Feb 6, 2018 - to Don Pavlasek and Joe Boka. (McGill University) who ..... T. H. Szymanski and H. S. Hinton, "Reconfigwable intelligent optical back- plane for ...
Design, Implementation, and Characterizationof an Optical Power Supply Spot Array Generator for a Four Stage Free-Space Optical Backplane
Rajiv Iyer
Deparunent of Electrical Engineering McGill University Montréal, Canada March, 1997
A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements of the degree of Master of Engineering
Q Rajiv Iyer, 1997
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Abstract
In order to alleviate the throughput bottlenecks being encountered by high speed electronic computing and switdUng systems, research is turning its attention toward freespace photonics technology. Of particular interest currently is the use of Hybrid/SEED devices implemented as smart pixel arrays to encoae the electronic data ont0 an array of constant power b e a m of light. This paper presents the design and implementation of a robust, scalable and modular optical power supply spot array generator for a modulator based free-space optical backplane demonstrator. Four arrays of 8 by 4 spots of (l/e2 irradiance) 6 A 7 p radi pitched at 1 2 5 in~ the vertical direction and 2 5 0 p in the horizontal were required to provide the light for the optical intercomect. Tight system tolerances demanded careful optical design, elegant optomechanics, and simple but effective alignment techniques. Issues such as spot array generation, polarization, power efficiency, and power uniformity are discussed and characterizaiion results are presented.
Sommaire
Les processeurs à haute performance nécessitent d'ore et déjà d'ëtre reliés par le biais de connections pouvant supporter des débits d'information extrêmement élevés. La technologie actuelle ne suffisant plus, l'effort de recherche se tourne vers l'utilisation de liens optiques fonctionnant à l'air libre pour remplacer les traces de cuivre actuellement utilisées. L'utilisation de puces optoélectroniques incorporant une matrice de pixels basée sur la technologie Hybrid/SEED pour moduler la lumière semble être particulièrement prometteuse. La conception et la construction d'un module d'alimentation optique générant une matrice de faisceaux nécessaires au fonctionnement d'un démonstrateur de bus photonique fonctionnant A l'air libre est présentée. Quatre matrices de faisceaux de 6.47pm de rayon (rayon défini A l/e2 d'intensité) de dimension 8 par 4 séparés de 1 2 5 verticalement ~ et 2 5 O p hor&ontalement sont requises afin d'alimenter optiquement les quatre étages du démonstrateur. Un minutieux travail de conception optique et optomécanique fût nécessaire afin de rencontrer les exigences du système. Les notions d'efficacité et d'uniformité de puissance de la matrice ainsi que les façons de la générer seront introduites. Des résultats expérimentaux seront également présentés.
Manuscript-Based Thesis - Note for the External Examiner The following five paragraphs have been reproduced in order to inform the external examiner of the Faculty of Graduate Studies & Research regulations: Candidates have the option of induding, as part of the thesis, the text of one or more papers submitted or to be submitted for pubiication, or the dearly-duplicated text of one or more published papers. These texts must be bound as an integral part of the thesis. If this option is chosen, comecting texts that provide logical bridges between the different papers are mandatory. The thesis m u t be written in such a way that it is more than a mere collection of manuscripts; in other words, results of a series of papers must be integrated. The thesis must still confonn to al1 other requirements of the "Guidelines for Thesis Preparation". The thesis must include: A Table of Contents, an abstract in English and French, an introduction which clearly States the rationale and objectives of the study, a review of the iiterature, a final conclusion and summary, and a thorough bibliography or reference list. Additional material must be provided where appropriate (e.g. in appendices) and in sufficient detail to allow a clear and precise judgement to be made of the importance and originality of the research reported in the thesis. In the case of manuscripts CO-authoredby the candidate and others, the candidate is required to make an expiiut statement in the thesis as to who contributed to such work and to what extent. Since the task of the examiners is made more difficult in these cases, it is in the candidate's interest to make perfectly clear the responsibilitiesof al1 authors of the CO-authoredpapers.
The manuscript upon which this thesis was based appears following this thesis. This manuscript was submitted to Applied Optics (Optical Society of Arnerica -OSA) in March 1997. It is currently in the process of review for acceptance for publication. Acopyright waiver from Applied Optics is not necessary as per OSA publication regulations.
iii
Acknowledgments The author would like to extend his heartfelt gratitude to his supervisor Professor David V. Plant for his support and guidance over two years of the best education the author has ever had. The initial design of the Optical Power Supply was done by Dr. Dominic J. GoodwiU (University of Colorado), whose tedinical guidance proved time and again to be an invaluable resource. As well, the author wouid like to thank the assistance by Dr. William Robertson (Middle Tennessee State University), who designed the Multiple Phase Grating, the "heart" of the Optical Power Supply. Editorial input was generously provided by Dr. David V. Plant. Appreciation is given to the following for their assistance: George Smith (Heriot-Watt University) who machined a subset of the optomechanics for the optical power supply, Heinz Nenhvich (NORTEL) who sawed the Multiple Phase Gratings to chip level accuracy, and speual th&
to Don Pavlasek and Joe Boka
(McGill University) who not only machined the majority of the optomechanics for the OPS, but provided invaluable assistance in their design. The author wouid also like to extend a speual th&
.
to Frank Tooley, M i e
Ayliffe, David Kabal, Yongsheng Liu, Guillaume Boisset, Fred Lacroix, and Pritha Khurana for their contributions toward this paper (translation, technical advice, assistance during experiments, analysis of data, etc.). As weU, a global thanks to the entire Photonics Systems Group of McGiU University for their support and patience for the time-sharing of resources, and for my occasional short fuse! Also a very warm thanks to my parents, Bah and Gita, who have provided the best education, love, and support throughout my life, my family in Montreal: Uncle, Aunty, Tara, Vieet, Dileep, Deepa, and baby Arjun, who have kept me very well nourished (physically and spirihially) over the past two years, Saraswati, and Gurumayi Chidvilasananda.
ïï-k work was supported by the Canadian Institute for Telecommunica-
tions Research under the National Centre for Excellence program of the Govemment of Canada, by NSERC (i#X;PO155159) and FCAR (#NC-1415). This work was also supported by the Nortel/NSERC Chair in Photonic Systems. Acknowledgrnent is given to the ARPA/CO-OP/Honeywell DOE Workshop for the manufacture of the multiple phase grating. The author gratefully achowledges funding from NSERC (PSGA).
Table of Contents Chapter 1 Introduction ....................................... 1 1.1 1.2 1.3 1.4 1.5 1.6
Motivation ............................................ 1 The Electronic Bottieneck................................ 1 The Optical Solution.................................... 4 The Optical Backplane ..................................5 Thesis Outline ......................................... 6 References............................................. 7
Chapter 2 System Overview and Optical Power Supply Requirements .................................... 12 2.1 2.2 2.2.1 2.2.2
2.3 2.4
System Overview ..................................... 12 Optical Power Supply Requirements..................... 18 Optical Requirements of the Optical Power Supply ................... 18 Optomechanical Requirements .....................................20
Summary ............................................20 References............................................21
Chapter 3 Light Distribution System .......................... 22 Light Source Distribution System Description .............22 Optimally Launching Light into a Polarization Maintaining Fiber . . . . . . . 24
Characterization of Light Distribution System.............26
Laser Characterization ...........................1................26 Characteriration of the Fiben and the Fiber Splitters ..................29 Characteriration of the Pellicles .................................... 30 Optical Power Budget for the Pellicle Light Distribution ............... 30
Summary ............................................31 References............................................32
Chapter 4 Optical Design .................................... 33 4.1 4.1.1 4.1.2 4.1.2.1 4.13
4.2 4.3 4.4 4.5 4.6
Optical Power Supply Optical Design ....................33 Gaussian Beam Propagation Mode1 ................................. 34 Two-Element Compound Lenses ................................... 39 Variabilityof Focal Length With a ThickCompound Lens .............40 Optical and Optomechanical Degrees of Freedom ....................45
Multiple Level Phase Grating Design ....................46 Design Tolerancing, Modeling and Simulation ............49 Optical Power Budget .................................53 Summary ............................................ 54 References............................................ 54
Chapter 5 OptomedianicalDesign............................ 56 5.1 5.2 5.3 5.4 5.5
OPSBarrel ........................................... 56 CeUholders ..........................................60 fibermount .......................................... 62 Summary ............................................ 63 References............................................ 64
Chapter 6 Assembly and Alignment .......................... 65 6.1 6.2 6.2.1
6.3 6.4
Assembly of the OPS ..................................65 Alignment of the OPS..................................66 Collimation Check Experimental Setup..............................69
Summary ............................................70 References............................................ 70
Chapter 7 Characterization .................................. 72 7.1 7.2 7.3 7.4 7.5 7.5.1 79.2
7.6 1.7
Spots and Spot Array ..................................73 Spectral Behavior .....................................75 Polarization ..........................................75 Beam Steering ........................................ï7 Optical Power Budget ................................. 77 Measurement of O B Throughput Efficiency .........................78 Diffraction Efficiency ............................................. 82
Summary ............................................83 References............................................. 83
Chapter 8 Discussion ....................................... 84 8.1 8.2. 8.3
Increased Optical Power through the System.............. 84 Array Scalability ......................................87 References............................................87
Chapter 9 Conclusion ....................................... 89 9.1
References............................................ 91
Appendix A OSLO Design Specs............................. 92 Appendix B Gaussian Fit Program ........................... 95 Appendix C Notes ......................................... 99
vii
CHAPTER 1
Introduction
Motivation
1.1
Computing and switching systems these days are placing heavier demands on their supporting technology than ever before. High speed data processing and handling systems, such as ATM switches and parallel computing systems, for years have been based upon an electronic foundation. However, the cal1 for higher bandwidth, lower power consumption, lower latency, and higher connectivity, represent a set of growing requirements that are exceediig the practical physical limitations of electronics technology, more specificaiiy, the actual interconnects from board-to-board.
The Electronic Bottleneck
1.2
Shown below in Table Tl-1 are the projections from the Semiconductor Industry Association (SM)for Silicon integrated circuits [Il.
Yeu.-
Feature Size (microns)
1995
0.35
1998 2001
Pin-out Number (Rent's Rule)
On-Chip
Off-Chip
Clock
Clock
800K
200MHz
100MHz
980
0.25
2M
350 MHz
175 MHz
1640
0.18
5M
500MHz
250MHz
2735
-
1
2004
2007
(
0.12 O.I
(
10M 20M
( 700 MHz ( 350MHz (
1
IGHZ
1
SWMHZ
1
Table Ti-i: Semiconductor lndusùy Association Rojections for IC Rates
Chapter 1
4026
(
59281
On-chip dock rates for current highspeed processor chips are typically 150MHz, resulting in huge aggregate bit rates of almost 100s of Gigabits per second (Gbps) per printed circuit board [2,3,4,5]. Graph G1-1 shows the trend for aggregate throughput for several high-speed processors.
-
-+- DEC-ALPHA
Pentiurn
...................
.
-
,-: :
i.................................
i
1975
1980
1985
1990
1995
Year Graph Gl-1: Aggregate ïhmughput for Selected Processors
The SIA projections indicate that with the increase of off-chip dock rates and of the nurnber of pin-outs per chip, within a few years, aggregate data rates will be in excess of 1Terabit per second (61. Because the aggregate bandwidth of the integrated circuits inside these systems continues to increase, so m u t the capabilities of the intercomection nehvork [7][3]. One of the fundamental problems with electrical intercomects is the bandwidth. Quantitatively, fast GaAs transistors have switching times below l0psec. However, due to parasitic effects caused by packaging, these rise and faIl times increase to the order of 100psec.Once the signal is sent along the board, and then to other boards via an electrical bus within a backplane, these times inaease to the order of more than l0nsec - an overall inaease by 3 orders of magnitude. Other
Chapter 1
2
problems encountered with board-to-board eledrical intercomects are power consumption (over distances of greater than Imm), aosstalk, low fan-in and fanout, skewing, electro-magnetic interference (EMi), ground lwps, splitting losses [BI, and capacitive loading effects [9]. An illustration of a typical electrical backplane is shown in figure FI-1.
Printed circuit boards (PCBs) housing high-speed electronic integrated circuits (1Cs) are comected to one another via an electrical bus comprised of îypicauy 32 high speed transmission lines. The total number of lines is limited by size of the chassis, the minimum allowable iine separation,and the dimensions of the electrical connectors on the PCB.
/ Printed /
Circuit
Electrical & Mechanical Support Structure
Figure FI-1: Standard Elechical Backplane
Backplane target specifications [IO] forecast 1000-5000
bus comections
benveen 10-50 boards, bit error rate of 10-14,bus dock speed of lGb/sec, greater
than lTbit/sec aggregate throughput, and a latency per c o ~ e c t i o nof less than h e c . Based on the limitations of electrical backplanes, an alternative needs to be found. Chapter 1
3
1.3
The Optical Solution
The intrinsic limitations of electrical intercomection networks has led system designers to consider short-distance optical intercomects (Ois) as a way of increasing their performance [Il-211. The advantages of OIS over electrical interconnects in terms of bandwidth, comectivity, power consurnption, and skew, along with some furthe: benefits are described below. Electronic interconnects have a physical limit on the communication bandwidth because of the inherent resistances in the transmission line, the capacitive load, and the inductive coupling between adjacent lines and devices [9] [22]. Light, with its inherently high temporal bandwidth (of approximately 1014 Hz) can accommodate the projected high data rates (of approximately 1012Hz). Exploiting the third dimension not available to electrical busses, a 2 dimensional array of optical data signals can be transrnitted from board to board, increasing the comectivity of the intercomect (i.e. spatial bandwidth). As irnpedance matching and capacitive loading are no longer issues in Ois, the only power concems deal with the optical losses within the interconnect, and electrical-to-optical and optical-to-e!ectrical conversions in the transmitter and in the receiver respectively. [23] Because the speed of light is constant (3.3ps/mm), skew problems associated with variations of signal speeds in electrical connections (between 6.8 to 10.2 ps/mm) are avoided. [22] EMI is not a problem Intercomection architectural rnaps (e.g. fan-in, fan-out, projection, perfect shuffle) can easily be realized with simple optical componenh (lenses, gratings, etc.). Reconfigurable interconnections are sirnpler to design.
1.4
The Optical Backplane
The optical and optoelectronic technologies being considered for board-toboard intercomects indude two-dimensional arrays of both surface-emitting and modulator-based devices integrated with arrays of electronic processing elements, called Smart Pixel Arrays (SPAs) 1241. These smart pixels cari be implemented with processing electronics to build a new class of 'intelligent optical backplanes' [25]. An illustrationof an optical backplane (equivalently known as a photonic backplane) is shown in figure FI-2 below.
1-
Optomechanical Structure ,
\ 1/ Electronic I C S
Sm& Pixel CornmuNcation' Cham~ck Arrays
l
Figure FI-2: Schematic of a Opiical Backplane
The current state of technology dictates that modulator based optoelectronics be used for the OIS,as the level of sophisticationof large arrays of uniforrn surface-emiîfinglasers, namely Vertical Cavity Surface Ernitting Lasers (VCSELs), has as yet not reached the level of adequacy to be used in these systems. A com-
Chapter 1
parison between different transmitter technologi@is presented in [26], and has been recently reported in [IO]. A class of modulator based SPA5 well suited for optical backplane inter-
connection applications utilizes the Hybrid/SEED technology which combines Quantum Confined Stark Wect (QCSE)modulators and PIN photodiodes (GaAs) with underlying silicon processing electronics [271 [28]. Because this type of smart pixel operates (in the transmit mode) by modulating an incident beam, systems utilizing this technology require optical power supply beams in order to power these reflective devices. The current state of affairs shows, unfortmately, that there is a generation gap between the evolution of the sophisticated optoelectronics versus the optics necessary to drive thern. Anaiyzing the enabling technologies required to build an optical backplane [IO], it is easily seen that although optoelectronicVLÇI fabrication, and transceiver circuit design is highiy sophisticated, optical packaging, optomechanics, and assembly & alignment techniques are in their infancy.
Recently, the Photonics Systerns Group at McGill University has constructed an optical backplane demonstration system utilizing Hybrid/SEED SPAs [15] to address the problem of bridging the generation gap that exists between
optoelectronics and optics. This thesis describes the design, implementation and characterization of an optical power supply spot array generation system that was used to opticaiiy power the SPAs in a four stage freespace optical backplane. The m l description of the optical design for the system was described in [29], and the optomechanical design in [30]. The paper begins by describing the requirements for the optical power supply (OPS) in Chapter 2. The light source and distribution are explained in Chapter 3. Chapter 4 describes in detail the optical desi@, and Chapter 5 the
Chapter 1
6
optomediania. The assembly and afigrunent methodology, and the characterization results are provided in Chapters 6 and 7 respectively. Chapter 8 discusses some higher level issues of opticai power in modulator based systerns. This paper is an elaboration of a manuscript submitted to Applied Optics (Optical Society of Arnerica) by R. Iyer, et al., [31]. Many sections have been expanded to provide mathematical justifications of models used, and to provide details on the experimental techniques employed. This thesis is written to provide a road-map for future engineers and engineering students building optical/optomechanical systerns.
1.6
References
The National T e s h n o l o g v m a p for Semiconductorç. Semiconductor Industry Association, (1994). T. H. Szymanski and H. S. Hinton, "Reconfigwable intelligent optical back-
plane for parallel computing and communications", Appl. Opt. 35,12531268, (1996). R A. Nordin, F. J. Levi, R. N. Nottenbwg, J. O'Gorman, T. Tanbun-Ek, and
R A. Logan, "A systems perspective on digital interconnections technol-
ogy," J. Lightwave Technol. 10,811-827 (1992). T. Koinuma and N. Miyaho, "ATM in 8-ISDN Communication Systems and VI51 Realization,", IEEE Journal of Solid State Circuits, 30,341-347, (1995).
D. Bertsekas and R. Gallager, Data N e t w o r k s . Prentice Hall, New
Jersey, 52-56 and 128-139, (1992). H. S. Hinton and T. H. Szymanski, "intelligent Optical Backplanes", Proceedings of the Conference on Masçioely ParaIlel Processing with Optical Inter-
connects, Oct 23-24, San Antonio, Texas, 1995.
Chapter 1
H. S. Hinton, A
n
n
z Fabrics. Plennurn
Press, hW (1993). D. H. Hartman, "Digital high speed interconnects: a study of the optical alternative", Optical Engineering, 25,1086-1102,
(1986).
A. Louri, and H. K. Sung, "3D optical interconnects for high speed interchip and intraboard communications, IEEE Cornputer Magazine, 27, October 1994. D. V. Plant, "Constructing a free-space optical backplane: Challenges and dioices", presented at the Optics in Compiiting Meeting at the 1997 Spnng
Topiral Meetings, OThB1, March 18-22, Lake Tahoe, Nevada, 1997. D. V. Plant, B. Robertson, H. S. Hinton, W. M. Robertson, G. C. Boisset, N. H. Kim, Y. S. Liu, M. R Otazo, D. R. Rolçton, and A. Z. Shang, "An optical backplane dernonstrator systern based on FET-SEED smart pixel arrays and diffractive lenslet arrays," EEE Photon Tech. Lett. 7, 1057-1059 (1995). T. Sakano, T. Matsurnoto, and K Noguchi, "Three-dimensional board-toboard free-space optical interconnects and their application to the prototype rnultiprocessor system-COSINE-m," Appl. Opt. 34, 1815-1822 (1995). D. 2.Tsang and T. J. Goblick, "Free-space optical interconnection technology in parailel processing systerns," Opt. Eng, 33,15244531 (1994). D. V. Plant, B. Robertson, H. S. Hinton, M. H. Ayliffe, G. C. Boisset, W. Hsiao, D. Kabal, N. H. Kim, Y. S. Liu, M. R. Otazo, D. Pavlasek, A. Z. Shang, J. Simmons, and W. M. Robertson, "A 4 X 4 VCSEL/MSM optical backplane dernonstrator systern," in Proceedings of the IEEE-LEOS Annual
Meeting 1995 (institute of Electrical and Electronics Engineers-Lasers and Electro-Optics Society, New York, 1995),Postdeadline paper PD2.4.
Chapter 1
[15] D. V. Plant, B. Robertson, H. S. Hinton, M. H. Ayliffe, G. C. Boiset, D. J. Gwdwill, D. N. Kabal, R Iyer, Y. S. Liu, D. R Rolston, W. M. Robertson, and M. R Taghizadeh, "A multistage CMOS-SEED optical backplane demonstrator system," Opt. Comput 96,14-15 (1996). [16] F. B. McCormick, F. A. P.Twley, T. J. Cloonan, J. L. Bmbaker, A. L. Lentine, R L. Morrison, S. J. Hinterlong, M. J. Herron, S. L. Walker, and J. M. Sasian, "Experimental investigation of a free-space optical switching network by using symmeiric self-electro-optic-effectdevices," Appl. Opt., 31, 54315446 (1992). [17] F. B. McCormick, T. J. Cloonan, A. L. Lentine, J. M. Sasian, R L. Morrison, M. G. Beckman, S. L. Walker, M. J. Wojcik, S. J. Hinterlong, R. J. Crisci, R A. Novomy, and H. S. Hinton, "Five-stage free-space optical switching network with field-effect transistor self-electro-optic-effect-devicesmart-pixel arrays," Appl. Opt., 33,1601-1618 (1994). [18] F. B. McCormick, A. L. Lentine, R L. Morrison, J. M. Sasian, T. J. Clwnan, R A. Novomy, M. G. Beckman, M. J. Wojcik, S. J. Hinterlong, and D. B. Buchholz, "155 Mb/s operation of a FET-SEED free-space switching network", Photonics Technology Letters, 6,1479-1481 (1994). [19] F. B. McCormick, A. L. Lentine, R L. Morrison, J. M. Sasian, T. J. Clwnan, R.A. Novomy, M. G. Beckman, M. J. Wojcik, S. J. Hinterlong, and D. B. Buchholz, "Free-space optical switching using FET-SEED smart-pixel arrays", Jnst. Phys. Conf. Ser. No 139:Part II, 131-136 (1994). [20] D. V. Plant, B. Robertson, H. S. Hinton, W. M. Robertson, G. C. Boisset, N. H. Kim, Y. S. Liu, M. R. Otazo, D. R. Rolston, A. Z. Shang, and L. Sun, "A FET-SEED Smart Pixel Based Optical Backplane Demonstrator", Jnst. Phys. Conf. Ser. No 139:Part 4 145-148 (1994). [21] S. Araki, M. Kajita, K. Kasahara, K. Kubota, K. Kurihara, 1. Redmond, E. Sdienfeld, and T. Suzaki, "Experimental free-space optical network for massively parallei computers", Appl. Opt., 35,1269-1281 (1996).
B. E. A. Saleh, M. C. Teich, Fundamentals Wiley and Sons, New York, Ch. 21.4,855-871 (1991). D. Z. Tsang, "High speed optical interconnections for digital systems", The Lincoln Laboratory Journal, 4 , 3 1 4 ,(1991). T. H. Szymanski, and H. S. Hinton, "A Smart Pixel Design for a Dynamic Free-Space Optical Backplane", IEEE/LEOS Sumrner Topical Meeting on Smart Pixels - 94,Lake Tahoe, NV,July 11-13,85-86(1994). H. S. Hinton, T. H. Szymanski, "Intelligent Optical Backplanes", Proceedings of the Conference on Massively Parallel Processing with Optical Intercono, 1995. nections, Oct 23-24, San A ~ ~ o NTexas, C. Fan, B. Mansoorian, D. A. Van Blerkom, M. W. Hansen, V. H. Ozguz, S. C. Esener, and G. C. Marsden, "Digital free-space optical interconnections: a comparison of transmitter technologies", Appl. Opt. 34, 3103-3115 (1995). K. W. Goosen, J. A. Walker, L. A. D'Asam, S. P. Hui, B. Tseng, R. Leibenguth, D. Kossives, D. D. Bacon, D. Dahringer, L. M. F. Chirovsky, A. L. Lentine, and D. A. B. Miller, "GaAs MQW modulators inteeated with silicon CMOS," IEEE Photon. Technol. Lett., 7,360-362 (1995). D. R Rolston, D. V. Plant, T. H. Szymanski,H. S. Hinton, W. S. Hsiao, M. H. Ayliffe, D. Kabal, M. B. Venditti, P. Desai, A. V. Kriihnamoorthy, K. W. Goosen, J. A. Walker, B. Tseng, S. P. Hui, J. C. Cunningham, and W. Y. Jan, "A hybrid-SEED smart pixel anay for a four-stage intelligent optical backplane demonstrator," IEEE Journal of Selected Topics in Quantum Electronics, 597-105 (1996). B. Robertson, Y. S. Liu, G. C. Boisset, D. J. Goodwill, M. H. Ayliffe, W. H. Hsiao, R Iyer, D. Kabal, D. Pavlasek, M. R. Taghizadeh, H. S. Hinton, and D. V. Plant, "Optical design and characterization of a compact free-space
Chapter 1
photonic backplane demonstrator," presented at the 1996 OSA Anmial Meeting, Rochester, NY,paper MLL5 (1996). (301 G. C. Boisset, M. H. Ayliife, D.J. Goodwiii, B. Robertson, R Iyer, Y. S. Liu, D. Kabal, D. Pavlasek, W. M. Robertson, D. R Rolston, H. S. Hinton, and D.
V. Plant, "Design, fabrication and characterization of optomechanics for a hybrid fourstage free-space optical backplane demonstrator," presented at the 1996 OSA Annual Meeting, Rochester, NY,paper MBBB4 (1996). 131) R Iyer, D. J. Goodwill, W. M. Robertson, D. V. Plant, M. H. Ayiiie, G. C. Boisset, D. Kabal, F. Lacroix, k: S. Liu, B. Robertson, "Design, Implementation, and Characterization of an Optical Power Supply Spot Array Generator for a Four Stage Free-Space Optical Backplane", manuscript submitted to Applied Optics, Mardi 1997.
Chapter 1
CHAPTER 2
System Overview and Optical Power Supply Requirements
In order to establish the teclmical context of this paper, a brief description of the system demonstrator is provided in section 2.1. With this overview, the functional utility of the OPS will have been established, resulting in a set of well defined requirements, which are presented in section 2.2.
2.1
System Overview
The system that was built by the Photonics System Group (McGillU~versity) was called the Phase II demonstrator. The system was built to demonstrate the possibiiity and feasibility of building a complex photonic (i.e. optical & electronic) system compact and robust enough to be assembled in an industrial housing (a standard VME commercial backplane chassis) to optically interconnect four (electrical) data nodes via modulator based optoelectronics. The four-stage system demonstrator was built in a three-dimensional layout, intercomecting four Hybrid/SEED smart pixel arrays in a unidirectional ring (11.The chips were obtained through the ARPAICO-OP/AT&T workshop [2] and
a layout schematic of the modulators and detectors is shown in figure F2-1. Sixteen smart-pixels operating in dual-rail were arranged on the chip in interleaving col-
of detectors and modulators. The modulators of the smart pixel arrays
(SPAs) were laid out on an 8 by 4 grid pitched 1 2 5 p in the vertical direction and 2 5 0 p in the horizontal direction.The 32 modulator windows had a dimension of 2 0 p by 2 0 ~A .full description of the chip design is given in 131. A photograph
of the Hybrid/SEED chip is shown in figure F2-2.
Chapter 2
Smart Pixel
U
Modulators 1 2 5 ~ Detectors (Modulator and Detector windows not to scale) Figure F2-1: layout of the modulators and transmitterson the HybndlSEED chip
I
Figure F2-2: Photograph of the HybridSEED Chip
Chapter 2
A schematic of the unfolded optical layout of the systern is shown below in
figure F2-3. (This figure is slightly misleading since the printed circuit boards should lie in the plane of the page and the optical power supplies perpendicular to the page).
Figure FZ-3:Schematic of the unfolded system
Chapter 2
The optical interconnect was polarization based, and routed the optically encoded data from one stage to the next via polarization optics. A dose-up of one stage is iilustrated in figure F2-4.
Figure F2-4:Close up of one stage
The focused spot array generated by the optical power supply was first collimated by the ( 1 2 5 p x 1 2 5 p ) micro-lenses of the first pixelated-mirror/diffractive lenslet array @Al). The light comprising the spot array needed to be right-hand circularly-polarized such that after passing through the first quarterwave plate (QWP1) (oriented at 45' in the x-y plane with respect to the axis of the polariziig beam spiitter (which has as yet not been introduced), it became iinearly
Chapter 2
15
(p-) polarized. After passing through the polarizing beam splitter (PBS), and the second quartenvave plate (QWP2) (also oriented at 45' in the x-y plane with respect to the axis of the PBS) which re-circularized the polarization, the beam array was then focused ont0 the modulators on the Hybnd/SEED smart pixel device array residing on the printed circuit board (PCB) by the second diffractive lenslet array (LM). The primary physical difference behveen the pixellated-mirror/lenslet array (LAI) and the second lenslet array (LA2) is illustrated in figure F2-5. Note that 4 columns of (pixellated) mirrors are interlaced behveen 4 columns of diffractive lenslets, while the entire LA2 is comprised of an 8 by 8 array of lenslets. As will be described below, the mirrors were used to route incoming beams from the previous stage toward the Hybrid/SEED SPA.
LAI - Pixellated-Miror/ Lenset Array
Pixellated Mirrors
LA2 - Second Lenslet Array
Lenslets
Figure F2-5:Schematic ciifferencesbetween the LAI and LA2
Chapter 2
The reflected (modulated) light off of the modulators on the Hybrid/ SEED chip was then re-coiiimated through the lenslet array ( W ) , and its polarization liiearized to s-polarization through QWPZ. Entering the PBS, the s-polarized light then reflected off the PBS mirror, to be routed to the next stage. Figure F2-4 also illustrates the light relayed from the previous stage. ïhis incoming light, stiil s-polarized, was reflected off the PBS mirror surface toward LAI, after passing through the QWPl which circularized its polarization. The
beams then hit the pixellated M o r s on LAI, and bounced back toward the device array, passing through the same optical path as the light from the O S (as described above). The relayed beams however were displaced (in the x direction) 1 2 5 p away from the OPS beams, thus impinging detectors (as opposed to mod-
uiators) o n the Hybrid/SEED SPA. It shouid be noted future reference that the QWP1, PBS, and QWP2 were pre-glued @y the supplier Meadowlark Optics) into what was collectively called the PBSQWF assembly. The PBSQWP assembly, LA1 and W were mounted ont0 an optomechanical housing calied the lenslet barrel, which, along with the OPS module resided within a larger housing called the outer barrel. A picture of the assembled system is shown below in figure F2-6.
Populated Outer Barr with OPS and Lenslet Barrel
Vertically mounted baseplate
Commercial VME backplane chassis Figure FZ-6:Photograph of assembled system
Chapter 2
Optical Power Supply Requirements
2.2
The optical and optomedianical requirements analysis of the system demonstrator directed a modularized approach to the design. The following subsecbons embody the results of this analysis pertaining to the OPS module to provide a full and comprehensive foundation for its design.
2.2.1 Optical Requirements of the Optical Power
S~PP~Y The 32 modulator windows ( 2 0 p by 2 0 p ) o n the device array (figure F21) represent the targets for the spot array having passed though the PBSQWF
assembly and the lenslets from the OPS (refer to figure F2-3). The requirements of the spot array at the output of the OPS, in order to hit the target modulators on the chip is given in Table T2-1, and a sdiematic of the desired spot array (looking
in the direction of light propagation) is shown in figure F2-7. It should be noted that in the figure, the 8 by 4 central grid represent the signal spots (i.e. those impinging upon modulators on the device array), while the those on the periphery correspond to alignment spots used to facilitate the integration of the system demonstrator. OPS Spot m y requirement 8 by 4 focused spots on a uniform grid of 125pm (vertical) by 250 pm (horizontal) 8 additional peripheral spots
Spot array positioned between 18.345 0.82rnm away from the output of the OPS l/e2 irradiance spot radii of 6.47pm
1 Slower than V6 beams generating spot array Stable right-hmd circularly polarized light Minimal field curvature of spot array Table TZ-2 OPS Spot Array Requirements
Chapter 2
1
Beam steenng capabilities of better than: I400pm lateral translation 0.46' angular deviation
+
Spectral tolet-ance of 850elnm Table TZ-k OPS Spot Array Requirements
Although the requirements listed in Table T2-1 suffice for the Phase II system demonstrator, it was also desired that the optical design be flexible to accommodate a larger array of target modulators for scalability. Signal spots
Alignrnent spots
Spot Size o= 6 . 4 7 ~
Figure F2-7: Schematicof the desired spot array at the output of the OPS
Chapter 2
2.22 OptomechanicalRequirements The system demonstrator was built upon a vertically mounted baseplate housed in a standard 19"6U VME commercial backplane chassis [4]. Based on the high level of integration, the optical power supply modules needed the foliowing features: compactness
ease of machinability ease of assembly ease of alignment modulariîy
2.3
Summary
In order to facilitate the integration of the system, a modularized approach was adopted, such that each unit of the system could be pre-assembled and aligned. With this mind-set, the interconnection scheme for 4 optoelectronic Hybrid/SEED SPAs was envisaged employing a fairly complex optical interconnect, ho~isedwithin a standard commercial backplane VME chassis. From this analysis emerged a set of well defined requirements for the OPS. In the foliowing chapters, a dissection of the optical and optomechanical design of the OPS will be provided. Preceding which, however, a discussion of the light provision will be presented.
Chapter 2
2.4 [l]
References
D. V. Plant, B. Robertson, H. S. Hinton, M. H. Ayliffe, G. C. Boisset, D. J. Goodwill, D. N. Kabal, R Iyer, Y. S. Liu, D. R Rolston, W. M. Robertson, and M. R Taghizadeh, "Amultistage CMOS-SEED optical backplane demonstrator system," Opt. Comput. 96,14-15
[2]
(1996).
ARPA/AT&T Hybnd SEED Workshop, July 18-21, 1995, George Mason University, Virginia (1995).
[3]
D. R Rolston, D. V. Plant, T. H. Szymanski, H. S. Hinton, W. S. Hsiao, M. H. Aylife, D. Kabal, M. B. Venditti, P. Desai, A. V. Krishnamoorthy, K. W. Goosen, J. A. Walker, B. Tseng, S. P. Hui, J. C. Cunningham, and W. Y. Jan, "A hybrid-SEED smart pixel array for a four-stage intelligent optical backplane demonçtrator," IEEE Journal of Selected Topio in Quantum Electronics, 2,97-105 (1996).
[4]
IEEE Standard 1014 for a Versatile Backplane Bus: VMEbus (1987).
CHAPTER 3
Light Distribution System
In order for the optical power supply to provide the desired output, it was imperative that the light provided at its input be exhemely weil behaved. A light distribution system was designed and characterized to provide the light to the system demomtrator. This chapter describes two methods that were attempted. The first using polarization maintaining fiber splitters, was rejected, and replaced with the second employing pellicles. A description of the light distribution systems are presented in section 3.1. Characterization results are presented in section 3.2.
3.1
Light Source Distribution System Description
As was shown in figure F2-3, the system was a four stage optical backplane, with each stage requiring an OPS to provide the array of constant optical power beams to illuminate the modulators on the respective Hybrid/SEED chip. For simplicity, optomechanical compactness, and ease of pre-alignment, light was launchedinto the OPS via a single mode polarization maintaining fiber. For practicality purposes, a single 500mW tunable laser with an extemal grating for wavelength selection and stabilization (Spectra Diode Labs Mode1 # SDL 8630 Tunable Laser Diode System) was used to provide the light for al1 four stages (see figures F3-1 or F3-2). A 500mm focal length lem was used to squeeze the beam through the 4.8mm aperture of a Faraday isolator (OFR Part # IO-5-TiSZ), and re-collimated to a (l/e2 irradiance) beam diameter of 1.2mm through a 200mm lem. The Faraday isolator was used to eliminate backreflections into the laser, and a Glan Laser polarizer (OFR Part # PEH-8-TEP) was used to improve the extinction ratio to better than 40dB (which was beyond the measurement capabilities of the New-
Chapter 3
22
port Model # 2832-c dual-channel power meter equipped with Model # 818-ST/ CM detector heads). The first approach of light distribution employed the use of a tree of three 1:2 fiber splitters (iDS Fitel Part # AC-PM11-850-FP) as shown in figure F3-1. However, due to power loss and polarization instabilities, this arrangement was rejected. Halfwave plate hlator
\
1:2 Fiber Splitters
Figure F3-1: Light Distribution System Using Fiber Splitters
A second arrangement was employed using three thin membrane (linearpolarization preserving) pellicles (National Photocolor Order Spec: 1" Pellicle
ETP Coated 50/50 for p-pol 8850nm
@4S0),
as shown in figure F3-2, which
incurred no significant power losses nor polarization instabilities. Characterization results will be provided in the next subsection for both of these systems.
1
Fiber Coupler Polarization Maintaining Sinale Mode Fibers Figure F3-2: Light Distribution System Using Pellicles
Chapter 3
In the pellicle arrangement, each beam was subsequently coupled into a 1 meter polarization maintaining (PM) single mode fiber (Fujikura PANDAm 850nm (see Note [A] in Append'ix C) - supplied by JDSFitel Part # A0101564) using a fiber coupler (Oz Optics Part # HPUC-23-850-P-6.2AS-11) to provide the optical inputs to each OPS module. Spectral stability was maintained by the laser to 850.M.05nm which was within the I l n m spectral tolerance demanded by the SEEDs.
3.1.1 Optimally Launching Light into a Polariza-
tion Maintaining Fiber Following the very simple alignment technique provided by the supplier of the fiber coupler (Oz Optics), 70% of the incoming light was launched into the fiber. Aligning the linearly polarized light along lhe PM fiber's fast axis was experimentally verified to provide better polarization stabiiity at the output compared to launching along its slow axis. The determination of this conclusion was dependent on the experimental method used to optimally align the polarization into the PM fiber. Several methods of optimally orienting the polarization of the incoming light into a PM fiber have been reported [1][2]. These techniques being too timeinefficient and unnecessarily complicated were replaced with an extremely easy and quick method which produced excellent results. The experimental setup is illustrated in figure F3-3. Two halfwave plates (HWP1 and HWPZ), a collimating lem and a polarizing beamsplitter (PBS), in
conjunction with a Newport dual-channel power meter (Mode1 # 2832-c) were used to perform the alignment. By actively monitoring the real-time data acquisition (via a GPIB interface to a computer) of the ratio of the powers read from channels A and B of the meter, Lie optimal orientation of HWPl was achieved by the following iterative method:
Adjust HWPl Adjut H W P ~ to maximize the ratio: P A / ( P + ~ PB) Repeat
r
1
HWPl
Collimating Lens
Computer
HWPZ
PBS
Dual Channel Power Meter
Figure F3-3: Experimental Setup to Orient the Polariution of the Light
Results showed that aligning the linear light along the PM fiber's slow axis resulted in variations of f5% of the ratio PA/(PA+ PB), while launching light along the fast axis resulted in only a M.05% variation (Le. a negligible variation), indicating that the polarization stability of the light launched along the fast axis was far superior. For more information on PM fibers, the following references provide an e-rsllent description: [3], [4]. The polarization extinction ratio of the light emitted from the fiber was then measured to be 28dB. Note that this was the linearify of the light entering each OPS.
Chapter 3
3.2
Characterization of Light Distribution System
Measurements were performed on the laser, the fibers, the fiber spiitters and peliides. Analysis of these results stated that the fiber splitting arrangement was clearly not adequate for the application. However, with the implementation of the pellide setup, it was shown that excellent performance was achieved. A final subsection on the characterization of the optical power budget of the pellicle system is also presented.
3.2.1 Laser Characterization
-
The characterized date for the SDL 8630 tunable laser diode system was
given from the manufacturer (Spectra Diode Labs) as follows: 500mW8 1.92 Amps (821.0 OC maintained by a thermo-electric coder) Diffraction limited, collimated beam 20nm tuning range (845 -865 nm) Center wavelength 855nm 92%
Beam steering capabilities of grcater than: f 400pm lateral translation f 0.46' angular dcviation
Beam steering capabilities of: f685pm lateral translation 0.49' angular deviation
1 Spectral tolerance of 850klnm
+ 1 850.0 IT 0.05m
1
Table Tg-1: OPS Spot Array Requirements and Characterization Results
Future optical intercomects will most probably employ the use of source based transmitters (e.g. VCSELs) rather than modulators. However, this is only foreseen t o occur after a few years once VCSEL techology manages to produce large (64 x64) arrays of lasers CO-integratedwith CMOS, interlaced with detectors, with high optical uniformity, and low electrical drive requirements [Il. Thus, in the interim, the necessity for the advancement of R&D in this field requires simple, elegant, and practical optical solutions to meet the current state of affairs. This paper has shown that for the first t h e , a compact modularized spot array generator has been built for use in a modulator based optical interconnect, successfully taking the first step in bndging the generation gap behveen sophisticated Hybrid/SEED optoelectronics and optics.
Chapter 9
9.1 [l]
References
D. V. Plant, "Constructing a free-space optical backplane: Challenges and
choices", presented at the Optics in Compiiting Meeting at the 1997 Spnns
Topicnl Meetings, OThB1, March 1û-22, Lake Tahoe,Nevada, 1997.
Chapter 9
APPENDIX A OSLO Design Specs Domuiic Goodvill
-
University of Colorado (1995)
.LDiS DATA
M f f i i l l Fiber to spot acray plane
AIR
AIR
SE56 C
.
AIR
AIR
AIR
-56
C
AIR
AIR
BK7 C
AIR
Quarter wave plate
13
--
AIR
.
14 15
-
B E active
10.000000
SIUVL C
10.o0o000
AIR
2.000000
10.000000
AIR
2.000000
10.000000
1.587500
--
.
BPG active surface
16
--
17
ELmIFXI
18
Risley p r i s
19
--
20
ELmmn
21
Risley p r i s
22
23 24
-lZEmXI T l l t plate
--
SFIO C
10.000000
AIR
2.000000
10.000000
AIR
2.000000 P
10.000000
-7.000000
3 .O00000
--
.Tl0 P
10.000000
AIR
10.000000
AIR
10.000000 10.000000
SFlO C AIR
T i l t plate t o correct chief ray angle e n o r a t spot array plane
--
7.230600
10.000000
26
25.483000
5.000000
9.000000
BK7 C
27
-21.909000
1.500000
9.0000nO
F2 C
28
-139.240000
25
29
--
--
5.240000
AIR
9.000000
AIR
11.250000
AIR
' Risley prism
' Risley prism
.
.
F2 C AIR
AIR
S
m OF LENS
AlmTYEE h t r a n c e beam radius :
Innge axial ray slope:
Object num. aperture:
F-number :
Image num. aperture:
Working F-number:
5.067612
Field angle:
Object height :
0.100000
Gaussian image height:
Chief ray ims height:
-0.216467
Object distance:
Srf 1 to p r i n . p t . 1:
-127.817751
Gaussian i m g e d i s t . :
Srf 33 t o Win. p t . 2:
Overall lens length:
Total track lenqth:
94.636317
Paraxial nngnification:
Sr: 33 t o image s r f :
18.325217
-0.100971 -34.163358
EIELD
c0mGAm
281.580594
çmEaATA h t r a n c e pupil radius:
Srf 1 t o a t r a n c e pup.:
Eiit pupil radius:
Srf 33 t o exit pupil:
kgrange invariant:
Petzval radius :
Effective focal length:
-41.193372 -10.460685
APPENDIX B Gaussian Fit Program %
This program takes a 2 dimensional array called 'A'
% %
This program is a modification to the program originally written by Yong Sheng Liu
% %
Modified by Rajiv Iyer Latest modification by Rajiv Iyer on Feb 6. 1997
% % % %
The A matrix is set by exporting the frame-grabbed spot as a text file, editting it with the characters : A=[ prior to al1 the data, and : 1; following al1 the data. The text file should be a .m file, so that it can be executed by MATLAB
% % % %
cla; cl£; w0=6;% wo is the first guess at the beamwaist wl=l.S*wO;% wl is a final guess at the beamwaist delt-x=0:% force the gaussian fit to shift --> shift to the left! deltj=O;% delt < O %
write the text file into matrix A
peak = max(max(A) )
;
B=A/peak;% normalization
%
find the peak position (rw~max,clm_mnx)£rom the B;
for i=l:m for j=l:n if B(i,jl > peak rw-max = i; cl-ma% = j;
peak = B(i,j); end end end %
normalize the matrix
% pick up the row and column which contains the Peak rw = B(w-max, : ) ; clm = B(:, clmmax); %
Gaussian curve fitting
fitting for row---x! for w = wO : w0/200 : wl for i=l:n j=-clm-max-delt-x+i;
%-----%
gauss_x(i)=exp(-2*j^2./(~.^2));
end errfit-new-x=stdigauss-x-rw);
if errfit-new-x < errfit-x errfit-x = errfit-new-x; w-best-x = w; end end best Gaussian fitting for the row-cut: for i=l:n j=-clm-max-delt-x+i;
%
gauss-best-x(i)=expi-2*jA2/(w-best-xA2));
end % Gaussian fit metric % ks stands for the Kolmogorov-Smirnov Test
%?zef: p617 of 'Numerical Recipies in Fortzan % - The art of Scientific Computing 2ndEd" %William H. Press Saul A Teukolsky, William T. Vetterling %Brian P. Flannery % This book was borrowed from Professor Peter Kabal. ks=O; sum=o; for i=l:n diff=gauss-best-x(i)-rw(i); sum=sum+diffA2;
if abs (di£f)>ks ks=absidiff); end end tmp=(sum)^(O.S); x-metricl=(l-tmp)'100; x-metric2=(l-sum)*lOO; x-metric3=(1-ks)*100;
fitting for column---y! for w = wO : w0/200 : Wl for k=l:m 1=-rw-mx-delty+k; tpy(k)=l; gauss~(k)=exp(-2~1^2. / (w.^2)) ; %-----O
end errfit-newy=std(gaussj-clm'); if errfit-newy c errfitj errfity = errfit-newy; w-besty = w; end end
best Gaussian fitting for the column-cut: for k=l:m l=-rw-mx-deltj+k;
%
gauss-bestj(k)=exp(-2*1^2/(w-besty^2));
end
% Gaussian fit metric ks=O; sum=o; for i=l:m
diff=gauss-besty(i)-clm(i);
sum=sum+diff"2; if abs(diff ) > ks ks=abs(diff) ; end end tmp=(sum)^(0.5); y-metricl=(l-tmp)*100; y-metric2=(1-sum)*100; y-metric3=(1-ks)*100;
%fprintf('xfitl: %g.xfit2: %g. xfit3: %g.best-w-x: %g.\n', x-metric2, x-metric3, w-best-xi fprintf ('xfit: %g %g %g. best-w-x: %g.\n', xmetricl. x-metric3.w-best-x) %fprintf('yfitl: %g.yfit2: %g. yfit3: %g.best-wj: %g.\n'. y-metricl, y-metricl, w-bestji fprintf ('yfit: %g %g %g. b e s t w j : %g.\nS, y-metricl. y-metric3, w-bestj) w-avg=(w-best-x+w_bestj)/2;
fprintf('average w= %g\na.w-avg) x-calib = 2.509; y-calib = 2.4529;
fprintf('with x calib %g, wx= %g\n', x-calib, wx) fprintf('with y calib %g, wy= %g\n'. y-calib, wy) fprintf('average : %g\nS,wavg)
% plot the curve of the row and column subplot(2,l,l), plot(rw, 'g') grid on hold on subplot(2,1,2),plot(c1m. 'r') grid on hold on
subplot(2.1.1). plot(gauss-best-x. 'y') subplot(2,1,2), plot(gauss-bestj. 'y') xx=I0:0.1:3001; yy=sin(xx); sound(yy) break
% END
xmetricl, x-metric2. y-metricl, y-netricl,
APPENDIX C Notes [A]
PANDA'^, Polarcortm,and Delrintmare trademarked names. The mention of these brand names in this paper is for information purposes only and does not constitute an endorsement of the products by the author or McGill University.
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