Multiple transmitter laser link

A system and method for establishing a laser link communications system in free space includes first and second terminals which are distanced from each other on an optical path. Each terminal includes a plurality of laser transmitters which together generate a plurality of laser beams, with each of the laser beams carrying a communication signal. Further, each terminal directs its independently generated laser beams along substantially parallel paths. Though their paths are substantially parallel, the transmitted beams generally overlap in the far field, at the other terminal, where they are collected and incoherently summed for reception and analysis of the communication signal.

This application claims priority on U.S. Provisional application Ser. No. 
60/003,397 filed on Sep. 1, 1995. 
FIELD OF THE INVENTION 
The present invention pertains generally to electronic communications 
systems. More specifically, the present invention pertains to 
communications systems which transmit and receive information signals that 
are carried on laser light beams. The present invention is particularly, 
but not exclusively useful, for establishing a laser terrestial 
communications link, or a ground-satellite communications link which 
compensates for the scintillation pattern caused by wind or turbulence in 
the atmosphere. 
BACKGROUND OF THE INVENTION 
It is well known that a communication signal can be imposed upon a beam of 
laser light. Further, it is known that, a particular signal-carrying beam 
of laser light can be separated and distinguished from other beams of 
laser light which have a different wavelength or are in a different state 
of polarization. Not surprisingly, however, the use of a laser light beam 
as a communications carrier can present some difficulties. A significant 
such difficulty arises from what is known in the art as scintillation. 
It happens that a single laser beam propagating through the atmosphere 
encounters variations in the index of refraction of the air which cause 
the beam to break up into separate beamlets, or turbules. More 
specifically, the beamlets or turbules in the laser beam are caused by 
wind or turbulence in the atmosphere, and the resultant pattern of 
beamlets is called a scintillation pattern. The scintillation pattern of 
turbules on a screen changes with a time scale of milliseconds or slower 
and varies from bright spots to dark spots over a distance scale which 
depends on various atmospheric parameters. For example, over a horizontal 
path of about 10 km distance at sea level using light at a wavelength of 
780 nm, the size scale is about 10 cm. Interestingly, a similar size scale 
is encountered for light coming down from a star or a satellite to a 
telescope on the ground. On the other hand, when transmitting up from the 
ground to a satellite the size scale is found to be much larger. (For 
example many meters or tens of meters). 
Due to the scintillating pattern, a receiver aperture which is small 
compared to the size of the turbules will see light intensity signals 
which vary between bright and dark. In general, this is unacceptable. 
Thus, although these variations will average out over time scales of many 
seconds, many applications require the signals to be detected on much 
faster time scales. For instance, a CCD acquisition camera trying to 
detect the direction to a laser beacon may need to collect its signal in 1 
to 10 milliseconds, and a detector used for communication at a data rate 
of 500 megabits per second may need to collect its signal in 1 to 2 
nanoseconds. If the communications detector needs a certain signal level 
Pmin to decode the data, and the communications channel is only allowed to 
decode the data incorrectly one time in a million, then enough margin must 
be included in the transmitted intensity so that the received signal is 
below Pmin only one time in a million. While an increase in the intensity 
is one way to improve the signal carrying characteristics of a laser beam, 
another solution is to increase the size of the receive aperture. 
It is well known that some spatial averaging of the signal to reduce the 
needed margin can be accomplished by having a large receive aperture. For 
instance, a sixteen inch receive aperture for communications from a 
satellite can reduce the margin needed for signal fluctuations to about a 
factor of 5. (Compare this to the worst case of saturated scintillation 
and a very small receive aperture where a margin factor of a million might 
be needed for a one in a million error rate). However, on an uplink to a 
satellite it is not practical to put a large enough receiver on the 
satellite to help reduce the signal fluctuations. Finally, on a 
terrestrial laser communications link of a distance of about 10 km at a 
wavelength of 780 nm using a 10 cm receive aperture, a transmit power 
margin factor of about 300 is needed (with a single transmit aperture). 
Apart from increasing the intensity of a laser beam or increasing the size 
of the receive aperture, a particular approach for reducing fading caused 
by scintillation has been suggested by Bruno et al. in an article entitled 
"Diode laser spatial diversity transmitter" SPIE Vol. 1044 Optomechanical 
Design of Laser Transmitters and Receivers (1989). In light of Bruno et 
al. the present invention recognizes that having multiple, non-coherent 
transmitters separated by about the turbule size scale allows for a great 
reduction in the transmitted power margin needed by reducing the detected 
intensity fluctuations. (Transmitters with a smaller spacing are still 
better than a single transmitter, but the advantageous effect is reduced). 
The reason this comes about is that the laser transmission paths are 
uncorrelated, and the probability that multiple lasers will have a dark 
spot in their scintillation patterns at the same place at the same time 
approaches zero rapidly as the number of lasers is increased. 
FIG. 1 shows the situation for different numbers of transmitting apertures 
in the case that the receive aperture is very small and the saturation is 
completely saturated. (This is a limiting worst case and over-emphasizes 
the severity of the problem for a small number of apertures, but seems to 
agree with more detailed models for a large number of apertures). FIG. 1 
also shows the probability of measuring a particular intensity plotted 
against that intensity, with the curves normalized so that the expected 
intensity is one, and the integrated probability is one. (This 
normalization means for instance that with 16 transmit apertures, each 
laser is operated at 1/16 the power for one aperture. Being able to 
operate each laser at full power only enhances the results described 
here). As can be seen in FIG. 1, with one transmitter there is a large 
probability of detecting very small intensities. With four transmit 
apertures, the probability of measuring a small intensity has been greatly 
reduced, and with 16 apertures, the probability is reduced still further. 
Using a more detailed model taking the finite receive aperture into 
account, and realizing that the atmospheric scintillation is not 
completely saturated, we determined that for the particular case of 
terrestrial laser communications at a range of 10 km we could expect to 
need the following transmit power margins for different numbers of 
uncorrelated apertures and an error rate of one in a million: 
______________________________________ 
Number 
of separate independent paths 
Needed power margin for flucuations 
______________________________________ 
1 300 
2 60 
4 20 
8 8 
16 5 
32 3 
______________________________________ 
For the horizontal link, using a 20 cm receive aperture (covering about 
2.times.2=4 turbules) and 4 separate transmitters, we expect to see the 
benefits of 4.times.4=16 independent paths, bringing the required power 
margin for scintillation down to a factor of about 5. For an uplink from 
the ground to a satellite, where no receiver averaging is possible, using 
16 separate transmitters should achieve this same reduction in power 
margin needed to overcome atmosphere induced signal fluctuations. 
In light of the above it is an object of the present invention to provide a 
system for two-way laser link communications through free space which uses 
multiple lasers transmitting from different spatial locations (separated 
by a few to tens or hundreds of centimeters) as a source for a beacon or 
communications signal. It is also an object of the present invention to 
provide a system for two-way laser link communications through free space 
which uses multiple lasers transmitting from different spatial locations 
to reduce fluctuations at a receive aperture located at a large distance 
from the transmitters (typically 100 meters to thousands of kilometers). 
Another object of the present invention is to provide a system for two-way 
laser link communications through free space which uses multiple transmit 
apertures for the combined purposes of summing laser powers, multiplexing 
different channels without complicated beam combining optics, and 
achieving transmit intensities within eye safety limits. Further, an 
object of the present invention is to provide a system for two-way laser 
link communication through free space which reduces fluctuations in the 
received signal caused by the intervening atmosphere. Still another object 
of the present invention is to provide a system for two-way laser link 
communication through free space which is relatively easy to manufacture 
and simple to operate and which is comparatively cost effective. 
SUMMARY OF THE PREFERRED EMBODIMENTS 
A system and method for establishing a two-way laser link communications 
connection through free space includes two terminals which are positioned 
at a distance from each other and which are located on the same optical 
path. Preferably, the optical path is along a line-of-sight. Each terminal 
in the system includes a plurality of laser transmitters. These 
transmitters are separated from each other at the terminal and are placed 
in an array to generate a plurality of non-coherent laser beams. For 
purposes of the present invention, the transmitted laser beams all emanate 
from the terminal and travel therefrom along substantially parallel paths. 
Although the transmitted laser beams are substantially parallel, due to 
divergence in each of the transmitted beams they will, at least to some 
extent, overlap one another in a far field. At least two of the 
non-coherent over-lapping transmitted laser beams carry the same 
communications signal. 
A receiver at each terminal includes a reflector which has a primary mirror 
and a secondary mirror that work in concert to collect and focus the 
incoming light received from the other terminal. Specifically, the 
constituents of this received light are the non-coherent overlapping beams 
that are transmitted from the other terminal. The received light is then 
analyzed by a computer and the communications signal that is carried on 
the transmitted laser beams is reconstructed. For purposes of the present 
invention the computer can be any electrical device, simple or complex, 
which converts signals as required for operation of the system. 
As contemplated by the present invention, the system can be used either for 
establishing a terrestrial communications link or a satellite 
communications link. Thus, in order to meet the specific needs of a 
particular application, several versions of the system for the present 
invention are contemplated. First, in a version that is perhaps best 
suited for use as a satellite communications link, the reflector is 
mounted on a telescope barrel-shaped base. This base has an aperture which 
collects the incoming light for the reflector. For this first version, the 
plurality of laser beam transmitters are arrayed around the periphery of 
the aperture. In a second, more compact, terrestrial version of the system 
for the present invention, the plurality of laser beam transmitters are 
positioned to transmit their respective beams through the reflector. For 
this second version, the separation and spacing of the transmitted laser 
beams are realized by optical manipulation of the transmitted laser beams. 
In all versions of the system of the present invention, the transmit 
wavelength of one terminal can differ from the transmit wavelength of the 
other terminal. Consequently, the respective receiver of the two terminals 
will be compatible with the wavelength transmitted by the other terminal. 
Further, although the two transmit wavelengths can be actually the same, 
they can have different circular polarizations. Additionally, combinations 
of different wavelengths and different polarizations can be used.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A system for a multiple transmitter laser link in accordance with the 
present invention is shown in FIG. 2 and is generally designated 10. As 
shown, the system 10 include a first terminal 12 and a second terminal 14 
which is essentially the same as the first terminal 12. In both, there is 
a transceiver 16 and a computer 18 which is electrically connected to the 
transceiver 16 for purposes to be subsequently disclosed. FIG. 1 also 
shows that each of the terminals 12, 14 transmit laser beams 20 to each 
other through a distance 22. As intended for the present invention, the 
distance 22 may be somewhere on the order of one to fifty miles for a 
terrestrial link and, of course, much greater for a satellite link. 
In FIG. 3 a one-way communication path, as established by the system 10, is 
shown for purposes of disclosure. There it will be seen that within a 
particular terminal (e.g. terminal 12) laser drive electronics 24 provide 
drive current for separate laser transmitters 26, of which the laser 
transmitters 26a, 26b and 26c are exemplary. These laser transmitters 
26a-c are pointed by respective steering assemblies 28a, 28b and 28c, also 
mounted on support member 27a, so that the respective emanating laser 
beams 20a, 20b and 20c are all pointed approximately in the same 
direction. For purposes of the present invention the laser beams 20 are 
pointed along substantially parallel paths, recognizing that in a far 
field 30 they will, at least to some extent, overlap one another. The 
laser transmitters 26a-c can be either semiconductor diode lasers or of 
some other type of laser well known in the art. The respective steering 
assemblies 28a-c can be either independent gimbals which are useful for 
steering the lasers transmitters 26a-c separately, or there can be a 
single steering assembly 28 which has a single gimbal for collectively 
holding and steering all of the multiple laser transmitters 26a-c. In this 
latter case, lenses or mirrors (not shown) can be used for fine pointing 
of the laser beams 20a-c. The drive electronics 24 can provide a 
communications signal to be modulated on the laser beams 20a-c, or the 
drive electronics 24 can provide a constant, non-modulated signal. 
Preferably, as indicated in FIG. 3, the laser beams 20a-c are separated 
when they leave their respective transmitters 26a-c. The exact amount of 
separation depends on the particular application, and is related to the 
atmospheric scale size r0. As indicated above the separation of the 
various transmitters 26 should be about the turbule size expected for the 
particular application. For communications links across a 10 km horizontal 
path a separation of 10 cm between the various laser transmitter 26 would 
be typical. On the other hand, for providing laser tracing beacons or 
communications signals for the ground to satellites, a slightly greater 
separation of 10 to 20 cm would be typical. 
Still referring to FIG. 3, it will be seen that after leaving the 
transmitters 26a-c, the laser beams 20a-c travel along respective 
propagation paths through the intervening atmosphere 32. For the present 
invention, the laser beams 20a-c are pointed in a manner such that the 
light from each laser transmitter 26 overlaps in the far field beam region 
30. Although the amount of light in this far field region 30 from any one 
laser transmitter 26 may have large fluctuations in intensity due to 
scintillation caused by the intervening atmosphere 32, the fluctuations in 
the different laser beams 20 will be largely uncorrelated and the overall 
fluctuation in the intensity from the overlapped lasers beams 20 will be 
greatly reduced. The combined laser light from laser transmitters 26a-c is 
detected by a receiver 34 mounted on the base support member 27b in the 
other transceiver 16 of the terminal (e.g. terminal 14). 
As shown in FIG. 3, the receiver 34 is pointed in the direction of the 
laser transmitters 26a-c by a steering assembly 36. The received signal 
from terminal 12, which may be used as a pointing beacon or may be a 
communications signal, is processed by receiver electronics 38 in terminal 
14. Although the various components for system 10 are shown schematically 
in FIGS. 2 and 3 in a general form as boxes, it will be appreciated by the 
skilled artisan that these components may be produced by many methods 
which are well understood by persons skilled in the art of making laser 
links. It is important for the present invention, however, that more than 
one separate laser transmitter 26 be used. Further, it is important that 
all of the laser beams 20 which emanate from separate spatial locations in 
terminal 12 overlap in the far field 30 to achieve a reduction in signal 
fluctuations at the receiver 34 in terminal 14. Thus, the required power 
of each laser transmitter 26a-c is reduced by far more than would be 
expected from just summing the laser output powers. 
A preferred embodiment for a laser communications terminal 12, 14 in 
accordance with the present invention is shown in FIGS. 4A and 4B. In 
particular, the components for a terminal 12 as shown in FIGS. 4A and 4B 
would be appropriate for the ground end of a satellite-to-ground laser 
communications link where the signal fluctuation problems associated with 
atmospheric scintillation are caused by the atmospheric path near the 
ground terminal. As shown, this embodiment utilizes sixteen separate laser 
assemblies 40a-p of which only laser transmitter assemblies 40a and 40i 
are seen in FIG. 4A. These transmitter assemblies 40 respectively contain 
laser transmitters 26a and 26i, which expand and are approximately 
collimated by focusing optics 42a and 42i in a manner that will be 
appreciated by the skilled artisan. The focusing optics 42 may be a single 
lens, as shown, or multiple lenses of a type well known in the pertinent 
art. The laser transmit assemblies 40 may also contain other components, 
such as polarization rotators (not shown). 
As intended for the embodiment of the terminal 12 shown in FIG. 4A, the 
transmitted laser beams 20a-p emanate from different locations in the 
terminal 12. They are, however, pointed in approximately the same 
direction so that they will overlap and reduce fluctuations at a large 
distance in the far field 30 where the receiver 34 is located. Due to the 
long distances "d" which are involved, it will be appreciated that the 
laser beams 20a-p which emanate from the laser assemblies 40 of terminal 
12 will be substantially parallel to each other. As so oriented, the 
transmitted beams 20 can provide a beacon to the satellite (not shown), 
indicating where it should point lasers back to the ground, and/or can be 
used to transmit communications signals to the satellite. 
The ground laser communications terminal 12 described above can also be 
used as a receiver 34 for signals coming from a satellite. For this 
purpose, the incoming received light 44 is collected through receive 
aperture 46 that is formed by a telescope base 48. Specifically, the 
received light 44 first hits telescope primary mirror 50 and is reflected 
toward telescope secondary mirror 52. In turn, secondary mirror 52 
reflects the received light 44 towards a beam splitter 54. At beam 
splitter 54, some of the received light 44 is reflected to focus on 
tracking detector assembly 56, while the remainder of the received light 
44 passes through the beam splitter 54 to focus on communications detector 
assembly 58. 
FIG. 4B shows a front view of the relative orientation of the different 
transmit laser assemblies 40a-p arrayed around the receiving aperture 46 
of telescope base 48 of receiver 34. Even with the partial obscuration of 
aperture 46 that is caused by the secondary mirror 52, a size of sixteen 
inches for aperture 46 is large enough to enable direct averaging over 
many scintillation cells for the laser signal transmitted on received 
light 44 from the satellite. Further, this size is effective even though 
received light 44 is aberrated by the atmosphere 32 in the last 10-20 
kilometers before reaching the receiver 34. At the same time a sixteen 
inch receiver aperture 46 is small enough to be available commercially at 
relatively low cost. On the other hand, the transmit aperture sizes 
established by focusing optics 42 of about two inches for each of the 
laser assemblies 40 are large enough to provide laser divergences down to 
about forty microradians (for communications uplinks) but can also be used 
for beacon uplink divergences of about one or two milliradians. Finally, 
the use of two inch transmit apertures for laser assemblies 40a-p allows 
the use of individual transmit laser powers up to 25 mW average while 
maintaining intensities in the eye safe region below 2 mW per square 
centimeter for near infrared lasers (for instance 700-900 nm wavelength). 
Larger transmit apertures can also be used in this configuration, allowing 
higher eye-safe powers. Higher power lasers, such as semiconductor lasers 
at 150 mW or amplified semiconductor lasers at 1 W can also be used, but 
then eye safety is not reached until the lasers have propagated and 
expanded over some distance. 
Arraying the laser transmitters 26 around the receive aperture 46 allows 
for sufficient separation to allow significant fluctuation reduction at 
the satellite. As indicated above, the whole receiver/transmitter assembly 
of terminal 12 can be steered by a single gimbal apparatus. 
FIGS. 5A and 5B show another embodiment for a laser communications terminal 
12, 14 utilizing the invention which is appropriate for terrestrial laser 
communications (for instance between buildings) up to a range of about ten 
kilometers (10 km). In this case the transmitted laser beams 20 use the 
same telescope base 60 as is used for the receiver 34. More specifically, 
the laser beams 20 are combined spatially at the back 62 of the telescope 
60 so that they emanate from different separated spatial positions at the 
front 64 of the telescope 60. Specifically, in FIG. 5A it will be seen 
that the laser beams 20 which are generated by laser transmitters 26a and 
26b are collimated by respective focusing optics 66a and 66b. The 
collimated laser beams 20 are then combined together and with two other 
collimated laser beams (not shown) using a spatial combining prism 68 of a 
type well known in the pertinent art, such as a many faceted mirror. All 
of the combined transmitted laser beams 20 are then combine into the 
telescope beam path using transmit/receive beam splitter 70. In our 
preferred embodiment, the beam splitter is dichroic, separating 780 nm 
wavelength from 860 nm wavelength. Thus, while terminal 12 transmits 780 
nm and receives 860 nm, terminal 14 transmits 860 nm and receives 780 nm. 
The transmitted lasers are steered with pointing mirror 72, which can be 
controlled automatically by computer 18 to maintain alignment if desired. 
The collimated laser beams 20 then transmit through lens 74, expanding to 
secondary mirror 52 and primary mirror 50 before being transmitted out 
through the telescope aperture 46. Received signal light 44 (coming from 
the right in FIG. 5A) passes receive aperture 46, reflects off of primary 
mirror 50 and secondary mirror 52 and is collimated by lens 74. The 
receive light then reflects off of pointing mirror 72 and passes through 
transmit/receive beam splitter 70 and into receiver assembly 76, which 
contains both a communications detector and a pointing alignment detector. 
FIG. 5B shows the beam locations 78 at the front of the eight inch 
telescope aperture 46 used in this embodiment. As implied above, the laser 
beams 20a-b will then pass through aperture 46 toward the other terminal. 
In general, if diode lasers are used, the transmitted beams may be 
elliptical. Using 50 mW peak power communications transmit lasers with a 
duty factor of 50%, the beams can be expanded to meet eye safety 
standards. At the same time, the received light signal 44 passes through 
receive aperture 46 except for the part obscured by secondary mirror 52. 
FIGS. 6A and 6B show still another preferred embodiment for a laser 
communications terminal 12 utilizing the invention of system 10 in which 
more than one set of laser transmitters 26 which are combined in one 
apparatus for detection by different devices at terminal 14. In 
particular, this embodiment employs ten laser transmitters 26 of which two 
are used for an acquisition and tracking function, four are used for one 
high data rate communications channel, and the remaining four are used for 
a different high data rate communication channel. For this embodiment, the 
acquisition lasers are set at a wavelength near 850 nm, while the 
communications lasers are at wavelength near 810 nm. Further, the two 
communications channels propagate on different polarizations (i.e. left 
circular and right circular). 
Referring to FIG. 6A, by way of example, laser transmit assemblies 40a and 
40b respectively contain laser transmitters 26a and 26b, which are 
approximately collimated by respective focusing lenses 80a and 80b. The 
transmitted laser beams 20a and 20b, along with other laser beams 20 (not 
shown) are approximately co-aligned to overlap in the far field 30 at the 
receive aperture 46 of the receiver 34. Received light 44 enters the 
telescope from the right through receive aperture 46. This received light 
44 then reflects off of primary mirror 50 and secondary mirror 52 as it 
propagates to dichroic communications/tracking beam splitter 82. The 
acquisition/tracking signal at wavelength 850 nm passes through the beam 
splitter 82, through narrowband filter 84 and into tracking detector 86. 
Preferably the tracking detector 86 is a CCD camera but could also be 
another position sensitive device such as a quadrant detector which is 
known in the art. 
Unlike the tracking signals, the communications signals at 810 nm in 
received light 44 reflect off of beam splitter 82, and pass through a hole 
88 in the secondary mirror 52. At this point, the two communications 
channels are distinguished from each other in that one is right circular 
polarized and the other is left circular polarized. After passing through 
hole 88, the two communications channels are separated by a channel 
one/two beam splitter 90. For the present invention, the beam splitter 90 
preferably consists of a quarter wave plate which turns the circular 
polarizations into respective linear polarizations, and a linear 
polarization beam splitter. The separated communications channel signals 
are then respectively focused by lenses 92a and 92b onto a channel one 
communications detector 94 and a channel two communications detector 96. 
FIG. 6B provides a front view of the transmit and receive apertures for the 
preferred embodiment shown in FIG. 6A. As stated above, received light 44 
comes into the terminal 12, 14 through receive aperture 46 which though 
partially obscured by the back of secondary mirror 52 is effective for 
this purpose. FIG. 6B also shows how the transmitter 26 might be arranged 
for, this embodiment of a terminal 12 for system 10 to establish two 
different communications channels as well as a tracking capability. By way 
of example, a beacon laser light (for acquisition and tracking) is 
transmitted by two separate beacon lasers from locations such as is 
indicated for laser transmitters 26a and 26b. It should be noted here that 
even two co-aligned beacon lasers will provide a substantial reduction in 
far field intensity fluctuations as compared to one. For the 
communications channels, signals for communications channel one can be 
transmitted with right hand circular polarization from channel one 
emanation locations such as indicated for the four separate laser 
transmitters 26a', 26b', 26c' and 26d'. Signals for communications channel 
two are then transmitted on left hand circular polarization from four 
separate channel two emanation locations such as are indicated for laser 
transmitters 26a", 26b", 26c" and 26d". In this preferred embodiment, the 
data rate of each channel is around 500 megabits per second, for a 
combined data rate of one gigabit per second. This preferred embodiment 
shows that the multiple laser transmitter invention can be used more than 
once in the same device with different channels, each channel having 
multiple transmitters to reduce signal fluctuations in the far field 30 at 
the receiver 34. For the present invention, various combinations of the 
optical components disclosed above are contemplated. 
OPERATION 
In the above disclosure, there has been mention of the divergence of the 
laser beams 20 as they emanate from a terminal 12, 14. The importance of 
this divergence is, of course, based on the intention of the system 10 
that all of the laser beams 20 overlap in the far field 30. This 
divergence and overlap, however, can not be haphazard. For the present 
invention, these characteristics of the laser beams 20 must be somehow 
controlled and, therefore, made predictable. 
FIG. 7 shows a schematic of optical components which can be used to shape 
the laser beams 20 as they emanate from a terminal 12, 14. For simplicity, 
only the transmitted laser beams 20 will be considered. It is to be 
appreciated, however, that received light 44 is also present. To discuss 
the transmitted laser beams, consider the three laser transmitters shown 
in FIG. 7 and designated 26a, 26b and 26c. Consistent with earlier 
disclosure, each of these laser transmitters 26a, 26b and 26c generates a 
respective separate laser beam 20a, 20b and 20c. In this version of the 
system 20, however, the laser beam 20c is bifurcated. 
With reference to FIG. 8, it can be seen that after the laser beam 20c has 
passed through focusing optics 66c, the laser beam 20c is bifurcated by a 
beam splitter assembly 98. Specifically, a prism 100, which is mounted in 
the beam splitter assembly 98, diverts half of the beam 20c that is 
generated by laser transmitter 26c from its path. This diverted half is 
then directed toward another prism 102 where it is redirected along a path 
that is substantially parallel to the original path of laser beam 20c. 
Thus, a bifurcated laser beam is created which includes a beam 20c' that 
is separate and independent from its companion beam 20c". Importantly, 
there are now four different beams which pass through the prism 68 (shown 
in FIG. 7). These are; beam 20a, beam 20b, beam 20c' and beam 20c". The 
pattern which the paths of these beams make relative to each other is best 
seen in FIG. 9A. 
Still referring to FIG. 7, it is to be seen that the laser beams 20a, 20b, 
20c' and 20c" which pass through the prism 68, are directed from prism 68 
to next pass through a diffuser 104. As implied above, the pattern in 
which these beams hit the diffuser 104 is shown in FIG. 9A. This pattern 
is important in that it separates the beam 20 in a manner which avoids 
blockage by the secondary mirror 52 as the beams 20 collectively pass 
outwardly through aperture 46. Also important, however, is the fact that 
the diffuser 104 establishes a divergence for the individual beams 20a, 
20b, 20c' and 20c" which will cause them to overlap in the far field 30. 
A divergence for the individually transmitted laser beams 20a, 20b, 20c' 
and 20c" is established by the diffuser 104 by effectively dividing each 
of the respective laser beams 20a, 20b, 20c' and 20c" into a very large 
number of micro-beams. To do this, the diffuser 104 effectively employs a 
multitude of micro-lenses. Specifically, as best seen in FIG. 9B, the 
diffuser 104 includes a glass casing 106 which is formed with a chamber 
108. A preselected fluid is placed in this chamber 108, and a very large 
number of glass microspheres 110, each having a diameter on the order of 
about four hundred microns (400 .mu.m), are immersed in the fluid in the 
chamber 108. For purposes of the present invention, the particular fluid 
that is used in chamber 108 should be selected to be optically compatible 
with the microspheres 110. More specifically, as will be appreciated by 
the skilled artisan, the fluid is chosen to nearly match the index of 
refraction of the microspheres 110. By proper selection of this fluid, it 
is possible to obtain a divergence for the laser beams 20a, 20b, 20c' and 
20c' which is on the order of approximately one degree, or about 20 
milliradians. 
At this point, consider the characteristics of the laser beams 20a, 20b, 
20c' and 20c". As they emerge from the diffuser 104, these beams 
collectively establish the pattern shown in FIG. 9A and have a collective 
aperture on the order of ten millimeters (10 mm). Also, as just disclosed, 
the beams will have a divergence about 20 milliradians. All of this 
predictably changes as the beams 20a, 20b, 20c' and 20c" continue to pass 
through the system 10. 
After passing through diffuser 104, the laser beams 20 are turned by 
turning mirror 72 and directed through the lens 74. They are then 
reflected from secondary mirror 52 toward primary mirror 50, where they 
are reflected out through aperture 46. As the beams 20 pass through 
aperture 46, it happens that they have been expanded from an aperture of 
approximately ten millimeters (10 mm) to an aperture of approximately two 
hundred millimeters (200 mm). By optics well known by those skilled in the 
art, this nearly twenty-fold expansion in the aperture of the beams 20 
also results in an inverse diminution of the divergence in these same 
beams 20 by a factor of twenty. Consequently, the divergence angle 112 for 
beams 20a, 20b, 20c' and 20c" as they emanate from the terminal 12, 14 
will be around one milliradian (1 mrad). For most applications, this 
divergence angle 112 will ensure overlap in the far field 30. It is to be 
appreciated, however, that divergence can be controlled simply by 
constituting diffuser 104 with appropriately selected microspheres 110 and 
the proper emersion fluid. 
While the particular system for two-way laser link communications through 
free space as herein shown and disclosed in detail is fully capable of 
obtaining the objects and providing the advantages herein before stated, 
it is to be understood that it is merely illustrative of the presently 
preferred embodiments of the invention and that no limitations are 
intended to the details of construction or design herein shown other than 
as described in the appended claims.