We want to measure the focusing efficiency of the Fresnel zone plate with a very small FWHM.
"The focusing efficiency is defined as the ratio of the total power in a circular aperture with a radius three times the FWHM of the focal spot to the total power of light transmitted through the lens."
There are two points confusing me:
1>. Due to the FWHM is about 1.5 um, when using a circular aperture with a radius three times the FWHM, diffraction happens, will the diffraction affect the measured result? how can I solve this problem?
2>."total power of light transmitted through the lens" means the power should be measured before or after the lens? How much should the spot size of the incident light be larger than the lens?
You can obtain magnified image of Fresnel lens focal plane using conventional lens, then measure light intensity distribution there by image sensor with sufficient resolution.
Diffraction is not important, because total power passed through circular aperture does not changes. More important are inconviniences related to very small required aperture diameter. Again, optical magnification will simplify the measurement.
According the definition of focusing efficiency you cite, the light power should be measured after Fresnel lens. Input beam size is not important (supposed Fresnel lens holder reject light outside lens aperture.
Thank you very much.
I think I have understood.
Have a good time@ Aleksandr A. Tarasov
To describe the quality of your Fresnel lens, it is better to use a more rigorous Strehl ratio concept rather than quite arbitrary defined focusing efficiency.
Using optics to magnify your focal plane zone please take the aperture to be large enough not to lose the scattered radiation as well. Otherwise, you will get an overestimated value of the Strehl factor of your Fresnel lens.
Shiru Jiang, to find the experimental answer for your question you need to put into the focal plane of your lens optical knife, (straight, thin, non-transparent object, like a razor blade), move it by micrometric translation stage in direction, normal to the optical axis, and simultaneously recording light intensity "I" that passes to photodetector as a function translation value "x". Then if you calculate the value dI(x)/dx as a function x, you will have measured the profile of intensity into the lens's focal plane.
The term focusing efficiency is widely used, but it is ambiguous, and I suggest not to use it. You can see that most of the replies you received assume you want to measure the spot size/resolution. For this I agree that measuring the spot intensity profile and obtaining the Strehl ratio from it is best. However, what I believe you are looking for is what is known as the diffraction efficiency. i.e. what percentage of the incident light goes to the desired order of diffraction (usually the first order). This can be measured using an appropriately sized pinhole, as Arbabi did, but I can suggest a more convenient method detailed in my paper:
https://www.degruyter.com/view/journals/nanoph/9/2/article-p361.xml
We have done something similar to what Aleksandr A. Tarasov has described above. We used a high-resolution camera and took an image at the focal plane. Integrate over the first disc and divided by the image with dimensions equal to the EDP. The last image was taken away from the metalens to account for the normal light incident on the metalens.
- Badges
- Science topic