Servo control circuit and frequency locking method

Abstract

The present invention provides a servo control circuit and a frequency locking method, which can be applied to the field of servo control technology. The servo control circuit includes: a filter for filtering an error signal to obtain a filtered signal; a common-phase input end of a first amplifier stage is connected to an output end of the filter, for amplifying the filtered signal to obtain an amplified signal; an input end of a slow feedback servo control unit is connected to an output end of the first amplifier stage, for determining a slow feedback control signal according to the amplified signal; a fast feedback servo control subcircuit, for determining a fast feedback control signal according to the error signal; the output ends of the slow feedback servo control unit and the fast feedback servo control subcircuit are connected to an input end of a device to be locked, for frequency modulating the device to be locked according to the slow feedback control signal and the fast feedback control signal, until the error signal meets a preset condition, so as to achieve frequency locking of the device to be locked.

Description

Technical Field

[0001] The present invention relates to the technical field of servo control, and in particular to a servo control circuit and a frequency locking method. Background Art

[0002] The requirements for laser linewidth in the field of precision measurement are getting higher and higher. Currently, the most commonly used method to narrow the laser linewidth and reduce the laser frequency noise is the PDH method (Pound-Drever-Hall, laser frequency stabilization method). The free-running laser is locked to the FP cavity (Fabry-Perot cavity) through the servo system PID (Proportional, Integral, Derivative control). The locked laser is generally called an ultra-stable laser. Ultra-stable lasers are used in the fields of optical clocks, gravitational wave detection, laser interferometry, and optical combs.

[0003] The influence of the servo system in PDH on the frequency noise of ultra-stable laser is second only to that of FP cavity. The equivalent input noise and gain of the servo system will affect the frequency noise of ultra-stable laser. At present, the servo control system generally used as PDH loop for ultra-stable laser generally has the disadvantages of large equivalent input noise, small low-frequency gain, poor temperature drift, etc., which can only meet the servo control requirements of ultra-stable laser of the order of 1e-16. Moreover, with the decrease of thermal noise of FP cavity, ultra-stable laser will definitely enter the order of 1e-18. The subsequent ultra-stable laser will enter FNC (Fiber Network Control, optical fiber transmission system control) and optical phase-locked loop for transmission. The noise in FNC and optical phase-locked loop also has an impact on ultra-stable laser. Therefore, the servo system in FDH, FNC and optical phase-locked loop needs to meet the ultra-stable laser of the order of 1e-19 at least, but these servo control systems cannot meet the servo control requirements of ultra-stable laser. Therefore, how to realize a servo control circuit with ultra-low noise, ultra-high gain and ultra-low temperature drift is a problem that needs to be solved in the prior art. Summary of the invention

[0004] In view of the above problems, the present invention provides a servo control circuit and a frequency locking method.

[0005] According to a first aspect of the present invention, there is provided a servo control circuit, comprising:

[0006] A slow feedback servo control subcircuit, the slow feedback servo control subcircuit comprising a filter, a first amplification stage and a slow feedback servo control unit;

[0007] The filter is used to filter the error signal to obtain a filtered signal, wherein the error signal represents the frequency deviation between the locking device and the device to be locked;

[0008] The in-phase input terminal of the first amplifier stage is connected to the output terminal of the filter, and the first amplifier stage is used to amplify the filtered signal to obtain an amplified signal, wherein the first amplifier stage includes a plurality of parallel operational amplifiers;

[0009] The input end of the slow feedback servo control unit is connected to the output end of the first amplifier stage, and the slow feedback servo control unit is used to determine a slow feedback control signal according to the amplified signal;

[0010] a fast feedback servo control subcircuit, the fast feedback servo control subcircuit being used to determine a fast feedback control signal according to the error signal; and

[0011] The output ends of the slow feedback servo control unit and the fast feedback servo control subcircuit are connected to the input ends of the device to be locked, and are used to frequency modulate the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal meets the preset conditions, so as to achieve frequency locking of the device to be locked.

[0012] According to an embodiment of the present invention, the first amplification stage includes an input resistor and a plurality of parallel operational amplification units, the operational amplification unit includes a first resistor, a second resistor, an operational amplifier and a third resistor, the first port of the first resistor is grounded, the second port of the first resistor is connected to the inverting input terminal of the operational amplifier, the first port of the second resistor is connected to the inverting input terminal of the operational amplifier, the second port of the second resistor is connected to the output terminal of the operational amplifier, and the first port of the third resistor is connected to the output terminal of the operational amplifier; the in-phase input terminals of the plurality of operational amplifiers are connected, and the second ports of the plurality of third resistors are connected to realize the parallel connection of the plurality of operational amplification units; the first port of the input resistor is connected to the output terminal of the filter, and the second port of the input resistor is connected to the in-phase input terminals of the plurality of operational amplifiers.

[0013] According to an embodiment of the present invention, the number of parallel stages of the operational amplifier units in the first amplifier stage is determined based on the resistance values ​​of the input resistor, the first resistor, the second resistor and the third resistor, and the voltage noise and current noise of the operational amplifier.

[0014] According to an embodiment of the present invention, the slow feedback servo control unit comprises an integral control subunit and a plurality of proportional integral control subunits connected in series, and the slow feedback servo control unit is further used to increase the gain of the servo control circuit.

[0015] According to an embodiment of the present invention, the above-mentioned slow feedback servo control subcircuit also includes a slow feedback output polarity switch, and the above-mentioned slow feedback output polarity switch includes a single-pole double-throw switch and an inverting amplifier follower. The output end of the last proportional-integral control subunit in the above-mentioned slow feedback servo control unit is connected to the first port of the above-mentioned single-pole double-throw switch, the output end of the above-mentioned last proportional-integral control subunit is connected to the input end of the above-mentioned inverting amplifier follower, and the second port of the above-mentioned single-pole double-throw switch is connected to the output end of the above-mentioned inverting amplifier follower.

[0016] According to an embodiment of the present invention, the fast feedback servo control subcircuit comprises at least one dual-zero dual-pole compensation unit and a fast feedback output polarity switch connected in series, and the fast feedback servo control subcircuit is also used to increase the bandwidth of the fast feedback control signal.

[0017] According to an embodiment of the present invention, the servo control circuit further includes a second amplifier stage, a common-phase input terminal of the second amplifier stage is connected to the error signal, an output terminal of the second amplifier stage is connected to a noise tester, so as to measure the actual equivalent input noise of the servo control circuit through the noise tester, wherein the first amplifier stage is the same as the second amplifier stage.

[0018] According to an embodiment of the present invention, in the case where the device to be locked includes at least two input terminals, the circuit further includes: an output terminal of the slow feedback servo control unit is connected to a first input terminal of the device to be locked, and an output terminal of the fast feedback servo control subcircuit is connected to a second input terminal of the device to be locked, for frequency modulating the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal satisfies the preset condition.

[0019] According to an embodiment of the present invention, in the case where the device to be locked includes an input terminal, the circuit further includes an adder, the first input terminal of the adder is connected to the output terminal of the slow feedback servo control unit, the second input terminal of the adder is connected to the output terminal of the fast feedback servo control subcircuit, the output terminal of the adder is connected to the input terminal of the device to be locked, and the adder is used to add the slow feedback control signal and the fast feedback control signal to obtain a control signal, which is used to perform frequency modulation on the device to be locked according to the control signal until the error signal meets the preset condition. A second aspect of the present invention provides a frequency locking method based on a servo control circuit, wherein the slow feedback servo control subcircuit includes a filter, a first amplifier stage and a slow feedback servo control unit, and the method includes:

[0020] Using the filter, the error signal is filtered to obtain a filtered signal, wherein the error signal represents the frequency deviation between the locking device and the device to be locked;

[0021] Amplifying the filtered signal using the first amplifier stage to obtain an amplified signal, wherein the first amplifier stage includes a plurality of parallel operational amplifiers;

[0022] Determine a slow feedback control signal according to the amplified signal by using the slow feedback servo control unit;

[0023] Determine a fast feedback control signal according to the error signal using a fast feedback servo control subcircuit; and

[0024] The slow feedback servo control unit and the fast feedback servo control subcircuit are used to frequency modulate the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal meets the preset conditions, so as to achieve the frequency locking of the device to be locked. According to the servo control circuit and frequency locking method provided by the present invention, the slow feedback servo control unit uses a filter to eliminate the influence of high-frequency noise on low-frequency noise, and the fast feedback servo control subcircuit directly accesses the error signal without considering the filter to ensure that the bandwidth is not affected by the filter, and ensures that the output fast feedback control signal has a high bandwidth; since the equivalent input noise and gain of the servo control circuit will affect the ultra-stable laser frequency noise, the first amplifier stage further reduces the equivalent input noise of the servo control circuit by using a multi-stage operational amplifier in parallel; due to the use of the first amplifier stage, based on the amplification factor of the first amplifier stage, the noise in the subsequent slow feedback servo control unit is offset and the gain of the servo control circuit is increased, so that the equivalent input noise of the servo control circuit mainly comes from the first amplifier stage, thereby, the servo control circuit has the advantages of small equivalent input noise and large gain, so as to meet the servo requirements of the ultra-stable laser of the order of 1e-18. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The above contents and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0026] Figure 1 A schematic diagram of the structure of a laser frequency stabilization circuit according to an embodiment of the present invention is shown;

[0027] Figure 2 A schematic diagram of a servo control circuit according to an embodiment of the present invention is shown;

[0028] FIG3 (a) shows a schematic diagram of the internal structure of the first amplifier stage according to an embodiment of the present invention;

[0029] FIG3( b ) is a schematic diagram showing a noise model of a first amplifier stage according to an embodiment of the present invention;

[0030] Figure 4 A schematic diagram showing a comparison between the equivalent input noise of FALC110 and the equivalent input noise of AD8130 according to an embodiment of the present invention is shown;

[0031] Figure 5 A schematic diagram showing a comparison between the equivalent input noise of the first amplifier stage according to an embodiment of the present invention and the equivalent input noise of a single ADA4523 is shown;

[0032] Figure 6 A schematic diagram showing a comparison between an equivalent input noise of a servo control circuit according to an embodiment of the present invention and an equivalent input noise of a first amplifier stage is shown;

[0033] Figure 7 A schematic diagram showing a comparison of equivalent input noises of first amplifier stages with different numbers of parallel stages according to an embodiment of the present invention is shown;

[0034] Figure 8 A schematic structural diagram of a servo control circuit according to another embodiment of the present invention is shown;

[0035] Fig. 9 A schematic structural diagram of a servo control circuit according to another embodiment of the present invention is shown;

[0036] Fig.10 A schematic diagram showing a comparison of frequency responses of a servo control circuit according to an embodiment of the present invention and a FALC110 fast feedback circuit is shown;

[0037] Fig.11 A schematic diagram showing a comparison between the instrument noise floor for testing equivalent input noise in a servo control circuit according to an embodiment of the present invention and the noise floor of other types of testing instruments;

[0038] FIG. 12 ( a ) is a schematic diagram showing a PDH closed-loop error signal noise generated by a servo control circuit of an ultra-stable laser U3 at 6.56e-4V / Hz according to an embodiment of the present invention;

[0039] FIG. 12 ( b ) is a schematic diagram showing the PDH closed-loop error signal noise generated by the ultra-stable laser U3 using a servo control circuit at 2.4e-4 V / Hz according to an embodiment of the present invention;

[0040] FIG. 12 ( c ) is a schematic diagram showing the PDH closed-loop error signal noise generated by the ultra-stable laser U3 using a servo control circuit at 3.2e-5 V / Hz according to an embodiment of the present invention;

[0041] FIG12( d ) is a schematic diagram showing a comparison between the free-running frequency noise of the ultra-stable laser U3 and the self-closed-loop error signal noise of the error detection board according to an embodiment of the present invention;

[0042] Fig.13 A schematic diagram showing a comparison of equivalent input noises of a servo control circuit according to an embodiment of the present invention and FALC110 is shown;

[0043] Fig.14 A schematic diagram showing a comparison of the gains of the servo control circuit and the FALC110 according to an embodiment of the present invention is shown;

[0044] Fig.15 A schematic diagram showing a comparison of temperature drifts of a servo control circuit and FALC110 according to an embodiment of the present invention is shown;

[0045] Fig.16 A schematic diagram showing a comparison of PDH closed-loop error signals of a servo control circuit according to an embodiment of the present invention and FALC110 is shown;

[0046] Fig.17 A circuit schematic diagram showing a servo control circuit according to an embodiment of the present invention applied to a phase-locked loop;

[0047] Fig.18 A schematic diagram showing a comparison of phase noise between a mixer phase-locked loop according to an embodiment of the present invention and a commercial SMA100B is shown;

[0048] Fig.19 A schematic diagram showing an application of a servo control circuit in an FNC according to an embodiment of the present invention is shown;

[0049] Fig. 20 A schematic diagram of phase noise comparison between a servo control circuit according to an embodiment of the present invention and FNC under a commercial SIM960 is shown;

[0050] Fig.21 A circuit diagram of an optical phase-locked loop using a servo control circuit according to an embodiment of the present invention is shown;

[0051] Fig. 22 A schematic diagram of phase noise of an optical phase-locked loop using a servo control circuit according to an embodiment of the present invention is shown;

[0052] Fig.23 A schematic diagram of error signal noise of an optical phase-locked loop using a servo control circuit according to an embodiment of the present invention is shown;

[0053] Fig.24 A flow chart of a frequency locking method according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0054] Below, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of concepts of the present invention.

[0055] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0056] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0057] When using expressions such as "at least one of A, B, and C, etc.", they should generally be interpreted according to the meaning of the expression commonly understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0058] In the technical solution of the present invention, the user information (including but not limited to user personal information, user image information, user device information, such as location information, etc.) and data (including but not limited to data used for analysis, stored data, displayed data, etc.) involved are all information and data authorized by the user or fully authorized by all parties, and the collection, storage, use, processing, transmission, provision, disclosure and application of the relevant data comply with relevant laws, regulations and standards, take necessary confidentiality measures, do not violate public order and good morals, and provide corresponding operation entrances for users to choose to authorize or refuse.

[0059] In the scenario of using personal information for automated decision-making, the methods, devices, and systems provided by the embodiments of the present invention provide users with corresponding operation portals for users to choose to agree or reject the automated decision-making results; if the user chooses to reject, the expert decision-making process will be entered. The expression "automated decision-making" here refers to the activity of automatically analyzing and evaluating an individual's behavioral habits, interests and hobbies, or economic, health, credit status, etc. through computer programs, and making decisions. The expression "expert decision-making" here refers to the activity of making decisions by people who specialize in a certain field, have specialized experience, knowledge and skills, and have reached a certain level of professionalism.

[0060] In the process of implementing the present invention, it is found that the core of PDH is to lock the free-running laser to the ultra-stable FP cavity, wherein the FP cavity is equivalent to a device that can identify the frequency jitter of the laser.

[0061] Figure 1 A schematic structural diagram of a laser frequency stabilization circuit according to an embodiment of the present invention is shown.

[0062] like Figure 1 As shown, the PDH (laser frequency stabilization circuit) 100 includes a laser (Laser) 110, an electro-optic modulator (EOM, Electro-optic Modulator) 120, a polarizing beam splitter prism (PBS, Polarizing Beam Splitter) 130, an FP cavity (Cavity) 140, a photodetector (PD, Photodetector) 150, a mixer (Mixer) 160, a radio frequency (RF, Radio Frequency) 170, a low-pass filter (LP Filter, Low-pass Filter) 180 and a servo control system (Servo) 190.

[0063] According to an embodiment of the present invention, the laser 110 can generate laser light, and the laser light phase-modulated by the electro-optic modulator 120 is then injected into the FP cavity 140 after passing through the polarization beam splitter prism 130. Due to the polarization beam splitter prism 130, the emitted light reflected from the FP cavity 140 will return to the photodetector 150, so that the photodetector 150 can be used to test the reflected light of the FP cavity 140. The signal measured by the photodetector 150 is demodulated by the radio frequency 170 and the mixer 160, and the demodulated signal is an ultra-stable error signal. Locking the demodulated error signal to 0 is equivalent to locking the laser to the FSR (Free-spectral-range) of the FP cavity 140, thereby achieving the locking of the laser frequency of the laser.

[0064] Among them, the error signal can represent the deviation between the laser frequency of the laser 110 and the stable reference signal frequency, and the stable reference signal frequency can refer to the free spectrum frequency range of the FP cavity 140, and the free spectrum frequency range of the FP cavity 140 is FSR; the signal used for demodulation by the mixer 160 and the signal modulated by the electro-optical modulator 120 must be of the same source and the difference between the two must be 90° to ensure that the demodulation slope of the obtained error signal is the largest.

[0065] According to an embodiment of the present invention, the laser frequency locking process requires the use of a low-pass filter 180 and a servo control system 190 to frequency modulate the laser 110 based on an error signal. The frequency control port of the laser 110 currently used is divided into PZT (piezoelectric ceramic driver) frequency modulation and current frequency modulation (Cur), which requires the servo control system 190 to also have two outputs. The part of the servo control system 190 that controls the PZT frequency modulation is called slow feedback, and the part of the servo control system 190 that controls the current frequency modulation is called fast feedback. After the parameters of the servo control system 190 are designed to appropriate values, the PDH locking can be completed.

[0066] According to an embodiment of the present invention, the influence of the servo control system 190 on the ultra-stable laser frequency noise is second only to the FP cavity 140. The equivalent input noise and gain of the servo control system 190 will affect the ultra-stable laser frequency noise, which can be converted into frequency noise by dividing the error signal noise of the PDH closed loop by the demodulation slope to evaluate the influence of the servo control system 190 on the ultra-stable laser frequency noise.

[0067] According to the embodiment of the present invention, the laser free-running frequency noise can be suppressed to the equivalent input noise of the servo control system 190 only if the gain of the servo control system 190 is large enough, so that the PDH closed-loop error signal noise is equal to the equivalent input noise of the servo control system. If the gain of the servo control system 190 is not large enough, the laser free-running noise is not suppressed to the equivalent input noise of the servo control system 190, and the PDH closed-loop error signal noise is greater than the equivalent input noise of the servo control system 190.

[0068] At present, ultra-stable lasers generally use commercial FALC110, SRS SIM960, etc. as the servo control system of the PDH loop, but these commercial servo control systems generally have the disadvantages of large equivalent input noise, small low-frequency gain, poor temperature drift, etc., and can only meet the current ultra-stable laser servo control requirements of 1e-16 seconds stability. Therefore, how to realize an analog servo control circuit with ultra-low noise, ultra-high gain and ultra-low temperature drift is a problem that needs to be solved in the prior art. To this end, an embodiment of the present invention provides a servo control circuit with ultra-low noise, ultra-high gain and ultra-low temperature drift.

[0069] The following Figure 2~Figure 22The servo control circuit and the frequency locking method according to the embodiment of the present invention are described in detail.

[0070] Figure 2 FIG. 4 is a schematic diagram showing a servo control circuit according to an embodiment of the present invention.

[0071] like Figure 2 As shown, the servo control circuit 200 may include a slow feedback servo control subcircuit 210 and a fast feedback servo control subcircuit 220 , and the slow feedback servo control subcircuit 210 includes a filter 211 , a first amplification stage 212 and a slow feedback servo control unit 213 .

[0072] According to an embodiment of the present invention, in the process of locking the laser of a laser, that is, the device to be locked is a laser, the servo control circuit 200 can be used to replace the above Figure 1 The low-pass filter 180 and the servo control system 190 in the laser frequency stabilization circuit 100 are connected. The input end of the filter 211 is connected to the output end of the mixer 160 in the laser frequency stabilization circuit 100, that is, the mixer 160 sends the output error signal to the filter 211 in the servo control circuit 200, that is, the input of the servo control circuit 200 is the error signal.

[0073] According to an embodiment of the present invention, the filter 211 can be used to filter the error signal output by the mixer 160 to obtain a filtered signal, wherein the error signal can characterize the frequency deviation between the reference signal and the device to be locked, that is, the deviation between the laser frequency of the laser 110 and the resonant frequency of the optical resonant cavity, the optical resonant cavity is the FP cavity 140, and the resonant frequency is the free spectrum frequency range of the FP cavity, and the reference signal is stable.

[0074] According to an embodiment of the present invention, the in-phase input terminal of the first amplifier stage 212 is connected to the output terminal of the filter 211, and the first amplifier stage 212 is used to amplify the filtered signal to obtain an amplified signal, wherein the first amplifier stage 212 may include multiple parallel operational amplifiers.

[0075] According to an embodiment of the present invention, an input end of the slow feedback servo control unit 213 is connected to an output end of the first amplifier stage 212 , and the slow feedback servo control unit 213 is used to determine a slow feedback control signal PZT according to the amplified signal.

[0076] According to an embodiment of the present invention, the input end of the fast feedback servo control sub-circuit 220 is connected to the output end of the mixer 160 , and the fast feedback servo control sub-circuit 220 is used to determine the fast feedback control signal Cur according to the error signal.

[0077] According to an embodiment of the present invention, the output end of the slow feedback servo control unit 213 can be connected to the voltage frequency modulation end (PZT) of the laser 110, and the output end of the fast feedback servo control subcircuit 220 can be connected to the current frequency modulation end (Cur) of the laser 110, and is used to frequency modulate the laser 110 according to the slow feedback control signal and the fast feedback control signal until the error signal meets the preset condition, so as to achieve the locking of the laser frequency of the laser. The preset condition can represent that the error signal is locked to near 0, that is, the deviation between the laser frequency of the laser generated by the laser 110 and the resonance frequency of the FP cavity is substantially 0.

[0078] According to an embodiment of the present invention, a filter 211 is used before the slow feedback servo control unit 213 to eliminate the influence of high-frequency noise on low-frequency noise, and the fast feedback servo control subcircuit 220 is directly connected to the error signal without passing through the filter 211 to ensure that the bandwidth is not affected by the filter 211, so as to ensure that the output fast feedback control signal has a high bandwidth; since the equivalent input noise of the servo control circuit 200 will affect the ultra-stable laser frequency noise, the first amplifier stage 212 further reduces the equivalent input noise of the servo control circuit by using a multi-stage parallel connection of an operational amplifier; due to the use of the first amplifier stage 212, based on the amplification factor of the first amplifier stage 212, the noise in the subsequent slow feedback servo control unit 213 is offset and the gain of the servo control circuit is increased, so that the equivalent input noise of the servo control circuit 200 mainly comes from the first amplifier stage 212, thereby, the servo control circuit has the advantages of small equivalent input noise and large gain, so as to meet the servo requirements of the ultra-stable laser of the order of 1e-18.

[0079] FIG3( a ) shows a schematic diagram of the internal structure of a first amplifier stage according to an embodiment of the present invention.

[0080] FIG3( b ) shows a schematic diagram of a noise model of a first amplifier stage according to an embodiment of the present invention.

[0081] According to an embodiment of the present invention, in FIG. 3( a ), the first amplifier stage 212 may include an input resistor and a plurality of parallel operational amplifier units, the parallel number of the operational amplifier units is n, where n is a positive integer; the first operational amplifier unit may include a first resistor , the second resistor , operational amplifier and third resistor .

[0082] According to an embodiment of the present invention, taking the first operational amplifier unit as an example, the first resistor The first port is grounded, the first resistor The second port is connected to the inverting input terminal of the operational amplifier, and the second resistor The first port is connected to the inverting input terminal of the operational amplifier, and the second resistor The second port is connected to the output terminal of the operational amplifier, and the third resistor The first port is connected to the output terminal of the operational amplifier.

[0083] According to an embodiment of the present invention, the same-phase input terminals of a plurality of operational amplifiers are connected, and a plurality of third resistors ~ The second port is connected to realize the parallel connection of multiple operational amplifier units; the input resistor The first port is connected to the output end of the filter 211, and the input resistor The second port is connected to the non-inverting input terminals of the plurality of operational amplifiers.

[0084] According to an embodiment of the present invention, for the input resistance Since the resistor itself has thermal noise, a 50 ohm resistor can be set to ground at the in-phase input of the operational amplifier, so that the input impedance is the input resistor The value of the resistor connected in parallel with the ground resistor to limit the input resistance , that is, the final input resistance value of the first amplifier stage 212 is less than or equal to 50 ohms, thereby ensuring that the input resistance of the first amplifier stage 212 is less than or equal to 50 ohms and limiting the corresponding resistance thermal noise.

[0085] Figure 4 A schematic diagram showing a comparison between the equivalent input noise of FALC110 and the equivalent input noise of AD8130 according to an embodiment of the present invention is shown.

[0086] like Figure 4 As shown in the figure, the ordinate is the voltage noise spectral density. The equivalent input noise of FALC110 is exactly the same as the equivalent input noise of AD8130, where AD8130 is the input stage amplifier of FALC110. Therefore, the equivalent input noise of FALC110 comes from the input stage amplifier of FALC110.

[0087] According to the embodiment of the present invention, in order to meet the requirements of low equivalent input noise of the constructed servo control circuit, since the principle of the chopper amplifier is to modulate and demodulate the input signal and then sum it with the input bias of the amplifier itself after low-pass filtering, the low-frequency noise, input bias voltage and temperature drift of the chopper amplifier are the smallest among all types of amplifiers. By comparing the low-frequency noise and temperature drift of chopper amplifiers of various models, it is determined that the chopper amplifier ADA4523 is selected as the operational amplifier in the first amplifier stage 212.

[0088] Figure 5A schematic diagram showing a comparison between the equivalent input noise of the first amplifier stage according to an embodiment of the present invention and the equivalent input noise of a single ADA4523 is shown.

[0089] like Figure 5 As shown in the figure, taking the example of 16 ADA4523s in parallel in the first amplifier stage, the equivalent input noise of a single ADA4523 is compared with the equivalent input noise of 16 ADA4523s in parallel. becomes That is, the equivalent input noise of the first amplifier stage composed of multiple operational amplifiers in parallel is smaller than the equivalent input noise of the first amplifier stage composed of a single operational amplifier. Among them, the open-loop noise of a single ADA4523 is the equivalent input noise of a single ADA4523, and the open-loop noise of 16 ADA4523s is the equivalent input noise of 16 ADA4523s in parallel.

[0090] According to an embodiment of the present invention, since the first amplifier stage adopts the form of multiple operational amplifiers connected in parallel and ADA4523 itself has advantages such as low low-frequency noise, the equivalent input noise of the first amplifier stage is effectively reduced, thereby achieving ultra-low noise in the servo control circuit.

[0091] According to an embodiment of the present invention, the amplification factor of the operational amplifier in the first amplification stage is determined based on the ratio of the resistance values ​​of the first resistor to the second resistor; when the amplification factor of the operational amplifier meets the preset factor condition, the equivalent input noise of the servo control circuit is the same as the equivalent input noise of the first amplification stage, and the error signal noise corresponding to the error signal under the closed loop of the laser frequency stabilization circuit comes from the equivalent input noise of the servo control circuit.

[0092] According to an embodiment of the present invention, as shown in FIG3 (a), the amplification factor of each operational amplifier in the first amplifier stage 212 is the same, and the resistance values ​​of the first resistor and the second resistor in the operational amplifier unit can be adjusted according to the required amplification factor. The resistance of the second resistor is 50 ohms. The resistance value of the third resistor is 5K ohms. Taking the resistance value of 1K ohm as an example, the amplification factor of the operational amplifier is 5K / 50+1=101, that is, the amplification factor of the operational amplifier is 101 times. and the second resistor A negative feedback network can be constructed to achieve 101 times in-phase amplification.

[0093] According to the embodiment of the present invention, if the amplification factor of the first amplifier stage 212 is not increased, it is difficult to ensure that the error signal noise corresponding to the error signal in the closed loop of the laser frequency stabilization circuit 100 all comes from the equivalent input noise, that is, the premise that the error signal noise all comes from the equivalent input noise is that the gain of the first amplifier stage must be large enough, that is, a relatively high amplification factor is required, so that the error signal noise all comes from the equivalent input noise.

[0094] The preset multiple condition can be characterized as the amplification multiple required by the first amplifier stage when the error signal noise corresponding to the error signal in the closed loop of the laser frequency stabilization circuit all comes from the equivalent input noise of the first amplifier stage.

[0095] Figure 6 A schematic diagram showing a comparison between an equivalent input noise of a servo control circuit according to an embodiment of the present invention and an equivalent input noise of a first amplifier stage is shown.

[0096] like Figure 6 As shown, taking the parallel number of the first amplifier stage 212 as 16 as an example, the open-loop equivalent input noise of 16 ADA4523s is the equivalent input noise of the first amplifier stage, and the equivalent input noise of the ultra-low noise and ultra-high gain model servo control circuit is the equivalent input noise of the servo control circuit 220 of the present invention, that is, the equivalent input noise of the servo control circuit 200 is the same as the equivalent input noise of the first amplifier stage 212.

[0097] According to the embodiment of the present invention, the signal filtered by the filter 211 is connected to the in-phase input terminal of the first amplifier stage 212, that is, the first amplifier stage 212 adopts in-phase amplification. After the multiple operational amplifiers in the first amplifier stage 212 are connected in parallel, the input impedance is equal to the input impedance of a single operational amplifier divided by n. The operational amplifier in-phase amplified signal is directly connected to the in-phase terminal of the operational amplifier, and the input impedance of the in-phase terminal of the operational amplifier is high impedance, and even if it is divided by n, it can still ensure that the input impedance is high impedance. If inverting amplification is adopted, taking a single input impedance of 50 ohms and a parallel stage number of 16 as an example, the input impedance of 16 operational amplifiers in parallel is 3 ohms, which will cause a large attenuation of the input signal and reduce the signal-to-noise ratio.

[0098] According to an embodiment of the present invention, by setting the amplification factor of the first amplifier stage 212, the amplification factor satisfies a preset factor condition, thereby ensuring that the error signal noise corresponding to the error signal under the closed loop of the laser frequency stabilization circuit all comes from the equivalent input noise of the servo control circuit, thereby achieving the goal of suppressing the laser free-running frequency noise to the equivalent input noise of the servo control circuit while reducing the equivalent input noise, so as to reduce the influence of the servo control circuit on the ultra-stable laser frequency noise.

[0099] According to an embodiment of the present invention, the number of parallel-connected operational amplifier units in the first amplifier stage is determined according to the resistance values ​​of the input resistor, the first resistor, the second resistor and the third resistor, and the voltage noise and current noise of the operational amplifier.

[0100] According to an embodiment of the present invention, the equivalent input noise of the first amplifier stage is related to the number of parallel stages of the operational amplifier units, the thermal noise of the resistors of each operational amplifier unit, the voltage noise and the current noise of the operational amplifier. The current noise may include the current noise of the in-phase terminal and the current noise of the inverting terminal.

[0101] According to an embodiment of the present invention, based on FIG. 3( a ) and FIG. 3( b ), taking the first operational amplifier unit in the first amplifier stage as an example, The first resistor and the second resistor Equivalent resistance; It can be expressed as the current noise at the non-inverting terminal of the first amplifier stage, It can be expressed as The current of the operational amplifier unit, is the resistor The thermal noise of the resistor, ~ are the equivalent voltage noises corresponding to the 1st to nth operational amplifiers in the first amplifier stage 320, ~ Respectively, the resistance ~ The corresponding resistor thermal noise is ~ They are the current noises corresponding to the inverting terminals of the 1st to nth operational amplifiers, =…= is the gain of the operational amplifier, Respectively, the resistance ~ The corresponding resistor thermal noise.

[0102] According to an embodiment of the present invention, based on FIG. 3( b ), the equivalent input noise of the first amplifier stage 212 can be expressed as the following formula (1).

[0103] (1)

[0104] Where K is a constant, T is the temperature, for ~ The value after parallel connection, is the sum of the squares of the currents in each parallel branch, for ~ sum; ~ It is the current noise at the inverting end, which can be eliminated by connecting multiple stages in parallel. = .

[0105] According to the embodiment of the present invention, the resistor thermal noise in the equivalent input noise of the first amplifier stage 212 is relatively small, that is, the influence of the resistor thermal noise is also relatively small.

[0106] According to an embodiment of the present invention, based on formula (1), increasing the number n of parallel operational amplifiers will reduce the voltage noise of the operational amplifiers, but will increase the current noise of the operational amplifiers. Therefore, the number of parallel stages is limited and cannot be increased indefinitely.

[0107] Figure 7 A schematic diagram showing a comparison of equivalent input noises of first amplifier stages with different numbers of parallel stages according to an embodiment of the present invention is shown.

[0108] like Figure 7 As shown, when the number n of the transport amplifiers connected in parallel in the first amplifier stage 212 is 16, 32, 64, 128, and 256, respectively, the equivalent input noise of the first amplifier stage corresponding to each of them.

[0109] According to an embodiment of the present invention, there is a turning point in the number of parallel operational amplifiers n. After this turning point, the continued increase of n will cause the equivalent input noise of multiple parallel operational amplifiers to begin to increase. The voltage noise, current noise and in-phase input impedance of AD4523 are Substituting (50 Ω) into formula (1), we get the turning point of n as 128. Figure 7 However, considering the marginal diminishing effect and power dissipation, the number of parallel operational amplifiers n is finally determined to be 16.

[0110] According to an embodiment of the present invention, through the above formula (1), according to the resistance values ​​of the input resistor, the first resistor, the second resistor and the third resistor, and the voltage noise and current noise of the operational amplifier, combined with Figure 7 , the number of parallel-connected operational amplifier units in the first amplifier stage can be determined to ensure that the equivalent input noise of multiple operational amplifier units connected in parallel is relatively small.

[0111] According to an embodiment of the present invention, the slow feedback servo control unit includes an integral control subunit and a plurality of proportional integral control subunits connected in series, and the slow feedback servo control unit is also used to increase the gain of the servo control circuit.

[0112] According to an embodiment of the present invention, the slow feedback servo control unit 213 may include an integral control subunit (I, Integral) and multiple proportional integral control subunits (PI, Proportional Integral Controller), and the integral control subunit and the proportional integral control subunit are connected in series. For example, the slow feedback servo control unit 213 may adopt a four-level integration, that is, a first-level integral control subunit plus a third-level proportional integral control subunit.

[0113] According to an embodiment of the present invention, since the laser free-running frequency noise is very large at low frequencies, approximately , which puts forward very stringent requirements on the low-frequency gain of the slow feedback servo control unit 213 in the servo control circuit. In addition, since the I gain (integral gain) of the slow feedback servo control unit 213 is limited by the loop bandwidth limitation of the fast feedback servo control sub-circuit, the servo control circuit 200 uses multiple levels of I instead of one level of I, so that the low-frequency slow feedback gain is changed from -20dB per decade to -80dB per decade, which greatly increases the low-frequency gain without changing the size of the first level I, and can do a very good job of suppressing the free-running frequency noise of the low-frequency laser.

[0114] According to an embodiment of the present invention, the slow feedback servo control unit 213 can quickly adjust the error signal to meet preset conditions due to the settings of the integral control subunit and the proportional integral control subunit, thereby improving the control accuracy and stability of the servo control circuit, while also improving the low-frequency gain of the servo control circuit to suppress the free-running frequency noise of the laser at low frequency to the equivalent input noise.

[0115] According to an embodiment of the present invention, the slow feedback servo control subcircuit also includes a slow feedback output polarity switch, the slow feedback output polarity switch includes a single-pole double-throw switch and an inverting amplifier follower, the output end of the last proportional-integral control subunit in the slow feedback servo control unit is connected to the first port of the single-pole double-throw switch, the output end of the last proportional-integral control subunit is connected to the input end of the inverting amplifier follower, and the second port of the single-pole double-throw switch is connected to the output end of the inverting amplifier follower.

[0116] According to an embodiment of the present invention, the slow feedback servo control unit 213 further includes a slow feedback output polarity switch, which is composed of a single-pole double-throw switch and an inverting amplifier follower.

[0117] According to the embodiment of the present invention, by regulating the slow feedback output polarity switch, the slow feedback output polarity switch can be used to ensure the establishment of the negative feedback loop of the slow feedback servo control unit 213 .

[0118] According to an embodiment of the present invention, the fast feedback servo control subcircuit includes at least one dual-zero dual-pole compensation unit and a fast feedback output polarity switch connected in series, and the fast feedback servo control subcircuit is also used to increase the bandwidth of the fast feedback control signal.

[0119] According to an embodiment of the present invention, the fast feedback servo control subcircuit 220 includes at least one dual-zero dual-pole compensation unit connected in series, wherein the dual-zero dual-pole compensation unit is a unit similar to the Type 3 PID, and the operational amplifier in the unit needs to be selected with a high bandwidth, for example, the operational amplifier in the unit can be OPA820.

[0120] According to an embodiment of the present invention, the fast feedback output polarity switch also includes a single-pole double-throw switch and an inverting amplifier follower, and its connection method is consistent with the slow feedback output polarity switch. The fast feedback output polarity switch and the slow feedback output polarity switch can ensure that the negative feedback corresponding to the slow feedback and the fast feedback is established.

[0121] According to the embodiment of the present invention, with the help of the fast feedback servo control sub-circuit 220, the bandwidth of the fast feedback control signal can be increased.

[0122] According to an embodiment of the present invention, the servo control circuit also includes a second amplifier stage, the in-phase input terminal of the second amplifier stage is connected to the error signal, and the output terminal of the second amplifier stage is connected to a noise tester to measure the actual equivalent input noise of the servo control circuit through the noise tester, wherein the first amplifier stage is the same as the second amplifier stage.

[0123] Figure 8 FIG. 4 is a schematic structural diagram of a servo control circuit according to another embodiment of the present invention.

[0124] like Figure 8 As shown, in the servo control circuit 800 of this embodiment, the slow feedback servo control unit 213 may include an integral control subunit 811, multiple proportional integral control subunits 812~814 and a slow feedback output polarity switch 815; the fast feedback servo control subcircuit 220 may include multiple double-zero double-pole compensation units 221~224 and a fast feedback output polarity switch 225.

[0125] According to an embodiment of the present invention, the servo control circuit 800 may further include a second amplifier stage 820, wherein the in-phase input terminal of the second amplifier stage 820 is connected to the output terminal of the filter 211, and the output terminal of the second amplifier stage 820 is connected to a noise tester 830 to measure the measured equivalent input noise of the servo control circuit, that is, the equivalent input noise of the second amplifier stage, wherein the first amplifier stage is the same as the second amplifier stage, that is, the structural schematic diagram of the second amplifier stage is also shown in FIG3 (a).

[0126] According to the embodiment of the present invention, the second amplifier stage 820 is added to facilitate the measurement of equivalent input noise, and the servo control circuit may not include the second amplifier stage. However, since the equivalent input noise is too low, exceeding the noise floor of all noise test instruments, such low noise cannot be measured by the instrument without the second amplifier stage, so the second amplifier stage 820 needs to be added to amplify the noise through the second amplifier stage 820 and then connected to the noise tester 830 for noise testing.

[0127] According to an embodiment of the present invention, the introduction of the second amplifier stage can amplify the equivalent input noise so that the connected noise tester can measure the equivalent input noise, which is convenient for the subsequent evaluation of the impact of the servo control circuit on the laser frequency noise of the laser based on the measured equivalent input noise.

[0128] According to an embodiment of the present invention, when the device to be locked includes at least two input terminals, the servo control circuit also includes: an output terminal of a slow feedback servo control unit is connected to a first input terminal of the device to be locked, and an output terminal of a fast feedback servo control subcircuit is connected to a second input terminal of the device to be locked, and is used to frequency modulate the device to be locked according to a slow feedback control signal and a fast feedback control signal until the error signal meets a preset condition.

[0129] Fig. 9 FIG. 4 is a schematic structural diagram of a servo control circuit according to another embodiment of the present invention.

[0130] like Fig. 9 As shown, the filter in the servo control circuit shown in the embodiment 900 is Figure 8 The filter 211 shown in the figure can be, for example, a 50KHz sixth-order low-pass passive filter constructed by an inductor, a capacitor and a resistor. The filter is used to eliminate the influence of high-frequency noise on low-frequency noise and to prevent the high-frequency noise from exceeding the rail voltage of the ADA4523 op amp after being amplified 101 times by the first and second amplifier stages, thereby preventing the output saturation of the amplifier stage and the input stage. The filter part can be adaptively adjusted.

[0131] According to an embodiment of the present invention, the input stage 1 shown in the embodiment 900 is Figure 8 The first amplifier stage 212 shown in the figure, the amplifier stage 2 is the second amplifier stage 820, and the test instrument is the noise tester 830.

[0132] According to an embodiment of the present invention, in the embodiment 900 That is the integral control subunit 811, That is the proportional integral control subunit 812, That is the proportional integral control subunit 813, That is the proportional-integral control subunit 814 , and the polarity switch is the slow feedback output polarity switch 815 .

[0133] According to an embodiment of the present invention, the operational amplifier used in the slow feedback servo control unit 213 in the servo control circuit shown in the embodiment 900 is also ADA4523. ADA4523 is selected because ADA4523 itself has low low-frequency noise, but other types of operational amplifiers can also be selected for use, because the noise existing in the slow feedback servo control subcircuit has a relatively small effect on the equivalent input noise of the first amplifier stage. Due to the setting of a larger amplification factor in the first amplifier stage 212, the size of the noise in the slow feedback servo control unit 213 is not greatly affected, because the noise in the slow feedback servo control unit 213 needs to be divided by the amplification factor to be equivalent to the input stage, so that the equivalent noise is relatively small, which is equivalent to being offset. Therefore, the noise in the slow feedback servo control unit 213 does not affect the error signal noise from the equivalent input noise of the first amplifier stage 212.

[0134] According to the embodiment of the present invention, in order to offset the noise existing in the slow feedback servo control unit 213, it is necessary to set a larger amplification factor of the first amplifier stage 212. The first amplifier stage 212 can also be used as a filter. The higher the amplification factor, the more high-frequency noise can be filtered out.

[0135] According to the embodiment of the present invention, the slow feedback servo control unit 213 is divided into four levels. The first level is an integral control subunit. For example, it can be an integration circuit built by a 1K ohm resistor, a 1uF capacitor and an ADA4523 op amp, and the last three stages are proportional integral control subunits. For example, it can be a proportional integral circuit constructed by a 1K ohm resistor, a 1K ohm resistor, a 100nF capacitor and an ADA4523 operational amplifier. The first-stage integral control subunit and the subsequent three-stage proportional integral control subunits are cascaded to form a slow feedback servo control unit 213.

[0136] According to an embodiment of the present invention, the fast feedback servo control subcircuit 220 is composed of four levels of Type 3 PID cascade, such as Fig. 9 As shown in Figure 8 In the double-zero double-pole compensation units 221-224 shown in FIG, the operational amplifier of each stage may use a high-bandwidth operational amplifier OPA820, for example.

[0137] According to an embodiment of the present invention, each stage in the fast feedback servo control subcircuit is constructed based on the Type 3 PID. Since the slow feedback servo control unit already includes an integration circuit, the fast feedback servo control subcircuit does not need an integration circuit. The low-frequency integration is replaced with a pole, so that each stage of the circuit has two pairs of zero poles.

[0138] Among them, the pole will increase the low-frequency gain (which is beneficial to the suppression of low-frequency noise) but will bring phase attenuation at high frequencies (reduce the phase margin of the loop gain), which is not conducive to the stability of the system. The zero point will reduce the low-frequency gain and increase the high-frequency phase (increase the phase margin of the loop gain), which is beneficial to the stability of the system.

[0139] According to an embodiment of the present invention, when the device to be locked includes at least two input terminals, the output terminal of the slow feedback servo control unit 213 can be connected to the first input terminal of the device to be locked, and the output terminal of the fast feedback servo control subcircuit 220 can be connected to the second output terminal of the device to be locked, so that the frequency of the device to be locked is modulated according to the slow feedback control signal and the fast feedback control signal, thereby achieving frequency locking of the device to be locked.

[0140] According to an embodiment of the present invention, when the device to be locked includes an input terminal, the circuit also includes an adder, a first input terminal of the adder is connected to the output terminal of the slow feedback servo control unit, a second input terminal of the adder is connected to the output terminal of the fast feedback servo control subcircuit, and the output terminal of the adder is connected to the input terminal of the device to be locked. The adder is used to add the slow feedback control signal and the fast feedback control signal to obtain a control signal, so as to perform frequency modulation on the device to be locked according to the control signal until the error signal meets a preset condition.

[0141] According to an embodiment of the present invention, when the device to be locked includes only one input terminal, such as Fig. 9 As shown, an adder 920 can be additionally added to the servo control circuit to add the outputs of the fast feedback servo control subcircuit 220 and the slow feedback servo control unit 213. The first input end of the adder 920 is connected to the output end of the slow feedback servo control unit 213, the second input end of the adder 920 is connected to the output end of the fast feedback servo control subcircuit 220, and the output end of the adder 920 is connected to the input end of the device to be locked. The adder 920 can add the slow feedback control signal and the fast feedback control signal to obtain a control signal, which is used to perform frequency modulation on the device to be locked according to the control signal until the error signal meets the preset condition.

[0142] The single feedback output of the adder has been verified using only the current modulation port of the laser, but the adder 920 is suitable for some lasers that have only one modulation port, such as a frequency-aligned laser.

[0143] According to the embodiment of the present invention, when the device to be locked includes only one input terminal, the PDH can be locked by implementing a single feedback output through the adder 920 .

[0144] Fig.10 A frequency response comparison diagram of a servo control circuit according to an embodiment of the present invention and a FALC110 fast feedback circuit is shown.

[0145] like Fig.10 As shown in the figure above, the fast feedback servo control subcircuit 220 uses a four-stage cascade method to make the amplitude-frequency response 3dB bandwidth exceed 20MHz, that is, it exceeds the test range of the loop analyzer, while the amplitude-frequency response 3dB bandwidth of the FALC110 fast feedback is only 11MHz. It can be seen that the present invention achieves an improvement in bandwidth.

[0146] According to an embodiment of the present invention, the fast feedback servo control subcircuit 220 adopts a four-stage cascade mode in order to increase the fast feedback bandwidth. Fig.10 In the figure below, it can be seen that the bandwidth of the four-stage cascaded fast feedback servo control sub-circuit is about 10 to the seventh power when the phase is reduced by about 45 degrees, but theoretically it should be able to reach a bandwidth of tens of megabytes or even 100 megabytes. It can be seen that the bandwidth corresponding to the four-stage cascaded fast feedback servo control sub-circuit does not achieve an effective improvement in the fast feedback bandwidth. It is suspected that there is phase attenuation caused by the delay of each stage of the op amp, and the single-stage effect may be better.

[0147] Based on the above, due to suspicion Fig.10 The phase attenuation of the mid-phase frequency response comes from the operational amplifier OPA820. The four-stage cascade of the fast feedback servo control subcircuit 220 can be changed to a single-stage output, that is, Fig. 9 The single-stage output 910 shown in the figure is to add a single-stage fast feedback output port to the output end of the first stage in the original fast feedback servo control sub-circuit 220, thereby reducing the delay and the phase attenuation of the fast feedback.

[0148] Fig.11 A schematic diagram showing a comparison between the noise floor of an instrument for testing equivalent input noise in a servo control circuit according to an embodiment of the present invention and the noise floor of other types of testing instruments is shown.

[0149] like Fig.11 As shown, the noise floor of the test system (second amplifier stage) of the equivalent input noise of the ultra-low noise and ultra-high gain analog servo control circuit of the present invention is , that is, the ultra-low noise analog PID noise floor is the noise floor of the noise tester 830, which is at least one order of magnitude lower than the noise floors of the Propylene 9812, SR785, and oscilloscope pico. Only this noise test system (second amplifier stage) can barely meet the requirements of testing the equivalent input noise of the ultra-low noise ultra-high gain analog servo control circuit. Therefore, due to the high noise floor of each test instrument, it is necessary to amplify the equivalent input noise through the second amplifier stage and then connect it to the noise tester to accurately measure the corresponding equivalent input noise.

[0150] FIG. 12( a ) is a schematic diagram showing the PDH closed-loop error signal noise generated by the ultra-stable laser U3 using a servo control circuit at 6.56e-4 V / Hz according to an embodiment of the present invention.

[0151] FIG12( b ) is a schematic diagram showing the PDH closed-loop error signal noise generated by the ultra-stable laser U3 using a servo control circuit at 2.4e-4 V / Hz according to an embodiment of the present invention.

[0152] FIG12( c ) is a schematic diagram showing the PDH closed-loop error signal noise generated by the ultra-stable laser U3 using a servo control circuit at 3.2e-5 V / Hz according to an embodiment of the present invention.

[0153] FIG12( d ) is a schematic diagram showing a comparison between the free-running frequency noise of the ultra-stable laser U3 and the self-closed-loop error signal noise of the error detection board according to an embodiment of the present invention.

[0154] As shown in Figure 12 (a) to Figure 12 (d), the self-closed loop refers to the equivalent input noise of the servo control circuit, the laser noise contribution refers to the laser free-running frequency noise, and the U3 measured error refers to the error signal noise of the PDH closed loop.

[0155] According to an embodiment of the present invention, the three figures of Figure 12 (a) to Figure 12 (c) are obtained under the conditions of 6.56e-4V / Hz, 2.4e-4V / Hz, and 3.2e-5V / Hz, respectively, and it can be seen that at low frequency, the PDH closed-loop error signal noise comes from the equivalent input noise of the servo control circuit, and at high frequency, the PDH closed-loop error signal noise comes from the laser free-running frequency noise.

[0156] According to an embodiment of the present invention, the error detection board self-closed loop in FIG12( d ) refers to the equivalent input noise of the servo control circuit, the U3 laser free-running noise refers to the laser free-running frequency noise, and the ordinate is the Frequency noise spectral density.

[0157] According to an embodiment of the present invention, the influence of the servo system on the free-running noise of the laser is accurately evaluated by the error signal noise of the PDH closed loop. The greater the gain of the servo system, the greater the suppression of the free-running frequency of the laser. However, there is a limit to the suppression of the free-suppression frequency noise of the laser, which is equal to the equivalent input noise of the servo control circuit. When the frequency is less than 10 Hz in FIG. 12 (a) to FIG. 12 (c), the free-running frequency noise of the laser has been suppressed to a very small level, but the measured PDH closed-loop error signal noise has not continued to decrease, which is limited by the equivalent input noise of the servo control circuit.

[0158] According to the embodiment of the present invention, when the gain is large enough and the equivalent input noise is very high, the error signal noise is also large; when the equivalent input noise is small but the gain is large enough, the error signal noise is still large.

[0159] Fig.13 FIG. 4 is a schematic diagram showing a comparison of equivalent input noises of a servo control circuit according to an embodiment of the present invention and FALC 110 .

[0160] like Fig.13 As shown, the equivalent input noise of the first amplifier stage in the servo control circuit is 300 times that of the equivalent input noise of FALC110. It is two orders of magnitude lower, which also indicates that the equivalent input noise of the ultra-low noise and ultra-high gain analog servo control circuit of the present invention is lower than the equivalent input noise of FALC110 in the entire frequency band.

[0161] Fig.14 FIG. 4 is a schematic diagram showing a comparison of the gains of the servo control circuit according to an embodiment of the present invention and the FALC 110 .

[0162] According to the embodiment of the present invention, since the slow feedback servo control unit 213 in the servo control circuit adopts a four-stage integration circuit cascade, the first stage is an integration circuit, and the last three stages are three-stage proportional integration circuit cascades, which makes the low-frequency gain of the slow feedback of the ultra-low noise ultra-high gain analog servo control circuit much higher than the slow feedback gain of FALC110, such as Fig.14 As shown, the slow feedback gain of the servo control circuit of the present invention can reach 200dB@1Hz. Fig.14 The resolution of the test instrument loop analyzer Bode100 is limited to 100dB, but the TINA simulation results show that it can reach 200dB at 1Hz, while the FALC110 slow feedback gain is only 50dB@1Hz.

[0163] According to the embodiment of the present invention, the gain of the servo control circuit 200 is relatively high. At 10 Hz, it has reached more than 200 dB, which exceeds the resolution limit of the test instrument and cannot measure a gain that is too high. That is, the gain of the servo control circuit is based on calculation within 20 Hz and on actual measurement outside 20 Hz.

[0164] Fig.15 FIG. 1 is a schematic diagram showing a comparison of the temperature drift of the servo control circuit according to an embodiment of the present invention and the FALC 110 .

[0165] like Fig.15 As shown, the operational amplifiers in the first and second amplifier stages use chopper amplifiers ADA4523, which makes the temperature drift of the servo control circuit , as a comparison, the temperature drift of FALC110 using AD8130 as the input stage op amp is It can be seen that the temperature drift of the servo control circuit is nearly four orders of magnitude lower than that of FALC110, achieving ultra-low temperature drift of the servo control circuit.

[0166] According to an embodiment of the present invention, the influence of the servo control circuit on the laser frequency noise of the laser is evaluated based on the measured equivalent input noise and the discrimination slope of the servo control circuit.

[0167] Fig.16 A schematic diagram showing a comparison of the PDH closed-loop error signal of the servo control circuit according to an embodiment of the present invention and the FALC110 is shown.

[0168] like Fig.16 As shown, Fig.16 The first curve from the left coordinate axis from top to bottom in the middle left figure represents the error signal noise of FALC110, and the second and third curves from top to bottom represent the error signal noise of the servo control circuit of the present invention at different gains. At low frequencies within about 100 Hz, the error signal noise of the present invention is significantly lower than that of FALC110, about 2 to 3 orders of magnitude lower.

[0169] According to the embodiment of the present invention, to evaluate the influence of the servo control circuit on the laser frequency noise of the laser, it is necessary to convert the measured PDH closed loop error signal noise into frequency noise by dividing it by the frequency discrimination slope. This frequency noise is the influence of the servo system on the ultra-stable laser frequency noise. Then, the allan deviation of the frequency noise can be calculated to calculate the contribution of the servo system to the ultra-stable laser second stability. Fig.16 In the middle right figure, the stabilization per second contributed by the servo control circuit of the present invention to the ultra-stable laser is 250 times lower than that contributed by the commercial servo system FALC110 to the ultra-stable laser. Fig.16 In the middle right figure, the servo control circuit of the present invention contributes to a stability of 4E-20 seconds, which indicates that the servo control circuit of the present invention can be fully applied to ultra-stable laser servo control with a stability of 1E-18 or even 1E-19 seconds.

[0170] Fig.17 A circuit schematic diagram of a servo control circuit according to an embodiment of the present invention applied to a phase-locked loop is shown.

[0171] like Fig.17 As shown, the servo control circuit of the present invention can also be applied to a phase-locked loop. When there is no need to increase the bandwidth, only the slow feedback servo control subcircuit in the servo control circuit of the present invention can be applied to the phase-locked loop.

[0172] Among them, signal source b is a stable reference signal. Signal source b can be a Rohde & Schwarz signal source or a very stable 10MHz crystal oscillator.

[0173] According to the embodiment of the present invention, the phase-locked loop mainly consists of four parts: a master signal source, a slave signal source, a mixer and a servo control.

[0174] Among them, the main signal source is signal source b, which is used to provide a stable reference signal; the slave signal source, that is, signal source a can also be called VCO, which controls the output frequency of the signal source through the input voltage, similar to the laser current modulation port and PZT modulation port, and also controls the output frequency of the device through the input voltage, but the output frequency of the signal source is not as high as the laser frequency; the servo control is generally called a loop filter in the phase-locked loop, that is, the slow feedback servo control unit 213.

[0175] According to an embodiment of the present invention, the purpose of the phase-locked loop is to lock the phase noise of the slave signal source to the reference signal source (master signal source), thereby greatly reducing the phase noise of the slave signal source, wherein the phase noise and the frequency noise can be converted into each other.

[0176] According to the embodiment of the present invention, based on the slow feedback servo control subcircuit 213 of the present invention, the phase noise of the slave signal source can be locked to 140dBc / Hz within 100Hz, which has reached the 10MHz phase noise of the Rohde & Schwarz signal source SMA100B, which has the lowest phase noise among the current commercial signal sources.

[0177] Fig.18 A schematic diagram showing a comparison of phase noises of a mixer phase-locked loop according to an embodiment of the present invention and a commercial SMA100B is shown.

[0178] According to the embodiment of the present invention, the signal source is essentially a phase-locked loop, and the low-frequency phase noise of its output signal is proportional to the 10MHz reference of the signal source itself. For example, the phase noise of the signal source outputting a 100MHz carrier frequency signal is 10 times that of its own 10MHz reference phase noise, and 1GHz is 100 times that of its own 10MHz reference phase noise. Therefore, in order to verify the excellent performance of the servo control circuit of the present invention, the reference frequency of the phase-locked loop experiment can be set to 10MHz. The final locking result is compared with the 10MHz output of SMA100B. Fig.18 shown.

[0179] According to an embodiment of the present invention, Fig.18 The poor performance within 1 Hz shown in the figure is due to the mixer phase-locked loop using a 10 MHz reference, not a problem with the servo control board.

[0180] Fig.19 A schematic diagram showing an application of a servo control circuit in an FNC according to an embodiment of the present invention is shown.

[0181] According to an embodiment of the present invention, for an ultra-stable laser, since the low-frequency noise of the ultra-stable laser is very low, the phase noise caused by the optical fiber several meters long is unacceptable to the ultra-stable laser and will affect the ultra-stable laser noise. Among them, the optical fiber phase noise is caused by the influence of factors such as vibration and temperature on the optical fiber.

[0182] like Fig.19 As shown, by locking the laser frequency of the laser through the above servo control circuit, an ultra-stable laser can be obtained. The ultra-stable laser can be input as follows Fig.19 The FNC circuit 1910 shown is used to suppress the noise caused by the optical fiber during the transmission process.

[0183] Among them, the fiber PD is used for convenience, but the fiber PD will bring additional unsuppressible fiber phase noise, so it is better to use the free space PD.

[0184] According to an embodiment of the present invention, after the ultra-stable laser enters the FNC circuit 1910, the ultra-stable laser is divided into two paths through the PBS. One path only passes through the Faraday mirror and is reflected by the Faraday mirror to the PBS, and the other path passes through the AOM, the optical fiber and the Faraday mirror in sequence, and is emitted by the Faraday mirror and then passes through the optical fiber and the AOM back to the PBS, which is equivalent to passing through the AOM twice. In the PBS, the two returning lights can be combined, and passed through the PD, and input into the mixer with a stable reference signal, and through the slow feedback servo control unit 213, the slow feedback modulated signal is input through the frequency modulation end of the signal source, so as to suppress the noise caused by the optical fiber during the transmission process. Among them, the signal source can be 80MHz, and the signal output by the PD is 160MHz. The AOM is used to shift the frequency of the laser, and the reference signal is a very stable 160MHz signal.

[0185] Fig. 20 A schematic diagram of phase noise comparison between a servo control circuit according to an embodiment of the present invention and FNC under a commercial SIM960 is shown.

[0186] like Fig. 20 As shown, Fig.19 The low-frequency phase noise and frequency noise of the FNC under servo control are better than those of the commercial SIM960.

[0187] Fig.21 A circuit diagram of an optical phase-locked loop using a servo control circuit according to an embodiment of the present invention is shown.

[0188] According to an embodiment of the present invention, the laser is locked to the FP cavity by the PDH method. However, in some application scenarios, the ultra-stable laser power after locking is not large enough. It is necessary to maintain the ultra-low frequency noise of the original ultra-stable laser while increasing the laser power. This requires the use of an optical phase-locked loop, because it is necessary to ensure that the laser frequency noise after the optical phase-locked loop is locked is completely locked to the reference ultra-stable laser. This puts forward strict requirements on the gain and noise of the servo control circuit. The optical phase-locked loop locked by the servo control circuit of the present invention can meet this requirement.

[0189] like Fig.21 As shown in the figure, the free-running frequency noise of the laser is too large, far exceeding the phase-detection range of the mixer. The mixer needs to be replaced with a frequency detector, for example, using the HMC4069 with a two-way frequency division.

[0190] According to the embodiment of the present invention, the principle of the optical phase-locked loop is essentially a phase-locked loop, but the signal to be locked becomes the beat signal (from the PD) of the slave laser and the master laser (ultra-stable laser). The phase noise (or frequency noise) of the beat signal represents the phase noise difference between the slave laser and the master laser. If the phase noise of the beat signal of the PD can be suppressed to a very low level, it means that the noise of the slave laser and the master laser is basically the same.

[0191] Fig. 22 A phase noise schematic diagram of an optical phase-locked loop using a servo control circuit according to an embodiment of the present invention is shown.

[0192] like Fig. 22 As described above, the reference signal uses SMA100B (configured with B711 option) with the lowest phase noise among commercial signal sources. After using the servo control board of the present invention, the optical phase-locked loop can completely lock the free-running noise of the low-frequency (100 MHz~10 Hz) laser to the reference, and the phase noise is reduced by 10 orders of magnitude.

[0193] Fig.23 A schematic diagram of error signal noise of an optical phase-locked loop using a servo control circuit according to an embodiment of the present invention is shown.

[0194] like Fig.23 As shown, compared with the commercial servo system FALC110, after using the servo control board of the present invention, the error signal noise of the optical phase-locked loop is reduced.

[0195] Fig.24 A flow chart of a frequency locking method according to an embodiment of the present invention is shown.

[0196] like Fig.24 As shown, the method 2400 includes operations S2410 to S2450.

[0197] In operation S2410, a filter is used to filter the error signal to obtain a filtered signal.

[0198] The error signal represents the frequency deviation between the locking device and the device to be locked.

[0199] In operation S2420, the filtered signal is amplified by using a first amplification stage to obtain an amplified signal.

[0200] The first amplification stage includes a plurality of operational amplifiers connected in parallel.

[0201] In operation S2430, a slow feedback control signal is determined according to the amplified signal by the slow feedback servo control unit.

[0202] In operation S2440, a fast feedback control signal is determined according to the error signal using the fast feedback servo control sub-circuit.

[0203] In operation S2450, the slow feedback servo control unit and the fast feedback servo control subcircuit are used to frequency modulate the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal meets a preset condition to achieve frequency locking of the device to be locked.

[0204] According to an embodiment of the present invention, based on the servo control circuit 200, the filter 211 is used before the slow feedback servo control unit 213 to eliminate the influence of high-frequency noise on low-frequency noise, and the fast feedback servo control sub-circuit 220 is directly connected to the error signal without considering the filter to ensure that the bandwidth is not affected by the filter 211, and to ensure that the output fast feedback control signal has a high bandwidth; because the equivalent input noise and gain of the servo control circuit will affect the ultra-stable laser frequency noise, the first amplifier stage further reduces the equivalent input noise by using an operational amplifier in parallel; due to the use of the first amplifier stage, based on the amplification factor of the first amplifier stage, the noise in the subsequent slow feedback servo control unit 213 is offset and the gain of the servo control circuit is increased, so that the equivalent input noise of the servo control circuit mainly comes from the first amplifier stage, thereby, the servo control circuit has the advantages of small equivalent input noise and large gain, so as to meet the servo requirements of ultra-stable lasers of the order of 1e-18.

[0205] According to an embodiment of the present invention, the servo control circuit 200 can be applied not only to servo control for ultra-stable lasers, but also to various other high-precision servo control scenarios to meet the needs of precision measurement physical experiments.

[0206] It will be appreciated by those skilled in the art that the features described in the various embodiments of the present invention may be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in the present invention. In particular, without departing from the spirit and teachings of the present invention, the features described in the various embodiments of the present invention may be combined and / or combined in various ways. All of these combinations and / or combinations fall within the scope of the present invention.

[0207] The embodiments of the present invention are described above. However, these embodiments are only for the purpose of illustration, and are not intended to limit the scope of the present invention. Although each embodiment is described above, it does not mean that the measures in each embodiment cannot be used in combination advantageously. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which should all fall within the scope of the present invention.

Claims

1. A servo control circuit, characterized in that: include: a slow feedback servo control subcircuit, the slow feedback servo control subcircuit comprising a filter, a first amplification stage and a slow feedback servo control unit; The filter is used to filter the error signal to obtain a filtered signal, wherein the error signal represents the frequency deviation between the reference signal and the device to be locked; The in-phase input terminal of the first amplifier stage is connected to the output terminal of the filter, and the first amplifier stage is used to amplify the filtered signal to obtain an amplified signal, wherein the first amplifier stage includes a plurality of parallel operational amplifiers; The input end of the slow feedback servo control unit is connected to the output end of the first amplifier stage, and the slow feedback servo control unit is used to determine a slow feedback control signal according to the amplified signal; a fast feedback servo control subcircuit, the fast feedback servo control subcircuit being used to determine a fast feedback control signal according to the error signal; and The output ends of the slow feedback servo control unit and the fast feedback servo control subcircuit are connected to the input end of the device to be locked, and are used to frequency modulate the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal meets a preset condition, so as to achieve frequency locking of the device to be locked.

2. The servo control circuit according to claim 1, characterized in that: The first amplifier stage includes an input resistor and a plurality of parallel operational amplifier units, wherein the operational amplifier unit includes a first resistor, a second resistor, an operational amplifier and a third resistor, wherein the first port of the first resistor is grounded, the second port of the first resistor is connected to the inverting input terminal of the operational amplifier, the first port of the second resistor is connected to the inverting input terminal of the operational amplifier, the second port of the second resistor is connected to the output terminal of the operational amplifier, and the first port of the third resistor is connected to the output terminal of the operational amplifier; the in-phase input terminals of the plurality of operational amplifiers are connected, and the second ports of the plurality of third resistors are connected to realize the parallel connection of the plurality of operational amplifier units; the first port of the input resistor is connected to the output terminal of the filter, and the second port of the input resistor is connected to the in-phase input terminals of the plurality of operational amplifiers.

3. The servo control circuit according to claim 2, characterized in that: The number of parallel-connected operational amplifier units in the first amplifier stage is determined according to the resistance values ​​of the input resistor, the first resistor, the second resistor and the third resistor, and the voltage noise and current noise of the operational amplifier.

4. The servo control circuit according to claim 1, characterized in that: The slow feedback servo control unit comprises an integral control subunit and a plurality of proportional integral control subunits connected in series. The slow feedback servo control unit is also used to increase the gain of the servo control circuit.

5. The servo control circuit according to claim 4, characterized in that: The slow feedback servo control subcircuit also includes a slow feedback output polarity switch, which includes a single-pole double-throw switch and an inverting amplifier follower. The output end of the last proportional-integral control subunit in the slow feedback servo control unit is connected to the first port of the single-pole double-throw switch, the output end of the last proportional-integral control subunit is connected to the input end of the inverting amplifier follower, and the second port of the single-pole double-throw switch is connected to the output end of the inverting amplifier follower.

6. The servo control circuit according to claim 1, characterized in that: The fast feedback servo control subcircuit comprises at least one double-zero double-pole compensation unit and a fast feedback output polarity switch connected in series. The fast feedback servo control subcircuit is also used to increase the bandwidth of the fast feedback control signal.

7. The servo control circuit according to claim 1, characterized in that: The servo control circuit also includes a second amplifier stage, a common-phase input terminal of the second amplifier stage is connected to the error signal, and an output terminal of the second amplifier stage is connected to a noise tester so as to measure the actual equivalent input noise of the servo control circuit through the noise tester, wherein the first amplifier stage is the same as the second amplifier stage.

8. The servo control circuit according to claim 1, characterized in that: In the case where the device to be locked includes at least two input terminals, the circuit further includes: The output end of the slow feedback servo control unit is connected to the first input end of the device to be locked, and the output end of the fast feedback servo control subcircuit is connected to the second input end of the device to be locked, and is used to frequency modulate the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal meets the preset condition.

9. The servo control circuit according to claim 1, characterized in that: In the case where the device to be locked includes an input terminal, the circuit also includes an adder, a first input terminal of the adder is connected to the output terminal of the slow feedback servo control unit, a second input terminal of the adder is connected to the output terminal of the fast feedback servo control subcircuit, and the output terminal of the adder is connected to the input terminal of the device to be locked, and the adder is used to add the slow feedback control signal and the fast feedback control signal to obtain a control signal, so as to perform frequency modulation on the device to be locked according to the control signal until the error signal meets the preset condition.

10. A frequency locking method based on a servo control circuit, characterized in that: The slow feedback servo control subcircuit includes a filter, a first amplifier stage and a slow feedback servo control unit, and the method includes: Using the filter, filtering the error signal to obtain a filtered signal, wherein the error signal represents the frequency deviation between the locking device and the device to be locked; amplifying the filtered signal using the first amplifier stage to obtain an amplified signal, wherein the first amplifier stage includes a plurality of parallel operational amplifiers; Determining a slow feedback control signal according to the amplified signal using the slow feedback servo control unit; Determining a fast feedback control signal according to the error signal using a fast feedback servo control subcircuit; and The slow feedback servo control unit and the fast feedback servo control subcircuit are used to frequency modulate the device to be locked according to the slow feedback control signal and the fast feedback control signal until the error signal meets a preset condition, so as to achieve frequency locking of the device to be locked.

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