This thesis describes applications of the spectrum-sliced incoherent light source (SILS) for a long-haul transmission system. The probability distribution function (PDF) and the signal-to-noise ratio (SNR) of SILS have been calculated and measured. BER characteristics of optical transmission systems using SILS also have been studied in terms of the channel bandwidth and the bit rate. An all-optical bandwidth expansion technique using the intra-channel four wave mixing (FWM) has been newly developed for high bit rate long-haul transmissions of SILS channels. The narrowest optical bandwidth (0.1 nm) and the longest transmission distance (300 km) have been obtained successfully for a 2.5 Gbit/s SILS channel. Also, we demonstrate the transmission of 2.5 Gbit/s x 4 channels with 0.45 nm channel bandwidth and 1.6 nm channel spacing over a 240 km dispersion shifted fiber (DSF).
The SNR of SILS channel is proportional to its optical bandwidth and inversely proportional to electrical bandwidth of the optical receiver. Unfortunately, the optical bandwidth required for the spectrum-sliced incoherent light transmission grows linearly with the bit rate in order to maintain high SNR. Thus, there are severe dispersion penalties especially in high-speed and long-distance applications. In this thesis, a simple all-optical technique to reduce the dispersion penalty of SILS channel is proposed.
First, PDF of a spectrum-sliced channel has been derived for two kinds of optical filters used to slice the amplified spontaneous emission (ASE) light from the erbium-doped fiber amplifier (EDFA). The calculated PDF characteristics are in good agreements with the measured values. If a spectrum-sliced channel bandwidth increases more than 0.5 nm, PDF of SILS approaches to Gaussian distribution. Thus, BER of a spectrum-sliced channel is derived using the standard Gaussian noise approximation.
Then, the external modulation of SILS using the LiNbO3 Mach-Zehnder modulator is explained in detail. The measured extinction ratio was 12 dB, the rise and fall times were about 30 ps at 10 Gbit/s operation. Using this SILS transmitter, we demonstrated a 2.5 Gbit/s transmission of SILS channel over 200 km of DSF. The channel center was tuned at the zero-dispersion wavelength of the DSF and the total power penalty was as low as 0.2 dB for a bandwidth of 1.8 nm. We also transmitted a 10 Gbit/s spectrum-sliced channel over 100 km of DSF with a 4.3 nm channel bandwidth.
Finally, a simple all-optical technique to reduce the dispersion penalty of SILS is demonstrated. During the transmission, the optical bandwidth is kept to a minimum value much less than the conventional value. Although, SNR of the transmitted signal is very poor, SNR can be fully restored at the receiver using the intra-channel fiber four-wave mixing that greatly expands the optical bandwidth of the received signal. We have successfully demonstrated the transmission of 2.5 Gbit/s NRZ signal with a bandwidth of 0.1 nm over a 300 km DSF. Without the optical bandwidth expansion, an error floor occurred at 1 x $10^{-5}$ BER. With the optical bandwidth expansion, however, the error floor was decreased to less than 1 x $10^{-10}$ BER. The transmission penalty was less than 0.5 dB at 1 x $10^{-10}$ BER. Using this technique, we also demonstrated the transmission of 2.5 Gbit/s x 4 channels with 0.45 nm channel bandwidth and 1.6 nm channel spacing over a 240 km DSF.
