per second vertical cavity surface emitting laser (VCSEL)56,57,58,59,60 transmitters became the dominant transmitters due to the low-drive voltage and low manufacturing cost. At present, 10 Gb/s VCSELs are being deployed and 40 Gb/s have been demonstrated in laboratories.61,62 Research efforts toward wavelength-tunable and 100 Gb/s direct modulated lasers will be important for next-generation broadband communications.63,64
High-Speed Electronic Circuits. Electronic circuits are an “optical system’s best friend.” High-speed, high-output-power and linear electronics are clearly needed to drive the optical modulators, and high-speed logic circuits are crucial to achieving the signal processing in coherent, MIMO, and OFDM systems.65,66 Good electronics can help mitigate many of the problems produced in the optical domain, thereby enabling better system performance.
Photonic Integration. In general, a system with better performance requires more components, which might make the system even more complex. For example, the transceiver in a system using higher-order modulation formats is much more complicated
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