HIGH-ENERGY HIGH-POWER DIODE PUMPED BROADBAND LASER

Abstract

A laser amplifier includes a volume configured to receive pump light from an array of laser diodes pump source, and a gain medium arranged within the volume and configured to amplify light in response to receiving the pump light. The gain medium comprises a first solid-state element configured to emit a first laser radiation having a peak centered at a first peak fluorescence wavelength and a second solid-state element configured to emit a second laser radiation having a peak centered at a second peak fluorescence wavelength. Each of the first and the second solid-state elements contain respective active laser ions. The difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm. The first solid-state element and the second solid-state element are cooled, for instance fluid-cooled.

Claims

1. A laser amplifier comprising: a volume configured to receive pump light from an array of laser diodes pump source, a gain medium arranged within the volume and configured to amplify light in response to receiving the pump light, the gain medium comprising a first solid-state element configured to emit a first laser radiation having a peak centered at a first peak fluorescence wavelength and a second solid-state element configured to emit a second laser radiation having a peak centered at a second peak fluorescence wavelength, each of the first and the second solid-state elements containing respective active laser ions, wherein a difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm, and wherein the first solid-state element and the second solid-state element are cooled.

2. The laser amplifier of claim 1 configured to receive input seed radiation from a seed radiation source, wherein the seed radiation comprises a first peak and a second peak respectively in correspondence of the first peak fluorescence wavelength and of the second peak fluorescence wavelength.

3. The laser amplifier of claim 1 wherein the gain medium comprises a third cooled solid-state element containing active laser ions and configured to emit a third laser radiation having a peak centered at a third peak fluorescence wavelength, wherein the third peak fluorescence wavelength is comprised between the first peak fluorescence wavelength and the second peak fluorescence wavelength.

4. The laser amplifier of claim 1, further comprising a cooling fluid directly contacting the solid-state elements.

5. The laser amplifier of claim 3, wherein at least one of the solid-state elements comprises a plurality of solid-state elements.

6. The laser amplifier of claim 5 wherein each one of the first solid-state element, the second solid-state element, and the third solid-state element comprises a respective plurality of solid-state elements.

7. The laser amplifier of claim 1, wherein at least one of the solid-state elements comprises a plurality of solid-state elements.

8. The laser amplifier of claim 1, wherein the first peak fluorescence wavelength is larger than 1000 nm and smaller than 1060 nm.

9. The laser amplifier of claim 1, wherein the first peak fluorescence wavelength is centered around 1053 nm.

10. The laser amplifier of claim 1, wherein the difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 20 nm and smaller than or equal to 30 nm.

11. The laser amplifier of claim 3, wherein the difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 20 nm and smaller than or equal to 30 nm.

12. The laser amplifier of any claim 1, wherein first solid-state element and the second solid-state element are arranged in different cooling enclosures and cooling cycles.

13. A laser device comprising: a laser amplifier comprising: a volume configured to receive pump light from an array of laser diodes pump source, a gain medium arranged within the volume and configured to amplify light in response to receiving the pump light, the gain medium comprising a first solid-state element configured to emit a first laser radiation having a peak centered at a first peak fluorescence wavelength and a second solid-state element configured to emit a second laser radiation having a peak centered at a second peak fluorescence wavelength, each of the first and the second solid-state elements containing respective active laser ions, wherein a difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm, and wherein the first solid-state element and the second solid-state element are cooled; a light pump source configured to generate pump light; and a seed radiation source configured to input to the laser amplifier seed radiation having a spectral shape comprising peaks in correspondence of each one of the first and second peak fluorescence wavelengths of the first and second solid-state elements, respectively.

14. The laser device of claim 13, further comprising a self-phase modulation unit configured to perform self-phase modulation of chirped pulse amplification (CPA) compressed short pulse laser radiation emitted by the laser device and a subsequent compression unit.

15. The laser device of claim 13, wherein the spectral shape comprises a further peak between the peaks corresponding to the first and second peak fluorescence wavelengths.

16. A method of amplifying light, comprising: providing a gain medium comprising a first solid-state element configured to emit a first laser radiation having a peak centered at a first peak fluorescence wavelength and a second solid-state element configured to emit a second laser radiation having a peak centered at a second peak fluorescence wavelength, each of the first and the second solid-state elements containing respective active laser ions, wherein, a difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm and the first solid-state element and the second solid-state element are cooled; including the gain medium in a volume of a laser amplifier; generating pump light by an array of laser diodes pump source; providing the generated pump light to the volume of the laser amplifier; and amplifying, by the gain medium in the volume, light in response to receiving the pump light.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0031] Embodiments of the present disclosure, which are presented for better understanding the inventive concepts, but which are not to be seen as being limiting, are now described with reference to the figures in which:

[0032] FIG. 1 shows a laser amplifier according to the known art.

[0033] FIG. 2 shows a laser amplifier according to a first embodiment of the present disclosure.

[0034] FIG. 3 shows a laser amplifier according to a variation of the first embodiment of the present disclosure.

[0035] FIG. 4A shows the fluorescence spectra of the solid-state elements according to an example of the first embodiment of the present disclosure.

[0036] FIG. 4B shows simulated laser radiation and seed radiation according to a second embodiment of the present disclosure.

[0037] FIG. 5 shows a laser amplifier according to a third embodiment of the present disclosure.

[0038] FIG. 6 shows the fluorescence spectra of the solid-state elements according to a third embodiment of the present disclosure.

[0039] FIG. 7 shows a block diagram of an implementation example of a laser according to an embodiment of the present disclosure and including the laser amplifier of the present disclosure.

[0040] FIG. 8 shows a block diagram relative to a method for lasing and amplifying according to a fourth embodiment of the present disclosure.

[0041] FIG. 9 shows the spectral profile of additional solid-state elements which can be employed according to additional embodiments of the present disclosure.

DETAILED DESCRIPTION

[0042] FIG. 1 shows a laser amplifier according to the known art. According to FIG. 1, there is provided a laser amplifier 100 comprising a volume 1 which includes a distributed gain medium. The volume 1 is part of a laser chamber. The gain medium comprises an input surface and an output surface which generally opposes the input surface.

[0043] The distributed gain medium also comprises a plurality of solid-state elements 2 (i.e., the gain material). The volume 1 is configured to include a cooling fluid 4, for instance a cooling liquid to mitigate the thermal stress.

[0044] The cooling fluid directly contacts the solid-state elements. For instance, the fluid flows about, or around, the solid-state elements so that the radiation generated by the solid-state elements directly crosses the cooling liquid.

[0045] The solid-state elements of the plurality of solid-state elements 2 are of a same type. For instance, all of the solid-state elements 2 are rare-earth (e.g., neodymium) doped glass, or yttrium lithium fluoride (Nd:YLF).

[0046] The solid-state elements may be held in respective fixed position within the volume 1.

[0047] Pump light 3 produced by a light pump source (not shown in the figure) impinges on the gain material, that is, on the plurality of the solid-state elements 2. A diode laser pump is an example of a light pump source.

[0048] The gain material can be pumped by the light pump source to produce excited state ions. Amplified pulses can then be produced by directing a plurality of light pulses to be amplified (e.g., the seed laser), through the gain medium. The pulses to be amplified can pass more than one time through the medium and hence can be further amplified.

[0049] FIG. 2 shows a laser amplifier according to a first embodiment of the present disclosure. The first embodiment may comprise components or elements similar to, or corresponding to, those of the laser amplifier of the known art described above. For the sake of brevity, description of similar or corresponding components or elements is not repeated. It will however be readily understood by those skilled in the art that said components or elements may be employed also in connection with the first embodiment, except when indicated otherwise.

[0050] According to the first embodiment, the laser amplifier comprises a volume configured to receive pump light from an array of laser diodes pump source. It also comprises a gain medium arranged within the volume and configured to amplify light in response to receiving the pump light.

[0051] The gain medium comprises two different types of solid-state elements. That is, it comprises a first solid-state element and a second solid-state element.

[0052] According to a variation of the first embodiment, one or more of the solid-state elements may be a glass. However, the present disclosure is not limited thereto, and other types of solid-state elements may be employed. For instance, crystals and/or ceramics may be employed.

[0053] A solid state-element of a given type is characterized by the properties of the emitted fluorescence spectra, for example by the position of the peak (or maximum) of the fluorescence emission spectra. This peak may also be referred to as fluorescence maximum. The peak position may be expressed by the wavelength at which the peak is centered and may be referred to as peak fluorescence wavelength. The peak may be an absolute peak or a relative peak (i.e., a local maximum).

[0054] Therefore, solid-state elements of different types are characterized by different fluorescence emission spectra and for instance by different positions of the peak of the fluorescence emission spectra.

[0055] It will be understood that different type of solid-state elements might be characterized by properties different than the position of the peak. For instance, they might be characterized by the respective full width half maximum (FWHM), so that even if two solid-state elements have the peak at a same spectral position (for instance at a same wavelength), they are of a different type if their FWHM is different.

[0056] The (fluorescence) emission spectrum is the radiation emitted by the solid-state element, for instance, in response to receiving pump light.

[0057] According to the first embodiment the gain medium comprises a first solid-state element 2A (i.e., a first type of solid-state element) and a second solid-state element 2B (i.e., a second type of solid-state element). The first solid-state element 2A is configured to emit a first laser radiation having a peak centered at a first peak fluorescence wavelength 1. The second solid-state element 2B is configured to emit a second laser radiation having a peak centered at a second peak fluorescence wavelength 2.

[0058] Each of the first and the second solid-state elements contain respective active laser ions.

[0059] The first solid-state element and the second solid-state element are selected so that the difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm. In particular, it has been determined that a pair of solid-state materials bound by this spectral relationship are suited to produce the desired pulse duration. On the other side, materials lying outside this range result in less favorable pulse properties.

[0060] Moreover, the first solid-state element and the second solid-state element are cooled, for instance liquid cooled.

[0061] According to an embodiment, the first solid-state element may be a LG-770 glass and the second solid-state element may be a K-824 glass. This combination is particularly advantageous since in view of the larger than 10 nm shift in the respective emission peaks, it allows to achieve FWHM spectral output larger than 16 nm with a peak wavelength between 1050 nm and 1070 nm. Such a spectral content can produce a pulse with duration shorter than 100 fs.

[0062] The properties of these material are well-known in the field of endeavor of the present disclosure and are, for instance, described in the book Nd-doped Laser Glass Spectroscopic and Physical Properties, Band 1 M-95, Rev. 2, Stanley Edward Stokowski, Lawrence Livermore National Laboratory, University of California, 1981.

[0063] However, the present disclosure is not limited thereto and according to another example, the first element may be a LG-770 glass and the second solid-state element may be a L-65. The L-65 is a glass which is also well-known in the field of endeavor of the present patent application and is also described in the above-referenced book.

[0064] It will however be understood that the present disclosure is not limited by the specific examples of solid-state materials given and other solid-state material can readily be employed as long as the respective peak fluorescence wavelengths satisfy the above spectral condition.

[0065] The solid-state elements may have the form of a disk or of a thin slab. The thickness of the solid-state elements can be chosen based on the desired cooling effect.

[0066] A laser including the laser amplifier of the first embodiment can reach energy larger than or equal to 100 J, a pulse duration equal to or smaller than 100 fs and a pulse repetition frequency of 10 Hz. The laser may also be scalable to higher average and peak power.

[0067] The diode pumping enables to improve efficiency and to reduce heat delivery to the active medium. The cooling provides together with diode pumping high average power capabilities.

[0068] The laser amplifier of the present disclosure, in view of the combination of mixed solid-state elements, diode pumping, and cooling, enables to obtain laser radiation having high efficiency, high average power, high peak power capabilities allowing high repetition rate operation of the system.

[0069] For instance, the laser amplifier of the present disclosure enables to obtain laser radiation having (i) energy after compression equal to or larger than 50 J and scalable up to or larger than 100 J, (ii) peak power of about 1 PW, and (iii) average power of 500 W or higher.

[0070] According to a variation of the first embodiment, the laser radiation can be stretched out temporally and spectrally. Afterwards, the pulse may be amplified and then compressed according to the chirped pulse amplification (CPA) technique. However, the present disclosure is not limited thereto and other techniques, different from the CPA technique, may also be applied. For instance, also the Optical Parametric Chirped Pulse Amplification (OPCPA) technique may be applied.

[0071] Accordingly, the laser parameters can be stretched to energy larger or equal to 200 J, pulse duration equal to or smaller than 50 fs, and a pulse repetition frequency substantially larger than or equal to 10 Hz.

[0072] While the figures show pump light crossing the solid-state elements perpendicularly, it will be apparent that the present disclosure is not limited thereto and configurations wherein the pump light does not impinge perpendicularly on the solid-state elements are also possible and within the scope of the present disclosure.

[0073] FIG. 3 shows a laser amplifier according to a variation of the first embodiment of the present disclosure.

[0074] In particular, according to FIG. 3, each of the first solid-state element 2A and the second solid-state element 2B comprises two solid-state elements.

[0075] More generally, according to the present disclosure, the first type of solid-state element may comprise a first plurality M of solid-state elements and the second-solid state element may comprise a second plurality N of solid-state elements. M may be equal to N. Alternatively M and N may be different. M and N are both integer number equal to, or larger than, 2.

[0076] Moreover, FIG. 3 shows a case wherein the first type of solid-state element 2A and the second type of solid-state element 2B are placed on different sides of the volume 1. However, the present disclosure is not limited thereto, and according to a further variation of the embodiment, the first solid-state element 2A and the second solid-state element 2B may be interlaced. That is, for example, a given second solid-state element 2B may be placed between two first type of solid-state element 2A or vice versa.

[0077] FIG. 4A shows the spectra of the solid-state elements according to an implementation of the first embodiment of the present disclosure.

[0078] The x-axis shows the wavelength of the emission spectra and the y-axis the normalized intensity of the spectra.

[0079] In particular, it shows laser radiation emitted by a laser comprising a laser amplifier employing LG-770 glass (an example of the first solid state element) and K-824 glass (an example of the second solid-state element). As it can be seen, in the example provided, the emission peak (or peak fluorescence wavelength) of the first solid-state element is located at approximately 1053 nm. Further, in the example provided, the emission peak (peak fluorescence wavelength of the second solid-state element) is located at approximately 1065 nm.

[0080] As it will be apparent to those skilled in the art, the emission peaks can be measured, for instance, via a spectrometer which may have an error margin of approximately 1 nm.

[0081] FIG. 4B shows simulated laser radiation and simulated seed radiation according to a second embodiment of the present disclosure. This embodiment is described also with reference to the first embodiment and by placing special emphasis on the differences.

[0082] The solid line in FIG. 4B shows the amplified shaped seed which is fed to the laser amplifier. The seed is produced by a seed radiation source. It will be understood that the seed radiation depicted has been amplified for illustration purposes and is hence not in scale with the actual seed inputted in the laser amplifier.

[0083] The dashed line shows instead the laser radiation emitted by a laser comprising the laser amplifier according to the first embodiment of the present disclosure, wherein LG-770 is used as the first solid-state element and K-824 is used as the second solid-state element.

[0084] In the figure, the wavelength, expressed in nm, is reported along the x-axis, whereas the normalized fluorescence spectrum with respect to peak fluorescence is reported in the y-axis. As indicated by the double arrow, the laser emission spectrum has a broad band of about 30 nm (FWHM).

[0085] According to the second embodiment, the laser amplifier may be configured to receive as input shaped seed radiation by a seed radiation source. The seed radiation may have a predetermined spectral shape comprising peaks in correspondence of each one of the peak fluorescence wavelength of the solid-state elements and also preferably have an additional peak comprised therebetween. The seed shape is such that the output radiation of the laser is optimized.

[0086] As used herein, the phrase in correspondence of the peak fluorescence wavelength may for instance mean that the seed peak and the corresponding fluorescence peak overlap within the error margins. Alternatively, it may mean that the seed peak and the fluorescence peak are within a given predetermined range in the wavelength domain. For instance, the predetermined range may be within 5% of the peak fluorescence wavelength.

[0087] In other words, the pre-shaped seed radiation may have a predetermined spectral shape comprising three peaks of which, the first peak being in correspondence of the first peak fluorescence wavelength of the first solid-state elements and the second peak being in correspondence of the second peak fluorescence wavelength of the second solid-state elements. The third peak is preferable.

[0088] Pre-shaped means that the shape, or form of the seed radiation has been determined prior to its employment with (i.e., prior to the seed to) the laser amplifier. In particular, the shape has been determined a priori on the basis of the gain medium.

[0089] In view of the shaped seed, it is possible to enhance the overall output spectrum of the laser amplifier according to the first embodiment in order to achieve even shorter pulses. According to a specific, non-limiting example, the shaped seed can be generated with the method of Optical Parametric Chirped Pulse Amplification (OPCPA) balance where the seed pulse is chirped in frequency and the spectral content is shaped by the intensity of the overlapping pump in the parametric amplification process. In this connection, reference can also be made to Spectral pulse shaping of a 5 Hz, multi-joule, broadband optical parametric chirped pulse amplification frontend for a 10 PW laser system, BATYSTA et al., Vol. 43, No. 16, 15 Aug. 2018, Optics Letters.

[0090] The pump pulse is in turn shaped electronically.

[0091] FIG. 5 shows a laser amplifier according to a third embodiment of the present disclosure. This embodiment is described also with reference to the first and/or second embodiment and by placing special emphasis on the differences. For the sake of brevity, the description of the corresponding components is not repeated.

[0092] According to the third embodiment, there are provided three different types of solid-state elements.

[0093] An example of the third solid-state element may be the silicate LG-680 produced by the company Schott (registered trademark). As also depicted in FIG. 6, the silicate LG-680 has an emission peak of about 1062 nm. That is, the emission peak of the third solid-state material is comprised between the emission peak of the first solid-state material and the second solid-state material.

[0094] Incidentally, and as schematically shown in FIG. 5 by the dashed lines, the first solid-state element, the second solid-state element and the third solid-state element may be included in separate cooling enclosures, 5A, 5B, and 5C. In this case, the solid state-elements may have separate, i.e., independent, cooling cycles. This may be advantageous since it may allow to design the cooling so as to take into account the different properties of the different solid-state materials.

[0095] However, the present disclosure is not limited thereto, and the solid-state elements may also be in a same cooling enclosure.

[0096] Moreover, the solid-state materials can be pumped by the same diode array or can be pumped by separate diode array.

[0097] FIG. 6 corresponds to FIG. 4A wherein additionally the fluorescence emission spectrum of the silicate LG-680 is represented.

[0098] According to the third embodiment, it is possible to improve the shape of the spectral profile of the radiation emitted by the laser amplifier. In particular, also depending on the specific spectral properties of the selected first solid-state element and second solid-state element, the third solid state element might prevent the formation of two humps (or local peaks) in the spectral profile of the emitted radiation.

[0099] The two humps might occur for instance if the emission peak of the first and second solid state material are too far apart in the wavelength space. In other words, the third solid state element may improve the spectral characteristics by spectrally filling the gaps between the emission spectrum of the first solid-state element and the second solid-state element.

[0100] Hence, it becomes possible to obtain a broadband spectrum (and hence short laser pulses) while at the same time preventing deterioration of the desired gaussian-like shape of spectrum of the emitted laser pulse.

[0101] While according to the third embodiment, only three different types of solid-states elements have been discussed, the present disclosure is not limited to this. In particular, configurations wherein the gain medium comprises, four, five or more solid-state elements is also possible.

[0102] In case, the gain medium comprises a third solid-state element, the seed shape may also comprise an additional fourth peak in correspondence of the third peak fluorescence wavelength of the third solid-state elements.

[0103] More generally, the seed shape may comprise a number of peaks equal to or larger than the number of solid-state elements.

[0104] A given solid-state element may have more than a single peak fluorescence wavelength (for instance an absolute peak and a relative peak). Therefore, and even more generally, the shaped seed may comprise a number of peaks larger than the total count of peak fluorescence wavelengths of the solid-state elements of the laser amplifier.

[0105] FIG. 7 shows a block diagram of an implementation example of a laser according to an embodiment of the present disclosure.

[0106] The laser comprises a CPA stage 1 denoted by reference 200, a OPCPA stage pump laser, denoted by reference 210 an OPCPA stage denoted by reference 220 a glass amplifier stage denoted by reference 230 and a compressor 240.

[0107] The CPA stage 1 may include a plurality of units and in particular a fs source 201, a pulse stretcher 202, an amplifier 203, a compressor 204 and a contrast filter 205. At the end of the CPA stage 1, and according to a specific, non-limiting example, the radiation may have the following properties: Pulse duration 30 fs, central wavelength 1060 nm, BW>50 nm, Energy 10 mJ, and being characterized by a high contrast.

[0108] The OPCPA stage pump laser may include a plurality of units and in particular, a cw fiber source 211, a temporal pulse shaper 212, a DPSSL Power Amplifier Nd:YAG 213, DP Regen amplifier 214 a spatial shaping unit 215, and a SHG unit. At the end of the OPCPA stage pump laser, according to a specific, non-limiting example, the pulse may have the following properties: energy 3 J, central wavelength 532 nm, pulse duration 5 ns, and it may be spatially and temporally shaped.

[0109] At the OPCPA stage the pulse is characterized by Stretcher, 5 ns chirped, 0.5 J, 50 nm, spectrally pre-shaped.

[0110] The glass amplifier stage may include a silicate glass cooled disk amplifier 231 and characterized by the following properties, x20(25 mm)2, 10 J, a phosphate glass cooled disk amplifier 232, and characterized by the following properties, x12, (50 mm)2, 120 J, diode pump 233 having 880 nm and 0.6 MW, and diode pump 234 having 880 nm, 21.2 MW.

[0111] That is, at the glass amplifier stage, the glasses are cooled by means of a cooling liquid.

[0112] At the compressor 240, and according to a specific, non-limiting example, the pulse may have the following characteristics: sample of: 100 J, <100 fs, 10 Hz.

[0113] FIG. 8 shows a block diagram relative to a method for lasing and amplifying light according to a fourth embodiment of the present disclosure.

[0114] The method of lasing and amplifying light comprises the following steps.

[0115] A first step (S1) comprises providing a gain medium comprising a first solid-state element configured to emit a first laser radiation having a peak centered at a first peak fluorescence wavelength and a second solid-state element configured to emit a second laser radiation having a peak centered at a second peak fluorescence wavelength, each of the first and the second solid-state elements containing respective active laser ions, wherein the difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm.

[0116] A second step (S2) comprises cooling the first solid-state element and the second solid-state element.

[0117] A third step (S3) comprises including the gain medium in a volume of the laser amplifier.

[0118] A fourth step (S4) comprises generating pump light by an array of laser diodes pump source.

[0119] A fifth step (S5) comprises providing the generated pump light to the volume of the laser amplifier.

[0120] A sixth step (S6) comprises amplifying, by the gain medium, light in response to receiving the pump light.

[0121] It will be understood that the steps of the method need not to be necessarily performed in the order described and certain steps may be performed before others. For instance, the second step of cooling may be performed after the third step of including the gain medium in the volume.

OTHER EMBODIMENTS

[0122] According to a variation of the embodiments described above, the laser system my further comprise a self-phase modulation unit configured to perform self-phase modulation (SPM) of the laser radiation emitted by the laser amplifier. Performing SPM enables achieving subsequent additional pulse compression in an additional compression unit.

[0123] In view of the additional SPM, the light emitted by the laser amplifier can be compressed to even shorter pulses.

[0124] The laser radiation comprises, or consists of, a respective wavelength or a respective range of wavelengths. Similarly, the first, second, and third laser radiation each comprise or consist of a respective first, second, and third wavelength, or a respective first, second, and third range of wavelengths.

[0125] FIG. 9 shows the spectral profile of solid-state elements which can be employed according to embodiments of the present disclosure. While in the above description reference has been made to LG-770, K-824 and silicate LG-680, the present disclosure is not limited thereto an additional solid-state material may be used, as for instance the glass 9016, L-65 and APG-1. The first glass is described by Nd-doped Laser Glass Spectroscopic and Physical Properties, Volume 1, M-95, Rev. 2. The second glass is described by Nd-doped Laser Glass Spectroscopic and Physical Properties, Volume 2, M-95, Rev. 2. The third glass is commercialized by the company Schott (registered trademark).

[0126] Any pair of these material can be selected as long as the difference between the first peak fluorescence wavelength and the second peak fluorescence wavelength is larger than or equal to 10 nm and smaller than or equal to 60 nm.

[0127] FIG. 9 corresponds to FIG. 6 (and hence to FIG. 4A) wherein additional fluorescence emission spectra, corresponding to additional solid-state elements, are shown.

[0128] In view of the large shift, an advantageous combination of solid-state materials may be LG-770 and 9016.

[0129] Moreover, any of the material shown in the figure can be used as the third (or the fourth) solid-state material as long as the respective fluorescence peak is comprised between the first fluorescence peak and the second fluorescence peak.

[0130] Incidentally, as it can be seen from the figure, the peak fluorescence wavelength of LG-770 is at around 1053 nm, the peak fluorescence wavelength of APG-1 is at around 1056 nm, the peak fluorescence wavelength of silicate LG-680 is at around 1062 nm, the peak fluorescence wavelength of K-824 is at around than 1065 nm, the peak fluorescence wavelength of L-65 is at around 1070 nm, and the peak fluorescence wavelength of 9016 is at around 1090 nm.

[0131] These materials are well-known and described in the literature or commercially available (e.g., Schott, registered trademark).

[0132] According to an embodiment, the emission spectrum of the laser amplifier extends beyond the range comprised between the lowest peak fluorescence wavelength (e.g., the first peak fluorescence wavelength) and highest peak fluorescence wavelength (e.g., the second peak fluorescence wavelength) of the selected solid-state materials. A suitable range is the peak-to-peak range extended, on the right of the highest peak side, of the range where the fluorescence of the second solid-state material drops to 80% of its maximum and extended, on the left of the lowest peak side, of the range where the fluorescence of the first solid-state material drops to 80% of its maximum.

[0133] If the intensity of the fluorescence spectrum at a given wavelength is 80% of the intensity at the peak fluorescence wavelength, the corresponding gain is lower than that corresponding to the peak fluorescence wavelength. Such lower gain can be compensated by appropriately seeding the gain medium. In particular, the seed radiation may be spectrally shaped to provide more energy at the wavelengths comprised between (less than) 100% of the peak maximum and 80% of the peak maximum.

[0134] Appropriately selecting the spectral shape of the seed radiation allows hence to achieve a broad and flat output spectrum within the range.

[0135] In other words, the optimum seed shape is a function of the fluorescence spectrum of the solid-state materials.

[0136] Although detailed embodiments have been described, these only serve to provide a better understanding of the disclosure and the independent claims and are not to be seen as limiting. Rather, the particular embodiments disclosed must be interpreted as representing the concepts that are the basis of this disclosure. As it will be immediately apparent to a person of ordinary skill in the art, the aspects and the features disclosed in conjunction with an embodiment can be freely combined with other embodiments without thereby departing from the teachings of the present disclosure.

[0137] All of the patents, applications and publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.

[0138] These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.