Erbium-doped fiber amplifiers (EDFA's) are considered as one of the most important technologies which enable the recent development of wavelength-division-multiplexed (WDM) fiber-optic communication systems. This is because EDFA's provide high gain, low noise figure, broad bandwidth, and operate in the 1.55-mm window where the fiber exhibits minimum loss. However, the performance of the amplified WDM system is often limited by the amplified spontaneous emission (ASE) noise. Thus, there has been a substantial efforts to suppress the deleterious effects of ASE by using various interstage components within EDFA's such as optical isolators, optical bandpass filters, fiber Bragg gratings, and optical circulators, etc.
In this dissertation, we propose and demonstrate various types of EDFA's by using novel interstage components such as synchronized etalon filters and WDM multiplexers. Our focus was mainly to develop EDFA's for the use in bidirectional transmission systems with improved performance and functionality. The proposed EDFA's were used to demonstrate high-capacity bidirectional WDM transmission systems and bidirectional self-healing ring networks. The scalabilities of these networks have been analyzed thoroughly based on the performances of the proposed EDFA's.
First, we have demonstrated a two-stage optical limiting amplifier (OLA) using a synchronized etalon filter as an interstage component. The synchronized etalon filter provides a comb of equally spaced passbands at the standardized wavelengths. Thus, by inserting the synchronized etalon filter within an EDFA, it would be possible to enhance the pump efficiency by reducing the ASE noise. The results show that the proposed EDFA provides higher gain, lower noise figure, and wider dynamic range (>30 dB) than the conventional two-stage EDFA's with an interstage isolator.
Bidirectional WDM transmission system could suffer from the relative intensity noise (RIN) caused by Rayleigh backscattering and optical reflection. Thus, it is essential to suppress the effects of RIN to implement an efficient bidirectional WDM transmission system. For this purpose, we have developed a bidirectional EDFA by using spectrally interleaved synchronized etalon filters. The performance of this EDFA was evaluated in a 2.5 Gb/s x 16-channel bidirectional WDM transmission system. The long-haul transmission experiment was simulated by using a recirculating loop. The results show that it would be possible to use the proposed EDFA for the transmission of bidirectional WDM signals over 7 EDFA spans with a span loss of 20 dB (~560 km).
Bidirectional WDM self-healing ring networks based on bidirectional transmission are attractive since they require only two fibers instead of four fibers. To implement such a network, we have first developed a simple bidirectional add/drop amplifier (BADA) module by using only one N x N arrayed-waveguide grating (AWG) multiplexer in a fold-back configuration. In this BADA module, the effect of RIN was suppressed simply by using two optical bandpass filters. Thus, there was no degradation in the receiver sensitivity when these BADA modules were used for the bidirectional transmission and add/drop multiplexing of WDM signals. The proposed BADA modules were used to implement a two-fiber bidirectional WDM self-healing ring network. The restoration time of this network was measured to be less than 2 ms. The power penalty caused by RIN was negligible. Thus, the scalability of the proposed network was limited by the accumulated ASE noise. We estimated that, when the node spacing is 80 km, the proposed network could accommodate about 14 nodes.
