Direct modulation of a semiconductor laser is the most convenient way of using these devices in communication circuits. However, there are certain fundamental limitations for a direct-current modulation approach. In digital data transport, this limitation starts to become important as the speed of modulation increases beyond 10 GBPS. The main limitations result from the wavelength chirping of the laser output as well as relaxationâoscillation. The first one causes jitter whereas the other parameters result in overshooting the eye pattern. Jitter phenomena of fiber dispersion due to wavelength chirp affects the digital transport eye quality because of group delay. When dealing with laser analysis, there are two parameters that are examined, intensity modulation (IM) in Secs. 7.1 and 7.2 and frequency modulation (FM) known as wavelength chirp in Sec. 7.3.
In analog community access television (CATV) transmitters, the bandwidth (BW) or modulation frequencies are much lower; however, the key problem there is nonlinear distortions (NLD) due to the interaction between the chirp phenomena of the distributed feedback (DFB) 1550-nm laser and the dispersion of a standard single-mode fiber (SMF). Additionally, analog channels at microwave frequencies and millimeter waves may be affected by relaxation-oscillation BW limitation due to reduction in distortion performance. In CATV, transmission at 1550 nm is desirable due to the advantage of low fiber losses and the use of erbium-doped fiber amplifiers (EDFAs). The power required from a 1310-nm DFB laser is higher due to the inherent higher losses of the SMF. These constraints impose more limitations. Moreover, the performance of a direct modulated laser (DML) is affected by multiple optical reflections and chirp. These problems limit the transmission distance over standard SMF.
The solution to these problems both in digital and in multicarrier analog CATV modulation schemes is using external modulation (EM) techniques. When EM is used, the laser itself is biased on continuous mode with an external device modulating the optical output.
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