IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 4, APRIL 2003
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238 Micromechanical Optical Cross Connect
V. A. Aksyuk, S. Arney, N. R. Basavanhally, Member, IEEE, D. J. Bishop, C. A. Bolle, C. C. Chang, R. Frahm, A. Gasparyan, J. V. Gates, R. George, C. R. Giles, J. Kim, P. R. Kolodner, T. M. Lee, David T. Neilson, Senior Member, IEEE, C. Nijander, C. J. Nuzman, M. Paczkowski, A. R. Papazian, F. Pardo, D. A. Ramsey, Member, IEEE, R. Ryf, R. E. Scotti, Member, IEEE, H. Shea, and M. E. Simon
Abstract—This letter describes a 238 238 beam-steering optical cross connect constructed using surface micromachined mirrors. Its innovative optical configuration resulted in superior optical performance, achieving a mean fiber-to-fiber insertion loss of 1.33 dB and a maximum insertion loss for all 56 644 connections of 2 dB. Index Terms—Microelectromechanical systems micromechanics, optical cross connect (OXC).
(MEMS),
I. INTRODUCTION
T
HERE HAS BEEN increasing interest in applying switching at the optical layer of communications networks to provide additional network management options so that features such as load balancing and traffic routing can be achieved economically. Cross-connect switches reduce the deployment times for new services and provide additional network robustness by offering comprehensive protection switching and restoration. These switches are generally referred to as optical cross connects (OXCs) irrespective of whether the switching is optical or electrical, since they are operating at the same network layer as the optical transport system. With increasing port count and data rate, electronics-based switching becomes increasingly difficult due to the limited scalability of electrical interconnects illustrated by the use of optical interconnect back planes in many electronic switches. One solution to this is to move to the use of optical transparent switches. A transparent or photonic optical cross connect (PXC) can be built using micromachines [microelectromechanical systems (MEMS)] that are not subject to the scalability issues of electrical interconnects [1]–[4]. Additionally, with transparent cross connects, since they carry the native optical traffic, the fabric is data-rate and format independent and can handle multiple wavelengths per port [1], [5]. In the longer term, they are critical to optical networking: building end-to-end transparent networks. Achieving low insertion loss is necessary to the success of OXCs in order to reduce the optical power requirements and, hence, cost of the transmitters and receivers in the network. If the insertion loss is too large, then this may require the addition of amplifiers or regenerators, which can negate much of Manuscript received November 6, 2002; revised December 10, 2002. V. A. Aksyuk, S. Arney, N. R. Basavanhally, D. J. Bishop, C. A. Bolle, C. C. Chang, R. Frahm, A. Gasparyan, J. V. Gates, R. George, J. Kim, P. R. Kolodner, T. M. Lee, C. Nijander, C. J. Nuzman, M. Paczkowski, A. R. Papazian, F. Pardo, D. A. Ramsey, R. E. Scotti, H. Shea, and M. E. Simon are with Bell Labs, Lucent Technologies, Murray Hill, NJ 07974 USA. C. R. Giles, D. T. Neilson, and R. Ryf are with Bell Labs, Lucent Technologies, Holmdel, NJ 07733 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2003.809261
Fig. 1. Optical system layout of cross connect, showing input and output fiber arrays, two micromechanical tilt mirror arrays (MEMS), and the Fourier lens. Extreme beam paths are also shown.
the advantage of the transparent cross connect. In addition to low optical loss, low loss variation is also important. Low loss variation allows improved receiver dynamic range requirements and improved performance when protection switching between connections or different OXC fabrics. The low loss requirement makes the option of multistage switch fabrics impractical due to the losses associated with coupling to fiber and connectors. Since in most applications the switch must scale to port counts 64 in a single stage, the only viable approach is that of free space optical beam steering. The use of arrays of micromechanical tilt mirrors allows such a switch to be compact and have fast 10 ms switching times. II. DESCRIPTION OF CROSS-CONNECT SWITCH FABRIC Previous analyses of transparent OXCs using micromechanical tilt mirrors have assumed that the beam simply propagates from one array to the next subject to diffraction [2], [6]. While these schemes can be locally optimized [6], they are globally suboptimal since the angular range required by mirrors in the center of the array is significantly different from those at its edge. The optical system used for our switch fabric differs from designs that have previously been considered. As illustrated in mm is placed between the two Fig. 1, a Fourier lens arrays of MEMS mirrors, which are spaced at a distance of twice the focal length of the Fourier lens. The Rayleigh range of the beams is chosen to be equal to the focal length of the lens. This provides several advantages for the system. It means that all mirrors in the array tilt to both sides on each axis, reducing the required tilt angle from normal incidence, while actually increasing the angle between connections from mirrors in the center of the array. This results in lower switch crosstalk since
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 4, APRIL 2003
Fig. 3. Chip with array of 256 surface micromachined, two-axis beam steering mirrors in a hermetic package. The schematic shows the layout of the mirrors, which are 0.6-mm diameter on a 1-mm pitch with wiring arteries every fourth row and column. One axis is stretched by 3.5% to account for the 15 skew of the chip in the system.
Fig. 2. Optical subsystem of the MEMS-based OXC showing MEMS mirror array, fiber array, and Fourier lens in optical housing.
the crosstalk is a function of the angular separation of the beams. It allows the beam waist as opposed to the beam at one Rayleigh range to be placed on the MEMS mirror array, which allows a smaller spot size on the mirrors, than is possible with systems using simple beam propagation [2], [6] from mirror array to mirror array. The smaller spot size is advantageous in that it reduces the size of MEMS mirror that is required to achieve low loss due to optical clipping. It also makes the insertion loss less sensitive to any residual mirror curvature, with a mirror radius of curvature of 200 mm or greater resulting in 0.3-dB excess insertion loss. The waist being at the tilt mirror also means that the tilt axis of the MEMS mirrors produce orthogonal changes in coupling efficiency, making the optimization of a connection simpler since the logarithm of the insertion loss is proportional to the sum of the squares of the mirror tilt angles, i.e., . The low skew angle (15 ) Z configuration of the optical switch ensures that polarization-dependent loss effects are minimized. The optical configuration also ensures the path length variation of the system is minimized, resulting in no measurable path-dependent loss. In order to obtain a low loss system, it is necessary to have , low wave microlenses with very low aberrations front errors across entire arrays, and high focal-length unifor. The silicon microlens array is made by a reflow mity process and has excess losses from 0.1 to 0.3 dB, which is comparable to the best individual collimators available today. The microlenses are aligned to ensure minimum double-pass insertion loss and beam registration of better than 50 m and attached to the fiber arrays, which are shown as part of the assembled switch fabric in Fig. 2. Fig. 3 shows the MEMS mirror array mounted in a hermetic package that includes an antireflection-coated sapphire window and a backside electrical pin-grid array. The mirror-to-mirror spacing of the MEMS mirror array is 1 mm with wiring arteries every fourth row or column. The MEMS mirrors [7], [8] are electrostatically actuated and the maximum angular deviation from normal used by the mirror is 4.4 . It achieves port-to-port switching in less than 10 ms. The individual mirrors are beam size on the MEMS mirror 600- m diameter, and the is 372 m. The parameters ensure that 99 of the light will
Fig. 4. Insertion loss distribution for all the 56 644 connections (solid line), and cumulative distribution (dashed line) in the 238 238 OXC. The mean fiber-to-fiber insertion loss is 1.3 dB.
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hit the mirror even with a 30- m error in spot position. The light enters the system from a two-dimensional fiber array with a 1-mm pitch, and the beams are relayed to produce the correct size of waist at the MEMS mirrors by the array of the refractive silicon micro lenses. III. EXPERIMENTAL RESULTS The cross-connect performance was verified by measuring the insertion loss on all 56 644 connections that the 238 238 ports support. The histogram (and cumulative distribution) of the insertion loss is shown in Fig. 4, for measurements at 1.54- m wavelength. The breakdown of the sources of loss in the fabric is summarized in Table I. The minimum and maximum variability of the losses are given for each contributing source. The loss contributions from the micro lenses, the optical coatings, the mirror curvature, and beam clipping add to a worst case of 1.3 0.4 dB. This loss is convolved, i.e., adds as root mean square (rms), with 0.4 dB the variability of the single-mode fiber connectors in the measurement to give the total measured variability of 1.3 0.6 dB. One of the significant issues relating to the fabrication and test of low loss fabrics is the variability of the six connectors in the test equipment, as well as the two connectors on the switch fabric itself. The use of eight LC connectors in the fabric and test set results in a 0.4-dB variability of which only 0.15 dB may be considered as being intrinsic to the two LC connectors associated with the fabric itself. The rest is due to the variability of the LC connectors in the test set and calibration procedure. This variability has been verified by measuring the fabric
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238 MICROMECHANICAL OPTICAL CROSS CONNECT
TABLE I SOURCES OF LOSSES AND MAXIMUM AND MINIMUM RANGE IN OXC FABRIC. THE COMPONENT LOSS ADDS AS WORST CASE WHILE CONNECTOR LOSS CONVOLVES, I.E., ADDS AS RMS, WITH THE INTRINSIC LOSS OF FABRIC
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chip and replaces the Fourier lens with a curved mirror, as used in [1]. REFERENCES [1] D. T. Neilson, V. A. Aksyuk, S. Arney, N. R. Basavanhally, K. S. Bhalla, D. J. Bishop, B. A. Boie, C. A. Bolle, J. V. Gates, A. M. Gottlieb, J. P. Hickey, N. A. Jackman, P. R. Kolodner, S. K. Korotky, B. Mikkelsen, F. Pardo, G. Raybon, R. Ruel, R. E. Scotti, T. W. VanBlarcum, L. Zhang, and C. R. Giles, “Fully provisioned 112 112 micro-mechanical optical cross connect with 35.8 Tb/s demonstrated capacity,” in Proc. OFC Conf., vol. 4, 2000, pp. 202–204. [2] P. M. Hagelin, U. Krishnamoorthy, J. P. Heritage, and O. Solgaard, “Scalable optical cross-connect switch using micromachined mirrors,” IEEE Photon. Technol. Lett., vol. 12, pp. 882–884, July 2000. [3] A. Neukermans, “MEMS devices for all optical networks,” Proc. SPIE, vol. 4561, pp. 1–10, 2001. [4] A. Keating, “Optical MEMS in switching systems,” in Proc. IEEE/LEOS Annu. Meeting 2001, CA, 2001, pp. 8–9. [5] R. Ryf, J. Kim, J. P. Hickey, A. Gnauck, D. Carr, F. Pardo, C. Bolle, R. Frahm, N. Basavanhally, C. Yoh, D. Ramsey, R. Boie, R. George, J. Kraus, C. Lichtenwalner, R. Papazian, J. Gates, H. R. Shea, A. Gasparyan, V. Muratov, J. E. Griffith, J. A. Prybyla, S. Goyal, C. D. White, M. T. Lin, R. Ruel, C. Nijander, S. Arney, D. T. Neilson, D. J. Bishop, P. Kolodner, S. Pau, C. Nuzman, A. Weis, B. Kumar, D. Lieuwen, V. Aksyuk, D. S. Greywall, T. C. Lee, H. T. Soh, W. M. Mansfield, S. Jin, W. Y. Lai, H. A. Huggins, D. L. Barr, R. A. Cirelli, G. R. Bogart, K. Teffeau, R. Vella, H. Mavoori, A. Ramirez, N. A. Ciampa, F. P. Klemens, M. D. Morris, T. Boone, J. Q. Liu, J. M. Rosamilia, and C. R. Giles, “1296-port MEMS transparent optical crossconnect with 2.07 Petabit/s switch capacity,” in Proc. OFC Conf. Exhibit, vol. 4, 2001, Paper PD28-P1-3. [6] R. R. A. Syms, “Scaling laws for MEMS mirror-rotation optical cross connect switches,” J. Lightwave Technol., vol. 20, pp. 1084–1094, July 2002. [7] V. A. Aksyuk, D. J. Bishop, C. A. Bolle, R. C. Giles, and F. Pardo, “Micro-Electro-Mechanical Optical Device,” U.S. Patent 6 300 619, Oct. 9, 2001. [8] V. A. Aksyuk, F. Pardo, D. Carr, D. Greywall, H. B. Chan, M. E. Simon, A. Gasparyan, H. Shea, V. Lifton, C. Bolle, S. Arney, R. Frahm, M. Paczkowski, M. Haueis, R. Ryf, D. T. Neilson, J. Kim, R. Giles, J. Gates, and D. Bishop, “Beam-Steering micromirrors for large optical crossconnects,” J. Lightwave Technol., to be published.
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on several test sets. The measured loss distribution can be deconvolved from the test set variability to yield a better estimate of the actual intrinsic loss distribution associated with the MEMS fabric. This distribution indicates an intrinsic switch loss of 0.95 to 1.75 dB, i.e., 1.35 0.4-dB variability across 56 644 connections. The PDL is 0.1 dB and the polarization-mode dispersion (PMD) and chromatic dispersion are less than 0.1 ps and 0.2 ps/nm, respectively. The worst case crosstalk occurs for nearest neighbors when only one mirror is deflected, and there is 44-dB isolation for this case. IV. SUMMARY We have described a previously undisclosed configuration for a micromechanical OXC that uses a Fourier lens to reduce the tilt angle and beam size requirements for low loss switch fabrics. The mean and maximum insertion losses of 1.3 and 2.0 dB, respectively, were demonstrated in a 238 238 OXC built using two MEMS mirror arrays arranged in a Z configuration incorporating a Fourier lens. It is also possible to modify the configuration into a folded OXC that uses a single MEMS mirror array
