Design, Fabrication, and Evaluation of High-Speed Vertical-Cavity Surface-Emitting Lasers

Licentiate thesis, 2009

The vertical-cavity surface-emitting laser (VCSEL) is a low cost light source with inherent high speed capabilities. These properties, along with low power consumption and a circular output beam, have made the VCSEL a well established component in digital communication networks, particularly in short reach optical data transmission where links consisting of GaAs-based 850nm VCSELs and multimode fiber have become the standard solution. Development of higher speed VCSELs is therefore crucial to meet the market's need for higher speed digital communication services in the near future. The scope of this work has been to extend the possible data transmission rate for 850nm VCSELs beyond the 10 Gbit/s bit-rate limit of today's commercially available devices. By tailoring the device design for high speed performance, we have demonstrated that 10 Gbit/s is far from the endpoint of 850nm VCSEL technology. Thermal conductivity is improved through the use of binary compounds in the bottom mirror and parasitics are kept at a minimum by incorporating a large diameter double layered oxide aperture in the design. It is shown that significant improvements of the high speed performance can be obtained by replacing the traditional GaAs quantum wells (QWs) with strained InGaAs QWs in the active region. The best overall performance in the VCSELs developed and fabricated for this work is demonstrated for a device with 9 μm diameter double oxide aperture and strained InGaAs QWs, with a threshold current of 0.6 mA, a maximum output power exceeding 9mW, a thermal resistance of 1.9 C/mW and a differential resistance of 80 ­Ohm. This type of component has a parasitically limited 3dB bandwidth exceeding 20 GHz and is shown to be capable of error-free transmission (BER<10^-12) under direct modulation at a record-high bit-rate of 32 Gbit/s over 50m of OM3 fiber at room temperature, and at 25 Gbit/s over 100m of OM3 fiber at 85 C. These results are obtained with the VCSEL biased at current densities in the range 11-14 kA/cm2, which is in the range of the 10 kA/cm2 industry benchmark for reliability.