Intensity of Q-switched 20W laser ???

[jjangdori]

I use 20W laser q-switched at 200KHz.
question 1)
If something is exposured to the laser during 10 sec, how much energy does it absorb ? ignore the absorptance and emissivity.
question 2)
What does it-20W- mean ? Does it mean summation of energy of unit area in laser beam ?

 

[likewhat]

Well, 200 kHz is the frequency that it emits pulses of light. So you have some amount of energy emitted every 1/200,000 = 50 microseconds. 20 watts is 20 Joules/second so you you have 20/200,000 = 100 microjoules per pulse.

In order to answer your question you need to know the pulse length and the spot size on the target. I can just make a guess that it is around 10 nanoseconds and 1 mm and run the numbers.

100 uJ/10 ns = 10000 watts

so if you have a beam diameter of 1 mm the area of the spot is

(0.05 cm)^2*pi = 0.0078 cm^2

so the intensity would be 10000 watts/.0078 cm^2 ~ 1.3 MW/cm^2 about 1.3 megawatts per square centimeter.


If you expose something for 10 seconds and it is a 20 watt or 20 joule/second laser it would absorb 20 J/s * 10 s = 200 Joules.

Hope that helps

 

[jjangdori]

Thank you so much for your great answer.....
I know that the spot size is 50 micrometers. but I coun't find the pulse length... is it duration? if it is...
How can I obtain the value of duration... the frequency of laser is 1064 nm....

 

[likewhat]

Ok, so Qswitched lasers typically have pulse lengths on the order of a few nanoseconds. I picked 10 for fun, but you can try to look up your laser online or something to see if someone who uses it mentions the pulse length.

If the spot size is 50 microns assuming a pulse length of 10 ns gives the intensity of about 0.5 GW/cm^2. That is quite high and whatever you are hitting with it is most likely being ablated rather absorbing the energy into the bulk material.

 

[jjangdori]

You are so kind!!!!!
Ok. my laser gives the intensity of about 0.5 GW/cm^2..
so, anywhere in laser spot, is it equl? I know that TEM00 laser has gaussian distribution...
absolutely... The intensity at center of beam spot is higher than 0.5GW/cm^2 ?????

 

[likewhat]

Yeah, so the spot will have a gaussian intensity profile, the thing i said is averaged over that, also the pulse is gaussian in time which i just averaged over. So if you are super interested in exact numbers you probably can solve for those analytically, but I am not going to do it for you for fun.