Optical information recording/reproducing apparatus

The present is to provide an optical information recording/reproducing apparatus which includes a multi-beam optical head extremely reducible of generation of a tracking offset caused by the inclination of a disc or track follow-up, so that the spot-wobbling method and the push-pull method are used to fetch a tracking error signal with respect to one reading light beam, the reading light beam being tracking-controlled by use of the tracking error signal obtained by the spot-wobbling method and then the tracking control for other light beams is performed by use of both the tracking error signal obtained by the push-pull method and that obtained by the push-pull method with respect to the reproducing light beam.

FIELD OF THE INVENTION 
The present invention relates to an optical information 
recording/reproducing apparatus for irradiating a reproducing beam and a 
recording beam onto an information recording medium, and more particularly 
to an optical information recording/reproducing apparatus improving the 
tracking performance. 
BACKGROUND OF THE INVENTION 
FIG. 9 is a structural view of the conventional optical information 
recording/reproducing apparatus described on Pages 58 through 62--"LD 
Array Head For DRAW"--of "Micro-Optics News" (Vol. 3, No. 1, edited on 
Feb. 4, 1985) by Ito and Ohta. 
Referring to FIG. 9, reference numeral 1 designates a semiconductor laser 
array emitting two light beams of recording beam L1 of high intensity of 
light and reproducing beam L2 of low intensity of light, 2 designates a 
collimator lens for collimating the beams L1 and L2 in the parallel light 
beams emitted from the semiconductor laser array 1, and 3 designates a 
beam shaping prism for correcting distribution of elliptical strength of 
the respective collimated beams L1 and L2. 
Reference numeral 4 designates a polarizing beam splitter, which is adapted 
to transmit therethrough the beams L1 and L2 having passed a beam shaping 
prism, toward an information recording medium (to be discussed below) and 
to reflect toward an error detecting system (to be discussed below) a 
recording reflected beam L1' and a reproducing reflected beam L2' from the 
information recording medium. 
Reference numeral 5 designates a reflecting mirror, and 6 designates a 1/4 
wavelength plate, which are disposed on an optical path at the 
transmission side of the polarizing beam splitter 4. 
Reference numeral 7 designates an objective lens for focusing on the 
information recording medium the beams L1 and L2 having passed the 
reflecting mirror 5 and 1/4 wavelength plate 6 respectively, 8 designates 
an information recording medium comprising an optical disc rotatable 
around a rotary shaft 8a, and 9 designates an information track formed 
concentrically or spirally in the information recording medium 8. 
Reference numeral 10 designates a convex lens disposed on an optical path 
at the reflection side of the polarizing beam splitter 4, and 11 
designates a spatial filter disposed at the focal point of convex lens 10, 
which cuts off the recording reflected beam L1' reflected from the 
information recording medium 8 and allows only the reproducing reflected 
beam L2' to transmit through the lens 11. 
Reference numeral 12 designates a beam splitter for dividing the 
reproducing reflected beam L2' having passed the spatial filter 11, toward 
a tracking error detection system and a focusing error detection system 
(both to be discussed below), 13 designates a two-divided photodetector 
for receiving the reproducing reflected beam L2' having passed the beam 
splitter 12, 14 designates a convex lens, 15 designates a knife edge, and 
16 designates a two-divided photodetector for receiving the reproducing 
reflected beam L2' through the knife edge 15, which are disposed on the 
optical path at the reflection side of the beam splitter 12. 
Reference 17 designates a differential amplifier which takes in a 
difference between the two signals output from the two-divided 
photodetector 13, 18 designates a tracking actuator for driving the 
objective lens 7 in the traversing direction (in the direction of the 
arrow T) with respect to the information track 9 on the basis of an output 
signal of the differential amplifier 17, 19 designates a differential 
amplifier which takes in a difference between the two signals output from 
the two-divided photodetector 16, and 20 designates a focusing actuator 
for driving the objective lens 7 in the vertical direction (in the 
direction of the arrow F) with respect to the surface of the information 
recording medium 8 on the basis of the output signal of the differential 
amplifier 19. 
In addition, the two-divided photodetector 13 and differential amplifier 17 
constitute the tracking error detection system for detecting a tracking 
error of the reproducing beam L2 irradiated to the information recording 
medium 8. The convex lens 14, knife edge 15, two-divided photodetector 16 
and differential amplifier 19, constitute the focusing error detection 
system for detecting a focusing error of the reproducing beam L2. 
FIG. 10 is an illustration showing the irradiation states of the respective 
light beams L1 and L2 with respect to the information track 9 in FIG. 9, 
in which reference numerals P1 and P2 designate two light spots, in other 
words, a recording spot and a reproducing spot, formed of the condensed 
beams L1 and L2, the arrow D designates the traveling direction by 
rotation of the information recording medium 8, and 21 designates a pit 
formed on the information track 9 by the recording spot P1. 
Next, explanation will be given on operation of the conventional optical 
information recording/reproducing apparatus shown in FIGS. 9 and 10. 
The recording beam L1 and reproducing beam L2 emitted from the 
semiconductor laser array 1 are collimated by the collimator lens 2 to the 
parallel light beams and further formed by the beam shaping prism 3 into 
two light beams having nearly symmetrical intensity-distribution with 
respect to the optic axis rotationally. 
Next, the recording beam L1 and reproducing beam L2 are incident on the 
objective lens 7 through the polarizing beam splitter 4, reflection mirror 
5 and 1/4 wavelength plate 6 and focused on the information track 9 at the 
information recording medium 8 to be the recording spot P1 and reproducing 
spot P2. 
The recording spot P1 in advance with respect to the rotation direction of 
the information recording medium 8 forms in order on the information track 
9 the pits 21 modulated corresponding to the contents of information, the 
lagging reproducing spot P2 reproducing at the same time the content of 
information included in the recorded pit. 
Continuously, the recording reflected beam L1' and reproducing reflected 
beam L2' reflected from the information recording medium 8 are again 
incident on the polarizing beam splitter 4 through the objective lens 7, 
1/4 wavelength plate 6 and reflection mirror 5. The reflected beams L1' 
and L2', which reciprocate through the 1/4 wavelength plate 6 so as to 
rotate in the polarizing direction of 90.degree., are reflected at the 
polarizing beam splitter 4 and image-formed on the spatial filter 11 by 
the convex lens 10. In this case, only the reproducing reflected beam L2' 
passes through the spatial filter 11, is divided by the beam splitter 12, 
and received by the two-divided photodetectors 13 and 16. 
Accordingly, the tracking error signal is detected by the push-pull method 
using the two-divided photodetector 13, the focusing error signal being 
detected by the knife-edge method using the knife edge 15 and the 
two-divided photodetector 16. The tracking error signal and focusing error 
signal thus obtained are amplified by the differential amplifiers 17 and 
19 so as to drive the tracking actuator 18 and focusing actuator 20 
respectively. 
Also, the sum of output signals (not shown) of two-divided photodetector 13 
is gained to detect the quantity of light of reproducing reflected beam 
L2' so as to reproduce the information signal recorded on the information 
track 9 at the information recording medium 8. 
The conventional optical information recording/reproducing apparatus 
records and reproduces the information as the above-mentioned, in which it 
is generally known that when the push-pull method detects the tracking 
error signal, a tracking offset becomes larger. 
FIG. 11 is a characteristic graph showing the relation between the 
follow-up quantity of the objective lens 7 with respect to the information 
track 9 and the tracking offset quantity, which is described in, for 
example, "Optical head for write-once disk with two perpendicular axes" of 
"Optical Memory Symposium" (in 1985, Pages 97 through 102), and which 
shows that, when a track follow-up amount of the objective lens 7 is 100 
.mu.m, the tracking offset is generated by only about 0.08 .mu.m. Usually, 
a tolerance of tracking offset is about 0.05 to 0.1 .mu.m so that it is 
seen that an offset value of 0.08 .mu.m is about the limit of the 
tolerance. 
FIG. 12 is a characteristic graph showing the relation between the 
inclination of the information recording medium 8 and the tracking offset 
quantity, which is described in Pages 224 through 229 of, for example, 
"On-land Composite Pregroove Method for High Track Density Recording" (by 
Y. Tsunoda et al), SPIE, Vol. 695, 1986. In this case, it is shown that 
the information recording medium 8 inclines at an angle of 1.degree. to 
generate a tracking offset of 0.11 .mu.m to exceed the aforesaid 
tolerance. 
Now, when the radial wobbling method, instead of the push-pull method, is 
used for the tracking control, it is well-known that the aforesaid 
tracking offset problem is almost solved, the radial wobbling method 
including a spot-wobbling method of vibrating the reproducing light spot 
P2 and a track wobbling method of providing the wobble pit at the 
information track 9, which are quite equivalent in principle. 
The spot-wobbling method has been described in the "Principles of Optical 
Disc System" [by G. Bouwhuis et al, Adam Hilger Ltd., 1985, Pages 73 
through 75]. Also, it is reported by, for example, the "Composite Wobbled 
Tracking in the Optical Disc System" (by Ohtake et al, Pages 181 through 
188, Optical Memory Symposium '85, 1985) that the tracking control by such 
radial wobbling method is hard to be affected by the inclination or the 
like of the information recording medium 8. 
However, it is difficult to adopt the spot-wobbling method with respect to 
the conventional apparatus which emits the recording beam L1 and 
reproducing beam L2 from one light source 1 as shown in FIG. 9. Even when 
the track wobbling method is adopted, the wobble pits are periodically 
provided, whereby the tracking state of the light spot cannot always be 
detected. 
Furthermore, the relative-positional relationship of the recording spot P1 
is mechanically adjusted to coincide with the position of the reproducing 
spot P2, but in the conventional apparatus in FIG. 9, in spite of that one 
objective lens 7 focuses the recording beam L1 and reproducing beam L2 on 
the information recording medium 8, the focusing error signal and tracking 
error signal are obtained only from the reflected light of the reproducing 
spot P2. Accordingly, when the information recording medium 8 becomes 
eccentric to even slightly deteriorate the parallelism between the line 
connecting two light spots P1 and P2 and the information track 9, the 
tracking offset is generated at the recording spot P1. 
The conventional optical information recording/reproducing apparatus, as 
above-mentioned, detects the tracking error signal by use of the push-pull 
method, thereby creating the problem in that the tracking offset generated 
by the track follow-up of objective lens 7 or the inclination of 
information recording medium 8 becomes larger. Also, since the focusing 
error signal and tracking error signal are obtained only from the 
reflected light of the reproducing spot P2, the information recording 
medium 8 is eccentric to deteriorate the parallelism between the line 
connecting the light spots P1 and P2 and the information track 9, thereby 
creating the problem in that the tracking offset is generated to make 
impossible the stable tracking control. 
SUMMARY OF THE INVENTION 
The invention has been achieved to solve the above-mentioned problems. 
An object of the invention is to provide an optical information 
recording/reproducing apparatus which can reduce a tracking offset of the 
light spot caused by a track follow-up of an objective lens or an 
inclination of an information recording medium and perform stable tracking 
control. 
Another object of the invention is to provide an optical information 
recording/reproducing apparatus which reduces a tracking offset of the 
light spot caused by a track follow-up of the objective lens, an 
inclination of the information recording medium, or eccentricity of the 
information recording medium, thereby enabling the tracking control to be 
stably performed. 
The optical information recording/reproducing apparatus of the invention is 
provided with a plurality of light sources for separately emitting the 
recording beam and reproducing beam, wobbling means for minutely vibrating 
the reproducing light spot in the transverse direction of the information 
track, beam composing means for composing the recording beam and 
reproducing beam, and a tracking error detection system for obtaining the 
tracking error signal from the reproducing reflected beam by the 
spot-wobbling method. 
An optical information recording/reproducing apparatus of another of the 
present invention is provided with a plurality of light sources, wobbling 
means, beam composing means, a first tracking error detection system for 
obtaining a tracking error signal St(R) from the reproducing reflected 
beam by the spot-wobbling method, and a second tracking error detection 
system for obtaining a tracking error signal St(W) on the basis of a 
tracking error signal S't(R) obtained from the reproducing reflected beam 
by the push-pull method and a tracking error signal S't(W) obtained from 
the recording reflected beam by the push-pull method. 
In the present invention, the tracking control for the reproducing beam is 
performed on the basis of the tracking error signal. 
In another embodiment of the present invention, the tracking control for 
the reproducing beam is performed on the basis of the tracking error 
signal St(R) and that for the recording beam is performed on the basis of 
the tracking error signal St(W). 
The optical information recording/reproducing apparatus of the present 
invention is provided with the plurality of light sources for separately 
emitting the recording beam and reproducing beam, wobbling means for 
finely vibrating the reproducing light spot, beam composing means for 
composing the recording beam and reproducing beam, and tracking error 
detection systems for obtaining the tracking error signal from the 
reproducing reflected beam by the spot-wobbling method, so that the 
tracking control for the reproducing beam is performed on the basis of the 
tracking error signal, thereby having the effect that the tracking offset 
of the light spot caused by the track follow-up or the inclination of 
information recording medium, is reduced so as to stabilize the tracking 
control. 
The optical information recording/reproducing apparatus of another of the 
invention is provided with a plurality of light sources, the wobbling 
means, beam composing means, a first tracking error detection system for 
obtaining the tracking error signal St(R) from the reproducing reflected 
beam by the spot-wobbling method, and a second tracking error detection 
system for obtaining the tracking error signal St(W) on the basis of the 
tracking error signal S't(R) obtained from the reproducing reflected beam 
by the push-pull method and of the tracking error signal S't(W) obtained 
from the recording reflected beam by the push-pull method, so that the 
tracking control for the reproducing beam on the basis of the tracking 
error signal St(R) and that for the recording beam is performed on the 
basis of the tracking error signal St(W), thereby being effective to 
reduce the tracking offset of the light spot caused by eccentricity or the 
like of the information recording medium and further stabilize the 
tracking control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 is an embodiment of an optical information recording/reproducing 
apparatus of the present invention, in which reference numerals 2A and 2B 
correspond to the aforesaid collimator lens 2 respectively, 4A and 4B to 
the aforesaid polarizing beam splitter 4 respectively, 6A and 6B to the 
aforesaid wavelength plate 6 respectively, 7 through 9 and 20 are the same 
as the aforementioned. Also, light spots P1 and P2 formed in the 
information recording medium 8 by the light beams L1 and L2 are in the 
state as shown in FIG. 10. 
Reference numeral 1A designates a light source of a semiconductor laser 
emitting the recording beam L1 of a wavelength .lambda..sub.1, and 1B 
designates a light source of a semiconductor laser emitting the 
reproducing beam L2 of a wavelength .lambda..sub.2. 
Reference numeral 26 designates a wobbling mirror, which is disposed on an 
optical path of the reproducing beam L2 passing the 1/4 wavelength plate 
6B and adopted to finely vibrate in the direction of the arrow WB 
corresponding to the transverse direction of information track 9. 
Reference numeral 28 designates a dichroic beam splitter, which transmits 
therethrough the recording beam L1 of wavelength .lambda..sub.1, and 
reflects the reproducing beam L2 of wavelength .lambda..sub.2, is disposed 
at an intersection of the optical paths of the recording beam L1 passing 
the 1/4 wavelength plate 6A and reproducing beam L2 reflected by the 
wobbling mirror 26, and constitutes beam composing means for composing the 
recording beam L1 and reproducing beam L2 to travel toward the information 
recording medium 8. 
Reference numeral 29 designates a tracking mirror, which is disposed on the 
optical path of the two light beams L1 and L2 composed by the dichroic 
beam splitter 28, leads the light beams L1 and L2 toward the information 
recording medium 8, and rotates in the direction of the arrow T1 so as to 
tracking-control the reproducing beam L2. 
Reference numeral 30 designates a wavelength filter for transmitting only 
the recording reflected beam L1' of wavelength .lambda..sub.1, 31 
designates a photodetector for receiving the recording reflected beam L1' 
passing the wavelength filter 30, 32 designates a wavelength filter for 
transmitting only the reproducing reflected beam L2' of wavelength 
.lambda..sub.2, and 33 designates a two-divided photodetector for 
receiving the reproducing reflected beam L2' passing the wavelength filter 
32. 
Reference numeral 34 designates an adder which obtains the sum of signals 
from the two-divided photodetector 33 to output the sum signal E 
representing the total quantity of light, 35 designates an oscillator for 
outputting a predetermined frequency signal C to vibrate the wobbling 
mirror 26, and 36 designates a multiplier for multiplying the frequency 
signal C by the sum signal E. 
Reference numeral 40 designates a low-pass filter which transmits the DC to 
low frequency component of the output of multiplier 36 and outputs the 
tracking error signal St(R) to the tracking mirror 29 and which 
constitutes a tracking error detection system for the reproducing beam L2 
by the spot-wobbling method together with the two-divided photodetector 
33, adder 34, oscillator 35 and multiplier 36. 
Also, the wobbling mirror 26 and oscillator 35 constitute wobbling means 
for finely vibrating the reproducing light spot P2 in the transverse 
direction of the information track 9. 
In addition, the focusing error detection system, which can be constituted 
on the basis of the well-known knife-edge method (refer to FIG. 9) or the 
astigmatism method, is not shown. 
Next, explanation will be given on operation of the embodiment of the 
present invention shown in FIG. 1, with reference to FIG. 10. 
In a case of usual information recording/reproducing operation, the 
recording beam L1 of wavelength .lambda..sub.1 emitted from the light 
source 1A passes the collimator lens 2A, polarizing beam splitter 4A, 1/4 
wavelength plate 6A and dichroic beam splitter 28, and then is reflected 
by the tracking mirror 29 and further is focused on the information track 
9 of the information recording medium 8 through the objective lens 7 to be 
a recording light spot P1, thereby forming a pit 21 to record the 
information. 
Simultaneously, the recording beam L1 is reflected from the information 
recording medium 8 so as to be a recording reflected beam L1', which again 
passes the aforesaid optical system and is reflected by the polarizing 
beam splitter 4A and then passes the wavelength filter 30 to be received 
by the photodetector 31. The signal obtained therefrom may be used for 
real-time monitoring, for example, the recording state. 
Also, the reproducing beam L2 of wavelength .lambda..sub.2 emitted from the 
light source 1B passes the collimator lens 2B, polarizing beam splitter 4B 
and 1/4 wavelength plate 6B and is reflected by the wobbling mirror 26 and 
further reflected by the dichroic beam splitter 28 so as to be composed 
with the recording beam L1. Thereafter, the composed reflected beam is 
focused through the same optical system on the information track 9 after 
recorded so as to be the reproducing light spot P2. 
The reproducing beam L2' from the light spot P2 passes the aforesaid 
optical system and is reflected by the polarizing beam splitter 4B and 
received by the two-divided photodetector 33 through the wavelength filter 
32, at which time the sum signal E from the adder 34 serves as a 
reproducing signal for the recorded information and is used to check 
simultaneously whether or not the recording spot P1 properly records the 
information during the information recording. 
On the other hand, the sum signal E is input to the multiplier 36 and 
multiplied by the wobbling signal C to thereby be a tracking error signal 
St(R) through the low-pass filter 40, the tracking error signal St(R) 
driving the tracking mirror 29 which operates as the tracking actuator for 
the reproducing light spot P2, and tracking-controlling the reproducing 
beam L2. 
Thus, since the spot-wobbling method carries out the tracking control for 
the reproducing light spot P2, no tracking offset occurs by the track 
follow-up of the objective lens 7 or the inclination of information 
recording medium 8. 
In addition, in the embodiment of the present invention in FIG. 1, the 
tracking offset cannot be eliminated which is caused by deviation of 
parallelism between the line connecting the recording light spot P1 and 
the reproducing light spot P2 and the information track 9, whereby it is 
required that a relative positional error of the recording light spot P1 
and reproducing light spot P2 is held at mechanical accuracy within the 
allowable range. 
Next, explanation will be given on an embodiment of another of the present 
invention with reference to FIG. 2. 
FIG. 2 is a structural view thereof, in which reference numerals 26, 28 
through 30, 32 through 36 and 40 are the same as those in the FIG. 1 
embodiment. 
Reference numeral 31A designates a second two-divided photodetector 
corresponding to the photodetector 31 in FIG. 1, 37 designates a tracking 
mirror which is disposed on the optical path of the recording beam L1 
passing a 1/4 wavelength plate 6A and rotates in the direction of the 
arrow T2 corresponding to the transverse direction of an information track 
9 so as to tracking-control the recording beam L1, and 38 designates a 
reflector for deflecting toward the dichroic beam splitter 28 the 
recording beam L1 reflected by the tracking mirror 37. 
Reference numeral 41 designates a first differential amplifier which 
amplifies a difference between the signals from the first two-divided 
photodetector 33 to output a tracking error signal S't(R) by the push-pull 
method, 42 designates a second differential amplifier which amplifies a 
difference between the signals from the second two-divided photodetector 
31A so as to output a tracking error signal S't(W) by the push-pull 
method, 43 designates a third differential amplifier which amplifies a 
difference between the tracking error signals S't(R) and S't(W) so as to 
output a true tracking error signal St(W) of the recording light spot P1, 
and 45 designates a low-pass filter which allows the DC to low frequency 
component to transmit therethrough from the tracking error signal St(W) to 
thereby drive the tracking mirror 37. 
The wavelength filters 30 and 32, two-divided photodetectors 31A and 33, 
and differential amplifiers 41 through 43, constitute a second tracking 
error detection system for obtaining the tracking error signal St(W) with 
respect to the recording light spot P1 on the basis of the tracking error 
signal S't(R) obtained from the reproducing reflected beam L2' and of the 
tracking error signal S't(W) obtained from the recording reflected beam 
L1'. Also, the differential amplifier 43, low-pass filter 45 and tracking 
mirror 37, constitute a tracking control system for the recording beam L1. 
Next, explanation will be given on operation of the FIG. 2 embodiment with 
reference to FIG. 10. 
The fundamental operation of the information recording and reproducing is 
the same as in FIG. 1 except that the tracking mirror 37 and reflector 38 
are interposed on the optical paths of the recording beam L1 and recording 
reflected beam L1'. 
Since the tracking control for the reproducing beam L2 is performed by the 
spot-wobbling method through the first tracking error detection system and 
tracking mirror 29, as the same as the above-mentioned, no tracking offset 
is created by the track follow-up of the objective lens 7 or the 
inclination of the information recording medium 8. 
Meanwhile, the tracking control for the recording beam L1 is carried out as 
follows: 
At first, the differential amplifier 42 amplifies a difference between the 
signals from the two-divided photodetector 31A so as to output the 
tracking error signal S't(W) regarding the recording beam L1 by the 
push-pull method, the tracking error signal S't(W), as shown in FIGS. 11 
and 12, including the tracking offset component caused by the track 
follow-up of objective lens 7 and the inclination of information recording 
medium 8. Accordingly, when a tracking error component of the recording 
light spot P1 is represented by .DELTA.t(W) and a tracking offset 
component caused by the track follow-up or the inclination of information 
recording medium 8 is represented by .delta., the tracking error signal 
S't(W) is given in the following equation: 
EQU S't(W)=.DELTA.t(W)+.delta. 1 
Also, the differential amplifier 41 outputs the tracking error signal 
S't(R) of the reproducing beam L2 by the push-pull method from the 
reproducing reflected beam L2', the tracking error signal S't(R) also 
including the tracking offset component the same as the above-mentioned. 
However, since the reproducing light spot P2 is properly controlled on the 
information track 9 by the tracking control due to the wobbling method, 
the tracking error signal S't(R) includes only the tracking offset 
component .delta. caused by the inclination of information recording 
medium 8. Accordingly, the signal S't(R) is given as follows: 
EQU S't(R)=.delta. 2 
Next, the tracking error signals S't(W) and S't(R) are input to the 
differential amplifier 43 so as to be the tracking error signal St(W), at 
which time the tracking error signal S't(R) based on the reproducing 
reflected beam L'2 is applied to the negative input terminal (-) of 
differential amplifier 43 so as to be the reference for the tracking error 
signal St(W) with respect to the recording beam L1. Accordingly, assuming 
that the tracking error signals S't(W) and S't(R) are equal in sensitivity 
to each other with respect to the track follow-up or the inclination of 
information recording medium 8, the tracking error signal St(W) output 
from the differential amplifier 43 is given in the following equation: 
##EQU1## 
Hence, the tracking error signal St(W) depending only on the true tracking 
error quantity .DELTA.t(W) of the recording light spot P1 is obtained. 
The tracking error signal St(W) is supplied to the tracking mirror 37 
through the low-pass filter 45 so as to drivingly control in the direction 
of the arrow T2 the tracking mirror 37 functioning as the tracking 
actuator for the recording beam L1. 
Generally, since the relative positional deviation of the recording light 
spot P1 and reproducing light spot P2 is caused by a change with the lapse 
of time in the mechanical system and optical system, the temperature 
change, or variation in mechanical dimension of the information recording 
mediums 8, the frequency component of the relative positional deviation 
following the rotation of information recording medium 8 may be deemed 
like DC. Accordingly, for stabilizing the servo loop, the low-pass filter 
45 limits the band of tracking servo loop to be low. 
Also, since the recording beam L1 and reproducing beam L2 use the objective 
lens 7 in common, the recording light spot P1 is controlled also by the 
tracking servo for the reproducing light spot P2 through the tracking 
mirror 29. Accordingly, for the recording beam L1, the tracking error 
signal St(W) according to the equation 3 need only be used to control the 
relative positional deviation of the recording light spot P1 with respect 
to the reproducing light spot P2 only perpendicular to the information 
track 9. 
Another of the present invention can obtain the tracking error signals 
St(W) and St(R) which depend only on the tracking error with respect to 
the recording beam L1 and reproducing beam L2 and are scarcely affected by 
the inclination of information recording medium 8 or the track follow-up, 
whereby the tracking offset generated by the inclination of information 
recording medium 8 or the track follow-up is remarkably reducible. 
Also, since the tracking servo is always applied to the recording beam L1 
and reproducing beam L2, there is no fear that one light spot P1 or P2 
causes the tracking error due to variation in the mechanical dimension 
between the information recording mediums 8, whereby the tracking offset 
generated by eccentricity of the information recording medium 8 is 
remarkably reducible. 
In addition, in the aforesaid embodiments, it is described that the 
tracking error signals S't(W) and S't(R) are equal in sensitivity to each 
other with respect to the track follow-up or the inclination of 
information recording medium 8, the sensitivity actually varying in 
proportion to quantities of light receiving of photodetector 31A and 34. 
In order that this variation is suppressed and the sensitivity with 
respect to the tracking error or the inclination of information recording 
medium 8 depends not on the quantity of light receiving of the 
photodetectors 31A and 33, an automatic gain control unit, as shown in 
FIG. 3, is required to be inserted into the tracking servo loop. 
In FIG. 3, reference numeral 49 designates an adder which fetches the sum 
of signals from the two-divided photodetector 31A, 50 designates a divider 
which divides an output signal of a differential amplifier 42 applied to 
the input terminal A by the output signal of the adder 49 applied to the 
input terminal B and outputs the result of division as the tracking error 
signal S't(W), 51 designates an adder which fetches the sum of signals 
from a two-divided photodetector 33, and 52 designates a divider which 
divides by the output signal of the adder 51 applied to the input terminal 
B the output signal of the differential amplifier 41 applied to the input 
terminal A and which outputs the result of division as the tracking error 
signal S't(R). 
Thus, the automatic gain control unit comprising the adders 49 and 51 and 
dividers 50 and 52 is inserted to enable the tracking error signals S't(W) 
and S't(R) depending not on the quantity of light receiving to be 
obtained. 
Also, in the embodiments in FIGS. 1 and 2, the occurrences of the tracking 
offsets of light spots P1 and P2 are sufficiently reduced with respect to 
deviation of the optical axes between the irradiation light and reflected 
light caused by the inclination thereof or the track follow-up, so that, 
as shown in FIG. 4, the objective lens 7, focusing actuator 20 and 
tracking mirror 29 may alternatively be separated from the optical system 
body and formed in a movable unit 54 of integral construction and movable 
through a rotary shaft 56 with respect to base member 55. Therefore, since 
the movable unit 54 is reduced in weight, the light spots P1 and P2 are 
movable at high speed in the transverse direction of the information track 
9, thereby reducing an access time in comparison with the case where the 
entire optical system is driven. 
In the FIG. 2 embodiment, the tracking mirror 37 for the recording beam L1 
and the wobbling mirror 26 for the reproducing beam L2 are provided 
separately from each other. Alternatively, an adder 46 may be inserted as 
shown in FIG. 5, so that the tracking mirror 37 may be omitted to allow 
the wobbling mirror 26 to function also as the tracking mirror 37. The 
reason for this is that the tracking control for the recording beam L1 is 
to control the relative positional deviation between the recording light 
spot P1 and the reproducing light spot P2 and its control region is an 
about DC region, but the wobbling frequency is several 10 kHz and higher 
at one digit than the frequency region of tracking control. 
In FIG. 5, the adder 46 takes in the sum of wobbling signal C and a signal 
of low-pass filter 45 to drive the wobbling mirror 26. Accordingly, the 
wobbling mirror 26 is driven in the direction of the arrow (WB+T2) of 
adding the wobbling drive and relative tracking drive for the recording 
beam L1. 
In this case, the tracking error signal St(W) having passed the low-pass 
filter 45 controls the reproducing beam L2, the tracking error signal 
St(W) corresponding to deviation of parallelism of the line connecting the 
recording light spot P1 and reproducing light spot P2 with respect to the 
information track 9, whereby the control for the reproducing beam L2 is 
substantially equal to control of the recording beam L1. Furthermore, 
quantity of error of the reproducing beam L2 is automatically corrected by 
the tracking mirror 29, thereby creating no problem. 
Also, in the FIG. 2 embodiment, the tracking error signal St(R) for 
drivingly controlling the tracking mirror 29 is obtained from the low-pass 
filter 40. Alternatively, an adder 47 may be inserted as shown in FIG. 6 
to compose the spot-wobbling method and the push-pull method to thereby 
tracking-control the reproducing beam L2. 
In FIG. 6, the adder 47 sums the signal from the low-pass filter 40 and 
tracking error signal S't(R) from the differential amplifier 41 so as to 
use the sum signal as the tracking error signal St(R) for the reproducing 
beam L2. Accordingly, the band (zero to several 10 Hz) from DC to nearly 
the rotation frequency of information recording medium 8 is in charge of 
the tracking control system mainly for the spot wobbling, and a band 
(several 10 Hz to several kHz) more than the above is in charge mainly of 
the tracking control by the push-pull method. 
At this time, in the low band, the level of tracking error signal S't(R) is 
sufficiently low in comparison with the output signal of low-pass filter 
40, whereby the tracking control only by the spot-wobbling method is 
performed. Hence, the tracking offset caused by the inclination or 
eccentricity of information recording medium 8 or the track follow-up is 
suppressed by the track control system by the spot-wobbling method. 
In the FIG. 2 embodiment, two beams of recording beam L1 of wavelength 
.lambda..sub.1 and reproducing beam L2 of that .lambda..sub.2 are used. 
Alternatively, multi-beams of three or more may be used as shown in FIG. 
7. 
In FIG. 7, reference numeral 1C designates a light source, such as a 
semiconductor laser, for emitting a recording beam L3 of wavelength 
.lambda..sub.3, 2C, 4C and 6C designate a collimator lens, a polarizing 
beam splitter and a 1/4 wavelength plate, disposed in the order on the 
optical path of the recording beam L3 respectively, 59 designates a 
tracking mirror which rotates in the direction of the arrow T3 to 
tracking-control the recording beam L3, and 60 designates a reflector for 
converting the optical path of recording beam L3 through the tracking 
mirror 59. 
Reference numeral 61 designates a dichroic beam splitter, which is disposed 
on the optical paths of the recording beams L1 and L3 and adapted to 
transmit the recording beam L3 of wavelength .lambda..sub.3 and reflect 
the recording beam L1 of wavelength .lambda..sub.1, 62 designates a 
dichroic beam splitter, which composes the recording beams L1 and L3 and 
reproducing beam L2, transmits the recording beams L1 of wavelength 
.lambda..sub.1 and L3 of that .lambda..sub.3, and reflects the reproducing 
beam L2 of wavelength .lambda..sub.2. 
Reference numeral 63 designates a wavelength filter which is disposed at 
the reflecting side of the polarizing beam splitter 4C and adapted to 
selectively transmit only a recording reflected beam L3' of wavelength 
.lambda..sub.3, 64 designates a two-divided photodetector for detecting 
the tracking error signal for the recording beam L3, 65 designates a 
differential amplifier for taking in a difference between the signals from 
the two-divided photodetector 64, 66 designates a differential amplifier 
for taking in a difference between the tracking error signals S't(W3) and 
St'(R) from the differential amplifiers 41 and 65, and 67 designates a 
low-pass filter for passing the DC to low frequency component from the 
tracking error signal St(W3) obtained by the differential amplifier 66 to 
thereby drive the tracking mirror 59. 
In this case, the wavelength filter 63, two-divided photodetector 64, and 
differential amplifier 65, constitute the tracking error detection system, 
the differential amplifier 66 and low-pass filter 67 constituting the 
tracking control system. The differential amplifiers 65 and 66 correspond 
to the differential amplifiers 42 and 43, the low-pass filter 67 
corresponding to the low-pass filter 45, the tracking mirror 59 being 
drivingly controlled as the same as the tracking mirror 37. 
Also, in the FIGS. 1 and 2 embodiments, the beams L1 and L2 from the 
recording optical system and reproducing optical system respectively are 
composed with each other by use of one dichroic beam splitter 28, so that 
the polarizing beam splitters 4A and 4B provided at the optical systems 
separate the irradiation light of the recording beam L1 and reproducing 
beam L2 from the reflected light of the recording reflected beam L1' and 
reproducing reflected beam L2'. Alternatively, as shown in FIG. 8, one 
polarizing beam splitter for separating the reflected light may be 
disposed on the optical path of the composed light beams L1 and L2 emitted 
from the light sources 1A and 1B respectively. 
Referring to FIG. 8, reference numeral 68 designates a dichroic beam 
splitter, which composes the recording beam L1 and reproducing beam L2, 
transmits the recording beam L1 of wavelength .lambda..sub.1, and reflects 
the reproducing beam L2 of wavelength .lambda..sub.2, and 69 designates a 
polarizing beam splitter for separating the irradiation light from the 
reflected light with respect to the information recording medium 8, which 
is adapted to transmit the irradiation light and reflect the reflected 
light and constitutes, together with the dichroic beam splitter 68, beam 
composing means. 
Reference numeral 70 designates a 1/4 wavelength plate disposed on the 
optical path between the polarizing beam splitter 69 and the tracking 
mirror 29, and 71 designates a dichroic beam splitter, which is disposed 
on the optical path at the reflecting side of the polarizing beam splitter 
69, transmits the recording reflected beam L1', and reflects the 
reproducing reflected beam L2', thereby separately deflecting the 
reflected beams L1' and L2' from each other toward the tracking error 
detection systems thereof respectively. 
In this case, since one polarizing beam splitter 69 is used in common, the 
polarizing directions of the irradiation lights L1 and L2, as shown in the 
arrows on the optical paths, are set in parallel to the drawing plane and 
the polarizing directions of the reflected lights L1' and L2' are 
perpendicular to the same respectively. 
In construction shown in FIG. 8, the common section of the optical path is 
long, whereby relative variation in every light beam is reduced to enable 
the tracking control of high stability and reliability. 
Furthermore, the embodiments in FIGS. 1 and 2 use the tracking controlling 
tracking mirror 29. Alternatively, a two-dimensional actuator capable of 
being driven also perpendicularly to the information track 9 may be used 
instead of the focusing actuator 20 and the tracking mirror 29 may be 
omitted. 
POSSIBILITY OF INDUSTRIAL UTILIZATION 
As seen from the above, the present invention is applicable not only to the 
optical information recording/reproducing apparatus using an optical disc 
but also to an optical information recording/reproducing apparatus using 
an optical card or other optical recording media.