1. INTRODUCTION

Optical frequency combs have revolutionized the fields of high-resolution and precision atomic spectroscopy due to their high coherence, wide spectral bandwidth, and absolute traceability [1,2]. Initially developed in the near-infrared (NIR) spectral region, frequency combs are now being extended to other parts of the spectrum. In particular, extending the spectral range of frequency combs into the mid-infrared (MIR) and terahertz (THz) regions will open new possibilities in the fields of frequency metrology, molecular spectroscopy, chemical analysis, and medical diagnosis [3], as the fundamental roto-vibrational absorption lines of a variety of molecules lie in this spectral region.

Different schemes have been investigated for generating MIR frequency combs. A well-established approach consists of transferring frequency combs from the NIR region into the MIR region through nonlinear processes using, for example, optical parametric oscillators [4,5] or difference frequency generation in fiber-based NIR combs [6–8]. Other examples include MIR combs generated by transition metals incorporated into chalcogenide hosts [9,10] or thulium-doped silica fiber lasers [11]. These sources are now well established, and applications such as MIR high-resolution spectroscopy are possible. These methods guarantee good spectral coverage and coherence, but usually require delicate experimental setups with large footprints.

Significant effort has recently been made for achieving chip-based MIR frequency combs. Microresonator frequency combs (Kerr-combs) have been significantly improved [12–15] and have been extended to the MIR region [16–19]. Although Kerr-combs can be produced in different material platforms, they still require a high-power continuous-wave (CW) laser as well as an evanescent coupling system, especially difficult to achieve in the MIR and THz regions.

Quantum cascade lasers (QCLs) have proven to be semiconductor lasers capable of generating comb radiation in the MIR and THz regions [20–23]. As the comb formation takes place directly in the QCL active region, QCL frequency combs (QCL-combs) offer the unique possibility of a completely integrated chip-based system capable of performing broadband high-resolution spectroscopy. Such a compact system is ideal for applications requiring the detection of several different molecules masked by a complex background matrix.

Meanwhile, dual-comb spectroscopy using QCL-combs has been demonstrated [24,25], and a theoretical description of the comb formation has recently been developed [26,27]. However, key characteristics of QCL-combs—such as optical bandwidth and power-per-mode distribution—still need to be improved in order to better address spectroscopy applications.

Group delay dispersion (GDD) plays an important role in the formation of QCL-combs [20,21,27]. In this work, we investigate how the comb operation of QCLs is influenced by the device dispersion. We measure the GDD of such a device while in operation just below threshold, and we investigate a scheme for controlling the dispersion in MIR QCL-combs. We demonstrate that a dispersion compensation scheme based on a Gires–Tournois interferometer [28] (GTI) directly integrated into the QCL-comb improves the comb performance. In particular, we show that the current range where the comb operates increases, effectively suppressing the high-phase-noise regime usually observed in QCL-combs [20,21,24,29,30]. Additionally, the power-per-mode distribution is improved. The CW output power of these combs can be as high as $\simeq 150\mathrm{mW}$, and their optical spectra are centered at $1330{\mathrm{cm}}^{-1}$ (7.52 μm) with up to $70{\mathrm{cm}}^{-1}$ of optical bandwidth.

2. INTEGRATED GIRES–TOURNOIS INTERFEROMETER FOR DISPERSION COMPENSATION

Optical frequency combs are generated when the different longitudinal modes of a laser are phase-locked [1,2], creating an array of equidistantly spaced modes. As previously demonstrated [20,31], broadband QCLs can achieve frequency comb operation by using four-wave mixing (FWM) as a phase-locking mechanism. Combined with the short gain recovery time ($\tau \simeq 0.3\mathrm{ps}$) inherent of intersubband transitions, QCL-combs show a phase signature comparable to a frequency-modulated laser [20,26].

An efficient FWM process only occurs if the phase-mismatch $\mathrm{\Delta }k$ between the modes involved in the FWM process nearly vanishes [32,33], i.e.,

On this basis, we design QCL-comb sources with a nearly zero GDD. As QCLs are based on heterostructures where the composition can be tailored, the material dispersion can be controlled. The MIR QCL-combs used in this study are based on ${\mathrm{In}}_{0.60}{\mathrm{Ga}}_{0.40}\mathrm{As}/{\mathrm{In}}_{0.355}{\mathrm{Al}}_{0.665}\mathrm{As}$ heterostructures grown on InP. Also, we optimize the mode profile in the waveguide for reducing the contribution of waveguide dispersion. This optimization is achieved by adjusting the number of periods of the heterostructure and the doping profile of the InP cladding layer grown on top of the active region. Finally, the laser gain design is based on two different bound-to-continuum strain-balanced designs in the active regions, which are designed in order to minimize the dispersion introduced by the gain [20].

Figure 1 shows the typical performance of MIR QCL-combs engineered in order to operate near the zero-dispersion region. The comb optical spectra and repetition frequency are measured simultaneously [cf. Fig. 1(a) and Section 1 of Supplement 1]. The device operates at room temperature emitting $>10\mathrm{mW}$ of output power in CW operation [cf. Fig. 1(b)]. More importantly, we also report in Fig. 1(b) the three different regimes that are typically observed in QCL-combs [20,21,24,29,30]. The laser emits single-mode radiation after the laser threshold. After a second threshold, the laser operates in a comb regime. Finally, at higher values of current, we observe a third regime, called the high-phase-noise regime hereafter. These three different regimes are well observed when measuring the radio-frequency (RF) beatnote and the optical spectrum, as shown in Figs. 1(c) and 1(d), respectively. In the single-mode regime, no RF beatnote is observed. The comb regime is characterized by a single low-noise beatnote, corresponding to a regime where all the modes are phase-locked [20] and equidistantly spaced [24]. In contrast, the high-phase-noise regime is identified by a broader beatnote. In this high-phase-noise regime, both the amplitude and phase noise of the laser are significantly higher than in the comb regime [30].

 

Fig. 1. Standard QCL-comb performances. (a) Setup used for characterizing the QCL-comb. The optical spectrum is measured with a FTIR (Bruker IFS 66/S, $0.12{\mathrm{cm}}^{-1}$ resolution). A bias-tee is inserted between the low-noise current driver (Wavelength Electronics) and the QCL-comb. The radio-frequency (RF) port of the bias-tee is connected to a spectrum analyzer (Rhode & Schwarz FSU50). BS, beam-splitter; FTIR, Fourier transform infrared spectrometer. (b) Power-current-voltage of a QCL-comb (4.5 mm long, standard HR coating on the back-facet, episide-down mounted on AlN submount) in CW operation at different temperatures. Single-mode, comb, and high-phase-noise regimes are highlighted. (c) Electrical RF spectra acquired at $T=-15°\mathrm{C}$ at different values of current, measured with a spectrum analyzer [$\mathrm{span}=50\mathrm{MHz}$, resolution bandwidth $(\mathrm{RBW})=30\mathrm{kHz}$, acquisition $\mathrm{time}=20\mathrm{ms}$]. The RF spectra are centered at 9.95 GHz, corresponding to the RF beatnote created by a 4.5 mm long device. Comb and high-phase-noise regimes are highlighted. (d) Optical spectra acquired at $T=-15°\mathrm{C}$ at the same values of current as in (c) and measured with a FTIR ($0.12{\mathrm{cm}}^{-1}$ resolution). The QCL-comb spectrum is centered at $1325{\mathrm{cm}}^{-1}$ and spans over $60{\mathrm{cm}}^{-1}$ in the comb regime. (e) Intermode beat spectrogram generated by a QCL-comb source (6.0 mm long, standard HR coating on the back-facet) acquired in the high-phase-noise regime ($I=950\mathrm{mA}$, $T=-15°\mathrm{C}$). The FTIR is driven in step-scan mode with a resolution of $0.25{\mathrm{cm}}^{-1}$. (f) Cuts of the spectrogram shown in (e) at 1286.1, 1305.1, and $1342.1{\mathrm{cm}}^{-1}$ ($\mathrm{span}=200\mathrm{MHz}$, $\mathrm{RBW}=100\mathrm{kHz}$, acquisition $\mathrm{time}=200\mathrm{ms}$).

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Moreover, it is crucial to understand how the comb is destabilized when operated in the high-phase-noise regime. In essence, one would like to know whether there is a region of the optical spectrum that is mainly responsible for destabilizing the comb. To answer this question, we have performed the so-called intermode beat spectroscopy technique [20]. By measuring the RF beatnote through a Fourier transform infrared spectrometer (FTIR), this technique allows us to know which region of the optical spectrum is generating which part of the RF beatnote. Figure 1(e) shows the intermode beat spectrogram of a device coated with a high-reflectivity (HR), acquired in the high-phase-noise regime. We observe that two broad regions of the optical spectrum (between $1280–1300{\mathrm{cm}}^{-1}$ and $1320–1360{\mathrm{cm}}^{-1}$) are generating a broad beatnote, therefore destabilizing the comb (see Supplement 1 for a detailed characterization of this device). This can also be observed in Fig. 1(f), where cuts of the spectrogram of Fig. 1(e) are shown at different optical frequencies. As a result, when the comb is destabilized by dispersion, the phase noise is the result of beatings across the entire spectrum and there is no evidence for cohabitation of locked and unlocked spectral regions.

Comb operation is therefore achieved only for a narrow range of the dynamic range of the laser operation. Moreover, comb operation is observed in regions relatively close to the laser threshold, where both output power and optical spectral bandwidth are small compared to roll-over, as shown in Fig. 1(b). Finally, we also observe in Fig. 1(d) that the power distribution between the modes is highly inhomogeneous, which is not optimal for spectroscopy applications based on frequency combs [24].

For further controlling the dispersion of QCL-combs, we integrate a GTI mirror [28] on the back-facet of the QCL-comb. Extensively used in solid state based mode-locked lasers [37], GTI mirrors are optical cavities specifically designed for introducing dispersion. Figure 2(a) shows a cross section of a QCL-comb coated with a GTI mirror taken with a scanning electron microscope (SEM), and Fig. 2(b) shows a schematic of the integration of a GTI mirror on a QCL-comb. The GTI mirror is directly deposited on the back-facet of the device and is composed of several layers of ${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{SiO}}_2$ and terminated with a gold layer (see Sections 2 and 3 of Supplement 1). Assuming no absorption is present on the coating, a GTI mirror usually constitutes a broadband HR coating. In addition, dispersion is introduced as the phase of the reflected light becomes frequency dependent due to the resonance effect introduced by the optical cavity. The dispersion introduced by a GTI is periodic with a period dependent on the length and on the refractive index of the material. By careful control of these parameters, a GTI mirror can introduce positive or negative dispersion to the QCL-comb (see Sections 2 and 3 of Supplement 1).

 

Fig. 2. GTI mirrors for dispersion compensation. (a) SEM picture of a cross section parallel to the laser ridge of the QCL-comb, which is coated with a GTI mirror. The upper left side shows the laser active region. The different layers of the GTI mirror can be observed as the vertical lines on the right side of the picture. (b) Schematic view of GTI mirror coated either on the back-facet of a QCL-comb or on a substrate (InP, 320 μm thick) to be used for dispersion characterization. The GTI acts as a high-reflectivity mirror but adds a frequency-dependent group delay, therefore introducing dispersion. (c) Setup used for the characterization of the dispersion introduced by the GTI mirror. The GTI mirrors coated on a substrate are measured in reflection on the sample compartment of the FTIR (Section 2 of Supplement 1). DUT, device under test. (d) Measured and simulated value of the GDD created by a GTI mirror. The GDD is measured over a wide spectral range in order to observe the GDD oscillations introduced by the GTI. The spectral region where the QCL-comb operates is highlighted. The GDD of a standard HR coating (300 nm of ${\mathrm{Al}}_2{\mathrm{O}}_3$, 150 nm of gold) is also represented. Inset: zoom on the spectral region where the QCL-comb operates, showing the negative GDD introduced due to the presence of the residual absorption of ${\mathrm{SiO}}_2$.

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Different types of GTIs were evaporated on the back-facet of several devices. During each evaporation, an InP substrate was coated as a reference sample so the dispersion introduced by the GTI mirror can be independently characterized. A FTIR was used to measure the complex reflection spectrum of the coating [see Fig. 2(c) and Section 2 of Supplement 1]. Figure 2(d) shows the measured values of the GDD introduced by the GTI mirror, as well as the GDD obtained by simulation (see Section 2 of Supplement 1). The periodic variations of the GDD of a GTI are observed. If the introduction of negative dispersion is desired, the GTI is designed such that one of its negative resonances lies in the spectral region of the QCL-comb. This is depicted in Fig. 2(d), where the section of the spectrum where the comb operates is highlighted. At this particular resonance, situated around $1300{\mathrm{cm}}^{-1}$, we observe a disagreement between the simulated value of GDD and its measured value. This disagreement is attributed to the fact that ${\mathrm{SiO}}_2$ starts to be absorbing in this spectral region (see Sections 2 and 3 of Supplement 1). This absorption adds a contribution to the total GDD introduced by the GTI. The value introduced at the minimum of this resonance is measured to be $\simeq -7000{\mathrm{fs}}^2$ [see inset in Fig. 2(d)]. Additionally, the absorption introduced by the GTI layer is also characterized (see Section 3 of Supplement 1). In particular, an increase of the absorption introduced by the GTI coating is observed for wavenumbers below $\sim 1315{\mathrm{cm}}^{-1}$. This has the direct effect of suppressing the lasing in the spectral region below $\sim 1315{\mathrm{cm}}^{-1}$ for devices coated with GTI mirrors. We also use the same measurement technique for characterizing the dispersion introduced by a standard HR coating (300 nm of ${\mathrm{Al}}_2{\mathrm{O}}_3$, 150 nm of gold). As expected, the GDD of a standard HR coating does not show any resonance effect and does not add any significant dispersion to the device.

3. DISPERSION MEASUREMENTS IN QCL-COMBS

For further evaluating the dispersion compensation technique, we also measure the dispersion of QCL-comb sources after being coated with a GTI mirror. GTI mirrors introducing positive and negative dispersion were evaporated on different QCL-comb sources. Particular attention was given to use close to identical devices by using lasers with the same dimensions (ridge width and length) and from the same fabrication process. In the previous section, a QCL-comb source coated with a standard HR coating was characterized (see Fig. 1), and we use this device as a reference sample.

The dispersion of QCL-combs is measured by driving the QCL-comb close to but below threshold and acquiring the interferogram generated by a FTIR, as shown schematically in Fig. 3(a). By careful analysis of the interferogram, the relative phase accumulated through a round-trip on the device can be extracted and the GDD of the QCL-comb can be measured (see Section 2 of Supplement 1). Figure 3(b) shows the relative phase of a device coated with a GTI mirror introducing negative dispersion, when the device is biased 2% below threshold. The dispersion of devices coated with different GTI mirrors is shown in Fig. 3(c). The determination of the dispersion is limited to the spectral range of the active region gain bandwidth (typically from 1250 to $1460{\mathrm{cm}}^{-1}$), as this method is based on sub-threshold measurements (see Section 2 of Supplement 1). The QCL-comb coated with a standard HR coating is operating with a total positive GDD of $4131{\mathrm{fs}}^2$ (measured at $1330{\mathrm{cm}}^{-1}$). A similar device coated with a GTI mirror introducing positive GDD shows a total dispersion of $10,602{\mathrm{fs}}^2$. The difference between the values of GDD of these two devices corresponds to the value of the GDD introduced by the GTI mirror, thus substantiating the claim that the added GDD is due to the engineered GTI coating. Finally, we also measure the dispersion of the device coated with a GTI mirror designed to introduce negative dispersion. This device is operating with a total negative GDD of $-3546{\mathrm{fs}}^2$.

 

Fig. 3. Dispersion measurements of QCL-combs. (a) Setup used to acquire the interferogram generated by the QCL-comb biased below threshold on a FTIR. This interferogram is used to retrieve the relative phase accumulated through a round-trip on the device (see Section 2 of Supplement 1). (b) Relative phase accumulated through a round-trip on a QCL coated with a GTI mirror introducing negative dispersion ($T=-15°\mathrm{C}$, $I=770\mathrm{mA}$ corresponding to a value $\sim 2\%$ below threshold). The measured device corresponds to the device characterized in Fig. 4 (4.5 mm long). (c) Measurement of the GDD of QCL-combs. Three different coatings were evaporated on the back-facet of three different devices (4.5 mm long devices cleaved together, $T=-15°\mathrm{C}$, current being set to $\sim 2\%$ below threshold for the three devices). The device showing negative GDD (green curve) corresponds to the device shown in Fig. 4. (d) Measurement of the GDD of the QCL showing negative GDD [green curve of (c)] as a function of the laser current ($T=-15°\mathrm{C}$).

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For justifying the introduction of the term $\mathrm{\Delta }{k}_{\mathrm{gain}}$ to the phase-matching condition for QCL-combs, the effect of gain on the dispersion was investigated by measuring the GDD of a QCL-comb as a function of the laser current. The device is always driven below threshold, as the intention is to study the dispersion of the device with no effect of gain clamping. Figure 3(d) shows the GDD of the device coated with a GTI mirror introducing negative dispersion for different driving currents (sub-threshold measurements). The peak GDD value changes from $2591{\mathrm{fs}}^2$ at $I=530\mathrm{mA}$ to $-26{\mathrm{fs}}^2$ at $I=770\mathrm{mA}$, demonstrating that the effect of gain on the total device dispersion is significant and that the gain-induced dispersion has to be considered when designing QCL-combs.

4. SUPPRESSION OF THE HIGH-PHASE-NOISE REGIME

QCL-combs where the dispersion was controlled by GTI mirrors are now characterized. As shown in Fig. 3(c), devices coated with a standard HR coating show small but positive dispersion values. Moreover, devices coated with GTI mirrors introducing positive dispersion showed the same performance as QCL-combs coated with standard HR coatings. Therefore, only the devices coated with GTI mirrors introducing negative dispersion are investigated here.

The performance of a QCL-comb coated with a GDD mirror introducing negative GDD ($-6814{\mathrm{fs}}^2$ introduced at $1258{\mathrm{cm}}^{-1}$) is shown in Fig. 4. We use the same characterization setup as already described in Fig. 1(a), where optical spectra and RF spectra can be acquired simultaneously. Figure 4(a) shows the power-current-voltage measured in CW operation for a GTI-coated QCL-comb with negative dispersion. As GTI mirrors also act as HR coatings, we observe a decrease of the threshold current as well as an increase of the slope efficiency when compared to uncoated devices. The device emits 142 mW at $T=-20°\mathrm{C}$ in CW operation. We observe in the RF spectra [see Fig. 4(b)] that the beatnotes generated at the comb repetition frequency are extremely narrow (FWHM $<500\mathrm{Hz}$) for all the different values of current. Therefore, the comb regime—which was present over a small range of the dynamical range of the QCL-comb without a GTI mirror—is now observed over a large dynamic range of the QCL-comb. More importantly, we observe that the device operates in the comb regime until the laser roll-over and that no high-phase-noise regime is observed. Finally, as observed in the optical spectra shown in Fig. 4(c), the power-per-mode distribution is more homogeneous when compared to the QCL-comb with no dispersion compensation [see Fig. 1(c)]. These findings were observed in several similar devices (same length, same laser fabrication process) that were coated with the same GTI mirror (see Section 4 of Supplement 1). However, the GTI structure developed in this work introduces some absorption in the spectral region below $1315{\mathrm{cm}}^{-1}$. Therefore, as shown in Fig. 4(d), no lasing is observed in the region of 1280–$1315{\mathrm{cm}}^{-1}$ for devices coated with GTI mirrors, although the optical spectrum of the reference device coated with a HR coating [see Fig. 1(d)] spans from 1280 to $1370{\mathrm{cm}}^{-1}$. This issue can be addressed by choosing transparent materials in this spectral region (see Section 3 of Supplement 1)

 

Fig. 4. Dispersion compensated QCL-combs. (a) Power-current-voltage of a QCL-comb (4.5 mm long, episide-down mounted on AlN submount) coated with a GTI mirror introducing negative dispersion. The measurements are done in CW operation at different temperatures. Single-mode and comb regimes are highlighted. (b) Electrical RF spectra acquired at $T=-10°\mathrm{C}$ for different values of current, measured with a spectrum analyzer ($\mathrm{span}=200\mathrm{kHz}$, $\mathrm{RBW}=500\mathrm{Hz}$, acquisition $\mathrm{time}=20\mathrm{ms}$). The RF spectra are centered at 9.814 GHz, corresponding to the RF beatnote created by a 4.5 mm long device. The measured RF spectra show single and narrow beatnotes (FWHM $<500\mathrm{Hz}$). No high-phase-noise regime is observed. (c) Optical spectra acquired at $T=-10°\mathrm{C}$ at the same values of current as in (b) and acquired with a FTIR ($0.12{\mathrm{cm}}^{-1}$ resolution). The QCL-comb spectrum is centered at $1335{\mathrm{cm}}^{-1}$ and spans over $45{\mathrm{cm}}^{-1}$ in the comb regime.

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High-performance QCL-combs are obtained when compensating the dispersion of a 6 mm long device with a GTI mirror introducing negative dispersion ($-6814{\mathrm{fs}}^2$ introduced at $1258{\mathrm{cm}}^{-1}$). Figure 5(a) shows the optical spectrum of such a QCL-comb, acquired when the laser is close to roll-over. The RF spectrum is also measured and is shown in Fig. 5(b). The power-per-mode distribution on the optical spectrum shows a normalized standard deviation of 31%. More importantly, the RF spectrum shows that the laser is operating in a comb regime, characterized by a single and narrow RF beatnote (FWHM $<30\mathrm{kHz}$). Although a broad pedestal on the RF spectrum is observed, this pedestal is at a level 40 dB lower than the carrier. Moreover, the RF beatnote signal-to-noise ratio [40 dB for the beatnote shown in Fig. 5(b)] is significantly higher than the ones observed for QCL-combs with a standard HR coating [see Fig. 1(c)]. Again, no high-phase-noise regime was observed on this device.

 

Fig. 5. High-performance QCL-combs. (a) Optical spectrum of a high-performance QCL-comb (6.0 mm long, GTI mirror on the back-facet introducing negative dispersion) acquired at $T=-6°\mathrm{C}$, $\mathrm{I}=1560\mathrm{mA}$, emitting $\simeq 150\mathrm{mW}$ of output power in these conditions. The power-per-mode distribution shows a normalized standard deviation of 31%. (b) RF spectrum measured at the same value of current as in (a), acquired with a spectrum analyzer ($\mathrm{span}=50\mathrm{MHz},\mathrm{RBW}=30\mathrm{kHz}$, acquisition $\mathrm{time}=20\mathrm{ms}$). The RF spectrum shows a narrow beatnote, characteristic of comb operation, together with a pedestal observed at a level 40 dB lower than the carrier. The signal-to-noise ratio of the RF beatnote is more than 40 dB.

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5. DISCUSSION AND CONCLUSION

Our experimental results show that QCLs can be precisely designed in order to achieve high-performance MIR semiconductor-based frequency combs. A detailed experimental analysis of the dispersion of QCL-comb sources is performed, and the concept of an integrated GTI mirror for dispersion control of QCL-comb sources is introduced. GTI mirrors were designed in order to introduce either positive or negative GDD. Improved designs of GTI mirrors based on different materials (Ge, ${\mathrm{YF}}_3$) could also be implemented in case higher values of dispersion compensation are needed. By fine characterization of the dispersion introduced by the GTI mirror, QCL-combs operating at negative GDD values were obtained. This led to QCL-combs showing a comb regime spanning over a wide current range, and no signature of the high-phase-noise regime was observed. Moreover, the power-per-mode distribution on these QCL-combs is more homogeneous compared to previously designed QCL-combs. These devices are ideal for systems using QCL-combs for spectroscopy applications, where a highly inhomogeneous power distribution along the comb modes is detrimental for high accuracy spectroscopy, as important values of signal-to-noise ratio are needed over the entire spectrum [24].

In conclusion, we have demonstrated a high-performance MIR QCL-comb obtained by dispersion compensation. By operating in the negative dispersion regime, the QCL-comb performance was dramatically improved. We achieved high-power QCL-combs ($\simeq 150\mathrm{mW}$) spanning over $\simeq 70{\mathrm{cm}}^{-1}$, where the comb operation regime is extended over a wide current range and where no signature of the high-phase-noise regime is observed. The spectral coverage of the QCL-combs is only limited by the bandwidth of the gain medium. Therefore, by using GTI mirrors to compensate the dispersion of multi-stack QCL designs with broader spectrum, QCL-combs as broad as $300{\mathrm{cm}}^{-1}$ could in principle be fabricated. For compensating the dispersion in a wider range, GTI mirrors terminated with dielectric HR coatings could be realized or double GTI designs could also be implemented [38]. Conversely, the comb structure changes dramatically when operating in the negative dispersion region, as shown by the increase of the comb operation regime and also by the modification of the power distribution spectrum. This is a signature that the control of the dispersion can induce a change in the phase distribution between the comb modes. By measuring the relative phases of the comb modes as well as by measuring an ultrashort temporal profile of the laser intensity, utilizing a frequency-resolved optical gating technique [39] or an ultrafast temporal magnifier [40,41]—as recently done on the field of Kerr-combs [14,42] and also for QCL-combs [29,43]—the structure of a QCL-comb can be fully understood. The control of the comb phases could potentially lead to the creation of QCL-combs operating in comb states not observed to date.