Integrated photodetector monitors output of surface-emitting laser

Integrated photodetector monitors output of surface-emitting laser

Integrating an intracavity quantum-well photodetector into a vertical-cavity surface-emitting laser (VCSEL) has enabled researchers at the University of California, Berkeley, to monitor VCSEL output without bulky external instrumentation. The device consists of 20.5 n-doped aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) distributed Bragg reflectors (DBRs) on a GaAs substrate, topped by a spacer region containing two indium gallium arsenide (InGaAs) quantum wells. The upper reflector for the active region is a p-doped AlGaAs/GaAs DBR stack containing eight mirror pairs and an aluminum arsenide oxidation layer for current confinement, topped by a spacer region with an InGaAs quantum well that acts as the detector, followed by 24.5 n-doped AlGaAs/GaAs DBR pairs with a to¥phase-matching layer.

To minimize detection of broadband spontaneous emission, the detector layer was placed at the peak of the VCSEL internal optical intensity distribution. The detector has a responsivity of about 9 A/W--compared with the 0.5-A/W response of a typical external silicon detector--and it introduced absorption losses of about 5.6 cm-1 compared with total system losses of 83 cm-1. In tests, results from the internal detector corresponded closely to results from an external detector, and the detector sensed less spontaneous emission below threshold.