Transmission and reception module for a bidirectional optical communication and signal transmission

A transmission and reception module for a bidirectional optical communication and signal transmission includes a light transmitter having a first lens coupling optical element, and a light receiver. A fiber connection has a second lens coupling optical element for a common optical waveguide. A beam splitter is disposed in a free beam path. A common housing surrounds the foregoing. The light transmitter, the beam splitter and the fiber connection as well as the light receiver which is orthogonal thereto, are disposed axially symmetrically. An optical axis of the first lens coupling optical element is axially offset from an optical axis of the light transmitter, an optical axis of the second lens coupling optical element is axially offset from the optical axis of the fiber connection, the end surface of the fiber connection, given optimal coupling-in of light, has an angle of inclination relative to its optical axis, and the beam splitter is inclined relative to an axis of symmetry of the configuration, such that backreflected radiation strikes neither a radiation-active part of the light transmitter nor a radiation-sensitive part of the light receiver.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a transmission and reception module for a 
bidirectional optical communication and signal transmission, having a 
light transmitter with a first lens coupling optical element, a light 
receiver, a fiber connection with a second lens coupling optical element 
for a common optical waveguide, and a beam splitter disposed in a free 
beam path, which are all surrounded by a common housing, the light 
transmitter, the beam splitter and the fiber connection, as well as the 
light receiver which is orthogonal to them, are disposed axially 
symmetrically. 
One such bidirectional module with free beam guidance is known from 
European Patent Application 0 238 977 A2, corresponding to U.S. Pat. No. 
4,767,171. In that transmission and reception module for a bidirectional 
communication network, two spherical lenses are spaced apart and disposed 
essentially between a laser diode and one end of an optical fiber in order 
to focus the laser light on the fiber end. Disposed between the spherical 
lenses is a beam separator or a beam splitter, which splits light, 
projected from the fiber end divergently toward the lens remote from the 
transmitter, which light is formed into a beam by that lens, at a 
wavelength other than the wavelength of the laser light, from the beam 
path. The split path is directed it to a detector or light receiver. 
A special embodiment of such a module, in which the light transmitter and 
the light receiver, each forming an independent component, are surrounded 
by a hermetic capsule and inserted into a common housing with the beam 
splitter, is described in European Patent Application 0 463 214 A1, 
corresponding to U.S. Pat. No. 5,127,075. 
It is also known from European Patent Application 0 400 161 A1, 
corresponding to U.S. Pat. No. 5,066,089, to reduce troublesome 
backreflections, in a configuration for optical coupling of an 
electrooptical converter module with an optical waveguide, without 
reducing the coupling efficiency of the configuration, through the use of 
two lenses, the first of which is disposed upstream of the converter 
module and the second of which is disposed upstream of the optical 
waveguide. The optical axis of the first lens is axially offset from the 
optical axis of the converter module, the optical axis of the second lens 
is axially offset from the optical axis of the optical waveguide, and the 
optical axes of the first lens and the second lens are axially offset from 
one another. 
It is known that the proportion of reflection, which repeatedly occurs at 
the optical boundary surfaces in modules with optical free beam guides, 
and which reduces the transported light output, can also be reduced to 
proportions of .ltoreq.1%, by coating the optical boundary surfaces. 
Nevertheless, in an electrooptical bidirectional module, two problems of a 
special type still occur, which negatively affect the output capacity of 
the module. First, high-power laser diodes as light transmitters are 
extremely sensitive to the smallest proportions of negative feedback light 
(.ltoreq.0.01% =-40 dB) and react with increased noise. Second, in 
bidirectional modules and especially modules that use a wavelength, all of 
the reflected proportions of light (&lt;1%=&lt;-20 dB) that arrive at the 
optical beam path contribute substantially to undesirable crosstalk 
between the transmission and reception channels. 
An attempt has already been made to counteract this problem by providing 
all of the optical boundary surfaces with the best possible, and therefore 
expensive, coating. However, those coatings are very difficult or 
expensive to apply, especially to fiber boundary surfaces. Among other 
reasons, the fiber boundary surfaces are in particular located precisely 
in the most optically sensitive focused beam region, where backreflection 
into the optical beam path is highest. Moreover, the reduced reflection 
(&lt;1%) achieved by the coating is not enough for the full power of the 
particular module to be reached. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a transmission and 
reception module for a bidirectional optical communication and signal 
transmission, which overcomes the hereinafore-mentioned disadvantages of 
the heretofore-known devices of this general type and in which, at an 
optimal optical coupling efficiency, both troublesome backreflections and 
undesired crosstalk are substantially reduced. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a transmission and reception module for a 
bidirectional optical communication and signal transmission, comprising a 
light transmitter with a radiation-active part and an optical axis, the 
light transmitter having a first lens coupling optical element with an 
optical axis; a fiber connection with an end surface and an optical axis, 
the fiber connection having a second lens coupling optical element with an 
optical axis for a common optical waveguide; a beam splitter disposed in a 
free beam path; a light receiver wish a radiation-sensitive part, the 
light receiver being orthogonal to the light transmitter, the beam 
splitter and the fiber connection; and a common housing defining an axis 
of symmetry and surrounding the light transmitter, the first optical 
element, the fiber connection, the second optical element, the beam 
splitter and the light receiver; the light transmitter, the beam splitter, 
the fiber connection and the light receiver being disposed axially 
symmetrically; the optical axis of the first lens coupling optical element 
being axially offset from the optical axis of the light transmitter, and 
the optical axis of the second lens coupling optical element being axially 
offset from the optical axis of the fiber connection, said end surface of 
the fiber connection having, at a given optimal coupling-in of light, an 
angle of inclination relative to the optical axis of the fiber connection, 
and the beam splitter being inclined relative to the axis of symmetry, so 
that backreflected radiation strikes neither the radiation-active part of 
the light transmitter nor the radiation-sensitive part of the light 
receiver. 
In accordance with another feature of the invention, the housing has an 
inner wall surface to which the beam splitter is secured. 
In accordance with a further feature of the invention, the second lens 
coupling optical element is a spherical lens being adjusted and fixed to 
the inner wall surface of the housing in front of the fiber connection. 
In accordance with an added feature of the invention, there are provided 
hermetically sealed capsules, the light transmitter and the light receiver 
each forming an independent component being inserted into the common 
housing and surrounded by a respective one of the hermetically sealed 
capsules. 
In accordance with an additional feature of the invention, the first lens 
coupling optical element is a lens being integrated into a carrier chip 
and is secured in front of the light transmitter. 
In accordance with a concomitant feature of the invention, the optical axis 
of the first lens coupling optical element is axially offset relative to 
the optical axis of the light transmitter, and the optical axis of the 
second lens coupling optical element is axially offset relative to the 
optical axis of the fiber connection, causing divergent radiation 
generated by the light transmitter to emerge from the first lens coupling 
optical element at a first tilt angle of approximately 3.degree. relative 
to the axis of symmetry and to emerge from the second lens coupling 
optical element at a second tilt angle of approximately 5.5.degree. 
relative to the axis of symmetry; and the end surface of the fiber 
connection is adapted to the second tilt angle and is inclined at an angle 
of inclination of approximately 12.degree. from the vertical relative to 
the optical axis. 
The advantages attained with the invention are in particular that on one 
hand the noise of the light transmitter from backreflection is prevented, 
and that on the other hand the problem of crosstalk between the light 
transmitter and the light receiver is minimized (less than -30 dB). These 
two substantial advantages complement one another. In other words, if the 
non-axial beam geometry in the light transmission direction is adjusted to 
optimal light coupling-in, then at the same time the proportion of light 
fed back to the receiver diode is minimal. Another advantage is that 
because of the optical beam guidance provided, a symmetrical and therefore 
more economical modular construction of the individual elements in the 
interior of the modular housing is possible. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
transmission and reception module for a bidirectional optical 
communication and signal transmission, it is nevertheless not intended to 
be limited to the details shown, since various modifications and 
structural changes may be made therein without departing from the spirit 
of the invention and within the scope and range of equivalents of the 
claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the single figure of the drawing in detail, there is seen 
a bidirectional transmission and reception module that substantially 
includes a light transmitter 3 which has a first lens coupling optical 
element 1 and is preferably a laser diode, a light receiver 6 which, for 
instance, is a photodiode, a fiber connection 4 which has a second lens 
coupling optical element 2 for a common optical waveguide, and a beam 
splitter 5 disposed in a free beam path, all of which are surrounded by a 
common housing 7. The light transmitter 3, the beam splitter 5 and the 
fiber connection 4, as well as the light receiver 6 which is disposed 
orthogonally to them, are all disposed axially symmetrically. 
The beam splitter 5 is suitably secured to an inner wall surface of the 
housing 7. The second lens coupling optical element 2, which preferably is 
a spherical lens, is adjusted and secured upstream of the fiber connection 
4 on the inner wall surface of the housing 7. 
The light transmitter 3 and the light receiver 6, preferably each forming 
an independent component and being surrounded by a hermetically sealed 
capsule 8, are built into the common housing 7. 
In this preferred exemplary embodiment, the first lens coupling optical 
element 1 which is in the form of a convex lens integrated into a carrier 
chip, such as a silicon chip, is secured in front of the light transmitter 
3. 
The optical axis of the first lens coupling optical element 1 is axially 
offset from the optical axis of the light transmitter 3, and the optical 
axis of the second lens coupling optical element 2 is axially offset from 
the optical axis of the fiber connection 4, the end surfaces of the fiber 
connection 4 form an angle of inclination to its optical axis, and the 
beam splitter 5 is inclined to the axis of symmetry of the configuration 
in such a way that, backreflected radiation strikes neither a 
radiation-active part of the light transmitter 3 nor a radiation-sensitive 
part of the light receiver 6. 
A free beam guidance between the electrooptical components, such as the 
light transmitter 3, the light receiver 6 and the fiber connection 4, is 
accordingly not located in the optical axis of the individual components 
that is determined by their geometrical symmetry. This results in the lens 
coupling optical element 1 in front of the light transmitter 3 being 
maladjusted laterally out of the beam axis and fixed, so that the 
radiation emerging from the lens coupling optical element 1 leaves an 
independent transmitter housing part 9 at a tilt angle of preferably 
approximately 3.degree. from the mechanical axis. This radiation thus also 
no longer strikes a window 10 of the transmitter housing part 9 
perpendicularly, nor is it reflected back upon the light transmitter 3 
itself. This radiation, which is extended onward at the tilt angle of 
approximately 3.degree., then strikes the beam splitter 5 at such an 
orientation that it is diffracted back again to the axis of the common 
housing 7. As a result, an expensive, non-symmetrical housing is not 
necessitated by the preselected tilt angle. In other words, the common 
housing 7 can remain symmetrical, with the optical elements indicated. 
In further beam guidance for coupling in or projection by the lens coupling 
optical element 2, the radiation is guided through the lens 2 in such a 
way that in the beam direction downstream of this lens 2, it undergoes a 
negative tilt angle from the mechanical axis (axis of symmetry) of 
approximately 5.5.degree.. The radiation is thus deflected back toward the 
axis of symmetry to its focal point and at that location strikes the 
intensive-reflection end surface of the fiber connection 4. This surface, 
which is then adapted to the tilt angle of approximately -5.5.degree., is 
inclined from the optical axis by approximately 12.degree. from the 
vertical. Thus in obedience to the law of refraction, the maximum 
proportion of the light or laser beam is coupled into the fibers, that is 
nearly the same proportion of light as in the case of vertical arrival at 
the end surface of the fiber. 
The oblique or inclined end surface of the fiber connection 4, which 
conventionally, even in other optical waveguide components, prevents 
backreflection to the laser diode or back into the path, in this case in a 
bidirectional module not only contributes to eliminating backreflection 
but also to preventing crosstalk. Firstly, due to the angle of inclination 
of the fiber end surface, the arriving oblique radiation is reflected by 
twice its angle from the optical axis and thus is effectively moved away 
from the optical axis toward the light transmitter 3. Secondly, however, 
the reflected portion is also reflected out of the reception range of the 
light receiver 3 by twice the reflection angle as compared with the 
incident angle. On one hand, this prevents noise of the light transmitter 
from backreflection, and on the other hand it minimizes crosstalk between 
the light transmitter 3 and the light receiver 6.