The variational method is a useful tool that can be used for design and optimization of dispersion-managed communication systems. Using this powerful tool, we evaluate the characteristics of a carrier signal for certain system parameters and describe several features of a dispersion-managed soliton.
Fiber optic based analog and/or digital CATV systems experience composite second order distortion (CSO) caused by the interaction between the gain tilt of the doped fiber amplifier and the laser chirp due to modulation. The gain tilt for an analog/digital transmission system using a high power Er-Yb co-doped fiber amplifier has been experimentally measured and its contribution to the CSO of the transmission system is evaluated. The results are in good agreement when compared with the direct measurement of the CSO of the system with and without the amplifier. The dependence of the gain tilt on the input light modulation frequency and the input light wavelength is also investigated.
Fiber optic based CATV distribution systems may need very high powers prior to distribution. High optical powers can be obtained using a cladding pumped fiber amplifier. The performance of an analog transmission system using a high power cladding pumped fiber amplifier has been investigated. Analog signals from a 120-channel CATV signal generator was used to directly modulate 1550 nm distributed feed back laser. The output was amplified to ~1W using a cladding pumped fiber amplifier. The carrier to noise ratio (CNR), composite second order (CSO) and composite triple beat (CTB) of the composite analog signals were measured before and after amplification. No significant change in CNR, CSO and CTB signals were observed which suggests that the amplifier does not introduce any additional non-linearity. Similar experiment was also carried out using a hybrid digital/analog system where the carrier at 223.25 MHz was modulated at 2Mb/s using the BPSK modulation format. Bit error rate (BER) of <10-9 was obtained for a modulation depth of 2.8% of the BPSK modulated channel. No significant changes in CNR, CSO and CTB were observed for this mixed mode transmission after amplification.