Long-distance dispersion-fading-resistant linearized microwave photonic link

Abstract

The invention belongs to the technical field of microwave photonic links, and particularly relates to a long-distance dispersion-fading-resistant linear microwave photonic link. The link comprises a transmitting end and a receiving end; the transmitting end comprises a laser, a dual-drive Mach-Zehnder modulator DDMZM and a 90-degree phase shifter; the receiving end comprises a photoelectric detector, a DC block, an analog-to-digital converter and a digital processing unit; an optical signal emitted by the laser serves as a carrier wave and is input into the DDMZM. An input target radio frequency signal is divided into two paths after passing through the power divider, one path is directly input to a radio frequency port of the DDMZM, and the other path is input to the other radio frequency port of the DDMZM after passing through the 90-degree phase shifter; and the modulated optical signal is transmitted through a long-distance optical fiber, is sequentially subjected to photoelectric conversion, blocking processing and analog-to-digital conversion and then enters a digital processing unit, digital post-compensation is carried out on nonlinear distortion according to a transmission response in a single-sideband modulation optical link, and a target radio-frequency signal is finally output.

Description

Technical Field

[0001] The invention belongs to the technical field of microwave photon links, and in particular relates to a long-distance dispersion-fading-resistant linearized microwave photon link. Background Art

[0002] At present, microwave photonic links play an important role in modern communications, radars, satellite communications and other scenarios that require high-frequency bands and large-capacity information transmission. In microwave photonic links, RF signals are electro-optically modulated using an intensity modulator and then transmitted to a remote receiving end through an optical fiber, where they are restored to RF signals after photoelectric conversion. Microwave photonic links have the advantages of high bandwidth, strong anti-interference ability, long transmission distance, light weight, and low power consumption, and have been widely used. In traditional microwave photonic links, affected by the dispersion effect of long-distance optical fibers, there will be a phase difference between the double-sideband and the optical carrier, which is related to the optical fiber dispersion coefficient, transmission distance, and RF frequency. When the phase difference between the optical carrier and the sideband is π, the RF signal power will decay. Therefore, when transmitting broadband RF signals in long-distance optical fibers, the spectrum of the received signal will have periodic power decay, resulting in the loss or misjudgment of the RF signal. Therefore, it is necessary to use anti-dispersion fading technology to compensate for the phase difference between the double-sideband and the carrier. This technology has been widely studied in long-distance microwave photonic links and has important application value.

[0003] like Figure 1 The figure shows a typical linearized anti-dispersion fading link structure based on dual polarization modulation. This solution uses a dual polarization modulator to achieve single-sideband modulation of the RF signal, which can achieve broadband power fading compensation in long optical fiber links, effectively improve the broadband reconfigurability of the link, and by adjusting the polarizer angle, the third-order nonlinear distortion is suppressed, greatly improving the spurious-free dynamic range of the link. This solution has good application prospects in broadband optical links, but it still has shortcomings: First, the core device of the system is the dual-polarization modulator, but there are many bias control ports in the dual-polarization modulator. The drift of the bias point will greatly reduce the transmission performance of the link. Secondly, the system suppresses third-order intermodulation distortion by precisely controlling the angle of the polarizer. However, slight changes in the optical fiber (thermal expansion and contraction, etc.) will cause changes in the polarization state of the transmitted light, and the angle of the polarizer cannot be controlled by real-time feedback. Therefore, the system's suppression effect on nonlinear distortion is not stable. Finally, the cost of a dual-polarization modulator is several times or even ten times that of a common electro-optical modulator. Using a dual-polarization modulator greatly increases the cost of using the system.

[0004] Another example Figure 2The figure shows a typical linearized anti-dispersion fading link structure based on optical carrier phase control. This solution uses a phase modulator to reduce the system performance degradation caused by bias point drift, and uses a waveform shaper to control the optical carrier phase, while compensating for power fading caused by dispersion, and suppressing nonlinear distortion. This technology has good innovation and simplicity in improving the linearization of anti-dispersion fading links, but it also has obvious shortcomings: First, this technology relies on changing the phase of the optical carrier to achieve power fading compensation caused by dispersion. However, since the phase relationship between the sideband and the optical carrier is different at each frequency, fading compensation can only be achieved at a specific frequency, which limits the application of broadband systems and the reconfigurability of multi-band systems. Secondly, the waveform shaper is bulky and expensive, which increases the cost of the actual application of the system; Finally, nonlinear distortion suppression is achieved by precisely controlling the phase of the optical carrier. However, the phase control accuracy of current commercial waveform shapers is not high, making it difficult to achieve a good level of nonlinear distortion suppression.

[0005] Therefore, it is very important to design a long-distance dispersion-resistant linearized microwave photonic link that can solve the problems of broadband / multi-band power fading and third-order intermodulation distortion interference and improve transmission performance. Summary of the invention

[0006] The present invention aims to overcome the problems of poor transmission performance, poor nonlinear distortion suppression effect and high system cost in the existing linearized anti-dispersion fading links in the prior art, and provides a long-distance anti-dispersion fading linearized microwave photonic link that can solve the problems of broadband / multi-band power fading and third-order intermodulation distortion interference and improve transmission performance.

[0007] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions: A long-distance dispersion-resistant linearized microwave photon link comprises a transmitting end and a receiving end; the transmitting end comprises a laser, a dual-drive Mach-Zehnder modulator DDMZM and a 90° phase shifter; the receiving end comprises a photodetector, a DC block, an analog-to-digital converter and a digital processing unit; The optical signal emitted by the laser is used as a carrier and input into a dual-drive Mach-Zehnder modulator DDMZM; the input target radio frequency signal is divided into two paths with equal power and coherent phase after passing through a power divider, one of which is directly input into the radio frequency port of the DDMZM, and the other is input into another radio frequency port of the DDMZM after passing through a 90° phase shifter, so as to realize single-sideband modulation; the modulated optical signal is input into a photoelectric detector after being transmitted through a long-distance optical fiber, and the optical signal enters a digital processing unit after undergoing photoelectric conversion, direct current isolation processing and analog-to-digital conversion in sequence, and at the same time, nonlinear distortion is digitally compensated according to the transmission response in the single-sideband modulated optical link, and finally the target radio frequency signal is output.

[0008] Preferably, the electric field of the optical signal generated by the laser is expressed as: (1); in, is the optical carrier amplitude, reflecting the initial strength of the optical signal; is the angular frequency of the signal, is the central frequency of light; j represents the imaginary unit, and t is the time variable.

[0009] Preferably, the target RF signal amplitude loaded on the two arms of the dual-drive Mach-Zehnder modulator is consistent, the phase difference is 90°, the target RF signal bias voltage of one arm is 0, and the target RF signal bias voltage of the other arm is , the expression of the single-sideband optical carrier light field output by the modulator is obtained as: (2); in, is the angular frequency of the RF signal, is the modulation depth, represents a type of Bessel function of order n; Assume that after a section of optical fiber transmission, the light field expression obtained is , then the corresponding Fourier transform is: ; in, and Respectively express and Perform Fourier transform; When only considering the fiber dispersion without considering the fiber loss, the fiber transmission function is: ; in , is the fiber dispersion coefficient, is the wavelength, represents the sideband angular frequency, c is the speed of light, and L is the length of the optical fiber.

[0010] Preferably, the current received on the photodetector is specifically expressed as: (3); Where r is the responsivity of the detector and L is the length of the optical fiber.

[0011] Preferably, the upper arm input signal of the dual-drive Mach-Zehnder modulator is set to a dual-tone signal, that is: ; The lower arm input signal is: ; The bias is set at the orthogonal point, and the expression of the modulated output light field is: (4); in, represents the modulator output light field, is the amplitude of the modulating signal, and is the frequency of the two-tone RF signal, is the half-wave voltage of the modulator; the output light field is expanded by Bessel function to obtain: (5).

[0012] Preferably, after the optical signal reaches the receiving end, the optical signal is converted into an electrical signal by a photodetector, and the optical power expression of the output single sideband signal is: (6); in, , , is the input optical power; finally, the transmission function at this time is: (7); The optical signal is passed through a DC block to remove the DC component, and a bandpass filter is used to retain the baseband and third-order intermodulation distortion components near the baseband. To simplify the derivation, let the voltage It is expressed as: ; Performing Taylor series expansion on the result and ignoring the higher harmonic components, the photocurrent can be further expressed as: (8); Among them, the symbol Represents a higher-order infinitesimal.

[0013] As a preference, considering that the nonlinear distortion compensation of the intensity modulator is limited to odd orders, the compensation function is an odd function, and the estimated transfer characteristic function is: ; in, and are expressed as coefficients of the nonlinear compensation function; the detected photocurrent signal Substituting into the nonlinear transfer function, the final signal after compensation is as follows: (9); The final signal includes a fundamental frequency signal component Fundamentals, a third-order harmonic component HD3 and a third-order intermodulation distortion IMD3; From formula (9), it can be concluded that when the cubic coefficient is 0, the third-order intermodulation distortion of the modulator is suppressed, that is: (10); Therefore, the coefficients of the nonlinear compensation function should satisfy: (11).

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) System cost reduction: The present invention uses a dual-drive Mach-Zehnder modulator to directly generate a single-sideband modulated signal, which solves the problem of dispersion attenuation in optical fiber transmission without the need for complex modulation devices or waveform shaping devices; (2) Dynamic compensation capability: The present invention uses digital post-processing technology to suppress nonlinear intermodulation distortion, and can dynamically acquire and adjust nonlinear compensation coefficients in real time, optimize the compensation effect according to different working conditions and signal characteristics, and effectively improve the system's spurious-free dynamic range; (3) Broadband adaptability: The present invention applies flexible digital compensation technology in a single-sideband modulation system, and can achieve power fading compensation and nonlinear suppression under conditions of higher frequency, wider bandwidth and longer distance, and is suitable for long-distance, large-bandwidth and multi-functional application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A schematic diagram of a typical architecture of a linearized anti-dispersion fading link based on dual-polarization modulation in the prior art; Figure 2 A schematic diagram of a typical linearized anti-dispersion fading link based on optical carrier phase control in the prior art; Figure 3 A schematic diagram of a long-distance dispersion-resistant linearized microwave photon link according to the present invention; Figure 4A schematic diagram comparing the dispersion fading of traditional double-sideband modulation and single-sideband modulation in optical fiber transmission provided by an embodiment of the present invention; Figure 5 A schematic diagram of spectrum results of a traditional DDMZM double-sideband modulation link; Figure 6 A schematic diagram of a spectrum result of a long-distance dispersion-resistant linearized microwave photon link of the present invention; FIG. 7 is a schematic diagram of SFDR test results of a long-distance dispersion-resistant linearized microwave photonic link provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0016] In order to more clearly illustrate the embodiments of the present invention, the specific implementation methods of the present invention will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other accompanying drawings and other implementation methods can be obtained based on these accompanying drawings without creative work.

[0017] like Figure 3 As shown, the present invention provides a long-distance dispersion-resistant linearized microwave photon link, comprising a transmitting end and a receiving end; the transmitting end comprises a laser, a dual-drive Mach-Zehnder modulator DDMZM and a 90° phase shifter; the receiving end comprises a photodetector, a DC block, an analog-to-digital converter and a digital processing unit; The optical signal emitted by the laser is used as a carrier and input into a dual-drive Mach-Zehnder modulator DDMZM; the input target radio frequency signal is divided into two paths with equal power and coherent phase after passing through a power divider, one of which is directly input into the radio frequency port of the DDMZM, and the other is input into another radio frequency port of the DDMZM after passing through a 90° phase shifter, so as to realize single-sideband modulation; the modulated optical signal is input into a photoelectric detector after being transmitted through a long-distance optical fiber, and the optical signal enters a digital processing unit after undergoing photoelectric conversion, direct current isolation processing and analog-to-digital conversion in sequence, and at the same time, nonlinear distortion is digitally compensated according to the transmission response in the single-sideband modulated optical link, and finally the target radio frequency signal is output.

[0018] The detailed steps are as follows: First, the laser generates a stable optical signal as a carrier. This optical signal is used for subsequent electro-optical modulation, and the electric field of the input optical signal is expressed as: (1); in, is the optical carrier amplitude, reflecting the initial strength of the optical signal; is the angular frequency of the signal, is the central frequency of light; j represents the imaginary unit, and t is the time variable.

[0019] In order to realize single-sideband modulation of the RF signal, it is necessary to set the modulator drive and bias structure, so that the target RF signal amplitude loaded on the two arms of the dual-drive Mach-Zehnder modulator is consistent, the phase difference is 90°, the target RF signal bias voltage of one arm is 0, and the target RF signal bias voltage of the other arm is , the expression of the single-sideband optical carrier light field output by the modulator is obtained as: (2); in, is the angular frequency of the RF signal, is the modulation depth, represents a first-order Bessel function of order n; from formula (2), it can be seen that the main components of the single-sideband signal are the center carrier and the first-order lower sideband; Assume that after a section of optical fiber transmission, the light field expression obtained is , then the corresponding Fourier transform is: ; in, and Respectively express and Perform Fourier transform; When only considering the fiber dispersion without considering the fiber loss, the fiber transmission function is: ; in , is the fiber dispersion coefficient, is the wavelength, represents the sideband angular frequency, c is the speed of light, and L is the length of the optical fiber.

[0020] The current received on the photodetector is specifically expressed as: (3); Among them, r is the detector's responsivity, and L is the fiber length. From the above formula, it can be seen that at the receiving end, there are only microwave millimeter waves generated by the first-order sideband and the center carrier beat frequency, and the power of the microwave millimeter wave is , so it is related to It does not matter, thus avoiding the influence of fiber dispersion effect on signal power.

[0021] The commonly used single-tone modulation output signal includes the base frequency and the second harmonic, but does not include intermodulation distortion, and cannot accurately measure the linearity of the system link, so dual-tone modulation is used. When the input signal is a dual-tone signal, the system output contains third-order intermodulation distortion items, which are closest to the input base frequency signal and have the greatest impact on the linearity of the signal. Therefore, to improve the linearity of the link, the third-order intermodulation distortion must be suppressed.

[0022] The upper arm input signal of the dual-drive Mach-Zehnder modulator is set to a dual-tone signal, that is: ; The lower arm input signal is: ; The bias is set at the orthogonal point, and the expression of the modulated output light field is: (4); in, represents the modulator output light field, is the amplitude of the modulating signal, and is the frequency of the two-tone RF signal, is the half-wave voltage of the modulator; the output light field is expanded by Bessel function to obtain: (5).

[0023] Considering the small signal modulation, the above formula only retains the optical carrier and the first-order signal. From the above formula, we can see that the main components of the single-sideband signal are the DC component and the frequency and Compared with double-sideband modulation signals, the power of the signal will oscillate periodically with the length of the optical fiber. However, single-sideband modulation reduces one sideband on the basis of double-sideband modulation. At the receiving end, only one sideband signal beats with the center carrier, thus avoiding the influence of optical fiber dispersion effect on signal power.

[0024] After the optical signal reaches the receiving end, it is converted into an electrical signal through a photodetector. The optical power expression of the output single-sideband signal is: (6); in, , , is the input optical power; finally, the transmission function at this time is: (7); The optical signal is passed through a DC block to remove the DC component, and a bandpass filter is used to retain the baseband and third-order intermodulation distortion components near the baseband. To simplify the derivation, replace the contents in the brackets with voltage Indicates that: ; Performing Taylor series expansion on the result and ignoring the higher harmonic components, the photocurrent can be further expressed as: (8); Among them, the symbol Represents a higher-order infinitesimal.

[0025] When the modulator is biased at the quadrature point, there is no even-order distortion, and only the third-order distortion term needs to be removed. Since the nonlinear distortion compensation of the intensity modulator is limited to odd orders, the compensation function is an odd function, and the estimated transfer characteristic function is: ; Among them, the coefficients of the nonlinear compensation function and It needs to be determined according to the actual situation; the detected signal Substituting into the nonlinear transfer function and ignoring some low-power terms, we can approximate the final signal after compensation: (9); The final signal includes a fundamental frequency signal component Fundamentals, a third-order harmonic component HD3 and a third-order intermodulation distortion IMD3; From formula (9), it can be concluded that when the cubic coefficient is 0, the third-order intermodulation distortion of the modulator is suppressed, that is: (10); Therefore, the coefficients of the nonlinear compensation function should satisfy: (11).

[0026] When the coefficients satisfy the above relationship, the third-order intermodulation distortion can be effectively suppressed.

[0027] The long-distance dispersion-resistant linearized microwave photonic link of the present invention is described below by taking a typical link system as an example. The system not only overcomes the dispersion fading in optical fiber transmission, but also suppresses nonlinear distortion: The laser outputs an optical signal with a wavelength of 1550nm and a power of 10mW, which is connected to the input of the dual-drive Mach-Zehnder modulator (Dual-Drive MZM, DDMZM). In the modulation part, the system uses two signal generators to generate microwave signals with frequencies of 10GHz and 10.01GHz respectively. After the two signals are combined into one by the RF combiner, one of them passes through a 90° phase shifter and serves as the two RF inputs of the DDMZM. ​​At the same time, the DDMZM is set at an orthogonal bias point to achieve single-sideband modulation of the signal. The modulated optical signal is then filtered. Due to the nonlinearity near the optical carrier, signal attenuation will occur after the beat frequency, so the second harmonic and nonlinear terms near the carrier are removed by the filter. The output signal is transmitted through a single-mode optical fiber with a length of 30km, and its dispersion parameter is During long-distance transmission, Figure 4The figure shows the dispersion fading of traditional double-sideband modulation and single-sideband modulation in optical fiber transmission. It can be seen that single-sideband modulation technology effectively solves the signal fading problem caused by optical fiber dispersion and ensures the integrity and stability of the signal.

[0028] The transmitted optical signal is received by a photodetector with a responsivity of 0.69A / W, which converts the optical signal into an electrical signal. In order to suppress the nonlinear distortion in the system and improve the linearity of the link, the system performs post-distortion compensation processing after photodetection. By performing a nonlinear compensation algorithm on the received electrical signal, the nonlinear distortion components introduced by the device and the transmission process are suppressed. Figure 5 This is the spectrum result of the traditional DDMZM double-sideband modulation link. Figure 6 This is the spectrum result of the long-distance dispersion-resistant linearized microwave photonic link of the present invention. From the spectrum result, it can be seen that the third-order intermodulation distortion of the entire link system is suppressed by 32 dBm.

[0029] Figure 7 The SFDR test result of the long-distance dispersion-resistant linearized microwave photonic link based on DDMZM in the present invention. The spurious-free dynamic range is used to measure the impact of nonlinear distortion on the system link. It is defined as the input power range where the output signal power is greater than the system noise and the output power of the third-order intermodulation distortion signal is less than the output power of the noise. In this experiment, the noise floor is -140dBm / Hz, and the SFDR measured by the traditional microwave photonic link is After linearization compensation, the dynamic range increases to , improved by about 10.6dB.

[0030] The present invention comprehensively considers the advantages and disadvantages of the existing technical solutions and proposes a long-distance linearized dispersion-resistant microwave photonic link. By setting the RF drive and DC bias voltage of the dual-drive Mach-Zehnder modulator, single-sideband modulation of the RF signal is achieved, the power fading of the broadband signal caused by the dispersion effect during long-distance transmission is compensated, and the post-distortion compensation technology is used to effectively suppress the third-order intermodulation distortion at the receiving end. The present invention effectively solves the problems of broadband / multi-band power fading and third-order intermodulation distortion interference, and significantly improves the transmission performance of long-distance transmission microwave photonic links.

[0031] The above description is only a detailed description of the preferred embodiments and principles of the present invention. For ordinary technicians in this field, according to the ideas provided by the present invention, there will be changes in the specific implementation methods, and these changes should also be regarded as the protection scope of the present invention.

Claims

1. Long-distance linearized microwave photonic links that are resistant to dispersion fading, It is characterized in that It includes a transmitting end and a receiving end; the transmitting end includes a laser, a dual-drive Mach-Zehnder modulator DDMZM and a 90° phase shifter; the receiving end includes a photodetector, a DC block, an analog-to-digital converter and a digital processing unit; The optical signal emitted by the laser is used as a carrier and input into a dual-drive Mach-Zehnder modulator DDMZM; the input target radio frequency signal is divided into two paths with equal power and coherent phase after passing through a power divider, one of which is directly input into the radio frequency port of the DDMZM, and the other is input into another radio frequency port of the DDMZM after passing through a 90° phase shifter, so as to realize single-sideband modulation; the modulated optical signal is input into a photoelectric detector after being transmitted through a long-distance optical fiber, and the optical signal enters a digital processing unit after undergoing photoelectric conversion, direct current isolation processing and analog-to-digital conversion in sequence, and at the same time, nonlinear distortion is digitally compensated according to the transmission response in the single-sideband modulated optical link, and finally the target radio frequency signal is output.

2. The long-distance dispersion-resistant linearized microwave photonic link according to claim 1, It is characterized in that The electric field of the optical signal generated by the laser is expressed as: （1）； in, is the optical carrier amplitude, reflecting the initial strength of the optical signal; is the angular frequency of the signal, is the central frequency of light; j represents the imaginary unit, and t is the time variable.

3. The long-distance dispersion-resistant linearized microwave photon link according to claim 2, It is characterized in that The target RF signal amplitude loaded on the two arms of the dual-drive Mach-Zehnder modulator is consistent, the phase difference is 90°, the target RF signal bias voltage of one arm is 0, and the target RF signal bias voltage of the other arm is , the expression of the single-sideband optical carrier light field output by the modulator is obtained as: （2）； in, is the angular frequency of the RF signal, is the modulation depth, represents a type of Bessel function of order n; Assume that after a section of optical fiber transmission, the light field expression obtained is , then the corresponding Fourier transform is: ； in, and Respectively express and Perform Fourier transform; When only considering the fiber dispersion without considering the fiber loss, the fiber transmission function is: ； in , is the fiber dispersion coefficient, is the wavelength, represents the sideband angular frequency, c is the speed of light, and L is the length of the optical fiber.

4. The long-distance dispersion-resistant linearized microwave photonic link according to claim 3, It is characterized in that The current received on the photodetector is specifically expressed as: （3）； Where r is the responsivity of the detector and L is the length of the optical fiber.

5. The long-distance dispersion-resistant linearized microwave photonic link according to claim 4, It is characterized in that The upper arm input signal of the dual-drive Mach-Zehnder modulator is set to a dual-tone signal, that is: ； The lower arm input signal is: ； The bias is set at the orthogonal point, and the expression of the modulated output light field is: （4）； in, represents the modulator output light field, is the amplitude of the modulating signal, and is the frequency of the two-tone RF signal, is the half-wave voltage of the modulator; the output light field is expanded by Bessel function to obtain: （5）。 6. The long-distance dispersion-resistant linearized microwave photonic link according to claim 5, It is characterized in that After the optical signal reaches the receiving end, it is converted into an electrical signal through a photodetector. The optical power expression of the output single-sideband signal is: （6）； in, , , is the input optical power; finally, the transmission function at this time is: （7）； The optical signal is passed through a DC block to remove the DC component, and a bandpass filter is used to retain the baseband and third-order intermodulation distortion components near the baseband. To simplify the derivation, let the voltage It is expressed as: ； Performing Taylor series expansion on the result and ignoring the higher harmonic components, the photocurrent can be further expressed as: （8）； Among them, the symbol Represents a higher-order infinitesimal.

7. The long-distance dispersion-resistant linearized microwave photonic link according to claim 6, It is characterized in that Considering that the nonlinear distortion compensation of the intensity modulator is limited to odd orders, the compensation function is an odd function, and the estimated transfer characteristic function is: ； in, and are expressed as coefficients of the nonlinear compensation function; the detected photocurrent signal Substituting into the nonlinear transfer function, the final signal after compensation is as follows: （9）； The final signal includes a fundamental frequency signal component Fundamentals, a third-order harmonic component HD3 and a third-order intermodulation distortion IMD3; From formula (9), it can be concluded that when the cubic coefficient is 0, the third-order intermodulation distortion of the modulator is suppressed, that is: （10）； Therefore, the coefficients of the nonlinear compensation function should satisfy: （11）。

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