OPTICAL INFORMATION RECORDING APPARATUS PROVIDED WITH MEANS FOR SAMPLING AND HOLDING A SERVO SIGNAL WHEN THE INTENSITY OF A LIGHT BEAM IS VARIED

Abstract: An optical information recording apparatus comprises a recording device for condensing into a spot-­ like shape a light beam intensity-modulated between at least two levels which are not zero, scanning the light beam on a recording medium and recording information thereon, a detector for detecting a signal indicative of the position error between the light beam spot and the medium in the focusing direction perpendicular to the surface of the medium or the tracking direction in a plane parallel to the surface of the medium which is perpendicular to tracks formed on the medium, a servo device for correcting the position error on the basis of the detected signal, means for varying the gain of the servo device in conformity with the intensity level of the light beam, and a device for sampling and holding the detected signal as long as at least the intensity of the light beam is halfway of the shift between different levels.

Description: BACKGROUND OF THE INVENTION This invention relates to an apparatus for recording information by scanning a recording medium by a light beam condensed into a spot-like shape, and in particular to an optical information recording apparatus provided with focusing and/or tracking servo means for the light beam. As the forms of a medium using a light to record information thereon and read out the recorded information, there are known various forms such as a disc-like form, a card-like form and a tape-like form. Among these, an optical information recording medium formed into a card-like shape (hereinafter referred to as the "optical card") has a great estimated demand as a medium of great recording capacity which is compact and light in weight and convenient to carry. Referring to Figure 1 of the accompanying drawings which is a schematic plan view of such an optical card 101, the reference numeral 102 designates an information recording area, the reference numeral 103 denotes information tracks, the reference numerals 104 and 104′ designate track selecting areas, and the reference numeral 105 denotes the home position of a light beam spot. On the optical card 101, information is optically detectably recorded as a record bit row (information tracks) by scanning the optical card by a light beam modulated in accordance with record information and stopped down into a minute spot. At that time, to record the information accurately without causing a trouble such as intersection between the information tracks, it is necessary to control the position of application of the light beam spot in the surface of the optical card in a direction perpendicular to the scanning direction (the lengthwise direction of the tracks) (auto tracking which will hereinafter be referred to as "AT"). It is also necessary to control the light beam in a direction perpendicular to the surface of the optical card (auto focusing which will hereinafter be referred to as "AF") to apply the light beam as a minute spot of stable size in spite of the bending or mechanical error of the optical card. Also, said AT and AF are necessary during reproduction. Referring now to Figure 2 of the accompanying drawings which shows the construction of an apparatus for recording and reproducing information on the optical card, the reference numeral 106 designates a motor for driving the optical card 101 in the direction of arrow parallel to the lengthwise direction of the tracks, the reference numeral 107 denotes a light source such as a semiconductor laser, the reference numeral 108 designates a collimator lens for collimating the light from the light source 107, the reference numeral 109 denotes a beam splitter, the reference numeral 110 designates an objective lens, the reference numeral 111 denotes a coil for tracking, the reference numeral 112 designates a coil for focusing, the reference numerals 113 and 114 denote condensing lenses, the reference numerals 115 and 116 designate photoelectric conversion elements, the reference numeral 117 denotes a tracking control circuit, and the reference numeral 118 designates a focusing control circuit. Electric currents are flowed to the coil 111 for tracking and the coil 112 for focusing by the commands from the control circuits 117 and 118 on the basis of a tracking signal and a focusing signal detected by the photoelectric conversion elements 115 and 116 to thereby move the objective lens 110 and accomplish AT and AF. The reference numeral 119 designates a system controller for controlling the recording-reproducing apparatus, and the reference numeral 120 denotes a group of various control signals output from the system controller. Other signals than the signals 120 are also output from the controller 119, but they are not shown. The reference numeral 122 designates an optical head, and the reference numeral 122 denotes a drive motor for moving the optical head in the direction of arrow u in Figure 1. The light from the light source 107 is collimated by the collimator lens 108, passes through the beam splitter 109 and thereafter is condensed on a recording track on the optical card 101 by the objective lens 110. The light reflected by the recording track is now transmitted through the beam splitter 109 and is divided into two light beams by the beam splitter 109, and the divided light beams are condensed on the tracking signal detecting photoelectric conversion element 115 and the focusing signal detecting photoelectric conversion element 116, respectively, by the condensing lenses 113 and 114, respectively. Signals obtained by the photoelectric conversion elements 115 and 116 are made into a tracking error signal and a focusing error signal by the tracking control circuit 117 and the focusing control circuit 118, respectively, and electric currents are flowed to the coil 111 for tracking and the coil 112 for focusing, whereby the objective lens 110 is moved to thereby accomplish AT and AF. Figure 3 of the accompanying drawings is a detailed diagram of the tracking control circuit 117. In Figure 3, the reference numeral 301 designates a reflected light resulting from the light from the light source being reflected by the medium, the reference characters 115-A and 115-B denote photoelectric conversion elements divided into two by a dividing line extending in a direction corresponding to the lengthwise direction of the tracks for detecting a tracking error, and the reference numeral 207 designates a subtraction circuit for subtracting signals D and E which are signals from the photoelectric conversion elements 115-A and 115-B adn outputting a tracking error signal. The reference numeral 206 denotes an adder circuit for adding the signals D and E together, the reference numeral 203 designates a dividing circuit for dividing the output G of the subtraction circuit 207 and the output F of the adder circuit 206, the reference numeral 202 denotes a phase compensator circuit for achieving the stabilization of AT servo, the reference numeral 204 designates a driver for flowing a drive current to the objective lens, and the reference numeral 205 denteos a tracking controller for receiving the signal 120 from the system controller 119 and controlling the entire tracking control circuit 117. Figures 4A-4D of the accompanying drawings are timing charts of the signals, at various positions of the Figure 3 circuit during recording. Figure 4A shows a record information signal, Figure 4B shows a variation in the power P of the light source modulated by said signal, Figure 4C shows a variation in the voltages of tracking signals D and E obtained by the power of the light source via the photoelectric conversion elements 115-A and 115-B, and Figure 4D shows a variation in the voltage of a signal F which is the addition signal of the tracking signals D and E. By "1" and "0" of the record information, the power P of the light source is varied into two stages P H and P L , and setting is made so that when the power is P H , a pit is formed and when the power is P L , no pit is formed. At this time, the signals D and E are also varied into two stages and therefore, if these signals D and E are only subtracted, the open loop gain of AT servo when the power of the light source is P H becomes P H /P L times as great as the open loop gain of AT servo when the power of the light source is P L . This has led to the problem that oscillation becomes liable to occur and the servo becomes unstable. Particularly in the case of an optical card in which the line speed is low and the zone of record information is near the zone of AT servo, the component of the record information mixes with the AT servo with a result that the objective lens is unnecessarily moved and thus, stable AT servo becomes impossible. Consequently, as an example for making the open loop gain of AT servo constant both when the power of the light source is P L and when the power of the light source is P H , there is a method using the dividing circuit 203 as shown in Figure 3. That is, the output G of the subtraction circuit 207 is input to the numerator side input terminal Y of the dividing circuit 203 and the output signal F of the adder circuit 206 is input to the denominator side input terminal X of the dividing circuit 203. Thus, the variation by record modulation is offset because even if the signals D and E are varied into V L and V H as shown in Figure 4C, the signal F is likewise varied into V DL and V DH as shown in Figure 4D, and the open loop gain of AT servo can always be made constant and it becomes possible to accomplish a stable AT servo operation. This method is also applicable to AF servo. For example, a focusing signal is detected by using an anamorphic optical system like a toric lens as the lens 114 of Figure 2 and using as the photoelectric conversion element 116 an element whose light-receiving surface is divided into four portions 116-A to 116-D as shown in Figure 5. A spot condensed on this light-receiving surface by the lens 114 is circular when the light beam is focused on the medium, but when out-of-focus occurs, the spot is deformed in conformity with the direction of the out-of-focus as indicated by broken line and dot-and-dash line in Figure 5 of the accompanying drawings. Accordingly, the sum signal O of the outputs of portions 116-A and 116-C which are at diagonal positions and the sum signal P of the outputs of portions 116-B and 116-D are calculated by adders 130 and 131, respectively, and these are differentiated by a differential amplifier 132 to thereby obtain a focusing signal. The basic construction of the optical card recording-reproducing apparatus as described above is disclosed, for example, in GB-A-2 172 138. Also, the dividing circuit of focusing servo is described in Japanese Laid-Open Patent Application No. 52-134704. However, the dividing circuit 203 in the above-mentioned example of the prior art effects complicated analog processing therein and therefore is expensive and lacks in precision. This has led to the expensiveness of the servo circuit as a whole and to the disadvantage that the accuracy of the information recording as during recording is reduced. On the other hand, an optical information recording-reproducing apparatus in which the gain of focusing servo is changed over between the time of recording and the time of reproduction during which the intensity of the light beam applied to the medium differs is disclosed in Japanese Laid-Open Patent Application No. 52-80802. However, again in this apparatus, there has been the problem that during recording, the gain is always constant and particularly at a point of time whereat the intensity of the light beam is varied, servo becomes unstable. SUMMARY OF THE INVENTION It is the object of the present invention to solve the above-noted problems peculiar to the prior art and to provide an optical information recording apparatus which is simple in construction and which accomplishes stable servo operation even at a point of time whereat the intensity of a light beam is varied. According to a first aspect of the present invention, an optical information processing apparatus comprises: means for condensing into a spot-like shape, a light beam intensity modulated between at least two discrete non-zero levels, scanning the light beam on a recording medium; means for detecting a signal indicative of a position error between said light beam spot and said medium in at least one of a focusing direction perpendicular to the surface of said medium and a tracking direction in a plane parallel to the surface of said medium and which direction is perpendicular to tracks formed on said medium; servo means for correcting said position error on the basis of said signal detected by said detecting means; means for varying gain of said servo means in accordance with the intensity level of said light beam; is characterised in that there are means for sampling and holding said detected signal while the intensity of said light beam is adjusted from one level to another. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic plan view showing an example of an optical card. Figure 2 is a schematic diagram showing an example of the construction of an apparatus for recording information on the optical card shown in Figure 1. Figure 3 is a block diagram showing an example of the prior-art tracking control circuit used in the apparatus shown in Figure 2. Figures 4A-4D are timing charts of signals in various portions of the circuit shown in Figure 3. Figure 5 is a block diagram showing an example of a circuit for obtaining a focusing signal in the apparatus shown in Figure 2. Figuer 6 is a block diagram showing an embodiment of a tracking control circuit used in an optical information recording apparatus according to the present invention. Figures 7A-7G are timing charts of signals in various portions of the circuit shown in Figure 6. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will hereinafter be described in detail with respect to an optical card recording apparatus. Figure 6 is a block diagram showing an embodiment of the tracking control circuit of an optical information recording apparatus according to the present invention. This circuit is intactly incorporated in the construction shown in Figure 2. In Figure 6, members identical to those in Figure 3 are given identical reference numerals and need not be described in detail. In Figure 6, the reference characters 302-A and 302-B designate amplifiers which can change over the amplification degree into at least two stages by a signal M from a tracking controller 305. The reference characters 303-A and 303-B denote circuits for sampling and holding output signals A and B from the amplifiers 302-A and 302-B, respectively, by a signal N from the tracking controller 305. The reference numeral 319 designates a system controller for controlling the entire system, and the reference numeral 320 denotes a signal output from the system controller 319 to the tracking controller 305. The tracking controller 305 according to the present invention controls the amplification degrees of the amplifiers 302-A and 302-B and also controls sampling and holding circuits 303-A and 303-B on the basis of a predetermined signal. The reference numeral 201 designates a subtraction circuit for subtracting signals A′ and B′ which are the output signals of the sampling and holding circuits 303-A and 303-B and outputting a tracking error. Figures 7A-7G are timing charts of respective signals at the respective positions of the circuit of Figuer 6. More specifically, Figure 7A is a chart of a record information signal, Figure 7B is a chart showing a variation in the power P of a light source modulated by said signal, Figure 7C is a chart showing a variation in the voltages of tracking signals D and E obtained by the power P of the light source via photoelectric conversion elements 115-A and 115-B, Figure 7D is a chart showing the manner in which the amplification degrees are changed over to G H and G L correspondingly to the record information signal shown in Figure 7A, Figure 7E is a chart showing the voltages at points A and B in Figure 6, Figure 7F shows a sampling and holding drive signal in the signal D, and Figure 7G is a chart showing the voltages at points A′ and B′. The operation of the tracking control circuit according to the present invention will now be described with reference to Figures 6 and 7A-7G. When the record signal varies in accordance with binary information indicated by "1" and "0" as shown in Figure 7A, the power of the light source also varies as indicated by "P H " and P L " in Figure 7B. The apparatus conditions are set so that when the power is "P L ", no pit is formed and when the power is "P H ", a pit is formed. Thus, in Figure 6, the signals D and E from the photoelectric conversion elements 115-A and 115-B, respectively, change to "V L′ " and "V H′ " in proportion to the power P of the light source (see Figure 7C). At this time, the gain G of the variable gain amplifiers 302-A and 302-B is changed over to two stages "G L " and "G H " by the signal M to keep the open loop gain of AT servo constant. The signal M corresponds to said record information "1" and "0". At this time, the gains G L and G H are preset so as to satisfy the following condition: P H ·G L = P L ·G H = k (constant) Thus, the output signals A and B from the amplifiers 302-A and 302-B become V AB which is constant both during the recording of the pit and during the non-recording of the pit. That is, V AB is V AB = V H′ ·G L = V L′ ·G H and the open loop gain of AT servo can always be kept constant, and accurate recording becomes possible. However, when as shown in Figure 7B, the state of variation in the power P of the light source (for example, the state of rising and falling) is not identical to the state of variation in the amplification degree G of the amplifiers, P·G ≠ k is brought about at t₁, t₂, t₃, t₄, ... which are the points of variation in the power of the light source. For example, at t₁ and t₃, the open loop gain of tracking servo becomes lower than the usual value k and becomes weak to disturbance, and conversely, at t₂ and t₄, the open loop gain becomes greater than the usual value k and oscillation becomes ready to occur. Thus, tracking servo becomes unstable (see Figure 7E). So, in the present invention, prior to the point of time t₁ at which the record information varies, the signal A and the signal B are sampled by the signal N (see Figure 7F). Thereafter, they are held for a time T H and are output as a signal A′ and a signal B′ (see Figure 7F). By repeating this, the variation in the open loop gain can be eliminated and the tracking servo can be stabilized, and accurate recording becomes possible (see Figure 7G). Also, as compared with the conventional sample-hold control, in the present embodiment, the hold time can be made short and the through time can be made long and therefore, the phase delay of the tracking servo by the sample-hold becomes small, and this is effective. While AT servo has been described, the present invention is equally applicable to AF servo also. For Example, if in the circuit shown in Figure 6, signals O and P, instead of signals D and E, are input and the output signal of a driver 204 is input to a coil 112 for focusing shown in Figure 2, stable AF servo can be accomplished. The present invention is not restricted to the above-described embodiment, but permits various modifications and applications. For example, in the above-described embodiment, the signal A and the signal B are sampled and held, whereas the present invention is not restricted thereto. The signal of the other portion in the servo loop may be sampled and held to achieve a similar effect. Also, the above-described embodiment has been shown with respect to an example in which sample-hold is effected both when the intensity of the light beam emitted from the light source increases and when said intensity decreases, but effecting sample-hold either when said intensity increases or when said intensity decreases is also effective. Further, the above embodiment has been described with respect to an apparatus in which the amplification degree of the amplifiers is changed over to a predetermined value, whereas the present invention can also be applied to an apparatus in which the gain varies in an analog fashion in conformity with the intensity of the light beam. Also, in the above-described embodiment, sample-hold has been effected when the intensity of the light beam conforming to the record signal varies, but where recording and reproduction are effected by the same apparatus, sample-hold may be effected when the intensity of the light beam is changed over between the recording mode and the reproducing mode. Also, in other case where the intensity of the light beam is varied, for example, in a case where the intensity is further reduced below the level during reproduction in order to prevent the destruction of the medium by the long-time standstill of the light spot during the standby for the recording or reproducing operation, sample-hold may be executed. Further, the present invention is applicable not only to an apparatus using a postscript type recording medium, but also to an apparatus using a rewritable type recording medium such as a magneto-optical recording medium. Further, the shape of the medium may be any shape such as a card-like shape, a disc-like shape or a tape-like shape. The present invention covers all such applications without departing from the scope thereof as defined in the appended claims.

Claims: An optical information processing apparatus of the kind comprising: means (112) for condensing into a spot-like shape, a light beam intensity modulated between at least two discrete non-zero levels, scanning the light beam on a recording medium; means (117, 118) for detecting a signal indicative of a position error between said light beam spot and said medium in at least one of a focusing direction perpendicular to the surface of said medium and a tracking direction in a plane parallel to the surface of said medium and which direction is perpendicular to tracks formed on said medium; servo means (106, 122) for correcting said position error on the basis of said signal detected by said detecting means; means (203,206,207) for varying gain of said servo means in accordance with the intensity level of said light beam; characterised in that there are means (303,305) for sampling and holding said detected signal while the intensity of said light beam is adjusted from one level to another. An optical information recording apparatus according to Claim 1, wherein said sampling and holding means (303, 305) samples and holds said detected signal before the intensity of the light beam starts the shift to another level, and releases the hold after said shift is terminated and before the next shift is started. An optical information recording apparatus according to Claim 1, wherein said signal detecting means (117, 118) comprises a photoelectric conversion element which receives the reflected light of the light beam by said recording medium (101). An optical information recording apparatus according to Claim 1, wherein said means for varying the gain varies the gain so that the product of the power of said light beam and the gain becomes constant. An optical information recording apparatus according to Claim 1, wherein said apparatus comprises an optical head including a light source (107) and an objective lens (110) for condensing a light beam emitted from said light source, and a motor (106) for moving said recording medium (101) relative to said optical head in the lengthwise direction of the tracks. An optical information recording apparatus according to Claim 5, wherein said servo means comprises means (122) for driving said objective lens in at least one of the focusing and tracking direction, and an electric circuit for feeding back said detected signal to said driving means.

Inventors: KANDA, SHIGETO, SHIKICHI, SATOSHI
Applicants: CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B7
IPC codes: G02B7, G11B7