Full Range Swept-Source Optical Coherence Tomography with Ultra Small Fiber Probes for Biomedical Imaging 27 Full Range Swept-Source Optical Coherence Tomography with Ultra Small Fiber Probes for Biomedical Imaging

Optical coherence tomography (OCT) (Huang et al., 1991) is becoming an increasingly important imaging tool for many applications in biology and medicine, such as diagnosis and guided surgery. Due to its high resolution and fiber catheter capability, OCT is more attractive than current imaging technologies, such as ultrasound. An OCT system with higher sensitivity is essentially important for imaging the biomedical turbid tissue because the backscattered optical signal from the tissue is extremely weak. In the earlier stages of OCT imaging, axial (depth) ranging is provided by linearly scanned low-coherence interferometry (Youngquist et al., 1987; Takada et al., 1987). This method of OCT, referred to as time-domain OCT (TD-OCT), has a relatively slow sensitivity and imaging speed because its sensitivity is inversely proportional to the imaging speed. Fourier domain techniques in OCT have received much attention in recent years due to its significant sensitivity and speed advantages over TD-OCT (Leitgeb et al., 2003; aChoma et al., 2003; De Boer et al., 2003). Fourier domain methods include spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT). In SD-OCT, individual spectral components of low coherence light are detected separately by the use of a spectrometer and a charge-coupled device (CCD) array (Fercher et al., 1995; Hausler & Lindner, 1998) CCD arrays however may introduce phase washout problems during the pixel integration time. Furthermore, detection using a spectrometer and CCD array cannot implement differential optical detection. SS-OCT uses a wavelengthswept laser source and photodetectors based on optical frequency-domain reflectometry for imaging (Chinn et al., 1997; aYun et al., 2003). SS-OCT is particularly important for imaging in the 1.3 m wavelength range, where low-cost detector arrays are not available. The larger penetration depth of the OCT image by using the 1.3 m wavelength light source is important for the biomedical turbid tissues, such as human skin and arterial plaque, in comparison to that by using 1.0 m or shorter wavelength light source. SS-OCT could also make possible for a quadrature interferometry based on multi-port fiber couplers, for example, 3x3 quadrature interferometer (bChoma et al., 2003; aMao et al., 2008). Due to its ability to have instantaneous complex signals with stable phase information, OCT with a 3x3 quadrature interferometer could suppress the complex conjugate artifact naturally, therefore 2