Fiber optics numerical aperture

Most optical fibers have numerical apertures between 0.15 and 0.4, and these correspond to light acceptance half-angles of about 8 and 23 degrees. Typically, fibers having high NAs exhibit greater loss and lower bandwidth. 

A variety of apertures have been used to deliver nanometer-sized spots of light. While early NSOM tips were fabricated out of etched quartz crystals and micro-pipettes, tapered optical fibers with tip diameters of ca. 100 nm are now typically used. A metallic thin film such as aluminum is usually applied around the sides of the tapered region of the NSOM tip to focus the light toward the sample. For optical fibers, the numerical aperture is related to the difference in the indices of refraction of the cladding and core (Eq. 2) ... 

A diagram of an infrared fiber is shown in Figure 15.20. Fibers are clad with a material of a lower index of refraction than the fiber itself. For example, chalcogen-ide fibers, which are fabricated from mixtures of Ge, As, Te, Sb, and S, have a refractive index of approximately 3.8 and are typically clad with a polyurethane or a polyamide coating (refractive index 1.5). Based on Eq. 15.2, this yields a critical angle of 23° inside the fiber as shown in Figure 15.20. For optical fibers the numerical aperture (NA) can be shown to be... 

Table 2.1 shows the efficiencies for the generation of FSB (f b) for various optical fibers, together with information about their manufacturer, product number, core, cladding and outer diameters, numerical aperture (NA), and material [28]. Data for some other optical fibers have been added to the table [28]. 

There is no inherent sample size restriction, large or small, but is fixed by the optical components used in the instrument. The diffraction limit of light, roughly a few cubic micrometers depending on the numerical aperture of the optics used and the laser s wavelength, sets the lower bound.7 In a process application, the type of fiber optics used also affects sample volume examined. Macroscopic to microscopic samples can be measured with the appropriate selections of laser wavelength, laser power, and optics. 

The properties of optical fibers are often characterized by the numerical aperture NA = n sin0. It is given by ... 

The range of acceptance angles for an optical fiber is generally stated as the numerical aperture (NA). For the case of the square-cut fiber shown in... 

There are two types of optical fibers SM and MM fibers. The important parameters characterizing fiber properties are the core and cladding indices, core diameter, attenuation ot, and numerical aperture (NA). The attenuation represents optical losses caused by absorption or scattering of light propagating through the fiber per unit length, as ... 

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