Optical Transmitter

Abstract

This optical transmitter comprises an optical transmission unit, a drive unit, an arithmetic circuit, and a bias supply circuit. The optical transmission unit includes a laser element. The drive unit drives the laser element by means of a first transmission signal. The arithmetic circuit generates a second transmission signal. The bias supply circuit superimposes the second transmission signal on a bias current of the laser element. An output of the arithmetic circuit including the second transmission signal is a rectangular wave-shaped digital signal based on a reference clock lower in frequency than a reference clock frequency of the first transmission signal. The bias supply circuit includes a circuit element for inclining a rise part and a fall part of the output of the arithmetic circuit.

Description

[Technical Field]

[0001] This application claims priority to Japanese Patent Application No. 2020-056207, filed on March 26, 2020, and incorporates by reference all of the contents of that application. [Background technology]

[0002] Patent Document 1 discloses a technique relating to a light amount control method in an optical communication device. The optical communication device described in this document has a light-emitting element, a light amount control means, a light-receiving element, a means for determining information, and a control signal generation means. The light-emitting element generates an optical signal. The light amount control means controls the amount of light transmitted from the light-emitting element. The light-receiving element receives an optical signal from a communication partner. The means for determining information decodes the optical signal from the communication partner and determines information related to the amount of light transmitted from the communication partner. The control signal generation means generates a control signal for controlling the light amount control means based on the determined information related to the amount of light. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-205239 Summary of the Invention

[0004] An optical transmitter according to one embodiment includes an optical transmitting unit, a driver, an arithmetic circuit, and a bias supply circuit. The optical transmitting unit includes a laser element. The driver drives the laser element with a first transmission signal. The arithmetic circuit generates a second transmission signal. The bias supply circuit superimposes the second transmission signal on a bias current for the laser element. The output of the arithmetic circuit, including the second transmission signal, is a square-wave digital signal based on a reference clock whose frequency is lower than the reference clock frequency of the first transmission signal. The bias supply circuit includes circuit elements for tilting the rising and falling portions of the output of the arithmetic circuit. [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is a block diagram schematically illustrating a configuration of an optical transceiver according to a first embodiment of the present disclosure. [Figure 2A] FIG. 2A is a diagram schematically illustrating an example of a bias supply circuit in a conventional optical transceiver. [Figure 2B] FIG. 2B is a diagram schematically illustrating a bias supply circuit included in the optical transceiver according to the first embodiment. [Figure 3] Fig. 3 shows an example of the time waveform of a low-frequency signal output from a shaping block. The upper part of Fig. 3 shows an example of the rectangular time waveform of a low-frequency signal before it is input to the shaping block. The middle and lower parts of Fig. 3 show an example of the time waveform of a low-frequency signal output from the shaping block corresponding to the low-frequency signal shown in the upper part. [Figure 4] Fig. 4 shows an example of the time waveform of a low-frequency signal output from a shaping block. The upper part of Fig. 4 shows an example of the rectangular time waveform of a low-frequency signal before it is input to the shaping block. The middle and lower parts of Fig. 4 show an example of the time waveform of a low-frequency signal output from the shaping block corresponding to the low-frequency signal shown in the upper part. [Figure 5] Fig. 5 shows an example of the time waveform of a low-frequency signal output from a shaping block. The upper part of Fig. 5 shows an example of the rectangular time waveform of a low-frequency signal before it is input to the shaping block. The middle and lower parts of Fig. 5 show an example of the time waveform of a low-frequency signal output from the shaping block corresponding to the low-frequency signal shown in the upper part. [Figure 6] FIG. 6 is a circuit diagram showing a specific example of a bias supply circuit. [Figure 7] FIG. 7 is a circuit diagram showing another specific example of the bias supply circuit. [Figure 8] FIG. 8 is a graph showing an example of a signal waveform output from the integrating circuit shown in FIG. [Figure 9] FIG. 9 is a graph showing another example of a signal waveform output from the integrating circuit shown in FIG. [Figure 10]FIG. 10 is a diagram schematically illustrating a low-frequency signal amplifier and its peripheral circuits included in the optical transceiver of the first embodiment. [Figure 11] FIG. 11 is a circuit diagram showing a specific example of the configuration of the amplifier circuit of the low-frequency signal amplifier. [Figure 12] FIG. 12 is a circuit diagram showing another specific example of the configuration of the amplifier circuit of the low-frequency signal amplifier. [Figure 13] 13 is a graph showing the results of performing a fast Fourier transform (FFT) on a rectangular time waveform of a 50 kHz low-frequency signal, in a comparative example in which the shaping block of the first embodiment is not provided. [Figure 14] Fig. 14 is a graph showing the results of performing a fast Fourier transform (FFT) on a rectangular waveform of a 50 kHz low-frequency signal, where the rising and falling edges of the low-frequency signal are sinusoidal. [Figure 15] Fig. 15 is a graph showing the results of performing a fast Fourier transform (FFT) on a rectangular time waveform of a 50 kHz low-frequency signal, where the falling edge and rising edge of the low-frequency signal are linear. [Figure 16] FIG. 16 is a diagram schematically showing a configuration used to evaluate the desensitization of low-frequency signals. [Figure 17] FIG. 17 is a graph showing the measurement results of the bit error rate. [Figure 18] FIG. 18 is a diagram schematically illustrating a configuration of an optical transceiver according to the second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0006] [Problem to be solved by this disclosure] In recent years, in optical communication systems, a technique has been used in which a high-frequency optical signal (hereinafter referred to as an optical main signal) mainly containing information to be communicated is superimposed with a low-frequency optical signal (hereinafter referred to as an optical sub-signal) containing information used for controlling the optical communication device itself. For example, in order for a main base station to manage a large number of wireless communication base stations, an optical sub-signal for managing the wireless communication base station is superimposed on an optical main signal exchanged between the wireless communication base stations. In such a technique, it is desirable to reduce as much as possible the degree of degradation of the time waveform of the optical main signal caused by the optical sub-signal.

[0007] [Effects of this disclosure] According to the present disclosure, it is possible to provide an optical transmitter that can reduce the degree of degradation of the time waveform of an optical main signal caused by an optical sub-signal.

[0008] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. An optical transmitter according to one embodiment includes an optical transmitting unit, a driver, an arithmetic circuit, and a bias supply circuit. The optical transmitting unit includes a laser element. The driver drives the laser element with a first transmission signal. The arithmetic circuit generates a second transmission signal. The bias supply circuit superimposes the second transmission signal on a bias current for the laser element. The output of the arithmetic circuit, including the second transmission signal, is a square-wave digital signal based on a reference clock whose frequency is lower than the reference clock frequency of the first transmission signal. The bias supply circuit includes circuit elements for tilting the rising and falling portions of the output of the arithmetic circuit.

[0009] In this optical transmitter, the optical signal based on the first transmission signal corresponds to the aforementioned optical main signal, and the optical signal based on the second transmission signal corresponds to the aforementioned optical sub-signal. The optical sub-signal is typically expressed as an envelope signal of the optical main signal. The influence of the optical sub-signal on the time waveform of the optical main signal is mainly due to the high-frequency components contained in the optical sub-signal. That is, the steeper the rise and fall of the optical sub-signal, the more high-frequency components the optical sub-signal contains in addition to its inherent low-frequency components. If the frequency of these high-frequency components is close to the frequency of the optical main signal, they act as noise on the optical main signal. As a result, the time waveform of the optical main signal is degraded. In contrast, in the above optical transmitter, the bias supply circuit includes circuit elements for tilting the rising and falling portions of the output of the calculation circuit including the second transmission signal. This lowers the frequency of the high-frequency components contained in the optical sub-signal based on the second transmission signal, moving away from the frequency of the optical main signal. Therefore, the above optical transmitter can reduce the degree of degradation of the time waveform of the optical main signal caused by the optical sub-signal.

[0010] In the optical transmitter, the laser element may include a light emitting unit and a modulator. The first transmission signal may drive the modulator. Alternatively, in the optical transmitter, the laser element may be a directly modulated type. The first transmission signal may be input to the laser element as a directly modulated signal.

[0011] In the optical transmitter, the time waveform of the second transmission signal output from the circuit element may be a triangular wave. Alternatively, the rising and falling portions of the time waveform of the second transmission signal output from the circuit element may be sinusoidal. Alternatively, the rising and falling portions of the time waveform of the second transmission signal output from the circuit element may have delays. For example, in these cases, the frequency of high-frequency components contained in the optical sub-signal based on the second transmission signal is lowered. Therefore, the degree of degradation of the time waveform of the optical main signal caused by the optical sub-signal can be effectively reduced. When the time waveform of the second transmission signal is a triangular wave, the time waveform may include a flat portion at least at one of the top and bottom of the triangular wave. When the rising and falling portions of the time waveform of the second transmission signal are sinusoidal or have delays, a flat portion may or may not exist between the rising and falling portions.

[0012] In the optical transmitter, the bias supply circuit may include an n-th order lag circuit and a current generation circuit. The n-th order lag circuit is the circuit element that applies an n-th order lag to the second transmission signal, where n is an integer greater than or equal to 1. The current generation circuit superimposes the second transmission signal that has passed through the n-th order lag circuit on the bias current. In this case, the n-th order lag circuit acts on the second transmission signal input to the current generation circuit, thereby tilting the rising and falling portions of the second transmission signal. In this case, the n-th order lag circuit may include a capacitor. One electrode of the capacitor is electrically connected to the signal path of the second transmission signal. The other electrode of the capacitor is electrically connected to a constant potential line. For example, with this configuration, a first order lag circuit can be realized as an n-th order lag circuit having the above-mentioned effect.

[0013] In each of the optical transmitters, the bias supply circuit may include an integrating circuit and a current generating circuit. The integrating circuit is one of the circuit elements and time-integrates the second transmission signal. The current generating circuit superimposes the second transmission signal that has passed through the integrating circuit on the bias current. In this case, the integrating circuit acts on the second transmission signal input to the current generating circuit. Therefore, the rising and falling portions of the second transmission signal can be inclined.

[0014] The optical transmitter may be used in an optical transmission system in which the optical intensity in the transmission path from the input end to the output end is 6 dBm or less, and is particularly effective in such an optical transmission system in which the maximum optical intensity in the transmission path is relatively small, in other words, in an optical transmission system in which the optical transmission path is short.

[0015] [Details of the embodiments of the present disclosure] Specific examples of optical transmitters according to the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the following description, the same elements in the drawings will be designated by the same reference numerals, and duplicate explanations will be omitted.

[0016] (First embodiment) FIG. 1 is a block diagram illustrating a schematic configuration of an optical transceiver 1A according to a first embodiment of the present disclosure. As illustrated in FIG. 1, the optical transceiver 1A according to this embodiment includes an optical transmitter 3, an optical receiver 4, and an arithmetic processing circuit 14. The optical transceiver 1A is connected to another optical transceiver 1A having the same configuration via an optical transmission path such as an optical fiber, and transmits and receives optical signals between the other optical transceiver 1A. Specifically, the optical signal Lt transmitted from the optical transmitter 3 of the optical transceiver 1A is received by the optical receiver 4 of the other optical transceiver 1A. The optical signal Lr transmitted from the optical transmitter 3 of the other optical transceiver 1A is received by the optical receiver 4 of the optical transceiver 1A. The optical transceiver 1A is used in a relatively short-distance optical transmission system, for example, where the optical intensity within the transmission path is 6 dBm or less. The transmission path refers to the range of the transmission path from the input end to the output end.

[0017] The optical transmitter 3 includes a clock data recovery (CDR) circuit 11, a driver circuit (driver) 12, an optical transmission module (optical transmitter) 13, and a bias supply circuit 15. The optical transmission module 13 includes a laser element 13e. The laser element 13e has a light-emitting unit 13a and a modulator 13b. The light-emitting unit 13a is, for example, a laser diode, and in one embodiment, a distributed feedback (DFB) laser diode. The modulator 13b is, for example, an electroabsorption (EA) optical modulator. When the light-emitting unit 13a is a laser diode, the light-emitting unit 13a has an active layer extending in the laser resonance direction along the surface of the semiconductor substrate and a pair of cladding layers provided above and below the active layer. Furthermore, the light-emitting unit 13a has an anode electrode and a cathode electrode for receiving a bias current Jb. When the modulation unit 13b is an EA-type optical modulator, the modulation unit 13b has an optical waveguide layer and a pair of clad layers provided above and below the optical waveguide layer. Furthermore, the modulation unit 13b has an anode electrode and a cathode electrode for receiving a drive voltage (modulation voltage) Vd. The light emitting unit 13a and the modulation unit 13b are monolithically integrated on a common semiconductor substrate and formed into a single chip as an electro-absorption modulator integrated laser (EML). The active layer of the light emitting unit 13a and the optical waveguide layer of the modulation unit 13b are optically coupled to each other using a butt joint technique or the like. One of the anode electrode and the cathode electrode may be common to the light emitting unit 13a and the modulation unit 13b. The light output from the light emitting unit 13a has its optical intensity modulated in the modulation unit 13b and is then output from the optical output port 13c of the optical transmission module 13. The light emitting section 13a and the modulation section 13b may be configured as separate elements.

[0018] The input end of the CDR circuit 11 of the optical transmitter 3 is electrically connected to the input terminal 31 of the optical transmitter 3. The input terminal 31 is connected to a host board (not shown) and receives the high-frequency signal Sa output from the host board. The high-frequency signal Sa is a digital signal (serial signal) generated at a high-frequency reference clock frequency. The high-frequency signal Sa contains the main information (data) to be transmitted from this optical transceiver 1A to the other optical transceiver 1A. The CDR circuit 11 outputs a digital signal obtained by shaping the high-frequency signal Sa based on a predetermined timing.

[0019] The input terminal of the driver circuit 12 is electrically connected to the output terminal of the CDR circuit 11. The output terminal of the driver circuit 12 is electrically connected to the anode electrode or cathode electrode of the modulator 13b. The driver circuit 12 receives a high-frequency signal Sa from the CDR circuit 11, amplifies the high-frequency signal Sa, and supplies a drive voltage Vd (first transmission signal) to the modulator 13b to drive the modulator 13b. The frequency of the drive voltage Vd is, for example, 1 Gbps or higher, and in one embodiment, 25.78 Gbps. Inside or outside the driver circuit 12, a bias voltage Vb for adjusting the magnitude of the low level of the drive voltage Vd is superimposed on the drive voltage Vd.

[0020] The arithmetic processing circuit (arithmetic circuit) 14 is electrically connected to the input / output terminal 10 of the optical transceiver 1A. The arithmetic processing circuit 14 receives control data Db, including an I2C (Inter-Integrated Circuit) signal, from a host board (not shown) via the input / output terminal 10. The control data Db includes management information that the optical transceiver 1A transmits to the other optical transceiver 1A. The management information is information for controlling and managing the operation of the optical transceiver 1A or the other optical transceiver 1A. The arithmetic processing circuit 14 generates a low-frequency signal Sb (second transmission signal) based on the control data Db and data collected by the arithmetic processing circuit 14. The low-frequency signal Sb is a square-wave digital signal. The reference clock frequency of the low-frequency signal Sb is sufficiently lower than the frequency of the drive voltage Vd, i.e., the reference clock frequency of the high-frequency signal Sa. The frequency of the low-frequency signal Sb is, for example, 50 kHz. The arithmetic processing circuit 14 has an output terminal electrically connected to the input terminal of the bias supply circuit 15 and outputs a low-frequency signal Sb to the bias supply circuit 15. The arithmetic processing circuit 14 is a circuit for controlling the operation of the optical transceiver 1A. The arithmetic processing circuit 14 can be configured with various large-scale integrated circuits, such as a memory control unit (MCU) or an FPGA (Field Programmable Gate Array). In an MCU, a computer system consisting of a CPU, memory, etc. is incorporated into a single integrated circuit. An FPGA is a device that makes it possible to program the connections between multiple logic circuits.

[0021] The bias supply circuit 15 has an input terminal and an output terminal. The input terminal of the bias supply circuit 15 is electrically connected to the arithmetic processing circuit 14. The output terminal of the bias supply circuit 15 is electrically connected to the anode electrode or the cathode electrode of the light emitting unit 13a. The bias supply circuit 15 supplies a bias current Jb, which is obtained by superimposing a DC bias B on a low-frequency signal Sb, to the light emitting unit 13a. FIG. 2A is a diagram schematically illustrating an example of a bias supply circuit 150 in a conventional optical transceiver. In the conventional optical transceiver, the bias supply circuit 150 includes a current conversion circuit block 150a. The DC bias B is input to one input terminal of the current conversion circuit block 150a, and the low-frequency signal Sb is input to the other input terminal. The current conversion circuit block 150a outputs a bias current Jb, which is obtained by superimposing the low-frequency signal Sb and the DC bias B, from its output terminal.

[0022] 2B is a schematic diagram illustrating the bias supply circuit 15 included in the optical transceiver 1A of this embodiment. As shown in the figure, the bias supply circuit 15 of this embodiment includes a current conversion circuit block 15a and a shaping block 15b. The shaping block 15b is a circuit element for tilting the rising and falling edges of the time waveform of the low-frequency signal Sb output from the arithmetic processing circuit 14. The shaping block 15b has an input terminal and an output terminal. The input terminal of the shaping block 15b inputs the low-frequency signal Sb from the arithmetic processing circuit 14. The output terminal of the shaping block 15b is electrically connected to another input terminal of the current conversion circuit block 15a. The shaping block 15b tilts the rising and falling edges of the time waveform of the low-frequency signal Sb input from the arithmetic processing circuit 14, and then outputs the low-frequency signal Sb from its output terminal to the current conversion circuit block 15a. The current conversion circuit block 15a outputs a bias current Jb, which is a combination of a low-frequency signal Sb and a DC bias B, from an output terminal.

[0023] 3, 4, and 5 are diagrams showing examples of the time waveform of the low-frequency signal Sb output from the shaping block 15b. In these diagrams, the upper part shows an example of a rectangular time waveform of the low-frequency signal Sb output from the arithmetic processing circuit 14 before being input to the shaping block 15b. The middle and lower parts show examples of the time waveform of the low-frequency signal Sb output from the shaping block 15b, corresponding to the low-frequency signal Sb shown in the upper part. As shown in the middle part of FIG. 3, the time waveform of the low-frequency signal Sb output from the shaping block 15b is a triangular wave, which may include a flat portion Wc at at least one of the top and bottom of the triangular wave. Here, a triangular wave refers to a waveform mainly composed of a rising portion Wa and a falling portion Wb, and the slopes of the rising portion Wa and the falling portion Wb are approximately constant. The flat portions at the top and bottom refer to portions with constant values. As shown in the lower part of Fig. 3, the time waveform of the low-frequency signal Sb output from the shaping block 15b may be trapezoidal, with a flat portion Wc between a sloping rising portion Wa and a falling portion Wb. The time waveforms shown in the middle and lower parts of Fig. 3 can be obtained, for example, by time integrating the rectangular-wave-shaped low-frequency signal Sb shown in the upper part.

[0024] As shown in the middle and bottom of Figure 4, the time waveform of the low-frequency signal Sb output from the shaping block 15b may have a rising portion Wd and a falling portion We each having a sine wave shape. In this case, as shown in the middle of Figure 4, the time waveform of the low-frequency signal Sb may include a pulse waveform in which there is no flat portion We between the rising portion Wd and the falling portion We. Alternatively, as shown in the bottom of Figure 4, there may always be a flat portion We between the rising portion Wd and the falling portion We. The middle of Figure 4 shows a case in which the period of the sine wave is relatively long. The bottom of Figure 4 shows a case in which the period of the sine wave is relatively short.

[0025] As shown in the middle and bottom of Figure 5, the time waveform of the low-frequency signal Sb output from the shaping block 15b may have a delay in each of the rising portion Wf and the falling portion Wg, i.e., a waveform that has been subjected to low-pass filtering. In this case, as shown in the middle of Figure 5, the time waveform of the low-frequency signal Sb may include a pulse waveform in which there is no flat portion Wc between the rising portion Wf and the falling portion Wg. Alternatively, as shown in the bottom of Figure 5, there may always be a flat portion Wc between the rising portion Wf and the falling portion Wg. The time waveforms shown in the middle and bottom of Figure 5 can be obtained, for example, by adding an nth-order delay to the rectangular-wave-shaped low-frequency signal Sb shown in the top, where n is an integer greater than or equal to 1. The middle part shows a case in which the time constant of the nth-order delay is relatively large. The bottom part shows a case in which the time constant of the nth-order delay is relatively small.

[0026] FIG. 6 is a circuit diagram showing a bias supply circuit 15A as a specific example of the bias supply circuit 15. As shown in FIG. 6, the bias supply circuit 15A includes a current conversion circuit block 15a, a shaping block 15ba, and an amplifier block 15c. The amplifier block 15c is a common-source circuit and includes a logic inverter circuit 151 and a transistor (FET) 152. A low-frequency signal Sb is input to the gate of the transistor 152 from the arithmetic processing circuit 14 via the logic inverter circuit 151. One current terminal of the transistor 152, for example, the drain, is connected to a constant potential line 159 via a resistor R1. The other current terminal of the transistor 152, for example, the source, is connected to a reference potential line 155. The potential of the constant potential line 159 is higher than the potential of the reference potential line 155. With this configuration, an amplified low-frequency signal Sb is output from one current terminal of the transistor 152.

[0027] The shaping block 15ba corresponds to the shaping block 15b shown in FIG. 2B. The shaping block 15ba is connected to the rear stage of the amplification block 15c. In this example, the shaping block 15ba is a first-order lag circuit, in other words, a low-pass filter circuit. In the illustrated example, the shaping block 15ba includes a resistor R2 and a capacitor C2. The resistor R2 is provided on a signal path extending from one current terminal of the transistor 152. One electrode of the capacitor C2 is connected to the signal path. The other electrode of the capacitor C2 is connected to the reference potential line 155. More specifically, one end of the resistor R2 is electrically connected to one current terminal of the transistor 152. The other end of the resistor R2 is electrically connected to one electrode of the capacitor C2.

[0028] The current conversion circuit block 15a includes an operational amplifier 153 and a transistor (FET) 154. The inverting input terminal of the operational amplifier 153 is connected to a node N1 between a resistor R2 and a capacitor C2 in the shaping block 15ba. Therefore, the low-frequency signal Sb that has passed through the shaping block 15ba is input to the inverting input terminal of the operational amplifier 153. The node N1 is electrically connected to a power supply potential line 156 via a resistor R4 and to a reference potential line 155 via a resistor R5. The potential of the power supply potential line 156 is higher than the potential of the reference potential line 155. The non-inverting input terminal of the operational amplifier 153 is connected to a bias potential line 157 via a resistor R3. The potential of the bias potential line 157 is higher than the potential of the reference potential line 155. This generates a voltage Vg in which a bias is superimposed on the low-frequency signal Sb. The resistors R3, R4, and R5 are provided to control the voltage-to-current conversion of the current conversion circuit block 15a. Specifically, resistors R3, R4, and R5 are provided to increase the input impedance when transmitting the voltage across resistor R7 to the control circuit, to control the voltage-current conversion magnification determined by the resistance ratio, and to adjust the amount of current flowing through resistor R7. If the resistance values ​​of resistors R3, R4, R5, and R6 are R3, R4, R5, and R6, respectively, the resistance values ​​are set to satisfy the relationship R4:R5=R6:R3. In this case, by setting one end of resistor R5 to the reference potential, the amount of current flowing through resistor R7 can be set to 0 mA when the potential of bias potential line 157 is at the reference potential. Furthermore, the amount of current flowing through resistor R7 can be adjusted by changing the voltage of bias potential line 157.

[0029] The output terminal of the operational amplifier 153 is electrically connected to the gate of the transistor 154. Therefore, a voltage Vg including the low-frequency signal Sb is applied to the gate of the transistor 154. A node N2 between the output terminal of the operational amplifier 153 and the gate of the transistor 154 is electrically connected to the inverting input terminal of the operational amplifier 153 via a capacitor C1 acting as a coupling capacitor. The capacitor C1 is provided to suppress the response of the operational amplifier 153, i.e., to prevent it from responding faster than necessary. The optical transceiver has a Tx Disable function, and when the Disable function is released, the light must be quickly turned back on. However, if the response is released too quickly, excessive current will flow, resulting in excessive light emission. To prevent this, the capacitor C1 is provided. One current terminal of the transistor 154, for example, the drain, is connected to the power supply line 156 via a resistor R7. Furthermore, one current terminal of the transistor 154 is connected to a node N3 via a resistor R6. The node N3 is a node between the non-inverting input terminal of the operational amplifier 153 and the resistor R3. The other current terminal of the transistor 154, for example the source, is electrically connected to the light emitting section 13a of the optical transmission module 13 (see FIG. 1).

[0030] In the bias supply circuit 15A shown in FIG. 6, a current having a magnitude corresponding to the gate voltage of the transistor 154 is supplied from the power supply potential line 156 to the light-emitting unit 13a. A voltage Vg including a low-frequency signal Sb that has passed through a shaping block 15ba, which is a first-order lag circuit, is input to the gate of the transistor 154. Therefore, the rising and falling portions of the low-frequency signal Sb are sloped by the shaping block 15ba, which is also a first-order lag circuit (see the middle and bottom parts of FIG. 5). The degree of slope is determined by the product of the resistance value of resistor R2 and the capacitance value of capacitor C2, i.e., the time constant.

[0031] FIG. 7 is a circuit diagram showing a bias supply circuit 15B as another specific example of the bias supply circuit 15. As shown in FIG. 7, this bias supply circuit 15B includes a current conversion circuit block 15a, a shaping block 15bb, and an amplifier block 15c. The configurations of the current conversion circuit block 15a and the amplifier block 15c are the same as those in FIG. 6, and therefore will not be described here. The shaping block 15bb corresponds to the shaping block 15b shown in FIG. 2B. The shaping block 15bb is connected after the amplifier block 15c. In this example, the shaping block 15bb is an integrator circuit. In the illustrated example, the shaping block 15bb includes a resistor R8 and an operational amplifier 158. The resistor R8 is provided on a signal path extending from one current terminal of the transistor 152. The inverting input terminal of the operational amplifier 158 is connected to the signal path. The non-inverting input terminal of the operational amplifier 158 is connected to a node N5 between resistors R9 and R10. Resistors R9 and R10 are connected in series between the constant potential line 159 and the reference potential line 155 and divide the voltage between the constant potential line 159 and the reference potential line 155. The voltage divided by the resistors R9 and R10 is input to the non-inverting input terminal of the operational amplifier 158. A resistor R11 and a capacitor C3 are connected in parallel between the output terminal and the inverting input terminal of the operational amplifier 158. The output terminal of the operational amplifier 158 is electrically connected to the inverting input terminal of the operational amplifier 153 in the current conversion circuit block 15a via a resistor R2.

[0032] In the bias supply circuit 15B shown in FIG. 7, a current having a magnitude corresponding to the gate voltage of the transistor 154 is also supplied from the power supply potential line 156 to the light-emitting unit 13a. A voltage Vg including a low-frequency signal Sb that has passed through a shaping block 15bb, which is an integrating circuit, is input to the gate of the transistor 154. Therefore, the rising and falling portions of the low-frequency signal Sb are sloped by the shaping block 15bb, which is also an integrating circuit (see the middle and bottom parts of FIG. 3). The degree of slope is determined mainly by the capacitance value of the capacitor C3.

[0033] 8 and 9 are graphs showing examples of signal waveforms output from shaping block 15bb, which is an integrating circuit shown in FIG. 7. In these figures, the vertical axis represents voltage and the horizontal axis represents time. The vertical axis is in volts. The horizontal axis is in milliseconds. FIG. 8 shows the case where the frequency of low-frequency signal Sb is 10 kHz. FIG. 9 shows the case where the frequency of low-frequency signal Sb is 5 kHz. As shown in these figures, shaping block 15bb, which is an integrating circuit, causes the rising and falling edges of the time waveform of low-frequency signal Sb to slope linearly. If the capacitance value of capacitor C2 in shaping block 15bb, which is an integrating circuit, is the same, the degree of slope, i.e., the gradient, will be approximately the same regardless of the frequency of low-frequency signal Sb.

[0034] Referring back to FIG. 1, the optical receiver 4 includes a low-frequency signal amplifier 16, an optical receiving module (optical receiving unit) 17, and a CDR circuit 18. The optical receiving module 17 receives received light Lr, an optical signal transmitted from the other optical transceiver 1A. This received light Lr is formed by superimposing an optical signal based on a high-frequency signal Sa and an optical signal based on a low-frequency signal Sb. The optical receiving module 17 includes a photodetector 17a and a transimpedance amplifier (TIA) 17b. The photodetector 17a is, for example, a photodiode. The photodetector 17a receives the received light Lr and converts it into a current signal. One terminal of the photodetector 17a, for example, an anode, is electrically connected to the input terminal of the TIA 17b. The TIA 17b converts frequency components of the current signal output from the photodetector 17a that are equal to or higher than a low frequency cutoff (LFC), i.e., frequency components including the high-frequency signal Sa, into a voltage signal. The voltage signal is output from the TIA 17b as a received signal Sr1 including the high-frequency signal Sa. The CDR circuit 18 is electrically connected to the output terminal of the TIA 17b. The CDR circuit 18 separates the clock from the received signal Sr1 output from the TIA 17b and extracts the received data.

[0035] One or the other terminal of the photodetector 17a, for example, the cathode, is electrically connected to the input terminal of the low-frequency signal amplifier 16. The concept of electrically connecting one or the other terminal of the photodetector 17a to the input terminal of the low-frequency signal amplifier 16 also includes, for example, the following configuration. That is, when the other terminal of the photodetector 17a is led out of the optical receiving module 17, a circuit for detecting the current from the photodetector 17a is provided outside the optical receiving module 17, for example, on the control board of the optical transceiver 1A. An example of a case where the other terminal of the photodetector 17a is led out of the optical receiving module 17 is when the photodetector 17a is an APD. The circuit for detecting the current from the photodetector 17a is, for example, a current mirror circuit. The output of this detection circuit is then connected to the input terminal of the low-frequency signal amplifier 16. Alternatively, if a circuit for detecting the current from the light receiving element 17a is provided inside the TIA 17b, the current detection signal output from the TIA 17b is provided to the input terminal of the low-frequency signal amplifier 16. For example, the case where the circuit for detecting the current from the light receiving element 17a is provided inside the TIA 17b is when the light receiving element 17a is a PIN photodiode. The current detection signal output from the TIA 17b is usually a received signal strength indicator (RSSI).

[0036] The low-frequency signal amplifier 16 converts the current signal output from the light-receiving element 17a into a voltage signal. The current signal input to the low-frequency signal amplifier 16 mainly contains frequency components below a predetermined frequency, i.e., frequency components including the low-frequency signal Sb. This voltage signal is output from the low-frequency signal amplifier 16 as a received signal Sr2 containing the low-frequency signal Sb. The current signal output from the other terminal of the light-receiving element 17a can also be used to monitor the optical intensity of the received light Lr. The output terminal of the low-frequency signal amplifier 16 is electrically connected to the arithmetic processing circuit 14. The arithmetic processing circuit 14 receives the received signal Sr2 from the low-frequency signal amplifier 16 and adjusts the bias current of the DC bias B to an appropriate value.

[0037] FIG. 10 is a schematic diagram illustrating the low-frequency signal amplifier 16 and its peripheral circuitry included in the optical transceiver 1A of this embodiment. As shown in FIG. 10, the low-frequency signal amplifier 16 of this embodiment includes a capacitor 21, a resistor 23, and an amplifier circuit 24. The resistor 23 is connected between the other terminal of the light-receiving element 17a and the reference potential line 162 and converts the current signal output from the light-receiving element 17a into a voltage signal, i.e., a reception signal including the low-frequency signal Sb. A node N6 between the other terminal of the light-receiving element 17a and the resistor 23 is connected to the input terminal of the amplifier circuit 24 via a capacitor 21, which functions as a coupling capacitor. The capacitor 21 passes components of the voltage signal above a certain frequency and blocks components below that frequency. The frequency is set lower than the frequency of the low-frequency signal Sb. Therefore, the low-frequency signal Sb passes through the capacitor 21 and is input to the amplifier circuit 24. The amplifier circuit 24 amplifies the low-frequency signal Sb and outputs it to the arithmetic processing circuit 14. The arithmetic processing circuit 14 inputs this low frequency signal Sb to a comparator to determine whether it is at a high level or a low level.

[0038] Node N6 is connected to the input terminal of low-pass filter circuit 22. Low-pass filter circuit 22 passes components below a certain frequency and blocks components above that frequency. This frequency is set lower than the frequency of low-frequency signal Sb. Therefore, low-frequency signal Sb is blocked by low-pass filter circuit 22. Only the DC bias component contained in the voltage signal passes through low-pass filter circuit 22 and is output to a light intensity monitor circuit (not shown).

[0039] FIG. 11 is a circuit diagram showing a low-frequency signal amplifier 16A as a specific example of the configuration of the low-frequency signal amplifier 16. As shown in FIG. 11, the low-frequency signal amplifier 16A has an operational amplifier 161 as the amplifier circuit 24. The non-inverting input terminal of the operational amplifier 161 is electrically connected to a node N6 via a capacitor 21. In addition, the non-inverting input terminal of the operational amplifier 161 is connected to a node N7 between resistors R13 and R14. The resistors R13 and R14 are connected in series between a constant potential line 163 and a reference potential line 162 and divide the voltage between the constant potential line 163 and the reference potential line 162. The divided voltage is input to the non-inverting input terminal of the operational amplifier 161 and superimposed on the low-frequency signal Sb. The inverting input terminal of the operational amplifier 161 is electrically connected to the output terminal of the operational amplifier 161 via a resistor R16. Additionally, the inverting input terminal of the operational amplifier 161 is electrically connected to the reference potential line 162 via a series circuit of a resistor R15 and a capacitor C5. The resistor R16 determines the gain of the low-frequency signal amplifier 16A. The gain of the low-frequency signal amplifier 16A is, for example, 900 times or more.

[0040] FIG. 12 is a circuit diagram showing a low-frequency signal amplifier 16B as another specific example of the amplifier circuit configuration of the low-frequency signal amplifier 16. As shown in FIG. 12, the low-frequency signal amplifier 16B is configured by configuring the amplifier circuit 24 shown in FIG. 11 in two stages. Specifically, the low-frequency signal amplifier 16B includes amplifier circuits 24A and 24B. The amplifier circuit 24A includes an operational amplifier 161A. The non-inverting input terminal of the operational amplifier 161A is electrically connected to a node N6A via a capacitor 21A. In addition, the non-inverting input terminal of the operational amplifier 161A is connected to a node N7A between resistors R13A and R14A. The resistors R13A and R14A are connected in series between a constant potential line 163 and a reference potential line 162 and divide the voltage between the constant potential line 163 and the reference potential line 162. The divided voltage is superimposed on the low-frequency signal Sb and input to the non-inverting input terminal of the operational amplifier 161A. The inverting input terminal of the operational amplifier 161A is electrically connected to the output terminal of the operational amplifier 161A via a resistor R16A. In addition, the inverting input terminal of the operational amplifier 161A is electrically connected to the reference potential line 162 via a series circuit of a resistor R15A and a capacitor C5A. The resistor R16A determines the gain of the amplifier circuit 24A. The amplifier circuit 24B includes an operational amplifier 161B. The non-inverting input terminal of the operational amplifier 161B is electrically connected to the output terminal of the operational amplifier 161A via a capacitor 21B. The non-inverting input terminal of the operational amplifier 161B is connected to a node N7B between the resistors R13B and R14B. Resistors R13B and R14B are connected in series between the constant potential line 163 and the reference potential line 162 and divide the voltage between the constant potential line 163 and the reference potential line 162. The divided voltage is superimposed on the output signal from the amplifier circuit 24A and input to the non-inverting input terminal of the operational amplifier 161B. The inverting input terminal of the operational amplifier 161B is electrically connected to the output terminal of the operational amplifier 161B via a resistor R16B. In addition, the inverting input terminal of the operational amplifier 161B is electrically connected to the reference potential line 162 via a series circuit of a resistor R15B and a capacitor C5B. The resistor R16B determines the gain of the amplifier circuit 24B.

[0041] 12, the gain of the low-frequency signal amplifier 16B is, for example, 900 times. The gains of the two-stage amplifier circuits 24A and 24B that make up this low-frequency signal amplifier 16B may be equal to each other. In this case, if the gain required for the low-frequency signal amplifier 16B is 900 times, the gain of each amplifier circuit 24A and 24B is set to 30 times.

[0042] The effects obtained by the optical transmitter 3 and optical receiver 4 of this embodiment having the above configuration will be described along with the conventional problems. Optical communication systems use a technology for superimposing an optical sub-signal on an optical main signal. The optical main signal is a high-frequency optical signal that mainly contains information to be communicated. The optical sub-signal is a low-frequency optical signal that contains information used for controlling the optical communication device itself. For example, in recent years, the increase in wireless communication capacity due to mobile communication terminals such as smartphones and the Internet of Things (IoT) has led to the need for a large number of wireless communication base stations. To efficiently expand the number of wireless base stations, a centralized wireless access network called a C-RAN (Centralized Radio Access Network) is being constructed. In a C-RAN, a wireless base station is equipped with only a receiving device. Control of the wireless base station and connection to a wide-area optical network are centrally performed at the main base station. Due to the recent increase in wireless communication capacity, wireless base stations are generally connected to the main base station via an optical communication system.

[0043] The Message Channel, specified in G698.4, is a method by which a main base station manages radio base stations. The Message Channel is a method in which a low-speed 50-kbps optical signal is superimposed on a high-speed optical signal, such as 10 Gbps or 25 Gbps, and intensity-modulated (ASK / Amplitude Shift Keying). G989.2 specifies the Auxiliary Management and Control Channel (AMCC), which includes overmodulation, which performs 115-kbps intensity modulation (ASK) on a high-speed optical signal. In these technologies that superimpose a low-frequency optical sub-signal on a high-frequency optical main signal, it is desirable to minimize the degree of degradation of the time waveform of the optical main signal caused by the optical sub-signal.

[0044] In the optical transmitter 3 of this embodiment, the optical signal based on the high-frequency signal Sa corresponds to the optical main signal, and the optical signal based on the low-frequency signal Sb corresponds to the optical sub-signal. The optical sub-signal is typically expressed as an envelope signal of the optical main signal. According to the inventor's findings, the influence of the optical sub-signal on the time waveform of the optical main signal is mainly due to the high-frequency components contained in the optical sub-signal. That is, when the optical sub-signal is a rectangular wave, the steeper its rising and falling edges are, the more high-frequency components the optical sub-signal contains in addition to the inherent low-frequency components. Furthermore, if the frequency of the high-frequency components is close to the frequency of the optical main signal, these frequency components act as noise on the optical main signal. As a result, the time waveform of the optical main signal is degraded. In contrast, in the optical transmitter 3 of this embodiment, the bias supply circuit 15 includes a shaping block 15b as a circuit element for tilting the rising and falling edges of the output of the arithmetic processing circuit 14, including the low-frequency signal Sb. This lowers the frequency of the high-frequency components contained in the optical sub-signal based on the low-frequency signal Sb, moving away from the frequency of the optical main signal. Therefore, the optical transmitter 3 of this embodiment can reduce the degree of degradation of the time waveform of the optical main signal caused by the optical sub-signal.

[0045] 13, 14, and 15 are graphs showing the results of performing a fast Fourier transform (FFT) on a rectangular time waveform of a 50 kHz low-frequency signal Sb. FIG. 13 shows the results of a comparative example in which the shaping block 15b of this embodiment is not provided. FIG. 14 shows the results when the falling edge and falling edge of the low-frequency signal Sb are sinusoidal (see FIG. 4). FIG. 15 shows the results when the falling edge and falling edge of the low-frequency signal Sb are linear (see FIG. 3). As shown in these graphs, odd-order harmonics appear with the frequency (50 kHz) of the low-frequency signal Sb as the fundamental frequency. For example, when the LFC of the TIA 17b is 100 kHz, third-order and higher harmonics of the fundamental frequency (50 kHz) appear as noise in the high-frequency signal Sa, which is thought to be a factor in degrading the sensitivity of the high-frequency signal Sa. The third harmonic of the fundamental frequency, i.e., the peak value of 150 kHz, is approximately −16 dB in the comparative example of Fig. 13, −44 dB in the example of Fig. 14, and −30 dB in the example of Fig. 15. Thus, compared to the comparative example, this embodiment achieves a harmonic reduction effect of approximately 14 dB to 28 dB.

[0046] The inventors actually fabricated an optical transceiver 1A according to this embodiment and evaluated the degree of sensitivity degradation of the low-frequency signal Sb. FIG. 16 is a schematic diagram illustrating the configuration used for the evaluation. At the optical transmitter 3 side, a low-frequency signal Sb with a frequency of 50 kHz generated by a function generator (FG) 42 was input to the bias supply circuit 15 via an attenuator 43 and superimposed on a DC bias B. The function generator 42 was selected to generate a square wave, a sine wave, and a triangular wave. The time waveform and modulation depth of the low-frequency signal Sb were observed by receiving light Lm from the light-emitting unit 13a at an RFM (Rear Facet Monitor) 49 and displaying the waveform of the output signal from the RFM 49 on an oscilloscope 44. A 12-km-long single-mode optical fiber 41 (200 ps dispersion) was used to transmit the signal light from the optical transmitter 3 to the optical receiver 4. On the optical receiver 4 side, the output current (RSSI) from the photodetector 17a was extracted by a current mirror circuit, amplified by an amplifier 50, and then observed by a real-time oscilloscope 47. At the same time, the waveform of the signal light propagating through the single-mode optical fiber 41 was observed by a sampling oscilloscope 46. The speed of the optical main signal was set to 25.78 Gbps. A pseudorandom signal (PRBS-31) was generated by a pulse pattern generator (PPG) 45 and received by an error detector (ED) 48, and the bit error rate (BER) was measured.

[0047] FIG. 17 is a graph showing BER measurement results when the low-frequency signal Sb is a square wave, a sine wave, and a triangular wave. In FIG. 17, graph G1 shows the case when the low-frequency signal Sb is a square wave. Graph G2 shows the case when the low-frequency signal Sb is a sine wave. Graph G3 shows the case when the low-frequency signal Sb is a triangular wave. The vertical axis represents the sensitivity penalty due to MSG modulation (unit: dB), and the horizontal axis represents the modulation depth (unit: %). Region A in the graph shows the allowable range of modulation depth (6% to 8.5%) specified in G698.4. Referring to FIG. 17, in this region A, when the low-frequency signal Sb is a sine wave or triangular wave (graphs G2 and G3), sensitivity degradation was suppressed by up to about 3 dB compared to when the low-frequency signal Sb is a square wave (graph G1).

[0048] As in this embodiment, the time waveform of the low-frequency signal Sb output from the shaping block 15b may be a triangular wave. In this case, for example, the frequency of the high-frequency component included in the optical sub-signal based on the low-frequency signal Sb is lowered. Therefore, the degree of degradation of the time waveform of the optical main signal caused by the optical sub-signal can be effectively reduced.

[0049] As shown in FIG. 6, the bias supply circuit 15 may include an n-th order lag circuit (shaping block 15ba) as a circuit element that imparts an n-th order lag to the low-frequency signal Sb, and a current generation circuit (current conversion circuit block 15a) that generates a bias current Jb by superimposing the low-frequency signal Sb that has passed through the n-th order lag circuit on a DC bias B. In this case, the n-th order lag circuit acts on the low-frequency signal Sb input to the current generation circuit, thereby tilting the rising and falling edges of the low-frequency signal Sb. In this case, the n-th order lag circuit (shaping block 15ba) may include a capacitor C2, one electrode of which is electrically connected to the signal path of the low-frequency signal Sb and the other electrode of which is electrically connected to the reference potential line 155. For example, such a configuration can realize an n-th order lag circuit (first order lag circuit) that functions as described above.

[0050] 7, the bias supply circuit 15 may have an integrating circuit (shaping block 15bb) as a circuit element that time-integrates the low-frequency signal Sb, and a current generating circuit (current converting circuit block 15a) that generates a bias current Jb by superimposing the low-frequency signal Sb that has passed through the integrating circuit on a DC bias B. In this case, the integrating circuit acts on the low-frequency signal Sb input to the current generating circuit, and the rising and falling portions of the low-frequency signal Sb can be inclined.

[0051] As in this embodiment, the optical transmitter 3 may be used in an optical transmission system in which the optical intensity in the transmission path from the input end to the output end is 6 dBm or less. The optical transmitter 3 of this embodiment is particularly effective in such an optical transmission system in which the maximum optical intensity in the transmission path is relatively small, in other words, in an optical transmission system in which the optical transmission path is short.

[0052] According to the optical receiver 4 of this embodiment, the optical sub-signal based on the low-frequency signal Sb can be separated from the optical main signal based on the high-frequency signal Sa and received.

[0053] (Second embodiment) FIG. 18 is a diagram schematically illustrating the configuration of an optical transceiver 1B according to a second embodiment of the present disclosure. This embodiment differs from the first embodiment in the configuration of an optical transmission module (optical transmitter) 13A. Specifically, the optical transmission module 13A of this embodiment includes a directly modulated laser element 13d instead of the laser element 13e of the first embodiment. A drive current Ja including a high-frequency signal Sa (first transmission signal) is input to the laser element 13d as a directly modulated signal from a driver circuit 19. At the same time, a bias current Jb obtained by superimposing a low-frequency signal Sb (second transmission signal) on a DC bias B is supplied to the laser element 13d from a bias supply circuit 15. Specifically, a drive current including the low-frequency signal Sb and the high-frequency signal Sa is supplied to the laser element 13d. Except for the above, the configuration is the same as that of the first embodiment.

[0054] In the first embodiment, the light emitting unit 13a is coupled to the modulation unit 13b, which is an external modulator, but as in the present embodiment, the optical transceiver 1B may also be configured to directly drive the laser element 13d. Even in this case, the same effects as those of the first embodiment can be obtained.

[0055] The optical transmitter according to the present disclosure is not limited to the above-described embodiment, and various other modifications are possible. For example, in the above-described embodiment, as examples of inclining the rising and falling portions of the time waveform of the low-frequency signal (second transmission signal), triangular waves, sinusoidal waves, and n-th order delay waves are given. The inclined waveforms of the rising and falling portions are not limited to these, and various other inclined waveforms may be adopted. [Explanation of symbols]

[0056] 1A, 1B...Optical transceiver 3...Optical transmitter 4...Optical receiver 10...Input / output terminal 11...CDR circuit 12...Drive circuit 13, 13A...Optical transmitter module 13a...Light emitting part 13b...Modulation section 13c...Optical output port 13d, 13e...Laser element 14...Arithmetic processing circuit 15, 15A, 15B...Bias supply circuit 15a...Current conversion circuit block 15b, 15ba, 15bb...Shaped blocks 15c...Amplification block 16, 16A, 16B...Low frequency signal amplifier 17...Optical receiving module 17a...light receiving element 17b...Transimpedance amplifier (TIA) 18...CDR circuit 19...Drive circuit 21, 21A, 21B...Capacitors 22...Low-pass filter circuit 23...Resistance 24, 24A, 24B...Amplifier circuit 31...Input terminal 41...Single-mode optical fiber 42...Function generator 43...Attenuator 44...Oscilloscope 45...Pulse Pattern Generator 46...Sampling oscilloscope 47...Real-time oscilloscope 48...Error detector 49...RFM 50...Amplifier 151...Logic inversion circuit 152, 154...Transistors 153...Operational amplifier 155, 162...Reference potential line 156...Power supply potential line 157...Bias potential line 158, 161, 161A, 161B... operational amplifiers 159,163...Constant potential line B: DC bias Db...Control data Ja...Drive current Jb: Bias current Lm…light Lr...Received light Lt…Signal light N1,N2,N3,N5,N6,N7,N7A,N7B…Node Sa...high frequency signal Sb…Low frequency signal Sr1, Sr2...Received signal Vb: Bias voltage Vd: Drive voltage Vg: Voltage Wa, Wd, Wf... rising parts Wb, We, Wg...Falling part

Claims

1. An optical transmitter, an optical transmitter including a laser element; a driver that drives the laser element with a first transmission signal; an arithmetic circuit that generates a second transmission signal; a bias supply circuit that superimposes the second transmission signal on a bias current of the laser element; Equipped with an output of the arithmetic circuit including the second transmission signal is a square-wave digital signal based on a reference clock having a frequency lower than a reference clock frequency of the first transmission signal; the bias supply circuit includes circuit elements for ramping rising and falling portions of the output of the arithmetic circuit; An optical transmitter, wherein the second transmission signal includes information for controlling the operation of the optical transmitter.

2. the laser element includes a light emitting section and a modulation section; 2. The optical transmitter according to claim 1, wherein the first transmission signal drives the modulation section.

3. the laser element is a directly modulated type; 2. The optical transmitter according to claim 1, wherein the first transmission signal is input to the laser element as a directly modulated signal.

4. 4. The optical transmitter according to claim 1, wherein the time waveform of the second transmission signal output from the circuit element is a triangular wave.

5. 5. The optical transmitter according to claim 4, wherein the time waveform includes a flat portion at least on one of the top and bottom of the triangular wave.

6. 4. The optical transmitter according to claim 1, wherein the rising and falling portions of the time waveform of the second transmission signal output from the circuit element are sinusoidal.

7. 4. The optical transmitter according to claim 1, wherein a rising edge and a falling edge of a time waveform of the second transmission signal output from the circuit element have a delay.

8. 8. The optical transmitter according to claim 6, wherein a flat portion exists between the rising portion and the falling portion in the time waveform.

9. 8. The optical transmitter according to claim 6, wherein the time waveform has no flat portion between the rising portion and the falling portion.

10. The bias supply circuit includes: an n-th order delay circuit as the circuit element that gives an n-th order delay (n is an integer equal to or greater than 1) to the second transmission signal; a current generating circuit that superimposes the second transmission signal that has passed through the n-th order delay circuit on the bias current; 4. The optical transmitter according to claim 1, further comprising:

11. 11. The optical transmitter according to claim 10, wherein the n-th order lag circuit includes a capacitor having one electrode electrically connected to a signal path of the second transmission signal and the other electrode electrically connected to a constant potential line.

12. The bias supply circuit includes: an integrating circuit as the circuit element that time-integrates the second transmission signal; a current generating circuit that superimposes the second transmission signal that has passed through the integrating circuit on the bias current; 4. The optical transmitter according to claim 1, further comprising:

13. 13. The optical transmitter according to claim 1, which is used in an optical transmission system in which the optical intensity in the transmission path from the input end to the output end is 6 dBm or less.

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