JP2023163890A - Dispersion compensation method and dispersion compensation device of optical fiber communication signal - Google Patents

Abstract

To provide a dispersion compensation method and a dispersion compensation device of an optical fiber communication signal which can contain a short/middle-range high-capacity IM-DD optical fiber transmission system in a realistic circuit scale.SOLUTION: A dispersion compensation method of an optical fiber communication signal by a dispersion compensation device 1 of the optical fiber communication signal comprises: an optical pre-processing step in which an optical pre-processing circuit 3 branches an optical signal transmitted through an optical fiber 5 and causes the branched optical signals to interfere with each other to obtain the optical signals after pre-processing; and a post-processing step in which an electric dispersion compensation circuit 7 performs dispersion compensation processing to the optical signals after pre-processing.SELECTED DRAWING: Figure 1

Description

The present invention relates to a dispersion compensation method and a dispersion compensation device for optical fiber communication signals.

In recent years, there has been an increasing demand for short- to medium-distance (20 to 80 km) high-capacity optical fiber transmission systems for inter-data center networks and mobile access network fronthaul. For this reason, extensions of optical Ethernet (registered trademark) standards such as 400GBASE-ER/ZR are being actively discussed. In intensity modulation-direct detection (IM-DD) type optical fiber transmission over 20 km, it is desirable to use the wavelength band of 1550 nm, which has small fiber propagation loss. On the other hand, in such optical fiber transmission, signal distortion caused by fiber chromatic dispersion becomes a problem. Since the influence of chromatic dispersion is proportional to the square of the optical signal bandwidth, it becomes the main limiting factor for transmission distance, especially in wideband IM-DD transmission exceeding 50 Gbps/lane. For this reason, various dispersion compensation methods have been considered in the discussion of stretching mentioned above.

Electric dispersion compensation technology is known as a dispersion compensation method for optical fiber communication signals. For example, Japanese Patent Publication No. 2019-514312 describes an electrical dispersion compensation system and a dispersion compensation method using the system. Electrical dispersion compensation technology equalizes the communication path using digital signal processing. However, the electrical dispersion compensation system tends to significantly increase the circuit size in broadband IM-DD transmission of 400GBASE-ER or higher. In electrical dispersion compensation systems, equalizers with close to 100 taps are often used in papers.

For this reason, it is possible to implement short- to medium-distance, high-capacity IM-DD optical fiber transmission systems on a realistic circuit scale, such as broadband transmission exceeding 50 Gbps/lane and 40 km (ER)/80 km (ZR). A method and apparatus for dispersion compensation of fiber communication signals is desired.

Special table 2019-514312 publication

SUMMARY OF THE INVENTION An object of the present invention is to provide a dispersion compensation method and a dispersion compensation device for optical fiber communication signals that can implement a near-medium-distance large-capacity IM-DD optical fiber transmission system on a practical circuit scale.

Basically, the above problem is solved by using a simple optical pre-processing circuit to remove in advance only the signal distortion components due to dispersion, which are particularly the cause of the increase in the scale of the electrical dispersion compensation circuit, and to compensate for the electrical dispersion compensation circuit. This problem can be solved by simplifying the circuit.

The first invention relates to a dispersion compensation method for optical fiber communication signals. This method includes a photo-pretreatment step and a post-treatment step.
The optical pre-processing step is a step in which the optical pre-processing circuit 3 branches and interferes the optical signal transmitted through the optical fiber 5 to obtain a pre-processed optical signal.
The post-processing step is a step in which the electrical dispersion compensation circuit 7 performs dispersion compensation processing on the pre-processed optical signal.

The next invention relates to a dispersion compensation device 1 for optical fiber communication signals.
The dispersion compensation device 1 for optical fiber communication signals includes an optical pre-processing circuit 3, an electrical dispersion compensation circuit 7, and a photoelectric processing circuit optimization section 9.
The optical preprocessing circuit 3 is an element for branching and interfering the optical signal transmitted through the optical fiber 5 to obtain a preprocessed optical signal.
The electric dispersion compensation circuit 7 is an element for performing dispersion compensation processing on the preprocessed optical signal.
The photoelectric processing circuit optimization unit 9 determines the driving conditions of the optical pre-processing circuit 3 and the electric dispersion compensation circuit 7 based on the quality of the output signal from the electric dispersion compensation circuit 7 or the prediction of the quality of the output signal by an analytical model. This is an element for optimization.

The present invention can provide a dispersion compensation method and dispersion compensation device for optical fiber communication signals that can implement a near-medium-distance large-capacity optical fiber transmission system on a practical circuit scale.

FIG. 1 is a block diagram of a dispersion compensator for optical fiber communication signals. FIG. 2 is a flowchart for explaining a dispersion compensation method for optical fiber communication signals. FIG. 3(a) is a schematic diagram of a system for opto-electronic integrated dispersion compensation. FIG. 3(b) shows a single-tap delay line which is an example of an optical pre-processing circuit. φ is the optical phase shift and D is the optical delay. FIG. 4 is a conceptual diagram showing the experimental system in the example. In the embodiment, a single-tap delay line is used as the optical pre-processing circuit, and a feedforward equalizer is used as the electrical dispersion compensation circuit. In the figure, PPG indicates a pulse pattern generator, MZM indicates a Mach-Zehnder modulator, SMF indicates a single mode optical fiber, VOA indicates a variable optical attenuator, EDFA indicates an optical amplifier, and FSR indicates a free spectrum. OBPF indicates an optical bandpass filter, PD indicates a photodetector, ADC indicates analog-to-digital conversion, and FFE indicates a feedforward equalizer. FIG. 5(a) is a graph replacing a drawing showing the frequency response of a 50 km IM-DD optical fiber transmission system with and without a single-tap delay line. FIG. 5(b) is a graph in place of a drawing showing a pole-zero plot of a 50 km IM-DD optical fiber transmission system without a single-tap delay line. FIG. 5(c) is a graph in place of a drawing showing a pole-zero plot of a 50 km IM-DD optical fiber transmission system using a single-tap delay line. Figure 6 shows the experimental results. FIG. 6(a) shows the bit error rate (BER) versus the number of electrical compensation circuit taps after 50 km SMF transmission of a 112 Gb/s PAM4 signal using an opto-electronic dispersion compensation circuit. FIG. 6(b) shows the BER vs. PD input optical power after 50 km SMF transmission of a 112 Gb/s PAM4 signal with and without an opto-electronic dispersion compensation circuit. FIG. 6(c) shows the BER versus the number of electrical compensation circuit taps for 50 km and 80 km 60 Gb/s PAM2 transmission using the opto-electrical integrated dispersion compensation circuit.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated using drawing. The present invention is not limited to the embodiments described below, but also includes modifications made from the following embodiments as appropriate within the range obvious to those skilled in the art.

FIG. 1 is a block diagram of a dispersion compensator for optical fiber communication signals. As shown in FIG. 1, the dispersion compensation device 1 for optical fiber communication signals includes an optical preprocessing circuit 3, an electrical dispersion compensation circuit 7, and a photoelectric processing circuit optimization section 9.

Dispersion compensation device for optical fiber communication signals 1
A dispersion compensation device 1 for optical fiber communication signals is a device for compensating for signal distortion caused by fiber chromatic dispersion. The dispersion compensator 1 for optical fiber communication signals can be preferably used for optical fiber transmission systems that are higher than short-range, medium-distance, and large-capacity optical fiber transmission systems.
Examples of the transmission distance by optical fiber are 20 km or more and 1000 km or less, 40 km or more and 500 km or less, 40 km or more and 100 km or less, or 40 km or more and 80 km or less.
Examples of the bandwidth of optical fiber communication are 10 Gbps/lane to 1000 Gbps/lane, 20 Gbps/lane to 500 Gbps/lane, 40 Gbps/lane to 500 Gbps/lane, and 50 Gbps/lane to 1000 Gbps/lane. It may be less than 50 Gbps/lane, it may be more than 50 Gbps/lane and less than 500 Gbps/lane, it may be more than 100 Gbps/lane and less than 500 Gbps/lane.
Examples of the wavelength of light used in optical fiber communication are 256 nm or more and 3200 nm or less, 500 nm or more and 2500 nm or less, or 1000 nm or more and 2000 nm or less.
A specific example of an optical signal is an optical signal transmitted over an optical fiber of 20 km or more by broadband transmission of 50 Gbps/lane or more based on the IM-DD system.

Photo pre-processing circuit 3
The optical preprocessing circuit 3 is an element for branching and interfering the optical signal transmitted through the optical fiber 5 to obtain a preprocessed optical signal. In order to realize the above functions, the optical preprocessing circuit 3 includes, for example, an intensity separation section 11, a phase adjustment section 13, a delay adjustment section 15, and a multiplexing section 17. Examples of such an optical preprocessing circuit 3 are an optical delay interferometer and a Mach-Zehnder type optical modulator.

The intensity separation section 11 is an element for separating the optical signals transmitted through the optical fiber 5 at a desired intensity ratio. For example, a coupler functions as the intensity separation section 11. An example of the number of branched waveguides is two. However, the number of branched waveguides may be two or more. A method for adjusting the intensity ratio of light separated into each waveguide is known.

The phase adjustment section 13 is an element for adjusting the phase of the optical signal separated by the intensity separation section 11. Usually, a phase modulator provided in any of the waveguides functions as the phase adjustment section 13. The phase adjustment section 13 adjusts the relative phase of the demultiplexed optical signal. The phase modulator can adjust the phase of an optical signal using a drive signal from a signal source, and thereby can adjust the relative phase of two demultiplexed optical signals.

The delay adjustment unit 15 is an element for adjusting the delay of the optical signal separated by the intensity separation unit 11. Normally, a waveguide path length difference adjustment circuit functions as the delay adjustment section 15. The delay adjustment unit 15 adjusts the relative time difference between the optical signals separated by the intensity separation unit 11. The delay adjustment section 15 can adjust the delay of the optical signal using the drive signal from the signal source, and thereby can adjust the relative delay difference between the two separated optical signals.

The multiplexer 17 is an element for multiplexing the optical signals separated by the intensity separator 11 and whose phases and delays have been adjusted by the phase adjuster 13 and the delay adjuster 15. Most importantly, an optical signal in which no phase difference and delay difference is given is also included in the optical signal whose phase and delay have been adjusted.

Since the optical pre-processing circuit 3 has the above-described configuration, components that become unstable zero points or extreme points after photo-electrical conversion can be removed in advance. In addition, by optimizing the driving conditions of the optical pre-processing circuit 3 and the driving conditions of the electrical dispersion compensation circuit 7, which will be described later, the electrical dispersion compensation circuit can be simplified and a medium-distance, high-capacity optical fiber transmission system can be realized. This means that it can be implemented on a realistic circuit scale.

Electric dispersion compensation circuit 7
The electric dispersion compensation circuit 7 is an element for performing dispersion compensation processing on the preprocessed optical signal. The basic configuration of the electrical dispersion compensation circuit 7 is well known. Examples of the electric dispersion compensation circuit 7 include the electric dispersion compensator (an electric dispersion compensator composed of a reflective microstrip line having group delay characteristics of electric signals) described in Japanese Patent No. 5658991, and the electric dispersion compensator described in Japanese Patent Application Laid-open No. 2010 This is an electrical dispersion compensation circuit described in Publication No. 278528. As the electrical dispersion compensation circuit 7, any one of a feedback equalizer, a feedforward equalizer, and a signal determination circuit may be used. An example of a feedback equalizer is a decision feedback equalizer. An example of a feedforward equalizer is
FIR filter, frequency domain equalizer, and Volterra filter. An example of a signal decision circuit is a maximum likelihood estimation circuit.

Photoelectric processing circuit optimization section 9
The photoelectric processing circuit optimization unit 9 optimizes the driving conditions of the optical pre-processing circuit 3 and the electric dispersion compensation circuit 7 based on the quality of the output signal from the electric dispersion compensation circuit 7 or the prediction by the analytical model of the quality of the output signal. It is an element to make it a reality. Optimization may be performed adaptively based on the actual output signal, or may be performed in advance using an analytical solution regarding the quality of the output signal. For example, the photoelectric processing circuit optimization section 9 determines the drive signal for the phase adjustment section 13 and the delay adjustment section 15 based on the bit error rate characteristics of the output signal from the electric dispersion compensation circuit 7 and the predicted value by the analysis model. , the tap coefficients and number of taps of the electrical dispersion compensation circuit 7 are adaptively optimized. Analytical models for predicting the quality (eg, bit error rate characteristics) of output signals are known. Therefore, a predicted value may be obtained using a known analytical model. The photoelectric processing circuit optimization unit 9 receives measurement values from, for example, a sensor that measures parameters of an output signal. Then, the photoelectric processing circuit optimization section 9 reads out the program stored in the storage section, causes the calculation section to perform various calculations using the measured value of the output signal, and controls the optical pre-processing circuit 3 and the electrical dispersion compensation circuit 7. It is only necessary to optimize the driving conditions. The program may include, for example, a known machine learning algorithm.

Design of optical pre-processing circuit 3 and electrical dispersion compensation circuit 7 (design optimization process)
The photoelectric processing circuit optimization unit 9 determines the number of optical signal branches and the interference method in the optical pre-processing circuit 3 and the electric dispersion compensation circuit 7 based on the quality of the output signal in the post-processing process or the prediction by the analytical model of the output signal. The number of addition/subtraction circuits and the number of multiplication/division circuits may also be determined. In this manner, the photoelectric processing circuit optimization unit 9 can design the optical pre-processing circuit 3 and the electrical dispersion compensation circuit 7. This aspect can also be said to be a method for constructing (manufacturing) the dispersion compensator 1 for optical fiber communication signals. The photoelectric processing circuit optimization unit 9 receives measurement values from, for example, a sensor that measures parameters of an output signal. Then, the photoelectric processing circuit optimization section 9 reads out the program stored in the storage section, uses the measured value of the output signal, causes the calculation section to perform various calculations, and calculates the number of optical signal branches in the optical pre-processing circuit 3. The interference method and the number of adding/subtracting circuits and the number of multiplying/dividing circuits of the electric dispersion compensation circuit 7 can be determined. The program may include, for example, a known machine learning algorithm. An example of determining the interference method is determining the driving conditions of the phase modulator 13 and delay adjustment section 15 in each waveguide.

Dispersion Compensation Method for Optical Fiber Communication Signals Next, a method for dispersion compensation for optical fiber communication signals using the above device will be described. FIG. 2 is a flowchart for explaining a dispersion compensation method for optical fiber communication signals. As shown in FIG. 2, this method includes a photopretreatment step (S101) and a posttreatment step (S102). This method may further include a drive condition optimization step (S103).

The optical pre-processing step (S101) is a step in which the optical pre-processing circuit 3 branches and interferes the optical signal transmitted through the optical fiber 5 to obtain a pre-processed optical signal.

The post-processing step (S102) is a step in which the electric dispersion compensation circuit 7 performs dispersion compensation processing on the pre-processed optical signal.

In the drive condition optimization step (S103), the photoelectric processing circuit optimization unit 9 optimizes the drive conditions of the optical pre-processing circuit 3 and the electric dispersion compensation circuit 7 based on the quality of the output signal of the post-processing step. This is the process. This process simultaneously optimizes the driving conditions of the optical pre-processing circuit 3 and the electrical dispersion compensation circuit 7, so dispersion compensation for optical fiber communication signals can be implemented in a short-medium distance high-capacity optical fiber transmission system on a realistic circuit scale. This means that we can provide a method. After optimizing the driving conditions, the optical pre-treatment step (S101) and post-treatment step (S102) may be performed using the optimized conditions.

Implementation system concept FIG. 3(a) is a diagram showing the concept of an opto-electronic dispersion compensation circuit for near-medium distance IM-DD optical fiber transmission. The purpose of the optical pre-processing circuit in the opto-electronic dispersion compensation circuit is to remove in the optical domain, among the signal distortion components due to fiber dispersion, components that would lead to an increase in the scale of the electrical dispersion compensation circuit. It is desirable that the optical circuit be simple and low-cost, and if possible, be composed only of passive optical elements. The complexity of the compensation circuit as a whole can be reduced by optimizing the parameters of the optical pre-processing circuit and the electrical dispersion compensation circuit adaptively or in advance based on the signal quality at the output of the optoelectronic dispersion compensation circuit and its prediction by an analytical model. It is possible to significantly reduce power consumption. of electrical dispersion compensation circuit.
FIG. 3(b) shows a single-tap delay line, which is one of the simplest implementations of an optical preprocessing circuit. This optical component is composed of a 1x2 optical demultiplexer, a phase modulator, an optical delay, and a 2x1 optical multiplexer, and can be integrated as a waveguide type optical circuit if necessary.

Experimental System Figure 4 is a diagram showing an experimental setup for high-speed optical PAM transmission in the 1550 nm band. On the transmitter side, a 56 GBd electrical PAM4 signal is generated from a pulse pattern generator (PPG, Anritsu), and a 1547 nm optical carrier (NKT Coheras Basik) is generated using a Mach-Zehnder modulator (MZM, 3 dB bandwidth approximately 25 GHz). was modulated using the PAM method. The modulated signal was transmitted over a 50 km single-mode fiber (SMF), amplified by an optical amplifier (EDFA), and then preprocessed by a single-tap delay line with a delay (D) of approximately 8 ps. The optical pre-processing circuit output optical signal was detected by a 50 GHz photodetector (PD) and measured by a 160 GSa/s analog-to-digital converter (ADC, Agilent real-time oscilloscope). Electrical dispersion compensation and PAM demodulation were performed in offline processing. A particularly simple feedforward equalizer (FFE) was used for electrical dispersion compensation. The characteristics were evaluated based on the bit error rate (BER) after demodulation.

Results Figure 5(a) is a graph replacing a drawing showing the frequency response of the entire system with and without a single-tap delay line in intensity modulation-direct detection (IM-DD) 50 km optical fiber transmission. be. FIG. 5(b) is a graph replacing a drawing showing a pole-zero plot of a 50 km IM-DD optical fiber transmission system without using a single-tap delay line. FIG. 5(c) is a graph in place of a drawing showing a pole-zero plot of a 50 km IM-DD optical fiber transmission system using a single-tap delay line.

In FIG. 5(a), there are five frequency notches caused by fiber chromatic dispersion in the frequency response of the entire system. Furthermore, in FIG. 5(b), five unstable pole-zero points corresponding to these five notches exist on the upper half-plane unit circumference. This indicates that the IM-DD system is a non-minimum phase system with unstable zero-poles due to the influence of fiber dispersion. In a non-minimum phase system, normal electrical dispersion compensation circuits such as FFE cause noise enhancement and cannot perform dispersion compensation efficiently. On the other hand, as shown in Figure 5(c), when a single-tap delay line is used, unstable zero-pole points (part of them) are removed, so signal distortion caused by dispersion can be efficiently reduced using FFE, etc. It becomes possible to compensate for

FIG. 6(a) shows the relationship between the BER characteristics of 56 Gbaud (112 Gb/s) PAM4 transmission and the number of FFE taps when the PD input optical power is 3 dBm. By using an optical pre-processing circuit, a 31-tap FFE achieves the threshold BER required for error-free transmission using a hard-decision error correction code (HD-FEC) with 6.7% redundancy.

FIG. 6(b) shows the BER versus received optical power of 50 km PAM4 transmission employing an optoelectronic integrated dispersion compensation circuit (single-tap delay line and 31-tap FFE). For comparison, FIG. 6(b) also shows the BER characteristics in the case of only electrical dispersion compensation without using a single-tap delay line. When optoelectronic integrated dispersion compensation is used, the threshold BER of 6.7% HD-FEC can be achieved with an input power of 3 dBm or more. On the other hand, when optical preprocessing is not used, the threshold BER cannot be achieved even if a DFE with three times the circuit size is used.

Further, FIG. 6(c) shows the BER versus FFE tap length of 80 km (ZR) 60 Gb/s PAM2 transmission using optoelectronic integrated dispersion compensation. By combining a single-tap delay line and electrical dispersion compensation, the threshold BER of KP4 FEC is achieved with only a 23-tap FFE. Furthermore, FIG. 6(c) also shows the BER characteristics of 50 km transmission and 60 Gb/s PAM2 transmission using the same optical pre-processing circuit as that for 80 km. The threshold BER was achieved only with a 13-tap FFE without any particular changes to the optical system.

Discussion and experiments have shown that the occurrence of unstable zero-pole points due to fiber dispersion, which is a factor in complicating electrical dispersion compensation circuits, can be efficiently avoided by using a simple single-tap delay line, and that it is possible to efficiently avoid the occurrence of unstable zero-pole points caused by fiber dispersion, which is a factor that complicates electrical dispersion compensation circuits. It was demonstrated that the desired BER characteristics can be achieved with a lightweight FFE, and the effectiveness of the optoelectronic integrated dispersion compensation technology of the present invention was demonstrated. Since it is relatively easy to derive analytical solutions for the output signal characteristics of optical delay lines and FFEs used in optoelectronic dispersion compensation, the optimal design of optoelectronic dispersion compensation circuits should be done in advance based on the analytical solutions. It is also possible.

This invention can be used in the information and communication industry.
The electrical dispersion compensation circuit in the dispersion compensator of the present invention has many parts in common with the signal processing circuit of an existing PAM4 transceiver for Ethernet (registered trademark). Further, the optical pre-processing circuit 3 in the dispersion compensator can be implemented with a passive optical waveguide of about several mm. Therefore, the dispersion compensator of the present invention is expected to be used to realize extension without major modification of existing transceivers by simply attaching a delay interferometer module to the fiber connector part of an existing transceiver. Ru.

1 Dispersion compensation device for optical fiber communication signals 3 Optical preprocessing circuit 5 Optical fiber 7 Electrical dispersion compensation circuit 9 Photoelectric processing circuit optimization section 11 Intensity separation section 13 Phase adjustment section 15 Delay adjustment section 17 Multiplexing section

Claims (6)

A dispersion compensation method for optical fiber communication signals, the method comprising:
an optical pre-processing step in which the optical pre-processing circuit (3) branches and interferes the optical signal transmitted through the optical fiber (5) to obtain a pre-processed optical signal;
a post-processing step in which the electrical dispersion compensation circuit (7) performs dispersion compensation processing on the optical signal after the pre-processing;
Including, method. The method according to claim 1,
Driving conditions under which the photoelectric processing circuit optimization unit (9) optimizes the driving conditions of the optical pre-processing circuit (3) and the electric dispersion compensation circuit (7) based on the quality of the output signal of the post-processing step. Further includes an optimization process,
Method. The method according to claim 2,
The photoelectric processing circuit optimization unit (9) determines the number of optical signal branches and the interference method in the optical pre-processing circuit (3) based on the quality of the output signal of the post-processing step or the prediction of the quality by an analytical model; and a method further comprising a design optimization step of determining the number of addition/subtraction circuits and the number of multiplication/division circuits of the electric dispersion compensation circuit (7). 2. The method according to claim 1, wherein the optical signal transmitted through the optical fiber (5) is transmitted over the optical fiber (5) over a distance of 20 km or more by broadband transmission of 50 Gbps/lane or more based on an optical intensity modulation method. A method that is an optical signal. A dispersion compensation device (1) for optical fiber communication signals,
an optical preprocessing circuit (3) for branching and interfering the optical signal transmitted through the optical fiber (5) to obtain a preprocessed optical signal;
an electric dispersion compensation circuit (7) for performing dispersion compensation processing on the preprocessed optical signal;
Optimizing the driving conditions of the optical pre-processing circuit (3) and the electrical dispersion compensation circuit (7) based on the quality of the output signal from the electrical dispersion compensation circuit (7) or the prediction of the quality by an analytical model. a photoelectric processing circuit optimization section (9);
Dispersion compensator (1). The dispersion compensator (1) according to claim 5,
The optical pre-processing circuit (3) includes:
an intensity separation unit (11) that separates the optical signal transmitted through the optical fiber (5) at a desired intensity ratio;
a phase adjustment section (13) that adjusts the phase of the optical signal separated by the intensity separation section (11);
a delay adjustment section (15) that adjusts the delay of the optical signal separated by the intensity separation section (11);
The intensity separation unit (11) is separated and has a combining unit (17) that combines optical signals whose phases and delays have been adjusted by the phase adjustment unit (13) and the delay adjustment unit (15).
Dispersion compensator (1).

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