167 research outputs found
Physique des Lasers
Master / M2Première partie du cours de physique des lasers du M2 'Lasers et Matière'. A partir d'une approche semi-classique de l'interaction matière-rayonnement, on dérive les équations du laser monomode. On étudie ensuite le comportement des lasers à élargissement homogène et inhomogène. Ensuite, on étudie les régimes transitoires et déclenchés de ces lasers, avant d'étudier leurs bruits d'intensité et de fréquence
Intensity and phase noise correlations in a dual-frequency VECSEL operating at telecom wavelength
The amplitude and phase noises of a dual-frequency vertical-external-cavity
surface-emitting laser (DF-VECSEL) operating at telecom wavelength are
theoretically and experimentally investigated in detail. In particular, the
spectral behavior of the correlation between the intensity noises of the two
modes of the DF-VECSEL is measured. Moreover, the correlation between the phase
noise of the radio-frequency (RF) beatnote generated by optical mixing of the
two laser modes with the intensity noises of the two modes is investigated. All
these spectral behaviors of noise correlations are analyzed for two different
values of the nonlinear coupling between the laser modes. We find that to
describe the spectral behavior of noise correlations between the laser modes,
it is of utmost importance to have a precise knowledge about the spectral
behavior of the pump noise, which is the dominant source of noise in the
frequency range of our interest (10 kHz to 35 MHz). Moreover, it is found that
the noise correlation also depends on how the spatially separated laser modes
of the DF-VECSEL intercept the noise from a multi-mode fiber-coupled laser
diode used for pumping both the laser modes. To this aim, a specific experiment
is reported, which aims at measuring the correlations between different spatial
regions of the pump beam. The experimental results are in excellent agreement
with a theoretical model based on modified rate equations
Frequency stabilization at the kilohertz level of a continuous intracavity frequency-doubled singly resonant optical parametric oscillator
A continuous intracavity frequency-doubled singly resonant optical parametric oscillator (OPO) is stabilized to the side of the transmission peak of a medium finesse Fabry–Perot cavity. The narrow bandwidth of the frequency noise of this OPO allows this simple scheme to lead to a stability of a few kilohertz with respect to the locking etalon. The system, operating in the visible domain, remains locked for more than 1h
Processus cohérents et applications des phénomènes de lumière lente et rapide dans l'hélium métastable à température ambiante
L'interaction entre des champs électromagnétiques et des systèmes à plusieurs niveaux peut donner lieu à différents processus cohérents. La transparence électromagnétiquement induite (EIT) ou les oscillations cohérentes de populations (CPO) sont des phénomènes résonnants ayant pour conséquence d'annuler l'absorption du système pour un champ sonde. L'EIT se produit dans les systèmes à trois niveaux et met en jeu une cohérence entre niveaux non couplés optiquement, alors que les systèmes à deux niveaux suffisent pour faire apparaître le CPO qui ne met pas en jeu la dynamique des cohérences.Il est possible dans un milieu constitué d'atomes d'hélium métastable à température ambiante d'extraire un système à trois niveaux en qui, lorsqu'il est excité par des polarisations circulaires, fait apparaître des fenêtres EIT de l'ordre de quelques dizaines de kHz de large. Lorsque ce système est excité par des polarisations linéaires et soumis à un champ magnétique de faible amplitude, c'est l'association des deux phénomènes de CPO et d'EIT qui détermine la réponse du système. Une fenêtre de transparence CPO de quelques dizaines de kHz de large a en particulier été observée.Ce type de phénomènes résonnants est associé à de grandes variations de l'indice de réfraction avec la fréquence du champ sonde, ayant pour conséquence de profondes modifications de la vitesse de groupe d'une impulsion se propageant dans le milieu. Notre système expérimental permet ainsi d'observer de la lumière lente, de la lumière rapide ainsi que des vitesses de groupe négatives. L'insertion de tels milieux dispersifs en cavité optique a été suggérée pour augmenter la précision de senseurs comme les gyroscopes lasers, mais leur bruit fondamental dépend de la durée de vie des photons dans la cavité. C est pourquoi l influence des milieux hautement dispersifs sur la durée de vie des photons dans une cavité est étudiée expérimentalement et théoriquement.Many coherent processes can be observed when electromagnetic fields are applied to multi-level systems. Electromagnetically induced transparency (EIT) or coherent population oscillations (CPO) are resonant phenomena resulting in the cancellation of the absorption of the system for a probe field. EIT occurs in three-level systems and involves the coherence between two optically uncoupled levels, whereas two-level systems are sufficient to produce CPO which does not involve the dynamics of the coherences.A three-level system can be isolated in a gas of metastable helium atoms at room temperature. When excited with circular polarizations, this system exhibits EIT resonances of a few tens of kHz. If this system is excited by linear polarizations and submitted to a weak magnetic field, the response of the system is given by the combination of these two phenomena, namely EIT and CPO. A narrow CPO transmission window of a few tens of kHz has then been observed.This kind of resonant phenomena is associated with very strong variations of the refractive index at the probe frequency, leading to major changes of the group velocity of a pulse propagating in the medium. Our experimental set-up allows us to observe slow light, fast light, and even negative group velocities. Insertion of such dispersive media inside an optical cavity has been suggested to increase the sensitivity of sensors such as laser gyroscopes, but their fundamental noise depends on the lifetime of photons inside the cavity. This is why the influence of highly dispersive media on the photon lifetime inside a cavity is studied experimentally and theoretically.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
Noise investigation of a dual-frequency VECSEL for application to Cesium clocks
We theoretically and experimentally study the noise of a class-A
dual-frequency vertical external cavity surface emitting laser operating at
Cesium clock wavelength. The intensity noises of the two orthogonally polarized
modes and the phase noise of their beatnote are investigated. The intensity
noises of the two modes and their correlations are well predicted by a theory
based on coupled rate equations. The phase noise of the beatnote is well
described by considering both thermal effects and the effect of phase-amplitude
coupling. The good agreement between theory and experiment indicates possible
ways to further decrease the laser noises
Fully-correlated pumping for dual-frequency VECSELs dedicated to cesium CPT clocks
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
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