By: Steve Rummel, Gary Herrit, Alan Hedges
One of the problems that high power fiber, direct-diode, and disk laser users experience is thermal lensing of their beam delivery optics and debris windows. This is placing increased demands on the optics used in 1 micron laser beam delivery systems. Direct-diode lasers, meanwhile, continue to show improved beam quality and may eventually be used for industrial metal cutting applications, placing increased demands on the optics used in these lasers.
High power fiber and disc lasers start with good quality beams; however, thermal lensing of optics in the beam delivery train may become significant as new 1 micron laser applications develop. In addition to thermal lensing, aberrations must also be minimized to prevent degrading beam quality. Fused silica makes balancing aberrations difficult without using multiple lens elements, which increases thermal lensing problems and system cost.
Direct-diode lasers are now being used in the high power laser heat treating and cladding industries. At this stage in their development, direct-diode laser applications can benefit from special beam shaping optics. And, as their beam quality improves for cutting applications, they will also require special optics that minimize thermal lensing and reduce optical aberrations.
New applications for high quality 1 micron lasers will require the best beam modes and the least amount of thermal lensing. F-theta scanning for welding and cutting are just two of the applications that will demand precise control of beam quality and tightly-focused spot size. Certain applications require just the opposite of small spot size, such as cladding and heat treating. Fiber and disc lasers produce too small a spot size for either application . To minimize thermal lensing and maintain tightly-focused spot size for certain applications, and to shape a 1 micron laser beam for others, new and novel optics made with non-traditional methods and materials such as ZnS MS and copper are required.
Fused silica has been the optic of choice for high power 1 micron laser users. The key advantages of using fused silica are low cost and low bulk absorption. The key disadvantages are very poor thermal conductivity and low refractive index. Also, fused silica cannot be diamond turned, a technique that can machine novel optics such as faceted beam integrators and “fly eye” type optics useful for diverging a 1 micron beam for cladding and heat treating. There is an alternative material to fused silica, however, that can be diamond turned: ZnS MS. This material has many other advantages as well.
ZnS MS offers good thermal conductivity, a relatively high index of refraction, and the ability to be diamond turned. Thermal conductivity is important because it enables the lens to more efficiently dissipate heat generated by the laser beam. This keeps the lens cooler and produces less thermal lensing. A high refractive index means that ZnS MS lenses can be designed with less spherical aberration, which is the dominate aberration in laser beam systems. ZnS MS optics can also benefit from low absorption 1 µm coatings, further increasing optical performance while further reducing thermal lensing.