High-brightness broadband mid-infrared supercontinuum sources are highly demanded for many applications such as remote sensing, environmental monitoring, manufacturing industry, medical surgery and thermal imaging. For fulfilling these applications, high average power output is required. Compared with all other mid-infrared glass fibers, chalcogenide glass fiber possesses low phonon energy, long wavelength transmission edge, and high Kerr nonlinearity, thereby becoming a uniquely ideal nonlinear optical material for generating broadband mid-infrared supercontinuum. Unfortunately, due to weak chemical bonds forming the glass network, the commonly used As-S chalcogenide glass has a relatively low laser damage threshold. Thus from the material aspect, it limits high power yielded from a chalcogenide fiber based mid-infrared supercontinuum source. A chalcogenide glass host with enhanced laser damage threshold is therefore needed for further power scaling up of such a mid-infrared fiber supercontinuum.

In this work, we introduce germanium into a traditional As-S glass system. The laser damage threshold of Ge-As-S glass is investigated systematically. A 3.6-μm femtosecond laser is employed as an excitation source. The relationship between the laser damage threshold and the glass composition indicates that of the studied Ge-As-S chalcogenide glasses, stoichiometric Ge_0.25As_0.1S_0.65 glass possesses the highest laser damage threshold.

In the following fiber design and fabrication, the optimized stoichiometric Ge_0.25As_0.1S_0.65 glass therefore is chosen as a core material of the designed fiber, while a compatible Ge_0.26As_0.08S_0.66 glass is selected as a cladding material. A step-index nonlinear fiber with a core diameter of 15 μm is fabricated by the traditional rod-in-tube method. The numerical aperture and the background loss of the fabricated Ge_0.25As_0.1S_0.65/Ge_0.26As_0.08S_0.66 fiber are ~0.24 and < 2 dB/m, respectively.

Broadband mid-infrared supercontinuum is generated in the fiber by using an anomalous-dispersion pumping scheme. A 4.8-μm femtosecond laser with a pulse duration of 170 femtosecond and a repetition rate of 100 kHz is adopted as a pump source. The guidance of the fundamental mode is confirmed under low pump power level. With the increase of the pump power, the supercontinuum shows to be significantly broadened. Broadband supercontinuum ranging from 2.5 μm to 7.5 μm is generated in an only 10-cm-long fiber, when the maximum coupled pump power is 15 mW, equivalent to a peak power of 882 kW. The power output of the supercontinuum is 5.5 mW.

All in all, the results indicate that the Ge-As-S chalcogenide glass fiber is a promising nonlinear medium for broadband mid-infrared supercontinuum sources with high brightness.