Design of Precision Control of Laser Current Using 51 Single Chip Microcomputer

    introduction

　　In recent years, with the rapid development of optoelectronic technology, lasers have been widely used in various fields such as medicine, national defense, and measurement. The change of ambient temperature will directly affect the wavelength of the laser. Limiting the local temperature of key components (such as high-performance crystal oscillators, SAW filters, optical amplifiers, and laser diodes) within a narrow range can improve the accuracy of electronic systems. Generally, the temperature needs to be controlled within 0.1℃ so that the working accuracy of the laser can be well maintained within 0.1nm. The design scheme in this article can provide effective support for high-power semiconductor lasers, with a maximum current of 2.5A.

　　1 Design of semiconductor laser controller

　　The laser controller consists of a controlled constant current source, a temperature monitoring and control circuit, a main controller and a display. The overall structural principle is shown in Figure 1.

    1. 1 Controlled constant current source:

　　In order to make the laser output stable laser, the current flowing through the laser is very strict, and the power supply circuit must be a low-noise stable constant current source. The constant current source can be continuously adjusted from 0A to 2.5A to adapt to semiconductor lasers of different specifications. The constant current source is based on a high-power MOS tube as the core, and the laser is connected in series with it as a load. The laser current is controlled by controlling the gate of the MOS tube. However, the MOS tube is a nonlinear device and is difficult to control directly, so it must be converted into linear control.

　　As shown in Figure 2, a 0.1Ω resistor is connected in series with the MOS tube for sampling feedback. The current range of the MOS tube is 0A to 2.

5A, the voltage range     of the input control signal is 0V～5V, and the voltage of the sampling resistor is amplified by 20 times to match the input voltage. In this way, a linear correspondence is established between the control voltage 0V～5V and the current 0A～2.5A. However, since the entire feedback is an open-loop system, it is very easy to generate self-excitation. Therefore, a 1μF capacitor is connected to the sampling resistor to destroy the self-excitation conditions and eliminate self-excitation, and a stable power supply should be used to reduce voltage fluctuations.

　　1.2 Temperature detection and control circuit

　　Since temperature has a great influence on the quality of laser, when the current is constant, the laser wavelength will increase by about 0.1nm for every 1°C increase in temperature, and too high temperature will cause the laser to age or even be damaged.

　　In addition, the laser is an electrically sensitive and expensive device, so the controller must provide monitoring, limiting and overload protection capabilities.

　　Including: self-start and overcurrent protection, thermoelectric cooler ( TEC ) voltage , current and temperature sensing. Abnormal working circuit shutdown to avoid damage to laser components. It is worth noting that the impact of changes in ambient temperature on the laser requires the controller to have cooling and heating capabilities. Usually, in order to keep the temperature of the component stable, the component will be enclosed in a constant temperature bath with a fixed temperature. In order to provide a certain adjustment tolerance, the selected temperature should be higher than the ambient temperature under all conditions. This method has been widely used, especially in the design of ultra-stable clocks (such as crystal oscillators controlled by constant temperature baths). However, this method has the following disadvantages for high-temperature applications: performance (such as noise factor, speed and life) is reduced; the regulator consumes heating power when the ambient temperature is in the middle range, and requires twice as much power when the ambient temperature is at the low end; the time required to reach a stable temperature may be quite long.

　　At present, semiconductor TEC is used to achieve this because it can choose to adjust the temperature value in the middle of the operating temperature range. TEC can be used as a heat pump or as a heat source, depending on the direction of the current. Some systems (such as refrigerators and high-power processor cooling) only use the cooling characteristics of TEC. Other applications (such as crystal oscillators and SAW  filters ) use both modes of heat flow. And the controller is truly bidirectional, so that there is no dead zone between the temperature from the cold end to the hot end. The driving circuit of TEC usually adopts the "H" bridge type, which is composed of two complementary Darlington tubes or MOS tubes.

　　The H-bridge should be driven by a switch-type drive, which has low power consumption and high efficiency. For the switch-type drive, a dedicated chip such as LTC1923 can be used. The principle is shown in Figure 3.

　　DRV592 is a high-efficiency, high-power H-bridge power driver IC produced by TI (Texas Instruments). The output voltage range is from 2.8V to 5.5V, and the maximum output current is 3A. DRV592 requires an external PWM trigger (compatible with TTL logic level ), built-in overcurrent, undervoltage and overheat (130℃) protection and level indication. The industry's smallest package (9mm×9mm32-pin PowerPADTM flat package mode) has an industrial temperature range standard of -40℃ to 85℃. It is worth mentioning that the chip integrates 4 high-power MOSFETs and overload protection circuits, which simplifies the design by 80% compared with the design using discrete components (see Figure 3). And it is easy to form an accurate temperature control loop to stabilize the laser diode system by adding only a few external components. The power driver circuit of the semiconductor TEC based on DRV592 is shown in Figure 4. Compared with Figure 3, it can be seen that the design of the TEC power drive circuit based on DRV592 is greatly simplified, and DRV592 also has built-in overcurrent, undervoltage and overheating (130°C) protection level indication. The pin functions are shown in Table 1.

Since high current switching circuits will generate a lot of noise interference, in order to reduce the interference, the switching time of the switch tube can be appropriately increased to reduce the high-frequency switching noise. Although this will reduce the switching efficiency, it is worth it to use this price in exchange for a significant improvement in noise.

　　In addition, due to the thermal inertia of TEC, there will be a certain delay in changing the state, which will cause oscillation in the system. In order to eliminate the oscillation, an integrating circuit can be connected in parallel at both ends of the amplifier to increase the delay and eliminate the oscillation. It should be noted that the stable temperature is determined by the feedback of the thermistor, so the TEC and the thermistor should be packaged in one module to make them tightly coupled.

　　The accuracy of the temperature detector directly affects the effect of temperature control.

　　The temperature detection circuit is similar to the constant current source, using NTC (negative temperature coefficient ) thermistors as temperature detectors. Among them, NTC components made with ceramic powder technology have the largest resistance change for small changes in temperature. In particular, some ceramic NTCs have a stability of 0.05°C over their life (after proper aging). And compared with other temperature sensors, the size of ceramic NTCs is particularly small. Then the thermistor is connected in series to a constant current source, the voltage across the thermistor is sampled, and the temperature is converted into an electrical signal. The principle is shown in Figure 5.

　　The temperature detection circuit uses the CMOS single-power, low-power dual operational amplifier TLC2252 produced by TI. The TLC225x series has the advantages of high input impedance , micro-power consumption, and low noise, and is suitable for handheld mobile devices. The noise at 1k Hz is only 19nV, which is 1/4 of similar products.