Measured group birefringence and group velocity dispersion of elliptical-core ZBLAN fibres for mid-infrared supercontinuum generation

Abstract

Polarisation-maintaining ZBLAN optical fibres with small elliptical cores are attractive for nonlinear frequency conversion, for example in supercontinuum generation (SCG), and for dispersion management in ultrafast fibre laser systems operating in the mid-infrared (MIR) spectral region. Accurate characterisation of group birefringence and group velocity dispersion (GVD) is essential for modelling ultrafast pulse propagation in such fibres, yet experimental data remain scarce. Here, we present broadband measurements of group birefringence and GVD in four elliptical-core ZBLAN fibres, selected for their potential for low-noise MIR SCG, over the wavelength range 1.4 to 4.3 $μ$m. Using polarisation-resolved white-light spectral interferometry, we quantify the dispersion characteristics of both fundamental modes in each fibre. Measurement uncertainties are evaluated using a standard deviation analysis to ensure reliability. This work aims to support the development of MIR supercontinuum sources and to facilitate the understanding of nonlinear optical phenomena in ZBLAN fibres.

Figures (3)

• Figure 1: 100$\times$ magnified cross-sections of the four elliptical-core fibres.
• Figure 2: (a) Schematic of the group birefringence measurement setup. (b) Example interference spectra for the group birefringence measurement, with the output polariser crossed in green and aligned in blue relative to the input polariser. (c) Schematic of the white light spectral interferometry setup used for GVD measurement. (d) Example spectral interference pattern I$_{Interf}$, for the GVD measurement showing fringes around the phase-matching point in green. The corresponding fringe order for this measurement is shown in blue with respect to the right y-axis
• Figure 3: (a)--(d) Mean group birefringence values in blue for the four elliptical-core PM-ZBLAN fibres, along with the $\pm$2$\times$ standard deviation error band shown in shades of orange. (e--h) Group velocity dispersion D curves for mode I of the four elliptical-core PM-ZBLAN fibres, along with the $\pm$4$\times$ standard deviation error band shown in shades of orange. Blue curves correspond to one of the three independent measurements conducted for each fibre. The dashed black line indicates zero dispersion.