Reflective Phase Retarders

Reflective Phase Retarders

Metal cutting and other critical laser operations are sensitive to any variation in kerf width or cross-section. The kerf’s quality depends on the polarization orientation relative to the cut direction. This is illustrated in Figure 1.

Current theory suggests that the assumption of a focused beam striking the work piece at normal incidence is only true at the cut’s beginning. Once the kerf forms, the beam encounters metal at some large angle of incidence, Θ, as shown in Figure 2. Light which is s-polarized with reference to such a surface is reflected much more than light which is p-polarized, leading to the difference in cut quality.

Introducing a quarter-wave (90°) reflective phase retarder into the beam delivery path eliminates kerf variations by converting linear polarization to circular polarization. Circular polarization consists of equal amounts of s-polarization and p-polarization for any beam orientation, therefore all axes encounter the same composition of polarization, and material is removed uniformly regardless of cut direction. This is illustrated in the photos below.

Ragged Cut: produced by linearly polarized light

Clean Cut: produced by circularly polarized light

A linearly polarized beam is oriented so that the plane of polarization is 45° to the plane of incidence and strikes the RPR at 45° to the normal, as shown in Figure 3. The reflected beam is circularly polarized.

The substrate choice depends upon the power level at which the laser operates. Alternate substrates, including water-cooled copper, are available. Eighth-wave and sixteenth-wave RPR designs, and designs for peak wavelengths other than 10.6µm are also available. Please contact a II-VI sales representative to obtain a quotation.