Ultranarrow-linewidth lasers on a silicon chip and their applications

We demonstrate diode-pumped distributed-feedback (DFB) and distributed-Bragg-reflector (DBR) channel waveguide lasers in Al₂O₃:Er³⁺ and Al₂O₃:Yb³⁺ on silicon substrates. Uniform surface-relief Bragg gratings are patterned by laser-interference lithography and etched into the SiO₂ top cladding, providing reflectivities exceeding 99%. Monolithic DFB and DBR cavities with Q-factors of up to 1.35×10⁶ are realized. The Er³⁺-doped DFB laser delivers 3 mW of output power at 1545 nm with 41% slope efficiency versus absorbed pump power. Single-longitudinal-mode operation at a wavelength of 1545.2 nm is achieved with an emission linewidth of 1.7 kHz, corresponding to a laser Q-factor of 1.14×10¹¹. Yb³⁺-doped DFB and DBR lasers operate with output powers of 55 mW near 1020 nm and 67% slope efficiency versus launched pump power. A dual-wavelength laser utilizing two local phase shifts in the DFB structure is achieved. A stable microwave signal at ~15 GHz with a -3-dB width of 9 kHz and a long-term frequency stability of ±2.5 MHz is created via heterodyne photo-detection of the two laser wavelengths. Measuring changes in the microwave beat signal, as the intra-laser-cavity evanescent field interacts with micro-particles on the waveguide surface, enables real-time detection and accurate size measurement of single micro-particles with diameters of 1-20 μm.