The present invention relates to a power controller for controlling the power of a light beam emitted from a light source such as a semiconductor laser.
The present application claims priority from Japanese Application No. 2002-157373, the disclosure of which is incorporated herein by reference.
Conventionally known as a power controller of this type is one for controlling a semiconductor laser in a pickup incorporated in an information read/write apparatus.
For example, known is a power controller for controlling the aforementioned semiconductor laser in order to illuminate an information storage medium such as CDs (Compact Discs), DVDs (Digital Versatile Discs), or other so-called optical discs with laser beams each of which is set to an appropriate power to write, read, and erase data thereon.
As shown in the block diagram shown in FIG. 8, this conventional power controller includes a target value setting unit 1, a subtractor 2, an integrator 3, a V-I (voltage to current) converter 4, a light receiving device 8, and an I-V (current to voltage) converter 9. The power controller controllably drives a semiconductor laser 7 which is provided in a pickup.
In this configuration, a half mirror HM provided in the pickup reflects a laser beam Po emitted from the semiconductor laser 7 for the aforementioned data writing operation or the like to illuminate the recording surface of an optical disc DISC via an optical system (not shown) and as well transmit xcex94Po (part of the laser beam Po) impinging on the light receiving device 8.
Accordingly, while the laser beam Po for the writing operation or the like is being emitted, the light receiving device 8 successively detects the power of the laser beam Po to supply a detection current xcex94I proportional to the detected power to the I-V converter 9, thereby causing the I-V converter 9 to successively output a detection voltage xcex94V proportional to the detection current xcex94I.
The target value setting unit 1 is formed of a variable voltage source. An externally entered target value for setting the power of the laser beam Po will cause the target value setting unit 1 to output a target voltage Vri proportional to the target value entered.
In this configuration, when the target value setting unit 1 outputs the target voltage Vri, the subtractor 2 determines the voltage difference (Vrixe2x88x92xcex94V) between the target voltage Vri and the detection voltage xcex94V and then the integrator 3 smoothes the voltage difference (Vrixe2x88x92xcex94V), which is in turn converted into a DC voltage. Meanwhile, the V-I converter 4 generates a feedback current Ic proportional to the DC voltage and then supplies it as a drive current Idv to the semiconductor laser 7, thereby causing the semiconductor laser 7 to emit the laser beam Po having a power corresponding to the current value of the drive current Idv.
Accordingly, the conventional power controller allows the target voltage Vri to set the emission power of the semiconductor laser 7. Additionally, the power controller determines a variation in the power of the laser beam Po as the voltage difference (Vrixe2x88x92xcex94V) output from the subtractor 2 to provide negative feedback control so that the power of the laser beam Po is adjusted to a constant power corresponding to the target voltage Vri.
For example, suppose that one of a target value Pr1 for writing data, a target value Pr2 for reading data, and a target value Pr3 for erasing data is externally entered as appropriate. In this case, the conventional power controller outputs the target voltage Vri proportional to the entered target value to set the laser beam Po emitted by the semiconductor laser 7 to the power for writing data, reading data, or erasing data, which is suited, e.g., for use with a phase change disc (or a rewritable optical disc). Additionally, the power controller employs the aforementioned negative feedback control, thereby preventing variations in the power of the laser beam Po.
In recent years, with the development of multifunctional and various types of information storage media, there have also been developed a write once optical disc on which data can be written only once and a rewritable optical disc on which data can be written, erased, and rewritten as many times as desired, in addition to read only optical discs.
In this background, for the information read/write apparatus employing these optical discs, there has been an increasing demand for further improved functions and multifunctional capabilities to perform writing operations, reading operations or the like at higher speeds.
As a specific example, take an information read/write apparatus which utilizes the aforementioned rewritable optical disc. In this case, there is a demand for a flexibility which enables high speed and continuous data writing processing, data reading processing, and data erasing processing, in which television broadcasting data or digital video data or the like is reproduced by streaming while recorded in real time and unnecessary data is quickly erased to provide a recording area for the subsequent data writing operation. Additionally, it is critical to develop a power controller which can provide optimal control to the emission power of the semiconductor laser in order to enable high-quality writing and reading operations without any degradation in data even when these processing procedures are carried out.
However, for example, suppose that the aforementioned target value is changed at a high speed from Pr1 to Pr2 for a high speed transition from the data writing processing to the data reading processing in the conventional power controller. In this case, there was a problem that the power of the laser beam Po emitted by the semiconductor laser 7 could not quickly reach the power corresponding to the target value Pr2 during the transition from the target value Pr1 to the Pr2.
In other words, suppose that the power levels for the data writing, reading, and erasing operations are different from each other, and the target voltage Vri output by the target value setting unit 1 is switched to different values in response to the target value Pr1 , Pr2, or Pr3. In this case, the transient event caused phenomena such as overshoot or undershoot to occur in the laser beam Po emitted by the semiconductor laser 7, thereby exerting an adverse effect on the high-quality writing or reading operation. This raised a problem of making it difficult to satisfy the demand for further improved functions and multifunctional capabilities to perform the aforementioned writing or reading operation at higher speeds.
The present invention was developed in view of those problems. It is therefore an object of the present invention to provide a power controller which controllably drives a light source such as a semiconductor laser to implement, e.g., high-quality writing and reading operations.
The power controller according to a first aspect of the present invention is a power controller for controlling the power of light emitted from a light source. The power controller includes: target value setting unit for setting a plurality of target values; a plurality of feedforward unit, provided corresponding to each of the plurality of target values, for generating feed forward currents corresponding to each of the target values; first switching unit for allowing the respective feedforward currents generated by the plurality of feedforward unit to be exclusively switched and then supplied to the light source; second switching unit for allowing the plurality of target values to be exclusively switched for output; and feedback unit for supplying a feedback current to the light source, the feedback current having been adjusted such that a difference between the power of light emitted from the light source and a target value output by the second switching unit is stabilized at a predetermined value. Upon changing each of the target values, the first and second switching unit are switched simultaneously corresponding to a target value changed.
With this configuration, the power controller allows the feedforward currents to be generated corresponding to a target value and supplied to the light source via the first switching unit, thereby causing the light source to emit light at the power corresponding to the target value. The feedback unit adjusts the feedback current such that the difference between the power of light emitted from the light source and the target value output via the second switching unit is stabilized at a predetermined value, and then supplies the feedback current adjusted to the light source. Thus, the adjustment to the feedback current prevents variations or the like in the power of light emitted in accordance with the feedforward current.
Upon changing the target values, the first and second switching unit are switched simultaneously corresponding to a target value which has been changed. This prevents variations or the like in the power of light, which occur when there is a change in the target value.