A scanning differential optical system for simultaneous phase and amplitude measurement

A scanning optical system has been developed which can simultaneously and independently measure the differential amplitude and phase of light reflected off an object surface. The central element of such a system is an acousto-optic modulator which splits the incoming light into two beams of different optical frequencies, amplitude modulated in phase quadrature. These are focused with close proximity on the sample surface. The theoretical sensitivity of this type of differential optical system is 1x10−3mrad in phase and 1 in 105in reflectivity variation. In this paper we will look at the first implementation of the technique and practical problems encountered with this system. A second system is presented in which two first order beams from the Bragg cell are used to interrogate the sample. An analysis is made of the interpretation of the detected signals when different types of amplitude modulation is used. This latter system is shown to have an improved differential amplitude response. A number of samples have been looked at to assess the performance of the system. These demonstrate the high sensitivity of this system and its capability in separating amplitude and phase information. The advantages of extracting differential amplitude as well as phase information is discussed using a simple layered model. Potential applications include film thickness metrology, surface profilometry and measurement of refractive index variation. In addition it can be used to image low contrast objects such as biological specimens and defects on crystalline structures.