Coupling of a laser diode to single mode circular core graded index fiber via parabolic microlens on the fiber tip and identification of the suitable refractive index profile with consideration for possible misalignments

Abstract

We present a theoretical investigation of coupling optics involving a laser diode and circular core graded index single mode fiber via a parabolic microlens on the fiber tip to predict the nature of suitable refractive index profile for the first time in connection with optimum coupling. The study is carried out in absence and presence of possible transverse and angular misalignments. By employing Gaussian field distributions for both the source and the fiber and also ABCD matrix for parabolic 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 parabolic microlens either directly on the circular core graded index single mode fiber tip or such fiber attached to single mode fiber to achieve long working distance.