Abstract:
An optical rotary transmitter which ensures the reliable transmission of optical signals in conjunction with a comparatively simple construction comprises two parts spaced apart from one another which are rotatable relative to one another about a common centre axis and the first of which comprises a first circular light coupler ( 10 ) and the second of which comprises a second circular light coupler ( 20 ). The light entrance and light exit surfaces of the two light couplers face one another. Each of the light couplers ( 10, 20 ) consists of collimators ( 11, 21 ) combined to form a respective collimator arrangement ( 16, 26 ), optical coupling elements ( 15, 25 ) being connected to said collimators.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. § 371 National Stage Patent Application of International PCT Application Serial Number PCT/EP2011/02057 having an International filing date of 21 Apr. 2011, which claims priority to German Patent Application Serial No. 102010016773.8 that was filed on 4 May 2010 and German Patent Application Serial No. 102010036174.7 that was filed on 2 Sep. 2010. This Application claims priority to and incorporates by reference the above applications in their entirety for all purposes. 
     BACKGROUND 
     The invention relates to an optical rotary transmitter having the features of the preamble of claim  1 . Such a rotary transmitter may be part of a rotary coupling, for example, or some other device having rotationally movable parts spaced a distance apart from one another and permits unidirectional or bidirectional transmission of optical signals between parts that are rotatable relative to one another independently of their rotational position and angular velocity. The wavelength range may extend from long-wavelength IR radiation to short-wavelength UV radiation. The terms “light” and “optical” are to be understood in this sense below. 
     Such optical rotary transmitters are known from DE 603 14 028 T2 and GB 2 247 089 A. They permit high signal transmission reliability, regardless of the angular velocity of the parts that are rotatable in relation to one another, and use commercial semiconductor transmitter and receiver bit rates of up to 10 Gbit/s. However, multichannel operation is possible only when using time multiplex methods or wavelength multiplex methods. 
     JP 62-028704 A discloses an optical rotary transmitter having two parts that are spaced a distance apart from one another and rotate relative to one another about a common central axis, the first part of which has collimators arranged tightly in a circle around this central axis and the second part has second collimators that are spaced further apart radially from this central axis, the parallel beam bundles being focused on the first collimators by a Fresnel lens. 
     JP59-017526 A discloses a scanner for rotary scanning of the beam exit surfaces of waveguide fibers arranged on the circumference of a circle. This purpose is served by another waveguide fiber having a longitudinal axis running through the midpoint of the circle, rotating about same and having an end bent at an angle by the amount of the midpoint distance of the beam exit surfaces of the first waveguide fibers. 
     US-A-4 943 137 discloses a rotary transmitter whose light is directed by an optical transmitter, which is assigned to one of the two parts that is rotatable in relation to the others, via optical fibers and collimators to an optical coupling module and from there further to an optical receiver, which is assigned to the other rotatable part. The coupling module serves the purpose of derotation of the light, so it rotates at half the angular velocity of the two parts that are rotatable in relation to one another. Consequently, this design is very complex. 
     US-A-4 027 945 discloses a rotary transmitter, in which the optical signals of a transmitter assigned to the first of the parts rotatable in relation to one another are fed into a fiber bundle. This fiber bundle is arranged in a circle about the axis rotation; likewise a similar fiber bundle is arranged on the second part which includes an optical receiver. This requires a large number of optical fibers, i.e., waveguide fibers. 
     AS-4-6 128 426 describes a rotary transmitter in which light from an optical transmitter, for example, which rotates together with the first part, is transmitted to a plurality of optical receivers or detectors arranged in a circle on the second part. This rotary transmission requires angle decoding of the relative positions of the rotary parts to one another. 
     DE-A-10 2008 030 187 describes a rotary transmitter in which an optical transmitter assigned to the first part feeds optical signals into a waveguide on the second part. The waveguide therefore has a specially processed surface, which permits this light signal feed. Such waveguides are not available commercially and are therefore expensive. 
     DE-A-10 2006 054 052 describes a rotary transmitter having two light-conducting hollow bodies coaxial with one another into which light signals from a transmitter are fed by means of waveguides distributed around the circumference of the one hollow body, the signals being emitted through the end face thereof in the direction of the end face of the other hollow body and output by the same principle via waveguides of the other part and sent to a receiver. With this design, there is high signal attenuation due to uniform distribution of the signals over the entire circumference of the hollow bodies. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to make available a rotary transmitter that will ensure reliable transmission of optical signals in physically separate channels between the parts that are rotatable in relation to one another, and doing so with a comparatively simple design. 
     This object is achieved according to the present invention by the fact that at least one additional optical coupling element is coupled to each collimator in such a way that the collimator generates an additional parallel beam bundle, which is assigned to this second coupling element and forms an angle with a parallel beam bundle corresponding to the first optical element. 
     This rotary transmitter may operate unidirectionally or bidirectionally. The terms “send” and “receive” as used below are therefore interchangeable. 
     Due to its simple and largely symmetrical design, the rotary transmitter can be manufactured inexpensively while at the same time ensuring a high signal transmission reliability that does not depend on the angular velocity of the parts that are rotatable relative to one another. Depending on the dimensions of the collimators, distances on the order of up to 100 mm can be bridged between the light couplers. 
     The angle between these parallel beam bundles must expediently be such that the parallel beam bundles of each collimator of the one light coupler corresponding to the different optical coupling elements strike neighboring collimators of the opposing light coupler. 
     Each collimator may consist of at least one (focusing) lens followed by at least one light guide and connecting devices for optical coupling elements. More complex designs for adjusting the numeric apertures and/or the diameters of the lens and of the optical coupling element, e.g., successive concentration stages, are also possible. Depending on the specific application, conventional spherical lenses made of glass or plastic, GRIN lenses, Fresnel lenses or other special shapes may be used in one or more stages. 
     The collimators of each collimator arrangement are preferably arranged adjacent to one another with essentially no gap. “With essentially no gap” means that collimators mounted in mechanical holders cannot be installed with their optically active surfaces directly abutting against one another. 
     The optical coupling elements preferably consist of waveguides in the form of waveguide fibers, which are expediently combined to form a bundle at their input and/or output ends. One of the conventional optical semiconductor transmitters and/or receivers may be coupled to the input and/or output end either directly or by way of concentrator. 
     An alternative design, although more complex, has each waveguide fiber ending in a transmission and/or reception element assigned to it. In this case, all the transmission elements must be triggered in parallel, and the reception signals must be sent together to the reception elements. The higher light transmission power improves the signal-to-noise ratio on the reception end. 
     When using just one transmission and/or reception element shared by all the optical coupling elements in the form of waveguide fibers, it may be expedient toward the goal of achieving a uniform distribution of signals over the circumferences of the respective circles if the waveguide fibers of each collimator arrangement are guided jointly to a shared light mixer. 
     For their part, the light mixers may he optical waveguides to which the respective light transmitter and/or light receiver is then connected, optionally by way of a concentrator. 
     In addition, the light mixers may at the same time adjust the numeric aperture and/or the cross section of the light transmitters and/or light receivers to those of the optical coupling elements, e.g., the waveguide fibers. 
     In the normal case, the collimators are designed so that their transmission and/or reception characteristics correspond essentially to those of a parallel beam bundle. One embodiment of the rotary transmitter, which makes it possible to bridge a different diameter of the first light coupler and of the second light coupler, which is due to the further design of the rotary transmitter and thus of the first collimator arrangement and of the second collimator arrangement, consists of the fact that the optical axes of opposite collimators of the first collimator arrangement and of the second collimator arrangement are aligned with one another but form an angle with the shared central axis. 
     If the parts of the rotary transmitter are arranged coaxially and concentrically with one another, i.e., if they consist of two coaxial hollow shafts, for example, then it is also possible to have an embodiment in which the first light coupler and the second coupler are arranged concentrically around one another and the respective essentially parallel beam bundles of their collimators run at least approximately orthogonally to the shared central axis, i.e., radially to it. 
     Another embodiment of the rotary transmitter consists of the fact that the first light coupler and the second light coupler each comprise at least one additional collimator arrangement in which the collimators are arranged on a circle that is concentric with the circle of the first collimator arrangement and has a diameter different from the diameter of the first circle. In other words, when there are two concentric collimator arrangements per light coupler, multiple first signal channels can be guided over the first collimator arrangement and multiple second signal channels can be guided over the second collimator arrangement without having to use a multiplex method. 
     Another embodiment of the rotary transmitter consists of the fact that the first light coupler and the second light coupler each comprise at least one additional collimator arrangement in which the collimators are arranged on a circle that is concentric with the circle of the first collimator arrangement and has a diameter different from the diameter of the first circle. In other words, when there are two concentric collimator arrangements per light coupler, multiple first signal channels can be guided over the first collimator arrangement and multiple second signal channels can be guided over the second collimator arrangement without having to use a multiplex method. 
     An improved coupling between the first and second light coupler through a more uniform distribution of the reception radiation over the circular ring on which the collimators are arranged is achieved if the number of collimators of the first light coupler is different, in particular different by one, from the number of collimators of the second light coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is explained below with reference to the drawings, in which simplified exemplary embodiments of the essential parts of the rotary transmitter are shown schematically namely: 
         FIG. 1  shows an optical rotary transmitter according to the prior art, 
         FIG. 2  shows a top view of the optical rotary transmitter&#39;s first light coupler, 
         FIG. 3  shows an exemplary embodiment of a collimator, 
         FIG. 4  shows a side view of a collimator in a three-channel embodiment, 
         FIG. 5  shows multiple collimators according to  FIG. 4  and the respective beam paths, 
         FIG. 6  shows a top view of two collimator arrangements projected one above the other with a different number of collimators by one, and 
         FIG. 7  shows a diagram of the coupling efficiency in a three-channel embodiment of the rotary transmitter according to  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the essential part of fundamentally any rotator transmitter. It includes a first light coupler  10  and a second light coupler  20 . The two light couplers  10 ,  20  are arranged opposite one another. The first light coupler  10  is assigned to a first part (now shown) of the rotary transmitter, while the second light coupler  20  is assigned to a second part of this rotary transmitter, which can rotate about a shared central axis  30  in relation to the first part. The first light coupler  10  comprises a first collimator arrangement  16  in a housing  12 . Similarly, the second light coupler  20  comprises a second collimator arrangement  26  in a housing  22 . The first collimator arrangement  16  consists of collimators  11  distributed uniformly and adjacent to one another on the circumference of a circle. The second collimator arrangement  26  consists of similarly arranged collimators  21 . The collimators send and receive light essentially in parallel with the shared central axis  30 , which is also the axis of rotation at the same time. The direction of the beam is merely indicated as an example in  FIG. 1  through the individual arrows labeled as  31  on the whole; in this case, the light coupler  10  is the transmitting light coupler and the light coupler  20  is the receiving light coupler. This relationship is of course reciprocal. One waveguide fiber  15  is connected to each light coupler  11  and one waveguide fiber  25  is connected to each light coupler  21 , 
     The waveguide fibers  15 ,  25  are each combined into a bundle and end in a block  13  or  23 , respectively. The blocks  13  and  23  may stand for a light transmitter, e.g., a VCSEL, and a light receiver, e.g., a PIN diode. Instead of that, the blocks  13 ,  23  may also be components between a light transmitter and/or a light receiver and the respective waveguide fiber bundle, e.g., light mixers in the form of an optical waveguide that distributes the transmitted light signal over the waveguide fibers  15  as uniformly as possible to all collimators  11  and combines the light signals received by the collimators  21  and couples them to the light receiver. The light mixers are therefore also designed so that they adjust the numeric aperture and/or the cross section of the light transmitter and/or the light receiver to that of the waveguide fibers  15  and  25 , respectively. 
     The collimators  11  and/or  21  are designed so that they convert the light signal input via the respective waveguide fiber into a parallel beam bundle an/or they feed the received parallel beam bundle into the connected waveguide fiber. 
     The circular or annular shape of the collimator arrangements  16  and/or  25  exposes an internal diameter of the light couplers  10  and/or  20 , so that other parts of the rotary transmitter that are not shown here, e.g., a drive shaft, can be passed through boreholes  14 ,  24 , for example, or rotary bearings (not shown here) can be arranged in the boreholes  14 ,  24 . 
       FIG. 3  illustrates the design of a collimator for a rotary transmitter according to the invention merely as an example, but this shows only a centrally connected waveguide fiber. 
     The collimator comprises a lens  51  through whose imaging surface  50  a parallel beam bundle  60  arrives. The lens is mounted in a holder  52 . A conical body  53  for guiding the light in the direction of a surface  59  comes after the lens  51  and is in turn followed by the optical concentrators  54 ,  55 , which adjust the diameter and the aperture of the light bundle to the input diameter and the aperture of a waveguide fiber  57 . The waveguide fiber  57  is connected to the collimator, preferably directly i.e., without an air interspace by means of an essentially known connection device (not shown here) in the form of a connection and coupling site  56 . 
       FIG. 4  Shows a collimator  40  to which a waveguide fiber  42  is connected centrally and waveguide fibers  41 ,  43  are connected eccentrically on both sides. The collimator  40  generates (or receives) three beam bundles  44 ,  45 ,  46  accordingly, as are shown in idealized form, but are also assigned individually to the waveguide fibers  41 ,  42 ,  43  in the real beam path, namely beam bundle  44  being assigned to waveguide fiber  42  and the beam bundle  46  being assigned to the waveguide fiber  43 . Therefore, the waveguide fibers  41 ,  42 ,  43  can be operated as independent signal channels. The waveguide fibers  41 ,  42 ,  43  are coupled locally to the collimator  40 , so that the beam bundles  44 ,  45 ,  46  strike different but neighboring collimators on the opposite side. 
     This is diagrammed schematically in  FIG. 5  in the form of a partial developed view of the opposite collimator arrangements. Collimators  80   a  through  80   e  are illustrated are illustrated on the left side like  FIG. 4  with first waveguides  81   a  to  81   e , second waveguides  82   a  to  82   e , third waveguides  83   a  to  83   e  and the respective beam paths  44   a  to  44   e ,  45   a  to  45   e  and  46   a  to  46   e . These are opposite the collimators  90   a  to  90   e  with the first waveguides  91   a  to  91   e , the second waveguides  92   a  to  92   e  and the third waveguides  93   a  to  93   e , namely on the right side of the drawing here. Light from the first eccentrically coupled waveguides  81   a, b, c, d  is guided through the collimators  80   a, b, c, d  via the beam bundles  44   a, b, c, d  through the collimators  90   b, c, d, e  and into the waveguides  91   b, c, d, e . Likewise, light from the first centrally coupled waveguides  82   a, b, c, d, e  is guided through the collimators  80   a, b, c, d, e  via the beam bundle  45   a, b, c, d  and through the collimators  90   a, b, c, d, e  into the waveguides  92   a, b, c, d, e . Accordingly, light from the first waveguides  83   b, c, d, e  is guided through the collimators  80   b, c, d, e , via the beam bundles  46   b, c, d, e  and the collimators  90   a, b, c, d  into the waveguides  93   a, b, c, d . Thus light is always being transmitted from the waveguides  81  to the waveguides  91 , from the waveguides  82  to the waveguides and from the waveguides  83  to the waveguides  93 . Each of these waveguides pairs may be used as a separate optical signal channel that is decoupled from the other channels, apart from diffraction-related scattered light in particular.  FIG. 6  shows schematically the collimators  11  of the first collimator arrangement illustrated one above the other and the collimators  21  of the second collimator arrangement in one embodiment, in which the second collimator arrangement has one less collimator than the first collimator arrangement. 
     The first collimator arrangement here has thirty-two collimators  11 , which the second collimator arrangement has thirty-one collimators  21 , resulting in a uniformly progressive coverage of the collimators during the rotational movement of the respective parts of the rotary transmitter, for example, of the second collimator arrangement, by the beam bundles created by the collimators of the first collimator arrangement and vice-versa in the reciprocal case. Consequently, the optical attenuation is averaged over the entire annular arrangement of the collimators. Even if the respective parts of the rotary transmitter are stationary in some cases at different rotational angle positions in relation to one another, this ensures a largely angle-independent transmission and/or signal strength. 
     Exemplary Embodiment 
     A VCSEL with a wavelength of 850 nm is used as the transmitter. 
     The output fiber bundle consists of 61 fibers with as fiber diameter of 125 μm; the diameter of the bundle is 1.125 mm. 
     The numeric aperture of the fibers is 0.20. 
     The lenses are 2.5;2.5 mm in size. The numeric aperture is reduced to 0.01. 
     The diameter of the collimator rings is approximately 50 mm. 
     If a beam widening of mm is allowed, the distance between the transmitter and receiver collimator rings may be as to 10 cm. 
     A Si-PIN diode with a 1.2 mm diameter is used as the photodiode, so a bit rate of up to 1 Gbit/s is possible. 
     Alternatively, a concentrator is used so that the photodiode diameter can be reduced to 300 μm, thereby achieving bit rates of up to 10 Gbit/s. 
       FIG. 7  shows the coupling efficiency of a rotary transmitter having three channels according to  FIG. 5 , starting from as center-to-center spacing of the collimators of 3.5 mm. The lateral shift in the collimators  80  with respect to the collimators  90  is plotted in mm on the abscissa. The received signal power standardized to as value of 1.0 is plotted on the ordinate. For example, collimators  80   a  and  90   a  are exactly opposite one another at the zero point. Then the collimators  80   a  and  90   b  are exactly opposite one another at the end abscissa value of 3.5 mm, i.e., after rotation of the one collimator arrangement relative to the other collimator arrangement by one increment corresponding to an arc of 3.5 mm. 
     Curve  71  shows the attenuation curve of the added signals that are received, for example, by the optical fibers  91  starting from the waveguides  81 . The curve  72  shows the attenuation curve of the added signals of the waveguides  92  received by the waveguides  82 . The curve  73  shows the corresponding attenuation between the waveguides  93  and  83 . In other words, the curve  72  shows the attenuation of the total signal transmitted over the central light bundle, while curves  71  and  73  illustrate the corresponding attenuations of the total signals of the neighboring “eccentric” light bundles, similar to the diagram in  FIG. 4 . 
     A coupling attenuation of 2.4 dB has been ascertained for the signal of the central beam bundles and channels, and a coupling attention of 5.1 dB has been found for the signals of the outer channels.