A laser diode precision driving circuit

0 Preface

Since the advent of semiconductor lasers in the 1960s, after half a century of development, due to its series of outstanding advantages, such as small size, low price, high conversion efficiency, easy modulation, high reliability, wide radiation wavelength range, etc., it has been widely used in many fields such as communication, sensing, laser processing, and medical treatment, becoming the most widely used type of laser in the world. Compared with other types of lasers, semiconductor lasers have some of their own characteristics, such as short cavity length, low quality factor, and solid gain medium is greatly affected by changes in carrier concentration; especially near the threshold current, the transient change of injected carrier concentration will cause the fluctuation of the phase of the spontaneous emission light field; and the spontaneous emission photons generated by the incomplete population inversion in the semiconductor laser will increase the fluctuation of the field intensity, etc.; these characteristics make the laser line width of semiconductor lasers relatively wide, and the frequency is significantly affected by environmental factors such as current and temperature. These characteristics restrict its application in some precision testing fields, such as precision interferometry and metrology, high-resolution spectroscopy, etc. This also makes the research on line width narrowing and frequency stabilization technology of semiconductor lasers a hot topic.

Frequency-stable narrow-linewidth semiconductor lasers have broad application prospects in many fields such as atomic and molecular spectroscopy, laser cooling, optical communications, optical sensors, laser interferometry, laser Raman spectroscopy, gas analysis and detection, etc. To carry out frequency stabilization and linewidth narrowing of semiconductor lasers, it is necessary to first solve the problem of its precision drive. To this end, this circuit, while maintaining its small size, uses a dedicated constant current and constant temperature control chip combined with digital control technology to monitor and precisely control the injection current and operating temperature of the semiconductor laser in real time, so that the frequency stability of the semiconductor laser is greatly improved, and it is expected to be used in the future semiconductor laser linewidth narrowing and frequency stabilization technology.

1 Precision drive circuit design

From the study of working mode and characteristics, it can be known that laser diode is a device with high power density and extremely high quantum efficiency. A slight change in current will lead to a significant change in the lasing wavelength and changes in other device parameters (such as output optical power, noise performance, mode stability, etc.). At the same time, the PN junction of the laser diode is greatly affected by temperature. A slight change in temperature will not only affect the characteristics of the semiconductor laser such as the emission wavelength, output power and threshold current, but also increase the laser output noise and even affect the normal operation of the laser. At the same time, due to the large loss, part of the electric power of the laser diode will be converted into heat. If constant temperature heat dissipation measures are not taken, the life of the laser will be greatly shortened. Based on the above considerations, the precision drive circuit designed in this paper mainly includes two modules: constant current (constant power) and constant temperature. Its structure is shown in Figure 1.

In the design, in order to stabilize the working performance of the laser diode (such as working wavelength and output optical power), the driving circuit is required to accurately monitor and control the temperature and current respectively; and in order to ensure the safe operation of the entire driving circuit, the current control module is required to have functions such as overcurrent protection, surge protection, and delayed soft start. In addition, in order to set and monitor the working temperature and working current of the laser diode in real time, the circuit is also designed with a central processing module with a single-chip microcomputer as the core. This module (with AD and DA conversion functions) controls and monitors the performance of various parameters of the laser diode in real time, such as working temperature, injection current, optical power, etc. Thermoelectric coolers (TECs) are most commonly used for temperature control of small components such as laser diodes. The production of TEC is based on the Peltier effect, that is, when current flows through the interface of two different conductors, it releases heat to the outside world or absorbs heat from the outside world.

2 Circuit Component Selection

According to the above design scheme, the selection of each device and circuit design are carried out. First, the central control module. According to its power requirements, C8051F007 is selected in this circuit. This type of microcontroller is a fully integrated low-power mixed-signal system-on-chip MCU. At the same time, C8051F007 is relatively low-priced among the current microcontrollers. In terms of its clock frequency and expansion performance, it is better than ordinary microcontrollers and can fully meet the requirements of applications such as drive circuit data acquisition and system control. More importantly, C8051F007 has the following important interfaces: 12-bit multi-channel ADC, programmable gain amplifier and 2 12-bit DACs.

The constant current control module in the circuit uses ATLS100MA103 from Anshan Core Electronics Co., Ltd. This chip is an electronic chip designed for driving laser diodes. It is small in size, heat sink-free, and has the characteristics of ultra-low noise (<2 μA), large current (100mA), high precision (<0.1%), high stability (<100ppm/℃), and full shielding. The chip contains a current limiter, a temperature sensor, a shutdown and soft-start circuit, a current sensor, and a low-noise driver, and has a soft-start function. It meets the circuit's requirements for laser diode current control.

The constant temperature control module in the circuit is also a product of Anshan Core Electronics Co., Ltd., model TECA1-5V-5V-D. This chip is a small electronic chip designed specifically for driving TEC, with the characteristics of small size and zero electromagnetic interference. The temperature control circuit of the TECA1-5V-5V-D controller is a temperature control technology based on the principle of closed-loop negative feedback. It reduces the deviation between the output value and the set value through negative feedback, thereby achieving the purpose of real-time and precise control of the temperature. Based on the TECA1-5V-5V-D chip, this paper designs a temperature control circuit. The circuit is powered by +5V, with a theoretical efficiency of ≥90%, a maximum output current of 2.5A, and a temperature stability that is better than 0.01℃ or even 0.001℃. [page]

In order to meet the requirements of the circuit in data acquisition and control, the single-chip computer C8051F007 is used as the microprocessor chip for data acquisition and control of the driving circuit. The program can be downloaded through the connector to the downloader to control the operation of the single-chip computer. The AD and DA conversion interfaces are respectively connected to the current control chip and the temperature control chip to set and monitor the output current and operating temperature. The single-chip computer can send the measured laser diode parameters to the computer through the serial port for reception and display.

3 Circuit Testing and Result Analysis

After the circuit design and production is completed, experimental tests are carried out to verify its working performance. The laser diode selected in the experiment is the GaAlAs red laser diode DL-

3148-025, center wavelength 635nm, rated output power 5mW, threshold current 20mA. Combined with the heat conduction mode of the laser diode, this paper designs a temperature control device, which consists of a copper heat sink, an aluminum heat sink, thermal conductive silicone, TEC and a thermistor. As shown in Figure 3, the laser diode is fixed on the heat sink through insulating thermal conductive silicone. The thermistor attached to the heat sink feeds the temperature back to the temperature control chip. The temperature control chip controls the TEC to cool the heat sink and indirectly achieves temperature control of the laser through heat dissipation. The divergence angle of the output laser can be changed by adjusting the collimating lens installed in front of the laser diode. The entire temperature control device is installed on the optical adjustment frame and placed on a stable optical platform. The wires of the laser diode, the thermistor and the TEC are connected to the drive circuit. Since the control accuracy of current and temperature is ultimately reflected in the frequency (or wavelength) stability of the laser diode, after completing the circuit production and setting the relevant parameters, the circuit can be used to drive the laser diode to test its frequency stability.

In the experiment, the wavelength stability of the laser diode was measured using a WA-1500 wavelength meter produced by Canada's EXFO Burleigh Company, with an absolute accuracy of ±0.2×10-3nm. The laser diode was turned on for preheating, and after it stabilized, the laser was coupled to the wavelength meter. At this time, the laser diode outputted a single-mode laser, and its central wavelength was measured. When the laser operates in a constant temperature and constant current mode, its laser frequency stability depends on the performance of the drive circuit, so the performance of the drive circuit can be inverted by measuring the stability of the laser frequency. The wavelength of the output laser was measured continuously for 500s at a frequency of 1Hz, and the experimental results are shown in Figure 4.

Figure 4 (a) shows the measured data and the fitting curve, and Figure (b) shows the residual curve obtained by data processing. After analyzing the measured data, it can be seen that within 500s, the average value of the laser wavelength measurement is 637.4nm, the mean square error is 2.4×10-4nm, and the drift of the laser wavelength is 7.2×10-4nm according to the fitting curve. The wavelength-current coefficient of the semiconductor laser with a central wavelength of 635nm from SANYO used in the experiment is 0.01~0.02nm/mA, and the wavelength-temperature coefficient is 0.2nm/℃. The current stability of the circuit is calculated to be 1.2~2.4×10-2mm, and the temperature stability is 1.2×10-3℃.

4 Conclusion

This paper introduces the laser diode digital drive circuit designed and manufactured in detail. This circuit uses the single-chip microcomputer C8051F007 as the control core, combined with the laser diode dedicated constant current control chip ATLS100MA103 and the temperature control chip TECA1-5V-5V-D, which can achieve high-precision control of the laser diode injection current and operating temperature. The actual test of the 635nm laser diode shows that the laser diode output laser wavelength stability driven by this circuit reaches the order of 10-4nm, which is converted into the stability of current and temperature of 1.2~2.4×10-2mA and 1.2×10-3℃ respectively. This circuit can be used for the study of semiconductor laser frequency stabilization and line width narrowing, and can also be used in cavity ring-down measurement systems, etc.