Optimized design and epitaxy growth of high speed 850 nm vertical-cavity surface-emitting lasers

Abstract

Using transfer matrix method and TFcalc thin film design software,the reflectance spectrum of distributed Bragg reflector (DBR) and vertical cavity surface emitting laser (VCSEL) are simulated.The reflectance spectra from the cavity and surface are compared with each other,thus providing the basis for white light source (WLS) optical reflectance spectrum of the VCSEL epitaxial wafer.When using WLS to characterize VCSEL wafer,it is necessary to combine the simulation results and the shape of optical reflectance spectrum to speculate the reflectance seen from the cavity.The influences of different cap layers on the reflectance of DBRs are discussed theoretically and experimentally.With a 1/4 GaAs cap layer,the reflectance reaches up to 97.8% seen from the cavity.This design can make the wavelength of the VCSEL etalon picked easily because of avoiding the influence of test noise. The active region has higher heat accumulation due to the small area and poor thermal conductivity.The characteristics of the gain spectrum of InGaAs/AlGaAs strained quantum well (QW) under different temperatures and the temperature distribution in VCSEL are simulated by Crosslight software.The gain-to-cavity wavelength detuning is used to improve the slope efficiency and the temperature stability.The temperature in active region ranges from 360 K to 370 K.The gain peak wavelength and the Fabry-Perot cavity wavelength are designed in the ranges of 829-832 nm and 845-847 nm,respectively.Epitaxial wafer with top-emitting VCSEL structure grown by metal-organic chemical vapor deposition is characterized.The room temperature photoluminescence peak is at 827.5 nm and the etalon cavity wavelength measured by optical reflectance is 847.7 nm,which are consistent with designed values. The oxide restricted VCSELs with 7.5 m oxide aperture are fabricated.The image of the infrared light source CCD shows that the oxide aperture is circular.A passivation layer of 120 nm SiO2 is finally deposited to insulate water vapor.The threshold current is 0.8 mA,and the maximum output power reaches up to 9 mW at 13.5 mA.The optical spectrum at 6.0 mA reveals multiple transverse modes.The center wavelength is 852.3 nm and the root mean square (RMS) spectrum width is 0.6 nm,meeting the high-speed Datacom standards.Shannon theory indicates that the maximum data rate is not only proportional to bandwidth but also related to signal-to-noise ratio (SNR).It is effective to reduce relative intensity noise and enhance the SNR by increasing output power.From the eye diagram of 25 Gbit/s on-off key VCSEL,it is demonstrated that fall time is 38.66 ps,rise time is 41.54 ps,SNR is 5.6,and jitter RMS is 1.57 ps.Clear eye opening is observed from eye diagram of 25 GBaud/s PAM-4 VCSEL,which indicates the qualified 50 Gbit/s high speed performance.