Subaperture coarse tracking optical system for optical data storage devices

A subaperture optical system for preventing interference between light inadvertently reflected from the protective overcoat of an optical recording disk from disruptively interfering with light reflected from the active layer of the optical recording disk, thereby causing coarse servo tracking errors as the coarse servo actuator carriage translates over the coarse servo tracks on the disk service. The optical system is comprised of a laser light source, a collimating lens system, an astigmatizing lens system, a subaperture mirror, a carriage actuator, a beam relaying telescope, an objective lens, an objective lens focus actuator, a reflected coarse servo beam focusing lens, and a coarse servo detector. The laser issues a beam which follows a first subaperture path that is parallel to, but off center from, the optical axis of an objective lens of the optical system. The beam is formed into a line focused spot which is focused on the disk surface at a non normal angle. The beam reflected by the active layer and the protective overcoat are spatially separated, and therefore do not interfere. The reflected beams follow a second subaperture path to the coarse servo detector, where the reflected signal is detected unaffected by any interference between the reflected beams.

BACKGROUND OF THE INVENTION 
The disclosed invention relates to the field of the optical beam 
manipulation in optical disk storage devices, and in particular to 
preventing undesirable reflections from the disk surface from interfering 
with the proper detection of the coarse servo tracking signal reflected 
from the disk surface. 
In optical storage, information is stored on media which generally consists 
of a reflecting substrate, a phase layer, an active layer, and a 
protective overcoat. A protective overcoat is used because the active 
layer is generally rather fragile and easily damaged. In some devices, the 
protective overcoat also serves as a dust defocusing layer. The 
information is initially recorded as changes in reflectivity between the 
"marked" and the "unmarked" regions of the media. The unreflective region 
exists as a result of destructive interference between the incident beam 
reflected at the active layer and the beam reflected off of the substrate. 
When an optical hole is created in the active layer, no light is reflected 
at the active layer, so that the light reflected by the substrate returns 
through the system with out interference, causing a bright "spot" to be 
detected. 
It is well known in the art to use a focused radiation beam, usually a 
laser, to record and read both digital and analog information on such a 
recording media. With such focused beams, the light is generally focused 
to as small a spot as feasible in order to store data as densely as 
possible on the recording surface. In rotating disk storage devices, 
information is stored in either concentric or spiral recording tracks. For 
proper device operation, the light beam must be properly focused on the 
disk surface. The focused beam must be properly tracked across the disk 
surface. There have been a number of different methods taught for properly 
focusing the beams on the disk surface, all requiring a tightly focused 
spot on the disk media. 
Likewise, tracking of the device beams over the disk surface has generally 
followed well known techniques. For fine tracking of the focused beam over 
the disk surface, prior devices have taught the focusing of a pair of 
tracking spots on opposite edges of a data track on the disk surface, and 
comparing the strength of the reflected signals. Thus, tracking also 
requires tightly focused spots. 
However, in random access devices using disk shaped carriers, it has 
generally been necessary to provide for the gross (coarse) translation of 
the optical elements over the disk surface from one area of the disk to 
another. The coarse translation of the beams over the disk surface has 
generally been accomplished by mounting some portion of the optics in a 
carriage actuator, and moving the carriage actuator over the disk surface 
to the area to be read or written. Relatively widely spaced coarse servo 
tracks are then used to determine actuator and optical element position as 
the actuator moves across the disk surface. To detect these relatively 
wide spaced tracks, a broad spot is generally preferred. 
Because two fundamentally differnt types of spots are needed at the disk 
surface, there has been increased interest in the use of multi-laser 
systems. The first laser is used to supply the coarse servo beam. The 
second laser is used to supply to fine tracking focusing and data reading 
beam. 
However, in systems having the protective overcoats, unwanted reflections 
from the protective overcoat can create signal detection problems. The 
beam reflected from the protective layer can interfere with the beam 
reflected from the active layer. Since the protective overcoat will vary 
slightly in thickness, the phase difference between the light reflected 
from the active layer and the light reflected from the protective overcoat 
will also vary as a beam is translated over the disk surface. Sometimes 
the beams reflected from the protective overcoat will constructively 
interfere with the beams reflected from the active layer, and sometimes 
the beams reflected from the protective layer will destructively 
interfere. This random variation in interference can result in serious 
signal detection problems. 
Both the read beams and the coarse servo beams have some portion of their 
light reflected by the protective overcoat. However, the read beams are 
focused onto the disk active layer in diffraction limited spots. With a 
diffraction limited spot, a beam reflected from the protective overcoat 
will have a different radii of curvature than one reflected from the 
active layer, so the constructive and destructive interferences tend to 
cancel out at the detector, not severely effecting the read beam signal 
sensed by the read detectors. 
However, because the coarse servo beam is focused on the disk surface as a 
broad spot in at least one of its optical axes, the radius of curvature of 
that portion of the coarse servo beam reflected from the protective layer 
will much more closely match that of a beam reflected from the active 
layer, so that the respective constructive and destructive interferences 
can result in unreliable coarse servo signals falling on the coarse servo 
detectors. 
What is needed then is a means for efficiently and completely preventing 
light reflected from the protective overcoat from interfering with light 
reflected from the active layer. The present invention discloses such a 
means. The present invention discloses a subaperture coarse servo optical 
system, wherein the coarse servo beam can be efficiently reflected from 
the disk so that reflection from the protective overcoat does not 
interfere with the proper detection of the coarse servo beam. 
It is an object of the disclosed invention to provide a system for 
delivering a coarse servo optical beam in a random access optical disk 
storage unit. 
It is another object of the disclosed invention to provide a coarse servo 
optical beam which does not create undesirable interference between beams 
reflected from the various interfaces of the optical recording media. 
It is yet another object of the disclosed invention to provide a means for 
preventing undesired interference between the coarse servo beams reflected 
from the media protective overcoat and those reflected from the active 
layer. 
BRIEF SUMMARY OF THE INVENTION 
The present invention discloses a means for selectively preventing coarse 
servo light reflected from the protective overcoat of an optical 
information storage disk from interfering with the proper detection of the 
coarse servo beam. The disclosed invention provides a means whereby the 
coarse servo beams reflected from the media active layer and those 
reflected from the media protective overcoat follow a spatially separated 
optical paths, but still share common optical elements. Thus, the two 
reflected beams do not interfere with one another and thereby altering the 
amplitude of the total signal reflected from the disk surface. 
The invention is comprised of: a laser light source, a collimating lens 
systems, a subaperture mirror, a carriage actuator, a beam relaying 
telescope, an objective lens, an objective lens focus actuator, a 
reflected coarse servo beam focusing lens, and a coarse servo detector. 
The laser emits a beam which follows a first subaperture path that is 
parallel to, but off center from, the optical axis of the object lens of 
the optical system. The coarse servo beam passes through a collimating 
lens system, which collimates the beam emitted by the laser. The incident 
coarse servo beam then passes through a cylindrical lens which 
astigmatizes the beam. 
The collimated incident coarse servo beam then impinges upon the 
subaperture mirror. In the preferred embodiment, the subaperture mirror is 
positioned in a first subaperture path, so that the incident beam impinges 
upon, and is deflected by, the subaperture mirror, but will allow a beam 
in a second subaperture region to pass undeflected. 
The incident coarse servo beam then enters the carriage actuator. In the 
preferred embodiment, the carriage actuator contains the beam relaying 
telescope, the objective lens and the objective lens focus actuator. The 
incident coarse servo beam passes through the first subaperture of the 
lenses in the beam relaying telescope and the objective lens respectively. 
The objective lens is positioned by the read optics and the focus actuator 
to focus the coarse servo beam on the active layer. The coarse servo 
optical system is arranged so that a radially oriented line focused beam 
falls on the disk surface. However, because the coarse servo beam is 
transmitted through the off center axis subaperture of the system, the 
coarse servo beam strikes the media at a non-normal angle. Coarse servo 
beam is still reflected from both the active layer and the protective 
overcoat. However, because the beam is incident on the detector at a 
non-normal angle, the reflected beams are spatially separated and thus do 
not interfere. 
The reflected coarse servo beams pass back through the the objective lens, 
and beam relaying telescope lenses. The beams pass undeflected under the 
subaperture mirror, and pass through the coarse servo focusing lens, which 
focuses the reflected coarse servo beams on the coarse servo detector. All 
coarse servo beams reflected from the disk surface are focused on the 
coarse servo detector. However, since they are spatially separated, they 
do not improperly interfere with one another. Consequently, as the coarse 
servo beam tracks over the the coarse servo track, no variation in the 
signal due to interference between the two reflected beams is generated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, the disclosed invention, a subaperture coarse servo 
system 1 for an optical disk storage device, is comprised of: a laser 
light source 2, a collimating lens system 3, an astigmatizing lens 4 for 
forming a line focused beam on the disk surface, a subaperture mirror 5, a 
carriage actuator 6, a beam relaying telescope 7, an objective lens 8, an 
objective lens focus actuator 9, a reflected coarse servo beam focusing 
lens 10, and a coarse servo detector 11. 
As shown in FIG. 2, in the optical storage device for which the coarse 
servo subaperture system is contemplated for use, the rotating optical 
storage disk 12 has a plurality of coarse servo tracks 13, between which 
user data is to be written on user data tracks 14. As shown in FIG. 3, in 
the contemplated system 1, the disk 12 media has an active layer 15 and a 
protective overcoat 16 which are of interest. 
As shown in FIG. 4, in a properly operating system not having any undesired 
reflected beams from the disk 2 surface, an example reflected coarse servo 
spot 17 would impinge upon the coarse servo detector 11. When a coarse 
servo beam, is correctly centered over a coarse servo track 13, a null 
signal is generated by the detector 11. However, when a coarse servo beam 
is not centered over the coarse servo track 13, a position error signal is 
generated by the detector 11, which is sent to a servo control device (not 
shown), which in turn drives the carriage actuator 6 in the direction 
appropriate for proper centering over the coarse servo track 13. 
However, as shown in FIG. 5, when there is random interference between a 
beam 18 reflected from the active layer 15 of the disk 12 surface and a 
beam 19 reflected from the protective overcoat 16 of the disk 12, a signal 
20 of randomly varying amplitude is received by the detector 11. 
As shown in FIG. 6, the optical system has a first subaperture optical 
region 21 which includes the upper portion of the objective lens 8 
aperture and a second subaperture region 22 which includes the lower 
portion of the objective lens 8 aperture. Any beam whose entire bundle of 
rays travel only in a given subaperture region is said to follow that 
subaperture path. As shown in FIGS. 1, 6 and 7, the incident 23 and 
reflected beams 18 and 19 pass through the objective lens 8, the incident 
coarse servo beam 23 emitted by the laser 2 follows a path through a first 
subaperture region 21 of the device optical system 1. 
As shown in FIG. 1, in the preferred embodiment, the laser 2 is positioned 
such that the coarse servo beam emitted by the laser follows a first path 
that is parallel to, but off center from, the system optical axis. Since 
some lasers emit diverging beams, in the preferred embodiment, the 
collimating lens system 3 is included to provide a collimated beam 23 for 
passage through the coarse servo optical system 1. The coarse servo beam 
23 then passes through an astigmatizing cylindrical lens 4. The incident 
coarse servo beam 23 is astigmatized so that a radial line focused beam 
(not shown) falls on the disk 12 surface. Upon exiting the cylindrical 
lens 4, the collimated coarse servo beam 23 then impinges upon the 
subaperture mirror 5. 
In the preferred embodiment, the subaperture mirror 5 is positioned in the 
first subaperture region 21, so that the incident coarse servo beam 23 
impinges upon, and is deflected by, the subaperture mirror 5 toward the 
objective lens carriage actuator 6. 
As discussed above, the coarse servo movement across the disk 12 surface is 
controlled by the carriage actuator 6. In the preferred embodiment, the 
carriage actuator 6 contains the beam relaying telescope 7, the objective 
lens 8 and the objective lens focus actuator 9, which all move as a unit. 
In the preferred embodiment, the beam relaying telescope 7 is comprised of 
two infinite conjugate lens 25 and 26. As Shown in FIG. 6, since the 
incident coarse servo beam 23 cross section is narrow, only the upper 
"subaperture" of the beam relaying telescope lens 25 and 26, and the 
objective lens 8 are used by the incident coarse servo beam 23. 
The objective lens 8, is mounted in an objective lens focus actuator 9, 
which translates the objective lens 8 in a direction normal to the disk 12 
surface in response to a signal form servo control means (not shown). In 
the preferred embodiment, a voice coil motor is used, and the objective 
lens is a compound spherical lens. 
As Shown in FIGS. 1 and 7, the objective lens 8 is positioned by the focus 
actuator 9 to focus the incident coarse servo beam 23 on the active layer 
15. However, because the incident coarse servo beam 23 is transmitted 
through the off axis first subaperture region 21 of the system 1, the 
incident coarse servo beam 23 strikes the active layer 15 and the 
protective overcoat 16, at a non-normal angle. 
As shown in FIG. 7, light is still reflected from both the active layer 15 
and the protective overcoat 16. However, because the incident coarse seek 
beam 23 is incident on the disk 12 at a non-normal angle, the reflected 
beams 18 and 19 are now spatially separated. This spatial separation 
prevents the beam 19 reflected from the protective overcoat 16 from either 
constructively or destructively interfering with the beam 18 reflected 
from the active layer 15. 
As shown in FIGS. 6 and 7, the reflected active layer and protective 
overcoat beams 18 and 19 pass back through the system 1, passing through 
the lower subaperture 22 of the objective lens 8, and beam relaying 
telescope lenses 25 and 26. 
As shown in FIG. 1, the subaperture mirror 5 is positioned only in the 
first subaperture region 21, so that only light traveling in the first 
subaperture path will be deflected by the subaperture mirror 5. Therefore, 
because the reflected beams 18 and 19 are propagating through the second 
subaperture region 22, the beams 18 and 19 pass undeflected under the 
subaperture mirror 8. It can thus be seen that this subaperture optical 
system 1 can also act as a beam splitter, splitting an incident beam from 
the reflected beam. 
The undeflected coarse servo beams 18 and 19 then impinge on the coarse 
servo focusing lens 10, which focuses the reflected coarse servo beams 18 
and 19 on the coarse servo detector 11. In the preferred embodiment, the 
coarse servo detector 11 is a one dimensional, position sensing, detector. 
Because both of the beams 18 and 19 reflected from the active layer 15 and 
the protective overcoat 16 respectively are collimated when incident on 
the coarse servo focusing lens 10 and are following parallel paths, both 
beams 18 and 19 are focuses on the coarse servo detector 11. However, the 
beam 19 reflected from the protective overcoat 16 does not interfering 
with the beam 18 reflected from the active layer 15, because of the 
spatial separation. Therefore, as shown in FIG. 8, as the coarse servo 
beam 23 tracks over the the coarse servo track 13, no variation in the 
signal 27 received by the coarse servo detector 11 is generated as a 
result of reflective interference between the beams 18 and 19 reflected 
from the active and the protective layers 15 and 16.