A femtosecond laser and method for generating clusters of GHz high energy laser pulses

Abstract

The application 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 laser pulse cluster generation method; the laser comprises a seed light source, a half-wave plate arranged on an outgoing light path of the seed light source, a first polarization beam splitter, an electro-optic pulse selection system and a second polarization beam splitter, a first laser isolation system and a pulse expansion system are arranged on a transmission light path of the second polarization beam splitter; a third polarization beam splitter is arranged on a reflection light path of the second polarization beam splitter, a second laser isolation system is arranged on a reflection light path of the third polarization beam splitter, a regenerative amplifier is arranged on an outgoing light path of the second laser isolation system, a first 1 / 4 wave plate and a pulse compression system are arranged on the transmission light path of the third polarization beam splitter; and the application further provides a method for generating laser pulse clusters.

Description

TECHNICAL FIELD

[0001] The application relates to a femtosecond laser and an application method, in particular to a femtosecond laser and a method for generating a GHz high-energy laser pulse cluster. BACKGROUND

[0002] The main feature of femtosecond laser processing is that the femtosecond pulse width endows the light and material with extremely short interaction time, so that the heat affected zone can be effectively suppressed to realize ultra-high precision laser micro-nano processing. At present, femtosecond laser technology has been widely used in industrial micro-processing, strong field physics and biomedicine.

[0003] In order to further expand the application prospect of femtosecond laser in commercial and industrial fields, it is the main direction of current scientific researchers to continue to break through the technical difficulties of femtosecond laser processing; the main technical bottlenecks limiting the development of femtosecond laser processing are plasma shielding and nonlinear effect; because femtosecond laser has extremely high peak power, plasma will be formed on the material surface in the processing process, which will cause the energy attenuation of the subsequent light pulse and reduce the processing efficiency; similarly, the extremely high laser peak power also aggravates the nonlinear effect of the material, thereby reducing the ablation rate of the femtosecond laser.

[0004] In the long development history of laser processing, many scholars have carried out their own research on the mode of laser output pulse, and have obtained many interesting results. The burst mode is one of the most novel and practical ones. In essence, the burst mode represents the time-domain transmission of the superfast laser pulse cluster. In this mode, the pulses are no longer output at fixed time intervals, but are divided into pulse clusters composed of multiple pulses by a controllable optical switch device, thereby realizing the expected index of pulse burst output. The common burst parameters correspond to the pulse cluster repetition frequency in the range of Hz~KHz, and two or more laser pulses are distributed at equal intervals in the cluster, and the pulse interval in the cluster is nanoseconds or shorter. Compared with the high repetition rate output mode of emitting laser pulses at fixed time intervals, the burst mode can reduce the number of pulses impacting the processed substrate per unit time under the premise that the average output power of the laser is unchanged, weaken the damage to the substrate surface caused by the thermal effect, and thus ensure the processing quality.

[0005] In 2022, Francesc Caballero-Lucas et al. compared the processing effect of metal ablation using single pulse and Burst pulse cluster. Under the condition of single pulse energy of 0.53 μJ, the metal ablation rate using Burst mode (5 pulses in the cluster, 5 GHz intra-cluster pulse repetition frequency) is 4.5 times that of single pulse; In 2023, Shota Kawabata et al. used the Burst mode of the laser, with parameters of pulse cluster repetition frequency of 200 KHz and intra-cluster pulse repetition frequency of 4.88 GHz, to realize the precise micro-machining of periodic structures on the surface of crystal materials; In 2023, Pierre Balage et al. used a pulse cluster output form of a cluster of 50 pulses, with the intra-cluster pulse repetition frequency set to 1 GHz and the pulse cluster repetition frequency set to 1 KHz, and used a top-down drilling method with a step size of 10 ms to drill internal holes in fused silica materials within a time scale of 20-100 ms, to explore the effect of laser action time on drilling depth, hole diameter and quality, and to reveal the physical mechanism of laser drilling in GHz-Burst mode.

[0006] Under the above technical background, the laser system can be combined with an oscillator and a regenerative amplifier, and high-intensity laser pulse clusters can be generated through special regenerative amplifier cavity design and high-speed pulse selection system.

[0007] Chinese patent CN109599741A proposes a high-repetition-frequency ultrafast laser pulse cluster generation device and its control method, which uses a linear resonant cavity or a ring resonant cavity, and the output mirror is a partially transmitting and partially reflecting mirror. Each time the pulse laser reaches the output mirror, an output pulse is output, forming an output pulse cluster. By controlling the duration of the high voltage of the Pockels cell, the required number of output pulse clusters is obtained. The pulse envelope frequency is determined by the Pockels cell period, and the intra-cluster pulse frequency is determined by the ring cavity length; In 2020, US patent US20200067260 proposes to load a step voltage to the Pockels cell to control one or more pulses into the regenerative amplifier, and then use an electro-optic modulator to amplify the injected pulse cluster multiple times. The interval of the intra-cluster pulse is related to the cavity length of the regenerative amplifier.

[0008] In summary, the current method for efficiently generating laser pulse clusters can be classified as using a step voltage to control an electro-optic modulator to select a certain number of laser pulse strings, and combining a regenerative amplifier with an accurate cavity length to accurately control the number of pulse clusters in the envelope and the frequency of the envelope, and to generate femtosecond laser pulse clusters with high energy. However, this method is complex in structure, requires precise control of the voltage loaded to the electro-optic modulator, and has a strict matching relationship for the cavity length design of the regenerative amplifier, which does not meet the requirements of low cost and simple operation and maintenance for engineering production. SUMMARY

[0009] The application aims at solving the problems of complex device structure and high technical difficulty in the prior art of generating laser pulse cluster, and provides a femtosecond laser and a method for generating GHz high-energy laser pulse cluster.

[0010] The application adopts the technical scheme of:

[0011] A femtosecond laser for generating GHz high-energy laser pulse cluster, which is characterized in that:

[0012] The femtosecond laser comprises a seed light source for emitting GHz laser pulses, a half-wave plate, a first polarization beam splitter, an electro-optic pulse selection system and a second polarization beam splitter which are sequentially arranged on an emission light path of the seed light source, and a first laser isolation system, a pulse stretching 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.

[0013] The first polarization beam splitter, the second polarization beam splitter and the third polarization beam splitter are all coated with an anti-reflection film for transmitting p-linear polarization laser and reflecting s-linear polarization laser, or transmitting s-linear polarization laser and reflecting p-linear polarization laser.

[0014] After the GHz laser pulses emitted by the seed light source pass through 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 on the electro-optic pulse selection system, and by intermittently loading a half-wave voltage on the electro-optic pulse selection system, a laser pulse string with an envelope pulse cluster structure is formed and emitted to the second polarization beam splitter.

[0015] 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 introduced into the pulse stretching system for pulse stretching, the laser pulse string emitted after pulse stretching passes through the first laser isolation system again, at this time, the polarization state of the laser pulse string is changed, and then the laser pulse string is reflected by the second polarization beam splitter.

[0016] 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 stretching is reflected by the third polarization beam splitter and then passes through the second laser isolation system, at this time, the polarization state of the laser pulse string is changed, and the laser pulse string is introduced into the regenerative amplifier for amplification, after amplification, the laser pulse string amplified by the regenerative amplifier passes through the second laser isolation system again, at this time, the polarization state of the laser pulse string is unchanged, and the laser pulse string is transmitted after returning to the third polarization beam splitter.

[0017] The first 1 / 4 wave plate, the pulse compression system are sequentially arranged on the transmission light path of the third polarization beam splitter, the amplified laser pulse train is transmitted through the third polarization beam splitter, enters the pulse compression system through the first 1 / 4 wave plate, and returns after the pulse width of the amplified laser pulse train is restored to the width before the expansion, passes through the first 1 / 4 wave plate again, the polarization state of the laser pulse train changes, and the laser pulse train is emitted again after being reflected by the third polarization beam splitter, so that the high-energy laser pulse cluster is obtained.

[0018] Further, the regenerative amplifier comprises a first thin film polarizer, an electro-optic modulator, a second 1 / 4 wave plate, a first mirror, a pump dichroic mirror, a laser crystal, a second mirror, a fiber-coupled semiconductor pump source, and a pump light coupling system.

[0019] The first thin film polarizer is used for transmitting p-linear polarized laser and reflecting s-linear polarized laser, or transmitting s-linear polarized laser and reflecting p-linear polarized laser.

[0020] The fiber-coupled semiconductor pump source emits pump light to the pump light coupling system, the pump light 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 polarizer is arranged on the output light path of the second laser isolation system, and the laser pulse train is transmitted after passing through the first thin film polarizer; the electro-optic modulator, the second 1 / 4 wave plate, and the first mirror are sequentially arranged on the transmission light path of the first thin film polarizer, the laser pulse train is reflected by the first mirror after passing through the electro-optic modulator and the second 1 / 4 wave plate, passes through the second 1 / 4 wave plate again, the polarization state of the laser pulse train changes, and the laser pulse train is reflected by the first thin film polarizer after passing through the electro-optic modulator.

[0021] The pump dichroic mirror, the laser crystal, and the second mirror are sequentially arranged on the reflection light path of the first thin film polarizer, the laser pulse train is incident to the laser crystal after passing through the pump dichroic mirror, extracts energy at the laser crystal, and is reflected by the second mirror, and then sequentially passes through the laser crystal and the pump dichroic mirror and is reflected by the first thin film polarizer; the polarization state of the laser pulse train is maintained by loading a voltage on the electro-optic modulator, so that the laser pulse train oscillates in the regenerative amplifier until the energy of the laser pulse train reaches gain saturation or meets the requirements, the voltage loaded on the electro-optic modulator is removed, the laser pulse train passes through the electro-optic modulator, the second 1 / 4 wave plate, and the first mirror again, is transmitted through the first thin film polarizer, and is emitted through the second laser isolation system.

[0022] Further, the regenerative amplifier further comprises a second thin film polarizer, a third mirror, and a fourth mirror.

[0023] The second thin film polarizer is the same as the first thin film polarizer.

[0024] The second thin film polarizer, the third mirror are sequentially arranged on the reflection light path of the first thin film polarizer and located between the first thin film polarizer and the pump dichroic mirror, and the fourth mirror is located between the laser crystal and the second mirror.

[0025] Further, the laser gain medium used in the laser crystal is an ytterbium-doped laser medium or a neodymium-doped laser medium.

[0026] Further, the seed light source is a fiber mode-locked ultrashort pulse seed light source or a solid mode-locked ultrashort pulse oscillator, and the pulse width is femtosecond level.

[0027] Further, the pulse stretcher is a chirped volume Bragg grating, a Martin-Nez-type concentric stretcher or a bulk material stretcher, which is used to stretch the femtosecond-level GHz laser pulse train to picosecond level.

[0028] Further, the pulse compressor is a Tracy-type single-grating compressor, a Tracy-type double-grating compressor, a chirped volume Bragg grating or a grating pair compressor.

[0029] The application further provides a method for generating a GHz high-energy laser pulse cluster, which is characterized in that the method comprises the following steps based on the above-mentioned GHz high-energy laser pulse cluster femtosecond laser:

[0030] Step 1: setting the repetition interval of the laser pulse according to the cavity length of the regenerative amplifier; setting the polarization state of the laser after passing through the first laser isolation system and the second laser isolation system;

[0031] Step 2: the seed light source emits a GHz laser pulse, the GHz laser pulse sequentially passes through a half-wave plate and a first polarization beam splitter, then forms a laser pulse train with a certain number of laser pulses and an envelope by intermittently loading a half-wave voltage to an electro-optic pulse selection system, and finally is emitted to the first laser isolation system through a second polarization beam splitter;

[0032] Step 3: the laser pulse train passes through the first laser isolation system, enters a pulse stretcher, is stretched by the pulse stretcher, again passes through the first laser isolation system, changes the polarization state of the laser pulse train, is then reflected by a second polarization beam splitter and a third polarization beam splitter in sequence, passes through a second laser isolation system, changes the polarization state of the laser pulse train, and is injected into a regenerative amplifier;

[0033] Step 4: the laser pulse train is oscillated back and forth in the regenerative amplifier to amplify the pulse energy until the gain saturation is reached or the requirement is met, is led out of the regenerative amplifier, and the amplified laser pulse train is obtained.

[0034] Step 5: After the amplified laser pulse train passes through the second laser isolation system again, the polarization state of the laser pulse train is unchanged, sequentially passes through the third polarization beam splitter and the first 1 / 4 wave plate, is introduced into the pulse compression system, the pulse compression system restores the laser pulse train to the original pulse width, passes through the first 1 / 4 wave plate again, the polarization state of the laser pulse train is changed, and the laser pulse train is emitted after being reflected by the third polarization beam splitter again, thereby obtaining the high-energy laser pulse cluster.

[0035] Further, in the step 3, the GHz laser pulse is femtosecond level, and the pulse expansion system expands the laser pulse train to picosecond level.

[0036] The beneficial effects of the present application are:

[0037] 1. The present application designs a femtosecond laser and a method for generating GHz high-energy laser pulse cluster based on GHz seed light source combined with regenerative amplifier, the seed light source of the emitted GHz laser pulse is used in combination with a half wave plate, a first polarization beam splitter and an electro-optic pulse selection system to form a laser pulse train with envelope pulse cluster structure, and the electro-optic pulse selection system does not need to load a step voltage, and has the advantages of simple device structure, convenient operation and the like.

[0038] 2. Compared with the method for generating laser pulse cluster in the prior art, the laser pulse interval in the laser pulse train is consistent with the time interval of the seed pulse, the time of loading high voltage of the pulse selector can be adjusted to provide a single to dozens of number of laser pulses to form a laser pulse cluster, and the number of pulses in the laser pulse cluster can be flexibly controlled, and after the laser pulse cluster is amplified by the regenerative amplifier, the output of the GHz laser pulse cluster with large energy can be realized.

[0039] 3. Compared with the regenerative amplifier for generating pulse cluster in the prior art, the present application has the advantages of controllable number of pulses in the pulse cluster, controllable envelope frequency of the pulse train and the like, and also has the advantages of simple device, high output pulse energy and the like, and has good application prospect and commercial value.

[0040] 4. The laser designed in the present application can be used for industrial laser micro-nano processing, laser spectroscopy research and biomedical treatment. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is a principle schematic diagram of a femtosecond laser for generating GHz high-energy laser pulse cluster according to an embodiment of the present application;

[0042] Figure 2 is a timing diagram of an electro-optic pulse selection system in an embodiment of the present application;

[0043] Figure 3 is a timing diagram of a regenerative amplifier in operation in an embodiment of the present application.

[0044] In the figure, 1, seed light source; 2, half-wave plate; 3, first polarization beam splitter; 4, electro-optic pulse selection system; 5, second polarization beam splitter; 6, first laser isolation system; 7, pulse stretching system; 8, third polarization beam splitter; 9, second laser isolation system; 10, first thin film polarizer; 11, electro-optic modulator; 12, second 1 / 4 wave plate; 13, first mirror; 14, second thin film polarizer; 15, third mirror; 16, pump dichroic mirror; 17, laser crystal; 18, fourth mirror; 19, second mirror; 20, fiber-coupled semiconductor pump source; 21, pump light coupling system; 22, first 1 / 4 wave plate; 23, pulse compression system; 24, regenerative amplifier. DETAILED DESCRIPTION

[0045] The application will be described in detail below in conjunction with the drawings and specific embodiments.

[0046] In the text, a term similar to "seed light source" will be used, which represents a class of mode-locked ultra-short pulse lasers with fixed repetition frequency, which can be solid mode-locked ultra-short pulse lasers or fiber mode-locked ultra-short pulse lasers, with a repetition frequency of GHz and higher. The interval between two adjacent pulses generated by such a seed light source is usually in the order of nanoseconds.

[0047] "Laser pulse train" refers to a collection of seed laser pulses with envelope structure formed after the seed light source 1 passes through the electro-optic pulse selection system 4. The interval between the pulses in the envelope is consistent with the interval between the pulses emitted by the seed light source 1.

[0048] For the sake of uniformity, in this embodiment, the seed pulses generated by the fiber seed light source 1 or the solid oscillator are defined as p-linear polarization with high extinction ratio; the polarization perpendicular to the seed pulse (linear polarization orthogonal to the seed pulse polarization) is defined as s-linear polarization. It is hereby stated that the definition of the polarization state of the seed pulse is irrelevant to the control principle of the laser system.

[0049] The present application proposes a femtosecond laser for generating GHz high-energy laser pulse clusters, as shown in Figure 1 which includes a seed light source 1, a half-wave plate 2, a first polarization beam splitter 3, an electro-optic pulse selection system 4, a second polarization beam splitter 5, a first laser isolation system 6, a pulse stretching 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;

[0050] 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 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, a fiber-coupled semiconductor pump source 20, and a pump light coupling system 21.

[0051] The functions of the components are as follows:

[0052] The pulse time interval provided by the seed light source 1 is t osc The repetition frequency is GHz and above, the period is nanosecond or smaller, the single pulse energy is nJ and above, the output laser pulse of the seed light source 1 satisfies the p-linear polarization of high extinction ratio, can be a mode-locked ultra-short pulse laser with fixed repetition frequency, can be a solid mode-locked ultra-short pulse laser or a fiber mode-locked ultra-short pulse laser, and in the embodiment, the seed light source 1 is based on a ytterbium-doped laser medium and is used to provide GHz Burst laser pulses.

[0053] The working wavelength of the half wave plate 2 is 1020-1070 nm, which is used to adjust the polarization state of the output laser (GHz Burst laser pulse) of the seed light source 1; the first polarization beam splitter 3, the second polarization beam splitter 5, and the third polarization beam splitter 8 are coated with a 1028-1064 nm anti-reflection film, and can pass p-polarized light and reflect s-polarized light, wherein the second polarization beam splitter 5 is used to separate the original pulse laser train and the pulse stretched pulse laser train; the third polarization beam splitter 8 is used to separate the amplified pulse laser train and the pulse compressed pulse laser train; in the 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.

[0054] The half wave plate 2 and the first polarization beam splitter 3 cooperate to control the power of the seed light source 1 injected into the regenerative amplifier 24 and improve the polarization extinction ratio of the seed light source 1.

[0055] The electro-optic pulse selection system 4 is set in the embodiment to keep the polarization direction of the laser unchanged when no high voltage is applied, and to rotate the polarization direction of the laser beam by 90° each time the laser beam passes through the electro-optic pulse selection system 4 when a half-wave voltage is applied, so as to select a certain number of laser pulses (the polarization state of the laser pulses changes when the high voltage of the electro-optic pulse selection system 4 is applied, and the laser pulses cannot pass through the second polarization beam splitter 5, and the polarization state of the laser pulses does not change when no half-wave voltage is applied, so the laser pulses pass through the second polarization beam splitter 5, i.e., the laser pulses generated by the seed light source 1 pass through the second polarization beam splitter 5 to form a laser pulse train with an envelope pulse cluster structure after passing through the electro-optic pulse selection system 4, the interval between the laser pulses in the laser pulse train is consistent with the period of the laser pulses emitted by the seed light source, the envelope repetition frequency is consistent with the period of the high voltage generated by the electro-optic pulse selection system 4, and the interval between the pulses in the laser pulse train generated by the electro-optic pulse selection system 4 is equal to the time required for the pulses to make a round trip in the regenerative amplifier cavity).

[0056] Specifically, the electro-optic pulse selection system 4 is composed of a Pockels cell, a high-voltage power supply, and a synchronization circuit, the rising edge and falling edge of the output voltage of the high-voltage power supply should be less than 8 ns, and the number of pulses contained in the laser pulse train generated by the electro-optic pulse selection system 4 is determined by the number of pulses injected into the electro-optic pulse selection system 4 within the range of the rising edge time of the high voltage of the electro-optic pulse selection system 4, the high-voltage duration, and the falling edge time corresponding to the removal of the high voltage, as shown in the following formula: Figure 2 The rising edge time and falling edge time of the output voltage of the high-voltage power supply and the minimum number of pulses in the laser pulse train satisfy the following formula: wherein N is the number of pulses contained in the pulse train, t is the rising time of the high voltage of the electro-optic pulse selection system, t is the duration of the high voltage, t is the falling time of the high voltage, and t is the interval between the laser pulses emitted by the seed light source. pulse rise gate fall osc

[0057] The first laser isolation system 6 is a polarization gating element, which is set in the embodiment to keep the polarization direction unchanged when passing in the forward direction and to rotate the polarization direction by 90° when passing in the reverse direction, so that the linear polarization states of the incident light and the outgoing light are orthogonal to each other; and the second laser isolation system 9 is a polarization gating element, which is set in the embodiment to rotate the polarization direction by 90° when passing in the forward direction and to keep the polarization direction unchanged when passing in the reverse direction, so that the linear polarization states of the incident light and the outgoing light are orthogonal to each other. o o

[0058] ​​​​​​​The pulse stretching system 7 is, but is not limited to, a chirped volume Bragg grating, a Martin-Netz concentric stretcher, and a bulk material stretcher, etc., and its function is to stretch femtosecond-level seed pulses to picosecond-level pulses.

[0059] The first thin-film polarizer 10 and the second thin-film polarizer 14 have high transmittance (T>98%) for p-linearly polarized lasers and low transmittance (T<0.2%) for s-linearly polarized lasers, with an incident angle of 65°. The first thin-film polarizer 10 is used to introduce and export laser pulse trains to the regenerative amplifier 24, and the second thin-film polarizer 14 is used to transmit the laser pulse trains within the cavity of the regenerative amplifier 24.

[0060] In this embodiment, the electro-optic modulator 11 is configured to either a zero-voltage state or a high-voltage state. In the zero-voltage state, the electro-optic modulator 11 is transparent to the laser pulse train and has no modulation effect, allowing the laser pulse train to enter the regenerative amplifier 24. In the high-voltage state, after passing through the electro-optic modulator 11, the linear polarization direction of the laser pulse train rotates or its phase changes. The laser pulse train is then confined within the regenerative amplifier 24, oscillating back and forth to amplify the laser pulse energy. Specifically, when the electro-optic modulator 11 is loaded with a quarter-wave voltage (λ / 4), the phase of the laser pulse train is delayed by π / 4 after passing through the electro-optic modulator 11, corresponding to the conversion of linearly polarized light into circularly polarized light. When the electro-optic modulator 11 is loaded with a half-wave voltage (λ / 2), the linear polarization direction of the laser pulse train rotates by 90 degrees after passing through the electro-optic modulator 11. o The electro-optic modulator 11 consists of a Pockel cell, a high-voltage power supply, and a synchronization circuit. The rise / fall time of the output voltage of the high-voltage power supply should be less than 8 ns. The duration of the high voltage output by the high-voltage power supply is related to the number of times the regenerative amplifier 24 amplifies the laser pulse train. Figure 3 As shown, the duration T of the λ / 4 voltage applied to the electro-optic modulator 11 gate With regenerative amplifier 24 times N RA 24-cavity regenerative amplifier, length L RA The relationship between the speed of light c and the speed of light c is:

[0061] .

[0062] The electro-optic modulator 11 is used in conjunction with the second quarter-wave plate 12 to adjust the linear polarization state of the laser within the cavity of the regenerative amplifier 24: when the electro-optic modulator 11 is at zero voltage, the linearly polarized light passes through the electro-optic modulator 11 and the second quarter-wave plate 12 twice, after which the polarization direction rotates by 90 degrees. o When the electro-optic modulator 11 is at a high voltage, such as a quarter-wave voltage (λ / 4), the polarization direction of the linearly polarized light remains unchanged after passing through the electro-optic modulator 11 and the second 1 / 4 wave plate 12 twice.

[0063] The front surface s1 of the pump dichroic mirror 16 is coated with a high-transmission film (T>98%) for 940-990 nm for transmitting the pump light, and the rear surface s2 is coated with a high-reflectivity film (R>99.8%) for 1020-1070 nm for reflecting the laser pulse train.

[0064] The fiber-coupled semiconductor pump source 20 is used for pumping the laser crystal 17, and has an output center wavelength of 981 nm, an output power of 140 W, a numerical aperture of 0.22, and a fiber core diameter of 200 μm; the pump light coupling system 21 is an optical imaging system with a magnification of 1:4, which is used for transmitting and focusing the pump light spot to the laser crystal 17 with a certain magnification.

[0065] The laser crystal 17 includes but is not limited to Yb:KGW, Yb:KYW, Yb:CALGO, Yb:CYA, etc.

[0066] The first mirror 13 and the second mirror 19 are both planar mirrors, and the s2 surfaces are coated with a high-reflectivity film (R>99.8%) for 1020-1070 nm.

[0067] The third mirror 15 and the fourth mirror 18 are both planar-concave mirrors, and the s2 surfaces are coated with a high-reflectivity film (R>99.8%) for 1020-1070 nm.

[0068] The third mirror 15 and the fourth mirror 18 are used for limiting the size of the light spot mode in the regenerative amplifier 24, while satisfying the mode matching of the pump light spot at the position of the laser crystal 17 and the oscillation light spot of the regenerative amplifier 24.

[0069] The pulse compression system 23 includes but is not limited to a Tracy type single / dual grating compressor, a chirped volume Bragg grating or a prism pair compressor, which is used for compressing the laser pulses in the pulse train to femtosecond level. In this embodiment, it is a single grating pulse compressor, and the grating line density is 1600 L / mm, which is used for compressing the pulse duration of the laser pulses in the pulse train after amplification, i.e., for compressing the laser pulses in the pulse train after amplification by the regenerative amplifier 24.

[0070] 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, which can change the polarization state or phase of the transmitted laser after a voltage is applied to both ends of the Pockels cell.

[0071] The setting directions of the components are as follows:

[0072] The first polarizing beam splitter 3 includes A1 and A2 surfaces arranged opposite to each other; the second polarizing beam splitter 5 includes B1 and B2 surfaces arranged opposite to each other; the third polarizing beam splitter 8 includes C1 and C2 surfaces arranged opposite to each other; the first thin-film polarizer 10 includes D1 and D2 surfaces arranged opposite to each other; the second thin-film polarizer 14 includes E1 and E2 surfaces arranged opposite to each other; and the pump dichroic mirror 16 includes s1 and s2 surfaces arranged opposite to each other.

[0073] Half-wave plate 2, first polarizing beam splitter 3, electro-optic pulse selection system 4, second polarizing beam splitter 5, first laser isolation system 6, and pulse broadening system 7 are arranged sequentially along the output optical path of seed light source 1, with surface A1 facing half-wave plate 2 and surface B1 facing electro-optic pulse selection system 4. Surface C1 of third polarizing beam splitter 8 is located on the reflected optical path of surface B2. Second laser isolation system 9 is located on the output optical path of surface C1 of third polarizing beam splitter 8. First thin-film polarizer 10, electro-optic modulator 11, second quarter-wave plate 12, and first reflector 13 are arranged sequentially along the output optical path of second laser isolation system 9, with surface D1 of first thin-film polarizer 10 close to second laser isolation system 9. Surface E1 of second thin-film polarizer 14... The first thin-film polarizer 10D2 surface is located on the reflected light path. The third reflector 15, the pump dichroic mirror 16, the laser crystal 17, the fourth reflector 18, and the second reflector 19 are arranged sequentially along the light path of the second thin-film polarizer 14E1 surface. The s2 surface of the pump dichroic mirror 16 faces the laser crystal 17. The fiber-coupled semiconductor pump source 20 and the pump optical coupling system 21 are arranged sequentially. The pump coupled light emitted from the pump optical coupling system 21 is transmitted through the pump dichroic mirror 16 and enters the laser crystal 17. The first quarter wave plate 22 and the pulse compression system 23 are arranged sequentially along the C2 surface of the third polarization beam splitter 8. The incident light path of the first quarter wave plate 22 is parallel to the reflected light path of the C1 surface of the third polarization beam splitter 8.

[0074] The first thin-film polarizer 10 and the second reflector 19 form the amplification cavity of the regenerative amplifier 24.

[0075] This invention also proposes a method for generating GHz high-energy laser pulse clusters using the aforementioned femtosecond laser, comprising the following steps:

[0076] Step 1: Set the repetition interval of the femtosecond laser pulses according to the cavity length of the regenerative amplifier 24; set the polarization direction to remain unchanged when the laser passes through the first laser isolation system 6 in the forward direction, and rotate the polarization direction by 90 degrees when passing through in the reverse direction. o When the laser passes through the second laser isolation system 9 in the positive direction, the polarization direction is rotated by 90 degrees. o The polarization direction remains unchanged when passing in the opposite direction;

[0077] Step 2: Adjust the polarization state of the GHz Burst laser pulse after passing through the half-wave plate 2 and the first polarization beam splitter 3, and change the polarization state of the laser pulse by intermittently loading a half-wave voltage to the electro-optic pulse selection system 4. During the duration of loading the half-wave voltage, the laser pulse is reflected and transmitted by the second polarization beam splitter 5 to form a laser pulse train with a certain number of laser pulses and a pulse cluster structure with an envelope, and the laser pulse train is output to the first laser isolation system 6;

[0078] Specifically, the linearly polarized GHz laser pulse output by the seed light source 1 passes through the half-wave plate 2 and the first polarization beam splitter 3, and the polarization state is p-linear polarization. Then, a high half-wave voltage is intermittently loaded to the electro-optic pulse selection system 4 (when the electro-optic pulse selection system 4 is not loaded with a high voltage, the polarization direction of the laser pulse passing through the electro-optic pulse selection system 4 remains unchanged, still p-linear polarization, and when the half-wave voltage is applied, the polarization direction of the laser beam rotates 90° every time it passes through the electro-optic pulse selection system 4, that is, s-linear polarization). Then, the laser pulse is filtered by the second polarization beam splitter 5 (p-linear polarization is transmitted and s-linear polarization is reflected), and a laser pulse train with a certain number of laser pulses and a pulse cluster structure with an envelope is output to the first laser isolation system 6. The interval between the sub-pulses in the laser pulse train is t osc As shown in Figure 2

[0079] Step 3: After the laser pulse train passes through the first laser isolation system 6 in the forward direction, it enters the pulse stretching system 7, which expands the laser pulse train to the picosecond level. Then, the laser pulse train passes through the first laser isolation system 6 in the reverse direction again, and the polarization state of the laser pulse train changes from p-linear polarization to s-linear polarization. Then, the laser pulse train is reflected by the second polarization beam splitter 5 and the third polarization beam splitter 8 in turn, and passes through the second laser isolation system 9 in the forward direction. At this time, the laser pulse train changes from s-linear polarization to p-linear polarization, and then enters the regenerative amplifier 24;

[0080] Specifically, the laser pulse train with p-linear polarization passes through the first laser isolation system 6 in the forward direction, and still has p-linear polarization when it enters the pulse stretching system 7. After the laser pulse train is stretched, it returns along the incident light path. After passing through the first laser isolation system 6 in the reverse direction again, the laser pulse train changes to s-linear polarization. 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 turn, the laser pulse train enters the second laser isolation system 9 in the forward direction. After passing through the second laser isolation system 9, the polarization state of the laser pulse train changes from s-linear polarization to p-linear polarization, and then enters the regenerative amplifier 24 through the D1 surface of the first thin film polarizer 10;

[0081] Step 4: Inject the picosecond laser pulse train into the regenerative amplifier 24 to oscillate back and forth to amplify the pulse energy until the gain saturation is reached or the demand is met, and then output the regenerative amplifier 24 to obtain the amplified picosecond laser pulse train; ​

[0082] Specifically: the laser pulse train entering the regenerative amplifier 24 passes through the electro-optic modulator 11 and the second 1 / 4 wave plate 12 without high voltage applied, linear polarization is converted into circular polarization, after being reflected by the first mirror 13, it passes through the second 1 / 4 wave plate 12 and the electro-optic modulator 11 without high voltage again, and the polarization state is s-linearly polarized light (the second 1 / 4 wave plate 12 is equivalent to passing through a half wave plate 2 twice, which causes the laser linear polarization state to change by 90 oAfter the first time, the laser pulse train is amplified twice in the laser crystal 17, and then the laser pulse train is reflected by the fourth mirror 18, and then reflected by the second mirror 19, and then reflected by the fourth mirror 18 again, and then the laser pulse train is amplified again in the laser crystal 17, and then the laser pulse train is reflected by the pump dichroic mirror 16, the third mirror 15, the second film polarizer 14, and the first film polarizer 10, and then the laser pulse train passes through the electro-optical modulator 11 again, and the electro-optical modulator 11 is loaded with a 1 / 4-wave voltage at this time, and at this time, the laser pulse train passes through the electro-optical modulator 11 and the second 1 / 4-wave plate 12 twice, and the process is equivalent to passing through the half-wave plate 2 twice, and the polarization direction of the laser line does not change, and at this time, the laser pulse train is still s-linearly polarized, and then the laser pulse train passes through the first film polarizer 10, the second film polarizer 14, the third mirror 15, and the pump dichroic mirror 16, and then the laser pulse train is amplified in the laser crystal 17, and then the laser pulse train is reflected by the fourth mirror 18, and then the laser pulse train is reflected by the second mirror 19, and then the laser pulse train is reflected by the fourth mirror 18 again, and then the laser pulse train is amplified again in the laser crystal 17, and then the laser pulse train passes through the pump dichroic mirror 16, the third mirror 15, the second film polarizer 14, and the first film polarizer 10, and then the laser pulse train passes through the electro-optical modulator 11 again, and the electro-optical modulator 11 is loaded with a 1 / 4-wave voltage at this time, and at this time, the polarization of the laser line does not change after the laser pulse train passes through the electro-optical modulator 11 and the second 1 / 4-wave plate 12 twice, and at this time, the laser pulse train is still s-linearly polarized, and therefore, the laser pulse train can be amplified in the laser crystal 17 multiple times after the electro-optical modulator 11 is loaded with a 1 / 4-wave voltage, and when the energy of the laser pulse train reaches the gain saturation or meets the current needs, the 1 / 4-wave voltage loaded on the electro-optical modulator 11 is removed, and then the amplified seed light pulse passes through the second 1 / 4-wave plate 12, and then the amplified seed light pulse is reflected by the plane mirror and then passes through the second 1 / 4-wave plate 12 again, and at this time, the polarization state of the amplified seed light pulse is changed to p-linearly polarized, and then the amplified seed light pulse is output from the amplification cavity of the regenerative amplifier 24 through the first film polarizer 10; (if the laser pulse train passes through the electro-optical modulator 11 again, and the laser pulse train passes through the electro-optical modulator 11 and the second 1 / 4-wave plate 12 twice, and the process is equivalent to passing through the half-wave plate 2 once, and the polarization direction of the laser line changes at this time, and at this time, the linear polarization is p-linearly polarized, and then the amplified seed light pulse is output from the amplification cavity of the regenerative amplifier 24 through the first film polarizer 10, and since the laser pulse train is amplified only twice before being output from the regenerative amplifier 24, the energy is small, and this operation is invalid amplification);

[0083] Step 5: After the amplified laser pulse train passes through the second laser isolation system 9, the polarization state remains unchanged, and after passing through the third polarization beam splitter 8, it is introduced into the pulse compression system 23. The pulse compression system 23 restores the laser pulse train to the femtosecond level, and then passes through the second laser isolation system 9 again. The laser pulse train is converted from s-linear polarization to p-linear polarization. After being reflected by the third polarization beam splitter 8, the high-energy laser pulse cluster is obtained.

[0084] Specifically: After the amplified laser pulse train passes through the third polarization beam splitter 8, it reverses through the second laser isolation system 9, and the polarization state remains p-linear polarization. After passing through the third polarization beam splitter 8, it is introduced into the pulse compression system 23 through the first 1 / 4 wave plate 22. The pulse compression system 23 is set to have a higher incident height than the outgoing beam height, restores the laser pulse train to the femtosecond level, and finally the outgoing light after the pulse compression system 23 is compressed again through the first 1 / 4 wave plate 22 (two times through the first 1 / 4 wave plate 22 is equivalent to once through the half wave plate 2, and the polarization direction changes). It is converted to s-linear polarization, reflected by the third polarization beam splitter 8, and the high-energy laser pulse cluster is obtained.

Claims

1. A femtosecond laser for generating GHz high-energy laser pulse clusters, characterized in that: it comprises a seed light source (1) emitting GHz laser pulses, a half-wave plate (2), a first polarization beam splitter (3), an electro-optic pulse selection system (4), and a second polarization beam splitter (5) arranged in sequence on the light path of the seed light source (1), and a first laser isolation system (6), a pulse stretching 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 first polarization beam splitter (3), the second polarization beam splitter (5), and the third polarization beam splitter (8) are each coated with an anti-reflection film for transmitting p-linearly polarized light and reflecting s-linearly polarized light, or for transmitting s-linearly polarized light and reflecting p-linearly polarized light; after the GHz laser pulses emitted by the seed light source (1) pass through the half-wave plate (2), the polarization state of the GHz laser pulses changes, and the GHz laser pulses transmitted by the first polarization beam splitter (3) are incident on the electro-optic pulse selection system (4); by intermittently applying a half-wave voltage to the electro-optic pulse selection system (4), a laser pulse train with an envelope pulse cluster structure is emitted to the second polarization beam splitter (5); the first laser isolation system (6) and the pulse stretching system (7) are arranged in sequence on the transmission light path of the second polarization beam splitter (5), and the laser pulse train passes through the second polarization beam splitter (5) and the first laser isolation system (6) in sequence and is then introduced into the pulse stretching system (7) for pulse stretching; after pulse stretching, the emitted laser pulse train passes through the first laser isolation system (6) again, at which time the polarization state of the laser pulse train changes, and the laser pulse train is then reflected by the second polarization beam splitter (5) in sequence; the third polarization beam splitter (8) is arranged on the reflection light path of the second polarization beam splitter (5), and the second laser isolation system (9) is arranged on the reflection light path of the third polarization beam splitter (8); after the laser pulse train after pulse stretching is reflected by the third polarization beam splitter (8), it passes through the second laser isolation system (9), at which time the polarization state of the laser pulse train changes, and the laser pulse train is then introduced into the regenerative amplifier (24) for amplification; after amplification, the amplified laser pulse train is emitted by the regenerative amplifier (24) and then passes through the second laser isolation system (9) again, at which time the polarization state of the laser pulse train does not change, and the laser pulse train is then transmitted by the third polarization beam splitter (8) after returning; the first 1 / 4 wave plate (22) and the pulse compression system (23) are arranged in sequence on the transmission light path of the third polarization beam splitter (8); after the amplified laser pulse train is transmitted by the third polarization beam splitter (8), it passes through the first 1 / 4 wave plate (22) and then enters the pulse compression system (23); after the pulse width of the amplified laser pulse train is restored to the width before pulse stretching, it returns and then passes through the first 1 / 4 wave plate (22) again, at which time the polarization state of the laser pulse train changes, and the laser pulse train is then emitted after being reflected by the third polarization beam splitter (8) again, thereby obtaining a high-energy laser pulse cluster.

2. The femtosecond laser for generating GHz high-energy laser pulse clusters according to claim 1, characterized in that: ​ ​ ​ ​ ​ ​ The regenerative amplifier (24) comprises a first thin film polarizer (10), an electro-optic modulator (11), a second 1 / 4 wave plate (12), a first mirror (13), a pump dichroic mirror (16), a laser crystal (17), a second mirror (19), a fiber-coupled semiconductor pump source (20), and a pump light coupling system (21); The first thin film polarizer (10) is used to transmit p-linear polarized laser and reflect s-linear polarized laser, or transmit s-linear polarized laser and reflect p-linear polarized laser; The fiber-coupled semiconductor pump source (20) emits pump light to the pump light coupling system (21), which is used to collimate and focus the pump light and then emit the pump light to the laser crystal (17) through the pump dichroic mirror (16); the first thin film polarizer (10) is arranged on the output light path of the second laser isolation system (9), and the laser pulse train is transmitted after passing through the first thin film polarizer (10); the electro-optic modulator (11), the second 1 / 4 wave plate (12), and the first mirror (13) are sequentially arranged on the transmission light path of the first thin film polarizer (10), and the laser pulse train is reflected by the first mirror (13) after passing through the electro-optic modulator (11) and the second 1 / 4 wave plate (12), and then passes through the second 1 / 4 wave plate (12) again, so that the polarization state of the laser pulse train is changed, and the laser pulse train is reflected by the first thin film polarizer (10) after passing through the electro-optic modulator (11); The pump dichroic mirror (16), the laser crystal (17), and the second mirror (19) are sequentially arranged on the reflection light path of the first thin film polarizer (10), the laser pulse train is incident to the laser crystal (17) after passing through the pump dichroic mirror (16), extracts energy at the laser crystal (17), is reflected by the second mirror (19), and then sequentially passes through the laser crystal (17) and the pump dichroic mirror (16) and is reflected by the first thin film polarizer (10); by loading a voltage on the electro-optic modulator (11), the polarization state of the laser pulse train is maintained, so that the laser pulse train oscillates in the regenerative amplifier (24) until the energy of the laser pulse train reaches gain saturation or meets the requirements, and then the voltage loaded on the electro-optic modulator (11) is removed, the laser pulse train passes through the electro-optic modulator (11), the second 1 / 4 wave plate (12), and the first mirror (13) for one round trip, is transmitted by the first thin film polarizer (10), and is then emitted through the second laser isolation system (9).

3. The femtosecond laser of 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 thin film polarizer (14) is the same as the first thin film polarizer (10); The second thin film polarizer (14), the third mirror (15) are sequentially arranged on the reflection light path of the first thin film polarizer (10) and located between the first thin film polarizer (10) and the pump dichroic mirror (16), and the fourth mirror (18) is located between the laser crystal (17) and the second mirror (19).

4. The femtosecond laser of claim 2 or 3, wherein: The laser gain medium used in the laser crystal (17) is an ytterbium-doped laser medium or a neodymium-doped laser medium.

5. The femtosecond laser of claim 4, wherein: The seed light source (1) is a fiber mode-locked ultrashort pulse seed light source or a solid-state mode-locked ultrashort pulse oscillator, and the pulse width is femtosecond level.

6. The femtosecond laser of claim 5, wherein: The pulse stretcher (7) is a chirped volume Bragg grating, a Martin-Nietz-type concentric stretcher, or a bulk material stretcher, which is used to stretch the GHz laser pulse train of femtosecond level to picosecond level.

7. The femtosecond laser of claim 6, wherein: The pulse compressor is a Tracy-type single-grating compressor, a Tracy-type double-grating compressor, a chirped volume Bragg grating, or a grating pair compressor.

8. A method of generating a cluster of GHz high energy laser pulses, characterized by, The femtosecond laser of any one of claims 1-7, comprising the following steps: Step 1: setting the repetition interval of the laser pulse according to the cavity length of the regenerative amplifier (24); setting the polarization state of the laser after passing through the first laser isolation system (6) and the second laser isolation system (9); Step 2: the GHz laser pulse emitted by the seed light source (1) passes through the half-wave plate (2) and the first polarization beam splitter (3), then through the electro-optic pulse selection system (4) by intermittently loading a half-wave voltage, forms a laser pulse train with a certain number of laser pulses and a pulse cluster structure with an envelope, and is then emitted to the first laser isolation system (6) through the second polarization beam splitter (5); Step 3: the laser pulse train passes through the first laser isolation system (6), enters the pulse stretcher (7), and is then stretched by the pulse stretcher (7). After passing through the first laser isolation system (6) again, the polarization state of the laser pulse train changes, and then the laser pulse train is reflected by the second polarization beam splitter (5) and the third polarization beam splitter (8) in turn, passes through the second laser isolation system (9), and is injected into the regenerative amplifier (24) after the polarization state changes; Step 4: the laser pulse train oscillates back and forth in the regenerative amplifier (24) to amplify the pulse energy until the gain saturation is reached or the required condition is met, and then the laser pulse train is extracted from the regenerative amplifier (24) to obtain an amplified laser pulse train; Step 5: the amplified laser pulse train passes through the second laser isolation system (9) again, and the polarization state of the laser pulse train remains unchanged. After passing through the third polarization beam splitter (8) and the first 1 / 4 wave plate (22) in turn, the laser pulse train is introduced into the pulse compression system (23). The pulse compression system (23) restores the laser pulse train to the original pulse width. After passing through the first 1 / 4 wave plate (22) again, the polarization state of the laser pulse train changes, and the laser pulse train is emitted again after being reflected by the third polarization beam splitter (8) to obtain a high-energy laser pulse cluster.

9. The method of claim 8, wherein: in step 3, the GHz laser pulses are femtosecond-level, and a pulse stretching system (7) stretches the laser pulse train to picosecond-level.

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