Advances in photonics, silicon electronics and digital signal processing (DSP) have converged to enable highly efficient transmission across fiber optic channels. Single wavelength data rates of 112 Gb/s are sought for wide deployment based on QPSK transmission, coherent detection, and digital demodulation. Here we examine continuous phase modulation (CPM) as a means to enhance performance and reach of coherent optical links. We quantify the robustness of the constant amplitude CPM format to spectral filtering and nonlinearities in comparison to QPSK. The challenges of generating and receiving the CPM waveform are considered and a novel CPM transmitter architecture is proposed.