Data storage and readout optical head using a single substrate having an electrooptic converging portion for adjustment of the light beam focal point

An optical head for optical storage and readout of information on or from an optical recording medium using a light beam comprises a single substrate having a waveguide on one of its opposite surfaces, a deflector portion disposed on the substrate to deflect the light beam passing through the waveguide at an adjustable angle, and a converging portion for converging the light beam to be emitted from the waveguide. The converging portion comprises a plurality of convergence electrodes which are disposed on a part of the waveguide. The convergence electrodes are spaced apart from each other in a direction perpendicular to the direction of propagation of the light beam through the part of the waveguide such that the part of the waveguide possesses, as a consequence of an electrooptic effect, a distribution of refractive index which permits adjustment of the focal point of the converged light beam.

BACKGROUND OF THE INVENTION 
1. Field of the Art 
The present invention relates to an optical head used for an optical 
recording apparatus. 
2. Related Art Statement 
An optical head is known, which emits a light beam toward an optical 
recording medium, for storing information on the recording medium, or 
reproducing information stored on the recording medium. For example, an 
optical head employs an optical system for directing a laser beam from a 
laser source to a recording track on an optical or magneto-optical 
recording disk, and/or for receiving and sensing a light beam reflected 
from the recording track. The optical system includes an object lens for 
focusing the laser beam on the surface of the recording disk, and a 
tracking mirror for deflecting the laser beam so as to position the spot 
of the laser beam at the center of a recording track of the disk. 
3. Problem Solved by the Invention 
However, there are various factors which cause a variation in the position 
of a recording disk relative to the focal point of a beam emitted from an 
optical head. More specifically, the positions of the recording tracks of 
the disk may be varied in a direction parallel to the optical axis of the 
optical head and in a direction perpendicular to the optical axis. To 
compensate for this positional variation, the focal point of the optical 
system should be adjusted in both directions indicated above. To this end, 
a known optical head of the type indicated above incorporates a device for 
moving the object lens along the optical axis, and a device for changing 
the angular position of the tracking mirror. If a tracking mirror is not 
employed, the optical head requires a device for moving the object lens in 
the direction perpendicular to the optical axis. Therefore, the 
conventional optical head incorporating such driving devices tends to be 
heavy, large-sized and complicated in structure. Therefore, the 
conventional optical head is not satisfactory in its access speed. 
Further, the use of a relatively large number of optical components leads 
to increased loss of light during propagation through the optical system. 
Moreover, the optical components must be aligned with each other with high 
precision, which makes it difficult to assemble the optical head. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide an optical 
head, which is comparatively lightweight and simple in construction, and 
which permits fast and accurate focusing of a light beam at a 
predetermined point on an optical recording medium. 
According to the present invention, there is provided an optical head for 
optical storage and readout of information on or from an optical recording 
medium, by means of emission of a light beam, comprising: (a) a single 
substrate having a waveguide on one of its opposite surfaces; (b) a 
deflector portion disposed on the substrate to deflect the light beam 
passing through the waveguide, at an adjustable angle; and (d) a 
converging portion for converging the light beam to be emitted from the 
waveguide, the converging portion comprising a plurality of convergence 
electrodes which are disposed on a part of the waveguide, the plurality of 
convergence electrodes being spaced apart from each other in a direction 
perpendicular to a direction of propagation of the light beam through the 
above-indicated part of the waveguide, such that this part of the 
waveguide possesses, as a consequence of an electro-optic effect, a 
distribution of refractive index which permits adjustment of a focal point 
of the converged light beam. 
In the optical head of the invention constructed as described above, the 
light beam passing through the waveguide is deflected at a suitable angle 
by the deflector portion provided integrally on the substrate, and the 
deflected light beam is converged by the converging portion also provided 
integrally on the substrate. Further, the focal point of the converged 
light beam is adjusted by the distribution of refractive index established 
by the convergnece electrodes. With this arrangement, a possible variation 
in the axial and radial positions of the recording medium relative to the 
optical head may be compensated for by the deflection of the light beam by 
the deflector portion, and by the focusing adjustment of the converged 
light beam by the converging portion. In other words, the instant optical 
head is capable of adjusting the positions of the beam spot in a direction 
parallel to the optical axis of the optical head and in a direction 
perpendicular to the optical axis. Hence the optical head according to the 
invention does not require a device for moving an object lens along its 
optical axis, and a device for changing an angular position of a tracking 
mirror or a device for moving the object lens perpendicularly to its 
optical axis. Consequently, the size and weight of the optical head are 
significantly reduced, whereby the speed of positioning the beam spot at a 
target on the recording medium is improved, with a resulting increase in 
the access speed of the optical head. Further, the present optical head 
uses a considerably reduced number of optical components, as compared with 
the conventional counterpart. This reduction in the number of components 
results in a correspondingly reduced loss of the light energy during 
propagation through the optical system. Furthermore, the fewer number of 
optical components means that assembly of the optical head is easier and 
that the cost of manufacture is reduced. 
According to one advantageous embodiment of the invention, the deflector 
portion comprises an oscillator which generates elastic surface waves to 
which another part of the waveguide is exposed, so that a refractive index 
of the another part of the waveguide is periodically varied at a frequency 
corresponding to a frequency of the elastic surface waves, to diffract the 
light beam at a varying angle, whereby the angle of deflection of the 
light by the deflector portion is varied as the frequency of the elastic 
surface waves is changed. 
According to another advantageous embodiment of the invention, the 
deflector portion comprises a plurality of deflection electrodes disposed 
on another part of the waveguide. The deflection electrodes are spaced 
apart from each other such that the above-indicated another part of the 
waveguide possesses, as a consequence of an electro-optic effect, a 
distribution of refractive index corresponding to a distribution of 
magnitude of an electric field produced by the deflection electrodes. 
Therefore, the distribution of magnitude of the electric field is varied 
as a function of time, to thereby vary the angle of deflection of the 
light beam by the deflector portion. 
In accordance with a further advantageous embodiment of the invention, the 
converging portion comprises a first converging portion including a 
plurality of first convergence electrodes disposed on a first section of 
the above-identified part of the waveguide, and a second converging 
portion including two second convergence electrodes disposed in parallel 
on a second section of the above part of the waveguide which is adjacent 
to an end of the substrate from which the light beam is emitted. In this 
embodiment, the plurality of first convergence electrodes are spaced apart 
from each other in a direction perpendicular to a line of propagation of 
the light beam through the first section of the waveguide, such that the 
first section possesses, as a consequence of an electro-optic effect, a 
distribution of refractive index which causes the light beam to be 
converged in a direction parallel to the one surface of the substrate. In 
the meantime, the two second convergence electrodes are arranged so as to 
extend perpendicularly to the direction of propagation of the light means, 
and are spaced from each other in the direction of propagation such that 
the second section possesses, as a consequence of an electro-optic effect, 
a distribution of refractive index which causes the light beam to be 
converged in a direction of thickness of the substrate. 
In one preferred form of the above embodiment, one of the second 
convergence electrodes is disposed on a top surface of the second section 
of the part of the waveguide, while the other second convergence electrode 
is disposed on an end face of the second section from which the light beam 
is emitted. 
In another preferred form of the above embodiment, the first section of the 
part of the waveguide has a distribution of refractive index which permits 
the light beam to be converged at least in the direction parallel to the 
one surface of the substrate, while the second section of the part of the 
waveguide has a distribution of refractive index which causes the light 
beam to be converged in the direction of thickness of the substrate. In 
this case, the first section of the part of the waveguide may be provided 
as a ridge formed on the previously indicated one surface of the 
substrate. 
According to a still further embodiment of the invention, the converging 
portion comprises a first converging portion which possesses, as a 
consequence of an electro-optic effect, a distribution of refractive index 
which permits the light beam passing through the part of the waveguide to 
be converged in a direction parallel to the surface of the substrate, and 
a second converging portion which possesses, as a consequence of an 
electro-optic effect, a distribution of refractive index which permits the 
light beam passing through the part of the waveguide to be converged in a 
direction of thickness of the substrate. The deflector portion is disposed 
between the first and second converging portions. 
In one form of the above embodiment, the waveguide extends, in order, 
through the first converging portion, the deflector portion and the second 
converging portion, and terminates at an end face of the substrate. The 
substrate has another waveguide for receiving a light beam which is 
reflected from the recording medium toward the end face. The 
above-indicated another waveguide guides the reflected light beam to a 
light-sensitive element attached to a side face of the substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be described in 
detail, referring to the accompanying drawings. 
An optical had embodying the invention is shown in fragmentary plan and 
side elevational views of FIGS. 1 and 2. The optical head comprises a 
substrate 10 made of an electro-optic and acousto-optical material, for 
example, a single crystal of LiNbO.sub.3. On one of the major opposite 
surfaces of the substrate 110, there is formed a waveguide 12, which is a 
layer formed adjacent to the surface of the substrate 10 by thermal 
diffusion of a suitable diffusion material such as Ti (titanium), so that 
the layer is given a refractive index higher than that of the substrate 
10. The substrate 10 has a deflector portion 14 for deflecting a light 
beam passing through the waveguide 12, and a converging portion consisting 
of a first and a second converging portion 16, 18 for converging the 
deflected light beam. With this arrangement of the optical head, a laser 
beam 20 radiated from a suitable laser source (not shown) and guided by 
the waveguide 12 is emitted from an end face 11 of the substrate 10 such 
that the emitted light is focused on a predetermined point on a currently 
selected track of an optical recording medium such as an optical or 
magneto-optical recording disk. As indicated above, the layer constituting 
the waveguide 12 is formed as an integral part of the substrate 10, and 
the refractive index is varied continuously across the interface or 
boundary of the substrate 10 and the waveguide layer 12. The boundary is 
indicated in broken line in FIG. 2. 
At one end of the deflector portion 14, there are provided a pair of 
deflection electrodes in the form of comb-like electrodes 24, 26 which are 
fixed on the surface of the substrate 10. By applying of a voltage between 
the comb-like electrodes 24, 26 under the control of a tracking controller 
(not shown), a portion of the substrate 10 between the comb-like 
electrodes 24, 26 is oscillated, whereby elastic surface waves 28 are 
generated. Thus, the comb-like electrodes 24, 28 cooperate with the 
above-indicated portion of the substrate 10 to constitute an oscillator to 
generate the elastic surface waves 28. As a result, the refractive index 
of a part of the waveguide 12 exposed to the elastic surface waves 28 is 
periodically varied in a direction intersecting an optical axis Lo of the 
laser beam 20, whereby the laser beam 20 is diffracted at the 
above-indicated part of the waveguide 12, in accordance with the Bragg 
diffraction principle. Described in more detail, the part of the waveguide 
12 corresponding to the deflector portion 14 is given a distribution of 
refractive index in the direction of propagation of the elastic surface 
waves 28, which distribution is expressed by the following formula (1): 
EQU n(x, t)=N+.DELTA.n sin(.OMEGA.t-kx) (1) 
where, 
k=2.pi./.LAMBDA. 
x: direction of propagation of elastic waves 28 
.OMEGA.: angular frequency of elastic waves 28 
.lambda.: wavelength of laser beam 20 
n: refractive index of waveguide 12 
.LAMBDA.: wavelength of elastic waves 28 
Further, an angle 2.theta..sub.B of Bragg diffraction of the laser beam 20 
is obtained from the following formula (2): 
EQU 2.theta..sub.B =.lambda./n.LAMBDA. (2) 
If the frequency of the elastic surface waves 28 is varied by .DELTA.f by 
the tracking controller, the laser beam 20 is deflected by an angle of 
.DELTA..theta..sub.B according to the following formula (3): 
EQU .DELTA..theta..sub.B =.lambda..multidot..DELTA.f/v (3) 
where, v: propagation velocity of elastic waves 28 While the elastic 
surface waves 28 are not actually visible, they are indicated in FIG. 1 
for ease of understanding. 
A part of the waveguide 12 corresponding to the first converging portion 16 
is formed on the substrate 10, by diffusion of Ti or other suitable 
diffusion material, such that the concentration of the diffused material 
increases in opposite directions toward and perpendicularly to an optical 
axis L1 of the laser beam 20 diffracted by the deflector portion 14. The 
first converging portion 16 comprises three first convergence electrodes 
30a, 30b and 30c which are disposed on a buffer layer 22 on the substrate 
10. The three electrodes 30a, 30b and 30c are spaced apart from each other 
in the direction perpendicular to the optical axis L1. In this arrangement 
of the first converging portion 16, the part of the waveguide 12 
corresponding to the first converging portion 16 serves as a waveguide 29 
which has a distribution of refractive index as shown in FIG. 3, in which 
the refractive index increases toward the center of the waveguide 29 
(toward the optical axis L1). Consequently, the first converging portion 
16 provides a light converging property as indicated in FIG. 4, i.e., an 
optical property to cause the laser beam 20 to be converged on the optical 
axis L1. In addition, the first converging portion 16 possesses a 
distribution of refractive index as a consequence of an electro-optic 
effect upon application of a voltage to the first convergence electrodes 
30a, 30b, 30c, whereby the convergence of the laser beam 20 can be 
adjusted primarily in the direction parallel to the surface of the 
substrate 10. In this connection, it is noted that spaced-apart parallel 
straight lines at the first and second converging portions 16, 18 in FIGS. 
1 through 3 indicate the distributions of the refractive index, which are 
not actually visible. The refractive index increases with an increase in 
the density of the straight lines. For example, such refractive index 
distributions may be established by vapor deposition of a diffusion 
material such as Ti as indicated at 31 in FIG. 5, and subsequent diffusion 
of the material 31 through the substrate 10. 
When voltages are applied from a focusing controller (not shown) between 
the electrode 30a on the optical axis L1 and the electrode 30b, and 
between the electrode 30a and the electrode 30c, an electric field is 
produced, as indicated in FIG. 6 for illustrative purpose only. According 
to an electro-optic effect, the refractive index of the first converging 
portion 16 is higher at a portion 50a under the electrode 30a, than at 
portions 50b and 50c under the electrodes 30b, 30c. In other words, the 
refractive index increases toward the optical axis L1. Generally, an 
electro-optic material has a refractive index which is varied as a 
function of the magnitude of an electric field to which the photoelectric 
material is exposed. For instance, if the substrate 10 is made of 
LiNbO.sub.3, the refractive index n of the portion of the waveguide 29 
located under the electrodes 30a, 30b, 30c is changed according to the 
following formula (4): 
EQU n=n.sub.e -(1/2)n.sub.e.sup.3 r.sub.33 .multidot.E (4) 
where, 
n.sub.e : refractive index of the substrate 10 with respect to abnormal 
light 
r.sub.33 : electro-optic constant of the substrate 10 in the direction of 
thickness 
Therefore, the light converging property of the first converging portion 16 
is varied as the voltages applied to the electrodes 30a, 30b, 30c are 
changed, whereby the focal point of the laser beam 30 is adjusted. 
A part of the waveguide 12 corresponding to the second converging portion 
18 is formed on the substrate 10, by diffusion of Ti or other diffusion 
material such that the concentration of the diffused material increases in 
opposite directions toward a line l which is perpendicular to the optical 
axis L1 of the laser beam 20. The second converging portion 18 comprises 
two second convergence electrodes 32, 34 extending in parallel with 
respect to the line l. The electrode 32 is disposed on the top surface of 
the waveguide 12, more precisely, on the previously described buffer layer 
22, and located adjacent to the end face 11 of the substrate 10. The 
electrode 34 is disposed on the end face 11 of the substrate 10. In this 
arrangement of the second converging portion 18, the part of the waveguide 
12 corresponding to the second converging portion 18 serves as a waveguide 
33 which has a refractive index that increases toward the line l as 
indicated in FIG. 2. Like a cylindrical lens (or semicylindrical lens), 
this waveguide 33 has a light converging property so as to converge the 
light in the direction of thickness of the substrate 10. In addition, the 
second converging portion 18 possesses a distribution of refractive index 
as a consequence of an electro-optic effect upon application of a voltage 
to the second convergence electrodes 32, 34, whereby the convergence of 
the laser beam 20 can be adjusted primarily in the direction of thickness 
of the substrate 10. 
In the present embodiment of the optical head which has just been 
described, the laser beam 20 which is eventually emitted from the end face 
11 of the substrate 10 is deflected at the deflector portion 14, by an 
angle corresponding to the frequency of voltages applied to the comb-like 
deflection electrodes 24, 26 under the control of the tracking controller. 
Further, the positions of the focal point of the emitted laser beam 20 are 
adjusted by the amounts determined by the levels of the voltages applied 
between the electrodes 30a and 30b and between the electrodes 30a and 30c, 
and between the electrodes 32 and 34, under the control of the focusing 
controller. Hence, the spot (focal point) of the emitted laser beam 30 can 
be aimed at a predetermined point on a target track of an optical 
recording medium, and can be caused to accurately follow possible 
variations in the position of the target track in the direction of the 
optical axis of the laser beam 20 and in the direction perpendicular to 
the optical axis. Thus, the present optical head is capable of controlling 
the emission of a light beam without using a device for moving an object 
lens along the optical axis, a tracking mirror and a device for actuating 
the tracking mirror, or a device for moving the object lens in the 
direction perpendicular to the optical axis. The elimination of these 
devices provides significant reduction in the size and weight of the 
optical head, which enables the optical head to record and read 
information on or from a selected track of an optical recording medium, 
with a shorter response to an access command and a higher speed of access 
of the beam spot to the appropriate recording track. Further, considerable 
reduction in the number of optical components incorporated in the optical 
head contributes to minimization of a loss of light energy within the 
optical system, and results in a reduced need for accurate alignment of 
the optical elements relative to each other, which leads to easier 
assembling of the optical head. 
Modified embodiments of the invention will now be described. The same 
reference numerals as used in the preceding embodiment will be used in 
these modified embodiments to identify the corresponding elements, and no 
repeated description of these elements is provided in the interest of 
brevity. 
The deflector portion 14 may be disposed between the first converging 
portion 16 and the second converging portion 18, as indicated in FIG. 7. 
This modified arrangement is free from an interference between an electric 
field produced by the first convergence electrodes 30a, 30b, 30c and the 
second convergence electrodes 32, 34, facilitating the focusing of the 
laser beam 20 by means of an electro-optic effect. This is an advantage of 
the present embodiment. 
As another modification, it is possible to utilize an electro-optic effect, 
rather than the elastic surface waves 28, for the deflector portion 14. 
Described more particularly, the deflector portion 14 may comprise a 
plurality of deflection electrodes 36 which are spaced from each other in 
a direction intersecting the optical axis Lo, as illustrated in FIG. 8. In 
this arrangement, a voltage is applied to each pair of the electrodes 36, 
so that a diffraction grating due to periodic variation in refractive 
index is established under the array of the electrodes 36. An angle 
.theta..sub.D of diffraction of the laser beam 20 by the diffraction 
grating is changed by varying the level of the voltages applied to the 
electrodes 36. 
A modified form of the first converging portion 16 is illustrated in FIG. 
9, which comprises a projection or ridge 38 which is formed on the surface 
of the substrate 10 such that the refractive index increases in the radial 
directions of the ridge 38, toward its center (toward the optical axis 
L1). In other words, the refractive index distribution is represented by 
concentric circles as viewed along the optical axis. Alternatively, the 
first converging portion 16 may be adapted such that a part of the 
waveguide 12 located under the central electrode 30a is given a refractive 
index distribution in the form of an ellipse as viewed along the optical 
axis L1, as indicated in FIG. 10, wherein the refractive index increases 
toward the center of the ellipse which is placed on the optical axis L1. 
The refractive index distributions as shown in FIGS. 9 and 10 may be 
established by ion bombardment. 
The second converging portion 18 may be formed to have a refractive index 
distribution as indicted in FIGS. 11(a) and 11(b), which is represented by 
a semicircle as viewed along the optical axis L1. 
Further, the second converging portion 18 may be formed by disposing two 
electrodes 40, 42 on a substrate made of a Y-cut crystal of LiNbO.sub.3, 
as shown in FIG. 12, in order to control the convergence of the light beam 
in the direction of thickness of the substrate. 
A further embodiment of the invention is illustrated in FIG. 13, wherein 
the substrate 10 has a waveguide 44 which extends through the first 
converging portion 16, the deflector portion 14 and the second converging 
portion 18, and terminates in the end face 11 of the substrate 10. In 
addition, the substrate 10 has a pair of waveguides 46, 46 which are 
adapted to receive at the end face 11 a light beam reflected from an 
optical recording medium, and to guide the received beam to 
light-sensitive elements attached to opposite side faces 48, 48 of the 
substrate 10. 
While the present invention has been described in its preferred 
embodiments, it is to be understood that the invention may be otherwise 
embodied. 
For example, the ultrasonic oscillator (comb-like electrodes 24, 26) of the 
deflector portion 14, which is controlled by a tracking controller, may be 
disposed at other locations, for example, on the side faces of the 
substrate 10. In this case, the substrate 10 need not be made of 
LiNbO.sub.3. 
While the deflector portion 14 is adapted to diffract the laser beam 20 
according to Bragg diffraction, it is possible to utilize the principle of 
Raman-Nath diffraction. 
Although the converging portion in the illustrated embodiments consists of 
the first and second converging portions 16, 18, the second converging 
portion may be eliminated if the convergence by the first converging 
portion is sufficient. 
As previously described, the converging portions 16, 18 of the illustrated 
embodiments are given refractive index distributions established by 
diffusion of a suitable material, and the focal point of the light beam 
converged by these refractive index distributions is adjusted by changing 
the distributions by utilization of an electro-optic effect. However, the 
converging portions 16, 18 may be adapted to exhibit a refractive index 
distribution equivalent to that of a convex lens, by means of an 
electro-optic effect. In this case, it is not essential to give the 
converging portions a refractive index distribution by diffusion of a 
suitable material. 
The previously indicated buffer layer 22 is a layer of several microns made 
of a transparent material such as SiO.sub.2 having a lower refractive 
index than the material of the waveguide 12. The buffer layer 22 is 
provided for preventing the electrodes 30a, 30b, 30c from absorbing the 
energy of a light beam. However, the buffer layer 22 may be removed. 
Further, it is noted that the shape and number of the electrodes of the 
converging portions 16, 18 are not confined to those of the illustrated 
embodiments. 
It is to be understood that the forms of the present invention herein shown 
and described are to be taken as preferred examples of the same, and that 
various changes may be made within the scope of the invention defined in 
the appended claims.