System for simultaneously inhibiting second-order nonlinearity and third-order nonlinearity in DML

Abstract

The invention discloses a system for simultaneously suppressing second-order nonlinearity and third-order nonlinearity in a direct modulation laser (DML), which belongs to the field of optical communication, and can simultaneously suppress the second-order nonlinearity and the third-order nonlinearity in the system under the condition of DSP-FREE through the combined action of a broadband pre-distortion circuit and an optical path push-pull structure. Wherein at a transmitting end, a set of Schottky diodes and Chebyshev gradient transmission lines which are connected in parallel reversely are adopted to build a DC-10GHz broadband pre-distortion module, and a radio frequency signal generates a reverse third-order intermodulation (IMD3) component through an analog pre-distortion circuit so as to counteract forward third-order nonlinearity of a laser; at a receiving end, a second-order intermodulation (IMD2) component is differentially eliminated through an optical path push-pull modulation architecture. Compared with a traditional DSP scheme, digital signal processing is not needed, the method has the advantages of being low in complexity and wide in bandwidth, and nonlinear accurate compensation of an optical communication system can be achieved.

Description

Technical Field

[0001] The present invention belongs to the field of optical communications, and more particularly, relates to a system for simultaneously suppressing second-order and third-order nonlinearities in a DML. Background Art

[0002] Analog radio over fiber (A-RoF) technology, due to its high spectral efficiency and simple system architecture, is considered a potential solution for next-generation mobile fronthaul systems. However, optical analog signals are particularly sensitive to intermodulation distortion (IMD). With the development of mobile communications, the bandwidth used for signal transmission is increasing, and more frequency bands are being put into use. This exacerbates the impact of IMD on signal quality, making third-order intermodulation distortion (IMD3) and second-order intermodulation distortion (IMD2) the primary factors limiting the dynamic range of multi-band A-RoF systems.

[0003] Currently, there are some solutions to improve dynamic range, such as analog predistortion (APD) based on Schottky diodes and pre- and post-compensation based on digital signal processing (DSP). However, these methods primarily target IMD3 suppression, with insufficient attention paid to IMD2. Furthermore, APD methods can only process signals with relatively small bandwidths, while DSP methods suffer from high complexity.

[0004] Therefore, it is crucial to develop a low-complexity, broadband solution to suppress both IMD2 and IMD3 in direct modulation systems. Summary of the Invention

[0005] In response to the above-mentioned defects or improvement needs of the prior art, the present invention provides a system for simultaneously suppressing second-order and third-order nonlinearities in DML, which can simultaneously suppress IMD2 and IMD3 in the direct modulation and direct detection link, thereby improving the linearity of the direct modulation and direct detection link, reducing the bit error rate of the signal, and improving the signal quality.

[0006] To achieve the above objectives, according to a first aspect of the present invention, a system for simultaneously suppressing second-order and third-order nonlinearities in a DML is provided, comprising a broadband analog predistortion module, a power splitter, a phase shifter, and an optical push-pull structure;

[0007] The broadband analog predistortion module includes first and second Schottky diode branches and an RF signal path; the first and second branches each include a Schottky diode, a resistor, a first capacitor, and an inductor, and one end of the first capacitor is connected in parallel with one end of the inductor and then in series with one end of the resistor, with the other end being grounded; in the first branch, the other end of the resistor is connected to the cathode of the Schottky diode, and the other end of the inductor is used to input a bias voltage V0 applied to the Schottky diode; in the second branch, the other end of the resistor is connected to the anode of the Schottky diode, and the other end of the inductor is grounded; the Schottky diode of the first branch is connected in anti-parallel to the Schottky diode of the second branch; the RF signal path includes an RF signal input terminal, second and third capacitors, and an RF signal output terminal connected in sequence, the path between the second and third capacitors is connected to the node after the anti-parallel connection, and the RF transmission line between the third capacitor and the RF signal output terminal is a Chebyshev gradient line, whose line resistance gradually changes from a first preset value to a second preset value along the RF signal transmission direction, and the first preset value is greater than the second preset value; the capacitance of the first to third capacitors is the same;

[0008] The power divider is used to convert the RF signal v output by the broadband analog predistortion module RF Divided equally into the first and second RF signals v RF1 、v RF2 ;

[0009] The phase shifter is used to convert v RF1 Perform 180° phase shift to obtain the first RF signal v after phase shift RF1 `;

[0010] The optical push-pull structure includes a first DML transmission link DML1 and a second DML transmission link DML2, and DML1 and DML2 both include a DML laser, an optical fiber and a PD; RF1 `、v RF2 One-to-one correspondence input to DML1 and DML2;

[0011] The value of V0 satisfies the formula To eliminate the third-order nonlinearity; among them, C1 and C3 are the first-order and third-order nonlinear coefficients of DML1 and DML2 respectively. R is the resistance value of the resistor, I s is the saturation current, α=q / nkT, q is the electron charge, k is the Boltzmann constant, T is the ambient temperature, and n is the ideality factor;

[0012] The attenuation coefficients α1, α2 and modulation depths m1, m2 of DML1 and DML2 satisfy the formula m1α1η1A n =m2α2η2B nTo eliminate the second-order nonlinearity; where η1 and η2 are the response sensitivities of PD in DML1 and DML2 respectively, A n 、B n are the nth-order nonlinear coefficients of the transfer functions of the lasers in DML1 and DML2 respectively.

[0013] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0014] The system provided by the present invention can simultaneously suppress IMD2 and IMD3 in the direct modulation and direct detection system through the synergistic effect of the broadband analog pre-distortion module and the optoelectronic combined push-pull structure, significantly improve the linearity of the system, and achieve simultaneous suppression of second-order and third-order nonlinearities in the system under DSP-free conditions. The system complexity is very low and the bandwidth is large, which greatly improves the spurious-free dynamic range of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A system structure diagram provided for an embodiment of the present invention;

[0016] Figure 2 A schematic diagram of a broadband analog predistortion module provided in an embodiment of the present invention;

[0017] Figure 3 A comparison chart of the bandwidth of the broadband analog predistortion module provided by an embodiment of the present invention and the bandwidth of the equal-width line solution;

[0018] Figure 4 This is a schematic diagram of the effect of broadband analog circuits on improving dynamic range;

[0019] Figure 5 A schematic diagram of the suppression effect of the optoelectronic combined push-pull structure on second-order and third-order nonlinearity provided by an embodiment of the present invention;

[0020] Figure 6 Schematic diagram of the average EVM under four conditions with different incident RF powers. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0022] The embodiment of the present invention provides a system for simultaneously suppressing the second-order and third-order nonlinearity in DML, such as Figure 1As shown, it includes: a broadband analog pre-distortion module, a power divider, a phase shifter and an optical push-pull structure;

[0023] The broadband analog predistortion module includes first and second Schottky diode branches and an RF signal path; the first and second branches each include a Schottky diode, a resistor, a first capacitor, and an inductor, and one end of the first capacitor is connected in parallel with one end of the inductor and then in series with one end of the resistor, and the other end of the first capacitor is grounded; in the first branch, the other end of the resistor is connected to the cathode of the Schottky diode, and the other end of the inductor is used to input an applied voltage V0; in the second branch, the other end of the resistor is connected to the anode of the Schottky diode, and the other end of the inductor is grounded; the Schottky diode of the first branch is connected in anti-parallel to the Schottky diode of the second branch; the RF signal path includes an RF signal input terminal, second and third capacitors, and an RF signal output terminal connected in sequence, the path between the second and third capacitors is connected to the node after the anti-parallel connection, and the RF transmission line between the third capacitor and the RF signal output terminal is a Chebyshev gradient line, whose line resistance gradually changes from a first preset value to a second preset value along the RF signal transmission direction, and the first preset value is greater than the second preset value; the capacitance of the first to third capacitors is the same;

[0024] The power divider is used to convert the RF signal v output by the broadband analog predistortion module RF Divided equally into the first and second RF signals v RF1 、v RF2 ;

[0025] The phase shifter is used to convert v RF1 Perform 180° phase shift to obtain the first RF signal v after phase shift RF1 `;

[0026] The optical push-pull structure includes two direct modulation and direct detection links: the first and second DML transmission links DML1 and DML2; DML1 and DML2 both include DML lasers, optical fibers and PDs; RF1 `、v RF2 One-to-one correspondence input to DML1 and DML2;

[0027] The value of V0 satisfies the formula To eliminate the third-order nonlinearity; among them, C1 and C3 are the first-order and third-order nonlinear coefficients of DML1 and DML2 respectively. V0 is the bias voltage applied to the Schottky diode, R is the resistance of the resistor, and I s is the saturation current, α=q / nkT, q is the electron charge, k is the Boltzmann constant, T is the ambient temperature, and n is the ideality factor;

[0028] The attenuation coefficients α1, α2 and modulation depths m1, m2 of DML1 and DML2 satisfy the formula m1α1η1A n =m2α2η2B n The second-order nonlinearity is eliminated when the RF current signals detected by the PDs in DML1 and DML2 are subtracted at the receiving end; where η1 and η2 are the response sensitivities of the PDs in DML1 and DML2, respectively. n 、B n are the nth-order nonlinear coefficients of the transfer functions of the lasers in DML1 and DML2 respectively.

[0029] The system provided by the present invention has a broadband analog pre-distortion module (hereinafter referred to as APD module) designed at the transmitting end. This module generates third-order intermodulation products opposite to the laser through two anti-parallel Schottky diodes. At the same time, by adopting the impedance matching design of Chebyshev gradient line, the APD module can cover the ultra-wideband frequency range of DC-10GHz.

[0030] The present invention uses Chebyshev gradient lines to perform impedance matching, and can obtain ideal matching characteristics over a wider frequency band by increasing the number of sections of the impedance transformer.

[0031] The method for selecting the series resistance of the Schottky diode and the first preset value of the gradient line is as follows: by adjusting the series resistance of the Schottky diode and the design of the Chebyshev gradient line, the line resistance of the node after the series resistance of the two Schottky diodes is connected in parallel and the first preset value line resistance of the Chebyshev gradient line is equal to the second preset value, which is the line resistance of the input RF signal.

[0032] The line resistance of the node after the two anti-parallel Schottky diodes are connected in parallel is set to a second preset value, which can effectively broaden the working bandwidth of the module.

[0033] Those skilled in the art know that the impedance matching of the input and output signals of the radio frequency signal transmission link is usually 50Ω, so the second preset value is usually set to 50Ω.

[0034] For example, when the series resistance of the two Schottky diodes is 200Ω, the internal resistance of the Schottky diode is too small to be ignored. The line resistance of the node after the two Schottky diodes are connected in parallel is 100Ω. The first preset value is set to 100Ω, so that the line resistance of the parallel node and the first preset value line resistance of the Chebyshev gradient line in parallel is the second preset value, i.e. 50Ω, thereby completing impedance matching with the input RF signal. In this way, the reflection of the RF signal can be reduced, thereby effectively broadening the working bandwidth of the module.

[0035] For a single Schottky diode, when the signal and bias voltage are loaded on it, the output current signal of the Schottky diode can be expressed as:

[0036]

[0037] Among them, I s is the saturation current, q is the electron charge, k is the Boltzmann constant, T is the temperature, and n is the ideality factor of approximately 1.2. The diode bias voltage U is composed of the DC bias voltage (i.e., the bias voltage applied to the diode) V0 and the RF signal voltage v. It can be expanded in a Taylor series about V0. The Schottky diode current can be expressed as follows:

[0038]

[0039] α=q / nkT, m is the order. Since components higher than the third order have low power consumption and can be ignored, the current of the RF signal output by a single Schottky diode can be expressed as:

[0040]

[0041] In the above derivation, only IMD3 is considered a third-order component. Research has shown that Schottky diodes have a nonlinearity opposite to that of the ROF link transmission, so they can be used as devices to improve the link nonlinearity. When two Schottky diodes are connected in anti-parallel, the currents generated by the two Schottky diodes can be expressed as:

[0042]

[0043] The forward transmitted RF signal can be expressed as:

[0044]

[0045] That is, after the RF signal v passes through the APD module, the output RF signal can be expressed as:

[0046]

[0047] Where g1 and g3 are coefficients related to the bias voltage, R is the resistance value of the resistor in the APD circuit, and v RF Indicates the RF voltage output by the APD module.

[0048] Then, use a power divider to divide v RF Divide into two signals v RF1 and v RF2 , v RF1 After being phase shifted by 180° phase shifter, it becomes v RF1 ` and v RF2 They are modulated to two lasers of optical push-pull structure respectively.

[0049] That is, the signal output by the APD is converted into two RF signals v after passing through the power divider and the 180° coupler. RF1 `、v RF2 The lasers input to DML1 and DML2 can be expressed as follows:

[0050]

[0051] x1(t) and x2(t) are the radio frequency signals input to the lasers in the first and second DML transmission links, respectively, and x1(t)=-x2(t).

[0052] At the receiving end, an optical push-pull structure is used to eliminate the second-order nonlinearity in the link. This structure includes a dual directly modulated laser (DML) parallel modulation architecture. By adjusting the link attenuation coefficient and modulation depth of the laser, the nonlinear characteristics of the two links tend to be symmetrical. The time domain difference method is used at the photoelectric detection end to offset the even-order nonlinear components.

[0053] The two opposite RF signals are modulated onto the two lasers LD1 and LD2 in a push-pull optical structure, generating upper and lower optical signals. After being transmitted through a single-mode optical fiber, the optical signals are converted into electrical signals I1(t) and I2(t) at the receiving end using a balanced detector PD:

[0054]

[0055] Where α1 and α2 are the attenuation coefficients of the upper and lower ROF transmission links (i.e., the first and second DML transmission links), η1 and η2 are the response sensitivities of the balanced detectors PD1 and PD2, respectively. b1 and I b2 are the bias currents of lasers LD1 and LD2 respectively, m1 and m2 are the modulation depths of the upper and lower signals, A n (n=1,2,3...) and B n (n=1,2,3...,∞) are the n-order nonlinear coefficients of the transfer functions of the upper and lower lasers respectively. It can be understood that A n ,B n It is a function with the input RF signal amplitude as the independent variable.

[0056] By adjusting the link attenuation and modulation depth, the n-order nonlinearity of the two links can be made the same, that is, by adjusting the parameters α1, α2, m1, and m2, m1α1η1α n =m2α2η2b n At this time, subtract the RF signals of PD1 and PD2 to get:

[0057]

[0058] Where, x1(t)=-x2(t)=x(t); when n is an odd number, e inπ =-1, when n is an even number, e inπ =1;

[0059] It can be seen that by properly adjusting the parameters α1, α2, m1, and m2, m1α1η1α n =m2α2η2b n , when n is an even number, e inπ =1,m1α1η1α n -m2α2η2b n e inπ = 0, the even-order nonlinearity can be offset, and the DC signal component (α1η1I b1 -α2η2I b2 ) is filtered out, at this time,

[0060] Considering the upper and lower ROF links as a nonlinear system and only considering the third-order nonlinearity, the expression of one of the links can be written as where v out is the RF signal output by the PD in the link, v in is the RF signal input to the laser in the link, C n (n=1, 2, 3...) is the n-order nonlinear coefficient input to the link.

[0061] As can be seen from the previous text, the RF signals input to the lasers in DML1 and DML2 are:

[0062]

[0063] That is, for the entire nonlinear system, v in1 =(g1v+g3v 3 ) / 2,v in2 =-(g1v+g3v 3 ) / 2, the signals above the third order are not taken into consideration because their amplitude is too small, so the output of the entire nonlinear system can be expressed as:

[0064]

[0065] v out1 、v out2 That is, they are the signals output by the PDs in the first and second DML transmission links, that is, the radio frequency signals output by PD1 and PD2 respectively.

[0066] V out1 With v out2 Subtracting them gives:

[0067]

[0068] Since the coefficients C1 and C3 are fixed for a certain system, we only need to adjust the Schottky diode bias voltage to make Thus, the IMD3 of the link is eliminated. At this time, v out =g1C1v, thereby achieving the joint suppression of the second-order and third-order nonlinearities in the DML, improving the system’s spurious-free dynamic range, and extending the RF input power range.

[0069] In order to reduce resonance and further improve bandwidth, preferably, the first to third capacitors are all high-frequency capacitors, and the inductors are all high-frequency inductors.

[0070] Preferably, the system further comprises a radio frequency source for providing a radio frequency signal.

[0071] Preferably, the system further comprises a radio frequency signal collector for collecting the signal output by the optical push-pull structure.

[0072] The schematic diagram of the APD module provided by the present invention is as follows Figure 2 As shown in Figure 2, since the nonlinearity exhibited by the Schottky diode is opposite to that of the RoF system, we can adapt to different laser characteristics by applying different bias voltages to the Schottky diode. Figure 3 Demonstrates the bandwidth improvement effect brought by using Chebyshev gradient lines.

[0073] To achieve optimal nonlinearity suppression, the bias voltages of the two lasers and the link attenuation are first adjusted to ensure that the nonlinearity of the two DML links is the same. During this process, the voltages are adjusted to the optimal bias point by performing time-domain subtraction and decoding on the two received signals. The same method is then used to scan the optimal bias point of the Schottky diode.

[0074] In the experiment, dual-tone signals with frequencies of 1.995 GHz and 2.005 GHz were first generated to test the third-order spurious-free dynamic range (SFDR3) enhancement capability of the proposed improved APD device. Figure 4 The relationship between input signal power and output signal power is shown, demonstrating the ability of the proposed APD module to suppress IMD3. The results show that SFDR3 is reduced from 102dB·Hz 2 / 3 Increased to 115.6dB·Hz 3 / 3 , successfully improving the SFDR3 of the DML link by 13.6dB.

[0075] Figure 5This study demonstrates the effectiveness of the present invention in suppressing IMD2 and IMD3 at 3G frequencies. Two 500MHz bandwidth 64QAM OFDM intermediate frequency (IF) signals were generated, with carrier frequencies of 1GHz and 2GHz, respectively. In this case, the IMD2 of the first IF signal falls within the frequency range of the second IF signal. The IMD3 of the two dual-frequency signals also affects the two frequency bands, respectively. This dual-frequency signal was transmitted over a 10km SSMF. The peak-to-peak voltage (Vpp) of the dual-frequency signal was set to 1000mV, and the received optical power (ROP) was 3dBm. Figure 5 The power spectral density (PSD) of two signals with and without the optoelectronic push-pull structure is shown. It can be seen that the proposed structure effectively reduces IMD3 and IMD2 by more than 7.5dB. Figure 6 The EVM performance measured under different Vpp conditions at an incident power of 4.5dBm is shown, including four cases: no compensation, compensation for third-order nonlinearity only, compensation for second-order nonlinearity only, and compensation for second-order and third-order nonlinearity at the same time. By using the optical push-pull structure and the proposed APD device in the 2GHz band, the EVM is reduced by 2% and 1%, respectively. However, when the optoelectronic push-pull structure is deployed, the EVM will be reduced by 3%. When the ROP is 4.5dBm, the received signal constellation diagram with and without the optical push-pull structure is shown in the figure below. Figure 6 The experimental results demonstrate the necessity of simultaneously suppressing second-order and third-order nonlinearities. Utilizing the proposed optoelectronic push-pull structure, the EVM values ​​at all RF input powers are successfully reduced to below 8%, greatly expanding the RF input power range.

[0076] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A system for simultaneously suppressing second-order and third-order nonlinearities in a DML, characterized in that: It includes a broadband analog pre-distortion module, a power splitter, a phase shifter and an optical push-pull structure; The broadband analog predistortion module includes first and second Schottky diode branches and an RF signal path; the first and second Schottky diode branches each include a Schottky diode, a resistor, a first capacitor, and an inductor, and one end of the first capacitor is connected in parallel with one end of the inductor and then in series with one end of the resistor, with the other end being grounded; in the first branch, the other end of the resistor is connected to the cathode of the Schottky diode, and the other end of the inductor is used to input a bias voltage V0 applied to the Schottky diode; in the second branch, the other end of the resistor is connected to the anode of the Schottky diode, and the other end of the inductor is grounded; the Schottky diode of the first branch is connected in anti-parallel to the Schottky diode of the second branch; the RF signal path includes an RF signal input terminal, second and third capacitors, and an RF signal output terminal connected in sequence, the path between the second and third capacitors is connected to the node after the anti-parallel connection, and the RF transmission line between the third capacitor and the RF signal output terminal is a Chebyshev gradient line, whose line resistance gradually changes from a first preset value to a second preset value along the RF signal transmission direction, and the first preset value is greater than the second preset value; the capacitance of the first to third capacitors is the same; The power divider is used to convert the RF signal v output by the broadband analog predistortion module RF Divided equally into the first and second RF signals v RF1 、v RF2 ; The phase shifter is used to convert v RF1 Perform 180° phase shift to obtain the first RF signal v after phase shift RF1 `; The optical push-pull structure includes a first DML transmission link DML1 and a second DML transmission link DML2, and DML1 and DML2 both include a DML laser, an optical fiber and a PD; RF1 `、v RF2 One-to-one correspondence input to DML1 and DML2; The value of V0 satisfies the formula To eliminate the third-order nonlinearity; among them, C1 and C3 are the first-order and third-order nonlinear coefficients of DML1 and DML2 respectively. R is the resistance value of the resistor, I s is the saturation current, α=q / nkT, q is the electron charge, k is the Boltzmann constant, T is the ambient temperature, and n is the ideality factor; The attenuation coefficients α1, α2 and modulation depths m1, m2 of DML1 and DML2 satisfy the formula m1α1η1A n =m2α2η2B n To eliminate the second-order nonlinearity; where η1 and η2 are the response sensitivities of PD in DML1 and DML2 respectively, A n 、B n are the nth-order nonlinear coefficients of the transfer functions of the lasers in DML1 and DML2 respectively.

2. The system according to claim 1, wherein The first to third capacitors are all high-frequency capacitors, and the inductors are all high-frequency inductors.

3. The system according to claim 1, wherein: The RF signal v output by the broadband analog predistortion module RF =g1v+g3v 3 , v is the radio frequency signal input to the broadband analog predistortion module.

4. The system according to claim 1, wherein: The calculation formulas for the electrical signals I1(t) and I2(t) detected by the PDs in the first and second DML transmission links are: Among them, I b1 and I b2 are the bias currents of the lasers in DML1 and DML2, respectively; x1(t) and x2(t) are the RF signals input to the lasers in DML1 and DML2, respectively.

5. The system according to claim 1, wherein: The signal v output by the optical push-pull structure out =g1C1v, v is the radio frequency signal input to the broadband analog predistortion module.

6. The system according to claim 1, wherein: A radio frequency source is also included for providing a radio frequency signal.

7. The system according to claim 1, wherein: It also includes a radio frequency signal collector for collecting the signal output by the optical path push-pull structure.

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