Efficient and ultra-narrow-linewidth integrated waveguide lasers in Al₂O₃:Yb and Al₂O₃:Er

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, laser operation of Yb3+-doped DBR cavities with record-high slope efficiency, and laser operation of Er3+-doped DFB cavities with ultra-narrow linewidth. Undoped, Yb3+-doped, and Er3+-doped Al2O3 layers were deposited on thermally oxidized silicon wafers by reactive co-sputtering, and microstructured channel waveguides were fabricated by standard photolithography and subsequent chlorine-based reactive ion etching. 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. 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, resulting in a record-high Q-factor of 1.02x10e6. 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 1.14×10e11, which was centered at a wavelength of 1545.2 nm.