Study On Polarization Control Of Vertical-Cavity Surface-Emitting Lasers And Two-modes VCSEL

Current research indicates that there are many physical mechanisms that affect the polarization characteristics of the output light of vertical cavity surface emitting semiconductor lasers(VCSELs).In gas and solid-state lasers,the polarization of the emitted electromagnetic radiation is also not always determined,but can be easily selected by adding a Brewster window.In edge-emitting lasers(EEL),the main polarization direction is determined by the anisotropic gain of the active cavity and quantum well.However,in VCSELs,due to the circular symmetry of the active cavity and the isotropic gain of the quantum well,it is difficult to stabilize the polarization direction of the VCSEL output light in a fixed direction.And the linear polarization direction of each independent transverse mode is also uncertain,so that the polarization direction of the laser will also switch during operation.The vertical cavity surface emitting lasers have superior characteristics than edge emitting lasers,such as very low threshold currents,circular output spots,small divergence angles,and easy integration of two-dimensional arrays.Thus the VCSEL is applied to more and more applications,for example optical interconnects,parallel processing of optical signals,pumped solid-state lasers,laser displays and so on.However,when the VCSEL was applied to optical data communication,spectrum detection,optical mouse,the stable polarization of the laser has become an urgent problem to be solved.At present,some popular solutions were already proposed: growing chip structure,external cavity feedback structure,semiconductor grating,surface relief structure,and grown on high crystal phase(311)B.Among the current methods of polarization stability,the semiconductor grating structure is integrated on the surface of the VCSEL.However,the semiconductor grating etched technology was mainly utilized with some expensive and time-consuming manufacturing methods,such as electron beam exposure which is difficult for mass production.This paper proposes an economical,time-saving,and reliable double exposure technique to prepare the surface grating to realize the polarization control of VCSEL.This technology combines the nanometer exposure precision of Talbot displacement exposure technology to fabricate grating lines and a conventional I-line lithography technique to obtain a microscale circular grating pattern above the output window of VCSEL.The two-step exposure technoloy can effectively reduce the fabrication cost of the grating VCSEL and shorten the preparation time.The accuracy can reach the nanometer level of the electron beam thchonlogy.With the increasing demand for data communication capacity in the information society,optical fiber communication multiplexing technology has developed rapidly.The existing multiplexing technology mainly utilizes the characteristics of light,such as the wavelength of light,the polarization state of light,and the time domain of light,to realize the multiplexing of optical information.The most widely used multiplexing technologies are wavelength division multiplexing,polarization multiplexing,code division multiplexing,time division multiplexing,and recently developed mode multiplexing technology,etc.At present,VCSELs are mainly applied in the form of array light sources,and the light of each laser is input to a bundle of single-mode fibers or multi-core fibers at the output end for transmission.However,this trapping multiple fibers together,or multi-core fibers inevitably increases the volume of the transmission path.In order to apply the VCSEL more efficiently to applications such as small-mode fiber in the communication field,to expand the independent mode capacity of the single-channel output and reduce the volume of the transmitting end,a novel dielectric film relief structure is proposed to prepare the two-mode VCSEL.In order to solve the problem of the current immature few-mode VCSEL technology,it provides a direct-use few-mode light source for the mode multiplexing technology,and improves the transmission capacity of the fiber-optic communication of the few-mode fiber.This relief structure avoids the requirment for extremely high etch depth requirements and secondary epitaxial requirements of inverted relief structure by directly etched on own materials of VCSEL.And the use of PECVD technology in growing such relief dielectric film structure can enable precise control of the thickness.The research in this paper is mainly divided into the following aspects:First,there is a brief introduction to the basics of VCSEL and its polarization.The implementation scheme of controlling the polarization of VCSEL at home and abroad was investigated.According to the research conditions of the laboratory,the scheme of etching the semiconductor grating was defined to realize the polarization control of VCSEL.Secondly,the polarization behavior and mode distribution of the VCSEL with integrated surface grating were theoretically analyzed.The threshold gains of the VCSEL fundamental mode along and orthogonal to the direction of the grating strip were calculated respectively.The polarization characteristics of the single-mode surface grating VCSEL are simulated and further analyzed.The polarization performance of the multimode surface grating VCSEL with different periods were also analyzed.The effect of grating etching depth and grating period on the overall performance of the surface grating VCSEL were also given.Then,the process technology of the surface grating VCSEL preparation was deeply studied.Utilizing Talbot displacement lithography and two-step exposure method to prepare surface gratings for VCSELs innovatively.The method is economical and suitable for high volume production.To promote the grating etched precision of surface grating VCSEL,some fabrication steps were reasonably improved based on the original process of VCSEL.And single-mode,multi-mode and surface grating VCSELs with different different periods were successfully fabricated and tested.The single-mode single-polarization grating VCSEL prepared has a main polarization direction along the [0-11] crystal axis direction in the entire current test range,whether it is at room temperature of 20 °C or high temperature of 80 °C.The orthogonal polarization ratio OPSR value reaches 30 dB.The threshold current is 0.3 mA at room temperature 20 °C,and the threshold current is 0.5 mA at 80 °C,which proves that the device can still guarantee the high performance of the threshold current below 1 mA at high temperature.At room temperature 20 ° C and high temperature 80 ° C,the total power of the device at 1.5 mA is 0.55 mW and 0.4 mW,respectively.Such surface grating VCSELs are expected to be used in applications requiring single mode single polarization,such as atomic clocks.The prepared multimode grating VCSELs are also stable in polarization and have an orthogonal polarization ratio of over 17 dB.Finally,to apply the VCSEL to few mode fiber applications in the communication field more effectively,a two-modes VCSEL was proposed take advantage of the dielectric relief structure above the VCSEL.By depositing dielectric relief structure to control the modes,VCSEL can output two modes steadily.The reasonable relief size was designed by theoretical simulated with the mirror loss of VCSEL under different etching depths and the influence on the gain of different modes.The two-mode VCSEL was fabricated successfully.The output performance was measured.