An optical mode converter comprising: a coupling waveguide having an input end, the coupling waveguide including a tapered core (41; 402) of a substantially constant refractive index n 1 with a substantially square cross section at the input end (5; 405), having a size that tapers down to a width of about 0 moving away from the input end, the coupling waveguide having a cladding (32; 403) at least partially surrounding the tapered core; and a receiving waveguide having an output end, the receiving waveguide comprising a core (31; 404) of a substantially constant refractive index n 2, greater than the refractive index n 1 of the tapered core (41; 402) of the coupling waveguide, and a cladding (32; 403) at least partially surrounding the core, a side surface of the tapered core of the coupling waveguide being optically in contact, in a coupling portion thereof, with the receiving waveguide so as to allow optical coupling between the coupling waveguide and the receiving waveguide, the output end of the receiving waveguide being not in contact with the coupling waveguide, wherein for a fundamental TM or TE mode at an operating frequency propagating through a combined waveguide resulting from the coupling of the coupling waveguide and the receiving waveguide, the coupling waveguide has a first effective refractive index n 1eff at the input end for the propagating fundamental mode, and the receiving waveguide has a second effective refractive index n 2eff at the output end for the propagating fundamental mode, wherein the refractive index n 1 of the tapered core of the coupling waveguide is selected so that the first effective refractive index n 1eff and the second effective refractive index n 2eff differ from each other in absolute value less than 30% of the difference (n 2 - n 2eff) between the core refractive index of the receiving waveguide and the second effective refractive index, wherein the operating wavelength is within a wavelength band between 1290 nm and 1650 nm. a coupling waveguide having an input end, the coupling waveguide including a tapered core (41; 402) of a substantially constant refractive index n 1 with a substantially square cross section at the input end (5; 405), having a size that tapers down to a width of about 0 moving away from the input end, the coupling waveguide having a cladding (32; 403) at least partially surrounding the tapered core; and a receiving waveguide having an output end, the receiving waveguide comprising a core (31; 404) of a substantially constant refractive index n 2, greater than the refractive index n 1 of the tapered core (41; 402) of the coupling waveguide, and a cladding (32; 403) at least partially surrounding the core, a side surface of the tapered core of the coupling waveguide being optically in contact, in a coupling portion thereof, with the receiving waveguide so as to allow optical coupling between the coupling waveguide and the receiving waveguide, the output end of the receiving waveguide being not in contact with the coupling waveguide, wherein for a fundamental TM or TE mode at an operating frequency propagating through a combined waveguide resulting from the coupling of the coupling waveguide and the receiving waveguide, the coupling waveguide has a first effective refractive index n 1eff at the input end for the propagating fundamental mode, and the receiving waveguide has a second effective refractive index n 2eff at the output end for the propagating fundamental mode, wherein the refractive index n 1 of the tapered core of the coupling waveguide is selected so that the first effective refractive index n 1eff and the second effective refractive index n 2eff differ from each other in absolute value less than 30% of the difference (n 2 - n 2eff) between the core refractive index of the receiving waveguide and the second effective refractive index, wherein the operating wavelength is within a wavelength band between 1290 nm and 1650 nm.