In this paper, we investigate the optimal design of optically gain-clamped (GC) erbium-doped fiber amplifier (EDFA). Three configurations under discussion, the first uses two optical circulators (OC), one optical band pass filter (OBPF), and one variable optical attenuator (VOA), to form ring cavity. Such ring cavity can regulate itself: when the channel numbers increase, the amounts of optical feedback decrease; contrarily, when the channel numbers decrease, the amounts of optical feedback increase. So it has the gain-clamped ability. The second employs one or double fiber Bragg grating (FBG) to reflect the residual Amplified Spontaneous Emission (ASE) for regulating the signal gain, such configurations have the same self-regulation as before. The choice of center wavelength and bandwidth of FBG can control the signal gain to reach the optimal gain and fairly low noise figure (NF). The third configuration is similar to the first; the difference is that the third with figure-8 cavity, which uses a common OBPF and VOA, can regulate the gain of C- and L-bands at the same time. The choice of OBPF is just located within the dead-zone between the C- and L-bands, where no WDM channels can be transmitted. If we choose suitable loop attenuation, we can get an equal signal gain of C- and L-bands¡¦ channels.

Recently, the long-wavelength band (L-band, 1570-1600 nm) erbium-doped fiber amplifiers (EDFAs) has received much attention. By combining the gain bandwidth of a conventional C-band EDFA and L-band EDFA in parallel, the available gain bandwidth can be increased by a factor of two. The change in the input power and the number of channels leads to the variation of the channel output power, which, in turn, changes the gain spectrum and flatness. To cope with this problem, the gain-clamped (GC) technique has been proposed. The gain-clamping techniques have been extensively explored for C-band EDFAs, but fewer for L-band EDFAs. In this thesis, we experimentally investigate an optically gain-clamped L-band EDFA with different lasing lights (1568 nm and 1600 nm) and different loop attenuation. The characteristics of such L-band GC-EDFAs measured in a simulated add-drop operation are examined and compared. We find that the 1568-nm lasing light with 0-dB loop attenuation is the better selection for L-band GC-EDFA to offer good channel gain (19.4 dB) and satisfied noise figure characteristics. Compared to the gain variation observed for GC-EDFA with an open loop, the GC-EDFA was effective in reducing the total gain variation of 17 dB and increasing the dynamic range of 25 dB.