Evaluation of optical thickness and refractive index of a layered sample by low coherence optical tomography

Following coupled mode approach, we report ab initio calculations of the signal intensity obtainable at the output end of a fibre optic coupler in a standard OCT technique and the results have been verified experimentally. The calculated intensity output varies periodically with respect to the path length difference between the sample and the reference ann and represent the interference signal, which sustains within the coherence length of the source. In the experimental set up, we have used 670 nm semiconductor diode laser possessing coherence length of 60 micron as a source. We have used a sample consisting of thin layer of glass plate and mirror. Reference mirror of 80% reflectivity was chosen. The output from the coupler was read on the Digital storage oscilloscope. A comparison of the theoretical plot of signal amplitude as a function of reference mirror position with the trace of the signal obtained on DSO exhibits good matching. The refractive index of the glass plate as calculated experimentally was 1.50 +/- .0125 which agrees well with the standard value. The experimental setup can enable one to determine the refractive index as well as optical thickness of any thin film simultaneously. The signal-to-noise ratio was found to be similar to 20dB.