What should be the easiest way to measure laser beam waist at focus and How?
Hi Aamir,
There are many nice practical descriptions. The best way is to scan along the propagation axis and measure with a camera the beamsize. You can then make a fit based on Gaussian propagation equations and tell the beamsize at the focus (eg. http://www.ophiropt.com/laser-measurement-instruments/beam-profilers/knowledge-center/tutorial/m2-beam-propagation).
If the beam is not Gaussian then the ISO standard description for beamsize is 4sigma, see different descriptions here:
https://en.wikipedia.org/wiki/Beam_diameter
If you don't have a camera or one which is sensitive to the wavelength you can measure with a real knife edge attached to a micrometer precision delay to scan across the beam horizontally and vertically an measure the energy passing.
As a bit of practical advice, assuming your beam waist is small, you will likely need to use a microscope objective to magnify the spot on the CCD, as Marta pointed out, for your equivalent plain imaging. To know your magnification you can either put a diffuser in the laser beam to back illuminate a known standard (like a NIST grid) at the lens imaging plane, or your can use a micrometer stage on the lens to move a known distance right-left ,up-down and track the focal spot that known distance as a calibration. This is the easiest way. Then you can scan through the focus (adjusting the beam intensity to keep it on scale (a 12-bit CCD or CMOS is more useful than an 8-bit CCD. Point Grey makes some cheap 12-bit CMOSs, and if your beam is in the IR, remove the cover glass that is present on most 8-bit CCDs as this glass does not let IR through). Most CCDs and CMOSs cannot see much below the bandgap of Si (1.1 eV) or around 1.2 micron light. If you don't have a wave-plate to adjust the beam intensity, filter the beam well before the focus with high quality (optical) filters. You can of course get more fancy and complicated with beam profile software from most of the laser companies (which can calculate the diffraction limited waist based on phase-fronts.) Attached are some links and a paper using the technique for some physics.
Good Luck!
https://www.coherent.com/products/?785/Laser-Beam-Diagnostics
https://books.google.com/books?id=6Rrx6HvBtVkC&pg=PA223&lpg=PA223&dq=lens+equivalent+plane+optics&source=bl&ots=YL_uEj8-cW&sig=4aWH3C-Vz1isM_7ni__Aa6nwoBw&hl=en&sa=X&ved=0ahUKEwjX7sagyqLLAhVJeSYKHW5lBAc4ChDoAQgxMAQ#v=onepage&q=lens%20equivalent%20plane%20optics&f=false
https://www.ptgrey.com/
Dear Marta Divall and Kirk Flippo.. what i think is to mark a circle on a spot at focus then from that mark we can calculate diameter of the spot.is it not a ryt way i dnt knw whether am wrong or ryt ...comment please
Dear Amir first thing is we cannot circle the spot at focus because its spot size will be in microns. another thing is cirlce over the beam spot will give spot size only for hat beam profile. But in case of Gaussian beam, spot size is given by intensity of beam where it is equal to 1/e^2 times the peak intensity in the transverse direction.
spot size at focusing lens position can be measured by knife edge/pinhole method and then the spot size at beam waist is given by
w0=lamda*f/pi*wl
f is focal length of lens
wl is spot size at lens
Yes, in principle drawing a circle around the beamwaist profile can be a technique, but you still have to quantify where you draw this circle. You could follow the D86 method. You could put the image into a software like MatLab and integrate the pixel intensity in the area of the circle. See below the official way to do this.
D86 width
The D86 width is defined as the diameter of the circle that is centered at the centroid of the beam profile and contains 86% of the beam power. The solution for D86 is found by computing the area of increasingly larger circles around the centroid until the area contains 0.86 of the total power. Unlike the previous beam width definitions, the D86 width is not derived from marginal distributions. The percentage of 86, rather than 50, 80, or 90, is chosen because a circular Gaussian beam profile integrated down to 1/e2 of its peak value contains 86% of its total power. The D86 width is often used in applications that are concerned with knowing exactly how much power is in a given area.
it is nice to continue this discussion, I came to know one more technique from dear Marta Divall. if it possible can suggest some articles or links about D86 width technique.
for more information there are some other techniques also there for spot size measurements like burn spot, Slit, quadrant photodiode, and periodic ruling technique.
THANKS DEAR Marta Divall FOR YOUR VALUABLE INFORMATION..
CAN YOU PROVIDE ME WITH THE DETAILED DESCRIPTION OF THE D86 METHOD..
SOME THING THAT DESCRIBE THE PROCESS IN DETAIL...
While CCD and knife-edge techniques are well-established, I would also suggest a scanning-slit beam profiler (such as the one linked below). SSBP devices usually have a much higher sampling rate than CCDs, are not limited by pixel size, and can handle greater incident power without the use of filters. The modern ones also measure both vertical and horizontal profiles at once.
Ideally, you would measure beam diameter at several positions along the beam. Then you can fit Gaussian beam parameters to these data, including waist size, position, and the M2 parameter.
http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=804
The D86 is also called encircled energy (but is in fact encircled intensity that is measured and energy is computed), this is generally the way that all short-pulse laser systems measure the intensity on target. This is the technique used in my post above to calculated the spot size from a CCD as you need some point of reference to know where to stop your contour, and this number can be essentially arbitrary because the spot may deviate significantly from Gaussian. Also, this number can be manipulated to make your laser look better than it is, i.e. taking the 50% contour instead of the 86% contour can yield a much higher average intensity, thus as a best practice one should not quote a number, but rather show the encircled energy curve. See attached Fig 2, encircled energy at best focus, and attached image of the focal spot through focus.
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