Laser system comprising an optical fiber unit, and method for assessing an input coupling quality of the input coupling of useful light into an optical fiber unit

A laser system comprising:

• a laser radiation source for emitting light,
• an optical fiber unit that comprises an optical fiber, wherein the optical fiber comprises a light guiding region configured for guiding the light through the optical fiber, an input coupling end having a first fiber end surface for coupling the light into the light guiding region, and an output coupling end having a second fiber end surface for coupling the light out of the light guiding region,
• a coupling device for coupling the light emitted by the laser radiation source into the optical fiber unit,
• a reflection element that is arranged downstream of the second fiber end surface in a direction of propagation of the light and is configured to reflect the light coupled out of the light guiding region back toward the second fiber end surface to be coupled back into the light guiding region via the second fiber end surface, and
• a measuring device that is arranged upstream of the first fiber end surface in the direction of propagation and is configured to capture the light reflected by the reflection element,
• wherein the reflection element is provided with a first antireflection coating having a first reflectance at a front end face with respect to the direction of propagation of the light, and is provided with a second antireflection coating having a second reflectance at a rear end face with respect to the direction of propagation of the light, and wherein the first reflectance is greater than the second reflectance.