Abstract-The layout of folded spiral erbium-doped planar waveguide optical amplifiers (EDWAs) is considered. A quasi-continuous function to describe the curvature of a folded-spiral layout is presented, based upon the use of cosine and raised-cosine variations in bend curvature. This layout provides greater gain in a given area than the corresponding fixed-radii layout. Analysis is initially based on simple geometric principles and the reduction of bending losses for 180 waveguide bends. Fold-back bends and complete EDWA layouts are then considered. Simulations are carried out using an algorithm that combines five-level rate equations with propagation by the method of lines.Index Terms-Erbium-doped waveguide amplifier (EDWA), optical amplifier.
WITH THE evolution of optical systems, the market now requires compact low-cost optical amplifiers that meet the performance demands of metro and core/access networks. Erbium-doped waveguide amplifiers (EDWAs) have many advantages over erbium-doped fiber amplifiers, including reliability and the potential for integration with other optical components [1], [2]. The main limitation of EDWAs is their relatively low gain, which is restricted by the cooperative up-conversion that accompanies heavy Er doping [1]. As a result, most EDWAs require a complex layout [3].EDWAs based on continuous and folded spiral layouts, as shown in Fig. 1, have already been considered and ranked according to their ability to maximize gain in a given chip area [4]. This investigation was restricted to layouts using straight sections and right-angle bends of constant radius, and the optimum layout was identified as a continuous spiral containing waveguide intersections. However, this layout is unsuitable for arrangement as an EDWA array, which would allow integration costs to be reduced. In this letter, we show that the folded-spiral layout (which can be used in a nested array) may be further improved using continuously varying bends. To do so, we combine a number of well-known curvature variations in a novel way to describe the whole of a complex EDWA layout.Optical simulations are carried out using a numerical algorithm, which links a five-level rate equation model with beam propagation by the method of lines [5], [6]. Full details of the model, which does not suffer from paraxial approximations, are given in [7]. Here, bends with constant curvature were modeled by solving a discretised form of the Helmholtz equation in cylindrical coordinates. In the present work, bends with varying curvature are modeled by cascading sections of constant curva- ture, matching the waveguide position and slope at the interfaces between sections. The number of sections required for convergence is small; typically up to nine sections were used for a 180 bend.Simulations were performed for example parameters, as follows. The waveguide itself is a step-index planar guide with a core width of 3.2 m and core and cladding indexes of and , respectively, so that the guide is single-moded at 980-nm wavelength. The Er concent...