We present results from our new generation of high performance 850 nm oxide confined vertical cavity surface-emitting lasers (VCSELs). With devices optimized for high-speed operation under direct modulation, we achieve record high 3dB modulation bandwidths of 28 GHz for ~4 μm oxide aperture diameter VCSELs, and 27 GHz for devices with a ~7 μm oxide aperture diameter. Combined with a high-speed photoreceiver, the ~7 μm VCSEL enables error-free transmission at data rates up to 47 Gbit/s at room temperature, and up to 40 Gbit/s at 85°C.
The impedance characteristics and the effects of photon lifetime reduction on the performance of high-speed 850 nm
VCSELs are investigated. Through S<sub>11</sub> measurements and equivalent circuit modeling we show that the parasitic mesa
capacitance can be significantly reduced by using multiple oxide layers. By performing a shallow surface etch (25 -
55 nm) on the fabricated VCSELs, we are able to reduce the photon lifetime by up to 80% and thereby significantly
improve both static and dynamic properties of the VCSELs. By optimizing the photon lifetime we are able to enhance
the 3dB modulation bandwidth of 7 μm oxide aperture VCSELs from 15 GHz to 23 GHz and finally demonstrate errorfree
transmission at up to 40 Gbit/s.
We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond
the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design
for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal
conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a
minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic
high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in
the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in
a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential
resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is
capable of error-free transmission (BER<10<sup>-12</sup>) under direct modulation at a record-high bit-rate of 32 Gb/s over 50 m of
OM3 fiber at room temperature, and at 25 Gb/s over 100 m of OM3 fiber at 85 °C. We also demonstrate transmission at
40 Gb/s over 200 m of OM3+ fiber at room temperature using a subcarrier multiplexing scheme with a spectrally
efficient 16 QAM modulation format. All transmission results were obtained with the VCSEL biased at current densities
between 11-14 kA/cm<sup>2</sup>, which is close to the 10 kA/cm<sup>2</sup> industry benchmark for reliability. Finally, we show that by a
further reduction of the oxide capacitance and by reducing the photon lifetime using a shallow surface etch, a record
bandwidth of 23 GHz for 850 nm VCSELs can be reached.