## 4 Experimental Results
### LR-DOCT with 1% Intralipid Flow
The Doppler phase shift at the capillary center is measured to be 1.9 rad, which conforms well to the estimation by means of the contour plot in Figure 1b and the transformed Doppler angle \(\vartheta^{\prime}\) of 29\({}^{\circ}\). The result of the forward lateral scan (FL-scan) presented by the red points conforms to a quadratic parabolic profile, with the expected maximum velocity of 69 mm/s at the capillary center. In spite of the increased phase noise at the upper and lower capillary borders, the Doppler phase shift of the tracked Intralipid particles is not limited, and consequently, is truly measured by the linear relation between \(\Delta\varphi\) and \(v_{z}\) referred to in Equation (2). The non-resonant imaging at the border of the capillary has no influence on the shape. For the backward lateral scan, the flow profile seems to be a flattened plug profile with a fitted exponent of the power law of 2.6. Despite the strongly increased phase noise \(\sigma_{\Delta\varphi}\), an erroneous mean velocity profile is measurable from 512 single complex A-scans. The correlation quotient \(CQ\) and the standard deviation \(\sigma_{\Delta\varphi}\) against the radial position \(r\) inside the capillary in Figure 7b,c, show what can be visually expected from Figure 6c,d. For the M-scan (blue points), the backscattering signals of the Intralipid scatterers at the capillary border are highly correlated and show a small standard deviation of \(\sigma_{\Delta\varphi}\) = 0.4 rad, in comparison to the capillary center with a reduced correlation of \(CQ\) = 0.73 and a phase shift noise of \(\sigma_{\Delta\varphi}\) = 1.15 rad. For the forward lateral scan (red points), the correlation at the capillary center is strongly increased to \(CQ\) = 0.97 and the phase noise of the fast moving particles is highly reduced to \(\sigma_{\Delta\varphi}\) = 0.4 rad compared to the M-scan. The experimentally measured \(CQ\) in Figure 7b shows that the maximum correlation of one is not achieved, possibly because of the Brownian motion of the Intralipid. However, it is evident that the velocity of the \(X\)-galvanometer scanner is matched very well to the oil particles for the FL-scan, since the behavior of the phase noise \(\sigma_{\Delta\varphi}\) of the M-scan (blue points) and the FL-scan (red points) is inverted, as presented in Figure 7c. The result of the BL-scan (green points) shows a strongly damped correlation and a high noise of the Doppler shift \(\sigma_{\Delta\varphi}\) over the entire capillary, as expected. The relation of \(\sigma_{\Delta\varphi}\) and \(CQ\) are empirically determined in [20] and presented as a solid line in Figure 8 besides the very well fitting measurement values. Since the correlation quotient \(CQ\) is indirectly proportional to the Doppler noise \(\sigma_{\Delta\varphi}\), \(CQ\) provides no added value for the analysis of LR-DOCT, for which reason, only \(\sigma_{\Delta\varphi}\) is presented for the following in vitro blood flow measurements. Figure 7: (**a**) Calculated and fitted flow profiles of the flowing 1% Intralipid as a function of the radial position r inside the 312 \(\upmu\)m capillary for the M-scan (blue points), the forward (FL-scan, red points) and backward lateral scan (BL-scan, green points). Additionally, the correlation quotient CQ (**b**) and the standard deviation \(\sigma_{\Delta\varphi}\) (**c**) are shown versus r for the detected M-scan, the FL-scan and the BL-scan.