Introduzione alle fibre ottiche Edoardo Milotti Corso di - PowerPoint PPT Presentation

Introduzione alle fibre ottiche Edoardo Milotti Corso di Fondamenti Fisici di Tecnologia Moderna A. A. 2019-20

1.2 Bit rate basso 1.0 Larghezza di banda piccola 0.8 0.6 0.4 0.2 0.0 0 2 4 6 8 10 1.2 Bit rate alto 1.0 Larghezza di banda grande 0.8 0.6 0.4 0.2 0.0 0 2 4 6 8 10

µ Figure 7-3 A typical fiber optic communication system: T, transmitter; C, connector; S, splice; R, repeater; D, detector Le fibre ottiche permettono di stabilire canali di telecomunicazione a larga banda

Problema: un anello misterioso …

n 1 > 1 Riflessione totale: se non tutti gli angoli di rifrazione n 2 sono possibili, infatti sin θ 2 = n 1 sin θ 1 n 2 allora esiste un angolo limite tale che (lim) = 1 (lim) = arcsin n 2 n 1 sin θ 1 θ 1 n 2 n 1

(lim) = arcsin n 2 θ 1 n 1 nel caso dell � interfaccia aria-acqua n 1 ≈ 1.33, n 2 ≈ 1, allora (lim) ≈ 48 ° .75 θ 1

Figure 7-2 Schematic of the photophone invented by Bell. In this system, sunlight was modulated by a vibrating diaphragm and transmitted through a distance of about 200 meters in air to a receiver containing a selenium cell connected to the earphone. vedi: Ghatak and Thyagarajan, "Optical Waveguides and Fibers", Fundamentals of Photonics, Module 1.7 https://spie.org/publications/fundamentals-of-photonics-modules

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= < = > < ∆ ∆ ≡ ∆ + ∆ = ≈ ≈ For a typical (multimode) fiber, a ≈ 25 µ m, n 2 ≈ 1.45 (pure silica), and ∆ ≈ 0.01, giving a core index of n 1 ≈ 1.465. The cladding is usually pure silica while the core is usually silica doped with germanium. Doping by germanium results in a typical increase of refractive index from n 2 to n 1 . Now, for a ray entering the fiber core at its end, if the angle of incidence φ at the internal core- φ

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λ λ λ Attenuazione nelle fibre ottiche Figure 7-10 Typical wavelength dependence of attenuation for a silica fiber. Notice that the lowest attenuation occurs at 1550 nm [adapted from Miya, Hasaka, and Miyashita].

Example 7-3 Calculation of losses using the dB scale become easy. For example, if we have a 40-km fiber link (with a loss of 0.4 dB/km) having 3 connectors in its path and if each connector has a loss of 1.8 dB, the total loss will be the sum of all the losses in dB; or 0.4 dB/km × 40 km + 3 × 1.8 dB = 21.4 dB. Example 7-4 Let us assume that the input power of a 5-mW laser decreases to 30 µ W after traversing through 40 km of an optical fiber. Using Equation 7-12, attenuation of the fiber in dB/km is therefore [10 log (166.7)]/40 ≈ 0.56 dB/km.

µ ≈ Dispersione degli impulsi nelle fibre ottiche 1. Different rays take different times to propagate through a given length of the fiber. We will discuss this for a step-index multimode fiber and for a parabolic-index fiber in this and the following sections. In the language of wave optics, this is known as intermodal dispersion because it arises due to different modes traveling with different speeds. 4 2. Any given light source emits over a range of wavelengths, and, because of the intrinsic property of the material of the fiber, different wavelengths take different amounts of time to propagate along the same path. This is known as material dispersion and will be discussed in Section IX. 3. Apart from intermodal and material dispersions, there is yet another mechanism—referred to as waveguide dispersion and important only in single-mode fibers. We will briefly discuss this in Section XI. In the fiber shown in Figure 7-7, the rays making larger angles with the axis (those shown as

Figure 7-11 Pulses separated by 100 ns at the input end would be resolvable at the output end of 1 km of the fiber. The same pulses would not be resolvable at the output end of 2 km of the same fiber. θ + θ = = = θ = θ θ θ θ θ θ =