Optical transmitter and control method of optical transmitter

Abstract

An optical transmitter includes a control section that generates a multilevel amplitude modulation signal based on binary transmission data, a drive section that generates a drive signal in accordance with the multilevel amplitude modulation signal, and a light-emitting section that generates an optical signal in accordance with the drive signal. The control section selects either a first encoding scheme or a second encoding scheme in accordance with a switching signal. The control section converts an M (M is an integer of 2 or more) bit string included in the transmission data into a pulse signal of 2 M logical levels using the selected encoding scheme, thereby generating the multilevel amplitude modulation signal. The control section sets voltage values of the 2 M logical levels in accordance with the selected encoding scheme.

Description

Technical Field

[0001] This disclosure relates to an optical transmitter and a method for controlling the optical transmitter. Background Technology

[0002] In recent years, with the widespread adoption of smartphones and cloud computing, communication capacity has been continuously increasing. To achieve greater communication capacity, multi-level modulation methods such as PAM4 (4-level Pulse Amplitude Modulation) are sometimes used to replace binary modulation methods such as NRZ (Non-Return to Zero). In optical transmission using such multi-level modulation methods, noise can sometimes be mixed into the optical signal, causing the optical receiver to be unable to correctly determine the signal level.

[0003] Patent Document 1 describes an optical transmitter that sets each level in such a way that, in order to make the S / N (Signal to Noise) ratio equal at each level, the greater the extinction ratio of the multi-value modulated optical signal, the greater the interval between two adjacent levels.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2008-113386 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] For example, in PAM4, 2 bits of data are assigned to each level. For an optical receiver, when a PAM4 signal is received, a level determination is performed, and the 2 bits of data are decoded. Each bit is then transmitted to the host device through different channels. If the PAM4 signal is misjudged as a level adjacent to its intended level, an error occurs in at least one of the 2 bits (channels). The channel where the error occurs varies depending on which logic level the misjudgment occurs between. When a misjudgment occurs between two adjacent logic levels, the probability of an error occurring in the channel (error probability) varies depending on the encoding method. For example, in the case of Gray code encoding, the error probability of the channel corresponding to the first bit is two-thirds, and the error probability of the channel corresponding to the second bit is one-third. Therefore, in the optical transmitter described in Patent Document 1, the S / N ratio of each level can be homogenized, but the receiving sensitivity (bit error rate) between the two channels may become uneven.

[0009] This disclosure describes an optical transmitter that can reduce the difference in receiving sensitivity between channels and a method for controlling the optical transmitter.

[0010] Solution for solving the problem

[0011] One aspect of this disclosure is an optical transmitter that outputs a multi-valued amplitude modulated optical signal. The optical transmitter includes: a control unit that generates a multi-valued amplitude modulated signal based on binary transmission data; a driving unit that generates a driving signal based on the multi-valued amplitude modulated signal; and a light-emitting unit that generates an optical signal based on the driving signal. The control unit selects either a first encoding method or a second encoding method based on a switching signal. The control unit uses the selected encoding method to convert an M-bit bit string included in the transmission data into a 2D binary format. M A pulse signal of logic level is generated, thereby producing a multi-value amplitude modulation signal. The control unit sets 2 according to the selected encoding method. M A logic level voltage value. M is an integer greater than or equal to 2.

[0012] Invention Effects

[0013] According to this disclosure, the difference in receiving sensitivity between channels can be reduced. Attached Figure Description

[0014] Figure 1 This is a schematic diagram illustrating the configuration of an optical transceiver including one embodiment of an optical transmitter.

[0015] Figure 2 This is an example of an eye diagram representing a PAM4 signal.

[0016] Figure 3 It is a diagram used to illustrate the values ​​of each channel in binary code and Gray code.

[0017] Figure 4 It is used for Figure 1 The diagram illustrates the four-valued pulse amplitude modulation performed by the DSP (Digital Signal Processor).

[0018] Figure 5 This is a diagram representing a channel where an error occurs due to a misjudgment between logic levels.

[0019] Figure 6 It is a diagram representing the combination of voltage values ​​for each logic level.

[0020] Figure 7 It means that it was used. Figure 6 The graph shows the bit error rate for the voltage values ​​shown.

[0021] Figure 8It means that it was used. Figure 6 The graph shows the linearity for the voltage values ​​shown.

[0022] Figure 9 It means that it was used. Figure 6 The diagram shows the SECQ (Stressed Eye Closure for PAM4) under the voltage values ​​shown.

[0023] Figure 10 It means Figure 1 The flowchart shown illustrates the setup process performed by the optical transceiver.

[0024] Figure 11 It means Figure 1 The flowchart shown illustrates the transmission control process performed by the optical transceiver.

[0025] Figure 12 It means Figure 1 The flowchart shown illustrates the receive control processing performed by the optical transceiver. Detailed Implementation

[0026] [Description of embodiments of this disclosure]

[0027] First, the contents of the embodiments of this disclosure will be listed for explanation.

[0028] One aspect of this disclosure is an optical transmitter that outputs a multi-valued amplitude modulated optical signal. The optical transmitter includes: a control unit that generates a multi-valued amplitude modulated signal based on binary transmission data; a driving unit that generates a driving signal based on the multi-valued amplitude modulated signal; and a light-emitting unit that generates an optical signal based on the driving signal. The control unit selects either a first encoding method or a second encoding method based on a switching signal. The control unit uses the selected encoding method to convert an M-bit bit string included in the transmission data into a 2D binary format. M A pulse signal of logic level is generated, thereby producing a multi-value amplitude modulation signal. The control unit sets 2 according to the selected encoding method. M A logic level voltage value. M is an integer greater than or equal to 2.

[0029] In this optical transmitter, the M-bit bit string included in the transmitted data is converted into 2... M A pulse signal of logic level is generated, thereby producing a multi-valued amplitude modulation signal. A drive signal is generated based on the multi-valued amplitude modulation signal, and an optical signal is generated based on the drive signal. In the optical receiver, the optical signal is converted into a multi-valued amplitude modulation signal, which is then decoded into an M-bit bit string, and transmitted through different channels. In the optical transmitter, the encoding method selected from the first and second encoding methods is set to 2... MThe voltage values ​​of each logic level. For example, by setting the voltage values ​​of each logic level according to the error probability between channels, the difference in bit error rate between M channels can be reduced. As a result, the difference in receiver sensitivity between channels can be reduced. Moreover, two encoding methods can be used without increasing the difference in receiver sensitivity between channels.

[0030] Alternatively, the control unit can set 2 such that the bit error rates of the first to the Mth bits included in the M bits are equal. M A voltage value of logic levels. In an optical receiver, the optical signal can be converted into a multi-valued amplitude modulation signal, and the multi-valued amplitude modulation signal can be decoded into an M-bit bit string, which is then transmitted through different channels. In this case, in the above configuration, the bit error rate of each of the first to the Mth bits is set to be equal. M The voltage values ​​of each logic level are used, thus reducing the difference in bit error rate between the M channels. Therefore, the difference in receiver sensitivity between channels can be reduced.

[0031] Alternatively, the control unit can set 2 based on the probability of error occurrence. M The voltage value of each logic level, where the probability of this error occurring is 2. M This refers to the probability of an error occurring in each bit of the bit string when a misjudgment occurs between two adjacent logic levels. If the voltage difference between two adjacent logic levels increases, the probability of a misjudgment between those two logic levels decreases. For example, by increasing the voltage difference between two logic levels that affect a bit with a high error probability, the bit error rate of that bit is reduced. Thus, by setting 2 based on the error probability... M A voltage value at each logic level can reduce the difference in receiving sensitivity between channels.

[0032] Alternatively, if the error probability of the first bit of the bit string is higher than that of the second bit, the control unit sets the voltage difference between the two logic levels where the first bit changes to be greater than the voltage difference between the other two logic levels where the second bit changes. M The voltage value of each logic level. If the voltage difference between two adjacent logic levels increases, the probability of misjudgment between these two logic levels decreases. Therefore, by making the voltage difference between the two logic levels where the first bit changes greater than the voltage difference between the other two logic levels where the second bit changes, the difference between the bit error rate of the first bit and the bit error rate of the second bit is reduced. As a result, the difference in receiver sensitivity between channels can be reduced.

[0033] It could also be that the first encoding method is Gray code. Alternatively, it could be 2... MEach logic level includes a minimum first logic level, a minimum second logic level, and a minimum third logic level. Alternatively, the control unit can set the logic level such that the voltage difference between the first and second logic levels when the first encoding method is selected is greater than the voltage difference between the second and third logic levels. M The voltage value of each logic level. In Gray code, compared to bits that change between the second and third logic levels, bits that change between the first and second logic levels change more frequently between adjacent logic levels. Therefore, when the voltage difference between the first and second logic levels is the same, and the voltage difference between the second and third logic levels is the same, the bit error rate of bits that change between the first and second logic levels is higher than that of bits that change between the second and third logic levels. Based on the above configuration, the probability of misjudgment between the first and second logic levels can be reduced compared to the probability of misjudgment between the second and third logic levels. Therefore, the difference between the bit error rates of bits that change between the first and second logic levels and those that change between the second and third logic levels can be reduced. As a result, the difference in receiver sensitivity between channels can be reduced.

[0034] Alternatively, the first encoding method could be Gray code, or the second encoding method could be binary code. In either case, an optical transmitter conforming to IEEE (Institute of Electrical and Electronics Engineers) P802.3bs could be provided.

[0035] It could also be, 2 M Each logic level includes a minimum first logic level, a minimum second logic level, and a minimum third logic level. Alternatively, the control unit can set the logic level such that the voltage difference between the second and third logic levels when the first encoding method is selected is less than the voltage difference between the second and third logic levels when the second encoding method is selected. M The voltage value of each logic level.

[0036] In binary code, when the voltage difference between the first and second logic levels and the voltage difference between the second and third logic levels are the same, the bit error rate of the first to Mth bits in the bit string can be homogenized. In Gray code, compared to bits that change between the second and third logic levels, bits that change between the first and second logic levels change between adjacent logic levels more frequently. Therefore, in Gray code, when the voltage difference between the first and second logic levels and the voltage difference between the second and third logic levels are the same, the bit error rate of bits that change between the first and second logic levels is higher than that of bits that change between the second and third logic levels. Therefore, based on the above configuration, when Gray code is chosen as the encoding method, the probability of misjudgment between the first and second logic levels is reduced compared to the probability of misjudgment between the second and third logic levels. Thus, the difference between the bit error rate of bits that change between the first and second logic levels and the bit error rate of bits that change between the second and third logic levels can be reduced. As a result, when Gray code is chosen as the encoding method, the difference in receiving sensitivity between channels can be reduced.

[0037] Another aspect of the optical transmitter control method disclosed herein is a control method for an optical transmitter that outputs a multi-valued amplitude modulated optical signal. This control method includes the following steps: generating a multi-valued amplitude modulated signal based on binary transmitted data; generating a drive signal based on the multi-valued amplitude modulated signal; and generating an optical signal based on the drive signal. In the step of generating the multi-valued amplitude modulated signal, either a first encoding method or a second encoding method is selected based on a switching signal, and the selected encoding method is used to convert the M-bit bit string included in the transmitted data into a 2D binary signal. M A pulse signal at each logic level is used to generate a multi-value amplitude-modulated signal. 2 M The voltage value of each logic level is set according to the selected encoding method. M is an integer greater than 2.

[0038] In the control method of this optical transmitter, the M-bit bit string included in the transmitted data is converted into 2... M A pulse signal of logic level is generated, thereby producing a multi-valued amplitude modulation signal. A drive signal is generated based on the multi-valued amplitude modulation signal, and an optical signal is generated based on the drive signal. In the optical receiver, the optical signal is converted into a multi-valued amplitude modulation signal, which is then decoded into an M-bit bit string, and transmitted through different channels. In the optical transmitter, the encoding method selected from the first and second encoding methods is set to 2... MThe voltage values ​​of each logic level. For example, by setting the voltage values ​​of each logic level according to the error probability between channels, the difference in bit error rate between M channels can be reduced. As a result, the difference in receiver sensitivity between channels can be reduced. Moreover, two encoding methods can be used without increasing the difference in receiver sensitivity between channels.

[0039] [Details of the embodiments of this disclosure]

[0040] Hereinafter, specific examples of an optical transmitter and a control method for the optical transmitter according to embodiments of the present disclosure will be described with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to these examples, but is illustrated by the claims and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0041] Figure 1 This is a schematic diagram illustrating the configuration of an optical transceiver including one embodiment of an optical transmitter. Figure 2 This is an example of an eye diagram representing a PAM4 signal. Figure 3 It is a diagram used to illustrate the values ​​of each channel in binary code and Gray code. Figure 4 It is used for Figure 1 The diagram illustrates the four-value pulse amplitude modulation performed by the DSP.

[0042] Figure 1 The optical transceiver 1 (optical transmitter) shown is, for example, an optical transceiver that implements 53Gbps transmission and conforms to the QSFP (Quad Small Form-factor Pluggable) 28 standard. Optical transceiver 1 is communicatively connected to another optical transceiver via an optical fiber. Optical transceiver 1 transmits multi-level amplitude modulated optical signals to and receives multi-level amplitude modulated optical signals from the other optical transceiver. In this embodiment, PAM4 (4-level Pulse Amplitude Modulation) is used as the multi-level amplitude modulation. Optical transceiver 1 is communicatively connected to host device 2.

[0043] The host device 2 is the upstream device, responsible for monitoring and controlling the optical transceiver 1. The host device 2 includes a host board 21 for communicating with the optical transceiver 1. The host board 21 transmits data to the optical transceiver 1 via channels Tx1 and Tx2, which serve as transmit channels. The host board 21 receives data from the optical transceiver 1 via channels Rx1 and Rx2, which serve as receive channels. The transmit data transmitted in channels Tx1 and Tx2 and the receive data transmitted in channels Rx1 and Rx2 are binary modulated electrical signals. For example, NRZ (Non-Return to Zero) is used as the binary modulation method.

[0044] Specifically, the motherboard 21 transmits data synchronously bit-by-bit through channels Tx1 and Tx2. The motherboard 21 receives data synchronously bit-by-bit from the optical transceiver 1 through channels Rx1 and Rx2, which serve as receive channels. Each channel (channels Tx1, Tx2, Rx1, Rx2) transmits data at a speed of 26.5 Gbps. To achieve a transmission speed of 400 Gbps, eight sets of channels Tx1, Tx2, Rx1, and Rx2 can be configured. The motherboard 21 sends a switching signal to the optical transceiver 1 via serial communication. The switching signal indicates whether Gray code is used. Examples of serial communication include I... 2 C (Inter-Integrated Circuit) and MDIO (Management Data Input / Output).

[0045] The optical transceiver 1 includes a DSP 11 (control unit), a drive circuit 12 (drive unit), a TOSA (Transmitter Optical Sub-Assembly) 13 (light-emitting unit), a bias circuit 14, a ROSA (Receiver Optical Sub-Assembly) 15, and a CPU (Central Processing Unit) 16.

[0046] DSP11 is a device for signal processing. DSP11 generates a PAM4 signal (Multi-Value Amplitude Modulation signal) as an electrical signal based on the transmitted data received from the host device 2. The PAM4 signal is transmitted, for example, at 26.5 Gbaud. Figure 2 As shown, the PAM4 signal has four logic levels (voltage levels): -0, -1, -2, and -3. Logic levels are sometimes referred to as symbols. Levels -0 to -3 increase in the order of -0, -1, -2, and -3. That is, -0 is the lowest logic level, -1 is the second lowest, -2 is the third lowest, and -3 is the highest.

[0047] DSP11 uses a prescribed encoding method to convert the 2-channel transmitted data into a 1-channel PAM4 signal. In this embodiment, DSP11 can use Gray code (first encoding method) and binary code (second encoding method) as encoding methods. The CPU16 sets which encoding method to use, Gray code or binary code. DSP11 selects either Gray code or binary code as the encoding method based on the switching signal output from the host board 21. Gray code is an encoding method where the Hamming distance between adjacent symbols (bit strings) is 1. DSP11 converts the combination (B2, B1) of bit B2 of channel Tx2 and bit B1 of channel Tx1 simultaneously received from the host board 21 into a pulse of any one of the following logic levels: -0, -1, -2, and -3.

[0048] The following explains the case where binary code is chosen as the encoding method. For example... Figure 3 and Figure 4 As shown, DSP11 generates a pulse of level -0 when the combination (B2, B1) is (0, 0). DSP11 generates a pulse of level -1 when the combination (B2, B1) is (0, 1). DSP11 generates a pulse of level -2 when the combination (B2, B1) is (1, 0). DSP11 generates a pulse of level -3 when the combination (B2, B1) is (1, 1).

[0049] The following explains the case where Gray code was chosen as the encoding method. For example... Figure 3 As shown, DSP11 generates a pulse of level -0 when the combination (B2, B1) is (0, 0). DSP11 generates a pulse of level -1 when the combination (B2, B1) is (0, 1). DSP11 generates a pulse of level -2 when the combination (B2, B1) is (1, 1). DSP11 generates a pulse of level -3 when the combination (B2, B1) is (1, 0).

[0050] DSP11 performs the above conversion whenever it receives one bit from each channel, thereby generating a PAM4 signal. DSP11 outputs the PAM4 signal to driver circuit 12. It should be noted that CPU16 sets the voltage values ​​(output amplitudes) of each logic level corresponding to the selected encoding method in DSP11's registers. DSP11 generates pulses with amplitudes corresponding to the voltage values ​​set in the registers.

[0051] DSP11 generates received data based on the PAM4 signal received as an electrical signal from ROSA15. Specifically, DSP11 uses a prescribed encoding method to convert the PAM4 signal of channel 1 into received data of channel 2. As described above, in this embodiment, DSP11 can utilize Gray code and binary code as encoding methods. DSP11 determines the logic level for each symbol (pulse) of the PAM4 signal received from ROSA15 and generates a combination (B2, B1) of bit B2 of channel Rx2 and bit B1 of channel Rx1 based on the logic level. DSP11 determines which logic level a symbol belongs to by comparing a preset threshold voltage with the voltage value of the symbol.

[0052] The following explains the case where binary code is chosen as the encoding method. For example... Figure 3 As shown, when the logic level is -0, DSP11 generates (0, 0) as the combination (B2, B1). When the logic level is -1, DSP11 generates (0, 1) as the combination (B2, B1). When the logic level is -2, DSP11 generates (1, 0) as the combination (B2, B1). When the logic level is -3, DSP11 generates (1, 1) as the combination (B2, B1).

[0053] The following explains the case where Gray code was chosen as the encoding method. For example... Figure 3 As shown, when the logic level is -0, DSP11 generates (0, 0) as the combination (B2, B1). When the logic level is -1, DSP11 generates (0, 1) as the combination (B2, B1). When the logic level is -2, DSP11 generates (1, 1) as the combination (B2, B1). When the logic level is -3, DSP11 generates (1, 0) as the combination (B2, B1).

[0054] DSP11 sends bit B1 to host device 2 via channel Rx1 and bit B2 to host device 2 via channel Rx2. DSP11 synchronizes the transmission of bit B1 with the transmission of bit B2.

[0055] The driving circuit 12 is a circuit that drives the laser diode included in the TOSA 13. The driving circuit 12 generates a driving signal as an electrical signal based on the PAM4 signal and outputs the driving signal to the TOSA 13. The driving circuit 12 can also be built into the TOSA 13.

[0056] The TOSA13 is an optical transceiver module that generates optical signals based on a drive signal and transmits these signals via an optical fiber to another optical transceiver. The TOSA13 includes a laser diode as the light-emitting element for generating the optical signal. The laser diode is driven by the drive signal and outputs the optical signal accordingly. Examples of laser diodes include directly modulated laser diodes (DMLs) and electro-absorption modulator integrated laser diodes (EMLs). Here, the use of an EML will be explained as an example. An EML consists of a laser diode and an EA (electro-absorption) modulator. A direct current is supplied to the anode of the laser diode, causing it to generate CW (Continuous Wave) light. The EA modulator modulates the CW light based on the drive signal.

[0057] The bias circuit 14 is a circuit that applies a DC (Direct Current) bias voltage to the EA modulator included in the TOSA 13 for waveform adjustment. The bias circuit 14 adjusts the DC bias voltage based on a control signal output from the CPU 16. It should be noted that when using DML as the laser diode of the TOSA 13, the bias circuit 14 adjusts the bias current supplied to DML based on the control signal output from the CPU 16 for waveform adjustment.

[0058] The ROSA15 is an optical receiver module that receives optical signals from a separate optical transceiver (TOSA) via optical fiber and converts them into a PAM4 signal as an electrical signal. The optical signal is a 26.5 Gbaud PAM4 optical signal. The ROSA15 includes photoelectric conversion elements and an amplification section for converting the optical signal into an electrical signal. Examples of photoelectric conversion elements include PIN-PD (PhotoDiode) and APD (Avalanche PhotoDiode). The amplification section is a circuit that generates a PAM4 signal by amplifying the electrical signal output from the photoelectric conversion element. An example of an amplification section is a linear TIA (Transimpedance Amplifier).

[0059] CPU 16 is a device that controls the entire optical transceiver 1. CPU 16 performs settings corresponding to the encoding method. CPU 16 receives a switching signal from the host board 21 and sets the encoding method and corresponding logic level voltage values ​​for the DSP 11 according to the switching signal. The voltage values ​​for each logic level are predetermined for each encoding method and stored in a memory (not shown). CPU 16 reads the voltage value corresponding to the selected encoding method from the memory and sets the read voltage value. Specifically, when the switching signal indicates the use of Gray code, CPU 16 sets Gray code as the encoding method for DSP 11, reads the corresponding logic level voltage values ​​from the memory, and sets them for DSP 11. When the switching signal indicates the use of non-Gray code, CPU 16 sets binary code as the encoding method for DSP 11, reads the corresponding logic level voltage values ​​from the memory, and sets them for DSP 11.

[0060] Next, the method for determining the optimal voltage value of each logic level corresponding to the encoding method will be explained. Figure 5 This is a diagram representing a channel where an error occurs due to a misjudgment between logic levels. Figure 6 It is a diagram representing the combination of voltage values ​​for each logic level. Figure 7 It means that it was used Figure 6 A graph showing the bit error rate for the voltage values ​​shown. Figure 8 It means that it was used Figure 6 The graph shows the linearity for the voltage values ​​shown. Figure 9 It means that it was used Figure 6 The diagram shows the SECQ under the voltage values ​​shown.

[0061] As described above, optical transceiver 1 transmits a four-valued pulse amplitude modulated optical signal to another optical transceiver. For the other optical transceiver, upon receiving the optical signal, it converts the optical signal into an electrical signal using a photoelectric conversion element, and then amplifies the electrical signal using an amplification unit, thereby generating a PAM4 signal. Then, the DSP of the other optical transceiver determines the logic level of each symbol (pulse) of the PAM4 signal. At this time, the DSP may sometimes determine the logic level of a symbol to be adjacent to its original logic level. In this case, at least one of bits B2 and B1 becomes an incorrect value.

[0062] The specific explanation is given for cases where binary code is chosen as the encoding method. For example... Figure 3 As shown, the combination (B2, B1) with a logic level of -0 is (0, 0), and the combination (B2, B1) with a logic level of -1 is (0, 1). Therefore, as Figure 5As shown, in the misjudgment between level -0 and level -1, bit B1 becomes an incorrect value, thus causing an error in channel Rx1. It should be noted that misjudgments between level -0 and level -1 occur when level -0 is judged as level -1 and when level -1 is judged as level -0. Similarly, in the misjudgment between level -1 and level -2, both bits B1 and B2 become incorrect values, thus causing errors in channels Rx1 and Rx2. In the misjudgment between level -2 and level -3, bit B1 becomes an incorrect value, thus causing an error in channel Rx1.

[0063] The specific cases where Gray code is chosen as the encoding method are explained in detail. For example... Figure 5 As shown, in the misjudgment between level -0 and level -1, bit B1 becomes an incorrect value, thus causing an error in channel Rx1. In the misjudgment between level -1 and level -2, bit B2 becomes an incorrect value, thus causing an error in channel Rx2. In the misjudgment between level -2 and level -3, bit B1 becomes an incorrect value, thus causing an error in channel Rx1.

[0064] As mentioned above, the channel at which an error occurs varies depending on which logic level the misjudgment occurs. For example, assuming a constant S / N (Signal to Noise) ratio across all logic levels and uniform symbol occurrences, in binary code, the error probability for bit B2 (channel Rx2) is one-third, and the error probability for bit B1 (channel Rx1) is 1. It should be noted that the error probability for a bit (channel) refers to the probability of an error occurring in that bit (channel) given a misjudgment between logic levels. Under the same conditions, in Gray code, the error probability for bit B2 (channel Rx2) is one-third, and the error probability for bit B1 (channel Rx1) is two-thirds.

[0065] When the voltage differences V(0-1) between level -0 and level -1, V(1-2) between level -1 and level -2, and V(2-3) between level -2 and level -3 are equal, the ratio of the bit error rate of channel Rx1 to the bit error rate of channel Rx2 is approximately the same as the ratio of the probability of error occurrence. If the voltage difference between two adjacent logic levels decreases, the probability of misjudgment between these two logic levels increases. Therefore, the bit error rate of each channel varies according to the voltage difference between two adjacent logic levels.

[0066] The receiver sensitivity is defined by the sum of the bit error rates of channels Rx1 and Rx2. However, in practical applications, if the difference in bit error rate (receiver sensitivity) between channels Rx1 and Rx2 increases, the likelihood of RxLOL (Loss of Lock) and Synchronize Loss occurring in the channel with the worse receiver sensitivity in the host device increases. LOL refers to the state where a PLL (Phase Locked Loop) such as an ASIC (Application Specific Integrated Circuit) and CDR (Clock Data Recovery) on the motherboard cannot lock the signal. Therefore, it is desirable to homogenize the receiver sensitivity between channels Rx1 and Rx2.

[0067] Therefore, use Figure 6 The bit error rate of channels Rx1 and Rx2 was determined by using combinations of voltage values ​​for each logic level (combinations P1 to P7), and the optimal combination was selected. The supply voltage to optical transceiver 1 was set to 3.3V, the ambient temperature was set to 35 degrees Celsius, and the extinction ratio of the optical signal was set to 8dB. It should be noted that the supply voltage to optical transceiver 1 was converted to a voltage suitable for DSP11 via a DC / DC converter within optical transceiver 1. TOSA13 and ROSA15 are connected back-to-back. The optical power (intensity) of the optical signal was set to an arbitrary value.

[0068] Here, the voltage value of level -3 is set to a fixed value, and the voltage value of each logic level is represented by its proportion (in %) relative to the voltage value of level -3. That is, the voltage value of level -3 is 100%. Figure 6 In the combinations shown, the voltage value of level -0 is also set to a fixed value (18%). Therefore, the voltage difference V(0-3) between the voltage values ​​of level -0 and level -3 is constant. The voltage values ​​of level -1 and level -2 are set such that the eye opening between level -1 and level -2 decreases as the combination number increases. Therefore, in combination P1, the eye opening between level -1 and level -2 is the largest, and in combination P7, the eye opening between level -1 and level -2 is the smallest. It should be noted that the voltage values ​​of level -1 and level -2 in each combination are set to be approximately symmetrical with respect to the line of symmetry between level -0 and level -3.

[0069] like Figure 7As shown, when Gray code is chosen as the encoding method, as the combination number increases, the bit error rate of channel Rx2 increases, while the bit error rate of channel Rx1 decreases. In other words, the smaller the voltage difference V(0-1) and voltage difference V(2-3), and the larger the voltage difference V(1-2), the smaller the bit error rate of channel Rx2 and the larger the bit error rate of channel Rx1. On the other hand, the larger the voltage difference V(0-1) and voltage difference V(2-3), and the smaller the voltage difference V(1-2), the larger the bit error rate of channel Rx2 and the smaller the bit error rate of channel Rx1.

[0070] As mentioned above, the error probability of channel Rx2 is lower than that of channel Rx1. Therefore, by making the voltage difference V(1-2) smaller than the voltage differences V(0-1) and V(2-3), the difference between the bit error rates of channel Rx1 and channel Rx2 is reduced. Figure 7 In the experimental results, the bit error rate of channel Rx1 in combination P5 is approximately equal to that of channel Rx2. In combination P5, the voltage difference V(1-2) is about 3% smaller than the voltage differences V(0-1) and V(2-3). Therefore, when Gray code is selected as the encoding method, the receiver sensitivity can be optimized by setting the voltage value of combination P5.

[0071] When binary code is selected as the encoding method, in combination P4 where voltage differences V(0-1), V(1-2), and V(2-3) are approximately equal, the bit error rate of channel Rx1 is approximately equal to that of channel Rx2. Therefore, when binary code is selected as the encoding method, the receiver sensitivity is optimized by setting the voltage value of combination P4. Thus, the voltage value of level -1 when Gray code is selected is approximately 1% to 4% higher than the voltage value of level -1 when binary code is selected. The voltage value of level -2 when Gray code is selected is approximately 1% to 4% lower than the voltage value of level -2 when binary code is selected.

[0072] It should be noted that the PAM4 signal waveform is required to have linearity and SECQ. Linearity is one of the indicators of waveform quality, representing the linearity of the eye diagram. Linearity is a positive value below 1; the closer to 1, the more ideal the eye diagram. SECQ is also one of the indicators of waveform quality, representing the waveform quality before transmission. The smaller the SECQ value, the less noise. Linearity is required to be greater than 0.75dB, and SECQ is required to be less than 3.2dB. Figure 8 and Figure 9 As shown, combinations P1 to P6 meet the linearity and SECQ specifications.

[0073] As described above, the voltage values ​​for each logic level are pre-determined experimentally for each encoding method in a way that optimizes sensitivity. These voltage values ​​are stored in a memory not shown in the diagram. It should be noted that... Figure 6 In the example shown, the voltage values ​​of level -0 and level -3 are fixed, but all voltage values ​​from level -0 to level -3 can be changed.

[0074] Next, the various processes performed by optical transceiver 1 will be explained. Figure 10 It means Figure 1 The flowchart shows the setup process performed by the optical transceiver. Figure 11 It means Figure 1 The flowchart shown illustrates the transmission control process performed by the optical transceiver. Figure 12 It means Figure 1 The flowchart shown illustrates the receive control processing performed by the optical transceiver.

[0075] exist Figure 10 In the setup process shown, firstly, the CPU 16 determines whether a switching signal has been received from the motherboard 21 (step S11). If the CPU 16 determines that no switching signal has been received (step S11; No), the determination process in step S11 is repeated until a switching signal is received. In step S11, if the CPU 16 determines that a switching signal has been received (step S11; Yes), it reads the voltage value of the logic level corresponding to the encoding method selected according to the switching signal from the unillustrated memory. Then, the CPU 16 sets the encoding method in the registers of the DSP 11, and simultaneously sets the voltage values ​​of each logic level in the registers of the DSP 11 (step S12). In step S12, the CPU 16 may output a control signal to the bias circuit 14 to make the output DC bias voltage corresponding to the selected encoding method. It should be noted that when using DML as the laser diode of TOSA 13, the CPU 16 may output a control signal to the bias circuit 14 to make the output bias current corresponding to the selected encoding method. The setup process ends with the above.

[0076] exist Figure 11 In the transmission control process shown, firstly, DSP11 determines whether it has received transmission data from the host board 21 for channel 2 (step S21). If DSP11 determines that no transmission data has been received (step S21; no), it repeatedly performs the determination process in step S21 until transmission data is received. In step S21, if DSP11 determines that transmission data has been received (step S21; yes), it generates a PAM4 signal (step S22).

[0077] In step S22, DSP11 uses the encoding method set by CPU16 to convert the transmitted data into a PAM4 signal. Specifically, DSP11 serializes the combination (B2, B1) of bit B2 of channel Tx2 and bit B1 of channel Tx1 simultaneously received from the host board 21 into a pulse of any one of logic levels: -0, -1, -2, and -3. At this time, DSP11 generates a pulse with a voltage value of the logic level set by CPU16. DSP11 performs the above conversion whenever it receives one bit from each channel, thereby generating a PAM4 signal. Then, DSP11 sequentially outputs the PAM4 signal to the driver circuit 12.

[0078] Next, for the drive circuit 12, when a PAM4 signal is received from the DSP 11, a drive signal is generated based on the PAM4 signal (step S23). Then, the drive circuit 12 outputs the drive signal to the TOSA 13. Next, for the TOSA 13, when a drive signal is received from the drive circuit 12, an optical signal is generated based on the drive signal (step S24). In step S24, the laser diode is driven by the drive signal to generate an optical signal. Then, the TOSA 13 transmits the optical signal to another optical transceiver via an optical cable. The transmission control process ends here.

[0079] exist Figure 12 In the received control process shown, firstly, ROSA15 determines whether an optical signal has been received from another optical transceiver via an optical cable (step S31). If ROSA15 determines that no optical signal has been received (step S31; No), it repeats the determination process in step S31 until an optical signal is received. In step S31, if ROSA15 determines that an optical signal has been received (step S31; Yes), it converts the optical signal into a PAM4 signal (step S32). In step S32, the photoelectric conversion element converts the optical signal into an electrical signal and outputs the electrical signal to the amplification unit. Then, the amplification unit amplifies the electrical signal output from the photoelectric conversion element, thereby generating a PAM4 signal. Then, ROSA15 outputs the PAM4 signal to DSP11.

[0080] Next, for DSP11, when a PAM4 signal is received from ROSA15, received data is generated based on the PAM4 signal (step S33). In step S33, DSP11 uses the encoding method set by CPU16 to deserialize the PAM4 signal into 2-channel received data. Specifically, DSP11 determines which logic level a symbol represents by comparing the threshold voltage with the voltage value of each symbol of the PAM4 signal. Then, DSP11 generates a combination (B2, B1) of bit B2 of channel Rx2 and bit B1 of channel Rx1 based on the determined logic level. Then, DSP11 sends a pulse representing bit B1 to the host board 21 via channel Rx1 and a pulse representing bit B2 to the host board 21 via channel Rx2. The receive control process ends thereafter.

[0081] In the optical transceiver 1 and its control method described above, the 2-bit bit string (bits B2, B1) included in the transmitted data is converted into a pulse signal of any one of the four logic levels, thereby generating a PAM4 signal. A drive signal is generated based on the PAM4 signal, and an optical signal is generated based on the drive signal. In another optical transceiver, the optical signal is converted into a PAM4 signal, and the PAM4 signal is decoded into a 2-bit bit string (bits B2, B1), which is then transmitted through channels Rx1 and Rx2, respectively. In optical transceiver 1, the voltage values ​​of the four logic levels (levels -0 to -3) are set according to the encoding method. For example, the voltage values ​​of each logic level are set according to the error probability between channels, thereby reducing the difference in bit error rate between channels Rx1 and Rx2. As a result, the difference in receiving sensitivity between channels Rx1 and Rx2 can be reduced.

[0082] Specifically, DSP11 sets the voltage values ​​from level -0 to level -3 based on the error occurrence probability. If the voltage difference between two adjacent logic levels increases, the probability of misjudgment between those two logic levels decreases. For example, by increasing the voltage difference between two logic levels that change a bit with a high error occurrence probability, the bit error rate of that bit is reduced. Thus, by setting the voltage values ​​from level -0 to level -3 based on the error occurrence probability, the difference in receiver sensitivity between channel Rx1 and channel Rx2 can be reduced.

[0083] In IEEE (Institute of Electrical and Electronics Engineers) P802.3bs, switching between binary and Gray codes is required to maintain low power consumption and versatility. The DSP11 selects either Gray code or binary code as the encoding method based on the switching signal. The DSP11 sets voltage values ​​from -0 to -3 according to the selected encoding method. Based on this configuration, an optical transceiver 1 conforming to IEEE P802.3bs can be provided. The voltage value of the logic level at which the receiving sensitivity is optimal when binary code is selected is different from the voltage value at which the receiving sensitivity is optimal when Gray code is selected. Therefore, if the voltage value of the logic level optimal in one encoding method is set, the receiving sensitivity will degrade in the other encoding method. To address this, in the optical transceiver 1, the voltage values ​​of each logic level are set according to the selected encoding method. Therefore, both Gray code and binary code encoding methods can be used without increasing the difference in receiving sensitivity between channel Rx1 and channel Rx2.

[0084] In Gray code, bit B1 varies between level -0 and level -1, and between level -2 and level -3. Bit B2, on the other hand, varies between level -1 and level -2. Therefore, compared to bit B2, bit B1 varies more frequently between two adjacent logic levels. Consequently, when voltage differences V(0-1), V(2-3), and V(1-2) are the same, the bit error rate of bit B1 is higher than that of bit B2. Therefore, when Gray code is chosen as the encoding method, voltage differences V(0-1) and V(2-3) are set to be greater than voltage difference V(1-2). This reduces the probability of misjudging between level -0 and level -1, and between level -2 and level -3, compared to the probability of misjudging between level -1 and level -2. Therefore, the difference between the bit error rates of bit B1 and bit B2 can be reduced. As a result, the difference in receiving sensitivity between channel Rx1 and channel Rx2 can be reduced.

[0085] In other words, when Gray code is selected as the encoding method, the error probability of bit B1 is higher than that of bit B2. In this case, DSP11 sets the voltage values ​​from level -0 to level -3 such that the voltage difference between the two logic levels varying in bit B1 (between level -0 and level -1, and between level -2 and level -3) is greater than the voltage difference between the other two logic levels varying in bit B2 (between level -1 and level -2). Therefore, the difference between the bit error rate of bit B1 and the bit error rate of bit B2 is reduced. As a result, the difference in receiver sensitivity between channels Rx1 and Rx2 can be reduced.

[0086] In binary code, when the voltage differences V(0-1), V(1-2), and V(2-3) are all the same, the bit error rates of bits B1 and B2 can be homogenized. Therefore, DSP11 sets the voltage values ​​from level -0 to level -3 such that the voltage difference V(1-2) when Gray code is selected is less than the voltage difference V(1-2) when binary code is selected. Based on this configuration, when Gray code is selected as the encoding method, the probability of misjudging between level -0 and level -1 and between level -2 and level -3 is reduced compared to the probability of misjudging between level -1 and level -2. Therefore, the difference between the bit error rates of bit B1 and bit B2 can be reduced. As a result, when Gray code is selected as the encoding method, the difference in receiving sensitivity between channels Rx1 and Rx2 can be reduced.

[0087] DSP11 sets the voltage values ​​from -0 to -3 such that the bit error rates of bits B1 and B2 are equal. Therefore, the difference in bit error rates between channels Rx1 and Rx2 is reduced. Consequently, the difference in receiver sensitivity between channels Rx1 and Rx2 can be reduced.

[0088] A misjudgment between a level of -1 and a level of -2 results in a 2-bit error in binary code, but a 1-bit error in Gray code. Therefore, by using Gray code, bit errors in optical transmission can be reduced.

[0089] It should be noted that the optical transmitter and the control method of the optical transmitter disclosed herein are not limited to the above-described embodiments.

[0090] Optical transceiver 1 may also be without ROSA15. In this case, optical transceiver 1 functions as an optical transmitter.

[0091] The DSP11 can utilize encoding methods not limited to Gray code and binary code. The DSP11 can also utilize other encoding methods instead of Gray code. The DSP11 can also utilize other encoding methods instead of binary code. The DSP11 can also be configured to utilize more than three encoding methods.

[0092] In the above embodiments, the DSP11 may also utilize only one encoding method. For example, the DSP11 may use only Gray code or only binary code.

[0093] The driver circuit 12 can also have a level shifting function. In this case, the CPU 16 can also set the voltage values ​​of each logic level for the driver circuit 12.

[0094] PAM8 and PAM16 can also be used for multi-valued amplitude modulation. In other words, the DSP11 uses a specified encoding method to convert the M (M is an integer greater than 2) bits included in the transmitted data into a bit string with 2... M A pulse signal of logic level is generated, thereby producing a multi-value amplitude modulated signal. DSP11 sets 2 according to the above encoding method. M The voltage value of each logic level. In this case, the DSP11 also sets the voltage value based on the error probability of each bit included in the bit string. M The voltage value of each logic level. For example, if the probability of an error occurring in the first bit of the bit string is higher than the probability of an error occurring in the second bit of the bit string, the DSP11 sets the voltage value of the two logic levels where the first bit changes to be greater than the voltage difference between the other two logic levels where the second bit changes. M The voltage value of each logic level. DSP11 sets the 2 logic level by ensuring that the bit error rates of the first to the Mth bits in the M-bit array are equal. M The voltage value is set to a logic level. Even when using multi-value amplitude modulation other than PAM4, the DSP11 can set the voltage value from level -0 to level -3 in the same way as PAM4.

[0095] Explanation of reference numerals in the attached figures

[0096] 1: Optical transceiver (optical transmitter)

[0097] 2: Main unit

[0098] 11: DSP (Control Unit)

[0099] 12: Drive circuit (drive unit)

[0100] 13: TOSA (Light-emitting part)

[0101] 14: Bias Circuit

[0102] 15: ROSA

[0103] 16: CPU

[0104] 21: Motherboard

[0105] B1: Position (First Position)

[0106] B2: Second position

[0107] Rx1: Channel

[0108] Rx2: Channel

[0109] Tx1: Channel

[0110] Tx2: Channel.

Claims

1. An optical transmitter that outputs a multi-value amplitude-modulated optical signal, the optical transmitter comprising: The control unit generates a multi-value amplitude modulation signal based on the binary transmitted data; The driving unit generates a driving signal based on the multi-value amplitude modulation signal; as well as The light-emitting unit generates the light signal according to the driving signal. The control unit selects either the first encoding method or the second encoding method based on the switching signal. The control unit uses the selected encoding method to convert the M-bit bit string included in the transmitted data into 2-bit encoding. M A pulse signal of logic level is generated thereby to produce the multi-value amplitude modulation signal. The control unit sets the 2 according to the selected encoding method. M The voltage value of each logic level. M is an integer greater than or equal to 2. The control unit sets the 2 based on the error occurrence probability. M The voltage values ​​of logic levels, wherein the probability of error occurrence is in the 2 M In the event of a misjudgment between two adjacent logic levels in a given logic level sequence, the probability of an error occurring in each bit of the bit string is as follows: When the error probability of the first bit of the bit string is higher than the error probability of the second bit of the bit string, the control unit sets the second bit to a value such that the voltage difference between the two logic levels where the first bit changes is greater than the voltage difference between the other two logic levels where the second bit changes. M The voltage value of each logic level.

2. The optical transmitter according to claim 1, wherein, The control unit sets the bit error rate of each of the first to M bits included in the M bits to be equal to that of each bit. M The voltage value of each logic level.

3. The optical transmitter according to claim 1 or 2, wherein, The first encoding method is Gray code. The 2 M Each logic level includes the smallest first logic level, the second smallest second logic level, and the third smallest third logic level. The control unit sets the voltage difference between the first logic level and the second logic level when the first encoding method is selected to be greater than the voltage difference between the second logic level and the third logic level. M The voltage value of each logic level.

4. The optical transmitter according to claim 1 or 2, wherein, The first encoding method is Gray code. The second encoding method is binary code.

5. The optical transmitter according to claim 4, wherein, The 2 M Each logic level includes the smallest first logic level, the second smallest second logic level, and the third smallest third logic level. The control unit sets the voltage difference between the second logic level and the third logic level when the first encoding method is selected to be less than the voltage difference between the second logic level and the third logic level when the second encoding method is selected. M The voltage value of each logic level.

6. A control method for an optical transmitter, comprising the following steps: Multi-value amplitude modulation signals are generated based on binary transmitted data; A drive signal is generated based on the multi-value amplitude modulation signal; as well as The optical signal is generated based on the driving signal. In the step of generating the multi-value amplitude modulation signal, either a first encoding method or a second encoding method is selected according to the switching signal, and the selected encoding method is used to convert the M-bit bit string included in the transmitted data into 22... M A pulse signal of logic level is generated thereby to produce the multi-value amplitude modulation signal. The 2 M The voltage value of each logic level is set according to the selected encoding method. M is an integer greater than or equal to 2. The 2 is set according to the probability of error occurrence. M The voltage values ​​of logic levels, wherein the probability of error occurrence is in the 2 M In the event of a misjudgment between two adjacent logic levels in a given logic level sequence, the probability of an error occurring in each bit of the bit string is as follows: When the error probability of the first bit of the bit string is higher than the error probability of the second bit of the bit string, the second bit is set such that the voltage difference between the two logic levels where the first bit changes is greater than the voltage difference between the other two logic levels where the second bit changes. M The voltage value of each logic level.

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