Application of aluminum mirror in infrared field

For future space-based infrared astronomical coronagraphic observations, aluminum mirrors are employed in coronagraphs. Broadband mid-infrared observations in space require cooled reflective optics, while coronagraphy demands high-precision Optical Components. For example, the coronagraph initially proposed for the next-generation infrared astronomical satellite project SPICA (SCI: SPICA Coronagraph Instrument) involved the fabrication and evaluation of an Optical System comprising high-precision aluminum off-axis mirrors with diamond-turned surfaces. A coronagraphic optical demonstration experiment with a coronagraph mask was conducted. First, the wavefront error (WFE) of the aluminum mirrors was measured using a He-Ne Fizeau interferometer to confirm that the power spectral density of the WFE met SCI requirements. Subsequently, the mirrors were integrated into the optical system, and the overall performance of the system was evaluated. The total WFE of the optical components was estimated to be 33 nm (rms), with each mirror contributing 10–20 nm (rms) to the central 14 mm region of the optical component. A contrast of 10−5.410−5.4 was achieved for the coronagraph in visible light. Based on model calculations and measured optical performance, the coronagraphic imaging system is projected to achieve a contrast of approximately 10−710−7 at a wavelength of 5 µm.

Application in the ARIEL Mission:
The ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission describes the design, analysis, and development of a 1-meter-diameter aluminum prototype mirror for its telescope. The European Space Agency (ESA) has selected ARIEL as its next medium-class science mission (M4), scheduled for launch in 2028. The mission aims to study the atmospheres of selected exoplanets. The payload is based on a 1-meter-class telescope preceded by a suite of instruments. The telescope configuration is defined as a classical Cassegrain design with an eccentric pupil, two-mirror layout, and a three-axis off-axis parabolic mirror. A trade-off analysis was conducted for materials to fabricate the 1-meter-diameter primary mirror (M1), and aluminum alloy was selected as the baseline material for both the telescope mirrors and structure. Today, metals such as aluminum alloys are frequently considered for manufacturing space telescopes operating in the infrared wavelength range. Producing large aluminum mirrors like those for ARIEL is challenging, and dedicated research and development programs have been initiated to demonstrate feasibility. A prototype mirror, identical in size to the M1 flight model but with a simpler surface profile, has been fabricated and tested.

Applications in Future Infrared Astronomical Satellites:

Cooled Optics for Space Infrared Missions:
For space infrared missions, cooling the entire instrument is critical to suppress infrared background and detector noise. In this context, aluminum is suitable for cryogenic optics because its excellent machinability allows the same material to be used for the entire instrument structure, including optical components, which helps mitigate optical misalignment at low temperatures. Aluminum mirrors were fabricated via ultra-precision machining, and their wavefront error (WFE) was measured using a Fizeau interferometer. Based on the power spectral density of the WFE, the surface accuracy of all mirrors was confirmed to meet the requirements of the SPICA Coronagraph Instrument. The mirrors were then integrated into the optical system, and the system’s image quality was inspected using an optical laser. The total WFE was estimated to be 33 nm (rms) based on the Strehl ratio, consistent with WFE values derived from individual mirror measurements.

Applications in Mid-Infrared Cryogenic Optics:

Deformation Constraints and Corrosion Protection:
In mid-infrared instruments, gold-coated aluminum mirrors are used for cryogenic optics. To evaluate thermal contraction-induced deformation of aluminum mirrors, surface monitoring measurements were performed during cooling cycles from room temperature to 100 K. Results showed that deformation effects were reduced to one-fourth when the mirrors were secured with spring washers. An effective method to prevent electrochemical corrosion of the mirrors was also explored. Multiple samples were prepared by varying coating conditions, such as inserting insulating layers, forming multilayer moisture-blocking coatings, or performing precision cleaning prior to coating. Precision cleaning before depositing the gold layer and covering it with an SiO protective layer proved effective in inhibiting aluminum corrosion. SiO-overcoated mirrors survived cooling tests for mid-infrared applications, exhibiting a reflectance reduction of approximately 1% in the 6–25 µm range compared to uncoated gold-plated mirrors.

Applications in Infrared Laser Optics:

Fabrication of Laser-Durable and Environmentally Stable Dielectric-Enhanced IR Mirrors:
HfO22​/SiO22​ multilayers were deposited on single-point diamond-turned aluminum substrates via modified reactive plasma ion-assisted deposition to form laser-durable and environmentally stable dielectric-enhanced IR mirrors at a wavelength of 1064 nm. The impact of the surface quality of diamond-turned aluminum on the optical performance of the dielectric-enhanced mirrors was evaluated. A laser-induced damage threshold (LIDT) of up to 11 J/cm22 was achieved for the enhanced aluminum mirror tested in pulsed mode at 1064 nm with a pulse duration of 20 ns and a repetition rate of 20 Hz. Laser damage morphology was revealed using scanning electron microscopy (SEM). The damage mechanism was attributed to nodule defects caused by particles embedded in the aluminum substrate surface.