Why do different laser fluence calculations give 0.18 to 0.7 J/cm²?

[Tah]

Calculating laser fluence -- so confused

Hi,

I'm now endeavoring to calculate laser fluence using some laser parameters.

The required parameters to solve the fluence are given as:

Spot size: 10mm diameter
Pulse duration: 200ms
Repetition rate: 2-3 (Hz)
Laser Power: 200mW (when measured by a power meter)
Wavelength: 660nm

I have several different results ranging from 0.18J/cm^2 to 0.7J/cm^2

I'm always so confused with fluence calculation.

Anybody please can help me clearly?

 

[Simon Bridge]

Please show your best attempt.

 

[Claude Bile]

Fluence is defined as Joules/m^2.

So, divide the energy of your laser pulse by the spot size.

Claude.

 

[Simon Bridge]

That's the units, not the definition ... but this is what I was wondering too.
If that was all there was to the question, then whence the confusion?

Maybe OP wants the particle flux rather than the energy flux (divide by energy per particle).
Maybe some averaging is needed over several pulses.
Need more information to be sure.

660nm 200mW sounds like a familiar spec for a laser pointer - but the spot size seems a bit big.

 

[Claude Bile]

Ah, yes, correct - it is the units not the definition.

Perhaps it's not clear that "energy" refers specifically to the energy of the laser pulse, which is understandable, since it is not explicit in the units.

2-3 Hz is a strange repetition rate for a laser pointer. Smells like a homework q. to me.

Claude.

 

[UltrafastPED]

Definition of laser fluence: http://lidaris.com/glossary-2/fluence/

It is usually given in joules/cm^2.

The average power is 200 mW, with a rep rate of 2-3 Hz, so the pulse energy is E2=200 mW/2 Hz or E3=200 mW/3 Hz. Thus E2=100 mJ, E3=67 mJ.

The spot size is 10 mm diameter, or 0.5 cm radius ... thus spot size is A=pi (0.5)^2 cm^2 = pi/4

So the fluence is F2=400/pi mJ/cm^2 and F3=266/pi mJ/cm^2.

So the fluence varies from F2=85.8 mJ/cm^2 to F3=127 mJ/cm^2.So: fluence = (average power)/rep rate/area of spot
Just be sure to use consistent units throughout the calculation. I usually do this via a spreadsheet, entering the parameters in their own cells.