3D Printing in Micro-Optics: Femtosecond Laser Writing Allows for Improvement Using Phase Masks

As 3D printing technology stamps its mark of innovation, magic, and miracle virtually everywhere, from the sectors we are all well aware of like aerospace and art and from automotive to medical, there are of course other areas you may not put a lot of thought to which tend to operate behind the scenes, and are steadily being improved with 3D printing.

Optics would be a great example, and 3D printing is definitely making an impact there, as we’ve followed from new high speed processes to new design and fabrication partnerships. Now, taking it one step further into micro-optics, strides are being made through ‘writing with light’ via repeatable femtosecond direct laser writing. According to researchers, this allows for ‘phase masks with unprecedented feature sizes in the submicrometer range.’ This also should allow for great potential in miniaturizing other optical elements used in applications ranging from sensing to telecommunications.

Their recent study is outlined in their paper, ‘Spatial beam intensity shaping using phase masks on single mode optical fibers fabricated by femtosecond direct laser writing,’ by Timo Gissibl, Michael Schmid, and Harald Giessn. Published in The Optical Society (OSA)’s journal, Optica, the paper discusses how they were able to 3D print these items directly onto the end facets of optical fibers with their new process.

Phase plates, which are known as diffractive elements, offer numerous benefits due to the limited capabilities in miniaturizing lenses. They are able to allow for a spatial shift, and because of this can be used for focusing or beam shaping elements. Traditionally, phase plates are made with lithographic processes and then followed with etching. With femtosecond direct laser writing, researchers are able to make phase masks with ‘unprecedented feature sizes.’ This will also extend to metamaterials and surfaces.

“Although femtosecond laser writing has been demonstrated in the lab, we have shown that it can be used to make high performance micro-optics in a manner that is highly repeatable and reliable,” said Harald Giessen, chair for ultrafast nano-optics, University of Stuttgart, who led the research team. “We believe our approach can be scaled up for volume manufacturing and used to directly print almost any type of optical element on a tiny scale, opening up a new era of integrated micro- and nano-optics.”

This style of ‘laser writing’ employs a laser beam to put out short light pulses that ultimately cure the material; however, that only occurs where the light focuses. Whatever has not been cured is washed away, and the desired 3D print is left. Other researchers have created their own similar laser writing processes but an industrial version was needed for nanometer-sized structures. Nanoscribe GmbH created the 3D printer with three elements in mind: stability, reliability and quality.

“We have something like a pen that we can move in 3D through a material to create structures in a manner similar to a 3D printer but at very tiny scales,” explained Giessen. “If you punch a set of parameters into our system and into a system in some other place in the world, you will get the same exact result. Even months from now, it will come out the same every time.”

This allows them to make the phase masks, which are responsible for shaping the light exiting a fiber. In using this process, they are able to eliminate the use of large, bulky lenses, and with them right on the end of the fiber, the phase masks are able to manipulate the light into a flat-top profile or a donut shape. Previously, just the smallest error would cause issue, but now they have solved one of the biggest challenges in placing a mask with ‘submicron accuracy’ right in the middle of the single-mode fiber.

“Our phase plates are fabricated by femtosecond two-photon direct laser writing using a dip-in approach,” state the researchers in their paper. “The surface patterns of the beam shaping phase plates are calculated using Fresnel-Huygens diffraction theory. By analyzing the optical performance of the diffractive optical elements we demonstrate the viability of the fabrication technique for producing compact integrated optical elements.”

 

“Femtosecond three-dimensional direct laser writing is additionally well suited for the manufacturing of refractive free-form surfaces, compound lenses, and photonic crystal structures with submicrometer feature size. As a result it is possible to manufacture optical elements in order to control the light propagation including polarization and intensity.”

This method is meant to offer a faster and easier technique over interference lithography, photolithography, electron-beam lithography, and focused ion-beam milling.

“As this technique enables feature sizes below 100 nm, unprecedented diffractive elements with diameters in the low micrometer range are feasible. Thus, the intensity distribution emerged from an optical fiber can be spatially redistributed. Additionally, direct laser writing provides a fabrication technique which allows for the fabrication of other optical elements, such as refractive free-form, reflective or polarization controlling optics,” state the researchers in their paper.

 

“Our method extends the realm of additive optical manufacturing into micro- and nano-optics and opens a new field for integrated fiber optical or complex lab-on-a-chip devices. Also, specific spatial modes and higher orbital angular momentum states in multimode fibers can easily be generated.”

So far the team of researchers reports that they were able to make optical elements as small as 4.4 microns onto the exact center of an optical fiber only 125 microns in diameter. For reference, that’s only a tiny big thicker than one human hair.

“Our results prove that 3D printing of diffractive micro-optics can achieve sufficient performance in order to enable compact devices,” concluded the researchers.

What this means in the industry is that we should see much higher performance as the direct writing method allows for new applications, along with a higher volume in manufacturing of micro-optics. Discuss in the 3D Printing & Optics forum over at 3DPB.com.