CN113472445B - Dual-band RoF system based on PDM-DPMZM and adjusting method - Google Patents

Abstract

The invention provides a double-frequency-band RoF system based on a PDM-DPMZM and an adjusting method. The invention realizes the separation of the low-frequency radio-frequency signal and the high-frequency radio-frequency signal through polarization control, realizes the single-sideband modulation of the high-frequency radio-frequency signal and the nonlinear distortion suppression of the low-frequency radio-frequency signal through reasonably setting the direct-current bias angle of the modulator, and finally obtains the high-performance dual-band RoF system. The invention has simple structure and strong operability; the invention has strong practicability and can be widely applied to the current 5G network deployment.

Description

Dual-band RoF system and adjustment method based on PDM-DPMZM

technical field

The invention relates to the technical field of optical communication and microwave, in particular to a radio-over-fiber communication system and an adjustment method.

Background technique

3.5GHz has been regarded as the pioneer frequency band of 5G network by many countries around the world. Its industrial chain is relatively mature and the networking cost is low. However, in order to meet the needs of different 5G application scenarios, it is necessary to adopt a full-band layout for the candidate frequency bands of the 5G system. Among them, in order to achieve the goal of high-speed transmission in 5G communication, the focus is on the application of high-frequency millimeter waves. 28GHz, as the 5G high-band spectrum chosen by many countries, has the advantage of high network speed, but the disadvantage is that the coverage area is very small, and the deployment requires more cost and time. Therefore, only when base stations in the 3.5GHz and 28GHz frequency bands are deployed at the same time, can people truly experience the speed and performance of 5G. At present, RoF systems have attracted much attention in the simultaneous transmission of low-RF and high-frequency RF signals due to their inherent advantages of high frequency band, large bandwidth, low loss, and anti-electromagnetic interference.

At present, a dual-band RoF transmission system has been reported in the industry, but this system can only realize the transmission of low-frequency radio frequency and high-frequency radio frequency signals, and cannot solve the problem of nonlinear distortion and high frequency introduced by the modulator during the transmission of low-frequency radio frequency signals. The problem of periodic power fading caused by fiber dispersion during the transmission of long-distance optical fiber links.

SUMMARY OF THE INVENTION

In order to overcome the deficiencies of the prior art, the present invention provides a PDM-DPMZM-based dual-band RoF system and an adjustment method. The present invention utilizes polarization multiplexing dual parallel Mach-Zehnder modulator (Polarization Division Multiplexing Dual-Parallel Mach-Zehnder Modulator, PDM-DPMZM), electrical phase shifter, electrical power splitter, optical beam splitter, polarization controller (Polarization Controller) , PC), polarizer (Polarizer) and photodetector (Photodetector, PD) to achieve a high-performance dual-band RoF system, by constructing a high-performance dual-band RoF system, in addition to simultaneously realizing low-frequency RF signals and high-frequency RF signals In addition to the transmission in the medium, it can also solve the nonlinear distortion problem and power fading problem of low-frequency radio frequency signals and high-frequency radio frequency signals in the transmission process, which can further improve the signal quality in addition to improving the spectrum utilization rate and saving the cost of physical links. transmission quality.

The technical scheme adopted by the present invention to solve its technical problems is:

A dual-band RoF system based on PDM-DPMZM, including a laser, a PDM-DPMZM, a 90° electrical phase shifter, an electrical power splitter, a section of optical fiber, an optical beam splitter, two polarization controllers, Two polarizers and two photodetectors, the output port of the laser is connected to the optical input port of the PDM-DPMZM, the output port of the PDM-DPMZM is connected to the input port of the optical fiber, and the output port of the optical fiber is connected to the input port of the optical beam splitter, An output port of the optical beam splitter is connected to the input port of the polarization controller 1, the output port of the polarization controller 1 is connected to the input port of the polarizer 1, and the output port of the polarizer 1 is connected to the optical input port of the photodetector 1; The other output port of the optical beam splitter is connected to the input port of the polarization controller 2 , the output port of the PC2 is connected to the input port of the polarizer 2 , and the output port of the polarizer 2 is connected to the optical input port of the photodetector 2 .

The PDM-DPMZM includes a Y-type optical splitter, two parallel DPMZMs (denoted as X-DPMZM and Y-DPMZM), a 90-degree polarization rotator (PR) and a polarization beam combiner ( Polarization Beam Combiner, PBC); where X-DPMZM contains two parallel sub-modulators (denoted as Xa and Xb), Y-DPMZM contains two parallel sub-modulators (denoted as Ya and Yb), Y- The optical signal output by the DPMZM modulator undergoes a 90-degree polarization rotation through PR, and is then input into the PBC together with the optical signal output by the X-DPMZM modulator.

In the polarization multiplexing dual-parallel Mach-Zehnder modulator PDM-DPMZM, the radio frequency signal 1 (f ₁ =28GHz) is divided into two paths of equal power by an electrical power divider, and one path is directly connected to the radio frequency port of the sub-modulator Xa, The other path is connected to the radio frequency port of the sub-modulator Xb after passing through the 90° electrical phase shifter; the radio frequency signal 2 (f ₂ =3.5GHz) is directly connected to the radio frequency port of the sub-modulator Yb, and the sub-modulator Ya is vacant; X-DPMZM works in The negative orthogonal point, Yb and Y-DPMZM both work at the maximum point, the RF signal 1 modulated on the X-DPMZM and the RF signal 2 modulated on the Y-DPMZM are located on the laser beam with orthogonal polarization states, respectively. unaffected by each other.

The present invention also provides a method for adjusting the dual-band RoF system based on the PDM-DPMZM, and the detailed steps are as follows:

The expressions of RF signal 1 and RF signal 2 are respectively VRF1 (t)= _VRF1 sin(ω _RF1 t) and _VRF2 (t)= _VRF2 sin(ω _RF2 t), where _VRF1 and _{VRF2 are} respectively is the amplitude of RF signal 1 and RF signal 2, ω _RF1 and ω _RF2 are the angular frequencies of RF signal 1 and RF signal 2 respectively, then the expression of the PDM-DPMZM output optical signal is:

和

分别为子调制器Xa、Xb、Ya和Yb的直流偏置角，

和

分别为X-DPMZM和Y-DPMZM的主偏置角；m₁和m₂分别为射频信号1和射频信号2对调制器的调制指数；

和

分别表示光场TE模和TM模的单位矢量；J_n(·)表示第一类n阶贝塞尔函数，在小信号调制下，可以忽略高阶边带；通过设置

时，令X-DPMZM输出单边带调制信号。Among them, E _c (t) is the laser output signal; μ is the modulator loss;

and

are the DC bias angles of the sub-modulators Xa, Xb, Ya and Yb, respectively,

and

are the main bias angles of X-DPMZM and Y-DPMZM respectively; m ₁ and m ₂ are the modulation indices of RF signal 1 and RF signal 2 to the modulator, respectively;

and

Represent the unit vector of the TE mode and TM mode of the light field, respectively; J _n ( ) represents the first kind of n-order Bessel function, under small-signal modulation, the high-order sidebands can be ignored; by setting

When , let X-DPMZM output SSB modulation signal.

After the output signal of the PDM-DPMZM passes through the optical fiber of length L, the expression is:

Among them, α _SMF is the attenuation coefficient of the fiber; θ is the phase shift introduced by the second-order dispersion to the first-order optical sideband of the modulated signal; β ₂ (ω _c ) is the second-order propagation coefficient of the laser signal, and ω _c is the laser signal’s Angular frequency, meanwhile, sets

The optical signal output by the fiber is divided into two beams of equal power by the optical beam splitter. After the polarization controller and the polarizer are used for polarization control in turn, the polarized light containing only the radio frequency signal 1 and the polarization containing only the radio frequency signal 2 are obtained respectively. and input the polarized light containing only radio frequency signal 1 into the high-speed PD (PD1), and input the polarized light containing only radio frequency signal 2 into the low-speed PD (PD2) to obtain the photocurrent, i _PD1 is the one containing only radio frequency signal 1 The photocurrent obtained by demodulation after polarized light enters PD1, i _PD2 is the photocurrent obtained by demodulation after polarized light containing only RF signal 2 enters PD2, respectively:

Among them, γ _PD is the responsivity of the photodetector; from equation (3), it can be found that due to the single sideband modulation, the final demodulated high-frequency radio frequency signal does not have power fading phenomenon;

Expanding the Bessel coefficients in Eq. (4) and ignoring higher-order terms, we get:

时，公式(5)中信号i_PD2(t)中代表非线性失真的三阶交调失真分量项(

相关项)被消除，而有用的基频分量项(m₂相关项)被保留，当设置

公式(5)中代表非线性失真的量被完全消除，解决了低频射频信号和高频射频信号在传输过程中的非线性失真问题和功率衰落问题。It can be found that when

When , the third-order _{intermodulation} distortion component term (

correlation term) is eliminated, while the useful fundamental frequency component term ₍ m correlation term) is retained, when setting

The amount representing the nonlinear distortion in the formula (5) is completely eliminated, which solves the problem of nonlinear distortion and power fading in the transmission process of the low-frequency radio frequency signal and the high-frequency radio frequency signal.

The beneficial effect of the present invention lies in that an integrated PDM-DPMZM is used to realize the simultaneous modulation of the dual-band radio frequency signal, the separation of the low frequency radio frequency signal and the high frequency radio frequency signal is realized through polarization control, and the DC bias of the modulator is reasonably set. The angle realizes the single-sideband modulation of the high-frequency RF signal and the nonlinear distortion suppression of the low-frequency RF signal, and finally obtains a high-performance dual-band RoF system. The invention has a simple structure and strong operability; the invention has strong practicability and can be widely used in current 5G network deployment.

Description of drawings

FIG. 1 is a device diagram of a high-performance dual-band RoF system based on PDM-DPMZM according to the present invention.

Figure 2 is a spectrum diagram of a 28GHz high-frequency radio frequency signal obtained after polarization control.

Figure 3 is a spectrum diagram of a 3.5GHz low-frequency radio frequency signal obtained after polarization control.

Figure 4 shows the system frequency response curves of the experimental group and the control group.

Figure 5 shows the system dynamic range of the experimental group and the control group.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and provides detailed implementation manner and specific operation process, but the protection scope of the present invention is not limited to the following examples:

In this example, the device includes: laser, RF signal source 1, RF signal source 2, DC source, PDM-DPMZM, electrical power splitter, 90° electrical phase shifter, optical fiber, optical beam splitter, PC1, PC2, Pol1 , Pol2, PD1 and PD2. The output end of the laser is connected to the optical input port of the PDM-DPMZM, the output end of the PDM-DPMZM is connected to the input end of the optical fiber, and the output end of the optical fiber is connected to the input end of the optical beam splitter. One output port of the optical beam splitter is connected to the input terminal of PC1, the output terminal of PC1 is connected to the input terminal of Pol1, the output terminal of Pol1 is connected to PD1, and the output port of PD1 is connected to the spectrum analyzer; the other output port of the optical beam splitter is connected to the input terminal of Pol1. The input terminal of PC2, the output terminal of PC2 is connected to the input terminal of Pol2, the output terminal of Pol2 is connected to PD2, and the output terminal of PD2 is connected to the spectrum analyzer.

In this example, the specific implementation steps of the method are:

Step 1: The laser generates an optical carrier with an operating wavelength of 1550nm and an optical power of 16dBm; RF signal source 1 generates a high-frequency RF signal with a frequency of 28GHz and a power of 0dBm; RF signal source 2 generates a frequency of 3.5GHz and a power of 0dBm. Low-frequency radio frequency signal; the half-wave voltage of PDM-DPMZM is 3.5V, and the extinction ratio is 35dB; the length of the optical fiber is 20Km; the responsivity of PD1 and PD2 are both 0.7A/W.

Step 2: Set the DC bias angles of the four sub-modulators (Xa, Xb, Ya and Yb) of the PDM-DPMZM to 135°, 135°, 199.2° and 0° respectively, and set the main modulation of X-DPMZM and Y-DPMZM The DC bias angles of the converters are -90° and 0°, respectively.

Step 3: Adjust the polarization control angle of PC1 to 0°, and the tunable phase difference is arbitrary; adjust the polarization control angle of PC2 to 90°, and the tunable phase difference to 0°, and observe the output spectrum of PD1 and PD2 respectively. Figure 2 shows the 28GHz high frequency radio frequency signal spectrum obtained after polarization control in the simulation, and Figure 3 shows the 3.5GHz low frequency radio frequency signal spectrum obtained after polarization control in the simulation.

Step 4: Keep the polarization control settings in Step 2 unchanged, change the frequency of the high-frequency RF signal when the 3.5GHz low-frequency RF signal is normally input, and observe the RF signal power output by PD1. At the same time, a quadrature biased MZM was used as a control group, and the frequency of the high-frequency RF signal was also changed to observe the final output signal power. Figure 4 shows the system frequency response curves of the experimental group and the control group in the simulation. It can be found that the system gain of the control group has obvious periodic fading phenomenon, while the system gain of the experimental group is relatively stable.

Step 5: Keep the polarization control settings in Step 2 unchanged, and adjust the low-frequency RF signal to a dual-tone signal when the 28GHz high-frequency RF signal is normally input, with a frequency of 3.5GHz/3.6GHz. The power of the input low-frequency two-tone signal was changed, and the fundamental frequency component, IMD3 component and noise power of the PD2 output signal were measured; at the same time, a quadrature biased MZM was used as a control group to keep the frequency of the RF signal unchanged. Also change the power of the input two-tone signal, and measure the fundamental frequency component, IMD3 component and noise power of the PD output signal respectively. Figure 5 shows the system dynamic range of the experimental group and the control group. It can be found that the Spurious-Free Dynamic Range (SFDR) of the control group is 102.9dB·Hz ^2/3 , and the SFDR of the experimental group is 123.4 dB·Hz ^2/3 . Compared with the control group, the SFDR of the experimental group increased by about 20.5dB.

To sum up, this scheme uses PDM-DPMZM to perform parallel modulation of low-frequency radio frequency signals and high-frequency radio frequency signals, and realizes independent transmission and polarization demodulation of signals in the two frequency bands through polarization control. In this process, the single-sideband modulation of high-frequency RF signals and the nonlinear distortion suppression of low-frequency RF signals are achieved by setting the appropriate DC bias point. This solution can obtain high-performance dual-band RoF transmission, with simple structure and easy implementation, flexible operation, and potential application value in today's 5G network deployment.

In a word, the above-mentioned embodiments are only examples of the present invention, and are not only used to limit the protection scope of the present invention. Several equivalent deformations and substitutions can be made. The frequency of high-frequency RF signal, the frequency of low-frequency RF signal, the wavelength of optical carrier, the power of optical carrier, the length of optical fiber, the power of RF signal, the polarization control angle and the DC bias angle of the modulator can all be changed. . These equivalent deformations and substitutions as well as the adjustment of the frequency range should also be regarded as the protection scope of the present invention.

Claims (1)

1. a kind of adjustment method based on the dual-band RoF system of PDM-DPMZM, the dual-band RoF system of described PDM-DPMZM, comprises a laser, a PDM-DPMZM, a 90 ° electrical phase shifter, an electrical power divider , a section of optical fiber, an optical beam splitter, two polarization controllers, two polarizers and two photodetectors, the output port of the laser is connected to the optical input port of the PDM-DPMZM, and the output port of the PDM-DPMZM is connected to the optical input port of the optical fiber. The input port, the output port of the optical fiber is connected to the input port of the optical beam splitter, one output port of the optical beam splitter is connected to the input port of the polarization controller 1, the output port of the polarization controller 1 is connected to the input port of the polarizer 1, and the polarization controller 1 is connected to the input port of the polarizer 1. The output port of the polarizer 1 is connected to the optical input port of the photodetector 1; the other output port of the optical beam splitter is connected to the input port of the polarization controller 2, and the output port of the polarization controller 2 is connected to the input port of the polarizer 2, The output port of the polarizer 2 is connected to the optical input port of the photodetector 2; the PDM-DPMZM includes a Y-type optical splitter, two parallel DPMZMs, a 90-degree polarization rotator and a polarization beam combiner, The X-DPMZM contains two parallel sub-modulators, and the Y-DPMZM contains two parallel sub-modulators. The optical signal output by the Y-DPMZM modulator undergoes a 90-degree polarization rotation through PR, and then modulates with the X-DPMZM. The optical signals output by the PBC are jointly input to the PBC, and the optical signals are combined in the PBC to be called polarization multiplexed signals and output from the PDM-DPMZM modulator; in the polarization multiplexed dual parallel Mach Zehnder modulator PDM-DPMZM, the radio frequency signal 1 is The electrical power divider is divided into two paths with equal power, one path is directly connected to the radio frequency port of the sub-modulator Xa, and the other path is connected to the radio frequency port of the sub-modulator Xb after passing through a 90° electrical phase shifter; the radio frequency signal 2 is directly connected to the sub-modulator. The RF port of Yb, the sub-modulator Ya is vacant; X-DPMZM works at the negative quadrature point, Yb and Y-DPMZM both work at the maximum point, the RF signal 1 modulated on X-DPMZM and the modulated RF signal on Y-DPMZM The radio frequency signals 2 are respectively located on the laser beams with orthogonal polarization states, and the two are not affected by each other, and is characterized in that it includes the following steps: The expressions of RF signal 1 and RF signal 2 are respectively VRF1 (t)= _VRF1 sin(ω _RF1 t) and _VRF2 (t)= _VRF2 sin(ω _RF2 t), where _VRF1 and _{VRF2 are} respectively is the amplitude of RF signal 1 and RF signal 2, ω _RF1 and ω _RF2 are the angular frequencies of RF signal 1 and RF signal 2 respectively, then the expression of the PDM-DPMZM output optical signal is:

Among them, E _c (t) is the laser output signal; μ is the modulator loss;

and

are the DC bias angles of the sub-modulators Xa, Xb, Ya and Yb, respectively,

and

are the main bias angles of X-DPMZM and Y-DPMZM respectively; m ₁ and m ₂ are the modulation indices of RF signal 1 and RF signal 2 to the modulator, respectively;

and

Represent the unit vectors of the TE mode and TM mode of the light field, respectively; J _n ( ) represents the first kind of n-order Bessel function, under small-signal modulation, ignoring high-order sidebands; by setting

When , make X-DPMZM output SSB modulation signal; After the output signal of the PDM-DPMZM passes through the optical fiber of length L, the expression is:

Among them, α _SMF is the attenuation coefficient of the fiber; θ is the phase shift introduced by the second-order dispersion to the first-order optical sideband of the modulated signal; β ₂ (ω _c ) is the second-order propagation coefficient of the laser signal, and ω _c is the laser signal Angular frequency, meanwhile, sets

The optical signal output by the fiber is divided into two beams of equal power by the optical beam splitter. After the polarization controller and the polarizer are used for polarization control in turn, the polarized light containing only the radio frequency signal 1 and the polarization containing only the radio frequency signal 2 are obtained respectively. and input the polarized light containing only radio frequency signal 1 into PD1, and input the polarized light containing only radio frequency signal 2 into PD2 to obtain the photocurrent, i _PD1 is obtained by demodulation after the polarized light containing only radio frequency signal 1 enters PD1 The photocurrent of i _PD2 is the photocurrent obtained by demodulation after the polarized light containing only the radio frequency signal 2 enters PD2, respectively:

Among them, γ _PD is the responsivity of the photodetector; it is found from equation (3) that due to the single sideband modulation, the final demodulated high-frequency radio frequency signal has no power fading phenomenon; Expanding the Bessel coefficients in Eq. (4) and ignoring higher-order terms, we get:

when

, the third-order intermodulation distortion component term representing nonlinear distortion in the signal i _PD2 (t) in formula (5) is eliminated, and the useful fundamental frequency component term is retained. When setting

The amount representing the nonlinear distortion in equation (5) is completely eliminated.

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