A High-Stability Photogenerated Millimeter Wave System Based on Phase-Locked Loop

Abstract

This invention belongs to the field of optical communication technology, specifically relating to a high-stability optically generated millimeter-wave system based on a phase-locked loop (PLL), comprising a first laser, a second laser, an acousto-optic frequency shifter, a dense wavelength division multiplexer (DWDM), an inner-loop feedback control loop, and an outer-loop signal output branch. The inner-loop feedback control loop includes, in series, a phase modulator, an optical filter, a photodetector, a frequency divider, a frequency and phase detector, a loop filter, a voltage-controlled oscillator (VCO), and a first power amplifier, wherein the output of the first power amplifier is connected to the acousto-optic frequency shifter. The inner-loop feedback control loop also includes a cesium atomic clock, a microwave source, and a second power amplifier; wherein the cesium atomic clock is connected to the microwave source and the frequency and phase detector, and the output of the microwave source is connected to the phase modulator after passing through the second power amplifier.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, specifically relating to a high-stability optically generated millimeter-wave system based on a phase-locked loop. Background Technology

[0002] Millimeter waves refer to electromagnetic waves with wavelengths ranging from 1 to 10 mm and frequencies ranging from 30 to 300 GHz. Millimeter wave technology was initially applied in the military field, finding widespread use in short-range air defense, battlefield surveillance, missile guidance, fire control and tracking, high-resolution imaging, and IFF (Identification Friend or Foe) systems. With the advancement of human civilization, millimeter wave technology has also gradually been applied to civilian fields such as wireless mobile communication, bio-detection, and healthcare. Microwave oscillators based on traditional electronics often use vacuum tubes and solid-state power sources as basic units, utilizing multi-stage frequency doubling to obtain microwave signals. However, the performance of these units degrades sharply at frequencies above GHz, resulting in high-frequency microwaves with significant phase noise and low spectral purity—a phenomenon known as the electronic bottleneck. In contrast, photonic methods are not limited by this electronic bottleneck, can generate microwave signals with very high carrier frequencies, and offer advantages such as resistance to electromagnetic interference, lightweight design, and compact structure. Furthermore, millimeter-wave photonic generators based on photonics are naturally compatible with fiber optic transmission systems, requiring no additional electro-optical or photoelectric conversion equipment.

[0003] Several optically generated millimeter-wave schemes have been disclosed in the prior art:

[0004] (1) Millimeter wave generation technology based on optical modulation

[0005] Highly stable, low-phase-noise millimeter-wave signals can be generated through optical modulation. Figure 1 The diagram shows the basic structure and transmission curve of a Mach-Zehnder modulator (MZM), whose two interference arms are each composed of two optical phase modulators. Under the influence of an applied electric field, the phase of the optical signal transmitted in the two interference arms changes. Since the electric fields applied to the two branches are in opposite directions, the refractive indices and phase changes of the two branches are also opposite. Therefore, if a phase change of π / 2 is introduced in the upper branch, a phase change of -π / 2 is introduced in the lower branch, and the total phase change of the optical signal in both arms is π. After interference, this is converted into an output optical signal with modulated light intensity. By appropriately adjusting the DC bias point of the MZM, a frequency-doubled modulated signal can be generated. For example, under carrier suppression, an 18 GHz modulated signal can generate a 36 GHz millimeter-wave signal. The main advantage of the optically modulated millimeter-wave generation method is its simple structure; its disadvantage is that it is limited by the bandwidth of the microwave amplifier and modulator, making it difficult to generate higher frequency signals.

[0006] (2) Millimeter wave generation technology based on four-wave mixing effect

[0007] To overcome the bandwidth limitations of optical modulation generation methods, researchers have proposed a millimeter-wave generation technique based on four-wave mixing (FWM). In this approach, two phase-locked optical carriers, f1 and f2, generated by MZM are used as pump light inputs into a highly nonlinear fiber (HNLF). The four-wave mixing effect of the HNLF generates phase-locked optical carriers 2f1-f2 and 2f2-f1. Then, a millimeter-wave signal with a frequency six times that of the MZM driving signal can be obtained through heterodyne beat frequency. The drawbacks of this approach are: firstly, it requires high pump power and a sufficiently long HNLF, increasing system loss, complexity, and cost. Secondly, to avoid the effects of fiber-induced Brillouin scattering (SBS), the pump power cannot be too high, limiting the four-wave mixing efficiency.

[0008] (3) Millimeter wave generation technology based on optical frequency comb

[0009] Extracting the heterodyne beat frequency of two optical carrier components from an OFC (Optical Frequency Comb) to generate a millimeter-wave signal is the most direct method. The system mainly consists of three parts: an optical frequency comb generator (OFCG), an optical filter, and a broadband photodetector. The OFCG generates a frequency spacing of f. comb The optical frequency signal is filtered out by an optical filter, which extracts two optical frequency components, and a millimeter-wave signal is generated by broadband photoelectric detection beat frequency.

[0010] (4) Millimeter wave generation technology based on optical phase-locked loop

[0011] The method of generating millimeter-wave signals using phase-locked laser heterodyne beat frequency offers advantages such as high frequency and power, and ease of tuning. The phase noise of the beat frequency-generated signal can be improved using an optical phase-locked loop (OPLL) and feedback techniques. It utilizes negative feedback to synchronize the frequency and phase of the slave laser with the master laser. When the slave and master lasers are out of phase, the phase difference is reflected in the phase of the beat frequency signal. The beat frequency signal is compared with a phase-stable radio frequency reference source to obtain a phase error signal. This error signal is then fed back to control the slave laser after passing through a loop filter, ensuring the steady-state error is zero, thus achieving phase synchronization between the master and slave lasers.

[0012] As mentioned earlier, optical modulation-based methods for generating millimeter waves are difficult to achieve signals above 100 GHz; the nonlinear effects of four-wave mixing introduce additional phase noise during the mixing process; and millimeter wave generation methods based on optical frequency combs (OFCs) typically generate millimeter waves with low power, which is difficult to meet the requirements. Summary of the Invention

[0013] In view of this, the present invention provides a high-stability optically generated millimeter-wave system based on a phase-locked loop. With the help of an optical phase-locked loop, the generation of high-frequency, high-power, and high-stability millimeter-wave signals can be achieved relatively simply.

[0014] The technical solution for implementing the present invention is as follows:

[0015] A high-stability optically generated millimeter-wave system based on a phase-locked loop includes a first laser, a second laser, an acousto-optic frequency shifter, a dense wavelength division multiplexer (DWDM), an inner-loop feedback control loop, and an outer-loop signal output branch. The DWDM is positioned on the output optical paths of the two lasers, the acousto-optic frequency shifter is located between the second laser and the DWDM, and the output of the DWDM is connected to both the inner-loop feedback control loop and the outer-loop signal output branch.

[0016] The inner loop feedback control circuit includes a phase modulator, an optical filter, a photodetector, a frequency divider, a frequency and phase detector, a loop filter, a voltage-controlled oscillator, and a first power amplifier connected in series. The output of the first power amplifier is connected to an acousto-optic frequency shifter. The inner loop feedback control circuit also includes a cesium atomic clock, a microwave source, and a second power amplifier. The cesium atomic clock provides a frequency reference for the microwave source and the frequency and phase detector. The output of the microwave source is connected to the phase modulator after passing through the second power amplifier.

[0017] Furthermore, the outer loop signal output branch of the present invention includes a phase modulator, an optical filter, and a photodetector connected in series.

[0018] Furthermore, the inner loop feedback control loop of the present invention also includes a microcontroller, the microcontroller analog-to-digital conversion module samples the output of the loop filter, and the output terminal of the microcontroller is connected to a second laser.

[0019] Furthermore, in this invention, the two laser beams pass through a phase modulator simultaneously to obtain multiple sidebands. By pre-controlling the wavelengths of the two lasers and the frequency of the radio frequency signal, the frequency interval between the +3rd sideband of the first laser and the -3rd sideband of the second laser is approximately 40MHz.

[0020] Furthermore, the present invention also includes a time interval analyzer connected to the output terminal of the photodetector for measuring the output of the photodetector.

[0021] Beneficial effects:

[0022] First, this invention uses the output frequency of a microwave source as a reference and locks the higher-order sidebands of two lasers to achieve stable millimeter-wave signal generation.

[0023] Secondly, by changing the microwave source frequency and selecting different order sidebands for locking, the present invention can obtain millimeter wave signals of different frequencies.

[0024] Third, the present invention can easily change the millimeter-wave signal power by changing the laser power. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 A schematic diagram of a Mach-Zehnder modulator;

[0027] Figure 2 This is a system structure diagram of the present invention;

[0028] Figure 3 This is the spectrum diagram of the IF signal;

[0029] Figure 4 This is a schematic diagram illustrating the frequency instability of beat frequency signals.

[0030] Figure 5 This is a schematic diagram of the phase noise of the beat frequency signal. Detailed Implementation

[0031] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0032] It should be noted that, in the absence of conflict, the following embodiments and features can be combined with each other; and, based on the embodiments of this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0033] It should be noted that various aspects of embodiments within the scope of the appended claims are described below. It will be apparent that the aspects described herein can be embodied in a wide variety of forms, and any particular structure and / or function described herein is merely illustrative. Based on this disclosure, those skilled in the art will understand that one aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects set forth herein can be used to implement the device and / or practice the method. Additionally, this device and / or method can be implemented using structures and / or functionalities other than one or more of the aspects set forth herein.

[0034] like Figure 2 As shown, the present invention discloses a high-stability optically generated millimeter-wave system based on a phase-locked loop, comprising a first laser, a second laser, an acousto-optic frequency shifter (AOFS), a dense wavelength division multiplexer (DWDM), an inner loop feedback control loop, and an outer loop signal output; wherein the DWDM is disposed in the output optical path of the two lasers, the AOFS is located between the second laser and the DWDM, and the output of the DWDM is connected to the inner loop feedback control loop and the outer loop signal output branch;

[0035] The inner loop feedback control circuit includes a phase modulator (PM), an optical filter (OBPF), a photodetector (PD), a frequency divider (1 / N), a phase-frequency detector (PFD), a loop filter, a voltage-controlled oscillator (VCO), and a first power amplifier (PA) connected in series. The output of the first power amplifier is connected to an acousto-optic frequency shifter (AOFS). The inner loop feedback control circuit also includes a cesium atomic clock (Ces), a microwave source (MS), and a second power amplifier (PA). The cesium atomic clock (Ces) is connected to the microwave source (MS) and the phase-frequency detector (PFD), providing a frequency reference for the microwave source (MS) and the phase-frequency detector (PFD). The output of the microwave source (MS) is connected to the phase modulator (PM) after passing through the second power amplifier (PA).

[0036] The outer loop signal output branch includes a phase modulator (PM), an optical filter (OBPF), and a photodetector (PD) connected in series.

[0037] To obtain higher frequency millimeter-wave signals, this invention requires relative frequency locking between two lasers with a large frequency interval. A phase modulator (PM) can be used to modulate the two lasers to obtain multiple sidebands, and frequency locking is achieved by locking the higher-order sidebands of the two lasers.

[0038] The specific working process in this embodiment is as follows: Two lasers, a first laser 1 and a second laser 2, with different wavelengths are used as light sources (assuming the wavelength of the first laser 1 is shorter than that of the second laser 2). After the second laser 2 is frequency-shifted by an acousto-optic frequency shifter (AOFS), it is combined with the output of the first laser 1 by a dense wavelength division multiplexer (DWDM) to split into two paths: an in-loop feedback control path and an out-of-loop output path, used for feedback control and as a millimeter-wave signal output, respectively. The two laser beams in the loop simultaneously pass through a phase modulator (PM). The radio frequency signal from the microwave source (MS) is amplified by a power amplifier (PA) and modulated onto two optical carriers, respectively obtaining multiple sidebands. By pre-controlling the wavelengths of the two lasers and the frequency of the radio frequency signal, the +3rd order sideband of the first laser 1 and the -3rd order sideband of the second laser 2 are made close in frequency (approximately 40MHz apart). Subsequently, the signal enters an optical filter (OBPF) for filtering, retaining only the two middle third-order sidebands. The above signal is input into a photodetector (PD). After photodetection, a beat frequency signal of about 40 MHz is obtained. The beat frequency signal is divided and mixed with a 10 MHz reference signal from a cesium atomic clock (Ces) using a phase detector (PFD). After loop filtering, an error voltage characterizing the relative frequency drift of the two lasers is obtained. This error voltage drives a voltage-controlled oscillator (VCO) to control an acousto-optic frequency shifter to perform fast feedback control of the lasers.

[0039] In this embodiment, the inner-loop feedback control circuit also includes a microcontroller, whose input is connected to a loop filter. The microcontroller (MCU) monitors the error voltage. When the laser drift exceeds the compensation range of the voltage-controlled oscillator-acoustic-optic frequency shifter, the laser's bias voltage is changed to shift the laser's frequency, thereby achieving wide-range feedback control. When the system is locked, the relative frequencies of the two optical frequency signals are locked to the radio frequency signal output by the microwave source.

[0040] In this embodiment, two laser beams within the loop simultaneously pass through a phase modulator (PM), each acquiring multiple sidebands. By pre-controlling the wavelengths of the two lasers and the frequency of the radio frequency signal, the +3rd order sideband of laser 1 is made close to the -3rd order sideband frequency of laser 2 (approximately 40MHz apart). The signal then enters an optical filter (OBPF) for filtering, retaining only the two middle third-order sidebands. This signal is input to a photodetector (PD), where the beat frequency signal is mixed with a reference signal for phase detection, yielding an error voltage characterizing the relative frequency drift of the two lasers. This error voltage drives the VCO to control the AOFS for acousto-optic frequency shifting of the lasers. Simultaneously, the error voltage is monitored; when the laser drift exceeds the VCO-AOFS compensation range, the laser bias voltage is changed to shift the laser frequency, thus achieving feedback control. When the system is locked, the relative frequencies of the two optical signals are locked to the radio frequency signal output from the microwave source.

[0041] This embodiment uses the output frequency of the microwave source as a reference. By locking the higher-order sidebands of the two lasers, the frequency of the secondary laser is locked to the main laser, ensuring the relative frequency stability of the two lasers. Compared with existing technologies, the solution proposed in this embodiment does not require ensuring the absolute frequency stability of the lasers, resulting in a simpler structure. Different orders of modulation sidebands or changes in the microwave source output frequency can be selected as needed to obtain millimeter-wave signals of different frequencies. Furthermore, higher millimeter-wave output power can be obtained by increasing the laser power.

[0042] like Figure 3 As shown, the performance of the OPLL directly determines the stability of the system. To evaluate the performance of the OPLL in this system, the stability of the 40MHz IF signal was measured in both locked and unlocked states. The wavelengths of the two lasers were set to 1549.50nm and 1550.60nm, respectively, and the microwave source frequency was set to approximately 23GHz, generating a probe signal at approximately 140GHz. In the unlocked state, the IF signal linewidth was wider due to the rapid relative drift of the two lasers. When the OPLL was locked, the locked loop significantly suppressed noise within a loop bandwidth of approximately 13kHz and achieved millimeter-wave signal generation, thus confirming the effectiveness of the OPLL.

[0043] like Figure 4 As shown, after verifying the in-loop stability of the OPLL, the out-of-loop stability of the probe signal was tested using a Time Interval Analyzer (TIA). The TIA detected the zero-crossing of the signal and calculated its frequency. Due to limitations in PD bandwidth and TIA detection range, directly measuring the relative frequency instability of the two lasers was not feasible. Therefore, the output signal was processed in the same way as inside the control loop, using another phase modulator to modulate the output optical probe signal. Two third-order sidebands were selected again and sent to the second PD. When the system was running freely, the frequency variation of the beat signal reached tens of MHz within 4500s due to external environmental factors such as temperature changes and vibration. However, when the system was locked, the frequency jitter of the beat signal dropped to below 20Hz peak-to-peak, with a root mean square jitter of only 1.7Hz. This indicates a significant improvement in the frequency stability of the probe signal after system locking.

[0044] like Figure 5As shown, phase noise is an important indicator for evaluating signal stability because it directly reflects the type and intensity of noise in the transmission system within the frequency domain. Using a 10MHz clock frequency as a reference, the phase noise of the 40MHz beat frequency signal outside the loop was measured using a phase noise analyzer. It is worth noting that due to significant laser frequency fluctuations, phase noise cannot be measured when the OPLL is not locked. When the system is locked, the phase noise of the beat frequency signal is effectively suppressed within the loop bandwidth. Specifically, at frequency offsets of 10Hz and 1kHz, the phase noise levels are -82dBc / Hz and -95dBc / Hz, respectively. The spike observed near 100Hz mainly originates from the bias voltage applied to the PZT inside the laser.

[0045] This invention addresses three key aspects: First, it solves signal quality issues (low phase noise and long-term stability) by suppressing laser phase and frequency noise through a phase-locked loop (PLL), resulting in a highly stable millimeter-wave signal. Second, it enables the generation of high-frequency and frequency-tunable millimeter-wave signals by locking the higher-order sidebands of two lasers, allowing for easy generation of higher-frequency (hundreds of GHz to THz) millimeter-wave signals. Third, it achieves high-power millimeter-wave signal generation because it is not limited by the efficiency of photoelectric conversion devices, enabling the generation of high-power millimeter-wave signals.

[0046] 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 high-stability optically generated millimeter-wave system based on a phase-locked loop, characterized in that, It includes a first laser, a second laser, an acousto-optic frequency shifter, a dense wavelength division multiplexer, an inner loop feedback control loop, and an outer loop signal output branch; wherein the dense wavelength division multiplexer is disposed on the output optical path of the two lasers, the acousto-optic frequency shifter is located between the second laser and the dense wavelength division multiplexer, and the output of the dense wavelength division multiplexer is connected to the inner loop feedback control loop and the outer loop signal output branch; The inner loop feedback control circuit includes a phase modulator, an optical filter, a photodetector, a frequency divider, a frequency and phase detector, a loop filter, a voltage-controlled oscillator, and a first power amplifier connected in series. The output of the first power amplifier is connected to an acousto-optic frequency shifter. The inner loop feedback control circuit also includes a cesium atomic clock, a microwave source, and a second power amplifier. The cesium atomic clock provides a frequency reference for the microwave source and the frequency and phase detector. The output of the microwave source is connected to the phase modulator after passing through the second power amplifier.

2. The high-stability optically generated millimeter-wave system based on a phase-locked loop according to claim 1, characterized in that, The outer loop signal output branch includes a phase modulator, an optical filter, and a photodetector connected in series.

3. The high-stability optically generated millimeter-wave system based on a phase-locked loop according to claim 1, characterized in that, The inner loop feedback control circuit also includes a microcontroller. The input terminal of the microcontroller is connected to the loop filter. The microcontroller's analog-to-digital conversion module samples the output of the loop filter. The output terminal of the microcontroller is connected to a second laser.

4. The high-stability optically generated millimeter-wave system based on a phase-locked loop according to claim 1, characterized in that, Two laser beams pass through a phase modulator simultaneously to obtain multiple sidebands. By pre-controlling the wavelengths of the two lasers and the frequency of the radio frequency signal, the +3rd sideband of the first laser and the -3rd sideband of the second laser are spaced 40MHz apart.

5. The high-stability optically generated millimeter-wave system based on a phase-locked loop according to claim 1, characterized in that, It also includes a time interval analyzer, which is connected to the output of the photodetector and is used to measure the output of the photodetector.

Images