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Introduction To Damage To Optical Components

Laser damage principle

1. Definition: Laser damage refers to the process by which the performance of optical materials or systems degrades due to the intensity of laser light.

2. Targets of Damage: Optical windows, functional materials, laser substrates, semiconductor materials, optical thin films, and other optical components or systems.

3. Manifestations:

 

                   

 

4. Types of damage:

 

    

 

5. Types of Laser-Related Materials Operations:

5.1 Thermal Effects → Leading to → Thermal Damage;

5.2 Mechanical Effects → Leading to → Mechanical Damage;

5.3 Nonlinear Effects → Leading to → Field-Induced Damage.

Generally, narrow-pulse (<30ns) lasers cause field-induced damage due to electric field breakdown, while long-pulse (>1ms) lasers exhibit thermal damage.

 

6. Factors Affecting Laser Damage:

6.1 Laser characteristics include: wavelength and bandwidth; single-pulse energy; peak power; pulse width; number of pulses; pulse repetition frequency; pulse interval; incident angle; polarization characteristics; transverse mode characteristics; spot size and shape; pre-treatment of the substrate and post-treatment of the film layer.

6.2 Target Objects: Substrate material, shape, and structure; surface film material and structure (including: refractive index, absorption and scattering, stress, adhesion, uniformity, defects); surface roughness; surface cleanliness; surface physicochemical properties.

6.3 Environmental Factors: Ambient temperature, humidity, and cleanliness; characteristics of the environmental medium.

 

7. Thin Film Laser Damage Mechanism:

Regarding the damage mechanism, the primary cause is thermal damage. Laser energy deposition occurs within the thin film, converting light energy into heat energy, thus raising the film temperature. Once the temperature exceeds the film's allowable limit, damage will occur. When the intrinsic absorption of the thin film is high, light energy can be directly converted into heat energy; however, when the intrinsic absorption is low, light energy cannot be directly converted and is insufficient to cause film damage. In this case, field damage, primarily caused by electron ionization and multiphoton absorption, will be the main factors leading to film damage.

Thin film laser-induced damage is related to three processes: the thin film absorbs energy from the laser beam; this absorbed energy is transported through localized absorption points; and due to the thermodynamic processes of the thin film material, heat concentrates in certain localized areas. When these localized areas exceed a certain critical temperature, film damage occurs. Thin film damage manifests in two main ways: first, damage to the thin film itself, manifested as ablation or detachment; and second, due to poor thermal conductivity of the substrate, the substrate may crack even before the thin film is destroyed. Thin film damage can occur in several ways:

Melting damage: Melting failure is the most fundamental damage mechanism for thin films. Melting failure occurs when the film temperature reaches its melting point. The melting point temperature of a thin film material is called its critical temperature for failure.

Thermal stress damage: Under laser irradiation, the thermal stress caused by the increase in film temperature is: =ET (1-2), where is the thermally induced stress (N/cm²), is Poisson's ratio, and is the linear expansion coefficient (C⁻¹). If the film temperature rises to this point, the surface adhesion force is overcome, and the film begins to detach, resulting in catastrophic damage. Kreidl and Rood recommend that the thermally induced stress should not exceed 1379 N/cm².

Thermal shock damage: If there is a large temperature difference between the first film pair and the substrate surface, it will cause film detachment and cracking, forming thermal shock damage to the film.

Defect damage: The defect absorption of a material is usually greater than its intrinsic absorption. Under laser irradiation, impurities absorb heat, causing a temperature rise. If the temperature reaches the material's melting point, it leads to melting and damage. Alternatively, the instantaneous high-temperature distribution of impurities under laser irradiation causes minute damage to the thin film. Under continuous laser irradiation, this damage accumulates until it causes significant laser damage. Soft damage to the film layer: Although there may be no visible morphological damage after laser irradiation, the optical properties are altered, affecting normal use.

 

8. Differences in laser damage resistance among different types of films:

AR film: Because the substrate surface is where the field strength peaks, the interface between the film layer and the substrate is most susceptible to laser damage. The surface smoothness of the substrate has a significant impact on laser damage. Therefore, for antireflective films, a highly smooth substrate with an appropriate inner coating and a high-refractive-index film layer with a higher damage threshold than the interface are required. Antireflective films are generally double-sided coated, with two types: front and back surfaces. The damage threshold of the back surface is generally 1.5 times lower than that of the front surface, mainly due to the standing wave electric field formed between the laser and the substrate on the back surface, which determines the damage threshold of the antireflective film.

High-reflectivity (HR) films: High-reflectivity films are generally more susceptible to surface damage; therefore, a protective film of half-wavelength thickness (or an integer multiple thereof) is often deposited on the surface. Under the same substrate and film material conditions, high-reflectivity films have the highest resistance to laser damage, followed by polarizing or partially reflective films, with anti-reflective films having the lowest.

 

9 typical

(1) HR laser mirror:Laser damage threshold: 15J/cm2@10ns pulse, 1064nm.

(2) Nd:YAG and Nd:glass laser high reflectors: >40 J/cm2 at 1064nm.

(3) Dual wavelength YAG coating: Damage threshold: 10J/cm2, 10ns pulse.

(4) Borosilicate and fused silica substrates narrowband anti-reflection coatings: LIDT>10MW/cm2 CW radiation at 1064 nm.

LIDT>20J/cm2 for fiber laser wavelengths on AR coating lens, at 1064 nm.

CVI Technical Optics Laser Damage Threshold Data：

    

    

    

* Multiple shot (200 shots) with 20Hz repetition rate.** Multiple shot(10 shots) with 0.2Hz repetition rate.

*** Multiple shot with 100Hz repetition rate.

**** Multiple shot(100 shots), LDT value is arithmetic average of lowest damage and highest survival power.

(5) High laser damage threshold:500MW/cm2 for Calcite;1GW/cm2 for-BBO.