Objective Narrow linewidth semiconductor lasers are crucial in coherent optical communications due to their low phase noise.The rapid growth in network capacity demands advanced modulation formats with stringent phase stability requirements,necessitating lasers with linewidths of 100 kHz or lower.The square-Fabry-Perot(FP)coupled cavity consists of a square microcavity and an FP cavity directly connected.The FP cavity acts as the main gain component,while the square microcavity serves as the selective reflective end facet.The strong coupling between the whispering gallery mode and the FP mode achieves a high side-mode rejection ratio and efficient coupling output,though the laser linewidth is typically in the MHz range.We aim to drastically narrow the linewidth of the square-FP coupled-cavity laser for coherent optical communications.Methods Using the finite element method,we simulate the reflectance of an 18 μm side-length square microcavity,identifying seven transverse modes with reflectance greater than 0.7 within twice the longitudinal mode spacing.We also simulate the mode characteristics of a coupled cavity with an FP cavity length of 550 μm and a width of 2 μm.Analyzing the square-cavity reflectance,coupled-mode Q-factor,and the fundamental transverse mode proportion in the FP cavity,we determine that the coupled mode corresponding to the sixth-order whispering gallery mode is preferentially excited in the actual device.Subsequently,we fabricate square-FP coupled-cavity lasers using a 3 quantum-well AlGaInAs/InP epitaxial wafer with dimensions of side length a=18 μm,FP cavity width w=2 μm,and length L=550 μm.The deeply etched laser waveguide's optical confinement factor is approximately 0.34％,effectively reducing the Lorentzian linewidths.Results and Discussions We measure the single-mode fiber-coupled output power and voltage characteristics of the square-FP coupled-cavity laser,finding a resistance of about 7 Ω and a single-mode fiber-coupled output power of 13 mW.The main mode is at 1550.5 nm with a side-mode suppression ratio of 47 dB for injection currents ISQ=35 mA and IFP=165 mA.The spectrum's envelope aligns with the square microcavity's reflectance spectrum.With a fixed square microcavity injection current ISQ=18 mA,we observe the lasing spectrum variation with the FP cavity injection current IFP.Below the threshold current,the fundamental mode of the square microcavity at 1519.2 nm is visible.As IFP increases,the coupled mode lases at about 1521 nm,separated from the fundamental whispering gallery mode by 1.6 nm.The anti-symmetric fundamental mode in the square microcavity,having the highest Q-factor,is preferentially excited.The lasing mode forms by coupling the sixth-order whispering gallery mode with the FP mode.Near IFP=70 mA,the lasing mode hops from 1521 nm to 1548 nm,corresponding to twice the longitudinal mode spacing of the square microcavity.We measure the frequency noise power spectral density of this laser using the self-homodyne optical coherent receiver method and obtain a Lorentzian linewidth as low as 233 kHz,maintaining around 300 kHz with stable single mode lasing.Conclusions We design a narrow linewidth square-FP coupled-cavity laser with a 3 quantum-well AlGaInAs/InP epitaxial wafer and a low transverse optical confinement factor.The square microcavity has a side length of 18 μm,and the FP cavity measures 2 μm in width and 550 μm in length.Simulations and experiments indicate that the coupling mode corresponding to the higher-order whispering gallery mode of the square microcavity is excited.The laser's maximum single-mode fiber-coupled output power is 13 mW,with a maximum side-mode suppression ratio of 47 dB,and a Lorentzian linewidth of 233 kHz.
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