Basic working principle of laser diode

- 2021-07-14-

Before we talk about laser diode, we should first understand stimulated radiation. There are three radiation processes in the light radiation,

I: the spontaneous transition from high energy state to low energy state is called spontaneous emission;

II: it is the transition of particles in high energy state to low energy state under the excitation of external light, which is called stimulated radiation;

III: it is the transition from the energy of external light absorbed by the particles in the low energy state to the high energy state, which is called stimulated absorption.

Spontaneous emission: even if two particles transition from a certain high-energy state to a low-energy state at the same time, the phase, polarization state and emission direction of the light emitted by them may be different, but the stimulated emission is different. When the particles in the high-energy state are excited by foreign photons, they transition to a low-energy state, and emit light that is exactly the same as the foreign photons in terms of frequency, phase and polarization state.

In the laser, the radiation is stimulated radiation, and the laser emitted by it is exactly the same in frequency, phase, polarization state and so on.

There are both stimulated radiation and stimulated absorption in any stimulated luminescence system. Only when the stimulated radiation is dominant, the external light can be amplified to emit laser. However, in general light sources, stimulated absorption is dominant. Only when the equilibrium state of particles is broken and the number of particles in high-energy state is larger than that in low-energy state (this situation is called ion number inversion) can laser be emitted.

Principle and structure of laser diode

The physical structure of the laser diode is to place a layer of semiconductor with photoactivity between the junctions of the light-emitting diode. After polishing, the end face of the semiconductor has partial reflection function, thus forming an optical resonant cavity.

In the case of forward bias, the LED junction emits light and interacts with the optical resonator, which further excites the single wavelength light emitted from the junction. The physical properties of the light are related to the material.

Principle of laser diode -- working principle

The crystal diode is a p-n junction formed by p-type semiconductor and n-type semiconductor. A space charge layer is formed on both sides of the interface, and a self built laser diode electric field is built.

When there is no applied voltage, the diffusion current caused by the carrier concentration difference on both sides of the p-n junction is equal to the drift current caused by the self built electric field, so it is in an electrical equilibrium state.

When there is a positive voltage bias, the external electric field and the self built electric field suppress each other and increase the carrier diffusion current, resulting in the positive current.

When there is a reverse voltage bias, the external electric field and the self built electric field are further strengthened to form a reverse saturation current I0 which is independent of the reverse bias voltage in a certain range of reverse voltage.

When the applied reverse voltage is high to a certain extent, the electric field strength in the space charge layer of p-n junction reaches the critical value, resulting in the multiplication process of carriers, a large number of electron hole pairs and a large number of reverse breakdown current, which is called diode breakdown phenomenon.

Principle of laser diode -- how to detect

I: resistance measurement method: remove the laser diode and use the multimeter R × 1K or R × The forward and reverse resistance values are measured at 10K gear. Under normal condition, the forward resistance is 20 ~ 40K Ω, and the reverse resistance is ∞ (infinity). If the measured forward resistance value has exceeded 50K Ω, the performance of the laser diode has decreased. If the measured forward resistance is greater than 90K Ω, the diode is seriously aged and cannot be used any more.

II: current measurement method: use a multimeter to measure the voltage drop at both ends of the load resistor in the laser diode driving circuit, and then estimate the current value through the tube according to Ohm's law. When the current exceeds 100mA, if the laser power potentiometer is adjusted and the current has no obvious change, the serious aging of the laser diode can be judged. If the current increases sharply and is out of control, the optical resonator of the laser diode is damaged.