We propose overlapping multi-pulse pulse position modulation (OMPPM) in optical direct-detection channel to improve information rate and efficiency over pulse position modulation (PPM) or multi-pulse PPM (MPPM). The error probability of uncoded OMPPM system is investigated in optical direct-detection channel in quantum limited case, and both capacity and cutoff rate are evaluated in this channel. Moreover, Reed-Solomon (RS) codes are employed for OMPPM to achieve high energy information efficiency in the high throughput range. It is shown that OMPPM outperforms OOK, PPM, and MPPM in terms of both capacity and cutoff rate for the same bandwidth and the same probability of pulse occurrence. It is also shown that RS coded OMPPM achieves high energy information efficiency even in the range of high throughput.