Transmission Characteristics of Optical FibersTransmission Characteristics of Optical Fiber Bundles
High quality optical glass (crown and flint glass) is used for the light transmitting core and an optical glass with a different refractive index for the cladding. Wavelengths between 400 and 900 nm are transmitted uniformly, with only minor variations. 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. For improved transmission over the entire range from 250 nm up to 3.0 μm quartz glass (fused silica) fibers are the best choice, but have a lower NA.
Transmission Characteristics of Optical Fiber BundlesAlthough specific information on the performance of a single fiber is valuable, it is important to understand how optical fibers perform when manufactured into bundles. Due to total reflection, a portion of the light will be reflected at the polished glass surface of the fiber at the entrance as well as the exit. In addition, the interstitial gaps between the fibers, usually filled with epoxy glue, will not transmit any light. The losses due to these two effects can be estimated at approximately 25 % -30 %, depending on the polishing quality. The loss of the interstitial gaps can be reduced by hot fusing the entrance end instead of gluing the fibers together. Thus, transmission can be increased up to 15 %. In addition, transmission loss will be caused with increasing length of the lightguide. A 3-foot/1m lightguide will transmit approximately 60% of the light emitted by the lamp towards the fiber bundle within the NA. A 10-foot/3m light guide will transmit about 55% of the light and a 30-foot/10 meter lightguide roughly of 40 %.