WO2013152447A2

WO2013152447A2 - Pulsed semiconductor laser

Info

Publication number: WO2013152447A2
Application number: PCT/CH2013/000058
Authority: WO
Inventors: Thomas SÜDMEYER; Deran J.H.C. MAAS; Christian Anton ZAUGG; Wolfgang Peter PALLMANN; Martin Hoffmann; Kurt Weingarten; Ursula Keller
Original assignee: Time Bandwidth Products AG
Current assignee: Time Bandwidth Products AG
Priority date: 2012-04-11
Filing date: 2013-04-10
Publication date: 2013-10-17
Anticipated expiration: 2014-10-11
Also published as: WO2013152447A3

Description

PULSED SEMICONDUCTOR LASER

FIELD OF THE INVENTION

The invention is in the field of pulsed laser systems.

BACKGROUND OF THE INVENTION

Pulsed lasers with short pulses in the picosecond or femtosecond (i.e. less than 1 ps) range with at least 1 nJ of pulse energy and with peak powers of 1 kW or more are interesting for scientific and industrial applications, for example biomedical imaging, frequency comb applications or material processing. Such high pulse powers are often only achievable if the pulse repetition rate is accordingly reduced, the laser output power being restricted by boundary conditions. Furthermore, many applications intrinsically require repetition rates of less than 1 GHz.

According to the state of the art, such low repetition rates - for example repetition rates below about 0.5 GHz - and/or such high peak powers typically require ion- doped laser materials such as solid-state glass or crystal lasers or fiber laser or amplifier systems. - ? -

Semiconductor lasers, in contrast, are often more cost efficient. Their laser gain medium comprises of a semiconductor material, such as a material of the InGaAs family of materials or materials based on GaN. By using appropriate composition, almost any desired bandgap corresponding to wavelength in the near infrared and visible range of the electromagnetic spectrum can be achieved. Most semiconductor lasers do not require an external cavity because end faces of the semiconductor material function as resonator mirrors. For special applications, external cavity semiconductor lasers were proposed, in which the semiconductor component is placed as laser gain element in a laser resonator comprising an external reflector. There are both, semiconductor lasers that are pumped by a voltage applied across the semiconductor material, and optically pumped semiconductor lasers.

Semiconductoi" lasers, while being able to provide femtosecond or picosecond pulses, typically operate in the GHz regime (i.e. at repetition rates around 1 GHz or higher; currently up to 50 GHz). Edge-emitting semiconductor lasers further are limited in peak powers due to high nonlinearities. Semiconductor disc lasers (SDLs; also termed Vertical External Cavity Surface-emitting lasers VECSELs) do not have this issue. A pulsed VECSEL is disclosed in US 6,735,234. Such pulsed VECSELs are modelocked using saturable semiconductor absorbers, such as devices available on the market under the trademark SESAM®, that are mirrors incorporating a saturable absorber. These lasers have turned out to be suitable for applications such as biomedical imaging and nonlinear frequency conversion. However, currently both, peak power and pulse energy are limited and are substantially lower than obtained for solid-state lasers so that other applications such as frequency-comb generation or some material processing applications are not feasible.

An approach for increasing the pulse peak power is by reducing the pulse duration. An alternative approach is by increasing the laser output power. Such an increase, however, is often not possible to an arbitrary extent due to boundary conditions (heat - J -

load, pump power, etc.). Yet an other approach is to reduce the pulse repetition rates by increasing the cavity length.

However, it has been found that for low repetition rates of substantially less than 1 GHz, pulse generation - by modelocking - becomes unstable and a boundary seems to be met. This severely limits the use of pulsed semiconductor lasers to applications that need pulses with repetition rates of at least 1 GHz. Furthermore, in fundamental modelocked operation, the peak power and pulse energy are inversely proportional to the repetition rate. The limitation in achievable repetition rates therefore corresponds to a limit in pulse energy peak power, which prevents numerous application areas.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a pulsed semiconductor laser that overcomes drawbacks of prior art pulsed semiconductor lasers and that especially is suitable for producing laser pulses of lower repetition rates compared to prior art semiconductor lasers while preferably remaining cost efficient.

In accordance with an aspect of the invention, an apparatus for emitting pulsed electromagnetic laser radiation is provided, the apparatus comprising a radiation directing arrangement comprising a plurality of reflectors, the radiation directing arrangement directing pulsed laser radiation on a closed path or on a non-closed path, o wherein if the laser radiation is directed on a closed path, the radiation direction arrangement comprises a first end reflector and a second end reflector and defines a laser resonator, and if the laser radiation is directed on a non-closed path, the apparatus comprises an input and an output, the apparatus further comprising: o an essentially plane surface-emitting semiconductor gain structure having a surface plane for emitting said laser radiation: o a pump for exciting said semiconductor gain structure to emit the laser radiation from the surface plane; o wherein the radiation direction arrangement is configured to direct the laser radiation on its path from the first end reflector or input to the second end reflector or output, respectively, a plurality of times onto the semiconductor gain structure to stimulate emission of the laser radiation from the semiconductor gain structure and thereby amplify the laser radiation.

If the path is a closed path and a resonator is defined, the apparatus is a laser (sometimes referred to as 'laser oscillator'). The apparatus may then comprise a means for causing the radiation to be pulsed, for example a mode locker. If the path is a non-closed path, the apparatus is an amplifier that uses stimulated emission.

In accordance with an other aspect of the invention, therefore, a laser for emitting pulsed electromagnetic laser radiation is provided, the laser being a vertical external cavity surface-emitting laser and comprising an optical resonator being defined by a radiation direction arrangement comprising two end reflectors, and the optical resonator defining a laser radiation beam path; the laser further comprising: - an essentially plane semiconductor gain structure having a surface plane for emitting said laser radiation; pump for exciting said semiconductor gain structure to emit the laser radiation from the surface plane; a mode locker arranged in the beam path for mode locking the laser radiation; wherein the radiation direction arrangement is configured to direct the laser radiation on its path from one of the end reflectors to the other one of the end reflectors a plurality of times onto the semiconductor gain structure surface plane to stimulate the emission of the laser radiation from the semiconductor gain structure.

The following discussion refers to both, laser oscillators and amplifiers unless explicitly characteristics of only one of these are addressed.

The essentially plane gain structure (the gain element or gain chip) will preferably reflect the incident laser radiation in addition to amplifying it. It may contain a Bragg mirror or other mirror with an active region on it, for example with several quantum wells. The gain structure may be mounted on a heat sink, for example an active cooler or a passive cooler.

The radiation may be directed onto a same location of the gain structure, i.e. the beam spots of the different beam path branches incident on the gain structure overlap on the gain structure surface. Especially, the beam spots may overlap to an extent of at least 50%, at least 60%, at least 70% or at least 80% up to a complete coincidence.

It is an advantage of a VECSEL that the beam spot on the gain structure can be relatively large so that the intensity on the semiconductor is comparably low, and high pulse intensities can be achieved. Especially, the gain structure can have a diameter of between a few microns and tens of millimeters. For example, the spot size on the gain structure can be between 1 μηι and 10 mm, especially between 10 μηι and 1 mm. Other, smaller and larger spot sizes are not excluded.

In accordance with the definition used herein, a mirror, on which the amplifying gain layers are mounted, belongs to the gain structure, so that incidence of a laser beam onto the gain structure and reflection thereby counts as one pass of the gain structure (and not for example as a first pass of the gain material by the incident beam and a second pass by the reflected beam; in a multilayer structure of thin layers of the order of magnitude of the radiation wavelength there is no defined location where the radiation is reflected, and there need not be a clear separation between mirror and gain material layers). The fact that the radiation direction arrangement is configured to direct the laser radiation on its path from one of the end reflectors to the other one of the end reflectors a plurality of times onto a same location of the semiconductor gain structure in most embodiments means that the beam between the two end reflectors undergoes an according plurality of reflections by the gain structure, which comprises the gain material and is reflecting.

In laser oscillator configurations, it is a possibility that the gain structure also acts as one of the end reflectors in a configuration in which one branch of the laser beam in the resonator is incident on the gain structure at a right angle to the mirror plane (i.e. in general to the surface plane).

In addition or as an alternative, in a laser oscillator with a mode locker, the mode locker may constitute one of the end reflectors. It may for example be a mirror comprising a saturable absorber. The mode locker in embodiments is a passive mode locker, because passive mode lockers are generally better suited for shaping and stabilizing short pulses, whereas active mode lockers have limited switching speeds. Passive modelockers generally have a saturable absorber that has an absorption decreasing with increasing radiation intensity. The saturable absorber in embodiments is integrated in a mirror of the optical resonator, for example, as mentioned, an end mirror. This latter design has the advantage that the laser radiation in the resonator is incident on the saturable absorber only once per resonator roundtrip, so that the losses caused by the saturable absorber can be minimized. In an alternative configuration, where these losses are not of primary importance, a saturable absorber may also be integrated in the gain structure or alternatively in a folding mirror.

The invention is based on a surprising insight. It has been found by the inventors that the seemingly intrinsic lower limit of the fundamental pulse repetition rate of semiconductor lasers can be overcome if the intracavity laser beam is incident on the semiconductor gain element several times per cavity roundtrip in a multi-pass configuration. Due to this construction, the frequency of pulse incidences on the gain structure is not limited by two times the pulse frequency as in ordinary laser resonators but can be - depending on the number of passes of the gain structure - higher than the latter by an arbitrary factor.

The inventors have also found that despite the seeming limitations, a cost-efficient semiconductor amplifier for providing amplified pulses of high energies of for example at least 3 nJ or also substantially more can be made by a configuration as described herein, namely by a configuration with a surface-emitting gains structure on which the radiation is incident several times. Multi-pass lasers have been known from thin-disk solid state lasers where the intracavity laser beam is incident on an ion-doped crystal laser gain element several times each round trip. An example for this is found in EP 1 286 434. However, in these lasers, the multipass cavity has the purpose of enhancing the amplification per roundtrip to make a higher output coupling (of for example up to 70%) possible, so that for a desired - high - output pulse energy, the intracavity pulse energy is comparably lower. This is useful for high output energies to avoid nonlinearities coming about with high pulse energies, for example the nonlinearity of air. The ion- doped solid state lasers do not, however, have the limitations of the pulsed semiconductor lasers in terms of limited pulse repetition rates, and the VECSELs are not operatable in a regime in which the effects of nonlinearity of air have substantial significance.

The surprising solution to the problem of limited minimal fundamental pulse repetition rate in semiconductor lasers is attributed to the limited upper-state lifetime in the semiconductor gain medium. If the pulse repetition rate is too long, spontaneous emission by the gain medium compared to the emission stimulated by the impinging laser pulse becomes significant, and as a consequence the gain is reduced and becomes insufficient for compensating for the losses of the modelocker and other components in the cavity below a certain pulse repetition rate. Another way to view this is to say that the gain due to the excited states in the semiconductor gain medium dissipates through spontaneous emission, when the time between impinging pulses is longer or substantially longer than the spontaneous emission lifetime. This problem does not arise without modelocker in cw operation - in a cw mode the VECSEL lases also for long cavities where the roundtrip time is longer than the upper-state lifetime.

In contrast the multi-pass approach according to the present invention ensures that the time span between two subsequent incidences of the laser pulse on the gain - Si -

structure is smaller than the cavity roundtrip time divided by two, so that the proportion of the stimulated emission comparing to the spontaneous emission can be kept high.

Therefore, in embodiments of a laser or amplifier according to the invention, the radiation direction arrangement is configured so that a time duration between two subsequent incidences on the gain structure is less a few times (for example five, three, two times) the upper-state lifetime τ (sometimes referred to as 'carrier lifetime' in a semiconductor gain structure) of the gain structure. Especially, a most stable operation can be achieved if the time duration between two subsequent incidences is less than the upper-state lifetime.

In this, this criterion applies to all beam path sections between subsequent incidences or at least to most beam path sections.

In semiconductor gain structures, the upper-state lifetime is considerably shorter than in gain media with a more narrow upper-state energy level. It is an insight of the inventors of the present application that this (well-known) fact is a limitation of the achievable pulse repetition rates from laser oscillators and of efficiency in semiconductor amplifiers, which limitations can be overcome by a multi-pass approach as proposed herein.

If the apparatus is a laser oscillator, the output pulse repetition rate preferably corresponds to the fundamental repetition rate. The laser does preferably not comprise any intracavity active switching means. In an embodiment, the pulsed laser has a repetition rate of less than 500 MHz. Especially, the repetition rate is below 300 MHz, or even below 150 MHz or 100 MHz. Especially, the pulse repetition rate may be between 1 MHz and 300 MHz.

The number of incidences of the beam path on the gain structure is preferably determined by the resonator/amplifier design and is thus a resonator/ampli ier specific fixed number. However, constructions where this number is a choosable parameter - for example by adjustment of passive optical components in the resonator/radiation directing arrangement - are not excluded. The number of incidences of the beam path on the gain structure (between one of the end reflectors to the other one of the end reflectors) is at least 2, and it may be 3, 4, 5, 6, ... or any even number; even dozens or a hundred passes are possible. Especially, the number of incidences may in embodiments be at least 4 or at least 6.

The beam path in the resonator or on the non-closed path may be essentially in a plane, with the possible exception of a branch that is incident on one of the end mirrors or both branches that are incident on the end mirrors. Alternatively, also 3D- resonator configurations are possible.

The optical beam path length between two incidences may be chosen depending on the requirements and need not be identical between all incidences. In a preferable arrangement, at least for some beam path sections between two subsequent incidences, the optical beam path length of the section is at most 30 cm. Especially, in a preferred configuration for most beam paths sections or even for all beam path sections, the optical path length is lower than 45 cm or lower than 30 cm, for example between 2 cm and 45 cm or between 5 cm and 45 cm, especially between 10 cm and 30 cm. It has been found by the inventors that this distance region the balance between a too long time between two incidences on the gain structure (so that the time would exceed the upper-state lifetime) and a too short time between two incidences may be especially favorable. Even lower distances than 5 cm or 2 cm are possible also, especially in situations where the pump is not an optical pump but where the gain structure is electrically pumped.

Depending on the specific requirements and on obtainable material properties of the semiconductor gain material (materials with a comparably high upper-state lifetime may be feasible), the distance between two subsequent incidences may also be larger than 45 cm for some or even all of the incidences. More in general, the said distance is preferably at most 3 m, at most 2 m or at most 1 m, while set-ups with this distance being at most 70 cm or at most 45 cm are often preferred, and for special applications set-ups with this distance being even larger are not excluded.

In certain configurations, it may be advantageous to approximately equally space the incidences on the beam path from one end reflector to the other one, for example so that the optical beam paths of the different sections differs by at most 50% or at most 30%, possibly with the exception of the first and/or last section.

The laser/amplifier can be configured to operate in nearly single transverse mode, for example having an M^~ parameter of 3 or less, especially 2 or less or even at most 1.5.

The laser/amplifier can further be configured and dimensioned to provide an output power of at least 1 W, especially at least 3 W or even at least 10 W.

The pulse energy can be at least 1 nJ or at least 3 nJ or at least 10 nJ. The pulse length is for example at most 100 ps, especially at most 20 ps, at most 10 ps, or at most 1 ps. Especially, the pulse duration may be between 100 fs and 20 ps, or also below 100 fs.

In addition to the end reflectors (if any), the gain element, the pump (especially an optical pump, for example a laser diode or laser diode bar), beam re-directors and the mode locker (if any; wherein, as discussed, the gain element and/or the mode locker may constitute one or both of the end reflectors), the laser/amplifier may comprise an additional element or additional elements. A first example of such additional element is a Brewster plate. A category of additional elements is dispersive elements, such as a Gires-Tournois-Interferometer (GTI) or an other dispersive mirror. Especially, a dispersive element exhibiting positive dispersion may be advantageous in case short pulses are to be generated. Yet an other possible additional element is an etalon.

The pump for exciting the semiconductor gain structure is preferably an optical pump. Pumping radiation, for example produced by at least one high-power laser diode or at least one bar of high-power laser diodes, impinges on the location of the gain structure on which also the different branches of the laser beam are incident. Also other radiation sources may be suitable as optical pumps, for example superluminescent diodes, or lasers or superluminescent diodes with an amplifier, etc. The pump may be a continuous-wave pump or a pulsed pump, the pulse repetition rate then for example being adapted to the laser resonator dimensions.

While optical pumping is favored for many applications, because the advantages of the invention will in most cases be more pronounced for optical pumping (also in view of power limitations for electrical pumping), electrical pumping is not excluded. A particular advantage of the pulsed VECSEL according to embodiments of the invention is that it that can achieve repetition rates and pulse energies comparable to embodiments of thin-disk lasers but without the restrictions of the latter concerning the radiation wavelength. Whereas solid-state lasers like the thin-disk lasers have wavelengths that are set by the energy levels of ion-doped materials available (for example Yb for Yb lasers, Nd for Nd lasers, etc.), VECSELs do not have this restriction. Rather, for almost any desired laser wavelength, there exists a material and/or quantum well arrangement that provides has a bandgap of according energy so that the VECSEL lases at the desired wavelength.

Applications of embodiments of the laser or amplifier described and claimed herein include biomedical application. Other applications of embodiments of the laser comprise wavelength dependent applications in biology, chemistry, and physics because in which applications large pulse energies and/or small repetition rates are desire.

The laser is - in combination with devices within the laser resonator or outside of the laser resonator - also suitable for generating ultrashort pulses of for example less than 200 fs, even down to pulses of less than 50 fs or less. An example is the combination with a fiber in a fiber compression system, where a high peak power is a requirement.

Further applications of embodiments of the laser or amplifier include supercontinuum generation (SCG), for example for medical or other applications. A high peak power enables low-noise SCG which enables broader tunability when combined with photonic-crystal fibers, highly nonlinear fibers, etc. Even further applications include the generation of frequency combs. Frequency comb generation requires both, a very large bandwidth and high pulse powers.

Yet other applications include machining, especially where high pulse energies are required. Even further applications comprise any application including 2-plioton- processes or that otherwise rely on optical nonlinearities.

BRIEF DESCRIPTION OF THE DRAWINGS

Flereinafter, embodiments of the present invention are described referring to drawings. In the drawings, same reference numbers denote same or analogous elements. The drawings show:

Figure 1 a first embodiment of a laser according to the invention;

Figure 2 a second embodiment of a laser according to the invention;

Figure 3 a third embodiment of a laser according to the invention;

Figure 4 a fourth embodiment of a laser according to the invention;

Figure 5 a fifth embodiment according to the invention; and

Figure 6 measurements made using a laser in accordance with Figure 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The laser 1 schematically depicted in Fig. 1 is an example of a vertical external cavity surface-emitting laser. It comprises an optical resonator defined two end reflectors and radiation deflectors that direct a laser radiation beam on a beam path 2 within the optical resonator. The radiation deflectors in the depicted configuration comprise a plurality of resonator mirrors 6 that are illustrated as plane mirrors but can, depending on collimation/focusing requirements, also comprise curved mirrors.

A first one of the end reflectors is a mirror comprising a saturable semiconductor absorber, for example sold under the trademark SESAM®. This saturable absorber mirror 4 in addition to defining one end of the resonator also serves for mode-locking the laser radiation to generate pulses.

Pertaining to all embodiments, an example of a suitable saturable absorber mirror is disclosed in US 6,735,234 in Fig. 3 and its description. Further examples are shown in WO 96/36906. As a further alternative, a modification containing a quantum dot saturable absorber layer may be used.

The second one of the end reflectors is an outcoupling mirror 5 that couples a portion of the laser radiation out to yield an output laser radiation 20 being a train of laser pulses.

The gain structure 10 comprises at least one semiconductor layer that can be pumped to have population inversion and so that an incident laser beam stimulates emission. For example, the gain structure may comprise a plurality of quantum wells. In addition, the gain structure is configured to reflect the incident laser radiation in addition to amplifying it. For example, the gain structure 10 may comprise a series of layers satisfying the conditions of being a Bragg reflector for the laser radiation, the Bragg reflector for example being underneath the multiple quantum well structure. An example of a gain structure of a kind also usable for the present invention can be found in US 6,735,234, incorporated herein by reference in its entirety. Especially, Figure 2 of US 6,735,234 and its description show an example of a gain structure.

The gain structure is mounted on a cooler 15. The cooler may comprise a passive cooler such as a block of heat conducting material, optionally comprising cooling ribs or other structures promoting heat dissipation. In addition or as an alternative, the cooler may comprise an active cooler such as a water cooler through which cooling water is pumped to absorb heat.

The gain structure is optically pumped. For example a bar of laser diodes (not shown) or an other radiation generator generates pumping radiation 12 directed onto the gain structure. In the shown embodiment, a pumping radiation focuser 13 is also illustrated schematically. The pump arrangement may comprise pumping radiation incident from different directions and/or multi-pass pumping where reflected pumping radiation is re-directed onto the gain structure to enhance the efficiency.

The resonator is configured to direct the radiation a plurality of times onto the gain structure 10 on its path from one of the end reflectors to the other one. In the configuration of Fig. 1, the radiation coming from the saturable absorber mirror 4 being the first end reflector impinges on the gain structure 10 that reflects the radiation. Radiation re-directors, namely the resonator mirrors 6 direct it again on the same spot of the gain structure, but under a different angle, so that the gain structure reflects the radiation to get to the outcoupling mirror 5. The outcoupling mirror retro- reflects a portion (for example between 80% and 99.98%) of the laser radiation back onto the gain structure.

While this is not a requirement, beam path lengths between the different incidences on the gain structure may be chosen to be approximately equal, for example to differ at most by 40% or at most by 30%.

In this, as well as in other embodiments such as the ones described hereinafter, further optical components may be present in the resonator, for example a Brewster plate. Also, the laser may comprise means for balancing the dispersion to ensure generation of very short pulses. Nonlinearities arising in the semiconductor gain structures may for example be compensated by positive dispersion. An element having positive dispersion may for example be a Gire-Toumois-Interferometer- (GTI-) like optical element, or an other dispersive resonator mirror. The compensation of nonlinearities (albeit with opposed signs) is for example known from soliton modelocking.

In the description of the further embodiments hereinafter, those elements or features are described which are substantially different from the embodiment of Fig. 1.

In the variant of Figure 2, the gain structure 10 is also hit two times on its path from one end reflector to the other one of the end reflectors. In contrast to the embodiment of Fig. 1 , the beam path is, however, folded a few more times. Also, the curved collimating resonator mirrors 6, 7 are shown in Fig. 2, and one of the resonator mirrors 7 is hit by two branches of the beam 2 in the resonator. Also, a Brewster plate 18 is shown. Using the configuration of Fig. 2, a proof-of-principle 257 MHz modelocked VECSEL was implemented. The gain structure comprised quantum well on a Bragg reflector, and different quantum dot saturably absorbing mirrors were used. Different outcoupling mirrors with transmissions between 0.1% and 2% were used. In addition, an intra-cavity etalon was used to set the wavelength. Pulses as short as 3.2 ps at an average output power of 30 mW as well as average output powers of 120 mW with a pulse length of 9 ps were measured. A typical measurement set of the four-pass VECSEL output is shown in Figure 6. Panel a) depicts the autocorrelation signal (thick line) and an ideal sech² (hyperbolic-secant-squared function) fit (thin line; the coincidence is so good that the lines are almost not discernible in the figure). Panel b) shows the RF peak at a repetition rate of 256.6 MHz. Panel c), finally, depicts the measured optical spectrum, i.e. the spectral intensity as a function of the wavelength. The autocorrelation signal shows a pulse duration of 3.2 ps at a center wavelength of about 951 nm. The spectrum analyzer shows a 60 dB peak at the fundamental repetition rate of 256.6 MHz which is extraordinarily low for a modelocked VECSEL. No side bands were observed between the higher harmonic orders of the repetition rate. The total cavity length amounted to 58.6 cm. The absence of multiple pulses in the resonator was verified by the autocorrelation covering a short time-range (60 ps) around the pulse and by a sampling oscilloscope with a fast photodiode (response time 25 ps) covering the dark-time between two pulses. Single pulses separated by 3.9 ns were identified, matching the period of the fundamental repetition rate. These measurements fully confirm fundamental modelocked operation.

Figure 3 shows an example of an embodiment of what could be called a Herriott- cell-like design. The resonator is delimited by the saturable absorber mirror 4 and the outcoupling mirror 5. A curved mirror 30, for example being an elliptical or ellipsoidal mirror, is arranged to direct radiation incident from a re-directing mirror 7 onto the gain structure 10. One branch of the beam reflected by the curved mirror 30 is incident on the saturable absorber mirror 4 and retro-reflected by the latter. In Fig. 3, the radiation is directed three times on the gain structure on its path from one end reflector to the other one, but the configuration is suited also for larger numbers of passes of the gain structure.

The curved mirror 30 of the embodiment shown in Figure 4 is for example a parabolic mirror that directs radiation coming from the active spot on the gain structure 10 onto parallel paths. A beam re-directing element may for example comprise a plurality of the folding mirrors 7 at right angles to each other so that they together constitute a retro-reflector comprising a plurality of mirrors so that radiation incident on parallel branches of the beam is reflected back on again parallel paths. Also this configuration can be extended to other number of passes of the gain structure 10.

One feature of the embodiment of Fig. 4 - that can be implemented independent of the us of parabolic or elliptical/ellipsoid curved mirror 30 is that the gain structure 10 at the same time acts as a cavity end mirror because one branch of the beam is incident perpendicularly on the gain structure and is therefore retro-reflected. Instead of this, like in Fig. 3, the saturable absorber mirror 4 or an other re-directing mirror could be placed to serve as cavity end mirror.

In the embodiment of Fig. 4, two possibilities of outcoupling are illustrated by dashed lines. One possibility is to provide the saturably absorbing mirror 4 with a partial transmissivity so that it also serves as outcoupling mirror. An other possibility is to place a partial reflector 35 within the resonator. In the depicted variant, the partial reflector for example has a low reflectivity (corresponding to the desired outcoupling), and the reflected portion is coupled out. In an other variant, the partial reflector has a correspondingly high reflectivity and serves as a folding mirror in the resonator, whereas the transmitted portion is the outcoupled portion. While the embodiments with an partial reflector within the resonator may bring about additional losses (due to radiation portions on the 'backward' path reflected into possibly undesired directions), this need not always be relevant.

Further variants of outcoupling comprise using an outcoupling mirror 5 of the kind taught referring to other embodiments of the present invention while placing the saturable absorber elsewhere, for example as folding mirror, as the other end mirror as in Fig. 3, or, as absorbing layer, within the gain structure. The latter (integration of the saturable absorber in the gain structure) corresponds to a gain structure sometimes referred to as MIXSEL, see for example Appl. Phys. B. 88, 493 (2007), and it is an alternative for other embodiments, too, i.e. also for embodiments where the gain structure does not act as end reflector.

Also the embodiment of Figure 5 comprises a parabolic mirror. More in particular, the laser has two parabolic mirrors 30.1 , 30.2 focusing incoming parallel branches of the beam onto the pumped location on the gain structure. The parabolic mirrors 30.1 , 30.2 are arranged approximately symmetrically with respect to the normal onto the gain structure surface plane.

The embodiment of Figure 5 has a further feature that can be implemented independent of the other features of the embodiment of Fig. 5. Namely, whereas in the other depicted embodiments, the pumping radiation has been shown to be incident from a rather flat angle of about 45°, this need not be the case. In Fig. 5 the pumping radiation 12 is incident on the gain element from an approximately right angle. The embodiment of Fig. 5 can, however, also be implemented with pumping radiation incident from smaller angles, for example approximately 45° or more or less than 45°. In Herriot-cell-like designs with a curved mirror (or several curved mirrors) like the ones of Figures 3, 4, and 5 there may also be folding mirrors between the curved mirror(s) and the gain structure to make the laser more compact for a given resonator optical beam path length.

Possible further embodiments include embodiments in which the outcoupling mirror is not an end mirror but a folding mirror. In such a configuration, the outcoupling mirror may for example be a mirror that is only hit once on the path from one end mirror to the other end mirror.

Further, while the beam paths in the above-describe embodiments has been assumed to be on a plane - corresponding to the drawing plane in the figures - this need not be the case. Rather, also 3D-resonator configurations are possible.

The skilled person having understood the present invention will be able to come up with any number of other resonator designs, including designs with more sophisticated beam foldings.

Further, independent of the Laser resonator design, different modlockers can be used. Especially, instead of saturable absorbers with saturably absorbing semiconductor material, it is also possible to use carbon iianotube or Graphene based saturably absorbing material, for example also on a mirror.

An amplifier can for example be obtained if in any one of the laser designs of Figures 1 -3 or 5 the end mirrors 4, 5 are left away or in the design of Fig. 4 the end mirror 4 is left away or replaced by a re-directing element (mirror, fiber etc.) that does not retro-reflect. The locations of the end mirrors are then possible input and output locations of the amplifier. It is possible to provide fiber couplers at these locations that couple seed laser radiation out and onto the beam path shown in the figures, and/or into which amplified radiation is coupled after the multiple passes, respectively.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for emitting pulsed electromagnetic laser radiation, o the apparatus comprising a radiation directing arrangement comprising a plurality of reflectors, the radiation directing arrangement directing pulsed laser radiation on a closed path or on a non-closed path, o wherein if the laser radiation is directed on a closed path, the radiation direction arrangement comprises a first end reflector and a second end reflector and defines a laser resonator, and if the laser radiation is directed on a non-closed path, the apparatus comprises an input and an output, the apparatus further comprising: o an essentially plane surface-emitting semiconductor gain structure having a surface plane for emitting said laser radiation; o a pump for exciting said semiconductor gain structure to emit the laser radiation from the surface plane; o wherein the radiation direction arrangement is configured to direct the laser radiation on its path from the first end reflector or input to the second end reflector or output, respectively, a plurality of times onto the semiconductor gain structure to stimulate emission of the laser radiation from the semiconductor gain structure and thereby amplify the laser radiation.

2. The apparatus according to claim 1 , the radiation directing arrangement being configured to direct the laser radiation on a closed path, thereby defining a laser resonator, the apparatus further comprising a mode locker arranged in the beam path for mode locking the laser radiation.

The apparatus according to claim 2, wherein the mode locker is a passive mode locker.

The apparatus according to claim 3, wherein the mode locker comprises saturably absorbing material.

The apparatus according to claim 4, wherein the mode locker is a mirror comprising saturably absorbing material.

The apparatus according to claim 5, wherein the mirror comprising saturably absorbing material serves as one of the end reflectors.

The apparatus according to any one of the previous claims, the radiation directing arrangement being configured to direct the laser radiation on a closed path, thereby defining a laser resonator, wherein one of the end reflectors is an outcoupling mirror that transmits a fraction of the incident laser radiation.

The apparatus according to claim 1 , the radiation directing arrangement configured to direct pulsed laser radiation on a non-closed path, whereby the apparatus comprises a laser amplifier, wherein the input is configured to receive pulsed seed laser radiation.

9. The apparatus according to claim 8, further comprising a seed laser configured to emit pulsed seed laser radiation.

10. The apparatus according to claim 9, wherein the seed laser is a vertical external cavity surface-emitting laser and comprises a seed laser optical resonator being defined by a seed laser radiation direction arrangement comprising two seed laser end reflectors, and the seed laser optical resonator defining a seed laser radiation beam path; the seed laser further comprising:

- an essentially plane seed laser semiconductor gain structure having a surface plane for emitting said seed laser radiation;

- a seed laser pump for exciting said semiconductor gain structure to emit the seed laser radiation from the surface plane; - a mode locker arranged in the beam path for mode locking the seed laser radiation;

- wherein the seed laser radiation direction arrangement is configured to direct the seed laser radiation on its path from one of the seed laser end reflectors to the other one of the seed laser end reflectors a plurality of times onto the seed laser semiconductor gain structure to stimulate the emission of the seed laser radiation from the seed laser semiconductor gain structure.

1 1. The apparatus according to claim 9, wherein the seed laser is a fiber laser or a solid state laser.

The apparatus according to any one of the previous claims, wherein the radiation direction arrangement is configured so that a time duration between two subsequent incidences on the gain structure is less than five times an upper-state lifetime τ of the gain structure.

13. The apparatus according to claim 12, wherein the radiation direction arrangement is configured so that the time duration between two subsequent incidences on the gain structure is less than two times the upper-state lifetime τ of the gain structure.

14. The apparatus according to claim 13, wherein the radiation direction an-angement is configured so that the time duration between two subsequent incidences on the gain structure is less than the upper-state lifetime τ of the gain structure.

15. The apparatus according to any one of the previous claims, wherein an optical beam path length between two subsequent incidences on the gain structure is less than 45 cm.

16. The apparatus according to any one of the previous claims, wherein the gain structure is mounted on an active and/or passive cooler.

17. The apparatus according to any one of the previous claims, having a pulse repetition rate of less than 500 MHz.

18. The apparatus according to claim 17 having a pulse repetition rate of less than 150 MHz.

19. The apparatus according to any one of the previous claims being configured to produce pulses of a pulse energy of at least 3 nJ.

The apparatus according to any one of the previous claims being configured to emit pulses with a pulse duration of less than 100 ps.

The apparatus according to any one of the previous comprising a dispersive element arranged so that the laser radiation circulating in the laser resonator is influenced by the dispersive element.

The apparatus according to claim 21 wherein the dispersive element has a positive dispersion.

The apparatus according to any one if the previous claims, wherein the pump is an optical pump comprising a pumping radiation source arranged to cause pumping radiation impinging on the gain structure.

A laser for emitting pulsed electromagnetic laser radiation, the laser being a vertical external cavity surface-emitting laser and comprising an optical resonator being defined by a radiation direction arrangement comprising two end reflectors, and the optical resonator defining a laser radiation beam path; the laser further comprising: an essentially plane semiconductor gain structure having a surface plane for emitting said laser radiation; a pump for exciting said semiconductor gain structure to emit the laser radiation from the surface plane; a mode locker arranged in the beam path for mode locking the laser radiation; wherein the radiation direction arrangement is configured to direct the laser radiation on its path from one of the end reflectors to the other one of the end reflectors a plurality of times onto the semiconductor gain structure to stimulate the emission of the laser radiation from the semiconductor gain structure.

25. The laser according to claim 24, wherein the gain structure is configured to reflect the laser radiation directed onto the gain structure.

26. The laser according to claim 24 or 25, wherein beam spots of the different beam path branches incident on the gain structure overlap on the gain structure surface plane to an extent of at least at least 70%.

27. The laser according to any one of claims 24-26, wherein the mode locker is a passive mode locker.^"

28. The laser according to claim 27, wherein the mode locker comprises saturably absorbing material.

29. The laser according to claim 28, wherein the mode locker is a mirror comprising saturably absorbing material.

30. The laser according to claim 29, wherein the mirror comprising saturably absorbing material serves as one of the end reflectors.

The laser according to any one of claims 24-30, wherein one of the end reflectors is an outcoupling mirror that transmits a fraction of the incident laser radiation.

The laser according to any one of claims 24-31 , wherein an optical beam path length between two subsequent incidences on the gain structure is less than 45 cm.

The laser according to any one of claims 24-32, wherein the pump optical pump comprising a pumping radiation source arranged to pumping radiation impinging on the gain structure.

The laser according to any one of claims 24-33, wherein the gain structure is mounted on an active and/or passive cooler.

The laser according to any one of claims 24-34, having a pulse repetition rate of less than 500 MHz.

The laser according to claim 35 having a pulse repetition rate of less than 150 MHz.

37. The laser according to any one of claims 24-36 being configured to produce pulses of a pulse energy of at least 3 J.

The laser according to any one of claims 24-37 comprising a dispersive element arranged so that the laser radiation circulating in the laser resonator is influenced by the dispersive element.

The laser according to claim 38, wherein the dispersive element has a positive dispersion.

The laser according to any one of claims 24-39, wherein the radiation direction arrangement is configured so that a time duration between two subsequent incidences on the gain structure is less than five times an upper- state lifetime τ of the gain structure.

The laser according to claim 40, wherein the radiation direction arrangement is configured so that the time duration between two subsequent incidences on the gain structure is less than two times the upper-state lifetime τ of the gain structure.

The laser according to claim 41, wherein the radiation direction arrangement is configured so that the time duration between two subsequent incidences on the gain structure is less than the upper-state lifetime τ of the gain structure.

43. A laser amplifier for emitting pulsed electromagnetic laser radiati amplifier comprising: a radiation directing arrangement comprising a plurality of reflectors arranged to direct incident laser radiation from an input to an output, an essentially plane surface-emitting semiconductor gain structure having a surface plane for emitting said laser radiation; a pump for exciting said semiconductor gain structure to emit the laser radiation from the surface plane; wherein the radiation direction arrangement is configured to direct the laser radiation on its path from the input to the output a plurality of times onto the semiconductor gain structure to stimulate emission of the laser radiation from the semiconductor gain structure and thereby amplify the laser radiation.