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Coherently-averaged dual comb spectrometer at 7.7 µm with master and follower quantum cascade lasers
We demonstrate coherent averaging of the multi-heterodyne beat signal between two quantum cascade laser frequency combs in a master-follower configuration. The two combs are mutually locked by acting on the drive current to control their relative offset frequency and by radio-frequency extraction and injection locking of their intermode beat signal to stabilize their mode spacing difference. By implementing an analog common-noise subtraction scheme, a reduction of the linewidth of all heterodyne beat notes by five orders of magnitude is achieved compared to the free-running lasers. We compare stabilization and post-processing corrections in terms of amplitude noise. While they give similar performances in terms of signal-to-noise ratio, real-time processing of the stabilized signal is less demanding in terms of computational power. Lastly, a proof-of-principle spectroscopic measurement was performed, showing the possibility to reduce the amount of data to be processed by three orders of magnitude, compared to the free-running system.
Frequency noise correlation between the offset frequency and the mode spacing in a mid-infrared quantum cascade laser frequency comb
The generation of frequency combs in the mid-infrared (MIR) spectral range by quantum cascade lasers (QCLs) has the potential for revolutionizing dual-comb multi-heterodyne spectroscopy in the molecular fingerprint region. However, in contrast to frequency combs based on passively mode-locked ultrafast lasers, their operation relies on a completely different mechanism resulting from a four-wave mixing process occurring in the semiconductor gain medium that locks the modes together. As a result, these lasers do not emit pulses and no direct self-referencing of a QCL comb spectrum has been achieved so far. Here, we present a detailed frequency noise characterization of a MIR QCL frequency comb operating at a wavelength of 8 μm with a mode spacing of ~ 7.4 GHz. Using a beat measurement with a narrow-linewidth single-mode QCL in combination with a dedicated electrical scheme, we measured the frequency noise properties of an optical mode of the QCL comb, and indirectly of its offset frequency for the first time, without detecting it by the standard approach of nonlinear interferometry applied to ultrafast mode-locked lasers. In addition, we also separately measured the noise of the comb mode spacing extracted electrically from the QCL. We observed a strong anti-correlation between the frequency fluctuations of the offset frequency and mode spacing, leading to optical modes with a linewidth slightly below 1 MHz in the free-running QCL comb (at 1-s integration time), which is narrower than the individual contributions of the offset frequency and mode spacing that are at least 2 MHz each.
Electrically-driven pure amplitude and frequency modulation in a quantum cascade laser
We present pure amplitude modulation (AM) and frequency modulation (FM) achieved electrically in a quantum cascade laser (QCL) equipped with an integrated resistive heater (IH). The QCL output power scales linearly with the current applied to the active region (AR), but decreases with the IH current, while the emission frequency decreases with both currents. Hence, a simultaneous modulation applied to the current of the AR and IH sections with a proper relative amplitude and phase can suppress the AM, resulting in a pure FM, or vice-versa. The adequate modulation parameters depend on the applied modulation frequency. Therefore, they were first determined from the individual measurements of the AM and FM transfer functions obtained for a modulation applied to the current of the AR or IH section, respectively. By optimizing the parameters of the two modulations, we demonstrate a reduction of the spurious AM or FM by almost two orders of magnitude at characteristic frequencies of 1 and 10 kHz compared to the use of the AR current only.
XUV Sources Based on Intra-Oscillator High Harmonic Generation with Thin-Disk Lasers: Current Status and Prospects
Ultrafast thin-disk laser (TDL) oscillators provide higher intracavity pulse energy, average power, and peak power levels than any other femtosecond laser oscillator technology. They are suitable for driving extreme nonlinear interactions directly inside the laser oscillator. High harmonic generation (HHG) driven inside ultrafast TDL oscillators is a very recent approach for the generation of coherent extreme ultraviolet (XUV) light at multi-megahertz repetition rates. In this paper, we review the current state of the development, discuss the technological potential, and give an outlook toward the future developments. We compare the current performance to established technologies and evalu-ate possible limitations. We discuss future improvements, such as reduction of the driving pulse duration and increase of the intracavity peak power, efficient extraction of the XUV light from the cavity, and carrier-envelope offset frequency stabilization of the generated XUV light. Due to the power scalability of the TDL concept and the possibility to operate in a spectrally broadened regime with pulse durations below the gain bandwidth limitation, intra-oscillator HHG with TDLs has a high potential for powerful table-top multi-megahertz coherent XUV light sources for science and applications.
Towards compact ultralow phase noise lasers and microwave signals based on new approaches
Aujourd'hui, les signaux micro-ondes à bruit de phase le plus faible sont générés optiquement par division de fréquence d'une référence optique ultra-stable utilisant un peigne de fréquence femtoseconde. Dans l'approche couramment utilisée, la référence optique ultra-stable est obtenue par stabilisation en fréquence d'un laser sur une cavité optique à très faible coefficient d’expansion thermique, et la division en fréquence est effectuée en stabilisant optiquement un laser à verrouillage de mode au laser ultra-stable. Ces deux sous-systèmes sont assez complexes et encombrants, mais ils ont démontré des performances de pointe.
Dans cette thèse, des approches alternatives ont été étudiées pour la génération de micro-ondes à faible bruit basées sur un schéma d'oscillateur de transfert. Dans une première partie, une nouvelle méthode inspirée du concept de l'oscillateur de transfert a été développée et validée pour caractériser la fréquence d’offset (décalage de phase entre la porteuse et l’enveloppe, carrier-envelope offset en anglais, CEO) d’un peigne de fréquence optique sans s'appuyer sur la méthode traditionnelle d'auto-référencement et donc sans le besoin d’un spectre optique couvrant une octave de fréquence, qui est difficile à générer notamment avec des peignes à fréquence de répétition élevée. Cette méthode a ensuite été appliquée avec succès pour caractériser trois différents types de peignes de fréquence optiques générés à partir d'un laser à semi-conducteur à verrouillage de mode, d'un laser à l'état solide pompé par diode avec un taux de répétition de 25 GHz et d'un laser à cascade quantique émettant dans la région spectrale de l’infrarouge moyen.
En modifiant et en améliorant cette technique, on a démontré et caractérisé la génération d'un signal hyperfréquence à bruit de phase ultra-faible basé sur un oscillateur de transfert. La méthode a également été mise en œuvre à l'aide d'un micro-résonateur à peigne de Kerr pour une première démonstration de principe.
En outre, la stabilisation en fréquence d'un laser continu à cascade quantique émettant dans l'infrarouge moyen sur une ligne à retard optique est présentée pour la première fois et conduit à une largeur de raie inférieure à 10 kHz en utilisant un montage en espace libre. La même approche peut être appliquée dans le proche infrarouge avec un long délai utilisant des fibres optiques, donnant la possibilité d'atteindre une largeur de raie au niveau du hertz.
Les technologies développées dans cette thèse sont des composants attrayants pour les futurs générateurs de micro-ondes compacts à très faible bruit., Today, the lowest phase noise microwave signals are generated optically by frequency division of an ultra-stable optical reference using a femtosecond frequency comb. In the commonly used approach, the ultra-stable optical reference is obtained by frequency-stabilizing a laser to a high-finesse ultra-low expansion optical cavity, and the frequency division is performed by optically locking a mode-locked laser to the ultra-stable laser. Both sub-systems are fairly complex and cumbersome, but have demonstrated state-of-the-art performance.
In this thesis, alternative approaches have been investigated for low-noise microwave generation based on a transfer oscillator scheme. In a first part, a novel method inspired by the transfer oscillator concept has been developed and validated to characterize the offset frequency of a comb spectrum without relying on the traditional self-referencing method and, thus, without requiring an octave-spanning spectrum that is challenging to be generated, especially with high repetition rate frequency combs. This method has then been successfully applied to characterize three different types of comb spectra from a semiconductor mode-locked laser, a diode-pumped solid-state laser with 25-GHz repetition rate, and a quantum cascade laser frequency comb emitting in the mid-infrared spectral region.
By modifying and improving this scheme, ultra-low phase noise microwave signal generation based on a transfer oscillator was demonstrated and characterized. The method was also implemented with a micro-resonator Kerr-comb for a first proof-of-principle demonstration of frequency division performed with a Kerr comb.
In addition, frequency stabilization of a mid-infrared quantum cascade laser to an optical delay-line is presented for the first time and led to a sub-10-kHz linewidth using only a meter-scale free-space delay-line. The same approach can be applied in the near-infrared with a long fiber delay with the potential to achieve Hz-level linewidth.
The technologies developed in this thesis are attractive components for future compact ultra-low noise microwave generators.
Frequency Noise Characterization of a 25-GHz Diode-Pumped Mode-Locked Laser With Indirect Carrier-Envelope Offset Noise Assessment
We present a detailed frequency noise characterization of an ultrafast diode-pumped solid-state laser operating at 25-GHz repetition rate. The laser is based on the gain material Er:Yb:glass and operates at a wavelength of 1.55 μm. Using a beating measure-ment with an ultralow-noise continuous-wave laser in combination with a dedicated electrical scheme, we measured the frequency noise properties of an optical mode of the 25-GHz laser, of its repetition rate and indirectly of its carrier-envelope offset (CEO) signal without detecting the CEO frequency by the standard approach of nonlinear interferometry. We ob-served a strong anticorrelation between the frequency noise of the indirect CEO signal and of the repetition rate in our laser, leading to optical modes with a linewidth below 300 kHz in the free-running laser (at 100-ms integration time), much narrower than the individual contributions of the carrier envelope offset and repetition rate. We explain this behavior by the presence of a fixed point located close to the optical carrier in the laser spectrum for the dominant noise source.
Ultra-low noise microwave generation with a free-running optical frequency comb transfer oscillator
We present ultra-low noise microwave synthesis by optical to radio-frequency (RF) division realized with a free-running or RF-locked optical frequency comb (OFC) acting as a transfer oscillator. The method does not require any optical lock of the OFC and circumvents the need for a high-bandwidth actuator. Instead, the OFC phase noise is electrically removed from a beat-note signal with an optical reference, leading to a broadband noise division. The phase noise of the ∼15 GHz RF signal generated in this proof-of-principle demonstration is limited by a shot-noise level below −150 dBc/Hz at high Fourier frequencies and by a measurement noise floor of −60 dBc/Hz at 1 Hz offset frequency when performing 1,100 cross-correlations. The method is attractive for high-repetition-rate OFCs that lead to a lower shot-noise, but are generally more difficult to tightly lock. It may also simplify the noise evaluation by enabling the generation of two or more distinct ultra-low noise RF signals from different optical references using a single OFC and their direct comparison to assess their individual noise.
Rb-stabilized laser at 1572 nm for CO2 monitoring
We have developed a compact rubidium-stabilized laser system to serve as optical frequency reference in the 1.55-m wavelength region, in particular for CO2 monitoring at 1572 nm. The light of a fiber-pigtailed distributed feedback (DFB) laser emitting at 1560 nm is frequency-doubled and locked to a sub-Doppler rubidium transition at 780 nm using a 2-cm long vapor glass cell. Part of the DFB laser light is modulated with an electro-optical modula-tor enclosed in a Fabry-Perot cavity, generating an optical frequency comb with spectral cover-age extending from 1540 nm to 1580 nm. A second slave DFB laser emitting at 1572 nm and offset-locked to one line of the frequency comb shows a relative frequency stability of 1·10-11at 1 s averaging time and <4·10-12 from 1 hour up to 3 days.
10 kHz linewidth mid-infrared quantum cascade laser by stabilization to an optical delay line
We present a mid-infrared quantum cascade laser (QCL) with a sub-10 kHz full width at half-maximum linewidth (at 1 s integration time) achieved by stabilization to a free-space optical delay line. The linear range in the center of a fringe detected at the output of an imbalanced Mach–Zehnder interferometer implemented with a short free-space pathlength difference of only 1 m is used as a frequency discriminator to detect the frequency fluctuations of the QCL. Feedback is applied to the QCL current to lock the laser frequency to the delay line. The application of this method in the mid-infrared is reported for the first time, to the best of our knowledge. By implementing it in a simple self-homodyne configuration, we have been able to reduce the frequency noise power spectral density of the QCL by almost 40 dB below 10 kHz Fourier frequency, leading to a linewidth reduction by a factor of almost 60 compared to the free-running laser. The present limits of the setup are assessed and discussed.
Full stabilization and characterization of an optical frequency comb from a diode-pumped solid-state laser with GHz repetition rate
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 ƒrep, of the CEO frequency ƒCEO, 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 ƒrep and ƒCEO are identified.