Bandwidth demand of the arriving information age has driven a greater need
for the supply of optoelectronic components. Implementation of optical
amplification using EDFAs and of DWDM has significantly increased the demand
for qualified optoelectronic component suppliers. The transformation of
bandwidth availability to a commodity is driving component price
compression. This requires higher process yields from wafer foundaries all
the way to packaging optical alignment and is rewarding suppliers with
higher levels of process automation.
Optical coupling of optoelectronic devices to optical fiber remains the
process that distinguishes the optoelectronics manufacturing industry from
the electronics industry. Precision placement and maintenance of optical
coupling through Bellcore and Telcordia requirements has these general
dimensional placement tolerances:
Multimode fiber MMF: 5 micrometer
Singlemode fiber SMF: 0.25 micrometer
Polarization maintaining fiber PMF: 0.25 micrometer and 1 degree rotation
Methods of optical fiber attachment determine the alignment tolerance
achieved. UV cured epoxy allows snap cure of components that can be
mechanically strengthened using thermally cured epoxies. While highly
filled epoxies are available to lower thermal expansion to 30 ppm/oC,
thermal related dimensional shift severely limits product yields for SMF
integration. Additionally, telecom performance requirements generally
prohibits their use inside hermetically sealed packages.
Fixing alignment positions using eutectic solder attachment suffers from
solder solidification position shift precluding it from high yields in SMF
integration. Solder attachment also requires metallization of optical
fibers, lenses, filters and other free standing optics or their holders.
The only present method to achieve optical alignment with submicron
tolerances over the lifetime of optoelectronic components is by laser weld
attachment followed by post-weld shift correction. This shift correction is
made by bending the optical train or by a laser hammering process where an
external force is applied during optical tracking optimization. Laser
hammering has been commercially successful for axial optical train
configurations but not for planar optical train configurations which
generally use metal clips to hold metallized optical fiber soldered into a
metal ferrule. Successful post-weld shift correction relies upon
predictable mechanical shifts in the optical train geometries. Commercial
success in laser welding alignment requires a high degree of process
automation and generally lays beyond the mere capital reach of small players
in the market.
Optical submount assemblies of thermal electrical coolers (TECs), optical
benches, thermistors, back facet diodes, filters, and free standing optics
require generally either solder or epoxy attachment. Solder attachment
without flux or no clean flux processes are commonly sought to reduce
organic contamination and consequent deterioration of laser die facets.
Package housing cleaning, including plasma pre-wire bonding cleaning, is
generally not performed once optoelectronic die have been integrated into
the assembly. However, there are examples of successful solvent cleaning of
packages with integrated laser and photodiode dies when appropriate process
controls are used.
Required optical coupling efficiencies are application dependent.
Expectations for practical optical coupling efficiencies range from 60% with
lensed tip SMF to 10-20% for freestanding optical trains. Antireflection AR
coatings on laser die, collimating lenses, focusing lenses and optical fiber
tip surfaces can improve optical coupling efficiencies from under 20% to
Hermetic seal requirements for telecom products are met either by solder
attachment of metal housing to a metallized optical fiber or by using a
metal housing with a preintegrated optical window. Product design choices
whether to perform all optical alignments outside of the package prior to
integration into the housing, or to perform the alignment with optical
submounts inside a packaging housing can significantly affect processing
yield and cost.
Channel waveguide integration with optical fiber arrays presents optical
alignment challenges solved with UV cured epoxy, thermal cured epoxy and
index matching epoxy attachment of optical fibers prepositioned in v-groove
mounts polished prior to attachment.
As the bandwidth demand increases, the data transmission rate is increasing
from 2.5 Gbps per channel (OC-48) to 10Gbps (OC-192) and very soon to 40Gbps
(OC-768). New packaging configurations for electrical design and optical
integration will be embraced by the telecom community for the higher data
rates. To realize these high bandwidths will require accounting for
impedance matching circuitry, interface matching, and understanding of
parasitics. Three dimensional microwave field simulation, circuit analysis
and electromagnetic radiation modeling will be necessary tools in
optoelectronic packaging design in order to realize device bandwidth
capability at these higher data rates. Highly integrated electronics may
soon become the norm for many components such as transmitters, laser
drivers, modulators, amplifiers, receivers, mux, demux, clock recovery
circuitry and switches. Products of higher levels of integration in the
future from integrated reconfigurable add/drop multiplexers to integrated
optical switches and to transceivers will require more challenging packaging
Telecom product qualification requirements of long time storage testing,
2000 hours, and accelerated aging testing, 5000 hours, has led to extreme
market demand of time compression for product development and qualification
and for qualification of production lines. Risk sharing to commence
manufacturing at 1000 hours is common. This qualification time constraint
has led to a resistance to explore design changes that could impact
manufacturing yield, automation and cost due to immediate product delivery
The future will reward manufacturers that embrace higher levels of
automation to meet production capacity demands and to compete with component
price compression. Also rewards will be for those with products of high
levels of integration that address bandwidth performance demands for higher
data transmission rates.
Edward J. Palen, Ph.D., P.E. was Director of Advanced Process Development at Teledyne Optoelectronics,|
a provider of high volume optoelectronic outsource manufacturing and optoelectronic packaging design services.
Teledyne Optoelectronics provides all of the services described in this article to their customers.
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