## 3 Experiments and Results
From Figure 5, the separation between the two sensitivity peaks can be used to calibrate the actual relative OPD between the two reference arms, and it can be shown that the actual relative OPD is about 3 mm. The benefit from the use of the spatial frequency domain multiplexing SDOCT system is also demonstrated, and the effective imaging depth range for the proposed system is increased from about 3.4 to 6.4 mm, compared with a conventional full-range SDOCT without depth range enhancement. The depth enhancement factor is 1.9. In this demonstration, the two sensitivity curvesare crossed at the \(-5.5\) dB from the sensitivity peaks. Therefore, the effective imaging depth range would be doubled if the relative optical path delay was precisely tuned to make the two sensitivity curves cross at the \(-6\) dB point from the sensitivity peaks. To demonstrate the ranging depth enhancement, we compare image results of the OCT images acquired by the proposed two-reference-arm spatial frequency domain multiplexing approach and the conventional single-reference-arm approach. Firstly, an artificial phantom is imaged by the system. Figure 6a shows the photograph of the phantom taken with a digital camera. The phantom is made by sticking two blocks of sellotape to generate a step 3 mm high. Figure 6b-d are the complex conjugate ambiguity free OCT images of the phantom reconstructed by the data processing algorithm described previously. Figure 6b shows the image acquired with Reference Arm I open while blocking Reference Arm II. As we can see, the upper part of the step is clearly imaged, while the image of the lower part is hard to be discerned. The OPD between the upper part and Reference Arm I resides in the effective imaging range, while that of the lower part and Reference Arm I is larger than 6 mm, out of the effective imaging range. Figure 6c shows the OCT image acquired with Reference Arm II open while blocking Reference Arm I. Likewise, the lower part of the step can be clearly imaged, while the upper part is not clear. Figure 6d shows the reconstructed OCT image of the phantom acquired with both Reference Arms I and II open. The whole phantom including the upper and lower part of the step can be clearly seen. The imaging depth-enhanced OCT image of the artificial phantom demonstrates the feasibility of the proposed method for extending the OCT ranging depth. The biological tissue of the swine adipose is also in vitro, imaged and compared with the results acquired by the proposed two-reference-arm spatial frequency domain multiplexing approach and the conventional single-reference-arm approach. Figure 7a,b show the images recorded using the two Reference Arms I and II separately. In each case, the area of the tissue around the DC signal component has maximum sensitivity. Figure 7c shows the OCT image of the tissue acquired with both of Reference Arms I and II open. As we can see, the upper and lower part of the biological sample is clearly appeared at the same time. Figure 7d plots an A-scan profile from the same sample position indicated by the red arrows in Figure 7a,b. It can be seen that the signal is about 13 dB stronger than that of the signal 3 mm away due to the sensitivity roll-off. The extended range OCT image of the biological tissue obtained with the system demonstrates the feasibility of the proposed method for extending the OCT ranging depth. Because the galvanometer scanners in the system are used for phase modulation of the lateral interference spectral signal, the phase noise does not affect the quality of the results. In future, the proposed method for extending the OCT ranging depth can be combined with the depth of the focus extension method to improve the quality of the extended range OCT image. Figure 5: The measured sensitivity curves corresponding to the Reference Arm I and Reference Arm II. Figure 6: **(a)** The photograph of the artificial phantom of a step comprised of two blocks of sellotape.