Light source for optically pumping a laser-active medium

Abstract

The invention relates to a light source for optically pumping a laser-active medium (7). It is an object of the invention to provide a pump light source, which is improved, in particular is more efficient, compared with the prior art, the pump light of which is generated by non-linear frequency conversion. For this purpose, according to the invention, the light source comprises a multi-core optical waveguide (1), which comprises a plurality of light-guiding cores (2) extending along the longitudinal extent of the multi-core optical waveguide (1). A frequency converter (4) is connected downstream of the multi-core optical waveguide (1) in the beam path, which is designed to convert individual beams (5) emitted from the cores (2) of the multi-core optical waveguide (1) into frequency-converted individual beams (6) in separate non-linear elements of the frequency converter (4) each assigned to the respective individual beams (5). A superposing optical unit is provided, to superimpose the frequency converted individual beams (6) in the laser-active medium (7), in order to optically pump the latter. Finally, the invention relates to a laser system, which uses such a light source as a pump light source, and to a method for optically pumping a laser-active medium (7).

Description

[0001] Light source for optical pumping of a laser-active medium

[0002] The invention relates to a light source for optically pumping a laser-active medium.

[0003] Furthermore, the invention relates to a laser system that uses such a light source.

[0004] Pump light sources are known in various designs from the prior art. Laser-active media in lasers or optical amplifiers often require a pump light source to generate optical amplification. The absorption of the pump light supplied by the pump light source in the laser-active medium results in a population inversion of electronic energy levels. Suitable pump light sources for doped solids as laser-active media include flash lamps, semiconductor light sources (light-emitting diodes or laser diodes), or solid-state laser systems with frequency conversion. The following requirements are placed on the pump light source:

[0005] • The spectral emission characteristics of the pump light source must correspond to the absorption spectrum of the laser-active medium.

[0006] A spatial distribution of the population inversion that is as homogeneous as possible is desirable in order to achieve the most homogeneous optical amplification possible. • When using pump light sources with frequency conversion of the pump light, the conversion efficiency of the pump light source is of great importance for high-performance applications.

[0007] Laser diodes have established themselves as pump light sources for laser systems due to their high efficiency and power. However, certain wavelength ranges combined with high power and brilliance of the pump light, e.g., in the visible spectral range, have not yet been accessible. This severely limits the use of laser diodes to generate population inversion in, for example, titanium:sapphire-based laser systems. Instead, flash lamps or (pulsed) solid-state lasers in combination with frequency doubling, e.g., based on Nd:YAG, are commonly used as pump light sources. However, these offer only low efficiency in converting electrical power into optical power, or they are limited in their pulse repetition frequency.

[0008] Against this background, it is the object of the invention to provide an improved pump light source whose pump light is generated by nonlinear frequency conversion.

[0009] The invention achieves this object by a light source for optically pumping a laser-active medium, with a multi-core optical waveguide which comprises a plurality of light-guiding cores running along the longitudinal extent of the multi-core optical waveguide, a frequency converter connected downstream of the multi-core optical waveguide in the beam path, configured to convert individual beams emitted from the cores of the multi-core optical waveguide into frequency-converted individual beams in separate non-linear elements of the frequency converter which are respectively assigned to the individual individual beams, and superposition optics configured to superpose the frequency-converted individual beams in the laser-active medium.

[0010] The approach of the invention is based on subjecting the individual beams, i.e., the individual emissions from the cores of the multi-core waveguide (e.g., realized as a multi-core fiber), to spatially separated frequency conversion (e.g., by second harmonic generation (SHG) in a nonlinear crystal). The frequency-converted individual beams thus generated separately are then superimposed in the laser-active medium to optically pump the medium, i.e., to generate the desired population inversion. The total available energy of the pump light results from the sum of the pulse energies of the individual beams due to the superposition.

[0011] Fiber optic systems are characterized by high efficiency and a simple, compact design. Furthermore, the combination of fiber geometry and light guidance allows operation at high average powers and thus, in pulsed operation, high pulse repetition frequencies. On the other hand, the size of the light-guiding region, i.e., the core of the optical fiber, limits the achievable power or pulse energy through destruction, nonlinear effects, or other limiting effects. Increasing the diameter of the optical fiber reduces beam quality due to the occurrence of a larger number of transverse modes in the optical fiber. In amplifier mode, i.e., with optical amplification in the optical fiber, additional instabilities (so-called mode instabilities) can occur due to the heat introduced.Due to these properties, pump light sources based on fiber optics with frequency conversion are not yet optimally suited for optically pumping a laser amplifier at high power (and pulse energy). The necessary frequency conversion requires good beam quality, particularly beam stability and beam homogeneity, for high conversion efficiency, which cannot be achieved with conventional systems.

[0012] To counteract these problems, the invention proposes parallelization through the use of a multi-core optical waveguide (multi-core fiber), which comprises a plurality of light-guiding cores. Each of these cores only has to withstand a portion of the total (average) power and pulse energy. Accordingly, a high quality of the individual beams emitted by the individual cores can be achieved. According to the invention, the individual beams are fed to the various nonlinear elements for frequency conversion. A separate nonlinear element is provided for each individual beam, i.e., one assigned only to this individual beam (1:1 assignment). Due to the high beam quality of the individual beams, the efficiency of the frequency conversion in the individual nonlinear elements is high.

[0013] Due to the separate frequency conversion in the nonlinear elements, any limitations of the respective element (e.g., due to heat input or destruction) are restricted to the individual beam. Consequently, power scaling can be achieved with the number of cores. One advantage of the multi-core optical fiber compared to the use of a multimode optical fiber with equivalent achievable power / pulse energy is the significantly longer Rayleigh length of the individual beams, which enables the maintenance of high intensities over longer distances in the nonlinear elements and thus, in turn, higher conversion efficiency.

[0014] As already mentioned, the generated frequency-converted individual beams are superimposed in the laser-active medium to optically pump it. This is achieved by the superposition optics of the light source according to the invention. The superposition optics can be implemented using suitable optical components or simply by a free beam guide, which does not require any dedicated optical components. The superposition of the independent frequency-converted individual beams results in an intensity profile in the laser-active medium as the sum of the spatial and temporal intensity distributions of the frequency-converted individual beams, independent of the phase of the individual beams. This reduces temporal fluctuations and avoids the need for phase control in the individual beams.The superposition can be adapted to the shape and dimension of the desired pumping region in the laser-active medium using the superposition optics.

[0015] Since the individual beams in the multi-core optical fiber are generated independently of one another, the beam profile superimposed from the individual beams can be homogenized or adapted to a desired beam profile in spatial and temporal terms, even if the beam profiles of the individual beams deviate from one another randomly or in a controlled manner. This means that in principle the use of larger, multi-mode light-guiding cores in the multi-core optical fiber is possible, with the resulting lower beam quality of the individual beams. The individual beam profiles of the individual beams before and after the spatially separate frequency conversion can also deviate from one another. Likewise, the individual emissions of the individual beams can have different temporal characteristics. The use of multi-mode light-guiding cores allows both the individual cores and the frequency conversion to be carried out at higher power orto operate with pulse energy, although a certain loss of efficiency in frequency conversion can be accepted.

[0016] The aforementioned deviations in the frequency-converted individual beams can also be deliberately introduced (e.g., by emission at different wavelengths from the different nuclei, different polarizations from nucleus to nucleus, pulse durations, or pulse energies) in order to manipulate the properties of the superposition of the frequency-converted individual beams and, for example, adapt them to a desired state. For this purpose, a modulator can be provided, for example, which is designed to modulate the individual beams independently of one another with regard to amplitude, phase, and / or polarization. A controller can be provided to control the modulator according to a predetermined desired illumination of the laser-active medium by the individual beams superimposed therein.

[0017] In one possible embodiment, the nonlinear elements are formed by essentially spatially separate regions of a nonlinear medium, in particular by various spaced-apart sections of a (single, ie, monolithic) nonlinear crystal. However, it is also conceivable to provide several separate nonlinear crystals, ie, a separate nonlinear crystal for each individual beam.

[0018] In a practical implementation of the light source, an imaging optics system can be provided in the beam path between the multi-core optical fiber and the frequency converter. This system is designed to image the individual beams emitted from the cores of the multi-core optical fiber onto the nonlinear elements assigned to them. In one possible embodiment, the cores of the multi-core optical fiber can be doped with rare earth ions, particularly ytterbium ions or neodymium ions, with the multi-core optical fiber being designed to optically amplify the light propagating along the cores. For this purpose, the multi-core optical fiber must itself be optically pumped, for example, by light from a laser diode coupled into a cladding of the multi-core optical fiber surrounding the cores.

[0019] When the cores of the multi-core optical fiber are doped with ytterbium ions, the wavelength of the light propagating in the multi-core optical fiber can be 1000 nm to 1100 nm, preferably 1030 nm. The wavelength of the individual beams generated by frequency doubling can be 500 nm to 550 nm, preferably 515 nm. The light generated in this way, for example at 515 nm, is ideally suited for optically pumping a titanium:sapphire crystal as a laser-active medium (e.g., a Ti:Sa laser) at high power and high efficiency.

[0020] In one possible embodiment, the light source can be implemented as a laser, in whose laser resonator the multi-core optical waveguide is located, wherein the cores of the multi-core optical waveguide form the laser medium of the laser. In this case, an optical amplifier for amplifying the light can be connected downstream of the laser, which comprises a further multi-core waveguide with a plurality of cores, wherein the cores of the further multi-core waveguide, which are doped with rare earth ions and guide the individual beams emitted by the laser, form the amplification medium of the additional optical amplifier. The laser can emit pulsed laser radiation as light, in particular by Q-switching, by cavity dumping or by amplitude modulation taking place outside the laser resonator. The pulse duration of the pulsed laser radiation is, for example, in the nanosecond range, in particular in the range from 10 ns to 100 ns.The temporal pulse shape of the laser pulses can be essentially rectangular. Such pulsed laser radiation (generated in the laser, for example, at approximately 1030 nm and frequency-doubled to approximately 515 nm) is ideally suited for optically pumping a titanium:sapphire crystal at high power. The light source of the invention can be used as a pump light source in a laser system comprising a laser resonator with a laser-active medium located therein, wherein the pump light source or several such pump light sources optically pump the laser-active medium in parallel.

[0021] Finally, the invention also relates to a method for optically pumping a laser-active medium, comprising the following method steps: optically amplifying light in a multi-core optical waveguide comprising a plurality of cores doped with rare earth ions running along the longitudinal extent of the multi-core optical waveguide, which cores guide the light,

[0022] Conversion of individual beams emitted from the cores of the multi-core optical waveguide into frequency-converted individual beams in separate non-linear elements of a frequency converter, each assigned to the individual beams, and

[0023] Superposition of the frequency-converted individual beams in the laser-active medium.

[0024] Embodiments of the invention are explained in more detail below with reference to the drawings. They show:

[0025] Fig. 1 : schematic representation of a pump light source according to the invention as a block diagram;

[0026] Fig. 2: schematic representation of a pump light source according to the invention as a block diagram with modulator;

[0027] Fig. 3: Schematic representation of a pump light source according to the invention as a block diagram with modulator and amplifier; Fig. 4: Refractive index profile of a core in a multi-core waveguide of a pump light source according to the invention and the resulting spatial beam profile.

[0028] Fig. 1 shows a pump light source according to the invention with a multi-core optical waveguide 1, which comprises a plurality of laser light-emitting cores 2 running along the longitudinal extent of the multi-core optical waveguide 1. The cores 2 are embedded in a common cladding 3 of the multi-core optical waveguide 1 and spaced apart from one another. Outside the cladding 3, the multi-core optical waveguide can be surrounded by further layers, not shown (e.g., as protection or to ensure mechanical stability). A frequency converter 4 in the form of a non-linear crystal (e.g., made of beta-barium borate or lithium borate) is connected downstream of the multi-core optical waveguide 1 in the beam path, which frequency converter converts the individual beams 5 emitted from the cores 2 of the multi-core optical waveguide 1 into frequency-converted individual beams 6 separately from one another, i.e., in essentially spatially spaced-apart regions of the non-linear crystal 4.The distance between two adjacent individual beams 5 (measured from center to center of the respective beam cross-sections) should be greater than the diameter (1 / e. 2-diameter) of the individual beams 5. Due to the resulting free beam guidance of the frequency-converted individual beams 6, the nonlinear crystal 4 simultaneously forms, in the example shown, a superposition optics system through which the frequency-converted individual beams 6 are superimposed in a laser-active medium 7, which is (optionally) located in a laser resonator formed by end mirrors 8, in order to optically pump the laser-active medium 7. In the process, a laser beam 9 is generated. In the beam path between the multi-core optical fiber 1 and the frequency converter 4, an imaging optics system 10, consisting of two lenses, is provided to image the individual beams 5 emitted from the cores 2 of the multi-core optical fiber 1 onto the regions of the nonlinear crystal 4 assigned to them. The imaging scale defines the size and thus also the light intensity in the spatially separated regions in the nonlinear crystal 4.This design degree of freedom can be used to optimize conversion efficiency. After the crystal-based frequency conversion, a separation of the primary radiation, i.e., the radiation at the wavelength of the individual beams 5 before frequency conversion, from the frequency-converted radiation 6 before the superposition of the spatially separated frequency-converted individual beams 6 by free beam propagation is expediently carried out (not shown for reasons of clarity). The superposition of two temporally synchronized frequency-converted arrays of individual beams 6 of orthogonal polarization on a polarizer (not shown) is also conceivable. Additional optical imaging by suitable superposition optics can be used to generate the beam profile appropriate for the laser-active medium 7 and thus to create an optimal population inversion therein.

[0029] In the embodiment of Fig. 2, the pump light source comprises a pulsed seed laser 11. The seed laser 11 is based on a multi-core waveguide 1 doped with ytterbium ions as the laser medium. Pulsed operation is achieved by Q-switching, cavity dumping, or external modulation. A modulator 12 can be integrated into the seed laser 11. Alternatively or additionally, an external modulator 13 can be provided. With the seed laser 11, spatially separated, independent individual beams at a wavelength of approximately 1 pm can be generated in the individual cores 2 of the multi-core waveguide 1 with a pulse duration of a few nanoseconds, but also longer pulses in the millisecond range.The external modulator 13 can be advantageous for temporally shaping the individual pump light pulses, thereby compensating for saturation-induced pulse shaping in any subsequent optical amplifier and ultimately having temporal rectangular pulses available for frequency conversion in the frequency converter 4. This increases the conversion efficiency. A spatial flat-top profile of the spatially separated regions in the nonlinear crystal 4 (see Fig. 1 ) is advantageous for the efficiency of frequency conversion. This can be achieved by adapting the refractive index profile of the individual cores 2 of the multi-core optical waveguide 1. The left-hand diagram in Fig. 4 outlines a possible refractive index profile (refractive index n as a function of the radius r of an individual core) and the transverse eigensolution propagating therein (right-hand diagram illustrating the light intensity in the cross-section of the individual beam).The beam profile is close to a flat-top profile. The spatially separated individual beams emitted by the seed laser 11 can optionally be amplified in an optical amplifier 14 with an additional laser-active multi-core optical fiber 1 to achieve high pulse energy and high average power, as shown in Fig. 3.

[0030] In one possible embodiment, the cores 2 of the multi-core optical waveguide 1 guide the laser light in a multi-mode fashion, which is designed so that the quality of the emitted individual beams 5 is nevertheless virtually diffraction-limited. A taper (a narrowing of the individual cores 2 (not shown) at the input of the multi-core waveguide 1 in the seed laser 11 or in the optical amplifier 14) can achieve a dominant amplification of the transverse fundamental mode in each core 2.

[0031] Even if in Fig. 2 and Fig. 3 the individual beams 5 and the frequency-converted individual beams 6 are schematically represented as a whole by a single wide connecting line between the individual components 11, 13, 14 and 4 or behind the frequency converter 4, they are, as in Fig. 1, separate beams which are only superimposed in the laser-active medium (not shown in Fig. 2 and Fig. 3) in order to optically pump it.

[0032] It should be noted that the light source according to the invention can be used not only as a pump light source, but also for other purposes, e.g. for laser material processing.

[0033] Furthermore, the frequency conversion by generating the second harmonic described in connection with the exemplary embodiments is a selected example adapted to the application of "pumping a Ti:Sa laser crystal." Likewise, the approach of the invention also allows for other frequency conversion processes, e.g., the generation of the third harmonic (THG) and thus the generation of UV light.

Claims

Patent claims 1 . A light source for optically pumping a laser-active medium (7), comprising a multi-core optical waveguide (1) which comprises a plurality of light-guiding cores (2) running along the longitudinal extent of the multi-core optical waveguide (1), a frequency converter (4) connected downstream of the multi-core optical waveguide (1) in the beam path, configured to convert individual beams (5) emitted from the cores (2) of the multi-core optical waveguide (1) into frequency-converted individual beams (6) in separate non-linear elements of the frequency converter (4) which are respectively assigned to the individual individual beams (5), and superimposing optics configured to superimpose the frequency-converted individual beams (6) in the laser-active medium (7).

2. Light source according to claim 1, wherein the non-linear elements are formed by spatially separate regions of a non-linear medium, in particular a non-linear crystal.

3. Light source according to claim 1 or 2, wherein an imaging optics (10) is provided in the beam path between the multi-core optical waveguide (1) and the frequency converter (4), adapted to image the individual beams (5) emitted from the cores (2) of the multi-core optical waveguide (1) onto the non-linear elements respectively associated therewith.

4. Light source according to one of claims 1 to 3, wherein the cores (2) of the multi-core optical waveguide (1) are doped with rare earth ions, in particular with ytterbium ions, wherein the multi-core optical waveguide (1) is designed for optical amplification of the light propagating along the cores (2).

5. Light source according to claim 4, wherein the light source comprises a laser (11) in whose laser resonator the multi-core optical waveguide (1) is located, wherein the cores (2) of the multi-core optical waveguide (1) form the laser medium of the laser (11).

6. Light source according to claim 5, wherein the laser (11) is followed by an additional optical amplifier (14) which comprises a further multi-core waveguide (1) with a plurality of cores (2), wherein the rare earth ion-doped cores (2) of the further multi-core waveguide (1) guide the individual beams (5) emitted by the laser (11) and form the gain medium of the additional optical amplifier (14).

7. Light source according to claim 5 or 6, wherein the laser (11) emits pulsed laser radiation, in particular by Q-switching, by cavity dumping or by amplitude modulation taking place outside the laser resonator.

8. Light source according to claim 7, wherein the pulse duration of the pulsed laser radiation is in the nanosecond range, in particular in the range from 10 ns to 100 ns.

9. Light source according to claim 7 or 8, wherein the temporal pulse shape of the laser pulses is substantially rectangular.

10. Light source according to one of claims 1 to 9, wherein the spatial beam profile of the individual beams (5, 6) has substantially a flat-top intensity distribution.

11. Light source according to one of claims 1 to 10, wherein at least one individual beam (5, 6) differs from one another with regard to wavelength, temporal profile, spatial intensity profile, spatial phase profile and / or polarization.

12. Light source according to one of claims 1 to 11, wherein a modulator (12, 13) is provided, adapted to modulate the individual beams (5) independently of one another with regard to amplitude, phase and / or polarization.

13. Light source according to claim 12, wherein a controller is provided, configured to control the modulator (12, 13) in accordance with a predetermined desired illumination of the laser-active medium (7) by the individual beams (6) superimposed therein.

14. Light source according to one of claims 1 to 13, wherein the cores (2) of the multi-core optical waveguide (1) guide the light in a single-mode or multi-mode manner.

15. Laser system with a laser resonator (8), a laser-active medium (7) located therein and one or more light sources according to one of claims 1 to 14, wherein this light source(s) optically pumps the laser-active medium (7).

16. Laser system according to claim 15, wherein the wavelength of the light propagating in the multi-core optical waveguide(s) (1) of the light source(s) is 1000 nm to 1100 nm, preferably 1030 nm, wherein the wavelength of the frequency-converted individual beams (6) is 500 nm to 550 nm, preferably 515 nm.

17. Laser system according to claim 15 or 16, wherein the laser-active medium (7) is a titanium:sapphire crystal.

18. Method for optically pumping a laser-active medium (7), comprising the following method steps: optically amplifying light in a multi-core optical waveguide (1 ) comprising a plurality of cores (2) doped with rare earth ions, which extend along the longitudinal extent of the multi-core optical waveguide (1 ) and guide the light, Conversion of individual beams (5) emitted from the cores (2) of the multi-core optical waveguide (1) into frequency-converted individual beams (6) in separate non-linear elements of a frequency converter (4) each assigned to the individual beams (5), and Superposition of the frequency-converted individual beams (6) in the laser-active medium (7).

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