Recently, multigigabit long-span optical fiber transmission experiments have been reported(1-4) following investigations on feasibility for future trunk line systems. An 8Gb/s-68.3km transmission at 1.53μm wavelength, has been carried out using an external modulator and a normal single-mode fiber1). However, high-speed transmission systems using directly modulated LDs and dispersion shifted (DS) fibers are also attractive. The reason is that higher fiber input power for the transmitter and small dispersion in DS fibers provide longer span transmission at 1.55μm wavelength.
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