FIELD OF INVENTION

The invention relates to a method and a device for

transmitting data between at least two members being in moveable engagement to each other. Devices of the type mentioned may be used in turrets, machine tools, or computer tomography scanners to transmit data between a movable and a fixed member.

BACKGROUND OF INVENTION

From WO 99/04309 a device for transmitting optical signals between two dynamically decoupled systems is known. This known device comprises at least one emitter unit including a first light source and a receiver unit having at least one optical fiber. The light beam from the emitter is directed onto the optical fiber. A photorefractive layer on the optical fiber surface acts as a movable grating and ensures that the light from the emitter couples into the optical fiber .

This known device has the disadvantage that two laser sources are needed. The first laser transmits the data. The second laser stimulates the photorefractive layer to form a grating. Both lasers are rotating so that the grating moves on the surface of the fixed waveguide.

Therefore, it is an object of the invention to provide a device for transmitting data between at least two members allowing a higher data rate and/or an easier and more reliable setup.

SUMMARY OF INVENTION

The object is solved by a device according to claim 1, a computer tomography scanner according to claim 19 and a method according to claim 20. Further embodiments of the invention are described in the depending claims and the description .

According to the invention, a device for transmitting data between at least two members is disclosed. The device may comprise at least one first member and at least one second member. The first and second members are in moveable

engagement to each other, i.e. one of said members or both may be rotatable around their common axis. In one embodiment of the invention, at least one member may be in a fixed position and at least one member may be movable.

In one embodiment, the at least one first member can be a fixed part of a bearing or a rotary joint. Accordingly, the at least one second member may be a rotatable part of a rotary joint or a bearing.

The disclosed device is adapted to transmit data between said first and second members. In some embodiments of the invention, data may be transmitted from a moving part to a fixed part. In order to achieve this data transmission, the first and second members each comprise at least one first and second waveguide. Said waveguides are arranged substantially parallel to each other in at least one

longitudinal section when the first and second members are in mutually cooperating engagement to each other. In one embodiment of the invention, the longitudinal portion may comprise the full length of the first and second waveguide. In other embodiments of the invention, the longitudinal portion may be defined by a respective subsection of the first and second waveguides. In still another embodiment of the invention, the first and second waveguides may have different lengths, so that the longitudinal portion is constituted by a subsection of one waveguide and the full length of the other waveguide. The relative distance of the first and second waveguide in the longitudinal portion is selected such that an optical signal guided in one waveguide may evanescently couple to the other waveguide. A parallel arrangement of the waveguides within the scope of this description shall mean that the waveguides have a relative distance to each other that is constant within a

predefinable range. However, the waveguides must not necessarily extend linearly but may be bend, i.e. the two parallel waveguides may be arranged concentric.

In order to transmit data between the first and the second member, one of said waveguides is coupled to at least one optical sender. The optical sender is adapted to receive data and modulate an optical carrier such as a laser beam. The modulated light is coupled to one of said waveguides. Thus, the signal is propagating inside said waveguide. This waveguide may be referenced as a sending waveguide.

By evanescent coupling, at least a portion of said optical signal is transferred to the other waveguide. This waveguide may be referenced as a receiving waveguide. The portion of the signal is able to propagate in said other waveguide. This waveguide is coupled to an optical receiver which is adapted to receive said portion of the optical signal propagating in said other waveguide. The receiver may comprise a photodiode being adapted to convert the optical signal into an electrical signal . The electrical signal may be demodulated for restoring the data originally being supplied to the sender.

The evanescent coupling of the optical signal may be

incomplete, i.e. only a small portion of the originally produced intensity coupled to the sending waveguide is received by the optical receiver. This is a known behavior in data communication where only a fraction of sending power is received by an antenna or any other receiving device. Accordingly, known receivers used in optical

telecommunication are adapted to restore the transmitted data notwithstanding that only a small portion of the signal intensity originally produced by the optical sender is received, as long as the signal-to-noise ratio is

sufficient. The intensity of the signal propagating in the receiving waveguide may be less than 10 %, less than 5 % or less than 1 %, of the intensity originally produced by the optical sender, depending on the quality of the receiver and the intensity of light originally coupled into the sending waveguide .

Some embodiments of the invention may allow a data transfer rate from the first and the second member or vice versa of more than 1 GBit/s, more than 10 GBit/s or more than

100 GBit/s.

In one embodiment of the invention any of the first and/or second members may comprise a groove and the first and/or the second waveguide may be arranged at least partly inside said groove. This embodiment may advantageously protect the waveguides from mechanical damage. Furthermore, a precise positioning of said waveguides may be achieved as the position of the waveguide relative to the member is defined precisely by the position of the groove. In some embodiments of the invention any of the first and/or the second waveguide is embedded in the groove by means of an adhesive. In some embodiments, the adhesive may comprise a synthetic resin. The synthetic resin may comprise a thermosetting material. The adhesive will allow for a secure fixing of the waveguides in a predefined position without inducing mechanical stress onto the waveguide which may degenerate the signal or add unwanted artefacts.

In some embodiments of the invention, at least one waveguide may be a step- index fiber comprising a core and a cladding. In some embodiments of the invention, the cladding may be removed at least partly along the longitudinal portion. The use of a step-index fiber ensures reliable transportation of the optical signal with low losses along the fiber. Removing or at least thinning the cladding along the longitudinal portion may result in better coupling of the optical signal between the first waveguide and the second waveguide.

In some embodiments of the invention, the cladding may comprise a dopant being adapted to cause a predefinable refractive index at least along a subsection of the

longitudinal portion of the waveguide. By approximating the refractive index of the cladding to the refractive index of the core in a predefined section of the waveguide, the total reflection of the optical signal in the core is reduced and the field strength outside the waveguide is increased. At the receiving waveguide, the evanescent coupling of the optical signal may be improved by reducing the difference between the refractive indexes of the core and the cladding. This may result in a bigger portion of the signal to couple from the sending waveguide into the receiving waveguide.

In some embodiments of the invention at least a part of the surface of at least one waveguide is optically rough so that light may be scattered and scattered light couples from the first waveguide to the second waveguide as the wave guiding condition resulting from the difference of the refractive index is bypassed locally by scattering. A rough surface may be obtainable by etching, micromachining or laser ablation of a part of the surface of a waveguide .

In one embodiment of the invention the first and second waveguides may be arranged at a relative distance being selected from 5 μπι to 100 μπι at least in said longitudinal portion. In one embodiment of the invention the first and second waveguides may be arranged at a relative distance being selected from 10 μπι to 100 μπι at least in said

longitudinal portion. In one embodiment of the invention the first and second waveguides may be arranged at a relative distance being selected from 10 \im to 50 \im at least in said longitudinal portion. The smaller the distance the better the evanescent coupling of the optical signal will be. The range mentioned may ensure a sufficient signal strength at the optical receiver and nevertheless a relative movement of the two members without mechanical interference.

In some embodiments of the invention, the first member and the second member may be coupled by a hydrodynamic bearing. In one embodiment of the invention, the at least two members may be constituted by at least two parts of such a

hydrodynamic bearing. The term hydrodynamic bearing refers to the bearing allowing rotational movement of two members which are sliding on a thin film of fluid. The fluid may be selected from a liquid or a gas. A liquid may be any of water, an alcohol or an oil. A gas may be compressed air or ambient air. Due to the non-contact sliding of the two members, abrasive wear is minimized but the relative

distance between the two members is held constant, thereby allowing good evanescent coupling of the optical signal.

If the hydrodynamic bearing is filled with an oil, said oil may have an index of refraction from approximately 1.4 to approximately 1.6. In one embodiment of the invention, the index of refraction of the oil may be matched to the index of refraction of the core of said waveguide. This may improve the coupling of the optical signal from the sending waveguide to the receiving waveguide as the wave guiding condition resulting from the difference of the refractive index is bypassed locally by index matching.

In some embodiments of the invention, the first member may have a toroidal shape with an inner and an outer diameter and a polygonal cross -section and the second member may have a toroidal shape with an inner and an outer diameter and a polygonal cross-section, wherein the second member is received with its outer diameter inside the inner diameter of the first member, and the first waveguide is arranged at the inner side of the first member and the second waveguide is arranged at the outer side of the second member, and wherein the first member is rotatable and the second member is in a fixed position. As the waveguide is arranged at the inner side of the rotating member, it is forced against the surface of the mechanical component by the centripetal force, so that the waveguide is securely fixed onto the first member and the relative distance between the first and second waveguides is less affected by rotational forces. In one embodiment of the invention, the polygonal cross section may have the form of a rectangle. In one embodiment of the invention, the polygonal cross section may have the form of a square .

In still another embodiment of the invention, the first and second members may a have a toroidal shape with an inner and an outer diameter and a polygonal cross-section, wherein both members are facing each other with opposing front faces and wherein the first and second waveguides are arranged at the respective front faces of the first and second members.

In some embodiments of the invention, at least one of said first and second members comprises at least one position transducer. By measuring the relative distance of the first and second member, the position of at least one member may be corrected by means of a closed loop control. This ensures a constant width of the gap between the waveguides, so that the signal strength coupled between both waveguides is merely constant.

In some embodiments of the invention, a plurality of optical senders or a plurality of optical receivers is provided which are coupled to a waveguide by means of at least one multiplexer, e.g. an arrayed waveguide grating. In one embodiment, the use of a multiplexer may allow an increasing data rate by using a plurality of optical carrier signals having different wavelength. In other embodiments of the invention, a sender and a receiver may be coupled to each waveguide to allow for a bidirectional data transfer.

The device according to the invention may be in particular useful for a computer tomography scanner to transfer

acquired data from the rotating x-ray detector to a non- rotating receiver being coupled to the data acquisition system. The higher data rate of the device according to the invention compared to known devices for transmitting data allows for shorter scanning times which may decrease the exposure dose of x-ray radiation for the patient.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

Fig. 1 shows a plane view to a first embodiment of a

device according to the invention. Fig. 2 shows a cross-sectional view of the device

according to Fig. 1.

Fig. 3 shows a part of Fig. 2 in greater detail.

Fig . 4 shows a detail of a second embodiment of the

invention .

Fig . 5 shows a detail of a third embodiment of the

invention .

Fig . 6 shows a cross-sectional view of a fourth

embodiment of the invention.

Fig. 7 explains the use of multiple optical senders

and/or receivers with the invention.

DETAILED DESCRIPTION

Fig. 1 shows a plan view of a first embodiment of the invention. In Fig. 1, a first member 11 can be seen. The first member has a basically toroidal shape. The first member 11 is surrounding a second member 12 having a

toroidal shape as well. The first and second members 11 and 12 are rotatably relative to each other around their common axis 4. In one embodiment, one member is rotating and the other is fixed. In another embodiment, both members 11 and 12 are rotating around the central axis 4.

The device 1 may be part of a computer tomography scanner acquiring data with a rotating detector opposing a rotating x-ray source. An object to be scanned is located inside the second member 12 near the axis 4. Data are acquired from multiple directions and transferred to the data acquisition station which is adapted to deconvolve the data and

restoring a three-dimensional picture of the object being located inside the inner diameter d of the second member 12. In some embodiments of the invention, the object being scanned may be a human body. In some embodiments of the invention, the inner diameter d may amount between 300 mm and 2000 mm. The rotating speed of either the first or the second member 11 or 12 may be selected from the range between approximately 60 revolutions/minute to approximately 400 revolutions/minute.

The first waveguide 21 is fitted to the first member 11. In the first embodiment shown in Figs. 1 - 3, the first

waveguide 21 is located at the inner surface of the first member 11. A groove 110 is provided in the first member 11 being adapted to receive the first waveguide 21. The first waveguide 21 has a length elongating between 20° up to 360° of the circumference of the first member 11. In other embodiments of the invention, the waveguide 21 may have a different length and elongate between 10° and 90° of the circumference .

The first waveguide 21 has a first end which is coupled to an optical receiver 31. The optical receiver 31 is adapted to receive an optical signal and to convert said optical signal into an electrical signal. For this purpose, the optical receiver 31 may comprise at least one photodiode . The receiver 31 may be of the same type as known from telecommunication equipment.

The first waveguide 21 has a second opposing end which may be configured as a beam dump, i.e. light incident to this second end is absorbed by the waveguide or the adjacent part of the first member 11.

The first waveguide 21 may be embodied as a polymer optical fiber or a glass fiber. It may comprise a step-index fiber as can be seen from Figs. 2 and 3 or a gradient fiber. It may be a multimode or a monomode fiber. The second waveguide 22 is arranged on the outer circumference of the second member 12. For this purpose, a groove 120 is provided being located on the outer

circumference of the second member 12. The second

waveguide 22 may have a length elongating approximately 10° to 90° of the circumference of the second member 12. The second waveguide 22 has two opposing ends. The first end is coupled to an optical sender 32. The optical sender 32 is adapted to receive an electrical signal representing data to be transferred. Furthermore, the optical sender 32 comprises a light source such as a light emitting diode or a

semiconductor laser to provide an optical carrier signal . Furthermore, the optical sender 32 comprises some circuitry to modulate the optical carrier signal so that the emitted modulated light represents the data to be transferred. In some embodiments of the invention, the optical sender 32 may be of a type known from optical telecommunication systems.

The opposing second end of the second waveguide 22 is configured as a beam dump, i.e. light reaching the second end of the second waveguide 22 is absorbed either by the waveguide and/or by the adjacent material of the second member 12.

The second waveguide 22 may be embodied as a polymer optical fiber or a glass fiber. It may comprise a step-index fiber as can be seen from Figs. 2 and 3 or a gradient fiber. It may be a multimode or a monomode fiber.

It's clear from the description that the first embodiment of the invention is described such that the second waveguide 22 is configured to be a sending waveguide and the first waveguide 21 is configured to be a receiving waveguide. In other embodiments of the invention, the first waveguide 21 may be configured as a sending waveguide and the second waveguide 22 may be configured as a receiving waveguide. In still another embodiment, a bidirectional data transmission may be realized either by time division multiplexing or wavelength division multiplexing or the first waveguide 21 may be configured as a sending waveguide and the second waveguide 22 may be configured as a receiving waveguide.

The first waveguide 21 and the second waveguide 22 are arranged substantially parallel in a longitudinal

portion 15. The longitudinal portion 15 is configured to allow an evanescent coupling of the optical signal from the second waveguide 22 to the first waveguide 21. As can best be seen from Fig. 3, the evanescent coupling may be improved by at least partly removing the cladding 212 and 222 from the waveguides 21 and 22 in at least a subsection of the longitudinal portion 15. As total reflection of the optical signal guided in the core 221 is reduced or even does not occur if the cladding 222 is removed, the evanescent field of the optical signal outside of the core 121 is increased in the subsection missing the cladding 222 at least partly. Therefore, an increased fraction of the optical signal may couple to the core 211 of the first waveguide 21. This may increase the signal strength received at the optical

receiver 31 which may result in higher data rate or lower bit error rate.

Furthermore, it can be seen from Fig. 3 that the

waveguide 21 is embedded in the groove 110 by means of a synthetic resin 111. The resin 111 may limit the movement of the waveguide 21 inside the groove 110 so that the gap 10 may be decreased and/or a constant width of the gap 10 between the first and second member may be achieved.

In a similar manner, the waveguide 22 is embedded in the groove 120 by means of a synthetic resin 121. The resin 121 may limit the movement of the waveguide 22 inside the groove 120 so that the gap 10 may be decreased and/or a constant width of the gap 10 between the first and second member may be achieved. In order to allow for a good evanescent coupling of the optical signal, the gap 10 may amount between 5 μπι to 100 μπι or between 10 μπι up to 50 μπι. The first member 11 and the second member 12 may be part of a hydrodynamic bearing, i.e. the gap 10 may be filled by an oil or a gas. This allows easy and secure movement between the first member 11 and the second member 12 and minimizes the risk of damage to any of the first and second member or the first or second

waveguides .

The circular cross section of the waveguides 21 and 22 and the circular cross section of their cores 211 and 221 is shown as an exemplary embodiment only. In other embodiments, other cross sections may be used such as a polygonal cross section .

Fig. 4 shows the cross-sectional view through a second embodiment of the invention. Same parts of the invention are denoted with the same reference numbers so that the

following description is limited to the main differences between the first and the second embodiment.

Fig. 4 shows basically the same detail as Fig. 3. This means that a device 1 according to the second embodiment comprises also a first member 11 and a second member 12 having a basically toroidal shape. The second member 12 and the first member 11 have basically the same inner and outer diameter. The first waveguide 21 and the second waveguide 22 are located inside a first groove 110 and the second groove 120 respectively. The grooves 110 and 120 are located in

opposing front faces of the toroidal members 11 and 12. The waveguides 21 and 22 may be embodied as step-index fiber which is fixed inside the respective groove by means of a synthetic resin.

As can be seen from Fig. 4, the cladding 122 of the

waveguide 21 has been removed at the longitudinal portion 15 in order to promote the evanescent coupling of the optical signal .

Furthermore, Fig. 4 shows a position transducer 40 which is adapted to measure the width of the gap 10. This may be done by a capacitive measurement, an optical measurement or any other position transducer known from the art. Supplying the values measured by the position transducer 40 to a closed loop control may allow in some embodiments of the invention to actively control the width of the gap 10 and thereby avoiding contact between the first member 11 and the second member 12 and nevertheless to guarantee a constant fraction of light being coupled from the sending waveguide to the receiving waveguide.

Fig. 5 shows a third embodiment of the invention in cross- section. Same parts of the invention are denoted with the same reference numbers so that the following description is limited to the main differences between the third embodiment and the embodiments explained earlier.

As can be seen from Fig. 5, the embodiment shown is

basically the same as detailed with respect to Fig. 4. The second member 12 has a groove 120 which is machined with approximately the double width, so that two waveguides 22a and 22b may be located adjacent to each other inside the groove 120. The relative position of the waveguides 22a and 22b may be ensured by means of a synthetic resin filling the remaining cross-section of the groove 120.

The first member 11 is fitted with two independent

grooves 110a and 110b, each groove receiving a single optical fiber 21a and 21b. The relative position of the waveguides 21a and 21b with respect to the first member 11 may be ensured by means of a synthetic resin filling the remaining cross-section of the grooves 110a and 110b. Fig. 5 shows two different embodiments for including a plurality of waveguides into a first or second member 11 or 12. Of course the invention does not rely on using both embodiments simultaneously. A plurality of grooves may be designed in both members 11 and 12 or a single groove 120 may be machined in both members 11 and 12.

Applying a plurality of waveguides to each member 11 and 12 may be advantageously for increasing the data rate to be transmitted between the members or may allow for a

bidirectional data transmission if the first member 11 and the second member 12 each comprise a sending waveguide and a receiving waveguide, respectively.

The core has not been removed from the waveguides as

explained previously. The cladding of the waveguide

comprises a dopant. The dopant may be selected such as to cause a predefinable refractive index of at least a part of the cladding 222. This refractive index is smaller than the refractive index of the core of the waveguide 22 so that a total reflection between core and cladding occurs. To promote the evaneszent coupling, an undoped region 223 facing the other waveguide is provided so that a fraction of the intensity of the optical signal is allowed to couple from the second waveguide 22 to the first waveguide 21 or vice versa. Thus, the core has at least in a subsection not a circular cross section.

Fig. 6 shows a fourth embodiment of the invention. The embodiment shown in figure 6 corresponds basically to the embodiment shown in figure 1. Same parts are assigned the same reference numbers, so that the description will be restricted to the main differences.

The device shown in figure 6 comprises two second waveguides 22a and 22b. As the first waveguide 21 does not extend to the full 360° of the circumference of the first member 11, the device according to figure 6 avoids that data transmission is interrupted at a position where the sending waveguide 22 does not oppose the receiving waveguide 21. This embodiment is in particular use full when the

application of the device does not allow a continuous rotation, e.g. if the device is included in a turret of a tank .

Fig. 7 explains schematically the bidirectional data

transfer with a single fiber or an increased data rate by means of a wavelength division multiplexing method.

Fig. 7 shows the first waveguide 21 and the second waveguide 22. In order to increase the intelligibility of the Figure, the first member 11 and the second member 12 are not shown. It is clear that the principle explained with respect to figure 7 may be included in any of the examples detailed above. The first waveguide 21 and the second waveguide 22 are arranged substantially parallel to each other in a longitudinal portion 15.

Each waveguide 21 and 22 is coupled to a respective

multiplexer which is embodied as an arrayed waveguide grating 30a and 30b. In other embodiments of the invention, another multiplexer may be used such as a dichroic mirror, a grating, prism or a 3db coupler. In principle, any device is suitable that is adapted to focus different wavelengths to a single spot or vice versa.

The arrayed waveguide grating 30 comprises an input 310. Incoming light is allowed to propagate freely and to couple to a plurality of waveguides 330 having different length. At the second end of the waveguides 330, a second propagating zone 340 is provided. The light interferes which results in different wavelengths arriving at different outputs 350. In the embodiment shown, five different outputs 350 are

provided, each output sending or receiving light of a different wavelength. The number of outputs may vary in other embodiments of the invention.

Any of the outputs 350 may be coupled to a respective optical sender 32 or optical receiver 31 in order to send or receive data at the respective wavelength. This allows either an increases data rate in an unidirectional data transfer system or a bidirectional data transfer between the first and second member.

The illustrative embodiments and modifications thereto described hereinabove are merely exemplary. It is understood that other modifications to the illustrative embodiments will readily occur to persons of ordinary skill in the art. All such modifications and variations are deemed to be within the scope and spirit of the present invention as will be defined in the accompanying claims. The above described embodiments may be used alone or combined for use.