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Abstract
In this paper, two noteworthy issues of mode dispersion and lamination stability of multimode polymer waveguides for optical backplane are investigated. In the case of center launching by 50-µm graded-index (GI) multimode fiber (MMF), mode dispersion of polymer waveguides with different widths is analyzed theoretically and measured in the view of bit error rate (BER) curves. Compared with the waveguide with the width of 40 µm, 1-dB power penalty is observed by the 70-µm-width waveguide due to its larger mode dispersion. On the other hand, waveguide stability after laminating process with high temperature and pressure is measured experimentally. No significant changes in core shape and size are observed. The average insertion loss of 80 channels before and after lamination are 0.137 dB/cm and 0.192 dB/cm, respectively. Error-free transmission at 25 Gb/s is obtained by laminated waveguides. The results imply the feasibility and potential of multimode waveguides for optical backplane.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Data-intensive companies in industries such as social media, cloud computing, and ecommerce are investing their own hyperscale data centers, in which tens of thousands of servers are housed to provide the scalability for a series of online businesses. Because of the explosion and complexity of data communication in data centers, demands for short-reach interconnect bandwidth grow dramatically [1,2]. Copper connection plays an important role in short-reach interconnects due to their high conductivity, flexibility and low cost [3]. Current architecture in data centers deploys copper-based electrical interconnects among server racks, and optical interconnects with active optical cables (AOCs) that combining vertical cavity surface emitting lasers (VCSELs) at 850 nm and MMFs have been widely used in rack-to-rack communication and beyond [4–7]. However, to continue the bandwidth scaling, the high-speed transmission for copper cables to connect signals from electrical chips to optical transceivers at the edge of board causes large power consumption and high cost [8]. With the comparison of electrical interconnects, optical interconnects exhibit better performance in bandwidth, integration density and cost. Therefore, it is the trend that optical interconnects extending to board-level communications [9–11].
In board-level optical interconnects, the optical engines are located very close to the application specific integrated circuit (ASIC) and ultimately the opto-electronic modules are packaged with switch ASIC on a common substrate as co-packaged optics (CPO) to reduce the distance of electrical interconnects [12,13]. For on-board optical interconnects, optical signals can be guided by optical fibers, which exhibits a low transmission [14,15]. Optical fiber is a uniform solution as in the present short-reach optical interconnects. Therefore, the coupling loss from on-board to off-board transmission is low. But, it limits the architecture to “point-to-point” transmission between multiple modules on the board. Using optical fibers is not an integrated solution and fiber coiling problems need to be solved.
Polymer waveguide provides an integrated solution and it covers the transmission distance from cm to m, which can satisfy the requirement for board-level optical interconnects [16]. Meanwhile, as the transmission medium from optical engines to MMFs, polymer waveguides have the similar relative refractive index difference with MMFs, and they can be coupled to MMFs with low coupling loss [17]. Large core dimensions of multimode polymer waveguides relax the coupling requirement from VCSELs and MMFs to polymer waveguides [18,19]. Moreover, polymer materials are compatible with multiple substrates [20–22]. Significantly, they can be fabricated on the FR4 substrate, which means that polymer waveguides can be laminated with the printed circuit board (PCB) as optical printed circuit board (OPCB) [23]. Various complicated functions such as power splitting, wavelength (de)multiplexing and optical routing can be realized easily by polymer components, which would help to overcome the “point-to-point” transmission limitation that happens when using optical fibers [24–26]. Therefore, Multimode polymer waveguide plays an important part in on-board optical interconnects.
However, the multimode nature limits the transmission bandwidth of multimode waveguides. It is the mode dispersion rather than the chromatic dispersion that limits their bandwidth. Much attentions have been paid to explore their transmission capacities [27–29]. It is well known that with the decrease of the core size, the number of guided modes decreases, and therefore the bandwidth should be larger. But it is a trade-off between the mode dispersion and coupling loss with MMFs. The relationship between waveguide core size and bandwidth, as well as the coupling loss should be studied carefully. On the other hand, to form OPCBs, the laminating processes are indispensable, during which the polymer waveguides on FR4 substrate will suffer high temperature and high pressure. Thus, the effects of lamination on waveguides performances need to be studied.
In this paper, firstly, we introduce the two fabrication techniques used in this paper. Then, the bandwidths of step-index (SI) multimode waveguides with different core widths are analyzed both theoretically and experimentally. To evaluate the effects of board lamination on waveguides, the core shape, insertion loss and transmission performance are measured before and after laminating process.
2. Fabrication of polymer waveguides
Polymer waveguides can be fabricated by several fabrication methods, such as photolithography, laser direct writing, nanoimprinting and 3D direct writing [30–32]. In this paper, we mainly used the laser direct writing and photolithography technique for waveguide fabrication. Figure 1 shows the process of photolithography, which is compatible with the traditional fabrication technique of PCBs. Using photolithography method, polymer waveguides and devices with high uniformity can be manufactured easily in a large scale [33,34]. However, the mask is necessary for photolithography technique and its flexibility is limited. Moreover, the waveguide lengths for meter-level backplane are constrained by the exposure area of mask aligner. Figure 2 illustrates the schematics of the fabrication process using laser direct writing technique. Compared with the photolithography method, no mask is needed when applying laser direct writing technique. It has advantages of fast, efficient and low cost. It is noteworthy that the laser direct writing technique can realize the fabrication of waveguides with long lengths [8,35]. In this paper, straight multimode polymer waveguides with lengths of 26 cm were fabricated by the laser direct writing technique.
3. Effects of multimode dispersion on waveguide bandwidth
For multimode waveguides, it’s the mode dispersion that mainly dominates the transmission bandwidth. Mode dispersion is caused by the different velocities between different modes and it results in pulse broadening, which is usually described by differential mode delay (DMD) [36]. The multimode dispersion is dominated by the waveguide size and launching condition simultaneously.
In practical transmission system, not all guided modes can be stimulated on the specific launching condition, and the launched power is concentrated in a few low-order modes. The mode power distribution (MPD) determines the modes stimulated and then determines the DMD. We calculated the stimulated mode numbers and the corresponding mode power portion of diverse waveguides with different sizes on the condition of center launching by 50-µm GI MMF. The waveguide height is fixed to 50 µm, and waveguide widths are in the range of 30 to 80 µm. In our simulation, modes with power that smaller than the 1 percent of the highest mode power are ignored. The relationship between MPD and waveguide width is shown in Fig. 3. The x, y and z axis express the stimulated mode number, the waveguide width and the corresponding power portion, respectively. It can be seen that with the specific launching condition of 50-µm GI MMF, the mode power concentrated on the lower order modes when the waveguide width is small. As the increase of the waveguide width, the stimulated modes increase and the power distribution becomes more uniform. It can be concluded that on the condition of center launching by 50-µm GI MMF, the smaller the waveguide width is, the smaller the DMD is and thus the larger the bandwidth-length product is. Considering the coupling loss between multimode waveguides and MMFs, it is a trade-off between coupling loss and bandwidth. We calculated the coupling loss between waveguides with different widths and 50-µm GI MMF using the Beam propagation method. The waveguide height is fixed to 50 µm and the results are shown in Fig. 4. The coupling loss from MMF to waveguide is less than 0.5 dB when the waveguide width is larger than 30 µm. Therefore, after trading off between the bandwidth and the coupling loss, the multimode waveguide width can be optimized to 30 to 40 µm.
Fig. 3. Mode power distribution of diverse waveguides on the specific condition of 50-µm GI MMF center launching.
In order to verify above calculation results, multimode waveguides with different widths are fabricated using UV-curable polymer materials provided by Panasonic Industry Co., Ltd., and laser direct writing technique is used. Figure 5(a) shows the whole picture of the waveguides on FR4 substrate and the waveguide length is 26 cm. Figure 5(b) illustrates the cross-sectional view of the waveguides. The waveguide height is 50 µm, which is consistent with the simulation. The waveguide widths are set to 40, 50, 60 and 70 µm. In the experiment, the input fiber uses 50-µm GI MMF and the output fiber uses 62.5-µm GI MMF. The measured insertion loss that including the coupling loss is in the range of 0.25 to 0.3 dB/cm.
Fig. 5. (a) Picture of the fabricated waveguide on FR4 substrate; (b) Cross-sectional view of the waveguide.
We measured the transmission performance of fabricated waveguides at the data rate of 10 Gb/s. The schematic of the experimental system is demonstrated in Fig. 6. PRBS signal of 10 Gb/s generated by the bit error ratio tester (BERT) with a proper voltage bias was modulated by the VCSEL. The amplitude modulated optical signal from VCSEL was coupled to the polymer waveguide through 50-µm GI MMF. At the output port, the signal was probed by a 62.5-µm GI MMF. The received optical signal was demodulated by the photodetector (PD) and then the electrical signal was amplified by a radio frequency amplifier (RAF). The eye-diagram is obtained by the oscilloscope (OSC) and the bit error ratio (BER) is measured by feeding the received electrical signal back to BERT.
The measured BER curves and eye-diagrams of polymer waveguides with different widths are shown in Fig. 7. It can be seen that on the specific launching condition of 50-µm GI MMF, error-free transmission can be obtained for all waveguides. With the decrease of the waveguide width, the power budget caused by the insertion of the waveguide decreases and the BER curve shifts to the left, which means the decrease of mode dispersion. Compared with the 40-µm waveguide, 1-dB power penalty is observed by the transmission of 70-µm waveguide due to its larger mode dispersion. Eye-diagrams at the received power of -9 dBm of different waveguides are shown in Fig. 7(b). Waveguide with a core width of 40 um exhibits an open eye-diagram. Under the certain launching condition, we can see that the smaller the waveguide width is, the lager the transmission bandwidth becomes. It can be seen that the experimental results are consistent with the theoretical analysis.
4. Effects of lamination on waveguides performances
To analyze the impact of lamination process on polymer waveguides, multimode waveguides were fabricated using UV-curable epoxy resins (NTT Advanced Technology Corporation, E3129 and E3135) by photolithography method. The optical characteristics and transmission performance before and after laminating process are evaluated. Figure 8(a) demonstrated the structure of the laminated backplane and the optical layer fabricated by the lithography method are inserted into two copper clad laminate (CCL). Considering the lamination quality and waveguide loss simultaneously, the temperature and pressure should be as low as possible while preventing the OPCBs from delaminating and bulging. In our experiment, the lamination parameters are optimized for OPCBs and the highest temperature is decreased to 200 °C. The pressure and temperature profiles that the sample was subjected is illustrated in Fig. 8(b). We can see that the highest temperature of 200 °C and pressure up to 370 PSI were imposed on the polymer waveguides during the laminating process.
Fig. 8. (a) Schematic diagram of optical backplane; (b) Temperature and pressure profiles during laminating process.
Figure. 9(a) is the photography of waveguides before lamination and Fig. 9(b) shows the overview of the optical backplane with CCL covered. Because the facet of the waveguide is cut and polished after lamination, the length reduces comparing with the original waveguide. Waveguide lengths before and after lamination are 7.9 cm and 7.3 cm, respectively. Using the optimized laminating parameters above, the laminated OPCB shows a good configuration without delaminating.
Firstly, we compared the core shape before and after lamination, and the micrographs are demonstrated in Fig. 10(a) and 10(b), respectively. The core shape and size did not change significantly after lamination. The core size remains to be about 45 × 39 µm2.
Secondly, the insertion loss was evaluated and the results are shown in Fig. 11. All 80 channels are considered and measured at 850 nm. Due to the board warp before laminating process, the coupling loss of channels at the edge of the board is high and it causes a higher insertion loss. The average insertion loss including coupling loss of 80 channels that before and after lamination are 0.137 dB/cm and 0.192 dB/cm, respectively. Regardless of the difference in coupling loss between the two measurements, the lamination process with high temperature and high pressure leads to the increase of the insertion loss by about 40%.
Although high-temperature stability of polymer waveguide under condition of solder reflow has been evaluated, the high temperature process in solder reflow lasts only for several minutes [16]. In the lamination process, the high temperature treatment will last for 100 minutes. We can see that the long heat treatment leads to about 40% increase in insertion loss, which demonstrates that the lamination impact is an important factor that need to be considered in reducing the transmission loss. We provided a set of laminating parameters that can be referred. At the same time, the waveguide loss caused by the lamination process are measured quantitatively. The results imply that to reduce the transmission loss, the laminating parameters can be optimized further, more importantly, the long-time stability of polymer materials under the condition of high temperature (> 200 °C) and high pressure for several hours should be improved and studied further.
Finally, to evaluate the transmission performance of the laminated multimode waveguide at 850 nm, both eye-diagrams and BER were measured at the data rate of 10 Gb/s and 25 Gb/s. We applied the external modulation method and the experimental setup is shown in Fig. 12. Distributed Bragg reflector laser diode (DBR-LD) and Mach-Zehnder modulator working at 850 nm were used. The input and output fibers were both 50-µm GI MMFs.
The measured eye-diagrams are shown in Fig. 13. The voltage and time scale are annotated and the received optical power is -12 dBm and -7 dBm at 10 Gb/s and 25 Gb/s, respectively. Although the multimode dispersion limits the bandwidth-length product, due to the short length of the tested waveguide, no obvious degradation in eye-diagrams due to the insertion of the waveguide is observed for both data rates. Then the BER curves are obtained as shown in Fig. 14 and error-free transmission is achieved. The power penalty for BER of 10−9 by the waveguide are 0.5 dB and 1.0 dB at the data rate of 10 Gb/s and 25 Gb/s, respectively.
5. Conclusion
We investigated two matters of concern in multimode optical backplane of mode dispersion and lamination stability. Mode dispersion of multimode waveguide is analyzed theoretically. Meanwhile, multimode polymer waveguides with different core widths in the length of 26 cm were fabricated by laser direct writing technique and the mode dispersion was measured in the sense of BER curves. Lower BER is obtained by the waveguide with smaller core size. Considering the trade-off between bandwidth and coupling loss, waveguide widths in the range of 30 to 40 µm are suggested in multimode optical backplane. Furthermore, waveguides with length of 7.9 cm were fabricated by the lithography method and their performances before and after lamination were compared to evaluate the impact of laminating process on polymer waveguides. No obvious changes on core size and shape are observed after lamination. The high temperature and high pressure during the laminating process causes that the insertion loss increases by about 40%. High-speed transmission was performed on the laminated optical backplane and error free transmission on 25 Gb/s was obtained. The results indicate the feasibility and potential of optical backplane with multimode waveguides for on-board interconnects.
Funding
National Key Research and Development Program of China (2018YFA0209001); National Natural Science Foundation of China (62105183); Natural Science Foundation of Shandong Province (ZR2021QF052).
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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