Transmission / reception circuit, transmission / reception system, and transmission circuit

Abstract

A transmission / reception system 1 comprises an optical transmission module 10, an optical reception module 20, and an optical fiber 30, and transmits, from a transmission-side circuit 40 to a reception-side circuit 50, a multi-value signal having three or more values. The optical transmission module 10 comprises a laser diode 11 and a Tx circuit 12. The Tx circuit 12 generates a multi-value drive signal that is pre-distorted in a direction for correcting distortion of a multi-value optical signal relative to the multi-value drive signal in the laser diode 11, and provides this pre-distorted multi-value drive signal to the laser diode 11.

Description

Transmitting and receiving circuit, transmitting and receiving system, and transmitting circuit

[0001] This invention relates to a transmitting and receiving circuit, a transmitting and receiving system, and a transmitting circuit.

[0002] In recent years, there has been a growing demand for generative AI and services that utilize it. Generative AI services are provided to users via a network from data centers. Data centers are facilities that house, store, and operate servers and other equipment, and they operate continuously under strict environmental control, including temperature. A server consists of multiple racks, each containing multiple types of computing resources such as CPUs (Central Processing Units), GPUs (Graphics Processing Units), and memory. To meet the growing demand for generative AI in recent years, the performance of the servers themselves in data centers has improved, as has the speed of data transmission.

[0003] Because the distances between servers and racks are relatively long, Ethernet® is generally used for data transmission between them. Furthermore, to accommodate high-speed data transmission, optical cables (Active Optical Cable, AOC) are used, and data is transmitted as an optical signal from a laser diode (preferably a Vertical Cavity Surface Emitting Laser, VCSEL) through an optical fiber to a photodiode. In addition, due to the need for even faster data transmission and limitations of optical components, Pulse Amplitude Modulation 4 (PAM4) is used for data transmission.

[0004] The transmission of multi-level optical signals, such as PAM4, is more susceptible to noise than the transmission of binary signals, resulting in signal quality degradation and a worse error rate. To address this problem, the Ethernet AOC standard specifies the adoption of KP4 FEC, a high-performance forward error correction (FEC) using a high-performance optical DSP (oDSP) within the AOC (Non-Patent Literature 1).

[0005] As the demand for generative AI services increases, the amount of data is expected to grow, and data transmission delays will become a problem. Data transmission delays can lead to problems such as delays in services (especially responses) and delays in learning. Therefore, minimizing data transmission delays is required.

[0006] Furthermore, the increasing demand for generational AI services is expected to lead to increased power demand at data centers. Increased power consumption at data centers will not only lead to environmental problems and greenhouse gas emissions, but also to increased operating costs, potentially extending the payback period for investments and making it difficult to adequately meet service demand. Therefore, it is necessary to curb the increase in power consumption at data centers. In order to reduce power consumption at data centers, it is necessary to reduce power consumption not only in servers but also in data transmission.

[0007] Non-patent document 2 describes an invention intended to meet the demands for lower latency and lower power consumption in data transmission. The invention described in this document is called Linear Optics, and it aims to reduce latency and lower power consumption by reducing the number of digital signal processors (DSPs) used in the AOC (Automatic Operation Center) in Ethernet-based data transmission.

[0008] Apama Prasad, et al., “Advanced 3D Packaging of 3.2Tbs Optical Engine for Co-packaged Optics (CPO) in Hyperscale Data Center Networks,” 2024 IEEE 74th Electronic Components and Technology Conference (ECTC 2024), Denver, Colorado, USA, 2024, pp.101-106.

[0009] Rang Chen Yu, et al., “Energy Efficient Optical Solutions for GAI” OCT 2024, SAN JOSE, CA, USA, 2024.

[0010] To address the further increase in demand for generative AI in the future, the low latency and power saving achieved by the invention described in Non-Patent Document 2 will not be sufficient, and further reductions in data transmission latency and power saving will be required. Furthermore, these reductions in data transmission latency and power saving will be required in a variety of applications, not limited to this one.

[0011] Compute Express Link (CXL) (registered trademark) is a new data communication protocol standard proposed to achieve further low latency and power savings in data transmission, utilizing the physical layer specifications of PCI Express (registered trademark). The CXL communication protocol enables lower latency than the communication protocol defined by Ethernet. Furthermore, the PCI Express Optical Cable Workgroup was established to realize optical communication using PCI Express for long-distance transmission between servers and between racks using CXL. PCI Express also adopts PAM4 transmission from its latest generation, the 6th generation.

[0012] However, adopting Linear Optics, the invention described in Non-Patent Document 2, is difficult in achieving low power consumption and low latency PCI Express optical communication. This is because, instead of using a DSP within the AOC, Linear Optics relies on the high-performance FEC of the oDSP built into the endpoint ASIC (transmitter and receiver circuits) to deal with the high error rate that is a problem in PAM4 optical communication. However, high-performance FECs such as KP4 cause significant latency, making it impossible to meet the low latency requirement for the entire system as defined in the PCI Express standard.

[0013] To meet the low-latency system-wide requirement stipulated by PCI Express, and to achieve low-power and low-latency PCI Express optical communication using Linear Optics, the invention described in Non-Patent Literature 2, without relying on the high-performance FEC of oDSPs that cause significant delays, it is necessary to solve the high error rate problem in PAM4 optical communication. This is because the PCI Express standard requires achieving a lower error rate than the Ethernet standard in order to establish communication without relying on the high-performance FEC of oDSPs such as KP4. To solve the high error rate, improving the signal-to-noise ratio (SNR) at the input of the optical receiver module is effective, so an optical transmitter module that outputs signals with larger modulation amplitudes is required. However, outputting with larger modulation amplitudes makes it easier for eye distortion to occur due to the nonlinearity of the photoelectric conversion element. In particular, in VCSELs, which are suitable from the viewpoint of low power consumption, the occurrence of DC distortion and AC distortion in the eye is unavoidable when driven with large amplitudes. This eye distortion is a fatal problem in Linear Optics. This is because, in Linear Optics, the optical receiver module is a linear receiver, and therefore the eye distortion generated in the optical transmitter module is not corrected and appears directly in the output of the optical receiver module. As a result, the eye of the AOC output signal received by the endpoint ASIC (receiving circuit) does not meet the eye aperture requirement specified in the PCI Express standard, making it difficult to establish communication.

[0014] The inventors focused on both the Tx circuit used for transmitting optical signals and the Rx circuit used for receiving optical signals, and conducted diligent research with the aim of further reducing the latency and power consumption of data transmission as a whole system, which led to the invention of this invention.

[0015] The present invention aims to provide a transmitting and receiving circuit equipped with a Tx circuit and an Rx circuit that enable further reduction in data transmission latency and power saving in a system that transmits and receives optical signals with three or more levels, as well as a transmitting and receiving system equipped with this transmitting and receiving circuit.

[0016] The transmitting and receiving circuit of the present invention is used together with a laser diode and a photodiode that transmit and receive multi-level optical signals of three or more levels. The transmitting and receiving circuit of the present invention includes (1) a drive circuit that provides a multi-level drive signal to the laser diode to output a multi-level optical signal from the laser diode, and a Tx circuit that generates a multi-level drive signal that is pre-distorted in a direction that corrects the distortion of the multi-level optical signal with respect to the multi-level drive signal in the laser diode, and provides this pre-distorted multi-level drive signal from the drive circuit to the laser diode; and (2) an Rx circuit that receives a multi-level electrical signal output from a photodiode that has received the multi-level optical signal output from the laser diode, amplifies this multi-level electrical signal as a multi-level electrical signal, and outputs it.

[0017] The Tx circuit preferably further includes a pre-circuit provided before the drive circuit. The pre-circuit generates a multi-level electrical signal that has been pre-distorted in a direction that corrects the DC distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode, based on a multi-level input electrical signal, and outputs it to the drive circuit. The drive circuit generates a multi-level drive signal that has been pre-distorted in a direction that corrects the AC distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode, based on the multi-level electrical signal output from the pre-circuit, and supplies it to the laser diode.

[0018] The transmitting and receiving system of the present invention comprises the transmitting and receiving circuit of the present invention described above, a laser diode that outputs a multi-level optical signal based on a multi-level drive signal provided from the Tx circuit, and a photodiode that receives the multi-level optical signal output from the laser diode and outputs a multi-level electrical signal to the Rx circuit.

[0019] The present invention preferably further comprises an optical fiber that inputs a multi-level optical signal output from a laser diode to one end, guides the light, and outputs it from the other end to a photodiode. It is also preferable to use PCI Express® or CXL® as the communication protocol, and to use a vertical cavity surface-emitting laser (VCSEL) as the laser diode.

[0020] According to the present invention, in a system that transmits and receives multi-level optical signals with three or more levels, an optical transmission module capable of outputting with a larger modulation amplitude can be realized without causing eye distortion problems due to the nonlinearity of the photoelectric conversion element. This improves the signal-to-noise ratio at the input of the optical reception module and achieves a sufficiently low error rate without relying on a high-performance FEC that causes large delays, thereby enabling further reductions in data transmission latency and power consumption for the entire system.

[0021] Figure 1 shows the configuration of the transmitting / receiving system 1. Figure 2 shows an example configuration of the optical transmitting module 10. Figure 3 shows an example configuration of the optical receiving module 20. Figure 4(a) is a graph showing the relationship between the input current and output optical power of the laser diode 11, Figure 4(b) is a schematic diagram showing the eye pattern of the four-level drive signal (input current) supplied to the laser diode 11, and Figure 4(c) is a schematic diagram showing the eye pattern of the four-level optical signal (output optical power) output from the laser diode 11. Figure 5(a) is a graph showing the relationship between the input current value and output optical power of the laser diode 11, Figure 5(b) is a schematic diagram showing the eye pattern of the multi-level drive signal (input current) supplied to the laser diode 11, and Figure 5(c) is a schematic diagram showing the eye pattern of the multi-level optical signal (output optical power) output from the laser diode 11. Figure 6(a) schematically shows the eye pattern of the four-value drive signal (input current) supplied to the laser diode 11, and Figure 6(b) schematically shows the eye pattern of the four-value optical signal (output optical power) output from the laser diode 11. Figure 7(a) schematically shows the eye pattern of the four-value drive signal (input current) supplied to the laser diode 11, and Figure 7(b) schematically shows the eye pattern of the four-value optical signal (output optical power) output from the laser diode 11.

[0022] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the attached drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted.

[0023] Figure 1 shows the configuration of the transmitting / receiving system 1. The transmitting / receiving system 1 comprises an optical transmitting module 10, an optical receiving module 20, and an optical fiber 30. The transmitting / receiving system 1 is installed between the transmitting circuit 40 and the receiving circuit 50, and transmits a multi-level signal from the transmitting circuit 40 to the receiving circuit 50. The multi-level signal is a signal with three or more levels, for example, a four-level signal of PAM4.

[0024] The optical transmission module 10 includes a laser diode 11 and a Tx circuit 12 (transmission circuit). The Tx circuit 12 receives a multi-level electrical signal (voltage signal) output from the transmitting circuit 40, and generates and outputs a multi-level drive signal (current signal) to be supplied to the laser diode 11 based on this multi-level input electrical signal. The laser diode 11 outputs a multi-level optical signal based on the multi-level drive signal supplied from the Tx circuit 12.

[0025] The laser diode 11 is preferably a VCSEL. In laser diodes, not limited to VCSELs, the power of the optical signal generally increases as the drive signal increases, but the relationship between the magnitude of the drive signal and the power of the optical signal is not linear. Therefore, the multi-level optical signal output in response to a given multi-level drive signal is distorted.

[0026] The Tx circuit 12 generates a multi-level drive signal that has been pre-distorted in a direction that corrects the distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode 11, and supplies this pre-distorted multi-level drive signal to the laser diode 11.

[0027] The optical receiving module 20 includes a photodiode 21 and an Rx circuit 22 (receiving circuit). The Rx circuit 22 receives a multi-level electrical signal (current signal) output from the photodiode 21 that has received a multi-level optical signal, amplifies it, and outputs a multi-level electrical signal (voltage signal) to the receiving circuit 50.

[0028] The optical fiber 30 receives the multi-level optical signal output from the laser diode 11 at one end, guides the light, and outputs the light to the photodiode 21 at the other end. The transmitting and receiving system 1, which includes the optical transmitting module 10, the optical receiving module 20, and the optical fiber 30, is preferably integrated as an optical cable (AOC). The multi-level optical signal output from the laser diode 11 may be received directly by the photodiode 21 without being guided by the optical fiber 30, but it is preferable that it be guided by the optical fiber 30.

[0029] Figure 2 shows an example configuration of the optical transmission module 10. The Tx circuit 12 includes a CDR circuit 13 (Clock Data Recovery circuit), a DA conversion circuit 14 (Digital-Analog conversion circuit), and a drive circuit 15. The CDR circuit 13 receives a multi-level electrical signal (voltage signal) output from the transmitting circuit 40, converts this multi-level input electrical signal into a binary electrical signal (digital signal), and outputs this binary electrical signal to the DA conversion circuit 14. The DA conversion circuit 14 receives the binary electrical signal output from the CDR circuit 13, converts this binary electrical signal into a multi-level electrical signal (voltage signal), and outputs this multi-level electrical signal to the drive circuit 15. The drive circuit 15 receives the multi-level electrical signal output from the DA conversion circuit 14, generates a multi-level drive signal (current signal) to be supplied to the laser diode 11, and outputs it.

[0030] The pre-stage circuit (CDR circuit 13 and DA conversion circuit 14) provided in the front stage of the drive circuit 15 generates a multi-valued electrical signal that is pre-distorted in a direction to correct the DC distortion of the distortion of the multi-valued optical signal with respect to the multi-valued drive signal in the laser diode 11, and outputs this multi-valued electrical signal to the drive circuit 15. The CDR circuit 13 and the DA conversion circuit 14 generate a multi-valued electrical signal that is pre-distorted in a direction to correct the DC distortion as follows. The multi-valued electrical signal output from the transmission-side circuit 40 is converted into a binary electrical signal by the CDR circuit 13, and then this binary electrical signal is converted into a multi-valued electrical signal by the DA conversion circuit 14. At the time of this DA conversion, the relationship between the value represented by the input binary electrical signal (the value represented by a fixed number of bits) and the value of the output electrical signal is made non-linear, and due to the non-linearity of this relationship, the non-linearity of the DC distortion of the distortion of the multi-valued optical signal with respect to the multi-valued drive signal in the laser diode 11 can be canceled out. This will be described again later using FIGS. 4 and 5.

[0031] Suppose that the level (output optical power) of the optical signal in the laser diode is H1, H2, H3, or H4. Suppose that a binary digital signal indicating the level HN (N = 1, 2, 3, 4) is output from the CDR circuit 13, and a multi-valued analog signal indicating the level HN is output from the DA conversion circuit 14. Regarding the correction of the DC distortion, for example, when the digital signal obtained by the CDR circuit 13 is subjected to analog conversion by the DAC circuit 14 to generate an analog signal, the correspondence (level mapping) between the digital signal and the output voltage value of the analog signal is intentionally made non-linear (predistortion).

[0032] When the input current I to the laser diode and the output optical power P have a relationship of P = f(I), the input current I １ , I ２ , I ３ , I ４ are present, when changing from I １ to I ２ , the change amount ΔP12 of the output optical power P (= P ２ - P １ ), and when changing from I ２ to I ３When changed to this, the change in output optical power P is ΔP23 (=P ３ -P ２ ) and I ３ From I ４ When changed to this, the change in output optical power P is ΔP34 = (P ４ -P ３ ) make equal. ΔP12 = f(I ２ ) - f(I １ ), ΔP23 = f(I ３ ) - f(I ２ ), ΔP34 = f(I ４ ) - f(I ３ ) Let P = f(I) be nonlinear, and if we want the intervals between the four levels of ΔP to be equal, the discrete condition is to choose the intervals between the input currents to be unequal (mapping with inverse characteristics) such that ΔP12 = ΔP23 = ΔP34. When a binary digital signal indicating level HN (N = 1, 2, 3, 4) is output from the CDR circuit 13, the DA conversion circuit 14 determines the value of the multi-level analog signal indicating level HN that represents I １ , I ２ , I ３ , I ４ Select the values ​​of as shown above at unequal intervals.

[0033] In other words, the intervals between the levels (H1, H2, H3, H4) corresponding to multiple values ​​of the output optical power P of the laser diode are substantially equal. １ , P ２ , P ３ , P ４ ) are determined, and these target values ​​(P １ , P ２ , P ３ , P ４ The drive current I (I) in the drive circuit 15 corresponding to ) １ , I ２ , I ３ , I ４ The drive current I is determined based on the relationship between output power P and drive current I (P = f(I)). The intervals are considered to be substantially equal if the difference between each interval is within ±10% of the difference between two intervals. The DA conversion circuit receives signals indicating the levels corresponding to the multi-level (H1, H2, H3, H4), and the drive current I is determined in accordance with the level of the received signal (I１ , I ２ , I ３ , I ４ A control signal is output to the drive circuit 15 that causes the drive circuit 15 to output ).

[0034] In other words, the target value P is set so that the four levels of output optical power are equally spaced. １ ~P ４ Determine the target value and the corresponding drive current I １ ~I ４ This is determined by the inverse characteristic of the static characteristic P = f(I) of the output light and drive current (inverse function (P) of I = f). The DA conversion circuit 14 receives the symbol value (level number) from the CDR / digital processing unit and outputs the corresponding analog signal (current or voltage) based on the lookup table, and the drive circuit 15 sets a predetermined drive current I １ ~I ４ You may control the process to generate it.

[0035] This allows for the cancellation of static nonlinearity (level spacing distortion) in the optical output drive signal of the laser diode 11. The relationship between the input current I and the output optical power P, P = f(I), can be determined by measuring the actual output optical power.

[0036] Correction processing to compensate for DC distortion (static and amplitude-direction nonlinearity) can be implemented digitally or analogously. For example, in the DA conversion circuit 14, or in the preceding stage, level mapping using a lookup table method is performed to pre-distortion. A target value for the output optical power is set so that each level of the PAM4 is equally spaced, the intensity of the laser light is measured using a monitoring photodiode, and the relationship between the input current and the output optical power is mapped in the table so as to minimize the difference between the target value and the measured value. An analog amplifier having an input / output relationship for correction designed with a digital circuit may be connected to the drive circuit 15. Such an analog amplifier can also be realized by appropriately combining multiple amplifiers with different amplification characteristics.

[0037] Furthermore, the drive circuit 15 generates a multi-level drive signal that has been pre-distorted in a direction that corrects the AC distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode 11, based on the multi-level electrical signal output from the DA conversion circuit 14, and provides this multi-level drive signal to the laser diode 11. The value of the drive signal output from the drive circuit 15 has a linear relationship with the value of the electrical signal input to the drive circuit 15. The rise time and fall time of the multi-level drive signal output from the drive circuit 15 are made different from each other, thereby enabling the cancellation of the AC distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode 11. For example, the drive circuit 15 generates a drive signal whose waveform has been shaped in a direction that reduces distortion in the time direction. This will be explained in more detail later using Figures 6 and 7.

[0038] Figure 3 shows an example configuration of the optical receiving module 20. The Rx circuit 22 includes a transimpedance amplifier 23 and a variable gain amplifier 24. The transimpedance amplifier 23 receives a multi-level electrical signal (current signal) output from the photodiode 21 that receives a multi-level optical signal, converts it to a current-voltage, and outputs the multi-level electrical signal (voltage signal) to the variable gain amplifier 24. The value of the electrical signal output from the transimpedance amplifier 23 has a linear relationship with the value of the electrical signal input to the transimpedance amplifier 23. The variable gain amplifier 24 amplifies the multi-level electrical signal output from the transimpedance amplifier 23 and outputs it. The gain of the variable gain amplifier 24 is controlled so that the amplitude of the electrical signal output from the variable gain amplifier 24 is constant. The value of the electrical signal output from the variable gain amplifier 24 has a linear relationship with the value of the electrical signal input to the variable gain amplifier 24. Therefore, the transimpedance amplifier 23 and the variable gain amplifier 24 do not binarize the electrical signal, but output it as a multi-level electrical signal. Furthermore, the Rx circuit 22 does not include the aforementioned CDR circuit that converts the input multi-level electrical signal into a digital signal.

[0039] Next, we will explain the DC distortion of the multi-level optical signal in the laser diode 11 with respect to the multi-level drive signal using Figure 4, and then explain the correction of this DC distortion using Figure 5. Here, the multi-level signal is treated as a quaternary signal, and the value of the quaternary drive signal (input current value) input to the laser diode 11 is I １ ~I ４ The power of the four-value optical signal output from the laser diode 11 (output optical power) for each of these input current values ​​is P １ ~P ４ Let it be so. However, I １ <I ２ <I ３ <I ４ P １ <P ２ <P ３ <P ４ That is the case.

[0040] Figure 4 illustrates the DC distortion in the multi-level optical signal in response to the multi-level drive signal in the laser diode 11. Figure 4(a) shows the relationship between the input current value and the output optical power of the laser diode 11. Figure 4(b) schematically shows the eye pattern of the four-level drive signal applied to the laser diode 11. Figure 4(c) schematically shows the eye pattern of the four-level optical signal output from the laser diode 11. These figures illustrate the case where DC distortion correction is not performed.

[0041] As shown in Figure 4, the larger the input current value, the greater the output optical power, but the ratio of the increase in output optical power to the increase in input current value becomes smaller. Therefore, the input current value I, which is one of the values ​​of the drive signal, １ ~I ４ Even if the intervals are equal, the output optical power P is the value of the optical signal corresponding to each of these intervals. １ ~P ４ The intervals are not equal. That is, P ２ and P ３ Compared to the interval between P １ and P ２ The gap between them widens, P ３ and P ４The gap between the two points narrows, causing the eye of the four-level optical signal to become distorted. This is the DC distortion of the laser diode 11. This DC distortion of the laser diode 11 increases the error rate.

[0042] Figure 5 illustrates the correction of DC distortion in the multi-level optical signal for the multi-level drive signal in the laser diode 11. Figure 5(a) shows the relationship between the input current value and the output optical power of the laser diode 11. Figure 5(b) schematically shows the eye pattern of the multi-level drive signal applied to the laser diode 11. Figure 5(c) schematically shows the eye pattern of the multi-level optical signal output from the laser diode 11. These figures illustrate the case when DC distortion correction is performed.

[0043] As shown in Figure 5, in order to perform DC distortion correction, the output optical power P, which is each value of the optical signal, is used. １ ~P ４ The input current values ​​I, which are the values ​​of the drive signal input to the laser diode 11, are set such that the intervals between the eyepieces of the optical signal are equal, that is, so that the height of the eyepiece of the optical signal is constant. １ ~I ４ The intervals between them are made different from each other. That is, I ２ and I ３ Compared to the interval between, I １ and I ２ Narrow the gap between them, I ３ and I ４ Widen the gap between them.

[0044] In this embodiment, a multi-level signal was described as a four-level signal, but it can be applied to signals with three or more levels. That is, the value of the drive signal (input current value) is I １ ~I ｎ (n is an integer greater than or equal to 3) １ <I ２ …<I ｎ When that is the case, I n-2 and I n-1 Compared to the interval between I n-1 and I n The gap between them should be widened.

[0045] As shown in Figure 5, the CDR circuit 13 and the DA conversion circuit 14 generate a multi-level electrical signal that is pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode 11, and output this multi-level electrical signal to the drive circuit 15. This cancels out the nonlinearity of the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode 11.

[0046] Next, we will explain the AC distortion, which is part of the distortion of the multi-level optical signal with respect to the multi-level drive signal in the laser diode 11, using Figure 6, and then explain the correction of this AC distortion using Figure 7. Here, as with Figures 4 and 5, the multi-level signal is treated as a quaternary signal.

[0047] Figure 6 illustrates AC distortion, a type of distortion in the multi-level optical signal in response to a multi-level drive signal in the laser diode 11. Figure 6(a) schematically shows the eye pattern of the four-level drive signal applied to the laser diode 11. Figure 6(b) schematically shows the eye pattern of the four-level optical signal output from the laser diode 11. These figures illustrate the case where AC distortion correction is not performed.

[0048] As shown in Figure 6, even if the rise time and fall time are equal in the transition of the drive signal value (input current value) input to the laser diode 11, the rise time and fall time are not equal in the power transition of the optical signal output from the laser diode 11. As a result, the time positions of the three eyes of the quadrant optical signal are different from each other, causing skew, narrowing the width of the eyes, and distorting the eyes of the quadrant optical signal. This is the AC distortion of the laser diode 11. This AC distortion of the laser diode 11 further increases the error rate.

[0049] Figure 7 illustrates the correction of AC distortion, one of the distortions in the multi-level optical signal for the multi-level drive signal in the laser diode 11. Figure 7(a) schematically shows the eye pattern of the four-level drive signal applied to the laser diode 11. Figure 7(b) schematically shows the eye pattern of the four-level optical signal output from the laser diode 11. These figures illustrate the case when AC distortion correction is performed.

[0050] As shown in Figure 7, in order to perform AC distortion correction, the rise time and fall time of the drive signal input to the laser diode 11 are made to differ from each other in the transition of the power of the optical signal output from the laser diode 11, that is, in order to make the rise time and fall time substantially equal to each other in the power transition of the optical signal output from the laser diode 11, i.e., to make the eye width of the optical signal constant. In other words, the rise time is made slower than the fall time in the transition of the value of the drive signal input to the laser diode 11. The rise time tr and fall time tf of the power of the optical signal are said to be substantially equal to each other if the difference between the rise time tr and the fall time tf (Δt = tr - tf) is within ±10% of the rise time (tr × 90% ≤ Δt ≤ tr × 110%). The rise time tr refers to the time it takes for the waveform to reach a different percentage from a certain percentage of its amplitude when it transitions from a low level to a high level, and can be taken as the time from the time when each value is at 10% to the time when it is at 90%. The fall time tf can be defined as the time from when each value reaches 90% of its high level to when it reaches 10%.

[0051] For example, the waveform edges of the drive signal applied to the laser diode 11 are made asymmetrical, so that the rising edge from "low level to high level" is gentler than the falling edge from "high level to low level," and the rise time tr is made longer than the falling time tf (tr > tf).

[0052] There are several circuit structures that make the rise time tr relatively longer than the fall time tf. For example, in a current-mode drive circuit, the driving force acting in the increasing direction (up) is made smaller, and the driving force acting in the decreasing direction (down) is made larger. When the drive circuit is constructed using a field-effect transistor (FET), the source current on the up side should be set to be small, and the source current on the down side should be set to be large. In addition, circuit configurations that control the slew rate can also be considered. For example, by connecting a resistor in series with the gate of a MOSFET and a switching element in parallel, the rise time of the drive current becomes longer due to the time constant of the resistor when the FET turns ON, and the gate charge is rapidly extracted by bypassing with the switching element when the FET turns OFF. In addition, it is also possible to control the rise time tr by changing the resistance value using a digitally controlled variable resistor. Another method is to use a digital FFE in the drive circuit to perform negative pre-emphasis and reduce the high-frequency component of the up edge.

[0053] In this embodiment, a multi-level signal was described as a quaternary signal, but it can be applied to signals with three or more levels. That is, the rise time should be slower than the fall time in the value transition of the drive signal input to the laser diode 11.

[0054] As shown in Figure 7, the drive circuit 15 generates a multi-level drive signal that is pre-distorted in a direction that corrects the AC distortion among the distortions of the multi-level optical signal relative to the multi-level drive signal in the laser diode 11, and outputs this multi-level drive signal to the laser diode 11. This cancels out the nonlinearity of the AC distortion among the distortions of the multi-level optical signal relative to the multi-level drive signal in the laser diode 11.

[0055] In the explanation so far, the CDR circuit 13 and DA conversion circuit 14 have been assumed to perform DC distortion correction, and the drive circuit 15 has performed AC distortion correction (rise time correction), but this is not limited to this. For example, the CDR circuit 13 and DA conversion circuit 14 may perform both DC distortion correction and AC distortion correction, or the drive circuit 15 may perform both DC distortion correction and AC distortion correction. The Tx circuit 12 may have other configurations. Also, the CDR circuit 13, DA conversion circuit 14, and drive circuit 15 may be implemented on the same semiconductor chip, or on different semiconductor chips. Furthermore, all or part of each of the three circuits within the Tx circuit 12 may be included in the transmitter circuit 40 of Figure 1. If the DA conversion circuit 14 is included in the transmitter circuit 40 of Figure 1, the CDR circuit 13 becomes unnecessary.

[0056] Conventionally, the problem of increased error rates due to DC distortion (Figure 4) and AC distortion (Figure 6) of the laser diode 11 was addressed by digital signal processing by an optical DSP provided on the receiving side. In contrast, in this embodiment, the Tx circuit 12 generates a multi-level drive signal that is pre-distorted in a direction that corrects the distortion of the multi-level optical signal with respect to the multi-level drive signal in the laser diode 11. By supplying this pre-distorted multi-level drive signal to the laser diode 11, the eye height of the multi-level optical signal output from the laser diode 11 can be kept constant (Figure 5), and the eye width can also be kept constant (Figure 7). As a result, in this embodiment, the increase in error rates can be suppressed without requiring digital signal processing by a DSP on the receiving side.

[0057] In this embodiment, the inclusion of the CDR circuit 13 in the Tx circuit 12 leads to increased delay and power consumption. However, compared to the optical DSPs that were conventionally provided on the transmitting and receiving sides for advanced digital signal processing and FEC, the CDR circuit 13 has low delay and low power consumption. Therefore, in this embodiment, where there is no need to provide optical DSPs on the transmitting and receiving sides, it is possible to reduce delay and save power for the entire system including the transmitting circuit 40, optical transmitting module 10, optical receiving module 20, and receiving circuit 50. Furthermore, in this embodiment, the Rx circuit 22 is equipped only with a transimpedance amplifier 23 and a variable gain amplifier 24, and does not include a CDR or DSP for binarization or conversion to logic signals, which is preferable from the viewpoint of low delay and low power consumption.

[0058] Next, we will further explain the challenges in transmitting multi-level signals such as PAM4. Compared to the transmission of binary signals, the transmission of multi-level signals presents a problem due to the increase in bit error rate (BER) caused by the deterioration of the signal-to-noise ratio. Conventionally, in order to reduce the BER and improve the communication quality of the system, FEC was performed using a DSP on the receiving side. The execution of FEC in this DSP was the dominant factor in delay.

[0059] To eliminate the DSP on the receiving side, it is necessary to make FEC (Field Emission Control) unnecessary. To achieve this, it is necessary to reduce the BER (Block Emission Rate) of multi-level signal transmission without performing FEC. In this embodiment, the transimpedance amplifier 23 and variable gain amplifier 24 of the Rx circuit 22 output multi-level electrical signals with wide bandwidth and large amplitude output, and with a sufficiently large eye height. Furthermore, in this embodiment, it is preferable that the Tx circuit 12 and the transmitting circuit 40, and the Rx circuit 22 and the receiving circuit 50 are connected by an OBO (On Board Optics) module to shorten the electrical transmission distance.

[0060] In the receiving circuit 50, the eye height and width of the receivable multi-level signal may be defined by the absolute voltage value and the absolute time value. To ensure a sufficient eye width for the received multi-level signal, the entire transmitting and receiving system 1 needs to have a high bandwidth. Furthermore, to ensure a sufficient eye height for the received multi-level signal, if the signal-to-noise ratio of the multi-level signal is constant, a larger signal amplitude is advantageous. Therefore, in this embodiment, the transimpedance amplifier 23 and variable gain amplifier 24 of the Rx circuit 22 are configured as broadband and large-amplitude linear amplifiers to output a multi-level electrical signal with sufficiently large eye height and width. It is preferable that the amplitude of the multi-level signal output from the Rx circuit 22 is larger than that of conventional systems.

[0061] Furthermore, improving the signal-to-noise ratio (SNR) at the input of the Rx circuit 22 is also effective in reducing the BER of multi-level signal transmission. This can be achieved by increasing the amplitude of the multi-level drive signal supplied to the laser diode 11. Conventionally, increasing the amplitude of the multi-level drive signal increases the distortion of the multi-level optical signal, which actually worsens the BER of multi-level signal transmission. However, in this embodiment, even with an increased amplitude of the multi-level drive signal, the distortion of the multi-level optical signal can be suppressed, thereby reducing the BER of multi-level signal transmission.

[0062] To ensure sufficient eye height and width of the multi-level signal input to the receiving circuit 50, it is preferable to minimize signal loss in the electrical wiring between the Rx circuit 22 and the receiving circuit 50. By minimizing this signal loss, deterioration of the eye due to inter-symbol interference (ISI) can be suppressed. The equalizer that corrects ISI in the receiving circuit 50 can be implemented using an analog circuit rather than a DSP, and a continuous-time linear equalizer (CTLE) is commonly used. Since a CTLE has the characteristic of attenuating the low-frequency components of a multi-level signal and boosting the high-frequency components, the stronger the correction by the CTLE, the lower the DC amplitude of the received signal, which is disadvantageous for ensuring the absolute voltage value of the eye height. Therefore, it is still advantageous to have less ISI. Therefore, in order to reduce ISI, it is preferable to connect the Rx circuit 22 and the receiving circuit 50 to each other using an OBO module, thereby shortening the electrical transmission distance between the Rx circuit 22 and the receiving circuit 50 and reducing signal loss.

[0063] In contrast, when connecting the Rx circuit 22 and the receiving circuit 50 using conventional pluggable modules, the Rx circuit 22 must be placed at the edge of the board, and depending on the position of the receiving circuit 50 on the board, the wiring distance between the Rx circuit 22 and the receiving circuit 50 can be long, for example, several tens of centimeters or more. When connecting the Rx circuit 22 and the receiving circuit 50 using an OBO module, the degree of freedom in the position of the Rx circuit 22 on the board is increased, and the wiring distance between the Rx circuit 22 and the receiving circuit 50 can be shortened.

[0064] The transmitting / receiving system 1 of this embodiment can be implemented using Ethernet (registered trademark), but it is preferable to implement it using PCI Express (Peripheral Component Interconnect Express, registered trademark). The first reason is that by combining the transmitting / receiving system 1, which aims to reduce the latency of the physical layer, with PCI Express, which employs a communication protocol with lower latency compared to Ethernet, the objective of reducing latency can be effectively achieved. The second reason is that PCI Express, with its sixth generation released in 2022, adopted PAM4 for the first time and is a standard that does not use an optical DSP (i.e., it does not employ advanced digital signal processing or high-performance FECs with high latency such as KP4 FEC). Therefore, there is no problem in removing the optical DSP from the receiving circuit 50 when implementing the transmitting / receiving system 1, and in fact, there is a problem in adopting an optical DSP (e.g., enormous investment is required to develop new PCI Express communication IP that supports optical DSP).

[0065] Furthermore, the current Ethernet standard assumes the use of an optical DSP to perform digital FFE (Feed Forward Equalizer) with dozens of taps or more, and heavy FEC (KP4, etc.) with high latency. Since the receiving circuit 50 specifies the use of an optical DSP, it is difficult to remove the optical DSP from it under the current standard. Also, since the current Ethernet standard specifies the use of an optical DSP in the receiving circuit 50, there is no motivation to deliberately correct the distortion of the multi-level optical signal in the Tx circuit 12 and remove the optical DSP from the receiving circuit 50.

[0066] PCI Express has been widely used as a standard for short-distance communication between CPUs and peripherals, and is attracting attention as a low-latency transmission standard for the future evolution of AI. In order to connect many computing resources (CPU, GPU, memory) with low latency, overcoming spatial constraints, the application of PCI Express to optical communication has been actively discussed in recent years. In order to realize optical communication of PCI Express, the transmission and reception system 1 of this embodiment is preferable to existing technologies in terms of lower power consumption and lower latency.

[0067] The transmission / reception system 1 of this embodiment can be particularly suitable for data transmission in data centers. In data centers, there is a demand for large-capacity memory to store data used for generating AI, and in addition to boards equipped with a CPU, GPU, and memory, boards that integrate only memory may be used in the future. As a protocol for data transmission between these boards, it is expected that CXL (Compute Express Link, registered trademark), which was developed based on the physical layer of PCI Express, will be used. Compared to PCI Express, CXL achieves significantly higher bandwidth, enables high-speed transmission of large amounts of data, has low latency, and is highly scalable. The transmission / reception system 1 of this embodiment can also be implemented using CXL.

[0068] As described above, the transmitting and receiving circuit according to the first embodiment is a transmitting and receiving circuit used together with a laser diode 11 and a photodiode 21 that transmit and receive multi-level optical signals of three or more levels, and includes a drive circuit 15 that provides a multi-level drive signal to the laser diode 11 to output a multi-level optical signal from the laser diode 11, a Tx circuit 12 that generates a multi-level drive signal that has been pre-distorted in a direction that corrects the distortion of the multi-level optical signal with respect to the multi-level drive signal in the laser diode 11, and provides this pre-distorted multi-level drive signal from the drive circuit 15 to the laser diode 11, and an Rx circuit 22 that receives a multi-level electrical signal output from the photodiode 21 that has received the multi-level optical signal output from the laser diode 11, amplifies this multi-level electrical signal as a multi-level electrical signal and outputs it.

[0069] In the second embodiment, the transmitting and receiving circuit, in the first embodiment, generates a multi-level drive signal that has been pre-distorted in a direction that corrects the AC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode 11, based on the input multi-level electrical signal, and supplies it to the laser diode 11.

[0070] In the third embodiment, the transmitting and receiving circuit, in the first embodiment, generates a multi-level drive signal that has been pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal relative to the multi-level drive signal in the laser diode 11, based on the input multi-level electrical signal, and supplies it to the laser diode 11.

[0071] The fourth embodiment of the transmitting and receiving circuit is the same as the second embodiment of the transmitting and receiving circuit, in which the Tx circuit 12 generates a multi-level electrical signal that has been pre-distorted in a direction that corrects the DC distortion of the multi-level optical signal with respect to the multi-level driving signal in the laser diode 11, based on the input multi-level electrical signal, and generates a multi-level driving signal that has been pre-distorted in a direction that corrects the AC distortion of the multi-level optical signal with respect to the multi-level driving signal in the laser diode, based on this electrical signal, and supplies it to the laser diode 11.

[0072] The fifth embodiment of the transmitting and receiving circuit further includes a pre-circuit provided before the drive circuit 15 in the transmitting and receiving circuit of the first embodiment, wherein the Tx circuit 12 is located before the drive circuit 15.

[0073] The transmitting and receiving circuit according to the sixth embodiment further includes a pre-circuit provided before the drive circuit 15 in the transmitting and receiving circuit of the second embodiment.

[0074] The seventh embodiment of the transmitting and receiving circuit is a transmission and receiving circuit according to the fifth embodiment, in which the pre-amplifier circuit generates a multi-level electrical signal that has been pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level driving signal in the laser diode 11, based on the multi-level input electrical signal, and outputs it to the driving circuit 15.

[0075] In the eighth embodiment, the transmitting and receiving circuit, in the sixth embodiment, has a pre-amplifier that generates a multi-level electrical signal that has been pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level driving signal in the laser diode 11, based on the multi-level input electrical signal, and outputs it to the driving circuit 15.

[0076] The transmission and reception system according to the ninth embodiment includes the above-described transmission and reception circuit, a laser diode 11 that outputs a multi-level optical signal based on a multi-level drive signal provided by the Tx circuit 12, and a photodiode 21 that receives the multi-level optical signal output from the laser diode 11 and outputs a multi-level electrical signal to the Rx circuit 22.

[0077] The tenth embodiment of the transmitting and receiving system further includes an optical fiber 30 that inputs a multi-level optical signal output from the laser diode 11 to one end, guides the light, and outputs it from the other end to the photodiode 21.

[0078] The transmission and reception system according to the eleventh embodiment uses PCI Express® or CXL® as the communication protocol.

[0079] In the transmission / reception system according to the 12th embodiment, the laser diode 11 is a vertical cavity surface-emitting laser.

[0080] The transmission circuit according to the 13th embodiment further includes a pre-circuit provided in front of the drive circuit 15.

[0081] The transmission circuit according to the 14th embodiment includes a drive circuit 15 that supplies a multi-level drive signal to the laser diode 11. The drive circuit 15 supplies the laser diode 11 with a drive signal (drive current) whose rise time has been corrected so that the rise time and fall time are substantially equal in the power transition of the optical signal output from the laser diode 11.

[0082] In the transmission circuit according to the 15th embodiment, the preamplifier includes a clock data recovery circuit (CDR circuit 13) and a digital-to-analog conversion circuit (DA conversion circuit 14) connected downstream of the clock data recovery circuit, and the drive circuit 15 is connected downstream of the digital-to-analog conversion circuit.

[0083] In the transmission circuit according to the 16th aspect, a plurality of target values (P １ , P ２ , P ３ , P ４ ) of the output optical power P are determined such that the intervals between the levels (H1, H2, H3, H4) corresponding to the multi-values of the output optical power P of the laser diode 11 are substantially equal. The drive currents I (I １ , I ２ , I ３ , I ４ ) in the drive circuit 15 corresponding to these target values (P １ , I ２ , I ３ , I ４ ) are determined based on the relationship between the output power P and the drive current I (P = f(I)). The DA conversion circuit 14 receives a signal indicating the levels (H1, H2, H3, H4) corresponding to the multi-values, and outputs a control signal to the drive circuit 15 that causes the drive circuit 15 to output the determined drive current I (I １ , I ２ , I ３ , I ４ ).

[0084] Incidentally, an example of DC distortion correction is correction of the drive current corresponding to the multi-value levels, and an example of AC distortion correction is rise time correction. As described above, the pre-stage circuits (CDR circuit 13 and DA conversion circuit 14) perform DC distortion correction and AC distortion correction, and the drive circuit 15 may not perform correction. The pre-stage circuit may perform DC distortion correction and the drive circuit 15 may perform AC distortion correction. The pre-stage circuit may not perform correction and the drive circuit 15 may perform DC distortion correction and AC distortion correction. The pre-stage circuit may perform DC distortion correction and the drive circuit 15 may not perform correction. The pre-stage circuit may not perform correction and the drive circuit 15 may perform AC distortion correction. The configuration of the pre-stage circuit is not limited to the above-described circuit, and may be a circuit including an amplifier or the like.

[0085] The present invention is not limited to the above examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0086] 1...Transmitting and receiving system, 10...Optical transmitting module, 11...Laser diode, 12...Tx circuit, 13...CDR circuit, 14...DA conversion circuit, 15...Drive circuit, 20...Optical receiving module, 21...Photodiode, 22...Rx circuit, 23...Transimpedance amplifier, 24...Variable gain amplifier, 30...Optical fiber, 40...Transmitting circuit, 50...Receiving circuit.

Claims

1. A transmitting and receiving circuit used with a laser diode and a photodiode that transmit and receive multi-level optical signals of three or more levels, comprising: a drive circuit that provides a multi-level drive signal to the laser diode to output a multi-level optical signal from the laser diode; a Tx circuit that generates a multi-level drive signal that is pre-distorted in a direction that corrects the distortion of the multi-level optical signal with respect to the multi-level drive signal in the laser diode, and provides this pre-distorted multi-level drive signal from the drive circuit to the laser diode; and an Rx circuit that receives a multi-level electrical signal output from the photodiode that has received the multi-level optical signal output from the laser diode, amplifies this multi-level electrical signal as a multi-level electrical signal, and outputs it.

2. The transceiver circuit according to claim 1, wherein the Tx circuit generates a multi-level drive signal that has been pre-distorted in a direction that corrects the AC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode, based on the input multi-level electrical signal, and provides it to the laser diode.

3. The transceiver circuit according to claim 1, wherein the Tx circuit generates a multi-level drive signal that has been pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode, based on the input multi-level electrical signal, and provides it to the laser diode.

4. The transceiver circuit according to claim 1, wherein the Tx circuit generates a multi-level electrical signal that is pre-distorted in a direction that corrects the DC distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode, based on an input multi-level electrical signal, and generates a multi-level drive signal that is pre-distorted in a direction that corrects the AC distortion of the multi-level optical signal relative to the multi-level drive signal in the laser diode, based on this electrical signal, and supplies it to the laser diode.

5. The transmitting and receiving circuit according to claim 1, wherein the Tx circuit further includes a pre-circuit provided in front of the drive circuit.

6. The transmitting and receiving circuit according to claim 2, wherein the Tx circuit further includes a pre-circuit provided in front of the drive circuit.

7. The preamplifier circuit generates a multi-level electrical signal that has been pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode, based on the input multi-level electrical signal, and outputs it to the drive circuit, as described in claim 5.

8. The preamplifier circuit generates a multi-level electrical signal that has been pre-distorted in a direction that corrects the DC distortion among the distortions of the multi-level optical signal with respect to the multi-level drive signal in the laser diode, based on the input multi-level electrical signal, and outputs it to the drive circuit, as described in claim 6.

9. A transmitting and receiving system comprising: a transmitting and receiving circuit as described in claim 1; a laser diode that outputs a multi-level optical signal based on a multi-level drive signal provided from the Tx circuit; and a photodiode that receives the multi-level optical signal output from the laser diode and outputs a multi-level electrical signal to the Rx circuit.

10. The transmitting and receiving system according to claim 9, further comprising an optical fiber that inputs a multi-level optical signal output from the laser diode to one end, guides the light, and outputs it to the photodiode from the other end.

11. The transmission / reception system according to claim 9, wherein PCI Express® or CXL® is used as the communication protocol.

12. The transmitting and receiving system according to claim 9, wherein the laser diode is a vertical cavity surface-emitting laser.

13. A transmission circuit comprising a drive circuit that supplies a multi-level drive signal to a laser diode, wherein the drive circuit supplies the drive signal to the laser diode with a corrected rise time such that the rise time and fall time are substantially equal in the power transition of the optical signal output from the laser diode.

14. The transmitting circuit according to claim 13, further comprising a pre-circuit provided in front of the drive circuit.

15. The transmission circuit according to claim 14, wherein the preamplifier circuit comprises a clock data recovery circuit and a digital-to-analog conversion circuit connected downstream of the clock data recovery circuit, and the drive circuit is connected downstream of the digital-to-analog conversion circuit.

16. A transmission circuit according to claim 15, wherein a plurality of target values ​​for the output optical power P of the laser diode are determined such that the intervals between the levels corresponding to the multiple values ​​of the output optical power P are substantially equal, the drive current I in the drive circuit corresponding to these target values ​​is determined based on the relationship between the output power P and the drive current I, and the digital-to-analog conversion circuit receives a signal indicating the level corresponding to the multiple values, and outputs a control signal to the drive circuit that causes the drive circuit to output the determined drive current I corresponding to the level of the received signal.

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