1. Introduction

Near-infrared (NIR) and mid-infrared supercontinuum (MIR-SC) sources are widely used in spectroscopy [1], optical coherence tomography [2], multi-spectral tissue imaging [3] and the early detection of many diseases [4], since the vibrational frequencies of many important molecules, so-called molecular fingerprints, are located at the NIR and MIR region. To generate SC, two key components should be highlighted, namely the pump source and the nonlinear medium. In recent years, the dominant pump sources for generating MIR radiation are the optical parametric amplifier (OPA) system and solid-state laser [5–7]. These lasers have several advantages, including high peak power, ultra-short pulse duration, excellent beam quality and high spectral contrast ratio, which all contribute to the generation of high-quality SC spectra. However, from a practical perspective, the complex structure, large footprint, and low robustness of these systems severely limit their potentials for miniaturization and integration. In contrast, fiber lasers have emerged as a promising alternative due to their high brightness, compact structure, good beam quality, high power stability and cost-effectiveness. In particular, fiber lasers are well-suited for meeting the requirements of low average power and high compactness, as the fibers used in the experiments can be spooled to a centimeter-scale ring with negligible bending loss, finally these lasers can be easily scaled down to a centimeter level.

On the other hand, high-compactness and ultra-broadband SC generation requires a medium with a high nonlinear coefficient and a suitable dispersion profile that can produce rich nonlinear effects over relatively short transmission distances. Previous studies have explored the use of nonlinear fibers [8–10] and waveguides [11–13] made from different materials, among which chalcogenide glass (ChG) can yield favorable results for SC generation due to their exceptional properties, such as a high nonlinear refractive index, a wide transmission window in the MIR and far-infrared (FIR) regions, and a low two-photon absorption (TPA). SC generation pumped by fiber laser in various ChG waveguides has been demonstrated in the number of the previous reports. For instance, Lamont et al. [14] utilized a 1.55-µm, 610-fs, fiber-based femtosecond laser at a repetition rate of 10 MHz to pump a 60 mm-long As₂S₃ waveguide with two zero-dispersion wavelengths (ZDWs) of around 1.5 µm and 2.15 µm, generating a SC spanning from 1.25 to 2 µm. Choi et al. [15] reported a SC spectrum spanning from 1.28 to 2.12 µm in a 3.5 mm-long Ge₂₃Sb₇S₇₀ ridge waveguide pumped by a 1.55 µm femtosecond fiber laser. Tremblay et al. [16] used the similar pump scheme to generate an octave-spanning SC covering 1.03-2.08 µm in a 20 mm-long Ge₂₃Sb₇S₇₀ rib waveguide. Du et al. [17] employed a 1.56-µm, palm-sized, home-built femtosecond Er³⁺-doped fiber laser (EDFL) to pump the well-designed Ge₂₂Sb₁₈Se₆₀ waveguides and obtain a SC spectrum spanning from 1.38 to 2.05 µm (at 20 dB level). Hwang et al. [18] used a 1.56 µm, 135-fs mode-locked (ML) EDFL running at 100 MHz to pump a 1 cm-long, dispersion well engineered waveguide, and achieved a SC with a long-wavelength edge (LWE) spanning over 1.5 octaves. Additionally, in the past decade, an effective approach of generating broadband MIR-SC in As₂S₃-silica nanospikes (NS) waveguide pumped by advanced fiber lasers has attracted much attention. For example, Granzow et al. [19], for the first time, achieved a MIR-SC with spectral LWE extending out to 4 µm in the spike-based nonlinear medium. In 2014, they demonstrated a SC spectrum covering from 0.8 to 2.5 µm in As₂S₃-silica double-nanospike (DNS) waveguide [20]. In 2021, a continuous coherent MIR-SC spectrum spanning from 1.1 to 4.8 µm (at 30 dB level) was obtained in the similar NS structured waveguide by using a 2.8-µm Er³⁺-doped ZBLAN fiber laser as a pump source [21]. More recently, SC spectrum covering a wavelength range from 1.1 to 2.5 µm at 10 dB level in As₂S₃-silica hybrid NS waveguide pumped by a femtosecond Tm³⁺-doped fiber laser (TDFL) was also reported [22].

In nonlinear applications, waveguide offers numerous advantages, including strong light confinement ability, high nonlinear coefficient and easy integration. However, larger linear refractive index in the ChG generally leads to a large ZDW. Although waveguide dispersion can be tuned via waveguide design [23–26], blue-shift of the ZDW that can match the emission wavelength of the fiber laser is still challenging, since most of the commercially available fiber lasers work around 1.55 µm, and the commercial fiber lasers with working wavelengths beyond 2 µm are still scarce. Emerging applications of SC requires the development of a low-power ultra-short pulsed laser with wavelengths beyond 1.55 µm, enabling the flexibility of the integration with the waveguide for broadband SC generation. The combination of fiber-based ultra-short pulsed lasers and ChG waveguides is thus highly promising for compact, on-chip-scaled SC generation. However, the fiber lasers with longer wavelengths as a pump source for SC generation have seldom been explored.

In this study, we constructed an all-fiber structured, wavelength-tunable fiber laser with a dual-femtosecond soliton pulse at 2 µm and 2.3 µm, respectively. We used such a laser to pump As₂S₃ waveguide with different widths and various ZDW locations. We also analyzed the spectral evolution of pulses in this As₂S₃ waveguide via simulation, and the simulated SC spectra matched well with the experimental results. A fiber-laser integrated on-chip SC spanning from 1.5-2.4 µm was demonstrated.

2. Experimental design and setup

Figure 1 is the overall experimental schematic of generating SC in As₂S₃ waveguide pumped by the dual-femtosecond solitons pulse laser. The seed is a semiconductor saturable absorption mirror (SESAM) ML soliton laser running at 50 MHz, with a central wavelength of 1.97 µm and a pulse duration of 480 fs. Its maximum output average power is around 20 mW. Then, by the adoption of a fiber-based chirped pulse amplification (CPA) system, 182 fs pulse with a power of 1 W was successfully obtained via a fiber soliton compression mechanism. Then, an optical isolator (OI) is used to prevent the back reflection light from the follow-up double-cladding Tm³⁺-doped fiber amplifier (DC-TDFA). Within the DC-TDFA, due to the elaborated structure design, signal amplification and soliton self-frequency shift (SSFS) effects are occurred simultaneously. The details of the soliton evolution process in the DC-TDFA system will be analyzed in the following sections. Thanks to the SSFS effect in amplifier, a dual-femtosecond soliton pulse output from the DC-TDFA can be obtained, which are located at 1.97 µm and 2.3 µm, respectively, as shown in Fig. 1(b). The light is then collimated by a fiberized optical collimator, and the residual pump light is reflected by a 45°mounted dichroic mirror (DM) with reflection at 793 nm and transmission at 2 µm. A pair of half-wave plate (HWP) and a polarization beam splitter (PBS) are used to precisely control the polarization state of the incident laser to match the fundamental quasi-TE modes of As₂S₃ waveguide. For a detailed characterization of the 2.3-µm soliton pulse, we used a polarization independent 50/50 beam splitter (BS) to split the light into two replicas, the transmitted beam was used to pump the As₂S₃ waveguide, and the reflected counterpart was for pulse checking. The reflected beam was delivered to a band-pass filter (BPF) with a central wavelength of 2.3 µm and a bandwidth of 50 nm to filter the 2.3-µm spectral component out. The filtered light was then characterized in detail. To protect the As₂S₃ waveguide from damage, a neutral density filter (NDF) was used to tune the incident power for pumping the waveguide, and a fiber tip lens was used to facilitate the coupling of laser into a sub-wavelength scale waveguide, as shown in Fig. 1(c), the coupling loss of the waveguides was estimated as 9 dB, 7 dB and 6 dB, corresponding to the waveguide widths of 1 µm, 2 µm and 3 µm, respectively. For a better light coupling between the tapered fiber lens and As₂S₃ waveguide, a five-dimensional translation stage matched by a high-resolution microscope were adopted to optimize the alignment precision and improve the coupling efficiency, the propagation loss of the As₂S₃ waveguide was measured as 1 dB/cm in a waveguide with a thickness of 870 nm and a width of 2 µm, by the cut-back measurement at wavelength of 1.96 µm. The cross-sectional scanning electron microscope (SEM) image of the waveguide was displayed in Fig. 1(d), showing that the vertical sidewalls free of etching residue. The signal output from the waveguide was collected by a 1 m-long fluoride multi-mode fiber (MMF) with a core/cladding diameter ratio of 100/160 µm, and an optical numerical aperture (NA) of 0.2 and an attenuation of 0.1 dB/m at 4.5 µm for output signal characterization.

 

Fig. 1. The experiment setup of As₂S₃ waveguide SC generation. (a) the schematic of overall experimental setup; (b) the SSFS process in the TDFA system (TDFA: Tm³⁺-doped fiber amplifier, SSFS: soliton self-frequency shift); (c) optical microscope image showing the states of fiber to chip coupling; (d) SEM cross-sectional image of the As₂S₃ waveguide. TDF-SS-ML: Tm³⁺-doped fiber-based SESAM mode-locked laser; OI: optical isolator; Col: collimator; DM: dichroic mirror; RP: residual pump; HWP: half-wave plate; PBS: polarization beam splitter; BS: beam splitter; BPF: band-pass filter; NDF: neutral density filter; SMF: single-mode fiber; MMF: multi-mode fiber; FROG: frequency-resolved optical gating pulse checking system; OSA: optical spectrum analyzer.

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In the experiments, the output spectrum was measured by an optical spectrum analyzer (OSA, Yokogawa Inc., AQ6375) with a resolution of 0.05 nm and an available spectral bandwidth from 1.2 to 2.4 µm; the pulse duration and phase were measured with a frequency-resolved optical gating (FROG) device (Mesa Photonics Inc., FS-Ultra 2); the pulse radio-frequency (RF) spectrum was measured by a signal analyzer (Rigol Inc., DSA875); the output power was monitored by a thermal power meter, which was assembled by a set of power sensors (Thorlabs Inc., S148C and S405C) and power meter (Thorlabs Inc., PM100D and PM400); the output beam quality of pulse was characterized by a set of scanning slit beam profile system (Ophir Inc., NMS-NS2s-Pyro5).

3. Results and discussion

The seed spectrum was measured at output power of near 1 W as shown in Fig. 2(a), the spectral width (full width at half maximum, FWHM) and the central wavelength are 38 nm and 1.97 µm, respectively. Figure 2(b) presents the pulse phase and temporal measured by FROG. We can see that the measured seed pulse duration is 182 fs (red solid line), and its time-bandwidth product (TBP) is 0.534, and the corresponding Fourier-transform-limited (FTL) pulse duration is about 107 fs (black solid line). From the temporal phase distribution (blue dotted line), we can read that the pulse has a minor negative chirp, therefore potential to be compressed shorter. Figure 2(c) shows that, the measured radio frequency (RF) of the pulse signal-to-noise ratio (SNR) is as high as 75 dB at fundamental frequency of 49.825 MHz. The RF spectral distribution in a high frequency level and a broadband frequency span (1 GHz, resolution bandwidth (RBW)) reveals the superior stability of the pulse operation state.

 

Fig. 2. The characteristic parameters of the seed laser. (a) the spectrum; (b) the red solid line indicates the measured pulse duration, the black solid lines represent the calculated Fourier-transform-limited (FTL) pulse duration and the blue dotted line plots pulse temporal phase; (c) the RF spectrum (resolution bandwidth (RBW), 3 Hz). Inset: the RF spectrum measured in a scanning range of 1 GHz and a RBW of 3 kHz.

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To obtain a dual-femtosecond soliton pulse laser, we had tried the technique of SSFS in DC-TDFA system. By optimizing a number of key parameters including the peak power, pre-chirp of incident pulse and fiber length in the amplifier (the detailed parameters from Ref. [27]), we finally acquired the dual-femtosecond soliton pulse located at a central wavelength of 2 µm and 2.3 µm, respectively. Figure 3(a) presents the details of the soliton evolution in DC-TDFA system with the increase of the pump power. As we can see that, under the low pump power condition, the primary soliton appears at the beginning, and shifts to 2.02 µm, and the corresponding energy conversion efficiency is up to 90%. With the increase of the pump power to 2 W, the soliton is split and the second order soliton comes out. Then, obviously, the red-shifting of the primary soliton was dramatically affected by the scaling of the pump power. However, impeded by the gain bandwidth of the Tm³⁺ ion and the loss of the silica fiber, the most red-shifted soliton slows down at a pump power from 4 W to 6 W, and finally stops at 2.3 µm. The detailed analysis on the variation of the primary soliton power and conversion efficiency with soliton positions are presented in Fig. 3(b). We can see that the overall tendency of the soliton energy conversion efficiency is negatively while the soliton energy is positively correlated with the soliton wavelength. The value of conversion efficiency and output power of the 2.3-µm soliton at a pump power of 6 W are 58.63% and 0.99 W, respectively. In addition, the long-term power stability of the dual-femtosecond solitons system was also measured as shown in Fig. 3(c), the measured power fluctuation root mean square (RMS) value is as low as 0.32% at an average power of 1.497 W, confirming a high-power stability of the dual-femtosecond solitons system The inset in Fig. 3(c) shows the normal distribution of the power value, where the peak is located at 1.497 W.

 

Fig. 3. (a) Spectral evolution in DC-TDFA system; (b) power (left) and SSFS efficiency (right) evolution of fundamental soliton in DC-TDFA system; (c) power stability of the DC-TDFA system with average power is 1.497 W in 60 minutes. Inset: plots the histogram of the power.

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We further used a 2.3-µm BPF with bandwidth of 50 nm to filter the soliton out, as shown in Fig. 4(a), where the spectral FWHM of this soliton is 37 nm. Then we examined the pulse duration of the soliton using FROG, it is around 229 fs (red solid line) as is plotted in Fig. 4(b), which has a perfect sech² pulse shape, and the corresponding TBP value is 0.48. Comparing with the corresponding calculated FTL pulse duration of 150 fs (black solid line). This implies a minor normal chirp existing in the 2.3-µm soliton, which is further confirmed by the measured temporal phase (blue dotted line) of this pulse in Fig. 4(b). Moreover, we also measured the far-field beam-spot intensity distribution profile and M² value at an average power of 0.99 W, and the results are shown in Fig. 4(c), the M² factor values are 1.17, and 1.05 in the x (red solid line) and y (blue solid line) direction, respectively, indicating an excellent beam quality of the 2.3-µm soliton laser.

 

Fig. 4. The characteristic parameters of 2.3-µm soliton. (a) the spectrum; (b) the red solid line shows the measured pulse duration, the black solid line is the calculated FTL pulse duration, the blue dotted line plots the pulse temporal phase; (c) measurement of the beam propagation factor of the output beam along the transverse x (red) and y (blue) axes. Inset: far-field intensity profile.

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Before generating high-quality As₂S₃ waveguide SC spectra, the design of waveguide structure and characterization of waveguide parameters such as dispersion distribution and nonlinear coefficients are essential. The general layout of our waveguide structure is presented in Fig. 5(a), the waveguide core is As₂S₃, cladded with air on the top and side and silicon dioxide on the bottom. Figure 5(b) is the electric field distribution of the quasi-TE and quasi-TM modes at 2 µm, where the quasi-TE and quasi-TM modes are well-confined within the waveguide. To fabricate the waveguide, an 870 nm-thick As₂S₃ film was deposited onto a 4-inch silicon wafer with a 2 µm-thick thermal oxide layer using thermal evaporation. The As₂S₃ waveguide with varying widths was patterned using standard UV lithography and fully etched using inductively coupled plasma etching. The waveguide chip was then coated in a protective coating layer of inorganic polymer glass (IPG) after removing the resist residue. The fabrication process of the waveguide is detailed in Ref. [14]. The smooth and clear waveguide morphology in Fig. 5(c), which confirms the high quality of the waveguide fabrication.

 

Fig. 5. (a) Structure of the dispersion engineered As₂S₃ waveguide; (b) the mode field for fundamental quasi-TM and quasi-TE modes as calculated using beam transmission analysis method; (c) the picture of As₂S₃ waveguide and top-view optical micro-graph of As₂S₃ waveguide structure.

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The refractive index distribution of the As₂S₃ glass film was characterized by an ellipsometer (wine red solid line in Fig. 6(a)). It exhibits an exponential decrease from 2.453 to 2.423 at a wavelength range from 1.3 to 3.0 µm. We calculated the As₂S₃ material dispersion (black dash line) and dispersion D of the fundamental quasi-TE mode of As₂S₃ waveguide with varying widths (W) and a fixed core thickness of 870 nm (H) by combining the Sellmeier equation with the waveguide structure, and the results were shown in Fig. 6(a). The dispersion curve depicts a ZDW located at near 1.63 µm when the waveguide width is W = 1 µm. With increasing waveguide width, such a ZDW is red-shifted. To achieve broadband SC spectra, it is essential to match the pump laser wavelength with ZDW of the waveguide. Therefore, the waveguides with the widths of 1 µm, 2 µm, and 3 µm were designed. In Fig. 6(b), the simulated nonlinear coefficients of As₂S₃ waveguide with different widths were displayed. As anticipated, the nonlinear coefficient decreases as the waveguide width and the effective mode area (A_eff) increase. The impact of the wavelength on the nonlinear coefficient follows an exponential decay trend. For the waveguide with the width of 1 µm, the nonlinearity coefficients γ are 11.2/W/m and 8/W/m at the pump wavelengths of 2.0 µm and 2.3 µm, respectively.

 

Fig. 6. (a) The refractive index of the As₂S₃ glass film measured using ellipsometry (right), As₂S₃ material dispersion and the total dispersion of quasi-TE mode in the As₂S₃ waveguide with changing widths (W) and a fixed core thickness H = 870 nm, λ_P1 and λ_P2 corresponds two pump laser wavelengths in the paper (left); (b) simulated nonlinear coefficient of As₂S₃ waveguide with various widths of W = 1, 2, 3 µm (λ_P1 and λ_P2 are two pump wavelengths).

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Before the experimental investigations on the SC generation of such a high-nonlinearity As₂S₃ waveguide, a detailed numerical simulation was performed using the classical generalized nonlinear Schrodinger equations (GNLSE) [28],

The first term on the left-hand side of this equation shows the pulse slowly varying amplitude A over transmission distance, the second term and the third term reflect the effect of waveguide loss and different orders of dispersion on the pulse evolution in the waveguide, respectively. The right-hand side of this equation describes the wide variety of the third-order nonlinear effects in the femtosecond pulse transmission. Additionally, α(ω₀) is waveguide propagation loss, ω₀ is center frequency of incident pulse, z is propagation distance, t is field transmission time, β_n is the n^th order dispersion, γ represents the nonlinearity parameter, R(t’) denotes the nonlinear response function.

Based on GNLSE model together with the split-step Fourier method (SSFM), we simulated the dual-femtosecond soliton pulse evolution process in the As₂S₃ waveguide with the different widths. In the pursuit of more accurate simulation result, the number of sampling points and the time window width were set as 2¹⁶ and 50 ps, respectively, and the corresponding step length in the time domain is 7.6 × 10⁻¹⁶ s. In our simulation, the dispersion coefficients of the waveguide were considered up to the 10^th order, As₂S₃ material optical parameters including nonlinear refractive index (n₂), the nonlinear TPA coefficient (α₂) and Raman effect related parameters were referred from [14] and [29], being n₂ = 3 × 10⁻¹⁸ m²/W, α₂ = 6.2 × 10⁻¹⁵ m/W, f_R =0.11, τ₁ = 15.5 fs and τ₂ = 230.5 fs, respectively. The length of the As₂S₃ waveguide in simulation was the same as the actual size of 1 cm. To balance the efficiency and precision in simulation, the step length value was finally identified as 50 µm. The pumping wavelengths were set as 2.0 µm and 2.3 µm, and the pulse duration were set as 233 fs and 229 fs, respectively. The pulse peak powers were well optimized according to the experimental results.

For comparison, the experimental and simulated results of the SC generation in As₂S₃ waveguide with different size and pump condition are presented in Fig. 7, where the gray dashed lines in each figure represent the profile of the pump pulse. Figure 7(a) shows the spectral evolution in the waveguide with the width of 1 µm. We can see that, the SC spectrum broadening occurred in both long and short wavelength simultaneously covering from 1.5 to 2.2 µm at a low pump power of 44.1 mW, although there is a large gap resulted from the dispersive wave (DW) generation around 1.5 µm. We assume that the intense SC spanning is benefited from the high nonlinearity of As₂S₃ waveguide, the dual-soliton pump mechanism and the well-suited dispersion engineering of waveguide. The emergence of spectral components at 1.55 µm indicates a great contribution of the DW to the SC broadening in short-wavelength region. Further increasing the pump power to 67.2 mW, the spectral components above 2.1 µm in the long wavelength region has been emerged. While the pump power is scaled up to 83.4 mW, the spectral LWE is extended up and terminated to 2.4 µm.

 

Fig. 7. The spectral evolution of the dual-soliton pulse in the As₂S₃ waveguide with changing widths and incident pulse powers. (a), (c), (e) are the experimental results, (P_o and P_i are the measured average powers from MMF and tapered fiber lenses, respectively). (b), (d), (f) show the simulated results, (P_s is the output average power, P_p is the peak power set in simulation). The gray dashed lines in these figures show the profile of pump pulse.

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Nevertheless, limited by the coupling efficiency and high loss of these 1-µm waveguide, the generated spectral flatness in both short and long-wavelength regions is poor. Moreover, comparing the measured curves with the corresponding simulated results in Fig. 7(b), the overall spectral shape and flatness in low pump conditions and spectral terminate wavelengths in both short and long-wavelength regions has quite difference, especially in high pump power conditions, the long and short wavelength edges are stopped at around 2.48 µm and 1.42 µm, respectively. This may due to the possible inaccuracy of estimating the coupling efficiency and waveguide loss in simulation, as well as the power fluctuations stemmed from the instability of the coupling-in and coupling-out systems. In addition, we can find that the average power in the simulation is lower than the corresponding experimental one, the difference may occur from the imperfection of the incident laser beam in the experiments and the estimation errors of the waveguide optical parameters including the nonlinear refractive index, nonlinear coefficient and the value of waveguide dispersion in the simulation.

Figure 7(c) depicts the SC spectral evolution in the waveguide with a width of 2 µm. We find that the DW spectral components in the short wavelength region vanish in comparison with that in Fig. 7(a). This may due to the changed dispersion profile and the variation of waveguide ZDW, as shown in Fig. 6(a). The waveguide dispersion in this case is shifted to 2 µm, which is close to wavelength of the short pump pulse, and thus the DW and the pump pulse are well overlapped. However, since the optimization of the coupling efficiency and the red-shift of ZDW, the overall spectral flatness is well improved. The gap in SC spectra in Fig. 7(c) is vanished. The experimental spectra in Fig. 7(c) are well agreed with the corresponding simulation spectra in Fig. 7(d). Nevertheless, with the increase of the waveguide width, the nonlinearity of waveguide is undoubtedly decreased, its spectral coverage bandwidth getting narrow although the generated SC spectral curves getting flattened. This tendency become more obvious in 3-µm width conditions in Fig. 7(e). The spectra are similar to that in Fig. 7(c), but its output power can be scaled up to 2.7 mW. The simulated results in the waveguide with a width of 3 µm is shown in Fig. 7(f), which are also agreed well with the experimental results, the minor differences in spectral flatness and spectral LWE mainly from the precision of pump power estimation.

As mentioned above, we have successfully obtained the SC spectra in As₂S₃ waveguide with different widths by using our home-made dual-femtosecond soliton laser. Compared to single-wavelength pumping mechanism, dual-wavelength pumping has many obvious advantages, mainly because there is pulse distribution in the short and long-wavelength region, making it easier to excite rich nonlinear effects and generate a broadband SC spectrum. As shown the results from 1-µm waveguide, the dual-femtosecond solitons pulse wavelength is located in anomalous dispersion region of the waveguide, generating DW in the short-wavelength direction, resulting in an extension of the SC spectrum. On the other hand, the small size of the waveguide end facet and the large refractive index of the ChG, result in low final output power and overall poor flatness of SC spectra in the long-wavelength region. In the comparison of 2-µm and 3-µm condition, due to the continuously red-shift operation of the ZDW towards the long-wavelength direction, the dual pulse wavelengths are both located at near zero and positive dispersion region, and the spectral expansion in the short-wavelength direction is limited.

The present results represent a promising approach to the development of an integration SC generation system, but the further improvements are still needed. Firstly, the LWE of SC spectra still limited to 2.4 µm, the distinguished optical properties of ChG were not fully exploited. Secondly, the pulse duration of dual-femtosecond solitons pulse is relatively long, around 200 fs, resulted in a low peak power, which is undesirable to excite rich nonlinear effects in the waveguide. Thirdly, excessive As₂S₃ waveguide transmission loss, low efficiency between tapered fiber lenses and waveguide, lead to a limited output power at milliwatt-level. Therefore, to improve the output performance of the generated SC spectra in As₂S₃ waveguide, further nonlinear compression of the pulse duration to around or less than 100 fs, elaborate improvement of the coupling technology between taper fiber lenses and waveguide to minimize coupling loss, and reducing As₂S₃ waveguide transmission loss in MIR region are needed.

4. Conclusion

In summary, we developed an all-fiber structured, power-stable, dual-wavelength soliton-based all-fiber structured femtosecond laser and fabricated a high-quality As₂S₃ waveguide. By taking the advantages of dual-wavelength femtosecond laser and As₂S₃ waveguide, we have achieved a broadband SC source in As₂S₃ waveguides. The results confirmed that the combination of high-nonlinearity and dispersion-controllable As₂S₃ waveguide with multi-wavelength pumping mechanism would be a promising candidate to realize on-chip SC sources. To further broaden the SC bandwidth, the stimulation of long-wavelength side DW and the application of shorter pulse as pump could be the key. Nevertheless, this successful approach has laid a solid foundation for further development of ChG waveguide-based integrated SC sources.