The laser beam is first expanded to get a elliptical shape beam using a cylindrical lens. This elliptical shaped beam is then passed through a variable attenuator in order to obtain a change in intensity with time. This attenuated elliptical beam is then incident on a fiber of approximately 0.36 mm width, clamped vertically. I would like to know an effective way of measuring what INTENSITY of the laser beam is actually hitting the sample.
I f you want to measure the intensity of the incident laser pulse you can use a suitable photo detector which is fast enough to respond for the time variation s of the pulse. Knowing the external quantum efficiency ETAe one can can get the photo flux pulse assuming that the photocurrent Iph= qETAe phi
where q is the electron charge, phi is the the indecent photonflux in photons/ sec.
If you have a pulse then you have to integrate its flux with with time along the duration of the pulse and also you integrate the photocurrent along the same time.
Best wishes
If you want power of the excitation source just use a power meter before the sample. If you want intensity in W/cm2 unit you need to calculate the beam diameter at your sample. If your laser is pulsed then you can also calculate the energy per pulse and subsequently peak power.
https://www.rp-photonics.com/peak_power.html
Previous discussions might help you
https://www.researchgate.net/post/How_can_I_calculate_the_Laser_Intensity_in_terms_of_W_m2_by_using_number_of_photons_or_photon_counts/1
https://www.researchgate.net/post/How_can_I_measure_Laser_power_using_a_power_meter
You probably need a photodiode sensor: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3328
Simple particularly, can be measured as the energy of the source dividing by the focal spot area on the sample.
Considering the amount of optics and filters, the best bet is to actually measure the laser intensity at the fiber. My first approach would be to measure the laser intensity passing thru a hole (the same diameter as the fiber) in a metal plate at the position of the fiber. Measuring the total power and the area of the hole, you can get the W/cm^2. This assumes that the intensity is uniform over that area. If not the you would need to photograph the laser spot at that point and then normalize the distribution with the measure power.
Can you substitute your sample fiber with an undoped length. You could then measure the power coming out the other end (shielding out any power not entering the fiber). This would give you a very good measure of the power interacting with the doped fiber.
First you must determine the laser intensity before exposing the sample.
You can then measure the laser intensity after exposing the sample, and you will notice the difference in the intensity value.
The most step to chose the suitable photo multiplier for the the wavelength of out put laser
and make the best coupling laser with fiber
insert the detector very close befor the sample and measure the power, beam spot and intensity
thank for this very useful open discussions .
Regard
Raad
https://www.researchgate.net/post/How_can_I_calculate_the_Laser_Intensity_in_terms_of_W_m2_by_using_number_of_photons_or_photon_counts
I f you want to measure the intensity of the incident laser pulse you can use a suitable photo detector which is fast enough to respond for the time variation s of the pulse. Knowing the external quantum efficiency ETAe one can can get the photo flux pulse assuming that the photocurrent Iph= qETAe phi
where q is the electron charge, phi is the the indecent photonflux in photons/ sec.
If you have a pulse then you have to integrate its flux with with time along the duration of the pulse and also you integrate the photocurrent along the same time.
Best wishes
Ome simple way , It is by Measure the intensity direct by photodetector befor insert the sample.
The laser mode should be taken into account because the intensity of laser is not normally homogenous, so on the spot incident laser on the sample, each part won’t receive the same value of the intensity.
laser intensity I0 can be calculated from:
Io= 2Ppeak /A, where A the area of the laser spot
where ωo: - the beam radius at the focal point,
P peak: - the peak power,
Z0=kω0
Z0 is the diffraction length of the beam,
k=2π/λ, the wavenumber, and λ, the
the wavelength of the laser light
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