Study Of In-phase Coupled Vertical Cavity Surface Emitting Laser Array And Electrical Beam Steering

Vertical cavity surface emitting laser?VCSEL?is an attractive surface emitted light source.Because of its high beam quality,single longitudinal mode,low threshold and wafer test,it has been widely used in many areas such as data communication,sensing,optical interconnection and signal processing.VCSELs are suitable to be integrated into 2-D arrays for high output power.However,the laser from every element is not coherent with each other.There is no certain phase relationship among elements in thus arrays,leading to no improvement on beam quality.If the optical fields from the elements are coupled with each other,a coherent beam can be obtained.Especially,if the entire array is oscillating in the in-phase mode,the far-field radiation pattern will consist of a narrow diffraction-limited central lobe,where the beam width varies as the inverse of the entire array size.If the scale of the array is larger,the beam quality will be better.The coherently coupled array may be used in many areas,such as laser radar,optical communications,pumping where high power and image processing,spectrum sensing where low power are demanded.In addition,based on coherent VCSEL arrays,independent injection current into every element can vary the relative phase between elements and causes electronic beam steering.Compared to mechanical methods,non-mechanical methods are more sensitive and more reliable,which is significant for laser radar application and steering optical source.2-Dimensional coherently coupled array and beam steering were systematically studies in this dissertation supported by the Nation Natural Foundation of China.The coupling mechnical,in-phase mode design,implanted VCSEL array fabrication and test method were studied.We designed and fabricated coherent VCSEL arrays with high beam quality,achieving near-diffraction limited single mode operation and high efficiency 2-D beam steering.The main research works are as follows:1.The coupling mechanism and the design of in-phase implant-defined VCSELarraysThe modal property of 1×2 coupled array was only investigated by an international group.The impact of thermal and carriers on material refractive is analyzed to establish the weak antiguided modal.The array mode dependent on 2-D array structure was analyzed experimentally and simulatedly.The array structure of in-phase mode and out-of-phase mode were confirmed.An array model based on the finite-difference method is established to simulate the far-field profile of the in-phase coherent arrays.Some important parameters including divergence,coupling factor and separation between lobes dependent on array structure and emitted wavelength in far-field profiles were studied.Considering these restraining factors,the optimization design for array elements and structure was obtained.2.Fabrication process technologyVCSEL array based on proton implantation technology was investigated.Considering the threshold and output power,proper implant energy and dose were confirmed experimentally and theoretically.The masks of small size array were fabricated successfully via etching SiO₂ by Ni.The fabricated masks are of good verticality and uniformity.The VCSEL coherent arrays were fabricated by both electrical isolation and optical coupling.The processes such as photolithography,etching and sputtering were optimized.A complete fabrication process for implant-defined VCSEL arrays was formed.3.The design and fabrication for high output power and wide current rangein-phase coupled VCSEL arraysThe resistance of device surface will increase after proton implantation.The output power was affected because of large thermal loss.The output power was only about 1mW according to some international reports.The uniformity of current injection was improved via depositing Au nano layer.Because of reduced thermal loss,in-phase VCSEL array with relative high output power of 4mW was obtained.Most structures of implanted VCSEL arrays are square according to the reports.In-phase mode can only operate in a narrow current range.7-element in-phase array was designed and fabricated.Compared to square array,thus array has stronger coupling because of more neighbors closest to the central element.The array can maintain an in-phase mode from threshold to 35 mA,showing a considerably wide in-phase single mode operation range.The near-field profiles,far-field profiles,and emission spectra under different injection currents were tested and analyzed.The divergence angle of the array is only 2.5°in a far-field profile.Approximately 29%of the total power is localized in the central lobe.The beam quality and brightness in the array have a great improvement.4.The study of beam steering in coupled VCSEL arraysIn beam steering coupled VCSEL arrays,the beam deflects via controlling relative phase difference between elements.The phase of element is controlled via current injection into every element separately.The increased resistance between elements is usually from etching DBRs in the inter-element region.However,the isolation is bad.The deflected efficiency is affected.We designed novel separate electrodes.The complete isolation was achieved by multi-implant with decreased energy.1×2 and3-element arrays were designed and fabricated.1-D and 2-D effective beam steerings were achieved.The maximum deflection angles are 2.1^o and 1.6^o,respectively.The array still keeps strong coupling between elements when the beam deflects.The output power,divergence and spectra were analyzed systematically when the beam deflected.The mode switching between in-phase mode and out-of-phase mode was achieved.