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Abstract
We demonstrate the development of optical printed circuit boards (OPCBs) containing multimode polymer waveguides and pluggable optical connectors. The basic optical characteristics of the PCB-embedded waveguides, waveguide connectors, and high-speed performance were comprehensively evaluated. The fabricated OPCB comprises eight electrical layers and one optical layer. Waveguides are terminated at both ends with MT/MPO connectors. The optical channels comprising 10 cm-long waveguides embedded in OPCBs with two connectors show an average insertion loss of 6.42 dB. The resulting coupling loss is 0.77 dB per interface, which is very low and to our knowledge is among the lowest reported to date for waveguides embedded in rigid PCBs. 30 Gbps per channel NRZ data transmission was demonstrated with a measured waveguide bandwidth of 23 GHz × m, which gives a possible data traffic of 720 Gb/s for such 24-channel parallel optical link. Our efforts lay the foundation for the further development of OPCBs with higher performance.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the mainstream technologies of data communication in the future, optical interconnection technologies will be widely used in supercomputers, data centers and other information fields. Various optical technologies have been proposed, including free-space optical interconnects [1,2], silicon photonics [3,4], optical fiber wiring technology [5,6] and planar waveguide technology [7–13]. Among them, many efforts of board-level optical interconnection technology focus on fiber wiring and planar waveguide technology, because they can realize direct low-loss coupling with external optical cables in existing optical networks due to the compatible interface parameters. However, fibers are usually routed on the surface of the boards and cannot be laminated as such into the printed circuit boards (PCBs), which limits their practical applications.
Optical printed circuit boards (OPCBs) which integrate planar optical waveguides into PCBs are regarded as one of the most promising methods owing to their great potential in board-to-board and chip-to-chip optical interconnects. Planar optical waveguides can be divided into single-mode and multimode waveguide technologies according to the number of guided modes. Multimode waveguides combined with on-board optical engines located close to host chip are considered to be a near term solution for chip-to-chip interconnects. Single-mode waveguides are presently limited by their higher material losses inherent to polymers in longer wavelengths, and much tighter requirement on alignments of connectors and devices. For that, they are considered to be a long-term solution [14,15]. According to the materials, planar optical waveguides embedded in OPCBs mainly include polymer waveguides and glass waveguides. In recent years, glass waveguides have emerged as a promising transmission medium for single mode application due to their inherent lower material absorption loss at wavelength regions of 1310 nm and 1550 nm (O- and C-band) compared with polymer waveguides. Furthermore, multi-mode graded-index glass waveguides by applying a thermal ion-exchange process have been fabricated and an OPCB up to four glass layers has been demonstrated [16]. However, in order to eliminate the built-in stress under normal lamination processes with high temperature up to 250 °C, a special glass substrate bonding technology with process temperature of 40-°C temperature needed to be developed [17].
Polymer waveguides have the advantages to be producible with various methods, more controllable and diversified waveguide shapes, and compatibility with traditional PCB process, which promote them to be a leading candidate for optical routing on the PCBs. Some polymer waveguides also exhibit excellent flexibility [18,19]. They can also be employed as interposers to bridge and access numerous photonic platforms, including silicon photonic chips, ion-exchange glass waveguides, and optical fiber arrays [20–22]. Specifically, the UV photolithography [23], laser direct writing [18,24], mosquito [25,26], and nano-imprint lithography [27] methods have been developed and demonstrated to build polymer waveguides with different refractive index distributions and waveguide shapes for OPCB applications. It has been demonstrated that recently developed optical polymer materials can withstand all the standard PCB processing steps including embedded, high-temperature lamination, and solder reflow while maintaining a consistent low optical loss [28–31]. Process compatibility and high-volume production readiness are key concerns to meet commercial viability of a new technology. The optical material must withstand lamination process to become part of the optical electrical stack, and all of the remaining process steps required to complete the product. Moreover, mechanical and optical properties must withstand solder reflow and assembly process conditions. In our previous papers [32,33], we studied OPCB process compliance including solder reflow process. Besides, in order to reduce the absorption loss induced by C-H bond vibration at wavelength of O and C band, polymer waveguide materials with high transparency such as fluorinated polyimide, perfluorinated polyimide, perfluorinated methacrylate, and deuterated polyfluoromethacrylate, have been synthesized [34–36].
As a key enabler for OPCBs, multimode polymer waveguides can be embedded inside or laminated on the surface of PCBs and coupled with multimode fibers (MMF) and vertical-cavity surface-emitting lasers (VCSELs) with a low coupling loss. The large core dimensions relax the alignment tolerances, and therefore, reduce cost for packaging and connection.
In this paper, we report the development of OPCBs containing pluggable edge connectors for multimode polymer waveguides. The fabricated OPCB comprises eight electrical PCB layers and one optical layer. Waveguides are terminated at both ends with MT/MPO connectors. The basic optical characteristics of the embedded waveguides and waveguide connectors including waveguide loss, crosstalk, misalignment tolerance, coupling loss of connectors, and the impact of connector assembly process on the whole optical loss were comprehensively evaluated. Bandwidth studies and demonstration of high-speed data transmission show that the developed OPCB can meet the requirements of high-speed optical interconnects applications.
2. Fabrication of polymer waveguides
The overall waveguide design is shown in Fig. 1(a). Three groups of waveguides are assembled with connectors, and each group contains 8 channels. Four waveguide channels in group 1 and group 2 are crossed respectively and the crossing angle is set to be 90 degrees to minimize crosstalk. In order to ensure the high positional accuracy of the connectors, markers on the mask and the corresponding position features in PCB are added in advance. The remaining board area is filled with non-functional optical core patterns are to ensure the consistent development etching conditions and board planarity.
Fig. 1. (a) The waveguide design. (b) Photograph of the fabricated polymer waveguide. (c) Waveguide end facet and (d) waveguide pitch.
Polymer optical waveguides were fabricated by photoimaging method using optical polymers with upper cladding: TC-447-20, core: PCW250-50, and top cladding: TC-447-60 from Panasonic. The refractive indexes of the core material and the claddings are 1.569 and 1.544, respectively resulting numerical aperture (N.A.) of 0.28. Material loss (as bulk polymer) is 0.05-0.07 dB/cm at 850 nm. We used 150 × 150 mm2 FR-4 with a thickness of 2 mm as the substrate. A 30 µm-thick bottom cladding was laminated on the substrate followed by UV curing and baking process. A 50 µm-thick core layer was laminated and then UV patterned using a mask aligner which was followed by a baking process. The core pattern was developed and dried with nitrogen flow. Finally, the top cladding with a thickness of about 25 µm was laminated and UV cured.
The waveguides were cut by a dicing saw. The waveguide pitch was designed as 250 µm to match the separation of the ribbon fiber as shown in Fig. 1(c). To reduce roughness, the waveguide end facets were polished by a polishing cloth. The core size was measured to be 46 × 50 µm2 as shown in Fig. 1(d).
3. Fabrication and characterization of OPCBs
In addition to the fabrication of waveguides, the main processes of the developed OPCB included: routing slots for MT pins on waveguide sub-cores, fabrication of opening window in electrical PCB for optical connector assembly, integration of optical waveguide sub-cores with electrical PCB layers, solder masking, and assembly of the connector receptacles as shown in Fig. 2. Figure 3(a) and (b) respectively show the designed overall and partial schematic diagrams of the slots on waveguide board, and Fig. 3(c) show the fabricated slots on boards. The process sequence was as follows: After fabrication of electrical PCB layers, the waveguide sub-cores were laminated to the boards. Then solder mask was coated on the surface (Fig. 4(a)). Finally, fiber ferrule receptacles were assembled in the OPCB.
Fig. 3. (a) The overall and (b) partial schematic diagrams of the routed slots for connectors on waveguide board. (c) The fabricated slots on the board.
Fig. 4. (a) OPCB prototype before assembly of fiber ferrule receptacles. (b) Active alignment of reference pins to board. (c) Practical assembled MT pins and ferrule receptacle.
Figure 4(a) shows the OPCB prototype before assembly of fiber ferrule receptacles. Assembly of MT fiber ferrule receptacles onto the waveguide array interfaces was carried out by Reichle & De Massari AG (R&M). In order to eliminate misalignment and optimize the coupling efficiency between fiber cables and waveguide arrays, the reference pins were assembled to board using active alignment assembly routine in a moving stage (Fig. 4(b)). By detecting the optical power at the receiving terminal in real time, the positions of the input and output MT connectors were adjusted to an optimal coupling state. In an optimum position, the housing pins were cured with glue and the MPO ferrule receptacle was fastened with screws. The assembly process was repeated to install all 6 receptacles on the optical circuit board. Fig. 4(c) shows the assembly steps for MT pins and ferrule receptacle. The final size of each ferrule receptacle is about 24.5 mm × 24.5 mm × 12.6 mm. It can be seen that the connector structure has no obvious cracking and is in good consistency with the design structure.
The final fabricated OPCB is shown in Fig. 5. The middle part contains embedded polymer waveguides terminated with MT/MPO connectors, and the remaining board area is used for electrical signal transmission. Prototype constructs of 8 electrical layers and one embedded optical waveguide layer. The PCB material M6G was used in electrical layers. The final board thickness is 3.2 mm, the thickness of each copper layer was 0.5 oz. The size of the finished Optical PCB is 200 × 400 mm2, with a window of 130 × 160 mm2 in the middle for the integration of optical connectors to the waveguides. The final length of the waveguide is 10 cm. The optical transceiver and receiver devices were not installed to the sockets designed in the demonstrator PCB in the first stage as the full process development and demonstration become the primary priority in this stage.
Fig. 5. Fabricated OPCB demonstrator with MPO fiber cable. Three waveguides are illuminated with red light to make the waveguides visible.
4. Optical characteristics
Coupling losses for optical connectors are due to the connector misalignment, rough waveguide end facet, waveguide registration errors or non-uniformity of the cross-section size of the waveguides. Misalignment tolerances at both the input and output side were measured using the cleaved 50-µm MMF and moving stages. The experimental setup is shown in Fig. 6. The index matching oil was applied to the end facets to reduce the coupling loss caused by roughness. The received power was recorded using scanning step of 1 µm. The normalized received power as a function of the misalignment of the input fiber and output fiber is shown in Fig. 7(a) and (b), respectively. For the input side, the 3-dB misalignment tolerances for the horizontal and vertical direction are (-20 um, + 17 um) and (-21 um, + 25 um), respectively, and are (-21 um, + 21 um) and (-27 um, + 22 um), respectively, for those of output side. It can be observed that although the symmetry of the misalignment tolerance in the vertical direction is slightly less compared with horizontal direction, the waveguides have large misalignment tolerance in general.
Fig. 7. Normalized received power as functions of misalignment in both horizontal and vertical directions for (a) input and (b) output fiber.
Based the same experimental setup and input conditions, the inter-channel crosstalk was measured with the scanning step of 10 µm. The measured results are shown in Fig. 8. For scanning the input fiber, the scan of background scattered light coupled into the waveguide ranging from -34 to -36 dB when the light is launched directly into the region between the waveguides and to the level of -60 dB in the center of the adjacent waveguide. For scanning the output fiber, the measured power rapidly attenuate and maintain to the level of -54 dB due to the good confinement of the input light, and then continue to attenuate to the level of -60 dB in the center of the adjacent waveguide. The center alignment of waveguide not only helps to reduce the coupling loss, but also helps to reduce the crosstalk between adjacent waveguides. Overall, the crosstalk in the adjacent waveguide of less than -44 dB meets the requirements of on-board optical interconnect applications.
Fig. 8. Normalized received optical power at the output side as a function of the horizontal offset of (a) input and (b) output fiber.
A connectorized OPCB was characterized of overall optical channel insertion loss and further cut back to separate waveguide loss and connector coupling losses. MPO fiber cables were used to connect the ferrule receptacles directly as the input and output fibers of waveguides as shown in Fig. 9(a). Index matching oil was not used to evaluate the real performance of waveguides in practical use case application. Fig. 9(c) shows the insertion losses of all straight waveguides in group 1 and group 2. The average insertion loss of optical channel i.e. waveguide and two connectors was 6.42 dB and the standard deviation is 0.34 dB. In order to decouple the waveguide loss and coupling loss of MPO-MT connectors, the OPCB sample was cut along the dotted line in Fig. 9(a). The cut sample diagram and the experimental setup using moving stages are shown in Fig. 9(b). The index matching oil was applied to the end facets. The corresponding measurement results are shown in Fig. 9(c). By subtracting the insertion losses obtained by MPO fiber cables from the insertion losses obtained by active alignment with moving stages, the coupling loss of waveguides including both input and output end faces can be obtained, and the results are shown in Fig. 9(d). It can be observed that the average insertion loss captured using the active alignment of fiber is 4.85 dB and the standard deviation is 0.22 dB. The coupling loss induced by one MPO-MT connectors is 0.77 dB with standard deviation of 0.11 dB. The resulting connector coupling loss is very low and to our knowledge is among the lowest reported to date for terminated waveguides embedded in rigid PCBs [37,38]. This low coupling loss meets the loss requirements for most practical OPCB applications. Furthermore, the coupling loss is consistent at each pair of connectors indicating robust and repeatable connector assembly process.
Fig. 9. Insertion loss measurement of (a) MPO fiber cables and (b) active alignment of fiber to MPO with moving stage. (c) Insertion loss optical channel (10-cm long waveguide with two connectors) and (d) coupling loss results for one connector.
In order to study whether the connector assembly process will cause additional insertion loss, such as the loss induced by pressure during assembly, we separated connector receptacles from the measured OPCB board unit along the dotted line in the Fig. 10(a). The insertion loss of the remaining waveguide board was measured repeating the same active alignment coupling with the experiment setup shown in Fig. 10(b). The length of the cut waveguides without connector receptacles was measured to be 6.55 cm. The index matching oil was also applied to the end facets. The obtained insertion losses were compared with the losses of including receptacle structure above, and the measured results are shown in Fig. 10(c). Since the end coupling loss containing the index matching oil is very small, insertion losses were normalized with waveguide lengths in order to make the comparison data more intuitive. It can be observed that the compared insertion losses show good consistency. We can conclude that the assembly process of the connector receptacles has no significant effect on the overall insertion loss and the designed optical coupling interface, and its fabrication process is very reliable.
Fig. 10. Insertion loss measurement (a) with and (b) without connectors. (c) Measured insertion loss results (dB/cm) for both cases.
5. High-speed performance
5.1 Bandwidth studies
The bandwidth of the optical channel i.e. connectorized polymer waveguides on OPCB were estimated using our previously proposed method, direct time-domain measurement based on an optical sampling technique [39]. The experimental setup is shown in Fig. 11(a). Two 1560-nm femtosecond pulse lasers and two Periodically Poled Lithium Niobate crystals were used to generate local laser and signal laser at the wavelength around 780 nm. Their time-domain pulse widths are 100 fs and 800 fs, respectively. The pulse repetition rates of both lasers are around 250 MHz and there is a small difference about 0.3 kHz between them. The dispersive pulses interfered with local pulses at coupler and then detected by a Si balanced photodetector (BPD). A low pass filter (LPF) and a DC block were applied in order to reduce the high-frequency noise and remove the DC bias, respectively. The 3-dB bandwidth of the connectorized waveguides was obtained by subtracting the Fourier transform of envelops of back-to-back (B2B) and waveguide pulses. The measured bandwidth of 10 cm-long straight optical channel is shown in Fig. 11(b). The dotted line shows the averaged bandwidth of terminated waveguides at 780 nm, which is 234 GHz.
Fig. 11. (a) Experiment setup for bandwidth measurement. (b) Measured bandwidth of 10 cm-long optical channel i.e. waveguides with two connectors on OPCB.
5.2 Demonstration of high-speed data transmission
A 30 Gbps high-speed interconnection demonstration was conducted as shown in Fig. 12. A pseudo-random binary electrical sequence with a length cycle of 27-1 was generated using PPG to simulate the actual signal. The generated electrical signal was modulated by an intensity modulator on the 850-nm DBR laser and sent to the connectorized polymer waveguide in the OPCB through an MPO-MMF jumper. At the receiving end, the MPO-MMF jumper was used to send the transmitted optical signal to PD for demodulation into electrical signal, which was then amplified by an RF amplifier and sent to an oscilloscope to obtain eye diagrams or to the bit error rate (BER) equipment for signal quality analysis.
Figure 13(a) shows the 30 Gbps NRZ eye diagrams of B2B link and all connectorized waveguide channels in Group 1 of the OPCB sample. It can be seen that although the amplitude of the eye diagrams after waveguide transmission is relatively lower than that of the B2B eye diagrams, all eye diagrams achieve a significantly open level. Fig. 13(b) shows the error curves of channel 1 and channel 5 in Group 1 and the back-to-back curve. The signals after waveguide transmission with two connectors can achieve error-free transmission and have sufficient power margin. Compared with back-to-back link, the signal after waveguide transmission brings about a power penalty of approximately 1.5 dB. The developed OPCB can meet the requirements of high-speed optical signal transmission applications.
6. Conclusion
We report the development of optical printed circuit boards containing multimode polymer waveguides with pluggable optical MPO edge connectors. The basic optical characteristics of the embedded waveguides, edge-coupled waveguide connectors, and high-speed demonstration of the OPCB were comprehensively evaluated. The optical channels comprising 10 cm-long waveguides embedded in OPCB with two connectors show average insertion loss of 6.42 dB. The resulting coupling loss is 0.77 dB per interface, which is very low and to our knowledge among the lowest reported to date for waveguides embedded in rigid PCBs. High-speed data transmission with data rate of 30 Gbps NRZ per channel was demonstrated with a measured waveguide bandwidth of 23 GHz × m, which gives a possible data traffic of 720 Gb/s for the demonstrated 24-channel parallel optical link.
Funding
National Natural Science Foundation of China (62275150); Open Research Projects of Zhejiang Lab (No. 2021QA0AB01).
Acknowledgment
The authors would like to thank Doctor Blanca Ruiz of Reichle & De-Massari Holding AG for providing the connectorization service and thank Mr. Jinhua Wu of Shanghai Optoweave Technology, Co., Ltd. for his advice and help on OPCB design and fabrication.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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