JP7783538B2 - Optical transmitter - Google Patents

Description

The present invention relates to an optical transmitter that can operate at higher speeds while minimizing increases in power consumption.

With the advancement and spread of communications technology, the increase in content capacity, and the growing need for data centers, communications capacity is continuing to increase at an astonishing pace. In trunk lines, from short to long distances, the development of optical fiber communications technology is supporting this explosive increase in capacity. Optical transmitters required for optical fiber communications include direct drive systems that directly drive the semiconductor laser that outputs light, and external modulation systems that use an optical modulator to modulate the continuous light output from the laser. Optical transmitters are equipped with a driver IC for driving the laser or a driver IC for driving the optical modulator.

In general, external modulation methods enable 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 above are being considered. While this 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.

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 driver amplifiers use a differential amplifier configuration that continuously flows a constant current, flowing a larger 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 large voltages at high speed, and the driver amplifier is unable to keep up with the large-amplitude high-frequency signals input to it, 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.

One known method for compensating for the nonlinear output signal of a driver amplifier is the technology described in Patent Document 1. In addition to the analog signal output converted by the digital-to-analog converter, a compensation current is added from the nonlinear compensation unit, and this current drives the optical modulator via the drive circuit. This compensates for the nonlinear extinction characteristics of the optical transmitter, resulting in a linear modulation result. Conventional methods for compensating for nonlinear characteristics have involved digitally or analogically compensating for static nonlinear characteristics. However, as the frequency of the input signal increases, not only does the nonlinearity with respect 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.

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 produce large voltage changes even in low-impedance optical modulators. This method reduces frequency-dependent nonlinearity. However, driving large currents at high speeds not only requires devices capable of high-speed operation, but also requires devices with high voltage resistance for large voltages. This not only increases power consumption, but also poses the problem that the higher the speed, the more difficult it becomes to increase the current.

Patent No. 6491129

The object of the present invention is to provide an optical transmitter that can operate at higher speeds and higher voltages while minimizing increases in power consumption and that can suppress nonlinearity at high frequencies.

In order to achieve this objective, one embodiment of the present invention is characterized in that an optical transmitter including a digital signal processing unit is provided with a differentiation circuit that calculates a differential value of a digital signal using the digital signal processing unit, a compensation circuit that calculates a compensation value corresponding 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.

FIG. 1 is a diagram showing the configuration of an optical transmitter according to an embodiment of the present invention; FIG. 2 is a diagram for explaining a method for compensating for nonlinearity in the optical transmitter of this embodiment; FIG. 3 is a diagram showing a configuration for determining a compensation value in the optical transmitter of this embodiment; FIG. 4 is a diagram showing another configuration for determining a compensation value in the optical transmitter of this embodiment.

Below, we will explain in detail the embodiments of the present invention with reference to the drawings.

Figure 1 shows the configuration of an optical transmitter according to one embodiment of the present invention. The optical transmitter 10 comprises 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 to an analog signal, and a driver amplifier 13 that drives an optical modulator 14, connected in this order. The optical modulator 14 modulates the output light from a laser 15, which serves as a light source, and outputs the modulated light to an optical fiber connected to the optical transmitter 10. The 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 the compensation circuit 22 is added in an adder circuit 24 to the digital input signal, the timing of which has been adjusted by a delay circuit 23. The digital signal to which the compensation value has been added is input to a digital-to-analog converter 12, converted to an analog signal, and output to the driver amplifier 13.

In this embodiment, an example will be described in which digital modulation is performed using the PAM4 modulation method. The differentiation circuit 31 calculates a differential value in response 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 in response to relative changes in the absolute value of the differential value. In other words, a compensation value is calculated based on changes in the signal level in response 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 differential value as a table, or may store a calculation formula for determining the compensation value. Details will be described later.

The compensation value may be determined by taking into account the amount of compensation obtained in advance through 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 differential values to be compensated may be limited, or the amount of compensation may be determined according to the specifications of the driver amplifier 13 and the optical modulator 14.

Referring to Figure 2, a method for compensating for nonlinearity in the optical transmitter of this embodiment will be described. This figure shows a case in which the original signal of the PAM4 modulation format changes in signal level from 0-3-2-0-1-3-0 at time steps 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 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->3 transition, or 0.6 is subtracted (-0.6 is added) for a 3->0 transition. For other differences, no compensation value is added or subtracted.

In addition, at the next transition after adding or subtracting a 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.

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 based on 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 based on the transition between adjacent time steps, but it is also possible to take into account 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.

In addition, when performing equalization processing in DSP 11, it is preferable to perform the equalization processing on the digitally modulated signal after equalization to which a compensation value has been added.

Figure 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. Therefore, it is possible to detect signals affected by nonlinearity. Based on the detection results by the peak detector 25, an external data processing device 41 determines a compensation value to compensate for the nonlinear output signal of the driver amplifier 13.

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 a change in the digitally modulated signal by the DSP 11, or a formula is created to calculate the compensation value. Finally, the created table or formula is stored in the compensation circuit 22 of the optical transmitter 10.

Figure 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 as the intensity modulation point, 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.

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.

In this way, in optical transmitters that operate at higher speeds and higher voltages, nonlinearity at high frequencies can be suppressed by adding or subtracting a compensation value to the digitally modulated signal.

Claims (5)

An optical transmitter including a digital signal processing unit configured to perform digital modulation in a PAM4 modulation format ,
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. 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. 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. An optical transmitter as described in claim 1, 2, or 3, characterized in that the digital signal processing unit further performs equalization processing on the digital signal to which the compensation value has been added. An optical transmitter as described in claim 1, 2, or 3, characterized in that the compensation circuit corrects the compensation value taking into account transitions between three or more time steps of the digital signal.