CA2244070A1 - Device for optical free space transmission systems - Google Patents
Device for optical free space transmission systems Download PDFInfo
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- CA2244070A1 CA2244070A1 CA 2244070 CA2244070A CA2244070A1 CA 2244070 A1 CA2244070 A1 CA 2244070A1 CA 2244070 CA2244070 CA 2244070 CA 2244070 A CA2244070 A CA 2244070A CA 2244070 A1 CA2244070 A1 CA 2244070A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
- H04B10/118—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum specially adapted for satellite communication
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B23/00—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices
- G02B23/02—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices involving prisms or mirrors
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B23/00—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices
- G02B23/02—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices involving prisms or mirrors
- G02B23/08—Periscopes
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- Astronomy & Astrophysics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Optical Communication System (AREA)
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Abstract
Devices for optical message transmission in free space are optimized for the respective use, in particular for operation in a satellite. The described device is intended for employment in satellites for covering distances of several thousands of kilometers, wherein a large number of devices is operated simultaneously withoutessential mutual coverage of the detection range. This is achieved by means of arecessed embodiment of a telescope (2) and the arrangement in the manner of a periscope of only a single, level mirror, which can be rotated around two axes and is used for rough alignment. A diffractive optical element optimizes the illumination of the aperture, while leaving the surface covered by a central collecting mirror (14) free.
Description
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, CA 02244070 l998-09-09 -.2.-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).
CA 02244070 l998-09-09 -.3.-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.
CA 02244070 l998-09-09 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.
-.5.-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, -.6.-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 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 CA 02244070 l998-09-09 -.8.' 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).
CA 02244070 l998-09-09 _ 9._ 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.
CA 02244070 l998-09-09 -.10.-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.
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, CA 02244070 l998-09-09 -.2.-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).
CA 02244070 l998-09-09 -.3.-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.
CA 02244070 l998-09-09 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.
-.5.-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, -.6.-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 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 CA 02244070 l998-09-09 -.8.' 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).
CA 02244070 l998-09-09 _ 9._ 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.
CA 02244070 l998-09-09 -.10.-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.
Claims (15)
1. 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, consisting of at least one telescope (2), a receiving unit (16), and a transmitting unit (10) and a mirror (44), wherein the mirror (44) is rotatably seated around two axes.
2. A device for radiating light, consisting of a telescope (2), characterized in that a light beam (18) conducted into the telescope (2) is changed into a radiated light beam (4), which is matched to the form of the radiating aperture.
3. The device in accordance with claim 1, characterized in that the mirror (44) can be rotated completely around the axis (46), the normal line of its reflecting layer is inclined by 45 degrees in relation to the axis (46), and this angle can additionally deviate from 45 degrees by at least a few degrees by rotating around the axis (48).
4. The device in accordance with claim 1, characterized in that a collimator (54) used for radiating beacon light radiates light arranged coaxially or parallel with the optical axis of the telescope (2) by reflection at the mirror (44).
5. The device in accordance with claim 1, characterized in that a collimator (54) used for radiating beacon light, arranged parallel with the optical axis (46) of the telescope (2), radiates light by reflection at the mirror (44).
6. The device in accordance with one of the preceding claims, characterized in that a collecting mirror (24) is fastened on a plate (58).
7. The device in accordance with claim 4, characterized in that the collimator (54) is attached back of the collecting mirror (24).
8. The device in accordance with one of the preceding claims, characterized in that in the shaded area (6) of the light beam (4), the mirror (44) has a bore (62).
9. The device in accordance with claim 2, characterized in that the adaptation of the light beam (4) is achieved by means of an axicon (26), whose surface is cone-shaped.
10. The device in accordance with claim 2, characterized in that the adaptation of the light beam (4) is achieved by means of a holographic phasegrating (64).
-.13.-
-.13.-
11. The device in accordance with claim 2, characterized in that the adaptation of the light beam (4) is achieved by means of the conversion of the collimated light beam (20) to be transmitted, in that the latter is radiated through an optical system (27), which converts a collimated beam (68) by means of double, dynamically balanced diffraction at its conical border surfaces into a collimated beam (70), which is provided with a dynamically balanced central recess (72).
12. The device in accordance with claim 2, characterized in that the adaptation of the light beam (4) is achieved by means of the conversion of the collimated light beam (20) to be transmitted, in that the latter is radiated through an optical system (27), which splits a collimated beam into several collimated partial beams divided from each other by means of double diffraction at an arrangement of plane-parallel plates (74).
13. The device in accordance with claim 1, characterized in that only a single rotatable top element (38) is located above a mounting plane (60).
14. The device in accordance with claim 1, characterized in that the acquisition unit (14) and the receiving unit (16) are interchanged.
15. The device in accordance with claim 1, characterized in that the light to be detected is coupled in the receiving unit (16) into an optical waveguide (82) which can be aligned by means of actuators (86).
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH19980160/98 | 1998-01-23 | ||
| CH16098 | 1998-01-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2244070A1 true CA2244070A1 (en) | 1999-07-23 |
Family
ID=4180626
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2244070 Abandoned CA2244070A1 (en) | 1998-01-23 | 1998-09-09 | Device for optical free space transmission systems |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0886160B1 (en) |
| JP (1) | JPH11261492A (en) |
| CA (1) | CA2244070A1 (en) |
| DE (1) | DE59800748D1 (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4689798B2 (en) * | 2000-07-26 | 2011-05-25 | Ｎｅｃ東芝スペースシステム株式会社 | Reflector driving device |
| US6608708B1 (en) * | 2000-07-28 | 2003-08-19 | Terabeam Corporation | System and method for using a holographic optical element in a wireless telecommunication system receiver |
| EP1199822A3 (en) * | 2000-10-05 | 2002-05-02 | Lucent Technologies Inc. | Telescope for a free-space wireless optical communication system |
| DE102006030421A1 (en) * | 2006-06-29 | 2008-01-03 | Carl Zeiss Optronics Gmbh | Device for transmitting optical signals |
| DE202007012193U1 (en) | 2007-08-30 | 2009-01-08 | Carl Zeiss Optronics Gmbh | Device for a free space optical transmission system and free space transmission system |
| US9274344B2 (en) * | 2013-07-15 | 2016-03-01 | The Boeing Company | Method for extracting optical energy from an optical beam |
| CN104393932B (en) * | 2014-11-20 | 2017-02-01 | 中国科学院光电技术研究所 | Real-time correction method for optical axis of telescope of quantum communication ground station |
| WO2023060450A1 (en) * | 2021-10-12 | 2023-04-20 | Beijing Sinaero Information And Communication Technology Limited | Optical coarse pointing assembly |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2538916A1 (en) * | 1982-12-30 | 1984-07-06 | Thomson Csf | Device and method for a collective preparation of optical fibres by a heat treatment |
| US5142400A (en) * | 1989-12-26 | 1992-08-25 | Cubic Corporation | Method and apparatus for automatic acquisition and alignment of an optical beam communication link |
| JPH0820510B2 (en) * | 1993-01-19 | 1996-03-04 | 株式会社エイ・ティ・アール光電波通信研究所 | Optical communication system Optical system alignment adjustment system |
| US5390040A (en) * | 1994-02-04 | 1995-02-14 | Martin Marietta Corporation | Optical transceiver for free-space communication links |
| US5517016A (en) * | 1994-03-31 | 1996-05-14 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Lasercom system architecture with reduced complexity |
| ES2138802T3 (en) * | 1995-09-26 | 2000-01-16 | Fiat Ricerche | LIGHTING SYSTEM WITH A MICROTELESCOPE BUILT IN A TRANSPARENT PLATE. |
-
1998
- 1998-09-03 DE DE59800748T patent/DE59800748D1/en not_active Expired - Lifetime
- 1998-09-03 EP EP19980116702 patent/EP0886160B1/en not_active Expired - Lifetime
- 1998-09-09 CA CA 2244070 patent/CA2244070A1/en not_active Abandoned
- 1998-10-01 JP JP10279744A patent/JPH11261492A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0886160A3 (en) | 1999-09-15 |
| JPH11261492A (en) | 1999-09-24 |
| DE59800748D1 (en) | 2001-06-28 |
| EP0886160B1 (en) | 2001-05-23 |
| EP0886160A2 (en) | 1998-12-23 |
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