Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2002-01-17
2003-02-25
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S016000, C385S025000
Reexamination Certificate
active
06526198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally concerns optical switches; particularly optomechanical switches; and still more particularly micromachined, or Micro Electro Mechanical Systems (MEMS), optomechanical switches.
The present invention particularly concerns micromachined, or Micro Electro Mechanical Systems (MEMS), optomechanical switches having a MEMS torsion plate—which serves to mount a micromirror—that is (i) electrostatically or electromagnetically moved in angular position, and/or (ii) hinged for angularly pivoting movement by surface micromachined hinges, and/or (iii) mechanically biased in angular privoting movement by torsion springs that also serve to conduct electricity.
2. Description of the Prior Art
2.1 General Prior Art Optical Switching, and Micromachined Optical Switches
Optical switching plays a very important role in telecommunication networks, optical instrumentation, and optical signal processing systems. In telecommunication networks, fiber optic switches are used for network restoration, reconfiguration, and dynamic bandwidth allocation.
There are many different types of optical switches. In terms of the switching mechanism, the switches can be divided into two general categories. A first type, called “optomechanical switches”, involves physical motion of some optical elements. The second type, which will be referred to here as “electro-optic switches”, employs a change of refractive index to perform optical switching. This refractive index change can be induced by electro-optic, thermal-optic, acousto-optic, or free-carrier effects. The electro-optic-type switch generally needs to be implemented in coupled optical waveguides.
Optomechanical switches offer many advantages over electro-optic switches. They have lower insertion loss, lower crosstalk, and higher isolation between ON and OFF state. They are bidirectional and independent of optical wavelength, polarization, and data modulation format. The crosstalk of electro-optic waveguide switches is limited to a range above −30 dB, and is often in the range of −10 to −15 dB, while optomechanical switches can routinely achieve crosstalk <−50 dB.
An optomechanical switch can be implemented either in free-space or in waveguides (or in fibers). The free-space approach is more scalable, and offers lower coupling loss to optical fibers. Currently, macro-scale optomechanical switches employing external actuators are available commercially. For example, conventional optomechanical switches are available from JDS, DiCon, AMP, HP, etc. Most, of these switches are, however, bulky and require extensive manual assembly. Their speed is also slow, the switching time ranging from 10 milliseconds to several hundred milliseconds. Worse, the switching time often depends on the switching path, i.e., how far is the next output port from the current output port. This is very undesirable for system design. Response times below 1 millisecond are desirable for network applications.
Meanwhile, micromachining technology, also known as Micro Electro Mechanical Systems (MEMS), offers many advantages for building optomechanical switches. MEMS technology is a batch processing technique for fabricating movable microstructures and microactuators. See, for example, K. E. Petersen, “Silicon as a Mechanical Material”, Proc. IEEE 70 (1982) 420-457; and M. F. Dautartas, A. M. Benzoni, Y. C. Chen, G. E. Blonder, B. H. Johnson, C. R. Paola, E. Rice, and Y. H. Wong, “A Silicon-Based Moving Mirror Optical Switch” J. Lightwave Technol. 8 (1992) 1078-1085.
Micromachining technology, or MEMS, can significantly reduce the size, weight, and cost of optomechanical switches. The switching time can also be reduced because of the higher resonant frequency of the smaller optomechanical switches. Furthermore, the MEMS optomechanical switch is more rugged than macro-scale switches of equivalent design because the inertial forces are much smaller in the micro-scale switches. MEMS technology has previously been employed to realize various types of optomechanical switches.
2.1 Specific Prior Art Micromachined Optical Switches—Free-Space Optomechanical Switches
There has been some demonstration of MEMS fiber optic switches. Both bulk-micromachining and surface-micromachining techniques have been employed. However, none of the reported switches fully satisfy all the requirements for large scale network applications. The bulk micromachined 2×2 fiber optic switch was reported by AT&T Bell Labs in 1992. See M. F. Dautartas, A. M. Benzoni, Y. C. Chen, G. E. Blonder, B. H. Johnson, C. R. Paola, E. Rice, and Y. H. Wong, “A Silicon-Based Moving Mirror Optical Switch,” in J. Lightwave Technol, 8 (1992) 1078-1085. The Bell Labs switch employs two separate silicon wafers (vertical micromirrors in the top <110> silicon wafer, and V-grooves grooves in bottom <100> silicon substrate). The two wafers are joined together manually and external actuators are employed. An insertion loss of 0.7 dB and a switching time less than 10 ms have been obtained. This switch, however, still requires substantial manual assembly and the cost is very high. It does not appear to be scalable to large arrays.
Toshiyoshi and Fujita of the University of Tokyo reported a 2×2 matrix switch using bulk micromachined torsion mirrors. See Toshiyoshi, H.; Fujita, H. “Electrostatic micro torsion mirrors for an optical switch matrix,” J. Microelectromechanical Systems, vol. 5 , p. 231-7, 1996. Torsion mirrors are suspended by thin polysilicon beams over through holes etched on silicon substrate. The mirror substrate is then bonded to another silicon substrate, on which bias electrodes and mechanical stops are patterned and etched. When the mirror is attracted downward by the electrostatic force, light is reflected to the orthogonal output fibers. Large switching contrast (>60 dB), small crosstalk (<60 dB), and a fairly high insertion loss of 7.6 dB were reported. One limitation of this approach is that the mirror angle in the ON state is determined by the mechanical stop on another wafer and cannot be accurately controlled or reproduced. This results in the high insertion loss reported in their paper.
Marxer et al., from University of Neuchatel reported a bulk micromachined 2×2 fiber optic switches using deep reactive ion etching (DRIE) process on silicon-on-insulator (SOI) wafer. See C. Marxer, M.-A. Gretillat, N. F. de Rooij, R. Battig,
0
. Anthamatten, B. Valk, and P. Vogel, “Vertical Mirrors Fabricated by Reactive Ion Etching for Fiber Optical Switching Applications,” in Proc. 10
th
Workshop on Micro Electro Mechanical Systems (MEMS), pp. 49-54, 1997. The 75-&mgr;m-thick silicon layer above SiO
2
is etched through by DRIE. A 2.3-&mgr;m-thick vertical mirror as well as the electrostatic microactuator are created by the same etching step. Switching time below 0.2 ms, coupling loss of 2.5 dB and 4 dB, and switching voltage of 28 V have been achieved. A similar process has recently been reported by the Michigan group. See W. H. Juan and S. W. Pang, “Batch-Micromachined, High Aspect Ratio Si Mirror Arrays for Optical Switching Applications,” in Proc. International Conf. Solid-State Sensors and Actuators (TRANSDUCERS 97), Paper 1A4. 09P, 1997.
There are two limitations of this approach: (1) the mirror angle and quality are determined by the etching process. Mirror angle <1° is difficult to achieve. The rough etched surface also introduces scattering losses. It is also difficult to polish the sidewalls or coat metals on the mirrors. (2) The displacement of the mirror is small because the comb drive actuator has limited displacement. In Marxer's paper, tapered fibers were placed very closed to the mirror (<20 &mgr;m), which reduces the required mirror movement. However, such configuration is not scalable to switches with dimension larger than 2×2 arrays. Michigan's group did not report the optical characterization data of their switch.
Miller and Tai of Caltech reported an ele
Fan Li
Husain Anis
Wu Ming C.
Ferrell Arien
Fuess William
OMM, Inc.
Palmer Phan T. H.
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