Branch connector for light waveguides

A branch connector for light waveguides which utilizes a beam divider mirror characterized by a carrier member aligning a pair of light waveguides on a common axis, a beam divider which is arranged on a plane slanted to the common axis on an angle which is larger than 45 degrees and is positioned between the ends of the waveguides and flush therewith and a second carrier plate mounting an additional waveguide adjacent one of the pair of aligned waveguides on a second axis extending at an angle to the common axis and intersecting the common axis at the beam divider mirror with the second axis being oriented to receive light reflected along the given direction.

BACKGROUND OF THE INVENTION 
The present invention is directed to a branch connector for light 
waveguides which connector utilizes a beam divider principal. The 
connector comprises a pair of light waveguides on a common axis; a beam 
divider, which is arranged on a plane slanted to the common axis and 
extends between the ends of the waveguides which are flushed therewith, to 
reflect a beam traveling parallel to the common axis along a given 
direction, and at least one additional waveguide adjacent one of the pair 
of aligned waveguides on a second axis extending at an angle to the common 
axis and intersecting the common axis at the beam divider mirror with the 
second axis being oriented to extend along the given direction. 
Branch connectors of the above mentioned type have already been proposed in 
optical communications for use as optical multiplexers and demultiplexers. 
These branch connectors have been proposed in various compact structural 
shapes which do not require lenses. Their characterstic features are the 
use of the beam divider principal with a spectrally selective beam divider 
mirror instead of the conventional beam divider mirror. It has turned out 
that in the case of the spectrally selective beam divider mirror with an 
angular radiation of incidence, the polarization effect occurs which 
becomes larger as the angle of incidence becomes larger. These 
polarization effects limit the channel spacing which is obtainable with 
the branch connector. Articles in IEEE Transactions on Communications, 
Vol. Com-26, No. 7, July, 1978, pages 1082-1087; Fiber Integr. Opt., 2, 
1979, page 1, and Electronics Letters, Vol. 15, No. 14, July, 1979, pages 
414 and 415, disclose the work of various Japanese groups which use 
complicated arangements with lenses and reduce the disturbing polarization 
effect by means of using almost perpendicular radiation incidence or 
radiation incidence of small angles. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing a branch connector for light 
waveguides which utilizes the beam divider principal and which has reduced 
polarization effects without requiring the use of lenses. 
This task is accomplished by an improvement in a branch connector for light 
waveguides utilizing the beam divider principal, said connector comprising 
means aligning a pair of light waveguides on a common axis, a beam divider 
mirror being arranged on a plane slanted to the common axis and extending 
between the ends of the waveguides which are flushed therewith, said beam 
divider mirror reflecting a beam of light traveling parallel to the common 
axis along a given direction, and means for mounting at least one 
additional waveguide adjacent one of the pair of aligned waveguide on a 
second axis extending at an angle to the common axis and intersecting said 
common axis at the beam divider mirror, said second axis being oriented to 
extend along the given direction. The improvement comprises the beam 
divider mirror being inclined toward the common axis at an angle .beta., 
with the angle .beta. being larger than 45 degrees. 
This surprisingly simple and apparently obvious solution leads to a 
significant advantage. The polarization effects are reduced to such an 
insignificant degree that in most of the practical cases, they can be 
disregarded. Thus, the simple structure is preserved which has been 
successful in the structure with the mirror being inclined towards the 
common axis at an angle of 45 degrees. The preservation of the basic 
concept of the structure, however, has the further result that the 
proposed branched connector can be just as easily manufactured as those 
branch connectors with the beam divider mirror inclined at 45 degrees. The 
proposed branch connector can thus be manufactured just as cheaply and in 
mass production as the previously known connectors. 
As in the case of the previously known connectors, the improved branch 
connector uses glass fibers as the light waveguides and thus above all, 
both the customary core-cladded glass fibers and also gradient index 
fibers can be used. So that the radiation, which is reflected at the beam 
divider mirror, is not reflected back into the feeding fiber but rather 
can penetrate its sheath or cladding and then proceed into the additional 
light waveguide, additional conditions must be fulfilled in the proposed 
branch connector, which conditions are that the angle .beta. also 
satisfies the condition 
EQU arc sin (A.sub.N /n.sub.0)&lt;B&lt;90.degree.-arc sin (A.sub.N /n.sub.0), 
wherein A.sub.N signfies the numerical aperture of the light waveguide and 
n.sub.0 signfies the index of refraction on the axis of the light 
waveguide. This additional condition as a rule can be easily fulfilled. 
It can however also be advantageous to proceed by removing the cladding 
from the one waveguide at the location of the contact with the additional 
waveguide. However, the additional waveguide must engage the one aligned 
waveguide, which is advantageous and practical in most cases. It is also 
desirable that the end face of the additional waveguide be on a plane 
which is slanted to the axis of the waveguide and is parallel to the 
common axis of the aligned waveguides. These last two features are 
favorable features for most cases. 
In view of the ability to produce branch connectors as proposed 
hereinabove, it is as simple and practical as possible to form embodiments 
in which the means for mounting the aligned pair is a carrier body which 
mounts a plurality of aligned pairs which have equal spacing and extend 
parallel to one another and the means for mounting the additional 
waveguide is a carrier body which mounts a plurality of additional 
waveguide all extending parallel to each other at the same spacing so that 
an additional waveguide is aligned with each of the aligned pairs and the 
one beam divider mirror is common with each of the aligned pairs. It is 
also possible to subdivide either one of the pair of aligned waveguides by 
an additional beam divider mirror and to provide this additional beam 
divider mirror with further waveguides which extend on an axis for 
receiving light reflected by the additional beam divider mirror which is 
also at an angle other than 45 degrees with the fibers. In this additional 
case, a plurality of elements can be formed side-by-side. 
All manufacturing methods, which are known and which have been proposed in 
connection with light waveguide branch connectors with a beam divider 
mirror which was inclined at 45 degrees to the aligned pair of waveguides, 
can be used in the proposed branched connector. The only changes would be 
due to the newly determined angle of inclination of the guide grooves with 
respect to one another and the necessary changes of the inclination of the 
cut edges of the relative carrier bodies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The principles of the present invention are particularly useful in a branch 
connector generally indicated at 100 in FIG. 1. The connector 100 includes 
a pair of core-cladded fiber 1 and 1', which are aligned on a common axis 
10. The two glass fibers 1 and 1' are separated from one another by means 
of a beam divider mirror 3 which is arranged on a plane slanted relative 
to the common axis 10. The beam divider mirror 3 is spectrally selective 
beam divider mirror, for example an optical cut-off filter. The branch 
connector 100 according to FIG. 1 is illustrated with the plane of the 
beam divider mirror 3 being perpendicular to the drawing plane so that 
only a trace of the mirror 3 is visible. It can be seen from FIG. 1, that 
the angle .beta. by which the mirror 3 is inclined to the common axis 10 
is larger than 45 degrees which means that the mirror is inclined 
relatively steeply towards the common axis 10. 
A light beam traveling in the direction of arrow 41 in the waveguide 1 
along the axis 10 will strike the mirror 3 whereby a portion will pass 
through the mirror into the waveguide 1' as indicated by the arrow 42. 
Another portion is reflected by the mirror 3 in the direction as indicated 
by arrow 43 along a dot-line 20 which line 20 cuts the common axis 10 at 
the mirror 3 and forms an angle .alpha. with the axis 10, which angle 
.alpha., as illustrated, is less than 90.degree.. 
Adjacent to the glass fibers 1 and 1', and slanted with respect to them is 
an additional core-cladded glass fiber 2 which is an additional light 
waveguide. This additional glass fiber 2 is to be arranged so that its 
axis extends parallel to the line 20 and preferably the axis of the fiber 
2 coincides with the line 20. 
In the manufacture of the branch connector illustrated in FIG. 1, one 
should attempt to have the axis of the additional light waveguide 2 
coincide with the line 20 to the greatest extent possible. In practice, 
however, this is possible only within allowable working tolerances which 
can be relatively large. The same applies in general for the aligned pairs 
of glass fibers 1 and 1'. Therefore, when the terms or statements aligned 
waveguides, common axis, intersection of the two axes in a point and 
orienting the axis in a specific direction are used, this signifies that 
these apply within allowable work tolerances. Within these tolerances, 
relative displacement and inclinations are allowable. In the device of 
FIG. 1 the ideal case is depicted that is the line 20 is simultaneously 
the the axis of the additional fiber 2 and is thus also designated as such 
in the following. 
An end face 4 of the additional fiber 2, which end face is turned towards 
the pair of aligned fibers 1 and 1', is cut or provided with a slanted 
surface and specifically in such a manner that the end face 4 lies in a 
plane that extends parallel to the common axis 10 of the aligned fibers 1 
and 1'. Thus, favorable relationships are created for the coupling over of 
the reflected beam into the additional fiber 2. 
The branch connector 100 can be used as a multiplexer or as a demultiplexer 
for two wavelengths .lambda..sup.(1) and .lambda..sup.(2). With 
.lambda..sup.(1) is designated as a wavelength which will pass through the 
mirror 3 whereas the .lambda..sup.(2) is reflected by the mirror 3. If the 
mirror 3 for example is a short wave transmitting cut-off filter, then the 
wavelength .lambda..sup.(1) is shorter than the wavelength 
.lambda..sup.(2). If the mirror 3 is a long wavelength transmitting 
cut-off filter, then the wavelength for .lambda..sup.(1) is longer than 
the wavelength .lambda..sup.(2). 
In FIG. 1, the laws of refraction are disregarded. The sheath or cladding 
of the fiber 1 has an index of refraction which is lower than the index of 
refraction for the core of the fiber. Thus, a light being reflected by the 
mirror is refracted at the inner face or boundary layer between the core 
and the cladding. The second refraction proceeds at the transition from 
the cladding of the fiber 1 to the core of the additional fiber 2. If the 
cores of the fibers 1 and 2 have the same index of refraction, then the 
reflected axial beam continues in the core of the additional fiber 2 in 
the same direction as in the core of the fiber 1. If this is not the case, 
then the axis of the fiber 2 would have to be oriented in the direction in 
which the reflected beam would spread in the case of the assumption of a 
different core index of refraction of the fiber 2. 
So that the radiation approaching from the left in the fiber 1 which is 
reflected at the mirror 3 is not reflected back into the fiber 1 but 
rather penetrates its cladding and can proceed into the additional fiber 
2, the angle .beta. must additionally satisfy the following condition: 
EQU arc sin (A.sub.N /n.sub.0)&lt;.beta.&lt;90.degree.-arc sin (A.sub.N /n.sub.0) 
wherein A.sub.N signfies the numerical aperture of the fiber and n.sub.0 
signifies its index of refraction on the axis. 
For the fiber types which are of interest at the present time, there 
results the following maximum permitted values for the angle .beta.: 
______________________________________ 
Fiber Type A.sub.N n.sub.0 
.beta..sub.max 
______________________________________ 
Gradient fiber 
0.18 1.46 83.degree. 
Thick core fiber 
0.4 1.6 73.5.degree. 
______________________________________ 
If the glass fiber cladding of the fiber 1 or 1' causes interference at the 
location of the contact with the additional fiber 2, then it can be 
partially or completely etched away. 
In FIG. 2 a practical construction of several branch connectors according 
to FIG. 1 is illustrated and generally indicted at 30. The device 30 
includes a carrier body 31 which forms means for aligning several pairs of 
glass fibers 1 and 1' along common axes which are parallel to one another 
and lie in the same plane. The aligned fibers 1 and 1' of each pair are 
separated by one common mirror layer 3' which also is arranged in the 
device 30. The mirror layer 3' forms an angle .beta. with the plane in 
which the pairs of aligned glass fibers lie. Several additional glass 
fibers 2 are arranged next to one another in the device 30 to lie in a 
different plane and to extend parallel to one another. The additional 
glass fibers 2 in each case meet a pair of aligned glass fibers 1 and 1' 
with the plane of the additional glass fibers forming an angle .alpha. 
with the plane of the aligned pairs and intersecting the plane of the 
aligned pairs in the mirror layer 3'. The angle .alpha. is determined by 
the angle .beta. and is an angle between a beam traveling in the aligned 
waveguides 1 and the reflected beam by the mirror 3'. 
The device 30 can be formed by the method steps which are illustrated in 
FIGS. 3a-3d. In the first step, a silicon sheet such as 6, (FIGS. 3a and 
3aa) is anisotropically etched to form V-shaped grooves 16 in which the 
glass fibers such as 5 are cemented. In a similar manner, a silicon sheet 
6a has V-shaped grooves 16a formed in a surface and fibers 5a are cemented 
therein. The silicon sheet 6 is cemented to a glass plate 7 in such a 
manner that the furrow side or the side having the groove 16 is turned 
away from the plate. Thus, the initial part shown in FIGS. 3a and 3aa is 
created. As illustrated in FIG. 3b, a pair of glass prisms 8 and 9 which 
have surfaces extending at an angle .alpha. are provided and arranged with 
the sheath 6a cemented therebetween. This forms an initial part 17 (FIGS. 
3b) with the grooved side of the silicon sheet 6a is facing the prism 9. 
The initial part 17 is cut on a plane formed by the lines I--I into two 
parts 18 and 19. After the cutting step, the cut surfaces of the upper 
part 18 are polished. The upper part 18 with its polished surface is 
cemented to the sheet 6 specifically in such a manner that the glass 
fibers 5a which lie in parallel grooves 16a meet and engage the glass 
fibers 5 which also lie in parallel grooves 16 to produce the unit or 
immediate part 20 of FIG. 3c. The part 20 is then cut along a plane II--II 
to form two parts whose cut surfaces are then polished. Then a partially 
permeable mirror layer 3' is applied. This layer may be applied for 
example, by a vacuum deposition process of a wavelength-selective 
dielectric multilayer system. The two parts are then brought together in 
the final stages illustrated in FIG. 3d and cemented to form the part 30 
with the waveguides aligned on common axes. The part containing the sheet 
6a with the fibers 5a corresponds to the fibers 2 in FIG. 2 and the 
aligned fibers 5 in the sheath 6 corresponding to the aligning pairs of 
fibers 1 and 1'. 
If the part 30 of FIG. 3d is cut by means of vertical cuts which extend 
parallel to the sheet of the drawing between the pair of aligned fibers, 
then individual branch connectors such as illustrated in FIG. 1 will be 
obtained. With the method just described, ten to twenty branch connectors 
or more can be manufactured without difficulty. It is important that in 
the case of the above method, that the grooves in the two silicon sheets 6 
and 6a are manufactured with the same method. 
In FIG. 4, a cascade of three branch connections similar to those of FIG. 
3d are illustrated. Losses occur in the transmitting channels only at each 
of the beam divider mirrors 3". As illustrated, the fibers extending on 
the common axis have been subdivided into sub-waveguides by the beam 
divider mirrors 3" and each of the beam divider mirrors 3" has been 
provided with an additional waveguide extending at an angle therefrom. 
Several branch connectors are arranged in reflection, for example, as 
illustrated in FIG. 5 as a device generally indicated at 50. A first 
connector 51 has an additional waveguide 52 which is connected at 13 to 
one of the aligned waveguides 53 of second connector 54. An additional 
waveguide 55 of the connector 54 is connected at 12 to one of the aligned 
waveguides 56 of a third connector 57. In this device 50, additional 
losses cannot be avoided at the transition points such as 12 and 13 
between the reversals of the orientation of the V-shaped grooves which 
will occur. 
Through a combination of arrangements depicted in both FIGS. 4 and 5, a 
tree-like structure can be obtained which is particularly desirable for 
use as a multiplexer and/or demultiplexer. It should be noted that in all 
complex arrangements, for reasons of space, it is practical to work with 
fiber tips or tails. 
Although various minor modifications may be suggested by those versed in 
the art, it should be understood that we wish to embody within the scope 
of the patent granted hereon, all such modifications as reasonably and 
properly come within the scope of our contribution to the art.