2 Fiber Optics & Optical Fiber Low -OH and High -OH Optical Fiber The optical attenuation characteristics are quite different for High -OH and Low -OH optical fiber core material. The -OH content of the core fused silica must be formulated into the raw boule material before the optical fiber is made. The choice is dependent on the user’s application. The Low -OH type optical fiber has very low attenuation throughout the near-IR wavelength range from 700nm to beyond 1800nm, except for a small peak at 1385nm. The normal absorption peaks at 726nm, 880nm, 950nm, 1136nm and longer, that exist in the High -OH material used in UV applications are not present in Low -OH material. Conversely, the Low attenuation of the High -OH in the UV is significantly better than the Low -OH material. Typical curves for Polymicro’s type FI fiber (ultra Low -OH) used for visible and NIR and FV fiber (High -OH core) used primarily for UV applications follow. Polymicro has recently introduced a new broad-spectrum FB fiber that operates between 275-2100nm. Please refer to FBP specification sheet under Optical Fiber in the Product Reference section for more information.
Figure 2-18 Attenuation, Type FI - Low -OH
Figure 2-19 Attenuation FV - High -OH
Internal & External Losses There are several sources of attenuation in optical fibers. In addition to losses due to mechanical variations in the fiber dimensions, there will be losses due to:
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R a y l e i g h s c a t t e r i n g ... microscopic irregularities in the index of refraction of the glass are the most important mechanism for attenuation in modern fiber. Rayleigh scattering is wavelength dependent (proportional to 1/λ4) and is thus less significant at longer wavelengths.
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A b s o r p t i o n ... in silica can be significant at short-wavelengths due to UV (electronic) absorption, and at long-wavelengths (infrared) due to multiphoton (atomic vibrational) absorption. In addition, unwanted impurities and intentional dopants in the fiber absorb optical energy around specific wavelengths. State-of-the-art manufacturing methods have reduced impurity levels to the point that their effect is almost insignificant.
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