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Characteristic of 850-nm InGaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers

Characteristic of 850-nm InGaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers. Master’s thesis of Yuni Chang Speaker:Han-Yi Chu National Changhua University of Education. Introduction.

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Characteristic of 850-nm InGaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers

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  1. Characteristic of 850-nm InGaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers Master’s thesis of Yuni Chang Speaker:Han-Yi Chu National Changhua University of Education

  2. Introduction • Although the InGaAsP lasers operating at 1.3 and 1.5 mmhave long been used for long-haul optical fiber communication, laser diodes operating near 850 and 650 nm find important applications in short-distance PMMA-based optical fiber communication. • The laser beam emitted from a vertical-cavity surface-emitting laser is circular in shape and hence can be easily coupled into an optical fiber with a coupling efficiency up to 90%. • The vertical-cavity surface-emitting laser usually have small diverging angle, low threshold current, single laser mode output.

  3. The vertical-cavity surface-emitting lasers operating in the spectral range near 850 nm usually utilize GaAs/AlGaAs as the active layer materials. • In a vertical-cavity surface-emitting laser, the distributed Bragg reflectors (DBR) are usually used as laser mirrors. • The laser cavity consists of the active region and two spacer regions (n spacer and p spacer ). In order to improve the carrier confinement and optical confinement, the bandgap energy of the spacers is usually higher than that of the active region. • In order to maximize the overlap of the electrons inside the active region and the standing wave, a laser cavity of one wavelength in length is usually utilized.

  4. 20 30 Simulation and Discussion Al0.15Ga0.85As p-doping=2×1018 (cm-3) Al0.9Ga0.1As p-doping=2×1018 (cm-3) Al0.6Ga0.4As undoped GaAs/Al0.35Ga0.65As or In0.1Ga0.9As/Al0.35Ga0.65As 3-Quantum wells undoped Al0.6Ga0.4As undoped Al0.9Ga0.1As n-doping=1×1018 (cm-3) Al0.15Ga0.85As n-doping=1×1018 (cm-3)

  5. The DBR mirrors consist of Al0.9Ga0.1As / Al0.15Ga0.85As that has an index of refraction and thickness of 2.97/71.55 and 3.5/60.7 nm respectively. • Since the use of indium in well of the In0.1Ga0.9As/Al0.35Ga0.65As active layer will result in a decrease of the bandgap energy, the width of the quantum well needs to be smaller than that of the GaAs/Al0.35Ga0.65As. • The widths of the quantum wells of GaAs/Al0.35Ga0.65As and In0.1Ga0.9As/Al0.35Ga0.65As active layers are assumed to be 120Å and 40Å respectively. • In the In0.1Ga0.9As/Al0.35Ga0.65As active region, the well is under a compressive strain of 0.7%.

  6. Spectra of Spontaneous Emission Rate In addition to the emission peak at 850 nm, there exists emission peaks at shorter wavelength (at 815 nm for GaAs/Al0.35Ga0.65As and 807 nm for In0.1Ga0.9As/Al0.35Ga0.65As) that are due to the transitions from the second energy level in the conduction band to the second energy level in the valence band.

  7. Energy band diagrams of the (a) GaAs/Al0.35Ga0.65As and (b) In0.1Ga0.9As/Al0.35Ga0.65As VCSEL under study.

  8. (a) (b) Distribution of electron concentration near active region at 330K and 2.0 mA.

  9. (a) (b) Stimulated Recombination Rate

  10. Laser output power as a function of input current • The In0.1Ga0.9As/Al0.35Ga0.65As and GaAs/Al0.35Ga0.65As quantum well structures have a threshold current of 0.37 mA and 0.77 mA.

  11. (a) (b) Mode spectra of the GaAs/Al0.35Ga0.65As VCSEL at 0.56 mA (below the laser threshold) and 1.18 mA (above the laser threshold)

  12. Side Mode Suppression Ratio • The main mode to first side mode suppression ratio increases dramatically near the laser threshold. • In0.1Ga0.9As/Al0.35Ga0.65As VCSEL has a larger main mode to first side mode suppression ratio than the GaAs/Al0.35Ga0.65As VCSEL for all current input levels, which is beneficial for single laser mode operation.

  13. Characteristic Temperature • These curves can be best-fitted with the following formula to corresponding characteristic temperature: • A high characteristic temperature is desired for a VCSEL since the threshold current is less sensitive to temperature.

  14. Threshold current of the InXGa1-XAs/Al0.35Ga0.65As VCSEL as a function of temperature for different indium composition,x. • From the results, the characteristic temperature increases when the indium composition in InXGa1-XAs/Al0.35Ga0.65As increases.

  15. Conclusion • The strained InGaAs/AlGaAs VCSEL has a higher stimulated recombination rate, a lower threshold current , a higher main-side mode suppression ratio, and a higher characteristic temperature, which might be owing to its narrower well width and smaller carrier effective masses.

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