The measurement of average refractive index with substrate-calibrating by using complex frequency-domain optical coherence tomography method

Abstract

Optical Coherence Tomography (OCT) was successfully applied in the microstructure imaging of biological tissue after being proposed firstly in 1991 by the researchers of MIT. As a novel optical imaging technology, it mainly uses interference principles to achieve noninvasive and high resolution visualization of samples. OCT works analogously to an ultrasound scanner, the major difference is that ultrasound pulses are replaced by broadband light. According to whether need for mechanical axial scan in the depth direction, it can be classified into the time-domain OCT (TD-OCT) and frequency-domain OCT (FD-OCT). The FD-OCT system overmatches the TD-OCT in imaging speed because of its depth collection advantage. But in the reconstructive image of FD-OCT detection, the complex-conjugate ambiguity will seriously deteriorate the imaging effect of tomogram. So the technique of removing the complex-conjugate image is employed that is called complex FD-OCT. The complex FD-OCT has widely application in many fields, especially in the refractive index measurement. The refractive index is an important parameter characterizing light propagation in the medium. In the paper, we present a method to measure the average refractive index of the sample with substrate calibration by using complex FD-OCT method, in which we can calculate it without depending on the parameters of system such as spectral width of light source. Due to the measurement of average refractive index relative to the actual thickness and optical length, it is necessary to obtain them of the sample experimentally. The complex FD-OCT method can easily achieved the optical length via measuring the positions of the sample’s front and rear surfaces. In the experiment, the coverslip (the borosilicate glass) is chosen as the sample and the calibration substrate. We make use of the substrate to load the sample on it, and then the tomogram of the sample can be achieved by means of OCT’s lateral scan in the edge of the sample and complex FD-OCT method. According to the experimental results, we can acquire the sample’s tomographic information and position of the substrate. The ratio of actual thickness and optical length can be indirectly calculated out with the pixel number obtained by analyzing the image data. So with only one time scan, we can complete the measurement of average refractive index of the sample without aid of other instruments.