Focus lens reflection

[jjl1210]

I would like to get peoples thoughts and opinions on this matter it’s not a test so please don’t guess. Is it possible for the focus lens to reflect the beam backwards while cutting a non-reflective material like mild steel? aluminum, brass and copper for sure but mild steel.

 

[laserarts]

focus lens would usually have AR(anti-reflective) coating on them. Ask your lens manufacturer what's AR rating of your lens.
PS: any material will bounce some of the beam back, even mild steel(especially metals if you are using 1064nm laser).

 

[laserengine1]

even for a very good coated ar lens, you can get ~.1 to .5% back reflections. for metal cutting lasers, (500+ W) lasers, this can be very significant back reflection. If your feed fiber is long enough, this(back reflected light) can be taken by cladding(especially in multimode fiber lasers). If this is free space, it might hit metal joints, lens holders in the relay telescope and unglue things or burn parts. The most often used solution is to pitch the last focusing lens 2-3 deg so the retro reflection takes a different path and hopefully hit a sidewall before it does major damage. (fyi if you have solidworks, try ray tracing in sketchmode using snell law )

 

[GordonGould]

Copper is ~98% Reflective to Fiber Laser Wavelength
Aluminum ~85%
Steel ~70%

Piercing through is the biggest problem time as molten material is highly reflective and launching light in all directions. It is important to pierce through quickly when dealing with Copper.

For MS, you have nearly zero worry. most AR coatings with a Normal Angle of Incidence are </= 0.1% per optical surface so the optical reflectivity should never be a concern. as you move away from normal AOI, you start to get higher and higher reflectivity.