Optimizing 1.55-µm VCSELs yields thinner DBRs
Vertical-cavity surface-emitting lasers (VCSELs) emitting at telecom wavelengths offer low threshold currents, single-mode operation, high coupling efficiencies into fiber, and high-speed modulation. Maria Linnick and Aris Christou at University of Maryland (College Park) reported results from an optimized 1.55-µm VCSEL design at the recent IEEE LEOS Summer Topical Meetings (Mont Tremblant, Quebec).1
The researchers sought materials with crystal lattices that match the InP substrate while offering a large index difference from InP. They chose to make their DBRs from alternating layers of Al0.05Ga0.43In0.53As and InP, which yields a refractive index difference of 0.63. The top (p-doped) DBR consists of 16 layers with an expected reflectivity of 97%, and the bottom (n-doped) DBR consists of 22 layers with an expected reflectivity of 99%.
Their design focuses on providing excellent performance while minimizing the number of layers that must be grown, which they expect to improve device reliability. They used MBE to grow a VCSEL with an unstrained multiple-quantum-well active layer, consisting of eight Ga0.43In0.57As0.92P0.08 wells, each 6 nm thick, separated by seven Ga0.23In0.77As0.5P0.5 barriers, each 9 nm thick. The quantum wells were located between cladding layers to place them at the peak of the electric field standing wave in order to achieve matched gain.
A 70-nm-thick layer of AlInAsP right next the cladding layers acts as the lowest layer of the p-doped DBR, and was selectively oxidized to form a 7-µm-diameter aperture. The oxide layer efficiently confines the charge carriers into the laser active region while the reduced refractive index of the oxide transversely confines the laser emission.
Average threshold current was 3 mA, threshold voltage was typically less than 2 V, and the power output exceeded 1 mW. The output spectrum showed a single mode at 1.54 µm. Contact Aris Christou at firstname.lastname@example.org.
- M. Linnik and A. Christou, LEOS Summer Topical Meetings 2002, Invited paper WG2 (July 5–17, 2002).