Abstract:
This invention relates to an acousto-optical tunable filter generally of the kind described in U.S. Pat. No. 6,266,462. More specifically, the invention relates to a filter and its construction, the filter including a support, first and second mounts at spaced locations on the support, an optical fiber having first and second mounted portions secured to the first and second mounts respectively and a filtering section between the first and second mounted portions, a signal generator operable to generate a periodic signal, and an electro-acoustic transducer having a terminal connected to the signal generator and an actuating portion, the electric signal causing vibration of the actuating portion, and the actuating portion being connected to the filtering section so that the vibration generates a transverse wave traveling along the filtering section. The filter has an improved damper to more effectively dampen waves traveling along the filtering section.

Description:
CROSS-REFERENCES 
   Priority is claimed from U.S. Provisional Patent Application No. 60/276,753 filed on Mar. 16, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an acousto-optic tunable filter. 
   2. Discussion of Related Art 
   As an optical signal traverses an optical network, the signal is subject to losses and nonlinear effects that result in signal attenuation and distortion. Amplifiers, such as erbium-doped fiber amplifiers (“EDFA&#39;s”), are typically placed approximately every 80 kilometers along an optical fiber to boost signal strength. However, such amplifiers impose their own distortions on the signal power spectral distribution (as a function of wavelength). One of the major distortions is caused by the non-uniform gain profile (as a function of wavelength) of the amplifiers, which imposes a non-uniform spectral distribution on the amplified signals. It is especially important in wavelength division multiplexed (“WDM”) networks to maintain a uniform spectral distribution across all channels. 
   Static filters are often used to attenuate the signal power as a function of wavelength to achieve a substantially uniform power distribution. Static filters, however, cannot adapt to dynamically changing conditions such as amplifier aging, temperature variations, channel add/drop, fiber loss and other changes in components along the transmission line. Moreover, the required filter shape is dependent upon system configuration, e.g., the spacing between amplifiers. Static filter characteristics cannot be modified to compensate for these changes without replacing the filter itself. 
   To overcome these problems, it is known in the art to employ dynamic wavelength tunable filters to flatten or equalize the signal spectrum, as well as to obtain any desired spectral shape. One such filter is an all-fiber acousto-optic tunable filter (“AOTF”) described in U.S. Pat. No. 6,233,379, entitled “Acousto-optic filter, ” which is assigned to the assignee of the present invention and incorporated by reference herein. As described in the patent, the all-fiber AOTF is a multiple-notch filter, with a transfer function characterized by notch depth and center frequency (or wavelength). 
   The ATOF has a piezoelectric transducer that vibrates a conical wave propagation member. A tip of the conical wave propagation member vibrates an optical fiber. Transverse flexural waves are created in the optical fiber that filter certain wavelengths of light from a core into a cladding layer of the optical fiber. 
   One problem with the AOTF is that the wave is reflected back to the conical wave propagation member. Such a reflection interferes with the wave traveling from the conical wave propagation member, resulting in a modification microbending of the optical fiber. The modified microbending, in turn, creates modifications in wavelengths and/or magnitude of light that couples from the core into the cladding, and results in undesirable filtering characteristics. 
   SUMMARY OF THE INVENTION 
   This invention relates to an acousto-optic tunable filter generally of the kind described in U.S. Pat. No. 6,266,462. More specifically an optical fiber has first and second mounted portions secured to first and second mounts respectively and an interaction length between the first and second mounted portions. A signal generator is operable to generate a periodic signal. An electro-acoustic transducer has a terminal connected to the signal generator and an actuating portion. The electric signal causes vibration of the actuating portion, and the actuating portion is connected to the interaction length so that the vibration generates an acoustic wave traveling along the interaction length. Such a filter has the ability to reduce an amplitude of one or more selected wavelengths of light as it travels through the interaction length. 
   A damper may be located at an end of the interaction length. The damper serves to at least partially dampen a wave or waves traveling through the interaction length. 
   According to one aspect of the invention, the damper has a continuous slanted surface at an angle other than 90° relative to a longitudinal axis of the optical fiber. Such a slanted surface is easy to create by depositing a damping material and allowing the damping material to flow under capillary action and gravity. An advantage of such a slanted surface is that it is more effective in not reflecting a wave than the surface located at right angles relative to a longitudinal axis of the interaction length. 
   According to a further aspect of the invention, a first reflected wave is created by a transition from the interaction length to the damper component and at least a second reflected wave is created. The second reflected wave may have an amplitude and phase which are selected so that it substantially cancels the first reflected wave. A damping effect is created by a cancellation of the waves. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described by way of example with reference to the accompanying drawings wherein: 
       FIG. 1  is a cross-sectional side view of an acousto-optic tunable filter according to an embodiment of the invention; 
       FIG. 2  is a side view illustrating functioning of the filter; 
       FIG. 3  is a cross-sectional side view of an end of the filter having a damper; 
       FIG. 4  is a perspective view of some of the components shown at the end shown in  FIG. 3 ; 
       FIG. 5  is an enlarged side view of the damper; 
       FIGS. 6A-D  are cross-sectional end views at (a), (b), (c), and (d) in  FIG. 5 , respectively; 
       FIGS. 7A and 7B  are cross sectional end views of a filter with a damper according to an alternative embodiment; 
       FIGS. 8A and 8B  are cross-sectional end views of a filter with a damper according to a further embodiment; 
       FIG. 9  is a perspective side view of an optical fiber, damper, and jacket shown in  FIG. 3 , further illustrating how they create three reflecting waves which cancel one another; 
       FIG. 10  is a perspective side view of an optical fiber having a damper and a jacket cause two reflected waves that cancel one another; 
       FIG. 11  is a perspective side view of an optical fiber, a damper, and another component having a larger diameter than the damper, wherein two reflecting waves are created; and 
       FIG. 12  is an illustration of waves traveling into, through, and out of the damper of FIG  10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  of the accompanying drawings illustrates an acousto-optic tunable filter  120  constructed according to an embodiment of the invention. The filter  120  is of the kind described in the U.S. Pat. No. 6,266,462 filed on Oct. 22, 1999, the subject matter of which is incorporated herein by reference. The filter  120  includes a mounting construction  122 , an optical fiber construction  24 , and an electrical signal generator  130 . 
   The mounting construction  122  includes a heat sink  132 , a piezo-electric transducer  134 , an acoustic wave propagation member  136 , an outer tube arrangement  138 , and an end plug  140 . 
   Metal electrode terminals are formed on opposing surfaces of the piezo-electric transducer  134 . One terminal is located against the heat sink  132  and attached to the heat sink  132 . A base of the acoustic wave propagation member  136  is then attached to an opposing terminal of the piezo-electric transducer  134 . 
   The optical fiber construction  24  includes an optical fiber  142  consisting of a core and a surrounding cladding layer (not shown), which is covered with a jacket  144 . A central section of the jacket  144  is removed so that only portions  144 A and  144 B of the jacket at opposing ends of the optical fiber  142  remain. 
   Openings are made in the heat sink  132 , piezo-electric transducer  134 , and acoustic wave propagation member  136  to form a continuous passage. The end of the optical fiber construction  24  having the first portion  144 A of the jacket is inserted through the opening of the acoustic wave propagation member  136 , whereafter it is inserted through the openings in the piezo-electric transducer  134  and the heat sink  132 . 
   The second portion  144 B of the jacket is then located in a groove in the end plug  140 . A resin is then placed in the groove and allowed to cure, thereby securing the second portion  144 B of the jacket to the end plug  140   
   Resin is also applied to the fiber  24  where it protrudes from a tip  150  of the acoustic wave propagation member  136 , and flows into the tip  150  of the acoustic wave propagation member  136 . The resin then cures and secures the fiber  24  to the tip  150  of the acoustic wave propagation member  136 . 
   A damper  52  is located on the optical fiber  142 . The damper  52  is coaxially disposed on the optical fiber construction  24  adjacent to the second portion  144 B of the jacket. The length of exposed fiber from the tip  150  to the end of the damper  52  nearest the tip  150  is the “interaction length  37 ” of the filter. Generally, the interaction length or “interaction region” is the length of fiber in which light is coupled from one mode to another, and, more particularly in this case, the portion of the optical fiber  142  not covered by the damper  52 . 
   An end  154  of the outer tube arrangement  138  is then located over the second portion  144 B of the jacket and moved over the end plug  140  until it contacts a surface of the heat sink  132 . A second, opposing end  156  of the outer tube arrangement  138  is located over the end plug  140 . The positioning of the end plug  140  is then adjusted within the end  156 . By adjusting the positioning of the end plug  140 , the interaction length  37  of the optical fiber  142  is tensioned by about 0.2 N to eliminate slack, while simultaneously compressing the outer tube arrangement  138 . When a predetermined tension in the interaction length  37  is reached, a resin is applied to an interface between the end plug  140  and the end  156 . The resin is allowed to cure, thereby securing the end plug  140  stationarily within the end  156 . The tension of the interaction length  37  is thereby set. 
   The signal generator is connected to the transducer  134  through leads  160  and  162 . The lead  160  couples to the heat sink  132 , which is itself electrically coupled to a terminal on one face of the transducer  134 . The lead  162  is electrically connected to the opposing face of the transducer  134 , either directly to the terminal on the opposing face, or indirectly through the acoustic wave propagation member  136 . The heat sink  132  and the acoustic wave propagation member  136  can be made of conductive aluminum so that the terminals on the opposing sides of the piezoelectric transducer  134  are at the voltages of the leads  160  and  162 , respectively. A voltage potential is thereby created across the piezoelectric transducer  134 . 
   The signal generator  130  applies across the piezoelectric transducer  134  a voltage at one or more frequencies in the range of 0-20 MHz, or more particularly, 0-3 MHz. The voltage signal applied across the piezo-electric transducer  134  causes opposing surfaces of the piezoelectric transducer  134  to vibrate relative to one another in a direction transverse to a longitudinal axis of the interaction length  37 . Adjusting the frequency and amplitude of the electrical signal applied to the transducer results in a corresponding change in the frequency and amplitude, respectively, of the mechanical vibration of the transducer. Those skilled in the art will recognize that the invention may employ acoustic wave exciters other than the acoustic wave exciter formed from the combination of the signal generator  130 , transducer  134 , and acoustic wave propagation member  136  described herein. 
   Vibrations of opposing surfaces of the piezo-electric transducer  134  are transferred through the acoustic wave propagation member  136  to the tip  150  thereof. The tip  150  vibrates periodically in response to the change in the voltage. Movement of the tip  150  is transferred to the end of the interaction length  37  closest to the tip  150 . 
     FIG. 8  illustrates how vibration of the tip  150  imparts acoustic waves in the interaction length  37 . In the present example, the waves are y-direction transverse flexural waves that travel along the interaction length  37  from the tip  150  to the damper  52 . The damper  52  is designed to absorb the waves or otherwise minimize reflection of the waves back to the tip  150 . The creation of a standing wave is thereby prevented. 
   In use, the filter  120  is inserted into a fiber optic transmission line. A light signal is transmitted through the core of the optical fiber  142 . The light signal may be modulated as a WDM signal having many channels, each at a different wavelength. For various reasons, including the non-uniform gain profiles of amplifiers along the fiber optic transmission line, the intensity of light may differ from channel to channel at the point where the light enters the optical fiber  142  of the filter  120 . 
   The effect of the acoustic waves in the interaction length  37  is that the intensity of selected wavelengths of light traveling through the interaction length  37  is attenuated by coupling these wavelengths from a mode in the core into one or more modes in the cladding of the interaction length  37 . This coupling creates a notch in the transmission spectrum centered at each selected wavelength. By changing the frequency of the applied electrical signal, and thus the frequency of the acoustic waves in the interaction length  37 , the center wavelength of the notch can be altered. Furthermore, by changing the magnitude of the applied voltage (and thereby the magnitude of the acoustic wave), the depth of the notch (representing the amount of light coupled to the other mode) can be changed. By cascading multiple acoustic exciter/interaction length combinations and/or applying multiple acoustic frequencies with each exciter, a combination of notches of different optical center frequencies and depths may be achieved, thereby allowing creation of a desired filter transfer-function as described in U.S. Pat. No. 6,266,462. Such a filter may be employed for gain equalization purposes. Those skilled in the art will recognize that, as an alternative to coupling light between core and cladding modes, an AOTF may also couple light between different core modes. Further details of the functioning of the filter  120  are described in U.S. Pat. No. 6,266,462 referenced above. 
     FIGS. 3 and 4  illustrate in more detail the filter in the region of the end  156 . The end plug  140  has a generally circular shape. A V-notch groove  66  is formed in an axial direction along one side thereof. The optical fiber  142  is located in the V-notch groove  66  without touching the end plug  140 . A section of the portion  144 B of the jacket is located in the V-notch groove  66  and another section of the optical fiber  142  not covered by the jacket  144  is also located in the V-notch groove  66 . 
   A resin  68  is located over the portion  144  of the jacket in the V-notch groove  66  and cured. The resin  68  is selected for its ability to securely attach the portion  144 B of the jacket to the end plug  140 . 
   The damper  52  is deposited through an open upper portion of the V-notch groove  66  over a portion of the optical fiber  142 . The damper  52  is selected for its ability to absorb the waves in the optical fiber  142  and is made of silicone, having a refractive index substantially the same as the optical fiber  142 . In another embodiment, the damper may be made from a porous material of a glass matrix containing voids. The air voids reduce the acoustic impedance of the glass itself, thereby reducing the ability of the glass to reflect a wave. 
   The damper  52  flows under capillary action and under gravity and is then cured so as to be longer in a base of the V-notch than above the optical fiber  142 . The damper  52  has a slanted surface  70  on a side thereof facing towards the interaction length  37  and the transducer  134 . As seen in  FIG. 3 , the slanted surface  70  extends continuously at an average angle  72  of about 5° (shown exaggerated) relative to a longitudinal axis of the optical fiber  142  from a base of the V-notch groove  66  to near the top of the V-notch groove  66 . The optical fiber  142  has a longitudinal axis that extends in a direction  64 . The optical fiber  142  extends into the continuously extending slanted surface  70 . There is an acute oblique angle of 5° measured clockwise from the surface  70  to the direction  64  in an area above the optical fiber  142 . 
   The damper  52  also has a slanted surface  74  on a side thereof opposing the transducer  134 . The slanted surface  74  also extends at an angle of about 20° from a base of the V-notch groove  66  to near the top of the V-notch groove  66 . There is an obtuse oblique angle of 160° measured counterclockwise from the surface  74  to the direction  64  above the optical fiber  142  and an acute oblique angle of 70° measured counterclockwise from the surface  74  to the direction  64  below the optical fiber  142 . 
   Furthermore, the resin  68  has a surface  76  facing the damper  52  which extends at an angle of about 20° relative to the direction  64  of the longitudinal axis of the optical fiber  142 . The surface  76  also extends continuously from a lower side of the optical fiber  142  to an upper side of the optical fiber  142  and is similar to the surface  70 . 
   In another embodiment, it may be possible that these angles may be 5°, 10°, or even 15° from the present embodiment, while still providing at least some of the advantages of the angles of the present embodiment. 
   Because the surface  70  is at an angle other than 90° relative to a longitudinal axis of the optical fiber  142 , the damper  52  is more effective in absorbing, and not reflecting, a wave traveling along the interaction length  37 . Any reflections by the surface  70  will be away from the optical fiber  142  into the air above the optical fiber  142 , but such reflection hardly occurs because of the large difference in the impedance between the optical fiber  142  and the air. 
   A portion of the wave may travel through the damper  52  to the surface  74 . Such a portion of the wave is primarily dampened by the material of the damper  52 . The surface  74  is also located at an angle other than 90° relative to the direction  64  of the longitudinal axis of the optical fiber  142 . The angle of the surface  74  further assists in damping the wave because of the same reasons as the surface  70 , and the angle of the surface  76  yet further assists in not reflecting any wave traveling from the surface  74  to the surface  76  because of the same reasons as the surface  70 . 
   It can thus be seen that the surfaces  70 ,  74 , and  76  more effectively dampen waves traveling through the optical fiber  142 . Moreover, the surfaces  70  and  74  are easily formed by depositing the material of the damper  52 , which flows under gravity and capillary action to form the surfaces  70  and  74 . 
     FIG. 5  shows the damper region in enlarged detail, and  FIGS. 6A-D  are cross-sections at (a), (b), (c) and (d), respectively. In the plane (a), the damper  52  touches the optical fiber  142  from the bottom. The surface where the surrounding damper  52  contacts the optical fiber  142  gradually increases through (b) and (c), and (d). The slant angle of damper  52  from the regions (a) to (c) is about 1-3 degrees. Therefore, the length of the region between (a) and (b) is about 5-10 mm This arrangement is effective in minimizing the back reflection of the wave, because the amount of perturbation on the wave, which is proportional to the mass load attached to the optical fiber, is small from the region (a) to (b). 
   Since the damper  52  touches only a fraction of the surface of the optical fiber  142 , the amount of acoustic reflection at the entrance is small compared to the perpendicular surface. The reflectivity increases in proportion to the contact area of the damper, i.e., from (a) through (d), and the reflections effectively cancel one another out. 
   The damping characteristic is dependent upon the direction of the vibration of the wave with respect to the orientation of the damper structure.  FIGS. 7A and 7B  show two cases where the vibration direction of the acoustic wave is horizontal ( FIG. 7A ) and vertical ( FIG. 7B ) with respect to the interface between the damper  52  and optical fiber  142 . The horizontal case as in  FIG. 7A  may be more desirable than the vertical case in  FIG. 7B  because the effective mass load in the case of  FIG. 7A  is smaller than the case  FIG. 7B , thereby causing smaller back reflection. 
   In the case where it is important to attenuate cladding-mode light in the damper, it is preferred to match the refractive index of the damper  52 , or at least a portion of the damper  52 , to the refractive index of the cladding layer. Index-matching should be done within 10 −3  difference. 
     FIGS. 8A and B  illustrate another possible embodiment. A long narrow ridge support  40   ii  is located on top of a flat mount  40   i . A damper  52   iii  is formed by injecting silicone between the optical fiber  142  and the narrow ridge  40   ii . The material of the ridge  40   ii  is preferably a metal such as aluminum, for purposes of conducting heat. The functioning of the damper  52   iii  is similar to the damper of FIG.  7 B. The material of the damper  52   iii  is preferably silicone with a refractive index substantially the same as the optical fiber  142 . 
   Further details, described hereafter, relate to cancellation of multiple back-reflected waves by destructive interference between them. As shown in  FIG. 9 , some of the wave traveling through the interaction length  37  is reflected, as represented by R 1 , because of the transition from the interaction length  37  to the damper  52 . A further reflection, represented by R 2 , occurs at a transition in acoustic impedance from the damper  52  to a section  78  of the optical fiber  142  between the damper  52  and the portion  144 B of the jacket. Yet a further reflection R 3  occurs at a transition from the section  78  to the portion  144 B of the jacket. 
   The reflections R 2  and R 3  can be used to cancel out reflection R 1 . The reflection R 2  causes a reflected wave which is out of phase with a reflected wave caused by the reflection R 1  and out of phase with a reflected wave caused by the reflection R 3 . The phase angles are chosen so that the magnitude of the vector sum of the waves due to reflections R 1 , R 2 , and R 3  is zero. The phase of the wave created by the reflection R 2  depends on the material of the damper  52  and the optical fiber  142  and can also be adjusted by adjusting the thickness and the length of the damper  52 . Similarly, the phase of the reflection R 3  depends on the material of the optical fiber  142  and its diameter, and can be adjusted by adjusting the length of the section  78 . 
   The degree to which the waves caused by the reflections R 2  and R 3  cancel the wave R 1  depends on the amplitudes of the reflected waves R 2  and R 3 . The amplitudes of the reflected waves depend on the angles of the surfaces  70 ,  74 , and  76  in  FIG. 3 , the materials of the optical fiber  142 , damper  52 , and jacket  144 , and thicknesses of the damper  52  and jacket  144 . These variables can all be altered to create a desired cancellation effect. 
     FIG. 10  illustrates how in another embodiment a cancellation effect can be created using two reflections. In the example given, a damper  52   i  is located directly against an end of a portion  144 Bi of a jacket. Both the jacket  144 Bi and the damper  52   i  have slanted surfaces which reduce reflection to a required degree. A first reflection R 1  occurs at a transition from the interactive length  37   i  to the damper  52   i , and a second reflection R 2  occurs at a transition from the damper  52   i  to the portion  144 Bi of the jacket. According to design, the phase of the wave due to the reflection R 2  is out of phase relative to a wave created by the first reflection R 1  by 180°. The materials of the damping material  52   i  and the portion  144 Bi of the jacket can be selected so that an amplitude of a wave due to the reflection R 1  is equal to an amplitude of a wave due to the reflection R 2 . 
     FIG. 11  illustrates an embodiment with two reflections, R 1  and R 2  respectively. The first reflection R 1  is caused due to a transition from a length  37   ii  to a damper  52   ii . The second reflection R 2  is caused due to a transition from a damper  52   ii  to a component  80  having a larger diameter than the damper  52   ii . Both the damper  52   ii  and the component  80  have slanted surfaces to control an amplitude of a respective reflected wave. The component  80  may be a portion of a jacket, or the component  80  may be a portion of a resin used for attaching a jacket, or any other component. 
     FIG. 12  illustrates waves as they travel through fibers and a double-reflection damper such as the double-reflection damper  52   i  shown in  FIG. 10. A  forward-traveling wave F 1  travels in a direction  64  through the interaction length  37  of the fiber. The wave F 1  is partially reflected in a direction  84 , opposing the direction  64 , as the reflected wave R 1 . An amplitude of the reflected wave R 1  is typically about 1% of the amplitude of the forward-traveling wave F 1 . 
   A portion of the forward-traveling wave F 1  also travels through the damper  52   i  as a forward-traveling wave F 2 . The forward-traveling wave F 2  initially has an amplitude which is a fraction of the forward-traveling wave F 1  and is then further dampened as it travels through the damper  52   i . The forward-traveling wave F 2  is then partially reflected in the direction  84  as the reflected wave of R 2 . An amplitude of the reflected wave R 2  is initially approximately 2% of an amplitude of the forward-traveling wave F 2  just before it is reflected. The reflected wave R 2  is then further dampened while it travels in the direction  84  back through the damper  52   i . When the reflected wave R 2  enters the interaction length  37 , the reflected wave R 2  has an amplitude which is approximately equal to an amplitude of the reflected wave R 1 . The reflected waves R 1  and R 2  are 180° out of phase so that their sum substantially equals zero. The reflected wave R 1  is thus cancelled by the reflected wave R 2 . 
   A number of factors contribute to the canceling of the reflected wave R 1  by the reflected wave R 2 . These factors include the length and material of the damper  52   i , the amount of reflection by surfaces of the damper  52   i , and the amount of attenuation of the wave F 1  at an interface of the damper  52   i . The number of reflections, in this case two, also plays a role. 
   While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described, since modifications may occur to those ordinarily skilled in the art.