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From the invention of OCT technology, the majority of OCT interferometers are Michelson configurations implemented in fibre-optics or in bulk-optics [1, 69, 148]. The Michelson interferometer is the heart of a typical OCT system.

The design of Michelson interferometer is based on two-beam interferometer with amplitude division. It is of simple form to make measurement of sample structure using interferometry. A beamsplitter is contained in the configuration to divide (the intensity) an incoming beam into two equal (intensity) beams that travel along separate arms. After being reflected from mirrors on the two arms, the two beams are recombined at the beamsplitter. A condition is created until which interference can take place. This is largely dependent on the

relative phase or the difference between the optical path lengths (OPL) of the two recombined beams. Here, OPL is the product of the geometric length of the path light follows through the system, and the index of refraction of the medium through which it propagates. If the difference in OPL between the two beams, i.e. optical path difference (OPD), is within half a coherence length, interference between the beams can occur. Under above condition, the output interference beam intensity can be varied along with the changes of the OPD. The variations can thus be measured with a detector as a function of the OPD.

Fig. 2.2 Schematic diagram of a Michelson interferometer. M2: 50 % beam-splitter, M1, M3: dielectric mirrors. Adapted from Luckiet al [149].

b. Basic OCT Components

Fig. 2.3 presents general components integrated with the Michelson configuration in a typical OCT systems. A low-coherence light source is used to illuminate the interferometer. The light beam is divided into two separate arms by the beamsplitter. Reflections or scattering would occur at both arms when beam wavefronts encounter the reference mirror and the sample. The two returned beams are reunited and combine their intensities at the beam-splitter. The resulting beam is typically directed to a photoelectric detector with the instantaneous light intensity recorded. To make measurements of the optical interference event, the returning sample and reference beams are made to interfere on condition that their OPD is within half the coherence length, where the output intensity is recorded by the detector.

The ideal OCT light source would yield strong enough (high power) radiation with temporally low coherence or incoherence [69]. Generally, a low coherent light source is required in an OCT system mainly to promote the depth resolution. However, the choices of

Fig. 2.3 Basic configuration and components of an OCT system. Light source: a low temporal-coherence light source. BS: beam-splitter to split source radiation into two arms with equal intensity. Reference Mirror: a plain mirror or window on the reference arm (it usually has similar reflectivity with the sample). Sample: the sample under test on the sample arm. GS: galvo scanner; PL: probing lens; PD: photo-detector; SP: spectrometer; CCD: charge coupled device;

source central wavelength and spectral width are crucial to the OCT imaging performance. According to the Beer-Lambert law, optical penetration depth is related to the material absorbance [150], which is strongly wavelength dependent [151]. Hence, OCT employing different spectral ranges leads to various OCT probing depths, for example, OCT applying NIR radiation can penetrate deeper than that with the white light in biomedical tissues. Besides, OCT depth resolution is defined by the temporal coherence of the light source, which is proportional to the central wavelength, and inverse proportional to the spectral width. Popular light sources in OCT include SLD [1], femtosecond Titanium-sapphire laser [49], photonic crystal fibre based light source [152, 153] and thermal source [154], etc. Note that, all optical components including the beam-splitter and detector should have a corresponding spectral range to the chosen light source.

The OCT detector is required to have superior performance characteristics including fast data acquisition speed and excellent sensitivity, noise and homogeneity [4]. Photodetectors equipped with charge coupled device (CCD) sensors and complementary metal-oxide- semiconductor (CMOS) sensors are used for high quality imaging performance [155]. Modern progresses made in CCD and CMOS technologies has led to faster acquisition speed and enable real-time imaging [156, 157].

OCT operating single-point scheme requires to perform depth scanning whilst laterally scanning the beam in either one or two orthogonal directions, in order to obtain a 2-D or 3-D image [158]. A control module should be included to operate all mechanical scans and trigger the data acquisition of the photodetector. A process module is also required to perform on-line or off-line data processing for the purpose of further characterisation, analysis and presentation of the OCT images.

In addition, OCT systems are generally composed of either bulk-optics or fibre-optics or a mixture of both setups. Traditional bulk-optics comprise discrete optical elements, which offers high flexibility and convenience in development, testing and maintenance, as one can easily remove or replace optical components and access optical powers or beam profiles [159]. However, fibre-based OCT systems are particularly adaptable to industrial environments [160]. They can access interiors of hard-to-reach spaces [161], and operate in hostile environments - whether radioactive, cryogenic, or very hot [162].

c. OCT Optics

The time-domain OCT and Fourier-domain OCT are two physical alternatives that exist for the OCT technique.

In time-domain OCT, sample depth structures are recorded as a sequence of partial temporal coherence interference signals [163]. These interference signals are generated by modulating the OPL of either one interferometer beam using optical delay lines, while their signal intensities are based on the complex of the back scattered sample beam and the back reflected reference beam. With this detection scheme, the weak structural signal from the sample is dramatically amplified by interfering with a strong coherent carrier wave [164].

Unlike the time-domain OCT with an OPL modulated beam, Fourier-domain OCT is operated in the frequency domain [50]. It can be implemented either based on a spectrometer detector (spectral-domain OCT) or on a tuneable laser source (swept-source OCT) [4]. While both interferometer arms remain fixed OPLs, the sample depth structural information can be derived by the inverse Fourier transform (FT) of the output spectrum of the interferometer. Both time-domain OCT and Fourier-domain OCT have been shown to be equivalent in terms of depth resolution [165].

Standard A-scan signals are derived from OCT interference signals after a depth scan in time-domain OCT, otherwise, they are obtained from the FT of OCT spectrums in Fourier- domain OCT. For both cases, A-scan signals convey single point structural information along the depth direction. OCT B-scan images are cross-sectional images synthesised from laterally adjacent OCT A-scans, analogous to ultrasound A-scans and B-scan images [4]. B-scan images present depth oriented 2-D structural distributions, which provide important information regarding the layer structure (or thickness) and media distribution, etc. OCT volume image is thus the combination of a series of laterally adjacent B-scan images, offering a straightforward presentation of the spatial or 3-D structural distribution. The spatial structure variations over time are also assessable with OCT.

To obtain A-scan signals and B-scan images, the depth scan is implemented with the linear translation stage in time-domain OCT. Fourier-domain OCT eliminates the needs of depth scanning, but requires inversely Fourier transformed spectrum to derive the A-scan signals. Lateral scans in single-point OCT systems are usually performed by laterally moving the sample under test or moving focus of the probing beam with the help of, e.g. a galvanometer scanner or a laterally scanning mirror [3]. Parallel OCT systems adopt a 2-D detector array, thus avoiding the needs of lateral scanning for the spatial evaluation of the sample, but it still requires OPL modulated depth-scan to record longitudinally distributed OCT signals.

d. Other OCT Configurations

OCT systems are commonly set up by using Michelson interferometers, which use a beam- splitter to divide the source light into separate paths and combine intensities of back reflected

light interferometrically. Other configurations based OCT systems, e.g. Fizeau [166], Mach–Zehnder [155], Fabry-Perot [68], and Linnik interferometers [8], have been proposed to serve performance enhancement and specific requirements. In addition, different beam probes have been incorporated to standard OCT systems for macroscopic, microscopic and endoscopic imaging purposes [69].

Optical coherence microscopy (OCM) aims at tomographic imaging with high transverse resolution [167]. It is generally implemented in a Linnik interferometer [8], which is essentially equipped with high NA lenses (such as microscopy objectives) in both interferometer arms to focus the probe beam at the sample or reference. Extremely high transverse resolution is achievable with the adoption of water / oil immersed objectives. As the depth resolution is decoupled of the transverse resolution, high depth resolution can be achieved simultaneously in OCM by the use of broadband light sources, like femtosecond lasers, photonic crystal fibre based or thermal light sources, etc.. Besides, OCM presents more effective optical sectioning over the conventional confocal scanning microscopy (CSM) in highly scattered samples, by providing improved rejection of scattering light at defocused planes [69]. It is potential to be used for real-time optical biopsy [32].

Endoscopic OCT is proposed for its superior spatial resolution and higher sensitivity compared to CPEUS for diagnosis of intra-abdominal organs [125]. By integrating a catheter with fibre-optic sample probe arm, the OCT instrument can enable imaging of intraluminal disorders and optical biopsy into organs for cancer diagnosis. The technique has been applied for imaging in GI tract and vascular systems clinically, while there remains a wide range of promising clinical applications, such as the respiratory tract, the ovary, the breast duct and the liver, where OCT endoscopes can have significant impact [168].