Abstract:
A tunable electromagnetic field frequency filter having an input waveguide which carries a signal including at least one desired frequency, and an output waveguide. A resonator-system is coupled to the input and output waveguides and is operable for the selective transfer of the at least one desired frequency to the output waveguide. The resonator-system supports at least two system modes, and includes at least three reflectors with at least two different reflectivity spectra. At least one of the reflectivity spectra is tuned such that at least two of the system-modes have substantially the same frequency when the transfer occurs substantially.

Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 60/162,177 filed Oct. 28, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of tunable add/drop filters using side-coupled resonant tunneling. 
     Add/drop filters play a vital role in wavelength division multiplexed (WDM) lightwave communications systems. They allow different wavelengths to be added or removed from an optical transmission line. Add/drop filters come in one of two classes: static or dynamic. Static devices are designed to operate at a given fixed wavelength predetermined during the fabrication process. Dynamic devices are designed to be switched on and off to add or remove selected wavelengths from a transmission line and, in some cases, are also designed to be tunable, i.e., single devices can be made to add or remove any one of a number of different wavelengths. 
     Examples of dynamic filters include acousto-optic tunable filters (AOTFs), polymer-based thermo-optic switches, and micro electromechanical systems (MEMS). These filters suffer from a variety of physical limitations such as high crosstalk, frequency insensitivity, and overall large size. This latter feature makes some of the filters unsuitable for very large scale integration (VLSI). In fact, a single AOTF can be as large as one square inch. 
     Recently, an optical switch based on side-coupled resonant tunneling suitable for VLSI has been designed and described in U.S. Pat. No. 6,101,300 entitled “HIGH EFFICIENCY CHANNEL DROP FILTER WITH ABSORPTION”, of common assignee. The switch is activated by changing the internal decaying rate of a resonant coupling element located between two adjacent waveguides. While being very small in size, the switch was designed mostly to add or drop a specific wavelength, determined during fabrication, with limited post-fabrication tunability, i.e., the user could select the wavelength to be added or dropped within only a small range. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a tunable add/drop filter capable of selectively adding or dropping one or more wavelength over a wide range of wavelengths. The tuning range can be made large enough to cover the entire bandwidth of a WDM system. The specific wavelength can be selected after fabrication. Post-fabrication tunability over a wide range is a highly desirable feature. It allows, for example, devices to be adjusted during installation. Control signals are adjusted to select a wavelength and the device is left to operate continuously at the selected wavelength, or the device could be electronically and remotely driven to dynamically switch between different wavelengths (such as in optical add/drop multiplexers) allowing fast reconfigurability of optical networks. 
     The add/drop filter includes two waveguides coupled by an element containing at least two resonators. It is the coupling element that determines the transfer properties of the add/drop filter such as the wavelength of the transferred electromagnetic field. The wavelength of the coupling element in the invention is tunable over a wide range of wavelengths. 
     In accordance with one embodiment of the invention there is provided a tunable electromagnetic field frequency filter having an input waveguide which carries a signal including at least one desired frequency, and an output waveguide. A resonator-system is coupled to the input and output waveguides operable for the selective transfer of the at least one desired frequency to the output waveguide. The resonator-system supports at least two system modes, and includes at least three reflectors with at least two different reflectivity spectra. At least one of the reflectivity spectra is tuned such that at least two of the system-modes have substantially the same frequency when the transfer occurs substantially. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a tunable add/drop filter in accordance with the invention; 
     FIGS. 2A-2C are schematic graphs of the reflectivity spectra of the three gratings shown in FIG. 1; 
     FIG. 3 is a plan view of a first exemplary embodiment of the invention; 
     FIG. 4 is a plan view of a second exemplary embodiment of the invention; 
     FIG. 5 is a cross sectional view of the waveguide shown in FIG. 4 along the dashed line  5 — 5  in which the device is tuned using the electro-optic effect; 
     FIG. 6 is a cross sectional view of the waveguide shown in FIG. 4 along the dashed line  5 — 5  in which the device is tuned using the injection of charge carriers; and 
     FIG. 7 is a cross sectional view of the waveguide shown in FIG. 4 along the dashed line  5 — 5  in which the device is tuned using the thermo-optic effect. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic diagram of a tunable add/drop filter  100  in accordance with the invention. The filter includes two waveguides that are labeled “bus”  102  and “receiver”  104 . A coupling element  106  has a resonator system comprising two resonators, labeled R 1  ( 108 ) and R 2  ( 110 ). The resonators are defined using three gratings, labeled G 1  ( 112 ), G 2  ( 114 ), and G 3  ( 116 ). The reflectivity spectra of the gratings consist of combs of reflection peaks, i.e., series of discrete frequency regions of high reflectivity separated by regions of low reflectivity. Examples of gratings include, but are not limited to, sampled gratings, chirped gratings, and super-structure gratings. Some examples are shown in the following two references: “Tunable Laser Diodes” by M. C. Amann and J. Buus, published by Artech, Boston (1998); and “Design of Widely Tunable Semiconductor Lasers and Concept of Binary Superimposed Gratings (BSG&#39;s)” by I. Avrutsky, D. Ellis, A. Tager, H. Anis and J. Xu, published in IEEE J. Quant. Elect., vol. 34, no. 4, pp. 729-741 (1998), both of which are incorporated herein by reference. 
     The two resonators are coupled through grating G 2  and are further coupled through the waveguides. Each resonator supports at least one mode and the resonator system supports at least two system modes. The system modes are eigenmodes of the resonator system formed in part by a linear combination of the resonator modes. Substantial transfer occurs between the bus and the receiver when the two system modes have substantially the same resonant frequency and the same overall decay rate. However, the invention does not necessarily have a plane of symmetry perpendicular to the waveguides. The wavelength of the transferred signal is selected by changing the resonant frequency of the resonators. This is accomplished by changing the reflectivity spectrum of the gratings and by adjusting the round-trip path length inside the resonators to insure resonance. The reflectivity and the round-trip path length can be adjusted, for example, using a variety of physical phenomena such as carrier injection, thermal heating, or the electro-optic effect. 
     The comb-like reflectivity spectra of the three gratings are shown schematically in the graphs of FIGS. 2A-2C. Each spectrum consists of a series of reflection peaks separated by a different frequency spacing Δf i (i=1, 2, or 3). The features of the reflectivity spectra depend on the physical parameters of the gratings; each grating has a different set of physical parameters. Since each grating has a different frequency spacing, it is possible to align one reflection peak from each grating, i.e., all three gratings have a reflection peak at the same frequency, while keeping all other reflection peaks misaligned. Tuning is accomplished by frequency shifting at least one of the reflection spectra. 
     Only small frequency shifts are needed to misalign the reflectivity peaks. Moreover, other peaks can be made to coincide resulting in the resonators being resonant at a different frequency. This effect, called the Vernier effect, is used to tune the filter over a wide frequency range while using only small frequency shifts. It is used in certain tunable laser sources for WDM applications. See, for example, “Widely tunable 1.55-μm lasers for wavelength-division-multiplexed optical fiber communications”, F. Delorme, IEEE J. Quant. Electron., Vol. 34, pp.1706-1716 (1998), incorporated herein by reference. 
     The coupling of the two resonators through G 2  (the “direct” coupling) causes frequency splitting of the two system modes. This direct coupling is compensated by “indirect” coupling through the waveguides. The indirect coupling depends on the optical path length between the resonators which in turn depends on the frequency of the resonant modes. In order to guarantee cancellation of direct and indirect coupling for any frequency, it is necessary to adjust the phase of the signal in the waveguides. This can be accomplished, for example, using the electro-optic effect. 
     FIG. 3 is a plan view of an exemplary embodiment of a tunable add/drop filter  300  of the invention. The filter includes two waveguides that are labeled bus  302  and receiver  304 . The filter includes a coupling element  306  having a resonator system comprising two resonators, labeled R 1  and R 2 . The resonators are defined using three gratings, labeled G 1 , G 2 , and G 3 . In this embodiment, the waveguides (both the bus and the receiver) are curved to minimize the transfer of non-resonant channels occurring through waveguide coupling, and back reflection from the gratings. The cross-hatched regions correspond to metal electrodes  308 ,  310  located above the waveguides, and metal electrodes  312 ,  314 ,  316 ,  318 ,  320  located above the resonators and gratings, respectively. In this exemplary embodiment, a voltage is applied to each electrode to change the optical properties of the materials. There are six control signals: 
     S G1 , S G2 , and S G3  are used to shift the reflectivity spectra of G 1 , G 2 , and G 3 , respectively, hence are used to select (tune) the desired channel; 
     S φ1  and S φ2  are used to satisfy the round-trip resonant condition in each resonator; and 
     S ind  is used to satisfy the cancellation of direct and indirect coupling between the resonators. 
     Alternatively, the filter could be operated by shifting only two reflectivity spectra, leaving the third unaffected. One reflectivity spectrum could be aligned with the standard International Telecommunication Union (ITU) grid, though this is not necessary to meet ITU standards. Also, S ind  could be driven with two separate sources. 
     In the exemplary embodiment shown in FIG. 3, each resonator has a single resonant mode within the frequency range δf i , where δf i  is defined as the width of the reflection peaks of grating i. The optical resonant modes extend spatially in (and beyond) the resonators over a total length L. L is the effective length of the resonators and is different than the physical length of the resonator. The frequency spacing Df between two adjacent resonant frequencies is given by: 
     
       
           Df=c /( Ln   eff ) 
       
     
     where n eff  is the effective index of refraction of the materials in the resonator region. The product Ln eff  is defined as the optical length of the resonators. Since the resonators support only one resonant mode within the frequency range δf i , δf i  must be smaller than Df. This sets an upper limit on the effective length of the resonators: 
     
       
           L&lt;c /(2 n   eff   δf   i ) 
       
     
     In the specific case where n eff =3 and δf 1 =δf 2 =δf 3 =10 GHz, the effective length of the resonator must be less than 5 mm. 
     The frequency spacing Δf i  of the different reflection peaks is given by: 
     
       
         Δ f   i   =c /(2 l   i   n   eff,i ) 
       
     
     where l i  is the length of the repeating unit of grating i, n eff,i  is the effective index of grating i, and l i n eff,i  defines the optical length of the i-th reflector. In the specific case where n eff,2 =3 and Δf 2 =100 GHz, the length of the repeating unit of grating  2  is 500 μm. The frequency spacing can be modified by changing the length of the grating. 
     In another embodiment, the width of the reflection peaks is made larger such that it overlaps with more than one frequency channel. In this embodiment, multiple channels are transferred between the two waveguides simultaneously. 
     FIG. 4 is a plan view of another exemplary embodiment of a tunable add/drop filter  400 . The filter includes two waveguides that are labeled bus  402  and receiver  404 . The filter includes a coupling element  406  having a resonator system comprising four resonators, labeled R 1 -R 4 . The resonators are defined using six gratings, labeled G 1 -G 6 . In this embodiment, the waveguides (both the bus and the receiver) are curved to minimize the transfer of non-resonant channels occurring through waveguide coupling, and back reflection from the gratings. The cross-hatched regions correspond to metal electrodes  408 ,  410  located above the waveguides, and metal electrodes  412 ,  414 ,  416 ,  418 ,  420  located above the resonators and gratings, respectively. In this exemplary embodiment, a voltage is applied to each electrode to change the optical properties of the materials. 
     As in the previous embodiment there is a total of seven metal electrodes. While the use of two resonators in the previous embodiment resulted in a Lorentzian-shaped transfer function, the presence of four resonators in the current embodiment results in a non-Lorentzian transfer lineshape. The use of four resonators has the advantage of generating “flat top” and “sharp sidewall” response characteristics. Additional resonator pairs could be added to this embodiment to further modify the transfer lineshape. 
     The tunable add/drop filter presented in this disclosure can be fabricated in any of a large number of material systems such as III-V or II-VI compound semiconductors, or Si-based material systems. FIG. 5 shows the cross section of an exemplary embodiment of a waveguide  500  as represented along dashed line  5 — 5  of FIG.  4 . FIG. 5 shows an InGaAsP ridge waveguide  502  buried in an InP substrate  504 , with a backside metal contact  506 . However, it will be appreciated by those skilled in the art that other waveguide geometries and other material systems can also be used. A metal electrode  508  is deposited on top of an insulating layer  510 . FIG. 5 illustrates a configuration for adjusting the device using the electro-optic effect. 
     FIG. 6 is a cross section of another exemplary embodiment of a waveguide  600  taken along dashed line  5 — 5  of FIG.  4 . FIG. 6 illustrates a configuration for tuning the device using the injection of charge carriers. Waveguide  600  includes an InGaAsP waveguide  602  buried in an InP substrate  604 , with a backside metal contact  606 . In this embodiment, an uppermost insulating layer is omitted and optional current-confinement layers  608  are added to help confine the carriers in the area of the waveguide. Furthermore, in this specific embodiment, doped semiconductors are used to promote current injection. A metal electrode  610  is deposited on the surface. 
     In yet another embodiment, the device is tuned using the thermo-optic effect. FIG. 7 is a cross section of an exemplary embodiment of a waveguide  700  taken along dashed line  5 — 5  of FIG. 4. A waveguide  702  of doped SiO 2  is buried in a layer  704  of undoped SiO 2 , which is provided on a Si substrate  706 . In this embodiment, resistive metal heaters  708  are used to locally change the material temperature, hence the index of refraction. Although the thermal effect is generally slower than the electroabsorption and electro-optic effects, i.e., the tuning speed is lower, the device retains its large range of tunability. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.