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Happy’s Tech Talk #36: The LEGO Principle of Optical Assembly

Dr. Steven Case of Avanti Optics Corporation once advocated a new approach to photonic assembly, based on the LEGO principle. He compared today’s erbium-doped fiber amplifier (EDFA) technology to the crude point-to-point wiring of early model circuit boards, which gave way to modern circuit boards that could be considered prefabricated interconnects.

Optoelectronics’ future solutions may involve silicon-based optical micro-benches with component “onsertion” (placement from above) and ideal kinematic mount. Case provided an example of such a component mount: Three V-grooves at 120° angles in the base, matched by three ball bearings on the component mount. The prototype would be a fiber-to-fiber coupler (without fusion splicing, i.e., space gap).

LEGOs are great toys. I bought them for my sons. When I was traveling in Europe on business, I would bring back a box of advanced technics or the pneumatic cylinders from France or Germany because they weren’t available in the U.S. The LEGOs are now in storage, waiting for the next generation. They are never really obsolete. I have been adding some of the more recent LEGOs, like Mindstorm robots and their microprocessors. They endure because of the modularity of their three-dimensional structure; all the pieces fit together in any dimension. That applies even if the pieces are 30 years old or brand new.

The LEGO Principle
Joe Fjelstad has also used the LEGO principle in his I-Connect007 columns to emphasize the flexibility of both components and high-density boards¹. Order and predictability are leading factors in both MEMS and photonic assembly.

Case’s idea was to precisely align each optical element, such as a fiber, a lens, a filter, a modulator, etc., and secure them to a standardized platform. The component platform is inserted into the optical printed circuit board (OCB), where the kinematic mounts are already positioned. Figure 1 shows the kinematic and component mounts and their assembled keying alignment.

The mounts in this new assembly system allow:

1. Submicron compensation for optical component variations, such as fiber core and lens focal positions.
2. A rapid method of securing mounts to the OCB.
3. Surface mounting or onsertion to use existing automated assembly equipment.

This approach allows the infrastructure built up by the circuit board assembly industry to be applied to photonic assembly (Figure 2). Karl Dietz once estimated the optoelectronic component market to be $59 billion and the optoelectronic equipment market at $150 billion annually.^2,3

Why will optoelectronics grow so much? Photons are to optical conduits and waveguides what electrons are to copper traces (wires), only they move 10,000 times faster. The current electrical protocol, Gigabit Ethernet, is a 1 Gbit/sec data stream at about 1.4 GHz. This already presents problems for signal attenuation (losses) and noise (EMI, crosstalk, ground loops, power supply noise). While a high-speed electronic signal can be multiplexed to carry many signals, an optical signal multiplexed on a single laser beam can be mixed without interference with other frequencies of lasers to carry 1000 times more information, and without the losses or noise of electronic signals.

Currently, first generation -1l-optical systems are being installed. These carry thousands of channels of the 1 Gb/s Ethernets (terabit routers). Next phase, -2l, will be 10Gb/s and then -4l (40 Gb/s). The upper limit may be -2048l systems optronics (photonics), which has four levels of packaging:

Level 0: Optical Device Fabrication

• Lens, filters, fibers, waveguides, chips, VCSELs, MEMs
• Materials and semiconductor processing, deposition, patterning

Level 1: Component Assembly (Integrated O-E Devices)

• Transmitters, receivers, lasers (two or more devices)
• Sub-micron positioning, active alignment, bonding, welding

Level 2: Module and Board Assembly and Test

• Optical amplifiers, mixed O-E, line cards, switches, ADM
• Fiber splicing and management, connectorization, solder attach
• Optical power, eye/mask test, BER, SONET frame

Level 3: System Assembly and Test

• Optical backplanes, box-level test, network traffic

The first optical backplanes are already in commercial use. Figure 3 shows the projected growth of the optical circuit board market. This projection was done by ElectroniCast of San Mateo, California.

References

1. Flexible Circuit Technology, 4th Edition, by Joe Fjelstad.
2. “Tech Talk #215: Optical Interconnects” by Karl Dietz, CircuiTree Magazine, March 2006.
3. “Waveguide and packaging technology for optical backplanes and hybrid electrical-optical circuit boards,” by Henning Schröder, et all, SPIE, February 2006.
    

Happy Holden has worked in printed circuit technology since 1970 with Hewlett-Packard, NanYa Westwood, Merix, Foxconn, and Gentex. He is currently a contributing technical editor with I-Connect007, and the author of Automation and Advanced Procedures in PCB Fabrication, and 24 Essential Skills for Engineers. 

This column originally appeared in the December 2024 issue of PCB007 Magazine.