An optical coupler (200, 500, 600, 700, 800) connectable between a first optical waveguide and a second optical waveguide for coupling modulated light guided by the first optical waveguide into multiple modes of the second optical waveguide, the optical coupler comprising: a graded-index waveguide comprising a core material orientated with respect to an optical axis of the optical coupler, the core material having a refractive index decreasing with increasing distance from the optical axis of the optical coupler, wherein a refractive index profile of the core material is designed in accordance to an optimality criterion with respect to conversion of the modulated light of the first waveguide into the multiple modes of the second waveguide, such that an energy coupling between modes of the first optical waveguide and the second optical waveguide which modes have different indices is eliminated; wherein the refractive index profile of the core material is designed to provide perfect mode matching of the modulated light of the first waveguide into the multiple modes of the second waveguide; wherein the perfect mode matching means that the optical coupler does not cause a redistribution of energy between the modes of the first optical waveguide and the second optical waveguide; the first optical waveguide is one of the following: a multi-mode graded index fiber, single-mode fiber, a laser diode; characterized in that: the second optical waveguide is a multi-mode graded index fiber; and wherein the first optical waveguide has a first gradient parameter and the second optical waveguide has a second gradient parameter; and wherein a gradient parameter of the refractive index profile corresponds to a geometric mean of the first gradient parameter of the first optical waveguide and the second gradient parameter of the second optical waveguide, in particular according to the relation: ω 0 = ω − ω +, where ω 0 denotes the gradient parameter of the refractive index profile, ω - denotes the first gradient parameter of the first optical waveguide and ω + denotes the second gradient parameter of the second optical waveguide; and wherein a length of the graded-index waveguide depends on the gradient parameter of the refractive index profile, in particular according to the relation: L = π 2 n 0 ω 0 2 p + 1, p = 0, 1, 2,..., where ω 0 is the gradient parameter of the refractive index profile and n 0 is a refractive index on the optical axis. a graded-index waveguide comprising a core material orientated with respect to an optical axis of the optical coupler, the core material having a refractive index decreasing with increasing distance from the optical axis of the optical coupler, wherein a refractive index profile of the core material is designed in accordance to an optimality criterion with respect to conversion of the modulated light of the first waveguide into the multiple modes of the second waveguide, such that an energy coupling between modes of the first optical waveguide and the second optical waveguide which modes have different indices is eliminated; wherein the refractive index profile of the core material is designed to provide perfect mode matching of the modulated light of the first waveguide into the multiple modes of the second waveguide; wherein the perfect mode matching means that the optical coupler does not cause a redistribution of energy between the modes of the first optical waveguide and the second optical waveguide; the first optical waveguide is one of the following: a multi-mode graded index fiber, single-mode fiber, a laser diode; characterized in that: the second optical waveguide is a multi-mode graded index fiber; and wherein the first optical waveguide has a first gradient parameter and the second optical waveguide has a second gradient parameter; and wherein a gradient parameter of the refractive index profile corresponds to a geometric mean of the first gradient parameter of the first optical waveguide and the second gradient parameter of the second optical waveguide, in particular according to the relation: ω 0 = ω − ω +, where ω 0 denotes the gradient parameter of the refractive index profile, ω - denotes the first gradient parameter of the first optical waveguide and ω + denotes the second gradient parameter of the second optical waveguide; and wherein a length of the graded-index waveguide depends on the gradient parameter of the refractive index profile, in particular according to the relation: L = π 2 n 0 ω 0 2 p + 1, p = 0, 1, 2,..., where ω 0 is the gradient parameter of the refractive index profile and n 0 is a refractive index on the optical axis. the second optical waveguide is a multi-mode graded index fiber; and wherein the first optical waveguide has a first gradient parameter and the second optical waveguide has a second gradient parameter; and wherein a gradient parameter of the refractive index profile corresponds to a geometric mean of the first gradient parameter of the first optical waveguide and the second gradient parameter of the second optical waveguide, in particular according to the relation: ω 0 = ω − ω +, where ω 0 denotes the gradient parameter of the refractive index profile, ω - denotes the first gradient parameter of the first optical waveguide and ω + denotes the second gradient parameter of the second optical waveguide; and wherein a length of the graded-index waveguide depends on the gradient parameter of the refractive index profile, in particular according to the relation: L = π 2 n 0 ω 0 2 p + 1, p = 0, 1, 2,..., where ω 0 is the gradient parameter of the refractive index profile and n 0 is a refractive index on the optical axis.