In several fields, such as biology, chemistry, combustion and environmental science, laser absorption spectroscopy represents an invaluable tool for the detection and identification of a variety of molecular species in the gas phase. For this detection to be quantitative, it is of paramount importance to rely on accurate spectroscopic parameters for the involved absorption lines in terms of line strength, line center frequency, pressure broadening, and pressure shift coefficients. The mid-infrared region offers the most favorable conditions for sensitive and chemically selective detection. The sensitivity derives from the presence of intense fundamental ro-vibrational transitions of molecules, whereas chemical selectivity relates to the unique absorption spectrum that molecules possess in the mid-IR region, thereby known as the fingerprint region. In this thesis, we combine the accelerating technology of optical frequency combs (OFC), which are powerful tools for accurate optical frequency measurements, with the wide tunability and single line emission in the mid-IR of extended cavity quantum cascade lasers (EC-QCL), to perform highly resolved, accurate and sensitive measurements in the fingerprint region, from 7.25 to 8 μm. Specifically, we have been able to lock for the first time the optical frequency of an EC-QCL to an OFC by utilizing nonlinear optics in the form of sum frequency generation (SFG) (Lamperti, AlSaif et al., 2018) and have exploited this comb-locked EC-QCL for an accurate survey of the entire ν1 ro-vibrational band of one of the most important greenhouse gases, nitrous oxide (N2O). The developed spectrometer is able to operate over a wide region of ~ 100 cm-1, in a fully automated fashion, while affording a 63 kHz uncertainty on the retrieved line center frequencies. The measurement allowed us to determine very accurately rotational constants of both ground and excited states of the ν1 band of N2O through the measurements of tens of lines of the P and R branches (AlSaif et al., JQSRT 2018). The spectrometer was then upgraded with a more recent and narrower linewidth EC-QCL to perform sub-Doppler saturated spectroscopy on the same N2O sample at a spectral resolution below 1 MHz, the sharpest ever observed with this type of laser. Finally, we worked at adding high sensitivity to the apparatus by introducing the gas in a high-finesse passive resonator and by developing a system to measure the intra-cavity absorption with cavity ring-down spectroscopy (CRDS) together with comb calibration.

The thesis deals with a design of a comparing interferometer. It concerns a setup of two interferometers, one of them is a Michelson and another a Fabry-Pérot type. This set-up is made to compare results of length measurements simultaneously performed by each of them. In the Theory, basic principles of Michelson and Fabry-Pérot interferometers are described. A special attention is paid to an innovative method of length measurement with tunable lasers and optical frequency comb. In the Practical Part, so-called Pilot Experimental Setup is described. It is a prototype that has been used to perform basic experiments on comparing of the two above mentioned methods. Based on experimental results and practical experience with the Pilot Experimental Setup, a Final Setup is designed. It comes in a form of a stand-alone instrument.

Le principal objectif de ce travail de thèse a été de développer de nouvelles techniques de spectroscopie moléculaire afin obtenir de nouvelles données spectroscopiques dans le visible et le proche infrarouge pour des molécules, radicaux ou ions d’intérêt astrophysiques. La première partie de ce travail a consisté en l’étude d’harmoniques élevés, très faibles en intensité, de bandes vibrationnelles de la molécule HC₃N (cyanoacetylène), par la technique de spectroscopie CRDS. La deuxième partie a consisté en l’étude spectroscopique d’espèces instables radicalaires. Pour cela, une décharge inductive Radio Fréquence a été couplée au montage CRDS. Le plasma ainsi créé a été caractérisé et sa capacité à produire des anions a été démontrée via notamment la mesure quantitative du rapport C₂ / C₂⁻pour une grande variété de conditions expérimentales. De même, une analyse spectroscopique quantitative a été menée sur les radicaux isotopomères ¹⁴NH₂ et ¹⁵NH₂. Cette étude valide certaines hypothèses faites dans de précédents travaux ayant permis de mesurer le rapport isotopique ¹⁵N/¹⁴N dans les comètes, pour la première fois à partir de raies d’émission de ces 2 isotopomères portant un groupe amino. La troisième et dernière partie de ce travail a consisté en le développement d’une nouvelle technique expérimentale appelée Broad-Band Dual Etalon Frequency COMb. Cette technique expérimentale basée sur l’utilisation de 2 cavités optiques de haute finesse devrait permettre d’obtenir une spectroscopie avec une résolution spectrale ultime non limitée par la source laser nanoseconde large bande mais par l’intervalle spectral libre des cavités de haute finesse utilisées. / The main objective of this PhD was to develop spectroscopic techniques using high finesse optical cavities. These were applied to the measurement of quantitative spectroscopic data for neutral, radical and ionic molecular species of astrophysical interest in the near infrared and visible spectral range. The first part was devoted to the measurement of the oscillator strength of high vibrational overtone bands of the cyanoacetylene (HC₃N) molecule with the Cavity Ring Down Spectroscopy (CRDS) technique. The second part was devoted to the study of the spectroscopy of transient neutral and ionic species. For that, an inductively radio frequency (RF) discharged has been coupled to the CRDS set up. The pertinence of this plasma to efficiently produce anions was demonstrated via the quantitative measurement of the C₂ / C₂⁻ ratio in a wide variety of conditions. A quantitative spectral analysis of the radical isotopomers ¹⁴NH₂ and ¹⁵NH₂ was also performed for the first time. This study provides experimental data that will allow to better constraint the ¹⁵N/¹⁴N isotopic ratio in comets through the emission lines of these two amino bearing isotopomers. The third and last part of the work was devoted to the development of a CRDS scheme called Broad Band Dual Etalon Frequency Comb Ring Down Spectroscopy. This new heterodyne technique, based on the use of the microsecond frequency combs generated by two high finesse optical cavities, should allow performing molecular spectroscopy with ultrahigh spectral resolution. First proof experiments were performed and perspective’s for improvement of the method is provided.

Optische Frequenzkämme (OFC) haben eine Vielzahl von Anwendungen in den angewandten Wissenschaften und der Grundlagenforschung, die auf der Bestimmung von absoluten Frequenzen und Frequenzdifferenzen beruhen, revolutioniert. Für letzteres wird nur die Stabilisierung des spektralen Abstandes der individuellen Kammlinien des OFCs benötigt, was erlaubt, den OFCG auf die Anwendung anzupassen und die Systemkomplexität zu reduzieren. Eine solche Anwendung ist der Quantentest der Universalität des Freien Falls (UFF) im Rahmen der Experimentserie QUANTUS. Mit diesem Test soll der Freie Fall zweier atomarer Spezies, Rubidium (Rb) und Kalium (K), in Mikrogravitation vergleichen werden. Das Ziel dieser Doktorarbeit war die Entwicklung eines hochkompakten, robusten, und weltraum-tauglichen diodenlaser-basierten OFCG mit einem modengekoppelten optischen Spektrum im Wellenlängenbereich um 780 nm. Es wurde ein diodenlaser-basierter OFCG entwickelt, der mit einer spektrale Bandbreite von mehr als 16 nm bei 20 dBc, einer optischen Leistung der Kammlinien > 650 nW (bei 20 dBc), einer Pulswiederholrate von 3.4 GHz, und einer RF-Linienbreite der frei-laufenden Pulswiederholrate < 10 kHz die Anforderungen übertrifft. Um ein Proof-of-Concept Demonstratormodul zu realisieren, wurde der diodenlaser-basierte OFCG in eine weltraum-taugliche Technologieplattform, die für die Anwendung in zukünftigen QUANTUS-Experimenten entwickelt wurde, hybrid-integriert. Der Nachweis einer ausreichend hohen RF-Stabilität des OFCGs wurde durch Stabilisierung der Pulswiederholrate auf eine externe RF Referenz erbracht. Dies ermöglichte eine stabilisierte Pulswiederholrate mit einer RF-Linienbreite von weniger als 1.4 Hz (auflösungsbegrenzt), was die die Anforderung übertrifft. Der entwickelte diodenlaser-basierten OFCG ist wichtiger Schritt in Richtung eines verbesserten Vergleichs des Freien Falls von Rb- und K-Quantengasen innerhalb der QUANTUS-Experimente in Mikrogravitation. / Optical frequency combs (OFC) have revolutionized various applications in applied and fundamental sciences that rely on the determination of absolute optical frequencies and frequency differences. The latter requires only stabilization of the spectral distance between the individual comb lines of the OFC, allowing to tailor and reduce system complexity of the OFC generator (OFCG). One such application is the quantum test of the universality of free fall within the QUANTUS experimental series. Within the test, the rate of free fall of two atomic species, Rb and K, in micro-gravity will be compared. The aim of this thesis was the development of a highly compact, robust, and space-suitable diode laser-based OFCG with a mode-locked optical spectrum in the wavelength range around 780 nm. A diode laser-based OFCG was developed, which exceeds the requirements with a spectral bandwidth > 16 nm at 20 dBc, a comb line optical power > 650 nW (at 20 dBc), a pulse repetition rate of 3.4 GHz, and an RF linewidth of the free-running pulse repetition rate < 10 kHz. To realize a proof-of-concept demonstrator module, the diode laser-based OFCG was hybrid-integrated into a space-suitable technology platform that has been developed for future QUANTUS experiments. Proof of sufficient RF stability of the OFCG was provided by stabilizing the pulse repetition rate to an external RF reference. This resulted in a stabilized pulse repetition rate with an RF linewidth smaller than 1.4 Hz (resolution limited), thus exceeding the requirement. The developed diode laser-based OFCG represents an important step towards an improved comparison of the rate of free fall of Rb and K quantum gases within the QUANTUS experiments in micro-gravity.

Every gas molecule has a unique absorption spectrum that can be captured using optical spectroscopy to identify an unknown sample's composition. Frequency combs systems can provide an extremely broad mid-infrared spectrum that is very useful for molecular detection. A degenerate optical parametric oscillator (OPO) was built to generate the down-converted and shifted frequency comb spectrum. This system utilizes an ultra-short pulse 1.56µm pump laser and a never before used orientation patterned gallium-phosphide crystal. Periodically polled lithium niobate (PPLN), Gallium Arsenide (GaAs) and Gallium Phosphide are all crystals used to accomplish this task. GaP, in comparison to PPLN, has (i) a larger nonlinear coefficient, (ii) much deeper infrared transparency, and (iii) smaller group dispersion – to allow for achieving broad spectral coverage. GaP also has a larger band gap than GaAs; therefore it can still be pumped with a standard telecom C-band laser. An octave-wide spanning frequency comb system was achieved and the characterization of the system is presented. This system is specifically designed to be compact and portable for initial experimental testing in the applications of medical breath analysis and combustion gas investigation.

Master of Science / Department of Physics / Brian R. Washburn / This thesis reports our attempt towards achieving a phase stabilized free-space nonlinear polarization rotation (NPR) mode locked erbium doped fiber laser frequency comb system. Optical frequency combs generated by mode-locked femtosecond fiber lasers are vital tools for ultra-precision frequency metrology and molecular spectroscopy. However, the comb bandwidth and average output power become the two main limiting elements in the application of femtosecond optical frequency combs. We have specifically investigated the free-space mode locking dynamics of erbium-doped fiber (EDF) mode-locked ultrafast lasers via nonlinear polarization rotation (NPR) in the normal dispersion regime. To do so, we built a passively mode-locked fiber laser based on NPR with a repetition rate of 89 MHz producing an octave-spanning spectrum due to supercontinuum (SC) generation in highly nonlinear fiber (HNLF). Most significantly, we have achieved highly stable self-starting NPR mode-locked femtosecond fiber laser based frequency comb which has been running mode locked for the past one year without any need to redo the mode locking. By using the free-space NPR comb scheme, we have not only shortened the cavity length, but also have obtained 5 to 10 times higher output power (more than 30 mW at central wavelength of 1570 nm) and much broader spectral comb bandwidth (about 54 nm) compared to conventional all-fiber cavity structure with less than 1 mW average output power and only 10 nm spectral bandwidth. The pulse output from the NPR comb is amplified through a 1 m long EDF, then compressed by a length of anomalous dispersion fiber to a near transform limited pulse duration. The amplified transform limited pulse, with an average power of 180 mW and pulse duration of 70 fs, is used to generate a supercontinuum of 140 mW. SC generation via propagation in HNLF is optimized for specific polling period and heating temperature of PPLN crystal for SHG around 1030 nm. At last, we will also discuss the attempt of second harmonic generation (SHG) by quasi phase matching in the periodically polled lithium niobate (PPLN) crystal due to nonlinear effect corresponding to different polling period and heating temperature.

In der vorliegenden Doktorarbeit wird ein Dauerstrich Optisch Parametrischer Oszillator (cw OPO) vorgestellt, der speziell für die hochauflösende Dopplerfreie Molekülspektroskopie und Metrologie entwickelt wurde. Der kontrollierte Zugang zu jeder beliebigen Wellenlänge im breiten Emissionsspektrum von OPOs wie auch das präzise Abstimmen seiner Ausgangsfrequenz über zu untersuchende molekulare und atomare Übergänge stellten lange Zeit Probleme dar, deren Lösung die Grundzielsetzung dieser Arbeit war. Das im Laufe dieser Arbeit entwickelte System hat diese Ziele vollständig erreicht, was durch verschiedene Messungen und Anwendungen demonstriert wurde. Zu diesem Zweck wurde ein neues OPO-Design mit einem Intracavity-Etalon entwickelt und aufgebaut, wobei der OPO auf dem Konzept eines einfach-resonanten cw OPOs mit resonanter Pumpwelle basiert. Die OPO-Ausgangsstrahlung zeigt sehr gute Langzeitstabilität und Spektraleigenschaften, welche durch direkte Frequenzvergleichsmessungen mit einem optischen Methan-Frequenzstandard im Infraroten bestimmt wurden. Eine Idler-Linienbreite von 12 kHz und ein Modensprung-freier Betrieb des OPOs über einen Zeitraum von einigen Tagen wurde beobachtet. Außerdem wurde gezeigt, dass ein OPO zu einer hochstabilen optischen Referenz phasengelockt und somit seine Frequenz sehr genau kontrolliert und durchgestimmt werden kann. Als erste erfolgreiche Anwendung eines OPOs in der Dopplerfreien Spektroskopie wurde ein Aufbau zur Frequenz-Modulationsspektroskopie in Methan realisiert. Weiterhin, wurde der entwickelte cw OPO mit einem femtosekunden optischen Frequenzkamm kombiniert, um eine neue Idee für eine kohärente Verbindung zwischen dem sichtbaren und dem infraroten Spektralbereich zu realisieren. Als erste Demonstration dieser Technologie wurde ein direkter absoluter Frequenzvergleich zwischen einem Jod-stabilisierten Laser bei 532 nm und einem Methan-stabilisierten Laser bei 3390 nm durchgeführt. / This thesis presents a continuous-wave optical parametric oscillator (cw OPO), specially developed for high-resolution Doppler-free molecular spectroscopy and metrology. The basic objective was to solve the long-standing problem of controlled access to any desired wavelength in the wide emission range of OPOs, including the ability to precisely tune the output frequency over the molecular and atomic transitions of interest. The system implemented during this work fully achieves these goals and its performance was demonstrated in various measurements and applications. For this aim, a new design for the OPO cavity with an intracavity etalon was implemented, extending the concept of a cw singly resonant OPO with resonated pump wave. The newly developed device demonstrates very good long-term stability and spectral properties, which were determined in direct beat frequency measurements with a methane infrared optical frequency standard. Thus, an idler radiation linewidth of 12 kHz and mode-hop-free operation of the OPO over several days were observed. Furthermore, it was shown that an OPO can be phase locked to a highly stable optical reference and thus much more precisely controlled and tuned. As the first successful application of OPOs in Doppler-free spectroscopy, a frequency modulation spectroscopy setup for detection of sub-Doppler resonances in methane was implemented. Furthermore, the developed cw OPO was integrated with a femtosecond optical frequency comb to realize a new concept for a coherent link between the visible and infrared spectral ranges. As a first demonstration of this technique, a direct absolute frequency comparison between an iodine stabilized laser at 532 nm and a methane stabilized laser at 3390 nm was performed.

Dans cette thèse, nous avons mis en oeuvre un système de mesure de distance absolue (Absolute Distance Measurement, ADM) de haute précision utilisant l'interférométrie à balayage de fréquence (Frequency Sweeping Interferometry, FSI). La technique FSI exige que la plage de réglage de fréquence du laser balayé soit mesurée avec une précision élevée, ce qui est difficile en raison de l'absence d'un moyen simple de mesurer la haute fréquence d'un laser en temps réel. Dans cette thèse, un peigne de fréquence a été utilisé comme règle de fréquence lumière pour mesurer la plage de réglage de la fréquence du laser à balayage. Un peigne de fréquence formé par un laser femtoseconde est constitué de millions de lignes de peigne régulièrement espacées, ce qui permet de le considérer comme une règle de fréquence de la lumière. La calibration de fréquence a été réalisée en filtrant le signal hétérodyne entre le laser à balayage et les lignes de peigne en utilisant un filtre passe-bande étroit. Cette approche nous permet de détecter le signal d'étalonnage lorsque la fréquence du laser à balayage est proche d'une ligne en peigne. Etant donné que l’intervalle de fréquence entre les lignes de peigne peut être mesuré avec précision ou activement verrouillé en phase par rapport à un oscillateur radiofréquence (RF) stable, la plage d ’ accord du laser à balayage peut être mesurée avec une grande précision. En particulier, chacun des deux pics d’étalonnage peut être utilisé dans le calcul de la distance, ce que nous appelons des «sous-mesures» en un seul balayage. Combinée au grand nombre de lignes de peigne, la moyenne des sous-mesures améliore considérablement la précision des mesures sans balayage multiple. Dans la thèse, la condition de détection et les caractéristiques du signal hétérodyne entre le laser à balayage et la ligne de peigne sont présentées. Une conception de filtre pour filtrer le signal hétérodyne est réalisée. Un travail de modélisation concernant l'effet du bruit de phase des lasers sur la distorsion d'enveloppe du pic d'étalonnage a été présenté. Des travaux expérimentaux basés sur les concepts de mesure ont été réalisés. Il montre que l'utilisation du schéma de mesure proposé peut considérablement améliorer la précision de la mesure de distance. Dans l’une des mesures, une précision de 30 nm pour une distance d’environ 0,8 m, correspondant à une incertitude relative de 37 ppm (part-perbillion) a été obtenue. Le résultat a été obtenu sur la base d'une méthode de traitement du signal de comptage de franges. La grande précision a été obtenue grâce au grand nombre de sous-mesures et à la stabilité des lignes de peigne régulièrement espacées. Nous avons constaté que la mesure de vibration de la cible peut également être effectuée en prenant avantage des lignes de peigne denses. Une sensibilité élevée, limitée à 1,7 nm efficace en bruit, de la mesure des vibrations a été atteinte. Ce résultat nous permet de surveiller la vibration de la cible, ce qui est un problème important de la technique FSI. / In this thesis, we implemented a high-precision absolute distance measurement (ADM) system using frequency sweeping interferometry (FSI). The FSI technique requires the frequency tuning range of the swept laser to be measured with high accuracy and precision, which is challenging due to the lack of an easy way to measure the high frequency of a laser in real time. In this thesis, a frequency comb has been used as the light frequency ruler for measuring the frequency tuning range of the sweeping laser. A frequency comb formed by a femtosecond laser consists millions of evenly spaced comb lines so that can be regarded as a light frequency ruler. The frequency calibration was realized by filtering the heterodyne signal between the sweeping laser and the comb lines using a narrow bandpass filter. This approach allows us to detect the calibration signal when the frequency of the sweeping laser is in the vicinity of a comb line. As the frequency interval between the comb lines space can be precisely measured or actively phase-locked against a stable radio-frequency (RF) oscillator, the tuning range of the sweeping laser could be measured with high accuracy. Especially, each two calibration peaks can be used in the calculation of distance, which we call sub-measurements in a single sweeping. Combined with the large number of the comb lines, averaging of the sub-measurements improves greatly the measurement precision without multiple sweeping. In the thesis, the condition of detecting and the characteristics of the heterodyne signal between the sweeping laser and the comb line are presented. A filter design for filtering the heterodyne signal is performed. A modeling work concerning the effect of the phase noise of lasers on the envelope distortion of the calibration peak has been presented. Experimental works based on the measurement concepts have been carried out. It shows that using the proposed measurement scheme can greatly improve the distance measurement precision. In one of the measurements, a precision of 30 nm for a distance around 0.8 m, corresponding to 37 ppb (part-per-billion) relative uncertainty has been achieved. The result was obtained based on a fringe counting signal processing method. The high precision was obtained thanks to the large number of sub-measurements and the stability of the evenly spaced comb lines. We have found that vibration measurement of the target can be also performed taking the advantage of the dense comb lines. A high sensitivity, limited by 1.7 nm noise RMS, of vibration measurement has been achieved. This result allows us to monitor the vibration of the target, which is an important issue of FSI technique.

An optical cavity comprises a set of mirrors between which light can be reflected a number of times. The selectivity and stability of optical cavities make them extremely useful as frequency references or discri­mi­nators. With light coupled into the cavity, a sample placed inside a cavity will experience a significantly increased interaction length. Hence, they can be used also as amplifiers for sensing purposes. In the field of laser spectroscopy, some of the most sensitive techniques are therefore built upon optical cavities. In this work optical cavities are used to measure properties of gas samples, i.e. absorption, dispersion, and refractivity, with unprecedented precision. The most sensitive detection technique of all, Doppler-broadened noise-immune cavity enhanced optical heterodyne molecular spectrometry (Db NICE-OHMS), has in this work been developed to an ultra-sensitive spectroscopic technique with unprecedented detection sensitivity. By identifying limiting factors, realizing new experimental setups, and deter­mining optimal detection conditions, the sensitivity of the technique has been improved several orders of magnitude, from 8 × 10-11 to 9 × 10-14 cm-1. The pressure interval in which NICE-OHMS can be applied has been extended by deri­vation and verification of dispersions equations for so-called Dicke narrowing and speed dependent broadening effects. The theoretical description of NICE-OHMS has been expanded through the development of a formalism that can be applied to the situations when the cavity absorption cannot be considered to be small, which has expanded the dynamic range of the technique. In order to enable analysis of a large number of molecules at their most sensitive transitions (mainly their funda­mental CH vibrational transitions) NICE-OHMS instrumentation has also been developed for measurements in the mid-infrared (MIR) region. While it has been difficult to realize this in the past due to a lack of optical modulators in the MIR range, the system has been based on an optical para­metric oscillator, which can be modulated in the near-infrared (NIR) range. As the index of refraction can be related to density, it is possible to retrieve gas density from measurements of the index of refraction. Two such instru­men­tations have been realized. The first one is based on a laser locked to a measure­ment cavity whose frequency is measured by compassion with an optical frequency comb. The second one is based on two lasers locked to a dual-cavity (i.e. one reference and one measurement cavity). By these methods changes in gas density down to 1 × 10-9 kg/m3 can be detected. All instrumentations presented in this work have pushed forward the limits of what previously has been considered measurable. The knowledge acquired will be of great use for future ultrasensitive cavity-based detection methods.

Submitted by Irene Nascimento (irene.kessia@ufpe.br) on 2017-04-25T19:25:57Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) ACGW_Dissertação.pdf: 2393143 bytes, checksum: 1dc4bede5ad081bffd11e546c86a753a (MD5) / Made available in DSpace on 2017-04-25T19:25:58Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) ACGW_Dissertação.pdf: 2393143 bytes, checksum: 1dc4bede5ad081bffd11e546c86a753a (MD5) Previous issue date: 2016-07-26 / CNPQ / Apresentamos um estudo da interação de um laser de femtosegundos de Ti:safira, com alta taxa de repetição, com vapor atômico de Rubídio. Trabalhamos no regime de acumulação coerente, em que o tempo de relaxação dos átomos do meio é maior que o tempo entre os pulsos. Para sondar a excitação produzida pelo laser pulsado sobre os vários grupos de velocidades atômicos usamos um laser contínuo de diodo. Estudamos a transmissão do laser de diodo em função de sua frequência, na presença e na ausência do laser pulsado, quando ambos os lasers estão sintonizados na transição 5S1=2 !5P3=2. Os resultados experimentais mostram que a transmissão do laser de diodo aumenta ou diminui dependendo do modo do pente de frequências que está interagindo com o grupo de átomos. Descrevemos os resultados experimentais trabalhando no domínio da frequência e modelando o meio atômico por um conjunto de dois sistemas de três níveis tipo L e um sistema de dois níveis, todos independentes entre si, interagindo com os modos do pente de frequência. Assim, a modelagem teórica considera os vários níveis hiperfinos e discrimina a interação com a transição cíclica do efeito de bombeio ótico, apresentado um resultado em concordância com os dados experimentais obtidos. O experimento foi realizado sem travar a taxa de repetição nem a frequência de off-set do laser de femtosegundos. / We present a study of the interaction of a high repetition rate femtosecond Ti:sapphire laser with a vapor atomic rubidium. We work on coherent accumulation regime, in which the atomic relaxation time is greater than the time interval between pulses. The action of the ultrashortpulsed laser over different atomic velocity groups is probed by a diode laser. We studied the transmission of the diode laser as a function of frequency, with and without the presence of the ultrashort-pulsed laser, when both lasers are tuned to 5S1=2!5P3=2 transition. The experimental results show that the diode laser transmission increases or decreases depending on the mode of the frequency comb that interacts with different velocity groups. The experimental results are described in the frequency domain and the atomic system is modeled by a set of two threelevel L systems and a system of two-level all independent and interacting with the frequency comb modes. The theoretical treatment considers the hyperfine transitions and distinguishes the interaction between the cyclic transitions from optical pumping effect, showing results in good agreement with the experimental data. The experiment is performed without lock repetition rate or the off-set frequency of the femtosecond Ti:sapphire laser.