direction through a second line focus 38 approximately coincident with an axis 39 of the waveguide 10 both the line focus 38 and the axis 39 of the waveguide 10 extend in the u201c x u201d orthogonal direction in contrast to the focused convergence in the u201c y u201d coordinate direction the beam 18 continues to diverge in the u201c x u201d coordinate direction through the focusing lens 36 until the beam 18 strikes a phase mask 40 at angles of incidence that vary as a function of distance from a center of the beam 18 a total divergence of the beam 18 from the first focal line 26 to the phase mask 40 takes place through a distance u201c z u201d an adjustable mount 42 for the phase mask 40 translates in the u201c z u201d coordinate direction to adjust the distance u201c z u201d the phase mask 40 which is itself a diffraction grating diffracts the beam 18 into two interfering beams 44 and 46 through two different preferably opposite sign diffraction orders e g 1 and u2212 1 orders other combinations of orders can also be used including a combination of zero and first orders but the two first orders are preferred instead of positioning the phase mask 40 directly against the exposed portion 12 of the waveguide 10 in accordance with usual practices the phase mask 40 is spaced apart.