The fiber-based supercontinuum (SC) sources offer both a broad bandwidth and a good beam quality. Particularly, SC sources in the mid-infrared (MIR) region attract enormous interest due to its potential applications in many fields, such as spectroscopic imaging,^[1] trace gas detection,^[2] and early-cancer diagnosis.^[3] Consequently, there have been a growing number of experiments aiming at generating MIR SC using a wide variety of nonlinear fibers. Traditionally, hindered by infrared absorption of silica glass, the long-wavelength edge of SC generated in silica fibers is limited at .^[4] Germania-core fibers have a lower phonon energy compared with silica fibers, which could further extend the long-wavelength edge of SC to about .^[5] For further broadening the SC spectrum, soft-glass fibers, such as fluoride fibers, tellurite fibers, and chalcogenide fibers, are much more suitable. The most common fluoride glasses are fluorozirconate glass ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) and fluoroindate glass (InF₃). The ZBLAN glass is a mixture of ZrF₄ (53 mol.%), BaF₂ (20 mol.%), LaF₃ (4 mol.%), AlF₃ (3 mol.%), and NaF (20 mol.%).^[6] To date, W MIR SCs have been reported in ZBLAN glass fibers,^[7–9] but the long-wavelength edge is limited at around due to multiphonon absorption edge when using long fibers. The InF₃ glasses own lower phonon energy compared with ZBLAN glasses, thus the long-wavelength edge of SC generated in InF₃ fibers has been extended up to with the output power reaching watt level.^[10–12] To further boost output power and increase long term stability of fluorite fiber-based SC sources, protective methods like an AlF₃ endcap^[13] need to be applied for preventing OH diffusion. Tellurite-based (TeO₂) glasses yield a higher resistance to moisture exposure than fluorides glass and it has a similar transmission band with fluorozirconate glass.^[6] A watt-level MIR SC source covering to is obtained in a dispersion-zero shifted tellurite fiber with an output power of 1.2 W.^[14] Recently, to improve the stability of fluoride glass fiber, a kind of fluorotellurite glass (TeO₂–BaF₂–Y₂O₃) fiber with improved chemical and thermal properties has been reported.^[15] Pumped by a homemade 1980-nm chirped pulse amplification, a 10.4-W SC spanning from 947 nm to 3934 nm was obtained without obvious damage observed on the end surface of the fluorotellurite fiber for more than 10 h. Chalcogenide fibers are a kind of MIR fiber, which is based on the chalcogen elements S, Se, and Te with transmission windows of , , and , respectively. These fibers naturally yield a wide transmission window in the MIR region and a stable chemical property.^[16–19]

Chalcogenide fibers usually have high zero-dispersion wavelengths (ZDWs)^[16] which are far away from general fiber laser. Due to the lack of mature and reliable MIR fiber lasers, free-space pump lasers like optical parametric amplifiers with ultra-short MIR pulses are widely employed in the chalcogenide fiber-based SC systems^[20–25] and great progresses have been achieved using this method. The broadest MIR SC generated in a nonlinear fiber is achieved in a piece of low-loss chalcogenide fiber, which covers at the −40-dB level.^[20] Practical applications require portable and stable SC sources which can work beyond the laboratory environment. Thus, the fiber laser with robust construction is the ideal pump source for SC generation. Cascaded SC generation in the chalcogenide fiber was proposed in 2014^[26] and realized in 2016.^[27] In this method, MIR soliton pulses generated in the ZBLAN fiber can further broaden toward the long-wavelength region in the chalcogenide fiber. This method can meet the need of MIR pump pulse for chalcogenide fibers and it naturally has the potential to develop an all-fiber MIR SC system. Adopting this method, an all-fiber SC laser covering to was obtained in an As₂S₃ step index fiber with an output power of 97.1 mW.^[28] And an MIR SC up to was obtained in a micro-structured As₂Se₃ fiber with an output power of about 6.5 mW.^[27]

In this paper, an SC spanning from to is demonstrated in cascaded ZBLAN and As₂Se₃ step-index fibers. The ZBLAN fiber is used to build an MIR SC laser with abundant high-peak-power soliton pulses between 3000 nm and 4200 nm. By coupling solitons generated in ZBLAN fiber into the following step-index As₂Se₃ fiber, MIR pulses further broaden toward long-wavelength region in the normal dispersion region of As₂Se₃ fiber. The spectral behavior of the cascaded SC generation is investigated by changing the pump power and pump repetition rate, which shows that the long-wavelength power generated in the ZBLAN fiber plays a critical role for further spectral broadening in the As₂Se₃ fiber.

The experimental setup for SC generation is shown in Fig. 1(a). The 1550-nm seed with 1-ns pulse duration and 10-kHz repetition rate is amplified in an erbium/ytterbium-codoped fiber amplifier (EYDFA) first and then injected into a 15-m-long single-mode fiber (SMF). An effective spectral broadening covering to is obtained during propagating in the SMF. Then spectral components from to are absorbed in a large-mode-area thulium-doped fiber (LMA-TDF) with a core/cladding diameter of and numerical aperture (NA) of 0.11/0.46, while the spectral components above 1.9 are amplified. Two mode field adapters (MFAs) are adopted in the thulium-doped fiber amplifier (TDFA) stage. The pigtail of the MFA2 has a core/cladding diameter of and a core NA of 0.2. The amplified pulses generated in LMA-TDF are sequentially shifted towards long wavelength region in the MFA and eventually lead to a SC laser.

Fig. 1.

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	Fig. 1. (a) Experimental set-up for SC generation in cascaded ZBLAN and As2Se3 step-index fibers. LMA-TDF: Large mode area thulium-doped fiber; SM: Single-mode; MFA: mode field adapter. (b) Microscope and (c) photograph of fusion splicing between silica fiber and ZBLAN fiber.

To effectively broaden the MIR spectrum, a 12-m-long ZBLAN fiber with a core/cladding diameter of and an NA of 0.27 is used. Low-loss fusion splice is achieved between the ZBLAN fiber and the pigtail of MFA2 by asymmetric heating method.^[29] Figures 1(b) and 1(c) depict the microscope and photograph of fusion splicing between silica fiber and ZBLAN fiber, respectively. This shows that the silica fiber is wrapped by the melted ZBLAN fiber, which forms a robust fusion splicing between silica fiber and ZBLAN fiber. For protecting the fusion-spliced joint, it was mounted into an aluminum V-groove and fixed with ultraviolet curing adhesive, as shown in the inset of Fig. 1(a). The fusion splicing loss is estimated to be less than 0.1 dB after considering the linear propagation loss at around (0.06 dB/m) and the mode mismatch loss between the pigtail of MFA2 and the ZBLAN fiber of about 0.024 dB. Based on the commercial finite element method software COMSOL Multiphysics and Sellmeier equation with the same coefficients as in Ref. [30], the calculated group velocity dispersion of the ZBLAN fiber is depicted in Fig. 2(a), which shows that it exhibits anomalous dispersion from around to its multiphonon absorption edge near .

Fig. 2.

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	Fig. 2. (a) Calculated group velocity dispersions of the ZBLAN fiber and As2Se3 fiber. (b) The relationship between the cutting angle of the As2Se3 fiber and Fresnel reflection proportion that meets the total reflection condition in the core of the As2Se3 fiber. Rp, Rs, and Rn represents reflection of P-light, S-light, and total light, respectively.

To effectively confine MIR wavelengths in the core of As₂Se₃ fiber and achieve a sufficient nonlinear interaction length, a 5-m-long As₂Se₃ fiber with large core diameter of and NA of 0.46 is used. Based on the Sellmeier equation with the same coefficients as in Ref. [31], the ZDW of the As₂Se₃ fiber is calculated at above , as shown in Fig. 2(a). Selenium glass fibers own high refractive indices in the MIR region (n = 2.77 at ), thus there is about 22% optical feedback resulting from Fresnel reflection at the end facet of As₂Se₃ fiber. Angle-cleaved As₂Se₃ fiber end can prevent backward propagated light to some extent. Figure 2(b) depicts the relationship between the cutting angle of the As₂Se₃ fiber and the proportion of optical feedback that meets the total reflection condition in the core of the As₂Se₃ fiber. Due to the large NA of the As₂Se₃ fiber, a large cutting angle is required at the end facet of As₂Se₃ fiber. A cutting angle of about 12° is obtained in the experiment, which means that about 0.4% SC feedback propagates backwards in the core of As₂Se₃ fiber. The As₂Se₃ fiber is placed on an aluminum plate without applying any active cooling method. Both end facets of As₂Se₃ fiber are placed in a pair of parallel V-grooves and indium-gallium alloy is applied to strip possible cladding light.

To flexibly control the light spot incident into the As₂Se₃ fiber, a pair of Ge₂₈Sb₁₂Se₆₀ aspheric lenses with antireflection coating covering is adopted to achieve light coupling between the ZBLAN fiber and the As₂Se₃ fiber. The focal lengths of the lenses are 6 mm and 4 mm, respectively. A monochromator with liquid-nitrogen-cooled InSb detector and a thermal power meter are used for the measurement of spectrum and power, respectively. With the aid of a pyro-electric array camera, fundamental mode operation is ensured during the coupling alignment.

The measured spectral evolution in the ZBLAN fiber pumped at 10-kHz repetition rate is depicted in Fig. 3(a) and the legend represents the corresponding output power. As TDFA pump power increases, the spectra of short subpulses generated in the LMA-TDF further broaden in the anomalous dispersion region of ZBLAN fiber and gradually reach . Abundant high-peak-power soliton pulses located between 3000 nm and 4200 nm are the dominant components of SC generated in ZBLAN fiber. Then, the output MIR SC generated in the ZBLAN fiber is coupled into the following As₂Se₃ fiber with the pair of aspheric lenses. Due to the antireflection coating covering , the coupling efficiency is only ∼50% before the pump spectrum reaches beyond . With the pump spectrum generated in the ZBLAN fiber reaching , the coupling efficiency gradually increases to about 80%.

Fig. 3.

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	Fig. 3. Measured spectral evolution of SC generated in the (a) 12-m-long ZBLAN fiber and (b) 5-m-long As2Se3 fiber pumped at 10-kHz repetition rate. The legend represents output power and the insets show the corresponded SC beam profiles.

The spectral evolution in the following As₂Se₃ fiber is depicted in Fig. 3(b) and the legend represents output power corresponding to the pump power in Fig. 3(a). For a clear visibility, each spectrum is shifted upward by 4 dB. This shows that at the beginning, the output solitons generated in the ZBLAN fiber are too weak to induce continue SC extension in As₂Se₃ fiber. Until the output power of ZBLAN fiber is about 2.49 mW, a significant spectral broadening is observed in the As₂Se₃ fiber. As the pump power is further increased, a cascaded spectral broadening of ∼1000 nm is achieved in the As₂Se₃ fiber and finally an MIR SC spanning from is obtained with corresponding −10-dBm points from to . Self-phase modulation and Raman-induced frequency red-shift in the normal dispersion regime of the As₂Se₃ fiber are regarded as the dominant mechanisms responsible for cascaded spectral broadening. Obvious Stokes peaks are hard to be distinguished in the output spectra of As₂Se₃ fiber due to pump soliton pulses with different center wavelengths. Two spectral dips beyond can be observed in the output spectrum of As₂Se₃ fiber. The spectral dip at is caused by the CO₂ absorption during free space propagation of spectral measurement. The Se–H bond in the fiber causes the dip at around . Near-field beam profiles detected at the output end of ZBLAN fiber and As₂Se₃ fiber are shown in the inset of Fig. 3. Although the single-mode cutoff wavelength of As₂Se₃ fiber is calculated as , the fundamental mode can be excited by proper alignment.

The spectral behaviors of MIR SC are investigated by changing the pump repetition rate. Figures 4(a) and 4(b) depict the comparison of SC spectra generated in ZBLAN fiber and As₂Se₃ fiber pumped at 10 kHz, 25 kHz, and 50 kHz, respectively. It shows that although SCs are obtained in the ZBLAN fiber at all pump repetition rates, the broadest cascading SC is generated in the As₂Se₃ fiber when pumped at 10 kHz. A possible explanation for this comes from the fact that solitons cannot propagate undistorted in the normal dispersion region of the As₂Se₃ fiber for a long distance. When solitons are incident into the As₂Se₃ fiber, self-phase modulation along with Raman-induced frequency red-shift leads to spectral broadening of SC. Meanwhile, time domain broadening of solitons occurs due to group dispersion. Along with the decrease of temporal intensity of pump pulse, self-phase modulation and Raman-induced frequency red-shift gradually stop in the As₂Se₃ fiber. Consequently, long-wavelength solitons with low group velocity dispersions indices in the As₂Se₃ fiber are beneficial for continuing SC broadening. Since most intense MIR solitons are obtained in the ZBLAN fiber at a repetition rate of 10 kHz, the broadest SC is generated in the As₂Se₃ fiber at 10 kHz. For effective obtaining MIR SC extension in As₂Se₃ fiber, the energy of long wavelength solitons generated in ZBLAN fiber needs to be boosted. For example, InF3 fibers with lower phonon energy in MIR region can be used to replace the ZBLAN fiber in the cascading configuration.^[32] Or a piece of As₂S₃ fiber can be added as an intermediate fiber between the ZBLAN fiber and the As₂Se₃ fiber for further bridging the matching-gap between the pump wavelength and ZDW of the As₂Se₃ fiber, which contributes to a SC laser source in a recent report.^[33]

Fig. 4.

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	Fig. 4. Comparison of the SC spectra generated in (a) ZBLAN fiber and (b) As2Se3 fiber pumped at 10 kHz, 25 kHz, and 50 kHz, respectively. The legend represents the output power of the ZBLAN fiber and As2Se3 fiber, respectively.

Further SC extension is prohibited in As₂Se₃ fiber at a repetition rate of 50-kHz through raising the pump power. When the pump power is increased above 263 mW, the output power of As₂Se₃ fiber suddenly decreases unexpectedly. The optical microscope image of the incident facet of As₂Se₃ fiber shows that there is a distinct cavity like a ‘crater’ with a similar dimension as the core diameter of As₂Se₃ fiber, as shown in Fig. 5(a). Figure 5(b) depicts the optical microscope image of longitudinal input segment of the As₂Se₃ fiber. A ∼200- -long deformation and an intra-core hollow are detected in the As₂Se₃ fiber, which indicates that an eruption of the core of As₂Se₃ fiber has occurred. According to Ref. [34], a core material blast from the input facet is one of the distinct features of fiber fuse, in contrast with other fiber damage mechanisms. It is inferred that fiber fuse has occurred in the As₂Se₃ fiber.

Fig. 5.

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	Fig. 5. (a) Damaged vertical incident facet of As2Se3 fiber pumped at 50-kHz repetition rate with maximum pump power of 263 mW; (b) optical microscope image of the longitudinal input segment of damaged As2Se3 fiber.

In this paper, an MIR SC spanning from to is achieved in cascaded ZBLAN and As₂Se₃ step-index fibers with an output power of 9.28 mW. The spectral behaviors of the SC source are investigated by changing the pump repetition rate and the pump power, which shows that the long-wavelength power generated in ZBLAN fiber plays a critical role for further spectral broadening in As₂Se₃ fiber. A damaged cross section is observed on the incident end facet of As₂Se₃ fiber while increasing the laser repetition rate, which suggests that in order to acquire a high-power MIR SC, improved fabrication of chalcogenide fiber along with a better understanding of the damage mechanism is needed.