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With the continuous penetration of infrared technology in security, medical, industrial, autonomous driving and other fields, multi-band sensing systems (such as SWIR + MWIR, 850nm + 1550nm and other combinations) are becoming the core trend of the new generation of sensor modules. However, in the actual landing process, the optical subsystem often becomes one of the "bottlenecks" of the performance of the whole machine, especially the natural conflict between the spectral response of the lens material, the surface design and the coating process.

From an engineering point of view, this paper analyzes three core technical challenges around the current mainstream polymer infrared lens system.

First, the transmittance performance of polymer materials in the infrared multi-band When designing a multi-band infrared system, the first consideration is not the structure, but the material.

Different plastic materials have great differences in transmittance, refractive index, and absorption peak position in the near-infrared to mid-infrared (700nm~ 14μm) band. For example:

Material Transmittance Band Range 940nm Transmittance 1550nm Transmittance Refractive Index (940nm) Absorption Peak HDPE2~ 12μm1400nmZeonex E48R400~ 1550nm > 90% > 85%~ 1.53 Stable COP/COC Modified Materials 400~ 1650nm85~ 95% Adjustable Medium Absorption Peak

Summary:

Common PMMA and HDPE materials in the market, although the processing is stable, the high-band transmittance is poor, making it difficult to use in imaging/ranging systems above 1550nm.

Zeonex, Cyclo Olefin Copolymer (COC) materials are recommended, which have better transmission performance in the NIR/SWIR segment and support high refr

In actual projects, we usually need to do "material pre-screening + actual spectral testing" based on customer band requirements to ensure the efficiency of the lens system.

2. Geometric limitations of non-spherical/free-form surfaces in large FOV infrared systems Infrared sensor modules gradually evolve from small angle center detection to "full field of view coverage + edge compensation", which puts forward higher requirements for the lens structure. Aubor summarizes the following trends through Zemax and CodeV joint simulation:

Traditional spherical Fresnel structure: easy to injection molding, serious distortion of edge light spots, and serious uneven energy distribution after FOV > 90.

Aspherical design: supports more uniform illumination and lower MTF loss, suitable for 65~ 100 application scenarios; but the manufacturing tolerance requirements are improved, and the mold needs to support sub-micron accuracy.

Freeform Optics: The field of view distribution can be customized according to the layout of the area array detector, but the difficulty lies in:

Designing asymmetric surface requires multi-axis CNC or diamond turning;

Surface shape detection can only rely on interferometer + surface profiler composite analysis;

Warpage control difficulty is significantly improved during multi-cavity molding.

Example:
In a ToF + PIR dual-mode detection system, we designed a 135 wide-angle freeform structure lens for a customer. The FWHM deviation was 23% when using standard PMMA. After replacing with COC material, the thermal stability of the system was improved by 47% and the MTF uniformity was improved by 18.6%.

3. Compatibility limitations of the coating process in a wide-band system Infrared lenses need to support two bands at the same time (such as 940nm + 1550nm), and need to make a trade-off in spectral reflection control:

Single-layer AR: Low cost, but only one main band can be optimized, such as the center lambda = 940nm.

Multi-layer broadband AR: The width of the band can be expanded, but the number of layers (> 6 layers) is prone to stress fission, especially on high-temperature injection molded substrates.

Nanostructure anti-reflection (Moth-eye): Ideal solution, but it has extremely high requirements for injection molding accuracy and post-treatment film protection, and has not yet been popularized in large-scale lenses.

Aubor Coping strategy:

Customize the coating curve according to the customer's band priority, using the "dual-band compatibility + weight tuning" method.

In the ToF project, the 940nm main-band AR coating is superimposed with bandpass filter support; in the laser communication (1550nm) project, the insertion loss is preferentially controlled < 0.5dB.

All infrared coating processes are detected by the AOI automatic detection system for film thickness deviation < 5%.

Summary and advice For engineers and product developers, we recommend that when starting any mid-to-far infrared project, work from the following three front perspectives:

Material first: Test the actual spectral response of the material in the target band to avoid "reasonable design but material absorption" resulting in wasted performance.

Surface segmentation synergy: Especially for ultra-wide-angle and multi-focal systems, non-spherical or free-form surface structure modeling should be given priority, and its dependence on tolerances should be estimated.

Spectral coating strategy integration: Incorporate coating design into the early planning of structures and systems to avoid cost and schedule risks caused by later adjustments.

Aubor is committed to being a co-developer of polymer infrared lenses and micro optical systems. We not only make lenses, but also participate more deeply in the "early optical decisions" of customer systems.

If you have infrared system development needs involving bands such as 850nm/940nm/1550nm/1650nm, we welcome in-depth technical dialogue between engineers.