CN113300207A - Asynchronous locking method and device for repetition frequency of ultrashort pulse laser - Google Patents
Asynchronous locking method and device for repetition frequency of ultrashort pulse laser Download PDFInfo
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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
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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
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- 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
- H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
Abstract
The invention provides an asynchronous locking method and device of repetition frequency of ultrashort pulse laser, wherein the method comprises the following steps: detecting a signal output by a main laser, extracting a higher harmonic signal of the signal, mixing the signal with a signal source to generate a corresponding error control signal, generating a corresponding feedback voltage according to the error control signal, and controlling the cavity length of a laser resonant cavity or the refractive index of a medium in the cavity by using the feedback voltage so as to control the repetition frequency of the main laser; taking the locked main laser as a source, dividing the output higher harmonic signal into two paths, wherein one path is subjected to frequency division, and the other path is used for triggering a DDS; the output of the master laser and the frequency-divided signal are mixed, the signal output from the slave laser and the signal generated by the DDS are mixed, then the mixed signal and the signal are mixed and converted into an error control signal, corresponding feedback voltage is generated, and the cavity length of the resonant cavity of the laser or the refractive index of the medium in the cavity is controlled by the feedback voltage, so that the repetition frequency of the slave laser is controlled.
Description
Technical Field
The invention relates to the technical field of ultrafast optics, in particular to an asynchronous locking method and device for repetition frequency of ultrashort pulse laser.
Background
A femtosecond laser frequency comb is an ultrashort pulse laser that appears in the time domain as a sequence of pulses with fixed time intervals and in the frequency domain as a collection of millions of frequency "combs". Femtosecond laser frequency combs have important applications in many fields as bridges for coupling between optical and microwave frequencies, such as: precision spectroscopy, absolute distance measurement, etc. If the femtosecond laser frequency comb is used for the applications, the long-term stable operation is the most basic condition. The locking of the repetition frequency and the repetition frequency difference (the difference of the repetition frequencies of two femtosecond laser frequency combs) has important application in many fields. Taking the absolute distance measurement of the double-femtosecond laser frequency comb as an example, the Coddington doctor of the us standard measuring office (NIST) in 2009 proposed the asynchronous optical sampling ranging principle of the double-femtosecond optical comb, and two femtosecond laser frequency combs with a small repetition frequency difference are respectively used as a measuring optical comb and a local oscillator optical comb to construct a ranging system. In the measurement process, the repetition frequency difference between the local oscillator optical comb and the measurement optical comb determines the density of asynchronous optical sampling, and the uncertainty of the repetition frequency and the repetition frequency difference affects the error of distance measurement, so that the repetition frequency and the repetition frequency difference need to be precisely locked.
In the femtosecond laser frequency comb, the formula f is c/nL (f is the repetition frequency of the optical comb, c is the optical speed, L is the cavity length of the laser resonant cavity, and n is the refractive index of the medium in the cavity)Refers to the optical path length that a pulse in the resonant cavity experiences after one cycle. The repetition frequency of the optical comb is mainly related to the cavity length of the resonant cavity of the laser and the refractive index of the medium in the resonant cavity, so that feedback control needs to be performed on the cavity length of the resonant cavity and the refractive index of the medium in the resonant cavity to lock the repetition frequency. A common solution is to use piezoelectric ceramics for feedback regulation. For example, the piezoelectric ceramic is installed on an end face of an optical reflector in the resonant cavity, the piezoelectric ceramic is extended or shortened and drives the optical reflector to move by controlling the loading voltage at two ends of the piezoelectric ceramic, so that the length of the resonant cavity is changed, the optical fiber can also be fixed on the piezoelectric ceramic, the length of the optical fiber in the laser is changed by changing the length of the piezoelectric ceramic, or the refractive index of a medium in the cavity is changed by changing the loading voltage of the photoelectric crystal, and finally the purpose of controlling the repetition frequency of the output pulse is achieved. At present, the locking technology of the repetition frequency and the repetition frequency difference of the double-femtosecond laser frequency comb is mainly realized by respectively and synchronously locking two femtosecond lasers to two high-precision high-output-frequency signal sources which are linked by the same external reference source, but the technology needs a high-precision common reference source and two high-precision signal sources with high output frequency and is expensive; and the two femtosecond lasers are precisely locked by an external signal source respectively, and the final relative precision is realized by using a single signal source
And the noise is larger.
Disclosure of Invention
The invention provides an asynchronous locking method of the repetition frequency of the ultrashort pulse laser and an asynchronous locking device of the repetition frequency of the ultrashort pulse laser, aiming at overcoming the defects of complex structure and low precision of a system adopted for locking the repetition frequency and the repetition frequency difference in the prior art.
In order to solve the technical problems, the technical scheme of the invention is as follows:
an asynchronous locking method of ultra-short pulse laser repetition frequency is applied to an optical system comprising a master femtosecond laser and a slave femtosecond laser, and comprises the following steps:
locking the repetition frequency of the main femtosecond laser: detecting a repetition frequency signal output by the main femtosecond laser, and extracting a higher harmonic signal from the repetition frequency signal obtained by detection; mixing the higher harmonic signal with a signal with high stability to obtain a first mixing signal, and generating a corresponding error control signal by the first mixing signal through a PI (proportional-integral) controller; generating corresponding feedback voltage according to the error control signal, and controlling the cavity length of the laser resonant cavity or the refractive index of the medium in the cavity by using the feedback voltage so as to control the repetition frequency of the main femtosecond laser;
locking the repetition frequency from the femtosecond laser: the locked main femtosecond laser is used as a stable signal source, and the output higher harmonic signals with the repetition frequency are divided into two paths, wherein one path of higher harmonic signals is subjected to frequency division, and the other path of higher harmonic signals triggers a DDS signal generator; mixing the higher harmonic signal output by the main femtosecond laser with the frequency-divided signal to obtain a second mixing signal, and mixing the higher harmonic signal output by the secondary femtosecond laser with a signal generated after triggering the DDS signal generator to obtain a third mixing signal; performing frequency mixing processing on the second frequency mixing signal and the third frequency mixing signal to obtain a fourth frequency mixing signal; and converting the fourth mixing signal into an error control signal through a PI controller, generating corresponding feedback voltage according to the error control signal, and controlling the cavity length of the resonant cavity of the laser or the refractive index of the medium in the resonant cavity by using the feedback voltage so as to control the repetition frequency of the slave femtosecond laser and realize asynchronous locking.
In this technical solution, signal processing is performed according to a repetition frequency signal output by the master femtosecond laser and a repetition frequency output by the slave femtosecond laser, respectively, to generate an error control signal, and generate a corresponding feedback voltage according to the error control signal, where the generated feedback voltage is used to control the repetition frequency of the slave femtosecond laser, specifically, the following means are included and not limited: loading the feedback voltage on a piezoelectric ceramic arranged on the end face of an optical reflector in the resonant cavity to extend or shorten the piezoelectric ceramic and drive the optical reflector to move, thereby changing the length of the resonant cavity; or the optical fiber is fixed on the piezoelectric ceramic, and the length of the optical fiber in the laser is changed by changing the length of the piezoelectric ceramic according to the feedback voltage loaded on the piezoelectric ceramic; or the magnitude of the refractive index of the optical fiber is changed by changing the loading voltage of the photoelectric crystal arranged in the resonant cavity. The technical scheme aims to achieve the purpose of controlling the repetition frequency of the laser and realize asynchronous locking by generating corresponding feedback voltage according to the error control signal and controlling the cavity length of the resonant cavity of the laser or the refractive index of a medium in the resonant cavity by using the feedback voltage.
Preferably, in the locking of the repetition frequency of the slave femtosecond laser, the output value of the DDS digital signal generator is set according to a difference between the repetition frequencies of the desired master femtosecond laser and the slave femtosecond laser.
Preferably, in the step of locking the repetition frequency of the femtosecond laser, the difference frequency signal of the second mixing signal is filtered out, and the sum frequency signal of the second mixing signal is reserved; filtering the difference frequency signal of the third mixing signal, and reserving the sum frequency signal of the third mixing signal; and then, performing frequency mixing processing on the sum frequency signal of the second mixing signal and the sum frequency signal of the third mixing signal to obtain a fourth mixing signal, filtering the sum frequency signal of the fourth mixing signal to obtain a difference frequency signal of the fourth mixing signal, and converting the difference frequency signal of the fourth mixing signal through a PI (proportional-integral) controller to obtain an error control signal.
Preferably, the method further comprises the following steps: mixing the repetition frequency signal output by the main femtosecond laser and the repetition frequency signal output by the slave femtosecond laser, and filtering a sum frequency signal to obtain a difference frequency signal, namely an actual repetition frequency difference signal; and inputting the difference frequency signal into a frequency counter for real-time monitoring.
Preferably, the error control signal is used for generating a corresponding feedback voltage, and the means for controlling the repetition frequency of the master femtosecond laser and the slave femtosecond laser includes but is not limited to:
1) loading feedback voltage on a power supply end of piezoelectric ceramics of a lens in resonant cavities of the master femtosecond laser and the slave femtosecond laser, and changing the cavity lengths of the resonant cavities of the master femtosecond laser and the slave femtosecond laser;
2) loading feedback voltage on power supply ends of piezoelectric ceramics with optical fibers fixed in resonant cavities of the master femtosecond laser and the slave femtosecond laser, and changing the lengths of the optical fibers in the resonant cavities of the master femtosecond laser and the slave femtosecond laser;
3) and loading feedback voltage on two sides of the electro-optical crystal in the resonant cavity of the master femtosecond laser and the slave femtosecond laser to change the refractive index of the electro-optical crystal.
The invention also provides an asynchronous locking device of the repetition frequency of the ultrashort pulse laser, which is applied to the asynchronous locking method of the repetition frequency of the ultrashort pulse laser in the technical scheme, and comprises a master femtosecond laser, a slave femtosecond laser, a first PI controller, a second PI controller and a repetition frequency locking device, wherein the first output end of the repetition frequency locking device is connected with the input end of the first PI controller; a second output end of the repetition frequency locking device is connected with an input end of a second PI controller; the repetition frequency locking device is used for processing according to repetition frequency signals respectively output by the main femtosecond laser and the slave femtosecond laser, generating corresponding error control signals respectively through the first PI controller and the second PI controller, respectively generating corresponding feedback voltages according to the error control signals, and controlling the cavity length of the resonant cavity of the laser or the refractive index of a medium in the resonant cavity by using the feedback voltages, thereby controlling the laser repetition frequency of the main femtosecond laser and the slave femtosecond laser.
Preferably, the repetition frequency locking device includes a first photodetector, a second photodetector, a first bandpass filter, a second bandpass filter, a third bandpass filter, a fourth bandpass filter, a first amplifier, a second amplifier, a first power divider, a second power divider, a third power divider, a frequency divider, a signal generator, a DDS signal generator, a first mixer, a second mixer, a third mixer, a fourth mixer, a first low-pass filter, and a second low-pass filter;
the repetition frequency signal output by the main femtosecond laser is detected by the first photoelectric detector and converted into an electric signal, the electric signal output by the first photoelectric detector is filtered out of a higher harmonic signal by a first band-pass filter, the higher harmonic signal is divided into two paths by a first power divider after passing through the first amplifier, one path of the higher harmonic signal is input into the first mixer and mixed with a signal with high stability output by a signal generator to obtain a first mixed signal, the first mixed signal is input into a first low-pass filter to be filtered out of a sum frequency signal, a difference frequency signal of the first mixed signal is reserved, and the difference frequency signal is input into the first PI controller to generate a corresponding error control signal;
the repetition frequency signal output from the femtosecond laser is detected by the second photoelectric detector and converted into an electric signal, the electric signal output by the second photoelectric detector is filtered out a higher harmonic signal by the second band-pass filter, and the higher harmonic signal is input into the second mixer after passing through the second amplifier;
the other path of higher harmonic signal distributed by the first power divider is divided into two paths by a second power divider:
one path of high-order harmonic signal is input into a third mixer; and the other path of higher harmonic signal is divided into two paths through a third power divider:
one path of higher harmonic signal is input into a frequency divider for frequency division, then is input into the third mixer for frequency mixing with the higher harmonic signal of the main femtosecond laser to obtain a second mixing signal, and then is input into a fourth mixer after the difference frequency signal of the second mixing signal is filtered by the third band-pass filter;
the other path of higher harmonic signal is input into the DDS signal generator to be used as an external reference signal, a signal output by the DDS signal generator is input into the second mixer to be mixed with the higher harmonic signal of the femtosecond laser to obtain a third mixed signal, and then the third mixed signal is input into the fourth mixer after the difference frequency signal of the third mixed signal is filtered by the fourth band-pass filter to obtain a fourth mixed signal;
and filtering the sum frequency signal of the fourth mixing signal by a second low-pass filter, and inputting the reserved difference frequency signal into the second PI controller to generate a corresponding error control signal.
Preferably, the output frequency of the DDS signal generator may not be equal to the frequency divided by the output of the frequency divider.
As a preferable scheme, the optical fiber power divider further includes a fourth power divider, a fifth mixer, a third low-pass filter, and a frequency counter, where an input end of the fourth power divider is connected to an output end of the first photodetector, a first output end of the fourth power divider is connected to an input end of the first band-pass filter, and a second output end of the fourth power divider is connected to an input end of the fifth mixer; an input end of the fifth power divider is connected to an input end of the second photodetector, a first output end of the fifth power divider is connected to an input end of the second band-pass filter, and a second output end of the fifth power divider is connected to an input end of the fifth mixer; and the fifth mixer mixes the input repetition frequency signal output by the main femtosecond laser and the repetition frequency signal output by the femtosecond laser, then filters the sum frequency signal through the third low-pass filter to obtain the difference frequency signal, namely the actual repetition frequency difference signal, and then inputs the difference frequency signal into the frequency counter for real-time monitoring.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that: the invention simplifies the scheme of repeating the frequency asynchronous locking of the double-flying-second laser, abandons the design of the traditional scheme that a common reference source is adopted to connect two expensive high-output-frequency high-precision signal sources, only adopts one high-output-frequency high-precision signal source as the reference of the whole system, and utilizes a low-cost DDS signal source with lower output frequency as the generation mechanism of the asynchronous signal, thereby greatly reducing the cost of the double-flying-second laser asynchronous locking system. Meanwhile, the output signal of the master femtosecond laser is used as a reference source of the slave femtosecond laser, and a repeating frequency difference frequency signal which does not carry the frequency residue noise of the master femtosecond laser or the slave femtosecond laser is obtained through a series of frequency mixing filtering, so that a higher-precision asynchronous locking result can be obtained.
Drawings
Fig. 1 is a flowchart of an asynchronous locking method of repetition frequency of ultrashort pulse laser in embodiment 1.
Fig. 2 is a schematic structural diagram of an asynchronous locking device of an ultrashort pulse laser repetition frequency in embodiment 3.
Fig. 3 is a schematic structural diagram of an asynchronous locking device of an ultrashort pulse laser repetition frequency in embodiment 4.
Fig. 4 is a schematic structural view of a femtosecond laser frequency comb of embodiment 4.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;
it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
Example 1
The present embodiment provides an asynchronous locking method of repetition rate of an ultrashort pulse laser, and as shown in fig. 1, is a flowchart of the asynchronous locking method of repetition rate of an ultrashort pulse laser of the present embodiment.
In the asynchronous locking method of the repetition frequency of the ultrashort pulse laser proposed by the embodiment, the method is applied to an ultrashort pulse laser system comprising a master femtosecond laser 1 and a slave femtosecond laser 2. The method comprises the following steps:
step 1: the repetition frequency of the main femtosecond laser 1 is locked:
detecting a repetition frequency signal output by the main femtosecond laser 1, and extracting a higher harmonic signal from the repetition frequency signal obtained by detection;
mixing the higher harmonic signal with a signal with high stability to obtain a first mixing signal, and generating a corresponding error control signal by the first mixing signal through a PI (proportional-integral) controller;
generating corresponding feedback voltage according to the error control signal, and controlling the cavity length of the laser resonant cavity or the refractive index of the medium in the cavity by using the feedback voltage so as to control the repetition frequency of the main femtosecond laser 1;
step 2: locking the repetition frequency from the femtosecond laser 2:
taking the locked main femtosecond laser 1 as a stable signal source, and dividing the output higher harmonic signals with the repetition frequency into two paths, wherein one path of higher harmonic signals is subjected to four-frequency division, and the other path of higher harmonic signals triggers a DDS signal generator;
mixing the higher harmonic signal output by the main femtosecond laser 1 with the frequency-divided signal to obtain a second mixing signal, and mixing the higher harmonic signal output by the secondary femtosecond laser 2 with a DDS signal generated after triggering a DDS signal generator to obtain a third mixing signal;
performing frequency mixing processing on the second frequency mixing signal and the third frequency mixing signal to obtain a fourth frequency mixing signal;
and after the fourth mixing signal is converted into an error control signal by a PI controller, generating corresponding feedback voltage according to the error control signal, and controlling the cavity length of the resonant cavity of the laser or the refractive index of the medium in the resonant cavity by using the feedback voltage, so that the repetition frequency of the slave femtosecond laser 2 is controlled, and asynchronous locking is realized.
Wherein in the locking step of the repetition frequency of the slave femtosecond laser 2, the output value of the DDS digital signal generator is set according to the difference of the repetition frequencies of the desired master femtosecond laser 1 and the slave femtosecond laser 2.
Further, in the step of locking the repetition frequency of the femtosecond laser 2, the difference frequency signal of the second mixing signal is filtered, and the sum frequency signal of the second mixing signal is reserved; filtering the difference frequency signal of the third mixing signal, and reserving the sum frequency signal of the third mixing signal; and then, performing frequency mixing processing on the sum frequency signal of the second mixing signal and the sum frequency signal of the third mixing signal to obtain a fourth mixing signal, filtering the sum frequency signal of the fourth mixing signal to obtain a difference frequency signal of the fourth mixing signal, and converting the difference frequency signal of the fourth mixing signal through a PI (proportional-integral) controller to obtain an error control signal.
Further, mixing the repetition frequency signal output by the main femtosecond laser 1 and the repetition frequency signal output by the slave femtosecond laser 2, and filtering the sum frequency signal to obtain a difference frequency signal, namely an actual repetition frequency difference signal; and inputting the difference frequency signal into a frequency counter for real-time monitoring.
The embodiment aims to achieve the purpose of controlling the repetition frequency and realize asynchronous locking by means of changing the cavity length of a laser resonant cavity, or changing the length of an optical fiber in a laser, or changing the equivalent optical path length experienced by optical pulses in the laser resonant cavity and the like according to corresponding feedback voltage generated by an error control signal. In addition, in the embodiment, the output signal of the master femtosecond laser is used as a reference source of the slave femtosecond laser, and through a series of frequency mixing filtering, a repetition frequency difference frequency signal which does not carry the frequency residual noise of the master femtosecond laser or the slave femtosecond laser is obtained, so that a higher-precision asynchronous locking result can be obtained.
Example 2
In this embodiment, on the basis of the asynchronous locking method for repetition frequency of ultrashort pulse laser proposed in embodiment 1, the following implementation means for controlling repetition frequency of laser according to corresponding feedback voltage generated by an error control signal are proposed, including:
1) loading a feedback voltage on a power supply end of piezoelectric ceramics of a lens in the resonant cavities of the master femtosecond laser 1 and the slave femtosecond laser 2, and changing the cavity lengths of the resonant cavities of the master femtosecond laser 1 and the slave femtosecond laser 2;
2) loading feedback voltage on a power supply end of piezoelectric ceramics with optical fibers fixed in resonant cavities of the main femtosecond laser 1 and the slave femtosecond laser 2, and changing the lengths of the optical fibers in the resonant cavities of the main femtosecond laser 1 and the slave femtosecond laser 2;
3) and the feedback voltage is loaded on two sides of the electro-optical crystal in the resonant cavity of the master femtosecond laser 1 and the slave femtosecond laser 2, so that the refractive index of the electro-optical crystal is changed.
In a specific implementation process, the method is applied to an ultrashort pulse laser system comprising a master femtosecond laser 1 and a slave femtosecond laser 2, wherein a first piezoelectric ceramic 301 is adhered to the end face of an optical lens in the laser cavity of the master femtosecond laser 1, and a second piezoelectric ceramic 302 is adhered to the end face of an optical lens in the laser cavity of the slave femtosecond laser 2.
In the process of locking the repetition frequency of the main femtosecond laser 1, detecting the repetition frequency signal output by the main femtosecond laser 1, and extracting a higher harmonic signal from the detected repetition frequency signal; mixing the higher harmonic signal with a signal with high stability to obtain a first mixing signal, and generating a corresponding error control signal by the first mixing signal through a PI (proportional-integral) controller; the error control signal is input into the piezoelectric controller to control the loading voltage of the first piezoelectric ceramic 301 adhered to the optical lens, so that the first piezoelectric ceramic 301 is extended or shortened and drives the optical lens to move, thereby changing the length of the resonant cavity of the main femtosecond laser 1 and realizing the control of the repetition frequency of the main femtosecond laser.
In the process of locking the repetition frequency of the slave femtosecond laser 2, the locked master femtosecond laser 1 is used as a stable signal source, and the output higher harmonic signals of the repetition frequency are divided into two paths, wherein one path of higher harmonic signals is subjected to frequency division, and the other path of higher harmonic signals triggers the DDS signal generator; mixing the higher harmonic signal output by the main femtosecond laser 1 with the frequency-divided signal to obtain a second mixing signal, and mixing the higher harmonic signal output by the secondary femtosecond laser 2 with a DDS signal generated after triggering a DDS signal generator to obtain a third mixing signal; performing frequency mixing processing on the second frequency mixing signal and the third frequency mixing signal to obtain a fourth frequency mixing signal; the fourth mixing signal is converted into an error control signal by the PI controller and then input into the piezoelectric controller, and the loading voltage of the second piezoelectric ceramic 302 attached to the optical lens is controlled, so that the second piezoelectric ceramic 302 is extended or shortened and the optical lens is driven to move, thereby changing the length of the resonant cavity of the slave femtosecond laser 2 and realizing the control of the repetition frequency of the slave femtosecond laser 2.
In another specific implementation process, the method is applied to an ultrashort pulse laser system comprising a master femtosecond laser 1 and a slave femtosecond laser 2, wherein the first piezoelectric ceramic 301 and the second piezoelectric ceramic 302 are respectively fixed on optical fibers in resonant cavities of the master femtosecond laser 1 and the slave femtosecond laser 2.
In the process of locking the repetition frequency of the master femtosecond laser 1/the slave femtosecond laser 2, the generated error control signal is input into the piezoelectric controller, the loading voltage of the first piezoelectric ceramic 301/the second piezoelectric ceramic 302 fixed on the optical fiber in the resonant cavity of the master femtosecond laser 1/the slave femtosecond laser 2 is controlled, the first piezoelectric ceramic 301/the second piezoelectric ceramic 302 is extended or shortened, the length of the optical fiber in the master femtosecond laser 1/the slave femtosecond laser 2 is changed, the length of the resonant cavity of the master femtosecond laser 1/the slave femtosecond laser 2 is changed, and the control of the repetition frequency of the master femtosecond laser 1/the slave femtosecond laser 2 is realized.
In another specific implementation process, the method is applied to an ultrashort pulse laser system comprising a master femtosecond laser 1 and a slave femtosecond laser 2, wherein optical fibers are arranged in resonant cavities of the master femtosecond laser 1 and the slave femtosecond laser 2. In the process of locking the repetition frequency of the master femtosecond laser 1/the slave femtosecond laser 2, the generated error control signal is input into the electro-optical modulator, and the adjustment of the refractive index of the optical fiber is realized by changing the control voltage, so that the control of the repetition frequency of the master femtosecond laser 1/the slave femtosecond laser 2 is realized.
Example 3
The present embodiment provides an asynchronous locking device for repetition frequency of ultrashort pulse laser, which is applied to the asynchronous locking method for repetition frequency of ultrashort pulse laser provided in embodiment 1. Fig. 2 is a schematic structural diagram of an asynchronous locking device for repetition frequency of ultrashort pulse laser in this embodiment.
The asynchronous locking device for the repetition frequency of the ultrashort pulse laser provided by the embodiment comprises a master femtosecond laser 1, a slave femtosecond laser 2, a first PI controller 4, a second PI controller 5 and a repetition frequency locking device, wherein the repetition frequency locking device is used for processing according to repetition frequency signals output by the master femtosecond laser 1 and the slave femtosecond laser 2 respectively, generating corresponding error control signals through the first PI controller 4 and the second PI controller 5 respectively, generating corresponding feedback voltages according to the error control signals respectively, and controlling the cavity length of a laser resonant cavity or the refractive index of a medium in the cavity by using the feedback voltages, so as to control the lengths of laser cavities in the master femtosecond laser 1 and the slave femtosecond laser 2 and control the laser repetition frequency.
The repetition frequency locking device in this embodiment includes a first photodetector 8, a second photodetector 15, a first band pass filter 9, a second band pass filter 16, a third band pass filter 23, a fourth band pass filter 26, a first amplifier 10, a second amplifier 17, a first power divider 11, a second power divider 19, a third power divider 21, a frequency divider 22, a signal generator 13, a DDS signal generator 25, a first mixer 12, a second mixer 18, a third mixer 20, a fourth mixer 24, a first low pass filter 14, and a second low pass filter 27, where:
the repetition frequency signal output by the main femtosecond laser 1 is detected and converted into an electric signal by the first photodetector 8, the electric signal output by the first photodetector 8 passes through the first band-pass filter 9 to filter out its higher harmonic signal, the higher harmonic signal passes through the first amplifier 10 and is divided into two paths by the first power divider 11, wherein one path of the higher harmonic signal is input into the first mixer 12 to be mixed with the signal with high stability output by the signal generator 13 to obtain a first mixed signal, the first mixed signal is input into the first low-pass filter 14 to filter out its sum frequency signal, the difference frequency signal of the first mixed signal is retained and input into the first PI controller 4 to generate a corresponding error control signal, and the error control signal is used for feedback control of the cavity length or the refractive index of the medium in the cavity of the laser resonant cavity, thereby controlling the repetition frequency of the main femtosecond laser 1;
the repetition frequency signal output from the femtosecond laser 2 is detected by the second photodetector 15 and converted into an electrical signal, the electrical signal output from the second photodetector 15 is filtered out its higher harmonic signal by the second band-pass filter 16, and the higher harmonic signal is input into the second mixer 18 after passing through the second amplifier 17;
the other path of higher harmonic signal distributed by the first power divider 11 is divided into two paths by the second power divider 19:
one path of the high-order harmonic signal is input into the third mixer 20; the other path of higher harmonic signal is divided into two paths by the third power divider 21:
one path of high-order harmonic signal is input into a frequency divider 22 for frequency division, then is input into the third mixer 20 for frequency mixing with the high-order harmonic signal of the main femtosecond laser 1 to obtain a second mixing signal, and then is input into a fourth mixer 24 after a difference frequency signal of the second mixing signal is filtered by a third band-pass filter 23;
the other path of higher harmonic signal is input into the DDS signal generator 25 as an external reference signal, the DDS signal output by the DDS signal generator 25 is input into the second mixer 18 to be mixed with the higher harmonic signal from the femtosecond laser 2 to obtain a third mixed signal, and then the third mixed signal is input into the fourth mixer 24 after the difference frequency signal is filtered by the fourth band-pass filter 26 to obtain a fourth mixed signal;
and filtering the sum frequency signal of the fourth mixing signal by a second low-pass filter 27, inputting the reserved difference frequency signal into the second PI controller 5 to generate a corresponding error control signal, and controlling the cavity length of the laser resonant cavity or the refractive index of the medium in the cavity by using the error control signal in a feedback mode, thereby controlling the repetition frequency of the slave femtosecond laser 2.
Further, the output frequency of the DDS signal generator in this embodiment is not equal to the frequency division frequency output by the frequency divider, so that the repetition frequency of the master femtosecond laser 1 is not equal to the repetition frequency of the slave femtosecond laser 2, and asynchronous locking is achieved.
Further, the asynchronous locking device for repetition frequency of ultrashort pulse laser in this embodiment further includes a fourth power divider 28, a fifth power divider 29, a fifth mixer 30, a third low-pass filter 31, and a frequency counter 32, where an input end of the fourth power divider 28 is connected to the output end of the first photodetector 8, and an output end of the fourth power divider 28 is connected to an input end of the fifth mixer 30; an input end of the fifth power divider 29 is connected to an input end of the second photodetector 15, and an output end of the fifth power divider 29 is connected to an input end of a fifth mixer 30; the fifth mixer 30 mixes the input repetition frequency signal output by the main femtosecond laser 1 and the repetition frequency signal output by the femtosecond laser 2, then filters the sum frequency signal through the third low-pass filter 31 to obtain the difference frequency signal, i.e. the actual repetition frequency difference signal, and then inputs the difference frequency signal into the frequency counter 32 for real-time monitoring.
To illustrate the principle of the asynchronous locking device for the repetition frequency of the ultrashort pulse laser in this embodiment, in a specific implementation process, a desired repetition frequency difference Δ f is setrep.setIs 0.25 KHz.
Step 1: the main femtosecond laser 1 is turned on, and the first photoelectric detector 8 is used to receive the repetition frequency signal f output by the main femtosecond laser 1rep1About 250MHz and converted into an electrical signal.
Step 2: the signal output by the first photodetector 8 is introduced into a fourth power divider 28, the fourth power divider 28 divides the signal into two paths, one path is used for monitoring the change of the repetition frequency difference, and the other path is introduced into a first band-pass filter 9 to filter out a 4 th harmonic signal 4frep1≈1GHz。
And step 3: 4 th harmonic signal 4f of the repetition frequency signal of the main femtosecond laser 1 output by the first band-pass filter 9rep1The signal is led into a first power divider 11 after passing through a first amplifier 10, the first power divider 11 divides the signal into two paths, one path is led into a second power divider 19, the other path is led into a first mixer 12 and is mixed with a signal f sent by a signal generator 13signalMixing at 1GHz to obtain a first mixed signal including sum frequency fsignal-4frep1And the difference frequency fsignal+4frep1。
And 4, step 4: the first mixed signal output from the first mixer 12 is introduced into a first low pass filter 14, which filters out the sum frequency of the two signals and preserves the difference frequency signalfsignal-4frep1Then, the difference frequency signal is output to the first PI controller 4 to obtain an error control signal, the error control signal is input to the first piezoelectric controller 6, the first piezoelectric controller 6 outputs an electric signal to control the first piezoelectric ceramic 301 arranged in the laser cavity of the main femtosecond laser 1 in a feedback manner, and the expansion and contraction of the first piezoelectric ceramic 301 can change the length of the laser cavity of the main femtosecond laser 1, so that the repetition frequency is changed.
After the above steps are finished, the repetition frequency of the main femtosecond laser 1 is locked, and the 4-th harmonic value of the repetition frequency and the signal frequency f sent by the signal generator 13 at the momentsignalIn agreement, i.e. 4frep1＝fsignal＝1GHz。
And 5: one of the two signals split by the second power divider 19 is input into the third power divider 21, the two signals are split, and one of the signals split by the third power divider 21 is input into the frequency divider 22 for frequency division to generate 1/4 × 4f signalsrep1＝frep1Another path of the output signal divided by the third power divider 21 is used as an external reference signal of the DDS signal generator 25 at 250MHz, so that the DDS signal generator 25 generates the DDS signal fDDS＝249.999MHz(frep1And fDDSThe signals are slightly different, and the difference frequency of the two signals is 4 times of the difference value of the preset repetition frequencies of the double-femtosecond laser, namely 4 delta frep.set＝|frep1-fDDS1KHz, the magnitude of the difference can be determined by adjusting the output of the DDS signal generator 25); the other signal divided by the second power divider 19 is introduced into the third mixer 20 to be divided by the frequency divider 22 by fourrep1Mixing to obtain a second mixed signal including sum frequency 4frep1+frep1And a difference frequency of 4frep1-frep1。
Step 6: the slave femtosecond laser 2 is turned on, and the repetition frequency signal f from the femtosecond laser 2 is received by the second photodetector 15rep2And converted into an electrical signal. The signal output by the second photodetector 15 is introduced into the fifth power divider 29, the fifth power divider 29 divides the signal into two paths, one path is used for monitoring the change of the repetition frequency difference, and the other path is introducedThe second band-pass filter 16 filters out its 4 th harmonic signal 4frep2。
And 7: 4 th harmonic signal 4f of repetition frequency from the femtosecond laser 2 output in the second band-pass filter 16rep2After passing through a second amplifier 17, is introduced into a second mixer 18 to mix with the DDS signal fDDSMixing to obtain a third mixed signal including sum frequency 4frep2+fDDSAnd a difference frequency of 4frep2-fDDS(ii) a The third mixed signal output by the second mixer 18 is introduced into a fourth band-pass filter 26, so as to filter the difference frequency of the two signals and retain the sum frequency signal thereof; at the same time, the second mixing signal outputted from the third mixer 20 in step 5 is introduced into the third band-pass filter 23 to filter the difference frequency of the two signals and retain the sum frequency signal, i.e. 4frep1+frep1(ii) a Then, the two sum frequency signals output by the third band-pass filter 23 and the fourth band-pass filter 26 are respectively introduced into the fourth mixer 24 for mixing to obtain a fourth mixed signal, which includes the sum frequency 4frep1+frep1+4frep2+fDDSAnd a difference frequency of 4frep1+frep1-4frep2-fDDSThen, the sum frequency signal in the fourth mixing signal is filtered by the second low pass filter 27, and the value of the reserved difference frequency signal is the actual repetition frequency difference Δ f of the 4-times double-femtosecond laserrep.actualDifference of frequency Δ f from 4 times the preset repetition frequencyrep.setI.e. 4 deltafrep.set-4Δfrep.actual。
And 8: outputting the difference frequency signal outputted from the second low pass filter 27 in step 7 to the second PI controller 5 to obtain an error control signal, inputting the error control signal to the second piezoelectric controller 7, outputting an electric signal by the second piezoelectric controller 7 to feedback control the second piezoelectric ceramic 302 arranged in the laser cavity of the slave femtosecond laser 2, wherein the expansion and contraction of the second piezoelectric ceramic 302 changes the length of the laser cavity of the slave femtosecond laser 2, thereby changing the repetition frequency.
After this step is finished, the repetition frequency of the slave femtosecond laser 2 is locked, and the value of the repetition frequency is frep2＝frep1+Δfrep.actual. At this time,. DELTA.frep.actual＝Δfrep.setThat is, the actual repetition frequency difference of the double-femtosecond laser frequency comb is consistent with the preset repetition frequency difference value.
And step 9: introducing the signals for repeating frequency difference monitoring divided by the fourth power divider 28 and the fifth power divider 29 in the above steps 2 and 6 into a fifth mixer 30 for mixing, including the sum frequency frep1+frep2And the difference frequency frep1-frep2Then, the sum frequency signal is filtered by a third low pass filter 31 to obtain a difference frequency signal frep1-frep2＝Δfrep.actual. The difference frequency signal is the actual repetition frequency difference, and the difference frequency signal is input into the frequency counter 32 for real-time monitoring.
In another specific implementation, to illustrate the error transmission principle of the asynchronous locking device for the repetition frequency of the ultrashort pulse laser of this embodiment, let the repetition frequency of the main femtosecond laser 1 be f1250MHz, repetition frequency f from the femtosecond laser 22250MHz (close to 250MHz), target weight difference Δ f (assuming 1kHz), then target f2＝f1- Δ f. Let the output frequency of the signal generator 13 be fs。
Suppose that the first band-pass filter selects the repetition frequency f of the main femtosecond laser1Fourth harmonic 4f of1Then the mixing result at the first mixer 12 is 4f1-fs. A feedback control loop formed by the low-pass filter 14, the first PI controller 4, the first piezoelectric controller 6 and the first piezoelectric ceramic 301 locks the first mixing result to be zero and has residual locking noise delta f of the feedback control loop1(typically of the order of mHz or less, very small), i.e.
4f1-fs＝δf1 (1)
Namely: f. of1＝fs/4-δf1/4 (2)
So if it is locked f1Fourth harmonic of (f), thensIs required to be set to 4f1Target value of (1GHz in this example). The second power divider 19 outputs after lockingThe signal frequencies of (a) are:
4f1＝fs+δf1 (3)
the frequency divider 22 is set to divide by four (or other division multiples), so that the frequency of the signal output thereafter is:
f1＝fs/4+δf1/4 (4)
therefore, the frequency of the signal output after the third mixer 20 is (taking the sum frequency component after mixing):
f1+4f1＝fs/4+δf1/4+fs+δf1＝5fs/4+5δf1/4 (5)
if the difference between the repetition frequencies of the asynchronous signals is Δ f, the setting frequency of the DDS signal generator 25 should be:
fDDS device＝fs/4+4Δf (6)
Since 4 Δ f is relative to fs/4 is small, so the frequency of the DDS signal generator 25 output is very close to fs/4, i.e. f1. Since the DDS signal generator 25 adopts 4f1As an external reference source, but with an output at f1Nearby, and therefore the actual frequency of its output with δ f1Noise of/4 (assuming that the noise of the DDS itself is much smaller than the locking noise), i.e.:
fDDS fruit＝fs/4+4Δf+δf1/4 (7)
The sum frequency output signal after the second mixer 18 is therefore:
4f2+fDDS fruit＝4f2+fs/4+4Δf+δf1/4 (8)
When the sum frequency signals of the above two mixers, i.e., the left-hand frequencies of equation (5) and equation (8), are inputted to the fourth mixer 24, and their difference frequencies are filtered, it is obtained
f1+4f1-4f2-fDDS fruit＝5fs/4+5δf1/4-4f2-4Δf-δf1/4
＝fs+δf1-4f2-4Δf (9)
Suppose the lock-in noise, via a feedback control loop from the laser, is δ f2Then, by equation (9), it can be obtained:
fs+δf1-4f2-4Δf＝δf2 (10)
namely:
f2＝fs/4-Δf+δf1/4-δf2/4 (11)
therefore, when the repetition frequencies of the master and slave lasers are input to the fifth mixer 30 and the difference frequency signal is obtained by the third low-pass filter 31, the frequency of the difference frequency signal is as follows according to equations (4) and (11):
f1-f2＝fs/4+δf1/4-fs/4+Δf-δf1/4+δf2/4
＝Δf+δf2/4 (12)
equation (12) represents the finally obtained difference between the two laser repetition frequencies. It can be seen that the lock-in noise δ f of the main femtosecond laser 1 is eliminated by common mode rejection1The lock noise component from the femtosecond laser 2 is contained only, and since the lock noise component is the fourth harmonic of the repetition frequency of the lock laser, the noise contained in the difference between the repetition frequencies is one fourth of the lock noise, that is, δ f2/4。
If the master laser and the slave laser adopt two independent sets of repetition frequency locking systems, the noises of the two sets of systems are independent of each other, and delta f in the situation can be assumed1＝δf2According to the error transfer formula, the noise of the repetition frequency difference between the two in this case is:
rather than the resulting δ f of the present invention2/4。
Therefore, compared with the traditional scheme of independently locking two femtosecond lasers, the invention saves an expensive high-frequency (1GHz) high-precision signal generator which is originally required by the repeated frequency locking of the lasers, and only needs to be usedA much lower frequency DDS (250MHz) is used, and the locking noise of the asynchronous repeat frequency difference is reduced
And (4) doubling.
Example 4
In this embodiment, on the basis of the asynchronous locking device for repetition frequency of ultrashort pulse laser proposed in embodiment 3, a specific implementation process is proposed in which the device is applied to control the position of a lens in a resonant cavity by using piezoelectric ceramic to realize laser repetition frequency control.
The asynchronous locking device for the repetition frequency of the ultrashort pulse laser in this embodiment further includes a first piezoelectric ceramic 301 and a second piezoelectric ceramic 302, which are respectively adhered to end faces of an optical lens in the laser cavity of the master femtosecond laser 1 and the slave femtosecond laser 2. In addition, the device also comprises a first piezoelectric controller 6 and a second piezoelectric controller 7, wherein a first output end of the repetition frequency locking device is connected with a power supply end of the first piezoelectric ceramic 301 through the first PI controller 4 and the first piezoelectric controller 6 in sequence; and a second output end of the repetition frequency locking device is connected with a power supply end of the second piezoelectric ceramic 302 sequentially through a second PI controller 5 and a second piezoelectric controller 7. Fig. 3 is a schematic structural diagram of an asynchronous locking device for repetition frequency of ultrashort pulse laser in this embodiment.
The repetition frequency locking device is used for processing according to repetition frequency signals respectively output by the master femtosecond laser 1 and the slave femtosecond laser 2, generating corresponding error control signals respectively through the first PI controller 4 and the second PI controller 5, respectively inputting the error control signals into the first piezoelectric controller 6 and the second piezoelectric controller 7, respectively controlling loading voltages at two ends of the first piezoelectric ceramic 301 and the second piezoelectric ceramic 302, respectively changing the lengths of laser cavities of the master femtosecond laser 1 and the slave femtosecond laser 2, and controlling laser repetition frequency.
The master femtosecond laser 1 and the slave femtosecond laser 2 in this embodiment have the same structure, and as shown in fig. 4, are a schematic structural view of the femtosecond laser frequency comb in this embodiment, taking the primary femtosecond laser 1 as an example, the laser comprises a pumping source 101, a wavelength division multiplexer 102, a first quarter wave plate 103, a second quarter wave plate 104, a third quarter wave plate 105, a half wave plate 106, a polarization beam splitter prism 107, a reflector 108, an isolator 109, a collimator 110 and an erbium-doped fiber 111, wherein the pump source 101, the wavelength division multiplexer 102, the first quarter wave plate 103, the half wave plate 106, the polarization beam splitter prism 107, the second quarter wave plate 104 and the reflector 108 are connected in turn via an optical path, the polarization beam splitter prism 107 splits the light path and then is connected with the isolator 109, the third quarter-wave plate 105, the collimator 110 and one end of the erbium-doped fiber 111 in sequence; the other end of the erbium-doped fiber 111 is connected with the input end of the wavelength division multiplexer 102; the reflector 108 is bonded to the first piezoelectric ceramic 301. The pump source 101 in this embodiment is a laser with a wavelength of 980 nm.
The components in the slave femtosecond laser 2 and their connection relationship in this embodiment are the same as those in the master femtosecond laser 1, and the mirror 108 is bonded to the second piezoelectric ceramic 302.
In the specific implementation process, the repetition frequency of the main femtosecond laser 1 is firstly locked:
detecting a repetition frequency signal output by the main femtosecond laser 1, and extracting a higher harmonic signal from the repetition frequency signal obtained by detection; mixing the higher harmonic signal with a signal with high stability to obtain a first mixing signal, and generating a corresponding error control signal by the first mixing signal through a PI (proportional-integral) controller; inputting an error control signal into the piezoelectric controller, controlling the loading voltage of the first piezoelectric ceramic 301, and changing the length of the laser cavity of the main femtosecond laser 1, thereby controlling the repetition frequency of the main femtosecond laser 1;
locking the repetition frequency from the femtosecond laser 2: taking the locked main femtosecond laser 1 as a stable source, and dividing the output higher harmonic signals with the repetition frequency into two paths, wherein one path of higher harmonic signals is subjected to frequency division, and the other path of higher harmonic signals triggers a DDS signal generator; mixing the higher harmonic signal output by the main femtosecond laser 1 with the frequency-divided signal to obtain a second mixing signal, and mixing the higher harmonic signal output by the secondary femtosecond laser 2 with a signal generated after triggering the DDS signal generator to obtain a third mixing signal; performing frequency mixing processing on the second frequency mixing signal and the third frequency mixing signal to obtain a fourth frequency mixing signal; and converting the fourth mixing signal into an error control signal through a PI controller, inputting the error control signal into a piezoelectric controller, controlling the loading voltage of the second piezoelectric ceramic 302, and changing the length of the laser cavity of the slave femtosecond laser 2 so as to control the repetition frequency of the slave femtosecond laser 2.
Similarly, the asynchronous locking device for the repetition rate of the ultrashort pulse laser proposed in embodiment 3 can also be applied to other scenarios that can realize the adjustment of the repetition rate of the laser by changing the control voltage. For example, a feedback voltage is loaded on a power supply end of piezoelectric ceramics with optical fibers fixed in resonant cavities of the master femtosecond laser 1 and the slave femtosecond laser 2, and the lengths of the optical fibers in the resonant cavities of the master femtosecond laser 1 and the slave femtosecond laser 2 are changed, so that the repetition frequencies of the master femtosecond laser 1 and the slave femtosecond laser 2 are changed; or, the feedback voltage is loaded on two sides of the electro-optical crystal in the resonant cavity of the master femtosecond laser 1 and the slave femtosecond laser 2, and the refractive index of the electro-optical crystal is changed, so that the repetition frequency of the master femtosecond laser 1 and the slave femtosecond laser 2 is changed.
The same or similar reference numerals correspond to the same or similar parts;
the terms describing positional relationships in the drawings are for illustrative purposes only and are not to be construed as limiting the patent;
it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (9)
1. An asynchronous locking method of the repetition frequency of ultrashort pulse laser is applied to an optical system consisting of a master femtosecond laser (1) and a slave femtosecond laser (2), and is characterized by comprising the following steps:
locking the repetition frequency of the main femtosecond laser (1): detecting a repetition frequency signal output by the main femtosecond laser (1), and extracting a higher harmonic signal from the repetition frequency signal obtained by detection; mixing the higher harmonic signal with a signal with high stability to obtain a first mixing signal, and generating a corresponding error control signal by the first mixing signal through a PI (proportional-integral) controller; generating corresponding feedback voltage according to the error control signal, and controlling the cavity length of the laser resonant cavity or the refractive index of the medium in the cavity by using the feedback voltage so as to control the repetition frequency of the main femtosecond laser (1);
locking the repetition frequency from the femtosecond laser (2): the locked main femtosecond laser (1) is used as a stable source, and the output higher harmonic signals with the repetition frequency are divided into two paths, wherein one path of higher harmonic signals is subjected to frequency division, and the other path of higher harmonic signals triggers a DDS signal generator; mixing the higher harmonic signal output by the main femtosecond laser (1) with a signal subjected to four-frequency division to obtain a second mixing signal, and mixing the higher harmonic signal output by the secondary femtosecond laser (2) with a signal generated after triggering the DDS signal generator to obtain a third mixing signal; performing frequency mixing processing on the second frequency mixing signal and the third frequency mixing signal to obtain a fourth frequency mixing signal; and converting the fourth mixing signal into an error control signal through a PI controller, generating corresponding feedback voltage according to the error control signal, and controlling the cavity length of the laser resonant cavity or the refractive index of the medium in the cavity by using the feedback voltage so as to control the repetition frequency of the slave femtosecond laser (2) and realize asynchronous locking.
2. The asynchronous locking method of repetition rate of ultra short pulse laser as claimed in claim 1, wherein in the locking step of the repetition rate of the slave femtosecond laser (2), the output value of said digital signal generator is set according to the difference of the desired repetition rate of the master femtosecond laser (1) and the slave femtosecond laser (2).
3. The asynchronous locking method of repetition frequency of ultra short pulse laser as claimed in claim 2, wherein in the step of locking the repetition frequency from the femtosecond laser (2), the second mixing signal is filtered to remove its difference frequency signal and keep its sum frequency signal; filtering the difference frequency signal of the third mixing signal, and reserving the sum frequency signal of the third mixing signal; and then, performing frequency mixing processing on the sum frequency signal of the second mixing signal and the sum frequency signal of the third mixing signal to obtain a fourth mixing signal, filtering the sum frequency signal of the fourth mixing signal to obtain a difference frequency signal of the fourth mixing signal, and converting the difference frequency signal of the fourth mixing signal through a PI (proportional-integral) controller to obtain an error control signal.
4. The asynchronous locking method of repetition rate of ultrashort pulse laser as claimed in claim 1, further comprising the steps of: mixing the repetition frequency signal output by the main femtosecond laser (1) and the repetition frequency signal output by the slave femtosecond laser (2), and filtering a sum frequency signal to obtain a difference frequency signal, namely an actual repetition frequency difference signal; and inputting the difference frequency signal into a frequency counter for real-time monitoring.
5. The asynchronous locking method of repetition frequency of ultra-short pulse laser as claimed in any claim 1 to 4, wherein a corresponding feedback voltage is generated according to the error control signal, and the means for controlling the repetition frequency of the master femtosecond laser (1) and the slave femtosecond laser (2) include but not limited to:
1) loading feedback voltage on a power supply end of piezoelectric ceramics of one lens in the resonant cavities of the master femtosecond laser (1) and the slave femtosecond laser (2) to change the cavity lengths of the resonant cavities of the master femtosecond laser (1) and the slave femtosecond laser (2);
2) loading feedback voltage on power supply ends of piezoelectric ceramics with optical fibers fixed in resonant cavities of the main femtosecond laser (1) and the slave femtosecond laser (2), and changing the lengths of the optical fibers in the resonant cavities of the main femtosecond laser (1) and the slave femtosecond laser (2);
3) and a feedback voltage is loaded on two sides of the electro-optical crystal in the resonant cavity of the master femtosecond laser (1) and the slave femtosecond laser (2), so that the refractive index of the electro-optical crystal is changed.
6. An asynchronous locking device of repetition frequency of ultrashort pulse laser, applied to the asynchronous locking method of repetition frequency of ultrashort pulse laser as claimed in any one of claims 1 to 5, comprising a master femtosecond laser (1), a slave femtosecond laser (2), and further comprising a first PI controller (4), a second PI controller (5) and a repetition frequency locking device, wherein:
a first output end of the repetition frequency locking device is connected with an input end of the first PI controller (4); the second output end of the repetition frequency locking device is connected with the input end of the second PI controller (5);
the repetition frequency locking device is used for processing according to repetition frequency signals respectively output by the main femtosecond laser (1) and the slave femtosecond laser (2), generating corresponding error control signals respectively through the first PI controller (4) and the second PI controller (5), generating corresponding feedback voltages respectively according to the error control signals, and controlling the cavity length of a laser resonant cavity or the refractive index of a medium in the cavity by using the feedback voltages so as to change the laser repetition frequency of the main femtosecond laser (1) and the slave femtosecond laser (2).
7. The asynchronous locking device of repetition frequency of ultrashort pulse laser as claimed in claim 6, wherein the repetition frequency locking device comprises a first photodetector (8), a second photodetector (15), a first band-pass filter (9), a second band-pass filter (16), a third band-pass filter (23), a fourth band-pass filter (26), a first amplifier (10), a second amplifier (17), a first power divider (11), a second power divider (19), a third power divider (21), a frequency divider (22), a signal generator (13), a DDS signal generator (25), a first mixer (12), a second mixer (18), a third mixer (20), a fourth mixer (24), a first low-pass filter (14), a second low-pass filter (27);
the repetition frequency signal output by the main femtosecond laser (1) is detected by the first photoelectric detector (8) and converted into an electric signal, the electric signal output by the first photoelectric detector (8) is filtered out a higher harmonic signal by a first band-pass filter (9), the higher harmonic signal is divided into two paths by a first power divider (11) after passing through the first amplifier (10), one path of the higher harmonic signal is input into the first mixer (12) and is mixed with a signal with high stability output by a signal generator (13) to obtain a first mixed signal, the first mixed signal is input into a first low-pass filter (14) to be filtered out a sum frequency signal, a difference frequency signal of the first mixed signal is reserved, and the first mixed signal is input into the first PI controller (4) to generate a corresponding error control signal;
the repetition frequency signal output from the femtosecond laser (2) is detected by the second photoelectric detector (15) and converted into an electric signal, the electric signal output by the second photoelectric detector (15) is filtered out a higher harmonic signal by a second band-pass filter (16), and the higher harmonic signal is input into a second mixer (18) after passing through the second amplifier (17);
the other path of higher harmonic signal distributed by the first power divider (11) is divided into two paths by a second power divider (19):
one path of high-order harmonic signal is input into a third mixer (20); the other path of higher harmonic signal is divided into two paths through a third power divider (21):
one path of high-order harmonic signal is input into a frequency divider (22) for frequency division, then is input into a third frequency mixer (20) for frequency mixing with the high-order harmonic signal of the main femtosecond laser (1) to obtain a second frequency mixing signal, and then is input into a fourth frequency mixer (24) after a difference frequency signal of the second frequency mixing signal is filtered by a third band-pass filter (23);
the other path of higher harmonic signal is input into a DDS signal generator (25) as an external reference signal, the DDS signal output by the DDS signal generator (25) is input into the second mixer (18) to be mixed with the higher harmonic signal from the femtosecond laser (2) to obtain a third mixed signal, and then the third mixed signal is input into a fourth mixer (24) after a difference frequency signal of the third mixed signal is filtered by a fourth band-pass filter (26) to obtain a fourth mixed signal;
and filtering the sum frequency signal of the fourth mixing signal by a second low-pass filter (27), and inputting the reserved difference frequency signal into the second PI controller (5) to generate a corresponding error control signal.
8. The asynchronous locking device of repetition frequency of ultrashort pulse laser as claimed in claim 7, wherein the output frequency of the DDS signal generator (25) is not equal to the frequency-divided frequency output by the frequency divider (22).
9. The asynchronous locking device for repetition frequency of ultrashort pulse laser according to claim 7, further comprising a fourth power divider (28), a fifth power divider (29), a fifth mixer (30), a third low-pass filter (31) and a frequency counter (32), wherein an input of the fourth power divider (28) is connected to the output of the first photodetector (8), a first output of the fourth power divider (28) is connected to the input of the first band-pass filter (9), and a second output of the fourth power divider (28) is connected to the input of the fifth mixer (30); an input end of the fifth power divider (29) is connected to an input end of the second photodetector (15), a first output end of the fifth power divider (29) is connected to an input end of the second band-pass filter (16), and a second output end of the fifth power divider (29) is connected to an input end of the fifth mixer (30);
the fifth mixer (30) mixes the input repetition frequency signal output by the main femtosecond laser (1) and the repetition frequency signal output by the femtosecond laser (2), then filters the sum frequency signal through the third low-pass filter (31) to obtain the difference frequency signal, namely the actual repetition frequency difference signal, and then inputs the difference frequency signal into the frequency counter (32) for real-time monitoring.
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| US20100225897A1 (en) * | 2009-03-06 | 2010-09-09 | Imra America, Inc. | Optical scanning and imaging systems based on dual pulsed laser systems |
| US20110085573A1 (en) * | 2009-09-14 | 2011-04-14 | Albrecht Bartels | Method for coupling two pulsed lasers having an adjustable difference of the pulse frequencies, which is not equal to zero |
| CN103148940A (en) * | 2013-02-28 | 2013-06-12 | 北京航空航天大学 | Light asynchronous sampling signal measurement method and system |
| US20150357788A1 (en) * | 2014-06-05 | 2015-12-10 | Toptica Photonics Ag | Optical Scanning |
| CN105762631A (en) * | 2016-04-29 | 2016-07-13 | 武汉虹拓新技术有限责任公司 | Method and device for improving repetition frequency of laser |
| CN109346913A (en) * | 2018-09-06 | 2019-02-15 | 天津大学 | A kind of round trip flight second laser optics frequency comb locking device based on fibre delay line |
| CN111082300A (en) * | 2019-12-31 | 2020-04-28 | 华中科技大学 | Cavity length locking method and system of double-resonance optical parametric oscillator |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN115412169A (en) * | 2022-11-01 | 2022-11-29 | 济南量子技术研究院 | System and method for simultaneously transmitting optical frequency, microwave and time signals of optical fiber |
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