A low phase noise arbitrary waveform generation system based on injection-locked optoelectronic oscillator

Abstract

This invention discloses a low-phase-noise arbitrary waveform generation system based on an injection-locked optoelectronic oscillator. It utilizes a dual-polarization Mach-Zehnder modulator to simultaneously realize the injection-locked optoelectronic oscillator and optical domain frequency quadruple, enabling simultaneous microwave signal generation and signal frequency multiplication. The frequency bandwidth of the generated signal is not limited by the injected signal, thus realizing the generation of ultra-wideband signals.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic information technology, specifically relating to a low phase noise arbitrary waveform generation system based on an injection-locked optoelectronic oscillator. Background Technology

[0002] With the continuous development of modern wireless communication and radar systems, arbitrary waveform generation technology has become an indispensable key technology in various fields. Traditional arbitrary waveform generation relies on electrical devices, which, due to limitations, often result in signals with low frequency, small bandwidth, and high phase noise. Photonics technology, with its inherent characteristics of high bandwidth, low transmission loss, and resistance to electromagnetic interference, can effectively solve electronic bottleneck problems. Among these technologies, the optoelectronic oscillator (OEO), as an important optically generated microwave system, can generate high-frequency, low-phase-noise microwave signals by using long-distance, low-loss optical fibers as energy storage elements. However, traditional OEOs, due to mode competition effects, cannot achieve stable multi-mode oscillation and can only generate single-frequency signals. Therefore, controllably adjusting the oscillation mode of the OEO to achieve stable oscillation of different output modes is crucial for generating arbitrary waveforms using OEOs.

[0003] Currently, optoelectronic oscillators based on Fourier domain mode-locking (OEO) and active mode-locking (AMO) technologies can generate linear frequency modulated (LFM) signals and pulse signals, respectively. Fourier domain mode-locked OEOs utilize a fast-sweeping microwave photonic filter. By setting the filter's sweep period to match the cavity's ring time, Fourier domain mode-locking is achieved, allowing all modes within the sweep range to oscillate stably within the cavity simultaneously. This overcomes the limitations of traditional OEO mode setup time, enabling the generation of LFM signals. However, due to the nonlinear response of the laser sweep, the imperfect match between the filter sweep period and the loop delay, and the susceptibility of optical fibers to temperature fluctuations, the output signal exhibits poor linearity and frequency stability. Active mode-locked OEOs introduce an external periodic signal through an electro-optic modulator, achieving active modulation of the light wave. This establishes a fixed phase relationship between multiple longitudinal modes within the cavity, generating periodic pulse trains with an equally spaced comb-like spectrum. However, the output signal suffers from poor power flatness, and overmode noise exists during harmonic mode-locking, affecting the signal-to-noise ratio and stability.

[0004] To achieve low phase noise, optoelectronic oscillators typically use long optical fibers to improve the loop Q value, resulting in small oscillation mode spacing, increasing the difficulty of single-mode selection, and leading to the emergence of side modes. Injection-locking technology has been introduced into optoelectronic oscillators to improve their side-mode rejection ratio. When an external radio frequency signal is injected, longitudinal modes with frequencies close to the injected signal are locked to it. The addition of injected signal energy enhances the competitiveness of the locked oscillation modes, causing the optoelectronic oscillator to ultimately select that mode for oscillation, while other modes are suppressed. In addition to generating single-frequency signals, researchers have proposed using injection-locking optoelectronic oscillators to generate linear frequency modulated (LFM) signals [Liu M, Liu S, Zhu N, et al. Low phase noise wideband LFM signal generation by injection-locking anoptoelectronic oscillator[C] / / Optoelectronics and Communications Conference. Optica Publishing Group, 2021: JS3D.5.]. This scheme achieves broadband signal injection locking by setting the period of the injected broadband signal to match the OEO loop delay and adjusting the phase shifter in the loop. However, this scheme produces a single type of signal, making it difficult to generate linear frequency modulated pulse signals, and the frequency bandwidth of the output signal is limited by the externally injected signal.

[0005] In summary, achieving arbitrary waveform generation technology with low phase noise, large bandwidth, and high stability remains a significant challenge. Summary of the Invention

[0006] In view of this, the present invention provides a low phase noise arbitrary waveform generation system based on an injection-locked optoelectronic oscillator, which can generate arbitrary waveform signals and has the advantages of low phase noise and high stability.

[0007] The technical solution for implementing the present invention is as follows:

[0008] A low-phase-noise arbitrary waveform generation system based on an injection-locked opto-oscillator includes an injection-locked opto-oscillator and an optical domain fourth harmonic generation system.

[0009] The injection-locked opto-oscillator consists of a laser (LD), polarization controllers (PC1 and PC2), an x-MZM in a dual-polarization Mach-Zehnder modulator (DP-MZM), a polarization beam splitter (PBS), an erbium-doped fiber amplifier (EDFA1), a non-zero dispersion-shifted fiber (NZ-DSF), a photodetector (PD1), an electrical phase shifter, an electrical amplifier (EA), a power divider (EC1 and EC2), an arbitrary waveform generator (AWG), and an arbitrary function generator (AFG). The single-frequency optical carrier output from the laser is input to the optical input interface of the DP-MZM through PC1. The DP-MZM includes two dual-drive MZMs (x-MZM and y-MZM). At the output of the DP-MZM, the modulated optical signals from the two dual-drive MZMs are orthogonally polarized and are separated using PC2 and PBS. The bias voltage of the x-MZM is adjusted using the signal output from the AFG to control its operating point and adjust the loop gain. The optical signal modulated by x-MZM is amplified by EDFA1 and transmitted to PD1 via NZ-DSF to complete photoelectric conversion. The output RF signal is amplified by EA through an electrical phase shifter and split into two paths by EC1. One path is input to the RF input terminal of y-MZM, and the other path is combined with the signal output by AWG through EC2 and fed back to one of the RF input terminals of x-MZM, forming an OEO loop.

[0010] The optical domain fourth harmonic is used to achieve a fourth harmonic output of microwave signals.

[0011] Furthermore, the optical domain fourth harmonic generation includes a laser (LD), polarization controllers (PC1 and PC2), a y-MZM in a dual-polarization Mach-Zehnder modulator (DP-MZM), a polarization beam splitter (PBS), a phase-shifted Bragg grating (PS-FBG), an erbium-doped fiber amplifier (EDFA2), and a photodetector (PD2). The single-frequency optical carrier output from the laser is input to the optical input interface of the DP-MZM via PC1. A separate RF signal from EC1 is input to the RF input of the y-MZM. The bias voltage of the y-MZM is adjusted to operate at its maximum transmission point, thereby outputting the optical carrier and ±2nd-order sidebands. By adjusting the center wavelength of the PS-FBG, the optical carrier is filtered out using the notch of its reflection spectrum. The retained ±2nd-order sidebands are amplified by EDFA2 and then photoelectrically converted by PD2 to achieve the fourth harmonic output of the microwave signal.

[0012] Beneficial effects:

[0013] This invention utilizes a dual-polarization Mach-Zehnder modulator to simultaneously achieve an injection-locked optoelectronic oscillator and a fourth-harmonic generation in the optical domain. By changing the type of the injected signal, it is possible to generate arbitrary waveform signals with high-frequency broadband characteristics, and the phase noise performance is superior to that of the injected signal.

[0014] This invention controls the bias voltage of the modulator using a square wave to regulate the loop gain, enabling not only the output of continuous signals but also the generation of pulse signals. Attached Figure Description

[0015] Figure 1 The block diagram of the arbitrary waveform generation system based on the injection-locked optoelectronic oscillator provided by the present invention is shown.

[0016] Among them, 1-laser, 2-polarization controller PC1, 3-dual polarization Mach-Zehnder modulator DP-MZM, 4-polarization controller PC2, 5-polarization beam splitter PBS, 6-erbium-doped fiber amplifier EDFA1, 7-non-zero dispersion shifted fiber NZ-DSF, 8-photodetector PD1, 9-electric phase shifter, 10-electric amplifier EA, 11-power divider EC1, 12-power divider EC2, 13-arbitrary waveform generator AWG, 14-arbitrary function generator AFG, 15-phase shift Bragg grating PS-FBG, 16-erbium-doped fiber amplifier EDFA2, 17-photodetector PD2.

[0017] Figure 2 This is a schematic diagram of broadband signal injection locking in this invention; wherein Figure 2 (a) and (b) indicate that the frequency interval between the injected signal frequency component and the adjacent OEO oscillation mode exceeds and approaches the lockout range, respectively. Figure 2 (c) represents the signal output after the injection lock is implemented.

[0018] Figure 3 This is a schematic diagram showing the loop gain and output signal during continuous and pulse signal generation in this invention; wherein... Figure 3 (a) and (b) are schematic diagrams of the open-loop gain and corresponding output signal of the OEO when a continuous signal is generated, respectively. Figure 3 (c) and (d) are schematic diagrams of the open-loop gain of OEO and the corresponding output signal when the pulse signal is generated.

[0019] Figure 4 A comparison diagram of the phase noise of the output signal and the injected signal provided for this invention.

[0020] Figure 5 The resulting diagrams for generating different waveform signals provided by this invention; wherein Figure 5 (a) and (c) are the spectrum diagrams of the 5-8 GHz LFM signal generated based on injection-locked OEO and the 20-32 GHz LFM signal generated after the corresponding fourfold frequency harmonics, respectively. Figure 5 (b) and (d) are time-frequency diagrams obtained by performing short-time Fourier transform analysis on the time-domain waveforms of the corresponding LFM signals, respectively. Figure 5 (e) and (f) are the spectrum and time-frequency diagram of the 5-8 GHz pulse LFM signal, respectively; Figure 5 (g) is the time-domain plot of the generated phase-coded signal. Figure 5 (h) shows the phase information recovered from the corresponding phase-encoded signal within the first 0.2 μs. Detailed Implementation

[0021] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] This invention proposes a high-bandwidth, low-phase-noise arbitrary waveform generation system based on an injection-locked optoelectronic oscillator, such as... Figure 1 As shown. The single-frequency optical carrier output from the laser (1) is input to the optical input interface of the DP-MZM (3) through PC1 (2). The DP-MZM (3) includes two dual-drive MZMs (x-MZM and y-MZM). At the output of the DP-MZM, the modulated optical signals from the two dual-drive MZMs are polarized orthogonally. When the control PC2 (4) aligns the polarization direction of the optical signal with the main axis of the PBS (5), the two modulated optical signals leave with a high extinction ratio. The optical signal modulated by the x-MZM is amplified by EDFA1 (6) and transmitted to PD1 (8) through a 4km NZ-DSF (7) to complete the photoelectric conversion. The output RF signal passes through an electrical phase shifter (9). The electrical phase shifter (9) fine-tunes the loop delay to ensure that the injected signal and oscillation mode are matched. The signal is then amplified by EA (10) and split into two paths by EC1 (11). One path is combined with the RF signal output by AWG (13) through EC2 (12) and fed back to one RF input of x-MZM to form an OEO loop. AWG (13) provides different types of injection signals, including envelope single-frequency signals, linear frequency modulation signals, frequency hopping signals, linear frequency modulation pulse signals, phase-coded signals, etc. When injecting broadband signals, the signal period should be set to be consistent with the loop delay, which is 19.9232μs. The DC signal output by AFG (14) is injected into another RF input of x-MZM. Changing the voltage of the output DC signal can control the working state of x-MZM and realize the generation of continuous signals or pulse signals. The other path is input to the RF input of y-MZM. Adjusting the bias voltage of y-MZM makes it work at the maximum transmission point, thereby outputting optical carrier and ±2nd order sidebands. By adjusting the center wavelength of PS-FBG (15), the optical carrier is filtered out by the notch of its reflection spectrum. The retained ±2nd order sidebands are amplified by EDFA2 (16) and then photoelectric conversion is completed by PD2 (17) to achieve the quadruple frequency output of the injected signal.

[0023] Figure 2 A schematic diagram of broadband signal injection locking is given, in which... Figure 2 (a) and (b) indicate that the frequency interval between the injected signal frequency component and the adjacent OEO oscillation mode exceeds and approaches the lockout range, respectively. Figure 2(c) represents the signal output after injection locking is achieved. To generate a wideband signal, the signal period output by AWG(13) needs to be set to match the OEO loop delay, meaning the frequency interval of the injected LFM signal should be equal to the longitudinal mode spacing of the OEO. For example... Figure 2 As shown in (a), the frequency component of the injected signal does not match the OEO oscillation mode, and injection lock is not achieved. By adjusting the electrical phase shifter (11) in the loop, the loop delay is finely adjusted so that the OEO oscillation mode is close to the injected signal, as shown in (a). Figure 2 As shown in (b), when the value is less than the injection locking range, injection locking is achieved. Due to frequency pull, the frequency of the signal output by the injection locking OEO is consistent with the frequency of the injection signal, as shown in (b). Figure 2 As shown in (c).

[0024] Figure 3 The diagrams showing the loop gain and output signal during continuous and pulse signal generation are provided. Figure 3 (a) and (b) are schematic diagrams of the open-loop gain and corresponding output signal of the OEO when a continuous signal is generated, respectively. Figure 3 (c) and (d) are schematic diagrams of the open-loop gain of the OEO and the corresponding output signal when the pulse signal is generated, respectively. When the output voltage of AFG(14) is a stable DC signal, the open-loop gain of the OEO loop remains unchanged, and the output signal is a continuous signal, such as... Figure 3 As shown in (a) and (b). At this time, the continuous signal output by AWG(13) can be injected into OEO to achieve continuous signal injection lock. Since OEO will oscillate freely without an injection signal, when the pulse signal output by AWG(13) is injected into OEO, the output of the OEO under injection lock is not a pulse signal. Therefore, in order to achieve the generation of pulse signal, the bias voltage of x-MZM is controlled by the square wave signal output by AFG(14) to change the operating point and achieve open-loop gain control of the loop. The square wave signal should be set to be synchronized with the period of the injected signal. During the time when there is signal injection, the square wave is at a high level. At this time, the loss of x-MZM is small and the open-loop gain is high, which can achieve the locking of the injected signal; during the time when there is no signal injection, the square wave is at a low level. At this time, the loss of x-MZM is large and the open-loop gain is low. OEO cannot oscillate freely. At this time, OEO has no output signal, thus realizing the generation of pulse signal, such as Figure 3 As shown in (c) and (d).

[0025] In the experiment, the phase noise of the single-frequency signal output by AWG(13) and the single-frequency signal output by the injected locked OEO were tested by a phase noise analyzer. Figure 4A comparison of the phase noise of the output signal and the injected signal at a frequency of 5 GHz is given. The dashed line represents the phase noise curve of the injected signal, and the solid line represents the phase noise curve of the output signal. The phase noise at a frequency offset of 10 kHz is -101.46 dBc / Hz and -123.18 dBc / Hz, respectively. After injection locking, the phase noise is improved by more than 20 dB. Figure 5 The results of generating different waveform signals are shown in the figure. Figure 5 (a) and (c) are the spectrum diagrams of the 5-8 GHz LFM signal generated based on injection-locked OEO and the 20-32 GHz LFM signal generated after the corresponding fourfold frequency harmonics, respectively. Figure 5 (b) and (d) are time-frequency diagrams obtained by performing short-time Fourier transform analysis on the time-domain waveforms of the corresponding LFM signals, respectively. Figure 5 (e) and (f) are the spectrum and time-frequency diagram of the 5-8 GHz pulse LFM signal, respectively; Figure 5 (g) is the time-domain plot of the generated phase-coded signal, which transmits 3900 bits within one period (19.9232 μs). Figure 5 (h) shows the phase information recovered from the corresponding phase-encoded signal within the first 0.2 μs.

[0026] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-phase-noise arbitrary waveform generation system based on an injection-locked optoelectronic oscillator, characterized in that, This includes injection-locked photoelectric oscillators and optical domain fourth harmonics; The injection-locked opto-oscillator consists of a laser, polarization controllers PC1 and PC2, and a dual-polarization Mach-Zehnder modulator DP-MZM. x The DP-MZM consists of a laser beam splitter (PBS), an erbium-doped fiber amplifier (EDFA1), a non-zero dispersion-shifted fiber (NZ-DSF), a photodetector (PD1), an electrical phase shifter (EA), an electrical amplifier (EA), power dividers (EC1 and EC2), an arbitrary waveform generator (AWG), and an arbitrary function generator (AFG). The single-frequency optical carrier output from the laser is input to the optical input interface of the DP-MZM via PC1. The DP-MZM includes two dual-drive MZMs. At the output of the DP-MZM, the modulated optical signals from the two dual-drive MZMs are orthogonally polarized and are separated using PC2 and PBS. Adjusting the signal output by AFG x - The bias voltage of the MZM controls its operating point and adjusts the loop gain; after x The MZM modulated optical signal is amplified by EDFA1 and transmitted to PD1 via NZ-DSF to complete photoelectric conversion; the output RF signal is passed through an electrical phase shifter, amplified by EA, and split into two paths by EC1, one path being input to... y -The MZM's RF input terminal, and another channel, combined with the AWG output signal via EC2, are fed back. x -One of the RF input terminals of the MZM forms the OEO loop; The optical domain fourth harmonic is used to achieve a fourth harmonic output of microwave signals; the optical domain fourth harmonic includes a laser, polarization controllers PC1 and PC2, and a dual-polarization Mach-Zehnder modulator DP-MZM. y The system consists of a DP-MZM, a polarization beam splitter (PBS), a phase-shifted Bragg grating (PS-FBG), an erbium-doped fiber amplifier (EDFA2), and a photodetector (PD2). The single-frequency optical carrier output from the laser is input to the optical input interface of the DP-MZM via PC1, and a radio frequency signal is output via EC1 to... y -MZM's RF input terminal, adjust y The bias voltage of the -MZM allows it to operate at the maximum transmission point, thereby outputting an optical carrier and ±2nd order sidebands. By adjusting the center wavelength of the PS-FBG, the optical carrier is filtered out using the notch of its reflection spectrum. The retained ±2nd order sidebands are amplified by EDFA2 and then photoelectrically converted by PD2 to achieve a fourth-harmonic output of the microwave signal.

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