EThe sketch above shows a typical fiber. The core has a refractive index of N1 and the cladding an index of N2. Light enters the fiber at angle A and is transmitted through the fiber. If the angle A is too large, the light will not be transmitted. We call the angle beyond which the light cannot be carried through the fiber the Critical Angle. This is calculated using the two refraction indices. The sine of the Critical Angle is the Numerical Aperture or NA The Acceptance Angle of the fiber is two times the Critical Angle.
EXAMPLE: If N1 is 1.62 and N2 is 1.52, the NA will be .56 which equals a Critical Angle of 34° and an Acceptance Angle of .68°. The f number/equivalent will be f/0.89.
Optical fibers tend to preserve the angle of incidence during the light transmission and therefore in the figure above, the angle A is shown at both the entrance and exit ends of the fiber. The sketch below shows a typical projecting lamp illuminating a fiber bundle. The angle A is the Acceptance Angle of a .25 NA fiber (29°). Angle B is the Incident Angle from the lamp and angle C is the Acceptance Angle of a .66 NA fiber (83°).
The calculated minimum NA. required for the 45° Angle of Incidence is .38. Therefore, a fiber with an NA. of .66 will accept all of the light from the lamp, but the output angle will only be 45° and not the 83° which might be expected. However, the .25 NA. fiber which cannot accept all of the light, will have an output angle of 20°. Using a low NA. fiber will not focus the light from a lamp because it can’t receive any light beyond its Critical Angle and therefore has a narrow output cone. Multicomponent glass fibers typically reach NA values up to 0.9, whereas quartz silica fibers typically do not exceed 0.4 NA values.
In this range SCHOTT's standard multicomponent glass optical fibers (GOF70 and GOF85) have attenuation levels between 150 and 300 dB/km. While featuring improved transmission for wavelengths between 350 and 450 nm, transmission for the deeper UV range is very low and wavelengths below 350 nm are not transmitted. Glass optical fibers feature as well transmission in the near infrared range (0.8 µm to 1.3 µm). At 1.4 micron, all fibers except those specifically designed for IR transmission, show a significant drop in transmission due to OH-Absorption within the glass. In the range from 1.4 up to 2.0 µm specifically designed glass optical fibers for IR-transmission can be used.
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