Several launching techniques, such as center launching, offset launching, mode-field diameter- matched center launching, twin-spot launching, etc., have been employed in the past to reduce modal dispersion in multimode fibers (MMFs). Unfortunately, all these methods require stringent alignment accuracy of an input beam with the fiber axis. In this paper, we propose a new optical launching method, viz., the mode-field diameter- matched twin-spot launching technique, which combines the desirable features of two earlier techniques so as to excite only the fundamental mode in an MMF with much higher misalignment tolerance compared to existing launching techniques. Also, we derive for the first time an analytical expression for power coupling coefficients of the excited modes in an infinite parabolic refractive index profiled MMF under twin-spot launching. The proposed technique can excite the fundamental mode with more than 9 dB extinction ratio at 1300 nm, even though the two beams are at a lateral offset position of 5 mu m from the axis (i.e., 10 mu m apart). Additionally, it is tolerant of variations in the mode-field diameter, relative positions, power, and phase of the input beams. The comprehensive analysis shows cancellation of the odd/even symmetrical mode groups in an MMF depending on the relative phase of the input beams that results in reduced group-delay differences of the excited mode groups. (C) 2017 Optical Society of America
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