We used cheap FDM 3D printers to make fibre-optic components that, with some refinement, could be used to make optical sensors and devices. (Full paper here.)

3D printers are revolutionizing manufacturing by offering fast and cheap fabrication of complicated or bespoke parts. As the capabilities of 3D printers expands, and the costs of the technology comes down, 3D printers are finding uses in an ever wider range of applications.

Cheap hobby-grade FDM printers are becoming more common in scientific laboratories where researchers are finding innovative uses for 3D printing in their research. These uses range from developing teaching and instruction aids to printing anatomically accurate personalized “phantoms” that can be used to test medical imaging machines and cancer treatment methods.

Optical physicists, such as myself, would usually assume that the low resolution of FDM 3D printers (limited to a few tens of micrometres by the size of the hot end nozzle) would prevent them from being of any use for making optical components (which need to be accurate to a few nanometres). However, several research groups around the world have come up with clever ways to get useful optical components out of cheap 3D printers.

A team from Rice University in the U.S. created custom g-code to lay down beads of clear filament in a carefully controlled grid which, when the ends were polished, created a faceplate — a device for guiding optical images for displays or sensors.

3D printed faceplate from Rice University. [Full paper here.]

A team from the University of Sydney, Australia, have shown how cheap FDM 3D printers can be used to make a range of specialized optical components including structured optical fibres made by 3D printing a preform which is then drawn into fibre by a dedicated drawing machine.

Step index fibre preform (prior drawing) from the University of Sydney. [Full paper here.]

The Sydney team also found that the hot end of common FDM 3D printers makes an almost ideal microfurnace for drawing clear filament into thin optical fibres.

We wanted to see if we could combine the methods developed by the Sydney team into one process — using the 3D printer to print the preform out of clear filament, which is then run through the printer again to draw the preform into optical fibre. This would allow us to directly print the optical fibres into larger objects, to make embedded sensors and interactive devices.

We wanted to make “step-index” fibre, meaning the centre of the fibre has a higher refractive index than the rest of the fibre around it, because this change in refractive index is needed to make “single mode fibre” for use in high-precision sensors. ABS and PETG are two 3D printer filaments that can be bought completely clear, and they have different refractive indices, just what we needed to make a preform for step-index fibre.

We made our own custom g-code that printed the lower half of a preform in PETG, laid-in a single bead of ABS, and then finished the rest of the preform in PETG over the top of the ABS.

Our step-index fibre preform (prior to drawing) with an orange-coloured core for visual inspection.

The diameter of the preform was 2.75 mm, which is small enough to be run through the extruder of printers like the Ultimaker. We commanded the Ultimaker to extrude at a fixed rate, with the extruded filament being taken up by a spool rotated by a small motor. Setting the speed of the motor higher than the extrusion rate of the nozzle meant that the preform could be drawn down to fibres only a few tens of micrometres across.

Our 3D printer-drawn step-index optical fibre guiding red laser light.

We also used the preforms to print fibre-optic couplers (optical power splitters and combiners) which are the basis for many high-precision optical sensors. (Specifically Mach-Zehnder interferometers.)

One of our 3D printed optical fibre couplers, showing how it splits the red laser light into two equal signals.

While the performance (optical loss) of our fibres was very close to that of the Sydney team’s fibres made using a specialized drawing machine, unfortunately, scattering of the laser light in the cladding of the fibre was too much for us to be able to tell whether we had succeeded in making single-mode fibre, and whether or not our 3D printed devices would make effective high-precision sensors.

We succeeded in making step-index optical fibre using only a hobby-grade FDM 3D printer. If we can refine the process to make single-mode fibre, this technique will allow us to print optical sensors and devices directly into larger 3D printed objects. The next steps will be to test other clear filaments, and carefully optimize the temperatures and flow rates at the various printing stages to achieve a better-quality preform and drawn fibre. We will then start expanding the repertoire of optical sensors and devices that can be made with cheap 3D printers.

More details of our process and testing can be found in the full paper here.

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