EP2064784A1

EP2064784A1 - Stable picosecond laser at high repetition rate

Info

Publication number: EP2064784A1
Application number: EP06769358A
Authority: EP
Inventors: Mikhail Grishin; Andrejus MICHAILOVAS
Original assignee: UAB "Ekspla"
Current assignee: UAB "Ekspla"
Priority date: 2006-08-03
Filing date: 2006-08-03
Publication date: 2009-06-03
Also published as: WO2008016287A1

Abstract

A laser system employing regenerative amplification, which generates a highly stable train of short laser pulses (11) even at repetition rates well above the inverse fluorescence lifetime of the gain medium. Said laser system includes a low power master oscillator (5), pulse picker (7), low power pre-amplifier (14) and continuously pumped regenerative amplifier (9). The pre-amplifier (14) is incorporated into said laser system to boost energy of initial pulses (8) and generate seed pulses (13) with energies exceeding the stability threshold of said regenerative amplifier (9). The use of the low power master oscillator and the low power pre-amplifier is a cost-effective solution of the bistability problem in cw pumped RAs at high repetition rates.

Description

STABLE PICOSECOND LASER AT HIGH REPETITION RATE

FIELD OF THE INVENTION

The present invention relates to solid-state laser systems for generation of high stability short pulses, and more particularly relates to short pulse generation in laser sources using mode-locking and consecutive regenerative amplification.

BACKGROUND OF THE INVENTION

Pulsed laser beam is a versatile tool for processing of materials. Nanosecond pulses are widely used for industrial machining or surface engineering [see for example, U.S. Pat. No. 5,635,089, issued to Singh et al; 6,316,191, issued to Pτwczyk et al; and 6,969,822, issued to Pollard], however, they cannot produce features with a high degree of precision and control [see e.g., U.S. Pat. No. RE37,585, issued to Mourou et al. ; or Breitling et al, SPIE 5339, pp. 49-63 (2004)].

Ultrashort laser pulses have a strong benefit for high precision micro-machining, that combines high ablation quality (no thermal side effects) with high processing speed and reliability, and, therefore, are preferred in numerous applications: selective drilling and cutting of various materials [see U.S. Pat. No. RE37,585; or No. 6,150,630, issued to Perry et al; and Stoical et al, Phys. Rev. Lett. 88, 097603/1-4 (2002)], and more particularly in structuring of semiconductor devices [see U.S. Pat. No. 6,979,798, issued to Gu et al; U.S. Pat. App. No. 2005/0274702, issued to Deshi].

Recent experiments show that picosecond lasers are prospective tools for low-cost micro- machining, while featuring nearly the same quality and efficiency as had been achieved with femtosecond lasers [see Dausinger et al, SPIE 5147, pp. 106-115 (2003); Nebel et al, SPIE 6108, pp. 226- 233 (2006); U.S. Pat. No. 5,720,894, issued to Neev et al; 6,720,519, issued to Liu et al; 6,879,605, issued to Kyusho et al; Ml. Pub. No. 2005/038994, issued to Gu et a/.].

The standard technique for generating short laser pulses with high energy is based on the technique of a master oscillator and a power amplifier (MOPA).

Master oscillators (MO) generating short or ultrashort pulses employ a well-known technique of mode locking, where an intra-cavity modulation forces all of the laser modes to operate at a constant phase [W.Koechner, "Solid state laser engineering", 4th ed. (Springer-Verlag, Berlin 1996), pp. 506-533}. Active mode locking relies on externally controlled modulation, comprising an acousto-optical (AOM, Bragg cell) or electro-optical (EOM, Pockels cell) modulator periodically governed by a radio-frequency signal [e.g., U.S. Pat. No.4,375,684, issued to Everett; 5,040,182, issued to Spinelli et al; 6,016,324, issued to Rieger et al.]., while passive mode locking relies on a saturable absorber mechanism [see U.S. Pat. No. 3,978,429, issued to Ippen et al; 6,259,719, issued to Cunningham et al; and 6,393,035, issued to Weingarten et al.].

The pulsewidths of widely used Nd-doped solid-state lasers vary in the range of a few picoseconds up to lOOps. More particularly, passively mode-locked Nd:YVO4 lasers produce pulses of 5-20ps.

The pulse repetition rate from these devices depends on the cavity length and usually is in the range of 50 - 200 MHz. Since the average output power of oscillators is up to 10W, the single pulse energy falls in the range of a few nJ to a few hundred nJ. However, many technological applications require pulses with energies from a few μj to a few hundred μj [see, U.S. Pat. No. 6,552,301, issued to Herman et al.; mdNebel et al., SPIE 6108, pp. 226-233 (2006)]. Only a consecutive regenerative amplification scheme is able to boost pulse energy by up to 3 orders of magnitude.

The principal of regenerative amplifier (RA) is revealed in U.S. Pat. No.3,597,695, issued to Swain et al; and 4,191,928, issued to Emmett: RA comprises an active element within an optically resonant cavity, capturing means for switching seed pulses into the cavity, and ejecting means for switching amplified pulses out of the cavity. Input pulse circulates along a single optical path between end mirrors a number of times and multiple passes through a gain medium are used for efficient extraction of gain.

The cavity of regenerative amplifier may include two separate switches for pulse injection and ejection or one combining both functions as in U.S. Pat. Nos. 3,597,695 and 4,191,928, respectively. These switches maybe electro-optical or acousto-optical modulators, see U.S. Pat. Nos. 3,597,695; 4,191,928; or Ruggiero et al, J. Opt. Soc. Am. B 8, pp. 2061-2067 (1991); U.S. Pat. App. No. 2002/0109911, issued to Fukuda; respectively.

The typical regenerative amplifier is a linear two- or three-mirror resonator with an active element and a single optical switch inside. The switch is usually realized with a polarizer, quarter- wave plate and a Pockel's cell. It is governed by a high voltage signal and acts as a Q-switch and a cavity dumper simultaneously. The operation of regenerative amplifier can be explained as follows. First, no voltage is applied to the PC, thus the resonator has low quality and energy of a pump source is accumulated by the gain medium. Once the PC is switched to a quarter-wave voltage, the cavity has high quality and pulse arriving from the oscillator can circulate between end mirrors. At every round-trip the pulse acquires the stored energy from the gain medium. After a predetermined number of round-trips, when the trapped pulse is amplified by several orders of magnitude, the voltage is switched off (or raised to a half-wave value) to dump the cavity. For more details please refer W.Koechner book, pp.541-548. The build-up time for the circulating pulse to reach maximum energy depends on the gain in the system and the characteristic of the Q-switch envelope. The higher the gain, the shorter the build-up time.

In the past, solid-state regenerative amplifiers have been limited to frequencies of approximately IkHz. Limitations arise from the maximum rate of the pumping system [see Dawson et al, Opt. Lett. 13, pp. 990-992 (1988); Selker et al, Opt. Lett. 19, 551-553 (1994)] or intracavity optical switch [Bado et al, Opt. Lett. 12, pp. 319-321 (1987); Sizer II et al, IEEE J. QE-24, pp. 404-410 (1988)].

Techniques suppressing piezoelectric effects, novel electro-optical crystals, improved electronic drivers and use of continuous pumping enable RA to operate at repetition rates from ten to few hundred of kHz [see Wang et al, Opt. Lett. 15, pp. 839-841 (1990); Siebold et al, Appl. Phys. B 78, pp. 287-290 (2004)].

When pulse repetition rate is less than the inverse fluorescence lifetime of the gain medium, high energy per pulse may be obtained, which is independent of pulse repetition rate. The average power grows monotonically with the increasing pulse repetition rate. Both pulsed or cw pumping schemes are used at rates below the inverse fluorescence lifetime [Bado et al. (1987); and Tun et al, Opt. Lett. 20, pp. 154-156 (1995), Braun et al, Appl. Opt. 36, pp. 4163-4167 (1997)].

Pulse repetition rate above the inverse fluorescence lifetime of the gain medium leads to a constant high average power of regenerative amplifier, while the pulse energy becomes low and decreases monotonically with the growing rate [Ruggiero et al (1991); Wang et al (1990); Braun et al (1997); Siebold et al (2004)]. Pulsed pumping is no longer meaningful, because population inversion has no time to decay toward zero before the arrival of the next pump pulse. Thus, if the fluorescence decay rate is slow in comparison to the pulse repetition rate, cw pumping is preferred. Besides, regenerative amplifier cw pumped, allow for variable repetition rate without cavity adjustments as presented in U.S. Pat. No. 4,896,119, issued to Williamson et al..

Lasers ensuring a good pulse-to-pulse energy stability while keeping a high repetition rate (e.g., lOOkHz) are required to meet the needs of industrial high volume manufacturing applications. Laser stability is very essential in obtaining uniform machining quality (ablated feature size) over the entire scan field.

Pulse repetition rate of, e.g., 100kHz is faster than the inverse of upper-state lifetime for most known gain media. Thus, continuous pumping is preferred. Although many authors declare a very stable output of cw pumped regenerative amplifiers due to a constant thermal lens, elimination of pump pulse instabilities, etc. [see U.S. Pat. No. 4,896,119, also Bado et al (1987); OJdshev et al, Appl. Opt. 43, pp. 6180-6186 (2004)], still there are sets of parameters that cause unstable output. Wang et al. (1990) observe a decreasing pulse-to-pulse stability with the growing pulse repetition rate. At some particular rate value the pulse amplification becomes too erratic to be useful. Siebold et al. (2004) observe a double period operation mode at high repetition rates.

Numerical simulations and experimental results in Dδrring et al, "Period doubling and deterministic chaos in continuously pumped regenerative amplifiers", Opt. Express 12, pp. 759-1768 (2004) show a multi-stable behaviour of continuously pumped regenerative amplifier. The route to multi-energy regimes depended on a pump power, dumping rate and gate length, and the instability occurred coincidentally in the parameter range where the highest output pulse energy was expected. More results on multi-stability in said RA are given in Intl. Pub. Nos. 2004/107513 and 2005/053118, issued to Kopfet al.

Double-energy behaviour of cw pumped regenerative amplifier was theoretically predicted and experimentally observed by inventors of current invention as well. At pulse repetition rate range from 20 to 40OkHz cw pumped regenerative amplifier based on Nd:vanadate produces a train of pulses altering between two stable energy values. 2OkHz is just above the value of inverse fluorescence lifetime equal to lOkHz. 400kHz is close to quasi- continuous operational mode.

A numerical modelling performed by inventors of current invention predicted that seed pulse energy is also among parameters that govern said phenomenon. High and low seed pulse energy values cause a different behaviour of said regenerative amplifier. High seed pulse energy leads to a single-energy output, while low seed pulse energy leads to a double-energy output of RA. Besides, double-energy output demonstrates a superior reproducibility of every second pulse.

Reasons of aforesaid different behaviour of RA as a function of seed pulse energy and bistability when seed pulse energy is low, can be explained after tracing processes acting in the gain medium: Before seed pulse injection (low-Q phase) pump energy is stored in the gain medium, i.e. population inversion is created. As soon as seed pulse enters said gain medium, high-Q phase and amplification process begins. At the peak of the build-up, after the inversion has been depleted and the gain saturates, RA is switched to the low-Q phase again and the amplified pulse is ejected. The main feature of cw pumped RA is that restoring of population inversion starts immediately after said cavity dumping.

During the amplification process, seed pulses with high energy strongly deplete initial inversion. At low-Q phase, pumping restores population inversion to the initial value before arrival of the next pulse. A new amplification cycle is exactly the same leading to the single-energy RA output.

In the case of low energy seed pulses, initial inversion is depleted weakly and there is still a significant amount of residual energy stored in the gain medium at the end of high-Q phase (amplification phase). During the next step, said residual inversion adds to inversion being restored by the pump. This leads to a higher inversion at the end of low-Q phase (pumping phase) and, therefore, larger value of the small-signal gain that would otherwise be expected. The new amplification cycle differs from the preceding one: due to the larger gain value circulating pulse is amplified to a higher energy. Strong amplification depletes the medium to a lower or zero residual energy at the end of high-Q phase (amplification phase). During the following pumping phase, inversion is restored back to a lower value leading to a smaller small-signal gain at the end of this phase. As a result, two consecutive amplification cycles produce different pulse energies.

The bistability problem does not appear, if number of round-trips that the circulating pulse makes inside a cavity is chosen small. This is because neither high-energy, nor low-energy seed pulses totally deplete initial inversion. The gain saturation is not reached during the amplification phase for both cases. The difference between high and low energy of seed pulses is only reflected in a regenerative amplifier's total efficiency.

Additionally, the bistability problem disappears, if number of round-trips is very large, because both high-energy and low-energy seed pulses deplete initial inversion. The gain saturates during the amplification phase in both cases. This operational mode is not meaningful due to a very low efficiency.

A block diagram of a traditional solid-state laser system employing regenerative amplification is given in Fig. 2 A. A train of pulses 6 at the output of master oscillator 5 passes into a pulse picker 7. There an initial pulse repetition rate of, e.g., 100MHz is divided by some factor. Single pulses in a train 8 are seeds for regenerative amplifier 9. The seed pulse travels multiple round-trips in the cavity of RA until they grow to a predetermined (or maximum available) energy level and are ejected then.

hi the case of a low power master oscillator 5 as schematically shown in Fig. 2A, seed pulses 8 feature low energy and, therefore, at high repetition rate, e.g. lOOkHz, said bistability problem occurs: regenerative amplifier at expected Q-switch envelope maximum produces a bistable output 4. Energy of every second pulse drastically differs (by up to several orders) from the preceding one. However, the stability between each second pulse is very high (0.5 %). Besides, such bistable behaviour of the system is very steady. Due to the considerable difference of energies between the two following pulses such laser system is not suitable for many applications requiring reproducibility.

The first known solution to overcome pulse-to-pulse instabilities at high repetition rates is disclosed in Ml. Pub. No. 2004/107513, issued to Kopfet al. It comprises a means for an active modulation of the pump diode current or the gate length of RA as a function of the detected output pulse energy.

A block diagram of MOPA with the active feedback mechanism is shown in Fig. 2B. The traditional scheme (see Fig. 2A) is supplemented by a control unit (CU) 10, which registers at least one error signal based on the extracted pulse energy and regulates at least one adjustable variable, e.g. LD current, in such a way that an unstable behaviour of RA emission is prevented or suppressed. Hence, a stable train of amplified pulses 11 is generated at the output of RA 9.

Although bifurcations are suppressed or prevented from occurring from the start, there are several drawbacks of this design. The dynamical modulation of the - often high- LD current, requires a sophisticated negative-feedback controller. Besides, adjustment of the pump power changes thermal conditions of the gain media, therefore, causes new instabilities. Selection of the gate length below bifurcation point (small number of round trips) means that an accumulated pump power is not efficiently extracted.

Another possible solution might be employment of a powerful master oscillator. There is evidence [Knappe et al, CLEO/Europe 2003, paper CP2-3-THU, p. 89 (2003)] that RAs, when seeded with pulses exceeding 5OnJ, operate in an inherently stable manner even at high repetition rates well above the inverse fluorescence lifetime.

As schematically shown in Fig. 2C, a more powerful oscillator 5 generates a pulse train 12 with higher energy. After the pulse picker 7 selects separate pulses (and dumps out all the residual ones!), a new train 13 is formed. Now single pulses in the train 13 are seeds for regenerative amplifier 9. Due to the higher seed pulse energy said bistability problem is overcome, i.e. RA 9 emits the stable train 11.

The evident drawback of this design is low efficiency because only a small part of the initial power is optimally used. E.g., to get 1OmW of seed power at 100kHz, 1OW of initial MO power are required at 100MHz. It means that nearly 30 Watts of cw laser diode (LD) pump power is necessary.

Besides, there are many disadvantages of working with a powerful master oscillator:

1. Oscillator optical elements operate at quite high thermal and optical load; 2. Pulse picker optical elements operate at quite high thermal and optical load;

3. Due to limited pulse picker and polarizer contrast ratio, undesired background radiation bears high power (e.g., 1OmW);

4. Low efficiency leads to high cost of a laser system;

5. High cost of maintenance due to expensive high power laser diodes.

There is a need to have a stable laser without aforesaid drawbacks of the previous art. The present invention offers a distinct method and laser system design for generation of high repetition rate high stability short laser pulses, while operating in the highest efficiency mode.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and a solid-state laser system for generation of high stability short laser pulses, running at 20-50OkHz repetition rate range.

Another object of the present invention is to provide a solid-state laser system for material processing with pulse energy up to several hundred of μj.

A further object of the present invention is to provide such a solid-state laser system, where the accumulated pump power is efficiently extracted.

In carrying out the above objects of the present invention, the following steps were made. Firstly, the rule of bistability dependence on seed pulse energy was formulated and the stability threshold was defined. Secondly, a laser system, comprising low power master oscillator, pulse picker, followed by pre-amplifier and regenerative amplifier, was designed. Said additional linear pre-amplifier is a means for boosting seed pulse energy to exceed the stability threshold. Seed pulses with energies above the stability threshold lead to a stable operation and a single-energy output of regenerative amplifier. The use of the low power master oscillator and extra pre-amplifier is a cost-effective solution of bistability problem in RAs at high repetition rates.

Implementation details and advantages of the present invention are readily apparent from the following detailed description and review of the accompanying drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph illustrating a bistability problem of cw pumped regenerative amplifier; Fig. 2A is a block diagram schematically illustrating said bistability problem; Fig. 2B is a block diagram showing the first prior art solution of said problem; Fig. 2C is a block diagram showing the second prior art solution of said problem; Fig. 2D is a block diagram showing a novel solution of said problem, according to this invention;

Fig. 3 is a principal optical scheme of a solid-state laser system, according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to the Fig. 1, there is depicted an influence of seed pulse energy to continuously pumped regenerative amplifier's output pulse energy at pulse repetition rates exceeding the inverse fluorescence lifetime of the gain medium. This is a numerical modelling performed by inventors of the present invention and it is a theoretical provement of bistability in cw pumped RA, when seed pulse has low energy.

In the diagram (main graph of Fig. 1), energy of amplified pulse is plotted versus number of round-trips that pulse runs inside the cavity of RA before ejection. Single-energy and double-energy regions of the diagram may be distinguished. RA output pulse has one energy value, when number of round-trips N_RJ is "small" or "very large". At intermediate region, where number of round-trips is referred to as "large", amplified pulse has two energy values. The inlay of Fig. 1 illustrates an actual sequence of dumped pulses 4 at this double-energy region. Two subsequent pulses have different amplitudes, but reproducibility of every second pulse is high, i.e. train of pulses exhibits a steady bistability.

Three plots 1, 2 and 3 in the main graph of Fig. 1 represent three cases: if RA was seeded by pulses with 3nJ, 5OnJ and 25OnJ of energy, respectively, when other parameters were fixed. Plots 1-3 are simulated for Nd: vanadate, but qualitatively suit for other gain media.

A typical shape of intracavity energy in RA during a high-Q phase has increasing and decaying parts [see e.g., p.547 of W.Koechner book]. The former one is due to the dominance of gain and the latter one is due to the dominance of losses. At the maximum gain is exactly compensated by losses. It is expected that energy of circulating pulse would trace this shape. In order to efficiently extract the stored energy of regenerative amplifier the amplified pulse should be ejected exactly at the maximum point. For a weak initial pulse more round-trips are needed to reach gain saturation.

It is evident, that plots 1 and 2 reveal some irregular behaviour. Lower seed energy (plot 1) leads to a worse situation. At a "small" number of round-trips (below 16-17) pulse energy follows the typical increment, but before reaching the expected maximum value (at N₃ = 23 of round-trips) a plot splits into two branches, corresponding to different energies of two subsequent output pulses. Energy gap is widest in the vicinity of N₃. For the weaker initial pulse this energy gap is wider and, therefore, bistability is bigger. This simple observation brings an idea that higher energy of seed pulse enables to make said energy difference minimal.

Plot 3 depicts energy of amplified pulse as a function of round-trips, when seed energy is high. There are several changes in results, as compared with previous cases. As follows from the theory, less round-trips are needed to reach gain saturation, i.e. the point of expected maximum energy shifts to the region of "small" number of round-trips. The energy gap decreased by several orders and the envelope of the diagram looks much like the typical shape known in the art [p.547 of W.Koechner book]. The distinct difference from plots 1 and 2, is that double-energy region is still near N₃, but not in the vicinity of expected maximum energy (from N₁ = 12 to N₂ = 15 of round-trips).

It is still possible to operate with weak seed pulse, if number of round-trips is below 17. The output of RA would have one energy value, but the stored energy would not be optimally extracted. For the highest efficiency seed pulses with high energies are strictly desired. As example, with 1OnJ seed pulse energy one obtains 7W output power, while 10OnJ seed pulse energy allows achieving 1OW output power of RA. Increase in RA output power when starting from higher initial pulse is significant - over 30 percent.

To conclude, the bistability problem may be overcome not because of the minimized energy gap, but due to said shift of .gain saturation point. It is not necessary to have 25OnJ of initial energy as in plot 3. The empirical value of 10OnJ is sufficient to make energy maximum be in the single-energy region. Energy of 10OnJ or average power of 1OmW (if pulse repetition rate is 10OkHz) hereinafter is called a single-energy or stability threshold. Energies below said 10OnJ will be referred to as low.

Fig. 2D is a block diagram showing a novel solution of said bistability problem, according to this invention.

In order to get said stable train of amplified pulses 11, it is more efficient to act as in present design. The low power master oscillator 5 (as in Fig. 2A) generates a first train of pulses 6. Further, pulses are selected by pulse picker 7, and a second train of pulses 8 is formed. In comparison with the prior art design of Fig. 2C, power losses in pulse picker are less. Said second train of pulses 8 is then amplified by an additional amplifier 14. This amplifier 14 boosts pulse energy by up to 2 orders of magnitude and is referred to as preamplifier (PreA). Energy of resulting seed pulses 13 exceeds regenerative amplifier's 9 stability threshold, therefore, single-energy output laser pulses 11 are generated.

The trade-off is that a power needed to pump the powerful master oscillator of Fig. 2C may be many times higher than a sum of powers needed to pump the low power master oscillator and pre-amplifier of Fig. 2D. E.g., to get said 1OmW at lOOkHz, two standard cw laser diodes with 3 Watts of output power are enough to pump MO and PreA. The first 3W laser diode pumps said master oscillator 5 to generate 10OmW at lOOMHz. This corresponds to O.lmW at 100kHz after pulse selection. The second 3 W laser diode pumps said pre-amplifier 14 to raise the average power of O.lmW up to 1OmW.

Furthermore, a use of low power master oscillator and low power pre-amplifier has many advantages:

1. Optical elements of master oscillator operate at moderate thermal and optical load;

2. Optical elements of pre-amplifier operate at moderate thermal and optical load;

3. Reliability; 4. Lower cost as compared with the prior art design;

5. Low cost of maintenance due to cheap low power laser diodes. Fig. 3 shows a principal optical scheme of a solid state laser system, including master oscillator (MO), pre-amplifier (PreA) and regenerative amplifier (RA). This is a realization of the novel solution said in Fig. 2D, according to this invention, illustrated in a greater detail.

Diode pumped passive mode-locked laser, well known in the art (see References), can be used as master oscillator for seeding regenerative amplifier. Generally, a master oscillator 5 comprises end mirrors 15 and 19, mode locker 16 and gain medium 18, aligned to generate and mode-lock the light. A dichroic mirror 17 can be used to direct pump laser radiation into the gain medium.

The end mirror 15 is HR coated and the end mirror 19 is an output coupler. The mode locker 16 can be an active modulator or a passive saturable absorber. In one of embodiments of MO the end mirror 15 and mode locker 16 may be a single element - saturable absorber mirror (SAM), e.g. SESAM.

The gain medium 18 may be of any kind from the variety of solid state active media. In one of embodiments of MO output coupler 19 is coated on the gain medium 18. The gain medium may be end-pumped.

When lasing conditions are satisfied and mode locker synchronizes modes, a single picosecond pulse circulates in a cavity of MO along an optical axis 20. A part of intracavity light is outcoupled at every round-trip and forms a train of pulses of the type 6 in Fig. 2D described above.

Output radiation of said MO is divided into two beams by abeam splitter 21. The beam reflected by said beam splitter propagates along an optical path 22 and hits a photodiode 23 that, further, bears a synchro-signal in synchronization pulse generator 24.

The beam transmitted by said beam splitter propagates along an optical path 25 and goes to a pulse picker 7, which selects pulses in a predetermined repetition rate. The pulse picker (PP) is controlled by said synchronization pulse generator and performs an active optical switch. It directs a single pulse to an optical path 26 (switch status is ON). AU the residual stream of pulses is directed via a path 27 to a dump 28 (switch status is OFF). Typically the pulse rep. rate in the output 26 of PP, as compared to the input 25, is divided by a factor of -1000. The new pulse train is presented as 8 in Fig. 2D.

Optical paths 25 and 26 may include additional optical elements, for example, a collimator to expand the beam and, hence, make it less divergent.

The pulse picker 7 may consist of a polarizing beam splitter, Pockel's cell and waveplate.

The selected pulse of type 8 goes through the optical path 26, polarization isolator 29 and optical path 30, then enters a pre-amplifier 14.

The pre-amplifier (PreA) is linear and may be of any configuration: one or several active elements, single or multiple passes, etc. Gain medium 31 of PreA may be laser diode end- pumped. Redirecting mirrors 32, collimators, lenses can be necessary in certain designs.

When example configuration of PreA, where the pulse passes gain medium 31 twice (as shown in Fig. 3), is used, polarization isolator 29 is necessary to separate input and output beams. Pre-amplified pulses are redirected from an optical path 33 to an optical path 34 there. Pulse train is presented as 13 in Fig. 2D. Single pulse in this train is referred to as seed for regenerative amplifier.

The polarization isolator may be a Faraday isolator comprising a polarizing beam splitter, Faraday rotator and waveplate.

Optical path 34 may include additional optical elements, for example, a collimator to satisfy mode-matching condition between input seed beam and RA' s intracavity eigenmode.

Design of regenerative amplifier is well known in the art (see References). Generally, a regenerative amplifier 9 consists of the following elements: end mirrors 40 and 43, gain medium 42, polarizer 37 and cavity dumper 38. Gain medium 42 of RA may be laser diode end-pumped.

In one of embodiments of RA the gain medium 42 is placed at a centre of the cavity and is pumped by two pump sources from both ends. Depending on the configuration of the cavity other elements: dichroic mirrors, collimators, apertures, etc., are needed.

The cavity dumper 38 is controlled by said synchronization pulse generator 24 and performs a Q-switch and cavity dumper simultaneously. It may comprise an electro-optical modulator. While switch status is OFF, the regenerative amplifier is a laser cavity with low quality, i.e. any spontaneous emission is lost through polarizer 37 and so RA cannot lase. At a predetermined instant of time, slightly before said seed pulse enters the cavity, switch is turned ON. This results in the cavity quality change from low to high, i.e. polarization is electro-optically changed by 90 degrees and so the seed can circulate inside RA cavity along axes 39 and 41. At every round-trip seed pulse is amplified by gain medium 42. After preset number of round-trips and when the amplified pulse is at the axis 39, electro- optical switch is returned to status OFF. This leads to polarization change by 90 degrees and so the light is ejected through polarizer 37.

In configuration of RA where the output pulse propagates counter the input pulse (optical paths 44 and 34 in Fig. 3, respectively), polarization isolator 35 is required to redirect the amplified pulse from optical path 44 to optical path 45. The train of amplified pulses is presented as 11 in Fig. 2D.

In example embodiment of the present invention, Nd: vanadate master oscillator was pumped by cw laser diode with output 3 W power. As a result of passive mode locking employing saturable Bragg reflector (SBR), continuous train (82MHz) of 6ps and 5nJ per pulse was produced at the output of MO. Pulse picker based on BBO-cell was exploited in a double-pass configuration and featured a high contrast (over 2500). Pulse picker selected pulses at repetition rate of lOOkHz and was followed by two stages of power amplifiers. Nd:vanadate based pre-amplifier, continuously pumped by 3W laser diode, raised the energy per pulse up to the level of 50OnJ in a double-pass design. Further, pulses with increased energy were injected into and trapped inside the cavity of regenerative amplifier by means of BBO electro-optical switch. Two cw 2OW laser diodes were used to pump the active Nd: vanadate crystal. The cavity was dumped after 7-14 round-trips, when circulating pulse saturated the gain medium, and highly stable train (10OkHz) of pulses was produced at the output of RA. Energy of approximately 100/J per pulse demonstrated stability better than 1%, and the pulsewidth lengthened up to 9.5ps.

Claims

1. A pulse laser system for generating train of short laser pulses comprising:

- a master oscillator subsystem (5) producing a first pulse laser beam (6) featuring a very high pulse repetition rate and low energy per pulse;

- a pulse picker means (7) producing a second pulse laser beam (8) comprising a train of pulses selected from said first pulse laser beam (6) at a lower pulse repetition rate; a regenerative amplifier subsystem (9) receiving a seed pulse laser beam (13) and amplifying its energy by at least 3 orders of magnitude; synchronization means for controlling temporal separations among processes of said pulse laser system; characterized in that it further comprises a pre-amplifier (14), incorporated into said pulse laser system to generate high repetition rate high stability short laser pulses (11), while operating in the highest efficiency mode.

2. A pulse laser system according to claim 1, characterized in that said pre-amplifier (14) is placed between said pulse picker (7) and said regenerative amplifier (9) receiving said second pulse laser beam (8) and amplifying pulse energy by 1 to 2 orders of magnitude.

3. A pulse laser system according to claim 1, characterized in that said pre-amplifier (14) boosts energy of said second pulse laser beam (8) to a predetermined value exceeding the stability threshold of said regenerative amplifier (9) and produces said seed pulse laser beam (13).

4. A pulse laser system according to claim 1, characterized in that pulses of said seed pulse laser beam (13) with energies exceeding the stability threshold of said regenerative amplifier (9) result in a highly stable output (11) of regenerative amplifier even at high pulse repetition rate.

5. A pulse laser system according to claim 1, characterized by producing a highly stable train of output laser pulses (11) repeating at 20-500 kHz rate.

6. A pulse laser system according to claim 1, characterized in that a width of said output laser pulses (11) is less than Ins, preferably in the picosecond range.

7. A pulse laser system according to claim 1, characterized in that an energy of said output laser pulses (11) is in excess of several hundred of microjoules.

8. A pulse laser system according to claim 1, characterized in that energy variations within said output laser pulses (11) in the train are not exceeding 1%.

9. A pulse laser system according to claim 1, characterized in that a gain medium (18) of said master oscillator (5) is cw pumped, preferably by a laser diode pumping device.

10. A pulse laser system according to claim 9, characterized in that said gain medium (18) of said master oscillator (5) is any solid-state laser medium, preferably Nd: vanadate.

11. A pulse laser system according to claims 9 to 10, characterized in that energy per pulse at the output of said master oscillator (5) is not exceeding 5nJ.

12. A pulse laser system according to claim 1, characterized in that said pre-amplifier (14) includes at least one solid-state gain medium (31), means for exciting, means for cooling said at least one laser gain medium and means (32) for directing laser light to make at least one passage through said at least one laser gain medium.

13. A pulse laser system according to claim 12, characterized in that said at least one gain medium (31) of said pre-amplifier (14) is cw pumped, preferably by a laser diode pumping device.

14. A pulse laser system according to claims 12 to 13, characterized in that said at least one gain medium (31) of said pre-amplifϊer (14) is any solid-state laser medium, preferably Nd:vanadate.

15. A pulse laser system according to claims 12 to 14, characterized in that energy per pulse at the output of said pre-amplifier (14) is in excess of 100 nJ.

16. A pulse laser system according to claim 1, characterized in that the pre-amplified pulse is directed to a pulse picker for contrast ratio enhancement.

17. A pulse laser system according to claim 16, characterized in that the system uses a single pulse picker (7) for pulse selection and contrast ratio enhancement.

18. A pulse laser system according to claim 1, characterized in that a gain medium (42) of said regenerative amplifier (9) is cw pumped, preferably by a laser diode pumping device.

19. A pulse laser system according to claim 18, characterized in that said gain medium (42) of said regenerative amplifier (9) is any solid-state laser medium, preferably Nd:vanadate.

20. A pulse laser system according to claim 18 to 19, characterized in that energy per pulse at the output of said regenerative amplifier (9) is in excess of several hundred of microjoules.