Manufacturing customized micro lenses with optical smooth surfaces using fuzzy tomography technology

Additive manufacturing, also known as 3D printing, has completely changed many industries with its speed, flexibility, and unparalleled design freedom. However, previous attempts to manufacture high-quality optical components using additive manufacturing methods often encountered a series of obstacles. Now, researchers from the National Research Council of Canada have turned to fuzzy tomography (an extension of the volume additive manufacturing (VAM) method of tomography) to create customized optical components.

"3D printing is changing every sector of the manufacturing industry," said lead author Daniel Webber. I have always been interested in 3D printed optical devices because they have the potential to completely change the design of optical systems. I saw a postdoctoral position at NRC, and they want to do volume 3D printing in micro optics. The rest is history.

Additive manufacturing challenges
In the past, technologies such as digital light processing, stereolithography, inkjet printing, and two-photon polymerization (2PP) have been used to construct optical components through layer by layer methods. However, the manufacturing process is often slow and it is difficult to manufacture optical components with curvature - which is required for many components - and the surface that is not parallel to the substrate has a height step defined by the layer thickness.

VAM also faces challenges due to the self writing waveguide effect and poor part quality (such as ridges on the surface called stripes), where the narrow writing beam used in VAM leads to an increase in printing speed on the plane parallel to the beam. Usually, post-processing methods are needed to improve part quality and smooth surfaces, but a direct VAM method that does not require additional steps has been sought.

Overcoming the challenge of blurred CT scans
In their latest research, Webber and his team have completed this direct VAM method while maintaining the design freedom provided by additive manufacturing for rapid prototyping.

Tomography VAM uses photosensitive resin that projects light to cure specific areas, allowing parts to be manufactured without supporting structures. Although the pencil shaped beam used in traditional tomography VAM methods can cause fringes, the new technology can produce commercial grade quality microlenses. It is called blurred tomography because it uses a wide range (more "dispersed") of sources to intentionally blur lines and reduce stripes.
The blurring of optical writing beams helps to generate surface roughness in the sub nanometer range, making it essentially molecular smooth. In contrast, other VAM methods have good collimation and low delay writing beams, so they do not blur in design.

By intentionally blurring the beam and coupling it with the scattered light introduced by a cylindrical photoresist bottle (a bath without refractive index matching), blurring can be achieved throughout the entire printing volume. In addition to its fast processing speed, another decisive feature of the fuzzy tomography method is that it does not require additional processing, making it a direct method for producing smooth optical components.
"The most important discovery of this work is that we can directly manufacture optically smooth surfaces and have free form ready to use optical components within 30 minutes," Webber said.

Although the entire processing time takes about 30 minutes, the actual printing time of the lens is less than one minute. This is similar to other VAM technologies (but does not require additional surface treatment steps). In contrast, a previous study found that printing a hemispherical lens with similar dimensions (2 and 3 millimeters), curvature error (3.9% to 5.4%), and surface roughness (2.9 and 0.53 nm) using 2PP takes 23 hours - indicating that the speed of blurred tomography scanning is much faster and produces finer surface features.

The research team demonstrated the potential of this new technology by manufacturing a millimeter sized flat convex optical lens with imaging performance comparable to commercial glass lenses. The inherent degree of freedom design provided by additive manufacturing has also helped researchers create biconvex microlens arrays (double-sided manufacturing) and overlay lenses onto optical fibers.

Like many fields of additive manufacturing, it is believed that VAM can provide a method for producing low-cost and rapid prototyping parts, especially free-form optical components. "We have demonstrated that fuzzy tomography can quickly manufacture a range of micro optical components. Looking ahead, we hope to extend these functions to larger part sizes and new materials," Weber said.

Source: Laser Net