amplifier 4, and inputs it to the receiver 6.

[0018] The receiver 6 receives the second signal light (Sout) output from the second optical fiber 5. The receiver 6 separates the signal light for each channel (wavelength) included in the second signal light (Sout) using an optical demultiplexer, and detects the signal light for each channel using a photodetector such as a photodiode. Since each signal light is pulse amplitude modulated using a method such as PAM4, the signal output from the photodetector is demodulated to obtain the same digital signal as the sender.

[0019] In the first fiber configuration, the first optical fiber 2 and the second optical fiber 5 are each a single-mode optical fiber (SMF), and in the second fiber configuration, the first optical fiber 2 and the second optical fiber 5 are each a dispersion-flattened optical fiber (DFF).

[0020] FIG. 2 is a diagram showing an example of the structure of the transmitter 1. As shown in FIG.

[0021] The transmitter 1 includes a digital signal processor (DSP) that performs pulse amplitude modulation (e.g., PAM4) on input digital signals (D1, D2, D3, D4). In this example, there are four digital signals (D1, D2, D3, D4) (four channels), and each digital signal may be a non-return-to-zero (NRZ) signal. The digital signals (D1, D2, D3, D4) are pulse amplitude modulated by digital signal processors (11, 12, 13, 14), respectively. Each pulse amplitude modulated signal is input to a laser diode module (LD1, LD2, LD3, LD4). The laser diode modules (LD1, LD2, LD3, LD4) include a laser diode and a laser driver circuit. Each pulse amplitude modulated signal is input to the corresponding laser driver circuit, and a pulse amplitude modulated signal light is output from each laser diode. The wavelengths of the laser beams output from the laser diodes included in the laser diode modules (LD1, LD2, LD3, LD4) are different from one another. The laser beams output from the laser diode modules (LD1, LD2, LD3, LD4) are multiplexed by an optical multiplexer 10 and output toward a first optical fiber 2.

[0022] Pulse amplitude modulation can also be performed on the laser light output from a laser diode module by using a device that modulates the light intensity in response to an external input signal. Such devices include, for example, electro-optic modulators, acousto-optic modulators, microelectromechanical system (MEMS) mirrors, and optical amplifiers. While amplitude modulation can be achieved by directly controlling the intensity of the laser light, it is also possible to achieve amplitude modulation by deflecting the light in response to an external input signal and controlling the amount of light incident on the optical fiber.

[0023] FIG. 3 is a diagram showing an optical phase conjugator 3 according to a first example.

[0024] The optical phase conjugator 3 includes an optical coupler 30, a nonlinear optical element 31, an optical filter 32, a pump light source 33, and a driver circuit .

[0025] The optical coupler 30 has a first optical input terminal IN to which a first signal light (Sin) is input. A and the pump light (S PUMP ) is input to the second optical input terminal IN B The first signal light (Sin) and the pump light (S PUMP ) is input to a nonlinear optical element 31 via an optical coupler 30. The optical coupler 30 divides the first signal light (Sin) and the pump light (S PUMP ) into the nonlinear optical element 31. The pump light may be introduced into the nonlinear optical element 31 without passing through the optical coupler 30. The traveling direction of the pump light may be set to be opposite to the traveling direction of the first signal light.

[0026] The nonlinear optical element 31 in this example is a highly nonlinear fiber (HNLF). The shape of the nonlinear optical element 31 can be other than an optical fiber, but in order to maintain the properties of the signal light, an optical fiber is preferable. The nonlinear coefficient γ of the highly nonlinear fiber is higher than the nonlinear coefficient γ of a standard single-mode optical fiber, and is illustratively N times the nonlinear coefficient γ of the standard single-mode optical fiber (e.g., 5≦N≦15). The nonlinear optical element 31 is a fiber optic cable that couples a first signal light (Sin) and a pump light (S PUMP ) and generates an idler light for the first signal light (Sin).

[0027] The optical filter 32 selectively transmits idler light as second signal light (Sout) from the light output from the nonlinear optical element 31. A second optical fiber 5 is optically coupled to the light output end OUT of the optical filter 32. The second signal light (Sout) is input to the optical amplifier 4 and amplified.

[0028] The pump light source 33 emits pump light (S PUMP ) is generated. PUMP The wavelength λp of the first signal light (Sin) is separated from the wavelength band of the first signal light (Sin) by Δλ=|λp−λs|. The structure of the pump light source 33 may be the same as the structure of the pump light source used in the optical amplifier 4, for example.

[0029] The drive circuit 34 supplies drive power to the pump light source 33. When the intensity of the pump light output from the pump light source 33 is to be constant, the drive circuit 34 outputs constant power. The drive circuit 34 in this example supplies constant power to the pump light source 33.

[0030] FIG. 4 shows the first signal light (Sin), the pump light (S PUMP ), and idler light (S IDLER ) is a graph showing the relationship between frequency (f) and intensity (I). Frequency is correlated with wavelength.

[0031] A case will be described in which single-mode optical fibers are used as the first and second optical fibers. The first signal light (Sin) has a plurality of channels (ch1 to ch4) within its frequency band (wavelength band), and each channel contains pulse amplitude modulated signal light. When a single-mode optical fiber is used, the frequency interval Δfch between adjacent channels (ch1 to ch4) satisfies the following relationship (condition A1). In addition, the pump light (S PUMP ) satisfies the following relationship (condition B1) with respect to the wavelength λs (channel ch1) that is closest to the wavelength λp within the frequency band (wavelength band) of the first signal light (Sin): ·Condition A1: 50(GHz)≦Δfch≦76.25(GHz) ·Condition B1: 5(nm)≦|λp-λs|≦10(nm) In addition, the idler light (S IDLER ) has multiple channels (ch1* to ch4*) within its frequency band (wavelength band), and each channel contains pulse amplitude modulated signal light. When a single-mode optical fiber is used, the frequency interval Δfch* between adjacent channels (ch1* to ch4*) satisfies the same condition as Δfch.

[0032] When using a single-mode optical fiber, the pump light (S PUMP ) wavelength λp is the idler light (SIDLER With respect to the wavelength λs* (channel ch1*) closest to the wavelength λp within the frequency band (wavelength band) of the optical fiber 100 (or 100 MHz), |λp-λs*| satisfies the same condition as |λp-λs|. Note that when the frequency band of the signal light is wider than the frequency of the pump light, the frequency band of the idler light is narrower than the frequency of the pump light.

[0033] In optical phase conjugate transmission, signal light is transmitted through a first optical fiber over a certain distance, and the transmitted signal is converted to phase conjugate light at the midpoint. Optical phase conjugate conversion inverts the sign of the accumulated phase information generated by transmission, inverting the spectrum of the signal light. The optical phase conjugate converted signal is then transmitted using a second optical fiber with the same parameters and length. This spectral inversion transmission can compensate for group velocity dispersion in optical phase conjugate transmission, but it cannot compensate for third-order dispersion by itself.

[0034] Signal light and pump light are simultaneously input into the optical phase conjugator, where cross-phase modulation (XPM) causes the phase of the pump light to change depending on the signal light intensity. This broadens the spectrum of the pump light depending on the bandwidth of the WDM-PAM4 signal light. Furthermore, if the wavelength spacing between the signal light and the pump light is narrow, the broadened pump light component interferes with the signal light, potentially causing distortion in the signal light. Furthermore, if the WDM wavelength channel spacing is narrow, distortion occurs in the signal light due to the effects of inter-channel four-wave mixing (FWM) in the PAM4 signal. This distortion limits the transmission distance and the number of WDM channels that can be transmitted. By satisfying the above-mentioned configuration and conditions, this embodiment significantly improves distortion due to third-order dispersion and nonlinear distortion during optical phase conjugation.

[0035] Furthermore, the modulation method for the optical signal described above is pulse amplitude modulation (PAM), which consumes less power than coherent modulation such as QAM16. Pulse amplitude modulation tends to have a shorter transmission distance compared to coherent modulation, but the inventors of the present application have discovered that even when pulse amplitude modulation is used, the transmission distance L in an optical communication system can be significantly increased by using an optical phase conjugator and ensuring that the parameters (Δfch, |λp-λs|) satisfy the above conditions A1 and B1. This is explained in detail below.

[0036] Figure 5 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance L (km) when using a standard single-mode optical fiber, where L is the total transmission distance between the transmitter and receiver.

[0037] The transmission distance L over which a signal can be transmitted is long if the quality of the optical signal does not deteriorate. The quality of the optical signal correlates with the degree of eye opening in the signal waveform's eye pattern, and for pulse amplitude modulation signals such as PAM4, the Transmitter and Dispersion Eye Closure Quaternary (TDECQ) is used as an evaluation index. In the following explanation, TDECQ represents the worst value among all channels.

[0038] In this example, a 26.6-Gbaud WDM-PAM4 signal with 1 to 16 channels was generated in the transmitter. The generated PAM4 signal was sent to a standard single-mode optical fiber (SSMF), and an optical phase conjugator was placed at the midpoint of the transmission path. Degenerate FWM in a highly nonlinear fiber (HNLF) was used for the optical phase conjugation. The TDECQ measurement method is specified in IEEE802.3bs (IEEE: Institute of Electrical and Electronics Engineers). The smaller the TDECQ value, the higher the quality of the signal light. In optical communication systems, signal light transmission is possible when the TDECQ is 3.3 dB or less. The signal frequency band Δfin of each channel included in the first signal light (Sin) output from the transmitter and input to the optical phase conjugator is 0 GHz < Δfin ≦ 50 GHz. The signal frequency band Δfout of each channel included in the second signal light (Sout) output from the optical phase conjugator is 0 (GHz) < Δfout ≦ 50 (GHz). Note that the influence of optical amplifier noise was ignored when acquiring the data.

[0039] The first optical fiber arranged on the transmitter side and the second optical fiber arranged on the receiver side are both standard single-mode optical fibers. The standard ITU-T G.652 can be adopted for the standard single-mode optical fiber in this example. Note that, with a single-mode optical fiber, results similar to those of the present disclosure can be obtained from the viewpoint of extending the transmission distance. The following parameters can be used to indicate the characteristics of this optical fiber: Group velocity dispersion coefficient β2 = -21.7(ps 2 / km), third-order dispersion coefficient β3 = 0.182 (ps 2 / km), nonlinear coefficient γ = 1.2(W -1 / km), transmission loss α dB =0.2 (dB / km). The characteristics of all optical fibers in the explanation are set in the signal light band (1.55 μm). Single-mode optical fibers with a cladding diameter of 125 μm are made from quartz (silica glass) and are commercially available.

[0040] The highly nonlinear fiber (HNLF) constituting the nonlinear optical element arranged in the optical phase conjugator has a higher nonlinear coefficient γ than the first optical fiber and the second optical fiber. The parameters that indicate the characteristics of the highly nonlinear fiber are the group velocity dispersion coefficient β2=0(ps 2 / km), third-order dispersion coefficient β3 = 0.146 (ps 2 / km), nonlinear coefficient γ=13(W -1 / km), transmission loss α dB =0.24 (dB / km). HNLF with a cladding diameter of 125 μm is made from quartz (silica glass), is commercially available, and can be easily spliced ​​to standard single-mode optical fiber. Note that the amount of germanium and fluorine doping in HNLF can be increased to increase the nonlinearity.

[0041] The pump light is a continuous wave, its wavelength (λp) is 1550 (nm), and the power supplied per channel is 20 (dBm / ch). In Fig. 5, the wavelength (λs) of the first signal light is changed from 1552.5 (nm) to 1560 (nm), and |λp-λs| is changed from 2.5 (nm) to 10 (nm).

[0042] Figure 6 is a graph showing the relationship between channel frequency spacing (Δfch (GHz)) and TDECQ (dB) when using standard single-mode optical fiber (SSMF). This graph was created from the data in Figure 5. Figure 7 is a graph showing the relationship between channel frequency spacing (Δfch (GHz)), |λp - λs| (nm), and TDECQ (dB) when using single-mode optical fiber, using data interpolation. The data interpolation is linear, connecting adjacent data points with line segments and interpolating data obtained by dividing each line segment into 10 equal parts. Data with a TDECQ (dB) of 3.3 (dB) or less is shown as a black circle, and data exceeding this is shown as a white circle.

[0043] The graph shows |λp-λs| values ​​of 10 nm, 7.5 nm, 5.0 nm, and 2.5 nm. Note that when |λp-λs| is 2.5 nm, the TDECQ value falls outside the graph. At least when |λp-λs| is between 5 nm and 10 nm, TDECQ is 3.3 dB or less when Δfch is 50 GHz ≦ Δfch ≦ 75 GHz, ensuring sufficient signal transmission. When the values ​​between the data points shown in Figure 5 are linearly interpolated as shown in Figure 7, TDECQ is 3.3 dB or less when Δfch is 50 GHz ≦ Δfch ≦ 76.25 GHz.

[0044] As is clear from the charts and graphs shown in FIGS. 5 to 7, when the above parameters satisfy the above conditions A1 and B1, the TDECQ is 3.3 dB or less when the transmission distance L is 80 km.

[0045] (TDECQ≦3.0(dB)) By satisfying the following conditions A2 and B2, the TDECQ is 3.0 dB or less when the transmission distance L is 80 km. ·Condition A2: 50(GHz)≦Δfch≦76.25(GHz) ·Condition B2: 5(nm)≦|λp-λs|≦10(nm) This range is a value found from the table shown in FIG. 5, but if the upper limit of Δfch is found by data interpolation, the upper limit can be made the same as condition A1.

[0046] (TDECQ≦2.7(dB)) By satisfying the following conditions A3 and B3, the TDECQ will be 2.7 dB or less when the transmission distance L is 80 km. This range is the value found from the table shown in FIG. ·Condition A3: 50(GHz)≦Δfch≦62.5(GHz) ·Condition B3: 5(nm)≦|λp-λs|≦10(nm)

[0047] When Δfch exceeds the upper limit of condition A1, TDECQ exceeds 3.3 dB, and the transmission distance becomes shorter than 80 km. It is believed that increasing Δfch increases TDECQ due to the effects of third-order dispersion in the optical fiber. Increasing |λp-λs| causes the spread of pump light due to cross-phase modulation (XPM) to interfere with the signal light, degrading the eye opening. A good eye opening was obtained when |λp-λs| = 7.5 nm and Δfch = 50 GHz. Note that signal light with eight or fewer channels can be transmitted with a TDECQ of 3.3 dB or less under the above conditions.

[0048] Figure 8 is a chart showing the relationship between various parameters and TDECQ (dB) and transmission distance (km) when a standard single-mode optical fiber is used. Figure 9 is a graph showing the relationship between transmission distance L (km) and TDECQ (dB) when a standard single-mode optical fiber is used. The graph in Figure 9 was created using the data in Figure 8. The wavelength (λp) of the pump light is 1550 (nm), the wavelength (λs) of the first signal light is 1555 (nm), and |λp - λs| is 5 (nm).

[0049] The case shown is where |λp-λs| = 5.0 (nm) and Δfch = 62.5 (GHz). The number of channels in the first signal light is 8, but when the channel numbers are 1st channel Ch.1, 4th channel Ch.4, 5th channel Ch.5, and 8th channel Ch.8, and the transmission distance L is 150 (km) or less, the TDECQ is 3.3 (dB) or less, and sufficient communication can be performed.

[0050] Fig. 10 is a chart showing the relationship between various parameters and TDECQ (dB) and transmission distance (km) when a standard single-mode optical fiber is used. Fig. 11 is a graph showing the relationship between transmission distance L (km) and TDECQ (dB) when a standard single-mode optical fiber is used. The graph in Fig. 11 was created using the data in Figs. 8 and 10. The wavelength (λp) of the pump light is 1550 (nm), the wavelength (λs) of the first signal light is 1555 (nm) to 1557.5 (nm), and |λp - λs| is 5 (nm) to 7.5 (nm). The number of channels in the data shown in Fig. 11 is 8. In Fig. 11, the transmission distance L for each wavelength allocation is as follows:

[0051] Wavelength allocation-1 shows the case where |λp-λs| = 5.0 (nm) and Δfch = 62.5 (GHz). In wavelength allocation-1, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 120 (km) or more.

[0052] Wavelength allocation-2 shows the case where |λp-λs| = 5.0 (nm) and Δfch = 75 (GHz). In wavelength allocation-2, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 120 (km) or more.

[0053] Wavelength allocation 3 shows the case where |λp-λs| = 7.5 (nm) and Δfch = 62.5 (GHz). In wavelength allocation 3, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 80 (km) or more.

[0054] Wavelength allocation-4 shows the case where |λp-λs| = 7.5 (nm) and Δfch = 75 (GHz). In wavelength allocation-4, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 80 (km) or more.

[0055] Fig. 12 is a chart showing the relationship between various parameters and TDECQ (dB) and transmission distance (km) when a standard single-mode optical fiber is used. Fig. 13 is a graph showing the relationship between transmission distance L (km) and TDECQ (dB) when a standard single-mode optical fiber is used. The graph in Fig. 13 was created using the data in Fig. 12. The wavelength (λp) of the pump light is 1550 (nm), the wavelength (λs) of the first signal light is 1557.5 (nm) to 1560 (nm), and |λp - λs| is 7.5 (nm) to 10 (nm). The number of channels in the data in Fig. 13 is 16. In Fig. 13, the transmission distance L for each wavelength allocation is as follows:

[0056] Wavelength allocation-1 shows the case where |λp-λs| = 7.5 (nm) and Δfch = 50 (GHz). In wavelength allocation-1, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 80 (km) or more.

[0057] Wavelength allocation-2 shows the case where |λp-λs| = 7.5 (nm) and Δfch = 62.5 (GHz). In wavelength allocation-2, the transmission distance L over which optical signals can be transmitted while keeping TDECQ (dB) below 3.3 (dB) is less than 40 (km), but transmission of approximately 20 (km) is possible.

[0058] Wavelength allocation 3 shows the case where |λp-λs| = 10 (nm) and Δfch = 50 (GHz). In wavelength allocation 3, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 40 (km) or more.

[0059] Wavelength allocation-4 shows the case where |λp-λs| = 10 (nm) and Δfch = 62.5 (GHz). In wavelength allocation-4, the transmission distance L that can transmit optical signals while keeping TDECQ (dB) below 3.3 (dB) is 40 (km) or more.

[0060] In 16-channel signal light transmission, when the transmission distance L is 20 km, transmission is possible without degradation of the signal light even under the above conditions. When the TDECQ (dB) is kept to 3.3 dB or less and the transmission distance L is 40 km or more or 80 km or more, it is preferable to satisfy the following conditions A4 and B4. Note that this range was determined by linearly interpolating the data in Figure 12 in the same way as above, to satisfy the condition that TDECQ (dB) is 3.3 dB or less. ·Condition A4: 50(GHz)≦Δfch≦52(GHz) ·Condition B4: 7.5(nm)≦|λp-λs|≦9.4(nm)

[0061] Figure 14 is a chart showing the relationship between various parameters and TDECQ (dB) and transmission distance (km) when a standard single-mode optical fiber is used. Figure 15 is a graph showing the relationship between transmission distance L (km) and TDECQ (dB) when a standard single-mode optical fiber is used. The graph in Figure 15 was created using the data in Figure 14. The wavelength (λp) of the pump light is 1550 (nm), the wavelength (λs) of the first signal light is 1557.5 (nm), and |λp - λs| is 7.5 (nm).

[0062] The cases shown are |λp-λs| = 7.5 (nm) and Δfch = 50 (GHz). In all cases where the number of channels in the first signal light is 4, 8, or 16 (4ch, 8ch, 16ch), the TDECQ is 3.3 (dB) or less when the transmission distance L is 80 (km) or less, ensuring sufficient communication. When using single-mode optical fiber, it is thought that the effects of third-order dispersion will appear as the transmission distance increases, but by satisfying the above conditions, transmission up to 80 (km) is possible even when 16 channels are multiplexed.

[0063] Furthermore, within the range shown in Figure 15, when the number of channels of the first signal light is 8 or less (8ch, 4ch), and the transmission distance L is 120 (km) or less, the TDECQ is 3.3 (dB) or less, and sufficient communication can be performed.Even when the number of channels is 8 or more (8ch, 16ch), and the transmission distance L is 80 (km) or less, the TDECQ is 3.3 (dB) or less, and sufficient communication can be performed.

[0064] Next, a case where dispersion-flattened optical fibers are used as the first and second optical fibers will be described.

[0065] Referring again to FIG. 4, when dispersion-flattened optical fibers are used as the first and second optical fibers, the frequency interval Δfch between adjacent channels (ch1 to ch4) satisfies the following relationship (condition A5): PUMP ) satisfies the following relationship (condition B5) with respect to the wavelength λs (channel ch1) that is closest to the wavelength λp within the wavelength band of the first signal light (Sin): ·Condition A5: 100(GHz)≦Δfch≦Δfmax ·Condition B5: Δλmin(nm)≦|λp-λs|≦Δλmax

[0066] The value of Δfmax is at least 300 (GHz) or more, and since there is no influence of third-order dispersion, the theoretical upper limit is quite high (an exemplary upper limit is 1000 (GHz)). The value of Δλmax is at least 12.5 (nm) or more, and since there is no influence of third-order dispersion, the theoretical upper limit is quite high (an exemplary upper limit is 20 (nm)). The value of Δλmin needs to be at least close to 12.5 (nm), and can be set to 11 (nm) to 12 (nm) taking into account the margin of error. Furthermore, when linearly interpolating the data in FIG. 18, the value of Δλmin is considered to be at least approximately 11 (nm). Therefore, Conditions A5 and B5 at least satisfy 100 (GHz)≦Δfch≦300 (GHz) and 11 (nm)≦|λp-λs|≦12.5 (nm). Condition A5 and condition B5 illustratively satisfy 100 (GHz)≦Δfch≦500 (GHz) and 11 (nm)≦|λp−λs|≦20 (nm).

[0067] In addition, when a dispersion-flattened optical fiber is used, the idler light (S IDLER ) has multiple channels (ch1* to ch4*) within its wavelength band, and each channel contains pulse amplitude modulated signal light. The frequency interval Δfch* between adjacent channels (ch1* to ch4*) satisfies the same conditions as Δfch.

[0068] When a dispersion-flattened optical fiber is used, the pump light (S PUMP ) wavelength λp is the idler light (S IDLER With respect to the wavelength λs* (channel ch1*) closest to the wavelength λp in the wavelength band of |λp−λs|, |λp−λs*| satisfies the same conditions as |λp−λs|.

[0069] The modulation method for the optical signal described above is pulse amplitude modulation (PAM), which consumes less power than coherent modulation such as QAM16. Even when an optical phase conjugator is used and a dispersion-flattened optical fiber is used, the transmission distance L in the optical communication system can be significantly extended if the parameters (Δfch, |λp-λs|) satisfy the above conditions A5 and B5. This is explained in detail below.

[0070] 16 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a dispersion-flattened optical fiber is used. The wavelength (λp) of the pump light is 1550 (nm), the wavelength (λs) of the first signal light is 1562.5 (nm), |λp - λs| is 12.5 (nm), and the number of channels of the first signal light is 16.

[0071] The signal frequency band of each channel of the first signal light (Sin) output from the transmitter and input to the optical phase conjugator and the signal frequency band of each channel of the second signal light (Sout) output from the optical phase conjugator can be set to the same as when the above-mentioned single-mode optical fiber is used.

[0072] The first optical fiber located on the transmitter side and the second optical fiber located on the receiver side are both dispersion-flattened optical fibers. The parameters that indicate the characteristics of this optical fiber are the group velocity dispersion coefficient β2 = -4.0(ps 2 / km), third-order dispersion coefficient β3 = 0.01 (ps 2 / km), nonlinear coefficient γ = 1.2(W -1 / km), transmission loss α dB = 0.2 (dB / km). Dispersion-flattened optical fibers with a cladding diameter of 125 μm are made from quartz (silica glass) and are commercially available.

[0073] In particular, dispersion-flattened optical fiber has a dispersion of 5 to 6 (ps / nm / km) and a dispersion slope of 0.0 (ps / nm 2 / km), effective area A of signal light transmission eff is about 45 (μm 2) and 41 to 46 (μm 2 ) can be achieved. Due to the zero dispersion slope, the zero dispersion wavelength (λ0) is approximately 1325 (nm) and is set between 1300 and 1350 (nm). The PMD (polarization mode dispersion) is 0.02 to 0.03 (ps / km 1 / 2 ) More specifically, in the wavelength band of 1460 nm to 1625 m, the dispersion is 5 to 6 (ps / nm / km), and more specifically, the dispersion fluctuation width is 1 (ps / nm / km). The dispersion fluctuation width in this wavelength band can be set to 0.9 to 1.1 (ps / nm / km), allowing for a 10% error.

[0074] 16, when the above parameters satisfy the above conditions A5 and B5, the transmission distance L at which the TDECQ is 3.3 dB or less is at least 160 km or more, even for 16 channels. In other words, if the number of channels of the first signal light is 8 or less, transmission of at least 160 km or more is possible under the same conditions.

[0075] 17 is a graph showing the relationship between transmission distance L (km) and TDECQ (dB) when a dispersion-flattened optical fiber is used. The graph in FIG. 17 was created from the data in FIG.

[0076] The case shown is one where |λp-λs| = 12.5 (nm) and Δfch = 112.5 (GHz). When the number of channels in the first signal light is 16, and the transmission distance L is at least 200 (km) or less, the TDECQ is 3.3 (dB) or less, and sufficient communication is possible. When the number of channels is less than 16, signal degradation is expected to be even lower. Using a dispersion-flattened optical fiber enables even higher-speed medium-distance transmission of PAM4 signals.

[0077] 18 is a chart showing the relationship between various parameters, TDECQ (dB), and transmission distance (km) when a dispersion-flattened optical fiber is used. The wavelength (λp) of the pump light is 1550 (nm), the wavelength (λs) of the first signal light is 1562.5 (nm), |λp-λs| is 5 (nm) to 12.5 (nm), Δfch is 100 (GHz) to 300 (GHz), and the number of channels of the first signal light is 8.

[0078] When |λp-λs| is 12.5 nm, the transmission distance L at which TDECQ is 3.3 dB or less when Δfch is 100 GHz to 300 GHz is at least 160 km, even for eight channels. Furthermore, for TDECQ to be 3.3 dB or less when the transmission distance L is 80 km, the above-mentioned conditions A5 and B5 must be met. The lower limit of |λp-λs|, Δλmin, can be calculated by linearly interpolating the data shown in FIG. 18. By satisfying at least 100 GHz≦Δfch≦300 GHz and 11 nm≦|λp-λs|≦12.5 nm, TDECQ at 80 km can be kept 3.3 dB or less.

[0079] 19 is a graph showing the relationship between transmission distance L (km) and TDECQ (dB) when a dispersion-flattened optical fiber is used. The graph in FIG. 19 was created from the data in FIG.

[0080] Wavelength allocation-1 shows the case where |λp-λs| = 12.5 (nm) and Δfch = 100 (GHz). In wavelength allocation-1, even when the transmission distance L is 200 (km), the TDECQ (dB) is kept below 3.3 (dB).

[0081] Wavelength allocation-2 shows the case where |λp-λs| = 12.5 (nm) and Δfch = 200 (GHz). In the case of wavelength allocation-2, even when the transmission distance L is 200 (km), the TDECQ (dB) is kept below 3.3 (dB).

[0082] Wavelength allocation-3 shows the case where |λp-λs| = 12.5 (nm) and Δfch = 300 (GHz). In the case of wavelength allocation-3, even when the transmission distance L is 200 (km), the TDECQ (dB) is kept below 3.3 (dB).

[0083] Wavelength allocation-4 shows the case where |λp-λs| = 5 (nm) and Δfch = 100 (GHz). In the case of wavelength allocation-4, if the transmission distance L is 14 (km), the TDECQ (dB) is kept below 3.3 (dB). In order to extend the transmission distance L, |λp-λs| must be greater than 5 (nm).

[0084] FIG. 20 is a diagram showing an optical phase conjugator 3 according to a second example.

[0085] The optical phase conjugator 3 includes an optical coupler 30 , a nonlinear optical element 31 , an optical filter 32 , a pump light source 33 , a driver circuit 34 , and an optical phase modulator 35 .

[0086] The structure and function of the optical coupler 30, nonlinear optical element 31, optical filter 32, pump light source 33, and driver circuit 34 are the same as those shown in FIG. 3, and therefore will not be described again for clarity.

[0087] The optical phase modulator 35 receives the pump light generated by the pump light source 33, phase-modulates the pump light, and inputs the pump light to the optical coupler 30. That is, the first signal light (Sin) and the phase-modulated pump light are input to the nonlinear optical element 31 via the optical coupler 30.

[0088] The optical phase modulator 35 can be configured to apply a control voltage to a waveguide or substrate made of a ferroelectric crystal material such as lithium niobium oxide (LiNbO3) or an electro-optic material. Optical phase modulators using waveguides made of compound semiconductors such as InP and GaAs are also known. The optical phase modulator undergoes phase modulation, in which the phase of the input light shifts depending on the magnitude of the applied voltage of the modulation signal.

[0089] In the non-linear optical element 31, if the intensity Ip of the pump light is increased to impart an optical amplification effect such as parametric amplification, the subsequent optical amplifier can be omitted. An exemplary condition for the intensity Ip of the pump light in this case is 10 (dBm) ≤ Ip ≤ 30 (dBm).

[0090] When performing parametric amplification using a continuous-wave pump light, in the non-linear optical element 31 (highly non-linear fiber), stimulated Brillouin scattering (SBS) may occur. Since stimulated Brillouin scattering degrades the quality of the signal light, it is preferable to suppress its occurrence. When the intensity Ip of the pump light is high as described above, by performing phase modulation of the pump light with the optical phase modulator 35, the occurrence of stimulated Brillouin scattering can be suppressed and the degradation of the signal light can be suppressed. To suppress the occurrence of stimulated Brillouin scattering, the pump light is modulated by the optical phase modulator 35, and the frequency fp of the suitable modulation signal can be set higher than the upper limit frequency (fBmax) of the Brillouin scattering band in the non-linear optical element 31 (fBmax < fp). Since the fBmax of a general non-linear optical element 31 is about several tens of MHz, a suitable example of fp can be set to 50 (MHz) ≤ fp ≤ 10 (GHz). When the Brillouin scattering band is smaller than 10 (MHz), the lower limit value of fp may be set to 10 (MHz). Exemplarily, a sine wave of 350 MHz is used for the modulation signal of the optical phase modulator 35. The output of the synthesizer (wave source) that generates the modulation signal is set to -3 (dBm), and the output may be amplified using a 1W RF driver. If the amplitude of the voltage of the modulation signal of the optical phase modulator 35 is Vπ, the phase of the input light is shifted by π, so this voltage can be set to about Vπ.

[0091] As described above, in the above optical communication system, pulse amplitude modulation is applied to the signal light. Pulse amplitude modulation has higher resistance to phase modulation of the pump light than QPSK, QAM, etc. that apply phase modulation to the signal light, and can suppress the degradation of the signal light.

[0092] The transmission distance and number of channels of high-speed PAM4 signals tend to be limited by the effects of dispersion distortion, distortion during optical phase conjugation, and third-order dispersion, but these issues can be resolved by satisfying the above-mentioned configuration and conditions. Such an optical communication system is particularly suitable for optical communications between data centers over transmission distances of 100 km or less, but can also be applied to optical communications over distances of 100 km or more.

[0093] As described above, the optical phase conjugator 3 when using a single-mode optical fiber comprises a nonlinear optical element 31, a pump light source 33 optically coupled to the nonlinear optical element 31, and a first optical input terminal IN for inputting a first signal light Sin to the nonlinear optical element 31. A and an optical output terminal OUT that outputs second signal light Sout that has been phase conjugated by a nonlinear optical element 31 via an optical filter 32 that selectively transmits the second signal light Sout, wherein the first optical fiber 2 that transmits the first signal light and the second optical fiber 5 that transmits the second signal light are each single-mode optical fibers, and the first signal light Sin has a plurality of channels within its wavelength band, and each channel includes pulse-amplitude-modulated signal light, and the frequency interval Δfch between adjacent channels satisfies 50 (GHz)≦Δfch≦76.25 (GHz), the wavelength λp of the pump light output from the pump light source 33 satisfies the relationship 5 (nm)≦|λp-λs|≦10 (nm) with respect to the wavelength λs that is closest to the wavelength λp within the wavelength band of the first signal light. Use of this optical phase conjugator 3 can extend the transmission distance and reduce power consumption.

[0094] Furthermore, the optical phase conjugator 3 in the case of using a dispersion-flattened optical fiber includes a nonlinear optical element 31, a pump light source 33 optically coupled to the nonlinear optical element 31, and a first optical input terminal IN for inputting a first signal light Sin to the nonlinear optical element 31. Aand an optical output terminal OUT that outputs the second signal light Sout phase-conjugated by the nonlinear optical element 31 via an optical filter 32 that selectively transmits the second signal light Sout, wherein the first optical fiber 2 that transmits the first signal light Sin and the second optical fiber 5 that transmits the second signal light Sout are each dispersion-flattened optical fibers, and when the first signal light Sin has a plurality of channels within its wavelength band and includes signal light that is pulse amplitude modulated in each channel, and the frequency interval Δfch between adjacent channels satisfies 100 (GHz)≦Δfch≦Δfmax (Δfmax is 300 (GHz) or more), the wavelength λp of the pump light output from the pump light source 33 satisfies the relationship Δλmin≦|λp-λs|≦Δλmax (Δλmin is a value of 12 (nm) or less, e.g., 11 (nm), and Δλmax is 12.5 (nm) or more) with respect to the wavelength λs that is closest to the wavelength λp within the wavelength band of the first signal light Sin. When this optical phase conjugator 3 is used, it is possible to extend the transmission distance and reduce power consumption.

[0095] Furthermore, when single-mode optical fibers or dispersion-flattened optical fibers are used as the first and second optical fibers, the optical phase conjugator 3 can further include an optical phase modulator 35 interposed between the pump light source 33 and the nonlinear optical element 31. In the optical phase conjugator 3, the intensity Ip of the pump light and the phase modulation by the optical phase modulator 35 can satisfy the above-mentioned conditions, thereby reducing the number of parts such as optical amplifiers and suppressing degradation of the signal light.

[0096] The above-mentioned optical communication system includes an optical phase conjugator 3 that satisfies the above-mentioned predetermined relationship, a transmitter 1 that transmits a first signal light Sin to a first optical fiber 2, and a receiver 6 that receives a second signal light Sout output from a second optical fiber 5. The optical communication system can extend the transmission distance and reduce power consumption.

[0097] In addition, in the range of various parameters, the range of an arbitrary parameter P is P min ≦P≦P max If given by (P min+ΔP)≦P≦(P max -ΔP), ΔP=(P max -P min )×R%, R may be set to 10, or R may be set to 20, R may be set to 30, or R may be set to 40. [Explanation of symbols]

[0098] 1... transmitter, 2... first optical fiber, 3... optical phase conjugator, 4... optical amplifier, 5... second optical fiber, 6... receiver, 31... nonlinear optical element, 32... optical filter, 33... pump light source, 35... optical phase modulator, IN A ...first optical input terminal, IN B ...second optical input terminal, OUT...optical output terminal, Sin...first signal light, Sout...second signal light, S PUMP ...pump light, 10...optical multiplexer, 30...optical coupler, 34...drive circuit, 100...optical communication system.

Claims

1. a nonlinear optical element; a pump light source optically coupled to the nonlinear optical element; an optical input terminal for inputting a first signal light to the nonlinear optical element; an optical output terminal that outputs the second signal light that has been phase conjugated by the nonlinear optical element via an optical filter that selectively transmits the second signal light; Equipped with the first optical fiber transmitting the first signal light and the second optical fiber transmitting the second signal light are each a single-mode optical fiber; The first signal light has a plurality of channels within its wavelength band, and includes signal light pulse-amplitude-modulated in each channel, and the frequency interval Δfch between adjacent channels satisfies 50 (GHz)≦Δfch≦76.25 (GHz), The wavelength λp of the pump light output from the pump light source is, with respect to the wavelength λs that is closest to the wavelength λp within the wavelength band of the first signal light, 5 (nm)≦|λp−λs|≦10 (nm), An optical phase conjugator that satisfies the relationship:

2. The interval Δfch is 50 (GHz)≦Δfch≦75 (GHz), 2. The optical phase conjugator according to claim 1, wherein the following relationship is satisfied:

3. The interval Δfch is 50 (GHz)≦Δfch≦62.5 (GHz), 2. The optical phase conjugator according to claim 1, wherein the following relationship is satisfied:

4. a nonlinear optical element; a pump light source optically coupled to the nonlinear optical element; an optical input terminal for inputting a first signal light to the nonlinear optical element; an optical output terminal that outputs the second signal light that has been phase conjugated by the nonlinear optical element via an optical filter that selectively transmits the second signal light; Equipped with the first optical fiber transmitting the first signal light and the second optical fiber transmitting the second signal light are each a dispersion-flattened optical fiber; The first signal light has a plurality of channels within its wavelength band, and includes signal light pulse-amplitude-modulated in each channel, and the frequency interval Δfch between adjacent channels satisfies 100 (GHz)≦Δfch≦300 (GHz), The wavelength λp of the pump light output from the pump light source is, with respect to the wavelength λs that is closest to the wavelength λp within the wavelength band of the first signal light, 11 (nm)≦|λp−λs|≦12.5 (nm), An optical phase conjugator that satisfies the relationship:

5. further comprising an optical phase modulator interposed between the pump light source and the nonlinear optical element; 5. The optical phase conjugator according to claim 1 or 4.

6. The intensity Ip of the pump light and the frequency fp of the modulation signal in the optical phase modulator satisfy the following conditions, where fBmax is the upper limit frequency of the band of Brillouin scattering in the nonlinear optical element: 10 (dBm)≦Ip≦30 (dBm), fBmax≦fp≦10 (GHz), 6. The optical phase conjugator according to claim 5, which satisfies the following:

7. an optical phase conjugator according to claim 1 or 4; a transmitter for transmitting the first signal light to the first optical fiber; a receiver for receiving the second signal light output from the second optical fiber; An optical communication system comprising:

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