Optical coherence tomography (OCT) is a high-resolution imaging technique that is actively used for various biomedical applications, such as medical diagnosis. This technique provides cross-sectional images of biological tissues over penetration depths of typically 1 to 3 mm. Although OCT is a unique and useful imaging modality, the relatively shallow penetration depth is considered a serious limitation for many applications [1
]. The image formation of OCT relies on the measurement of backscattered photons from various depths and scattering sites in the tissue. These photons could be classified into least scattered or singly-reflected ballistic photons, and multiple scattered photons [2
]. Singly scattered photons contribute to the meaningful OCT signal, while multiple scattered photons essentially add to the background noise. The penetration depth of OCT is fundamentally limited by the attenuation of ballistic light propagation via scattering and absorption. Besides the scattering and absorption losses, the phenomenon of multiple scattering also makes it difficult to achieve meaningful structural information at deeper penetration depths [4
]. The fraction of multiply-scattered photons that contributes to the OCT image increases with the depth (optical delay) in tissue, resulting in reduced signal localization (both lateral and longitudinal) and decreased image contrast at larger depths [4
]. While scattering is the major contrast mechanism in OCT, a reduction of light scattering in a sample may actually increase the penetration depth and contrast in the image.
Several theoretical and experimental studies have been conducted to get insights into the phenomenon of multiple scattering in tissues and its detrimental effects on OCT imaging [2
]. Monte Carlo simulation has been used to study and ultimately compensate for the effect of multiple scattering. However, this approach has been limited to samples involving homogeneous media or layered tissues with simple geometry [3
]. Post-processing correction of multiple scattering has been ineffective without knowing a priori
the detailed optical properties of heterogeneous tissues. Optical clearing, a method involving chemical administration (such as glycerol and dimethyl sulfoxide), can enhance penetration depth by reducing optical scattering in tissues [11
] and may be used in limited applications.
Optical properties of tissues tend to vary significantly as a function of wavelength; therefore, an optimal spectral window may be chosen for maximizing light penetration and enhancing image contrast at deeper depths. A clear improvement in the penetration depth was demonstrated by using 1.3 µm versus 0.85 µm OCT systems [13
] and using 1.06 µm over 0.85 µm in the retina [15
]. OCT systems at 1.55 µm and 1.8 µm have also shown comparable penetration depth to that of 1.3 µm systems [17
]. Other studies suggested that scattering coefficients of bulk skin and adipose tissues are ~20% lower at 1.7 µm compared to 1.3 µm [19
]. Optical absorption by water content (~70%) in a biological tissue causes a propagation loss of 2–3 dB/mm around the wavelength of 1.7 µm, which in fact is less than the loss caused by Rayleigh and Mie scattering. However, the water absorption loss increase significantly at around 1.5 µm and beyond 1.85 µm [21
]. The inferences drawn from these studies suggest that the spectral window of 1.6–1.8 µm could provide optimal performance for enhancing image contrast at larger penetration depths. However, this remains controversial, to the best of our knowledge.
In this paper, we describe an experimental investigation of the long-wavelength window for OCT imaging. For this study, we develop a wavelength swept laser and realize swept-source OCT or optical frequency domain imaging (OFDI), for the first time in the 1.7 µm spectral range. We image several phantom and biological samples using the 1.7-µm OFDI system and compare the images directly with those obtained from a state-of-the-art 1.3-µm OFDI system with similar sensitivity and resolution. We show that the spectral range around 1.7 µm is a promising window for OCT imaging.