Single-mode square-grid MOFs with enlarged mode field intended for the middle infrared

Abstract In this paper, we modeled and fabricated a single-mode microstructured fibre (MOF) based on the solid solution crystals of AgBr–TlI and AgBr–TlBr0.46I0.54 systems. For this purpose, we gained the refractive index dispersion of the given systems in the wavelength range of 19–25 μm. It served as a basis for fibre simulations. The obtained MOF model was 700 μm in diameter; it included 200-μm central insert and 40 quasi-square peripheral inserts with the cross-section of 20 × 20 μm and the pitch of 80 μm and operated in a single-mode regime within the wavelength range of 22–25 μm, while the transmitting range was 2.5–25 μm. It possessed an enlarged mode field when compared to a corresponding single-mode step-index fibre (SIF). This MOF was fabricated using a method, which combines hot pressing and extruding. It was shown that the fundamental mode is tightly confined in the fibre core region. The developed MOF structure may be suitable for producing outputs, splitters, and couplers for various types of solid-state lasers based on semiconductors and gas steam lasers, including ones deployed in a harsh environment.