Precise recursive formula for calculating spot size in optical waveguides and accurate evaluation of splice loss

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Abstract
The spot size of a single-mode waveguide was defined from the viewpoint of a least-squares fit of the field profile to a Gaussian profile. The field profile was expanded in terms of Hermite–Gaussian functions, and a new precise recursive formula for calculating the spot size was derived. It was shown that our formula is equivalent to the best fitting of the offset coupling loss to that of a Gaussian profile and keeps its form against the Fourier transform that corresponds to the diffraction in the same manner as the Gaussian profile. The accuracy of conventional formulas and our new spot-size formula was compared with the exact value defined from the viewpoint of a least-squares fit to a Gaussian profile, and it was shown that our recursive formula is the most accurate of the approximate formulas. Next we proposed a new formula for calculating the splice loss between two waveguides and showed that our formula is more accurate than the conventional one, which involves only the spot size.