Distortion due to gain tilt of erbium-doped fiber amplifiers

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Abstract
It was found experimentally and theoretically that the second-order intermodulation product, namely composite second-order (CSO) distortion, is caused by coupling of the gain tilt of an erbium-doped fiber amplifier (EDFA) with laser chirp. When the signal light is not at the gain peak of the EDFA, gain tilt exists and EDFA cannot be used in an AM-FDM optical analog video transmission system because of CSO degradation. To counteract the gain tilt induced CSO, the authors demonstrated that codoping Al and setting the laser oscillation wavelength at the gain peak are useful. Er-Al-codoped fiber amplifiers have better linearity than Er-doped fiber amplifiers because the gain tilt of the former is smaller.