Study On Grating Assisted Microcylinder Surface Emitting Lasers

Low-power consumption,low-cost and high-speed optical devices are highly desirable for optical interconnections.Surface-emitting lasers have the advantages of low cost,small size,low power consumption,high direct modulation rate,easy to realize two-dimensional array integration,easy to couple with optical fibers and easy to test so as become ideal optical communication sources.This dissertation mainly focus on lasers based on the whisperinggallery modes(WGMs)microcavities,which are added with gratings to realize single-mode operation and surface emission.The design-wavelength of the lasers is at O-band.Actually,the lasers can be easily applied to other wavelength bands in theory.The research achievements are mainly as the following:(1)Several methods for analyzing properties of WGMs microcavities are introduced.Intrinsic equations and the scattering matrix methods are used to analyze the characteristics of mode distributions,mode wavelengths,longitudinal mode spacings and quality(Q)factors of WGMs of the typical two-dimensional micro-cavities.By using the effective refractive index method,three-dimensional microcylinder cavity structures are simplified to twodimensional microcavity models.We represent the time-domain finite-difference(FDTD)method as well,which can be used for analyzing irregular microcavities flexibly and universally.In addition,the Floquet-Bloch(F-B)method is as well used to analyze the interactions of the gratings and microcavities in two-dimensional quickly and conveniently.(2)From the bottleneck encountered by long-wavelength vertical-cavity surfaceemitting lasers currently,we propose a horizontal cavity surface-emitting semiconductor laser.The laser,a grating-assisted microcylinder surface-emitting laser,is based on a microcylinder cavity etched with two second-order gratings so as to realize single-mode operation and surface emission.Therein,the side grating etched at the periphery of the microcylinder can select a whispering gallery mode(WGM)with a specific azimuthal mode number with a proper emission pattern to lase.While the top grating etched on top of its microcylinder cavity scatters the WGMs vertically so as to form the surface emission.We analyzed the cases of whether the number of periods of the side grating and the top grating are equal or not.The output emission is radially polarized beams which can easily couple with TM01 mode of a few-mode or multi-mode fiber when the number of period of the gratings are euqal.The interactions of the WGMs with gratings are analyzed in detail.We also investigate the highspeed modulation characteristics of the laser theoretically,indicating that the laser can achieve high-speed directly modulated characteristic because of its small cavity volume.(3)We also propose a grating-assisted microcylinder surface-emitting laser based on transverse magnetic(TM)modes.The top grating etched on top of its microcavity can select a WGM with proper emission pattern and can simultaneously scatters the WGM vertically to form surface emission.The laser is based on a TM-mode cavity and the active region consists of tensile strained quantum wells.The lasing mode selected by the top grating is a specific WGM with the azimuthal mode number equal to the number of period of the top grating.The top grating interacts with the lasing mode effectively to realize the surface emission.The output emission is radially polarized vector beams and can easily couple with the TM01 mode of a few-mode or multi-mode fiber.Microcavity lasers based on TM modes can be realized with very small size even when their radii are reduced down to micron range therefore they are easier to achieve high-speed modulation.(4)Based on the above analysis,we propose a single-mode surface-emitting orbital angular momentum(OAM)laser can be modulated directly with high speed.Similar to the lasers mentioned previously,the OAM laser is also a lateral cavity surface emitting laser based on a microcylinder cavity.The laser can realize single-mode operation by etching a second-order grating on the side of the cavity.The side grating can select a standing wave WGM with a specific azimuthal mode number to lase.Unlike previous lasers,we would like to have traveling wave emissions carrying OAM beams instead of standing waves.Complex refractive gratings consisting of a grating modulate the real part of the refractive and a grating modulate the image part of the refractive are added on the top of the microcylinder cavity to achieve a vertical scattering traveling wave.The output emission is radially polarized vortex beams.We also analyze the characteristics of small signal modulation and the 3d B bandwidth is about 29 GHz at 8m A.(5)We design a complete fabrication process of grating-assisted microcylinder surfaceemitting lasers on account of planar semiconductor manufactural technology.The lasers with the number of period of the side grating equal to the top grating are mainly fabricated here so that the output emission is radially polarized zero-order vector beams.The main difficulty of the fabrication lies in the side grating,which not only requires to pattern with high-precision but also deeply etched for acting as the cavity boundary.The laser is measured for static performance after finishing the fabrication.The lasers have realized continuous-wave(CW)operation for different cavity radii(from 6 micrometers to 25 micrometers).Lasers with larger cavity-radius have achieved C-W operation at room temperature and the maximal output power reached up to 1 m A.Two-lens and three-lens systems are used to measure the near-field and far-field of the output emission of the laser,respectively.It is proved that the output emission is radially polarized zero-order vector beams,which is in agreement with theoretical calculations.