Südmeyer, Thomas

2017, Gürel, Kutan, Südmeyer, Thomas

Les lasers ont révolutionné le domaine de la photonique en tant que source de lumière intense et cohérente capable de réaliser de nombreuses fonctionnalités. Ils sont largement utilisés dans les applications industrielles et médicales, ainsi que pour la recherche scientifique. Deux domaines d'application importants sont la métrologie optique et la spectroscopie. Les lasers à cascade quantique (QCL pour « quantum cascade laser ») couvrent une vaste gamme spectrale dans l'infrarouge moyen où la plupart des molécules présentent de fortes raies d'absorption caractéristiques, agissant telle une empreinte digitale des molécules. Les QCL sont particulièrement adaptés à la spectroscopie moléculaire de précision, cependant leur gamme d'accordabilité en longueur d'onde est limitée. De meilleurs mécanismes permettant de moduler leur fréquence permettraient d'améliorer de nombreuses applications. D'autre part, les peignes de fréquences optiques produits par des lasers à verrouillage de mode couvrent une large bande spectrale et s'avèrent être des outils idéaux pour la métrologie optique et la spectroscopie. Cependant, de nombreuses sources de peignes optiques utilisées de nos jours présentent certains inconvénients, notamment au niveau de leur complexité, coût et taille. Cette thèse présente de nouvelles directions pour améliorer d'une part les QCLs et d'autre part les peignes de fréquences optiques pour la spectroscopie et la métrologie.
La première partie de ce travail est consacrée à un peigne de fréquences optique généré par un laser titane:saphir. Les solutions habituelles de pompage dans le bleu-vert pour les lasers titane:saphir sont volumineuses, coûteuses, peu efficaces et contraignantes en termes de maintenance. De plus, leur puissance optique ne peut généralement pas être directement modulée, ce qui empêche de mettre en œuvre la méthode la plus simple pour l'auto-référencement d'un peigne, c'est-à-dire la stabilisation du décalage de phase entre la porteuse et l'enveloppe des impulsions (carrier-envelope offset en anglais, CEO). Dans cette thèse, l'utilisation de nouvelles diodes lasers émettant dans le vert comme sources de pompage est démontrée. Ce travail a donné lieu à des rendements optiques records pour les lasers titane:saphir pompés par diode. Un rendement de plus de 20% a ainsi été obtenu en régime continu avec une puissance de sortie de 650 mW, et supérieur à 15% en fonctionnement en verrouillage de mode, avec une puissance optique moyenne de 450 mW. Les diodes de pompe vertes présentent de moins bonnes caractéristiques spectrales et spatiales par rapport aux autres sources de pompage utilisées habituellement dans les peignes de fréquence titane:saphir. Dans ce travail, ce défi a pu être surmonté, menant à la première détection et stabilisation de la fréquence CEO d'un laser titane:saphir pompé par diode. Ce travail démontre le haut potentiel du pompage direct par diodes qui peut mener à une nouvelle génération de lasers titane:saphir économiques et compacts.
Dans la deuxième partie de ce travail, une nouvelle méthode de stabilisation est démontrée pour les peignes de fréquence générés par des lasers à fibre. De nos jours, les lasers à fibre représentent la majorité des peignes de fréquence commerciaux, car ils sont compacts, faciles à utiliser et polyvalents. Cependant, la méthode traditionnelle de stabilisation de la fréquence CEO par modulation du laser de pompe est fortement limitée dans sa bande passante par la longue durée de vie de l’état excité du gain. Par conséquent, des modulateurs supplémentaires sont nécessaires pour réaliser des peignes à faible bruit. Ici, la première stabilisation de la fréquence CEO d'un laser à fibre est démontrée par modulation opto-optique d'un composant semiconducteur. Par rapport à la modulation standard du courant du laser de pompe, la bande passante de modulation est améliorée d'un facteur supérieur à 60, ce qui permet d'atteindre un régime à bas bruit pour les peignes de fréquence basés sur des lasers à fibre.
Dans l'étude finale de cette thèse, une nouvelle génération de QCLs est évaluée, qui présente des propriétés d'accordabilité en fréquence améliorées. Les mécanismes existants de balayage en longueur d'onde utilisent d'une part la température du laser et d'autre part le courant d'injection. Le réglage par la température est lent, tandis que celui par le courant d'injection influence aussi la puissance optique du laser. Ici les propriétés d'accord de fréquence et de modulation d'un QCL par l’intermédiaire d'un élément résistif intégré à proximité de la zone active du laser sont étudiées. Ce nouvel élément permet un contrôle rapide de la fréquence du laser avec des effets minimaux sur sa puissance de sortie. Dans une expérience de spectroscopie simple, le bénéfice de ce nouvel actuateur est démontré. Abstract: Lasers have revolutionized the photonics field as a coherent intense light source, capable of performing many tasks. They are widely used in industrial and medical applications, as well as for scientific research. Two important areas of applications are frequency metrology and spectroscopy. Quantum cascade lasers (QCL) cover a wide spectral region in the mid-infrared where most molecules exhibit strong characteristic absorption lines, the so-called molecular fingerprint. QCLs are highly suited for precise molecular spectroscopy, but in their standard configuration, their tuning range is limited. Better frequency tuning mechanisms are required, which can improve many applications. Frequency combs from mode-locked lasers cover a large optical bandwidth and are proven to be ideal tools for frequency metrology. However, many commonly used comb sources have strong drawbacks, in particular with respect to complexity, cost and size. This thesis presents novel directions to improve both QCLs and frequency combs for spectroscopy and metrology.
In the first part of this work, I focus on simple frequency combs generated by Ti:Sapphire lasers. The standard blue-green pumping solutions for Ti:Sapphire lasers are bulky, expensive, have low efficiency, and require a high degree of maintance. Moreover, they usually lack direct power modulation, which is an advantage for simple implementation of carrier-envelope offset (CEO) stabilization. In this thesis, I investigated the suitability of newly-emerged green laser diodes as pump sources. This work resulted in record-high optical-to-optical efficiencies for diode-pumped Ti:Sapphire lasers. More than 20% with 650 mW of output power for continuous wave, and 15% with 450 mW of output power for mode-locked operation of the laser have been obtained. Green pump diodes exhibit poor spectral and spatial properties compared to other previously used pump sources for Ti:Sapphire frequency combs. In this work, I showed that this challenge can be overcome and demonstrated the first CEO frequency detection and stabilization of a diode-pumped mode-locked Ti:Sapphire laser. Our work indicates the high potential of direct diode-pumping, and might lead to a new generation of cost-efficient and compact Ti:Sapphire lasers.
In the second part of this work, I investigate a novel comb stabilization method for fiber lasers. Today, fiber lasers constitute the majority of commercial frequency combs, because they are compact, easy-to-use and versatile. However, the traditional method of pump modulation is severely limited in feedback bandwidth due to the long gain lifetime, and low-noise sources require additional modulators. Here I demonstrate the first CEO frequency stabilization of an ultrafast fiber laser using opto-optical modulation of a semiconductor chip. Compared to standard pump modulation, the modulation bandwidth is improved by at least a factor of 60, enabling low-noise operation of the fiber laser frequency comb.
In the final study of this thesis, I evaluate a new generation of QCLs with improved frequency tuning properties. Existing frequency tuning mechanisms involve laser mount temperature and laser injection current tuning. Temperature tuning suffers from lack of speed and injection current tuning affects the optical output power due to the cross-talk. Here, I study the frequency tuning and modulation properties of a QCL with an integrated resistive heater placed in close proximity to the active region of the laser. This new heater enables fast actuation of the laser frequency with minimal effects on the laser output power. In a simple spectroscopy experiment, I clearly show that this new technique is a highly promising frequency actuator.

, Modsching, Norbert, Paradis, Clément, Brochard, Pierre, Jornod, Nayara, Gürel, Kutan, Kränkel, Christian, Schilt, Stephane, Wittwer, Valentin, Südmeyer, Thomas

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.

, Hakobyan, Sargis, Wittwer, Valentin, Brochard, Pierre, Gürel, Kutan, Schilt, Stephane, Mayer, Aline S, Keller, Ursula, Südmeyer, Thomas

We demonstrate the first self-referenced full stabilization of a diode-pumped solid-state laser (DPSSL) frequency comb with a GHz repetition rate. The Yb:CALGO DPSSL delivers an average output power of up to 2.1 W with a typical pulse duration of 96 fs and a center wavelength of 1055 nm. A carrier-envelope offset (CEO) beat with a signal-to-noise ratio of 40 dB (in 10-kHz resolution bandwidth) is detected after supercontinuum generation and ƒ-to-2ƒ interferometry directly from the output of the oscillator, without any external amplification or pulse compression. The repetition rate is stabilized to a reference synthesizer with a residual integrated timing jitter of 249 fs [10 Hz – 1 MHz] and a relative frequency stability of 10⁻¹²/s. The CEO frequency is phase-locked to an external reference via pump current feedback using home-built modulation electronics. It achieves a loop bandwidth of ∼150 kHz, which results in a tight CEO lock with a residual integrated phase noise of 680 mrad [1 Hz – 1 MHz]. We present a detailed characterization of the GHz frequency comb that combines a noise analysis of the repetition rate ƒ_rep, of the CEO frequency ƒ_CEO, and of an optical comb line at 1030 nm obtained from a virtual beat with a narrow-linewidth laser at 1557 nm using a transfer oscillator. An optical comb linewidth of about 800 kHz is assessed at 1-s observation time, for which the dominant noise sources of ƒ_rep and ƒ_CEO are identified.

, Gürel, Kutan, Schilt, Stephane, Südmeyer, Thomas

We present the first carrier-envelope offset (CEO) frequency stabilization of a fiber laser by cross gain modulation. The Yb-doped fiber laser is mode-locked by nonlinear polarization evolution and emits 32-nm wide dissipative solitons at a repetition rate of 125 MHz with 150 mW of average output power. A continuous wave laser signal at a wavelength of 1025 nm is used as an intracavity power modulator. A low power of only 200 μW of modulator signal is coupled into the fiber laser and amplified in the gain segment. This signal cross modulates the laser gain, achieving 40 times larger modulation bandwidth of the intracavity laser power than with standard pump-current control. A tight CEO lock is demonstrated with 361 mrad of residual integrated phase noise (from 1 Hz to 1 MHz). The method allows easy implementation in many existing fiber laser frequency combs based on various saturable absorbers and fiber configurations.

, Gürel, Kutan, Wittwer, Valentin, Hakobyan, Sargis, Schilt, Stephane, Südmeyer, Thomas

We demonstrate the first diode-pumped Ti:sapphire laser frequency comb. It is pumped by two green laser diodes with a total pump power of 3 W. The Ti:sapphire laser generates 250 mW of average output power in 61-fs pulses at a repetition rate of 216 MHz. We generated an octave-spanning supercontinuum spectrum in a photonic-crystal fiber and detected the carrier envelope offset (CEO) frequency in a standard ƒ-to-2ƒ interferometer setup. We stabilized the CEO-frequency through direct current modulation of one of the green pump diodes with a feedback bandwidth of 55 kHz limited by the pump diode driver used in this experiment. We achieved a reduction of the CEO phase noise power spectral density by 140 dB at 1 Hz offset frequency. An advantage of diode pumping is the ability for high-bandwidth modulation of the pump power via direct current modulation. After this experiment, we studied the modulation capabilities and noise properties of green pump laser diodes with improved driver electronics. The current-to-output-power modulation transfer function shows a bandwidth larger than 1 MHz, which should be sufficient to fully exploit the modulation bandwidth of the Ti:sapphire gain for CEO stabilization in future experiments.

, Jornod, Nayara, Gürel, Kutan, Wittwer, Valentin, Brochard, Pierre, Hakobyan, Sargis, Schilt, Stephane, Waldburger, Dominik, Keller, Ursula, Südmeyer, Thomas

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.

, Gürel, Kutan, Hakobyan, Sargis, Wittwer, Valentin Johannes, Schilt, Stephane, Südmeyer, Thomas

In this paper, we review the current state and discuss new developments in opto-optical modulation (OOM) of semiconductor elements for frequency comb self-referenced stabilization of ultrafast lasers. This method has been successfully used for carrier-envelope offset (CEO) frequency stabilization of diodepumped solid-state lasers operating in 1-μm and 1.5-μm regimes, providing high feedback bandwidth and resulting in low noise performance. We compare the achieved results for Er- and Yb-based laser materials and in different regimes of repetition rates up to 1 GHz. In addition, we present the first semiconductor OOM for CEO stabilization in an ultrafast fiber laser. Moreover, we discuss requirements and design guidelines for OOM chips. In most demonstrations, semiconductor saturable absorber mirrors have been used for OOM, which in parallel were also responsible for pulse formation. By separating the OOM functionality from the pulse formation, we expect that it will enable low-noise CEO stabilization in other types of ultrafast lasers, such as, for example, high-power Kerr-lens mode-locked thin disk lasers.

, Hakobyan, Sargis, Wittwer, Valentin, Gürel, Kutan, Mayer, Aline S, Schilt, Stephane, Südmeyer, Thomas

We demonstrate, to the best of our knowledge, the first carrier-envelope offset (CEO) frequency stabilization of a GHz femtosecond laser based on opto-optical modulation (OOM) of a semiconductor saturable absorber mirror (SESAM). The 1.05-GHz laser is based on a Yb:CALGO gain crystal and emits sub-100-fs pulses with 2.1-W average power at a center wavelength of 1055 nm. The SESAM plays two key roles: it starts and stabilizes the mode-locking operation and is simultaneously used as an actuator to control the CEO frequency. This second functionality is implemented by pumping the SESAM with a continuous-wave 980-nm laser diode in order to slightly modify its nonlinear reflectivity. We use the standard ƒ-to-2ƒ method for detection of the CEO frequency, which is stabilized by applying a feedback signal to the current of the SESAM pump diode. We compare the SESAM-OOM stabilization with the traditional method of gain modulation via control of the pump power of the Yb:CALGO gain crystal. While the bandwidth for gain modulation is intrinsically limited to ∼250  kHz by the laser cavity dynamics, we show that the OOM provides a feedback bandwidth above 500 kHz. Hence, we were able to obtain a residual integrated phase noise of 430 mrad for the stabilized CEO beat, which represents an improvement of more than 30% compared to gain modulation stabilization.

, Gürel, Kutan, Schilt, Stephane, Bismuto, Alfredo, Bidaux, Yves, Tardy, Camille, Blaser, Stéphane, Gresch, Tobias, Südmeyer, Thomas

We present a detailed experimental investigation of the use of a novel actuator for frequency tuning and modulation in a quantum cascade laser (QCL) based on a resistive integrated heater (IH) placed close to the active region. This new actuator is attractive for molecular spectroscopy applications as it enables fast tuning of the QCL wavelength with a minor influence on the optical output power, and is electrically-controlled. Using a spectroscopic setup comprising a low-pressure gas cell, we measured the tuning and modulation properties of a QCL emitting at 7.8 μm as a function of the active region and IH currents. We show that a current step applied to the IH enables the laser frequency to be switched by 500 MHz in a few milliseconds, as fast as for a step of the current in the active region, and limited by heat dissipation towards the laser sub-mount. The QCL optical frequency can be modulated up to ∼100 kHz with the IH current, which is one order of magnitude slower than for the QCL current, but sufficient for many spectroscopic applications. We discuss the experimental results using a thermal model of the heat transfer in terms of cascaded low-pass filters and extract the respective cut-off frequencies. Finally, we present a proof-of-principle experiment of wavelength modulation spectroscopy of a N₂O transition performed with a modulation of the IH current and show some potential benefits in comparison to QCL current modulation, which results from the reduced associated amplitude modulation.

, Gürel, Kutan, Wittwer, Valentin, Hoffmann, Martin, Saraceno, Clara J, Hakobyan, Sargis, Resan, B, Rohrbacher, A, Weingarten, K, Schilt, Stephane, Südmeyer, Thomas

We investigate power-scaling of green-diode-pumped Ti:Sapphire lasers in continuous-wave (CW) and mode-locked operation. In a first configuration with a total pump power of up to 2 W incident onto the crystal, we achieved a CW power of up to 440 mW and self-starting mode-locking with up to 200 mW average power in 68-fs pulses using semiconductor saturable absorber mirror (SESAM) as saturable absorber. In a second configuration with up to 3 W of pump power incident onto the crystal, we achieved up to 650 mW in CW operation and up to 450 mW in 58-fs pulses using Kerr-lens mode-locking (KLM). The shortest pulse duration was 39 fs, which was achieved at 350 mW average power using KLM. The mode-locked laser generates a pulse train at repetition rates around 400 MHz. No complex cooling system is required: neither the SESAM nor the Ti:Sapphire crystal is actively cooled, only air cooling is applied to the pump diodes using a small fan. Because of mass production for laser displays, we expect that prices for green laser diodes will become very favorable in the near future, opening the door for low-cost Ti:Sapphire lasers. This will be highly attractive for potential mass applications such as biomedical imaging and sensing.