Method for manufacturing a mode converter ensuring adiabatic coupling between a first waveguide and a second waveguide, the mode converter being formed from a first coupler of the first waveguide and from a second coupler of the second waveguide, the first and second couplers (1c, 2c) being superposed one on the other and having a width, W 1 and W 2 respectively, that varies over a length L along a longitudinal axis, the method comprising the following steps: o defining on the one hand a longitudinal variation W 1 = f 1 ( z), in the width W 1 of the first coupler, between positions z 0 and z f along the longitudinal axis such that z f = z 0 + L, from a maximum value W 1,max at z 0 to a minimum value W 1,min at z f, and on the other hand minimum and maximum values W 2,pmin and W 2,pmax, referred to as provisional values, of the width W 2 of the second coupler; o determining (221; 222) propagation constants β e and β 0 associated with even and odd supermodes supported by the two couplers (1c, 2c), respectively, for each position z between z 0 and z f depending: • on the local value W 1 ( z) in z of the width W 1 of the first coupler, and • on a local value Ŵ 2 ( z) in z of the width W 2 of the second coupler, - for which value the propagation constants β 1 ( W 1 ( z)) and β 2 ( Ŵ 2 ( z)) respectively associated with the fundamental modes supported by the first coupler of local width W 1 (z) and by the second coupler of local width Ŵ 2 ( z) are equal, the two couplers (1c, 2c) then being isolated from each other; o determining (231; 232) a longitudinal variation κ = h ( z) between z 0 and z f in the coupling parameter κ of the coupling between the two couplers (1c, 2c), on the basis of a difference between the determined propagation constants β e and β 0; o determining a longitudinal variation W 2 = f 2 ( z) between z 0 and z f in the width W 2 of the second coupler such that the adiabaticity criterion is met at each position z, this adiabaticity criterion relating the position z in question, the predefined length L, the predetermined coupling parameter κ ( z), said propagation constant β 1 ( W 1 ( z)), and a propagation constant β 2 ( W 2 ( z)) associated with a local value W 2 ( z) to be determined; o manufacturing the mode converter, in which mode converter the first coupler has the defined longitudinal variation W 1 = f 1 ( z), and the second coupler has the determined longitudinal variation W 2 = f 2 ( z). o defining on the one hand a longitudinal variation W 1 = f 1 ( z), in the width W 1 of the first coupler, between positions z 0 and z f along the longitudinal axis such that z f = z 0 + L, from a maximum value W 1,max at z 0 to a minimum value W 1,min at z f, and on the other hand minimum and maximum values W 2,pmin and W 2,pmax, referred to as provisional values, of the width W 2 of the second coupler; o determining (221; 222) propagation constants β e and β 0 associated with even and odd supermodes supported by the two couplers (1c, 2c), respectively, for each position z between z 0 and z f depending: • on the local value W 1 ( z) in z of the width W 1 of the first coupler, and • on a local value Ŵ 2 ( z) in z of the width W 2 of the second coupler, - for which value the propagation constants β 1 ( W 1 ( z)) and β 2 ( Ŵ 2 ( z)) respectively associated with the fundamental modes supported by the first coupler of local width W 1 (z) and by the second coupler of local width Ŵ 2 ( z) are equal, the two couplers (1c, 2c) then being isolated from each other; • on the local value W 1 ( z) in z of the width W 1 of the first coupler, and • on a local value Ŵ 2 ( z) in z of the width W 2 of the second coupler, - for which value the propagation constants β 1 ( W 1 ( z)) and β 2 ( Ŵ 2 ( z)) respectively associated with the fundamental modes supported by the first coupler of local width W 1 (z) and by the second coupler of local width Ŵ 2 ( z) are equal, the two couplers (1c, 2c) then being isolated from each other; - for which value the propagation constants β 1 ( W 1 ( z)) and β 2 ( Ŵ 2 ( z)) respectively associated with the fundamental modes supported by the first coupler of local width W 1 (z) and by the second coupler of local width Ŵ 2 ( z) are equal, the two couplers (1c, 2c) then being isolated from each other; o determining (231; 232) a longitudinal variation κ = h ( z) between z 0 and z f in the coupling parameter κ of the coupling between the two couplers (1c, 2c), on the basis of a difference between the determined propagation constants β e and β 0; o determining a longitudinal variation W 2 = f 2 ( z) between z 0 and z f in the width W 2 of the second coupler such that the adiabaticity criterion is met at each position z, this adiabaticity criterion relating the position z in question, the predefined length L, the predetermined coupling parameter κ ( z), said propagation constant β 1 ( W 1 ( z)), and a propagation constant β 2 ( W 2 ( z)) associated with a local value W 2 ( z) to be determined; o manufacturing the mode converter, in which mode converter the first coupler has the defined longitudinal variation W 1 = f 1 ( z), and the second coupler has the determined longitudinal variation W 2 = f 2 ( z).