Why does the laser power decay?

Laser power attenuation is a common phenomenon, which is usually the result of a variety of factors. These causes can be divided into two broad categories: inevitable inherent attenuation and external factors that can be slowed or avoided. The following are the main causes of laser power decay, arranged from primary to secondary:

Aging and damage (inherent attenuation) of laser core components

This is the root cause of power decay, especially for high power lasers.

1. Pump source attenuation (taking semiconductor laser LD as an example)

Working principle: LD is the “engine” of most solid-state lasers and fiber lasers “.

Attenuation mechanism:

Defect growth: The lattice defects inside the LD chip will continue to grow and multiply under long-term electrical and thermal stress, resulting in an increase in non-radiative recombination and a decrease in electro-optical conversion efficiency.

Optical catastrophic damage of cavity surface: under extremely high power density, tiny pollution or defects of the output cavity surface will absorb laser light, causing local heating and causing cavity surface melting, which is an irreversible avalanche process.

Electrode metallization degradation: the contact between the electrode and the semiconductor material will undergo intermetallic diffusion and oxidation at high temperature, resulting in increased contact resistance and more serious heating.

Performance: The threshold current of the pump source will gradually increase, and the slope efficiency will decrease, resulting in a decrease in output power.

2. Gain medium performance degradation

Solid/fiber lasers (e. g. YAG, YV04 crystals, Yb/Er doped fiber):

Color center formation: Under long-term irradiation of pump light (especially ult

raviolet components), “color centers” (absorption centers) will be generated inside the crystal or glass fiber. These color cores absorb the pump light and the generated laser light, converting it into heat energy instead of being used for stimulated radiation.

Thermal lens effect and thermal stress damage: Although the thermal lens effect itself is reversible, long-term, severe temperature cycling and thermal stress can cause microcracks in the crystal, permanent changes in refractive index, or darkening of the fiber.

Gas laser (such as cO2 laser): The gas mixture will gradually decompose, be adsorbed by the electrode or react with the inner wall material of the tube, resulting in the imbalance of the working gas ratio and the decrease of the gain.

3. Optical element damage

Resonant cavity lens/window: the full mirror, output mirror and window inside the laser are the places where the power is most concentrated.

Film damage: high-energy laser will make the lens on the antireflection film or high anti-film ablation, cracking or off.

Pollution: Small organic matter in the environment (such as oil vapor) will adhere to the surface of the lens, carbonized under the action of the laser, forming a permanent absorption point, causing further damage.

Material absorption: Even high-quality optical materials have a small intrinsic absorption of laser light at a specific wavelength. Long-term effects can lead to slow changes in material properties.

External conditions of use and improper operation

These factors can significantly accelerate the aging process of the core components described above.

1. Thermal management failure

The efficiency of the cooling system decreases: the water quality of the cooling water deteriorates and causes the waterway to scale and block; the failure of the water cooler causes the water temperature to be unstable or too high; the dust deposition of the fan causes poor heat dissipation.

Consequences: The increase in the operating temperature of the laser will greatly accelerate the aging rate of all semiconductors and optical components. For LD, the lifetime may be reduced by half for every 10°C increase in junction temperature (following the Arrhenius model).

2. Working environment and pollution

Dust and pollutants: dust, smoke, oil, etc. in the air enter the laser, which will pollute the optical surface and circuit, causing local heat absorption, short circuit or arc.

Vibration and shock: Unstable installation foundation or external vibration will cause the alignment of optical components to deviate from the optimal state, reduce the output efficiency, and may cause the mechanical structure to loosen in severe cases.

Humidity: Excessive humidity will cause condensation on the surface of the lens, which can easily cause the film to burst under laser irradiation. At the same time, humidity can also corrode electronic components.

3. Improper mode of operation

Frequent hard switching: Frequent instantaneous full power on and off of the laser will bring huge electrical and thermal impact to the pump source and power system, shortening its life.

Long-term operation under the limit parameters: Let the laser continue to run at more than 90% of the nominal maximum power or even overload, which will make its core components in a high stress state and accelerate aging.

Mismatched Loads: For some types of lasers, a load-reflectivity mismatch can cause some of the energy to be reflected back into the cavity, causing instability or even damage.

Degradation of power and control systems

The performance of the drive power supply is degraded: the output current/voltage stability of the power supply becomes worse, and the ripple increases, which cannot provide a stable and pure drive for the pump source, affecting its output performance and life.

Aging of control circuit: The parameters of capacitors, resistors and other components drift with time and temperature, resulting in a decrease in the accuracy of the control system.

Summary and preventive measures

The power decay of a laser is a process that cannot be completely avoided but can be effectively managed and slowed down.

In order to delay the power decay to the greatest extent and maintain the stable operation of the laser, it is recommended that:

1. Provide excellent working environment: keep clean, constant temperature, low humidity, no vibration.

2. Ensure efficient thermal management: regularly maintain the cooling system to ensure water quality and flow.

3. Follow the standard operation process: avoid frequent switching and long-term operation under extreme conditions.

4. Perform regular preventive maintenance: regularly check the optical components, clean the optical path, and replace consumables (such as gas and filter elements) according to the manufacturer’s requirements.

5. Use a stable power supply: ensure that the power supply voltage is stable, and use a voltage stabilizer when necessary.

By understanding these attenuation mechanisms and taking appropriate precautions, the life of the laser can be significantly extended and the output performance maintained over the life cycle is more stable.