An optical device, comprising: a first optical coupler including a first pair of waveguides (12a, 12b; 56a, 56b) separated by a first gap, the first pair of waveguides (56a, 56b) defining a first coupling region, wherein the first pair of waveguides includes a first waveguide and a second waveguide, the first waveguide having a first width and the second waveguide having a second width, the first width being different than the second width; a second optical coupler including a second pair of waveguides (12a, 12b) separated by a second gap, the second pair of waveguides defining a second coupling region; a third optical coupler including a third pair of waveguides (12a, 12b) separated by a third gap, the third pair of waveguides defining a third coupling region, the first, second and third gaps being varied in width such that the first, second and third couplers (52a, 52b, 52c) have different net coupling values (T1, T2, T3); a first differential phase cell provided between the first and second optical couplers (52a, 52b), the first differential phase cell including first and second waveguides (56a, 56b), and imparting a first differential phase shift between first optical signals propagating in the first and second waveguides (56a, 56b) of the first differential phase cell; and a second differential phase cell provided between the second and third optical couplers (52b, 52c), the second differential phase cell including first and second waveguides (56a, 56b), and imparting a second differential phase shift between second optical signals propagating in the first and second waveguides of the second differential phase cell. a first optical coupler including a first pair of waveguides (12a, 12b; 56a, 56b) separated by a first gap, the first pair of waveguides (56a, 56b) defining a first coupling region, wherein the first pair of waveguides includes a first waveguide and a second waveguide, the first waveguide having a first width and the second waveguide having a second width, the first width being different than the second width; a second optical coupler including a second pair of waveguides (12a, 12b) separated by a second gap, the second pair of waveguides defining a second coupling region; a third optical coupler including a third pair of waveguides (12a, 12b) separated by a third gap, the third pair of waveguides defining a third coupling region, the first, second and third gaps being varied in width such that the first, second and third couplers (52a, 52b, 52c) have different net coupling values (T1, T2, T3); a first differential phase cell provided between the first and second optical couplers (52a, 52b), the first differential phase cell including first and second waveguides (56a, 56b), and imparting a first differential phase shift between first optical signals propagating in the first and second waveguides (56a, 56b) of the first differential phase cell; and a second differential phase cell provided between the second and third optical couplers (52b, 52c), the second differential phase cell including first and second waveguides (56a, 56b), and imparting a second differential phase shift between second optical signals propagating in the first and second waveguides of the second differential phase cell.