The guiding of intense laser pulses in plasma channels is necessary to maximize the energy of electrons accelerated in a laser wakefield accelerator. A significant fraction of the energy in the laser pulse may be lost during and after coupling from vacuum into a channel. For example, imperfect coupling can lead to enhanced leakage of laser energy transversely through the channel walls. We present 2D particle-in-cell (PIC) simulations, using the VORPAL code, of an example problem. We present a numerical diagnostic, based on simultaneous FFT s in space and time, which enables quantitative measurements of forward and backward propagating pulse energy andits spectra. Future work will be directed toward VORPAL parameter studies designed to optimize the amount of laser energy that couples into a channel.