The hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) technique only utilizes the odd subcarriers and the imaginary part of the even subcarriers for symbol transmission,leading to the problem of low spectral efficiency.To address this problem,an adaptively biased HACO-OFDM (AB-HACO-OFDM) method is proposed.In the proposed method,a pulse-amplitude modulated discrete multitone (PAM-DMT) signal is designed for the unused real part of the even subcarriers,and is further superimposed on the HACO-OFDM signal for hybrid transmission to enhance the subcarrier utilization.Meanwhile,a power-efficient adaptive bias signal is introduced to guarantee the non-negativity.The simulation results demonstrate that compared to HACO-OFDM,the proposed AB-HACO-OFDM is capable of effectively improving the spectral efficiency of the transmission,while at the same time,achieving better performance in bit-error rate (BER).Additionally,compared to the conventional orthogonal frequency division multiplexing (OFDM) methods,the proposed method requires much lower signal-to-noise ratio to achieve the BER of target,thus exhibiting higher power efficiency.