CN113285342B - Optical frequency comb carrier envelope phase signal frequency multi-frequency point locking method and system - Google Patents
Optical frequency comb carrier envelope phase signal frequency multi-frequency point locking method and system Download PDFInfo
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- H—ELECTRICITY
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
- H01S3/1106—Mode locking
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
- H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
- H01S3/094042—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a fibre laser
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
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- H—ELECTRICITY
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10053—Phase control
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- H—ELECTRICITY
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
- H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
- H01S3/109—Frequency multiplication, e.g. harmonic generation
Abstract
The invention provides a frequency multi-frequency point locking method and a frequency multi-frequency point locking system for an optical frequency comb carrier envelope phase signal, which are characterized in that a frequency sweep output signal of a frequency synthesizer and an electric signal which is output by an optical frequency comb optical system and contains carrier envelope phase signal frequency phase information are subjected to frequency mixing and then low-pass filtering, the frequency deviation of the frequency of the optical frequency comb carrier envelope phase signal is quickly judged by measuring the power and the frequency of the signal subjected to low-pass filtering, the output frequency of the frequency synthesizer is fixed to a frequency set value of the optical frequency comb carrier envelope phase signal by accurately setting the pumping current of a seed laser, and a PI (proportional integral) controller is started to realize the quick automatic locking of the frequency multi-frequency point of the optical frequency comb carrier envelope phase signal.
Description
Technical Field
The invention relates to the technical field of frequency control, in particular to a method and a system for locking multiple frequency points of a carrier envelope phase signal frequency of an optical frequency comb.
Background
The optical frequency comb is an important means for connecting optical frequency and radio frequency, is the most effective tool for measuring absolute optical frequency so far, can accurately and simply connect a microwave atomic frequency standard and an optical frequency standard, provides a carrier for developing a frequency standard with high resolution, high precision and high accuracy, also provides a relatively ideal research tool for scientific research directions such as precise spectrum, astronomical physics, quantum control and the like, and has wide application space in the fields of optical frequency precise measurement, measurement of atomic ion transition energy level, remote signal clock synchronization, satellite navigation and the like.
High precision locking of the frequency of the optical-frequency comb carrier envelope phase signal is the basis for optical-frequency comb applications. For the frequency locking of the carrier envelope phase signal of the optical frequency comb, because the signal-to-noise ratio is often poor, a band-pass filter with a narrow pass band is needed, and the frequency of the carrier envelope phase signal is difficult to enter the pass band of the band-pass filter, and generally needs manual judgment, so that the complexity of locking is increased, and long time is also needed for completing the locking. Once the lock is lost and the lock is carried out again, the complex locking process needs to be repeated, and the difficulty is brought to the use of the optical frequency comb. If the frequency point of the carrier envelope phase signal needs to be changed, the center frequency of the filter may need to be changed, which also brings certain difficulty to the design of the band-pass filter.
Although there is a scheme for automatically locking the frequency of the carrier envelope phase signal at present, parameters such as pumping current need to be scanned for many times, and it is difficult to quickly lock multiple frequency points of the frequency of the carrier envelope phase signal.
For example, patent document CN101764346A discloses a method for high-power laser pulse carrier envelope phase locking. The method utilizes a double-light-path interference technology of a self-reference measurement method to obtain a beat frequency signal, the beat frequency signal feeds back and controls a laser through a phase-locked circuit, and a laser amplification system is arranged in one laser light path for generating frequency doubling light in the double-light-path interference technology to amplify the frequency doubling laser at high power. However, the laser amplification system adopted in the technical scheme is complex and difficult to adjust.
As another example, in patent document CN103633537A, a low-noise fiber laser frequency comb device with controllable carrier envelope phase shift frequency is disclosed, which includes an optical path structure and a circuit structure, wherein the optical path structure includes an oscillator, an acousto-optic frequency shifter, an optical fiber amplifier, a pulse compressor, a fiber spread spectrum device, and a coherent heterodyne beat frequency device; the circuit structure comprises a front feedback circuit phase control device and a phase-locked loop circuit repetition frequency control device. The fiber laser oscillator can ensure that the system runs for a long time, and the stability of the system is superior to that of a solid laser oscillator; the low-noise fiber laser frequency comb device is realized by optimizing the intra-cavity net dispersion of the fiber oscillator, introducing an intra-cavity modulator into the oscillator, adopting a front feedback acousto-optic frequency shifter and other technologies. However, the technical scheme is mainly used for long-time measurement, so that the whole system can operate more stably.
Aiming at the technical problem, the invention provides a frequency-comb carrier-envelope phase signal frequency multi-frequency-point locking method and a system, frequency-sweep output signals of a frequency synthesizer and signals output by a photoelectric detector are mixed and then subjected to low-pass filtering and amplification, frequency deviation of carrier-envelope phase signal frequency is rapidly judged by measuring power and frequency of the signals subjected to low-pass filtering, and multi-frequency-point rapid automatic locking of carrier-envelope phase signal frequency is realized by closed-loop control of seed laser pumping current.
Disclosure of Invention
The invention provides a frequency multi-frequency point locking method for an optical frequency comb carrier envelope phase signal, which comprises the steps of mixing a frequency sweeping output signal of a frequency synthesizer with a signal output by a photoelectric detector, then entering a low-pass amplifier for low-pass filtering and amplifying, quickly judging the frequency deviation of the carrier envelope phase signal frequency by measuring the power and the frequency of the signal amplified by the low-pass filtering, fixing the output frequency of the frequency synthesizer to a set value of the carrier envelope phase signal frequency by accurately setting the pumping current of a seed laser, and starting fast PI control and slow PI control to realize the quick and automatic locking of the multi-frequency point of the carrier envelope phase signal frequency.
Specifically, the method for rapidly and automatically locking the multi-frequency point of the carrier envelope phase signal frequency of the optical frequency comb comprises the following steps:
converting an optical signal which is output by an optical frequency comb optical system and contains carrier envelope phase signal frequency phase information into an electric signal;
performing frequency mixing processing on the electric signal and the radio frequency signal, and outputting an output signal after the frequency mixing processing as an error signal after low-pass amplification;
dividing the error signals into three paths for output, respectively measuring the power of a first path of error signals, measuring the frequency of a second path of error signals, and performing fast PI control on a third path of error signals;
step three, the output frequency f of the radio frequency signal is obtainediSetting the minimum value in a frequency scanning range, and reading the power value of the first path of error signal after delta t time;
step four, increasing the output frequency f of the radio frequency signal by taking delta f as the stepping frequencyiReading the power value of the first path of error signal after delta t time, and judging whether the power value reaches threshold power;
step five, when the power value of the first path of error signal reaches or exceeds the threshold value power, the frequency value f of the second path of error signal is readerrAt this time, the real-time frequency value of the frequency of the optical frequency comb carrier envelope phase signal is Fceort=fi+ferrWherein f isiFor this purpose, the output frequency f of the radio-frequency signali；
Step six, setting the set value of the output frequency of the frequency synthesizer as FceosetCalculating the offset delta DA of the digital-to-analog converter for controlling the frequency offset value of the optical frequency comb carrier envelope phase signal:
ΔDA=（Fceort-Fceoset）/k；
k is the variation of the frequency of the optical frequency comb carrier envelope phase signal corresponding to the unit offset;
step seven, increasing the output value of the digital-analog converter in the step six by delta DA, and detecting whether the power value of the first path of error signal reaches or exceeds the threshold power; if so, starting fast PI control and slow PI control to realize the locking of the frequency of the optical frequency comb carrier envelope phase signal; and if not, reducing the output value of the digital-analog converter by 2 multiplied by delta DA, and starting fast PI control and slow PI control through a PI control switch to realize the locking of the frequency of the optical frequency comb carrier envelope phase signal.
Further, when changing the set value F of the output frequency of the frequency synthesizerceosetAnd in time, hardware parameters are not required to be changed, the third step to the seventh step are executed, and the fast automatic locking of the frequency of the optical frequency comb carrier envelope phase signal is realized again.
Further, the cut-off frequency f of the filtering of the low-pass amplification process of the second stepcLarger than lockThe loop bandwidth is fixed by 3 times.
Further, the frequency sweep range is set from 200MHz to 300MHz, and Δ t is larger than the sum of the response time of the frequency synthesizer and the response time of the power measurement module.
Further, the step frequency Δ f is the cut-off frequency f of the filtering of the low-pass amplification processc1/4 of (1).
The invention also provides a locking system for realizing the frequency multi-frequency point locking method of the optical frequency comb carrier envelope phase signal, which comprises the following steps:
an optical frequency comb optical system for outputting an optical signal containing carrier envelope phase signal frequency phase information; the optical-frequency comb optical system includes: the system comprises a seed laser and an f-2f module, wherein the f-2f module receives a laser light source provided by the seed laser and generates an optical signal containing carrier envelope phase signal frequency phase information;
the photoelectric detector is used for converting the optical signal containing the frequency phase information of the carrier envelope phase signal into an electric signal and outputting the electric signal to the mixer;
the frequency synthesizer is used for outputting a radio frequency signal and outputting the radio frequency signal to the mixer;
the frequency mixer is used for mixing the electric signal output by the photoelectric detector and the radio frequency signal output by the frequency synthesizer;
the low-pass amplifier is used for low-pass filtering and amplifying the output signal subjected to the frequency mixing processing by the frequency mixer, outputting an error signal, and dividing the error signal output by the low-pass amplifier into three paths; the first path is output to a power measurement module, the second path is output to a frequency measurement module, and the third path is output to a fast PI controller;
the output end of the fast PI controller is connected with the pumping source and the slow PI controller, and the fast PI controller and the slow PI controller are both connected with the pumping source and used for modulating the pumping source to form a carrier envelope phase signal locking loop.
The output end of the digital-analog converter is connected with the pumping source and is used for controlling the offset value of the frequency of the optical frequency comb carrier envelope phase signal;
and the operation controller is used for setting the output frequency of the radio frequency signal output by the frequency synthesizer, reading the power value measured by the power measuring module and the frequency value measured by the frequency measuring module, calculating the offset of the digital-analog converter and controlling the output value of the digital-analog converter.
Further, the operation controller controls the opening and closing of the fast PI controller and the slow PI controller through the PI control switch.
Further, the cut-off frequency f of the low-pass filteringcGreater than 3 times the locked loop bandwidth.
Further, the frequency sweep range of the frequency synthesizer is set to a sweep range from 200MHz to 300 MHz.
Further, the operation controller is based on a set value FceosetAnd the real-time frequency value F of the frequency measurement moduleceortCalculating and controlling the frequency F of the optical frequency comb carrier envelope phase signalceoOffset of the digital-to-analog converter of the offset value, thereby controlling the digital-to-analog converter output value.
Has the advantages that: the invention utilizes the sweep frequency output signal of the frequency synthesizer and the signal output by the photoelectric detector to carry out low-pass filtering after frequency mixing, and quickly judges the frequency F of the optical frequency comb carrier envelope phase signal by measuring the power and the frequency of the signal after the low-pass filteringceoThe frequency deviation of the optical frequency comb carrier envelope phase signal frequency F is realized through the closed-loop control of the seed laser pumping currentceoThe multi-frequency point is locked automatically and rapidly. The invention solves the problem of changing the frequency F of the carrier envelope phase signal of the optical frequency comb in the prior artceoThe locking frequency point also needs to change the center frequency of the band-pass filter, thereby simplifying the frequency F of the carrier envelope phase signal of the optical frequency combceoThe locking process shortens the locking time and realizes the frequency F of the optical frequency comb carrier envelope phase signalceoThe multi-frequency point quick automatic locking improves the locking reliability and simplifies the use of the optical frequency comb, and is beneficial to the popularization and the use of the optical frequency comb. The invention can realize the optical frequency comb carrier envelope phase signal frequency F superior to 5E-18/sceoLocking stability, and the time for recovering locking after losing the lock is less than 2 s.
The terms used herein are explained as follows:
the optical frequency comb is based on the mode-locked femtosecond pulse laser technology, and can conveniently, reliably and accurately lock the optical frequency to the reference frequency. The basic principle is as follows: the output of the pulse laser is a series of ultrashort pulses with equal intervals in the time domain, the pulse width is generally several to dozens of femtoseconds, the repetition frequency is several hundred MHz to several GHz, and the output of the pulse laser is an optical comb consisting of a series of spectral lines with equal intervals in the frequency domain, and the interval between every two comb teeth is equal to the repetition frequency of the femtosecond laser.
Mode locking, a technique used in optics to generate laser pulses for very short periods of time, typically in picoseconds or even femtoseconds. The theoretical basis of this technique is to introduce a fixed phase relationship between the different modes in the laser cavity, and the laser thus produced is called a phase-locked laser or mode-locked laser.
The carrier envelope phase is a very important parameter of the periodic-magnitude laser pulse, and is a relative phase between the maximum value of the periodic-magnitude pulse envelope and the maximum value of electric field oscillation under the envelope.
And the pumping source is used for exciting the laser working substance and pumping the activated particles from a ground state to a high energy level so as to realize the population inversion. Depending on the working substance and the operating conditions of the laser. Different actuation modes and actuation means may be employed. Optical excitation (optical pumping), gas discharge excitation, chemical excitation, nuclear energy excitation are common.
The PI (operation) controller is a linear controller, which forms a control deviation according to a given value and an actual output value, linearly combines the proportion and the integral of the deviation to form a control quantity, and controls a controlled object to improve the steady-state performance of a control system.
Drawings
FIG. 1 is a flow chart of a method for frequency multi-frequency point locking of an optical frequency comb carrier envelope phase signal according to the present invention;
FIG. 2 is a frequency spectrum diagram of a carrier envelope phase signal after frequency locking;
FIG. 3 is a schematic diagram of the overall structure of the optical-frequency comb carrier-envelope phase signal frequency multi-frequency point locking system of the present invention;
10, an optical frequency comb optical system; 11. a seed laser; 12. an f-2f module; 13. 14, a reflector; 20. a pump source; 30. a frequency synthesizer; 41. a power measurement module; 42. a frequency measurement module; 50. an arithmetic controller; 60. a photodetector PD; 71. a fast PI controller; 72. a slow PI controller; 73. a PI control switch; 74. a digital-to-analog converter DA; 80. a mixer; 90. a low pass amplifier.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the following is a more detailed description of the present invention with reference to the accompanying drawings by way of examples, but the embodiments of the present invention are not limited thereto. It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The term "comprising", without further limitation, means that the element so defined is not excluded from the group consisting of additional identical elements in the process, method, article, or apparatus that comprises the element.
As shown in fig. 1, which is a schematic flow chart of a frequency multi-frequency point locking method for an optical-frequency comb carrier-envelope phase signal of the present invention, the implementation of the frequency multi-frequency point locking method for the optical-frequency comb carrier-envelope phase signal includes the following steps:
outputting an optical signal containing carrier envelope phase signal frequency phase information of an optical frequency comb optical system by using an f-2f module, and converting the optical signal containing the carrier envelope phase signal frequency phase information output by the optical frequency comb optical system into an electric signal by using a photoelectric detector;
and secondly, inputting the electric signal output by the photoelectric detector and the radio frequency signal output by the frequency synthesizer into a mixer for mixing, performing low-pass filtering and amplification on the output signal of the mixer through a low-pass amplifier, outputting an error signal, dividing the output error signal into three paths, outputting the first path of error signal to a power measurement module for power measurement, outputting the second path of error signal to a frequency measurement module for frequency measurement, and outputting the third path of error signal to a fast PI controller for fast calculation control.
The cut-off frequency f of the low-pass filter of the low-pass amplifier in the step twocIt may be more than 3 times the locking bandwidth of the carrier envelope phase signal, preferably 2 MHz.
Step three, the operation controller firstly outputs the frequency f of the frequency synthesizeriSet to a minimum value within the frequency sweep range, the frequency sweep range of the frequency synthesizer is preferably set from 200MHz to 300 MHz; after waiting for delta t time, reading the power value of the first path of error signal measured by the power measurement module in the step two;
the latency Δ t needs to be larger than the sum of the response time of the frequency synthesizer and the response time of the power measurement module, preferably 1ms ± 0.5 ms.
Step four, increasing the output frequency f of the frequency synthesizer by taking the delta f as the stepping frequencyiAnd after waiting for delta t time, reading the power value measured by the power measurement module, and judging whether the power value reaches the threshold power.
The method for determining the threshold power in the fourth step is as follows: when the output frequency of the frequency synthesizer is set to the minimum value in the frequency scanning range, the power value P measured by the power measurement module is recordedfminWhen the output frequency of the frequency synthesizer is set to the maximum value in the frequency scanning range, the power value P measured by the power measuring module is recordedfmaxTaking PfminAnd PfmaxThe smaller of the two is taken as the platform power PpltDue to the real-time frequency value F of the frequency of the carrier-envelope phase signal as the optical frequency combceortOutput frequency f of frequency synthesizeriIs less than the cut-off frequency f of the low-pass filtering of the low-pass amplifier in the step twocAnd the power value measured by the power measuring module is higher than the platform power value by more than 10 dB. Thus, the threshold power P is setthr=Pplt+10, where the unit of power is dBm.
The step frequency delta f is preferably the cut-off frequency f when the low-pass amplifier performs low-pass filtering in the second stepc1/4 of (1).
Step five, when the power value detected by the power measurement module reaches or exceeds the threshold power, reading the frequency value f measured by the frequency measurement module on the second path of error signalserr。
At this time, the real-time frequency value of the carrier envelope phase signal frequency of the optical frequency comb is as follows:
Fceort=fi+ferr；
wherein f isiIs the frequency value of the radio frequency signal output by the frequency synthesizer at this time.
Step six, setting the frequency value output by the frequency synthesizer as FceosetAccording to a set value FceosetWith a real-time frequency value FceortCalculating the offset of a digital-analog converter DA for controlling the frequency offset value of the optical frequency comb carrier envelope phase signal, wherein the offset delta DA of the DA is calculated by the following method:
ΔDA=（Fceort-Fceoset）/k
k is the variation of the frequency of the optical frequency comb carrier envelope phase signal corresponding to the unit offset, and can be measured through experiments.
Step seven, increasing the output value of the digital analog converter DA in the step six by delta DA, simultaneously detecting the power value of the first path of error signal by the power measurement module, and if the detected power value reaches or exceeds the threshold power, starting fast PI control and slow PI control by the PI control switch to realize the frequency F of the optical frequency comb carrier envelope phase signalceoLocking of (2); if the measured power value does not reach the threshold power, the control direction of the digital-analog converter DA is opposite, the output value of the digital-analog converter DA needs to be reduced by 2 xDeltaDA, and the fast PI control and the slow PI control are started through the PI control switch to realize the frequency F of the optical frequency comb carrier envelope phase signalceoLocking of (2).
In the seventh step, the fast PI control and the slow PI control indicate the bandwidth of the value PI control. In the invention, the bandwidth of the fast PI controller for realizing the fast PI control is more than 1MHz, and the bandwidth of the slow PI controller for realizing the slow PI control is less than 100 kHz. The error signal output by the low-pass amplifier passes through the fast PI controller and then fast controls the current of the pumping source, so that the jitter of the carrier envelope phase signal frequency is inhibited and the stability of the carrier envelope phase signal is improved. However, the control range of the fast PI controller is small and only 1-5 mA, so that the fast PI controller is suitable for inhibiting the jitter of the carrier envelope phase signal frequency in a small range, and the frequency drift of the carrier envelope phase signal is difficult to compensate. In the invention, the output signal of the fast PI controller is used as the input signal of the slow PI controller, and the slow PI controller has a large control range which can reach 100-200 mA, so that the slow PI controller is used for controlling the current of a pumping source in a large range to compensate the frequency drift of a carrier envelope phase signal. Therefore, the seventh step combines the characteristics of a fast PI controller with a large bandwidth in a small range and a slow PI controller with a small bandwidth in a large range, and realizes the long-term stable control of the frequency of the carrier envelope phase signal.
Therefore, the fast automatic locking of the frequency of the optical frequency comb carrier envelope phase signal is realized. If the frequency F of the optical frequency comb carrier envelope phase signal is changedceoSet value of (F)ceosetAnd the quick automatic locking can be realized again only by repeating the third step, the seventh step and the fourth step without changing hardware parameters. Fig. 2 is a frequency spectrum diagram of a carrier envelope phase signal after frequency locking.
In a preferred embodiment, the step frequency Δ F =500kHz, the width of the sweep range is 100MHz, each frequency point needs 1ms, and then the sweep only needs 200ms to realize the sweep, and the frequency F of the optical frequency comb carrier envelope phase signalceoThe time required by the automatic locking process is less than 2s, the method for rapidly and automatically locking the multi-frequency point of the frequency of the optical frequency comb carrier envelope phase signal realizes the automation of the whole rapid frequency locking process, and the frequency locking time can reach less than 1 s.
Referring to fig. 3, there is shown an overall structural diagram of a locking system for implementing a multi-frequency-point fast automatic locking method for an optical-frequency comb carrier-envelope phase signal frequency according to the present invention, where the locking system includes:
an optical-frequency comb optical system 10 for outputting an optical signal containing carrier-envelope phase signal frequency phase information; the optical-frequency comb optical system 10 includes:
the seed laser 11 is used for providing a laser light source for the optical frequency comb optical system, the seed laser 11 is connected with a pumping source 20, and the pumping source 20 provides sufficient pumping light power for the optical frequency comb optical system; in a preferred embodiment, the seed laser 11 may be an erbium doped fiber pumped by a 980nm fiber laser, the central wavelength of the output light is 1550nm, the output power is 10mW, and the pulse width of the output light is 100 fs.
And an f-2f module 12, which receives the laser light source provided by the seed laser 11, and is configured to frequency-multiply and beat a portion of the laser beam with a wavelength of 1550nm ± 20nm to generate an optical signal including frequency phase information of the carrier envelope phase signal.
And mirrors 13, 14 for reflecting the laser beam.
The locking system further comprises: and the photoelectric detector 60, the photoelectric detector 60 is connected to the output end of the f-2f module 12 of the optical frequency comb optical system 10, and is configured to convert the phase information including the carrier envelope phase signal frequency output by the f-2f module 12 into an electrical signal, and output the electrical signal to the mixer 80.
A frequency synthesizer 30 for outputting a radio frequency signal to the mixer 80; when the locking system starts to work, the output frequency f of the radio frequency signal output by the frequency synthesizer 30 is firstly outputiSetting the frequency of the RF signal to be a minimum value in a frequency scanning range, and gradually increasing the output frequency f of the RF signal by taking deltaf as a stepping frequencyiThe frequency sweep range of the frequency synthesizer 30 is preferably set from 200MHz to 300 MHz.
A mixer 80, configured to mix the electrical signal output by the photodetector 60 with the radio frequency signal output by the frequency synthesizer 30; and the low-pass amplifier 90 is used for low-pass filtering and amplifying the output signal subjected to the frequency mixing processing by the frequency mixer 80, and the low-pass filtering is performed after the frequency mixing, so that variable band-pass filtering is realized, a real-time frequency value of the carrier envelope phase signal frequency signal is found by scanning the center frequency of the band-pass filtering, the signal output by the low-pass amplifier is an error signal, and the error signal output by the low-pass amplifier 90 is divided into three paths.
The first path is output to the power measurement module 41, and the power measurement module 41 measures the power of the first path of error signal and outputs the measured power value to the operation controller 50.
The second path is output to the frequency measurement module 42, and the frequency measurement module 42 measures the frequency of the second path of error signal and outputs the measured frequency value to the operation controller 50.
The third path is output to the fast PI controller 71; the fast PI controller 71 performs fast PI control on the third error signal.
An operation controller 50 for receiving the power value measured by the power measuring module 41 and the frequency value measured by the frequency measuring module, and controlling the frequency of the RF signal output by the frequency synthesizer 30 according to FceoSet value of (F)ceosetWith a real-time frequency value FceortCalculating and controlling the frequency F of the optical frequency comb carrier envelope phase signalceoThe offset of the digital-analog converter DA74 of the offset value, thereby controlling the output value of the digital-analog converter DA74, and connecting the PI control switch 73 for controlling the switches of the fast PI controller 71 and the slow PI controller 72; specifically, the output frequency f of the frequency synthesizer is firstlyiSetting the minimum value in the frequency scanning range, and reading the power value measured by the power measurement module 41 after waiting for delta t; if the power value reaches or exceeds the threshold power, the fast PI controller 71 and the slow PI controller 72 are started through the PI control switch 73 to realize FceoLocking of (2). If the power value measured by the power measurement module does not reach the threshold power, the output value of the digital analog converter DA74 at the moment is reduced by delta DA which is 2 times, and the fast PI controller and the slow PI controller are started through the PI control switch 73, so that the locking of the carrier envelope phase signal frequency is realized.
The fast PI controller is connected with the slow PI controller, and the fast PI controller and the slow PI controller are connected with the pumping source together and used for controlling the current of the pumping source and changing the frequency of the carrier envelope phase signal. Specifically, the fast PI controller is used for performing proportional integral PI operation according to a frequency phase error between an acquired carrier envelope phase frequency signal and a set frequency signal, outputting a control signal and controlling a laser pumping current in real time, and accordingly realizing closed-loop control of the carrier envelope phase signal frequency phase. When the system is in a steady state, the slow PI controller takes the average value of the control signals of the output port of the fast PI controller as an error signal to perform proportional-integral PI operation, and outputs the control signals to control the average value of the pumping current of the laser, so that the output signals of the fast PI controller are kept in the optimal working state of 0V +/-1V on average.
Just as the method part states, the fast PI controller has small control range, only 1-5 mA, but wider control bandwidth, more than 1MHz, and is suitable for inhibiting the jitter of the carrier envelope phase signal frequency and improving the stability; the slow PI controller is large in control range and has a current control range of 100-200 mA, the bandwidth of the slow PI controller is set below 100kHz in order to guarantee stability of a control loop, and the slow PI controller is suitable for controlling current of a pumping source in a large range to compensate frequency drift of a carrier envelope phase signal.
In a preferred embodiment, the latency Δ t is greater than the sum of the response time of the frequency synthesizer and the response time of the power measurement module, preferably 1ms ± 0.5 ms.
The invention utilizes the sweep frequency output signal of the frequency synthesizer and the signal output by the photoelectric detector to carry out low-pass filtering after frequency mixing, quickly judges the frequency deviation of the carrier envelope phase signal by measuring the power and the frequency of the signal amplified by the low-pass filtering, and realizes the quick automatic locking of the multi-frequency point of the carrier envelope phase signal frequency by the closed-loop control of the pumping current of the seed laser. The invention solves the problem that the change of the locking frequency point of the carrier envelope phase signal frequency also needs to change the center frequency of the band-pass filter in the prior art, simplifies the locking process of the carrier envelope phase signal frequency, shortens the locking time of the carrier envelope phase signal frequency, realizes the quick automatic locking of the multi-frequency point of the carrier envelope phase signal frequency, improves the locking reliability, simplifies the use of the optical frequency comb, and is beneficial to the popularization and the use of the optical frequency comb. The invention can realize the carrier envelope phase signal frequency locking stability better than 5E-18/s, and the recovery locking time after losing the lock is less than 2 s.
For the system embodiment, since it basically corresponds to the method embodiment, reference may be made to the partial description of the method embodiment for relevant points. The above-described system embodiments are merely illustrative, and the units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment. One of ordinary skill in the art can understand and implement it without inventive effort.
The foregoing is directed to embodiments of the present invention, and it is understood that various modifications and improvements can be made by those skilled in the art without departing from the spirit of the invention.
Claims (10)
1. A multi-frequency point locking method for the frequency of an optical frequency comb carrier envelope phase signal is characterized by comprising the following steps:
converting an optical signal which is output by an optical frequency comb optical system and contains carrier envelope phase signal frequency phase information into an electric signal;
step two, performing frequency mixing processing on the electric signal and a radio frequency signal output by a frequency synthesizer, and outputting an output signal after the frequency mixing processing as an error signal after low-pass filtering and amplification;
dividing the error signals into three paths for output, respectively measuring the power of a first path of error signals, measuring the frequency of a second path of error signals, and performing fast PI control on a third path of error signals;
step three, the output frequency f of the radio frequency signal is obtainediSetting the minimum value in a frequency scanning range, and reading the power value of the first path of error signal after delta t time;
step four, increasing the output frequency f of the radio frequency signal by taking delta f as the stepping frequencyiReading the first way error after delta t timeThe power value of the difference signal is judged whether the power value reaches the threshold power;
step five, when the power value of the first path of error signal reaches or exceeds the threshold value power, the frequency value f of the second path of error signal is readerrAt this time, the real-time frequency value of the frequency of the optical frequency comb carrier envelope phase signal is Fceort=fi+ferrWherein f isiThe output frequency of the radio frequency signal at this time;
step six, setting the set value of the output frequency of the frequency synthesizer as FceosetCalculating the offset delta DA of the digital-to-analog converter for controlling the frequency offset value of the optical frequency comb carrier envelope phase signal:
ΔDA=（Fceort-Fceoset）/k；
k is the variation of the frequency of the optical frequency comb carrier envelope phase signal corresponding to the unit offset;
step seven, increasing the output value of the digital-analog converter in the step six by delta DA, and detecting whether the power value of the first path of error signal reaches or exceeds the threshold power; if so, starting fast PI control and slow PI control to realize the locking of the frequency of the optical frequency comb carrier envelope phase signal; and if not, reducing the output value of the digital-analog converter by 2 multiplied by delta DA, and starting fast PI control and slow PI control to realize the locking of the frequency of the optical frequency comb carrier envelope phase signal.
2. The method as claimed in claim 1, wherein the frequency of the output frequency of the frequency synthesizer is changed by changing a setting value FceosetAnd then, executing the third step to the seventh step, and realizing the quick automatic locking of the frequency of the optical frequency comb carrier envelope phase signal again.
3. The method as claimed in claim 1, wherein the cut-off frequency f of the low-pass filtering in the second step iscGreater than 3 times the locked bandwidth of the carrier envelope phase signal.
4. The method of claim 1, wherein the frequency sweep range is set from 200MHz to 300MHz, and Δ t is greater than the sum of the response time of the frequency synthesizer and the response time of the power measurement module.
5. The method as claimed in claim 3, wherein the step frequency Δ f is the cut-off frequency fc1/4 of (1).
6. A locking system for implementing the frequency multi-frequency point locking method of the optical-frequency comb carrier-envelope phase signal according to any one of claims 1 to 5, comprising:
an optical frequency comb optical system for outputting an optical signal containing carrier envelope phase signal frequency phase information; the optical-frequency comb optical system includes: the system comprises a seed laser and an f-2f module, wherein the f-2f module receives a laser light source provided by the seed laser and generates an optical signal containing carrier envelope phase signal frequency phase information;
the photoelectric detector is used for converting the optical signal containing the frequency phase information of the carrier envelope phase signal into an electric signal and outputting the electric signal to the mixer;
the frequency synthesizer is used for outputting a radio frequency signal and outputting the radio frequency signal to the mixer;
the frequency mixer is used for mixing the electric signal output by the photoelectric detector and the radio frequency signal output by the frequency synthesizer;
the low-pass amplifier is used for low-pass filtering and amplifying the output signal subjected to the frequency mixing processing by the frequency mixer, outputting an error signal, and dividing the error signal output by the low-pass amplifier into three paths; the first path is output to a power measurement module, the second path is output to a frequency measurement module, and the third path is output to a fast PI controller;
the output end of the fast PI controller is connected with the pumping source and the slow PI controller, and the fast PI controller and the slow PI controller are both connected with the pumping source and used for modulating the pumping source to form a carrier envelope phase signal locking loop;
the output end of the digital-analog converter is connected with the pumping source and is used for controlling the offset value of the frequency of the optical frequency comb carrier envelope phase signal;
and the operation controller is used for setting a set value of the output frequency of the frequency synthesizer, reading the power value measured by the power measuring module and the frequency value measured by the frequency measuring module, calculating the offset of the digital-analog converter and controlling the output value of the digital-analog converter.
7. The locking system of claim 6, wherein the operational controller controls the opening and closing of the fast PI controller and the slow PI controller via a PI control switch.
8. Locking system according to claim 6, characterized in that the cut-off frequency f of the low-pass filteringcGreater than 3 times the locked loop bandwidth.
9. The locking system of claim 6, wherein the frequency sweep range of the frequency synthesizer is set from 200MHz to 300 MHz.
10. Locking system according to claim 6, characterized in that said calculation controller is based on a set value FceosetAnd the real-time frequency value F of the frequency measurement moduleceortAnd calculating the offset of the digital-analog converter for controlling the frequency offset value of the optical frequency comb carrier envelope phase signal, thereby controlling the output value of the digital-analog converter.
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| CN114142925B (en) * | 2021-11-29 | 2023-05-12 | 西安空间无线电技术研究所 | Optical carrier radio frequency carrier frequency measuring system |
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