3D printed optical chip coupled microlenses

Wafer-level mass production photonic integrated circuits (PICs) have become the backbone of optics and photonics and have revolutionized the way many existing applications work. Despite the strong application advantages of PIC-based solutions, scalable photonic packaging and system assembly remain a big challenge, which severely hinders the commercial application of PIC. For example, chip-to-chip and fiber-to-chip connections often require the use of active alignment techniques to continuously measure and optimize coupling efficiency during assembly. This technical complexity and high cost inevitably leads to the significant erosion of PIC’s inherent advantages in scalability.

Researchers from the Karlsruhe Institute of Technology in Germany recently published a research paper entitled “3D-printed facet-attached microlenses for advanced photonic system assembly” at Light: Advanced Manufacturing.

In this paper, the researchers propose and validate a facet-attached microlens system (FaML) and realize its direct assembly with a highly scalable photonic system through 3D printing technology. In addition, the researchers also demonstrated the feasibility and versatility of the scheme through a series of interface coupling experiments. This FaML optical system assembly solution, which can combine the unique advantages of different photonic integration platforms, will open up a promising commercialization path for PIC-based system architectures.

Face microlens design

FaML can be printed onto the facets of optical components with high precision using multiphoton lithography, providing the possibility of shaping emitted beams by freely designed refracted or reflective surfaces. Specifically, the emitted beam can be aligned up to a relatively large diameter, which is independent of the device-specific mode field. This relaxes axial and lateral alignment tolerances and can be replaced by passive assembly techniques based on machine vision or simple mechanical stops. In addition, the FaML concept opens up the possibility of inserting discrete optical elements such as optical isolators into the free-space beam path between PIC surfaces compared to direct docking coupling.

Figure 1 illustrates the concept of assembling an integrated optical system using a surface-mount microlens FaML. A typical example of an optical emitter includes an InP laser array with an inclined plane, an optoisolator block, and a modulator array located on a silicon photonic chip (SiP). The output of the SiP chip is connected to a pluggable single-mode fiber (SMF) array via mechanically transferred ferrules and positioned by mechanical alignment pins. Illustrations (i), (ii), and (iii) show magnified views of different free-form FaMLs that can be designed to focus free-space beams up to 60 μm in diameter, greatly relaxing the translation alignment tolerance.

Figure 1: Schematic diagram of different types of FaML optical components based on 3D printing. Source: Light: Advanced Manufacturing 4, 3 (2023)

FaML is integrated with SiP chips

The researchers demonstrated low-loss coupling to edge-emitting SiP chips via FaML, as well as pluggable optical connections based on a simple mechanical alignment structure, as shown in Figure 2. The researchers printed the FaML array to the edge of the SiP chip and tested its coupling efficiency with the lens single-mode fiber array (FA) and the associated alignment tolerances. Experiments show that the insertion loss per interface is 1.4 dB, and the translation lateral 1 dB alignment tolerance is ± 6 μm, which is the lowest loss of edge-emitting SiP waveguide interfaces with micron-level alignment tolerances that have been experimentally demonstrated to date.

Figure 2: FaML microlens sets are used to couple single-mode fiber arrays with edge-transmitting SiP waveguide arrays. Source: Light: Advanced Manufacturing 4, 3 (2023)

At the same time, due to the excellent alignment tolerance allowable range of the scheme, the researchers demonstrated the possibility of implementing contactless pluggable fiber optic chip interfaces using low-cost, mass-produced injection molded plastic parts, such as LEGO LEGO bricks. As shown in Figure 3, the LEGO connection consists of a fixed (yellow) brick and a removable (dark transparent) brick with a flat aluminum cover plate (gray) attached to the brick. Start by gluing the SiP chip to the base, then actively align and glue the fiber array to the cover plate on the right while gluing the Lego bricks together. Finally, after disassembling and re-establishing the LEGO connection a total of 50 times, the insertion loss is measured. The loss per connection is between 1.41 dB and 2.46 dB, with an average loss of 1.9 dB, which is about 0.5 dB higher than the value of the active alignment originally discovered.

Figure 3: Contactless pluggable fiber chip interface coupling via LEGO bricks. Source: Light: Advanced Manufacturing 4, 3 (2023)

Long-distance passive positioning optical coupling

At the same time, the researchers used standard machine vision technology for alignment, demonstrating long-distance free space transmission and passive positioning optical coupling in the millimeter range. The design results in a larger beam diameter, thereby reducing beam divergence and increasing the distance between the associated coupling interface FaMLs, allowing discrete microoptics such as optical isolators to be inserted into the beam path between optical chips, as shown in Figures 3 and 4.

Figure 4: FaML for fiber array output beam collimation and long-distance transmission coupling. Source: Light: Advanced Manufacturing 4, 3 (2023)

In the experiment, the researchers used standard SMF arrays and edge-coupled InP photodiode arrays by 3D printing FaML to fiber optic array FA and photodiode arrays (PDA) chips. Using these components, the researchers designed a passive SMF array-to-chip component with a free-space coupling distance of up to 3.3 mm. The lens on the FA converts a 10 μm SMF mode field diameter into a free-space Gaussian beam with a beam diameter of 60 μm at a distance of 1.65 mm from the FaML apex, i.e. at the center of the free-space beam path. The PDA chip is designed to be docked and coupled to the SMF and contains an on-chip tapered spot size converter that generates a mode field diameter of 10 μm at the die end face. The researchers printed the lens onto a chip with the same optical design as the lens printed to FA, and converted the incident Gaussian beam with a beam diameter of 60 μm into a Gaussian spot with a diameter of 10 μm at the end face of the PDA chip.

Further, using the same types of FA and PDA chips, the researchers also demonstrated that discrete optics can be inserted into a collimated free-space beam path, as shown in Figure 4. In the proof-of-principle assembly, the researchers inserted an optical polarization beam splitter (PBS) into the beam path between the single-mode fiber array and the edge-coupled indium phosphide photodetector array. The PBS consists of two right-angled glass prisms and a dielectrically polarized-sensitive reflective surface in between, and the beam path is shown as a colored dashed line in the figure. Experiments have shown that FaML-based optical microsystems can achieve levels of accuracy that are comparable to or exceed those provided by standard discrete microoptical components.

Figure 5: FaML coupling experiment with polarized beam-splitting optical components. Source: Light: Advanced Manufacturing 4, 3 (2023)

Tilted end-face device coupling

At the same time, the researchers demonstrated FaML components printed onto angled chip facets, as shown in Figure 6, which effectively suppress unnecessary backreflections from semiconductor lasers and amplifiers. In the experiment, the researchers used dedicated FaML on the active chip and fiber side to couple a Distributed-feedback-feedback (DFB)-based laser array to a single-mode fiber array. FaML is specifically designed to produce a non-planar beam path between two faces that contains only inclined or strongly curved optical surfaces S1, S2, S3, and S4, greatly reducing the back-reflection into the DFB chip. This non-planar beam path is designed so that the overhead projection of the FaML is perpendicular to the chip and FA edges. By top-down camera vision combined with linear panning parallel to the edge of the chip, the difficulty of device alignment during assembly can be greatly simplified.

Figure 6: Coupling a DFB laser array to a single-mode fiber array using FaML. Source: Light: Advanced Manufacturing 4, 3 (2023)


In summary, this paper demonstrates the great potential of 3D printed FaML microlens components in the assembly of integrated photonic systems. Multiphoton lithography is printed onto the small face of the optics with high precision, providing the possibility of shaping the emitted beam by freely designing the refractive surface. The beam can be aligned up to relatively large diameters that are independent of the device-specific mode field, allowing the relaxation of axial and lateral alignment tolerances, making it possible to insert discrete optics such as optical isolators between PIC planes. Advanced photonic system components based on the FaML concept can overcome most of the current limitations and will open up a promising application path for photonic integrated circuits.

Related paper information:

(Source: Advanced Manufacturing WeChat public account)
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