Researchers from the University of Stuttgart in Germany are using 3D printing to produce optical lenses the size of a human hair, which could have applications in endoscopy, aerial drone observation and sensors for autonomous vehicles.
The researchers used a microscope to focus a femtosecond laser – with pulse durations less than 100 femtoseconds – into liquid photoresist which rests on a glass substrate or an optical fibre. The laser beam is directed with either a scanner or by movement of the substrate. Two 785nm wavelength photons of the red laser beam are absorbed simultaneously during focusing and expose the photoresist. This crosslinks and hardens the polymer. After exposure, the unexposed photoresist is washed away with a solvent. Only the hardened transparent polymer remains and forms the optical element.
Using this method, optical freeform surfaces can be created with sub-micrometre accuracy. The precision of the 3D laser writing enables not only the construction of common spherical lenses, but also more ideal surfaces such as paraboloids or aspheres of higher order. In particular, optical lens systems with two or more lenses can be created for the first time with this method. This opens the door to aberration correction and microoptical imaging systems with unprecedented quality.
The Stuttgart researchers believe that the 3D printing of optics will open an entire new era of optics manufacturing. ‘The time from the idea, the optics design, a CAD model, to the finished, 3D printed micro-objectives is going to be less than a day’, stated Professor Harald Giessen. ‘We are going to open potentials just like computer-aided design and computer-integrated manufacturing did in mechanical engineering a few years ago’.
PhD student Timo Gissibl, part of Professor Giessen's group at the University's 4th Physics Institute, printed microobjectives with a diameter and height of only 125µm, even on optical fibres. This could enable the construction of extremely small endoscopes that can pass through tiny entrances into the body during a procedure, or into a machine for inspection purposes.
Gissibl also printed optical freeform surfaces and miniature objectives directly onto CMOS image chips, creating an extremely compact sensor. Such optics make smaller cameras – about the size of a bee – possible for attaching onto aerial drones. Smaller sensors for autonomous cars and robots are also conceivable. Even smaller sensors for the body and surround-cameras for mobile phones could be developed.
The researchers were able to combine their optics with illumination systems, enabling extremely small optics for LEDs. Additionally, illumination of ring-shaped areas, triangles, or elongated rectangles are possible with such miniature free-form optics.