Fully–correlated multi–mode pumping for low–noise dual–frequency VECSELs

Abstract

We report a fully-correlated multi-mode pumping architecture optimized for dramatic noise reduction of a class-A dual-frequency Vertical External Cavity Surface Emitting Laser (VECSEL). Thanks to amplitude division of a laser diode, the two orthogonally polarized modes emitted by the VECSEL oscillating at 852 nm are separately pumped by two beams exhibiting fully in-phase correlated intensity noises. This is shown to lead to very strong and in-phase correlations between the two lasing modes intensities. As a result, the phase noise power spectral density of the RF beat note generated by the two modes undergoes a drastic reduction of about 10 to 20 dB throughout the whole frequency range from 10 kHz to 20 MHz and falls below the detection floor above a few MHz. A good agreement is found with a model which uses the framework of rate equations coupled by cross-saturation. The remaining phase noise is attributed to thermal effects and additional technical noises and lies mainly within the bandwidth of a phase-locked-loop.

Optimization of laser dynamics for active stabilization of DF-VECSELs dedicated to cesium CPT clocks

“…Besides, the high-frequency component of the beat-note phase noise comes from the conversion of the intensity noise of the two modes into optical phase noise due to the large Henry factor α of such lasers. The pumping architecture 2 ○ has proven to be efficient for minimizing the pump-to-laser intensity noise transfer, thus limiting the bandwidth of the free running beat-note phase noise to about 100 kHz [12]. Such a limited noise bandwidth allows us to consider the implementation of an optical phase-locked loop (OPLL) to mitigate the beatnote phase noise, as is now going to be investigated.…”

“…In particular, reducing the noises of the DF-VECSEL requires both a strong in-phase correlation of the pump noises seen by the two modes and a low nonlinear coupling between these modes. Such conditions can be met using a fully-correlated multi-mode pumping architecture with a relatively large spatial separation between the two modes arXiv:2009.05046v1 [physics.ins-det] 10 Sep 2020 in the active medium [12].…”

Thermal phase fluctuations in optically pumped dual-frequency vertical external-cavity surface-emitting lasers for cesium clocks based on coherent population trapping

“…Moreover, a disagreement of this model with experimental data has recently been reported. 9 In this example, the phase noise that was measured corresponded to the beatnote between the two modes emitted by a DF-VECSEL operating at 852 nm. It was observed that the measured phase-noise spectrum of this beatnote exhibits a f À3 slope below 200 kHz instead of the f À4 behavior predicted by the usual simple model.…”

“…Indeed, DF-VECSELs operating at 852 nm are promising sources to build clocks combining both a good relative frequency stability and compactness. 6,9 This wavelength of 852 nm corresponds to the D 2 transition of cesium. The two emitted orthogonal linear polarizations of a DF-VECSEL can be used to probe a double lambda system along this line, which leads to a coherent-population-trapping (CPT) resonance.…”

“…Noise investigations in VECSELs have led to a quantitative understanding and description of at least the two first sources of noise. 4,9,[11][12][13][14][15][16][17][18] The description of the two other sources of noise deserves more effort, especially the thermal noise. These fluctuations of the local temperature induce changes of the effective optical length of the laser cavity and thus of the laser phase.…”