J. Eur. Opt. Society-Rapid Publ. 2026, 22, 26 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026029 Available online at: https:// jeos. edpsciences. org
EOSAM 2025 Guest editors: Omar El Gawhary, Stefan Witte, Ignacio Moreno
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
Study of tapered microfibers by line-field confocal optical coherence tomography
Arnaud Dubois, Aloïs Baudry, and Sylvie Lebrun * Université Paris-Saclay, Institut d’ Optique Graduate School, CNRS, Laboratoire Charles Fabry, 91127, Palaiseau, France
Received 19 January 2026 / Accepted 14 March 2026
Abstract. We present for the first time to our knowledge measurements of the evolution of the core and cladding diameters in tapered silica microfibers using LC-OCT. The results could help refine models of propagation of optical modes in tapers.
Keywords: Silica microfibers, Optical tapers, Line-field confocal optical coherence tomography, Silica optical fibers.
1 Introduction and motivations
Optical microfibers are typically produced by tapering standard silica fibers – commonly used in telecommunications – until their diameters become comparable to or smaller than the wavelength of the propagating light. The uniform section of the resulting structure, referred to as the microfiber, is connected to the untapered fiber parts through two conical sections known as tapers( see Fig. 1). These devices have been extensively used for over three decades in both scientific research and engineering, serving as fundamental optical devices that are inherently compatible with all-fiber networks [ 1 ]. Owing to their unique properties, microfiberbased technologies have proven to be versatile across a wide range of applications, from fundamental studies to advanced implementations such as quantum information systems [ 2, 3 ], remote sensing devices [ 4 ], and nonlinear optics in the silica itself [ 5, 6 ] or in the surrounding medium via the evanescent field [ 7, 8 ].
A key challenge in microfiber fabrication is ensuring the adiabaticity of the taper regions. An adiabatic taper is characterized by a sufficiently small taper angle to suppress coupling between the guided fundamental mode and unwanted higher-order modes, thereby preserving high transmission efficiency of the desired mode. The standard adiabaticity criterion is based on the comparison of two characteristic lengths. The first one is the beat length z b between the fundamental LP 01 mode and the nearest higher-order mode with the same azimuthal symmetry, namely the LP 02 mode. This beat length is obtained from the difference between the longitudinal propagation constants of the two modes b 01 ðÞ z and b 02 ðÞalong z the taper and is given by z b ¼
2p. b 01 ðÞ�b z 02 ðÞ z
* Corresponding author: sylvie. lebrun @ institutoptique. fr
The second characteristic length is the local taper length z t, which can be approximated as z t XðÞ z, where rz ðÞ rz ðÞ
denotes the local cladding radius and( z) the local taper angle [ 9 ]. When the condition z t z b is satisfied at every position z along the taper – implying negligible intermodal coupling – the taper can be considered adiabatic for the fundamental mode. Consequently, designing an adiabatic taper profile requires calculating the propagation constants of the relevant modes throughout the taper as functions of z, implying a precise knowledge of the refractive index profile.
In this study, we consider conventional step-index optical fibers consisting of a Ge: SiO 2-doped core with refractive index n core, surrounded by a pure SiO 2 cladding with refractive index n clad. Current theoretical models for the calculation of the propagation constants in such fibers rely on the following assumptions:( 1) the ratio between the cladding and core diameters remains constant throughout the taper;( 2) the refractive indices of both the core and the cladding are uniform along the taper;( 3) the core effectively vanishes at a critical diameter, beyond which light guidance transitions from the core-cladding interface to the cladding – external medium interface. In general, for step-index telecommunication fibers, the critical cladding diameter is typically calculated to be around 40 lm [ 9 ]. Typically, to design optimized tapers, three steps should be considered: thedecreaseofthediametershouldbefirstly sharp until reaching the critical diameter, then much smoother when passing this diameter to limit the coupling with higher order modes, then sharp again. This highlights the importance to determine also the position and the value of this diameter.
In this article, for the first time to our knowledge, we report on measurements of the tapered section of a silica microfiber performed using Line-field Confocal Optical
This is an Open Access article distributed under the terms of the Creative Commons Attribution License( https:// creativecommons. org / licenses / by / 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.