Pembroke Instruments, LLC
Question
Asked 1st Feb, 2016
How to determine laser beam diameter manually?
I need to determine laser beam diameter manually just by firing laser spot to paper/arcylic and changing focal length and laser power, then observe using microscope to determine laser beam diameter. Is the pulse width and repitation rate can be constant by taking the minimum value? Is pulse width and repitation rate give any effect to beam diameter?.
Most recent answer
I have a related question regarding using a CCD or CMOS camera to measure the diameter of a focused laser beam from pulsed green laser (Thorlabs mode NPL52B). I measured beam diameter using KEM and get about 3 um which is expected. But if I measure with CD camera with pixel size 15 um the spot appears much larger. I believe this is due to over saturation. The laser has huge peak power and using ND filter does not help that much as peak power is still quite large. Is there a workaround to measure 3 um diameter laser with sensor with 15 um square pixel size?
Popular answers (1)
Polyus Research & Development Institute
Dear Ardi!
The diameter of the laser beam can be measured manually four most well-known ways (in the examples, the diameter is measured at the level of 1/e of the maximum intensity; it is also assumed linearity of the readings of a sensor):
1. The method of "copy paper".
Gradually weaken laser radiation, for example, using a set of neutral filters, until there is no longer observed in the microscope the mark on the paper. Thus the previous power level must exceed the value obtained is not more than 10%. Then increase the radiated power in "e" times. Measured the diameter of the mark is equal to the diameter of the laser spot. It is assumed that the laser works on a basic transverse mode that is also verified by the characteristic appearance of the imprint near threshold for the paper value. The imprint must have the correct round shape with no "hot spots".
2. The method of "razor blade" or knife.
The blade should be fixed in the vertical (horizontal) position on the table with micrometric flow in two mutually perpendicular positions. At the table put the radiation receiver (head power meter). Moving the blade with the micrometer screws across the laser beam, record the readings of the power meter depending on the position of the blade (the indications on the scale of the micrometer screw). As you move the blade the readings of the recording instrument will change from maximum value to zero. Do the same in mutually perpendicular direction In this case, you both check and the mode composition of the radiation. If the readings obtained are symmetrical and placed on a circle, the diameter of the laser beam will get if you use the readings on the scale of the micrometer screw, when the power value in "e" times smaller than the maximum (diameter of the laser beam at 1/e intensity).
3. The method of "aperture".
On a micrometric table in front of the photodetector is set, for example, revolving diaphragm with a kit diaphragm with calibrated holes. First set in the holder the aperture with a hole diameter that is certainly smaller than the spot diameter. Centre the aperture relative to the laser beam, that is, achieve the coincidence of the geometric centers of the aperture and the laser beam. Then insert obviously larger aperture to measure the total power of laser radiation. Inserted into the holder aperture with a smaller size and at the same time fix the meter reading. On the chart, build the dependence of the measured power of the laser radiation on the diameter of the aperture. The diameter of the laser beam is adopted, the size of the length of the interval on the chart connecting two points at height 1/e from the maximum.
4. The method of "reference sample".
If you have a reference sample with known laser damage threshold and the passport on it, indicating the dependence of the damage threshold on the duration of the laser pulse and the diameter of the laser spot, then you can also measure the diameter of the laser spot. To do this by using lenses to focus the laser beam on the surface of the reference sample and measure in accordance with his method, the threshold of laser damage. Then, knowing the focal length of the lens, you can easily find the laser spot diameter in the unfocused single-mode laser beam.
It is noteworthy that the method of "reference sample" allows estimating the size of the most "hot" region in the laser beam of a multimode laser.
Influence of laser pulse duration. To change it, you need to significantly change the photon lifetime in the resonator. That is to change the resonator losses, Q-switching parameters, forming a giant pulse, or a resonator configuration. In General, all this may lead to a change of a thermal mode of laser operation. This changes the parameters of the thermal lens in the active element that will change the diameter of the laser beam. But it is - in the general case. In fact, the spot diameter at good as the active element is weakly dependent on the above parameters. Its size is mainly determined by the diameter of the diaphragm (L. Skvortsov et al. Influence of the surface layer on the optical strength of lithium niobate, Quantum Electronics 7(9) · September 1977).
Laser pulse repetition rate may also indirectly affect the diameter of the laser beam. This is because it affects the average power laser. The average amount of power parameters affect the thermal lens in the active element, which leads to a change in beam diameter.
Best regards, Leonid Skvortsov.
5 Recommendations
All Answers (7)
University of Lisbon
You don´t specify the type of laser, but by the choices of targets, I will presume it is a infrared laser. In this case, and assuming that pulse duration is higher than ps, the effect on paper or acrylic is mainly thermal. So, the energy density over the sample will influence the interaction of the beam with the material. If you change the set pulse width - repetition rate, you change energy density and thus change the "footprint" on the target, and consequently your measurement. The best way is, in fact, keep the parameters constant for the different measurments. Nevertheless, you will always have just a qualitative analysis since the measurement is always affected by the interaction process. Unless you can calibrate using another more precise method...
Does pulse width and repetition rate affect the beam diameter? I think it depends on the laser technology being considered. Perhaps best ask the manufacturer...
1 Recommendation
Indian Institute of Technology Guwahati
There is a very simple method for getting the laser beam diameter. You focus your laser on a paper. Once it fall on a paper will make hole in it which is equal to the laser beam diameter.
The Boeing Company
Maybe, in some vague sort of way, this is a macro version of Heisenberg's uncertainty principle. Whatever you use to measure beam diameter will affect beam diameter.
The diameter of any hole the beam is allowed to burn will vary according to power density impacting the medium. Or introducing some sort of particles in the space traveled by the laser beam, to make the beam visible so it can be measured, will cause the beam to scatter.
Can't fool mother nature.
Polyus Research & Development Institute
Dear Ardi!
The diameter of the laser beam can be measured manually four most well-known ways (in the examples, the diameter is measured at the level of 1/e of the maximum intensity; it is also assumed linearity of the readings of a sensor):
1. The method of "copy paper".
Gradually weaken laser radiation, for example, using a set of neutral filters, until there is no longer observed in the microscope the mark on the paper. Thus the previous power level must exceed the value obtained is not more than 10%. Then increase the radiated power in "e" times. Measured the diameter of the mark is equal to the diameter of the laser spot. It is assumed that the laser works on a basic transverse mode that is also verified by the characteristic appearance of the imprint near threshold for the paper value. The imprint must have the correct round shape with no "hot spots".
2. The method of "razor blade" or knife.
The blade should be fixed in the vertical (horizontal) position on the table with micrometric flow in two mutually perpendicular positions. At the table put the radiation receiver (head power meter). Moving the blade with the micrometer screws across the laser beam, record the readings of the power meter depending on the position of the blade (the indications on the scale of the micrometer screw). As you move the blade the readings of the recording instrument will change from maximum value to zero. Do the same in mutually perpendicular direction In this case, you both check and the mode composition of the radiation. If the readings obtained are symmetrical and placed on a circle, the diameter of the laser beam will get if you use the readings on the scale of the micrometer screw, when the power value in "e" times smaller than the maximum (diameter of the laser beam at 1/e intensity).
3. The method of "aperture".
On a micrometric table in front of the photodetector is set, for example, revolving diaphragm with a kit diaphragm with calibrated holes. First set in the holder the aperture with a hole diameter that is certainly smaller than the spot diameter. Centre the aperture relative to the laser beam, that is, achieve the coincidence of the geometric centers of the aperture and the laser beam. Then insert obviously larger aperture to measure the total power of laser radiation. Inserted into the holder aperture with a smaller size and at the same time fix the meter reading. On the chart, build the dependence of the measured power of the laser radiation on the diameter of the aperture. The diameter of the laser beam is adopted, the size of the length of the interval on the chart connecting two points at height 1/e from the maximum.
4. The method of "reference sample".
If you have a reference sample with known laser damage threshold and the passport on it, indicating the dependence of the damage threshold on the duration of the laser pulse and the diameter of the laser spot, then you can also measure the diameter of the laser spot. To do this by using lenses to focus the laser beam on the surface of the reference sample and measure in accordance with his method, the threshold of laser damage. Then, knowing the focal length of the lens, you can easily find the laser spot diameter in the unfocused single-mode laser beam.
It is noteworthy that the method of "reference sample" allows estimating the size of the most "hot" region in the laser beam of a multimode laser.
Influence of laser pulse duration. To change it, you need to significantly change the photon lifetime in the resonator. That is to change the resonator losses, Q-switching parameters, forming a giant pulse, or a resonator configuration. In General, all this may lead to a change of a thermal mode of laser operation. This changes the parameters of the thermal lens in the active element that will change the diameter of the laser beam. But it is - in the general case. In fact, the spot diameter at good as the active element is weakly dependent on the above parameters. Its size is mainly determined by the diameter of the diaphragm (L. Skvortsov et al. Influence of the surface layer on the optical strength of lithium niobate, Quantum Electronics 7(9) · September 1977).
Laser pulse repetition rate may also indirectly affect the diameter of the laser beam. This is because it affects the average power laser. The average amount of power parameters affect the thermal lens in the active element, which leads to a change in beam diameter.
Best regards, Leonid Skvortsov.
5 Recommendations
KU Leuven
From your question I am assuming you are focusing a pulsed laser beam. Knife edge and thermal paper measurements will be difficult in this case, after focusing. Pulse duration will definitely affect the diameter on your sample due to thermal diffusion term which depends on pulse duration.
Ruhr-Universität Bochum
very simple way of determining the laser beam diameter is by using CCD camera. First you can determine the number of pixels in terms of unit length. Then you can take the laser beam image and from pixel measurement you know that how many pixels are in terms of length. So in that way you can determine the laser beam diameter or also called Clear aperture
Pembroke Instruments, LLC
I have a related question regarding using a CCD or CMOS camera to measure the diameter of a focused laser beam from pulsed green laser (Thorlabs mode NPL52B). I measured beam diameter using KEM and get about 3 um which is expected. But if I measure with CD camera with pixel size 15 um the spot appears much larger. I believe this is due to over saturation. The laser has huge peak power and using ND filter does not help that much as peak power is still quite large. Is there a workaround to measure 3 um diameter laser with sensor with 15 um square pixel size?
Similar questions and discussions
What is the best way to couple a pulsed laser source into a fibre?
I am trying to optimally couple the light from a pulsed Nd:YAG laser source into a 2mm core fibre. The wavelength range is between 680nm - 1050nm.
The output beam from the laser is large so currently we have been focusing the laser onto the fibre using a plano-convex lens, however I don't think this is the most optimal way of coupling the light. I have been looking at aspheric lens (such as https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3811) and also FibrePort Couplers (such as https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2940), however I am unsure if these will work with our system since we are focusing the light to a smaller spot size before coupling to the fibre. Is it possible to focus the light from the laser onto a FibrePort coupler, or does the light need to be collimated when entering the fibreport?
Any help/tips/info on this will be greatly appreciated!
Laser: Surelite OPO Plus (http://www.continuumlasers.com/index.php?option=com_content&view=article&id=652&Itemid=581)
Fibre: CeramOptec 2mm core, 0.22 NA (yes, the core is really 2mm in diameter - http://www.ceramoptec.com/products/fiber-cables.html)
Thanks!
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