基于As_(2)S_(3)玻璃拉锥光子晶体光纤的超连续谱技术

Supercontinuum Generation from Tapered As_(2)S_(3) Glass Photonic Crystal Fiber

为了探索硫系玻璃光纤器件在中红外波段超连续光源的潜在应用,自主制备了一种硫系玻璃光子晶体光纤,该光纤由组分为As_(2)S_(3)的纤芯和呈六边形排列的空气孔的包层所组成。利用波长为2.87μm,重复频率为42 MHz,脉冲宽度为173 fs的中红外光纤激光器为泵源,利用拉锥硫系玻璃光子晶体光纤研制了中红外超连续谱。经过优化As_(2)S_(3)光子晶体光纤的拉锥直径后,其腰身直径为55μm,长度为3 cm。在该拉锥光纤中实现了-20 dB水平的光谱覆盖范围为2 000~5 500 nm的超连续光谱,实验结果和理论计算结果一致性较好。

Chalcogenide glass material has an ultra-broad infrared transmission window,ultrafast nonlinear response time and ultra-high third-order nonlinearity.The As_(2)S_(3)material has lower cost,higher nonlinearity and a broader transmission span than other chalcogenide materials,which is a supporting factor for supercontinuum generation.In this paper,a chalcogenide As_(2)S_(3)glass based Photonic Crystal Fiber(PCF) with an As_(2)S_(3)glass fiber core and air-holes as the microstructure cladding was theoretically designed,and the optical performance of the As_(2)S_(3)glass PCF was studied using a commercial software of CO_(2)MSOL Multiphysics.The proposed As_(2)S_(3)glass PCF preform was then experimentally fabricated using an improved molding method along with a chemical etching method.The As_(2)S_(3)glass PCF was drawn at the temperature of 350oC under the protection of dry Ngas.The fabricated As_(2)S_(3)glass PCF has a solid hole in the center and its cladding consists of four layers of air holes arranged in regular hexagonal order.The solid core diameter of the fabricated As_(2)S_(3)glass photonic crystal fiber is 10 μm,the diameter of the airholes is 3.3 μm and the air hole pitch between the centers of proximal holes is 7.2 μm.For the fiber tapering process,a micro-tapering system using a CO_(2)laser along with a scanning mirror and two high precision translation stages was established,all of which are computer programming controlled.The use of a CO_(2)laser to heat the fiber is advantageous over standard oxyhydrogen flame-tapering systems since it allows greater control over the tapering parameters,namely the size of the irradiated zone over the sapphire capillary,the heating rate and the exposure time,and also this avoids potential further pollution of OH^(-)-and HO into the As_(2)S_(3)glass PCF.By mounting the As_(2)S_(3)glass PCF on computer-controlled translation stages gives programmable dynamic control over the fiber tension,as well as the ability to control the position of the tapered section with an accuracy of ±0.5 μm.Using this tapering system,taper regions as long as 5 cm were achieved with a tapering fiber diameter reduction of 56%.The diameter of the As_(2)S_(3)glass photonic crystal fiber can be tapered down to 40 μm based on the tapering method.To fabricate a pump source of supercontinuum generation,a customized mode-locked femtosecond(fs)fiber laser based on nonlinear polarization rotation effect in a home-made Ho^(3+)/Pr^(3+)codoped(Ho^(3+)∶2 mol.%,Pr^(3+)∶0.2 mol.%)ZBLAN glass fiber(the core diameter is 12 μm,the cladding diameter is 125 μm and the NA is 0.16)was achieved and it generates 173 fs pulses at the wavelength of 2.87 μm with an estimated peak power of 25 kW and a repetition frequency of 42 MHz.Then,the tapered chalcogenide glass photonic crystal fibers were pumped using the above ZBLAN fs fiber laser to generate the mid-infrared supercontinuum spectra.After optimizing the tapered diameters of the tapered As_(2)S_(3)glass photonic crystal fiber,the tapered As_(2)S_(3)glass photonic crystal fiber with a waist diameter of 55 μm and a waist length of 3 cm can generate a supercontinuum spectral coverage range of 2 000 nm to 5 500 nm at the loss level of -20 dB.A theoretical model based on the well-known generalized non-linear Schr?dinger equation was also established for simulating the evolution of the proposed supercontinuum generation in the tapered As_(2)S_(3)glass photonic crystal fiber over the length of 3 cm.The experimental results have a good agreement with the theoretically calculated results.This investigation constitutes a major step toward devoloping an efficient chalcogenide glass photonic crystal fiber based broadband supercontinuum light source operating in the mid-infrared region.