Rare-earth-ion Doped Waveguide Amplifiers and Lasers in Alumina and Polymers M. Pollnau, J. D. B. Bradley, J. Yang, E. H. Bernhardi, R. M. de Ridder, and K. Wörhoff Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
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Abstract: 170 Gbit/s data transmission, microring lasers operating across the telecom C-band, and narrow-linewidth distributed-feedback lasers in Al2O3:Er waveguides on silicon, as well as amplifiers and continuous-wave lasers in Nd-doped polymer waveguides on silicon are presented. OCIS codes: (130.3130) Integrated Optics Materials; (140.4480) Optical Amplifiers; (140.5680) Rare Earth and Transition Metal Solid-State Lasers
1. Introduction Optical integration will pave the way for numerous applications of photonics in computing and data communication, chemical/biological sensing, biomedical instrumentation, space applications et cetera. For tight light confinement as well as integration with electronic functionality, the silicon-on-insulator (SOI) technology has emerged as a viable platform. Nevertheless, there are serious drawbacks associated with this approach, such as considerable waveguide propagation losses, strong interband absorption in the visible spectral range, and lack of the functions of light generation and detection (in the infrared spectral range). The latter two problems, light generation and detection, can be addressed by integration with other materials that provide such functionality. Here we present our recent results on amplifiers and lasers in rare-earth-ion doped amorphous alumina and polymers integrated on thermally oxidized Si wafers. 2. Al2O3:Er3+ waveguide amplifiers and lasers Erbium-doped amorphous alumina (a-Al2O3:Er3+) was deposited onto thermally oxidized Si wafers by reactive cosputtering [1] and channel waveguides with low propagation loss (~0.2 dB/cm at 1533 nm) were fabricated by reactive ion etching [2]. The relatively high refractive index of 1.65 in Al2O3 ensures tighter light confinement compared to other dielectric materials, such as silica or phosphate glass. Internal net gain with a bandwidth of 80 nm (1500-1580 nm) was achieved, with a peak gain of 2.0 dB/cm at 1533 nm [3]. In Al2O3:Nd3+ even 6.3 dB/cm gain was achieved [4]. Such amplifier waveguides were monolithically integrated with SOI waveguides at wafer scale by direct deposition and etching of Al2O3:Er3+ waveguides onto SOI waveguides and use of inverted-taper couplers [5].
Fig. 1. Schematic of a SOI on-chip optical circuit including monolithically integrated Al2O3:Er3+ waveguide amplifiers and lasers (red sections). Figure taken from [5].
In the visionary picture sketched in Fig. 1, different active functions are enabled by this few-processing-steps, wafer-scale monolithic integration of an active gain material with SOI technology. Firstly, there exists the option of producing loss-less fiber-chip couplers by adding an erbium-doped section between the fiber and the SOI waveguide. Besides, in Al2O3:Er3+ a loss-less 1×2 splitter, compensating the 3-dB splitter loss across the entire telecom C-band (1525-1565 nm) has been achieved [6]; also a loss-less 1×4 splitter seems feasible. Furthermore, we have demonstrated data transmission with 170 Gbit/s through such an amplifier [7] without additional bit error rate or patterning effects, a feature that makes such an amplifier attractive compared to semiconductor optical amplifiers. Also lasing devices have been obtained in Al2O3:Er3+. A ring laser operating across most of the telecom C-band was achieved [8], and a 1.7-kHz-linewidth distributed feedback laser with 41% slope efficiency vs. absorbed pump power and 3 mW output power has recently been demonstrated [9]. Also ring lasers with high slope efficiency and tunability across the telecom C-band seem feasible. 3. Nd3+-doped polymer waveguide amplifiers and lasers Alternatively, when low-cost fabrication of active gain structures is a key issue, e.g. for disposable opto-fluidic devices, polymers have significant advantages. However, up to now solid polymer lasers could be operated only in the pulsed regime. We developed a polymer host material based on a cycloaliphatic diepoxy cured with a fluorinated dianhydride [10]. When activated with a Nd3+-doped complex, luminescence quenching due to high-energy vibrations from O–H and C–H chemical bonds is avoided and the typical emission lines of the Nd3+ ion are detected. By spin-coating onto a thermally oxidized silicon wafer and photodefining a cycloaliphatic epoxy prepolymer, inverted channels in the low-refractive-index polymer were obtained. The Nd3+-doped polymer material was then backfilled via spin-coating to obtain Nd3+-doped channel waveguides. An additional cladding layer was spin-coated on top of the channels [10]. Internal net gain at 865-930 nm and 1064 nm was demonstrated [11]. The small-signal gain measured in samples with a Nd3+ concentration of 1.0×1020 cm-3 was 2.0 dB/cm and 5.7 dB/cm at 873 nm and 1064 nm, respectively. Ease of fabrication, compatibility with other materials, and low cost make such rare-earth-ion-doped polymer waveguide amplifiers suitable for providing gain in many integrated optical devices. Optimization of the fabrication procedure of both, host material and optical structure, lead to steady-state laser emission from a channel waveguide near 1060 nm [12], providing a slope efficiency of 2.1% and up to 1 mW of output power [13]. The laser device was tested for continuous operation over two hours. Also lasing at the threelevel-transition near 880 nm was demonstrated [13]. To the best of our knowledge, this result represents the first steady-state laser in a solid polymer host. 4. References [1] K. Wörhoff, J. D. B. Bradley, F. Ay, D. Geskus, T. P. Blauwendraat, and M. Pollnau, "Reliable low-cost fabrication of low-loss Al2O3:Er3+ waveguides with 5.4-dB optical gain", IEEE J. Quantum Electron. 45, 454-461 (2009). [2] J. D. B. Bradley, F. Ay, K. Wörhoff, and M. Pollnau, "Fabrication of low-loss channel waveguides in Al2O3 and Y2O3 layers by inductively coupled plasma reactive ion etching," Appl. Phys. B 89, 311-318 (2007). [3] J. D. B. Bradley, L. Agazzi, D. Geskus, F. Ay, K. Wörhoff, and M. Pollnau, "Gain bandwidth of 80 nm and 2 dB/cm peak gain in Al2O3:Er3+ optical amplifiers on silicon," J. Opt. Soc. Am. B 27, 187-196 (2010). [4] J. Yang, K. van Dalfsen, K. Wörhoff, F. Ay, and M. Pollnau, "High-gain Al2O3:Nd3+ channel waveguide amplifiers at 880 nm, 1060 nm and 1330 nm," Appl. Phys. B, in press (2010). [5] L. Agazzi, J. D. B. Bradley, F. Ay, G. Roelkens, R. Baets, K. Wörhoff, and M. Pollnau, "Monolithic integration of erbium-doped amplifiers with silicon waveguides," submitted (2010). [6] J. D. B. Bradley, R. Stoffer, A. Bakker, L. Agazzi, F. Ay, K. Wörhoff, and M. Pollnau, "Integrated Al2O3:Er3+ zero-loss optical amplifier and power splitter with 40 nm bandwidth," Photon. Technol. Lett. 22, 278-280 (2010). [7] J. D. B. Bradley, M. Costa e Silva, M. Gay, L. Bramerie, A. Driessen, K. Wörhoff, J. C. Simon, and M. Pollnau, "170 GBit/s transmission in an erbium-doped waveguide amplifier on silicon," Opt. Express 17, 22201-22208 (2009). [8] J. D. B. Bradley, R. Stoffer, L. Agazzi, F. Ay, K. Wörhoff, and M. Pollnau, "Integrated Al2O3:Er3+ ring laser on silicon with wide wavelength selectivity," Opt. Lett. 35, 73-75 (2010). [9] E. H. Bernhardi, H. A. G. M. van Wolferen, L. Agazzi, M. R. H. Khan, C. G. H. Roeloffzen, K. Wörhoff, M. Pollnau, and R. M. de Ridder, "Ultra-narrow-linewidth, single-frequency distributed feedback waveguide laser in Al2O3:Er3+ on silicon," Opt. Lett., accepted (2010). [10] J. Yang, M. B. J. Diemeer, D. Geskus, G. Sengo, M. Pollnau, and A. Driessen, "Neodymium-complex-doped, photo-defined polymer channel waveguide amplifiers," Opt. Lett. 34, 473-475 (2009). [11] J. Yang, M. B. J. Diemeer, G. Sengo, M. Pollnau, and A. Driessen, "Nd-doped polymer waveguide amplifiers," IEEE J. Quantum Electron. 46, 1043-1050 (2010). [12] J. Yang, M. B. J. Diemeer, C. Grivas, G. Sengo, A. Driessen, and M. Pollnau, "Steady-state lasing in a solid polymer," Laser Phys. Lett., accepted (2010). [13] C. Grivas, J. Yang, M. B. J. Diemeer, A. Driessen, and M. Pollnau, "Continuous-wave Nd-doped polymer laser," Opt. Lett., accepted (2010).
