Abstract

Erbium-doped fiber amplifiers (EDFA’s) are becoming increasingly important in long-haul telecommunications because of their high optical gain and compatibility with fiber cables. It has been proposed that EDFAs be used in other applications, such as wavelength-multiplexed networks or cable television. However, the irregular erbium gain spectrum may constitute a severe limitation for these future developments. It is well known that by using 1.48-μm pumping, smoothing of the gain spectrum can be obtained in certain conditions but at the expense of low pump efficiency and a high noise figure.¹ When pumped at the wavelength of 980 nm, which is widely recognized to be the best choice when high efficiency and low noise are sought, the EDFA gain curve is peaked at around 1531 or 1555 nm; a signal whose wavelength is displaced from the emission peak experiences a gain penalty of 8-12 dB with respect to the maximum figure of 30-35 dB routinely measured at the optimum wavelength for low-level input signals in diode-pumped amplifiers. A detuned signal has to compete with the spontaneous emission contribution 1531 nm, which is efficiently amplified during its propagation in the inverted medium. It would be advisable to make the gain curve more uniform, reducing the effect of amplified spontaneous emission buildup but maximizing the mean gain obtainable from the amplifier. This can be achieved by the use inside the EDFA of a spectral filter with high absorption at 1531 nm and negligible loss elsewhere. Filters based on mechanical corrugations of the fiber surface have been reported.² In this paper we propose an all-fiber-optic filter that leads to a rugged and reliable wideband EDFA module that can be used in field applications.

© 1991 Optical Society of America