AGC-based optical frequency phase-locked loop frequency modulation system and frequency modulation method thereof
By introducing an AGC (Automatic Gaussian Generative Circuit) link between the optical detector and the phase detector, the phase-locked error caused by the amplitude variation of the echo signal is eliminated. By using the signal processing methods of DDS and MZ interferometers, the phase-locked error problem in the optical frequency phase-locked loop system is solved, and high-precision frequency-modulated laser ranging is achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SOUTHEAST UNIV
- Filing Date
- 2025-11-25
- Publication Date
- 2026-06-12
AI Technical Summary
Existing linear frequency modulation systems based on optical frequency phase-locked loops fail to effectively eliminate phase-locked errors introduced by changes in echo signal amplitude, resulting in variations in the amplitude of the phase detector's output error signal. This makes it impossible to accurately compensate for the nonlinearity of the frequency modulation signal source, thus affecting ranging accuracy.
An AGC (Automatic Gain Control) stage is introduced between the optical detector and the phase detector. The amplitude of the detection signal is kept constant through automatic gain amplification, eliminating the phase-locked error caused by amplitude variation. A triangular wave modulation signal is generated by DDS to modulate the DFB laser, and high-precision compensation control of the signal is achieved through beat frequency of the MZ interferometer and integral amplifier.
It improves phase-locked loop accuracy and frequency calculation accuracy, significantly enhancing the overall accuracy and reliability of frequency-modulated laser ranging. The frequency calculation accuracy is improved by 5 times, achieving high-precision real-time compensation control for frequency modulation nonlinearity error.
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Figure CN122194099A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of frequency-modulated laser ranging technology, specifically relating to an optical frequency phase-locked loop linear frequency modulation system based on AGC and its frequency modulation method. Background Technology
[0002] When using a DFB laser as the measurement light source, the optical frequency modulation of the laser is achieved by changing the laser's driving current, which in turn changes the laser's wavelength. The modulation current is directly proportional to the wavelength, while the laser wavelength is inversely proportional to the optical frequency. Therefore, when the laser is modulated using a linear modulation current, its output optical frequency will inevitably change nonlinearly. Thus, the modulation current of the laser needs to be corrected to reduce the nonlinearity of the laser modulation. However, linear frequency modulated laser ranging has a large modulation bandwidth and a high tuning rate, so the modulation nonlinearity cannot be completely eliminated. Usually, an opto-phase-locked loop is used to lock the output optical frequency of the linear frequency modulated laser in real time to reduce the modulation nonlinearity.
[0003] A typical linear frequency modulation system based on an optical frequency phase-locked loop is as follows: Figure 1 As shown (excluding the AGC step), in Figure 1 In this circuit, the integrator, DFB laser, fiber optic path, and optical detector constitute a voltage-controlled oscillator (VCO), whose output signal is coupled with the phase-locked signal U. R (t) is compared, and the phase error signal between the two signals is output. After passing through a loop filter and an integrating amplifier, it is fed back to the control signal of the laser, causing the oscillation frequency of the VCO to shift towards the phase-locked signal U. R The frequencies of (t) converge until they are the same, so that the phase difference between the output signal and the reference signal reaches a fixed value, causing the system to enter a "locked" state, and the DFB laser emits periodic linear frequency modulated laser.
[0004] However, during the modulation of the laser, the change in the driving current causes a change in the carrier concentration of the semiconductor laser. Therefore, the optical frequency modulation process is inevitably accompanied by laser intensity modulation, so the amplitude of the signal also shows a linear change, resulting in uneven echo signal power. The power change trend is basically consistent with the modulation signal.
[0005] Variations in the echo signal amplitude have a significant impact on linear frequency modulation systems based on optical frequency phase-locked loops. These amplitude variations will cause changes in the amplitude of the error signal output by the phase detector, introducing phase-locked errors and making it impossible to accurately compensate for the nonlinearity of the frequency modulation signal source.
[0006] It is evident that, among the existing methods, the linear frequency modulation system based on optical frequency phase-locked loop does not consider the influence of the echo signal amplitude, which leads to changes in the amplitude of the error signal output by the phase detector, introducing phase-locked error and failing to accurately compensate for the nonlinearity of the frequency modulation signal source. Summary of the Invention
[0007] In view of this, the present invention provides an optical frequency phase-locked loop linear frequency modulation system based on AGC and its frequency modulation method, which can eliminate the phase-locked error introduced by amplitude variation, realize high-precision real-time compensation control of frequency modulation nonlinear error, thereby improving the accuracy of frequency-modulated laser ranging.
[0008] To achieve the objectives of this invention, the following technical solutions are provided.
[0009] An AGC-based optical phase-locked loop linear frequency modulation system includes: an integrator, a DFB laser, an optical fiber path, an optical detector, an AGC stage, a phase detector, a loop filter, and an integrating amplifier; wherein, the integrator, DFB laser, optical fiber path, and optical detector constitute a voltage-controlled oscillator; the output terminal of the optical detector is connected to the input terminal of the AGC stage, the output terminal of the AGC stage is connected to the first input terminal of the phase detector, and the second input terminal of the phase detector receives a reference signal; the output terminal of the phase detector is connected to the input terminal of the loop filter, the output terminal of the loop filter is connected to the input terminal of the integrating amplifier, and the output terminal of the integrating amplifier is connected to the input terminal of the integrator; the output terminal of the integrator is connected to the modulation input terminal of the DFB laser.
[0010] The AGC stage includes an adjustable gain amplifier, which adjusts the gain by adjusting a sliding rheostat to amplify the amplitude of the input signal to a constant value.
[0011] The fiber optic path includes an MZ interferometer, which is used to beat the optical signal output by the DFB laser to generate an interference signal.
[0012] The reference signal is a phase-locked signal, and its frequency is set to be consistent with the frequency of the voltage-controlled oscillator output signal so that the system enters a locked state.
[0013] A linear frequency modulation method for optical frequency phase-locked loop based on AGC, using the system described in this invention, includes the following steps: (1) A triangular wave modulation signal is generated using a DDS, and the modulation signal is converted into a current signal by a voltage-current converter to modulate the DFB laser; (2) The DFB laser output signal is beat-frequencyd by an MZ interferometer to obtain an interference signal, which is then detected and received by an optical detector and converted into a voltage signal; (3) Perform AGC automatic gain amplification on the voltage signal output by the optical detector to obtain a detection signal with flat amplitude; (4) The detection signal after automatic gain amplification is mixed with the reference signal and then filtered by a low-pass filter to obtain a DC signal containing phase error information. (5) The DC signal is amplified by an integrating amplifier and superimposed with a triangular wave modulation signal to form a driving signal, which continues to drive the DFB laser to realize the linear frequency modulation of the optical frequency phase-locked loop.
[0014] In step (3), the automatic gain control (AGC) operation is achieved by adjusting the sliding rheostat to make the amplitude of the detection signal constant and consistent with the amplitude of the reference signal.
[0015] In step (4), the frequency of the reference signal is set to be the same as the frequency of the voltage-controlled oscillator output signal to ensure that the phase-locked loop is locked.
[0016] Beneficial effects (1) This invention eliminates the phase-locked error introduced by amplitude variation by performing AGC (automatic gain amplification) on the detected power uneven signal, thereby achieving high-precision real-time compensation control of frequency modulation nonlinear error and improving the accuracy of frequency modulation laser ranging.
[0017] (2) By adjusting the gain of the AGC stage, the amplitude of the detection signal is stabilized at a constant level, avoiding amplitude changes in the output error signal of the phase detector, thereby improving the phase-locked loop accuracy and frequency calculation accuracy. Experimental results show that adding the AGC stage significantly improves the phase-locked loop accuracy and increases the frequency calculation accuracy by 5 times, thus enhancing the overall accuracy and reliability of frequency-modulated laser ranging.
[0018] (3) By introducing an AGC (Automatic Gain Control) stage, this invention automatically amplifies the voltage signal output by the optical detector, effectively eliminating signal amplitude fluctuations caused by changes in light intensity during DFB laser modulation. This design solves the phase-locked loop error problem caused by echo signal amplitude changes in traditional optical frequency phase-locked loop systems, and achieves high-precision real-time compensation control for frequency modulation nonlinearity errors.
[0019] (4) This invention not only improves system performance, but also has a simple structure and is easy to implement, and has broad application prospects. Attached Figure Description
[0020] Figure 1 This is a schematic diagram of the principle of an existing linear frequency modulation system based on an optical frequency phase-locked loop.
[0021] Figure 2 This is a schematic diagram of the optical frequency phase-locked loop linear frequency modulation system based on AGC of the present invention.
[0022] Figure 3 This is a schematic diagram of the AGC (Automatic Generative Control) process of the present invention. Detailed Implementation
[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.
[0024] This invention proposes an optical frequency phase-locked loop linear frequency modulation system based on AGC, such as... Figure 2 As shown, to eliminate the influence of echo signal amplitude variations, an AGC (Automatic Gain Control) stage is added between the optical detector and the phase detector in this invention system to flatten the gain amplification of the detected reference signal. The integrator, DFB laser, fiber optic path, and optical detector together form a voltage-controlled oscillator (VCO), whose output signal is coupled with the phase-locked signal U. R (t) is compared, and the phase error signal between the two signals is output. After passing through a loop filter and an integrating amplifier, it is fed back to the control signal of the laser, causing the oscillation frequency of the VCO to shift towards the phase-locked signal U. R The frequencies of (t) converge until they are the same, so that the phase difference between the output signal and the reference signal reaches a fixed value, causing the system to enter a "locked" state, and the DFB laser emits periodic linear frequency modulated laser.
[0025] Laser output light frequency It can be represented as: (1) in, k The modulation rate of the laser. Let be the initial output frequency of the laser, then the instantaneous phase of the output optical signal at time t is... It can be represented as: (2) After the laser signal passes through the power splitter, one path does not pass through the delay fiber, and the other path does. The expression for the signal that does not pass through the delay fiber is: (3) The laser travels through a length of L The time required for delay fiber is The signal expression after passing through the delayed fiber is: (4) In formulas (3) and (4), S ( t The amplitude of the laser signal is related to the modulation current of the laser, and is therefore a nonlinear function.
[0026] S 1( t )and S 2( t After passing through the coupler, the signals are combined to form a new laser signal. S c ( t ): (5) The voltage signal generated after the above signal passes through the photodetector is: (6) In the above formula, Let be the conversion efficiency of the photodetector. After removing the DC component and the high-frequency components that the detector cannot respond to, the above equation can be simplified to: (7) In the above formula, if amplitude variation is ignored, it is a standard sinusoidal signal with a frequency of... Phase is This signal, along with the reference voltage signal, enters the phase detector for mixing. Reference voltage signal The expression is: (8) in S R The amplitude of the reference signal, The frequency of the reference signal, As the initial phase of the reference signal, according to the phase-locked loop principle, it is necessary to... The value is set to The output signal after passing through the phase detector and low-pass filter is: (9) in, K pf This represents the gain value of the low-pass loop filter. K H , where is the gain value of the phase detector. According to formula (9), the amplitude of the mixed signal is not a constant value, so it will generate additional phase modulation on the signal, causing the phase information of the mixed signal to be distorted. Finally, after being superimposed on the preset triangular wave voltage drive signal, it cannot achieve the effect of photoelectric closed-loop feedback, and ultimately cannot make the system enter the "locked state".
[0027] Therefore, this invention proposes adding an "AGC" stage before the phase detector stage to automatically amplify the detection signal output by the photodetector, thereby amplifying the signal in formula (7). The transformation is made into a constant value, eliminating the phase-addition modulation problem introduced by amplitude changes. The specific implementation principle diagram of the AGC stage is shown below. Figure 3 As shown.
[0028] exist Figure 3 In this process, the input signal gain can be adjusted by changing the resistance value of the sliding rheostat R10. The signal after passing through the AGC stage changes according to formula (7): (10) in, It is a constant. The gain function of the AGC stage is given. Therefore, after passing through the AGC stage, the output signal of the photodetector becomes a signal with a constant amplitude, and the phase-addition modulation problem is solved.
[0029] Frequency-modulated laser ranging is a coherent laser ranging technology. This invention achieves accurate distance measurement through linear modulation of the laser frequency and coherent detection. In frequency-modulated laser ranging, the distance information of the target can be obtained by measuring the frequency of the beat frequency signal returned by the target, which contains distance information. This invention also provides a linear frequency modulation method based on an AGC-based optical frequency phase-locked loop. The specific steps of this system implementation are as follows: (1) Construct the linear frequency modulation system described in this invention, use DDS technology to generate a triangular wave modulation signal, and convert the modulation signal into a current signal through a voltage-current converter to modulate the DFB laser; (2) An MZ interferometer is set up to beat the laser output signal to obtain an interference signal. The interference signal is detected and received by a detector to obtain a voltage signal whose amplitude varies with the amplitude of the modulation signal. (3) Perform AGC (Automatic Gain Amplification) operation on the voltage signal output by the detector to obtain a detection signal with flat amplitude, and the signal amplitude is consistent with the reference signal amplitude; (4) The probe signal after automatic gain amplification is passed through a phase detector and a low-pass filter to obtain a DC signal containing phase error information; (5) The DC signal is amplified by the integrating amplifier and superimposed with the modulated triangular wave signal to continue driving the laser; The above process achieves photoelectric phase-locked linear frequency modulation with AGC component, eliminating the influence of echo signal amplitude variation and improving the linearity of DFB laser output frequency.
[0030] This invention includes, but is not limited to, the above embodiments. Any equivalent substitutions or partial improvements made under the spirit and principles of this invention shall be considered within the scope of protection of this invention.
Claims
1. A linear frequency modulation system based on AGC (Automatic Generation Control) optical phase-locked loop, characterized in that, include: The system comprises an integrator, a DFB laser, an optical fiber path, an optical detector, an AGC stage, a phase detector, a loop filter, and an integrating amplifier; wherein the integrator, DFB laser, optical fiber path, and optical detector constitute a voltage-controlled oscillator. The output of the optical detector is connected to the input of the AGC circuit, the output of the AGC circuit is connected to the first input of the phase detector, and the second input of the phase detector receives a reference signal. The output of the phase detector is connected to the input of the loop filter, the output of the loop filter is connected to the input of the integrating amplifier, and the output of the integrating amplifier is connected to the input of the integrator. The output of the integrator is connected to the modulation input of the DFB laser.
2. The system according to claim 1, characterized in that, The AGC stage includes an adjustable gain amplifier, which adjusts the gain by adjusting a sliding rheostat to amplify the amplitude of the input signal to a constant value.
3. The system according to claim 1, characterized in that, The fiber optic path includes an MZ interferometer, which is used to beat the optical signal output by the DFB laser to generate an interference signal.
4. The system according to any one of claims 1-3, characterized in that, The reference signal is a phase-locked signal, and its frequency is set to be consistent with the frequency of the voltage-controlled oscillator output signal so that the system enters a locked state.
5. A linear frequency modulation method for optical frequency phase-locked loops based on AGC, characterized in that, The system according to claim 1 includes the following steps: (1) A triangular wave modulation signal is generated using a DDS, and the modulation signal is converted into a current signal by a voltage-current converter to modulate the DFB laser; (2) The DFB laser output signal is beat-frequencyd by an MZ interferometer to obtain an interference signal, which is then detected and received by an optical detector and converted into a voltage signal; (3) Perform AGC automatic gain amplification on the voltage signal output by the optical detector to obtain a detection signal with flat amplitude; (4) The detection signal after automatic gain amplification is mixed with the reference signal and then filtered by a low-pass filter to obtain a DC signal containing phase error information. (5) The DC signal is amplified by an integrating amplifier and superimposed with a triangular wave modulation signal to form a driving signal, which continues to drive the DFB laser to realize the linear frequency modulation of the optical frequency phase-locked loop.
6. The method according to claim 5, characterized in that, In step (3), the automatic gain control (AGC) operation is achieved by adjusting the sliding rheostat to make the amplitude of the detection signal constant and consistent with the amplitude of the reference signal.
7. The method according to claim 5 or 6, characterized in that, In step (4), the frequency of the reference signal is set to be the same as the frequency of the voltage-controlled oscillator output signal to ensure that the phase-locked loop is locked.