Industrial Ytterbium ~1 micron wavelength near infrared fibre lasers first emerged in the late 1990's. At this time, they were less than 10W of optical power and standard coated BK7 could be used for both reflective and transmissive beam delivery optics. During the 1990's fibre laser development moved at a very rapid rate with 100W and then 200W lasers becoming industrially available in the early 2000's. At these power density levels, in some applications, BK7 became infeasible due to thermal effects. There are two main effects: ? Thermal lensing due to refractive index shift as well as uneven thermal expansion.
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