In optical manufacturing, additive manufacturing (AM), as an advanced manufacturing method, has been gaining interest because of its capabilities in the fabrication of extremely complex shapes. Applications of AM in the fabrication of single optical components (e.g. optical crystal) or systems (e.g. the making of an eagle eye) at the microscale or macroscale level have been the focus of recent developments. AM is an especially good fit for products that require a high level of customization.

AM techniques that have been used in optical manufacturing include selective laser melting (SLM), fused deposition modeling (FDM), stereolithography (SLA), multiphoton stereolithography (MPS), direct inkjet writing, and inkjet printing. Since laser writing methods can create optical patterns on a substrate (one layer or more), they can be classified as AM as well. Other less popular AM techniques, such as laser-induced forward transfer fabrication and microcontact printing, have also been tested.

Selective Laser Melting

Figure 2. 3-D printed optical component: out of a laser spot, a laser line is generated to illuminate a part for optical metrology. (Reprinted with permission from Heinrich A and Rank M, 3D Printing of Optics, SPIE Press, Volume SL39, August 3, 2018

SLM is based on melting specific regions of a thin layer of powder to form a layer of a desired object by laser beam scanning. Although a wide range of materials can be used to build a component using SLM, its application in the manufacturing of metallic reflective optical components, such as mirrors, is more common. Since roughness on the surface created with this technique, however, is not normally suitable for optical components, post-processing, such as sand blasting, milling, and polishing, is required.

The study showed that printed surfaces with 6 nm roughness can be obtained after post-processing. In the end, after comparison to conventionally manufactured parts, it was concluded that all of the mirrors fabricated showed excellent density, dimension, stability, and homogeneity of thermal expansion over temperature in the scale of optical measurement.

Stereolithography

This AM technique is based on solidifying curable polymer materials by UV light. Two main approaches to UV exposure are laser beam spot scanning and light pattern projections. Since each layer is fabricated by a single exposure in light pattern projection using a digital micromirror device (DMD), the printing speed is significantly enhanced compared to the single spot scanning method. Although the quality of the optical components made by stereolithography appears to be adequate, in most cases, improvements are still needed in both vertical and lateral resolution. However, SLA has good potential as a highly efficient manufacturing method for nonlinear optical lenses (NOL).

Multiphoton Stereolithography

This technique is based on curing resins using an ultrafast laser. Once the laser, with a pulse duration in the range of tens or hundreds of femtoseconds, hits the photosensitive material, a multiphoton absorption phenomenon may happen in the very center of the focus beam region, roughly on the order of tens of nanometers. The main advantage of multiphoton stereo lithography over SLA is the possibility of achieving nanometer resolution. Using continuous beam movement, the optical component is created via a spot-by-spot curation process. The excellent smoothness of the surface parts fabricated using this method and the high optical clarity are the reasons why this technique is a fascinating approach to fabricating stereolithography precision optical components.

MPS is utilized to manufacture a wide range of precision optical components, such as microlens arrays (MLAs}, waveguides, and photonic crystals. As an important optical component in photonic systems, microlenses (MLs) have made it possible to build high-performance, miniaturized systems for a wide range of applications, such as imaging, sensing, and optical communication. MPS using a femtosecond laser is a common method of fabricating MLAs.

Figure 3. Microlens manufacturing. (Image by sakkmesterke/Shutterstock)
Figure 4. (a), (c) Optical performance spherical microlenses and (b), (d) micro-Fresnel lenses fabricated by MPS. (Reprinted with permission from Guo R, Xiao S, Zhai X, Li J, Xia A, and Huang W 2006 Micro lens fabrication by means of femtosecond two photon photopolymerization Opt. Express 14 810–6, © 2006 Optical Society of America.)

Using two-photon polymerization (TPP), Guo et al[1] fabricated a 2 × 2 spherical microlens array (diameter: 15 μm) and a Fresnel lens (diameter: 17 μm). Larger MLAs on the scale of hundreds of microns are reported by Chung et al.[2] In the same manner, cylinder and axicon MLs were fabricated by Li et al[3] with a resolution of up to 1.5 μm. Laser beam scanning is the common method used in MPS experiment setups. In this technique, a combination of linear stages and galvano mirrors are normally used.

Since MPS has a higher resolution compared to other AM fabrication methods, it has been used to manufacture mesoscale structures and large lenses. Mesoscale structures have an overall size of up to centimeters but with micro-or nanosized features. To demonstrate MPS capabilities in the fabrication of mesoscale structures, a large butterfly (in the range of a millimeter) with micro- and nanoscale features is shown in Figure 5[4] Mesoscale lenses have a wide range of applications, such as ophthalmology and imaging systems.

Figure 5. (a) Scanning electron microscope (SEM) image of the whole structure. (b) Enlarged view of the antennas. (c) Close-up view of the microlenses on the head, and (d) embedded nanolattice of a wing. (reprinted with permission from Jonušauskas L, Gailevi ius D, Rekštyt S, Baldacchini T, Juodkazis S, and Malinauskas M 2018 Mesoscale laser 3D printing Preprints 2018 2018100384)

Among 3D printing technologies, it has been demonstrated that multiple photon polymerization (MPP) has the highest resolution in comparison to other printing techniques (around 100 nm or less). Research in resolution improvement of mesoscale lenses fabricated by MPP is limited to process improvements by controlling parameters and developing new resins, new systems, or laser scanning algorithms.

One of the main advantages of AM over conventional optical manufacturing is its capability of creating multicompound systems at different scales in a single setting without assembly. Recently, Thiele et al[5] demonstrated 3D printing of multilens objectives directly onto a complementary metal-oxide semiconductor image sensor to produce a foveated imaging system. Multilens optical systems, combining both refractive and diffractive optics, are among the wide range of multicomponents that have been manufactured using MPS.

Inkjet Printing

Inkjet printing works by ejecting droplets from a nozzle (by pressure) and depositing them onto the substrate. The major challenge is how to attach the droplets to each other to form continuous lines and surfaces. In spite of this tough challenge, this technique has recently gained interest because of its good capability in controlling ejected droplets and depositing volume on the substrate. Since the ink can be selected from a wide range of solvents, a large number of materials are possible for use in optical fabrication. This method has been used to fabricate optical components, such as waveguides, sensors, MLs, and detectors. It is also capable of manufacturing large lenses with a surface roughness in the range of injection-molded lenses.

AM of Optics with Features at Nanoscale

AM has already been used for microscale rapid fabrication, and some methods have already been commercialized. However, AM methods at nanoscale are not widely adopted as yet, especially in commercial products. Photolithography is the primary conventional process for fabricating microstructures. However, at nanoscale, the diffraction limit of light restricts the application of UV photolithography. Electron beam lithography can be used; however, this method is expensive and has an extremely low production rate. In addition, arbitrary 3D shapes cannot be made using the electron beam process without a major effort. As a possible alternative, AM can effectively reduce the cost and improve the fabrication efficiency and complexity. These unique features of AM have been the focus of recent research and development activities.

Different from microscale AM, additive nanomanufacturing (ANM) relies on some unique methods. There are two groups of ANM methods: Direct writing (DW) and single particle placement. Usually, single particle placement methods are applied in the fabrication of atom-sized features. In the optical range, DW methods are more widely used.

Conclusions

AM processes have shown promising results in the manufacturing of high-performance optical components. The devices and systems consisting of these components have also demonstrated unique features and performance. The exact capability of this exciting technology is difficult to determine based on the existing information. Nevertheless, this review clearly described a promising group of processes with mounting evidence that AM could potentially revolutionize optical fabrication in the near future.

Based on the current research and development in 3D AM for optical fabrication, much remains to be done in the near future, particularly in the manufacturing of regular size optical elements, e.g. optics in the size of a few inches. Even at micro and nanoscale levels, the most promising results appear to only have occurred in lab settings rather than production environments. There are many unanswered questions and issues before this technology can be widely adopted. These issues include, but are not limited to, things such as index distribution, geometry, and volume shrinkage of the optical elements.

References

  1. Guo R, Xiao S, Zhai X, Li J, Xia A and Huang W 2006 Micro lens fabrication by means of femtosecond two photon photopolymerization Opt. Express 14 810–6
  2. Chung T-T, Tu Y-T, Hsueh Y-H, Chen S-Y and Li W-J 2013 Micro- lens array fabrication by two photon polymerization technology Int. J. Autom. Smart Technol. 3 131–5
  3. Li S, Jiao J and Kim Y-J 2018 3D printing of polymeric optical components by two-photon polymerization 3rd Int. Conf. Progress in Additive Manufacturing
  4. Jonusauskas L, Gailevicius D, Rekstyte S, Baldacchini T, Juodkazis S and Malinauskas M 2018 Mesoscale laser 3D printing Preprints 2018 2018100384
  5. Thiele S, Arzenbacher K, Gissibl T, Giessen H and Herkommer A M 2017 3D-printed eagle eye: compound microlens system for foveated imaging Sci. Adv. 3 e1602655

This article was excerpted under a CCBY (website ) license from Zolfaghari A, Chen T, and Yi, A-Y, Additive Manufacturing of Precision Optics at Micro and Nanoscale, Int. J. Ext. Manuf., Volume 1, Number 1, 15 April 2019.

Photonics & Imaging Technology Magazine

This article first appeared in the January, 2020 issue of Photonics & Imaging Technology Magazine.

Read more articles from this issue here.

Read more articles from the archives here.