Transient voltage suppression for electro-optic modules

An improved electro-optic module able to sustain high voltage transients is disclosed. The module is able to eliminate unwanted signals by providing a discharge path for high voltage transients, away from the electronic and opto-devices. The module is comprised of an optical connector having a connecting end adapted to receive and secure an optical fiber cable, a circuit board attached to the optical connector and which is electrically isolated therefrom and means for isolating the circuit board from the connecting end of the optical connector.

FIELD OF THE INVENTION 
This invention relates to fiber optic modules for fiber optic links used to 
connect various system elements in a telephone switching system. 
BACKGROUND OF THE INVENTION 
In a telephone switching system using fiber optic links between various 
system elements, electro-optic transmitter and receiver modules are used 
to terminate each end of the optical link. Each serve as the 
electronic/optical interfaces between the equipment and the fiber. 
Electronic equipment must be protected from electro-static voltage (ESD), 
or transient voltages which it may be exposed to, by adjacent equipment, 
or craftsperson servicing the equipment. Such destructive voltages may be 
in the range of a few hundred volts to thousands of volts. 
Because total protection from ESD is very difficult, separation of the 
electronic ground return path became a common practice. Any hits from ESD 
may then be conducted on metallic surfaces to the frame ground, which is 
in turn solidly connected to the regulatory building ground. Although 
optical signals are not affected by ESD, the electrical components can be. 
Physically, the modules are located on a circuit card which is attached to 
one side of the backplane. In some instances, up to twelve modules 
including associated clock recovery circuits are located on such a card. 
Because of the small size of the card, which can be (10.5.times.3.5) inch, 
and the need to locate the modules close to the faceplate to make the 
optical receptacle accessible, small module size is required. In addition, 
the module should be able to relieve strain to the hybrid from forces 
associated with the fiber optic connector and cable, provide general 
mechanical protection, EMI shielding with grounding, heat transfer away 
from electronic components, and provide minimal resistance to the general 
flow of cooling air over the PCB. 
Although electro-optic modules of small size already exist, these have an 
optical connector with a metallic liner or shell to provide increased 
rigidity. Even with the connector grounded to the equipment ground, it was 
found that unwanted signals and high voltage transients were affecting the 
operation of the internal components of the module, and other electronic 
circuits on the circuit card. 
There is accordingly a need for an improved electro-optic module able to 
sustain high voltage transients, able to eliminate unwanted signals while 
retaining its small size and rigidity as well as meeting the above 
requirements. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
electro-optic module and grounding scheme which will provide a discharge 
path for high voltage transients, away from the module containing the 
electronic and opto-devices, while meeting the above design requirements. 
According to a first embodiment of the invention, there is provided, an 
electro-optic module for connecting to an optical fiber and which is 
comprised of an optical connector having a connecting end adapted to 
receive and secure an optical fiber cable, a circuit board attached to the 
optical connector and which is electrically isolated therefrom and means 
for isolating the circuit board from the connecting end of the optical 
connector. 
According to a second embodiment of the invention, there is provided, an 
electro-optic module for connecting to an optical fiber and which is 
comprised of an optical connector having a connecting end adapted to 
receive and secure an optical fiber cable, a circuit board attached to the 
optical connector and which is electrically isolated therefrom, and a 
conductive outer shell disposed around the optical connector and extending 
from the connecting end to a point along the connector, which is disposed 
away from the circuit board, such that the conductive outer shell provides 
a discharge path for voltage transients. 
According to a third embodiment of the invention, there is provided, an 
electro-optic module for connecting to an optical fiber and which is 
comprised of a circuit board having electro-optic devices thereon, an 
optical connector with a first portion having an outer shell with an 
optical fiber receptacle disposed centrally thereof, the portion having 
means for receiving and securing the optical fiber, a second portion for 
attaching to the circuit board and an isolator separating the first 
portion from the second portion to isolate the circuit board from the 
first portion, such that high voltage transients are directed away from 
the circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1a and 1b, we have shown a front and side view of a 
typical connecting card 10 used on backplanes of telephone switching 
systems. As indicated above, advanced telephone switches now make use of 
electro-optic modules 11 and 12 to send and receive high speed 
information. For example, module 11 could be used as a receiving module, 
whereas module 12 could be used as a transmitting module. Card 10 supports 
various electronic components 13 and is connected to the backplane of the 
switch using multi-pin connectors 14 and 15. A face plate 16 separates the 
optical connectors 17 and 18 of modules 11 and 12 from the modules' 
enclosures 19 and 20 and from the other electronic components 13. Optical 
fibers can be connected to modules 11 and 12 using the necessary optical 
connectors adapted to match connectors 17 and 18. 
In FIGS. 2a and 2b, we have shown a front and side view of an electro-optic 
module according to a first embodiment of the invention. The module 30 is 
comprised of an optical connector 31 having a first portion 32 used as a 
connecting end and which is adapted to receive and secure an optical fiber 
cable and an enclosure 33 containing a circuit board with various 
electro-optic components. Enclosure 33 can be a conventional housing for 
electronic components. It is however, not required for the purpose of 
transient suppression, but is normally used to shield the electronic and 
optical devices from electromagnetic interference, or to reduce 
electromagnetic radiation. The enclosure 33 is attached to the optical 
connector 31, but is electrically isolated therefrom by means of an 
isolator 34 separating the first portion 32 of the connector 31 from a 
second portion 35 attached to the enclosure 33. A series of connecting 
pins 36 are used to secure and electrically connect the circuit board and 
module 30 to the support card shown in FIG. 1. 
FIG. 3 is a sectional view of the electro-optic module of FIG. 2a. As seen 
in FIG. 3, the first portion 32 of connector 31, is isolated from the 
second portion 35 and enclosure 33 by means of a ring-shaped isolator 34. 
The ring-shaped isolator allows light travelling through optical channel 
37 to reach electro-optic transducer 38. Light may be focused on the 
transducer 38 by suitable means, such as a GRIN (graded index) lens 39 or 
equivalent. The electro-optic transducer 38 may contain an LED or LASER 
diode, or it may have an optical detector. Various electro-optic 
components 40 are supported on circuit board 41 and attached to the 
connector 31 via the second portion 35, thereby isolating the 
electro-optic components from high voltage transients, which may affect 
these as well as other electronic components on the card. However, the 
optical channel 37 is not affected by transient voltages, electromagnetic 
interference or radiation. 
FIG. 4 is a grounding scheme representation using the electro-optic module 
of the present invention. In a typical telephone switch, a plurality of 
cards are placed side by side on a mounting rack to form a series of 
shelves. When placed on the shelves, the cards are slid into individual 
slots to become connected to the backplane. FIGS. 4 and 5 are simple 
representations of the grounding scheme used with the electro-optic module 
embodiments of FIGS. 3 and 5. In the embodiment of FIG. 4, reference 
numeral 50 represents the removable card, having face plate 51. The face 
plate is conventionally present in equipment design practice. The plate 51 
has a conductive surface, if it is not made of metal. The electro-optic 
module has its optical connector 52 extending therethrough. The enclosure 
53 is secured to card 50 and electrically connected thereto by means of 
pins 54. The frame of the telephone switch is represented by side walls 55 
and 56 which are connected to an earth ground 59. If a high voltage 
transient is carried along the metallic liner of the optic fiber, it will 
be discharged via a physical conductive path 57 or 58 to the equipment 
frame 55 or 56 and eventually to the earth ground 59. Isolator 60 isolates 
the circuit card from any harmful effects of the transient voltages, 
without affecting the operation of the electro-optic module and its 
components. A power supply or logic ground 61 is provided for the circuit 
board and the electro-optic components. The practice of using an earth 
ground separated from a logic ground eliminates the effects of unwanted 
frame ground induced currents from affecting the supply ground and 
electro-optic components. 
FIG. 5 is a sectional view of an electro-optic module according to a second 
embodiment of the invention. As seen in FIG. 5, the module 70 is also 
comprised of an optical connector 71 and an enclosure 72 disposed over a 
circuit board 73 having connecting pins 74. Various electro-optic 
components 75 are used to convert optical energy to electrical energy, or 
vice-versa with transducer 76. However, in this embodiment, the optical 
connector 71 is comprised of a single connecting element 77 made of 
nonconductive material. A discharge path away from the enclosure 72 and 
circuit board 73 is provided by means of a conductive outer shell or liner 
78 disposed around the connecting element 77. The liner extends from the 
connecting end 79 to a point 80 along the connector away from the 
enclosure 72 and circuit board 73. Accordingly, the enclosure 72 and 
circuit board 73 are isolated from the connector by the gap created 
between the liner 78 and the edge of the enclosure. 
FIG. 6 is a grounding scheme representation using the electro-optic module 
of FIG. 5. Similarly, as with the grounding scheme arrangement of FIG. 4, 
the physical juxtaposition of elements is the same. That is, reference 
numeral 90 represents the removable card, having face plate 91. The 
electro-optic module has its optical connector 92 extending therethrough. 
The enclosure 93 is secured to card 90 and electrically connected thereto 
by means of pins 94. The frame of the telephone switch is represented by 
side walls 95 and 96 which are connected to an earth ground 99. However, 
in this embodiment, if a high voltage transient or unwanted signal is 
carried along the metallic liner of the optic fiber, it will be discharged 
via the metallic liner 100 of connector 92 through physical conductive 
path 97 or 98 to the equipment frame 95 or 96 and eventually to the earth 
ground 99. The gap 101 created between liner 100 and enclosure 93 will 
isolate the circuit card from any damaging effects unwanted signals might 
have on the module, when carried along the fiber. A power supply or logic 
ground 102 is provided for the circuit board and the electro-optic 
components. This practice eliminates the effect of unwanted frame ground 
induced currents from affecting the supply ground and electro-optic 
components.