Femtosecond lasers have emerged as a versatile and powerful instrument, capable of three-dimensional processing within dielectric materials. Among their applications, they have been employed to precisely inscribe Bragg grating structures in various types of glass fibers. This method achieves both arbitrary grating periods and high temperature stability. However, the application of direct femtosecond laser writing in Chalcogenide (ChG) fiber has been limited. This limitation stems from the material's high linear and nonlinear refractive indices, along with its low threshold for laser-induced damage.
Despite these challenges, Prof. GUO Haitao’s group at the Xi’an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences, have successfully fabricated ChG fiber Bragg grating (FBG) with remarkable properties: a high reflectivity of 20.5 dB and a narrow 3-dB bandwidth of 164 pm, using femtosecond laser direct writing. To navigate the challenges posed by ChG's high refractive indices, a novel double-D shaped ChG fiber was developed. This shape mitigates aberrations, facilitating the creation of superior grating morphologies. Moreover, the team discovered an optimal range of femtosecond laser power for direct FBG writing, addressing the material's low laser-induced damage threshold. Additionally, their work in characterizing the laser-induced damage threshold (LIDT) of ChG offers valuable insights into the process. Details of the work was published in Optics and Laser Technology.
This work not only advances the direct femtosecond laser writing of ChG FBGs but also highlights the potential of Chalcogenide glasses. With the material’s wide transparency window ranging from 0.6 to 12 μm, ChGs stand out as a prime mid-infrared material for applications in fiber lasers and sensors. The successful direct writing of ChG FBGs paves the way for the development of monolithic infrared fiber lasers and fiber sensors, marking a significant milestone in the field.
Schematic of the femtosecond laser direct inscription, and FBG morphologies(Image by LIU et al.)
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