Infrared Optical System Structure
Traditional infrared optical structures can be divided into three types: refractive, reflective, and catadioptric. Modern new infrared optical structures can be divided into three types: refractive-diffractive hybrid system, off-axis three-mirror system, and dual-FOV optical system.
1、Traditional infrared optical system——refractive
The refractive optical system can be composed of a single lens or multiple lenses, with the lens materials primarily including germanium, silicon, zinc selenide, and chalcogenide glass. By employing three-stage imaging in refractive systems, compact optical system can be designed.
Advantages: stable image quality, low stray light, large FOV
Disadvantages: relatively small aperture, narrow working band, and aberrations
Applications: Suitable for infrared search and tracking systems for pan-tilt search in larger spaces, optoelectronic turrets, infrared pods, infrared seekers and other systems.
2、Traditional infrared optical system——reflective
Reflective optical system consists of a primary mirror and a secondary mirror. The secondary mirror is convex, called Cassegrain system, the secondary mirror is concave, called Gregorian system. Compared to refractive structures, reflective infrared optical system requires fewer optical elements and uses the principle of mirror reflection to fold the light path.
Advantages: no chromatic aberration, fewer elements, light weight, wide working band, easy to eliminate thermal difference, etc.
Disadvantages: complex processing and assembly, small FOV, weak aberration correction ability, can only eliminate two aberrations at the same time (such as eliminating spherical aberration and coma at the same time)
Applications: radiometers, forward-looking infrared imagers, lidar, large aperture telescopes, and missile guidance systems, etc.
3、Traditional infrared optical system——catadioptric
The catadioptric optical system is based on spherical mirrors combined with refractive elements. Since the primary and secondary mirrors in the catadioptric optical system share most of the optical focal power, it is conducive to athermalized design. In addition, the folded optical path reduces the lens size and mass, making the system length shorter than its focal length.
Advantages: compact structure, relatively large aperture, long focal length, good image quality, conducive to athermalization, etc.
Disadvantages: obstruction in the center, complex inspection and adjustment
Application: can be used for large- aperture, long focal-length infrared optical systems, such as the anti-interference design of infrared seekers, tank target tracking and identification infrared optical systems, etc.
4、Modern new infrared optical system——refractive-diffractive hybrid system
The refractive-diffractive hybrid system is an optical system that combines diffractive optical elements with refractive optical elements. Diffractive optical elements（DOE）are fabricated using micro-nano etching techniques on a lens surface to etch a binary optical pattern, creating a two-dimensional distribution of diffractive units, forming a dual-surface diffractive-refractive lens..
Advantages: small size, light weight, high design freedom, many optimization variables, good elimination of thermal and chromatic aberration
Application: The dual-band zoom optical system is designed to achieve zoom in different bands, providing greater flexibility and application range, and is suitable for scanning imaging and forward-looking infrared systems (FLIR).
※DOE can usually be divided into beam shaping, beam splitting, structured light, multi-focus, and other special beam generation types.
5、Modern new infrared optical system——off-axis three-mirror system
The design of off-axis three-mirror optical systems is based on Gaussian optical theory. The design starts with a coaxial three-mirror system and achieves a central obstruction-free system by off-aixs of the aperture or field of view, or a combination of the two.
There are two main methods to solve the center blocking: placing the aperture on the secondary mirror and tilting the field to avoid the obstruction, where the aperture remains on-axis.; or placing the aperture on the primary mirror, where the aperture is off-axis.
Advantages: no chromatic aberration, no secondary spectrum, wide working band, easy to achieve large aperture, large FOV, good thermal resistance performance, simple structure, lightweight, effectively address the issue of central obstruction and offer many optimization variables, improving both the FOV and image quality
Applications: space exploration, ground-based remote sensing, aviation, aerospace, lighting, display and other fields
6、Modern new infrared optical system——dual-FOV system
There are two mainstream design methods for infrared dual-FOV imaging systems: switchable-type (insertable) and axial motion.
(1) Switchable-type (insertable) optical system: different FOV systems will switch different zoom lens groups to change the focal length of the optical system. With the increasing application of diffraction elements, diffraction elements can be introduced into infrared dual-FOV optical systems to simplify the system structure, reduce weight and make the whole system more compact. Lightweight switchable-type zoom systems offer diverse configurations, including transmissive, coaxial reflective, and off-axis reflective components. To meet the demands of high-precision and high-speed zooming, high-speed rotary motors and mechanical limiters are used to achieve the insertion and removal of the optical lens groups.
Advantages: Rapid field-of-view switching, higher optical axis accuracy, and better transmission
Disadvantages: Larger lateral dimensions, leading to a less compact overall structure, and lower optical axis consistency
（2）Axial motion optical system: the system changes its focal length by adjusting the axial spacing of lens groups. When the motion of component is in the solid-line position, the system is in a wide FOV mode with a short effective focal length. Conversely, when in the dotted-line position, the system is in a narrow field of view mode with a long effective focal length.
Advantages: compact and simple optical system, stable optical axis consistency
Disadvantages: image plane blurring during zooming, easy to lose tracking object
The dual-FOV system is actually a two-speed zoom system with a fast switching of the FOV. In the short-focus position, the large FOV can search for targets in the scene; in the long-focus position, the small FOV can capture and observe targets.
Applications: Suitable for dual-band surveillance, forward-looking infrared systems(FLIR), target detection, and tracking. In practical applications, the axial motion dual-FOV system is widely adopted due to its good imaging quality and applicability.
Traditional infrared optical systems share the common characteristic of simple structures. With the continuous development of infrared systems, new and higher requirements such as large aperture, long focal length, high spatial resolution, and athermalization have been introduced, revealing the limitations of traditional systems.
Modern new infrared optical systems incorporate new technologies such as binary optics, adaptive optics, the rational use of aspherical or diffractive surfaces, or changing the existing system structure. It is greatly significant to select the appropriate new infrared optical system structure according to different technical requirements in the future.