## 1 Introduction  
Fourier domain optical coherence tomography (FDOCT) is a high-speed, high-resolution and non-invasive biomedical optical imaging technique [1]. It has been applied in the field of biomedical research, clinical diagnostics, and non-destructive material inspection. FDOCT can be categorized into two embodiments. One is based on a wideband laser source and a spectrometer which is called spectral domain OCT (SDOCT), and the other one is based on a frequency sweeping laser source and a balanced photon detector which is called swept source OCT (SSOCT, also called optical frequency domain imaging, OFDI). For both types of the FDOCT, the achievable imaging depth range is an important performance indicator. In some situations, a long ranging depth is highly desirable. One such example is for the endoscopic application where the distance between the distal end of the endoscopic probe and the tissue surface cannot be accurately controlled. Another example is for imaging the whole eye from anterior to retina. For FDOCT, the ranging depth can be calculated using the formula \(\Delta z={\lambda_{0}}^{2}/4n\delta\lambda\) and is fundamentally limited by the center wavelength \(\lambda_{0}\) of the light source, the spectral resolution \(\delta\lambda\), and the refractive index \(n\). The spectral resolution is \(\delta\lambda=\Delta\lambda/N\) when the spectrometer is pixel-limited. In this equation, \(\Delta\lambda\) represents the spectral bandwidth of the light source and \(N\) is the data number of the digitized axial spectral interference signal that covers the bandwidth. In fact, there is a sensitivity roll-off issue for FDOCT, which poses a further limitation to the practical imaging depth range. For the case of SDOCT, the sensitivity roll-off effect is due to the finite pixel number of the line scanning camera. Due to this effect, the visibility of the spectral interference fringe is maximized when the optical path difference (OPD) is zero and decays as the path length difference increases. Thus, the effective ranging depth is defined as the imaging depth at which the sensitivity has decayed \(-6\) dB from the peak value [2]. For FDOCT, the complex conjugate ambiguity further limits the imaging depth to one half of the full-range imaging space. Another limitation on the imaging depth of the FDOCT is posed by the finite depth of focus of the objective lens. For extending the effective imaging depth range, various techniques have been proposed. A novel MEMS-Vertical Cavity Surface Emitting Laser (VCSEL) source-based [3] SSOCT system was developed for in vivo high-speed imaging of the eye with an unprecedented imaging depth of 50 mm in air and little sensitivity loss. The SDOCT system based on the orthogonal dispersion spectrometer [4] or the frequency comb approach [5] was proposed for large ranging depth imaging through enhancing spectral resolution. To solve the complex conjugate ambiguity, many complex Optical Coherence Tomography (OCT) techniques were proposed. Among these complex OCT methods, an approach involving spatial phase modulation induced by the pivot-offset galvo scanner (GS) in the sample arm is regarded as a widely accepted way [6; 7; 8; 9]. The technique was initially introduced for en face OCT imaging [10]. In the approach, a slight eccentricity of the laser beam incident upon the galvo scanner causes a path length modulation during lateral scanning, generating the required carrier frequency. Recently, the optimization of galvo scanning for OCT was investigated [11]. Alternatively, based on implementing multiple reference arms in the FDOCT system the extended ranging depth can also be achieved [12; 13; 14; 15; 16]. These multiple-reference-arm approaches can be roughly categorized into two sorts: simultaneous acquisition approach and sequential acquisition approach. For the simultaneous acquisition approach, [PERSON] et al.