I'm trying to focus light from a flashing light lamp throught two lens into a optical fiber cable which is around $0.8 \ \mathrm{cm}$ in diameter.
- flashing light : a bulb and then covered by a glass of $7.5 \ \mathrm{cm \times 7\ cm}$
- Fresnel Lens $310 \times 310 \ \mathrm{mm}$ , Thickness: $2 \ \mathrm{mm}$ , Focal length: $220 \ \mathrm{mm}$
I tried to use a piece of white paper to focus the image, however, my teacher said that the focal point might have more intense light. But focal point is not somewhere I can see. Is his theory right about focal point have more intense light than the image point?
I wonder if I could use more convex lens to focus the spot of light into a smaller diameter?
Best Answer
Ok, first of all, if you are dealing with fiber optics things are little bit more difficult than just shine with bulb on fiber frontface. Talking about standard single mode telecommunication fiber, core diameter (e.g. this part of fiber, where is the light guided) is around 9 micrometers. If you have multimode, core will be like 50 micrometers, so this is the diameter you need to have in focus. Structure of single multimode fiber optical cable is concentric like this: core (50 um) - cladding (250um) - primary protection (1 mm) - secondary protection (rest of diameter). If you have fiber with larger diameter of core and cladding, it is not called "fiber" but "light guide", used for example in endoscopy. Because fibers are guiding light due to total internal reflection effect, crucial characteristic is numerical aperture of fiber (NA), which defines the broadness of acceptance "cone" where you can achieve total internal reflection. Obviously, to confine maximum light into fiber you need to match the cone of focusing element with acceptance cone of the fiber. Common approach is to use microscope objective with similar NA as fiber and micrometric mount to launch the light. If you need to confine broad spectra, you can use GRIN objective with abberation compensation, but ussually you need just to pick objective for the strongest wavelength and you are fine.