3D Printed Monolithic Optomechanical Chassis for an Unbalanced Field-Widened Interferometer

Light, when split and recombined, can reveal the deepest secrets of the universe, from the distance to a star to the presence of a single atom. This delicate dance of light waves relies on instruments called interferometers, which measure tiny differences in the distance light travels. To work, these machines need a frame that holds mirrors and lenses with absolute stillness. If the frame expands even a fraction of a hair's width because the room got warmer, or if it vibrates from a passing footstep, the measurement fails. For decades, scientists have built these frames by bolting together heavy metal parts, a method that is reliable but bulky and rigid. As the need for smaller, more sensitive devices grows—especially for quantum technology that could revolutionize computing and sensing—engineers are looking for a way to build these delicate frames differently, perhaps by printing them as a single, solid piece.

A team of researchers at the University of Waterloo has taken a significant step toward this goal by creating a fully printed chassis for a complex optical instrument. Instead of assembling many separate metal parts, they used a high-powered laser to melt and fuse layers of a special metal alloy called Invar36, building the entire support structure in one go. This alloy is chosen for its unique ability to resist expanding or contracting when temperatures change, a property essential for keeping light waves in sync. The resulting object is a monolithic, or single-piece, frame that holds the mirrors and includes built-in flexible joints, known as flexures, which allow for precise adjustments without the need for screws or separate mounts. This approach demonstrates that additive manufacturing, often called 3D printing, can produce complex, high-precision optical systems that are lighter and more compact than those made with traditional methods.

The researchers designed their device to house a specific type of interferometer known as a field-widened Offner relay, which is capable of handling complex beams of light with many different modes. In a traditional setup, this instrument would require a large collection of separate brackets, screws, and adjustment knobs, creating a heavy and space-consuming assembly. The team's design folds the path of the light into a tight, compact shape and integrates the mounting points directly into the metal structure. To make this possible, they had to navigate the limitations of the printing process. Metal 3D printing works by building up layers, which means that parts of the design cannot hang in mid-air without support. The team engineered a series of internal pillars, which they called "Loft Islands," that grow upward from the base to support the mirrors and the flexible joints. These pillars are hollow to save weight and are shaped with gentle slopes to prevent the metal from collapsing under its own weight during the printing process.

The material chosen for this experiment, Invar36, is a nickel-iron alloy known for its incredibly low thermal expansion. In the world of precision optics, even a tiny change in temperature can cause a metal frame to grow or shrink, throwing off the alignment of the light. By using this alloy, the researchers ensured that the frame would remain dimensionally stable even as the environment around it shifted. They printed the entire chassis, including the delicate flexure joints, on a Renishaw AM 400 machine using a laser that melted metal powder layer by layer. The process involved carefully controlling the speed of the laser and the thickness of the powder layers to ensure the metal fused correctly without creating weak spots or holes. The final print was a single, solid block of metal with the optical mounts and adjustment mechanisms built directly into its geometry, requiring only the removal of temporary support structures and the insertion of a few screws to secure the mirrors.

Once the printing was complete, the team tested how well the new chassis performed compared to standard, commercially available optical mounts. They measured how much the device vibrated and how much it drifted over time. The results showed that the printed flexures were surprisingly flexible, moving more than expected without breaking, which suggested that the printed metal had slightly different properties than the solid metal usually used in engineering. However, this extra flexibility came with a trade-off: the printed mounts were more sensitive to low-frequency vibrations, causing the light to wobble slightly more than in a traditional, stiffer mount. To fix this, the researchers added small, compressible dampers between the flexible parts, which significantly reduced the wobble and brought the stability closer to that of standard equipment. Over a twelve-hour period, the printed device held its position nearly as well as the commercial standard in the horizontal direction, though it showed slightly more drift vertically due to gravity pulling on the less stiff structure.

The team also simulated how the device would behave under different temperature changes to see if the choice of base material mattered. They compared a base made of the same Invar alloy against bases made of stainless steel and aluminum. The simulations revealed that using a different metal for the base would cause the entire structure to warp and shift as it heated up or cooled down, ruining the precision of the instrument. Only when the base and the rest of the structure were made of the same low-expansion alloy did the system remain stable. This confirmed that for such delicate instruments, the entire frame must be made of a material that reacts to heat in the same way, or the instrument will fail.

Finally, the researchers tested the optical performance of the complete system by shining a laser through it. They measured how well the light waves interfered with each other, a key indicator of how well the mirrors were aligned. While the printed device did not achieve the perfect clarity of a lab-made instrument, it successfully produced clear interference patterns with both simple and complex light beams. The visibility of the interference was high enough to prove the concept works, even though the print was not perfect; a printing error near the end of the process had left the top of one of the support structures incomplete, which slightly reduced the stiffness and alignment precision. Despite this flaw, the experiment proved that a fully 3D printed, monolithic optical chassis is a viable path forward. It offers a way to build custom, lightweight instruments that are difficult or impossible to make with traditional machining, opening the door to new generations of quantum sensors and portable optical devices. The work suggests that with further refinement of the printing process and the design of the flexible joints, these 3D printed optical systems could become a standard tool for the future of precision science.