Optical pickup device for use in a video and/or audio disc player

An optical pickup device is placed in a video and/or audio disc player operative to reproduce information recorded in the form of an optical pattern such as a train of pits on a recording disc, in which the direction of light beam emitted from a light source is parallel to the propagation path of light impinging onto and reflected from the recording disc toward a photosensor. A polarization prism is used to deflect the light from the light source toward the recording disc, which prism cooperates with a quarter wave plate to act as a beam splitter.

FIELD OF THE INVENTION 
The present invention relates to an optical pickup device for optically 
detecting an information recorded in the form of an optical pattern such 
as a series of pits or bosses on a recording medium. The optical pickup 
device herein proposed may be preferably used in an audio and/or video 
disc player. 
BACKGROUND OF THE INVENTION 
A recently developed optical video and/or audio disc player is adapted to 
play a recording medium in the form of a disc having thereon a helically 
wound train of discrete pits or the like representing video and/or audio 
information. 
In order to pick up the information on the recording disc, the player has 
an optical pickup device which has a light source for generating a 
plane-polarized light beam to be directed toward the recording disc, and a 
photosensor for detecting the light originated from the light source and 
reflected from the recording surface of the recording disc. A polarizer 
prism is provided between the photosensor and the recording disc for 
deflecting the light from the light source toward the recording disc and 
passing therethrough the light reflected from the recording disc. A 
quarter wave plate is provided between the polarizer and the recording 
disc. 
In a prior art optical pickup device of the abovementioned type, the light 
source means including a light emissive element and a concentrating lens 
must be situated largely apart from the path of light reflected from the 
recording disc to the photosensor and thus is so disposed as to project 
transversely to a considerable extent from the linearly disposed assembly 
constituted by the quarter wave plate, the polarizer and the photosensor. 
Therefore, the above-mentioned pickup device has a drawback that it has a 
considerably large dimension and requires a large space in the pickup 
device. In addition, the propagation direction of the light from the light 
source toward the polarizer is perpendicular to the propagation directions 
in the assembly of the three parts, namely, the wave plate, the polarizer 
and the photosensor, with the deleterious result that errors of mounting 
of the polarizer cause the reflecting angle to deviate and degrades the 
pickup characteristic of the device. 
SUMMARY OF THE INVENTION 
The present invention was made to eliminate the above drawbacks. It is thus 
the prime object of the present invention to provide an optical pickup 
device which is compact in construction. 
It is another object of the present invention to provide an optical pickup 
device which can be fabricated precisely with negligible deviation of the 
propagation direction of light. 
In order to achieve the above objects, an optical pickup device according 
to the present invention comprises light source means for generating a 
plane-polarized light beam having a polarization direction and directed to 
a first predetermined direction, a polarizer prism having a reflecting 
surface therein disposed in the path of the light beam so as to deflect 
the light beam to a second predetermined direction and having a polarizing 
plane therein which is inclined to the second predetermined direction and 
has such a polarizing property as to pass therethrough a light beam having 
a polarization direction different from that of the deflected light beam 
so that the deflected light beam is reflected at the polarizing plane 
toward a third predetermined direction, a quarter wave plate disposed 
transversely in the path of the light beam reflected at the polarizing 
plane of the polarizer prism, photosensor means disposed opposite to said 
quater wave plate with respect to said polarizing plane of the polarizer 
prism and in the third predetermined direction.

DESCRIPTION OF A PRIOR ART 
In FIG. 1 there is shown a prior art optical pickup device which comprises 
a light source 10. The light source 10 may be preferably made of a 
semiconductor laser device. The light source 10 emits a light beam which 
is polarized in only one plane parallel to propagation direction thereof. 
The light beam from the light source 10 passes through a concentrating 
lens 12 and enters a side surface 14a of a polarizing prism 14. The 
surface 14a is perpendicular to the propagation direction of the light 
beam passed through the concentrating lens 12. The light beam enterred the 
prism 14 is reflected at an oblique polarizing plane or surface 16 to a 
front surface 14b of the prism 14 the polarizing plane 16 is coated with a 
suitable material. The surfaces 14a and 14b are perpendicular to each 
other and the polarization surface 16 is inclined from those surfaces 14a 
and 14b by 45.degree., respectively. The reflected light beam passes 
through the side surface 14b and enters a quarter wave plate 18 which 
passes therethrough the light beam. The quarter wave plate 18 applies a 
phase shift to the light beam passing therethrough so that the light beam 
passed through the quarter wave plate 18 contains two components 
respectively shifted in phase from the original light beam .+-.45.degree.. 
The light beam passed through the quater wave plate 18 is concentrated 
onto a recording surface of recording disc 20 by means of an object lens 
22. The light beam concentrated onto the recording disc 20 is reflected 
from the disc 20 and returns along the same propagation path through the 
object lens 22 and the quarter wave plate 18 to the polarization surface 
16 of the prism 14. The returned light beam is polarized in a direction 
shifted by 90.degree. from the direction in which the original light beam 
emitted from the light source 10 is polarized so that the returned light 
beam passes through the polarization surface 16 and passes through a rear 
surface 14c. The light beam passed through the rear surface 14c enters a 
photosensor 24 which produces an electric signal having a magnitude 
according to changes in the intensity or phase of the entered light beam. 
The electric signal thus represents the information recorded on the 
recording disc 20. 
As mentioned above, the light source 10 and the concentrating lens 12 form 
a light path perpendicular to that of light passing through the object 
lens 22, the quater wave plate 18, the polarization prism 16 and the 
photosensor 24. Thus, the light source 10 and the concentrating lens 12 
are necessarily largely apart from the light path through the object lens 
22, the quarter wave plate 18, the prism 14 and the photosensor 24. It is 
now apparent that the prior art pickup device mentioned above is 
relatively large in construction thereby to make large the overall pickup 
device. 
In order to solve this drawback in the prior art pickup device, there is 
provided a novel and improved pickup device according to the present 
invention as described hereinbelow. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 2 there is shown an optical pickup device according to the present 
invention which comprises a light source 10 which is preferably a 
semiconductor laser source emitting a plane-polarized light beam. The 
light beam from the light source 10 passes through a concentrating lens 12 
and enters a prism 26 through a front surface 26a thereof. The surface 26a 
is perpendicular to the propagation direction of the light emitted from 
the light source 10. The light beam entered the prism 26 is reflected at 
an oblique reflecting surface 28 toward a polarizing plane or surface 16. 
The reflecting surface 28 and the polarizing plane 16 are both inclined 
from the front surface 26a by 45.degree., respectively, and perpendicular 
to each other. The front surface 26a, the reflecting surface 28 and the 
polarizing plane 16 are preferably so disposed that there exists such a 
plane as to be perpendicular to these surfaces 26a and 28 and the plane 16 
and parallel to the propagation direction of the light beam emitted from 
the light source 10. The light source 10 is so disposed that the light 
emitted therefrom has a polarization direction parallel with the the light 
beam reflected by the reflecting surface 28 is further reflected by the 
polarizing plane 16 and passes through the front surface 26a. Because of 
such geometrical relationship between the surfaces 26a and 28 and the 
plane 16 as described above, the propagation direction of the light beam 
emitted from the light source 10 toward the reflecting surface 28 is 
parallel, but reverse in sense, to that of the light beam reflected from 
the polarization plane 16. 
In this embodiment, the prism 26 preferably comprises two independent prism 
elements 30 and 32 respectively having cross sections of right-angled 
equilateral triangular shape. One side surface of the prism 30 is securely 
attached to an oblique surface, i.e. the hypotenuse of the other prism 
element 32 which forms the polarizing plane 16. 
In the propagation path of the light reflected from the polarization plane 
16 there is transversely positioned a quarter wave plate 18. This wave 
plate 18 is adapted to convert the plane-polarized light from the 
polarization plane 16 into a circularly polarized light, as already 
mentioned. The light beam passes through the quarter wave plate 18 is 
concentrated onto a recording surface of a recording disc 20 by means of 
an object lens 22. 
On the recording surface there is formed a train of pits representing the 
information to be picked up. While the disc 20 is rotated, the pickup 
device is so moved the recording surface of the disc 20 that the spot of 
light traces the train of pits. Thus, while the recording disc 20 is 
rotated, the incident strictly coherent light originated from the light 
source 10 is modulated by the train of pits of the disc 20 and is 
reflected back therefrom toward the polarizing plane 16 through the lens 
22 and the plate 18. 
The light beam reflected from the recording surface of the recording disc 
20 is collimated by the object lens 22 and then passes through the quarter 
wave plate 18. The circularly polarized light beam returned from the disc 
20 is reconverted by the quarter wave plate 18 into a plane-polarized 
light beam which is polarized in a direction shifted by 90.degree. from 
the direction in which the original light beam emitted from the light 
source 10 is polarized so that the returned light beam passes through the 
polarization plane 16. 
Thus, the polarizing plane 16 and the quarter wave plate 18 cooperate as a 
beam splitter in fact. 
The light passed through the prism 26 enters a photosensor 24 which 
produces an electric signal having a magnitude according to changes in the 
intensity or phase of the entered light beam. The electric signal thus 
represents the information recorded on the recording disc 20. 
Whereas, the embodiment described above has a light source disposed 
opposite to the photosensor 24 with respect to the polarizer prism 26, a 
second embodiment as shown in FIG. 3 has a light source disposed in the 
same side with the photosensor with respect to the polarizer prism. 
In the embodiment shown in FIG. 3, the polarizer prism 34 comprises two 
prism elements 36 and 38. The cross section of one of the two prism 
elements 36 is of a parallelogram having interior angles of 45.degree. and 
135.degree., while the cross section of the other prism element 38 is of a 
right-angled equilateral triangle. The prism element 38 is securely 
attached at its oblique surface forming a polarizing plane 16 to one side 
surface of the prism element 36 in such a manner that a side surface 34a, 
which is inclined from the plane 16 by 135.degree., of the 
parallelogram-shaped prism element 36 is registered in a level to a side 
surface 34b of the triangular prism element 38. In this embodiment, the 
light source 10 emits light toward the side surface 34a of the prism 
element 36 in a direction which is normal to the surface 34a and is 
reverse to the emission direction of the light source in the first 
embodiment as shown in FIG. 2. The embodiment shown in FIG. 3 is the same 
as the embodiment shown in FIG. 2 except the disposition of the light 
source and the construction of the polarization prism, and therefore no 
description will be given here with respect to the other parts. 
It should be noted that the photosensor 24 and the assembly constituted by 
the light source 10 and the concentrating lens 12 are exchangeable with 
each other with respect to their positions in the two embodiments 
described above and shown in FIGS. 2 and 3. FIGS. 4 and 5 show other 
embodiments respectively corresponding to the embodiments of FIGS. 2 and 
3, wherein the light source assembly 10, 12 and the photosensor 24 are 
replaced by each other. 
As will be understood from the above description, in an optical pickup 
device according to the present invention the light source and the 
concentrating lens associated therewith can be so disposed that the light 
beam emitted from the light source propagates toward a polarizer prism in 
a direction substantially parallel to the path of light within the 
asssembly constituted by an object lens, a quarter wave plate, a a 
polarizer prism and a photosensor. Consequently, in accordance with the 
present invention, it is possible to accomodate all of the said members 
including the light source in a compact, generally cylindrical housing. 
In addition, in an optical pickup device according to the present 
invention, the deviation of path of light due to the errors of mounting of 
the prism can be reduced, since, according to the present invention, the 
oblique reflecting surface 28 and the polarizing plane 16 of the prism are 
disposed parallel or perpendicular to each other. 
It will be apparent that various changes and modifications may be envisaged 
by those skilled in the art without exceeding the scope of the present 
invention as claimed in the appended claims.