Meaning
Refractive index variations caused by temperature gradients create a parasitic focusing effect that is measured against the ideal beam quality. Optical thermal lensing occurs when a medium absorbs energy from a light beam, causing the center to heat up more than the edges. It acts like a temporary lens that distorts the path of the light.
This effect is a common problem in high power laser amplifiers.
Refractive Shift
Gradient of the temperature creates a profile that mimics a convex or concave lens. The optical thermal lensing changes the divergence of the beam as the power increases. If the effect is not compensated, the beam can focus too early and damage the next optical component.
Beam Distortion
Uniformity of the wavefront is compromised by the uneven heating of the material. Within a system suffering from optical thermal lensing, the focal spot becomes enlarged or asymmetrical. A pilot result from a cold system will not match the production yield of a system at full power.
Performance Degradation
Quality of the laser process decreases as the thermal effects stabilize. When optical thermal lensing is present, the cutting or welding speed must be reduced to maintain precision. Calling a design production ready without a thermal stabilization run leads to inconsistent results between batches.
The focal shift is measured against the Rayleigh range of the laser beam.
