## 3 Experiments and Results  
For experimental verification of the method, we constructed a fiber-based spatial frequency domain multiplexing SDOCT system as depicted in Figure 1. The imaging system employs a super luminescent diode light source (SLD-371-HP2, Superlum Inc., Carrigtwohill, Ireland) with a center wavelength at 837 nm and a full width at half maximum (FWHM) of 54 nm. The beam diameter \(d\) in the sample arm is 1.8 mm, which will produce a 44 \(\upmu\)m 1/e\({}^{2}\) diameter focal spot on the sample with a confocal parameter of 3.7 mm. A 50/50 fiber coupler splits the light beam into the sample arm and the reference arms. In the sample arm, the light beam is deflected by a 2D galvo scanner and focused via a 75 mm achromatic lens L0 to the sample. Within the reference arm, a wide bandwidth non-polarizing beam splitter (NPBS, Daheng Optics Inc., Beijing, China) is used to divide the reference optical power into the two separate reference arms. The fiber collimator on a translation stage in the reference arm is used to adjust the overall delay of the two channels. The single-pass OPD between Reference Arms I and II is set to 3 mm. The GS1 and GS2 are the \(X\) and \(Y\) mirror of a pair of large beam 2D GS system (GVS012, Thorlabs Inc., Newton, NJ, USA), respectively. The focus lengths of achromatic lens L1 and L2 are both 30 mm. The plane mirror M1 and M2 are the wide bandwidth dielectric coated mirrors covering the wavelength range of 800 to 900 nm. The spectral interference signal is detected by a custom-built spectrometer, which consists of a collimator (\(f\) = 60 mm, OZ optics Inc., Ottawa, ON, Canada), a transmissive diffraction grating (1800 lines/mm), a camera lens (\(f\) =105 mm, Nikon Inc., Tokyo, Japan), and a line-scan CMOS camera (sp2048-70 km, Basler AG Inc., Ahrensburg, German). The spectral resolution of the spectrometer is 0.065 nm, allowing for a theoretical effective imaging depth range of 3.5 mm. In view of avoiding aliasing terms and according to Equation (9), the number of A-scans per B-scan is set to 500, and the lateral scanning range is set to 1.3 mm. The highest positive spatial frequency calculated by Equation (6) is 192 mm\({}^{-1}\). According to the values of the sample beam diameter \(d\), the center wavelength, the focus length of the sample objective and Equation (7), the theoretical BW of the spatial frequency content is calculated to be 22.5 mm\({}^{-1}\). In order to suppress the crosstalk of the two spatial frequency contents of the two depths signal, the difference of the two center spatial carrier frequencies must be greater than 67.5 mm\({}^{-1}\) according to Equation (8). Considering that the optimal conjugate suppression ratio occurs when \(\delta\varphi\) is \(\pi/2\)[9], each of the two center spatial carrier frequencies needs to be close to the central region of the positive spatial frequency range. Thus, the two center spatial carrier frequencies \(\
u_{cl}\) and \(\
u_{c2}\) are set to 56.5 and 123.5 mm\({}^{-1}\), respectively. According to Equation (4), the phase shift \(\delta\varphi_{1}\) and \(\delta\varphi_{2}\) between successive axial scans are calculated to be 0.3\(\pi\) and 0.7\(\pi\). The angular velocity of the galvo scanners is set to 0.93 rad/s. The exposure time of the CMOS camera is set to 18 \(\upmu\)s. According to Equation (3) and the above parameters, the pivot-offset distances on the GS1 and GS2 mirrors are calculated to be 1.8 and 3.9 mm, respectively. The pivot-offset distance is adjusted via the diagonal translation of the scanners to keep the light beam at the center of the focusing optics. To experimentally characterize the extension of the imaging depth range of the spatial frequency domain multiplexing SDOCT system, the point spread function is measured by using a plane mirror at various depths in the sample arm. Figure 5 shows the experimentally measured normalized sensitivity as a function of depth for the two reference arms. The measured value of the absolute sensitivity is 59.7 dB.