Retrieval of phase relation and emission profile of quantum cascade laser frequency combs

Cappelli, Francesco; Consolino, Luigi; Campo, Giulio; Galli, Iacopo; Mazzotti, D.; Campa, Annamaria; Cumis, Mario Siciliani de; Pastor, P. Cancio; Eramo, Roberto; Rösch, Markus; Beck, Mattias; Scalari, Giacomo; Faist, Jérôme; Natale, P. De; Bartalini, S.

doi:10.1038/s41566-019-0451-1

Cited by 95 publications

(77 citation statements)

References 49 publications

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“…The shifted‐wave interference Fourier‐transform spectroscopy (SWIFTS) measures the intermodal phase relation of the QCL comb using a coherent beatnote spectroscopy [ 3 ] and the Fourier‐transform analysis of comb emission (FACE) study of the dual‐comb multi‐heterodyne detection and Fourier‐transform analysis. [ 4 ] Both cases are consistent and show a nontrivial phase relation among the modes translating the complex mode‐locking mechanism. Indeed, the strong dipole matrix elements of ISBT allow giant second and third order susceptibilities responsible for the generation of four‐wave mixing (FWM) in QCLs.…”

Section: Introductionmentioning

confidence: 62%

Tunability of the Free‐Spectral Range by Microwave Injection into a Mid‐Infrared Quantum Cascade Laser

Rodríguez

Mottaghizadeh

Gacemi

et al. 2020

Laser & Photonics Reviews

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The free‐spectral range (FSR) of quantum cascade lasers (QCLs) emitting at 4.7 µm can be tuned through direct microwave modulation. The intrinsic short carrier lifetime of inter‐subband transitions, along with impedance matched electrical packaging, allows a high‐speed modulation up to more than 30 GHz. A significant broadening and flattening of the lasing spectrum is observed under radio frequency (RF) injection with frequencies close to the round‐trip cavity. An accurate analysis of the high resolution spectra of the laser shows a comb‐like regime for both free‐running and RF modulated QCLs, if the modulation frequency is within the locking‐range of the device. One of the main advantages of collecting high‐resolution mid‐infrared spectra, over the plain investigation of the beatnote in the microwave region, is the access to all the longitudinal modes and thus the accurate measure of the FSR over the whole optical spectrum. The use of high‐resolution spectroscopy provides an in‐depth and comprehensive analysis of lasing spectra under microwave modulations.

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Section: Introductionmentioning

confidence: 62%

Tunability of the Free‐Spectral Range by Microwave Injection into a Mid‐Infrared Quantum Cascade Laser

Rodríguez

Mottaghizadeh

Gacemi

et al. 2020

Laser & Photonics Reviews

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“…The implemented stabilization scheme removed the common noise between the optical modes of the QCL comb that includes the combined contribution of the offset frequency and of the mode spacing, but no information about the noise of the offset frequency of the free-running QCL comb was assessed. In the recent work of Cappelli et al [12], the amplitude and phase of the different modes of THz and MIR QCL combs were retrieved by a Fourier-transform analysis of the multi-heterodyne spectrum obtained in a dual-comb setup with a metrological comb. The relative frequency noise between the QCL comb modes was analyzed by implementing a scheme that combines a phase-lock of the mode spacing to an external reference oscillator by feedback to the QCL current, and a noise compensation scheme to remove the common noise between all modes of the multi-heterodyne beat signal with the metrological comb.…”

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confidence: 99%

Frequency noise correlation between the offset frequency and the mode spacing in a mid-infrared quantum cascade laser frequency comb

Shehzad¹,

Brochard²,

Matthey³

et al. 2020

Opt. Express

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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. using a high-bandwidth photodiode. On the other hand, the standard method to detect the CEO frequency is the self-referencing scheme, which in the usual implementation requires a coherent octave-spanning spectrum and nonlinear f -to-2f interferometry [1].A different type of frequency combs has been demonstrated in the mid-infrared (MIR) spectral region that is important for molecular spectroscopy, based on the occurrence of four-wave mixing in the semiconductor gain medium of broadband quantum cascade lasers (QCLs) [4]. These lasers produce a frequency comb in some particular conditions where all emitted modes become equidistant as a result of the parametric four-wave mixing process. QCL combs are very attractive for dual-comb spectroscopy [5][6][7] in the MIR spectral range that is of particular importance for high-resolution molecular spectroscopy and trace gas sensing. As a result of the very short upper state lifetime of the gain medium, these comb sources do not generate optical pulses, but deliver a fairly constant output power [8] that is associated to a frequency-modulated (FM) spectrum. A recent model proposed by Opacak and Schwarz describes the formation of FM combs with the combined contributions of spatial hole burning, gain saturation and a minimum group velocity dispersion or Kerr nonlinearity due to gain asymmetry [9]. The FM nature of QCL combs has made the self-referencing method not possible so far and no detection of t...

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“…The temporal profile of modelocked or frequency comb QCLs operating on the fundamental frequency has very recently garnered considerable attention, with a variety of new techniques applied, ranging from SWIFT 19 , phase stablization 20,21 to intensity autocorrelations 22 . The time and spectral characteristics of the sample were investigated in this work using the established technique of injection seeding, which permits the amplitude and phase of the QCL emission to be determined on femtosecond time scales, from the build-up of the electric field to steady-state laser action (see Materials and methods section) 10,11,23 .…”

Section: Resultsmentioning

confidence: 99%

Ultrafast response of harmonic modelocked THz lasers

et al. 2020

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The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity's fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

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Retrieval of phase relation and emission profile of quantum cascade laser frequency combs

Cited by 95 publications

References 49 publications

Tunability of the Free‐Spectral Range by Microwave Injection into a Mid‐Infrared Quantum Cascade Laser

Tunability of the Free‐Spectral Range by Microwave Injection into a Mid‐Infrared Quantum Cascade Laser

Frequency noise correlation between the offset frequency and the mode spacing in a mid-infrared quantum cascade laser frequency comb

Ultrafast response of harmonic modelocked THz lasers

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