Abstract:
An optical disk drive has a pick-up head for generating a laser beam to record data on an optical disk and for detecting corresponding power of the laser beam, and a power control unit for adjusting power of the laser beam generated by the pick-up head. The power control method includes inputting a plurality of first control signals into the power control unit in order so that the power control unit generates a plurality of first control voltages related to the first control signals for sequentially driving the pick-up head to output a plurality of first test powers, and using the plurality of first test powers and the plurality of first control signals to establish a first mapping function. The optical disk drive uses the first mapping function to get a first predetermined output power and a corresponding first predetermined control signal according to the first mapping function.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a power control method of a pick-up head of an optical disk drive. In particular, the present invention discloses a method of establishing a mapping function related to laser powers of the pick-up head and corresponding control signals for driving the pick-up head, and using the established mapping function to control laser powers of the pick-up head.  
           [0003]    2. Description of the Prior Art  
           [0004]    For companies or the end users, the management and storage of documents is regarded as an important task. In the past, documents were printed or written on paper. Therefore, when a user deals with a huge amount of documents, it is not convenient for the user to manage those documents because of a great size or a heavy weight. With the development of computer technology, digital data and digital documents are widely stored in a plurality of data storage media. Many kinds of data storage media are developed to help users with those digital data conveniently. An optical disk recorder such as a CD-RW drive takes advantage of recordable compact disks to record data. The compact disk has a low production cost, a small size, and a great storage capacity. Therefore, the user can easily keep data through the compact disk. Generally speaking, the optical disk recorder has to perform an optimum power control (OPC) process to find a most adequate laser power before burning data onto the compact disk. The selected laser power is used to etch the compact disk for forming pits that are used to record binary bits “0”s.  
           [0005]    Please refer to FIG. 1, which is a block diagram of a prior art CD-R drive  10 . The CD-R drive  10  is used to record data on a CD-R disk. The CD-R drive  10  has a controller  12 , a power control unit  14 , a pick-up head  16 , and a signal converter  18 . The controller  12  is used to control operation of the CD-R drive  10 . The power control unit  14  is used to adjust laser powers outputted from the pick-up head  16  when the pick-up head  16  records data on the CD-R disk. In addition, the power control unit  14  includes a sample/hold circuit  20 , a digital-to-analog converter (DAC)  20 , and a driving circuit  24 . The sample/hole circuit  20  is used to determine whether a closed loop configuration or an open loop configuration is established between the power control unit  14  and the pick-up head  16 . When the sample/hold circuit  20  is enabled, an output port of the sample/hold circuit  20  is capable of holding the signal sampled at an input port of the sample/hold circuit  20 . The DAC  22  is used to convert a digital control signal  26  into a corresponding analog control voltage  28 . The driving circuit  24  then generates a control voltage  30  according to the control voltage  28  for driving the pick-up head  16  to output a laser beam with a predetermined power. The pick-up head  16 , therefore, adjusts the laser power according to the control voltage  30  generated from the power control unit  14 . While recording data on the CD-R disk, the pick-up head  16  detects a magnitude of the outputted laser power. With regard to the CD-R drive  10 , the pick-up head  10  outputs an incident pulse to etch the CD-R disk, and the incident pulse is reflected from the surface of the CD-R disk to generate a corresponding reflective pulse. A reflected pulse level (B-level defined in a prior art Orange Book) of the reflective pulse stands for deepness of the etched pit. According to the reflected pulse level of the reflective pulse, that is, the actual laser power of the pick-up head  10 , the signal converter  18  generates a corresponding feedback signal  32  to the power control unit  14  for further tuning the laser power of the pick-up head  16 . If a control signal  26  corresponds to an optimum write power, the closed loop configuration established between the pick-up head  16  and the power control unit  14  will force the laser power of the pick-up head  16  to approach the optimum write power eventually. Writing operation of the CD-R drive  10  is described as follows.  
           [0006]    The pick-up head  16  of the CD-R drive  10  is capable of generating laser beams to etch a recording layer on the CD-R disk. The etched area (pit) represents “0”, and the non-etched area (land) represents “1”. Therefore, digital data are recorded on the CD-R disk. However, different CD-R disks have different recording layer characteristics. In other words, different CD-R disks have different power absorption capacities. Therefore, when the same laser power is applied on different CD-R disks, etching deepness of each CD-R disk varies. Based on the above-mentioned reason, each disk manufacturer will record a proposed write power on a lead-in area of the CD-R disk as a reference laser power when manufacturing the CD-R disk. Based on the reference laser power, the CD-R drive  10  then performs the optimum power control to acquire a write power P1 that is suitable for the CD-R disk. After obtaining the write power P1, the CD-R drive  10  begins writing data on the recording layer of the CD-R disk with the help of the write power P1. According to hardware specifications of the pick-up head  16  and the DAC  22 , mapping relation between the laser powers outputted from the pick-up head  16  and the control signal  26  driving the pick-up head  16  is determined. For example, when the laser power of the pick-up head  16  equals P1, the required control signal  26  corresponds to a value DAC1 (that is, the control voltage  28  has a corresponding voltage level V1). If the laser power generated from the pick-up head  16  is equal to P2, the signal converter  18  generates a feedback signal  32  corresponding a voltage level V2 according to the laser power P2 detected by the pick-up head  16 . When the pick-up head  16  is driven to etch the CD-R disk for recording data, the controller  12  enables the sample/hold circuit  20  to form a closed loop configuration between the pick-up head  16  and the power control unit  14 . Because the voltage level V2 is different from the target voltage level V1, the driving circuit  24  adjusts the control voltage  30  according to the voltage levels V1, V2 until the feedback signal  32  and the control voltage  28  have the same voltage level. Then, the laser power P1 of the pick-up head  16  is stabilized to etch the CD-R disk correctly. In other words, each time when the write power of the pick-up head  16  deviates from the target laser power P1, the closed loop configuration established between the pick-up head  16  and the power control unit  14  forces the driving circuit  24  to automatically tuning the erroneous power for recovering the target laser power P1. However, physical characteristics of the pick-up head  16  change after a long period of usage. The mapping relation between the laser power and the corresponding control voltage is not always fixed, so that control voltage used to drive the pick-up head  16  for outputting the target laser power in the beginning does not work over a long period of time. In addition, various circuits disposed in the CD-R drive  10  with decay effects on transmitted signals alter original physical characteristics after a long period of usage. If the CD-R drive  10  uses the control signal DAC T1  to drive the pick-up head  16  to output a laser power P T1  at time T1, the same control signal DAC T1  inputted to the DAC  22  at time T2 will make the laser power outputted from the pick-up head  16  deviate from the previous laser power P1. If the laser power P T1  is the optimum write power required by the CD-R drive  10  at time T2, the CD-R drive  10  has to repeatedly perform tuning operations for adjusting the value of the control signal until the outputted laser power becomes P T1 . It is well-known that the tuning operations take a long period of time, and the writing efficiency of the CD-R drive  10  is greatly deteriorated. Besides, if the laser power P T1  corresponds to high power, the pick-up head  16  might be damaged while performing the repeated tuning operations to detect the outputted high power. Therefore, the life span of the CD-R drive  10  is reduced.  
           [0007]    Please refer to FIG. 2, which is a block diagram of a prior art CD-RW drive  40 . The CD-RW drive  40  has a controller  42 , a power control unit  44 , a pick-up head  46 , and a signal converter  48 . The controller  42  is used to control operation of the CD-RW drive  40 . The power control unit  44  is used to adjust laser power outputted from the pick-up head  46  when the pick-up head  46  records data on a CD-RW disk. The power control unit  44  includes a sample/hold circuit  50 , a digital-to-analog converter (DAC)  52 , a driving circuit  54 , and a power amplifier  56 . The sample/hold circuit  50  is used to determine whether a closed loop configuration or an open loop configuration is established between the pick-up head  46  and the power control unit  44 . The DAC  52  is used to convert a digital control signal  58  into a corresponding analog control voltage  60 . The driving circuit  54  generates a control voltage  62  according to the control voltage  60  so as to drive the pick-up head  46  to output laser beams with an erase power. The power amplifier  56  with a gain setting is capable of generating a control voltage  66  according to the control voltage  62 . The control voltage  62  and the control voltage  66  are both used to drive the pick-up head  46  to etch the CD-RW disk through a write power. The pick-up head  46 , therefore, adjusts its laser power according to the control voltages  62 ,  66  outputted from the power control unit  44 . The writing operation of the CD-RW drive  40  is briefly described as follows.  
           [0008]    Please refer to FIG. 3, FIG. 4, and FIG. 5. FIG. 3 is a first equivalent circuit  70  of the CD-RW drive  40  shown in FIG. 2. FIG. 4 is a second equivalent circuit  80  of the CD-RW drive  40  shown in FIG. 2. FIG. 5 is a third equivalent circuit  90  of the CD-RW drive  40  shown in FIG. 2. Because the CD-RW drive  40  is capable of performing writing operations and erasing operations on the same CD-RW disk, the pick-up head  46  of the CD-RW drive  40  requires a write power for generating pits used to represent “0”s, and an erase power for removing pits and forming lands used to represent “1”s. The erase power is capable of clearing data recorded on the CD-RW disk. In other words, the erase power heats the recording layer of the CD-RW disk so that the surface of the whole recording layer is polished without any existing pits. Therefore, recorded data are cleared by the erase power. According to a limitation defined in the Orange Book, the erase power Pe is proportional to the write power Pw according to a predetermined ratio #, that is, Pe=#*Pw. Because the write power Pw is greater than the erase power Pe, the CD-RW drive  40  generally searches for a stable erase power Pe first through the optimum power control. Then, the desired write power Pw is obtained directly by the erase power Pe and the equation (Pe=#*Pw). In other words, if the erase power Pe is successfully set, the required write power Pw is easily obtained without any additional tuning operations. Generally speaking, the write power used by the CD-R drive is equivalent to the erase power used by the CD-RW drive. As mentioned above, the CD-RW drive  40  has to find the suitable erase power Pe first. The first equivalent circuit  70  shown in FIG. 3 is similar to the block diagram of the CD-R drive  10  shown in FIG. 1. The sample/hold circuit is enabled so that the closed loop configuration is established between the pick-up head  46  and the power control unit  44 . The output port of the sample/hold circuit  50  holds the signal sampled at the input port of the sample/hold circuit  50 . After the pick-up head  46  detects its laser power, the signal converter  48  converts the detected power into a corresponding feedback signal, and transmits the feedback signal to the sample/hold circuit  50 . The first equivalent circuit  70 , therefore, is mainly used to the erase power, and its operation is identical to that of the CD-R drive  10  for acquiring the write power. The lengthy description is skipped for simplicity. Because the CD-RW drive  40  does not perform the optimum power control upon the write power Pw, the sample/hold circuit  50  is disabled when the CD-RW drive  40  starts etching the CD-RW disk with the write power Pw. Therefore, the open loop configuration is then established between the pick-up head  46  and the power control unit  44 . The closed loop configuration and the open loop configuration are different operating conditions for the CD-RW drive  40 . Generally speaking, when the pick-up head  46  outputs predetermined laser power, the value required by the control signal  58  to drive the pick-up head  46  to output the predetermined laser power under the open loop configuration is greater than the value under the closed loop configuration. As shown in FIG. 4, because the erase power Pe is obtained in the closed loop configuration, the CD-RW drive  40  under the open loop configuration has to adjust the control signal  58  corresponding to the erase power Pe through the second equivalent circuit  80  until the control voltage  62  can drive the pick-up head  46  to output the erase power Pe. As mentioned above, the erase power Pe is proportional to the write power Pw according to a predetermined ratio #, that is, Pe=#*Pw. The write power Pw then is acquired by the above equation. In the third equivalent circuit  90  shown in FIG. 5, the control voltage  62  willpass to the power amplifier  56  so that the control voltage  62  is amplified according to a gain value for generating the control voltage  66 . Owing to the decay effect upon signals, the gain value of the power amplifier  56  set according to the predetermined ratio # is not capable of generating a correct control voltage  66  to force the pick-up head  46  to output the target write power Pw. Therefore, the gain value is adjusted repeatedly, and the pick-up head  46  also continuously detects its output power until the laser power outputted from the pick-up head  46  is equal to the write power Pw. Then, the control voltages  62 ,  66  simultaneously drive the pick-up head  46  to output the write power Pw for forming pits on the CD-RW disk. However, the write power Pw is high power. If the pick-up head  46  detects the write power Pw, the corresponding reflective pulse will damage the pick-up head  46  so that the pick-up head  46  malfunctions. In addition, physical characteristics of the pick-up head  46  change after a long period of usage. The mapping relation between the laser power and the corresponding control voltage is not always fixed so that control voltage used to drive the pick-up head  46  for outputting the target laser power in the beginning does not work over a long period of time. In addition, various circuits disposed in the CD-RW drive  40  alter original physical characteristics, decay effect upon signals for example, after a long period of usage. Therefore, the CD-RW drive  40  needs a longer period of time to tune the gain value of the power amplifier  56 . Because the tuning operations take a long period of time, and the writing efficiency of the CD-RW drive  40  is greatly deteriorated.  
         SUMMARY OF INVENTION  
         [0009]    It is therefore a primary objective of the claimed invention to provide a power control method of testing the pick-up head under a low power condition and establishing a mapping function for representing actual laser power characteristics of the pick-up head to solve the above-mentioned problem.  
           [0010]    Briefly summarized, the preferred embodiment of the claimed invention discloses a power control method for controlling a laser power used by an optical disk drive to record data on an optical disk. The optical disk drive has a pick-up head for generating a laser beam to record data on the optical disk and measuring the laser power corresponding to the laser beam, a converter for converting the laser power measured by the pick-up head into a feedback signal, and a power control unit for adjusting the laser power outputted from the pick-up head. The power control unit includes a digital-to-analog converter (DAC) for converting a first control signal into a first control voltage, and a sample/hold circuit for controlling whether the feedback signal is fed back to the power control unit. The power control method includes sequentially inputting a plurality of first control signals into the DAC so that the DAC sequentially outputs a plurality of first control voltages to the pick-up head for driving the pick-up head to sequentially output a plurality of first test laser powers after enabling the sample/hold circuit, using the pick-up head for measuring the first test laser powers, and using the first control signals and the first test laser powers for establishing a first mapping function. The optical disk drive uses the first mapping function for calculating a first predetermined laser power and a first predetermined control signal, and the first predetermined control is used for driving the pick-up head to output the first predetermined laser power.  
           [0011]    It is an advantage of the claimed invention that power tests are performed under a low power condition to figure out mapping functions and to prevent the pick-up head from being damaged. The value of the control signal for driving the pick-up head to output a predetermined power is quickly and easily obtained through the mapping function. The data recording performance of the optical disk drive is greatly improved.  
           [0012]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a prior art CD-R drive.  
         [0014]    [0014]FIG. 2 is a block diagram of a prior art CD-RW drive.  
         [0015]    [0015]FIG. 3 is a first equivalent circuit of the CD-RW drive shown in FIG. 2.  
         [0016]    [0016]FIG. 4 is a second equivalent circuit of the CD-RW drive shown in FIG. 2.  
         [0017]    [0017]FIG. 5 is a third equivalent circuit of the CD-RW drive shown in FIG. 2.  
         [0018]    [0018]FIG. 6 is a flow chart of a first power control method according to the present invention.  
         [0019]    [0019]FIG. 7 is a laser power diagram of a pick-up head shown in FIG. 1 according to the first power control method.  
         [0020]    [0020]FIG. 8 is a flow chart of a second power control method.  
         [0021]    [0021]FIG. 9 is a laser power diagram of a pick-up head shown in FIG. 2 according to the second power control method.  
         [0022]    [0022]FIG. 10 is a gain value diagram of a power amplifier shown in FIG. 2 according to the second power control method. 
     
    
     DETAILED DESCRIPTION  
       [0023]    Please refer to FIG. 6 in conjunction with FIG. 1. FIG. 6 is a flow chart of a first power control method according to the present invention. The first power control method includes following steps.  
         [0024]    Step 102: Enable the sample/hold circuit  20 ;  
         [0025]    Step 104: Input a plurality of control signals  26  to the DAC  22 ;  
         [0026]    Step 106: The DAC  22  sequentially converts the control signals  26  into corresponding control voltages  28  for sequentially driving the pick-up head  16 ;  
         [0027]    Step 108: The pick-up head  16  sequentially detects a plurality of test laser powers;  
         [0028]    Step 110: Establish a mapping function according to the test laser powers; and  
         [0029]    Step 112: Decide a control signal corresponding to a laser power according to the mapping function.  
         [0030]    Operation of the power control method according to the present invention is described as follows. With regard to the CD-R drive  10  shown in FIG. 1, the sample/hold circuit  20  is first enabled to establish a closed loop configuration between the pick-up head  16  and the power control unit  14  (step 102). The output port of the sample/hold circuit  20  holds the signal sampled at the input port of the sample/hold circuit  20 . Then, a plurality of control signals  26  corresponding to different values are sequentially inputted to the DAC  22  (step 104). The DAC  22 , therefore, sequentially converts the received control signals  26  into corresponding control voltages  28 , and the control voltages  28  are transmitted to the driving circuit  24 . The driving circuit  24  sequentially generates control voltages  30  according to the received control voltages  28  for driving the pick-up head  16  (step 106). Each control signal  26  drives the pick-up head  16  to generate laser beams with a corresponding laser power for etch the CD-R disk. At the same time, the pick-up head  16  detects magnitude of the laser power. In addition, the closed loop configuration makes the pick-up head  16  output stable laser power corresponding to the control signal  26  (step 108). The pick-up head  16  sequentially detects a laser power corresponding to each of the control signals  26 . The detected laser power is the actual power outputted from the pick-up head  16 . Aging of the pick-up head  16  and influences of the circuits disposed in the CD-R drive  10  make the actual laser power driven by the control signal  26  deviate from an ideal laser power, but the preferred embodiment generates a mapping function according to the control signals  26  and corresponding actual laser powers without considering the ideal laser powers. That is, the established mapping function is capable of representing a function relation between the control signal  26  and the actual laser power of the pick-up head  16  when the CD-R drive operates. The actual behavior of the CD-R drive is obtained. Generally speaking, a prior art polynomial curve fitting method can be used to generate the mapping function (step 110). After the mapping function is obtained, the CD-R drive  10  is capable of easily determining value of the control signal  26  corresponding to a desired write power, and drives the pick-up head  16  to record data on the CD-R disk according to the calculated control signal  26 . Please refer to FIG. 7, which is a laser power diagram of the pick-up head  16  shown in FIG. 1 according to the first power control method. The horizontal axis represents values of the control  26 , and the vertical axis represents laser powers of the pick-up head  16 . When the value of control signal  26  equals DAC1, the actual laser power of the pick-up head equals P1. When the value of control signal  26  equals DAC2, the actual laser power of the pick-up head equals P2. The preferred embodiment, therefore, is capable of quickly figuring out the mapping function  100  through the values DAC1, DAC2 of the control signal  26  and the corresponding laser powers P1, P2. Please note that the preferred embodiment only uses two test results to quickly obtain the mapping function  100  by a straight-line relation. However, if more tests are performed, the prior art polynomial curve fitting can be used to calculate the mapping function  100 . Therefore, when the CD-R drive  10  needs the laser power P3 to record data on the CD-R disk. With the help of the mapping function  100 , the required value DAC3 of the control signal  26  is quickly got. In the preferred embodiment, the laser powers P1, P2 are both low powers, and the pick-up head  16  is not damaged during the testing process. In addition, when the mapping function  100  is established under a low power condition, the mapping function  100  then can be used to get value of control signal  26  under a high power condition. Therefore, when the CD-R drives  10  requires a high power (P3 for example) to record data, the corresponding control signal  26  having a value DAC3 is directly obtained through the mapping function  100  for driving the pick-up head  16 . In other words, the preferred embodiment simplifies prior art tuning operations and related operation time. The writing efficiency is greatly improved, and the damage to the pick-up head  16  is prevented because the pick-up head  16  is not driven under a high laser power condition.  
         [0031]    Please refer to FIG. 2, FIG. 8, FIG. 9, and FIG. 10. FIG. 8 is a flow chart of a second power control method. FIG. 9 is a laser power diagram of the pick-up head  46  shown in FIG. 2 according to the second power control method. FIG. 10 is a gain value diagram of the power amplifier  56  shown in FIG. 2 according to the second power control method. With regard to FIG. 9, the horizontal axis stands for values of the control signal  58 , and the vertical axis stands for laser power of the pick-up head  46 . With regard to FIG. 10, the horizontal axis stands for values of the control signal  58 , and the vertical axis stands for gain values of the power amplifier  58 . The second power control method of the present invention includes the following steps.  
         [0032]    Step 202: Enable the sample/hold circuit  50 ;  
         [0033]    Step 204: Input a plurality of control signals  58  to the DAC  52 ;  
         [0034]    Step 206: The DAC  52  sequentially converts the received control signals  58  into corresponding control voltages  60  for sequentially driving the pick-up head  46 ;  
         [0035]    Step 208: The pick-up head  46  sequentially detects a plurality of first test powers;  
         [0036]    Step 210: Establish a first mapping function according to the first test powers;  
         [0037]    Step 212: Disable the sample/hold circuit  50 ;  
         [0038]    Step 214: Input a plurality of control signals  58  to the DAC  52 ;  
         [0039]    Step 216: The DAC  52  sequentially converts the received control signals  58  into corresponding control voltages  60  for sequentially driving the pick-up head  46 ;  
         [0040]    Step 218: The pick-up head  46  sequentially detects a plurality of second test powers;  
         [0041]    Step 220: Establish a second mapping function according to the second test powers;  
         [0042]    Step 222: Determine third test powers corresponding to a plurality of control signals  58  with the help of the second mapping function;  
         [0043]    Step 224: Enable the power amplifier  56 ;  
         [0044]    Step 226: Input the control signals  58  to the DAC  52 ;  
         [0045]    Step 228: The DAC  52  sequentially converts the received control signals  52  into a plurality of control voltages  60  for driving the pick-up head  46 ;  
         [0046]    Step 230: Sequentially adjust the gain values of the power amplifier  56  to make each laser power of the pick-up head  46  equal each third test power, which corresponds to each of the inputted control signals  58 , multiplied by a predetermined coefficient;  
         [0047]    Step 232: Establish a third mapping function according to the control signals  58  and corresponding gain values of the power amplifier  56 ;  
         [0048]    Step 234: Set the value of the control signal  58  used to drive the pick-up head  46  through the first, second, and third mapping functions.  
         [0049]    The second power control method according to the present invention is described as follows. The above-mentioned procedure for calculating a mapping function related to write powers of the CD-R drive can be also applied to the CD-RW drive  40 . It is well-known that the write power of the CD-R drive  10  corresponds to the erase power of the CD-RW drive  40 . The CD-R drive  10  utilizes a closed loop configuration to steadily output the write power. The CD-RW drive  40 , similarly, adopts a closed loop configuration to steadily output the erase power for forming land. But, the CD-RW drive  40  utilizes an open loop configuration to drive the pick-up head  46  to output a write power. Therefore, the CD-RW drive  40  has to use the closed loop configuration to acquire an adequate erase power of the CD-RW disk in the beginning. Then, the CD-RW drive  40  uses the open loop configuration to decide a control signal corresponding to a write power based on the erase power, and tunes a gain value of the power amplifier  56  to generate the required write power used to etch the CD-RW disk for recording “0”s. However, if the CD-RW drive  40  wants to record “1”s, the erase power is then used to polish the CD-RW disk for recording “1”s. In addition, the closed loop configuration is switched on to stabilize the outputted erase power. In other words, the pick-up head  46  will detect its laser power while polishing the CD-RW disk by the outputted laser power. If the laser power deviates from the desired erase power, the closed loop configuration is automatically actuated to adjust the laser power of the pick-up head  46  until the laser power is equal to the erase power. The CD-RW drive  40  is similar to the CD-R driver  10  so that the sample/hold circuit  50  is first disabled to figure out the mapping function related to the erase power and the control signal  58 . The detailed operation is described as follows.  
         [0050]    First of all, the CD-RW drive  40  enables the sample/hold circuit  50  so that a closed loop configuration is established between the pick-up head  46  and the power control unit  44  (step 202). A plurality of control signals  58  with different values are sequentially inputted to the DAC  52  (step 204). The DAC  52 , therefore, sequentially receives the control signals  58 , and sequentially outputs a plurality of corresponding control voltages  60  to the driving circuit  54 . The driving circuit  54  then outputs control voltages  60  with different voltage levels according to the received control voltages  60  for sequentially driving the pick-up head  46  (step 206). Each control signal  58  drives the pick-up head  46  to output laser beams with a predetermined power for etching the CD-RW disk. At the same time, the pick-up head  46  detects the predetermined power outputted from the pick-up head  46  itself, and the closed loop configuration automatically operates to force the pick-up head  46  to generate a stable laser power (step 208). Because the pick-up head  46  detects an outputted laser power, which is an actual power of the pick-up head  46  corresponding to each inputted control signal  58 , the aging of the pick-up head  46  and the decay effect caused by the circuits disposed in the CD-RW drive  40  are considered. In other words, the output powers of the pick-up head and the corresponding control signals  58  are utilized to establish a first mapping function  300  that represents a relation between the control signal  58  and the output power of the pick-up head  46 . The first mapping function  300  reveals actual operation behavior of the CD-RW drive  40 . As shown in FIG. 9, when the value of the control signal  58  corresponds to DAC1, the laser power of the pick-up head  46  equal P1, and when the value of the control signal  58  corresponds to DAC2, the laser power of the pick-up head  46  equal P2. The preferred embodiment, therefore, is capable of obtaining the first mapping function  300  quickly by a straight-line relation. With a plurality of test operations, the first mapping function  300 , similarly, can be figured out by the prior art polynomial curve fitting.  
         [0051]    After acquiring the first mapping function  300 , the CD-RW drive  40  then disables the sample/hold circuit  50  for establishing an open loop configuration between the pick-up head  46  and the power control unit  44  (step 212). A plurality of control signals with different values are sequentially inputted to the DAC  52  (step 214). The DAC  52 , therefore, sequentially receives the inputted control signals  58 , and outputs a plurality of corresponding control voltages  60  to the driving circuit  54  at the same time. The driving circuit  54  then generates control voltages  62  according to the inputted control voltages  60  for driving the pick-up head  46  (step 216). Each control signal  58  drives the pick-up head  46  to output laser beams with a predetermined power for etching the CD-RW disk. At the same time, the pick-up head  46  detects magnitude of the predetermined power (step 218). Because the pick-up head  46  detects an outputted laser power, which is an actual power of the pick-up head  46 , corresponding to each inputted control signal  58 , the aging of the pick-up head  46  and the decay effect caused by the circuits disposed in the CD-RW drive are considered. In other words, the output powers of the pick-up head and the corresponding control signals  58  are utilized to establish a second mapping function  302  that represents a relation between the control signal  58  and the output power (erase power) of the pick-up head  46  (step 220). The second mapping function  302  reveals actual behavior of the CD-RW drive  40  under the open loop configuration. As shown in FIG. 9, when the value of the control signal  58  corresponds to DAC3, the laser power of the pick-up head  46  equal P1, and when the value of the control signal  58  corresponds to DAC4, the laser power of the pick-up head  46  equal P2. The preferred embodiment, therefore, is capable of obtaining the second mapping function  300  quickly by a straight-line relation. With a plurality of test operations, the second mapping function  302 , similarly, can be figured out by the prior art polynomial curve fitting. Concerning the same laser power P1 of the pick-up head  46 , the CD-RW drive  40  with the open loop configuration requires a greater value of the control signal  58  (DAC3&gt;DAC1).  
         [0052]    As mentioned above, the erase power Pe is proportional to the write power Pw according to a predetermined ration # (Pe=#*Pw). The erase power Pe and the write power Pw are respectively used to record “l”s (lands) and “0”s (pits). It is well-known that the write power Pw can be acquired with the help of the erase power Pe. The operation is described as follows. Referring to the second mapping function  302 , the control signals having values DAC 5 , DAC6 correspond to laser powers P5, P6 (erase powers). It is obvious that the corresponding ideal write powers are P5/# and P6/# respectively (step 222). Then, the power amplifier  56  is actuated (step 224). The control signals having values DAC5, DAC6 are inputted to the DAC  52  (step 226). Therefore, the control signal having the value DAC5 makes the driving circuit  54  output the control voltage  62 , and the control voltage  62  then introduces the control voltage  66  through the power amplifier  56 . Similarly, the control signal having the value DAC6 makes the driving circuit  54  output the control voltage  62 , and the control voltage  62  then introduces the control voltage  66  through the power amplifier  56  (step 228). For the control signal having a value DAC5, the control voltage  62  is capable of driving the pick-up head  46  to generate laser power P5. The added control voltage  66 , however, will make the pick-up head  46  generate laser power P5/#. In other words, the ideal gain value of the power amplifier  56  is (1−#)/#. Because the decay effect caused by the circuits disposed in the CD-RW drive  40  deviates a required gain value from the ideal gain value (1−#)/#, the gain value of the power amplifier  56  has to be adjusted until the laser power of the pick-up head  46  is equal to P5/#. For example, the gain value of the power amplifier  56  is tuned to be G1. For the control signal having a value DAC6, the gain value of the power amplifier  56 , similarly, has to be adjusted until the laser power of the pick-up head  46  is equal to P6/#. For example, the gain value of the power amplifier  56  is tuned to be G2 (step 230). A third mapping function  304  is established according to the control signals having values DAC5, DAC6, and the corresponding gain values G1, G2. The third mapping function  304  represents a relation between the control signal  58  and the gain value required by the power amplifier  56 . With a plurality of test operations, the third mapping function  304 , similarly, can be figured out by the prior art polynomial curve fitting.  
         [0053]    It is noteworthy that the pick-up head  46  in the preferred embodiment has to detect its write power during the process of establishing the third mapping function  304 . Because the write power is greater than the erase power, the preferred embodiment utilizes control signals corresponding to small value DAC5, DAC6 to protect the pick-up head  46  from being damaged. That is, the preferred embodiment adopts small laser powers P5/#, P6/# to get the required gain values G1, G2. Therefore, the third mapping function  304  is calculated under a low power condition, and is used to get the parameters such as the gain value of the power amplifier  56  for the high power condition. For instance, when the CD-RW drives  40  needs an erase power equaling P7, the value of the control signal  58  for recording “1”s (lands) is DAC7 according to the first mapping function  300 . Similarly, when the CD-RW drive  40  wants to record “0”s (pits), the required value of the control signal  58  is equal to DAC8 according to the second mapping function  302 , and the required gain value of the power amplifier  56  for obtaining the desired write power P7/# is equal to G3. In addition, the preferred embodiment utilizes a power calibration area (PCA) on the CD-RW disk for performing above-mentioned laser power tests to obtain the mapping functions.  
         [0054]    In contrast to the prior art, the claimed power control method establishes mapping functions related to the outputted laser powers and the corresponding control signals. The mapping function stands for actual behavior of the pick-up head. Influence such as aging of the pick-up head or decay on the transmitting signals for the outputted laser powers is considered and represented by the mapping functions through the actual laser power measurement. Therefore, an actual value of the control signal for driving the pick-up head to output a predetermined power is quickly figured out through the corresponding mapping function. In addition, the claimed power control method performs power tests under a lower power condition. When the wanted mapping function is acquired, various parameters related to the high power condition are directly derived from the acquired mapping function. The claimed power control method, therefore, prevents the pick-up head from being damaged owing to power tests under a high power condition. In other words, the life span of the CD-R drive or the CD-RW drive is increased.  
         [0055]    Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.