Abstract:
A variable attenuator includes a rotatable actuator and cam arrangement arranged to vary a length of an air gap between ends of fibers in a transmission line. The cam arrangement includes a cylindrically shaped cam member having an end surface cut to fit a curve representing normalized attenuation-to-gap-length data, the cam follower(s) being fitted in a sliding member that holds the end of the optical fiber and moves it relative to a fixed end of another fiber connector member. The cam surface includes multiple identical cam surface sections extending less than 180° around the cam to define movement of the cam follower(s) and the sliding member between a maximum position and a minimum position. The cam surface sections may be connected by ramps that permit a pair of cam followers to move from the maximum position to the minimum position and back in a continuous path that allows an actuator to be turned beyond the maximum and minimum positions and automatically reset without damage to any of the components of the attenuator.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a variable attenuator for fiber optic transmission cables, and in particular to a variable attenuator of the type in which attenuation is achieved by varying an air gap between ends of two optical fibers. A cam arrangement is included to achieve a linear response between turning of an actuator and attenuation of an optical signal transmitted through the gap between the fiber optic transmission cables. 
     2. Description of Related Art 
     Attenuators are used to regulate the strength of an optical signal in a fiber optic transmission system. It is desirable to maintain the strength of an optical signal within a certain range to avoid receiver saturation, to compensate for variable distances of various receivers from the source, or to compensate for aging or other changes in the system. 
     The present invention involves a variable attenuator that is in the form of a fiber optic coupler arranged such that a controllable air gap exists between the ends of the fibers in the transmission cables coupled by the coupler. By mechanically varying the air gap length, the amount of attenuation can be controlled. Examples of variable attenuators of this type are disclosed in U.S. Pat. Nos. 4,145,110 and 5,066,094. A similar arrangement, involving interception of the air gap by a screw rather than direct adjustment of the air gap length is disclosed in U.S. Pat. No. 5,734,778. 
     FIG. 1 illustrates one such conventional variable attenuator of the type in which the air gap length in varied by turning a screw, and which is available from Amphenol Corporation, Fiber Optics Products Division, Lisle, Ill. The variable attenuator couples two transmission cables  1 , 2 , each of which is terminated in conventional fashion by: (1) stripping the respective cable jackets  3 , 4 , and strength members  5 , 6  to expose fiber buffers  7 , 8 ; (2) further stripping portions of the fiber buffers  7 , 8  to expose the fibers and inserting the exposed fibers in alignment ferrules  9 , 10 ; (3) polishing ends of alignment ferrules  9 , 10  so that the ends of the exposed fibers are flush with the facing surfaces of the alignment ferrules; (4) capturing the alignment ferrule holding members  11 , 12  and bias springs  13 , 14  within externally threaded front portions  15 , 16  of rear bodies  17 , 18  by securing the front portions to internally threaded rear portions  19 , 20  of front bodies  21 , 22 ; (5) crimping exposed portions of strength members  7 , 8  between rear portions  23 , 24  of rear bodies  17 , 18  and crimp ferrules  25 , 26 ; and securing boots  27 , 28  to the rear bodies  17 , 18 . 
     Front body  21  of the first transmission cable is arranged in conventional fashion to support a coupling nut  29 , but slide body  22  of the second transmission cable termination has been modified to be cylindrical in shape and to include external threading for cooperation with an internally threaded thumb wheel  30 . Thumb wheel  30  includes a collar  31  that is captured between a front housing  32  and a rear housing  33  which together form the attenuator housing upon threading of rear housing  33  onto an externally threaded extension  34  of front housing  32 , and as a result thumb wheel  30  can rotate relative to front housing  32  but cannot move axially. On the other hand, slide body  22  is free to slide axially within front housing  32 , causing alignment ferrule  10  to also move axially in response to rotation of thumb wheel  30 . Rotation of slide body  22  is prevented by a dowel  35  extending inwardly from front housing  32  and slidably fitted in a slot in slide body  22 , while an attenuation spring  36  captured between the front housing  32  and slide body  22  eliminates tolerances between the threading of the thumb wheel  30  and slide body  22 . 
     Alignment of ferrules  9 , 10  is provided by an alignment sleeve  37  fitted in a holder  38 , which in turn is captured between front body  32  and an adapter  39  upon threading of adapter  39  into the front housing. Extending from adapter  39  is a cylindrical coupling section  40  to which the coupling nut  29  is secured to thereby fix cable  1  relative to the housing. In order to vary the length of the air gap between the ends of ferrules  9 , 10 , it is simply necessary to turn thumb wheel  30 , causing corresponding axial movement of slide body  22  and ferrule  10  relative to the fixed position of ferrule  9  and front housing  32 . 
     A problem with attenuators of the type illustrated in FIG. 1 is that the relationship between air gap length and attenuation is non-linear, making it difficult to predict the attenuation that will result for a given number of turns of the thumb wheel  30 . For example, adjustment of the attenuation from 10 dB to 15 dB might require 1.3 turns of the wheel, while adjustment of the attenuations from 20 dB to 25 dB might require 0.8 turns of the thumb wheel. Adjustment of the attenuation would be much easier to achieve if each turn of the thumb wheel resulted in the same variation in attenuation, i.e., if the relationship between turns of the wheel and attenuation were linear, but such a linear relationship is not possible in conventional screw arrangements of the type illustrated in FIG. 1 due to the non-linear relationship between the air gap length and attenuation of the light signal across the air gap. 
     A second problem with attenuators of the type illustrated in FIG. 1 is that the thumb wheel or adjustment screw can easily be turned beyond the minimum and maximum attenuation points, causing damage to the attenuator. 
     The present invention solves these problems by replacing just five parts of the conventional variable attenuator mechanism illustrated in FIG.  1 . The five parts are the slide body  22 , thumb wheel  30 , rear housing  33 , a wave washer (not shown) which fits between the slide body  22  and front housing  32 , and a Teflon™ washer (not shown) which fits between the thumb wheel  30  and rear housing  33 . These five parts are replaced by a modified slide body, a cam body, a worm and worm gear mechanism, and a modified rear housing which cooperate to cause the air gap length to be adjusted in such a way that each turn of an actuator will result in a proportional change in the attenuation, as will be described below, thereby simplifying the adjustment procedure without an undue increase in complexity of the attenuator. The cam surface is arranged such that the position of the slide body will reset when the actuator is turned beyond the maximum or minimum positions, making it impossible to damage the attenuator by over-turning of the actuator. 
     SUMMARY OF THE INVENTION 
     It is accordingly an objective of the invention to provide a variable attenuator for fiber optic transmission cables that achieves a linear response between turning of an actuator and attenuation of a light signal across a air gap. 
     It is a further objective of the invention to provide a variable attenuator for fiber optic transmission cables in which attenuation is achieved by mechanically varying a air gap between ends of respective transmission cables in response to turning of an actuator, the relationship between air gap length and attenuation being non-linear, and yet in which the relationship between turning of the actuator and the resulting attenuation is linear. 
     It is a still further objective of the invention to provide a variable attenuator for fiber optic transmission cables in which a linear relationship is achieved between a mechanical input and the resulting attenuation, and yet which is easily assembled and which uses standard fiber optic transmission cable terminations. 
     It is yet another objective of the invention to provide a variable optical attenuator with a non-destructive feature which allows the input actuator to be turned beyond the maximum range of the attenuator without damaging the attenuator. 
     These objectives are achieved, in accordance with the principles of the invention, by providing a linear attenuator of the type in which a rotatable actuator is used to vary a length of the air gap between ends of fibers in a transmission line, but in which instead of directly driving the sliding mechanism that moves the fiber ends relative to each other, the rotatable actuator is used to turn a cam having a surface that causes the air gap length to change in non-linear fashion in response to turning of the actuator, and the attenuation to thus vary in a linear fashion. 
     In a preferred embodiment of the invention, the cam is a cylindrically shaped cam member having an end surface cut to fit a curve representing normalized attenuation-to-gap-length data, and the follower is fitted in a slide body that holds the end of the optical fiber and moves it relative to a fixed end of a second fiber connector member. The cam is turned by a screw drive that provides multiple turns of the screw for each rotation of the cam member, the screw being connected to the cam by a worm and a worm gear that mesh and turn the cam. A sliding body is arranged to transport an end of a cable terminated to a ferrule, rear body, and the slide body in the conventional manner described above in connection with FIG. 1, the slide body having an interior structure identical to that of the prior sliding front bodies. Two cam followers are arranged at ends of the slide body to face a cam surface at one end of the cam member, and a spring is arranged between the front housing and the sliding body to ensure positive engagement between the cam followers and the cam surface. 
     In an especially advantageous version of the preferred embodiment, the cam has two identical cam surfaces extending less than 180° around the cam and connected by ramps, the cam surfaces defining movement of the sliding member between a maximum and a minimum and the ramps permitting a pair of cam followers to move from a maximum position to a minimum position and back in a continuous path to thereby allow the actuator to be turned beyond the maximum and minimum positions and automatically reset without damage to any of the components of the attenuator. 
     Preferably, the housing is arranged so that the worm gear can easily be accessed by removing a worm gear plate at the rear of the rear housing, the worm can be accessed by shaft plates at opposite sides of the rear housing, and the front of the housing is arranged to receive a front housing and adapter corresponding to the front housing  32  and adapter  39  shown in FIG. 1, so that a second fiber optic transmission cable having a conventional termination and coupling nut can be coupled thereto and fixed in position relative to the movable fiber optic transmission cable, removal of the front housing and worm gear plate providing complete access to the sliding body and cam member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional side view of a conventional fiber optic transmission cable attenuator arrangement having a screw drive. 
     FIG. 2 is a cross-sectional side view of a linear-response fiber optic transmission cable attenuator arrangement constructed in accordance with the principles of a preferred embodiment of the invention. 
     FIG. 3 is a cross-sectional rear view of the linear response attenuator arrangement illustrated in FIG. 2, without the transmission cables. 
     FIG. 4 shows the same cross-sectional side view as in FIG. 2, without the transmission cables. 
     FIGS. 5 and 6 are respective cross-sectional side views of the slide body and cam member illustrated in FIGS. 2-4. 
     FIG. 7A is a graph illustrating the attenuation provided by the conventional attenuator of FIG. 1 as a function of the air gap length. 
     FIG. 7B is a graph illustrating the desired linear response between rotation of the cam member and the resulting attenuation. 
     FIG. 7C is a graph of air gaps as a function of attenuation, which is used to determine the shape of the cam surface. 
     FIG. 8 illustrates the manner in which the air gap and attenuation values are normalized to account for tolerances in the conventional attenuator used to generate the graph illustrated in FIG.  7 C. 
     FIG. 9 is a graph of actual attenuation data taken at 1550 nm and 1310 nm. 
     FIG. 10 is a graph representing a curve fit to the mean of the attenuation data shown in FIG.  9 . 
     FIG. 11 is a table converting the data represented by the graph shown in FIG. 10 into cutting parameters for an exemplary cam surface. 
     FIG. 12 is an idealized schematic representation of the effect of cam follower shape on movement of the cam follower along the cam surface. 
     FIGS. 12 and 13 are, respectively, a side view of the cam surface portion of a preferred cam member and a projection of the circumference of the cam member illustrated in FIG. 12 onto a plane for the purpose of better showing the shape of the cam surface. 
     FIG. 14 is a projection of a cam surface corresponding to that of FIG. 13, but modified to include a non-destructive reset feature. 
     FIG. 15 is a projection of a preferred modification of the cam surface of FIGS.  13  and  14 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in FIGS. 2-4, a linear variable attenuator constructed in accordance with the principles of a preferred embodiment of the invention includes a front housing  32  corresponding to front housing  32  illustrated in FIG. 1, and a rear housing  100  having a front opening  101  into which front housing  32  may be threaded. Because front housing  32  may be identical to front housing  32  of FIG. 1, transmission cable  1  may conveniently be terminated to the housing via the same adapter  39  illustrated in FIG. 1, adapter  39  extending into opening  41  to abut inwardly extending collar  42  of front housing  32 , with coupling nut  29  being coupled to adapter  39  in exactly the same manner as described above in connection with FIG.  1 . 
     Because the parts are interchangeable, the corresponding elements of the termination and coupling mechanism for the first fiber optic transmission cable  1  have been given the same reference numerals as corresponding parts in FIG.  1  and will not be further described herein. Similarly, the second fiber optic transmission cable  2  is terminated and coupled to the attenuator in the same manner as illustrated in FIG.  1  and therefore the corresponding elements of the second transmission cable termination have been given the same reference numerals as corresponding parts in FIG.  1  and will not be further described herein, with the exception of the slide body  102 , which fits into central opening  43  and rear opening  44  of front housing  32 , and has been modified to cooperate with various drive elements for causing the slide body to move with respect to the attenuator housing in non-linear fashion. It is noted that openings and a collar corresponding to openings  41 ,  43 , and  44 , and collar  42  illustrated in FIG. 4, are present in the front housing illustrated in FIG. 1 but not numbered therein. 
     Slide body  102  has an internal configuration identical to that of slide body  22  illustrated in FIG. 1 so as to accommodate the conventional fiber optic cable termination, including a central opening  46  for receiving alignment ferrule  10  and a communicating central opening  47  for receiving the front section  16  of rear body  18 . However, the exterior of slide body  102  has been modified to enable it to cooperate with cam member  103  to move the second transmission cable  2  relative to the first transmission cable  1 . Although the cable termination and coupling elements as well as front housing  32  are conveniently identical to those illustrated in FIG. 1, those skilled in the art will appreciate that the details of the cable termination and couplings form no part of the present invention, and that any of the illustrated termination and coupling elements may be modified so long as they can be moved in a manner which linearizes the output of the attenuator, as described below. 
     Extending from the outside surface of slide body  102  are two cam follower supports  104  arranged to support cam follower pins  105  and  106 , which extend rearwardly in an axial direction relative to the cylinder axis of slide body  102  so as to float on the surface of the cam. An outer surface of cylindrical front section  107  of slide body  102  is received in opening  43  of the front housing  32  in such a manner that dowel  35  extends into slot  108  to prevent rotation of the slide body as it moves back and forth. Although not shown, a second dowel may be added to help prevent the slide body from rotating. In addition, those skilled in the art will appreciate that the cam surface should be made as smooth as possible to prevent imparting torque to the slide body, and dimensions and materials of the slide body, cam member, and any other parts may be adjusted accordingly to prevent torque on the slide body and consequent deformation. Lubrication, consistent with environmental specifications, may be added to the cam surface so that torque is further reduced. 
     An opening  109  in the slide body  102  receives an attenuator spring  110 , which may be identical to the corresponding spring  36  illustrated in FIG. 1, and which biases the slide body in the direction of engaging the cam follower. Rear section  48  of slide body  102  fits into opening  132  in the cam member, while the front surface  111  of the cam follower supports faces a shoulder of the front housing to capture the slide body between the cam member  103  and the front housing  32 , with both the slide body  102  and cam member  103  being fully accessible upon removal of the front body  32  from rear body  100 . 
     Cam member  103  has attached to its end a cam piece  125 , illustrated in FIGS. 12 and 13 and described in more detail below, which includes a cam surface  112  arranged to face and be engaged by the cam followers  105 , 106 . A connector element  128  fits into slots  129  in the cam member  103  and cam piece  125  to secure the cam piece to the cam member. Those skilled in the art will appreciate that the cam surface  112  could alternatively be cut directly into the cam member rather than being provided on a separate cam piece. 
     As indicated above, and described in further detail below, the basic principle involved in designing the cam piece  125  and cam surface  112  is to ensure that rotation of the cam member  103  causes a linear movement of the slide body  102  by an amount which is proportional to the angle of rotation or the number of turns of an actuator. In a conventional screw drive, the change in air gap length is proportional to the angle of rotation, but the resulting attenuation does not vary by a constant amount because attenuation is not proportional to the air gap length. The cam surface of the present invention is intended to compensate for the this non-linear relationship between air gap length and attenuation of light across the air gap. 
     Rotation of the cam member could be achieved by any suitable mechanism, including a direct drive or thumb screw mechanism, but the preferred embodiment of the invention utilizes a worm and worm gear mechanism in order to achieve a more compact and efficient mechanism that is easily assembled and disassembled. One example of such a worm and worm gear can be found in the W.M. Berg B2000 Master Catalog, p. B149 W48B29-D40 (worm gear) and W48S-2D (worm). It will nevertheless be appreciated by those skilled in the art that the present invention is not to be limited in any way to a particular drive mechanism for causing rotation of the cam member. 
     The worm drive of the embodiment illustrated in FIGS. 2-4 includes a worm gear  113  and worm  114 . Considerations for selecting the worm and worm gear include the absence of significant backlash, a favorable drive ratio of, for example, 20-1, which gives an effective drive ration of 10-1 for a cam surface with two cam followers, and standardization so that off-the-shelf worm and worm drive gears of the type used in other smaller applications can be used. 
     As illustrated in FIGS. 2-4, worm gear  113  is fitted over rear section  133  of the cam gear and is held in place by a worm gear plate  115  which slips against the worm gear, the worm gear being sandwiched between the worm gear plate  115  and a shoulder  134  on the cam member to constrain the worm gear and cam body, thereby pressing the cam against the slide body  102 . Attenuator spring  110  pushes the slide body back toward the worm gear plate while the central opening in the worm gear centers the cam body, which is also centered by the housing  100 . 
     The worm  114  is mounted within an opening  123  on a shaft  116  supported by plates  117  and  118  at each end, the front end including a slotted shaft head  119  which can be turned by a screwdriver to rotate the worm. Shaft  116  is secured to the shaft head plate  117  by a collar  120  and plates  115 ,  117 , and  118  are attached to the housing  100  by respective sets of screws  121 ,  122 , and  123 , not all of which are shown. There are many ways to support the shaft and worm. Those skilled in the art will appreciate that the slotted shaft head could be replaced by a thumb wheel or other actuator member to facilitate turning of the worm and worm gear, and that the shaft could also be turned by a motor to make possible automatic adjustment of the attenuation. 
     An advantage of the illustrated attenuator structure is that it can easily be assembled by terminating a cable to the slide body  102  in the manner described above, coupling a cable to the adapter  39  and securing the front housing  32  to the rear housing  100  (or securing the front housing to the rear housing and then coupling a cable to the adapter), inserting the slide body into the housing  100  from the rear, inserting the cam body  103  from the rear, fitting the worm gear  112  over the cam body  103 , attaching worm gear plate  114  to the housing  100  using screws  120 , attaching shaft  115  to the shaft head plate  116  and fitting the worm into opening  123 , and securing plates  116  and  117  to the rear housing  110  using screws  122  and  123 . 
     The manner in which the response curve of a variable attenuator is linearized is illustrated in FIGS. 7A-7C and  8 - 11 . Initially, the response of a variable attenuator to air gap length must be measured using a conventional variable attenuator of, for example, the type illustrated in FIG.  1 . FIG. 7A shows the change in attenuation as the length of the air gap is varied across the operating range of the attenuator. 
     In order to linearize the response of the variable attenuator, the invention seeks to vary the air gap in non-linear fashion so that the attenuation becomes proportional to movement of the actuator, as illustrated in FIG.  7 B. This is accomplished by plotting the air gap as a function of the attenuation, which can then be used to determine the amount by which the surface cam  112  must be cut to linearize the air gap. In the example shown in FIG. 7C, which is subject to the adjustments described below, the air gap is increased from a minimum, the air gap needs to be increased at a relatively rapid rate, which slows down in the middle of the attenuation response curve and increases at towards the maximum. In principle, a corresponding cam would have approximately the same shape as the response curve, thereby providing an air gap adjusting mechanism that varies with each turn in the manner indicated in FIG. 7B to achieve a linear relationship between the attenuation and turning of the actuator. 
     In practice, however, the attenuation data must be adjusted to account for the delay in separation of the air gap due to the connector springs that maintain contact between the two terminations. Furthermore, the attenuation data must be normalized so that, at the point where the air gap starts to separate, the attenuation is zero, and finally the data must be collected for the multiple wavelengths that must be carried in any bi-directional transmission cable. 
     Normalization of the response data may be accomplished, as illustrated in FIG. 8, by defining a line a+bz through the two points (z 3 ,A 3 ) and (z 4 ,A 4 ) where the attenuation is just starting to change, calculating the slope b=(A 4 −A 3 )/(z 4 −z 4 ) and a=A 4 −bz 4 , and then calculating the intersection point z 0 ,A 0  by calculating z 0 (A 0 −a/b where A 0 =A 1 =A 2 . The starting point z 0  can then be subtracted from each raw z in the data to obtain a table or graph of adjusted response data. 
     A typical bi-directional fiber optic transmission cable carries light at wavelengths of 1310 nm and 1550 nm and, therefore, in order to provide a practical attenuator, response curves must be generated for both wavelengths. An example of a graph of the normalized mean attenuation values for 1550 nm and 1310 nm is found in FIG.  9 . The mean of the two separate response curves illustrated in FIG. 9 is then used to design the surface of the cam, for example by employing a curve fitting program such as TableCurve 2D™, ver. 3, to generate a function that matches the data shown in the graph, as illustrated in FIG.  10 . The curve of FIG. 10 can then be converted into a table of values expressed in terms of cam angle and drops or cuts, as illustrated in FIG. 11 (which uses slightly different curve fit data than the graph of FIG.  10 ). Theta in FIG. 11 is the cam surface angle relative to the z=0 reference line shown in FIGS. 14 and 15. 
     The data in FIGS. 9-11 does not take into account the radius of curvature of the cam followers, whose effect is illustrated in FIG.  12 . In a practical application of the preferred embodiment of the invention, it will generally be necessary to further adjust the response data curve using simple trigonometry to convert points (x,y) representing the cam surface derived form the response curves to points (x′,y′) representing the actual cam surface necessary to compensate for the effect of the radius of curvature of the cam followers, which are in the form of radiused pins. 
     The rotary cam piece  125  illustrated in FIGS. 13 and 14 has a shape determined by the table of FIG.  11 . FIG. 13 is a cross-section of the cam, with dashed lines corresponding to points on the projection shown in FIG.  14 . The projection shown in FIG. 14 covers slightly more than a 180° section of the cam surface. As the cam is rotated over a distance of 152° from point A to point B, the cam followers  105 ,  106  will move from the z=0 reference line a distance of 0.0504 inches, to the position indicated by reference numerals  105 ′, 106 ′. Further rotation of the cam piece is prevented by stop  126 , while over-rotation in the opposite direction is prevented by stop  127 . 
     In a preferred modification of the cam surface illustrated in FIGS. 13 and 14, as shown in FIG. 15, the pin stops  126  and  127  are removed and replaced by ramps  130  and  131  to allow the cam followers  105 , 106  to move beyond the range of 0 to 152° and reset. This prevents the variable attenuator from being damaged by adjusting the actuator too far beyond its endpoints, causing the stops  126  and  127  to apply torque to the cam followers. In the illustrated example of a 10:1 turn ratio, the ramps permit the cam pins to move about 1.5 turns of the screw drive from minimum attenuation to maximum attenuation and back so that input turns can be made without restriction, the attenuator simply resetting itself without any damage as the cam is turned passed the minimum or maximum points at the ends of the ramps. 
     Although the cam surfaces shown in FIGS. 13-15 are illustrated as including two identical sections arranged to cooperate with two cam followers, those skilled in the art will appreciate that the cam could include any number of sections, including one, three, four, or more sections, all connected by pin stops of ramps, with one cam follower is included for each section. 
     Having thus described a preferred embodiment of the invention with sufficient particularity to enable those skilled in the art to easily make and use the invention, and having described several possible variations and modifications of the preferred embodiment, it should nevertheless be appreciated that still further variations and modifications of the invention are possible, and that all such variations and modifications should be considered to be within the scope of the invention. Accordingly, the scope of the invention should not be limited by the above description, but rather should be interpreted solely in accordance with the appended claims.