PAM-4 optical signal generation system based on digital modulation to suppress nonlinear effects

Abstract

The present application belongs to the field of short distance optical interconnection technology, and specifically relates to a PAM-4 optical signal generation system based on digital modulation to inhibit nonlinear effect. The present application only utilizes a single laser and a single photodiode to generate PAM-4 optical signals suitable for short distance optical interconnection. Based on the principle of digital modulation, a single laser is used to respectively send two routes of binary on-off keying signals with a power difference of about 14 dB into a Mach-Zehnder optical modulator for intensity modulation. The output two routes of optical signals are multiplexed in one optical path through an optical multiplexer to generate a PAM-4 optical signal with a transmission rate of 100 Gbps. The optical signal generates a PAM-4 signal after inputting into a photodiode and a filter through an optical fiber. The system transmission end of the present application has low structural complexity, does not need digital-to-analog conversion, has large input signal power, and can be widely applied in the field of short distance optical interconnection by flexibly adjusting the pre-distortion of the amplitude of two routes of OOK signals to inhibit the nonlinear effect of the optical fiber in the system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of short-distance optical interconnection, and particularly relates to a PAM-4 optical signal generation system based on digital modulation to suppress nonlinear effects. BACKGROUND

[0002] With the development of high-speed optical communication and telecommunication data networks, the traffic demand is growing rapidly, and it is urgent to adopt complex modulation methods. For many years, non-return-to-zero (NRZ) modulation has been the main technology of incoherent optical communication, and most of the current optical communication systems use NRZ modulation. When continuously improving the transmission rate, the transmission process will generate higher crosstalk to the high-rate signal. If a higher-order modulation method is used, more data can be carried at the same bandwidth as NRZ. At present, 100Gbit / s optical transmission systems are being commercialized. When the single-channel rate exceeds 10Gbit / s, the NRZ technology will be difficult to meet the needs of long-distance transmission. In the upgrading scheme of the NRZ modulation method, 4-level pulse amplitude modulation (PAM-4) has attracted widespread attention. The use of PAM-4 technology can improve the problems of NRZ modulation in optical-electric conversion bandwidth, cost, etc. There are many ways to generate PAM-4 signals. In the early stage, there was a method of passively synthesizing a PAM-4 signal from two NRZ signals. However, the synthesized PAM-4 index will be severely deteriorated, and more compensation is required. There is also a method of actively synthesizing a PAM-4 signal from two NRZ signals. This method uses a DAC converter. Compared with passive synthesis, the output signal of this scheme has high bandwidth and fast rise time, but it also has limitations and a complex structure. PAM-4 signals can also be output based on an AWG arbitrary waveform generator, but the hardware is relatively expensive, and the system structure is complex. There are also high-integration PAM-4 optical transmission modules, which have a narrow application range and can only provide the required PAM-4 signal for a few specific scenarios. The modular structure has high complexity and high cost. The two OOK signals used in the conventional PAM-4 generation method have a small power difference, which is better for generating PAM-4 signals with equal amplitude and interval. However, when the input signal power is large, there is a nonlinear effect in the optical system, which makes the generated PAM-4 signal distorted. Therefore, it is necessary to study how to realize high-power PAM-4 optical signal generation and transmission based on a simple architecture to suppress nonlinear effects, so as to apply it to short-distance optical interconnection systems.

[0003] In view of the above, the present application provides a PAM-4 optical signal generation system based on digital modulation to suppress nonlinear effects. The system uses a single laser, and two binary on-off keying (OOK) signals with large amplitude difference are sent into a Mach-Zehnder optical modulator (MZM) for intensity modulation. The output two optical signals are multiplexed in one optical path by an optical multiplexer to generate a PAM-4 optical signal with a rate of 100 Gbps. The PAM-4 optical signal generates a PAM-4 signal after inputting into a photodiode and a filter through an optical fiber. The system has low complexity, does not need digital-to-analog conversion, has low cost, and has large input signal power. The nonlinear effects of the optical fiber in the system are suppressed by a pre-distortion method of flexibly adjusting the amplitude of the two OOK signals, and the system can be widely applied in short-distance optical interconnection field. SUMMARY

[0004] The purpose of the present application is to provide a PAM-4 optical signal generation system based on digital modulation to suppress nonlinear effects. The nonlinear effects of the optical fiber in the system are suppressed by a pre-distortion method of flexibly adjusting the amplitude of the two OOK signals, and the PAM-4 optical signal is generated in the optical domain, so that it can be widely applied in short-distance optical interconnection scenarios.

[0005] The PAM-4 optical signal generation system based on digital modulation to suppress nonlinear effects provided by the present application specifically comprises:

[0006] A free single-mode laser for providing continuous wave optical waves, with a frequency of fc;

[0007] Two digital signal generators for generating OOK signals with a power difference of about 14 dB by digital modulation;

[0008] Two Mach-Zehnder optical modulators (MZM) in push-pull mode for generating optical signals carrying OOK information;

[0009] Two digital DC sources for providing DC bias voltage for the optical modulator;

[0010] Two adjustable optical attenuators for adjusting the optical signal power entering the optical fiber;

[0011] An optical multiplexer for multiplexing the optical signals of the two optical paths in one branch to generate a PAM-4 optical signal;

[0012] A 2km optical fiber for transmitting the PAM-4 optical signal;

[0013] A photodiode for converting the transmitted PAM-4 optical signal in the system into an electrical signal;

[0014] A low-pass filter for filtering the PAM-4 electrical signal output by the photodiode;

[0015] An oscilloscope for observing the PAM-4 signal formed.

[0016] The PAM-4 optical signal generation system based on digital modulation for suppressing nonlinear effects provided by the application, at the sending end, the system uses two OOK signals with a power difference of about 14 dB generated directly by digital modulation to pass through a Mach-Zehnder optical modulator respectively, and cooperates an external cavity modulation laser and a direct current power supply to modulate the signal in intensity, and through an optical multiplexer, the two generated optical signals are multiplexed in one optical path to generate a 100 Gbps rate PAM-4 optical signal, the PAM-4 optical signal is transmitted to a photodetector through an optical fiber and filtered, and finally enters an oscilloscope for reception, and the PAM-4 signal can be obtained.

[0017] The PAM-4 optical signal generation system based on digital modulation for suppressing nonlinear effects, the two OOK signals with a power difference of about 14 dB are directly generated by digital modulation.

[0018] The PAM-4 optical signal generation system based on digital modulation for suppressing nonlinear effects, the direct current power supply and the OOK signal can be adjusted to make the optical modulator work in a push-pull mode.

[0019] The PAM-4 optical signal generation system based on digital modulation for suppressing nonlinear effects, only a single laser and two optical modulators with the same parameters are used, and two optical signals generated by intensity modulation can be transmitted in the system.

[0020] The PAM-4 optical signal generation system based on digital modulation for suppressing nonlinear effects, the nonlinear effect of the optical fiber in the system is suppressed by making the power difference of the two OOK signals about 14 dB.

[0021] The system based on the digital modulation principle, the system uses a single laser, and two binary on-off keying (OOK) signals with a large amplitude difference are sent into a Mach-Zehnder optical modulator (MZM) respectively, and the two output optical signals are multiplexed in one optical path through an optical multiplexer to generate a 100 Gbps rate PAM-4 optical signal, and the PAM-4 optical signal is input into a photodiode and a filter through an optical fiber to generate a PAM-4 signal.

[0022] The system of the application has a simple structure at the transmitting end, does not need digital-to-analog conversion, can input a signal with a relatively large power, suppresses the nonlinear effect of the optical fiber in the system through a flexible pre-distortion method of adjusting the amplitude of the two OOK signals, and can be widely applied in the field of short-distance optical interconnection.

[0023] The PAM-4 optical signal generation system of the application can avoid using an expensive arbitrary waveform generator, and only uses a laser and an optical modulator to realize generation of a high-power PAM-4 optical signal, can effectively suppress the nonlinear effect of an optical fiber in the system, and has the advantages of simple system structure, low cost and strong stability. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The PAM-4 optical signal generation method system architecture based on digital modulation to suppress nonlinear effects is provided by the application.

[0025] Figure 2 The simulation system is built in VPI software.

[0026] Figure 3 The PAM4 signal eye diagram obtained at the receiving end when the input amplitude of the OOK signal is 2.5V and 0.5V.

[0027] Figure 4 The waveform diagram of the OOK signal with input amplitudes of 2.5V and 0.5V after superposition in the electrical domain.

[0028] Figure 5 The waveform diagram of the OOK signal with input amplitudes of 1V and 0.5V after superposition in the electrical domain.

[0029] Figure 6 The PAM4 signal eye diagram obtained at the receiving end when the input amplitude of the OOK signal is 1V and 0.5V.

[0030] In the figure, 1 is an external cavity modulated laser, 2 is a first OOK signal generated by digital modulation, 3 is a second OOK signal generated by digital modulation, 4 is a first direct current power supply, 5 is a direct current power supply, 6 is a first Mach-Zehnder optical modulator, 7 is a second Mach-Zehnder optical modulator, 8 is a first adjustable optical attenuator, 9 is a second adjustable optical attenuator, 10 is an optical multiplexer, 11 is a short-distance optical fiber, 12 is a photodiode, 13 is a low-pass filter, and 14 is an oscilloscope. EMBODIMENT

[0031] The application will be specifically described below with reference to the drawings.

[0032] Figure 1 The PAM-4 optical signal generation system architecture based on digital modulation to suppress nonlinear effects comprises:

[0033] At the transmitting end, a first OOK signal 2 and a second OOK signal 3, with a power difference of approximately 14 dB, are generated by digital modulation and input to a first Mach-Zehnder optical modulator 6 and a second Mach-Zehnder optical modulator 7, respectively. A first DC power supply 4 and a first DC power supply 5 provide the required bias voltages for the first optical modulator 6 and the second optical modulator 7, respectively, putting them in push-pull mode. A continuous wave of light with a frequency of fc from an external cavity modulated laser 1 is modulated by the first optical modulator 6 and the second optical modulator 7, respectively. The two modulated optical signals enter a first adjustable optical attenuator 8 and a second adjustable optical attenuator 9, respectively, and are then multiplexed by an optical multiplexer 10 to output a single PAM-4 optical signal. The PAM-4 optical signal is input to a photodiode 12 via a short-distance optical fiber 11 and converted into an electrical signal. After passing through a low-pass filter 13, it is connected to an oscilloscope 14 to observe that the output signal is a PAM-4 signal.

[0034] The system of this invention was simulated and tested using VPI. The system framework is as follows: Figure 2 As shown, with Figure 1 Maintain consistency. Figure 2 The system framework diagram of the VPI software is given. The two small figures represent simulators integrated with MATLAB. The OOK bitstream generated by MATLAB is converted into electrical signals and input to the system. OOK signals with amplitudes of 2.5V and 0.5V are input to the MZM for intensity modulation and then fed into the optical fiber. The two signals are then multiplexed in the optical path. At the receiving end, they enter a photodetector to achieve light-to-electricity conversion. After passing through a filter, the final output PAM4 signal eye diagram is shown below. Figure 3 As shown. It's worth noting that the two OOK signals in the electrical domain were added together in the simulation. It can be seen that the superposition of the two signals in the electrical domain results in four voltage levels: -3V, -2V, 2V, and 3V, as shown below. Figure 4 As shown. To verify that this method can effectively suppress the nonlinear effects of the optical fiber in the system, OOK signals with amplitudes of 1V and 0.5V were input in the same manner. The superposition of the two signals in the electrical domain forms PAM4 signals with four levels: -1.5V, -0.5V, 0.5V, and 1.5V, as shown. Figure 5 As shown. Finally, the PAM4 eye diagram obtained at the receiving end through the optical path is as follows. Figure 6 As shown. By observation Figure 3 and Figure 6 As can be seen, when two OOK signals with significantly different amplitudes are input, the system can effectively suppress the nonlinear effects of optical fibers, while simultaneously increasing the input power of the signals.

[0035] In summary, the PAM-4 optical signal generation method based on digital modulation to suppress nonlinear effects can realize the generation of 100 Gbps rate PAM-4 optical signals with low complexity and cost, can use higher power input signals, can suppress the nonlinear effects of optical fibers in the system, and is suitable for application in the future short-distance optical interconnection field.

Claims

1. A PAM-4 optical signal generation system based on digital modulation to suppress nonlinear effects, characterized in that, include: A free single-mode laser that provides continuous wave light at a frequency of fc; Two digital signal generators each use digital modulation to generate two OOK signals with a power difference of 14dB. Two Mach-Zehnder optical modulators (MZMs) are used to convert electrical signals in the system into optical signals; Two digital DC sources are used to provide a suitable DC bias voltage for the optical modulator; Two adjustable optical attenuators are used to flexibly adjust the power of the optical signal entering the optical fiber; An optical multiplexer is used to multiplex optical signals from two optical paths into one branch to generate a PAM-4 optical signal; A 2km section of optical fiber is used to transmit PAM-4 optical signals; A photodiode is used to convert the PAM-4 optical signal transmitted in the system into an electrical signal; A low-pass filter is used to filter the PAM-4 electrical signal output by the photodiode; An oscilloscope is used to observe the generated PAM-4 signal; In the transmitting end, a first OOK signal (2) and a second OOK signal (3) with a power difference of 14dB are generated by digital modulation and input to the first Mach-Zehnder optical modulator (6) and the second Mach-Zehnder optical modulator (7), respectively. The first DC power supply (4) and the first DC power supply (5) provide the bias voltage required by the system for the first Mach-Zehnder optical modulator (6) and the second Mach-Zehnder optical modulator (7), so that the first Mach-Zehnder optical modulator (6) and the second Mach-Zehnder optical modulator (7) are in push-pull mode; from the external cavity modulated laser (1) The continuous wave light with frequency fc is modulated by the first Mach-Zehnder optical modulator (6) and the second Mach-Zehnder optical modulator (7). The two modulated optical signals enter the first adjustable optical attenuator (8) and the second adjustable optical attenuator (9) respectively. The two optical signals are then multiplexed by the optical multiplexer (10) to output a PAM-4 optical signal. The PAM-4 optical signal is input to the photodiode (12) through the short-distance optical fiber (11) and converted into an electrical signal. After passing through the low-pass filter (13), it is connected to the oscilloscope (14) to observe that the output signal is a PAM-4 signal.

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