Laser Diode to Circular Core Graded Index Single Mode Fiber Excitation via Upside Down Tapered Microlens on the Fiber Tip and Identification of the Suitable Refractive Index Profile

Abstract

We investigate theoretically the coupling optics involving a laser diode and circular core graded index single mode fiber via upside down tapered microlens on the fiber tip in absence of possible transverse and angular misalignments to predict the nature of suitable refractive index profile for the first time in connection with optimum coupling. By employing Gaussian field distributions for both the source and the fiber and also ABCD matrix for upside down tapered microlens under paraxial approximation, we formulate analytical expressions for the concerned coupling efficiencies. The investigations are performed for two different light-emitting wavelengths of 1.3 µm and 1.5 µm for such fibers with different refractive index profile exponents. Further, it is observed that out of the studied refractive index profiles, triangular index profile having the dispersion-shifted merit comes out to be the most suitable profile to couple laser diode to such abovementioned fiber for two wavelengths of practical interest. The analysis should find use in ongoing investigations for optimum launch optics for the design of upside down tapered microlens either directly on the circular core graded index single mode fiber tip or such fiber attached to single mode fiber