Fresnel lens apparatus for optically coupling a plurality of data channels

Disclosed is fresnel lens apparatus for optically coupling a plurality of data channels between stationary and rotating systems. Each data channel of the apparatus includes an optical data transmitter, an optical data receiver and a fresnel lens optical element. The plurality of fresnel lens optical elements are preferably annular and arranged concentrically in a plane. Each fresnel lens optical element focuses the optical data from a transmitter to a respective receiver which is positioned off axis with respect to the central axis of the concentric optical elements.

CROSS-REFERENCE TO A RELATED APPLICATION 
Reference is made to commonly assigned, copending U.S. patent application 
Ser. No. 249,820; filed Sept. 27, 1988; entitled "Optical Data Signal 
Apparatus For Optically Coupling A Plurality of Data Channels Between 
Stationary and Rotating Systems"; Inventors M. F. Estes and A. W. 
Lungershausen. 
BACKGROUND OF THE INVENTION 
This invention relates in general to apparatus for coupling data between 
stationary and rotary systems. More particularly this invention relates to 
fresnel lens apparatus for optically coupling a plurality of data channels 
between the rotating and stationary structure of the rotary head scanner 
of a magnetic tape recorder. 
Advanced magnetic tape recording and reproducing systems require ever 
higher data transfer rates and increased bandwidths. In helical scan 
magnetic tape recorders using rotary head scanners, there must be some 
means for transmitting signals between the rapidly rotating head wheels 
upon which the recording and reproducing heads are mounted and stationary 
signal processing circuitry. Typically, rotary transformers are used to 
transmit both the record and reproduce signals from and to the magnetic 
heads carried by the rotating head wheel. Rotary transformers, however, 
have a predicted upper frequency limit of approximately 150 megahertz. 
Moreover, at such high signal frequencies, dimensional tolerances between 
the stationary and rotary coils of the rotary transformer are severe and 
cross talk between adjacent rotary transformers is difficult to eliminate. 
In order to mitigate the disadvantages of rotary transformers, several 
proposals have been made to optically transmit signals between the 
stationary and rotating structure of a rotary head scanner. Optical 
transmission has an upper frequency limit of six gigahertz at the present 
time, with an unknown limit in the future. Moreover, crosstalk between 
record and reproduce signals and crosstalk between adjacent channels is 
virtually eliminated and dimensional tolerances are less severe. Where 
only one or two signals (for example, record and reproduce signals) are to 
be optically transmitted between the rotating and stationary structure of 
a rotary head scanner, the simplest technique is to have the optical 
signal transmission axis coincide with the axis of rotation of the head 
wheel or to have, at least either the optical signal transmitter or 
receiver coincident with the axis of rotation of the rotating head wheel. 
Such a technique is disclosed, for example, in Japanese Kokai Patent No. 
53-21912, published Feb. 28, 1978, Inventors, Koshimoto et al.; in U.S. 
Pat. No. 4,401,360, issued Aug. 30, 1983, Inventors Streckmann et al.; and 
in U.S. Pat. No. 4,511,934, issued Apr. 16, 1985, Inventors Ohira et al. 
The arrangements disclosed in these patents are generally not easily 
adaptable to the transmission of multiple optical signals over separate 
optical paths. In order to minimize interference between the signals, 
either a half mirror or filter is used to separate two optical signals 
transmitted over the same optical axis which coincides with the axis of 
rotation of the head wheel. 
Although multiplexing techniques (for example, time division multiplexing, 
frequency division multiplexing or wavelength division multiplexing) may 
be used to optically transmit multiple signals along the same optical 
axis, the circuitry required to multiplex and demultiplex such signals is 
complex and costly and susceptible to crosstalk and signal degradation. 
Thus, it is desirable to optically transmit each signal along a separate 
optical path in order to minimize such difficulties. Several techniques 
have been proposed for providing optical transmission between a first 
array of optical elements mounted on a rotating body and a second 
complementary array of optical elements mounted on a stationary or 
rotating body. Thus, in U.S. Pat. No. 4,447,114, issued May 8, 1984, 
Inventor Koene, there is disclosed an optical coupling body which is 
disposed between first and second arrays of optical conductors mounted for 
rotation at equal but opposite rates relative to the optical coupling 
body. Either light reflective or light transmissive means is arranged in 
the body to effect constant coupling of individual conductors in the first 
array with respective conductors in the second array. A similar technique 
is disclosed in U.S. Pat. No. 4,109,998, issued Aug. 29, 1978, Inventor 
Iverson and U.S. Pat. No. 4,258,976 issued Mar. 31, 1981, Inventors Scott 
et al. As disclosed, a derotation assembly is located between a stationary 
body and a rotating body having respective complementary arrays of optical 
transmitting and optical receiving elements. The derotation plate is 
rotated at half the speed of the rotating body. The optical transmission 
arrangements of the latter three patents are disadvantageous because of 
mechanical and electrical complexity, cost, and unreliability. 
U.S. Pat. No. 4,519,670, issued May 28, 1985, Inventors Spinner et al., 
discloses a light rotation coupling for the transmission of a plurality of 
light channels between two parts which rotate relative to each other. A 
plurality of radially arrayed light transmitters rotate about an axis 
which coincides with the optical axis of a multiple refractive or 
reflective light transmitting optical assembly. A plurality of light 
receivers are axially arrayed along the optical/rotation axis. This 
technique is disadvantageous because of the use of complex, expensive and 
heavy optical transmission assemblies. Moreover, locating the light 
receivers on the axis of rotation is disadvantageous for several reasons. 
First, the coupling system is bulky and not suitable for applications 
where space is at a premium. Moreover, bidirectional transmission of data 
is difficult because the optical data channels are interfered with by the 
electrical conductors and hardware associated with the other optical data 
receivers. Thus, data cannot be transmitted at all times. 
In another proposed optical signal transmission technique, individual 
optical slip rings are stacked along the axis of rotation of a moving 
body. Thus, U.S. Pat. No. 4,278,323, issued July 14, 1981, Inventor 
Waldman, discloses an optical signal transmission system which includes a 
plurality of axially spaced optical fiber rings rotatably mounted on a 
spindle. The rings are mounted in a fixed casing having a number of 
separate chambers corresponding to the number of optical fiber rings. 
Fixed fiber optic blocks surround each fiber optic ring. Complementary 
LEDs and photodiodes are respectively embedded in each optic fiber ring 
and block to effect optical signal transmission between the stationary 
optic fiber block and the rotating optical fiber ring. In U.S. Pat. No. 
4,444,459, issued Apr. 24, 1984, Inventor Woodwell, a fiber optic slip 
ring comprises a toroidal optic wave guide which is uncoated along a 
circumferential window extending along the inner or outer circumference of 
the wave guide. An electronic device for transmitting an optical signal is 
connected to one end of the wave guide. A ring of radiation is produced so 
that an optical signal receiver may be positioned at any location around 
the circumference of the wave guide to receive the transmitted optical 
signal. As disclosed in the latter patent, a plurality of slip rings may 
be arrayed along the rotational axis of the receiver in order to effect 
transmission of a plurality of separate optical signals. The optical slip 
ring systems of the latter two patents are disadvantageous, among other 
reasons, because of the inefficiency in producing a 360.degree. ring of 
radiation. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided optical data signal 
apparatus which obviates the disadvantages of the prior art. According to 
an aspect of the present invention, a plurality of optical data signals 
may be coupled between stationary and rotating members by means of an 
optical coupling assembly which is simple, light-weight and inexpensive. 
Moreover, crosstalk between coupled optical data signals is substantially 
minimized. 
According to an aspect of the invention, an optical data signal apparatus 
includes an optical data signal transmitter and an optical data signal 
receiver which are rotatable with respect to one another about a 
rotational axis. The receiver and the transmitter are both spaced from the 
rotational axis. The apparatus includes a preferably annular, 
substantially planar, fresnel lens optical coupling element which has a 
central axis coincident with the rotational axis. The fresnel lens optical 
coupling element causes a collimated optical data signal, transmitted by 
said transmitter along a path, preferably perpendicular, to said optical 
element, to be focused along a path to said receiver. 
According to another aspect of the present invention, the optical data 
signal receiver is stationary, the optical data signal transmitter rotates 
about a rotational axis relative to said stationary receiver, and the 
fresnel lens optical coupling element is stationary and causes an optical 
data signal transmitted by said rotating transmitter to be focused to said 
stationary receiver. 
According to a further aspect of the present invention, the optical data 
signal transmitter is stationary, the optical data signal receiver rotates 
about a rotational axis relative to said transmitter, and the fresnel lens 
optical coupling element rotates about the rotational axis in synchronism 
with the receiver and causes an optical data signal transmitted by the 
stationary transmitter to be focused to the rotating receiver. 
According to still another aspect of the present invention, a plurality of 
optical data signals are transmitted and received between stationary and 
rotating members which may comprise the stationary and rotating components 
of a rotary magnetic head scanner of a helical magnetic tape recorder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following description of preferred embodiments of the present 
invention, specific application will be described relating to rotary head 
scanners in helical scan magnetic tape recording/reproducing apparatus. It 
will be understood, however, that the present invention may be used in any 
application where data signals are to be optically coupled between a 
rotating body and a stationary body. Other such applications include, for 
example, coupling of optical data signals between rotating signal 
processing equipment and stationary signal processing equipment, such as 
in radar equipment or the like. 
Referring now to FIG. 1A, there is shown a diagrammatic perspective view 
illustrating a general embodiment of the present invention. As shown, 
optical data signal transmitters 10A, 10B, 10C, 10D mounted on member 12 
rotate about rotational axis 14. Transmitters 10A-10D (which may, for 
example, be laser diodes) transmit collimated optical data signals along 
respective paths 16A, 16B, 16C, 16D which are perpendicular to fresnel 
lens optical coupling elements 18A, 18B, 18C, 18D. Optical coupling 
elements 18A-18D comprise concentric annular, substantially planar, 
fresnel lens optical elements. Optical coupling elements 18A-18D are 
stationary and cause the collimated data signals to be focused along paths 
20A-20D to respective optical data signal receivers 22A-22D which are 
spaced from rotational axis 14. Receivers 22A-22D may, for example, be 
photodiodes or the like. 
Each fresnel lens optical coupling element 18A-18D has a central axis which 
is coincident with rotational axis 14. Each of elements 18A-18D is 
dimensioned to be coextensive with the circular path of radiation 
transmitted by its respective transmitter 10A-10D, in completing a 
revolution about axis 14. An optical element 18A-18D has a width which is 
determined by the size of the beam of optical signal transmitted by a 
transmitter 10A-10D. The fresnel optical characteristics of an optical 
coupling element 18A-18D are such as to focus radiation which is 
perpendicular to element 18 at any point around its length to a single 
point which is off of the rotational axis 14. The optical data signal 
receiver is placed at this point in space. 
Optical coupling elements 18A-18D are fresnel optical elements which may be 
made according to the technique illustrated in FIG. 6. It will be 
understood that any other technique known to the those skilled in the art 
may be used to form the fresnel lens. As shown, a fresnel lens is 
configured as a planar sheet 24 with the center of the lens at the center 
26 of the sheet. Light rays parallel to a central axis passing through 
center 24 will be focused at a point on the axis. The fresnel lens 
includes a plano side and concentric sections of convex lens surfaces each 
of which is equidistant from the plano side. The surface angle of each 
section increases as its radial distance increases from the central axis. 
The annular fresnel optical elements 18A-18D (FIG. 1A) may be prepared by 
cutting a circular piece 28 from sheet 24. The circle is defined by using 
two adjacent sides 30A and 30B and center 26 of sheet 24. The center 31 of 
this circle is used to cut out annular elements 18A-18D. The annular 
elements are respectively rotated by an angle Nx phi. Thus, annular 
fresnel optical element 18D is rotated by an angle phi, annular fresnel 
optical element 18C is rotated by an angle 2.times. phi, annular fresnel 
optical element 18B is rotated an angle 3.times. phi; and annular fresnel 
optical element 18A is rotated by an angle 4.times. phi. By this 
technique, light rays which impinge perpendicularly on an annular fresnel 
optical element are caused to be focused to the focal point of lens 24 
offset by the angle of rotation of the element (i.e., phi, 2.times. phi, 
etc.). Thus, optical signal data receivers 22A-22D are located in a circle 
circumferentially displaced by the appropriate offset angles. 
The width of the annular ring need only be equal to the width of the 
collimated radiation beam impinging on it. Thus, a plurality of optical 
signal data channels may be provided by assembling a plurality of 
concentric annular fresnel optical elements in a plane. Each such element 
has a different radius and focuses an optical data signal to a different 
point in space which is off of the central axis of the annular optical 
element. By using optical techniques, the individual data channels can 
have an extremely high bandwidth. The optical element is small, 
light-weight and occupies little volume since the element is substantially 
planar. Moreover, crosstalk between adjacent channels is substantially 
eliminated. It will be understood that the fresnel lens may take any 
configuration other than annular as described herein. 
Referring now to FIG. 1B, there is shown another embodiment of the present 
invention. As shown, optical data signal transmitter 32, mounted on 
stationary member 34, transmits an optical data signal along a path 36 
which is perpendicular to rotating fresnel optical coupling element 38. 
Element 38 comprises an annular ring which focuses optical data signal 36 
along a path 40 to optical data signal receiver 42 mounted on rotating 
member 44. Element 38 and receiver 42 are rotated in synchronism about 
rotational axis 46. Fresnel optical coupling element 38 has a central axis 
which is coincident with rotational axis 46. Receiver 42 and transmitter 
32 are spaced from rotational axis 32, so that optical coupling element 38 
focuses perpendicular radiation impinging on it to a point (receiver 42) 
which is off of axis 46. 
Referring now to FIGS. 2-5, there is shown another embodiment of the 
present invention, as incorporated in a rotary magnetic head scanner of a 
helical scan magnetic tape recorder. As shown, rotary head scanner 48 
includes upper and lower stationary drums 50 and 52, and a rotating head 
wheel 54 upon which are mounted a plurality of magnetic heads, such as, 
heads 56 and 58. Scanner 48 is mounted in a helical scan, magnetic tape 
recording/reproducing apparatus, in which a magnetic tape 60 is 
transported between supply and take-up reels (not shown) around scanner 
48. Tape 60 is wrapped around scanner 48 in a helical path so that as head 
wheel 54 rotates, record/reproduce heads 56 and 58 record to and reproduce 
from slant tracks on tape 60. Head wheel 54 is rotatably mounted on a 
shaft 62 which is rotated by head wheel drive circuit 64. Circuit 64 
includes a motor (not shown) mechanically linked to shaft 62 and also 
includes a tachometer (not shown) which is linked to shaft 62 to provide 
speed and phase control signals to control the rotational speed and phase 
of the motor. 
According to the present invention, record and reproduce signals are 
optically coupled between rotary record/reproduce circuit 66 and 
stationary record/reproduce circuit 68 by means of an optical data signal 
apparatus. Such apparatus includes rotating assembly 70 mounted on head 
wheel 54 and stationary assembly 72 mounted on stationary drum 52. The 
optical data signal apparatus of the invention is capable of transmitting 
far greater bandwidth and higher frequency signals between the stationary 
and rotating members of scanner 48 than is possible with conventional 
rotary signal transformers. Thus, whereas the upper frequency of a rotary 
transformer is limited to less than 200 megahartz, optical signals may be 
transmitted in the several gigahartz range. Optical signal transmission 
provides excellent signal to noise ratio; minimizes signal degradation 
during transmission; and eliminates electromagnetic and radio frequency 
interference from external and internal sources. Moreover, by providing a 
separate optical coupling link for each record and reproduce channel, 
crosstalk between channels is virtually eliminated. The optical data 
signal apparatus of the invention is simple in design and construction, 
light-weight, inexpensive and may be manufactured at low cost, by 
well-known mass production manufacturing techniques due to less severe 
dimensional tolerances than rotary transformers. 
Referring to FIG. 2, stationary assembly 72 includes first and second 
stationary optical data signal transmitters 74 and 76 and first and second 
stationary optical data signal receivers 78 and 80. Assembly 72 also 
includes stationary fresnel optical coupling element 82. Rotary assembly 
70 includes first and second rotating optical data signal transmitters 84 
and 86 and first and second rotating optical data signal receivers 88 and 
90. Rotating assembly 70 also includes a rotating fresnel optical coupling 
assembly 92. 
Rotating transmitters 84 and 86 are radially spaced from rotational axis 94 
of head wheel 54. Stationary transmitters 74 and 76 are also radially 
spaced from axis 94, but at different radial distances than transmitters 
84 and 86. This staggered spacing assures that collimated optical data 
signals 96 and 98 from stationary transmitters 74 and 76, respectively, do 
not interfere with collimated optical data signals 100 and 102 transmitted 
by rotating transmitters 84 and 86. Thus, there is substantially no 
crosstalk between adjacent channels and bidirectional communications may 
be effected between the rotating and stationary systems of rotary scanner 
48. 
Referring now to FIGS. 3-5, the construction and operation of assembly 70 
and assembly 72 will be described in greater detail. As shown, stationary 
fresnel optical coupling assembly 82 includes substantially planar, 
annular fresnel optical coupling elements 104 and 106 which are concentric 
about central axes which coincide with rotational axis 94 of head wheel 
54. Assembly 82 also has opaque areas 107 and 108 and transparent annular 
rings 110 and 112. Rotating fresnel optical coupling assembly 92 includes 
substantially planar, annular fresnel optical coupling elements 114 and 
116 having central axes which are coincident with the rotational axis 94. 
Elements 114 and 116 are concentric and rotatable about axis 94. Assembly 
92 also includes opaque regions 118 and 120 and optically transparent 
annular rings 122 and 124. 
Assemblies 82 and 92 are parallel to each other and perpendicular to axis 
94. Transparent rings 122 and 124 of assembly 92 are axially aligned with 
fresnel optical coupling rings 104 and 106, respectively, of assembly 82. 
Thus, collimated optical data signals transmitted by rotating transmitters 
84 and 86, will pass through rings 122 and 124 and impinge perpendicularly 
on fresnel optical coupling elements 104 and 106. Fresnel optical element 
rings 104 and 106 cause the optical data signals to be focused to 
stationary optical data signal receivers 78 and 80. Similarly, the 
optically transparent rings 110 and 112 of stationary assembly 82 are 
axially aligned with fresnel optical coupling elements 114 and 116 of 
rotating assembly 92. Thus, collimated optical data signals transmitted by 
stationary transmitters 74 and 76, will pass through rings 110 and 112 and 
impinge perpendicularly on optical coupling elements 114 and 116. Elements 
114 and 116 cause the data signals to be focused to rotating optical data 
signal receivers 88 and 90. 
Although the optical data signals transmitted by transmitters 74 and 76 and 
84 and 86 may comprise a single signal, it will be appreciated that such 
signals may be multiplexed according to well known multiplexing 
techniques, so that a plurality of signals may be transmitted over a 
single channel. Thus, time-division multiplexing, frequency division 
multiplexing and radiation-wave division multiplexing may be utilized to 
increase the number of separate signals transmitted over a single channel 
to and from head wheel 54. It will also be appreciated that although the 
apparatus of FIG. 2 is described as having two sending and two receiving 
channels, any number of optical data signal channels may be between the 
stationary and rotating systems of rotary scanner 48. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.