A specifically designed SMA connector with jacket signal pin configuration is used to connect the transistor outline can (TO-can) packaged Fabry–Pérot laser diode (FPLD) for enhancing its direct modulation bandwidth beyond 6 GHz. The direct modulation of such an TO-can/jacket-SMA packaged FPLD with a 256 quadrature amplitude modulation (QAM) and single subcarrier optical orthogonal frequency-division multiplexing (OOFDM) format data stream at carrier frequency up to 6 GHz is approached with 10 MSa/s symbol rate. The optimized 256-QAM data with error vector magnitude (EVM) of 1.5% and signal-to-noise ratio (SNR) of 32.5 dB are observed, corresponding to a bit error rate as low as $ 1.1 × 10-6 and a highest carrier-to-noise ratio of 43.5 dB. Under injection locking, the tradeoff between the lengthened photon lifetime and the enlarged threshold current reduction of such an M-QAM-N-OOFDM modulated FPLD is theoretically and experimentally investigated to explain the saturation of SNR at 31.5 dB and EVM at 1.7%. Both 16-QAM and 64-QAM 200-subcarrier OOFDM formats with total transmission bit rates up to 15.6 and 23.4 Gb/s, respectively, are performed at carrier frequency of 2 GHz. The corresponding EVM and SNR for the 16-QAM OOFDM are determined as 11.67% and 20 dB, respectively. Under 64-QAM and 200-subcarrier OOFDM modulation, the received data stream shows a slightly degraded EVM and SNR of 12.85% and 19 dB, respectively.
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