Holographic Optical Elements for optical backplane bus targeted at high speed data transfer Jonathan Ellis a, Robert Mays, Jr.b, Dale Griffithsc a Advanced Communications Concepts, Inc., 9430 Research Blvd. Building IV, Suite 433, Austin, Texas, USA 98759-5769 b R & D M Foundation, 1439 Cardinal Creek Dr., Duncanville, Texas 75137 c Information Management Technology, 109 Oak Street, Vienna, VA, USA 22180-6318 ABSTRACT The primary technical challenge for optical backplanes involve the alignment and optical isolation of multiple data channels. Since most backplanes require data transfer rates greater than a single optical channel can cost-effectively provide, multiple data channels is the common solution for higher aggregate transfer rates. Established optical alignment and isolation techniques include spatial separation of optical channels, use of lensing elements to focus specific transmitter outputs to specific receiver areas, use of differing wavelengths for adjacent channels with appropriate frequency filtering on receivers, and the use of “light guide tubes” for each channel. This presentation will examine another promising option, the use of “matched” Holographic Optical Elements (HOEs) to provide both cross channel optical isolation and to significantly relax traditional optical alignment requirements. Matched HOEs can both induce upon a transmitted optical stream, and then filter upon a received optical stream, a number of distinguishing characteristics such as wavelength, polarization, phase, and amplitude. Thus the use of a unique “matched HOE” pair with each transmitter-receiver pair of multiple optical data channels can provide an efficient mechanism to isolate individual data streams even when they may be physically coincident, such as in a length of fibre optic or when multiple free space data transmitters illuminate several channel’s receiver elements. Thus, the alignment issue is relaxed from the usual constraint of attempting to physically separate channels to one where, as long as the receiver is within the optical cone of it’s matched transmitting element, cross channel interference can be effectively eliminated. Keywords: Holographic Optical Element, free space optics, optical backplane, optical bus, high speed bus, optical isolation, optical alignment 1. INTRODUCTION The data transfer speed , cross talk and length limitations of copper based electronic busses for very high speed backplanes have long been well know and documented.1,2 As backplane capacity requirements move forward to the tens of terabits per second range, optical pathways are widely expected to be the technology of choice for bus interconnect in the increasingly multiprocessor-based high productivity computing systems. 3, 4 These optical backplane systems are anticipated to be comprised of multiple parallel optical data channels transmitted in free space or waveguide based architectures.5, 6 A key implementation requirement of all of these proposed multi-channel optical backplane architectures is some method of providing optical alignment between the individual sets of transmitter/receiver elements and at the same time providing for optical signal isolation to reduce cross talk between the various channels. 4, 7, 8 1.1 Methods of optical alignment and cross channel isolation We may summarize the architectural approaches to providing backplane optical alignment and cross channel isolation into four major implementational methodologies, used individually or in some combinations.
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1.1.1 Spatial separation This approach can be as simple as a linear array of receiver elements with sufficient length between them that there is minimal cross talk from adjacent channels due to spacing being greater than the cast “spot size” of the optical transmitters on either side. While this approach has the merit of simplicity, it does not lend itself to the geometries of multi-element VCSEL or PIN Diode arrays produced from a single wafer, and therefore generally involve additional steps, and expense, for individual element packaging, alignment and final placement. Often the physical distances required for effective optical isolation imply packaging dimensions not suitable for commercial massively parallel computer designs. Multi-dimensional arrays of transmitters/receivers can help minimize the requirement for the array along any single axis, but may add additional complications to the packaging alignment constraints. 1.1.2 Lensing elements Lensing elements can be applied to the transmitting optical element, the optical receiving element or both. Generally the objective is to minimize the “spot size” of the transmitting element such that it converges on only the appropriate single receiving element which is paired to this transmitter. In a sense, this is an active approach to otherwise passive spatial separation method described immediately above. The goal is the same, to eliminate optical channel cross-talk by means of physical signal channel separation so that the optical signal-to-noise ratio of the channel of interest is not lowered due to “noise” from adjacent channels. This approach can also aid in the alignment objective by providing an additional means for redirecting the transmitters beam to the receiver in the event that the physical position of the transmitter/receiver pair can not be guaranteed to be in line with the transmitter’s beam configuration. The primary drawback to this approach would be the effort and cost of alignment of the various lensing elements during manufacture, and the associated impact on the mechanical robustness of the system due to the tight optical alignment requirements of these various elements. 1.1.3 Differing wavelengths Wavelength differentiation can be an extremely efficient channel isolation approach if transmitters of differing wavelengths are a practical packaging option. With channels at differing wavelengths, then optical isolation is a relatively simple matter of placing a suitable frequency filtering element in front of the appropriate receiver so as to remove the potential cross talk optical energy before it can impinge, as noise, upon the receiver. Depending upon the frequencies available, and their harmonics, this can provide relatively high optical isolation. The general problem with this approach is the limited number of frequencies in commercially available packaging. While VCSELs in a number of frequencies can be fabricated, low cost packaging is generally available only in the commonly used fibre optics oriented wavelengths of 850, 1340 and 1550 nanometers. When wavelength only filtering is employed, this severely limits the number of data channels that can be effectively employed. Depending upon the backplane packaging density required, there can also be some impact of preparing the multi-wavelength filtering materials into a commercially cost effective format. 1.1.4 Light guide tubes Light guides can range from lengths of fibre optic materials9, 10 to etched or formed tunnels through some appropriate media such as glass, plastic or gel materials. There are a wide range of methodologies for forming and using such light guides, with varying claims as to process maturity, ease of production, cost effectiveness, mechanical robustness and availability of size packaging suitable to differing interconnect distances needed. 11, 12 Varying forms of multi-channel fiber optics bundles are used in a number of systems13, 14 for interconnect of racks and intra-rack communications, but there use in actual backplane bus products has been much more limited. It might be fair to characterize the marketplace as still considering the cost-benefit aspects of light guides for bus interconnect schemes. 1.2 Another approach to optical channel beam alignment and cross channel optical signal isolation In this paper, we examine the use of Holographic Optical Elements (HOEs) for optical alignment and cross channel signal isolation in a multi-channel free space optical backplane bus. The primary attribute of HOEs we will explore is their application in “matched HOE” sets, that is, the use of the same HOE in line with the channel data transmit element to modulate certain characteristics of the optical beam and an identical HOE to filter the impingement beam striking the channel data receiver element so as to reduce any incident optical energy except that of the matched transmitter.
2. METHODOLOGY 2.1 Theory of “Matched HOEs” Holographic Optical Elements have the property of interest that two identical HOEs will act as “inverse Fourier Transforms” of one another. That is to say, if a light beam is transmitted through a HOE, it will be “scattered” in accordance with the wavefront properties recorded in that hologram. If the light rays emanating from that first HOE encounter then strike a second HOE, identical to the first, those various rays will be recollimated, providing a pure representation of the initial optical beam. If rays from some other, non-matched, HOE, or rays emanating from some other light source, were to strike the second HOE, those rays would simply be further “scattered” by the HOE, thus attenuating the portion which would travel on to strike the receiver. This can be illustrated in the following Figures:
Figure 1. Ideal optical alignment and channel isolation
In Figure 1, we see the ideal situation, where there is perfect optical alignment between the each transmitter and receiver pair and there is complete focusing of all transmitter output directly upon its intended receiver element’s active light reactive area. In this case, 100% of the each transmitter’s optical energy is collected by its paired receiving element and absolutely no optical energy from any other transmitter impinges upon that receiver.
Figure 2. The reality of optical element arrays – Overlap of transmitter beams
Figure 2 shows the reality of most optical arrays, where the output of multiple transmitters beams impinge upon various receiver elements. In a two dimensional array of transceivers, output from a dozen or more transmitters may strike a given receiver, with an attendant increase in the “noise” component of the critical optical signal-to-noise ratio. In this environment, in may be impossible to extract an error free data stream at any commercially acceptable data rate.
Figure 3. Matched HOEs provide optical isolation of each transmitter-receiver data channel
Figure 3 shows the effect of having differing matched sets of HOEs between each data channel’s transmitter-receiver element pair. The HOE in line with the transmitter modulates that data channel with its wavefront characteristics, let us say in this example wavelength specific filtering and a particular polarity. Even though a given receiver element may have the light from multiple transmitters impinging upon its area, the HOE in front of the receiver will filter out all light not of the wavelength and polarity for which it is designed, namely the light from its specific transmitter/HOE. In this way, a very high proportion of the appropriate transmitter’s light energy is collected by the receiver while an overwhelming portion of any other transmitter’s light, or any other incidental source’s light, will be attenuated. This will provide an optical signal-to-noise ratio that will support a significant data transfer rate with a minimal BER, just the result needed for a viable multi-channel optical backplane system. 2.2 Demonstration system optical architecture Figure 4 shows the layout of the optical assembly portion of the test systems being prepared as part of our company’s Holographic Optical Backplane Bus Interconnect Technology (HOBBIT) program to demonstrate the effectiveness of matched HOEs as an optical isolation mechanism. The HOBBIT system comprises a three slot backplane, with each slot’s optical backplane consisting of 16 VCSEL elements and 16 PIN Diode (PD) elements. Mechanically each VCSEL and PD is packaged in an individual TO-46 can, with the cans held in a precision drilled aluminum base plate arranged as 4 rows of 8 can positions. The VCSELs and PDs are interlaced within each row and the rows are interlaced so that each VCSEL is adjacent on a prime ordinal with a PD element at one can-to-can distance, and the nearest adjacent VCSEL is at least 1.4 can-to-can distance units away. Beneath each slot housing is a thin film HOE sandwiched between it and the free space optical wave guide, in this case a glass slab. The HOE is designed provides appropriate diffraction to the normal light path emanating from the VCSELs such that each VCSELs light is “bent” so as to reflect through the slab at the correct angle to strike the PDs of the adjacent slot’s array. This is illustrated in more detail in Figure 5.
Figure 4 – Layout of optical portion of HOBBIT demonstration system
The diffractive portion of the respective HOE’s wavefront design is arranged so that each of the three slot’s VCSEL output is equally received by the other two slots. His is accomplished by having the transmissive and reflective characteristics of each of the slots HOEs separately programmed to divide the light striking it according to it’s position relative to the other two slots. In the case of the center slot, each of it’s VCSELs’ output is evenly divided to reflect both right and left in equal amounts at an angle appropriate to strike the adjacent end slots. The right and left slots are each programmed to direct their 100% of their internally produced optical energies left and right respectively, so as to strike the center slot. The center slot’s HOE reflectivity to incident light is adjusted so that it will pass one half of light incident upon it, and reflect one half at an angle equal to the incident angle, thus passing ½ of the incident light up to the center slot’s receivers and passing ½ the incident light on the slot opposite of the transmission initiating slot. Each of the end slot’s reflectivity are programmed to pass 100% of light incident upon them up through to their respective receiver PDs. In this way, each slot receives ½ of the light transmitted by either of the other two slots.
HOE Array glass Figure 5. Optic reflection pathways from the three slots on the backplane
2.3 Demonstration system electronic architecture The four boards sticking upward from the top of the slot base plates in Figure 5 are the electronics driver boards for the respective slots’ VCSEL and PD assemblies, as detailed in Figure 6 below.
Figure 6. Four electronics driver boards per slot – 4 VCSELs & 4 PDs per board
Each of the four electronics boards per slot contains the driver circuits for one of the four rows of TO-46 cans per slot base plate. Each row contains four VCSEL and four PIN Diode cans, so each corresponding of the driver electronic board contains the electronic drivers for the same. Figures 7 & 8 are the electronic circuits for the individual VCSEL and PIN Diode drivers, respectively.
Figure 7 – VCSEL driver circuit schematic
Figure 8 – PIN Diode/ TIA / Limiting Amplifier circuit schematic
The four electronics driver boards for each slot are in turn connected to a slot connector, which provides the “slot interface” to the higher order system boards which plug into the respective slots. In the current implementation, the higher order cards which plug into the HOBBIT slots are standard Xilinx 410 development boards, each with a wide assortment of I/O peripherals including 10/100/100 Ethernet connections, USB 2.0 slots, Serial ATA interfaced for hard drive and optical mass storage devices, and the usual collection of serial RS-232/485 and parallel electronic interfaces. 2.4 HOE based optical isolation & alignment A key aspect of the HOBBIT system design is the use of Holographic Optical Elements to provide for both optical alignment and channel filtering and isolation. To this end, the HOEs that are sandwiched between the slot base plates and the glass free space waveguide medium are designed to simultaneously provide multiple optical effects, including a diffractive property to align the normal VCSEL light path with the appropriate angle to cause reflection to the adjacent slot base plate holders, and a wavelength filtering property to isolate subgroups of channels from other wavelength based subgroups, and also a polarization property to filter each wavelength subgroup into specific data channels. These HOE optical properties, and how they are utilized, are discussed in the sub-sections below. 2.4.1 Spatial alignment and co-channel optical isolation crosstalk
A
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2 Figure 9 – Holographic diffraction to align slot output to adjacent slot receivers
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As seen in Figure 9 above, the HOE is used to diffract the normal VCSEL output to reflect from the lower surface of the glass waveguide up into the receiver array of the adjacent slot. The angel of diffraction must be carefully controlled such that the output of a given VCSEL will impinge primarily upon its associated PD in the adjacent slot and not the PDs to the immediate diagonals of the intended PD. We are aided in the optical isolation aspects of this effort by an interesting beam forming characteristic of the HOE, namely a horizontal “flattening” of the diffracted beam. This unexpected, but fortunate, optical characteristic has been observed and subsequently modeled. Discussion of it’s impact and mathematics are left to the Data/Results section of this paper below, but it is interesting to note the form of the distortion as seen in Figures 10 & 11 immediately below.
Figure 10 – Original VCSEL output spot profile
Figure 11- Spot profile after passage through HOE
2.4.2 HOE based wavelength co-channel optical isolation We are currently experimenting with use of two and three wavelengths of VCSELs in each slots transmitter mix, utilizing 850 , 1340 and 1550 nm devices, with appropriately tuned PIN Diode receivers. Since the response pattern of PIN Diodes are substantially wider than the transmit width of VCSELs, we utilize the wavelength filter properties of the HOE materials to selectively restrict light passage to a narrow bandwidth of interest for each PD. By alternating the VCSEL wavelength in a transverse manor relative to the VCSEL/PD interlace, we can maximize the spatial distance between adjacent PDs illuminated by the same frequency, as show in Figure 12.
Figure 12- Interlaced VCSELs (rectangles) and PIN Diodes (circles) in two wavelengths
2.4.3 HOE based polarization & wavelength co-channel optical isolation IN addition to wavelength filtering, the HOEs can be programmed to perform filtering upon polarization of the transmitted data channels. By alternating simple Horizontal and Vertical polarization planes on alternate columns of VCSELs and their associated PDs, we can further improve the distance between PDs which are illuminated by identical wavelengths and polarizations. This is illustrated in Figure 13 below. V
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Figure 13 – HOE optical isolation when both alternating column Polarization and two wavelengths of VCSELs are used
3. DATA/RESULTS 3.2 Spatial separation due to HOE diffraction alignment The HOE, sandwiched between the slot base plate holder of the VCSEL and PD arrays and the glass substrate, is programmed with a wavefront to provide diffraction grating like properties to vector the VCSEL optical output at an angle designed to strike the adjacent slot base plates. The calculation of the phase matching condition needed for such programming is shown in Figure 14.
Figure 14 - Phase-matching condition of a Hologram for the surface-normal coupling
The calculations for lateral mismatch between the intended diffraction angle and resultant angle for is given per :
with the calculated results for the test system as shown in Figure 15 below
Figure 15 – Lateral and Angular Mismatch for test system
As noted in section 2.4.1 above, there was an additional spatial separation factor observed with the use of HOE filtering, a horizontal “flattening” of the beam spot which provided the serendipitous effect of reducing cross talk between laterally adjacent receiver elements. The magnitude of this effect can be seen in Figure 16.
Figure 16 – Observed Beam profile resulting from HOE processing of VCSEL output
The effect has been mathematically modeled in a preliminary investigation, with the initial results demonstrated in Figure 17 below. Additional investigation of the optical properties responsible for this flattening are proceeding, and a separate report is expected to be published in the near term on this potentially useful property of HOE filtering.
Figure 17 – Log scale 3D profile of HOE beam flattening
3.3 Optical isolation due to HOE polarization filtering The HOE can additionally be programmed to provide polarization to the transmitted beam and to provide matching polarization filtering to the receiver optical element. The degree of crosstalk signal attenuation is depend upon the type of polarization applied, as well as the thickness of the polarizing HOE material. For simple vertical and horizontal polarization, the reduction in cross talk signal strength is given in Figure 18. The equivalent reduction in the designated channel signal of interest, for the same thickness of HOE, is plotted in Figure 19.
Figure 18 – Crosstalk reduction due to Polarized HOE material thickness
Figure 19 – Signal of interest strength reduction due to due to Polarized HOE material thickness
The resultant “Extinction Ratio”, the net crosstalk reduction, in dB of optical signal strength minus the reduction of signal of interest in dB, is plotted in Figure 20 below. As can be seen, a HOE thickness of 10 to 30 μm seems an appropriate range for optimal signal to noise maximization. A thickness of 20 μm has been chosen for use for all HOEs.
Figure 20 – Net (crosstalk minus signal) reduction verses HOE material thickness
The cross talk reduction impact of simple horizontal verses vertical polarization was tested as setup shown in Figure 21. The results of these tests are graphed in Figure 22, with the result that the net optical strength reduction of a direct signal is reduced over 10 dB, which equates to over 40 dB of cross talk reduction for an adjacent transmitter transmitter’s signal after spatial reduction for fall off in bean strength per center spot distance is calculated.
Figure 21 – Polarization impact on signal strength test set up
Figure 22 – Results of Polarization Impact
4. CONCLUSIONS While the HOBBIT program is continuing to acquire and analyze data, certain conclusions can be confidently drawn from the work thus far. These include the key confirmation that matched Holographic Optical Elements (HOEs) can provide high levels of optical isolation between adjacent, or even spatially coincident, free space data channels. This capability is based upon their ability, when used in unique matched sets per optical channel, to distinguish between channels based upon optical signal characteristics of wavelength, polarization, phase and amplitude. The HOEs can also simultaneously impart diffractive and reflective optical properties to each channel bean so as to provide optical alignment and beam steering to aid in signal distribution and optical cross talk reduction. Inherent in the design of the HOE based optical beam dispersion, the test system demonstrates a significant degree beam alignment robustness. This is expected to equate directly to significant alignment margin allowing for simplified construction tolerances and operational robustness in the presence of mechanical shock and vibration environments. While data rate testing is still in progress with the test system described herein, at this point, no technical obstacle has been determined which would cast doubt on the program objective of providing terabit and above aggregate backplane transfer rates for HOE based bus architectures.
ACKNOWLEDGEMENTS This program is partially supported in its demonstration phase by the Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC, under a contract per provisions of BAA-76-05-01. Portions of the test system construction, test data collection and test data analysis were performed under contract by the University of Texas at Austin’s Optical Interconnect Group, under the direction of Dr. Ray T. Chen, PhD, and including Dr. Wei Jiang, Hai Bi, and Jinho Choi.
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