Optical transmitter

Abstract

An optical transmitter according to the present invention comprises a broadband signal generation unit including: a switch circuit that alternately selects a first narrow band signal or a second narrow band signal from among the first narrow band signal and the second narrow band signal by synchronizing with a clock signal; a frequency shifter that shifts the frequency of a third narrow band signal using a signal phase-shifted 90 degrees relative to the clock signal; and a first adder that adds the signal selected from among the first narrow band signal and the second narrow band signal and the frequency-shifted third narrow band signal. The broadband signal generation unit may further include an addition and subtraction circuit that updates the first narrow band signal by adding the second narrow band signal multiplied by a predetermined coefficient and the first narrow band signal, updates the second narrow band signal by subtracting the second narrow band signal multiplied by the predetermined coefficient from the first narrow band signal prior to being updated, and inputs the updated first narrow band signal and second narrow band signal to the switch circuit.

Description

Optical transmitter

[0001] The present invention relates to an optical transmitter.

[0002] Due to an increase in the traffic volume of client signals in the backbone networks of optical communication systems, there is a demand for an increase in the transmission capacity per channel. In optical communication systems, a digital coherent optical transmission method using DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) is sometimes adopted as the modulation method. Also, Figure 25 of Patent Document 1 shows an example of the configuration of a wideband signal generator that generates a wideband analog signal in a band doubler optical transmitter.

[0003] International Publication No. 2019 / 031447

[0004] 13 is a diagram showing an example of the configuration of a wideband signal generator 101. In an optical transmitter of an optical communication system, the wideband signal generator 101 generates a wideband analog signal based on narrowband analog signals "a(t)," "b(t)," and "c(t)." The wideband signal generator 101 includes a frequency shifter "#1," a frequency shifter "#2," a phase shifter "(π / 2)," an adder 102 (wideband combiner circuit), and an adder 103 (wideband combiner circuit).

[0005] The adder 102 adds the narrowband analog signal output from the frequency shifter "#1" and the narrowband analog signal output from the frequency shifter "#2". The adder 103 adds the result of the addition by the adder 102 to the narrowband analog signal "a(t)". ​​This generates a wideband analog signal.

[0006] Thus, unless the wideband signal generating section is provided with at least two adders for adding a plurality of narrowband analog signals, there is a problem that a wideband analog signal cannot be generated.

[0007] In view of the above circumstances, an object of the present invention is to provide an optical transmitter that is capable of generating a wideband analog signal even when the wideband signal generating section is provided with only one adder that adds multiple narrowband analog signals.

[0008] One aspect of the present invention is an optical transmitter comprising a wideband signal generating unit having: a switch circuit that alternately selects the first narrowband signal or the second narrowband signal from among a first narrowband signal and a second narrowband signal in synchronization with a clock signal; a frequency shifter that shifts the frequency of a third narrowband signal with a signal that is phase-shifted by 90 degrees relative to the phase of the clock signal; and a first adder that adds the signal selected from the first narrowband signal and the second narrowband signal and the frequency-shifted third narrowband signal.

[0009] According to the present invention, even if the wideband signal generating section is provided with only one adder for adding a plurality of narrowband analog signals, it is possible to generate a wideband analog signal.

[0010] FIG. 1 is a diagram showing an example of the configuration of an optical communication system in a first embodiment. FIG. 2 is a diagram showing an example of the configuration of a wideband signal generating device in a first embodiment. FIG. 3 is a diagram showing an example of signal processing of a band dividing unit in a first embodiment. FIG. 4 is a diagram showing an example of the configuration of a narrowband signal processing unit in a first embodiment. FIG. 5 is a diagram showing an example of the configuration of a wideband signal generating unit in a first embodiment. FIG. 6 is a diagram showing an example of signal processing of a wideband signal generating unit in a first embodiment. FIG. 7 is a diagram showing an example of the configuration of a switch circuit in a first modified example of the first embodiment. FIG. 8 is a diagram showing an example of the configuration of a frequency shifter in a second modified example of the first embodiment. FIG. 9 is a diagram showing an example of the configuration of a wideband signal generating unit in a second embodiment. FIG. 10 is a diagram showing an example of the configuration of a narrowband signal processing unit in a second embodiment. FIG. 11 is a diagram showing an example of signal processing of a wideband signal generating unit in a second embodiment. FIG. 12 is a diagram showing an example of the configuration of an optical communication system in a third embodiment. FIG. 13 is a diagram showing an example of the configuration of a wideband signal generating unit.

[0011]

[0023] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) Fig. 1 is a diagram showing an example of the configuration of an optical communication system 1a in the first embodiment. The optical communication system 1a includes an optical transmitter 2a, an optical transmission line 3, and an optical receiver 4a. The optical transmitter 2a includes a digital signal processing unit 21, a broadband signal generating device 22a, and an intensity modulator 23.

[0012] Some functions of the digital signal processing unit 21 and the wideband signal generating device 22a are realized as software by a processor such as a CPU (Central Processing Unit) executing a program stored in a storage unit having a non-volatile storage medium (non-transitory storage medium). The program may be recorded on a computer-readable storage medium or may be recorded on the storage unit via a network. Examples of computer-readable storage media include portable media such as flexible disks, magneto-optical disks, ROMs (Read Only Memory), and CD-ROMs (Compact Disc Read Only Memory), and non-transitory storage devices such as hard disks or solid-state drives (SSDs) built into a computer system.

[0013] Some functions of the digital signal processing unit 21 and the wideband signal generating device 22a may be realized using hardware (accelerator) including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0014] The optical transmitter 2a transmits an optical signal based on a transmission data sequence (binary information) to the optical receiver 4a via the optical transmission path 3. The digital signal processing unit 21 generates a wideband digital signal by performing predetermined digital signal processing on the transmission data sequence. The digital signal processing unit 21 outputs the wideband digital signal (high-speed digital signal) to the wideband signal generating device 22a. The wideband signal generating device 22a generates a wideband analog signal (high-speed analog signal) based on the wideband digital signal. The intensity modulator 23 generates an optical signal by performing intensity modulation on the wideband analog signal. The intensity modulator 23 transmits the optical signal to the optical transmission path 3.

[0015] The optical transmission path 3 transmits an optical signal. The optical receiver 4a receives the transmitted optical signal. The optical receiver 4a performs predetermined reception processing on the received optical signal to generate a received signal. The optical receiver 4a performs predetermined demodulation processing on the received signal to generate a received data sequence (binary information).

[0016] Next, the wideband signal generation device 22a will be described in detail. Fig. 2 is a diagram showing an example of the configuration of the wideband signal generation device 22a in the first embodiment. The wideband signal generation device 22a includes a band division unit 221, a narrowband signal processing unit 222, a digital-to-analog conversion unit 223, a clock generation unit 224, and a wideband signal generation unit 225a.

[0017] The wideband digital signal is input to the band dividing section 221 from the digital signal processing section 21. The band dividing section 221 divides the wideband digital signal into bands to generate narrowband digital signals for a plurality of systems.

[0018] The narrowband signal processing unit 222 performs predetermined signal processing on the narrowband digital signals of the multiple systems, thereby generating processed narrowband digital signals for each system. The digital-to-analog conversion unit 223 converts the processed narrowband digital signals into narrowband analog signals for each system (digital-to-analog conversion). The clock generation unit 224 outputs a clock signal to the wideband signal generation unit 225a. The wideband signal generation unit 225a generates a wideband analog signal by combining the narrowband analog signals of the multiple systems using the clock signal.

[0019] In the following formulas, "f" represents the frequency of the clock signal. "ω" represents the angular frequency (angular velocity) of the clock signal. The angular frequency "ω" is equal to "2πf". "(.) ＊ " represents a complex conjugate signal. "j" represents an imaginary unit. "r" represents a Fourier coefficient when the clock signal is expanded into a Fourier series.

[0020] 3 is a diagram showing an example of signal processing by the band dividing unit 221 in the first embodiment. In FIG. 3, the band dividing unit 221 divides a narrowband digital signal “X ０ (f)" and narrowband digital signal "X １ (f)" and narrowband digital signal "X ２ (f)) and (f)).

[0021] 4 is a diagram showing an example of the configuration of the narrowband signal processing unit 222 in the first embodiment. The narrowband signal processing unit 222 includes three folding signal generating units 201, nine coefficient multiplying units 202, nine coefficient multiplying units 203, and three adders 204.

[0022] The folding signal generating unit 201-1 (first folding signal generating unit) generates a narrowband digital signal “X ０ The folded signal generator 201-2 (second folded signal generator) folds the frequency of the narrowband digital signal "X (f)" (first digital signal). １ The folded signal generating unit 201-3 (third folded signal generating unit) folds the frequency of the narrowband digital signal "X (f)" (second digital signal). ２ (f)" (third digital signal) is folded back in frequency.

[0023] When a non-ideal state (e.g., imbalance, skew, and crosstalk) exists between the narrowband analog signals in the digital-analog conversion unit 223 and the wideband signal generation unit 225a, the coefficients of each coefficient multiplication unit 202 are determined in advance so that the wideband analog signals of each system generated by the wideband signal generation unit 225a approach each target waveform. The following describes the coefficients of each coefficient multiplication unit 202 when an ideal state exists between the narrowband analog signals in the digital-analog conversion unit 223 and the wideband signal generation unit 225a.

[0024] The coefficient multiplication unit 202-1 (first coefficient multiplication unit) multiplies the narrowband digital signal “X ０ The coefficient multiplication unit 202-2 (second coefficient multiplication unit) multiplies the narrowband digital signal "X １The coefficient multiplication unit 202-3 (third coefficient multiplication unit) multiplies the narrowband digital signal "X ２ (f) is multiplied by the coefficient "1 / r" (third coefficient).

[0025] The coefficient multiplication unit 203-1 (fourth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal “X ０ ＊ The coefficient multiplication unit 203-2 (fifth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X １ ＊ The coefficient multiplication unit 203-3 (sixth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X ２ ＊ (-f)" is multiplied by the coefficient "0" (the sixth coefficient).

[0026] The coefficient multiplication unit 202-4 (first coefficient multiplication unit) multiplies the narrowband digital signal "X ０ The coefficient multiplication unit 202-5 (second coefficient multiplication unit) multiplies the narrowband digital signal "X １ The coefficient multiplication unit 202-6 (third coefficient multiplication unit) multiplies the narrowband digital signal "X ２ (f)" is multiplied by the coefficient "-1 / r".

[0027] The coefficient multiplication unit 203-4 (fourth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X ０ ＊ The coefficient multiplication unit 203-5 (fifth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X １ ＊ The coefficient multiplication unit 203-6 (sixth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "-1 / r". ２ ＊ (-f)" is multiplied by the coefficient "0".

[0028] The coefficient multiplication unit 202-7 (first coefficient multiplication unit) multiplies the narrowband digital signal "X ０The coefficient multiplication unit 202-8 (second coefficient multiplication unit) multiplies the narrowband digital signal "X １ The coefficient multiplication unit 202-9 (third coefficient multiplication unit) multiplies the narrowband digital signal "X ２ (f)" is multiplied by the coefficient "j".

[0029] The coefficient multiplication unit 203-7 (fourth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X ０ ＊ The coefficient multiplication unit 203-8 (fth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X １ ＊ The coefficient multiplication unit 203-9 (sixth coefficient multiplication unit) multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "-j". ２ ＊ (-f)" is multiplied by the coefficient "0".

[0030] The adder 204-1 outputs the narrowband digital signal "X ０ (f)" and the narrowband digital signal "X １ (f)" and the narrowband digital signal "X ２ (f)" and a folded frequency narrowband digital signal "X ０ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "1 / r". １ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". ２ ＊ (-f)" is added.

[0031] When a non-ideal state exists between the narrowband analog signal in the digital-to-analog conversion unit 223 and the wideband signal generation unit 225a, these coefficients are predetermined so that the addition result by the adder 204-1 approaches the narrowband analog signal (target waveform) exemplified in equation (1).

[0032]

[0033] The adder 204-2 outputs the narrowband digital signal "X ０ (f)" and the narrowband digital signal "X １ (f)" and the narrowband digital signal "X ２ (f)" and a folded frequency narrowband digital signal "X ０ ＊ (-f)" and the folded frequency narrowband digital signal "X １ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". ２ ＊ (-f)" is added.

[0034] When a non-ideal state exists between the narrowband analog signal in the digital-to-analog conversion unit 223 and the wideband signal generation unit 225a, these coefficients are predetermined so that the addition result by the adder 204-2 approaches the narrowband analog signal (target waveform) exemplified in equation (2).

[0035]

[0036] The adder 204-3 outputs the narrowband digital signal "X ０ (f)" and the narrowband digital signal "X １ (f)" and the narrowband digital signal "X ２ (f)" and a folded frequency narrowband digital signal "X ０ ＊ (-f)" and the folded frequency narrowband digital signal "X １ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". ２ ＊ (-f)" is added.

[0037] When a non-ideal state exists between the narrowband analog signal in the digital-to-analog conversion unit 223 and the wideband signal generation unit 225a, these coefficients are predetermined so that the addition result by the adder 204-3 approaches the narrowband analog signal (target waveform) exemplified in equation (3).

[0038]

[0039] 5 is a diagram showing an example of the configuration of the wideband signal generator 225a in the first embodiment. The wideband signal generator 225a includes a switch circuit 205, a phase shifter 206, a frequency shifter 207, and an adder 208.

[0040] A clock signal is input to the switch circuit 205 from the clock generating unit 224. The switch circuit 205 alternately selects the first narrowband analog signal “a(t)” or the second narrowband analog signal “b(t)” in synchronization with the clock signal.

[0041] The phase shifter 206 generates a signal that is shifted by 90 degrees relative to the phase of the clock signal. The frequency shifter 207 shifts the frequency of the third narrowband signal by the signal that is shifted by 90 degrees relative to the phase of the clock signal. That is, the frequency shifter 207 multiplies the third narrowband analog signal "c(t)" by the signal that is shifted by 90 degrees relative to the phase of the clock signal.

[0042] The adder 208 (first adder) adds three narrowband analog signals. That is, the adder 208 adds a signal selected from the first narrowband analog signal and the second narrowband analog signal and a frequency-shifted third narrowband analog signal. In FIG. 5 , the addition result is expressed as "d(t)=a(t)+b(t)+r{a(t)-b(t)}cos(ωt)+c(t)sin(ωt)." Here, "a(t)+b(t)" may be expressed as "a'(t)." Furthermore, "r{a(t)-b(t)}" may be expressed as "b'(t)."

[0043] 6 is a diagram illustrating an example of signal processing by the wideband signal generating unit 225a in the first embodiment. As an example, the frequency "ω" of the clock signal is twice the bandwidth "ω / 2" of each narrowband analog signal input to the wideband signal generating unit 225a. That is, the frequency "ω" of the clock signal is twice the bandwidth "ω / 2" of each of the first narrowband signal, the second narrowband signal, and the third narrowband signal. Furthermore, the bandwidth "3ω / 2" of the wideband analog signal generated by the wideband signal generating unit 225a is three times the bandwidth "ω / 2" of the narrowband analog signal.

[0044] The narrowband analog signal exemplified by equation (1) and the narrowband analog signal exemplified by equation (2) are input to the switch circuit 205. The coefficient "1 / r" in these narrowband analog signals is canceled by the Fourier coefficient "r" of the clock signal input to the switch circuit 205. The narrowband analog signal exemplified by equation (3) is input to the frequency shifter 207. The adder 208 outputs the wideband analog signal exemplified by equation (4) to the intensity modulator 23.

[0045]

[0046] Here, equations (5) and (6) hold.

[0047]

[0048]

[0049] Therefore, the intensity of the spectral component modulated with the sine wave "sin(ωt)" or the cosine wave "cos(ωt)" is halved.

[0050] As described above, the switch circuit 205 alternately selects the first narrowband signal "a(t)" or the second narrowband signal "b(t)" in synchronization with the clock signal. The frequency shifter 207 shifts the frequency of the third narrowband signal "c(t)" with a signal that is phase-shifted by 90 degrees relative to the phase of the clock signal. The adder 208 (first adder) adds the signal selected from the first narrowband signal and the second narrowband signal to the frequency-shifted third narrowband signal. The adder 208 outputs the addition result "d(t)" to the intensity modulator 23.

[0051] This makes it possible to generate a wideband analog signal even if the wideband signal generating unit 225a is provided with only one adder 208 (wideband adder) that adds multiple narrowband analog signals (three systems of narrowband analog signals).

[0052] 7 is a diagram showing an example of the configuration of a switch circuit 211 (equivalent circuit of the switch circuit) in a first modification of the first embodiment. The switch circuit 211 is provided in a wideband signal generating unit 225a, instead of the switch circuit 205 illustrated in FIG. 5. The switch circuit 211 includes a multiplier 212, a multiplier 213, and an adder 214.

[0053] In FIG. 7, the switch operation using the clock signal is expanded into a Fourier series. Higher-order components are ignored. Multiplier 212 multiplies narrowband analog signal "a(t)" by narrowband analog signal "1+rcos(ωt)." Multiplier 213 multiplies narrowband analog signal "b(t)" by narrowband analog signal "1-rcos(ωt)." If the waveform of the clock signal is a square wave, the Fourier coefficient "r" is 4 / π. If the waveform of the clock signal is a sine wave, the Fourier coefficient "r" is 1. Adder 214 outputs narrowband analog signal "a(t)+b(t)+r{a(t)-b(t)}cos(ωt)" to adder 208.

[0054] 8 is a diagram showing an example of the configuration of a frequency shifter 215 in a second modification of the first embodiment. The frequency shifter 215 is provided in a wideband signal generating unit 225a instead of the frequency shifter 207 illustrated in FIG. 5. The frequency shifter 215 includes a differential signal output circuit 216 and a switch circuit 217.

[0055] The differential signal output circuit 216 outputs the narrowband analog signal "c(t)" and the narrowband analog signal "c(t)" multiplied by the coefficient "-1" to the switch circuit 217. In other words, the differential signal output circuit 216 outputs the narrowband analog signal "c(t)" and the narrowband analog signal "c(t)" with its polarity (sign) inverted to the switch circuit 217.

[0056] The switch circuit 217 selects an analog signal in synchronization with the clock signal from the narrowband analog signal “c(t)” and the narrowband analog signal “c(t)” multiplied by the coefficient “−1.” The switch circuit 217 outputs the selected analog signal to the adder 208.

[0057] Second Embodiment The second embodiment differs from the first embodiment mainly in that the wideband signal generating unit includes an adder-subtractor circuit. The second embodiment will be described focusing on the differences from the first embodiment.

[0058] 9 is a diagram showing an example of the configuration of a wideband signal generator 225b in the second embodiment. The wideband signal generator 225b includes a switch circuit 205, a phase shifter 206, a frequency shifter 207, an adder 208, and an addition / subtraction circuit 218.

[0059] The addition / subtraction circuit 218 receives the narrowband analog signal "a(t)" and the narrowband analog signal "b(t)." The addition / subtraction circuit 218 performs part of the function of the narrowband signal processing unit 222. That is, the addition / subtraction circuit 218 updates the narrowband analog signal by adding the narrowband analog signal "b(t)," which has been multiplied by a predetermined coefficient "1 / r," to the narrowband analog signal "a(t)." The addition / subtraction circuit 218 inputs the updated narrowband analog signal "a(t)+b(t) / r" to the switch circuit 205. The addition / subtraction circuit 218 updates the narrowband analog signal by adding the narrowband analog signal "b(t)," which has been multiplied by a predetermined coefficient "-1 / r," to the narrowband analog signal "a(t)." The addition / subtraction circuit 218 inputs the updated narrowband analog signal "a(t)-b(t) / r" to the switch circuit 205.

[0060] 10 is a diagram showing an example of the configuration of the narrowband signal processing unit 222 in the second embodiment. The narrowband signal processing unit 222 includes three folding signal generating units 201, nine coefficient multiplying units 202, nine coefficient multiplying units 203, and three adders 204 (second adders).

[0061] When a non-ideal state (e.g., imbalance, skew, and crosstalk) exists between the narrowband analog signals in the digital-analog conversion unit 223 and the wideband signal generation unit 225b, the coefficients of each coefficient multiplication unit 202 are determined in advance so that the wideband analog signals of each system generated by the wideband signal generation unit 225b approach each target waveform. The following describes the coefficients of each coefficient multiplication unit 202 when an ideal state exists between the narrowband analog signals in the digital-analog conversion unit 223 and the wideband signal generation unit 225b.

[0062] The coefficient multiplication unit 202-1 multiplies the narrowband digital signal "X ０ The coefficient multiplication unit 202-2 multiplies the narrowband digital signal "X １ The coefficient multiplication unit 202-3 multiplies the narrowband digital signal "X (f)" by the coefficient "0". ２ (f)" is multiplied by the coefficient "0".

[0063] The coefficient multiplication unit 203-1 multiplies the folded frequency narrowband digital signal "X ０ ＊ The coefficient multiplication unit 203-2 multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "0". １ ＊ The coefficient multiplication unit 203-3 multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "0". ２ ＊ (-f)" is multiplied by the coefficient "0".

[0064] The coefficient multiplication unit 202-4 multiplies the narrowband digital signal "X ０ The coefficient multiplication unit 202-5 multiplies the narrowband digital signal "X １ The coefficient multiplication unit 202-6 multiplies the narrowband digital signal "X (f)" by the coefficient "0". ２ (f)) is multiplied by a coefficient of 1.

[0065] The coefficient multiplication unit 203-4 multiplies the folded frequency narrowband digital signal "X ０ ＊ The coefficient multiplication unit 203-5 multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "0". １ ＊ The coefficient multiplier 203-6 multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "1". ２ ＊ (-f)" is multiplied by the coefficient "0".

[0066] The coefficient multiplication unit 202-7 multiplies the narrowband digital signal "X ０ The coefficient multiplication unit 202-8 multiplies the narrowband digital signal "X (f)" by the coefficient "0". １ The coefficient multiplier 202-9 multiplies the narrowband digital signal "X (f)" by the coefficient "0". ２ (f)" is multiplied by the coefficient "j".

[0067] The coefficient multiplication unit 203-7 multiplies the folded frequency narrowband digital signal "X ０ ＊The coefficient multiplication unit 203-8 multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "0". １ ＊ The coefficient multiplier 203-9 multiplies the folded frequency narrowband digital signal "X (-f)" by the coefficient "-j". ２ ＊ (-f)" is multiplied by the coefficient "0".

[0068] The adder 204-1 outputs the narrowband digital signal "X ０ (f)" and the narrowband digital signal "X １ (f)" and the narrowband digital signal "X ２ (f)" and a folded frequency narrowband digital signal "X ０ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". １ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". ２ ＊ (-f)" is added.

[0069] When a non-ideal state exists between the digital-to-analog conversion unit 223 and the narrowband analog signal in the wideband signal generation unit 225b, these coefficients are predetermined so that the addition result by the adder 204-1 approaches the narrowband analog signal (target waveform) exemplified in equation (7).

[0070]

[0071] The adder 204-2 outputs the narrowband digital signal "X ０ (f)" and the narrowband digital signal "X １ (f)" and the narrowband digital signal "X ２ (f)" and a folded frequency narrowband digital signal "X ０ ＊ (-f)" and a folded frequency narrowband digital signal "X１ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". ２ ＊ (-f)" is added.

[0072] When a non-ideal state exists between the digital-to-analog conversion unit 223 and the narrowband analog signal in the wideband signal generation unit 225b, these coefficients are predetermined so that the addition result by the adder 204-2 approaches the narrowband analog signal (target waveform) exemplified in equation (8).

[0073]

[0074] The adder 204-3 outputs the narrowband digital signal "X ０ (f)" and the narrowband digital signal "X １ (f)" and the narrowband digital signal "X ２ (f)" and a folded frequency narrowband digital signal "X ０ ＊ (-f)" and the folded frequency narrowband digital signal "X １ ＊ (-f)" and a folded frequency narrowband digital signal "X" multiplied by the coefficient "0". ２ ＊ (-f)" is added.

[0075] When a non-ideal state exists between the digital-to-analog conversion unit 223 and the narrowband analog signal in the wideband signal generation unit 225b, these coefficients are predetermined so that the addition result by the adder 204-3 approaches the narrowband analog signal (target waveform) exemplified in equation (9).

[0076]

[0077] 11 is a diagram illustrating an example of signal processing by the wideband signal generating unit 225b in the second embodiment. As an example, the frequency "ω" of the clock signal is twice the bandwidth "ω / 2" of each narrowband analog signal input to the wideband signal generating unit 225b. That is, the frequency "ω" of the clock signal is twice the bandwidth "ω / 2" of each of the first narrowband signal, the second narrowband signal, and the third narrowband signal. Furthermore, the bandwidth "3ω / 2" of the wideband analog signal generated by the wideband signal generating unit 225b is three times the bandwidth "ω / 2" of the narrowband analog signal.

[0078] The narrowband analog signal exemplified by equation (7) and the narrowband analog signal exemplified by equation (8) are input to the adder / subtractor circuit 218. The narrowband analog signal exemplified by equation (9) is input to the frequency shifter 207. The adder 208 outputs the wideband analog signal exemplified by equation (10) to the intensity modulator 23.

[0079]

[0080] Here, the above-mentioned formulas (5) and (6) hold true. Therefore, the intensity of the spectral component modulated with the sine wave "sin(ωt)" or the cosine wave "cos(ωt)" is halved.

[0081] As described above, the adder-subtractor circuit 218 updates the first narrowband signal "a(t)" by adding the second narrowband signal "b(t)" multiplied by the predetermined coefficient "1 / r" to the first narrowband signal "a(t)". ​​The adder-subtractor circuit 218 updates the second narrowband signal "b(t)" by subtracting the second narrowband signal "b(t)" multiplied by the predetermined coefficient "1 / r" from the first narrowband signal "a(t)" before updating. The adder-subtractor circuit 218 inputs the updated first and second narrowband signals to the switch circuit 205.

[0082] This makes it possible to generate a wideband analog signal even if the wideband signal generating section is provided with only one adder (wideband adder) that adds a plurality of narrowband analog signals.

[0083] (Third Embodiment) In the third embodiment, the main difference from the first and second embodiments is that the optical transmitter includes an IQ modulator that performs quadrature modulation (complex modulation). In the third embodiment, the differences from the first and second embodiments will be mainly described.

[0084] 12 is a diagram showing an example of the configuration of an optical communication system 1c according to the third embodiment. The optical communication system 1c includes an optical transmitter 2c, an optical transmission line 3, and an optical receiver 4a. The optical transmitter 2c includes a digital signal processing unit 21, two broadband signal generators 22c, and an IQ modulator 24.

[0085] The digital signal processing unit 21 performs predetermined digital signal processing on the transmission data sequence to generate a wideband digital signal. The digital signal processing unit 21 outputs the in-phase wideband digital signal to the wideband signal generating device 22c-1. The digital signal processing unit 21 outputs the quadrature wideband digital signal to the wideband signal generating device 22c-2.

[0086] The wideband signal generator 22c-1 outputs an in-phase component wideband analog signal "I(t)" to the IQ modulator 24. The wideband signal generator 22c-2 outputs a quadrature component wideband analog signal "Q(t)" to the IQ modulator 24.

[0087] The IQ modulator 24 generates an optical signal by performing quadrature modulation (complex modulation) on the in-phase component wideband analog signal “I(t)” and the quadrature component wideband analog signal “Q(t).” The IQ modulator 24 transmits the optical signal to the optical transmission line 3.

[0088] As described above, in the wideband signal generating device 22c-1, the switch circuit 205 alternately selects the first narrowband signal "a(t)" or the second narrowband signal "b(t)" in synchronization with the clock signal. The frequency shifter 207 shifts the frequency of the third narrowband signal "c(t)" with a signal that is phase-shifted by 90 degrees relative to the phase of the clock signal. The adder 208 adds the signal selected from the first narrowband signal and the second narrowband signal to the frequency-shifted third narrowband signal. The adder 208 outputs the addition result "d(t)" to the IQ modulator 24. The same applies to the wideband signal generating device 22c-2.

[0089] This makes it possible to generate a wideband analog signal even if the wideband signal generating section is provided with only one adder (wideband adder) that adds a plurality of narrowband analog signals.

[0090] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0091] The present invention is applicable to optical communication systems.

[0092] 1a, 1c...optical communication system, 2a, 2c...optical transmitter, 3...optical transmission path, 4a, 4c...optical receiver, 21...digital signal processing unit, 22a, 22c...wideband signal generating device, 23...intensity modulator, 24...IQ modulator, 101...wideband signal generating unit, 102...adder, 103...adder, 201...folded signal generating unit, 202...coefficient multiplier, 203...coefficient multiplier, 204...adder, 205...switch circuit, 2 06...phase shifter, 207...frequency shifter, 208...adder, 211...switch circuit, 212...multiplier, 213...multiplier, 214...adder, 215...frequency shifter, 216...differential signal output circuit, 217...switch circuit, 218...adder / subtractor circuit, 221...band division section, 222...narrowband signal processing section, 223...digital-analog conversion section, 224...clock generation section, 225a, 225b...wideband signal generation section

Claims

1. An optical transmitter equipped with a wideband signal generating unit having: a switch circuit that alternately selects the first narrowband signal or the second narrowband signal from among a first narrowband signal and a second narrowband signal in synchronization with a clock signal; a frequency shifter that shifts the frequency of a third narrowband signal with a signal that is phase-shifted by 90 degrees relative to the phase of the clock signal; and a first adder that adds the signal selected from the first narrowband signal and the second narrowband signal and the frequency-shifted third narrowband signal.

2. An optical transmitter as described in claim 1, wherein the frequency of said clock signal is twice the bandwidth of each of said first narrowband signal, said second narrowband signal and said third narrowband signal.

3. The optical transmitter according to claim 1, further comprising an adder-subtractor circuit, wherein the wideband signal generating unit updates the first narrowband signal by adding the second narrowband signal multiplied by a predetermined coefficient to the first narrowband signal, updates the second narrowband signal by subtracting the second narrowband signal multiplied by the predetermined coefficient from the first narrowband signal before updating, and inputs the updated first narrowband signal and second narrowband signal to the switch circuit.

4. A first folded signal generation unit that generates a first folded signal by folding back the frequency of a first digital signal, a first folded signal generation unit that generates a second folded signal by folding back the frequency of a second digital signal, a first folded signal generation unit that generates a third folded signal by folding back the frequency of a third digital signal, a first coefficient multiplication unit that multiplies the first digital signal of a frequency before folding back by a first coefficient, a second coefficient multiplication unit that multiplies the second digital signal of a frequency before folding back by a second coefficient, a third coefficient multiplication unit that multiplies the third digital signal of a frequency before folding back by a third coefficient, a fourth coefficient multiplication unit that multiplies the first folded signal by a fourth coefficient, a fifth coefficient multiplication unit that multiplies the second folded signal by a fifth coefficient, and a sixth coefficient multiplication unit that multiplies the third folded signal by a sixth coefficient.

4. The optical transmitter according to claim 1, further comprising: a narrowband signal processing unit having a second adder that adds the first digital signal multiplied by the first coefficient, the second digital signal multiplied by the second coefficient, the third digital signal multiplied by the third coefficient, the first folded signal multiplied by the fourth coefficient, the second folded signal multiplied by the fifth coefficient, and the third folded signal multiplied by the sixth coefficient; and a digital-to-analog conversion unit that generates the first narrowband signal, the second narrowband signal, or the third narrowband signal by performing digital-to-analog conversion on a result of the addition by the second adder.

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