Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers

LPF Donations

Links below open in new window

ArcticMyst Security by Avery | Browser Hide by Avery

# Optic damage threshold? 445nm

#### likevvii

##### Member
I have been looking around regarding the laser power tolerance of AR coated lens and mirrors.
However, I don't know how to do the math to really find out. There are a bunch of numbers thrown around but nothing I can actually use. They do have some info for UV-fused silica, but that base material is not typically what a hobbyist uses.

In W/mm^2 what is the damage threshold a AR coated lens and mirrors in continuous operation?

Wavelength: 445nm
Material: BK7
Coating: AR Broadband (400~700) - HR Dielectric (400-750)

#### gazer101

##### Well-known member
I don't know any specifics, but you'd need to figure out the % of your wavelength absorbed by the lens, and then see if (that)x(the wattage of your) laser exceeds the fracturing temperature of (that glass)x(the amount area the laser is shone upon). Then you can use the 3 variables to solve for the one you want:

Given:
Area Laser is Shown Upon=A (mm^2)
Shattering Temperature of Lens=S (degrees C/mm^2)
Power of Laser=P (W)
Energy Converted to Heat by Lens=E (degrees C/W)

(S*A)=(E*P)
Peak Laser Power before Shattering=(S*A)/E

Not sure if that's what you need ¯\_(ツ)_/¯

#### Anthony P

##### Well-known member
Edmund Optics has several articles and webinars on the topic.

#### Cyparagon

##### 0
445nm? So you're talking diode laser type powers? CW? As far as I know, you're not going to reach damage threshold with any blue diode. Maybe if you focus the beam to a point on the optic, it'll cause problems when dust lands there.

Just keep your optics clean, and you'll be fine. I've never cracked a lens in my decade of shenanigans, and I've poked around in the dozens of watts area.

#### likevvii

##### Member
Thank you for the replies everyone. I have come across equations and try to convert them to W/mm^2 but never got anything reasonable so I must be doing something wrong.

Thanks for the reassurance Cyparagon.

I would still like to know the estimated W/mm^2 if anyone gets the chance to calculate it.

_____
I am in the process of purchasing a few lens and mirrors for ~4W blue diodes. I wont be focusing to a point so it seems I will be fine!

#### kecked

##### 0
Let’s put it this way. I routinely put MW pulses into fibers from a focused tisap. Your fine. Glass. Plastic cheap fiber not so sure.

random link but in the ballpark. 480GW/cm2

interesting my sub ps pulses actual have a higher damage threshold. Didn’t know that

Last edited:

#### likevvii

##### Member
thorlabs.com/newgrouppage9.cfm?objectgroup_id=5840
Going to the "HR Coating" tab, we see some CW laser thresholds.

I understand the first part "W/cm" but after with the "(532 nm, Ø1.000 mm)" does that mean that is the value they tested at and then did the math to convert it to the standard unit of W/cm?

At about 300W/cm, it is about 3W per mm which is pretty good, but not completely safe if I accidentally focus too much; which can happen in many ways.

#### Cyparagon

##### 0
Ø is common notation for diameter, so that's how I would read it, yes.

I was about to lecture you on omitting that superscript 2 and how that changes the meaning entirely, but I actually read the page and it seems Thorlabs is coming up with their own units here I guess. They justify it as follows: "Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size "

So no, 300W/cm is NOT "about 3W per mm", it's 30W/mm of beam diameter.

#### likevvii

##### Member
Ahhhh, it totally slipped my mind that I am dealing with squares and circles here. Makes alot of sense now. Thanks!!!