Femtosecond pulses and laser resonators

In a mode-locked laser, a wave packet of light of transverse dimension of the order of a mm, and longitudinal dimension of only a few micron, travels back and forth in a resonator of the order of one or two meter. It is difficult to conceive why a light bullet, six orders of magnitude shorter than the cavity, would care whether its central wavelength would fit as a sub-multiple of the cavity length. As the length of the resonator changes constantly because of vibrations, thermal drifts., the "central wavelength" of the intracavity fs pulse should also change constantly to follow the cavity resonances. We present evidence that this is indeed the case, and that the micron long wave packet travelling in the cavity does indeed keep record of cavity motion, with subwavelength accuracy. Applications range from distance measurements with a spatial resolution of 0.01 pm, and fs temporal resolution, to inertial navigation (measurement of acceleration and rotation). Stabilization of the mode-locked laser can enhance the resolution of these measurements by at least three orders of magnitude.