Design of Laser Diffraction Grating Tester Based on SPCE061A

O Introduction
Laser diffraction grating is a key part of laser read/write heads in computer optical drives and consumer electronics. At present, instruments specifically used to measure laser diffraction grating parameters are relatively rare. The laser wavelength used in DVD read/write heads is usually 650 nm. This design uses a 650 nm laser diode LD (Laser Diode) as the diffraction grating source, combined with a high-performance 16-bit SPCE061A microcontroller, to design a laser diffraction grating tester, which mainly tests the brightness of the 0th and 1st order spots and the grating transmittance of the diffraction grating to improve the accuracy and efficiency of the diffraction grating quality test.

1 Working principle of the tester
The working principle block diagram of the laser diffraction grating tester is shown in Figure 1. The hardware of the tester consists of four parts: the driving circuit of the laser diode, the small signal sampling and amplification circuit, the LED display circuit and the minimum peripheral circuit of the single-chip microcomputer. The system display part uses two 4-bit LEDs, adopts a dynamic scanning method, and uses one LED each to display the grating spot brightness ratio and transmittance.

Test principle: The laser diode drive circuit provides a stable and appropriate voltage and current for the laser diode to drive the laser diode to emit light normally. The photodetector converts the light spot after diffraction by the grating into a weak current, which is sampled and amplified by the current circuit to obtain a voltage that meets the A/D sampling range. The voltage is converted by A/D to obtain the corresponding digital signal, which is processed to calculate the brightness ratio and transmittance of the light spot, and finally displayed by the LED. At the same time, control buttons such as restart, clear, start/hold, and alarm range setting are set to cooperate with the test.

2 System Hardware Design
2.1 Minimum Peripheral Circuit of SPCE061A MCU
The tester MCU uses a high-performance 16-bit SPCE061A MCU, which has a 7-channel 10-bit voltage analog/digital converter (ADC) and 32 programmable I/O ports, and can be debugged online through the built-in online simulation circuit ICE (In-Circuit Emulator) interface. The development of SPCE061A is realized through the online debugger PROBE. It is both a programmer (i.e., a program burner) and a real-time online debugger. It can replace the commonly used software tools in the development process of single-chip microcomputer application projects - hardware online real-time emulators and program burners. The 10-bit single-ended ADC converts the linear voltage between GND and VREF into 2n different digital quantities, that is, 1 LSB=VREF/2n. In this system, 1 LSB=2.5 V/1 024=2.4 mV. For example: Assuming that the current coming out of the photodetector is 0.100 mA, and after sampling with a 10 kΩ resistor, a voltage of 1 V is obtained, the A/D conversion accuracy is 2.4/1×100%=0.24%. Considering the current-voltage conversion error, AD sampling error, etc., the final accuracy can reach 2% after testing, which can basically meet the test accuracy.
2.2 Laser diode drive circuit
The laser diode model selected in the design is SDL6161RL from Sony Corporation of Japan, and the operating parameters are: output wavelength is 650 nm, operating voltage is DC 5 V, threshold current is 55 mA, operating current is 65 mA, and exit pupil power is 7 mw. The laser diode drive circuit is shown in Figure 2.

In Figure 2, the voltage regulator uses TL431, which generates 2.5 V voltage, and then passes through the voltage follower AR1 and the voltage regulator AR2 to obtain a stable voltage VREF, which is sent to the positive input terminal of AR3. The current signal from AR3 is amplified by the Darlington tube to generate a sufficiently large driving current to meet the working current requirements of the laser diode. In the figure, R2=R3, and AR3 works in a deep negative feedback state. From the concept of virtual short, it can be seen that the voltages of the positive input terminal and the negative input terminal of AR3 are equal, that is:

Since the first stage of the Darlington tube works in saturation,

Therefore, as long as appropriate R6, R7, R10 and R11 are given, the required output voltage and current can be obtained.
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2.3 Small signal sampling and amplification circuit
The system has two sampling channels, one for sampling the signal of the 0-level spot and the laser directly irradiating the detector when no grating is added, and the other for sampling the signal after the photoelectric conversion of the 1-level spot. The weak current signal of 0.01-0.5 mA comes out from the photodetector, while the ADC conversion range of the microcontroller is 0-2.5 V. In order to improve the accuracy of A/D conversion, the current signal must be converted from current to voltage and the signal must be amplified.
Generally speaking, the conversion error of the ADC converter is the smallest around the middle value within the range. Therefore, all current signals are converted into voltage signals of about 1 V during design. The parameters that need to be tested in this system are the spot brightness ratio and the grating transmittance, so it is necessary to measure the detector current before the grating is added and the current generated by the two bright spots after the grating is added. The current before adding the grating is about 0.4-0.5 mA, the current of the 0th order spot is about 0.1-0.2 mA, and the current of the 1st order spot is about 0.01-0.05 mA, so their magnifications will be very different. The light detector for measuring the 0th order spot and the laser without the grating is the same. In order to make the voltage generated by both of them about 1 V, a single-pole double-throw switch is designed to achieve different magnifications.

3 Software Design of Tester
The main program flow chart of the tester is shown in Figure 3. It includes A/D conversion subroutine, LED display subroutine, and external key interrupt subroutine.

The voltage signals converted from the 0th and 1st level spots are sampled through two A/D channels. In order to improve the stability and reliability of the sampled data, digital filtering is used. Each channel of data is sampled 150 times, and the average value is taken after removing the maximum and minimum values. For each qualified grating, the values ​​of the spot brightness ratio and transmittance are within a certain range. If it exceeds this range, it means that the grating parameters are unqualified. The instrument will display U or inverted U to indicate whether the parameter is overflow or underflow, which is convenient for users.

4 Test results and analysis
For 10 grating samples, the data of the brightness ratio of level 1 and level 0 are shown in Table 1. Five sets of data were measured for each identical sample. The reference value is the standard sample value provided by the grating manufacturer.

In formula (5) and formula (6), δ is the relative error; D is the mean square error; Ri is the measured value; R is the average value; N is the number of items. The test results show that the test error of the tester is less than 2%, and the mean square error is less than 2.2×10-3, which fully meets the design requirements.

5 Conclusion
This design uses the high-performance 16-bit SPCE061A single-chip microcomputer to realize the laser diode diffraction grating tester for DVD and other devices, which is very suitable for testing small batches of diffraction gratings. After actual test verification, it has high stability and accuracy. It has an overflow alarm function. At the same time, the instrument has low cost, small size, high detection efficiency, and high promotion value.