Semiconductor laser diode driver

Semiconductor laser diode, which can directly convert electrical energy into light energy, has the characteristics of high brightness, high efficiency, long life, small size, and direct modulation.

The difference between semiconductor laser diode LD and ordinary light-emitting diode LED is that LD emits light by stimulated emission recombination, and the photons emitted are in the same direction and in the same phase; while LED uses spontaneous emission recombination of carriers injected into the active area to emit photons. The direction and phase are random.

So essentially the laser diode LD is driven by current just like the ordinary light-emitting diode, but the laser diode requires a larger current.

Low-power laser diodes can be used as light sources (seed sources, optical modules), and commonly used packages include TO56, butterfly packages, etc.

High-power laser diodes can be used directly as lasers or as pump sources for amplifiers.

Laser diode LD driver instructions:

1. Constant current drive: Due to the volt-ampere characteristics of the diode, the conduction voltage at both ends is relatively less affected by changes in current, so it is not suitable for voltage sources to drive laser diodes. DC constant current is required to drive laser diodes. When used as a light source, the driving current is generally ≤500mA. When used as a pump source, the driving current is usually about 10A.

2. ATC control (automatic temperature control): The threshold current of the light source, especially the laser, will change with changes in temperature, which will cause the output optical power to change. ATC acts directly on the light source, making the output optical power of the light source stable and not affected by sudden changes in temperature. At the same time, the wavelength spectrum characteristics of laser diodes are also affected by temperature. The wavelength spectrum temperature coefficient of FP laser diodes is usually 0.35nm/℃, and the wavelength spectrum temperature coefficient of DFB laser diodes is usually 0.06nm/℃. For details, see the basics of fiber-coupled semiconductor lasers. The temperature range is generally 10~45℃. Taking the butterfly package as an example, pins 1 and 2 are thermistors to monitor the temperature of the laser tube, usually 10K-B3950 thermistors, which feed back to the ATC control system to drive the TEC cooling chip on pins 6 and 7 to control the temperature of the laser tube. , forward voltage cooling, negative voltage heating

3. APC control (automatic power control): The laser diode will age after a period of use, which will reduce the output optical power. APC control can ensure that the optical power is within a certain range, which not only prevents optical power from attenuating, but also prevents constant current circuit failures from causing damage to the laser tube due to excessive optical power.

Taking the butterfly package as an example, pins 4 and 5 are PD diodes, which are combined with a transimpedance amplifier as a photodetector to monitor the optical power of the laser diode. If the optical power decreases, increase the constant current driving current; otherwise, decrease the driving current.

Although both ATC and APC aim to stabilize the output optical power of the light source, they target different factors. APC targets the decrease in optical power caused by the aging of the light source device. APC ensures that the optical power remains as high as before. Stable output state, and ATC is for the power of the light source to rise and fall due to the influence of temperature. After passing the ATC, it is ensured that the light source still outputs a stable optical power.