We present a review of research performed in the area of coherent access technologies employing vertical cavity surface emitting lasers (VCSELs). Experimental demonstrations of optical transmission over a passive fiber link with coherent detection using VCSEL local oscillators and directly modulated VCSEL transmitters at bit rates up to 10 Gbps in the C-band as well as in the O-band are presented. The broad linewidth and frequency chirp associated with directly modulated VCSELs are utilized in

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... n envelope detection receiver scheme which is demonstrated digitally (off-line) as well as analog (real-time). Additionally, it is shown that in the optical front-end of a coherent receiver for access networks, the 90 • hybrid can be replaced by a 3-dB coupler. The achieved results show that VCSELs are attractive light source candidates for transmitter as well as local oscillator for coherent detection PONs Index Terms-Access networks, coherent detection, optical communication systems, passive optical networks (PONs), vertical cavity surface emitting lasers (VCSEL).