System Characteristics

The phase-shifting FF-OCT system is beneficial into directly obtaining fringe-free tomo- graphic images during the measurement. The resultant image volume occupies a much smaller storage space in the PC than the simple system for the investigation of the same sample with the same imaging depth.

The depth-resolution and transverse resolution of the phase-shifting system is identical to that of the simple system. This is because the phase-shifting system was developed based upon the simple system and there were no modifications to the light source, the CMOS camera, and the beam spread. According to Equations 2.21 and 2.22 (see above), the imaging resolution stays the same with the fixed source central wavelengthλ0, the spectral bandwidth

∆λ, the NA and the pixel size. As described in Section 3.3.5 (see above), the depth-resolution was measured as 3.6 µm and the transverse resolution was measured as 10.3 µm.

The sensitivity of the simple FF-OCT system is determined by the high sampling rate of the interference patterns. For the phase-shifting system, the sensitivity is enhanced by the averaging of a large number of acquired phase-shift images.

During the measurement, a pre-set numberN of images at each step of phase-shifting was acquired and stored temporarily in the PC buffer. By averaging these images, the system sensitivity in dB can be enhanced by 10 log₁₀N:

Σavg=10 log10 ⟨NI_OCT⟩2 √ Nσ_noise2 =10 log₁₀⟨IOCT⟩ 2 σ_noise2 +10 log₁₀N. (3.23)

To compare the sensitivity between the two developed systems, the same window sample was measured by the phase-shifting system with a mirror reference. Each resultant fringe- free image was averaged by 90 phase-shift images. Fig. 3.18 (see below) shows both the measurements of the window sample by using the phase-shifting system and the simple system, respectively. The reflectivity variation obtained from the phase-shifting system is described by the dark blue dots; the background light blue signals are taken from Fig. 3.12 (see above) for the sensitivity evaluation of the simple system.

(a) (b)

μ μ

Fig. 3.18 Sensitivity comparison between the phase-shifting system and the simple system by measuring a window sample; the dark blue dots were obtained from the phase-shifting system and the background light blue signals were obtained from the simple system; (a) A-scan signals in a range of 0 to 0.04 to describe the window reflectivity as a function of depth; (b) the A-scan signals in dB obtained by using the phase-shifting system with a minimum identifiable reflectivity approaching−70 dB.

As the window has a reflectivity of 0.04 or −28 dB, Fig. 3.18 (a) demonstrates the variation of the reflectivity within the range of 0 to 0.04 as a function of the penetration depth. The retrieved envelope points from the phase-shifting system agree with the envelope signal obtained from the simple system, but the number of envelope points is largely reduced in the phase-shifting system, as there are only 32 sampling points within a 20 µm depth range. Fig. 3.18 (b) identifies the smallest resolvable reflectivity approaching−70 dB for the phase-shifting measurement. Compared to the minimum identifiable reflectivity below

window sample has a 10 dB advantage over the simple system. The DR is also obtained to have a wider range from−70 dB to−28 dB.

As mentioned in Section 3.3.5.3 (see above), the system’s achievable sensitivity can be calculated by the reflectivity attenuation plus ten times the base 10 logarithm of the ratio of the square of the A-scan peak intensity to the STD of the reference arm noise floor. The STD of the reference noise was calculated as 3.4×10−5based on the reference arm contribution to the A-scan noise floor. By substituting the envelope peak reflectivity 0.04 and the reference noise variance 3.4×10−5into Equation 3.19 (see above), the achievable sensitivity of the phase-shifting FF-OCT system was obtained as 89 dB, which has a 15 dB advantage over the simple system.

The superiority of the phase-shifting FF-OCT system thus is that it directly retrieves fringe-freeen-faceimages during the measurement and envelope signals are reconstructed with fewer sampling points. The total space in the PC for storing the image volume is hence greatly reduced. It is also found that the imaging sensitivity of the phase-shifting system can be better than the simple system, if the averaging over a large number of phase-shift images are applied during the phase-shifting. The averaging enhances the system sensitivity, but also prolongs the measurement duration. A larger depth step-size, e.g. 5 µm between adjacent acquired images, would lead to the loss of image precision. It could be unrealistic to measure a 3-D data cube on biological samples, as the sample would dry out during the measurement. The system can permit a singleen-facetomographic image to be acquired in real-time, in a similar manner to the histology (biopsy), which would definitely require more sophisticated processing and more efficient system components.