Apparatus and method for focus control

Control apparatus and method are disclosed for shifting focus on a multilayered optical medium using closed loop servo operation. The outputs of an astigmatic optical pickup are independently modified by a control signal generator according to a predetermined time varying pattern to produce a deceptive error signal that changes the apparent focal offset seen by a servo circuit. While in closed loop mode, the servo tracks the shifting apparent focal plane from a first information layer of the medium toward a second information layer. Thereafter the control signals are removed, and the servo locks focus on the second information region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the control of a beam of energy. More 
particularly this invention relates to the controlled motion of a focal 
point of a beam of light to selected positions in a multilayered optical 
information medium. 
2. Description of the Related Art 
Digital optical media such as optical discs and optical tapes are now 
commonly used for mass storage of information, for example compressed MPEG 
encoded audio and video signals. The information storage capacity of the 
discs and tapes can be enhanced by layering a plurality of information 
containing layers on a substrate. To read a multilayered optical medium, a 
focal point of light is selectively positioned on the layers, and is 
shifted from layer to layer in accordance with the format in which the 
media has been written. Shifting of the focal point is generally 
accomplished by arrangements requiring mechanical movement of the optics 
relative to the medium. This requires a large separation between layers to 
assure differentiation of the individual layers by an optomechanical link 
and its associated servo circuitry. 
Focus acquisition is conventionally accomplished by various servo 
arrangements initially operating in an open loop mode. The feedback loop 
of the focus servo includes a switch that is initially open, during which 
time the focus servo is driven by an oscillating waveform, such as a 
sawtooth waveform, causing an objective lens to be displaced toward and 
away from the optical medium, and a beam of light passing through the 
objective lens goes in and out of focus on the medium. At some point, when 
the beam is near a desired focal position, the sawtooth waveform is 
removed and the switch closed, thereby closing the feedback loop. Typical 
is the disclosure of Wachi, U.S. Pat. No. 5,379,282, which proposes the 
use of detectors that detect maxima and minima of return light from the 
optical medium, and maxima and minima of a focus error signal. These 
maximum and minimum signals are processed by a servo, which drives a focus 
actuator. A focus servo operation then executes to lock in the light focus 
at a desired level. 
In Millar et al., U.S. Pat. No. 4,607,157, it is proposed to intentionally 
defocus the light spot on an optical disk after focus acquisition has been 
achieved and while the servo is operating in closed loop mode. The 
resulting variation in the read-back signal is utilized by a synchronous 
detection circuit to extract magnitude and polarity information of the 
focus offset. This is fed back to the focus servo signal to null out the 
defocussing effect, and restore focus lock. 
A conventional focus control circuit 10 is illustrated in FIG. 1, wherein a 
astigmatic optical pickup 12 comprises a matrix of four photoelectric 
transducers 12a-12d, arranged to detect a light beam that returns from an 
optical information medium through an objective lens (not shown). It will 
be understood herein that the objective lens is a component of a known 
optomechanical link 28 between the focus control circuit 10 and the 
optical pickup 12. Signals from paired, diagonally opposed transducers 
(pair 12a, 12d and pair 12b, 12c) are combined on lines 13a, 13b 
respectively, and amplified respectively by operational amplifiers 14a, 
14b. The paired signals on lines 13a, 13b independently vary as the focal 
point of the objective lens transits the information layer of an optical 
medium, and these signals are responsive to the focus offset of the 
objective lens from the information layer. The outputs of the operational 
amplifiers 14a, 14b drive a differential amplifier 16, which outputs a 
focus error signal on line 32. The focus error signal on line 32 is 
representative of the difference between the signals on lines 13a, 13b. In 
closed loop operation, the focus error signal on line 32 is coupled to 
conventional phase and gain compensating circuitry, referred to herein as 
servo circuitry 18. Servo circuitry is disclosed, for example, in 
Ceshkovsky et al., U.S. Pat. No. 4,332,022. The focus error signal on line 
32 is an input to the servo circuitry 18, and causes a modification in its 
behavior in accordance with the loop design. The output of servo circuitry 
18 is summed with the output of focus acquire control circuitry 20 in 
summing circuit 22. The output of the summing circuit 22 is amplified in a 
drive amplifier 24, and coupled to a focus actuator, represented as 
actuator coil 26. The optomechanical link 28 between the actuator coil 26 
and the optical pickup 12 is indicated by a dashed line. 
Initially switch 30 is opened by a control means (not shown), so that the 
focus error signal on line 32 is disengaged from the servo circuitry 18, 
but remains coupled to the focus acquire control circuitry 20 via line 34. 
In this circumstance, the focus actuator coil 26 is driven by an 
oscillating waveform added on summing junction 22, and the optomechanical 
link 28 moves an objective lens (not shown) generally toward and away from 
the surface of the optical medium. The output of the optical pickup 20 
varies as the focal point of the objective lens approaches an information 
layer of the optical medium. When the lens is approximately in focus on 
the information layer, the switch 30 is closed, and the servo circuitry 18 
begins closed loop operation. 
When a light beam is perfectly focused on an information layer of an 
optical medium, the light intensity on the paired photodetector elements 
12a, 12d and 12b,12c of the optical pickup 12 is equal. The signals on 
lines 13a, 13b, and the signals developed by the operational amplifiers 
14a, 14b are also equal, and the output of the differential amplifier 16 
is nominally zero. As the focal point of the objective lens drifts away 
from the information layer, the intensity of light measured by the pairs 
of photodetector elements varies, so that the signals on lines 13a and 13b 
become unequal, and the differential amplifier 16 generates a focus error 
signal on line 32 that has a voltage level either greater than zero, or 
less than zero, depending on the direction the focal point of the 
objective lens has moved from the information layer. 
Fundamental principles underlying the invention are also disclosed in 
copending application Ser. No. 08/474,424, of common assignee herewith. 
A typical waveform plot of a focus error signal according to the circuit of 
FIG. 1 is shown as waveform 50 in FIG. 3, wherein F1 and F2 indicate the 
positions of two information bearing layers on a multilayered optical 
medium. When the focal point of the objective lens is remote from the 
information layer of the optical medium, for example at the left side of 
focus error waveform 50, the focus error signal has a baseline value. As 
the focal point of the objective lens approaches the first information 
layer F1, in a direction indicated by arrow A, the differential amplifier 
16 begins to develop a positive signal, which is approximately sinusoidal, 
and which returns to the baseline value when the objective lens focal 
point actually crosses the first information layer F1 at point 52. As the 
objective lens continues traveling beyond the first layer, the 
differential amplifier 16 produces a signal which is less than the 
baseline value. When the objective lens is sufficiently remote from the 
first information layer F1, the focus error signal again returns to 
baseline. The above sequence is repeated as the focal point of the 
objective lens transits a second information layer F2, with a zero 
crossing occurring at point 54. 
With the above noted approaches it is necessary to return to an open loop 
mode of operation when it is desired to shift focus from a first 
information layer to a second information layer, and to reclose the loop 
in order to lock focus on the second layer. Otherwise the servo loop would 
initially resist movement to the second information layer, and eventually 
be overcome, after which the focus would move in an uncontrolled manner. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide an improved 
apparatus and method for focus control in a multilayered optical 
information medium that allows the control loop to remain closed while 
focus is shifted from layer to layer. 
It is another object of the invention to provide precise, reliable control 
of an energy beam in an apparatus that reads or writes a multilayered 
information medium wherein the spacing between layers is very small. 
These and other objects of the present invention are attained by an 
apparatus for controlling a focal point of a focused beam of light that is 
directed onto a multilayered optical medium. The apparatus has an optical 
pickup which has a plurality of outputs and is responsive to light 
returning from the medium. A control signal generator generates first and 
second control signals. A first multiplier multiplies a first output of 
the optical pickup and the first control signal, and a second multiplier 
multiplies a second output of the optical pickup and the second control 
signal. The multipliers provide input to a difference amplifier which 
produces a focus error signal. A servo responsive to the focus error 
signal controls an actuator driver for effecting movement of the focal 
point toward and away from the medium. 
Preferably the first and second control signals comprise smoothly 
continuous waveforms offset from one another by a predetermined phase 
angle, optimally 90 degrees. Most preferably the smoothly continuous 
waveforms are substantially sinusoidal. 
In one aspect of the invention the control signal generator, the first 
multiplier, the second multiplier, the circuit, and the focus error servo 
are realized in a digital signal processor. 
The invention provides a method of controlling a beam of radiant energy 
that is directed onto a medium having a plurality of information regions. 
The medium has a characteristic that varies according to an offset from 
the information regions. A beam of radiant energy is directed onto the 
medium and interacts with the medium. Energy resulting from the 
interaction between the medium and the beam is sensed, and the sensed 
energy is representative of the characteristic. First and second signals 
responsive to the sensed energy are generated, preferably in quadrature, 
and are modified in a predetermined manner. A servo is coupled to the 
modified first signal and to the modified second signal, wherein the servo 
operates in closed loop mode. The servo is linked to a beam adjusting 
means, with which it cooperates to vary an optimization of the beam with 
respect to a first information region of the medium toward an optimization 
thereof with respect to a second information region of the medium. 
In one aspect of the invention the first signal is generated independently 
of the second signal. 
In another aspect of the invention the first and second signals have 
smoothly continuous waveforms offset from one another by a predetermined 
phase angle, preferably about 90 degrees. The smoothly continuous 
waveforms are preferably substantially sinusoidal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to FIG. 2, there is shown a focus control circuit in accordance 
with a preferred embodiment of the invention, wherein parts identical with 
those of FIG. 1 are given like reference numerals. The arrangement of 
optical pickup 12, and operational amplifiers 14a, 14b are the same as 
previously described, except now the outputs of the operational amplifiers 
14a, 14b are each coupled to the inverting input of operational amplifiers 
40a and 40b respectively. The noninverting inputs of the operational 
amplifiers 40a and 40b are connected to a voltage V.sub.CM. V.sub.CM can 
be a fixed reference voltage or can be derived from a characteristic of 
the optical medium. The output signals on lines 47a, 47b of the 
operational amplifiers 40a, 40b (representing focus error components of 
the output of the optical pickup 12) are respectively connected to 
multiplier circuits 44a, 44b, which are preferably analog multipliers. The 
multiplier circuits 44a, 44b are also respectively coupled to the control 
signals on lines 49a, 49b of a control signal generator 42. The multiplier 
circuits 44a, 44b drive another differential amplifier 46, which outputs a 
focus error signal on line 48. The focus error signal on line 48 
represents the difference between the outputs on lines 43a, 43b of the 
multiplier circuits 44a, 44b, and propagates through switch 30 to servo 
circuitry 18. The switch 30, servo circuitry 18, focus acquire control 
circuitry 20, summing circuit 22, drive amplifier 24, actuator coil 26, 
and the optomechanical link between the operation of the focus actuator 
and the operation of the optical pickup 12 indicated by the dashed line 28 
are identical with the arrangement described with reference to FIG. 1. 
In operation, when the focus error signal on line 48 is not at baseline, a 
focus offset is interpreted as being present. The servo circuitry 18 
responds to this circumstance by varying the current flowing through the 
actuator coil 26 in a manner that effects a motion of the objective lens 
so as to null out the focus offset. The response of the optical pickup 12 
changes accordingly, and the focus error signal is restored to its 
baseline level. This, as is well known to those skilled in the art, is a 
fundamental aspect of the operation of a servo control loop. 
The inventor has discovered that the apparent position of the focal plane 
of the first information layer, as evaluated by the servo circuitry 18, 
can be shifted continuously toward the second information layer, and that 
the servo circuitry 18 will accurately track the motion of a "phantom" 
focal plane from one actual information layer to another, while 
maintaining closed loop operation. 
A focus shifting operation will now be described with reference to FIGS. 2, 
3, and 4. According to the invention, when it is desired to shift focus of 
the objective lens from a first information layer F1 of the optical medium 
to a second information layer F2 in a closed loop mode of servo operation, 
a phantom focal plane is created by multiplying the two activated control 
signals on lines 49a, 49b with the output signals on lines 47a, 47b to 
produce two modified focus error components on lines 43a, 43b. The phantom 
focal plane is shifted toward the second information layer F2 by 
independently varying the voltages of the two control signals on lines 
49a, 49b in a predefined manner. The two modified focus error components 
on lines 43a, 43b are compared to each other by a differential amplifier 
46 to generate a focus error signal on line 48 that is supplied to the 
servo circuitry 18. The error signal on line 48 has the effect of 
attempting to cause the servo circuitry 18 to displace the objective lens 
to follow the continually shifting phantom focal plane. This interaction 
thus causes the focal point of the objective lens to shift from the first 
information layer F1 to the second information layer F2. When the focal 
point of the objective lens is effectively spaced apart from the first 
information layer F1, the influence on the focus error signal on line 48 
is removed by returning the control signals on lines 49a, 49b to a nominal 
value of +1. The servo circuitry 18 then continues to maintain focus on 
the second information layer F2. 
During a focus shifting operation the control signal generator 42 develops 
time varying control signals on lines 49a, 49b that range in voltage 
levels from -1 to +1, which are explained with reference to FIG. 4. The 
range of -1 to +1 volts is used for descriptive purposes only, and many 
actual voltage ranges can be used. 
For purpose of this explanation, it is assumed that focus has been acquired 
on a first information layer, and the magnitudes of the output signals on 
lines 47a and 47b are equal. The control signals on lines 49a and 49b are 
represented by waveforms 60a, 60b respectively, and have initial values of 
+1. Consequently the multiplier outputs on lines 43a, 43b are initially 
equal, and are also equal to the output signals on lines 47a and 47b 
respectively. The focus error signal on line 48 is initially at a baseline 
value of zero. An idealized time plot of the focus error signal on line 48 
is shown by waveform 62. The signals on lines 47a and 47b are represented 
by idealized waveforms 68a and 68b respectively. The signals on lines 43a, 
43b are represented by idealized waveforms 69a and 69b respectively. 
The focus shifting operation is begun at a time indicated by reference 
numeral 64. When the control signal generator 42 receives a motion control 
signal on line 29, and a direction control signal on line 31, indicating 
that focus is to be shifted to the second information layer F2, the 
control signal on line 49a begins a sinusoidal transition from a value of 
+1 to a value of -1 at time 64. When the control signal on line 49a has 
reached a potential of zero volts at a subsequent time 66, the control 
signal on line 49b begins a sinusoidal transition in the same manner, so 
that waveform 60b lags waveform 60a. It will be evident from inspection of 
the waveforms 60a, 60b that in the interval between times 64 and 74, the 
control signals on lines 49a and 49b never are simultaneously at zero 
volts. The outputs of the multiplier circuits 44a, 44b differ during this 
interval, and the focus error signal on line 48 (waveform 62) begins to 
rise as a result of differential amplification by the amplifier 46. In 
practice the focus error signal on line 48 has high frequency components, 
which are not shown in waveform 62, and is also affected by the outputs on 
lines 13a, 13b of the optical pickup link 12. Between times 64 and 74 
there is a net rise in the waveform 62 from its baseline value. In 
practice the outputs on lines 13a, 13b depend on the characteristics of 
the particular optical medium being read. 
The objective lens attempts to track the apparent focal plane in response 
to the focus error signal on line 48 (waveform 62), and becomes defocused 
with respect to the information layer F1. As the reflectance of an optical 
medium differs between the information layer F2 and the non-information 
bearing regions of the medium, the amplified astigmatic outputs on lines 
47a, 47b (FIG. 2) of the optical pickup link 12 diverge (waveforms 68a, 
68b). The waveforms 69a, 69b, which respectively represent the products of 
the signals on line 47a, 47b, and the control signals on lines 49a, 49b 
also diverge. 
Treating the interval defined by times 64, 74 as a 360 degree cycle, the 
phase angular offset between the waveforms 60a, 60b is optimally 90 
degrees, which matches the phase offset of the signals on lines 13a, 13b. 
While the invention can be practiced with a relatively wide range of phase 
offsets, if the phase angular offset between the waveforms 60a, 60b is 
reduced too much, the differential amplifier 46 will sense input values 
that are close to zero, and the system will become unstable. On the other 
hand, if the phase angular offset is increased too far above the preferred 
value, the wave form 62 will take on increasingly sinusoidal 
characteristics, sometimes with zero crossings, and the system will again 
not operate effectively. 
The phase offsets of the signals on lines 49a, 49b (waveforms 60a, 60b) 
should be adjusted to match the phase offset of the outputs of the optical 
pickup 12 for optimum performance. 
As explained above, during the focus shifting operation the servo circuitry 
18 receives an intentionally incorrect representation of the location of 
the information layer F1, and attempts to compensate by driving the 
actuator coil 26 such that the objective lens moves in a direction of the 
second information layer F2. After the control signals on lines 49a, 49b 
have both returned to values of +1, the servo circuitry 18 continues in 
closed loop operation to maintain a focus lock on the second information 
layer F2. The focus error signal on line 48 has returned to its baseline 
value. 
The interval defined by times 64, 74 is selected according to the response 
of the servo system, and the characteristics of the optomechanical link 
between the servo circuitry 18 and the optical pickup 12. 
Sinusoidal control waveforms are preferred because they are symmetric, 
smoothly continuous, and therefore avoid abrupt motions of the actuator. 
However other waveforms that produce a transition from a level of +1 to -1 
could be used, for example triangular waves. It is important that during 
the focus shifting operation the control signals on lines 49a, 49b never 
are simultaneously zero, in order to avoid a condition wherein the servo 
circuitry 18 would sense a value of zero, and become unstable. 
It is unnecessary to adjust the duration of the sinusoidal control signals 
on lines 49a, 49b to correspond with the travel time of the optomechanical 
link 28. In the event the focal point lies between information layers, for 
example in region 53 FIG. 3 when both control signals on lines 49a, 49b 
have returned to baseline, the servo circuitry 18 continues moving the 
objective lens until a higher voltage level of the focus error signal on 
line 48 is sensed in region 55. The servo circuitry 18 will then continue 
to focus the focal point of the objective lens on the information layer 
F2. 
To shift focus from the first information layer F1 to a third information 
layer (not shown) which is disposed in the opposite direction from the 
second information layer F2, it is only necessary that the waveform 
described by the control signal 49b lead rather than lag the waveform 
described by the control signal 49a. This is illustrated in FIG. 5, 
wherein the control signals on lines 49a and 49b are represented by 
waveforms 70a, 70b respectively, and have initial values of +1. When the 
focus shifting operation is initiated at time 64, the control signal 49b 
begins a sinusoidal transition from a value of +1 to a value of -1. The 
control signal 49a likewise begins a sinusoidal transition at subsequent 
time 66, so that waveform 70a lags waveform 70b. The focus error signal on 
line 48 now describes the waveform 72, and is depressed below its baseline 
value during the interval defined by times 64, 74. The response of the 
servo circuitry 18 is to drive the actuator coil 26 and thereby move the 
objective lens in a direction of the third information layer. 
It should be noted that throughout the focus shifting operation the 
position of the phantom focal plane corresponds to a focus error signal 
that is within the closed loop operating range of the servo circuitry 18. 
The above explanation is given with reference to an astigmatic optical 
pickup 12 as a non-limiting example. The invention can be practiced with 
other types of optical pickups, so long as the output can be resolved into 
at least two components, for example a well known knife edge detector, or 
the concentric ring detector disclosed in Elliott, U.S. Pat. No. 
4,152,586. 
The control signal generator 42 is any conventional signal generator 
capable of producing two phased sinusoidal signals or other time-varying 
signal outputs in a defined range of amplitudes, preferably -1 to +1 
volts. Control signal generator 42 could be a digital signal processor, or 
even an analog device. It is responsive to a direction control signal on 
line 31 and a motion control signal on line 29 from another control means, 
for example a microprocessor (not shown), which generates seek commands in 
accordance with the requirements of an information reading application and 
the format of a particular optical medium. A seek command is initiated by 
issuing the motion control signal over line 29. The optical pickup link 28 
then operates in a direction specified by the direction control signal on 
line 31. 
In another mode of operation, when it is not desired to shift focus from 
one information layer to another, the control signals on lines 49a, 49b 
are held at +1 volts. The outputs of the multiplier circuits 44a, 44b on 
lines 43a, 43b are the same as the signals on lines 47a, 47b, and the 
circuit functions in the same manner as the circuit illustrated in FIG. 1. 
As can be appreciated from the foregoing discussion, the invention provides 
an apparatus for controlling the focal point of a focused beam of light 
that is directed onto a multilayered optical medium. The apparatus has an 
optical pickup 12 responsive to light returning from the medium and has 
first and second outputs 13a, 13b, and a control signal generator 42 for 
producing first and second control signals 49a, 49b. A first multiplier 
44a is coupled to the first output 13a of the optical pickup 12 and the 
first control signal on line 49a. A second multiplier 44b is coupled to 
the second output 13b of the optical pickup 12 and the second control 
signal on line 49b. A circuit is coupled to the output of the first 
multiplier 44a and the output of the second multiplier 44b and generates a 
focus error signal on line 48. A focus error servo 18 is responsive to the 
focus error signal, and an actuator driver, represented as actuator coil 
26, is responsive to the servo circuitry 18 for effecting movement of the 
focal point toward and away from the medium. Preferably the optical pickup 
12 is an astigmatic optical pickup, but it can be a knife edge detector, 
or a concentric ring detector. The first output 13a is optimally in a 
quadrature relationship with the second output 13b. Preferably the first 
and second control signals on lines 49a, 49b comprise smoothly continuous 
waveforms offset from one another by a predetermined phase angle, and 
optimally are substantially sinusoidal waveforms separated by a phase 
angle of about 90 degrees. 
A first alternate embodiment of the invention will now be disclosed with 
reference to FIG. 6, wherein parts identical with those of the previous 
embodiment are indicated by the same reference numerals. As explained 
above, it is a feature of the invention that the output of the optical 
pickup link 12 includes a plurality of components, and that these 
components are modified in a predetermined manner. In FIG. 6 the 
multipliers that multiply the output signals on lines 47a, 47b of the 
operational amplifiers 40a, 40b and the control signals on lines 49a, 49b 
have been replaced by variable resistance elements 77a, 77b. The control 
signals on lines 49a, 49b now control the variable resistance elements 
77a, 77b to independently modify the output signals on lines 47a, 47b in 
accordance with a predetermined pattern, preferably the waveforms 60a, 60b 
(FIG. 4) and 70a, 70b (FIG. 5). A resistor 78 has been placed across the 
amplifier 46, such that the signal on line 43a connected to one input of 
the amplifier 46 represents the output of a voltage divider comprising 
variable resistor 77a and resistor 78. Similarly a resistor 79 connects 
the other input of the amplifier 46 and ground, so that the signal on line 
43b represents the output of a voltage divider comprising variable 
resistor 77b and resistor 79. In other respects this embodiment is 
identical to the previous embodiment. 
The invention provides a method of manufacturing an apparatus for 
controlling a beam of radiant energy that is directed onto a optical 
medium which has a plurality of focal planes and an optical characteristic 
that varies according to an offset from the focal planes. A source is 
provided for emitting a focused beam of radiant energy. The beam has a 
focal point on the medium and returns therefrom. A sensor responsive to 
the returning beam is provided, wherein the sensor has an output 
responsive to a focus offset from a first focal plane of the medium. The 
output of the sensor is resolved into a plurality of components, 
preferably in quadrature. A first control means is provided for modifying 
a first component of the output of the sensor, and a second control means 
for modifying a second component of the output of the sensor is provided. 
A servo is coupled to the first control means and the second control 
means, wherein the servo operates in a closed loop mode. An optomechanical 
link is coupled to the servo for varying the focal point. In operation the 
first control means and second control means are operative, and the focal 
point moves from a first focal plane of the medium toward a second focal 
plane of the medium and the servo locks focus on the second focal plane. 
A second alternate embodiment of the invention is shown in FIG. 7., wherein 
parts identical with those of the previous embodiment are indicated by the 
same reference numerals. In this embodiment, the control signal generator, 
multipliers, electronics for producing a focus error signal, focus 
acquisition circuitry, and the servo circuitry are all realized as a 
digital signal processor 200. The digital signal processor 200 includes a 
control signal section 242, a focus acquire section 220, and a servo 
section 218. Operational amplifiers 14a, 14b are coupled to 
analog-to-digital converters 80a, 80b respectively, and the signals output 
by the analog-to-digital converters are independently modified in the 
control signal section 242, using a predetermined pattern as in the 
previous embodiment. An error signal is delivered by the control signal 
section 242 to the servo section 218. The output of the servo section 218 
is reconverted to an analog signal and suitably conditioned in a 
digital-to-analog conversion section 222. The output of the digital signal 
processor 200 is amplified in drive amplifier 24 and then handled as 
described in the first embodiment. The focus acquire section cooperates 
with the servo section 218 in initially acquiring focus on an information 
layer of the optical medium being read. 
The invention can also be practiced in applications where the sensor 
measures optical characteristics other than beam intensity, so long as the 
sensor produces an output having two components, preferably in quadrature 
relationship. For example it can be utilized in arrangements which the 
sensor detects diffraction patterns produced by an information layer of a 
multilayered medium, or which use interferometry to direct an 
interrogating beam. In such arrangements more than one beam may be 
directed onto the medium. In general the invention provides a method of 
controlling a beam of radiant energy that is directed onto a medium having 
a plurality of information regions, and the medium has a characteristic 
that varies according to an offset from the information regions. A beam of 
radiant energy is directed onto the medium and interacts with the medium. 
Energy resulting from the interaction between the medium and the beam is 
sensed, and the sensed energy is representative of the characteristic. 
First and second signals responsive to the sensed energy are generated, 
preferably in quadrature, and are modified in a predetermined manner. A 
servo is coupled to the modified first signal and to the modified second 
signal, wherein the servo operates in closed loop mode. The servo is 
linked to a beam adjusting means, with which it cooperates to vary an 
optimization of the beam with respect to a first information region of the 
medium toward an optimization thereof with respect to a second information 
region of the medium. The first signal may be generated independently of 
the second signal, and preferably have smoothly continuous waveforms 
offset from one another by a predetermined phase angle, optimally about 90 
degrees. The smoothly continuous waveforms are preferably substantially 
sinusoidal. 
The embodiments disclosed herein utilize light beams by way of example; 
however the application of the invention is not limited to light. The 
invention can be practiced in applications employing other radiant 
energies that interact with a multi-segmented medium, in which the radiant 
energies are required to be adjusted to optimally interact with different 
regions of the medium using a servo feedback loop. Such energies include, 
but are not limited to radiation in the infrared, ultraviolet, or 
microwave spectra. 
For example chrominance variation can be measured in multicolored optical 
media in which color varies with an offset from the information layers. 
The invention may equally be practiced in applications wherein the light 
beam that is incident on the sensor is transmitted through the media, 
rather than back-scattered or reflected. 
The radiation reaching the sensor need not be the same radiation as was 
directed to the medium. For example in a particular application, the 
energy received by the sensor could be a secondary emission from materials 
in the optical media, or in the information layer thereof, that are 
excited by a primary beam and possess photochemical properties. It is only 
necessary that the primary beam interact with a medium to produce a signal 
that reaches the sensor, and that the sensor produce an output having more 
than one component, such as two outputs in quadrature. 
While this invention has been explained with reference to the structure 
disclosed herein, it is not confined to the details set forth and this 
application is intended to cover any modifications and changes as may come 
within the scope of the following claims: