Optical transmitting device, optical communication system, and optical transmitting method

Abstract

According to the present invention, an optical transmission device comprises: a modulated signal generation unit that generates an intensity modulated signal and a continuous phase frequency shift keying (CPFSK) signal; a light source that outputs a signal modulated by the CPFSK signal; and an intensity modulation unit that performs intensity modulation, which cancels an intensity modulation component generated through the modulation by the CPFSK signal and intensity modulation using the intensity modulated signal, on the signal output from the light source.

Description

[Technical Field]

[0001] The present invention relates to an optical transmitting device, an optical communication system, and an optical transmitting method. [Background technology]

[0002] Currently, optical subscriber networks offer a system called a PON system, in which an optical line terminal (OLT) and a portion of the optical fiber transmission line are shared by multiple optical subscriber units (ONUs) in order to provide users with high-speed communication services economically.

[0003] For example, the All-Photonics Network (APN) has been proposed as a future network in Non-Patent Document 1. The APN is expected to accommodate communications between users using a direct optical connection network that minimizes photoelectric conversion and electrical routing processing on the route.

[0004] In optical direct-connect networks, the challenge for both OLT and ONU is to increase the speed and extend the transmission distance while maintaining a simple and economical ONU configuration.

[0005] As a means for solving this problem, Non-Patent Document 2 proposes a method of using an EA modulator integrated direct modulation diode on the ONU side. In the proposed method, the ONU generates a continuous phase frequency shift keyed (CPFSK) signal using an EA modulator integrated direct modulation diode for upstream communication, and transmits the modulated signal.

[0006] Non-Patent Document 3 proposes a communication method that uses an EA modulator integrated direct modulation diode in an APN. In the proposed communication method, intensity modulation (IM) signals are sent and received for communication between devices over short distances, and direct communication is achieved by utilizing the loopback function of a photonic gateway (PhGW), which is an optical node in the APN. On the other hand, CPFSK signals are used for communication between repeaters for communication between devices over long distances.

[0007] As a method for increasing the speed of a CPFSK signal, Non-Patent Document 4 proposes a configuration in which the multilevel level of the signal applied to the directly modulated laser is increased to improve the number of information bits that can be transmitted in one symbol. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Kawahara et al., NTT Technical Review, Vol. 32, No. 3, 2020, pp. 14-21. [Non-patent document 2] M. Fujiwara, R. Koma, J. -i. Kani, K. -I. Suzuki and A. Otaka, "Performance evaluation of CPFSK transmitters for TDM-based digital coherent PON upstream," 2017 Optical Fiber Communications Conference and Exhibition (OFC), 2017, pp. 1-3. [Non-patent document 3] R. Koma, K. Hara, T. Kanai, J. -i. Kani and T. Yoshida, "Novel EA-DFB Mode-Switching Transmitter Supporting Continuous Phase Frequency Shift Keying and Intensity Modulation for All-Photonics Network," 2021 European Conference on Optical Communication (ECOC), 2021, pp. 1-4, doi: 10.1109 / ECOC52684.2021.9605834. [Non-patent document 4] D. Che, F. Yuan, Q. Hu and W. Shieh, "Frequency Chirp Supported Complex Modulation of Directly Modulated Lasers," in Journal of Lightwave Technology, vol. 34, no. 8, pp. 1831-1836, 15 April15, 2016, doi: 10.1109 / JLT.2015.2512298. Summary of the Invention [Problem to be solved by the invention]

[0009] However, the speed-up method proposed in Non-Patent Document 4 increases the speed in the phase direction, so as with multi-level PSK modulation, the distance between signal points decreases when the multi-level is increased. As a result, the required SNR (Signal-to-Noise Ratio) to ensure signal quality increases. While it is possible to prevent SNR degradation by using M-level quadrature phase amplitude modulation (M-QAM), this method has the disadvantage that the intensity modulation component associated with CPFSK signal generation degrades the SNR because the frequency-modulated signal is generated using a directly modulated laser.

[0010] In view of the above circumstances, an object of the present invention is to reduce degradation of CPFSK signals and increase the transmission speed. [Means for solving the problem]

[0011] One aspect of the present invention is an optical transmission device comprising: a modulation signal generation unit that generates an intensity-modulated signal and a continuous phase frequency shift keying (CPFSK) signal; a light source that outputs a signal modulated by the CPFSK signal; and an intensity modulation unit that performs intensity modulation on the signal output from the light source to cancel out intensity modulation components generated by modulation by the CPFSK signal and intensity modulation by the intensity-modulated signal.

[0012] One aspect of the present invention is an optical communication system comprising: an optical transmitting device including a modulation signal generating unit that generates an intensity-modulated signal and a continuous phase frequency shift keying (CPFSK) signal; a light source that outputs a signal modulated by the CPFSK signal; and an intensity modulating unit that performs intensity modulation on the signal output from the light source to cancel out intensity modulation components generated by modulation by the CPFSK signal and intensity modulates the signal by the intensity-modulated signal; and an optical receiving device including a receiving unit that performs polarization separation and phase separation on a signal received from the optical transmitting device; and a signal processing unit that decodes the intensity-modulated signal and the CPFSK signal based on the polarization separation and phase separation signals.

[0013] One aspect of the present invention is an optical transmission method including: a modulated signal generation step of generating an intensity-modulated signal and a continuous phase frequency shift keying (CPFSK) signal; an output step of outputting a signal modulated by the CPFSK signal; and an intensity modulation step of intensity-modulating the signal output from the light source to cancel out intensity-modulated components generated by modulation by the CPFSK signal and to intensity-modulate the signal by the intensity-modulated signal. [Effects of the Invention]

[0014] According to the present invention, it is possible to reduce the degradation of the CPFSK signal and increase the transmission speed. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a diagram illustrating an example of a configuration of an optical communication system 1 according to a first embodiment. [Figure 2] 1 is a diagram illustrating an example of the configuration of an optical transmitter 2 according to a first embodiment. [Figure 3] FIG. 2 shows the electric field of the signal generated by the light source 24. [Figure 4] 10 is a diagram showing the electric field of the signal output by the intensity modulation section 26. FIG. [Figure 5] 4 is a flowchart showing the operation of the optical transmitter 2. [Figure 6] 1 is a diagram illustrating an example of the configuration of an optical receiving device 3 according to a first embodiment. [Figure 7] FIG. 2 is a diagram illustrating an example of the configuration of a signal processing unit 33. [Figure 8] 4 is a flowchart showing the operation of the optical receiving device 3. [Figure 9] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmitter 2 according to a second embodiment. [Figure 10] 10 is a diagram illustrating an example of the relationship between the modulation degree of an intensity-modulated signal and reception sensitivity. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0017] (First embodiment) 1 is a diagram showing an example of the configuration of an optical communication system 1 according to the first embodiment. The optical communication system 1 includes an optical transmitting device 2 and an optical receiving device 3. The optical transmitting device 2 transmits an optical signal to the optical receiving device 3, and the optical receiving device 3 receives the transmitted optical signal.

[0018] (Optical transmitter) 2 is a diagram showing an example of the configuration of the optical transmitter 2 according to the first embodiment. The optical transmitter 2 includes a modulated signal generator 20, a DA converter 22, a light source 24, and an intensity modulator .

[0019] The modulation signal generation unit 20 generates a modulation signal. The DA conversion unit 22 converts the modulation signal into an analog signal. The modulation signal generation unit 20 and the DA conversion unit 22 generate a modulation signal to be input to the light source 24 and the intensity modulation unit 26. The modulation signal generation unit 20 and the DA conversion unit 22 may be analog signal generators that generate analog signals.

[0020] The modulated signal includes an intensity modulated signal (DATA_IM), a CPFSK signal (DATA_CPFSK), and a CPFSK cancellation signal (Equation 1).

[0021]

number

[0022] Hereinafter, DATA_CPFSK and DATA_IM will be described as binary amplitude modulated signals, but DATA_CPFSK and DATA_IM may be ternary or more amplitude modulated signals, and the number of each value may be independent.

[0023] The light source 24 is a directly modulated laser (for example, a distributed feedback (DFB) laser) and outputs a signal modulated based on the CPFSK signal.

[0024] 3 is a diagram showing the electric field of the signal generated by the light source 24. Here, it is assumed that the polarization of the signal output from the light source 24 is linearly polarized, and the output linearly polarized wave is defined as X polarization, and the polarization axis orthogonal to the X polarization is defined as Y polarization. sig1 is expressed by equation (2).

[0025]

number

[0026] In equation (2), E sig is the electric field, A m_CPFSK is the intensity modulation component generated by the light source 24, ω m_CPFSK is the angular frequency of the frequency-modulated signal light, t is time, and θ0 is the phase that does not change over time.

[0027] A m_CPFSK The amplitude of the CPFSK signal may vary depending on the value of the CPFSK signal. In other words, the CPFSK modulation may result in intensity modulation of different magnitudes depending on the value of the CPFSK signal.

[0028] Based on the cancelled CPFSK signal and the intensity modulation signal, the intensity modulation unit 26 intensity-modulates the signal output from the light source 24. The intensity modulation unit 26 performs intensity modulation based on the modulation signal expressed by equation (3).

[0029]

number

[0030] Here, α is a coefficient that sets the degree of modulation of the signal applied to the external intensity modulator to cancel out the intensity modulation component by the light source 24. Here, β is a coefficient that sets the modulation depth of the intensity modulation signal to an arbitrary value. The amplitude of the signal output from the light source 24 becomes a constant value due to the intensity modulation based on the cancellation CPFSK signal. The intensity modulation based on the intensity modulation signal causes the frequency modulation component to remain and is intensity modulated.

[0031] 4 is a diagram showing the electric field of the signal output by the intensity modulation unit 26. When the CPFSK signal and the intensity modulation signal are binary signals, a four-level modulation signal is applied to the intensity modulation unit 26.

[0032] The electric field E of the signal output from the intensity modulation unit 26 sig2 is expressed by equation (4).

[0033]

number

[0034] In equation (4), A m_IM is a component that is intensity modulated by the intensity modulation signal.

[0035] 5 is a flowchart showing the operation of the optical transmitting device 2. First, the modulated signal generating unit 20 generates a modulated signal (step S11). The DA converting unit 22 converts the modulated signal into an analog signal (step S12). The light source 24 modulates the signal based on the CPFSK signal (step S13). The intensity modulating unit 26 intensity-modulates the signal output from the light source 24 based on the cancel CPFSK signal and the intensity-modulated signal (step S14). Thereafter, the intensity modulating unit 26 outputs the intensity-modulated signal to the optical receiving device 3 (step S15).

[0036] (Optical receiving device) 6 is a diagram showing an example of the configuration of the optical receiving device 3 according to the first embodiment. The optical receiving device 3 includes a receiving unit 31, an AD conversion unit 32, and a signal processing unit 33.

[0037] The receiving unit 31 is a general polarization / phase diversity receiver, and performs polarization separation and phase separation on the signal received from the optical transmitting device 2. The AD conversion unit 32 converts the signal separated by the receiving unit 31 into a digital signal. The signal processing unit 33 processes the signal converted by the AD conversion unit 32.

[0038] 7 is a diagram showing an example of the configuration of the signal processing unit 33. The signal processing unit 33 includes a chromatic dispersion compensator 331, a polarization estimator / compensator 332, an intensity signal processor 333, and a CPFSK signal processor 334.

[0039] The chromatic dispersion compensator 331 estimates and compensates for chromatic dispersion that occurs when a signal propagates through a fiber. The polarization estimation and compensation unit 332 estimates and compensates for a polarization rotation component that occurs when the signal compensated for by the chromatic dispersion compensator 331 propagates through a fiber. The intensity signal processor 333 processes the signal compensated for by the polarization estimation and compensation unit 332. The CPFSK signal processor 334 processes the signal compensated for by the polarization estimation and compensation unit 332.

[0040] The intensity signal processing unit 333 includes an absolute value calculation unit 3331, a DC component removal unit 3332, an adaptive equalization filter 3333, and a decoding unit 3334. The absolute value calculation unit 3331 calculates the absolute value of the complex signal. As a result, the signal contains only intensity information. The DC component removal unit 3332 removes the DC component of the absolute value calculated by the absolute value calculation unit 3331. The adaptive equalization filter 3333 compensates for waveform deterioration of the signal from which the DC component has been removed by the DC component removal unit 3332. The decoding unit 3334 decodes the signal compensated by the adaptive equalization filter 3333.

[0041] CPFSK signal processing unit 334 includes 1-bit differential detection unit 2241, adaptive equalization filter 3342, phase compensation unit 3343, and decoding unit 3344. 1-bit differential detection unit 2241 performs 1-bit differential detection of the signal. Adaptive equalization filter 3342 compensates for waveform degradation of the signal in which a 1-bit delay has been detected. Phase compensation unit 3343 compensates for the phase of the signal compensated by adaptive equalization filter 3342. Decoding unit 3344 decodes the phase-compensated signal. The processing by CPFSK signal processing unit 334 is a normal CPFSK signal processing method described in Non-Patent Document 2.

[0042] 8 is a flowchart showing the operation of the optical receiving device 3. First, the receiving unit 31 receives a signal from the optical transmitting device 2 (step S21). Next, the AD conversion unit 32 converts the analog signal into a digital signal (step S22). Next, the signal processing unit 33 processes the intensity signal (step S23) and processes the CPFSK signal (step S24).

[0043] As described above, in the optical transmitter 2, the light source 24 modulates the signal based on the CPFSK signal, and the intensity modulation unit 26 performs intensity modulation based on the intensity-modulated signal and cancels out the intensity modulation component generated by the modulation based on the CPFSK signal. This makes it possible to use not only the frequency per wavelength of the signal but also the magnitude of the intensity for signal transmission, thereby increasing the transmission speed per wavelength of the signal.

[0044] (Second embodiment) 9 is a diagram showing an example of the configuration of an optical transmission device 2 according to the second embodiment. The optical transmission device 2 according to the second embodiment includes a modulation index changing unit 28 and a receiving sensitivity table storage unit 29 in addition to the components of the optical transmission device 2 according to the first embodiment.

[0045] The modulation degree changing unit 28 changes the modulation degree of the intensity-modulated signal generated by the modulation signal generating unit 20. The modulation degree changing unit 28 changes the modulation degree of the intensity-modulated signal based on, for example, a reception sensitivity table stored in a reception sensitivity table storage unit 29. The reception sensitivity table indicates reception sensitivity for each combination of modulation method, modulation multilevel degree, symbol rate, modulation degree, receiver configuration, and transmission distance.

[0046] 10 is a diagram showing an example of the relationship between the modulation degree of an intensity-modulated signal and the receiving sensitivity. Increasing the modulation degree of an intensity-modulated signal improves the receiving sensitivity when receiving the intensity-modulated signal, but deteriorates the receiving sensitivity when receiving a CPFSK signal. For example, when a digital coherent receiver such as the optical receiving device 3 simultaneously receives an intensity-modulated signal and a CPFSK signal, the intensity-modulated signal and the CPFSK signal must have equivalent receiving sensitivity. In other words, a modulation degree of A is desirable.

[0047] Furthermore, for example, in the case where an intensity-modulated receiver that receives only the intensity-modulated signal and a coherent receiver that receives the intensity-modulated signal and the CPFSK signal receive signals simultaneously, and there is a large signal loss between the transmitter and the intensity-modulated receiver and there is a margin in the receiving sensitivity of the CPFSK signal, the modulation depth may be set to B to improve the sensitivity of the intensity-modulated signal.

[0048] Furthermore, for example, in the case where an intensity-modulated receiver that receives only the intensity-modulated signal and a coherent receiver that receives the intensity-modulated signal and the CPFSK signal receive the signals simultaneously, and there is a margin in the receiving sensitivity of the intensity-modulated signal, the modulation index may be set to C, thereby improving the sensitivity of the CPFSK signal.

[0049] In other words, based on the receiving sensitivity indicated in the receiving sensitivity table, the modulation degree changing unit 28 can change the modulation degree so that, for example, the receiving sensitivity of the intensity-modulated signal and the receiving sensitivity of the CPFSK signal are equivalent, and can also change the modulation degree so that the intensity-modulated signal or the CPFSK signal has any desired receiving sensitivity.

[0050] Other Embodiments One embodiment of the present invention has been described in detail above with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes and the like are possible within the scope that does not deviate from the gist of the present invention.

[0051] For example, in the above-described embodiment, the modulation signal generator 20 generates an intensity modulation signal, a CPFSK signal, and a CPFSK cancellation signal, but this is not limited to this. For example, a plurality of modulation signal generators 20 may each generate an intensity modulation signal, a CPFSK signal, and a CPFSK cancellation signal. Furthermore, the optical transmitter 2 may be provided with a plurality of DA converters 22 corresponding to the plurality of modulation signal generators 20, respectively.

[0052] Alternatively, the optical transmitter 2 may be provided with two intensity modulation sections 26 corresponding to the intensity-modulated signal and the CPFSK cancellation signal, and each intensity modulation section 26 may perform intensity modulation based on the intensity-modulated signal and the CPFSK cancellation signal. [Explanation of symbols]

[0053] REFERENCE SIGNS LIST 1 Optical communication system, 2 Optical transmitter, 20 Modulation signal generator, 22 DA converter, 24 Light source, 26 Intensity modulator, 28 Modulation index changer, 29 Receiving sensitivity table memory, 3 Optical receiver, 31 Receiver, 32 AD converter, 33 Signal processor, 331 Wavelength dispersion compensator, 332 Polarization estimation and compensation unit, 333 Intensity signal processor, 3331 Absolute value calculator, 3332 DC component remover, 3333 Adaptive equalization filter, 3334 Decoder, 3341 1-bit delay detector, 3342 Adaptive equalization filter, 3343 Phase compensation unit, 3344 Decoder

Claims

1. a modulation signal generator that generates an intensity modulated signal and a continuous phase frequency shift keying (CPFSK) signal; a light source that outputs a signal modulated by the CPFSK signal; an intensity modulation unit that performs intensity modulation on the signal output from the light source to cancel out intensity modulation components generated by modulation with the CPFSK signal and intensity modulation with the intensity modulation signal; a modulation degree changing unit that changes the modulation degree of the intensity-modulated signal; An optical transmitting device comprising:

2. a modulation signal generator that generates an intensity modulated signal and a continuous phase frequency shift keying (CPFSK) signal; a light source that outputs a signal modulated by the CPFSK signal; an intensity modulation unit that performs intensity modulation on the signal output from the light source to cancel out intensity modulation components generated by modulation with the CPFSK signal and intensity modulation with the intensity modulation signal; an optical transmitter comprising: a receiving unit that performs polarization separation and phase separation on the signal received from the optical transmitting device; a signal processing unit that decodes the intensity modulated signal and the CPFSK signal based on the polarization separated and phase separated signals; an optical receiving device comprising: An optical communication system comprising:

3. a modulated signal generating step of generating an intensity modulated signal and a continuous phase frequency shift keying (CPFSK) signal; an output step of outputting a signal modulated by the CPFSK signal; an intensity modulation step of performing intensity modulation on the signal outputted in the output step to cancel out intensity modulation components generated by modulation with the CPFSK signal and intensity modulation with the intensity modulation signal; a modulation degree changing step of changing the modulation degree of the intensity-modulated signal; An optical transmission method comprising:

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