An easier connection for multiple optical wiring layers

As the speed of optical signal processing and transmission by information technology equipment has increased, metal wiring has become a bottleneck for high-capacity data transfer systems and large parallel-processing computer systems. Its drawbacks are large crosstalk between adjacent lines, high signal reflection, low-density mounting, and high electricity consumption. By contrast, optical wiring has attracted much attention as a possible board-to-board interconnection,¹ and could solve these problems.

The structure of optoelectronic printed wiring boards (OE-PWBs) has progressed from a single to multiple optical wiring layers,² but it is difficult to connect all the optical wirings at once. To make this easier, we have proposed a new ‘optical socket’ that makes it simple to align the interconnections. The socket consists of optical waveguide plugs (OWPs) and a microhole array (MHA). The MHA is formed as an array of tapered holes, and the OWPs can easily be plugged into it (see Figure 1). Optical waveguide wirings are buried in the OE-PWBs, and optoelectronic surface mount devices (OE-SMDs), on which lasers and photo detectors are mounted, are passively aligned and connected to the optical waveguide wirings.

Figure 1. Schematic of an optical socket interconnection using a microhole array (MHA) and optical waveguide plugs (OWPs). OE-PWD: Optoelectronic printed wiring board. OE-SMD: Optoelectronic surface-mount device. PD: Photodetector.

Figure 2. Schematic of the mask transfer method for the MHA fabrication. Injecting resin into a gap and irradiating UV light for a few seconds through a photomask (a). Once the uncured resin is removed by ethanol, the MHA structure is revealed (b).

We use a mask transfer method to form the OWPs optically at the face of the optical waveguide channels of the OE-PWB. This method involves contact exposure of the UV-curable resin to UV light through a photomask.³ To form the OWPs, we mask all of the substrate except for four circular windows, illuminate with UV, and remove the uncured regions to reveal cylindrical prongs where the windows were. We use the same method to form the MHA, but as an individual optical element (see Figure 2), masking four circular areas that remain uncured so that treating with ethanol reveals microholes. By careful alignment of the optical waveguide wirings with the unmasked windows, we can easily and precisely align the OWPs with the optical waveguide wirings. After that, we can interconnect both OE-PWBs by inserting the OWPs to the MHA (see Figure 3).

The photomask for the MHA was four circular opaque patterns 60μm in diameter, and two circular opaque patterns 700μm in diameter, arranged in a rectangular window on the MHA substrate. We injected UV-curable acrylic resin into a gap between the photomask and a slide glass. The 0.54mm thickness of the MHA was determined by the gap. After irradiating with 365nm UV light, we removed uncured regions with ethanol to reveal precisely positioned tapered microholes.⁴ Our MHA had a rectangular shape of 6.4×2.5mm, a thickness of 0.54mm, and a diameter, pitch and microhole taper angle of 60μm, 0.25mm, and 1.06^°, respectively (see Figure 4). We could control the microhole taper angle between 0.4 and 1.6^° by changing the irradiation power of the UV light.

We formed cylindrical 60μm OWPs with pitch 0.25mm and controlled the OWP height to be 0.43mm by adjusting the gap between the faces of the OE-PWB and the photomask. It would be possible to expand the technique to fabricate an optical interconnection of multiple optical layers and optical waveguide channels simply by using an array of circle photomasks.

Figure 3. Optical microscope photograph of the fabricated OWPs at the face of the OE-PWB.

Figure 4. Optical microscope photograph of the fabricated MHA for the optical socket.

Figure 5. Optical microscope photograph of the demonstrated plug-in alignment with the optical socket.

We positioned and mounted the MHA plate onto the face of the OE-PWB under an optical microscope. We used four GI50/125 optical fibers as the optical waveguide wirings. We used an 850nm laser light source to measure the insertion loss of the optical socket interconnection. We connected a pair of OE-PWBs with four optical waveguide wirings (see Figure 5), and evaluated the gap by the difference between the thickness of the MHA and the height of the OWPs. We evaluated the insertion loss as 1.5dB at a gap of 0.03mm and 4.8dB at a gap of 0.11mm on average. This is the same as measurements of loss for a single optical waveguide wiring. In neither case did we observe crosstalk. From simulation results, we conclude that if the gap is less than 0.008mm, insertion loss can be maintained at less than 0.5dB.

In summary, we have developed a multichannel optical interconnection using a novel optical socket that can be easily connected and aligned. Our results show that our socket is one of the best solutions available for OE-PWB optical interconnections. We are now working to develop a mechanical structure like a cylindrical pin for the optical socket. It will be formed simultaneously with the OWPs for an easy, stable, and cost-effective board-to-board optical interconnection.