Optical Coating Mirror for Infrared

I. Working Principle of Infrared OC Mirrors

Role of Optical Coatings: Infrared OC Mirrors typically achieve their specific optical properties by coating one or more layers of optical thin films on a substrate material. These films control light propagation through optical phenomena such as interference, reflection, and absorption. For example, by selecting appropriate materials and layer thicknesses, high reflection or high transmission of light within specific wavelength ranges can be achieved.

Characteristics in the Infrared Band: For Infrared OC Mirrors, the primary goal is to exhibit excellent optical performance in the infrared band. This may include high reflectivity, low absorption, and specific bandwidth characteristics. For instance, some Infrared OC Mirrors are designed for specific infrared wavelength ranges, such as 800-2532 nm, or achieve near-total reflection in specific infrared bands. An example is an omnidirectional mirror based on a Fibonacci-sequence quasi-periodic dielectric stack design, which achieves 99.5% reflectivity in the 1437-1618 nm infrared band, fully covering the telecom wavelength region and the S, C, and L bands of the erbium-doped fiber amplifier gain region.

II. Applications of Infrared OC Mirrors

Astronomical Observation: In astronomical observation, high-performance optical equipment is required to collect and analyze faint signals from celestial objects. Large space telescopes such as LUVOIR and HabEx, targeting observation ranges from far-ultraviolet to near-infrared, require advanced coating technologies to achieve efficient light collection, especially in the far-ultraviolet region (down to 90 nm). New and improved coatings are sought to protect aluminum from oxidation in normal environments, maintaining high reflectivity in the deep ultraviolet region without degrading performance in the visible and near-infrared regions.

Precision Laser Experiments: In many precision laser experiments, the performance requirements for Optical Components are extremely stringent, such as low optical absorption. Thermal noise from highly reflective mirror coatings is a major limiting factor in the sensitivity of many precision laser experiments. Research shows that amorphous silicon and silicon nitride can serve as alternatives to the currently used combination of silica and tantala coatings, with amorphous silicon's optical absorption at near-infrared wavelengths improved by approximately 55 times compared to the lowest reported values. Silicon-based coatings reduce thermal noise by a factor of 12 compared to silica and tantala at 20 K.

Solar Thermal Devices: In solar thermal devices, reducing heat loss is key to improving conversion efficiency. Adding infrared mirror coatings to the inner surface of glass in encapsulated high-vacuum insulated flat solar thermal panels can reflect radiation emitted by the absorber back to itself, enabling external photon recycling on the cold side, thereby reducing radiative losses and improving panel efficiency. Thermal modeling studies show that using infrared mirrors can achieve efficiency improvements of over 50% at operating temperatures above 300°C.

Optical Measurement Instruments: In optical measurement instruments, such as infrared thermal imaging optical lens group axial deviation measuring instruments, specific light sources and optical systems are required to achieve high-precision measurements. These systems include CO2 lasers, adjustable-focus telescopes, TGS pyroelectric thermal imagers, and computer-assisted data readout and processing, enabling axial deviation detection for 8-14 μm infrared optical lens groups. By changing the light source, they can also be used for measurements in 3-5 μm infrared optical systems.

Modern Optical and Optoelectronic Devices: In various modern optical and optoelectronic devices, optical mirror coatings with high reflectivity are required. For example, a reflective coating based on aluminum and copper alloys, covered with a lutetium oxide protective layer, can enhance reflectivity across the entire working spectral range, making it suitable for optical and optoelectronic devices in different spectral ranges.

III. Manufacturing Techniques for Infrared OC Mirrors

Material selection: one of the keys to manufacturing IR OC mirrors is to choose the right material. They are mainly divided into substrate materials and coating materials. Common substrate materials include silicon, germanium, zinc selenide, zinc sulfide, calcium fluoride, magnesium fluoride, sapphire, quartz, gallium arsenide and diamond. Common coating materials include aluminum, copper alloy, lutetium oxide, amorphous silicon, silicon nitride, silicon dioxide, and lithium tantalate. Different materials have different optical properties and physical characteristics, and the choice of material depends on the specific application requirements and operating bands. For example, in some applications it is necessary to select materials with high reflectivity in the infrared band, such as aluminum and copper alloys; in other applications it is necessary to select materials with low absorptivity, such as amorphous silicon and silicon nitride.

Coating Techniques: Common coating techniques include vacuum deposition and magnetron sputtering. For instance, a reflective coating based on aluminum and copper alloys, covered with a lutetium oxide protective layer, is manufactured using vacuum deposition technology (VU-1A apparatus), with layer thickness controlled by a quartz control system during deposition. An omnidirectional mirror coating is deposited and characterized using microwave-assisted DC magnetron sputtering technology, which allows precise control over coating performance.

IV. Performance Optimization of Infrared OC Mirrors

Design Optimization: Through rational optical design, the performance of Infrared OC Mirrors can be optimized. For example, a dual-detector miniature near-infrared spectrometer using a self-made integrated scanning grating micromirror as the core spectroscopic component avoids interference between different optical paths by utilizing the spatial layout of two focusing mirrors and two InGaAs single-tube detectors, enabling dual-channel simultaneous and independent operation. Additionally, bandpass filters with different cutoff wavelengths are placed in front of the dual-channel detectors to eliminate spectral overlap. A theoretical model is established using ray tracing to calculate the initial structural parameters of the Optical System, and ZEMAX is used to optimize the optical system design and provide optimized structural parameters. This instrument offers advantages such as broad spectrum, small size, and low cost.

Process Optimization: By optimizing coating process parameters such as temperature, pressure, and deposition rate, the performance of Infrared OC Mirrors can be improved. For example, studies show that heat treatment of amorphous silicon, final operation at low temperatures, and using a 2 μm wavelength instead of the more commonly used 1550 nm wavelength can improve the optical absorption of amorphous silicon at near-infrared wavelengths by approximately 55 times compared to the lowest reported values.

In summary, Infrared OC Mirrors have important applications in multiple fields. Ongoing research into their working principles, manufacturing techniques, and performance optimization provides strong support for the development of related areas.