Observing the Intrinsic Linewidth of a Quantum-Cascade Laser: Beyond the Schawlow-Townes Limit

Bartalini, S.; Borri, Simone; Cancio, P.; Castrillo, A.; Galli, Iacopo; Giusfredi, G.; Mazzotti, D.; Gianfrani, L.; Natale, P. De

doi:10.1103/physrevlett.104.083904

Cited by 151 publications

(117 citation statements)

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“…The QCLs frequency noise was measured using a spectroscopic setup as implemented in former studies [7,[10][11][12]. The side of a molecular transition was used as a frequency-to-intensity converter (a so-called frequency Here, a 10-cm-long sealed glass cell filled with 2 mbar of pure N 2 O was used with QCLs emitting in the 7.6-8 μm wavelength range.…”

Section: Methodsmentioning

confidence: 99%

“…Novel applications of QCLs in very high-resolution spectroscopy and optical metrology have emerged in the last years, in particular in combination with optical frequency combs [2][3][4][5], which are generally more demanding in terms of low frequency-noise and nar-row spectral linewidth. QCLs have the potential to achieve very narrow linewidths with an intrinsic value of a few hundreds hertz only [6,7] resulting from their close-to-zero Henry's linewidth enhancement factor [8]. However, a narrow linewidth is generally not achieved in practice in freerunning QCLs as a result of the presence of undesired noise that compromises their spectral properties.…”

Section: Introductionmentioning

confidence: 99%

“…Hence, the char-acterization and understanding of the origin of frequency noise spectrum of free-running QCLs gained a significant interest in the recent years. A few research groups reported experimental frequency noise spectra for QCLs produced by different manufacturers and operated at temperatures ranging from cryogenic [7,9] up to room temperature [6,10,11]. Significantly different levels of noise have been observed in the considered QCLs, leading to linewidths in the range of sub-100 kHz [6] to many megahertz [7].…”

Section: Introductionmentioning

confidence: 99%

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An experimental study of noise in mid-infrared quantum cascade lasers of different designs

et al. 2015

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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An experimental study of noise in mid-infrared quantum cascade lasers of different designs

et al. 2015

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“…2 shows the resulting frequency noise power spectral density (PSD) of the QCL. It has a 1/f trend at low frequency, followed by a steeper slope above ∼ 300 kHz, as observed in 21,22 . Note however that the measured frequency noise PSD is roughly one order of magnitude lower than previously published characterizations of free-running cw-mode near-RT DFB QCLs 20-24 .…”

mentioning

confidence: 72%

A widely tunable 10-μm quantum cascade laser phase-locked to a state-of-the-art mid-infrared reference for precision molecular spectroscopy

Sow

Mejri

Tokunaga

et al. 2014

Applied Physics Letters

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We report the coherent phase-locking of a quantum cascade laser (QCL) at 10-µm to the secondary frequency standard of this spectral region, a CO 2 laser stabilized on a saturated absorption line of OsO 4 . The stability and accuracy of the standard are transferred to the QCL resulting in a line width of the order of 10 Hz, and leading to our knowledge to the narrowest QCL to date. The locked QCL is then used to perform absorption spectroscopy spanning 6 GHz of NH 3 and methyltrioxorhenium, two species of interest for applications in precision measurements.With their rich internal structure, molecules can play a decisive role in precision tests of fundamental physics. They are being used to test fundamental symmetries such as parity 1-3 or parity and time reversal 4 , to measure absolute values of fundamental constants 5-7 and their possible temporal variation [8][9][10] . Many of these experiments can be cast as measurements of resonance frequencies of molecular transitions, for which ultra-stable and accurate sources in the mid-infrared (mid-IR) are highly desirable, since most rovibrational transitions are to be found in that region.Our group has a long-standing interest in performing spectroscopic precision measurements on molecules at extreme resolutions around 10 µm 1,9,11 . We are currently working on two such measurements: the determination of the Boltzmann constant, k B , by Doppler spectroscopy of ammonia 6,12 and the first observation of parity violation by Ramsey interferometry of a beam of chiral molecules 3,13 . For these experiments, we currently use spectrometers based on custom built ultra-stable CO 2 lasers. We obtain the required metrological frequency stability and accuracy -10 Hz line width, 1 Hz stability at 1 s, accuracy of a few tens of hertz 14,15 -by stabilizing these lasers to saturated absorption lines of molecules such as OsO 4 . CO 2 lasers have a major shortcoming: a lack of tunability. They emit at CO 2 molecular resonances. An emission line is found every 30 to 50 GHz in the 9-11 µm wavelength range, and each line is tunable over about 100 MHz. Although, as in our spectrometers, this range can be extended a few gigahertz using electrooptical modulators (EOMs), this is done at the expense of power (EOMs at these wavelengths have an efficiency of 10 −4 ) and necessitates subsequent spectral filtering. Overcoming these difficulties without the loss of stability is key to enabling precision measurements in the mid-IR.One solution would be to use frequency combreferenced continuous-wave (cw) [16][17][18] or femtosecond mid-IR sources. These are based on frequency mixing in nonlinear crystals and provide absolute-frequency referencing, reasonable line widths and tunability, but are very complex and often exhibit limited power. By comparison, cw quantum cascade lasers (QCLs) are a new mature and robust technology that offer broad and continuous tuning over several hundred gigahertz at 100 mW-level powers. Several can be combined giving access to the whole mid-IR region. Recent studies of...

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“…Optical frequency discriminators directly convert optical frequency fluctuations of the laser into intensity fluctuations that are detected by a photodetector. Optical discriminators are typically devices displaying a frequency-dependent transmission in a restricted frequency range, such as gas-filled cells near an atomic/molecular resonance (Doppler-broadened [10][11][12] or sub-Doppler 13 ), Fabry-Perot resonators 14 or unbalanced twobeam interferometers. 15 As it is not always possible to have a proper optical discriminator at the considered laser wavelength, another approach consists in heterodyning the laser under test with a second laser, either similar to the first one or with a negligible frequency noise, and subsequently analyzing the generated RF beat signal.…”

Section: Frequency Discriminatorsmentioning

confidence: 99%

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

Schilt

Bucalović

Tombez

et al. 2011

Review of Scientific Instruments

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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 Hz 2 /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.

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Observing the Intrinsic Linewidth of a Quantum-Cascade Laser: Beyond the Schawlow-Townes Limit

Cited by 151 publications

References 20 publications

An experimental study of noise in mid-infrared quantum cascade lasers of different designs

An experimental study of noise in mid-infrared quantum cascade lasers of different designs

A widely tunable 10-μm quantum cascade laser phase-locked to a state-of-the-art mid-infrared reference for precision molecular spectroscopy

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

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