Study On Highly Coherent Supercontinuum Generation In Chalcogenide Tapered Fibers

Supercontinuum(SC)is a kind of spectrum that narrow-band ultrashort laser pulse spreads in the nonlinear optical medium,through the combination of dispersion and nonlinear optical effects of the medium,making the spectrum of the incident pulse broadened.SC sources are widely applied in many fields,such as frequency metering and spectroscopy due to their advantages of good spatial coherence,high brightness and wide spectral rang.In addition,infrared band of3?16mm has important application value in communication systems and military fields.Chalcogenide glasses(ChGs)show very low phonon energy and ultra-wide infrared transmission range(up to 20 mm),and extremely high nonlinear refractive index(2 to 3 orders of magnitude higher than silica glass),thus it have become one of the excellent candidates for midinfrared SC generation.Many ultrabroadband SCs in step-index Ch G fibers are generated by pumping in the anomalous dispersion regime.In the cases above,SC generation is mainly due to soliton dynamics,which is sensitive to laser shot noise,leading to coherent degradation of SCs.A common approach to obtain coherent SCs is to pump fibers with an all-normal dispersion profile by using ultrashort lasers.In this case,despite the SC spectral broadening process can eliminate noise-sensitive soliton dynamics,the width of SC is relatively narrow due to the stimulated Raman scattering and self-phase modulation.Tapering fiber,which is a well-known post-processing technology,can engineer dispersion profiles and increase the optical nonlinearity by reducing the fiber diameter.So,tapered fiber can achieve ultrabroadband coherent SC.In this paper,we fabricated a Te-based ChG tapered fiber with all-normal dispersion characteristic.A mid-infrared supercontinuum generation that covers a 1.7–12.7 ?m region was generated in a 7-cm-long fiber pumped at 5.5 ?m using a femtosecond laser pulse.The results of the simulations confirmed that the SC spectrum is highly coherent in the region of 2 –12 mm.In addition,we fabricated a novel all-normal dispersion Ge-As-Se-Te double cladding tapered fiber,and the SC spectra with different transition region lengths were also investigated.The spectrum spanning 1.5–14.3 ?m was obtained in a 10-cm-long double cladding tapered fiber when transition region length was 9.8 mm pumped at 6 ?m using a femtosecond laser pulse.