278 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 5, MARCH 1, 2010 Integrated Al O :Er Zero-Loss Optical Ampliﬁer and Power Splitter With 40-nm Bandwidth Jonathan D. B. Bradley, Student Member, IEEE, Remco Stoffer, Arjen Bakker, Laura Agazzi, Student Member, IEEE, Feridun Ay, Kerstin Wörhoff, and Markus Pollnau Abstract—A combined planar lossless optical ampliﬁer and 1 2 power splitter device has been realized in Al O :Er on silicon. Net internal gain was measured over a wavelength range of 40 nm across the complete telecom -band (1525–1565 nm). Calculations predict net gain in a combined ampliﬁer and 1 4 power splitter device over the same wavelength range for a total injected pump power as low as 30 mW. Index Terms—Aluminum oxide, erbium, integrated waveguide device, integrated optics, optical ampliﬁer. I. INTRODUCTION I NTEGRATED optical ampliﬁers allow for loss compensa- tion within photonic circuits. Typically in such an integrated photonic circuit the optical signal power must be split into two or more separate branches, resulting in a 50% power decrease in each branch or more. In addition, the waveguide scattering losses and losses as a result of various other device functions must be compensated for. In the past, integrated Er-doped planar lossless splitters have been demonstrated in Er-doped silicate glass [1]–[3] and phosphate glass hosts [4], [5]. Net gain was measured over wavelength ranges of up to 16 nm for launched pump powers in excess of 100 mW. Compared to these other materials, Er-doped aluminum oxide (Al O :Er ) offers advantages due to its higher refractive index contrast, which allows for smaller bend radii and higher integration density. The higher refractive index contrast also reduces the typical waveguide cross-section, resulting in higher pump intensities in the Er-doped waveguide core, thus lower pump power required for net gain. Further- more, the broad Al O :Er emission spectrum allows for ampliﬁcation over a wide wavelength range. We have recently demonstrated a peak gain of 2 dB/cm at 1533 nm and a net gain over a wavelength range of 80 nm in this material [6]. In this letter, we describe a combined integrated Al O :Er ampliﬁer and 1 2 power splitter device. Zero-loss on-chip Manuscript received October 12, 2009; revised November 12, 2009; accepted November 19, 2009. Current version published February 05, 2010. This work was supported by the European Union’s Sixth Framework Programme (Speciﬁc Targeted Research Project “PI-OXIDE”, Contract 017501) and by the Smartmix Memphis programme of the Dutch Ministry of Economic Affairs. J. D. B. Bradley, L. Agazzi, F. Ay, K. Wörhoff, and M. Pollnau are with the Integrated Optical Microsystems Group, MESA + Institute for Nanotech- nology, University of Twente, 7500 AE Enschede, The Netherlands (e-mail: J.D.B.Bradley@ewi.utwente.nl). R. Stoffer and A. Bakker are with the PhoeniX BV, 7500 AM Enschede, The Netherlands. Color versions of one or more of the ﬁgures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/LPT.2009.2037927 Fig. 1. Al O :Er on-chip lossless ampliﬁer and splitter. power splitting over a wavelength range of 40 nm was demon- strated. II. EXPERIMENTAL DETAILS A. Fabrication A 0.5- m-thick Al O :Er ﬁlm was reactively cosput- tered on a thermally oxidized silicon substrate [7]. Channel waveguides were then deﬁned by standard lithography and reactive ion etching [8]. The resulting core of the Al O :Er waveguides was 0.5 m high by 1.5 m wide. A 5- m-thick SiO top-cladding layer was deposited by plasma-enhanced chemical vapor deposition, and end facets were prepared by dicing. B. Lossless Ampliﬁer/Splitter Design An illustration of the combined Al O :Er ampliﬁer and splitter device is shown in Fig. 1. The device includes two sep- arately pumped 8.6-cm-long Al O :Er waveguide sections. This was to avoid the signiﬁcant reduction in pump intensity due to Er absorption and radiation and scattering of higher order guided modes which would be expected to occur towards the end of a single, 17.2-cm-long waveguide. By dividing the am- pliﬁer into two pumped sections, it was possible to ensure sufﬁ- cient pump intensity and Er ion excitation over the total active waveguide length, based on the available pump power in the ex- perimental setup. To facilitate the two pump inputs, two direc- tional couplers were applied which were designed to selectively couple signal light, while coupling minimal pump light. Each coupler consisted of a straight coupling region with a length of 500 m, a gap of 2 m, and adiabatic sine bend transitions at the input and output. Each sine bend had a lateral offset of 20 m and a length of 250 m [9]. The lengths of the pump input sec- tions in front of the couplers (2.3 mm) and the length of the bend section between the couplers (bend radius m, length mm) were minimized to avoid pump absorption and reabsorption of signal light, respectively. The two pumped 1041-1135/$26.00 © 2010 IEEE Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on February 6, 2010 at 14:28 from IEEE Xplore. Restrictions apply.