Vertical-cavity surface-emitting lasers (VCSELs) have replaced edge-emitting laser diodes as the preferred light sources for short-reach optical interconnects (OIs) due to their significant advantages, including high modulation bit rates, low energy consumption, high beam quality, low manufacturing cost, and more. Considering cost, long-term system sustainability, and reliability, future OIs must be suited for operation without extra cooling, implying the VCSELs must be capable of operating perpetually and reliably at elevated temperatures. Future exaflops-scale supercomputers will require billions of OIs and are predicted to require high bit rate interconnects operating at least at 25 Gbit/s per channel. This leads to the firm requirement for future OI systems of increased bit rate and lower energy dissipation. This work experimentally demonstrate that 980 nm VCSELs can achieve high bit-rate, temperature-stable, and energy-efficient operation concurrently for the first time. It is shown that this is a result of high-speed fabrication and the epitaxial wafer design, including the active region design, the quantum well gain-to-etalon wavelength offset design, the distributed Bragg reflector design, and a careful thermal design. Systematic experimental temperature-dependent and oxide-aperture diameter-dependent characterization are presented, including static measurements, small-signal analysis, and data transmission experiments. It is also demonstrate that VCSELs with oxide-aperture diameters between ~3 and ~4 µm are most suitable to achieve energy-efficient, temperature-stable, and high bit rate operation at the same time. Error-free data transmissions at 38 Gb/s at 25, 45, 65 and 85 °C are achieved without any change of working point and modulation condition by using VCSELs with oxide-aperture diameters smaller than 5 μm. Moreover, error-free data transmission at a bit rate of 42 Gb/s at room temperature is achieved, and 38 Gb/s at 85 °C by using small oxide-aperture VCSELs. These maximum achievable data transmission bit rates match very well with the prediction from small-signal analysis. Record low energy dissipation of 139 and 177 fJ/bit for 35 and 38 Gb/s error-free data transmission at 85 °C are achieved by using a ~3 μm oxide-aperture diameter VCSELs. To date, these VCSELs are the most energy-efficient VCSELs operating at 85 °C at any wavelength. At room temperature, only 145, 147, and 217 fJ/bit of dissipated heat energy per transferred bit are needed for 35, 38, and 42 Gb/s error-free data transmission by using a ~3 μm oxide-aperture diameter VCSEL, which are all record low energy dissipation for 980 nm VCSELs. A temperature-dependent and oxide-aperture diameter-dependent impedance analysis are performed to better understand the bit rate limitations and to understand what improvements should and can be made for the next generation 980 nm VCSEL device design. Relative intensity noise values are also given, which are low enough to satisfy the application requirements of the 32GFC Fiber Channel standard. During the course of this dissertation, small oxide-aperture (diameter smaller than 5 μm) 980 nm VCSELs are demonstrated to be especially well suited for use in short-reach optical interconnects in high performance computers, and in board-to-board and chip-to-chip integrated photonics systems.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Keywords:||Vertical-cavity surface-emitting lasers|
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