An optical frequency comb repetition frequency locking device and method based on automatic power control

Abstract

The application discloses an optical frequency comb repetition frequency locking device and method based on automatic power control, and relates to the technical field of optical frequency combs. The device comprises an optical frequency comb optical system, a fiber length automatic control system and an automatic power control system. The optical frequency comb optical system is used for generating an optical frequency comb repetition frequency. The fiber length automatic control system locks the optical frequency comb repetition frequency output by the optical frequency comb optical system. The automatic power control system is used for adaptively adjusting the signal power size in the corresponding link when the optical frequency comb light power or the reference source power changes, so that the input power of the local oscillator and the radio frequency port of the mixer is maintained at a predetermined power. The technical scheme of the application improves the stability and accuracy of the input power of the radio frequency and local oscillator ports of the mixer, strengthens the suppression of noise in the link, enables the phase-locked loop of the fiber length automatic control system to be maintained in the best working state for a long time, and improves the practicability of the optical frequency comb product.

Description

Technical Field

[0001] This invention relates to the technical field of optical frequency combs, and more specifically, to a device and method for locking the repetition frequency of an optical frequency comb based on automatic power control. Background Technology

[0002] The emergence and development of optical frequency combs have greatly promoted the advancement of science and technology such as ultrafast optics, precision spectral measurement, optical frequency synthesizers, and optical frequency standards. With its ultra-stable and precise comb spectrum, the optical frequency comb acts like a transmission gear, easily coupling optical frequencies with microwave frequencies, thus ending the era of difficulty in accurately and stably measuring optical frequencies. It also fostered the development and application of optical clocks. Furthermore, due to the specific relationship between frequency and parameters such as time and spatial distance, optical frequency combs have wide applications in precision ranging and optical frequency transmission, and possess enormous market potential in satellite launch and recovery, navigation, electronic countermeasures, and precision CNC manufacturing.

[0003] The effective application of optical frequency combs relies on their stable and precise repetition frequency control. Currently, both domestic and international precision optical frequency comb repetition frequency control uses traditional analog phase-locked loops, and their block diagram is attached. Figure 2As shown. Traditional analog phase-locked loops (PLLs) typically use photodetectors to convert the optical frequency comb signal into an radio frequency (RF) signal with a repetition frequency. This RF signal is then amplified by a low-pass amplifier and transmitted to a mixer. Simultaneously, a reference signal from an ultra-stable reference source is amplified by a low-noise amplifier and also transmitted to the mixer. The mixer discriminates the RF and reference signals to obtain a frequency difference signal, which is used to adjust the repetition frequency output by the optical frequency comb system, thus locking the optical frequency comb's repetition frequency. The inventors noted that the transfer function of a traditional analog PLL is calculated based on loop transfer parameters measured at a specific moment to achieve optimal loop performance. When the optical frequency comb's operating environment changes, components age, or the reference source is replaced, the loop transfer parameters change, causing the PLL to deviate from its optimal state and even lose lock due to instability. This affects the use of the optical frequency comb and the accuracy of measurement results. Furthermore, instability introduces more amplitude noise into the PLL, which directly affects the optical frequency comb's repetition frequency through the PLL's output control signal, severely degrading the stability and accuracy of the repetition frequency. Currently, those skilled in the art typically maintain and adjust frequency locking devices by replacing aging components, adjusting phase-locked loop (PLL) control parameters, and controlling the operating temperature to reduce parameter drift caused by environmental changes. However, these methods can only play an auxiliary role in maintaining the stability of the locking device, and require real-time monitoring of the locking device's status and corresponding adjustments based on the status. This process is cumbersome and cannot fundamentally solve the problem of poor PLL performance. Furthermore, the causes of PLL performance degradation are extremely complex, and no effective way to improve the repetition frequency locking capability of optical frequency combs has yet been found. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for locking the repetition frequency of an optical frequency comb based on automatic power control, which solves the problems of large amplitude noise and low stability and accuracy of the repetition frequency in the closed-loop control of the optical frequency comb repetition frequency in the prior art.

[0005] The technical solution of the present invention is: to provide an optical frequency comb repetition frequency locking device based on automatic power control, the device comprising:

[0006] The optical frequency comb optical system is used to generate the optical frequency comb repetition frequency and output the optical frequency comb repetition frequency to the fiber length automatic control system.

[0007] The fiber optic length automatic control system is used to lock the repetition frequency of the optical frequency comb output by the optical frequency comb optical system.

[0008] The automatic fiber optic length control system includes a photodetector, an ultra-stable reference source, a mixer, and a PID controller. The photodetector converts the optical frequency comb signal into an RF electrical signal with a frequency matching the repetition frequency of the optical signal, and provides it to the RF port of the mixer. The ultra-stable reference source generates a reference electrical signal with high frequency stability and provides it to the local oscillator port of the mixer. The mixer discriminates the RF electrical signal and the reference electrical signal to obtain a frequency difference signal. The PID controller performs proportional-integral-differential operations on the frequency difference signal to convert it into a smooth first control signal. This first control signal is used to adjust the repetition frequency of the optical frequency comb, and the voltage of the control signal is inversely proportional to the repetition frequency of the optical frequency comb.

[0009] The first automatic power control system is used to adjust the input signal power at the mixer's RF port to maintain it at a predetermined power value;

[0010] The second automatic power control system is used to adjust the input signal power at the local oscillator port of the mixer to maintain it at a predetermined power value.

[0011] Furthermore, the automatic fiber length control system also includes an impedance matching network, a first low-pass amplifier module, an adjustable frequency divider, and a second low-pass amplifier module.

[0012] Impedance matching networks are used to adjust the output impedance of a photodetector to a preset impedance value so that it matches the input impedance of subsequent circuits.

[0013] The first low-pass amplifier module is used to extract and amplify the fundamental component of the first radio frequency signal output by the photodetector, and filter out the remaining higher harmonic components.

[0014] An adjustable frequency divider is used to divide the fundamental component of the first radio frequency electrical signal to obtain a second radio frequency electrical signal with the same frequency as the output signal of the ultra-stable reference source.

[0015] The second low-pass amplifier module is used to extract and amplify the fundamental component in the second radio frequency signal and filter out the remaining higher harmonic components.

[0016] Furthermore, the first automatic power control system is used to automatically control the fundamental component of the second radio frequency electrical signal output by the second low-pass amplifier module, so that the input power at the RF port of the mixer remains unchanged. The first automatic power control system includes a voltage-controlled attenuator, an RF coupler, a power detector, a voltage comparator, an adjustable reference voltage source, and a PI controller.

[0017] The voltage-controlled attenuator is used to adjust the attenuation of the fundamental component of the second radio frequency signal according to the signal fed back by the PI controller, so that the input signal power of the mixer RF port is constant. The attenuation of the voltage-controlled attenuator is proportional to the voltage value of the feedback signal from the PI controller.

[0018] The radio frequency coupler is used to couple the power of the fundamental component of the second electrical signal to the power detector for subsequent power detection.

[0019] The power detector is used to receive the radio frequency power signal output by the radio frequency coupler and convert the radio frequency power signal into a DC voltage signal;

[0020] The voltage comparator receives the ultra-stable reference voltage from the adjustable reference voltage source, compares the DC voltage signal with the ultra-stable reference voltage, and outputs a voltage difference signal.

[0021] An adjustable reference voltage source is used to provide an adjustable, ultra-stable reference voltage for a voltage comparator;

[0022] The PI controller is used to perform proportional-integral calculations on the voltage difference signal, converting the voltage difference signal into a smooth second control signal. The second control signal is used to adjust the degree of attenuation of the fundamental component of the second radio frequency signal by the voltage-controlled attenuator.

[0023] Furthermore, the automatic fiber length control system also includes a low-noise amplifier, which is used to amplify the power of the reference electrical signal output by the ultra-stable reference source.

[0024] The second automatic power control system is used to automatically control the power of the reference electrical signal output by the low-noise amplifier, so that the input power at the local oscillator port of the mixer remains constant.

[0025] Furthermore, the automatic fiber length control system also includes a high-voltage drive module and an optical pump source;

[0026] The high-voltage drive module is used to amplify the voltage of the first control signal to a predetermined high voltage, so that the voltage-amplified first control signal can fully drive the piezoelectric ceramic in the optical frequency comb system.

[0027] An optical pump source is used to provide a short-wavelength pump source for an optical frequency comb optical system, which uses the short-wavelength pump source to generate the optical frequency comb repetition frequency.

[0028] The technical solution of the present invention also provides a method for locking the repetition frequency of an optical frequency comb for use in the above-mentioned optical frequency comb repetition frequency locking device based on automatic power control, characterized in that the method includes:

[0029] Step 1: Use an optical pump source to provide short-wavelength pump light to the optical frequency comb optical system. The optical frequency comb optical system generates an optical frequency comb signal based on the short-wavelength pump light.

[0030] Step 2: Use a photodetector to convert the optical frequency comb signal into a first radio frequency electrical signal with a repetition frequency. Divide the fundamental component of the first radio frequency electrical signal to obtain a second radio frequency electrical signal with the same frequency as the output signal of the ultra-stable reference source. Input the fundamental component of the second radio frequency electrical signal into the RF port of the mixer through the first automatic power control system.

[0031] Step 3: Use the first automatic power control system to adaptively adjust the input power at the RF port of the mixer so that the input power at the RF port remains constant.

[0032] Step 4: Generate a reference electrical signal using an ultra-stable reference source, amplify the power of the reference electrical signal using a noise amplifier, and input the amplified reference electrical signal into the local oscillator port of the mixer through the second automatic power control system.

[0033] Step 5: Use the second automatic power control system to adaptively adjust the input power at the local oscillator port of the mixer so that the input power at the local oscillator port remains constant.

[0034] Step 6: Use a mixer to discriminate the fundamental component of the second radio frequency signal and the reference signal to obtain the frequency difference signal. The frequency difference signal is shaped and filtered by a PID controller into a smooth first control signal. The first control signal is amplified by a high-voltage drive module and then input into the optical frequency comb optical system.

[0035] Step 7: The piezoelectric ceramic inside the optical frequency comb system adjusts the length of the fiber ring cavity according to the amplified first control signal, wherein the voltage of the first control signal is inversely proportional to the repetition frequency of the optical frequency comb.

[0036] Furthermore, step 3 specifically includes the following steps:

[0037] Step 3.1: The voltage-controlled attenuator attenuates the fundamental component of the second radio frequency signal according to the second control signal output by the PI controller, and uses the radio frequency coupler to separate the fundamental component of the second radio frequency signal with a predetermined power ratio, and couples it to the power detector as a radio frequency power signal;

[0038] Step 3.2: The radio frequency power signal is converted into a DC voltage signal using a power detector, and the DC voltage signal is transmitted to a voltage comparator. The voltage comparator receives the ultra-stable reference voltage output from the adjustable reference voltage source, and subtracts the DC voltage signal from the ultra-stable reference voltage to obtain the voltage difference signal, which is then transmitted to the PI controller.

[0039] Step 3.3: Use the PI controller to smooth and filter the voltage difference signal to obtain the second control signal. Use the second control signal to adjust the attenuation of the voltage-controlled attenuator. The attenuation of the voltage-controlled attenuator is proportional to the voltage value of the feedback signal from the PI controller.

[0040] Furthermore, the expression for the power attenuation of the fundamental component of the second radio frequency signal by the voltage-controlled attenuator in step 3.1 is as follows:

[0041] AT = αRV T

[0042] In the formula, AT represents the attenuation power in dB, α is the attenuation coefficient of the voltage-controlled attenuator, α = 5.2 dB / V, and V T This is the voltage value of the second control signal output by the PI controller.

[0043] Furthermore, in step 3.2, the radio frequency power signal is converted into a DC voltage signal using a power detector, as shown below:

[0044] V P =γ·(P i -P s )

[0045] In the formula, V P P represents the converted DC voltage, γ is the conversion factor of the power detector, γ = 0.032V / dBm. i The power of the radio frequency power signal is expressed in dBm (P). s This represents the power of the radio frequency power signal when the conversion voltage is 0V, in dBm.

[0046] Furthermore, step 3.3 specifically includes:

[0047] The PI controller performs proportional and integral operations on the voltage difference signal, specifically:

[0048]

[0049] In the formula, V c (s) represents the Laplace transform of the second control signal, V e (s) represents the Laplace transform of the voltage difference signal, G is the scaling factor, p is the differential operator, s is the complex frequency variable in the Laplace transform, and represents the dynamic characteristic integral bandwidth of the proportional-integral controller as 1 / 2πp.

[0050] The attenuation level of the voltage-controlled attenuator is adjusted by the second control signal. When the voltage value of the feedback signal of the PI controller increases, the attenuation of the voltage-controlled attenuator increases, and the signal power of the fundamental component of the attenuated second radio frequency signal decreases. When the voltage value of the feedback signal of the PI controller decreases, the attenuation of the voltage-controlled attenuator decreases, and the signal power of the fundamental component of the attenuated second radio frequency signal increases.

[0051] Explanation of the principle:

[0052] By monitoring changes in internal parameters and radio frequency signals of various modules during the operation of existing optical frequency comb repetition frequency locking devices, the technicians of this invention discovered that changes in the optical frequency comb's operating environment, device aging, and reference source replacement cause variations in optical power, radio frequency amplifier device gain, reference source power, and radio frequency signal power. These parameter changes lead to variations in loop transmission parameters, especially the reference source power and radio frequency signal power. When their power is unstable, phase noise increases, which interferes with the mixer's output, preventing the phase-locked loop (PLL) from accurately tracking frequency errors. This results in unstable PLL performance, making it difficult to maintain optimal operating conditions and ultimately leading to loss of lock-in. This is a significant factor affecting the use of the optical frequency comb and the accuracy of measurement results. Therefore, to ensure that the PLL can accurately detect frequency errors, it is necessary to ensure the stability of the reference source power and radio frequency signal power input to the mixer. Stable reference source power and radio frequency signal power allow the PLL of the fiber length automatic control system to maintain optimal loop characteristics over a long period, providing greater tolerance to changes in the operating environment, product aging, and reference source replacement.

[0053] The beneficial effects of this invention are:

[0054] First, the technical solution of this invention sets up an automatic power control system in the front stage of both the local oscillator port and the radio frequency port of the mixer. The automatic power control system automatically controls the power of the fundamental component of the input second electrical signal and the reference power, so that the signal power output to the local oscillator port and the radio frequency port of the mixer is maintained at a predetermined power value, which improves the stability of the input power at the radio frequency port and the local oscillator port of the mixer. Compared with the existing technical solutions, the closed-loop adjustment process of the technical solution of this invention is more stable, which enables the phase-locked loop of the fiber length automatic control system to maintain the working state of optimal loop characteristics for a long time. It has a stronger tolerance to changes in the working environment, product aging, and replacement of the reference source, and can be better applied to field or sports scenarios, making the optical frequency comb product more practical.

[0055] Secondly, the automatic power control system in this invention can also suppress amplitude noise of the input signal at the local oscillator port and the radio frequency port, thus reducing amplitude noise. Furthermore, the automatic fiber length control system incorporates an impedance matching network, an adjustable frequency divider, and a low-pass amplifier module. The impedance matching network adjusts the output impedance of the photodetector to match the input impedance of subsequent circuits, preventing radio frequency signal reflection and improving transmission efficiency. The adjustable frequency divider and low-pass amplifier module work together to further adjust the radio frequency signal, ensuring it matches the frequency of the ultra-stable reference source output signal. Simultaneously, they filter out high-order harmonics in the link, further enhancing noise suppression and improving the overall performance of the radio frequency link. Compared to existing solutions, this invention enables a more stable and accurate optical frequency comb repetition frequency, making it suitable for high-precision measurement applications. Attached Figure Description

[0056] The advantages of the above and / or additional aspects of the present invention will become apparent and readily understood in the description of the embodiments taken in conjunction with the following drawings, wherein:

[0057] Figure 1 This is a schematic diagram of the overall structure of an optical frequency comb repetition frequency locking device based on automatic power control according to an embodiment of the present invention;

[0058] Figure 2 This is a schematic diagram of the overall structure of a conventional analog phase-locked loop according to an embodiment of the present invention. Detailed Implementation

[0059] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0060] In the following description, many specific details are set forth in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0061] like Figure 1 As shown, this embodiment provides an optical frequency comb repetition frequency locking device based on automatic power control. The device includes: an optical pump source, an optical frequency comb optical system, an automatic fiber length control system, a first automatic power control system, and a second automatic power control system.

[0062] Optical pump sources are used to provide short-wavelength pump sources for optical frequency comb optical systems.

[0063] An optical frequency comb system is used to generate the repetition frequency of an optical frequency comb using a short-wavelength pump light source (the repetition frequency is a performance indicator of the optical frequency comb signal, which varies with the length of the optical fiber), and outputs the optical frequency comb repetition frequency to an automatic optical fiber length control system.

[0064] The optical frequency comb optical system includes piezoelectric ceramics (PZT), gain fiber, and fiber ring cavity; the fiber ring cavity is formed by a ring-shaped fiber loop, the gain fiber is disposed in the fiber loop forming the fiber ring cavity, and the piezoelectric ceramic is bonded to the outer surface of one section of fiber in the fiber ring cavity.

[0065] The piezoelectric ceramic is used to receive the first control signal output by the high-voltage drive module and adjust the length of the optical fiber bonded to it according to the first control signal, thereby changing the length of the optical fiber ring cavity in the optical frequency comb optical system, so that the repetition frequency of the optical frequency comb generated by the optical frequency comb optical system changes to a preset frequency, which is the frequency of the reference electrical signal output by the ultra-stable reference source.

[0066] Specifically, the first control signal is ≥0V. The voltage of the first control signal is inversely proportional to the repetition frequency of the optical frequency comb. When the voltage of the first control signal increases, the piezoelectric ceramic elongates, the length of the fiber optic ring cavity increases, and the repetition frequency of the optical frequency comb generated by the optical frequency comb optical system decreases. When the voltage of the first control signal decreases, the piezoelectric ceramic shortens, the length of the fiber optic ring cavity decreases, and the repetition frequency of the optical frequency comb generated by the optical frequency comb optical system increases.

[0067] In this embodiment, the piezoelectric ceramic can adjust the length of the optical fiber bonded to it by electrostriction based on the first control signal output by the high voltage drive module, thereby changing the length of the optical fiber ring cavity in the optical frequency comb system. The optical frequency comb optical system can form a closed loop with the optical fiber length automatic control system. Through this closed loop, the repetition frequency of the optical frequency comb can be adaptively adjusted and locked to a preset frequency that is the same as the output signal of the ultra-stable reference source.

[0068] Gain fiber is used to convert short-wavelength pump light provided by an optical pump source into long-wavelength light, and at the same time enhance the long-wavelength light propagating in the fiber ring cavity to compensate for the loss caused by light transmission and reflection, and ensure that the intensity of the long-wavelength light signal remains stable during multiple reflections in the ring cavity.

[0069] Fiber optic ring cavities are used to cyclically transmit long-wavelength light, forming an optical frequency comb signal with a fixed comb tooth spacing (i.e., repetition frequency).

[0070] The fiber optic length automatic control system is used to lock the repetition frequency of the optical frequency comb output by the optical frequency comb optical system.

[0071] The automatic fiber optic length control system includes a photodetector, an impedance matching network, a first low-pass amplifier module, an adjustable digital divider, a second low-pass amplifier module, an ultra-stable reference source, a low-noise amplifier, a mixer, a PID controller, and a high-voltage drive module.

[0072] The output of the optical frequency comb optical system is connected to the input of the photodetector. The photodetector, impedance matching network, first low-pass amplifier module, adjustable frequency divider, and second low-pass amplifier module are connected in sequence. The output of the second low-pass amplifier module is connected to the RF port of the mixer through the first automatic power control system. The output of the ultra-stable reference source is connected to the input of the low-noise amplifier. The output of the low-noise amplifier is connected to the local oscillator port of the mixer through the second automatic power control system. The output of the mixer is connected to the piezoelectric ceramic through a PID controller and a high-voltage drive module in sequence.

[0073] A photodetector is used to convert the optical frequency comb light signal output by the optical frequency comb optical system into a first radio frequency electrical signal with the same frequency as the repetition frequency of the light signal.

[0074] In this embodiment, the photodetector can be a silicon photodiode, an InGaAs photodiode, or the like.

[0075] Impedance matching networks are used to adjust the output impedance of a photodetector to a preset impedance value so that it matches the input impedance of subsequent circuits.

[0076] In this embodiment, the preset impedance value is 50 ohms. The impedance matching network can adjust the output impedance of the photodetector to 50 ohms so that it matches the input impedance of the first low-pass amplifier module at the back end. This allows the first radio frequency signal to be transmitted efficiently in the subsequent circuit, avoids reflection of the first radio frequency signal, and thus improves the performance of the entire radio frequency link.

[0077] In this embodiment, the impedance matching network adjusts the transmission conditions of the first radio frequency (RF) signal based on the output characteristics of the photodetector, thereby improving the quality of subsequent signal processing. The RF link is the circuitry that generates and processes the RF signal, such as... Figure 1 As shown, the RF link includes a first automatic power control system, a photodetector, an impedance matching network, a first low-pass amplifier module, an adjustable digital divider, a second low-pass amplifier module, a mixer, a PID controller, and a high-voltage drive module.

[0078] The first low-pass amplifier module is used to extract and amplify the fundamental component in the first radio frequency signal and filter out the remaining higher harmonic components.

[0079] An adjustable frequency divider is used to divide the fundamental component of the first radio frequency signal to obtain a second radio frequency signal with the same frequency as the output signal of the ultra-stable reference source. In other words, the divided signal is used as the second radio frequency signal so that the frequency of the divided signal is consistent with the frequency of the output signal of the ultra-stable reference source.

[0080] It should be noted that in specific operations, frequency division can be achieved by setting the division ratio of the adjustable digital divider. For example, if the repetition frequency of the optical frequency comb is about 100MHz and the frequency of the ultra-stable reference source is about 10MHz, then the division ratio of the adjustable digital divider can be set to 10.

[0081] The second low-pass amplifier module is used to extract and amplify the fundamental component in the second radio frequency signal and filter out the remaining higher harmonic components.

[0082] In this embodiment, when using an adjustable frequency divider for frequency division, high-order harmonics will be generated due to its nonlinear characteristics. The second low-pass amplifier module can filter out the high-order harmonics generated by the frequency division and amplify the fundamental component of the second radio frequency signal.

[0083] Ultra-stable reference sources are used to provide a reference electrical signal with high frequency stability for radio frequency links.

[0084] In this embodiment, the ultra-stable reference source is a device that can generate a very stable and accurate signal frequency. It is typically used in high-precision applications, such as communications, measurement, metrology, and frequency standards, to provide a reliable reference signal; the ultra-stable reference source can be a hydrogen clock, a rubidium clock, etc.

[0085] To achieve efficient signal processing and system stability, the frequency of the reference electrical signal output by the ultra-stable reference source needs to be close to the frequency of the fundamental component of the second radio frequency electrical signal. This matching frequency relationship is an important basis for selecting or customizing a reference source. That is, after determining the frequency of the fundamental component of the second radio frequency electrical signal by measurement, the ultra-stable reference source is selected or customized based on the frequency of the fundamental component of the second radio frequency electrical signal.

[0086] Low-noise amplifiers are used to amplify the power of the reference electrical signal output from an ultra-stable reference source.

[0087] The mixer is used to discriminate the fundamental component of the second radio frequency signal and the reference signal to obtain the frequency difference signal (that is, to mix the two signals and identify and extract the frequency information in the two signals to obtain the frequency difference between the two signals).

[0088] A PID controller (i.e., proportional-integral-derivative controller) is used to perform proportional-integral-derivative operations on the frequency difference signal, converting the frequency difference signal into a smooth first control signal.

[0089] The high-voltage drive module is used to amplify the voltage of the first control signal to a predetermined high voltage, so that the voltage-amplified first control signal can fully drive the piezoelectric ceramic in the optical frequency comb system, wherein the predetermined high voltage is 150V.

[0090] The first automatic power control system is used to automatically control the power of the fundamental component of the second radio frequency signal output by the second low-pass amplifier module, so that the input power at the mixer's radio frequency port remains unchanged. Specifically, the fundamental component of the second radio frequency signal is transmitted to the mixer's radio frequency port through the first automatic power control system. The first automatic power control system can adaptively adjust the fundamental component of the second radio frequency signal to a predetermined power value, which is consistent with the input power value required at the mixer's radio frequency port.

[0091] In this embodiment, the first automatic power control system is set at the front end of the mixer's RF port. When the optical frequency comb power changes, it can timely and adaptively adjust the power value of the fundamental component of the second RF electrical signal, which is equivalent to adaptively adjusting the gain of the RF link. This keeps the input signal power at the mixer's RF port with frequency discrimination function constant, thereby ensuring that the transmission parameters of the phase-locked loop do not change.

[0092] The first automatic power control system includes a voltage-controlled attenuator, an RF coupler, a power detector, a voltage comparator, an adjustable reference voltage source, and a PI controller; the voltage-controlled attenuator, RF coupler, power detector, voltage comparator, and PI controller are connected in sequence to form a loop, and the adjustable reference voltage source is connected to the voltage comparator.

[0093] The voltage-controlled attenuator is used to adjust the attenuation of the fundamental component of the second radio frequency signal (i.e., adjust the radio frequency link gain) according to the signal fed back by the PI controller, so as to keep the power of the fundamental component of the second radio frequency signal constant, and thus keep the input signal power of the mixer radio frequency port constant.

[0094] Specifically, the attenuation of the voltage-controlled attenuator is directly proportional to the voltage value of the feedback signal from the PI controller. When the voltage value of the feedback signal from the PI controller increases, the attenuation of the voltage-controlled attenuator increases, and the signal power of the fundamental component of the second radio frequency signal decreases. When the voltage value of the feedback signal from the PI controller decreases, the attenuation of the voltage-controlled attenuator decreases, and the signal power of the fundamental component of the second radio frequency signal increases.

[0095] The radio frequency coupler is used to couple the power of the fundamental component of the second electrical signal to a power detector for subsequent power detection.

[0096] In this embodiment, the radio frequency coupler extracts a portion of the signal from the main signal path through coupling. This portion is usually a part of the fundamental component of the second electrical signal. This extraction does not affect the transmission process of the main signal; it simply separates a small part of the signal and transmits this part of the signal to the power detector.

[0097] The power detector is used to receive the radio frequency power signal output by the radio frequency coupler and convert the radio frequency power signal into a DC voltage signal.

[0098] The voltage comparator is used to receive the ultra-stable reference voltage output from the adjustable reference voltage source, compare the DC voltage signal with the ultra-stable reference voltage, and output the voltage difference signal.

[0099] An adjustable reference voltage source is used to provide an adjustable, ultra-stable reference voltage for a voltage comparator.

[0100] It should be noted that the ultra-stable reference voltage corresponds to the magnitude of the signal power input to the mixer's RF port. That is, when the signal power input to the mixer's RF port is determined, the voltage of the adjustable reference voltage source is also determined. In this way, after the automatic power control of the first automatic power control system, the power value of the second RF signal fundamental component can remain unchanged when it is transmitted to the mixer's RF port.

[0101] The PI controller is used to perform proportional-integral calculations on the voltage difference signal, converting the voltage difference signal into a smooth second control signal. This second control signal is used to adaptively adjust the attenuation level of the voltage-controlled attenuator, that is, to adjust the attenuation level applied by the voltage-controlled attenuator when attenuating the power of the fundamental component of the second electrical signal.

[0102] The second automatic power control system is used to automatically control the power of the reference electrical signal output by the low-noise amplifier, so that the input power at the local oscillator port of the mixer remains unchanged. Specifically, the reference electrical signal is transmitted to the local oscillator port of the mixer through the second automatic power control system, which can adaptively adjust the reference electrical signal to a predetermined power value that is consistent with the input power value required at the local oscillator port of the mixer.

[0103] In this embodiment, similarly, the automatic power control system is set at the front end of the mixer local oscillator port. When the reference source power changes, it can adjust the power value of the reference electrical signal in a timely and adaptive manner, so that the input signal power at the mixer local oscillator port remains constant, thereby ensuring that the transmission parameters of the phase-locked loop do not change.

[0104] The second automatic power control system shares the same structural system as the first automatic power control system, including a voltage-controlled attenuator, an RF coupler, a power detector, a voltage comparator, an adjustable reference voltage source, and a PI controller; its signal processing process is the same as that of the first automatic power control system, and will not be described in detail here.

[0105] This application also provides a method for locking the repetition frequency of an optical frequency comb based on automatic power control. This method further includes automatic control of the input power at the local oscillator port and the RF port of the mixer. Specifically, this method for locking the repetition frequency of an optical frequency comb based on automatic power control includes the following steps:

[0106] Step 1: Use an optical pump source to provide short-wavelength pump light to the optical frequency comb optical system. The optical frequency comb optical system generates an optical frequency comb signal based on the short-wavelength pump light.

[0107] Specifically, the optical pump source inputs short-wavelength pump light into the optical frequency comb system. The short-wavelength pump light is pumped into enhanced long-wavelength light by the gain fiber. The long-wavelength light circulates in the fiber ring cavity to form an optical frequency comb with a fixed comb tooth spacing (i.e., repetition frequency).

[0108] Step 2: Using a photodetector, the optical frequency comb signal is converted into a first radio frequency (RF) signal with a repetition frequency. The fundamental component of the first RF signal is divided to obtain a second RF signal with the same frequency as the output signal of the ultra-stable reference source. The fundamental component of the second RF signal is then input to the RF port of the mixer through a first automatic power control system. Specifically, this includes:

[0109] The optical frequency comb signal output by the optical frequency comb optical system is converted into a first radio frequency (RF) signal with a repetition frequency by a photodetector. This first RF signal is transmitted to a first low-pass amplifier module via an impedance matching network. The impedance matching network adjusts the output impedance of the photodetector to a preset impedance value, matching it with the input impedance of the first low-pass amplifier module. This is expressed as:

[0110]

[0111] In the formula, R′ P R is the output impedance of the photodetector after adjustment by the impedance matching network. P The output impedance of the photodetector before adjustment is preset to 50Ω.

[0112] In this embodiment, the photodiode in the photodetector operates in a reverse-biased state, and its output impedance is in a high-impedance state. Let its output impedance be R. p R pThe impedance value is 50Ω, which needs to be adjusted to the preset impedance value of 50Ω to match the subsequent circuit modules. The input impedance of the low-pass amplifier module is 50 ohms, thus achieving impedance matching. When setting up the impedance matching network, a 50Ω resistor can be connected in parallel at the output port of the photodetector and grounded to adjust the output impedance of the photodetector. The first radio frequency signal is impedance matched with the low-pass amplifier module through the impedance matching network, which can avoid signal reflection, improve the transmission efficiency of the first radio frequency signal in the subsequent circuit, and improve the performance of the entire radio frequency link.

[0113] After impedance matching is completed, the fundamental component of the first radio frequency signal is amplified by the first low-pass filter amplification module, and the remaining higher harmonic components are filtered out to obtain the fundamental component of the first radio frequency signal. The fundamental component of the first radio frequency signal is divided by an adjustable frequency divider to obtain a second radio frequency signal with the same frequency as the output signal of the ultra-stable reference source. The fundamental component of the second radio frequency signal is then amplified by the second low-pass amplification module, and the higher harmonics generated by the nonlinearity of the adjustable frequency divider are filtered out to obtain the fundamental component of the second radio frequency signal. The fundamental component of the second radio frequency signal is then input to the RF port of the mixer after passing through the first automatic power control system.

[0114] Step 3: Adaptively adjust the input power at the mixer's RF port using the first automatic power control system to keep the input power at the RF port constant. Specifically, this includes:

[0115] Step 3.1: The voltage-controlled attenuator attenuates the fundamental component of the second radio frequency signal according to the second control signal output by the PI controller, and uses the radio frequency coupler to separate the fundamental component of the second radio frequency signal with a predetermined power ratio, and couples it to the power detector as a radio frequency power signal;

[0116] The expression for the power attenuation of the fundamental component of the second radio frequency signal by the voltage-controlled attenuator is as follows:

[0117] AT=α·V T

[0118] In the formula, AT represents the attenuation power in dB, α is the attenuation coefficient of the voltage-controlled attenuator, α = 5.2 dB / V, and V T The voltage value of the second control signal output by the PI controller (i.e., the input control voltage received by the voltage-controlled attenuator).

[0119] In this embodiment, the RF coupler is a three-port RF device, including one input port and two output ports. The input port is used to receive the attenuated fundamental component signal of the second RF electrical signal output by the voltage-controlled attenuator. One output port is used to couple a portion of the energy separated from the fundamental component signal of the second RF electrical signal to the power detector. The other output port is used to input the separated fundamental component signal of the second RF electrical signal to the RF port of the mixer. The RF coupler couples a portion of the energy of the input RF signal to the power detector based on the coupling of electromagnetic fields and electromagnetic induction. The coupling ratio (i.e., the predetermined power ratio) is 1:10, that is, the power of the signal output by the coupling port of the RF coupler is 1 / 10 of the power of the input signal.

[0120] In this embodiment, the power detector can convert the power of the second radio frequency signal into a DC voltage signal according to a certain proportional coefficient. In actual use, the coupling ratio of the radio frequency coupler and the conversion ratio coefficient of the power detector are specifically determined according to the loop characteristics required by the automatic power control system. After determination, the power detector is selected.

[0121] Step 3.2: The radio frequency power signal is converted into a DC voltage signal using a power detector, and the DC voltage signal is transmitted to a voltage comparator. The voltage comparator receives the ultra-stable reference voltage output from the adjustable reference voltage source, and subtracts the DC voltage signal from the ultra-stable reference voltage to obtain the voltage difference signal, which is then transmitted to the PI controller.

[0122] In this embodiment, a logarithmic power detector is used, which can convert the logarithmic relationship between power and voltage into a linear relationship. The power detector is used to convert the radio frequency power signal into a DC voltage signal, as shown below:

[0123] V P =γ·(P i -P s )

[0124] In the formula, V P P represents the converted DC voltage, γ is the conversion factor of the power detector, γ = 0.032V / dBm. i The power of the radio frequency power signal (i.e., the input signal power of the power detector), measured in dBm, P. s This represents the power of the radio frequency power signal when the conversion voltage is 0V, in dBm.

[0125] In this embodiment, the selection of the ultra-stable reference voltage of the adjustable reference voltage source can be determined by the measurement data of the power detector. Specifically, there is a one-to-one correspondence between the input and output of the power detector. This one-to-one correspondence table can be obtained through experimental measurement. When the required stable power is known, the output voltage of the power detector can be obtained accordingly, and the ultra-stable reference voltage is set to this output voltage.

[0126] Step 3.3: Use the PI controller to smooth and filter the voltage difference signal to obtain the second control signal. Use the second control signal to adjust the attenuation of the voltage-controlled attenuator. The attenuation of the voltage-controlled attenuator is proportional to the voltage value of the feedback signal from the PI controller.

[0127] The PI controller performs proportional and integral operations on the voltage difference signal, specifically:

[0128]

[0129] In the formula, V c (s) represents the Laplace transform of the second control signal, V e (s) represents the Laplace transform of the voltage difference signal, G is the scaling factor, p is the differential operator, and s is the complex frequency variable in the Laplace transform, used to analyze the frequency domain behavior of the control system. The transfer function of the PI controller has an integral bandwidth of 1 / 2πp. In this embodiment, the PI controller performs proportional and integral operations on the voltage difference signal, which is equivalent to amplifying and low-pass filtering the voltage difference signal (i.e., smoothing filtering).

[0130] The attenuation level of the voltage-controlled attenuator is adjusted using the second control signal. Specifically, when the voltage value of the feedback signal from the PI controller increases, the attenuation of the voltage-controlled attenuator increases, and the signal power of the fundamental component of the attenuated second radio frequency signal decreases. When the voltage value of the feedback signal from the PI controller decreases, the attenuation of the voltage-controlled attenuator decreases, and the signal power of the fundamental component of the attenuated second radio frequency signal increases.

[0131] In this embodiment, when the second control signal increases by 1V, the attenuation of the voltage-controlled attenuator increases by 5.2dB; when the second control signal decreases by 1V, the attenuation of the voltage-controlled attenuator decreases by 5.2dB.

[0132] In this embodiment, the voltage-controlled attenuator is typically positive voltage controlled, while the voltage difference signal is in the positive and negative voltage range. Therefore, in a specific embodiment, a positive voltage bias can be introduced into the PI controller so that the output control signal is in the positive voltage range.

[0133] In this embodiment, the fundamental component of the input second electrical signal is automatically controlled by the first automatic power control system, so that the signal power output to the mixer RF port remains unchanged according to the set power value.

[0134] Step 4: Generate a reference electrical signal using an ultra-stable reference source, amplify the power of the reference electrical signal using a noise amplifier, and input the amplified reference electrical signal into the local oscillator port of the mixer through the second automatic power control system.

[0135] Step 5: Use the second automatic power control system to adaptively adjust the input power at the local oscillator port of the mixer so that the input power at the local oscillator port remains constant.

[0136] In this embodiment, the ultra-stable reference source outputs a high-frequency stable reference electrical signal to a low-noise amplifier for power amplification; the amplified reference electrical signal is then output as a constant-power electrical signal to the local oscillator port of the mixer via the second automatic power control system. The signal processing method in step 5 is the same as that in step 2, and will not be described again here.

[0137] Step 6: Use a mixer to discriminate the fundamental component of the second radio frequency signal and the reference signal to obtain a frequency difference signal. The frequency difference signal is shaped and filtered by a PID controller into a smooth first control signal. The first control signal is amplified by a high-voltage drive module and then input into the optical frequency comb optical system.

[0138] Step 7: The piezoelectric ceramic inside the optical frequency comb optical system adjusts the length of the fiber optic ring cavity according to the amplified first control signal. When the voltage of the first control signal increases, the piezoelectric ceramic elongates, the length of the fiber optic ring cavity increases, and the repetition frequency of the optical frequency comb generated by the optical frequency comb optical system decreases. When the voltage of the first control signal decreases, the piezoelectric ceramic shortens, the length of the fiber optic ring cavity decreases, and the repetition frequency of the optical frequency comb generated by the optical frequency comb optical system increases.

[0139] In this embodiment, the first control signal is amplified into a high-voltage control signal by the high-voltage drive module and then input into the optical frequency comb optical system to drive the piezoelectric ceramic (PZT) to adjust the length of the fiber ring cavity, thereby adjusting the repetition frequency and forming a closed-loop control. When the repetition frequency of the optical frequency comb is equal to the frequency of the ultra-stable reference source, the frequency difference signal is zero, and the fiber length automatic control system enters a balanced state. If this balanced state is broken by a disturbance, the fiber length control system continues to enter the same working process to complete the closed-loop adjustment until it is balanced again.

[0140] Example: Using a conventional optical frequency comb repetition frequency locking device without automatic power control (APC) and the optical frequency comb repetition frequency locking device with automatic power control of the present invention, closed-loop adjustment of the repetition frequency was performed respectively. The amplitude noise of the mixer RF port input signal was detected by a phase noise analyzer during the adjustment of the two devices. The detection results are shown in Table 1 below:

[0141] Table 1

[0142]

[0143] As can be seen from Table 1, when using the existing optical frequency comb repetition lock device without automatic power control for closed-loop adjustment, the amplitude noise of the input signal at the mixer's RF port is relatively large. However, when using the optical frequency comb repetition lock device with automatic power control in this invention for closed-loop adjustment, the amplitude noise of the input signal at the mixer's RF port is relatively small. The device of this invention has a suppression effect of about 20dB on amplitude noise and an optimization of power stability by nearly two orders of magnitude. Compared with the device of the existing technology, the device of this invention is more conducive to noise suppression and improves the stability of the input power at the mixer's RF port.

[0144] In this invention, the terms "installation," "connection," "linking," and "fixing" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; "linking" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of these terms in this invention according to the specific circumstances.

[0145] The shapes of the components in the accompanying drawings are schematic and may differ from their actual shapes. The drawings are only used to illustrate the principles of the present invention and are not intended to limit the present invention.

[0146] Although the invention has been disclosed in detail with reference to the accompanying drawings, it should be understood that these descriptions are merely exemplary and not intended to limit the application of the invention. The scope of protection of the invention is defined by the appended claims and may include various modifications, alterations, and equivalents made to the invention without departing from the scope and spirit of the invention.

Claims

1. A repetition frequency locking device for an optical frequency comb based on automatic power control, characterized in that, The device includes: The optical frequency comb optical system is used to generate the optical frequency comb repetition frequency and output the optical frequency comb repetition frequency to the fiber length automatic control system. The fiber optic length automatic control system is used to lock the repetition frequency of the optical frequency comb output by the optical frequency comb optical system. The automatic fiber length control system includes a photodetector, an ultra-stable reference source, a mixer, and a PID controller. The photodetector converts the optical frequency comb signal into a radio frequency electrical signal with a frequency matching the repetition frequency of the optical signal, and provides it to the radio frequency port of the mixer. The ultra-stable reference source generates a reference electrical signal with high frequency stability and provides it to the local oscillator port of the mixer. The mixer discriminates the radio frequency electrical signal and the reference electrical signal to obtain a frequency difference signal. The PID controller performs proportional-integral-differential operations on the frequency difference signal to convert it into a smooth first control signal. This first control signal is used to adjust the repetition frequency of the optical frequency comb, and the voltage of the control signal is inversely proportional to the repetition frequency of the optical frequency comb. The first automatic power control system is used to adjust the input signal power at the RF port of the mixer to keep it at a predetermined power value. Specifically, the first automatic power control system is used to automatically control the power of the fundamental component of the RF signal so that the input power at the RF port of the mixer remains unchanged. The first automatic power control system includes a voltage-controlled attenuator, an RF coupler, a power detector, a voltage comparator, an adjustable reference voltage source, and a PI controller. A voltage-controlled attenuator (VCO) adjusts the attenuation of the fundamental component of the radio frequency (RF) signal based on the feedback signal from the PI controller, ensuring a constant input signal power at the mixer's RF port. The attenuation level of the VCO is proportional to the voltage value of the feedback signal from the PI controller. An RF coupler couples the power of the fundamental component of the RF signal to a power detector for subsequent power detection. The power detector receives the RF power signal output from the RF coupler and converts it into a DC voltage signal. A voltage comparator receives a highly stable reference voltage from an adjustable reference voltage source and compares the DC voltage signal with the highly stable reference voltage, outputting a voltage difference signal. The adjustable reference voltage source provides an adjustable, highly stable reference voltage to the voltage comparator. The PI controller performs proportional-integral (PI) calculations on the voltage difference signal, converting it into a smooth second control signal. This second control signal adjusts the attenuation level of the fundamental component of the RF signal by the VCO. The second automatic power control system is used to adjust the input signal power at the local oscillator port of the mixer to maintain it at a predetermined power value. Specifically, the second automatic power control system is used to automatically control the power of the reference electrical signal so that the input power at the local oscillator port of the mixer remains unchanged.

2. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 1, characterized in that, The automatic fiber length control system also includes an impedance matching network, a first low-pass amplifier module, an adjustable frequency divider, and a second low-pass amplifier module. Impedance matching networks are used to adjust the output impedance of a photodetector to a preset impedance value so that it matches the input impedance of subsequent circuits. The first low-pass amplifier module is used to extract and amplify the fundamental component of the first radio frequency signal output by the photodetector, and filter out the remaining higher harmonic components. An adjustable frequency divider is used to divide the fundamental component of the first radio frequency electrical signal to obtain a second radio frequency electrical signal with the same frequency as the output signal of the ultra-stable reference source. The second low-pass amplifier module is used to extract and amplify the fundamental component in the second radio frequency signal and filter out the remaining higher harmonic components.

3. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 2, characterized in that, The first automatic power control system is used to automatically control the fundamental component of the second radio frequency electrical signal output by the second low-pass amplifier module, so that the input power at the RF port of the mixer remains unchanged.

4. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 1, characterized in that, The automatic fiber length control system also includes a low-noise amplifier, which is used to amplify the power of the reference electrical signal output by the ultra-stable reference source. The second automatic power control system is used to automatically control the power of the reference electrical signal output by the low-noise amplifier, so that the input power at the local oscillator port of the mixer remains unchanged.

5. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 1, characterized in that, The automatic fiber length control system also includes a high-voltage drive module and an optical pump source; The high-voltage drive module is used to amplify the voltage of the first control signal to a predetermined high voltage, so that the voltage-amplified first control signal can fully drive the piezoelectric ceramic in the optical frequency comb system. An optical pump source is used to provide a short-wavelength pump source for an optical frequency comb optical system, which uses the short-wavelength pump source to generate the optical frequency comb repetition frequency.

6. A method for locking the repetition frequency of an optical frequency comb in the optical frequency comb repetition frequency locking device based on automatic power control according to any one of claims 1-5, characterized in that, The method includes: Step 1: Use an optical pump source to provide short-wavelength pump light to the optical frequency comb optical system. The optical frequency comb optical system generates an optical frequency comb signal based on the short-wavelength pump light. Step 2: Use a photodetector to convert the optical frequency comb signal into a first radio frequency electrical signal with a repetition frequency. Divide the fundamental component of the first radio frequency electrical signal to obtain a second radio frequency electrical signal with the same frequency as the output signal of the ultra-stable reference source. Input the fundamental component of the second radio frequency electrical signal into the RF port of the mixer through the first automatic power control system. Step 3: Use the first automatic power control system to adaptively adjust the input power at the RF port of the mixer so that the input power at the RF port remains constant. Step 4: Generate a reference electrical signal using an ultra-stable reference source, amplify the power of the reference electrical signal using a noise amplifier, and input the amplified reference electrical signal into the local oscillator port of the mixer through the second automatic power control system. Step 5: Use the second automatic power control system to adaptively adjust the input power at the local oscillator port of the mixer so that the input power at the local oscillator port remains constant. Step 6: Use a mixer to discriminate the fundamental component of the second radio frequency signal and the reference signal to obtain the frequency difference signal. The frequency difference signal is shaped and filtered by a PID controller into a smooth first control signal. The first control signal is amplified by a high-voltage drive module and then input into the optical frequency comb optical system. Step 7: The piezoelectric ceramic inside the optical frequency comb system adjusts the length of the fiber ring cavity according to the amplified first control signal, wherein the voltage of the first control signal is inversely proportional to the repetition frequency of the optical frequency comb.

7. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 6, characterized in that, Step 3 specifically includes the following steps: Step 3.1: The voltage-controlled attenuator attenuates the fundamental component of the second radio frequency signal according to the second control signal output by the PI controller, and uses the radio frequency coupler to separate the fundamental component of the second radio frequency signal with a predetermined power ratio, and couples it to the power detector as a radio frequency power signal; Step 3.2: The radio frequency power signal is converted into a DC voltage signal using a power detector, and the DC voltage signal is transmitted to a voltage comparator. The voltage comparator receives the ultra-stable reference voltage output from the adjustable reference voltage source, and subtracts the DC voltage signal from the ultra-stable reference voltage to obtain the voltage difference signal, which is then transmitted to the PI controller. Step 3.3: Use the PI controller to smooth and filter the voltage difference signal to obtain the second control signal. Use the second control signal to adjust the attenuation of the voltage-controlled attenuator. The attenuation of the voltage-controlled attenuator is proportional to the voltage value of the feedback signal from the PI controller.

8. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 7, characterized in that, The expression for the power attenuation of the fundamental component of the second radio frequency signal by the voltage-controlled attenuator in step 3.1 is as follows: ； In the formula, The power attenuation is expressed in dB. The attenuation coefficient of the voltage-controlled attenuator. =5.2 dB / V, This is the voltage value of the second control signal output by the PI controller.

9. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 7, characterized in that, In step 3.2, the radio frequency power signal is converted into a DC voltage signal using a power detector, as shown below: ； In the formula, The converted DC voltage. The conversion factor of the power detector. =0.032 V / dBm, This refers to the power of the radio frequency power signal, measured in dBm. This represents the power of the radio frequency power signal when the conversion voltage is 0 V, in dBm.

10. The optical frequency comb repetition frequency locking device based on automatic power control as described in claim 7, characterized in that, Step 3.3 specifically includes: The PI controller performs proportional and integral operations on the voltage difference signal, specifically: ； In the formula, The Laplace transform of the second control signal, For the Laplace transform of the voltage difference signal, As a scaling factor, Let be the differential operator, s be the complex frequency variable in the Laplace transform, and let represent the integral bandwidth of the proportional-integral controller's dynamic characteristics. ; The attenuation level of the voltage-controlled attenuator is adjusted by the second control signal. When the voltage value of the feedback signal of the PI controller increases, the attenuation of the voltage-controlled attenuator increases, and the signal power of the fundamental component of the attenuated second radio frequency signal decreases. When the voltage value of the feedback signal of the PI controller decreases, the attenuation of the voltage-controlled attenuator decreases, and the signal power of the fundamental component of the attenuated second radio frequency signal increases.

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