CN116667109A - Femtosecond laser and method for generating GHz Burst high-energy laser pulse cluster - Google Patents
Femtosecond laser and method for generating GHz Burst high-energy laser pulse cluster 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
- H01S3/005—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
- H01S3/0064—Anti-reflection devices, e.g. optical isolaters
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- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—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
- H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—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
- H01S3/0071—Beam steering, e.g. whereby a mirror outside the cavity is present to change the beam direction
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—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
- H01S3/0085—Modulating the output, i.e. the laser beam is modulated outside the laser cavity
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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
- H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
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- H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
- H01S3/10023—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by functional association of additional optical elements, e.g. filters, gratings, reflectors
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Abstract
The invention relates to a femtosecond laser and a method for generating GHz Burst high-energy laser pulse clusters; the method solves the problems of complex structure and high technical difficulty of the existing method for generating the laser pulse clusters; the laser comprises a seed light source, a half wave plate arranged on an emergent light path of the seed light source, a first polarization beam splitter, an electro-optical pulse selecting system and a second polarization beam splitter, wherein the first laser isolating system and the pulse widening system are arranged on a transmission light path of the second polarization beam splitter; the third polarization beam splitter is arranged on a reflection light path of the second polarization beam splitter, the second laser isolation system is arranged on a reflection light path of the third polarization beam splitter, the regeneration amplifier is arranged on an emergent light path of the second laser isolation system, and the first 1/4 wave plate and the pulse compression system are arranged on a transmission light path of the third polarization beam splitter; the invention also proposes a method of generating clusters of laser pulses.
Description
Technical Field
The invention relates to a femtosecond laser and an application method, in particular to a femtosecond laser and a method for generating GHz Burst high-energy laser pulse clusters.
Background
The main characteristic of the femtosecond laser processing is that the pulse width of the femtosecond magnitude gives the extremely short interaction time of light and material, and the ultra-high precision laser micro-nano processing can be realized by inhibiting the heat affected zone effectively. The femtosecond laser technology is widely applied to the fields of industrial micromachining, intense field physics, biomedicine and the like.
In order to further expand the application prospect of the femtosecond laser in the commercial and industrial fields, the technical difficulty of continuously breaking through the femtosecond laser processing is the main attack direction of the current scientific researchers; the technical bottlenecks that limit the development of femtosecond laser processing mainly include: plasma shielding and nonlinear effects, etc.; because the femtosecond laser has extremely high peak power, plasma can be formed on the surface of a material in the processing process, the energy attenuation of subsequent light pulses can be caused, and the processing efficiency is reduced; similarly, extremely high peak laser powers also exacerbate nonlinear effects of the material, thereby reducing the ablation rate of the femtosecond laser.
In the long development history of laser processing, many scholars develop their own researches on the mode of outputting pulses by a laser and obtain a lot of interesting results, and a pulse cluster mode (Burst mode) is one of the most novel and practical modes, essentially, the Burst mode characterizes the time domain transmission situation of an ultrafast laser pulse cluster, in this mode, the pulses are not output at fixed time intervals any more, but are divided into pulse clusters consisting of a plurality of pulses by a controllable optical switching device, and further, the expected indexes of pulse serial output are realized; the repetition frequency of the Burst parameter corresponding to the pulse cluster is adjustable within the range of Hz-KHz, two or more laser pulses are distributed in the cluster at equal intervals, and the pulse interval in the cluster is nanosecond or shorter. Compared with a high-repetition-frequency output mode of transmitting laser pulses at fixed time intervals, the Burst mode can reduce the number of pulses of the processed substrate impacted in unit time on the premise that the average output power of the laser is unchanged, reduce damage to the surface of the substrate caused by thermal effect, and further guarantee processing quality.
The Francicloro-Lucas et al experiment in 2022 contrasts the processing effects for metal burn-out with a single pulse and with a Burst pulse cluster. Under the unified condition of single pulse energy of 0.53 mu J, the metal burning rate of adopting a Burst mode (5 pulses in a cluster and 5GHz pulse repetition frequency in a cluster) is 4.5 times of that of a single pulse; the laser Burst mode was adopted by ShotaKawabata et al in 2023, the parameters were: the pulse cluster repetition frequency is 200KHz, and the pulse repetition frequency in the cluster is 4.88GHz, so that the precise micromachining of the periodic structure of the surface of the crystal material is realized; 2023 Pierrebaage et al used a pulse cluster output form of 50 pulses and set the repetition frequency of the pulses in the cluster to 1GHz, the pulse cluster repetition frequency was 1KHz, and in the top-down drilling mode, the fused silica material was internally drilled in a time scale of 20-100 ms with 10ms as the step length, and the influence of the laser action time on drilling depth, aperture and quality was explored, and the physical mechanism of laser drilling in GHz-Burst mode was further revealed.
In the above technical background, the laser system can be combined with a regenerative amplifier by an oscillator, and a high-intensity laser pulse cluster can be generated by a special regenerative amplifier cavity design and a high-speed pulse selection system.
The chinese patent with publication number CN109599741a proposes a high repetition frequency ultrafast laser pulse cluster generating device and a control method thereof, which adopts a linear resonant cavity or a ring resonant cavity, wherein an output mirror is a partially transmissive and partially reflective mirror, and outputs an output pulse whenever a pulse laser reaches the output mirror, forming an output pulse cluster, and the number of output pulse clusters meeting the requirement is obtained by controlling the high voltage duration of a pockels cell. Wherein the pulse envelope frequency is determined by the pockels cell period, and the sub-pulse frequency in the pulse cluster is determined by the annular cavity length; united states patent 2020 (US 20200067260) proposes controlling one or more pulses into a regenerative amplifier by applying a stepped voltage to a pockels cell, followed by multiple laser amplification of the injected pulse train by an electro-optic modulator, the spacing of the sub-pulses within the pulse train being related to the cavity length of the regenerative amplifier.
In summary, the current method for efficiently generating laser pulse clusters can be categorized into a method for controlling the electro-optical modulation device to select a certain number of laser pulse clusters by using a step voltage, and combining with accurate cavity length matching of the regenerative amplifier, so as to accurately control the number of pulse clusters in the envelope and the frequency of the envelope, and generate a large-energy femtosecond laser pulse cluster. However, the method has a complex structure, needs to accurately control the voltage loaded by the electro-optical modulator, has a strict matching relation for the cavity length design of the regenerative amplifier, and does not meet the requirements of engineering production on low cost and simplicity in operation and maintenance.
Disclosure of Invention
The invention aims to solve the problems of complex device structure, high technical difficulty and the like of the traditional method for generating laser pulse clusters, and provides a femtosecond laser and a method for generating GHz Burst high-energy laser pulse clusters.
The technical scheme adopted by the invention is as follows:
a femtosecond laser generating a GHz Burst high-energy laser pulse cluster, which is characterized in that:
the device comprises a seed light source for emitting GHz laser pulses, a half wave plate, a first polarization beam splitter, an electro-optical pulse selecting system, a second polarization beam splitter, a first laser isolation system, a pulse widening system, a third polarization beam splitter, a second laser isolation system, a first 1/4 wave plate, a pulse compression system and a regenerative amplifier, wherein the half wave plate, the first polarization beam splitter, the electro-optical pulse selecting system and the second polarization beam splitter are sequentially arranged on an emitting light path of the seed light source;
the surfaces of the first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter are plated with antireflection films, and the antireflection films are used for passing p-linear polarized laser and reflecting s-linear polarized laser or passing s-linear polarized laser and reflecting p-linear polarized laser;
after the seed light source emits GHz laser pulses to the half-wave plate, the polarization state of the GHz laser pulses is changed, the GHz laser pulses transmitted by the first polarization beam splitter are incident to the electro-optical pulse selecting system, and laser pulse strings with envelope pulse cluster structures are formed to be emitted to the second polarization beam splitter in a mode of intermittently loading half-wave voltages to the electro-optical pulse selecting system;
the first laser isolation system and the pulse stretching system are sequentially arranged on a transmission light path of the second polarization beam splitter, the laser pulse string sequentially passes through the second polarization beam splitter and the first laser isolation system and then is led into the pulse stretching system for pulse stretching, the laser pulse string is emitted after pulse stretching and returns to pass through the first laser isolation system again, at the moment, the polarization state of the laser pulse string is changed and then is sequentially reflected by the second polarization beam splitter;
the third polarization beam splitter is arranged on a reflection light path of the second polarization beam splitter, the second laser isolation system is arranged on a reflection light path of the third polarization beam splitter, the laser pulse string after pulse broadening passes through the second laser isolation system after being reflected by the third polarization beam splitter, the polarization state of the laser pulse string is changed at the moment, the laser pulse string is led into the regenerative amplifier to be amplified, after amplification is completed, the laser pulse string after the emergent amplification of the regenerative amplifier passes through the second laser isolation system again, the polarization state of the laser pulse string is unchanged at the moment, and the laser pulse string returns to the third polarization beam splitter to be transmitted;
the first 1/4 wave plate and the pulse compression system are sequentially arranged on a transmission light path of the third polarization beam splitter, amplified laser pulse trains are transmitted by the third polarization beam splitter, enter the pulse compression system through the first 1/4 wave plate, recover the pulse width of the amplified laser pulse trains to the width before widening and return, pass through the first 1/4 wave plate again, change the polarization state of the laser pulse trains, and are emitted after being reflected again by the third polarization beam splitter, so that high-energy laser pulse clusters are obtained.
Further, the regenerative amplifier comprises a first thin film polarizer, an electro-optical modulator, a second 1/4 wave plate, a first reflecting mirror, a pumping dichroic mirror, a laser crystal, a second reflecting mirror, an optical fiber coupling semiconductor pumping source and a pumping optical coupling system;
the first film polaroid is used for passing p-linearly polarized laser and reflecting s-linearly polarized laser or passing s-linearly polarized laser and reflecting p-linearly polarized laser;
the optical fiber coupling semiconductor pump source emits pump light to the pump optical coupling system, and the pump optical coupling system is used for collimating and focusing the pump light and then emitting the pump light to the laser crystal through the pump dichroic mirror; the first thin film polaroid is arranged on an output light path of the second laser isolation system, a laser pulse string is transmitted after passing through the first thin film polaroid, the electro-optical modulator, the second 1/4 wave plate and the first reflecting mirror are sequentially arranged on the transmission light path of the first thin film polaroid, the laser pulse string is reflected by the first reflecting mirror after passing through the electro-optical modulator and the second 1/4 wave plate, the polarization state of the laser pulse string is changed after passing through the electro-optical modulator, and the laser pulse string is reflected by the first thin film polaroid;
the pumping dichroic mirror, the laser crystal and the second reflecting mirror are sequentially arranged on a reflecting light path of the first thin film polaroid, the laser pulse string enters the laser crystal after passing through the pumping dichroic mirror, energy is extracted at the laser crystal and then emitted out, the laser pulse string is reflected by the second reflecting mirror, then passes through the laser crystal and the pumping dichroic mirror in sequence and then is reflected by the first thin film polaroid, the polarization state of the laser pulse string is kept by loading voltage on the electro-optical modulator, so that the laser pulse string oscillates in the regenerative amplifier until the energy of the laser pulse string reaches the gain saturation or meets the requirement, the voltage loaded on the electro-optical modulator is withdrawn, the laser pulse string is transmitted through the first thin film polaroid and then emitted out through the second laser isolation system after going back and forth for a circle at the electro-optical modulator, the second 1/4 wave plate and the first reflecting mirror.
Further, the regenerative amplifier further comprises a second thin film polarizer, a third mirror, and a fourth mirror;
the second film polarizer is the same as the first film polarizer;
the second thin film polarizer and the third reflector are sequentially arranged on the reflection light path of the first thin film polarizer and positioned between the first thin film polarizer and the pumping dichroic mirror, and the fourth reflector is positioned between the laser crystal and the second reflector.
Further, the laser gain medium used by the laser crystal is ytterbium-doped laser medium or neodymium-doped laser medium.
Further, the seed light source is an optical fiber mode locking ultrashort pulse seed light source or a solid mode locking ultrashort pulse oscillator, and the pulse width is in the femtosecond level.
Further, the pulse stretching system is a chirped Bragg grating, a Martin-Netzt concentric stretcher or a bulk material stretcher, and is used for stretching the GHz laser pulse train of the femtosecond level to the picosecond level.
Further, the pulse compressor is a Tracy type single-grating compressor, a Tracy type double-grating compressor, a chirped Bragg grating or a prismatic grating pair compressor.
The invention also provides a method for generating the GHz Burst high-energy laser pulse cluster, which is characterized by comprising the following steps of:
step 1: setting a repetition interval of laser pulses according to the cavity length of the regenerative amplifier; setting the polarization state of laser after passing through the first laser isolation system and the second laser isolation system;
step 2: the seed light source emits GHz laser pulses, the GHz laser pulses sequentially pass through the half-wave plate and the first polarization beam splitter, laser pulse strings which have a certain number of laser pulse numbers and have an enveloped pulse cluster structure are formed by intermittently loading half-wave electricity to the electro-optical pulse selecting system, and the GHz laser pulses are emitted to the first laser isolation system through the second polarization beam splitter;
step 3: after passing through the first laser isolation system, the laser pulse string enters the pulse stretching system, the pulse stretching system spreads the laser pulse string, then the laser pulse string passes through the first laser isolation system again, the polarization state of the laser pulse string changes, then the laser pulse string is reflected by the second polarization beam splitter and the third polarization beam splitter in sequence, and the laser pulse string is injected into the regenerative amplifier after the polarization state of the laser pulse string changes through the second laser isolation system;
step 4: the laser pulse string oscillates back and forth in the regenerating amplifier to amplify the pulse energy until reaching the saturated gain or meeting the requirement, and the regenerating amplifier is led out to obtain amplified laser pulse string;
step 5: after the amplified laser pulse string passes through the second laser isolation system again, the polarization state of the laser pulse string is unchanged, the laser pulse string sequentially passes through the third polarization beam splitter and the first 1/4 wave plate and is led into the pulse compression system, the pulse compression system restores the laser pulse string to the original pulse width, the laser pulse string passes through the first 1/4 wave plate again, the polarization state of the laser pulse string is changed, and the laser pulse string is emitted after being reflected again through the third polarization beam splitter, so that the high-energy laser pulse cluster is obtained.
Further, in the step 3, the GHz laser pulse is in the femto-second level, and the pulse stretching system spreads the laser pulse train to the pico-second level.
The beneficial effects of the invention are as follows:
1. the invention designs a femtosecond laser and a method for generating GHz Burst high-energy laser pulse clusters based on a GHz seed light source combined with a regenerative amplifier, and the laser pulse clusters with an envelope pulse cluster structure are formed by using the seed light source for emitting GHz laser pulses, a half wave plate, a first polarization beam splitter and an electro-optical pulse selection system, and the electro-optical pulse selection system does not need to load step voltage, and has the advantages of simple device structure, convenience in operation and the like.
2. Compared with the method for generating the laser pulse clusters in the prior art, the method has the advantages that the laser pulse intervals in the laser pulse clusters are consistent with the time intervals of the seed pulses, the laser pulse clusters can be formed by single to dozens of laser pulses by adjusting the time of loading high voltage by the pulse selector, the number of the laser pulses in the laser pulse clusters can be flexibly controlled, and the laser pulse clusters can realize the output of the large-energy GHz Burst laser pulse clusters after being amplified by the regenerative amplifier.
3. Compared with the existing regenerative amplifier for generating pulse clusters, the invention has the advantages of simple device, high output pulse energy and the like on the premise of having the advantages of controllable pulse number in the pulse clusters, controllable pulse envelope frequency and the like, and has good application prospect and commercial value.
4. The laser designed by the invention can be used for industrial laser micro-nano processing, laser spectroscopy research and biomedical treatment.
Drawings
FIG. 1 is a schematic diagram of an embodiment of a femtosecond laser that generates GHz Burst high-energy laser pulse clusters according to the present invention;
FIG. 2 is a timing diagram of an electro-optic pulse selection system in accordance with an embodiment of the present invention;
FIG. 3 is a timing diagram illustrating operation of a regenerative amplifier in accordance with an embodiment of the present invention.
In the figure, 1, a seed light source; 2. a half-wave plate; 3. a first polarizing beamsplitter; 4. an electro-optic pulse selection system; 5. a second polarizing beamsplitter; 6. a first laser isolation system; 7. a pulse stretching system; 8. a third polarizing beamsplitter; 9. a second laser isolation system; 10. a first thin film polarizer; 11. an electro-optic modulator; 12. a second 1/4 wave plate; 13. a first mirror; 14. a second thin film polarizer; 15. a third mirror; 16. pumping the dichroic mirror; 17. a laser crystal; 18. a fourth mirror; 19. a second mirror; 20. the optical fiber is coupled with a semiconductor pumping source; 21. a pump light coupling system; 22. a first 1/4 wave plate; 23. a pulse compression system; 24. and a regenerative amplifier.
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
A term similar to a "seed light source" will be used herein to refer to a type of mode-locked ultrashort pulse laser having a fixed repetition rate, which may be a solid-mode-locked ultrashort pulse laser or a fiber-mode-locked ultrashort pulse laser, at a repetition rate of GHz and higher, and two adjacent pulses generated by such a seed light source are spaced apart by a repetition period, typically on the order of nanoseconds.
The laser pulse train refers to a set of seed laser pulses with an envelope structure formed by the seed light source 1 after passing through the electro-optical pulse selecting system 4, and the interval between the pulses in the envelope is consistent with the pulse interval emitted by the seed light source 1.
For the sake of uniform description, in the present embodiment, the seed pulse generated by the fiber seed light source 1 or the solid oscillator is defined as p-linear polarization of high extinction ratio; the polarization perpendicular to the seed pulse (the linear polarization orthogonal to the seed pulse polarization) is defined as s-linear polarization, and it is explained here that the definition of the seed pulse polarization state is independent of the control principle of the laser system.
The invention provides a femtosecond laser for generating GHz Burst high-energy laser pulse clusters, which is shown in fig. 1, and comprises a seed light source 1, a half-wave plate 2, a first polarization beam splitter 3, an electro-optical pulse selection system 4, a second polarization beam splitter 5, a first laser isolation system 6, a pulse widening system 7, a third polarization beam splitter 8, a second laser isolation system 9, a first 1/4 wave plate 22, a pulse compression system 23 and a regenerative amplifier 24;
the regenerative amplifier 24 includes a first thin film polarizer 10, an electro-optic modulator 11, a second 1/4 wave plate 12, a first mirror 13, a second thin film polarizer 14, a third mirror 15, a pump dichroic mirror 16, a laser crystal 17, a fourth mirror 18, a second mirror 19, an optical fiber coupled semiconductor pump source 20, and a pump optical coupling system 21;
the functions of the components are as follows:
the pulse time interval provided by the seed light source 1 is t osc The repetition frequency is GHz and higher, the period is a pulse period of nanosecond or less, the single pulse energy is nJ or more, the seed light source 1 outputs laser pulse to meet the p-linear polarization of high extinction ratio, the laser can be a mode-locked ultrashort pulse laser with fixed repetition frequency, the laser can be a solid mode-locked ultrashort pulse laser or an optical fiber mode-locked ultrashort pulse laser, and in the embodiment, the seed light source 1 is based on ytterbium-doped laser medium and is used for providing GHz Burst laser pulse.
The working wavelength of the half wave plate 2 is 1020nm-1070nm, and the half wave plate is used for adjusting the polarization state of laser (GHz Burst laser pulse) output by the seed light source 1; the surfaces of the first polarization beam splitter 3, the second polarization beam splitter 5 and the third polarization beam splitter 8 are plated with 1028-1064nm antireflection films, and can pass p-polarized light and reflect s-polarized light, wherein the second polarization beam splitter 5 is used for separating an original pulse laser string and a pulse laser string after pulse broadening; the third polarization beam splitter 8 is used for separating the amplified pulse laser string and the pulse laser string after pulse compression; in the present embodiment, the first polarization beam splitter 3, the second polarization beam splitter 5, and the third polarization beam splitter 8 are all polarization beam splitting prisms.
The half wave plate 2 is matched with the first polarization beam splitter 3 and is used for controlling the power of the seed light source 1 injected into the regenerative amplifier 24 and improving the polarization extinction ratio of the seed light source 1.
The electro-optical pulse selection system 4 is in this embodiment arranged to: when no high voltage is applied, the polarization direction of the laser is kept unchanged through the electro-optical pulse selection system 4, when half-wave voltage is applied, the polarization direction rotates 90 degrees every time the laser beam passes through the electro-optical pulse selection system 4, so as to select a certain number of laser pulses (in the time of the high voltage duration of the electro-optical pulse selection system 4, the polarization state of the laser pulses is changed and cannot penetrate through the second polarization beam splitter 5, when no half-wave voltage is applied, the polarization state of the laser pulses is unchanged, the laser pulses penetrate through the second polarization beam splitter 5, namely, after the laser pulses generated by the seed light source 1 pass through the electro-optical pulse selection system 4, the laser pulse string with an envelope pulse cluster structure is formed through the second polarization beam splitter 5, the laser pulse interval in the laser pulse string is consistent with the laser pulse period emitted by the seed light source, the envelope repetition frequency is consistent with the period of the high voltage generated by the electro-optical pulse selection system 4, and the pulse interval in the laser pulse string is equal to the time required by the round trip of the pulse in the regenerative amplifier cavity;
specific: the electro-optical pulse selecting system 4 is composed of a pockels cell, a high-voltage power supply and a synchronous circuit, the rising edge/falling edge of the output voltage of the high-voltage power supply is required to be smaller than 8ns, the number of pulses contained in a laser pulse string generated by the electro-optical pulse selecting system 4 is determined by the number of pulses injected into the electro-optical pulse selecting system 4 in the rising edge time, the high-voltage duration time and the falling edge time range corresponding to the removal of the high voltage of the electro-optical pulse selecting system 4, and as shown in fig. 2, the rising edge time and the falling edge time of the output voltage of the high-voltage power supply and the minimum number of pulses in the laser pulse string are required to be satisfied: n (N) pulse ＝(t rise +t gate +t fall )/t osc Wherein N is pulse T is the number of pulses contained in the pulse train rise For the high-voltage rise time of the electro-optical pulse selection system, t gate For high pressure duration, t fall For high voltage drop time, t osc Is the interval between the laser pulses emitted by the seed light source.
The first laser isolation system 6 is a polarization gating element, which in this embodiment is arranged to: the polarization direction is kept unchanged during forward passing, and the polarization direction is rotated by 90 degrees during reverse passing, so that the two linear polarization states of the incident light and the emergent light are orthogonal to each other; the second laser isolation system 9 is a polarization gating element, which in this embodiment is arranged to: the polarization direction rotates by 90 degrees in the forward direction and remains unchanged in the reverse direction, so that the two linear polarization states of the incident light and the emergent light are orthogonal to each other.
The pulse stretching system 7 is, but not limited to, a chirped volume bragg grating, a martin-litz concentric stretcher, a bulk stretcher, and the like, which functions to stretch seed pulses on the order of femtoseconds to the order of picoseconds.
The first and second thin film polarizers 10 and 14 have high transmittance (T > 98%) for p-linearly polarized laser light and low transmittance (T < 0.2%) for s-linearly polarized laser light, and have an incident angle of 65 °; the first thin film polarizer 10 is used to direct laser bursts into and out of the regenerative amplifier 24 and the second thin film polarizer 14 is used to transmit laser bursts within the cavity of the regenerative amplifier 24.
The electro-optical modulator 11 is set to a zero voltage state or a high voltage state in this embodiment, in which the electro-optical modulator 11 has no modulation effect on the transparency of the laser pulse train, allowing the laser pulse train to enter the regenerative amplifier 24; when the laser pulse train is in a high-voltage state, after passing through the electro-optical modulator 11, the linear polarization direction is rotated or the phase is changed, and the laser pulse train is constrained in the regenerative amplifier 24 to oscillate back and forth to realize laser pulse energy amplification, specifically: when the electro-optical modulator 11 is loaded with a quarter voltage (lambda/4), the phase delay of pi/4 occurs after the laser pulse string passes through the electro-optical modulator 11, the corresponding linearly polarized light is converted into circularly polarized light, and when the electro-optical modulator 11 is loaded with a half-wave voltage (lambda/2), the laser pulse string rotates by 90 degrees after passing through the electro-optical modulator 11; electro-optical deviceThe modulator 11 is composed of a Prkerr box, a high-voltage power supply and a synchronous circuit, the rising/falling edge of the output voltage of the high-voltage power supply is smaller than 8ns, the duration of the output high-voltage of the high-voltage power supply is related to the amplifying times of the regenerating amplifier 24 to the laser pulse train, as shown in fig. 3, the duration T of the electro-optic modulator 11 loading lambda/4 voltage gate And the number of times N of amplification by the regenerative amplifier 24 RA Length L of 24 cavity of regenerative amplifier RA The relation of the light velocity c is as follows:
the electro-optical modulator 11 is used together with the second 1/4 wave plate 12 to adjust the linear polarization state of the laser in the cavity of the regenerative amplifier 24: when the electro-optical modulator 11 is at zero voltage, the polarization direction is rotated by 90 degrees after linearly polarized light passes through the electro-optical modulator 11 piece and the second 1/4 wave plate 12 twice; when the electro-optical modulator 11 is at a high voltage, such as a quarter wave voltage (lambda/4), the polarization direction remains unchanged after the linearly polarized light passes back and forth through the electro-optical modulator 11 and the second 1/4 wave plate 12 twice.
The front surface s1 of the pump dichroic mirror 16 is coated with a 940-990nm high-transmittance film (T > 98%) for transmitting pump light, and the rear surface s2 is coated with a 1020-1070nm high-reflectance film (R > 99.8%) for reflecting laser pulse trains.
The optical fiber coupling semiconductor pump source 20 is used for pumping the laser crystal 17, and has the output center wavelength of 981nm, the output power of 140W, the numerical aperture of 0.22 and the optical fiber core diameter of 200 mu m; the pump optical coupling system 21 is an optical imaging system with an amplification ratio of 1:4, and is used for transmitting and focusing the pump light spot to the laser crystal 17 with a certain amplification ratio.
Laser crystal 17 includes, but is not limited to, yb: KGW, yb: KYW, yb: CALGO, yb: CYA, etc.
The first reflecting mirror 13 and the second reflecting mirror 19 are plane reflecting mirrors, and the s2 surface is plated with 1020-1070nm high-reflectivity film with the reflectivity R more than 99.8%.
The third reflecting mirror 15 and the fourth reflecting mirror 18 are plano-concave reflecting mirrors, and the s2 surface is plated with 1020-1070nm high-reflectivity film with the reflectivity R of more than 99.8%.
The third and fourth mirrors 15 and 18 are used to limit the size of the spot mode in the cavity of the regenerative amplifier 24, and at the same time, the mode matching between the pump spot at the position of the laser crystal 17 and the oscillating spot of the regenerative amplifier 24 is satisfied.
The pulse compression system 23 includes, but is not limited to, a Tracy single/double grating compressor, a chirped bragg grating or a prismatic grating pair compressor for compressing laser pulses in pulse clusters to the order of femtoseconds, in this embodiment, a single grating pulse compressor, with a grating line density of 1600L/mm for compressing amplified laser pulses to the order of femtoseconds, i.e., for compressing the pulse duration of laser pulses in a laser pulse train amplified by the regenerative amplifier 24.
In this embodiment, the pockels cell includes, but is not limited to, a high-speed electro-optical modulation crystal such as an RTP or BBO crystal, and the polarization state or phase of the transmitted laser light can be changed after a voltage is applied across the pockels cell.
The arrangement direction of each component is as follows:
definition the first polarizing beam splitter 3 comprises oppositely arranged A1 and A2 faces, the second polarizing beam splitter 5 comprises oppositely arranged B1 and B2 faces, the third polarizing beam splitter 8 comprises oppositely arranged C1 and C2 faces, the first thin film polarizer 10 comprises oppositely arranged D1 and D2 faces, the second thin film polarizer 14 comprises oppositely arranged E1 and E2 faces, and the pump dichroic mirror 16 comprises oppositely arranged s1 and s2 faces.
The half wave plate 2, the first polarization beam splitter 3, the electro-optical pulse selection system 4, the second polarization beam splitter 5, the first laser isolation system 6 and the pulse widening system 7 are sequentially arranged along the emergent light path of the seed light source 1, the A1 surface faces the half wave plate 2, the B1 surface faces the electro-optical pulse selection system 4, the C1 surface of the third polarization beam splitter 8 is positioned on the reflection light path of the B2 surface, the second laser isolation system 9 is positioned on the emergent light path of the C1 surface of the third polarization beam splitter 8, the first thin film polarizer 10, the electro-optical modulator 11, the second 1/4 wave plate 12 and the first reflecting mirror 13 are sequentially arranged along the emergent light path of the second laser isolation system 9, the D1 surface of the first thin film polarizer 10 is close to the second laser isolation system 9, the E1 surface of the second thin film polarizer 14 is located on the reflection light path of the D2 surface of the first thin film polarizer 10, the third mirror 15, the pump dichroic mirror 16, the laser crystal 17, the fourth mirror 18 and the second mirror 19 are sequentially arranged along the light path of the E1 surface of the second thin film polarizer 14, the s2 surface of the pump dichroic mirror 16 is arranged towards the laser crystal 17, the optical fiber coupling semiconductor pump source 20 and the pump optical coupling system 21 are sequentially arranged, and pump coupling light emitted by the pump optical coupling system 21 is transmitted through the pump dichroic mirror 16 and enters the laser crystal 17; the first 1/4 wave plate 22 and the pulse compression system 23 are sequentially arranged along the C2 plane of the third polarization beam splitter 8, and the incident light path of the first 1/4 wave plate 22 is parallel to the reflected light path of the C1 plane of the third polarization beam splitter 8.
The first thin film polarizer 10 and the second mirror 19 form an amplifying cavity of the regenerative amplifier 24.
The invention also provides a method for generating GHz Burst high-energy laser pulse clusters by the femtosecond laser, which comprises the following steps:
step 1: setting a repetition interval of the femtosecond laser pulses according to the cavity length of the regenerative amplifier 24; the polarization direction is kept unchanged when the laser passes through the first laser isolation system 6 in the forward direction, the polarization direction is rotated by 90 degrees when the laser passes through the second laser isolation system 9 in the reverse direction, and the polarization direction is kept unchanged when the laser passes through the second laser isolation system in the forward direction;
step 2: the polarization state of the GHz Burst laser pulse is adjusted through the half-wave plate 2 and the first polarization beam splitter 3, half-wave voltage is intermittently loaded to the electro-optical pulse selection system 4, the polarization state of the laser pulse is changed within the duration of the loading of the half-wave voltage, and the laser pulse string which has a certain number of laser pulses and has an enveloped pulse cluster structure is formed through the reflection and transmission of the second polarization beam splitter 5 and is emitted to the first laser isolation system 6;
specific: after the linear polarization GHz laser pulse emitted by the seed light source 1 passes through the half-wave plate 2 and the first polarization beam splitter 3, the polarization state is p-linear polarization, then high half-wave voltage is intermittently loaded to the electro-optic pulse selecting system 4 (when the electro-optic pulse selecting system 4 is not loaded with high voltage, the polarization direction of the laser pulse passing through the electro-optic pulse selecting system 4 is kept unchanged, the laser pulse is still p-linear polarization, and half-wave is appliedWhen the voltage is applied, the laser beam passes through the electro-optical pulse selecting system 4 once, the polarization direction is rotated by 90 degrees, namely s-linear polarization), and then passes through the screening of the second polarization beam splitter 5 (through p-linear polarization and reflecting s-linear polarization), a laser pulse string with a certain number of laser pulses and an envelope pulse cluster structure is formed and emitted to the first laser isolating system 6, and the interval of sub-pulses in the laser pulse string is t osc As shown in fig. 2;
step 3: after passing through the first laser isolation system 6 in the forward direction, the laser pulse string enters the pulse stretching system 7, the pulse stretching system 7 expands the laser pulse string to picosecond magnitude, then passes through the first laser isolation system 6 again in the reverse direction, the laser pulse string is converted from p-linear polarization to s-linear polarization, then is reflected by the second polarization beam splitter 5 and the third polarization beam splitter 8 in sequence, passes through the second laser isolation system 9 in the forward direction, and enters the regenerative amplifier 24 after being converted from s-linear polarization to p-linear polarization;
specific: after passing through the first laser isolation system 6 in the forward direction, the p-linear polarization laser pulse string enters the pulse widening system 7, returns along the original path of the incident light after being widened, and after passing through the first laser isolation system 6 in the reverse direction again, the laser pulse string is converted into s-linear polarization, and after being reflected by the B2 surface of the second polarization beam splitter 5 and the C1 surface of the third polarization beam splitter 8 in sequence, the laser pulse string enters the second laser isolation system 9 in the forward direction, and after passing through the second laser isolation system 9, the polarization state of the laser pulse string is converted from s-linear polarization to p-linear polarization, and then enters the regenerative amplifier 24 through the D1 surface of the first film polarizer 10;
step 4: injecting the picosecond-level laser pulse train into the regenerative amplifier 24 to perform reciprocating oscillation for pulse energy amplification until reaching gain saturation or meeting the requirement, and leading out the regenerative amplifier 24 to obtain an amplified picosecond-level laser pulse train;
specific: the laser pulse string entering the regenerating amplifier 24 is converted into circular polarization by the electro-optical modulator 11 without high voltage and the second 1/4 wave plate 12, the linear polarization is reflected by the first reflecting mirror 13, then the laser pulse string passes through the second 1/4 wave plate 12 and the electro-optical modulator 11 without high voltage again, the polarization state is s-linear polarized light (the two passes through the second 1/4 wave plate 12 are equivalent to the 90 DEG rotation of the linear polarization state of the laser light caused by passing through the half wave plate 2), then the laser pulse string is amplified by the regenerating amplifier 24 and then is reflected by the first film polarizing plate 10, the second film polarizing plate 14, the third reflecting mirror 15 and the pumping dichroic mirror 16 in sequence, then is incident to the laser crystal 17 for amplification, passes through the laser crystal 17 again after being reflected by the fourth reflecting mirror 18 and the second reflecting mirror 19, and is amplified twice at the laser crystal 17, then the laser pulse train passes through the electro-optical modulator 11 and the second 1/4 wave plate 12 twice when the laser pulse train passes through the electro-optical modulator 11 and the second 1/4 wave plate 12 after being reflected by the pumping dichroic mirror 16, the third reflecting mirror 15, the second thin film polarizer 14 and the first thin film polarizer 10 again, the laser pulse train is still s-linear polarized when the polarization direction of the laser pulse train is unchanged when the laser pulse train passes through the half wave plate 2 twice, then the energy is extracted at the laser crystal 17 by sequentially passing through the first thin film polarizer 10, the second thin film polarizer 14, the third reflecting mirror 15 and the pumping dichroic mirror 16 to realize the energy amplification of the laser pulse train, passes through the fourth reflecting mirror 18 and then is reflected by the second reflecting mirror 19, passes through the fourth reflecting mirror 18 again and is amplified by the laser crystal 17 again, after passing through the pumping dichroic mirror 16, the third reflecting mirror 15 and the second thin film polarizer 14 in turn, the laser pulse string is reflected by the first thin film polarizer 10 and then passes through the electro-optical modulator 11 loaded with 1/4 wave voltage again, and the laser linear polarization is still s-linear polarization after passing through the electro-optical modulator 11 and the second 1/4 wave plate 12 twice, so that after the electro-optical modulator 11 loads 1/4 wave voltage, the laser pulse string can extract energy through the laser crystal 17 for multiple times to realize the energy amplification of the laser pulse string, when the energy of the laser pulse string reaches the gain saturation or meets the current requirement, only the 1/4 wave voltage loaded on the electro-optical modulator 11 is needed to be removed, the amplified seed light pulse passes through the second 1/4 wave plate 12 and then passes through the second 1/4 wave plate 12 again after being reflected by the plane reflecting mirror, and then the amplified seed light pulse polarization state is converted into p-linear polarization, and is led out of the amplifying cavity of the regenerative amplifier 24 through the first thin film polarizer 10; (if the laser pulse passes through the electro-optic modulator 11 again, the laser pulse passes through the electro-optic modulator 11 and the second 1/4 wave plate 12 twice, the process is equivalent to the process of changing the polarization direction of the stress line by passing through the half wave plate 2 once, the linear polarization is p-linear polarization, and the laser pulse passes through the first film polarizer 10 to be led out of the amplifying cavity of the regenerative amplifier 24, and the laser pulse is amplified only twice before being led out of the regenerative amplifier 24, so that the energy is smaller, and the operation is represented as ineffective amplification);
step 5: the amplified laser pulse string passes through the second laser isolation system 9, the polarization state is unchanged, the amplified laser pulse string is transmitted through the third polarization beam splitter 8 and then is led into the pulse compression system 23, the pulse compression system 23 restores the laser pulse string to the femto-second level, then the amplified laser pulse string reversely passes through the second laser isolation system 9 again, the laser pulse string is converted from s-linear polarization to p-linear polarization, and the laser pulse string is reflected by the third polarization beam splitter 8 and is led out, so that a high-energy laser pulse cluster is obtained.
Specific: after passing through the third polarization beam splitter 8, the amplified laser pulse string passes through the second laser isolation system 9 in the opposite direction and still has p-linear polarization, after passing through the third polarization beam splitter 8, the amplified laser pulse string is led into the pulse compression system 23 through the first 1/4 wave plate 22, the pulse compression system 23 is set to have the incident height higher than the emergent beam height, the laser pulse string is restored to the femto-second level, finally, the emergent light compressed by the pulse compression system 23 passes through the first 1/4 wave plate 22 again (the two passes through the first 1/4 wave plate 22 are equivalent to the one pass through the half wave plate 2, the polarization direction is changed), and the s-linear polarization is converted, and the s-linear polarization is reflected by the third polarization beam splitter 8 and is led out, so that the high-energy laser pulse cluster is obtained.
Claims (9)
1. A femtosecond laser generating a GHz Burst high energy laser pulse cluster, characterized by:
the device comprises a seed light source (1) for emitting GHz laser pulses, a half-wave plate (2), a first polarization beam splitter (3), an electro-optical pulse selecting system (4) and a second polarization beam splitter (5), a first laser isolation system (6), a pulse widening system (7), a third polarization beam splitter (8), a second laser isolation system (9), a first 1/4 wave plate (22), a pulse compression system (23) and a regeneration amplifier (24), wherein the half-wave plate (2), the first polarization beam splitter (3), the electro-optical pulse selecting system (4) and the second polarization beam splitter (5) are sequentially arranged on an emitting light path of the seed light source (1);
the surfaces of the first polarization beam splitter (3), the second polarization beam splitter (5) and the third polarization beam splitter (8) are plated with antireflection films, and the antireflection films are used for passing p-linear polarized laser and reflecting s-linear polarized laser or passing s-linear polarized laser and reflecting p-linear polarized laser;
after the seed light source (1) emits GHz laser pulses to the half-wave plate (2), the polarization state of the GHz laser pulses is changed, the GHz laser pulses transmitted by the first polarization beam splitter (3) are incident to the electro-optic pulse selecting system (4), and laser pulse strings with envelope pulse cluster structures are formed by intermittently loading half-wave voltages to the electro-optic pulse selecting system (4) and emitted to the second polarization beam splitter (5);
the first laser isolation system (6) and the pulse widening system (7) are sequentially arranged on a transmission light path of the second polarization beam splitter (5), a laser pulse string sequentially passes through the second polarization beam splitter (5) and the first laser isolation system (6) and then is led into the pulse widening system (7) to be subjected to pulse widening, the laser pulse string is emitted after the pulse widening and returns to pass through the first laser isolation system (6) again, at the moment, the polarization state of the laser pulse string is changed and then is sequentially reflected by the second polarization beam splitter (5);
the third polarization beam splitter (8) is arranged on a reflection light path of the second polarization beam splitter (5), the second laser isolation system (9) is arranged on a reflection light path of the third polarization beam splitter (8), the laser pulse string after pulse widening passes through the second laser isolation system (9) after being reflected by the third polarization beam splitter (8), at the moment, the polarization state of the laser pulse string is changed and is led into the regenerative amplifier (24) to be amplified, after amplification is completed, the laser pulse string after the emergent amplification of the regenerative amplifier (24) passes through the second laser isolation system (9) again, at the moment, the polarization state of the laser pulse string is unchanged, and the laser pulse string returns to the third polarization beam splitter (8) to be transmitted;
the first 1/4 wave plate (22) and the pulse compression system (23) are sequentially arranged on a transmission light path of the third polarization beam splitter (8), amplified laser pulse trains are transmitted by the third polarization beam splitter (8), enter the pulse compression system (23) through the first 1/4 wave plate (22), restore the pulse width of the amplified laser pulse trains to the width before widening, return the pulse width, pass through the first 1/4 wave plate (22) again, change the polarization state of the laser pulse trains, and exit the laser pulse trains after being reflected again by the third polarization beam splitter (8), so that high-energy laser pulse clusters are obtained.
2. A femtosecond laser generating a GHz Burst high energy laser pulse cluster as set forth in claim 1 wherein:
the regenerative amplifier (24) comprises a first thin film polarizer (10), an electro-optical modulator (11), a second 1/4 wave plate (12), a first reflecting mirror (13), a pumping dichroic mirror (16), a laser crystal (17), a second reflecting mirror (19), an optical fiber coupling semiconductor pumping source (20) and a pumping optical coupling system (21);
the first thin film polarizer (10) is used for passing p-linearly polarized laser light and reflecting s-linearly polarized laser light or passing s-linearly polarized laser light and reflecting p-linearly polarized laser light;
the optical fiber coupling semiconductor pump source (20) emits pump light to the pump optical coupling system (21), and the pump optical coupling system (21) is used for collimating and focusing the pump light and then emitting the pump light to the laser crystal (17) through the pump dichroic mirror (16); the first thin film polaroid (10) is arranged on the output light path of the second laser isolation system (9), the laser pulse string is transmitted after passing through the first thin film polaroid (10), the electro-optical modulator (11), the second 1/4 wave plate (12) and the first reflecting mirror (13) are sequentially arranged on the transmission light path of the first thin film polaroid (10), the laser pulse string is reflected by the first reflecting mirror (13) after passing through the electro-optical modulator (11) and the second 1/4 wave plate (12), the polarization state of the laser pulse string is changed after passing through the electro-optical modulator (11) and then reflected by the first thin film polaroid (10);
the pumping dichroic mirror (16), the laser crystal (17) and the second reflecting mirror (19) are sequentially arranged on a reflection light path of the first thin film polaroid (10), a laser pulse string enters the laser crystal (17) after passing through the pumping dichroic mirror (16), energy is extracted at the laser crystal (17) and then emitted, the laser pulse string is reflected by the second reflecting mirror (19), then sequentially passes through the laser crystal (17) and the pumping dichroic mirror (16) and then is reflected by the first thin film polaroid (10), the polarization state of the laser pulse string is kept in a mode of loading voltage on the electro-optical modulator (11), the laser pulse string is made to oscillate in the regenerative amplifier (24) until the energy of the laser pulse string reaches gain saturation or meets the requirement, the voltage loaded on the electro-optical modulator (11) is withdrawn, the laser pulse string is back and forth for a circle again at the electro-optical modulator (11), the second 1/4 wave plate (12) and the first reflecting mirror (13), and then emitted through the second isolation system (9) after being transmitted by the first thin film polaroid (10).
3. A femtosecond laser generating a GHz Burst high energy laser pulse cluster as defined in claim 2 wherein:
the regenerative amplifier (24) further comprises a second thin film polarizer (14), a third mirror (15) and a fourth mirror (18);
the second film polarizer (14) is identical to the first film polarizer (10);
the second thin film polarizer (14) and the third reflector (15) are sequentially arranged on the reflection light path of the first thin film polarizer (10) and positioned between the first thin film polarizer (10) and the pumping dichroic mirror (16), and the fourth reflector (18) is positioned between the laser crystal (17) and the second reflector (19).
4. A femtosecond laser generating a GHz Burst high energy laser pulse cluster as claimed in claim 2 or 3, wherein:
the laser gain medium used by the laser crystal (17) is ytterbium-doped laser medium or neodymium-doped laser medium.
5. A femtosecond laser generating a GHz Burst high energy laser pulse cluster as recited in claim 4 wherein: the seed light source (1) is an optical fiber mode locking ultrashort pulse seed light source or a solid mode locking ultrashort pulse oscillator, and the pulse width is in the femtosecond level.
6. A femtosecond laser generating a GHz Burst high energy laser pulse cluster as recited in claim 5 wherein: the pulse stretching system (7) is a chirped Bragg grating, a Martin-Netzt concentric stretcher or a bulk material stretcher and is used for stretching the GHz laser pulse train of the femtosecond level to the picosecond level.
7. A femtosecond laser generating a GHz Burst high energy laser pulse cluster as recited in claim 6 wherein: the pulse compressor is a Tracy type single-grating compressor, a Tracy type double-grating compressor, a chirped Bragg grating or a prismatic grating pair compressor.
8. A method of generating GHz Burst high energy laser pulse clusters, comprising the steps of:
step 1: setting a repetition interval of the laser pulses according to a cavity length of the regenerative amplifier (24); setting the polarization state of laser after passing through a first laser isolation system (6) and a second laser isolation system (9);
step 2: the seed light source (1) emits GHz laser pulses, the GHz laser pulses sequentially pass through the half-wave plate (2) and the first polarization beam splitter (3), laser pulse strings which have a certain number of laser pulse numbers and have an enveloped pulse cluster structure are formed by intermittently loading half-wave electricity to the electro-optic pulse selecting system (4), and the GHz laser pulses are emitted to the first laser isolation system (6) through the second polarization beam splitter (5);
step 3: after passing through the first laser isolation system (6), the laser pulse string enters the pulse stretching system (7), the pulse stretching system (7) spreads the laser pulse string, the laser pulse string passes through the first laser isolation system (6) again, the polarization state of the laser pulse string is changed, then the laser pulse string is reflected by the second polarization beam splitter (5) and the third polarization beam splitter (8) in sequence, and the laser pulse string is injected into the regenerative amplifier (24) after the polarization state of the laser pulse string is changed through the second laser isolation system (9);
step 4: the laser pulse string oscillates back and forth in the regenerative amplifier (24) to amplify the pulse energy until reaching the saturation of gain or meeting the requirement, and the regenerative amplifier (24) is led out to obtain the amplified laser pulse string;
step 5: after the amplified laser pulse string passes through the second laser isolation system (9) again, the polarization state of the laser pulse string is unchanged, the laser pulse string sequentially passes through the third polarization beam splitter (8) and the first 1/4 wave plate (22) and then is led into the pulse compression system (23), the pulse compression system (23) restores the laser pulse string to the original pulse width, the laser pulse string passes through the first 1/4 wave plate (22) again, the polarization state of the laser pulse string is changed, and the laser pulse string is emitted after being reflected again through the third polarization beam splitter (8) to obtain the high-energy laser pulse string.
9. A method of generating GHz Burst high energy laser pulse train as in claim 8, wherein:
in the step 3, the GHz laser pulse is in the femto-second level, and the pulse stretching system (7) spreads the laser pulse train to the pico-second level.
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