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Part I: Optical Assembly Manufacturing Evolution
December 31, 1969 |Estimated reading time: 3 minutes
By Rob Suurmann
The substantial growth of network traffic in recent years left OEMs competing to release the next generation of network products to boost the long haul capacity of communication networks. Time-to-market and pricing pressures drove OEMs to outsource optical module assembly to reduce cost and remain competitive. EMS providers, with skills and infrastructure in areas ranging from supply chain management to design for manufacturing (DFM) and test (DFT), integrated optical assembly into mainstream electronics manufacturing and set out to expedite the global release of these products.
While optical component and assembly production has entered mainstream manufacturing, the processes have remained mostly manual. Fiber splicing illustrates the intricacies involved in one aspect of optical module assembly while demonstrating the industry's focus on cost reduction through component design and automation — potentially opening new markets for optical technologies. Part 1 of this article gives an overview of fiber and fusion splicing. Part 2 in the next issue of SMT reviews automated fusion splicing.
General single-mode fiber (SMF) construction consists of three elements. An outer coating, sometimes referred to as buffer or jacket, protects the glass from abrasion and moisture. The outer cladding and glass core, with distinct refractive indexes, guide light through the fiber.
Today, one must address the challenges of optical interconnects. Optical connectors offer solutions for many situations; however, applications exist where performance requirements cannot be achieved with this interconnect technology. For instance, optical power balancing of multiplexing products involves creating specified attenuations at each connection. For this and similar applications, fusion splicing offers many advantages. The losses experienced during fusion splicing generally are lower, more consistent and controllable, in addition to offering reduced size and cost.
Fusion splicing provides a method of fusing fibers together using an electrical arc. To join the fibers, the polymer coating is removed, exposing the underlying glass. The glass fibers are cleaned of residue and cleaved, creating a smooth, flat end-face suitable for fusion splicing. After aligning the fiber cores at the micron level, the splicing process results in a single fused fiber.
Fusion splicing success generally is measured by power loss (dB) and tensile strength (kpsi). Typical minimum requirements for splice strength range from 100 to 200 kpsi, depending on the protection method and reliability requirements. However, splice losses typically span values from tenths to hundredths of a dB, depending on the fiber and splice type. While a loss of this level may seem trivial to a system outfitted with several dBm of optical power, one must consider the overall loss budget. With products carrying up to 100 splices, an unoptimized splicing process can exceed the total budget — decreasing production yields and increasing product cost.
Each step of this process requires careful consideration to ensure that the loss and strength of the splice is not compromised. Frequently, significant attention is given to the fusion splicer, as it drives most of the equipment cost. However, fiber preparation equipment also plays an equally important role. Each stage of fiber preparation requires thorough design to prevent negative impacts on splice loss or strength.
In the fusion splicer, alignment typically is performed through profile alignment systems that use light emitting diodes and video cameras to monitor images of fiber cores during the process. While this is suitable for many applications, some demand greater accuracy and verification. In these cases, an optical source and power meter actively monitor, align and verify the splicing process.
However, selecting a fusion-splicing system is only half the battle. Equally important is how equipment is applied to create a robust process. The dB between two single-mode fibers can occur for several reasons. More specifically, erbium-doped fibers, used to provide optical amplifier gain, possess mode field diameters approximately one half that of standard SMF. This difference in mode fields, which describes each fiber's beam intensity distribution, results in additional losses. The fusion splicer can partially compensate for this by diffusing core dopants into the cladding, but more than a dozen parameters during optimization must be considered. A design of experiment approach can reduce this number to a manageable few key parameters, streamlining the optimization process.
As the fundamental splicing processes develop and become understood and controlled, the next manufacturing step is automation, which will be explored in Part 2 of this article.
Rob Suurmann,process engineer, Optoelec-tronics Group, may be contacted at Celestica Inc., (416) 448-5800, ext. 8034; Fax: (416) 448-4736; E-mail: rsuurman@celestica.com.