Title: Relaxed tolerance optical interconnect system capable of providing an array of sub-images
Abstract: An optical interconnect system having a GRIN rod lens, the GRIN rod lens having a first end and a second end, and further having a preselected length, a preselected width and a preselected index of refraction. Means are fixedly secured to the first end of the GRIN rod lens for emitting electromagnetic radiation and means are fixedly secured to said second end of said GRIN rod lens for receiving the emitted electromagnetic radiation. The GRIN rod lens forms an image of the emitting means onto the receiving means and overcomes problems associated with misalignment. Further embodiments of this optical interconnect system are capable of use in even further applications in an environment where alignment problems are at issue.
Patent Number: 7,015,454 Issued on 03/21/2006 to Stone
| Inventors:
|
Stone; Thomas W. (Hellertown, PA)
|
| Assignee:
|
Wavefront Research, Inc. (Bethlehem, PA)
|
| Appl. No.:
|
675873 |
| Filed:
|
September 29, 2003 |
| Current U.S. Class: |
250/216; 250/208.1 |
| Current Intern'l Class: |
H01J 3/14 (20060101) |
| Field of Search: |
250/22711,227.24
385/34
|
References Cited [Referenced By]
U.S. Patent Documents
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| |
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| 5050954 | Sep., 1991 | Gardner et al.
| |
| 5071216 | Dec., 1991 | Sullivan.
| |
| 5093879 | Mar., 1992 | Bregman et al.
| |
| 5245680 | Sep., 1993 | Sauter.
| |
| 5266794 | Nov., 1993 | Olbright et al.
| |
| 5291324 | Mar., 1994 | Hinterlong.
| |
| 5384874 | Jan., 1995 | Hirai et al.
| |
| 6253004 | Jun., 2001 | Lee et al.
| |
Other References
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Hugo Thienpont, et al. "Free Space Optical Interconnect and Processing Demonstrators
With Arrays of Light-Emitting Thyristors", Proceedings of the SPIE, vol. 3002,
156-167, Conference date Feb. 13-14, 1997 [probably published several months later].
Andrew Kirk, et al. "Compact Optical Imaging System for Arrays of Optical Thyristors",
Applied Optics 36, No. 14, 3070-3078, May 10, 1997.
V. Baukens, et al. "An Optical Interconnection System for Arrays of MicroEmitters
and Detectors: Combining Printed Microlenses and Large Diameter GRINs", Proceedings
of the SPIE, vol. 3490, 155-158, Conference date (Belgium) Jun. 17-20, 1998
[probably published several months later].
Tomasz Maj, et al. "Interconnection of a Two-Dimensional Array of Vertical-Cavity
Surface-Emitting Lasers to a Receiver Array by Means of a Fiber Image Guide", Applied
Optics vol. 39, No. 5, 683-689, Feb. 10, 2000.
Donald M. Chiarulli, et al. "Demonstration of a Multichannel Optical Interconnection
by Use of Imaging Fiber Bundles Butt Coupled to Optoelectronic Circuits", Applied
Optics vol. 39, No. 5, 698-703, Feb. 10, 2000.
Donald M. Chiarulli, et al."Optoelectronic Multi-Chip Modules Based on Imaging
Fiber Bundle Structures", Proceedings of the SPIE, vol. 4089, 80-85, Conference
date Jun. 18-23, 2000 [probably published several months later].
Valerie Baukens, et al. "Free Space Optical Interconnection Modules for 2-D Photonic-VLSI
Circuitry Based on Microlenses and GRINs", Proceedings of the SPIE, vol.
4114, 169-181, Conference date Aug. 2-3, 2000 [probably published several months later].
Mohammad R. Taghizadeh, et al. "Microoptical Elements and Optoelectronic Devices
for Optical Interconnect Applications", Proceedings of the SPIE, vol. 4455,
119-130, Conference date Jul. 29-31, 2001 [probably published several months later].
|
Primary Examiner: Porta; David
Assistant Examiner: Lu; Tony
Attorney, Agent or Firm: Burns & Levinson LLP, Erlich; Jacob N., Lopez; Orlando
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
This invention was made with U.S. Government support under Contract Nos. F08630-97-C-0048,
and F08630-98-C-0027 awarded by the U.S. Air Force. The Government has certain
rights in the invention.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent application Ser. No.
09/425,551 entitled RELAXED TOLERANCE OPTICAL INTERCONNECT SYSTEMS, filed on Oct.
22, 1999 now U.S. Pat. No. 6,635,861, which claims priority of U.S. provisional
application Ser. No. 60/105,251 entitled OPTICAL DATA PIPE filed Oct. 22, 1998
by the present inventor.
Claims
What is claimed is:
1. An optical interconnect system, comprising:
a first GRIN rod lens and a second GRIN rod lens;
said first GRIN rod lens having a first end and a second end, and further having
a preselected length, a preselected width and a preselected index of refraction;
said second GRIN rod lens having a first end and a second end, and further having
a preselected length, a preselected width and a preselected index of refraction;
means fixedly secured to said first end of said first GRIN rod lens for emitting
electromagnetic radiation, said emitting means comprising an array of emitters;
means fixedly secured to said second end of said second GRIN rod lens for receiving
said emitted electromagnetic radiation, said receiving means comprising an array
of detectors, each detector from said array of detectors having substantially a
preselected size;
said first GRIN rod lens forming an image of said emitting means substantially
at infinity;
said second GRIN rod lens having its first end spaced apart and proximate said
second end of said first GRIN rod lens;
said first end of said second GRIN rod lens and said second end of said first
GRIN rod lens defining a space therebetween, wherein said space being substantially
void of any optical component; and
said second GRIN rod lens forming an image of said emitting means onto said receiving
means, said image formed onto said receiving means comprising an array of sub-images,
each sub-image from said array of sub-images having substantially a preselected size;
wherein said preselected size of each detector from said array of detectors being
larger than said preselected size of each sub-image from the array of sub-images.
2. The optical interconnect system as defined in claim 1 wherein said preselected
length and said preselected width of said first and said second GRIN rod lens,
respectively, is defined to permit each of said first GRIN rod lens and said second
GRIN rod lens to have some degree of flexibility.
3. The optical interconnect system as defined in claim 1 wherein at least one
of said GRIN rod lenses is of a curved configuration.
4. The optical interconnect system as defined in claim 1 wherein said array of
emitters comprises a hexagonal array of emitters; and, wherein said array of detectors
comprises a hexagonal array of detectors.
5. The optical interconnect system as defined in claim 1 further comprising,
in combination therewith:
means for providing electrical signals and converting said electrical signals
into optical signals;
said electrical signal providing and converting means being operably connected
to said means for emitting said electromagnetic radiation, said electromagnetic
radiation being in the form of optical signals.
6. An optical interconnect system, comprising:
a first GRIN rod lens, said first GRIN rod lens having a first end and a second
end, and further having a preselected length, a preselected width and a preselected
index of refraction;
a second GRIN rod lens, said second GRIN rod lens having a first end and a second
end, and further having a preselected length, a preselected width and a preselected
index of refraction;
means fixedly secured to said first end of said first GRIN rod lens for emitting
electromagnetic radiation, said emitting means comprising an array of emitters;
means fixedly secured to said second end of said second GRIN rod lens for receiving
said emitted electromagnetic radiation, said receiving means comprising an array
of detectors, each detector from said array of detectors having substantially a
preselected size;
said first GRIN rod lens and said emitting means being attached to a first circuit board;
said second GRIN lens and said receiving means being attached to a second circuit board;
said first GRIN rod lens forming an image of said emitted radiation substantially
at infinity; and
said second GRIN rod lens forming an image of said emitted electromagnetic radiation
from said first GRIN rod lens onto said receiving means, said image formed onto
said receiving means comprising an array of sub-images, each sub-image from said
array of sub-images having substantially a preselected size; wherein said preselected
size of each detector from said array of detectors being larger than said preselected
size of each sub-image from the array of sub-images.
7. The optical interconnect system as defined in claim 6 wherein said preselected
length and said preselected width of said first and said second GRIN rod lens,
respectively, is defined to permit each of said first GRIN rod lens and said second
GRIN rod lens to have some degree of flexibility.
8. The optical interconnect system as defined in claim 6 wherein at least one
of said GRIN rod lenses is of a curved configuration.
9. The optical interconnect system as defined in claim 6 wherein said array of
emitters comprises a hexagonal array of emitters; and, wherein said array of detectors
comprises a hexagonal array of detectors.
10. The optical interconnect system as defined in claim 6 further comprising,
in combination therewith:
means for providing electrical signals and converting said electrical signals
into optical signals;
said electrical signal providing and converting means being operably connected
to said means for emitting said electromagnetic radiation, said electromagnetic
radiation being in the form of optical signals.
11. The optical interconnect system as defined in claim 6 wherein said preselected
length and said preselected width of said first and said second GRIN rod lens,
respectively, is defined to permit each of said first GRIN rod lens and said second
GRIN rod lens to have some degree of flexibility.
12. The optical interconnect system as defined in claim 6 wherein at least one
of said GRIN rod lenses is of a curved configuration.
13. The optical interconnect system as defined in claim 6 wherein
said array of emitters comprises a hexagonal array of emitters; and,
wherein said array of detectors comprises a hexagonal array of detectors.
14. The optical interconnect system as defined in claim 6 further comprising,
in combination therewith:
means for providing electrical signals and converting said electrical signals
into optical signals;
said electrical signal providing and converting means being operably connected
to said means for emitting said electromagnetic radiation, said electromagnetic
radiation being in the form of optical signals.
15. An optical interconnect system, comprising:
a first GRIN rod lens, said first GRIN rod lens having a first end and a second
end, and further having a preselected length, a preselected width and a preselected
index of refraction;
a second GRIN rod lens, said second GRIN rod lens having a first end and a second
end, and further having a preselected length, a preselected width and a preselected
index of refraction;
means fixedly secured to said first end of said first GRIN rod lens for emitting
electromagnetic radiation, said emitting means comprising an array of emitters;
means fixedly secured to said second end of said second GRIN rod lens for receiving
said emitted electromagnetic radiation, said receiving means comprising an array
of detectors, each detector from said array of detectors having substantially a
preselected size;
said first GRIN rod lens and said emitting means being attached to a first multi-chip module;
said second GRIN lens and said receiving means being attached to a second multi-chip module;
said first GRIN rod lens forming an image of said emitted radiation substantially
at infinity; and
said second GRIN rod lens forming an image of said emitted electromagnetic radiation
from said first GRIN rod lens onto said receiving means, said image formed onto
said receiving means comprising an array of sub-images, each sub-image from said
array of sub-images having substantially a preselected size; wherein said preselected
size of each detector from said array of detectors being larger than said preselected
size of each sub-image from the array of sub-images.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to interconnection systems, and, more
particularly, to alignment tolerant dense optical interconnect systems which incorporate
the use of rod lenses.
With the advent of substantial new performance levels in high bandwidth digital
and analog electro-optic systems, there exists a greater need to provide dense,
alignment tolerant interconnection capability. This is especially true in digital
computing systems; in analog systems such as phased array radar; and in high bandwidth
optical carriers in communication systems. However, it should be realized that
these are just several of numerous systems which benefit from application of high-bandwidth
electro-optic interconnection.
In many current and future systems light beams are modulated in a digital and/or
analog fashion and used as "optical carriers" of information. There are many reasons
why light beams or optical carriers are preferred in these applications. For example,
as the data rate required of such channels increases, the high optical frequencies
provide a tremendous improvement in available bandwidth over conventional electrical
channels such as formed by wires and coaxial cables. In addition, the energy required
to drive and carry high bandwidth signals can be reduced at optical frequencies.
Further, optical channels, even those propagating in free space (without waveguides
such as optical fibers) can be packed closely and even intersect in space with
greatly reduced crosstalk between channels.
Conventional electrical interconnection over wires or traces is reaching
severe performance limits due to density, power, crosstalk, time delay, and complexity.
For example, chip scaling continues to provide for a doubling of transistors on
a chip every 18 months. A 2 cm×2 cm chip currently requires 2 km of wires
or traces for interconnection with 6 layers of metal and the complexity exponentiates
with the number of metal layers. With designs using 0.18 micron wires, 60% of the
delay is from the interconnects themselves. In shrinking from 0.5 micron wires
to 0.18 micron wires on chip, the rc time constant increases by a factor of 10.
Using optical interconnection, the power dissipation does not scale with the length
of interconnection, and optical interconnects are superior for short signal rise
times. Similar advantages of optical interconnection over electrical interconnection
pertain to longer range interconnection, e.g., from chip-to-chip, intra-board,
inter-board, and computer-to-peripheral.
Other optical interconnect approaches suffer from critical alignment tolerances;
restrictive focusing, component separation and vibration tolerance requirements;
insertion loss which limits speed and power efficiency; bulky and large-footprint
optical systems; limited density and scalability; lack of physical flexibility
and compliance of the interconnect, and the need to provide an excessively protective
environment in order to maintain optical alignment over time.
It is therefore an object of this invention to provide a high density (many parallel
interconnected channels in a small volume) optical interconnect system that can
optically interconnect tens, hundreds, or thousands of high-bandwidth channels
with a technology that is flexible and alignment tolerant.
It is another object of this invention to provide a high density optical interconnect
system in which the component optical and electro-optical components are arranged
in a single monolithic unit.
It is another object of this invention to provide a high density optical interconnect
system that provides a nearly lossless one-to-one optical interconnection from
a set of input channels to a set of output channels.
It is another object of this invention to provide a high density optical interconnect
system that, by virtue of its low insertion loss, can provide very high-speed bidirectional
interconnection among dense two-dimensionally packed interconnected channels.
It is another object of this invention to provide a high density optical interconnect
system that can interconnect over distances spanning millimeters to tens of meters
or longer.
It is further an object of this invention to provide a high density optical interconnect
system that can be pre-aligned during manufacture and is thus readily fieldable
by non-optically trained personnel.
It is further an object of this invention to provide a high density optical interconnect
across single die or integrated circuit substrate, inside multi-chip modules, between
die, between modules or components, between boards, between computers, between
computers and peripherals, or between peripherals.
It is still further an object of this invention to provide a high density optical
interconnect system that uses a small fraction of device area that would be required
for arrays of traces, wires, or waveguides.
It is still further an object of this invention to provide a high density optical
interconnect system that is capable of bending and flexing to accommodate components
that shift relative to each other while maintaining interconnection of many parallel
optical channels among the shifting components.
It is still further an object of this invention to provide a high density optical
interconnect system that interconnects many parallel optical channels among components
while maintaining relative insensitivity to longitudinal and lateral shifts between
the interconnected components.
It is still further an object of this invention to provide a high density optical
interconnect system that can be disconnected and reconnected simply while maintaining
alignment among many parallel optical channels.
It is still further an object of this invention to provide a high density optical
interconnect system that provides for a uniform delay for all channels, thus minimizing
relative signal skews.
SUMMARY OF THE INVENTION
The objects set forth above as well as further and other objects and advantages
of the present invention are achieved by the embodiments of the invention described hereinbelow.
The present invention overcomes problems associated with size, power, crosstalk,
and speed associated with conventional electrical interconnects, and overcomes
problems with alignment tolerance in conventional optical interconnection. In a
preferred embodiment of The optical interconnect system or optical data pipe approach
of the present invention, although not limited thereto, mating emitter and detector
arrays are pre-aligned and fixed on or near the ends of a gradient index rod imager,
and this flexible pre-aligned structure is then mounted to the host. Using this
technology, which includes the various embodiments of this invention, hundreds
or thousands of high bandwidth channels can be interconnected for short distances
(intra-die, between neighboring chips or MCMS) or over relatively long distances
(full board wrap-around, board-to-board, computer to peripheral, computer to computer,
etc.). The optical interconnect system of this invention provides a nearly lossless
one-to-one optical interconnection from a set of input channels to a set of output
channels, and supports extreme density, low power, and low crosstalk for high bandwidth signals.
One of many advantages of the system of this invention is that it can be pre-aligned
and fixed during manufacture (e.g., using automated alignment and cementing procedures)
to produce optical interconnects that have greatly relaxed alignment tolerances
and are thus readily usable in the field by non-optical personnel. The interconnection
systems of the present invention are thus tolerant of handling, bending and displacements
among interconnected components without losing their function of interconnecting
many closely packed (dense) optical channels. Other advantages of this invention
relate to the fact that it is tolerant of misalignments, vibrations, etc.
For a better understanding of the present invention, together with other and
further objects, reference is made to the following description taken in conjunction
with the accompanying drawings, and its scope will be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of the problem addressed by the optical
interconnect systems of the present invention;
FIGS. 2 and 3 represent prior art approaches;
FIG. 4 is a schematic representation of a fixed point-to-point monolithic optical
interconnect system of the present invention;
FIG. 5 is a schematic representation of an embodiment of an interconnection
of The optical interconnect system of this invention;
FIGS. 6A and 6B schematically and pictorially, respectively represent a dense
hexagonally packed emitter array utilized with the present invention, with FIG.
6B being an exploded view thereof;
FIG. 7 is a schematic representation of a prism used with one embodiment of
the optical interconnect system of this invention;
FIGS. 8A-8D are photographs of alignment of channels during bending of The
optical interconnect system of the present invention;
FIG. 9 is a schematic representation of a circuit board application of the optical
interconnect system of this invention;
FIG. 10 is a schematic representation of an embodiment of this invention illustrating
a curved optical interconnect (optical data pipe) used with this invention;
FIG. 11 is a schematic representation of micro-optic lens arrays used with the
optical interconnect systems of this invention;
FIG. 12 is a schematic representation of another embodiment of the optical interconnect
system of this invention;
FIG. 13 is a schematic representation of a relaxed tolerance interconnect system
of this invention linking channels between boards or MCM's;
FIG. 14 is a schematic representation of yet another embodiment of the optical
interconnect system of this invention;
FIG. 15 is a schematic representation of an emitter array and a receiver array
utilized in a detailed embodiment of the optical interconnect system of this invention;
FIG. 16
a is a schematic representation of an optical raytrace for an
embodiment of the optical interconnect system of this invention including substantially
aligned components;
FIG. 16
b-d are schematic representations of an image of one emitter from
emitter array projected onto a detector from the receiver array;
FIG. 17 is a schematic representation of an embodiment of the optical interconnect
system of this invention including slightly misaligned components;
FIG. 18
a is a schematic representation of an optical raytrace for an
embodiment of the optical interconnect system of this invention including slightly
misaligned components;
FIG. 18
b-d are schematic representations of an image of one emitter from
emitter array projected onto a detector from the receiver array for the embodiment
of FIG. 17; and,
FIG. 19 is a schematic representation of hexagonally packed arrays of emitters
and detectors (receivers).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
If an optical interconnect approach is to have a significant impact in practice,
it must possess a variety of desirable characteristics including in particular
a tolerance for misalignments and alignment variations among the interconnected
components. These areas are highlighted and described briefly hereafter:
- Alignment Tolerance, with the ability to function in a practical computer
environment which is subject to vibrations, thermal variations, misalignments between
interconnected components and devices, etc.
- High channel density: allowing for hundreds of parallel channels to
be interconnected with a small footprint.
- High channel bandwidth: allowing for data rates in the megahertz and
gigahertz regimes.
- Low insertion loss: allowing for high speeds with low power consumption.
- Uniform delay for all data channels, therefore introducing little or
no relative skew in the switched signals.
- Miniaturized opto-electronics interconnecting many parallel optical
channels in a compact package. The ability to densely pack channels in a volume
(e.g., a slender flexible rod) rather than along planar substrate or circuit board
surface will be a distinct advantage.
- Low crosstalk between neighboring optical data channels.
- Monolithic packaging for ruggedness and low insertion loss.
- Scalability to large numbers of data channels.
The terms detectors and receivers are used interchangeably hereinbelow.
The general optical interconnection problem addressed by the present invention
is shown in FIG. 1. Here there are two planes A
1 and A
2 of densely
packed emitters and detectors that are interconnected by an optical interconnect
20. For clarity, not shown are the drivers for the emitters and amplifiers
for the detectors. In general, each of the planes contain both emitters and detectors
to enable bi-directional communication. The emitters are sources of electromagnetic
radiation (in this application equivalently referred to as optical radiation, optical
signals, or light). The electromagnetic radiation can be modulated to carry information
that originated, for example, on electrical signals, and the electromagnetic radiation
has a higher frequency than that of the information it carries. For example, the
emitters can be sources of electromagnetic radiation in the infrared, visible,
or ultraviolet spectral bands. This emitted electromagnetic radiation can then
be modulated with information including frequencies and data rates ranging from
DC to many gigahertz and higher. The detectors then receive the electromagnetic
radiation and extract the information usually in the form of electrical signals.
The terms electromagnetic radiation, optical signals, and light are used to refer
to the high frequency electromagnetic radiation carrier described above and are
distinguished from the information carried by the optical interconnect system and
the input and output electrical signals, which can also be electromagnetic in nature.
The optical interconnect systems described below of the present invention couple
electromagnetic radiation from corresponding optical channels or among mating emitters
and detectors with the features described above.
One of the two primary limiting approaches to this optical interconnect problem
is the macro-optical approach. In macro-optics, a single optical system is used
to simultaneously image the entire array of optical channels from emitter array
to detector array. A typical prior art macro-optical imaging approach is illustrated
in FIG. 2. Here a single lens system
30 is used to simultaneously image
many optical channels
42 between planes
51 and
52. The optical
channels are arrayed in the optical field extent
40 of width W. Key parameters
for such a system are the field extent W, field angle
44 of width β,
spot size or resolution
46 or d, and stop
48 of diameter D.
In the classic or prior art macro-optic approach, the tradeoffs are very well
defined. The lens required to maintain small spot sizes d over a large device field
W grows rapidly in complexity. For example, the spot size d of even a perfect lens
is limited by diffractive spreading of the light through the finite aperture of
the lens. If there were no aberrations in the lens, the spot size can be shrunk
as far as to the order of a wavelength of the imaged light by increasing the stop
diameter D, or equivalently by increasing the numerical aperture of the system
(which is proportional to the sine of θ).
However, in practice there are always aberrations present. A major goal
of the design of this prior art type of interconnect lens system is to reduce the
magnitude of sum of the aberrations present to the order or magnitude of the desired
spot size. Rather than attempting to eliminate each of the aberrations, they are
balanced so that they form a net aberration magnitude that is acceptable. In order
to accomplish this difficult task, many degrees of freedom are introduced including
split and multiple elements, refractive surface curvatures, spacings, glass choice,
apertures, etc.
Perhaps the largest limitation in the macro-optic approach is the lack of
scalability of a given lens system. If there is a diffraction limited lens system
that performs acceptably with spot size d over a field of extent W, a natural approach
to increasing the covered field (and thus the number of optical channels that can
be interconnected) is to scale up the size of the lens system by a factor s. In
this process, linear dimensions are scaled and angles are preserved. Accordingly,
the larger field with the same spot size should, in principle, provide for more
independent optical channels. After scaling the lens, the diffraction limited spot
size remains the same. However when scaling is performed, the forms of the aberrations
remain the same but their magnitude is scaled by s. As a result, the balance of
aberrations that was acceptable prior to scaling is no longer adequate. The net
aberration magnitudes (which were in balance with the spot size prior to scaling)
can scale larger than the desired spot size. As a result of the larger actual spot
sizes, the scaled up field extent W can in practice eliminate any net increase
of optical channels that are interconnected. The only alternative is to redesign
the scaled lens in hope of reducing the magnitude of aberrations of the larger
lens to the same magnitude which the lens had prior to scaling.
A further drawback for the macro-optic approach remains. If a single lens is
used
to image, using finite conjugates, arrays of emitters and detectors, there is still
the unreasonable alignment sensitivity between the two planes. Relative motions
between either of the planes, on the order of the detector sizes, will be catastrophic.
The other major limiting approach to the optical interconnect problem described
above is the micro-optic approach. With micro-optics, many much-simpler parallel
optical systems are used to image the many optical channels one at a time. These
micro-optic elements are therefore arrayed in the parallel optical channels. This
prior art approach is illustrated in FIG. 3, where two arrays of devices A
1
and A
2 are imaged with an array of simple refractive or diffractive micro
lenses
56. The channels are interconnected by many isolated optical beams
58.
In this micro-optic limit, there is an imaging lens for each device. Since the
device can be placed on the axis of symmetry of the simple lens elements, the performance
of the lens does not need to be maintained over an extended field as was the case
with the macro-optic limit. As a result, the complexity and size of the lens can
be greatly scaled down. Further, another distinct advantage here is that there
is a trivial scaling requirement, which now simply amounts to extending the size
of the device array and corresponding size of the microlens array. This is in sharp
contrast to the complex scaling problems in the macro-optic approach described previously.
There remains a significant problem, however, with the micro-optic approach
to high density optical interconnection. This is due to the presence of diffractive
crosstalk among neighboring optical channels. Diffractive spreading from the aperture
of each of the microlenses causes light to couple into neighboring channels, resulting
in crosstalk. The larger the aperture of the microlenses, the smaller is this diffraction
spreading effect. However, since the neighboring lenses cannot overlap, reducing
diffractive crosstalk forms a major performance tradeoff with density of the interconnection.
From a geometrical analysis augmented by results from diffraction theory, it
can be shown that the optical signals in the parallel channels can propagate a
critical distance L
c before the beam, augmented by diffraction, will
cross over into the neighboring channel—i.e., until the crosstalk becomes
significant. This critical distance is given by:
Lc=D2/2λ, (1)
where D is the microlens aperture and λ is the communication wavelength.
Thus for a device spacing and microlens aperture of 100 microns, the beams can
only propagate for a few millimeters before crosstalk is significant. Increasing
the aperture of the microlenses results in a squared effect on the crosstalk free
propagation distance, but if a large relay distance is required for the signals,
the sought after high density of the optical channels must be sacrificed in order
to prevent diffractive crosstalk.
A first preferred embodiment of the relaxed tolerance optical interconnect system
of the present invention is the fixed point-to-point monolithic optical data pipe
100 illustrated in FIG. 4, where the term optical data pipe may also be
referred to herein on occasion as optical interconnect
100. Here mating
interconnection planes
10 and
14 are affixed preferably by an adhesive
"cement" on the ends of a gradient index (GRIN) rod imager
102, and this
flexible pre-aligned structure is then mounted to the components
106 and
108 of host
104 which provides dense interconnection. The interconnection
planes
10 and
14 can contain emitters, detectors, or general optical
channel ports such as arrays of free-space channels or guided wave (fiber) channels,
or the like. This device is capable of very high channel densities, on the order
of hundreds along a cross section of only a few millimeters. Using demonstrated
Vertical Cavity Surface Emitting Microlaser (VCSEL) technology, each of these channels
is capable of multi-gigahertz data rates at these high spatial densities. The small
insertion loss allows high data rates and low power consumption. Negligible temporal
skews are generated once the optical signals are generated in the optical data
pipe
100, independent of the length of the link (millimeters to feet). A
distinct advantage of the monolithic construction of the optical data pipe
100
is that it is very tolerant of flexure and relative misalignments between the components
being interconnected.
In the optical interconnect system of the present invention, also referred to
as an optical data pipe
100, a gradient index (GRINO rod
102 is used
as a data pipe for conducting hundreds of high bandwidth optical interconnections
with little crosstalk. This high density optical data pipe
100 is formed
by pre-aligning and permanently affixing mated emitter and detector arrays to or
near the ends of a gradient index (GRIN) rod lens imager. This rod lens images
the optical channels, emitters, or detectors onto each other as conjugate image
planes. The magnification can be unity or non-unity in this imaging operation.
The monolithic end-to-end connection of device planes
10 and
14 and
rod lens
102 forms a flexible pre-aligned structure capable of interconnecting
hundreds of high bandwidth optical channels in a digital computer environment.
The gradient index rod lens
102 forms the backbone of the relaxed tolerance
optical interconnect system
100 of the present invention. The rod lens
102
can be made with a broad range of diameters and lengths by controlling the gradient
of the refractive index profile. The rod lens
102 is typically 0.2 mm-5.0
mm in diameter and can image with high resolution from rod face-to-rod face over
distances from millimeters to many meters.
In the optical data pipe
100, the interconnected planes
10 and
14
of optical channels are rigidly fixed as an integral part of the system. This link
can then be mounted to interconnect multichip modules (MCMs), dies, or boards.
This type of interconnection is illustrated in FIG. 5 where MCMs
112 and
114 are interconnected with optical data pipe
100. Similarly, drivers
amplifiers, and supporting electronics can be grouped in place of MCMs
112
and
114 to form a plug-replaceable optical interconnect component
110
which is flexible and alignment tolerant. This flexible device offers relaxed alignment
sensitivities with very high density of interconnected channels. For example, hundreds
of multi-GHz channels can be interconnected through a cylinder that is ˜2
mm in diameter and which can be several millimeters to meters long. The optical
data pipe can be used for short-range or long-range (e.g., board wrap-around) high
density communication that is established with transceiver modules
110 that
include drivers, amplifiers, and other support for the pre-aligned emitter and
detector arrays and rod lens.
The data pipe or optical interconnect system of the present invention has the
benefit of relaxed alignment sensitivity since the critical elements are rigidly
pre-aligned on the gradient index rod lens. Further, since the rod is flex-tolerant
as shown below, misalignments and vibrations can be tolerated without interrupting
the optical data communication. Additional benefits include a high channel density
of high-bandwidth optical channels with negligible crosstalk or optically added
signal skews, and sparse use of board real estate. This device can be used for
dense short-range and long-range interconnection. An important part of the present
invention is to pre-align and rigidly couple the emitters and detectors to ends
only of the flexible gradient index (GRIN) rod lens thus forming a robust data
pipe which is tolerant of stresses, vibrations, and misalignments typical in high
performance computer and application environments.
The optical data pipe
100, while only typically on the order of several
mm's in diameter, can relay hundreds of channels over distances spanning millimeters
(for MCM-to-MCM communication); several tens of centimeters (for processor array
wrap-around across a board, etc.); or even meters for applications such as linking
supercomputers to external memory. This high density interconnection is accomplished
with low loss, and clean imaging, and extreme densities.
The channels in the optical data pipe
100 can be packed with extreme density.
For example, the optical channel pitch for emitters such as vertical cavity surface
emitting microlasers (VCSELs) can be 125 microns or closer and still permit simultaneous
operation due to recent VCSEL technology innovations. If a 32×32 VCSEL array
has a 62.5 micron pitch, 1024 optical channels could fit in a 2 mm square. The
key to these higher densities lies in reducing the heat and thermal crosstalk between
neighboring channels. It is expected that simultaneous CW operation of arrays such
as these with smaller pitches, e.g., 125 or 62.5 micron pitches are possible.
Higher densities may also be achieved through arranging the VCSEL devices
on a hexagonally packed grid. The VCSEL array may be hexagonally packed in a circular
array, and for bi-directional links, emitters and detectors may be tiled in some
fashion on a single die.
FIGS. 6A and 6B illustrate a dense circular hexagonally packed emitter array
120. Relaxed alignment tolerances are provided by either the natural flexibility
of the slender GRIN rod lens, or flexure mountings
122 can be used to absorb
misalignments between the MCMs and the pre-aligned optical components.
The fieldability and ruggedness of the optical data pipe or optical interconnect
system of the present invention can be increased in several ways. For example,
the ends of the GRIN rod lens may be rigidly affixed to the hosting MCMs. In this
way, the natural flexibility of the data pipe can be used to absorb misalignments
and vibrations among the circuit component ends. Accordingly the strain relief
clamps
124 shown in FIG. 6A can be used to fix the GRIN rod lens rigidly
to the transceiver modules, and since the GRIN rod lens is flexible, it can be
used to absorb flexures and misalignments.
For many applications it is natural for the optical data pipe or optical interconnect
system of this invention to lie parallel to a circuit board, and in those cases
it may be more practical to have the emitter and detector arrays mounted parallel
to the board rather than at an angle to it. This need can be accommodated by modifying
the gradient profile of the rod lens so that conjugate planes are imaged off the
end face of the rod. This permits the device arrays to be mounted on prisms or
sub-assemblies as shown in FIG. 7. Here the end of the rod lens
102 is affixed
to a prism/mirror
136 which directs the light toward the board. While the
optical channel plane
14 is remote from the end of the rod
102 it
is still fixed with respect to the rod end such as by cementing it to the prism/mirror
136. An array of drivers and receivers
130 can also be attached with
interleaving flexible electrical connectors
132 optionally used to provide
further alignment tolerance to component contact pads
134.
In FIG. 7, a prism
136 is used to fold the optical path at a right angle
so the die can be more easily electrically connected to the host. Flexure connections
132 such as "s" springs can optionally be used to allow for small displacements
as may be generated thermally.
Making use of the natural flexibility of the GRIN rod lens
102 is an
important feature of this invention. The GRIN rod lens
102 of The optical
interconnect system
100 of this invention can be bent and the alignment
of the channels is maintained as illustrated in the photograph sequence of FIG.
8. Optical channels
140 remain fixed with respect to the rod lens end
142
as the long rod lens is bent with end-displacements of 0.1, 0.4, 0.7, and 1.0 mm
in photographs
8A,
8B,
8C, and
8D, respectively. As
the GRIN rod lens is deflected while carrying the image of an array of 8 VCSELs,
the output face of the lens was photographed with a CCD camera and the output image
remains essentially fixed in position on the output face. Thus when a detector
array is pre-aligned and affixed on the face, deflections and misalignments of
this order of magnitude can be tolerated without appreciable deleterious effects
on the dense interconnection of the optical channels.
Quantitative data was also obtained where it was shown that for a deflection
of 6 mm, the optical channels are deflected by less than 10 microns, if at all.
Insensitivity of optical channel location with vibrations and misalignments are
accommodated by the present optical interconnect system is a major feature of this invention.
A typical circuit board application for The optical interconnect system of this
invention is illustrated schematically in FIG. 9. Here remote MCMs or devices
152
are interconnected with hundreds of high bandwidth, low crosstalk, low skew channels
using optical data pipes
100 on board
150, and with very efficient
use of board real-estate. In each case, the optical data pipe
100 is flexible
and connected rigidly at its ends only to the interconnected components. Other
board placement variations which can be accomplished by the optical interconnect
system of this invention similarly include, for example, board-to-board side connection,
board-to-board edge connection, and system-to-system interconnection.
A further embodiment
160 of this invention involves the use of a GRIN
rod
lens
102 which is curved and still interconnect optical channel planes
10
and
14 as shown in FIG. 10. This curving rod may be obtained by heating
a straight rod lens.
In FIG. 11 micro-optic lens arrays are shown to reduce the divergence angle (numerical
aperture) of the light beams (channels)
172 emanating from optical channel
plane
10. The smaller numerical apertures of the light beams
174
incident on the rod lens
102 can improve the transmission through the rod
lens and lower the required numerical aperture of the rod lens used in the optical
data pipe
100. This same technique is useful for an optical interconnect
system of this invention where the planes
10 and
14 are spaced apart
from the ends of the rod lens. In such as case, the light beams
172 can
be nearly collimated and directed toward the rod lens
102 face thereby reducing
or eliminating light loss.
Another embodiment of the relaxed tolerance interconnect system
198
of the present invention is shown in FIG. 12, which relaxes alignment tolerances
by mounting symmetric infinite conjugate rod lens imagers
200 and
202
(which may be in the form of GRIN rod lenses) in pairs in front of mating optical
channel planes
10 and
14. This inherently relaxes alignment sensitivities
to gap width and lateral translation because beams from each channel are wide,
collimated plane waves in the gap region
210. Lateral shifts of the order
of channel spacing in plane
10 are usually devastating in micro-optic interconnection
schemes, but result in very little loss with this embodiment of the present invention.
Similarly, longitudinal motions that increase or decrease the gap region
210
produce only a slow walk-off of channel throughput and do not alter the tightly
focused conjugate channel imaging in planes
10 and
14. An optional
alignment sleeve or collar
220 can be used to make the interconnect easily
disconnectable and reconnectable. A keyed groove
222 in collar
220
can be used to prevent rotational misalignment.
The relaxed tolerance approach of this invention as shown in FIG. 12 results
in wide insensitivity to lateral and longitudinal misalignments (parallel and perpendicular
to the direction of the alignment gap), which is critical for practical application.
However there is still pronounced sensitivity to tilts between the two interconnection
planes. Spacers may be used (see below) in some configurations to reduce the presence
of tilts. Alternatively if the collar
220 is used but kept much shorter
than the rod lens lengths, the same type of flexure benefit obtained in the optical
data pipes described in earlier embodiments is retained since the rods can flex.
Such flexure here too is with no ill effects since the collar maintains the alignment
at the infinite conjugate interface region where gap width tolerances are also
relaxed. This "cutting" of the optical data pipe or optical interconnect system
198 at infinite conjugate locations of the internal imaging adds an additional
longitudinal alignment compliance to the system described earlier.
The relaxed tolerance interconnect
198 of FIG. 12 can be used to provide
parallel optical interconnection from board to board and similar scenarios. For
example, FIG. 13 illustrates the relaxed tolerance interconnect system
198
linking channels between boards or MCMs
232 and
234. If no collar
220 is used, spacers
230 may be used to keep the boards parallel
and thus angularly aligned.
Another embodiment of the present invention is the infinite-conjugate relaxed-tolerance
optical interconnect system
300 shown in FIG. 14. This system includes a
pair of transceiver modules each comprising a carrier
312, transceiver die
310, and infinite conjugate rod lens
330. The rod lens
330
is pre-aligned over the transceiver array
310 and cemented in place using
an optical polymer
314 or similar cement, which also acts as a structural
potting compound. The transceiver die may be wirebonded, solder bump bonded, or
electrically contacted by similar means to the carrier. The carrier may be a ceramic
pin grid array, ball grid array, or leadless chip carrier, or other means for electrically
connecting the transceiver die. There is a gap
318 between the two modules.
These mating modules can be mounted on separate boards for board-to-board interconnection,
or alternatively they may be mounted on neighboring circuits, chips, MCMs (multi-chip
modules), etc., in which case the carrier
312 may be eliminated or replaced
with a carrier also holding the circuits to be interconnected (e.g., incorporated
directly in a MCM).
A breakthrough degree of angular tolerance between the optical data pipe transceiver
modules of system
300 can be achieved by proper choice of detector element
sizes with respect to the imaged spot size on the detectors in this optical interconnect
system. A detailed example of the wide angular tolerance available in this infinite-conjugate
relaxed-tolerance optical interconnect system
300 is described in FIGS. 14-18.
The transceiver die
310 of FIG. 14 can consist of tiled arrays of emitters
and detectors together with supporting circuitry for driving of the emitters and
amplifying the output of the detectors (See FIG. 19). These transceiver die consisting
of both emitters and detectors allows for bi-directional interconnection. For simplicity
in the following angular alignment tolerance discussion, however, FIG. 15 illustrates
the uni-directional case of an emitter array
410 which is imaged onto a
detector (receiver) array
420.
Other parameters chosen for this case study include: commercial off-the-shelf
gradient index rod lenses with a diameter of 4 mm; rod lens lengths of 10.24 mm;
VCSEL wavelength 830 nm; VCSEL emission cone angle +/-15 degrees (air). A gap has
been provided between die and lens surfaces to allow wire bond relief. The boards
on which the transceiver modules of FIG. 14 are mounted are not shown in FIG.
16-FIG. 18.
The operation of this Broad Angle Tolerant Optical Data Pipe is illustrated in
FIG.
16-FIG. 18. Additional parameters chosen for this numerical case study
are shown in FIG. 15. The VCSEL array
410 consists of individual VCSELs
412 that are located on an array pitch
430 of 250-microns. The lateral
extent of the array is 2 mm full width, sup-porting 9 VCSELs in that dimension.
This can represent, for example, a cut through a circularly apertured hexagonally
packed array of elements or alternatively a 9×9 array of VCSELs. As described
earlier, the VCSELs and receivers may be tiled in the corresponding 2-D array for
bi-directional interconnection. In FIG. 15 the emitters and detectors are shown
to scale in 1 dimension, and are located in arrays with a 250-micron pitch. The
active VCSEL apertures have been exaggerated in the figure.
The detectors are modeled with a 240-micron wide active area, and a 10 micron
dead space separating the active areas. For reference later in the example, six
special elements have been identified in FIG. 15: the top edge VCSEL
414
and top edge detector
424; the bottom edge VCSEL
418 and bottom edge
detector
428; and the central VCSEL
416 and central detector
426.
The size and location of imaged spots, defined through raytrace calculations, will
be illustrated in FIGS. 16 and 18 for each of these three limiting cases.
The parameters of this case study were chosen as a typical example with which
to demonstrate the very large angular alignment tolerances. Related scenarios with
other parameters, e.g., with larger or smaller detector sizes, follow accordingly.
For example, if very large single channel bandwidths are required, smaller detector
active areas may be desirable and will be accompanied by reduced angular alignment
tolerances. Experiments were performed, however, illustrating a wide angular alignment
tolerance even with 100 micron detector apertures.
FIG. 16A illustrates real optical raytrace data for aligned (no tilt) rod lenses
330, and an air gap
318 of 2.6 mm between Optical Data Pipe module
ends. On-axis VCSEL
416 emits a fan of rays
460 that are imaged onto
spot
450. Similarly top edge VCSEL
414 emits a fan of rays
465
that are imaged on to spot
454. Due to symmetry, the spot
452 on
the top-edge detector element
424 shown in FIG. 16(
b) is identical
to the bottom-edge spot
454 imaged onto detector element
428 as shown
in FIG. 16(
d). FIG. 16(
c) shows the spot
450 on detector element
426. The locations of the spots and their sizes are shown to scale in FIG.
16. The corresponding detector size is also shown to scale. Here the sizes of the
spots represent the dimension enclosing 95% of the optical energy in the imaged
spot. Clearly the channels are linked with negligible crosstalk. Further, it is
seen in this calculation that there are no vignetted rays even with an air gap
of 2.6 mm. When the modules are separated with larger air gaps than these (for
this case), the throughput of the edge optical channels will begin to slowly degrade.
Now the case where one of the modules is tilted with respect to the other is
considered. This tilt represents an angular alignment error between the modules
or the boards that they are mounted on. FIG. 17 illustrates the board-to-board
ODP modules used to interconnect circuit boards that are tilted by 1.4 degrees.
The air gap
318 between the modules is maintained at 2.6 mm, and the second
module is tilted with respect to the first by 1.4 degrees as shown in FIG. 17.
In this figure, the 1.4-degree tilt between the modules is clearly visible.
As a result of the tilt, the spots imaged by the interconnect translate across
the detector plane. This translation of the imaged spots is shown to scale in FIG.
18, and is based on real raytrace data. The tilt angle of 1.4 degrees was chosen
since it still allows for essentially all the light from the optical channels to
fall on their respective detector elements (i.e., no vignetting). This tilt represents
a total tilt budget. In practice, this 1.4-degree angular alignment budget can
be distributed between tilt of the boards themselves, and other tilt sources including
tilt of the carriers in their sockets, etc.
FIG. 18(
a) illustrates real optical raytrace data for rod lenses
330
which are misaligned (tilted) by 1.4 degrees with respect to each other. Air gap
318 of 2.6 mm between Optical Data Pipe module ends is maintained. On-axis
VCSEL
416 emits a fan of rays
460 that are imaged onto spot
450.
Similarly top edge VCSEL
414 emits a fan of rays
465 that are imaged
on to spot
454. Further, bottom-edge VCSEL
418 emits the fan of rays
470 which are imaged onto top-edge detector spot
452. The tilt has
broken the symmetry and spots
452 and
454 are no longer identical.
FIG. 18(
b) shows that the top-edge detector spot
452 has been
displaced 100 microns from the no-tilt case, and has a 95% energy full-width of
55 microns. It clearly lands on detector element
424 as required. Similarly,
FIG. 18(
c) shows that the center element spot
450 also clearly falls
on its targeted detector element
426 and is displaced by 99.8 microns with
a 95% energy full width of 30 microns. Finally, FIG. 18(
d) shows the spot
454 also resides on the targeted bottom-edge detector element
428.
Here the spot center is displaced by 93 microns, with a 95% full-width of 50 microns.
The locations of the spots and their sizes are shown to scale in FIGS. 18(
b)-
18(
d),
where the sizes of the spots shown represents the dimension enclosing 95% of the
optical energy in the images spot. The size of the corresponding detector is also
shown to scale. As can be seen from FIGS. 16(
b)-
16(
d) and
18(
b)-!
8(
d) the detector size is larger than the spot
size. As expected, the on-axis VCSEL is imaged to the smallest spot, while aberrations
increase the spot sizes of the edge elements. Clearly the channels are still linked
with negligible crosstalk. There is an edge of the edge spot that falls on the
10-micron guard band separating detector elements, but very little crosstalk into
the neighboring detector is evident. The crosstalk should be ver
