Nearly collimated 940-nm buried-photonic-crystal surface-emitting laser for lens-free discrete multitone transmission at 22  Gbit/s over 20  m

By using a high-power surface-emitting laser configured with a buried two-dimensional hollow pyramid photonic-crystal layer (referred to as PhCSEL) with a low-divergent-angle (0.25 deg) and large-emission-aperture beam at 940 nm, a lensless free-space-optical point-to-point link with either NRZ-OOK or QAM-DMT data format carried by the nearly collimated beam is demonstrated over 0.3-m short-reach and 20-m long-reach schemes. The buried 2D hollow pyramid photonic-crystal layer configures the band-edge lasing effect to induce zero group velocity for all coupled light waves and provide an almost collimated beam across a broad aperture. Under directly encoded back-to-back transmission, the PhCSEL offers a −3 dB bandwidth of 3.25 GHz to enable either the NRZ-OOK data rate up to 5.75 Gbit/s or the 3-GBaud 64-ary QAM DMT data rate of 18 Gbit/s over 0.3 m in free space. When using the bit-loading DMT algorithm for spectral usage efficiency optimization within 4.09-GBaud bandwidth, the multiband bit-loaded QAM-DMT format can elevate the total data rate up to 22 Gbit/s. Lengthening the free-space distance to 20 m slightly degrades the received signal-to-noise ratio by the inevitable diverged beam spot. Nevertheless, the PhCSEL still performs 1-GBaud broadband DMT streaming with allowable 16-QAM at 4-Gbit/s or 64-QAM at 6-Gbit/s over the 20-m lensless free-space link.