Frequency noise of free-running 46 μm distributed feedback quantum cascade lasers near room temperature

Tombez, Lionel; Francesco, J. di; Schilt, Stéphane; Domenico, G. Di; Faist, Jérôme; Thomann, P.; Hofstetter, Daniel

doi:10.1364/ol.36.003109

Cited by 54 publications

(63 citation statements)

References 11 publications

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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 .…”

supporting

confidence: 52%

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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supporting

confidence: 52%

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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“…4,5 In order to push the limits of those highresolution experiments, narrow-linewidth sources of coherent light which can be achieved by active stabilization of DFB-QCLs to optical references with high-bandwidth servoloops are required. 6,7 For the most demanding applications in the field of high-resolution spectroscopy, frequency-noise analysis revealed that feedback loop bandwidths of several hundred of kHz are necessary for linewidth narrowing of DFB-QCLs. 7,8 While the picosecond carrier lifetime in QCLs allows a very fast modulation of the intensity above 10 GHz, 9,10 the modulation of the optical frequency-or wavelength-is limited by the thermal dynamics of the device.…”

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confidence: 99%

“…6,7 For the most demanding applications in the field of high-resolution spectroscopy, frequency-noise analysis revealed that feedback loop bandwidths of several hundred of kHz are necessary for linewidth narrowing of DFB-QCLs. 7,8 While the picosecond carrier lifetime in QCLs allows a very fast modulation of the intensity above 10 GHz, 9,10 the modulation of the optical frequency-or wavelength-is limited by the thermal dynamics of the device. Indeed, unlike interband semiconductor laser diodes whose wavelength can be modulated at high speed through carrier density modulation, 11,12 the latter has no effect in QCLs because of the symmetric gain curve and associated independence of the refractive index at the gain peak (zero alpha parameter).…”

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confidence: 99%

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Wavelength tuning and thermal dynamics of continuous-wave mid-infrared distributed feedback quantum cascade lasers

Tombez

Cappelli

Schilt

et al. 2013

Applied Physics Letters

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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.Since the first demonstration of quantum cascade lasers (QCLs) in 1994, 1 the number of promising application in the field of chemical sensing for biomedical and environmental sciences has been constantly rising during the last years. Indeed, thanks to the ability to tailor their emission wavelength and to target precisely selected ro-vibrational molecular transitions in the fingerprint region, QCLs were demonstrated to be very sensitive probes for a wide variety of molecules.2 Single-frequency QCLs are generally required for trace gas sensing and a common approach is to use a distributed feedback (DFB) grating etched at the surface of the QCL active region in order to force laser operation at a precise wavelength.3 Moreover, promising developments in the field of high-resolution spectroscopy can lead to the possibility of performing measurements with unprecedented precision, especially by linking spectrally narrow QCLs to optical frequency combs. 4,5 In order to push the limits of those highresolution experiments, narrow-linewidth sources of coherent light which can be achieved by active stabilization of DFB-QCLs to optical references with high-bandwidth servoloops are required. 6,7 For the most demanding applications in the field of high-resolution spectroscopy, frequency-noise analysis revealed that feedback loop bandwidths of several hundred of kHz are necessary for linewidth narrowing of DFB-QCLs. 7,8 While the picosecond carrier lifetime in QCLs allows a very fast modulation of the intensity above 10 GHz, 9,10 the modulation of the optical frequency-or wavelength-is limited by the thermal dynamics of the device. Indeed, unlike interband semiconductor laser diodes whose wavelength can be modulated at high speed through carrier density modulation, 11,12 the latter has no effect in QCLs because of the symmetric gain curve and associated independence of the refractive index at the gain peak (zero alpha parameter). 13 The wavelength tuning in DFB-QCLs is therefore mainly governed by the temperature dependence of the refractive index with a tuning rate of 1/k dk/dT % 7 Â 10 À5

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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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Frequency noise of free-running 46 μm distributed feedback quantum cascade lasers near room temperature

Cited by 54 publications

References 11 publications

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

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

Wavelength tuning and thermal dynamics of continuous-wave mid-infrared distributed feedback quantum cascade lasers

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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