![]() ![]() Indeed, a second-generation commercial telecommunication system in 1987 was capable of operating 1.7 Gb/s with a repeater spacing of 40-50 km, demonstrating the potential of single-mode fibers for optical communications. The BL product of such lightwave systems can exceed 100 (Gb/s)-km. For a semiconductor laser the spectral width Δλ is 2-4 nm even when the laser operates in several longitudinal modes. It is found that D ~ 1 ps/(km-nm) can be obtained in the wavelength region near 1.3 μm. In the next two sections the wavelength dependence of D is studied. This equation provides an order-of-magnitude estimate of the bit rate-distance product BL offered by single-mode fibers. By using ΔT from last equation this condition becomes The effect of dispersion on the bit rate B can be estimated by using the criterion BΔT < 1 in a manner similar to that used previously. By using ω = 2πc/λ and Δ ω = (-2πc/λ 2)Δλ, we can rewrite the equation asĭ is called the dispersion parameter and is expressed in units of ps/(km-nm). It is customary to use Δλ in place of Δ ω. In optical communication systems the frequency spread Δ ω is often determined by the range of wavelengths Δλ emitted by the optical source. It determines how much an impulse would broaden on propagation inside the fiber. The parameter β 2 = d 2β/dω 2 is known as the GVD parameter. If Δω is the spectral width of the pulse, the extent of the pulse broadening is governed by The frequency dependence of the group velocity (or GVD) leads to pulse broadening simply because different spectral components disperse during propagation and become desynchronized at the fiber output. A specific spectral component at the frequency ω would arrive at the output end of the fiber after a time delay T = L/v g, where v g is the group velocity defined asīy using, one can show that, where is the group index given by Group-Velocity DispersionĬonsider a single-mode fiber of length L. This tutorial considers both of them and discusses how GVD limits the performance of lightwave systems employing single-mode fibers. Intramodal dispersion has two contributions known as material dispersion and waveguide dispersion. As a result, different spectral components of the pulse travel at slightly different group velocities, a phenomenon referred to as group-velocity dispersion ( GVD), intramodal dispersion, or simply fiber dispersion. The group velocity associated with the fundamental mode is frequency dependent because of chromatic dispersion. However, pulse broadening does not disappear altogether. The main advantage of single-mode fibers is that intermodal dispersion is absent simply because the energy of the injected pulse is transported by a single mode. In the modal description it is related to the different mode indices (or group velocities) associated with different modes. In the geometrical-optics description such a broadening was attributed to different paths followed by different rays. We have seen that intermodal dispersion in multimode fibers leads to considerable broadening of short optical pulses (- 10 ns/km). ![]()
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