Erbium-doped fiber amplifiers (EDFA’s) offer very attractive prospects for ultra-long-distance nonregenerated systems1 and long-distance unrepeated systems.2 In this paper we present intensity-modulation direct-detection (IM-DD) transmission experiments by using an erbium-doped fiber preamplifier pumped by a 1480-nm laser diode, and we investigate the influence of amplifier gain, type of photodiode [positive-intrinsic-negative (P-I-N) or avalanche photodiode (APD)I, and optical filter bandwidth.

In the system considered, the postamplifier, with an output power of +9.6 dBm, did not degrade the very high linearity of the isolated distributed-feedback (DFB) laser transmitter. A 51.2-dB weighted signal-to-noise ratio was obtained, and an optical power budget of 16 dB was achieved with a weighted signal-to-noise ratio higher than 48 dB. A larger power budget with a higher signal-to-noise ratio (SNR) is possible with an increase of DFB laser output power and erbium-doped fiber amplifier (EDFA) saturated output power, and with a minimization of the EDFA noise contribution.<>

Some experiments have already been presented for long haul transmission using in-line Erbium-Doped Fiber Amplifiers (EDFA) [1,2]. However, the question is to determine the actual system limitations for transoceanic links (up to 10,000 km), such as EDFA maximum output power, amplifier spacing, optical filter design. Some theoretical [3] and experimental [4] works have been published mainly with a recirculating loop.

An optimisation of the erbium-doped fibre greatly improves the 1.48 mu m pumping efficiency. The amplifier exhibits 26.2 dB fibre gain for 8 mW launched pump power and 20.2 dB gain for only 5.2 mW pump power, giving an outstanding 3.9 dB/mW gain coefficient.<>

Laser-diode-pumped erbium-doped fiber amplifiers (EDFA) have recently demonstrated all their potential for very long haul transmissions [1]. In-line EDFA application is clearly identified for long undersea links. However, repeaterless transmissions have many applications, especially over 250 km lengths. Compared to direct detection, heterodyne detection allows to achieve a very high receiver sensitivity, increasing the overall link budget [2], Optical amplifiers highly improve this budget, particularly by compensating for the insertion loss of the external phase modulator in DPSK systems. We report a 565 Mbit/s DPSK transmission experiment at 1.532 μm using an EDFA module at the emission, operating in gain saturation regime. Pumped by laser diodes, this optical post-amplifier delivers an amplified output power exceeding +13 dBm, leading to a repeaterless power budget of 62.9 dB. This gives the potentiality to design over 350 km repeaterless links by use of a pure silica core fiber.

We report on a parametric study to optimize the Er3+-doped fiber amplifiers performances. A 24 dB efficient gain in Er3+- doped silica-based fibers has been achieved for small signal regime at 1.553 μm with a 40 mW pump power in the 1.48-1.49 μm range. The saturation output signal power was up to 20 mW for a gain of 17 dB. For the signal wavelength a 3dB-bandwidth of 30 nm has been obtained always with a pump around 1.48 4m. We achieved these results by optimizing the relevant parameters which are the pump and signal characteristics (wavelength and power), the Er3±-concentration and length for the doped fiber. We report on the main parameters which determine the amplifier efficiency (gain, saturation output power, saturation gain, bandwidth, amplified spontaneous emission,...). It is worth noticing that an outstanding amplification is obtained at 1.55 μm wavelength where the gain value is not optimal in order to demonstrate the capability of such amplifiers.