In this paper, a 2×2 single-mode coupler based on indium fluoride optical fibers from Le Verre Fluoré (Bruz, France) is designed and characterized in the mid-infrared wavelength range. Coupled mode theory and finite element method are employed for its design. The 2×2 optical fiber coupler is fabricated via fused biconical tapering technique, employing a Vytran® GPX-2400 glass processing system. The primary constraint associated with the limited temperature range for processing indium fluoride optical fibers has been successfully addressed. Two identical fluoroindate (InF3) step-index optical fibers having a core diameter dco = 7.5 μm, cladding diameter dco = 125 μm, and numerical aperture NA = 0.30 are inserted into a fluoroindate capillary with a lower refractive index. The whole structure is tapered down ~ 2.4 times the initial diameter for a waist length Lw = 21.6 mm to achieve power coupling between the two optical fibers. The device is characterized at the wavelength λ = 3.34 μm, employing an interband cascade laser pigtailed with a single-mode fluoroindate optical fiber. The 2×2 optical fiber coupler is characterized in terms of through port and cross port powers, showing perfect agreement with the numerical results. A coupling ratio CR = 48.1:51.9 is measured at the wavelength λ = 3.34 μm, with a reduced excess loss EL < 1.2 dB. These results pave the way for reliable fabrication of highperformance fused optical fiber components that can boost research toward the development of all-in-fiber mid-infrared systems, such as in-band pumped mid-infrared amplifiers.
In this paper, a fiber amplifier based on ZBLAN fiber doped with dysprosium is designed and optimized considering an in-band pumping scheme. The model is validated by comparing the simulated amplified spontaneous emission with the experimental curves reported in the literature. It allows to investigate the amplification of the signal of a continuous-wave fiber laser emitting in the wavelength range from 2.9 μm to 3.25 μm. The numerical analysis is carried out via home-made code that accurately takes into account the rate equations and the power propagation equations for the signal, pump, and amplified spontaneous emission. The finite element method (FEM) is used to calculate the modal overlap in the designed pump fiber combiner with the Dy3+-doped core. By employing an input pump power Pp = 5 W at the wavelength λ = 2.82 μm, a signal power Ps = 2 mW at the wavelength λ = 2.95 μm, a fiber length L = 3 m an amplifier output power of 0.5 W and an optical gain of about 24 dB are achieved. The obtained results are attractive for feasible innovative applications, e.g. the development of all-in-fiber systems. For instance, the pump and signal beams can be obtained via an Er:ZBLAN fiber laser and coupled with the dysprosium fiber through a single-mode fluoride coupler.
An optical fiber amplifier based on a fluoroindate fiber doped with praseodymium (Pr3+:InF3) has been designed. The chosen fiber has a double-cladding and a 2-D shape. The electromagnetic behavior of the fiber has been simulated via a Finite Element Method (FEM) software, and the design of the fiber amplifier has been performed via a computer code, solving the rate-equations and power propagation equations. The gain G and the Amplified Spontaneous Emission (ASE) noise have been investigated as a function of different input parameters as the input signal power Ps0, the fiber length Lfiber , and the signal wavelength λs. The simulated fiber amplifier exhibits a bandwidth BG close to BG = 100 nm around the central signal wavelength λg = 4 μm, and a gain G close to G = 30.7 dB, when an input signal power PS = 10 μW and a pump power PP = 75 mW are considered. This pump value seems particularly low and further investigation will be performed to better understand this unexpected promising value.
A high sensitivity temperature sensor exploiting indium fluoride optical fibers is designed and characterized. It is based on a non-adiabatic tapered optical fiber, acting as a Mach-Zender interferometer. The sensitivity of the sensor is predicted via mode analysis, performed with Finite Element Method, and then computing the phase delay between the LP01 mode and the LP02 mode. By considering the effect of the thermal expansion and of the thermo-optical properties of the glass, respectively on the waist length and on the core and the cladding refractive indices, the sensing mechanism is explained. The non-adiabatic tapered optical fiber (Le Verre Fluoré IFG SM [2.95] 7.5/125) sensor is fabricated with Vytran GPX-2400 glass processing system, addressing the difficulties of indium fluoride glass, including its inclination to crystallize, its limited temperature range for fabrication, and its low glass transition temperature. The sensor is characterized in the mid-infrared spectral range with an interband cascade laser, emitting at the wavelength λ = 3.34 µm.
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