applied sciences

Review Chalcogenide Microstructured Optical Fibers for Mid- Supercontinuum Generation: Interest, Fabrication, and Applications

Yiming Wu, Marcello Meneghetti, Johann Troles * and Jean-Luc Adam

CNRS, ISCR-UMR 6226, Univ Rennes, F-35000 Rennes, France; [email protected] (Y.W.); [email protected] (M.M.); [email protected] (J.-L.A.) * Correspondence: [email protected]; Tel.: +33-(0)2-2323-6733

 Received: 25 July 2018; Accepted: 11 September 2018; Published: 13 September 2018 

Abstract: The mid-infrared spectral region is of great technical and scientific importance in a variety of research fields and applications. Among these studies, mid-infrared supercontinuum generation has attracted strong interest in the last decade, because of unique properties such as broad wavelength coverage and high coherence, among others. In this paper, the intrinsic optical properties of different types of and fibers are presented. It turns out that microstructured chalcogenide fibers are ideal choices for the generation of mid-infrared supercontinua. The fabrication procedures of chalcogenide microstructured fibers are introduced, including purification methods of the , rod synthesis processes, and preform realization techniques. In addition, supercontinua generated in chalcogenide microstructured fibers employing diverse pump sources and configurations are enumerated. Finally, the potential of supercontinua for applications in mid-infrared imaging and spectroscopy is shown.

Keywords: chalcogenide glasses; chalcogenide fibers; mid-infrared; supercontinuum generation

1. Introduction The search for supercontinuum (SC) sources reaching into the mid-infrared (mid-IR) region (which covers the 2–20 µm electromagnetic spectral range) has drawn much attention in recent years. This is mainly because the atmospheric transparent windows (3–5 µm and 8–12 µm) and the molecular fingerprint region are included in this range. Thanks to these features, Mid-IR SC is considered to be a new source highly applicable in the fields of spectroscopy, sensing, biology, metrology, and defense [1,2]. Limited in terms of infrared absorption, silica-based SC generation could merely extend their spectral domain above ~2.7 µm [1,3–5]. Meanwhile, several other alternative materials were selected for the realization of Mid-IR SC generation, some of which are soft glasses such as tellurite [6,7], fluoride glasses [8,9], and chalcogenide glasses (ChG) [10–12]. As an example, Martinez et al. reported an all-fiber configuration SC generation with a continuous spectrum from 1.6 to >11 µm and 417 mW on-time average output power, by pumping a cascade of Zr–Ba–La–Al–Na fluoride (ZBLAN) fiber, arsenic sulfide (As2S3) fiber, and arsenic selenide (As2Se3) fiber with a master oscillator power amplifier (MOPA) [13]. Considering the optical and chemical properties of these materials, chalcogenide glass with its wide transparency in the Mid-IR range and several orders of magnitude higher nonlinearity than standard fused silica [14] surpasses the other glasses. Furthermore, by combining the characteristics of single-mode propagating and more flexible chromatic modification of microstructured optical fibers (MOF), chalcogenide MOFs would be optimal for Mid-IR SC generation. In this paper, optical transmissions of different fibers are compared, and the advantages of chalcogenide photonic fiber (PCF) for generating Mid-IR supercontinua are introduced and

Appl. Sci. 2018, 8, 1637; doi:10.3390/app8091637 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 11

Mid-IR supercontinua are introduced and discussed. Fabrication methods of chalcogenide fibers andAppl. Sci.chalcogenide2018, 8, 1637 PCF are described, and finally, examples of Mid-IR supercontinua generated2 of 11 from these fibers and applications are presented. discussed. Fabrication methods of chalcogenide fibers and chalcogenide PCF are described, and finally, 2. Why Choose Chalcogenide Microstructured Optical Fibers for Mid-Infrared Supercontinuum examples of Mid-IR supercontinua generated from these fibers and applications are presented. Generation? 2. WhyFor Choose the generation Chalcogenide of supercontinua Microstructured in Optical the Mid- FibersIR region, for Mid-Infrared the gain Supercontinuummedium or the Generation?conversion mediumFor theis of generation great importance, of supercontinua besides the in pump the Mid-IR source. region, Generally, the gain there medium are two or sorts the conversion of Mid-IR fibers,medium which is of greatare passive importance, fibers besidesand active the pumpfibers,source. respectively. Generally, Passive there fibers are two are sorts applied of Mid-IR to transmit fibers, infraredwhich are information, passive fibers whereas and active active fibers, fibers respectively. generate PassiveMid-IR fiberslight areby appliedusing nonlinear to transmit effects infrared or rare-earthinformation, doping whereas [15]. activeFigure fibers 1 lists generate and compares Mid-IR the light transmission by using nonlinear coverage effectsof several or rare-earth different fibers.doping [15]. Figure1 lists and compares the transmission coverage of several different fibers.

FigureFigure 1. 1. Mid-infraredMid-infrared transmissions transmissions of of different different types types of of optical optical fibers. fibers. Reproduced Reproduced with with permission permission fromfrom [15], [15], copyright copyright Informa Informa PLC, 2017.

AsAs cancan bebe seen,seen, among among glass glass fibers, fibers, chalcogenide chalcogenide glass glass is the is only the oneonly with one a with transparent a transparent domain domainpaving thepaving Mid-IR the region. Mid-IR The region. transmission The transmi varies withssion the varies constituting with the constituting elements, chalcogen such as elements,, , such as andsulfur, selenium, [16 and], as tellurium shown in [16], Figure as shown2. Thus, in As–SFigure fiber 2. Thus, can transmitAs–S fiber from can transmit1 to 6.5 µfromm [ 171 to], As–Se6.5 µm fiber[17], As–Se from 1.5fiber to from 10 µ m1.5 [to18 ],10 andµm [18], Te-based and Te-based fibers, due fibers, to heavydue to atomicheavy atomicweight, weight, can transmit can transmit further further than 14 thanµm. 14 In µm. terms In ofterms optical of optical nonlinearity, nonlinearity, chalcogenide chalcogenide glasses glassespossess possess a nonlinear a nonlinear refractive refractive index that index is 2 that to 3 ordersis 2 to of3 orders magnitude of magnitude higher than higher that than of silica that [16 of]. silicaTheseAppl. Sci.[16]. nonlinear 2018 These, 8, x FOR opticalnonlinear PEER properties REVIEW optical properties are even enhanced are even inenhanced small-core in small-core single-mode single-mode fibers. fibers.3 of 11 In microstructured optical fibers (MOF), the mode of the light propagating in the fiber is determined by the diameter of the air holes (d) and the distance between air holes (Λ), as depicted in Figure 3. Thus, single-mode guiding regardless of the wavelength can be achieved in MOF, as long as the d/Λ ratio is less than 0.42 [19,20]. Also, by reducing the diameter of the core in MOF, the nonlinearity can be increased and the zero-dispersion wavelength (ZDW) can be controlled over a wide range of wavelengths [21]. Indeed, in order to obtain an efficient supercontinuum source, the fiber has to be pumped close to the ZDW. One of the main advantages of MOFs is to obtain strong flexibility in dispersion modification. The intrinsic ZDWs of chalcogenide glasses are normally located beyond 5 µm, where it is difficult to find available powerful pump sources, not to mention fiber . Then, it would be beneficial to shift the ZDW to a shorter wavelength: 1.55 µm for the telecom wavelength, 2 µm, or at least less than 4 µm.

FigureFigure 2.2. Mid-infraredMid-infrared attenuationattenuation ofof chalcogenidechalcogenide opticaloptical fibersfibers inin comparisoncomparisonto tosilica silicaattenuation. attenuation.

For example, by altering the core diameter and with the microstructured geometry, a suspended-core chalcogenide As2S3 fiber with a ZDW blue-shifted to around 2 µm was realized [11], whereas the ZDW of the bulk glass is around 5 µm. In this study, the simulated results were confirmed by experimental measurements. Besides, in a Ge–As–Se system [22], where the ZDW of the bulk material is around 7 µm, a microstructured fiber with a ZDW blue-shifted to 3.6 µm was achieved by tapering the fiber core diameter from 12 to 6 µm [22]. Therefore, chalcogenide MOFs combine a broad Mid-IR transmission, high nonlinearity, single-mode guiding, and dispersion tunability, which open up the way to the generation of Mid-IR supercontinua.

Figure 3. Cross section of a chalcogenide microstructured . d: hole size, Λ: distance between the hole (also called “pitch”).

3. Fabrications of Chalcogenide Fibers In order to achieve a broad-bandwidth Mid-IR supercontinuum generation, chalcogenide glass has to be synthesized under vacuumed conditions and purified to avoid optical absorption caused by the presence of chemical bonds such as O–H, Se–H, and As–O, or impurities such as CO2 and H2O. To date, several purification methods have been reported, such as the purification of raw material before synthesis [23] or using microwave treatment before synthesis [24]. Also, distillation of the glass after synthesis in the presence of chemical getters proved to be efficient in reducing optical losses in fibers [25]. This last process consists of the addition of halides (such as TeCl4) and metals (such as Mg or Al) to the charge before synthesis. During the melting of the charge, metals will react with -based pollutants, while halides will react with hydrogen and carbon impurities. The byproducts of this reaction are either refractory or volatile, allowing for their removal by distillation of the glass. Standard chalcogenide fibers exhibit a step-index profile that can be achieved by implementing two different fabrication methods: the “rod-in-tube” method and the double-crucible method [26– Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 11

Appl. Sci. 2018, 8, 1637 3 of 11

In microstructured optical fibers (MOF), the mode of the light propagating in the fiber is Figure 2. Mid-infrared attenuation of chalcogenide optical fibers in comparison to silica attenuation. determined by the diameter of the air holes (d) and the distance between air holes (Λ), as depicted in Figure3. Thus, single-mode guiding regardless of the wavelength can be achieved in MOF, For example, by altering the core diameter and with the microstructured geometry, a as long as the d/Λ ratio is less than 0.42 [19,20]. Also, by reducing the diameter of the core in suspended-core chalcogenide As2S3 fiber with a ZDW blue-shifted to around 2 µm was realized [11], MOF, the nonlinearity can be increased and the zero-dispersion wavelength (ZDW) can be controlled whereas the ZDW of the bulk glass is around 5 µm. In this study, the simulated results were over a wide range of wavelengths [21]. Indeed, in order to obtain an efficient supercontinuum source, confirmed by experimental measurements. Besides, in a Ge–As–Se system [22], where the ZDW of the fiber has to be pumped close to the ZDW. One of the main advantages of MOFs is to obtain strong the bulk material is around 7 µm, a microstructured fiber with a ZDW blue-shifted to 3.6 µm was flexibility in dispersion modification. The intrinsic ZDWs of chalcogenide glasses are normally located achieved by tapering the fiber core diameter from 12 to 6 µm [22]. beyond 5 µm, where it is difficult to find available powerful laser pump sources, not to mention fiber Therefore, chalcogenide MOFs combine a broad Mid-IR transmission, high nonlinearity, lasers. Then, it would be beneficial to shift the ZDW to a shorter wavelength: 1.55 µm for the telecom single-mode guiding, and dispersion tunability, which open up the way to the generation of Mid-IR wavelength, 2 µm, or at least less than 4 µm. supercontinua.

Figure 3. Cross section of aa chalcogenidechalcogenide microstructuredmicrostructured opticaloptical fiber.fiber. d:d: hole size,size, ΛΛ:: distancedistance between the hole (also called “pitch”).

3. FabricationsFor example, of Chalcogenide by altering Fibers the core diameter and with the microstructured geometry, µ a suspended-coreIn order to achieve chalcogenide a broad-bandwidth As2S3 fiber with Mid-IR a ZDW supercontinuum blue-shifted to generati around 2on,m chalcogenide was realized glass [11], µ whereashas to be the synthesized ZDW of the under bulk glassvacuumed is around conditions 5 m. In and this purified study, the to simulatedavoid optical results absorption were confirmed caused by experimental measurements. Besides, in a Ge–As–Se system [22], where the ZDW of the bulk by the presence of chemical bonds such as O–H, Se–H, and As–O, or impurities such as CO2 and material is around 7 µm, a microstructured fiber with a ZDW blue-shifted to 3.6 µm was achieved by H2O. To date, several purification methods have been reported, such as the purification of raw µ taperingmaterial thebefore fiber synthesis core diameter [23] or from using 12 microwave to 6 m [22 treatment]. before synthesis [24]. Also, distillation of theTherefore, glass after chalcogenide synthesis in MOFsthe presence combine of ch aemical broad getters Mid-IR proved transmission, to be efficient high nonlinearity,in reducing single-mode guiding, and dispersion tunability, which open up the way to the generation of optical losses in fibers [25]. This last process consists of the addition of halides (such as TeCl4) and Mid-IRmetals (such supercontinua. as Mg or Al) to the charge before synthesis. During the melting of the charge, metals will react with oxygen-based pollutants, while halides will react with hydrogen and carbon 3. Fabrications of Chalcogenide Fibers impurities. The byproducts of this reaction are either refractory or volatile, allowing for their removalIn order by distillation to achieve of a the broad-bandwidth glass. Mid-IR supercontinuum generation, chalcogenide glass has toStandard be synthesized chalcogenide under vacuumedfibers exhibit conditions a step-index and purifiedprofile that to avoid can be optical achieved absorption by implementing caused by thetwo presence different offabrication chemical bondsmethods: such the as “rod-in-tube” O–H, Se–H, and method As–O, and or impuritiesthe double-crucible such as CO method2 and H[26–2O. To date, several purification methods have been reported, such as the purification of raw material before synthesis [23] or using microwave treatment before synthesis [24]. Also, distillation of the glass after synthesis in the presence of chemical getters proved to be efficient in reducing optical losses in fibers [25]. This last process consists of the addition of halides (such as TeCl4) and metals (such as Mg or Al) to the charge before synthesis. During the melting of the charge, metals will react with oxygen-based pollutants, while halides will react with hydrogen and carbon impurities. The byproducts of this reaction are either refractory or volatile, allowing for their removal by distillation of the glass. Standard chalcogenide fibers exhibit a step-index profile that can be achieved by implementing two different fabrication methods: the “rod-in-tube” method and the double-crucible method [26–28]. The first technique allows a better control of the fiber-core size, and consequently of the core-clad Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 11

28]. The first technique allows a better control of the fiber-core size, and consequently of the core-clad ratio [21,29,30]. The second one permits a better control of the core–clad interface [31,32], and commercial step-index chalcogenide fibers have been fabricated by using this technique. Microstructured optical fibers have been obtained with a variety of different glasses, including chalcogenide glasses [33,34]. In 2000, the first chalcogenide MOF, with a Ga–La–S composition, was successfully fabricated by using the “stack-and-draw” technique originally utilized for making silica MOF,Appl. yet Sci. 2018 guidance, 8, 1637 of light was not observed [33]. Later on, light propagation in chalcogenide4 of 11 MOFs based on sulfur and selenide was realized [35,36]. Then, in 2006, single-mode propagation in a chalcogenideratio [21, 29MOF,30]. The with second Ge–Ga–Sb–S one permits acomposition better control of was the core–clad reported interface [34]. [ 31Propagation,32], and commercial losses of the fiber werestep-index quite high, chalcogenide between fibers 15 haveand been20 dB fabricated m−1 at by1.55 using µm; this this technique. was attributed Microstructured to the optical poor quality of chalcogenidefibers have glass been preforms obtained with prepared a variety by of the different “stack-and-draw” glasses, including method chalcogenide [37]. glasses Indeed, [33 ,34it ].has been shown thatIn 2000, important the first defects chalcogenide at the MOF, interfaces with a Ga–La–Sbetween composition, the chalcogenide was successfully capillaries fabricated constituting by the using the “stack-and-draw” technique originally utilized for making silica MOF, yet guidance of stack induce strong optical losses [37]. light was not observed [33]. Later on, light propagation in chalcogenide MOFs based on sulfur and Consequently,selenide was realizedother fabrication [35,36]. Then, methods in 2006, single-mode had to be propagation developed in afor chalcogenide chalcogenide MOF withglasses. For example,Ge–Ga–Sb–S the geometry composition of chalcogenide was reported MOF [34]. Propagation can be realized losses of by the the fiber “drilling were quite method” high, between or “casting method”.15 The and 20“drilling dB m−1 at method” 1.55 µm; this exploits was attributed mechanical to the poor drilling quality in of chalcogenide chalcogenide glass glass preforms preforms to create variousprepared geometrical by the “stack-and-draw” structures. method In order [37]. Indeed, to avoid it has structure been shown destruction, that important the defects position at the of the interfaces between the chalcogenide capillaries constituting the stack induce strong optical losses [37]. holes and the friction between glass and drills have to be precisely controlled. Besides, for the Consequently, other fabrication methods had to be developed for chalcogenide glasses. protectionFor of example, the inner the side geometry of the of holes, chalcogenide it is ne MOFcessary can to be optimize realized bythe the rotation “drilling speed method” and or the force applied “castingby the method”.drills [38]. The For “drilling the method”“casting exploits method”, mechanical a mold drilling made in chalcogenide of silica capillaries glass preforms threaded to in hexagonalcreate silica various guides geometrical is designed structures. with In orderthe desi to avoidred structuredistribution destruction, of holes. the position Then, ofthe the chalcogenide holes glass, whichand the has friction been between heated glass to a andquasi-liquid drills have tostate, be precisely is poured controlled. into the Besides, mold. for Afterward, the protection the of mold– the inner side of the holes, it is necessary to optimize the rotation speed and the force applied by the glass ensemble is quenched in air and annealed. The silica tube and capillaries, which are still inside drills [38]. For the “casting method”, a mold made of silica capillaries threaded in hexagonal silica the MOFguides preform is designed at this withstage, the are desired removed distribution with ofa holes.diamond Then, tool the chalcogenideand soaking glass, in 40% which concentrated has hydrofluoricbeen heatedacid [39]. to a quasi-liquidBoth methods state, have is poured been into tested the mold. for Afterward,making monolithic the mold–glass preforms ensemble suitable is for obtainingquenched microstructured in air and annealed. optical Thefibers silica [38–40]. tube and capillaries, which are still inside the MOF preform at this stage, are removed with a diamond tool and soaking in 40% concentrated hydrofluoric acid [39]. 4. SupercontinuumBoth methods haveGeneration been tested in for Chalcogenide making monolithic MOFs preforms suitable for obtaining microstructured optical fibers [38–40]. As one of the promising applications of chalcogenide fibers, Mid-IR supercontinuum 4. Supercontinuum Generation in Chalcogenide MOFs generation has been investigated extensively with both step-index and microstructured chalcogenideAs fibers one of [41–44]. the promising applications of chalcogenide fibers, Mid-IR supercontinuum generation has been investigated extensively with both step-index and microstructured chalcogenide fibers [41–44]. The first generation of supercontinuum was demonstrated in 2006 for chalcogenide MOFs [36]. The first generation of supercontinuum was demonstrated in 2006 for chalcogenide MOFs [36]. One meterOne of meter a selenide-based of a selenide-based chalcogenide chalcogenide MOF MOF was was pumped pumped by a Ti:sapphire by a Ti:sapphire laser, with an laser, output with an output wavelengthwavelength atat 2.52.5µ µmm and and pulse pulse duration duration and and pulse pulse energy energy of 100 of fs 100 and fs 100 and pJ, 100 respectively. pJ, respectively. The experimentalThe experimental results results displayed displayed in inFigure Figure 4 showshow that that effective effective wavelength wavelength broadening broadening can be can be achievedachieved through through pumping pumping around around the the ZDW ZDW or or the anomalousanomalous region region [1] of [1] the of fiber. the fiber.

Figure 4.Figure Output 4. Output spectrum spectrum of ofsupercontinuum supercontinuum generated genera inted chalcogenide in chalcogenide microstructured microstructured optical fibers optical fibers mademade by by thethe stack stack and and draw draw technique. technique. Reproduced Reproduced with permission with from permission [36], copyright from INOE, [36], 2006. copyright INOE, 2006. Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 11

Later in 2010, M. El-Amraoui et al. reported the fabrication of the first As2S3 chalcogenide MOFs with a suspended core. Instead of the “stack-and-draw” method, mechanical drilling was used to prepare the fiber preform. By pumping a 45-m-long section of MOF with an 8-ps mode-locked laser at around 1.55 µm, a continuum covering more than 200 nm from 1450 to over 1700 nm was obtained [11]. Several other pumping schemes have been investigated to generate Mid-IR supercontinua. In 2016, Petersen et al. used a two-cascading configuration, based on a 1.55-µm laser-diode-pumped thulium-doped silica fiber and a Zr–Ba–La–Al–Na fluoride (ZBLAN) fiber, to pump a chalcogenide MOF, which generated a Mid-IR supercontinuum up to 4.4 µm [45]. By comparing the output spectra of ZBLAN fibers with different chromatic dispersions, it was concluded that solitons located Appl. Sci. 2018, 8, 1637 5 of 11 in the long-wavelength part of the pump are essential for the effective generation of the supercontinuum. This was corroborated by a series of simulations. The spectrum of the supercontinuumLater in 2010, was M. El-Amraouiextended to et 7 al. µm reported with thea total fabrication output ofpower the first of As6.52S 3mW.chalcogenide Then, in MOFs2017, Petersenwith a suspended et al. demonstrated core. Instead a Mid-IR of the SC “stack-and-draw” generation from method, a tapered mechanical large-mode-area drilling chalcogenide was used to MOFprepare [22]. the The fiber tunable preform. Mid-IR By pumping pump light a 45-m-long utilized sectionin the experiment of MOF with was an achieved 8-ps mode-locked by combining laser ata tunablearound 1.55seedµ m,laser a continuum and a 1.04-µm covering mode-locke more thand 200 Yb:KYW nm from (ytterbium-doped 1450 to over 1700 nm potassium was obtained yttrium [11]. tungstateSeveral crystal) other pumpingsolid-state schemes laser into have periodically-poled been investigated toMgO:LiNbO generate Mid-IR3 supercontinua. to obtain quasi-phase-matchedIn 2016, Petersen et al. parametric used a two-cascading anti-Stokes configuration,generation from based 3.7 to on 4.5 a 1.55-µm.µ Inm order laser-diode-pumped to increase the spectralthulium-doped broadening silica and fiber reduce and a the Zr–Ba–La–Al–Na confinement loss fluoridees, tapered (ZBLAN) fibers fiber, with to a pumpdiameter a chalcogenide of 15.1 µm andMOF, shorter which lengths generated before a Mid-IR and after supercontinuum the taper, with up tolength 4.4 µ mbefore [45]. Bytaper comparing (Lbt) of 7.5 the cm output and spectralength afterof ZBLAN taper (L fibersat) of with4.5 cm, different was utilized chromatic in the dispersions, experiment. it wasAt the concluded end, an output that solitons spectrum located from in 1 the to 11.5long-wavelength µm with an average part of output the pump power are of essential 35.4 mW for was the obtained, effective generationas shown in of Figure the supercontinuum. 5. ThisBesides, was corroborated hybrid chalcogenide by a series of MOFs simulations. were Thefabricated spectrum by ofCheng the supercontinuum et al. to generate was a extended Mid-IR supercontinuumto 7 µm with a total [46]. output The hybrid power MO of 6.5F has mW. been Then, made in 2017,with four Petersen AsSe et2 capillaries al. demonstrated and inserted a Mid-IR into anSC As generation2S5 glass tube from (Figure a tapered 6a). large-mode-areaThe high refractive chalcogenide index difference MOF [(around22]. The 0.61) tunable between Mid-IR the pump core andlight the utilized cladding in thewould experiment improve waslight achievedconfinement by combiningin the fiber, awhich tunable was seed the original laser and intent a 1.04- of µthem fibermode-locked designing. Yb:KYW As for the (ytterbium-doped pump light, a tunable potassium optical yttrium parametric tungstate oscillator crystal) (OPO) solid-state emitting laser 200-fs into pulsesperiodically-poled at a repetition MgO:LiNbO of 80 MHz3 wascrystals employed. to obtain For quasi-phase-matchedthe purpose of extending parametric the output anti-Stokes spectra, differentgeneration pump from 3.7wavelengths: to 4.5 µm. In3062 order nm, to increase3241 nm, the and spectral 3389 broadeningnm, located and at reducepositions the far confinement from the ZDWlosses, in tapered the normal fibers dispersion with a diameter range, close of 15.1to theµm ZDW and in shorter the normal lengths dispersion before andrange, after and the close taper, to ZDWwith lengthin the beforeanomalous taper dispersion (Lbt) of 7.5 range, cm and respecti lengthvely, after were taper used (Lat to) of pump 4.5 cm, the was 2-cm-long utilized hybrid in the MOF.experiment. Compared At the to end,the other an output wavelengths spectrum used, from the 1 broader to 11.5 µ spectrumm with an from average 1250 outputto 5370 powernm was of obtained35.4 mW by was pumping obtained, at as 3.389 shown µm in(Figure Figure 6b).5.

Figure 5. SupercontinuumSupercontinuum spectra spectra obtain obtaineded from from a a tapered tapered chalcogeni chalcogenidede MOF, MOF, for for three different configurationsconfigurations of of lengths lengths of of nont nontaperedapered zones zones at at the the input and output of the fiber fiber (in brown color).color)

(a)) L Lbtbt = = 25 25 cm, cm, L Latat = 7.5 = 7.5 cm; cm; (b) ( bL)bt Lbt = 7.5 = cm, 7.5 cm,Lat = Lat 25 =cm; 25 ( cm;c) Lbt (c =) Lbt7.5 cm, = 7.5 Lat cm, = 4 Latcm. = (With 4 cm. L (Withbt : length Lbt: beforelength beforetaper, taper,Lat length Lat: lengthafter aftertaper taperand andPout P=out average: average integrated integrated out out power) power). Adapted Adapted with with permission from ref [22], [22], copyright The Optical Society, 2017.2017.

Besides, hybrid chalcogenide MOFs were fabricated by Cheng et al. to generate a Mid-IR supercontinuum [46]. The hybrid MOF has been made with four AsSe2 capillaries and inserted into an As2S5 glass tube (Figure6a). The high refractive index difference (around 0.61) between the core and the cladding would improve light confinement in the fiber, which was the original intent of the fiber designing. As for the pump light, a tunable optical parametric oscillator (OPO) emitting 200-fs pulses at a repetition of 80 MHz was employed. For the purpose of extending the output spectra, different pump wavelengths: 3062 nm, 3241 nm, and 3389 nm, located at positions far from the ZDW in the normal dispersion range, close to the ZDW in the normal dispersion range, and close to ZDW in the anomalous dispersion range, respectively, were used to pump the 2-cm-long hybrid MOF. Compared Appl. Sci. 2018, 8, 1637 6 of 11 to the other wavelengths used, the broader spectrum from 1250 to 5370 nm was obtained by pumping at 3.389 µm (Figure6b). Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11

(b)

FigureFigure 6. 6.(a )(a Cross) Cross section section of of the the AsSe AsSe2–As2–As2S25Shybrid5 hybrid MOF. MOF. (b ()b Output) Output spectrum spectrum of of the the mid-infrared mid-infrared (Mid-IR)Mid-IR supercontinuumsupercontinuum obtainedobtained byby pumping pumping at at 3389 3389 nm. nm. Adapted Adapted with with permission permission from from ref ref [46 [46],], copyrightcopyright The The Optical Optical Society, Society, 2014. 2014.

AA summary summary of supercontinuaof supercontinua generated generated in chalcogenide in chalcogenide MOF with differentMOF with glass different compositions, glass geometries,compositions, and geometries, pumping regimes and pumping discussed regimes in this discu articlessed are in presentedthis article in are Table presented1. The in broader Table 1. supercontinuumThe broader supercontinuum spectrum obtained spectrum in a chalcogenide obtained in MOF a chalcogenide presents a spectral MOF coveragepresents froma spectral 1 to 11.5coverageµm with from an average 1 to 11.5 output µm with power an of average 35 mW. Theoutput higher power output of 35 power mW. obtained The higher in a chalcogenideoutput power MOFobtained reaches in morea chalcogenide than 57 mW MOF in thereaches 1–8 µ morem wavelength than 57 mW range. in the 1–8 µm wavelength range.

TableTable 1. 1.Mid-IR Mid-IR supercontinuum supercontinuum generation generation obtained obtained in in chalcogenide chalcogenide MOFs. MOFs. Spectral Pump Wavelength Output Average FiberFiber Composition Composition Spectral Coverage Pump Wavelength (Pulse Duration) Output Average Power ReferencesReferences Coverage (Pulse Duration) Power As2S3 2.1–3.2 µm 2.5 µm (100 fs) - [36] As2S3 2.1–3.2 µm 2.5 µm (100 fs) - [36] As2S3 1–2.6 µm 1.55 µm (400 fs) - [38] AsSeAs22-AsS3 2S5 1–2.61.2–5.37 µm µm 1.55 3.3 µ µmm (200 (400 fs) fs) 214 mW (input) - [46] [38] AsSeAs382-AsSe622S5 1.2–5.371.9–7.1 µmµ m Cascading 3.3 from µm 1.55 (200 to 4.5fs) µm (3 ns) 6.5 214 mW mW (input) [45] [46] µ µ GeAs10As38Se2262Se 68 1.9–7.11–11.5 µm m Cascading from 4 m 1.55 (252 to fs) 4.5 µm (3 ns) 35.4 mW 6.5 mW [22] [45] Ge10As22Se68 1–8 µm 4 µm (252 fs) 57.3 mW [22] Ge10As22Se68 1–11.5 µm 4 µm (252 fs) 35.4 mW [22] Ge10As22Se68 1–8 µm 4 µm (252 fs) 57.3 mW [22]

TheThe supercontinuum supercontinuum obtained obtained by by cascading cascading SC SC in in different different materials materials [45 [45]] is is probably probably the the most most promisingpromising way way for for the the future. future. Indeed, Indeed, this all-fiberthis all-fiber approach approach can lead can to lead more to stable, more versatile, stable, versatile, robust, androbust, compact and Mid-IRcompact sources Mid-IR with sources easy with handling. easy handli Also,ng. it is Also, well it known is well that known more that stable more and stable better and couplingbetter coupling efficiencies efficiencies can be obtained can be obtained with fibers with in fibers comparison in comparison to free space to free coupling. space coupling. 5. Applications of Mid-IR Supercontinuum Generation 5. Applications of Mid-IR Supercontinuum Generation Thanks to a broad Mid-IR range coverage, high coherence, and good beam quality, Mid-IR Thanks to a broad Mid-IR range coverage, high coherence, and good beam quality, Mid-IR supercontinuum sources are potential candidates for a variety of applications in domains such as supercontinuum sources are potential candidates for a variety of applications in domains such as spectroscopy, sensing, biology, metrology, and spectral imaging [43,47]. spectroscopy, sensing, biology, metrology, and spectral imaging [43,47]. As an example of this, the IR signature of propanol and acetone has been detected by fibers As an example of this, the IR signature of propanol and acetone has been detected by fibers evanescent wave spectroscopy (FEWS) using a chalcogenide MOF [47]. In this experiment, it was evanescent wave spectroscopy (FEWS) using a chalcogenide MOF [47]. In this experiment, it was demonstrated that an exposed-core chalcogenide MOF, as shown in Figure7, can be more sensitive to demonstrated that an exposed-core chalcogenide MOF, as shown in Figure 7, can be more sensitive the environment than classical single-index fibers, such as those used in ref. [23]. to the environment than classical single-index fibers, such as those used in ref. [23].

Figure 7. Scanning electron microscope image of an exposed-core chalcogenide microstructured optical fiber (MOF). Reproduced with permission from [47], copyright Elsevier, 2013. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11

(b)

Figure 6. (a) Cross section of the AsSe2–As2S5 hybrid MOF. (b) Output spectrum of the mid-infrared Mid-IR supercontinuum obtained by pumping at 3389 nm. Adapted with permission from ref [46], copyright The Optical Society, 2014.

A summary of supercontinua generated in chalcogenide MOF with different glass compositions, geometries, and pumping regimes discussed in this article are presented in Table 1. The broader supercontinuum spectrum obtained in a chalcogenide MOF presents a spectral coverage from 1 to 11.5 µm with an average output power of 35 mW. The higher output power obtained in a chalcogenide MOF reaches more than 57 mW in the 1–8 µm wavelength range.

Table 1. Mid-IR supercontinuum generation obtained in chalcogenide MOFs.

Spectral Pump Wavelength Output Average Fiber Composition References Coverage (Pulse Duration) Power As2S3 2.1–3.2 µm 2.5 µm (100 fs) - [36] As2S3 1–2.6 µm 1.55 µm (400 fs) - [38] AsSe2-As2S5 1.2–5.37 µm 3.3 µm (200 fs) 214 mW (input) [46] As38Se62 1.9–7.1 µm Cascading from 1.55 to 4.5 µm (3 ns) 6.5 mW [45] Ge10As22Se68 1–11.5 µm 4 µm (252 fs) 35.4 mW [22] Ge10As22Se68 1–8 µm 4 µm (252 fs) 57.3 mW [22]

The supercontinuum obtained by cascading SC in different materials [45] is probably the most promising way for the future. Indeed, this all-fiber approach can lead to more stable, versatile, robust, and compact Mid-IR sources with easy handling. Also, it is well known that more stable and better coupling efficiencies can be obtained with fibers in comparison to free space coupling.

5. Applications of Mid-IR Supercontinuum Generation Thanks to a broad Mid-IR range coverage, high coherence, and good beam quality, Mid-IR supercontinuum sources are potential candidates for a variety of applications in domains such as spectroscopy, sensing, biology, metrology, and spectral imaging [43,47]. As an example of this, the IR signature of propanol and acetone has been detected by fibers evanescent wave spectroscopy (FEWS) using a chalcogenide MOF [47]. In this experiment, it was demonstratedAppl. Sci. 2018, 8, 1637that an exposed-core chalcogenide MOF, as shown in Figure 7, can be more sensitive7 of 11 to the environment than classical single-index fibers, such as those used in ref. [23].

Figure 7. Scanning electron microscope image of an exposed-core chalcogenide microstructured Figure 7. Scanning electron microscope image of an exposed-core chalcogenide microstructured optical optical fiber (MOF). Reproduced with permission from [47], copyright Elsevier, 2013. Appl. Sci.fiber 2018 (MOF)., 8, x FOR Reproduced PEER REVIEW with permission from [47], copyright Elsevier, 2013. 7 of 11

Mid-IR spectralspectral imaging,imaging, combinedcombined with with data data mining mining algorithms, algorithms, has has been been utilized utilized as anas aidan aid for diagnosingfor diagnosing some some types types of cancers of cancers [48–51 ].[48–51]. The acquisition The acquisition speed and speed penetration and penetration depth of traditional depth of Mid-IRtraditional spectral Mid-IR imaging spectral is subject imaging to low isbrightness subject to and low lack brightness of flexibility and for lack the deliveryof flexibility and detection for the ofdelivery light [ 52and]. Therefore,detection of intense light [52]. laser Therefore, sources with intense high laser signal-to-noise sources with ratios high become signal-to-noise an ideal choice ratios forbecome rapid an acquisition ideal choice through for rapid the Mid-IRacquisition range. through Quantum the cascadeMid-IR range. lasers (QCL)Quantum were cascade once used lasers in Mid-IR(QCL) were spectral once imaging used in systems Mid-IR tospectral reduce imagin the acquisitiong systems time, to reduce but the the limited acquisition frequency time, coverage but the oflimited QCLs frequency cannot satisfy coverage the diagnostic of QCLs cannot requirements satisfy the and diagnostic maintain, requirements at the same time, and themaintain, simplicity at the of thesame system time, atthe a reasonablesimplicity of cost. the Thus, system supercontinuum at a reasonable sources cost. Thus, were selectedsupercontinuum for broadband sources Mid-IR were spectralselected imaging.for broadband For example, Mid-IR ZBLAN-fluoride-fiber-based spectral imaging. For supercontinuumexample, ZBLAN-fluoride-fiber-based sources, with a 2–4.5 µm spectralsupercontinuum range, were sources, selected with to a be 2–4.5 integrated µm spectral into Mid-IR range, spectralwere selected imaging to be systems integrated demonstrated into Mid-IR in thespectral past fewimaging years systems [53–55]. demonstrated in the past few years [53–55]. In 2012,2012, DupontDupont etet al.al. presented, presented, for for the the firs firstt time, a high-resolutionhigh-resolution contact-freecontact-free infraredinfrared microscope. By By using using a a1900-nm 1900-nm fiber fiber laser laser to to pump pump 10 10 m m of ZBLAN fiber,fiber, the generatedgenerated supercontinuum spanned from 1.4 to 4.0 µµm.m. The system was tested by a mixture of oil and water and resolutions of 35 µµmm andand 2525 µµmm forfor thethe waterwater absorptionabsorption imageimage andand oiloil absorptionabsorption imageimage were obtained, respectively, asas shownshown inin FigureFigure8 [853 [53].].

(a) (b) (c)

Figure 8. (a) OpticalOptical microscopemicroscope image of oil–wateroil–water mixture; (b,c) IRIR microscopemicroscope imageimage obtainedobtained atat wavelengths correspondingcorresponding to highto hi absorptiongh absorption of water of andwater oil, and respectively. oil, respectively. Adapted withAdapted permission with frompermission ref [53 ],from copyright ref [53], The copyright Optical The Society, Optical 2012. Society, 2012.

In 2017,2017, FarriesFarries etet al.al. reported aa mid-infraredmid-infrared spectral imaging system for the rapidrapid assessmentassessment of cells forfor cytologicalcytological diagnosis,diagnosis, consisting of a ZBLAN-fiber-basedZBLAN-fiber-based supercontinuumsupercontinuum source, a fast acousto-optic tunable filterfilter (AOTF), and aa high-resolutionhigh-resolution thermalthermal camera.camera. The AOTF enables the system toto recordrecord aa 100-wavelengths 100-wavelengths image image cube cube and and 300 300 k pixelsk pixels in in 2 s,2 sos, thatso that it can it can be usedbe used to test to test the cellsthe cells of a of living a living person. person. By comparison, By comparison, the systemthe system proved proved to have to have a higher a higher spectral spectral resolution resolution than athan Fourier a Fourier transform transform infrared infrared (FTIR) (FTIR) system. system. Restricted Restricted by the filterby the inside filterthe inside thermal the thermal camera andcamera the rangeand the of therange supercontinuum of the supercontinuum source, collection source, ofcollec samplestion of colonsamples cells of could colon be cells imaged, could nevertheless, be imaged, innevertheless, the 2.87–3.7 inµ mthe spectral 2.87–3.7 range µm spectral [54]. range [54]. Hence, in the later studies, chalcogenide MOFs were chosen to replace ZBLAN fibers. In 2018, Peterson et al. designed the first Mid-IR spectral imaging system in the long-wavelength region using SC generation [52]. The system consisted of a point scanning device and a chalcogenide fiber-based supercontinuum. The supercontinuum source could deliver light from 2 to 7.5 µm with an output power of 25 mW, which enabled the system to collect sample information from 5.7 to 7.3 µm, in the diagnostic fingerprint region [52]. Figure 9 presents the first mid-IR spectral image obtained with an SC containing wavelengths longer than 4 µm. The Mid-IR image obtained at 6.03 µm (Figure 9c) is compared to classical histologic analysis (visible images, Figure 9a,b) [52]. Appl. Sci. 2018, 8, 1637 8 of 11

Hence, in the later studies, chalcogenide MOFs were chosen to replace ZBLAN fibers. In 2018, Peterson et al. designed the first Mid-IR spectral imaging system in the long-wavelength region using SC generation [52]. The system consisted of a point scanning device and a chalcogenide fiber-based supercontinuum. The supercontinuum source could deliver light from 2 to 7.5 µm with an output power of 25 mW, which enabled the system to collect sample information from 5.7 to 7.3 µm, in the diagnostic fingerprint region [52]. Figure9 presents the first mid-IR spectral image obtained with an SC containing wavelengths longer than 4 µm. The Mid-IR image obtained at 6.03 µm (Figure9c) is comparedAppl. Sci. 2018 to, classical8, x FOR PEER histologic REVIEW analysis (visible images, Figure9a,b) [52]. 8 of 11

Figure 9. (a) Confocal image from histological analysis using gold standard hematoxylin and eosin Figure 9. (a) Confocal image from histological analysis using gold standard hematoxylin and eosin (H&E); (b) visible light transmission image of the sample; (c) Mid-IR absorbance image. Adapted with (H&E); (b) visible light transmission image of the sample; (c) Mid-IR absorbance image. Adapted permission from ref [52], copyright The Optical Society, 2018. with permission from ref [52], copyright The Optical Society, 2018. 6. Conclusions 6. Conclusions The main interest in chalcogenide MOFs is to combine the broad Mid-IR transmission range of chalcogenideThe main glasses interest with in chalcogenide the highly tunable MOFs opticalis to combine properties the broad of microstructured Mid-IR transmission fibers. Viarange the of comparisonchalcogenide of glasses different with classes the ofhighly glasses tunable and fiberoptical structures, properties we of can micros establishtructured that chalcogenidefibers. Via the MOFscomparison are surely of different an optimal classes medium of glasses for efficient and fiber supercontinuum structures, we generation can establish in the that Mid-IR chalcogenide region. MOFsSince are the surely first an generations optimal medium of supercontinua for efficient in chalcogenidesupercontinuum fibers, generation the power in andthe theMid-IR bandwidth region. obtainedSince have the increased first generations dramatically, of supercontinua now reaching 400in chalcogenide mW in step-index fibers, fibers the power and 55 mWand inthe chalcogenidebandwidth obtained MOFs, with have mid-IR increased wavelengths dramatically, generated now reaching up to 12 µ400m. mW Consequently, in step-index we canfibers say and that 55 themW concept in chalcogenide of generating MOFs, supercontinua with mid-IR in chalcogenidewavelengths fibersgenerated has beenup to proved. 12 µm. It Consequently, has been shown we alsocan thatsay thethat power the concept obtained of generating is now sufficient supercontinu for applicationsa in chalcogenide such as spectroscopy fibers has been or imaging. proved. It has beenHowever, shown also some that strong the power challenges obtained remain is tonow be tackledsufficient before for applications compact and such robust as all-fiberspectroscopy sources or areimaging. achieved. Setting innovative and efficient pump schemes, achieving fiber splicing, and improving the damageHowever, threshold some are strong examples challenges of those remain challenges. to be When tackled overcome, before compact it will be possibleand robust to take all-fiber full advantagesources are of achieved. the unique Setting and versatile innovative optical and properties efficient ofpump chalcogenide schemes, microstructuredachieving fiber splicing, fibers in theand infraredimproving spectrum, the damage such as threshold endless single-modeare examples propagation, of those challenges. high numerical When aperture, overcome, and it readilywill be tunablepossible dispersion. to take full advantage of the unique and versatile optical properties of chalcogenide microstructured fibers in the infrared spectrum, such as endless single-mode propagation, high Authornumerical Contributions: aperture, andY.W., readily J.T., and tunable J.-L.A. dispersion. have written the paper together and M.M. has contributed for constituting the bibliography. Author Contributions: Y.W., J.T., and J.-L.A. have written the paper together and M.M. has contributed for Funding: The article has been supported by the SUPUVIR ITN project. This project has received funding from the Europeanconstituting Union’s the bibliography. Horizon 2020 research and innovation program under grant agreement No. 722380. ConflictsFunding: of The Interest: articleThe hasauthors been supported declare no by conflict the SUPUVIR of interest. ITN project. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 722380.

ReferencesConflicts of Interest: The authors declare no conflict of interest. 1. Dudley, J.M.; Genty, G.; Coen, S. Supercontinuum generation in photonics crystal fiber. Rev. Mod. Phys. References2006, 78, 1135–1184. [CrossRef] 2.1. Schliesser,Dudley, J.M.; A.; PicquGenty,é, N.;G.; Coen, Hänsch, S. T.W.Supercontinuum Mid-infrared generation frequency in combs. photonicsNat. crystal Photonics fiber.2012 Rev., 6 ,Mod. 440–449. Phys. [CrossRef2006, 78,] 1135–1184. 2. Schliesser, A.; Picqué, N.; Hänsch, T.W. Mid-infrared frequency combs. Nat. Photonics 2012, 6, 440–449. 3. Swiderski, J.; Michalska, M. Mid-infrared supercontinuum generation in a single-mode thulium-doped fiber amplifier. Laser Phys. Lett. 2013, 10, 035105. 4. Yin, K.; Zhu, R.; Zhang, B.; Jiang, T.; Chen, S.; Hou, J. Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications. Opt. Express 2016, 24, 20010– 20020. 5. Lægsgaard, J.; Tu, H. How long wavelengths can one extract from silica-core fibers? Opt. Lett. 2013, 38, 4518–4521. 6. Savelii, I.; Mouawad, O.; Fatome, J.; Kibler, B.; Désévédavy, F.; Gadret, G.; Jules, J.-C.; Bony, P.-Y.; Kawashima, H.; Gao, W.; et al. Mid-infrared 2000-nm bandwidth supercontinuum generation in suspended–core microstructuered Sulfide and Tellurite optical fibers. Opt. Express 2012, 20, 27083–27093. Appl. Sci. 2018, 8, 1637 9 of 11

3. Swiderski, J.; Michalska, M. Mid-infrared supercontinuum generation in a single-mode thulium-doped fiber amplifier. Laser Phys. Lett. 2013, 10, 035105. [CrossRef] 4. Yin, K.; Zhu, R.; Zhang, B.; Jiang, T.; Chen, S.; Hou, J. Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications. Opt. Express 2016, 24, 20010–20020. [CrossRef][PubMed] 5. Lægsgaard, J.; Tu, H. How long wavelengths can one extract from silica-core fibers? Opt. Lett. 2013, 38, 4518–4521. [CrossRef] [PubMed] 6. Savelii, I.; Mouawad, O.; Fatome, J.; Kibler, B.; Désévédavy, F.; Gadret, G.; Jules, J.-C.; Bony, P.-Y.; Kawashima, H.; Gao, W.; et al. Mid-infrared 2000-nm bandwidth supercontinuum generation in suspended–core microstructuered Sulfide and Tellurite optical fibers. Opt. Express 2012, 20, 27083–27093. [CrossRef][PubMed] 7. Domachuk, P.; Wolchover, N.A.; Cronin-Golomb, M.; Wang, A.; George, A.K.; Cordeiro, C.M.B.; Knight, J.C.; Omenetto, F.G. Over 4000 nm Bandwidth of Mid-IR Supercontinuum Generation in sub-centimeter Segments of Highly Nonlinear Tellurite PCFs. Opt. Express 2008, 16, 7161–7168. [CrossRef][PubMed] 8. Théberge, F.; Daigle, J.-F.; Vincent, D.; Mathieu, P.; Fortin, J.; Schmidt, B.E.; Thiré, N.; Légaré, F. Mid-infrared supercontinuum generation in fluoroindate fiber. Opt. Lett. 2013, 38, 4683–4685. [CrossRef][PubMed] 9. Qin, G.; Yan, X.; Kito, C.; Liao, M.; Chaudhari, C.; Suzuki, T.; Ohishi, Y. Ultrabroadband supercontinuum generation from ultraviolet to 6.28 µm in a fluoride fiber. Appl. Phys. Lett. 2009, 95, 161103. [CrossRef] 10. Petersen, C.R.; Møller, U.; Kubat, I.; Zhou, B.; Dupont, S.; Ramsay, J.; Benson, T.; Sujecki, S.; Abdel-Moneim, N.; Tang, Z.; et al. Mid-infrared supercontinuum covering the 1.4–13.3 µm molecular fingerprint region using ultra-high NA chalcogenide step-index fibre. Nat. Photonics 2014, 8, 830–834. [CrossRef] 11. El-Amraoui, M.; Fatome, J.; Jules, J.C.; Kibler, B.; Gadret, G.; Fortier, C.; Smektala, F.; Skripatchev, I.; Polacchini, C.F.; Messaddeq, Y.; et al. Strong infrared spectral broadening in low-loss As-S chalcogenide suspended core microstructured optical fibers. Opt. Express 2010, 18, 4547–4556. [CrossRef][PubMed] 12. Cheng, T.; Kanou, Y.; Deng, D.; Xue, X.; Matsumoto, M.; Misumi, T.; Suzuki, T.; Ohishi, Y. Fabrication and

characterization of a hybrid four-hole AsSe2-As2S5 microstructured optical fiber with a large refractive index difference. Opt. Express 2014, 22, 13322–13329. [CrossRef][PubMed] 13. Martinez, R.A.; Plant, G.; Guo, K.; Janiszewski, B.; Freeman, M.J.; Maynard, R.L.; Islam, M.N.; Terry, F.L.; Alvarez, O.; Chenard, F.; et al. Mid-infrared supercontinuum generation from 1.6 to >11 um using concatenated step-index fluoride and chalcogenide fibers. Opt. Lett. 2018, 43, 296–299. [CrossRef][PubMed] 14. Yang, W.; Zhang, B.; Xue, G.; Yin, K.; Hou, J. Thirteen watt all-fiber mid-infrared supercontinuum generation in a single mode ZBLAN fiber pumped by a 2 µm MOPA system. Opt. Lett. 2014, 39, 1849–1852. [CrossRef] [PubMed] 15. Troles, J.; Brilland, L.; Caillaud, C.; Adam, J.L. Original designs of chalcogenide microstuctured optical fibers. Adv. Device Mater. 2017, 3, 7–13. [CrossRef] 16. Savage, J.A. Optical properties of chalcogenide glasses. J. Non-Cryst. Solids 1982, 47, 101–116. [CrossRef] 17. Snopatin, G.; Shiryaev, V.; Plotnichenko, V.; Dianov, E.; Churbanov, M. High-purity chalcogenide glasses for fiber . Inorg. Mater. 2009, 45, 1439–1460. [CrossRef] 18. Churbanov,M.F. High-purity chalcogenide glasses as materials for fiber optics. J. Non-Cryst. Solids 1995, 184, 25–29. [CrossRef] 19. Birks, T.A.; Knight, J.C.; Russell, P.S. Endlessly single-mode photonic crystal fiber. Opt. Lett. 1997, 22, 961–963. [CrossRef][PubMed] 20. Renversez, G.; Bordas, F.; Kuhlmey, B.T. Second mode transition in microstructured optical fibers: Determination of the critical geometrical parameter and study of the matrix refractive index and effects of cladding size. Opt. Lett. 2005, 30, 1264–1266. [CrossRef][PubMed] 21. Ballato, J.; Ebendorff-Heidepriem, H.; Zhao, J.; Petit, L.; Troles, J. Glass and Process Development for the Next Generation of Optical Fibers: A Review. Fibers 2017, 5, 11. [CrossRef] 22. Petersen, C.; Engelsholm, R.D.; Markos, C.; Brilland, L.; Caillaud, C.; Trolès, J.; Bang, O. Increased mid-infrared supercontinuum bandwidth and average power by tapering large-mode-area chalcogenide photonic crystal fibers. Opt. Express 2017, 25, 15336–15347. [CrossRef][PubMed] 23. Hocdé, S.; Boussard-Plédel, C.; Fonteneau, G.; Lucas, J. based glasses for IR fiber chemical sensors. Solid State Sci. 2001, 3, 279–284. [CrossRef] Appl. Sci. 2018, 8, 1637 10 of 11

24. Danto, S.; Thompson, D.; Wachtel, P.; Musgraves, J.D.; Richardson, K.; Giroire, B. A comparative study of

purification routes for As2Se3 chalcogenide glass. Int. J. Appl. Glass Sci. 2013, 4, 31–41. [CrossRef] 25. Shiryaev, V.S.; Churbanov, M.F. Recent advances in preparation of high-purity chalcogenide glasses for mid-IR photonics. J. Non-Cryst. Solids 2017, 475, 1–9. [CrossRef] 26. Churbanov, M.F.; Shiryaev, V.S.; Scripachev, I.V.; Snopatin, G.E.; Gerasimenko, V.V.; Smetanin, S.V.; Fadin, I.E.; Plotnichenko, V.G. Optical fibers based on As-S-Se glass system. J. Non-Cryst. Solids 2001, 284, 146–152. [CrossRef] 27. Kim, W.H.; Nguyen, V.Q.; Shaw, L.B.; Busse, L.E.; Florea, C.; Gibson, D.J.; Gattass, R.R.; Bayya, S.S.; Kung, F.H.; Chin, G.D.; et al. Recent progress in chalcogenide fiber technology at NRL. J. Non-Cryst. Solids 2016, 431, 8–15. [CrossRef] 28. Kobelke, J.; Kirchhof, J.; Scheffler, M.; Schwuchow, A. Chalcogenide glass single mode fibres—Preparation and properties. J. Non-Cryst. Solids 1999, 256, 226–231. [CrossRef] 29. Houizot, P.; Smektala, F.; Couderc, V.; Troles, J.; Grossard, L. Selenide glass single mode optical fiber for nonlinear optics. Opt. Mater. 2007, 29, 651–656. 30. Troles, J.; Niu, Y.; Duverger-Arfuso, C.; Smektala, F.; Brilland, L.; Nazabal, V.; Moizan, V.; Desevedavy, F.; Houizot, P. Synthesis and characterization of chalcogenide glasses from the system Ga-Ge-Sb-S and preparation of a single-mode fiber at 1.55 µm. Mater. Res. Bull. 2008, 43, 976–982. [CrossRef] 31. Chenard, F.; Alvarez, O.; Moawad, H. MIR chalcogenide fiber and devices. In Proceedings of the SPIE Conference on Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XV, San Francisco, CA, USA, 7–12 February 2015; Volume 9317, p. 93170B. 32. Lafond, C.; Couillard, J.-F.; Delarosbil, J.-L.; Sylvain, F.; de Sandro, P. Recent improvements on mid-IR chalcogenide optical fibers. In Proceedings of the SPIE 40th Conference on Infrared Technology and Applications XL, Batimore, MD, USA, 5–9 May 2014; Volume 9070, p. 90701C. 33. Monro, T.M.; West, Y.D.; Hewak, D.W.; Broderick, N.G.R.; Richardson, D.J. Chalcogenide holey fibres. Electron. Lett. 2000, 36, 1998–2000. [CrossRef] 34. Brilland, L.; Smektala, F.; Renversez, G.; Chartier, T.; Troles, J.; Nguyen, T.N.; Traynor, N.; Monteville, A. Fabrication of complex structures of Holey fibers in chalcogenide glass. Opt. Express 2006, 14, 1280–1285. [CrossRef][PubMed] 35. Le Person, J.; Smektala, F.; Chartier, T.; Brilland, L.; Jouan, T.; Troles, J.; Bosc, D. Light guidance in new chalcogenide holey fibres from GeGaSbS glass. Mater. Res. Bull. 2006, 41, 1303–1309. [CrossRef] 36. Sanghera, J.S.; Aggarwal, I.D.; Shaw, L.B.; Florea, C.M.; Pureza, P.; Nguyen, V.Q.; Kung, F. Nonlinear properties of chalcogenide glass fibers. J. Optoelectron. Adv. Mater. 2006, 8, 2148–2155. 37. Brilland, L.; Troles, J.; Houizot, P.; Desevedavy, F.; Coulombier, Q.; Renversez, G.; Chartier, T.; Nguyen, T.N.; Adam, J.L.; Traynor, N. Interfaces impact on the transmission of chalcogenide photonic crystal fibres. J. Ceram. Soc. Jpn. 2008, 116, 1024–1027. [CrossRef] 38. El-Amraoui, M.; Gadret, G.; Jules, J.C.; Fatome, J.; Fortier, C.; Désévédavy, F.; Skripatchev, I.; Messaddeq, Y.;

Troles, J.; Brilland, L.; et al. Microstructured chalcogenide optical fibers from As2S3 glass: Towards new IR broadband sources. Opt. Express 2010, 18, 26655–26665. [CrossRef][PubMed] 39. Coulombier, Q.; Brilland, L.; Houizot, P.; Chartier, T.; Nguyen, T.N.; Smektala, F.; Renversez, G.; Monteville, A.; Méchin, D.; Pain, T.; et al. Casting method for producing low-loss chalcogenide microstructured optical fibers. Opt. Express 2010, 18, 9107–9112. [CrossRef][PubMed] 40. Zhang, P.; Zhang, J.; Yang, P.; Dai, S.; Wang, X.; Zhang, W. Fabrication of chalcogenide glass photonic crystal fibers with mechanical drilling. Opt. Fiber Technol. 2015, 26, 176–179. [CrossRef] 41. Hudson, D.D.; Antipov, S.; Li, L.; Alamgir, I.; Hu, T.; El Amraoui, M.; Messaddeq, Y.; Rochette, M.; Jackson, S.D.; Fuerbach, A. Toward all-fiber supercontinuum spanning the mid-infrared. Optica 2017, 4, 1163–1166. [CrossRef] 42. Deng, D.H.; Liu, L.; Tuan, H.T.; Kanou, Y.; Matsumoto, M.; Tezuka, H.; Suzuki, T.; Ohishi, Y. Mid-infrared

supercontinuum covering 3–10 µm using a As2Se3 core and As2S5 cladding step-index chalcogenide fiber. J. Ceram. Soc. Jpn. 2016, 124, 103–105. [CrossRef] 43. Mouawad, O.; Picot-Clemente, J.; Amrani, F.; Strutynski, C.; Fatome, J.; Kibler, B.; Desevedavy, F.; Gadret, G.; Jules, J.C.; Deng, D.; et al. Multioctave midinfrared supercontinuum generation in suspended-core chalcogenide fibers. Opt. Lett. 2014, 39, 2684–2687. [CrossRef][PubMed] Appl. Sci. 2018, 8, 1637 11 of 11

44. Gao, W.Q.; El Amraoui, M.; Liao, M.S.; Kawashima, H.; Duan, Z.C.; Deng, D.H.; Cheng, T.L.; Suzuki, T.;

Messaddeq, Y.; Ohishi, Y. Mid-infrared supercontinuum generation in a suspended-core As2S3 chalcogenide microstructured optical fiber. Opt. Express 2013, 21, 9573–9583. [CrossRef][PubMed] 45. Petersen, C.R.; Moselund, P.M.; Petersen, C.; Moller, U.; Bang, O. Spectral-temporal composition matters when cascading supercontinua into the mid-infrared. Opt. Express 2016, 24, 749–758. [CrossRef][PubMed] 46. Cheng, T.L.; Kanou, Y.; Xue, X.J.; Deng, D.H.; Matsumoto, M.; Misumi, T.; Suzuki, T.; Ohishi, Y. Mid-infrared

supercontinuum generation in a novel AsSe2-As2S5 hybrid microstructured optical fiber. Opt. Express 2014, 22, 23019–23025. [CrossRef][PubMed] 47. Toupin, P.; Brilland, L.; Boussard-Pledel, C.; Bureau, B.; Mechin, D.; Adam, J.-L.; Troles, J. Comparison between chalcogenide glass single index and microstructured exposed-core fibers for chemical sensing. J. Non-Cryst. Solids 2013, 377, 217–219. [CrossRef] 48. Baker, R.; Rogers, K.D.; Shepherd, N.; Stone, N. New relationships between breast microcalcifications and cancer. Br. J. Cancer 2010, 103, 1034–1039. [CrossRef][PubMed] 49. Kwak, J.T.; Kajdacsy-Balla, A.; Macias, V.; Walsh, M.; Sinha, S.; Bhargava, R. Improving Prediction of Prostate Cancer Recurrence using Chemical Imaging. Sci. Rep. 2015, 5, 8758. [CrossRef][PubMed] 50. Fernandez, D.C.; Bhargava, R.; Hewitt, S.M.; Levin, I.W. Infrared spectroscopic imaging for histopathologic recognition. Nat. Biotechnol. 2005, 23, 469–474. [CrossRef][PubMed] 51. Nallala, J.; Diebold, M.D.; Gobinet, C.; Bouche, O.; Sockalingum, G.D.; Piot, O.; Manfait, M. Infrared spectral histopathology for cancer diagnosis: A novel approach for automated pattern recognition of colon adenocarcinoma. Analyst 2014, 139, 4005–4015. [CrossRef][PubMed] 52. Peterson, C.R.; Prtljaga, N.; Farries, M.; Ward, J.; Napier, B.; Lloyd, G.R.; Nallala, J.; Stone, N.; Bang, O. Mid-infrared multispectral tissue imaging using a chalcogenide fiber supercontinuum source. Opt. Lett. 2018, 43, 999–1002. [CrossRef][PubMed] 53. Dupont, S.; Peterson, C.; Thogerson, J.; Agger, C.; Bang, O. IR microscopy utilizing intense supercontinuum light source. Opt. Express 2012, 20, 4887–4892. [CrossRef][PubMed] 54. Farries, M.; Ward, J.; Lindsay, I.; Nallala, J.; Moselund, P. Fast hyper-spectral imaging of cytological samples in the mid-infrared wavelength region. In Proceedings of the Conference on Optical Biopsy XV-Toward Real-Time Spectroscopic Imaging and Diagnosis, San Francisco, CA, USA, 28 January–2 February 2017; Volume 10060, p. 100600Y. 55. Borondics, F.; Jossent, M.; Sandt, C.; Lavoute, L.; Gaponov, D.; Hideur, A.; Dumas, P.; Février, S. Supercontinuum-based Fourier transform infrared spectromicroscopy. Optica 2018, 5, 378–381. [CrossRef]

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