Abstract:
In accordance with the invention, a tunable, reconfigurable optical add-drop filter comprises a pair of optical waveguides optically coupled by a microring or microdisk resonator wherein the coupling distance between the resonator and at least one of the waveguides is micromechanically controllable. With this arrangement, the degree of coupling can be tuned after fabrication to provide high level extinction of dropped wavelengths and the filter can be dynamically reconfigured. Advantageously, laser radiation is provided to tune the resonant wavelength.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical add-drop filters and, in particular, to micromechanically active, reconfigurable add-drop filters. 
     BACKGROUND OF THE INVENTION 
     Optical communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical communication system comprises a source of light, a modulator for impressing information on the light to produce optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels. 
     Add/drop devices are important components of WDM optical communication systems. Such devices are typically disposed at various intermediate points along the transmission fiber (called nodes) to permit adding or dropping of signal channels at the nodes. Thus, for illustration, an add/drop device would permit a transmission line from New York to Los Angeles to drop off at Chicago signal channels intended for Chicago and to add at Chicago signal channels for New York and Los Angeles. As the number of nodes increases, the number of add/drop devices increases, and their cost and effect on the system become appreciable. 
     FIG. 1 schematically illustrates a conventional optical add-drop filter  10  known as a microring add-drop filter. The filter  10  comprises, in essence, a pair of optical waveguides  11  and  12  optically coupled by a microscale resonator  13  comprising a waveguide ring closely adjacent each of the wave guides  11 ,  12 . The ring  13  is optically resonant for optical wavelengths λ i  such that nλ i =C, where C is the circumference of the ring and n is an integer. 
     In operation, if a set of wavelengths λ 1 , λ 2 , . . . λ N  is incident on input port I of waveguide  11 , any of the wavelengths resonant with the microring resonator will couple across the resonator  13  to waveguide  12  and exit the filter  10  at drop port R. Nonresonant wavelengths will pass the ring structure unperturbed and exit the filter  10  at the through port T. In addition, resonant wavelengths can be added at the add port A and will exit at port T. 
     The diameter D of the ring is chosen sufficiently small to obtain a desired free spectral range. To obtain a free spectral range of the order of tens of nanometers, D must be less than about 10 micrometers. With such small diameters, the index contrast between the ring and its cladding (the lateral index contrast) must be high to avoid bending losses. Typically, the rings are fabricated with air cladding in the lateral direction. 
     In view of the high lateral index contrast, the coupling distances d 1  and d 2  between the ring  13  and waveguides  11 ,  12 , respectfully, must be small—typically less than 300 nanometers in order to obtain the necessary coupling. In alternative embodiments, the microring resonator  13  can be replaced by a microdisk resonating in whispering gallery modes. Further details concerning the structure and operation of conventional microring and microdisk add-drop filters are set forth in B. E. Little, et al, “Microring Resonator Channel Dropping Filters”, 15  Journal of Lightwave Technology  998 (1997); B. E. Little, et al., “Ultracompact Si—SiO 2  Microring Resonator Optical Channel Dropping Filters, 10  IEEE Photonics Technology Letters  549 (1998); and D. Radfizadeh, et al., “Wave-Guide-Coupled AlGaAs/GaAs Microcavity Ring and Disk Resonators . . . ”, 22  Optics Letters  1244 (1997), each of which is incorporated herein by reference. 
     While theoretically promising, microring and microdisk add-drop filters are difficult to fabricate with necessary precision. For example, a good quality add-drop filter must essentially eliminate a dropped wavelength so that it does not reach the port T. (The filter must achieve a high extinction ratio for the dropped wavelength.) This elimination requires precise control of the coupling distances d 1 , d 2 . But due to their small sizes (less than 300 nm), these distances are difficult to fabricate with the necessary precision. Published results to date have shown only slightly better than 10 dB extinction for the best individual devices. 
     Another challenge in fabrication is to make microrings or microdisks with precise resonant frequencies. An add-drop filter for telecommunications would need rings or disks with diameters specified and fabricated to better than 1 part in 1500 in order to overlap a dense WDM grid (100 GHz spacing). Moreover, sidewall roughness of the ring adds a further degree of uncertainty to the precise value of the diameter. 
     Finally it should be noted that the conventional microring and microdisk add-drop filters are fixed in configuration. Once fabricated, the filter will always add and drop the same respective wavelengths. However, in contemplated systems it would be highly advantageous if add-drop filters could be dynamically reconfigured to select and change which wavelength channels are added and dropped. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a tunable, reconfigurable optical add-drop filter comprises a pair of optical waveguides optically coupled by a microring or microdisk resonator wherein the coupling distance between the resonator and at least one of the waveguides is micromechanically controllable. With this arrangement, the degree of coupling can be tuned after fabrication to provide high level extinction of dropped wavelengths and the filter can be dynamically reconfigured. Advantageously, laser radiation is provided to tune the resonant wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in connection with the accompanying drawings. In the drawings: 
     FIG. 1 schematically illustrates a conventional optical add-drop filter. 
     FIG. 2 is a schematic top view of an exemplary tunable, reconfigurable optical add-drop filter in accordance with the invention; 
     FIG. 3 is a schematic cross section of the filter of FIG. 2; and 
     FIG. 4 is a schematic top view of a multi-wavelength reconfigurable add-drop filter. 
    
    
     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
     DETAILED DESCRIPTION 
     Referring to the drawing, FIG. 1 illustrates a conventional add-drop filter and was described in the Background of the Invention hereinabove. 
     FIG. 2 is a schematic top view of a tunable, reconfigurable optical add-drop filter  20  comprising a pair of locally non-intersecting optical waveguides  21 ,  22  having an optical resonator  23  (here a microring resonator) disposed between them. At least one of the waveguides e.g.  21  is micromechanically movable toward or away from the resonator  23  to control the optical coupling distance d, between the waveguide and the resonator  23 . One or more actuating electrodes  24 A,  24 B can be provided to move the waveguides  21 ,  22  toward or away from the resonator  23 . 
     FIG. 3 is a schematic cross section of the filter of FIG.  2 . The waveguide  21 , the resonator  23  and the waveguide  22  are advantageously low-loss rib waveguides (single mode or multimode). They can be conveniently formed on a silicon-on-insulator workpiece  30  comprising a base layer  31  of silicon, a middle insulating layer  32  comprising several micrometers of SiO 2  and a top layer  33  comprising several micrometers of single crystal silicon. The waveguides, resonator and actuating electrodes are preferably formed in the top layer  33  using techniques well known in the art. The top silicon layer  33  possesses good mechanical properties, and the underlying oxide layer  32  can function as a sacrificial layer for releasing the mechanically active region. See, for example, R. A. Soref, et al., “Large single-mode rib waveguides in GeSi—Si and Si-on-SiO 2 ,” 27  IEEE J. Quant. Elec . 1971 (1991) and B. Jalali, et al., “Guided-Wave Optics In Silicon-on-Insulator Technology,” 143  IEE Proceedings - Optoelectronics  307 (1996), which are incorporated herein by reference. 
     The device can be fabricated with the initial waveguide-resonator spacing d o  at a larger value than the spacing required for coupling (e.g. d o =0.5 micrometer). In this manner, the device can be made using optical lithography with relatively loose tolerances on the precise value of d o . Referring back to FIG. 2, the central strips of silicon  34  surrounding the resonator  23  are electrically grounded, and different voltages (V 1  and V 2 ) can be applied to the two waveguides, drawing them towards the resonator. Waveguide  21  in a position drawn toward the resonator is shown as  21 A. The coupling can be adjusted independently for the two waveguides  21 ,  22 , permitting optimization of throughput extinction. In addition, separate voltages (V 3  and V 4 ) can be applied to the outlying electrodes  24 A and  24 B to pull the waveguides away from the resonator and completely shut off the coupling. 
     The precise shape and dimensions of the waveguides and the ring will depend upon the optical and mechanical design constraints. The relative shapes and aspect ratios shown in FIG. 2 are merely illustrative and do not necessarily reflect what the final device would look like. The dimensions of the top two layers will be on the order of one to several microns, and the spacing between the waveguide and the ring will be less than about 300 nm. The optical constraint is that the distance a should be large enough to allow for “shutting off” the interaction. Motion is obtained by electrostatic actuation: the waveguide  21  can be electrically grounded (at a point adjacent to the mechanically active region) and a voltage is applied to the Si layer pictured to the right of the released waveguide. The waveguide will move away from the resonator. For lower drive voltages, smaller w, larger h, and smaller a are preferred. The mechanically active region  35  will extend beyond the waveguide-resonator interaction region by ten to several tens of microns to allow for a wide bending length. Outside this region the waveguide will be anchored on the SiO 2  underlayer  32 . Adiabatic tapering can be used to modify the waveguide parameters and to allow for good coupling to an optical fiber. 
     Advantageously radiation  36  from a laser (not shown) can be shone selectively on the ring resonator  23  to tune the resonant wavelength. The radiation provides tuning by increasing the temperature of the resonator and increasing the density of charge carriers in the resonator. The wavelength of the radiation is preferably less than 1 micrometer and typically 0.85 micrometer. 
     To illustrate the feasibility of this design, one can calculate the voltage required to move the waveguide. For simplicity, consider a waveguide with a rectangular cross-section, and assume w=1 micron and h=3 microns. Also assume the length l of the mechanically released section of the waveguide to be l=40 microns. The displacement δ of a beam of length l anchored on both ends can be written:        δ   =       5      q                   l   4         384      E                 I                              
     where q is the force per unit length applied to the beam (the force is uniform across the whole beam), I is the moment of inertia of the beam cross-section, and E is Young&#39;s modulus. The force per unit length generated on the beam by applying a voltage V can be written:        q   =         ɛ   o        h                   V   2         2        d   2                                
     where ε o  is the premitivity of free space, and d is the separation between the two surfaces defining the capacitor. Assuming an initial (as fabricated) separation a=1 micron, and calculating the voltage required to displace the beam by 0.5 micron, we arrive at a value of V=57.6 volts. Current MEMs devices routinely operate at tens of volts, so this value is feasible. The L-shaped waveguide pictured in the diagram above will have a larger moment of inertia than was assumed for this calculation, but the larger moment can be compensated by increasing the length l of the mechanically released section. 
     FIG. 4 is a schematic top view of a multi-wavelength reconfigurable add-drop filter  40  formed by cascading a plurality of tunable, reconfigurable add-drop filters  20 A,  20 B,  20 C, with resonator stages independently addressable actuating electrodes (not shown for simplicity of stimulation). As many wavelengths as desired can thus be added or dropped. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.