Liquid-Crystal Phase-Shifting Shearing Interferometer

John H. Glenn Research Center, Cleveland, Ohio

The liquid-crystal phase-shifting shearing interferometer is a common-path interferometer based partly on a lateral-shear-plate interferometer. It bears a partial similarity to the liquid-crystal point-diffraction interferometer (LCPDI), which is also of the common-path type. The phase-shifting nature of this interferometer is expected to increase (relative to prior shearing interferometers) resolution by up to two orders of magnitude. The liquid-crystal phase-shifting shearing interferometer is expected to be useful for measuring spatially varying optical density in a laboratory or manufacturing setting; examples of such densities include the index of refraction of a liquid in a production process, the density and/or temperature of a gas in a combustion system, and the temperature of a fluid in a boiler.

The Liquid-Crystal Layer in the Shear Plate is used to vary the phase of the light reflected from its rear surface.

The proper functioning of the LCPDI depends on focusing a laser beam onto a transparent microsphere and, hence, depends on the critical adjustment of a focusing lens by a highly trained technician. Unlike the LCPDI and many other interferometers, the liquid-crystal phase-shifting shearing interferometer can be aligned easily, with no need for critical adjustments by a highly trained technician. Moreover, elimination of the need for focusing on a microsphere also eliminates phase noise associated with inhomogeneities in a focusing lens.

In the liquid-crystal phase-shifting shearing interferometer, collimated light from a laser is incident on a shear plate oriented at an angle of 45° (see figure). Unlike a traditional shear plate, this shear plate contains a liquid-crystal layer that can be used to vary the phase of the light reflected from its rear surface. The amount of shear depends on the effective optical thickness of the shear plate, which is comparable to the combined optical thicknesses of its glass layers. When a voltage is applied across the liquid-crystal layer, the index of refraction of the layer changes, causing the phase of the portion of the incident light reflected from the rear surface to be stepped relative to the phase of this portion when the voltage is not present. Calibration of the phase shift as a function of voltage can produce the phase steps required to implement common phases-stepping algorithms.

The amount of shear varies roughly inversely with index of refraction and thus with the phase. However, calculations for the case of a total shift of one wavelength have shown that the total shift in beam position can be safely neglected because it is more than an order of magnitude below the size of a typical pixel in a charge-coupled-device camera that would be used in implementing a practical version of this interferometer.

In a prototype of this liquid-crystal phase-shifting shearing interferometer, the shear plate is a commercial liquid-crystal phase retarder originally intended for use as an extended-range, variable-retardance wave plate rather than an interferometer component. The manufacturer's specification for reflectance at each surface is less than half of one percent. The device would perform optimally if its front surface were coated with a partially reflective film and its rear surface with a totally reflecting film.

This work was done by DeVon W. Griffin of Glenn Research Center.

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