Optical Circuit Packaging Technology
BY OSAMU MIKAMI
To serve a new seamless telecom and multimedia communications society, the smooth, fast, intelligent transmission and processing of large data files is necessary. Electrical signal transmission using conventional metallic wiring is "bottlenecking" performance improvements. To solve the problem of large data transmission, optical wiring eventually will replace conventional copper; however, there's no direct method for accomplishing this. Cost-effective optoelectronic packaging is needed before the technology transfer can begin.
High-density Packaging R&D Project
Optical fiber communications have exhibited success in the high-end telecom area. Photons can handle high-speed signal transmission, and using optoelectronics also reduces electromagnetic interference (EMI) and electromagnetic compatibility (EMC). However, there are difficulties in handling the last meter of optoelectronic transmission in assembling optical devices. Precision positioning has been a problem, and high costs have also slowed the necessary technological development. Assembly of optical devices, so different from electronics in general, requires precision positioning and high hands-on labor costs. Prohibitively high cost has slowed optoelectronics development in the consumer market.
In Japan, the Association of Super-advanced Electronics Technologies (ASET) recently concluded a 5-year national consortium project, Ultrahigh-density Electronic System Integration (E-SI). This project included collaboration between universities and companies, focusing on high-density 3-D packaging, optoelectronics packaging and circuit design.
Optoelectronics packaging technologies were developed by concentrating on the "last 1-meter" area, including the OE multichip module (OE-MCM), OE-conversion module called the "active interposer" for chip-level optical interconnects between LSIs, OE board and backplane associated with multichannel optical connectors. Self-aligned chip mounting, waveguide film lamination and self-written waveguide formation were studied while considering cost-effective assembly. OE-MCM and other devices were installed on the OE-board and then assembled into the OE-sub-rack and server system cabinet, reaching to more than Tbit/sec transmission rate performance.
Optical Surface Mount Technology
In 1992, O-SMT was proposed in response to expanding optoelectronic assembly needs. Because of automated, low-cost, mass assembly techniques used in SMT, this same concept was applied to optoelectronics assembly.
Figure 1. Optical soldering between optical fibers.
An optoelectronics surface mount device (OE-SMD) was mounted on an optoelectronics printed circuit board (OE-PCB). OE-SMD and OE-PCB have standard interfaces so that effective optical coupling, as well as electrical circuit links between them, can be easily achieved. In optical SMT, the beam direction must be redirected from a horizontal to a vertical plane, then back again, to allow for coupling between optical waveguide and OE-SMD board orientation.
In most cases, a 90° optical path conversion using a micromirror trench with a 45° angle has been studied. However, conventional mirror trenches have the following problems. First, it is difficult to form micromirrors in arbitrary locations on the PCB. Forming a micromirror might risk disconnecting other optical wiring in the vicinity. Second, it is difficult to apply metal coating to mirror faces to obtain higher optical coupling efficiency.
A method using an optical pin has been investigated that enables alignment-free coupling between optical wiring and devices. First, a number of through-holes are processed at positions in the OE-PCB as designated by dry etching and laser ablation. Next, the optical pin with a 45° micromirror at the point of insertion in the through-hole is inserted. Then, these optical pins are fixed with an optical adhesive. Most optoelectronic assembly methods use connecting pins in a conventionally applied method; this is a different optical pin method.
To confirm the feasibility of using optical pins, theoretical studies were done on optical coupling between pins and optical waveguides. A 3-D optical ray tracing method was used for analysis, with a multimode fiber as an optical pin. Its core diameter is 50 µm, and the clad diameter is 125 µm. The optical waveguide has a rectangular cross section of 40 × 40 µm2.
Engineers manufactured through-holes on polymeric waveguides using excimer laser ablation. Core and clad materials of polymeric waveguides were d-PMMA and UV-cured epoxy. Optical pins were inserted into holes on polymeric waveguides and coupling efficiency was measured. Coupling efficiencies of Au film-coated pins were measured at –2.84 and –2.76 dB for transmission and reception. These values improved to –1.0 dB when matching oil was immersed into the hole. The results show that a 90° optical path converter using an optical pin is feasible.
In the next step, the engineers used an optical coupling method using a self-written waveguide. This technique, referred to as optical soldering because of its self-alignment effect, is analogous to metal alloy soldering in electronic board assembly. Optical solder has attracted attention because it is cost effective and offers easier coupling of optical components. The alignment between waveguide and fiber becomes unnecessary, because the waveguide is directly grown from the fiber core. This formation mechanism using a self-written waveguide is based on fundamental principles of photopolymerization.
We developed optical solder with self-written waveguides formed using a green laser beam, a method useful for many polymer waveguides with strong UV absorption. A green-light-curable resin was prepared by using a UV-curable epoxy mixed by the Rhodamine 6G dye. A multimode fiber (MMF) and a vertical cavity surface-emitting laser (VCSEL) were faced with a spacing gap of 1 mm. The optical coupling efficiency between the MMF and the VCSEL was –18dB. When a self-written waveguide, fabricated by introducing the green laser beam into the gap through the MMF, the optical coupling efficiency significantly improved to –1.8 dB. The engineers confirmed the optical solder effect of the self-written waveguide using a green laser.
Optical surface mount technology based on optical pins shows great potential for drastically reducing man-hours in optical device mounting on boards. Optical solder using self-written waveguides promises to reduce alignment failure. Electronic and optical wiring may co-exist on boards in the near future.
The author expresses gratitude to Professor Teiji Uchida of Tokai University. This work was partly performed in the METT's R&D program, supported by NEDO.
For a complete list of references, please contact the author.
OSAMU MIKAMI, professor, may be contacted at the Department of Communications Engineering, School of Information Technology and Electronics, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa, 259-1292 Japan; e-mail: email@example.com.