Narrow-linewidth lasers on a silicon chip

Diode-pumped distributed-feedback (DFB) channel waveguide lasers were demonstrated in Er3+-doped and Yb3+-doped Al2O3 on standard thermally ox-idized silicon substrates. Uniform surface-relief Bragg gratings were patterned by laser-interference lithography and etched into the SiO2 top cladding. The maxi-mum grating reflectivity exceeded 99%. Monolithic DFB cavities with Q-factors of up to 1.35×10^6 were realized. The Er3+-doped DFB laser delivered 3 mW of output power with a slope efficiency of 41% versus absorbed pump power. Single-longitudinal-mode operation at a wavelength of 1545.2 nm was achieved with an emission line width of 1.70 ± 0.58 kHz, corresponding to a laser Q-factor of 1.14×10^11. Yb3+-doped DFB lasers were demonstrated at wavelengths near 1020 nm with output powers of 55 mW and a slope efficiency of 67% versus launched pump power. An Yb3+-doped dual-wavelength laser was achieved based on the optical resonances induced by two local phase shifts in the DFB structure. 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 was created via the heterodyne photo-detection of the two laser wavelengths. Interaction of the intra-cavity evanescent laser field with micro-particles in contact with the grating surface induces changes in the microwave beat signal, whose detection enabled real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 μm and 20 μm, which represents the typical size of many fungal and bacterial patho-gens. A limit of detection of ~500 nm was deduced.