Proposed is a broad supercontinuum spectrum generated highly nonlinear photonic crystal fibre which can be used in ultra-high-resolution optical coherence tomography and optical transmission systems. Investigation showed that it is possible to obtain longitudinal resolution in a biological tissue of 1.3, 1.2, and 1.1 mm by using picosecond continuum light at centre wavelengths 1.06, 1.31, and 1.55 mm, respectively.

Introduction: The design freedom of photonic crystal fibres (PCFs) can be used to tailor and extend the range of optical parameters such as dispersion and nonlinearity [1]. Owing to the high index difference between silica core and air hole cladding, PCFs allow much stronger mode confinement, and thereby much higher nonlinearities. The reduced effective area Aeff is achieved by stronger mode confinement in the core with a small core diameter, as a result nonlinearity g can be increased. From the nonlinearity equation g ¼ 2pn/lAeff, it is clearly shown that nonlinearity is inversely proportional to the fibre’s effective area. The broadband supercontinuum (SC) generation in optical fibres currently attracts much attention because of the high potential for applications in the fields of optical communications, optical coherence tomography (OCT), optical metrology, time resolved absorption and spectroscopy [1, 2]. OCT enables micron-scale, cross-sectional and three-dimensional imaging of biological tissues in situ and in real time. The spectral region from 1.0 to 1.6 mm is of particular interest for OCT because it penetrates deeply into biological tissue and permits spectrally resolved imaging of water absorption bands. In this spectral region, attenuation is minimum due to absorption and scattering. It should be noted that scattering decreases at longer wavelengths in proportion to 1/l4, indicating that the scattering magnitude at 1.0–1.6 mm wavelengths is lower than at the visible wavelengths [3]. Superluminescent diodes (SLDs) are often used for OCT imaging and typically have 10–15 mm longitudinal resolution [4] and this resolution is insufficient for identifying individual cells or assessing subcellular structures such as nuclei. Ultra-high-resolution OCT in biological tissue, achieving high longitudinal resolution at centre wavelengths near 1.0 mm [2, 5], 1.3 mm [6, 7] and 1.55 mm [8], has been demonstrated with femtosecond lasers as low coherence light sources. On the other hand, the telecommunication window (around 1.55 mm) is the most attractive window in optical communication systems, dispersion compensation and nonlinear optics because of the minimum transmission loss of the fibre.