Source: {"pile_set_name": "USPTO Backgrounds"}

The present invention relates to a laser control apparatus, more particularly, an apparatus which carries out power control of a semiconductor laser employed for an optical recording/reproducing apparatus, and can control the optical power stably even when an output light is modulated at high speed at the high-speed recording.
A semiconductor laser is generally employed as an optical pick up and the like in an optical recording/reproducing apparatus such as a CD player, a writable CD-R drive, an erasable CD-RW drive and the like. The power of a semiconductor laser varies greatly in accordance with a temperature change or changes with passage of time(lifetime), and therefore, it is necessary to carry out power control for stabilizing the power when employing a semiconductor laser as a light source in an apparatus for performing recording/reproduction into/from an optical recording medium such as an optical disk and the like.
Such prior art semiconductor laser power control apparatus is disclosed in Japanese Published Patent Application No. Hei. 1-204224 in detail.
FIG. 9 is a block diagram for illustrating a prior art semiconductor laser power control apparatus. In figure, numeral 10 denotes a semiconductor laser for radiating laser light to an optical disk; numeral 1 denotes a photodiode for receiving the irradiation light of the semiconductor laser 10; numeral 2 denotes a monitor circuit for monitoring the output of the photodiode 1; numeral 20 denotes a bottom hold circuit for holding a bottom level of the output from the monitor circuit 2; numeral 21 denotes a sample-hold circuit for sample-holding the output from the monitor circuit 2; numeral 22 denotes a peak hold circuit for holding a peak level of the output from the monitor circuit 2; numeral 19 denotes a control circuit for outputting a first, a second and a third digital signals corresponding to a bias reference voltage, an erase power reference voltage and a peak power reference voltage, respectively; numeral 26 denotes a D/A converter for converting the first digital signal outputted by the control circuit 19 to a bias reference voltage; numeral 27 denotes a D/A converter for converting the second digital signal outputted by the control circuit 19 to an erase reference voltage; numeral 28 denotes a D/A converter for converting the third digital signal outputted by the control circuit 19 to a peak power reference voltage; numeral 23 denotes a servo amplifier for comparing the bias reference voltage outputted by the D/A converter 26 with the bottom level which is held in the bottom hold circuit 20 to amplify the error; numeral 24 denotes a servo amplifier for comparing the erase reference voltage outputted by the D/A converter 27 with a sample-hold level which is held in the sample-hold circuit 21 to amplify the error; numeral 25 denotes a servo amplifier for comparing the peak power reference voltage outputted by the D/A converter 28 with the peak hold level which is held in the peak hold circuit 22 to amplify the error; numerals 7,8 and 9 denote current sources for generating the currents corresponding to the outputs of the servo amplifiers 23,24 and 25, respectively; numeral 11 denotes a switch means for disconnecting the current source 8 with the semiconductor laser 10 in accordance with data EFM1; numeral 12 denotes a switch means for disconnecting the current source 9 with the semiconductor laser 10 in accordance with data EFM2.
Next, the operation will be described. A portion of the output light from the semiconductor laser 10 is received by the photodiode 1 and the generated photoelectric current is converted to a voltage in the monitor circuit 2.
At reproduction from the optical disk, the reproduction power reference voltage VR which is outputted by the D/A (Read/Bias) converter 26 serving as a reference voltage source and a voltage which is obtained by the bottom hold circuit 20 by sample-holding the output of the monitor circuit 2 are compared by the servo amplifier 23 and a current is passed through the semiconductor laser 10 by the current source 7 which is controlled by the comparison result of servo amplifier 23 so that a reproduction power PR corresponding to the reproduction power reference voltage VR is constantly outputted.
At recording into an optical disk, as shown in FIG. 2, the power of the semiconductor laser 10 is modulated into the three values of bias power PB, erase power PE, and peak power PP, thereby recording is performed into the optical disk. Then, a portion of the output light from the semiconductor laser 10 is received by the photodiode 1, and a signal corresponding to the optical output is outputted from the monitor circuit 2. The signal from the monitor circuit 2 is held by the bottom hold circuit 20, the sample-hold circuit 21, and the peak hold circuit 22 respectively, and the bottom level, the erase level and the peak level of the optical output are detected respectively. Initially, the bias power PB can be obtained by switching the output of the D/A (Read/Bias) converter 26 serving as a reference voltage source in FIG. 9 to switch over from the VR to the bias power reference voltage VB corresponding to the bias power PB. The bias power reference voltage VB and the voltage which is held in the bottom hold circuit 20 are compared by the servo amplifier 23, and a current is passed through the semiconductor laser 10 by the current source 7 which is controlled by the comparison result of servo amplifier 23 so that the bias power PB corresponding to the bias power reference voltage VB is constantly outputted.
Next, the erase power PE is obtained by comparing the erase power reference voltage VE outputted by the D/A(Erase) converter 27 serving as a reference voltage source and the output from the sample-hold circuit 21 by the servo amplifier 24 and by passing an erase power current through the semiconductor laser 10 with the erase power current superposed on the bias power current by the current source 8 which is controlled by the comparison result of servo amplifier 24.
Further, the peak power PP is obtained by comparing the peak power reference voltage VP outputted by the D/A(Peak) converter 28 serving as a reference voltage source and the output from the peak hold circuit 22 by the servo amplifier 25 and by passing a peak power current through the semiconductor laser 10 with the peak power current further superposed on the erase power current by the current source 9 which is controlled by the comparison result of servo amplifier 25.
Meanwhile, the erase power PE and the peak power PP are turned on and off by the switch means 11 and 12 which are switched by data EFM1 and EFM2, respectively, and therefore the power of the semiconductor laser is modulated between the bias power PB, the erase power PE and the peak power PP. As shown in FIG. 3, the interval at which the modulation is performed between the bias power PB and the peak power PP is the interval at which pits are formed on the track, and when the switch means 11 and 12 are in the state of on and off, respectively, the intervals at which the value of the erase power PE is maintained are the intervals where the space between pits are erased and spaces are formed. The three power values (the value of the bias power PB, the value of the erase power PE, and the value of the peak power PP) can obtain the desired power values by changing the respective reference voltages of D/A converters 26, 27 and 28 serving as the reference voltage sources.
The conventional semiconductor laser power control apparatus is constructed as above, and it can perform such a control that the powers of peak and bottom of the semiconductor laser should be predetermined by detecting the powers of peak and bottom of the semiconductor laser.
By the way, in a CD-R drive, a CD-RW drive or the like, the recording of data into an optical disk is performed at high speed by such as so-called double-speed recording which performs recording of data at an average rotation speed which amounts to N times (N is an integer of 2 or above) of the reproduction rotation speed of a music CD player as a reference, thereby, it is necessary to increase the modulation speed of the laser to a higher one, and for controlling the laser power at such high-speed modulation, the conventional construction requires to detect the bias power PB, the erase power PE, and the peak power PP, respectively, and therefore a monitor circuit, a bottom hold circuit, and a peak hold circuit which can perform high-speed responses are required, thereby resulting in an increase in costs.
That is, even when the laser is modulated at high speed and the high speed modulated light is incident on a monitor photodiode, if a photodiode and a monitor circuit having low response speeds are used, delays or unsharpening of signals would be generated therein and unsharpening of a monitor output signal would also occur, and therefore, as shown in FIG. 3, the optical output waveform cannot be correctly reflected on the monitor output and even if an output signal of the monitor circuit has become of high speed, as the signal becomes of higher speed, the detection efficiency of the bottom hold circuit and the peak hold circuit are lowered, thereby making it difficult that the peak output level and the bias output level should be detected correctly. In addition, in order to avoid these problems, it is required for a monitor circuit, a bottom hold circuit, and a peak hold circuit to have the capability of high-speed response, thereby resulting in an increase in costs.
The present invention is made to solve the above-mentioned problems and has for its object to provide a laser control apparatus which can perform a stable power control even at a high-speed modulation even though employing a monitor photodiode and a monitor circuit which operate at a relatively low speed.
According to Claim 1 of the present invention, there is provided a laser control apparatus which controls a power of a semiconductor laser in an optical recording/reproducing apparatus for performing recording/reproduction into/from an optical recording medium by a semiconductor laser, comprising: a reproduction power monitor means for monitoring an output of the semiconductor laser at reproduction just before recording;