Design of a highly parallel board-level-interconnection with 320 Gbps capacity U. Lohmanna , J. Jahnsa , S. Limmerb , D. Feyb and H. Bauerc a University
of Hagen, Universitaetsstr 27/PRG, 58097 Hagen, Germany; of Erlangen, Martensstr. 3, 91058 Erlangen; c Microsens GmbH, Kueferstr. 16, 59067 Hamm, Germany b University
ABSTRACT A parallel board-level interconnection design is presented consisting of 32 channels, each operating at 10 Gbps. The hardware uses available optoelectronic components (VCSEL, TIA, pin-diodes) and a combination of planarintegrated free-space optics, fiber-bundles and available MEMS-components, like the DMD TM from Texas Instruments. As a specific feature, we present a new modular inter-board interconnect, realized by 3D fiber-matrix connectors. The performance of the interconnect is evaluated with regard to optical properties and power consumption. Finally, we discuss the application of the interconnect for strongly distributed system architectures, as, for example, in high performance embedded computing systems and data centers. Keywords: optical interconnection, high density I/O, parallel computing, integrated free-space optics, modeling, MEMS, DMD, evolutionary algorithm, multi-core architectures, data center
1. INTRODUCTION In the architecture of future power efficient computer and communication systems, the use of optical interconnection technology can alleviate bottlenecks and enhance performance. For example, optical transmission offers huge bandwidth and transmission distance can be very large without losing significant amounts of power. A specific aspect of optics is that one can use the third dimension as an additional degree of freedom. Thus it is possible to enhance performance and implement modular architectures.1 The spatial enhancement of the system can be used for example, to optimize the critical thermal management for high-performance multi-core systems. Furthermore, the shown 3D-integrated micro optics in section 2 and 3 offers the possibility of novel architectures and novel implementation of certain functions. Here, we report on specific new implementations of crossbar, one based on fiber-connector, one based on integrated free-space optical system.In section 4 a matrix based raytracing method for calculating beam propagation inside 3D anamorphotical optical systems is shown.
1.1 Energy efficient optoelectronical media converting With regards to the increasing demand of power in telecommunication networks due to the exponential growth in recent years, it is important to find ways to limit (or even reduce) the power consumption of the critical components and (sub-)systems. An important aspect is the dissipated power for the media conversion between the electronic and optical domain and vice versa. Here you will find current worldwide research activities taking place, which tend to reduce the optoelectronic conversion energy consumption by designing new more powerefficiency optoelectronical interface with a total power consumption of below 5mW/Gb/s at data rates up to 10 Gb/s (with actually available VCSEL diodes).2 But the importance of low power consumption of information systems exists also for mobile and autonomous information systems, like strongly distributed embedded systems for aircrafts or automotive systems. Here, fiber optical information system shows an additional advantage, because reducing weight by a fiber optical system comparing to multi channel copper cables becomes also an important criterion.2 Further author information: (Send correspondence to U.Lohmann) U.Lohmann: E-mail:
[email protected], Telephone: 0049-2331-987-1120 Optoelectronic Interconnects XII, edited by Alexei L. Glebov, Ray T. Chen, Proc. of SPIE Vol. 8267, 826706 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.905888
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1.2 Modularization of board-level-network architecture for distributed embedded systems and multi-core architectures In a strongly distributed information architecture, the use of high density multi-channel interfaces between electronical and optical interconnection lines are a key feature, because of upcoming complex interconnection geometries and the need for latency-free clock distribution.3 Currently, there exists a strong demand for integrating optoelectronical board-level-interconnections into shorter distances for intra-board- and chip-level-interconnections. The planar-integrated free-space-optics (PIFSO) approach has several advantages for this application, regarding to mechanical and optical robustness.4 In this section we show the design of a new vertically-coupled fiber-optical inter-board-interconnection approach via fiber optical matrices based on the PIFSO-approach to realize a high density multi-channel inter-board-interconnections with up to 32 optical channels.
z
x
z
y
x
y
Figure 1: On the left, the scheme of a modularized multi-channel optical Gb/s board-level interconnection for strongly distributed embedded systems and multi-core architectures is shown. On the right, the used vertical-coupled fiber matrix modules for optoelectronical media converting are depicted. An advantage of the massively parallel Gb/s-coupling of multi-core systems is the capability to spatially enhance the architecture of these systems regarding to reduce critical thermal issues. A remarkable aspect lies in the fact that by spreading out the system in space, the thermal management can become much easier to handle.2
2. VERTICAL FIBER-COUPLING BASED ON PLANAR-OPTICAL FIBER MATRICES The new fiber matrix approach enables the coupling of fiberoptical channels into planar-integrated free-space optical systems (PIFSO) with combining the advantages of 2D coplanar high density fiber-coupling with flexible 3D free space optical interconnection scheme. By flexible positioning the drilling holes onto the 2D-plane and therefore onto a 3D-cube, for example from one glass, it is possible to realize various complex optical interconnection functions like fan-out or cross-connections inside the PIFSO cubus. Due to the nature of light, free space optical channels have no interaction with each other. This fact offers the possibility of various complex 3D interconnections inside the planar-optical cubus, without the classical geometrical cross connection problems of classical copper wire- or microstrip-based interconnections. In what follows, the design of a optoelectronical media conversion based on PIFSO approach is shown. The coplanar geometry of the fiber matrix provides a mechanical robust coupling of the fibers to the PIFSO glass
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cubus and further to the coplanar design of the VCSEL- and PIN-arrays inside this cubus. A microlens-layer for collimating the 32 optical beams is implemented directly beneath the top surface.
z
y
x
Figure 2: Design of a planar-integrated optoelectronical media conversion for up to 32 optical channels with 10 Gb/s data rate capacity The VCSEL diodes emit light orthogonally to the top surface of the substrate and on the receiver-side, the beams propagate through the fibers into the PIFSO-module and further vertical to the PIN-arrays onto the bottom electronic plane. The VCSEL driver and TIA are positioned in front of the PIFSO cubus in the direction of in- and out coming micro stripe lines of the GB/s board. In an earlier project funded by ESA,2 various analyses were carried out with the aim to optimize different aspects of optical board-level interconnection. As one simulation result, the combination of VCSEL and driver into one chip offers minimized power consumption. Because of missing available components, the shown design prefer the conventional distributed components.
Figure 3: Realization of a 16-channel micro optical fiber matrix for vertical multi mode fiber coupling For vertical fiber coupling into the micro optical PIFSO module, a prototype of a fiber matrix was manufactured. The shown fiber matrix contains 16 multi mode fibers with a core diameter of 50μm and ensure an exactly vertical propagation of the beams into the glass cubus. The needed holes are drilled mechanically into the metal with a core diameter of 128μm. The position accuracy of the drilling is actually ±5μm.
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3. MEMS-BASED PLANAR-INTEGRATED FREE-SPACE OPTICAL SWITCHING In a free-space optical design with the possibility of beams crossing each other without interaction, nearly arbitrary interconnection schemes can be implemented, in principle. Certain limitations, of course, stem from the specific geometry and from device limitations. In a PIFSO implementation, one can use lithographically fabricated off-axis microlenses to implement a large variety of optical paths.5
3.1 Design of planar-integrated free space optical MEMS based switching modules In this section the PIFSO approach is used to realize a intra-board-level cross connect based on MEMS-devices. The MEMS-devices for integrated optical beam switching are realized with Digital Mirror Devices from TI, because this device is available, robust6 and easy to adapt in coplanar constellations. Analog to section 1 the planar-optical model is useful to handle multiple optical channels with VCSEL-transmitter-arrays in a spacedivision-multiplex (SDM) constellation. Figure 4 shows a 4x12 channel solution as an example.
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y
x
Figure 4: Scheme of a planar-integrated free space optical based MEMS switching module with 4x12 optical channel For the design of an arbitrary switching network using a PIFSO implementation, an analytical model for the layout of the optical paths will now be presented. Several numerical raytracing tools offer modeling of the geometry inside the planar cubus. One resulting geometry is the so called ’honeycombed switching cells’ as simulated in former works.7 Here the optical routing inside this network is realized with the separately switched micro mirrors of the DMD. In opposite to that approach here we focus on a ’one-touch-micromirror’ solution to reduce the attenuation because of the refection.
4. MATRIX-BASED ANAMORPHOTICAL MODELING OF PLANAR INTEGRATED FREE SPACE OPTICAL SYSTEMS In planar-integrated free-space optics, the 3D anamorphotical modeling of beam-propagation is a helpful method for asymetrical (nonrotations-symetrical) designing of micro optical systems.As remark, the discussed raytracing model contains only geometrical optical aspect, not wave-optical aspects like diffraction or aberration.But for first designing a planed planar-optical systems, this algebraic 4x4 matrix method is very useful. In this section, the mathematical basis of the 4x4 beam transfer matrices is described and the resulting formulas for a PIFSO-MEMS combination are worked out.
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4.1 Mathematical description of 3D anamorphotical nonrotations-symetrical raytracing For the mathematical modeling, we use a description based on 4x4 matrices. This approach may be viewed as an extension of the well-known ABCD matrix concept when going from two to three dimensions. Our approach turns out to be very helpful for the modeling of systems that are not rotationally-symmetric, as it is the case for PIFSO systems8 .9
z
P2 α x
P1
β
y
Figure 5: Geometrical scheme for the 4x4 beam transfer matrix calculation as a enhancement of the well known 2D ABCD-matrices into 3D anamorphotical systems
The geometrical scheme of the method is shown in Figure 5. In a starting Point P1 at a X-Y-plane (Z = 0) of a given 3D space, the description of the beam ν is given by the vector with the parameters of the beam position x, y and the polar-angles α and β analog to the method of the 2x2 beam transfer matrices. ⎛ ⎞ x ⎜y ⎟ ⎜ ν=⎝ ⎟ α⎠ β
(1)
For a more global design approach with consideration the outer-axial-area, we further cancel the linearity approximation of the angles α and β and write the beam vector ν to: ⎛
⎞ x ⎜ y ⎟ ⎟ ν=⎜ ⎝tan(α)⎠ tan(β)
(2)
Similar to the ABCD-matrices of 2D spaces, the transmission of the beam from P1 to P2 can described with a multiplication of a translation matrix T and the beam vector ν as follows: ⎛
νP 2 = T · νP 1
⎞ xP 2 ⎜ yP 2 ⎟ A ⎜ ⎟ = =⎝ C tan(αP 2 )⎠ tan(βP 2 )
⎛
⎞ xP 1 ⎜ yP 1 ⎟ B ⎟ ·⎜ D ⎝tan(αP 1 )⎠ tan(βP 1 )
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(3)
The transmission of the beam can be described with the change of the beam position in a destination X-Yplane (Z = ΔZ) depends on the distance ΔZ and the both angles α and β. The description of the given 4x4 translation matrix T can reduced for further operations to: 0 T = 0
B 0
(4)
The 4x4 translation matrix T includes the block matrix B. B=
ΔZ 0
0 ΔZ
(5)
So we can calculate the beam vector ν at point P2 with the translation matrix T to: ⎛
⎞ ⎛ xP 2 1 ⎜ yP 2 ⎟ ⎜0 ⎟ ⎜ ν2 = ⎜ ⎝tan(αP 2 )⎠ = ⎝0 tan(βP 2 ) 0
0 ΔZ 1 0 0 1 0 0
⎞ ⎛ ⎞ ⎞ ⎛ 0 xP 1 + ΔZ · tan(αP 1 ) xP 1 ⎟ ⎜ ⎟ ⎜ ΔZ ⎟ ⎟ · ⎜ yP 1 ⎟ = ⎜ yP 1 + ΔZ · tan(βP 1 ) ⎟ ⎠ tan(αP 1 ) 0 ⎠ ⎝tan(αP 1 )⎠ ⎝ tan(βP 1 ) tan(βP 1 ) 1
(6)
The resulting beam vector ν2 shows only a change of the beam position at the X-Y-plane in Z = ΔZ,but the same beam angles α and β as expected with a beam transmission function.
4.2 Mathematical description of the 3D planar-integrated PIFSO-MEMS model In the case of the PIFSO-MEMS combination from figure 4 the planar integrated free space optical beam was additional deflected by the used digital mirror devices at the bottom of the constellation. So the matrix method has to be enhanced by a reflectance component. In the following drawing the optical path is shown for one beam. The beam starts at point P1 and is transmitted to point P2 , here the beam was defected by the planar micro mirrors of the DMDTM to the Point P3 . The tilt angle of the micro mirror is .
z
P1 P3
x
P2
y
Figure 6: Geometrical scheme of the PIFSO-MEMS combination for a 3D beam transfer matrix representation
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For the mathematical matrix representation of the scheme in figure 6, a combination of the following optical functions: transmission from pint P1 to point P2 , reflection by the tilted micro mirror surface in point P2 and transmission from point P2 to point P3 is performed by matrix multiplication in the following way:
To calculate the resulting beam position and angles it it necessary to build up a rotations transformation as a result of the tilting beam deflection by the micro mirror. As in picture 6 is shown the plan micro mirror is tilted by the angle around the Y-axis. The needed transformation matrix E can described by following matrix representation: E =
RT 0
0 RT
A C
B D
RT 0
0 RT
=
RT AR RT CR
RT BR RT DR
(7)
Here the rotation matrix R is rotated the cartesian coordinate by the angle . R=
cos − sin
sin cos
(8)
The related transposed matrix RT is : RT =
cos sin
− sin cos
(9)
Further the block matrix D for the reflection of the light beam at the digital micro mirror can to: D=
1 0
0 −1
(10)
We can write the product of the matrix multiplication to: T
R DR =
cos sin
− sin cos
1 0 0 −1
cos − sin
sin cos
=
cos2 − sin2 2 sin cos
2 sin cos sin2 − cos2
(11)
With the usage of additions theorems this expression can reduced to: − cos 2 R DR = − sin 2 T
− sin 2 cos 2
(12)
To build up the needed system matrix E the resulting block matrix RT DR has to be implemented into a 4x4 matrix. ⎛ 1 ⎜0 E = ⎜ ⎝0 0
0 0 1 0 0 − cos 2 0 − sin 2
⎞ 0 0 ⎟ ⎟ − sin 2⎠ cos 2
(13)
Now, we can calculate the beam vector ν2 in point P2 as the reflection of the beam vector ν1 to: ⎛
⎞⎛ xν2 1 ⎜ yν2 ⎟ ⎜0 ⎟⎜ ν 2 = E ν 1 = ⎜ ⎝tan(αν2 )⎠ ⎝0 tan(βν2 ) 0
0 1 0 0
0 0 − cos 2 − sin 2
⎞ ⎞⎛ 0 xν1 ⎟ ⎜ 0 ⎟ ⎟ ⎜ yν1 ⎟ − sin 2⎠ ⎝tan(αν1 )⎠ tan(βν1 ) cos 2
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(14)
The result of this matrix multiplication is:
⎛
⎞ xν1 ⎜ ⎟ yν1 ⎟ ν2 = ⎜ ⎝tan(αν1 )(− cos 2) + tan(βν1 )(− sin 2)⎠ tan(αν1 )(− sin 2) + tan(βν1 )(cos 2)
(15)
As expected, the position of the beam is not changed, only the angles are changed depending on the input angle αν1 and βν1 and the mirror tilting angle .In additional steps of the calculation, now the translation matrix from point P1 to point P2 is multiplied with the reflexion matrix in point P2 and multiplied further with the translation matrix from P2 to point P3 .
Figure 7: Geometrical simulation of a high density MEMS based switchable micro optical 8x8 cross connect. Each input-beam is able to hit each output-position by micro mirror reflection.
The algebraic result can further be used for design optimization, as shown in figure 7. Here a high density micro optical cross connect based on the MEMS-PIFSO approach is simulated with 8x8 channels. Each optical input is connected with each optical output by deflected free space beams. The positions and angles of each beam is optimized to realize a planar integrated free space optical cross connect. Each switching cell contains ≈ 100 micro mirrors and is separated spatially to ensure that neighboured beams are seperated from each other.
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5. SUMMARY AND OUTLOOK In this paper we showed the design of a planar-integrated micro optical inter-board-interconnection based on available optoelectronic components (VCSEL, TIA, pin-diodes) and a combination of planar-integrated free-space optics, fiber-bundles and available MEMS-components, like the DMD TM from Texas Instruments.Furthermore the mathematical approach of a 3D beam transfer matrix for anamorphotical systems is described by extending the well known 2D approach for rotation-symetrical systems of ABCD matrices. The result is the base for further system design and optimizations for complex free space optical functions like multi channel cross connects as shown in figure 7.
Figure 8: Picture of the DMD TM -based demonstrator systems for the analysis of the beam switching by micro mirrors. A couple of switched laser beams are seen onto the MEMS device. The next step wil be the realization of described micro optical system as an integrated monolithic demonstrator based on the simulated geometry to show the capacity of such devices for future greater efficient communication systems.
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