Study Of High Output Power Single Fundamental Moda Vertical Cavity Surface Emitting Lasers

The vertical-cavity surface-emitting laser (VCSEL) is an attractive light sourcefor an expanding range of applications. Due to low cost manufacturing, low powerconsumption, intrinsically low diverging circular beam and excellent high-speedmodulation characteristics at low currents, it has been extensively used in short-reachoptical communication, optical interconnects, and optical storage. However, theVCSELs’ disadvantages of multi-transverse mode emission and the unstablepolarization limit its application in the new areas such as sensing and atomic clocks.Therefore, improving the mode characteristics and increasing the stability of itspolarization state are becoming critical issues in the field of VCSELs’ research. In thisarticle, photonic crystal and other optical confinement structures combined with theoxide structure are introduced into the VCSELs to control the mode characteristicsand polarization states. The combined structures were also applied to reduce thethreshold current and enhance the efficiency of the conventional optical confinementstructures VCSELs. After that, Low threshold current, high output power, singlefundamental mode, and stable polarization VCSELs were fabricated and followingresults have been obtained:1. Model,simulation, and optimization of the DevicesIn devices simulation, we found that the commonly used photonic crystal fibermodel can only be used to analyze accurately the mode characteristics affected by thephotonic crystal structure when the oxide aperture diameter of the devices is largerthan15μm. However, such a diameter of the low threshold current single fundamentalmode devices is usually less than10μm and the mode characteristics of the devicesare determined by both the photonic crystal and the oxide aperture structure. So that,an accurate optical micro-cavity model including the effects of both photonic crystaland oxide apertures was proposed, which can overcome limitations of the photoniccrystal fiber model. At the same time, the near-and far-field distribution, cavity modeintensity distribution, and spectral curves of the devices can be shown by the model.Therefore, it is significant for the optimization of the device. On the other hand, byusing the full three dimensions Finite-difference time-domain (FDTD) algorithm, themicro-cavities quality factor (Q factor) of the devices has been calculated to analyzethe mode loss of the VCSELs. According to the optimization, several kinds of singlefundamental mode (SFM) VCSELs with low threshold current were designed (IEEEPhotonics Technology Letters vol.24, no.6, pp.464-466).2. Design and fabrication of low threshold current SFM-VCSELsBoth the photonic crystal and oxide aperture structures were optimized by usingoptical micro-cavity model. For the photonic crystal structure, the optimal value of the oxide aperture diameter was obtained. After the optimization of the selectively wetoxidation, electron beam lithography (EBL), and inductively coupled plasma etching(Inductively Coupled Plasma ICP) processes, single fundamental mode PhC-VCSELwith3.1mW of output power,0.9mA of threshold current, over35dB of side modesuppression ratio (SMSR), and less than0.1nm of full width at halfmaximum (FWHM) of the spectrum was obtained.(Reported in the37thOpticalFiber Communication (OFC/NFOEC) conference and published in the SPIENewsroom). This method was also used to optimize the multi-holey VCSELs,SFM-VCSELs with0.7mA of threshold current and2.6mW of output power wasalso obtained.3. Design and fabrication of polarization stable SFM-VCSELsWe found that the polarization and the mode of the devices are dependent on theshape of rhombus oxide aperture and photonic crystal structure. According to that thepolarization-stable single fundamental mode PhC-VCSELs were fabricatedFurthermore, the optical micro-cavity model was used to optimize the rhombusshaped PhC-VCSELs and the polarization-stable devices with1.3mW of singlefundamental mode output power,0.6mA of threshold current, and more than14dB oforthogonal polarization suppression ratio (OPSR) was obtained. The method was alsoapplied to the multi-holey VCSELs to achieve the polarization-stable singlefundamental mode devices. The birefringence effect and polarization dependent modeloss caused by the elliptical shaped air hole of the photonic crystal were also used inour experiments, and polarization stable device with1.6mW of single fundamentalmode output power, lower than1mA of threshold current, and more than10dB ofOPSR was obtained.4. Design and fabrication of different photonic crystal structures PhC-VCSELsMany PhC-VCSELs with different photonic crystal structures were designed andfabricated to analyze the mode dependence on photonic crystal structures. Amongthese devices, triangular lattice single-point defect PhC-VCSELs has a singlefundamental mode output power of1.7mW with the SMSR larger than35dB(Chinese Physics Letter vol.27, no.2, pp.0242061-3); Square lattice single-pointdefect device has a single fundamental mode output power of3.7mW with the SMSRlarger than40dB; Triangular lattice seven point-defect device has a singlefundamental mode output power of2.3mW, threshold current of1.2mA, farfielddivergence angle less than6degree, and SMSR over35dB (Optic&laser Technologyvol.50,pp.130-133).