Hi !
I tryed to calculate the fiber coupling with FICL and POPD and find it results very different.
In POPD I used Gaussian Angle for 0.1 Sna and Rna FICL fibers aperture. For 0.1 Na Gaussian Angle I used 5.74 degree in POPD setting dialog for Beam definition and Fiber Data.
In POPD I got fiber coupling 0.999
In FICL fiber coupling is 0.74
What data I can trust ?)
Best answer by Jeff.Wilde
In general, FICL should be okay as long as diffraction in collimated space can be ignored. A good way to quantify this is based on the Fresnel number of the collimated beam at the output of the first lens as viewed from the input to the second lens. If the Fresnel number is on the order of 10 or larger, then diffraction can be neglected. The Fresnel number is given by Fn = r^2/(lambda*z), where r is the radius of the collimated beam leaving the first lens, lambda is the wavelength, and z is the propagation distance to the second lens. In your case it looks like Fn is about 8, which is probably just large enough that diffraction is minimal, which is why the FICL and POP coupling values are comparable (again with an appropriate aperture setting). In Paul Colbourne’s example where FICL and POP report fundamentally different values, Fn = 0.26, so diffraction cannot be neglected and FICL is no longer valid unless you use his clever skew ray scheme.
Here is my version of your system.
Aperture: Object Space NA = 0.09
Aperture: Object Space NA = 0.15
+2Hi Arkadiy,
The answer here is “it depends” because you’re really looking at 2 different input data sets. The POPD is using Physical Optics with a finite beam waist at the starting surface and diverging in a Gaussian envelope. The FICL is using an infinitely small point source at the starting surface and linearly diverges (this is what the Layout shows). Therefore, without extra consideration, POP doesn't inherently match any other analysis you would have in OpticStudio, but often has the correct results for laser applications.
The webinar that Paul Colbourne gave yesterday has a really good discussion about the differences between Physical Optics & ray-based analysis and how you can get the TEM00 modes to more closely align. I would suggest registering & watching his webinar:
[Webinar] Using Skew Rays to Model Laser Beams [Q&A] | Zemax Community
+3In general, FICL should be okay as long as diffraction in collimated space can be ignored. A good way to quantify this is based on the Fresnel number of the collimated beam at the output of the first lens as viewed from the input to the second lens. If the Fresnel number is on the order of 10 or larger, then diffraction can be neglected. The Fresnel number is given by Fn = r^2/(lambda*z), where r is the radius of the collimated beam leaving the first lens, lambda is the wavelength, and z is the propagation distance to the second lens. In your case it looks like Fn is about 8, which is probably just large enough that diffraction is minimal, which is why the FICL and POP coupling values are comparable (again with an appropriate aperture setting). In Paul Colbourne’s example where FICL and POP report fundamentally different values, Fn = 0.26, so diffraction cannot be neglected and FICL is no longer valid unless you use his clever skew ray scheme.
Here is my version of your system.
Aperture: Object Space NA = 0.09
Aperture: Object Space NA = 0.15
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