Asservissement de deux lasers séparés d'une fréquence micro-onde sur un interféromètre à fibre

National audienceNous présentons une méthode permettant d'asservir simultanément sur un interféromètre deux lasers séparés d'une fréquence micro-onde. Cette méthode repose sur l’utilisation de la technique Pound- Drever-Hall [2] qui est très largement utilisée pour l’asservissement sur les cavités Fabry-Perot. Les deux lasers sont des diodes lasers bas bruit (modèle ORION), émettant autour de 1542 nm et séparés de 20 GHz. La sortie de chacun des lasers est modulée en phase par un modulateur électrooptique fibré (EOM) alimenté par un signal RF de fréquence différente (ν1=14 MHz et ν2=34 MHz)

Ultralow frequency noise laser by locking to an optical fiber delay line

International audienceWe report the frequency stabilization of an erbium-doped fiber distributed-feedback laser using an all-fiber based Michelson interferometer of large arm imbalance. The interferometer uses a 1 km SMF-28 optical fiber spool and an acousto optic modulator allowing heterodyne detection. The frequency noise power spectral density is reduced by more than 40 dB for Fourier frequencies ranging from 1 Hz to 10 kHz, corresponding to a level well below 1 Hz^2/Hz over the whole range. It reaches 10^{-2} Hz^2/Hz at 1 kHz. Between 40 Hz and 30 kHz, the frequency noise is shown to be comparable to the one obtained by Pound-Drever-Hall locking to a high finesse Fabry-Perot cavity. Locking to a fiber delay line could consequently represent a reliable, simple and compact alternative to cavity stabilization for short term linewidth reduction

Cascaded multiplexed optical link on a telecommunication network for frequency dissemination

We demonstrate a cascaded optical link for ultrastable frequency dissemination comprised of two compensated links of 150 km and a repeater station. Each link includes 114 km of Internet fiber simultaneously carrying data traffic through a dense wavelength division multiplexing technology, and passes through two routing centers of the telecommunication network. The optical reference signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station operates autonomously ensuring noise compensation on the two links and the ultra-stable signal optical regeneration. The compensated link shows a fractional frequency instability of 3 \times 10-15 at one second measurement time and 5 \times 10-20 at 20 hours. This work paves the way to a wide dissemination of ultra-stable optical clock signals between distant laboratories via the Internet network

High-resolution optical frequency dissemination on a telecommunication network with data traffic

We transferred the frequency of an ultra-stable laser over a 108 km urban fiber link comprising 22 km of optical communications network fiber simultaneously carrying Internet data traffic. The metrological signal and the digital data signal are transferred on two different frequency channels in a dense wavelength division multiplexing scheme. The metrological signal is inserted into and extracted from the communications network by using bidirectional off-the-shelf optical add-drop multiplexers. The link-induced phase noise is measured and cancelled with round-trip technique using an all-fiber-based interferometer. The compensated link shows an Allan deviation of a few 10-16 at one second and below 10-19 at 10,000 seconds. This opens the way to a wide dissemination of ultra stable optical clock signals between distant laboratories via the Internet network

An agile laser with ultra-low frequency noise and high sweep linearity

We report on a fiber-stabilized agile laser with ultra-low frequency noise. The frequency noise power spectral density is comparable to that of an ultra-stable cavity stabilized laser at Fourier frequencies higher than 30 Hz. When it is chirped at a constant rate of ~ 40 MHz/s, the max non-linearity frequency error is about 50 Hz peak-to-peak over more than 600 MHz tuning range. The Rayleigh backscattering is found to be a significant frequency noise source dependent on fiber length, chirping rate and the power imbalance of the interferometer arms. We analyze this effect both theoretically and experimentally and put forward techniques to reduce this noise contribution.Comment: 14 pages, 11 figures, accepted by Optics Expres

Corrélation du bruit de phase de lasers à réseau de Bragg par injection optique. Application à la génération et au transport sur fibre de signaux radiofréquence

Travail effectué de novembre 2002 à novembre 2005.Mixing of two laser beams on a photo-detector is a way to generate radiofrequency signals up to THz band. By correlation of both optical sources, this signal can reach the required spectral purity for radio-over-fiber communications systems. Our work deals with the method of correlation by optical locking on a frequency comb. Injection locking is used to transfer the phase noise of a reference master laser to a slave laser. Using two harmonics line of a frequency modulated laser as two distinct master sources, the phase noises of both slave lasers are correlated and the frequencies difference become a multiple of the primary frequency. We have realized a general theoretical study of the injection into semi-conductor laser with complex cavity, in particular distributed feedback lasers, showing in particular the geometrical asymmetry of the noise properties. The correlation degree between the lasers has been analytically linked to the injection parameters and the phase noise characteristics. Measurements concerning interferences fringes contrast and heterodyne photocurrent spectrum have confirmed the expression. The temporal beating phenomenon has been compared with Fourier optics and speckle. Spectral purity of the beat-note has been studied and fundamental limits of this technique derived in function of the quality of the primary oscillator, the spectral proprieties of the lasers, the parameters of injection and fiber transportation. Phase noise measurements on the experimentally generated radio-frequency signal, for different conditions of injection, are in good agreement with analytical expressions.Le mélange de deux faisceaux laser sur un photo-détecteur permet de générer un signal radiofréquence jusqu'au THz. Par corrélation des deux sources optiques, le signal obtenu peut acquérir la pureté spectrale requise pour les réseaux de communications radio sur fibre. Notre travail porte sur la méthode de corrélation par accrochage optique sur un peigne de fréquences. L'injection optique permet de transférer le bruit de phase d'un laser maître, pris comme référence, à un laser esclave. En utilisant deux harmoniques d'un laser modulé en fréquence comme sources distinctes d'injection, les bruits de phase des deux lasers esclaves sont corrélés et la différence de fréquences est multiple de la fréquence primaire. Nous avons réalisé une étude théorique générale de l'injection dans les lasers semi-conducteur à cavité complexe, en particulier les lasers DFB, en mettant notamment en évidence l'asymétrie géométrique du bruit. Nous avons relié théoriquement le degré de corrélation entre les deux lasers aux paramètres d'injection et au bruit de phase. L'expression a été confirmée par des mesures sur le contraste de franges d'interférences et le spectre du photo-courant hétérodyne. Ces battements temporels ont été mis en regard avec l'optique de Fourier et le speckle. Nous avons étudié la pureté spectrale du battement et établi les limites fondamentales de cette technique en fonction de la qualité de l'oscillateur primaire, des propriétés spectrales des lasers, des paramètres d'injection et de transport sur fibre. Les mesures de bruit de phase sur le signal généré expérimentalement, pour différentes conditions d'injection, sont en très bon accord avec les expressions analytiques

Corrélation du bruit de phase de lasers à réseau de Bragg par injection optique. Application à la génération et au transport sur fibre de signaux radiofréquence

Travail effectué de novembre 2002 à novembre 2005.Mixing of two laser beams on a photo-detector is a way to generate radiofrequency signals up to THz band. By correlation of both optical sources, this signal can reach the required spectral purity for radio-over-fiber communications systems. Our work deals with the method of correlation by optical locking on a frequency comb. Injection locking is used to transfer the phase noise of a reference master laser to a slave laser. Using two harmonics line of a frequency modulated laser as two distinct master sources, the phase noises of both slave lasers are correlated and the frequencies difference become a multiple of the primary frequency. We have realized a general theoretical study of the injection into semi-conductor laser with complex cavity, in particular distributed feedback lasers, showing in particular the geometrical asymmetry of the noise properties. The correlation degree between the lasers has been analytically linked to the injection parameters and the phase noise characteristics. Measurements concerning interferences fringes contrast and heterodyne photocurrent spectrum have confirmed the expression. The temporal beating phenomenon has been compared with Fourier optics and speckle. Spectral purity of the beat-note has been studied and fundamental limits of this technique derived in function of the quality of the primary oscillator, the spectral proprieties of the lasers, the parameters of injection and fiber transportation. Phase noise measurements on the experimentally generated radio-frequency signal, for different conditions of injection, are in good agreement with analytical expressions.Le mélange de deux faisceaux laser sur un photo-détecteur permet de générer un signal radiofréquence jusqu'au THz. Par corrélation des deux sources optiques, le signal obtenu peut acquérir la pureté spectrale requise pour les réseaux de communications radio sur fibre. Notre travail porte sur la méthode de corrélation par accrochage optique sur un peigne de fréquences. L'injection optique permet de transférer le bruit de phase d'un laser maître, pris comme référence, à un laser esclave. En utilisant deux harmoniques d'un laser modulé en fréquence comme sources distinctes d'injection, les bruits de phase des deux lasers esclaves sont corrélés et la différence de fréquences est multiple de la fréquence primaire. Nous avons réalisé une étude théorique générale de l'injection dans les lasers semi-conducteur à cavité complexe, en particulier les lasers DFB, en mettant notamment en évidence l'asymétrie géométrique du bruit. Nous avons relié théoriquement le degré de corrélation entre les deux lasers aux paramètres d'injection et au bruit de phase. L'expression a été confirmée par des mesures sur le contraste de franges d'interférences et le spectre du photo-courant hétérodyne. Ces battements temporels ont été mis en regard avec l'optique de Fourier et le speckle. Nous avons étudié la pureté spectrale du battement et établi les limites fondamentales de cette technique en fonction de la qualité de l'oscillateur primaire, des propriétés spectrales des lasers, des paramètres d'injection et de transport sur fibre. Les mesures de bruit de phase sur le signal généré expérimentalement, pour différentes conditions d'injection, sont en très bon accord avec les expressions analytiques

Locking and noise properties of multisection semiconductor lasers with optical injection Application to Fabry-Perot and DFB cavities

International audienceAn analytical theory is presented for the study of injection locking in multisection semiconductor lasers. The Helmoltz equation for the electric field is solved using the Green's function method and the injected fields are included via the boundary conditions. Two cases are distinguished, injection through the front facet of the laser and injection through the rear facet. In both cases, an equation of evolution for the envelope of the electric field is established, taking into account the longitudinal distribution of the carrier and photon densities and the nonlinear gain. The expressions of the intensity, phase and carrier density noise spectra are derived using a matrix formulation. Comparison to classical equations used for Fabry-Perot lasers is discussed. The locking properties of a DFB laser with an anti-reflection coated front facet are studied in detail. Results demonstrate the strong sensitivity of the locking properties on the phase grating and rear facet reflectivity

Characteristics of the phase noise correlation of injection locked lasers for RF signal generation and transmission

International audienceTheoretical and experimental results on phase noise characteristics of an optical radio-frequencygeneration and transmission system using heterodyne photo-detection and side band injection locking isreported. Optical phase noise correlation of the lasers is achieved by noise transfer from a modulatedmaster laser. Our investigation takes into account phase noise enhancement in optical comb generation,injection dynamic, path delay, and fiber dispersion. Theoretical expressions are derived from an adiabaticapproximation and compared with experimental measures

Experimental evidence of a fundamental noise floor at the tens of millihertz level in laser locking onto unbalanced fibre-based Michelson interferometer

International audienceWe show the existence and explain the origin of a fundamental frequency noise floor at the tens of millihertz level when a low noise laser is locked on an unbalanced long-fiber-based Michelson interferometer