Apparatus and method for a single return path signal sensor system

The radiation resulting from interaction with a data track or groove on a storage surface of an optical information storage and retrieval system is separated into three components and detected to provide tracking, focussing, and data signals. The separation is performed using a dual diffraction grating in a single optical path. The division between grating elements in the dual diffraction grating is oriented perpendicular to the data track or groove projected on the grating element. Diffraction radiation components generated by the dual diffraction grating are applied to a first and a second dual sensor elements. The first and second dual sensor elements provide a focusing signal. The undiffracted radiation component transmitted by the dual grating is applied to a third dual sensor. The division between sensors of the third dual senor is perpendicular to the division of the dual grating. Signals from the third dual sensor elements provide the tracking signal and the data signal. Several embodiments of the basic configuration are disclosed including a variety of configurations for defocusing the undiffracted transmitted radiation on the third dual sensor. In addition, a cylindrical lens can be used to defocus the radiation components from the diffraction grating in a single dimension.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates generally to the read/write heads in optical 
information storage and retrieval systems and, more particularly, to the 
read/write head optical and sensor component configuration which generates 
the data, tracking, and focusing signals as a result of processing the 
resulting radiation beam, the radiation beam resulting from interaction 
with the radiation beam with a data track or grooves in the storage 
medium. 
2. Description of the Related Art 
Referring to FIG. 1, one configuration for an optical information storage 
and retrieval system, according to the related art, is shown. A radiation 
source 11, typically a laser diode, provides a radiation beam which is 
collimated by collimating lens 12. The collimated radiation beam is 
transmitted through polarization beam splitter 13 and applied to quarter 
wave plate 14. The polarization beam splitter 13 provides a linear 
polarization for the radiation beam and the quarter wave plate 14 provides 
a circular polarization to the radiation beam. The circularly polarized 
radiation beam transmitted by the quarter wave plate 14 is focused by 
objective lens 15 on the information storage surface 10A of the storage 
medium 10. The storage medium 10 is typically a disk with a surface which 
interacts with the circularly polarized radiation beam. The interaction 
with the storage medium surface 10A causes the radiation beam to be 
reflected and diffracted therefrom. The resulting information beam is 
collimated by objective lens 15 and the collimated resulting radiation 
beam transmitted through the quarter wave plate 14. The quarter wave B 
plate restores the linear polarization of the radiation beam. However, a 
component polarization perpendicular to the original polarization of the 
radiation beam will typically be present as a result of the interaction of 
the radiation beam with the storage surface. When the radiation beam is 
applied to the polarization beam splitter 13, the perpendicular component 
resulting from the interaction of the radiation beam with the storage 
surface 10A will be reflected by the beam splitter 13 while the 
polarization component parallel to the polarization component generated as 
a result of transmission through the polarization beam splitter 13 will 
pass through the polarization beam splitter. The reflected radiation beam 
is applied to sensor focusing lens 16. The lens 16 converges the reflected 
radiation beam on sensor array 5. The reflected beam has imposed thereon 
modulation that can be processed to provide the information (or data) 
which is stored on the disk. In addition, the reflected beam can be 
processed in such a manner as to provide tracking and focusing signals 
which can be used to activate apparatus which controls the position of the 
focused radiation beam on the storage surface 10A (i.e., the tracking in 
one dimension) and which controls the distance of the objective lens 15 
from the storage surface 10A, (i.e., the focusing of the radiation beam on 
the storage surface). In this type of optical information storage and 
retrieval system, the quarter wave plate imparts, to the radiation beam 
illuminating the storage surface 10A, a circular polarization. After 
interaction with the storage surface, the quarter wave plate restores the 
linear polarization, however, the linear polarization will be rotated from 
the plane of polarization originally established by the polarization beam 
splitter 13. The rotated linearly polarized radiation component of the 
radiation resulting from interaction with the storage surface 10A is 
reflected by the beam splitter 13 and applied to sensor array 5. 
Referring to FIG. 2, an example of the use of the processing of the 
radiation beam to provide tracking and focusing signals, according to the 
related art, is shown. This example is taken from European Patent 
Application 0,177,108 A1, issued in the name of A. Smid, P. F. Grave, and 
H 't Lam, entitled "Opto-Electronic Focussing-Error Detection System", and 
filed on Feb. 10, 1985. In this Figure, the path of the resulting 
radiation beam, the radiation beam which has already interacted with data 
track 21, is shown. (The quarter wave plate 14 and the beam splitter 13 
have been omitted to emphasize certain important aspects of the 
configuration.) The data track or groove 21 is the path on the storage 
surface (10A of FIG. 1) along which the radiation beam will move in 
accessing or storing the information. A dual prism 25 is shown interposed 
between the objective lens 15 and the sensor focusing lens 16. The dual 
prism divides the resulting radiation beam into two radiation components. 
The two radiation components are essentially 1.), the radiation component 
reflected and radiation component diffracted from a first side of the 
storage medium and 2.) the radiation reflected and radiation diffracted by 
a second side of the storage medium, the two sides being separated by a 
median line of the data track. The first radiation beam component is 
focused on dual sensor elements A and B of the sensor array 5, while the 
second radiation beam component is focused on dual sensor elements C and D 
of sensor array 5. As will be known to those skilled in the art of 
processing resulting radiation beams, the data signal DS, the focusing 
signal FS, and the tracking signal TS are given respectively by: 
EQU DS=A+B+C+D 1.) 
EQU FS=(A+D)-(B+C) 2.) 
EQU TS=(A+B)-(C+D) 3.) 
where A, B, C, and D of the Equations 1-3 represent the voltages developed 
by the equivalently designated sensor element when radiation is applied 
thereto. The data signal DS is the sum of voltages developed by all of the 
sensors elements. The focusing signal FS is the difference between the sum 
of the voltages resulting from the radiation applied to a first pair of 
diagonal sensor elements, i.e., A and D, and the sum of voltages resulting 
from the complementary diagonal pair of sensors, i.e., B and C. When the 
absolute value of the focusing signal FS is minimized, the radius of the 
radiation beam on the storage surface 10A will be minimized, i.e., the 
radiation beam will be focused on the storage surface. The tracking signal 
TS is minimized when the radiation reflected and diffracted from one side 
the center of the data track and from the other side below the center of 
the data track are equal. In order to understand how the tracking signal 
is derived, the role of the diffraction of the radiation beam must be 
understood. 
Referring to FIG. 3A, the objective lens 15 is shown focusing the 
circularly polarized radiation beam on the storage surface 10A of storage 
medium 10. The storage surface 10A is shown as having a multiplicity of 
grooves, or equivalently, a multiplicity of data tracks 10B fabricated 
therein. The grooves 10B have dimensions relative to the wavelength of the 
radiation beam whereby diffraction patterns are formed. The data tracks 
10B can be replaced with series of raised regions which are not connected, 
can be replaced with regions of appropriate dimension and refractive 
index, or any other structure which provides diffraction patterns in 
response to an impinging radiation beam without departing from the scope 
of the present invention. Referring to FIG. 3B, the resulting radiation 
beam after interaction with the storage surface is shown. The resulting 
radiation beam includes a zeroth order (reflected) component and a 
positive and a negative diffracted component. As will be clear, higher 
order diffraction components can be present, however, the present 
invention can be understood without further consideration of these 
components. The impinging radiation beam is shown as being off center and 
therefore closer to one edge of the data track or groove which is 
currently being tracked. This asymmetric positioning causes a wavefront 
phase shift in the diffracted orders and, consequently, an asymmetric 
interference between each of the diffracted components and the 
undiffracted (i.e., reflected or zeroth order radiation component). As a 
consequence, constructive interference occurs in one region, e.g., the 
region of overlap between the reflected radiation component and the + 
diffracted radiation component, while destructive interference occurs 
between the reflected radiation component and the -1 diffracted component. 
The magnitude of the resulting signal depends on the amount of shift of 
the impinging beam relative to the center of the data track or groove. In 
FIG. 3C, the difference between the intensities of the regions of 
interference is illustrated by region 32 (wherein the undiffracted 
radiation component and the +1 first order interference component 
interfere) and region 34 (wherein the undiffracted radiation beam 
component and the -1 first order diffracted radiation beam interfere). The 
polarity depends on whether the tracking of the radiation beam occurs for 
the data tracks (or grooves) or for the lands, i.e., the regions between 
the data tracks or grooves. Note that in the preferred embodiment, the two 
first order diffraction components are contiguous with the optic axis of 
the radiation beam. As a consequence, the two first order diffraction 
components will be superimposed on and will interfere with the reflected 
radiation beam. Referring once again to FIG. 2, the projection of the 
first order diffraction patterns 29A and 29B are shown on objective lens 
15 and on dual prism 25. The difference in intensities of the resulting 
radiation components separated by dual prism 25 is determined by the 
relative intensities of the radiation components resulting from the 
interference between the undiffracted (reflected) radiation component and 
the first order diffraction components. It will be clear that the groove 
can be replaced by a diffracted and undiffracted radiation components 
resulting from applying a radiation beam to a data track without an 
associated groove, the data track implemented to provide the requisite 
diffracted and undiffracted radiation components. 
The configuration for providing tracking signals and focusing signals, as 
disclosed by the Smid, suffers from a significant amount of optical 
cross-talk, generally originating from ever-present wavefront aberrations 
and the diffraction radiation components. Referring to FIG. 4, presence of 
optical cross-talk between the tracking signal and the focusing signal is 
illustrated. The presence of this optical cross-talk becomes particularly 
important in high performance signals such as are required in the 
information storage and retrieval systems. 
In U.S. Patent Application Ser. No. 07/998,179 filed on Dec. 29, 1992, now 
abandoned in the name of David B. Kay, entitled APATUS AND METHOD FOR A 
DUAL HALF APERTURE FOCUS SENSOR, and assigned to the assignee of the 
present invention, a read/write head configuration is disclosed which 
minimizes the cross-talk between the tracking signal and the focusing 
signal. Referring to FIG. 5, the configuration of optical and electrical 
components which provide data, tracking, and focusing signals while 
reducing the optical cross-talk, according to the Kay application, is 
shown. As in FIG. 2, the apparatus interacts with the resulting radiation 
beam, i.e., the radiation beam which has interacted with the storage 
medium 10. Other components, such as the quarter wave plate shown in FIG. 
1, have been omitted for clarity. The resulting radiation beam is 
recollimated by objective lens 15. The first order diffraction components 
29A and 29B are shown in FIG. 5 by shadowing on objective lens 15. As will 
be clear, the reflected radiation component is also present and collimated 
by the objective lens 15. The collimated radiation beam is applied to beam 
splitter 52 where a portion of the collimated radiation beam is reflected 
and applied to dual element sensor 51, the dual element sensor having 
sensor elements E and F. Each of the sensor elements E and F have applied 
thereto a portion of the collimated and reflected radiation beam which 
includes only one of the two first order diffraction components. The 
remainder of the collimated radiation beam transmitted by beam splitter 52 
is applied to dual prism 55. The dual prism 55 divides the applied 
radiation component into two focusing radiation beam components. Comparing 
dual prism 55 which dual prism 25 of FIG. 2, dual prism 55 is rotated 
90.degree. with respect to a projection of the data track 21 on the prism. 
Therefore, the focusing radiation components include portions of both 
first order diffraction components as illustrated by the shadowing shown 
on the dual prism 55. Sensor focusing lens 16 focuses the radiation 
component from each prism element of the dual element prism 55 on one of 
the dual element sensors 5. The first dual element sensor has elements A 
and B associated therewith, while the second dual element sensor has 
sensor elements C and D associated therewith. The disclosed configuration, 
as shown by inspection of FIG. 5, includes in a separate path for the 
tracking and for the focusing signals. The separate paths diminish the 
intensity of the radiation beam at the detectors and require additional 
space and components. In typical optical storage systems, having a 
read/write head, the space on the read write/head is typically limited. 
In U.S. Pat. No. 4,665,310 entitled "Apparatus For Optically Scanning An 
Information Plane Wherein A Diffraction Grating Splits The Beam Into Two 
Sub-Beams" and issued on May 12, 1987 in the name of Heemskert, a dual 
diffraction grating has been used in place of the dual prism to separate 
the radiation beam into two components. The separated components are 
thereafter used to provide the focusing and tracking signals. However, the 
cross-talk (coupling) between the tracking signal and the focusing signal 
has been found to limit high performance operation of an optical 
read/write head. 
A need has therefore been felt for apparatus and an associated method for 
an improved optical read/write head in which diffraction gratings are used 
to process components of a radiation beam resulting from interaction with 
a storage medium. After processing, the radiation beam components are 
typically applied to radiation sensors and used for the generation of 
tracking signals, focusing signals, and the data (or information) signals. 
In this type of grating based read/write head in an optical information 
storage system, a need has been felt for a read/write head in which the 
cross-talk between the tracking and the focusing signals are minimized and 
in which a single return path is used for processing of the resulting 
radiation, i.e., the radiation which has interacted with a data track in 
the storage medium. 
SUMMARY OF THE INVENTION 
The present invention is directed to overcoming one or more of the problems 
set forth above. Briefly summarized, according to one aspect of the 
present invention, the resulting radiation beam, i.e., the portion of the 
radiation beam which has interacted with the storage medium, is applied to 
a dual diffraction grating. One grating element of the dual diffraction 
grating applies a first order diffraction component of the radiation beam 
to a first dual sensor, while a second grating of the radiation beam 
applies a first order diffraction component to a second dual sensor. The 
transmitted portion or undiffracted component of the radiation beam 
transmitted by the dual grating is applied to a third dual sensor. Signals 
from the two sensor elements of the third dual sensor can be added to 
provide a data signal, while the signals from the two sensor elements of 
the third dual sensor can be subtracted to provide a tracking signal. The 
dual grating is positioned with the division between the two gratings at 
an angle of approximately 90.degree. with respect to the division between 
the sensor elements of the third dual sensor. In this manner, the signals 
from the sensor elements of the first and second dual sensors can be 
combined to minimize the optical cross-talk introduced by the diffraction 
components generated through the interaction of the radiation beam with 
the data track on the storage medium. Several embodiments and 
configurations are disclosed which permit flexibility in the design of the 
apparatus. 
The present invention advantageously provides that the radiation beam 
resulting from an interaction with a storage medium can be separated into 
components which can be used, once reduced to electrical signals, to 
provide the tracking, focusing, and data (information) signals. The 
cross-talk between the tracking and focusing signals is minimized, thereby 
providing for higher performance of the read/write head in the optical 
information storage and retrieval system. The disclosed embodiments 
require only one return radiation path, thereby reducing the physical 
space and the quantity of optical components required for implementation. 
These and other aspects, objects, features and advantages of the present 
invention will be more clearly understood and appreciated from a review of 
the following detailed description of the preferred embodiments and 
appended claims, and by reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
1. Detailed Description of the Figures 
Referring next to FIG. 6A through FIG. 6E, a block diagram of the 
components implementing a first embodiment of a grating-based focusing and 
tracking actuator, according to the present invention, is shown. The 
resulting radiation, i.e., the radiation which has interacted with the 
storage medium, is transmitted through dual blazed grating 61. The dual 
blazed grating 61 provides a zeroth order radiation component (the 
radiation beam directly transmitted by the dual grating 61), a first order 
diffraction component 60A and a first order diffraction component 60B, the 
radiation beam components 60A and 60B being the positive first order 
diffraction components for the blazed gratings. One grating is blazed to 
the right, while the other grating is blazed to the left. The three 
components from the dual grating are focused by sensor array lens 16 on 
the sensor array 5. The sensor array 5 is supported by substrate 62. In 
addition, a transparent protective coating 63 protects the sensor array 5. 
Protective coating 63 has a protective coating lens 63A formed therein to 
defocus the radiation component 60 applied to the third dual sensor. 
Referring to FIG. 6B, a top view of the dual grating 61, according to a 
first embodiment, is shown. The dual grating has two gratings 61A and 6lB 
each blazed to provide a substantially greater intensity of the (first) 
diffraction component in opposite directions when assembled into the dual 
grating 61. Also shown on the top view of the grating 61 are the 
projections of the zeroth order (i.e., the reflected or transmitted) 
resulting radiation beam component 29 and the two (first) order resulting 
radiation beam diffraction components 29A and 29B generated by interaction 
with the data track. Referring next to FIG. 6C, a top view of the sensor 
array 5 is shown. The sensor array 5 includes a first dual sensor array 
having sensor elements A and B, a second dual sensor array having sensor 
elements C and D, and a third dual sensor array having sensor elements E 
and F. The radiation beam component transmitted undiffracted through dual 
grating 61 is applied to the third dual sensor array with sensor elements 
E and F. The first order diffraction radiation, 60A(61A) from grating 61A 
is focused on the first dual sensor array and the first order diffraction 
pattern 60B(61B) from grating 61B is focused on the second sensor array. 
In FIG. 6D, a top view of a second embodiment of the dual grating 61, in 
which a non-blazed dual diffraction grating is provided, is shown. The 
rulings on the grating 6lB are at a positive angle .theta. with respect a 
perpendicular to the plane separating each grating of the dual grating 61 
while the rulings of grating 61A are at a negative angle .theta. with 
respect to the plane of separation of the two gratings of dual grating 61. 
Referring next to FIG. 6E, a top view the sensor array 5' used with the 
dual grating 61' of FIG. 6D is shown. As with the sensor array shown in 
FIG. 6C, a first, a second and third dual sensor array is included. The 
difference is that the first dual grating having elements A and B is 
rotated about a center of third dual sensor array by an negative angle 
.theta. while the second dual sensor array having elements C and D is 
rotated a positive angle .theta. with respect to the center of the third 
sensor array. In this manner, a first order diffraction component (60B) 
from grating 61A will be focused on the second sensor array, while a first 
order diffraction component 60A from grating 61 B will be focused on the 
first dual sensor array. 
Referring next to FIG. 7, a second embodiment for the apparatus for 
providing the tracking, focusing, and data signals is shown. In this 
embodiment, the sensor focusing lens 16 is placed prior to dual grating 61 
in the optical path of the resulting radiation. Either of the grating and 
detector array configurations of FIG. 6B and 6C or of FIG. 6D and 6F can 
be used with this embodiment. Because the radiation beam impinging on the 
dual grating 61 is, in general, no longer perpendicular to the ruled 
surface, optical compensation can be made in the focusing of the 
transmitted radiation beam component and the first order diffraction 
radiation beam components the detector array. The surface of the 
protective coating can be flat or can be tilted at an angle .phi. with 
respect to the plane of the sensor array. In either configuration, the 
lens 63A is fabricated in the surface of the protective coating to defocus 
the radiation beam on the third dual sensor (E and F) of FIGS. 6C and 6E. 
Referring to FIG. 8A, a third implementation for the apparatus for 
providing tracking, focusing, and data signals is shown. This 
implementation is similar to FIG. 6A except that a cylindrical lens 81 has 
been added in the optical path. With respect to the detector array, the 
radiation beam components are defocused in a direction perpendicular to 
the axis of symmetry of the cylindrical lens 81. This defocusing is shown 
in FIG. 8B for the sensor array described with respect to FIG. 6C. Note 
that the radiation beam components 29, 60A(61A), and 60A(61B). By 
elongating the radiation components, the tracking signal is available at 
the sensor elements E and F. 
Referring to FIG. 9A, FIG. 9B, and FIG. 9C, each Figure illustrates a 
technique for solving the problem that the radiation beam transmitted by 
the dual grating 61 is preferably out-of-focus when applied to the third 
dual sensor (E, F) whereas the diffraction components from the dual 
grating 61 and applied to dual sensors (A, B) and (C, D) are preferably 
in-focus, at least in one dimension. A lens fabricated in the protective 
coating has been described previously, however, this solution to the 
defocus problem can be difficult to implement. In FIG. 9A, the substrate 
is built up with structure 91, thereby raising dual sensor (E, F) above 
the plane of the dual sensors (A, B) and (C, D). This difference in the 
distance between the lens 16 and dual sensor (E, F) as compared to dual 
sensors (A, B) and (C, D) can be used to defocus the radiation impinging 
upon the dual sensor (E, F). In FIG. 9B, the dual grating parameters are 
selected to provide a large angle for the diffraction components. As a 
result of this large angle, the distance between lens 16 and dual sensors 
(A, B) and (C, D) is significantly greater than the distance between lens 
16 and the dual sensor (E, F). This difference in distance can be used to 
defocus the transmitted radiation applied to dual sensor (E, F) while 
focusing the diffracted radiation on dual sensors (A, B) and (C, D). In 
FIG. 9C, a small aperture (which also serves as a spatial filter) is 
formed in the in the detector array which includes sensor elements A, B, 
C, and D, permitting the dual sensor (E, F) to be positioned beyond the 
lens 16 focal point. This positioning of the dual sensor (E, F) defocuses 
the radiation falling thereon. 
Referring next to FIG. 10, the results changing the configuration of the 
present invention into the configuration of the related art is shown. When 
the division between the grating elements of the dual grating is in the 
0.degree. position relative to a projection of the data track on the dual 
grating, i.e., the angle of the present invention, the cross-talk is at a 
minimum. As the dual grating division is rotated into a position that is 
similar to the configuration shown in the reference of Smid et al, i.e., 
at and angle of 90.degree. relative to the data track, the cross-talk 
noise is increased more than 5 times. This noise level for the parameters 
of the optical storage disk upon which the measurements were made amounts 
to a focus excursion of the read/write head of over 1 micron. As a 
consequence, high performance is difficult to achieve as a result of the 
optical coupling between the focusing signal and the tracking signal. 
2. Operation of the Preferred Embodiment(s) 
It will be now appreciated that there has been presented an apparatus and 
an associated method for providing tracking signals, focusing signals, and 
data (information) signals employing a single path for the resulting 
radiation and reducing the cross-talk between the between the tracking 
signal and the focusing signal. The invention relies on the result that 
the focused beam tracking a data track (or groove) in the optical storage 
medium generates a positive and negative first order diffraction pattern. 
The diffraction pattern radiation and the reflected (undiffracted) 
radiation form a first and a second interference patterns. One of the 
interference patters will be stronger depending on which side of the data 
track or groove the applied radiation beam is focused. This difference in 
interference pattern intensity is used to provide the difference signal 
which can control the tracking mechanisms. The sensors providing the 
focusing signal receive radiation beam components in which noise generated 
by the diffraction components has been substantially cancelled. 
Referring to FIG. 6A-FIG. 6C, the signals generated by dual sensors (A, B), 
(C, D), and (E, F) can be determined. For the focus signal (FS) 
EQU FS=(A+D)-(B+C), 1.) 
where A, B, C, and D are the electrical signals generated as a result of 
radiation falling on the respective sensor elements. Similarly, the 
tracking signal is given by 
EQU TS=(E-F). 2.) 
The information or data signal (IS) from the storage medium is given by 
EQU DS=(E+F) 3.) 
or 
EQU DS=(A+B+C+D+E+F) 4.) 
The TS signal and the FS signal are respectively used by the servomechanism 
system to track and to focus the radiation being applied to the storage 
medium automatically. The use of grating optical elements to separate the 
optical components provides a lighter read/write head than is possible 
with a dual prism component, for example. And the disclosed embodiments 
all have a single path for the resulting radiation. In addition, the 
orientation of the dual grating in combination with the orientation of the 
division between the dual sensors in the disclosed embodiments results in 
reduced cross-talk between the tracking signal and the focussing signal. 
In particular, for transmitted radiation, the presence of the two gratings 
is irrelevant. The dual sensor receiving the transmitted radiation is 
divided such that the positive and negative diffraction components, 
resulting from the interaction with the data track on the storage medium, 
are each applied to a different sensor element. Therefore, taking the 
difference between the two sensor elements provides a tracking signal. The 
orientation of the division of two gratings of the dual grating at 
approximately 90.degree. to the division of the dual sensor which receives 
the transmitted radiation results in portions of the both the positive and 
the negative data track-induced diffraction components being applied to 
both of the dual sensors which provide the focusing signal. In fact, the 
data track-induced diffraction components are applied to these dual 
sensors in such a manner that the noise in these diffraction components 
(i.e., the origin of the cross-talk) can be approximately cancelled. 
The apparatus includes a dual grating and a sensor focusing lens in the 
optical path of the resulting radiation. As will be clear, the order of 
the these elements in the optical path is unimportant. Similarly, a 
cylindrical lens can be added to the optical path to defocus the radiation 
in one dimension. The order of the cylindrical lens, the focusing lens and 
the dual grating on the optical path is similarly unimportant with the 
exception of the minor positional adjustments along the optical path. 
The radiation beam transmitted by the dual grating and applied to a dual 
sensor is defocused relative to the dual grating-induced diffraction 
components. Because the transmitted radiation beam is used in the tracking 
signal and in the data signal, a larger sensor area is required for the 
out-of-focus beam. The defocusing of the transmitted beam has been 
accomplished by several techniques in the present invention. These 
techniques include fabrication of a (negative) lens in the coating 
protecting the detector array, providing structure to control the relative 
position of the dual detectors, and adapting the parameters of the dual 
grating to cause an automatic defocusing between the radiation components. 
Operation of the present invention is believed to be apparent from the 
foregoing description and drawings, but a few words will be added for 
emphasis. The present invention is used most advantageously in an optical 
information storage and retrieval system, such as a system in which discs 
are used as the storage medium. The information on the disks is stored in 
the vicinity of groove or data track, the groove being specifically 
provided in some embodiments to permit automatic tracking of the stored 
information. The application of a focused radiation beam to the groove or 
data track results in modulation of the return radiation beam determined 
by the stored information. In addition, first order diffraction patterns 
are generated by the data track or groove which can be used to provide a 
tracking signal. By proper separation of the resulting radiation beam 
components, i.e., through the orientation of a split diffraction grating, 
the cross-talk between the focusing signal and the tracking signal can be 
reduced. 
While the invention has been described with reference to reflection from an 
optical storage medium, it will apparent that the invention can be adapted 
to other modes of interaction with the storage medium. By way of specific 
example, the radiation beam can interact with the storage medium during 
transmission through the storage medium. At present, however, the mode of 
operation wherein a radiation beam is reflected from a storage medium is 
the preferred mode of operation. 
While the invention has been described with particular reference to several 
embodiments, it will be understood by those skilled in the art that 
various changes may be made and equivalents may be substituted for 
elements of the disclosed embodiments without departing from invention. In 
addition, many modifications may be made to adapt a particular situation 
and material to a teaching of the invention without departing from the 
essential teachings of the present invention. 
As is evident from the foregoing description, certain aspects of the 
invention are not limited to the particular details of the examples 
illustrated, and it is therefore contemplated that other modifications and 
applications will occur to those skilled in the art. It is accordingly 
intended that the claims shall cover all such modifications and 
applications as do not depart from the true spirit and scope of the 
invention.