Ingenious Assembly of Micro-optical Components
New manufacturing methods
BY ELSBETH HEINZELMANN
Specialists in optics and communication are developing innovative interconnection and packaging technologies that make entirely new manufacturing methods for photonic components available to the industry. Photons are quick and not susceptible to interference. They do not interact with one another and are unaffected by external fields. These characteristics make them attractive for optical data transfer. Further applications are emerging from optical sensor systems, whether for controlling and monitoring automated processes in industrial production, in consumer goods, or medical technology. As commercial demand rises, so does pressure to keep cost down. There are increased requirements for gripper and positioning systems for rapid, flexible, and high-precision positioning and assembly of micro-optical components; and also for image processing, alignment control software, and optimized packaging technologies.
Optical Connection - A Sticking Point
One company* that manufactures optical components for different applications faced the challenge of developing new packaging designs. Their 980-nm pump laser diodes, which act as an excitation source for erbium-doped fiber amplifiers, are the basis for trans- and inter-continental optical data transmission networks. While it has been possible to reduce manufacturing costs for the laser chip, assembly and packaging remain the main cost factor for pump lasers. Researchers in the field investigated cost-reducing solutions.
In a first step, thermal characteristics of existing package design were evaluated using finite element methods (FEMs). Optimization of these characteristics is essential. Most pump lasers must be electrically cooled to run stable (at constant optical power and wavelength) and have a long lifetime. Managing without electrical cooling in the modules may be achieved by using materials with suitable thermomechanical characteristics. The FEM modeling led to guidelines for an optimized combination of materials, which improved the thermal characteristics of the pump laser modules.
The focus was on novel methods for fixing optical fibers in the module. The light emitted by the pump laser must be coupled as efficiently as possible into the optical fiber. This coupling between the laser chip and the optical fiber, which requires fiber alignment accuracy of around 0.2 µm, is one of the most critical points in pump laser modules, and is optimized by active alignment. An alternative, potentially lower cost method for aligning and fixing optical fibers with the desired accuracy uses an adhesive bonding process.
Pioneering Approach to Fiber Adhesive Bonding
A modular, semi-automated fiber alignment station was assembled, equipped with a high-precision optical inspection system and incorporated a fixing process using a UV-curing adhesive. A 6-axis positioning system with special grippers aligned the fiber (external diameter 125 µm, core diameter 9 µm) with an accuracy of < 0.2 µm relative to the laser chip. Fixing was achieved using a dispenser to apply a defined quantity of adhesive, which was then cured with UV light. The adhesive shrinks during this curing process, therefore disrupting the desired accuracy of alignment. Prior to fixing with UV light, the fibers are reproducibly and accurately misaligned. ‘As a result, on curing the adhesive, the fibers are displaced by the shrinkage into the optimum laser chip/fiber coupling position (Figure 1). This fixing technology is substantially less costly than conventional methods, as it requires no additional components. Nevertheless, the reliability of this connection technology is an extremely critical factor. Promising initial stability testing (temperature, photosensitivity) of adhesively bonded connections revealed, however, that the researchers had set out along the right path (Figure 2).
Figure 1. Top view of a fiber-coupled pump laser.
Figure 2. Stability of coupling efficiency of a fiber-coupled pump laser during temperature cycling for one week. The arrangement exhibits no degradation.
Although this adhesive bonding process works effectively in a laboratory setting, further development is required before industrial use. Problems with outgassing after application of the adhesive, which may result in residues on the optical system, is one of the issues that needs to be addressed.
Multifibers Seek Connection
One company** wanted to move on from single fiber to multifiber connector systems up to 12 fibers per connection. They were interested in technology for producing optical connection between the fibers of the connector and active components, such as light sources or detectors. In a first step towards this goal, multifiber connector systems with fiber collimator platform should be realized (Figures 3a and b).
Figure 3a. Prototype of multifiber connector system. Figure 3b. Multifiber connector system with collimator platform, consisting of V-groove and micro-lens array. Spacing between the lenses is 250 µm.
The fiber collimator platform consists of a fiber support (V-grooves) and a lens. The highly divergent beam needs to be converted from the optical fiber into a collimated beam. In this way, different optical components may be positioned behind the lens without requiring high-precision positioning, reducing manufacturing costs. The lack of available space in a compact multi-fiber connector requires the use of micro-lenses. After a thorough analysis, researchers identified a suitable micro-lens array, which fits in a compact housing and may be mass-produced. The components were handled with micro-manipulators which can pick up, transport, and place the minuscule components to a high degree of accuracy (Figure 4). The necessary high-precision fiber support required the development of special silicon substrates with structures into which the micro-lenses and optical fibers fit perfectly. Adhesive bonding technology was used to fix the lenses in place. The positioning and fixing method is based on a vision system and is less complex than methods in which the fibers have to be actively aligned.
Figure 4. Lens array held by a microgripper.
Prototypes of the multifiber connector system (Figure 3b), including the fiber collimator platform consisting of the optical coupling element (the micro-lenses) and the silicon substrate with alignment structures, have already been produced (Figure 5). Once these have been optically characterized, they will undergo the necessary reliability testing.
Figure 5. Silicon substrate with alignment structures for the glass fibers, on which a lens array has been aligned and fixed. Compare side view in Figure 3a.
Future of Photonics
The time is ripe for new manufacturing technologies for photonic components, and well-informed observers of the sector are talking about a massive boom in micro-optoelectronic devices in the coming years. However, key issues remain to be resolved, such as heat dissipation; the extremely high level of positioning accuracy of optical components; compatibility between different material systems; and automation. There is a clear trend towards uncooled modules, cost reduction by passive alignment of components, and higher levels of integration; for example, connectors that directly integrate photodiodes or lasers (“smart connectors”). Experts predict that although the demand for photonic devices based on micro-optical components is growing, they will only be able to achieve their full potential for the mass market if costs can be reduced. Assembly and packaging account for 60 to 80% of the costs for photonic and integrated optical devices. Innovative connection solutions, in combination with automation, are intended to eliminate the assembly and packaging bottleneck, and bring acceptance of photonic devices.
ELSBETH HEINZELMANN is a science and technology journalist. For technical questions, Dr. Christian Bosshard, head of optics and packaging, may be contacted at CSEM SA, Untere, Gründlistrasse 1, CH-6055 Alpnach-Dorf; +41/41 672 7528; E-mail: email@example.com.