Abstract:
A shield for fiber optic connectors and cables is provided. This shield minimizes the effects of EMI (electromagnetic interference) either radiating from or radiating into an electronic enclosure. Embodiments include this shield as an integral part of the enclosure chassis, a separate shield, or integrated into the fiber optic cable and connector.

Description:
[0001]    The present invention relates to a shield for fiber optic connectors and cables which minimizes the electromagnetic interference (EMI) energy entering or leaving an enclosure for associated electrical computer components.  
         BACKGROUND OF THE INVENTION  
         [0002]    As digital data and edge rates increase, multi-processing computer systems increasingly use fiber optic links for data transmission. Particularly, systems for transmitting large blocks of data in short intervals of time and systems for transmitting high speed data over relatively long distances use fiber optic links. A common high speed application is the connection between computer nodes in parallel processing computing and the high data rate connections between computers and data storage.  
           [0003]    A common misperception associated with the use of fiber optic links is that fiber optic links eliminate all problems with electromagnetic interference (EMI) regulatory certification compliance. This misperception (that use of fiber optics eliminates EMI concerns) likely stems from past fiber optic systems when either analog signals or low speed digital signals were transmitted by the fiber optics. Probably due to this misperception, most fiber optic connectors are not designed to minimize EMI effects.  
           [0004]    Currently, modern high data rate fiber optic data transmission systems present a range of significant EMI problems. While the fiber optic cable does not radiate electromagnetic energy, the electrical computer components that feed the optical transmission system can cause EMI to radiate. Additionally, EMI problems may arise due to susceptibility of the electronic system to electromagnetic noise created by neighboring equipment that can create component damage, system upset, data error or related problems.  
           [0005]    EMI may enter a computer system in various ways such as via various apertures and conducted points of entry associated with a typical electrical enclosure. Particularly, EMI may enter a computer system enclosure via the substantial electrical aperture associated with a fiber optic cable connector. This electrical aperture occurs between the fiber optic connector and the electrical enclosure because the fiber optic connector body is constructed from plastic, rather than metal (or any other electrically conductive material). This electrical aperture is a common point of entry or exit for EMI energy in current systems with fiber optic links.  
           [0006]    Accordingly, there is a need for a shield for fiber optic connectors and cables which minimizes the EMI energy entering or leaving an enclosure via the fiber optic connector electrical aperture.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with the teachings of the present invention, a shield for fiber optic connectors and cables is provided. This shield minimizes the effects of EMI either radiating from or radiating into an electronic enclosure. Embodiments include this shield as an integral part of the enclosure chassis, a separate shield, or integrated into the fiber optic cable and connector. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 shows a typical electronic enclosure including an integrated EMI shield in accordance with the present invention;  
         [0010]    [0010]FIG. 2A shows a side view and FIG. 2B shows an end view of the integrated EMI shield of FIG. 1;  
         [0011]    [0011]FIG. 3 shows a graphical example of attenuation achievable using an EMI shield in accordance with the present invention;  
         [0012]    [0012]FIG. 4 shows a side view of a first design of a separate EMI shield in accordance with the present invention; and  
         [0013]    [0013]FIG. 5A shows a side view and FIG. 5B shows an end view of a second design of a separate EMI shield in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring now to the drawings, in which like-referenced characters indicate corresponding elements throughout the several views, attention is first drawn to FIGS. 1, 2A and  2 B which show a typical electronic enclosure  10  including a chassis panel  20  for external connections. Electronic enclosure  10  houses electronic subsystems, such as components of a multiple processor computer system, which includes fiber optic links. Only the components necessary for an understanding of the present invention are shown and discussed herein.  
         [0015]    Chassis panel  20  includes an aperture for a panel connector  22  into which fiber optic cable and connector  24  may be mounted. Panel connector  22  may be secured to the surface of chassis panel  20 . According to a first embodiment of the present invention, an integrated EMI shield  26  is attached to the chassis panel  20  so that the aperture for the panel connector  22  is at the entrance to the integrated EMI shield  26 . The integrated EMI shield  26  may be formed of a metallic pipe and may have a circular cross section as shown in FIGS. 1 and 2B. The integrated EMI shield  26  acts as a waveguide through which electromagnetic waves propagate. It is important that the integrated EMI shield  26  is attached so that the aperture for the panel connector  22  is at the entrance to the waveguide and that there is virtually no other egress for the electromagnetic energy than through the waveguide.  
         [0016]    In general, electromagnetic waves propagate through a waveguide as long as the frequency of the wave is higher than the cutoff frequency of the waveguide. The geometry of the cross section of the waveguide determines the cutoff frequency of the waveguide. Below the cutoff frequency, electromagnetic waves do not propagate and are highly attenuated.  
         [0017]    Sample calculations for determining the cutoff frequency of a few common waveguide are as follows:  
         [0018]    Circular Cross Section Waveguide  
           f   cutoff     =     1.841     2      π                 a        ɛμ           ,   where                         
 
         [0019]    ƒ cutoff =frequency in Hertz  
         [0020]    α=diameter of the circular aperture in meters ε=permittivity of the media within the waveguide μ=permeability of the media within the waveguide  
         [0021]    Circular Cross Section Waveguide Filled With Air  
         f   cutoff     =         5.523   ×     10   8         2      π                 a                     Hz                           
 
         [0022]    Rectangular Cross Section Waveguide Filled With Air  
           f   cutoff     =         1.5   ×     10   8       b                   Hz       ,                         
 
         [0023]    where  
         [0024]    ƒ cutoff =frequency in Hertz  
         [0025]    b=width of waveguide in meters (width greater than height)  
         [0026]    The length of the waveguide determines the degree of attenuation of frequencies below the cutoff frequency. Although any length of waveguide operating below the cutoff frequency will provide shielding attenuation, in practice, lengths greater than the general waveguide width are usually required to achieve desired attenuations. For waveguides of circular cross section, lengths greater than the diameter are usually required to achieve desired attenuation. For waveguides of rectangular cross section, lengths greater than the largest cross sectional dimension are usually required to achieve desirable levels of attenuation. For waveguides of any other cross sectional shape, lengths greater than the maximum width dimension are usually required to achieve desired attenuation levels.  
         [0027]    Practical diameters of a circular waveguide shield exhibit cutoff frequencies that are very high, in the Gigahertz range (10 9 ) and higher. Thus, most frequencies associated with electromagnetic interference are below the cutoff frequency and the waveguide shield of the present invention acts as an EMI barrier.  
         [0028]    [0028]FIG. 3 shows a graphical example of the level of field attenuation achievable by the present invention at 1 GHz with a waveguide having a diameter of 2 cm (cutoff frequency of 7.5 GHz). The plot shows attenuation versus length of the waveguide tube, i.e. the number of diameters in the length of the guide. Attenuation is expressed in decibels (dB) which is a logarithmic scale, 20 dB corresponds to a factor of 10, 40 dB to a factor of 100, 60 dB to a factor of 1000, etc.  
         [0029]    In a second embodiment of the present invention, the waveguide shield is not a permanently integrated part of the chassis sheet metal but rather is a separate component. Attention is directed to FIG. 4 which shows a side view of a first design of a separate shield  126  and associated chassis panel  120 . Chassis panel  120  includes an aperture for the panel connector  122  for fiber optic cable and connector  124 .  
         [0030]    A tapered flange or collar  128  mounts onto the chassis panel  120  and provides a virtually continuous (360 degree for the circular cross-section waveguide) peripheral electrical bond when the sleeve-like shield is firmly fitted into place. This fitting may be press-fit, twist-on, or any other type of bond which provides the necessary substantially continuous electrical connection between the chassis panel  120  and the sleeve-like shield  126 . The tapered-shape of the flange or collar provides a means for forcing the mating surfaces tightly together to improve electrical conduction between the elements. The sleeve-like shield  126  may be formed of a metallic pipe and may have a circular cross section as shown.  
         [0031]    As shown in FIGS. 5A and 5B, a second design of a separate shield  226  includes screws  232  or other fasteners and, optionally, an EMI gasket  230  to insure the necessary continuous electrical connection is achieved. The EMI gasket is formed of an electrically conductive material and provides the electrical connection between the separate shield  226  and the chassis panel  220  in the area of the panel connector  222 . The EMI gasket may be eliminated in cases where very good metal-to-metal electrical contact is achieved between the shield and chassis panel. The fiber optic cable and connector  224  connects with panel connector  222  in the conventional manner.  
         [0032]    It is important to note that a good electrical connection must be maintained between the separate component and the chassis sheet metal in order for the shield to be effective. This good electrical connection causes the aperture to be located at the entrance to the waveguide and insures that there is virtually no other egress for the electromagnetic energy than through the waveguide. Having the shield as a separate component may facilitate easier manipulation of the fiber optic connectors and cables, particularly when connecting or disconnecting fiber optic cables.  
         [0033]    In yet another embodiment of the present invention, the fiber optic cable and connector may include an integrated shield which, when mated with the panel connector, provides an effective waveguide shield. In this embodiment, the shield surrounding the connector may be inserted during a typical molding process for making a fiber optic cable and connector and must include some means for electrically connecting the integrated shield in a substantially continuous manner with the associated enclosure.  
         [0034]    Although the above discussion has been directed to circular or rectangular waveguides, the cross section of the waveguide of the present invention may be any closed polygon. Although the above discussion has described the shield as metallic, any electrically conductive material which can provide the necessary electrical shielding can be used. One skilled in the art will recognize that some examples of electrically conductive materials include metallized plastic, metallized glass, and carbon coated plastic; however any electrically conductive material which can provide the necessary electrical shielding is contemplated within the scope of the present invention.  
         [0035]    Although the invention has been described with particular reference to certain preferred embodiments thereof, variations and modifications of the present invention can be effected within the spirit and scope of the following claims.