Optical transmitter

Abstract

The present invention suppresses non-linearity in high frequencies. Provided is an optical transmitter that includes a digital signal processing unit, and that comprises: a differential circuit that calculates a differential value of a digital signal from the digital signal processing unit; a compensation circuit that calculates a compensation value corresponding to relative changes in a relative value of the differential value; and an addition circuit that adds the compensation value to the digital signal.

Description

[Technical Field]

[0001] The present invention relates to an optical transmitter that can operate at a higher speed while suppressing an increase in power consumption. [Background technology]

[0002] With the advancement and spread of communication technology, the increase in content capacity, and the growing need for data centers, communication capacity is steadily increasing at an astonishing pace. In trunk lines, from short distances to long distances, the development of optical fiber communication technology is supporting this explosive increase in capacity. The optical transmitters required for optical fiber communication are classified into two types: a direct drive system that directly drives the semiconductor laser that outputs light, and an external modulation system that modulates the continuous light output from the laser using an optical modulator. Each optical transmitter is equipped with a driver IC to drive the laser or a driver IC to drive the optical modulator.

[0003] In general, external modulation methods allow for faster communication than direct drive methods and are used at higher baud rates. In recent years, 66 GBaud has been put into practical use, and modulation speeds of 100 Gbaud and higher are being considered. Although it depends on the type of optical modulator, it is often necessary to apply a large modulation voltage amplitude of 2 Vpp or more to drive the optical modulator. Whether the optical modulator uses a direct intensity modulation method such as PAM, or a coherent phase-intensity modulation method such as QPSK or QAM, a large voltage amplitude of around 2 to 4 Vpp is required.

[0004] When attempting to change such a large voltage at high speed, for example, to drive an optical modulator with a low input impedance of several tens of ohms, it is necessary to change the driver amplifier's voltage with a larger current as it approaches the driver amplifier's output stage. Because the driver amplifier uses a differential amplifier configuration that continuously flows a constant current, flowing a large current requires greater power consumption. In particular, the higher the operation speed, the larger this current becomes, resulting in excessive power consumption. On the other hand, limiting the current makes it impossible to change a large voltage at high speed, and the driver amplifier is unable to keep up with the large-amplitude high-frequency signals input, limiting the output voltage amplitude. As a result, the driver amplifier's output changes out of proportion to increases in voltage and current as the input signal becomes more frequent, resulting in a nonlinear output signal.

[0005] A known method for compensating for the nonlinear output signal of a driver amplifier is disclosed, for example, in Patent Document 1. In addition to the analog signal output converted by a digital-to-analog converter, a compensation current is added from a nonlinear compensation unit to drive an optical modulator via a driver circuit. This compensates for the nonlinear extinction characteristics of the optical transmitter, resulting in a linear modulation result. Conventional methods for compensating for static nonlinear characteristics have been used, either digitally or analogically. However, as the frequency of the input signal increases, not only does the nonlinearity relative to the amplitude of the output signal increase, but the degree of change in nonlinearity varies depending on the frequency. Therefore, conventional methods have had difficulty compensating for the entire frequency band of the input signal.

[0006] Furthermore, by passing a large current through the output stage of the differential amplifier in the driver amplifier and increasing the amount of modulating current, it is possible to quickly generate large voltage changes even in low-impedance optical modulators. This method reduces frequency-dependent nonlinearity. However, driving large currents at high speeds requires not only devices capable of high-speed operation, but also devices with high voltage resistance for large voltages. This not only increases power consumption, but also poses the problem of the increasing difficulty of increasing current as speed increases. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6491129 Summary of the Invention

[0008] An object of the present invention is to provide an optical transmitter that can operate at higher speeds and higher voltages while suppressing an increase in power consumption, and that can suppress nonlinearity at high frequencies.

[0009] In order to achieve the above object, one embodiment of the present invention is characterized in that an optical transmitter including a digital signal processing section is provided with: a differentiation circuit that calculates a differential value of a digital signal by the digital signal processing section; a compensation circuit that calculates a compensation value according to a relative change in the absolute value of the differential value; and an addition circuit that adds the compensation value to the digital signal. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of an optical transmitter according to one embodiment of the present invention; [Figure 2] FIG. 2 is a diagram for explaining a method for compensating for nonlinearity in the optical transmitter of this embodiment; [Figure 3] FIG. 3 is a diagram showing a configuration for determining a compensation value in the optical transmitter of this embodiment; [Figure 4] FIG. 4 is a diagram showing another configuration for determining a compensation value in the optical transmitter of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0012] FIG. 1 shows the configuration of an optical transmitter according to one embodiment of the present invention. Optical transmitter 10 includes a digital signal processor (DSP) 11 that performs digital modulation, such as PAM4 or QPSK, on ​​a digital input signal, a digital-to-analog converter 12 that converts the digitally modulated signal into an analog signal, and a driver amplifier 13 that drives an optical modulator 14, all connected in this order. Optical modulator 14 modulates the output light from laser 15, which serves as a light source, and outputs the modulated light to an optical fiber connected to optical transmitter 10. Optical transmitter 10 also includes a differentiation circuit 21 that calculates a differential value of the digitally modulated signal, and a compensation circuit 22 that calculates a compensation value from the differential value. The compensation value calculated by compensation circuit 22 is added in adder circuit 24 to a digital input signal whose timing has been adjusted by delay circuit 23. The digital signal to which the compensation value has been added is input to digital-to-analog converter 12, converted to an analog signal, and output to driver amplifier 13.

[0013] In this embodiment, a case where digital modulation is performed using the PAM4 modulation method will be described as an example. The differentiation circuit 31 calculates a differentiation value according to changes in the four-value signal of the PAM4 modulation method. Here, the four-value signal levels are expressed as 0-3. The compensation circuit 22 calculates a compensation value according to relative changes in the absolute value of the differentiation value. That is, the compensation value is calculated based on changes in the signal level according to the baud rate, and is added to the digital signal to compensate for the nonlinear output signal of the driver amplifier. The compensation circuit 22 may store compensation values ​​for relative changes in the absolute value of the differentiation value as a table, or may store a calculation formula for determining the compensation value. Details will be described later.

[0014] In addition, the compensation value may be determined by taking into account the compensation amount previously obtained by simulation or actual measurement so that the quality of the optical signal output from the optical transmitter is the desired quality. Furthermore, the range of the differential value to be compensated may be limited, or the compensation amount may be determined according to the specifications of the driver amplifier 13 and the optical modulator 14.

[0015] A method of compensating for nonlinearity in an optical transmitter according to this embodiment will be described with reference to Figure 2. This figure shows a case in which an original signal of PAM4 modulation format changes in signal level from 0-3-2-0-1-3-0 from time step t1 to t7 on the time axis. Of the 16 transitions between four-value signals, the signal changes most quickly when transitioning from 0 to 3 or from 3 to 0. Therefore, when the difference is 3, a compensation value of 0.6 is added to the signal after the transition for a 0-to-3 transition, or 0.6 is subtracted (-0.6 is added) for a 3-to-0 transition. For other differences, no compensation value is added or subtracted.

[0016] In the next transition after adding or subtracting the compensation value, the compensated amount is subtracted to return to the original signal level, and a decision is made as to whether to add or subtract a compensation value according to the difference. In this way, the differential value is calculated according to the change in the four-value signal, and the compensation value is calculated according to the relative change in the absolute value of the differential value.

[0017] The compensation value is determined in advance by adjusting the amount of compensation through simulation or actual measurement so as to improve the quality of the output optical signal. In the above example, the compensation value was added or subtracted only for the transition with the largest difference, but compensation values ​​according to the absolute value of the differential value may also be added or subtracted for other transitions. Furthermore, when using four values ​​per time step, the compensation value was determined according to the transition between adjacent time steps, but it is also possible to consider transitions between not only adjacent time steps but also three or more time steps, taking into account higher-order differential coefficients, and further correct the compensation value. Furthermore, when the signal change rate is above a certain level, it may be changed discretely, for example by adding a specific coefficient.

[0018] Furthermore, when equalizing is performed in the DSP 11, it is preferable to perform equalizing on the digitally modulated signal after adding the compensation value.

[0019] 3 shows an example of a configuration for determining a compensation value in the optical transmitter of this embodiment. A peak detector 25 is used to detect the peak amplitude of the output of the driver amplifier 13. The peak detector 25 detects the output after the digital signal to which the compensation value has been added is converted into an analog signal and amplified by the driver amplifier 13. This makes it possible to detect signals affected by nonlinearity. An external data processing device 41 determines a compensation value based on the detection result by the peak detector 25 so as to compensate for the nonlinear output signal of the driver amplifier 13.

[0020] Next, in the data processing device 41, a table is created as a compensation value for the relative change in the absolute value of the differential value in response to the change in the digitally modulated signal by the DSP 11, or a formula is created for calculating the compensation value. Finally, the created table or formula is stored in the compensation circuit 22 of the optical transmitter 10.

[0021] 4 shows another configuration for determining a compensation value in the optical transmitter of this embodiment. A photoelectric conversion element and a measuring instrument 42 such as an oscilloscope are connected to the output of the optical transmitter 10. With the bias point fixed for intensity modulation, the digital signal to which the compensation value has been added is converted into an optical signal, and the intensity of the optical signal output from the optical modulator 14 is measured. An external data processing device 41 determines the compensation value so that the quality of the optical signal is the desired quality.

[0022] Next, in the data processing device 41, a table of compensation values ​​for relative changes in the absolute value of the differential value in response to changes in the digitally modulated signal by the DSP 11 is created, or a formula for calculating the compensation values ​​is created. Finally, the created table or formula is stored in the compensation circuit 22 of the optical transmitter 10.

[0023] In this way, in an optical transmitter that operates at higher speeds and higher voltages, nonlinearity at high frequencies can be suppressed by adding or subtracting a compensation value to a digitally modulated signal.

Claims

1. An optical transmitter including a digital signal processing unit configured to perform digital modulation using a PAM4 modulation method, a differentiation circuit that calculates a differential value of a signal that exhibits the fastest signal level transition between adjacent time steps of the digital signal modulated by the digital signal processing unit; a compensation circuit that calculates a compensation value according to the differential value; an adder circuit that adds the compensation value to the digital signal; An optical transmitter comprising:

2. a digital-to-analog converter for converting the digital signal into an analog signal; a driver amplifier connected to an output of the digital-to-analog converter and configured to drive an optical modulator; a peak detector for detecting a peak amplitude of the output of the driver amplifier; 2. The optical transmitter according to claim 1, wherein the compensation circuit stores a table or a calculation formula for calculating a compensation value for compensating for a nonlinear output signal of the driver amplifier based on the detection result by the peak detector.

3. a digital-to-analog converter for converting the digital signal into an analog signal; a driver amplifier connected to an output of the digital-to-analog converter and configured to drive an optical modulator; 2. The optical transmitter according to claim 1, wherein the compensation circuit stores a table or a formula for calculating a compensation value that will result in a desired quality of the optical signal output from the optical modulator.

4. 4. The optical transmitter according to claim 1, wherein the digital signal processing unit further performs equalization processing on the digital signal to which the compensation value has been added.

5. 4. The optical transmitter according to claim 1, wherein the compensation circuit corrects the compensation value in consideration of transitions between three or more time steps of the digital signal.

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