Light source for optically pumping laser-active medium
By combining multi-core optical waveguides and frequency converters, the limitations of existing pump light sources in the high-power and brightness wavelength range are overcome, achieving efficient optical pumping and frequency conversion, which is suitable for high-power laser systems.
Patent Information
- Application Number
- CN202480022171.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-02-06
- Filing Date
- 2024-02-05
- Publication Date
- 2025-11-04
AI Technical Summary
Existing pump sources are not yet accessible in certain wavelength ranges with high power and brightness, and conventional systems are limited in frequency conversion efficiency and beam quality, making it difficult to achieve uniform population inversion and efficient optical amplification.
By employing a combination of multi-core optical waveguides and frequency converters, multiple independent beams are generated in the multi-core optical waveguides, and after frequency conversion in nonlinear elements, they are superimposed in a laser-active medium. This superimposed optical unit enables efficient optical pumping.
It achieves efficient optical pumping, enabling optical amplification at high average power and pulse repetition frequency, improving frequency conversion efficiency and beam quality, reducing heat input and damage limitations, and is suitable for high-power laser systems.
Smart Images
Figure CN120898341A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present invention relates to a light source for optically pumping a laser-active medium.
[0002] The present invention further relates to a laser system using such a light source. BACKGROUND
[0003] In various embodiments, pump light sources are known in the art. Laser-active media in a laser or in an optical amplifier generally require a pump light source to generate optical amplification. When pump light provided by a pump light source is absorbed in a laser-active medium, a population inversion of electronic energy levels occurs. Possible pump light sources for a doped solid as a laser-active medium include flashlamps, semiconductor light sources (light emitting diodes or laser diodes) or solid-state based laser systems with frequency conversion. Thus, the following requirements are made to a pump light source:
[0004] • The spectral emission properties of the pump light source must correspond to the absorption spectrum of the laser-active medium.
[0005] • A spatial distribution of the population inversion as uniform as possible is desirable in order to achieve an optical amplification as uniform as possible.
[0006] • When using a pump light source with frequency conversion of the pump light, the conversion efficiency of the pump light source is of great importance for high power applications.
[0007] Due to their high efficiency and power, laser diodes have established themselves as pump light sources for laser systems. However, certain wavelength ranges with high power and brightness of the pump light (e.g. in the visible spectral range) are not yet accessible. This severely limits, for example, the generation of a population inversion in a titanium:sapphire based laser system using laser diodes. Alternatively, flashlamps or (pulsed) solid-state lasers in combination with frequency doubling (e.g. based on Nd:YAG) are usually used as pump light sources. However, these only provide low efficiency in converting electrical power into optical power or they are limited in pulse repetition frequency.
[0008] Against this background, it is the task of the present invention to provide an improved pump light source, the pump light of which is generated by nonlinear frequency conversion. SUMMARY
[0009] The present invention solves this problem by means of a light source for optically pumping a laser-active medium, the light source having:
[0010] - a multicore optical waveguide comprising a plurality of light guiding cores extending along a longitudinal extent of the multicore optical waveguide,
[0011] - a frequency converter connected downstream of the multi-core optical waveguide in the beam path, designed to convert the individual light beams emitted from the cores of the multi-core optical waveguide into frequency-converted individual light beams in separate nonlinear elements of the frequency converter, each separate nonlinear element of the frequency converter being assigned to a respective individual light beam, and
[0012] - a superposition optical unit designed to superimpose the frequency-converted individual light beams in the laser-active medium.
[0013] The method of the present invention is thus based on a spatial separation of the individual light beams (i.e. the individual emission from the cores of a multi-core waveguide, e.g. realized as a multi-core fiber), a frequency conversion (e.g. by generating second harmonics (SHG) in a nonlinear crystal) and finally a superposition of the frequency-converted individual light beams generated separately from each other in the laser-active medium in order to optically pump it, i.e. to create the desired population inversion. Due to the superposition, the available total energy of the pump light results from the sum of the pulse energies of the individual light beams.
[0014] The fiber system is characterized by a high efficiency and a simple and compact design. Furthermore, the combination of fiber geometry and light guiding allows for operation at high average power and thus, in pulsed operation, at high pulse repetition frequency operation. On the other hand, the size of the light guiding region (i.e. the core of the optical waveguide) limits the achievable power or pulse energy due to destruction, nonlinear effects or other limiting effects. An increase of the diameter of the optical waveguide leads to a decrease of the beam quality due to a larger number of transverse modes occurring in the optical waveguide. In amplifier mode, i.e. when optical amplification is performed within the optical waveguide, additional instabilities (so-called mode instabilities) can occur due to the introduced heat. Due to these properties, a pump light source based on an optical waveguide with frequency conversion is not optimally suited for the optical pumping of a laser amplifier at high power (and pulse energy) either. For high conversion efficiency, the necessary frequency conversion requires a good beam quality, in particular beam stability and beam homogeneity, which is not achievable with conventional systems.
[0015] In order to overcome these problems, the present invention proposes a parallelization by using a multi-core optical waveguide (multi-core fiber) comprising a plurality of light guiding cores. Each of these cores has to bear only a fraction of the total (average) power and pulse energy. Thus, a high quality of the individual light beams emitted by the individual cores can be achieved. According to the present invention, the individual light beams are fed to individual nonlinear elements for the frequency conversion. Thus, a separate nonlinear element is provided for each individual light beam, i.e. is assigned to this individual light beam only (1 : 1 assignment). Due to the high beam quality of the individual single light beams, the efficiency of the frequency conversion in the individual nonlinear elements is high.
[0016] Due to the separate frequency conversion in the non-linear elements, any limitations of the respective element (e.g. due to thermal input or destruction) are limited to the individual single beams. As a result, power scaling can be achieved with the number of cores. One advantage of the multi-core optical waveguide over the use of multi-mode optical waveguides, which are equivalent in terms of achievable power / pulse energy, is the significantly larger Rayleigh length of the individual beams, which enables the maintenance of high intensities over longer distances in the non-linear elements and thus, in turn, higher conversion efficiencies.
[0017] As already mentioned, the generated frequency-converted individual beams are superimposed in the laser-active medium in order to optically pump it. The superimposition optics of the light source according to the application serve this purpose. The superimposition optics can be realized by suitable optical components or simply by free beam guidance, which does not require any optical components to be provided specifically for this purpose. The superposition of the frequency-converted individual beams independent of one another results in an intensity profile in the laser-active medium as a sum of the spatial and temporal intensity distribution of the frequency-converted individual beams, i.e. independent of the phase of the individual individual beams, which reduces temporal fluctuations and avoids the need for phase control in the individual beams. By means of the superimposition optics, the superimposition can be adapted to the shape and size of the desired pump region in the laser-active medium.
[0018] Because the individual beams are generated in the multi-core optical waveguide independent of one another, a homogenization or adjustment of the beam profile from the superimposition of the individual beams to the target beam profile can take place in the spatial and temporal range, even if the beam profiles of the individual beams deviate from one another randomly or in a controlled manner. In principle, this also makes it possible to use larger cores of the multi-core optical waveguide, which guide light in multiple modes and thus have a lower beam quality of the individual beams. The individual beam profiles of the individual beams before and after the spatial separation of the frequency conversion can also deviate from one another. The individual emission of the individual beams can also have different temporal characteristics. By using multi-mode light-guiding cores, it is possible to operate both the individual cores and the frequency conversion at higher power or pulse energy, which enables a certain efficiency loss of the frequency conversion to be accepted.
[0019] It is also possible to deliberately introduce the above-mentioned deviations of the frequency-converted individual beams (e.g. by different wavelengths emitted from different cores, different polarizations from core to core, pulse duration or pulse energy) in order to manipulate the properties of the superimposition of the frequency-converted individual beams and, for example, to adapt them to a target state. For this purpose, for example, modulators can be provided which are intended to modulate the individual beams independently of one another in terms of amplitude, phase and / or polarization. A controller can be provided to control the modulators in accordance with a specified target illumination of the laser-active medium by the individual beams superimposed in it.
[0020] In one possible embodiment, the nonlinear element is formed by substantially spatially separated regions of a nonlinear medium, in particular by segments of individual, i.e. monolithic, nonlinear crystals that are spaced apart from each other. However, it is also conceivable to provide several separate nonlinear crystals, i.e. a separate nonlinear crystal for each individual light beam.
[0021] In a practical implementation of the light source, imaging optics can be provided in the beam path between the multi-core optical waveguide and the frequency converter, which are designed to image the individual light beams emitted from the cores of the multi-core optical waveguide onto the nonlinear elements assigned to them.
[0022] In one possible embodiment, the cores of the multi-core optical waveguide can be doped with rare earth ions, in particular with ytterbium ions or neodymium ions, whereby the multi-core optical waveguide is designed to optically amplify light propagating along the cores. For this purpose, the multi-core optical waveguide itself must be optically pumped, for example by coupling light from a laser diode into the cladding of the multi-core optical waveguide surrounding the cores.
[0023] If the cores of the multi-core optical waveguide are doped with ytterbium ions, the wavelength of the light propagating in the multi-core optical waveguide can be 1000 nm to 1100 nm, preferably 1030 nm. The wavelength of the individual light beams generated by frequency doubling can then be 500 nm to 550 nm, preferably 515 nm. For example, light generated in this way at 515 nm is ideally suited for the optical pumping of a titanium:sapphire crystal as a laser-active medium, for example a Ti:Sa laser.
[0024] In one possible embodiment, the light source can be implemented as a laser, the multi-core optical waveguide being located in a laser resonator of the laser, whereby the cores of the multi-core optical waveguide form the laser medium of the laser. An optical amplifier can thus be connected downstream of the laser to amplify the light, the optical amplifier comprising a further multi-core waveguide having a plurality of cores, whereby the cores of the further multi-core waveguide doped with rare earth ions, which guide the individual light 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, cavity dumping or amplitude modulation outside the laser resonator. The pulse duration of the pulsed laser radiation is thus for example in the nanosecond range, in particular in the range of 10 ns to 100 ns. The temporal pulse shape of the laser pulses can be substantially rectangular. Such pulsed laser radiation (generated in the laser, for example at about 1030 nm and frequency-doubled to about 515 nm) is ideally suited for the optical pumping of a titanium:sapphire crystal at high power.
[0025] The light source according to the application can be used as a pump light source in a laser system having a laser resonator with a laser-active medium therein, whereby the pump light source or also several such pump light sources optically pump the laser-active medium or the laser-active medium in parallel.
[0026] Finally, the application also relates to a method for optically pumping a laser-active medium, the method having the following method steps:
[0027] - optically amplifying light in a multicore optical waveguide, the multicore optical waveguide comprising a plurality of rare earth doped cores extending along a longitudinal extent of the multicore optical waveguide, the rare earth doped cores guiding the light,
[0028] - converting individual light beams emitted from the cores of the multicore optical waveguide into frequency-converted individual light beams in separate nonlinear elements of a frequency converter, each separate nonlinear element of the frequency converter being assigned to a respective individual light beam, and
[0029] - superimposing the frequency-converted individual light beams in a laser-active medium. BRIEF DESCRIPTION OF DRAWINGS
[0030] Examples of embodiments of the application are explained in more detail below with reference to the accompanying drawings. The drawings show
[0031] Figure 1 : schematic representation of a pump light source according to the application as a block diagram;
[0032] Figure 2 : schematic representation of a pump light source according to the application as a block diagram with a modulator;
[0033] Figure 3 : schematic representation of a pump light source according to the application as a block diagram with a modulator and an amplifier;
[0034] Figure 4 : refractive index profile of a core in a multicore waveguide of a pump light source according to the application and resulting spatial beam profile. DETAILED DESCRIPTION
[0035] Figure 1A pump light source with a multicore optical waveguide 1 is shown according to the present application, the multicore optical waveguide 1 comprising a plurality of cores 2 emitting laser light along a longitudinal extension of the multicore optical waveguide 1. The cores 2 are embedded in a common cladding 3 of the multicore optical waveguide 1 and are spaced apart from each other. Outside the cladding 3, the multicore optical waveguide can be surrounded by further layers (not shown), for example, as protection or to ensure mechanical stability. A frequency converter 4 in the form of a nonlinear crystal (for example, made of beta-barium borate or lithium borate) is connected downstream of the multicore optical waveguide 1 in the beam path, which frequency converter 4 converts individual light beams 5 emitted from the cores 2 of the multicore optical waveguide 1 separately from each other into frequency-converted individual light beams 6, i.e., in spatially substantially separate regions of the nonlinear crystal 4. The distance between two adjacent individual light beams 5 (measured from the center of the respective beam cross section to the center) should be greater than the diameter (1 / e2 diameter) of the individual light beams 5. In the example shown, due to the resulting free-beam guidance of the frequency-converted individual light beams 6, the nonlinear crystal 4 simultaneously forms superposition optics by means of which the frequency-converted individual light beams 6 are superposed in a laser-active medium 7, which (optionally) is located in a laser resonator formed by end mirrors 8, in order to optically pump the laser-active medium 7. This results in a laser beam 9. In the beam path between the multicore optical waveguide 1 and the frequency converter 4, imaging optics 10, here consisting of two lenses, are provided in order to image the individual light beams 5 emitted from the cores 2 of the multicore optical waveguide 1 onto the regions of the nonlinear crystal 4 to which they are each assigned. The mapping scale thus defines the size in the spatially separated regions in the nonlinear crystal 4 and thus also the light intensity in the spatially separated regions in the nonlinear crystal 4. This design freedom can be used to optimize the conversion efficiency. Separation of the primary radiation (not shown for the sake of clarity) takes place expediently after the crystal-based frequency conversion, i.e., the radiation of the individual light beams 5 before the frequency conversion at the wavelength of the frequency-converted radiation 6 before superposition by free-space propagation of the spatially separated frequency-converted individual light beams 6. It is also conceivable to superpose two time-synchronous frequency-converted arrays of orthogonally polarized individual light beams 6 on a polarizer (not shown). Suitable superposition optics can use additional optical imaging to produce an appropriate beam profile for the laser-active medium 7 and to produce an optimum population inversion in the medium.
[0036] In Figure 2In the illustrated embodiment, the pump source includes 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 via Q-switching, cavity emptying, or external modulation. A modulator 12 can be integrated into the seed laser 11. Alternatively or additionally, an external modulator 13 can be provided. The seed laser 11 can be used to generate independent individual beams spatially separated from each other in individual cores 2 of the multi-core waveguide 1 at wavelengths of approximately 1 µm, with pulse durations of several nanoseconds, but also longer pulses in the millisecond range. An external modulator 13 can be advantageous here to temporally shape the individual pump light pulses, thereby compensating for saturation-related pulse shaping in any subsequent optical amplifier and ultimately producing time-square-wave pulses usable for frequency conversion in the frequency converter 4. This improves conversion efficiency. The spatially flat-top profile of the spatially separated regions in the nonlinear crystal 4 is beneficial to the efficiency of frequency conversion (see [link to relevant documentation]). Figure 1 This can be achieved by adjusting the refractive index profile of the individual core 2 of the multi-core optical waveguide 1. Figure 4 The left-hand diagram outlines the possible refractive index profile (refractive index n as a function of the radius r of a single core) and the lateral characteristics of propagation within it (the right-hand diagram illustrates the light intensity in the cross-section of each individual beam). The beam profile approximates a flat-top profile.
[0037] 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 waveguide 1 to produce high pulse energy and high average power, such as... Figure 3 As shown.
[0038] In one possible embodiment, the core 2 of the multi-core waveguide 1 guides the laser in multiple modes, thus being designed such that the quality of the emitted individual beam 5 remains almost diffraction-limited. The tapering (a reduction in the size of the individual core 2 (not shown)) can achieve dominant amplification of the lateral fundamental mode in each core 2 at the input of the multi-core waveguide 1 in the seed laser 11 or optical amplifier 14.
[0039] Even in Figure 2 and Figure 3 In the diagram, the individual beam 5 and the frequency-converted individual beam 6 are schematically shown as a whole via a single wide connecting line between individual components 11, 13, 14, 4 or after the frequency converter 4, as shown in the diagram. Figure 1 Beams that are separated from each other, these beams only Figure 2 and Figure 3 It is superimposed in a laser-active medium (not shown) to optically pump it.
[0040] It should be noted that the light source according to the present application can not only be used as a pump light source, but also in other ways, for example, for laser material processing.
[0041] Furthermore, the frequency conversion by generating the second harmonic described in connection with the example embodiment is a selected example suitable for applying a "pumped Ti:Sa laser crystal". Likewise, the method of the present application also allows other frequency conversion processes, for example, generating the third harmonic (THG) and thus generating UV light.
Claims
1. A light source for optically pumping a laser active medium (7), comprising: - Multi-core optical waveguide (1), the multi-core optical waveguide (1) includes a plurality of optical cores (2) extending along the longitudinal direction of the multi-core optical waveguide (1). - A frequency converter (4), connected downstream of the multi-core optical waveguide (1) in the beam path, is designed to convert individual beams (5) emitted from the core (2) of the multi-core optical waveguide (1) into frequency-converted individual beams (6) in separate nonlinear elements of the frequency converter (4), each separate nonlinear element of the frequency converter (4) being assigned to a corresponding individual beam (5), and - A superimposed optical unit, which is designed to superimpose the frequency-converted individual beam (6) in the laser-active medium (7).
2. The light source according to claim 1, wherein, The nonlinear element is formed through regions of a nonlinear medium that are spatially separated from each other, particularly a nonlinear crystal.
3. The light source according to claim 1 or 2, wherein, Imaging optics (10) are disposed in the beam path between the multi-core waveguide (1) and the frequency converter (4) and are designed to image individual beams (5) emitted from the core (2) of the multi-core waveguide (1) onto nonlinear elements that are assigned to the individual beams (5) in each case.
4. The light source according to any one of claims 1 to 3, wherein, The core (2) of the multi-core optical waveguide (1) is doped with rare earth ions, particularly ytterbium ions, wherein the multi-core optical waveguide (1) is designed to amplify light propagating along the core (2).
5. The light source according to claim 4, wherein, The light source includes a laser (11), and the multi-core optical waveguide (1) is located in the laser resonator of the laser (11), wherein the core (2) of the multi-core optical waveguide (1) forms the laser medium of the laser (11).
6. The light source according to claim 5, wherein, Following the laser (11) is an additional optical amplifier (14), which includes another multi-core waveguide (1) having multiple cores (2), wherein the rare-earth-doped cores (2) of the other multi-core waveguide (1) guide individual beams (5) emitted by the laser (11) and form the amplification medium of the additional optical amplifier (14).
7. The light source according to claim 5 or 6, wherein, The laser (11) emits pulsed laser radiation, in particular by Q-switching, by cavity emptying, or by amplitude modulation occurring 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 time pulse shape of the laser pulse is generally 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) generally have a 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 each other in wavelength, time profile, 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, which are designed 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 controller is provided, which is designed to control the modulator (12, 13) based on the predetermined target illumination of the laser active medium (7) by the separate beam (6) superimposed on the laser active medium (7).
14. The light source according to any one of claims 1 to 13, wherein, The core (2) of the multi-core optical waveguide (1) guides the light in a single-mode or multi-mode manner.
15. A laser system comprising a laser resonator (8), a laser-active medium (7) located in the laser resonator (8), and one or more light sources according to any one of claims 1 to 14, wherein, The light source optically pumps the laser active medium (7).
16. The laser system according to claim 15, wherein, The wavelength of the light propagating in the multi-core optical waveguide (1) of the light source is 1000 nm to 1100 nm, preferably 1030 nm, wherein the wavelength of the frequency-converted individual beam (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), the method comprising the following steps: - Optically amplify light in a multi-core optical waveguide (1), the multi-core optical waveguide (1) comprising a plurality of rare-earth-doped cores (2) extending along the longitudinal direction of the multi-core optical waveguide (1), the plurality of rare-earth-doped cores (2) guiding the light. - In the separate nonlinear elements of the frequency converter (4), the individual beams (5) emitted from the core (2) of the multi-core optical waveguide (1) are converted into individual frequency-converted beams (6), each separate nonlinear element of the frequency converter (4) being assigned to the corresponding individual beam (5), and - The frequency-converted individual beam (6) is superimposed on the laser-active medium (7).