Abstract:
A detunable Fabry-Perot interferometer, and method of tuning a Fabry-Perot interferometer are provided. The Fabry-Perot interferometer includes a first mirror, a second mirror oriented with respect to the first mirror so as to define a Fabry-Perot cavity therebetween, and an actuator configured to adjust a resonant wavelength of the Fabry-Perot cavity by varying a gap between the first and second mirrors, wherein the actuator is configured to selectively maintain the first and second mirrors in a substantially non-parallel relationship while the resonant wavelength of the Fabry-Perot interferometer is varied. The detunable Fabry-Perot interferometer can be employed in a multiplexer of a telecommunications system, as provided.

Description:
This is a Continuation-in-part (CIP) of International (PCT) Application No: PCT/US02/12496, filed Apr. 22, 2002, which is a CIP of U.S. patent application Ser. No. 10/085143, filed Mar. 1, 2002 now U.S. Pat. No. 6,665,109, which claims priority to U.S. Provisional Application Nos. 60/284,943, filed Apr. 20, 2001 and 60/303,772, filed Jul. 10, 2001 and is also a CIP of U.S. patent application Ser. No. 09/811,612, filed Mar. 20, 2001 now U.S. Pat. No. 6,519,074, which is a CIP of U.S. patent application Ser. No. 09/766,687, filed Jan. 19, 2001 now U.S. Pat. No. 6,597,461, which claims priority to U.S. Provisional Application Nos. 60/190,110, filed Mar. 20, 2000 and 60/211,529, filed Jun. 15, 2000. Incorporation By Reference. The entire disclosure of the prior application is considered as being part of the disclosure of the accompanying application and is hereby incorporated by reference therein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a detunable Fabry-Perot Interferometer, and a method of tuning a Fabry-Perot Interferometer. Further, the invention relates to a detunable Fabry-Perot Interferometer employed in a multiplexer of a telecommunications system. 
     2. Background of the Related Art 
     There is a continuing need for tunable optical components for various applications, such as optical networking, wavelength-division-multiplexing and other telecommunications applications. 
     Existing technologies for tunable optical components are either too costly, unreliable, or do not exbibit the performance needs for present and/or future systems requirements. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter. 
     The invention relates to a detunable Fabry-Perot Interferometer, and a method of tuning a Fabry-Perot Interferometer. Further, the invention relates to a detunable Fabry-Perot Interferometer employed in a multiplexer of a telecommunications system. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 is a schematic side cross-sectional view of a Fabry-Perot interferometer, according to an embodiment of the invention; 
     FIGS. 1A and 1B show a plan view of exemplary electrodes of an actuator according to an embodiment of the invention; 
     FIG. 2 is a schematic side cross-sectional view of a Fabry-Perot interferometer, according to an embodiment of the invention, showing the island of the compliant optical support in a tilted configuration; 
     FIGS. 3A-3D schematically shows the steps of de-tuning, scanning and then re-tuning a Fabry-Perot cavity, according to a method of the invention; 
     FIG. 4 schematically shows channel changing in a multiplexer; 
     FIGS. 5A-5D schematically show the theoretical application of the Fabry-Perot interferometer and method according to the invention in a multiplexer; 
     FIG. 6 schematically shows the general layout of a multiplexer employing a Fabry-Perot interferometer; and 
     FIG. 7 schematically shows a multiplexer employing the detunable Fabry-Perot interferometer and method according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention provides a detunable Fabry-Perot interferometer, and a method of tuning the Fabry-Perot interferometer. Fabry-Perot interferometers, or filters, transmit light of a predetermined wavelength and reflect the non-transmitted light back to the source. Generally, Fabry-Perot interferometers consist of an optical cavity formed by two parallel reflectors or mirrors. When the optical path length between the reflectors is an integer number of half waves, the structure becomes optically resonant, with zero electric field intensity at the boundaries, and energy is coupled through the interferometer. Generally, to make the interferometer tunable, one of the reflectors is fixed and the other is configured so that it is movable, with the distance between them controlled to “tune” the wavelength that will pass through the interferometer. 
     An example of a Fabry-Perot cavity interferometer is shown in FIG. 1. A tunable Fabry-Perot cavity is described in co-pending International (PCT) parent application Ser. No. PCT/US02/12476, filed Apr. 22, 2002, entitled “MEMS-based Tunable Fabry-Perot Filters and Method of Forming Same”. Any of the embodiments disclosed in PCT Application No. PCT/US02/12496 can be employed to realize the apparatus and methods according to the invention discussed herein. 
     The Fabry-Perot interferometer  1  of FIG. 1 includes a mirror support  10  and a compliant optical support  20 . A Fabry-Perot cavity  5  is formed by a first mirror  15  and a second mirror  25 . The first mirror  15  is attached to the mirror support  10  and, in a preferred embodiment, is fixed in place by the mirror support  10 . The mirror support  10  may further include an anti-reflective (AR) coating  55 . In this embodiment, the AR coating  55  is positioned on a surface  11 A of the mirror support  10  opposite to a surface  11 B of the mirror support  10 , on which the first mirror  15  is positioned. 
     The second mirror  25  is attached to the compliant optical support  20 . The compliant optical support  20  is formed of a frame  20 B, an island  20 A, and a compliant member  50 , which attaches the island  20 A to the frame  20 B, and provides flexibility therebetween. In one preferred embodiment, the second mirror  25 , which is affixed to the island  20 A of the compliant optical support  20 , is movable with respect to the first mirror  15 , which is affixed to the first layer  10 , via an actuator  60 , which will be further discussed hereafter. 
     The mirror support  10 , the frame  20 B, and the island portion  20 A of the compliant optical support  20  are preferably formed of a generally inflexible material, preferably a material that is compatible with micro-electro-mechanical systems fabrication processes, such as silicon. However, other materials, generally or partially flexible, may also be appropriate. The compliant member  50  is formed of a flexible material, preferably a highly compliant polymeric material, such as an elastomer. However, other materials may also be appropriate. 
     In operation, the actuator  60  can be controlled to apply a force to the island  20 A, thereby moving the island  20 A. The compliant member  50  exerts a restoring force to the island  20 A, which tends to urge the island  20 A back into alignment with the frame  20 B when the actuating force is removed. The actuator  60  functions to move at least the island  20 A, thereby varying a distance between the mirrors  15  and  25 , and thus varying the wavelength to which the Fabry-Perot cavity  5  is tuned. The actuator  60  can include any number and configuration of magnetic, electrostatic, or mechanical force transducers. 
     In a preferred embodiment, the actuator  60  includes a first set  40  of electrodes  40 A positioned on a surface  21 A of the island  20 A opposite to a surface  21 B on which the second mirror  25  is positioned. In one preferred embodiment, an AR coating  45  is provided between the surface  21 A of the island portion  20 A and the electrodes  40 A. 
     The actuator  60  further includes a common electrode  35 A positioned on a surface  31 A of an actuator support  30  of the Fabry-Perot interferometer  1 , according to an embodiment of the invention. The actuator support  30  includes a hole  325  for passing source light to the second mirror  25 . The actuator support  30  is preferably formed of a generally inflexible material, preferably a material that is compatible with micro-electro-mechanical systems fabrication processes, such as silicon. However, other materials, generally or partially flexible, may also be appropriate. The compliant optical support  20  and the actuator support  30  together form an actuated optical support  350 , which is described in detail in co-pending U.S. parent patent application Ser. No. 10/085,143, filed Mar. 1, 2002, entitled “Compliant Mechanism and Method of Forming Same”, which is hereby incorporated by reference. 
     FIGS. 1A and 1B show a plan view of the electrodes  40 A and  35 A. In this embodiment, three electrodes  40 A are provided on the compliant optical support  20  and one common electrode  35 A is provided on the actuator support  30 . However, this arrangement could be reversed. Further, a variety of other configurations of electrodes which cooperatively function together could be utilized. 
     The electrodes  40 A,  35 A are configured to generate an electrostatic force when a command signal is applied thereto. The command signal can be configured to create a repulsive or an attractive electrostatic force between the electrodes. 
     Traditional Fabry-Perot cavities are tuned by varying the distance between parallel partially reflective mirrors. Generally, one mirror is held fixed, while the other mirror is moved with respect to the fixed mirror to “tune” the Fabry-Perot cavity to a particular wavelength. 
     The Fabry-Perot interferometer according to the invention includes the compliant member  50 . The compliant member  50  allows the island  20 A to flex with respect to the frame  20 B of the compliant optical support  20 . By controlling the actuator  60 , the island  20 A can be flexed with respect to the frame  20 B to vary the distance between the first and second mirrors  15  and  25  to “tune” the Fabry-Perot cavity  5  to pass a desired wavelength of light so that the Fabry-Perot cavity  5  passes a predetermined or desired wavelength of light while reflecting substantially all other wavelengths of light. More importantly, by varying the voltage applied between the individual electrodes  35 A,  40 A, of the respective sets  35 ,  40  of electrodes, the island  20 A, and thus the second mirror  25 , can be tilted with respect to the first mirror  15  to “de-tune” the Fabry-Perot cavity  5 . That is, while the second mirror  25  is tilted with respect to the first mirror  15 , the Fabry-Perot cavity  5  reflects substantially all wavelengths of light independent of the spacing between mirrors  15  and  25 . 
     FIGS. 3A-3D schematically show the steps of detuning, and then retuning a Fabry-Perot cavity according to a method of the invention. It is noted that in FIGS. 3A-3D only the first and second mirrors  15 ,  25  and the cavity  5  are shown for simplicity of explanation. 
     As shown in FIG. 3A, the Fabry-Perot cavity  5  is initially tuned to a desired wavelength λ 1  by orienting the first and second mirrors  15 ,  25  parallel to one another a distance d λ1  apart, which corresponds to a cavity spacing that will pass the desired wavelength λ 1 . By tilting the second mirror  25  with respect to the first mirror  15 , as shown in FIG. 3B, the Fabry-Perot cavity  5  is detuned, thereby reflecting substantially all wavelengths of light. The second mirror  25  is then adjusted so that one end  25 A is a distance d λ2 , from the first mirror  15 , corresponding to a cavity spacing for the next desired wavelength λ 2  of light as shown in FIG. 3D, the second mirror  25  is oriented to be parallel to the first mirror at the distance d λ2 , corresponding to the cavity spacing for the next desired wavelength λ 2  of light. In this manner, while the distance between the first and second mirrors  15 ,  25  is varied, the Fabry-Perot cavity  5  is detuned so that it does not pass intermediate varying wavelengths of light during the time period in which the distance between the mirrors is varied. 
     The tunable Fabry-Perot cavity according to the invention has a variety of applications, and is particularly applicable in a multiplexer, for the reasons discussed below. An example of a tunable single channel add/drop multiplexer employing a detunable Fabry-Perot interferometer  1  according to the invention is shown in FIG.  7 . 
     In dense wavelength division multiplexing (DWDM) systems, which transmit numerous wavelengths of light simultaneously over a single optical fiber, Fabry-Perot interferometers used in add/drop multiplexers must exhibit high finesse, because the optical channels are spaced extremely close together in wavelength. Add/drop multiplexers are used to add and/or drop channels as necessary. Thus, it is important that the multiplexer be able to resolve the individual optical channels. 
     As shown in FIG. 6, generally, a multiplexer  100  receives an incoming signal, which includes light at different wavelengths, or channels, and is designated as “Traffic IN”, via an input path way  110 . A circulator re-directs the signal onto pathway  180 , which contains a tunable Fabry-Perot interferometer  170 . The tunable Fabry-Perot interferometer  170  allows channels to be added to the incoming signal via add pathway  130  and circulator  160 , or dropped from the incoming signal via circulator  160  and drop pathway  140 . The signal, now designated “Traffic OUT”, then exits the multiplexer via output pathway  120 . 
     When no channel is to be added to or dropped from the “Traffic IN” signal, the cavity spacing of the Fabry-Perot interferometer  170  is adjusted so that the resonant wave length of the cavity does not correspond to any of the optical channel wavelengths. Thus, any optical channels impinging on the Fabry-Perot interferometer  170  from circulator  150  are reflected by the Fabry-Perot interferometer  170 , as are any optical channels impinging on the Fabry-Perot interferometer  170  from pathway  180 , and any optical channels impinging on the Fabry-Perot interferometer  170  from add pathway  130 . Accordingly, the “Traffic IN” signal is reflected back to circulator  150 , without any additional optical channels being added, and is directed onto the output pathway  120 . 
     When an optical channel is to be added, the cavity spacing of the Fabry-Perot interferometer  170  is adjusted so as to pass the wavelength of the optical channel to be added. Thus, the optical channel to be added is received by circulator  160  from the add pathway  130 , and is directed to the Fabry-Perot interferometer  170 , where it is passed to circulator  150 , and directed to the output pathway  120 . 
     When an optical channel is to be dropped from the “Traffic IN” signal, the cavity spacing of the Fabry-Perot interferometer  170  is adjusted to pass the wavelength of the optical channel to be dropped. Thus, when the optical channel to be dropped impinges on the Fabry-Perot interferometer  170  via pathway  180 , it is passed by the Fabry-Perot interferometer  170  and directed to the drop pathway  140  by the circulator  160 . 
     However, in prior art multiplexers, as the Fabry-Perot interferometer was tuned from, for example, channel  1  to channel  5 , as shown in FIG. 4, the Fabry-Perot interferometer scanned through channel  2 , then channel  3 , then channel  4 , before the desired channel (channel  5 ) was reached. This resulted in optical channels being inadvertently added and/or dropped. 
     An add/drop multiplexer using the detunable Fabry-Perot interferometer of the invention can be tuned from one optical channel to another without interfering with the optical channels interposed therebetween. FIGS. 5A-5D schematically show the steps of detuning, and then retuning the Fabry-Perot interferometer of the present invention in the context of an add/drop multiplexer. It is noted that, in FIGS. 5A-5D, only the first and second mirrors  15 ,  25  and the Fabry-Perot cavity  5  are shown for simplicity of explanation. 
     As shown in FIG. 5A, the Fabry-Perot Cavity  5  is initially tuned to the wavelength λ ch1  of channel  1  by orienting the first and second mirrors  15 ,  25  parallel to one another a distance λ ch1  apart, which corresponds to the cavity spacing that will pass the desired wavelength λ ch1 . The second mirror  25  is tilted with respect to the first mirror  15 , by an amount sufficient to substantially detune the Fabry-Perot cavity  5 , as shown in FIG.  5 B. The second mirror  25  is then adjusted so that one end  25 A is a distance dλ ch5  from the first mirror  15 , which corresponds to the cavity spacing that will pass the wavelength of channel  5  (λ ch5 ) once the mirrors  15 ,  25  are made substantially parallel. Then, as shown in FIG. 5D, the second mirror  25  is oriented to be substantially parallel to the first mirror at the distance d λ ch5 , which corresponds to the cavity spacing that will pass wavelength λ ch5 . In this manner, while the distance between the first and second mirrors  15 ,  25  is adjusted, the Fabry-Perot cavity  5  is detuned and does not pass wavelengths corresponding to intervening optical channels  2 - 4 . 
     FIG. 7 schematically shows a multiplexer employing the detunable Fabry-Perot interferometer and method of the present invention. The multiplexer  200  includes input pathway  210 , a GRIN lens  215 , a circulator  250 , an output pathway  220 , a circulator  260 , an add pathway  230 , and a drop pathway  240 . Detunable Fabry-Perot interferometer  270  is provided on pathway  280  connecting circulators  250 ,  260 . The Fabry-Perot interferometer  270  is in electrical communication with both tunable filter drive electronics  276 , and capacitance sense electronics  235 . A transimpendence amp  245  and photodiode  255  are in communication with pathway  280 . 
     The principle of operation of the multiplexer of FIG. 7 is substantially the same as the multiplexer of FIG. 6, except that the Fabry-Perot interferometer  270 , under the control of tunable filter drive electronics  276 , is detuned during scanning between optical channels, as described above. The dropped optical channel is monitored by beam splitter  300 , photodiode  255 , and transimpendence amplifier  245 . The beam splitter  300  redirects a small portion of the optical signal coming from the Fabry-Perot interferometer  270 , and directs the signal portion to photodiode  255 . The photodiode  255  converts the optical signal to an electronic signal, and sends the electronic signal to transimpendence amplifier  245  for amplification. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.