Design of a portable multifunctional SLD digital measurement and control system

0 Introduction
    As the core device of the fiber optic gyroscope system, the working characteristics of SLD will affect the performance and reliability of the entire system. Therefore, it is of great practical significance to study how to quickly and accurately measure the characteristic parameters of SLD to complete the evaluation and screening of device performance. The existing characteristic test systems are mostly composed of discrete devices, which are large in size and expensive. They do not have the portability required for field testing, and the working mode is single.
    In view of the above problems, this paper proposes a design scheme for a portable SLD measurement and control system, briefly describes its overall design, and focuses on discussing the key technologies in the system implementation. Then, the actual system is tested for performance, and the stability of the injected current, optical power and temperature is tested respectively. Finally, the characteristic test results of the actual SLD device are given.

1 System working principle and design scheme
    The overall design of the system is shown in Figure 1. The system mainly uses the embedded microcontroller C8051F060 as the control core. The two 16-bit ADC modules, two 12-bit DAC modules and one 8-bit ADC module integrated inside it constitute a basic on-chip data control and acquisition system, which makes it possible to design a portable SLD measurement and control system with small size, low power consumption and high reliability, and also greatly reduces the cost. The whole system is mainly composed of a drive module, a temperature control module, a parameter detection module and a human-machine interface module. The drive module provides three drive modes for the device: constant current drive, constant power drive and LIV test; the temperature control module keeps the device working temperature stable by adjusting the current size and direction of the thermoelectric cooler; the parameter acquisition module detects the device's drive current, tube voltage drop, optical power, temperature control voltage and other data, and sends them to the ADC module of the microcontroller for preprocessing, which is displayed in real time by the LCD; at the same time, the system's working mode and parameter size can be set through the keyboard. If it is a LIV test, the above data can be remotely controlled by communicating with the computer through the serial port.

2. Design of driving module
    The driving module mainly consists of three parts: driving circuit, protection circuit and preamplifier circuit. The module can provide three working modes, namely constant current driving mode, constant light power driving mode and LIV test mode.
2.1 Constant current driving Constant
    current driving is a control mode for constant control of the injection current of SLD. It is essentially a voltage-controlled current source using current series negative feedback. Its schematic diagram is shown in Figure 2.
    A jitter-free voltage is set by the DAC0 of the microcontroller. This voltage is applied to the reverse input of the operational amplifier. The operational amplifier and the transistor form a VI converter, thereby obtaining the corresponding output current. The output current flows through the sampling resistor R to obtain the sampling voltage. The sampling voltage is amplified and fed back to the positive input of the operational amplifier. By comparing with the set voltage, the output current is controlled, thereby forming a dynamic balance of closed-loop feedback to make the output current constant. According to the virtual short-open principle, the output current value is the ratio of the voltage setting value to the sampling resistor value, that is:
    I = VDAC0 / R (1)
2.2 Constant optical power drive
    Constant power drive is a control method for the output optical power of the SLD to be constantly controlled. Figure 3 shows the schematic diagram of the constant power drive circuit. The output optical power of the device is monitored by an internally integrated photodetector (PD), whose splitting ratio is 5%. The sampled optical signal is converted into an electrical signal, and the monitored photocurrent signal is amplified by the preamplifier circuit. The amplified signal is transmitted to the monolithic 16-bit ADC0 module for analog/digital conversion. The converted digital quantity is compared with the set digital quantity, the deviation is compensated, and the set voltage value added to the constant current circuit is adjusted, thereby adjusting the injection current of the SLD. The entire control process forms a closed-loop dynamic balance, so that the output optical power is constant.

2.3 LIV test
    The LIV test is to test the optical power L output by the SLD and the forward voltage V across the SLD while changing the SLD injection current I under remote control. The collected data is displayed as the LIV characteristic identification curve, including the VI curve representing the reactance characteristic and the LI curve representing the photoelectric conversion characteristic. In the LIV test mode, after the test parameters are set by the remote computer, the driving module generates a step-by-step driving current. The parameter detection module automatically records the tube voltage drop V, driving current I, and optical power L at each step point, and draws the LIV curve. These data and curves can be used to analyze the characteristics of the SLD, such as external quantum efficiency, threshold current, etc.
2.4 Protection circuit
    SLD is an expensive semiconductor device, and most of its damage is caused by static electricity and surge breakdown. In order to eliminate the damage to the device caused by these electrical shocks and extend the service life of the device, an electrostatic protection circuit and a limiting circuit are designed.
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2.4.1 Electrostatic protection circuit
    A relay with very small contact resistance is connected in parallel with SLD to form a short-circuit protection switch. When SLD is not working, the switch is closed. In this way, when it is not turned on, the electrodes at both ends of SLD are short-circuited, thereby realizing the function of preventing electrostatic breakdown. In practical applications, in addition to the normally closed switch, a diode is connected in parallel between the two electrodes of SLD to prevent the device from being damaged due to reverse polarity connection; a capacitor is also connected between the anode and cathode of SLD. This capacitor can not only limit the voltage mutation at both ends of SLD, but also filter out the high-frequency interference current on the SLD drive current.
2.4.2 Limiting circuit
    Each SLD has a safe operating current range. If the current exceeds this range, SLD will be damaged. Therefore, the operating voltage of SLD must be limited to a given range. The design of the limiting circuit is based on the comparator principle. Figure 4 shows the designed limiting protection circuit, which is mainly composed of an integrated operational amplifier UA, an instrumentation amplifier UB and a diode D1.

    When working, the input voltage UI acts on the driving circuit, and the current flowing through SLD is converted into IV by the sampling resistor and then sampled by UB to obtain the voltage difference Usample of the sampling resistor. From formula (1), it can be seen that U1=Usample, which is compared with the upper limit value of the limiting circuit. When the input voltage UI value is less than the set value USET, UA outputs UO1>0, so D1 is cut off, and UI acts directly on the driving circuit; when the input voltage value UI is greater than the set value USET, UA outputs UO1<0, so D1 is turned on, and the input voltage UI is pulled down to USET, thereby realizing the function of limiting protection.

    USET is the upper voltage limit of the protection circuit. The voltage transfer characteristics of the limiting circuit are shown in Figure 5.

3. Design of temperature control module
    The change of ambient temperature will affect the working stability of SLD. In order to stabilize the output power of SLD, its working temperature must be kept constant. The core device of temperature control adopts ADN8830 integrated temperature controller. The chip is small in size and low in noise. It can provide bidirectional temperature control current to independently drive TEC. The long-term control accuracy can reach ±0.01℃, and it has current limiting protection and temperature setting functions. The actuator of temperature control is the semiconductor cooler integrated in SLD, also known as thermoelectric cooler (TEC), and the temperature sensor is the thermistor (NTC) integrated in it. The principle of temperature control module is shown in Figure 6.

    The temperature control voltage generated by the microcontroller's DAC1 is connected to the temperature setting terminal of the ADN8830 to set the device's operating temperature. The heating and cooling of the TEC is controlled by providing a bidirectional drive current to the TEC. When the SLD die temperature changes, the NTC senses the temperature change and converts the temperature change into an electrical signal, which is returned to the ADN8830 through the feedback network. The ADN8830 adjusts the direction and size of the input TEC electric filter through its own internal compensation network to form a closed-loop control process, thereby achieving constant temperature.
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4 Parameter Detection Module Design
    This module uses a high-precision instrumentation amplifier to measure the tube voltage drop, drive current, optical power, and temperature control voltage of the SLD in real time. The principle of the parameter detection module is shown in Figure 7.

    A high-precision instrument amplifier AD620 is selected to extract relevant parameters. Its amplification factor is G=1+(50 kΩ/RG), and the output voltage Vo=(V+-V-)×G. Generally, RG is disconnected. At this time, the output voltage of the op amp is the voltage difference between the positive input terminal and the reverse input terminal of the op amp. The signal output by AD620 is sent to the 8-channel 10-bit ADC2 module of the microcontroller for analog-to-digital conversion, data calculation and processing.
    Since the resistance value of the sampling resistor is fixed, the driving current value can be obtained by taking the quotient of the sampling resistor voltage drop and the sampling resistor resistance value.
    The voltage across the thermistor is:
   
    where VE is the power supply voltage of the thermistor bridge circuit; VE=1.25 V, and RT is the resistance value of the thermistor. It can be seen from formula (2) that the voltage drop of the NTC can be converted to the resistance value of the thermistor. It is also known that 1°C corresponds to a 500 Ω change in the thermistor, so the change in temperature can be determined by sampling the change in the voltage drop of the thermistor.
    In the optical power detection part, it is known that the input splitting ratio of the optical splitter is 5%, and the optical power and the photocurrent are in a linear relationship. The photocurrent signal is amplified and converted into a photovoltage signal in the preamplifier circuit, and the optical power and the photovoltage are in a linear relationship. The photovoltage value is sent to the 16-bit ADC0 module of the microcontroller for analog/digital conversion, so that the output optical power can be detected.
    After the measured parameters are processed by the microcontroller, they can be displayed in real time through the liquid crystal display (LCD), which is convenient for users to read the real-time working parameters. The measured data can also be sent to the computer through the serial port for storage and further analysis.

5 System Performance Test and Analysis
    To evaluate the performance of the system, the system was used to conduct 2 h constant current stability test, constant power stability test, temperature control stability test and LIV test on the SLD devices produced by the 44th Institute of Electronics. Stability is defined as the ratio of the change in output to the average value of the output, that is, stability = (maximum value - minimum value) / average value.
5.1 Drive current stability test
    At room temperature, the drive current was continuously measured for 2 h in constant current mode, with one point sampled every 2 min, and the drive current was set to 130 mA. The measurement data is shown in Figure 8.

    From the above figure, it can be seen that the maximum driving current is 130.71 mA, the minimum is 130.68 mA, and the average is 130.69 mA. Calculate its stability: stability = (130.71-130.68)/130.69 = 0.023%.
5.2 Output power stability test
    At room temperature, the output optical power is continuously measured for 2 hours using the constant optical power mode, with one point sampled every 2 minutes, and the output optical power is set to 445μW. The measurement data is shown in Figure 9.

    It can be concluded from the above figure that the maximum output optical power is 445.344μW, the minimum optical power is 445.222μW, and the average is 445.292μW. Calculate its stability: stability = (445.344 - 445.222) / 445.292 = 0.026%.
5.3 Temperature control stability test
    Since it is not possible to directly measure the internal temperature of the SLD component, the temperature control stability is indirectly evaluated by measuring the voltage across the thermistor. The test was carried out for 2 h at room temperature, where the control temperature was set at 25℃. The measurement data is shown in Figure 10.

    As can be seen from the figure above, the maximum value of the NTC voltage drop is 740.5 mV, the minimum value is 740.16 mV, and the change is 0.34 mV. From formula (1), it can be calculated that the change of the thermistor is 16Ω. According to the relationship between the resistance value of the thermistor and the temperature, the maximum temperature change can be calculated to be 0.03℃.
5.4 LIV characteristic test The
    LIV characteristic test of the device was also carried out using this system, where the step current is 5 mA and the driving range is 0-130 mA. The measured data is shown in Figure 11.
    After fitting the LI curve, the relationship between the optical power and the driving current is L=-250.040 57+5.453 51I. From this, the threshold current can be calculated to be 46 mA, and the external quantum efficiency is η=
dP/dI=5.45μW/mA.

6 Conclusion
    Based on the embedded microcontroller C8051F060, a SLD digital measurement and control system is designed. The system has high integration, small size, simple operation, and high performance while being portable. The system can not only provide multi-functional drive for SLD devices, including constant current drive, constant power drive and constant temperature control; it can also be used as a LIV test system to test and characterize the device.
    The test results show that the system has good performance, among which the long-term stability of constant current drive and constant power drive reaches 10-4 level, and the temperature control deviation is 0.03℃. At the same time, the system is also suitable for the drive and measurement of semiconductor light sources such as semiconductor lasers and LEDs.