Low-loss window

The residual hydrogen atoms in such matmals still give rise to their characteristic absorption bands. Although the intensities of these bands will be reduced, their influence in the near IR may still be sufficiently strong to interfere with the low-loss windows. By fluorination of alkyl methacrylate polymers, the refractive indexes of the fluorinated polymers reduce significantly. As a result, it becomes difficult to choose an appropriate cladding material that has lower refractive index and that maintains sufficient numerical aperture for the POF — one of the most important features for a POF. Styrene derivatives, on the other hand, are expected to maintain appropriate refractive indexes even after the introduction of fluorine because their refractive indexes are high (about 1.590). 

Fortunately, they are several species of low-loss dielectric ceramics with tailored temperature coefficient of dielectric constant, which can be made lower than 1 ppm/K for a certain temperature window around room temperature. Physically, this can be accomplished either by intrinsic compensation of the temperature dependence of thermal volume expansion V(T) and lattice polarizability a(T) via the Clausius-Mossotti relation ... 

Optical amplifiers are essential components for the development of high capacity telecommunication networks. Silica fibers are characterized by a low-loss optical window that ranges from 1.2 to 1.7qm, as shown in Figure 4. That window is divided into the ultrashort (XS), short (S), conventional (C) and long (L) bands. The first two bands are centered at 1.3 and 1.47 qm, while the C- and L-bands are equal to 1530-1565 run and 1565-1625 nm, respectively. Amplification in the C- and L-bands is commonly achieved with Er +-doped silica fibers. On the other hand, amplification at 1.47 qm and 1.3 qm with Tm + and Pr + ions requires the use of materials with lower phonon energy than silica. High gain optical amplification was demonstrated in the XS, S, and C, telecommunication windows with rare earth-doped... 

At microwave frequencies in the range 72-145 GHz, the critical parameters for high-power transmission are the dielectric characteristics of the window material the dielectric loss factor tan 5 and the permittivity e[. (or the refractive index n = because they affect power absorption and reflection [42]. The dielectric loss factor tan 8 in low loss samples is usually measured as the decrease in the Q factor of a resonant cavity [43]. Low dielectric loss materials find application as the output windows of high-power microwave tubes. A specific case is that of windows for Gyrotron tubes operating in the 70-170 GHz frequency region with output powers in excess of 1 MW, as will be discussed later. 

A very high degree of linear polarization can be obtained for a laser with low losses in one direction of oscillation and high losses in the perpendicular direction. For a laser with a gas as the active medium this is accomplished by placing the windows of the gas container at Brewster s angle to the optical axis of the laser (Fig.8.5). For other lasers some other type of polarizer can be placed inside the cavity. 

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