Apparatus for aligning optical axis of optical pick-up

An optical axis alignment apparatus of an optical pick-up device for measuring and adjusting a shift amount between a central axis of an objective lens and an optical axis of a laser beam emitted from a light source includes a first photodetector positioned to reciprocate along a central axis of the objective lens, for detecting a shift amount of an optical axis of the laser beam with respect to the central axis of the objective lens and for generating a first detection signal; a second photodetector for detecting a distribution of the laser beam emitted from the laser source and for generating a second detection signal; a beam splitter for splitting the laser beam at a predetermined angle to direct the laser beam to said first and said second photodetectors, respectively; a driving mechanism for reciprocating the first photodetector along the central axis of the objective lens; and mechanism for adjusting the position and the angle of the laser source in dependence upon reception of the first and second detection signals.

CROSS-REFERENCE TO RELATED APPLICATION 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.119 from an application 
for Apparatus For Aligning Optical Axis Of Optical Pick-up earlier filed 
in the Korean Industrial Property Office on 7 Apr. 1995 and there duly 
assigned Ser. No. 8115/1995. 
BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to an optical axis alignment apparatus for 
aligning an optical axis of an optical pick-up device so that an optical 
axis of a laser beam which enters an objective lens is in congruity with a 
central axis of the objective lens. 
2. Background Art 
An optical pick-up device of an optical disc memory system is well known in 
the art which includes an objective lens through which a laser beam is 
passed to form a small light spot on an optical information recording 
medium such as an optical disk in order to record, read or erase 
information to and from the optical disk. In such an optical disk memory 
system, a unit of information to be recorded on an optical disk is 
extremely small so that, in order to record and reproduce information to 
and from an optical disk accurately, it is necessary to control the 
focusing of an optical pick-up device by aligning an optical axis of a 
laser beam with a central axis of an objective lens through which the 
laser beam is irradiated onto an optical disk. 
Typically, conventional optical pick-up devices such as those disclosed, 
for example, in U.S. Pat. No. 5,218,586 for Optical Recording And 
Reproducing Apparatus issued to Tadokoro, U.S. Pat. No. 5,140,572 for 
Optical Pickup Device Having Fixed And Movable Optical System issued to 
Kibune et al., U.S. Pat. No. 5,175,718 for Optical Pickup Apparatus Having 
Set Positions Of Deflecting Prism And Objective Lens issued to Honda, U.S. 
Pat. No. 5,073,884 for Tracking Apparatus Having Movable And Stationary 
Parts issued to Kobayashi, U.S. Pat. No. 5,072,436 for Optical Device For 
Recording And Reproducing Information issued to Honda, and U.S. Pat. No. 
5,060,231 for Separation Type Optical Head issued to Kamisada, are known 
as separation types in which each device comprises a fixed optical system 
and a movable optical system. The fixed optical system usually includes a 
light source, typically a semiconductor laser and a collimating lens. A 
laser beam emitted from the laser source is collimated by the collimating 
lens and then propagates toward the movable optical system. The movable 
optical system comprises a carriage, a reflection prism and an objective 
lens mounted on the carriage. When the laser beam emitted from the fixed 
optical system enters into the movable optical system in which the beam is 
deflected perpendicularly by the reflection prism and converged by the 
objective lens to a point on the optical disk surface so that information 
data is recorded on or reproduced from the disk. 
In the separation type of optical pick-up device however, the optical axis 
of the laser beam transmitted from the fixed optical system to the movable 
optical system in which the laser beam is reflected by the reflection 
prism to the objective lens is inclined by a small angle with respect to a 
central axis of the objective lens due to many factors such as, for 
example, the errors in the mounting position or attitude of the reflection 
prism or the position of the light source in the fixed optical system 
emitting the laser beam when assembling the optical pick-up device. Such a 
misalignment of the optical axis of the laser beam and the central axis of 
the objective lens causes astigmatism and aberration so that the spot 
shape of the converged beam irradiated to the surface of the disk is 
deformed, which degraded the image state of the laser beam and lowers the 
reliability of signal detection. 
Accordingly, to avoid such a problem, it is necessary to align an optical 
axis of the laser beam with a central axis of the objective lens through 
which the laser beam is irradiated onto an optical disk during the 
manufacturing steps of an optical pick-up device so that, after the 
optical pick-up device is assembled, there will be no misalignment between 
the optical axis of the laser beam and the central axis of the objective 
lens. A typical optical axis alignment device for use in a separation-type 
optical pick-up device usually seeks adjustment of the slope and shift of 
the optical axis of a laser beam so that the optical axis of the laser 
beam may be aligned with a central axis of an objective lens. Other 
optical alignment devices require an additional driving mechanism 
including a step motor and a motor controller for shifting a photodetector 
in the optical axis direction instead of shifting the objective lens. 
Driving the photodetector upward and downward in the optical axis 
direction of the laser beam, as I have observed, is unsuitable for 
parallel optical system where the optical axis of the laser beam deviates 
from the central axis of an objective lens within a preset error range. 
SUMMARY OF THE INVENTION 
Accordingly, it is therefore an object of the present invention to provide 
an improved optical axis alignment apparatus. 
It is also an object of the invention to provide an optical axis alignment 
apparatus in which an optical axis of a laser beam emitted from a fixed 
optical system of an optical pick-up device is aligned with a central axis 
of an objective lens from a movable optical system. 
These and other objects of the present invention can be achieved by an 
optical axis alignment apparatus for aligning an optical axis of an 
impinging laser beam with a central axis of an objective lens in a 
separation-type optical pick-up device which includes a stationary section 
comprising a light source adapted to emit a light beam and a collimating 
lens for converting the light beam emitted from the light source into a 
parallel beam, a beam splitter positioned to split the parallel beam 
emitted from the stationary section into a first and second beam, and a 
movable section comprising a carriage adapted to translate in a radial 
direction of an information recording medium, a deflection prism fixedly 
mounted on the carriage for deflecting the first beam splitted from the 
beam splitter, an objective lens fixedly mounted on said carriage for 
converging the first beam. A first photodetector is mounted on a moveable 
member of a jig, and positioned spaced-apart from the stationary section 
and the movable section of the separation-type optical pick-up device to 
reciprocate along a central axis of the objective lens for detecting a 
shift amount of an optical axis of the first beam with respect to the 
central axis of the objective lens and generating a first detection 
signal. A second photodetector is mounted on the jig, positioned spaced 
apart from the stationary section and the movable section of the 
separation-type optical pick-up device, for detecting a distribution of 
the second beam splitted from the beam splitter and for generating a 
second detection signal. A driving mechanism is then used to reciprocate 
the first photodetector along the central axis of the objective lens; an 
adjustment mechanism is further used to adjust the position and the angle 
of the light source of said stationary section in dependence upon 
reception of the first and second detection signals. 
It is also preferred in the present invention that the first and the second 
photodetectors are supported by an unitary member such as, for example, a 
jig which comprises a moving member adjustable by a pair of adjustment 
screws. A first adjustment screw is used for shifting the first 
photodetector in the circumference direction, and a second adjustment 
screw is used for shifting the first photodetector in the radius 
direction. 
The present invention is more specifically described in the following 
paragraphs by reference to the drawings attached only by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings and particularly to FIG. 1, which illustrates 
a conventional optical axis alignment apparatus for use in a 
separation-type optical pick-up device comprising a fixed optical system 1 
for generating a laser beam and a movable optical system 2 for irradiating 
the laser beam onto a photodetector 22a for detecting a shift of an 
optical axis of the laser beam with respect to a central axis of an 
objective lens 21b. A laser beam emitted from a light source such as a 
semiconductor laser diode 1a of the fixed optical system 1 is converted to 
a parallel beam by a collimating lens 1b and is reflected at 90.degree. 
from a reflection mirror or prism 21a installed in a carriage 21 of the 
movable optical system 2. The reflected laser beam is then converged by an 
objective lens 21b to a photodetector 22a positioned in the same central 
axis as that of an objective lens 21b. The photodetector 22a is fixedly 
mounted on a jig 22 of the carriage 21 of the movable optical system 2. 
FIG. 2 illustrates a typical circuit representing the photodetector 22a 
used in the conventional optical axis alignment apparatus of FIG. 1. When 
there is a shift in the optical axis of the laser beam, the photodetector 
22a generates a differential signal to an oscilloscope 23. Alignment of 
the slope and shift of the optical axis is made by adjusting a light 
emission angle of the semiconductor laser diode 1a by means of a slope 
adjuster 4 and a shift adjuster 5 in response to the shift amount 
represented by the differential signal generated from the photodetector 
22a via an oscilloscope 23, while the movable optical system 2 
reciprocates along two guide rails 3 and 3' in a radius direction of a 
disc 6 within a preset alignment reference. 
FIGS. 3 and 4 illustrate waveforms of a differential signal of the 
photodetector 22a according to an optical axis shift in radius and 
circumference directions, respectively, while FIG. 5 illustrates an actual 
waveform of the differential signal of the photodetector 22a according to 
the optical axis shift in the conventional optical axis alignment 
apparatus of FIG. 1. In FIGS. 3 to 5, positions "I," "J" and "K" 
respectively represent relative positions of the photodetector 22a with 
respect to the objective lens 21b. As established in the separation type 
of optical pick-up device however, error occurring when the objective lens 
21b is assembled in the movable optical system 2 becomes an obstacle to an 
accurate operation. In order to reduce the operational error, the position 
of the objective lens 21b is adjusted by driving an actuator 21c. When the 
position of the objective lens 21b is shifted upward and downward by the 
actuator 21c, however, the waveform of the differential signal generated 
from the photodetector 22a appears near to a sine wave as shown in FIG. 5, 
and not a rectangular waveform of FIGS. 3 and 4. This is because, when 
being moved by the actuator 21c with respect to the photodetector 22a, the 
objective lens 21b does not move linearly along the optical axis but moves 
tracing a dotted line while drawing a circle which deviates in one side as 
shown in FIG. 6. Such a circular movement occurs due to a structure of the 
actuator 21c, i.e., a fixing member (not shown) of the actuator 21c, to 
one end of which the objective lens 21b is attached is shaken due to a 
magnetic field generated by the actuator 21c. Further, as the central axis 
of the objective lens 21b deviates as much as .DELTA.X, an offset is 
generated as shown in FIG. 5. The offset value relates to the deviation 
amount .DELTA.X of the objective lens 21b and serves as a factor in 
deviating the shift amount from the reference range. 
In order to improve such an offset problem, a more recent conventional 
optical alignment apparatus further includes an additional driving 
mechanism 100 including a step motor 100a and a motor controller 100b for 
shifting the photodetector 22a in the optical axis direction instead of 
shifting the objective lens 21b as shown in FIG. 7. While such a driving 
mechanism 100 is suitable for use in an infinite parallel optical system 
where the optical axis of the laser beam emitted from the laser diode 1a 
is congruous with the central axis of the objective lens 21b as shown in 
FIG. 8, it has been my observation that this type of optical axis 
alignment construction is not suitable for a limited parallel optical 
system where the optical axis deviates from the central axis of an 
objective lens within a preset error limit range or the divergent beam is 
input to the objective lens 21b while the optical axis of the laser diode 
1a deviates from the central axis of the objective lens 21b as much as 
.DELTA.X. Moreover, since most optical pick-up devices available are the 
limited parallel optical systems where the divergent beam is converged to 
the objective lens, the differential signal of the photodetector 22a can 
deviate from the reference scope though a light intensity peak position of 
the optical axis as the divergent beam passes through the center of the 
objective lens 21b as shown in FIG. 9, and alternatively, when the light 
intensity peak position is deviated toward one side as shown in FIG. 10. 
Turning now to FIG. 11 which illustrates an improved optical axis alignment 
apparatus constructed according to the principles of the present 
invention. The optical axis alignment apparatus constructed according to 
the present invention is intended to align an optical axis of an impinging 
laser beam with a central axis of an objective lens of an optical pick-up 
device. The optical pick-up device as contemplated is a separation-type 
which includes a stationary optical system comprising a light source 
adapted to emit a light beam and a collimating lens for converting the 
light beam emitted from a laser source 11a into a parallel beam, a beam 
splitter 17 positioned to split the parallel beam into a first and second 
beam, and a movable optical system comprising a carriage (not shown) 
adapted to translate in a radial direction of an information recording 
medium, a deflection prism 121a fixedly mounted on the carriage for 
deflecting the laser beam splitted from the beam splitter, an objective 
lens 121c fixedly mounted on the carriage for converging the laser beam 
onto an optical disk (not shown). However, before the separation-type 
optical pick-up device is finally assembled for use, the optical axis of 
an impinging laser beam is aligned with a central axis of the objective 
lens 121c by the optical axis alignment apparatus as shown in FIG. 11. 
Referring back to FIG. 11, the optical axis alignment apparatus constructed 
according to the present invention includes a jig 122 having a moveable 
member 122c mounted thereon a first photodetector 122a at the center of 
its bottom in order to reciprocate along a central axis of the objective 
lens 121c for detecting a shift amount of an optical axis of the laser 
beam with respect to the central axis of the objective lens 121c. A second 
photodetector is mounted on the same jig 122 but spaced apart from the 
stationary optical system and the movable optical system of the 
separation-type optical pick-up device, for detecting a distribution of 
the second beam splitted from the beam splitter 17. A pair of adjusting 
screws 113a, 113b are mounted to the movable member 122c of the jig 122 to 
allow adjustment of the position of the first photodetector 122a both in 
the radius or circumference directions once the initial position of the 
jig 122 where the photodetectors 122a and 122b are installed is 
determined. It should be noted here that, after an optical axis of an 
impinging laser beam is aligned with a central axis of an objective lens 
by the optical axis alignment apparatus as shown in FIG. 11, an optical 
disk may then be placed at the position corresponding to the first 
photodetector 122a in order to operate the optical pick-up device within 
an optical disk player. 
FIG. 12 is a block diagram of the optical axis alignment apparatus 
constructed according to the principles of the present invention for use 
in a separation-type optical pick-up device. As described above, the 
optical axis alignment apparatus includes a stationary optical system 11 
and a movable optical system 12. The distance between the stationary 
optical system 11 and the movable optical system 12 is typically adjusted 
by way of two guide rails 13 and 13' which enables a linear movement of 
the movable optical system 12. The stationary optical system 11 includes a 
semiconductor laser diode 11a for emitting a laser beam and a collimating 
lens 11b for converting the laser beam emitted from the semiconductor 
laser diode 11a into a parallel beam. 
The movable optical system 12 includes a carriage 121 moving in a radius 
direction of an optical disk 16 along the guide rail 13, a reflection 
mirror 121a and an objective lens 121b both installed inside the carriage 
121. An actuator 121c is also installed inside the carriage 121 for 
adjusting the movement of the objective lens 121b. A first photodetector 
122a as mounted on a movable member of the jig 122 is used for measuring 
optical axis shift amount from the light intensity of the laser beam 
passing through the objective lens 121b and installed to be capable of 
reciprocating along the central axis of the objective lens 121b by a 
driving portion 100. A second photodetector 122b is mounted on one side of 
the same jig 122 as shown in FIG. 11 such that the center of a quadrantal 
plate can be congruous with an optical axis (peak axis) of the laser beam 
emitted from the laser diode 11a. 
Once the first photodetector 122a is mounted on a movable member 122c of 
the jig 122, its movement can be adjusted both in the radius or 
circumference directions by means of adjustment screws 113a and 113b as 
shown in FIG. 11. That is, the adjustment screws 113a and 113b are used to 
adjust the first photodetector 122a attached at the bottom of the moving 
member 122c in the circumference direction and the radius direction, 
respectively. 
A slope adjuster 14 is electrically connected to the stationary optical 
system 11 for adjusting a beam emission angle of the semiconductor laser 
diode 11a. Similarly, a shift adjuster 15 is electrically connected to the 
stationary optical system 11 for adjusting horizontal or vertical shift of 
the laser diode 11a. The driving portion 100 is electrically connected to 
the movable optical system 12 for reciprocating the first photodetector 
122a along the central axis of the objective lens 121b comprises a step 
motor 100a and a motor controller 100b. The driving portion 100 is used to 
correct errors occurring during assembly of the objective lens 121b. 
In order to align the optical axis of the laser beam so that the optical 
axis is in congruity with the central axis of the objective lens 121b, the 
moving member 122c is primarily shifted in the circumference or radius 
direction by the adjustment screws 113a and 113b of the jig 122 such that 
the center of the first photodetector 122a attached at the bottom of the 
moving member 122c can be positioned on the same optical path as the 
central axis of the objective lens 121b. Next, the first photodetector 
122a is shifted by the driving portion 100 toward each of "I", "J" and "K" 
planes as described in reference to FIGS. 3 and 4 in order to detect 
deviation amount between the optical axis of the laser beam and the 
central axis of the objective lens 121b. The emission angle and position 
of the semiconductor laser diode 11a of the fixed optical system 11 are 
adjusted by means of the slope adjuster 14 and the shift adjuster 15 in 
order to maintain the deviation amount within a reference scope. 
To assure that any error that occurs when the central axis of the laser 
beam of the laser diode 11a and the central axis of the objective lens 
121b are not congruous with each other as described referring to FIGS. 9 
and 10, the second photodetector 122b is installed at the jig 122 where 
the first photodetector 122a is installed, to detect the incongruity 
between the center of the laser diode 11a and the central axis of the 
objective lens 121b, and thus, the above incongruity is adjusted such that 
the laser beam can be a parallel beam having beam distribution (spot size) 
within the reference scope. The adjustment is achieved by adjusting the 
angle and position of the laser diode 11a by the slope and shift adjusters 
14 and 15 according to the differential signal detected at the second 
photodetector 122b having the quadrantal structure. 
To detect both shift of the optical axis of the laser beam from the 
objective lens central axis and distribution variation due to the slope of 
the laser diode by using the two photodetectors 122a and 122b, the laser 
beam emitted from the laser diode 11a should be divided using a beam 
splitter 17 as shown in FIG. 13. That is, the beam splitter 17 divides the 
laser beam to transfer the divided laser beam to each of the first and 
second photodetectors 122a and 122b. 
Each of the described shift and distribution variation are detected by the 
first and second photodetectors 122a and 122b in the radius and 
circumference directions, respectively, and applied to a photodetector 
circuit as shown in FIG. 14 so that the optical axis alignment is 
performed. That is, the shift amount of the objective lens central axis 
from the laser beam optical axis and angular incongruity between the laser 
diode and the objective lens central axis which are detected by the first 
and second photodetectors 122a and 122b in the radius and circumference 
directions each are differentially amplified at the photodetector circuit 
of FIG. 14 and provided to the slope adjuster 14, the shift adjuster 15 
and the driving portion 100 as an adjustment amount. 
As described above, in the optical axis alignment apparatus according to 
the present invention, the shift of the objective lens central axis from 
the laser beam optical axis and the angular incongruity between the laser 
diode and the objective lens central axis each are adjusted by the first 
and second photodetectors. Thus, a parallel beam can always be input to 
the objective lens so that an accurate optical axis adjustment is 
obtained. 
While there have been illustrated and described what are considered to be 
preferred embodiments of the present invention, it will be understood by 
those skilled in the art that various changes and modifications may be 
made, and equivalents may be substituted for elements thereof without 
departing from the true scope of the present invention. In addition, many 
modifications may be made to adapt a particular situation to the teaching 
of the present invention without departing from the central scope thereof. 
Therefore, it is intended that the present invention not be limited to the 
particular embodiment disclosed as the best mode contemplated for carrying 
out the present invention, but that the present invention includes all 
embodiments falling within the scope of the appended claims.