| Chalcogenide glass is considered as one of the best candidate materials for the preparation of infrared optical devices and nonlinear optical devices,and have been widely concerned and deeply studied in the infrared transmission,infrared laser,nonlinear optics,semiconductor and other fields because of their wide transmittance in infrared range,low phonon energy,high linear and nonlinear refractive index,good chemical stability and thermal stability,and continuous adjustable glass composition and performance.Based on the advantages above,the mid-infrared supercontinuum generation in chalcogenide glass fibers has been realized.The supercontinuum source has high brightness and good spatial coherence like lasers except the wide transmittance in infrared compared with the ordinary light source,which allows it to be used in toxic gas detection,medical diagnosis and even infrared countermeasures.Therefore,the supercontinuum generation in chalcogenide glass fibers becomes gradually a research hotspot in the fields of optics and materials.Among the chalcogenide glasses,S-based ones have the best mechanical properties and resistance to laser induced damage,making the S-based glass fibers be the ideal nonlinear medium for generating the supercontinuum of high power.In this paper,the purpose of this study is to explore a suitable chalcogenide glass fiber for generating high brightness and wideband mid-infrared supercontinuum.The third-order nonlinearity,laser induced damage characteristics and mid-infrared optical properties of S-based glasses were systematically investigated.Moreover,the nonlinear chalcogenide glass fibers were drawn and the supercontinuum generation in those optical fibers was studied.The main work and the results obtained are as follows:?1?The third-order nonlinearity and laser induced damage characteristics of Ge-As-S glasses were investigated.The best glass composition was selected to make small-core step-index fiber and the supercontinuum generation in the fiber was tested.The results show that the Ge-As-S glass has higher third-order nonlinear refractive index?n2 is about 0.8×10-143.1×10-14 cm2/W?at 1.55?m.n2 could be approximated according to the glass linear refractive index n0 by 9)2=3.73×10-16?9?2/0-1)4?9)2/0 cm2/W.The figure of merit?FOM?of the system glass is more than 20,which means they are good material of nonlinear optics.Especially the glass with chemical composition of Ge12As24S64 has outstanding performance.Its FOM is as high as 67.The system glass also has a higher femtosecond laser induced damage threshold?LIDT?.With the decrease of the laser pulse frequency,the LIDT increases exponentially.When the glass is irradiated by femtosecond pulses with repetition frequency of 1 kHz and 10 kHz,the main mechanism of glass damage is the cold mechanism.When the pulse frequency reaches more than 100 kHz,the thermal effect becomes the main factor to induce the glass damage.When the Ge12As24S64/Ge18As10S72 fiber with a core diameter of 9?m and a length of 13.5 cm was pumped using170 fs pulses?1 MHz?at 4.5?m and 4?m,the supercontinua covering 1.38.1?m and 1.58?m spectral ranges were obtained.?2?Ga8Sb32S600 was selected as the most stable glass in the Ga-Sb-S system,and the structure and properties of the glass were investigated by gradually replacing Ga in the composition with Ge.The results show that in the Ga8Sb32S60 glass,the glass transition temperature?Tg?does not change significantly with the Ga being replaced by Ge gradually,and the glass crystallization temperature?Tx?has a maximum of376°C when x=4 such that the difference between Tx and Tg,?T,is maximum?135°C?,indicating that Ga4Ge4Sb32S60 glass has the best thermal stability.With the gradual introduction of Ge,the short-wavelength absorption limit??S?of glass has a significant red shift,the optical bandgap?Eg?decreases,and the long-wavelength cutoff edge??S?has a blue shift gradually.The changes of?S and Eg are related to the formation of Sb-Sb bond after the introduction of Ge,and the blue shift of?S is caused by the decrease of the effective glass phonon energy.With the increase of Ge content,the density of glass?,the linear refractive index n0 and the nonlinear refractive index n2 decrease gradually,which is attributed to the gradual decrease of the three-coordinated S atoms in the glass network structure.Compared with the two-coordinate S,the three-coordinated S makes the glass structure more compact.?3?In order to further improve the thermal stability against crystallization of Ga-Sb-S glass and achieve the glass suitable for drawing optical fibers,a small amount of As was substituted for Sb in the glass to study the thermal stability.Based on the optimized glass,a nonlinear fiber was obtained and the supercontinuum generation in the fiber was investigated.The results show that a small amount of substitution of As for Sb in Ga8Sb28S60 glass can lead to the decrease of glass density and linear refractive index,the blue shift of the short wavelength absorption limit and long wavelength cutoff edge,and the increase of effective phonon energy.When the molar content of As in the glass is 4%,the composition?Ga8Sb28As4S60?has the best thermal stability.When the fabricated Ga8Sb28As4S60/As38S62 optical fiber with a core diameter of 7?m and a length of 14.5 cm was pumped with a 4?m laser with a pulse width of170 fs and a repetition rate of 1 MHz,the supercontinuum spanning from 2 to 7.8?m was obtained.?4?The Ga-Sb-S system has very small glass forming regions.In order to control the refractive index of glass and other important optical parameters like optical bandgap and zero dispersion wavelength in a wide range,a novel chalcohalide glass system,Ga2S3-Sb2S3-CsCl,was developed by introducing CsCl into this system.The physical properties of the system glass were systematically studied and its application prospects were also evaluated.The results show that up to 65 at%CsCl could be introduced into the Ga-Sb-S system,and the Ga2S3-Sb2S3-CsCl system shows a larger glass-forming region.The glasses have Tg of 217250°C and excellent thermal stability with CsCl content?30 at%.The glasses have a wide transmission range?0.5613.4?m?,and the transmittance are higher than 56%at 112?m.Taking into account the sensitivity of CsCl to water,the glasses containing less than 40 at%CsCl are more suitable for practical applications.The glasses have a smaller zero dispersion wavelength??0,3.37-5.15?m?,high n0 and n2.The n0 are 1.99-2.58 at 10?m,n2 are 1.111.9×10-14 cm2/W at 1030 nm,densities???are 3.423.97 g/cm3,the microhardness?HV?are 1.121.88 Gpa.With the increase of CsCl content,more[GaS4-xClx]tetrahedra are formed in the glass,which makes the network structure of the glass looser,resulting in the decrease of the glass density?and the microhardness HV.The high n2,wide transmission range,and small?0 make the chalcohalide glasses have a good application prospect in the field of mid-infrared nonlinear optics. |
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