Abstract:
A dynamic optical spectral filter is implemented in a microelectromechanical system (MEMS). It comprises a frame. An array of mirrors is provided on a first portion of the frame, along with a second array of adjustment mirrors on a second portion of the frame. An array of variable beam splitters is provided on a middle portion of the frame, between the first array and the second array. Finally, to provide the filter functionality, optical delays are disposed in beam paths between the first mirror array the second mirror array. These elements are used to produce multiple Mach-Zehnder interferometers, such as cascaded or series interferometers. Implementations have advantages associated with speed of operation and can have polarization isotropy.

Description:
BACKGROUND OF THE INVENTION 
     Dynamic optical spectral filters are a class of filters that can dynamically change the shape of their spectral filter transmission/reflection function. This distinguishes them from single cavity Fabry-Perot tunable filters, for example, in which, while the passband can be tuned across a band of interest, the passband shape itself is static or relatively static. 
     One proposed application for these dynamic optical spectral filters is as gain flattening filters. These filters are deployed at various stages along the optical fiber communication link to control the relative powers in the channels of some wavelength or frequency band of interest in a wavelength division multiplexed (WDM) optical signal. Gain tilt from optical amplifiers, such as erbium-doped fiber amplifiers (EDFA), or wavelength dependent losses, for example, can be neutralized. 
     Dynamic filters based on Mach-Zehnder interferometers, and more general arrayed waveguide grating filters, have been proposed and fabricated using integrated waveguide technology. Combinations of Bragg gratings and thermo-optic phase shifters are used to realize cascaded Mach-Zehnder interferometers. These integrated waveguide dynamic spectral filters have advantages associated with fabrication. Using photolithographic wafer processing techniques, completely integrated systems have been made. 
     SUMMARY OF THE INVENTION 
     The problem with these waveguide dynamic filters, however, concerns their size, response time, and polarization anisotropy. Typically, only a few filters can be fabricated on a wafer. Further, the modulation of the thermo-optic components can be relatively slow. Although this problem can be mitigated with good design, polarization anisotropy inherent in integrated waveguides is a more pernicious problem. In effect, the operation of the filter under otherwise static conditions changes due to changes in the polarization of the input light. 
     Two general approaches exist for addressing polarization anisotropy. A polarization homogenizer or scrambler can be used upstream of the waveguide dynamic spectral filter. This converts an input signal having an arbitrary or random polarization into an unpolarized signal. Scramblers typically add three decibels (dB) of insertion loss, however. 
     A second option is to use a polarization beam splitter and two waveguide spectral filters, one for each polarization. The problem here, however, is the detrimental impact to the system size and power requirements. Moreover, the unified control of the two filters is now required. 
     The present invention is directed to a dynamic optical spectral filter. Different from previous such filters, the present invention is directed to a microelectromechanical system (MEMS) implementation. Such implementations can be small, operate high speed, and be made isotropic with respect to polarization. 
     In general, according to one aspect, the invention features a dynamic optical spectral filter. It comprises a frame. An array of mirrors is provided on a first portion of the frame, along with a second array of adjustment mirrors on a second portion of the frame. An array of variable beam splitters is provided on a middle portion of the frame, between the first array and the second array. Finally, optical delays are disposed in beam paths between the first mirror array the second mirror array. These components yield cascaded or series Mach-Zehnder interferometers that can be collectively tuned to provide an arbitrary net filter function. 
     According to a preferred embodiment, the first mirror array, the second mirror array, and the beam splitter array form successive stages. These stages are preferably organized in a cascade or serial configuration. The optical delay in each of these stages is different to thereby yield different spectral periods (free spectral ranges) for the interferometers of each stage. 
     According to a specific embodiment, the optical delays for each of the stages are integer multiples of the smallest delay. This provides for Fourier series-like behavior that helps in obtaining the desired filter transmission profiles using control algorithms. 
     Further, according to the preferred embodiment, the adjustable mirrors are separate deflectable mirrors. Preferably, these are implemented as out-of-plane deflecting mirrors, which are preferably deflected using electrostatic forces or voltages. The variable beam splitter array is preferably implemented as short-cavity tunable Fabry-Perot cavities. Low finesse cavities with a very large free spectral range can be used to yield a relatively uniform reflectivity across the wavelength band of interest. 
     In general, according to another aspect, the invention features a dynamic optical spectral filter comprising cascaded Mach-Zehnder interferometers. Each of these interferometers includes a beam splitter comprising a short-cavity tunable Fabry-Perot cavity, a first mirror, and a second adjustable mirror. Typically, at least some of these interferometers include a discrete optical delay on one of the interferometer arms. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1 is a plan view of a MEMS dynamic optical spectral filter according to the present invention; 
     FIG. 2 is a plot of transmission as a function of frequency for the interferometers of each of the stages of the dynamic filter; and 
     FIG. 3 is a schematic view showing a bi-directional implementation to counteract any polarization anisotropy in the filter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a MEMS dynamic optical spectral filter, which has been constructed according to the principles of the present invention. 
     Generally, the dynamic filter  100  comprises a frame  102 . The frame  102  has an upper portion  104 , a middle portion  106 , and a lower portion  108 , with reference to the arbitrary orientation of the figure. 
     An array  110  of deflectable mirrors  112  is installed on the upper portion  104  of the frame  102 . Each deflectable mirror  112  of the array  110  is preferably implemented as a discrete optical membrane that has been coated to be highly reflective. Reflective coatings are preferably implemented with thin film dielectric coatings. 
     U.S. patent application Ser. No. 09/797,529, filed on Mar. 1, 2001, entitled “Integrated Tunable Fabry-Perot Filter and Method of Making Same”, discloses the fabrication of a tunable MEMS optical membrane device that can be used as the deflectable mirrors  112 . In one implementation, these membranes are used in a partially singulated state when the membrane-to-membrane pitch on the handle wafer is appropriate for the design of the filter  100 . In this example, the upper frame portion  104  is formed from the handle wafer material of the unsingulated MEMS membrane bars, in some embodiments. In an alternative embodiment, the MEMS membranes are singulated and installed on a separate frame portion. 
     In the illustrated example, the upper frame portion  104  comprises an input port  114  and an output port  116 . 
     An array of variable beam splitters  118  is attached to the middle frame portion  106 . The individually controlled variable beam splitters  120  of the array  118  control balancing of the optical signals in the upper cavity  130  relative to the lower cavity  132 . 
     In the present implementation, the array of variable beam splitters  118  is implemented as discrete beam splitters. Each discrete beam splitter  120  comprises a Fabry-Perot tunable filter, comprising an upper mirror  124  and a lower mirror  122 . These mirrors  122 ,  124  have relatively low reflectivities to yield a relatively low finesse Fabry-Perot cavity and are spaced so that the free spectral range is much greater than the wavelength band of interest. 
     In one implementation, the finesse of the filters/beam splitters is less than 2.5 or 2.0. This yields an extinction ratio of 5 dB, which is greater than the minimum of 3 dB required. This minimum is achieved with a finesse of 1.6. A finesse range of 1.6 to about 1.8 is probably the minimum tolerable for good operation. Further, the free spectral ranges of the filters is much greater than four times the width of the band of interest to yield relatively uniform reflectance for the band. In one embodiment, the filters  118  operate in the first or second order in which the mirror spacing is between λ/2 and λ, wherein λ is the center frequency of the band of interest. 
     As a result, the spectral filter function of each Fabry-Perot filter  120  is relatively uniform in transmission/reflection across the band of interest. Modulating the distance between the upper and the lower reflectors  124 ,  122  changes the reflectivity of the filters  120  with little regard to the wavelength of the input signal. This configuration allows the Fabry-Perot filters  120  to function as variable beam splitters in the present implementation. 
     In the current implementation, the Fabry-Perot filters  120  are implemented as described in Ser. No. 09/797,529 application with a separate non-deflecting mirror structure. 
     In one implementation, the variable beam splitters  120  are attached to a separate middle portion  106 . In an alternative implementation, the handle wafer material of partially singulated filter membranes functions as the middle portion  106  of the frame  102 . 
     Further, in the illustrated embodiment, the middle frame portion  106  comprises separate optical port regions  128  that allow transmission between the upper cavity  130  and the lower cavity  132  of the filter  100 . Whether the optical port regions are implemented as simply antireflection (AR) coated regions or actual holes through the frame depends on the transmissivity of the material of the frame. Generally, however, actual holes are preferred to reduce scattering and lower insertion loss. 
     An array  140  of stationary mirrors  144  is provided on the lower portion  108  of the frame  102 . These function as fold mirrors to redirect the optical signals back to the variable beam splitter array  118 . Further, in the illustrated embodiment, ports  142  are provided in the lower portion  140  to enable rejected light to leave the filter  100 . 
     Distributed in the lower cavity  132  are a series of optical delays  150 - 1  to  150 - 7  for the Mach-Zehnder interferometer arms. In the illustrated implementation where a free space interconnect is provided between the arrays of mirrors  110 ,  140  and the variable beam splitters  118 , the optical delays  150  can be implemented as AR-coated glass blocks, for example. Generally, they can be implemented as any high refractive index, transparent material. 
     In the present embodiment, the blocks  150 - 1  to  150 - 7  all provide substantially the same amount of delay. They are distributed in the cavity, however, so that the stages, stage  1 -stage  4 , of the filter  100 , each see different amounts of delay. Specifically, delay  150 - 1  is provided on only one leg of the signal transmission between the variable beam splitter  120 - 1  and stationary mirror  144 - 1 . In contrast, in stage  2 , delay  150 - 2  provides essentially twice the delay. Finally, for stage  4 , for example, three blocks of delay material  150 - 5 ,  150 - 6 ,  150 - 7  provide essentially four times the delay for stage  4  relative to stage  1 . 
     FIG. 2 is a plot of transmission as a function of frequency for each of the four stages of filter  100  to thereby illustrate its operation. Specifically, the filter  100  is designed to operate across some band of interest. Typically, this is in the communication wavelengths between 1,000 and 2,000 nm. For example, the band of interest can be C, L, or S bands in the ITU grid. 
     The delay  150 - 1  for stage  1  of filter  100  is selected so that the spectral period of the filter function is roughly equivalent to the band of interest. 
     As a result, stage  2  has a period that is then relatively one-half the band of interest, stage  3  has a spectral period that is approximately one-third the band of interest, and stage  4  has a spectral period that is one-fourth the period of the band of interest. 
     The filter functions of stage  1 - 4  are combined to yield a net filter function based on the Fourier series composition of the contribution of each stage. This is accomplished by modulating the magnitude of the contribution to the net filter function of each the stages  1  through stage  4 , by the independent control of the reflectivity of the separate variable beam splitters  120  of the beam splitter array  118 . The spectral phase of the filter functions of each of the stages is controlled by modulating the tunable mirrors  112  of the tunable mirror array  110 . 
     FIG. 3 illustrates an embodiment that addresses polarization anisotropy. Generally, in some cases the elements in the filter  100  may have some polarization anisotropy. In such a situation, a circulator can provided in combination with a beam splitter. Signals are sent each way through the filter  100 . The filtered signal is separated using the circulator. 
     In still another embodiment, the requirement for the circulator  214  is removed by separating the incoming signal  210  and filtered light using a polarization beam splitter  216 , assuming that the polarization is not changed in the filter  100 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.