3342 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 16, AUGUST 15, 2009 Effect of Si-nc to Er Coupling Ratio in EDWAs Longitudinally Pumped by Visible Broad-Area Lasers Valentina Donzella, Student Member, IEEE, Stefano Faralli, Veronica Toccafondo, and Fabrizio Di Pasquale, Member, IEEE Abstract—We present a finite element based model for Si-nc sensitized Er doped waveguide amplifiers (EDWA), longitudi- nally pumped by a novel pumping scheme using broad-area visible lasers, which accurately describes the effect of the Si-nc to Er coupling ratio on the amplifier performance. We show that by pumping the active material resonantly with the Er absorption spectrum, the Si-nc sensitization is extremely beneficial even at relatively low fractions of Er ions coupled to the nanoclusters. On the other hand, when the pump light is only directly absorbed by the Si-nc, more than 80% of the Er ions should be coupled to the Si-nc in order to achieve significant net gain. In particular, numerical results based on realistic material pa- rameters, point out that resonant multimode pumping at 660 nm provides significant benefits in terms of gain enhancement, with re- spect to standard EDWAs, even at low fractions of Er ions cou- pled to the Si-nc (less than 50%). This feature suggests that further material optimization could lead to the realization of compact and cost-effective integrated am- plifiers and lasers. Index Terms—Erbium, integrated optics, optical amplifier, sil- icon photonics. I. INTRODUCTION S ILICON-NANOCLUSTER (Si-nc) sensitized Er doped silica has been recently proposed as an attractive mate- rial for realizing compact and cost-effective Er -doped wave- guide amplifiers (EDWAs) [1], compatible with CMOS tech- nology. Indeed the extremely high Si-nc absorption cross sec- tion in the visible region, combined with the highly efficient energy-transfer process from Si-nc to Er ions [2], make this material attractive in order to overcome the main limitations of standard EDWAs, such as long interaction distance and ex- pensive single-mode pump lasers required to achieve significant gain [3]. The previous pumping schemes that have been considered for Si-nc sensitized EDWAs [1], [4] were LED-based; the first, re- lying on a top-pumping scheme was shown not to exploit the Manuscript received July 29, 2008; revised November 19, 2008. First pub- lished April 17, 2009; current version published July 24, 2009. This work was supported in part by the European Commission via project LANCER (FP6- 033574). The authors are with Scuola Superiore Sant’Anna, 56127 Pisa, Italy (e-mail: v.donzella@sssup.it; sfaralli@sssup.it; v.toccafondo@sssup.it; f.dipasquale@sssup.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2008.2010627 pump intensity effectively and the amplifier efficiency was hin- dered by some of the intrinsic characteristics of the active mate- rial. Then longitudinal evanescent pumping for slab waveguide was proposed [4], but its performance, although better compared to [1], was still leading to the conclusion that the active ma- terial has not yet reached the required features to allow prac- tical deployment. In particular, two main problems need to be addressed: the confined carrier absorption (CCA) mechanism within the Si-nc, which can induce significant losses at the signal wavelength [5] and the low concentration of Er ions effi- ciently coupled to the nanoclusters [6]. The CCA-induced loss has been recently reduced to less than 0.2 dB/cm by engineering the Si-nc concentration and size, and by increasing the fraction of Er ions coupled to the Si-nc up to 25% [7]. Although the reduction of the Si-nc CCA is a fun- damental step in order to limit the signal attenuation during the propagation within the amplifier, further material optimization is however required to increase the fraction of Er ions cou- pled to the Si-nc in order to achieve net gain in feasible Si-nc sensitized EDWAs. In this paper, we present a model for Si-nc sensitized EDWAs, which accurately describes the effects of CCA mechanism and Si-nc to Er coupling ratio on the amplifier performance. In addition, we propose a new pumping scheme, based on the phys- ical mechanism of partial overlap between multimode pump and single-mode signal lights in the active core. The proposed pumping configuration can be applied in general to sensitize rare-earth-doped waveguide amplifiers, and is described in de- tail considering Si-nanoclusters as efficient sensitizers for Er ions pumped by high-power broad-area laser at around 660 nm. The use of laser diodes overcomes the difficulties of focusing and coupling incoherent emitting sources, such as LEDs, into multimode waveguide, and allows us to effectively exploit the high absorption cross section of Si-nc in the visible region. In particular, a 3-D finite element based model has been developed for modal analysis and propagation of both signal and pump lights. Propagation equations, directly derived from Maxwell’s equations, are coupled to population rate equations that describe the active material considering Er ions coupled to the Si-nc system as well as isolated but optically active Er ions. The developed model allows us to accurately compare the performance of indirect pumping condition, in which Er excitation is only possible by energy transfer from the Si-nc [2], with the performance of resonant pumping condition, in which the pump light can be absorbed either directly by the optically active Er ions or by the nanoclusters and then effectively transferred to the rare-earth ions. 0733-8724/$26.00 © 2009 IEEE