Photonic bandgap structures are very promising for the integration of optical function at the nanoscale level and particularly 2D structures perforated on a slab waveguide. In this context, photonic crystal waveguides obtained by removing rows of holes in the periodic structure are very interesting. We will present here, results of near field optical microscopy conducted on PCW. With a Fourier transform analysis of the image, we can image the Bloch waves propagating in the waveguide. The results are compared to FDTD simulations and we will show that the images obtained correspond to the electric field. Such result allows us to study the interaction of the optical tip with the electromagnetic field. 2D FDTD simulations of 1D photonic crystal interaction with a SNOM tip is presented. The results obtained confirm that the SNOM signal is mainly related to the electric field.