I have a pulsed laser system and I'm working with ablation of metal and carbonaceus materials. How can I calculate with a good precision the correct fluence(J/cm2)? I know the laser source emission height, power, frequency and pulse width.
If the NA going into the focus is not too great, you can use a wedged window to pick off a small part of the beam before reaching the high intensity focus. How much you get from the reflection depends on the coating. Typically one side will be uncoated and the other will be AR coated. The uncoated side will reflect a well known guaranteed fraction of the beam (Fresnel coefficient) which is typically ~4% depending on the wavelength, the index of refraction of the window material, and the angle of reflection. The wedge assures that the reflection from the second surface and subsequent multi-bounce reflections are separated from the front surface reflection. If the focus is still too bright, use a second wedge to reduce it further. If the second reflection is made orthogonal to the first, the net reflection is independent of the polarization. If the NA is too great the Fresnel coefficient will vary too much over the cone of the beam. It helps to work at a steep reflection angle where the Fresnel coefficients do not change rapidly with angle of incidence.
The resulting reflection is a guaranteed fraction of the unmodified beams’ intensity. Since the reflecting surfaces are flat, the shape of the reflected spot at the focus is also the same as the unmodified beam. You can use a camera or any other method to characterize the intensity profile of the reduced intensity focused spot.
Power/Frequency/S, S stands for the size of laser spot on the materials
Thank you all, but what I'm want to calculate is the energy in the focus of the laser. Because I'm working with the formation of nanoparticles and I need to know with a good accuracy the energy delivered by the laser to the surface of the target to be ablated.
I think you may misunderstand. By picking off a small fraction of the beam using a flat plate, the reflection continues to a focus exactly like the focal spot produced without the pickoff. Measuring the weak reflection is equivalent to measuring the full power focus but with the energy reduced by a known constant. Same focal spot size, same profile. Measure the weak focus with a camera or knife edge and then multiply the result by the known constant and you will have an excellent measurement of the full power focal spot.
From definition, fluence is laser pulse energy divided by spot area. However, in a real laser beam we have energy distribution and, indeed, the fluence is different at different places.
For your needs, the simple approximation may be sufficient. You may measure the size of the crater made by a pulse (or several pulses) onto your target and use its area in the formula. The pulse energy should be the same as during the experiment. You will need a simple optical microscope.
Anyway, the most important is repeatability of your experiment., so a simple check, as the above, is sometimes better than an elaborate method. The simple test can be easily repeated on demand and ensure you that conditions are the same as a year before.
The elaborated method determining the shape of pulse energy spatial distribution may be used once. With crater size measurements, it could be helpful in determining the threshold fluence of your target.
As I remember it is necessary to measure the power of output beam of laser by power gauge (detector) or power meter. The second step is the using optical microscope to estimate the size of spot on the surface. But usually the beam diameter is not greater than wavelength of radiation. For best laser focus it will be for visible light from 400 nm to 1000 nm. If you have ordinary laser beam it will be more than 1 micrometer. Because such size is visible in optical microscope and you can easy estimate its shape and size. The third step is dividing your value of power on area of laser spot on the target.
Thank you everyone. I will try to ablate the targets I have with some pulses and check the size of the crater under the optical microscope.
I was wondering if, in addition to checking the craters on the target, we could try using the laser height to estimate the laser focus height in the beam constancy region?
With a lens with a focal length not too long, it is practically impossible to determine the "constancy" region.
It is best to have a focal spot a bit below of the surface of the target. It is a way to minimize laser beam interaction with products of ablation.
To some degree, a shorter lens gives you a smaller focal area. Not too short, since spherical aberration reverses trend.
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