Two pulses waltzing in a laser cavity: exploiting the fs laser as the ultimate sensor (Prof. J.-C. Diels)

A passive interferometer such as a Michelson measures phase differences by converting them into an amplitude modulation. By contrast, in the Intracavity Phase Interferometer, a minute phase shift is converted into a frequency. The technique is to use a mode-locked laser as a differential interferometer between two frequency combs issued from the same cavity. Measurements of phase with a sensitivity of 1:108 has been demonstrated with an un-stabilized laser. This exquisite phase sensitivity leads to measurements of displacements, linear and nonlinear indices, magnetic fields, inertial measurements with unprecedented sensitivity. Implementation with various mode-locked lasers (dye, Ti:sapphire, optical parametric oscillators, fiber lasers) will be presented. In these various applications, the sensor properties is related to the phase velocities, while the envelope velocity is maintained. The envelope velocity of a pulse is not just defined by dispersive properties of a material or group index dk/dΩ in a medium with saturable gain or absorption. Therefore, the label “fast light” enhancement given to the amplification of the phase response by Intracavity dispersion control is a misnomer, as the phase response and its enhancement are related to phase velocities.