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Chapter 6:
Semiconductor Laser Diode Fundamentals
This chapter reviews laser physics and its concept of operation. A short introduction to the quantum phenomenon of stimulated emission is presented, explaining the process of lasing and emission types for general background. This is followed by modes of excitation and structure of gain-guided lasers versus index-guided lasers. Common lasers used by the industry are reviewed and background is provided about longitudinal modes, distributed feedback (DFB) lasers, Fabry—Perot (FP) lasers, quantum well lasers, tunable lasers, and vertical cavity surface-emitting lasers (VCSELs). After a basic solid foundation of laser physics is laid, a practical RF modeling of the laser is presented, followed by modulation terminologies for RF and digital schemes. An introduction to the optical isolator is provided at the end of this chapter to explain how to overcome and improve the reflection coefficient of the laser to the fiber. Reflections in both cases of digital and analog transport introduce distortions. In the case of digital transport, the observation might be double-eye transitions, implying deterministic jitter. For analog, reflections may cause a reduction in carrier-to-noise ratio (CNR) and intermodulation (IMD) performance is explained in Chapter 17.
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