Efficient and ultra-narrow-linewidth integrated waveguide lasers in Al
2O
3:Yb and Al
2O
3:Er
E. H. Bernhardi, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau
Integrated Optical MicroSystems (IOMS) Group, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands
phone: +31-53-4440 , fax: +31-53-489 3996 e-mail: e.h.bernhardi@ewi.utwente.nl
The ability to integrate Bragg grating structures with optical waveguides provides the opportunity to realize a variety of compact monolithic optical devices, such as distributed feedback (DFB) lasers, and distributed Bragg reflector (DBR) lasers. In this work, we report passive DBR cavities with record-high Q-factor [1], laser operation of Yb3+-doped DBR cavities with record-high slope efficiency [2], and laser operation of Er3+-doped DFB cavities with ultra-narrow linewidth [3].
Undoped, Yb3+-doped, and Er3+-doped Al2O3 layers were deposited on thermally oxidized silicon wafers by reactive co-sputtering [4], and microstructured channel waveguides were fabricated by standard photolithography and subsequent chlorine-based reactive ion etching [5]. After depositing a SiO2 upper cladding by plasma-enhanced chemical vapor deposition, Bragg gratings were patterned into a photoresist by laser inteference lithography and etched into the waveguide cladding [1]. Since the grating is located in the cladding, the spatial overlap between the guided mode and the grating is only ~0.15%. Transmission measurement performed on passive uniform Bragg gratings resulted in high reflectivities, exceeding 99%. DBR cavities formed by two such Bragg gratings generate a resonance within the reflection band (Fig. 2b), resulting in a record-high Q-factor of 1.02106 (Fig. 1).
Applying such distributed Bragg gratings to Al2O3:Yb3+ channel waveguides produces highly efficient laser emission. The DBR cavity was formed by two 3.75-mm-long integrated Bragg reflectors on either side of a 2.5-mm-long grating-free waveguide region, to form a total DBR cavity length of 1 cm. The device was pumped with a 976-nm laser diode. Laser operation was demonstrated at a wavelength of 1021.2 nm, with output powers of up to 47 mW and a launched pump power threshold of 10 mW, resulting in a slope efficiency of 67%. Also a distributed feedback channel waveguide laser was demonstrated. The diode-pumped Al2O3:Er3+ continuous-wave laser had a threshold of 2.2 mW absorbed pump power and a maximum output power of more than 3 mW, with a slope efficiency of 41.3% versus absorbed pump power. Single-longitudinal-mode and single-polarization operation was achieved with an emission linewidth of 1.70 ± 0.58 kHz, corresponding to a Q factor of 1.14×1011, which was centered at a wavelength of 1545.2 nm.
Fig. 1. Measured (points) and calculated
(dashed line) Q-factors of a DBR cavity in Al2O3.
Fig. 2. Measured power characteristics of
the Al2O3:Yb3+ DBR waveguide laser.
Fig. 3. Measured RF beat signal (circles) and
fitted theoretical RF power spectrum (solid lines) of a 1.70 kHz Lorentzian linewidth.
[1] E. H. Bernhardi, Q. Lu, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau, Photon. Nanostruct., in press (2011). DOI: 10.1016/j.photonics.2011.03.001.
[2] E. H. Bernhardi, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau, Opt. Lett. 36, 603-605 (2011).
[3] 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, Opt. Lett. 35, 2394-2396 (2010).
[4] K. Wörhoff, J. D. B. Bradley, F. Ay, D. Geskus, T. P. Blauwendraat, and M. Pollnau, IEEE J. Quantum Electron. 45, 454-461 (2009).
