Design of a low power semiconductor laser driving power supply

Design of a low power semiconductor laser driving power supply

With the semiconductor in communication, measurement and control, medical, integrated optics and other technical fields of wide application, its attention is also increasing, for the design of a high precision, reliable performance, economic, durable driving power has become our current most urgent problem, because of the semiconductor laser "delicate" characteristics, the use of power must be in the performance and quality of strict control.

Transfer characteristics of semiconductor lasers

At a certain temperature, when the driving current is lower than the threshold current, the output optical power P of the laser is approximately zero, and the semiconductor laser can only fluoresce. When the driving current is higher than the threshold, the laser is output, and the optical output power increases rapidly and linearly with the increase of the driving current. In practical application, two requirements must be put forward for laser diode, one is low threshold current, the other is stable P-I curve. In principle, in the case of a certain working substance, the output laser frequency of the semiconductor laser should be related to the length of the resonant cavity and the intensity of the excitation source, in other words, the output frequency of the semiconductor laser depends on the temperature of the PN junction and the size of the injection current. In addition, because the semiconductor PN junction is quite fragile, a little current impact will cause damage, so in the specific use of semiconductor laser, we have very strict requirements on its power supply circuit and modulation circuit.

We can reduce the threshold current by two orders of magnitude by replacing the homogenous junction with a heterojunction, and the stability problem can only be improved by external constant temperature and optical feedback. For general semiconductor laser, laser diode is a forward junction, photodiode is a reverse junction. The photocurrent converted by light reflects the light power in the form of voltage on the resistance. The control circuit can be added to achieve the purpose of controlling the luminous power.

Circuit design

The type of laser diode used in the test is HT670T5, which has a wavelength of 650nm and a rated power of 30mW.

The linear power supply of current source circuit has the characteristics of high precision, high stability but low efficiency. However, considering the strict requirement of semiconductor laser on the precision of power supply, we design a small selection linear power supply.

In order to meet the requirements of precision and stability, ripple suppression and noise reduction, a two-stage adjusting module is designed. The voltage regulator chip is used in the first stage, which is transmitted to the second stage adjusting module by expanding the current. The mains power enters the transformer through the power grid filter and drops to 21V (peak value). After the rectification filter (the specific circuit is omitted), it passes through the first-stage regulating circuit composed of the regulator integrated block and the current expanding circuit. After that, through the series - sampling - feedback - adjustment of the last stage, the final output.

In the design of the specific constant current source circuit, the load is not added to its output end, but to the input end of the voltage regulator LM317T. For the actual load, the input of the voltage regulator LM317T acts as a constant current source. Because the voltage regulator output terminal is connected to the false load R1, so regardless of the actual load at both ends of the true value of the voltage, it consumes a constant current. The voltage on the voltage regulator and spurious load R1 causes the total allowable voltage of the circuit to drop. The load current is set by R1, which is 1.25A/Ω x R1.

Ripple zeroing circuit In order to reduce the ripple voltage of steady current power supply, it is necessary to add ripple zeroing circuit to the circuit. In normal operation, adjusting ripple zeroing potentiometer can make the output ripple voltage very small. The ripple component is capacitantly coupled to the opamp's inverting input and amplified to the base of the regulating tube in a specific ripple zeroing circuit. Therefore, the above effect can be achieved.

In practical application, the laser is easily damaged by the surge current caused by the interference of other electrical appliances in the same circuit. In order to protect the laser from the impact of surge current, we can add a slow start circuit to the circuit. In addition, in order to better protect the laser, we can choose 2SA1015 and 2SC1815 and other types of suction tubes, in the production process of voltage source can basically protect the safe operation of the laser. Combined with the power grid filter, the circuit is made into a simple current-limiting protection circuit.

Experimental result

The driving power supply designed in this paper, through slow start, ripple zeroing and other circuits, has a good application effect in the laboratory, and better solves the problem of semiconductor diode output power instability in use. Considering the bandwidth limit of oscilloscope, the measurement results are as follows:

Current source: current ripple and noise: 0.1uA

Voltage source: Ripple: 0.01mV

Current ripple and noise: 0.5uA

Adjustment range: 0-500mA