A design scheme for a high-power semiconductor laser controller

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°C 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 up to 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.


Figure 1 Laser controller functional module Figure

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 adjustable 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~2.


Figure 2 Electronic Design

Mitsubishi FX Series PLC - Mitsubishi PLC Programming Manual (FX1S, FX1N, FX2N, FX2NC Series) 5A, and the voltage range of the input control signal is 0V~5V. 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 the laser, when the current is constant, the laser wavelength will increase by about 0.1nm for every 1℃ increase in temperature, and excessive temperature will cause the laser to age or even damage.

In addition, the laser is a highly 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 the laser component. 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 to be 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 two 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 an "H" bridge type, which is composed of two complementary Darlington tubes or MOS tubes.

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


Figure 3 TEC power drive circuit

DRV592 is a high-efficiency, high-power H-bridge power drive integrated block produced by TI (Texas Instruments). The output voltage range is from 2.8 V to 5.5 V, and the maximum output current is 3A. DRV592 requires an external PWM trigger (compatible with TTL logic level), built-in overcurrent, undervoltage and overheating (130℃) protection and level indication. The industry's smallest package (9mm×9 mm 32-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 discrete component design (see Figure 3). And just adding a few external components can easily form an accurate temperature control loop to stabilize the laser diode system. The power drive 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.


Figure 4 Typical TEC power drive



Since the high-current switching circuit 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 to exchange for a significant improvement in noise.

In addition, since TEC has thermal inertia, there will be a certain delay in changing the state, which will cause oscillation in the system. In order to eliminate oscillation, an integration 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 a 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, and an NTC (negative temperature coefficient) thermistor is used as the temperature detector. Among them, the NTC element made by ceramic powder technology has the largest resistance change for small changes in temperature. In particular, some ceramic NTCs have a stability of 0.05℃ during 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.


Figure 5 Temperature detection circuit

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

1.3 Main control and display part

The controller is based on the AT89C51 microcontroller. It directly controls the drive current and temperature of the laser, and can directly and accurately reflect the current temperature, current size, preset current and preset temperature of the system, and it is more convenient and accurate for instrument operation.

The entire microcontroller control part process is shown in Figure 6, and the program flow chart is shown in Figure 7.


Figure 7 Program flow chart

The control voltage of the constant current source is 0V~5V. If the input end is controlled by an 8-bit D/A, the resolution is 2.5A×1/2e8=0.01A. If a 12-bit D/A is used, it can be accurate to the milliampere level. The resistance of thermistor is nonlinear with temperature, which is roughly in the form of e exponential. Therefore, in the high temperature part, the resolution of temperature will be reduced, so the A/D converter should be above 12 bits to achieve better results. And organize a table of the relationship between thermistor temperature and voltage in the ROM of the single-chip microcomputer, and realize the temperature conversion and H-bridge temperature control after sampling the thermistor by looking up the table.

In addition, some devices such as D/A and A/D need to use -5V voltage as a reference. The 555 chip can be used as a square wave pulse generator, and its DC component can be filtered out. Then the forward voltage is short-circuited with a diode, and the negative voltage left is smoothed to obtain -5V voltage.

2 Summary

In recent years, with the rapid development of optoelectronic technology, lasers have been widely used in various fields such as medical treatment, national defense, and measurement. The laser controller designed in this scheme has the advantages of strong adaptability, large output current range, high temperature control accuracy, simple and intuitive operation, etc. It is a more feasible laser controller scheme.