I have a laser with a divergent beam passing through an aspherical lens. In this way, the laser will be focused after the focal point of the lens. However I would like to know the size of the laser spot at the focal point of the lens. The divergence of the laser beam is variable and therefore, I would like to know if there is a relation of the spot with the power of the laser so that it is practical to calculate the spot of the laser.
Hi Eduardo,
no, unless you get to extreme situations in which you start getting thermal effects due to high laser power, the light distribution is not dependent on the power.
However, be very careful how you measure the size of the spot, because while the light distribution does not change, the output of a detector can be misleading.
Only use a ruler and your eyes if the beam has very clear edges sharp edges (human eyes are very bad at estimating diameters of gaussian beams).
If you are using a camera, make sure you tune the exposure of the camera every time the divergence of your beam changes, in order to use as much of the dynamic range of the camera as possible, but without saturating. For example, if your camera returns images where pixels have values between 0 and 255, where 0 is black and 255 is white, the brightest pixels should be around 200.
Good luck!
If you run a multi mode laser the central modes may have a lower threshold than higher order modes. As a result neat the lasing threshold the beam quality of the laser may be much higher than listed in the datasheet. So the lasers beam parameter product may increase with increasing laser power (especially near the lasing threshold).
If you run a single mode laser the number of modes in1. As a result the spot size should not change with laser power. However at powers near the maximum output power the beam quality may be spoiled due to thermal lensing or the arising of higher order modes.
It is a single mode Laser with a free beam, i.e. without a fiber.
If I understand your problem, you would like to determine the size of the laser beam at the focal plane of the lens (which would indicate your true far field divergence) but you seem to only have a power meter.
With a single mode laser you are probably safe in assuming the beam is radially symmetric, so I think a knife edge scan would be a good solution. Put a razor blade on a micrometer and measure the power as a function of position of the blade as crosses into the beam. With a little math you can then back out the shape of the laser beam with the assumption that it is radially symmetric. The math is a little annoying because you are blocking arcs of a radially symmetric function, but it is a simpler measurement than using different sized pin holes.
Thank you all. Desta Mike. Vetores impinge the lens, the laser beam os filtered spatialy André spectraly by a ASE bragg filter. So, using a pinhole, as tou say, can bem solve the issue? Yes, i want to use a powermeter.
I know, due to the manufacture datasheet, the laser beam size and the power at the head exit of the laser.
The diameter of the spot does not depend on laser power but it is related to the coherence of light, types of channel and the focusing power of the lens.
First It should be clear that spot size of the laser beam does not depend on its power. The main point is to estimate the spot size. The spot size can be estimated as suggested by Prof. Aslam Baig. However, the measurement of the spot size can be performed by two ways one as suggested by the Dr. Mike Albert and the other simple method is to use exposed photographic film. Keep this film at the location where you want to measure the spot size. It will darken the area exposed by the laser. The dark area can be taken as the spot size of laser at that location. Now after measuring the laser power at that location one can find the power density. Dividing the power density by time duration of laser will provide laser intensity in that area.
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