Charles C. Zhou, Sean Sutton, Ray T. Chen, Member, IEEE, and Brian M. Davies, Member, IEEE. Abstractâ We present a 4 24 surface-normal wavelength-.
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Surface-Normal 4 4 Nonblocking Wavelength-Selective Optical Crossbar Interconnect Using Polymer-Based Volume Holograms and Substrate-Guided Waves Charles C. Zhou, Sean Sutton, Ray T. Chen, Member, IEEE, and Brian M. Davies, Member, IEEE
Abstract— We present a 4 2 4 surface-normal wavelengthselective nonblocking crossbar using polymer-based volume holograms and substrate-guided waves. A prototype device is demonstrated using the center wavelengths of 750, 780, 810, and 840 nm. The employment of wavelength-division multiplexer and of address coding reduce the required 16 wavelengths to four while maintaining the 4 2 4 interconnects. Diffraction efficiencies of 85%, 83%, 79%, and 82% are experimentally confirmed for randomly polarized light at 750, 780, 810, and 840 nm, respectively. The measured crosstalk is less than 024 dB. Index Terms—Graded index lens, holographic gratings, optical switches, optical interconnection.
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ROSSBAR-BASED optical interconnection represents the most desirable network switching configuration due to its fast switching speed and low latency in transmitting high speed signals [1]. In this letter, we report the formation of a surface-normal nonblocking 4 4 all optical crossbar interconnect based on wavelength-division multiplexing (WDM) and surface-normal holograms with demultiplexing and beam routing capabilities. A prototype polymer-based 4 4 crossbar is experimentally demonstrated at 750, 780, 810, and 840 nm. The unique wavelength demultiplexing and beam routing properties of the volume holograms, in combination with the WDM, reduce the required 16 wavelengths to four wavelengths while maintaining the required 16 (4 4) individual interconnects. Furthermore, the elimination of edge coupling significantly enhances the packaging reliability. The functionality of the 4 4 all optical crossbar interconnect is shown in Fig. 1. The four incoming wavelengths from each of the four input ports are routed to the designated output port according to the wavelength. By using different wavelengths, each port can communicate with any of the four outputs by self-routing through holographic gratings. Physically speaking, at one time, the fiber can act as a 1 4 switch. In a signal processing sense, this is a 4 4 crossbar Manuscript received June 22, 1998; revised August 3, 1998. This work was supported by the Office of Naval Research, by BMDO, by the U.S. Army Space and Strategic Defense Command (SSDC), by the Defense Advanced Research Projects Agency, and by the ATP program of the State of Texas. C. C. Zhou, S. Sutton, and R. T. Chen are with the Microelectronics Research Center, University of Texas at Austin, Austin, TX 78712 USA. B. M. Davies is with Radiant Research, Inc., Austin, TX 78759 USA. Publisher Item Identifier S 1041-1135(98)07933-6.
Fig. 1. Functional diagram of 4 length resolution.
2 4 crossbar interconnect based on wave-
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Fig. 2. Layout of the 4 4 crossbar interconnect based on volume holographic gratings and substrate-guided waves.
switch. A true 4 4 crossbar can be constructed using the same principle using 16 different wavelengths. The 16wavelengths from four input ports are routed to the four output ports accordingly. The 4 4 nonblocking crossbar interconnection is realized based on wavelength-dependent routing. In the present scheme, only four wavelengths are used for the four input ports to provide the 16 (4 4) interconnects. Based on the
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wavelength routing, the four input ports can communicate with the four output ports nonblockingly. The address of the sender in this case can be identified through the header encoded in the optical signal [2]. Due to wavelength self-routing and reconfiguration capabilities of this scheme, high throughput and low latency optical interconnects can be realized. The demonstrated device using holographic gratings and substrateguided waves is shown in Fig. 2. The four wavelengths of each input port are multiplexed into a fiber by the WDM technique [3]. The multiplexed wavelengths from the four input ports are further coupled into a single fiber using a broad-band coupler. A fiber collimator is used to collimate the incoming light. The collimated light is diffracted into substrate-guided waves with different propagation vectors by the holographic gratings. The volume phase gratings recorded in the photopolymer films are slanted. The 750-nm wavelength of the input surface-normal beam is designed to be diffracted with a maximum diffraction efficiency of 85% at the Bragg angle of 45 in our design. The second central wavelength of the input surface-normal beam, i.e., 780 nm, is designed to be diffracted with a maximum diffraction efficiency of 83% at the Bragg angle of 47 in our design. The third and fourth wavelengths are maximized at 79% and 82% with Bragg angles of 49 and 51 , respectively. Since the input angle deviation will cause the diffraction efficiency to drop sharply, the angles are designed to minimize the crosstalk. One phase-matched wavelength is strongly diffracted by one of the four holograms shown in Fig. 2 while the other three wavelengths are transparent to such a hologram. The beam routing is achieved by aligning the holographic grating vector to the routing direction. The diffracted beams generated having angles larger than the total internal reflection (TIR) angles are zigzagged within the substrate. An output coupler is used at the output location to couple the light out surface-normally. The schematic of the microstructure of the designed volume holograms is shown in the inset of Fig. 2. The grating structure induced by the refractive index mod. For ulation is slanted with grating spacing Bragg gratings, the diffraction efficiencies are described by Kolgenik’s coupled wave theory [4]. Given the wavelengths of the device, the design is to find optimal film thickness and refractive modulation so that the overall diffraction efficiencies are maximized. For the employed holographic film with a thickness of 20 m and maximum refractive index modulation of 0.02, the diffraction efficiencies of the four individual holograms are simulated and plotted in Fig. 3 [5]. From left to right in Fig. 3, the four diffraction efficiency curves correspond to holograms with diffraction angles of 45 , 47 , 49 , and 51 , respectively. The diffraction efficiencies are averaged over the TE and TM waves so that they represent randomly polarized incident light. The 3-dB spectral bandwidth is 36.5 nm for the wavelength 750 nm with a 45 diffraction angle. Each hologram only diffracts the phase-matched wavelength efficiently while passing other wavelengths. The use of different diffraction angles further reduces the crosstalk level, since different wavelengths will be guided in the substrate at a different angle. Given enough propagation distance, the different wavelengths will separate spatially like in a wavelength-
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 11, NOVEMBER 1998
Fig. 3. Simulation results of the average diffraction efficiencies for the four holograms used in the experiment covering 700–880-nm bandwidth.
Fig. 4. Optical spectrum of the wavelength multiplexed light after the WDM.
division demultiplexer (WDDM) [6]. The minimum diffraction angle and wavelength separation can be calculated using the volume holographic grating equation given by Tomlinson [7]. In our experiment, the four different wavelengths are multiplexed into a 62.5- m core multimode fiber using a WDM based on a Littrow grating [8]. To achieve the 4 4 nonblocking crossbar interconnect, 4 WDM’s are used. The 16 lightwave signals in four separate fibers are coupled into a fiber by a broad-band four-to-one coupler. The fiber containing 16 signals in four wavelengths is collimated by a planar-planar graded index (GRIN) lens. The planar surface of the GRIN lens facilitates the interfacing with the planar guiding substrate. After the light is collimated, it passes through the volume holographic gratings. Different wavelengths are diffracted into different directions according to the grating vectors. For surface-normal input light beams, the diffracted beams are guided in the substrate in four different directions. The 4 4 nonblocking crossbar is realized by only four wavelengths instead of 16 wavelengths. The four different wavelength input locations associated with each wavelength are determined by address coding. Broadcasting can be provided by launching four wavelengths simultaneously. The light sources are edge emitting laser diodes (Sharp CD quality lasers) packaged in ST receptacles. The laser output power is 3 mW with 0.5 mW coupled into a 62.5- m GRIN multimode fiber. The input wavelength is monitored by an optical spectrum analyzer shown in Fig. 4. The holographic grating is fabricated using Dupont photopolymer film (HRF-600) having a thickness of
ZHOU et al.: SURFACE-NORMAL 4
4 NONBLOCKING WAVELENGTH-SELECTIVE OPTICAL CROSSBAR INTERCONNECT
Fig. 5. Fanout spots of different wavelengths from the 4 interconnect.
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20 m, and the hologram is recorded at 514 nm using a Coherent high power argon ion laser. The volume holograms are fabricated using the two-beam interference method [9]. The current fabrication process involves four separate holographic films and four exposures due to limited refractive index modulation of DuPont photopolymer thin film. A multiplexed holographic grating is possible with thick photo sensitive materials with a large refractive index modulation such as dichromated gelatin (DCG) [10]. Diffraction efficiencies of 85%, 83%, 79%, and 82% are experimentally confirmed for randomly polarized light with wavelengths of 750, 780, 810 and 840 nm, respectively. The input multiplexed light is collimated and diffracted by the volume holographic gratings shown in Fig. 2 and is zigzagged inside the glass substrate in four different directions. It is subsequently coupled out by the output holographic couplers, which are complementary gratings with respect to the input gratings [11]. The fanout patterns can be arbitrarily designed by adjusting the input grating vector direction. Fig. 5 shows the fanout spots of a 4 4 optical interconnect using holographic gratings with orthogonal grating vectors. The fanout spots at a bouncing distance of 18 mm are shown in this figure. Using an 8-bit charge-coupled (CCD) camera imaging system, the crosstalk is measured to be less than 24 dB. In summary, we present a surface-normal nonblocking crossbar based on volume holographic gratings and substrate-
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guided waves. A 4 4 crossbar containing 16 interconnections has been successfully demonstrated with wavelengths of 750, 780, 810, and 840 nm. The use of WDM’s and broad-band couplers allows us to combine the four input ports into a single surface-normal input fiber collimator. Therefore the fabrication and packaging is simple and reliable. The unique beam routing property of the holographic gratings reduces the required 16 wavelengths to four while maintaining the 4 4 interconnects. Realizing the fact that the switching speed of semiconductor lasers can be as fast as 1 ns [12], a high throughput and low latency interconnect can be built for many high performance computer and communication applications. REFERENCES
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[1] C. C. Zhou and R. T. Chen, “Surface-normal 3 3 nonblocking wavelength-selective crossbar using polymer-based volume holograms,” Appl. Phys. Lett., vol. 69, no. 26, pp. 3990–3992, 1996. [2] IEEE Standard for Scalable Coherent Interface (SCI), IEEE Standard, pp. 1596–1992. [3] M. M. Li and R. T. Chen, “Five-channel surface-normal wavelengthdivision demultiplexer using substrate-guided waves in conjunction with a polymer-based Littrow hologram,” Opt. Lett., vol. 20, no. 7, pp. 797–799, 1995. [4] H. Kogelnik, “Coupled wave theory for thick hologram gratings,” The Bell Syst. Tech. J., vol. 48, no. 9, pp. 2909–2947, 1969. [5] M. R. Wang, G. J. Sonek, R. T. Chen, and T. Jannson, “Five-channel polymer waveguide wavelength division demultiplexer for the near infrared,” IEEE Photon. Technol. Lett., vol. 3, no. 1, pp. 36–38, 1991. [6] Y. Huang, D. Su, and Y. Tsai, “Wavelength-division-multiplexing and -demultiplexing by using a substrate-mode grating pair,” Opt. Lett., vol. 17, no. 22, pp. 1629–1631, 1992. [7] W. J. Tomlinson, “Wavelength-multiplexing in multimode optical fibers,” Appl. Opt., vol. 16, pp. 2180–2194, 1977. [8] J. Lipson, C. A. Young, P. D. Yeates, J. C. Maslad et al., “A fourchannel lightwave subsystem using wavelength division multiplexing,” J. Lightwave Technol., vol. LT-3, pp. 16–20, Jan. 1985. [9] R. T. Chen, S. Tang, M. M. Li, D. Gerald, and S. Natarajan, “1-to12 surface normal three-dimensional optical interconnects,” Appl. Phys. Lett., vol. 63, no. 14, pp. 1883–1885, 1993. [10] M. R. Wang, G. J. Sonek, R. T. Chen, and T. Jannson, “Large fanout optical interconnects using thick holographic gratings and substrate wave propagation,” Appl. Opt., vol. 31, no. 2, pp. 236–249, 1992. [11] R. K. Kostuk, M. Kato, and Y. T. Huang, “Polarization properties of substrate-mode holographic interconnects,” Appl. Opt., vol. 29, No. 26, pp. 3848–3854, 1990. [12] C. Chang, “Multiple wavelength tunable surface-emitting laser arrays,” IEEE J. Quantum Electron., vol. 27, pp. 1368–1376, 1991.
