| The goal of this research is to demonstrate the feasibility of a OPO ring, in which two pulses circulate in opposite directions with exactly the same group velocity, and do not interact with each other. The absence of interaction implies that the outputs corresponding to the opposite sense of circulation of these two pulses can be made to interfere, and show a low-frequency beat note representative of the difference in frequency between the combs of modes corresponding to these two pulse trains. This “beat note” can be used for measurements of rotation (laser gyro) displacements (with a sensitivity of less than 0.001 Å), index of refraction, electric fields, magnetic fields, magnetic susceptibility, etc.; A long series of approaches led to a particular configuration of a ring OPO pumped inside the cavity of a linear Ti:sapphire laser, which exhibited a low frequency beat note with a small beat note bandwidth. It is remarkable to note that, even though each output of the ring OPO has a bandwidth of 5 THz (the inverse of the 200 fs pulse duration), and fluctuations at all time scales, the beat note bandwidth is less than 10 Hz. The tolerance of the OPO to a mismatch with the pump cavity length being only of the order of only a few microns, the operation of the OPO could only be demonstrated for very short tunes. Nevertheless, the final goal of demonstrating the exceptional performance (in low beat note and narrow beat note bandwidth) of the OPO ring laser has been achieved. As compared to the standard cw lasers used for rotation sensing (laser gyros), the fundamental problem of lock-in (or dead band) and the gain competition problem has been solved.; The operation of a bi-directional femtosecond pulse ring OPO based on periodically poled lithium niobate (PPLN), pumped alternately with non-simultaneous pulses from a Ti:sapphire mode-locked laser was first demonstrated. A beat note between the two counter-propagating beams attested to a gyro response without dead band. The sensitivity of the device to differential phase changes was demonstrated by measuring the nonlinear index of lithium niobate n 2 = 8.5 × 10−15 cm2/W. (Abstract shortened by UMI.)... |
