Power calibration system

The present invention provides a high precision, low cost laser power calibration system. First, the photo signals output from a standard photo diode are received by a photo calibration system; then a set of standards consisting of the corresponding current signals and the power signals are saved in an EEPROM. Finally, the photo diode to be calibrated can be calibrated using the present system in accordance with the calibration standards.

This application claims the benefit of Taiwan application Ser. No. 091109098, filed May 1, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a power calibration system, and more particularly to a high precision, low cost power calibration system.

2. Description of the Related Art

As requirements for high volume storage mediums become greater, compact disks (CD) play a more important role. During the manufacturing process pits are created on the CD by the optical pickup unit (OPU) of the CD recorder when it emits a light beam on the dye layer; however, lands are formed on the CD when no light beam is emitted thereon. The pit has a lower reflectivity than the land and the pit and the land represent the information of 0 and 1 respectively.

However, pits produced by laser beams of different output power levels from different CD recorders are usually shaped differently, which will cause difficulty in the process of reading information. This is a result of the variation in the assembly of the OPU and inconsistencies in the photo diode properties. Therefore, the CD recorder has to have its laser power calibrated prior to the fab-out stage so that the OPU can provide laser beams of the correct power.

Traditionally, laser power is measured by a power meter and a sensor probe, which together cost about more than one thousands of US dollars. If each service center around the world must be equipped with such an apparatus, it increases manufacturing costs a great deal.

SUMMARY OF THE INVENTION

It is therefore an objective of the invention to provide a low cost, high precision power calibration system.

The power calibration system of the present invention comprises a computer, a laser diode, a photo detector, an current to voltage converter, an electrically erasable programmable read only memory (EEPROM), an A/D converter, an interface, and a microprocessor.

The photo detector is used to receive a laser beam from the laser diode and outputs a current signal to the current to voltage converter, which receives the current signal and outputs a voltage signal. The A/D converter receives the voltage signal and outputs a power signal input to the computer through the interface, and the interface is used for adjusting the communication protocol between the computer and the A/D converter. The microprocessor is used for controlling the transmission between the interface and the A/D converter.

First, a standard laser diode capable of emitting predetermined signal laser beams is used in the power calibration system first. The computer records this as a standard relation in the EEPROM, wherein the standard relation corresponds to the respective levels of the standard laser signals, standard current signals, and the standard power signals.

Then, in second stage, the computer reads the standard relation from EEPROM and the standard laser diode is replaced with a new laser diode which need to be calibrated. The computer commands the laser diode to be calibrated to emit a first laser signal, which will correspond to a first current signal and a first power signal. The computer compares the first current signal with the standard relation, and the first current signal is smaller than a second current signal and larger than a third current signal respectively. The second current signal and the third current signal respectively correspond to a second power signal and a third power signal. By using an interpolation method, the computer evaluates the corrected first power signal with the first current signal, the second current signal, the third current signal, the second power signal, and the third power signal.

Finally, the first laser signal is adjusted to fall within an acceptable range, and the corresponding corrected power signals are evaluated to complete the calibration process.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a high precision, low cost power calibration system. The power calibration system has a first module110, a second module120, and a computer130. The first module110is used for receiving the light beam output from the photo diode200. The second module120is used for digitizing the signal from the first module110, so that the user can easily measure the power of the light beam.

The first module110is positioned in a jig and consists of a photo detector101, an current to voltage converter102, and an electrically erasable programmable read only memory (EEPROM)103. The second module120consists of an A/D converter121, an interface122, and a microprocessor123, wherein the interface122can be a RS232 interface.

The light beam, output from the photo diode200, is received by the photo detector101, which converts the photo signal of the light beam to a current signal. The current to voltage converter102receives the current signal from the photo detector101, and converts the current signal to a voltage signal. The photo diode200can be a photo diode to be calibrated or a standard photo diode (golden sample), which can emit the light beam with correct power.

The analog voltage signal from the current to voltage converter102is sampled and converted to a digital signal by the A/D converter121. The digital signal is usually represented as a unit of power, for instance mW. The digital signal is input to the computer130by interface122. Interface122is used for adjusting the transmitting protocol between the computer130and the A/D converter121. The microprocessor123is used for controlling the transmission of the AD converter121and the interface122.

First, the characteristic of the photo detector101is recorded in EEPROM103by using a standard photo diode200, a golden sample photo diode, which emits a light beam with the accurate power, in the range from 0 to 50 mW. The procedure as following: The computer130instructs the standard photo diode200to emit light beams with different power levels, for example, a range of from 0 to 50 mW and with a 0.2 mW step. Therefore, these power levels, respectively, 0 mW, 0.2 mW, 0.4 mW, 0.6 mW . . . , 49.8 mW, and 50.0 mW. In the same time, photo detector101receives the light beam and generates a current whose amplitude related to the power levels instructed by the computer30. Each power level which instructed by computer130corresponds to a current level which comes from the current to voltage converter102. The computer130collects the standard relation of power level and current level, the characteristic of the photo detector101, and saves them in the EEPROM103.

The ideal power step is determined by the trade-off between precision and cost of memory. If a 0.05 mW step is selected so as to achieve a more precise power calibration, the volume of memory needed by EEPROM103to record the corresponding values as well as calibration cost would be increased. Hence, a 0.05 mW step is too precise to be practical. If a larger step, for instance 5 mW, is selected, precision will be adversely affected, especially in the low power range where there will be a decrease in linearity. If this step size were used, the ±5% precision requirement may not be achieved. Therefore, the calibration step is optimized when conducted between 0.2 mW to 0.6 mW to achieve the precision requirement of 5%.

After the standard relation is recorded in EEPROM103, the standard photo diode200is replaced with a photo diode to be calibrated. The computer130reads the standard relation from EEPROM103and instructs the photo diode to be calibrated to progressively emit light beams of increasing power. The photo detector101receives the light beam from the photo diode to be calibrated and produces a first current. The first current is received by current to voltage converter102, and converted to the first voltage. The level of the first voltage is digitalized and output from the interface122to the computer130. The level of the first voltage also represents the power level of the light beam. For each current level, the computer130identifies two sets of measurements from the standard relation, where the standard current is closest to the first current. Then, a corrected power, corresponding to the first current, is evaluated by an interpolation method. Afterwards, the computer130commands the photo diode to be calibrated to emit the next light beam with a larger power. Upon repeating the steps described above, the calibration procedure for calibrating the photo diode will be completed.

For example, the computer130commands the photo diode to be calibrated to emit a light beam of ideal power PtmW, such as 0.2 mW, and the corresponding real current measured by the power calibration system is itmA. Within the standard relation, two sets of current signals i1and i2are found, wherein i1and i2are closest in value to it, and obey the relation of i1<it<i2. Using an interpolation method, a corrected power Pt′ is evaluated according to the following formula and is recorded in the EEPROM103by computer130.Pt′=P1-P2i1-i2·it+(P1-P1-P2i1-i2·i1)

Then, the value of ideal power PtmW is changed to 0.4 mW, 0.6 mW . . . sequentially, and the non-standard relation between the real current and the corresponding corrected powers Pt′ is also recorded in the computer130. When a CD is being recorded, the photo diode emits light beams according to the corrected powers Pt′.

Therefore, the power calibration system of the present invention only needs to equip an EEPROM103, which has recorded a set of standard relation from a standard photo diode. When a photo diode to be calibrated is applied in this power calibration system, a non-standard relation, recording the relation of the ideal power Pt, real current itmA, and the corrected power Pt′, is recorded in the EEPROM by the computer130. Even though a photo detector101with poor linearity is used, the photo diode to be calibrated can be calibrated within the precision requirement of 5% in the present invention.

FIG. 2shows the power calibration method of the present invention. The procedures from steps11to13illustrate the application of a standard photo diode in the present power calibration system, wherein a characteristic curve of the photo detector101is recorded in the EEPROM103. The procedures from steps21to23illustrate the procedure of applying a photo diode to be calibrated in the present power calibration system.

First, in step11, a standard photo diode is used in the power calibration system. Next, in step12, the computer130instructs the standard photo diode to progressively emit a light beam with correct power. The light beam is received by the photo detector101to produce a standard current. In step13, the current to voltage converter102outputs a standard voltage, and the interface122outputs a digitalized standard power level, wherein EEPROM103records the reference standards including standard current and standard power.

Then, in step21, the standard photo diode is replaced by a photo diode to be calibrated. In step22, the computer130reads the reference standards and commands the photo diode being calibrated to progressively emit light beams of increasing power levels. A corrected power is evaluated by the interpolation method. In step23, the computer130records the non-standard relation in EEPROM103to complete the embodiment of the present invention.

The power calibration system and method thereof can be applied in any case calibration process is needed, such as for a laser diode. Referring toFIG. 3, it shows the power calibration system used for the laser diode200of the optical recording drive20, wherein the laser diode200is positioned within the optical recording drive20. The optical recording drive20comprises a CD plate21, which can move in and out of the optical recording drive20. The first module110is positioned upon the laser diode200in order to receive the laser beam from laser diode200. The second module120is coupled to the first module110and the computer130, and the computer130is coupled to the first module110and the optical recording drive20.

From the above description, the power calibration system can calibrate a photo diode with acceptable precision even though some optical elements may have poor characters, such as a photo detector with poor linearity. It also decreases calibration costs without decreasing calibration precision.