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Abstract
Low-cost, short-range optical interconnect technology plays an indispensable role in high-speed board-level data communications. In general, 3D printing technology can easily and quickly produce optical components with free-form shapes, while the traditional manufacturing process is complicated and time-consuming. Here, we present a direct ink writing 3D-printing technology to fabricate optical waveguides for optical interconnects. The waveguide core is 3D printed optical polymethylmethacrylate (PMMA) polymer, with propagation loss of 0.21 dB/cm at 980 nm, 0.42 dB/cm at 1310 nm, and 1.08 dB/cm at 1550 nm, respectively. Furthermore, a high-density multilayer waveguide arrays, including a four-layer waveguide arrays with a total of 144 waveguide channels, is demonstrated. Error-free data transmission at 30 Gb/s is achieved for each waveguide channel, indicating that the printing method can produce optical waveguides with excellent optical transmission performance. We believe this simple, low-cost, highly flexible, and environmentally friendly method has great potential for high-speed short-range optical interconnects.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Benefiting from the advantages of high bandwidth, low latency, and anti-electromagnetic interference, photonic applications have been greatly developed in many fields, such as optical interconnects [1], biophotonics [2,3], wearable strain sensors [4–8], and artistic lighting [9]. In today's era of big data, traditional electrical interconnects exhibit inherent disadvantages in terms of low bandwidth, high power consumption, and electromagnetic interference, making it difficult to keep up with the growing data demand [10]. To address these issues, optical interconnects are widely used between data processing devices, especially in high-speed board-level data communication [11], where the multimode optical waveguides are typically utilized [12], though the cost and rapid production remain challenging. Thus, developing methods for high-efficient, low-cost, and large-scale production of multimode optical waveguides is promising.
For years, many different materials and schemes to fabricate optical waveguides have been reported. Most of the reported work utilizes conventional photolithography techniques. To date, single-waveguide and single-layer waveguide array have been investigated and demonstrated. For instance, silicon waveguides are fabricated on silicon-on-insulator substrates using photolithography and reactive ion etching techniques [13]. Other schemes use ormocer polymer to prepare polymer optical waveguides by standard photolithography process [14] or employ siloxane-based polymer to fabricate single-layer waveguide array by UV-laser direct-writing [11]. Several studies use SU-8 polymer to create photonic wire bonds for inter-chip optical connections by two-photon lithography [15]. Some groups have also recently investigated 3D photonic waveguide integration. For example, optical waveguides created by the 3D two-photon polymerization (TPP) method have enabled large-scale 3D photonic interconnections [16,17]. Furthermore, 3D integration of optical waveguide arrays has been achieved by combining one-photon polymerization and 3D TPP techniques [18]. These methods, however, suffer from strict fabrication processes, high cost, and limited materials [13]. To overcome these challenges, 3D-printing technology [19–22], which can fabricate complex optical components, is proposed and demonstrated. For instance, some studies develop the mosquito method based on direct ink writing (DIW) to fabricate waveguides using UV-curable optical resins [23–26]. DIW attracts more attention because it allows rapid and cost-effective manufacturing of optical waveguides [27–31].
Here, we fabricate optical waveguides by a DIW 3D printing system under mild process conditions. Moreover, we report a scheme for fabricating spatial multilayer waveguide arrays (SMWA) with dense waveguide channels in three-dimensional space, as the schematic in Fig. 1 shown. It provides a new idea of photonic integration with high space utilization. The polymethylmethacrylate (PMMA) polymer is used as the printing material to fabricate an optical waveguide core, exhibiting a spectral absorption coefficient of 0.226/mm in the 900-1600 nm band, good mechanical properties, chemical and thermal stability, and low cost. The polydimethylsiloxane (PDMS) polymer is used as the cladding material. Four-layer waveguide arrays (each layer has 36 waveguide channels, a total of 144) are experimentally demonstrated, with the insertion loss of 3.08 dB at 980 nm, 4 dB at 1310 nm, and 6.67 dB at 1550 nm for a single waveguide channel with a length of 3.6 cm, respectively. Clear and open eye diagrams can be obtained at 30 Gb/s for modulated signal transmission. To the best of our knowledge, unlike previous studies that use UV lithography or near-field melt electrospinning, this is the first time that PMMA polymer has been used to fabricate optical waveguides through DIW 3D printing. The fabricated waveguides have potential applications for short-range data communication in data centers.
2. Experimental method
2.1 Direct ink writing 3D printing system
As shown in Fig. 2(a), the printing system consists of a motion controller (Lead Shine SMC606-BAS), a 3-axis motion platform (self-assembled), and a dispenser (Nordson EFD ULTMUS-V). It works by first loading the material into the syringe, then the dispenser provides pressure to extrude the material from the needle (as indicated in Fig. 2(b)). The motion controller controls the movement of the needle in three-dimensional space according to a predetermined trajectory.
Fig. 2. Schematic of DIW 3D printing system: (a) 3D printing devices and (b) schematic of dispensing.
We dissolve the granular PMMA in ethyl acetate to make a solution and seal it in a glass bottle for later use. Since the refractive index of PDMS (n = 1.405 at 1550 nm) is lower than that of PMMA (n = 1.474 at 1550 nm), it is utilized as the cladding material. By mixing PDMS and curing agents in a ratio of 10:1, a PDMS precursor is formed.
2.2 Printed structure
Unlike gel-like polymer materials that require a curing step after printing [27], the PMMA solution solidifies rapidly when squeezing from the needle, due to the fast evaporation of ethyl acetate (boiling point: 78°C). The printing performance is closely influenced by multiple factors such as solution concentration, needle inner diameter, dispensing pressure, dispensing height, and printing speed.
First, to optimize the printing process, a solution concentration of 600 mg/ml and a needle with 210 µm inner diameter are utilized to ensure that the waveguide core has a small width-to-height ratio, as the waveguide will spread out to both sides when printing on the substrate. Then, we gradually adjust the dispensing pressure to obtain an appropriate value of 380 kPa, enabling the solution to be extruded smoothly and uniformly. Experimentally, the dispensing height equals to 1-1.5 times of the needle inner diameter. The printing speed is another important parameter to obtain a good waveguide. A higher printing speed results in a smaller waveguide under the fixed printing parameters. However, if the printing speed is too fast (larger than 40 mm/s), the waveguide will be easily broken during the printing process. The resulting waveguide width as a function of the printing speed is investigated in Fig. 3(a).
Fig. 3. (a) The function of straight waveguide width and print speed, (b) straight waveguides, (c) cross-section of waveguide core, and (d) optical mode field distribution simulation.
Using the optimized printing parameters, we print the straight waveguides, as shown in Fig. 3(b). The cross-section of the waveguide core is quasi-quadrilateral, as shown in Fig. 3(c). As the waveguide core is printed on the PDMS substrate, the lower surface of the waveguide will be flattened. On the other hand, the needle scrapes flat the upper surface, and thus the waveguide core can be achieved in a quasi-quadrilateral shape. Experimentally, at a solution concentration of 600 mg/ml, a dispensing pressure of 380 kPa, a needle inner diameter of 210 µm, and a printing speed of 38 mm/s, a waveguide core with a size of 55 µm × 175 µm is obtained. The simulated optical mode field distribution of the waveguide at 1550 nm is presented in Fig. 3(d).
3. Optical properties
3.1 Transmission spectra and refractive indices
We evaluate the transmission spectra of the PMMA in the visible and near-infrared bands using a spectrophotometer. As shown in Fig. 4(a), the PMMA film with a thickness of approximately 0.16 mm exhibits 92% transmittance in the 900-1600 nm band, and its spectral absorption coefficient is 0.226/mm, indicating it is an excellent material for manufacturing optical components.
Fig. 4. (a) The transmission spectra of PMMA are measured for films with a thickness of approximately 0.16 mm and (b) refractive index of PMMA and PDMS.
The refractive index of PMMA and PDMS are further characterized using a spectroscopic ellipsometer. It can be seen from Fig. 4(b) that the PMMA has a higher refractive index than PDMS, and thus the optical confinement can be formed.
3.2 Propagation loss
The propagation loss of the waveguide is measured by the cut-back method [32]. The end face of the waveguide is cut flat with a scalpel, and the coupling fiber is aligned with the waveguide through the coupling platform.
Considering the waveguide core size is 55 µm x 175 µm, a 50 µm multimode fiber (MMF, Fiber Home OM3-50/125) with a small numerical aperture (NA = 0.2) is used to couple the light into the waveguide, while a 200 µm MMF (YOFC HP-200/230) with a large numerical aperture (NA = 0.37) is used to collect the light output from the waveguide, as shown in Fig. 5(a). After linear fitting of the measurement results, as shown in Fig. 5(b), the propagation loss of the waveguide is 0.21 dB/cm at 980 nm, 0.42 dB/cm at 1310 nm, and 1.08 dB/cm at 1550 nm, respectively. The total coupling losses between the MMF and the input and output ends of the waveguide are 2.32, 2.49, and 2.78 dB at different wavelengths, respectively. The waveguide loss is partly caused by scattering loss, it can be improved by more elaborate fabrication processing. The molecular vibration of PMMA is a factor contributing to waveguide attenuation, and the resulting loss at 1550 nm is greater than that at 1310 and 980 nm [33,34].
Fig. 5. Waveguide propagation loss measurement: (a) schematic of propagation loss measurement and (b) measurement results after linear fitting.
4. Spatial multilayer waveguide arrays
4.1 Design and Fabrication of SMWA
Previously, single-layer waveguide arrays and 3D waveguide arrays have been investigated and demonstrated [18,35,36]. Here, a scheme for fabricating SMWA with dense waveguide channels in three-dimensional space is demonstrated to realize high-density short-range multi-layer optical interconnects. It is printed on a PDMS substrate with a size of 4 cm × 4 cm. The spacing between waveguides is set to be 1 mm, while the spacing between different layers is approximately 450 µm. Totally, 144 waveguide channels locating in four layers are printed.
The manufacturing process is shown in Fig. 6, using the printing parameters in Section 2.2. First, spin coating a layer of PDMS as a substrate. Then, printing the first waveguide layer on the substrate and spin coating another PDMS layer to cover it. Repeating this procedure until the fourth waveguide layer and the cladding are formed. To be noted, it needs to be cured at 60 degrees for 5 hours, after spin coating each PDMS layer.
4.2 Characterization and discussion
As shown in Fig. 7(a), the end face of the SMWA is cut flat with a scalpel to expose the core of the waveguide. The size of the SMWA is 3.6 cm x 3.6 cm after this process and its height is approximately 2 mm. We couple two beams of red light into two orthogonal waveguides at different layers, as shown in the inset of Fig. 7(a). The light propagates along the respective waveguides. To quantitatively evaluate the crosstalk, a 50 µm MMF is employed to couple the light into one of the waveguides, and the output power of the adjacent waveguide is measured and recorded. Experiments show that the crosstalk between adjacent waveguide channels is -42.3 dB at 1550 nm, -46.8 dB at 1310 nm, and -53.4 dB at 980 nm, indicating low crosstalk occurs between waveguides. The waveguide cross-section is a quasi-quadrilateral, as shown in Fig. 7(b), and all the waveguide channels exhibit a high degree of homogeneity, as shown in Fig. 7(c). The width of the waveguide for each layer in the SMWA is presented in Fig. 7(d). As expected, the width of the waveguide is approximately 175 µm with a variation of less than 4%, indicating the high reproducibility of the printing system.
Fig. 7. Demonstration of SMWA: (a) coupling red light into the waveguide channel, (b) cross-section of the waveguide, (c) high consistency of waveguide channels, and (d) measurement results of waveguide width.
Then, we evaluate the insertion loss of the SMWA at 980, 1310, and 1550 nm, respectively. Thanks to the large waveguide core, it has a high alignment tolerance. We select 15 waveguides in each layer to measure the insertion loss, and the results are plotted in Fig. 8. The values vary slightly for different waveguide channels, mainly due to the inconsistent uniformity of each waveguide end face. It exhibits a larger insertion loss at 1550 nm, due to the larger propagation loss at this wavelength. Table 1 summarizes the average insertion loss and the lowest insertion loss statistics for each layer of the waveguide in SMWA. The insertion loss is 3.08 dB at 980 nm, 4 dB at 1310 nm, and 6.67 dB at 1550 nm for a single waveguide channel with a length of 3.6 cm, respectively.
Fig. 8. Insertion loss measurement of SMWA: (a) first layer, (b) second layer, (c) third layer, and (d) fourth layer.
Table 1. SMWA insertion loss statistics
Furthermore, the modulated signals at 10, 20, and 30 Gb/s are utilized to test the data transmission characteristics. The experimental setup is shown in Fig. 9. We use a 1550 nm laser as the light source. The signal is generated by a signal generator, amplified by an electric amplifier (EA), and then enters an optical intensity modulator for modulation. A direct current (DC) source is used to bias the modulator. A polarization controller (PC) is utilized to optimize the polarization state. Two adapters are employed to connect the single-mode fiber (SMF) and MMF. In case of the light transmits from SMF to the 50 µm MMF, the MMF can receive the light with the connection loss of 0.5 dB due to the larger core diameter of MMF than that of SMF, while when light is guided from 200 µm MMF to SMF, the connection loss is 19.2 dB, which can be attributed to the mismatch of the mode field. An erbium-doped fiber amplifier (EDFA) is used to ensure sufficient optical power at the receiving end to facilitate eye diagram and bit error rate (BER) measurements. A tunable filter is used to eliminate the amplifier spontaneous emission noise, and a variable optical attenuator (VOA) is utilized to control the received optical power. A photodetector (PD) is utilized to convert the optical signal into electrical one. An oscilloscope and a BER meter are employed to record the eye diagram and the BER value, respectively. Back-to-back (b2b) optical link without the samples is also tested to provide a reference.
Figure 10(a) shows the eye diagrams recorded at different bitrates for the b2b and waveguide optical links. The clear and open eye diagrams is obtained, thanks to the high-performance waveguide channels. As shown in Fig. 10(b), the BER results at 10 Gb/s indicate error-free transmissions.
5. Conclusion
In summary, we report a scheme to fabricate multimode optical waveguides using DIW 3D printing. The propagation loss of the waveguide is 0.21 dB/cm at 980 nm, 0.42 dB/cm at 1310 nm, and 1.08 dB/cm at 1550 nm, respectively. Furthermore, a scheme for fabricating SMWA with four-layer waveguide arrays to achieve high-density short-range multi-layer optical interconnects is demonstrated. Error-free data transmission is achieved for the waveguide channel in SMWA. The proposed method has great application prospects in high-speed and short-range board-level optical interconnects.
Funding
National Key Research and Development Program of China (Grant No. 2019YFB2203502); Key Research and Development Program of Hubei Province (2020BAA011, 2021BAA005); National Natural Science Foundation of China (Grant No. 62135004); Interdisciplinary Scientific Research Foundation of Guangxi University (Grant No. 2022JCC014).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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