A general method which extends quadrature techniques to Type I nonlinear optical parametric interactions. In one embodiment, a pair of either uniaxial or biaxial birefringent nonlinear optical crystal elements are serially arranged and oriented so that each crystallographic axis in one conversion means is parallel to the corresponding axis in the second conversion means. Two colinear input fundamental laser fields with parallel polarizations propagate through both crystals, generating a sum-frequency output field. Between the two crystals, a harmonic waveplate is inserted that rotates only the polarization of the sum-frequency field generated in the first conversion means by 90.degree. about its propagation axis. The net polarization rotation of each of the two residual fundamental waves which remain after the interaction in the first conversion means is zero. Therefore, the fundamental waves remain correctly polarized for efficient nonlinear optical conversion in the second conversion means, while the sum-frequency wave is no longer correctly phase-matched for back-conversion. Precise phase-matching may be accomplished either by tilting the assembly about its angularly sensitive axis ("critical" phase-matching) or by adjusting the temperature of the crystals ("non critical" phase-matching). In general, the polarization of the sum-frequency output field will be elliptical, but it can be linearized by tilting the assembly about its angularly insensitive axis.