Optical videodisc read unit with tracking and focussing wobblers

An optical read unit for reading radiation-reflecting video discs comprises a semiconductor laser which with the aid of a scan wobbler and a focussing wobbler is moved at high frequency in two directions relative to the video disc, a radial direction for tracking purposes and a direction normal to the disc for the purpose of automatic focussing. The semiconductor diode laser is mounted on a metal radiation source carrier which is connected to a heat sink for carrying off the heat with a low temperature gradient, and is driven by piezo-electric plates.

SUMMARY OF THE INVENTION 
The invention relates to an optical read unit for optically reading 
radiation-reflecting information tracks disposed in an information plane 
of an information carrier, which moves relative to the read unit, with the 
aid of a radiation beam which is focussed to a read spot in a focussing 
plane, and more in particular for optically reading a rotating video disc 
with the aid of a laser beam, which unit comprises: a low-mass radiation 
source, such as a semiconductor diode laser, for producing the radiation 
beam; a lens system for focussing the radiation beam to a read spot in the 
focussing plane; optical and electronic means for receiving a radiation 
beam which has been reflected by the information tracks and modulated by 
the information in the information tracks and comprising a 
radiation-sensitive electronic detector for the conversion of the 
radiation beam modulation into an electrical modulation; a scan wobbler 
for an automatic tracking system and comprising electromechanical drive 
means for periodically moving the radiation source so as to cause the read 
spot in the focussing plane to move periodically about a global position 
with low amplitude (scan wobble) in a direction transverse to the 
information track being read and a focussing wobbler for an automatic 
focussing system and comprising electromechanical drive means for 
periodically moving the radiation source so as to cause the focussing 
plane to move periodically about a global position with low amplitude 
(focussing wobble) in a direction normal to the information plane at the 
location of the read spot; and an electrical power supply circuit for 
supplying supply voltage to the electromechanical drive means. 
The Applicants previously filed Patent Application, which has not yet been 
laid open for public inspection, namely Netherlands Patent Application 
NL-PA 76 08561 corresponding to U.S. Pat. application Ser. No. 924,858, 
filed July 17, 1978, describes an optical read unit of the type mentioned 
in the preamble. As a radiation source a semiconductor diode laser is 
used. The radiation beam which has been modulated by the information in 
the information tracks is returned to the semiconductor diode laser by the 
lens system, the diode laser itself acting as radiation-sensitive 
electronic detector for the conversion of the radiation beam modulation 
into an electrical modulation, in that certain properties of the diode 
laser vary in dependence on the radiation beam modulation. Specifically, 
the quotient of the voltage across and the current through the diode laser 
varies at a specific current, or briefly the electrical resistance of the 
diode laser varies in step with the reflected modulated beam. 
The Applicant has already proposed various methods of detecting positional 
errors of the read spot relative to an information track to be read, see 
for example the previous Netherlands Patent Application NL-PA 72 06378 
corresponding to U.S. Pat. No. 3,876,842, and NL-PA 73 05517. The 
positional errors of the read spot may be deviations from the correct 
position of the read spot which are disposed in the information plane, 
so-called tracking errors, or deviations in the position of the focussing 
plane relative to the information plane, so-called focussing errors. The 
previously mentioned Netherlands Patent Application NL-PA 76 08561 also 
contains a proposal for a method of detecting positional errors. In 
accordance with this proposal the radiation source is periodically moved 
by a scan wobbler and/or a focussing wobbler with the aid of electro 
mechanical means, so that a periodic displacement of the read spot 
relative to its global position is obtained, the displacement having an 
amplitude which is smaller than the diameter of the read spot and having a 
frequency which is substantially lower than the frequency which 
corresponds to the average spacial frequency of the details in the 
information tracks. The automatic tracking system, or as the case may be 
the automatic focussing system, comprises an electronic circuit for 
processing the signal supplied by the semiconductor diode laser, the 
circuit including a filter for extracting a low frequency signal which is 
processed into a control signal which is applied to electro-magnetic means 
for correcting the position of the read spot relative to the information 
track to be read. For example, in accordance with an other Netherlands 
Patent Application NL-PA 77 03232, corresponding to U.S. Patent 
application Ser. No. 911,212, filed June 1, 1978, previously filed by the 
Applicant and not yet laid open for public inspection, it is possible 
under the influence of the control signals to tilt the entire optical read 
unit about an axis parallel to the information plane and normal to the 
axis of rotation of a video disc for the purpose of tracking, while for 
focussing the read unit can bodily be moved to and fro in the direction of 
its optical axis. 
It is the object of the invention to provide an optical read unit of the 
type mentioned in the preamble in which the electro-mechanical drive means 
for periodically moving the radiation source has small dimensions and has 
a satisfactory efficiency and which furthermore assures a satisfactory 
removal of heat from the radiation source. In accordance with the 
invention a read unit of the type mentioned in the preamble is therefore 
characterized in that at least one of the wobblers comprises: a metal 
resilient rod-shaped radiation source carrier with an end secured to the 
radiation source in a satisfactorily heat-conducting manner; a metal heat 
sink whose heat capacity and surface area are several times greater than 
those of the radiation source and to which the radiation source carrier is 
connected at some distance from the radiation source in a satisfactorily 
heat-conducting manner, so as to carry off heat, produced at the location 
of the radiation source, with a low temperature gradient to locations 
where it can be released to the environment over a sufficiently large 
surface area and piezo-electric drive means rigidly connected to the said 
radiation-source carrier and to parts of the read unit which are 
stationary relative thereto. 
In the read unit in accordance with the invention the wobblers make use of 
mechanical resonance with the aid of the metal resilient rod-shaped 
radiation-source carrier. As a result of this a relatively low drive power 
suffices and moreover excellent heat removal is possible via the metal 
rod-shaped carrier. Suitably, an embodiment of the invention may be 
employed which is characterized in that the resilient rod-shaped 
radiation-source carrier is integral with a metal mount and consists of a 
portion of the mount which is separated from the remaining portion by 
means of at least one slot. This embodiment prevents an undesired heat 
resistance at the location where the rod-shaped carrier is secured. 
The use of an optical read unit which is provided with both a scan wobbler 
and a focussing wobbler is readilly possible, namely when a read unit in 
accordance with the invention is characterized in that the piezo-electric 
drive means comprises drive means for the scan wobble disposed in a first 
direction and drive means for the focussing wobble disposed in a second 
direction perpendicular to the first direction. In such an embodiment the 
resilient metal rod-shaped radiation source carrier performs vibrations in 
two directions which are perpendicular to each other. This is found to 
work very well in practice. 
In order to achieve optimum efficiency, in accordance with a further 
embodiment of the invention, the electrical power supply circuit for the 
piezo-electric drive means for each wobbler is preferably included in a 
self-oscillating loop which includes the piezo-electric drive means for 
periodically moving the radiation source with a frequency corresponding to 
the resonant frequency of the resilient rod-shaped radiation source 
carrier in the relevant direction of movement. It is then possible to 
cause the radiation source carrier to perform two vibrations of different 
frequency and amplitude by giving the radiation source carrier in the one 
direction of vibration a different rigidity than in the other direction of 
vibration. 
As the piezo-electric drive means themselves also have a certain rigidity, 
which is certainly not negligible, these drive means influence the 
vibration and the vibration mode of the radiation source carrier. In a 
further embodiment of the invention the influence of the piezo-electric 
drive means on the movements of the radiation source carrier is minimized. 
This embodiment is characterized in that the metal resilient rod-shaped 
radiation source carrier consists of a rod spring with a free end and, 
disposed opposite thereto, a fixed end, the radiation source being secured 
to the free end and the fixed end being connected to the heat sink, and 
that the piezo-electric drive means is connected to the rod spring at such 
a location between the free end and the fixed end, that the ratio between 
the amplitude (s) of the periodic movements of the radiation source and 
the amplitude(s) of the supply voltage(s) applied to the piezo-electric 
drive means substantially has a maximum value. 
A still further embodiment of the invention gives the advantage that a 
satisfactory shielding is possible of the semiconductor diode laser 
against electromagnetic fields which are produced by the piezo-electric 
drive means, without adversely affecting the efficiency of the drive 
means. This embodiment is characterized in that the metal resilient 
rod-shaped radiation source carrier consists of a rod spring having a 
first free end on which the radiation source is mounted and a second free 
end which is disposed opposite the first free end; that between the two 
free ends the radiation source carrier is rigidly connected to the heat 
sink at a connection point; and that the piezo-electric drive means is 
connected to the rod spring near the second free end. The connection point 
between the rod spring and the heat sink in this embodiment then functions 
as a pivot for the movement of the radiation source carrier, the movements 
at the location of the pivot having a substantially negligible amplitude, 
so that a shielding envelope for the piezo-electric drive means may 
contact the rod spring at this location without having a significant 
influence on the movements thereof. Thus, the piezo-electric means may be 
encapsulated almost completely so as to prevent the occurrence of 
electro-magnetic interference. 
An optical read unit in accordance with the last-mentioned embodiment may 
advantageously be modified so as to be characterized in that the 
piezo-electric drive means of the scan wobbler and the drive means of the 
focussing wobbler are combined to single drive means which exert driving 
forces on the radiation source carrier in a direction which is disposed 
between the directions of the radiation-source movements necessary for the 
scan wobble and the focussing wobble respectively. This embodiment enables 
two wobble modes to be produced with the aid of a single piezo-electric 
drive element, namely the scan wobble and the focussing wobble.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an optical read unit in accordance with the previously cited 
Patent Application NL-PA 77 03232 which is adapted to scan information 
tracks in an information plane 3 of a rotating video disc 4 with the aid 
of a radiation beam 1, obtained from a unit 2 in accordance with FIG. 5, 
which is represented only symbolically, and which comprises a diode laser 
and a combined scan and focussing wobbler. The structure of the video disc 
will not be discussed in further detail, for this reference is made to 
series of articles published in "Philips Technical Review", Vol. 33, 1973, 
No. 7, pages 178-193. The video disc is rotatable about an axis of 
rotation 6. This is symbolically indicated by means of a curved arrow. The 
means which serve for rotating the video disc are irrelevant for the 
invention and are not shown in the drawing. 
The optical read unit comprises a frame 7 which is made of a 
non-magnetizable material such as a suitable plastic or aluminium. A 
recess in the frame accommodates an objective 8, comprising a tube 9 and a 
lens system which is symbolically represented by two lenses 10 and 11. A 
description of a a combination of means in the tube 9, which ensures that 
the radiation beam is generated, is focussed to a read spot of the desired 
dimension on the recording surface so that the modulation of the reflected 
beam is converted into an electrical signal falls beyond the scope of the 
present description. For a description of a combination of means suitable 
for the present read unit reference is made to the said Patent Application 
Nl-PA 76 08561 (which is incorporated herein by reference). In this 
respect "objective" is to be understood to mean the lens system as well as 
the combination of parts which are rigidly connected thereto and which are 
movable relative to the chassis 7. By means of the lens system the 
radiation beam 1 can be concentrated to a scanning spot 12 in a focussing 
plane. The optical axis of the lens system is designated by the reference 
numeral 13. 
Near the bottom of the objective 8 an annular focusing coil 14, which is 
wound from electrically conducting wire, is secured to the tube 9. This 
coil belongs to electrically controllable focusing means for electrically 
performing and controlling focussing movement parallel to the optical axis 
13 of the objective. The focussing coil 14 is disposed in an air gap 
between a soft-iron plate 15 and a soft-iron core 16 belonging to a 
permanent-magnetic stator. Between the soft-iron plate 15 and a soft-iron 
flange 17 which is connected to the soft-iron core 16 there is disposed an 
axially magnetized permanent-magnetic stator magnet 18. By applying a 
control signal to the focussing coil 14 by means, not shown, which are 
irrelevant for the present invention, an electro-magnetic Lorenz force is 
exerted on the focussing coil and thus on the tube, which force is 
directed in accordance with the optical axis 13 of the objective. 
Near the top of the objective there are disposed two coils 21 and 22, wound 
from an electrically conducting material, which coils cooperate with two 
permanent magnetic stators 25 and 26. These coils and stators belong to 
the electrically controllable means for electrically performing and 
controlling tracking movements of the objective 8. These means will be 
discussed in more detail further in the description. 
For the focussing movements and for the tracking movements there is 
provided a combination of a focussing bearing and a tracking bearing, 
comprising a single resilient objective suspension 8 near the bottom and 
made of an impregnated, undulated textile fiber diaphragm 27. This 
diaphragm is rigidly connected to the tube 9 by glueing and is also 
rigidly connected to the frame 7 with the aid of two rings 28 and 29 and 
by glueing. 
With the aid of electrically controllable tracking means the objective can 
be tilted about a tilting axis 30 which is substantially normal to the 
optical axis 13. The deformability of the diaphragm 27 allows the various 
movements to be performed. 
The tracking coils 21 and 22, which are connected to the objective near the 
top, are wound from electrically conducting wire, with turns which are 
situated in planes substantially parallel to the optical axis 13 of the 
lens system and which have an oblong shape. The longitudinal direction of 
the tracking coils is parallel to the optical axis. The tracking stators 
25 and 26 cooperate with the tracking coils. The tracking coils are 
located in an air gap of the associated permanent-magnetic stator. Owing 
to the oblong shape of the coils there is sufficient room for performing 
the focussing movements. 
FIG. 4 shows a stator 25 with the cooperating coil 21. The stator 25 
comprises two permanent magnets 32 and 33. Their direction of 
magnetization is indicated by means of double arrows and the letters N and 
S, the letter N denoting the north pole and the letter S the south pole. 
Between the magnetic 32 and 33 a soft-iron core 34 is glued. Furthermore, 
on both sides of the assembly consisting of the core 34 and the permanent 
magnets 32 and 33, there are disposed two soft-iron pole shoes 35 and 36. 
Between the ends of the core 34 and the pole shoes 35 and 36 a 
substantially homogeneous magnetic field exists whose lines of force are 
parallel to the direction of magnetization of the permanent magnets 32 and 
33 and are normal to the parts of the turns of the coil 19 disposed in the 
air gap between the core 34 and the pole shoes 35 and 36. The height 
dimension of the core 34 is smaller than the corresponding length 
dimension of the coil 19. As a result of this it is possible that the coil 
21 performs movements relative to the core 34 parallel to the optical axis 
13 of the lens system, i.e. focussing movements. Furthermore, the 
dimensions of the air gaps have also been selected in such a way that 
tilting movements about the axis 30 are possible without the coil 19 
coming into contact with parts of the permanent magnetic stator 25. 
The two coils 21 and 22 are jointly mounted on a coil mount 37 which is 
manufactured from a non-magnetic material such as aluminium or plastic. 
Said mount consists of a sleeve 38 which fits around the tube 9 of the 
objective 8, which sleeve for each coil is provided with two radially 
directed coil bases 39 and 40 for the fixation of the coils. The assembly 
consisting of the coil mount 37 and the two coils 21 and 22 is combined to 
one unit with the aid of a suitable glue or impregnating lacquer, so that 
no vibrations can occur between the various components. The connection 
between this assembly and the tube 9 of the objective is also made by 
glueing, so that the tube together with the coil mount 37 and the coils 21 
and 22 constitutes one solid unit. 
FIG. 5 shows the unit 2 used in the read unit of FIG. 1 on an enlarged 
scale, said unit comprising a diode laser 41 and a combined scan and 
focussing wobbler 42. This wobbler is glued onto a disc 43 of an 
insulating material at whose edge a plurality of metal connecting pins 34 
are provided for making electrical connections with the diode laser 41, 
and with two piezo-electric drive means in the form of two piezo-plates 45 
and 46. FIG. 1 shows that the disc 43 is connected to the tube 9 of the 
objective 8 with the aid of a cylindrical sleeve 47. 
The combined scan and focussing wobbler 42 comprises a metal resilient 
rod-shaped radiation source carrier in the form of a rod 48 with 
dimensions of substantially 0.4.times.0.7.times.3 mm. The radiation 
source, in the present example an aluminum-gallium-arsenide diode laser 
41, is soldered onto the free end 49 of the rod 48. Thus, a satisfactorily 
heat-conducting fixation of the diode laser 41 onto the rod 48 is 
obtained. At some distance from the diode laser 41 the rod 48 is connected 
to a metal heat sink 50 with dimensions of substantially 
4.5.times.5.times.3.5 mm. The thermal capacity and the surface area of the 
heat sink 50 are several times greater than those of the diode laser 41, 
so that the heat produced at the location of the diode laser can be 
carried off with a low temperature gradient to locations where it can be 
removed to the ambient air within the tube 9 over a sufficiently large 
surface area. If desired, the tube may be provided with vent holes. The 
two piezo-plates 45 and 46 are also soldered to the rod 48 and to the heat 
sink 50. They are disposed at an angle of 90.degree. relative to each 
other, so that by the application of voltage to the piezo-electric plates 
the rod 48 can be deflected in a direction F of the optical axis 13 of the 
lens system and in a direction S transverse thereto, so as to obtain the 
focussing wobble and the scan wobble respectively. 
The rod 48 is integral with the heat sink 50 manufactured from a solid 
block of brass by means of a spark-erosion operation. By the spark erosion 
operation two slots 51 and 52 are formed which separate the rod 48 from 
the remaining part of the component. As the rod 48 is integral with the 
component 50 excellent heat transfer from the rod to the other part of the 
component is guaranteed. 
FIG. 10 shows a block diagram of an electrical power supply circuit for the 
piezo-plate 45. The plate together with the capacitor C and the two 
resistors R1 and R2 is included in a bridge. The difference voltage across 
the bridge is amplified by a differential amplifier 53. The amplified 
signal is applied to an amplifier 55 via a bandpass filter 54, across 
which amplifier two diodes D1 and D2 are connected in opposite sense, so 
that the assembly functions as a limiter. The output voltage is applied to 
the amplifier 56 whose output voltage in its turn is applied to the bridge 
and thus to the piezo-plate 45. The band-pass filter 54 is turned to the 
resonant frequency of the rod 48. The circuit serves to apply the 
difference voltage across the bridge to the piezo-plate in the correct 
phase and after limitation, so that the rod 48 is excited at its natural 
frequency. As the circuit is fully conventional it will not be described 
in any more detail. In this respect it may be mentioned that the active 
length of the piezo plates, which are made of PXE5, is 2 mm at a thickness 
of 0.25 mm and a width of 0.5 mm. The frequencies for the scan wobble and 
the focussing wobble are 25 and 50 kHz respectively. At a difference 
voltage varying between -5 and +5V, the travel of the diode laser between 
its extreme position is 0.1.mu.. 
For a combined scan wobbler and focussing wobbler in accordance with FIG. 
5, it is of importance that the piezo-plates 45 and 46 are mounted at the 
correct location. When they are mounted too closely near the fixed end of 
the rod 48, the amplitude of the diode laser 41 will become too small. 
However, if the location of mounting is selected near the free end of the 
rod 48 the amplitude also becomes smaller than necessary, owing to the 
inherent, rigidity of the piezo-plates. The piezo-plates should be 
connected to the rod 48 at such a location between the free end and the 
fixed end that the ratio between the periodic movements of the diode laser 
41 and the amplitude of the supply voltages applied to the piezo-electric 
plates is approximately a maximum, by utilizing the excursion of the rod 
48 at resonance while the vibrating rod loads the piezo-plate in such a 
way that a maximum transfer of driving power is possible. The correct 
location is to be determined by experiment. It has been found that in an 
embodiment in accordance with FIG. 5 the piezo-plates 45 and 46 should be 
disposed slightly further in the direction of the fixed end of the rod 48 
than in the direction of the free end. 
FIG. 6 (also see FIG. 8) shows a different embodiment of a combined 
focussing and tracking wobbler. In this embodiment a semiconductor diode 
laser 57 is mounted on a free end of a rod 58. This rod has a second free 
end, designated 59, which is situated opposite the diode laser 57. Between 
these two free ends the rod is rigidly connected to a heat sink 60. Two 
piezo-electric drive means 61 and 62, again so-called piezo-plates, are 
connected to the free end 59 of the rod 58 by soldering. In this 
embodiment the rod 58 and the heat sink 60 are also integrally 
manufactured from a suitable material, for example brass. By means of a 
spark erosion process a recess 64 is formed in the flange 63, which recess 
has a depth which is smaller than the thickness of the flange 63, so that 
a wall portion 65 is left, which constitutes the connection between the 
rod 58 and the heat sink 60. The two piezo-plates 61 and 62 extend over 
two slots 66 and 67 which are also formed by spark erosion. At their ends 
opposite the end 59 of the rod 58 the two piezo-plates are soldered to the 
heat sink 60. The two piezo-electric plates are perpendicular to each 
other and are energized independently of each other. The forces which are 
exerted on the free end 59 of the rod 58 by the two piezo-plates 61 and 62 
deform the wall portion 65 and the rod 58, by means of which the 
displacements of the diode laser 57 are obtained. The two piezo-plates 61 
and 62 are connected in such a frequency that a maximum amplitude is 
obtained at the location of the diode laser 57, in that the rod 58 
resonates in the relevant direction of movement at its natural frequency. 
For electro-magnetic shielding of the piezo-plates there is provided a 
shield 68 which is made of a suitable material, such as for example 
mu-metal. 
FIG. 7 (also see FIG. 9) shows a modified combined focussing and tracking 
wobbler, employing a single piezo-plate 69 instead of two piezo-plates 61 
and 62. A diode laser 70 is mounted on a rod 71 with a free end 72, 
disposed opposite the diode laser 70, to which end the piezo-plate 69 is 
connected by soldering. The piezo-plate 69 extends over a slot 74 formed 
by spark erosion and is connected to the heat sink 73 by its end which is 
disposed opposite the end 72 of the rod 71. Owing to its oblique 
orientation the piezo-plate 69 exerts forces on the rod 71 in a direction 
between the directions S and F of the laser 70 for the scan wobble and the 
focus wobble. Thus, two vibrations can be imparted to the diode laser 70 
using a single piezo-plate only.