Transmitting device and signal generation method

Abstract

This transmission device comprises a direct modulating laser, a branching unit, a first polarized signal generation unit, a second polarized signal generation unit, and a polarized wave multiplexing unit. The branching unit branches a continuous phase frequency-modulated signal light generated by the directing laser by application of a first modulation signal, into a first branch light and a second branch light. The first signal generation unit generates a first polarized wave signal light in which the intensity modulated component generated by the modulating laser by application of the first modulation signal is removed from the first branch light. The second signal generation unit generates a second polarized wave signal light which is orthogonal to the first polarized wave, and in which the removal of an intensity modulated component and the addition of an intensity modulated component by application of a second modulation signal are performed on the second branch light. The polarized wave multiplexing unit multiplexes the first polarized wave and second polarized wave signal lights.

Description

Technical Field

[0001] The present invention relates to a transmission device and a signal generation method.

Background Art

[0002] Currently, in an optical subscriber network, a PON (Passive Optical Network) system is used to economically provide high-speed communication services to users. In the PON system, a plurality of subscriber devices (ONU: Optical Network Unit) share a part of the optical fiber transmission path with the central office device (OLT: Optical Line Terminal).

[0003] Looking further into the future, it has been proposed to use an APN (ALL-Photonics Network) for communication between users, including not only subscriber networks but also high-speed and low-latency wireless services such as 5G and 6G, and networks that require low latency such as data center networks (see, for example, Non-Patent Document 1). In the APN, it is assumed that communication is accommodated in a directly-connected optical network by minimizing optical-electrical conversion and electrical routing processing on the communication path between users.

[0004] How to increase the speed and extend the transmission distance while keeping the user device arranged on the user side in a simple and economical configuration is a common problem for both.

[0005] Therefore, in the PON system, there is a technology for increasing the speed and extending the transmission distance while keeping the user device in a simple configuration (see, for example, Non-Patent Document 2). FIG. 15 is a diagram showing the configuration of a PON system using this technology. The ONU, which is a transmitter in the upstream communication, uses an EA (Electro Absorption) modulator integrated direct modulation diode to generate and transmit a binary continuous phase frequency shift keying signal (CPFSK) using the chirp of a laser. The OLT, which is a receiver in the upstream communication, receives the signal transmitted from the ONU by digital coherent reception.

[0006] FIG. 16 is a diagram showing a configuration example of the APN (see, for example, Non-Patent Document 3). In the APN, in communication between short-distance user devices, an intensity modulation (IM) signal is transmitted and received using the same EA modulator integrated direct modulation diode as described above. These short-distance user devices communicate directly with each other by using the folding function of the PhGW (optical gateway), which is an optical node of the APN. On the other hand, during long-distance transmission, the user device uses CPFSK modulation for communication with repeaters arranged in the local network.

[0007] As a conventional study on speeding up the CPFSK signal, there is a configuration of a transmitter that improves the multiplicity of the signal applied to the directly modulated laser and improves the number of information bits that can be transmitted in one symbol (see, for example, Non-Patent Document 4). Assume that the multilevel modulation signal applied to the directly modulated laser of the transmitter is a 4-level pulse amplitude modulation signal (PAM4; Pulse Amplitude Modulation 4). FIG. 17 is a diagram showing an eye pattern of the modulation signal output from the directly modulated laser, FIG. 18 is a diagram showing the relationship between the applied current to the directly modulated laser and the center frequency and intensity, and FIG. 19 is a diagram showing an example of a received constellation. As shown in FIG. 18, since the center frequency of the laser shifts according to the applied current, a signal is superimposed on the fluctuating component of this frequency. Although speeding up is possible with this method, it only speeds up in the phase direction. Therefore, similar to the multilevel PSK (Phase Shift Keying) modulation method, when the multiplicity is increased, the distance between signal points decreases, and thus the increase in the required SNR for ensuring a predetermined signal quality becomes significant. In order to prevent SNR degradation due to narrowing of the distance between signal points caused by multilevel conversion only in the phase direction, there is also a method such as the M-ary quadrature amplitude modulation method (M-QAM method). However, since a frequency modulation signal is generated by a directly modulated laser, in addition, as shown in FIG. 19, there is also a drawback that the intensity modulation component generated during CPFSK signal generation deteriorates the signal quality (SNR).

[0008] In the technique of Non-Patent Document 4, in addition to multi-valued modulation in the phase direction, high-speed operation is achieved by polarization multiplexing. In Non-Patent Document 4, for principle verification, after generating a signal using a single LD (laser diode), the signal is branched and delayed on an optical fiber, and after orthogonalizing the polarizations, they are combined for verification. However, from the perspective of implementation, since independent frequency modulation must be applied to each polarization, a configuration using two LDs to generate signals of orthogonal polarizations is assumed.

[0009] FIG. 20 shows the configuration of a transmitter that transmits a binary CPFSK signal and the intensity of each polarization of the generated binary CPFSK signal. In FIG. 20, after applying signals independently to each directly modulated laser, the polarizations of those signals are orthogonalized using a polarization control element, and then the orthogonalized signals are combined by a polarization combiner. FIG. 21 shows another configuration of a transmitter that transmits a binary CPFSK signal and the intensity of each polarization of the generated binary CPFSK signal. As shown in FIG. 21, by combining with an intensity modulator such as an EA modulator in the same way as in Non-Patent Document 2, the intensity modulation component in the signal output from the directly modulated laser can be canceled. Thereby, although it is possible to prevent deterioration of the CPFSK signal, the configuration becomes somewhat complicated.

[0010] Also, in the case of the configuration of FIG. 21, it is assumed that each directly modulated laser operates independently. Therefore, as shown in FIG. 22, the center frequencies F and F' of each polarization vary independently. FIG. 23 is a diagram showing a configuration example of a receiver assumed when digitally coherently receiving a signal as shown in FIG. 21, and FIG. 24 is a diagram showing a configuration example of a general digital signal processing circuit section (DSP) used in the receiver shown in FIG. 23. The IQ components of the received XY polarizations are input to the DSP. The polarization state of the received signal changes randomly during fiber transmission. Therefore, after wavelength dispersion compensation, the DSP separates the signals for each polarization so as to be in the polarization state at the time of transmission by polarization separation and adaptive equalization processing. At this time, when the center frequencies are different for each polarization, since it is necessary to estimate the frequency difference Δf (frequency offset) between the signal and the LO (local oscillator) light independently for each polarization, the DSP scale increases.

Prior Art Documents

Non-Patent Documents

[0011]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

[0012] In the transmission and reception method of CPFSK signals using directly modulated lasers that have been studied so far, the configuration focuses only on the frequency modulation component without detecting the intensity modulation component associated with the bias modulation of the directly modulated laser. Also, although there are examples of achieving high speed through polarization multiplexing, since the transmitter uses two DFB lasers to realize polarization multiplexing, the circuit scale increases.

[0013] In view of the above circumstances, an object of the present invention is to provide a transmission device and a signal generation method capable of performing polarization multiplexing with a high transmission speed without increasing the circuit scale. Means for Solving the Problems

[0014] A transmission device according to an aspect of the present invention includes a directly modulated laser, a branching unit that branches the signal light of continuous phase frequency modulation generated by the directly modulated laser by applying a first modulation signal into first branched light and second branched light, a first signal generation unit that generates signal light of a first polarization from which the intensity modulation component generated by the directly modulated laser by applying the first modulation signal is removed from the first branched light, a second signal generation unit that generates signal light of a second polarization orthogonal to the first polarization, which is signal light obtained by removing the intensity modulation component from the second branched light and adding an intensity modulation component by applying a second modulation signal, and a polarization multiplexing unit that multiplexes the signal light of the first polarization and the signal light of the second polarization.

[0015] A signal generation method according to one aspect of the present invention includes a branching step of branching signal light of continuous phase frequency modulation generated by a directly modulated laser by applying a first modulation signal into first branched light and second branched light, a first signal generation step of generating signal light of a first polarization that removes an intensity modulation component generated by the directly modulated laser by applying the first modulation signal from the first branched light, a second signal generation step of generating signal light of a second polarization that is orthogonal to the first polarization, which is signal light obtained by removing the intensity modulation component from the second branched light and adding an intensity modulation component by applying a second modulation signal, and a polarization multiplexing step of multiplexing the signal light of the first polarization and the signal light of the second polarization.

Effect of the Invention

[0016] According to the present invention, it is possible to perform polarization multiplexing with a high transmission speed without increasing the circuit scale.

Brief Description of the Drawings

[0017]

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Mode for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the plurality of drawings, the same parts are denoted by the same reference numerals, and the description thereof will be omitted.

[0019] In this embodiment, the transmitter independently modulates the intensity modulation component and the frequency modulation component for each polarization and transmits them simultaneously to achieve a higher transmission rate per wavelength. Since the intensity modulation component and the frequency modulation component are transmitted separately for each polarization, the CPFSK signal for transmitting the frequency modulation component is not affected by the SNR degradation due to intensity modulation. Also, since the modulation signals for both polarizations are transmitted using a single laser, not only can the high speed be achieved with a simple configuration, but also the influence of independent frequency fluctuations for each polarization that occurs when using two lasers can be removed, and the DSP resources for frequency offset can be reduced.

[0020] (First Embodiment) FIG. 1 is a configuration diagram of a transmitter 100 according to the first embodiment. The transmitter 100 includes a digital signal processing circuit 101, a digital - analog (DA) converter 102, a directly modulated laser 103, a demultiplexer 104, a first intensity modulator 105, a second intensity modulator 106, a polarization rotation unit 107, and a polarization multiplexer 108. In FIG. 1, a configuration for applying a binary amplitude - shift - keying (ASK) signal is shown.

[0021] The digital signal processing circuit 101 is a signal generation circuit for digital signals. The digital signal processing circuit 101 generates a modulation signal and outputs the generated modulation signal to the DA converter 102. The digital signal processing circuit 101 generates DATA_CPFSK, α·￣DATA_CPFSK, and α·￣DATA_CPFSK + β·DATA_IM for the DA converter 102. "￣DATA_CPFSK" indicates that "￣" is described above "DATA_CPFSK". α and β are coefficients described later. The values of α and β are determined in advance. DATA_CPFSK is an amplitude - modulation signal for transmitting the first data. ￣DATA_CPFSK is a modulation signal for canceling the intensity modulation component generated in the directly modulated laser 103 by applying DATA_CPFSK. DATA_IM is an intensity - modulation (IM) signal for transmitting the second data.

[0022] The DA converter 102 converts the DATA_CPFSK, α·￣DATA_CPFSK, and α·￣DATA_CPFSK + β·DATA_IM of the modulation signal generated by the digital signal processing circuit 101 from a digital signal to an analog signal. The DA converter 102 applies the DATA_CPFSK directly to the directly modulated laser 103. Also, the DA converter 102 outputs the α·￣DATA_CPFSK to the first intensity modulator 105 and outputs the α·￣DATA_CPFSK + β·DATA_IM to the second intensity modulator 106.

[0023] The directly modulated laser 103 is, for example, a DFB (Distributed-FeedBack Laser) laser. By applying the DATA_CPFSK output from the DA converter 102 to the directly modulated laser 103, the directly modulated laser 103 generates a CPFSK (Continuous Phase Frequency-Shift Keying) signal.

[0024] The optical splitter 104 splits the CPFSK signal output from the directly modulated laser 103 into two branches while maintaining the same polarization. The optical splitter 104 outputs one of the split CPFSK signals to the first intensity modulator 105 and the other CPFSK signal to the second intensity modulator 106.

[0025] The first intensity modulator 105 applies the α·￣DATA_CPFSK of the intensity modulation component removal signal output from the DA converter 102 to the CPFSK signal input from the optical splitter 104. α is a coefficient that sets the degree of modulation of the signal applied to the first intensity modulator 105 in order to cancel the intensity modulation component of the light source of the directly modulated laser 103. The first intensity modulator 105 outputs the CPFSK signal with the intensity modulation component canceled by applying the α·￣DATA_CPFSK to the polarization multiplexing section 108.

[0026] The second intensity modulator 106 generates a modulated signal obtained by applying α·￣DATA_CPFSK + β·DATA_IM output from the DA converter 102 to the CPFSK signal input from the demultiplexer 104. α·￣DATA_CPFSK + β·DATA_IM is an applied modulation signal for adding a signal component to be transmitted by intensity modulation in addition to a modulation signal that cancels out the intensity modulation component generated in the directly modulated laser 103 by applying DATA_CPFSK. β is a coefficient that sets the modulation degree of the intensity modulation signal to an arbitrary value. For example, in the case of binary modulation, by changing the value of the coefficient α and the value of the coefficient β, the extinction ratio between the mark (1) and the space (0) of the signal can be changed. The second intensity modulator 106 outputs the generated intensity modulation signal to the polarization rotation unit 107.

[0027] The polarization rotation unit 107 rotates the polarization of the intensity modulation signal input from the second intensity modulator 106 and converts it into an intensity modulation signal orthogonal to the input intensity modulation signal. The polarization rotation unit 107 outputs the converted intensity modulation signal to the polarization multiplexer 108. The polarization multiplexer 108 multiplexes the CPFSK signal input from the first intensity modulator 105 and the intensity modulation signal input from the polarization rotation unit 107, and outputs the multiplexed signal light. The signal light output from the polarization multiplexer 108 is output to an optical transmission line (not shown). The optical transmission line is, for example, an optical fiber.

[0028] FIG. 2 is a diagram showing an example of the output signal light of the directly modulated laser 103 and its polarization state. Here, it is assumed that the directly modulated laser 103 is a DSP laser. Also, assume that the polarization of the signal output from the directly modulated laser 103 is linearly polarized. Let the linearly polarized wave output from the directly modulated laser 103 be the X polarization, and the polarization orthogonal to the X polarization be the Y polarization. FIG. 2(a) shows the correspondence between DATA_CPFSK and the electric field of the CPFSK signal output from the directly modulated laser 103. FIG. 2(b) shows the oscillation direction W1 of the polarization of the CPFSK signal output from the directly modulated laser 103.

[0029] A m_CPFSK is the intensity modulation component generated by the directly modulated laser 103, ω mLet ω be the angular frequency of the frequency-modulated signal light, t be time, and θ0 be the phase that does not change with time. The signal E, which is a CPFSK signal output by the direct modulation laser 103 sig is represented by Equation (1). Here, as an ideal state, the influence of the phase change (chirp) associated with external intensity modulation is not considered.

[0030]

Equation

[0031] Figure 3 is a diagram showing an example of the output signal light and polarization state of the first intensity modulator 105. Figure 3(a) shows the correspondence between DATA_CPFSK and the electric field of the output signal light (CPFSK signal) from the first intensity modulator 105. Figure 3(b) shows the vibration direction W2 of the polarization of the output signal light from the first intensity modulator 105. Since the intensity modulation component generated by the direct modulation laser 103 is removed, the signal amplitude of the output signal light from the first intensity modulator 105 becomes a constant value (A). The output amplitude E of the output signal light from the first intensity modulator 105 sig_X is represented by Equation (2).

[0032]

Equation

[0033] Figure 4 is a diagram showing an example of the output signal light and polarization state of the second intensity modulator 106. Figure 4(a) shows the correspondence between DATA_CPFSK and DATA_IM signals and the electric field of the output signal light (IM signal) from the second intensity modulator 106. Figure 4(b) shows the vibration direction W3 of the polarization after polarization rotation by the polarization rotation unit 107 of the output signal light output from the second intensity modulator 106. The second intensity modulator 106 removes the intensity modulation component generated by the direct modulation laser 103 from the CPFSK signal output by the direct modulation laser 103 while applying a new intensity modulation component. On the other hand, the frequency modulation component remains. Therefore, the output signal light E sig_Y is represented by Equation (3). Here, A m_IM is the intensity modulation component applied by the second intensity modulator 106.

[0034]

Number

[0035] The signal light after polarization multiplexing by the polarization multiplexing unit 108 is the sum of the orthogonal output signal lights E sig_X and the output signal light E sig_Y and.

[0036] Figure 5 is a flowchart showing the processing of the transmitter 100. The digital signal processing circuit 101 outputs the DATA_CPFSK of the amplitude modulation signal, the α·￣DATA_CPFSK of the intensity modulation component removal signal, and the α·￣DATA_CPFSK + β·DATA_IM of the applied signal to the DA converter 102. The DA converter 102 converts the DATA_CPFSK from a digital signal to an analog signal and applies it to the directly modulated laser 103 (step S11). The directly modulated laser 103 outputs a CPFSK signal generated by the application of DATA_CPFSK (step S12). The demultiplexer 104 branches the CPFSK signal output by the directly modulated laser 103 into two and outputs it to the first intensity modulator 105 and the second intensity modulator 106 (step S13).

[0037] The DA converter 102 converts the α·￣DATA_CPFSK from a digital signal to an analog signal and outputs it to the first intensity modulator 105 (step S14). The first intensity modulator 105 applies the α·￣DATA_CPFSK to the CPFSK signal input from the demultiplexer 104 and then outputs it to the polarization multiplexing unit 108 (step S15).

[0038] Also, the DA converter 102 converts the α·￣DATA_CPFSK + β·DATA_IM from a digital signal to an analog signal and outputs it to the second intensity modulator 106 (step S16). The second intensity modulator 106 applies the α·￣DATA_CPFSK + β·DATA_IM to the CPFSK signal input from the demultiplexer 104 to generate an intensity modulation signal, and outputs the generated intensity modulation signal to the polarization rotation unit 107 (step S17).

[0039] The polarization rotation unit 107 rotates the polarization of the intensity modulation signal input from the second intensity modulator 106 and converts it into an intensity modulation signal with a polarization orthogonal to the input intensity modulation signal (step S18). The polarization rotation unit 107 outputs the converted intensity modulation signal to the polarization multiplexing unit 108. The polarization multiplexing unit 108 multiplexes the CPFSK signal input from the first intensity modulator 105 and the intensity modulation signal input from the polarization rotation unit 107, and outputs the multiplexed signal light (step S19).

[0040] In the above, the polarization rotation unit 107 is provided at the subsequent stage of the second intensity modulator 106, but it may be provided at the subsequent stage of the first intensity modulator 105. Also, in the present embodiment, the signal generation unit is configured by the digital signal processing circuit 101 and the DA converter 102, but as the signal generation unit, an analog signal generator that does not go through digital processing may be used. Further, in FIG. 1, a configuration for applying a binary amplitude modulation signal (ASK) is described, but an amplitude modulation signal with an arbitrary number of levels can be used. Furthermore, the CPFSK signal and the IM signal may be modulated with independent numbers of levels respectively.

[0041] Also, as shown in FIG. 6, the direct modulation laser 103, the first intensity modulator 105, and the signal generation unit that generates the signals output to the second intensity modulator 106 may be configured by different devices respectively. Also, they may be used for information transmission and monitoring control as independent paths.

[0042] FIG. 6 is a configuration diagram of the transmitter 150 of the first embodiment. In FIG. 6, the same parts as those of the transmitter 100 shown in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. The difference between the transmitter 150 shown in FIG. 6 and the transmitter 100 shown in FIG. 1 is that instead of the digital signal processing circuit 101 and the DA converter 102, it includes digital signal processing circuits 151, 153, 155, and DA converters 152, 154, 156.

[0043] The digital signal processing circuit 151 outputs DATA_CPFSK of the digital signal. The DA converter 152 converts the DATA_CPFSK output from the digital signal processing circuit 151 from a digital signal into an analog signal of amplitude modulation and applies it to the direct modulation laser 103. The digital signal processing circuit 153 outputs α·￣DATA_CPFSK of the digital signal. The DA converter 154 converts the α·￣DATA_CPFSK output from the digital signal processing circuit 153 from a digital signal into an analog signal and outputs it to the first intensity modulator 105. The digital signal processing circuit 155 outputs α·￣DATA_CPFSK + β·DATA_IM. The DA converter 156 converts the α·￣DATA_CPFSK + β·DATA_IM output from the digital signal processing circuit 155 from a digital signal into an analog signal and outputs it to the second intensity modulator 106.

[0044] As a modification of the first embodiment, a configuration may be adopted in which polarization separation is performed using an element that separates polarization at an angle shifted by 45 degrees with respect to the output polarization axis of the direct modulation laser. FIG. 7 is a configuration diagram of the transmitter 200 of the modification. In FIG. 7, the same parts as those of the transmitter 100 shown in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. The difference between the transmitter 200 shown in FIG. 7 and the transmitter 100 shown in FIG. 1 is that it includes a polarization beam splitter 201 instead of the wavelength division multiplexer 104 and does not have the polarization rotator 107.

[0045] The polarization beam splitter 201 separates the CPFSK signal, which is the output signal light from the directly modulated laser 103, into two polarizations that are 45 degrees offset and -45 degrees offset from the output signal light. The polarization beam splitter 201 outputs the CPFSK signal of one polarization to the first intensity modulator 105 and the CPFSK signal of the other polarization to the second intensity modulator 106. The first intensity modulator 105 applies the α·￣DATA_CPFSK output by the DA converter 102 to the CPFSK signal input from the polarization beam splitter 201 to remove the intensity modulation component, and then outputs it to the polarization multiplexing section 108. The second intensity modulator 106 applies the α·￣DATA_CPFSK + β·DATA_IM output by the DA converter 102 to the CPFSK signal input from the polarization beam splitter 201 to generate an intensity modulation signal, and outputs the generated intensity modulation signal to the polarization multiplexing section 108. The polarization multiplexing section 108 multiplexes the output of the first intensity modulator 105 and the output of the second intensity modulator 106, and outputs the multiplexed signal light.

[0046] The processing of the transmitter 200 is the same as the processing flow shown in FIG. 5 except for the following. That is, in step S13, the polarization beam splitter 201 separates the CPFSK signal, which is the output signal light from the directly modulated laser 103, into two polarizations that are 45 degrees offset and -45 degrees offset, and outputs each polarization to the first intensity modulator 105 and the second intensity modulator 106. Also, the transmitter 200 does not perform the processing of step S18.

[0047] Also, as shown in FIG. 8, the signal generation units that generate the signals output to the directly modulated laser 103, the first intensity modulator 105, and the second intensity modulator 106 may be configured by different devices. Also, they may be used for information transmission and monitoring control as independent paths.

[0048] FIG. 8 is a configuration diagram of the transmitter 250. In FIG. 8, the same parts as those of the transmitter 200 shown in FIG. 7 are denoted by the same reference numerals, and the description thereof is omitted. The difference between the transmitter 250 shown in FIG. 8 and the transmitter 200 shown in FIG. 7 is that instead of the digital signal processing circuit 101 and the DA converter 102, the digital signal processing circuits 151, 153, 155 and the DA converters 152, 154, 156 are provided. The digital signal processing circuits 151, 153, 155 and the DA converters 152, 154, 156 of the transmitter 250 operate in the same manner as the digital signal processing circuits 151, 153, 155 and the DA converters 152, 154, 156 of the transmitter 150 shown in FIG. 6.

[0049] (Second Embodiment) In the second embodiment, as described in Reference 1, as a light source, a DFB (Distributed-FeedBack) laser that emits signal light in both directions with respect to the active layer by removing the antireflection film is used. A CPFSK signal is generated by direct modulation of the DFB laser, and removal and application of an intensity modulation component are performed by external intensity modulators disposed at both ends of the DFB laser.

[0050] (Reference 1) K. Zhong et al., "Double-Side EML for High Speed Optical Short Reach and Metro Applications," 2017 Opto-Electronics and Communications Conference (OECC) and Photonics Global Conference (PGC), 2017, doi: 10.1109 / OECC.2017.8115007.

[0051] FIG. 9 is a configuration diagram of the transmitter 300. In the figure, the same parts as those of the transmitter 100 according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. The transmitter 300 includes a digital signal processing circuit 101, a DA converter 102, a light source 301, a first intensity modulator 105, a second intensity modulator 106, a polarization rotation unit 107, and a polarization multiplexing unit 108.

[0052] The DA converter 102 applies DATA_CPFSK to the light source 301 in the same manner as in the first embodiment. Also, the DA converter 102 outputs α·￣DATA_CPFSK to the first intensity modulator 105 and outputs α·￣DATA_CPFSK + β·DATA_IM to the second intensity modulator 106 in the same manner as in the first embodiment.

[0053] The light source 301 is a DFB laser that removes the antireflection coating and emits signal light in both directions with respect to the active layer. The light source 301 to which DATA_CPFSK is applied outputs a CPFSK signal to the first intensity modulator 105 and the second intensity modulator 106.

[0054] The first intensity modulator 105 applies α·￣DATA_CPFSK to the CPFSK signal input from the light source 301. As a result, the first intensity modulator 105 outputs a CPFSK signal with the intensity modulation component canceled to the polarization multiplexing unit 108. On the other hand, the second intensity modulator 106 applies α·￣DATA_CPFSK + β·DATA_IM output from the DA converter 102 to the CPFSK signal input from the light source 301. As a result, the second intensity modulator 106 cancels the intensity modulation component generated in the directly modulated laser 103, and further adds the signal component to be transmitted by intensity modulation to generate an intensity modulation signal. The second intensity modulator 106 outputs the generated intensity modulation signal to the polarization rotation unit 107.

[0055] The polarization rotation unit 107 rotates the polarization of the intensity modulation signal input from the second intensity modulator 106 and converts it into an intensity modulation signal orthogonal to the input intensity modulation signal. The polarization rotation unit 107 outputs the converted intensity modulation signal to the polarization multiplexing unit 108. The polarization multiplexing unit 108 multiplexes the CPFSK signal input from the first intensity modulator 105 and the intensity modulation signal input from the polarization rotation unit 107, and outputs the multiplexed signal.

[0056] The processing of the transmitter 300 is the same as the processing flow shown in FIG. 5, except for the following. That is, in step S11, the DA converter 102 applies DATA_CPFSK to the light source 301. In step S12, the light source 301 outputs the CPFSK signal generated by the application of DATA_CPFSK to the first intensity modulator 105 and the second intensity modulator 106. Also, the transmitter 300 does not perform the processing of step S13.

[0057] In this configuration, since a wavelength division unit is not required as compared with the first embodiment, the configuration is simplified.

[0058] As shown in FIG. 10, the signal generation units that generate the signals output to the light source 301, the first intensity modulator 105, and the second intensity modulator 106 may be configured by separate devices. Further, they may be used for information transmission and monitoring control as independent paths.

[0059] FIG. 10 is a configuration diagram of the transmitter 350 of this embodiment. In the transmitter 350 shown in FIG. 10, the same parts as those of the transmitter 300 shown in FIG. 9 are denoted by the same reference numerals, and the description thereof is omitted. The transmitter 350 shown in FIG. 10 is different in that it includes digital signal processing circuits 151, 153, 155, and DA converters 152, 154, 156 instead of the digital signal processing circuit 101 and the DA converter 102. The digital signal processing circuits 151, 153, 155, and the DA converters 152, 154, 156 of the transmitter 350 operate in the same manner as the digital signal processing circuits 151, 153, 155, and the DA converters 152, 154, 156 of the transmitter 150 shown in FIG. 6.

[0060] (Third Embodiment) In this embodiment, a configuration example of the receiver is shown. In the above-described embodiments, the transmitter independently modulates the intensity modulation signal and the CPFSK signal for each polarization. Therefore, signal reception is possible by both a photoelectric converter that performs square-law detection of the signal optical electric field and a coherent receiver that can decode including phase information.

[0061] When the signal optical electric field is square - detected, the square component of the polarization - multiplexed signal electric field E, which is the received signal, is received. Therefore, the received signal i(t) after photoelectric conversion is as shown in Equation (4). sig The A in the first term on the right - hand side of Equation (4) is a DC component and does not vary with time. The A

[0062]

Equation

[0063] in the second term on the right - hand side indicates that only the intensity - modulation component can be received without being affected by the CPFSK component. m_IM Figure 11 is a configuration diagram of the receiver 400. The receiver 400 includes a photoelectric converter 401, an AD converter 402, and a digital signal processor 403. The photoelectric converter 401 receives the signal light transmitted from the transmitters 100, 150, 200, 250, 300, 350 and transmitted through the optical fiber. The photoelectric converter 401 square - detects the signal optical electric field of the received signal light and receives the received signal i(t) shown in Equation (4). The AD converter 402 converts the received signal from the photoelectric converter 401 from an analog signal to a digital signal. The digital signal processor 403 is a signal decoding unit. The digital signal processor 403 decodes the intensity - modulation component of the received signal converted by the AD converter 402 into a digital signal.

[0064] Figure 12 is a configuration diagram of the receiver 450. In the figure, the same parts as those of the receiver 400 shown in Figure 11 are denoted by the same reference numerals, and their descriptions are omitted. The receiver 450 includes a photoelectric converter 401, an amplifier 451, a clock data recovery 452, and an identification unit 453. The amplifier 451 amplifies the received signal from the photoelectric converter 401. The clock data recovery 452 and the identification unit 453 are analog processing units. The clock data recovery 452 detects the signal timing of the received signal amplified by the amplifier 451. The identification unit 453 decodes the intensity - modulation component of the received signal whose signal timing has been detected by the clock data recovery 452.

[0065]

[0066] ​ FIG. 13 is a configuration diagram of the receiver 500. The receiver 500 receives a signal by a coherent receiver. The receiver 500 includes an LO 501, a coherent receiver 502, an AD converter 503, and a decoding processing unit 504. The LO 501 generates light of a single frequency. The coherent receiver 502 is a general polarization and phase diversity receiver. The coherent receiver 502 receives the signal light transmitted from the transmitters 100, 150, 200, 250, 300, 350 and transmitted through the optical fiber. The coherent receiver 502 obtains a signal converted into an electrical signal by the beat between the received signal light and the local light emitted by the LO 501. The coherent receiver 502 performs polarization separation and phase separation of the received signal converted into an electrical signal to obtain an intensity modulation signal and a CPFSK signal. The coherent receiver 502 outputs the obtained intensity modulation signal and CPFSK signal to the AD converter 503. The AD converter 503 converts the intensity modulation signal and CPFSK signal input from the coherent receiver 502 from an analog signal to a digital signal. The decoding processing unit 504 decodes the data of each of the intensity modulation signal and the CPFSK signal.

[0067] FIG. 14 is a diagram showing a configuration example of the signal processing circuit 600. The signal processing circuit 600 is used as the decoding processing unit 504 shown in FIG. 13. The signal processing circuit 600 includes a wavelength dispersion compensation circuit 610, a polarization estimation and separation circuit 620, an intensity signal processing unit 630, and a CPFSK signal processing unit 640.

[0068] The wavelength dispersion compensation circuit 610 estimates the wavelength dispersion of the signal light due to fiber propagation and compensates the received signal for the estimated wavelength dispersion. The wavelength dispersion compensation circuit 610 outputs the received signal with compensated wavelength dispersion to the polarization estimation and separation circuit 620. When the polarization axis at the time of transmission is different from the default polarization axis of the coherent receiver, or when the polarization state of the signal light fluctuates due to polarization rotation during fiber propagation, etc., if polarization diversity is used to separate and receive the polarization, the intensity signal and the CPFSK signal may be mixed in the received signal corresponding to each polarization. Therefore, the polarization estimation and separation circuit 620 estimates the polarization state, compensates it to the polarization state at the time of transmission, and separates the signals of each polarization in which the intensity modulation component and the CPFSK modulation component are superimposed. The polarization estimation and separation circuit 620 outputs the polarization of the intensity modulation signal among the complex signals obtained by polarization separation to the intensity signal processing unit 630, and outputs the polarization of the CPFSK signal to the CPFSK signal processing unit 640.

[0069] The intensity signal processing unit 630 includes an absolute value acquisition unit 631, a DC (direct current) component removal unit 632, an adaptive equalization filter 633, and a decoder 634. The absolute value acquisition unit 631 of the intensity signal processing unit 630 converts the complex signal into a signal of intensity information by calculating the absolute value of each complex signal input from the polarization estimation and separation circuit 620. The DC component removal unit 632 inputs the signal converted by the absolute value acquisition unit 631 and removes the DC component from each input signal. The adaptive equalization filter 633 inputs the signal from which the DC component has been removed by the DC component removal unit 632 and compensates for the waveform degradation of the input signal. The decoder 634 decodes the signal whose waveform degradation has been compensated by the adaptive equalization filter 633 after performing downsampling processing. Thereby, the data transmitted by the intensity modulation (IM) signal is obtained.

[0070] The CPFSK signal processing unit 640 receives a CPFSK signal by the method described in Non-Patent Document 2. The CPFSK signal processing unit 640 includes a 1-bit delay detector 641, an adaptive equalization filter 642, a phase compensation unit 643, and a decoding unit 644. The 1-bit delay detector 641 performs 1-bit delay detection of the polarization signal. The adaptive equalization filter 642 applies an adaptive equalization filter to the signal obtained by the 1-bit delay detector 641 through 1-bit delay detection. The phase compensation unit 643 compensates the phase of the signal to which the adaptive equalization filter has been applied. The decoding unit 644 decodes the polarization whose phase has been compensated. Thereby, the data transmitted by the CPFSK signal is obtained.

[0071] According to the above-described embodiment, the transmission device includes a directly modulated laser, a branching unit, a first polarization signal generation unit, a second polarization signal generation unit, and a polarization multiplexing unit. The branching unit branches the signal light of continuous phase frequency modulation generated by the directly modulated laser by applying the first modulation signal into a first branched light and a second branched light. The first signal generation unit generates signal light of a first polarization obtained by removing the intensity modulation component generated by the directly modulated laser by applying the first modulation signal from the first branched light. The second signal generation unit generates signal light of a second polarization orthogonal to the first polarization, which is signal light obtained by removing the intensity modulation component from the second branched light and adding an intensity modulation component by applying the second modulation signal.

[0072] The branching unit branches the signal light of continuous phase frequency modulation of the first polarization into a first branched light and a second branched light. The branching unit is, for example, the optical splitter 104 of the embodiment. The second signal generation unit converts to the second polarization after removing the intensity modulation component generated by the directly modulated laser by applying the first modulation signal to the second branched light and adding an intensity modulation component by applying the second modulation signal. The second signal generation unit is, for example, the second intensity modulator 106 and the polarization rotation unit 107 of the embodiment.

[0073] The branching unit may branch the signal light of the continuous phase frequency modulation of the second polarization into first branched light and second branched light. The branching unit is, for example, the optical splitter 104 of the embodiment. The first signal generation unit removes the intensity modulation component generated by the directly modulated laser by applying the first modulation signal from the first branched light, and then converts it into the first polarization. The first signal generation unit is, for example, the first intensity modulator 105 of the embodiment and the polarization rotation unit 107 provided after the first intensity modulator 105.

[0074] The branching unit branches the signal light of the continuous phase frequency modulation into the first branched light of the first polarization and the second branched light of the second polarization. The branching unit is, for example, the polarization beam splitter 201 of the embodiment. The first signal generation unit removes the intensity modulation component generated by the directly modulated laser by applying the first modulation signal from the first branched light to generate the signal light of the first polarization. The second signal generation unit performs removal of the intensity modulation component from the second branched light and addition of the intensity modulation component by applying the second modulation signal to generate the signal light of the second polarization.

[0075] Instead of the directly modulated laser and the branching unit, the transmitter includes a light source that directly modulates by applying the first modulation signal and outputs the signal light of the continuous phase frequency modulation in two directions. The light source is, for example, the light source 301 of the embodiment.

[0076] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Description of Reference Numerals

[0077] 100, 150, 200, 250, 300, 350 Transmitter 101, 151, 153, 155 Digital Signal Processing Circuit 102, 152, 154, 156 DA Converter 102 Converter 103 Directly Modulated Laser 104 Optical Splitter 105 First Intensity Modulator 106 Second Intensity Modulator 107 Polarization Rotation Unit 108 Polarization multiplexing section 201 Polarization demultiplexer 301 Light source 400 Receiver 401 Photoelectric converter 402, 503 AD converter 403 Digital signal processor 450 Receiver 451 Amplifier 452 Clock data recovery 453 Identification section 500 Receiver 501 LO 502 Coherent receiver 504 Decoding processing section 600 Signal processing circuit 610 Chromatic dispersion compensation circuit 620 Polarization estimation and separation circuit 630 Intensity signal processing section 631 Absolute value acquisition section 632 DC component removal section 633 Adaptive equalization filter 634 Decoder 640 CPFSK signal processing section 641 1-bit delay detection section 642 Adaptive equalization filter 643 Phase compensation section 644 Decoding section

Claims

1. A directly modulated laser, a branching unit that branches the signal light of continuous phase frequency modulation generated by the directly modulated laser by applying a first modulation signal into a first branched light and a second branched light, a first signal generation unit that generates signal light of a first polarization that removes an intensity modulation component generated by the directly modulated laser by applying the first modulation signal from the first branched light, a second signal generation unit that generates signal light of a second polarization that is orthogonal to the first polarization, which is signal light obtained by removing the intensity modulation component from the second branched light and adding an intensity modulation component by applying a second modulation signal, a polarization multiplexing unit that multiplexes the signal light of the first polarization and the signal light of the second polarization, A transmission device comprising:

2. The branching unit branches the signal light of the continuous phase frequency modulation of the first polarization into the first branched light and the second branched light, The second signal generation unit converts the second branched light into the second polarization after removing the intensity modulation component and adding the intensity modulation component by applying the second modulation signal, The transmission device according to claim 1.

3. The branching unit branches the signal light of the continuous phase frequency modulation of the second polarization into the first branched light and the second branched light, The first signal generation unit converts the first branched light into the first polarization after removing the intensity modulation component generated by the directly modulated laser by applying the first modulation signal, The transmission device according to claim 1.

4. The branching unit branches the signal light of the continuous phase frequency modulation into the first branched light of the first polarization and the second branched light of the second polarization, The first signal generation unit removes the intensity modulation component generated by the directly modulated laser by applying a first modulation signal from the first branched light to generate signal light of the first polarization, The second signal generation unit generates signal light of the second polarization by removing the intensity modulation component from the second branched light and adding an intensity modulation component by applying a second modulation signal, The transmission device according to claim 1.

5. Instead of the directly modulated laser and the branching unit, a light source that performs direct modulation by applying the first modulation signal and outputs signal light of continuous phase frequency modulation in two directions is provided, The transmission device according to claim 1.

6. A branching step of branching the signal light of continuous phase frequency modulation generated by the directly modulated laser by applying a first modulation signal into a first branched light and a second branched light, A first signal generation step of generating signal light of a first polarization that removes an intensity modulation component generated by the directly modulated laser by application of a first modulation signal from the first branched light; A second signal generation step of generating signal light of a second polarization orthogonal to the first polarization, which is signal light obtained by removing the intensity modulation component from the second branched light and adding an intensity modulation component by application of a second modulation signal; A polarization multiplexing step of multiplexing the signal light of the first polarization and the signal light of the second polarization; A signal generation method comprising the above steps.

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