What is the temperature resistance of optical filters and is their performance stable at high temperatures?

Optical filters, as the core element for controlling the wavelength of light in optical systems, are widely used in imaging, sensing, laser technology and other fields. Its performance depends not only on optical parameters such as transmittance and cutoff wavelength, but also closely related to the usage environment, among which temperature stability is a key indicator. In high-temperature environments such as industrial furnace temperature monitoring and optical sensing in car engine compartments, if the optical performance of the filter drifts, it may lead to system measurement errors, signal distortion, and even failure. This article will analyze the temperature resistance performance of optical filters from dimensions such as material characteristics, structural design, and failure mechanisms, evaluate their stability performance in high-temperature environments, and provide targeted selection and usage recommendations.

1、 The material and temperature resistance basis of optical filters

The temperature resistance of optical filters is mainly determined by the substrate material and coating layer, and the high temperature resistance limit and performance stability of different materials vary significantly

（1） Base material: the foundation for bearing and heat resistance

glass substrate

Ordinary optical glass (such as K9 glass): It can withstand high temperatures of about 300 ℃. Beyond this temperature, internal stress will be generated due to uneven thermal expansion coefficient (α ≈ 7 × 10 ⁻⁶/℃), resulting in substrate deformation or cracking. It is suitable for normal or medium temperature (<200 ℃) scenarios.

High temperature resistant glass (such as quartz glass, borosilicate glass):

Quartz glass (SiO ₂ purity>99.9%): can withstand high temperatures of over 1000 ℃, has an extremely low coefficient of thermal expansion (α ≈ 5 × 10 ⁻⁷/℃), and can maintain structural stability even under drastic temperature changes (such as rising from room temperature to 800 ℃), making it an optional substrate for high-temperature environments.

Borosilicate glass (containing B ₂ O ∝ 10% -15%): resistant to high temperatures of about 600 ℃, with a lower cost than quartz glass, suitable for medium to high temperature scenarios (such as automotive headlight optical systems, working temperature of 150-300 ℃).

Crystal substrate

Sapphire (Al ₂ O ∝): melting point 2050 ℃, excellent high temperature resistance, and high mechanical strength (Mohs hardness 9), suitable for high temperature and mechanical impact environments (such as aircraft engine flame monitoring). But the cost is high (5-10 times that of quartz glass), and the light transmission range is limited (only transmitting wavelengths of 200-4000nm).

Silicon (Si) and germanium (Ge) crystals: They can withstand high temperatures of about 600 ℃, but their surfaces may deteriorate due to oxidation at high temperatures (such as silicon oxidizing to form SiO ₂ above 400 ℃, which changes optical properties). They need to be used in conjunction with anti-oxidation coatings and are commonly used in infrared filters (infrared thermometers).

（2） Coating layer: the 'weak link' at high temperatures

The filtering function of optical filters mainly relies on surface coating (such as dielectric film, metal film), and the coating layer is the "short board" of high temperature resistance:

Dielectric film (commonly used SiO ₂, TiO ₂, Ta ₂ O ₅, etc.)

High temperature resistance: A single-layer SiO ₂ film can withstand 1000 ℃, but multi-layer films (such as high reflectivity films with alternating stacking of SiO ₂ and TiO ₂) may experience interlayer delamination at 300-500 ℃ due to differences in thermal expansion coefficients of different materials (such as TiO ₂'s α ≈ 9 × 10 ⁻⁶/℃, SiO ₂'s α ≈ 0.5 × 10 ⁻⁶/℃, significant stress is generated at a temperature difference of 100 ℃).

Performance change: The refractive index of the dielectric film will drift at high temperatures (such as TiO ₂ decreasing from 2.3 to 2.1 at 600 ℃), resulting in a shift in the center wavelength of the filter (0.5-2nm shift for every 100 ℃ increase).

Metal film (such as aluminum, silver, gold film)

Aluminum film: resistant to high temperatures of about 300 ℃, above which it will oxidize to form Al ₂ O ∝, resulting in a decrease in reflectivity (from 90% to 60%);

Silver film: high reflectivity (visible light region>95%), but poor high temperature resistance (melting at>200 ℃), and prone to sulfurization and deterioration;

Gold film: can withstand high temperatures up to 500 ℃, has good chemical stability (oxidation resistance, corrosion resistance), but is only suitable for the infrared band (strong absorption in the visible light region), commonly used in high-temperature infrared filters.

Composite film (dielectric metal dielectric)

Such as the "SiO ₂ - silver SiO ₂" composite film, the SiO ₂ protective layer can delay the oxidation of silver, improve the temperature resistance to 250 ℃, but still lower than the pure dielectric film, suitable for room temperature scenes with high reflectivity requirements (such as projector lenses).

2、 The influence of high temperature on the performance of optical filters: from parameter drift to structural failure

In high temperature environments, the core performance of optical filters (center wavelength, transmittance, cutoff depth) will change, and in severe cases, it can lead to functional failure, specifically manifested as:

（1） Drift of optical parameters

Center wavelength offset

The center wavelength of a filter (such as the peak wavelength of a bandpass filter) is closely related to the thermal expansion of the substrate and coating layer. For example, the center wavelength of a quartz based bandpass filter will shift towards longer wavelengths by about 1nm at 100 ℃; when it rises to 500 ℃, the offset can reach 5-8nm, which may exceed the matching range of the optical system (such as a laser radar filter wavelength shift>3nm, which can lead to a 50% decrease in signal reception efficiency).

Decrease in transmittance and cut-off depth

The microstructural changes in the coating layer at high temperatures, such as grain growth and increased porosity, can lead to enhanced light scattering and decreased transmittance. For example, after heating at 400 ℃ for 100 hours, the visible light transmittance of TiO ₂/SiO ₂ multilayer film decreased from 95% to 88%.

The cut-off depth (the degree of light attenuation in the cut-off band) deteriorates: After high-temperature oxidation, the metal film's ability to block stray light decreases. For example, after using an aluminum film filter at 350 ℃, the optical density (OD value) in the cut-off zone decreases from 4.0 (only passing through 0.01%) to 2.0 (passing through 1%), which cannot effectively filter out stray light.

（2） Risk of structural failure

Coating layer peeling and cracking

The stress generated by the mismatch of thermal expansion coefficients in multilayer films will gradually accumulate during repeated high and low temperature cycles (such as rising from room temperature to 500 ℃ and then cooling), resulting in network cracks or local peeling of the coating layer. Experimental data shows that after 50 cycles from 300 ℃ to room temperature, the peeling area of TiO ₂/SiO ₂ film can reach over 15%.

Base deformation and explosion

Ordinary glass substrates may explode due to uneven thermal conductivity when subjected to sudden heating (such as local exposure to high-temperature light sources), resulting in temperature stress (for example, K9 glass has a 30% probability of exploding at a temperature difference of 200 ℃). Even if it does not explode, small deformations of the substrate can cause optical path deviation, affecting system accuracy.

Chemical metamorphism

In high-temperature environments containing sulfur and water vapor (such as industrial kiln flue gas), chemical reactions may occur in the coating layer: for example, silver film reacts with sulfur to produce Ag ₂ S (black silver sulfide), resulting in a sharp decrease in transmittance; SiO ₂ film will slowly hydrolyze in high-temperature water vapor, generating loose SiO ₂ · nH ₂ O and damaging the film structure.

3、 Technical solution for improving the temperature resistance of optical filters

In response to the demand for high-temperature environments, it is necessary to improve the temperature resistance of filters from three aspects: material selection, structural design, and process optimization:

（1） Selection and matching of high-temperature resistant materials

Thermal matching design of substrate and coating layer

Choosing a material combination with a similar coefficient of thermal expansion, such as a quartz substrate (α ≈ 5 × 10 ⁻⁷/℃) paired with SiO ₂/Ta ₂ O ₅ coating (Ta ₂ O ₅ α ≈ 5 × 10 ⁻⁷/℃), can reduce interlayer stress. Experiments have shown that the delamination rate of this combination at 600 ℃ is only 1/5 of that of the traditional SiO ₂/TiO ₂ combination.

Application of High Temperature Stable Coating Materials

Using high-temperature resistant dielectric materials (such as HfO ₂, ZrO ₂) instead of TiO ₂, with a melting point exceeding 2700 ℃ and a refractive index change rate of<1% at 800 ℃, is suitable for ultra-high temperature scenarios (such as aircraft engine exhaust flame monitoring, temperature>600 ℃).

（2） Process optimization and structural improvement

Upgrading the coating process

Using ion beam sputtering (IBS) instead of traditional electron beam evaporation: Ion beam sputtering can make the coating layer denser (porosity<1%) and improve high temperature resistance by 30% (such as increasing the temperature limit of SiO ₂ film from 800 ℃ to 1000 ℃).

Introduction of annealing treatment: After coating, annealing in a vacuum environment of 400-600 ℃ for 2-4 hours can release the internal stress of the film layer and reduce the risk of cracking during high-temperature use (the stress of the film layer can be reduced by 50% after annealing).

Structural protection design

Installing a sapphire protective window: Attach a 0.5mm thick sapphire plate (resistant to high temperatures above 1000 ℃) to the surface of the filter to block direct erosion of the coating layer by high-temperature airflow, without affecting optical performance (sapphire has a transmittance of>85% in the 200-4000nm wavelength range).

Adopting a water-cooled structure: For filters that are continuously exposed to high temperatures (such as>800 ℃), a water-cooled jacket is designed (with circulating water temperature controlled below 50 ℃) to reduce the actual operating temperature of the filter through thermal conduction (which can make the filter temperature 300-500 ℃ lower than the ambient temperature).