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Propriétés de bruit et dynamique de lasers à semiconducteurs en régime d'émission bifréquence pour l'optique micro-ondes et la métrologie

Abstract : Coherent emission of two optical tones with a radiofrequency (RF) tunable beatnote can spur many microwave photonics applications. Indeed, it can be useful for radars, remote sensing, or in the metrological context of ultra-stable RF frequency standards. A birefringent crystal can be inserted inside the external cavity of a VECSEL (vertical-external-cavity surface-emitting lasers) to produce a tunable very low-noise RF beatnote out of two orthogonal linear polarizations. Such dual-frequency semiconductor lasers operating along the D₂ line of cesium have been developed for CPT (coherent population trapping) atomic clocks. The development of these clocks corresponds to a need to find a trade-off between size and performance. First, intensity and RF beatnote phase noise investigations in these dual-frequency VECSELs are reported. A model based on coupled rate equations allows us to find minimization strategies for the noise transfer from the pump to the laser thanks to a new pumping architecture, which produces two correlated pump spots to feed the laser modes. The laser dynamics is then optimized for efficient noise active suppression. We predict a contribution to the short term relative frequency stability of the CPT clock compatible with an Allan deviation of 5x10⁻¹³ at 1s. Pump induced thermo-optic effects are dominating the low-frequency part of the dual-frequency VECSEL beatnote phase noise spectrum and require to be better understood. A new thermal model is thus established. This model takes into account the details of the gain medium, heat diffusion spatial anisotropy, thermal lens effects and also thermodynamic fluctuations. Besides, an analysis of the spectral intensity noise covariance matrix is initiated. We evidence that the eigen modes of the cross-polarized intensity fluctuations are close to in-phase and anti-phase noise mechanisms. Eventually, we elaborate a novel characterization and ranking method for non-stationary noises, which are often related to low-frequency spurious peaks.
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  • HAL Id : tel-02901722, version 1

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Grégory Gredat. Propriétés de bruit et dynamique de lasers à semiconducteurs en régime d'émission bifréquence pour l'optique micro-ondes et la métrologie. Optique [physics.optics]. Université Paris-Saclay, 2020. Français. ⟨NNT : 2020UPASS109⟩. ⟨tel-02901722⟩

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