A low phase noise microwave signal source

Abstract

This application relates to a low phase noise microwave signal source, belonging to the field of microwave signal sources. It includes a light source module for emitting light; a beam splitter module connected to the light source module, receiving the light source and splitting it into beams before outputting an optical signal; a conversion module connected to the beam splitter module, receiving the optical signal and converting it into an electrical signal for output; and a phase modulator connected to the conversion module, receiving the electrical signal and performing phase modulation processing before outputting it. This application effectively improves the RF phase noise degradation problem caused by long-distance optical fiber transmission in OEO (Optical Outline Optical) output.

Description

Technical Field

[0001] This application relates to the field of microwave signal sources, and in particular to a low phase noise microwave signal source. Background Technology

[0002] A high-quality microwave signal source is the foundation of all microwave applications. As the most basic part of various microwave optoelectronic devices, the quality of the signal generated by the microwave signal source directly affects the performance of various equipment in the microwave system.

[0003] An optoelectronic oscillator (OEO) is a high-performance microwave signal source. An OEO is mainly composed of optoelectronic devices such as a stable DC laser source, an electro-optic modulator, a long optical fiber, a photodetector, an RF amplifier, and an RF filter. Due to the high quality factor provided by the long optical fiber, the OEO can generate a spectrum-pure microwave signal with ultra-low phase noise.

[0004] Existing high-performance OEO optoelectronic oscillators all use long-distance optical fiber as the cavity medium to ensure a high Q value in the oscillation loop. During long-distance optical fiber transmission, the optical signal is affected by noise factors such as stimulated Brillouin scattering, Rayleigh scattering, relative intensity noise, dispersion, interference intensity noise, and mode segmentation noise (MPN), which degrade the amplitude and phase parameters of the optoelectronic detection output signal, leading to a degraded phase noise in the RF signal output by the optoelectronic oscillator. Summary of the Invention

[0005] This application provides a low phase noise microwave signal source, which improves the problem of OEO output RF phase noise degradation caused by long-distance optical fiber transmission.

[0006] The above-mentioned objective of this application is achieved through the following technical solution:

[0007] A low phase noise microwave signal source, comprising:

[0008] The light source module is used to emit the light source;

[0009] The beam splitting module connects to the light source module, receives the light source, splits the light source into beams, and outputs the light signal.

[0010] The conversion module connects to the beam splitter module, receives optical signals, and converts the optical signals into electrical signals for output.

[0011] The phase modulator connects to the conversion module, receives electrical signals, performs phase modulation processing on the electrical signals, and then outputs them.

[0012] The beam splitting module includes a polarization-maintaining beam splitter, a transmission fiber, and a polarization beam splitter; the input end of the polarization-maintaining beam splitter is connected to the light source module; the first output end of the polarization-maintaining beam splitter is connected to the input end of the transmission fiber; the output end of the transmission fiber is connected to the input end of the polarization beam splitter; and the output end of the polarization beam splitter is connected to the conversion module.

[0013] The conversion module includes a first photodetector, a second photodetector, and a third photodetector; the input terminal of the first photodetector is connected to the second output terminal of the polarization-maintaining beam splitter; the output terminal of the polarization beam splitter includes a first output terminal and a second output terminal; the input terminal of the second photodetector is connected to the first output terminal of the polarization beam splitter, and the input terminal of the third photodetector is connected to the second output terminal of the polarization beam splitter.

[0014] In a preferred embodiment, this application may be further configured to include a combiner / splitter and an amplifier; the input of the combiner / splitter is connected to the output of the second photodetector and the output of the third photodetector; the first output of the combiner / splitter is connected to the input of the amplifier; and the output of the amplifier is connected to a phase tuner.

[0015] In a preferred embodiment, this application may be further configured to include a beat frequency phase detection unit; the beat frequency phase detection unit is connected to the output terminal of the first photodetector and the second output terminal of the combiner / splitter; the beat frequency phase detection unit is also connected to a phase tuner.

[0016] In a preferred embodiment, this application may be further configured to include a filter and a power divider; the filter is connected to a phase modulator; and the power divider is connected to the filter.

[0017] In a preferred embodiment, this application may be further configured to include an intensity modulator; the intensity modulator is connected to the light source module and the beam splitter module, and is used to modulate the light source and output it to the beam splitter module; the intensity modulator is also connected to the power divider and receives the signal fed back by the power divider. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the system structure of a low phase noise microwave signal source in an embodiment of this application.

[0019] Explanation of reference numerals in the attached diagram: 1. Light source module; 2. Beam splitter module; 21. Polarization-maintaining beam splitter; 22. Transmission fiber; 23. Polarization beam splitter; 3. Conversion module; 31. First photodetector; 32. Second photodetector; 33. Third photodetector; 4. Combiner / splitter; 5. Amplifier; 6. Beat frequency phase detector unit; 7. Phase modulator; 8. Filter; 9. Power divider; 10. Intensity modulator. Detailed Implementation

[0020] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0023] Currently, high-performance OEO optoelectronic oscillators use long-distance optical fibers as the cavity medium to ensure a high Q value in the oscillation loop. During long-distance optical fiber transmission, the optical signal is subject to various noise factors, including stimulated Brillouin scattering, Rayleigh scattering, relative intensity noise, dispersion, interference intensity noise, and mode segmentation noise (MPN), which degrade the amplitude and phase parameters of the optoelectronic detection output signal. These combined factors lead to phase noise degradation in the RF signal output by the optoelectronic oscillator, especially in the near-end portion (below 1 kHz), where phase noise is relatively high. Existing OEO technology solutions cannot effectively address this issue, and the near-end phase noise degradation problem significantly impacts the practical application of OEO technology.

[0024] To address the aforementioned problems, this application also provides a low phase noise microwave signal source, such as... Figure 1 As shown, a low phase noise microwave signal source includes a light source module 1, a beam splitter module 2, a conversion module 3, and a phase modulator 7. The light source module 1 is used to emit light. The beam splitter module 2 is connected to the light source module 1 to split the light source into beams. The conversion module 3 is connected to the beam splitter module 2 to convert the signal. The phase modulator 7 is connected to the conversion module 3 to perform phase modulation processing on the signal. Through the coordinated use of the above modules, the improvement effect of OEO output RF phase noise degradation caused by long-distance optical fiber transmission is achieved.

[0025] In this embodiment, the light source module 1 includes a laser, which is used to emit a stable light source. The specific light source module 1 is not limited, and the laser is only used as an example in this embodiment.

[0026] The low phase noise microwave signal source also includes an intensity modulator 10; the intensity modulator 10 is connected to the laser and is used to modulate the laser source. Here, the intensity modulator 10 is an optical intensity modulator; the intensity modulator can adjust the optical power, and when connected to the laser, it can adjust the optical power of the laser emitted by the laser.

[0027] The beam splitting module 2 includes a polarization-maintaining beam splitter 21, a transmission fiber 22, and a polarization beam splitter 23; the conversion module 3 includes a first photodetector 31, a second photodetector 32, and a third photodetector 33; the input end of the polarization-maintaining beam splitter 21 is connected to the intensity modulator 10 to receive the modulated light source; the polarization-maintaining beam splitter 21 has two output ports, wherein the first output end of the polarization-maintaining beam splitter 21 is connected to the input end of the transmission fiber 22, and the second output end of the polarization-maintaining beam splitter 21 is connected to the input end of the first photodetector 31; the output end of the transmission fiber 22 is connected to the input end of the polarization beam splitter 23; the polarization beam splitter 23 also has two output ends, wherein the first output end of the polarization beam splitter 23 is connected to the input end of the second photodetector 32, and the second output end of the polarization beam splitter 23 is connected to the input end of the third photodetector 33.

[0028] In this embodiment, the polarization-maintaining beam splitter 21 and the transmission fiber 22 are connected by a 45-degree fusion splice. The polarization-maintaining beam splitter 21 includes a polarization-maintaining fiber ring. The 45-degree fusion splice refers to splicing the fast axis of the polarization-maintaining beam splitter 21 and the fast axis of the transmission fiber 22 at a 45-degree angle. The polarization-maintaining beam splitter 21 splits the optical signal into fast and slow axes and sends it into the polarization-maintaining fiber ring. The optical signal is transmitted separately through the fast and slow axes within the ring and enters the polarization beam splitter 23 to form two optical signals, which then enter two photodetectors respectively. Since the propagation paths of the two optical signals with different axes are different, this will result in two different oscillation modes. Only when the coupling vibration condition is met can a microwave signal with a good side-mode suppression ratio be obtained. By using the polarization-maintaining fiber fast and slow axes instead of the traditional dual-loop, the length of the polarization-maintaining fiber is saved, and the cost is reduced.

[0029] The function of PMFS (polarization-maintaining fiber beam splitter) is to split the optical power while maintaining the original polarization state of the light wave. It has the advantages of low WDL, low PDL, wide operating temperature range and wide operating wavelength range. The product is manufactured using micro-optics and mature glass tube technology, which has the advantages of strong environmental adaptability, excellent performance and suitability for mass production. The pigtail types include full polarization-maintaining and polarization-maintaining single-mode hybrid specifications.

[0030] The function of a polarization beam splitter is to split the power of an optical wave while maintaining its original polarization state. It boasts advantages such as low WDL (wavelength dropout ratio), low PDL (polarization-dependent dropout ratio), and a wide operating temperature and wavelength range. Manufactured using micro-optics and mature glass tube technology, it offers strong environmental adaptability, excellent performance, and suitability for mass production. Available fiber types include full polarization-maintaining and polarization-maintaining single-mode hybrid. The advantages of polarization beam splitters include low insertion loss, high stability and reliability, high return loss, and high extinction ratio. Polarization beam splitters are widely used in fiber optic current transformers, fiber optic gyroscopes, fiber optic sensing, and coherent communication.

[0031] The main difference between polarization beamsplitters and non-polarization beamsplitters is that polarization beamsplitters split incident light into two beams, with one beam vibrating perpendicular to the incident plane and the other vibrating parallel to the incident plane. They are often used in optical paths with special requirements for polarization states. The principle of polarization beam splitting is that when the incident angle is the polarization angle, the reflected light is linearly polarized light with the vibration direction perpendicular to the incident plane. The reflected light generally accounts for a single-digit percentage of the total light intensity. Therefore, a method of superimposing multiple layers of media is used to filter out the linearly polarized light component perpendicular to the incident plane in each incident light, so that the final two parts of the outgoing light are relatively pure vertically polarized light and the remaining relatively pure parallel linearly polarized light.

[0032] The low phase noise microwave signal source in this embodiment also includes a combiner / splitter 4, an amplifier 5, a beat frequency phase detector 6, a filter 8, and a power divider 9.

[0033] Among them, the input terminal of the combiner / splitter 4 is connected to the output terminal of the second photodetector 32 and the output terminal of the third photodetector 33, the first output terminal of the combiner / splitter 4 is connected to the input terminal of the amplifier 5, and the output terminal of the amplifier 5 is connected to the phase modulator 7; the beat frequency phase detection unit 6 is connected to the output terminal of the first photodetector 31 and the second output terminal of the combiner / splitter 4, and the beat frequency phase detection unit 6 is also connected to the phase modulator 7.

[0034] The term "combiner-splitter" here can be understood as a combination of a combiner and a splitter, meaning that the combination-splitter incorporates the functions of both devices.

[0035] The optical signal output from the second output end of the polarization-maintaining fiber beam splitter is converted into an electrical signal by the first photodetector 31. The optical signals output from the second and third output ends of the polarization beam splitter 23 are converted into electrical signals by the second photodetector 32 and the third photodetector 33. The two electrical signals are combined into one by the combiner / splitter 4. One part of the signal enters the beat frequency phase detection unit 6, where it is demodulated with the electrical signal from the first photodetector 31 to remove phase noise. The other part of the signal is output to the amplifier 5, where it is amplified and then enters the phase modulator 7. The phase modulation circuit in the phase modulator 7 compensates for the amplified electrical signal. In this way, the conversion and corresponding processing between photoelectric signals are realized.

[0036] In this embodiment, filter 8 is connected to phase modulator 7, and power divider 9 is connected to filter 8 and intensity modulator 10. The signal processed by phase modulator 7 enters filter 8 for filtering, and then enters power divider 9 for processing. Power divider 9 feeds back part of the electrical signal to intensity modulator 10, and outputs the other part. Intensity modulator 10 receives the electrical signal fed back by power divider 9. This part of the electrical signal can be understood as a modulation signal. Intensity modulator 10 will load the modulation signal onto the laser light wave emitted by the laser, thereby increasing the light intensity of the laser.

[0037] In this embodiment, the polarization maintaining beam splitter 21 includes a lens with a semi-transparent and semi-reflective structure, and the beam splitting ratio of the first output terminal and the second output terminal is controlled by coating.

[0038] In this embodiment, the beat frequency phase detector 6 can use devices such as a phase-locked loop to demodulate the phase noise introduced by the fiber optic loop; the demodulated phase noise can be compensated by the phase modulation circuit in the phase modulator 7; in this way, phase noise compensation of the main signal is achieved.

[0039] The low phase noise microwave signal source provided above, after experimental simulation, shortened the polarization-maintaining loop length by half and improved the phase noise at 1kHz by more than 10dB. Therefore, relevant experiments have proven that the low phase noise microwave signal source in this solution can effectively improve the near-end phase noise degradation of the RF signal of the OEO device and solve the noise degradation problem caused by long-distance optical fiber.

[0040] In one example, a signal processing module is also included. This module connects to the three photodetectors, receives the electrical signals output by the photodetectors, and analyzes and processes these signals, converting them into corresponding signal images: a first signal image, a second signal image, and a third signal image. The module stores preset electrical signal images and compares the first, second, and third signal images with these preset images to obtain similarity values. It then calculates the similarity difference between these values ​​and a preset similarity threshold, using this difference to determine if the corresponding photodetector is faulty. The module also sets a difference threshold, comparing the similarity difference with this threshold. If the difference is greater than the threshold, it indicates an anomaly in the corresponding signal image, suggesting a fault in the corresponding photodetector.

[0041] The above testing process proves that the photodetector is abnormal, but it does not necessarily mean that the photodetector itself is faulty, or that the problem occurred due to special circumstances. Therefore, further analysis of the relevant information is needed.

[0042] If the photodetector malfunctions, it needs to be tested to determine whether the malfunction is due to a fault in the photodetector itself or a special circumstance.

[0043] In this embodiment, the photodetector needs to convert the optical signal from the polarization-maintaining beam splitter 21 and the polarization beam splitter 23 into an electrical signal, which is then processed by a related module, and finally output by the power divider 9. Therefore, the photodetector is crucial in this solution, so it is necessary to analyze the photodetector to determine whether it can work properly.

[0044] When analyzing the photodetectors, the signal processing module obtains the numbers of the three photodetectors: the first photodetector 31 is numbered 1, the second photodetector 32 is numbered 2, and the third photodetector 33 is numbered 3. During the analysis, the first photodetector 31 is tested first to determine if it is malfunctioning. If the first photodetector 31 is not malfunctioning, the second photodetector 32 and the third photodetector 33 are then tested. In the scheme of this application, the polarization-maintaining beam splitter 21 processes the laser, and then the laser is further processed via the transmission fiber 22 and the polarization beam splitter 23. The first photodetector 31 is connected to the polarization-maintaining beam splitter 21, and the second photodetector 32 and the third photodetector 33 are both connected to the polarization beam splitter 23. The polarization beam splitter 23 is connected to the polarization-maintaining beam splitter 21 via the transmission fiber 22. Therefore, when an abnormality is detected in the first photodetector 31, it may be due to a fault in the first photodetector 31 itself, or it may be due to a fault in the polarization-maintaining beam splitter 21 to which the first photodetector 31 is connected.

[0045] In this embodiment, fault detection of the photodetector involves analyzing and processing the signal. The first photodetector 31 is connected to the polarization-maintaining beam splitter 21, receives the optical signal transmitted by the polarization-maintaining beam splitter 21, and converts the optical signal into an electrical signal. Therefore, if a problem is detected in the electrical signal, it may be that the transmitted optical signal has a problem, or that a problem occurred during the process of converting the optical signal into an electrical signal. If the problem is with the optical signal, it indicates that the polarization-maintaining beam splitter 21 is faulty. If the problem is with the process of converting the optical signal into an electrical signal, it indicates that the first photodetector 31 is faulty.

[0046] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of disclosure in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the foregoing disclosed concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A low phase noise microwave signal source, characterized in that, include: Light source module (1), used to emit light source; The beam splitting module (2) is connected to the light source module (1), receives the light source, splits the light source into beams, and outputs the light signal. The conversion module (3) is connected to the beam splitter module (2), receives optical signals, and converts the optical signals into electrical signals for output; Phase modulator (7) is connected to conversion module (3), receives electrical signals, performs phase modulation processing on the electrical signals, and outputs them; The beam splitting module (2) includes a polarization-maintaining beam splitter (21), a transmission fiber (22), and a polarization beam splitter (23); the input end of the polarization-maintaining beam splitter (21) is connected to the light source module (1); the first output end of the polarization-maintaining beam splitter (21) is connected to the input end of the transmission fiber (22); the output end of the transmission fiber (22) is connected to the input end of the polarization beam splitter (23); the output end of the polarization beam splitter (23) is connected to the conversion module (3); the polarization-maintaining beam splitter (21) and the transmission fiber... (22) are connected by a 45-degree fusion splice; the polarization-maintaining beam splitter (21) includes a polarization-maintaining fiber ring; the 45-degree fusion splice means that the fast axis of the polarization-maintaining beam splitter (21) is fused with the fast axis of the transmission fiber (22) at a 45-degree angle; the polarization-maintaining beam splitter (21) splits the optical signal into the polarization-maintaining fiber ring by the fast and slow axes, and the optical signal is transmitted through the fast and slow axes in the ring. After entering the polarization beam splitter (23), two optical signals are formed, and then they enter two photodetectors respectively; The conversion module (3) includes a first photodetector (31), a second photodetector (32), and a third photodetector (33); the input end of the first photodetector (31) is connected to the second output end of the polarization-maintaining beam splitter (21); the output end of the polarization beam splitter (23) includes a first output end and a second output end; the input end of the second photodetector (32) is connected to the first output end of the polarization beam splitter (23), and the input end of the third photodetector (33) is connected to the second output end of the polarization beam splitter (23).

2. The low phase noise microwave signal source according to claim 1, characterized in that, It also includes a combiner / splitter (4) and an amplifier (5); the input of the combiner / splitter (4) is connected to the output of the second photodetector (32) and the output of the third photodetector (33); the first output of the combiner / splitter (4) is connected to the input of the amplifier (5); the output of the amplifier (5) is connected to the phase tuner (7).

3. The low phase noise microwave signal source according to claim 2, characterized in that, It also includes a beat frequency phase detection unit (6); the beat frequency phase detection unit (6) is connected to the output terminal of the first photodetector (31) and the second output terminal of the combiner / splitter (4); the beat frequency phase detection unit (6) is also connected to a phase tuner (7).

4. The low phase noise microwave signal source according to claim 1, characterized in that, It also includes a filter (8) and a power divider (9); the filter (8) is connected to a phase modulator (7); the power divider (9) is connected to the filter (8).

5. The low phase noise microwave signal source according to claim 4, characterized in that, It also includes an intensity modulator (10); the intensity modulator (10) is connected to the light source module (1) and the beam splitter module (2) and is used to modulate the light source and output it to the beam splitter module (2); the intensity modulator (10) is also connected to the power divider (9) and receives the signal fed back by the power divider (9).

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