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Analysis and improvement of the spectral properties in mid-infrared semiconductor quantum cascade lasers

2014, Tombez, Lionel, Südmeyer, Thomas

Les lasers à cascade quantique (QCL) sont des lasers à semiconducteur basés sur des transitions inter-sous-bandes. Au contraire des diodes lasers traditionnelles, la longueur d'onde d'émission de ce type de laser n'est pas définie par la bande interdite (bandgap) du matériau semiconducteur, mais par l'écart d'énergie entre les niveaux discrets de plusieurs puits quantiques, ce qui permet la réalisation de sources laser compactes émettant dans l'infrarouge moyen (mid-IR) et dans l'infrarouge lointain (far-IR). Le moyen infrarouge comporte les bandes fondamentales de vibrations d'un grand nombre d'espèces moléculaires telles que le dioxyde de carbone (CO2) ou le méthane (CH4), et permet la détection d'espèces chimiques avec grande précision. Grâce à l'utilisation de techniques de fabrication standards et bien connues des lasers à semiconducteur utilisés pour les télécommunications optiques dans l'infrarouge proche, le laser à cascade quantique est un dispositif extrêmement compact et productible en masse, et constitue par conséquence un excellent candidat pour la réalisation de capteurs de gaz portables et extrêmement sensibles utilisant des techniques de spectroscopie par absorption dans le moyen infrarouge.
Dans cette thèse, les propriétés spectrales des lasers à cascade quantique à base d'InGaAs/InAlAs émettant dans l'infrarouge moyen et utilisant un réseau de Bragg distribué pour garantir une émission mono-mode ont été étudiées, avec pour but d'en évaluer et d'en améliorer la pureté spectrale. Les sources de lumière cohérentes à largeur de raie étroite et faible bruit de fréquence sont en effet d'une grande importance pour le développement de nouveau systèmes de mesure et d'instruments de haute précision dans la gamme spectrale du moyen infrarouge. Dans un premier temps, la dynamique de l'ajustement de la fréquence optique et des propriétés thermiques des lasers à cascade quantique, qui est essentielle à la compréhension du mécanisme de formation des instabilités de fréquence, a été étudiée et est présentée. Dans un deuxième temps, l'impact des conditions d'opération et de divers paramètres des lasers à cascade quantique sur leur bruit de fréquence a été évalué. L'impact du type de fabrication de la région active des lasers a notamment été étudié. Pour ce faire, une alimentation à faible bruit de courant a dû être développée afin de fournir un courant d'injection aussi stable que possible. Le bruit électrique dans les lasers à cascade quantique a également été étudié, et nous montrons un lien particulièrement intéressant entre les instabilités de fréquence du laser et les fluctuations de la puissance électrique dissipée dans ce dernier. Ces résultats ont permis la démonstration d'une nouvelle approche particulièrement simple visant à évaluer la pureté spectrale des lasers à cascade quantique à partir uniquement de mesures électriques. Finalement, une nouvelle méthode innovante de réduction active du bruit de fréquence et de diminution de la largeur de raie sans avoir recours à aucune référence de fréquence optique a été développée dans le cadre de cette thèse et a permis d'obtenir une réduction de 90% de la densité spectrale de puissance du bruit de fréquence., Quantum Cascade Lasers (QCLs) are semiconductor lasers based on intersubband transitions in semiconductor heterojunctions. Unlike conventional laser diodes, the emission wavelength of QCLs is not defined by the energy gap between the conduction and valance bands of the semiconductor material, but by the energy spacing between the discrete states of quantum wells, which enables the realization of compact semiconductor lasers in the mid-infrared and far-infrared spectral regions. The mid-IR spectral region contains the fundamental vibration bands of many molecular species, such as carbon dioxide (CO2) and methane (CH4), and enables high precision analysis of chemical species. Thanks to the use of well-known semiconductor fabrication techniques widely developed for optical telecommunication applications in the near-IR, QCLs are compact and suitable for mass-production, and therefore constitute a very interesting candidate for the development of portable and highly sensitive and selective trace-gas sensors by absorption spectroscopy in the mid-infrared.
In this thesis, the spectral properties of InGaAs/InAlAs distributed-feedback (DFB) QCLs emitting in the mid-IR spectral region were studied, with the aim of assessing and improving the spectral purity of these devices. Low frequency-noise and narrow-linewidth coherent light sources emitting in the mid-IR spectral region are indeed of prime interest for the future development of high-resolution spectroscopy systems. First of all, this thesis presents the frequency-tuning and thermal dynamics in DFB-QCLs, which are important to understand the underlying mechanisms of frequency noise generation. A simple thermal model is used to explain the observed thermal dynamics. In a second phase, the frequency-noise properties of different QCLs were studied upon operating conditions and devices parameters. The effect of the processing of the lasers active region, namely in ridge waveguide or buried-heterostructure was in particular investigated. The design of a low-noise power supply to provide a stable injection current as well as the impact on the spectral properties of QCLs is also presented. Then, the noise properties at the electrical level in the semiconductor laser chips were investigated. A particularly interesting outcome of the experiments is a clear link between instabilities of the emission frequency and electrical power fluctuations due to the electronic transport in the laser chip. The results enabled the demonstration of a novel and extremely simple method for assessing the spectral properties of QCLs from electrical measurements only. Finally, a novel and innovative active method for frequency-noise reduction and linewidth narrowing of QCLs without using any optical frequency reference was developed and yielded a 90% reduction of the frequency-noise power spectral density.

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Frequency Noise and Linewidth of Mid-infrared Continuous-Wave Quantum Cascade Lasers: An Overview

2013, Schilt, Stephane, Tombez, Lionel, Di Domenico, Gianni, Hofstetter, Daniel

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Frequency noise of free-running 4.6 um distributed feedback quantum cascade lasers near room temperature

2011-8-10, Tombez, Lionel, Di Francesco, Joab, Schilt, Stephane, Di Domenico, Gianni, Faist, J., Thomann, Pierre, Hofstetter, Daniel

The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 um range and operated in cw mode near room temperature (277K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz2/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.

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Frequency Noise and Linewidth of Mid-infrared Continuous-Wave Quantum Cascade Lasers: An Overview

, Schilt, Stephane, Tombez, Lionel, Di Domenico, Gianni, Hofstetter, Daniel

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Wavelength tuning and thermal dynamics of continuous-wave mid-IR distributed feedback quantum cascade laser

2013-7-17, Tombez, Lionel, Cappelli, Francesco, Schilt, Stephane, Di Domenico, Gianni, Bartalini, Saverio, Hofstetter, Daniel

We report on the wavelength tuning dynamics in continuous-wave distributed feedback quantum cascade lasers (QCLs). The wavelength tuning response for direct current modulation of two mid-IR QCLs from different suppliers was measured from 10 Hz up to several MHz using ro-vibrational molecular resonances as frequency-to-intensity converters. Unlike the output intensity, which can be modulated up to several gigahertz, the frequency-modulation bandwidth was found to be on the order of 200 kHz, limited by the laser thermal dynamics. A non-negligible roll-off and a significant phase shift are observed above a few hundred hertz already and explained by a thermal model.

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Linewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driver

2012-4-21, Tombez, Lionel, Schilt, Stephane, Di Francesco, Joab, Di Domenico, Gianni, Hofstetter, Daniel, Thomann, Pierre

We report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6 cm^−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ≈1 nA/√Hz allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ≈1.6 MHz/(nA/√Hz) for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several nA/√Hz , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only 350 pA/√Hz current noise, which is suitable to observe the ≈550 kHz linewidth of our QCL.

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Frequency noise of free-running 4.6 μm distributed feedback quantum cascade lasers near room temperature

2011, Tombez, Lionel, Di Francesco, Joab F., Schilt, Stephane, Di Domenico, Gianni, Thomann, Pierre, Hofstetter, Daniel, Faist, Jérôme

The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 μm range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz2/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.

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Active linewidth-narrowing of a mid-infrared quantum cascade laser without optical reference

2013, Tombez, Lionel, Schilt, Stephane, Hofstetter, D, Südmeyer, Thomas

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Temperature dependence of the frequency noise in a mid-IR DFB quantum cascade laser from cryogenic to room temperature

2012, Tombez, Lionel, Schilt, Stephane, Di Francesco, Joab F., Thomann, Pierre, Hofstetter, Daniel

We report on the measurement of the frequency noise power spectral density in a distributed feedback quantum cascade laser over a wide temperature range, from 128 K to 303 K. As a function of the device temperature, we show that the frequency noise behavior is characterized by two different regimes separated by a steep transition at ≈200 K. While the frequency noise is nearly unchanged ~200 K, it drastically increases at lower temperature with an exponential dependence. We also show that this increase is entirely induced by current noise intrinsic to the device. In contrast to earlier publications, a single laser is used here in a wide temperature range allowing the direct assessment of the temperature dependence of the frequency noise.

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Frequency discriminators for the characterization of narrow-spectrum heterodyne beat signals: Application to the measurement of a sub-hertz carrier-envelope-offset beat in an optical frequency comb

2011, Schilt, Stephane, Bucalovic, Nikola, Tombez, Lionel, Dolgovskiy, Vladimir, Schori, Christian, Di Domenico, Gianni, Zaffalon, Michele, Thomann, Pierre

We describe a radio-frequency (RF) discriminator, or frequency-to-voltage converter, based on a voltage-controlled oscillator phase-locked to the signal under test, which has been developed to analyze the frequency noise properties of an RF signal, e.g., a heterodyne optical beat signal between two lasers or between a laser and an optical frequency comb. We present a detailed characterization of the properties of this discriminator and we compare it to three other commercially available discriminators. Owing to its large linear frequency range of 7 MHz, its bandwidth of 200 kHz and its noise floor below 0.01 Hz2/Hz in a significant part of the spectrum, our frequency discriminator is able to fully characterize the frequency noise of a beat signal with a linewidth ranging from a couple of megahertz down to a few hertz. As an example of application, we present measurements of the frequency noise of the carrier envelope offset beat in a low-noise optical frequency comb.