BEAM CONTROLLER FOR MAGNETO-OPTICAL DISC MEMORY SYSTEM

Abstract: The beam controller for controlling a semiconductor laser provided in an optical recording-play-back apparatus com­ prises first current source for driving said semiconductor laser at a low level necessary for play-backing data memorized in an optical memory means, second current source for driving said semiconductor laser at a high level necessary for recording and/or erasing data into the optical memory means, first sample-hold circuit for holding data corresponding to a beam intensity from said semiconductor laser being det­ ected upon play-backing memorized data, first control means for controlling the output of the first current source according to the output from the first sample-­ hold circuit upon recording and/or erasing data in the optical memory means, second sample-hold circuit for holding data correspond­ ing to a beam intensity from said semiconductor laser being detected upon erasing memorized data, and second control means for controlling the output current of second current source according to the output from the second sample-hold circuit upon recording data in the optical memory means.

Description: The present invention relates to a beam control device for controlling laser beam of a semiconductor laser being provided in an optical memory system in which data can be recorded, playbacked and/or erased by applying laser beam onto a magneto-optical disc. As is well known to those skilled in the art, the magneto-optical disc has such a structure that an amorphous film of rare metal-ferroalloy is deposited on a substrate such as a glass substrate by spattering and is covered with a magnetic film having an axis of easy magnetization perpendicular to the surface of the magnetic film. The optical memory system has a recording head for recording data onto the disc, playbacking or erasing recorded data in which a semiconductor laser is provided for applying laser beam onto the disc. In this optical memory system, the recording of data is made according to a method as follows; laser beam focused to a spot of a diameter of about 1 µm is applied onto the magnetic film of the disc to raise the temperature of the portion to which the laser beam is applied, in order to reduce a coercive force thereat and, at the same time, the direction of magnetization is inverted by applying an auxilliary magnetic field externally to the portion having been raised the temperature thereof. The erasing method is essentially same to the recording method just mentioned. The play-back method for data recorded is as follows; linearly polarized light of the laser beam having been controlled to have an intensity of light weaker than that of the recording is applied to the magnetic film of the disc on which data has been recorded, the light having been reflected therefrom has a proper inclination of polarization due to the magneto-optical effect of the magnetic film (Kerr effect), the inclination of polarization is converted to the intensity of light by an analyzer and, then, play-back signals can be obtained as outputs from a photo-detector which detects said intensity of light. As is apparent from the mentioned above, the semiconductor laser should be driven at both of a high and low levels for recording and playbacking, respectively. Meanwhile, the semiconductor laser has such a temperature dependence that the intensity of laser beam is varied according to the ambient temperature due to which the thresh-hold current thereof is varied. However, if the intensity of laser beam is varied during the recording at a high level, informations are written into the disc wrongly. This lowers the credibility of the optical memory system. This is also similar to the case of play-back of recorded data. Namely, when the intensity of laser beam is varied during the play-back, S/N ratio of play-back signal to noises is lowered to give wrong informations. In order to solve these problems mentioned above, there have been proposed different laser driving systems. Patent abstract of Japan vol. 9, no. 157 (P-369) (1880) of 2nd July 1985 discloses a driving circuit of a semiconductor laser using a laser current control means to obtain a prescribed low level and a constant current control means which obtains a prescribed high level. Both current control means are supplied with sample and hold circuits one of which provides a higher control signal for producing a high writing current and the other provides a lower control signal for producing a low reading current for the semiconductor laser. EP-A-0 082 357 concerns a laser driving system, which has a laser light power control loop which is enabled or energized only in the read-out or reproduction mode. The control loop is disabled in the write or recording mode and a predetermined value of current is superposed on the read-out current. A further known laser beam controller for the optical memory system is shown in Fig. 5. According to this laser beam controller, there are provided first and second current sources (b) and (c) for supplying two different driving currents to a semiconductor laser (a) respectively. The first current source (b) is provided for supplying a low power driving current I₁ during the play-back of recorded data. Meanwhile, during the recording of data, the second current source (c) supplies a high power driving current I₂ to the semiconductor laser together with the first current source (b) in order to obtain a laser beam of a high intensity. When the laser is driven only by the first current source (b), the intensity of laser beam emitted therefrom is detected by a photodetector (d) and output signal thereof is input, via a pre-amplifier (e), into a sample-hold circuit (f). This sample-hold circuit (f) is controlled by a sample-hold signal S₁ in such a manner that, when the sample-hold signal S₁ has a high level, data entered is held therein and is passed therethrough when it has a low level. Data signal outputted from the sample hold circuit (f) is compared with a reference voltage given by a standard voltage source (g) at a differential amplifier (h). The output of of differential amplifier (h) is input into a low-pass filter (i) and low frequency components having been passed therethrough are input into a power amplifier (j). The power amplifier (j) controls the low current I₁ of the low power current source (b). If the sample-hold signal S₁ is set at the low level, according to the power control system mentioned above, the beam intensity of the semiconductor laser (a) is kept constant irrespective to the temperature dependence thereof. This control system is referred to APC (Auto-Power Control) hereafter. During the high power driving mode (recording mode), the sample hold signal S₁ is switched to a high level and, due to this, the sample hold circuit (f) holds the data signal and, therefore, APC is frozen. Further, when the high power driving mode is chosen, a data-record signal S D of a high level is applied to an AND gate (k) together with the high level sample-hold signal S₁ which is provided for controlling a switching circuit (1). This switching circuit (1) is turned on when the output of AND gate (k) becomes a high level and, therefore, the current I₂ supplied by the high power current source (c) is added to the current I₁ in order to drive the semiconductor laser (a) at the high power level. The reason for making APC freeze is to avoid possible drop of the beam intensity during the recording and/or erasing mode. However, APC operates even in the high power driving mode apparently. This is on the premises that only the thresh-hold value is varied according to the ambient temperature, when considered the driving current to the beam intensity curvature characteristic of the semiconductor laser, and that the gradient of said curvature above the thresh-hold value is not varied according to the ambient temperature. Namely, if the low power driving current I₁ is controlled so as to have a higher value than the thresh-hold value, APC can be realized even in the high power driving mode by superposing a constant current to the low power driving current. But these premises are not correct since the gradient of the curve above the thresh-hold is varied according to the ambient temperature and use-time. The sample-hold circuit (f) employed in the conventional APC shown in Fig. 6 is comprised of a low-pass filter (m) into which the output from a differential pre-amplifier (h) is input, a memory means (n) which can memorize the output V₀ of the low-pass filter (m) and a switching means (o) for switching either the low pass filter (m) or the memory circuit (n) against a low power driving current source (b). When the play-back mode is indicated, the switching means (o) is switched so as to connect the low-pass filter (m) to the low power driving current source (b) directly. The differential amplifier (h) outputs a signal V₁ proportional to the difference between the output signal Va from the photo-detector (d) and a predetermined reference voltage Vb. Therefore, APC is obtained as mentioned above. When the recording mode (high power driving mode) is selected, the switching circuit (o) is switched by the data recording signal S D so as to connect the memory circuit (n) to the low power driving current source (b). Accordingly, the sample hold circuit (f) outputs a voltage signal V m having been memorized in the memory circuit (n) due to the sample-hold signal S₁. As is clearly understood, the sample-hold circuit (f) has a first mode in which the output V₀ is outputted via the low pass filter (m) and a second mode during which the output V m memorized in the memory circuit (n) is outputted. However, this conventional sample-hold circuit (f) has an essential disadvantage in that it is difficult to obtain a quick response upon switching from the recording mode to the play-back mode or vice versa since the low pass filter (m) has a slow transition responsibility. As shown in Fig. 7, the low pass filter (m) has a relatively slow transition responsibility. When the mode is switched from the recording mode to the play-back mode, the output voltage Vo of the low pass filter (m) is raised delayed. Due to this delay in the transition responsibility, the low power driving current I₁ by the first current source (b) is also delayed and therefore, it takes a time until the low power driving current I₁ becomes stabilized. It is an object of the present invention to provide a beam controller for an optical memory system in which APC can be made not only in the low power driving mode (play-back mode) but also in the high power driving mode (recording mode) exactly. Another object of the present invention is to provide a beam controller having a sample-hold circuit which can operate with a quick responsibility upon switching from the recording mode to the play-back mode or vice versa. These objects are solved, according to the invention, with the features of claims 1 and 4, respectively. These and other objects and features of the present invention will become more apparent from the description of the preferred embodiments accompanying the drawings attached herein; in which Fig. 1 is a block diagram showing a beam controller according to the second preferred embodiment of the present invention, Fig. 2 is a time chart of the beam controller shown in Fig. 1, Fig. 3 is a block diagram showing a beam controller according to the first preferred embodiment of the present invention, fig. 4 is a time chart of the beam controller shown in Fig. 3, Fig. 5 is a block diagram showing one conventional beam controller, Fig. 6 is a block diagram showing another conventional beam controller, and Fig. 7 is a time chart of the conventional beam controller shown in Fig. 6. Second Preferred Embodiment Fig. 1 shows a block diagram of a beam controller according to the present invention. As is well known to those skilled in the art, a semiconductor laser device 1 is provided as a light source in a recording head H for recording data into a magneto-optical disc MOD and/or erasing or playing back data memorized therein. The beam controller for controlling the intensity of laser beam emitted from the semiconductor laser device 1 is comprised of a photodetector 2 which transforms the laser beam intensity P₁ into a voltage signal Va proportional thereto; a differential amplifier 3 which compares the voltage signal Va with a predetermined reference voltage Vb and outputs a difference signal Vc proportional to a difference between two voltage Va and Vb (Vc∝Vb-Va); a sample-hold circuit 4 for generating a control signal Ve in order to control a low power driving current source 8 and an adder 10 for adding a high power driving current I₂ supplied by a high power driving current source 9 to a low level driving current I₁ supplied by the low level driving current source 8. Said sample-hold circuit 4 is comprised of a switching means 5, a low-pass filter 6 and a memory circuit 7. The switching means 5 has a first input terminal 5a connected to the output of the differential amplifier 3, a second input terminal 5b connected to the output of the memory circuit 7 and one output 5c connected to the input of the low-pass filter 6. The output of the low-pass filter 6 is connected to the first driving current source 8 and the input of the memory circuit 7 is connected to the output of the low-pass filter 6. The memory circuit 7 is operated to memorize the output signal of the low-pass filter 6 when a sample-hold signal S₁ is applied thereto by a control circuit of the optical memory system (not shown). Also, the switching means 5 is switched from the first input terminal 5a to the second input terminal 5b when a recording signal S D is applied thereto by the control circuit which is continued during the recording mode and erasing mode of the system. Accordingly, the sample-hold circuit 4 operates so as to connect the differential amplifier 3 to the low-pass filter 6 directly during the play-back mode and to connect the memory circuit 7 to the low-pass filter 6 during the recording or erasing mode in order to apply the memorized output signal of the memory circuit 7 to the first driving current source 8. The second driving current source 9 is turned on by the recording signal S D during the recording mode and erasing mode and the adder 10 supplies the high level driving current (I₁ + I₂) to the semiconductor laser 1 during the recording mode or the erasing mode. Fig. 2 shows a timing chart of the beam controller. As shown in Fig. 2, the beam intensity P₁ is stablized at a predetermined low level during the play back mode by APC constituted from the photo detector 2, the differential amplifier 3 and the sample-hold circuit 4 as mentioned above. The beam intensity P₁ is transformed into voltage signal Va and the differential amplifier 3 outputs the voltage signal Vc proportional to the difference between the signal Va and the reference signal Vb. The sample-hold circuit 4 according to the present invention does not show any transition responsibility as explained herebelow. At first, during the play back mode, APC is done by the sample-hold circuit 4 since the differential amplifier 3 is directly connected to the low-pass filter 6 during this mode. Next, just before the recording (erasing) mode is started, the memory circuit 7 memorizes the filtered voltage signal Ve obtained during the play-back mode at the timing that the sample-hold signal S₁ is applied to the memory circuit 7. And, the switching means 5 is switched from the first input terminal 5a to the second input terminal 5b when the recording signal S D is turned from the high level to the low level. Therefore, when the recording mode is selected, voltage signal data memorized in the memory circuit 7 is input into the low-pass filter 6 through the second input terminal 5b. Accordingly, the output signal Ve of the low-pass filter 6 is kept unchanged since the memorized signal data Ve is input thereinto. Due to this, the output signal Ve does not show any transition responsibility. As the result of this, the low power driving current I₁ of the first current source 8 is kept constant without any affection by the transition responsibility of the low pass filter 6. Accordingly, a stable high level driving current (I₁ + I₂) can be obtained as soon as the mode is switched from the play-back mode to the recording (or erasing) mode. Even if the mode is switched from the recording (or erasing) mode to the play-back mode, the output signal Ve of the low-pass filter 6 is kept unchanged since the voltage signal Vc of the differential amplifier 3 is raised to a level substantialy equal to that in the preceding play-back mode as soon as the mode is switched from the recording mode to the play-back mode. First preferred Embodiment In this embodiment, one more APC is introduced in order for the high power driving mode namely the recording or erasing mode. As shown in Fig. 3, the output signal Va of the photodetector 2 which detects the beam intensity P₁ of the semiconductor laser 1 is input via a pre-amplifier 11, into a first sample-hold circuit 4A provided for APC during the low power driving mode and into a second sample-hold circuit 4B provided for APC during the high power driving mode. Said first sample-hold circuit 4A is controlled by a first control signal S A . As shown in Fig. 4, the first control signal S A is switched from "Low" level to "High" level just before the timing when the mode is switched from the play-back mode to the erasing mode, and is kept at "High" level during the erasing and recording modes, and is switched from "High" level to "Low" level just after the timing when the mode is switched from the recording mode to the play-back mode. The memory circuit 7A memorizes the output of the low-pass filter 6A at the timing of rise of the first control signal S A and, at the same time, the switching means 5A is switched from the first input terminal 5Aa to the second input terminal 5Ab. Namely, the first sample-hold circuit 4A freezes APC for the low power driving mode. On the contrary to the above, the second sample-hold circuit 4B is controlled by a second control signal S B . As shown in Fig. 4, the second control signal S B is switched from "High" level to "Low" level just after the timing when the mode is switched from the play-back mode to the erasing mode, and is switched from "Low" level to "High" level just before the timing when the mode is switched from the erasing mode to the recording mode. The memory circuit 7B of the second sample hold circuit 4B is set to hold the memorized data during the second control signal S B is kept at "High" level. And, the switching means 5B is switched from the second input terminal 5Bb to the first input terminal 5Ba at the timing of fall of the second control signal S B . In the play-back mode, namely the low power driving mode, APC for the low power driving current source 8 is carried out as follows. The photo-detector 2 detects the intensity P₁ of the laser beam emitted from the semiconductor laser 1 and the output voltage signal Va thereof is input, via the pre-amplifier 11, into the first sample-hold circuit 4A. The first control signal S A is kept at "High" level during this mode, and therefore, the output signal Va of the photodetector 2 is directly input to the low pass filter 6A and then, the output signal Vb of the low pass filter 6A is input into the first comparator 3A. The comparator 3A compares the input signal data Vb with a low level reference voltage V BA which is set by a first standard voltage source 12A and then, outputs a voltage signal Vc proportional to the difference between Vb and V BA (VC V BA -Vb). This output signal Vc is input into a power amplifier 13 provided for controlling the low power driving current source 8. Thus, APC for play-back mode is carried out by applying a stablized low power driving current I₁ to the semiconductor laser 1. During this mode, neither an erasing signal S E nor a recording signal S R is input to an OR gate 15. Accordingly, a switch means 16 provided as an adder 10 is kept in OFF state. On the contrary to the above, APC for the high power driving mode is carried out as follows. At first, as shown in Fig. 4, the first control signal S A is dropped to "Low" level just before the start of the erasing mode to freeze APC for the play-back mode. Then, the erasing signal S E is raised to "High" level to switch on the switch 16. Therefore, the second driving current I₂ supplied by the high power driving current source 9 is superposed to the low driving current I₁. Therefore, the semiconductor laser 1 is driven by a high power driving current (I₁+I₂) necessary for erasing data. Just after the mode is switched to the erasing mode, the second control signal S B is dropped to "Low" level as shown in Fig. 4. Due to this, the switching means of the second sample-hold circuit 4B is switched from the second input terminal 5Bb to the first input terminal 5Ba to pass signal outputted from the pre-amplifier 11 therethrough and to input the filtered signal directly to the second differential amplifier 3B. The second differential amplifier 3B compares the output signal Vb' from the low-pass filter 6B sith a second reference voltage V BB of a high level predetermined by a second standard voltage source 12B and outputs a voltage signal Vc' proportioanl to the difference between two voltages (Vc' ∝ V BB - Vc') to a second power amplifier provided for the high power driving current source 9. Thus, APC for the high power driving mode is obtained. Just before the mode is switched from the erasing mode to the recording mode, the second control signal S B is switched to "High" level. Due to this, the memory circuit 7B memorizes the output Vb' of the low-pass filter 6B and is connected to the input of the low-pass filter 6B. Namely, APC for the high driving mode is frozen and the output signal of the second sample-hold circuit 4B is fixed to a value obtained during APC of the erasing mode. Then, the erasing signal S E is dropped to "Low" level to start the recording mode. During the recording mode, data is recorded according to the recording signal S R . It is to be noted that APC is not done during the recording mode but it is done just before the recording mode, in other words during the erasing mode in which the semiconductor laser is driven by a high power driving current. This ensures the stable beam control during the recording mode. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art are possible without departing from the scope of the present invention, they should be construed as being included therein.

Claims: A beam controller for controlling a semiconductor laser (1) provided in an optical recording-play-back apparatus which comprises, a first current source (8) for driving said semiconductor laser (1) at a low level necessary for play-backing data memorized on an optical memory means, a second current source (9) for driving said semiconductor laser (1) at a high level necessary for recording and/or erasing data on the optical memory means, a first sample-hold circuit (4A) for holding data corresponding to a beam intensity from said semiconductor laser (1) being detected in a play-backing modus, a first control means for controlling the output of the first current source (8) according to the output from the first sample-hold circuit (4A) in an recording and/or erasing modus, characterized by a second sample-hold circuit (4B) for holding data corresponding to a beam intensity from said semiconductor laser (1) being detected in said erasing modus and a second control means for controlling the output current of the second current source (9) according to the output from the second sample-hold circuit (4B) in said recording and play-backing modus. A beam controller according to claim 1, said first sample-hold circuit (4A) is comprised of a switching means (5A) having first and second input terminals (5Aa,5Ab) and one output terminal, a low pass filter means (6A) connected to the output terminal of the switching means (5A) and a memory means (7A) for storing output signal from the low-pass filter means (6A) wherein first input terminal (5Aa) of the switching means (5A) is connected so as to input the data corresponding to the beam intensity from said semiconductor laser (1), second input terminal (5Ab) of the switching means (5A) is connected to the output of the memory means (7A) and said switching means (5A) is controlled so as to connect the first input terminal (5Aa) to the low-pass filter (6A) when the play-back mode is selected and, when the recording or erasing mode is selected, to connect the second input terminal (5Ab) to the low-pass filter (6A). A beam controller according to claim 1 or 2, said second sample-hold circuit (4B) is comprised of a switching means (5B) having first and second input terminals (5Ba,5Bb) and one output terminal, a low pass filter means (6B) connected to the output terminal of the switching means (5B) and a memory means (7B) for storing output signal from the low-pass filter means (6B) wherein first input terminal (5Ba) of the switching means (5B) is connected so as to input the data corresponding to the beam intensity from said semiconductor laser (1), second input terminal (5Bb) of the switching means (5B) is connected to the output of the memory means (7B) and said switching means (5B) is controlled so as to connect the first input terminal (5Ba) to the low-pass filter (6B) when the erasing mode is selected and, when the recording or play-backing mode is selected, to connect the second input terminal (5Bb) to the low-pass filter (6B). A beam controller for controlling a semiconductor laser (1) provided in an optical recording play-back apparatus which comprises, first current source (8) for driving said semiconductor laser (1) at a low level necessary for playbacking data memorized in an optical memory means, second current source (9) for driving said semiconductor laser (1) at a high level necessary for recording data in the optical memory means and/or erasing data memorized, a sample-hold circuit (4), characterized in that it comprises: switching means (5) having first and second input terminals (5a,5b) and one output terminal (5c), a low-pass filter means (6) connected to the output terminal (5c) of the switching means (5) and a memory means (7) for storing output signal from the low-pass filter means (6) wherein said first input terminal (5a) of the switching means (5) is connected so as to input the data corresponding to the beam intensity from said semiconductor laser (1), second input terminal (5b) of the switching means (5) is connected to the output of the memory means (7) and said switching means (5) is controlled so as to connected the first input terminal (5a) to the low-pass filter (6) when the play-back mode is selected and, when the recording or erasing mode is selected, to connect the second input terminal (5b) to the low-pass filter (6), and a control means for controlling said first current source (8) according to the output signal from the sample-hold circuit (4).

Inventors: DEGUCHI, TOSHIHISA C-722, DAIYA HAITSU GAKUENMAE, FUJI, HIROSHI, FUJIWARA, TSUNEO, IWAKI, TAKASHI, KOBAYASHI, SHOZOU
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1986
Main classification: G11B7
IPC codes: G11B19, G11B7, G11B11