A tunable microwave pulse generation system and method based on injection locking

Abstract

The application relates to a tunable microwave pulse generation system and method based on injection locking, and belongs to the technical field of microwave photon signal generation and processing. The system comprises a laser, a polarization controller, a closed-loop optoelectronic oscillator (OEO) loop and two microwave sources; wherein the closed-loop optoelectronic oscillator (OEO) loop is composed of a double-parallel Mach-Zehnder modulator, a single-mode optical fiber, an optical tunable delay line (OTDL), an optical detector, a broadband band-pass filter, an electric amplifier, a power divider and a coupler which are connected in sequence.

Description

TECHNICAL FIELD

[0001] The application relates to a tunable microwave pulse generation system and method based on injection locking, and belongs to the technical field of microwave photon signal generation and processing. BACKGROUND

[0002] The detection system and detection signal (radar, fuze, etc.) generation and processing technology has a wide range of applications in national defense and military. The detection system is mainly used for signal detection, early warning, interception and tracking, etc. Common detection signals include linear frequency modulation signals, pulse signals and phase-coded signals. Nowadays, the generation of detection signals mainly uses electronic methods, that is, electronic devices are used to directly generate detection signals. However, with the increasing requirements of detection systems on detection accuracy, detection imaging and other aspects, the requirements on the frequency of detection signals and low phase noise are also increasing. Microwave photon technology with ultra-large output bandwidth and ultra-low phase noise has gradually become a hot research direction of detection systems. With the development of microwave photonics, the detection system based on microwave photon method has attracted widespread attention. The microwave photon detection system combining microwave photon technology and the detection system can realize the generation and processing of large-bandwidth, high-frequency-band detection signals, achieve high-precision detection, and due to the characteristics of optical mixing of microwave photon technology, the influence of strong electromagnetic interference on the detection performance can be reduced, and the accuracy of the detection system can be improved.

[0003] Optoelectronic oscillators are representative technologies of microwave photonics. Low phase noise and high frequency stability microwave signals can be generated by using optoelectronic oscillators, which have been widely used in radar, sensing, communication and terahertz generation fields. In the generation of detection signals, phase-coded signal generation methods based on optoelectronic oscillators are constantly being proposed. In 2013, a team from the University of Ottawa in Canada first realized a phase-coded signal generation system based on an optoelectronic oscillator. The polarization state of the optical signal was used for phase selection, and a phase-coded signal that can be arbitrarily switched between 0, π / 2, π / 2 and 3π / 2 was realized.

[0004] In recent years, various research units have also proposed linear frequency modulation wave generation methods based on Fourier domain mode locking. In 2018, the Institute of Semiconductors of the Chinese Academy of Sciences first proposed using the delay characteristics of an optoelectronic oscillator to break through the limitation of the single-mode build-up time in a traditional optoelectronic oscillator. A fast-sweeping microwave photon filter was constructed in a ring cavity by using a sweeping light and an optical bandpass filter. When the period of the sweeping light meets the Fourier domain mode locking condition, the optoelectronic oscillator can output a high-quality, low-phase-noise linear frequency modulation signal. Subsequently, other institutions have also proposed double-chirp signal generation, multi-band chirp signal generation and injection locking chirp signal generation based on Fourier domain mode locking optoelectronic oscillators.

[0005] In 2020, a team from the University of Electronic Science and Technology of China proposed an actively mode-locked optoelectronic oscillator, which uses an injection of an actively mode-locked signal to periodically modulate the gain of the optoelectronic oscillator loop, thereby generating microwave frequency comb signals and microwave pulse signals. In recent years, research units have also designed actively mode-locked optoelectronic oscillators with multiple center frequencies, tunable actively mode-locked optoelectronic oscillators, and actively mode-locked optoelectronic oscillators with ultra-low phase noise.

[0006] The actively mode-locked optoelectronic oscillator uses an external actively mode-locked signal to modulate the optoelectronic oscillator loop, thereby controlling the loop gain. Due to the output characteristics of the optoelectronic oscillator, when the loop gain is less than a certain threshold, no signal is output. Therefore, after the loop gain is controlled, the output signal of the actively mode-locked optoelectronic oscillator is in the form of a pulse sequence. Due to the starting characteristics of the optoelectronic oscillator, the phase relationship between the mode components of the pulse signal is constant, so in the frequency domain, it is a coherent microwave frequency comb signal with fixed intervals, and in the time domain, it is a pulse signal after coherent superposition, which is suitable for detection systems.

[0007] Existing microwave pulse generation systems can be divided into two categories: one is electronic method, and the other is microwave photonics method. Microwave pulse generation systems based on electronic method are limited by electronic bottleneck, and have poor phase noise performance in high frequency signal generation. Therefore, microwave photonics method with high frequency and low phase noise is proposed. The core of the low phase noise microwave pulse generation system based on microwave photonics method is the actively mode-locked optoelectronic oscillator. However, the tuning range of the actively mode-locked optoelectronic oscillator is limited by the filter. Therefore, the present application proposes a tunable microwave pulse generation system based on injection locking, which can control the carrier frequency of the output microwave pulse by adjusting the frequency of the injection signal. SUMMARY

[0008] Therefore, the present application designs a tunable microwave pulse generation system based on injection locking, which uses an external microwave source to excite the optoelectronic oscillator loop to form stable oscillation, and injects an actively mode-locked signal to realize actively mode-locked modulation of the optoelectronic oscillator. The output frequency of the system can be tuned by adjusting the output frequency of the external microwave source.

[0009] The technical scheme of the present application is as follows:

[0010] In a first aspect, the present application provides a tunable microwave pulse generation system based on injection locking, which comprises a laser, a polarization controller, a closed-loop optoelectronic oscillator (OEO) loop, and two microwave sources. The closed-loop optoelectronic oscillator (OEO) loop is composed of a double parallel Mach-Zehnder modulator, a single-mode optical fiber, an optical tunable delay line (OTDL), an optical detector, a wideband bandpass filter, an electrical amplifier, a power divider, and a coupler connected in sequence.

[0011] laser: for generating a single frequency optical signal, and transmitting to the polarization controller;

[0012] polarization controller: for changing the polarization state of the laser output optical signal, so that the polarization state of the optical signal is aligned with the principal axis of the dual parallel Mach-Zehnder modulator;

[0013] dual parallel Mach-Zehnder modulator: comprising a main MZM, a sub-MZM1 and a sub-MZM2, the sub-MZM1 is used to close the OEO loop and receive the feedback signal from the closed OEO loop, the sub-MZM2 is used to receive the active mode-locked signal output by the second microwave source, and the main MZM is used to control the relative phase difference between the two sub-MZMs;

[0014] single mode fiber: for transmitting the optical signal output by the dual parallel Mach-Zehnder modulator to the optical tunable delay line.

[0015] optical tunable delay line (OTDL): for fine-tuning the delay of the loop signal, so that the signal injected by the first microwave source is transmitted to the photodetector after the phase matching of the loop is completed;

[0016] photodetector: for detecting the optical signal to generate an electrical signal;

[0017] wideband bandpass filter: for filtering the spurious noise of the electrical signal;

[0018] electrical amplifier: for amplifying the electrical signal output by the photodetector;

[0019] power divider: for distributing the loop signal, part of which enters the coupler, and part of which is output as an output port;

[0020] coupler: for combining the first microwave source injection locking signal with the power divider output signal, and transmitting the combined signal to the sub-MZM1 through the radio frequency input port.

[0021] Further, the sub-MZM1 in the application works at the minimum transmission point, the sub-MZM2 works at the maximum transmission point, and the main MZM works at the maximum transmission point.

[0022] Further, the phase matching condition in the application is ωτ=2nπ, where ω is the angular frequency of the oscillation signal in the system, τ is the time delay of the loop, and n is a positive integer.

[0023] Further, the electrical amplifier in the application is used for amplifying the electrical signal, so that the OEO loop gain is greater than the set output threshold.

[0024] Further, the frequency of the output signal of the second microwave source is equal to an integer multiple of the mode interval, the mode interval being the frequency interval of adjacent frequency components ω satisfying the condition ωτ=2nπ.

[0025] Further, the gain of the required power divider output signal is G S , and the active mode-locked signal injected by the second microwave source is calculated according to the following formula:

[0026]

[0027] wherein V ph is the optical voltage, V π is the half-wave voltage of the dual parallel Mach-Zehnder modulator, and m(t) is the injected active mode-locked signal.

[0028] In a second aspect, the embodiment of the application is a tunable microwave pulse generation method based on injection locking, and the specific process is as follows:

[0029] (1) A single-frequency signal is output by a laser, and the output wavelength is within the working wavelength of the dual parallel Mach-Zehnder modulator; after the output optical signal enters a polarization controller, the polarization state is controlled to be parallel to the principal axis of the dual parallel Mach-Zehnder modulator;

[0030] (2) The first microwave source is turned on, and a radio frequency signal is injected into the system through the dual parallel Mach-Zehnder modulator, and the frequency of the radio frequency signal is within the range of the wideband bandpass filter;

[0031] (3) The optical tunable delay line OTDL is adjusted to match the phase condition of the injected signal and the OEO loop; the frequency spectrum of the power divider output signal is observed by using an observation instrument, and each mode component is observed to calculate the mode interval, which is the frequency interval of adjacent frequency components satisfying the condition ωτ=2nπ;

[0032] (4) The second microwave source is set, and the frequency of the output signal is equal to an integer multiple of the mode interval obtained in step (3), and the second microwave source outputs an active mode-locked signal; the tunable microwave pulse is output at the output port of the power divider.

[0033] Advantages:

[0034] The application provides a tunable microwave pulse generation system based on injection locking, which realizes the closure of an optoelectronic oscillator loop and active mode locking modulation by using double parallel Mach-Zehnder modulators, realizes the tunable frequency of an output signal by using a first microwave source to injection lock a loop signal, and has low phase noise due to the phase noise characteristics of the output signal, which is related to the injection signal. BRIEF DESCRIPTION OF DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the application, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0036] Figure 1 FIG. 1 is a system block diagram of the tunable microwave pulse generation system based on injection locking.

[0037] Figure 2 FIG. 2 is the working state of the double parallel Mach-Zehnder modulator in the system.

[0038] Figure 3 FIG. 3 is the frequency domain pattern of the output signal when only the intermediate frequency locking signal is injected in the system.

[0039] FIG. 4 is the frequency domain pattern of the output signal when only the intermediate frequency locking signal is injected in the system (enlarged version). Figure 4a the carrier frequency of the signal is 4.5 GHz, Figure 4b the carrier frequency of the signal is 5.5 GHz.

[0040] FIG. 5 is the frequency domain pattern of the output signal after the intermediate frequency injection locking signal and the active mode locking signal are simultaneously injected in the system (multiple frequencies), Figure 5a the carrier frequency of the signal is 4.5 GHz, Figure 5b the carrier frequency of the signal is 5.5 GHz.

[0041] FIG. 6 is the time domain spectrum of the output signal of the system, Figure 6a the carrier frequency of the signal is 4.5 GHz, Figure 6b the carrier frequency of the signal is 5.5 GHz.

[0042] Figure 7 FIG. 7 is the phase noise curve of the output signal of the system. DETAILED DESCRIPTION

[0043] The embodiments of the application will be described in detail below with reference to the drawings.

[0044] It should be noted that the following embodiments and features of the embodiments can be combined with each other without conflict, and all other embodiments obtained by those skilled in the art based on the embodiments in the present disclosure without creative labor are within the scope of protection of the present disclosure.

[0045] It should be noted that various aspects of the embodiments described below are within the scope of the appended claims. It should be apparent that the aspects described herein can be embodied in a wide variety of forms and that any specific structure and / or function described herein is merely illustrative. Based on the teachings herein one skilled in the art should appreciate that an aspect described herein can be implemented independently of any other aspects and that an aspect described herein can be implemented both as any number of software and / or hardware structures. For example, an aspect can be implemented as a single structure or any number of software modules or hardware structures, and the aspects described herein can be combined in any number of ways. In addition, one skilled in the art should appreciate that an aspect can comprise any number of claims.

[0046] As shown in Figure 1 The present application proposes a tunable microwave pulse generation system based on injection locking, which comprises a laser, a polarization controller, a double parallel Mach-Zehnder modulator, a single-mode fiber, an optical tunable delay line (OTDL), a photodetector, a wideband bandpass filter, an electrical amplifier, a power divider, a coupler and two microwave sources. The double parallel Mach-Zehnder modulator, the single-mode fiber, the optical tunable delay line (OTDL), the photodetector, the wideband bandpass filter, the electrical amplifier, the power divider and the coupler are connected in sequence to form a closed-loop optoelectronic oscillator (OEO) loop, and the power divider is also used to output a signal.

[0047] The laser is used to generate a single-frequency optical signal, and the optical signal is transmitted to the polarization controller through an optical fiber;

[0048] The polarization controller is used to change the polarization state of the optical signal output by the laser, so that the polarization state of the optical signal is aligned with the principal axis of the double parallel Mach-Zehnder modulator, and at this time the double parallel Mach-Zehnder modulator has the highest modulation efficiency.

[0049] Dual parallel Mach-Zehnder modulator: used for modulating the optical signal output by the polarization controller. The dual parallel Mach-Zehnder modulator has two radio frequency input ports and three bias voltage control interfaces, which can be regarded as three Mach-Zehnder modulators (MZM), namely main MZM, sub MZM1 and sub MZM2. In the present application, the optical signal output by the polarization controller is divided into two paths and input into the sub MZM1 and the sub MZM2 respectively, the sub MZM1 is used to close the OEO loop and receives the feedback signal from the closed OEO loop through one radio frequency input port, the sub MZM2 is used to receive the active mode-locked signal output by the second microwave source 2 through the other radio frequency input port, and the main MZM is used to control the relative phase difference between the two sub MZMs, which works at the maximum transmission point to ensure the gain of the loop. Among them, the sub MZM1 works at the minimum transmission point, the sub MZM2 works at the maximum transmission point, and the main MZM works at the maximum transmission point, as shown in the principle Figure 2

[0050] Single-mode optical fiber: used for transmitting the optical signal output by the dual parallel Mach-Zehnder modulator to the optical tunable delay line. The higher the length of the optical fiber (within a certain range), the better the energy storage effect of the loop, and the higher the quality factor.

[0051] Optical tunable delay line (OTDL): used for fine-tuning the delay of the loop signal, so that the injected signal and the microwave photon loop of the system are phase matched. The phase matching condition is ωτ=2nπ, where ω is the angular frequency of the oscillation signal in the system, and τ is the time delay of the loop. Only the signal that satisfies this condition can start oscillation in the loop. By adjusting the loop time delay with the optical tunable delay line, the injected signal can reach the phase matching condition.

[0052] Photodetector: used for detecting the optical signal to generate an electrical signal.

[0053] Wideband bandpass filter: used for filtering the stray noise of the electrical signal. In the present application, the bandwidth of the bandpass filter determines the tuning range of the center frequency of the output microwave frequency comb.

[0054] Electric amplifier: used for amplifying the electrical signal output by the photodetector, so that the OEO (optoelectronic optical) loop gain is greater than the output threshold.

[0055] Power divider: used for distributing the loop signal, part of which enters the coupler, and part of which is output as an output port.

[0056] Coupler: used for injecting the locking signal from the first microwave source 1, combining with the output signal of the power divider, and transmitting the combined signal to the sub MZM1 through the radio frequency input port.

[0057] Two microwave sources: used for generating the injection locking signal and the active mode-locked signal respectively.​

[0058] The working process of the system is as follows: the laser emits a single-wavelength laser signal, which is subjected to electro-optical modulation in an electro-optical modulator (double parallel Mach-Zehnder modulator), and the output optical signal changes from single-wavelength to multi-wavelength; the output signal of the electro-optical modulator is subjected to beat frequency in a photodetector after transmission through a long optical fiber, and a microwave signal is obtained; the microwave signal is amplified in an electrical amplifier and filtered through a band-pass filter; the microwave signal is injected into the electro-optical modulator, and the process is repeated to continuously output the microwave signal. In the above steps, the output signal of the microwave source 1 is combined with the output signal at the output end of the electrical amplifier, and the output signal of the microwave source 2 is injected into one of the injection ports of the electro-optical modulator. Specifically:

[0059] (1) The laser outputs a single-frequency signal, and the output wavelength is within the operating wavelength of the double parallel Mach-Zehnder modulator; the output optical signal is subjected to polarization state control after entering the polarization controller, so that it is parallel to the principal axis of the double parallel Mach-Zehnder modulator, and in this state, the optical signal will obtain the maximum modulation efficiency in the double parallel Mach-Zehnder modulator.

[0060] (2) Turn on the microwave source 1, inject the radio frequency signal into the system through the double parallel Mach-Zehnder modulator, and the frequency of the radio frequency signal is within the range of the wideband band-pass filter.

[0061] (3) Adjust the optical tunable delay line OTDL so that the injection signal matches the phase condition of the OEO loop. At this time, the system outputs a stable single-frequency signal through injection locking. Use an observation instrument to observe the frequency spectrum of the output signal of the power divider, observe the mode components, and calculate the mode interval, which is the frequency interval of each frequency component that satisfies the condition ωτ=2πn. At this time, the output signal can be represented as V out =∑A k sin(2πf osc +kf c )t, where k is an integer, A k is the intensity of the kth component, f osc is the frequency component with the highest intensity, called the main mode, and f c is the mode interval. Therefore, the system outputs a multi-frequency signal at this time, and one of the frequencies has a higher intensity and the other frequencies have lower intensities.

[0062] (4) turn on the microwave source 2, set the frequency of the output signal to be an integer multiple of the mode interval obtained in step (3), connect the microwave source 2 and the radio frequency input port of the sub-MZM2 of the dual-drive parallel Mach-Zehnder modulator, and output the active mode-locked signal. wherein V ph is the optical voltage, V π is the half-wave voltage of the modulator, and m(t) is the injected active mode-locked signal, which can be expressed as m(t) = A1cos(ω1t).

[0063] The filter in the application is a wideband bandpass filter, and thus the output frequency of the microwave source 1 can be adjusted to change the carrier frequency of the output microwave pulse after exciting oscillation at other frequencies.

[0064] According to the characteristics of the application, a physical experiment is designed to test the performance of the frequency-tunable microwave frequency pulse and low near-carrier end phase noise generated by the design, and the final result is shown in Figure 3 , Fig. 4, Fig. 5, Fig. 6 and Figure 7 The experimental parameters of this test are as follows: the output power of the tunable laser is 12 dBm, the modulation bandwidth of the dual parallel Mach-Zehnder modulator is 40 GHz, the length of the single-mode fiber is 1 km, the 3dB bandwidth of the photodetector is 31 GHz, the 3dB bandwidth of the electrical amplifier is 20 GHz, and the passband of the wideband bandpass filter is 2-4 GHz.

[0065] It can be obtained that, after injection locking by the microwave source 1, the output signal of the optoelectronic oscillator changes with the change of the injected signal, and the output signal has a side mode suppression ratio of about 50 dB. Figure 3 When the microwave source 2 is turned on and the active mode-locked signal is injected into the loop, the loop changes from single-mode oscillation to multi-mode oscillation, and the specific result is shown in Fig. 4; in terms of frequency spectrum, the output signal changes from single-frequency signal to microwave frequency comb signal, and changes with the change of the output frequency of the microwave source 1; the time-domain spectrum of the output signal is measured by using an oscilloscope, and the result is shown in Fig. 6, which shows that the output signal is a microwave pulse, and the carrier frequency changes when the output frequency of the microwave source 1 changes. Figure 7 The phase noise of the above output signal is measured, and the specific result is shown in . It can be known from the result that the phase noise has good performance at the near-carrier end.

[0066] The above merely provides the specific implementation of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of the changes or replacements within the technical range disclosed by the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A tunable microwave pulse generation system based on injection locking, characterized in that, include: The system comprises a laser, a polarization controller, a closed-loop opto-oscillator (OEO) loop, and two microwave sources. The OEO loop is composed of a dual parallel Mach-Zehnder modulator, a single-mode fiber, an optically tunable delay line (OTDL), a photodetector, a broadband bandpass filter, an electrical amplifier, a power divider, and a coupler connected in sequence. Laser: Used to generate single-frequency optical signals and transmit them to a polarization controller; Polarization controller: Used to change the polarization state of the laser output light signal so that the polarization state of the light signal is aligned with the principal axis of the dual parallel Mach-Zehnder modulator; Dual parallel Mach-Zehnder modulator: includes a main MZM, sub-MZM1 and sub-MZM2. Sub-MZM1 is used to close the OEO loop and receive the feedback signal from the closed OEO loop. Sub-MZM2 is used to receive the active mode-locked signal output from the second microwave source. The main MZM is used to control the relative phase difference between the two sub-MZMs. Single-mode fiber: used to transmit the optical signal output from the dual parallel Mach-Zehnder modulator to the optically tunable delay line; OTDL: Used to fine-tune the delay of the loop signal so that the signal injected by the first microwave source is phase-matched with the loop and then transmitted to the photodetector; Photodetector: Used to detect optical signals and generate electrical signals; Broadband bandpass filter: used to filter stray noise from electrical signals; Electrical amplifier: Used to amplify the electrical signal output by a photodetector; Power divider: Distributes the loop signal, with one part going into the coupler and the other part being output as the output port; Coupler: Used to combine the first microwave source injection lock signal with the power divider output signal, and transmit the combined signal to sub-MZM1 through the RF input port.

2. The tunable microwave pulse generation system based on injection locking according to claim 1, characterized in that, The sub-MZM1 operates at the minimum transmission point, the sub-MZM2 operates at the maximum transmission point, and the main MZM operates at the maximum transmission point.

3. The tunable microwave pulse generation system based on injection locking according to claim 1, characterized in that, The phase matching condition is ωτ=2nπ, where ω is the angular frequency of the oscillation signal in the system, τ is the loop delay, and n is a positive integer.

4. The tunable microwave pulse generation system based on injection locking according to claim 1, characterized in that, The electrical amplifier is used to amplify the electrical signal so that the OEO loop gain is greater than the set output threshold.

5. The tunable microwave pulse generation system based on injection locking according to claim 3, characterized in that, The frequency of the active mode-locked signal output by the second microwave source is equal to an integer multiple of the mode interval, wherein the mode interval is the frequency interval of each frequency component that satisfies the condition ωτ=2nπ.

6. The tunable microwave pulse generation system based on injection locking according to claim 1 or 5, characterized in that, Let the gain of the desired power divider output signal be G. S The active mode-locking signal required to be injected by the second microwave source is calculated according to the following formula; Among them, V ph V is the photovoltage. π t is the half-wave voltage of the dual parallel Mach-Zehnder modulator, and m(t) is the injected active mode-locking signal.

7. A pulse generation method based on the tunable microwave pulse generation system based on injection locking according to any one of claims 1-6, characterized in that, The specific process is as follows: (1) A single-frequency signal is output from a laser, and the output wavelength is within the working wavelength of the dual parallel Mach-Zehnder modulator. After the output optical signal enters the polarization controller, it is controlled by polarization state to make it parallel to the main axis of the dual parallel Mach-Zehnder modulator. (2) Turn on the first microwave source and inject radio frequency signals into the system through the dual parallel Mach-Zehnder modulator. The frequency of the radio frequency signals is within the range of the broadband bandpass filter. (3) Adjust the optically adjustable delay line OTDL to match the phase condition of the injected signal with the OEO loop; The spectrum of the power divider output signal is observed using observation instruments. Each mode component is observed, and the mode interval is calculated. The mode interval is the frequency interval of each frequency component that satisfies the condition ωτ=2nπ. (4) Set the second microwave source, and the frequency of the output signal is equal to an integer multiple of the mode interval obtained in step (3). The second microwave source outputs an active mode-locked signal; the power divider output port outputs a tunable microwave pulse.

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