## 2 Common SWIR Imaging Technologies
### Optical Coherence Tomography (OCT)  
OCT is a method of interferometry that creates high-resolution 2D and 3D images of biological tissues at the micron level [28]. A prevalent type of OCT is Fourier-domain OCT, available in two configurations: swept source (Figure 3e) and spectral domain (Figure 3f). In both configurations, elastically back-reflected light from various tissue structures interfere with a reference light beam, generating a fringe pattern in the Fourier domain [28]. Applying Fourier analysis on this pattern transforms the \"wavenumber\" space (k-space) information into the \"physical length\" space (z-space). The absolute values of the complex numbers resulting from this Fourier transformation constitute the structural image of the tissue. In addition, the phase of the Fourier transformation result represents the differences in optical path lengths between tissue layers and the reference mirror. OCT functional modalities tied to phase, such as photothermal (PT)-OCT [29] and optical coherence elastography (OCE) [30], leverage phase variations over time to obtain additional information about tissue molecular and mechanical characteristics. In such modalities, the supplementary phase information automatically overlays on the high-resolution structural OCT amplitude images, creating a depth-resolved map for the additional properties of the tissue.  
OCT and its extensions find diverse applications in both preclinical and clinical realms, spanning ophthalmology, cardiology, dermatology, oncology, and beyond [28]. By far, the largest clinical impact and area of application of OCT is in ophthalmology that traditionally utilized NIR light in the 800-870 nm range. More recently, however, ophthalmic OCT systems take advantage of reduced scattering at longer wavelengths and employ SWIR wavelengths of 1050 nm to enhance imaging depth and image quality, specifically for the comprehensive visualization of deeper regions of the posterior eye. OCT images generated in the SWIR frequency range offer unprecedented clarity, capturing detailed cross-sectional views of retinal structures. These captivating images not only showcase the potential of SWIR-based OCT in early detection and monitoring of eye conditions, such as macular degeneration and diabetic retinopathy, but also emphasize its role in advancing our understanding of ocular health. By highlighting the immediate clinical impact of SWIR-based OCT on visualizing intricate retinal details, this application captures the large interest of readers, underlining its potential to revolutionize ophthalmic diagnostics and contribute to the broader landscape of biomedical imaging. In other areas of application of OCT (e.g., cardiology), these systems typically incorporate SWIR light sources operating at 1310 \(\pm\) 90 nm, to effectively mitigate light attenuation caused by scattering. This extension of the optical spectrum allows light to penetrate biological tissues to a depth of 2 to 4 mm, offering promising prospects for advanced medical imaging [28]. In cardiology, intravascular OCT (IV-OCT) systems, designed in a catheter-based configuration, scan arteries for detailed analysis of plaque characteristics in coronary disease and even to visualize the location of struts used in treatment [31]. OCT is also coupled with fluorescence [32; 33] and spectroscopy techniques [34] to unveil deeper insights into biological tissues. OCT outcomes are quantifiable and contribute to the measurement of tissue scattering properties [35].