窄线宽正方形-法布里-珀罗耦合腔激光器(特邀)

Narrow Linewidth Square Fabry-Perot Coupled-Cavity Lasers(Invited)

窄线宽半导体激光器在相干光通信领域具有重要的应用,利用较小光限制因子的AlGaInAs/InP外延片,研制了窄线宽的正方形-法布里-珀罗(FP)耦合腔激光器。利用有限元方法仿真了正方形-FP耦合腔的模式特性,采用投影式光刻工艺制备了正方形-FP耦合腔激光器,激射主模处于1550.5 nm,单模光纤耦合输出功率约为13 mW,边模抑制比约为47 dB。利用延时自零差相干接收方法测量频率噪声,得到洛伦兹线宽为233 kHz。根据实验和仿真结果,激射模式是由正方形微腔的高阶横模和FP模式耦合形成。

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 18μm 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 550μm and a width of 2μm.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=18μm,FP cavity width w=2μm,and length L=550μm.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 7Ωand 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 I_(SQ)=35 mA and I_(FP)=165 mA.The spectrum’s envelope aligns with the square microcavity’s reflectance spectrum.With a fixed square microcavity injection current I_(SQ)=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 IFPincreases,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 I_(FP)=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 18μm,and the FP cavity measures 2μm in width and 550μm 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.