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Description  |
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FIELD OF THE INVENTION
The present invention relates to routing, forming and connecting
(collectively, "configuring") optical fibers between optical ports such as
optical transmitters, receivers/detectors and additional fibers, and more
particularly, to configuring optical fibers between optical ports where
connection distances are short thus requiring small radius bends in the
routing of connection fibers. The present invention further relates to
optoelectronic devices involving such optical fiber configurations and
apparatus for manufacturing such optoelectronic devices.
BACKGROUND OF THE INVENTION
Optical fiber communication is an important mode of data transmission
around the world due in part to large bandwidth capabilities and freedom
from most forms of electromagnetic interference. Additionally, as computer
speeds approach one gigahertz (GHz) and beyond, parasitic Resistance,
Capacitance and Inductance (RCL) of connecting wires adversely influence
data transmission, making direct optical connections more desirable to,
between, and within computers and other communication devices.
Particularly desirable is the optical connection of all timed devices
within a computing device to deliver clock distributions between
chips/components in phase or otherwise with a known temporal relationship.
In addition, it is not unusual for different timed modules in a device to
require different voltages. In conventional wiring arrangements, timing
and data transmissions must first be converted to the proper voltage
before being relayed from one module to the next. This, along with the RCL
problems associated with high speed transfers can cause significant delays
of synchronized signals and thus, adversely affect system performance.
Optical connections can reduce or eliminate these problems. However,
connecting and routing optical fibers between optical ports presents an
array of additional problems. These problems include: shear stress at the
connection point, difficulty routing due to fiber stiffness, optical
distortion caused by adhesives, light leakage from sharp bends, micro
fractures, sharp fibers damaging components etc.
There have been numerous attempts to overcome the difficulties in
connecting optical ports. One method entails a non-contact alignment of a
connecting fiber and the corresponding optical port such that the signal
propagates through space. While this method solves several problems, such
as shear stress at the connection point, it causes others. For example,
components typically must be precisely mounted, directed, and aligned
within `eyesight` of the optical ports, e.g., in a pathway of the
transmitted optical beam, within the acceptance angle of an optical fiber
end, or in optical alignment with a detector surface of an optoelectric
transducer. Other attempts have used optical adhesives to `glue` optical
fibers to optical ports, however, such methods have proven difficult in
mass production and often the adhesive has an impact on optical
performance.
Another approach involves extruded optic transmission lines between optical
transmission ports. This eliminates certain inherent problems with
connecting ports with optical fibers. Since the optical pathways are
formed in place, they reduce or avoid internal stresses and micro
fractures that result from bending an existing optical fiber.
Additionally, the extruded optical pathways are easily routed since they
are formed in place in a fluid state. However, in order to obtain a good
surface adhesion bond between he extruded pathways and the optical port,
the temperature of the optical port must generally be elevated to ensure
proper bonding. Additionally, if the extruded pathway cools too quickly,
clogging can occur at the nozzle of the extruder head. These problems may
be addressed by performing these extrusions at an elevated ambient
temperature, e.g., between 170.degree. and 250.degree. Celsius. These
elevated temperatures may be problematic in certain contexts involving
sensitive components. Finally, such extruded optical pathways generally
only address a single connection, from one optical port to a second port,
and do not provide a method for forming a series of connections using a
continuous optical guide.
SUMMARY OF THE INVENTION
This invention addresses problems outlined above providing the desired
result of accurately connecting optical ports with an optical fiber in a
simple efficient manner and without unduly degrading the fiber or its
optical transmission qualities. The invention provides a simple process
and associated structure for forming optical connections without
intervening adhesives or free space pathways and with minimum heating of
the ambient environment. Additionally, the invention allows connecting
fibers to be simply lengthened and shortened and annealed in place.
Processes and apparatus for fiber alignment and forming multiple
connections with a single fiber are also described, as well as the
resulting optoelectronic devices.
As used herein, optical ports include active optical transmission elements
(such as, electronic chips with light emitters, including but not limited
to LEDs, Lasers, or VCSELs (Vertical Cavity Surface Emitting Laser))
active optical receiving elements such as photodiodes and other detectors
or photoelectric transducers) and passive transmission/detection elements
(such as ends of optical fibers which, in turn, may be associated with
active elements). Optical fibers include homogenous single strands and
fibers including a core and cladding like modem telecommunication fibers.
Preferably, the fiber will contain a core and cladding to improve its
optical characteristics. The optical fibers may be made of various
materials including quartz, glass or plastic.
According to one aspect of the present intention, a method and
corresponding apparatus are provided for connecting an optical fiber
between optical ports. The method involves heating an end portion of an
optical fiber to soften the end portion, contacting the softened end
portion to a first optical port, routing the optical fiber to a second
port and connecting the fiber to the second port. In addition, to increase
adhesion, the top surface of the optical port or associated structure may
also be heated. Preferably, all heating is done using a localized heating
process, (e.g. laser or ultrasound heating such that the potential for
heat damage to nearby components is reduced. Once softened, the end may be
contacted with the port to create an optical coupling. Feedback based on
an optical signal transmitted through the fiber may be used for accurate
fiber/port alignment as described in more detail below. The fiber is
preferably a preformed fiber, accordingly, the step of routing may involve
unspooling a length of fiber as well as forming, dimensioning and
annealing the fiber by heating as discussed below. The fiber may be
optically connected to the second port, for example, by physically
terminating the fiber at the second port or by bending and bonding the
fiber's longitudinal edge at the second port. In either case, the fiber
may be bonded at the second port by a heating, softening, and contacting
process.
As will be appreciated, the inventive optical connection method will work
with multiple fiber types including fibers made of glass, plastic or
quartz. More particularly, the method can utilize homogenous fibers as
well as optical fibers that containing a homogenous core surrounded by a
different refractive index light reflecting cladding.
With respect to the optical ports, at least two ports may be connected
using an optical fiber for signal transfer therebetween. The connection
generally will involve connecting at least one light emitting port with at
least one light detecting port. However, connections can also be made
between active components (emitters and receivers) and passive or
intermediate components such as the end of a telecommunications
fiber-optic cable and/or wave guides. As will be appreciated, connection
to an intermediate component such as a telecommunication fiber allows the
direct connection of an electronic chip to an optical network, especially
in contexts where it may be impractical to directly connect the
telecommunications fiber to the chip. Additionally, multiple optical ports
contained within a computer on optoelectronic chips may be interconnected
using the above method. For example, a clock chip within a computer
containing multiple optical ports may be connected to numerous time
dependent components using multiple fibers. Alternatively, multiple ports
may be connected using a single optical fiber creating chain-link
connections therebetween as will be further described below. Finally, many
fibers may be connected in parallel between optoelectronic chips creating
bus connections.
With respect to connecting the fiber to the second port, the fiber may be
softened and pressed into contact with the optical port. This may be done
using a mechanical pawl that holds the fiber on the port either
immediately after or while the localized heating is performed. Again this
heating will preferably be done with a focused source such as a laser.
Preferably, the pawl is made of a transparent material such that the laser
can heat the fiber while the pawl remains in place. As will be
appreciated, by using a localized source, not only can the fiber be
heated, but the port itself may also be heated to increase the adhesion
bond between the port and fiber. Generally, the pawl presses in a
direction transverse to the longitudinal axis of the connection fiber to
force the fiber into contact with the structure of the second port; this
in effect `bends` the connection fiber at the connection point. This bend
creates a narrowing or `neck` in the fiber's cross-section, thus,
increasing the angle at which photons within the fiber hit the core/clad
interface. Normally, the fiber cladding reflects the photons back into the
fiber, however, if the angle at which the photons hit the optical fiber
cladding is great enough, light will leak out of the core pass though the
cladding and thereby form a connection with the optical port. If the neck
angle is great enough, nearly all the photons will exit the fiber,
however, if the neck angle is reduced, fewer photons will exit the fiber
and the remainder can continue down the fiber. As will be appreciated,
this method works with homogenous fibers as well as core and cladding
optical fibers.
By utilizing a bending connection with a shallow angle, where some photons
continue being refracted within the fiber, multiple optical ports can be
connected using a single optical fiber. For example, a light emitting port
may be connected to three light detecting ports, where the middle two
detecting ports utilize a neck connection as discussed above allowing the
light emitting port to be coupled with all light detecting three ports. As
will be appreciated, this creates a chain-link connection between the
ports and several such fibers connected in parallel may be used to create
a bus connection. The above example envisions a simple chain-link or bus
connection, however, much more complicated connections are possible within
the scope of this invention. According to another aspect of the present
invention, an optoelectronic device includes a side-mounted fiber/port
interface. Specifically, the device includes an optical fiber and port for
transmitting and/or receiving optical signals relative to the optical
fiber. The fiber includes a longitudinal axis, along which optical signals
propagate, and a side wall defined by the circumference of a cross-section
transverse to the longitudinal axis. In accordance with the invention, a
connection is formed between the port and a portion of the fiber side wall
so that light is transmitted between the port and the fiber through the
side wall. The port may be an active port or a passive port.
In one embodiment, the port is a receiver or detector. Light transmitted
through the fiber is caused to penetrate the side wall at the location of
the port to form an optical connection. The fiber may be of homogenous
construction or non-homogenous (e.g., core and cladding) construction. In
the latter case, light may also penetrate the cladding to effect the
connection. Light may be caused to penetrate the side wall by bending or
forming a neck in the fiber so that light is incident on the side wall or
core/cladding interface at an angle greater than a critical angle for
retaining the light in the fiber which may be related to the fiber's
acceptance angle. The connection may be formed so that some light exits
the fiber at the port to form the connection and other light is retained
within the fiber and is available for forming serial connections using a
single fiber.
According to another aspect of the present invention, a method and
corresponding apparatus are provided for routing an existing optical fiber
between optical ports without unduly degrading the fiber's optical
transmission qualities. The inventive method includes applying heat to
soften the optical fiber as it is routed from one connection point to the
next to prevent fracturing the stiff fiber. Preferably, the heating is a
localized process, such that the potential for heat damage to nearby
components is reduced. A localized heat source as described above may be
used to soften the fiber route the fiber. Additionally, to prevent fiber
degradation, the method preferably involves retracing and heating the
entire length of the optical pathway after connection. As may be
appreciated, this post connection softening of the fiber relieves internal
stresses of the connected fiber. This softening in turn eliminates shear
forces located at the connection between the optical ports and the optical
fiber. Last, the post connection softening seals microfractures in the
optical connection fiber's surface preventing undue light leakage and
premature breakage.
According to another aspect of the invention, a method is provided for
active alignment of the optical fibers with their respective optical
ports. In certain cases, optical ports on optoelectronic chips are not
easily located by sight or alignment marks or are otherwise susceptible to
optical losses due to misalignment. For example, a common practice
involves mounting chips upside down on a ball and solder grid and emitting
or receiving optical signals through the chip's substrate. Generally,
chips do not contain alignment marks on their substrate making visual
location and alignment more difficult. The active alignment method
involves disposing a first end of a connecting fiber proximate to the
optical port, transmitting an optical signal through the fiber relative to
the port, monitoring the signal, and adjusting the position of the
connection fiber to maximize the signal transfer. More particularly, one
end of the connection fiber may be optically connected to an optical
component capable of sending and/or receiving optical signals. The other
end is brought into proximity with the optical port. Then, in the case of
an emitting port, the connection port is driven to produce a light output.
The light output signal is monitored through the attachment fiber. Once
the signal is maximized, the above described attachment method or another
attachment method is performed. Alternatively, a signal may be transmitted
through the fiber to the port, e.g., in the case of a light receiving or
passive port to perform the active alignment. In this regard, an output of
the port may be monitored to identify a maximum value of the received
signal to thereby optimize alignment.
According to a further aspect of the invention, a method is provided to
adjust the optical pathway length after connection. As will be
appreciated, this is particularly desirable in connecting multiple timed
modules to a clock chip, wherein variance in the length of the optical
pathways can affect synchronization. The method involves heating a section
of the connected optical pathway, causing or allowing a force to act upon
the fiber, and adjusting the length of the fiber. More particularly, when
increasing length, a section may be heated to a near molten state after
which a mechanical force is applied to stretch the fiber. When decreasing
the length, the applied force may be the surface tension of the molten
fiber, which draws the fiber shorter. As will be appreciated, multiple
adjustments may be made on a single fiber if needed.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and further
advantages thereof, reference is made to the following detailed
description, taken in conjunction with the drawings, in which:
FIG. 1 is a perspective view of a Multi Chip Module (MCM) in accordance
with the present invention.
FIGS. 2A and 2B are side views of an MCMs showing optical fiber connections
between optoelectronic chips in accordance with the present invention.
FIG. 3 is a plan view of an apparatus for making optical connections in
accordance with the present invention.
FIGS. 4A-4C are side views of connections between a fiber end and an
optical port surface in accordance with the present invention.
FIG. 5 is a close-up plan view of a fiber end connected to an
optoelectronic emitting port in accordance with the present invention.
FIG. 6 is a plan view showing an apparatus for routing a flexible optical
fiber between chips on an MCM in accordance with the present invention.
FIG. 7 is a plan view of an apparatus for routing a stiff optical fiber
between chips on an MCM in accordance with the present invention.
FIG. 8 is a plan view of an apparatus for forming optical connections
relative to chips on an MCM in accordance with the present invention.
FIGS. 9A-9C illustrate operation of a mechanical element to press an
optical fiber to a chip surface and severing the fiber in accordance with
the present invention.
FIG. 10 shows a side view of a connection of an optical fiber to an
optoelectronic detecting chip formed in accordance with the present
invention.
FIG. 11 is a perspective view of a Multi Chip Module (MCM) containing
optical bus interconnections between multiple chips in accordance with the
present invention.
FIG. 12 shows a side view of an intermediate fiber connection and a
terminating fiber connection on optoelectronic detecting chips in
accordance with the present invention.
FIG. 13 is a perspective view of a Multi Chip Module (MCM) containing a
master clock chip connected to multiple timed chips in accordance with the
present invention.
FIG. 14 is a perspective view of a Multi Chip Module (MCM) containing a
master clock chip connected to multiple timed chips with optical pathways
of equal distance in accordance with the present invention.
FIGS. 15A-15C illustrate lengthening an optical trombone and shortening an
optical trombone in accordance with the present invention.
FIG. 16 shows a fiber connecting apparatus configured to monitor light
emitted from an optoelectronic port in accordance with the present
invention.
FIGS. 17A and 17B show side views of an optoelectronic emitting VCSEL chip.
FIG. 18 is a side view of an optolelectronic chip attached to a
telecommunications fiber in accordance with the present invention.
FIG. 19 illustrates an apparatus for dispensing liquid material and
selectively curing the material to form optical wave lengths.
FIG. 20 is a flow chart illustrating a process for attaching time-dependent
chips to a master clock chip on a MCM in accordance with the present
invention.
DETAILED DESCRIPTION
Certain preferred embodiments of the present invention will be explained in
detail by referring to the accompanying drawings. Although the illustrated
embodiments of the invention are shown in conjunction with specific
multichip module (MCM) structures interconnected with fiber optic
connections, various aspects of the invention described below are
applicable in other contexts.
FIGS. 1, 2A and 2B, in which similar components are identified by the same
numerals, show various implementations of a multichip module (MCM) 5. The
MCM 5 generally includes a number of optoelectronic chips 6 mounted on a
carrier 9. The MCM 5 includes at a minimum of two chips and up to several
tens of chips or more. The optoelectronic chips 6 contained on the MCM 5
may be comprised of any type of electrical componentry such as CPUs,
memory chips, timing chips, etc. In accordance with the present invention,
the chips 6 contain either a light (radiation) emitter, such as an LED, a
laser, or a VCEL or a light-detecting element such as a photodiode, CCD
detector or the like. Generally, the light emitters or light detectors
convert electrical energy to optical energy or optical energy to
electrical energy. Additionally, a single chip may contain either or both
optical emitters and detectors.
Referring to FIGS. 2A-2B, in certain cases a separate optoelectronic chip 7
is used for light generation. For example, many chips are made from
silicon, whereas sources typically are fabricated using a variety of other
materials. In the illustrated embodiments, the separate optoelectronic
chip 7, which includes a light emitter, is electrically coupled to the
chips 6. As shown in FIGS. 2A and 2B, a light source chip 7 made of, for
example, GaAs is connected to silicon based chips 6. In FIG. 2A, chips and
optoelectronic source chip 7 are mounted on ball and grid arrays 8 on the
chip carrier 9. Chips 6 and optoelectronic source chip 7 are electrically
connected by circuitry not shown in the carrier 9. In FIG. 2B, source chip
7 is directly mounted on one of chips 6 using a ball and grid array 2. The
optoelectronic chips 6 and 7 of FIGS. 1-2B may be mounted on any suitable
carrier 9 such as, for example, an integrated circuit board, a ceramic
chip carrier etc.
To allow communication between various ones of the optoelectronic chips 6
and 7 of FIGS. 1-2B, they are interconnected via optical pathways to allow
for the transfer of signals (transmitted as photons) therebetween. In the
illustrated embodiment, each pathway is created using an existing optical
fiber 10. This fiber 10 may be made of glass, quartz or plastic and may be
homogenous in cross-section or may contain a core and cladding like a
typical single mode or multimode telecommunications fiber. Preferably,
cladded fibers are used since the clad provides near total internal
reflection of the light within the core reducing light leakage and
improving signal transfer. However, when using homogenous fibers, a white
or transparent (with a lower refractive index than the fiber) coating may
be applied to aid in light refraction.
In the embodiments discussed above, the fiber 10 connects an optical
emitter formed on one of the chips to an optical detector of another one
of the chips. As will be discussed below, fiber may also be used to couple
an emitter or detector of one chip to a component other than an emitter or
detector e.g., another fiber. Such emitters, detectors and other
components for optical coupling are collectively and separately referred
to hereafter as optical ports. As shown in FIG. 3, a routing head 20 may
be used to form the optical connections. The routing head 20 is preferably
able to freely move in three dimensions to make optical connections on the
electronic chips and route fibers therebetween. Additionally, the routing
head 20 can rotate about vertical and horizontal axes to permit
connections in the horizontal and vertical planes. In this regard, the
head 20 can accommodate whatever topological features may exist on the MCM
surface such as connections of uneven heights, vertical to horizontal,
etc. (See FIG. 2B). The head may be controllably driven by a number of
micro motors in response to control signals and accurate positioning may
be ensured by utilizing feedback signals from optical encoders or the
like. In the case of an assembly line process, where the same connections
are repeatedly made, a stored computer program is used to manipulate the
routing head between connection points. The routing head 20 contains a
spool 22 for holding a length of the preexisting fiber 24, which is laced
through a connecting tip 26. The spool 20 is preferably tensioned to keep
stiff fibers from unwinding. The connecting tip 26 unwinds fiber from the
spool 22 using optical fiber guides (not shown) for connection and
routing. Care should be taken when adjusting the spool tension such that
the fiber is not unduly stressed while fed through the connecting tip.
The preexisting fiber 24 is fed through the connecting tip 26 and extended
a predetermined distance beyond the end of the connecting tip 26. This
preexisting distance is determined by the connection fiber qualities, such
as fiber size, stiffness, etc. In addition, a heat source 30, proximal to
the routing head 20, is used for making the fiber connections as will be
discussed below. In particular, the heat source 30 is used to soften the
end of the fiber 24 for connection to a chip surface 38 or other structure
associated with an optical port. In the illustrated embodiment, the heat
source 30 is a laser. When using a laser heat source, the laser should be
chosen and operated to optimally correspond with fiber qualities. A
non-inclusive list of laser characteristics and operating parameters which
may be selected and adjusted based on characteristics of the fiber and the
particular application includes: optical intensity, wavelength, time
profile of laser power and focused spot size. For example, a 10 Micron
wavelength CO.sub.2 laser may be used to heat glass fibers since glass
readily absorbs 10 Micron wavelengths. Regardless the heating method used,
localized heating is preferable such that the connection fiber 24 may be
heated without unduly heating other nearby components that may be damaged
by high ambient temperatures. A 10 Micron wavelength CO.sub.2 laser, for
example, may easily be focused onto a 20-30 micron area minimizing the
area affected by the heating.
The illustrated laser is mounted proximally to the routing head 20 such
that it may be easily focused on the fiber end 27, the chip surface 38,
and the full length of the fiber 24 between optical ports after
connections are made, as will be more fully discussed herein. The
illustrated routing head 20 also contains a mechanical element or `pawl`
32 to force the longitudinal axis of the connection fiber 24 into contact
with optical ports as will further be described below.
INITIAL CONNECTION
Referring to FIGS. 3, and 4A-4C, for an initial connection, the end of the
preexisting fiber 27 is positioned so that it extends out of the routing
head connecting tip 26 and is disposed above the connection surface 38 as
shown in FIG. 4A. The chip surface 38 may be an upper or lower surface of
an optoelectronic chip 40. The fiber can be aligned above the optical port
39 of chip 40 using certain mechanisms and processes, as will be more
fully described below. Once correctly aligned, the end of the connection
fiber 27 is heated, using the localized heat source 30, to reduce the
fiber's viscosity (i.e. soften the fiber). The chip surface 38 may also be
heated to improve bonding between the fiber 24 and the optical port 39.
Alternatively, as shown in FIG. 4B, the connecting fiber end 27 may be
brought in contact with the chip surface 38 prior to heating in which case
the localized heating source 30 may be used to heat both the chip surface
38 and the connection fiber 24 simultaneously. As will be appreciated,
heating the chip surface 38 of the optical port 39 provides for a better
surface adhesion bond between the fiber and the port. Although the fiber
end 27 is shown as having a clean cut or `square` end, it will be
appreciated that the fiber end 27 need not be neatly trimmed before
connections are made. In the implementation of FIG. 4A, once the fiber 24
is softened to a predetermined point, the routing head 20 moves the fiber
24 into contact with the chip 40 and presses the fiber against the chip
connection surface 38. See FIG. 4C. The force exerted by the routing head
20 expands the end of the connection fiber 24 as shown in FIG. 4C.
FIG. 5 shows the resulting expanded section 50 as well as certain details
of the port 39. In FIG. 5, the fiber 24 includes a core 71 and a cladding
72. The fiber 24 establishes a connection to the optical port 39 (a VCSEL
in the illustration) incorporated in an optoelectronic chip 40. In the
illustrated embodiment, the VCSEL 39 has two mirror stacks 75 and an
active region 76, which emits light 78 through a substrate 77. As will be
appreciated, the expanded section increases the contact area between the
chip surface 38 and the fiber 24, thus increasing the adhesion bond
strength. The expanded section 50 also increases the optical
interconnection coupling area allowing more light 78 to enter into the
optical fiber 24 from the optical port 39. Typically, good coupling
efficiency can be obtained when the connection fiber has a core diameter
20-50% greater than the emitter diameter. For example, the expanded
section 50 will allow a connection fiber 24 with a core diameter of 12-15
microns to efficiently attach to a light emitter 76 with a 10-micron
diameter. As will be appreciated, the expanded area 50 allows for a small
margin of error when aligning the connection fiber with the optical port.
ROUTING
Referring to FIGS. 6 and 7, after attaching the fiber end 27 to the
optoelectronic chip 40, the optical fiber 24 is routed to another
electronic chip 70 in the illustrated application. In the case of an
assembly line type process, the routing will be performed automatically by
computerized control to make a predetermined pathway between two or more
predetermined coordinates on the MCM. As will be appreciated, the optical
pathway connection distances on | | |
