Noise Correlations in Dual Frequency VECSEL
Résumé
Optically-pumped vertical-external-cavity semiconductor lasers (OP-VECSELs) sustaining two orthogonally polarized modes of different frequencies (see Fig. 1), but belonging to the same longitudinal mode, may find many applications especially to generate optically carried pure millimeter waves tunable over a large frequency range. The main advantage of dual frequency centimeter-long-cavity OP-VECSELs over the familiar solid-state lasers is the low relative-intensity-noise (RIN) since such OP-VECSELs do not suffer from relaxation oscillations [1]. This is due to the class-A dynamical behavior of centimeter-long-cavity OP-VECSELs, which implies that the photon lifetime inside the cavity is much longer than the excited-carriers lifetime. The dual frequency operation of OP-VECSEL has also other advantages since the two oscillating polarized modes share the same cavity. As a result of that the two mode fields experience highly correlated frequency and intensity fluctuations and we can also tune the frequency difference between the two modes by changing the intra-cavity phase anisotropy. But the simultaneous oscillation of the two modes and the noise correlation between the modes depend on the nonlinear coupling constant between the modes, which must be less than unity. Now the purity of the RF beat note, generated by mixing the two orthogonally polarized laser modes, depends on the correlation of the phase and intensity fluctuations of the two modes inside the laser cavity. So in every respect it is worth to explore the noise correlation of the two laser modes. We report here the behavior of the intensity and phase noise correlations of the modes and their dependence on the coupling both experimentally and theoretically.