If we have a resonator composed of 2 partially transmitting mirrors/grids and we excite the resonator from one end, say the left side then on the right we expect to get the characterstic transmission comb and on the left the inverse (1-Transmission) reflection curve. Whats giving me a headache is this.... shouldn't we see the characteristic transmission curve at both ends of the resonator as the frequency that is being built up is dependent upon the length of the resonator not its orientation in space... it shouldn't matter to the resonant wave which direction its going in. So.. at the left side we'd see reflection curve + half transmission curve and at the right end just half the transmission curve. Put perhaps slightly more clearly. If we have a 1mW laser going in from the left we are going to get 1mW coming out from the right if the frequency is tuned correctly, but I don't see why we shoudn't get 0.5mW from each end leaving the resonator??