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Semiconductor disk laser-based frequency combs
Cette thèse présente une étude des premiers peignes de fréquences auto-référencés basés sur des lasers à disques semi-conducteurs (SDLs) à modes verrouillés en phase.
L’avènement des peignes de fréquences stabilisés basés sur des lasers à impulsions ultra-courtes a permis une avancée significative et de nombreuses applications dans divers domaines de la physique, de la spectroscopie et de la métrologie. Les peignes de fréquences optiques peuvent être utilisés comme une règle de mesure dans le domaine des fréquences fournissant un lien direct et cohérent entre les fréquences optiques et micro-ondes. Bien que les peignes de fréquences optiques aient révolutionné de nombreux domaines scientifiques, ils n'ont pas encore pénétré les marchés à grande échelle. Pour cela, les sources laser doivent être améliorées, en ciblant une haute fiabilité tout en gardant la source compacte et économiquement attractive.
Les SDLs à impulsions ultra-courtes, également appelés lasers à cavité verticale externe à émission de surface ou VECSELs (de l’anglais vertical external-cavity surface-emitting lasers), constituent une source laser très prometteuse à cet égard de par la technologie des semi-conducteurs qui permet une production de masse à faible coût. Ils rendent possibles des configurations de lasers très compactes et présentent une grande flexibilité dans leur longueur d'onde d’émission grâce à l'ingénierie de bandes. En outre, ils ne souffrent pas d'instabilités de mode déclenché (Q-switching) et un fonctionnement stable en verrouillage de mode a été démontré à des taux de répétition allant de 100 MHz à 100 GHz. Les peignes de fréquences à taux de répétition élevés ont une puissance accrue par mode, ce qui est bénéfique pour des applications telles que l'astronomie ou la génération de signaux micro-ondes à faible bruit.
La stabilisation des deux degrés de liberté du laser à verrouillage de mode, la fréquence de répétition et la fréquence du décalage de phase entre la porteuse et l'enveloppe (carrier-envelope offset en anglais, CEO) est nécessaire pour la plupart des applications. Cependant, la détection de la fréquence CEO est difficile et aucune stabilisation n'avait été obtenue auparavant pour un laser à semi-conducteur à impulsions ultra-courtes. La détection de la fréquence CEO est généralement effectuée à l'aide d'une méthode d'auto-référencement qui nécessite un spectre cohérent couvrant une octave de fréquence. Ce dernier peut être généré en utilisant des fibres optiques hautement non linéaires telles que des fibres à cristaux photoniques (photonic crystal fibers en anglais, PCFs). Afin de maintenir la cohérence durant le processus d'élargissement spectral, des impulsions ultra-courtes dans le domaine des femtosecondes (typiquement <200 fs) avec une puissance de crête de l’ordre du kilowatt sont nécessaires. Cependant, même si des puissances maximales allant jusqu'à 6.3 kW et des durées d'impulsion allant jusqu'à 96 fs ont été démontrées pour des SDLs, la combinaison des deux n'a pas encore été atteinte. Par conséquent, la génération d’un spectre d'une octave utilisant une PCF directement à partir de la sortie du laser n'a pas pu être démontrée jusqu'à présent et des étapes supplémentaires d'amplification et de compression des impulsions sont nécessaires.
Dans ce travail, un amplificateur à fibre a été développé permettant la première démonstration de stabilisation de la fréquence CEO d'un SDL à impulsions ultra-courtes. Les impulsions amplifiées ont été comprimées temporellement et couplées dans une PCF pour la génération cohérente d’un spectre supercontinuum couvrant une octave. Une technique d'auto-référencement permet la détection et la stabilisation de la fréquence CEO via une modulation de la puissance de la diode de pompe du laser. Ce résultat démontre la faisabilité d'un peigne de fréquences basé sur la technologie SDL et constitue une étape importante dans le développement des peignes de fréquences compacts.
Finalement, une conversion de longueur d'onde à l’aide d’un oscillateur paramétrique optique a été étudiée, permettant de surmonter les limites actuelles de la longueur d'onde d'émission atteignable avec des SDL à impulsions ultra-courtes. L'émission dans l'infrarouge moyen est très intéressante car un grand nombre de molécules ont de fortes transitions rotationnelles-vibrationnelles dans cette gamme spectrale et le développement de peignes de fréquences dans l'infrarouge moyen permet l’accès à des méthodes de détection de spectroscopie moléculaire simples, rapides et très sensibles., This thesis studies the first self-referenced frequency combs based on modelocked semiconductor disk lasers (SDLs).
The generation of stabilized frequency combs based on ultrafast lasers has been a significant breakthrough for many applications in various fields of physics, spectroscopy and metrology. Optical frequency combs can serve as a frequency ruler that provides a direct and phase-coherent link between optical and microwave frequencies. Despite the fact that optical frequency combs revolutionized numerous scientific areas, so far, they have not entered large-scale markets. For this, comb laser sources have to be improved, targeting high reliability, while keeping the source compact and cost-efficient.
Ultrafast SDLs, also referred to as vertical external-cavity surface-emitting lasers (VECSELs), are a very promising technology for this purpose as they are based on the semiconductor technology, allowing for low-cost wafer-scale mass-production. They enable very compact laser setups and have large emission wavelengths flexibility inherited from the band-gap engineering. In addition, they do not suffer from Q-switching instabilities and stable fundamental modelocking was demonstrated at repetition rates ranging from 100 MHz to 100 GHz. High repetition rate frequency combs have an increased power per comb line that is beneficial for applications such as astronomy or low-noise microwave generation.
The stabilization of the two degrees of freedom of the modelocked laser, the repetition frequency and the carrier-envelope offset (CEO) frequency is required for most comb applications. However, the detection of the CEO frequency is challenging and no stabilization was achieved before for any ultrafast semiconductor laser. The CEO detection is usually done using a self-referencing scheme that requires a coherent octave-spanning spectrum, which can be generated using highly nonlinear fibers such as photonic crystal fibers (PCFs). In order to maintain the coherence in the spectral broadening process, ultrashort femtosecond pulses (typically <200 fs) with kilowatt peak power are required. However, even though peak powers up to 6.3 kW and pulse durations down to 96 fs have been demonstrated in SDLs, the combination of both has still not been reached. Therefore, the octave-spanning spectrum generation in a PCF directly from the output of the laser could not be demonstrated until now and additional amplification and compression stages are necessary.
In this work, an efficient fiber amplifier has been developed, which led to the first demonstration of the CEO frequency stabilization of an ultrafast SDL. The amplified pulses were temporally compressed and sent to a commercially available PCF for the coherent octave-spanning supercontinuum spectrum generation. A self-referencing scheme enabled the CEO detection and stabilization via a modulation of the pump power. This result demonstrates the feasibility of a frequency comb based on the SDL technology and constitutes an important step in the further development of compact frequency combs.
Finally, wavelength conversion in an optical parametric oscillator is studied to overcome the current limitations in the emission wavelength of ultrafast SDLs. Emission in the mid-infrared is highly attractive since a large number of molecules have strong rotational-vibrational transitions in this spectral range and the development of mid-infrared frequency combs enables simple, fast and highly sensitive molecular spectroscopy sensing methods.
Carrier-envelope offset frequency stabilization of a gigahertz semiconductor disk laser
Optical frequency combs based on ultrafast lasers have enabled numerous scientific breakthroughs. However, their use for commercial applications is limited by the complexity and cost of femtosecond laser technology. Ultrafast semiconductor lasers might change this issue as they can be mass produced in a cost-efficient way while providing large spectral coverage from a single technology. However, it has not been proven to date if ultrafast semiconductor lasers are suitable for stabilization of their carrier-envelope offset (CEO) frequency. Here we present what we believe to be the first CEO frequency stabilization of an ultrafast semiconductor disk laser (SDL). The optically pumped SDL is passively modelocked by a semiconductor saturable absorber mirror. It operates at a repetition rate of 1.8 GHz and a center wavelength of 1034 nm. The 273 fs pulses of the oscillator are amplified to an average power level of 6 W and temporally compressed down to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a photonic crystal fiber. The CEO frequency is detected in a standard ƒ–to–2ƒ interferometer and phase locked to an external reference by feedback applied to the current of the SDL pump diode. This proof-of-principle demonstrates that ultrafast SDLs are suitable for CEO stabilization and constitutes a key step for further developments of this comb technology expected in the coming years.
Frequency comb metrology with an optical parametric oscillator
We report on the first demonstration of absolute frequency comb metrology with an optical parametric oscillator (OPO) frequency comb. The synchronously-pumped OPO operated in the 1.5-μm spectral region and was referenced to an H-maser atomic clock. Using different techniques, we thoroughly characterized the frequency noise power spectral density (PSD) of the repetition rate frep, of the carrier-envelope offset frequency fCEO, and of an optical comb line νN. The comb mode optical linewidth at 1557 nm was determined to be ~70 kHz for an observation time of 1 s from the measured frequency noise PSD, and was limited by the stability of the microwave frequency standard available for the stabilization of the comb repetition rate. We achieved a tight lock of the carrier envelope offset frequency with only ~300 mrad residual integrated phase noise, which makes its contribution to the optical linewidth negligible. The OPO comb was used to measure the absolute optical frequency of a near-infrared laser whose second-harmonic component was locked to the F = 2→3 transition of the 87Rb D2 line at 780 nm, leading to a measured transition frequency of νRb = 384,228,115,346 ± 16 kHz. We performed the same measurement with a commercial fiber-laser comb operating in the 1.5-μm region. Both the OPO comb and the commercial fiber comb achieved similar performance. The measurement accuracy was limited by interferometric noise in the fibered setup of the Rb-stabilized laser.
High-power amplification of a femtosecond vertical external-cavity surface-emitting laser in an Yb: YAG waveguide
We present the amplification of a mode-locked vertical external-cavity surfaceemitting laser (VECSEL) using an Yb:YAG crystalline waveguide as gain medium. The VECSEL seed laser operates at a center wavelength of 1030 nm and generates 300-fs pulses at a repetition rate of 1.77 GHz. An average seed power of 60 mW was launched onto a 8.3 mm long fs-laser written Yb:YAG waveguide pumped by 7.7 W from a 969-nm continuous-wave VECSEL. The amplifier achieves an average output power of up to 2.9 W, corresponding to an amplification factor of 17 dB. Due to gain narrowing, the pulse duration increases to 629 fs. Our results show that crystalline waveguides are a promising technique for the realization of compact multi-watt ultrafast amplifier systems.
Ultrafast optical parametric oscillator pumped by a vertical external-cavity surface-emitting laser (VECSEL)
We report the first optical parametric oscillator synchronously pumped by a SESAM modelocked vertical external-cavity surface-emitting laser (VECSEL). As a nonlinear medium, we use a periodically poled MgO:PPLN crystal. The VECSEL operates at a wavelength of 982 nm and a repetition rate of 198 MHz. The pump radiation is converted to signal and idler wavelengths tunable in the ranges of 1.4-1.8 μm and 2.2-3.5 μm, respectively, simply by a change of the poling period and crystal temperature. The signal pulses have a duration between 2 ps to 4 ps and an average output power up to 100 mW.
Carrier-envelope offset frequency stabilization of a thin-disk laser oscillator operating in the strongly self-phase modulation broadened regime
We demonstrate the carrier-envelope offset (CEO) frequency stabilization of a Kerr lens mode-locked Yb:Lu2O3 thin-disk laser oscillator operating in the strongly self-phase modulation (SPM) broadened regime. This novel approach allows overcoming the intrinsic gain bandwidth limit and is suited to support frequency combs from sub-100-fs pulse trains with very high output power. In this work, strong intra-oscillator SPM in the Kerr medium enables the optical spectrum of the oscillating pulse to exceed the bandwidth of the gain material Yb: Lu2O3 by a factor of two. This results in the direct generation of 50-fs pulses without the need for external pulse compression. The oscillator delivers an average power of 4.4 W at a repetition rate of 61 MHz. We investigated the cavity dynamics in this regime by characterizing the transfer function of the laser output power for pump power modulation, both in continuous-wave and mode-locked operations. The cavity dynamics in mode-locked operation limit the CEO modulation bandwidth to ~10 kHz. This value is sufficient to achieve a tight phase-lock of the CEO beat via active feedback to the pump current and yields a residual in-loop integrated CEO phase noise of 197 mrad integrated from 1 Hz to 1 MHz.
First investigation of the noise and modulation properties of the carrier-envelope offset in a modelocked semiconductor laser
We present the first characterization of the noise properties and modulation response of the carrier-envelope offset (CEO) frequency in a semiconductor modelocked laser. The CEO beat of an optically-pumped vertical external-cavity surface-emitting laser (VECSEL) at 1030 nm was characterized without standard ƒ-to-2ƒ interferometry. Instead, we used an appropriate combination of signals obtained from the modelocked oscillator and an auxiliary continuous-wave laser to extract information about the CEO signal. The estimated linewidth of the free-running CEO beat is approximately 1.5 MHz at 1-s observation time, and the feedback bandwidth to enable a tight CEO phase lock to be achieved in a future stabilization loop is in the order of 300 kHz. We also characterized the amplitude and phase of the pump current to CEO-frequency transfer function, which showed a 3-dB bandwidth of ∼300 kHz for the CEO frequency modulation. This fulfills the estimated required bandwidth and indicates that the first self-referenced phase-stabilization of a modelocked semiconductor laser should be feasible in the near future.