Bandwidth limitations due to harmonic distortion in single-mode optical fibers

Abstract

A b m t -T h e bandwidth of the singlemode fiber is discussed for the intensity modulation and detection system provided that the spectral width of the light source is much less than the modulation bandwidth. The harmonic distortions reduce the bandwidth by the factor 0.5 to 0.1 compared to the, amplitude modulation (AM) system.The single-mode optical fiber is promising as a very broad-band and low-loss transmission line. The bandwidth of the single-mode fiber is determined by the spectral width of the light source when the spectral width is larger than the modulation bandwidth [ 1 1. However, the spectral width of the light source can be reduced to the value much less than the modulation induced linewidth, for example, 0.3A at 10 GHz for the wavelength of 1 pm, by using the laser operating at single wavelength, for example, the distributed Bragg reflector [ 2 ) , the distributed feedback [ 31, and the dual integrated-twin-guide [4] semiconductor lasers.In this paper, the bandwidth of the singlemode fiber is discussed in which the spectral width of the light source is much less than the modulation bandwidth, and the light intensity is modulated by the signal, corresponding to the case of direct modulation of the injection laser and direct detection.In the intensity-modulation system, the envelope of the square of the amplitude of the electric field of the optical wave is modulated by a signal. Therefore, the electric field at the input of the fiber is as follows:where Eo is the constant, m is the modulation index, cos wt is the input s i g n a w is the angular frequency of modulation, and u is the angular frequency of the optical wave.Expanding the square root in (l), and using Taylor series of the propagation constant, the optical wave at the output of the fiber is determined. Then, in making the square of the electric field, the output signal of the intensity detection is obtained [ 51 .Besides the fundamental wave cos wt, there appear higher harmonic waves cos 2wt, cos 3wt,. . . in the output signal. According to the amplitude response of the fundamental H(w), and higher harmonic wave H(2w), H(3w), . . . , the higher harmonic waves disappear when the frequency dispersion is zero, that is, d2p/du2 = 0, where p is the propagation constant of the HEll mode in the single-mode fiber. Therefore, the harmonic distortions of the signal, H(2w), H(3w), . . . are generated by the frequency dispersion of the fiber. Such harmonic distortions do not appear in the AM and detection system.From the frequency response of the fundamental wave H(w), 3 dB bandwidth is almost the same as that in the AM system which is given as follows 161.The value of BAM for the fused silica fiber is more than 30 GHz 6[ 6 ] . However, in the intensity modulation system, the second and the third harmonic distortions 20 log IH(Zw)/H(w)l, and 20 log IH(3w)l H(w)l exceed -10 dB, and -30 dB, respectively, within the >dB bandwidth of the fundamental response. Therefore, the bandwidth available for the distortiorrfree transmission, for example, where th...

Nonlinear Transmission Behaviour of a Single-Mode Fiber Transmission Line due to Polarization Coupling

“…However, even when using a single-mode fiber, the material dispersion may yield nonlinear distortions [4][5][6][7] and also noise [8], the noise arising from the partition noise of semiconductor lasers. However, even when using a single-mode fiber, the material dispersion may yield nonlinear distortions [4][5][6][7] and also noise [8], the noise arising from the partition noise of semiconductor lasers.…”

“…The influence of material dispersion on the transmission properties, however, has been studied in [4][5][6][7][8]. The expansion is carried out up to first order, only, so that material dispersion effects are not included.…”

“…with (5) we write (5) with φ 0 = φ(ω=ω ί ), the delay differences Δτ^ Δτ 2 according to j, j = i,2 (6) and τ = dφ/dω| ω = ίll^. with (5) we write (5) with φ 0 = φ(ω=ω ί ), the delay differences Δτ^ Δτ 2 according to j, j = i,2 (6) and τ = dφ/dω| ω = ίll^.…”