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Optical fiber pigtails integration in co-package design

By Alexander Janta-Polczynski [IBM Canada]

T he optical fibers that exit a silicon photonic device length, there can be benefits for high-volume manufacturing

– also referred to as pigtails that are bundles or
ribbons of optical fibers – need appropriate fastening (HVM) to having certain lengths. The sets of parameters
for the manufacturing approach includes how to grab and
within the package to protect the light-coupling interface. A assemble the component into the package, with either short
strain relief element must be designed to protect the light- pigtails, or small fiber rolls for longer pigtails. In automated
coupling interface from deformations that can occur from manufacturing as proposed by IBM, the ribbon array is
various stresses, such as from downstream assembly processes, grabbed by a robotic handling head of a high-throughput
from the thermal expansion cycles experienced during device pick-and-place tool [3,4]. Two attachment points between the
lifetime, and from external forces applied to the pigtail. We picking head and the fiber ribbon are desired: one close to
will discuss here a parametric model developed to evaluate the fiber array that will be attached to the photonic die, and
latching configurations that use fiber bending as a strategy the other to support the weight of the ribbons and connector
to accommodate thermal and mechanical strain of the fiber termination [5]. This arrangement also enables the control of
ribbons. This model can help the design of optimal geometries the shape of the fiber ribbon while it is being anchored inside
to control within the limits the stress at both anchoring points: the module. The fibers remaining inside the package need to
the photonics interconnect and the ribbon strain relief area. be properly laid out to ease the manufacturability and increase
The work from [1] is resumed here, where we demonstrated the the robustness of the integration. In particular, proper layout is
benefits of using a free-fiber length of 8mm or more to incorporate needed to support the environmental and stress conditions that
the bends with low stress on our selected layouts. Also, we showed optoelectronics and photonics modules with fiber pigtails must
that an exit angle of a few degrees of the fiber ribbon provides withstand, such as: Telcordia, UPC, JEDEC, OIF, COBO, ITU,
advantages in controlling the fiber bending and buckling direction IEC, to name a few. Furthermore, this approach is suitable for
to reduce the fiber pistoning. (Pistoning refers to the fiber use in HVM environments where automated high-throughput
butting motion and forces on the photonic coupler.) Finally, the pick-and-place tools must handle the pigtails efficiently and
use of boots was also explored because they provide significant allow for scalability of fiber count. Co-package modules that
benefits with respect to controlling the radius of curvature and use multi-ribbon assemblies in close proximity maximize the
stress for the ribbon side pull stress tests; also, a novel boot optical bandwidth at the PIC interface, something that only
morphology study showed up to a 33% stress reduction. These direct-attach fiber pigtails provide [6].
results provide guidance for optimizing the layout of optical fibers When integrating optical pigtails within a package, the
inside a package and how to manage the strain on the photonic main challenge is to create a strain-relieving structure that
interconnect by leveraging fiber bending designs. protects the optical interface from the external loads applied
High-bandwidth silicon photonic modules require a multi- on the ribbon while accommodating the strain from the high
fiber interface, and fiber optic ribbons that exit the co-packaged thermal expansion coefficient mismatch between the optical
module need to be properly secured, especially to protect the fiber anchoring points and the package structure—throughout
photonic die’s connection where light coupling occurs. Several the temperature range of the application. The intent is to use
techniques can be used to secure the optical fibers to the fiber bending to accommodate the deformation (thermal and
photonic chip with adhesive. One cost-effective solution for mechanical) and control the stress at both anchoring points.
co-packaged components is to use automated self-alignment The deformations are either thermally-induced through
to assemble pigtails to V/U grooves and secure single-mode temperature excursions, or mechanical, such as pulling on
fibers to silicon photonic integrated circuits (PICs) [2]. The the ribbon. Optical fiber reliability and optical performance
geometrical design and methods of how the fiber ribbon is fixed is ensured with stringent control with respect to the radius of
within the module must protect this photonic interconnection curvature, which is a significant challenge when the ribbons
from the stresses that will occur during downstream assembly are exiting abruptly from a package edge.
steps and during environmental stress conditions that Fiber force must be controlled at the V-groove interface to
optoelectronic modules must withstand. Furthermore, this maintain the sub-micron alignment required by single-mode
solution must also provide a sufficient retention strength for the optics, and the fiber radius of curvature must be adequate
ribbon to allow the expected level of handling required by the for fiber reliability. Furthermore, the exiting ribbons must
pigtail. We resume here the model and results used to evaluate withstand various pull and retention tests, as well as provide
solutions that include optical fibers inside co-packaged optical proper strain relief for the protection of the photonic die
modules for ribbons connecting photonics devices. interface. To overcome those challenges, we analyzed the
The fiber pigtail length exiting the module is usually set design of the pigtail integration to reduce the impact of the
by the application requirement and how it will connect to the stresses at the various interfaces. Ribbons pulls and twists are
system, but in addition to such application requirements on the some of the most challenging tests to pass for optical modules.

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