High-stability photo-generated millimeter wave system based on phase-locked loop

Abstract

The invention belongs to the technical field of optical communication, and particularly relates to a high-stability photo-generated millimeter wave system based on a phase-locked loop, which comprises 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, the inner loop feedback control loop comprises a phase modulator, an optical filter, a photoelectric detector, a frequency division arithmetic unit, a phase frequency detector, a loop filter, a voltage-controlled oscillator and a first power amplifier which are sequentially connected in series, and the output of the first power amplifier is connected with an acousto-optic frequency shifter; the inner loop feedback control loop further comprises a cesium atomic clock, a microwave source and a second power amplifier; wherein the cesium atomic clock is connected with the microwave source and the phase frequency detector, and the output of the microwave source is connected with the phase modulator after passing through the second power amplifier.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical communication, and in particular relates to a high-stability optically generated millimeter wave system based on a phase-locked loop. Background Art

[0002] Millimeter waves refer to electromagnetic waves with a wavelength range of 1 to 10 mm and a frequency range of 30 to 300 GHz. Millimeter wave technology was first applied in the military field and has been widely used in many military fields such as short-range air defense, battlefield surveillance, missile guidance, fire control and tracking, high-resolution imaging, and friend-or-foe identification. With the civilization and development of human society, millimeter wave technology has also been gradually applied to civilian fields such as wireless mobile communications, biological detection, and health care. Microwave oscillators based on traditional electronics mostly use vacuum tubes and solid-state power sources as basic units, and use multi-level frequency multiplication to obtain microwave signals. When these units work in the frequency band above GHz, their performance will drop sharply, and the high-frequency microwaves generated have large phase noise and low spectral purity, that is, there is a so-called electronic bottleneck. In contrast, the photonic method is not limited by the electronic bottleneck and can generate microwave signals with very high carrier frequencies. In addition, the photonic method also has the advantages of anti-electromagnetic interference, light weight, and compact structure. At the same time, the millimeter wave photon generator based on photonics is naturally compatible with the optical fiber transmission system, and no additional electro-optical and photoelectric conversion equipment is required.

[0003] The prior art discloses several solutions for photogenerating millimeter waves:

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

[0005] Millimeter-wave signals with high stability and low phase noise can be generated through optical modulation. Figure 1 The figure shows the basic structure and transmission curve of the Mach-Zehnder Modulator (MZM). Its two interference arms are composed of two optical phase modulators. Under the action of an external electric field, the phase of the optical signal transmitted in the two interference arms changes. Since the electric fields loaded on the two branches are in opposite directions, the refractive index 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 signals in the two arms is π. After interference, it is converted into an output optical signal with modulated light intensity. A frequency-doubled modulation signal can be generated by appropriately adjusting the DC bias point of the MZM. For example, in the case of carrier suppression, an 18 GHz modulated signal can generate a 36 GHz millimeter wave signal. The main advantage of the optical generation of millimeter waves based on optical modulation is its simple structure. The disadvantage is that it is difficult to generate higher frequency signals due to the limitation of the bandwidth of microwave amplifiers and modulators.

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

[0007] In order to break through the bandwidth limitation of the optical modulation generation method, researchers proposed a millimeter wave generation technology based on the four-wave mixing effect (FWM). In this type of scheme, the two phase-locked optical carriers f1 and f2 generated by MZM are input into the highly nonlinear fiber (HNLF) as pump light, and the four-wave mixing effect of HNLF is used to generate phase-locked optical carriers 2f1-f2 and 2f2-f1. Then, the millimeter wave signal with a frequency of 6 times the MZM drive signal can be obtained by heterodyne beat frequency. The disadvantages of this scheme are: first, higher pump light power and sufficiently long HNLF are required, which increases the loss, complexity and cost of the system. Secondly, in order to avoid the influence of fiber stimulated Brillouin scattering (SBS), the pump power cannot be too large, which limits the efficiency of four-wave mixing.

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

[0009] The most direct method is to extract the heterodyne beat frequency of the two optical carrier components from OFC to generate millimeter wave signals. The system mainly includes three parts: Optical Frequency Comb Generator (OFCG), optical filter and broadband photodetector. OFCG generates a frequency spacing of f comb The optical filter filters out two optical frequency components from the optical frequency signal and generates a millimeter wave signal by broadband photoelectric detection of the beat frequency.

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

[0011] The method of generating millimeter wave signals by phase-locked laser heterodyne beat frequency has the advantages of high frequency and easy power tuning. The phase noise of the signal generated by the beat frequency can be improved by optical phase locked loop (OPLL) and feedback technology. It uses the principle of negative feedback to control the frequency and phase of the slave laser (Slave laser) to synchronize with the master laser (Master laser). When the phase between the slave laser and the master laser is not synchronized, the phase difference between the master and slave lasers will be reflected in the phase of the beat frequency signal. The beat frequency signal is compared with the phase-stable RF reference source to obtain the phase error signal. The error signal is fed back to control the slave laser after passing through the loop filter, so that the steady-state error is zero, achieving the purpose of phase synchronization between the master and slave lasers.

[0012] As mentioned above, the photogenerated millimeter-wave method based on optical modulation is difficult to generate signals above 100 GHz; the nonlinear effect based on the four-wave mixing method will introduce additional phase noise in the mixing process; and the millimeter-wave generation method based on the optical frequency comb (OFC) usually generates millimeter-waves with low power, which is difficult to meet the needs. 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, high-frequency, high-power, and high-stability millimeter wave signal generation 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 comprises 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 arranged 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;

[0016] The inner feedback control loop includes a phase modulator, an optical filter, a photodetector, a frequency division operator, a frequency detector, a loop filter, a voltage-controlled oscillator and a first power amplifier connected in series in sequence, wherein the output of the first power amplifier is connected to an acousto-optic frequency shifter; the inner feedback control loop also includes a cesium atomic clock, a microwave source and a second power amplifier; wherein the cesium atomic clock provides a frequency reference for the microwave source and the frequency detector, and 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 comprises a phase modulator, an optical filter and a photodetector which are sequentially connected in series.

[0018] Furthermore, the inner feedback control loop of the present invention further comprises a single chip microcomputer, the analog-to-digital conversion module of the single chip microcomputer samples the output of the loop filter, and the output end of the single chip microcomputer is connected to the second laser.

[0019] Furthermore, the two laser beams of the present invention pass through the phase modulator at the same time to obtain multi-order sidebands respectively, and by pre-controlling the wavelengths of the two lasers and the frequency of the RF signal, the frequency interval between the +3rd order sideband of the first laser and the -3rd order sideband of the second laser is about 40MHz.

[0020] Furthermore, the present invention also includes a time interval analyzer connected to the output end of the photoelectric detector and used for measuring the output of the photoelectric detector.

[0021] Beneficial effects:

[0022] First, the present invention uses the output frequency of the microwave source as a reference and achieves stable millimeter wave signal generation by locking the high-order sidebands of the two lasers.

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

[0024] Third, the present invention can easily change the millimeter wave signal power by changing the laser power. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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.

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

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

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

[0029] Figure 4 It is a schematic diagram of the frequency instability of the beat frequency signal;

[0030] Figure 5 Schematic diagram of the phase noise of the beat frequency signal. DETAILED DESCRIPTION

[0031] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0032] 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.

[0033] 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.

[0034] like Figure 2 As shown, the present invention is 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 dense wavelength division multiplexer (DWDM) is arranged on the output optical path of the two lasers, the acousto-optic frequency shifter (AOFS) is located between the second laser and the dense wavelength division multiplexer (DWDM), and the output of the dense wavelength division multiplexer (DWDM) is connected to the inner loop feedback control loop and the outer loop signal output branch;

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

[0037] In order to obtain a higher frequency millimeter wave signal, the present invention needs to lock the relative frequency of the two lasers at a larger frequency interval. A phase modulator (PM) can be used to modulate the two lasers to obtain multi-order sidebands, and the frequency of the two lasers can be locked by locking the high-order sidebands of the two lasers.

[0038] The specific working process in this embodiment is as follows: a first laser 1 and a second laser 2 with two different wavelengths are used as light sources (assuming that the wavelength of the first laser 1 is smaller 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) and then divided into two paths: feedback control within the loop and output outside the loop, which are used for feedback control and output as millimeter wave signals, respectively. The two laser beams in the loop pass through a phase modulator (PM) at the same time, and the radio frequency signal from the microwave source (MS) is amplified by a power amplifier (PA) and modulated onto two optical carriers to obtain multi-order sidebands, respectively. By pre-controlling the wavelengths of the two lasers and the frequency of the radio frequency signal, the +3-order sideband of the first laser 1 and the -3-order sideband of the second laser 2 are close in frequency (separated by about 40MHz). The signal then enters the optical filter (OBPF) for filtering, retaining only the two third-order sidebands in the middle. The above signal is input into a photodetector (PD), and a beat frequency signal of about 40MHz is obtained after photodetection. The beat frequency signal is divided and mixed with a 10MHz reference signal from a cesium atomic clock (Ces) using a phase frequency detector (PFD). After loop filtering, an error voltage representing the relative frequency drift of the two lasers is obtained. The error voltage will drive a voltage-controlled oscillator (VCO) to control the acousto-optic frequency shifter to perform rapid feedback control on the laser.

[0039] The inner feedback control loop of this embodiment also includes a single chip microcomputer, and the input end of the single chip microcomputer is connected to the loop filter. The single chip microcomputer (MCU) is used to monitor the error voltage. When the laser drift exceeds the compensation range of the voltage controlled oscillator-acoustic-optic frequency shifter, the bias voltage of the laser is changed to shift the laser frequency, thereby realizing a wide range of feedback control. When the system is locked, the relative frequency of the two optical frequency signals is locked to the radio frequency signal output by the microwave source.

[0040] In this embodiment, the two laser beams in the ring pass through the phase modulator (PM) at the same time to obtain multi-order sidebands. By pre-controlling the wavelengths of the two lasers and the frequency of the RF signal, the +3-order sideband of laser 1 is close to the -3-order sideband frequency of laser 2 (about 40MHz apart). The signal then enters the optical filter (OBPF) for filtering, retaining only the two third-order sidebands in the middle. The above signal is input into the photodetector (PD), and the beat signal is mixed and phase-detected with the reference signal to obtain an error voltage that characterizes the relative frequency drift of the two lasers. The error voltage will drive the VCO to control the AOFS to perform acousto-optic frequency shift on the laser. At the same time, the error voltage is monitored. When the laser drift exceeds the VCO-AOFS compensation range, the bias voltage of the laser will be changed to shift the laser frequency, thereby realizing feedback control. When the system is locked, the relative frequency of the two optical frequency signals is locked to the RF signal output by the microwave source.

[0041] This embodiment uses the output frequency of the microwave source as a reference, and by locking the high-order sidebands of the two lasers, the slave laser frequency is locked to the master laser, thereby ensuring the relative frequency stability of the two lasers. Compared with the prior art, the solution proposed in this embodiment does not need to ensure the absolute frequency stability of the laser, and the structure is simpler. According to needs, different orders of modulation sidebands can be selected or the output frequency of the microwave source can be changed to obtain millimeter wave signals of different frequencies. At the same time, higher millimeter wave output power can be obtained by increasing the laser power.

[0042] like Figure 3 As shown in the figure, the performance of OPLL directly determines the stability of the system. In order to evaluate the performance of OPLL of this system, the stability of the 40MHz IF signal is measured in the locked and unlocked states. The wavelengths of the two lasers are set to 1549.50nm and 1550.60nm respectively, and the frequency of the microwave source is set to about 23GHz, generating a detection signal with a frequency of about 140GHz. In the unlocked state, the IF signal linewidth is wide due to the rapid relative drift of the two lasers. When the OPLL is locked, the locked loop will significantly suppress the noise within the loop bandwidth of about 13kHz and realize the generation of millimeter-wave signals, thus confirming the effectiveness of the OPLL.

[0043] like Figure 4 As shown, after verifying the in-loop stability of the OPLL, a time interval analyzer (TIA) is used to test the out-of-loop stability of the detection signal. The TIA detects the zero crossing of the signal and calculates its frequency. Due to the limitations of the PD bandwidth and the TIA detection range, it is not feasible to directly measure the relative frequency instability of the two lasers. Therefore, the output signal is processed in the same way as inside the control loop, using another phase modulator to modulate the output optical detection signal. The two third-order sidebands are selected again and sent to the second PD. When the system is running freely, the frequency variation of the beat signal reaches tens of MHz within 4500s due to external environmental factors such as temperature changes and vibrations. However, when the system is locked, the frequency jitter of the beat signal drops to less than 20Hz peak-to-peak, and the root mean square jitter is only 1.7Hz. This shows that the frequency stability of the detection signal has been significantly improved after the system is locked.

[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 in the frequency domain. Using a 10MHz clock frequency as a reference, the phase noise of a 40MHz beat signal outside the loop is measured with a phase noise analyzer. It is worth noting that due to significant laser frequency fluctuations, the phase noise cannot be measured when the OPLL is not locked. When the system is locked, the phase noise of the beat 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 is mainly from the bias voltage applied to the PZT inside the laser.

[0045] The present invention: First, solve the signal quality problem (low phase noise, long-term stability): suppress the laser phase noise and frequency noise through the phase-locked loop to obtain a high-stability millimeter wave signal; second, realize the generation of high-frequency and frequency-adjustable millimeter wave signals: by locking the high-order sidebands of the two lasers, it is easy to achieve the generation of higher frequency (hundreds of GHz to THz) millimeter wave signals. Third, realize the generation of high-power millimeter wave signals: the present invention is not limited by the efficiency of the photoelectric conversion device, and can obtain high-power millimeter wave signals.

[0046] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-stability optically generated millimeter wave system based on a phase-locked loop, characterized in that: The invention comprises 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 arranged 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 feedback control loop includes a phase modulator, an optical filter, a photodetector, a frequency division operator, a frequency detector, a loop filter, a voltage-controlled oscillator and a first power amplifier connected in series in sequence, wherein the output of the first power amplifier is connected to an acousto-optic frequency shifter; the inner feedback control loop also includes a cesium atomic clock, a microwave source and a second power amplifier; wherein the cesium atomic clock provides a frequency reference for the microwave source and the frequency detector, and 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 comprises a phase modulator, an optical filter and a photodetector which are connected in series in sequence.

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

4. The high-stability optically generated millimeter wave system based on a phase-locked loop according to claim 1, characterized in that: The two laser beams pass through the phase modulator at the same time to obtain multi-order sidebands respectively. By pre-controlling the wavelengths of the two lasers and the frequency of the RF signal, the frequency interval between the +3rd order sideband of the first laser and the -3rd order sideband of the second laser is about 40MHz.

5. The high-stability optically generated millimeter wave system based on a phase-locked loop according to claim 1, characterized in that: The device also comprises a time interval analyzer which is connected to the output end of the photoelectric detector and is used for measuring the output of the photoelectric detector.

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