## 6 Conclusions  
In this paper, a method for improving SD-OCT performance based on a spectrometer using an aberration-corrected imaging lens is proposed. To do this, we designed a TFL with improved field curvature aberration characteristics. ADL was chosen as a reference for comparison because it is generally adopted in spectrometers in SD-OCT systems. The ADL used is a commercial product and the chromatic aberration is corrected well, but the field curvature aberration has not yet been addressed, thus the focal plane of ADL is not flat, but curved. These characteristics were observed in the optical simulation whereas the newly designed TFL exhibited an almost flat focal plane. The chromatic focal shift feature of the TFL was similar to the ADL with well-corrected chromatic aberration. The difference in the field curvature of the two imaging lenses was dramatic with regard to the change in spot size. The TFL showed an RMS spot radius of 5 \(\upmu\)m at the edge wavelength that matched the size of a single pixel whereas the ADL showed an RMS spot radius of 20 \(\upmu\)m at the same wavelength. The TFL showed better performance than the ADL in regard to MTF characteristics. Therefore, for a spectrometer with the ADL, the optical loss increases as the wavelength increases or decreases from the central wavelength. Figure 12: OCT images of the palm of a human hand obtained using (**a**) S-ADL and (**b**) S-TFL. (**c**) Depth profiles at the same A-line of each image. This results in reduced visibility of the spectral interference signal, which decreases the imaging depth that can be achieved. This fact well explains the experimental results for the degradation of axial resolution and the roll-off characteristics at deep depths. The S-FTL maintained an axial resolution of 7.8 \(\upmu\)m at 210 \(\upmu\)m even when the OPD increased, but when this axial resolution of 7.8 \(\upmu\)m was measured using the S-ADL, it increased to 12 \(\upmu\)m as the OPD increased to 2.5 mm. According to the roll-off curve, the imaging depth at 3 dB was 0.75 mm for the S-ADL and 0.89 mm for the S-TFL, and the ranging depth at 6 dB was extended to 1.16 mm for the S-ADL and 1.33 mm for the S-TFL. The S-TFL showed better imaging depth that the S-ADL. SD-OCT images obtained for non-biological samples and biological samples, which were assessed by considering the differences in the performance of the spectrometers. In the image obtained with the S-TFL, the boundary was distinguishable even at a deeper depth due to the roll-off characteristic. As can be seen from the simulation results for aberration and spot size, the results obtained with the S-TFL were better than those obtained with the S-ADL regarding signal amplitude and contrast, and according to depth. The overall brightness of the SD-OCT images may be affected by the MTF characteristics. From the experimental results obtained with the spectrometers, we can see that the field curvature aberration correction of the spectrometer's imaging lens contributes significantly to improving the SD-OCT imaging performance. [PERSON] performed simulation on imaging lens aberration and implemented the spectrometer using the imaging lenses and SD-OCT system with the spectrometers. Also, [PERSON] carried out all the experiments and signal processing for image reconstruction and contributed to the draft of the paper. [PERSON] conducted lens design with the help of the company and fabricated the imaging lens. [PERSON] also contributed to the draft of the paper. [PERSON] supervised the work including optical design, experiment, and data analysis and contributed to the draft of the paper. All authors have read and approved the final version of the manuscript. This work was supported by the National Research Foundation of Korea (NRF) grant (NRF-2017 RI1A2B2009732 & NRF-2015M3A9E2067140) funded by the Korea government (Ministry of Science, ICT, and Future Planning). We would like to thank WINUS Technology Co., Ltd. for developing and manufacturing the lenses. The authors declare no conflict of interest.