LASER DIODE DRIVING APPARATUS

Abstract: The drive system has a detector (14) measuring the intensity of light from the laser diode (13) for comparison (17), with a setting signal describing the wanted power of the laser. The comparator's output is modulated (19) with the signal being recorded on the optical disc (11). A drive circuit (72) uses the modulator's output to drive the laser diode. A biassing circuit (21) is located between the modulator and the drive circuit and sets the drive signals lowest value below the diode's threshold current. A mode switch (18) operates between erase and record for the setting signal being applied to the comparator (17).

Description: This invention relates to a laser diode driving apparatus useful when irradiating an optical disk, especially a write-once or an overwrite optical disk, with a laser beam. A laser diode driving apparatus according to the preamble of the claim of this invention is described in EP-A-0 256 827 as an apparatus to be used in cooperation with a magneto-optical disk. In this apparatus, the comparing means compares the output signal of the photodetector with the output signal of the reference power setting means. The low-pass filter receives the output signal of the comparator, which output signal is modulated by the modulating means. US-A-4,843,604 describes a similar laser diode driving apparatus for use in combination with a magnetooptical disk. As a reference power setting means, this apparatus comprises two channels, each having a low-pass filter and a comparing means, for setting the intensity of the light beam emitted from the laser diode. The comparing means in each channel compares the output signal of the photodetector with a predetermined, respective voltage. In the recording mode, the output signal of the comparing means in the high power driving channel is modulated in a modulating means according to recording data. In an adding means, the output signal of said modulating means is added to the output signal of a low power biasing means. The low power driving current of this biasing means is controlled so as to have a higher value than the threshold value of the laser diode. It is the object of the present invention to provide a laser diode driving apparatus which can be switched into different laser driving modes, and which provides for a long service life of the laser diode. The laser diode driving apparatus of the present invention is defined by the features of the appended claim. This invention will be better understood by the following description of preferred embodiments, with reference to the appended drawings. Fig. 1 is a block diagram showing a semiconductor laser diode driving apparatus embodying the present invention; Fig. 2 is a waveform diagram for illustrating the operation of the apparatus of Fig. 1; and Fig. 3 is a circuit diagram showing a specific embodiment of the invention. The block diagram of Fig. 1 shows the construction of a laser diode driving apparatus according to the present invention. This laser diode driving apparatus drives a semiconductor laser diode 13 radiating a laser beam for data recording and/or reproducing by an optical head 12 which performs data recording and/or reproducing on a overwrite optical disk 11 the recording surface of which is formed of a magneto-optically recordable material. The optical head 12 is arranged so that the light radiated by the semiconductor diode 13 is radiated by way of a beam splitter BS and an objective lens L on the recording surface of the optical disk 11, and that the back beam from the semiconductor laser diode 13 is detected by a monitoring detector 14. The monitoring detector 14 for the optical head 12 detects the laser power of the laser beam radiated by the semiconductor laser diode 13 on the recording surface of the optical disk 11 by a back beam and transmits the detection output signal from an amplifier 15 by way a low-pass filter 16 to comparator means 17. The comparator means 17 is supplied with a reference signal E₁ for the record mode, a reference signal E₂ for the reproducing mode and a reference signal E₃ for the erasure mode, as laser power setting means, by way of a mode switch 18, which is actuated in dependence upon the operating mode of the recording and/or reproducing apparatus for an optical disk and compares the detection output signal transmitted from the monitoring detector 14 by way of the low-pass filter 16 with the reference signals E₁, E₂ and E₃ to transmit the comparison output signal to modulating means 19. The modulating means 19 turns the comparison output signal produced at the comparator means 17 on and off with the recording data signal E D , and is formed by a switching circuit which is turned on and off by the record data signals E D "1" and "0", respectively. The modulation output signal obtained at the modulation signal 19 is transmitted to a summation circuit 20. The summation circuit 20 is supplied with a bias current i DC from a bias circuit 21, and sums the bias current i DC to a modulation output signal produced at the modulating means 19 to transmit the summation output signal to a driver circuit 22 of the semiconductor laser diode 13. The bias circuit 21 sets the bottom value of the driving signal supplied by the driving circuit 22 to the semiconductor laser diode 13 so as to be equal to or lesser than the threshold current I th of the semiconductor laser diode 13. However, the bias current may be zero, as later described, and thus the bias circuit 21 may be dispensed with. The operation of the laser diode driving apparatus of Fig. 1 according to the present invention will be explained with reference to the waveform diagram of Fig. 2. Assuming that the recording and/or reproducing apparatus is at the recording mode, and the peak value of the optimum laser power is P₀, for example, the reference signal E₁ selected by the mode switch 18 during the recording mode is so set that the peak value of the recording current I(mA) is equal to i L . Thus the difference between the detection output signal of the back beam of the semiconductor laser diode 13 detected by the monitoring detector 14 and integrated by the low-pass filter 16 (the peak value) and the reference signal E₁ is set so as to be equal to the recording current i L . This record current i L is turned on and off with the record data signal E D at the modulating means 19 and added to by the bias current I DC at the summation circuit 21 to produce a driving signal i A which is transmitted by way of the driving circuit 22 to the semiconductor laser diode 13. The record data signal E D is recorded as the record pattern in which the direction of magnetization is changed by that the direction of magnetization on the recording surface of the optical disk 11 is changed with rising of the temperature of the recording surface to higher than the Curie temperature. This rise in temperature occurs when the laser power P₁ during the recording mode reaches the peak in the course of changes of the laser power, as shown at P₁ in Fig. 4, of the light emission output of the semiconductor laser diode 13 driven by the driving circuit 22 with the driving signal i A . In the operation of the above described APC circuit, when the light emission output characteristics of the semiconductor laser diode 13 are changed from the characteristics A to the characteristics B as shown in Fig. 4, as a result of replacement of the semiconductor laser diode 13 or temperature changes, for example, the detection output signal by the monitoring detector 14, taken out at the amplifier 15 and the low-pass filter 16, is decreased, so that the output signal of the comparator means 17 is increased. The output signal of the comparator means 17 is turned on and off with the record data signal E D by modulating means 19, and added to by the bias current current I DC at the summating circuit 20, with the resulting signal being supplied with the peak value of i L1 to the driven circuit 22. By loop control of the APC circuit, a drive signal i B is formed which has such a peak value i L1 that the laser power P₁ during the recording mode of the semiconductor laser diode 13 will have an optical peak value P₀. Since the driving signal i B has its maximum amplitude value I DP controlled by the APC circuit, its bottom value is lesser than the threshold current I th of the semiconductor laser diode 13, with the laser power of the light emission output of the semiconductor laser diode 13 being on-off controlled sufficiently so that the recording pattern of a desired size and shape may be formed on the recording surface of the optical disk 11. By providing modulating means 19 for modulating the current driving the semiconductor laser diode 13 in the APC circuit loop, as in the above embodiment, it becomes possible to cover fluctuations in light emission characteristics of the semiconductor laser diode 13 over a wide range, to release the light emission characteristics of the semiconductor laser diode that may be employed as the semiconductor-laser diode 13 as well as to widen the range of the working temperature to improve the durability of the semiconductor laser diode. It is noted that, during the reproduction mode, the reference signal E₂ for the reproduction mode is selected by the mode switch 18, and the control by APC curcuit is performed so that the laser beam having a laser power low enough to permit data reading from the optical disk 11 is radiated from the semiconductor laser diode 13. During the erasure mode, the reference signal E₃ for the erasure mode is selected by the mode switch 18, and the control by APC curcuit is performed so that the laser beam having a laser power high enough to permit data erasure on the optical disk 11 is radiated from the semiconductor laser diode 13. The above embodiment is concerned with driving the semiconductor laser diode 13 irradiating the laser beam on an overwrite optical disk 1 which is a magneto-optical disk having its recording surface formed of a photomagnetically recordable material. However, a write-once optical disk having its recording surface formed of an amorphous alloy material, such as Sb₂, Sbe₃, Bi₂Te₃, that may be alloyed with rise in temperature, may be used as the optical disk 1. In this case, the erasure mode is not necessitated. Fig. 3 shows an example of the above described APC circuit, wherein T₁ denotes a terminal to which a laser power setting signal E reff is input, T₂ denotes an input terminal for a signal setting the bias current I DC , 30 denotes a signal source outputting the record data signal E D or an erasure period signal and 40 denotes a modulator for the semiconductor laser diode 13. This modulator 40 is comprised of a pair of transistors Q₂, Q₃ differentially connected to each other, and a transistor Q₄ having its collector connected to the emitters of the transistors Q₂, Q₃ and acting as a current source. The transistors Q₂, Q₃ of the modulator 40 are supplied with complementary signals from the signal source 30 and thereby on-off controlled alternately. Even during the turn-off state of the transistor Q₂, the constant bias current I DC is supplied by way of transistor Q₁ to the semiconductor laser diode 13. The output of the semiconductor device functioning as the monitoring detector 14 is amplified by an operational amplifier A₂ and supplied to a reverse phase input terminal of an operational amplifier A₁ functioning as comparator means. The operational amplifier A₁ has its forward phase input terminal supplied with the laser power setting signal E reff from the terminal T₁, this signal E reff setting the light emission output power of the semiconductor laser diode 13. The operational amplifier A₁ functions as comparator means for effecting level comparison between-the laser power setting signal E reff and the detection output signal by the monitoring detector 14. The output of the comparator means formed by the operational amplifier A₁ controlls the current flowing through the transistor Q₄ of the modulator 40, while controlling the maximum amplitude of the on/off current which is the driving signal for the semiconductor laser diode 13. A feedback circuit formed by a parallel circuit of a resistor R₁ and a capacitor C₁ is annexed to the operational amplifier A₁, and functions to eliminate the high frequency signal or data signal component of the output signal. It is noted that, when the modulator 40 has high through-rate switching characteristics, or when the semiconductor laser diode having an extremely low threshold current I th is used as the semiconductor laser diode 13, the circuit supplying the bias current I DC may be eliminated.

Claims: A laser diode driving apparatus comprising: a laser diode (3) driven by a driving signal from a driving means (9); a photo detector (4) for receiving and monitoring a laser light beam emitted from said laser diode; a reference power setting means (10) for outputting an intensity setting signal for setting the intensity of the light beam emitted from said laser diode according to the actually selected operation mode of said laser diode driving apparatus; and a low-pass filter (7), a comparing means (6), and a modulating means (11); characterized in that said low-pass filter (7) is arranged to receive the output signal from said photodetector; said comparing means (6) is arranged to compare the output signal of said low-pass filter and the output signal of said reference power setting means; said modulating means (11) is arranged to modulate the output signal from said comparing means according to recording data; a biasing means (12) is provided for setting the bottom value of the driving signal from said driving means to a value below the threshold value for said laser diode; and an adding means (8) is providing for adding the output signal from said modulating means and said biasing means; and for supplying its output signal to said driving means.

Inventors: IIMURA TOSHIAKI, TERAYAMA YASUNORI, YAMASAKI YOSHIMORI, IIMURA, TOSHIAKI, TERAYAMA, YASUNORI, YAMASAKI, YOSHIMORI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1989
Main classification: G11B7
IPC codes: G11B7