We propose and experimentally demonstrate a shift-multiplexing complex spectral-domain optical coherence tomography (shift-multiplexing CSD-OCT) method, in which the maximum detection depth of SD-OCT can be greatly extended by incorporating the shift-multiplexing of detection positions with CSD-OCT. The tomographic imaging with twofold or threefold microscopic slides as the target sample is performed. The experimental results show that the tomographic imaging with more uniform brightness and clarity for the different depth regions in a thick sample can be achieved by the shift-multiplexing CSD-OCT system. In particular, even while the sample’s depth is beyond the maximum imaging depth of CSD-OCT system, the tomographic imaging of this sample can still be realized by using the shift-multiplexing CSD-OCT method without the need for any replacement of the equipment, such as high spectral capacity grating or high resolution of CCD. The shift-multiplexing CSD-OCT system can perform the imaging with the optimization and less reduction of sensitivity for the deeper detection position in the sample.
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.
The three-step or many steps phase shifting method is usually employed to resolve the complex-conjugate ambiguity in Spectral-domain optical coherence tomography (SD-OCT). However it reduces the image quality and also the imaging speed is slow. In this paper two steps phase-shifting is used in digital image processing to resolve the complex-conjugate ambiguity and improves the quality of reconstructed image in SD-OCT. In the two-step phase shifting method the phase shifting operation is used only once which simplified the experiment and also the effect of relative error in SD-OCT on image quality is eliminated.