| CPC G02B 6/125 (2013.01) [G02B 6/26 (2013.01); G02B 6/44715 (2023.05); G01D 5/3538 (2013.01); G02B 6/02042 (2013.01)] | 6 Claims |

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1. A Fan-In Fan-Out (FIFO) device comprising:
a first device end surface;
a second device end surface opposing the first device end surface;
a waveguide that has a first waveguide end surface that matches the first device end surface and a second waveguide end surface that matches the second device end surface, and in which light beams of a plurality of modes having different orders are guided, and one or more bent portions are provided on an optical path from the first waveguide end surface to the second waveguide end surface; and
a cladding layer in which the waveguide is provided inside or on a surface, and has a refractive index lower than a refractive index of the waveguide,
wherein the waveguide has
a rectangular cross section,
a waveguide length L of 5×106 [nm] or more and 100×106 [nm] or less defined by an optical path length from the first waveguide end surface to the second waveguide end surface, and
a structure in which a group delay time difference Δβ1 between the plurality of modes satisfies a condition given by |Δβ1|≤2×10−12 [s]/L.
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