Laser system with an optical fiber unit and method for evaluating the coupling quality when coupling useful light into an optical fiber unit

Description

[0001] The invention relates to a laser system with an optical fiber unit and a method for assessing the coupling quality of the coupling of useful light into an optical fiber unit. US 2011 / 305249 A1 discloses a laser system according to the prior art.

[0002] A laser system of the type discussed here comprises an optical fiber assembly with an optical fiber that has a light-guiding area designed for guiding useful light through the optical fiber and, at one end of the fiber, designated as the coupling end, a first fiber end face for coupling laser light into the light-guiding area. At another end of the fiber, designated as the coupling end, the optical fiber assembly has a second fiber end face for coupling laser light out of the light-guiding area.

[0003] With such an optical fiber assembly, there is a fundamental requirement to couple laser light with the highest possible beam quality and the lowest possible losses into the assembly via the coupling end. However, it proves difficult to adequately evaluate and, in particular, monitor the coupling quality. It has been proposed to monitor the coupling quality by detecting leakage light exiting laterally from the optical fiber assembly. However, this typically results in a power plateau, meaning that even with poor beam quality, high power is still measured at the detector. This prevents optimal coupling adjustment. It has also been proposed to detect cladding light propagating within the fiber cladding of optical fiber assemblies that have a fiber sheath.However, this also does not allow for an optimal assessment of the coupling quality and is only relevant if the laser light coupled into the optical fiber unit has higher modes that propagate in the fiber cladding.

[0004] The invention is based on the objective of creating a laser system with an optical fiber unit and a method for assessing the coupling quality of the coupling of useful light into an optical fiber unit, wherein the aforementioned disadvantages are at least reduced, preferably avoided.

[0005] The problem is solved by providing the present technical teaching, in particular the teaching of the independent claims as well as the embodiments disclosed in the dependent claims and the description.

[0006] The problem is solved, in particular, by creating a laser system that includes a laser radiation source for emitting useful light. The laser system also includes an optical fiber assembly, which comprises an optical fiber. The optical fiber has a light-guiding area configured for guiding useful light, a first fiber end face at one end for coupling laser light into the light-guiding area, and a second fiber end face at the other end for coupling laser light out of the light-guiding area. Furthermore, the laser system includes a coupling device configured to couple the useful light into the optical fiber assembly.The laser system includes a reflective element positioned behind the second fiber end face in the propagation direction of the useful light. This element is designed to deflect a reflection of the useful light away from the propagation direction and couple it back into the light-guiding area via the second fiber end face, opposite to the propagation direction. The laser system also includes a measuring device positioned in front of the first fiber end face in the propagation direction. This device is designed to detect the reflection of the useful light deflected by the reflective element.

[0007] The measuring device is preferably configured to assess the coupling quality of the coupling of the useful light into the optical fiber unit based on the detected reflection of the useful light.

[0008] The reflective element is preferably arranged behind the output end. Alternatively or additionally, the measuring device is preferably arranged in front of the input end. In particular, the reflective element and the measuring device are not themselves part of the optical fiber unit, but are provided in the laser system in addition to the optical fiber unit and preferably arranged separately from it.

[0009] In the laser system proposed here, a reflection is generated behind the fiber exit of the optical fiber. This reflection is then coupled back into the fiber, travels back along it against the propagation direction, and can finally be measured spatially in the propagation direction upstream of the fiber input. This back reflection can then be advantageously used to adjust the coupling power. The power measurement of the fed-back light does not result in a power plateau. Therefore, the coupling quality can be determined reproducibly and with very high accuracy. The measurement is also particularly sensitive to deteriorating beam quality.

[0010] This detection of the back reflection advantageously enables, especially with a purely passive collimation module, the realization of an adjustment mode for optimizing the coupling state, mode monitoring of the expected coupled-out laser mode indirectly via the back reflection, and laser power control by measuring the back reflection.

[0011] In this context, a fiber end face is understood to be, in particular, an imaginary or physical surface that bounds the optical fiber in the direction of its longest extent, i.e., its longitudinal direction, which coincides in particular with the propagation direction of the useful light propagating in the optical fiber. The fiber end face can also be a partial surface of an arrangement of end faces composed of a plurality of end faces. In a preferred embodiment, the fiber end face is a fiber end facet.

[0012] The propagation direction is, in particular, the direction along which laser light spreads from the laser radiation source. The statement "in the propagation direction in front of" an element indicates a location that photons of the useful light, propagating in the direction of propagation, pass before the element to which the statement refers. Similarly, the statement "in the propagation direction behind" an element indicates a location that photons propagating in the direction of propagation pass after the element in question.

[0013] In this context, useful light is understood to be light that propagates along a predetermined beam path leading from a laser radiation source through the light guide to a target location. The target location is, in particular, a place where the useful light is applied as intended, for example, on or to a workpiece that is irradiated with laser light, such as during welding or cutting. Specifically, useful light is light that propagates along the propagation direction through the light guide as intended. In particular, useful light is the sum of all photons or light rays that would pass through the light guide if propagated undisturbed, regardless of whether they are located—in the propagation direction—before, within, or after the light guide and / or are branched off.For the classification of light as part of the useful light, it is therefore irrelevant whether the light has actually already passed through the light guide area; rather, it is sufficient that the light would pass through the light guide area as intended if it were not deflected beforehand by an element provided for this purpose. In particular, part of the useful light can be deflected in the propagation direction in front of or behind the light guide area. Specifically, useful light is that light which, when the optical fiber unit is used in a laser processing machine, propagates, particularly from the laser radiation source of the laser processing machine, along the beam path that leads through the light guide area to a workpiece as the light destination. Specifically, useful light is not cladding light and not leakage light.

[0014] According to a further development of the invention, the optical fiber has a cladding region that circumferentially surrounds the optical fiber. The optical fiber is, in particular, a fiber cladding or part of a fiber cladding. Such a cladding region can advantageously increase the efficiency of the optical fiber transmission. Depending on the modes of the laser light coupled into the optical fiber, cladding light may occur in the cladding region. Importantly, regardless of whether cladding light occurs in the cladding region, the cladding light is not used to assess the coupling quality according to the teaching proposed here, but rather a portion, i.e., the reflection, of the useful light propagating through the optical fiber.

[0015] According to a further development of the invention, the reflection element is designed as an optical beam shaping element for shaping the useful light beam. Such a beam shaping element can be, for example, a lens, in particular a collimating lens or diverging lens, a diffractive optical element, a wave plate, an axicon, or a wedge. In this embodiment, a back reflection of the beam shaping element is advantageously used, which allows the laser system to be designed compactly.

[0016] Alternatively, the reflector element is preferably arranged in a beam path of the useful light in addition to an optical beam-shaping element. Such a beam-shaping element can be, for example, a lens, in particular a collimating lens or diverging lens, a diffractive optical element, a wave plate, an axicon, or a wedge. In this embodiment, a back reflection of the beam-shaping element is not used, but rather a separate reflector element is provided, which is specifically designed to generate the back reflection. A beam-shaping element typically has a high-quality antireflective coating. In contrast, the reflector element preferably has a lower-quality antireflective coating on at least one surface in order to reflect a larger proportion of the useful light than the beam-shaping element.

[0017] According to a further development of the invention, the reflective element is designed as an element selected from a group consisting of: a plane-parallel plate and a window. If the reflective element is a plane-parallel plate or a window, it is preferably provided in addition to a beam-shaping element.

[0018] According to a further development of the invention, the optical fiber is designed as a hollow-core photonic crystal fiber (HC-PCF). Alternatively, the optical fiber is preferably designed as a band-gap photonic fiber. Alternatively, the optical fiber is preferably designed as an antiresonant coupling fiber. In particular, the optical fiber is preferably designed as a tubular fiber. Alternatively, the optical fiber is preferably designed as an inhibited coupling fiber, in particular as a kagomé fiber. Such fibers are particularly suitable for guiding ultrashort pulses, and thus for ultrashort pulse applications such as ultrashort pulse welding.

[0019] According to the invention, the reflective element is provided with a first antireflective coating on an end face facing forward with respect to the propagation direction of the useful light. This coating has a first reflectance. The reflective element is provided with a second antireflective coating on an end face facing backward with respect to the propagation direction of the useful light. This second reflectance has a higher reflectance than the second reflectance. This advantageously allows for the generation of a sufficiently intense back reflection for controlling the coupling, while simultaneously preventing the reflective element from exhibiting an unnecessarily low transmission overall, which would adversely attenuate the useful light.In a particularly advantageous embodiment, a balance is achieved between generating a meaningful back reflection on the one hand and ensuring the least possible attenuation of the useful light on the other. According to a preferred embodiment, the reflectance on the front face is increased, especially compared to typically used antireflective coatings, in order to generate a defined back reflection suitable for a meaningful measurement of the coupling quality. In contrast, the reflecting element on the rear face preferably exhibits a reflectance typical of antireflective coatings, thus advantageously minimizing power losses and, in particular, avoiding unnecessary power losses.According to the invention, the first reflectance is at least 0.3% to at most 2%, and in embodiments not covered by the claims, it is at least 0.1% to at most 1%, preferably at least 0.5% to at most 1%. The second reflectance, according to the invention, is at most 0.2%, preferably at most 0.1%.

[0020] According to a further development of the invention, the measuring device includes a light measuring device which is configured to detect a light power and / or a mode profile of the detected reflection of the useful light.

[0021] The light measuring device is preferably designed as a camera. This allows for precise measurement of the position of the reflection in the camera's image plane.

[0022] Alternatively, the light measuring device is preferably designed as a photodiode. In this way, the position of the reflection and thus the coupling quality can be assessed very precisely – possibly more cost-effectively than with a camera.

[0023] According to a further development of the invention, the measuring device includes a control unit, wherein the coupling device has a controllable adjustment unit configured to adjust the coupling of the useful light into the optical fiber unit. The control unit is operatively connected to the adjustment unit and configured to control the adjustment unit depending on the detected reflection of the useful light. In this way, an automatic or automated, preferably also continuous, adjustment of the coupling, and thus in particular a real-time adjustment of the coupling quality, is advantageously possible. In a preferred embodiment, the adjustment unit has two motorized mirror holders which can be controlled by the control unit in such a way as to optimize the coupling of the useful light into the optical fiber unit.Alternatively or additionally, it is preferably possible that the adjustment device has an adjustable, i.e., in particular, adjustable, coupling lens.

[0024] The laser system preferably includes a measuring reflective element arranged in the propagation direction upstream of the first fiber end face. This element is configured to deflect light returned by the light guide away from the beam path of the useful light, particularly towards the measuring device. This deflection can specifically be the reflection from the reflective element. Alternatively or additionally, the light returned by the light guide and deflected away from the beam path by the measuring reflective element is process light, allowing for an evaluation of the work process carried out with the useful light. "Deflecting" is understood to mean, in particular—especially with a partially transparent deflecting element and depending on the undisturbed beam path of the useful light—"transmitting" or "deflecting."

[0025] The measuring reflective element can be integrated into the coupling end of the optical fiber assembly, for example, into an end cap or connector of the optical fiber. However, it can also be a separate element, additional to and, in particular, separate from the optical fiber assembly.

[0026] If the measuring reflective element is used in addition to or as an alternative to guiding the reflection originating from the reflective element to implement process monitoring of the work process carried out with the useful light, then, in particular, the ratio of incident light power to reflected light power can serve as a measure of the feedback from the process. The measuring reflective element is then advantageously configured to direct process light reflected through the light guide area towards the measuring device.

[0027] In a preferred embodiment, the measuring reflective element is additionally configured to direct a further portion of the useful light incident in the propagation direction away from the beam path towards an additional light measuring device. This makes it possible to assess the incident power for process monitoring purposes and, in particular, to calculate the ratio of incident power to reflected power as a measure of the process's impact. The additional light measuring device is preferably part of the measuring device, which is additionally configured to assess the work process. In particular, a comparison of the measured values ​​of the light measuring device on the one hand and the additional light measuring device on the other can be used for optical fiber breakage monitoring, either in addition to or as an alternative to process monitoring.

[0028] However, it is also possible that instead of the measuring reflective element, an additional optical deflection element, in particular a beam splitter, is used to direct the further part of the useful light incident in the propagation direction towards the additional light measuring device on the coupling side, or an adjustment mirror is designed to be semi-transparent for this purpose.

[0029] The semi-transparent adjustment mirror can also be used additionally or alternatively to allow the reflection coming from the reflective element to pass through to the light measuring device.

[0030] According to a further development of the invention, the measuring device is additionally configured to detect the portion of the useful light diverted away from the beam path before coupling into the optical fiber unit. The measuring device is further configured to evaluate a processing operation carried out with the laser system using the useful light, based on the detected reflection of the useful light or the detected process light and the detected additional portion of the useful light. In particular, process monitoring can be performed in this way, preferably using the ratio of incident power to reflected power as a measure of the feedback effect from the process. Furthermore, a rapid shutdown can preferably be performed if reflected light is expected but none can be detected. In particular, optical fiber breakage monitoring can be implemented in this way.

[0031] In particular, the measuring reflective element, the additional optical deflection element, or a semi-transparent alignment mirror is preferably used to detect the further portion of the useful light that is diverted away from the beam path before being coupled into the optical fiber unit. If the measuring reflective element is used for this purpose, it advantageously also serves to direct the reflection of the useful light or the process light onto the measuring device.

[0032] The problem is ultimately solved by creating a method for assessing the coupling quality of useful light from a laser radiation source into an optical fiber assembly, wherein a laser system according to the invention or a laser system according to one of the previously described embodiments is used within the framework of the method. In connection with the method, the advantages already described previously in connection with the laser system are realized in particular.

[0033] In the process, in particular a reflection of the useful light deflected by the reflective element of the optical fiber unit is detected, whereby the coupling quality is assessed based on the detected reflection of the useful light.

[0034] Preferably, a luminous flux and / or a mode profile of the detected reflection of the useful light is detected.

[0035] Preferably, the coupling of the useful light into the optical fiber unit is automatically adjusted based on the detected reflection of the useful light.

[0036] Preferably, the emission of the useful light is suppressed when the intensity of the detected reflection of the useful light falls below a predetermined threshold.

[0037] Preferably, a further portion of the useful light, diverted from the beam path of the useful light before coupling it into the optical fiber unit, is detected. A processing operation carried out with the laser system using the useful light is preferably evaluated based on the detected reflection of the useful light or the detected process light and on the detected further portion of the useful light.

[0038] The invention will be explained in more detail below with reference to the drawing. The drawing shows: Figure 1 is a schematic representation of a first embodiment of a laser system with an embodiment of an optical fiber unit, and Figure 2 is a schematic representation of a second embodiment of a laser system.

[0039] Fig. 1Figure 1 shows a schematic representation of a first embodiment of a laser system 1, comprising a laser radiation source 3 for emitting useful light and an embodiment of an optical fiber assembly 5. The laser radiation source 3 is preferably a laser, ideally an ultrashort pulse laser. The laser system 1 also includes a coupling device 7 for coupling the useful light into the optical fiber assembly 5. Furthermore, the laser system 1 includes a measuring device 9 configured to detect a reflection of the useful light deflected by a reflective element 11 of the optical fiber assembly 5. The measuring device 9 is preferably configured to assess the coupling quality of the useful light into the optical fiber assembly 5 based on the detected reflection of the useful light.

[0040] The optical fiber unit 5 has an optical fiber 13, which has a light guide area 15 designed for guiding useful light through the optical fiber 13 and at a first fiber end 17, which is an input end 21, a first fiber end surface 25 for coupling laser light into the light guide area 15, and at a second fiber end 19, which is an output end 23, a second fiber end surface 27 for coupling laser light out of the light guide area 15.

[0041] The reflector 11 is arranged and configured in the propagation direction of the useful light downstream of the second fiber end face 27, in particular downstream of the output end 23, to deflect the reflection of the useful light away from the propagation direction and couple it back into the light guide area 15 via the second fiber end face 27, in the opposite direction to the propagation direction. The measuring device 9 is arranged upstream of the first fiber end face 25, in particular upstream of the input end 21, in the propagation direction of the useful light. The reflection of the useful light deflected by the reflector 11 and guided back through the light guide area 15 can be detected by the measuring device 9 and advantageously used to assess the coupling quality. In particular, a very stable and reproducible assessment of the coupling quality is possible, whereby the beam quality is also detected, and the formation of a power plateau is avoided.

[0042] In a preferred embodiment, the optical fiber 13 has a sheath area 33, shown only schematically here, which surrounds the optical fiber area 15 in the circumferential direction.

[0043] The laser system 1, preferably the optical fiber unit 5, also comprises an optical beam shaping element 35, here in the form of a lens, in particular a diverging lens. The optical beam shaping element 35 is provided for shaping the useful light beam, in particular for coupling it out of the optical fiber area 15 and for widening the useful light beam. In this first embodiment of the optical fiber unit 5, the reflector element 11 is designed as the beam shaping element 35 or is formed by the beam shaping element 35. In particular, it is designed as a lens, in particular a diverging lens. A separate reflector element 11 is then not required, making this design particularly compact. The beam shaping element 35 can alternatively also be designed as a diffractive optical element, as a wave plate, as an axicon, or as a wedge.

[0044] The optical fiber 13 is preferably designed as a photonic crystal fiber with a hollow core, as a photonic band gap fiber, as an antiresonant fiber, in particular a tubular fiber, or as an inhibited coupling fiber, in particular a kagomé fiber.

[0045] The reflective element 11 is provided with a first antireflective coating on a front end face 39, which has a first reflectance value, with respect to the propagation direction of the useful light. The reflective element is provided with a second antireflective coating on a rear end face 41, which has a second reflectance value. The first reflectance value is higher than the second reflectance value. Preferably, the reflectance value on the front end face 39 is increased, particularly compared to typically used antireflective coatings, in order to generate a defined back reflection suitable for a meaningful measurement of the coupling quality. In contrast, the reflective element 11 preferably has a reflectance value typical for antireflective coatings on the rear end face 41, thus advantageously minimizing power losses and, in particular, avoiding unnecessary power losses.According to the invention, the first reflectance is at least 0.3% to at most 2%, and in embodiments not covered by the claims, it is at least 0.1% to at most 1%, preferably at least 0.5% to at most 1%. The second reflectance, according to the invention, is at most 0.2%, preferably at most 0.1%.

[0046] The measuring device 9 preferably includes a light measuring device 43, which is configured to detect a luminous intensity and / or a mode profile of the detected reflection of the useful light. The light measuring device 43 is preferably designed as a camera or as a photodiode.

[0047] The measuring device 9 preferably also includes a control unit 45. The coupling device 7 preferably includes a controllable adjustment device 47 for adjusting the coupling of the useful light into the optical fiber unit 5. The control unit 45 is operatively connected to the adjustment device 47 and configured to control the adjustment device 47 depending on the detected reflection of the useful light. In a preferred embodiment, the adjustment device 47 has two motorized mirror holders 49, which can be controlled by the control unit 45 for automatic adjustment of the coupling. Alternatively, a coupling lens 50 can also be adjustable, in particular adjustable, and controllable by the control unit 45 for automatic adjustment of the coupling.

[0048] The measuring device 9 also includes a measuring reflective element 51 to direct the reflection of the useful light, guided back by the reflective element 11 through the light guide area 15, away from a beam path of the useful light, preferably at a specific angle, particularly in the direction of the measuring device 43. Here, the measuring reflective element 51 is formed in a particularly compact manner by a semi-transparent adjustment mirror held in the second mirror holder 49. However, it can also be provided separately and in addition to this.

[0049] The measuring reflective element 51 can additionally be used to measure the coupled power for process monitoring purposes by means of an additional light measuring device. Process monitoring is used in connection with Figure 2 explained in more detail.

[0050] Fig. 2Figure 1 shows a schematic representation of a second embodiment of the laser system 1 with a second embodiment of the optical fiber unit 5. Identical and functionally equivalent elements are provided with the same reference numerals in all figures, so that reference is made to the preceding description in each case.

[0051] In the second embodiment of the laser system 1, the reflection element 11 is provided separately from the optical beam shaping element 35, which is configured here as a collimation lens. The reflection element 11 is preferably configured as a plane-parallel plate. Alternatively, the reflection element 11 can be configured as a window.

[0052] The measuring reflective element 51, provided separately from the mirror holder 49, preferably also deflects a portion of the useful light incident in the propagation direction towards the coupling side in the direction of the additional light measuring device 55.

[0053] The measuring reflective element 51 can be used, in particular, to perform process monitoring by detecting process light through the light measuring device 43. Simultaneously, a measure of the coupled power can be acquired using the additional light measuring device 55, whereby a comparison of the detected process light power with the coupled power can be used to assess the work process. The additional light measuring device 55 is preferably part of the measuring device 9, which is additionally configured to assess the work process accordingly. A comparison of the measured values ​​of the light measuring device 43 on the one hand and the additional light measuring device 55 on the other can also be used for optical fiber breakage monitoring.

[0054] The measuring reflective element 51 can, in particular, be designed as a measuring mirror, wherein a suitable coating on a front and a back side of the measuring reflective element 51 ensures that the useful light can pass through the measuring mirror in the propagation direction almost undiminished, while the back reflection is directed with sufficient intensity onto the light measuring device 43. In particular, preferably 99% of the useful light is transmitted in the propagation direction, and a proportion of 1% of the back reflection is reflected onto the light measuring device 43. In an alternative embodiment, the measuring reflective element 51 can also be designed as a thin-film polarizer, wherein a 1 / 2 plate (not shown here) rotates the polarization of the back reflection on the return path.

[0055] In a method for assessing the coupling quality of the useful light from the laser radiation source 3 into the optical fiber unit 5, a laser system 1 according to one of the previously described embodiments is preferably used, wherein the emission of the useful light is preferably suppressed when the intensity of the detected reflection of the useful light falls below a predetermined threshold value. In particular, optical fiber break monitoring can be implemented in this way.

Claims

1. A laser system (1), comprising - a laser radiation source (3) for emitting useful light, - an optical fiber unit (5) having an optical fiber (13), wherein the optical fiber (13) has a light guiding region (15) configured to guide useful light through the optical fiber (13), and a first fiber end surface (25), at an input coupling end (21) as a fiber end (17, 19), for coupling laser light into the light guiding region (15), and a second fiber end surface (27), at an output coupling end (23) as another fiber end (17, 19), for coupling the laser light out of the light guiding region (15), - a coupling device (7) for coupling the useful light into the optical fiber unit (5), - a reflection element (11) that is arranged downstream of the second fiber end surface (27) in the propagation direction of the useful light and is configured to deflect a reflection of the useful light from the propagation direction and to couple it back into the light guiding region (15) opposite to the propagation direction via the second fiber end surface (27), and comprising - a measuring device (9) that is arranged upstream of the first fiber end surface (25) in the propagation direction and is configured to capture the reflection of the useful light deflected by the reflection element (11), wherein the reflection element (11) is provided with a first anti-reflection coating on a front end face (39), relative to the propagation direction of the useful light, the first anti-reflection coating having a first reflectance, characterized in that the reflection element (11) is provided with a second anti-reflection coating on a rear end face (41), relative to the propagation direction of the useful light, the second anti-reflection coating having a second reflectance, wherein the first reflectance is greater than the second reflectance, wherein the first reflectance is at least 0.3 % to at most 2 %, and wherein the second reflectance is at most 0.2 %.

2. The laser system according to claim 1, characterized in that the optical fiber (13) has a cladding region (33) that reaches around the light guiding region (15) in the circumferential direction.

3. The laser system according to any one of the preceding claims, characterized in that the reflection element - is designed as an optical beam shaping element (35) for performing beam shaping of the useful light or - is arranged in a beam path of the useful light in addition to being an optical beam shaping element (35).

4. The laser system according to any one of the preceding claims, characterized in that the reflection element (11) is designed as an element selected from a group consisting of: a plane-parallel plate and a window.

5. The laser system according to any one of the preceding claims, characterized in that the optical fiber (13) is designed as a hollow-core photonic crystal fiber, as a photonic band gap fiber, as an anti-resonant fiber, in particular a tubular fiber, or as an inhibited coupling fiber, in particular a Kagomé fiber.

6. The laser system according to any one of the preceding claims, characterized in that the measuring device is configured to assess an input coupling quality of a coupling of the useful light into the optical fiber unit (5) based on the captured reflection of the useful light.

7. The laser system (1) according to any one of the preceding claims, characterized in that the measuring device (9) has a light measuring device (43), in particular a camera or a photodiode, wherein the light measuring device (43) is configured to capture a light power and / or a mode profile of the captured reflection of the useful light.

8. The laser system (1) according to any one of the preceding claims, characterized in that the measuring device (9) has a control device (45), wherein the coupling device (7) has a controllable aligning device (47) for aligning the coupling of the useful light into the optical fiber unit (5), wherein the control device (45) is operatively connected to the aligning device (47) and is configured to control the aligning device (47) depending on the captured reflection of the useful light.

9. The laser system (1) according to any one of the preceding claims, characterized in that in addition, the measuring device (9) is configured to capture a further portion of the useful light that is guided away from a beam path of the useful light before the light is coupled into the optical fiber unit (5), wherein the measuring device (9) is further configured to assess a machining process performed with the laser system (1) using the useful light, the assessment being based on the captured reflection of the useful light or on the captured process light and on the captured further portion of the useful light.

10. A method for assessing an input coupling quality of the coupling of useful light of a laser radiation source (3) into an optical fiber unit (5), wherein a laser system (1) according to any one of claims 1 to 9 is used, and wherein the emission of the useful light is preferably suppressed if an intensity of the captured reflection of the useful light falls below a predetermined threshold value.