Patent Publication Number: US-6343165-B1

Title: Optical add drop multiplexer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. Ser. No. 09/426,060, filed Oct. 22, 1999, now U.S. Pat. No. 6,266,462 which is a continuation-in-part of U.S. Ser. No. 09/022,413 filed Feb. 12, 1998, now U.S. Pat. No. 6,021,237, issued Feb. 1, 2000, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to telecommunication systems and assemblies, and more particularly to an optical add/drop multiplexer. 
     2. Description of Related Art 
     In modem telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA&#39;s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, “Radiation-Hardened Optical Repeater”. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed. 
     Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1 microsecond, are polarization and temperature dependent. 
     Furthermore, the best AOTF consumes several watts of RF power. has spectral resolution about 3 nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions. 
     Another wavelength-sensitive optical switch may be implemented with a tunable Fabry Perot filter (TFPF). When the filter is aligned to a specific wavelength, it is transparent to the incoming optical power. Though the filter mirrors are almost 100% reflective no power is reflected back from the filter. With the wavelength changed or the filter detuned (for example, by tilting the back mirror), the filter becomes almost totally reflective. With the optical circulator in front of the filter, the reflected power may be redirected from the incident port. The most advanced TFPF with mirrors built into the fiber and PZT alignment actuators have only 0.8 dB loss. The disadvantage of these filters is a need for active feedback and a reference element for frequency stability. 
     There is a need for an improved optical add/drop multiplexers and improved narrowband AO converters that include AOTF&#39;s. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an improved optical add/drop multiplexer. 
     Another object of the present invention is to provide an improved narrowband AO converter. 
     These and other objects of the present invention are achieved in an optical add/drop multiplexer that includes a mode converter. A first mode coupler is coupled to an input of the mode converter. A second mode coupler is coupled to an output of the mode converter. The mode converter includes an optical fiber with multiple cladding modes and a single core mode guided along a core. An acoustic wave propagation member is coupled to the optical fiber. The acoustic wave propagation member propagates an acoustic wave from its proximal to its distal end. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 ( a ) is a schematic diagram of one embodiment of an AOTF of the present invention. 
     FIG. 1 ( b ) is a cross-sectional view of the optical fiber of the FIG. 1 AOTF. 
     FIG. 2 is a cross-sectional view of one embodiment of an acoustic wave propagation member that can be used with the AOTF of FIG.  1 . 
     FIG.  3 ( a ) is a cross-sectional view illustrating one embodiment of an interface created between an optical fiber and a channel formed in an acoustic wave propagation member of the FIG. 1 AOTF. 
     FIG.  3 ( b ) is a cross-sectional view illustrating an embodiment of an interface between an optical fiber and a channel formed in an acoustic wave propagation member of the FIG. 1 AOTF where a bonding material is used. 
     FIG. 4 is a schematic diagram of one embodiment of an AOTF of the present invention with an acoustic damper. 
     FIG. 5 is a cross-sectional view of one embodiment of an index profile of an optical fiber, useful with the AOTF of FIG. 1, that has a doubling cladding. 
     FIG. 6 is a cross-sectional view of an optical fiber with sections that have different diameters. 
     FIG. 7 is a cross-sectional view of an optical fiber with a tapered section. 
     FIG. 8 is a perspective view of one embodiment of an AOTF of the present invention that includes a heatsink and two mounts. 
     FIG. 9 is a perspective view of one embodiment of an AOTF of the present invention with a filter housing. 
     FIG. 10 is a block diagram of an optical communication system with one or more AOTF&#39;s of the present invention. 
     FIG. 11 is a schematic view showing the structure of an acousto-optic tunable filter according to one embodiment of the present invention. 
     FIG. 12 is a graph showing the coupling and transmittance of the filter of FIG.  1 . 
     FIG. 13 is a graph showing the transmittance of the filter of FIG.  11 . 
     FIG. 14 is a graph showing the center wavelength of filter of FIG. 1 as a function of the frequency applied to the acoustic wave generator. 
     FIGS.  15 ( a )-( d ) are graphs illustrating the transmissions of the filter of FIG. 11 when multiple frequencies are applied to the acoustic wave generator. 
     FIGS.  16 ( a )-( b ) are graphs showing the transmittance characteristics of the filter of FIG. 11 when varying an electric signal with a three frequency component applied to the filter. 
     FIGS.  17 ( a )-( d ) are graphs for comparing the mode converting characteristic of the filter according to an embodiment of the present invention with that of a conventional wavelength filter. 
     FIG.  18 ( a ) illustrate one embodiment of a transmission spectrum of the FIG. 11 filter. 
     FIG.  18 ( b ) illustrates the measured and the calculated center wavelengths of the notches as a function of acoustic frequency of an embodiment of the FIG. 11 filter. 
     FIG. 19 illustrates two examples of configurable spectral profiles with spectral tilt from the FIG. 11 filter. 
     FIG.  20 ( a ) is a filter assembly that includes two filters of FIG. 11 that are in series. 
     FIG.  20 ( b ) is a schematic diagram of a dual-stage EDFA with a filter of FIG.  20 ( a ). 
     FIG. 21 ( a ) is a graph of gain profiles of an EDFA with the filter of FIG.  20 ( a ). 
     FIG. 21 ( b ) is a graph illustrating filter profiles that produced the flat gain profiles shown in FIG.  21 ( a ). 
     FIG. 21 ( c ) is a graph illustrating filter profiles of the FIG.  20 ( a ) filter assembly. 
     FIGS.  22 ( a ) and  22 ( b ) are graphs illustrating the polarization dependence of one embodiment of the filter of the present invention. 
     FIG. 23 illustrates one embodiment of the present invention from FIG. 4 that has a reduction with a lower polarization dependent loss. 
     FIGS.  24 ( a ) and  24 ( b ) are graphs illustrating the polarization dependent loss profile of one embodiment of the invention, from FIG. 4, when the filter is operated to produce 10-dB attenuation at 1550 nm. 
     FIG. 25 illustrates an embodiment of the invention with two of the filters of FIG.  1 . 
     FIG. 26 is a graph illustrating the effects of a backward acoustic reflection at the damper of one embodiment of the present invention from FIG.  4 . 
     FIG.  27 ( a ) is a graph illustrating, in one embodiment of FIG. 4, the modulation depth at 10-dB attenuation level at both first- and second-harmonics of the acoustic frequency. 
     FIG.  27 ( b ) is a graph illustrating the modulation depth of first- and second-harmonics components from FIG.  27 ( a ). 
     FIG. 28 is a schematic diagram of one embodiment of an optical add/drop multiplexer of the present invention. 
     FIG. 29 is a schematic diagram of a second embodiment of an optical add/drop multiplexer of the present invention. 
     FIG. 30 is a schematic diagram of a narrowband AO converter of the present invention. 
     FIG. 31 is a schematic diagram of an interleaver embodiment of the present invention using the add/drop multiplexers of FIG.  29 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates one embodiment of an AOTF (hereafter filter  10 ) of the present invention. A non-birefringent single mode optical fiber  12  has a longitudinal axis, a core  14  and a cladding  16  in a surrounding relationship to core  14 . Optical fiber  12  has multiple cladding modes and a single core mode guided along core  14 . Optical fiber  12  provides fundamental and cladding mode propagation along a selected length of optical fiber  12 . Alternatively, optical fiber  12  does not have multiple cladding modes and a single core mode, and it can a birefringent single mode fiber. In one embodiment, optical fiber  12  is tensioned. Sufficient tensioning can be applied in order to reduce losses in a flexure wave propagated in optical fiber  12 . 
     The core of the non-birefringent fiber is substantially circular-symmetric. The circular symmetry ensures that the refractive index of the core mode is essentially insensitive to the state of optical polarization. In contrast, in hi-birefringent single mode fibers the effective refractive index of the core mode is substantially different between two principal polarization states. The effective refractive index difference between polarization modes in high birefringence single mode fibers is generally greater than 10 −4 . A highly elliptical core and stress-inducing members in the cladding region are two main techniques to induce large birefringence. In non-birefringent fibers, the effective index difference between polarization states is generally smaller than 10 −5 . 
     An acoustic wave propagation member  18  has a distal end  20  that is coupled to optical fiber  12 . Acoustic wave propagation member  18  propagates an acoustic wave from a proximal end  22  to distal end  20  and launches a flexural wave in optical fiber  12 . The flexural wave creates a periodic microbend structure in the optical fiber. The periodic microbend induces an antisymmetric refractive index change in the fiber and, thereby, couples light in the fiber from a core mode to cladding modes. For efficient mode coupling, the period of the microbending, or the acoustic wavelength, should match the beatlength between the coupled modes. The beatlength is defined by the optical wavelength divided by the effective refractive index difference between the two modes. 
     Acoustic wave propagation member  18  can be mechanically coupled to the optical fiber and minimizes acoustic coupling losses in between the optical fiber and the acoustic wave propagation member. In one embodiment, acoustic wave propagation member  18  is coupled to optical fiber  12  in a manner to create a lower order mode flexure wave in optical fiber  12 . In another embodiment, acoustic wave propagation member  18  is coupled to the optical fiber to match a generation of modes carried by optical fiber  12 . 
     Acoustic wave propagation member  18  can have a variety of different geometric configurations but is preferably elongated. In various embodiments, acoustic wave propagation member  18  is tapered proximal end  22  to distal end  20  and can be conical. Generally, acoustic wave propagation member  18  has a longitudinal axis that is parallel to a longitudinal axis of optical fiber  12 . 
     At least one acoustic wave generator  24  is coupled to proximal end  22  of acoustic wave propagation member. Acoustic wave generator  24  can be a shear transducer. 
     Acoustic wave generator  24  produces multiple acoustic signals with individual controllable strengths and frequencies. Each of the acoustic signals can provide a coupling between the core mode and a different cladding mode. Acoustic wave generator  24  can produce multiple acoustic signals with individual controllable strengths and frequencies. Each of the acoustic signals provides a coupling between the core mode and a different cladding mode of optical fiber  12 . A wavelength of an optical signal coupled to cladding  16  from core  14  is changed by varying the frequency of a signal applied the acoustic wave generator  24 . 
     Acoustic wave generator  24  can be made at least partially of a piezoelectric material whose physical size is changed in response to an applied electric voltage. Suitable piezoelectric materials include but are not limited to quartz, lithium niobate and PZT, a composite of lead, zinconate and titanate. Other suitable materials include but are not limited to zinc monoxide. Acoustic wave generator  24  can have a mechanical resonance at a frequency in the range of 1-20 MHz and be coupled to an RF signal generator. 
     Referring now to FIG. 2, one embodiment of acoustic wave propagation member  18  has an interior with an optical fiber receiving channel  26 . Channel  26  can be a capillary channel with an outer diameter slightly greater than the outer diameter of the fiber used and typically in the range of 80˜150 microns. The length of the capillary channel is preferably in the range of 5˜15 mm. The interior of acoustic wave propagation member  18  can be solid. Additionally, acoustic wave propagation member  18  can be a unitary structure. 
     Optical fiber  12  is coupled to acoustic wave propagation member  18 . As illustrated in FIG.  3 ( a ), the dimensions of channel  26  and an outer diameter of optical fiber  12  are sufficiently matched to place the two in a contacting relationship at their interface. In this embodiment, the relative sizes of optical fiber  12  and channel  26  need only be substantially the same at the interface. Further, in this embodiment, the difference in the diameter of optical fiber  12  and channel  26  are in the range of 1˜10 microns. 
     In another embodiment, illustrated in FIG.  3 ( b ), a coupling member  28  is positioned between optical fiber  12  and channel  26  at the interface. Suitable coupling members  28  including but are not limited to bonding materials, epoxy, glass solder, metal solder and the like. 
     The interface between channel  26  and optical fiber  12  is mechanically rigid for efficient transduction of the acoustic wave from the acoustic wave propagation member  18  to the optical fiber  12 . 
     Preferably, the interface between optical fiber  12  and channel  26  is sufficiently rigid to minimize back reflections of acoustic waves from optical fiber  12  to acoustic wave propagation member  18 . 
     In the embodiments of FIGS.  3 ( a ) and  3 ( b ), acoustic wave propagation member  18  is a horn that delivers the vibration motion of acoustic wave generator  24  to optical fiber  12 . The conical shape of acoustic wave propagation member  18 , as well as its focusing effect, provides magnification of the acoustic amplitude at distal end  20 , which is a sharp tip. Acoustic wave propagation member  18  can be made from a glass capillary, such as fused silica, a cylindrical rod with a central hole, and the like. 
     In one embodiment, a glass capillary is machined to form a cone and a flat bottom of the cone was bonded to a PZT acoustic wave generator  24 . Optical fiber  12  was bonded to channel  26 . Preferably, distal end  20  of acoustic wave generator  18  is as sharp as possible to minimize reflection of acoustic waves and to maximize acoustic transmission. Additionally, the exterior surface of acoustic wave generator  18  is smooth. In another embodiment, acoustic wave generator  18  is a horn with a diameter that decreases exponentially from proximal end  22  to distal end  20 . 
     As illustrated in FIG. 4, filter  10  can also include an acoustic damper  30  that is coupled to optical fiber  12 . Acoustic damper  30  includes a jacket  32  that is positioned in a surrounding relationship to optical fiber  12 . Acoustic damper  30  absorbs incoming acoustic waves and minimizes reflections of the acoustic wave. The reflected acoustic wave causes an intensity modulation of the optical signal passing through the filter by generating frequency sidebands in the optical signal. The intensity modulation is a problem in most applications. A proximal end  34  of the acoustic damper  30  can be tapered. Acoustic damper  30  can be made of a variety of materials. In one embodiment, acoustic damper  30  is made of a soft material that has a low acoustic impedance so that minimizes the reflection of the acoustic wave. Jacket  32  itself is a satisfactory damper and in another embodiment jacket  32  takes the place of acoustic damper  30 . Optionally, jacket  32  is removed from that portion of optical fiber  12  in a interactive region  36  and that portion of optical fiber  12  that is bonded to acoustic wave generator  24 . 
     The interactive region is where an optical signal is coupled to cladding  16  from core  14 . This coupling is changed by varying the frequency of a signal applied to acoustic wave generator  24 . In one embodiment, interactive region  36  extends from distal end  20  to at least a proximal portion within acoustic damper  30 . In another embodiment, interactive region  36  extends from distal end  20  and terminates at a proximal end of acoustic damper  30 . In one embodiment, the length of optical fiber  12  in interactive region is less than 1 meter, and preferably less than 20 cm. The nonuniformity of the fiber reduces the coupling efficiency and also causes large spectral sidebands in the transmission spectrum of the filter. Another problem of the long length is due to the mode instability. Both the polarization states of the core and cladding modes and the orientation of the symmetry axis of an antisymmetric cladding mode are not preserved as the light propagates over a long length greater than 1 m. This modal instability also reduces the coupling efficiency and causes large spectral sidebands. Preferably, the outer diameter of optical fiber  12 , with jacket  32 , is in the range of 60-150 microns. 
     The profile of the refractive index of the cross section of optical fiber  12  influences its filtering characteristics. One embodiment of optical fiber  12 , illustrated in FIG. 5, has a first and second cladding  16 ′ and  16 ″ with core  14  that has the highest refractive index at the center. First cladding  16 ′ has an intermediate index and second cladding  16 ″ has the lowest index. Most of the optical energy of several lowest-order cladding modes is confined both only in core  14  and first cladding  16 ′. The optical energy falls exponentially from the boundary between first and second claddings  16 ′ and  16 ″, respectively. 
     Optical fields are negligible at the interface between second cladding  16 ″ and the surrounding air, the birefringence in the cladding modes, due to polarization-induced charges, is much smaller than in conventional step-index fibers where second cladding  16 ″ does not exist. The outer diameter of first cladding  16 ′ is preferably smaller than that of second cladding  16 ″, and can be smaller by at least 5 microns. In one specific embodiment, core 14 is 8.5 microns, first cladding  16 ′ has an outer diameter of 100 microns and second cladding  16 ″ has an outer diameter of 125 microns. Preferably, the index difference between core  14  and first cladding  16 ′ is about 0.45%, and the index difference between first and second claddings  16 ′ and  16 ″ is about 0.45%. 
     In another embodiment, the outer diameter of first cladding  16 ′ is sufficiently small so that only one or a few cladding modes can be confined in first cladding  16 ′. One specific example of such an optical fiber  12  has a core  14  diameter of 4.5 microns, first cladding  16 ′ of 10 microns and second cladding  16 ″ of 80 microns, with the index difference between steps of about 0.45% each. 
     The optical and acoustic properties of optical fiber  12  can be changed by a variety of different methods including but not limited to, (i) fiber tapering, (ii) ultraviolet light exposure, (iii) thermal stress annealing and (iv) fiber etching. 
     One method of tapering optical fiber  12  is achieved by heating and pulling it. A illustration of tapered optical fiber  12  is illustrated in FIG.  6 . As shown, a uniform section  38  of narrower diameter is created and can be prepared by a variety of methods including but not limited to use of a traveling torch. Propagation constants of optical modes can be greatly changed by the diameter change of optical fiber  12 . The pulling process changes the diameter of core  14  and cladding  16  and also changes the relative core  14  size due to dopant diffusion. Additionally, the internal stress distribution is modified by stress annealing. Tapering optical fiber  12  also changes the acoustic velocity. 
     When certain doping materials of optical fiber  12  are exposed to ultraviolet light their refractive indices are changed. In one embodiment, Ge is used as a doping material in core  14  to increase the index higher than a pure SiO 2  cladding  16 . When a Ge-doped optical fiber  12  is exposed to ultraviolet light the index of core  14  can be changed as much as 0.1%. This process also modifies the internal stress field and in turn modifies the refractive index profile depending on the optical polarization state. As a result, the birefringence is changed and the amount of changes depends on optical modes. This results in changes of not only the filtered wavelength at a given acoustic frequency or vice versa but also the polarization dependence of the filter. Therefore, the UV exposure can be an effective way of trimming the operating acoustic frequency for a given filtering wavelength as well as the polarization dependence that should preferably be as small as possible in most applications. 
     Optical fiber  12  can be heated to a temperature of 800 to 1,300° C. or higher to change the internal stresses inside optical fiber  12 . This results in modification of the refractive index profile. The heat treatment is another way of controlling the operating acoustic frequency for a given filtering wavelength as well as the polarization dependence. 
     The propagation velocity of the acoustic wave can be changed by chemically etching cladding  16  of optical fiber  12 . In this case, the size of core  14  remains constant unless cladding is completely etched. Therefore, the optical property of core mode largely remains the same, however, that of a cladding mode is altered by a different cladding diameter. Appropriate etchants include but are not limited to hydro fluoride (HF) acid and BOE. 
     The phase matching of optical fiber  12  can be chirped. As illustrated in FIG. 6, a section  40  of optical fiber can have an outer diameter that changes along its longitudinal length. With section  40 , both the phase matching condition and the coupling strength are varied along its z-axis  42  and the phase matching conditions for different wavelengths satisfied at different positions along the axis. The coupling then can take place over a wide wavelength range. By controlling the outer diameter as a function of its longitudinal axis  42 , one can design various transmission spectrum of the filter. For example, uniform attenuation over a broad wavelength range is possible by an appropriate diameter control. 
     Chirping can also be achieved when the refractive index of core  14  is gradually changed along z-axis  42 . In one embodiment, the refractive index of core  14  is changed by exposing core  14  to ultraviolet light with an exposure time or intensity as a function of position along the longitudinal axis. As a result, the phase matching condition is varied along z-axis  42 . Therefore, various shapes of transmission spectrum of the filter can be obtained by controlling the variation of the refractive index as a function of the longitudinal axis  42 . 
     As illustrated in FIG. 8 a heatsink  44  can be included to cool acoustic wave generator. In one embodiment, heatsink  44  has a proximal face  46  and a distal face  48  that is coupled to the acoustic wave generator  24 . Preferably, acoustic wave generator  24  is bonded to distal face  48  by using a low-temperature-melting metal-alloy solder including but not limited to a combination of 95% zinc and 5% tin and indium-based solder materials. Other bonding material includes heat curable silver epoxy. The bonding material should preferably provide good heat and electrical conduction. Heatsink  44  provides a mount for the acoustic wave generator  24 . Heatsink can be made of a variety of materials including but not limited to aluminum, but preferably is made of a material with a high heat conductivity and a low acoustic impedance. 
     Acoustic reflections at proximal face can be advantageous if controlled. By introducing some amount of reflection, and choosing a right thickness of heatsink  44 , the RF response spectrum of acoustic wave generator  24  can be modified so the overall launching efficiency of the acoustic wave in optical fiber can be less dependent on the RF frequency. 
     In this case, the reflectivity and size of heatsink  44  is selected to provide a launching efficiency of the flexural wave into optical fiber  12  almost independent of an RF frequency applied to acoustic wave generator  24 . The thickness of heatsink  44  is selected to provide a travel time of an acoustic wave from distal face  48  to proximal face  46 , and from proximal face  46  to distal face  48  that substantially matches a travel time of the acoustic wave traveling through acoustic wave propagation member  24  from its proximal end to its distal end, and from its distal end to its proximal end. The heat sink material or the material for the attachment to the proximal face  46  is selected to provide the amount of back reflection from the heat sink that substantially matches the amount of back reflection from the acoustic wave propagation member. In various embodiments, the proximal and distal faces,  46 ,  48  of heatsink  44  have either rectangular or circular shapes with the following dimensions: 10×10 mm 2  for the rectangular shape and diameter of 10 mm for the cylindrical shaped heat sink. 
     However, acoustic back reflections due to proximal face  46  are preferably avoided. Acoustic reflections from the heat sink back to the acoustic wave generator are reduced by angling proximal face  46  at an angle greater than 45 degree or by roughing the face. The acoustic wave coming from the acoustic generator toward the angled proximal face  46  is reflected away from the acoustic generator, reducing the acoustic back reflection to the acoustic wave generator. The roughed face also reduces the acoustic reflection by scattering the acoustic wave to random directions. Preferably, the side faces of the heat sink are also roughened or grooved to scatter the acoustic wave and thereby to avoid the acoustic back reflection. Another method to reduce the back reflection is to attach an acoustic damping material at the proximal face  46 . Suitable materials that reduce back reflections include soft polymers, silicone, and the like that can be applied to proximal face  46 . 
     Referring again to FIG. 8, an acoustic damper mount  50  supports acoustic damper  30 . Acoustic damper mount  50  can be made of a variety of materials including but not limited to silica, invar, and the like. A filter mount  52  supports heatsink  44  and acoustic damper mount  50 . In one embodiment, filter mount is a plate-like structure. Preferably, filter mount  52  and optical fiber  12  have substantially the same thermal expansion coefficients. Filter mount  52  and fiber  12  can be made of the same materials. 
     Filter mount  52  and optical fiber  12  can have different thermal expansion coefficients and be made of different materials. In one embodiment, filter mount  52  has a lower thermal expansion coefficient than optical fiber  12 . Optical fiber  12  is tensioned when mounted and bonded to the filter mount  52 . The initial strain on optical fiber  12  is released when the temperate increases because the length of filter mount  52  is increased less than optical fiber  12 . On the other hand, when the temperature decreases optical fiber  12  is stretched further. When the amount of strain change according to temperature change is appropriately chosen by selecting proper material for mount the  52 , the filtering wavelength of filter  10  can be made almost independent of temperature. Without such mounting arrangement, the center wavelength of the filter increases with temperature. Additionally, interactive region  36  of is sufficiently tensioned to compensate for changes in temperature of the interactive region  36  and filter mount  52 . 
     In another embodiment, illustrated in FIG. 9, a filter housing  54  encloses interactive region  36 . Filter housing  54  can be made of a variety of materials, including but not limited to silica, invar and the like. Filter housing  54  eliminates the need for a separate filter mount  52 . Filter housing  54  extends from distal face  48  of heatsink  44  to acoustic damper  30  or to a jacketed portion  32  of optical fiber  12 . Acoustic wave propagation member  18 , acoustic wave generator  24  and the acoustic damper  30  can be totally or at least partially positioned in an interior of filter housing  54 . 
     In one embodiment, filter housing  54  and optical fiber  12  are made of materials with substantially similar thermal expansion coefficients. A suitable material is silica. Other materials are also suitable and include invar. Filter housing  54  and optical fiber  12  can have different thermal expansion coefficients and be made of different materials. In one embodiment, filter housing  54  has a lower thermal expansion coefficient than optical fiber  12 . 
     In one embodiment, interactive region  36  is sufficiently tensioned sufficiently to compensate for changes in temperature of interactive region  36  and filter housing  54 . 
     As illustrated in FIG. 10, filter  10  can be a component or subassembly of an optical communication system  56  that includes a transmitter  58  and a receiver  60 . Transmission  58  can include a power amplifier with filter  10  and receiver  60  can also include a pre amplifier that includes filter  10 . Additionally, optical communication system  56  may also have one or more line amplifiers that include filters  10 . 
     Referring now to FIG. 11, if an electric signal  57  with constant frequency “f” is applied to acoustic wave generator  24 , a flexural acoustic wave having the same frequency “f” is generated. The flexural acoustic wave is transferred to optical fiber  12  and propagates along optical fiber  12 , finally absorbed in acoustic damper  30 . The flexural acoustic wave propagating along optical fiber  12  produces periodic microbending along the fiber, resulting in the periodic change of effective refractive index which the optical wave propagating along optical fiber  12  experiences. The signal light propagating along optical fiber  12  in a core mode can be converted to a cladding mode by the change of effective refractive index in optical fiber  12 . 
     When signal light is introduced into filter  10  part of the signal light is converted to a cladding mode due to the effect of the acoustic wave and the remainder of the signal light propagates as a core mode while the signal light propagates along interactive region  36 . The signal light converted to a cladding mode cannot propagate any longer in optical fiber  12  with jacket  32  because the light is partly absorbed in optical fiber  12  or partly leaks from optical fiber  12 . A variety of mode selecting means, including a mode conversion means between core modes and cladding modes, can be incorporated in filter  10 . For example, the long-period grating described in the article “Long-period fiber-grating based gain equalizers” by A. M. Vengsarkar et al. in Optics Letters, Vol. 21, No. 5, p. 336, 1996 can be used as the mode selecting means. As another example, a mode coupler, which converts one or more cladding modes of one fiber to core modes of the same fiber or another fiber, can also be used. 
     A flexural acoustic wave generated by acoustic wave propagation member  18  propagates along interactive region  36 . The acoustic wave creates antisymmetric microbends that travel along interactive region  36 , introducing a periodic refractive-index perturbation along optical fiber  12 . The perturbation produces coupling of an input symmetric fundamental mode to an antisymmetric cladding mode when the phase-matching condition is satisfied in that the acoustic wavelength is the same as the beat length between the two modes. The coupled light in the cladding mode is attenuated in jacket  32 . For a given acoustic frequency, the coupling between the fundamental mode and one of the cladding modes takes place for a particular optical wavelength, because the beat length has considerable wavelength dispersion. Therefore, filter  10  can be operated as an optical notch filter. A center wavelength and the rejection efficiency are tunable by adjustment of the frequency and the voltage of RF signal applied to acoustic wave propagation member  18 , respectively. 
     The coupling amount converted to a cladding mode is dependent on the wavelength of the input signal light. FIG.  12 ( a ) shows the coupling amounts as functions of wavelength when flexural acoustic waves at the same frequency but with different amplitudes are induced in optical fiber  12 . As shown in FIG.  12 ( a ), the coupling amounts are symmetrical with same specific wavelength line (λ c ), i.e., center wavelength line, however they show different results  62  and  64  due to the amplitude difference of the flexural acoustic waves. Therefore, the transmittance of the output light which has passed through filter  12  is different depending on the wavelength of the input light. Filter  12  can act as a notch filter which filters out input light with specific wavelength as shown in FIG.  12 ( b ). 
     FIG.  12 ( b ) is a graph showing the transmittances as a function of wavelength when flexural acoustic waves with different amplitudes are induced in filter  12 . The respective transmittances have same center wavelength as does the coupling amount, but different transmittance characteristic  64  and  66  depending on the amplitude difference of the flexural acoustic waves can be shown. 
     The center wavelength λ c , of filter  10  satisfies the following equation. 
     
       
         β co (λ)−β cl (λ)=2π/λ a   
       
     
     In the above equation, β co (λ) and β cl (λ) are propagation constants of core mode and cladding mode in optical fiber  12  which are respectively dependent on the wavelength, and λ a  represents the wavelength of the flexural acoustic waves. 
     Accordingly, if the frequency of the electric signal applied to acoustic wave generator  24  varies, the wavelength of the acoustic wave generated in optical fiber  12  also varies, which results in the center wavelength change of filter  10 . In addition, since the transmission is dependent on the amplitude of the flexural acoustic wave, the transmission of signal light can be adjusted by varying the amplitude of the electric signal which is applied to acoustic wave generator  24 . 
     FIG. 13 is a graph showing the transmittance of filter  10  in one embodiment when different electric signal frequencies are applied. As shown in FIG. 13, each center wavelength (i.e., wavelength showing maximum attenuation) of filter  10  for different electric signals was 1530 nm, 1550 nm and 1570 nm. Therefore the center wavelength of filter  10 , according to the embodiment, is changed by varying the frequency of the electric signal which is applied to acoustic wave generator  24 . 
     As described above, since there are a plurality of cladding modes in interactive region  36  the core mode can be coupled to several cladding modes. FIG. 14 is a graph showing the center wavelength of filter  10  according to the embodiment of the invention as a function of the frequency applied to the flexural acoustic wave generator. In FIG. 14, straight lines  71 ,  72  and  73  represent the center wavelength of filter  10  resulting from the coupling of a core mode with three different cladding modes. 
     Referring to FIG. 14, there are three applied frequencies for any one optical wavelength in this case. Therefore the input signal light is converted to a plurality of cladding modes by applying multi-frequency electric signal to acoustic wave generator  24 . Moreover, it means transmission characteristics of filter  10  can be electrically controlled by adjusting the amplitude and each frequency component of the electric signal. 
     As shown in FIG.  15 ( a ), the respective transmission features  74 ,  75  and  76  of filter  10  can be provided by applied electric signals with different frequencies f 1 , f 2  and f 3 . In this example, assuming that f 1  couples the core mode of input signal light to a cladding mode (cladding mode A), f 2  couples the core mode to other cladding mode (cladding mode B) and f 3  couples the core mode to another cladding mode different from A or B (cladding mode C, the transmission feature is shown in FIG.  15 ( b ) as a curve  77  when electric signal with three frequency components f 1 , f 2  and S is applied to acoustic wave generator  24 . 
     As shown in FIG.  5 ( c ), if filter  10  has transmission feature curves  78 ,  79  and  80  corresponding to respective frequencies f 1 ′, f 2 ′ and f 3 ′ and electric signal having three frequency components f 1 ′, f 2 ′ and f 3 ′ is applied to the flexural acoustic wave generator, the transmission feature of filter  10  is shown as a curve  81  of FIG.  5 ( d ). 
     FIGS.  16 ( a ) and  16 ( b ) are graphs showing the transmittance of filter  10  according to an embodiment of the present invention, when varying electric signal having three frequency components is applied to filter  10 . When varying electric signal having a plurality of frequency components is applied to acoustic wave generator  24  various shapes of transmittance curves  82 ,  83  and  84  can be obtained. 
     Since conventional tunable wavelength filters utilize the coupling of only two modes, the difference between a plurality of applied frequencies naturally becomes small to obtain wide wavelength band filtering feature by applying a plurality of frequencies. In this case, as described under the article “Interchannel Interference in multiwavelength operation of integrated acousto-optical filters and switches” by F. Tian and H. Herman in Journal of Light wave technology 1995, Vol. 13, n 6, pp. 1146-1154, when signal light input to a filter is simultaneously converted into same (polarization) mode by various applied frequency components, the output signal light may undesirably be modulated with frequency corresponding to the difference between the applied frequency components. However, with filter  10  the above problem can be circumvented, because the respective frequency components convert the mode of input light into different cladding modes in filter  10 . 
     In one embodiment, the filtering feature shown in FIG.  17 ( a ) was obtained by applying adjacent frequencies 2.239 MHz and 2.220 MHz to reproduce the result of a conventional method. The applied two frequencies were such that convert the mode of input light into the same cladding mode. Under the condition, narrow wavelength-band signal light with a center wavelength of 1547 nm was input to filter  10  to measure output light. Referring to the measurement result shown in FIG.  17 ( b ), there is an undesirable modulated signal with frequency corresponding to the difference of the two applied frequencies. 
     In another embodiment, when adjacent frequencies 2.239 MHz and 2.220 MHz were applied to acoustic wave generator  24 , according to the embodiment of the invention, the two frequency components convert the mode of input light into mutually different cladding modes. FIG.  17 ( c ) shows the measurement result when the same signal light as the above experiment was input to filter  10  and output light was measured. 
     However, an undesirable modulated signal, which appeared in a conventional filter, practically disappeared as shown in FIG.  17 ( d ). 
     In optical communications or optical fiber-sensor systems, wavelength filters are required that has a wide tuning range and are capable of electrically controlling its filtering feature. 
     FIG.  18 ( a ) illustrates one embodiment of a transmission spectrum of filter  10  with a 15.5-cm-long interaction length for a broadband unpolarized input light from a LED. A conventional communication fiber was used with a nominal core diameter of 8.5 μm, a cladding outer diameter of 125 μm and a normalized index difference of 0.37%. The frequency of the applied RF signal was 2.33 MHz, and the corresponding acoustic wavelength was estimated to be ˜650 μm. The three notches shown in FIG.  8 ( a ) are from the coupling to three different cladding modes with the same beat length at the corresponding wavelengths. The coupled cladding modes were the LP 11   (cl) , the LP 12   (cl) , and the LP 13    (cl)  modes, which was confirmed from far-field radiation patterns. The center of each coupling wavelength was tunable over &gt;100 nm by tuning the acoustic frequency. 
     FIG.  18 ( b ) shows the measured and the calculated center wavelengths of the notches as a function of acoustic frequency. The fiber parameters used in the calculation for best fit with the experimental results are a core diameter of 8.82 μm, a cladding outer diameter of 125 μm, and a normalized index difference of 0.324%, in reasonable agreement with the experimental fiber parameters. 
     Referring again to FIG.  8 ( a ), coupling light of a given wavelength from the fundamental mode to different cladding modes requires acoustic frequencies that are separated from each other by a few hundred kilohertz. This separation is large enough to provide a wide wavelength-tuning range of almost 50 nm for each coupling mode pair without significant overlap with each other, thereby practically eliminating the coherent cross talk that is present in conventional counterparts. The tuning range is sufficient to cover the bandwidth of typical EDFA&#39;s. In one embodiment, filter  10  provides for a combination of independent tunable notch filters built into one device, and the number of involved cladding modes corresponds to the number of filters. The multifrequency acoustic signals can be generated by a single transducer, and the spectral profile of filter  10  is determined by the frequencies and amplitudes of the multiple acoustic signals. 
     FIG. 19 shows two examples of the configurable spectral profiles with spectral tilt, which can be used to recover the gain flatness in an EDFA with a gain tilt caused by signal saturation. In one embodiment, three cladding modes [LP 11   (cl ) , the LP 12   (cl) , and the LP 13   (cl) ] were used and three RF signals were simultaneously applied with different voltages and frequencies adjusted for the particular profile. The 3-dB bandwidth of the individual notch was ˜6 nm with a 10-cm-long interaction length. 
     A complex filter profile is required to flatten an uneven EDFA gain, which exhibits large peaks with different widths around 1530 and 1560 nm. The combination of three Gaussian shaped passive filters can produce a flat gain over a 30-nm wavelength range. 
     As illustrated in FIG.  20 ( a ), a filter assembly  12  of the present invention can include first and second filters  10 ′ and  10 ″ in series. Each filter  10 ′ and  10 ″ is driven by three radio frequency (RF) signals at different frequencies and amplitudes that produce acousto-optic mode conversion from the fundamental mode to different cladding modes. This approach eliminates the detrimental coherent crosstalk present in LiNbO 3 -based AOTF&#39;s . The 3-dB bandwidths of the first filter  10 ′ were 3.3, 4. 1, and 4.9 nm for the couplings to the cladding modes LP 12   (cl) , the LP 13   (cl) , and LP 14   (cl) , respectively. For second filter  10 ″, they were 8, 8.6, and 14.5 nm for the couplings to the cladding modes, LP 11   (cl) , the LP 12   (cl) , and the LP 13   (cl)  respectively. 
     The minimum separations of notches produced by single RF driving frequency were ˜50 nm for first filter  10 ′ and ˜150 nm for second filter  10 ″ , respectively, so that only one notch for each driving frequency falls into the gain-flattening range (35 nm). The large difference between filters  10 ′ and  10 ″ was due to the difference in optical fiber  12  outer diameters. The polarization splitting in the center wavelength of the notches as ˜0.2 nm for first filter  10 ′ and ˜1.5 nm for second filter  10 ″. The relatively large polarization dependence in second filter  10 ″ is due mainly due to the unwanted core elliptically and residual thermal stress in optical fiber  12 , that can be reduced to a negligible level by using a proper optical fiber. First and second filters  10 ′ and  10 ″ were used for the control of the EDFA gain shape around the wavelengths of 1530 and 1555 nm, respectively. The background loss of the gain flattening AOTF was less than 0.5 dB, which was mainly due to splicing of different single-mode fibers used in the two AOTF&#39;s  1010 . Adjusting the frequencies and voltages of the applied RF signals provided control of the positions and depths of the notches with great flexibility. The RF&#39;s were in the range between 1 and 3 MHz. 
     FIG.  20 ( b ) shows a schematic of a dual-stage EDFA employing gain flattening filter  10  along with a test setup. A 10-m-long EDF pumped by a 980-nm laser diode and a 24-m-long EDF pumped by a 1480-nm laser diode were used as the first and the second stage amplifiers, respectively. The peak absorption coefficients of both EDF&#39;s were ˜2.5 dB/m at 1530 nm. Filter  10  was inserted between the two stages along with an isolator. Total insertion loss of filter  10  and the isolator was less than 0.9 dB. Six synthesizers and two RF power amplifiers were used to drive filter  10 . 
     Gain profiles of the EDFA were measured using a saturating signal at the wavelength of 1547.4 nm and a broad-band light-emitting diode (LED) probing signal. The saturating signal from a distributed feedback (DFB) laser diode was launched into the EDFA after passing through a Fabry-Perot filter (optical bandwidth: 3 GHz, extinction ratio: 27 dB) to suppress the sidelobes of the laser diode. The total power of the probe signal in 1520-1570-nm range was 27 dBm, which is much smaller than that of the input saturating signal ranging from 13 to 7 dBm. 
     FIG. 21 ( a ) shows gain profiles before and after the gain flattening for two different saturating signal powers of 13 and 7 dBm when the second-stage pump power was 42 mW. The gain excursions before flattening were larger than 5 dB. By adjusting the filter profile, flat gain profiles within 0.7 dB were obtained over 35 nm for both cases. The flat gain region is shifted slightly toward the shorter wavelength for higher gain level, which is due to the intrinsic gain characteristics of the EDF. FIG.  21 ( b ) shows filter profiles that produced the flat gain profiles shown in FIG.  21 ( a ), where Profile 1 and Profile 2 are for-the cases of saturating tones of 13 and 7 dBm, respectively. For the measurements, EDFI was used as an ASE source, while the second pump diode (1480 nm) was turned off. The ASE signal leaked out of the second WDM coupler was monitored and the signals obtained when the filter was on and off were compared to yield the filter response. The attenuation coefficients for Profile 1 and Profile 2 at the saturating signal wavelength were 5.0 and 4.9 dB, respectively, and the average attenuation over the 35-nm range (1528-1563 nm) was 5 dB in both cases. The total RF electrical power consumption of the filter was less than 500 mW. Profile 2 could be obtained from Profile 1 by adjusting mainly the depths of notches, although fine adjustments of center wavelengths of notches within 0.5-nm range slightly improved the gain flatness. FIG.  21 ( c ) shows the filter profiles of first filter  10 ′ and second filter  10 ″ used to form Profile 1, and also the locations of center wavelengths of six notches. By adjusting first and second filters  10 ′ and  10 ″ spectral profiles electronically gain flatness of &lt;0.7 dB over 35-nm wavelength range were obtained at various levels of gain as well as input signal and pump power. 
     One important characteristic of filter  10  is polarization dependence. The shape of filter  10  can be dependent on the polarization state of input light. The polarization dependence originates from fiber birefringence. Fiber birefringence causes the effective propagation constant of a mode to be different between two eigen polarization states. Since the magnitude of birefringence is different from mode to mode, the fiber birefringence causes the beat length between two coupled modes to be different between the eigen polarization states, and, therefore, results in splitting of center wavelength of filter  10  for a given acoustic frequency. 
     FIG.  22 ( a ) illustrates the polarization dependence. Curve  90  represents the filter profile for light in one eigen polarization state, and filter profile  92  is when the input is in the other eigen state. The center wavelengths are split because of the birefringence. Moreover, since the field overlap between two coupled modes is also polarization dependent due to the birefringence, the attenuation depth can be different between filter profiles  90  and  92 . 
     A critical feature due to the polarization dependence is the polarization dependent loss (PDL) which is defined as the difference of the magnitude of attenuation between two eigen polarization states. Since polarization dependent loss is an absolute value, it increases with the attenuation depth. FIG.  22 ( b ) shows polarization dependent loss profile  93  associated with filter profiles  90  and  92 . In WDM communication system applications, the polarization dependent loss should be minimized. Most applications require the polarization dependent loss to be less than 0.1 dB. However, acousto-optic tunable filter  10  has exhibited a typical polarization dependent loss as large as 2 dB at 10-dB attenuation level. This is due to the large birefringence of the antisymmetric cladding modes. 
     FIG. 23 shows one possible configuration that can reduce the inherent polarization dependent loss of filter  10 . In FIG. 23, double-pass filter  100  consists of a 3-port circulator with input-, middle-, and output-port fibers,  12 ′,  12 ″ and  12 ″, respectively, and Faraday rotating mirror (FRM)  104 . The middle-port fiber  12 ″ is connected to acousto-optic tunable filter  10  and Faraday rotating mirror  104 . When light comes in through input-port fiber  12 ′, it is directed to filter  10 , through circulator  102 , and then refracted by Faraday rotating mirror  104 . Faraday rotating mirror  104  acts as a conjugate mirror with respect to optical polarization states. So, if the light pass through filter  10  in a specific polarization state, then on the way back after reflection it pass through filter  10  in its orthogonal polarization state. Since the light pass through filter  10  twice but in mutually orthogonal states, the total attenuation after the double pass becomes polarization-insensitive. Another benefit of the double pass configuration is that, since the filtering takes place twice in filter  10 , the drive RF power applied to filter  10  to obtain a certain attenuation depth is reduced half compared to single-pass configuration as in FIG.  4 . For instance, when filter  10  is operated at an attenuation depth of 5 dB, the overall attenuation depth of double-pass filter  100  becomes 10 dB. 
     Another embodiment of a device configuration for low polarization dependence is shown in FIG.  24 . In this embodiment, dual filter  110  consists of filters  10 ′ and  10 ″ in tandem and connected through mid fiber section  112 . Filters  10 ′ and  10 ″ are preferably operated at the same RF frequency. The birefringence of mid fiber section  112  is adjusted such that it acts as a half-wave plate aligned with 45-degree angle with respect to the eigen polarization axes of filters  10 ′ and  10 ″. In other words, the light passing through filter  10 ′ in one eigen polarization state enters filter  10 ″ in the other eigen polarization state. If the polarization dependent loss is the same loss for filters  10 ′and  10 ″, the overall attenuation after passing through filters  10 ′ and  10 ″ becomes polarization-insensitive. If filters  10 ′ and  10 ″ are not identical in terms of polarization dependent loss, the double filter  10  would exhibit residual polarization dependent loss that should be, however, smaller than the polarization dependent loss of individual filters,  10 ′ or  10 ″. Therefore, it is desirable that filters  10 ′ and  10 ″ are identical devices. Since the filtering takes place by two filters, the drive powers to individual filters are reduced, compared to using a single filter alone, to achieve the same attenuation depth. 
     In one embodiment, illustrated in FIG. 23, circulator  102  based on magneto-optic crystal has overall insertion loss and polarization dependent loss of 1.5 dB and 0.5 dB, respectively. Faraday rotating mirror  104  has insertion loss and polarization dependent loss of 0.5 dB and 0.5 dB, respectively. Curve  120  in FIG.  24 ( a ) shows the polarization dependent loss profile in one embodiment when filter  10  was operated to produce 10-dB attenuation at 1550 nm. The filter profile in this instance is shown by curve  124  in FIG.  24 ( b ). Optical fiber  12  used in the filter was a conventional communication-grade single mode fiber. When the filter was used in the double-pass configuration, the overall polarization dependent loss was reduced greatly as shown by curve  121  in FIG.  24 ( a ). 
     The polarization dependent loss was reduced down to less than 0.2 dB. The total insertion loss of double-pass filter was 3 dB, mainly due to the circulator and splices. In this embodiment, the drive power to filter  10  required to produce total 10-dB attenuation at 1550 nm, as shown by filter profile  125  in FIG.  24 ( b ), was decreased compared to the single-pass filter experiment. 
     In another embodiment, illustrated in FIG. 25, two filters were fabricated with a conventional circular-core single mode fiber. Each filter was operated with  5- dB attenuation at the same center wavelength, 1550 nm. The overall dual filter profile is shown by curve  126  in FIG.  24 ( b ). In these filters, the eigen polarization states are linear and their axes are parallel and orthogonal to the direction of the flexural acoustic wave vibration or the acoustic polarization axis. This is generally true with filters made of a circular-core fiber where the dominant birefringence axes are determined by the lobe orientation of the cladding mode, which is the same as the acoustic polarization axis. Linear axis orthogonal to acoustic polarization is the slow axis, and its orthogonal axis is the fast axis. In this embodiment, a polarization controller was used in mid fiber section  112  and controlled to minimize the overall polarization dependent loss of dual filter  110 . 
     The loss profile is shown by curve  122  in FIG.  24 ( a ). The total filter profile is shown by curve  126  in FIG.  24 ( b ). The residual polarization dependent loss as large as 0.6 dB is primarily due to different polarization dependent loss of filters  10 ′ and  10 ″, and could be reduced greatly if identical two filters were used. 
     Another important characteristic of filter  10  is the intensity modulation of an optical signal passing through the filter. One reason which gives rise to the intensity modulation of the output signal is static coupling between the core and cladding modes either by microbending of fiber  12  or imperfect splices, if present. Another reason is an acoustic wave propagating backward in interactive region  36  by an acoustic reflection at imperfect acoustic damper  30  and fiber jacket  32 . FIG. 26 shows an example of output signal  139  suffering from the intensity modulation by backward acoustic reflection at acoustic damper  30 . In this case, the major modulation frequency is equal to twice the acoustic frequency. The modulation depth is defined by the ratio of peak-to-peak AC voltage amplitude, V AC  to DC voltage, V DC . By static mode coupling, the major modulation frequency is equal to the acoustic frequency. When both static mode coupling and backward acoustic wave are present, the intensity of the output is modulated at frequencies of both first- and second-harmonics of the acoustic frequency. The modulation depth, when smaller than 20%, is, approximately, linearly proportional to the amount of attenuation in dB scale. In most WDM communication system applications, the modulation depth is generally required to be less than 3% at 10-dB attenuation level. 
     In one embodiment, illustrated in FIG. 4, filter  10  was fabricated by using a conventional single-mode fiber. The modulation depth at 10-dB attenuation level was about 10% at both first- and second-harmonics of the acoustic frequency, as shown by curves  140  and  141  in FIG.  27 ( a ), respectively. The same filter was used as filter  10  in another embodiment, illustrated in FIG.  23 . The RF drive power to the filter was controlled to produce 10-dB attenuation depth. In the first embodiment of double-pass filter  100 , the length of fiber section  106  was selected such that the round-trip travel time of fiber section  106  is equal to a quarter of the period of the acoustic wave. In this case, the second-harmonics component of the intensity modulation can be compensated out. Curves  142  and  143  in FIG.  27 ( a ) show the modulation depth of first- and second-harmonics components, respectively. The second-harmonics was eliminated almost completely. The first-harmonics was also reduced a little, which may be attributed to imperfect length matching of fiber section  106 . In the second embodiment of double-pass filter  100 , the length of fiber section  106  was such that the optical round-trip travel time of fiber section  106  is equal to a half of the period of the acoustic wave. In this case, the first-harmonics component of the intensity modulation can be reduced. Curves  147  and  148  in FIG.  27 ( b ) show the modulation depth of first- and second-harmonics components, respectively. The first-harmonics was eliminated almost completely. 
     Reduction of intensity modulation can also be achieved by dual filter  110  where the length of mid fiber section  112  is selected properly. For example, if the first-harmonic modulation component is to be compensated, the length of mid fiber section  112  is such that the optical travel time from one end of section  112  to the other end is equal to a half of the period of the acoustic wave. 
     Referring now to FIG. 28, an optical add/drop multiplexer (OADM)  150  includes filter  10  coupled to a first mode selective coupler  152  and a second mode selective coupler  154 . In this embodiment, filter  10  is a frequency selection AO mode converter. OADM  150  is reconfigurable in that it is channel tunable. Multiple wavelengths, such as WDM signals, come into the input port. An added channel is added at the add port. The second mode is added at the add port. Filter  10  is-frequency selective and cross-couples the frequency between the multiple waves and the added channel. 
     In FIG. 29, an OADM  156  includes a first filter  10 ′ and a second filter  10 ″, both of which are frequency selection AO mode converters First mode selective coupler  152  is coupled to an output of first filter  10 ′ while. Second mode selective coupler  154  is coupled to an input of second filter  10 ″. At least one channel is dropped at first mode couple  152 . At least one channel is added at second mode coupler  154 . 
     Referring now to FIG. 30, multiple OADM&#39;s  150  and  156  ar utilized. Each OADM  150  or  156  is coupled to an optical fiber. Each optical fiber carries single or multiple channel bands. An input of multiple channels and/or band is coupled to a splitter  158  and an output is coupled to a combiner  160 . Splitter  158  and combiner  160  can be AWG&#39;s. Splitter  158  splits channels and/or bands into multiple groups in the different fibers. The spacing between adjacent channels and/or bands in the same group can be the original channel spacing (e.g., 100 GHz) times the number of groups. 
     In FIG. 31, the input is coupled to a first wavelength interleaver  162  which separates odd channels on one fiber and even channels on another fiber. The fiber carrying the odd channels is coupled to a second interleaver  164  while the fiber carrying the even channels is coupled to a third interleaver  166 . An OADM  150  or  156  is coupled to interleaver  164  and  166 . Additionally, OADM&#39;s  150  or  156  can be coupled to interleaver  162 . 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.