Coupling for a light-conducting fiber

A coupling for a light-conducting fiber, including a metal coupling body, a first device for holding a light-conducting fiber disposed in one side of the metal body and, a second device for holding an opto-electronic transmitting element provided in the other side of the metal body. An opto-electronic transmitting element located in a housing is thermally coupled to the coupling body via a metal cap placed on the housing. As a result of this coupling, the thermal loading of the transmitting element is considerably reduced. For purposes of electrical isolation, an insulating layer can be additionally applied between the metal cap and the coupling body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a coupling for a light-conducting fiber, including 
a metal body, in which is disposed a first device for holding an end piece 
of a light conducting fiber, a housing for an opto-electronic transmitting 
element, and a second device for holding the opto-electronic transmitting 
element in its housing such that an exit opening of the element is 
opposite an end face of the fiber. 
2. Description of the Prior Art 
Couplings of the abovenoted type have been known for some time from the 
technology of optical data transmission. The opto-electronic transmitting 
element used has an optical axis extending in a direction which is the 
preferred direction for emitting the light signals. The light-conducting 
fiber, having an end face into which the optical signals are to be 
coupled, also has its own optical axis which is fixed by a device in the 
coupling. For protecting the active part of the opto-electronic 
transmitting element, the latter is built into a closed housing into which 
an exit opening for the light signals is recessed, mostly at the front. 
Because of the production conditions, displacements of up to 0.5 mm usually 
occur in the opto-electronic transmitting elements in the housing between 
the optical axis and the housing axis, which displacements must be taken 
into consideration when installing the completed element into a coupling. 
In order to achieve optimum coupling of light into the light-conducting 
fiber, it is essential, therefore, to align the transmitting element in 
the coupling with reference to the optical axis of the light-conducting 
fiber and, following that, fix the transmitting element in the aligned 
position. 
Usually, this is done by centering the transmitting element in its housing 
in the holding fixture of the coupling, which has sufficient clearance 
both in the axial and in the radial direction, having regard to optimum 
coupling and then potting it with an adhesive. 
In a coupling produced in this manner, the housing of the transmitting 
element is joined to the coupling body only via the adhesive or at the 
most via gas-filled cavities created during the potting. This adds to the, 
in most cases desired, electrical insulation of the transmitting element 
with respect to the coupling body, and to a thermal insulation which 
impedes dissipation of the heat, produced in the transmitting element, to 
the metallic coupling body. This structure can lead to thermal overloading 
of the transmitting element, especially under extreme environmental 
conditions. Since thermal overloading, however, considerably reduces the 
life of the transmitting element and thus causes outages in the operation 
of whole transmission sections, ways must be found for reducing this risk 
factor in optical data transmission. Since the couplings are used in 
greater numbers in a transmission network, the required improvements must 
also not have any significant effect on the production costs. 
SUMMARY OF THE INVENTION 
Accordingly, the objects of this invention are to provide a novel coupling 
for a light-conducting fiber which ensures improved heat dissipation from 
the opto-electronic transmitting element to the coupling body and the 
production of which is at the same time simple and inexpensive. 
These and other objects are achieved according to the invention by 
providing a novel coupling of the type mentioned initially, wherein the 
opto-electronic transmitting element is fitted into a metal cap, the metal 
cap has a through-hole for the optical connection between the exit opening 
of the housing for the opto-electronic transmitting element and end face 
of the fiber, the opto-electronic transmitting element is thermally 
coupled via the metal cap to the coupling body in order to improve heat 
dissipation, and the thermal resistance between the housing and the 
coupling body is less than 100.degree. C./W. 
The additional metal cap which absorbs the heat flow from the housing of 
the opto-electronic transmitting element and passes it on to the coupling 
body has the special advantage that it can be adapted to a plurality of 
commercially available transmitting-element housings and coupling bodies 
both with regard to geometric dimensions and with regard to the thermal 
properties of the material, and is very simple and inexpensive to produce. 
In addition, the metal cap can be used, in cases in which a specified 
distance is required between the end face of the light-conducting fiber 
and the transmitting element, directly for fixing this distance. 
In particular, the metal cap makes it possible to provide, independently of 
the nature of the transmitting-element housing, at the front face of the 
latter, a heat-transfer area which can be optimized with regard to heat 
transfer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 there is shown an illustrative embodiment of a 
coupling with improved heat dissipation. The coupling contains as the main 
part a metal coupling body 1 into which a first device for holding the end 
piece of a light-conducting fiber 4 is recessed. This first device is of 
different construction in the various couplings on the market and will 
not, therefore, be described in closer detail. An end face 14 of the 
light-conducting fiber 4 fixed in the first device ends inside the 
coupling body 1 at about the same level as the bottom 15 of a blind hole 
11 which extends from the other side into the coupling body 1. The hole 11 
makes the end face 14 optically accessible through an opening in the 
bottom 15. 
The blind hole 11 is used for accommodating an opto-electronic transmitting 
element 3 which is housed, for the protection of an active part thereof, 
in a largely closed housing 12. The housing 12 usually consists of sheet 
metal with a refined surface but can also be produced of another material, 
for example synthetic resin, in special cases. At the front of the housing 
12 an exit opening 13 is located for the light signals emitted. For the 
focussing of divergent light beams, a focussing lens 8 having a 13 the 
coupling focus of which lens is located outside the housing 12 in the end 
face 14 is frequently provided in the exit opening 13. Connecting wires 16 
for the electric supply of the opto-electronic transmitting element 3 are 
carried outside through a housing bottom 17 located opposite to the exit 
opening 13. 
The housing 12 of the opto-electronic transmitting element 3, preferably a 
transmitting diode, is fitted into a metal cap 2. This fitting is 
preferably executed as a light press fit in order to ensure good heat 
transfer between housing 12 and metal cap 2. In this arrangement, the 
housing 12 is recessed as far as possible into the metal cap so that the 
predominating part of the housing surface is in contact with the metal 
cap. It is also favorable to use a part of the housing end faces for heat 
dissipation. 
The metal cap 2 consists of a material which, on the one hand, has a heat 
conductivity which is as high as possible and, on the other hand, matches 
the material of the housing 12 with regard to heat expansion. Thus, if the 
housing 12 is manufactured, for example, of brass plate, good results are 
achieved with a metal cap 2 of brass or copper. The matching of the 
coefficients of expansion prevents the heat transfer between housing 12 
and metal cap 2 from deteriorating with greater temperature fluctuations. 
Towards the front, the metal cap 2 is provided with a through-hole 9 
through which the light signals can pass from the exit opening 13 to the 
end face 14 of the light-conducting fiber 4. The front face of the metal 
cap 2 is thermally coupled to the bottom 15 of the blind hole 11. In the 
illustrative embodiment of FIG. 1, this coupling takes place via an 
electrically insulating layer 6 and a subsequent layer 5 of a 
heat-conducting paste. In other cases, however, another layer of 
heat-conducting paste can also be provided between the insulating layer 6 
and the front face in order to even out uneveness in the adjacent 
surfaces. In every case, the thermal resistance between the housing 12 and 
the coupling body 1 should be less than 100.degree. C./W in order to 
ensure reliable dissipation of the heat from the transmitting element to 
the environment. 
The insulating layer 6 consists of an electrically insulating material 
having a comparatively good thermal conductivity such as, for example, 
beryllium oxide or mica. Such insulating layers are known to the expert 
from power electronics and are there used as an insulating intermediate 
layer between a power semi-conductor and its heat sink. Similarly, plastic 
foils can be used, which are commercially available under the name of 
"MYLAR". The use of foils sold by Thermalloy Inc. under the tradename 
"THERMAFILM" has been particularly successful and they meet the American 
Military Specification Mil-P 46 112. In applications in which a dielectric 
strength of the insulation of the order of magnitude of only a few volts 
is sufficient, it is particularly advantageous to construct the insulating 
layer 6 in a thickness of less than 0.1 mm in order to keep the influence 
of the layer on the heat transfer as small as possible. 
For the same reasons, the layer 5 of heat-conducting paste should also be 
as thin as possible and its thickness should be selected to be such that 
the surface roughnesses and unevennesses of the adjoining heat-transfer 
areas are reliably evened out and a good match is achieved over the whole 
available area. These requirements are met if, before assembly of the 
coupling, the side of the insulating layer 6 facing the light-conducting 
fiber 4 is thinly and evenly covered with the heat-conducting paste and 
after insertion of the metal cap 2 into the blind hole 11, the metal cap 
with the intermediate layers 5 and 6 is pressed with light pressure in the 
direction of the optical axis against the coupling body 1 and is fixed 
under pressure by means of an adhesive. 
As a heat-conducting paste, one of the pastes known to the expert from 
power electronics is again suitable. The silicon-based heat-conducting 
pastes are particularly advantageous because of their thermal and chemical 
properties. It is also possible to use a high-viscosity heat-conducting 
oil instead of a paste. 
The opto-electronic transmitting element 3 and the enclosing metal cap 2 
are fixed in the blind hole 11 of the coupling body 1 by filling adhesive 
7 into the spaces, remaining free after the alignment, between the metal 
cap and the coupling body. The selection of the adhesive with regard to 
thermal stability, thermal behavior and the necessary hardening process 
can be made by the expert in accordance with the requirements which are 
created by the later application of the coupling and which are affected by 
the limiting parameters of the opto-electronic transmitting element used. 
Thus, the possibility exists, for example, of using a metal-filled epoxy 
resin in order to achieve additional improvement in the lateral heat 
transfer to the coupling body. 
In the illustrative embodiment shown in FIG. 1 in which a focussing lens 8 
is used in the exit opening 13, optimum coupling of light into the end 
face 14 is achieved only if a specified distance is established between 
the end face and the plane of the lens. Whereas with the previously known 
couplings, this distance was first set during the alignment and then fixed 
by the adhesive, the setting is here advantageously already predetermined, 
and also fixed, by the specially selected dimensions of the metal cap 2, 
taking into consideration the thickness of any insulating layers 6. This 
limits the alignment process to the axes perpendicular to the optical axis 
and considerably facilitates this process. 
Overall, it has been found to be advantageous to select the thermal 
resistance between the housing 12 and the coupling body 1, which is 
determined by the type of the heat-transfer areas and the metal cap 2, to 
be smaller than or approximately equal to the thermal resistance between 
the opto-electronic transmitting element 3 and its housing 12. The total 
thermal conduction from the transmitting element to the environment is 
then essentially determined only by the predetermined, internal structure 
of the transmitting element in its housing and can no longer be decisively 
improved by external measures. 
The illustrative embodiment of FIG. 1 shows the special case of a coupling 
with electrical isolation between the housing 12 of the opto-electronic 
transmitting element 3 and the coupling body 1. However, if such 
electrical isolation is not necessary, the insulating layer 6 can be 
omitted and the metal cap 2 coupled to the coupling body only via the 
layer 5. Similarly, other shapes of the metal cap 2 are advantageous, in 
modification of the shape of FIG. 1, if the coupling body 1 and the 
housing 12 are constructed in a different manner. This modification can be 
carried out without difficulty by the expert within the context of the 
present invention. 
EXAMPLE 
Infrared transmitting diodes of the ASEA type 1a 124 and 1A 137, emitting 
light at 860 and 900 nm, respectively, were used to carry out tests in 
which the diodes, on the one hand, were molded in previously disclosed 
manner into the coupling body and, on the other hand, were thermally 
coupled to the coupling body via a metal cap, a 0.06 mm thick "Thermafilm" 
foil and an intermediate layer of heat-conducting paste. In all cases, the 
diodes were operated with a current of 100 mA and consumed a power of 
about 150 mW. 
Without metal cap and thermal coupling, the housing temperature rose to 
over 50.degree. C., starting from about 20.degree. C. at switch-on, and 
did not reach a steady-state value. 
With the thermal coupling via the metal cap and identical starting 
conditions, an increase in housing temperature by only 3.degree. C. was 
measured. This increase settled as a steady-state value. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.