Collimated light acoustooptic lateral shearing interferometer.

In this Letter, a collimated light acoustooptic lateral shearing interferometer (AO/LSI) capable of variable shear is described for use with either visual or ac electronic real-time phase measurements. The principle of operation of the AO/LSI is shown in Figs. 1 and 2. An input beam of diameter W is incident at Bragg’s angle upon two acoustooptic modulators (AOM’s) in series. They are shown schematically as thin gratings in Fig. 1, where the Bragg angles of the two AOM’s are indicated as θ B and θ 2. The first AOM is driven at an acoustic frequency f1 (typically in the tens of megahertz), the second AOM at frequency f2 = f1 + ∆ f. If ∆ f << f1, θ2 is nearly the same as θ B, as assumed in Fig. 2. It may be seen from Figs. 1 and 2 that some light is diffracted out of the zero-order beam by each AOM. The separation x between each AOM is adjusted until the two overlapping diffracted beams, which produce an interference pattern, are sheared (i.e., shifted) by an amount s. (A convenient value of s is 0.1 W.) The amount of shear can be varied by adjusting f1 too. In Fig. 2, it may be seen that the diffracted wave is assumed to originate from the center of each AOM. Actually, the wave is diffracted from the volume of the AOM, and the resultant wavefront exits the AOM at the angle θ B. However, for purposes of this discussion, it is convenient to assume that the central ray of the incident wavefront is reflected (i.e., diffracted) from the center of the AOM. From the geometry of Fig. 2 together with the Bragg diffraction equation, an equation relating the various parameters of the collimated light AO/LSI can be derived: