Study On Few-mode Vertical-Cavity Surface-Emitting Laser And VCSEL Array With Optimized Mesa Arrangement

Vertical-Cavity Surface-Emitting Laser(VCSEL)owns plenty of excellent optical and electric characteristics,such as single longitudinal mode,low threshold,narrow divergence angle,circle spot,high stability and high modulation speed,etc.making it the key component of 3D imaging,optical communications and so on.This dissertation mainly discuss works of 850 nm few-mode VCSEL with emission-controllable transverse modes and 980 nm,808nm VCSEL array with optimized mesa arrangement,including structure design,optimization algorithm,fabrication and performance evaluation.The major lines and achievements are as below:1.A novel,cost-effect VCSEL sources for future mode division multiplexing in optical communications system is proposed,which has promising potential in simplifying the optical setup or even eliminate the need of multiplexer and in the meantime can replace the VCSEL array.Making use of the large modal volume of VCSEL,we present new VCSEL devices that can be controlled to emit certain modes and polarizations states.By simply etching the upper mesa into several parts,the mode-control was achieved.Here is the principle: VCSEL with large oxide aperture can support multiple transverse mode to be excited.Therefore,by means of directly etching trenches in the p-doped distributed Bragg reflector,we can form air gaps and sub-mesas as waveguides above the oxide layer and individual contacts are deposited on each sub-mesa.As is injected into a single contact,current could only be transferred from the corresponding sub-mesa into the active region below,resulting a lasing region which carriers are densely distributing in and laser can only emit from.Meanwhile,because the air in the trenches has much lower refractive index than GaAs,optical restriction to the lasing mode is introduced.2.The current flux and distribution in the few-mode VCSEL was simulated using COMSOL Multiphsics.The effect of the oxide aperture and size of the trench on the current density distribution were investigated.As expected,the trenches resulted in highly inhomogeneous current density distribution and the influence of the oxide is more obvious than that of the trench on the current distribution.And mode analysis of individual sub-mesa was performed.The simulation results indicated that size of the lasing region determines the number of modes that can be excited,the shape of the sub-mesas influences the shape of modes and the direction of the sub-mesa determine the polarization.Consequently,the key of mode control is manipulating the geometry of the lasing region properly.3.After several improvement of fabrication process,two-sub-mesa,three-sub-mesa,four-sub-mesa and six-sub-mesa VCSEL with emission-controllable transverse mode(few-mode VCSEL in what follows)were successfully fabricated.By controlling the size of oxide aperture,independent control of two modes,three modes and four modes were achieved.However,the three-mode and four-mode control was operated under limited current scales.In the same time,because the polarizations of each mode could be different,such type of few-mode VCSEL can be regarded as VCSEL with multiple polarizations controlled by changing biased contact.However,because the packaging condition,six-sub-mesa VCSEL are still not fully tested.4.The influence of mesa arrangement of a VCSEL array on the thermal characteristics was studied and mesa arrangement was optimized.A VCSEL array with special mesa distribution was designed.Four type of 980 nm VCSEL array with different mesa distance were fabricated.Three of them were 4×4 square VCSEL array while one was specially designed VCSEL array.The experimental results proved that the mesa distribution did improve the thermal stability of the device.Power was increased and the thermal resistance was suppressed.Under increasing heat sink temperatures,the decline ratio of power of the optimized device was apparently smaller than the 4×4 square VCSEL array.5.According to the integral equation of temperature distribution of the VCSEL array,a heat coupling factor was defined,in which both the mesa distance and the whole area of the mesa were considered,to optimized the mesa distribution of circle VCSEL array.Our goal was still to improve the thermal stability of the device to improve the performance.The circle VCSEL array was defined of loops of mesas.The heat coupling factors of different combinations of loop radii were calculated by a traversal algorithm to find the combination with minimal heat coupling factor.The loop combination with the minimal heat coupling factor is considered the optimized one.Normal circle and square VCSEL arrays with the same number of mesas and whole areas with the optimized VCSEL arrays were fabricated as reference devices.Measurement results showed that the optimized devices had certain advantages over the normal devices.