We present a novel approach to gradient waveform design for diffusion weighted MRI. The approach is founded on the temporal frequency domain formulation of the signal attenuation. The objective is to obtain a set of circularly polarized oscillating gradient waveforms that are optimal in terms of b-value, frequency selectivity and in-plane rotational invariance. A new pore size estimation algorithm is presented. We show phantom and in vivo results based on MRI scans using an opimized waveform set.