Application of Nematic Liquid Crystals to Optical NDE

Nematic liquid crystals (NLC) exhibit a so-called ‘giant’ optical non-linearity, that is, relatively low optical powers (10 - 100mW) can induce large changes in the refractive index of the material. By applying both an optical field and an AC/DC voltage to an aligned NLC, a situation is created in which the induced refractive index of the NLC are extremely sensitive to changes in the optical field intensity. This effect serves as the foundation for the development of several new devices that have direct application to optical interferometric NDE techniques.

An example of such a device is an ‘optical phase amplifier (OPA).’ Inspection of surface deformations on the order of a fraction of a wavelength of light present problems when trying to achieve a high degree of accuracy in the measurements. By placing an OPA at the output of a surface scanning interferometry, the very slight changes in intensity, associated with the small deformation, are mapped to large NLC refractive index changes. These induced changes can then be retrieved using either interferometric or polarization techniques. This process yields projected displacement amplification factors of one to two orders of magnitude, depending on NLC material choice and device geometry, and could potentially be used in conjunction with such high resolution techniques as phase stepping or heterodyning.

Currently, our efforts are being focused upon theoretical modeling in order to optimize the performance of the OPA, as well as, the characterization of preliminary NLC device prototypes.

Practical Application

Various devices, NLC materials, and optical configurations are still under study, but among the applications being considered are increasing the accuracy of optical profilometry by at least an order of magnitude, increasing the depth resolution of full-field interferometry, and ‘low-frequency immune’ interferometry for industrial applications.