Fibres Gain Built-In Lenses Carved with 80-Nanometre Precision

A new method for fabricating highly precise micro-optical elements directly onto optical fibres using focused ion beam (FIB) machining has been presented by Raman Kumar and Sebastian Will at Brookhaven National Laboratory. The process achieves shape accuracies of approximately λ/80 and λ/50 at 780 nm, creating micro-spherical, micro-spiral, and micro-axicon structures with nanometre-scale precision. This monolithically integrated approach improves photon collection and beam shaping, potentially enabling scalable solutions for fibre micro-cavities, neutral atom trapping, and free-space quantum network links. The technique manipulates light at the microscopic level for advances in quantum technology and provides a key set of tools for these applications.

High precision fibre-integrated micro-optics enable advanced quantum applications

Shape accuracies of λ/80 and λ/50 have now been achieved in micro-optical elements directly integrated onto optical fibres, representing a significant improvement over previous fabrication methods. At a wavelength of 780 nm, this level of precision overcomes a long-standing limitation in creating high-performance, miniaturised optical components for quantum technologies. Prior techniques struggled to consistently produce such smooth surfaces and accurate geometries, hindering efficient light manipulation in fibre-based systems.

Micro-spherical, micro-spiral, and micro-axicon structures were successfully fabricated using focused ion beam machining, with performance verified through donut beam patterns and precise phase control. These results promise enhanced photon collection and beam shaping for applications including quantum cavities, atom trapping, and free-space quantum networks. Optical characterisation using a He-Ne laser emitting at 633 nm confirmed donut beam patterns from both micro-spiral and micro-axicon structures, demonstrating successful beam shaping capabilities.

Mach-Zehnder interferometry was used to measure the azimuthal and radial phase profiles of light emitted from the spiral and axicon fibres, confirming accurate phase control. Surface metrology showed that the FIB process preserves optical-grade surface quality, with no detectable increase in roughness at visible and near-infrared wavelengths. However, the long-term durability of these delicate structures under varying environmental conditions, as well as their scalability to multi-element systems, remains an open challenge.

FIB milling of high-accuracy micro-optics directly onto fibre cores

A single-step fabrication process, along with structural and optical characterisation, has been demonstrated for micro-optical elements fabricated directly on single-mode fibre cores using focused ion beam milling. Micro-concave, micro-convex, micro-spiral, and micro-axicon structures were created. To enable accurate centring of the structures with respect to the guided mode, a process was developed that exposes the fibre core using a buffered oxide etch (BOE) step and images it using in-situ electron microscopy.

Structure quality and optical performance were characterised using scanning electron microscopy (SEM), atomic force microscopy (AFM), focal-field measurements, far-field imaging, and Mach–Zehnder interferometry. Results show that FIB milling can reproducibly achieve shape accuracies better than λ/50, establishing this technique as a route toward quantum-grade fibre-integrated micro-optics.

Selective chemical etching of optical fibres in 20:1 BOE for 15 minutes reveals three distinct etch-contrast regions: a central core, an intermediate annular region, and an outer cladding, enabling deterministic alignment. A zoomed-in inset shows fibre core localisation after etching, while a schematic illustrates FIB processing directly on the fibre core with precise alignment. SEM images of representative micro-concave and micro-convex structures show sub-micron placement accuracy directly centred on the fibre core, enabling direct integration with guided modes. Commercial single-mode fibres with a nominal mode-field diameter of ~4 μm at 633 nm were used.

A short fibre segment (~4 inches) was cleaved using a Fujikura CT-50 cleaver and cleaned with isopropyl alcohol. Precise centring of micro-optical structures on the guided mode was achieved by etching the fibre tip with a hydrofluoric acid-based solution, exploiting differential etch rates of silica with different doping levels. The cleaved fibre was immersed in 20:1 BOE for 15 minutes, then rinsed in deionised water and dried.

After etching, the fibre facet shows three distinguishable regions: a central core, an intermediate annular region, and an outer cladding. The intermediate ring-like contrast corresponds to a differently doped region with a distinct etch rate. The central core, appearing as a raised pedestal (~100 nm height), is clearly visible in SEM images.

This precise localisation of the fibre core addresses a key limitation of earlier approaches, where misalignment could lead to increased insertion loss and degraded optical performance. Finally, fibres were coated with a thin (~10 nm) gold layer via ion sputter coating to ensure conductivity and prevent charging during FIB processing and SEM imaging. FIB milling was performed using a Thermo Fisher Helios G5 dual-beam instrument with a liquid gallium ion source.

The fibre was mounted in a custom holder to ensure the cleaved facet was normal to the ion beam. Achieving normal incidence is critical for accurate fabrication. The system’s electron and ion columns are oriented at 52°, requiring precise stage alignment to ensure perpendicular ion incidence on the fibre facet.

Alignment was performed by bringing the stage to eucentric height and adjusting tilt until the feature remained stationary in the electron beam image. The stage was then tilted to 52° so the fibre facet was perpendicular to the ion beam. This procedure was repeated for each fabrication session to ensure reproducibility.

The electron column (5 kV, 25 pA) was used for imaging, after which the fibre was aligned so that the FIB pattern was centred on the core. Micro-optical elements were defined using 512 × 512 pixel bitmap files, where grayscale values encode milling depth via dwell time modulation.

Spherical structures were generated from analytical height profiles, spiral phase plates encoded a 2π azimuthal phase ramp at 633 nm, and axicons were defined using conical profiles. Typical fabrication parameters included 30 kV beam voltage, 0.75 nA current for spiral/axicon structures, and 2.6 nA for concave/convex structures, with dwell times of 1–10 μs.

AFM measurements show that micro-concave and micro-convex surfaces achieve shape accuracies of approximately λ/80 and λ/50 at λ = 780 nm. SEM images of representative structures confirm high-fidelity fabrication. The FIB process preserves optical-grade surface quality, introducing no measurable increase in roughness at relevant spatial scales.

Three-dimensional AFM topography maps and line cuts fitted to spherical models yield residual errors over 45% and 50% of surface areas for concave and convex structures, respectively. The fitted radii of curvature are 106.13 μm (concave) and 28.13 μm (convex). Residual plots expressed in λ/10 units confirm optical-quality surfaces compatible with cavity-QED applications.

Wavelength discrepancies and fabrication precision in fibre optic quantum components

The fabrication of nanoscale optical components directly on fibres offers a promising route toward miniaturised quantum technologies. However, a mismatch exists between optical characterisation at 633 nm and fabrication accuracy reported at 780 nm, making it important to establish how precision translates across wavelengths. Even small geometric deviations, invisible at one wavelength, could significantly affect performance in quantum applications.

This work demonstrates a single-step FIB process to create micro-spherical, micro-spiral, and micro-axicon structures directly on fibre cores with nanometre precision. Achieving λ/80 and λ/50 accuracy confirms the potential for high-performance phase and intensity control, demonstrated through donut beam patterns and verified phase profiles.

Direct fabrication of micro-optical elements onto fibres represents a significant step toward integrated quantum technologies, simplifying optical systems and improving stability in applications such as atom trapping and quantum communication. The results confirm that FIB machining enables reproducible, high-precision optical structures directly integrated into fibre platforms, supporting the development of compact quantum devices.