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Dispersion reduces the bandwidth, or informationcarrying capacity, of an optical fiber. Dispersion causes the spreading of the light pulse as it travels along the fiber (see figure 220). Fiber dispersion mechanisms include intramodal (chromatic) dispersion and intermodal (modal) dispersion. Multimode fiber bandwidth is a measure of the intermodal dispersion of the multimode fiber. Intermodal dispersion is maximum when all fiber modes are excited. The source used for intermodal dispersion measurements must overfill the fiber. The optical source must also have a narrow spectral width to reduce the effects of chromatic dispersion in the measurement. There are two basic techniques for measuring the modal bandwidth of an optical fiber. The first technique characterizes dispersion by measuring the impulse response h(t) of the fiber in the time domain. The second technique characterizes modal dispersion by measuring the baseband Frequency response H( f) of the fiber in the frequency domain. H(f) is the power transfer function of the fiber at the baseband frequency (f). H(f) is also the Fourier transform of the power impulse response h(t). Only the Frequency response method is described here. The test method for measuring the bandwidth of multimode fibers in the frequency domain is EIA/TIA45530. Signals of varying frequencies (f) are launched into the test fiber and the power exiting the fiber at the launched fundamental frequency measured. This optical output power is denoted as P_{out}( f). The test fiber is then cut back or replaced with a short length of fiber of the same type. Signals of the same frequency are launched into the cutback fiber and the power exiting the cutback fiber at the launched fundamental frequency measured. The optical power exiting the cutback or replacement fiber is denoted as P_{in}(f). The magnitude of the optical fiber Frequency response is
The fiber bandwidth is defined as the lowest frequency at which the magnitude of the fiber Frequency response has decreased to onehalf its zerofrequency value. This is the 3 decibel (dB) optical power frequency (f_{3dB}). This frequency is referred to as the fiber bandwidth. Bandwidth is normally given in units of megahertzkilometers (MHzkm). Converting the 3 dB fiber bandwidth to a unit length assists in the analysis and comparison of optical fiber performance. For long lengths of fiber (>1km), the method for normalization is to multiply the length times the measured bandwidth. Q.9 The cutoff wavelength of matchedclad and depressedclad single mode fibers varies
according to the fiber's radius of curvature and length. The cutoff wavelength of which
single mode fiber type is more sensitive to length? 