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J. Eur. Opt. Society-Rapid Publ. 22, 26( 2026)
Fig. 1. Schematic diagram of a microfiber and its two tapers linking the uniform part to the untapered fiber. U clad and U core denote the cladding and core diameters, respectively, as indicated here for conventional single-mode step-index fibers used in telecommunications.
Coherence Tomography( LC-OCT), aiming to assess the validity of the existing models.
Observing the evolution of both the core and cladding over distances of a few centimeters with high resolution and sensitivity requires sophisticated techniques tailored to semi-transparent media. Compared with nano- and microtomography methods, which offer higher sensitivity [ 10 ], LC-OCT provides significantly faster acquisition times( on the order of tens of minutes to scan a 2 cm-long taper). Moreover, LC-OCT enables microscale characterization of macroscopic objects, whereas the above-mentioned methods are typically limited to lengths of only a few millimeters. Quantitative phase microscopy can also be used to obtain phase maps of the taper; however, unlike the approach presented here, the resulting images are not directly amenable to physical interpretation [ 11 ]. Altogether, these advantages make LC-OCT a complementary and powerful technique for investigating the evolution of the core and cladding in the tapered section of a microfiber, with a micrometer-scale resolution and high detection sensitivity. This article is organized as follows: After describing the experimental setup, we report results obtained from two different tapered sections fabricated from step-index fibers designed for telecommunications( Alcatel Clear Incolore and Corning SMF28e). Our measurements show that the assumptions commonly made for numerical modelling need to be refined for a more accurate description of the evolution of propagation modes in the tapered section.
2 Experimental setup
LC-OCT is an optical imaging technique that combines confocal detection with low-coherence interferometry to provide real-time, cross-sectional images of semi-transparent samples [ 12 ]. The LC-OCT system used in this work, shown schematically in Figure 2, is routinely employed for cellular-level skin imaging [ 13, 14 ]. Micrometer-scale lateral and axial resolution is achieved using two identical high numerical aperture( 0.5) immersion microscope objectives and broadband light with a central wavelength of 750 nm. Illumination is provided by a fiber-coupled supercontinuum light source, whose output is collimated and shaped into a line by a cylindrical lens before entering the interferometer via a beam splitter. The line of light is focused onto the sample in the object arm of the interferometer and onto a glass plate in the reference arm. The interference signal is detected by a line-scan camera, and a vertical cross-sectional image is reconstructed from a stack of lines acquired by the camera as the depth is scanned using a piezoelectric translation stage [ 12, 14 ]. The refractive index step Dn = n core � n clad between the fiber core and the cladding being very small( typically a few 10 �3), the resulting optical reflection to be detected is very weak( 10 �5). To enhance the interferometric signal contrast and thereby improve detection sensitivity, the reflection coefficient of the interferometer reference surface was reduced to 0:03 % by immersing it in water.
The microfibers are drawn using the classical pull-andbrush technique with a butane flame, producing high-optical-quality microfibers with symmetrical tapers in a very reproducible manner as was shown in previous studies [ 6, 7, 15 ]. Two silica step-index single-mode fibers designed for telecommunications are studied( Alcatel Clear Incolore and Corning SMF28e, respectively named fiber 1 and fiber 2). The initial cladding diameter of both fibers is 125 lm. The characteristics of fiber 2 are well-known. Its initial core diameter is 8.2 lm, and its step-index is Dn = 5.2 10 �3 [ 11 ]. Although the parameters of fiber 1( diameters and step-index) are less well characterized, they are known to be similar. In both cases, the targeted microfiber diameter is 1.71 lm. The length of the uniform part of the microfibers is 1 cm, and the length of the tapers is 1.8 cm. The microfiber and its two tapers are immersed in a tank filled with silicone oil( refractive index’ 1.45) to attenuate optical reflections at the fiber surface( see Fig. 2). Consequently, the contrast in optical reflection between the cladding surface and the core surface is reduced. A series of cross-sectional LC-OCT images of the taper is acquired at positions separated by 0.5 mm. Each cross-sectional image