Abstract:
An apparatus for generating electromagnetic radiation pulses comprises a mode-locked laser oscillator with an oscillator cavity defining an oscillator beam path, a first gain element, first pumping means for pumping said first gain element, and a mode locker, and being operable to produce a train of seed electromagnetic radiation pulses. Further an optical switch is placed outside of the oscillator beam path and is arranged in a beam path of radiation coupled out from the oscillator, the optical switch operable to couple radiation from a switch input into a switch output during a certain time period or certain time periods. A radiation amplifier is arranged in a beam path of radiation radiated from the switch output. The amplifier includes a second gain element and second pumping means, the second pumping means comprising a continuous-wave pump radiation source.

Description:
FIELD OF THE INVENTION 
     The invention is in the field of generating electromagnetic radiation pulses. It more particularly relates to an apparatus and a method for generating high energy electromagnetic radiation pulses including a master oscillator and at least one amplifier. The invention also relates to a radiation amplifier. 
     BACKGROUND OF THE INVENTION 
     Applications of pulsed electromagnetic radiation sources (primarily infrared light or visible light but also ultraviolet radiation) such as material processing or precision micro machining offer improved performance with higher fluence (energy per area) and peak intensity (energy per time per area). Such pulse parameters result in better laser micromachining performance, i.e. the material can be readily ablated with improved spatial precision and a reduction in material damage of nearby areas (so-called “cold ablation” resulting in a reduced heat-affected zone around the area of the ablated material). A further performance influencing parameter is the pulse repetition rate. In material processing applications, processing speed generally scales linearly with the repetition rate. 
     Therefore, radiation sources of coherent radiation combining the following properties would be desirable:
         short optical pulses, in the range of picoseconds (preferably around 10-15 ps, often more than 20 ps, but even down into femtoseconds)   high pulse energies (preferably 100 microjoules or more)   high repetition rate (ideally 100 kHz or more)   preferably near diffraction limited spatial profiles (to allow for near-diffraction limited focusing to beam spot size diameters on the order of the laser wavelength i.e. one micron approximately)   if possible, readily adjustable repetition rate, over a range of pulse repletion rates, preferably from a minimum to maximum ratio of at least 4× (i.e., for example 50 kHz to 200 kHz) without having to substantially re-align or re-adjust the laser or amplifier, and while still maintaining good spatial beam properties, preferably without substantially changing key beam properties (M2, beam waist, beam divergence).       

     These requirements are also beneficial if shorter wavelengths are required, because nonlinear frequency conversion efficiency increases with increasing peak intensity and with good spatial beam qualities. Shorter wavelengths (i.e. frequency doubling, tripling, quadrupling, or quintupling) decrease the potential spot size for diffraction-limited focused beams, and can improve the material processing performance for different materials, since the shorter-wavelength light has higher photon energy, resulting in different and improved ablative performance (for example in transparent materials). 
     Current state-of-the art diode-pumped solid-state lasers can be passively mode-locked to produce, in a simple and robust way, picosecond and femtosecond optical pulses, as, for example, disclosed in U.S. Pat. No. 5,987,049. These lasers typically produce optical pulses with low pulse energy (10-100 nJ) but at high repetition rates (e.g. 100 MHz). Also mode-locked high power lasers have been known, for example, from U.S. Pat. No. 6,834,064. However, the achievable pulse energies of these high power pulsed lasers are in the low microjoule range, which is still not enough for the initially mentioned applications. 
     As an alternative to high power lasers, it has been suggested to combine a mode-locked laser oscillator with an amplifier. Several embodiments of continuous-wave diode-pumped multi-pass amplifiers have been described, for example, in U.S. Pat. No. 5,546,222, U.S. Pat. No. 5,615,043, U.S. Pat. No. 5,774,489. By this technique, the average output power can be amplified considerably, even to many tens of Watts if a plurality of amplification stages are applied. However, due to the high repetition rate of the initial oscillator, the pulse energies remain below 1 μJ. An even higher average power is not desirable due to disadvantages entailed by high average power, such as potential thermal fracture, thermal lens effects, complex set up involving a multitude of pump diodes, a sophisticated heat management, high power consumption etc. Also, repetition rates exceeding some tens of MHz or hundreds of MHz may be disadvantageous since they are difficult to handle for some material processing applications. The most desirable range would be between 100 KHz and 4 to 10 MHz. However, decreasing the repetition rate of the initial oscillator would mean to increase the cavity length, which is usually not desired, since the oscillator would become physically very large or require a complex folding technique, which results in optical loss and reduced mechanical stability. 
     An alternative approach to decreasing the pulse repetition rate and at the same time increasing the pulse energy is “cavity dumping”. This technique comprises inserting an optical switch (typically electro-optic or acousto-optic) into the laser cavity and reducing the normal output coupling of the laser through the output-coupling mirror to as little as possible. This allows the intracavity pulse energy to increase. Occasionally, an intracavity pulse is switched out at a reduced repetition rate (typically over the range from single shot to megahertz pulse rates) but at increased pulse energy compared to the normal continuous operation of the laser. Such a cavity dumped laser can typically get ten times higher pulse energies, but the pulse energy typically becomes independent on the dumping frequency below a few MHz, so that further reductions in the repetition rate do not result in a pulse energy gain. Also, switching is inherently critical. Any misalignment of the intracavity beam with respect to the switching apparatus changes the output coupling and therefore the laser dynamics, the circulating pulse energy, etc. Further, cavity dumping perturbs the laser dynamics, since the leftover pulse has a smaller than equilibrium pulse energy, resulting in non-steady state performance. All this may lead to higher pulse-to-pulse fluctuations than in continuously mode-locked lasers, and even chaotic pulse performance. 
     Even if these stability problems are somehow overcome, the pulse energies are still not sufficient. It has therefore been proposed (for example Huber et al., Optics Letters 28, p. 2118 (2003)) to combine a cavity-dumped laser with a 2-pass pass continuously pumped amplifier. However, such an approach did also not result in sufficient maximum pulse energy, since the given gain material to be chosen under the given boundary conditions have a very low gain, as a consequence of which tight focusing is required. This leads to strong gain saturation (depletion) at low pulse energies. More in general, it has proven to be difficult to achieve a continuously pumped, high-gain multi-pass amplifier, and there are significant trade-offs between high total gain and high average power output. 
     Higher gain can be achieved in a spatial multi-pass approach if the pump power is increased. However, because of thermal problems, which are similar to the problems encountered in continuously pumped high-power lasers, this is only possible in a pulsed pump scheme at low repetition rate. An example of this state of the art may be found in Lenzner et al., Optics letters 20, p. 1397 (1995), where a TiSa mode-locked laser has been combined with a Pockels cell selecting single pulses from a 80-MHz-pulse train at a repetition rate between 1 kHz and 5 kHz and a pulsed-radiation pumped amplifier. The achieved radiation rates are not fast enough for high-speed material processing as required in industrial applications. Alternatively, systems have been proposed (for example in U.S. Pat. No. 5,812,308), where the amplifier does not have a high small-signal gain but is seeded with a high-average power oscillator and serves more as a power amplifier to increase the average power by a factor 2-4. 
     Yet another approach to achieve the high total gain is a regenerative amplifier, where a pulse is trapped in an amplifier cavity, and is re-circulated many times until the pulse energy has grown to where the gain material is effectively saturated. Such a regenerative amplifier has for example been disclosed in U.S. Pat. No. 4,896,119. However, since such regenerative amplifiers comprise a cavity, the misalignment sensitivity is comparably high and chaotic instabilities result in a limited range of repetition rates. Further, the optical switch has to be an electro-optic Pockels cell (acousto-optic modulators are normally not suitable due to the small beam size that would be required in them for fast switching, which would result in peak intensities, due to the high intracavity pulse energy, in the device that exceed their damage threshold). This brings about the necessity for high voltages in the system and as a consequence high-power electronics with all its disadvantages. Also, the switch is alignment-sensitive, the achievable repetition rates are limited, and it is not straightforward to change the repetition rate because the repetition rate influences the roundtrip gain and thermal lens effects, hence the optical performance. 
     SUMMARY OF THE INVENTION 
     In view of the state of the art, it is an object of the invention to provide an apparatus for generating pulsed electromagnetic radiation with high pulse energy. It is another object of the invention to provide an apparatus for generating pulsed electromagnetic radiation with high average output power and repetition rates substantially between 50 kHz and 8 MHz. Yet another object of the invention is to provide an apparatus for generating pulsed electromagnetic radiation with a tunable repetition rate where the tuning of the repetition rate does not necessitate the adjustment of geometrical parameters and does not alter the spatial beam parameters. 
     It is a further object to provide an apparatus for generating pulsed electromagnetic radiation, which apparatus is less sensitive to different parameters (such as outside temperature, chosen calibration parameters, chosen repetition rate etc.) than prior art apparatuses. It is yet a further object of the invention to provide a robust continuously pumped radiation power amplifier with a high small-signal gain. 
     According to a first aspect of the invention, there is provided an apparatus for generating electromagnetic radiation pulses comprising
         a mode-locked laser oscillator comprising an oscillator cavity defining an oscillator beam path, a first gain element, first pumping means for pumping said first gain element, and a mode locker, and being operable to produce a train of seed electromagnetic radiation pulses,   an optical switch placed outside of the oscillator beam path and being arranged in a beam path of radiation coupled out from the oscillator, the optical switch operable to couple radiation from a switch input into a switch output during a certain time period or certain time periods, and   a radiation amplifier including a second gain element, the radiation amplifier being arranged in a beam path of radiation radiated from the switch output, the radiation amplifier further including second pumping means, the second pumping means comprising a continuous-wave pump radiation source.       

     The inventors have surprisingly found that it is possible, by this combination of features, to obtain a pulsed radiation source with pulse energies high enough to fulfil many requirements of high energy pulse radiation sources, even with continuous-wave (cw) pumping. 
     The approach according to the invention allows for a compact set-up without too many folds of a cavity, since despite the comparably high pulse-to-pulse time period, no long optical paths are necessary (which would be necessary for a small repetition rate seed laser). The fact that the optical switch (the “picker”) is placed outside the cavity leads to the possibility of having a robust seed laser which is not influenced by any operating conditions of the apparatus. The robustness is supported by the pump radiation source being of the continuous-wave type. 
     Additionally, it has been found that this simple multi-pass approach to amplification results in very clean, high-performance spatial mode properties of the amplified output, which is a very important characteristic for nonlinear optics and material processing. Also, the approach substantially conserves the pulse nature of the input (for example picosecond) pulses produced by the laser oscillator. Some pulse broadening is expected from the well-known phenomena of “gain narrowing”. As an example, it has been observed seed pulses of 7 ps result in an output pulse of less than 12 ps, which is close to the expected value based on gain-narrowing calculations. 
     The mode locker may be any known or yet to be developed mode locking means. It may, for example, be a passive mode locker such as an element comprising a saturable absorber material or a Kerr lens mode locking means, possibly an additive-pulse mode locking means, or other. It may also be an active mode locker such as a mode locker comprising an acousto-optic or electro-optic coupler. 
     The first pumping means may be any known pump of a pulsed laser, such as an optical pump including at least one flashlamp, at least one non-lasing diode, at least one superluminescent diode, at least one laser diode or at least one other laser, or combinations of these. It may also be an electrical discharge pump, a gas dynamic expansion pump (if the first gain element is a gas), a chemical pump, or a current injection pump (if the first gain element is a semiconductor gain element), all these pumps being as such known in the art. Especially preferred is the situation where the first gain element is a solid state gain element and the pump is an optical pump, especially an optical pump comprising at least one diode laser. 
     The term “seed pulses” denoting the pulses output by the laser oscillator does not imply special properties of these pulses but merely relates to their function as being pulses selectively input to the radiation amplifier. The seed pulses may have any properties concerning wavelength, pulsewidth, pulse repetition rate, pulse shape, pulse energy etc. 
     The optical switch couples the incident radiation or a fraction thereof from an input to an output during a certain time period or certain time periods which period/periods are for example defined by switching signals. As an example, the optical switch may direct radiation either in a first or a second direction, the first direction corresponding to the switch output. The optical switch may be any known or yet to be developed switch, such as an electro-optical switch (including a Pockels cell), a magneto-optical switch or an acousto-optical switch. Especially preferred is the embodiment where the switch is acousto-optical. This is possible since the pulse energy and average power of the seed radiation pulses need not be excessive. Seed laser oscillator and optical switch can then produce any pulse repetition rate which is an integer division of the fundamental repetition rate of the seed laser oscillator. For example, an 80 MHz seed oscillator can be conveniently adjusted to produce pulses at 40/20/10/5/2.5/1.25 MHz etc. Other repetition rates can be generated by recognizing that the final pulse separation of the output pulse train must simply be some integer multiple of the pulse separation of the seed oscillator (for the case of 80 MHz corresponding to 12.5 ns). This allows for many selectable repetition rates, for example exactly 1.0 MHz, or 1.013 MHz above or 0.988 MHz below. Other seed oscillator repetition rates may be chosen to obtain other desired selectable repetition rates. 
     The radiation amplifier is preferably non-regenerative, i.e. does not form a cavity and is in most cases free of an optical switch. Preferably, it is a multi-pass amplifier. A multi-pass amplifier in this context denotes a spatial multi-pass amplifier. This is in contrast to temporal multi-pass amplifiers (regenerative amplifiers) where the amplifier comprises a cavity, in which the radiation pulsed circulate a number of times, and where the radiation is coupled out by an active switching means. In a spatial multi-pass amplifier the gain element is traversed by the beam amplifier beam path a plurality of times. In the spatial multi-pass amplifier, in contrast to regenerative amplifiers, the gain element is passed a plurality of times due to the geometrical set-up and possibly also due to polarization influencing means such as polarization filters and/or polarization rotators. This may be viewed as spatial multiplexing in contrast to the temporal multiplexing of regenerative amplifiers. The preferred number of passes is at least four, but it may also be only two or three passes, or at least five or six, at least seven or eight, or an uneven number etc. 
     The second gain element may be a gain element of any known or yet to be discovered kind but is preferably a solid-state gain element, especially preferred a Nd doped gain element. Nd doped gain elements were so far usually not considered to be suitable for amplifiers of pulsed radiation generated by mode-locked lasers, since they do not have the bandwidth to support ultrashort (femtosecond) pulses. However, it has been found that for the picosecond pulsewidths required for certain applications, they are suitable and may provide an excellent small signal gain. The currently preferred material is Nd:Vanadate. 
     The multi-pass amplifier approach according to the invention can lead a very-high small-signal gain, and allows for amplification of individual pulses from approximately 10 nJ input pulse energy to substantially above 1 microjoule or more. 
     The second pumping means may be any continuously radiating light source, for example one of the continuous-wave light sources mentioned above referring to the first pumping means. Especially preferred is a laser diode or a plurality of laser diodes. 
     The seed electromagnetic laser pulses produced by the mode-locked laser oscillator preferably have laser pulses of pulse lengths between 1 ps and 1 ns, although also shorter or longer pulse lengths are possible (for Q-switched seed lasers also considerably longer pulses, even up to 1 μs, are possible). The pulse repetition rate of the train of seed electromagnetic radiation pulses may be almost arbitrarily chosen to optimize the geometry of the laser oscillator according to any chosen criteria, such as required space, useable components etc. It may, for example, be in a region between 20 MHz and 200 MHz because these repetition rates are fundamental repetition rates of cavities that have easy-to-handle dimensions. The rate with which the optical switch couples a selected pulse into the amplifier may, for example, be between 50 kHz and 8 MHz. 
     According to a special embodiment of the invention, the apparatus comprises, downstream of the radiation amplifier, a power radiation amplifier. The power radiation amplifier comprises a third gain element and third pumping means. The third pumping means may again comprise an optical pump, such as a cw optical pump, especially at least one laser diode. The power radiation amplifier may also be a multi-pass amplifier, the number of passes being at least two or at least three or four. The power radiation amplifier may for example be laid out in a similar manner as the radiation amplifier or may even be identical with the latter. The wording “power radiation amplifier” does not imply special physical properties and is not a quantitative statement about the power of radiation of input in the power radiation amplifier or output therefrom. It merely relates to the function of the amplifier, i.e. to further increase the energy of radiation pulses already amplified by the radiation amplifier. 
     In one preferred embodiment, a high-gain multi-pass first amplifier, using 8-passes through the gain medium, is combined with a lower-gain power amplifier, using 4-passes through a second gain medium. Combining a pre-amplifier for high small-signal gain, and a post-amplifier for further gain at high pulse energy and high average power, one can achieve substantially more than 100 microjoule output pulse energy, at high pulse repetition rates (100 kHz or more). 
     Preferably, the apparatus comprises repetition frequency tuning means by which the user may select a desired repetition frequency out of a range of repetition frequencies. The frequency tuning means may include a user interface as well as a communication module operable to interact with a controller of the optical switch. The repetition rate may preferably be tuned without changing the position of cavity elements of the laser oscillator and without changing other geometrical parameters. In fact, in a preferred embodiment, the approach according to the invention allows tuning of the repetition rate without changing any parameters other than the switching rate of the optical switch followed by proportional change of the output pulse energy. This is especially the case in combination with the power radiation amplifier which may be operated in saturation where relatively large changes of the seed radiation average power causes only small changes of the output radiation power. For example, in a preferred embodiment, tuning of the pulse repetition rate from 100 kHz to 1000 kHz results in an increase of the seed beam power by a factor of ten, then directed by the optical switch towards the first radiation amplifier (1000% boost), while the amplified power after the power radiation amplifier is increased by only 10%. 
     Also according to the invention, a radiation pulse generator is provided, the radiation pulse generator comprising
         a pulsed seed laser operable to produce a train of seed electromagnetic radiation pulses,   an optical switch placed in a beam path of radiation output from the seed laser, the optical switch operable to couple radiation from a switch input into a switch output during a certain time period or certain time periods, and   a radiation amplifier including a second gain element being a solid-state gain element which includes an Nd dopant, the radiation amplifier being arranged in a beam path of radiation from the switch output, the radiation amplifier further including second pumping means, the second pumping means comprising a continuous-wave pump radiation source, the pump radiation source including at least one laser diode.       

     Further, the invention concerns an apparatus for generating electromagnetic radiation pulses including
         a mode-locked laser oscillator comprising an oscillator cavity defining an oscillator beam path, a first gain element, first pumping means for pumping said first gain element, and a mode locker, and being operable to produce a train of seed electromagnetic radiation pulses,   an optical switch placed outside of the oscillator beam path and being arranged in a beam path of radiation coupled out from the oscillator, the optical switch comprising an acousto-optical modulator and being operable to direct radiation either in a first or a second radiation direction,   a spatial multi-pass radiation amplifier including a second gain element being a Nd:Vanadate gain element, the radiation amplifier being arranged in a beam path of radiation radiated from the optical switch in the first direction, the radiation amplifier further including second pumping means, the second pumping means comprising a continuous-wave pump radiation source, the pump radiation source comprising at least one laser diode, and   a spatial multi-pass power radiation amplifier including a third gain element and a continuous-wave optical pump for pumping said third gain element, the power radiation amplifier being arranged in a beam path of radiation output from the radiation amplifier.       

     According to a second aspect of the invention an apparatus for generating electromagnetic radiation pulses includes
         a mode-locked laser oscillator comprising an oscillator cavity, a first gain element, first pumping means for pumping said first gain element, and a mode locker,   a radiation amplifier including a second gain element, which second gain element includes Nd doped gain material, the radiation amplifier further including second pumping means, the second pumping means including a continuous-wave pumping radiation source, and   an optical switch, the optical switch operable to couple seed radiation pulses from the laser oscillator into the radiation amplifier.       

     Again, the second pumping means may include any known cw optical pump but preferably includes at least one laser diode. The invention according to the second aspect thereof also distinguishes, according to preferred embodiments, from the state of the art in that the radiation pulses have a pulsewidth between 1 ps and 1 ns (or longer for Q-switched seed lasers) and in that the second gain material does not include any active cooling means (such as cooling with a flowing liquid or flowing gas, Peltier cooling etc.) in direct contact with the gain element; “absent any active cooling means” does not exclude the gain element being attached to a cooled element, such as a water cooled gain element holder. 
     A method of generating high-energy electromagnetic radiation pulses includes the following steps
         generating a train of seed electromagnetic radiation pulses, of a first pulse repetition frequency,   picking a fraction of pulses from said train of seed electromagnetic radiation pulses, said fraction of pulses having a second pulse repetition frequency being smaller than the first pulse repetition frequency,   continuously irradiating a doped solid by pump electromagnetic radiation,   directing said fraction of pulses onto said doped solid, so that said doped solid is traversed by said fraction of pulses, and   re-directing said fraction of pulses so that the doped solid is traversed by the fraction of pulses at least one further time,
 
wherein the fraction of pulses traverses the doped solid on a non-closed path.
       

     “Non-closed path” denotes setups where the beam is not directed in a cavity. A cavity—defining a closed path—is characterized in that radiation therein travels back and forth between end elements or travels around (circular cavity) an undefined number of times, so that light is only coupled out through partially transparent mirrors or upon incidence of certain events (cavity dumping, by an optical switch). On a non-closed path, the travelling light only passes the gain element a pre-defined number (which, however, may depend on parameters such as polarization, wavelength etc.) of times. Especially preferred is an embodiment, where—in contrast to set-ups according to the state of the art, the closed beam path is 2D, i.e. the beam on all passes lies in a single plane. 
     An amplifier for producing a train of amplified electromagnetic radiation pulses upon incidence of a train of seed electromagnetic radiation pulses includes a radiation directing arrangement defining a radiation entry and a radiation exit, a gain element and continuous-wave optical pumping means operable to optically pump the gain element, the radiation directing arrangement operable to direct incident radiation onto the gain element and to re-direct, on a non-closed path, radiation at least after a first traversal of the gain element onto the gain element, the gain element forming, in an operational state, a thermal lens, wherein a beam waist of a radiation beam directed by the radiation directing arrangement comprises a beam waist outside of the gain material. Preferably, thus, at least one beam waist of the radiation beam is at a place different from the place of focus of the pumping beam (which place of focus is preferably in the gain material). 
     The gain element is preferably an Nd doped solid, especially a Nd:Vanadate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiments of the invention are described with reference to schematic drawings. 
         FIG. 1  is a block diagram of an apparatus according to the invention. 
         FIG. 2  is a diagram of an embodiment of an apparatus according to the invention comprising two amplification stages. 
         FIG. 3  is a diagram of an amplifier according to the invention. 
         FIG. 4  is a clarifying drawing of the optical path scheme in an amplifier according to the invention. 
         FIG. 5  is a drawing of an unfolded optical path scheme. 
         FIG. 6  is a diagram of an alternative embodiment of an amplifier according to the invention. 
         FIG. 7  is a block diagram of an alternative apparatus according to the invention. 
         FIG. 8  is a diagram of an embodiment of the alternative apparatus according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of an apparatus in accordance with the first and the second aspect of the invention comprises the following components shown in FIG.  1 : A seed laser oscillator SL, an optical Switch OS, a first amplifier A 1 , and a second amplifier A 2 . A laser pulse produced by the seed laser traverses the optical switch serving as pulse picker. The optical switch either selects the pulse or discards it. The selected pulses then consecutively traverse the first and the second amplifier. 
     The seed laser may be a compact, industrial-style, 1 W average output power, 100 MHz-range repetition rate oscillator optically cw-pumped with a laser diode and mode-locked with semiconductor saturable absorber device. The seed laser gain element—the first gain element in this text—may be any gain element known for mode-locked pulsed lasers, especially lasers producing pulses of about 1 ps to 1 μs pulse length. It may be a laser Nd doped solid, for example a Vanadate (such as Nd:YVO4 or Nd:GdVO4) crystal. 
     The laser oscillator repetition rate is defined by the cavity length and optimized for the best performance together with subsequent optical switch. 
     The output TEM 00  beam is coupled out the cavity with an output coupler mirror of an outcoupling transparency of for example 5%. The pump diode and laser crystal are for example temperature stabilized. 
     Due to cw-pumping, temperature stabilization and lack of any movable and switchable components (both mechanically and electro-optically or acousto-optically) inside the cavity the seed laser generates a continuous train of the picosecond pulses which are very stable both in amplitude (&lt;0.1% rms typically) and repetition rate. Additionally the oscillator can be synchronized with an external clock with low timing jitter (&lt;200 fs rms typically). 
     Individual pulses are selected after the oscillator with an external fast optical switch, which can operate in a range from on a single-shot basis up to at least several megahertz. 
     Preferably, the optical switch comprises an acousto-optical modulator (rather than an electro-optic modulator) and is placed outside (rather than inside) the seed cavity as an optical switch (pulse picker) for the apparatus according to the invention. This brings about the advantages of compactness, no need for high voltage electronics, lack of influence on the seed laser performance in the whole range of possible pulse repetition rates, a wide range of the repetition rates from single shots up to for example 8 MHz. Further, the spatial separation of the high frequency beam from the seed cavity and low frequency beam deviated on the Bragg angle by the acousto-optical modulator towards an amplifier allows high contrast and minimizes a feedback between an amplifier and seed cavity. 
     The minimum distance between the seed cavity and the acousto-optical modulator is defined by:
         i) The requirement that the seed beam is preferably focused by means of focusing lens or focusing mirror into the acousto-optical modulator with certain waist diameter in order to reach as high efficiency of pulse picking as possible   ii) Certain minimum distance between the focusing lens or focusing mirror and acousto-optical modulator due to geometrical factors and constraints.   iii) The beam directed by acousto-optical modulator towards an amplifier may be diverging. If it is, it is advantageously collimated with a certain beam diameter defined by the input characteristics of the amplifier.       

     The pre-amplifier and the power amplifier are described in more detail further below. 
       FIG. 2  shows an example of an apparatus according to  FIG. 1  in somewhat more detail. The seed laser is shown including an optically pumped first gain element  2  including a coating  4  which is highly transparent for pump radiation  3  and partially transmittive for laser radiation  5 . An end mirror  6  of the cavity may comprise the saturable absorber material responsible for mode-locking. The seed pulses are coupled out through the partially transmittive facet of the first gain element and reach the optical switch  11 . The selected pulses are directed, via Mirrors M 1 , M 2 , to the first amplifier which comprises a Faraday isolator  12 , a radiation deflection arrangement comprising a plurality of mirrors M 3 -M 6 , and a lens L 1 , as well as a second gain element  14  coated by a reflection layer  13  and second pumping means including a laser diode or a bar or an array of laser diodes (not shown) and a pump radiation focusing lens  15  for focusing the pumping radiation  16  into the second gain element. The orientation of the second gain element (being a crystal) and the polarization of radiation hitting it may have to be adjusted to each other, either by adapting the gain element&#39;s orientation to the polarization provided after the Faraday isolator  12  or by providing separate (not shown) polarization influencing means such as half-wave plates etc. From the first amplifier, the pre-amplified pulses propagate to the second amplifier, via mirrors M 7 , M 8 , and M 9 . The second amplifier also comprises a light deflection arrangement including a plurality of mirrors M 10 -M 12  and a lens L 2  as well as a third gain element  24  coated by a reflection layer  23  and third pumping means including a laser diode or a bar or an array of laser diodes (not shown) and a pump radiation focusing lens  25  for focusing the pumping radiation  26  into the third gain element. 
     Output mirrors M 13 -M 15  serve for directing the output beam to the desired direction. 
     An especially preferred radiation amplifier was found to meet the following requirements:
         i) To keep the Gaussian beam diameters large enough in order to avoid any bulk or surface optical damage caused by the high radiation intensity (power per area);   ii) To keep input/output Gaussian beams preferably collimated with diameters small enough in order to use low aperture steering (mirrors) and transmitting (lenses, isolators etc.) optics;   iii) To use as little optical components as possible (no additional beam expanders, compressors etc.);   iv) To use as simple components as possible (no parabolic optics, no cemented lenses, no complex objectives, only spherical singlets, flat mirror etc.);   v) To have as compact and robust optical scheme as possible with as short working distances as possible;   vi) To keep a defined polarization state of the seed radiation corresponding to the spatial orientation of the optical axes of an anisotropic gain element employed;   vii) To employ when possible both spatial and polarization dependent separation of the beams in order to minimize overall dimensions of the amplifier; and   viii) To have easy, reasonable and predictable adjustment procedure.       

     The function principle of both, the first and the second amplifier is described in more detail referring to  FIGS. 3 ,  4  and  5 . Note that in  FIG. 4  as well as in  FIG. 5 , the elements are shown in an upside-down arrangement compared to  FIG. 3 . In these figures, elements that appear in both the first and the second amplifier are provided with the two reference numerals. Firstly, only the principle of the first amplifier is described. The first amplifier is an 8-pass amplifier and serves for boosting the energy of the pulses selected by the optical switch  11  from the level of a few nanojoules to a few microjoules with an overall gain coefficient of more than 500 or more than 1000, in some embodiments even more than 5000. The gain element  14  is a cw-end-pumped Nd:YVO 4  gain crystal. The crystal is provided with a high reflection coating  13  for the pulsed radiation wavelength, which coating has the effect of an antireflecting coating for the longer wavelength pumping radiation  16 . The coating  13  is on the pumping side, whereas the crystal is antireflection (AR) coated on the face side. 
     All elements of the shown amplifier are arranged in or along one plane (being the drawing plane, or the x-z plane coordinate system shown in  FIG. 3 ). 
     The beam formed by the incoming pulses (seed pulses) is assumed to be polarized in the specified plane, i.e. in the x-direction as indicated by the vertical lines. After its passing of the Faraday isolator  12 , the polarization remains x-oriented. The HR coating and the mirrors M 3 , M 4 , M 5  direct the beam on four passes P 1 , P 2 , P 3 , P 4  through the amplifier on in each case slightly different beam paths shown by the full black lines. After four passes, the beam hits the retro-reflecting mirror M 6 , whereafter the beam makes for more passes P 5 , P 6 , P 7 , P 8  through the crystal on the path way but backward. The Faraday isolator separates the incoming beam from the output beam: After the second pass through the Faraday rotator, the beam is polarized in y-direction as indicated by the dots shown for the output beam in the figure. 
     The waist position  31  of the beam, preferably for all passes, is outside the gain element. This is achieved by positioning the collimation lens L 1  at the particular distance from the second gain element as will be explained in more detail. 
     The second amplifier (or power radiation amplifier) may be set up in the same manner as the first amplifier. It may as an alternative be set up differently. In the shown embodiment, the second amplifier comprises a 4-pass optical scheme analogous to the scheme of the first amplifier, but without retro-reflecting mirror M 6  and, as a consequence, without the need for the Faraday isolator. 
     Referring to  FIG. 4 , the beam paths scheme is illustrated in somewhat more detail. The main idea behind is as follows: The collimated Gaussian beam hits the lens L 1 , L 2  at a certain small angle and at some distance from the lens center. The beam is pointed towards the pump area of the gain crystal. It converges behind the lens, with the waist being at the distance f L1,L2  from the lens. Due to initial non-90° angle of incidence, the waist of the seed Gaussian beam has an off-axis position with respect to the optical axis of the amplifier. Downstream of the focal point, the beam diverges, then hits the gain crystal. Passing through the gain crystal, the beam experiences the change of its wavefront due to a positive thermal lens effect. Approaching the HR facet of the crystal from inside, the beam is nearly collimated again and is reflected from the HR coating back into the crystal under a certain angle with respect to the incident beam. It now travels in the reverse direction, experiences the influence of the thermal lens again and hits the lens L 1 , L 2  on another side and at the same distance from the amplifier optical axis as compared to the input beam. Thus the beam geometrically reproduces itself with transverse displacement after two passes through the amplifier. The third and fourth passes do the same but with a larger transverse displacement at the lens L 1 , L 2 . Due to very low effective numerical aperture of the entire cone of rays between the lens L 1  and the gain element (less than 0.03 in a preferred embodiment) the Gaussian beam faces the thermal lens under a very small angle and, therefore, may experience only minor, insubstantial distortion while passing through the gain crystal that does not affect, noticeably, the (often desired) TEMoo beam quality. 
     The thermal lens formed in the gain element (which arises automatically when the gain element is optically pumped, due to the radial heat distribution in the element) is considered as an important part of the optical scheme. The distance between the collimating lens L 1 , L 2  and the main plane of the formed thermal lens tl is chosen to correspond approximately to the sum of the focal lengths f L1,L2  and f tl  of the collimating lens and of the thermal lens. Thus the beam waist position  31  is adjusted to be outside of the gain element at the distance from the gain crystal approximately equal to the focal length of the thermal lens. It has been found that the above described geometry of the amplifier with a Gaussian beam waist position outside the gain element and vertex of the ray cone coinciding with HR surface of the gain crystal to be an advantageous as compared to any other geometries of a spatial amplifier (for example described in Forget et al, OSA TOPS V.68, ASSL, 2002 p. 321-323, Müller et al., OSA TOPS V.83, ASSL, 2003 p. 278-284), since it compromises all the requirements listed previously herein. 
       FIG. 5  shows the beam path scheme of  FIG. 4  in a unfolded illustration (i.e., reflection by highly reflecting (HR) coating illustrated as transmission), where the gain element  14 ,  24  is shown twice. α denotes the angle of incidence for the first pass, β for the third pass. 
       FIG. 6  illustrates an alternative embodiment of the (first and/or second) amplifier where the gain element  14 ,  24  does not comprise an HR coating but AR coatings on both sides. Instead, a separate mirror element  41  is shown, which is highly reflecting for the radiation wavelength and is translucent for the pump radiation wavelength. The further elements of  FIG. 6  correspond to the elements of  FIG. 3  and are not described again here. 
     In an embodiment, where the amplifier gain element  14 ,  24  is a Nd:Vanadate (i.e. Nd:YVO 4 ), the central radiation wavelength may be 1064 nm, and the pump radiation wavelength approximately 808 nm. Of course, the seed laser has to operate at substantially the same center wavelength. Preferably, the seed laser&#39;s gain element (the first gain element) therefore comprises the same dopant. Especially preferred is an apparatus, where the first, the second, and if available, the third or further gain elements are of the same material. 
       FIG. 7  shows an apparatus in accordance with the second aspect of the invention. The seed laser SL comprises an optical switch OS integrated in the cavity and on a beam path in the seed laser cavity. The light pulses coupled out by the optical switch are directed to a first amplifier A 1 , and a second amplifier A 2 . 
       FIG. 8  shows a diagram of an embodiment of the apparatus of  FIG. 7 . The oscillator cavity of the seed laser  51  comprises an integrated optical switch  52 —which may be an electro-optical switch, an acousto-optical switch or a magneto-optical switch. The optical switch couples the pulse travelling back and forth in the cavity out at regular intervals and directs it to the first amplifier. The first and second amplifier in the embodiment of  FIG. 8  correspond to the first and second amplifier of  FIG. 2 . 
     It is to be noted that the above description merely shows examples of ways to carry out the invention and should by no means construed to be limiting. Especially, the shown geometrical arrangements as well as number and nature of optical appliances such as mirrors etc. may vary in many ways. The skilled person will for example know many ways to configure (concerning the folding etc.) a seed laser oscillator and will, given the teaching provided herein, find many ways to set up an amplifier or a plurality of amplifiers in accordance with the invention. It is also by no means a requirement that the seed oscillator need physically be separated. Rather, in order to conserve space, they can be amalgamated with crossing or nearly crossing beam paths etc. 
     Although all shown embodiments comprise two amplifiers, the second amplifier is optional and may be omitted. It is especially not necessary where the application does not demand the average power to be constant for different settings (such as pulse repetition frequency etc.) and where the pulse energy does not have to be necessarily maximized. Similarly, a third or more power amplifiers could be added for applications requiring higher average power. 
     In the figures, for reasons of simplicity, the coupling of the pump radiation source with the gain elements is not shown. In principle, any known or to be developed way of coupling pump radiation into the gain element is possible. A way to be mentioned in particular is the use of the fiber coupled laser diode bars as a pump source, since it allows very effective pumping with perfect spatial distribution of the pump radiation and simplified mode-matching technique. 
     Various other embodiments may be envisaged without departing from the spirit and scope of the invention. 
     The apparatus, amplifier and method according to the invention may be used in many contexts. Next to the mentioned material processing applications, also nonlinear optical devices are applications of choice, since they require high energy densities. An example is the combination of the apparatus according to the invention with nonlinear crystals to produce picosecond pulses at shorter or longer wavelengths than the seed laser wavelength.