Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR OPTICAL FREE SPACE TRANSMISSION SYSTEMS
FIELD OF THE INVENTION
The present invention relates to a device for optical free space transmission
systems, in particular for the optical transmission of messages in free space between a
message source and a message sink.
BACKGROUND OF THE INVENTION
Fiber-optical communications systems have revolutionized the wire-dependent datatransmission over large distances within a few years. In connection with directional
30 radio installations, which had been dominant up to that time, systems already in service
today can be considered to be superior in every respect in view of the availablebandwidth. Only mobile communications are able to profit indirectly from this advance
by means of efficient fixed networks, since cellular networks also need to utilize narrow-
band and trouble-prone radio on a portion of the transmission path. In connection with
35 the transmission via or between satellites, large distances still need to be overcome,
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which absolutely requires large transmission outputs and antennas, which in turn runs
counter to the desire for systems as compact and as light as possible for space travel.
For this reason efforts were undertaken fairly soon after the triumphal march of the
fiber-optical communication technology to also use its advantages for optical
5 communications in free space by means of suitable systems.
New factors in the fiber-optical communications technology, inherent in the system,
have shown themselves to be limiting, in particular in connection with bridging great
distances, especially the dispersion in the dielectric wave guide used for transmission,
10 and various non-linear effects of its material. Optical communication in free space
again meant the return of old limiting effects of radio technology and wire-dependent
communications. Here, the loss of signal output on the transmission path and theeffects of foreign signals dominated again. However, in fiber- optical communications
the extreme limits of the energy of a symbol used for transmission are not expressed by
15 the terms describing the phenomenon of thermal noise, but by means of photons per
bit.
For example, at an error quotient of 1/1,000,000,000, 10.5 photons per bit are
inherently required for the assured transmission of data by means of intensity
20 modulation (J.S. Senior, "Optical Fiber Communications, Principles and Practice",
second edition, Prentice Hall, pp. 469 to 471).
Better results can be achieved with pulse-position modulation, as well as various
coherent techniques, in particular methods with homodyne transmission. The best
25 realized results were obtained by means of homodyne superimposition (less than 30
photons per bit). Since there is a clear requirement for low energy consumption for
space-based systems, an optical system for data transmission between distant
geostationary satellites should transmit and receive light waves by the largest possible
and very accurately aligned aperture. This, in turn, can only be realized, starting at a
30 defined size and while maintaining a low weight, in the form of a reflecting telescope.
Reflecting telescopes in the so-called coaxial form are known in numerous designs, the
systems in accordance with Gregory, Cassegrain and Schmidt should be mentioned
(Eugene Hecht, "Optics", secQnd edition, Addison-Wesley Publishing Company,
Reading, MA, USA, pp. 197,198).
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Common to all of them is the system-related disadvantage of the partial central
covering of the aperture by the collecting mirrors and their suspension devices. In this
case a compromise between mechanical sturdiness and losses because of covering of
s the aperture must be found.
Generally, an additional baffle is required, which prevents the reflection of
scattered light, which is encouraged by the collecting mirror and its suspension, in the
direction of the light to be received. The simultaneous use of such a telescope for
10 radiating a light wave as well as for receiving an oppositely entering light wave
generally results in significant disadvantages, since the said collecting mirror and its
suspension reflects a portion of the high-output transmitted light in the direction of the
simultaneously entering light wave and results in interferences because of
superimposition. Accepting great losses regarding the imaging quality, this problem
15 can be bypassed by the use of an oblique reflecting telescope proposed by Kutter.
However, the mentioned imaging errors result in the waste of valuable transmission
output.
A solution of this problem is explained in Swiss Patent Application No. 2930/97, in
20 which an oblique mirror telescope is described which avoids imaging errors because
of the special shape of its mirrors. However, in general such an installation, which is
intended for bridging the greatest distances, in particular between satellites circling
the earth in geostationary orbits, requires large bodies, which are rotatable around at
least two axes in relation to the satellite body, which limits the number of such
25 installations which can be placed on a satellite.
A method for optical communications between satellites circling in the same
direction on parallel and comparatively low orbits, described in Swiss Patent
Application No. 1153/97, requires a large number of optical terminals on each satellite
30 which, in addition, should little reduce their respective detection ranges by mutual
coverage. Although the distances to be bridged do require the turning away from
systems based on optical lens devices, the weight of the optical mirror device should
be extremely low because of the large number of the systems installed on the
individual satellites.
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OBJECTAND SUMMARY OF THE INVENTION
It is therefore the object of the invention hereinafter described to meet the above
described requirements in a particularly advantageous manner in that both a low
weight as well as a narrow reduction of the detection range of neighboring installations
is achieved by means of a greatly reduced number of mirrors and a specially executed
construction .
This object is attained by means of the characteristics of claim 1.
The invention contains an optical communications system, whose optical
components include, among others, a telescope which is embodied in the conventional
manner as a reflecting telescope. The rough aiignment of the light beam to be
transmitted and of the reception direction takes place in accordance with the invention
by means of a single planar mirror which, in the manner of the construction of a15 periscope, is attached obliquely at the outer opening of the telescope and is seated,
fully rotatable around the axis of the telescope. The reflecting surface of the planar
mirror is attached, inclined by 45 degrees in respect to the optical axis of thetelescope, and can be adjusted to deviate by a few degrees from this inclination.
The mentioned disadvantages of a collecting mirror centrally attached in front of
the main mirror of the telescope can be circumvented with special optical devices, by
means of which a light beam coupled into the telescope for transmission is changed in
such a way that it falls into the spatial area between two cone surfaces with different
opening angles, and finally is changed by the main mirror of the telescope into a light
25 beam whioh has a recess in its central area, in which the centrally arranged collecting
mirror is placed.
Conversions of a light beam are provided in further development of this idea of
the invention, which also include the suspension device for such a collecting mirror.
Realization of such an optical device is possible in various ways. For example, a
specially formed component (axicon) can be used, or a holographic phase grating,which can be used in particular in connection with more complex adaptations of a light
beam to be transmitted. It is alternatively possible to employ an ensemble of four
tilted plane-parallel plates for generating four off-centered partial beams.
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The resulting advantage is the avoidance of the coupling of the light beam to betransmitted to the optical transmitter of the device and the interference occurring
because of this. Furthermore, another result is the economical use of the optical
5 transmission output generated by means of a large outlay, which is particularly
advantageous because it must be generated by a multiple of the optical pumping
output which, in turn, requires the multiple of the latter of the electrical output for
generation.
o A further advantage of the mechanical design in accordance with the invention
resides in the use of a single mirror, which can be mechanically rotated around two
axes, for the rough alignment of the light beam to be transmitted, as well as of the
reception direction. Together with a mirror of the telescope which is merely provided
with increased refractive power, this results in an extremely low weight of the partial
optical system, which can be reduced still further if the portion of the rotatably seated
mirror which, because of being obscured by the collecting mirror, is not illuminated by
light, is removed by means of a central recess. At the same time, this central recess
can receive the suspension device of the mirror or an optical beacon.
Further details, characteristics and advantages of the invention ensue not only
from the claims and the features which can be taken from them either individually or in
combination, but also from the following description of preferred exemplary
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a reflecting telescope together with further essential components of
an optical transmission system,
Fig. 2 is a schematic cross section through the systems represented in Fig. 1,
Fig. 3 is an exploded view of the essential optical and mechanical components ofan optical transmission system,
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Fig. 4 represents a sagittal section through the optical transmission systemrepresented in Fig. 3,
Fig. 5 represents a particularly advantageous embodiment of the rotatably seatedmirror used for rough alignment,
Fig. 6 shows an axicon,
o Fig. 7 shows a holographic phase grating,
Fig. 8 shows an optical system,
Fig. 9 shows an optical system made of plane-parallel plates,
Fig. 10 shows a further device for the fine tuning of the receiving direction of the
receiving unit in Fig. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 comprises several important modules of an optical transmission system in
an exploded view. A transmitting unit 10, an acquisition unit 14, a receiving unit 16 as
well as a core unit 12 are arranged around the body of the telescope 2, connected by
25 a support 8.
As represented in Fig.2, the core unit 12 causes the coupling in of a light beam20 to be transmitted into the telescope, as well as the division of the received light into
the receiving unit 16 and the acquisition unit 14. After the transfer to the light beam
18, which is congruent to the beam of received light waves, and passage through the
telescope 2, the light beam 20 to be transmitted is changed into a collimated light
beam 4, which has a ring-shaped intensity distribution perpendicular to its propagation
direction. The exterior diameter of the light beam, as well as the diameter of its inner
recess 6 are determined by the surface of a main mirror 22 and the baffle effect of a
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collecting mirror 24, independently of the effects of an axicon 26, whose purpose is to
maximize the illumination of the main mirror. Optimization is accomplished in that the
collimated light beam 18 is converted into a di~ergent light beam 28 by light refraction
or other methods which, in a dynamically balanced manner around the optical axis of
the telescope 2, illuminates that spatial angle area, which is free of large obstacles
following a reflection at the planar collecting mirror 24 and further reflection and
conversion at the main mirror 22. By means of this a particularly large portion of the
optical output conducted in the collimated beam 18 is radiated via the collimated beam
4, furthermore, very little backscattering of the light to be transmitted into the receiving
10 unit 16 takes place. In a further exemplary embodiment, an optical system 27, which
contains the axicon 26 or similarly acting devices, is already integrated into the
transmitting unit 10 and causes a light beam 20 to be transmitted, which is provided
with a central recess and makes a transition into the light beam 18 and takes on the
shape of the light beam 4 after passage through the telescope. However, an optical
device containing an axicon 26 or comparable component can be installed at any
arbitrary location in the beam path between the position represented in Fig. 2 with the
axicon and the transmitting unit 10 in order to achieve the above described desired
effect. In further embodiments, the transmitting unit 10, the acquisition unit 14 and the
receiving unit 16 are interchanged in any arbitrary manner, wherein the optical system
20 can also be positioned at any arbitrary location in the beam path between thetransmitting unit 10 and the location indicated in Fig. 2 by the axicon 26.
The integration of the modules represented in Figs. 1 and 2 into an optical
transmission system is represented in Fig. 3. The telescope 2 is flanged to a ring-
25 shaped housing 32, which simultaneously contains a cable drum 34 which, closed by a
cover 36, takes along lines leading into a azimuthally rotatable top element 38 during
corresponding rotating movements. The telescope unit 2 and partial systems
connected thereto are enclosed in a tub-shaped housing 30. Rotating movements ofthe top element 38 are initiated by an electric motor 40 and are controlled by means of
30 an angle encoder 42. The top element 38 contains a plane mirror 44 which, in the
neutral position, is inclined by 45 degrees in respect to the longitudinal axis 46 of the
housing 30, is used for the rough alignment of the light beam as well as the receiving
direction, and is seated, rotatable around a further axis 48, in the top element 38
transversely to the longitudinal axis 46 of the housing 30. Rotating movements
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around the axis 48 are initiated by a further electric motor 50 and controlled by means
of a further angle encoder 52. A collimator 54, which is also deflected and aligned by
means of the mirror 44, radiates a beacon light beam for assisting the establishment
of the connection with another optical transmission system. To avoid weakening of
5 the light beam 4 to be transmitted, the collimator 54 can be attached in the beam
direction in one line with the collecting mirror 24 or parallel with the light beam 4. To
prevent laterally scattered light, the top element 38 is provided with a baffle 56. The
telescope 2, as represented in a sagittal section in Fig. 4, is closed off by means of a
quartz glass plate 58, which is slightly angled to avoid backscattering toward the axis
10 46, in order to protect the optical device located under it from particle and gamma
radiation. As can be seen by means of a mirror 45, which is represented in Fig. 4
turned in relation to the mirror 44, the receiving direction and the direction of the
radiated light can be affected to a limited degree in relation to the elevational angle.
But the special purpose of the communications system of the invention permits this
15 limitation, which allows the simultaneous operation of a large number of terminals on a
mounting plane 60 of a satellite without cutting each other off to too large an extent in
their detection range. If the collimator 54 is attached in the position represented in
Fig. 3 and Fig. 4 for radiating the beacon light required for establishing the connection
with another optical system, it is possible to achieve a reduction of the mass of the
20 mirror 44 used for the rough alignment, taking into consideration its partial shading
occurring because of the collecting mirror 24, in that the centrally shaded area 62 in
accordance with Fig. 5 is completely removed.
Further than that, an additional reduction of the mass of the mirror 44 can take2S place in accordance with the method described in Swiss Patent Application No.2988/96, wherein the body of the mirror 44 is provided over its entire surface facing
away from the reflecting layer with blind bore-like recesses, which extend closely up to
the reflecting layer and whose structure comes close to the weight-saving honeycomb
structure known from aircraft construction. The completely removed shaded area 62
30 of the mirror 44 is in addition suitable for receiving a fit, in which the mirror 44 is
alternatively connected with the top element 38, rotatable around one or two axes.
The axicon 26, which is required to form the divergent light beam illuminating the
required spatial angle area, is embodied as a lens body, whose surface is made cone-
shaped (Fig. 6).
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By means of this and in combination with conventional lenses, the divergent light
beam 28 is generated, which is reflected via the collecting mirror 24 and is limited by
an inner and an outer cone. This takes place in that the light beam 18 is again
5 diffracted in cross section into two partial beams at the two surfaces of the axicon 26
which are plane in cross section wherein, however, in a spatial view a single light
beam results, which is bordered by two cones of the same opening angle offset inrespect to each other in the direction of the optical axis and illuminates an infinitely
small spatial angle without taking any diffraction effects into consideration. Here,
lO conventional lenses cause the limitation by two cones with different opening angle.
The integration of a conventional lens with an axicon 26 can be achieved in that the
spherical surface of a lens is conically distorted, or in that the front or back of the body
of such an element have a conical, or respectively spherical surface. But the axicon
26 can also be replaced by a holographic phase grating 64 in accordance with Fig. 7,
15 which is described in European PatentApplication EP 97109111.1. Its effect is based
on the controlled interference of the phase progression along the front of an originally
level light wave. As represented in Fig. 7, this interference is performed by radiating
through an optically denser medium 66, whose surface has dynamically balanced
recesses at constant periods. By means of this the originally level wave front is
20 divided into circular zones which are in opposite phase in respect to each other, which
in the first order interfere constructively, dynamically balanced in respect to the
optical axis, at an angle determined by the mutual distance between these zones and
by the wavelength, but destructively in the direction of the optical axis.
If, as described above, it is intended to merely convert a beam 20, which is to be
transmitted collimated, so that it leaves a corresponding optical system again as a
non-divergent beam with a central recess, devices in accordance with Fig. 8 and Fig.
9 should be used. The optical system 27 in accordance with Fig. 8 consists of a glass
body, which is delimited by two cones of equal opening angles, which are offset in
30 respect to each other, and which changes a collimated light beam 68 by means of
double dynamically balanced diffraction at its border surfaces into a dynamically
balanced collimated light beam 70 with a dynamically balanced central recess 72.
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ln contrast thereto, a system 27 represented in Fig. 9 consists of four
appropriately worked and joined plane-parallel plates 74, which generate collimated
partial beams separated in a clover-leaf like manner from each other in four angular
areas, which not only leave a square core area open, but also leave a cross-shaped
area unilluminated, by means of which the illumination of support strips provided for
the collecting mirror 24 can be prevented. The system represented in Fig. 9 can also
be put together from only three plane-parallel plates 74, or by any arbitrary number,
wherein an infinite number of plates 74 indicates the transition into the conical shape
in accordance with Fig. 8.
For the fine adjustment of the receiving device, in a further embodiment in
accordance with Fig. 10, the portion of the collimated light beam 18 coupled into the
receiving system 16 is coupled by means of a lens 76, or a more complex optical
device, into a monomode optical waveguide 82 which, by means of actuators 86
15 attached to the optical waveguide 82 orthogonally in respect to each other, is directed
into the light to be coupled in. The light of a local oscillator, which is provided through
a further optical waveguide 84, is superimposed on this light in a directional coupler 78
in order to be provided via the two remaining gates of the directional coupler 78 to two
photodiodes 80, whose photo flow is provided to a balanced receiver.
The devices in accordance with Fig. 7, Fig. 8 and Fig. 9 used for forming the light
beam are replaced in a further embodiment by a fixed arrangement of waveguides,
which respectively radiate a partial beam of a bundle of collimated parallel light
beams, wherein corresponding recesses in the total beam are created by means of
the arrangement of the waveguides.
