The mid-infrared region (MIR; defined as 3–50 µm in [1]) enables direct molecular sensing with high selectivity/specificity. MIR fiber lasers offer excellent beam quality of bright, spatially and temporally coherent light, routable in MIR fiber-optics for applications such as narrow-band sensing, clinical diagnostics, new medical laser wavelengths, and pulsed seeding of MIR-supercontinua for MIR broadband sensing [2].

The longest wavelength room temperature continuous wave (CW) fiber lasing to date is 3.92 µm in ${{\rm Ho}^{3 +}}$-doped fluoro-indate glass fiber [3], enabled by the lower phonon energy [4] (${509}\;{{\rm cm}^{- 1}}$) fluoro-indate glass host compared to prior fluoro-zirconate glass hosts. However, ${509}\;{{\rm cm}^{- 1}}$ is still too high for laser operation $\gt 4\; \unicode{x00B5}{\rm m}$ [5]; thus, chalcogenide glass hosts, with phonon energies down to ${200}\;{{\rm cm}^{- 1}}$, are prime candidates [6] to achieve this goal. Selenide-chalcogenide glasses sufficiently combine low phonon energy with good glass stability.

Covalent chalcogenide glasses exhibit large linear refractive indices, so large absorption/emission cross-sections of doped-in lanthanide-ions provide promising short, active devices. Chalcogenide glasses are based on sulfur S, selenium Se, and tellurium Te; thus, adding Groups 14 and 15 elements increases chemical/mechanical robustness. Chalcogenide fibers are weaker than silica fibers, exhibiting a Young’s modulus of $\sim 1/5$ x silica [7] and a Vickers’ hardness of $\sim{2}\;{\rm GPa}$ [8] (cf. window-glass: 5.5 GPa). Chalcogenide glasses/fiber are exceptionally stable in liquid water/water–vapor at ambient temperature, unlike fluoride glasses [9], and they are not oxidized in air below the glass transition temperature, beyond a protective oxide nanolayer [10] analogous to ambient silicon oxidation [11]. Plastic-coated/uncoated chalcogenide fibers older than 2 years, stored under ambient conditions, retained respectable ultimate fracture stress median of $\sim{80}\;{\rm MPa}$ [12]. Coated/uncoated fibers can maintain optical transmission for over 7 years. Furthermore, high optical damage thresholds have also been reported [13].

The MIR-PL (photoluminescence) emission of lanthanide ions in selenide glasses occurs across the wavelengths 3–10 µm [14]. Calculated non-radiative transition rates are orders of magnitude lower than fluoride glasses [15], and hence, they offer higher efficiencies and lower thermal problems. We reported first-step index ${{\rm Pr}^{3 +}}$-doped chalcogenide fiber MIR-PL emission, and long milli-second MIR-PL lifetime equivalent to bulk-glass, showing fiber processing had not compromised the lanthanide local environment [16]. With Churbanov and Shiryaev [17], we demonstrated record low optical loss GeAsSe fiber. Recently, we have announced gain in ${{\rm Pr}^{3 +}}$-doped selenide fiber [18]. In addition, ${{\rm Tb}^{3 +}}$ and ${{\rm Pr}^{3 +}}$ doped chalcogenide bulk glass lasers have been reported [19,20]. However, the multiple excitation levels of ${{\rm Tb}^{3 +}}$ and ${{\rm Pr}^{3 +}}$ give rise to excited state absorption at some wavelengths, and this can reduce population inversion.

In this Letter, we report MIR fiber lasing $\gt 5\; \unicode{x00B5}{\rm m}$ in a step-index selenide–chalcogenide fiber. The step index fiber comprised core glass: 500 ppmw (parts-per-million-by-weight) Ce -${{\rm Ge}_{15}}{{\rm As}_{21}}{\rm Ga}_1{{\rm Se}_{63}}$ at. % and cladding glass ${{\rm Ge}_{21}}{{\rm Sb}_{10}}{{\rm Se}_{69}}$ at. %. The ${{\rm Ce}^{3 +}}$ ion dopant was selected due to its simple energy level structure, which, in principle, excludes excited state absorption and co-operative up-conversion phenomena, while allowing efficient in-band pumping, with a small quantum defect. Thus, this choice mimics Yb³⁺-doped silica glass, both reducing heating in the cavity and with potential for becoming the MIR analogue of the ${{\rm Yb}^{3 +}}$-doped silica glass fiber laser. This contribution, besides reporting the MIR fiber lasing beyond 5 µm also displays results on ${{\rm Ce}^{3 +}}$ MIR-PL.

To create the step index lasing fiber, arsenic As (7N5, Furakawa Denshi), antimony Sb (5 N, Materion), and selenium Se (5N, Materion) were heated under vacuum (${{10}^{- 3}}\;{\rm Pa}$), to remove anionic impurities; germanium Ge (5N, Cerac) was used as supplied. Melt-containment was a silica glass ampoule, purified by heating at 1000°C/6 h (hours), first in air (removed carbonaceous impurities), then in vacuum (${{10}^{- 3}}\;{\rm Pa}$) (removed physi-/chemi-sorbed water). Precursors were batched under ${\rm N}_2$ gas (MBraun Glovebox: $\lt 1\;{\rm ppm}$ ${\rm O}_2$; $\lt 1\;{\rm ppm}$ ${\rm H}_2{\rm O}$). Cladding glass, ${{\rm Ge}_{21}}{{\rm Sb}_{10}}{{\rm Se}_{69}}$ at. %. (M287RC), was melted at 900°C/12 h. Core-glass constituents 0.99 (${{\rm Ge}_{15}}{{\rm As}_{21}}{{\rm Se}_{63}}$) at. % were pre-melted at 850°C/12 h. Glasses were quenched and annealed, in situ inside the ampoule. The ${{\rm Ge}_{21}}{{\rm Sb}_{10}}{{\rm Se}_{69}}$ at. %. cladding glass was then extruded [2] to form a tube (E093RC) of 10.22 mm outside diameter (OD), inside diameter 900 µm and 60 mm long. The pre-melted core glass 0.99 (${{\rm Ge}_{15}}{{\rm As}_{21}}{{\rm Se}_{63}}$) at. % had gallium Ga (1 at. %, 3 N purity, Alfa Aesar) plus 500 ppmw Ce foil (3 N, Alfa Aesar) batched on and remelted at 850°C/6 h, then quenched/annealed, to make the 500 ppmw Ce-doped ${{\rm Ge}_{15}}{{\rm As}_{21}}{\rm Ga}_1{{\rm Se}_{63}}$ at. % core-glass rod preform (M259REZQT), which was drawn to unstructured fiber (F109REZQT(RCJN)), termed intermediate fiber, and it had a 900 µm diameter cane. To fabricate the step index lasing fiber (F130RERC), a rod-in-tube [16] preform was fiber drawn in a radio frequency furnace on a customized heathway fiber-drawing tower within a class-10,000 cleanroom (e.g., see [16]).

Churbanov et al. in [21] reported absorption across ${1666 - 2857}\;{{\rm cm}^{- 1}}$ (3.5-6 µm) due to the $^2{{\rm F}_{5/2}}{\to ^2}{{\rm F}_{7/2}}$ transition of ${{\rm Ce}^{3 +}}$ doped in GeSbGaSe glass. In this work, an absorption spectrum of the core bulk glass [Fig. 1(a)] was collected, 500 ppmw Ce-GeAsGaSe using a Fourier transform infrared (FTIR) spectrometer (Bruker IFS 66/S). The sample pathlength was 2.45 mm.

 

Fig. 1. (a) MIR absorption spectrum of 500 ppmw Ce-doped GeAsGaSe bulk core glass, and (b) ${{\rm Ce}^{3 +}}$ electronic energy level diagram overlaid with the 4.15 µm QCL and 5.14 µm emission.

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From Fig. 1(a), the ${{\rm Ce}^{3 +}}$ absorption due to $^2{{\rm F}_{5/2}}{\to ^2}{{\rm F}_{7/2}}$ is centered at 4.63 µm (${2160}\;{{\rm cm}^{- 1}}$) and spans ${1660 - 2900}\;{{\rm cm}^{- 1}}$ (3.5–6 µm) agreeing well with [21]; the ${{\rm Ce}^{3 +}}$ absorption band was used to construct the corresponding ${{\rm Ce}^{3 +}}$ energy level diagram [Fig. 1(b)], which matches the classic Dieke and Crosswhite [22] energy level diagram of ${{\rm Ce}^{3 +}}$ [Xe] ${{4f}^1}$ with the $^2{{\rm F}_{5/2}}$ ground state and $^2{{\rm F}_{7/2}}$ first excited state separated by $\sim{2200}\;{{\rm cm}^{- 1}}$. The ${{\rm Ce}^{3 +}}$ electronic absorption overlies [-Se-H] host impurity vibrational absorption [23]. Churbanov et al. in [21] reported MIR PL emission due to $^2{{\rm F}_{7/2}}{\to ^2}{{\rm F}_{5/2}}$ of ${{\rm Ce}^{3 +}}$, in a ${{\rm Ce}^{3 +}}/{{\rm Dy}^{3 +}}$ co-doped GeSbGaSe bulk glass, which was broad with two maxima at 4.44 and 4.62 µm of very high absorption cross-section: ${4.1} \times {{10}^{- 20}}$ and ${3.4} \times {{10}^{- 20}}\;{{\rm cm}^2}$, respectively. Cross-sections in the present work are not included due to underlying [-Se-H] absorption and potential ${{\rm Ce}^{4 +}}$ presence. Further work is required to obtain the true ${{\rm Ce}^{3 +}}$ cross-section.

${{\rm Ce}^{3 +}}$ MIR-PL emission from the 500 ppmw Ce-doped GeAsGaSe intermediate fiber was evaluated using in-band pumping with a 4.15 µm quantum cascade laser (QCL), with the equipment configuration shown in Fig. 2(a). Further details on equipment and ${{\rm Ce}^{3 +}}$ MIR-PL will be discussed in a follow-up paper. From Fig. 2(b), ${{\rm Ce}^{3 +}}$ PL spanned 3.4–6 µm. A ${{\rm Ce}^{3 +}}$ MIR-PL lifetime of 3.6 ms was measured at wavelengths 3.8, 4.6, and 5.2 µm, compared to ${{\rm Ce}^{3 +}}$ PL lifetime 2.5 ms in [21].

 

Fig. 2. (a) Simplified equipment diagram used for ${{\rm Ce}^{3 +}}$ PL (photoluminescence) measurements, and (b) ${{\rm Ce}^{3 +}}$ MIR-PL emission spectrum of 500 ppmw Ce-doped “intermediate” fiber. Note: A 4.203 µm LWP filter was used $\geq{4.4}\;{\unicode{x00B5}{\rm m}}$, and no filter was used at $\leq 3.90\; \unicode{x00B5}{\rm m}$.

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The optical loss spectrum of the 500 ppmw Ce doped GeAsGaSe intermediate glass fiber with an OD ${246}\pm{8}\;{\unicode{x00B5}{\rm m}}$ and 2.75 m long was collected using the cutback method. From Fig. 3, the ${{\rm Ce}^{3 +}}$ electronic absorption due to $^2{{\rm F}_{5/2}}{\to ^2}{{\rm F}_{7/2}}$ overlays vibrational absorption due to hydride impurity in the glass host [cf. Fig. 1(a)]. The lowest optical loss was ${2.16}\;{\rm dB}\,{{\rm m}^{- 1}}$ across the 6.6–7.1 µm wavelength.

 

Fig. 3. MIR optical loss of 500 ppmw Ce-doped “intermediate” fiber with an ${\rm OD} = 246 \pm {8}\;{\unicode{x00B5}{\rm m}}$ and length of 2.75 m. The core of the step index lasing fiber has potentially the same optical loss.

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The refractive index dispersion of bulk annealed core and cladding glass samples, which constituted the core and cladding of the step index lasing fiber, were measured on an ellipsometer (Woollam IR-VASE Ellipsometer Mark II). Variable angle spectrometric ellipsometry (V.A.S.E) was employed on the bulk samples at angles of 55°, 65°, and 75°, and models fitted (see Fig. 4). The calculated NA of the lasing fiber was 0.471 at the pump wavelength and 0.476 in the lasing wavelength range.

 

Fig. 4. Refractive index dispersion and V.A.S.E. model plots of bulk glasses. Core: 500 ppmw Ce-doped GeAsGaSe; cladding: GeSbSe.

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An unannealed, step-index fiber, with a 500 ppmw Ce-doped GeAsGaSe core with a 9 µm diameter, and 180 µm GeSbSe OD cladding that was 64-mm-long with $\sim{90}^\circ$ cleaved-end faces, was prepared. It was mounted to a stainless-steel V-groove with ultraviolet cured polymer and coated with a metal alloy (Galinstan: InGaSn alloy) for thermal management and cladding mode stripping. A 4.421 µm short wave-pass (SWP; 87 % reflective, and 0.2% transmission at the lasing wavelengths) was abutted to the fiber, while a gold mirror (95.8% reflective at the lasing wavelengths) was abutted to the opposite end, forming the optical cavity (see Fig. 5). A 4.15 µm QCL beam was passed through a polarizer, and a one-fourth wave plate to prevent reflections and unpolarized ${{\rm Ce}^{3 +}}$ PL, emanating from the lasing fiber, influencing QCL output power. A 4.203 µm LWP filter was used as a dichroic mirror to reflect the 4.15 µm QCL pump toward the optical cavity. The QCL was then focused through the 4.421 µm SWP filter to the 9 µm core ($\sim{19}\%$ coupling efficiency). The output of the cavity was passed through a 4.630 LWP filter to block the 4.15 µm QCL pump light reflected from the optical cavity. The MIR output spectrum of the cavity was measured to a spectral resolution of $\sim{15}\;{\rm nm}$, using a monochromator and a mercury cadmium telluride detector (Vigo Systems, PVI-4TE-6). The total output power of the cavity was $\lt 100\; \unicode{x00B5}{\rm W}$ (due to poor transmission of 4.421 µm SWP filter), and it could not be accurately measured with a thermal power meter (Thorlabs, C-Series S302C). The QCL was operated in either CW output, to measure the MIR output spectrum, or electronically with a 50% duty cycle, to measure the cavity “fall time.” As the CW pump power increased to 40 mW, initially, CW lasing was observed at 5.13 µm, 5.17 µm, and 5.28 µm, that is, peaks A, B, and C, respectively.

 

Fig. 5. Unoptimized optical setup used to investigate lasing.

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Figure 6 shows the cavity emission spectrum with increasing pump power. Peaks A and B shifted slightly to longer wavelengths as pump power increased. Above the pump power of 86 mW, the intensity of peak A (but not B and C) grew roughly linearly with increasing pump power, while above 400 mW pump power, peak B plateaus and peak C drops to zero. A cavity fall time of 2.5 µs was measured at the 5.127 µm wavelength.

 

Fig. 6. Cavity emission intensity versus the 500 ppmw Ce-doped GeAsGaSe core that was 9 µm in diameter and 64 mm long for different CW 4.15 µm QCL pump powers.

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Figure 7 shows the area under laser peaks A, B, and C as the pump power increases. The area under “Peak A,” and Total area under peaks A, B, and C both exhibit a near-linear dependence with increasing pump power $\gt{86}\;{\rm mW}$.

 

Fig. 7. Areas under cavity emission peaks from Fig. 6 plotted versus the 4.15 µm QCL pump power (mW) of small-core, step index fiber excited with the 4.15 µm CW QCL between 0 and 616 mW.

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The MIR electronic configuration of ${{\rm Ce}^{3 +}}$ is [Xe] ${{4f}^1}$, with the $^2{{\rm F}_{5/2}}$ ground state and $^2{{\rm F}_{7/2}}$ excited state. Crystal field and spin-orbit interactions split these two states into three and four energy levels, respectively [24,25]. ${{\rm Ce}^{3 +}}$-doped glasses and crystals have been investigated as UV-pumped blue lasers [26]. Low-temperature, MIR absorption measurements of ${{\rm Ce}^{3 +}}$ doped into garnet [24] showed large splitting of the excited $^2{{\rm F}_{7/2}}$ up to $\sim{4000}\;{{\rm cm}^{- 1}}$. The theoretical crystal field analysis supported this [25] with 4f–4f intra-shell transitions of ${{\rm Ce}^{3 +}}$ ions spectrally extending to $\sim{3700}\;{{\rm cm}^{- 1}}$ due to the large splitting of the $^2{{\rm F}_{7/2}}$ excited state.

Thermal population of the second Stark level of the $^2{{\rm F}_{5/2}}$ ground state was shown to occur at ambient temperature, called “hot” transitions [25]. Here, we observe more than one MIR lasing line; the Stark intra-levels of the upper/lower states may account for the co-existent lasing lines. The ${{\rm Er}^{3 +}}$: ZBLAN operated on several wavelength lines in the 2.71–2.79 µm range [27].

In summary, the narrow peaks, short cavity lifetime, the threshold at the 86 mW pump power, and the near-linear dependence with increasing pump power are all evidence of fiber lasing $ \gt 5\;\unicode{x00B5}{\rm m}$.