Multi-octave two-color soliton frequency comb in integrated chalcogenide microresonators

Abstract

Mid-infrared (MIR) Kerr microcombs are of significant interest for portable dual-comb spectroscopy and precision molecular sensing due to strong molecular vibrational absorption in the MIR band. However, achieving a compact, octave-spanning MIR Kerr microcomb remains a challenge due to the lack of suitable MIR photonic materials for the core and cladding of integrated devices and appropriate MIR continuous-wave (CW) pump lasers. Here, we propose a novel slot concentric dual-ring (SCDR) microresonator based on an integrated chalcogenide glass chip, which offers excellent transmission performance and flexible dispersion engineering in the MIR band. This device achieves both phase-matching and group velocity matching in two separated anomalous dispersion regions, enabling phase-locked, two-color solitons in the MIR region with a commercial 2-μm CW laser as the pump source. Moreover, the spectral locking of the two-color soliton enhances pump wavelength selectivity, providing precise control over soliton dynamics. By leveraging the dispersion characteristics of the SCDR microresonator, we have demonstrated a multi-octave-spanning, two-color soliton microcomb, covering a spectral range from 1156.07 to 5054.95 nm (200 THz) at a −40 dB level, highlighting the versatility and broad applicability of our approach. And the proposed multi-octave MIR frequency comb is relevant for applications such as dual-comb spectroscopy and trace-gas sensing.

Graphical abstract

Keywords

Mid-infrared / Kerr microcombs / Two-color soliton / Multi-octave / Chalcogenide glasses

1 Introduction

Mid-infrared (MIR) spectral region is critically important as its two atmospheric windows and stronger absorption strength of many molecules than those in the near-infrared (NIR) spectrum [1], which are particularly attractive in applications ranging from materials science to environmental monitoring [2]. Optical frequency combs, which provide equidistant frequency markers in the infrared, visible, and ultraviolet regions, have triggered substantial advances in optical frequency metrology and precision measurements [3, 4]. Over the past decade, advances in microfabrication technology have enabled the development of microresonator-based Kerr frequency combs, which are compact size, broadband, and suitable for building a portable device [5–7]. However, the generation of compact, multi-octave-spanning MIR frequency combs continue to pose significant challenges due to the absence of suitable MIR continuous-wave (CW) pump lasers and appropriate photonic materials for the core and cladding of integrated devices [8, 9].

A promising solution lies in the development of multi-color solitons, which offers broad spectral bandwidths and flexible pump wavelength locations [10–12]. These solitons generate multiple soliton-like components through intersoliton Cherenkov radiation, each exhibiting a sech² envelope, while behaving as a single pulse in the time domain due to their similar group velocities [13]. Supporting a multicolor soliton state requires multiple separated anomalous dispersion regions, necessitating intricate dispersion engineering [9]. Chalcogenide glasses (ChG), comprising one or more chalcogen elements such as sulfur, selenium, and tellurium (S, Se, Te), along with various metals or non-metals, demonstrate exceptional optical transmission properties that span from the visible to the far-infrared regions (> 25 μm) and possess an amorphous structure conducive to multilayer deposition on silicon wafers via thermal evaporation [14, 15]. Moreover, their optical characteristics can be tuned by manipulating the elemental composition, which is advantageous for achieving flexible dispersion engineering in MIR photonic devices [16]. In recent years, integrated chalcogenide glass photonic devices have attracted significant interest due to their versatile applications in supercontinuum generation, Raman and Brillouin lasers, parametric oscillation, and integrated photonic computing, marking them as pivotal components in the advancement of photonic technologies [17–19]. In addition, the 2-μm band, at the leading position among the new wavelength bands, has developed mature devices, such as commercial 2-μm distributed feedback (DFB) lasers and thulium-doped fiber amplifiers (TDFA) with more than 240 nm gain bandwidth and low noise figure, which is utilized in various application including gas sensing, optical interconnection, and medical treatment [20–23].

In this work, we propose a novel microresonator architecture termed the slot-concentric-dual-ring (SCDR) based on ChGs with low absorption loss and high optical stability in the MIR region [7, 24], including Ge₁₅As₂₀Te₄₅Se₁₅ and Ge₁₅Sb₁₅Se₇₀. The SCDR microresonator promotes mode coupling, which enables group velocity matching and the formation of two distinct regions of anomalous dispersion, both essential for supporting a two-color soliton. The introduction of a slot structure within the microresonator allows for precise engineering of the integrated dispersion curve, thereby enabling the tunability for the phase-matching location of new-color and broadening the comb's bandwidth to span multi-octave. Utilizing a CW pump in the 2-μm band, a multi-octave spanning two-color soliton, extending from 1156.07 to 5054.95 nm (200 THz) has been achieved. Our simulations also demonstrate spectral locking, which stabilizes the generation of frequency combs and offers flexibility in pump wavelength selection. The SCDR microresonator provides a compact and efficient solution for generating broadband MIR frequency combs with customizable spectral profiles, suitable for advanced spectra for medical diagnosis, environmental monitoring and materials science.

2 Operation principle

2.1 Principle of two-color soliton generation and microresonator design

Two-color soliton state comprises dual soliton-like components that engage in energy exchange via inter-soliton Cherenkov radiation. These components maintain approximately equal amplitudes while exhibiting distinctly different central frequencies [10]. Such a soliton state is enabled by group-velocity matched co-propagation of two colors in two separated anomalous dispersion regions [25]. In this work, we propose the SCDR microresonator, consisting of concentric inner and outer rings, each featuring a horizontal GeSbSe slot. These are encapsulated by dual GeAsTeSe layers atop a ChG substrate (Ge₂₅Sb₁₀S₆₅, GeSbS), see Fig. 1a. Chalcogenide materials providing broad transparency windows, high refractive indices and nonlinear refractive index coefficients at 2 μm are selected in this work, detailed in Table 1. Mode hybridization arises when the optical path lengths (OPLs) of these concentric rings align, due to coupling between the rings, with the OPLs for each ring being independently calculable [26].

The hybridization couples the modes of the inner and outer rings, resulting in the formation of a pair of symmetric and antisymmetric modes, each exhibiting distinct free spectral range (FSR) behaviors as the wavelength increases. The antisymmetric mode, with its decreasing FSR, induces additional anomalous dispersion in the coupling region, a feature exploited to achieve multiple zero dispersion wavelengths [28]. Conversely, the symmetric mode exhibits an increased FSR, resulting in normal dispersion around the coupling region. To create two separate regions of anomalous dispersion, the symmetric mode is selected, as shown in Fig. 1b (I). Additionally, group-velocity matching for the co-propagation of two colors is required, which can be calculated by

\begin{array}{}\text{(1)}& {\nu }_{\text{g}}=c/n_{\text{g}}\mathrm{,}\end{array}

where n_g is the group index of modes given by

\begin{array}{}\text{(2)}& n_{\text{g}}=\frac{c}{2\text{π}\cdot \text{FSR}\cdot R}\mathrm{,}\end{array}

where R is the radius of the microresonator. The decrease of n_g induced by the symmetric mode coupling enables the match of n_g for the two separated anomalous regions, as shown in Fig. 1b (II). Moreover, to facilitate inter-soliton Cherenkov radiation, phase-matching between the pump and the new color is required [9, 10, 13], which can be approximated by

\begin{array}{}\text{(3)}& D_{\text{int}}={\omega }_{\mu }-{\omega }_0-D_1\mu ={\displaystyle \sum _{k=2}^{\infty }\frac{1}{k!}}{D}_k{\mu }^k\approx \mathrm{0,}\end{array}

where ω_μ are the resonance frequencies of the microresonators, determined by

\begin{array}{}\text{(4)}& {\omega }_{\mu }={\omega }_0+D_1\mu +\frac{1}{2}{D}_2{\mu }^2+\cdots \mathrm{,}\end{array}

where D₂ is the second-order dispersion. D_int represents the integrated dispersion, D_k presents the k-order of the dispersion coefficient, ω_0(μ) is the angular resonant frequency for pump mode and other modes, and μ is the relative mode number. The strong anomalous dispersion, induced by symmetric mode coupling, decrease the D_int profile so as to garner the phase-matching between the two colors, see Fig. 1b (III). When group velocity and phase for the new-color soliton are matched at the same wavelength, located in another anomalous dispersion, a new-color soliton is generated, which features roughly similar amplitudes but distinctly different center frequencies with the pump, as shown in Fig. 1b (IV).

For comparative purposes, the schematic for the generation of broadband MIR two-color soliton based on a Slot microresonator fabricated from the same materials is depicted in Fig. 1b. This microresonator, unaffected by mode hybridization, exhibits a smoother dispersion profile with a single anomalous dispersion region. The Slot microresonator's D_int profile escalates at longer wavelengths due to the accumulation of anomalous dispersion, precluding the support of a two-color soliton. Consequently, it generates only a traditional soliton with a sech² envelope, its bandwidth governed by the D₂ profile [29]. Moreover, the elevated D_int presents a barrier too high to sustain a high-amplitude dispersive wave (DW) at longer wavelengths [30, 31]. In contrast, the SCDR microresonator facilitates the generation of a new-color soliton, which not only matches the soliton supported by the pump in amplitude but also differs significantly in frequency, fulfilling the criteria for a two-color soliton. This new-color soliton, combined with a DW, is tailored for the MIR band, thereby effectively expanding the bandwidth of the comb and enhancing the energy at longer wavelengths.

We simulate the generation of a broadband MIR comb using a 2 μm pump by engineering the dispersion of the SCDR, see Fig. 2. Only TM modes are considered in this work, which are easier to realize group-velocity and phase matching conditions at the same wavelength. While the group-velocity and phase of TE modes matched far from each other, which is hard to realize two-color soliton. Meanwhile, the guided modes for TM modes are finely confined in the Slot layer, achieving flexible tunability. In general, the guided mode is mainly confined in the high-index Slot layer, resulting in the dispersion profile at short wavelengths similar to the isolate outer-ring [32]. After mode hybridization, it resembles the isolated inner ring's dispersion profile. Mode coupling is designed to occur around 2700 nm, featuring strong normal dispersion and separating the two anomalous dispersion regions. To verify the affection of the mode coupling, the inner and outer rings of the SCDR are simulated independently. Among the four modes, only the symmetric mode enables group index matching for the new-color soliton at 3360.5 nm, provided by the negative slope of n_g around 2700 nm due to mode hybridization.

For comparison, we simulate the D₂ and D_int profiles of traditional microresonators such as Strip, Slot, and Dualring microresonators, adjusted for the MIR region using a 2 μm pump (Fig. 2c, d). The Strip microresonator, with limited design flexibility, cannot maintain a low D_int barrier for high amplitude DW in the MIR band [30, 31], resulting in a narrowband soliton. The Slot microresonator offers additional design freedom to adjust dispersion via the anti-crossing effect caused by mode transition, extending the comb's bandwidth through DW [33–36]. However, its spectrum is still based on the sech²-shaped D₂ profile, preventing a broadband MIR comb with a 2 μm pump. The Dualring microresonator, enabled by mode hybridization, achieves group velocity and phase-matching for a two-color soliton. However, the introduced normal dispersion confines the new color close to the pump, providing narrow bandwidth [9, 37]. Our SCDR microresonator combines the advantages of Slot and Dualring microresonators, providing tunability and realizable two-color soliton. Consequently, group index and phase-matching of new-color are designed to coincide at 3360.5 and 3360.16 nm, respectively. Thus, a new-color soliton is expected around 3360 nm, with similar amplitudes to the pump-supported soliton. Additionally, the Slot structure's tunability achieves flat and broad D_int at longer wavelengths, enhancing the DW amplitude around 4487.64 nm, greatly extending the bandwidth and energy in the MIR region. The DW around 1312.22 nm also broadens the bandwidth.

2.2 Dispersion engineering

To obtain a new-color soliton with a high comb power in the MIR region, the D_int profile should be finely adjusted, since the phase-matching for new-color and the generation of DWs are approximated by D_int = 0. Meanwhile, a low D_int barrier is required for the generation of Cherenkov radiation [37], enabling the two-color soliton and DWs. Therefore, the tunability of our SCDR microresonator is necessary to be verified by simulating the six structural parameters individually. The standard profile (yellow) satisfies the requirements for two-color soliton discussed above, with the same geometric parameters in Fig. 2a.

As shown in Fig. 3b and f, increasing W₂ or the gap gradually shifts the D_int profile at longer wavelengths from negative to positive, allowing adjustment of the phase mismatch for the new-color without changing its position. While increasing W₁, H₁, H₂, and T regulate the phase-matching wavelength for the new-color to longer wavelength, as indicated by the red arrows in Fig. 3a, c, d, and e. The D_int barrier caused by mode coupling can be effectively reduced by increasing W₁ or T, or by decreasing W₂ and the gap, as illustrated in Fig. 3a, b, e, and f. Moreover, the DW before mode coupling can be shifted to shorter wavelengths, further broadening the comb's bandwidth, by decreasing H₁ and T, as seen in Fig. 3c and e. Conversely, the D_int profile, which determines the phase-matching location and barrier for Cherenkov radiation at longer wavelengths, can be independently tailored while preserving the profile at shorter wavelengths, as shown in Fig. 3a and f. In addition, as variation of T affects the D_int profile a lot, thickness of the Slot layer requires be precisely controlled with a resolution down to the few nanometers level, as seen in Fig. 3e. By adjusting these structural parameters, the locations of the phase-matching and the height of the D_int barrier can be precisely tuned.

3 Results and discussion

3.1 Two-color soliton generation

To study the generation of a two-color soliton comb, we simulate two-color soliton using the Lugiato-Lefever equation in the integrated chalcogenide SCDR microresonator [38],

\begin{array}{}\text{(5)}& \frac{\partial {\tilde{A}}_{\mu }(t)}{\partial t}=\left(-\frac{k}{2}+\text{i}\left(2\text{π}{\delta }_0\right)+\text{i}{D}_{\text{int}}(\mu )\right){\tilde{A}}_{\mu }-\text{i}g(\mu )F{\left[{\left|A\right|}^{2}A\right]}_{\mu }+\sqrt{k_{\text{c}}}{S}_{\text{in}}\mathrm{,}\end{array}

where A represents the temporal envelopes of the optical field in the microresonators. {\tilde{A}}_{\mu }=F[A(t)], the Fourier transform of the optical field in the azimuthal direction, μ is an integer representing the relative mode number from the pumped wavelength, k is the cavity total decay rate k=k_{\text{i}}+k_{\text{c}}=\frac{\omega }{Q_{\text{i}}}+\frac{\omega }{Q_{\text{c}}} composed of the intrinsic decay rate ki and the external coupling rate k_c. We set Q_i is equal to Q_c to obtain the critical coupling of the microresonator and Q_c is set as 1 × 10⁶ at 2 μm. δ₀ is the pump resonance detuning. g=\frac{h{\omega }^{2}c{n}_2}{n_{\text{g}}^2{V}_{\text{eff}}} represents the Kerr gain coefficient. P_in = |S_in|² = 60 mW is the input pump power and D_int(μ) means the integrated dispersion. Considering that the mode area A_eff will have an in-negligible variation in the MIR region and here to accurately model two-color cavity soliton in the MIR region, the nonlinear reduction induced by mode area (shown in Fig. 4b), high-order dispersion, and self-steepening are included in our model. The mode area A_eff and nonlinear coefficient γ can be calculated by

\begin{array}{}\text{(6)}& A_{\text{eff}}=\frac{\left({\displaystyle \iint }{\left|E(x\mathrm{,}y)\right|}^2\text{d}x\text{d}y\right)}{\left({\displaystyle {\iint }_{\text{core}}}{\left|E(x\mathrm{,}y)\right|}^4\text{d}x\text{d}y\right)}\mathrm{,}\end{array}

\begin{array}{}\text{(7)}& \gamma =\frac{n_2\omega }{c{A}_{\text{eff}}}\mathrm{,}\end{array}

where E(x, y) is the profile of the field, n₂ = 3.4 × 10⁻¹⁷ m²/W is the nonlinear Kerr index for GeSbSe at 2 μm. As shown in Fig. 4b, the nonlinear coefficient γ is inversely proportional to the mode area A_eff. Due to the mode coupling process, there is a bump in the A_eff curve, causing a heavy reduction of the nonlinear coefficient γ.

We begin by theoretically studying the dynamic of two-color soliton with the pump wavelength of 2 μm and the pump power of 60 mW, see Fig. 4. By uniformly tuning pump resonance detuning from – 10 κ to 15 κ, we observed the variation of intracavity power in the SCDR microresonator (Fig. 4a). As the pump detuning shifts from blue detuned to 1.41 κ (I), 5.73 κ (II), and 12.44 κ (III), the system transitions through sub-combs, modulation instability, and a phase-locked two-color soliton state, respectively. Initially, sub-combs form around the primary lines in the bluedetuned region (Fig. 4d (I)). As the pump wavelength redshifts, the sub-combs merge, leading to a chaotic modulation instability (Fig. 4d (II)). Further redshift reaches the reddetuned side, where the resonance exhibits bistable behavior, resulting in a phase-locked two-color soliton state (Fig. 4d (III)). Interestingly, additional anomalous dispersion in the MIR region can facilitate the exchange of energy between two-color soliton through Cherenkov radiation [13]. Hence, a broadband two-color soliton frequency comb can be generated, spanning from 1098.39 to 4770.35 nm at − 40 dB level, and the spectral profile shows two characteristic sech² envelopes at the two phase-matched regions. Meanwhile, two DWs are stimulated at 1202.36 and 4320.72 nm, meeting the satisfaction of the phase matching condition. The spectrum in the two-color regions shows a beating component, creating a modulated envelope for the temporal pulse of the two-color soliton (Fig. 4c) [9]. Furthermore, the two-color soliton temporal pulse is beneath the two backgrounds, further proving the emission of two DWs [39].

For comparison, we also numerically investigate the generation of the soliton comb state in the Slot, Strip, and Dualring microresonator with a pump power of 60 mW. As shown in Fig. 4e, the Strip microresonator, despite achieving phase matching in the MIR region, only produces a single sech² envelope at the pump wavelength due to a high D_int barrier, failing to extend into the MIR region (see Fig. 2). The Slot microresonator, while offering more design flexibility, can achieve another DW at 1273.39 nm but is still unable to flatten the D_int barrier in the MIR region. For the Dualring resonator, additional anomalous dispersion can also be achieved around 2.3 μm, resulting in phase-matching that allows Cherenkov radiation. However, the presence of a high D_int barrier in the MIR region limits its bandwidth expansion to the MIR region. By integrating the dispersion characteristics of the Slot and Dualring, the SCDR microresonator not only achieves a broadband, close-to-zero dispersion in the MIR region. This design supports a significantly broader MIR frequency comb, enabling coverage of MIR wavelengths that conventional structures cannot achieve.

3.2 Pump wavelength selectivity

We concentrate on a pump wavelength around 2 μm, supported by readily available commercial distributed DFB feedback lasers and TDFA. Considering the effect of the D_int barrier, here we select the D_int barrier within one FSR region in different pump wavelengths to effectively generate Cherenkov radiation, which corresponds to the pump wavelength (λ_p) adjusting from 1.90 to 2.15 μm, see Fig. 5a. Meanwhile, the requirement for group velocity-matching between the pump and the new-color soliton, which must reside in the anomalous dispersion regions, restricts the λ_p flexibility to a range from 1748.42 to 2173.37 nm, see Fig. 5b. When these conditions are both satisfied, inter-soliton Cherenkov radiation is facilitated within the spectral region, resulting in the generation of a two-color soliton [13]. To ensure a precise comparison of the two-color soliton in different λ_p, we employ nearly identical pump powers in the aforementioned above LLE model.

As λ_p is equal to 1.90 μm, D_int barrier in the MIR region is close to about one FSR, enabling effective Cherenkov radiation and energy exchange with this MIR color, leading to a two-color soliton with a bandwidth covering the MIR region. As λ_p redshifts from 1.90 to 2.05 μm, the D_int barrier in the MIR region can be reduced to about zero and the comb power of the MIR color raises, respectively. Beyond 2.05 μm, the D_int barrier in the MIR region boosts again, causing a slight reduction of the comb power. Furthermore, as a new color is generated, the spectral profiles of the two-color soliton exhibit spectral locking characteristics, meaning that the spectral profiles of soliton can be sustained even when the λ_p fluctuates in a large range (Fig. 5c). This stability is attributed to strong inter-soliton nonlinear interactions where phase and group velocity matching are optimally satisfied [9]. Therefore, the λ_p selectivity for generating two-color soliton can be robust, enabling the continued generation of the MIR color in the inner ring. Moreover, by combining a commercial DFB laser with thulium-doped fiber amplifiers, the selection of λ_p for generating two-color solitons can be flexibly managed with a pump power of approximately 60 mW.

3.3 On-demand tunable MIR frequency comb

In this section, we have investigated the influence of the geometries to the spectral profiles of the two-color soliton. However, by tailoring the geometries of the SCDR microresonator, the group velocity condition for generating a new-color can possibly be broken. As shown in Fig. 6a (I), two-DWs are generated without a new-color in the MIR region due to the break of the group velocity matching. Furthermore, the D_int in Fig. 6a (II) seems to support a new-color while its group velocity is not in the phase-matched region. Therefore, multi-DWs state is more competitive than the new-color state during the nonlinear process. It is noteworthy that a key dispersion characteristic of the SCDR microresonator allows for the independent tailoring of D_int in the MIR band, while preserving the D_int profiles in the short wavelength region. Thereby, under the conditions required for generating a new-color soliton, the phase-matched region in the MIR region can also be adjusted without affecting the D_int profiles of the short wavelength, which provides a convenient approach to expanding the bandwidth of the two-color soliton to the MIR region. By tailoring the geometries of the SCDR microresonator, the position of the new-color soliton can be varied from 2849.76 nm in (I) to 3399.49 nm in (II), without altering the spectrum at the short wavelength. Ultimately, a multi-octave MIR frequency comb spanning from 1156.07 to 5054.95 nm can be obtained by further modifying the position of the MIR color to longer wavelengths, see Fig. 6b (III).

3.4 Schematic of fabrication procedures

A feasible nano-fabrication procedure for our SCDR microresonator is demonstrated in Fig. 7. First, a 5 μm thick GeSbS lower layer is deposited by thermal evaporation as the substrate of the SCDR on a silicon with a 3 μm thick thermal oxidation layer to avoid the severe absorption of silica in MIR region. Then, a 0.4 μm thick GeAsTeSe lower layer and a 0.15 μm thick GeSbSe slot layer are deposited successively by thermal evaporation. Whereafter, a dry etch trimming approach is utilized to fine control of the thickness of the slot layer with a resolution down to the few nanometers level [29]. Subsequently, another 0.43 μm thick GeAs-TeSe layer is deposited by thermal evaporation as the upper layer of the core. After film deposition, the photoresist is spin-coated onto the wafer, and the waveguide pattern is transferred using a mask. Afterwards, ultraviolet lithography and inductively coupled plasma etching (ICP-RIE) are used to pattern the waveguide, and then the residual photoresist of the chip is removed. The upper layers and gap are etched in an ICP reactive ion etcher with CHF₃ gas. Moreover, a 3 nm thick Al₂O₃ layer on the top surface of the ChG film was deposited to prevent the ChG waveguide' top surface from oxidizing during the resist removing process. In final, our SCDR microresonators have been fabricated.

4 Conclusion

We compare the spectral bandwidth of the two-color soliton comb in this work with several reported frequency combs in the microresonator, as detailed in Table 3. Considering the bandwidth covering the MIR region, both experimental and numerical works relevant to the MIR are included [5–7, 28, 36, 37, 40, 41]. To the best of our knowledge, the state-of-the-art bandwidth of MIR frequency comb in numerical simulation is about 108 THz, with a spanning from 2.2 to 10.5 μm, which employed the generation of two-DWs. However, a MIR laser with a λ_p of 4.51 μm that combines both a narrow linewidth and high CW output power is hard to achieve. In addition to using DW generation to expand frequency comb bandwidth, the creation of multi-color solitons can also extend the frequency comb to a multi-octave span based on broadband inter-soliton Cherenkov radiation [13, 30]. However, the broadband frequency comb of [13, 30] do not cover the MIR region. Thus, to achieve a multi-octave-spanning MIR frequency comb, we combine a two-color soliton with two DWs in this work, ultimately achieving coverage of the MIR region with a λ_p of 2 μm. Our resulting bandwidth of the two-color soliton is comparable to those achieved in the visible and near-infrared regions [30, 31, 33–35, 42].

In summary, we have developed a novel dispersion-engineered SCDR microresonator that generates a two-color MIR frequency comb. By integrating the dispersion characteristics of Slot and Dualring structures, this microresonator achieves group velocity and phase matching, crucial for inter-soliton Cherenkov radiation and supporting dual-color soliton generation. Utilizing a 2 μm pump, we achieved a multi-octave-spanning spectrum ranging from 1156.07 to 5054.95 nm with a modest pump power of 60 mW. Our analysis of both spectral and temporal profiles reveals a spectral locking feature that enhances pump wavelength selectivity, crucial for stable two-color soliton generation. The SCDR microresonator not only facilitates the generation of multi-octave-spanning, tunable two-color solitons with readily accessible pump sources but also represents a significant advancement in designing tailored broadband MIR frequency combs, which pave the way for a range of applications, including multi-heterodyne [43] and dual-comb spectroscopy [44, 45] of characteristic molecular vibrational transitions and trace-gas sensing within the Earth's atmospheric transparency window [2].

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