Method and device for carrier synchronization of two-phase coded signals based on microwave photonic phase discrimination

Abstract

The present invention discloses a carrier synchronization method for binary phase-coded signals based on microwave photonic phase discrimination. Mix the binary phase-coded signal with the output signal of a voltage-controlled oscillator to generate a mixed-frequency signal; extract the down-converted signal in the mixed-frequency signal and perform square-law detection on it to obtain a single-frequency signal; use the microwave photonic phase discrimination method to obtain the phase difference signal between the single-frequency signal and the input optical pulse, and use this phase difference signal as a feedback signal to perform feedback control on the voltage-controlled oscillator, thereby realizing the carrier synchronization of the binary phase-coded signal. The present invention also discloses a carrier synchronization device for binary phase-coded signals based on microwave photonic phase discrimination. Compared with the prior art, the technical solution of the present invention has the advantages of high carrier synchronization accuracy, simple structure, and low implementation cost.

Description

Technical Field

[0001] The present invention relates to a carrier synchronization method, and particularly to a carrier synchronization method for a two-phase coded signal, belonging to the field of microwave photonics technology. Background Art

[0002] As a common spread spectrum signal, the two-phase coded signal has good pulse compression characteristics and can take into account both range resolution and operating range. However, the two-phase coded signal is susceptible to the Doppler effect, which reduces the main-to-side lobe ratio of the signal after pulse compression. Moreover, as the carrier frequency increases, it becomes more sensitive to the Doppler effect. However, traditional Doppler compensation methods are all based on electronics methods and are processed in the digital domain after analog-to-digital conversion. For the electronics method in the analog domain, it is difficult to achieve high-precision carrier. For the digital domain processing method, carrier synchronization can be stably achieved. However, as the signal bandwidth increases, higher requirements are imposed on analog-to-digital conversion, and for large-bandwidth signals, the processing delay of this scheme will increase. To solve the above problems, researchers have proposed a carrier synchronization scheme based on photonics methods, which can achieve carrier synchronization for high-frequency large-bandwidth signals.

[0003] Regarding carrier synchronization based on photonics methods, a number of research groups at home and abroad have conducted extensive research. In 2015, Nandakumar Nambath et al. proposed a coherent receiver based on a photonics solution (Nambath N, Raveendranath R K, Banerjee D, et al. Analog domain signal processing-based low-power 100-Gb / s DP-QPSK receiver[J]. Journal of Lightwave Technology, 2015, 33(15):3189-3197.). The microwave signal is modulated onto the optical signal, and orthogonal signals are obtained using photonics mixing technology. This method can reduce the power consumption, size, and cost of the coherent receiver. In 2021, Shalabh Gupta et al. proposed a Costas loop based on a photonics solution (Ashok R, Naaz S, Kamran R, et al. Analog domain carrier phase synchronization in coherent homodyne data center interconnects[J]. Journal of Lightwave Technology, 2021, 39(19):6204-6214.), which realizes signal processing in the all-analog domain. Compared with the digital signal processing solution, this solution can effectively reduce the delay, but the carrier synchronization accuracy of this solution is limited.

[0004] Currently, all existing photonics-based analog domain carrier synchronization schemes are implemented in the optical domain after electro-optic modulation based on the original electronic domain schemes. Some defects in the electronics schemes, such as the problem of delay matching between the two branches of the quadrature mixing signal, still exist in the photonics method. Therefore, there is an urgent need for a new carrier synchronization scheme to solve these problems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a carrier synchronization method for two-phase coded signals based on microwave photon phase discrimination, which has the advantages of high carrier synchronization accuracy, simple structure, and low implementation cost.

[0006] The present invention specifically adopts the following technical solutions to solve the above technical problems:

[0007] A carrier synchronization method for binary phase - coded signals based on microwave - photonic phase discrimination. Mix the binary phase - coded signal with the output signal of a voltage - controlled oscillator to generate a mixed - frequency signal. Extract the down - converted signal from the mixed - frequency signal and perform square - law detection on it to obtain a single - frequency signal. Use the microwave - photonic phase - discrimination method to obtain the phase - difference signal between the single - frequency signal and the input optical pulse, and use this phase - difference signal as a feedback signal to perform feedback control on the voltage - controlled oscillator, thereby achieving carrier synchronization of the binary phase - coded signal.

[0008] Preferably, the microwave - photonic phase - discrimination method is as follows: Divide the single - frequency signal into two paths, which are respectively used as the driving signals of two sub - modulators in a dual - parallel dual - drive Mach - Zehnder modulator to modulate the input optical pulse. Both sub - modulators operate in the push - pull mode and the bias phases are 0 and 90° respectively. Use a low - speed photodetector to perform beat - frequency detection on the output modulated optical signal of the dual - parallel dual - drive Mach - Zehnder modulator, and the phase - difference signal can be obtained.

[0009] Preferably, a PI controller is used to perform feedback control on the voltage - controlled oscillator.

[0010] Based on the same inventive concept, the following technical solutions can also be obtained:

[0011] A carrier synchronization device for binary phase - coded signals based on microwave - photonic phase discrimination, including:

[0012] A mixer, which is used to mix the binary phase - coded signal with the output signal of a voltage - controlled oscillator to generate a mixed - frequency signal; a filter, which is used to extract the down - converted signal from the mixed - frequency signal;

[0013] A square - law detector, which is used to perform square - law detection on the down - converted signal to obtain a single - frequency signal;

[0014] A microwave - photonic phase discriminator, which is used to obtain the phase - difference signal between the single - frequency signal and the input optical pulse;

[0015] A servo system, which is used to perform feedback control on the voltage - controlled oscillator with this phase - difference signal as a feedback signal, thereby achieving carrier synchronization of the binary phase - coded signal.

[0016] Preferably, the microwave - photonic phase discriminator includes:

[0017] A power splitter, which is used to divide the single - frequency signal into two paths;

[0018] A dual - parallel dual - drive Mach - Zehnder modulator, which is used to modulate the input optical pulse with the two paths of single - frequency signals divided by the power splitter as the driving signals of its two sub - modulators. Both of its sub - modulators operate in the push - pull mode and the bias phases are 0 and 90° respectively;

[0019] A low-speed photodetector is used to beat the output modulated optical signal of a dual-parallel dual-drive Mach-Zehnder modulator and output the phase difference signal.

[0020] Preferably, the servo system is a PI controller.

[0021] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0022] The present invention breaks through the bandwidth limitation of traditional electrical domain carrier synchronization, utilizes the ultra-narrow pulse width characteristic of optical pulses, and improves the carrier synchronization performance; the present invention realizes a new carrier synchronization scheme, converts a broadband phase-encoded signal into a single-frequency signal, and thus locks it with a certain harmonic of a femtosecond pulse to achieve carrier synchronization. Description of the Drawings

[0023] Figure 1 It is a schematic structural principle diagram of the two-phase encoded signal carrier synchronization device of the present invention;

[0024] Figure 2 It is a schematic structural diagram of a preferred embodiment of the two-phase encoded signal carrier synchronization device of the present invention;

[0025] Figure 3 It is a schematic diagram of the time domain relationship between the microwave signal input to the microwave photodetector and the femtosecond laser pulse. Detailed Embodiments

[0026] Aiming at the deficiencies of the prior art, the solution idea of the present invention is to transform the received two-phase encoded signal into a single-frequency signal, and lock the single-frequency signal with an optical pulse through a microwave photodetector, so as to achieve the carrier synchronization of the two-phase encoded signal.

[0027] The two-phase encoded signal carrier synchronization device based on microwave photon phase discrimination proposed by the present invention has a basic structure as Figure 1 shown, including:

[0028] A mixer is used to mix the two-phase encoded signal with the output signal of a voltage-controlled oscillator to generate a mixed signal;

[0029] A filter is used to extract the down-converted signal from the mixed signal;

[0030] A square-law detector is used to perform square-law detection on the down-converted signal to obtain a single-frequency signal;

[0031] A microwave photon phase discriminator is used to obtain the phase difference signal between the single-frequency signal and the input optical pulse;

[0032] A servo system is used to perform feedback control on the voltage-controlled oscillator with the phase difference signal as a feedback signal, so as to achieve carrier synchronization of the two-phase coded signal.

[0033] The microwave photonic phase detector in the above technical solution can adopt various existing microwave photonic phase detection schemes. For example, the through-type microwave photonic phase detector based on a polarization modulator proposed by Wei J et al. (Wei J, Zhang S, Kim J, et al. Compact phase detector for optical-microwave synchronization using polarization modulation[J]. Journal of Lightwave Technology, 2018, 36(19): 4267-4272.), and a fully polarization-maintaining microwave photonic phase detector proposed by Shao K (Shao K, Liu S, Gao P, et al. A microwave photonic phase detector based on a dual-polarization dual-drive Mach-Zehnder modulator[C] / / Optoelectronics and Communications Conference. Optica Publishing Group, 2021: M3E.3).

[0034] For the convenience of public understanding, the technical solution of the present invention will be described in detail below through a preferred embodiment:

[0035] As Figure 2 shown, the basic structure of the two-phase coded signal carrier synchronization device in this embodiment includes: a mode-locked laser, a dual-parallel dual-drive Mach-Zehnder modulator, a photodetector, a servo system, a voltage-controlled oscillator, a mixer, a filter, and a square-law detector; among them, the microwave photonic phase detector is composed of a mode-locked laser, a dual-parallel dual-drive Mach-Zehnder modulator, and a photodetector.

[0036] As Figure 1As shown, the two-phase coded signal input to the mixer is frequency-converted to an intermediate frequency signal by the output signal of the voltage-controlled oscillator. After passing through the filter, the down-converted signal is retained, and then it is input to the square-law detector for square-law detection to extract the single-frequency carrier signal. Then, this single-frequency carrier signal is input to the microwave photonic phase detector; after passing through the phase detector, the phase error signal between the single-frequency microwave signal and the femtosecond laser pulse is obtained. This signal is fed back to the voltage-controlled oscillator through the servo system to control the frequency of the voltage-controlled oscillator, realizing the synchronous locking between this single-frequency signal and a certain harmonic of the femtosecond pulsed laser. This single-frequency signal is obtained by square-law detection of the intermediate frequency signal. Therefore, it can be considered that the locking between the carrier of the intermediate frequency signal and a certain harmonic of the femtosecond pulsed laser is achieved. When this harmonic is used as the local oscillator, the synchronous locking between the carrier of the intermediate frequency signal and the local oscillator is achieved.

[0037] Figure 3 shows the schematic diagram of the time-domain relationship between the microwave signal input to the microwave photonic phase detector and the femtosecond laser pulse, as Figure 3 shown, the phase error between the microwave signal input to the microwave photonic phase detector and the femtosecond laser pulse is Δθ, and this error signal becomes the feedback control signal of the system after passing through the servo system.

[0038] The two-phase coded signal input to the mixer is:

[0039] E in =E 0 cos(ω 1 t+φ(t))

[0040] Φ(t) is the coded signal equal to 0 or π, and then this signal and the output signal of the voltage-controlled oscillator enter the mixer for mixing. The output signal is:

[0041] E 2 =E 1 *E in =E 01 sin(ω 2 t)*E 0 cos(ω 1 t+φ(t))

[0042] =E 02 [sin(ω 1 +ω 2 )t+φ(t))-sin(ω 1 -ω 2 )t+φ(t))]

[0043] where E 1 is the output signal of the voltage-controlled oscillator. The output signal of the mixer is filtered by the filter to remove the up-converted signal to obtain the down-converted signal:

[0044] E3 = E 02 [sin(ω 1 - ω 2 )t + φ(t))]

[0045] The intermediate-frequency signal obtained by down-conversion is input into a square-law detector to obtain a single-frequency signal:

[0046] E 4 = E RF [sin(2(ω 1 - ω 2 )t + Δθ)]

[0047] = E RF [sin(2πf RF t + Δθ)]

[0048] where E RF is the amplitude of the single-frequency signal, and Δθ is the phase error between the single-frequency signal and the femtosecond pulse. Since the pulse width of the optical pulse is extremely narrow, the output of the mode-locked laser can be written as:

[0049]

[0050] where A is the pulse amplitude, f rep is the repetition frequency of the pulse, and δ is the impulse function. And it satisfies ω 1 - ω 2 = 2πf RF = N * 2πf rep , the femtosecond laser pulse is input into a dual-parallel dual-drive Mach-Zehnder modulator, and the single-frequency signal obtained by square-law detection is used to modulate the femtosecond laser signal. The sub-modulators on the upper and lower branches both operate in the push-pull mode, β is the modulation coefficient, ΔΦ is the bias phase, and E in represents the field strength of the input optical signal:

[0051]

[0052] And the bias phases of the upper and lower arms are respectively:

[0053]

[0054] After modulation, the optical signals E x and E y are respectively:

[0055]

[0056] E x , E y are the two modulated optical signals respectively, and E in1represents the field strength of the input optical signal, and Δθ is the phase error between the single-frequency signal and the femtosecond pulse. Then, the signals of the upper and lower branches are input into a low-speed optoelectronic detector for beat frequency to obtain a phase difference signal:

[0057]

[0058] It can be seen that at this time, the phase error between the femtosecond pulse and the radio frequency signal has been converted into the amplitude imbalance output by the optoelectronic detector. When the frequency relationship: f RF = N·f rep is satisfied, in the case of small signals, the output of the optoelectronic detector has a linear relationship with the phase difference signal:

[0059] P out ∝ βΔθ

[0060] Taking this phase difference signal as feedback, the voltage-controlled oscillator is feedback-controlled through a servo system. This servo system is a PI controller, and its output function is:

[0061]

[0062] where K p and K i are the proportionality coefficient and the integral coefficient respectively. Then, this signal is fed back to the voltage-controlled oscillator to realize the control of the carrier of the intermediate-frequency signal, thereby realizing the control of the single-frequency signal input to the phase discriminator, realizing the synchronous locking between this single-frequency signal and a certain harmonic of the femtosecond pulse laser. This single-frequency signal is obtained by square-law detection of the intermediate-frequency signal. Therefore, it can be considered that the locking between the carrier of the intermediate-frequency signal and a certain harmonic of the femtosecond pulse laser is realized. When this harmonic is used as the local oscillator, the synchronous locking between the carrier of the intermediate-frequency signal and the local oscillator is realized.

Claims

1. A carrier synchronization method for binary phase - coded signals based on microwave - photonic phase discrimination, characterized in that, the binary phase - coded signal is mixed with the output signal of a voltage - controlled oscillator to generate a mixed - frequency signal; the down - converted signal in the mixed - frequency signal is extracted and subjected to square - law detection to obtain a single - frequency signal; the microwave - photonic phase - discrimination method is used to obtain the phase - difference signal between the single - frequency signal and the input optical pulse, and this phase - difference signal is used as a feedback signal to perform feedback control on the voltage - controlled oscillator, thereby realizing the carrier synchronization of the binary phase - coded signal.

2. The carrier synchronization method for binary phase - coded signals based on microwave - photonic phase discrimination according to claim 1, characterized in that, the microwave - photonic phase - discrimination method is specifically as follows: the single - frequency signal is divided into two paths, which are respectively used as the driving signals of the two sub - modulators in a dual - parallel dual - drive Mach - Zehnder modulator to modulate the input optical pulse. Both sub - modulators operate in a push - pull mode and the bias phases are 0 and 90° respectively. A low - speed photodetector is used to perform beat - frequency detection on the output modulated optical signal of the dual - parallel dual - drive Mach - Zehnder modulator, and thus the phase - difference signal is obtained.

3. The carrier synchronization method for binary phase - coded signals based on microwave - photonic phase discrimination according to claim 1, characterized in that, a PI controller is used to perform feedback control on the voltage - controlled oscillator.

4. A carrier synchronization device for binary phase - coded signals based on microwave - photonic phase discrimination, characterized in that, it includes: a mixer, which is used to mix the binary phase - coded signal with the output signal of a voltage - controlled oscillator to generate a mixed - frequency signal; a filter, which is used to extract the down - converted signal in the mixed - frequency signal; a square - law detector, which is used to perform square - law detection on the down - converted signal to obtain a single - frequency signal; a microwave - photonic phase discriminator, which is used to obtain the phase - difference signal between the single - frequency signal and the input optical pulse; a servo system, which is used to perform feedback control on the voltage - controlled oscillator with this phase - difference signal as a feedback signal, thereby realizing the carrier synchronization of the binary phase - coded signal.

5. The carrier synchronization device for binary phase - coded signals based on microwave - photonic phase discrimination according to claim 4, characterized in that, the microwave - photonic phase discriminator includes: a power splitter, which is used to divide the single - frequency signal into two paths; a dual - parallel dual - drive Mach - Zehnder modulator, which is used to modulate the input optical pulse with the two paths of single - frequency signals divided by the power splitter as the driving signals of its two sub - modulators. Both of its sub - modulators operate in a push - pull mode and the bias phases are 0 and 90° respectively; a low - speed photodetector, which is used to perform beat - frequency detection on the output modulated optical signal of the dual - parallel dual - drive Mach - Zehnder modulator and output the phase - difference signal.

6. The carrier synchronization device for binary phase - coded signals based on microwave - photonic phase discrimination according to claim 4, characterized in that, the servo system is a PI controller.

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