PURPOSE: To obtain an optical coupler which has less aberrations even in case of the use of various wavelength bands and has a controllable branch ratio and is suitable for mass production, by forming the pattern of a grating lens with a function to which an offset is given by a function which changes approximately periodically with respect to a function expressing the pattern of the grating lens where a diffracted light is converged at one point.

CONSTITUTION: xy coordinates and ξη coordinates are taken on a grating lens surface 4 and a focal surface 10 of the lens of the optical coupler respectively. This optical coupler 6 has a concentric circle-shaped grating pattern, and an offset is given to the phase function of this grating lens 4 by the periodically changing function. In this constitution, the intensity of the diffracted light of the optical coupler 6 at a point Q(ξ,η) on the focal surface 10 is a function having a peak at points (md, 0) (m=0, ±1, ±2...). That is, the diffracted light is converged at plural points placed at equal intervals of a length (d) on the ξηsurface. Thus, the optical coupler having the N:1 multiplexing function is realized if 1:N branch or arrangement is reversed.