Dear Alnaimi,

Usually, a photo detector/ energy meter and  a moving knife edge with rack and pinion setup is used for laser beam profiling. I think you could use the same by doing profiling at focal point.

For other methods, look at http://www.ophiropt.com/laser-measurement-instruments/beam-profilers/knowledge-center/white-paper/maximizing-beam-diagnostics

Hope this would help.

Hi,

in my opinion you can use an Optical Density (OD with values enough as to avoid damage in a CCD). The pixel size of a bare-CCD can be a good estimate for your measures.

Hi Narayanachari & Juan,

Many thanks for your answers. Yes  I'll go for ODs and look at the reflected beam of the polarizer to avoid high beam energy. However I guess one shall need to calibrate the CCD pixels with a naval slide or so to get the the actual size of the focus.

Cheers,

Rad

Hi Md, 

Thanks for your answer. I've already captured the profile with D-cam. However just to avoid any surprises with high power beam so I'll try a SMX camera with short focal length objective in front and damp the beam with ODS on the beam path before the lens which I hope it will work.

Cheers,

Rad

you can take a photo, using a filter in the camera and a reference bar, then in the image you can make the relation via pixels using your reference bar.

Dear Radhwan,

You have not indicated the power of your laser. First by optical means you can attenuate the laser beam to a safe level, Then, to measure the spot size, you can use simple scanning method to measure the intensity in terms of transverse sweep of the output beam at a suitable distance. That is the intensity at far filed. To check you experimental data, you can use Petermann formula for far field optical field. Just try and let see.

Hi Jose,

Yes excellent idea, however the calculated focus is around 25 microns which in this case one can either use the camera's  pixel size or calibrate with either Thorlabs resolution test targets (the so called Naval slides) or using Leitz  Wetzlar stage which is a 1mm divided into 100divisions,  then naturally calibrate the pixel size after this.

Cheers,

Rad

Hi Faramarz,

Many thanks for your answer. I've just managed to measure the focus spot size for a 75mm focal lens And used wedges as mirrors to reflect around 4% of the power and then the beam is attenuated through a wave plate before passing through the polarizer which brought  the power down to uJ value. On the camera SMX sits a short focal length objective ~2mm With nominated mag x100. Then captured the profile for both 1 and 200mJ (full power of the used Nd YAG). and after working out the actual magnification with and without objective so the measured diameter (30um) turned out to be 5 microns bigger than the calculated one.

Cheers,

Rad

Hi Aurelio,

It an 8mm Nd:YAG beam of 800ps and 50Hz rep. focused to a Calculated 24.5 um.

Cheers, 

Rad

Hi,

You can a use Spiricon beam profiler to measure your beam size:

http://www.ophiropt.com/laser/spiricon-laser-beam-profiles

I hope it helps,

Assaf

hi

you can use safe and accurate method via polarid film which was used in old cameras because the thermal diffusivity of this film is very low .Put it directly inront of the laser beam then measure the spot directly or under microscope depending on the spot size.

I hope it is useful

Hussein

If you measured 5 um wider than expected, it could be tail of the gaussian that is being detected for sensitive targets, i.e camera, or thermal paper etc.  

Was the diameter calculated a FWHM, or 90% power content diameter etc? when you image the beam the tails may or may not show up depending on the target.  For instance, meting a metal target only shows you the diameter of energy sufficient to melt, not the edges. 

This where a beam profiler really shines, it can measure the energy vs radius and you can specify which diameter is important for your application and to match your calculation.   

Hi John,

So the image obtained directly on the camera with low power 3mJ as I noticed no difference  between this measurement and the magnified one through short focal objective at high power say 200mJ. Both were giving  excellent  beam profile. Naturally it was attenuated through wave plate and polarizer as well as NDs prior to focusing lens to avoid any hot burns or camera damage.  The measured FWHM was utilised in obtaining thr radius which in turn corrected with guassian optics factor of 0.85.

Cheers,

Rad

you may use a SNOM tip to scan the spot, the Scanner can be a PZT tube.

I agree with Mr. Hussein

Though the camera, or Spiricon, or profiler are awesome once the power has been adequately attenuated, a quick and cheap alternative may be the use of a set of pinholes with receding apertures and a powermeter in tandem. Having found a power decrease to 86% at the best alignment location determines that the beam waist equals the pinhole diameter, assuming a Gaussian beam.  

You can scan the intensity of the laser spot across its diameter though a photodiode which has a pinhole in its center. Plot it vs distance. It will be a Gaussian curve. Then consider (1/e^2)th fraction of the maximum intensity on both the sides of the center of the curve. The distance between these two points will be the diameter of your spot. 

Hi Malcom,

Thank you for your answer. We solved the issue by using a CCD cam.

Also, I would recommend using both glass wedges and ND filters in order to reduce both the energy and intensity of the laser beam to avoid any damages to the camera's chip.

Best,

Rad