1. Introduction

In the domain of very-high-bandwidth short-range communications, light-based waveguides have consistently demonstrated higher placement density, more packaging flexibility, and superior alignment reliability than their electrical counterparts [1-3]. Two-dimensional optics-embedded printed circuit boards (PCBs) [4, 5], or hybrid electro-optical PCBs [6, 7] in which the optical and electrical layers are integrated monolithically, has both been researched and presented as their fabrication can take advantage of existing PCB manufacturing processes. Recently, polymer-based waveguides embedded into PCBs were shown to provide 120 Gb/s capabilities in PCB-to-PCB transmission [8].

Despite the advantages of 2D light-based waveguides, they are limited by substrate size in its degree of integration. Therefore, a 3D optical interconnect structure implemented between stacked optical waveguide layers is expected to significantly enhance functionality, lower power requirement, shorten interconnect length, and lower the cost of production. Several solutions have been proposed to facilitate the transverse-to-longitudinal propagation transfer, including (i) a 45° planar mirror/inclined plane/fused vertical coupler [9] for a 90°-deflection in the beam; (ii) a multimode interference (MMI) coupler [10]; and (iii) angled polymer waveguides [11]. (i) was found to be accompanied with large power loss and signal attenuation, and are difficult in fabrication by molding. (ii) was found to be effective in vertical coupling between waveguide layers. However, its coupling efficiency underperforms compared to (iii) and its fabrication process, consisting of spin coating, thermal curing, and dry etching, is far more complex. The angled polymeric interconnect, as proposed, was found to have a lower loss when used in complex 3D optical integrated circuits. Although loss at the vertical-bend structure is large, the fabrication method was found to be irrelevant to this loss, whether it be the relatively simple soft lithography process, or shadow mask reactive ion etching (RIE) and shadow photolithography, where precise control of mask displacement and process gas flow rate was demanded.

As an effort to solve the aforementioned problems, a novel structure for the vertical optical interconnect coupling between different layers on PCB is proposed in this paper. Its characteristics have been thoroughly researched and presented as well as a detailed fabrication process.

2. General layout

After experimenting a variety of waveguide curvatures capable of crossover between separately layered waveguides, we found that an S-shaped curve waveguide as shown in Fig. 1 could be adopted for gradient vertical optical coupling. The center line of the S-shaped curve waveguide consists of two identical but centro-symmetric arcs with its center point being located at the curve inflexion point. It is furthermore feasible to implement 3D molding by soft lithography, by which the transfer of 3D features onto polymers can be simplified to a mold-printing process while maintaining a surface resolution of as low as several tens of nanometers. As illustrated in Fig. 1, optical signals can be transmitted from the second layer to the fourth layer or vice versa through the red S-shaped waveguide I, and between the third and fifth layers through waveguide II. In-plane connections denoted by green straight waveguides in Fig. 1(left) and (right) provide intra-layer optical interconnection, while the S-shaped waveguide can provide vertical optical coupling between different layers.

 

Fig. 1. Coupling of interconnects on two separated layers by an S-shaped waveguide.

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3. Simulations

3.1 Parameter analysis

Given a particular molding procedure, the quality of transmission relies heavily on the spatial shape of the coupling waveguides, or the curvature function of the substrate. Figure 2 displays the cross-section of the coupling waveguide. The horizontal propagation distance L needs to be sufficiently long to maintain an upward or downward propagation of light with minimal loss, yet be restricted to a certain range for the purpose of simplified integration and compact lamination. On the other hand, a higher vertical stride-over height H enables the S-shaped waveguide to propagate the beam between two distant layers, allowing more stacked layers be incorporated to a substrate on PCB, realizing a high spatial integration level. The goal of our search has been for an optimized design with a maximized connection height (H) at a shortest possible horizontal propagation distance (L). As refractive index differences between widely-used core and cladding remains to be 1.3~2% at wavelength of 850nm (e.g. indices of refraction 1.47/1.50 from Chem Optics, or 1.50/1.52 from DSM Desotech), a search for spatial line function was conducted in an effort to minimize power leakage.

 

Fig. 2. Schematic of the vertical coupling optical interconnection circuit on PCB

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3.2 Stride-over adjacent layers

We investigated the relationship between propagation loss and L while maintaining the value of H where the S-shaped waveguide can effectively guide light between adjacent layers with an arbitrary interval of 50 μm. We utilized wide-angle BPM to simulate the efficiency of S-shaped curve waveguide by varying L while the output from the waveguide was monitored. Parameters used in the simulation are listed in Table 1.

Table 1. Specifications of optical components used in BPM simulations

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As shown in Fig. 3(left), propagation loss as a function of horizontal propagation distance L was calculated for four selected cross sections of 30×30μm², 50×50μm², 100×100μm², and 200×200μm², with distance L ranging from 0 to 7 mm for weak-confinement (core 1.52, cladding 1.50). It is found propagation loss decreases at a larger slope from 5dB/cm to 0.2dB/cm with L increasing from 1.7mm to 2.6mm for 100×100μm² light guides, and when L continues to increase from 2.6mm to 5.5mm, propagation loss shows approximate linearly, slowly decrease. The other three different cross-section light guides also display similar characteristic curves, except when L increasing from 4.8mm to 6mm, propagation loss almost remain the same for 200×200μm² light guides. The phenomenon is partly explained by Fig. 3(right), which shows part of leaking waves in S-shaped curve waveguide at 1.5mm, 2.1mm, and 3mm propagation distance. In summary, at a given vertical stride-over height H, propagation loss decreases rapidly with increasing L until it reaches threshold L₀ when loss equals 1dB/cm; then propagation loss will decrease very little or remain at a constant low even if L continues to increase. To satisfy the need of simplified integration when using the S-shaped curve waveguide between adjacent layers, we can set L₀ as the optimal horizontal propagation distance.

 

Fig. 3. (left) Propagation loss vs. L for a given H; (right)A portion of BPM simulation for propagation between adjacent layers: leaking waves in S-shaped waveguide with a 50×50μm² cross-section and L= (a)1.5mm, (b)2.1mm, (c) 3mm

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3.3 Stride-over multilayer

In addition, L₀, as determined in the above analysis, needs to increase with increased H in order to maintain low-loss propagation. Therefore, when designing interlayer-coupling optical interconnection circuits, it would be preferable to determine the optimum H/L value of the S-shaped waveguide to achieve low propagation loss in minimal space. Assuming that the distance offset between adjacent PCB layers is 50 μm and propagation loss limit 1dB, then for a 30×30μm² waveguide as exampled in Fig. 4, as the number of interlayers increase, i.e. a higher vertical, L₀ increases and demonstrates an approximately linear increase. The three other S-shaped curved waveguides with different cross sections have a similar H/L relationship as the sample 30×30μm² waveguide. The general trend of H as a function of L is described by two black dotted lines in Fig. 4, which illustrates that for the condition of H/L≈0.128, the S-shaped waveguide can provide optical signal transmission vertically up to six interlayers while maintaining relatively low propagation loss.

 

Fig. 4. BPM simulation result of S-shaped waveguide striding over multiple layers, here the maximum is 6 layers.

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4. Fabrication

By contrast, soft-lithography [12, 13], in which a flexible silicone elastomer stamp is used to print micro-structures with a high surface resolution, can print either 2D or 3D polymer waveguides onto substrates in a manner that bypasses the aforementioned difficulties, being able to form 3D structures for layer-to-layer connections owing to its excellent surface conformity as a flexible mold during multilayered stacked printing. Recent work in soft lithography demonstrated the fabrication of a monolithic integrated coupling-waveguide combination, including a polymer 45° reflector (TIR) and a beam duct to couple light beams between a vertical transmitter and a vertical receiver on PCB [14]. Furthermore, the printing of in-plane connections, such as cross-over and branching nodes, have also been demonstrated [15]. Our results illustrate how optical waveguides on separated layers can be joined with one another with the use of a scalable printing method to yield complex 3D integrated interconnection systems. The capabilities of this process are demonstrated by fabricating high-performance 3D polymer waveguides for coupling multilayer stacks of waveguides on PCB, by printing with a series of aligned elastomeric stamps.

Figure 5 illustrates representative steps for fabricating the aforementioned layer-to-layer coupling in detail. First, a series of straight waveguides were micro-engraved on a brass substrate using a computer controlled engraver with a step size of 0.8μm. The master was then surface-polished and a silicone elastomer (Sylgard® 184, Dow Corning) was applied in a vacuum container to eliminate possible bubbles during the process. The elastomer was cured at 80°C for two hours to form a low-shrinkage negative mold of the master. The elastomer negative mold is partially UV transparent, allowing it to be capable of pattern printing with UV polymers. In our fabrication process, a UV-curable polyurethane acrylate prepolymer (DeSolite® optical fiber secondary coating, DSM Desotech) with a refractive index of 1.52 was poured into the negative, and subsequent UV exposure cured the polymer. The finished structure may then be detached from the elastomer negative mold to form the desired straight waveguide structure, a precise replica of the original micro-machined positive master. The finished straight waveguide arrays can be directly placed on PCB substrate as one side of vertical coupling circuit and can also be used as in-plane circuits, as shown in Fig. 5(A). Triangular positioning keys together with straight waveguides were fabricated to ensure overlay registration during the following fabrication process. Fig. 5(B) shows how the S-shaped sloping substrate, which has the same structure parameters as the corresponding S-curved waveguide, was molded with another elastomer negative stamp, which is directly mounted to the same substrate as the straight waveguide arrays and aligned with the positioning keys. A UV-curable polyurethane acrylate prepolymer with a refractive index of 1.50 (cladding) [other than 1.52] was injected into the negative to form a curved cladding. After UV curing, the elastomer negative mold can be detached from the substrate and the desired S-curved structure, shown in purple in Fig. 5(B), is completed. Finally, as shown in Fig. 5(C), the critical S-curved waveguide arrays were fabricated with a third elastomeric negative stamp; by filling the negative molds with a UV-curable polyurethane acrylate prepolymer with a refractive index of 1.52, the molded curved waveguide arrays connect seamlessly to the straight waveguide arrays of the same polymer on the bottom layer with the help of the positioning keys. Because the S-shaped sloping surface structure and straight waveguides precisely matched the molds, only S-curved waveguide arrays were formed and, with their end surfaced precisely merged with those of the straight waveguide arrays, further reduced optical loss caused by the alignment error. The hardened polymer final product depicted in Fig. 5(C) was subsequently cladded with a coating with a refractive index of 1.50. Examples of prototypes for the above three procedures were displayed in Fig. 5(D).

 

Fig. 5. Fabrication procedures for 3D vertical coupling optical interconnect circuits. For clarity, the thicknesses were not drawn to scale and only waveguide cores are shown. A) Leads of waveguides on bottom layers were molded using a soft stamp with alignment marks, B) A slope is formed by the second stamp, using as a substrate for 3D coupling waveguide; C) Finally layer-to-layer coupling waveguide is printed on the slope by a soft mold; D) prototypes for each procedure..

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Over nine sets of these vertical coupling ports with varying H/L ratios were fabricated for two most popularly used dimensions, 50×50μm² and 100×100μm², and an example (cross-section=100×100μm², H=0.45mm for three layer, and L=4.5mm) in operation is presented in Fig. 6. Insertion loss measurement was conducted with a fiber-coupled 850nm LD source (Hlaser Opto Co.) and a power meter (Thorlab PM120). An index matching oil (n=1.48) was used for LD fiber and polymer prototype coupling to reduce insertion loss down to 0.02-0.03dB. The experimentally measured propagation losses for the two cross sections were compared with calculated results, as shown in Fig. 7, the measured general trend of propagation loss for both cross sections agrees with theoretical predictions reasonably well, except some observed excessive leakages, which may be attributed to surface roughness in some of locations where the chemical polishing of engraved master plate was not satisfactorily made. For fourteen fabricated samples same as shown in Fig. 6, it is found that the insertion loss is within 14% of an averaged value, showing a reasonably good repeatability of the soft-lithography process.

 

Fig. 6. working condition of one S-shaped waveguide

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Fig. 7. Part of comparison between experimentally measured (circle) and simulation (line) results

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5. Conclusion

Printed multilayer coupling structures provides novel approaches to integrated 3D electro-optical systems that could be implemented in various fields of applications, including dense multilayered chip-to-chip or board-to-board applications as suggested by the interconnect systems reported here. It can also pave the way for microfluidic devices with integrated electronics, chemical and biological sensor systems that incorporate varying functions on differing layers, or advanced optical computing of the next generation. Furthermore, the compatibility of this approach with a low-temperature, lightweight plastic substrate may create additional opportunities for devices featuring dense polymer-based photonic circuits.