A frequency conversion system capable of generating at least one converted beam (Fc) having a converted frequency (vc) from at least one signal beam (Fs) having a signal frequency (vs), using a plurality of n elementary pump beams F p,i, indexed i, with i varying from 1 to n, with n being greater than or equal to 2, having a pump frequency (vp), the system comprising: - a crystal comprising a material, the sign of which for the second order non-linear coefficient (χ) is periodically inverted along an axis Z, with said crystal being configured so that the conversion occurs by means of a second order non-linear effect in said crystal, - an optical device configured to generate said plurality of n elementary pump beams from an initial pump beam (Fpini), with an elementary pump beam F p,i+1, indexed i+1, being generated by redirecting the elementary pump beam F p,i, indexed i, towards the crystal after it has passed through the crystal, the optical device comprising a first optical component and a second optical component disposed on either side of the crystal, with said first and second optical components being configured to at least partially reflect the plurality of elementary pump beams, the first optical component comprising two flat mirrors (M1, M' 1) perpendicular to one another and joined along a first edge, and the second optical component comprising two flat mirrors (M2, M'2) perpendicular to each other and joined along a second edge, with the first and second edges (d1, d2) being perpendicular to Z and having a non-zero angle (α) between them in an XY plane, - the crystal and the optical device being configured so that at least one elementary pump beam, a signal beam and a converted beam fulfil a quasi-phase-matching condition when propagating in said crystal in a direction of propagation, the optical device thus being configured so that the plurality of n elementary pump beams are parallel to each other. the system comprising: - a crystal comprising a material, the sign of which for the second order non-linear coefficient (χ) is periodically inverted along an axis Z, with said crystal being configured so that the conversion occurs by means of a second order non-linear effect in said crystal, - an optical device configured to generate said plurality of n elementary pump beams from an initial pump beam (Fpini), with an elementary pump beam F p,i+1, indexed i+1, being generated by redirecting the elementary pump beam F p,i, indexed i, towards the crystal after it has passed through the crystal, - a crystal comprising a material, the sign of which for the second order non-linear coefficient (χ) is periodically inverted along an axis Z, with said crystal being configured so that the conversion occurs by means of a second order non-linear effect in said crystal, - an optical device configured to generate said plurality of n elementary pump beams from an initial pump beam (Fpini), with an elementary pump beam F p,i+1, indexed i+1, being generated by redirecting the elementary pump beam F p,i, indexed i, towards the crystal after it has passed through the crystal, the optical device comprising a first optical component and a second optical component disposed on either side of the crystal, with said first and second optical components being configured to at least partially reflect the plurality of elementary pump beams, the first optical component comprising two flat mirrors (M1, M' 1) perpendicular to one another and joined along a first edge, and the second optical component comprising two flat mirrors (M2, M'2) perpendicular to each other and joined along a second edge, with the first and second edges (d1, d2) being perpendicular to Z and having a non-zero angle (α) between them in an XY plane, - the crystal and the optical device being configured so that at least one elementary pump beam, a signal beam and a converted beam fulfil a quasi-phase-matching condition when propagating in said crystal in a direction of propagation, - the crystal and the optical device being configured so that at least one elementary pump beam, a signal beam and a converted beam fulfil a quasi-phase-matching condition when propagating in said crystal in a direction of propagation, the optical device thus being configured so that the plurality of n elementary pump beams are parallel to each other.