阅读说明：本技术 用于产生高重复率激光脉冲丛波的装置和方法 (Apparatus and method for generating high repetition rate laser pulse bursts ) 是由 马尔蒂纳斯·巴克斯卡斯 卡里奥利斯·内蒙塔斯 维塔斯·布特库斯 于 2018-04-19 设计创作，主要内容包括：一种用于在激光器装置中产生一系列激光脉冲(特别是单个丛波中的脉冲之间的最小时间距离为皮秒域的单脉冲丛波和多脉冲丛波)的方法和装置。该装置至少包括主振荡器和再生放大器。该方法至少包括以下步骤：将来自主振荡器的激光脉冲注入到再生放大器中,在再生放大器的光腔中多次往返行程期间对注入的脉冲丛波进行放大,从再生放大器的腔中排出放大后的脉冲丛波,其中注入步骤包括向光开关施加第一中间电压持续一段时间,在该一段时间内,将来自振荡器的一个或多个脉冲注入到放大器中,从而形成注入的种子脉冲的丛波,该种子脉冲在放大步骤中被进一步放大,在放大步骤中将光开关电压设置为锁定电压。(A method and apparatus for generating a series of laser pulses in a laser device, particularly single-pulse bursts and multi-pulse bursts with a minimum temporal distance between pulses in a single burst being in the picosecond region. The apparatus includes at least a master oscillator and a regenerative amplifier. The method at least comprises the following steps: injecting laser pulses from a main oscillator into a regenerative amplifier, amplifying the injected pulse bursts during a plurality of round trips in an optical cavity of the regenerative amplifier, discharging the amplified pulse bursts from the cavity of the regenerative amplifier, wherein the injecting step comprises applying a first intermediate voltage to an optical switch for a period of time during which one or more pulses from the oscillator are injected into the amplifier, thereby forming a burst of injected seed pulses, which are further amplified in the amplifying step, and setting the optical switch voltage to a locking voltage in the amplifying step.)

1. A method for generating a series of laser pulses in a laser device comprising at least one master oscillator and a regenerative amplifier, the method comprising at least the steps of:

Injecting laser pulses from the master oscillator into the regenerative amplifier,

Amplifying the injected pulses during multiple round trips in an optical cavity of the regenerative amplifier,

Discharging the amplified pulses from the cavity of the regenerative amplifier, wherein the injecting step comprises applying a first intermediate voltage generated by a first high voltage switch to an optical switch for a time period during which one or more pulses from the oscillator are injected into the amplifier to form a burst-seed burst of injected seed pulses, the seed pulses being further amplified in the amplifying step, and wherein the optical switch voltage is set to a locking voltage generated by the sum of the voltages from the first high voltage switch and a second high voltage switch in the amplifying step.

2. The method of claim 1, wherein the draining step comprises setting the optical switch to a second intermediate voltage at which the optical switch allows the amplified pulse burst to be partially drained out of the regenerative amplifier's cavity on each round trip of the pulse burst, thereby generating a multiple burst.

3. Method according to any of the preceding claims, characterized in that the optical switch is an electro-optical device capable of changing the polarization and/or the phase of the transmitted light, and the locking voltage of the optical switch corresponds to the gate voltage of the electro-optical device, in particular a λ/4 voltage or zero voltage.

4. A method according to any one of claims 1 to 3, wherein the injected pulses in a seed burst are separated in time by a time interval equal to the round trip time of the pulse in the cavity of the master oscillator.

5. A method according to any one of claims 1 to 3, wherein the amplified pulses are spaced apart in a single burst at a time interval which is less than the absolute value of the smallest difference between at least one round trip time of a pulse in the optical cavity of the regenerative amplifier and the time interval between seed pulses generated by the master oscillator.

6.A method according to any of claims 1 to 3, wherein the time interval between amplified laser pulses in a single burst can be adjusted by adjusting the round trip time of an injection pulse in the regenerative amplifier and/or adjusting the time interval between seed pulses of the master oscillator.

7. Method according to claim 6, characterized in that the round trip time of the pulses injected in the regenerative amplifier and/or the time interval between the seed pulses of the master oscillator are changed manually or actively (computer controlled) by adjusting the optical path of the master oscillator and/or at least one branch of the regenerative amplifier.

8. The method according to any of claims 1 to 7, characterized in that the intermediate voltage used in the injecting step is generated by the first fast switching high voltage switch.

9. The method according to any one of claims 1 to 7, wherein the intermediate voltage used in the burst exhaust step is generated by the first or second fast switching high voltage switch.

10. The method of claim 9, wherein the lockout voltage used in the amplifying step is generated by activating or deactivating first and second fast switching high voltage switches.

11. A method according to any one of claims 1 to 10, wherein the amplitude envelope of the pulses in a single burst is controlled by varying the amplitude of the first intermediate voltage.

12. The method of any one of claims 1 to 10, wherein the amplitude envelope of the pulses in a single burst is controlled by providing a slope to the first intermediate voltage.

13. The method according to any one of claims 2 to 12, wherein the amplitude envelope of the multiple plexuses is controlled by varying the amplitude of the second intermediate voltage.

14. The method according to any one of claims 2 to 12, wherein the amplitude envelope of the multiple plexuses is controlled by providing a slope to the second intermediate voltage.

15. The method of any one of claims 1 to 14, wherein the injecting step comprises controlling the duration of the first intermediate voltage, which determines the number of seed pulses injected into the regenerative amplifier and thus the number of pulses in a single burst.

16. The method of any one of claims 1 to 14, wherein the step of draining comprises controlling the duration of the second intermediate voltage, which determines the number of amplified plexuses drained from the regenerative amplifier and thereby the number of plexuses in a multiplicity of plexuses.

17. A laser device comprising at least a master oscillator and a regenerative amplifier, the amplifier comprising a fast optical switch arranged to be switched to at least a locked state, in the lock-in state, a seed pulse is locked in the regenerative amplifier to perform an amplifying step, wherein the laser device comprises two high voltage switches and the optical switch is arranged to be switched to a seed injection state by applying a first intermediate voltage generated by the first high voltage switch, in a seed injection state, the optical switch is partially transmissive and allows more than one seed pulse into the cavity of the regenerative amplifier, and the second high voltage switch is arranged to provide a voltage such that the sum of the voltages provided by the first and second high voltage switches equals the locking voltage of the optical switch.

18. A laser device as claimed in claim 17, wherein the optical switch is further arranged to be switched to a second intermediate state in which it is partially transmissive and allows multiple bursts of waves to be expelled from the cavity of the regenerative amplifier.

19. A laser device as claimed in claim 17, wherein the optical switch is arranged to be switched to an off state in which it is fully transmissive and allows a single pulse burst to be emitted.

20. A laser device as claimed in claims 17 to 19 wherein the amplified pulses are spaced apart in a single burst at a time interval which is less than the absolute value of the smallest difference between at least one round trip time of a pulse in the optical cavity of the regenerative amplifier and the time interval between seed pulses generated by the master oscillator.

21. a laser device as claimed in claims 17 to 20, wherein the optical switch is a pockels cell.

22. A laser device as claimed in claim 21, wherein the pockels cell comprises two separate independently controlled electro-optical cells.

23. A laser device as claimed in claim 21, wherein the pockels cell comprises one optical component and two independently controlled high voltage switching electronic units.

24. A laser device as claimed in claim 21, wherein the pockels cell comprises at least two optical components and two separate independently controlled high voltage switching electronic units.

25. a laser device as claimed in claims 17 to 24 wherein the optical switch is arranged to control the amplitude envelope of the pulses in a single burst by controlling the amplitude of the first intermediate voltage.

26. A laser device as claimed in claims 17 to 24 wherein the optical switch is arranged to control the amplitude envelope of the pulses in a single burst by providing a slope to the first intermediate voltage.

27. A laser device as claimed in claims 17 to 26, wherein the optical switch is arranged to control the amplitude envelope of the multiple plexuses by controlling the amplitude of the second intermediate voltage.

28. A laser device as claimed in claims 17 to 26, wherein the optical switch is arranged to control the amplitude envelope of the multiple plexus by providing a slope to the second intermediate voltage.

29. A laser device as claimed in any one of claims 17 to 28, wherein the optical switch is arranged to control the number of seed pulses injected into the regenerative amplifier by controlling the duration of the first intermediate voltage, thereby controlling the number of pulses in a single burst.

30. A laser device as claimed in any of claims 17 to 29, wherein the optical switch is arranged to control the number of amplified bursts emerging from the regenerative amplifier from a multiplicity of bursts by controlling the duration of the second intermediate voltage.

31. A laser device as claimed in claims 17 to 30, characterized in that the laser device is arranged for laser material processing.

32. A laser device as claimed in claims 17 to 30, characterized in that the laser device is arranged for laser-based spectroscopy.

33. A laser device as claimed in claims 17 to 30, characterized in that the laser device is arranged for medical treatment.

Technical Field

The present application relates to a method of generating a sequence of laser pulses, in particular a series of laser pulse bursts, by using electro-optical control in a laser system comprising a master oscillator and a regenerative amplifier.

Background

The use of lasers has become critical in many areas of industrial micromachining, science, and medicine. Various techniques for using laser-generated light in a wide range of applications are being intensively developed. Advantages of laser-assisted welding, cutting, marking, etc., include, but are not limited to, high speed and high precision, as compared to conventional mechanical techniques, so that simple or no post-processing of the material is possible.

In any of the mentioned and related laser-assisted techniques, the desired mechanical change is due to a related physical phenomenon that occurs after the deposition of optical energy to the material. During this ablation process, the solid phase material is converted to the gas phase and vaporized. The present state of the art laser systems provide different methods of laser generation and delivery, with their own advantages and disadvantages, which are well known to those skilled in the art.

One of the most common problems faced by laser assisted micromachining techniques is related to the heating of the pulse affected area, which may cause collateral damage. Material can be removed accurately using femtosecond laser pulses rather than picoseconds or longer, reducing damage due to localized heating.

For faster material processing, a powerful output laser radiation is required. However, the technique using the high power laser has a shielding effect. They are caused by plasma on the surface of the material, which leads to attenuation of subsequent light pulses due to defocusing, reflection, scattering, etc. Similarly, nonlinear effects also become important for high energy interactions. In this case, the ablation rate may be severely reduced. High repetition rate techniques are advantageous due to increased processing speeds and reduced thermal effects.

Material processing by bursts of laser pulses is a relatively new method (see, for example, patent US6552301(Herman et al, 2003)). Bursts of laser pulses are also referred to as finite length pulse trains, multiple beams, or large pulses. In general, a laser pulse burst is defined as two or more laser pulses that are equally separated in time. The time interval between successive pulses is called the inter-burst pulse separation, and the measurement opposite to the inter-burst pulse separation is called the intra-burst frequency.

From here we refer to the following operation, where the pulses in the burst are separated by a time: from 1 mus to 1ms as kilohertz (kHz) bursts, from 1ns to 1 mus as megahertz (MHz) bursts, and from 1 mus to 1ns as gigahertz (GHz) bursts.

When pulsed bursts are repeatedly generated at the burst frequency, the laser system may be operated in a continuous burst mode, the burst frequency being reciprocal of the time interval between the start of two adjacent bursts and less than the intra-burst frequency.

the laser system can operate in a burst-on-demand mode when the burst of pulses is fired at any time triggered either manually by the end user or by the electronic device driver.

Individual pulse lengths, intra-burst frequencies, and burst frequencies must be considered in characterizing the different burst modes and their performance in material processing.

For example, in Kerse et al, an exhaustive discussion of how to increase ablation rate by several orders of magnitude and reduce thermal effects in the laser burst mode can be found (Kerse et al 2016). Very promising results are obtained by replacing a single high-energy laser pulse with a GHz burst of low-energy pulses, while keeping the average power of the fiber laser radiation constant. The low energy pulses do not cause a shadowing effect but are closely spaced in time so that locally heated material is removed before thermal diffusion occurs.

Zimmermann et al demonstrated laser welding of megahertz bursts of low intensity femtosecond pulses to achieve a combined glass fracture stress of up to 96% uncut glass (Zimmermann et al, 2013). In the megahertz burst mode of high intensity femtosecond pulses, deeper holes were produced in BK7 glass compared to the single pulse mode (Rezaei, Li and Herman, 2015). The laser system operates in a picosecond pulsed kHz or MHz burst mode, performing significantly faster copper ablation rates than a single pulse at the same fluence (Hu, Shin and King, 2010). Simulations confirm this result, indicating that for high intra-burst frequencies, the cumulative energy not dissipated between pulses is critical for fast operation. Neuenschwander et al reported similar results with respect to copper ablation, but no increase in steel removal rate was observed in the same protocol (Neuenschwander et al, 2015).

Zhang et al have demonstrated that the use of laser pulse bursts may also be advantageous in the fabrication of bragg grating waveguides (Zhang, Eaton and Herman 2007).

The femtosecond pulsed MHz burst was shown to form more effective filamentary lesions in soda-lime glass under defocused conditions (Deladurantaye et al, 2011).

Biological tissue treatment with laser pulses in burst mode is advantageous because heat transfer to adjacent soft tissue can be minimized and collateral damage avoided (Forrester et al, 2006). It has also been shown that under laser treatment, cell necrosis depends on pulse intensity, rather than burst length, and therefore a safer treatment can be achieved (Qian et al, 2014).

State of the art laser systems include a master oscillator and a regenerative amplifier that generate bursts of high intensity laser pulses and utilize several techniques.

In some cases of the prior art, a burst of laser pulses is generated prior to amplification and then used as a seed pulse for the amplifier. The apparatus and driving method of the amplifier need not be any special and may be well known from the prior art (see for example patents US7649667(Bergmann et al, 2010), US9306370(Danielius 2016), US7016107(Kafka et al 2004) or US6882469(Tamaki et al 2005)).

For example, patent US9246303(Rockwell et al, 2016) discloses a method and apparatus for switching optical pulses provided by a master oscillator, generating a burst in a preamplifier gain medium, and providing such a shaped seed signal to a power amplifier; patent US9431436(Noh et al, 2016) discloses an apparatus and a method for generating unamplified laser pulse bursts by combining the irradiation of a pulsed and continuous laser source, followed by selective amplification and frequency conversion by non-linear elements. One of ordinary skill in the art will recognize that according to these and similar methods, the frequency within the burst is fixed and equal to the repetition rate of the pulses provided by the solid state master oscillator, i.e., on the order of tens of megahertz.

Patent US8798107(Deladurantaye et al, 2004) discloses an apparatus and method for generating picosecond seed burst pulses by applying periodic phase modulation and performing pulse selection by means of a pulse picker. In the given embodiment, the maximum intra-burst frequency is in the MHz range.

In some cases of the prior art, bursts of laser pulses are generated in a separate apparatus (the burst generator) after pulse amplification, wherein the pulse generation and amplification is performed by any kind of laser system known in the prior art.

For example, patent EP2250714(Hosseini et al, 2015) discloses an apparatus and method for generating laser bursts from amplified and stretched pulses. In a given embodiment, the burst frequency is fixed and equal to the repetition rate of the pulses provided by the laser system itself, and the intra-burst frequency is fixed and equal to the reciprocal optical period (tens of nanoseconds, i.e., megahertz burst mode) within the disclosed device.

Patent application US9525264(Courjaud 2016) discloses a method and system for generating a laser burst in which more than one pulse from a master oscillator is trapped in a multi-channel regenerative amplifier. After amplification, bursts of pulses are released, the number of pulses being related to the optical path of the amplifier, and the frequency within the bursts being fixed and equal to the repetition rate of the pulses provided by the master oscillator.

In some cases of the prior art, bursts of laser pulses are generated by injecting a single seed pulse from a master oscillator into an amplifier, which is then amplified in a gain medium. After a period of time, the pulse is partially extracted from the amplifier; the remaining part is further enlarged and repeatedly partially extracted, for example, in the next round trip. In this way, a burst of amplified laser pulses is generated.

The injection, trapping and ejection of pulses inside the amplifier can be done by controlling and switching the polarization of the pulses within the amplifier cavity. For this reason, at least one polarization switching device (e.g. a pockels cell) is placed in the cavity within the optical path of the light beam and its driving voltage is varied with time (e.g. US7649667(Bergmann et al, 2010), US9306370(Danielius 2016), US6882469(Tamaki et al, 2005).

Patent US9531151(Fuchs et al 2016) discloses a method of switching a single pockels cell within the cavity of a regenerative amplifier, such that a single pulse from a master oscillator is captured within the cavity of the regenerative amplifier for multi-pass amplification, and a plurality is further expelled by applying a particular sequence of voltage changes to the pockels cell. This method allows the generation of a laser burst whose frequency can be adjusted by the pockels cell driver voltage and whose frequency within the burst is fixed and equal to the reciprocal duration of one pulse period within the regenerative amplifier cavity (from 10ns to 200ns, i.e. in the megahertz range).

In all known laser burst generation devices and methods known in the art (including the master oscillator and the regenerative amplifier), the highest intra-burst frequency is related to the pulse generation frequency in the master oscillator or the round trip time of the pulses in the regenerative amplifier. Since the master oscillator operates at a frequency of several tens of MHz and the round-trip cycle time of the pulses in the regenerative amplifier is longer than 10ns, none of the laser burst generation devices and methods known in the art, which comprise a master oscillator and a regenerative amplifier, allow the generation of laser bursts with a frequency higher than several hundred MHz within a burst.

Disclosure of Invention

The implementation of the present invention takes the above circumstances into consideration and it is an object of the present invention to provide a method for generating bursts of laser pulses at a frequency within any burst in the GHz and THz ranges.

To achieve this object, a laser system includes a master oscillator and a regenerative amplifier, and a pulse generation period of the master oscillator and a laser pulse round trip time of the regenerative amplifier satisfy a certain relationship condition. The regenerative optical amplifier according to the present embodiment includes a resonator for amplifying light injected through an electro-optical switching system including at least one pockels cell; the method of operation of the electro-optical system enables the injection, locking/amplification and ejection of laser pulses by a process consisting of at least three stages corresponding to the injection of several laser pulses into the regenerative amplifier, their locking and amplification during the cycles in the regenerative amplifier and their ejection as part of the laser burst.

The seed pulses are generated by a master oscillator, the pulses being supplied with a time interval of period tau_OSCThe period is at least several nanoseconds to several tens of nanoseconds. The polarization state of the seed pulse is such that the pulse is transmitted completely through the polarization-selective optical element before the regenerative amplifier and injected into the cavity of the regenerative amplifier. Regenerating the cycle time (round trip time tau) of a laser pulse in an amplifier cavity_RA) In relation to its optical path and is typically a few nanoseconds to a few tens of nanoseconds. In this embodiment, τ may be adjusted by mechanically shortening or lengthening the optical path within the regenerative amplifier and/or oscillator_OSCAnd τ_RA。

Pulse injection is achieved by setting the electro-optical system of the regenerative amplifier to some intermediate state (intermediate voltage), which results in a partial transfer of the seed laser pulse from the master oscillator to the cavity of the regenerative amplifier. For pulses that have been injected into the cavity of the regenerative amplifier, they will be partially reflected back into the cavity and partially drained out by transmission through the polarization-selective optical element. As a result, during this phase, only a portion of the seed pulse energy is left circulating within the regenerative amplifier, while the remainder is original and drained from the regenerative amplifier.

At the same time, the seed pulses are continuously injected into the regenerative amplifier. Dependent on round trip time tau in regenerative amplifiers_RAAnd a seed pulse separation period tau_oscThe pulses newly injected into the regenerative amplifier are added before or after one or more pulses that have circulated in the cavity.

In this way, a laser pulse train is formed within the regenerative amplifier. The time interval delta tau between the pulses in the pulse train is then

Δτ＝|τ_oscNτ_RA|.

Where N is an integer greater than or equal to 1 to ensure

Δτ＜τ_RA.

round trip time tau in regenerative amplifiers_RAAnd a seed pulse separation period tau_oscSimilarly (i.e. less than 1 nanosecond apart), N is 1 and the time interval between successive pulses in the cyclic pulse train in the regenerative amplifier is

Δτ＝|τ_osc-τ_RA|.

By setting the electro-optical system of the regenerative amplifier to a state that does not cause any change in the polarization of the laser pulses (twice through the branch of the regenerative amplifier containing the electro-optical system and any other pulse polarization changing element), locking of the regenerative amplifier and amplification of the pulses can be achieved. This is equivalent to setting the voltage of the pockels cell to a lambda/2 voltage if the pockels cell is the only pulse polarization implementing element in the branch, and to a lambda/4 voltage if the branch additionally contains a quarter wave plate.

In such a mode of operation, any seed pulse supplied to the regenerative amplifier leaves the cavity after two passes in the branch containing the polarization-enforcing element, without amplification. The pulses injected into the regenerative amplifier during the injection phase remain cycled and further amplified when the regenerative amplifier is locked by the pulses supplied by the master oscillator.

The draining of pulses from the regenerative amplifier is achieved by setting the electro-optical system of the regenerative amplifier to some intermediate state, which causes the laser pulses within the regenerative amplifier to be transmitted only partially through the polarization-selective optical element at each round trip. As a result, a portion of the pulse energy remains circulating in the regenerative amplifier and is further amplified, while the remainder is drained from the regenerative amplifier with a burst of laser pulses, the intra-burst frequency f being equal to the inverse of the time interval between pulses circulating in the regenerative amplifier. Namely, it is

In contrast to the laser burst approach known in the prior art, in this embodiment, the frequency within the laser burst is related to the relative times of the round trip and seed pulse separation in the regenerative amplifier, respectively, rather than the fixed repetition rate of the master oscillator. Those of ordinary skill in the art will recognize that by adjusting the geometric parameters, e.g., cavity length of the regenerative amplifier and/or seed oscillator, the value of Δ τ may be adjusted to less than nanoseconds, thereby providing f in the range from several GHz to tens of GHz_in-burst(frequency within the burst).

Drawings

Fig. 1a to 1d show schematic diagrams of a laser system comprising a regenerative amplifier according to a given embodiment;

FIGS. 2 a-2 b illustrate the process of injection, lock/amplify and drain according to a given embodiment by a switching sequence of a polarization switching device;

Fig. 3a to 3c show a method of switching pockels cells with time dependence of the voltage provided by the respective high voltage driver;

4 a-4 b show calculated results of an exemplary operation of an electro-optical system for generating a laser burst;

FIG. 5 shows measured optical signals for an exemplary operation of an electro-optical system for generating a laser burst;

Attached drawings-reference numerals

110 laser system

112 master oscillator

114 polarizing beam splitter

116 half-wave plate

118 faraday isolator

120 reflecting mirror

122 mirror

124 first branch

126 second branch

128 polarization beam splitter

130 polarization switching device

132 gain medium

134 output

136 quarter wave plate

138 electro-optical cell

140 control element

142 quarter wave plate

144 electro-optical cell

146 electro-optical cell

148 control element

150 control element

152 quarter wave plate

154 two-crystal pockels cell

156 control element

158 control element

210 voltage time dependence

212 first time interval, corresponding to the injection of n pulses

214 second time interval corresponding to amplification of 1 burst

216 operating phase, corresponding to the discharge of 1 plexus

218 pulse time dependence of the discharge

220 voltage time dependence

222 first time interval corresponding to the implantation of n pulses

224 second time interval corresponding to amplification of 1 burst

226 third time interval, corresponding to discharge of m bursts

228 pulse time dependence of the discharge

Detailed Description

In this context and further we will use a term like "master oscillator", which means a mode locked short pulse laser with a fixed pulse repetition rate (typically in the tens of MHz range). Unless the oscillator includes means for actively changing the length of the optical cavity.

The term "RA" refers to regenerative amplifiers.

"seed pulse" refers to a plurality of pulses emitted by a master oscillator, wherein the temporal distance (e.g., for a master oscillator with an 80MHz pulse repetition rate) is in the nanosecond range; "seed burst" refers to a collection or burst of injected seed pulses that may be injected into the regenerative amplifier cavity but not yet amplified, the temporal distance between the pulses of the seed burst being in the picosecond range, since the pulses may be injected into the regenerative amplifier and added to the seed burst that has been circulated in a different round trip of the burst within the RA cavity.

The term "PC" refers to pockels cells.

single burst-i.e. a burst of pulses amplified from a seed burst.

The multiple-single cluster set is ejected one after another on different round trips of the amplified cluster within the RA cavity. .

description-FIGS. 1a to 1d

Fig. 1a to 1d show schematic diagrams of embodiments of a laser system 110, comprising: a seed pulse generator or master oscillator 112; a first polarization-selective element 114, such as a first polarizing beamsplitter; a first polarizing element 116, such as a half-wave plate; a polarization rotator 118, such as a Faraday isolator; a regenerative amplifier cavity confined between at least two reflective elements (e.g., mirrors 120 and 122); the regenerative amplifier cavity further includes at least two branches 124 and 126 separated by a second polarization-selective element 128 (e.g., a second polarization beam splitter); the first branch 124 further comprises a mirror 120 and a polarization switching device 130; the second branch 126 also includes a mirror 122 and a gain medium 132.

The gain medium 132 may be embodied, for example, as an ytterbium-or neodymium-doped laser medium (e.g., Yb: KYW, Yb: KGW, or Nd: YVO 4). These materials are provided as examples only. The choice of gain medium should not limit the scope of the invention.

The polarization switching means 130 comprises at least one electro-optical cell, preferably a pockels cell, and a corresponding voltage driving circuit, and may comprise one or more polarization elements, such as quarter-wave plates or half-wave plates.

Possible embodiments of the electro-optical device are shown in fig. 1b to 1 d.

Fig. 1b shows an embodiment of a polarization switching device known in the prior art. It comprises a quarter wave plate 136 and an electro-optical unit 138, e.g. a pockels cell, and its control elements (drivers) 140. The pockels cell may be a standard commercially available electro-optical component, while the driver is not a standard electronic component and its working principle is known from the prior art (US patent US 9531151).

Fig. 1c shows a possible embodiment of a polarization switching device, which can be applied to the present invention. It comprises a quarter wave plate 142 and two electro-optical cells 144 and 146 (e.g. pockels cells) and their control elements 148 and 150. In this embodiment, the pockels cell may be a standard commercially available electro-optical component, and the corresponding driver may be a standard commercially available high voltage switch.

Fig. 1d shows an alternative embodiment of a polarization switching device, which can be applied to the present invention. It includes a quarter wave plate 152, a two-crystal pockels cell 154, and two control elements 156 and 158. In this embodiment, the pockels cell is a non-standard electro-optical element, comprising two non-linear crystals, to which a voltage can be supplied separately. Alternatively, there may be a single nonlinear crystal with appropriate contacts (typically 4 contacts) for connecting the two high voltage switches. The corresponding control element used in this arrangement may be a standard commercially available high voltage switch.

In any of these embodiments, the optical switch may be any electro-optical device capable of changing the polarization and/or phase of transmitted light, and the locking voltage of the optical switch corresponds to the gate voltage of the electro-optical device, in particular a λ/4 voltage or zero voltage.

operation-FIGS. 1a to 1d

For simplicity, herein and further, the polarization of the input seed pulse will be referred to as "p-polarization"; the polarization that is perpendicular to the seed pulse polarization will be referred to as "s-polarization". Those skilled in the art will recognize that the principles of operation of this and other embodiments are independent of the assumed polarization state of the seed pulse polarization.

The first polarizing beam splitter 114, positioned between the master oscillator 112 and the quarter wave plate 116, is oriented such that it will transmit the p-polarized seed pulses arriving from the side of the master oscillator 112 and will reflect the s-polarized pulses arriving at the output 134 from the opposite side.

The laser pulse further passes through a half-wave plate 116 and a faraday isolator 118, which causes the polarization state of the pulse to switch to s-polarization. A second polarizing beamsplitter 128, oriented such that it will transmit s-polarized light, further transmits the pulse into the first branch 124 of the resonator cavity.

In the first branch, the pulse passes through the polarization switching device 130, is reflected from the mirror 120, and passes through the polarization switching device a second time before reaching the second polarization beam splitter 128. Since this polarizing beam splitter is oriented to transmit the s-polarized pulse towards the first polarizing beam splitter 114 and reflect the p-polarized pulse to the second branch 126 of the regenerative amplifier cavity, the transmittance/reflectance of the pulse at this point is completely defined by the mode of operation of the polarization switching device 130.

When polarization switching device 130 is operated in a mode that changes the linear polarization (i.e., p-polarization or s-polarization) of the pulse to circular polarization (i.e., operates as a single quarter wave plate), two passes through this element will result in the polarization of the pulse being switched from s-polarization to p-polarization. In this case, the pulse is reflected from the second beam splitter and directed to the second branch 126 of the regenerative amplifier cavity. During two passes of this branch, the pulse is amplified twice in the gain medium 132 and then reflected again in the polarizing beam splitter 128 to the first branch. In the first branch, the polarization state of the pulse is switched back to p-polarization again. The pulse is then transmitted through a polarizing beam splitter. Its polarization is then switched to s-polarization in faraday isolator 118 and half-wave plate 116. Finally, after reflection from the first polarizing beam splitter 114, the pulse is extracted from the laser system. Since each pulse is amplified only twice before being discharged, the mode of operation is indicated as inactive.

When the polarization switching device 130 is operated in a mode that switches the polarization state of the pulse from p-polarization to s-polarization (i.e., operates as a single half-wave plate), two passes through this element results in no change in the polarization of the pulse. Then, if the s-polarized seed pulse is injected into the first branch 124, it is transmitted through the second polarization beam splitter 128 later without amplification. With respect to the seed pulse, the regenerative amplifier operates in an inactive mode. However, if the pulse arrives from the second branch 126 (i.e. p-polarized), it is not transmitted through the second polarizing beam splitter 128, but is locked in the regenerative amplifier cavity. Thus, this mode of operation is denoted as locked.

When the polarization switching device 130 operates in a mode that converts the polarization state of the pulse to an intermediate state, reflection and refraction of the pulse of arbitrary polarization occurs at the second polarization beam splitter 128. That is, at each round trip, a portion of the pulse is locked into the regenerative amplifier cavity and a portion is extracted from the regenerative amplifier cavity. At the same time, a portion of any incoming seed pulse is also injected into the regenerative amplifier cavity. This mode of operation is denoted as intermediate, wherein the electro-optical cell is supplied with an intermediate switching voltage.

In other words, the injecting step involves applying the first intermediate voltage to the polarization switching device for a period of time during which one or more pulses from the master oscillator are injected into the regenerative amplifier, thereby forming a burst-seed burst of injected seed pulses. Such a sub-burst is further amplified in an amplification step in which the polarization switching device voltage is set to a locking voltage.

In yet another embodiment, the pulses from the master oscillator have a repetition rate in the tens of MHz range, and only some pulses are picked up by the additional electro-optical pulse picking means and injected into the RA chamber at the appropriate time to create a seed burst with the desired time pattern.

description-FIGS. 2a to 2b

Fig. 2 a-2 b illustrate the behavior of a laser system according to an embodiment, injection, amplification and ejection of any number of laser bursts with any number of pulses in a single burst.

operation-FIGS. 2 a-2 b

Referring to fig. 2a, a method of generating a single laser pulse burst is shown. On the left side, the time dependence of the voltage 210 applied to the electro-optical cell 130 of a given embodiment is shown. At a duration of T₁by applying a first intermediate switching voltage, the polarization switching device 130 is set to an intermediate operating mode and n-T is set to the intermediate operating mode during the first time interval 212₁/τ_OSCLaser pulses are injected into the regenerative amplifier. At a duration of T₂During a second time interval 214, the polarization switching device is set to the lock-in mode and the previously injected pulse is amplified, the temporal distance between successive pulses within the cavity being:

Δτ＝|τ_osc-Nτ_RA|.

Where N is an integer greater than or equal to 1 to ensure that Δ τ < τ_RA. Finally, the driving voltage is closed, and the polarization switching unit is switched to an invalid mode; this operational phase 216 corresponds to the ejection of a single burst of n pulses. The intra-burst frequency f at which the burst is generated is 1/Δ τ, as shown in fig. 2a, and the right side is the pulse time dependence 218 of the ejection.

In fig. 2b, a method of generating a plurality of bursts or bursts of laser pulses is shown. On the left side, the time dependence of the voltage 220 applied to the electro-optical cell of a given embodiment is shown. At a duration T₁By applying a first intermediate switching voltage, the polarization switching device 130 is set to an intermediate mode of operation, and n-T₁/τ_oscLaser pulses are injected into the regenerative amplifier cavity. At a duration T₂During a second time interval 224, the polarization switching device 130 is set to the lock-in mode by applying a lock-in voltage and the previously injected pulse is amplified. The time distance between successive pulses in the cavity is Δ τ ═ τ_osc-Nτ_RA|.

Here, N is an integer of 1 or more to ensure that Δ τ < τ_RA. At a duration T₃By applying a second intermediate voltage during a third time interval 226, the polarization switching device 130 is set to a second intermediate mode, which results in each round trip in the RA cavity partially discharging a burst of amplified laser pulses, thus forming a multiple burst. The number of clusters m generated is defined as m ═ T₃/τ_RA(ii) a The bursts being separated by tau_RAAnd the frequency in the burst is f 1/Δ τ, as shown in fig. 2b, with the discharged pulse time dependence 228 on the right.

In other words, in one embodiment, the step of draining includes setting the polarization switching device to a second intermediate voltage at which the optical switch allows the amplified pulse burst to be partially drained out of the cavity of the regenerative amplifier on each round trip of the pulse burst, thereby generating a multiple burst.

Description-figures 3a to 3c

Fig. 3a to 3c show the time dependence of the voltage provided by the respective high voltage driver.

In fig. 3a, the time dependence of the total voltage supplied to the entire polarization switching device 130 is shown in the first row. In the second and third row, the time dependence of the voltage provided by the first and second control elements (fast high voltage switches) 148 and 150 in the embodiment shown in fig. 1c is shown. For an alternative embodiment, shown in FIG. 1d, the corresponding high voltage switches are 156 and 158.

Referring to fig. 3b to 3c, an alternative time dependence of the supplied voltage is shown.

operation-FIGS. 3a to 3c

The pockels cell switching time dependency detailed in fig. 3a corresponds to the operation mode of single plexus generation previously disclosed and schematically shown in fig. 2 a.

The pockels cell switching time dependence detailed in fig. 3b and 3c corresponds to an alternative method of multiple plexus generation, previously disclosed and schematically shown in fig. 2 b.

At the position of FIG. 3bIn the case of the illustrated switching mode (intermediate switching voltage), the state of the polarization switching unit 130 during the injection phase 222 and the exhaust phase 226 is equal and corresponds to the voltage u provided by one of the active control elements (fast high voltage switch)₁。

In the case of the switching mode shown in FIG. 3c, the state of the polarization switching device 130 during the injection phase 222 and the exhaust phase 226 is different; the state during the injection phase 222 corresponds to the voltage U provided by one of the active drivers₁The state during the exhaust phase 226 corresponds to the voltage u₂Which is equal to the voltage of lambda/4 and u₁The difference between them.

Description-figures 4a to 4b

Fig. 4 a-4 b show calculated results of an exemplary operation of an electro-optical system for generating a laser burst.

In fig. 4a, the time dependence of the voltage applied to the polarization switching device 130 (i.e., the PC voltage), the energy at the gain medium 132, and the pulse energy at the output 134 is shown on the left, which shows a scheme of a single gigahertz burst generated by a laser pulse according to the scheme shown in fig. 2 a. The right side shows the time-dependent amplification region with laser pulses in the single cluster

The diagram of fig. 4b is similar to fig. 4a, showing a multiple gigahertz burst generation scheme in accordance with the scheme shown in fig. 2 b.

Operation-figures 4a to 4b

the time dependence of the laser pulse burst output shown in fig. 4 a-4 b is obtained by theoretically simulating seed pulse amplification in the laser system 110. It is assumed that the seed pulses generated by the master oscillator 112 each have an energy of 75.0nJ, and their loss-causing multiple interactions with the gain medium 132 and other optical elements are calculated. During each round trip of the regenerative amplifier, the energy amplification of each pulse at the gain medium is simulated using the Frantz-Nodvik equation,

Here E_satIs the saturation energy of the gain medium, set to 2 muJ, E_inIs the energy of the incident pulse, E_stIs the energy stored at the gain, consumed at each interaction, as shown below

Where t is the time between successive interactions and τ is the fluorescence lifetime of the gain medium, set at 500ns, G₀Is the gain, set to 2.

The injection, amplification and ejection of pulses in the regenerative amplifier is achieved by the operation of the polarization switching device 130 and the polarization beam splitter 128. Here, by assuming that the time-dependent transmission coefficient at the polarizing beam splitter 128 corresponds to the PC voltage u₁、u₂And u_λ/4To simulate the operation of these elements. In each round trip of the regenerative amplifier, 5% of the extra loss caused by the passive optical element is included.

The generation of the seed pulses in the regenerative amplifier and their propagation are simulated by assuming that the optical path in the master oscillator is equal to the time interval τ osc between successive seed pulses of 15.6ns and the round trip time τ RA in the regenerative amplifier of 15.8 ns. With this arrangement, bursts of laser pulses are generated with a pulse spacing of 200ps, corresponding to an intra-burst frequency of 5GHz

description-FIG. 5

Fig. 5 shows the measured optical signal of an electro-optical system for generating a laser burst.

the left side shows the light output of a single burst consisting of 11 femtosecond pulses. The right side shows the light output of a single burst consisting of 4 femtosecond pulses.

operation-FIG. 5

the spectrogram of fig. 5 was obtained by focusing the optical output of the electro-optical system onto a fast photodiode, and the electrical signal was measured with an oscilloscope.

In yet another embodiment, the time interval between amplified laser pulses in a single burst can be adjusted by adjusting the round trip time of the injected pulse in the regenerative amplifier and/or the time interval between seed pulses of the master oscillator. This can be achieved by manually or actively (computer controlled) altering the RA and/or the cavity length of the master oscillator. For example, in RA, this may be done by adjusting the optical path in at least one branch of the regenerative amplifier.

In yet another embodiment, the amplitude envelope of the pulses in the single burst is controlled by varying the amplitude of the first intermediate voltage. In addition, the slope of the first intermediate voltage (in other words, at time T)₁Gradual increase/decrease of voltage during) may be used to shape the amplitude envelope of the pulses in the single burst.

In yet another embodiment, the amplitude envelope of the multiple plexor waves is controlled by varying the magnitude of the second intermediate voltage. In addition, the slope of the second intermediate voltage (in other words, at time T)₃Gradual increase/decrease of voltage during) may be used to shape the amplitude envelope of the burst in the multiple burst.

Such a laser device as described above may be installed in a laser system dedicated to precision material processing, medical treatment, or for time-resolved spectroscopy.

The claims (modification according to treaty clause 19)

1. A method for generating a series of laser pulses in a laser device comprising at least one master oscillator and a regenerative amplifier, the method comprising at least the steps of:

Injecting laser pulses from the master oscillator into the regenerative amplifier,

Amplifying the injected pulses during multiple round trips in an optical cavity of the regenerative amplifier,

Draining the amplified pulses from the cavity of the regenerative amplifier,

wherein the injecting step comprises applying a first intermediate voltage directly to an electrode of an optical switch, wherein the intermediate voltage is adjustable in a range between a locking voltage for which the seed pulse is locked in the regenerative amplifier for an amplifying step, and an off state in which the seed pulse can be injected into the regenerative amplifier and the amplified pulse can be drained without loss, the intermediate voltage being generated by a first high voltage switch for a period of time during which a plurality of one pulse from the master oscillator is injected into the cavity of the regenerative amplifier and circulates in its partial intensity in the cavity of the regenerative amplifier, thereby forming a burst-seed burst of injected seed pulses, wherein the pulse frequency within the injected seed burst exceeds the pulse frequency of the master oscillator, the amplifying step includes applying a second voltage generated by a second high voltage switch, wherein the optical switch voltage is set to a locking voltage generated by a sum of voltages from the first high voltage switch and the second high voltage switch.

2. The method of claim 1, wherein the step of discharging comprises: setting the optical switch to a second intermediate voltage at which the optical switch allows the amplified seed bursts to be partially expelled in the cavity of the regenerative amplifier on each round trip of the pulse burst, thereby generating a multiple burst.

3. Method according to any of the preceding claims, characterized in that the optical switch is an electro-optical device capable of changing the polarization and/or the phase of the transmitted light, and the locking voltage of the optical switch corresponds to the gate voltage of the electro-optical device, in particular a λ/4 voltage or zero voltage.

4. A method according to any one of claims 1 to 3, wherein the injected pulses in a seed burst are separated in time by a time interval equal to the round trip time of the pulse in the cavity of the master oscillator.

5. A method according to any one of claims 1 to 3, wherein the amplified pulses are spaced apart in a single burst at a time interval which is less than the absolute value of the smallest difference between at least one round trip time of a pulse in the optical cavity of the regenerative amplifier and the time interval between seed pulses generated by the master oscillator.

6.A method according to any of claims 1 to 3, wherein the time interval between amplified laser pulses in a single burst can be adjusted by adjusting the round trip time of an injection pulse in the regenerative amplifier and/or adjusting the time interval between seed pulses of the master oscillator.

7. Method according to claim 6, characterized in that the round trip time of the pulses injected in the regenerative amplifier and/or the time interval between the seed pulses of the master oscillator are changed manually or actively (computer controlled) by adjusting the optical path of the master oscillator and/or at least one branch of the regenerative amplifier.

8. The method according to any of claims 1 to 7, characterized in that the intermediate voltage used in the injecting step is generated by the first fast switching high voltage switch.

9. The method according to any one of claims 1 to 7, wherein the intermediate voltage used in the burst exhaust step is generated by the first or second fast switching high voltage switch.

10. The method of claim 9, wherein the lockout voltage used in the amplifying step is generated by activating or deactivating first and second fast switching high voltage switches.

11. A method according to any one of claims 1 to 10, wherein the amplitude envelope of the pulses in a single burst is controlled by varying the amplitude of the first intermediate voltage.

12. The method of any one of claims 1 to 10, wherein the amplitude envelope of the pulses in a single burst is controlled by providing a slope to the first intermediate voltage.

13. The method according to any one of claims 2 to 12, wherein the amplitude envelope of the multiple plexuses is controlled by varying the amplitude of the second intermediate voltage.

14. The method according to any one of claims 2 to 12, wherein the amplitude envelope of the multiple plexuses is controlled by providing a slope to the second intermediate voltage.

15. The method of any one of claims 1 to 14, wherein the injecting step comprises controlling the duration of the first intermediate voltage, which determines the number of seed pulses injected into the regenerative amplifier and thus the number of pulses in a single burst.

16. The method of any one of claims 1 to 14, wherein the step of draining comprises controlling the duration of the second intermediate voltage, which determines the number of amplified plexuses drained from the regenerative amplifier and thereby the number of plexuses in a multiplicity of plexuses.

17. A laser device comprising at least a master oscillator and a regenerative amplifier, the regenerative amplifier comprising a fast optical switch arranged to be switched to at least a locked state in which more than one seed pulse is locked in the regenerative amplifier for an amplifying step, wherein the optical switch is arranged to be switched by a first high voltage switch having an intermediate voltage to a state in which the optical switch is partially transmissive and allows more than one seed pulse to enter a cavity of the regenerative amplifier, wherein the optical switch is arranged to be switched by a second high voltage switch having the locking voltage corresponding to the locked state of the optical switch and the locked state of the regenerative amplifier.

18. A laser device as claimed in claim 17, wherein the optical switch is further arranged to be switched to a second intermediate state in which it is partially transmissive and allows a plurality of the amplified seed bursts to be expelled from the cavity of the regenerative amplifier.

19. A laser device as claimed in claim 17, wherein the optical switch is arranged to be switched to an off state in which it is fully transmissive and allows a single amplified said seed burst to be expelled.

20. A laser device as claimed in claims 17 to 19 wherein the amplified pulses are spaced apart in a single burst at a time interval which is less than the absolute value of the smallest difference between at least one round trip time of a pulse in the optical cavity of the regenerative amplifier and the time interval between seed pulses generated by the master oscillator.

21. A laser device as claimed in claims 17 to 20, wherein the optical switch is a pockels cell.

22. A laser device as claimed in claim 21, wherein the pockels cell comprises two separate independently controlled electro-optical cells.

23. A laser device as claimed in claim 21, wherein the pockels cell comprises one optical component and two independently controlled high voltage switching electronic units.

24. A laser device as claimed in claim 21, wherein the pockels cell comprises at least two optical components and two separate independently controlled high voltage switching electronic units.

25. A laser device as claimed in claims 17 to 24 wherein the optical switch is arranged to control the amplitude envelope of the pulses in a single burst by controlling the amplitude of the first intermediate voltage.

26. A laser device as claimed in claims 17 to 24 wherein the optical switch is arranged to control the amplitude envelope of the pulses in a single burst by providing a slope to the first intermediate voltage.

27. A laser device as claimed in claims 17 to 26, wherein the optical switch is arranged to control the amplitude envelope of the multiple plexuses by controlling the amplitude of the second intermediate voltage.

28. A laser device as claimed in claims 17 to 26, wherein the optical switch is arranged to control the amplitude envelope of the multiple plexus by providing a slope to the second intermediate voltage.

29. A laser device as claimed in any one of claims 17 to 28, wherein the optical switch is arranged to control the number of seed pulses injected into the regenerative amplifier by controlling the duration of the first intermediate voltage, thereby controlling the number of pulses in a single burst.

30. A laser device as claimed in any of claims 17 to 29, wherein the optical switch is arranged to control the number of amplified bursts emerging from the regenerative amplifier from a multiplicity of bursts by controlling the duration of the second intermediate voltage.

31. A laser device as claimed in claims 17 to 30, characterized in that the laser device is arranged for laser material processing.

32. A laser device as claimed in claims 17 to 30, characterized in that the laser device is arranged for laser-based spectroscopy.

33. a laser device as claimed in claims 17 to 30, characterized in that the laser device is arranged for medical treatment.