Optical Amplifiers from Rare-Earth Co-Doped Glass Waveguides

Abstract

Optical amplifiers are of potential use in wide variety of optoelectronic and optical communication applications, particularly for Wavelength Division Multiplexing (WDM) to increase the number of channels and transmission capacity in optical network systems. For efficient performance of WDM systems, essential requirements are larger bandwidth, higher output power and flat gain over entire region of operation. Recent research is focused on design and development of fiber/MEMS-compatible optical amplifiers. Some examples of such sources are semiconductor quantum dot light-emitting diodes, super-luminescent diodes, Erbium doped fibre amplifier (EDFA, 1530-1625nm), Erbium doped planar amplifier (EDWAs), Fibre Raman amplifier, Thulium doped fibre amplifier (1460-1510nm). However, for many applications covering the total telecommunication window (1260-1700nm) is highly desirable and as such it is not yet realized. Typical attenuation spectrum for glassy host is shown in Figure 1. Specially the low loss region extending from 1450 to 1600 nm, deemed the 3rd telecommunication window, emerged as the most practical for long haul telecommunication systems. This window has been split into several distinct bands: Shortband (S-band), Centre-band (C-band) and Long-band (L-band). With several generations of development, the transmission rates have increased dramatically so that several Terabits per second data can be transmitted over a single optical fiber at carrier wavelengths near 1550 nm, a principal optical communication window in which propagation losses are minimum. EDFAs are attractive to WDM technology to compensate the losses introduced by WDM systems and hence has grown as a key to upgrade the transmission capacity of the present fiber links. EDFAs are widely used in long-haul fiber optic networks where the fiber losses are limited to 0.2 dB/km, is compensated periodically by placing EDFAs in the transmission link with spacing of up to 100 km. EDFAs make use of trivalent erbium (Er3+) ions to provide the optical amplification at wavelengths near 1550 nm, the long wavelength window dominantly used in optic networks since the fiber losses are found minimum around this wavelength. Light from an external energy source at a wavelength of 980 nm or 1450 nm, coupled along with the information signal, and is passed through the EDFA to excite the Er3+ ions in order to produce the optical amplification through stimulated emission of photons at the signal wavelength. Er3+ doped waveguides (EDWA) have