Light source for optically pumping a laser-active medium

Abstract

The present invention relates to a light source for optically pumping a laser active medium (7). The object of the present invention is to provide an improved, and in particular more efficient, pump light source compared to the prior art, in which the pump light is generated by nonlinear frequency conversion. To this end, the present invention proposes that the light source includes a multicore optical waveguide (1), the multicore optical waveguide (1) includes a plurality of photoconductive cores (2) extending along the longitudinal length of the multicore optical waveguide (1). A frequency converter (4) is connected downstream of the multicore optical waveguide (1) in the beampath, the frequency converter (4) is configured to convert a plurality of individual beams (5) emitted from the plurality of cores (2) of the multicore optical waveguide (1) into a plurality of frequency-converted individual beams (6) in a plurality of mutually distinct nonlinear elements of the frequency converter (4), each associated with an individual beam (5). A superimposed optical system is provided to superimpose the plurality of frequency-converted individual beams (6) in the laser active medium (7), thereby optically pumping the laser active medium (7). Finally, the present invention relates to a laser system using such a light source as a pump light source, and a method for optically pumping a laser active medium (7).

Description

Technical Field

[0001] The present invention relates to a light source for optically pumping a laser active medium.

[0002] Furthermore, the present invention relates to a laser system using such a light source.

Background Art

[0003] Pump light sources are known in various different configurations from the prior art. The laser active medium in a laser or an optical amplifier often requires a pump light source to generate optical amplification. When the pump light supplied from the pump light source is absorbed by the laser active medium, an inversion distribution of electron energy levels occurs. As pump light sources for doped solids as laser active media, for example, flash lamps, semiconductor light sources (light emitting diodes or laser diodes), or solid-state laser systems with frequency conversion are also considered. In this case, the following requirements are imposed on the pump light source: · The spectral emission characteristics of the pump light source must correspond to the absorption spectrum of the laser active medium.

[0004] · In order to achieve as uniform optical amplification as possible, a spatially uniform distribution of the inversion distribution is desirable.

[0005] · When using a pump light source involving frequency conversion of the pump light, the conversion efficiency of the pump light source is extremely important for high-power applications.

[0006] Laser diodes have established themselves as pump sources for laser systems due to their high efficiency and output. However, they have not been able to reach certain wavelength ranges, such as within the visible spectrum, where high output and brightness of the pump light are required. Therefore, the use of laser diodes to generate population inversion in laser systems, for example, titanium:sapphire-based systems, is severely limited. Instead, flash lamps or (pulsed) solid-state lasers, often combined with frequency doubling, such as Nd:YAG-based lasers, are typically used as pump sources. However, such pump sources offer only minimal efficiency in the conversion of power to optical output, or are limited in terms of pulse sequence frequency. [Overview of the project]

[0007] Given the above background, the object of the present invention is to provide an improved pump light source in which the pump light is generated by nonlinear frequency conversion.

[0008] The present invention is a light source for optically pumping a laser-active medium, The light source comprises a multicore optical waveguide, the multicore optical waveguide includes a plurality of photoconductive cores extending along the longitudinal length of the multicore optical waveguide, The light source includes a frequency converter connected downstream of a multicore optical waveguide in the beampath, the frequency converter being configured to convert multiple individual beams emitted from multiple cores of the multicore optical waveguide into multiple frequency-converted individual beams in multiple distinct nonlinear elements of the frequency converter, each associated with a separate individual beam. The light source includes a superposition optical system configured to superimpose multiple frequency-converted individual beams within a laser-active medium. The above problems are solved by using a light source.

[0009] In other words, the approach of the present invention is based on subjecting multiple individual beams, i.e., individual emissions from multiple cores of a multicore waveguide (implemented, for example, as a multicore fiber), to spatially separated frequency conversion (e.g., by generating second harmonics (SHGs) in a nonlinear crystal), and then superimposing the multiple frequency-converted individual beams thus generated separately from each other in a laser active medium, thereby optically pumping the laser active medium, i.e., generating a desired population inversion. In this case, the total available energy of the pump light is obtained from the sum of the pulse energies of the multiple individual beams based on the superposition.

[0010] Optical fiber systems excel in their high efficiency, simple structure, and compactness. Furthermore, the combination of fiber geometry and optical conduction enables operation at high average power, and consequently, high pulse sequence frequencies in pulsed operation. On the other hand, the size of the optical conduction region, i.e., the size of the optical waveguide core, limits the achievable power or pulse energy due to disruption, nonlinear effects, or other limiting influences. Increasing the diameter of the optical waveguide results in a decrease in beam quality due to the generation of more transverse modes within the waveguide. In amplifier operation, i.e., amplifier operation involving optical amplification within the optical waveguide, additional instability (so-called mode instability) can occur due to the introduced heat. Due to these characteristics, pump sources based on optical waveguides with frequency conversion have not been optimally suited for optical pumping of laser amplifiers at high power (and pulse energy) levels. The required frequency conversion presupposes good beam quality, particularly beam stability and beam uniformity, for high conversion efficiency, but such good beam quality is not achieved in conventional systems.

[0011] To address these challenges, the present invention proposes parallelization using a multicore optical waveguide (multicore fiber) containing multiple photoconductive cores. Each of these cores only needs to withstand a portion of the overall (average) power and pulse energy. Accordingly, high quality of individual beams emitted from each core can be achieved. The multiple individual beams are fed to different nonlinear elements for frequency conversion according to the present invention. In this case, each individual beam is provided with one dedicated nonlinear element, i.e., one dedicated nonlinear element associated only with that individual beam (1:1 correspondence). Due to the high beam quality of each individual beam, the efficiency of frequency conversion in each nonlinear element is increased.

[0012] The separate frequency conversion across multiple nonlinear elements limits any potential limitations on each element (e.g., due to thermal input or fracture) to individual beams. Therefore, output scaling can be achieved by increasing the number of cores. The advantage of multicore optical waveguides compared to multimode optical waveguides, which are equivalent in terms of achievable output / pulse energy, is the significantly longer Rayleigh length of the individual beams. This allows for maintaining high intensity over longer distances within the nonlinear elements, and consequently, higher conversion efficiency.

[0013] The generated frequency-converted individual beams are superimposed within the laser active medium as described above, thereby optically pumping the laser active medium. For this purpose, the superposition optical system of the light source according to the present invention is used. The superposition optical system may be implemented by appropriate optical components, or simply by a free-space transmission path that does not require optical components specifically provided for this purpose. By superimposing multiple independently frequency-converted individual beams, the intensity distribution within the laser active medium is obtained as the sum of the spatial and temporal intensity distributions of the multiple frequency-converted individual beams, and without depending on the phase of the individual beams, thereby reducing temporal fluctuations and avoiding the requirement of phase control in the individual beams. The superposition can be adapted using the superposition optical system to the shape and dimensions of the desired pumping region within the laser active medium.

[0014] Since multiple individual beams are generated independently within a multicore optical waveguide, it is possible to make the superimposed beam profile from multiple individual beams uniform, or to adapt it to a target beam profile in the spatial and temporal domains, even if the beam profiles of the individual beams deviate from each other by chance or under controlled conditions. This also makes it possible to utilize larger cores in the multicore optical waveguide that conduct light in multiple modes and therefore have lower beam quality for the individual beams. The individual beam profiles of the individual beams before or after spatially separate frequency conversions may deviate from each other. Similarly, the individual emissions of the individual beams may each have different temporal characteristics. By using cores that conduct light in multiple modes, both the individual cores and the frequency conversion can be operated at higher power or pulse energy, in which case some loss of efficiency in the frequency conversion can be tolerated.

[0015] The aforementioned deviations of multiple frequency-converted individual beams can also be intentionally introduced (for example, by emitting from multiple different cores at different wavelengths, with different polarizations, pulse durations, or pulse energies for each core), thereby manipulating the superposition characteristics of the multiple frequency-converted individual beams to be adapted, for example, to a target state. For this purpose, a modulator may be provided, for example, to modulate the individual beams independently of each other with respect to amplitude, phase, and / or polarization. A control device may be provided to control the modulator according to a predetermined target illumination of the laser active medium by the individual beams superimposed in the laser active medium.

[0016] In one possible embodiment, the multiple nonlinear elements are formed by multiple substantially spatially distinct regions of a single nonlinear medium, and in particular by multiple different, spaced-apart divisions of a (single, i.e., monolithic) nonlinear crystal. However, it is also conceivable to have multiple separate nonlinear crystals, i.e., one dedicated nonlinear crystal for each individual beam.

[0017] When actually realizing the light source, an imaging optical system may be provided in the beam path between the multicore optical waveguide and the frequency converter. The imaging optical system is provided to image multiple individual beams emitted from multiple cores of the multicore optical waveguide onto multiple nonlinear elements, each associated with a specific individual beam.

[0018] In one possible embodiment, the core of a multicore optical waveguide may be doped with rare earth ions, particularly ytterbium ions or neodymium ions, and the multicore optical waveguide is configured to optically amplify the light propagating along the core. To this end, the multicore optical waveguide itself must be optically pumped, for example, by light from a laser diode incident within a cladding surrounding the core of the multicore optical waveguide.

[0019] When the core of a multicore optical waveguide is doped with ytterbium ions, the wavelength of light propagating within the multicore optical waveguide may be 1000 nm to 1100 nm, preferably 1030 nm. The wavelength of the individual beams subsequently generated by frequency doubling may be correspondingly 500 nm to 550 nm, preferably 515 nm. For example, light thus generated at 515 nm is very suitable for optically pumping titanium:sapphire crystals as laser active media (e.g., Ti:Sa lasers) with high power and high efficiency.

[0020] In one possible embodiment, the light source may be implemented as a laser, with a multicore optical waveguide located within the laser resonator of the laser, where the cores of the multicore optical waveguide form the laser medium of the laser. In this case, an optical amplifier may be connected downstream of the laser to amplify the light, and the optical amplifier includes another multicore waveguide with multiple cores, where the cores of the other multicore waveguide, doped with rare earth ions, conduct the individual beams emitted by the laser and form the amplification medium of an additional optical amplifier. The laser can emit pulsed laser radiation as light, particularly by Q-switching, cavity damping, or amplitude modulation performed outside the laser resonator. In this case, the pulse duration of the pulsed laser radiation is, for example, in the range of nanoseconds, particularly in the range of 10 ns to 100 ns. In this case, the temporal pulse shape of the laser pulse may be substantially rectangular. Such pulsed laser radiation (generated, for example, in lasers with a wavelength of approximately 1030 nm, or down to approximately 515 nm when the frequency is doubled) is very suitable for optically pumping titanium:sapphire crystals at high power.

[0021] The light source of the present invention may be used as a pump light source in a laser system having a laser resonator containing a laser active medium, and the pump light source, or multiple such pump light sources in parallel, optically pumps the laser active medium.

[0022] Finally, the present invention relates to a method for optically pumping a laser active medium, comprising the following method steps, namely: · optically amplifying light in a multi-core optical waveguide, the multi-core optical waveguide comprising a plurality of cores, the plurality of cores extending along the longitudinal length of the multi-core optical waveguide, being doped with rare earth ions, and conducting light; · converting a plurality of individual beams emitted from the plurality of cores of the multi-core optical waveguide into a plurality of frequency-converted individual beams in a plurality of separate non-linear elements of a frequency converter, each of the individual beams being associated with a respective one of the plurality of separate non-linear elements; · superimposing the plurality of frequency-converted individual beams within the laser active medium and also relates to a method comprising these steps.

[0023] Hereinafter, embodiments of the present invention will be described in more detail based on the drawings.

Brief Description of the Drawings

[0024] [Figure 1] Schematic diagram of a pump light source according to the present invention as a block diagram. [Figure 2] Schematic diagram of a pump light source according to the present invention as a block diagram with a modulator. [Figure 3] Schematic diagram of a pump light source according to the present invention as a block diagram with a modulator and an amplifier. [Figure 4] Diagram showing the refractive index profile of the cores in a multi-core waveguide of a pump light source according to the present invention and the resulting spatial beam profile.

Embodiments for Carrying Out the Invention

[0025] Figure 1 shows a pump light source according to the present invention, comprising a multicore optical waveguide 1, which includes a plurality of cores 2, these cores 2 extending along the longitudinal length of the multicore optical waveguide 1 and emitting laser light. These cores 2 are embedded within a single common cladding 3 of the multicore optical waveguide 1 and are spaced apart from one another. Outside the cladding 3, the multicore optical waveguide may be surrounded by another layer (not shown) (for example, for protection or to ensure mechanical stability). Downstream of the multicore optical waveguide 1 in the beampath is connected a frequency converter 4 in the form of a nonlinear crystal (for example, made of barium betaborate or lithium borate), which converts a plurality of individual beams 5 emitted from the plurality of cores 2 of the multicore optical waveguide 1 into a plurality of frequency-converted individual beams 6, separated from one another, i.e., in substantially spatially spaced-apart regions of the nonlinear crystal 4. In this case, the distance between two adjacent individual beams 5 (measured from center to center of each beam cross-section) is the diameter of the individual beam 5 (1 / e 2The diameter should be larger than the specified value. The nonlinear crystal 4, through the resulting free-space transmission path for the frequency-converted individual beams 6, simultaneously forms a superimposed optical system in the illustrated example. This superimposed optical system superimposes the frequency-converted individual beams 6 within a laser active medium 7 located in a laser resonator formed (optionally) by end mirrors 8, thereby optically pumping the laser active medium 7. In this process, a laser beam 9 is generated. In the beam path between the multicore optical waveguide 1 and the frequency converter 4, an imaging optical system 10 consisting of two lenses is provided, thereby imaging the multiple individual beams 5 emitted from the multiple cores 2 of the multicore optical waveguide 1 into the respective regions of the nonlinear crystal 4. In this case, the imaging magnification defines the size of the multiple spatially separated regions within the nonlinear crystal 4, and consequently the optical intensity. This degree of design freedom can be used to optimize the conversion efficiency. After crystal-based frequency conversion, preferably, the primary radiation, i.e., the radiation at the wavelength of the individual beam 5 before frequency conversion, is separated from the frequency-converted beam 6 (not shown for clarity), and then multiple spatially separated frequency-converted individual beams 6 are superimposed by free-space propagation. It is also conceivable to superimpose two time-synchronized frequency-converted arrays of orthogonally polarized individual beams 6 in a polarizer (not shown). By utilizing additional optical imaging with a suitable superposition optical system, a beam profile suitable for the laser active medium 7 can be generated, thus creating an optimal population inversion within the laser active medium 7.

[0026] In the embodiment shown in Figure 2, the pump light source includes a pulsed seed laser 11. The seed laser 11 is based on a multicore waveguide 1 doped with ytterbium ions as the laser medium. Pulsed operation is achieved by Q-switching, cavity damping, or external modulation. A modulator 12 can be incorporated into the seed laser 11. Alternatively or additionally, an external modulator 13 can be provided. Using the seed laser 11, it is possible to generate spatially separated, independent individual beams with wavelengths of approximately 1 μm, with pulse durations of several nanoseconds, within each core 2 of the multicore waveguide 1, but it is also possible to generate longer pulses in the range of milliseconds. The external modulator 13 would be advantageous in this case to temporally shape the individual pump optical pulses, thereby compensating for pulse shaping due to saturation in the optical amplifier, which may follow, and ultimately making the temporal rectangular pulses available for frequency conversion in the frequency converter 4. This improves conversion efficiency. A spatially flat-top profile of multiple spatially separated regions within the nonlinear crystal 4 is advantageous for frequency conversion efficiency (see Figure 1). This flat-top profile can be achieved by fitting the refractive index profiles of the individual cores 2 of the multicore optical waveguide 1. The left-hand diagram of Figure 4 schematically shows the possible refractive index profiles (refractive index n as a function of the radius r of the individual cores) and the transverse eigensolutions propagating within them (the right-hand diagram illustrating the optical intensity in the cross-section of individual beams). The beam profile approaches the flat-top profile.

[0027] The spatially separated and emitted individual beams of the seed laser 11 can optionally be amplified to higher pulse energies and higher average power in an optical amplifier 14 equipped with another laser-activated multicore optical waveguide 1, as shown in Figure 3.

[0028] In one possible embodiment, the cores 2 of the multicore optical waveguide 1 conduct laser light in multiple modes, and the multicore optical waveguide 1 is configured such that the quality of the emitted individual beams 5 still approaches the diffraction limit. Tapers (fine tips of the individual cores 2 (not shown)) can achieve significant amplification of the lateral fundamental modes in each core 2 at the input side of the multicore waveguide 1 in the seed laser 11 or in the optical amplifier 14.

[0029] In Figures 2 and 3, even though the individual beam 5 and the frequency-converted individual beam 6 are schematically shown as a whole by a single broad connecting line between the individual components 11, 13, 14 and 4, or downstream of the frequency converter 4, these individual beams 5 and 6 are multiple separate beams as shown in Figure 1, and these multiple separate beams are superimposed for the first time in a laser-active medium not shown in Figures 2 and 3, thereby optically pumping this laser-active medium.

[0030] It should be added that the light source according to the present invention may be used not only as a pump light source but also in other ways, such as for laser material processing.

[0031] Furthermore, the frequency conversion by generating the second harmonic, as described in relation to multiple embodiments, is a selected example adapted to the application of "pumping Ti:Sa laser crystals." Similarly, the approach of the present invention enables other frequency conversion processes, such as the generation of the third harmonic (THG) and, consequently, the generation of UV light.

Claims

1. A light source for optically pumping the laser active medium (7), - The light source comprises a multicore optical waveguide (1), and the multicore optical waveguide (1) includes a plurality of photoconductive cores (2) that extend along the longitudinal length of the multicore optical waveguide (1). - The light source includes a frequency converter (4) connected downstream of the multicore optical waveguide (1) in the beam path, and the frequency converter (4) is configured to convert a plurality of individual beams (5) emitted from the plurality of cores (2) of the multicore optical waveguide (1) into a plurality of frequency-converted individual beams (6) in a plurality of mutually distinct nonlinear elements of the frequency converter (4) each associated with the individual beams (5). - The light source includes a superposition optical system configured to superimpose the plurality of frequency-converted individual beams (6) within the laser active medium (7). light source.

2. The light source according to claim 1, wherein the plurality of nonlinear elements are formed by a plurality of spatially distinct regions of a single nonlinear medium, in particular a single nonlinear crystal.

3. The light source according to claim 1 or 2, wherein an imaging optical system (10) is provided in the beam path between the multicore optical waveguide (1) and the frequency converter (4), and the imaging optical system (10) is configured to image the plurality of individual beams (5) emitted from the plurality of cores (2) of the multicore optical waveguide (1) onto the plurality of nonlinear elements each associated with the plurality of individual beams (5).

4. The light source according to any one of claims 1 to 3, wherein the core (2) of the multicore optical waveguide (1) is doped with rare earth ions, particularly ytterbium ions, and the multicore optical waveguide (1) is configured to optically amplify light propagating along the core (2).

5. The light source according to claim 4, wherein the light source includes a laser (11), the multicore optical waveguide (1) is located within the laser resonator of the laser (11), and the core (2) of the multicore optical waveguide (1) forms the laser medium of the laser (11).

6. The light source according to claim 5, wherein an additional optical amplifier (14) is connected downstream of the laser (11), the additional optical amplifier (14) includes another multicore waveguide (1) having a plurality of cores (2), the cores (2) of the other multicore waveguide (1), which are doped with rare earth ions, conduct individual beams (5) emitted by the laser (11) and form an amplification medium for the additional optical amplifier (14).

7. The light source according to claim 5 or 6, wherein the laser (11) emits pulsed laser radiation, particularly by a Q-switch, by cavity damping, or by amplitude modulation performed outside the laser resonator.

8. The light source according to claim 7, wherein the pulse duration of the pulsed laser radiation is in the nanosecond range, particularly in the range of 10 ns to 100 ns.

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

10. The light source according to any one of claims 1 to 9, wherein the spatial beam profiles of the individual beams (5, 6) have a substantially flat-top intensity distribution.

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

12. The light source according to any one of claims 1 to 11, wherein modulators (12, 13) are provided, and the modulators (12, 13) are configured to modulate the individual beams (5) independently of each other with respect to amplitude, phase, and / or polarization.

13. The light source according to claim 12, wherein a control device is provided, and the control device is configured to control the modulators (12, 13) according to a predetermined target illumination of the laser active medium (7) by the individual beams (6) superimposed in the laser active medium (7).

14. The light source according to any one of claims 1 to 13, wherein the core (2) of the multicore optical waveguide (1) conducts the light in single mode or multimode.

15. A laser system comprising a laser resonator (8), a laser active medium (7) located within the laser resonator (8), and one or more light sources according to any one of claims 1 to 14, wherein the light sources optically pump the laser active medium (7).

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

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

18. A method for optically pumping a laser-active medium (7), comprising the following method steps, namely, - A step of optically amplifying light within a multicore optical waveguide (1), wherein the multicore optical waveguide (1) includes a plurality of cores (2), the plurality of cores (2) extend along the longitudinal length of the multicore optical waveguide (1), are doped with rare earth ions, and conduct the light; - A step of converting a plurality of individual beams (5) emitted from the plurality of cores (2) of the multicore optical waveguide (1) into a plurality of frequency-converted individual beams (6) in a plurality of mutually distinct nonlinear elements of a frequency converter (4), each associated with each of the individual beams (5), - The step of superimposing the plurality of frequency-converted individual beams (6) within the laser active medium (7) A method that includes [a certain feature].

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