High-resolution spectrographs, the instruments used to measure the tiny velocity wobbles that reveal orbiting exoplanets, traditionally rely on optical fibres to pipe starlight from the telescope into the analysis chamber. Along the way, engineers use image scramblers and coupling optics to keep the illumination stable, because any instability translates into systematic errors in the measured wavelengths. Since the early 2000s, the emerging field of astrophotonics has transformed the humble fibre link from a passive light pipe into an active manipulation tool, spawning devices such as photonic lanterns, octagonal scrambling fibres, and fibre Bragg gratings that suppress contaminating atmospheric emission lines. Yet comparatively little attention has been paid to one of the simplest possible components: a single tapered fibre that gently squeezes a large light beam down to a smaller one.

The motivation for the new work comes from EXOhSPEC, a high-resolution spectrograph under development that requires its input light to arrive through a circular fibre no larger than roughly 10 micrometres across. Meeting that constraint while capturing as many photons as possible is the central challenge of fibre-feed design, because larger telescope fibres collect more starlight but deliver beams too wide for the spectrograph’s resolving power. A tapered fibre promises to bridge that gap: light enters through a wide multimode core and exits through a narrowed waist, potentially concentrating the field into a few-mode or near-single-mode beam. The question the team set out to answer was how much light survives that compression and what happens to the shape of the emerging beam.

The researchers tested two commercial graded-index fibre tapers fabricated by Thorlabs using a glass-processing machine, alongside an untapered 10-micrometre step-index fibre for comparison. One taper was drawn from a 50-micrometre graded-index fibre and the other from a 62.5-micrometre graded-index fibre, each compressed to a 5:1 taper ratio over a 25-millimetre length, yielding final tapered cores of about 10 and 12.5 micrometres respectively. Graded-index fibres differ from conventional step-index fibres in that their refractive index falls smoothly and parabolically from the centre of the core outward, a profile that continuously refracts light back toward the fibre axis and can strongly confine the guided mode.

To characterise the output, the team built a fibre microscope that images the emitting end face of each fibre onto a sensitive camera at a calibrated magnification of nearly 16, sufficient for a 10-micrometre core to span more than a hundred pixels. They illuminated the fibres in two ways. A coherent red laser diode at 635 nanometres was coupled through a single-mode fibre to excite primarily the fundamental mode, mimicking ideal conditions. In parallel, incoherent white light from a halogen lamp was butt-coupled through multimode fibres of either 10 or 50 micrometres, replicating the messy, mode-jumbled illumination typical of real astronomical feeds. Images were reduced with dark, bias and background calibration frames, and the intensity profiles were fitted with Gaussian functions to extract two complementary width measures: the full width at half maximum, which tracks the projected beam size seen by a spectrograph, and the mode field diameter, defined at the 1/e-squared intensity level, which captures the overall energy confinement of the guided light.

The headline result was counterintuitive. Simple step-index logic suggests that a smaller physical core should always produce a smaller output mode, so the 50-micrometre taper with its 10-micrometre waist ought to win. Instead, under coherent illumination the 62.5-micrometre graded-index taper, whose waist is actually larger at 12.5 micrometres, produced the tightest beam of all three fibres, with a full width at half maximum of just 2.85 micrometres and a mode field diameter of 4.84 micrometres. The 50-micrometre taper followed at 3.33 and 5.66 micrometres, while the untapered step-index fibre trailed at 5.07 and 8.61 micrometres. The explanation lies in the numerical aperture: the larger starting fibre gives the taper a higher effective numerical aperture, which confines light more concentrically and compresses the intensity profile into a narrower, brighter spot.

On the transmission side, the 62.5-micrometre taper delivered nearly 70 percent of the coupled light, essentially matching the throughput of the untapered step-index fibre, while the 50-micrometre taper reached about 49 percent. The researchers caution that their measured values may include some light propagating in the cladding, and that the experimental coupling cannot achieve a perfectly pure fundamental-mode launch, so measured transmissions are expected to exceed idealised predictions. Supporting finite-element simulations performed with COMSOL Multiphysics 6.3, run on the University of Hertfordshire’s high-performance computing facility, modelled the true-to-scale three-dimensional fibre geometry and confirmed the experimental trends, though the simulations predicted smaller mode sizes of 1.38 and 1.95 micrometres because they assume an ideal fundamental-mode launch free of the micro-bending and mode mixing inherent in bench experiments.

The simulations also probed how modes behave as the taper geometry changes. In the step-index fibre, supported modes uniformly fill the core, and transmission cuts off sharply near the fibre’s nominal numerical aperture, exactly as textbook theory predicts. In the graded-index tapers, by contrast, there is no sharp cutoff: isolated propagating modes persist at larger effective angles, partly owing to mode conversion, and the mode field diameters of higher-order modes never grow to fill the nominal core as they do in step-index designs. Averaging over the first ten propagating modes and across wavelengths of 450, 635 and 900 nanometres to approximate white light, the calculations showed that both the mode field diameter and the transmitted intensity increase with taper ratio, reinforcing the practical conclusion that gentler tapers preserve light more efficiently.

The practical message for instrument builders is striking. Because even modest taper ratios keep the output mode below 10 micrometres, designers may not need aggressive 5:1 compression at all; a gentler 4:1 taper or less could deliver the required beam size with higher throughput, provided the feeding fibre is chosen to match the taper’s numerical aperture, for example a narrow 10-micrometre step-index feed rather than a wide 50-micrometre multimode one. The team also found that conventional Gaussian fitting adequately described their measured profiles, with super-Gaussian and elliptical variants offering no significant improvement, simplifying future analyses. As observatories worldwide race to squeeze ever more photons through their spectrographs in pursuit of biosignatures on alien worlds, this unassuming piece of stretched glass may prove that elegant simplicity still has a place at the frontier of astronomical instrumentation.

Subject of Research: Experimental and simulated transmission and mode-field properties of graded-index tapered optical fibres for high-resolution astronomical spectrograph feeds.

Article Title: The high transmission of graded-index fibre tapers

Article References: Choochalerm, P., Martin, W. E., & Jones, H. R. (2026). The high transmission of graded-index fibre tapers. Results in Optics, Article 101138. https://doi.org/10.1016/j.rio.2026.101138

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101138

Keywords: graded-index fibre, fibre taper, astrophotonics, spectrograph, mode field diameter, FWHM, transmission, EXOhSPEC, COMSOL simulation, exoplanets, optical fibres, mode propagation