Tunable microwave pulse generation system and method based on injection locking

Abstract

The invention 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), a photoelectric 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 invention relates to a tunable microwave pulse generating system and method based on injection locking, belonging to the technical field of microwave photon signal generation and processing. Background Art

[0002] Detection systems and detection signal (radar, fuze, etc.) generation and processing technology are widely used in national defense and military. Detection systems are mainly used for signal detection, early warning, interception and tracking. 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 improvement of detection system requirements for detection accuracy, detection imaging, etc., the frequency requirements and low phase noise requirements for detection signals are also getting higher and higher. Microwave photon technology with ultra-large output bandwidth and ultra-low phase noise has gradually become a hot topic in the research of detection systems. With the development of microwave photonics, detection systems based on microwave photon methods have attracted widespread attention. Microwave photon detection systems that combine microwave photon technology with detection systems can realize the generation and processing of large-bandwidth, high-frequency detection signals and achieve high-precision detection. In addition, due to the characteristics of optical domain mixing of microwave photon technology, the influence of strong electromagnetic interference on detection performance can be reduced, and the accuracy of the detection system can be improved.

[0003] Optoelectronic oscillators are the representative technology of microwave photonics. They can generate microwave signals with low phase noise and high frequency stability. They are now widely used in radar, sensing, communication, and terahertz generation. In terms of detection signal generation, phase-coded signal generation methods based on optoelectronic oscillators have been proposed. In 2013, a team from the University of Ottawa in Canada first realized a phase-coded signal generation system based on optoelectronic oscillators, using the polarization state of the optical signal for phase selection, and realized a phase-coded signal that can be switched arbitrarily between 0, π / 2, π / 2, and 3π / 2.

[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 to use the delay characteristics of the optoelectronic oscillator to break through the limitation of the single-mode establishment time in the traditional optoelectronic oscillator, and use swept light and optical band-stop filters to construct a fast-sweeping microwave photon filter in the ring cavity. When the swept light period meets the Fourier domain mode-locking conditions, the optoelectronic oscillator can output high-quality, low-phase noise linear frequency modulation signals; subsequently, other institutions also proposed dual-chirp signal generation, multi-band chirp signal generation, injection-locked chirp signal generation, etc. based on Fourier domain mode-locked optoelectronic oscillators.

[0005] In 2020, a team from Hangzhou Dianzi University proposed an actively mode-locked optoelectronic oscillator, which uses the injected active mode-locked signal to periodically modulate the gain of the optoelectronic oscillator loop to achieve the generation of microwave frequency comb signals and microwave pulse signals; in recent years, various research units have also successively designed multi-center frequency active mode-locked optoelectronic oscillators, tunable active mode-locked optoelectronic oscillators, and ultra-low phase noise active mode-locked optoelectronic oscillators.

[0006] The active mode-locked optoelectronic oscillator uses an external active mode-locked signal to modulate the optoelectronic oscillator loop, thereby adjusting the loop gain. Due to the output characteristics of the optoelectronic oscillator, no signal will be output when the loop gain is less than a certain threshold. Therefore, after adjusting the loop gain, the output signal of the active 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 various mode components of the pulse signal is constant, so it is a coherent microwave frequency comb signal with a fixed interval in the frequency domain, and a pulse signal after coherent superposition in the time domain, which is suitable for detection systems.

[0007] Existing microwave pulse generation systems can be divided into two categories, one is the electronic method and the other is the microwave photonic method. The microwave pulse generation system based on the electronic method is limited by the electronic bottleneck, and the phase noise performance is poor in the high-frequency range of signal generation. Phase noise is a key indicator of signal quality, so a microwave photonic method with high frequency and low phase noise is proposed. The core of the low-phase noise microwave pulse generation system based on the microwave photonic method is the active mode-locked optoelectronic oscillator, but the tuning range of the active mode-locked optoelectronic oscillator is limited by the filter. Therefore, the present invention 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 of the invention

[0008] In view of this, the present invention designs a tunable microwave pulse generation system based on injection locking, which utilizes an external microwave source to excite the optoelectronic oscillator loop to form a stable oscillation, and simultaneously injects an active mode-locking signal to realize active mode-locking modulation of the optoelectronic oscillator, and realizes the tunability of the output signal center frequency by adjusting the output frequency of the external microwave source.

[0009] The technical solution for implementing the present invention is as follows:

[0010] In a first aspect, an embodiment of the present application provides a tunable microwave pulse generation system based on injection locking, comprising: 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 dual parallel Mach-Zehnder modulator, a single-mode optical fiber, an optical tunable delay line (OTDL), a photodetector, a broadband bandpass filter, an electrical amplifier, a power divider, and a coupler connected in sequence;

[0011] Laser: used to generate single-frequency optical signals and transmit them to the polarization controller;

[0012] Polarization controller: used to change the polarization state of the laser output optical signal so that the polarization state of the optical signal is aligned with the main 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, wherein the sub-MZM1 is used to close the OEO loop and receive a feedback signal from the closed OEO loop, the sub-MZM2 is used to receive an active mode-locking signal output by a second microwave light source, and the main MZM is used to control the relative phase difference between the two sub-MZMs;

[0014] Single-mode optical fiber: used to transmit the optical signal output by the dual parallel Mach-Zehnder modulator to the optical adjustable delay line.

[0015] Optical tunable delay line (OTDL): used to fine-tune the delay of the loop signal so that the signal injected by the first microwave light source is transmitted to the photodetector after completing phase matching with the loop;

[0016] Photodetector: used to detect light signals and generate electrical signals;

[0017] Wideband bandpass filter: used to filter stray noise of electrical signals;

[0018] Electrical amplifier: used to amplify the electrical signal output by the photodetector;

[0019] Power divider: distributes the loop signal, one part enters the coupler, and the other part is used as the output port for signal output;

[0020] Coupler: used to combine the first microwave source injection locking signal with the power divider output signal, and transmit the combined signal to the sub-MZM1 through the RF input port.

[0021] Furthermore, the sub-MZM1 of the present invention 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] Furthermore, the phase matching condition of the present invention 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] Furthermore, the electrical amplifier of the present invention is used to amplify the electrical signal so that the OEO loop gain is greater than a set output threshold.

[0024] Furthermore, the frequency of the output signal of the second microwave source of the present invention is equal to an integer multiple of the mode interval, and the mode interval is the frequency interval of adjacent frequency components ω that satisfies the condition ωτ=2nπ.

[0025] Furthermore, the gain of the desired output signal is G S , the active mode-locking signal that the second microwave source needs to generate and inject is calculated according to the following formula;

[0026]

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

[0028] In a second aspect, the present application provides a method for generating tunable microwave pulses based on injection locking, and the specific process is as follows:

[0029] (1) using a laser to output a single-frequency signal, the output wavelength of which 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 so that the output optical signal is parallel to the main axis of the dual parallel Mach-Zehnder modulator;

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

[0031] (3) Adjust the optical tunable delay line OTDL so that the injected signal matches the phase condition of the OEO loop; use an observation instrument to observe the spectrum of the power divider output signal, observe each mode component, and calculate the mode spacing, which is the frequency spacing between adjacent frequency components that satisfies the condition ωτ=2nπ;

[0032] (4) Setting the second microwave source, 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; and the output port of the power divider outputs a tunable microwave pulse.

[0033] Beneficial effects:

[0034] The present invention proposes a tunable microwave pulse generation system based on injection locking, which uses a dual parallel Mach-Zehnder modulator to complete the closure of the optoelectronic oscillator loop and active mode-locking modulation, and uses a first microwave source to perform injection locking on the loop signal, thereby achieving tunable output signal frequency. Due to the phase noise characteristics, the output signal phase noise is related to the injection signal and has the characteristic of low phase noise. Due to the above characteristics, the present invention can generate a microwave frequency pulse signal with an adjustable center frequency and low phase noise near the carrier end of the output signal, that is, the output signal has the advantage of good phase noise performance near the carrier end. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

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

[0037] Figure 2 This is the working state of the dual parallel Mach-Zehnder modulator in this system.

[0038] Figure 3 This is the frequency domain graph of the signal output by this system when only the intermediate frequency locking signal is injected.

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

[0040] Figure 5 shows the frequency domain graph (multiple frequencies) of the output signal after the system simultaneously injects the intermediate frequency injection locking signal and the active mode locking signal. Figure 5a The carrier frequency is 4.5GHz, Figure 5b The carrier frequency is 5.5GHz.

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

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

[0043] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0044] It should be noted that the following embodiments and features in the embodiments may be combined with each other in the absence of conflict; and, based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making any creative work are within the scope of protection of the present disclosure.

[0045] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described herein may be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on the present disclosure, it should be understood by those skilled in the art that an aspect described herein may be implemented independently of any other aspect, and two or more of these aspects may be combined in various ways. For example, any number of aspects described herein may be used to implement the device and / or practice the method. In addition, other structures and / or functionalities other than one or more of the aspects described herein may be used to implement this device and / or practice this method.

[0046] like Figure 1 As shown, the present invention proposes a tunable microwave pulse generation system based on injection locking, which includes: a laser, a polarization controller, a dual parallel Mach-Zehnder modulator, a single-mode optical fiber, an optically adjustable delay line (OTDL), a photodetector, a broadband bandpass filter, an electrical amplifier, a power divider, a coupler and two microwave sources. The dual parallel Mach-Zehnder modulator, the single-mode optical fiber, the optically adjustable delay line (OTDL), the photodetector, the broadband 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] Laser: used to generate a single-frequency optical signal and transmit the optical signal to the polarization controller through an optical fiber;

[0048] Polarization controller: used to change the polarization state of the laser output optical signal so that the polarization state of the optical signal is aligned with the main axis of the dual parallel Mach-Zehnder modulator. At this time, the dual parallel Mach-Zehnder modulator has the highest modulation efficiency.

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

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

[0051] Optical tunable delay line (OTDL): used to fine-tune the delay of the loop signal so that the injected signal can achieve phase matching with the microwave photonic loop of this system. The phase matching condition is ωτ=2nπ, where ω is the angular frequency of the oscillating signal in the system and τ is the delay of the loop. Only signals that meet this condition can start oscillation in the loop. By adjusting the loop delay using an optical tunable delay line, the injected signal can achieve phase matching conditions.

[0052] Photodetector: used to detect light signals and generate electrical signals.

[0053] Broadband bandpass filter: used to filter stray noise of electrical signals. In the present invention, the bandwidth of the bandpass filter determines the tuning range of the center frequency of the output microwave frequency comb.

[0054] Electrical amplifier: used to amplify the electrical signal output by the photodetector so that the OEO (opto-electrical-optical) loop gain is greater than the output threshold.

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

[0056] Coupler: used to inject a locking signal into the first microwave source 1 to combine it with the output signal of the power divider, and transmit the combined signal to the sub-MZM1 through the RF input port.

[0057] Two microwave sources: used to generate injection locking signal and active mode locking signal respectively.

[0058] The working process of this system is as follows: the laser emits a single-wavelength laser signal, which is electro-optically modulated in an electro-optic modulator (dual parallel Mach-Zehnder modulator), and the output optical signal changes from a single wavelength to multiple wavelengths; the output signal of the electro-optic modulator is transmitted through a long optical fiber and then beat in a photodetector to obtain a microwave signal; the microwave signal is amplified in an electrical amplifier and filtered through a bandpass filter; the microwave signal is injected into the electro-optic modulator, and the process is repeated in a cycle to continuously output the microwave signal. In the above steps, the output signal of microwave source 1 is combined with the output signal at the output end of the electrical amplifier, and the output signal of microwave source 2 is injected into an injection port of the electro-optic modulator. Specifically:

[0059] (1) A single-frequency signal is outputted 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 the polarization controller, the polarization state is controlled so that it is parallel to the main axis of the dual parallel Mach-Zehnder modulator. In this state, the optical signal will obtain the maximum modulation efficiency in the dual parallel Mach-Zehnder modulator.

[0060] (2) Turn on the microwave source 1 and inject a radio frequency signal into the system through a dual parallel Mach-Zehnder modulator. The frequency of the radio frequency signal is within the range of the broadband bandpass filter.

[0061] (3) Adjust the optical adjustable delay line OTDL so that the injection signal matches the phase condition of the OEO loop. At this time, injection locking is used to make the system output a stable single-frequency signal. Use an observation instrument to observe the spectrum of the power divider output signal, observe each mode component, and calculate the mode interval. The mode interval is the frequency interval of each frequency component that satisfies the condition ωτ=2nπ. At this time, the output signal can be expressed as V out =ΣA k sin(2πf osc +kf c )t, where k is an integer, A k is the intensity of the kth component, f osc The frequency component with the highest intensity is called the main mode, f c is the mode interval. Therefore, the system outputs a signal in the form of multi-frequency output at this time, and the intensity of one frequency is larger, while the other frequencies are lower.

[0062] (4) Turn on microwave source 2, set the frequency of the output signal to be equal to an integer multiple of the mode interval obtained in step (3), connect microwave source 2 and the RF input port of sub-MZM2 of the dual-drive parallel Mach-Zehnder modulator, and output the active mode-locking signal. In this step, the intensity modulation of the system optical signal intensity is completed by injecting the active mode-locking signal, thereby changing the system's gain for the output signal. Since this system is a positive feedback system, there is no signal output when the gain is less than a certain threshold, so the output signal is a microwave pulse signal. At this time, the small signal gain can be expressed as Among them, V ph is the photovoltage, V π is the half-wave voltage of the modulator, and m(t) is the injected active mode-locking signal, which can be expressed as m(t)=A1cos(ω1t). Active mode-locking realizes the periodic adjustment of the gain, thereby realizing the pulsed output of the output signal. Since the output signal of microwave source 2 periodically modulates the system gain, the pulse repetition rate is consistent with the output frequency of microwave source 2.

[0063] The filter in the present invention is a broadband bandpass filter, so the carrier frequency of the output microwave pulse can be changed by adjusting the output frequency of the microwave source 1 and exciting oscillations of other frequencies.

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

[0065] Depend on Figure 3 It can be obtained that after injection locking using microwave source 1, the output signal of the optoelectronic oscillator changes with the injection signal, and at this time the output signal has a side mode suppression ratio of about 50dB. When microwave source 2 is turned on and an active mode-locked signal is injected into the loop, the loop changes from single-mode oscillation to multi-mode oscillation. The specific results are shown in Figure 4; in terms of spectrum performance, the output signal changes from a single-frequency signal to a microwave frequency comb signal, and changes with the output frequency of microwave source 1; the time domain spectrum of the output signal is measured using an oscilloscope, and the results are shown in Figure 6. The output signal is in the form of a microwave pulse, and the output frequency of microwave source 1 is changed, and it can be seen that the carrier frequency changes. The phase noise of the above output signal is measured, and the specific results are as follows. Figure 7 As shown in the figure, it can be seen from the results that it has better phase noise performance at the close-to-carrier end.

[0066] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall 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: A laser, a polarization controller, a closed-loop optoelectronic oscillator (OEO) loop and two microwave sources; wherein the OEO loop is composed of a dual parallel Mach-Zehnder modulator, a single-mode optical fiber, an optically adjustable 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 the polarization controller; Polarization controller: used to change the polarization state of the laser output optical signal so that the polarization state of the optical signal is aligned with the main axis of the dual parallel Mach-Zehnder modulator; Dual parallel Mach-Zehnder modulator: comprising a main MZM, a sub-MZM1 and a sub-MZM2, wherein the sub-MZM1 is used to close the OEO loop and receive a feedback signal from the closed OEO loop, the sub-MZM2 is used to receive an active mode-locking signal output by a second microwave light source, and the main MZM is used to control the relative phase difference between the two sub-MZMs; Single-mode optical fiber: used to transmit the optical signal output by the dual parallel Mach-Zehnder modulator to the optical adjustable delay line; OTDL: used to fine-tune the delay of the loop signal so that the signal injected by the first microwave light source is phase-matched with the loop and then transmitted to the photodetector; Photodetector: used to detect light signals and generate electrical signals; Wideband bandpass filter: used to filter stray noise of electrical signals; Electrical amplifier: used to amplify the electrical signal output by the photodetector; Power divider: distributes the loop signal, one part enters the coupler, and the other part is used as the output port for signal output; Coupler: used to combine the first microwave source injection locking signal with the power divider output signal, and transmit the combined signal to the 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 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.

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 time delay of the loop, 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 a 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-locking signal output by the second microwave source is equal to an integral multiple of the mode interval, and 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: The gain of the desired output signal is G S , the active mode-locking signal that the second microwave source needs to generate and inject is calculated according to the following formula; Among them, V ph is the photovoltage, V π 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 a tunable microwave pulse generation system based on injection locking according to any one of claims 1 to 6, characterized in that: The specific process is: (1) using a laser to output a single-frequency signal, the output wavelength of which 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 so that the output optical signal is parallel to the main axis of the dual parallel Mach-Zehnder modulator; (2) turning on the first microwave source and injecting a radio frequency signal into the system through a dual parallel Mach-Zehnder modulator, wherein the frequency of the radio frequency signal is within the range of the broadband bandpass filter; (3) Adjust the optical tunable delay line OTDL so that the injected signal matches the phase condition of the OEO loop; Observe the spectrum of the output signal of the power divider using an observation instrument, observe each mode component, and calculate the mode interval, which is the frequency interval of each frequency component that satisfies the condition of ωτ=2nπ; (4) Setting the second microwave source, 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; and the output port of the power divider outputs a tunable microwave pulse.

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