Combination lens method for coupling a laser diode to a single-mode fiber

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Abstract
A new combination lens method using a relatively large spherical lens and a virtual fiber is proposed for coupling an InGaAsP laser diode to a single-mode fiber. The virtual fiber is formed by attaching a GRIN rod lens to the input endface of the single-mode fiber. Coupling efficiency of from −3 to −4 dB is easily obtained. Alignment sensitivities of the proposed optical coupling circuit are investigated both theoretically and experimentally. The lateral and axial misalignment tolerances for the virtual fiber are improved three and nine times, respectively, in comparison with those for the single-mode fiber. The measured dependence of the coupling efficiency on the misalignments for the optical components agrees relatively well with the theoretical results through the ray matrix approach.